EPA
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, Ohio 45268
EPA/625/8-89/016
Sept. 1989
Technology Transfer
Summary Report
In-Vessel Composting
of Municipal Wastewater
Sludge
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Technology Transfer
EPA/625/8-89/016
Summary Report
InA/essel Composting
of Municipal Wastewater
Sludge
September 1989
This report was developed by the
Risk Reduction Engineering Laboratory
Center for Environmental R^seaffch information
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268.
Printed on Recycled Paper
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NOTICE
This report has been reviewed by the US. Environmental Protection Agency and approved for publication. Mention
of trade names or commercial products does not constitute endorsement or recommendation for use.
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ACKNOWLEDGMENTS
The US. Environmental Protection Agency's (EPA's) Risk Reduction Engineering Laboratory (RREL) directed Camp
Dresser & McKee, Inc., Boston, Massachusetts, to conduct an in-depth survey of eight in-vessel composting facilities.
The case studies and other information developed from these facility surveys form the bases of this summary report.
Donald Brown of EPA-RREL provided overall direction, management, and input to the project. He was assisted by Dr.
James Smith, Jr. of EPA's Center for Environmental Research Information (CERI). John Donovan, John Johnston, and
Dr. Albert Pincince of Camp Dresser & McKee, Inc. developed and prepared the information for the report. Technical
writing and editorial work was provided by Leslie Beyer, Carol Merrill, Susan Richmond, and Heidi Schultz of Eastern
Research Group, Inc., Arlington, Massachusetts. Dr. Joel Alpert, E & A Environmental Consultants, provided technical
assistance. A panel of peer reviewers assisted EPA in defining the project scope and content and provided technical
review of the final report. The peer reviewers were:
Dr. Roger T Haug, Roger T Haug, Engineers, Torrence, California
Charles Murray, Washington Suburban Sanitary Commission, Silver Spring, Maryland
Dr. John Walker, EPA Office of Municipal Pollution Control (OMPC), Washington, D.C.
EPA gratefully acknowledges the assistance of the following public agency officials:
Akron, Ohio — Eric Exley (Fairfield Service Company), Terry Leslie (City of Akron), Dennis Meek (Burgess and
Niple, Limited)
Cape May County Municipal Utilities Authority, New Jersey — William Cathcart, Michael Riccio
Clayton County Water Authority, Georgia — Jeff Brandon, Melvin Newman
City of Newberg, Oregon — Robert Thompson
City of Pittsburgh, New York — George Miller, David Powell
City of Portland, Oregon — Gene Appel, Daniel Clark, Greg Hettman
City of Sarasota, Florida — Gilbert Fernandez, Ken Sosville, Douglas Taylor
City of Schenectady, New York — Douglas Connor
EPA also acknowledges the assistance of the operators and staff of the eight composting plants surveyed, and John
Laurenson, Jr., American Bio Tech; John O'Brien, Ashbrook-Simon-Hartley; Richard Gossett, Compost Systems
Company; Ake Norin, Purac Engineering; and Ronald Dye, Taulman Composting Systems in providing the information
contained in the document. The assistance of Richard Kuchenrither and other members of the Kansas City Black and
Veatch staff in planning the study and reviewing the report is also acknowledged.
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TABLE OF CONTENTS
NOTICE
ACKNOWLEDGMENTS
LIST OF FIGURES
LIST OF TABLES
CHAPTER ONE INTRODUCTION
1.1 Purpose and Scope
1.2 Organization of the Report
1.3 In-Vessel Composting
1.3.1 Materials
1.3.2 Materials Handling
1.3.3 Reactor System
1.3.4 Aeration System
1.3.5 Odor Control System
1.3.6 Exterior Curing/Storage Facilities
1.3.7 Marketing
CHAPTER TWO PROJECT PLANNING CONSIDERATIONS
2.1 Choosing In-Vessel Composting
2.2 Siting
2.2.1 Siting to Minimize Odor Effects
2.2.2 Siting at a Wastewater Treatment Plant
2.3 Public Participation
2.3.1 Involving the Public in Planning
2.3.2 Public Relations Programs
2.4 Procurement
2.4.1 Project Engineering
2.4.2 Prequalification of System Suppliers
2.4.3 The Request for Proposal
2.5 The Budget
2.5.1 Capital Costs
2.5.2 Operating and Maintenance Costs
2.5.3 Revenues from Product Sales
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2.6 Marketing
2.6.1 Identifying and Quantifying Compost Product Markets
2.6.2 Compliance with State Regulations
2.6.3 Choosing a Target Compost Market
2.6.4 Quality Criteria Required by Potential Users
2.6.5 Marketing Agents
2.6.6 Contingency Plans
2.7 Pilot Plants
2.8 References
CHAPTER THREE DESIGN CONSIDERATIONS
3.1 General System Considerations
3.1.1 Environmental Conditions
3.1.2 Operating Conditions
3.1.3 Equipment Access
3.2 Materials Handling
3.2.1 General System Considerations
3.2.2 Materials Receiving and Storage
3.2.3 Conveyor Systems
3.2.4 Mixers
3.2.5 Reactor Discharge Devices
3.3 Aeration
3.3.1 General System Considerations
3.3.2 Air Collection/Moisture Removal
3.3.3 Hardpan
3.3.4 Curing/Storage Aeration
3.4 Odor Control
3.4.1 Compost Process Control
3.4.2 Inventory of Potential Sources
3.4.3 Collection and Containment
3.4.4 Treatment
3.4.5 Dispersal
3.4.6 Additional Considerations
3.5 Process Monitoring and Control
3.5.1 Analytical Equipment for Process Control
3.5.2 Process Sensors
3.5.3 Instrumentation
3.6 Support Facilities
3.6.1 Maintenance Work Area
3.6.2 Onsite Spare Parts Storage
3.6.3 Staff Facilities
3.7 Fire Prevention and Control
3.8 References
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CHAPTER FOUR OPERATIONS CONSIDERATIONS
4.1 Startup Issues
4.1.1 Performance Test Plan
4.1.2 Conducting the Performance Test
4.1.3 Additional Personnel
4.2 Operating Procedures
4.2.1 System Supplier Specifications
4.2.2 Monitoring and Recording Process Variables
4.2.3 Monitoring Product Quality
4.2.4 Monitoring Feed Material Characteristics
4.2.5 Impact of Wastewater Treatment Plant Operations
4.3 Fire Prevention
4.3.1 Fire Detection
4.3.2 Fire Suppression
4.4 Staffing Issues
4.4.1 Operations Staff
4.4.2 Operator Training
4.4.3 Staff Rotations
4.4.4 Acquiring Necessary Maintenance and Repair Skills
4.4.5 Assigning Regular Maintenance Staff
4.5 Maintenance Issues
4.5.1 Preventive Maintenance
4.5.2 Operations and Maintenance Manual
4.5.3 Spare Parts Inventory
CHAPTER FIVE INTRODUCTION TO THE CASE STUDIES
5.1 Background
5.2 Accuracy of the Data
5.3 Nomenclature
5.4 Summary Data
5.4.1 Materials
5.4.2 Mixing
5.4.3 Materials-Handling Systems
5.4.4 Aeration
5.4.5 Odor Control Systems
5.4.6 Exterior Curing/Storage
CHAPTER SIX AKRON, OHIO
6.1 Introduction
6.2 Operating History
6.2.1 Procurement and Construction
6.2.2 Capital Costs
6.2.3 Operating History and Current Status
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6.3 Description of the Plant
6.3.1 Systems Overview
6.3.2 Feed and Mix Characteristics
6.3.3 Materials Handling
6.3.4 Reactor
6.3.5 Exterior Curing/Storage
6.3.6 Nonprocess Air Handling
6.3.7 Odor
6.3.8 Support Facilities
6.4 Monitoring and Performance
6.4.1 Reactor Control Strategy
6.4.2 Product Quality
6.4.3 Mass Balance and Reactor Detention Time
6.5 Operations
6.5.1 Staffing
6.5.2 Marketing and Distribution
6.5.3 Operating Costs
6.6 Update
CHAPTER SEVEN CAPE MAY, NEW JERSEY
7.1 Introduction
7.2 History of the Plant
7.2.1 Procurement and Construction
7.2.2 Capital Costs
7.2.3 Operating History and Current Status
7.3 Description of the Plant
7.3.1 Systems Overview
7.3.2 Feed and Mix Characteristics
7.3.3 Materials Handling
7.3.4 Reactors
7.3.5 Exterior Curing/Storage
7.3.6 Nonprocess Air Handling
7.3.7 Odor
7.3.8 Support Facilities
7.4 Monitoring and Performance
7.4.1 Reactor Control Strategy
7.4.2 Mass Balance and Reactor Detention Time
7.4.3 Product Quality
7.5 Operations
7.5.1 Staffing
7.5.2 Marketing and Distribution
7.5.3 Operating Costs
7.6 Update
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CHAPTER EIGHT CLAYTON COUNTY, GEORGIA
8.1 Introduction
8.2 History of the Plant
8.2.1 Procurement and Construction
8.2.2 Capital Costs
8.2.3 Operating History and Current Status
8,3 Description of the Plant
8.3.1 Systems Overview
8.3.2 Feed and Mix Characteristics
8.3.3 Materials Handling
8.3.4 Reactors
8.3.5 Exterior Curing/Storage
8.3.6 Nonprocess Air Handling
8.3.7 Odor
8.3.8 Support Facilities
8.4 Monitoring and Performance
8.4.1 Reactor Control Strategy
8.4.2 Mass Balance and Reactor Detention Time
8.4.3 Product Quality
8.5 Operations
8.5.1 Staffing
8.5.2 Marketing and Distribution
8.5.3 Operating Costs
8.6 Update
CHAPTER NINE NEWBERG, OREGON
9.1 Introduction
9.2 History of the Plant
9.2.1 Procurement
9.2.2 Capital Costs
9.2.3 Operating History and Current Status
9.3 Description of the Plant
9.3.1 Systems Overview
9.3.2 Feed and Mix Characteristics
9.3.3 Materials Handling
9.3.4 Reactor
9.3.5 Exterior Curing/Storage
9.3.6 Nonprocess Air Handling
9.3.7 Odor
9.3.8 Support Facilities
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9.4 Monitoring and Performance
9.4.1 Reactor Control Strategy
9.4.2 Mass Balance
9.4.3 Product Quality
9.5 Operations
9.5.1 Staffing
9.5.2 Marketing and Distribution
9.5.3 Operating Costs
9.6 Update
CHAPTER TEN PLATTSBURGH, NEW YORK
10.1 Introduction
10.2 History of the Plant
10.2.1 Procurement and Construction
10.2.2 Capital Costs
10.2.3 Operating History and Current Status
10.3 Description of the Plant
10.3.1 Systems Overview
10.3.2 Feed and Mix Characteristics
10.3.3 Materials Handling
10.3.4 Reactors
10.3.5 Exterior Curing/Storage
10.3.6 Nonprocess Air Handling
10.3.7 Odor
10.3.8 Support Facilities
10.4 Monitoring and Performance
10.4.1 Reactor Control Strategy
10.4.2 Mass Balance and Reactor Detention Time
10.4.3 Product Quality
10.5 Operations
10.5.1 Staffing
10.5.2 Marketing and Distribution
10.5.3 Operating Costs
10.6 Update
CHAPTER ELEVEN PORTLAND, OREGON
11.1 Introduction
11.2 History of the Plant
11.2.1 Procurement and Construction
11.2.2 Capital Costs
11.2.3 Operating History and Current Status
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11.3 Description of the Plant
11.3.1 Systems Overview
11.3.2 Feed and Mix Characteristics
11.3.3 Materials Handling
11.3.4 Reactors
11.3.5 Exterior Curing/Storage
11.3.6 Nonprocess Air Handling
11.3.7 Odor Control
11.3.8 Support Facilities
11.4 Monitoring and Performance
11.4.1 Reactor Control Strategy
11.4.2 Mass Balance and Reactor Detention Time
11.4.3 Product Quality
11.5 Operations
11.5.1 Staffing
11.5.2 Marketing and Distribution
11.5.3 Operating Costs
11.6 Update
CHAPTER TWELVE SARASOTA, FLORIDA
12.1 Introduction
12.2 History of the Plant
12.2.1 Procurement and Construction
12.2.2 Capital Costs
12.2.3 Operating History and Current Status
12.3 Description of the Plant
12.3.1 Systems Overview
12.3.2 Feed and Mix Characteristics
12.3.3 Materials Handling
12.3.4 Reactors
12.3.5 Exterior Curing/Storage
12.3.6 Nonprocess Air Handling
12.3.7 Odor
12.3.8 Support Facilities
12.4 Monitoring and Performance
12.4.1 Reactor Control Strategy
12.4.2 Mass Balance and Reactor Detention Time
12.4.3 Product Quality
12.5 Operations
12.5.1 Staffing
12.5.2 Marketing and Distribution
12.5.3 Operating Costs
12.6 Update
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CHAPTER THIRTEEN SCHENECTADY, NEW YORK
13.1 Introduction
13.2 History of the Plant
13.2.1 Procurement and Construction
13.2.2 Capital Costs
13.2.3 Operating History and Current Status
13.3 Description of the Plant
13.3.1 Systems Overview
13.3.2 Feed and Mix Characteristics
13.3.3 Materials Handling
13.3.4 Reactors
13.3.5 Exterior Curing/Storage
13.3.6 Nonprocess Air Handling
13.3.7 Odor
13.3.8 Support Facilities
13.4 Monitoring and Performance
13.4.1 Reactor Control Strategy
13.4.2 Mass Balance and Reactor Detention Time
13.4.3 Product Quality
13.5 Operations
13.5.1 Staffing
13.5.2 Marketing and Distribution
13.5.3 Operating Costs
13.6 Update
APPENDIX A GLOSSARY
APPENDIX B CONVERSION FACTORS
APPENDIX C
EXAMPLES OF TECHNICAL INFORMATION REQUIRED
OF SYSTEM SUPPLIERS AND SYSTEM SELECTION CRITERIA
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LIST OF FIGURES
Figure No.
1-1 Flow Diagram of In-Vessel Composting
1-2 Reactor Systems
2-1 The Effect of Sludge Dryness on Mix and Amendment Quantities
2-2 Stability as a Function of Detention Time
2-3 Operating Costs as a Function of Sludge Processing Rate
3-1 Pittsburgh Screw Conveyor
3-2 Top of Reactor at Portland
3-3 Sarasota Mixing Area
3-4 Minor Spillage Associated with Conveyors
3-5 Ventilation System at Sawdust Receiving Facility, Portland
3-6 Retrofit Bin to Contain Dust at Sarasota
3-7 Example of Screw Conveyors that Empty Bins at Pittsburgh
3-8 Shaftless Screw for Emptying the Sawdust Receiving Bin at Sarasota
3-9 "Flat" Belt Conveyor at Newberg
3-10 Drag Chain at Clayton County
3-11 Drag Chains at the Cape May Plant
3-12 Drag Chain Flights at the Portland and Cape May Facilities
3-13 Cleated Conveyor at the Akron Facility
3-14 Spilled Material at Akron
3-15 Two Kinds of Mixers Used at In-Vessel Facilities
3-16 Discharge Device Installation at Schenectady
3-17 Piece of "Hardpan" Material Removed from the Pittsburgh Reactors
3-18 Storage/Curing Piles
3-19 In-Vessel Scrubbers
3-20 Odor Control System at Clayton County
3-21 Spare Parts Storage on the Roof of the Bathroom at Newburg
4-1 Clayton County Daily Process Log
6-1 Akron, Ohio, Composting Process Train
6-2 Akron, Ohio, Materials Handling Train
6-3 Cleated Conveyor Modifications
6-4 Akron Composting Facility Building Plan
6-5 Reactor Room
6-6 Front of Extractoveyor
6-7 Extractoveyor at Akron Facility
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Figure No.
6-8 Air Handling System
6-9 Stacking Conveyor
6-10 Compost Biofilter for Odor Control
6-11 Time-Temperature Profile
6-12 Akron, Ohio, Mass Balance
7-1 Cape May, New Jersey, Sludge Composting Process Train
7-2 Cape May, New Jersey, Materials Handling System
7-3 Drain Chain/Conveyor to Top of Reactor
74 Purac Reactor, Cape May
7-5 Aeration and Ventilation Schematic
7-6 Aerated Static Curing/Storage Pile
7-7 Reactor Building Showing Headspace Exhaust System
7-8 Cape May, New Jersey, Mass Balance
8-1 Clayton County, Georgia, Composting Process Train
8-2 Overview of Facilities at Clayton County Plant
8-3 Schematic of Materials Handling System
8-4 Taulman Composting System Reactor
8-5 Section of Reactor Aeration Piping with Temperature Probe
8-6 Typical Temperature Profile
8-7 Clayton County, Georgia, Materials Flow
9-1 Newberg, Oregon, Composting Process Train
9-2 Newberg, Oregon, Materials Handling System
9-3 Ashbrook Composting System
94 Interior of Tunnel Reactor, Facing Discharge End
9-5 Air Blower Room Between the Two Reactors
9-6 Reactor Air Being Discharged from 18-in. Diameter Exhaust Pipe into Oxidation Ditches
10-1 Plattsburgh, New York, Composting Process Train
10-2 Sawdust Bins with Screw Conveyors
10-3 Feed Conveyor System
104 Cross Section of a Fairfield Reactor
10-5 Ventilation Duct Taking Air from the Reactors to the Odor Control Building
10-6 Air Handling System
10-7 Plattsburgh, New York, Mass Balance
11-1 Portland, Oregon, Composting Process Train
11-2 Portland, Oregon, Sludge Composting Process Train
11-3 Sawdust Receiving Bin and Sawdust Silo with Dust Control Ventilation System
114 Portland, Oregon, Materials Handling System
11-5 Side View of Vertical Drag Conveyor, Mixer Building, and Enclosed Belt Conveyor
11-6 Rrst Reactor from Portland Train
11-7 Portland Reactor Outfeed Device
11-8 Portland, Oregon, Materials Flow
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Figure No.
12-1 Sarasota, Florida, Sludge Composting Facility
12-2 Sarasota, Florida, Composting Process Train
12-3 Sarasota, Florida, Materials Handling System
12-4 Drag Chain Modifications
12-5 Top of the Reactor After Loading
12-6 Supply and Exhaust Distribution System
12-7 Purac Composting System, Sarasota
12-8 Worn Outfeed Screw
12-9 Temperature Profiles Average, May to June 1988
12-10 Sarasota, Florida, Materials Flow
13-1 Schenectady, New York, Composting Process Train and Materials Handling System
13-2 Schenectady, New York, Materials Handling Schematic
13-3 American Bio Tech Reactor at Schenectady Plant
13-4 Outfeed Screw and Air Lances in Empty Reactor
13-5 Drive End of Outfeed Screw at the Bottom of the Reactor
13-6 Outfeed Drive Tracks Where the Channel Bolts Were Breaking
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LIST OF TABLES
Table No.
1-1 System Status at Time of Site Visit
1-2 Information Contained in the Case Studies
1-3 Plant Configurations
1-4 Potential Sources of Odors
2-1 Factors to Consider in Siting a Composting Plant
2-2 Summary of Procurement Methods
2-3 Minimum Technical Information Needs for Evaluating In-Vessel Systems
2-4 Summary of Capital Costs
2-5 Operating Cost Data
2-6 Amendment Unit Costs
2-7 Comparison of Design and Actual Mix Ratios In Use at Time of Site Visit
2-8 Comparison of Solids Contents in Design and Actual Mixes
2-9 State Standards for Unrestricted Compost Use
2-10 Compost Characteristics That May Be Used in a Product Specification
2-11 Horticultural Compost Requirements
2-12 Compost Marketing Agents7
2-13 Potential Backup Measures for Various Composting Problems
3-1 In-Vessel Composting Design Considerations
3-2 Variations in Moisture Contents for Different Materials
3-3 Common Conveyor Problems and Modifications
3-4 Standby Blower Systems
3-5 Wet Scrubbe/ Installations
3-6 Odor Removal by Compost Filters at the Akron Composting Facility
3-7 Monitoring Needs
4-1 In-Vessel Operations Considerations
4-2 Example Amendment Specifications
4-3 Summary of Personnel Used at Composting Plants
5-1 Information Collected During the Site Visits
5-2 Solids Content of Compost Ingredients
5-3 Types of Material Conveying Equipment Used
5-4 Summary of Reactor Air-Handling Systems at Sites Visited
5-5 Summary of Blower Equipment
5-6 Odor Control Facilities
5-7 Exterior Curing/Storage Facilities
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Table No.
6-1 Summary of Odor Measurements
6-2 Aeration System Operation
6-3 Compost Composition
6-4 Compost Use Regulations and Standards
6-5 Detention Times and Capacities at Various Mix Ratioc
6-6 Staff Requirements
6-7 Operations and Maintenance Cost Estimates
6-8 Operating Costs
7-1 Properties and Ratios of Sludge and Amendments
7-2 Blower Capacities
7-3 Air Quality Measurements and Scrubber Removal Efficiencies
7-4 Heavy Metals Content After Curing
7-5 Operations and Maintenance Costs
7-6 Materials Processed
7-7 Operating Costs
8-1 Process Air Constituents
8-2 Heavy Metal Concentrations in Compost
8-3 Expenses and Revenues for Fiscal Year 1988
9-1 Bid Results
9-2 Oregon Administrative Rules for Acceptable Levels of Metal Content of Sludge for
General Application to Agricultural Land
9-3 Estimated First Year Operations and Maintenance Costs
10-1 Mix Ratios
10-2 Sludge and Compost Constituents
10-3 Operations and Maintenance Costs
10-4 Materials Processed
10-5 Operating Costs
11-1 Maximum Heavy Metal Loading Recommended for Sludge Applications to
Privately Owned Farmland
11-2 Product Quality
11-3 Operation and Maintenance Costs
12-1 City of Sarasota Compost Plant Production Calculations Spread Sheet
12-2 Compost Constituents
12-3 Dewatering and Composting Operations and Maintenance Costs
12-4 Materials Processed
12-5 Operating Costs
13-1 Sludge and Compost Constituents
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Chapter 1
Introduction
1.1 Purpose and Scope
Since the 1970s, composting has become a very impor-
tant method for stabilizing and processing municipal
sewage sludge in the United States. The development of
this technology has been extremely rapid, from less than
10 facilities operating in 1975 to nearly 200 under design
or in operation in 1989. Today much of the compost pro-
duced is being used in accordance with the U.S. Environ-
mental Protection Agency's (EPA's) policy that
encourages the beneficial use of sewage sludge (49 FR
24358, June 12,1984).
More recently, because of odor, labor, and materials-
handling problems, a greater number of composting sys-
tems are being designed and built to contain the
materials within a vessel and extensively use conveyors
and other materials-handling equipment. Although the
evolution of these "in-vessel" systems is very rapid,
municipalities continue to encounter serious problems in
dealing with odors, removing moisture, handling the
materials in the system, and consistently marketing the
product. Furthermore, guidance in the selection, design,
and operation of suitable systems is not yet readily avail-
able to municipalities and their engineers.
To learn from the experiences of these new, mostly first-
generation in-vessel sludge composting systems and
their operators, EPA studied representative in-vessel
facilities in the United States. The information collected
on system performance related to odor generation, mix
ratios, moisture removal, detention times, materials han-
dling, production of quality product for a market, and
costs have been analyzed and structured to present
accurate information to the engineering and sludge man-
agement communities on in-vessel
To gather the necessary information, onsite visits (sur-
veys) were conducted at 8 of the 13 full-scale facilities
operating in the United States in early 1988. Only two of
the facilities had operated for more than 2 years. The
eight plants studied included all six types of in-vessel
systems existing at the time. These six were the first
plants of their kind to be built in this country. Table 1.1
provides background data for each installation at the time
of the plant survey.
Table 1.1 System Status at Time of Site Visit
Location System Supplier
Akron, OH
Cape May
County, NJ
Clayton
County, GA
Newberg, OR
Pittsburgh, NY
Compost Systems Co.
Paygro System
Purac Engineering
Taulman Composting
Systems
Ashbrook-Simon-Hartley
Compost Systems Co.
Sludge Processing
Rate (average dt/
calendar day)b
Startup
Date
Dec. 86
May 85"
Aug. 86
Aug. 87
Mar. 86
System Status at the
Time of Site Visit3
Operating at reduced capacity
Operating at reduced capacity
Processing all the sludge available
Undergoing acceptance testing
Processing all the sludge available
Design
73
20°
2.9
3.5
34
Actual
25°
11
1.4
—
28
Portland, OR
Sarasota, FL
Schenectady, NY
Fairfield System
Taulman Composting
Systems
Purac Engineering
American Bio Tech
Mar. 85 Operating at reduced capacity 60
Aug. 87 Processing all the sludge available 6.3
July 87 Not operating while modifications 15
are being performed
26
4.3°
"The visits took place between April 27 and November 17,1988.
"Dion/day x 0.9072 = dt (metric ton).
"Actual loading is 44 dton/day, 4 day/week.
"Startup date for original configuration.
'Plant modifications made in 1987-1988 increased the capacity to 20 dton/day.
-------
The surveys revealed that while problems have been
encountered in these initial systems, much has been
learned from these problems. Of the eight plants sur-
veyed, most have had severe problems with odors and
materials handling (drag chains have been particularly
troublesome). Nevertheless, the survey also revealed
that newer plants have improved designs, which avoid
many of the problems found at first-generation plants.
Because in-vessel technology is changing rapidly and
many new facilities are being built, readers interested in
specific systems may wish to contact the system suppli-
ers to determine the location of currently operating
plants. It should also be noted that in-vessel systems
have been in use in Europe and Japan for some time, so
that prospective buyers may want to investigate the Euro-
pean and Japanese experience as well.
This report highlights design and operating consider-
ations for possible incorporation into future in-vessel and
other sludge composting systems. It is not meant to sin-
gle out one design as superior to another. The document
also alms to heighten awareness that a sludge in-vessel
composting system includes much more than the vessel
itself. Indeed, many of the difficulties encountered at in-
vessel facilities have resulted from a failure to recognize
the totality of the system that must be managed.
7.2 Organization of the Report
This summary report is divided into 13 chapters. Chapter
One presents the purpose and content of the report, an
introduction to how the surveys were conducted, and an
overview of in-vessel composting.
Chapters Two, Three, and Four summarize the "lessons
learned" from the successes and failures of the sites
visited. These chapters also present problems encoun-
tered during the processes of selecting, procuring,
designing, and operating an in-vessel composting facility,
as well as solutions developed in response to these
problems.
Chapters Five through Thirteen present detailed informa-
tion on the plants visited. Chapter Five introduces the
case studies and includes tables that summarize the
information gathered during the onsite investigations.
Chapters Six through Thirteen present the eight case
studies including a detailed description of each plant, and
information on the history, performance, and operations
of the facilities (see Table 1.2). The historical background
Is presented to provide a context for the site visit informa-
tion, but the factual data (such as mix ratios, loading
rates, and operating practices) are accurate only for the
time of the site visit. More recent 'information, where
available, is included in the "Update" section at the end
of each case study.
Appendix A is a glossary of sludge composting terms.
Appendix B lists the metric-to-English unit conversions
used in the document. Within the document text, numeri-
cal measurements are given in metric units, followed in
Table 1.2 Information Contained In the Case Studies
INTRODUCTION
HISTORY OF THE PLANT
Procurement
Capital Costs
Operating History and Current Status
DESCRIPTION OF THE PLANT
Systems Overview
Feed and Mix Characteristics
Materials Handling
Reactors
Exterior Curing/Storage
Nonprocess Air Handling
Odor
Support Facilities
PERFORMANCE
Reactor Control Strategy
Mass Balance and Reactor Detention Time
Product Quality
OPERATIONS
Staffing
Marketing and Distribution
Operating Costs
UPDATE
parentheses by their English unit equivalent. In tables
where data are presented in English units, the metric
conversions are included as footnotes. To distinguish
metric from English tons, metric tons are abbreviated dt
(dry ton) and wt (wet ton); English tons are abbreviated
dton and wton. Finally, a sample of a Request for Pro-
posal (RFP) used in a 1989 procurement of an in-vessel
composting system is included as Appendix C.
7.3 In-Vessel Composting
In-vessel composting is a total system that comprises a
number of integrally related components, including:
• Materials (sludge cake, amendment, and recycle)
• Materials handling (including storage, mixing, and
conveyance)
• Reactor system
• Aeration system
• Odor control system
• Exterior curing/storage facilities
• Marketing
A generic flow diagram of in-vessel composting is shown
in Figure 1.1.
The three ingredients — sludge cake, amendment (usu-
ally sawdust), and recycle — are mixed together and
placed into one or more aerated reactors for composting.
Air is diffused into the reactor system for temperature
control, moisture removal, and biological metabolism. Air
is then usually exhausted directly to an odor treatment
system before being dispersed into the atmosphere.
After the desired detention time, the material is removed
from the reactor for further curing/storage outside.
-------
Figure 1.1 Flow Diagram of In-Vessel Composting
-> ATMOSPHERE
SLUDGE '
AMENDMENT
MIXING
AND
CONVEYING
/
^
REACTOR
SYSTEM
1
/
f
\-
^» /
EXTERIOR CUI
MARKET
KEY
•5* = SAS FLOW
> - SOLID MATERIAL FLOW
Changing one component of an in-vessel system affects
the entire system. For example, if a component such as
odor control or product marketing is overlooked in the
design phase, the entire system may have to be shut
down due to odor problems or lack of compost storage
space. Furthermore, system suppliers do not usually pro-
vide all components for the system. Although they pro-
vide conveyors, they may not supply odor control
equipment or certain other key features. Also, the system
suppliers usually do not identify a market for the product
and may not install the monitoring devices (i.e., for mois-
ture content, stability, odor control, etc.) necessary to
assure that an aesthetically acceptable and marketable
product is produced. Therefore, it is up to the individuals
procuring the in-vessel system to ensure that all system
components work together effectively.
The following sections of this chapter discuss the overall
design of existing in-vessel composting systems. Subse-
quent chapters will present details on the various
systems and the problems that specific facilities have
encountered with equipment, design, and operations.
1.3.1 Materials
The materials required for in-vessel composting are
sludge cake, amendment, and recycled compost. Sludge
cake and recycled compost are usually referred to simply
as "sludge" and "recycle," respectively. The mixture of
sludge, amendment, and recycle is commonly referred to
as the "mix."
Before composting, sludge must be dewatered. All facili-
ties surveyed in this report used belt filter presses to
dewater sludge. Most in-vessel systems use sawdust as
amendment, but wood shavings, ground-up wood, or
shredded bark also are used. Some facilities that origi-
nally used sawdust have switched to coarser materials to
permit more air penetration. The source of recycle can be
-------
either material discharged from the reactor system or
material from exterior curing/storage piles.
1.3.2 Materials Handling
13.2.7 Storage
Since most compost reactors are loaded for only a por-
tion of each day, storage facilities for sludge, amendment,
and recycle are needed to match the volume of feed
delivered to that needed by reactor operations. A variety
of storage facilities are used, including piles, bins, cov-
ered sheds, and portable containers.
1.3.2.2 Mixing
Mixing appropriate proportions of sludge, amendment,
and recycle is essential to create a desirable compost
structure with respect to porosity, moisture content, and
energy balance. The mix ratio also directly affects the
composting process. Proper porosity promotes easier
materials handling and thorough aeration. Proper mois-
ture content supports the biodecpmposition of the
organic material present in the mix. Too little water
retards the process; too much water results in a loss of
porosity and development of septic conditions and
odors.
Amendment and recycle are added to the sludge to
increase the solids concentration of the mix. Most facili-
ties use as much recycle as possible because it is less
expensive than purchasing amendment. The exact
amount of recycle used, however, is limited by its poros-
ity, which decreases as the recycle ages. Also, as the
proportion of recycle in the mix increases, the overall
solids retention time in the system increases. The proper
moisture content and ratio of mix materials also depend
on the length of the air path in the system (see Chapter
Three).
The energy required for the latent heat of evaporation and
for heating the incoming solids, water, and air is gener-
ated by the aerobic biodegradation of the solids in the
mix. Composting ceases if the heat requirements and
energy balance are not maintained. Section 3.9 provides
more information on process monitoring and energy
requirements.
Most in-vessel composting systems use pugmill and
plow-blade mixers to create a homogeneous mix. Often,
existing systems have had to adjust these mixers to
enable effective and efficient composting (described in
more detail in Section 3.5.1).
1.3.2,3 Conveyance
Mechanically and operationally, conveyance systems are
the dominant physical features of in-vessel facilities.
Conveyors were the second biggest cause of system
problems at the sites (after odor production). Although a
high degree of automation for conveyor systems is not
required, most in-vessel composting facilities chose
automation to minimize labor costs.
Generally, in-vessel systems use three kinds of convey-
ors: belt conveyors (for horizontal or gently inclined trans-
fer), screw conveyors (for steeply inclined transfer), and
drag chain conveyors (for vertical or steeply inclined
transfer). Vertical conveyance can also be accomplished
using a cleated belt conveyor or a bucket conveyor. Saw-
dust is sometimes conveyed pneumatically. Specific
problems encountered with these conveyance systems
are discussed in later chapters, particularly in the case
studies.
1.3.3 Reactor System
13.3.7 Composting Stages
The composting system is often divided into two stages:
(1) a first-stage, high-rate phase and (2) a second-stage,
curing phase. The first stage is characterized by high
oxygen uptake rates, high temperatures, rapid degrada-
tion of biodegradable volatile solids, and high potential
for odor production. The second stage is characterized
by lower temperatures, reduced oxygen uptake rates,
and a lower, but significant, potential for odor production.
No precise definition or distinction exists between these
two stages, however. Moreover, although the first stage of
composting is performed in reactors at all in-vessel
facilities, the second stage can be performed in a reactor,
an exterior pile, or both. To avoid confusion, the term
"first reactor" or "first-step reactor" is used in this hand-
book to denote the first vessel that the compost enters.
"Second reactor" or "second-step reactor" is used to
denote the second vessel the compost enters in series.
(At many plants, there is no second reactor.) "Exterior
curing/storage piles" refer to any compost piles located
outside the reactors. Table 1.3 summarizes the compost-
ing system configurations at the sites visited.
7.3.3.2 Types of Reactors
There are two general classes of reactors: plug-flow reac-
tors (vertical and horizontal) and agitated bed reactors
-------
Table 1.3 Plant Configurations
Location Type of Reactor
Composting System
Akron, OH
Cape May, NJ
Clayton County, GA
Newberg, OR
Pittsburgh, NY
Portland, OR
Sarasota, FL
Schenectady, NY
Paygro
Purac
Taulman
Ashbrook-Simon-Hartley
Fairfield
Taulman
Purac
American Bio Tech
Reactor •
Reactor -
Reactor •
Reactor -
Reactor •
Reactor -
Reactor -
Reactor -
» Unaerated piles
* Aerated piles0 -
* Reactor
* Aerated piles
* Unaerated piles
* Reactor -
* Reactor •
* Reactor -
» Unaerated piles
* Unaerated piles
» Unaerated piles
* Unaerated piles
* Unaerated piles
•After 1987 modifications. Plant was originally constructed with two reactors in series followed by unaerated curing/storage piles.
Figure 1.2 Reactor Systems
Air Cycle
In/Out "**
Air Cycle
' In/Out
Outfeed
Device
Compost
American Bio Tech
Air out
Outfeed
Device
n
Air in compost
Purac Engineering
Mix
Air out
Outfeed
Device
Compost
Taulman Composting Systems
VERTICAL PLUG-FLOW REACTORS
-------
Figure 1.2 Reactor Systems (continued)
Mix
Hydraulic^
Ram
Air
in
tt H H H H tt
Air
ompost
out
Ashbrook-Simon-Hartley
HORIZONTAL PLUG-FLOW REACTORS
Mix
Mobile Device to Agitate
And Discharge
Air Material •
Reversible
Air Flow
Compost Systems Co. (Paygro)
* Reactors not operated in this fashion at Akron
See Case Study
Agitating
Augers on
Rotating
Bridge
Compost
Compost Systems Co. (Fairfield)
Compost
Air
Out Mix
AGITATED BED REACTORS
(circular and rectangular). Figure 1.2 illustrates reactor
systems from each of these classes and subclasses.
In a vertical plug-flow reactor, the mix is placed in the top
of the reactor and moves as a plug to the bottom, where it
is discharged by a rotating screw. Vertical plug-flow reac-
tors are manufactured by Taulman Composting Com-
pany (Taulman), Purac Engineering (Purac), and Ameri-
can Bio Tech (ABT). The primary differences among
these reactors are the configurations of the aeration sys-
tems and the discharge devices.
-------
Table 1.4 Potential Sources of Odors
Liquid Sludge Facilities
Sludge storage tanks
Pump rooms
Wet wells
Dewatered Sludge Facilities
Dewatering equipment
Sludge cake storage bins or portable containers
Conveyors between dewatering facilities and the composting
plant
Open doors
Composting Facilities
Sludge storage bins or portable containers
Recycle storage bins
Amendment storage binsa
Mixing room (conveyors, mixers, spillage)
Reactors - exhaust airstream from compost bed (process air)
- ventilation air for work space or building above
the reactor
- ventilation air for work space containing
discharge device
Recycle conveyors
Heat exchangers for supply air
Odor control systemb
Open doors
Exterior Curing/Storage .-.-.--
Discharge conveyors
Aerated piles
Unaerated piles
Product storage piles
Stacking or moving materials with mobile loaders
Miscellaneous Facilities
Runoff storage ponds
Amendment receiving facilitiesa
Publicly owned treatment work processes
Adjacent facilities, such as refineries, rendering plants, paper
mills, and other manufacturing activities
'Amendment (sawdust) facilities can emit an odor that some people consider objectionable.
"If not properly managed, wet scrubbers using sodium hypochlorite can emit a "chlorine" odor that some people consider objectionable.
In a horizontal plug-flow reactor, the mix is introduced at
one end of the tunnel, or rectangular tube, and travels the
length of the tube as a plug. The material is propelled by a
mechanism such as a moving floor or a hydraulic door.
The Ashbrook-Simon-Hartley (Ashbrook) reactor is an
example of this reactor type.
In agitated bed reactors, the mix is placed in the reactor
and mechanically mixed either in place or as it is moved
through the reactor. Agitating mechanisms and the point
at which mix is introduced to the reactor vary from sup-
plier to supplier. The Paygro and Fairfield systems (both
supplied by Compost Systems Co.) are examples of agi-
tated bed reactors.
1.3.4 Aeration System
There are three basic demands on the aeration system —
temperature control, moisture removal, and biological
metabolism. Determination of the proper air supply in the
compost mass is complex and depends on several inter-
related variables, including:
• The desired quality of the final compost product (which
depends on user-defined criteria, such as moisture
content and stability)
• Reactor-specific factors, such as air distribution and
collection system, agitation (if any), and potential for
short-circuiting
• The EPA requirement for pathogen destruction (i.e.,
the temperature must remain above 55°C (131 °F) for
at least 3 days)
• The porosity of the mix (which depends on particle size
and moisture content)
• The total volume of air supplied (which depends on
both the airflow rate and the detention time)
See Section 3.3 for further discussion of aeration.
1.3.5 Odor Control System
Odor control was the greatest problem found at most of
the sites surveyed, as will be discussed in later chapters.
There are many potential sources of odors at in-vessel
composting facilities (see Table 1.4). Although process air
is the most odorous, all potential odor sources must be
evaluated and controlled. If they are not, the odor prob-
lems may well result in the shutdown of the plant.
At in-vessel plants, four techniques have been used to
control odor:
• Diluting the odors with large volumes of air
• Bubbling odorous air through mixed liquor at an adja-
cent wastewater treatment plant
• Passing the air through compost ("earth") biofilters
• Treating the air chemically (either through ozone oxi-
dation or scrubbers)
The first method, dilution alone, does not control odors,
but can be used as a final step after other treatment
methods. The other three techniques have had limited
success. As will be discussed later, odor treatment facili-
ties are only one element in a comprehensive odor con-
trol strategy. Section 3.8 provides considerable detail on
odor identification, characterization, meteorological
modeling, and control.
1.3.6 Exterior Curing/Storage Facilities
Exterior curing/storage facilities are typically uncovered
paved areas located near the in-vessel facility. Pavement
enhances leachate collection and control. (The leachate
is usually collected and either treated at the adjacent
-------
wastewater treatment plant [WWTP] or sent to the local
sewer.) Some plants construct a roof over the paved
areas to control the moisture content of the compost.
Some plants also aerate curing/storage piles to avoid
anaerobic zones and increased odor production, and to
enhance product quality. The need for aerated curing/
storage piles will be discussed in more detail in Sec-
tions 2.7.4 and 3.3.3.1.
1.3.7 Marketing
Although in many areas the potential uses for "finished"
compost are great, such as in land reclamation areas or
where government agencies actively support compost
use, functioning markets for compost usually must be
developed. Thus, plant owners should not assume a mar-
ket exists, just because a facility can produce a product
that Is in compliance with Federal, State, and/or local
regulations.
First, product users are concerned with many product
quality criteria not covered by government regulations.
Such factors as product stability, anaerobic versus aero-
bic nature of the compost, pH, lime content, nutrient
content, and amendment content need to be carefully
monitored and controlled for specific user markets. For
example, some users demand a consistent product; for
others, product consistency is less important. During the
facility's planning stages, potential markets must be
explored and a marketing plan developed that ensures
that specifications and quality criteria for a marketable
product are incorporated into the design.
Second, even if a facility creates a well-developed mar-
keting plan, it should not assume that every batch of
compost will meet product specifications and be salable.
The facility must continuously monitor product quality to
make sure that the product actually meets specifications
and user needs.
Developing a marketing plan involves investigating
potential uses and demands for the compost, competing
products, applicable regulations and quality criteria for
various compost uses, packaging requirements for main-
taining product stability, marketing agents, purchasing
arrangements, contingency plans, and pricing structures.
See Section 2.7 for further discussion of compost
marketing.
-------
Chapter 2
Project Planning Considerations
This chapter catalogs the project planning experience
gained by the in-vessel composting facilities surveyed for
the report. The information is not a comprehensive list of
all factors to consider in planning an in-vessel compost-
ing system. Rather, the purpose of this chapter is to aid
readers in designing future in-vessel systems by describ-
ing specific problems and situations encountered by
these plants.
Throughout the section, the term "owner" generally
applies to the initiating public agency, although a private
enterprise may own the composting project. The term
"system supplier" applies to the in-vessel composting
equipment manufacturer and/or supplier. The term "engi-
neer" applies to the agency's engineering staff, a consult-
ing engineer hired by the agency, or a system supplier's
staff or consultant.
2.1 Choosing In-Vessel Composting
The purpose of composting is to beneficially reuse
sludge in a way that can not be accomplished with
untreated sludge — by creating a marketable product.
Composting is not commonly chosen merely as a predis-
posal treatment process. At the sites surveyed, in-vessel
composting was selected for the following reasons:
• State restrictions or bans on land application or
incineration
• More consistent product quality
• Fewer personnel required
• More effective odor control
• Better public acceptance due to aesthetics
• Space constraints
2.2 Siting
2.2.1 Siting to Minimize Odor Effects
The preeminent issue in choosing a site for an in-vessel
composting facility is its potential to minimize odor
impacts to local residents. If the plant has an odor prob-
lem, all other advantages associated with a site are unim-
portant. Because no odor treatment system is 100
percent effective, the odor dispersal characteristics of
any proposed site should be identified during project
planning. Local conditions such as meteorology, other
odor sources, and the proximity of a Publicly Owned
Treatment Works (POTW) influence the degree of odor
control to be designed for the facility. Section 3.4 further
discusses odor sources and control options.
2.2.2 Siting at a Wastewater Treatment Plant
It is common practice to site the compost plant on the
local wastewater treatment plant (WWTP) site for a num-
ber of reasons. First, the land is usually owned by the
public agency already. Secondly, locating the compost
plant near the source of sludge saves transportation
costs. Finally, the WWTP is often located in an appropri-
ately zoned area with few residential neighbors (an indus-
trial area or the rural fringe of the city), which minimizes
siting debates with local residents.
Of the eight plants surveyed for this report, six were
located at existing treatment plant sites and a seventh
(Newberg) was built concurrently with a new treatment
plant. The only facility not sited near a WWTP was the
Pittsburgh composting plant; it was built at a separate
site because there was not enough room at the WWTf?
Despite the apparent advantages of siting at a WWTP
plant owners need to consider a number of additional
factors, including general suitability, potential for site
development, legal and economic issues, and potential
for future problems (see Table 2.1).
2.3 Public Participation
2.3.1 Involving the Public in Planning
The public should.be involved in the planning process for
several reasons:
• The money for the facility will come from the tax
payers.
• The plant's construction and operation may affect local
residents.
• It is required by law.
• It can help promote interest in the compost product.
• It is cost-effective.
Even the most expensive and time-consuming public
participation process will, in the end, be less expensive
than building a fully operational facility only to have it sit
idle or be underutilized because of adverse public reac-
tion (usually related to odor problems).
-------
Table 2.1 Factors to Consider in Siting a Composting Plant
General Suitability for Intended Use
Size
Odor dispersal characteristics (micrometeorology)
Proximity to other uses
Site Development
Terrain (earth-moving required)
Foundation conditions (soils, geology, depths to ground water)
Drainage
Utilities at the site
Vehicular access (local roads)
Development costs
Operational Issues
Distance from the treatment plant(s)
Traffic impacts on local roads and residents
Buffer zones (to mitigate visual, noise, odor impacts)
Logal and Economic Issues
Acquisition costs
Encumberances
Ownership
Local zoning
Potential for Future Problems
Local growth and development
Expansion needs
Another key goal of a public participation program is to
build a local market for the compost. Local markets can
return the greatest revenues to the plant because of
lower transportation costs. Perhaps more importantly,
local residents may be more forgiving of minor problems
experienced by a composting facility if they receive
something tangible (the compost product) for their
trouble.
2.3.2 Public Relations Programs
The main goal of a public relations program is to build
credibility with the community. A program used at the
Montgomery County Composting Facility (in suburban
Washington, D.C.), a static pile facility that was not sur-
veyed, included the following elements (1):
• Speak to the local community near the facility and
meet with them monthly.
• Distribute flyers acknowledging community concerns,
presenting plant problems, and explaining how the
program addresses community concerns.
• Set up an objective odor evaluation process.
• Develop brochures and information packages to give
to visitors.
• Print posters. The Montgomery County Composting
Facility used the motto "A Plant You Can Live With."
• Keep the reception area looking professional.
• Publicly demonstrate the usefulness of the product
(compost).
• Visit local schools to educate students and get them
involved.
• Stage plant visits for members of the community,
school children, etc.
2.4 Procurement
Table 2.2 summarizes the procurement processes used
by the eight plants surveyed. As the table shows, the
methods ranged from turn-key (Portland) to multiple sets
of plans and specifications (Cape May). In all cases, a
system supplier prequalification step was included in the
procurement process. Sometimes the final choice of sys-
tems was made during the prequalification step. Other
times, the final choice was based on the construction
bids.
These projects were funded primarily by EPA grants with
the exception of the Portland and Clayton County facili-
ties, which were funded locally.
Table 2.2 Summary of Procurement Methods
Location Prequalification
Engineering
Akron, OH One system supplier chosen as sole source.
Cape May, NJ Two system suppliers prequalified and
"Or equal" clause.
Clayton County, GA One system supplier chosen as sole source.
Nawberg, OR Two system suppliers prequalified.
"Or equal" clause in bid documents.
Piattsburgh, NY One system supplier prequalified.
"Or equal" clause in bid documents.
Portland, OR One system supplier chosen for a turn-key
contract.
Sarasota, FL Three system suppliers prequalified.
Schenectady, NY One system supplier chosen as sole source
allowed after submitting binding equip-
ment bid.
Consultant based design on sole source.
Consultant prepared a design for each prequalified system supplier.
Consultant based design on sole source.
Consultant prepared a generic design that fit the two prequalified system
suppliers. The low bid included a third system supplier whose system
was accepted under the "or equal" clause. System supplier prepared
final design.
Consultant based design on prequalified system supplier. A second
system suppliers system was bid under the "or equal" clause. It was not
the low bid.
Design by system supplier.
Consultant prepared design based on preferred system supplier. Low bid
(accepted) included another prequalified system supplier. Final design
prepared by system supplier.
Consultant based design on sole source.
10
-------
The basic steps in the procurement process include
assigning responsibility for engineering and facility
design, prequalifying system suppliers, and writing and
issuing the Requests for Proposals (RFPs). Details of the
procurement experiences at the plants can be found in
the case studies.
2.4.1 Project Engineering
There are three potential sources of engineering exper-
tise: the facility owner, the system supplier, and indepen-
dent consultants. Usually, engineering responsibility is
divided among these three, because consultants cannot
design the proprietary reactor systems and system sup-
pliers may not have the capability to do the site, struc-
tural, and architectural designs. However, splitting or
sharing responsibility for the design can be risky, and
may lead to certain design elements not meshing well or
being overlooked. For these reasons, project responsibil-
ities must be clearly divided and assigned according to
expertise. For example, the owner might assign all pro-
cess responsibilities (materials-handling systems, reac-
tors, controls, and odor control systems) to the system
supplier and all structural and site design responsibilities
(building, foundations, and utilities) to the consulting
engineer.
The Schenectady plant experienced the type of project
coordination problem that can occur. When the odor con-
trol system proved to be too small, the city tried to identify
the responsible party. Odor control design was suppos-
edly the responsibility of the consulting engineer, but the
consultant claimed that the system supplier had underes-
timated the odor potential of the system and the system
supplier claimed that the consultant's design was flawed.
Owners should seek qualified consulting engineers who
possess general civil and environmental engineering
backgrounds and are experienced in all aspects of public
works procurement. More importantly, they should have
specific knowledge and experience with composting of
wastewater sludges, odor control, materials-handling
systems, and ventilation.
2.4.2 Prequalification of System Suppliers
2.4.2.1 The Prequalification Process
The procurement methods appropriate for in-vessel sys-
tems differ from normal public works procedures. The
heart of an in-vessel composting facility — the reactors,
feed devices, and discharge devices — is assembled
from proprietary equipment. Consequently, a system
supplier should be chosen early in the process. Without
prequalifying system suppliers, the public agency must
either prepare multiple designs or a generic design to fit
all possible bidders. Both options have disadvantages.
Preparation of multiple complete designs is costly and
inefficient, whereas generic designs leave many of the
design details up to the bidders, placing the agency in a
position to potentially lose control of the project.
Another advantage of prequalifying a system supplier is
to elicit system supplier protests early. System suppliers
protested the choice of system at most of the plants
surveyed. The Pittsburgh protest took 4 months to
resolve and required participation of the city and EPA.
Clayton County officials had to tour several European
composting facilities to satisfy a protesting system sup-
plier's concerns. Since system supplier protests are time-
consuming and expensive, it is better for both the public
agency and the system supplier to resolve disputes over
the system choice before either side commits large
resources to the project.
2.4.2.2 Single Prequalifiers
If, through a RFP/prequa1ification bid process, the
agency can choose a single system supplier before start-
ing the detailed design, it can create a design package
that satisfies operations and maintenance requirements,
life-cycle costs, and other concerns. The agency can
make the bid documents very specific, even to the choice
of standard equipment packages on which all construc-
tion companies would bid. At Clayton County, managers
credited this approach with a very smooth bidding and
construction process.
The City of Schenectady used an innovative two-bid
approach to include economic competition in the pre-
qualification process. After talking with system suppliers
and visiting existing plants in the United States and
Europe, city officials prequalified three systems. Then
they followed a two-step bidding process. The first step
was the bidding of an equipment contract which included
the reactors; feed and discharge devices; mixer; convey-
ance system; sludge and amendment storage facilities;
aeration system; and service provided during design,
construction, and startup (2). American Bio Tech (ABT)
was chosen on the basis of its low equipment bid. There-
after, plans and specifications were developed by the
city's consulting engineer with extensive input by ABT
The second step of the bidding process was for a stan-
dard construction and installation contract executed by a
general contractor.
2.4.2.3 Multiple Prequalifiers
The advantage of opening the bidding procedure to a
number of system suppliers is decreased costs due to
increased competition. On the other hand, there are sev-
eral disadvantages of accepting multiple system suppli-
ers. Engineering costs may be higher if the "equality" of
different bids must be evaluated. The probability is high
that a system supplier will protest a decision that his or
her system is not equal. Prequalification of multiple sys-
tem suppliers has been used at several locations, and in
some cases, all prequalified bidders went through the
entire bidding process. The Cape May Authority's con-
sulting engineer prepared two sets of plans because EPA
required the contractor to be selected based on bids
responding to design packages prepared by conven-
tional architectural engineering services.
11
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2.4.2.4 Use of "Or Equal" Clauses
The public agency should take care not to lose control of
the procurement process through imprecise "or equal"
specifications. The agency should select and include in
the bid package the criteria for judging whether an alter-
native bid is equal before advertising the job.
At Pittsburgh, the city chose agitated bed in-vessel
composting systems, and then prequalified the Fairfield
process. The plant was designed around the chosen
technology but the specifications allowed for submitted
of "equal" systems. The low bid for the construction
contract included the Fairfield system. Another system
supplier protested the award of the contract; the protest
was disallowed, but only after a 4-month review process
by EPA.
2.4.3 The Request for Proposal
The prequalification starts with a Request for Proposals
(RFP) or Request for Qualifications (RFQ). In this sec-
tion, "RFP" will be used to refer to either.
2.4.3.7 Defining System Requirements
The goal of the RFP is to elicit proposals that are com-
plete and can be evaluated on a comparable basis.
Exactly what information is required to evaluate different
proposals depends on the project's purposes, goals, and
functions. Table 2.3 lists information that should be
requested. Appendix C (City A) presents the technical
information section in a sample RFP Among the plants
surveyed, the following system requirements were
encountered:
Table 2.3 Minimum Technical Information Needs for Evaluating In-Vessel Systems (1,2)
1. Plant Configuration
2. Process Information
(a) Sludge, Amendment, and Mix Properties
(for range of conditions)
- Type and Amounts of Sludge, Amendment, and Recycle
- Volatiles Content - Sludge, Amendment, Mix
- Initial Solids Content-Sludge, Amendment, Mix
- Porosity/Bulk Density - Mix, Product
-Mix Ratio
(b) Reactor Shape, Dimensions, and Volume
(c) Detention Time in:
- Each Reactor
- Aerated Curing
- Product Storage
(d) Solids Retention Time in:
- Reactors
- Facility
(e) Aeration (both in and outside reactor)
- Method
- Rate (cfm)
- Volume of Air (cf/unit of sludge or mix)
- Static Head Pressure
- Range of Straight Line Airflow Distances
(0 Moisture Content of the Mix When Discharged from the:
- Reactor
- Composting Facility
(g) Monitoring and Control
- Temperature
- Oxygen Levels
(h)
(0
Mass Balance
Energy Balance
3. Product Quality
(a) Ability to meet pathogen reduction time and
temperature criteria
(b) Ability to meet other product quality standards
•4. Odor Control
(a) Inventory of all potential odor sources
(b) Control plan for each source, including:
- Capture and Containment System
- Treatment Facilities
- Method of Dispersal
5. Materials Handling
(a) Configuration of System
(b) Description of Components
- Physical Descriptions
- Materials of Construction
- Maintenance Requirements
- Operating History in this Application (reliability)
- Repair Procedures for Most Common Problems
- Energy Requirements
(c) Control System
6. Aeration and Exhaust Systems
(a) Configuration of System
(b) Description of Components
- Physical Descriptions
- Materials of Construction
- Maintenance Requirements
- Operating History in this Application (reliability)
- Repair Procedures for Most Common Problems
- Energy Requirements
(c) Control System
7. Reactor Systems (including discharge device)
(a) Configuration of System
(b) Description of Components
- Physical Descriptions
- Materials of Construction
- Maintenance Requirements
- Operating History in this Application (reliability)
- Repair Procedures for Most Common Problems
- Energy Requirements
(c) Control System
8. Operations
(a) Labor Needs
(b) Skills Required for:
- Operating the Equipment
- Controlling the Process
- Maintenance and Repair of Equipment
(c) Proposed Operating Schedule
12
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• Must be odor-free at all times
• Must use an existing site (usually at a WWTP)
• Must meet certain time constraints for construction
• Must be acceptable to the public
• Must minimize total costs
• Must be owned and operated by private enterprise
• Must be reliable
• Must produce a marketable product
Design requirements should be considered in the earliest
stages of the procurement process and be included in the
RFP In every design, there are tradeoffs between costs
and system capabilities.
To promote comparability among proposals, the RFP
should be as specific and clear as possible in describing
the information sought. Definitions should be provided for
terms like "solids retention time," "curing," and compost
"stability." Where appropriate, the RFP should contain
numerical standards to be met. In short, the RFP should
be written with the same care used in writing a construc-
tion specification (although the degree of detail will be
much less).
2.4.3.2 Developing Evaluation Criteria
Before issuing an RFP the public agency should thor-
oughly consider the basis for choosing one in-vessel
system over another. This approach serves four
purposes:
• It forces the public agency to decide how the choice
will be made.
• It helps the agency write a focused RFP so that the
proposals respond to those issues of importance to
the agency.
• It makes the selection criteria a matter of public record,
thereby decreasing the potential for system supplier
protests.
• It provides a common basis for judging different propri-
etary systems. System suppliers may not agree with
the criteria, but if their products are fairly judged
according to these criteria, they will have little ground
for protest.
Detailed selection criteria should be published in the
RFP These criteria, set for the public agencys vital inter-
ests, help system suppliers decide whether or not they
want to risk their resources in developing an alternative
design and bid, and form the technical basis for settling
potential disputes.
When developing criteria, the agency should consider
publishing a draft RFP and requesting comments from
system suppliers. This will help ensure that the RFP does
not inadvertently favor one system or eliminate another
from competition. Comments from system suppliers will
also help clarify ambiguities and identify important selec-
tion criteria that may have been omitted inadvertently.
The criteria should include clear definitions such as resi-
dence times, aeration, odor control, etc. The definitions
should include specific methods or formulas for calculat-
ing values such as detention time. The sample RFP in
Appendix C details the selection criteria used at several
plants.
2.4.3.3 Realistic Physical Parameters
RFP requests for designs and costs should be based on
realistic estimates of physical parameters such as sludge
production, sludge and amendment characteristics, and
allowable odor emissions rates. Sludge production rates
and cake solids are particularly important because the
size of the system depends heavily on these values. The
RFP should involve average values and daily and sea-
sonal variations.
Sludge Production
Most systems are modular in design so that sludge load-
ing variations can be accommodated by the use of multi-
ple reactors. Nevertheless, system suppliers need to
know these variations so that they can size their equip-
ment properly. The sizes of system components are
based on materials volumes, which in turn, are sensitive
to sludge solids contents. In areas where the population
is growing, in-vessel systems should be designed to
accommodate future sludge production rates or con-
structed in stages.
Sludge and Amendment Characteristics
Using the typical values for moisture contents, bulk
weights, and desired mix solids content, the mix weights
and amendment requirements for sludges with different
solids contents were calculated using the method pre-
sented by Haug (3) and Haug and Tortorici (4). As can be
seen in Figure 2.1 (a), approximately 6.7 wt mix per dt
sludge (7.4 wton/dton sludge) would be created if the
sludge cake solids content is 24 percent. If the solids
content drops to 16 percent, the quantity (8.7 wt/dt sludge
[9.6 wton/dton sludge]) of mix would increase to 11.0 wt/
dt (12.1 wton) sludge, a 63 percent increase. To maintain
the same plug-flow detention times, the reactor would
have to be 63 percent larger. The materials handling
system would have to convey materials 63 percent faster
or work 63 percent longer on a daily basis. As can be
seen in Figure 2.1 (b), amendment requirement triples
over this range of sludge solids. Thus, variations in
sludge cake solids content can have a large impact on
both the capital and operating costs of a composting
system.
At many plants, the dewatering systems were installed
concurrently with the composting facilities. Because of
the importance of the assumed sludge solids content to
the overall design, public agencies should consider build-
ing a temporary dewatering system to develop a perform-
ance data base. Equipment can be rented, or one unit of
the planned dewatering facility can be purchased and
housed in a temporary location.
13
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Figure 2.1 The Effect of Sludge Dryness on Mix
and Amendment Quantities
~ 12
3
•o"o 10
II
6
14 16 18 20 22 24 26
Sludge Solids Content (percent)
(»} Mix Quantities as a Function of Sludoe Solids Content
I
II
2.0
1.0
0.0
16 18 20 22 24
Sludge Solids Content (percent)
26
(b) Amendment Requirement as a Function of Sludge Solids Content
Note: In this example, the ratio of sludge to amendment was not
allowed to drop below 1 :0.6 by volume (or porosity reasons.
are not considered "stable" in the sense used here.)
Several indicators of stability have been proposed,
including temperature increase, oxygen consumption,
effects on seed germination, and chemical changes in
the composting material (4). To date, no one test has
gained universal acceptance, although an increasing
body of data is being collected using oxygen consump-
tion as the indicator. A respirometry-based test proce-
dure for compost oxygen consumption has been
developed (1). Commercial respirometers for this pur-
pose are also available.
Regardless of the measurement used, stability increases
with the compost solids retention time (SRT), which is the
average residence time of the solids in the system,
including the effect of recycle. The relationship between
oxygen consumption rate and SRT is illustrated in Fig-
Planners and designers must consider the SRT of the
whole compost system rather than just the SRT of the
reactors. At all eight plants surveyed, the compost emerg-
ing from the reactor system was still biologically active,
although to varying degrees depending on the SRT of the
reactor system.
The design SRT should be that necessary to produce the
desired product quality. This depends on the compost
system and amendments used, because the biological
degradation rates are affected by temperature, moisture,
and the availability of oxygen, nutrients, and organic
materials.
Unfortunately, few data are available relating stability
measurements with SRTs or different product uses. Of
the eight plants visited, only the Akron facility attempted
to measure stability (using oxygen uptake rate). The
enclosed static pile system at the Montgomery County
Composting Facility has measured oxygen uptake rate
Figure 2.2 Stability as a Function of Detention Time (6)
Odor Emissions Rates
The design of odor control systems should be based on
allowable odor emission rates established by dispersion
studies or modeling of realistic worst-case weather condi-
tions. None of the eight plants surveyed performed this
kind of analysis during project planning, although a study
was performed at Akron in support of the odor control
modifications plan (5).
Stability and Solids Retention Time (SRT)
One of the difficulties in writing a product specification for
compost is defining "stability." A stable product is one in
which the rate of biological activity is low because of a
relative lack of easily degraded organics. (Biological
activity is also slowed in materials that are too dry or are
sterilized by excessive temperatures, but these materials
oc
11
I
O
150
125
100
50
25
0
10
15
Time (d)
20
25
30
14
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for several years. Operators there have found that an
SRT of 51 days (with continuous aeration) is sufficient to
reduce the oxygen uptake rate of the product to about 25
mg/kgh, a rate low enough to allow bagging and retail
sale of the material (7). As a first approximation, Haug (8)
recommends a minimum system SRT of 60 days to pro-
duce a compost with sufficient stability to avoid reheating
and phytotoxic effects. Longer SRTs may be needed for
product uses that require highly stable materials.
2.5 The Budget
2.5.1 Capital Costs
Capital costs for the eight plants visited are shown in
Table 2.4. These costs show the order of magnitude of
dollars involved in building an in-vessel facility. These
figures, however, should not be used to compare different
systems. Since in-vessel composting is a new and
extremely competitive business, new technology will con-
tinue to be introduced and equipment prices may change
in response. Also, since most of the plants are first-
generation facilities (i.e., the first implementation of the
technology in this country), a number of plant modifica-
tions have been required to correct unexpected prob-
lems. The costs for these modifications, which range
from complete reconstructions of odor control systems to
minor changes in individual conveyors, were often
unavailable. Many of these costs were absorbed by sys-
tem suppliers, who made the changes in order to meet
theirwarrantiesorperformance test criteria.
Other factors that affect cost and make costs difficult to
compare are:
• Site-specific factors such as the physical layout,
geotechnical conditions, climate, and the tradeoff
between capital and operating costs
• Sludge type, cake solids
• Local construction market
• Local regulations
Finally, at several locations, the capital costs included
modifications to local wastewater treatment plants. It was
not always possible to separate all of these noncompost-
ing costs from the total construction costs.
The impact of odor control costs should be noted. Based
on the figures in Table 2.4, odor control systems can
account for as much as 25 percent of the total construc-
tion cost.
Table 2.4 Summary of Capital Costs
Location
Akron
Cape May
Clayton County
Newberg8
Pittsburgh
Original
Construction
($1,000,OOOs)
30.5, which
includes 8-9 for
dewatering
facilities
8.4
control upgrade
and increased
aeration
2.0
2.84
13.07
Subsequent
Modifications
$3M for odor
$80K for product
and spare parts
storage building
—
Future
Modifications
Anticipated in 1988
Odor control
system upgrade
($7.5M estimated by
odor consultant)
Rebuild reactors
because of structural
design error (cost not
available)
Rebuild reactor because
of structure design
error (cost not
available)
PrnuiHo frtwarc* r\n
Portland"
Sarasota
Schenectady
11.6
4.2
6.67
Amendment dust
control system
($55K)and
miscellaneous
work($150K).
reactors ($500K
budgeted). Odor
control upgrade
estimated at $4.8M
Compost plant
will share new
spare parts
storage building
withWWTP
Odor control
system upgrade
($200K budgeted)
Odor control
system upgrade
($470K budgeted)
"Plant still in startup.
"Modifications funded by system supplier.
15
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2.5.2 Operating and Maintenance Costs
Annual operations and maintenance cost estimates for
six of the eight plants visited are shown in Table 2.5.
Because of their limited operating experiences, costs are
not available for the Newberg and Schenectady plants.
Unless otherwise noted, the values reflect the 1988 budg-
ets of the various facilities. Based on the sludge process-
ing rates reported during the site visits, estimates were
made of the sludge quantity processed during the year.
This figure was used to calculate the operations cost per
dry ton of sludge processed. As the table shows, opera-
tion and maintenance costs varied from approximately
$91 to $345 dt/day ($100 to $380 dton/day) of sludge.
Figure 2.3 Operating Costs as a Function of Sludge Processing
Rate
Table 2.5 Operating Cost Data
Location
Akron
Cape May
Clayton County
Pittsburgh
Portland
Sarasota
Total
Costs
($1000/yr)
1,125-
1,038°
148
1,020-
1,100
600*
Costs per
DryTon($)
135"
215"
290
100"
115
380
Capacity
Dry Ton/Day
(design)(1988)
73
20
2.9
34
60
6.3
•1987 expenditures.
>$/dry ton X 0.9072=$/dt.
•1989 budget estimate.
•Based on expected sludge processing rate of 12.1 dt/day (13.3 dton/
day), assuming that the plant is allowed to move to the next stage of
operation. Based on actual expenditures of $474,000 (Jan.- Sept. 88)
and actual sludge processing of 1,100 dt (1,200 dton) (Apr.- Sept.), the
unltcostwas$435/dt($395/dton).
•Not Including sludge disposal costs.
These costs exhibited an important economy of scale
effect. In Figure 2.3, operating costs are plotted as a
function of the actual sludge processing rate. The plants
with actual throughputs of less than 13 dt/day (12 dton/
day) reported "per ton" costs that were significantly
greater than plants with throughput capacities of greater
than 28 dt/day (25 dton/day).
An owner should exercise caution in applying these costs
to other situations. The costs reflect a number of site-
specific factors such as the local unit costs for labor,
amendment, power, etc., that may not be applicable else-
where. Moreover, all of the plants are currently operating
at less than design capacity. Since their fixed costs are
not distributed over the total design amount of sludge
processed, the reported "per ton" costs are higher than
would be the case if these plants were operating at their
design capacities.
2.5.2.1 Labor
The high degree of mechanization in existing in-vessel
composting plants has, in most cases, resulted in rela-
tively small operating staffs. However, the staff must have
the skills to operate and maintain this complex equip-
ment Consequently, although labor requirements are
small, individual staff members must be skilled.
S i 200
O O
o I
£•
100
Clayton Co.
) Cape May
Akron
0 Portland
Pittsburgh
10 20 30 40
Sludge Processing Rate (dry ton/day*)
•dton/day x .9072 - English dt/day.
From a planning perspective, greater investments in
training and incentives to minimize staff turnover should
be instituted. The loss of staff can mean large disruptions
in operations. Disincentives to working at a composting
plant include unpleasant working conditions (high humid-
ity and temperatures, odors, dust) and limited upward
mobility. Composting staff requirements are discussed
further in Chapter Four.
Because of different bookkeeping systems (maintenance
labor sometimes is included with operations labor and
only the combined value reported), operating labor costs
are available for only three of the eight plants visited. At
these three plants, labor costs range from 18 to 43 per-
cent of the composting budget (i.e., approximately $33 to
$132 dt [$30 to $120/dton] of sludge processed).
2.5.2.2 Amendment
Amendment costs usually are the largest single cost item
after operating labor. At five of the eight plants visited (not
including Clayton County, which uses wood chips grown
at the Clayton County Water Authority's land treatment
system), amendment costs range from 18 to 41 percent
of the composting budget (i.e., approximately $22 to $143
dt [$20 to $130/dton] of sludge processed).
One reason for this wide cost variation is the range in unit
costs for sawdust. Unit costs for amendment at six of the
eight plants visited are reported in Table 2.6. On a dry
weight basis, amendment costs vary by nearly a factor of
four.
16
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Table 2.6 Amendment Unit Costs"
Location
Akron
-Shredded bark
-Sawdust
Cape May
Clayton County
-Authority grown chips
-Purchased chips
Pittsburgh
Portland
Sarasota
Amendment Cost
per Cubic Yard
$ 9.4"
5.0
14.25
1.0
2.7
4.5
3.0
11.5
"Costs reported are from the time of the site visit.
b$/cy x 0.7646 = $/m3
Table 2.8 Comparison of Solids Contents in Design
and Actual Mixes
When deciding, on the basis of costs, whether to use
sawdust or wood chips, the owner may find it useful to
consider the relationships between amendment, desired
porosity, aeration uniformity, and screening cost. Man-
agers should be aware that amendment usage has con-
sistently been greater than anticipated in the design.
Table 2.7 compares the volumetric mix ratios projected at
the time of design to the actual mix ratios. Except for the
Sarasota plant, which uses less amendment than antici-
pated, actual amendment usage at the eight plants vis-
ited was up to 300 percent greater than anticipated
during design.
Table 2.7 Comparison of Design and Actual Mix Ratios in Use
at Time of Site Visit
Location
Volumetric Mix Ratio
(sludge/amendment/recycle)
Design Actual
Akron, OH"
Cape May, N J
Clayton County, GA
Newberg, OR"
Plattsburgh, NY
Portland, OR
Sarasota, FL
Schenectady, NY
1/0.66/1.33
NA
1/1/1
1/0.35/5
1/0.67/1.67
1/0.4/0.6
1/1/2
NA
1/1/2
1/0.6/1
1/1.4/1.4
1/0.5/10
1/1.4/3.3
1/1.6/0.6
1/0.56/2.44
NA
*-« i IGI iui i ioi u 10 uuii 101 ueuuuu uarK fcinu sawausi. uesign. u.oo parts
each. Actual: 0.4 and 0.6 parts, respectively.
bData are from the time of the site visit when the plant was in startup.
Process not at steady state.
NA = Not available.
Two factors account for this increase: (1) in some plants,
the dewatered sludge and/or recycle have been wetter
than anticipated, requiring greater use of the driest mate-
rial to achieve the desired mix solids content; and (2) at
some plants, the design mix solids content was undesir-
able either because it caused materials-handling prob-
lems or because it led to a product that was too wet.
Table 2.8 compares design and actual mix solids
contents.
Solids Content of Mix
(percent)
Location
Akron, OH
Cape May, NJ
Clayton County, GA
Newberg, OR
Pittsburgh, NY
Portland, OR
Sarasota, FL
Schenectady, NY
Design
40
40
—
38
40
32
35
35
Actual'
44
37
34
38"
40
37
40
37
"Target values. Actual mix solids vary slightly from these values on a
day-to-day basis.
bPlant in startup; process not a steady state.
2.5.2.3 Power
In contrast to preconstruction amendment estimates,
estimates of power usage have been relatively reliable. In
fact, power or energy requirements are often included as
a performance criterion that must be met during the
acceptance test. In a few cases, motors have been
replaced with larger units during startup, but undersized
motors have not been a widespread problem.
Power costs at six of the eight plants visited range from
11 to 27 percent of the composting facility budget (i.e.,
approximately $17 to $83/dt [$15 to $75/dton] of sludge
processed). The electricity usage in kWh per unit of
sludge or product was not available at the time of the site
visit.
Variations in power usage can be attributed to several
factors. The air volume used and the air distribution
method, which affects headloss, vary from system to
system. In addition, the energy requirements of
materials-handling systems depend on the kinds and
numbers of conveyors used and on the length of time
they are operated.
2.5.2.4 Maintenance Budgets
Obtaining consistent maintenance cost data was difficult
because facilities have different bookkeeping systems.
At many plants, maintenance is provided by the WWTP
staff or some other central office. At other plants, mainte-
nance labor is combined with operations labor. Reliable
information was available from only three of the eight
plants visited. At these plants maintenance costs range
from 13 to 24 percent of the composting budget (i.e.,
approximately $28 to $88/dt [$25 to $80/dton] of sludge
processed). Although, maintenance costs at the eight
plants visited were highly variable, in almost all cases,
they were greater than expected.
The service life of compost equipment is not well known.
Most equipment that did not exhibit obvious problems
17
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during startup has not been operated long enough to
show problems caused by normal wear. Nevertheless, if
the experiences of drag chains and reactor discharge
devices are an indication (see Chapter Three), plant
managers can anticipate that equipment service life in
composting systems will not be as long as in other appli-
cations of the same equipment. Therefore, managers
should be conservative in estimating maintenance budg-
ets and consider purchasing extended warranties from
the system suppliers.
2.5.3 Revenues from Product Sales
There is little reliable information on which to base expec-
tations of revenues from compost sales. Not surprisingly,
the plants operating for longer periods of time have real-
ized more revenue gains. At most of the plants, however,
the demand for compost had not caught up with the
supply. Most plants were stockpiling excess product or
essentially giving it away as landfill cover material. At the
time of the site visits, Pittsburgh and Sarasota were
paying to have the product hauled away as a waste.
Only two of the eight plants reported an income from
sales that covered more than 5 percent of their compost-
ing operating costs. These plants were Clayton County
(about 25 percent) and Portland (about 35 percent). In
their original procurements, both plants required the sys-
tem supplier to purchase all compost produced. North
American Soils (NAS), a subsidiary of Taulman pays
$7.50/m3 ($5.75/cy) for Clayton County compost, and
$39/dt ($35/dton) for sludge composted at Portland. How-
ever, such an arrangement does not guarantee compost
sales. If the product is not adequate for the market, it will
inevitably accumulate.
Although marketing the compost may not increase reve-
nues substantially, product sales can represent signifi-
cant avoided costs when compared to alternative sludge
disposal options. At the Cape May plant, for instance,
operations and maintenance costs exceed $220/dt
($200/dton) of sludge. The alternatives, however, are
much more expensive — $441/dt ($400/dton) to inciner-
ate it locally and $880/dt ($800/dton) to land apply it in
Pennsylvania.
2.6 Marketing
2.6.1 Identifying and Quantifying Compost Product
Markets
A marketing study that investigates all of the potential
uses of the product as well as the criteria associated with
each use should be part of the planning effort of any
composting facility. Real and potential uses identified
from existing plants include:
• Large-scale landscaping (bulk sales to private land-
scapers, golf courses, public works projects, parks,
schoolyards, highway median strips, etc.)
• Local nursery industries (as potting material)
• Greenhouses
• Urban gardeners
• Land reclamation projects (strip mines, etc.)"
• Landfills (daily cover or permanent cover for sections
of landfills being closed)
The marketing study should verify that there is sufficient
market demand to absorb the anticipated amount of com-
post. Projections of market demand should be done with
some care; underestimating the market is vastly prefera-
ble to overestimating it. Competing products, if any,
should be identified. For example, at the present time,
landscapers, nurseries, greenhouse growers, and urban
gardeners use peat moss, pine bark, composted hard-
wood bark, and spent mushroom soil to improve soils
and blend with potting media. These materials frequently
are in short supply, are imported from other states or
countries, and typically cost more than $9/m3 ($12/cy).
Sludge compost might be an attractive alternative for this
market.
2.6.2 Compliance with State Regulations
State regulatory agencies generally have jurisdiction
over the sale and distribution of compost. All State agen-
cies require the composting process to meet EPA's path-
ogen reduction criteria — a 3-day residence time at a
temperature exceeding 55 °C (131°F) (9). Beyond this
minimum requirement, most States have regulations
designed to assure that toxins, particularly heavy metals,
do not accumulate in the food chain. Different uses are
permitted depending on the metals content of the com-
post. Some examples of State regulations are shown in
Table 2.9.
Table 2.9 State Standards for Unrestricted Compost Use
Maximum contaminant concentrations (mg/kg) allowed in
the least restricted use category
State
New New
Contaminant Florida' Jersey" York" Ohio0 Oregon'
Arsenic
Cadmium
Chromium
Copper
Lead
Mercury
Nickel
PCBs
Zinc
Cadmium/Zinc ratio
—
30
—
900
1,000
—
100
—
1,800
—
1
40
1,000
1,200
4,800
10
1,250
—
2,400
—
—
10
1,000
1,000
250
10
200
1
2,500
—
—
12.5
—
—
500
—
—
5
—
0.015
—
25
—
800
1,000
—
100
—
2,000
—
'General application to agricultural land.
"May not be used on food crops.
"No use restrictions.
Many States require a permit to market compost, but it
may take months or even years to obtain one. Product
accumulated at the Schenectady plant for months while
the city waited for a State permit to sell it. The Cape May
18
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Municipal Utilities Authority did not receive its license to
distribute compost until 3 years after the plant began
operations. Although much of the compost was sold to an
independent broker who had a distribution license, inven-
tories still built up to a year's worth of production. In
addition, some States, such as Massachusetts, consider
most compost products akin to sludge and require users
to have application site permits.
2.6.3 Choosing a Target Compost Market
There are two major approaches to marketing compost.
One is to build the facility and then look for a market for
the product. This is a high risk approach.
In the second approach, the compost use and target
market is chosen in advance. Then product characteris-
tics are used to guide the design of the plant and com-
posting process. For example, the product particle-size
distribution and metals content help define the allowable
characteristics of the feed materials. The target levels of
product stability and moisture content help determine the
solids retention time, aeration rate, and monitoring prac-
tices of the facility. Stability and moisture content targets
and the need to avoid septic conditions also affect how
the compost product is cured and stored. Additionally,
product characteristics define the need for postcompost-
ing processes such as screening or blending with other
materials.
If the intent of composting is to manufacture compost for
horticultural use, it is necessary to design, construct, and
manage composting facilities to ensure a quality product.
Unacceptable compost is almost as expensive to pro-
duce as quality compost, so facility owners should con-
sider the economic benefits of aiming their product at the
high end of the market.
2.6.4 Quality Criteria Required by Potential Users
Each compost use has associated with it particular qual-
ity requirements and standards. Horticultural outlets, for
example, can provide a ready market for high-quality
compost.
Because of the intimate relationship between product
quality and facility design, the importance of defining
process goals during facility planning cannot be overem-
phasized. Plant owners/operators should consider writ-
ing a specification for compost that reflects the qualities
desired by the expected users. In the past, this has not
been an easy task because "compost" has not been
easily defined. Table 2.10 lists characteristics that are
important to potential compost users. The lack of a strin-
gent definition of compost and tests to determine
whether or not a compost has achieved the desired
"quality" are endemic problems that need further study.
Finished compost should have a comparatively uniform
particle-size distribution and texture. Metals and salts
should be within State limits. If high metals concentra-
tions are a problem, source reduction through industrial
pretreatment or blending of the compost with cleaner
Table 2.10 Compost Characteristics that May Be Used in a Prod-
uct Specification
Pathogen Destruction
Concentrationsof Specific Constituents (metals, nutrients, etc.)
Particle Size
Texture
PH
Moisture Content
Odor
Oxygen Demand or Respiration Rate (index of stability)
Aerobic/Anaerobic State
Weed Seed Inactivation
Phytotoxicity
Reduction of Volatile Solids
Consistency of Product
materials may have to be implemented. Improper use of
hydrated lime or alum at the WWTP or materials high in
cellulose and low in lignins adversely affect the quality of
the product. The compost specifications required for vari-
ous horticultural uses are described by Gouin (10) and
shown in Table 2.11.
Table2.11 Horticultural Compost Requirements(IO)
Moisture
Particle Concen-
Odor pH Size tration Other
Container
nurseries,
landscape
diameter
contractors,
greenhouses
Minimal 6.0-7.0 <; 1/2 in. 50%
(.01 m)
No lime
added
Homegardeners Minimal 6.0-7.0 <,M2'm.
(.01 m)
diameter
40%-
No lime
unless
required
by state"
NS
Field grown NS 6.0-7.0 <1in. NS
nursery, (.03m)
plants and sod= diameter
"Especially important if product is bagged.
"If required by state, lime should be added after composting and not
before to allow the facility to manufacture two types of compost instead
of one.
"Compost levels should not exceed 50 dry tons per acre and soil testing
is highly recommended.
NS = Not specified.
Improper composting or storage can result in a wet, odor-
ous product. Testing conducted by Willson and Dalmat (6)
suggests that compost with a respiration rate of 100 mg/
kg hr is acceptable for most field applications. For horti-
cultural uses with sensitive plants, a respiration rate of
less than 20 mg/kg hr may be desirable.
Consumer demand may necessitate the implementation
of additional quality control criteria, including weed seed
inactivation, phytotoxicity (toxicity to plants), and volatile
solids content. Sewage sludge often contains seeds with
the potential to germinate in the product, if not inactivated
19
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during the composting process. Tomato seeds are partic-
ularly hardy in this respect. Meeting EPA time and tem-
perature requirements for pathogen reduction is usually
sufficient to eliminate weed seeds.
Phytotoxicity, particularly to germinating seeds, is a
major concern for the nursery industry. Excess ammonia,
salts, or certain chemicals in the compost may be harm-
ful to seedlings. The ammonia may be a residual from the
composting process, or may be produced by further bio-
logical degradation after sale of the compost. Thus, the
degree of stabilization of the organic materials in the
compost is important.
Changing the form of compost packaging may affect the
compost stabilization required. At the Portland plant, the
compost produced had a very slight ammonia odor that
was not a problem for bulk consumers because the
ammonia quickly dissipated after the compost was
applied to soil. When the plant switched to bagging,
however, the plastic bags prevented the ammonia from
evaporating. Consumers faced a strong and unpleasant
ammonia odor when opening bags of product. To avoid
this situation, the plant operation is being modified (lower
sludge loading rates, longer reactor detention times) to
provide more complete compost stabilization bagging.
2.6.5 Marketing Agents
Compost can be marketed by the public agency (owner),
the system supplier, or a third party (broker). The choice
is a tradeoff between risk and revenues. To the extent that
the owner passes off the risk to other parties, his or her
financial return usually is reduced. Marketing by the
owner maximizes return, but also requires the owner to
develop the necessary marketing capabilities. Few public
agencies wantthis responsibility.
As seen in Table 2.12, selling the compost to a broker or
other marketing agent is the most popular option. At
Clayton County, the system supplier agrees to buy all of
the compost at a set price. At Akron, the owner and
system supplier share the costs of marketing and the
profit from the sales.
Table 2.12 Compost Marketing Agents
Location Marketing Agents-
Akron, OH
Cape May, NJ
Clayton County, GA
Newberg, OR
Pittsburgh, NY
Portland, OR
Sarasota.FL
jScheneotady, NY
Same subsidiary of composting system
supplierthat is operating the plant
Owner
Subsidiary of composting system supplier
No product yet (plant in startup)
City
Subsidiary of composting system supplier
Owner (looking forbroker)
Owner (looking for broker)
•As of December 1988.
Marketing contracts usually specify minimum and maxi-
mum quantities to be sold, minimum quality criteria to be
met, and a fixed price over a defined time period. The
broker may have the ability to blend the compost with
other materials to make a product with the specific char-
acteristics that the market demands; this flexibility also
lessens the risk. It is not necessary to restrict sales to a
single broker.
2.6.6 Contingency Plans
Occasionally, the facility may have excess product due to
insufficient market demand or product that does not meet
the specifications. The metals content of the sludge may
increase temporarily, or unstable batches of compost
may be produced because of mechanical problems. Con-
tingency plans for disposing of sludge and unmarketable
compost should be developed during project planning.
Potential backup measures vary with the problem and
with the individual plant, but may include creating addi-
tional storage space, incineration, land application, and
giveaway programs. Table 2.13 summarizes backup mea-
sures appropriate for various problems. As the table indi-
cates, several backup measures require additional
equipment or space, which should be included in the
initial plans for the facility.
Table 2.13 Potential Backup Measuresfor Various
Composting Problems
(Locations where these backups have been
used are shown in parentheses.)
PROBLEM: Wetor understabilizedproduct
BACKUPS: a. Recyclematerialbackintothereactor(Allplants)
b. Temporary aerated static pile operations
(Possible at Cape May, contingency plan for
Newberg)
c. Landfilling or land application (Newberg, Sarasota)
d. Shift product to users who have lesser quality
standards (Plattsburgh - landfill cover material)
PROBLEM: Increase in metals or other constituent
(Not found to be a problem at plants surveyed)
BACKUPS: a. Compost storage and dilution with good material
b. Landfill or landfill cover
PROBLEM: Compost plant out of service
BACKUPS: a. Land application of sludge (Portland)
b. Landfilling of sludge
c. Storage of liquid sludge at the WWTP (Akron -
sludge storage tanks, Clayton County- DAF unit,
Portland - sludge lagoon, Sarasota - sludge storage
tanks)
d. Aerated static pile operations (Newberg)
e. Combustion of sludge in an existing incinerator
(Akron, Schenectady)
PROBLEM: Insufficient demand
BACKUPS: a-e. Same as for compost plant out of service
f. Compost storage on site (Akron, Plattsburgh,
Portland, Schenectady, Newberg)
2.7 PilotPlants
Pilot composting plants can be used to approximate pro-
duction of representative compost with the same sludge
and amendment that will be used in the full-scale unit.
20
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A product sample from the pilot facility also could be
useful during the market study. Potential users may not
know how to specify product quality requirements, but
know whether or not they can use a specific product. In
Portland, for example, the nursery industry was identified
as a potentially large market. After the plant was built,
however, nursery owners were unwilling to buy large
quantities of compost until they had conducted long-term
tests themselves. If sample product had been available
before plant construction, the nurseries could have
begun testing earlier. Using a pilot plant, the users and
the public agency together may be able to develop a
product.
A pilot plant also could be used as part of a public
education program to demonstrate the composting pro-
cess and show that compost is not simply a mixture of
sludge and sawdust. At several of the plants visited, oper-
ators gave away compost samples to interested mem-
bers of the public to try for themselves. The response
was overwhelmingly positive. This type of public relations
activity during the planning stage can be very effective in
building good will.
A pilot plant operation may also be useful for testing
certain process parameters. For example, the suitability
of potential amendment materials that are locally avail-
able and inexpensive can be tested with the actual
sludge. Such a test can identify undesirable amendments
before they are tried at full scale. At Schenectady, for
example, some of the sawdust originally specified con-
tained an excessive amount of hardwood powder. When
mixed with the sludge, it created a paste-like material with
little porosity that dried into hard balls in the reactor. A
pilot test might have identified this problem in advance.
At Pittsburgh, a 3.0 m (10 ft) high pilot reactor con-
structed from a 0.6I m (24 in.) diameter metal tube has
been used to test a variety of amendments for the New
York State Energy Research and Development Authority.
A pilot operation also can test the odor constituents of
reactor process air and provide information on odor emis-
sjon rates. This information then can be used in disper-
sion modeling and odor treatment facility design. The
original odor control system at the Pittsburgh facility
was designed to remove hydrogen sulfide, which was a
problem at the WWTP dewatering room. A pilot operation
might have revealed that ammonia would be a major
constituent of the process air at this site. Designers could
have included ammonia removal in the odor control sys-
tem from the start, ratherthan retrofitting the plant later.
A pilot operation may not reliably predict the amount of
odor that will be produced by a full-scale plant. At full
scale, a site's odor-dispersing capacity may be exceeded
during stagnant air conditions. Facilities should use
meteorological modeling in addition to odor emissions
data from the pilot plant to predict impacts at full scale.
The scale of the pilot plant is related to its use. If a pilot
plant will be used only for marketing and public relations,
a static pile or enclosed static pile operation will suffice.
On the other hand, testing of porosities, mix ratios, aera-
tion requirements, and degrees of drying will necessitate
a model pilot plant that simulates the full-scale reactor.
2.8 References
1. Murray, C. Managing Odor Control Programs. Presen-
tation at the 19th Annual Biocycle National Confer-
ence. Washington, DC. May 1989.
2. Connor, D.A. and J.A. Stearns. A Method for Evalua-
tion and Procurement of In-Vessel Composting Sys-
tems. Public Works. April 1986. p. 64.
3. Haug, R.T Composting Process Design Criteria, Part
I — Feed Conditioning. Biocycle. August 1986. p. 38.
4. Haug, R.T and L.D. Tortoriei. Composting Process
Design Criteria — Part IV — Case Study. Biocycle.
November/December 1986. p. 34
5. Akron Composting Facility Odor Study. Draft Final
Report. Odor Science and Engineering, Inc. July
1988.
6. Willson, G.B. and D. Dalmat. Measuring Compost
Stability. Biocycle. August 1986. p. 34
7. Murray, C. Personal communication. June 1989.
8. Haug, R.T Composting Process Design Criteria, Part
II — Detention Time. Biocycle. September 1986. p.
36.
9. US. EPA. Reductions of Pathogen Municipal Waste
Sludge. CFR207.1989.
10. Gouin, FR. Compost Standards for Horticultural
Industries. Biocycle.
21
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Chapter 3
Design Considerations
This chapter summarizes design considerations and
experiences at the eight in-vessel composting facilities
visited. It is not a comprehensive discussion of the issues
involved in designing an in-vessel composting facility.
Rather, the chapter's purpose is to present facility design-
ers with a number of design options, and to illustrate past
successes and failures for the benefit of future designs.
Table 3.1 lists the items covered in this chapter and the
sections where they are discussed.
Table 3.1 In-Vessel Composting Design Considerations
Topic Section
General System Considerations 3.1
Environmental Conditions 3.1.1
Operating Conditions 3.1.2
Equipment Access 3.1.3
Materials Handling 3.2
General System Considerations 3.2.1
Materials Receiving and Storage 3.2.2
Conveyor Systems 3.2.3
Mixers 3.2.4
Reactor Discharge Devices 3.2.5
Aeration 3.3
General System Considerations 3.3.1
Air Collection/Moisture Removal 3.3.2
Hardpan 3.3.3
Curing/Storage Aeration 3.3.4
Odor Control 3.4
Compost Process Control 3.4.1
Inventory of Potential Sources 3.4.2
Collection and Containment 3.4.3
Treatment 3.4.4
Dispersal 3.4.5
Additional Considerations 3.4.6
Process Monitoring and Control 3.5
Analytical Equipment for Process Control 3.5.1
Process Sensors 3.5.2
Instrumentation 3.5.3
Support Facilities 3.6
Maintenance Work Area 3.6.1
Onsite Spare Parts Storage 3.6.2
Staff Facilities 3.6.3
Fire Prevention and Control 3.7
Much of the chapter is devoted to conveyors and odor
control equipment because the majority of plant prob-
lems have been caused by failures of this equipment
rather than of the composting reactor system. Conveyor
problems have forced short-term shutdowns or otherwise
disrupted operations at nearly all of the plants surveyed,
and four plants have either temporarily shut down or
curtailed operations because of odor problems.
Reactors have also had problems, however. Both the
Newberg and Schenectady reactors have had structural
problems, and reactor discharge devices have experi-
enced problems at Portland, Schenectady, Clayton
County, Pittsburgh, and Sarasota. Formation of a com-
pacted "hard pan" layer at the bottom of the reactor
which blocks airflow has occurred at Akron, Cape May,
Clayton County, and Pittsburgh.
3.1 General System Considerations
3.1.1 Environmental Conditions
In-vessel composting plants experience extreme environ-
mental conditions inside the process building, such as
high temperatures, humidity, and dust levels; corrosive
atmosphere; and wet, sticky materials. In addition, con-
veyors at many plants connect hot, humid interiors to
cold exteriors. For example, Pittsburgh has experienced
ice build-up on conveyor belts that travel from the humid
interiors of the reactors to the mixing room via the exterior
of the buildings. Dust has been a problem at some plants,
including Portland, where it has caused bearing failures
in the motors that power the hydraulic system, and
Schenectady, where it has caused problems with motor
controllers.
Environmental conditions should be considered when
choosing construction materials and corrosion protection
systems. At Pittsburgh, the fog inside the reactor build-
ings is corroding the feed and agitation machinery; epoxy
coatings on the auger motors are peeling. Epoxy coat-
ings are also degrading in the hot and wet work space
over the Cape May reactors. At Cape May, galvanized
equipment seems to be performing better than epoxy-
coated equipment. Zinc-based paint might be another
option to deal with corrosion. Wilber (1) found thSt the
aromatic organic solvents that evolve during composting
do not attack zinc-based paints.
3.1.2 Operating Conditions
Sludge, amendment, recycle, and mix characteristics
such as moisture contents, volatile solids contents, bulk
23
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weights, porosities, and particle-size distributions are not
constant. These properties may change daily. Table 3.2
shows the ranges of moisture contents of sludge, amend-
ment, recycle, and mix observed at the eight plants vis-
ited. Composting systems must be designed to
accommodate all foreseeable materials conditions.
Materials-handling systems should be designed to move
materials with a range of bulk weights, consistencies, and
particle sizes. The transfer rates of conveyors used in
creating and moving the initial mix should be adjustable
over a wide range so that operators can make and trans-
port an acceptable mix.
Aeration systems should be able to respond to varying
conditions in the reactor. Wet mixes or sludge batches
with a high volatile organic fraction will require higher
airflow rates. Variable speed blowers or inlet throttling are
normally provided to facilitate airflow rate adjustment.
Blowers should also be designed so that their discharge
pressures can overcome headless, even when the mix
porosity is low.
3.1.3 Equipment Access
Designers should lay out and specify equipment to facili-
tate operations, monitoring, and preventive maintenance.
Critical locations, particularly wear points and places
where materials can jam, should be made as visible and
accessible as possible so that operators can observe
equipment operations and resolve problems before they
become catastrophic. At several plants, operators or sys-
tem suppliers have installed additional inspection and
access hatches, especially in materials-handling
systems.
In addition, operators should be provided easy access to
equipment wherever preventive maintenance activities
Table 3.2 Variations In Moisture Contents
for Different Materials
Solids Content (% by weight)
Location
Akron
Cape May
Clayton County
Newberg1
Pittsburgh
Portland
Sarasota
Schenectady
Sludge
20-26
20-30
16-18
12-20
20-25
23-28
12-16
20-28
Amendment
50-60
65-70
52-66
—
—
60-95
—
—
Recycle
60-68
53-65
45-52
35-45
42-55
38-40
50-55
—
Mix
40-50
—
34-35
30-40
—
—
34-46
35-39
'Data shown were collected in Sept. to Oct., 1988 during acceptance
testing.
or regular repairs take place (e.g., lubrication points,
Inspection ports, bearings, cleanouts, etc.). If the com-
posting equipment cannot be serviced from the floor,
elevated walkways, monorails, and other means of
access should be provided.
System layouts should allow sufficient space around
equipment for maintenance activities. Where appropri-
ate, enough space should be provided so that a unit can
be repaired while an adjacent unit is operating.
In the mixing room at Pittsburgh, the enclosed screw
cross conveyors are located about 4.5 m (15 ft) above the
floor (see Figure 3.1) with no built-in access. Moreover,
Figure 3.1 Plattsburgh Screw Conveyer
Screw conveyor
located 4.6m (15
ft) off floor with no
permanent opera-
tor access.
Mobile lift used
by operators
when screw
conveyor
requires
servicing.
access hatches were not provided in the screw housing.
To clear a simple materials jam, the operators had to
climb ladders and cut through the housing with a torch.
At the Portland plant, the hydraulically operated conical
"slingers," which distribute the compost mix as it is fed
into the reactors, are located at the apex of the domes
that cover the reactors. Only a narrow walkway leads to
them (see Figure 3.2). There is no mechanism to remove
the slinger from the reactor and no place to put it. Conse-
quently, when a problem develops in a slinger, the opera-
tors must approach it from inside the reactor by walking
over the top of the compost bed, a potentially hazardous
maneuver.
Accessibility is equally important for equipment that must
be monitored regularly, particularly mixers. The whole
composting process depends on the quality of the mix,
which operators must monitor regularly. At the Clayton
County and Portland plants, the mixers are located at the
tops of the reactors in small sheds, so operators, who are
stationed in the ground-level control room, cannot moni-
tor them. To test the mix, an operator must climb the
exterior stairs, take a sample, then descend to the ground
level, where the testing equipment is located. If the mixer
must be adjusted, the operator may have to make several
24
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trips. To avoid this situation, the mixer should be located
near the control room, and the testing equipment should
be located near the mixer, as it is at Sarasota (see Fig-
ure 3.3).
Figure 3.2 Top of Reactor at Portland
Note: No working space is provided above the "slinger," located
inside and just below the roof of the reactor.
3.2 Materials Handling
3.2.1 General System Considerations
3.2.17 Equipment Reliability
The materials-handling system must remain operable for
the composting plant to function. It usually is not eco-
nomical or necessary to provide full materials-handling
equipment redundancy (i.e., permanent standby units).
Because sludge can be stored, short-term interruptions
in operations can be tolerated.
Figure 3.3 Sarasota Mixing Area
Instruments
Note that the instruments for determining mix solids content are
located immediately adjacent to the mixer for operator convenience.
The key to maximizing the reliability of the system is
to minimize the frequency and duration of these inter-
ruptions. To accomplish this, materials-handling systems
should be built with equipment that is durable and easy to
repair. In addition, a fast response to mechanical failure
requires an adequate supply of easily accessible spare
parts and a commitment to provide adequate numbers of
appropriately trained staff.
3.2.1.2 Operational Flexibility
Based on the experiences at the sites visited, and despite
the best efforts to minimize downtime, there will probably
be occasions when a piece of equipment is out of service
for an extended period. Where possible, alternative path-
ways for materials should be provided so that the entire
system is not paralyzed. The Sarasota facility's materials-
handling system, shown in Figure 12-3, can transfer
materials from the bottom of either reactor to the top of
either reactor, and both reactors can be used.first or
second in the process. Additionally, the mixers can feed
either reactor. (Note that the Sarasota plant is a second-
generation facility.)
If multiple process trains are used, bypasses and cross-
overs should be provided so that alternate conveyance
routes can be used. In contrast to the Sarasota system,
the almost complete lack of connections between the two
parallel compost trains at the Portland plant has contrib-
uted to extended periods of downtime.
The designers of the Akron plant used an innovative
approach to circumvent mechanical problems. Rather
than providing numerous connecting conveyors to
bypass trouble spots, they provided the means to take
materials out of the system, move them with mobile
equipment (i.e., front-end loaders), and reinsert the mate-
rials at different point. In this way, broken conveyors can
be bypassed while they are repaired. If necessary, the
mix can even be prepared on the mixing room floor using
front-end loaders.
Some designers of materials-handling systems have
tried to reduce costs by using one piece of equipment to
perform two functions, but such layouts limit the flexibility
of the materials-handling system. For instance, at the
Clayton County plant, a single screw conveyor beneath
the reactors transfers recycle to the feed system when
turning in one direction, and transfers compost out of the
plant when turning in the other direction. Clayton County
operators cannot simultaneously remove product from
the second reactor and load the first reactor. The capital
cost savings from these kinds of installations must be
balanced against the potential for increased labor and
power costs.
3.2.1.3 System Bottlenecks
A bottleneck, or chokepoint, is a conveyance point where
the downstream conveyors have a lower capacity than
the upstream conveyors. Chokepoints limit the convey-
ance capacity of the entire system. To avoid chokepoints,
all conveying equipment should cover approximately the
25
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same range of capacities. At the Clayton County plant,
the discharge conveyor that transfers product outside the
building to waiting trucks cannot accommodate the full
flow rate of the screw conveyor that brings the product
from the second reactor.
Analysis for chokepoints should be done under all operat-
ing conditions. Again, at the Clayton County plant, the
elevator drag chain becomes a chokepoint when the
sludge is very wet. The sludge hopper discharge drag
chain runs at a constant speed. Therefore, when the
sludge is very wet, the large amounts of sawdust and
recycle needed to create a proper mix cause overloading
of the elevator drag chain.
3.2.1.4 Centralized Computer Control
Centralized computer control of the materials-handling
system enables a small number of people to operate a
large amount of equipment. A centralized computer can
also interlock the conveyors so that when a problem
develops, all upstream devices shut down. Sequential
startup of conveying systems is another feature provided
by a central computer.
The software should include capabilities to both execute
a completely programmed function (e.g., feed the first
reactor, transfer compost from one reactor to another,
etc.) and to run individual items of equipment from a
central location. Clear visual displays can also be very
helpful. Manual controls should be provided near the
individual conveyors.
Centralized systems of this kind have been installed at all
of the eight plants visited. Except for a few minor prob-
lems, operators have been unanimous in their approval.
3.2.7.5 Tracking Materials Flow
As part of process monitoring, a mass balance should be
created and updated regularly. To create a mass balance,
the flow of materials must be known. Scales (including
truck scales) and other measuring devices should be
provided to track the flow of materials throughout the
plant At some plants (e.g., Akron and Pittsburgh),
scales in the conveying system are tied into the computer
system, allowing the computer to control the mix prepara-
tion. One problem with scales experienced at Platts-
burgh, however, was that old sludge adhered to the
scales and belts became heavier over time.
3.2.7.6 Detecting and Cleaning Plugs at Transfer
Points
Funnels, chutes, and other transfer devices between con-
veyors have been problematic because of the sticky nat-
ure of sludge, mix, and recycle. Plugs occur even in
vertical funnels. Sensors should be provided at all key
transfer points to detect plugging. These sensors should
also be tied into the central computer so that upstream
conveyors shut down in the event of a plug.
Periodic cleaning of funnels and chutes by operators can
alleviate many plugging problems. Access doors should
be provided for this purpose.
3.2.7.7 Preventing and Containing Spillage
at Transfer Points
Spillage, particularly from belt conveyors, generates a
continual stream of cleanup chores and is a chronic prob-
lem at many plants (see Figure 3.4). Providing catch pans
and belt scrapers reduces the labor needed to keep plant
areas clean. The cleated belt conveyor at Akron had
severe spillage problems, which are discussed in detail in
Section 3.2.3.1. The Schenectady plant contains several
features that make the area easy to clean, including
sloping floors, fire hoses, and open drains that can
accept solids.
Figure 3.4 Minor Spillage Associated with Conveyors
Schenectady
Sarasota
26
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3.2.7.8 Operating Schedule
The materials-handling system schedule should be
designed so that the required amount of mix can be
loaded during the time when the operators are at the
plant. In most cases, composting plants operate only one
shift per day. Because operators must perform other
tasks, it is seldom possible to convey materials for an
entire shift. Variations in the amount of materials to be
loaded also must be considered. The materials-handling
system should be sized for reasonable worst-case com-
binations of operator availability and materials flow.
At the Cape May plant, the conveyors were sized to move
the average quantity of material in 8 hours. Because
operators cannot run the system for 8 hours per shift and
sludge quantities exceeding the average amount cannot
be handled in 8 hours, the plant must be staffed for two
shifts. It was estimated by local officials that $100,000
per year could be saved if a one-shift operation could be
established.
3.2.2 Materials Receiving and Storage
3.2.2.7 Dust Control at Amendment
Receiving Facilities
At some plants, dust generation has been a problem
during the delivery of sawdust. A hood around the receiv-
ing facility should be provided to contain fugitive dust. At
the Portland plant, both a hood and a vacuum system
were installed after the plant was built (see Figure 3.5)
because the receiving facility is so close to other equip-
ment. A simple containment bin installed at the Sarasota
plant is shown in Figure 3.6. Even with a receiving bin,
some dust inevitably is generated during delivery.
Figure 3.5 Ventilation System at Sawdust Receiving Facility,
Portland
3.2.2.2 Protective Screens
Several plants have found large chunks of wood, rocks, or
other debris in their sawdust deliveries. If a piece of
equipment such as a screw conveyor or funnel cannot
convey large particles, screens should be included in the
system to prevent their entry. The Cape May plant has a
simple vibrating mesh screen over its bulking agent
receiving hopper. At the Schenectady plant, a rotary disk
screen is located at the exit of the amendment storage
silo.
3.2.2.3 Amendment Storage
In almost all cases, the amendment materials are the
driest of the three feed materials; facilities should be
provided to keep these materials dry, such as silos, cov-
ered bins, and covered storage yards. Although covered
bins or storage yards are less expensive than silos, they
frequently have problems with wind-blown sawdust. They
are, however, satisfactory for wood chips and bark.
The Schenectady and Clayton County plants both have
aeration systems that dry stored amendments. To date,
neither system has been used much, since the amend-
ments are usually sufficiently dry when received.
3.2.2.4 Recycle Surge Bins
Some facilities are designed so that during loading, recy-
cle is discharged from the reactor and conveyed directly
to the mixer. This configuration requires that the reactor
discharge device operate at a rate that exactly matches
the recycle feed rate needed at the mixer. It is difficult to
match these rates and, in addition, if the compost is
compacted or excessively dry, it will not always discharge
at an even rate.
Providing a surge bin between the reactor and the feed
system adds flexibility (the reactor discharge rate doesn't
have to match the mixer requirements) and allows opera-
tors to run the discharge device and the reactor feed
system at different times. In addition, a surge bin can be
used as a loading hopper for recycle when filling a reactor
for the first time or after an annual inspection.
3.2.2.5 Curing/Storage Space
Exterior curing/storage space must be sized to accom-
odate the solids retention time needed to produce a prod-
uct that is stable enough to market, since only part of the
composting process is performed in the reactor(s).
The kind of aeration also should be considered. Win-
drows require more space than forced air systems. Piles
that are too high may not receive enough air, and may
produce acidic by-products and become phytotoxic.
State regulatory agencies also have storage time require^
ments that need to be considered. Finally, the curing/
storage space should be large enough to accommodate
low quality batches of compost that need extra curing
time, seasonal variations in the compost markets, and
slow development of the local market. Since market
development can be a slow process, it may be worthwhile
to arrange temporary offsite storage for the first few years
of plant operations.
Curing/storage yards should have impermeable surfaces
(concrete or asphalt). Some regulatory agencies require
that runoff be controlled and returned for treatment. Con-
crete or asphalt surfaces also reduce housekeeping
chores. The area should be covered depending on the
climate and the desired moisture content of the final
product.
27
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3.2.3 Conveyor Systems
Typical parameters used in evaluating various conveyor
systems include:
• Properties of conveyed materials (bulk weight, mois-
ture content)
Load evaluation
Load path (geometry)
Conveyor function
Conveyance rates required
Physical dimensions
Horsepower requirements
Appurtenances required
Operation and maintenance costs
Capital costs
Reliability
Ease and speed of typical repairs
Ease of cleaning
3.2.3.1 Conveyor Reliability
All of the facility conveyors have been used reliably in
industry for years; none are experimental. Yet due to
differences in materials characteristics, conveyors at in-
vessel plants have experienced problems with jamming,
breakage, excessive wear, and spillage. A poorly operat-
ing conveyor consumes time, labor, and money and can
effectively shut down the whole plant.
Table 5.3 lists the kinds of conveyors used at the eight
plants visited and their functions. Operating experiences
with these conveyors are presented in the case studies
(Chapters Six to Thirteen), and are summarized in
Table 3.3. Lessons learned from these experiences are
discussed below.
Screw Conveyors
Screw conveyors have been the most successful type of
conveyor at the eight plants surveyed. In different forms,
Table 3.3
Problem
Common Conveyor Problems and Modifications
Modifications Performed
Plugging/Compaction Problems
Build-up and compaction of material in drag chain conveyors caused
motor overload. (Newberg)
Vertical discharge chute from the mixer to belt conveyor becomes
plugged with mixture. (Pittsburgh)
Divider gate that directs compost to recycle or to storage conveyors
becomes plugged. (Plattsburgh)
Compaction of material in drag chain conveyors led to jamming.
(Sarasota)
BulM-up and compaction of material in drag chain conveyors caused
bending of steel flights. (Portland)
Enclosed screw conveyors clogged when sludge accumulated
between screws and housings.(Clayton County)
Spillage Problems
Belt conveyor contents spill at discharge points. (Plattsburgh)
Sludge sticks to the cleated belt conveyor and eventually falls to the
floor. (Akron)
Spillage occurs at transfer points between conveyors. (Akron)
Wear/Abrasion Problems
Positive displacement drag chain conveyors were designed with
close tolerances between flights and housing especially at turns and
bending of flights. (Cape May)
Cleated belts have become worn and occasionally tear because of
full width idlers (support rollers). (Akron)
Drag chain housing suffered severe abrasion at turns. (Portland)
Miscellaneous Problems
The free end of the shaftless screw conveyor in the sawdust bin rises
out of the sawdust. (Sarasota)
Dust is generated wherever compost is dropped onto belt conveyors.
(Portland)
Icing of exterior belt conveyors. (Plattsburgh, Portland, Schenectady)
Flight height reduced from 4 in.a to 1.5 in.
Installation of motion indicators at this and several other locations
being considered.
Gate is now manually cleaned several times daily. Permanent
mechanical modification being considered.
Every other flight removed.
Half the flights removed. The other half were replaced by smaller,
plastic flights. Over-size steel flights were installed at 15-ftb intervals
to clean the housing.
Screw conveyor housings were lined with plastic.
Changed head and tail pulleys to smooth (rather than cleated)
lagging.
Cleated belt is now "dusted" with sawdust before sludge is applied.
Installing drip pans is under consideration.
The flight area has been reduced. Chain, flight, and housing
replacement costs still approximate $100,000/year.
Stub idlers, which support the belt only on the flat portion of the belt
outside the cleat wall, have been installed.
Portions of the housing were replaced with abrasion resistant steel.
Horizontal hold-down bars have been installed.
Exhaust hoods were installed over strategic sections of the
conveyors.
Operation of conveyors during icing conditions, even when materials
transfer is not required.
•In. x 2.540 x 10s = m
ftx.3048 = m
28
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they serve many functions. They are used to empty stor-
age bins at almost all of the plants (see Figure 3.7);
shaftless, spiral-type screws (see Figure 3.8) empty both
amendment and sludge storage bins. Enclosed screws
are used to convey the different materials found at a
compost plant. Screws are also used as leveling devices
for reactors and sludge bins. Many of the reactor dis-
charge devices are based on screws as well. (These
devices are discussed in Section 3.2.5.)
Figure 3.6 Retrofit Bin to Contain Dust at Sarasota
Spilled sawdust
Retrofit bin
In these many applications, screw conveyors have been
relatively trouble-free. The few reported problems are
related to excessive wear and the accumulation of mate-
rial on flights and housings. Materials accumulate on the
Figure 3.7 Example of Screw Conveyors that Empty Bins
at Plattsburgh.
(Typical of other plants as well.)
flights at Plattsburgh, but regular cleaning prevents prob-
lems. At Clayton County, materials accumulated between
the housing and the screw flights, causing undesirable
vibration. This problem was eliminated by lining the
housings with ultra-high molecular weight plastic. At
Portland, the accumulators, which are double screw con-
veyors, wore out quickly, but replacing them with single
screw units solved the problem.
Belt Conveyors
Belt conveyors have worked successfully at many plants.
In almost all installations, belt conveyors are shaped into
troughs to better contain the materials (see Figure 3.9).
Figure 3.8 Shaftless Screw for Emptying the
Sawdust Receiving Bin at Sarasota
Spillage is a recurring problem with belt conveyors. Wet
materials, which adhere to the rubberized belts, do not
transfer completely. The remaining material is carried
past the transfer point and subsequently falls on the floor,
creating a cleanup problem. This problem can be mini-
mized by using belt scrapers. Another tactic is to reduce
the contact between wet materials and the belt fabric by
placing a layer of dry material, such as sawdust, onto the
belt before adding a wet material, such as sludge.
Screws and belt conveyors are limited to straight-line
paths and cannot move materials vertically without long
inclined runs. When faced with the need to move materi-
als vertically, designers have turned to other types of
conveyors, particularly drag chain conveyors.
Drag Chain Conveyors
Drag chain conveyors are versatile devices, particularly
suited for moving materials between different floors of the
compost plant. A typical installation is shown in Fig-
ure 3.10. In practice, drag chains have not performed
well, suffering from plugging and excessive wear. Com-
posting materials, particularly the wet mix, adhere to the
29
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Figure 3.9 "Flat" Belt Conveyor at Newberg
Figure 3.10 Drag Chain at Clayton County
Usually the conveyor belts are formed into a trough-like shape as
shown here.
metal walls of the drag chain housing. As these materials
dry, they create a rough surface. When the flights push
wet material over this high friction surface, the wet mate-
rial is compacted against the flight and between the walls
of the housing and the flight, particularly in curved sec-
tions of the conveyor. Eventually, so much compacted
material accumulates that the flights bend and the chains
break. This problem has occurred at Cape May, Portland,
and Sarasota.
Modifications to the drag-chain conveyors at the eight
plants surveyed have involved increasing the space
between flights, increasing the space between the flights
and the conveyor housing, and decreasing the area of the
flights themselves. These modifications allow the com-
posting material to push backwards over or around the
flight, so it won't be compacted. These modifications,
however, also reduce the capacity of the conveyor.
At the Cape May facility, the designs specified close
tolerances between the flights and the housing. These
tolerances were achieved by placing the "feed" chain in
the narrow housing normally used for the "return" and
placing the "return" chain in the wider housing normally
used for the "feed" chain (see Figure 3.11). Flights with
large cross-sectional areas were also specified (see Fig-
ure 3.12a). In the narrow "feed" housing, compaction of
compost materials and consequent jamming was partic-
ularly frequent. In 1988, plant staff estimated that drag
chain maintenance cost the equivalent of one and one-
half persons full-time and about $100,000 per year in
replacement costs for broken chains and flights and worn
housings. Modifying the flight shape alleviated the
problem.
At Portland, half of the flights were removed and the
remaining flights were trimmed to 25 percent of their
Flight
Conveyor pictured with cover removed.
original area (see Figure 3.12b). In addition, most flights
were rebuilt and every 4.6 m (15 ft) along the chain, a
reinforced steel scraper flight having a slightly larger area
than the other flights was installed. These measures
have greatly reduced the conveyor jamming.
Figure 3.11 Drag Chains at the Cape May Plant
Feed side
Return side
The Schenectady facility, which uses drag chains, has
not experienced problems. There, the drag chain is
equipped with 0.2- by 0.33-m (8- by 13-in.) double-leg
(open center) flights. This drag chain is primarily a verti-
cal lift device with no curved sections.
Another modification that helps prevent jamming is to
line the conveyor housing with plastic. Mix materials do
not adhere to the plastic and the housing surface stays
smooth, thereby reducing mix materials compaction.
This modification was used successfully on a drag chain
at the Clayton County plant.
30
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Figure 3.12 Drag Chain Flights at the Portland and
Cape May Facilities
Figure 3.13 Cleated Conveyor at the Akron Facility
(a) Cape May
(b) Portland
Drag chains with large flights (a) have experienced jamming and
breakage problems at several facilities. Cutting down the flight area
as pictured in (b) has generally alleviated these problems.
Drag chains have been highly problematic at in-vessel
plants and should be used with great caution. When
used, the cross-sectional areas of flights should be sub-
stantially reduced, the spacing between flights should be
increased, curves should be eliminated as much as pos-
sible, and operators should consider lining the drag chain
housings with plastic.
Cleated Belt Conveyors
Cleated belt conveyors are another means of moving
materials up steep inclines. In a cleated belt conveyor,
folds (or cleats) sewn into the belt fabric create a series of
buckets (see Figure 3.13). Cleated belt conveyors were
encountered only at the Akron plant.
The Akron cleated belts have had a number of problems.
The major problem was that wet material, especially
sludge, sticks to the belt fabric and does not discharge at
the transfer point. During the plant startup, approximately
10 percent of the feed, or more than 27 wt/day (30 wton/
day) spilled on the discharge side of the conveyor (see
Figure 3.14). Spillage has been reduced to 1 to 2 percent
by dusting the. belt with sawdust before applying the
sludge. This solution may apply to plants that have expe-
rienced similar problems with other types of conveyors.
A second problem at Akron was that the conveyor idlers
(support rollers) originally supplied with the cleated con-
veyors spanned the full width of the belts and caused
excessive wear on the cleat edges. When a cleat wall
tore, it caught on the idler, tearing the belt apart. These
problems have been solved by replacing the original
rollers with "stub" idlers that support the belt only on its
edges.
A third problem is excessive spillage when handling very
dry material. Apparently a static charge builds up on the
cleats, causing the dry materials to cling.
Although subsequent modifications have made the sys-
tem functional, cleated belts do not appear to be well-
suited to composting operations. Currently, the City of
Akron is seeking EPA funding to replace its cleated belt
conveyors.
3.2.4 Mixers
3.2.4.1 Mixing Equipment
Of the eight plants visited, four had mixers that used plow
blades mounted on a rotating shaft; the other four used
pugmill mixers (see Figure 3.15). In general, the mixers
have needed minor modifications after which they have
worked satisfactorily.
31
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Figure 3.14 Spilled Material at Akron
Figure 3.15 Two Kinds of Mixers Used at In-Vessel Facilities
Spilled mix
material
f Clean-up crew
Only Schenectady has reported problems with a plow-
blade mixer. An overloaded mixer caused jamming and
was thus unable to accept its rated throughput ratio. To
correct this problem, the mixer discharge hole was
enlarged. It is not known whether the problem has been
solved because the mixer has not been tested at capacity
since the modification was made.
Accumulation of the mix materials on mixer paddles and
consequent plugging of the mixer was reported at three
Pugmill (Cape May)
Shaft
Blade
Close-up of Plow Blade (Sarasota)
pugmill installations. At Plattsburgh, the plugging prob-
lem was greatly alleviated when the solids content of the
sludge was increased from 18 percent to 25 percent. At
both Akron and Cape May, the plugging problems were
resolved by removing half of the mixer flights. (At Akron,
the drive motors were also enlarged.)
3.2.5 Reactor Discharge Devices
Reactor discharge devices are key components of the
materials-handling system; their function cannot be eas-
ily duplicated. If a discharge device fails, the reactor
cannot be used.
Discharge devices are proprietary items supplied as a
package with the reactor. Many of the discharge devices
are adaptations of machinery used in the paper and pulp
industry, where they have a long history of reliable opera-
tion. In adapting these devices to in-vessel composting
operations, designers should make sure that the dis-
charge devices can move the relatively high bulk weight
of the compost under all foreseeable conditions.
32
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3.2.5.1 Discharge Device
The discharge devices at some plants have experienced
structural problems. At the Schenectady plant, the pins
connecting one of the screws to its drive mechanism
failed. Also, many of the bolts broke in the steel channels
that serve as tracks for the gear drive (which propels the
screw through the reactor) and are being replaced with
stronger bolts. At the Pittsburgh plant, four of the vertical
screws in the Fairfield reactor broke and have been
replaced.
3.2.5.2 Hard-Surfacing on Auger Flights
Compost is a surprisingly abrasive material, especially
when it is compressed at the bottoms of vertical reactors.
Excessive wear of auger flights has been observed at
several plants.
At Sarasota, the original screws were hard-surfaced by
hard steel welding on the flight edge only. The screw in
the first reactor lost 15 percent of its flight thickness in 6
months because of wear on the flight face. Additional
hard-surfacing needed to be added to the flight face. The
discharge screws at Cape May were originally surfaced
with Triten steel and have not suffered excessive wear. At
Portland, the screws, though originally not hard-surfaced,
were expected to last 7 years. After 14 months, however,
four of the screws were replaced due to excessive flight
wear; the replacement screws are hard-surfaced.
(Although the need for hard-surfacing is apparent from
the experiences at other plants, Portland operators
believe that the wear on the screws was exacerbated by
the ash from the Mt. St. Helens volcanic eruption.)
At Pittsburgh, the vertical augers are wearing out much
faster than expected despite being hard-faced. To date,
four augers have been replaced because of wear. Eight-
een others show excessive wear and the city is consider-
ing ceramic coating, ceramic wear strips, special hard
coatings, and polyurethane coatings as replacements or
additions to standard hard-surfacing.
3.2.5.3 Propelling the Discharge Device through
the Compost Bed
Occasionally, compost becomes either dry and hard or,
under wet conditions, gooey and viscous. Both condi-
tions make it difficult to move the compost with a dis-
charge device. Reactors must be provided with discharge
devices that work reliably, even under worst-case
conditions.
Although their discharge devices are slightly different,
both Portland and Clayton County have experienced diffi-
culties with their discharge augers. At Portland, if the
compost is too wet ("oozing compost"), the composting
material collapses around the screw and traps it. Oozing
compost also physically blocks the path of the peripheral
drive mechanism (see Chapter Eleven for a detailed
description of this problem.)
The oozing compost at Portland may be partly attribut-
able to the large diameter of Portland's reactors. Oozing
sludge has not been observed at the Clayton County
plant, whose reactor diameter is 7.6 m (25 ft) (compared
to the 14.3-m (47-ft) diameter of the Portland first reactors
and the 18-m (59-ft) diameter of the second reactor. The
pressure at the bottom of a large diameter reactor may be
greater than in a smaller diameter reactor because the
wall friction per unit volume (and therefore structural sup-
port) is less.
Despite the smaller reactor diameter, the auger at Clay-
ton County has not escaped getting stuck in the com-
post. (The screw turned, but would not advance.) At the
time, the mix and compost characteristics were not unu-
sual and the screw had not been stationary for an unu-
sual length of time. A hole was cut in the reactor wall so
that the screw could be examined. Although no definitive
cause for the sticking was found, the drive mechanism
that pushes the auger through the compost bed was
modified to increase and more evenly distribute the
driving torque. (See Chapter Eight for a detailed
description.)
3.2.5.4 Machinery Access
Consistent with the strategy of assuring reliability
through quick repair and preventive maintenance, opera-
tors should be given easy access to the discharge device
machinery. The work spaces around the machinery
should be large enough to allow maintenance personnel
to work comfortably. If possible, the discharge device
should be removable from the compost bed to facilitate
maintenance and inspection. The discharge device
installation at the Schenectady plant exemplifies these
recommendations (see Figure 3.16). The discharge
devices at the Akron, Cape May, and Sarasota facilities
can also be removed from their respective reactors with-
out disrupting operations.
At Pittsburgh, there is good access to the vertical auger
drives but the augers cannot be removed and must be
dug out of the compost bed by hand for repair or inspec-
tion. At the Portland and Clayton County plants, access to
the auger machinery is difficult due to the cramped facili-
ties. The discharge screws at these plants can be
removed through doors on the outside walls of the reactor
but at both plants, there have been times when the
augers could not be aligned with these doors. On one
occasion at Clayton County, the funnel that conveys com-
post out of the center pivot became blocked. Compost,
which is pushed into the center pivot housing for dis-
charge, backed up, lifting the screw off its drive mecha-
nism and pushing the screw into the wall of the reactor. A
hole had to be cut into the steel outer wall of the reactor in
order to remount the screw. At Portland, the flights on one
screw were so worn that the screw could not advance.
Unfortunately, this situation was not recognized in time to
stop the screw in front of one of the reactor doors; the
screw stopped between the doors and had to be dug out.
33
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Figure 3.16 Discharge Device Installation at Schenectady
Hoist provided
to assist
maintenance
Reactor
Space where
auger can be
parked outside of
the reactor for
easy access.
Auger
Drive carriage
Note the large amount of room around the equipment.
3.3 Aeration
3.3.1 General System Considerations
There are three basic demands on the aeration system:
(1) heat removal, (2) moisture removal, and (3) biological
metabolism. A comprehensive discussion of proper aera-
tion equipment selection is beyond the scope of this
report. However, aeration systems should be (1) flexible
to allow for changing conditions, (2) controllable to allow
for proper process control, and (3) well monitored to allow
operators to specifically identify how the aeration system
is performing.
The aeration systems at the sites visited have been rela-
tively trouble-free. (See Tables 5-4 and 5-5 for a summary
of the equipment found at these facilities.) The systems
appear to be adequately sized to meet the biological and
heat removal requirements; only Schenectady had prob-
lems with elevated temperatures. The major problems
have been related to moisture removal: condensation
and fog formation above the reactors and formation of a
"hardpan" layer. Inadequate aeration for curing/storage
piles was a third major problem. These three items are
discussed below. Other areas of concern can be summa-
rized as follows:
• Standby capacity: The oxygen level in an actively com-
posting material can drop to zero in a few hours or less
if air supply is stopped; therefore, standby capacity
with standby blowers and/or with standby power is
essential. Table 3.4 summarizes standby blower sys-
tems at the eight plants visited.
• Noise: At Akron, neighbors complained about the
noise from 32 roof fans; Akron is planning to add noise
attenuation.
• Preheating supply air: Akron, Plattsburg, and
Schenectady all preheat the air going into the reactor.
However, the systems that do not preheat have not
had specific problems.
• Temperature: The desired operating range depends
on site-specific conditions. Generally, temperatures
above 70°C (158°F) will slow biological metabolism.
The EPA pathogen reduction requirement calls for at
least 3 days at 55°C (131 °F).
• Reactor pressure requirements: The air supply system
must have sufficient pressure to move the air through
the distribution piping and composting mass in worst-
case (wet mix) conditions.
• Exhaust pressure requirements: The air exhaust sys-
tem must have sufficient pressure to move the air
through exhaust piping and odor control equipment,
including dispersion. Note that five of the eight plants
visited have modified or will modify their odor control
systems, requiring additional exhaust capacity.
• Work space ventilation: If the work space is too nox-
ious due to lack of ventilation or other breakdown,
operators will not be able to repair the problem in that
area.
3.3.2 Air Collection/Moisture Removal
In-yessel composting systems should provide for ade-
quate collection of air after it has passed through the
compost (2). Important considerations are the collection
system configuration in the vessel, the efficiency of mois-
ture removal, and the extent of condensation that could
occur.
Thermodynamically, air can hold four times as much
water at 60°C (140°F) than at 40°C (104°F). Therefore, if
the temperature drops before the air leaves the vessel, it
does not remove as much water, and condensation
occurs within the vessel and composting mass. If the air
collection system extends into the composting mass,
34
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Table 3.4 Standby Blower Systems
Location Air Supply
Exhaust
Akron
Cape May
Clayton County
Newberg
Pittsburgh
Portland
Sarasota
Schenectady
Two blowers per section of reactor. Except for high demand
early in the compost cycle, only one blower is used; the
other is standby.
No standby with original plant. Standby blowers added
later.
One standby blower for each reactor.
One standby blower for each reactor.
No standby.
One blower per reactor train that can act
as an auxilliary or standby blower.
No standby.
One standby blower for four reactors;
No exhaust blowers.
No standby for reactor exhaust in original plant. Standby
blowers added later. One standby for the reactor
head-space ventilation system.
No standby blower.
One blower supplied, but because it must be used in series
with the other exhaust blower because of odor control
system back pressure, there is effectively no standby
capacity.
No standby blowers.
No standby blowers.
No standby blowers.
One standby blower for four reactors;
•Not including the conditioning cell.
Note: Standby power capacity not investigated.
parts of the system can become exposed due to natural
settling and uneven subsidence. Such exposure can
cause odors to escape, saturated air to cool, and conden-
sate to form within the vessel.
Systems such as Akron and Pittsburgh discharge the
warm, moist air from the compost bed into a building or
work space above the reactor. In each case, the introduc-
tion of too much cold outside air into these same areas
results in the condensation of the water vapor and the
formation of fog. At Akron, the fog does not directly affect
operations, although its long-term corrosion effects are
not known. At Plattsburgh, the fog hampers the stabiliza-
tion of the product, promotes corrosion, and is so thick
that operators cannot see to work.
3.3.3 Hardpan
At several plants, leachate combined with compost and
other materials at the bottom of the reactor to form a
"hardpan" layer that was nearly impermeable to air. In
1987, plant operators at Plattsburgh discovered that the
layer of wood chips placed over the base gravel bed to
keep compost fines from entering the air distribution pip-
ing had turned into "hardpan." This hardpan prevented
the flow of air into the compost bed (see Figure 3.17). At
Clayton County, the annual inspection of the first reactor
interior revealed that leachate was seeping into the 0.01-
m (0.5-in.) unwashed river stone layer on the bottom of
the reactor, "cementing" the gravel particles together
and plugging the holes in the aeration piping. The gravel
was replaced with 0.02-m (0.75-in.) washed round river
rock, and the aeration piping was remounted in the mid-
dle (rather than the bottom) of the gravel layer, separating
it from the pooling leachate.
During the first year of operations at Cape May, a hard-
pan layer formed, which was attributed to excessive lea-
chate generation. The mix ratio and amendment
specifications were changed to obtain a more porous
initial compost mix, which significantly reduced leachate
generation. Also, the aeration system was modified to
give the operators better control of the airflow. The hard-
pan problem has not recurred since these changes were
made.
3.3.4 Curing/Storage Aeration
The curing/storage process is an important step in the
overall sludge composting process. The composting pro-
cess does not stop when material is removed from the
reactor(s); the compost continues to have an oxygen
demand.
Figure 3.17 Piece of "Hardpan" Material Removed from the
Plattsburgh Reactors
3.3.4.1 Extended Curing/Storage
Even where compost is processed in second reactors (at
Clayton County, Portland, Sarasota, and Schenectady),
the unaerated curing/storage piles have reheated.
Depending on the stability of the compost, oxygen
demand may be elevated to the point where anaerobic
35
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conditions develop, and moving the piles causes odors to
be emitted (see Figure 3.18). At Akron, Cape May, and
Newberg, the exterior curing/storage piles are aerated or
will be aerated in the future. The aeration systems at
Akron and Cape May were added due to odor problems;
Newbergfe system (unused at the time of the site visit)
was included as part of the original construction. At Plat-
tsburgh, the curing/storage piles are odorous, and plan-
ning is underway to determine whether they should be
aerated.
3.4 Odor Control
Effective odor control is a critical requirement for the
success of a composting facility. Fear of potential odors
engenders public opposition to proposed plant sites.
Odor complaints can cause a regulatory agency to shut
down or curtail the operations of an existing facility. Plant
odors can hurt product sales.
Figure 3.18 Storage/Curing Piles
-4
Steam and odors being released from a curing pile during moving.
(Akron)
Compost product (after curing) reheats when it is placed into storage
piles. (Portland)
Of the eight composting facilities surveyed in this project,
six had received odor complaints from the local commu-
nity. At five of these sites, the complaints were serious
enough to cause the plant owners to retrofit the facilities
with additional odor treatment equipment. Two plants
ceased operations while the new equipment was being
installed. A third plant was operated at reduced capacity
to limit odor production while the new treatment equip-
ment was piloted, designed, and installed. The last two
plants continued to operate but were proceeding with the
odor treatment system retrofits as quickly as possible.
Almost all of these problems can be attributed to incom-
plete odor control plans. To be effective, an odor control
plan must include at least five elements:
• Control of the composting process
• An inventory of potential odor sources
• An odor collection and containment system
• An odor treatment system
• Effective dispersal of residual odors
These elements are discussed below.
3.4.1 Compost Process Control
Controlling the composting process is crucial in minimiz-
ing odor production. This entails providing sufficient aer-
ation, keeping temperatures and moisture contents in the
appropriate ranges, maintaining the porosity of the com-
posting materials, and properly mixing the compost
ingredients. Anaerobic conditions, in particular, lead to
increased odor production.
Although process control is a necessary condition for
minimizing odor production, it is not sufficient in itself.
Even the plants surveyed provided adequate oxygen (as
evidenced by measurements of the gases from the reac-
tors) still produced some odors. The ammonia produced
at the Plattsburgh plant is one example. Therefore,
designers should not depend on process control to elimi-
nate the need for other odor control measures.
Stabilizing sludge before composting may affect odor
production. The two plants surveyed that have not experi-
enced odor problems are composting relatively stable
sludges. At Portland, the sludge is anaerobically digested
before composting, and at Newberg the sludge from an
underloaded oxidation ditch is partially aerobically
digested before composting. Nevertheless, there are
static pile systems (not surveyed) that compost anaerobi-
caily digested sludge yet still experience odor problems.
Chemical treatment of sludge can reduce nonprocess
odors in the dewatering and mixing areas of composting
plants. At Plattsburgh and Schenectady, potassium per-
manganate is added to the sludge before dewatering. At
Cape May, a masking agent is added to the sludge, saw-
dust, and recycle at the mixer. These chemical additions
have not significantly changed odor emissions from the
reactors.
36
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3.4.2 Inventory of Potential Sources
Although the reactor outlet air is the primary source of
odor asssociated with an in-vessel facility, it is not the
only source. Odors can be generated at numerous points
In the system. A list of potential sources of odor, based on
the experiences of the eight plants visited, is shown in
Table 1.4. During design, a complete inventory of poten-
tial odor sources, types, and amounts, should be made
and a comprehensive control strategy should be devel-
oped for the entire facility.
The odor inventory should also include offsite odors so
that after the composting plant is started up odor com-
plaints can be directed to the proper offending source.
Identifying the specific odor-causing compounds may be
important in designing treatment systems. At the Platts-
burgh plant, for instance, the single-stage wet scrubber
was designed with a caustic chemical feed system that
was completely ineffective in removing ammonia, the
primary odor-causing gas. This issue is discussed below
in relation to treatment systems.
3.4.3 Collection and Containment
All odor sources should be evaluated for inclusion in the
containment and collection system. Some emissions
may be small enough that they can be dispersed satisfac-
torily without treatment. (See discussion below on odor
dispersal.) All other odors sources should be contained
and the malodorous air collected for treatment.
The containment and collection system should be coordi-
nated with the building ventilation systems. If the work
space is inadequately ventilated, operators will open
doors to increase circulation and inadvertantly allow
odors to escape. At the Sarasota plant, for example,
sludge odors are released from the dewatering room
through a garage door that is opened in warm weather
because the ventilation system cannot sufficiently miti-
gate the internal heat, humidity, and odors generated by
the belt presses. Similarly, doors are left open around the
process building at the Portland plant to help dissipate
ammonia odors generated when the reactor discharge
devices are operating.
3.4.4 Treatment
Odors found at compost facilities are caused by a large
number of different compounds that originate in both the
sludge and the amendment. In-vessel systems may pro-
duce more amendment-related odors than static pile or
windrow systems because the sawdust used at most in-
vessel facilities has a greater surface area than the wood
chips usually used in the other systems.
These odor-causing compounds fall into a number of
classes that differ in their physical and chemical proper-
ties. Some are acidic; some are basic. Some can be
destroyed by oxidation; others cannot. These com-
pounds are not equally soluble in water, nor equally ame-
nable to adsorption. Moreover, the mix of compounds in
the odorous gas stream can change over time, depend-
ing on the composting materials and the reactor operat-
ing conditions. For this reason, treatment systems must
employ a broad spectrum of removal mechanisms to be
effective. Treatment systems that rely on only one or two
removal mechanisms such as single-stage wet scrub-
bers and ozone oxidation chambers generally have not
been successful.
Four basic odor treatment options were found at the eight
plants surveyed. A fifth option, activated carbon, was
tested at one location but was not part of any plant's odor
treatment system. All five options are discussed below.
3.4.4.1 Wet Chemical Scrubbers
At the time of the site visits, packed tower wet scrubbers
were in use at three of the eight plants visited: one-stage
units were operating at the Cape May, Pittsburgh, and
Schenectady plants; and a two-stage unit was in use at
the Cape May plant. Two-stage units will be installed as
part of the odor control system upgrades planned for the
Akron, Pittsburgh, and Schenectady plants. A two-stage
unit was constructed at the Sarasota plant in the spring of
1989 (after the site visit). The odor sources treated and
the chemicals used in these scrubbers are listed in
Table 3.5. Typical scrubbers are illustrated in Fig-
ure 3.19.
Table 3.5 Wet Scrubber Installations
Number of
Location
Cape May
Stages
1
1
2
Source of Odorous Air
Sludge storage building
Mixing building
Reactors
Chemicals Used
Sodium hypochlorite and sodium hydroxide.
Sodium hypochlorite and sodium hydroxide.
First stage: Sodium hypochlorite and sodium
hydroxide. Second stage: Sulfuric acid.
Pittsburgh
Sarasota
Schenectady
Reactors and mixing room after it passes through the
compost.
Reactors, mixing room, dewatering room
Reactors and mixing room
Originally sodium hypochlorite and sodium hydroxide.
Later, changed to sulfuric acid and DeAmine, a
proprietary chemical.
First stage: sulfuric acid Second stage: hypochlorite.
Sulfuric acid and DeAmine, a proprietary chemical.
•Existing ozone contact chamber is being used as a cooling tower upstream of the scrubber.
37
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One-stage scrubbers have not provided adequate treat-
ment of odors from the reactors. At the Cape May plant,
the single-stage scrubber, which was designed to control
odors from both the mixing building and the reactor
exhaust, was not successful. In August 1986, a two-stage
scrubber was installed to treat the reactor exhaust air,
while the original unit was retained for the mixing build-
ing. Replacement units are being designed for Platts-
burgh and Schenectady, as well.
There may be several reasons for the failures of the
single-stage scrubbers. However, the fundamental rea-
son for their ineffectiveness is that the odors in compost
offgases are caused by a variety of compounds that
require several removal mechanisms, some of which
cannot be combined in a single-stage scrubber. Addi-
tional problems have been caused by overloading the
scrubbers and using inappropriate chemicals. Single-
stage scrubbers may have a place at compost plants for
treating odors from sources in which hydrogen sulfide or
another single compound is the primary odor source. As
noted earlier, the mixing and sludge storage buiildings at
the Cape May plant are serviced by single-stage
scrubbers.
Because of the need to provide a spectrum of removal
mechanisms (including both acid and caustic scrub-
bing), two stages are the minimum number needed for
effective odor treatment. Packed tower wet scrubbers
were the only scrubbers observed at the eight plants
studied. Mist-type scrubbers have been used success-
fully at the Montgomery County, Maryland, facility, an
enclosed static pile operation. As discussed below, ade-
quate dispersion of the scrubber discharge is a key ele-
ment of a successful odor control device. (Note the lack of
a stack for dispersion on the Schenectady scrubber
shown in Figure 3-19.)
3.4.4.2 Compost Biofliters
At the Akron facility, odorous airstreams from the dewa-
tering room, the mixing room, and the negatively aerated
reactor cells were passed through compost beds. (See
Chapter Six for a description of this system.) Measure-
ments made at the site and summarized in Table 3.6
showed that the biofilters were able to remove large frac-
tions of these odors.
Biofilters (soil and compost filters) may have potential in
this application because within the biofilter a variety of
mechanisms — physical adsorption, chemical oxidation
and neutralization, and biological degradation — are
available to remove the odors. Nevertheless, design
parameters such as loading rates, residence times, and
moisture contents are not well established. Biofilters
require relatively large land areas and are discharged at
ground level, which makes dispersal of the odors not
removed in the biofilter difficult.
Figure 3.19 In-Vessel Scrubbers
Single-Stage
Scrubber at
Schenectady
Locating the
scrubber exhaust
near the building
wall and below
the roof line
prevents proper
dispersion.
Stack
Two-Stage Scubber at Cape May (Tanks to left are chemical storage
tanks)
3.4.4.3 Bubbling Odorous Air through Water
At the Portland, Newberg, and Clayton County facilities,
which are smaller facilities that use a more stabilized
sludge, the odorous air is bubbled through water for treat-
ment. At Clayton County, the reactor exhaust air is bub-
bled through 0.71 m (28 in.) of water in the wastewater
plant's emergency bypass holding pond (see Fig-
ure 3.20). Operators have received some odor com-
plaints, but the odors generally are not problematic, partly
because of the rural setting of the plant. The Portland and
Newberg facilities have not received any odor com-
plaints; both plants are located in outlying industrial
38
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areas and both compost relatively stable sludges. These
mitigating factors make it difficult to project the effective-
ness of this technique at other locations.
Table 3.6 Odor Removal by Compost Filters at the Akron
Composting Facility (3)
Odor Level(d/t)«
Time of Sampling
March 29, 1988
11:50
12:00
12:17
12:50
April 5, 1988
11:25
11:26
11:36
April 7, 1988
8:36
8:41
8:55
12:25
Inlet Air
281
—
—
—
346
269
— -
615
—
758
—
Outlet Air
<10
3
5
- ,
14
26
"
18
"d/t = dilutions-to-threshold.
Figure 3.20 Odor Control System at Clayton County
Reactor offgas bubbled through pond water.
Bubbling the odorous gases through biologically active
suspensions such as activated sludge aeration tanks
might be considered a combination of biofilter and wet
scrubber techniques. Like biofilters, the design parame-
ters associated with this technique are not well estab-
lished. Also like biofilters, a ground-level discharge is
created.
3.4.4.4 Ozone Contact Chambers
An ozone contact chamber was used only at the Sara-
sota facility. The ozone concentration was 1 to 2 ppm and
the detention time at the design airflow rate was about 20
seconds. The system was not effective at reducing
compost-generated odors.
3.4.4.5 Activated Carbon
None of the in-vessel composting plants surveyed use
activated carbon as part of their odor control systems.
Activated carbon was tested as a possible treatment
method at the Akron plant. Odor removal (measured as
dilutions-to-threshold) varied from 85 to 95 percent at the
beginning of the test and diminished thereafter as the
adsorption capacity of the carbon diminished. The test-
ing program was not sufficiently detailed to develop an
adsorption isotherm or establish system economic
parameters.
3.4.5 Dispersal
No treatment system is 100 percent effective; some con-
centration of odorous compounds will be discharged
after treatment. Likewise, no containment and collection
system is perfect. Fugitive releases of odors are very
likely to occur despite the best prevention efforts. To
avoid undesirable effects on local residents, the odorous
emissions must be diluted and/or dispersed so that ambi-
ent air concentrations of the odorous gases fall below
detection thresholds.
Sometimes, the dispersion characteristics of a site exac-
erbate odor problems. One such example is the Akron
facility. It is located in a narrow river valley enclosed by
60-m (200-ft) high ridges. During periods of atmospheric
stagnation, emissions from the plant are trapped in the
valley and accumulate. When the wind returns, it trans-
ports these accumulated odors through residential and
commercial areas also located in the valley.
Because of local conditions, odors may travel for miles. At
Akron, odor complaints have been received from resi-
dents 2.4 km (1.5 mi) away and odors have been detected
4.0 km (2.5 mi) away. Moreover, wind patterns can some-
times bypass local residents and transport odors to dis-
tant neighborhoods.
It is advisable to model the dispersion characteristics of
proposed plant sites during project planning. Dispersion
modeling should consider the effects of local meteorol-
ogy (including seasonal and diurnal variations and worst-
case weather conditions such as inversions), topography,
the location and proximity of neighbors, and the charac-
teristics of the odorous discharges (i.e., elevated, ground-
level, point source, or area source).
The modeling results can be used to screen sites with
poor dispersion characteristics. Modeling can also be
used to establish maximum allowable odor emissions for
treatment systems and the need for supplemental disper-
sion such as stacks or mechanically induced ventilation.
Finally, model results can be used to develop a flexible
odor control plan that varies according to the weather. An
example of one such plan is discussed in the Akron case
study (see Chapter Six).
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3.4.6 Additional Considerations
3.4.6.7 Curing/Storage Pile Aeration
Curing/storage piles are biologically active and without
enough air, anaerobic conditions may develop, which can
lead to odor emissions. See Section 3.3.4.1 for a detailed
discussion of curing/storage pile aeration.
3.4.6.2 Odor Control System Reliability
Odor control systems should not, under any circum-
stances, be inoperable for more than a day. If they fail to
function, odors will likely be released in sufficient
amounts to elicit neighborhood complaints. Therefore,
odor control systems should be designed with a high
degree of reliability. To ensure reliability, the strategies
discussed earlier — standby blowers, standby power,
and/or quickly reparable equipment — are applicable.
Chemical feed pumps, valves, instruments, and controls
should be easily accessible and replaceable. In addition,
appropriate spare parts should be kept on hand.
3.4.6.3 Backup Plan
A backup plan should be developed to reduce the poten-
tial for odors in the event of compost process problems or
mechanical breakdowns. The backup plan should
include moving the sludge or compost offsite (e.g., emer-
gency landfilling) or aerating the material onsite with tem-
porary odor treatment measures (e.g., compost
biofilters). Untreated sludge or incompletely composted
materials should not be stored at the plant. The majority
of odor complaints were received at the Clayton County
plant, for example, when the contents of one reactor were
piled outside while the reactor received its annual
inspection.
3.5 Process Monitoring and Control
Composting is a biological process that, to date, has
been operated on a semi-empirical basis although many
advances in quantitative analysis have been made (i.e.,
by Haug, Wilber, and Murray [4,5,1,6]). Designers should
provide generous monitoring and control capabilities to
aid operators in diagnosing problems and documenting
process performance. Table 3.7 lists the monitoring
needs of an in-vessel facility. The centralized computer
control system should be capable of recording the run-
ning times and speeds of various conveyors. The com-
puter should be programmed to convert these into
volumes and weights.
3.5.1 Analytical Equipment for Process Control
The solids content of the feed materials and the mix must
be determined on a regular basis. In the standard test,
the sample is dried in an oven to a constant weight
(usually overnight). This does not provide "real time"
process control, however.
Other tests include use of analytical microwave devices,
homemade microwaves, and heat lamps. The analytical
microwave devices produce results quickly (about 20
minutes), but can analyze only a very small sample (sev-
eral grams). Operators note that it is very hard to choose
a representative sample of this size. The operators at
Cape May have developed their own test using a kitchen
microwave oven. Other facilities use heat lamps to dry
the solids. The facilities using homemade tests or heat
lamps usually use the standard oven technique to ana-
lyze at least one representative sample from each batch
of material loaded into the reactors.
Another test that must be performed on a regular basis is
the determination of bulk weight. This test can be done
Table 3.7 Monitoring Needs
Purpose
Parameter
Process Control
Odor Control
Regulatory Requirements
Marketing or Product Quality
Temperature
Aeration rate
Backpressure
Oxygen
Mass balance
(materials flow)
Scrubber chemistry
Odor emissions
Meteorology
Temperature
Constituents (especially
C metals)
Moisture content
Stability
Constituents (especially
C metals)
Phytotoxicity
easily with a standard bucket and a scale. Bulk weights of
mixes are sometimes used as surrogate measurements
of porosity.
Operators at some plants attempt to measure compost
stability with oxygen uptake devices. Consideration
should be given to equipping the pjant with this or some
other type of equipment for measuring compost stability.
For odor control, the chemicals going in the scrubber
should be metered and the odors emitted from the scrub-
ber should be monitored. Oxygen and temperature
should also be monitored (see Section 3.5.2).
Designers should consider sampling and monitoring
needs while specifying and laying out equipment. Situa-
tions such as those at Clayton County and Portland,
where the mixers are at the top of the facilities and the
control room and analytical equipment are located on the
ground floor, should be avoided. Such logistics make
monitoring extremely difficult.
3.5.2 Process Sensors
3.5.2.7 Temperature and Oxygen Measuring Devices
Although most in-vessel plants use temperature to con-
trol aeration rates on a day-to-day basis, equipment to
measure oxygen should be provided for diagnostic pur-
40
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poses. An operator cannot tell if temperatures are low
simply because the volatile content of the mix is low or
because there is insufficient oxygen in the system. At
Sarasota, automatic controls that vary the aeration rate in
response to the temperature (i.e., as temperature
increases, the aeration rate is increased, and vice versa)
are not used because the operators are concerned that
the computer will misinterpret low temperatures caused
by insufficient oxygen, and exacerbate the situation by
decreasing the aeration rate. Oxygen probes at several
of the plants visited were inoperable because they were
not designed for the high humidity of the reactor exhaust
airstream.
3.5.2.2 Temperature Probes
It is difficult to define how many temperature probes are
"enough" as the number will vary with the geometry of
the reactor. The number and positioning of the probes,
however, should relate to the ultimate use of the data
collected. If, for example, the temperature data are used
to distribute the airflow among different sections of the
reactor, each reactor section should contain temperature
probes. The regulatory requirements for temperature
data should be considered when specifying the number
and location of temperature probes.
3.5.2.3 Airflow Rate Measuring Devices and
Pressure Sensors
For process control, an operator should know the actual
amount of air passing through the reactor as well as the
minimum, average, and peak aeration rates. For centrifu-
gal or centaxiaf blowers, blower speed is not a good
indicator; as reactor back pressure increases, the
amount of air delivered decreases, even though the
blower speed is constant. Flow rate measuring devices
must be provided for these types of blowers and throttling
valves must operate over the entire range of conditions.
For positive displacement blowers, however, blower
speed can be directly correlated with airflow rate, pro-
vided the outlet pressure is relatively constant. Proce-
dures presented by Haug (3) allow the designer to
estimate the quantity of air required and the minimum,
.average, and peak aeration rates for various process
conditions and feed substitutes.
The reactor pressure is useful for monitoring compost
porosity. Increasing pressures indicate a loss of porosity,
which may be due to wet conditions, a breakdown of
material structure, or improper aeration. Pressure sen-
sors can also be connected to alarms to protect aeration
equipment.
3.5.3 Instrumentation
3.5.3.7 High Temperature Alarms for Fire Detection
Fires in compost reactors have occurred at Cape May
and Schenectady. When the solids content of compost is
greater than 65 percent and the temperature exceeds
80°C (176°F), it is possible for a series of exothermic
reactions to result in full combustion. A temperature
alarm might warn operators of this potentially dangerous
condition.
Compost temperatures, however, are not good indicators
of fire conditions. Since compost is an excellent insulator,
temperatures can be quite different, even a few feet away
from a probe. At the Schenectady plant, for instance, the
temperature data did not indicate that the compost was
on fire. During design, every facility should develop a fire
prevention and firefighting plan.
3.5.3.2 Control Indicator Calibration
When possible, gauges, dials, and other control indica-
tors should be calibrated in actual units rather than surro-
gate variables and should be easy to read. For instance,
airflow rates should be indicated in appropriate volume
units, such as cfm, rather than as a percentage of maxi-
mum blower speed. Supplying correct units on dials facil-
itates data collection and intra-plant data comparison,
while decreasing conversion errors. The scales on the
dials should be adjustable or replaceable so that
changes in calibration parameters can be correctly indi-
cated on the dials.
3.5.3.3 Automated Data Logging
Automated data logging — the maintenance of a record
of plant conditions by the central computer — offers
several advantages over manual recording. Plant condi-
tions can be monitored even when the plant is not staffed.
This information is useful for diagnosing problems. More-
over, the data can be saved in a form that is compatible
with other computer systems so that it can be incorpo-
rated into reports for regulatory agencies, recorded for
budgeting purposes, or manipulated for diagnostic
purposes.
The kinds and frequency of data logging should be coor-
dinated with regulatory agencies during the design
phase. At the Cape May plant, the regulatory agency
wanted to use the actual computer records. Because the
computer monitored the temperature every 30 minutes,
minor temperature fluctuations were recorded and the
compost bed did not always appear to meet the time-
temperature criterion (i.e., the temperature did not always
exceed 55 °C [131 °F]). After 2.5 years of Salmonella test-
ing, which showed that a significant reduction in patho-
gens was taking place as long as the average 3-day
temperature exceeded 55 °C (131°F), the regulatory
agency revised its requirements.
3.6 Support Facilities
3.6.1 Maintenance Work Area
Even if the compost plant is located at a wastewater
treatment plant, a small maintenance work area (e.g., a
bench and set of tools) should be provided nearby. Oper-
ators need the tools and work space to do the everyday
jobs that are too small to justify calling in maintenance
personnel from the central shop. Even if maintenance
personnel are called in on the job, having a place to work
41
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at the site minimizes unnecessary transportation of parts
and equipment.
3.6.2 Onsite Spare Parts Storage
Every facility should have a maintenance management
program, of which a spare parts inventory is a key com-
ponent. Locating an adequate supply of spare parts on
site, or at least close by, is essential in minimizing down-
time. Yet a number of the plants surveyed had little or no
organized spare parts storage space. At the Cape May
plant, spare parts were stored in stairwells and in an old
truck trailer. At the Newberg plant, spare parts were
stored on the roof of the bathroom (see Figure 3.21).
Figure 3.21 Spare Parts Storage on the Roof of the Bathroom at
Newberg
The Portland, Sarasota, and Schenectady plants had no
spare parts storage at all, although in each case the
compost plant shared some of the wastewater plant's
storage space. The importance of spare parts storage
was recognized at Clayton County, where the first modifi-
cation to the plant after construction was the addition of a
storage facility.
3.6.3 Staff Facilities
An enclosed control room, with a separate ventilation
system to keep out dust and odors from the compost
building, should be provided. The room should be located
near the mixer to facilitate frequent observations of the
mix by the operator and should be large enough to double
as a staff meeting room. A separate office should be
provided to allow staff to do paperwork or use the tele-
phone. (The noise and activity in the control room inter-
feres with these functions.) The office ventilation system
should also be separate from that of the compost build-
ing. Air conditioning should be considered since many
parts of the compost plant are hot and operators may
need a cool place to rest.
Restroom facilities should be provided, including toilets,
showers, and lockers. If the compost plant is located at a
wastewater treatment plant, the showers and lockers
could be located in the main administrative building, but
restrooms should be provided at the compost plant for
purposes of hygiene and first aid.
3.7 Fire Prevention and Control
Fire prevention and control considerations affect many
aspects of plant design. Both spontaneous combustion
and accidental ignition (e.g., welding accidents) have
caused fires at the plants surveyed.
Spontaneous combustion fires occurred when the com-
posting materials were held in reactors for too long
because some other part of the system slowed or
stopped processing material through the plant. Materials-
handling systems should be reliable enough to prevent
this from happening. When sizing reactors to accommo-
date growth in sludge quantities or seasonal variations in
sludge production, excessive detention times must be
avoided. Several smaller reactors may be better than a
few large units.
A firefighting plan should be worked out with local offi-
cials during the design so that necessary equipment or
facilities, such as local water supplies or distribution pip-
ing, can be provided. Some equipment, especially reac-
tor discharge devices and ventilation systems, must
continue to function in a fire. These should be identified in
the firefighting plan and designed accordingly.
Other fire control considerations include the placement
and types of sensors and the choice of reactor materials
based on fire-resistance. Whether required or not, local
fire codes should be followed and plant designs should
be coordinated with local fire officials.
At the eight plants surveyed, design features to detect or
combat fires varied, and coordination with local officials
was rare.
42
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3.8 References
1. Wilber, C. Odor Source Evaluation and Control
Options. Lecture Transcript. Presented at Biocycle
Conference. June 6,1989.
2. Walker, J., N. Goldstein, and B. Chen. Evaluating the
In-Vessel Composting Option, Part I. Biocycle. May-
June 1986, pp. 23-27.
3. Odor Science and Engineering, Inc. Akron Compost-
ing Facility Odor Study, Draft Final Report. July 1988.
4. Haug, R.T Composting Process Design Criteria, Part
III: Aeration. Biocycle. October 1986.
5. Haug, R.T Composting Process Design Criteria, Part
II: Detention Time. Biocycle. September 1986.
6. Murray, C. Managing Odor Control Programs. Lecture
Transcript. Presented at Biocycle Conference. June 6,
1989.
43
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Chapter 4
Operations Considerations
The purpose of .this chapter is to highlight the operations
and maintenance issues that are peculiar to in-vessel
composting facilities and to summarize the operations
experiences of the eight plants surveyed. The chapter
focuses on management issues, including staffing and
training requirements, staff assignment policies, and
maintenance policies. In addition, strategies for carrying
out specific tasks, such as preventing and controlling
fires, specifying amendment characteristics, and moni-
toring and controlling the composting process, are pre-
sented. Table 4.1 lists operations considerations and the
sections in which they are discussed.
Table 4.1 In-Vessel Operations Considerations
Topic
Section
Startup Issues
Performance Test Plan
Conducting the Performance Test
Additional Personnel
Operating Procedures
System Suppliers Specifications
Monitoring and Recording Process Variables
Monitoring Product Quality
Monitoring Feed Material Characteristics
Impact of Wastewater Treatment Plant Operations
Fire Prevention
Fire Detection
Fire Suppression
Staffing Issues
Operations Staff
Operator Training
Staff Rotations
Acquiring Necessary Maintenance and Repair Skills
Assigning Regular Maintenance Staff
Maintenance Issues
Preventive Maintenance
Operations and Maintenance Manual
Spare Parts Inventory
4.1
4.1.1
4.1.2
4.1.3
4.2
4.2.1
4.2.2
4.2.3
4.2.4
4.2.5
4.3
4.3.1
4.3.2
4.4
4.4.1
4.4.2
4.4.3
4.4.4
4.4.5
4.5
4.5.1
4.5.2
4.5.3
4.1 Startup Issues
4.1.1 Performance Test Plan
The performance test plan defines the conditions under
which the system supplier will be freed from further finan-
cial obligation to the owner. The criteria for the perform-
ance test should be specified in detail in the bid
documents and might include criteria for acceptance,
testing protocols, and sampling frequencies. Regulatory
officials, who .are responsible for issuing an operating
permit for the plant, should be consulted during the prep-
aration of the plan to ensure that when the plant passes
the performance test, it will be licensable and able to sell
the product.
The test plan should specify the measurement parame-
ters and methods. Measuring devices required for the
performance test will need to be installed, at least for the
duration of the test. Electrical meters, for example, may
be needed to verify energy consumption and power
demand criteria. The plan should also take into account
that there will not be any recycle for the mix durinq
startup.
4.1.2 Conducting the Performance Test
All equipment that is operated under normal conditions
should be operated in the performance test. In addition,
the test should be conducted for a long enough period to
establish steady-state conditions in the composting pro-
cess. If the plant is designed to run under a variety of
conditions (e.g., ranges of sludge cake solids, sludge
quantities, or mix solids), all of these conditions should
be included or simulated in the test.
4.1.3 Additional Personnel
Additional personnel should be provided during startup.
Some parts of the system inevitably will not work as
planned and operating personnel will have to run the
plant (probably in a less-than-fully-automated fashion)
while simultaneously trouble-shooting problems. In addi-
tion, personnel will be needed to collect the data required
for verifying compliance with the terms of the perform-
ance test. In most of the plants surveyed, the additional
startup staff was provided by the system supplier or by
the wastewater treatment plant (WWTP).
4.2 Operating Procedures
4.2.1 System Supplier Specifications
During startup, operators should make every effort to run
the plant as specified by the system supplier. Situations
commonly arise in which operators claim that equipment
doesn't work properly while system suppliers claim that
systems are not being operated correctly. If the plant is
operated according to the system supplier's specifica-
tions, this kind of conflict will not be an issue.
45
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More often than not, individual plants must adapt to local
conditions, and departures from the original operating
procedures are inevitable. If a facility begins with a stan-
dard operating procedure, however, operators can judge
whether proposed changes are indeed improvements.
Since similar plants may face analogous problems, oper-
ators should become familiar with plants using the same
composting system. In-plant training at other facilities
and communication with colleagues help operators
expand their knowledge base. Exchanging information
and ideas among the different facilities may lead to
improved operations at all plants.
4.2.2 Monitoring and Recording Process Variables
Good records of process performance are extremely use-
ful. They can help detect and diagnose problems and can
also be used to determine whether the product is meet-
ing marketing requirements.
Data should be logged on a daily basis. If the computer
system does not provide automated data entry, operators
should fill out a daily process log. The Clayton County
plants log is shown in Figure 4.1; similar log sheets were
used at all eight plants. When automated data collection
Ffgure 4.1 Clayton County Daily Process Log
NORTHEAST COMPOST FACILITY
DAILY PROCESS LOG
TIUC
REACTOR DATA
)%
0. (EXHAUST AIR)
IIR FLOW (C.F.M)
:XAUST TEMP. («C)
TOP TEMP. CO
MIDDLE TEMP. (°C)
BOTTOM TEMP. (°C)
REACTOR LEVEL (% FULL)
NTENERAL PRESSURE
BACK
BACK
LuMUMMH
PRESSURE (SUPPLY)
PRESSURE (EXHAUST)
ZONES l-« (SETTINGS)
ZONES l-« (QUANTITY)
OUTFEED SCREW SETTING
OUTFEED ADVANCE SETTINC
OUTFEED RATE (FT VHR)
BIO
I
CURE
PO
DA
WER MFTER . .
TA ENTRY | 1
AUX. BLOWER
AUX. BLOWER
VL-VERY LITTLE (< I/2 GAL.)
L-LITTLE (I/2-IGAL.)
M-MEDIUM ( >IGAL.,< 2 GALS)
H- HEAVY O2GALS.)
SLUDGE BIN LEVEL {% FULL)
CARBON SILO LEVEL (% FULL)
CARBON FEED SETTING (AVG.)
FUNCTION TIMES
FILL B/C (KRSJ
TRANSFER B/C (HRS.)
DISCHARGE B/C (HRS.)
RECYCLE 810 (HRS.)
RECYCLE CURE (MRS.)
%
SOLIDS
SLUDGE
SAWDUST
RECYCLE B/C
MIX
PRODUCT 8/C
FEED SLUDGE
PH
46
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is provided, printing out a daily summary ensures the
records will not be lost if a computer fails.
The data should be stored and displayed in ways that
help the operator identify trends and relationships. Indi-
vidual log sheets stored in a binder are not very useful
because the data must be extracted and organized.
Graphic presentations are ideal.
Mass and energy balances, when updated on a regular
basis, can help analyze and track the performance of
various system parts. Using a mass balance, an operator
can determine whether wet product is caused by inade-
quate drying in the reactor, too much water in the mix, or
insufficient volatile organics (insufficient biological
energy) in the compost ingredients.
A computer is a great asset when managing data. An
inexpensive personal computer with a spreadsheet pro-
gram can perform all the data management activities
normally needed.
4.2.3 Monitoring Product Quality
Product quality must be monitored to meet regulatory
requirements, provide consumer information (i.e., to meet
product labeling requirements), establish a data base for
use in marketing, and protect the public agency from
liability claims.
The parameters measured depend on the State regula-
tory requirements and the characteristics important in
marketing the product. State regulations usually require
in-vessel plants to measure the concentration of metals
and other contaminants in the compost. For marketing
purposes, parameters of interest to potential customers,
such as phytotoxicity, nutrient content, stability, texture,
moisture content, consistency, aerobic state, and
particle-size distribution should be monitored.
4.2.4 Monitoring Feed Material Characteristics
The characteristics of the feed materials — degree of
contamination by trash or heavy metals, particle size,
moisture content, and biodegradability — strongly influ-
ence the compost process and product quality. Sludge
contaminants, particularly heavy metals, can result in
unmarketable compost. For this reason, heavy metals
should be monitored on a regular basis.
Contaminants in the sawdust, such as rocks, large
chunks of wood, or trash, can cause materials-handling
problems in the composting plant. Specifications, which
should be written and enforced, should prohibit the inclu-
sion of these materials. Protective devices such as
screens effectively prevent these materials from entering
the composting process.
The particle size of the amendment influences the struc-
ture and porosity of the mix. Several plants have had
problems with sawdust "flour" or sanding dust. When
mixed with sludge, these materials create a paste-like
substance that sticks to the materials-handling system
and does not have the porosity needed for good com-
posting. At the other extreme, particles that are too large
do not mix thoroughly with the sludge. The amendment
particle size specifications used at six of the eight in-
vessel facilities visited are listed in Table 4.2.
To create a mix with the proper moisture content, the
operator must know the moisture contents of the individ-
ual ingredients. Although amendment moisture content
does not change substantially during most types of stor-
age, it can vary from delivery to delivery; each batch
should be tested. Table 4.2 also lists the amendment
solids content specifications used at the surveyed in-
vessel facilities.
Finally, the biological degradability of the mix ingredients
should be considered. Biological degradation provides
the energy that raises the compost temperature and
evaporates the excess water. If amendments are not suf-
ficiently degradable, achieving the desired temperature
may be difficult. The Sarasota plant had this problem
when it used cypress, which is a hardwood resistant to
microbial decomposition, as an amendment. In general,
hardwoods tend to degrade too slowly and cannot be
visually identified in a batch of sawdust. This may be a
problem, depending on whether the compost product will
be screened or how it will be used. Several facilities
surveyed specify that amendment suppliers provide
softwood amendment.
If amendment materials are too easily degraded, how-
ever, the temperature inside the reactor may reach unde-
sirably high levels, requiring large amounts of air for
cooling.
47
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Table 4.2 Example Amendment Specifications
Location Solids Content (%)
Particle Size
Other
Cape May
Newberg
Pittsburgh
Portland
Sarasota
Sohenactady
min60
minSO
avg60
minSO
min 60b
mlnSO
<2% larger than 12.5 mm
< 20% smaller than 5 mm
Well graded.
0.125 in. max"
Must be able to be conveyed by
sawdust bin augers.
Well graded. 0.067 in. max. Sanding
dust is prohibited.
Nominal size 0.25 in.
Average size 0.25 in.
Kiln-dried
Bark allowed up to 50% by volume.
Coniferous species < (15% - . . ,
hardwoods).
Thin wood shavings allowed up to
50% by volume. Any combination of
soft woods and hardwoods.
Bark allowed up to 50% by volume.
May not deviate more than 20% in
ability to alter moisture content of
sludge.°
Kiln-dried. Softwoods.
Rough-cut pine.
•In. X 2.54 x 10-2 = m
"To receive full payment, sawdust must be at least 60% dry solids. Payment for sawdust between 50 and 60% solids may be made at the
discretion of the plant manager. No payment for sawdust less than 50% solids.
'Defined by an equation developed by plant operators. •
4.2.5 Impact of Wastewater Treatment Plant
Operations
Consistent with other treatment objectives, the wastewa-
ter treatment processes should be operated to create a
sludge that dewaters well. Efficient dewatering helps pro-
cess performance and is cost-effective; poorly dewa-
tered sludge is generally expensive to compost because
more space and amendment are required. (Figure 2.1
illustrates the effect of sludge dryness on mix and
amendment quantities.)
Wastewater treatment plant operators should be
instructed concerning their roles and impact on the com-
posting process. Arranging periodic meetings between
compost operators and treatment plant operators and
assigning Wastewater plant operators to the compost
plant for a short period of time may enhance this educa-
tional process.
4.3 Fire Prevention
Although one goal of composting is removing moisture,
producing an excessively dry product inhibits the biologi-
cal activity of the composting microbes, thereby hamper-
Ing sludge stabilization. Excessively dry compost is also
very dusty. The most important reason to avoid excessive
drying, however, is fire prevention. At both the Schenec-
tady and Cape May plants, spontaneous combustion fires
were caused, at least in part, by excessively dry compost.
Fires can start at surprisingly low temperatures in dry
compost. Exothermic chemical reactions that can start at
a low temperature liberate heat that elevates the temper-
ature to a point where true ignition and smoldering fires
occur. Tests at the Schenectady plant demonstrated that
exothermic reactions started at temperatures as low as
90°C (194°F) in dry materials.
4.3.1 Fire Detection
Fires at composting plants are not unusual. Among the
plants surveyed, nine fires occurred at three different
facilities (not including minor smoldering fires in curing
piles); six of the nine fires were related to composting
processes, the remaining three were caused by welding
work.
The best fire detector to date has been operator vigi-
lance. Automated fire detection in reactors is problem-
atic. As discussed in Chapter Three, monitoring the
temperature of the composting materials may not reliably
detect fires. A relatively small, smoldering fire deep in the
compost bed may not raise the temperature of the reactor
offgases sufficiently to trigger an alarm. Furthermore,
heat or smoke detectors may not work reliably in the hot,
steamy atmosphere of reactors.
4.3.2 Fire Suppression
Fire suppression plans must be formulated in advance of
plant startup. Plant operators and local fire officials
should develop these plans jointly.
One firefighting strategy is to disperse the compost as
quickly as possible. Once dispersed, the compost cools,
extinguishing the fire. Then, if necessary, water can be
added. Unless there are flames, however, adding water to
the compost in the reactor may not be necessary and will
create a mess. At the Schenectady plant, when firefigh-
ters poured water onto the smoldering compost in the
reactor, the compost turned to mud and was difficult to
remove from the reactor. In later fires at the same plant,
firefighters set up fans to remove the smoke from the
building and used misting hoses to contain the fires in the
reactors while operators emptied them. Damage from
these fires was minimal.
48
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To enhance fire suppression, the computer control sys-
tem should be programmed to respond to fires by halting
reactor feed systems, shutting off aeration systems, and
issuing alerts so that reactor loading equipment can be
moved to safe areas.
4.4 Staffing Issues
4.4.1 Operations Staff
During normal operations, process monitoring (watching
controls, taking samples of mix, etc.); equipment moni-
toring (watching the equipment, routinely cleaning
chutes, etc.); and general utility work (housekeeping, pre-
ventive maintenance activities, etc.) all must be con-
ducted. The number of employees needed to run the
plant depends on the size and layout of the plant, the
degree of mechanization, and the number of operating
shifts per week. The Clayton County plant, which is a
small, compact, highly mechanized facility, operates with
only two operators; the Cape May plant, which is larger
and less mechanized, employs nine operators. The com-
plexity of odor control equipment is also important in
determining staffing. Table 4.3 lists the personnel
employed at the eight facilities visited.
Table 4.3 Summary of Personnel Used at Composting Plants
Number of Personnel Used
Location
Akron, OH"
Cape May County, NJ
Clayton County, GA
Newberg, OR^
Pittsburgh, NY
Portland, OR
Sarasota, FL
Schenectady, NY
Operations
8
9"
2
1.5"
4
6"
3
3
Maintenance
4
3
Maintenance
by WWTP staff
Maintenance
by WWTP staff
3
3'
2.5'
Maintenance
by WWTP staff
Other
4
2
1
0
1
1
1
1
•Personnel needed to operate at current reduced capacity. The plant
manager estimates that operations at full design capacity would
require 24 people plus 6 for the dewatering operation.
"Not including 5 summer employees used for groundskeeping and
utility work.
'Includes dewatering operations.
"Plant undergoing acceptance testing.
•Operators rotate through composting plant. Value shown is an esti-
mate of equivalent personnel.
'Maintenance by WWTP staff. Value shown is an estimate of equiva-
lent personnel.
4.4.2 Operator Training
At present, most in-vessel composting plants are man-
aged by operators with mid-level wastewater treatment
plant certifications. Certified operators, because of their
extensive experience, have the planning, budgeting,
organizational, and equipment skills needed to run a
composting plant. Operator certification courses include
basic training on biological processes, but do not provide
training in composting. Only one operator at the eight
plants surveyed had previous composting experience.
Operators must be educated in the proper use, mainte-
nance, and calibration of the sophisticated monitoring
and controlling devices, as well as in the basic operations
of the plant. In general, system suppliers provide training
on how to operate and maintain the composting equip-
ment and how to create appropriate compost mixes. Typi-
cally, system supplier representatives spend weeks to
months working with plant operators during startup. Most
plant operators reported that this on-the-job training was
satisfactory.
Many operators expressed a need for more information
on composting principles; at many sites, operators
receive only 1 day of this type of training, at most. As a
result, most operators approach the composting process
in a relatively empirical manner, which makes it more
difficult to diagnose problems or react to changing
conditions.
Training in odor control has also been lacking. Operators
should have a knowledge of the principles and chemistry
of odor detection and treatment. The proper control of a
chemical scrubber requires this kind of training, even if
the mechanical operations are straightforward. Some
background in the principles of odor dispersion and
meteorology would also be helpful.
The public agency may want to include a provision in the
construction contract requiring system suppliers to train
operators in the principles of composting and in process
control theory. Alternatively, the public agency may
engage an outside consultant to provide this training.
After startup, the consultant's expertise might also be
valuable in assisting plant operators with process-related
questions.
A written record of composting principles and related
issues that will outlast the system supplier's tenure on the
site and survive future staff turnover is essential. At the
very least, the operations and maintenance manual
should contain a thorough treatment of process theory
and control (see Section 4.5.2).
Ongoing training programs allow operators to keep
abreast of developments in sludge composting. In-vessel
composting is relatively new and optimum operational
practices are not yet established. Professional contacts
with other composting plants, particularly "sister" plants,
would increase the flow of information.
4.4.3 Staff Rotations
A permanent operations staff is preferable to a rotating
one because its workers develop expertise in the com-
posting process and the workings of the plant. In addi-
tion, permanence may foster a sense of "ownership" and
pride in the composting plant and its operation.
49
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There may be constraints, however, on assigning perma-
nent staff. Compost plant duty may not be considered as
desirable as treatment plant duty because of the working
conditions at the compost plant (e.g., odors, dust, humid-
ity, and heat). Moreover, operators may not be able to
obtain higher certification grades without a greater
breadth of experience than can be gained at a compost
plant.
If a rotation policy is instituted, the rotation period should
be long enough so that operators do not have to relearn
the operation each time they return. At the Portland facil-
ity, operators rotate into the compost plant for staggered
2-week tours. Because there are so many other assign-
ments in the wastewater treatment plant, operators do not
return to the compost plant for months. As a result, at any
given time, the Portland compost plant is staffed by one
operator with 1 week of recent experience and one opera-
tor who must be reacquainted with the operation after a
long absence. The Pittsburgh plant started operations
with a similar rotational system but decided to recruit a
permanent staff.
4.4.4 Acquiring Necessary Maintenance
and Repair Skills
To minimize downtime, staff with the skills necessary to
maintain and repair compost equipment should be read-
ily available. Although WWTP and compost plant equip-
ment have similarities, many compost plant items are not
found in a wastewater treatment plant and require differ-
ent skills. For instance, maintenance and repair of
materials-handling equipment at an in-vessel compost-
ing plant requires at least one person with good welding
skills. In addition, control systems are more complex at
In-vessel plants. Although WWTP personnel possess
many of the skills needed to maintain the compost plant,
the skills and knowledge available should be inventoried
before something goes wrong so that appropriate train-
ing can be arranged. Careful planning and recruitment
procedures, combined with appropriate compensation,
are required to obtain and retain the necessary skilled
personnel.
4.4.5 Assigning Regular Maintenance Staff
if a composting plant is not big enough to justify its own
maintenance staff, or if the public agency prefers to run
its maintenance program out of a central office, the com-
posting plant should be maintained by a small, defined
group of personnel. Maintenance personnel should not
be randomly assigned to the plant.
One reason for developing "compost plant specialists" is
that it is more efficient to train a small group than to train
the entire maintenance staff. More importantly, through
observation and the performance of regular maintenance
activities, the compost plant maintenance staff will
develop a familiarity with the equipment that will allow
them to anticipate and diagnose problems more quickly
than personnel who see the equipment only occasionally.
An additional benefit of this approach might be the devel-
opment of a sense of "ownership" among the staff. Dedi-
cated personnel may do a better job because they care
how "their" plant is being maintained. Where mainte-
nance crews rotate through a plant or are supplied by a
central pool, maintenance tends to be reactive rather
than preventive.
4.5 Maintenance Issues
4.5.1 Preventive Maintenance
The reliability of an in-vessel composting plant depends
on the operations ability to prevent and quickly remedy
disruptive conditions. A preventive maintenance pro-
gram is an important element in this effort.
4.5.2 Operations and Maintenance Manual
An up-to-date operations and maintenance manual is an
important tool. The manual should contain descriptions
of composting principles, as applied to the in-vessel sys-
tem; operating procedures for all equipment; diagrams of
the equipment ("catalog cuts"); spare parts lists; and
maintenance requirements. The manual should be
updated regularly and every modification to the plant
should be recorded. In this way, the document will
become a record that outlasts the individual operators
and maintenance personnel.
4.5.3 Spare Parts Inventory
An adequate supply of spare parts is essential to respond
quickly to mechanical breakdowns. In general, it is better
to err on the side of having too many spare parts than not
enough.
In determining what parts to stock, two factors should be
considered: the importance of the part to the system, and
the length of time required to obtain a replacement. Parts
that take a long time to obtain should be kept in stock; it
may be unnecessary to stock locally available parts. The
difficulty of procuring parts should also be considered. If
it takes 3 weeks to process the paperwork through the
public agency's procurement process, it may be prudent
to stock the part, even if it is locally available.
50
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Chapter 5
Introduction to the Case Studies
5.1 Background
In-vessel composting technology is relatively new, and
operating procedures are still rapidly evolving. Conse-
quently, the engineering literature has not contained up-
to-date information on the performance and operation of
in-vessel composting facilities. Moreover, the literature is
rarely detailed enough to provide guidance on the selec-
tion, design, and operation of systems that take advan-
tage of technology advances and the lessons learned
from currently operating facilities.
The site visit approach was proposed as the best means
of establishing a data base that would allow developers
of in-vessel composting facilities to draw information and
benefit from past experiences. A peer review team of
experts helped select the sites to be visited. Visits were
planned early in 1988 to eight facilities representing the
six types of in-vessel composting systems operating in
the United States. These systems are:
• American Bio Tech (ABT)
• Ashbrook-Simon-Hartley (Ashbrook)
• Fairfield Process (Fairfield)
• Paygro Process (Paygro)
• Purac Engineering (Purac)
• Taulman Composting Systems (Taulman)
For the ABT, Ashbrook, Purac, and Taulman systems, the
name of the composting system is synonymous with the
corporate name of the supplier. The Fairfield and Paygro
systems are both marketed and supplied by Compost
Systems Company.
At least one example of each kind of in-vessel compost-
ing process was surveyed. The Newberg, Oregon; Platts-
burgh, New York; and Schenectady, New York, plants
were chosen because they were the only full-scale plants
using the Ashbrook, Fairfield, and ABT technologies,
respectively, to compost municipal wastewater sludge
exclusively. The Akron, Ohio, plant was chosen to repre-
sent the Paygro process because it had a longer operat-
ing record than its sister plant in Baltimore.
For both Purac and Taulman, there were several candi-
date plants. In each case, one large plant and one small
plant were chosen for site surveys. The large plants were
located in Cape May, New Jersey (Purac), and Portland,
Oregon, (Taulman); the small plants were located in Sara-
sota, Florida (Purac), and Clayton County, Georgia
(Taulman). One reason for choosing two plants for these
systems was to investigate any differences that could be
attributed to size alone (independent of the type of reac-
tor). The other reason was to investigate differences
between so-called "first-generation" facilities and those
built later.
The site visits were conducted between April and Novem-
ber of 1988. As shown in Table 1.1, not all of the plants
were fully operational at the time of the site visit. The
Newberg plant was undergoing acceptance testing. At
the Schenectady plant, only a small "conditioning" reac-
tor was operating while mechanical repairs and modifica-
tions to the odor control system were being performed.
Three other plants (Akron, Cape May, and Portland) were
operating at reduced capacities because of odor or
mechanical problems. Three of the facilities (Clayton
County, Pittsburgh, and Sarasota) were processing all
of the sludge available.
A survey team of at least two members conducted each
site visit over a 1.5 to 2-day period. The survey team
interviewed plant supervisors, operators, and others
familiar with each facility's procurement and operations.
Contract documents and operating records were also
examined when available. Table 5.1 lists the information
collected during the site visits.
After each visit, the survey team prepared a report sum-
marizing the information gathered and sent the report to
each plant and system supplier for comment. The com-
ments were then incorporated into revised site visit
reports, which became the basis for the eight case stud-
ies in this summary report. The case studies detail expe-
riences, problems, and unique situations at each facility.
This information, in part, also formed the basis for Chap-
ters Two through Four.
5.2 Accuracy of the Data
Each case study represents a "snapshot" of the plant
taken at the time of the site visit. The factual data pre-
sented, such as mix ratios, loading rates, and operating
practices, are accurate only for the time of the site visit
and may have subsequently changed. To identify some of
these changes, the survey team conducted telephone
interviews just before publication of this document. This
51
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Table 5.1 Information Collected During the Site Visits
• General Plant Information
• History of the Plant
- Procurement and construction
- Capital costs
-Operating history and current status
• Materials Characteristics
- Sludge characteristics
-Amendment characteristics
- Recycle characteristics
- Mix characteristics
• Materials Handling
-Storage facilities
-Mixerfeed system
- Reactor feed system
• Reactors
- Configuration
-Agitation (if any)
- Discharge device
• Exterior Curing/Storage
- Description of facilities
• Air Handling
- Aeration system in first and second reactors
-Aeration system for exterior curing/storage area
- Nonprocess air ventilation
• Odor Control
- Characterization of problem
- Process air odor control
- Nonprocess air odor control
• Support Facilities
- General site
- Buildings
• Compost Product
- Characterization
- Restrictions on use
- Marketing and distribution
• Operations
-Staffing
-Training
- Operating costs
Information appears in the "Update" section at the end of
each case study. It should also be noted that the onsite
plant personnel provided all of the information presented;
the survey team did not independently verify the data.
5.3 Nomenclature
To keep the case studies as uniform as possible, the
facilities were described as follows. First, because terms
such as "bioreactor" and "cure reactor" are not used by
all system suppliers, reactors are referred to only with
respect to their location in the process train. The terms
"first reactor" and "first-step reactor" are used for the
first reactor in the train. "Second reactor" and "second-
step" reactor refer to the reactor second in the train.
Second, because no consistent measure of compost sta-
bility was used at the facilities, the survey team was not
able to determine whether material in outside piles was
"curing" or just being "stored." Therefore, the terms
"curing/storage" and "cured/stored" are used through-
out the studies to describe any outside piles of material,
both aerated and unaerated.
5.4 Summary Data
This section summarizes the data collected during the
site surveys in the following areas:
• Materials
• Mixing
• Materials-Handling Systems
• Aeration Systems
• Odor Control Systems
• Exterior Curing/Storage
5.4.1 Materials
Both waste-activated sludge (WAS) and mixtures of pri-
mary sludge and WAS are processed at the in-vessel
facilities. Of the plants surveyed, only one (Portland) com-
posts anaerobically digested sludge.
All of the plants surveyed use sawdust as an amend-
ment; one plant (Akron) uses a combination of bark and
sawdust. The amendment is not recovered or reused
after composting at these plants.
Amendment materials are usually purchased from local
industry, but there are alternative sources. At Clayton
County, for instance, sawdust is made at the plant by
grinding wood chips in a harnmermill. The wood chips
are produced from timber grown on lands irrigated with
treatment plant effluent.
Table 5.2 summarizes the types of sludge composted
and the design (when available) and actual solids con-
tents of the sludge, sawdust, recycle, and mix. Table 4.2
shows amendment specifications used at six of the
facilities.
5.4.2 Mixing
Table 2.7 compares the design mix ratios to the mix ratios
in use during the site visits. Most plants use more saw-
dust and frequently more recycle in their actual mix ratio
than anticipated in the design.
At all plants, analytical procedures are used to take mix
samples and determine mix ratios. Some of the short-
term tests for solids content (i.e., the heat lamp test) are
approximated. Also, percentage of total solids in the
sludge changes over time, even within one loading cycle.
Thus, operators must use their judgment in determining
mix quality.
In some cases, the operators enter the solids contents
and other pertinent data into a computer. The computer
uses this information to achieve the proper mix ratio of
sludge, amendment, and recycle with respect to moisture
and porosity, by controlling the speed of one or more
conveyors supplying materials to the mixing device.
52
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Table 5.2 Solids Content of Compost Ingredients (Percent)
Sludge*
Sawdust
Recycle
Mix
Akron, OH
Cape May, NJ
Clayton County, GA
Newburg, OR1
Type of Sludge
1° + WAS
1 ° + WAS"
WAS'
(34-35)
WAS'
Design
— .
20
22
—
Actual
26
(20-26)°
23
16
17
Design
—
50-70
(20-30)
(15-17)
60
Actual
55
(50-60)
—
60
60
Design
—
>40
(65-70)
"""""
—
Actual
68
(60-68)
—
50
(52-66)
—
Design
—
40
(53-65)
"™"
38
Actual
42
(40-50)
37
34
(45-52)
—
Pittsburgh, NY
WAS"
20
(12-20)
(20-25)
>50
60
55
(35-45)
(42-55)
40
(30-40)
40
Portland, OR
Sarasota, FL
Schenectady, NY"
1° + WAS1
WAS
1° + WAS«
— 25
(23-28)
— 14
(50-55)
25 25
(20-28)
— 86
(60-95)
— • 90
(34-46)
>50 55
(38-40)
— —
s>45 — k
—
35
35
(35-39)
37
40
—
"Sludge = dewatered sludge.
"Numbers in parentheses represent ranges.
"Same for bark and amendment used here.
"Ferric chloride added to primary clarifiers.
"Aluminum sulfide is added.
'Data are from the time of the site visit when the plant was in startup;
process not at steady state.
"Partially aerobically digested during storage.
"Potassium permanganate is added.
1 Both 1 * and WAS are digested.
1 Limited operating data.
"Not using recycle at the time of the site visit.
— = Not available or not collected.
1" = Primary sludge.
WAS = Waste-activated sludge.
Four of the plants surveyed use pugmill mixers and four
use plow-blade mixers. Both types of mixers work satis-
factorily, although at two sites, the pugmill mixers were
modified to improve their performance. Akron replaced
the original motors, removed half the paddles, and modi-
fied the drive mechanisms; Cape May removed half the
paddles and replaced the remaining paddles with more
effective, custom paddles.
5.4.3 Materials-Handling Systems
Conveyor systems are designed to carry sludge cake,
amendment, mix, recycle, and finished compost. There-
fore, their design must take into account the different bulk
weights, moisture contents, temperatures, and other
characteristics of these materials. In addition, the sys-
tems must account for changes to the materials due to
mechanical action by the conveyance equipment itself.
Site restrictions and the physical design of the reactors,
which dictate the conveyor configurations, also influence
the choice of conveyor types. Table 5.3 summarizes the
different kinds of conveyors found at the in-vessel facili-
ties surveyed and the functions they perform.
Generally, three kinds of conveyors are used: drag chain
conveyors, belt conveyors, and screw conveyors. Belt
conveyors are the predominant type of conveyor used at
the plants surveyed and have experienced only minor
problems (i.e., sticking of materials and spillage). Drag
chains at in-vessel plants have not performed well and
have required numerous modifications and extensive
maintenance. In contrast, screw conveyors, including
those in live-bottom bins, experienced few problems at
the plants surveyed. Cleated belt conveyors, used at
Akron (unsuccessfully) for wet and dry materials, experi-
enced severe spillage problems because of materials
sticking to the belts. At some plants, pneumatic systems
are used to move sawdust; these have worked well.
Schenectady uses a bucket conveyor to move sawdust.
Conveyor successes and failures are discussed in more
detail in Chapter Three (see Table 3.3) and in the case
studies.
5.4.4 Aeration
At the facilities surveyed, all first and second reactors are
aerated, while some curing/storage piles are not. (Curing/
storage pile aeration is discussed in Section 4.3.6.) In
most reactors, air is introduced at the reactor bottom.
ABT reactors introduce and remove the air through
lances that extend vertically into the composting mate-
rial. In Purac reactors, most air is introduced from the
bottom, but air is also pulled in through the top of the
reactors, because the exhaust blowers draw a greater
volume of air than that provided by the supply blowers.
53
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Table 5.3 Types of Material Conveying Equipment Used
Location
Type
No. Function
Akron, OH
Cape May, NJ
Clayton County, GA
Newberg, OR
PJattsburgh, NY
Portland, OR
Sarasota, FL
Schenectady, NY
Belt conveyor
Cleated belt conveyor
Radial stacking conveyor
Belt conveyor
Drag chain conveyor
Drag chain conveyor
Screw conveyor
Pneumatic system
Belt conveyor
Drag chain conveyor
Screw conveyor
Pneumatic system
Belt conveyor
Screw conveyor
Radial stacking conveyor
Belt conveyor
Drag chain conveyor
Screw conveyor
Pneumatic system
Belt conveyor
Drag chain conveyor
Screw conveyor
Pneumatic system
Belt conveyor
Drag chain conveyor
Screw conveyor
Bucket conveyor
18 A,B,C,D,E,i7G
7 A,B,C,D,E,FG
1 G
7 B,D,E,FG
4 B.D.E.F
2 B,D,E,FG
3 . A, D, G
1 C
1 A
8 B,E,FG
1 D
1 C
13 B,D,E,FG
4 E
1 G
12 A,B,D,E,FG
3 E,G
6 B
1 C
6 B,D,E,FG
2 FG
5 B, D, E
1 C
8 A,B,D,E,FG
1 F
2 A, F
1 C
Legend for Materials Conveying Systems:
A. Transfer of sludge from dewatering to storage.
B. Transfer of sludge from storage to mixer.
C. Transferor amendment from receiving to storage.
D. Transfer of amendment from storage to mixer.
E. Transfer of sludge, amendment, and/or recycle to mixer.
F. Transfer of mix from mixer to reactor infeed.
G. Transfer of compost from outfeed device to recycle and/or
curing/storage.
Ashbrook reactors remove the air from the bottom of the
reactor, the two Purac reactors remove the air from 0.9 to
1.8 m (3 to 6 ft) below the top surface of the compost
(depending on the depth to which the reactor is filled),
and the rest of the facilities remove the air from above the
top surface of the compost.
Table 5.4 summarizes the reactor air-handling systems
by location, including the capacities of both the aeration
and exhaust systems and the straight-line distance the
air must f tow through the compost.
The types of air-moving equipment used at the plants
surveyed are listed in Table 5.5. For air supply, Cape May,
Clayton County, Newberg, Portland, and Sarasota all use
positive displacement blowers. Centrifugal blowers are
used at Akron, Pittsburgh, and Schenectady. The air
supply systems were designed to provide a range of flow
rates. Airflow rates can be varied by changing the speed
of the blower motor, throttling the inlet or outlet, or operat-
ing different numbers of blowers. (Table 2.4 describes
standby blower systems at the eight plants.)
With the exception of the Paygro system, the exhaust
blowers are all constant-speed centrifugal blowers that
direct the exhaust air to some kind of odor control
system.
5.4.5 Odor Control Systems
Odor has been the most serious environmental/public
relations problem at in- vessel facilities. All but one of the
facilities surveyed treat the air coming directly from the
reactors (the process air). Nonprocess air is sometimes
treated and sometimes vented directly to the atmo-
sphere. The odors from exterior curing/storage piles are
not controlled. Although recently found to be a problem,
odors from intermittent sources like these usually require
less treatment than process air.
Despite odor control systems, six of the eight plants sur-
veyed have received complaints from the local commu-
nity. At five of these plants, the complaints have been
serious enough to cause plant owners to decide to
upgrade or replace their existing odor control systems.
Although ammonia and organic sulfides have been found
at several plants, the spectrum of odor-causing com-
pounds in composting reactor exhaust is not well charac-
terized. At the time of the site visits, gas chromatographic
studies of odorous air had been performed only at Akron,
Cape May, and Sarasota. Pittsburgh has since per-
formed this kind of analysis. A number of odorous com-
pounds have been identified at the facilities, but the
concentrations of these compounds vary from plant to
plant. The causes of these variations are not understood
and little analytical work has been done to correlate odor
production with plant operating procedures.
At the eight plants surveyed, four strategies have been
employed to control process odors:
• Diluting the odors with large volumes of air
• Passing the air through water at an adjacent wastewa-
ter treatment plant
• Passing the air through compost boilfilters
• Chemically treating the air either with ozone or wet
chemical scrubbers
Table 5.6 summarizes the types of odor control systems
used in the plants surveyed. (For a more detailed discus-
sion of the odor control systems used at these plants, see
Chapter Three and the case studies.)
5.4.6 Exterior Curing/Storage
All of the plants have some kind of exterior curing/storage
facilities. Six of the plants have a paved area (to permit
collection of any leachate). Three of the plants have cov-
ered facilities and two aerate the curing/storage piles.
Table 5.7 summarizes the curing/storage facilities pro-
vided at the plants surveyed.
54
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Table 5.4 Summary of Reactor Air-Handling Systems at Sites Visited
Installed Aeration Capacity
Location
Akron, OH
Cape May, NJ
Clayton Co., GA
Newberg, OR
Pittsburgh, NY
Portland, OR
Sarasota, FL
Schenectady, NY
Equipment
Supplier
Compost Systems
(Paygro)
Purac
Taulman
Ashbrook-
Simon-Hartley
Compost Systems
(Fairfield)
Taulman
Purac
American
Bio Tech
Total
cfm"
240,000
16,500°
1,500
5,000
32,000
7,800
5,250
9,600
Average
Straight-Line
Airflow
Path Through
Compost (ft)"
8 to 10
21"
26
2 to 30"
8 to 10
26
,23d
6to7
Installed Exhaust Capacity
Area
Served
Reactors plus
building
Reactors plus
headspace
Reactors only
Reactors only
Reactors plus
Reactors only
Reactors only "
Reactors only
Total
cfm
870,000
18,000=
1,000
5,000'
184,000"
domes
6,000
5,700
12,000
"of m x .0005 = m3
6ftx.3048 = m
'After plant modification. Due to pipe size limitations, maximum rates achieveable are 12,000 cfm supply and 15,400 cfm exhaust
"From bottom of reactor to exhaust nozzles.
•Usually no more than two consecutive zones run in the same mode.
'Only 2,500 cfm currently available.
"Including roof fans on domes.
.Table 5.5 Summary of Blower Equipment
Location
Akron, OH
Cape May, NJ
Clayton County, GA
Newburg, OR
Pittsburgh, NY
Portland, OR
Sarasota, FL
Schenectady, NY
Type of
System
Paygro
Purac
Taulman
Ashbrook
Fairfield
Taulman
Purac
American
Bio Tech
#of
Reactors
'• . 4 •
2
2
2
2
6
2
4.
Air Supply
96 constant speed,
axial-flow blowers
3 variable speed,
positive displace-
ment blowers plus
2 constant speed,
centrifugal blowers
3 variable speed,
rotary lobe blowers
2 variable speed,
rotary lobe blowers
2 constant speed,
centrifugal blowers
6 variable speed, and
2 constant speed
positive displacement
3 variable speed,
positive displacement
blowers
3 constant speed,
centrifugal blowers
with inlet throttling
Exhaust
32 roof fans
5 constant speed,
centrifugal blowers
2 constant speed,
centrifugal blowers
2 constant speed,
centrifugal blowers
2 constant speed,
centrifugal blowers
plus 2 roof fans
6 constant speed,
centrifugal blowers
blowers
3 constant speed,
centrifugal blowers
3 constant speed,
centrifugal blowers
with suction
throttling
"Not including conditioning reactor.
55
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Table 5.6 Odor Control Facilities
Location
Odor Control System*
Air Dispersal System
Akron, OH
Cape May, NJ'
Clayton County, GA
Newberg, OR
Plattsburgh, NY
Portland, OR1
Sarasota, FL
Sohenectady, NY
Dilution of process air"
Compost filter for de-
watering and mixing rooms
Two-stage wet chemical
scrubber for process air
One-stage wet chemical scrubber
for mixing building
One-stage wet chemical scrubber
for sludge storage building
Bubble air through water in
storage pond
Bubble air through water in
oxidation ditch
One-stage wet chemical scrubber
Bubble air through primary
effluent
Ozone contact tower (replaced
by two-stage wet scrubber
in spring 1989)
One-stage wet chemical scrubber9
Roof fans on building
Ground-level release
100-fF stack
50-ft stack
20-ft stack
Ground-level release
Ground-level release
5-ft stack on top of
scrubber building
(35-ft above ground)
Ground-level release
Stack on top of ozone
contact tower (80 ft
above ground)
Released from top of
scrubber, 10 ft above
ground
•For process air, unless otherwise specified.
'Two-stage wet scrubbing planned.
•After 1987 modification.
*ftx.3048 = m
•Multiple-stage wet scrubbing planned.
'Anaaroblcally digested sludge.
"Two-stage wet scrubbing planned.
Note: Effectiveness of odor control and air dispersal systems is discussed in the case studies.
Table 5.7 Exterior Curing/Storage Facilities
Location
Covered
Paved
Aerated
Comments
Akron, OH
Cape May, NJ
Clayton County, GA
Newberg, OR
Partially
X
X
X
Aeration system
constructed but
no longer used.
Aeration system had not
been used by the time of
the site visit.
Ptattsburgh, NY
Portland, OR
Sarasota, FL
Sormectady, NY
—
—
X
Partially
X
X
X
X
— :
_
—
—
56
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Chapter 6
Akron, Ohio
6.7 Introduction
The City of Akron owns the in-vessel composting facility,
which is operated by Fairfield Service Company. The
system used is the Paygro rectangular agitated bin pro-
cess, supplied by Compost System Company (CSC).
Burgess and Niple, Limited was the design engineering
firm. The design capacity of the plant is 66 dt/day (73
dton/day), based on a two-shift operation, 7 days per
week (464 dt/week [511 dton/week]).
The plant is located on a 77,300 m2 (19.1-acre) site in the
Cuyahoga River Valley. The city's 75-year-old wastewater
treatment plant (WWTP) and landfill are to the east of the
plant, across the river. A restaurant and a shopping area
are to the south, and a large number of condominiums
and expensive homes are to the west, with the nearest
homes 91 to 152 m (300 to 500 ft) away. The Cuyahoga
River Valley National Recreation area is to the north.
Except for a small area north of the facility, there is little
room for expansion and no buffer zone.
The site visit took place on June 9 and 10, 1988. Unless
otherwise indicated, all information presented in this
case study is representative only of that time.
6.2 Operating History
6.2.1 Procurement and Construction
The city decided to pursue composting as a sludge dis-
posal option to supplement the existing incinerators,
which were approaching full utilization. Originally, the city
planned to install a static pile system at the site, which is
adjacent to the environmentally sensitive Cuyahoga
River Valley and National Recreation Area. Upon further
investigation, the city decided that a static pile system
would produce excessive odors and therefore would not
be suitable.
The Paygro in-vessel composting system was chosen as
an alternative because it demonstrated the ability to:
• Protect materials and equipment from the weather
• Compost municipal sludge with full-scale capability
• Provide adequate aeration (and thus prevent odor
production)
• Handle periodically wet mixes without "catastrophe"
• Successfully complete a pilot operation using Akron
sludge at a South Charleston, Ohio, facility
The city determined, with the approval of EPA, that CSC
was the only company capable of providing the desired
Paygro system. Consequently, CSC was selected as a
"sole source" vendor. To open up bidding as much as
possible, however, EPA minimized the scope of the work
specified under the sole-source portion of the contract.
As a result of this approach, the general contractor had to
integrate various parts of the project and pieces of equip-
ment, such as some of the materials-handling equipment,
the curing system, and the odor control system. Accord-
ing to both the city's construction superintendent and the
project engineer from Burgess and Niple, this integration
process greatly complicated the job and may have
resulted in higher rather than lower costs.
Startup of the facility was initiated in December 1986.
Individual component tests and a 6-week integrated sys-
tems test were conducted during the summer and fall of
1987; they were completed in November of that year. All
major tests were successfully concluded and the facility
was legally accepted under the construction contract.
Nevertheless, some of the cleated conveyors, the curing
system, and the odor control system were not performing
in accordance with the design criteria.
To cover the costs of improving these items, the city has
applied for an EPA Modification/Replacement (M/R)
Grant. Preliminary plans for the conveyor modifications
and alternative curing methods, and the pilot testing of
alternative odor control systems have been completed.
6.2.2 Capital Costs
The city based its original decision to invest in sludge
composting on cost estimates for operating a static pile
system, which was thought to be comparable to in-vessel
composting. The cost estimates used by the city when it
decided to pursue the in-vessel composting option were
not available at the time of the site visit.
The preliminary 1981 construction cost of the plant was
estimated to be $24.16 million, and the postdesign esti-
mate made in January 1984 was $28.6 million. Actual
construction costs were $30.5 million, including approxi-
mately $1.5 million in change order costs. All estimates
included the costs of the dewatering facilities, which were
57
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$8 to $9 million. An EPA Innovative/ Alternative (I/A)
Grant covered 85 percent of the costs; the City of Akron
paid for the remainder.
6.2.3 Operating History and Current Status
The plant has been operating continuously since January
1988. At the time of the site visit, the plant was process-
ing 40 dt/day (44 dton/day) of sludge, in one shift, 4 days
per week. This amount is equivalent to 160 dt/week (176
dton/week), or 34 percent of the design capacity and 50
percent of the WWTP sludge produced. All four of the
system5s reactors have been in use, but the facility has
operated at a reduced rate to decrease odor problems.
The WWTP incinerates the sludge that is not
composted.
6.3 Description of the Plant
6.3.1 Systems Overview
At the Akron facility, the dewatering, mixing, composting,
and administrative operations take place in one large
building (317 by 73 m, 23,000 m3 [1,040 by 240 ft, 5.7
acres]). Curing/storage takes place outside this building
in a 4,000 m2 (1-acre) covered and a 20,000 m2 (5-acre)
uncovered area. A 5,300 m2 (1.3-acre) odor control biofil-
ter is also located outside the main process building.
Figure 6.1 presents an overview of the Akron Paygro
composting process. Liquid sludge from the WWTP is
pumped into sludge wells in the process building and
dewatered by belt filter presses. The dewatered sludge,
bark and sawdust amendments, and recycle are stored in
piles and then in live-bottom surge bins prior to being
conveyed to the two pugmill mixers.
The mix is transferred by a series of conveyors to any one
of the four reactors. The reactors are long rectangular
bins adjacent to one another that share two mobile
excavation/conveyor ("extractoveyor") digging devices.
The extractoveyors periodically either agitate the com-
post or move it to a different section of the reactor. The
extractoveyors also transfer the compost from the reac-
tors to a series of conveyors, which carry the compost to
Figure 6.1 Akron, Ohio, Composting Process Train
RECYCLE STORAGE
flH
BARK
STORAGE
SURGE
BIN
flR^
WVTTP
SLUDGE WELLS
BELT FILTER PRESSES
MIXERS
REACTORS
RECYCLE
REACTORS
UNAERATED CURING/STORAGE
THIS SLUDGE STORAGE BIN IS TEMPORARILY BEING USED TO HOLD
SAWDUST TO DUST THE CLEATED CONVEYOR (SEE TEXT).
58
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the recycle storage area or to unaerated curing/storage
piles. Blowers supply air to the reactors via plenums
located in the bottom of the reactors. The airflow in the
reactors was designed to be reversible with a variable
flow rate, but due to processing difficulties, the negative
airflow mode is no longer use. Positive airflow occurs
when the air is blown up through the compost mass.
Process air from the positive airflow is discharged into
the reactor area where it mixes with ambient air and is
vented to the outside atmosphere by a series of roof
exhaust fans. Mixing and dewatering room air are ducted
to the compost biofilter.
6.3.2 Feed and Mix Characteristics
6.3.2.1 Sludge
The liquid sludge from the WWTP is an unstabilized mix
of primary sludge and waste-activated sludge (WAS). It
contains 4 to 6 percent total solids, depending on the
primary sludge to WAS ratio and a number of other pro-
cess variables. The volatile solids content of the sludge
exceeds 70 percent.
The liquid sludge is delivered to the composting facility
by pipeline from the WWTP It is then deposited into six
283,838 L (75,000 gal) receiving wells prior to dewatering
by 10 belt filter presses. Emergency storage of sludge
occurs at the WWTP in open-topped, aerated sludge
holding tanks with a combined capacity of 22,710,000 L
(6,000,000 gal). Sludge is not normally held in the holding
tanks for longer than 3 days.
Dewatered sludge was originally stored in two multiple-
screw live-bottom surge bins, each 3.7 by 3.7 by 5.5 m
high (75 m3) (12 by 12 by 18 ft high, 2,592 cf). The bins
hold 127 to 145 wt (140 to 160 wton) of sludge, which
correspond to 27 to 36 dt (30 to 40 dton). At the time of
the site visit, one of the sludge bins was used to store
sawdust (see Section 7.3.3.2).
The bulk weight of the dewatered sludge is typically 881
to 929 kg/m3 (55 to 58 Ib/cf). After storage in the surge
bins, the bulk weight of the dewatered sludge is 961 to
993 kg/m3 (60 to 62 Ib/cf).
6.3.2.2 Amendment
The Akron facility uses sawdust and shredded hardwood
bark as amendment. The total solids content in both the
bark and the sawdust materials is 50 to 60 percent, and
both materials contain 90 to 95 percent volatile solids.
The bulk weight of the bark is 320 to 401 kg/m3 (20 to 25
Ib/cf); the bulk weight of the sawdust is 288 to 352 kg/m3
(18 to 22 kg/m3).
The materials are delivered by truck to the covered stor-
age area, where they remain in storage piles for up to 2
months. A front-end loader places the materials into a
hopper, which then directs the amendment to a series of
conveyors. The conveyors transport the amendment to
storage bins in the mixing area (see Figure 6.2).
The facility has one bark storage and one sawdust stor-
age bin. In addition, one of the sludge bins is used to
store sawdust for dusting the sludge cleated conveyor
(see Section 6.3.3.1). The bark and sawdust bins are
each 3.7 by 4.9 by 61 m, 111 m3 (12 by 16 by 20 ft, 3,840
cf). They are duplex units, each divided by a partition wall
and equipped with a multiple-screw live-bottom dis-
charge unit.
6.3.2.3 Recycle
Recycle is transported by a series of belt and cleated
conveyors from one of the reactors or curing piles into
one of two recycle storage' bins located in the mixing
room. The bins are 3.7 by 4.9 by 6.1 m, 111 m3 (12 by 16
by 20 ft, 3,840 cf) and have a total capacity of 87 to 104 wt
(96 to 115 wton). The bins are equipped with multiple-
screw live-bottom discharge units.
The total solids content of the recycled compost is 60 to
68 percent. The volatile solids content exceeds 60 per-
cent and the bulk weight is 401 to 481 kg/m3 (25 to 30
Ib/cf).
6.3.2.4 Mix
According to the system operators, porosity of the mix is
a key variable in producing an acceptable compost prod-
uct, and the ratio of the mix components is adjusted daily
to achieve this desired porosity. A porosity test developed
by the design engineer is conducted daily (see
Section 6.4.1.4). The mix ratio depends on the varying
moisture contents and on whether recycle or product is
produced. The design volumetric mix ratio is 1/0.33/0.33/
1.33 (sludge to bark to sawdust to recycle). The actual
volumetric mix ratio at the time of the site visit was 1/0.4/
0.6/2,33 percent greater than the design value.
Since recycle is the largest fraction of the bulking agent,
the quality of the recycle used has a great impact on mix
porosity. When too much recycle is used in the mix, the
compost contains too many fine particles, which
decreases the porosity of the mix.
The target value for total solids in the mix is 43 to 45
percent, with an average of 42 percent and a range of 40
to 50 percent. This target value is higher than the design
rate of 40 to 42 percent because during early operations
the recycle was wetter than anticipated and, thus, more
sawdust was needed in the mix. The added sawdust
made it difficult to control the temperature and flow of air
in the reactors. As soon as the mix solids reached 40
percent or greater, the situation improved. At the time of
the site visit, the 43 to 45 percent target rate was consid-
ered the lowest solids content that could give a proper
mix porosity.
The volatile solids content of the mix is approximately 70
percent. The bulk weight is 561 to 641 kg/m3 (35 to 40 Ib/
cf) (609 to 673 kg/m3 [38 to 42 Ib/cf] by design). By adding
additional sawdust, bark, and recycle (to maintain the
proper porosity of the mix), the bulk weight of the mix can
59
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Figure 6.2 Akron, Ohio, Materials Handling Train
«
*
60
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generally be kept about 593 kg/m3 (37 Ib/cf). Additional
sawdust, bark, and recycle in the mix also help to reduce
odors through maintenance of desirable porosity.
If the mix is too dry, the work area can become very dusty,
especially near the reactor loading area during "fluffing"
or unloading (see Section 6.3.4.2). The dust is annoying
to the workers and requires additional cleanup efforts.
6.3.3 Materials Handling
The materials handling system is depicted in Figure 6.2.
A number of different types of conveyors perform a vari-
ety of functions and offer the operators a great deal of
flexibility in maneuvering the materials. For example, the
compost can be "fluffed" in place or transported to
another reactor at any time, and finished compost can be
set out for curing or sent to the recycle bin without han-
dling by a front-end loader.
All conveyors are interlocked through the central com-
puter. In addition, throughout the plant, a number of mate-
rial flow indicators are connected to the interlock
system.
In general, the system's flat belt conveyors have worked
well and convey mix material at a greater vertical angle
than the "standard" maximum values used in conveyor
design. Materials can be moved with portable equipment
in some areas (on the floor of the mixing room or in the
storage area outside the building) to bypass many of the
conveyors. Excessive spillage has occurred at some of
the transfer points between the flat belts and cleated
conveyors (see Section 6.3.4.4). In a plant performance
evaluation report, the city recommended that drip or spill-
age pans be installed at all cleated conveyor transfer
points.
6.3.3.1 Mixer Feed System .
The live-bottom multiple-screw system in the sludge,
recycle, and amendment surge bins regulates the dis-
charge of the materials from the bins onto an outfeed
screw conveyor. The sludge bin deposits the sludge onto
a cleated conveyor, and the recycle, bark, and sawdust
bins deposit material onto a trough-shaped belt conveyor
and then a cleated conveyor.
The discharge rates are controlled by a computer that
uses gamma-ray mass weigh scales to measure the vol-
ume of the materials on the belts (radiation attenuation is
correlated with the amount of material on the belt). The
operator provides the computer with the volumetric mix
ratio, which is chosen on the basis of moisture content
measurements taken the previous day and periodic bulk
density measurements taken during loading.
Cleated conveyors for the amendment, recycle, and
sludge transport the materials up a 45-degree incline to
the mixer feed conveyor arid then to the mixers. The
cleated conveyor is one item the city considers a "fail-
ure"; it is not well-suited for transporting dewatered
sludge because the sludge tends to stick to the belt and
does not completely discharge, even with the aid of the
belt "beater" furnished with the unit. It appears that the
sludge bin discharge screws change the sludge consist-
ency from a friable cake to a sticky gelatinous mass. As a
result of sludge stickiness, approximately 10 percent of
the feed, or 36 wt/day (40 wton/day), spilled off the dis-
charge end of the conveyor. Under the worst conditions,
cleanup required 1 to 2 laborers working full-time.
As an interim solution, one of the sludge surge bins was
converted to a sawdust bin, and the conveyor is now
dusted with sawdust before the sludge is applied, pre-
venting the sludge from sticking. This process has
reduced spillage to a temporarily acceptable 1 to 2 per-
cent. The operators are investigating the possibility of
applying the proper amount of sawdust for the mix via a
dusting mechanism rather than using the existing saw-
dust surge bin.
Another problem with the cleated conveyors was that the
idlers (support rollers) originally supplied with the con-
veyor spanned the full width of the belts (see Figure 6.3).
This configuration caused excess wear and vibration on
the belts (as the cleats ran over the rollers), and minor but
annoying spillage. More seriously, when one of the walls
of a cleat broke, the broken piece caught on the idler and
tore the belt. Replacing the original rollers with "stub"
idlers, which support the belt only on its flat portion out-
side the cleat walls, has reduced vibration and eliminated
the possibility of the belt getting caught on a worn cleat
wall (see Figure 6.3).
6.3.3.2 Mixers
The system has two pugmill mixers that are used one at a
tjme on an alternating basis. Either mixer can feed any
one of the four reactors.
Each mixer was originally equipped with two 15,000 W
(20 hp) motors. During startup, however, the electrical
drive motors and drive mechanisms would often over-
load because too much material was being forced
through the mixers. The 15,000 W (20 hp) motors were
replaced with 19,000 W (25 hp) units, half of the pugmill
mixer paddles were removed, and the drive mechanisms
were modified. During the site visit, the mixers were oper-
ating well at design capacity and there was no apparent
degradation of mix quality due to this modification.
In emergencies, mixing can be done by front-end loaders
on the mixing room floor.
6.3.4 Reactor
6.3.4.1 Reactor Feed System
The mix is discharged from the mixer to a trough-shaped
belt conveyor that transfers the material to the reactor
area via one of two flat belt conveyors. The belt convey-
ors are located between reactors 1 and 2 and reactors 3
and 4, and run the length of the process building (see
61
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Figure 6.3 Cleated Conveyor Modifications
±
BELT
CLEATS
SIDE
ORIGINAL RETURN IDLER
(FULL-WIDTH ROLLER)
SECTION THROUGH BELT
FAILURE OF BELT CAUSED BY
FULL-WIDTH ROLLER
SIDE
SECTION THROUGH BELT
REPLACEMENT STUB IDLERS
Rgure 6.4). The mixed material is transferred to a tripper
car and then to an indexing belt conveyor that spans the
width of the reactors and deposits the mix into the
desired portion of the reactor.
The feed conveyor and tripping system have a loading
capacity of 0.06 mVs (7,500 cf/hr). The loading system
can be operated either semiautomatically or manually. In
either case, the tripper car is controlled by an operator
stationed on site.
6.3.4.2 Configuration
The system is equipped with four concrete reactors, each
223 by 6.1 m (730 by 20 ft) with a composting depth of 3
m (10 ft) (see Figure 6.5). The total system volume is
16,500 m3 (584,000 cf). The four reactors share two
extractoveyor digging machines that span the reactor
width and move on rails mounted on the outside walls of
the reactors. Figure 6.6 shows the front of one of these
extractoveyors.
The extractoveyors use toothed rotating drums that
break up and mix ("fluff") the compost mass and drop the
material onto a wide table conveyor inclined at a variable
angle (see Figure 6.7). This conveyor either drops the
compost back into the reactor behind the extractoveyor
or lifts the compost out of the reactor (see
Section 6.3.4.4). Fluffing is usually done on a weekly
basis (i.e., at least twice during a typical 21-day detention
period). As part of this fluffing operation, the system can
move the composting mass to another reactor. The
extractoveyors operate semiautomatically and are con-
62
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Figure 6.4 Akron Composting Facility Building Plan
Scale 1 in. = 80 ft.
COVERED STORAGE AREA
BULKING AGENT BINS
RELOAD HOPPER
FINISHED COMPOST STORAGE/CURING
BULKING AGENT
LOADING
REACTOR NO. 1
REACTOR NO. 2
muz
REACTOR AREA
BLOWER GALLERY BELOW
COMPOST CONTROL
REACTOR NO. 3
CONVEYOR (2)
LOAD OUT AREA
DISCHARGE CONVEYOR
.COMPOST EQUIPMENT MAINTENANCE
SCALE ROOM
REACTOR NO. 4 [_JJ DIGGING MACHINE (2)
SLUDGE PROCESSING
SLUDGE VELLD
BELT FILTER PRESSES
CHEMICAL FEED (POLYMEN)
PIPE GALLERY
TUELVE AERATION ZONES; NOT SUBDIVIDED; EACH
CAN BE RUN IN POSITIVE OR NEGATIVE MODE.
ADMINISTRATION
OFFICES
LABORATORY (THIRD FLOOR)
PERSONNEL FACILITY
MAIN CONTROL ROOM (THIRD FLOOR)
MECHANICAL ROOM
AND VEHICLE MAINTENANCE
trolled by an operator situated in an enclosed cab on the
device.
Heat detectors and a dry standpipe system with hose
stations provide fire protection in the reactor area.
Figure 6.5 Reactor Room
6.3.4.3 Reactor Aeration System
The reactors are aerated by a series of centaxial blowers
via 0.46-m (18-in.) high concrete air plenums running
beneath the full width of each reactor (see Figure 6.8).
The blowers are located in long covered galleries
between reactors 2 and 3 and the outside of reactors 1
and 4.
Each reactor is configured in a series of twelve 18-m (60-
ft) long aeration sections. Although the compost mass
itself is not separated into individual aeration sections,
concrete block walls in each plenum separate the individ-
ual sections. Each aeration section is supplied with air
using two blowers, for a total of 24 blowers per reactor; 96
for the whole system. Each blower has a capacity
of 1.18 m3/s (2,500 cfm) at a pressure of 0.2 m (8 in.) water
column (W.C.) (see Section 6.4.1.2).
Galvanized steel grates which separate the plenums
from the reactors allow air to flow to or from the compost.
A 0.2-m (8-in) layer of washed gravel on top of the grating
Figure 6.6 Front of Extractoveyor
serves to distribute air below the compost bed; 0.15 to
0.23 m (6 to 9 in) of bark on top of the gravel provide an
additional support layer for the compost. Short-circuiting
of air in the compost does not appear to be a problem.
By design, air can be supplied to the reactors in either the
positive mode, in which air is blown up through the com-
post, or the negative mode, in which air is drawn down-
63
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Figure 6.7 Extractoveyor at Akron Facility
AIRFIOW
DIRECTION
AERATION
SYSTEM
DISCHARG
CONVEYOR
ward through the compost. In the positive mode, outside
air is pulled in from the roof of the process building
through long vertical intake ducts that connect to the
blowers in the blower gallery. Air is then forced up
through the reactors into the process building, and is
ventilated out the roof.
Thirty-two roof fans dilute the process air generated dur-
ing the positive aeration mode and draw it out of the
reactor building. The air is released 16 m (52 ft) off the
ground. The total volume of air moved is about 425 m3/s
(900,000 cfm) or 6 air changes per hour. This dilution air
enters the building through louvers on two sides of the
reactor building. The design was intended to provide a
10:1 ratio of fresh air to process air.
Noise from the reactor roof fans has been excessive
enough for neighbors to issue formal complaints, and
thus the use of the fans has been reduced. When the fans
are not operating, however, and the temperature of the
incoming air is less than 7.2° to 10°C (45° to 50°F)
(during winter), condensation in the building increases
significantly. Under the worst conditions, visibility inside
the building can drop to less than 6.0 m (20 ft). Moisture
appears to be well managed when the roof fans are
operating and the solids content of the compost is 15 to
25 percent higher than the incoming mix.
As designed, in the negative aeration mode, air is drawn
from the process building through the reactors, into the
plenum, and to the exhaust blowers, which send the air
into a common plenum running the length of the blower
gallery. This air is sent to a compost biofilter (see
Section 6.3.7.4). The system was designed to operate in
the negative aeration mode during the first few days of
the composting cycle, when the potential for odor produc-
tion is the highest. However, moist air drawn through the
blowers during the negative aeration mode accelerated
the wear on the blower bearings. Also, the bark layer
became saturated with moisture and impeded the air-.
flow. To avoid these problems, the negative aeration
mode has been abandoned and all aeration is run in the
positive mode. Section 6.3.7.4 describes the odor control
system used when the reactors are aerated in the posi-
tive mode.
During the coldest part of the winter, the process air is
preheated to remove additional moisture from the com-
post. Preheating is accomplished by using boiler-
supplied hot water and metal coils in the vertical air
intake duct. The system is designed to raise the incoming
air temperature by about 44°C (40°F).
6.3.4.4 Outfeed Device
The outfeed process involves the use of extractoveyors
64
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Figure 6.8 Air Handling System
£
u
65
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as well as several additional conveyors. During the out-
feed process, the compost is first discharged onto a
mobile transfer conveyor that couples to the back of the
extractoveyor. The transfer conveyor deposits the com-
post onto the main reactor conveyor, which is connected
to a common discharge conveyor located at the end of
each reactor. The discharge conveyor feeds another con-
veyor, which in turn goes outside the process building to
aflat belt storage conveyor (see Figure 6.2).
The storage conveyor runs the length of the building and
feeds a mobile cleated stacking conveyor, which deposits
the compost onto piles for curing. If desired, the compost
can be transferred from the storage conveyor to the recy-
cle surge bin using the storage cleated conveyor and the
amendment conveyor.
Excessive spillage has occurred at two different transfer
points between flat belt conveyors and the cieated con-
veyors. This spillage occurred even though the rated
capacity of the successive conveyors is the same.
6.3.4.5 Leachate/Condensate
The plenums and air ducts are equipped with drains that
were at one time used to handle the condensate pro-
duced during the negative aeration mode. The conden-
sate was piped to a pump and then pumped to the
WWTP
6.3.5 Exterior Curing/Storage
The facility was designed to provide covered curing/
storage for 2 months and uncovered curing/storage for
an additional 4 months. At the time of the site visit, the
piles were 3.0 to 4.0 m (10 to 12 ft) high and were built in
the uncovered storage yard using front-end loaders.
Uncovered piles can be built up to 7.6 m (25 ft high) using
stacking conveyors (see Figure 6.9). The piles have been
a source of odor, even after 6 months of curing/storage.
All runoff from the piles is captured and returned to the
WWTP
Figure 6.9 Stacking Conveyor
The curing/storage piles are not aerated. During the
design phase, it was expected that the compost leaving
the reactors after 21 days would be stable enough for a 3-
month unaerated curing/storage period, but the actual
compost has not been as stable as expected. After com-
posting for 19 to 21 days, the material reheated when
stored in high (7.6 m [(25 ft]) piles. Even compost that had
been cured/storage for 6 months reheated when placed
back in the reactors.
At the time of the site visit, the feasibility of providing
permanent aerated curing/storage was being studied.
Although, it appears that supplemental aeration may be
required in the higher piles to avoid anaerobic conditions,
it might be possible to maintain aerobic conditions in
smaller (3 to 3.7 m [10 to 12 ft]) unaerated curing piles or
windrows. The smaller piles would increase the storage
space requirements, however, and the available onsite
curing/storage space is already limited.
6.3.6 Nonprocess Air Handling
Air from the mixing room 9.4 m3/s (20,000 cfm) and the
dewatering room 8.5 m3/s (18,000 cfm) is vented out
exhaust plenums (see Figure 6.8) to the biofilter (see
Section 6.3.7.4). Large doors also naturally ventilate the
mixing areas. The sludge receiving well is vented through
an independent compost biofilter.
6.3.7 Odor
6.3.7.7 Predominant Site Characteristics and
Weather Patterns
At the plant site, the Cuyahoga Valley is about a third of a
mile wide. One mile north and south of the plant, the
valley is one mile wide. The ridges along each side of the
valley rise about 61 m (200 ft) above the plant site. This
location greatly affects the dominant weather patterns
and thus the dispersion of composting odors in the vicin-
ity of the plant.
The wind direction and velocity are monitored by two
weather stations located on the roof of the process build-
ing and at the top edge of the valley. Prevailing winds are
from the west. When the air is calm, an air inversion
occurs in the valley. The inversion traps the ventilation
exhaust odors close to the plant. The strongest odors
reach the community during the early morning hours, as
the winds increase, break up the nocturnal inversion, and
blow the odors trapped by the inversion into the neigh-
boring area.
Residents living on the adjacent ridge of the valley are
affected during the stagnant wind conditions that pre-
dominate for longer periods of time than the nocturnal
inversions. When sustained winds finally occur, they
move the odors that have accumulated in the restricted
airspace around the plant to the residential and commer-
cial areas throughout the valley; compost odors have
been documented up to 2.4 km (1.5 miles) from the
facility.
66
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6.3.7.2 Odor Complaints
From March through December 1987, the plant received
63 odor complaints; from January through August 1988,
73 complaints were received. Most of the complaints
were made by individuals residing or working in the
nearby area. Despite the presence of background odor
sources from the WWTr? local residents have distin-
guished a particular "compost odor" that they find objec-
tionable. Generally, however, no correlation has been
observed between community odor complaints and spe-
cific plant operations, except when the operators are
digging into curing/storage piles on windy days.
6.3.7.3 Odor Studies and Sources
Odor Science and Engineering, Inc. performed an odor
study for the city in mid-1987. Before the study was under-
taken, odor emissions had not been specifically mea-
sured, but odor complaints had been investigated and
addressed. The studys Draft Final Report (July 1988)
concluded the following:
1. The major source of odors reaching the residents is the
ventilation exhaust from the reactor building. (Odors in
the building ranged from about 0 to over 200 D/T
[dilutions-to-threshold].)
2. Other odor sources include the belt filter press room,
sludge storage well, and curing/storage piles.
3. The compost odor has a characteristic "pungent,
woody, earthy" odor that could be detected up to 4.0
km (2.5 miles) from the facility. Odor complaints have
been received from distances of up to 2.4 km (1.5
miles) away. Odors from the incinerator across the river
were detectable up to 3.2 km (2 miles) from the plant.
4. Community odor impacts depend primarily on wind
and weather conditions. Preventing ambient odor lev-
els from reaching nuisance levels during the inversion
or stagnant wind conditions in the valley requires a 90
percent reduction (during full operation at full capacity)
in odor emissions from the reactor room ventilation
system. Under all other wind conditions, a reduction of
about 50 percent is acceptable.
5. Curing/storage piles do not appear to contribute to the
detectable odors downwind except if the piles turn
septic.
Table 6.1 summarizes the measurements taken during
the study.
Another odor source was leaky process air pipes in the
vicinity of the compost filter, which have since been
sealed.
6.3.7.4 Odor Treatment
To control the sludge well odors, an airflow of about 0.5
m3/s (1,000 cfm) was directed at one time from the top of
the sludge wells through a 15.3 m3 (20 cy) pile of compost
that served as a biofilter. Currently, the odor treatment
system for the sludge well, mixing, and dewatering air is a
Table 6.1
Location
Summary of Odor Measurements*
n" Range
(dilutions-to-threshold)
Compost Building Exhaust Fans
Liquid Sludge Storage Tank
(after treatment by biofilter)
Dewatering Room (presses operating)
Sludge Pump Gallery
Large Biofilter
(Inlet)
(Outlet)
Curing/Storage Piles
19 11-228
2 1,316-2,698
4 144-315
1 397
1 1,280
5 281-758
6 3-26
6 3-126
•From Akron Composting Facility Odor Study, Draft Final Report, Odor
Science and Engineering, Inc. July, 1988.
bn = number of samples represented in reported range.
compost biofilter, 23m2 (250 ft2) in area by 1.1 m (3.5 ft)
deep (see Figure 6.10). The filter is divided into two sec-
tions, each of which contains 30 aeration lines made of
0.3 m (12 in.) corrugated and perforated polyethylene
tubing. Air is exhausted from the filter at ground level.
The compost filter decreases odor concentrations by
about 95 percent.
When the compost filter becomes dry, the compost piles
crack and must be raked. At the time of the site visit, the
compost had not been raked for 2 months. The filter also
must periodically be replaced; the original filter material
was replaced after 10 months of use. To prevent the
compost filter from cracking and to extend its life, opera-
tors are also considering wetting the compost filter.
Figure 6.10 Compost Biofilter for Odor Control
Diluting the reactor building (process) air with outdoor air
at a 10:1 ratio has failed to prevent odor impacts to the
local community. To improve control of odors from the
reactors, a 0.3-m (12-in.) layer of recycle is placed on top
of the composting mass. Consequently, the effective
reactor volume has been deceased by 10 percent. This
layer of recycle was not included in the recycle fraction of
67
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the initial mix discussed in Section 6.3.2.4. However, this
additional recycle is taken into account in inventory
records and mass balance calculations.
In general, inadequate process control (i.e., wet mixtures,
low porosities, and poor air control) during startup has
contributed to more odor production than expected. This
situation was partially alleviated when drier recycle was
produced. The operators have also increased the volu-
metric mix ratio by adding more amendment and have
instituted closer control over the mix porosity. When
proper porosity is attained by maintaining a mix bulk
weight of less than 641 kg/m3 (40 Ib/cf), a uniform airflow
can be achieved, leading to aerobic conditions in the
composting mass, good moisture removal, and lower (but
not eliminated) odor production.
At the time of the site visit, the city was exploring the
possibility of aerating the piles in the covered curing/
storage area for 1 to 3 weeks prior to long-term storage.
Permanent pile aeration facilities will be included in the
M/R grant application.
6.3.7.5 Odor Treatment Recommendations
Odor Science and Engineering, Inc. recommended the
following three-tiered approach to control the odors from
the composting areas:
• Tier 1: The present ventilation rate of the roof fans
[about 425 m3/s (900,000 cfm)] would be reduced to
142 mVs (300,000 cfm) (77.9 mVs [165,000 cfm] of
process air and 63.7 mVs [135,000 cfm] of additional
building exhaust air). This air would be treated in a
two-stage oxidizing wet scrubber. Tier 1 would be used
at all times. Under worst-case weather conditions, this
tier would be the only ventilation allowed, and odorous
process operations such as fluffing and excavating
compost beds would be prohibited.
• Tier 2: An additional 70.8 m3/s (150,000 cfm) from the
reactor building would be sent to the two-stage scrub-
bers, which are expected to remove 90 + percent of
the odors. Tier 2 ventilation would be used in addition
to Tier 1 under all but the worst weather conditions.
• Tier 3: The existing roof fans would be modified to
lower-noise units, and their capacity decreased to 212
mVs (450,000 cfm). This system would be used only to
maintain acceptable temperature and humidity levels
in the reactor room, and only if weather conditions
permitted.
Odor Science and Engineering, Inc. also recommended
the diversion of ventilation air from the dewatering, mix-
ing, and sludge well areas to a dedicated wet scrubber
that would operate under all weather conditions.
6.3.8 Support Facilities
The administration area in the main process building
contains an open reception area; a private office for the
general manager; a private office for the city plant super-
intendent; a shared office for several other supervisory
personnel; a lunchroom, conference room, and restroom
for administrative staff; and a restroom and showers for
operating personnel. Some unassigned storage space
was converted into the lunchroom for the operating per-
sonnel. Odors entering the administration area from the
dewatering area have been eliminated by careful sealing
of the wall between the administrative area and the pro-
cess area, and by modifying the ventilation system.
Maintenance facilities consist of a two-bay maintenance
shop. Spare parts are stored above the shop and in other
rooms nearby. There is one laboratory room with a hood
and equipment to perform the solids and polymer jar
testing for dewatering.
6.4 Monitoring and Performance
6.4.1 Reactor Control Strategy
Although the only regulatory requirement for the Akron
facility is that the compost maintain a temperature of
55°C (131 °F) or greater for 3 consecutive days, the oper-
ators' goals are to maintain aerobic conditions, minimize
odor production, and adequately remove moisture in the
compost by making sure that the mix is of sufficient
porosity. The system usually meets the regulatory crite-
rion and produces a compost product with a satisfactory
solids content. If a batch does not reach 55°C (131 °F), it
is used as recycle.
6.4.1.1 Temperature Monitoring and Control
The original temperature probes were strung across the
compost reactors and buried in the compost. Because
they were difficult to see and remove, however, they were
often accidentally dug up by the extractoveyor, and once
removed from the compost, they were difficult to replace.
Consequently, the original temperature probes have
been replaced by lance-type probes, which are easier to
use. The probes, which contain three thermistors, are
placed by hand in the compost mass, where they remain
except when the material is being fluffed. The probes
reach to the bottom third of the compost mass.
The central computer takes the average of the three
thermistor readings every 3 seconds and uses that tem-
perature value to control the blowers, based on the oxy-
gen requirements of the compost (see Section 6.4.1.3). A
daily average temperature reading is also entered into a
materials tracking database system. A temperature pro-
file created from these daily data is shown in Figure 6.11.
6.4.1.2 Aeration Quantities and Rates
The aeration system is capable of providing a total flow of
113 mVs (0.41 cfm/cf) of reactor volume, or 1.0 m3/s/dt
(2,350 cfm/dton) of sludge. Actual air provided varies
according to the temperature readings in the reactor.
Each reactor is divided into 12 independently controllable
aeration zones, and the blower schedules and tempera-
ture setpoints are varied by the operator. The total aera-
tion rate is varied by operating either one or two blowers
at a time.
68
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The following four different aeration levels can be used,
depending on temperature readings and the oxygen level
in the compost:
1. One blower per each aeration section operates
intermittently.
2. One blower operates continuously.
3. One blower operates continuously and one blower
operates intermittently.
4. Two blowers operate continuously.
Figure 6.11 Time-Temperature Profile
The temperature setpoints at which the four aeration
levels become activated are changed seasonally and are
based on prior experience. Level 1 aeration is usually
adequate to supply the needed process air except for the
early part of the composting cycle (see Section 6.3.4.3).
The setpoints and aeration schedule in place at the time
of the site visit are shown in Table 6.2.
Table 6.2 Aeration System Operation (June 1988)
Temperature
Setpoint
0°C«
35°C
45°C
57°C
8(°CX1.8) -
Blower #1
3 min on; 7 min off
Continuous
Continuous
Continuous
h 32 = "(=
Blower #2
Off
Off
As temperature rises,
3 min on; 7 min off
As temperature falls,
1 min on; 16 min off
Continuous
During the first 24 hours that the mix is in the reactor, the
compost temperature increases when the air supply
increases. In the summer, the aeration system is set to
introduce only enough air into the compost to maintain an
oxygen level of at least 10 to 12 percent at all times. In the
winter, because the air supply is colder, the oxygen level
is increased to raise the compost temperature, thereby
offsetting the cold incoming air. However, care is taken
not to add so much cold air that the compost becomes
chilled.
6.4.1.3 Oxygen Monitoring
Oxygen in the composting mass is measured manually
using a lance-type probe and oxygen meter. Daily mea-
surements are made during the first several days after a
batch of mix is loaded into the reactor. The results are
used to establish temperature setpoints or to override the
temperature control of the blowers, if necessary. After the
first few days, the biological activity of the compost mass
diminishes, and the oxygen levels are measured less
frequently.
6.4.7.4 Other Tests and Equipment
At the time of the site visit, the laboratory technician was
experimenting with two stability tests. One was a pres-
sure differential test that measures oxygen uptake by the
change in pressure in a sealed vessel containing com-
post. In the second test, a compost sample is wetted to
60 percent moisture and placed in a beaker. A dissolved
oxygen (DO) meter is then used to measure the oxygen
content of the beaker atmosphere.
The stability test results had not been correlated with
process parameters. The two tests did not always agree,
and according to the tests, compost that is stable will
reheat if placed in the reactors again. Since the prelimi-
nary data from these tests are inconclusive or mislead-
ing, they are not used for process control.
The design engineer developed a porosity test, which
measures the bulk weight of the mix or compost using a
standardized bucket. The operator compresses the mate-
rial in the bucket by standing on a plywood disk placed on
top the material, and measures the decrease in depth.
From the bulk weight and change in height after com-
pression, the "loose" and "compacted" porosities are
calculated by the database computer program. The
loose bulk weight is also checked against the measure-
ments estimated by the nuclear scale (see Sec-
tion 6.3.3.1).
Grab samples of the compost leaving the reactors are
collected each day and stored in a refrigerator, and a
composite sample is created once a month and sent to
an outside laboratory for detailed analysis. Example data
from three monthly composites are shown in Table 6.3. In
addition, two grab samples per 9.1 t (10 ton) of product
compost leaving the reactor are sent to the WWTP for
analysis as a backup to the outside laboratory analysis.
Dry solids are measured in the dewatered sludge, the
amendment, the mix as it is loaded into the reactor, and
the compost (once during fluffing and again during dis-
charge operations). The facility plans to obtain a micro-
69
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Table 6.3 Compost Composition
(Composites taken In period: February 2 to March 31,1988)
Constituent
Total Solids
Conductivity of 10% Solution
pH of 10% Solution
Total Organic Carbon (TOC)
Heat Content
Total KJeWahl Nitrogen (TKN)
Organic -Nitrogen
Ammonia -Nitrogen
Nitrate -Nitrogen
Total- Phosphorus
Calcium
Potassium
Arsenic
Cadmium
Chromium
Copper
Lead
Magnesium
Mercury
Molybdenum
Nickel
Selenium
Zinc
AWdn
Benzene
a-NHC
b-BHC
d-BHC
Concentration
Range
61-63.4
510-1,080
6.8-8.5
210,000-260,000
4,000-4,532
2,450-2,700
520-800
1,900-2,300
0.14-88
2.7-4,400
1,800-30,000
2.0-1,600
3.7-12
7.7-11
0.2-54
190-230
130-160
470-530 (only
two samples)
0.46-0.51
14 (only
one sample)
18-23
1.0-<1.3
110-900
<0.01-<0.1
<5
<0.01-<0.1
<0.01-<0.1
<0.01-<0.1
Units'
%
mhos/cm
mg/kg
Btu/lb"
mg/kg
n
n
n
n
a
tt
n
tt
n
tt
tt
n
tt
n
tt
n
n
n
/ig/kg
mg/kg
n
n
AR
—
—
AR
DW
n
tt
tt
n
n
n
n
tt
n
tt
tt
n
AR
n
II
Constituent
g-BHC (Lindane)
Carbon Tetrachloride
Chlorodane
Chloroform
Cyanide
4,4', -ODD
4,4', -DDE
4,4', -DDT
Dieldrin
Endosufan I
Endosulfan II
Endosulfan Sulfate
Endrin
Endrin Aldehyde
Heptachlor
Heptachlor Epoxide
Hexachlorobenzene
Hexachlorobutadiene
PCB 1016
PCB 1221
PCB 1232
PCB 1242
PCB 1248
PCB 1254
PCB 1260
Toxaphene
Trichloroethylene
Vinyl Chloride
Concentration
Range
< 0.01- < 0.1
<5
< 0.01- < 0.1
4-20
1.2-87
<0.01-<0.1
< 0.01- < 0.1
< 0.01- < 0.1
<0.01-<0.1
<0.01-<0.1
< 0.01- < 0.1
< 0.01- < 0.1
< 0.01- < 0.1
< 0.01- < 0.1
<0.01-<0.1
<0.1
<4-<15
<4.5-<14
< 0.02- < 0.2
< 0.02- < 0.2
< 0.02- < 0.2
< 0.02- < 0.2
< 0.02- < 0.2
< 0.02- < 0.2
1.7-2.9
<0.1-<3
<5
<5-<10
Units'
mg/kg AR
Mg/kg "
mg/kg "
Mg/kg "
mg/kg
tt tt
" "
" "
" "
H It
tt N
n tr
it tt
mg/kg AR
n n
" "
tt n
" "
it it
it tt
tr tt
n tr
ti n
tt n
it it
n n
Mg/kg "
n tt
•DW = dry weight; AR = as received.
»BtU/lbX 2.326 = kJ/kg.
wave solids instrument because oven-drying is too slow.
The metals content of the WWTP sludge is measured by
the city.
6.4.7.5 Data Management
Temperature, oxygen, and porosity data are collected
periodically for each batch of compost loaded into a reac-
tor. Data are tabulated on a computerized spreadsheet
and are used to decide whether a batch will be used as
recycle or as product. The data are also used to diagnose
process problems when a "bad" batch of compost is
produced.
The data collection and storage methods have been eas-
ily understood by the operators, and have greatly aided in
diagnosing process problems and in forecasting and
maintaining product quality.
6.4.2 Product Quality
Ohio regulations define three classes of compost/sludge
for reuse purposes. Table 6.4 shows the allowable uses
and associated standards. Akron compost is a Class II
product, primarily because it does not always meet the
cadmium/zinc standard. The product is of proper quality
for landscaping and other non-food chain uses.
No product is sold until after it has been cured/stored for
a minimum of 3 months. Also, to enhance the appear-
ance of the product and remove large particles, the prod-
uct can be run through a shredder before being sold.
6.4.3 Mass Balance and Reactor Detention Time
The mass balance for the Akron facility is shown in Fig-
ure 6.12. It was constructed based on loading data from
February 1 to December 1,1988, and on the correspond-
ing discharge data from March 14 to December 20,1988.
Table 6.4 Compost Use Regulations and Standards
Compost Sludge Class
Constituent I' \\° III0
Cadmium
Lead
Polychlorinated Biphenyls
(PCB)
Cadmium/Zinc (ratio)
£12.5
£500
£5
s 0.015
£25
£1,000
£10
>0.015
> 25 mg/kg
> 1,000 mg/kg
> 10 mg/kg
> 0.015
•Class I: No restrictions on use. Can be applied to food crops.
"Class II: Landscaping uses only. Can nor be applied to food crops.
Consumers must be warned of dangers and all sales must be logged.
cClasslll: Used for landfill or strip mine reclamation only.
70
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Figure 6.12 Akron, Ohio, Mass Balance
Average Daily Quantities, 2/1/88 to 12/1/88 (Based on 7-day/week)
o
ii
CM
O
05
£
s
CM
tJ
O
3
a
0)
| I
g£*
uJ *i in
a: csj co
v
g «n m. g •
S S3 ed !
g
/^
<55
2 -H «M
!&e
X " X
J
71
-------
The values shown in the figure are the daily averages
calculated by dividing the aggregate values for the entire
time period by the number Of days (305 days between
February 1 and December 1,1988). The batches loaded
during February 1 to December 1,1988, remained in the
reactor an average of 23 days with a range of 15 to 44
days.
Based on the values calculated in the mass balance, the
reactor detention times and plant capacities at various
mix ratios were calculated, as shown in Table 6.5. As
indicated by the table, the greater the volume of mix/dt of
sludge used, the shorter the detention time at the design
capacity or the lower the capacity at the design detention
time. Because the effective reactor volume has been
decreased by adding a layer of compost on top of the
compost bed for odor control, either the ultimate capacity
of the plant will be limited or the detention time in the
reactors at the design capacity will be shortened.
Table 6.5 Detention Times and Capacities at Various Mix Ratios
Design Current Mix Current Mix
Mix Ratio with- Ratio with
Ratio out Cover8 Cover
trained on the job in both the principles of composting
and the operation and maintenance of the equipment.
Continuing education is provided on safety issues.
Table 6.6 Staff Requirements'
Mix volume per dry ton of sludge
(ftVdton)
Detention time at design capacity
of 73 dt sludge/day
Plant capacity at a 21-day
detention time (dton sludge/day)
378"
21
73
504
16
55
561
14
50
•A layer of recycle Is placed on top of the compost bed for odor control
purposes.
»ftVdtonx.0311=mVdt.
6.5 Operations
6.5.1 Staffing
All staff are employees of the Fairfield Service Company
rather than the city. Table 6.6 lists the staff in place at the
time of the site visit, including the staff needed to operate
the dewatering equipment. The table also includes an
estimate of the staffing requirements for operating the
plant at design capacity (i.e., a two-shift operation). The
table does not reflect contracted services, such as night
and weekend guards.
All staff work Monday to Friday. The operations staff
works from 6:00 a.m. to 2:30 p.m. and the office staff
works from 8:00 a.m. to 4:30 p.m. Sludge is processed
four days per week, and the fifth day is used for mainte-
nance and housekeeping.
At the time of the site visit, the general manager had 5
years of experience working at a solid waste and sludge
composting facility. The operations manager is certified
as a Class IV (highest in Ohio) wastewater treatment
plant operator. The lab technician has a B.S. degree in
chemistry. Other staff personnel generally have no prior
compost or wastewater training. All staff members were
Position
General Manager
Operations Manager
Foreman
Lab Technician
Accountant
Secretary
Dewatering Operator
Dewatering Utility Worker
Loader Operator
Compost Operator
Maintenance Mechanic
Maintenance Helper
Utility Worker
Total
No. Employed
During
Site Visit
1
1
1
1
1
1
1
1
2
2
2
2
2
18
No. Needed for
Two-Shift (full
capacity)
Operation
(estimated by
manager)
1
1
2
1
1
1
3
3
4
3
3
3
4
30
•The staff are employed by Fairfield. The city employs a full-time site
superintendent in addition to the personnel listed above.
The city employs a full-time onsite superintendent to
oversee the facility for the city. He is a registered engineer
with a background in construction and was closely
involved in the plant construction.
The city continues to retain Burgess and Niple, Limited,
who at the time of the site visit, operated the odor control
pilot facilities and coordinated the technical aspects of
the planned M/R grant application.
6.5.2 Marketing and Distribution
The city completed a marketing study that identified top-
soil producers and landscapers, including nursery opera-
tors, as the major potential users of the compost. Direct
use by the general public is not expected, although the
public is the end user for many landscaping applications.
In addition to its operating contract, the Fairfield Service
Company has a 3-year marketing contract with the city. In
the first year of marketing, Fairfield is responsible for
product sales and receives all revenues from the sales
after marketing expenses are reimbursed. In the second
and third years, if revenues exceed marketing costs, the
city will receive 75 percent of the profit.
The compost will be priced on a sliding scale, depending
on the quantity ordered. For instance, a pickup truck-
sized load will cost $25/m3 ($19/cy), with larger orders
72
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receiving lower unit prices. As a goodwill gesture,
bagged compost is available to the general public for
$1/bag.
As of July 1988, the onsite inventory of product was about
46,000 m3 (60,000 cy). During the summer of 1988,8,000
m3 (10,000 cy) were sold to primary markets and 8,000 m3
(10,000 cy) were sold to secondary markets.
Revenues generated were not available.
6.5.3 Operating Costs
The total annual operation and maintenance costs esti-
mated in 1984 (postdesign) were $1,944,000, which
includes the cost of sludge dewatering.
The individual cost categories are shown in Table 6.7 in
both 1984 and 1987 dollars. In 1987, expenditures for the
composting process were estimated to be $1,398,000.
Table 6.7 Operations and Maintenance Cost Estimates ($1,000)
Estimated Actual
Preconstruction Expenditures (25
Estimate (73 dton/day dton/day sludge
sludge processing)' processing)0
1984 $° 1987$'
1987$'
Labor
Power and Fuel
Chemicals and Materials
Maintenance
Contracted Services
Subtotal
Management Fee
Total
$/dry ton
528
337
816
263
0
1,944
0
1,944
73'
564
360
871
281
0
2,075
0
2,075
78'
390
187
318
50
153
1,098
300
1,398
153
•7-day/week basis.
"dton/day x .9072 = dt/day.
'Includes dewatering.
d1984 costs updated by ratio of Engineering News Record cost
indexes for January 1984 (4109) and June 1987 (4386).
"Excludes dewatering.
'Based on 73 dt/day, 365 days/year.
"Based on 25 dt/day, 365 days/year.
Comparisons of actual with estimated costs are difficult
because the current plant budget includes the de-
watering operation and the plant is running at only partial
capacity. Power costs cannot be isolated to show the
amounts spent for aeration, materials handling, and
otheruses.
Unexpected costs include the additional sawdust
needed to dust the cleated conveyors, modifications
made to the odor control system and reactor roof fans (to
reduce noise), and the design of the plant modifications
to provide aerated curing/storage. Cost estimates for
these jobs were not available during the site visit.
6.6 Update
As of June 1989, no major changes had been made to
the facility or operations since the site visit. The plant
processed a total of 7,104 dry metric tons (7,825 dry
tons) in 1988, or an average of 19.3 dry metric tons (21.3
dry tons) per day. The actual operating costs shown in
Table 6.8 include dewatering costs, which are not kept
separately.
Table 6.8 Operating Costs*
(January 1 through December 31,1988)
Item
Labor
Utilities
Amendments
Polymer
Other"
TOTAL
Amount
$ 595,977
260,275
410,022
87,911
719,726
$2,073,91 1
% of Total
29
12
20
4
35
•Includes dewatering.
"Breakdown of these costs not given.
Akron plans to change the method of conveying sludge
from the storage bin to the mixer, where sludge sticking to
the existing cleated conveyor is a problem. At the time of
the site visit, the plan was to add a sawdust bin to "dust"
the conveyor prior to sludge placement. The current plan
is to replace the cleated conveyor with a flat belt "pinch"
conveyor, and rearrange the sludge feed to the belt to
give the conveyor a flatter slope.
The city feels compost marketing is going well; 1989
sales to date already meet 1988's total sales, Also, as a
promotion, bags of compost are being sold for one dollar
per bag; eight to nine thousand bags were sold "in the
past few weeks".
73
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-------
Chapter 7
Cape May, New Jersey
7.1 Introduction
The Cape May County, New Jersey, in-vessel composting
plant is owned by the Cape May County Municipal Utili-
ties Authority (CMCMUA). The facility uses a rectangular
vertical system supplied by Purac Engineering, Inc. The
original design capacity of the facility was 11.0 dt/day
(12.1 dton/day), but its maximum capacity has since been
increased to 18 dt/day (20 dton/day).
The facility is situated on a 20,000 m2 (5-acre) fenced-in
flat site, bordered by back bay waters leading to the
Atlantic Ocean on the east and by a highway on the west.
Homes are located on the far side of the highway, with the
nearest residence to the plant about 305 m (1,000 ft)
away. Some public office buildings are situated on the
south side of the facility, and there is an open area to the
north. Little room exists for expansion of the facility.
The site visit took place on June 22 and 23,1988. Except
as indicated, the information presented in this case study
is representative of that time only.
7.2 History of the Plant
7.2.1 Procurement and Construction
The CMCMUA hired PQA Engineering Company as a
consultant to evaluate in-vessel composting technolo-
gies and other methods of sludge disposal. The consult-
ant recommended in-vessel, plug-flow composting
because that method would allow the operators to have
greater control over the composting process, including
odor emissions. At the time the decision was made, the
plug-flow method was well established in Europe. In addi-
tion, in-vessel sludge composting was estimated to cost
less per dry ton ($341/dt [$310/dton]) than incinerating
the sludge ($400/dt [$400/dton]) or shipping it to Pennsyl-
vania for land application ($881/dt [$800/dt]).
The GMCMUA issued a prequalification RFP to which
Taulman-Weiss and Purac both responded. Both submis-
sions were judged to be acceptable, and the consulting
engineer designed the Cape May facility based on these
two systems. An advertisement was then issued for a
construction contractor, from which several bids were
received. Because the low bid exceeded the construction
cost estimates (see below), the engineer redesigned the
facility to lower the estimated costs, as follows:
• Deleted redundant conveyors and blowers, emer-
gency generators, a shredded newspaper amendment
system, a fire protection system for the amendment stor-
age area, and an office building, which was to include the
laboratory and lockers
• Incorporated additional drag chain conveyors and
compound curve configurations into the conveyance
system so that it would use less equipment
• Converted an amendment storage silo to open bins
• Reduced storage space
• Simplified instrumentation
• Minimized paving
• Reoriented the reactors to utilize less surface area
According to the chief of operations, some of the design
modifications, such as those made to the original instru-
mentation system, eliminated overly complicated and
unnecessary aspects of the system. Other changes,
including the addition of drag chains, the minimized pav-
ing, and the reorientation of the reactors, have adversely
impacted operations, as is described in the following
sections.
After redesign, the project was advertised again and an
adequately low bid was received from McElwee-Courbis
Construction Co. Purac was hired as a subcontractor to
the general contractor.
Purac's performance bond was still outstanding at the
time of the site visit because the company had not met
the amendment guarantee. The recycle, as specified,
was not yet available on a regular basis.
7.2.2 Capital Costs
Construction costs were $8.4 million; 95 percent of the
costs was paid for by an EPA Innovative/Alternative (I/A)
Grant and the State. Approximately $3 million has been
spent on modifications.
7.2.3 Operating History and Current Status
Regular operation began on April 29,1985, and a perma-
nent operating permit was issued in September 1987 by
the New Jersey Department of Environmental Protection
(DEP). Due to odor control problems and community
complaints, the DEP issued a consent order that required
the facility to reduce operations to a throughput that
would not produce nuisance odors. In August 1986, the
reactor exhaust system was redirected to a new two-
stage scrubber.
75
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Rve fires have occurred at the facility: one in August
1985; three in November 1986; and one in December
1987. The 1985 and 1987 fires were not attributed to the
composting process, but to poor fire prevention mea-
sures practiced by contractors working on site. The 1986
fires appeared to be caused by spontaneous combustion
in the compost (see Section 7.3.4.3). The damage from
the last fire in November 1986 put the plant out of opera-
tion until June 1987. Smoldering fires continue to occur in
the outdoor curing/storage piles.
Several other aspects of the system were modified,
including the aeration system, in mid-1987; and the con-
veyance, reactor, and external curing/storage systems,
from the winter of 1987 through April 1988. After the
modifications were completed, the facility was restarted
in April 1988.
At the time of the site visit, the plant was limited to
processing 11 dt/day (12.2 dton/day) of sludge, due to the
DEP consent order. Based on the ongoing system modifi-
cations and odor control monitoring, the plant is gradually
increasing its operating throughput. In general, the plant
operates at a peak rate during summer the when it pro-
cesses 18 dt/day (20 dton/day) of sludge. During the
winter, the plant processes sludge at a low rate of 7.3 dt/
day (8.0 dton/day).
7.3 Description of the Plant
7.3.1 Systems Overview
Figure 7.1 is a schematic of the Cape May composting
facility. The plant is composed of a storage shed for
dewatered sludge; storage bins for amendment and recy-
cle; an operations and maintenance building, where the
feed is conveyed and mixed in two mixers; a reactor
building, which houses the two reactors and blowers; an
open-air covered temporary holding area for the material
from the reactor; and an uncovered curing/storage area.
The sawdust and recycle storage bins are in adjacent
bays outside the operations and maintenance building.
Odor control scrubbers are outdoors.
Figure 7.2 shows the materials-handling train. Amend-
ment and recycle are pre-mixed on the ground and trans-
ported by a front-end loader to an amendment/recycle
mixture storage area. The sludge and amendment/
recycle are then transported by truck to live-bottom bins
in the operations and maintenance building. The materi-
als are then carried via conveyors to the mixers.
In the reactor aeration system, inlet air enters the bottom
of the compost mass in the reactor. Exhaust air is drawn
from beneath the surface of the upper layer of the mate-
rial and from the headspace above it. The exhaust air
from the reactors is vented into a two-stage odor control
scrubber. The operations and maintenance building is
ventilated by an exhaust blower that pulls the air through
a single-stage scrubber.
The mix is conveyed to the top of one of the reactors (see
Figure 7.3). At the bottom of each reactor, a moving out-
feed screw conveys material from the second reactor to a
series of conveyors, which deposit it in a temporary pile
outside the reactor building. A front-end loader moves the
material from this pile to either the recycle storage bins or
a concrete pad, which serves as the aerated curing/
storage area.
7.3.2 Feed and Mix Characteristics
7.3.2.1 Sludge
The composting facility serves four wastewater treatment
plants (WWTP) with a combined treatment capacity of 1.4
m3/s (31 mgd). The treatment train for all of the WWTPs
consists of the following stages: primary clarifiers (with
about 50 mg/L of ferric chloride added at three of the
facilities), rotating biological contactors, secondary clari-
fiers, gravity thickeners for combined primary and sec-
ondary sludges, and belt filter presses. While the sludge
is not stabilized at any of these facilities, it must meet
State regulations for land application before it can be
composted, and metals and organics levels in the sludge
must be tested before it can be sent to the composting
plant.
By design, the minimum dry solids content of the sludge
cake is 20 percent. The actual dry solids content is 20 to
30 percent (23 percent average). The actual bulk weight
is 740 kg/m3 (46 Ib/cf) average; 800 to 960 kg/m3 (50 to 60
Ib/cf), by design. Volatile solids in the sludge cake range
from 70 to 80 percent, depending on the plant, the sea-
son, and the amount of ferric chloride added to the
sludge. The greater the amount of ferric chloride, the
lower the volatile solids content in the incoming sludge.
7.3.2.2 Amendment
The facility uses hogged wood waste, which is a very
coarse sawdust or very fine woodchip, as amendment.
According to the specifications, the shavings must be
kiln-dried, the dry solids content must be 50 to 70 per-
cent, and the bulk weight must be 260 to 350 kg/m3 (16 to
22 Ib/cf). The actual dry solids content is 65 to 70 percent,
and the actual bulk weight is approximately 264 kg/m3
(16.5 Ib/cf).
At least 80 percent of the shavings must be larger than 5
mm (5 mesh) and 98 percent must be smaller than 12.5
mm (150 mesh). The operators developed the size speci-
fication so that the particles would be large enough to
maximize porosity in the mix, but small enough to elimi-
nate the need to screen compost. In the past, wood
"flour" in the amendment created a muddy consistency
in the mix, which tended to jam the conveyors. Stringy
materials such as bark also jammed the conveyors.
76
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Figure 7.1 Cape May, New Jersey, Sludge Composting Process Train
2'
3
77
-------
Figure 7.2 Cape May, New Jersey, Materials Handling System
dl t-
D
t
Q
I
IT
LJo
1"
n
o
.
I
I—I
•x.
UJ
UJ
3
\4
n
u
78
-------
Changing to a larger particle size in the amendment
significantly increased the porosity of the mix and
decreased the supply blower back pressures measured
Figure 7.3 Drain Chain/Conveyor to Top of Reactor
across the mass in the reactor (see Section 7.3.4.3). The
operator of the facility has had some difficulty obtaining
amendment that meets the size specifications, however,
because there is no local lumber industry.
7.3.2.3 Recycle
The dry solids content of recycle from the reactors is 53 to
65 percent. The bulk weight is 609 kg/m3 (38 Ib/cf)
(design); 480 to 640 kg/m3 (30 to 40 Ib/cf) (actual,
depending on the moisture content). Too many fine parti-
cles in the recycle create a sticky mix.
7.3.2.4 Mix
The design criteria for the mix is 40 percent dry solids;
705 kg/m3 (44 Ib/cf). At the time of the site visit, the
operating goal for the mix was 37 percent solids, or 401
kg/m3 (36 Ib/cf). The operator wanted to increase the
moisture content of the mix to maintain the moisture
levels emerging from the reactor because the process
appears to be moisture-limited during composting out-
side the reactor (see Section 7.3.5).
The overall design mix ratio by volume is 1/0.6/1, sludge
to sawdust to recycle. Between May 18 and 27,1988, the
actual mix ratios were 1/0.72/0.57 by volume, 1/0.22/0.37
by wet weight, and 1/0.74/0.87 by dry weight. Since that
time, a greater amount of recycle has been used. At the
time of the site visit, the mix ratio was closer to the design
ratio than it was during spring 1988. Table 7.1 summa-
rizes mix properties present on June 22,1988.
7.3.3 Materials Handling
All phases of the materials-handling process are con-
trolled by computer from a single location. Conveyors are
interlocked through the computer, and switches/
indicators are located on all discharge chutes.
The sludge is delivered in 15 m3 (20-cy) roll-off containers,
which are then stored in an enclosed loading dock
designed to hold a maximum of six containers. Sludge is
usually stored for less than 1 day. The containers are slid
off the truck into the sludge bays, accessed through
metal "garage" doors.
Table 7.1 Properties and Ratios of Sludge and Amendments
(Based on mix sample, June 22,1988)
% Solids Density (Ib/cf) Volume
Wet Ib/min (input for batch materials) Ratio
Sludge
Amendment/Recycle
Mixture
Water"
Mix
218a
760
40
418
24
63
37
52»
24
36
1C
1.6
0.15
•Ib/min x 7.5610-3 = kg/s
"Ib/cfx 16.02 = kg/m3
'Underlined values are those set by the operator.
"Water is added at the mixers.
A roll-off truck also dumps the sludge from the 15 m3 (20-
cy) roll-off containers into a 51 m3 (1,800-cf) live-bottom bin
containing six bottom screws and one collector screw. The
screws then direct the sludge to a flat belt conveyor.
Amendment and recycle are stored in covered concrete
bins that are open on one side so that the material can be
moved with a front-end loader.
The recycle and amendment are pre-mixed on the ground
with a front-end loader. The mixture is measured volumet-
rically by counting the "buckets" of recycle and amend-
ment in the mixture. The solids content is measured before
the material is fed into the system (see below).
The amendment/recycle mixture is dumped into and
stored in a live-bottom bin located in a bay adjacent to the
sludge bins and is accessed through a garage door. A
vibrating screen was installed over the opening of the bin
to remove large (0.03 to .04 m [1 to 1.5 in.]) chunks and
trash and protect the equipment downstream of the bins.
The bin contains four bottom screws and one cross collec-
tor screw.
The outfeed from the amendment/recycle mixture and
sludge storage bins passes a weigh belt and the data are
fed into the computer, which controls the feed rates. The
amendment/recycle mixture feed rate is usually held con-
stant; the sludge feed rate is varied.
The outfeed from each bin falls onto a separate flat belt
conveyor housed in the basement of the operations and
maintenance building. The belt conveyors feed drag chain
conveyors that lead to the mixers on the ground floor of the
building.
By design, the feed materials drop into the two mixers
through sliding plate doors located in the housing of the
conveyor. There is a sepal-ate door for each mixer. The
79
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original intent was to use the mixers on an alternating
basis. Because some of the mix sticks to the drag chain
flights and does not completely discharge in one pass
across an open door, however, both conveyor doors must
be opened, requiring that both mixers operate at the
same time. If both mixers are not run, feed materials that
stick to the flights could accumulate in the drag chain
housing and jam the conveyor. With the modified
arrangement, about 90 percent of the feed materials are
processed by one mixer and 10 percent by the other. The
solids content of the mix is checked manually every 30
minutes during loading.
The two .01442 m3/s (1,833 cf/hr) pugmill mixers contain
interlocking paddles. Water is almost always added
directly into the mixers to create the proper solids con-
tent. However, the mix sometimes becomes sticky when
water is added, which can plug the chutes. To keep the
mixer discharge chute clear, 58 of the original 116 mixer
paddles were removed and the 58 remaining paddles
were replaced with more effective custom paddles.
7.3.4 Reactors
7.3.4.1 Reactor Feed System
The mixers feed a drag chain conveyor that leads to the
top of the reactors. The specifications called for a
"positive-displacement" drag chain, which requires
close tolerances between the flights and the housing of
the chain. These close tolerances were created by build-
ing the system with a narrow flight housing for the feed
side of the chain and a wide housing for the return side.
(Customarily, the wide housing covers the feed side of the
chain and the narrow housing covers the return side. The
close tolerances and the absence of "wear bars" for the
flights to rest on, however, have caused abrasion of the
flights and the housing.
Another problem with the drag chain is that the mix can
compact into dense masses capable of bending back the
steel flights, and causing them to jam in the chain hous-
ing. The flights have been made smaller, but they still
require extensive maintenance (1.5 people full-time) and
about $100,000 per year to replace broken chains, flights,
and worn housings.
The drag chain leads to a flat belt conveyor that runs
along the length of the reactors. The flat belt conveyor
drops the mix into a tripper car that contains another flat
belt and movable plows. The car moves along the length
of the top of the reactor, while the plows move across its
width, so that all the material is pushed off the belt.
Although the intent is to provide an even bed, the mix is
not deposited evenly near the reactor walls; the depth of
the bed near the walls is not as deep as in the middle of
the reactor.
7.3.4.2 Configuration
In the original design, the reactors were operated in
series. To increase the capacity of the facility to 18 dt/day
(20 dton/day), the reactors were converted to parallel
operation. By design, the original second reactor is a
smaller version of the first reactor, with a volume of 850
m3 (30,000 cf) as opposed to 1,050 m3 (37,200 cf). They
share the same infeed device, have identical outfeed
conveyance systems, and have a composting mass
height of 8.0 m (26 ft) (see Figure 7.4).
The walls of each reactor slope slightly outward from top
to bottom (see Figure 7.1). The design intent was to
prevent bridging of the compost. However, as the com-
post moves down through the reactor, it shrinks and
separates from the walls, creating potential paths for the
short-circuiting of the aeration system (see Sec-
tion 7.3.4.3). Galvanized angles 0.2 by 0.2 by 0.01 m (8 by
8 by 0.5 in.) were bolted to.the walls of the reactors to
redirect the flow of air through the compost.
7.3.4.3 Reactor Aeration System
The aeration system has undergone two major modifica-
tions: the reactor exhaust was separated from the mixing
building ventilation system and directed to a new two-
stage scrubber; and additional supply and exhaust blow-
ers, a headspace ventilation system, and the
curing/storage pile aeration system were installed (see
Section 7.3.6). Figure 7.5 shows the original and the
modified systems.
The modified aeration system uses 11 blowers: 5 to sup-
ply air to the bottom of the reactors, 5 to exhaust the air
from beneath the surface at the top of the reactors, and 1
to exhaust air from the reactor headspace. Blower capac-
ities are summarized in Table 7.2. The plant operates with
a total back pressure of less than 6,897 Pa (1 psi) (3,448
Pa [0.5 psi] through the compost).
Each reactor contains three header pipes situated 2.8 m
(9 ft) apart on the bottom of the reactor. Each header pipe
has twenty-two 0.5-m (2-in.) PVC diffusers centered in a
bed of gravel 0.838 m (2.75 ft) thick. The configuration
creates three vertical aeration zones in each reactor. One
valve per header pipe in the smaller reactor and two
valves per header pipe in the larger reactor regulate the
amount and rate of flow of air into each reactor.
A 0.6-m (2-ft) layer of wood chips lies above the gravel
and beneath the compost and outfeed screw. The thick-
ness of the chips was chosen to prevent creating a hard-
pan in the bottom of the reactor. Masonry walls 0.15 m
(6-in.) thick separate the three aeration sections.
On one occasion during the first year of operation, a
hardpan formed on top of the gravel layer. Aeration tests
showed that air did not penetrate this layer, and picks and
shovels were required to remove it. In addition, portions
of the gravel layer were glued together by dried leachate.
This layer did not allow passage of air either. The gravel
layer was cleaned by filling it with caustic soda and turn-
ing on the aeration system for agitation. The problem has
not recurred. The operators believe that leachate produc-
tion is caused by improper aeration of the composting
mass, and the modifications that have been made to
ensure proper aeration have prevented the problem.
80
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Figure 7.4 Purac Reactor, Cape May
HEADSPACE
VENTILATION
PIPING
(to odor control
system)
FEED CONVEYOR
EXAUST
AIR
HEADER
TO ODOR CONTROL SYSTEM
AIRFLOW DIRECTION
w. -
' t. «? %.. f*.fv
'..• •..• •..-•/.• •.".• •.".• *.*.«'.'.• •.•.••/.• •.•_• :'•:•• fi •;••
DISHARGE
/SCREW
\ DISHARGE
x CONVEYOR
Carbon steel air supply lines were originally installed.
Due to corrosion, newly installed additional supply lines
are stainless steel. The original exhaust piping was made
of .05-m (2-in.) fiberglass, but these pipes broke, appar-
ently because the flow of compost twisted them. Flexible
joints, 0.15 m (6 in.) stainless steel stove pipes, and wire
reinforced rubber hose replaced the broken exhaust
pipes. At the time of the site visit, the facility planned to
replace the exhaust piping once again, this time with 0.3-
m (10-in.) stainless steel or galvanized pipes, and to build
a sturdier support system.
In November 1986, when the reactors were still run in
series, three fires occurred in the reactors. Two smolder-
ing fires occurred in the bottom of the second reactor on
November 6 and 7. Although the actual cause of the fires
is not known, spontaneous combustion apparently took
place in the compost.
The fires caused moderate physical damage, including
spalling of the concrete walls of the reactor. The fires
were extinguished by emptying the reactor and thinly
spreading out the hot material to allow it to cool.
A fire in the first reactor started several weeks later. There
is disagreement over the specific cause of the fire, but
overly long storage of material in the reactors lead to
excessive drying, which was a contributing factor. The
fire flared out the top of the reactor, causing extensive
damage to the flat conveyor belt, the tripper car-conveyor,
the movable plow, and the metal roof. The fire department
was called in to extinguish the fire.
After the fire, a number of modifications were made to the
system and new procedures were instituted, as follows:
• Compost will not remain in the reactor for an excessive
amount of time (i.e., the reactor is no longer used to
store compost).
• The mix is kept as wet as possible, consistent with
proper materials handling and product quality.
81
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Figure 7.5 Aeration and Ventilation Schematic
AS CONSTRUCTED ORIGINALLY
t t
T T
REACTOR
^^^^^t
EXHAUST AIR
t°~
K>
KD-1
r*
*+
^
T T T T
REACTOR
T
PROCESS AIR
MIXING
BUILDING
-»
ir
STACK
O
SINGLE STAGE SCRUBBER
POSITIVE DISPLACEMENT.
VARIABLE SPEED
CENTRIFUGAL.
CONSTANT SPEED
= STANDBY
V1TO HOOIFICATIONS
IONS
J
^
&
f \
fO-
f
>
/
f"
fO
HEADSPACE AIR
t
REACTOR
i '*•
EXHAUST AIR
K ^
K>
K5-1
^
-•
£_^^^
r^^v
T T T *?
REACTOR
T
PROCESS AIR
TWO-STAGE SCRUBBER
STACK
STACK
SINGLE-STAGE SCRUBBER
CURING/STORAGE PILE AERATION
Heat detectors and a sprinkler system were installed in
the headspace area, firefighting hoses and standpipes
were installed throughout the reactor building, and
heat sensors were installed in the outfeed device tun-
nel. As a secondary means of fire protection, a high
temperature 85°C (185°F) alarm was programmed
into the computer.
Equipment in the headspace, including the flat belt
conveyors, was retrofit with fire-resistant material. To
prevent potential fire damage, the tripper car is now
stored over the blower room when not in use, rather
than over the reactors.
The materials-handling system now provides for mate-
rial to be moved, if necessary, under any foreseeable
circumstance and to avoid a situation where material
must be stored in the reactor.
• State DEP approval of the compost distribution pro-
gram eliminated the need to store large quantities of
material on site.
7.3.4.4 Outfeed Device
The outfeed device, identical for both reactors, is a mov-
ing screw contained in a tunnel beneath the compost
mass, which deposits the reactor product onto a belt
conveyor. The screw is hard-surfaced with Triten steel.
The taper and the size of the flights on the screw and the
rotational and advance speeds of the outfeed device
were selected to optimize the outfeed rate and volume.
82
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Initial Construction
Supply to reactor
Exhaust from reactors3
Number
3
3
,cfm
1,500°
2,000
Pressure
6.7 psid
10in.W.C.b
Horsepower
75"
20
Number
3
2
3
2
Units Now in Place
cfm
1,500
6,000
2,000
7,000
Pressure
6.7 psi
4 psi
10in.W.C.
2 psi
Horsepower
75
200
20
150
Exhaust from headspace" None
Exhaust from container building 1 3,500
"Total exhaust from reactors and headspace is limited to 20,000
cfm by the capacity of the odor control scrubbers and other piping.
bW.C. = water column.
2 9,000
(1 is a standby)
1 3,500
ccfm x 4.719 x 10-" = m3/s
dpsi x 6,895 = Pa
•hpx 745.7 = W
An electronic control unit that senses pressure on the
outfeed carriage and the torque of the screw automati-
cally slow the speed of the carriage if preset limits are
reached.
7.3.4.5 Leachate and Condensate
Leachate drains at the bottom of the reactors are con-
nected to drip traps and a 6,250 L (1,650 gal) storage
tank, which is pumped out periodically. Very little, if any,
leachate is generated under current operations. A 1,140 L
(300 gal) receiving tank at the suction end of the new
exhaust blowers controls the accumulation of conden-
sate in nonoperating blowers. The estimated volume of
condensate is about 380 L/day (100 gal/day). Conden-
sate is mixed with the leachate, sanitary waste, and
scrubber blowdown water, all of which is pumped to the
adjacent WWTPThe scrubber blowdown water and run-
off from the curing/storage pad (see Section 7.3.5) are the
major sources of wastewater.
7.3.5 Exterior Curing/Storage
The plant was modified so that curing/storage takes
place outside of the reactors in aerated piles. To accom-
plish this, the material on the outfeed belt conveyor is
now transferred to a chain conveyor that deposits the
material in a temporary covered holding area outdoors. A
front-end loader or dump truck then moves the material to
either the recycle storage area or an uncovered concrete
pad where curing/storage takes place.
The working area of the uncovered pad is 49 by 110 m
(160 by 360 ft). The dimensions of the extended piles are
14 m wide by 2 m high by 6 m long (47 by 6 by 150 ft)
each. The area holds 90 days worth of unaerated product
at summer (high) production plus 30 days of aerated
curing/storage.
Each pile is aerated with an independently controlled
centrifugal, positive-flow blower that has a 0.03 m3/s (600
cfm) at 0.2 m (7 in.) water column (W.C.) capacity (see
Figure 7.6). PVC piping is used for the four main header
pipes in each pile; lateral pipes are polyethylene 0.1 m (4-
in.) in diameter with two rows of perforations. These holes
are 0.006 m (0.25 in.) in diameter on 0.15 m (6-in.) cen-
ters. There are no holes in the first 2 m (6 ft) of the lateral
pipes.
Figure 7.6 Aerated Static Curing/Storage Pile
Thermocouples are buried in newly created aerated piles
to measure temperature once per day. The goal is to
maintain the temperature at between 45° and 50°C (113°
and 131 °F) by adjusting the blower operating schedule.
The piles are aerated continuously for the first 1 to 3 days
and intermittently thereafter (by an electric timer) for the
duration of the 30-day minimum curing/storage time
required by the permit. Aerated piles are not agitated, but
unaerated piles are turned on a weekly basis.
The area is surrounded by a 9 m (30 ft) wide asphalt
apron. Both the apron and the concrete pad are sloped 3
percent for drainage. The drainage is channeled to a
holding pond that is periodically pumped to the adjacent
treatment plant.
One fire occurred at the end of a curing/storage pile and
smoldering fires occasionally occur. When the material
gets too dry, the biological reactions stop. Rain on the pile
can then restart the biological reactions without wetting
the material enough to prevent combustion. A consultant
recommended monitoring the moisture content in the
piles and changing the aeration pipe configuration so
that the perforated pipe extends the full length of the pile
rather than stopping 2.4 to 3 m (8 to 10 ft) from the end.
7.3.6 Nonprocess Air Handling
The headspace of the reactors was originally vented by
louvers only. In the modified system, ventilation of the
83
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process building is part of the odor control system (see
Section 7.3.7). Table 7.2 shows the size of the blowers
used for ventilation.
Fogging in the headspace has been reduced by the new
ventilation system but not completely eliminated during
cool, damp days. A consultant to the CMCMUA sug-
gested supplementing the capacity with an additional 4
m'/s (9,000 cfm) blower capacity, to be used only when
needed. Odor control for this intermittently operated sys-
tem would be provided by a compost filter. Exhausts from
the sludge container building and mixing building are
also collected for treatment.
7.3.7 Odor
The plant began operating with two odor control systems:
one system handled the exhaust from the reactors and
the operations and maintenance building, while the other
processed the exhaust from the sludge container build-
ing. Sodium hypochlorite was the only chemical used in
both scrubbers. Reactor headspace air was vented
directly into the atmosphere.
7.3.7.7 Odor Complaints
From the start of operations, individuals residing west of
the plant complained about the odors. All confirmed com-
plaints have been received when the winds were from the
east and southeast.
As a result of the complaints, the DEP negotiated an
administrative consent order with the CMCMUA. To com-
ply, the plant had to shut down operations, modify its odor
control equipment to eliminate the nuisance conditions
(see Section 7.3.7.3), and then reinitiate operations in
stages. At each level of increasing production, the facility
must prove it can operate without producing nuisance
odors before it is allowed to increase capacity. State and
local evaluation teams judge the plant's performance.
7.3.7.2 Odor Sources and Characterization
The CMCMUA hired Roy F Weston, Inc. to conduct three
odor control studies, which investigated complaints, iden-
tified and characterized odor sources, measured air qual-
ity, conducted air dispersion modeling, and evaluated the
newly installed equipment. The studies took place in
September 1985, and in August and September, 1987.
The studies found that the major odors detected were
from sulfur and nitrogen compounds. Analysis of the odor
complaints identified the following sources:
• Air that escapes through the open doors of the sludge
storage area and mixing building
• Sludge exposed to the atmosphere during transport
• Spilled sludge
• An overloaded single-stage scrubber that treats the
reactor and operations and maintenance building
• Reactor headspace air
• Sodium hypochlorite that escapes from the scrubber
stack
• Material from the reactors, exposed in transport from
the outfeed area to the recycle storage bins, or while
mixing with the amendment
• Compost piles that are incompletely stabilized in the
reactors (identified before the exterior aerated curing/
storage system was installed). (See Section 7.3.5.)
• Hydrogen sulfide that escapes from the mixers
During the site visit, a slightly "musty" odor was detected
when curing/storage piles were opened. These odors
were not detected off site. A "burnt" odor was noticed
whenever recycle was handled, i.e., mixed with sawdust,
loaded into the amendment/recycle mixture feed bin,
loaded onto a truck for transport to the curing/storage
area, etc. It is not known if this odor was detectable off
site. Subsequent to the site visit, operators reported that
the "burnt" odor disappeared after all the "old" compost
was removed from the facility.
At the Cape May facility, unaerated curing/storage results
in anaerobic pile centers and odor production, even after
compost has met the regulatory requirements for time
and temperature following 14 days in the reactors. In
general, when the compost is properly conditioned in the
reactors (i.e., correct detention time, aerobic conditions,
proper temperatures), aerated curing/storage does not
appear to be malodorous.
7.3.7.3 Odor Treatment
In summer, approximately 50 to 75 ppm of ferric chloride
is added to the influent at the WWTPs to aid primary
sedimentation. This has the additional benefit of greatly
reducing the sulfides in the raw sludge. During the non-
summer months, Chi-X is added to the mixer water to
control odors emanating from sludge in the mixing area.
The modifications to the reactor aeration system (see
Section 7.3.4.3) included changes to the odor treatment
system (see Figure 7.5). Three independent scrubber
systems are used to treat exhaust from the sludge con-
tainer building, the operations and maintenance building,
and the reactor and headspace.
Exhaust from the sludge container building is drawn
through a single-stage wet scrubber containing a solution
of sodium hypochlorite and sodium hydroxide. Nonpro-
cess air from the operations and maintenance building,
including the sawdust room, sludge room, mixer, and
basement, is treated in a single-stage scrubber with
sodium hypochlorite and sodium hydroxide. This
84
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scrubber system originally treated the air from the reac-
tor, as well as from the operations and maintenance
building.
With the modified system, all the louvers that originally
vented the reactor headspace air, except for one on the
west end of the building, have been closed; the reactor
and headspace exhausts (see Figure 7.7) are now treated
with a condenser and a two-stage scrubber. The scrub-
bing chemicals are sodium hydroxide and sodium hypo-
chlorite in the first stage, and sulfuric acid in the second
stage.
Figure 7.7 Reactor Building Showing
Headspace Exhaust System
7.3.7.4 Odor Control Monitoring and Performance
Sampling ports have been installed at several locations
in the odor control treatment train so that the operator
can spot check the odor levels by sniffing. In addition,
detailed checklists of system operations are completed
each day and in the event of an odor complaint.
Odor control consultants judged that the removal efficien-
cies for sulfur and nitrogen compounds from the two-
stage reactor scrubber were sufficient to avoid affecting
neighbors. Removal efficiencies for organic compounds
were not as successful, but these compounds are not
considered to be major causes of odor. When the odor
control consultants conducted their sampling, sulfuric
acid was used in the first stage and sodium hypochlorite
with sodium hydroxide was used in the second stage.
During subsequent experimentation by the operator, the
reverse sequence (sodium hypochlorite and sodium
hydroxide in the first stage and sulfuric acid [pH of 4] in
the second stage) was judged to be more effective in
controlling odors. At the time of the site visit, the operator
was investigating techniques to increase the efficiency of
the system, including using soil or compost biofilters as
polishing devices, refrigerating part of the system to con-
dense organics, and/or adding surfactants to increase
the solubility of organics.
Table 7.3 shows the results of a test that measured the
odors emitted from the first reactor, mixing building, and
sludge storage building scrubbers. Dispersion modeling
tests were also conducted to estimate the maximum 1-
hour ground-level concentrations of a number of com-
pounds and their distances from the two-stage odor
scrubber stack. The results indicated that the maximum
impact of the modified system occurs 270 m (870 ft) away
from the stack and that no odors exist at the property
boundary.
During the site visit, odors from the treated airstreams
were not detectable when the plant operated at reduced
capacity. The operators expected to receive approval to
increase the loading rate at that time, but odor complaints
were still being received. According to a monitoring
team's followup reports, the odors were mostly transitory
and not very strong. Since the odor control system has
been modified and odors reduced, some question exists
as to whether the complaint-causing odors originated
from the composting plant or the adjacent WWTP
7.3.8 Support Facilities
The primary support facilities are the laboratory area
located in the mixing building and two trailers, which
were added after construction and serve as the office for
the supervisor and operators, a meeting room, and a
lunchroom. The clerk's office is located in .the mainte-
nance building, which consists of a two-bay garage
space and a storage area for spare parts. Spare parts are
also stored in two old tractor-trailers parked on the edge
of the site. A shower, located in the operations and main-
tenance building, serves the whole facility.
7.4 Monitoring and Performance
7.4.1 Reactor Control Strategy
7.4.7.7 Temperature Monitoring and Control
The facility aims to attain the EPA pathogen-reduction
temperature criterion of 55°C (131 °F) for 3 consecutive
days.
Temperatures are recorded at different depths using dif-
ferent length stainless steel probes that are sealed on the
bottom. Each of the three vertical zones, created by the
aeration system pipes, contains five temperature probes.
A thermistor (electronic temperature measuring device)
sits in a pool of glycerine in each probe.
7.4.7.2 Aeration Quantities and Rates
The quantity of air fed to the reactor is modulated by
blower output and the air feed valve schedule. The
blower speed (for positive displacement blowers) or suc-
tion valves (for centrifugal blowers) is automatically regu-
lated to maintain about a 58 °C (136°F) average
temperature in the area 2 to 4 m (6 to 12 ft) down from the
top surface of the compost. This is the so-called "hot
85
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Table 7.3 Air Quality Measurements and Scrubber Removal Efficiencies
(Data taken from September, 1987, odor control report prepared by Roy F Weston, Inc.)*
Mixing Building
Reactor Scrubber (two-stage) Scrubber (one-stage)
Constituent (ppm/v)
Removed
Removed
Sludge Storage
Building Scrubber (one-stage)
Removed
In
Out
In
Out
In
Out
Total Reduced Sulfur, S
Total Nitrogen, N
Ammonia, NH3
Total Aldehydes
Total Volatile Organics
8.4
11.7
10.2
25.0
> 2.387"
> 3.443-
0.38
0.06
0.04
12.4
>3.212
96
99
>99
50
—
9.6
0.27
0.27
e
> 3.357
> 0.694
>0.11
0.13
<0.04
—
> 2.073
> 0.207
>99b
52
>85
—
—
0.88
0.43
0.42
—
0.377
0.21
0.11
0.05
<0.04
—
0.135
0.054
88
88
>90
—
64
74
•Single tests were conducted for all but the volatile organics for which duplicate tests were conducted. Individual organic compounds reported
Include acetone, chloroform, 1,2-dichloroethane, toluene, xylene, 1,2-dichlorobenzene, trichlorofluoromethane, dimethyl disulfide, C9/C10
hydrocarbons, aldehydes, methylene chloride, acetone, carbon disulfide, 2-butanone, 1,1,1-trichloroethane, styrene, C4/C5 hydrocarbons.
•Test results reported as " > " indicate tests where the mass spectrometer detector response capabilities were exceeded. ,
'"—" Indicates that no data were analyzed for these chemicals.
"Measurements on first line taken while mixing an "odorous" sludge.
•Measurements on second line taken while mixing a "normal" sludge.
zone" in the Cape May reactors. The average tempera-
ture of each aeration section controls the valve schedule,
with the valves opened for a minimum of 5 minutes out of
each 15-minute cycle. Aeration to each zone is set in
proportion to the amount the average temperature in the
zone is cooler than the hottest zone. The valves in the
hottest zone are open the full 15 minutes of each cycle.
The total air fed to each reactor is measured and
recorded daily. In addition, the computer displays all tem-
peratures in the reactor every 30 minutes. Although the
frequency of the temperature readings controls tempera-
tures to a relatively close tolerance, the bed does not
always appear to meet the EPA time-temperature crite-
rion. But, in general, the reactor produces a compost that
meets the criterion and has an acceptably low moisture
content (see Section 7.4.3).
7.4.7.3 Other Tests and Equipment
Carbon dioxide (CO2) is continuously measured in the
exhaust gas and recorded once per day. Based on low
CO2 values, the air needed for cooling is significantly
greater than the amount of air needed for biological
reactions.
A solids test was developed on site using a kitchen micro-
wave oven calibrated against the Standard Method.
Operators prefer using the kitchen microwave oven over
a microwave unit engineered for WWTP operations
because the kitchen oven is less expensive and can
handle larger samples.
Testing for chlorine residuals and pH in the scrubbers
and screening the particle-size of wood shavings are also
done on site. Metals are tested in the aerated piles by
taking three grab samples per pile. A monthly composite
is then made from the samples. The composite is sent to
an independent laboratory for analysis.
7.4.14 Data Management
The computer monitors the temperature probes and is
capable of printing out either instantaneous or 24-hr
average temperature reports. Outfeed batch quantities
are also printed out. Characteristics of each batch of mix
are printed out by the computer after the batch is fed into
the reactors. The operators keep this information in a
computerized data base.
The data are used to determine if a particular batch will
be used as recycle or sent to curing/storage. A batch will
be used as recycle if its temperature exceeds 75°C
(167°F) (the in-house criterion for a "bad" batch), or if it
does not reach 55°C (131 °F) for 3 days (the EPA
pathogen-reduction criterion).
The amount of sludge processed and the data on metals
content are reported to the State on a regular basis as a
condition of the permit. Temperature profiles are main-
tained as proof of significant pathogen reduction.
7.4.2 Mass Balance and Reactor Detention Time
A mass balance for the plant during May 16 to 27,1988, is
illustrated in Figure 7.8.
As specified in the operating permit, the minimum plug-
flow detention time in the reactors is 14 days. The actual
plug-flow detention time, based on the bulk weight of the
mix and the total feed volume, is estimated to be 17 days
when using both reactors at 18 dt/day (20 dton/day). The
operator believes that 9 to 10 days would probably pro-
duce a material acceptable for aerated curing/storage.
Based on the loading rate at the time of the site visit,
when only the larger reactor was used, the plug-flow
detention time was 17 days.
At the time of the site visit, the detention time in the
reactor was not sufficient to stabilize the compost so that
it could be cured/stored without aeration.
86
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Figure 7.8 Cape May, New Jersey, Mass Balance
Average Daily Quantities, May 16 to 27,1988
• SLUDGE: 23.5 WTON". 5.8 DTON, 25X TS*. 37.6 Cf
SAWDUST: 5.1 UTON. 4.3 DTON. 84X TS, 27.2 CY
H20: 1.7 T
MIXER
MIX
38.9 VTON
15.2 DTON
39% TS
RECYCLE
8.6 VTON
5.1 DTON
59X TS
21.6 CY
REACTOR
\>
TO ATMOSPHERE
17.9 T WATER
5.1 T TS
15.9 VTON
10.1 DTON
63% TS
_^ TO EXTERIOR CURING/STORAGE
7.3 WTOH
5.0 DTON
* VTON (DTON) X 0.9072
* TS - TOTAL SOLIDS
' CY X .765 - H3
WT(DT)
7.4.3 Product Quality
Project specifications require that the finished compost
have the following characteristics:
• Brown color
• Medium coarse to fine texture
• Friable
• Maximum of 10-mm particle size
• Minimum solids content of 40 percent
• Minimum volatile solids reduction of 40 percent
* Maximum of two odor units (ASTM D-1391-1978)
The product produced at the time of the site visit met
color, texture, solids content, and volatile solids reduction
specifications. It did not, however, meet the maximum
particle-size criterion because of the kind of sawdust
used. (The ASTM odor specification applies to gases and
has no provision for odors from solid materials; therefore,
it cannot be applied to the compost.) At full capacity, the
plant should produce about 5,700 mVyear (7,500 cy/year)
(2,700 to 3,600 dt/year [3,000 to 4,000 dton/year]) of
product.
Salmonella testing, conducted during plant startup,
showed that pathogens were significantly reduced as
long as the average "hot zone" temperature was greater
than 55°C (131 °F). After 2.5 years of testing, the DEP
now accepts average "hot zone" temperatures as dem-
onstration that the process meets EPA's criteria for PFRP
(Process to Further Reduce Pathogens).
Testing for metals is done by an outside laboratory.
Table 7.4 lists the metals content of a sample pile, along
with the New Jersey criteria for metals in sludge.
New Jersey State restrictions on the use of the compost
are as follows:
• No sludge-derived product can be used on food crops.
• Distribution to homeowners or the general public is
prohibited without DEP approval (see Section 7.5.2).
• Compost cannot be applied during the winter (mid
November through March 1).
87
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Thirty dt/year/acre or less can be applied to nonfood
crops if the nitrogen requirement of the crop and the
metals-loading criteria are less than the levels listed in
"State Guidelines for Land Application of Residuals" or
Federal Regulations 30 CFR, Part 257, Sec 257.3-5. Any
application greater than 27 dt/year/acre (30 dton/year/
acre) or 45 dt/acre (50 dton/acre) over the life of the site
requires a DEP permit. In addition, any project utilizing
more than 100 cy of compost requires a permit.
Table 7.4 Heavy Metals Content After Curing
(ppm, dry weight basis)
NJDEP"
Sludge Quality
Criteria
for Land
Average Application
Metal
Minimum Maximum
Arsenic, As
Cadmium, Cd
Chromium, Cr
Copper, Cu
Lead, Pb
Mercury, Hg
Nickel, Nl
Zinc, Zn
0.5
0.6
11.0
155.7
22.9
0.2
6.2
190.9
9.4
7.6
257.0
870.0
160.0
3.5
45.0
1,030.0
2.5
3.0
89.7
416.0
97.9
1.7
16.8
743.1
10
. 40
1,000
1,200
4,800
10
1,250
2,400
•New Jersey Department of Environmental Protection.
7.5 Operations
7.5.1 Staffing
Fourteen people are employed year-round by the Cape
May facility:
• 1 superintendent
• 1 foreman
• 2 operators
• 6 utility people
1 mechanic
1 assistant mechanic
1 electrician
1 clerk
In addition, there are five summer employees.
At the time of the site visit, the supervisor had just been
promoted from the foreman position. (Promotions are
generally made from within the organization.)
He had worked at the plant for 1.5 years, but had no prior
experience in composting or wastewater treatment oper-
ations. The two operators were studying for the entry
level WWTP operator State certification; there is no com-
post handling certification in New Jersey.
The system supplier provided "hands-on" training during
startup. In addition, the previous supervisor initiated
monthly training meetings that covered both composting
theory and operations. These monthly meetings were still
being held at the time of the site visit. The plant electri-
cian received special training on the use and mainte-
nance of the computer system. In addition, the staff are
trained to operate all phases of the system, although
each operator is generally assigned to operate the same
equipment on a regular basis.
The plant is operated 5 days per week according to the
following schedule:
Maintenance shift
Operations I
Operations II
— 5:00 a.m. to 1:30 p.m.
— 11:00 a.m. to7:30p.m.
— 7:00 p.m. to 3:00 a.m.
Supervisory personnel — 7:00 a.m. to 3:30 p.m.
The maintenance shift operates during the day so that
the staff can procure supplies and parts. Mixing and
loading operations are done at night to minimize odor
complaints and avoid interruptions due to daytime activ-
ities such as maintenance or construction. The shifts
overlap to maintain continuity of operations. The facility
has instituted the two shift operation shown above in
anticipation of time required to load and process the 20
dt/day (18 dton) that will eventually be fed into the
system.
7.5.2 Marketing and Distribution
No marketing study has been conducted except to ascer-
tain that the product is marketable. The costs associated
with composting (see Section 7.3.3) have influenced the
CMCMUA's approach to marketing, however. The facility
intends to keep a price for the compost so that it is not
considered to be "waste" product; nevertheless, the
CMCMUA claims to have no illusions about making
money with the product. The compost sells for $2.6/m3
($2/cy),$5.5/dt($5/dton).
In accordance with State regulations, the compost can
be sold in bulk only to "qualified" customers. "Qualified"
is an undefined term that implies that the State wants to
ensure that the customer will follow instructions about
allowable uses. All members of the New Jersey Nursery-
man's Association are "qualified," for example.
The plant received its license to distribute compost in
April 1988. Before that time, only a State-approved com-
post blender and distributer could dispose of the com-
post. Under such stipulations, the plant had to inventory 1
year's production of compost. Since April 1988, however,
all compost produced has been sold and the facility is not
concerned about flooding the market.
At the time of the site visit, most compost had been sold
as soil amendment to cover and promote revegetation of
a 65,000 m2 (16-acre) local landfill that was being closed.
This was a short-term, high-volume demand. Other major
users and uses of the compost include:
• Sand and gravel companies — topsoil component for
highway revegetation projects
• Municipalities — parks
• Earthlife (State-approved company) — compost
blending and distribution
88
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Table 7.5 Operations and Maintenance Costs
Estimate at Startup
(1,372 dry tons of sludge processed)"
Budget Estimate
(4,840 dry tons of sludge processed)
Category
Labor (number of employees)
Power
Sawdust, Chemicals, and Supplies
Other3
Repetitive Repair and
Replacement
(sinking fund reserve)
Total
(1985$)
105,000(4)
135,000
100,000
40,000
63,000
443,000
($323/ton)
0/0
23.7
30.5
22.6
9.0
14.2
100.0
(FY1989$)b
114,200(4)
146,800
108,700
43,500
68,500
481,700
($351 /ton)
o/o
23.7
30.5
22.6
9.0
14.2
100.0
FY 1989
418,074(15)
175,000
215,000=
230,325
136,000
1,174,000
($243/ton)
%
35.6
14.9
18.3
19.6
11.5
100.0
•The number of tons processed is artifically low and the price per ton is high because the figures reflect a combination of startup
and routine operation.
"1985 costs escalated by the ratio of Engineering News Record (4568/4201) construction cost indices for December, 1988, and June, 1985.
cChi-X masking agent costs about $8.60 per liter ($1,800 per 55 gallon). Amendment is $18.64/m3 ($14.25/cy).
<1dtonsx.9072 = dt
'This category is a "catch all," and items included in the startup and budget estimates are not necessarily the same.
The operators would prefer to distribute all of the material
locally, if possible, because local residents are subsidiz-
ing the manufacture of the compost.
7.5.3 Operating Costs
Table 7.5 shows the expected operations and mainte-
nance costs for 1985 which were estimated at startup
and updated to 1989, as well as the 1989 budget
estimate.
Through September 1988, the operations and mainte-
nance costs to process 1,090 dt (1,200 dton) of sludge
were $474,150 ($435/dt) ($474,009 I$395/dton]). The ton-
nage processed in this time period was limited by the
DEP order.
7.6 Update
As of June 1989, the plant had been running at full
capacity, 18 dt (20 dton) per day, since the end of the
summer, 1988. Because there has been only one odor
complaint since March 1988, the State has given the
plant permission to operate at full capacity all year. Actual
amounts of materials processed and operating costs are
shown in Tables 7.6 and 7.7.
The plant is temporarily running at half capacity now, due
to a broken outfeed device (screw) in the center of one
reactor. A new screw is being fabricated locally, and will
be used to dig out the broken screw. The plant should be
back at full capacity by July 1. It is not known why the
screw broke; no problems were seen at the last annual
inspection.
Cape May has spent a great deal of time in the past year
fine-tuning the odor treatment process. Sulfuric acid is
used in the first stage of the two-stage, packed tower wet
scrubbers. Hydrogen peroxide will be used instead of the
current hypochlorite in the second stage. Cape May will
be testing corn syrup as a surfactant in the first stage to
make aldehyde compounds more water soluble.
Table 7.6 Materials Processed
(April 1 through December 31,1988)
Material
Amount Percent Solids Mix
Processed- Average Range Ratio
Sludge
Amendment
Recycle
Compost Product
1,982"
1,594
2,020
2,394°
27.3
81.9
58.9
52.8
24-32 1
76-90 0.84
54-66 0.81
47-63
•Total dton (dton x 0.9078 = dt)
"Processing rate was limited during the summer due to an agreement
with State concerning odors.
C100 percent of the product was sold.
Table 7.7 Operating Costs
(April 1 through December 31,1988)
Item
•Includes 35 percent fringes.
"Cost of city water and chemicals.
"Breakdown of these costs not given.
Amount
% of Total
Labor (operations)
Labor (maintenance)
Power and Fuel
Amendment
Maintenance Materials
Odor Control*
Contracted Repairs
Other
TOTAL
Revenues
Product Disposal Costs
$194,127«
129,419-
130,750
123,686
93,646
36,218
29,195
39,866
$776,907
19,038
0
25
17
17
16
12
5
3
5
89
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Chapter 8
Clayton County, Georgia
8.1 Introduction
The Clayton County in-vessel composting facility is
owned by the Clayton County Water Authority (the
Authority). It is located on the same site as the Northeast
Clayton County Water Pollution Control Plant in Rex,
Georgia, about 48 km (30 miles) southeast of Atlanta.
The composting facility, designed by Taulman Compost
Systems (Taulman), has two reactors in series. It is a
second generation Taulman system and has a design
capacity of 2.6 dt/day (2.9 dton/day) sludge on a 7-day
basis.
The composting facility is situated in a rural area
between a creek and a county road. The nearest homes
are on the other side of the road, 183 m (200 yd) uphill
from the plant. There are also homes on the far side of the
creek, about 366 m (400 yd) from the plant.
The site visit took place on July 26 and 27,1988. Except
as indicated, all the information presented in this case
study is representative of that time only.
8.2 History of the Plant
8.2.1 Procurement and Construction
In 1982, the Authority, which is a quasi-governmental
special services agency, considered a number of sludge
management alternatives: landfilling, incineration, pallet-
izing, and composting. The Authority chose in-vessel
composting primarily to maximize odor control (the
Authority's existing pelletizing plant experienced many
odor problems at the time). The Authority also wanted to
produce a consistently high-quality product and felt that
keeping the compost system out of the weather (as com-
pared to outside static pile composting) would be a key
factor in achieving this goal.
The Authority solicited proposals from two equipment
suppliers who they felt had the most experience with in-
vessel composting. In choosing between the proposals,
the Authority gave the greatest weight to the suppliers'
experience and willingness to buy the product; cost was
a secondary consideration. The Authority chose the
Taulman in-vessel composting system. Although the
competitor protested (Taulman had projected a 20-year
cost about a million dollars higher than the competitor),
the Authority ultimately chose Taulman because of its
superior experience and willingness to purchase the
compost product.
The engineering firm of Robert and Company designed
the plant using the Taulman technology. The general
manager of the Authority described the process as a
"joint design" in which the supplier and the engineer
specified the equipment. These equipment specifica-
tions (and their associated costs) were included in the bid
documents, which made it easier for the Authority to
compare construction bids.
The general manager was satisfied with the procurement
process and results. He found that having an engineering
firm was an asset because of its expertise and objectivity,
especially in the prequalification stage. In addition, he felt
that the criteria used by the Authority in choosing an in-
vessel system-supplier experience and willingness to
buy the product — worked well.
8.2.2 Capital Costs
In November 1984, the design engineering firm esti-
mated that the capital costs for the facility would be
$2,756,300. The actual capital costs were $2,434,243
(after subtracting a $90,000 change order in the Author-
ity's favor). Although this value is very close to the 1984
estimate, the scope of the final project was smaller than
that envisioned in 1984. Since the Authority had a fixed
budget (a reserve bond issue), it trimmed a number of
items from the construction contract (including a covered
product storage building, spare parts storage, an
employee restroom, and an automatic sprinkler system)
in order to stay within the budget. As money becomes
available, some of these items are being constructed.
The product and equipment storage building, one of the
items deleted from the original scope, has since been
built at an additional cost of $80,000. A compost storage
building (30 m x 12 m [100 ft x 40 ft]) has also been
constructed.
8.2.3 Operating History and Current Status
The construction of the plant went well and the plant
began operating in August 1986. Both reactors were full
by December 1986, and, since then, the plant has been
operating continuously. In February 1987, the facility
began to use output from the second reactor for recycle.
As of July 1988, the plant was processing all of the
91
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Figure 8.1 Clayton County, Georgia, Composting Process Train
SLUDGE
EXTERNAL CURING/STORAGE
treatment plantb sludge; 1.2 dt/day (1.3 dton/day) on a
7-day basis (averaged from January to June 1988).
8.3 Description of the Plant
8.3.1 Systems Overview
The plant has two reactors in series; both are cylindrical
vertical reactors. The mix ratio is visually controlled and
the speed of the sawdust conveyor is varied to achieve
the proper mix. The mix is transported by conveyors to
the mixer, then dropped into the first reactor. The com-
post is removed from the first reactor by a screw con-
veyor, and transported to the second reactor. After the
compost is removed from the second reactor, some of it is
used as recycle, the remainder is cured/stored in a
curing/storage building. The process is shown in Fig-
ure 8.1. Because the plant is small with essentially a
single process train, there is no redundancy on most
major process components. Figure 8.2 provides an over-
view of the existing facilities.
While in the reactors, the compost is aerated by diffusers
located in the reactor bottom. The air is exhausted from
the top of the reactors and directed to the WWTr? where it
is bubbled through the wastewater emergency bypass
holding pond.
8.3.2 Feed and Mix Characteristics
8.3.2.7 Sludge
The facility composts waste-activated sludge (WAS) from
a 0.18 mYs (4 mgd) wastewater treatment plant (WWTP)
where ammonia, nitrates, and phosphorus are removed
from the sludge. Dissolved air flotation (DAF) units
receive the wastewater, which contains 0.75 to 1 percent
solids, and concentrate it to about 4 percent solids.
When the sludge enters the composting facility, it is
dewatered on a belt filter press. (Although the plant was
designed to have two belt presses, only one belt press
was installed during construction. There is space in the
building for another press to be added in the future.) The
dewatering operation is run 4 to 5 hours a day, 5 days a
week. An average 8.4 dt/week (9.2 dton/week) were
dewatered from January to June 1988.
Although the dewatering system is designed to increase
the solids content to a minimum of 18 percent (average
20 percent) solids, it has not been able to meet the 18
percent minimum consistently. In 1987, the solids content
ranged from 16.0 to 18.5 percent and averaged 17.7 per-
cent. In the first 6 months of 1988, the solids content
averaged 16.3 percent, ranging from 15.3 to 16.7 percent.
At 17 percent solids, the volatile solids of the dewatered
sludge range from 68 to 79 percent and the bulk density
ranges from 880 to 960 kg/m3 (55 to 60 Ib/cf).
A screw conveyor collects the sludge from the belt press
and conveys it to the sludge hopper. The sludge hopper is
open-topped and has a capacity of 26 m3 (920 cf). The
sludge is removed from the hopper by a live-bottom,
integral drag chain conveyor.
8.3.2.2 Amendment
Sawdust made on site in a hammermill from wood chips
(60 percent pine and 40 percent hardwood) is used as an
92
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amendment. The pine chips are produced from timber
grown on Authority land irrigated with treatment plant
effluent, while the hardwood chips are purchased from a
local supplier. There are only verbal quality control speci-
fications, and sometimes the supplier delivers hardwood
chips contaminated by metals and other trash. The bulk
density of the wood chip mix is 260 kg/m3 (16 Ib/cf); it
contains 56 percent solids.
The operators found that green chips were hard to grind
and move pneumatically. Now, the operators dry the
chips for 1 to 1.5 weeks on site before trucking them to a
live-bottom hopper. A dual screw conveyor takes the
chips to the hammermill, where they are made into saw-
dust. According to the design specifications, 90 percent
of the sawdust must be smaller than 0.006 m (0.25 in.).
The solids content of the sawdust averages 60 percent,
and ranges from 52 to 66 percent. The sawdust is pneu-
matically transported to a silo with an effective volume of
200 m3 (7,000 cf). An outfeed screw conveyor with an
adjustable feed rate removes the sawdust from the silo.
8.3.2.3 Recycle
The recycle comes from the second reactor. It has an
average solids content of 50 percent and an average bulk
weight of 590 kg/m3 (37 Ib/cf).
If the facility is inoperable, the DAF units can store 1-day5s
production of sludge. Additional storage is available in the
WWTPfe aeration tanks. Under Georgia Environmental
Protection Department regulations, in emergency situa-
tions the Authority can land-apply dewatered sludge at
one of its land treatment facilities.
8.3.2.4 Mix Ratio
Because the sludge is wetter than anticipated, 40 percent
more amendment and recycle are used than planned for
in the design. The design mix ratio is 1/1/1 sludge to
sawdust to recycle (by volume). In 1987, the actual mix
ratio was 1/1.4/1 sludge to sawdust to recycle (by vol-
ume); 1/3/1.7
-------
Figure 8.3 Schematic of Materials Handling System
I 1
FUTURE PRESS
BELT PRESS
SAWDUST STORAGE SILO
DRAG CHAIN CONVEYOR
SCREW CONVEYOR
PNEUMATIC CONVEYOR
Operators are based in the ground-level control room
where they cannot see the accumulator or the mixer, and
have to climb exterior stairs periodically to check the mix.
The mixer and accumulator are also difficult to service
since there is no elevator and the operators have to climb
exterior stairs to reach this equipment.
8.3.4 Reactors
8,3.4.1 Reactor Feed System
The materials from the mixer are conveyed by a drag
chain conveyor to the first reactor. There, the feed materi-
als are dropped onto a rotating conical distributor (a
"slinger") located in the apex of the reactor. This slinger
distributes the material uniformly only when it falls into
the center of the cone. Uneven distribution has been a
problem and the operator is considering building a funnel
to direct the feed to the center of the slinger.
8.3.4.2 Configuration
Both reactors are vertical cylinders made from insulated
steel and have steel bases; the inside walls are epoxy-
coated. The reactors are 8.8 m (29 ft) tall, and have 7.6 m
(25 ft) diameters and 400 m3 (14,125 cf) volumes (see Fig-.
ure 8.4). There is no fire protection system.
The composting mass is 8.2 m (27 ft) deep; the depth is
determined by a mechanical level indicator. Because the
surface of the compost is uneven, the depth reading is
sometimes incorrect, so the operators periodically check
the depth visually when charging the reactor in order to
determine when it is full.
8.3.4.3 Reactor Aeration System
The reactors are aerated by three positive displacement
blowers located in the process building. There is one
supply blower for each reactor plus a backup blower that
can feed either reactor. Each blower is rated at 0.09 to 0.3
94
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Figure 8.4 Taulman Composting System Reactor
TO ODOI
CONTROL
DISCHARGE
SCREWS
INFEED
CONVEYOR
DISTRIBUTOR
AIRFLOW
DIRECTION
COMPOST TO
DISCHARGE CONVEYOR
AERATION PIPING
m3/s (200 to 700 cfm). Normally, the blower for the first
reactor runs between 0.2 to 0.28 m3/s (500 to 600 cfm)
and the blower for the second reactor runs at 0.18 to 0.2
m3/s (400 to 500 cfm) with a 10,340 Pa (1.5 psi) back
pressure.
The aeration system is divided into quadrants inside the
reactor, with no walls between the quadrants to prevent
the air from short-circuiting. The blowers pull in air from
the outside and heat it (by compression). Then the blow-
ers direct the air through carbon steel pipes to a header
that distributes it into the quadrant-specific pipes that
feed the perforated plastic pipes in the bottom of the
reactor. The 50-mm (2 in.) diameter plastic piping is per-
forated and corrugated. The pipe is laid on top of 0.5 m (2
in.) of 0.02 m (0.75-in.) gravel in a 0.2-m (8-in.) deep
plenum, with 0.15 m (6 in.) of gravel covering it. Figure 8.5
shows a section of reactor aeration piping with tempera-
ture probes.
Originally, the aeration piping was mounted with no sup-
port between the span of the aeration headers, except
the gravel underneath, and covered with a 0.2 m (6-in.)
layer of 0.01 m (0.5 in.) gravel. This allowed the piping to
settle in the gravel (upon installation), rather than keep
the pipes level. During the annual inspection of the first
reactor, operators found leachate in the low areas where
the piping had settled, close to the floor of the reactor.
Also, during the annual inspection of the first reactor
interior, operators found a compressed, "hardpan-like"
material on top of the gravel layer. No back pressure
problems had been noticed, so they assumed that air had
95
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Figure 8.5 Section of Reactor Aeration Piping with
Temperature Probe
Aeration Pipe
Temperature Probe
been penetrating this layer. The lowest portion of the
gravel had been covered with a layer of dried leachate
that started to cement the gravel particles together and
plug the holes in the aeration piping.
As a result of these findings, the aeration piping in the
first reactor was modified. The gravel was replaced with
0.02 m (0.75 in.) washed round river rock, and the aera-
tion piping was remounted on pressure-treated hard-
wood boards located in the middle (rather than bottom) of
the gravel layer. In this way, the aeration piping was sepa-
rated from the collected leachate. The piping in the sec-
ond reactor had not been modified yet^but the Authority
planned to do so during the next inspection.
When either reactor discharges material, its aeration sys-
tem must be deactivated or material and air will be blown
from the discharge area. Originally, the aeration system
was arranged so that anytime a reactor was loaded or
discharged from, the aeration system (not exhaust) was
shut down. As a result, the operators could not aerate
either reactor while loading. In the case of the second
reactor, substantial amounts of leachate were drained
from the bottom of the reactor on a daily basis. The
aeration systems have since been wired differently, so
that both reactors can be aerated while being loaded, but
not while discharging. In response, leachate in the sec-
ond reactor has been virtually eliminated.
8.3.4.4 Outfeed Device
The outfeed device is a radial screw (Weiss type AST)
that is fixed at a center pivot and rotates around the
reactor while turning on its axis. As the screw turns, it
draws compost into the center of the reactor, where the
compost is dropped onto a reversible screw conveyor.
The screw drives are housed in the center pivot, and the
screw is turned on its axis by a hydraulic motor and is
rotated through the compost by a hydraulic piston. By
adjusting the gearing and hydraulic power system, the
screw can be turned at any speed. To date, the hydraulic
system has worked well.
The screw has never been trapped in the composting
mass, although sometimes the operator must use the
screw to break the crust that forms around it. The outfeed
screw is epoxy-coated and is not hard-surfaced. Exces-
sive wear lias not been a problem.
Beneath the outfeed device, a funnel conveys the com-
post to a reversible screw. In the first reactor, this funnel
has become blocked with compost that backs up into the
center pivot. On one occasion, the blockage or a dump
gate that failed to open pushed the screw off the splined
gear on the gearbox and into the wall of the reactor. To
remedy the situation, a hole was cut into the steel outer
wall of the reactor and "porta-power" units were used to
remount the screw. A detector has been installed in the
funnel to detect blockages, which are removed by hand
through an access door.
Because the reversible screw conveyor connecting the
outfeed ends of the two reactors serves two functions
(loading recycle to the reactor and discharging the com-
post), the operator cannot simultaneously load the first
reactor and discharge material from the plant (see Fig-
ure 8.3). The typical daily loading schedule from March to
July, 1988, was 1.7 hours of discharge from the second
reactor, 2.6 hours of transfer from the first reactor to the
second reactor, and 3.5 hours of first reactor fill. Typically,
the system operates 8 hours/day.
8.3.4.5 Leachate/Condensate
Leachate is collected from each quadrant, which the
operators check each morning. If the operators notice
excess leachate in any quadrant over a period of time,
they increase the aeration rate or time to that quadrant.
The quadrants are aerated in sequence, with each quad-
rant normally aerated for 6 minutes. Four and a half
minutes into the cycle, aeration in the next quadrant is
initiated, so that the aeration of adjacent quadrants over-
laps by 1.5 minutes. Since the operators changed the
aeration piping and gravel in the first reactor, excessive
leachate has not been a problem.
8.3.5 Exterior Curing/Storage
A drag chain conveyor takes the compost from the sec-
ond reactor to the exterior of the building, after which it is
loaded into trucks and taken to the storage building (372
nf [4,000 cf]) where compost and wood chips are stored.
This covered, open-sided building can hold a maximum
612 m (800 cy) of compost, or 5 to 7 weeks of compost,
depending on how much space is utilized for wood chip
storage. After storage (no minimum time required), a
front-end loader moves the compost onto trucks for trans-
port off site.
96
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A trench aeration system was built into the concrete floor
of the compost storage building. The operators, however,
found that the system would not pull air through the
compost, probably because of the complex configuration
of the system's 0.15 m (6 in.) tubing. Alternatively (or
additionally), compost fines may be infiltrating the aera-
tion trenches and limiting their aeration capacity. The
aeration system is presently used for drying wood chips.
8.3.6 Nonprocess Air Handling
All nonprocess air is vented directly to the atmosphere by
three roof fans. There are additional exhaust fans (that
also vent directly to the atmosphere) in the exhaust
blower room, the polymer room, the hammermill room,
and the dewatering room. Often the "garage" doors in
the dewatering area are left open to increase the airflow
through the process building.
8.3.7 Odor
The process air is exhausted from the reactors by two
exhaust blowers located just inside the building. The
blowers are ducted so that one exhaust blower can pull
from both reactors. This air is directed to the wastewater
emergency bypass holding pond, where it enters a perfo-
rated plastic pipe (0.02 m [0.75 in.] holes) and is released
into the water 0.71 m (28 in.) deep. After diffusing through
the water, the process air is released at ground level.
The process air was originally diffused into a sludge
storage tank located following the DAF units. This config-
uration was changed due to splashing of the sludge out of
the tank and an adverse effect on sludge dewaterability.
Odors from the plant are not excessive despite the rela-
tively simple odor control system. In the 8 months prior to
the site visit, there were 12 odor complaints from local
residents. The odors are worse at night and early morn-
ing when the air is relatively still. When the plant is oper-
ating normally, the odors do not correlate with plant
activities, but whenever the reactor vessel must be
cleaned out for inspection or repair, the composting
sludge must be stored outside, thereby generating odors
and complaints. The storage site for material removed
from the reactor during maintenance is closer (91 to 137
m [100 to 150 yd]) to the neighbors than the plant itself,
exacerbating the problem.
Neighbors who were brought on site to help locate
sources of odors identified the dewatering room, the pro-
cess air diffuser, and the curing pile as odor sources. The
odors at both the air diffuser and the curing piles were
verified by the survey team, who reported a musty odor
while standing on the edge of the pond near the process
air discharge and both musty and sickly-sweet odors
from the compost piles when they were moved.
Although there is no formal monitoring program, MacMil-
lan Research Limited tested the process air, which was
sampled before and after it was bubbled through the
holding pond. The results are shown in Table 8.1.
Table 8.1 Process Air Constituents
Constituent
Concentration (ppm)
Reactor Air Diffuser Air
Methane
Total Hydrocarbon
Hydrogen Sulfide
Carbon Monoxide
Sulfur Monoxide
< 1 ppm < 1 ppm
9.7
4.5
There are no plans to modify the odor control system.
8.3.8 Support Facilities
The process building, which is a one-story building
wrapped around the bottom of the reactors, houses the
control room, laboratory, and restroom. Solids testing is
done in the laboratory; all other testing is done at the
Authority's offsite centrarlaboratory. The operators'
"office" is a desk in the control room. The operators
desire a separate, quiet office suitable for meetings.
8A Monitoring and Performance
8.4.1 Reactor Control Strategy
At the time of the site visit, State regulations were not yet
in effect. The plant is operated to meet the EPA 3-day
55°C (131 °F) criterion for pathogen reduction. In addi-
tion, the operators aim for 60°C (140°F) at the top of the
first reactor. Strip chart recorders document the tempera-
tures of the compost in the reactors and the exhaust gas
from the reactors. Reactor temperatures are not currently
being monitored by the State Environmental Protection
Department. A single daily value taken at the beginning
of the workday is recorded in the daily log sheet. This
value is the primary basis for deciding whether or not to
change airflow rates.
The temperatures are measured inside the reactors by
probes, which are stainless steel thermocouples con-
tained in 0.02 m (0.75 in.) diameter stainless steel pipes
filled with lightweight oil. Each reactor has three probes
placed in three different locations: 2 m (6 ft) off the bot-
tom; 1.2 to 1.5 m (4 to 5 ft) into the compost from the top,
and at the midpoint of the compost. The temperature
profile in the first reactor shows an unexpected pattern:
the middle temperature probe records the lowest temper-
ature in the reactor (see Figure 8.6). This pattern is
reportedly stable.
The operators control the temperatures by manually
changing the airflow rates. Typically, the airflow rates are
0.24 to 0.28 m3/s (500 to 600 cfm) per reactor. When the
flow rate is 0.28 m3/s (600 cfm), decreasing the airflow
increases the temperature. If the flow is near 0.24 m3/s
(500 cfm), decreasing the airflow depresses the tempera-
ture. When the plant first started up and discharge from
the first reactor was used as recycle, the temperature
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was increased by decreasing the airflow and vice versa.
Now that discharge from the second reactor is used as
recycle, the first reactor does not operate as predictably:
the temperature at the top of the reactor changes most in
response to changes in airflow, the middle temperature is
the least sensitive, and the bottom temperature is inter-
mediately sensitive.
Figure 8.6 Typical Temperature Prof Me
(Average of Dally Readings for May 1987)
1WTOURE per
« 50
locf caran tig
scran RCACTO
To further complicate temperature control, changes in
airflow rates do not immediately affect reactor tempera-
tures. Usually the temperature changes 8 to 24 hours
after the airflow rates have been adjusted. To date, tem-
perature variations have not had a noticeable impact on
product moisture content.
There is no specific temperature goal set for the second
reactor. The aeration system is set at the maximum rate
possible without cooling the reactor to below 35° to 40° C
(95° to 104° F).
An oxygen feedback system was built into the control
system, but is not used because the operators have
successfully run the facility without it and the variation in
oxygen content is minimal. Typical concentrations in the
past have been 18 to 19 percent in the exhaust gas
stream.
A solids test is performed on each batch of sludge going
to the belt press, dewatered sludge, feed mix going to the
first reactor, material transferred from the first reactor to
the second reactor, material discharged from the second
reactor, and sawdust used in the mix.
Daily log sheets are kept by the operators, who also
prepare monthly summaries. There is no computer
logging capability yet, but the Authority is planning to
purchase some software. Computerized data storage
and manipulation capabilities will save operators a signif-
icant amount of time and facilitate long-term process
monitoring.
8.4.2 Mass Balance and Reactor Detention Time
A materials flow diagram, constructed from plant records
for July 1987 to June 1988 is shown in Figure 8.7. A mass
balance is not shown because for unknown reasons, the
calculations to close the balance yielded negative volatile
solids destruction in the reactors.
At the design mix ratio and average bulk density values,
the plug-flow detention time in the first reactor is 16 days.
At the current mix ratio (1/1.4/1.4), the actual plug-flow
detention time in the first reactor is only 12.6 days.
In the second reactor, the plug-flow time is 15 to 18 days
under design conditions. Under actual conditions, the
material takes 19 to 21 days to pass through the second
reactor.
8.4.3 Product Quality
The final product contains 50 percent solids. Typical
metals data from an April 1988 laboratory report are
shown in Table 8.2. There are no restrictions on the use
of the compost.
Table 8.2 Heavy Metal Concentrations in Compost
Concentration (mg/kg)
Metal
Cadmium
Chromium
Copper
Iron
Manganese
Nickel
Lead
Silver
Zinc
Sludge
Cake
3
80
150
16,500
650
28
3.4
1.5
590
Fresh
Compost
2
30
68
6,700
400
20
0.50
1.6
300
Cured
Compost
2
30
95
7,100
410
19
1.0
2.3
560
8.5 Operations
8.5.1 Staffing
The plant is staffed 5 days/week, 10 hours/day by three
staff members assigned exclusively to the dewatering/
composting facility: a supervisor, a foreman, and an oper-
ator. The control system alarms are tied into the WWTP
board so that during the second and third shifts, when the
compost operators are not working, WWTP operators
can monitor the compost plant. These operators, how-
ever, are not trained in the operation of the composting
plant. As a result, the composting supervisor must be
98
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Figure 8.7 Clayton County, Georgia, Materials Flow
July 1987 to June 1988
SLUDGE: 1.37 DTON". 16.7% TS*, 299 CF*
SAWDUST: 2.07 DTOH. 58.6% TS. 442 CF
MIX: 34.5% TS
FIRST REACTOR
42.7% TS
SECOND REACTOR
PRODUCT: 3.98 DTON, 48.7% TS, 440 CF
RECYCLE: 3.41 DTON. 48.6X TS, 378 CF
0 DTON (tfTON) X 0.9072 = DT(WT)
' TS = TOTAL SOLIDS
« CF X .02832 = H3
called whenever a problem occurs during the second or
third shift or on weekends.
The operating staff performs preventive maintenance
duties. Maintenance personnel supplied by the WWTP
staff or the central Authority are used whenever man-
power or knowledge limitations make the work impos-
sible for the operating staff to conduct. The intermittent
use of maintenance personnel has not caused any prob-
lems, but it was impossible to estimate their hours. The
plant is too small and is working too well to justify assign-
ing a full-time maintenance person to the staff.
When drawing maintenance personnel from the treat-
ment plant or central Authority staff, the priority given to
compost plant needs compared with other needs is
determined on a case-by-case basis. The operator feels
that the Authority management has a good understand-
ing of the operations at the compost plant. The plant has
never been idle for lack of maintenance.
The compost supervisor is a Class 2 wastewater opera-
tor. (Georgia has a 3-class licensing system.) He has 6
years of WWTP experience and some junior college edu-
cation. The foreman is also a Class 2 operator with 6 to 7
years WWTP experience. He is a high school graduate.
The operator is a high school graduate and has no
certification.
None of the operators had prior experience working at a
composting plant. Initial training by Taulman emphasized
the mechanical aspects of the plant. Although some pro-
cess questions were addressed during startup, the plant
supervisor would have preferred that more emphasis had
been given to composting process principles at the
beginning so that the operators would have been aware
of potential problems.
The plant supervisor has hosted other Taulman plant
operators, who visited Clayton County for orientation
prior to their own plant startups. The Clayton County
supervisor would have liked to have had an orientation
similar to this before starting up his own plant.
8.5.2 Marketing and Distribution
North American Soils (NAS) is committed to buying all of
the plant product at $7.67/m3 ($5.75/cy). The compost's
primary users are iandscapers and public agencies. In
addition, the Authority uses some compost for its own
facilities. Approximately 90 to 95 percent of the compost
produced is sold to NAS; 5 to 10 percent is used "in-
house." The product distribution system is working well,
with compost going directly to end users on a regular
basis.
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8.5.3 Operating Costs
A design cost estimate by Taulman in 1982 projected
operating and maintenance costs of $33/wt ($30/wton)
compost and revenues of $22/t ($20/ton). At 590 kg/m3
(1,000 Ib/cy), these values would be $20/m3 and $13/m3
($15/cy and $10/cy, respectively.) The following estimates
of annual costs (presented in 1988 dollars) were adapted
from the 1984 evaluation report by Robert and
Company:
Power:
Amendment:
Subtotal:
Revenues from product sale:
Net:
$27,000
49,800
$76,800
47,400
$29,400
Expenses for fiscal year 1988 (May 1987 to April 1988)
are summarized in Table 8-3. The actual amendment and
power costs were higher than predicted due to wetter
sludge cake; the revenues were lower than predicted.
The operations and maintenance costs are reasonably
close to those expected.
Table 8.3 Expenses and Revenues for Fiscal Year 1988
(May 1987 to April 1988)
$/dton
$/cy Sold Processed
Annual (6,366 cy" (503.1 dton"
Total Sold) Processed)
Operating Expenses:
Direct Labor and Benefits
Electricity
Other Utilities
Wood Chips
Chemicals (for Dewatering)
Supplies
Outside Services
Maintenance and Repairs
Miscellaneous and Overhead
Total Operating Expenses
Interest on Bond and
Depreciation of Plant1
Total Expenses
Revenue (sales):
Net lncome/(Loss)
59,482
37,610
607
13,307
9,884
5,820
1,370
17,467
3,051
148,598
279,081
427,679
36,604
(391,075)
9.34"
5.91
0.10
2.09
1.55
0.91
0.22
2.74
0.48
23.34
43.84
67.18
5.75
(61.43)
118.23=
74.76
1.21
26.45
19.65
11.57
2.72
34.72
6.06
295.37
554.72
850.09
72.76
(777.33)
•Compost plant assets total $2,268,300. All assets depreciated on
tha straight line method with 30-year life.
*$/cyx1.31=$/m;)
•S/dtonxl.102=$1/dt
«cyx.7646 = m5
•dton x.9702=dt
It costs the Authority $4.46 to produce and transport 1
wet ton of pine chips to the composting plant from its land
treatment operation. Since pine chips are supplied
"free" there is little economic pressure to reduce saw-
dust use. The wood chips from the land treatment system
also power the Authority's sludge pelletizing operation.
Therefore, the Authority must supplement its supply for
the composting plant by buying hardwood chips from
local sources for $12 to $13/wt ($11 to $12/wton.)
The operators have made a number of equipment modifi-
cations, all of which were unexpected costs. These modi-
fications included lining the discharge drag chain
conveyor with UHMW plastic and installing inspection
hatches on chutes. The aggregate cost for these modifi-
cations was not available during the site visit.
8.6 Update
In March 1989, the survey team received the following
information by phone from the facility. Since the site visit,
one gearbox that transfers rotational power to 'the out-
feed auger failed when the operators misread the sight
glass and did not recognize that the fluid level was too
low. (The fluid level sight glass was difficult to inspect and
had to be read in an awkward position using a mirror and
flashlight.) The operators had the gearbox rebuilt and
moved the fluid level indicator to a more accessible
location.
Another problem occurred when the outfeed auger got
stuck in the compost. The screw would turn, but it would
not advance. The mix and compost characteristics were
not unusual and the screw had not been stationary for an
unusual length of time. Thinking that a temperature
probe may have become entangled in the auger, the
operators cut a hole in the outer wall of the reactor to
inspect the auger. The inspection did not reveal any obvi-
ous cause for the hang-up. Subsequently, the operators
changed the angle of the hydraulic piston that turns the
central pivot of the auger and had the hydraulic motor
powering the piston rebuilt. Although these two modifica-
tions solved the problem, both reactors will be retrofitted
with a second advance mechanism (mirror image to the
existing one) to better distribute the torsional forces. The
system supplier has also observed problems with this
mechanism at European facilities.
During the annual inspection of the first reactor (the sec-
ond reactor has not been inspected yet), the following
conditions were noted and addressed as indicated:
• The epoxy coating on the walls was blistered; the
spots were recoated.
• The outfeed screw was minimally worn, so resurfacing
was not required.
• The rubber air seal between the reactor and the build-
ing was replaced.
As of June 1989, the faciity had no further changes to the
plant or operations to report.
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Chapter 9
Newberg, Oregon
9.1 Introduction
The city of Newberg, Oregon, owns and operates an in-
vessel composting facility located on the same site as the
city's wastewater treatment plant (WWTP). The WWTP
pumps sludge to the facility, where it is composted in two
reactors that were designed and constructed by
Ashbrook-Simon-Hartley (Ashbrook). This plant is the
first of its kind in the United States, although there is a
comparable plant that composts solid waste in Germany.
The design capacity of the system is 3.2 dt/day (3.5
dtons/day). At the time of the site visit, the facility was
processing 0.9 to 1.4 dt/day (1.0 to 1.5 dton/day).
The facility is located outside the city limits in a rural area
on a plateau approximately 30 m (100 ft) above the Willa-
mette River valley. There are a few homes located within
a one-half mile radius. Adjacent to the plant are a small
airport, a solid waste transfer station, an orchard, and a
large papermaking company.
The site visit took place on July 11 and 12, 1988. Except
as indicated, the equipment, processes, data, and issues
described in this case study are representative only of
that time.
9.2 History of the Plant
9.2.1 Procurement
Planning for the Newberg WWTP began in 1983. Initially,
the city's sludge management plan specified the use of
either land application or landfilling. But when the Oregon
Department of Environmental Quality gave the Newberg
plant low priority on the construction grant priority list, the
city investigated Innovative/Alternative (I/A) technologies
as a means of enhancing its position on the State's prior-
ity list. The city selected in-vessel composting, an I/A
technology, and was then given a high priority for con-
struction grants. The EPA subsequently funded 75 per-
cent of the wastewater treatment plant and 85 percent of
the composting facility.
The city's consultant, Kramer, Chin and Mayo, Inc., evalu-
ated several in-vessel composting technologies and
specified Taulman-Weiss, Purac, or an equal. On Octo-
ber 18,1985, the city received bids from Taulnian-Weiss,
Purac, and Ashbrook. The specifications required that
the bids separate capital costs from operating costs (e.g.,
electric power, power demand, sawdust, downtime, and
chemical costs). The city then added the capital and
operating costs to obtain tdtal life-cycle costs and used
these figures to compare bids.
9.2.2 Capital Costs
Ashbrook was the low bidder at $3.3 million, followed by
Purac at $4.6 million, and Taulman-Weiss at $5.1 million.
After evaluation by the city, the engineer, and the State,
the Ashbrook system was accepted as an equal. (See
Table 9.1 for a breakdown of Ashbrook's bid.) At the time
of the site visit, a change order for minor modifications
had resulted in the expenditure of an additional $15,000
by the city.
Table 9.1 Bid Results (Present Worth Costs)
October 18,1985
Item
1. Composting
Facilities
2. Electric
Power
(@ $950,000/
kWh/lb dry solids)
3. Power Demand
(@ $600/kW)
4. Sawdust
(Ib dry solids/
Ib dry solids @
$260,000)
5, Downtime
(@ $5,000/day/
year)
6. Chemicals
(@ $/16,000,000
Ib dry solids)
Total Life-Cycle Costs
Ashbrook
Estimate
Contract Price
(lump sum)
0.366 kwh/lb"
84 kW
0.35 Ib/lb"
0 day/yr
0
Ashbrook
Cost($)
$2,824,000
347,700
50,400
91,000
0
0
$3,313,100
"KWh/lb x 7.936 x 10* = kJ/kg.
"Ib x .4536 = kg.
The contract between the city and Ashbrook requires
Ashbrook to indemnify the city if the life-cycle costs
exceed those presented in the bid. The city will evaluate
how well the Ashbrook bid compares to the facility's
steady-state performance after the performance test is
conducted. If the calculated total life-cycle costs are less
than the bid, there will be no payments. If the costs
101
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exceed the bid by up to 30 percent, Ashbrook will pay the
difference to the city in a lump sum payment prior to the
release of the performance bond by the city. If the calcu-
lated life-cycle costs exceed the bid by 30 percent or
more, the facility will be considered unacceptable and the
composting process will be revised or replaced.
9.2.3 Operating History and Current Status
The in-vessel composting facility was built at the same
time as the new wastewater treatment facility. Construc-
tion of both started in October 1985 and the composting
facility began operating in August 1987. Until February
1988, the plant composted sludge from the nearby Hills-
boro wastewater treatment plant on an intermittent basis,
because sludge was not yet produced by the new plant.
From February to May 1988, the facility processed nearly
all sludge produced by the new treatment plant (approxi-
mately 0.9 to 1.4 dt/day [1 to 1.5 dton/day]). Since May
1988, some of the sludge produced by the plant-has been
applied to farm land owned by the plant manager.
In May 1988, the interior wall of one of the reactors buck-
led, causing structural damage. The reactor wall was
repaired and the reactor went back into operation in
August 1988. Then, in November 1988, the exterior side-
wall of the same reactor cracked. The reasons for these
failures are not known, although both were associated
with unusually wet sludge and, at least in the first
Instance, increased door pressures shortly before the
Incident.
At the time of the site visit, the composting facility was still
in the start-up mode. A performance test was initiated in
August 1988 but suspended in November 1988 when the
second wall section of the reactor buckled. As of Decem-
ber 1988, Ashbrook was studying the reactorfe structural
design and was planning to repair the damage and
repeat the performance test.
9.3 Description of the Plant
9.3.1 Systems Overview
The Newberg composting facility consists of two build-
ings: the dewatering building and the composting build-
ing. The composting building, which measures 24 by 30
m (80 by 100 ft), houses two reactors; a blower room; a
control room; and the conveyance systems for sludge,
sawdust, and recycled compost. Feed materials are con-
veyed into a mixer, and the mix is conveyed to the reac-
tors, which are operated in parallel. The compost is
pushed through and out of the reactors with a hydraulic
ram, and conveyed outside, where composting is com-
pleted in aerated piles. The reactors are aerated by air
supplied by equipment .in the blower room, located
between the two reactors. The air is exhausted by equip-
ment in the blower room and directed to the oxidation
ditches at the WWTPThis entire process is controlled by
a computer that is capable of running the plant automati-
cally. Figure 9.1 is a schematic of the process.
9.3.2 Feed and Mix Characteristics
Average dry weather flows of wastewater to the WWTP
range from 0.07 to 0.09 m3/s (1.5 to 2.0 mgd). The WWTP
generates between 0.9 to 1.9 dt (1 to 1.5 dton) of waste
sludge per day with 1 percent solids and 75 percent
volatile solids. This sludge is stored for about a week at
the WWTP in two aerated, uncovered tanks, each with a
303,000 L (80,000-gallon) capacity. Then the sludge is
pumped to the dewatering building, where it is polymer-
conditioned and thickened in two dissolved air flotation
(DAF) thickeners. After thickening, the waste-activated
sludge (WAS) contains 3 to 4 percent solids.
Next, the sludge is dewatered in two Ashbrook belt filter
presses where more polymer is added. Since startup,
cake solids have ranged from 12 to 20 percent dry solids
with an average of 16 to 18 percent; bulk density has
been approximately 880 kg/m3 (55 Ib/cf).
Sawdust is added to the dewatered sludge as an amend-
ment. The contract specifications for the sawdust call for
a minimum solids content of 50 percent (60 percent
weekly average); a product clean and free from dust, rot,
burned material, sand, grit, or other objectionable mate-
rial; a maximum particle size of 0.006 m (1/4 in.);
untreated softwood (no more than 15 percent hardwood);
and no more than 50 percent bark (by volume). Initially,
the sawdust used at the plant was frequently contami-
nated with rocks, bark, and oversized material that
jammed the amendment storage pneumatic loading sys-
tem. Subsequently, the city found a more reliable sup-
plier. The amendment used now has a dry solids content
of 90 percent and has a bulk density of approximately 176
kg/m3 (11 Ib/cf).
Recycle directly from the reactor output is also added to
the mix. The solids content of the recycle ranges
between 35 and 45 percent; bulk density is 721 kg/m3 (45
Ib/cf). Recycle is stored in a 25-m3 (900-cf) steel bin
equipped with a drag chain outfeed device.
The design specifications required a mix ratio appropri-
ate for the Taulman system. After Ashbrook was selected
as the supplier, its engineers changed the specified mix
ratio to one consistent with their process: 1.0/0.35/5.0
sludge to sawdust to recycle (by volume). During early
July 1988, the volumetric mix ratio was approximately
1.0/0.5/10 sludge to sawdust to recycle. Thus, both saw-
dust and recycle usage is higher than originally
anticipated.
Although Ashbrook recommended that the mix contain
38 percent solids with a minimum of 35 percent, actual
solids in the mix have ranged from 30 to 40 percent; the
bulk density is approximately 400 kg/m3 (25 Ib/cf). Within
the reactor, the bulk density increases to approximately
880 to 1,040 kg/m3 (55 to 65 Ib/cf) (based on limited
testing). At the reactor discharge, the solids content
ranges from 35 to 45 percent.
102
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Figure 9.1 Newberg, Oregon, Composting Process Train
SAWDUST STORAGE SILO
oo
DEWATERING FACILITY
SLUDGE STORAGE TANKS BELT FILTER PRESS
HIXER
RECYCLE STORAGE BIN
RECYCLE
COMPOSTING FACILITY
BUILDING LINE
AERATED CURING/
STORAGE PILES
9.3.3 Materials Handling
The feed systems for sludge, sawdust, and recycle differ
(see Figure 9.2 for a diagram of the entire conveyor
system). The dewatered sludge is transported on belt
conveyors from the dewatering building to an enclosed,
vented storage overhead hopper in the composting build-
ing. The storage hopper is a live-bottom steel bin with a
34 m31,200-cf capacity; the outfeed device is a variable
speed screw conveyor.
Sawdust is received on an open pad, then transferred to
a 156 m3 (5,500 cf) glass-lined steel storage silo. The
specifications require that suppliers dump the sawdust
next to the building and the operators use a suction hose
to "manually" transfer the sawdust into the storage silo.
Using this method, operators took 4 hours to transfer one
truckload. The city has recently constructed a temporary
1.5-m3 (2-cy) wooden receiving hopper to facilitate the
pneumatic transfer of sawdust to the silo. A permanent
receiving hopper is being planned The sawdust outfeed
device is a rotating chain with a capacity of 0.0006 to
0.008 m3/s (80 to 1,100 cf/hr). An enclosed screw con-
veyor meters sawdust to the mixer infeed device, an
enclosed drag chain conveyor.
The sludge and amendment are both taken to the mixer
by the drag chain conveyor. Recycle is discharged from
the recycle bin by drag chain conveyor directly to the
mixer. All other drag chain conveyors are constructed of a
high strength alloy and have galvanized steel housings.
The mix ratio is controlled by a computer, based on
information supplied by the operators. Every time the
operators load mix into the reactor, they record the per-
cent solids in the reactor infeed and outfeed and then
supply the computer with the solids content and required
volume. The computer adjusts the mixture ratio for the
next reactor charge by setting the speed of the convey-
ors, and hence, the feed rates of sludge, sawdust, and
recycle. Weight scales were not specified by the engi-
neer, so there are none.
The pugmill mixer is operated at constant speed. The mix
discharges to an inclined drag chain conveyor, and then
to a horizontal reactor infeed drag chain conveyor that
loads either of the two reactors in the order designated by
the operator.
Several of the drag chain conveyors have been modified
to reduce compaction of material and motor overload.
The flights on the compost outfeed conveyor were
reduced in height from 0.1 m to 0.04 m (4 in. to 1.5 in.),
allowing more room between the flights and the conveyor
103
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Figure 9.2 Newberg, Oregon, Materials Handling System
HYDRAULIC
UNIT "
REACTOR OUTFEED
CONVEYOR
TUNNEL REACTOR® NO. 2
TUNNEL REACTOR® NO. 1
CONTROL
ROOM
N- RECYCLE HOPPER
SLUDGE
HOPPER
AMENDMENT
SILO
FINISHED PRODUCT
CONVEYOR
SAWDUST
BLOWER ROOM
SLUDGE CONVEYOR
FROM SOLIDS BUILDING
ASHBROOK-SIMON-HARTLEY
housing. At the time of the site visit, occasional breaking
of flights and jamming of conveyors still occurred.
9.3.4 Reactor
9.3.4.1 Reactor Feed System
Each reactor has a hydraulically activated steel door
measuring 5.5 m wide by 3.7 m high (18-ft wide by 12-ft
high). To charge the reactor, the doors are retracted 0.76
m (30 in.) for approximately 15 minutes while the void
created by the retracted door is filled with mix from the
infeed conveyor. When a level probe, located at the door,
indicates that the void is filled, the door automatically
closes. During the performance test period, the probes
became fouled, causing the door to close prematurely.
Daily (or more frequent) probe cleaning has since solved
the problem.
Approximately 15 m (540 cf) of mixture is fed to the
reactor in one charge. After filling, the door closes, com-
pressing the infeed to approximately a 0.25 m (10-in.)
thickness. The entire contents of the reactor (about 3631
[400 tons]) then move forward and 0.25 m (10 in.) of
compost are pushed out at the discharge end.
While the reactor is charged, air is blown into the bottom
of the reactor to reduce the friction of the compost moving
along the reactor floor. During this time (40 seconds),
104
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each reactor receives 1.18 m3/s (2,500 cfm) of air at a
pressure of about 20,700 Pa (3 psi). As a result of this
pressurization, the door has to overcome only 8,074 Pa
(1,200 psi) instead of 13,790,000 Pa (2,000 psi) in order to
close.
9.3.4.2 Configuration
There are two totally enclosed plug-flow composting
reactors. Each unit is a large reinforced concrete cham-
ber with a volume of approximately 885 m3 (13,600 cf)
and inside dimensions of 5.5 m wide by 2.7 m high by 19
m long (18 ft wide by 12 ft high by 63 ft long). The walls are
0.23 m (9 in.) thick. Aeration diffusers are embedded in
the floor of the reactor (see Section 9.3.4.3). Two meters
(6 ft) in front of the discharge door, the reactor floor slopes
45 degrees to the outfeed conveyor, which is located 2 m
(6 ft) below the reactor floor (see Figure 9.3). Figure 9.4
shows the interior of the reactor.
Figure 9.3 Ashbrook Composting System
COMPOST
FEED CONVEYOR
DOOR
HYDRAULIC
CYLINDERS
DISCHARGE
CONVEYOR
'AIR HEADERS
FLOOR MOUNTED
DIFFUSERS
X
AIR
SUPPLY
VACUUM
Figure 9.4 Interior of Tunnel Reactor, Facing Discharge End
9.3.4.3 Reactor Aeration System
The reactors are aerated by air supplied to and with-
drawn from the bottom of the reactor. The air is moved by
blowers that are housed in the blower room, which is
located between the two reactors (see Figure 9.5). The
air is supplied by two 30,000 W (40 hp) variable speed
rotary positive displacement pressure blowers with cast
iron lobes. Each unit has a capacity of 1.18 m3/s (2,500
cfm) at 20,700 Pa (3 psi). The system was designed to
supply 0.6 m3/s (1,250 cfm) to each reactor, with one unit
supplying both reactors; the second unit was included as
a standby. Both units can be operated simultaneously,
however, so that one reactor theoretically can be sup-
plied with up to 2.4 m3/s (5,000 cfm).
The aeration system was designed to supply 0.001 m3/s
per 1,000 m3 (90 cfm per 1,000 cf) of reactor volume,
105
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Figure 9.5 Air Blower Room Between the Two Reactors
which is equivalent to about 0.56 mVs (1,200 cfm) per
reactor or 5.3 air changes per hour (empty-bed basis).
Typical rates are approximately 0.38 to 0.47 rri3/s (800 to
1,000 cfm) per reactor at 10,340 Pa (1.5 psi). During mid-
1988, with one reactor in operation, the rate was
increased to 1.18 mVs (2,500 cfm).
Air is supplied to or withdrawn from each reactor through
seven independent aeration zones. Each zone consists
of six diffuser channels in the floor of the reactor. The
diffusers consist of a 0.0009 m (3/8-in.) wide slot that
connects an embedded 0.08 m (3-in.) diameter stainless
steel pipe to the floor surface. The pipes run the 5.5 m
(18-ft) width of the reactor and are spaced about 0.46 m
(18 in.) apart. The seven aeration zones are valved and
connected to two 0.15 m (6-in.) diameter steel headers:
one is valved to the pressure blowers; the other to the
exhaust blowers. The seven aeration zones can be oper-
ated in any combination of positive pressure and vacuum
to regulate temperature and airflow in the reactor. Nor-
mally, of the seven aeration zones in each reactor, three
or four are in the vacuum mode.
Two 30,000 W (40 hp) constant speed centrifugal
exhaust blowers direct the air to the odor control system.
Each blower was originally sized for 1.18 mVs (2,500
cfm), 0.6 mVs (1,250 cfm) per reactor. However, the dis-
charge pressure to the odor control system exceeded
expectations and the two exhaust blowers are now oper-
ated in series to obtain a total airflow of 1.18 mVs (2,500
cfm) at 14,000 Pa (2 psi). Under existing conditions there
is no standby unit.
9.3.4.4 Outfeed Device
Both reactors are serviced by a single drag chain outfeed
conveyor rated at 0.0007 to 0.001 m3/s (100 to 200 cf/hr).
This outfeed conveyor discharges to an elevated drag
chain conveyor, which in turn discharges to a reversible
drag chain conveyor that either transfers compost to the
recycle bin or discharges compost outside the building.
9.3.4.5 Leachate/Condensate
Condensate and reactor leachate are collected in a mod-
ified in-line moisture trap in the suction line of the exhaust
blowers. The trap discharges into a dry sump in the
center of the aeration gallery that empties into the plant
drainage system. Nineteen to 38 L/day (5 to 10 gal/day) of
condensate and leachate are discharged by the blower
room sump. Condensate is also collected at the dis-
charge of the 0.46 m (18-in.) odor control line that runs to
the two oxidation ditches.
9.3.5 Exterior Curing/Storage
As designed, the material from the reactors would be
placed outside on a concrete pad, which measures 27 by
21 m (90 by 70 ft). This curing/storage area is approxi-
mately 2,800 m2 (30,000 ft2). At the time of the site visit,
nearly all the material produced since startup, 1,530 m to
1,900 m3 (approximately 2,000 to 2,500 cy), was stored at
the conveyor drop point on the ground in 2.4- to 3.7-m (8-
to 12-ft) high piles. Using a front-end loader (delivered
during the site visit), the facility plans to move the com-
post to the concrete pad, which is intended to accommo-
date two 450 m3 (16,000-cf) piles of compost. The piles
will be aerated with four 52,000 W (7.5 hp), 0.47 m3/s
(1,000 cfm) portable blowers operated in the suction
mode and regulated by timers. Exhaust from the piles will
be discharged to the oxidation ditch or to a second pile of
older, "finished" compost. Ashbrook engineers believe
that the odor control pile will be adequate and that the
oxidation ditch discharge system will not be required.
Condensate, leachate, and stormwater runoff are col-
lected from the curing/storage area and returned to the
wastewater treatment facility.
The plant may be operated without aerated piles because
the actual sludge quantities are less than anticipated,
which increases reactor detention time, and because the
currently unaerated piles do not appear to present odor
problems.
9.3.6 Nonprocess Air Handling
The dewatering building air is vented to the atmosphere
with roof fans rated at 0.71 m3/s (1,500 cfm), which pro-
vide five air changes per hour. In the mixing room area,
two roof fans rated at 2.3 m3/s (4,800 cfm) discharge to
the atmosphere and provide 24 air changes per hour. The
DAF units (which are outdoors, but covered), dewatered
sludge hopper, and enclosed conveyors are vented to the
inlet of the blowers that supply air to the reactors. These
supply blowers are also valved to allow the intake of fresh
air from the blower room. The reactor feed area, where
the reactor doors and their hydraulic mechanisms are
located, is vented to the atmosphere with two roof fans
rated at 2.3 m3/s (800 cfm). The fans provide 24 air
changes per hour.
9.3.7 Odor
To control odors, blowers discharge the air from the reac-
tors to the oxidation ditches through a 0.46-m (18-in.)
diameter pipe, 610 m (2,000 ft) in length. At each oxida-
tion ditch, the airstream is bubbled through approxi-
106
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mately 0.9 m (3 ft) of mixed liquor using coarse-bubble
diffusers (see Figure 9.6). No apparent "composting" or
ammonia odors are discernible, although there is a slight
musty odor characteristic of an activated sludge system.
As a standby measure, the exhaust blowers can be
vented to the cure piles.
Figure 9.6 Reactor Air Being Discharged from 18-in. Diameter
Exhaust Pipe into Oxidation Ditches
The odor control system appears to be effective,
although at the time of the site visit, slight ammonia odors
were noticeable at the sludge storage bin, at the dis-
charge end of the reactor, and in the curing/storage area
when a 1-month-old pile was disturbed. There were no
compost odors in the mixing room or around the plant
grounds. Several factors may be contributing to the good
odor record: the plant is surrounded by a large buffer
zone, the sludge production is only about 0.9 to 1.4 dt/day
(1 to 1.5 dtons/day) (30 percent of design), and the sludge
itself may be partially stabilized in the underloaded oxida-
tion ditch before it is dewatered. (Approximately 50 per-
cent of the wastewater is highly biodegradable food
process wastes.) No odor characterization studies have
been conducted.
9.3.8 Support Facilities
The mixing area has an enclosed control room separate
from the motor control center (MCC) with sufficient area
for the computer and a desk (see Figure 9-2). A separate
multipoint strip chart shows reactor temperatures, door
pressures, and pressure in the charging zone. In the
future, the pressure-sensing capability will be increased
to show wall pressures near the charging door.
The central WWTP control building is located near the
composting building. It contains administrative facilities,
a classroom, a laboratory, and a shop and vehicle mainte-
nance area. All facilities appear to be adequate.
9.4 Monitoring and Performance
9.4.1 Reactor Control Strategy
Newberg's composting goal is to produce a pathogeni-
cally safe, marketable product. The system was designed
for a 14-day reactor plug-flow detention time, followed by
exterior aerated curing/storage. Because the plant has
not operated consistently due to varying sludge solids
contents and quantities and reactor structural problems,
it has not been possible to compare performance with
design objectives.
9.4.1.1 Temperature Monitoring and Control
Wall-mounted temperature probes were supplied with
the system, but they were removed after numerous fail-
ures. Each reactor was also fitted with a 15 m (50-ft) long
temperature probe that was installed along the length of
the reactor core (about 2 m [6 ft] from the floor and 2.7 m
[9 ft] from the reactor walls). The probes were pushed into
the composting mass after the units were initially filled,
and attached to the charging door. As a consequence of
their location, the probes moved 0.76 m (30 in.) back and
forth every time the reactor was charged. This movement
broke the probes and, at the time of the site visit, they had
not yet been replaced.
New probes planned for the system will also be placed
longitudinally through the core of the reactor. They will,
however, be anchored to an outside wall approximately 3
m (10 ft) in back of the door. The door will have a hole and
gasket system so that the probe will not move with it. The
probe will be equipped with seven sets of thermocouples
that take three temperature readings in the same location
every 2.4 m (8 ft) along the reactor length. A computer will
then average the three readings, disregarding any
outliers.
The reactor is also equipped with ports for manually
recording temperatures: each of the seven aeration
zones has three ports along the sides at different eleva-
tions and two roof ports, for a total of 35 ports. (These
ports can also be used for sampling.) All of the ports are
accessed from the exterior of the reactor. During the site
visit, the automatic temperature probes were inoperable,
so a 0.9-m (3-ft) temperature probe was used to record
temperatures manually from the roof ports.
The roof of each compost reactor has been equipped
with temperature probes set for 85°C (185°F) to act as
fire indicators. According to Ashbrook, in the event of fire,
the sampling ports can be used for injection of water or
chemical retardants. Shutting the closed loop aeration
system might also be effective.
9.4.12 Aeration Quantities and Rates
The operator can control the temperature of the compost
by setting the airflow rate (0 to 0.6 rrWs [0 to 1,250 cfm])
per reactor and airflow direction (pressure or vacuum) for
each of a reactorfe seven aeration zones. If temperatures
in the first two or three zones are cool 30° to 50°C (86° to
122°F), the aeration system is set in the vacuum mode. If
the temperatures exceed 60° to 70°C (140° to 158°F),
either pressure is applied to cool the material or the zone
is kept in the vacuum mode and the airflow is increased.
107
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In the sixth and seventh aeration zones, the temperature
Is allowed to drop below 50°C (122°F) to enhance drying,
provided that the EPA time and temperature criteria have
been met The adequacy of the aeration system could not
be judged due to the lack of reliable temperature data, but
operators report that the system is aerobic and generally
meets EPA pathogen reduction requirements.
Oxygen meters, installed on exhaust lines from each
reactor were required in the specifications, but are not in
use and the operators have no plans to activate them.
Ashbrook supplied the equipment but believes that such
monitoring does not assure aerobic conditions in all
zones, since it is possible to have anaerobic pockets that
are undetectable. Furthermore, oxygen is rarely a limiting
factor in in-vessel composting systems.
9.4.1.3 Mix Ratio Control
The computer system controls the mix ratio by setting the
speed of the conveyors, based on information entered by
the operators.
9.4.7.4 Data Management
The computer system has a data logger for airflow, tem-
perature, and materials quantities. No further information
Is available due to the status of the facility.
9.4.2 Mass Balance
A mass balance is not available since the plant is still in
the startup phase.
9.4.3 Product Quality
The recommended and actual mix ratios both have an
unusually small proportion of sawdust and a very large
proportion of recycle (see Section 3.2). A low solids con-
tent in the mix could be corrected by adding less sawdust
than recycle, since the solids content of the sawdust (90
percent) is higher than the recycle (35 to 45 percent).
Because of the nature of their contract with the city,
however, Ashbrook cannot dramatically alter the percent
of sawdust required in the mix without risking monetary
penalties. As a result, they use large quantities of recycle
to improve the solids content of the mix. Because of the
large volumes of recycle, the reactor detention time has
been reduced. However, these ratios change weekly and
are not indicative of the potential performance of the
facility.
Compost quality is shown in Table 9.2. The compost
meets all of the State standards for application to agricul-
tural land. Analyses performed during the initial perform-
ance test period showed no salmonella and
approximately 130 fecal coliform bacteria per gram of dry
solids.
9.5 Operations
9.5.1 Staffing
The wastewater treatment facility has a staff of six opera-
tors including the manager. They all work the day shift, 5
Table 9.2 Oregon Administrative Rules for Acceptable Levels
of Metal Content of Sludge for General Application to
Agricultural Land
Element
Cadmium
Chromium
Copper
Lead
Mercury
Nickel
Zinc
Grab Sample
Concentration'
(mg/kg)
0.8
4.9
84
1.6
0.3
4.7
240
State Standard
(mg/kg)
25
—
800
1,000
100
2,000
•May 1988.
days per week. At the time of the site visit, one operator
was assigned to the sludge conditioning and dewatering
process and another to the composting process. The city
plans to assign a second person to the composting pro-
cess for 20 hours per week. During the site visit,
Ashbrook had a full-time process engineer on site.
The composting operators have a high school education,
wastewater treatment experience, and State wastewater
treatment certification. The State does not have a com-
posting operator certification program.
Ashbrook provided a combination of classroom and
hands-on training for the manager and selected opera-
tors. The topics covered in these sessions included the
composting process, operational control, and equipment
operation and maintenance.
9.5.2 Marketing and Distribution
Very little of the product has been utilized to date,
although some has been used by the city at the plant and
at a public library. A1987 marketing study recommended
that the city sell the product in bulk to a single end user
(nursery, landscapes or broker). City officials have also
discussed a potential giveaway program to city
residents.
9.5.3 Operating Costs
Prior to operation of the facility, the operator did not have
a complete estimate of operations and maintenance
costs. Ashbrook made the following estimates in its bid:
• Sawdust 0.16 kg/kg (0.35 Ib/lb) sludge
• Labor (one and one-half operators, 5 days/week)
• Power 0.003 kJ/kg (0.37 kWh/lb) dry solids and 84 kW
peak power demand)
The current sawdust usage is somewhat higher than
anticipated (it varies from 0.18 to 0.4 kg/kg [0.4 to 0.8 Ib/
Ib] sludge), even though the dry solids content is 90
percent as compared to the 60 percent specified.
An approximate first-year budget was developed during
the site visit with the assistance of the WWTP manager.
108
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He estimated that the annual operating costs were
$75,000 for the first year (August 1987 to August 1988).
(See Table 9.3.) There have been no unexpected costs
(except for the $15,000 change order), as Ashbrook has
made all the necessary repairs and modifications at their
own expense.
9.6 Jpdate
The plant was not operating as of June 1989. In Novem-
ber 1988, the exterior wall cracked for the same reactor
whose interior wall had buckled in May. The performance
test which was begun in August was suspended. It is
suspected, but not known, that the exterior wall was also
damaged in May, and the problems did not surface until
November. The system supplier is currently rebuilding
the reactor, and strengthening the walls of the other reac-
tor. The plant should be back in operation in September
1989.
Table 9.3 Estimated First Year Operations and
Maintenance Costs
Component
Labor
Sawdust
Power
Maintenance
Total
Cost($)
Basis
$42,000
3,000
25,000
5,000
$75,000
60 hours/week',
$28,000/year/operator
0.35 Ib/lb" sludge, 300 Ib/cy
$3/cy, 1 dt/day sludge
Two exhaust blowers,
continuous draw;
one aeration blower, 50% draw;
$0.04/kWh
Materials handling and
lighting additional 20%
First year, includes front-end
loader fuel
•Represents 1.5 operators each working 40 hours/week at the
composting facility (dewatering facility operator labor not included).
"Ib x 0.4336 = kg.
°lb/cy x 0.5933 = kg/m3.
109
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Chapter 10
Plattsburgh, New York
10.1 Introduction
The Clinton County in-vessel composting facility is
located in Plattsburgh, New York. Although owned by the
county, the facility is operated by the City of Plattsburgh
under an intermunicipal agreement whereby the city
assumes responsibility for all costs and operations. The
Water Pollution Control Plant (WPCP) in Plattsburgh
delivers dewatered sludge to the facility. The Plattsburgh
WPCP is an activated sludge plant that is jointly owned
by the city and three paper mills. In 1987, the paper mills
contributed 56 percent of the plant flow and 91 percent of
the biological oxygen demand (BOD); the mills pay their
share of annual operating costs through user charges.
The dewatered sludge is composted using a Fairfield
circular agitated bin process. The Clinton County plant is
the first Fairfield in-vessel sludge composting facility in
the United States designed for only municipal wastewa-
ter sludge. There is an older facility that co-composts
sludge and solid waste in Wilmington, Delaware. The
design capacity of the system is 31 dt/day (34 dton/day)
of sludge. At the time of the site visit, the facility was
processing all of the city's sludge, an average 25 dt/day
(28 dton/day).
The site visit took, place on April 27 and 28,1988. Except
as indicated, the equipment, processes, data, and issues
described in this case study are representive of that time
only.
10.2 History of the Plant
10.2.1 Procurement and Construction
Before the in-vessel facility was constructed, dewatered
sludge from the WPCP was trucked to a landfill site some
12 miles away. However, when it became clear that the
landfill would be full by mid-1986, the city investigated
other methods of sludge management. At first, the city
elected to co-incinerate the sludge with the county solid
waste. But when the solid waste incinerator was not
funded, the city quickly chose an alternative in order to
obtain funding as part of an upgrade of the WPCP
The city chose in-vessel composting over static pile com-
posting for two reasons: to better contain and control
odors and to more effectively stabilize the sludge in win-
ter. System suppliers were invited to make presentations
before city officials. Two reasons influenced the city's
choice of in-vessel system. First, the city's sludge is
coarse and fibrous. (Due to the paper mill waste, the city
was concerned about compaction and loss of porosity in
the reactor.) The shallow depth and mechanical mixing of
an agitated bin system appeared to ensure that the
sludge would be completely mixed and aerated. Second,
the capability to simultaneously load and unload material
from the reactor appeared to provide more operational
flexibility. After comparing the costs associated with rec-
tangular and circular agitated bin systems, the city chose
the Fairfield process.
The city hired the engineering firm of Metcalf and Eddy to
prepare the contract specifications and put them out for
bid. The specifications, which were based on the Fairfield
process, called for the Fairfield process or "an equal."
Fairfield was the low bidder and, in June 1984, the con-
tract was awarded to Fairfield.
10.2.2 Capital Costs
The projected capital cost of the facility was $10,340,000 .
(201 Wastewater Facilities Plan Amendment, February
1983). Construction costs (1985-86) were $12,086,000,
plus $530,000 for engineering and change orders total-
ing $451,000.
10.2.3 Operating History and Current Status
The plant started up in April 1986. The plant operators
and Fairfield Service Company conducted the perform-
ance test during July and August, 1986; the plant met the
specifications. Once the performance test was success-
fully completed, the supplier's engineer left the site and
the plant initiated "commercial" operations in August
1986. The facility, however, operated throughout 1987 at
reduced capacity, primarily due to severe odor problems
(see Section 10.3.7).
There have been other problems, too. In August 1987,
plant operators discovered that the layer of wood chips in
the bottom of the reactor had turned into "hardpan,"
preventing air from flowing up into the compost bed. (The
aeration system is described in detail in
Section 10.3.4.3.) The plant was taken out of service
while the hardpan was dug out by hand (reactor 1:
August to November 1988; reactor 2: November to Janu-
ary 1988).
111
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In January 1988, the hangers holding up the exhaust air
plenum in reactor 1 failed and the plenum fell into the
reactor. (See Section 10.3.6 for a complete description of
the ventilation system.) The plenum was removed and
the reactor returned to operation. Since February 5,1988,
both reactors have been running continuously, compost-
ing all of the WPCPs sludge. (In March 1988, the New
York Department of Conservation [DEC] prohibited fur-
ther landfilling of sludge except when a "dire mechanical
emergency exists whereby sludge cannot be transferred
through the system.") At the time of the site visit, the plant
was processing an average 25 dt/day (28 dton/day) of
sludge.
10.3 Description of the Plant
10.3.1 Systems Overview
The plant consists of four buildings: two reactors operat-
ing In parallel, the air pollution control building, and the
administrative/mixing room building. The feed (sludge,
amendment, and recycle) is dumped into bins located on
one side of the mixing room building and conveyed down
into the mixing room. Conveyors also transfer mix to the
reactors and material from the reactors to the curing/
storage area. The air from the reactors is usually
exhausted through a duct to the air pollution control build-
ing, where it is treated and released. Air can also be
released directly from each vessel into the atmosphere.
This process is shown schematically in Figure 10.1.
10.3.2 Feed and Mix Characteristics
The sludge from the WPCP is a mixture of primary sludge
and waste-activated sludge (WAS) that has been thick-
ened, then dewatered in belt filter presses at the WPCP In
early 1988, the average solids content was 20 to 25
percent. (The bulk density was typically 880 kg/m3 [55 Ib/
of].) To control odors, the WPCP adds potassium per-
manganate to the sludge. This treatment keeps sludge
odors under control for several hours, usually more than
enough time to put the sludge into containers, truck it to
the composting facility, mix it, and convey it into the
reactor.
When the sludge arrives at the composting facility, it is
dumped from the trucks into two of four live-bottom feed
bins. Each of the four feeo! bins holds 31 m (40 cy) and
has nine 0.3 m (12-in.) diameter screw conveyors span-
ning the bottom. Figure 10.2 shows the sawdust bins with
their screw conveyors. The bins are below grade and
covered by a set of double doors, which are at grade.
Sawdust is added as an amendment to increase the
solids content of the sludge and make it easier to handle.
The sawdust specifications require a solids content
greater than 50 percent; a product free from ice, dirt,
sand, stones, and other foreign materials; and a product
that can be moved by the existing screw conveyors.
Figure 10.1 Pittsburgh, New York, Composting Process Train
UNAERATED CURING/PRODUCT STORAGE
112
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"Thin wood shavings" can be mixed with the sawdust up
to 50 percent by volume. The sawdust used at Platts-
burgh has worked well and has been an important factor
Figure 10.2 Sawdust Bins with Screw Conveyors
in the success of the operation. The sawdust is procured
by an annual bidding procedure.
Currently, the sawdust supply, consisting mostly of
softwoods, has a solids content of about 60 percent (the
typical bulk density is 240 kg/m3 [15 Ib/cf]). The solids
content of each truckload is measured when it is deliv-
ered; the solids content of the material stored on site is
measured daily.
The sawdust is stored on a concrete pad in a fiberglass
shelter that is open on one side. A continuous roof vent
and open areas about 1.2 m (4 ft) high at the bottom of the
walls were provided to allow release of moist air. In prac-
tice, the openings allowed the wind to scatter the saw-
dust, so the operators covered them with plywood. The
sawdust is transferred from the shelter to one of the feed
bins by front-end loader.
The third feed material, recycle, comes directly from the
reactor output to one of the feed bins. Recycle from the
curing/storage area can also be loaded in the bin by
front-end loader. Its solids content varies from about 43
percent in the winter to about 55 percent in the summer.
The bulk density typically ranges from 400 to 481 kg/m3
(25 to 30 Ib/cf).
10.3.3 Materials Handling
The feed systems for sludge, sawdust, and recycle are
identical. At the end of each feed bin is a pair of twin 0.41
m (16-in.) diameter screw cross-conveyors that move the
materials to individual feed conveyor belts. (The feed
conveyor system is depicted in Figure 10.3.)
The four individual belts carry the sludge, sawdust, and
recycle to a reversible mixer feed belt that is set up to
feed either of the two pugmill mixers. The feed conveyors
and mixers are located in an underground mixing room
that is adjacent to the feed bins. In the past, the mixers
have experienced some plugging problems, which the
operators attribute to overly wet sludge. As the solids
content of the sludge has increased, mixer problems
have decreased and are now infrequent.
The sludge, sawdust, and recycle are mixed in the proper
ratios to achieve a solids content of 40 percent and a bulk
density of approximately 450 kg/m3 (28 Ib/cf). Table 10.1
summarizes the design and actual mix ratios. As these
Table 10.1 Mix Ratios
Design
Actual
Sludge/Sawdust/Recycle Sludge/Sawdust/Recycle
Dry Weight
Wet Weight
Volume
1/0.68/2.68
1/0.23/0.97
1/0.69/1.67
1/1/3.4
1/0.4/1.7
1/1.5/3.3
figures indicate, the actual mix contains more sawdust
and recycle than anticipated in the design.
The mix is controlled by a computer linked to belt scales.
The solids contents and bulk densities of the mix constit-
uents are measured several times during a shift. This
information is manually fed into the computer, which
adjusts the speeds of the feed conveyors to achieve the
proper mix. The solids content of the mix is also mea-
sured several times during each shift. The system is able
to consistently meet its solids content goal within a few
percentage points.
The computer system has worked well except for the belt
scales. These electronic devices quit frequently during
the first year of operation and were difficult to repair
because of a lack of local service support. When the
scales are inoperable, the conveyor speeds must be
determined by trial-and-error, a time-consuming task.
Once the electronics in the system were repaired and the
system was sealed better, the reliability of the belt scales
improved substantially.
The mix is discharged through a funnel chute to a con-
veyor belt. Even though the funnel is nearly vertical, it
sometimes plugs. Since there is no flow or plug indicator,
the operators are not aware of the problem until the
funnel is completely jammed.
The conveyor belt goes from the underground mixing
room to an outside reactor feed conveyor. This conveyor
is reversible and can feed either reactor, one at a time.
The reactors are loaded 7 days/week, 10 p.m. to 8 a.m.,
to take advantage of offpeak power rates.
113
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10.3.4 Reactors
10.3.4.1 Reactor Feed System
The reactor feed conveyor drops the mix into a hopper in
the center of the rotating reactor bridge (see
Section 10.3.4.2). The hopper then dumps the feed onto
a conveyor belt on the bridge that takes the mix to the
perimeter of the reactor, where it is discharged onto the
top of the composting bed as the bridge rotates.
10.3.4.2 Configuration
The plant has two reactors that operate in parallel. Each
reactor is a 35.4-m (116-ft) diameter concrete cylinder
placed below grade and covered with an aluminum geo-
desic dome. There is no fire protection system in the
reactors. There also are no railings separating the perim-
eter walkway from the compost bed below.
The unique feature of the Fairfield system is its rotating
bridge agitation system. (Figure 10.4 shows a cross sec-
tion of a reactor.) A rotating bridge spans the compost
bed and rides on a rail circling the bed. Twenty-seven
augers (each 0.51 [20 in.] in diameter) are mounted on
one half of the bridge, extending to above the bottom of
the compost bed. Each auger is made from steel; the
carrying side of the flights (the spirally arranged ridges
on an auger) are hard-faced on both the edge and the
outside 0.08 m (3 in.).
The top of the augers are angled towards the center of
the reactor so that as the compost is mixed, it moves
gradually from the perimeter to the center of the reactor.
As the augers mix the compost, they maintain its porosity
and create circular windrow-like piles on top of the bed. It
was discovered that if the bed is 10 ft deep (as designed),
these piles interfere with the bridge movement at the
present rotational speed of one turn per 2 hours (as
designed). Consequently, the bed depth is kept at 2.4 to
2.7m (8 to 9 ft.).
The flights are wearing down to a "knife-edge" faster
than expected (their design service life is 3 to 5 years). At
the time of the site visit, eight augers had been replaced
because of flight wear or broken or split shafts and an
additional 18 needed replacement due to flight wear. In
1988 to 1989, the city planned to rebuild six augers and
possibly buy six new ones. The replacement augers have
thicker flights (0.01 m [0.5 in.] thick on the outside edge).
Figure 10.3 Feed Conveyor System
SCREW CONVEYORS
SLUDGE BIN *1
-SLUDGE BIN fZ
RECYCLE/AMENDMENT BIN
r RECYCLE/AMENDMENT BIN fZ
CROSS CONVEYORS
FEED CONVEYORS
BELT SCALES
MIXER MIXER
FEED
CONVEYOR
(REVERSIBLE)
HIXER DISCHARGE CONVEYOR
REACTOR FEED CONVEYOR
(REVERSIBLE)
114
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10.3.4.3 Reactor Aeration System
System Configuration
The compost is aerated by pressurized air introduced at
the bottom of the reactor. This air is drawn from the
mixing room, blown into the plenum underneath the reac-
tor, and then distributed through perforated 0.25-m (10-
in.) diameter PVC pipes. The pipes are arranged in a
circular pattern, concentric to the reactor wall. These
pipes are separated from the compost by a layer of 0.03-
m (1-in.) diameter gravel, which in turn is covered with a
layer of #1 stone (formerly wood chips, see discussion
below). These layers are designed to prevent the com-
post from falling into the pipes.
A maximum of 7.6 m3/s (16,000 cfm) of air are pumped
through the compost in each reactor, and divided among
five concentric aeration zones according to temperature.
The amount of air going to each zone cannot be mea-
sured. Additional air enters the headspace of the reactor
(the space between the top of the compost and the
underside of the dome) through the side vents. These
vents are permanently open to the outside and are
located above the level of the compost. The air drawn in
through these vents dilutes the odorous process air. A
positive displacement blower exhausts a total of 28 m3/s
(60,000 cfm) from each reactor through a large doughnut-
shaped duct which hangs from the dome (see discussion
below). This air is directed to the odor control system in a
separate building (see Figure 10.5). Figure 10.6 is a dia-
gram of the air handling system.
Aeration Problems
In August 1987, plant operators noticed that air was flow-
ing up through the floor drains into the pipe gallery below
the reactors, blowing leachate onto the walls and ceiling.
When the operators bored a pilot hole into the compost
bed, they discovered that the layer of wood chips,'which
Figure 10.4 Cross Section of a Fail-field Reactor
,REACTOR FEED CONVEYOR
T/DOHE EL. 295'0
AAAAAAA7\AAAAA/\AAA/\/\AA
ROTATING FEED ACCESS BRIDGE BELT CONVEYOR
AIR DISTRIBUTION PIPING
i «r b a r\e> F) an .a af a n fl
RADIAL STACKER FEED CONVEYOR
115
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was on top of the gravel base, had turned into "hardpan."
The hardpan prevented the air from flowing up into the
Figure 10.5 Ventilation Duct Taking Air from the Reactors (on
extreme right and left) to the Odor Control Building
(In the middle)
compost bed. Consequently, the air was flowing into the
leachate drain system, pressurizing it. The plant was shut
down for 2 months and ran at one half of its capacity for
an additional 4 months while the hardpan was dug out by
hand, and the wood chips were replaced with #1 stone.
The reactors have also been plagued by excessive
amounts of moist air, which condenses when the outside
air temperature drops below about 10°C (50°F). This
problem is exacerbated by the absence of any insulation
in the domes. The excessive moisture is caused by top
much cold air coming in through the side vents; cold air
doesn't remove moisture, thus, fog and ammonia build
up. At even colder temperatures, the moisture freezes on
the equipment. In January 1988, the drain line for the
doughnut-shaped exhaust duct froze in reactor 1, caus-
ing ice and water to accumulate in the duct until it was so
heavy that the aluminum straps holding it to the dome
gave way and the duct fell into the reactor. The duct was
removed and the reactor returned to operation the follow-
ing month.
The excessive moisture has also increased the corrosion
of the auger drives and motors, the conveyor system,
electrical contacts, and other electrical equipment in the
reactor buildings. In addition, the condensation of mois-
ture reduces the rate of moisture removal from the com-
post because the condensed vapor falls back onto the
material. Consequently, more amendment must be
added, reducing the detention time and increasing costs.
Figure 10.6 Air Handling System
STACK
RECIRCULATION PUMP
WET WELL
PROCESS AIR
TO UPCP
116
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Other problems are also caused by the high moisture
levels inside the reactors. Ammonia builds up to poten-
tially unsafe levels and fog reduces visibility. As a result,
before maintenance work or manual temperature read-
ings can be conducted, the reactor must be aired out for
several hours by a roof fan. Supply air to the reactor is
shut off while the reactor air is cleared. Since the ventila-
tion system does not have the capacity to process all this
air in a timely fashion, the air is vented directly to the
atmosphere. This action results in odor complaints from
neighbors. At the present time, the operators load only
one reactor per night so that the other reactor is aerated
for all but the 1 to 2 hours it takes to clear the fog and take
temperature measurements.
System Modifications
To solve moisture related problems, the city is planning to
install a rotating cover in each reactor. The cover will be
mounted on the underside of the reactor bridge a few feet
above the compost but below the side vents, so there will
be no cold air pouring in through the side vents and much
less process air to remove and treat. A similar rotating
cover has been installed successfully at a full-scale Fair-
field facility in Toronto. In Pittsburgh, the cover would
have to be accompanied by modifications in the duct-
work so that moisture-laden process air from the reactors
could be separated from the air between the rotating
cover and the dome, and sent directly to the odor control
system. The air between the cover and the dome, which
should be relatively odorless, could then be diluted with
the air drawn in through the side vents and vented
directly to the atmosphere by the roof fan. The fog would
be eliminated and the decrease in the air volume han-
dled by the odor control system would increase the resi-
dence time in the scrubbers, thereby decreasing the odor
level of the exhausted air.
Funds for installing the reactor covers were requested
under the modification/replacement provisions of the
EPA I/A grant program, but DEC and EPA did not approve
this request. In the opinion of these agencies, this situa-
tion represents a design failure rather than a technology
failure. The city intends to proceed with the upgrade
using its own funds.
70.3.4.4 Outfeed Device
The innermost auger moves the compost from the reac-
tor over a weir and into a discharge chute (see Fig-
ure 10.4). The compost then falls onto a horizontal belt
conveyor and is transferred to a common discharge con-
veyor, which takes the compost to a diverter gate. Some
compost is diverted to the recycle feed bin; the remainder
is diverted outside for long-term curing/storage.
10.3.4.5 Leachate/Condensate
The leachate from the reactors drains down through the
#1 stone and gravel to the concrete pad below, where it is
collected in drains. All leachate from the curing/storage
piles is captured and directed to the WPCP; there is no
leachate from the piles except when it rains.
10.3.5 Exterior Curing/Storage
Outside, a radial stacker conveyor with a 150-degree
swing arc deposits compost, which is then moved by a
front-end loader to the curing/storage yard, a 11,300 m2
(2.8 acre) asphalt pad. The compost is cured/stored in
unaerated piles a minimum of 30 days, although no mini-
mum time is specified in the operating permit. Since
demand for compost is low at the present time, curing/
storage times are typically much longer. The piles are not
odorous when undisturbed, but can emit strong odors if
moved.
10.3.6 tlonprocess Air Handling
Mixing room air is ventilated at the rate of 0.76 m3/s
(16,000 cfm), representing four air changes per hour.
There is a supplemental roof fan that vents to the atmo-
sphere, but it is seldom used, Instead, the air is
exhausted into the plenum beneath the reactor.and is
used as process air. When the reactors are not being
aerated, the air from the mixing room bypasses the reac-
tors and goes directly to the odor control system. The
facility's designers anticipated that the odors in the mix-
ing room would be elevated, but the odor level has not
been problematic.
10.3.7 Odor
10.3.7.1 Odor Characteristics and Treatment
Because air exhausted from the reactor was anticipated
to have a very low odor level, the facility was designed to
vent most of the reactor air directly to the atmosphere,
although a backup scrubber system was also provided.
Once the facility came on line, however, it became imme-
diately obvious that odor produced in the reactors was a
major problem. To reduce odors, sludge inputs to the
plant were severely restricted during the first year of
operation.
For each reactor, there is a single-stage packed-bed wet
scrubber in the odor control building. The original odor
scrubbing system was designed to treat hydrogen sulfide
and utilized a combination of sodium hydroxide and
sodium hypochlorite. However, these chemicals were
ineffective against ammonia and other compost-
generated odors, so they were replaced with sulfuric acid
and DeAmine, a proprietary chemical. The empty-bed air
retention time in the scrubber is about 2.3 seconds.
Before exiting, the exhaust goes through a mist elimina-
tor into a short stack that protrudes about 1.5 m (5 ft)
above the roof of the odor control building.
10.3.7.2 Odor Complaints
Odor complaints are recorded in the city engineer's
office. In addition, an Odor Report Form is included in
every homeowner's city electricity bill. In spite of the
revamped odor control system, 29 odor reports were
received in November and December of 1987. The odors
were described as being "fecal and pungent (acidic)"
and smelling like "garbage, dead animals, dead fish, and
117
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ammonia." Most complaints were multiple reports of the
same incidents (i.e., 24 reports covered only 13 inci-
dents). During February and March 1988,78 odor reports
were received for 11 major odor incidents. Although the
closest residence is 910 m (3,000 ft) away, odor com-
plaints have originated from residential neighborhoods
an average of 1,500 m (5,000 ft) away, and some com-
plaints have come from residences more than 2.6 km (1.6
miles) away.
Correlation of complaints with plant activities, time of day,
and weather show that odor incidents occur almost
exclusively between 7:30 and 12:00 p.m., and between
6:00 and 8:00 a.m., when the process air is off and the
headspace air is being vented directly to the atmosphere.
Unfortunately, because of the fog problem, this situation
occurs whenever maintenance work is required under
the dome. Maintenance activities are needed frequently
(almost daily) and, in general, cannot be scheduled in
advance. Odors also correlate with wind direction when
the wind speed is less than 2 knots. (The plant does not
record meteorological conditions, but the nearby Air
Force base and Pittsburgh airport do.) When the wind is
greater than 2 knots, odor complaints drop, presumably
because of dilution. Odors are worst in the late summer
and early fall, when local overnight inversions trap emis-
sions. Winter odors are thought to result from valley-wide
stagnant air under high pressure conditions.
To date, limited air quality monitoring has been per-
formed. In November 1987, Environment One Testing
Laboratory from Albany, New York, measured 0.3 to 1.5
ppm of ammonia in the scrubber exhaust. The operators
measured 100 to 200 ppm of ammonia under the dome.
Nearly all sludge composting processes generate some
ammonia, but the concentrations documented at Platts-
burgh are higher than those found at other facilities. Th3
system supplier believes that much of the ammonia
released in the reactors is derived from the urea added to
the paper mill waste when it is treated at the WPCP
Because paper waste is nitrogen deficient, sludge bulk-
ing occurs unless nitrogen in the form of urea is added.
As a result of the urea addition, the sludge contains
nitrogen forms that are easily ammonified.
10.3.7.3 Odor Treatment
Funds for upgrading the odor control system were
requested under the modification/replacement provi-
sions of the EPA I/A grant program. This request was
approved by DEC and EPA. The upgrade may add aero-
sol scrubbers in front of the existing scrubbers, forming a
two-stage system: sulfuric acid and DeAmine may be
used in the first stage; sodium hypochlorite and caustic
soda may be used in the second.
There are no odor control measures for the curing/
storage piles. The piles are not odorous when stationary.
However, if they are opened or moved for any reason,
they can emit strong odors.
10.3.8 Support Facilities
The maintenance facility is located in the administrative
building and consists of a two-bay, two-story garage
space. The facility is meant to serve as both a workshop
and a garage, but there is no room to work when vehicles
are inside. There is also very little space available, partic-
ularly on the ground floor, for storing heavy spare parts.
The operators expressed a desire for chemical storage
space in or near the scrubber building. The current prac-
tice of storing barrels of DeAmine on the working floor is
not desirable, since they take up work space and restrict
movement.
Additional facilities at the plant include an office, control
room, lunch room, and locker rooms. The onsite labora-
tory has a microwave total solids instrument that mea-
sures the solids and water content of the sludge and
compost in about 20 minutes. All chemical determina-
tions are done at the WPCP laboratory or an outside
laboratory.
10.4 Monitoring and Performance
10.4.1 Reactor Control Strategy
Five temperature probes, one for each aeration zone, are
exhausted in lances that enter the reactor from below and
extend about 1.5 m (5 ft) into the compost bed. They are
tied into an electronic controller that opens or closes the
motorized valves on the air supply lines to the five zones.
If the temperature rises above the set range, the valves
open. The temperature control range for the outer three
concentric zones, Zones 3 to 5, is 52° to 60°C (125° to
140°F). The range for Zone 2 is 49° to 57°C (120° to
135° F); for Zone 1 (the innermost zone), it is 49° to 54°C
(120° to 130°.F)".
The system is operated to meet DEC regulations, which
require that the compost be maintained above 55°C
(131 °F) for 3 consecutive days. To ensure that these
requirements are met, the DEC requires the facility to
measure the temperature at six locations along the path
of the compost motion (i.e., radially in the reactor). The
operators take these readings manually twice a day with
a temperature probe mounted on a pole.
Oxygen content data are used as backup to the tempera-
ture data for control purposes. The system is operated to
maintain oxygen levels in the range of 12 to 18 percent.
The same lances that hold the temperature probes can
remove gas samples and direct them to an oxygen ana-
lyzer. Analyzing one gas sample at a time, all five zones
are sampled over the space of an hour and the data are
recorded on strip charts. Maximum and minimum oxygen
contents are recorded daily. Porosity is not measured on
a regular basis. A test column used during startup estab-
lished that the proposed mix ratio would allow passage of
the proper amount of air, as long as the mix density was
less than 48.1 kg/m3 (30 Ib/cf). A similar test will be
performed whenever drastic amendment changes are
proposed.
118
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Monthly summaries of daily log sheets are prepared
manually by the operator.
10.4.2 Mass Balance and Reactor Detention Time
The mass balances for average daily quantities taken
from February 5 through April 4 are shown in Figure 10.7.
Since the facility must take all of the sludge generated at
the WPCF? there is no operator control over the detention
time. Current detention times range from 7 days in the
winter to 10 days in the summer. The design detention
time is 14 days.
Figure 10.7 Pittsburgh, New York, Mass Balance Average Daily Quantities, 2/5/88 to 4/28/88
XTS'
22
52
43
ORGANIC SOLIDS:
2 T
REACTORS
43% TS
43X TS
* DTON (WTON) X 0.9072 = DT(UT)
* TS - TOTAL SOLIDS
119
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10.4.3 Product Quality
The design values for product bulk density and product
solids content are 561 kg/m3 (35 Ib/cf) and 55 percent,
respectively. The actual value for product solids content
is 43 percent; the actual value for bulk density was
unavailable. The State requires the city to monitor metals
and polychlorinated biphenyls (RGBs) on a regular basis,
with the frequency dependent on the production rate of
compost. The city analyzes both the sludge and compost
from the reactor for solids, nitrogen, and metals on a
weekly basis. The weekly composite is created from
three grab samples taken during the week. The concen-
trations of PCBs, phosphorus, and potassium are mea-
sured periodically. Stability and phytotoxicity are not
tested. New York State standards and average product
quality values for the characteristics measured (July to
December 1987) are shown in Table 10.2. '
During the first several months of operation, the compost
was tested after curing/storage, in addition to being
tested after leaving the reactor. The testing revealed that
little or no changes occurred during curing/storage so
this testing was discontinued. The compost is classified
as a Class I product, meaning that it can be used in all
applications except where food crops are grown. New
\brk State policy prohibits the application of sludge or
sludge-derived materials to any food crops.
70.5 Operations
10.5.1 Staffing
The plant is staffed by eight individuals: one supervisor,
three operators, three maintenance personnel, and one
laborer. Contract labor includes a New York State-
certified electrician and truck drivers. Processing the
nightly load of sludge regularly takes longer than the one
shift allocated.
The plant supervisor has a B.S. in geology; 15 years
experience in wastewater treatment, including 6 years at
the Pittsburgh WPCP; and is a certified Grade 1A (high-
est) operator. He has been associated with the compost-
ing facility since the start of construction. The operators
are certified Grade 2A WPCP operators. Maintenance
personnel and laborers are not certified, but maintenance
personnel are required to be high school graduates. Staff
was trained initially by "looking over the shoulder" of the
Fairfield personnel during startup (4 to 5 weeks) and
making an extended visit to the Fairfield System in
Wilmington, Delaware. There is no continuing training
program. When the plant started up, staff rotated
between the composting facility and the WPCP (3 weeks
at the WPCP; 1 week at the composting plant). However,
this practice was deemed inefficient since it took staff 2
or 3 days to reacquaint themselves with the plant at the
start of each rotation. At the time of the site visit, the staff
at the composting plant was assigned there permanently,
although for administrative purposes they are still consid-
ered part of the WPCP staff.
10.5.2 Marketing and Distribution
The city has attempted to negotiate sales/disposal
agreements. Unfortunately, these have fallen through
either because the terms offered were not attractive to
the city or because the other party experienced
materials-handling problems. The compost has been
marketed primarily by word of mouth. The city engineer
has attempted to increase interest by mailing a series of
flyers to landscapers, nurseries, golf courses, local gov-
ernmental bodies, and people holding gravel mining per-
mits. At the time of the site visit, the city was about to
initiate a public giveaway program. DEC requires the
plant to record all Customers taking more than 23 m3 per
load (30 cy/per load) or 110 t/year (100 ton/year). In 1987,
about 3,600 t (4,000 ton) were sold to customers; total
Table 10.2 Sludge and Compost Constituents
New York State
Component Standard-
Sludge"
Compost"
•Maximum contaminant concentrations for Class 1 distribution (i.e.,
restricted only from soils growing edible crops).
•Average of weekly composites taken'in the second half of 1987.
Remarks
Total Solids— TS(%)
Volatile Solids— VS(%)
TKN'(%)
NHN'(%)
Chromium (ug/g)
Cadmium (ug/g)
Copper (ug/g)
Nlckal(ug/g)
Lead (ug/g)
Zinc (ug/g)
Mercury (ug/g)
PCBs' (ug/g)
Phosphorus (ug/g)
Potassium (ug/g)
_
—
—
—
1,000
10
1,000
200
250
2,500
10
1
—
—
17
85
4.3
0.11
17
3.1
182
17
36
159
0.36
0.81
—
—
50
82
2.5
0.59
17
1.6
167
11
26
156
0.36
<0.17
2,700
1,000
Week of 12/1/87 only
Measured 11/24-12/22 only
'Polychlorinated biphenyls.
Total Kjeldahl nitrogen — organic nitrogen plus ammonia.
•Nitrogen in the form of ammonia.
120
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revenues were $1,120 (some of the 3,600 t [4,000 tons]
removed were taken by government agencies who were
not charged). That year, the city paid $20,000 to dispose
of 9,080 t (10,000 ton) of surplus product which was
mainly used as landfill cover. Even though revenues were
expected to increase to between $5,000 and $10,000 in
1988, an additional $20,000 (for a total of $40,000) was
budgeted for the removal of surplus product.
10.5.3 Operating Costs
The total 1988 budget was $1,060,000; the Facilities Plan
and the 1988 budget are summarized in Table 10.3. Cur-
rent unit costs are:
Sawdust = $24/wt ($22/wet ton) delivered
Electricity = $0.07/kWh onpeak
$0.04/kWh offpeak
$7.11/kW demand charge
($0.02/kWh estimated in Facilities Plan)
Sulfuric acid = $0.14/L ($0.54/gal) (75 percent solution)
DeAmine = $7/L ($26/gal)
Table 10.3 Operations and Maintenance Costs
($1,000's)
Facilities
Cost Item
Labor
Power
Amendment
and
Chemicals
Sawdust
Odor Control
Chemicals
Maintenance
Miscellaneous
Water and
Sewer
Sludge
Haulingc
Excess
Product
Disposal
Costs0
TOTAL
1983
Facilities
Plan
Estimate
122
60
173
45
20
420
Plan
Estimate
in 1988
Dollars8
139
68
198
51
23
479
1988
Plant
Budget
125"
250
225
120
140
60
100"
40
1,060
Percent
Of 1988
Budget
12
24
21
12
13
6
9
4
100
•Inflated by ratio of ENR cost indices (Facilities Plan: 3900,
January 1988; 4456).
"Estimate by city.
"Not anticipated in Facilities Plan.
Operations costs have been higher than projected in all
areas. The unit costs of power have been approximately
four times greater than projected. As a result, the plant's
operating schedule was changed to take advantage of
offpeak rates; this is expected to save $60,000 per year
in electricity costs. Maintenance costs have also been
greater than expected due to corrosion problems inside
the reactor. Sawdust costs are greater because the recy-
cle has been wetter than expected.
10.6 Update
The facility was called in June 1989, to determine if there
had been any major changes to the plant or operations
since the site visit. The plant has operated continuously
since February 1988, processing all of the sludge from
theWWPT
A hardpan was discovered again in February 1989, and
had to be dug out.
The facility is in the process of enclosing the electrical
system in the reactors to alleviate the problems due to the
excessive flog.
The actual amounts of materials processed from January
1 to December 31,1988 and operating costs are shown in
Tables 10.4 and 10.5.
Table 10.4 Materials Processed
(January 1 through December 31,1988)
Amount Percent Solids Mix
Material • Processed" Average Range Ratio
Sludge 33,454"
Amendment 11,940
Recycle 45,652
Compost
Product 25,41 5C
21.2
54
48
21-23
45-64
43-55
43-55
1
1.4
2.67
"Total wton x 0.9078 = wt.
"Equal to 6,664 dt (7,319 dton).
'About 25 percent of the product was sold.
Table 10.5 Operating Costs
(January 1 through December 31,1988)
Item Amount Percent of Total
Labor
Power and Fuel
Amendment
Maintenance Materials
Chemicals
Electrical Repair Service
Water and Sewer
Other
TOTAL
Revenues
Product Disposal Costs
$ 241,444"
255,479"
246,957
97,476
81,151
22,670
35,781
50,378
$1,031,336
13,219
8,522
23
25
24
9
8
2
3
5
"Comes from WPCP budget.
"Power = $252,565; fuel = $2,914.
'Breakdown of these costs not given.
121
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Chapter 11
Portland, Oregon
11.1 Introduction
The Portland, Oregon, in-vessel composting facility is
owned by the City of Portland. The facility, designed by
Taulman Composting Systems (Taulman), has two trains;
each consists of three cylindrical vertical reactors, two
"first-step" reactors, and a "second-step" reactor. It is
the first Taulman system in the United States, and design
improvements and modifications resulting from the expe-
rience at Portland have been incorporated into subse-
quent projects. The design capacity of the facility is 54
dt/day (60 dton/day) of sludge. At the time of the site visit,
the plant was processing 18 dt/day (20 dton/day) of
sludge.
The composting facility is situated in an urban area in the
Columbia River valley on a flat, 20,200 m2 (5-acre) site it
shares with the wastewater treatment plant (WWTP). The
total building area (including vehicular access) is 3,035
m2 (0.75 acres). The plant is bounded by the river on the
north, a boulevard on the south, and manufacturing
plants on the east and west.
The site visit took place on July 12 and 13,1988. Except
as indicated, the equipment, processes, data, and issues
described in this case study are representative of that
time only.
11.2 History of the Plant
11.2.1 Procurement and Construction
In response to Portland's RFPthe city received proposals
for a variety of sludge disposal systems. In May 1982, the
city judged that Taulman's in-vessel composting proposal
was technically and economically superior to the other
proposals. In June 1982, a consulting engineer con-
firmed the city's analysis. The city's primary reasons for
choosing an in-vessel system were the relatively small
size of the site (the plant had to be built at the existing
WWTP) and odor control. Taulman's guaranteed pur-
chase of the compost for 20 years (four 5-year agree-
ments) was another important factor.
The city awarded a turn-key contract to Taulman, who
designed and built the facility. The following performance
criteria were defined in the contract: product characteris-
tics (color, texture, odor, pH, solids content, germination,
and pathogen content); mix ratio; electrical usage; and
staff requirements for normal operations.
The performance test, however, was not well-defined and
became a matter of negotiation between Taulman and
the city. They agreed that during the performance test,
the city would operate only one train for 3 months and
cannibalize the other train for the parts necessary to
keep the first train operational. For the most part, the city
was satisfied with the turn-key approach but, in retro-
spect, the city would write tighter specifications, particu-
larly regarding the design and equipment.
11.2.2 Capital Costs
In their bid, Taulman proposed a capital cost of
$10,900,000. After negotiating for additional items, the
city issued the contract in October 1982 for $11,397,000.
These construction costs were paid by 20-year bonds
issued by the city.
In addition, there were four change orders in which the
costs were split between the city and Taulman. The city's
portion of these costs were:
Redesign of amendment-receiving facility to
enable it to utilize shredded newspaper
in the future $ 48,000
Upgrading exterior and interior finishes 38,000
Dust control at amendment-receiving facility 63,000
Miscellaneous 17,000
Total
186,000
Taulman spent $55,000 for the dust control system at the
amendment-receiving facility and an unknown amount
for changes to the outfeed device and conveyor system.
Taulman spent an additional $150,000 for warranty repair
or modifications to equipment to gain plant acceptance.
11.2.3 Operating History and Current Status
Facility construction began in February 1983; it began
operating in September 1984. In March 1985, the first
compost produced by the facility was sold to North Amer-
ican Soils (NAS), a wholly-owned subsidiary of Taulman.
123
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The actual capacity of the facility is less than design
capacity, primarily due to increased sawdust usage and
mechanical maintenance/repair problems. As of July
1988, only two first-step reactors (out of four) normally
operated at any one time. Each reactor processed about
9.1 dt/day (10 dton/day) sludge. At the time, the WWTP
was producing slightly more than 18 dt/day (20 dton/day)
sludge; the excess was stored in the adjacent lagoon until
it became full in July 1988. In July-August 1988, the plant
was shut down for 4 weeks to remove sludge from the
lagoon (under a temporary land application permit) and to
perform some maintenance tasks.
The operating goal is to maintain the current loading rate
of 9.1 dton/day (10 dton/day) per reactor and expand the
operating capacity to 27 dton/day (30 dton/day) by oper-
ating another first-step reactor. This capacity is expected
to be adequate for sludge production in the short-term
future.
77.3 Description of the Plant
11.3.1 Systems Overview
The plant has two independent trains, each with three
reactors; two reactors operate in parallel (first-step) and
are followed by a reactor in series (second-step). The
sludge, sawdust, and recycle requirements are computer
controlled; the speed of their respective conveyor belts is
set by the computer to achieve the proper mix. For each
train, the mix is transported on a vertical drag chain up to
the accumulator (a reversible screw) and then down into
the mixer. Once mixed, the material can be conveyed to
either first-step reactor. The compost is removed from the
first-step reactors by an outfeed device; some of it to be
used as recycle, the rest to be conveyed to the second-
step reactor. After removal from this reactor, the compost
is cured/stored outside. Figure 11.1 illustrates materials
flow for both process trains. An overview of the facilities
for one train is shown in Figure 11.2.
Figure 11.1 Portland, Oregon, Composting Process Train
SAVDUST
RECEIVING
SAWDUST
STORAGE
124
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Figure 11.2 Portland, Oregon, Sludge Composting
Process Train
Sludge Storage
Accumulator
Vertical Conveyor
Sawdust Silo / First-Step Reactors / Cross Conveyor
Process Building Second-Step Reactor
While in a reactor, the compost is aerated by air supply
piping located in the bottom of the reactor. The air is
exhausted from the top of the reactor and directed to the
WWTf? where it is bubbled up through the primary efflu-
ent collection channel.
There are no sprinkler systems for fire protection in the
compost plant.
11.3.2 Feed and Mix Characteristics
77.3.2.7 Sludge
In the feed process, anaerobically digested primary
sludge is blended with anaerobically digested waste-
activated sludge (WAS) that has been stored in a lagoon.
The volatile solids of the sludge range from 35 to 50
percent. Next, the sludge is dewatered in a belt filter
press to achieve a total solids content ranging from 23 to
28 percent, with an average of 25 percent. The bulk
weight ranges from 800 to 1,060 kg/m3 (55 to 66 Ib/cf).
The dewatered sludge is stored in rectangular steel stor-
age bins, one per train. Each bin has a working capacity
of 76m3 (2,692 cf).
11.3.2.2 Amendment
Sawdust is used as an amendment. The total solids
content of the sawdust varies from 60 to 95 percent,
averaging 86 percent between June 15 and 30,1988. The
bulk weight ranges from 128 to 224 kg/m3 (8 to 14 Ib/cf).
The city awards the sawdust contract based on the low-
est price for the sawdust needed to create a 37 percent
solids mixture when mixed with sludge containing 25
percent solids. The advantage of using this criteria is that
the city pays less for sawdust with a higher moisture
content (50 to 60 percent solids) than it does for sawdust
with a lower moisture content (greater than 60 percent
solids). The major drawback of this approach, however, is
that it does not encourage minimization of the sawdust
volume. Excess sawdust aggravates materials-handling
problems and shortens the plug-flow detention time in the
reactors.
Originally, the sawdust specification allowed up to 15
percent hardwoods, a maximum particle size of 0.006 m
(1/4 in.), and a maximum moisture content of 50 percent.
The sawdust may also contain up to 50 percent bark by
volume. However, the operators could not distinguish
hardwood sawdust from softwood sawdust, so the hard-
wood specification was deleted. In addition, the maxi-
mum particle size was reduced to 0.002 m (1/16 in.) to
eliminate wood shavings that did not mix well.
There is one sawdust storage silo for each train, from
which sawdust is removed by a center pivot outfeed
screw. Each silo holds 66 m3 (23,556 cf). Almost every
year over the December holidays, short-term closures of
lumber mills and the consequent cessation of sawdust
production shut down the plant for 2 weeks. Additional
amendment storage space would help carry the plant
through these periods.
Originally, the sawdust operation created a great deal of
airborne dust, which was a safety hazard. To remedy the
situation, the sawdust delivery system was retrofitted
with a ventilation system that captures the sawdust at the
delivery hopper, conveyor belt transfer points, and stor-
age silo outfeeds; then directs the dust to a baghouse.
(See Figure 11.3.)
Figure 11.3 Sawdust Receiving Bin and Sawdust Silo
with Oust Control Ventilation System
125
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11.3.2.3 Recycle
The recycle, which has a dry solids content of 38 to 40
percent, comes directly from either first-step reactor.
11.3.2.4 Mix
The design mix ratio is 1/0.4/0.6 sludge to sawdust to
recycle (by volume), a ratio that assumes a sludge solids
content of 25 percent, an amendment solids content of 60
percent, and a recycle solids content of at least 40 per-
cent. Solids content of the mix was not given at design,
other than as specified by the design mix ratio and solids
content of the mix ingredients. The actual mix ratio during
the site visit was 1/1.6/0.6 sludge to sawdust to recycle
(by volume). The solids content of the sludge and the
recycle were very close to the design values, but the
sawdust used actually contained more solids than antici-
pated. Nevertheless, the actual mix uses about four times
as much sawdust as the design primarily due to
materials-handling problems, especially "oozing" (see
Section 11.3.4.4).
The operators report that the optimal solids content of the
mix is 35 to 50 percent. The current operational goal is 37
percent, which the plant has been able to meet
consistently.
11.3.3 Materials Handling
Each train has its own highly automated materials han-
dling system (see Figure 11.4). Normal operations are
preprogrammed and the various conveying devices are
electrically interlocked. If necessary, however, the opera-
tors can run the system manually.
The feed materials are conveyed on a trough belt con-
veyor to a vertical drag chain that moves the material up
about 18 m (60 ft) into the accumulator, which is located
on the roof of the mixer shed. The accumulator accepts
materials from the drag chain conveyor and directs them,
via a feed funnel, into the mixer.
Both the accumulator and the vertical drag chains have
been problematic. The accumulator originally used
double screw augers, but they wore out in less than 2
years and were unable to move material fast enough,
thereby plugging the feed funnel. To correct these prob-
lems, the accumulator was modified to a one-screw sys-
tem, which appears to be working reliably. Bearing
failures caused by dust from the mix, however, are a
continuing problem.
The vertical drag chains have had a number of problems
including erosion and bent chain flights (due to material
jamming, particularly on the curves). To correct these
problems, the housings were rebuilt with abrasion-
resistant steel, half of the flights were removed, and the
remaining flights were trimmed so that they now block
only 25 percent of the housing cross section (the original
flights covered it almost completely). Every 4.6 m (15 ft)
along the chain, a reinforced steel scraper flight slightly
larger than the original flights has been installed. Since
these modifications were made, few problems have
occurred.
Nonetheless, the drag chains are wearing out faster than
expected. The chains and flights must be replaced every
year and the housings will probably have to be replaced
every 3 years.
There is one Littleford plow-type mixer per train, both of
which have had only minor problems. However, their
location complicates operations because the operators,
who depend heavily on mix appearance for quality con-
trol, are situated in the ground-level control room where
they cannot see the accumulators or the mixers. The
accumulators and mixers are also difficult to service
since there are no elevators, and the operators have to
climb a steel stair tower (sometimes covered with ice in
winter) to reach them.
11.3.4 Reactors
11.3.4.1 Reactor Feed System
The mix is conveyed to the reactors on enclosed belt
conveyors. (See Figure 11.5.) The feed materials enter
either first-step reactor from the belts through "slingers",
funnel-shaped devices that distribute the mix. In order to
evenly distribute the mix, the slingers have been modi-
fied to spin in opposite directions for equal lengths of
time. The slingers work well mechanically.
Repairing the slinger, however, is very difficult. To repair
the top of the slinger or unjam the feed conveyor, the
operator must access the top of the reactor on the outside
of the building by climbing the stair tower to a narrow
catwalk. From the catwalk, which is about 0.6 m (2 ft)
above the top of the reactor, the operator must reach
down into the top of the reactor to the slinger, an awkward
position for the operator. In addition, there is no working
space on the catwalk and the slinger cannot be removed
from the top because there is no hoist. The operator is
usually forced to repair the slinger by accessing the bot-
tom of the slinger from inside the reactor dome. To do this,
the operator must approach the slinger by walking across
the surface of the composting mass.
126
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Figure AV4 Portland, Oregon, Materials Handling System
127
-------
The feed system is configured so that only one first-step
reactor per train can be fed at a time. To make room for
feed material in the reactor, some of the existing material
in the reactor must first be removed. A typical loading
cycle starts with 3 hours of unloading compost from the
reactor and moving it to the second-step reactor. This is
followed by 4 hours of unloading material from the first
reactor and immediately recycling it as part of the fresh
mix for the same reactor.
Currently, there is no way to move material from one feed
train to the other until it is discharged from the first reac-
tor (i.e., the mixer on one train cannot service the reactors
on the other train). However, material from any of the first-
step reactors can be moved to either second-step reactor.
To give the facility more flexibility, adding a conveyor to
connect the two trains at a location downstream of the
mixers is being contemplated.
Figure 115 Side View of Vertical Drag Conveyor, Mixer
Building, and Enclosed Belt Conveyor
Mixer Building
Enclosed Belt
Conveyor
11.3.4.2 Configuration
There are two cylindrical vertical first-step reactors per
train, operated in parallel. Each one is 8.8 m (29 ft) tall
and 13.0 m (42.5) ft in diameter with an average compost
depth of 7.0 m (23 ft). The first-step reactors are con-
structed of steel and have concrete bases (see Fig-
ure 11.6).
There Is one second-step reactor per train. Its construc-
tion is similar to the first reactors and it has the same
height, 8.8 m (29 ft), but it has a larger diameter, 16 m (59
ft). Each cure reactor has a volume of 1,800 m3 (63,200
cf). The second reactors are operated like the first
reactors. After leaving the second-step reactor, the com-
post contains about 40 to 42 percent solids, and may
contain up to 45 percent solids if the mix contains 40
percent solids.
11.3.4.3 Reactor Aeration System
Each of the six reactors is aerated by a positive displace-
ment blower rated at 0.66 m3/s (1,400 cfm) at 69,000 Pa
(10 psi), 75,000 W (100 hp) variable speed. In addition,
two auxiliary fixed speed blowers (one per train), each
0.33 m3/s (700 cfm) at 69,000 Pa (10 psi); and 30,000 W
(40 hp), can supply any of the four first-step reactors. The
supply piping consists of 0.15 m (6-in) diameter PVC
headers feeding 0.03 m (1.25-in) perforated PVC tubing
laid beneath a 0.03 m (12-in) layer of 0.02 to 0.04 m (0.75-
to 1.5-in.) washed river gravel. The piping is divided into
four independent quadrants that are controlled by a pro-
grammable sequencer. There are no dividing walls in the
gravel bed to prevent short-circuiting between the quad-
rants. The blowers for the first-step reactors usually run at
90 percent capacity for 20 hours per day (there is no
aeration during reactor loading). The sequence for each
quadrant in the second-step reactors is 2 to 3 minutes
"on" followed by 6 minutes "off."
One centrifugal exhaust blower per reactor (six total),
rated at 1.1 kg m3/s (2,330 cfm) at 26,000 Pa (3.76 psi)
4,500 W (60 hp) removes the air from the headspace of
the reactors (process air). The ability of the exhaust sys-
tems to remove the air blown into the reactors is satisfac-
tory under normal circumstances. If all the blowers,
including the auxiliary units, are operating at maximum
capacity, however, a slight pressure builds up inside the
first-step reactors. Each adjustable reactor is fitted with
an adjustable pressure/vacuum relief valve that opens
automatically if the pressure exceeds 62,000 Pa (9 psi)
(factory setting).
11.3.4.4 Outfeed Device
The outfeed device for all reactors is a center pivot screw
that pivots around the center of the reactor much like the
hand of a clock. A cam operated brake assembly at the
end of the screw grips an I-beam rail, which runs around
the circumference of the reactor (see Figure 11.7). The
clamping assembly rotates the screw in a circular path
around the bottom of the reactor. To remove the compost
from the reactor, the clamp must be secured. Once it is
secured, a variable speed hydraulic piston powered by a
hydraulic motor located in the center pivot pushes the
outfeed screw into the compost. The screw then rotates
on its axis, feeding the compost through a discharge gate
located in the center of the reactor, where it drops onto a
reversible conveyor belt. From there, the material from
the first-step reactors goes up the vertical drag chain to
the accumulator where it is either used as recycle and
directed to a first-step reactor via a mixer, or directed to
the second-step reactor. Material from the second-step
reactor is conveyed to a drag chain that takes the com-
post outside the building to a waiting dump truck.
128
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The reactor outfeed device has experienced four prob-
lems, the first being excessive wear on the screw.
Although not hard-surfaced, the screws were expected to
last 7 years. Nevertheless, after 14 months of operation,
during an annual inspection, screw wear was noted as
"extreme." Four of the six screws had to be replaced and
all of the screws were subsequently hard-surfaced. Facil-
ity personnel believe the continued presence of ash (from
the Mt. St. Helens eruption) in the sludge contributes to
the excessive wear. This is supported by the observation
that wear on the end of the screws from the midpoint to
the center of the reactor is greater than wear on the outer
end.
Figure 11.6 First Reactor from Portland Train
TOODO
CONTROL
DISCHARGE
SCREWS \
FEED
CONVEYOR
DISTRIBUTOR
AIRFLOW
DIRECTION
COMPOST TO
DISCHARGE CONVEYOR
AERATION PIPING
129
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The second problem with the reactor outfeed device
Involved the clamp assembly. This mechanism would not
hold the I-beam rail securely enough to enable the piston
to rotate the screw into the carpet. The clamp was modi-
fled Into a mechanically operated cam that grips the rail
more firmly. This modification did not sufficiently improve
the reliability of the unit, however, so at the time of the site
visit, an engineering firm was preparing design improve-
ments. Access to this brake mechanism is very restricted
— a 0.91-m by 1.2-m (3-ft by 4-ft) space around the
outside edge of the reactors. A third, though less serious,
problem is that the filters in the hydraulic power units
must be replaced more often than anticipated because of
the dirty work environment.
The fourth problem is that the outfeed screw must move
regularly, even though it was designed to operate only
when outfeeding the compost. If it does not move regu-
larly, the screw becomes trapped in the composting
mass. If the compost is too dry, it forms a hard, tunnel-like
structure that is difficult to penetrate. If too wet, the mass
will collapse around the screw (so-called "oozing" com-
post), making the screw hard to turn. Oozing compost
can also move beneath the annular ring of the reactor
(see Figure 11.7), blocking the path of the drive mecha-
nism. To avoid this, a plow has been welded onto the
braking mechanism to clear material from between the
rail and the annular ring.
11.3.4.5 Leachate/Condensate
In each reactor, four leachate drains discharge the lea-
chate to the WWTP These drains are closed by valves to
prevent short-circuiting of air and must be manually
opened each day. The city is contemplating replacing
these valves with automatic valves. Typical leachate vol-
umes are 0 to 7.6 L/day (0 to 2 gal per day) per first-step
reactor. There is usually no leachate from the second-
step reactors.
11.3.5 Exterior Curing/Storage
North American Soils (NAS) assumes responsibility for
handling the material after it leaves the reactors. j
The compost is cured/stored short-term on an open
asphalt pad, then moved to unpaved ground for long-
term storage. The runoff from the curing/storage areas
drains into the ground. A new, larger pad is being planned
that will capture the runoff and return it to the treatment
plant. Once the new pad is constructed, all of the com-
post will be cured/stored on it. At the time of the site visit,
more than half of the compost was cured/stored directly
on the ground.
The curing/storage piles are not controlled and there are
no State regulations on minimum curing/storage times.
NAS sells the oldest material first. Generally they do not
Figure 11.7 Portland Reactor Outfeed Device
REACTOR
WALL
ANNULAR
/RING WALL
SCREW
ADVANCIN
MECHANISM
CENTER PIVOT
HOUSING
- ACCESS
HATCH
AERATION PIPING
''CRUSHED STONE
COMPOST DISCHARGE
TO CONVEYORS BELOW
130
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screen the compost,-but they do mill it if the particle size
is too large for the customer's intended use.
11.3.6 Nonprocess Air Handling
The mixing room, which is actually just an enclosure
around the mixer, is ventilated by a small wall fan. The
dewatering building is reportedly odor-free.
The process building air is vented directly to the atmo-
sphere by a number of small ventilating fans in the roof
and walls. (The overall exchange rate is not known.) To
increase airflow, operators commonly leave the doors of
the building open, even in winter.
The operators' control room/office is located in the pro-
cess building. Originally, this room did not have an air
conditioner, but the operators installed one. The ventila-
tion in this room is provided by fans that pull air out of the
office and exhaust it to the atmosphere. As a conse-
quence, the ammonia is pulled into the control room from
the rest of the building. The operators plan to modify the
ventilation system so that the fresh air is brought in from
the outside.
11.3.7 Odor Control
11.3.7.1 Characterization
The Columbia River forms the north boundary of the
plant site and North Columbia Boulevard forms the south
boundary. On the other side of the boulevard, there are
homes on sloping land approximately 305 m (1,000 ft)
from the plant and 150 m (500 ft) from the compost
curing/storage yard. The part of the yard nearest these
neighbors is reserved for the oldest and least odorous
compost. There are no neighbors on the river side of the
plant. Other industries, including a rendering plant and a
box factory, are located to the east and west of the site.
As of March 1,1989, there had been only one citizen odor
complaint, which was made when fresh compost was
cured/stored near North Columbia Boulevard. The fact
that the sludge is anaerobically digested may be an
important factor behind the lack of odors associated with
this plant.
Ammonia has been problematic in the process building,
where ammonia concentrations up to 38 ppm have been
measured. Although the level is below the EPA 8-hour
exposure limit of 25 ppm, the operators find the odor
objectionable. The source of the ammonia appears to be
freshly removed material as it is discharged from the first-
step reactors by the outfeed devices. There have been no
complaints of ammonia odors offsite.
11.3.7.2 Treatment
Process air is bubbled into the collection channel for the
primary clarifiers at the WWTP and then released at
ground level. The channel is covered except for open
grates at a slide gate installation. Near the grates, the
compost odor could be detected during the site visit, but it
was not offensive. The treatment system is not
monitored.
11.3.8 Support Facilities
The operators' control room is in the process building,
which is located on the first floor of the reactor complex.
The supervisor's office is in the WWTP building as are
restrooms, showers, lockers, lunch room, and laboratory.
The maintenance facilities are also shared with the
WWTP
11.4 Monitoring and Performance
11.4.1 Reactor Control Strategy
The composting system is operated to meet EPA time
and temperature criteria for pathogen reduction.
The operators use temperature probes (metal pipes con-
taining thermistors suspended in oil) to measure the tem-
perature manually in each reactor at nine vertical points.
They also measure the offgas temperature. The tempera-
tures are recorded on strip charts and noted in daily logs.
The temperature data are not used for process control or
reported to the State.
In the first-step reactors, the temperatures are typically
higher in the middle and bottom of the compost mass
than on the top. In the second reactors, the temperatures
are generally highest in the top and lowest in the bottom.
The aeration system for the reactors was designed for
oxygen feedback control based on the oxygen necessary
to maintain aerobic conditions to produce a stable fin-
ished product. The amount of oxygen supplied by the
oxygen control system was not enough to dry the com-
post to the desired level, given the moisture content of
the mix. Consequently, the oxygen control system was
abandoned. Now the compost in the first-step reactors is
aerated continuously at 90 to 95 percent blower capacity
to produce a drier product.
Operators have not found good correlation between the
total solids measured with a heat lamp and those mea-
sured in the lab with a drying oven. Consequently, they do
not use the heat lamp for "real-time" mix control. Instead,
the quality of the mix during loading is determined "by
eye." A sample of the mix from every batch is also col-
lected and analyzed with a drying oven for solids
content.
11.4.2 Mass Balance and Reactor Detention Time
A materials flow diagram, constructed from available
plant records, is shown in Figure 11.8. There was not
enough data available to construct a mass balance. The
system was designed for a 30-day plug-flow detention
time that encompasses the time in both first- and second-
step reactors. The estimated plug-flow detention time of
the system was about 19 days at the time of the site visit;
the split between time in the first- and second-step reac-
tors was not known.
131
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11.4.3 Product Quality
The compost from the Portland facility is meeting the EPA
pathogen requirements. When the compost emerges
from a first-step reactor, its solids content is 38 to 40
percent. Material discharged from the second-step reac-
tor has a solids content of 40 to 42 percent (with a maxi-
mum of 45 percent) and a bulk weight of about 590 kg/m3
(37lb/cf).
The compost can be used in accordance with the follow-
ing State regulations for sludge:
• No sludge may be applied to fruits or vegetables that
are to be eaten raw.
• Sludge applications shall not exceed the nitrogen
requirements of the crop. Metals loadings shall be
limited to those shown in Table 11.1.
• Crops grown for direct human consumption shall not
be planted for 18 months following sludge application.
(This restriction is waived if the edible parts of the crop
are not in contact with the soil or if the crop
is processed before consumption so that pathpgen
contamination is not a problem.) Grazing animals are
not allowed on land treated with sludge for 30 to 180
days following application.
• There are no restrictions on growing food chain crops
if the crop is not grown for direct human consumption,
if the part of the crop eaten by animals is not in direct
contact with the sludge, and if the concentrations of
metals in the sludge are less than those shown in
Table 11.1.
Table 11.1 Maximum Heavy Metal Loading Recommended for
Sludge Applications to Privately Owned Farmland
Maximum Metal Addition (kg/haa) with a
Soil Cation Exchange Capacity (meq/100 g)
Metal
Pb
Zn
Cu
Ni
Cd"
Less than 5
500
250
125
50
5
5-15
1,000
500
250
100
10
Greater than 15
2,000
1,000
500
200
20
•ha = hectare. Kg/ha is roughly equivalent to Ib/acre.
"The maximum application of Cadmium (Cd) for soils with pH values
of 6.5 or less is 4.5 Ib/acre of the cation exchange capacity.
Figure 11.8 Portland, Oregon, Materials Flow Average Daily Quantities, December 1987 to May 1988
SLUDGE: 26 Of, 26% TS*. 141 CY°
—*— SAWDUST: 23 DT, 78% TS, 228 CY
38.7% TS*
FIRST REACTOR
SECOND REACTOR
COMPOST:
RECYCLE: 41.0% TS*
* BASED ON DATA FROM APRIL 1988 TO JUNE 1988.
NOTE: PLANT STAFF REPORT THAT THE TYPICAL MIX RATIO FOR THIS PERIOD
WAS 1/16/0.6 BY VOLUME.
97.4 WTON, 43.6% TS*
" DTON (WTON) X 0.9072 = DT(WT)
* TS = TOTAL SOLIDS
c CY X .765 = M3
132
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The compost may be used without restriction on indoor
and outdoor ornamental plants, shrubs, trees, and grass.
Typical metals concentrations for Portland's sludge and
compost are reported in Table 11.2.
Table 11.2 Product Quality
Oregon State
Standard for Belt Press Cake
General (average of 3 Finished Compost
Application to composite (average of 3 grab
Constituent (unit) Agricultural Land samples) samples)
Zinc (mg/kg)
Lead (mg/kg)
Copper (mg/kg)
Nickel (mg/kg)
Cadmium
(mg/kg)
Chromium
(mg/kg)
Total Nitrogen
(%)
Total Phosporus
(%)
Potassium (%)
2,000
1,000
800
100
25
—
—
—
—
2,073
582
843
175
38
409
3.83
1.95
0.20
1,151
319
427
102
22
244
1.85
1.45
0.17
"Sampling conducted on September 17-28,1987; December 15-28,
April 5-18,1988.
11.5 Operations
11.5.1 Staffing
The plant is staffed by two operators per shift for two
shifts per day, 7 days per week. The lead compost opera-
tor on each shift has 2 to 3 years of experience and an
associate's (2-year) degree in wastewater technology.
Most assistant operators also have an associate's
degree, but do not have experience in wastewater treat-
ment. The city reportedly pays well and has no trouble
getting qualified operators.
Initially, the equipment suppliers gave the operators sev-
eral hours of classroom training followed by hands-on
training. Continuing training, which covers both process
and equipment operation, is organized in house and
delivered by the supervisor. The current supervisor is a
Grade IV (highest level) certified treatment plant operator
with 11 years experience. He has associate's degrees in
wastewater treatment technology and waterworks
technology.
The compost facility supervisor is a full-time position.
The operating staff rotates through the composting plant.
Because relatively little process control is being exer-
cised, short tours by the operators do not create prob-
lems, despite the fact that the mixture quality is
dependent on the operator's judgment. The maintenance
budget anticipates running one shift 5 days per week with
three persons. Maintenance personnel are supplied by
the central WWTP maintenance pool. The plant supervi-
sors feel that the present size of the maintenance staff is
adequate.
The rotating maintenance workers from the pool of work-
ers at the WWTP have no comprehensive training in
composting system maintenance. In addition, the mainte-
nance department's responses to problems at the com-
post plant have been lax, due to the unavailability of parts
and the department's assumption that there were redun-
dancies in the systems. The compost plant, however, has
little redundancy, and there have been times when the
plant has been shut down, waiting for maintenance per-
sonnel. This problem has recently been recognized by
the WWTP management, who are revising the mainte-
nance procedures and developing a preventive mainte-
nance program to forestall plant shutdowns. In addition,
the plant is trying to streamline parts procurement
procedures.
11.5.2 Marketing and Distribution
Although the compost meets State standards, the prod-
uct is relatively wet and the market has been slow to
develop. The primary customers have been golf courses,
landscapers, nurseries, and public works departments,
who use the compost on highway median strips. The
nursery industry is a growing market, but has been slow
to develop. Currently, nurseries are conducting long-term
compost testing before accepting the compost as a sub-
stitute for peat in their potting mixes.
Until June 1988, NAS bought compost for $11/t ($10/ton).
This price has been renegotiated to $39/dt ($35/dton) of
sludge processed under a joint marketing agreement
between the city and NAS. Under the new agreement,
which specifies profit sharing rather than revenue shar-
ing, the city will pay for the advertising. Previously, NAS
was committed to buying everything produced by the
system, and product quality was not an important issue to
the city. Now it is in the city's as well as NAS's interest to
maximize product marketability. Spring 1988 was the first
time NAS sold more than the plant produced during a
given period.
11.5.3 Operating Costs
Operation and maintenance costs for 1988 were esti-
mated by Taulman at the time of the 1982 proposal. The
estimated 1988 costs compared to actual 1988 budget
amounts are shown in Table 11.3. The table does not
include costs for replacement parts and materials and the
supervisor^ salary. The costs were higher than expected
in the following categories: operations (due to higher
labor requirements), sawdust, and maintenance labor.
The price for sawdust varies according to its quality (bulk
weight and moisture content), ranging from $3.4 to $4.97
m3 ($19 to $28 per 200 cf), with an average price of $3.77
m3 ($21/200 cf). Recent competition among sawdust
suppliers has driven the price down dramatically from
1985 levels, which exceeded $39/m3 ($30/cy). Even
though these prices dropped, total amendment costs
were, still higher than expected because more sawdust
was used than anticipated.
133
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Table 11.3 Operation and Maintenance Costs (Major Items)
Annual Cost
1982 Projection of
1988 Costs
1988 Budget ($)
Electricity
Sawdust
Laborand
Miscellaneous Materials
110,000*
197,000
234,000
541,000
179,000
335,000
331,000"
260,000"
1,105,000
Basis of Estimate
(dt/day of sludge)'
46
30
•Operations.
'Maintenance.
«d/tonx.9072 = dt
17.6 Update
In March 1989, the survey team received the following
information by phone from the facility.
Operations have changed substantially since the site
visit The lagoon system that previously stored waste
activated sludge (WAS) before composting has been
bypassed. In addition, the dewatering system now
receives digested primary and WAS (50/50 mixture)
directly from their respective anaerobic digesters. Also,
the tmperatures are higher in the first-step reactors now
(typically over 70°C for 6 days), presumably because the
volatile solids content of the sludge is higher.
The city is running the reactors at a lower throughput rate
(10 dt/day [11 dton/day] sludge) than design (15 dt/day
[16.5 dton/day]). The purpose of this change was to pro-
mote drying and improve product quality (fewer ammonia
odors, cooler, more friable). Based on the volumetric mix
ratio of 1/1.6/0.6, the current plug-flow detection time is
estimated to be 14 days in the first-step reactors.
The combination of higher temperatures and longer
detention times (aeration has not changed) has
increased the drying capability of the system. The mix is
37 percent total solids, as before, but the discharge solids
from the first-step reactors are about 45 percent total
solids (compared to 38 to 40 percent previously). The
second-step reactor discharge is 50 to 55 percent total
solids (compared to 40 to 42 percent previously). These
solids contents represent a substantial improvement over
performance at the time of the site visit. To further pro-
mote drying and improve product quality, MAS is install-
ing an aeration system and a cover for the curing/storage
area. NAS is also installing a bagging operation.
Numerous minor mechanical problems continue to
plague the plant and reduce its capacity. Most of these
problems are related to parts and equipment wearing out
faster than expected. For example, the city now expects
outfeed screws to last 18 months, drag chain flights and
chains to last 1 year, and drag chain housings to last 3
years.
A number of measures, in addition to those discussed in
Section 11.5.1, are being taken to reduce down time. A
new computerized maintenance management system is
being implemented to help the maintenance staff track
equipment performance, anticipate problems, and plan
preventive maintenance activities. A new spare parts
storage building is being built (which serves the entire
WWTP complex), and the compost supervisor is steadily
acquiring the needed stockpile of spare parts.
When called in June 1989, the Portland facility had noth-
ing to add to the information they gave in March 1989.
134
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Chapter 12
Sarasota, Florida
12.11ntroduction
The Sarasota, Florida, in-vessel composting facility is
owned by the city and managed by the Water Utilities
Department. The system uses two rectangular vertical
reactors supplied by Purac Engineering, Inc., in series.
The design capacity of the facility is 5.67 dt/day (6.25
dton/day) based on a 7 day per week operation.
The facility is located at the existing wastewater treat-
ment plant (WWTP), which is situated in the midst of a
commercial neighborhood. Businesses are built up to the
fenceline all around the facility, with no buffer zone and
no room for the compost facility to expand. The compost-
ing facility site area is 3,500 m2 (0.86 acres).
The site visit took place on June 29 and 30,1988. Except
as indicated, the equipment, processes, data, and issues
described in this case study are representative of that
period only. The plant supervisor feels that the only per-
iod in which the operations were stable enough to be
considered "normal" were in May and June of 1988.
Thus, much of the data presented below are based on
short-term averages.
12.2 History of the Plant
12.2.1 Procurement and Construction
A1982 sludge management study, conducted for the City
of Sarasota by Smith and Gillespie Engineers, Inc., rec-
ommended composting as a more energy-efficient
method of managing sludge than hauling liquid sludge or
pelletizing. Of the composting processes, Sarasota
chose in-vessel composting because the city wanted to
locate the plant at the existing WWTP and in-vessel
facilities require a relatively small area. A turn-key project
was not considered because the city wanted to be
involved throughout the process and be totally familiar
with the plant.
The city hired Smith and Gillespie Engineers to develop
the prequalification design criteria, plans, and specifica-
tions. The design specifications were based on a
Tauiman-Weiss (Taulman) system, and included an "or
equal" clause. To qualify, the contractors had to have
proven experience working with vertical plug-flow reac-
tors, composting sewage sludge, and handling compost
using an outfeed screw device. In addition, the system
had to contain three outfeed devices in a two-stage sys-
tem designed for flexibility, meeting space limitations,
and even distribution of compost over the reactor sur-
face. Estimates for motor loads with kilowatts per hour
consumption had to be included in the specifications.
In the conceptual plan, odor control was not an issue.
During the design phase, however, an ozone system was
added as a contingency measure. The ozone system
would control anaerobic odors occurring due to blower
failures or other unusual events.
The city issued an RFP for wastewater treatment plant
improvements, of which composting was 50 percent of
the estimated cost. Gulf Constructors, Inc. was awarded
the project; Purac was the compost system subcontrac-
tor. Several design changes occurred during the
submittal/construction process that affected the project
costs. These changes were a result of different interpreta-
tions of the plans and specifications by the engineer and
the system supplier. The issue is still being disputed.
12.2.2 Capital Costs
An EPA Innovative Alternative (I/A) Grant provided 75
percent of the funds for the project; the city funded the
remainder. The costs contained in the grant application
were estimated at $5.066 million, including the costs of
dewatering facilities.
Actual construction costs were $4.1 million, including the
dewatering system, which could not be separated from
the total capital costs. The costs related to change orders
totaled $86,000.
An odor control retrofit project, budgeted at $200,000, is
planned to replace the ozone system.
12.2.3 Operating History and Current Status
The facility began operating in September 1987.
Although it has been running continuously since then,
the first year was primarily "trial" operations. During this
period, the sludge flow rate, product quality, and opera-
tions were inconsistent due to various startup problems.
Also during this period, the contractor performed war-
ranty work, the computerized control system was
installed and debugged, and a variety of amendment
sawdusts were tried.
135
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At the time of the site visit, the facility was processing all
available sludge, but it was not running at full capacity
(5.7 dt/day [6.25 dton/day]). Based on annual sludge pro-
duction at the WWTr?the city estimates that the plant will
process 3.5 dt/day (3.9 dton/day) in summer (April to
September) and 4.3 dt/day (4.7 dton/day) in winter (Octo-
ber to March).
After startup, the Florida Department of Environmental
Regulation (DER) classified the plant as a "solid waste
volume reduction facility." The plant therefore must obtain
a solid waste operating permit to process and market its
compost. The facility has submitted a permit application.
72.3 Description of the Plant
12.3.1 Systems Overview
The Sarasota facility consists of a dewatering building, a
sawdust receiving station, a sawdust storage silo, two
reactor buildings, and a compost curing/storage building.
The reactor buildings also contain the mixing, blower, and
control/administrative rooms. Figure 12.1 provides an
overview of the facility.
The conveyance and mixing systems for the two reactors
are parallel in design. Both reactors can operate as the
first or second in the series, and materials can be fed
from either side of the materials train. Only one system
operates at any one time, however.
In the process flow of the Sarasota facility, conveyors
carry dewatered sludge from the dewatering building and
sawdust from the sawdust storage silo to one of the two
mixers in the mixing room. The sludge, amendment, and
Figure 12.1 Sarasota, Florida, Sludge Composting Facility
Sawdust Storage Silo / Reactor Buildings
Sawdust Hopper (Truck Drop)
recycle are combined in either mixer to create the raw
compost mixture. The mix is then conveyed to the top of
either reactor. An outfeed reclaiming screw, located at the
bottom of each reactor, loads the processed material onto
a conveyor which carries the material from either reactor
to the mixers. The material is then carried to and depos-
ited in the top of the curing/storage area. Figure 12.2
illustrates the process flow.
Figure 12.2 Sarasota, Florida, Composting Process Train
FIRST REACTOR
SECOND REACTOR
CURING/STORAGE
136
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The aeration/temperature control system contains supply
and exhaust air blowers, a heat exchanger, pneumati-
cally controlled air valves, flow meters, pressure meters,
and temperature meters. By design, the reactor inlet air is
drawn from the dewatering building. In practice, the sup-
ply air blowers also draw air from the blower room. Air is
supplied to the compost material through a series of
distribution pipes located in the bottom of the reactor
beds and regulated by a series of pneumatic valves. The
air is drawn out of the reactors by a series of exhaust air
blowers and an exhaust air piping system located within
the upper level of the compost material.
The process air drawn from the reactors can be ducted
through a heat exchanger to preheat supply air. It is then
channeled into an ozone contactor, where it is treated to
remove odors and released into the atmosphere. Nonpro-
cess air is ventilated throughout the facility with a number
of exhaust fans.
The facility does not have an automatic fire protection
system.
12.3.2 Feed and Mix Characteristics
72.3.2.7 Sludge
The facility receives waste-activated sludge (WAS) that
has been thickened in dissolved air flotation (DAF) units
to 3.0 to 3.5 percent solids. The sludge is stored in unaer-
ated liquid holding tanks at least 1 day every weekend.
This sludge turns septic in about 24 hours.
The liquid sludge is dewatered by belt filter presses 6
days/week to 15 to 16 percent solids. At the time of the
site visit, the volatile solids were about 76 percent and the
bulk weight was 918 kg/m3 (57.3 Ib/cf). However, although
measured during the startup phase, bulk weight is not
measured on a regular basis unless the solids content
deviates significantly.
72.3.2.2 Amendment
The sawdust is supplied by several kiln-dried lumber
operations. The only specifications for the sawdust are
that it must be "as per the sample provided" at the
beginning of the contract. The sawdust is from softwood,
probably pine, and has the following characteristics: both
the total solids and volatile solids contents are 90 per-
cent; the pH is about 4.5; the bulk weight is 290 kg/m3 (18
Ib/cf); and the nominal particle size is 0.006 m (0.25 in.).
The city has recently changed to a sawdust with an
increased particle size that is typical of a small chip.
When cypress sawdust was tried during startup, reactor
temperatures were lower and they peaked deeper in the
bed. According to the plant supervisor, this was probably
because cypress wood is more resistant to decay. This
unusual temperature profile was not considered desir-
able by the operator.
72.3.2.3 Recycle
The compost from the second reactor is most often used
for recycle. At the time of the site visit, the dry solids
content in the recycle from the first reactor was 50 to 55
percent, although it can range up to 70 percent. The
compost from the second reactor is generally 10 to 15
percent drier than the compost from the first reactor.
72.3.2.4 Mix
The 35 percent solids design mix ratio initially caused
mixer plugging problems; thus, during startup, the saw-
dust to recycle ratio was estimated, tested, and modified
until clogging in the mixer was reduced. The present
operational goal is to create 40 percent solids in the initial
mix. The average mix achieved from May through June
1988 was 40.4 percent, with a range of 33.8 to 46.4
percent.
The design volumetric mix ratio of sludge to sawdust to
recycle is 1/1/2, and 1/0.72/1.76 by dry weight. Actual
volumetric mix ratio at the time of the site visit was 1 /0.56/
2.44 and 1/1.13/5.18 by dry weight.
The facility is using more sawdust and recycle than antic-
ipated (dry weight), primarily because the present mix is
drier than the design mix, and the sludge cake is lower in
solids content. The problem has been somewhat allevi-
ated by using sawdust and recycle that are drier than
anticipated in the design. Also, sludge production is less
than expected.
12.3.3 Materials Handling
Figure 12.3 is a schematic of the materials handling sys-
tem. From the belt filter presses, the sludge drops directly
into an uncovered live-bottom sludge storage bin con-
taining open core (shaftless) discharge screws. Based on
the availability of the sludge, the reactors are loaded only
5 days/week. Because dewatering takes place 6 days/
week, dewatered sludge is stored in the bins at least 1
day/week, but usually for not more than 2 days/week.
Sawdust is delivered by bottom-feed trucks to a modified
hopper. A metal shroud built around the receiving bin
doors controls blowing dust during delivery. Two shaftless
screws then transfer the material from the hopper to a
pneumatic feed that conveys the sawdust to the top of a
26 ft diameter by 34 ft tall storage silo. The pneumatic
system shuts down if the air pressure in the sawdust
storage system becomes greater than 20,700 Pa (3 psi).
An auger circulates around the bottom of the sawdust
storage silo to move the sawdust onto a variable-speed
screw conveyor and then onto a belt conveyor for trans-
port to the mixing room.
The hollow-core sludge and sawdust discharge screws,
both shaped like springs with support at only one end,
137
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have worked very well. One or two horizontal bars
installed perpendicular to and just above the screws hold
down their free ends, which have a tendency to rise out of
the material.
The recycle material is not stored but brought directly
from the reactor to the mixer feed system.
Figure 12.3 Sarasota, Florida, Materials Handling System
DISTRIBUTION CARRIAGE
WITH LEVELING SCREWS
fr*3t->
PHttJHATIC FEED
BLOWER
COMPOST STORAGE
BUILDING
DISTRIBUTION CARRIAGE
WITH LEVELING SCREWS
SCREW
SCREW COHVEY
SAWDUST HOPPER
(TRUCK DROP) SCREW CONVEYOR
SCREW CONVEYOR
SCREW CONVEYOR
SCREW
CONVEYOR
BYPASS CONVEYORS
138
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Two variable-speed screws feed the sludge from the stor-
age bin to a cross conveyor and then to a transport screw,
where it is deposited on the belt conveyor with the saw-
dust. The sludge feed rate is varied to control the solids
content of the mix. The speed is usually controlled auto-
matically, but the operators have a manual control option.
Under normal operating conditions, only the sludge feed
rate
-------
The mix drops from the drag chain to either a reactor
distribution belt conveyor or to a reversible cross belt
conveyor that can shuttle the mix to the other reactor. A
reactor distribution carriage located on top of the reactor
places the mix in the reactor, where it is leveled with dual
leveling screws. Figure 12.5 shows the top of the reactor
after loading. Uneven settling of the compost overnight is
a normal occurrence.
The metal equipment in the reactor area has been
painted with vinyl chloride enamel for corrosion
protection.
12.3.4.2 Configuration
The first and second reactors are each 7.6 m wide by 15
m long (25 ft wide by 50 ft long), with a full volume of 990
m3 (35,000 cf). The average depth of the compost in both
reactors is 8.2 m (27 ft).
12.3.4.3 Reactor Aeration System
As designed, inlet air is drawn from the dewatering build-
ing to a heat exchanger and three variable-speed posi-
tive displacement aeration blowers. Two of the blowers
supply air to the first reactor; one supplies air to the
second reactor. At the time of the site visit, the dewatering
system did not provide air intake for the composting
operation because the operators felt that the heat and
humidity of this air would shorten the life of the blowers.
Inlet air is now obtained from the outside atmosphere,
except for a few days in winter when the outside air is too
cold.
Figure 12.5
Top of the Reactor After Loading. The uneven
settling of the compost overnight is a normal
occurrence.
u
The aeration blowers force air through a series of pipes
(located beneath the outfeed device) into the bottom of
the two reactors. The pipes are galvanized steel, perfo-
rated, and covered with approximately 0.3 m (12 in.) of
crushed stone, 0.01 m (0.5 in.) or less in size, to protect
the pipes from the compost and to disperse the air
through the reactor. Each reactor has three side-by-side
aeration zones based on the configuration of the piping
system (see Figure 12.6).
Figure 12.6 Supply and Exhaust Distribution System
REACTOR
EXHAUST BLOWERS
REACTOR
140
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A series of valves located on the supply pipes is used to
correct differential temperatures (hot and cool spots) in
the reactors (see Section 12.4.1). Each aeration zone has
two valves (i.e., six valves per reactor).
The exhaust piping system consists of a series of vertical
pipes extending 1.8 to 2.1 m (6 to 7 ft) down into the
compost mass, and connecting to a series of pipes lead-
ing to the exhaust blowers. Three fixed-speed centrifugal
exhaust blowers draw air up through the compost from
the bottom (50 to 65 percent) and down through the top
1.8 to 2.1 m (6 to 7 ft) of the compost from the top of the
reactors (35 to 50 percent). The blowers, which service
both reactors, contain fiberglass rotors to aid in corrosion
control. The system has no blower back-up.
By design, air from the exhaust blowers then moves to
either the heat exchanger (to heat inlet air) or to the ozone
contactor of the odor control system. The heat exchanger
condenses and collects the moisture in the airstream,
and the condensate drains to a sanitary sewer lift station.
Air from the heat exchanger and sludge feed bins also is
fed directly into the ozone contactor. At the time of the site
visit, inlet air bypassed the heat exchanger due to odor
leaks. Also, the exchanger was not effective because of
short residence times.
72.3.4.4 Outfeed Device
At the bottom of each reactor is a horizontally mounted
discharge screw, enclosed in a pressurized tunnel (see
Figure 12.7). An outfeed reclaiming screw conveyor
moves the compost from the bottom of the reactor to a
collecting belt. The collecting belt, in turn, carries the
compost to the mixing room for further processing. The
material from the first reactor is then either conveyed
from the mixer to the second reactor or used as recycle.
From the second reactor, the material is transferred to the
distribution conveying system, located at the top of the
reactors, for transport to the curing/storage area.
Figure 12.7 Purac Composting System, Sarasota
EXAUST AIR
HEADER
^ FEED CONVEYOR
^LEVELING SCREW
TO ODOR CONTROL SYSTEM
AIRFLOW DIRECTION
DISCHARGE
'SCREW
DISCHARGE
CONVEYOR
141
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The outfeed device is removed from the composting bed
after each use. The operators feel that if the device is left
in the bed too long, it will not restart because the com-
posted material will "set" around the screw and cause
excessive friction. According to the design specifica-
tions, however, the drive has enough torque to start while
embedded in the material, even if it has not been in
operation for a few days. To date of the site visit, the screw
had not gotten stuck in the compost pile. A spare screw
was supplied during construction, in case of failure.
One problem with the screw did occur when the motor
amperage sensor burned out. When the operators
received no amperage feedback, the screw was acciden-
tally overworked and the bearing was damaged.
The outfeed screws were originally hard-surfaced only on
the edge of the flight by hard steel welding, according to
system specifications. Despite this hardening, the first
reactor screw suffered heavy wear on the flight face (see
Rgure 12.8) (15 percent wear after 6 months') and was
replaced by a screw with additional Triten steel hard-
surface on the flight face. The wear on Triten steel is one-
tenth that on hard-surfaced flights. The spare screw and
the second reactor screw, which is not often used, were
both hard-surfaced with Triten steel in the field.
Figure 12.8 Worn Outfeed Screw
12.3.5 Exterior Curing/Storage
The distribution conveyor at the top of the second reactor
conveys the material to the top of the curing/storage
building. This conveyor connects to a reversible belt con-
veyor with a blade that can be manually moved across
the top of the building to create several compost piles. By
settling on the rails and piling up under the belt, the
compost, which is quite dusty, has caused the reversible
conveyor to misalign.
The curing/storage building is a three-sided, roofed steel
and precast concrete structure about 6.1 m (20 ft) high
and 293 m2 (0.06 acres) in area. Because of the dimen-
sions of the curing/storage area, high piles, about 4.6 m
(15 ft), need to be formed. The front wall of the curing/
.storage building is formed by tarpaulins that can be
raised or lowered with motorized rollers, but which are
usually kept down to control wind-blown dust. At the
design processing rate, this space is reportedly adequate
for 2 weeks of curing/storage in the summer and 1 week
in the winter. The operators plan to increase the height of
the walls to increase the curing/storage capacity. There is
no open curing/storage at the Sarasota facility, which
also increases the potential for anaerobic conditions and
odor production.
There is no space on site for emergency or long-term
curing/storage. This lack of space may force the city to
continue its current practice of hauling the compost for
land-spreading disposal, even if a sufficiently large but
seasonal market develops for the compost. A front-end
loader is used to load the finished product onto trucks for
removal.
12.3.6 Nonprocess Air Handling
Exhaust vents are situated above the belt filter presses
and the sludge storage bins. Ventilation air is piped to the
odor control system.
An exhaust fan in the mixing room roof, capable of 15 air
changes per hour, discharges air to the atmosphere. The
exhaust fan is not always used because it is a direct
source of odors. The doors in the mixing room are kept
open, however, because ventilation is inadequate, as evi-
denced by the prevailing wet and odorous conditions.
Air from the outfeed screw tunnel is vented only when the
operators enter the tunnel. A mixing fan was installed in
the tunnel to eliminate condensation that occurred on
equipment and walls. Three exhaust fans are used to
vent the curing/storage building.
12.3.7 Odor
12.3.7.1 Odor Sources and Characterization
Formal site studies have not been conducted to charac-
terize and measure any odors except for ammonia, which
is present in the outfeed screw tunnels at approximately
250 ppm. Uncontrolled sources of odors include the
exhaust fan and open doors in the mixing room, the
reactor headspace, the curing/storage building, and the
sludge storage tanks, which are particularly odorous
when sludge is stored for 3 to 4 days (a situation that has
only happened once since startup).
Competing odor sources include hydrogen sulfide and
other sulfate odors from the adjacent WWTP (due to
sulfur in the ground water) and unaerated stored sludge
from the WWTP Odors are worse in summer when the
142
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humidity is high and winds are light. In winter, however,
when the weather is generally dry and windy, condensate
from the odor control exhaust stack offers "visible proof"
of odors.
The plant and the DER have received many odor com-
plaints from local commercial establishments, including
one local weekly newspaper. The complaints started
immediately after the facility began operating. Staff mem-
bers have detected odors 1.6 to 3.2 km (1 to 2 miles)
away, but all complaints have originated in the local
neighborhood from individuals situated downwind of the
plant. There are fewer complaints in winter.
The plant has kept a log of odor complaints since it began
operations. Individuals with complaints are called back if
they so request. The plant also maintains contact with
certain individuals and solicits their opinions with respect
to the odor control system.
Local complaints have prompted DER to issue a consent
order addressing odor control. The order, which was
being prepared at the time of the site visit, will address
compliance activities and establish a schedule for elimi-
nating the nuisance conditions at the plant. The DER also
has the authority to limit operations or shut down the
plant.
12.3.7.2 Odor Control System
The system consists of ozone oxidation in a 88-m3 (3,100-
cp contact chamber containing simple baffles that deflect
air in a spiral pattern to increase the residence time in the
chamber. The design detention time is 19.9 seconds,
based on 4.42 mYs (9,364 cfm). The maximum airflow
rate detention time is 14.7 seconds, based on a flow rate
of 6.0 mVs (12,700 cfm): 2.4 m3/s (5,000 cfm) from the
dewatering room; 0.9 mVs (2,000 cfm) from the sludge
storage bin; and 2.7 m3/s (5,700 cfm) from the reactor
exhaust system.
A stack on top of the ozone tank with an elevation of 24 m
(80 ft) above ground serves as the dispersion system.
Convenient access to the top of the stack and a way to
sample the stack gases were not built into the system.
Except to keep the ozone generator working, operators
do not monitor the odor control system.
12.3.7.3 Odor Pilot Treatment
Because the ozone system was not effective in control-
ling odors, a two-stage scrubber (first stage, acid; second
stage, sodium hypochlorite with caustic soda pH control)
was tested on site. According to an informal odor panel
consisting of plant and city staff members, ammonia and
most other obnoxious odors were removed during the
pilot. The two-stage scrubber was scheduled for installa-
tion in the spring of 1989. With the odor control retrofit,
the existing ozone contact chamber will be used as a
cooling tower for air from the dewatering room, sludge
storage bin, and reactor exhaust before it enters the two-
stage system.
The facility also has plans to aerate the sludge storage
tanks. Past experience at the plant has shown that aero-
bic digestion is less odorous than anaerobic sludge
storage.
12.3.8 Support Facilities
The control room/administrative area is a combination
office, operating center, lunch room, and conference
room, which has proved to be inadequate for office work
because the area is noisy and lacks privacy. An office
area separate from the operations area has been
requested. The staff can also use the WWTP lunchroom
and showers. A bench near the mixer serves as the
laboratory area for the heat-lamp solids test. Other labo-
ratory tests, including the drying oven solids tests, are
conducted in the WWTP laboratory.
12.4 Monitoring and Performance
12.4.1 Reactor Control Strategy
12.4.1.1 Temperature Monitoring and Control
There are nine analog-to-digital thermometers in each
reactor, three per adjacent aeration zone, at depths of 1.5,
3.7, and 6.1 m (5,12, and 20 ft). The locations vary from
zone to zone. The probes are 1-in. diameter stainless
steel pipe sealed on the bottom, with each probe contain-
ing a resistive thermal device (RTD) suspended in a pool
of glycerine. A temperature probe also monitors the
exhaust temperature in the uppermost area of the bed.
The aim is to keep the temperature of the top probes in
the first reactor between 64° and 68°C (147 and 154°F)
and to meet EPA's 3-day, 55°C (131 °F) time-temperature
criterion. The temperatures are recorded by computer
every 30 minutes, stored on computer disk, and dis-
played in the control room. Only three digital readouts,
one from each zone, are displayed. Daily averages are
printed out as needed to meet reporting requirements.
There are several problems related to the temperature
monitoring system in the first reactor. First, although ade-
quate temperatures are most often maintained at the
bottom two probes of the first reactor without any opera-
tor intervention, the temperature at the upper probes is
more difficult to regulate. In addition, the three tempera-
ture probes per reactor section do not provide enough
information on temperatures in the reactor. Also, the auto-
matic temperature feedback program cannot differenti-
ate between low temperatures due to cooling and low
temperatures due to "bad" anaerobic conditions. This is
important because low temperatures due to cooling
require less air to correct than those due to poor anaero-
bic conditions. Since the operator feels that low tempera-
tures would cause the computer to decrease the airflow
allowing anaerobic conditions to develop, this program is
not used.
The temperature in the second reactor is not closely
monitored or controlled because the EPA time-
143
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temperature criterion is met in the first reactor, and the
second reactor is emptied only enough to provide room to
toad material from the first reactor. If the second reactor is
kept full, the exhaust vents are not exposed, and odorous
air cannot be released into the atmosphere.
Temperature profiles for the first and second reactors are
depicted in Figure 12.9. The temperatures are the mean
daily averages for May through June 1988, for the three
aeration sections in each reactor.
12.4.1.2 Aeration Quantities and Rates
To control temperatures, aeration is regulated by manu-
ally setting the speed of the blowers that input air to the
reactors. Generally, the valves on the air supply pipes are
operated in one aeration zone at a time. The valves open
automatically for 2 to 11 minutes, according to a schedule
set by the operator. Airflows are measured, recorded, and
compared to temperature readings to determine the opti-
mal operating conditions of the reactors.
During startup, before the proper air pressure could be
created within the reactors, the outfeed screw tunnel had
to be pressurized. At one time, the first reactor operated
at 165 rrVYs (3,500 cfm), but this rate caused tempera-
tures to be too low (mid-50s°C [mid 130s°F]). Since that
time, the first reactor has operated mainly at 1.32 m3/s
(2,800 cfm). Since startup, the second reactor has oper-
ated at 0.38 m3/s (800 cfm).
12.4.1.3 Oxygen Monitoring
Oxygen-measuring equipment was supplied but has
never worked properly because the probes were not
designed for use in a high humidity environment. For the
equipment to work properly, the air upstream of the
probes must be dried before reaching the probes.
12.4.1.4 Other Tests and Equipment
During every load, the operators measure the solids con-
tent in the reactor with a heat lamp. This measurement is
checked daily against the results of the drying oven tests
conducted in the WWTF? Other parameters are routinely
measured as follows: bulk density, twice per month;
heavy metals, quarterly; volatile solids, twice per month;
and pH, twice per month.
12.4.1.5 Data Management
Information from the daily logs kept by operators is fed
into a computerized spreadsheet once per month. A
summary spreadsheet generated by the facility is shown
In Table 12.1.
12.4.2 Mass Balance and Reactor Detention Time
A materials flow diagram, constructed from operator
records for May through June 1988, is shown in Fig-
ure 12.10. A mass balance is not shown because the
calculations to close the balance yielded negative volatile
solids destruction in the reactors. The operators have not
been able to determine the reason for this.
The design detention time in the first reactor is 16 days.
From May through June of 1988, plug-flow detention time
was calculated to be 12.5 days.
The design detention time in the second reactor is 20
days. This is 4 days longer than the detention time of the
first reactor because of the loss of mix volume created by
the composting process. That is, for every 911 (100 ton) of
mix that enter the first reactor, only 73 t (80 ton) are
removed from the second reactor. As a result, a greater
percentage of the first reactor compost batch can be
loaded into the equal-sized second reactor, and the plug
flow detention time is longer. From May through June
1988, the detention time in the second reactor was' calcu-
lated to be about 61 days.
12.4.3 Product Quality
Overall, the composting process regularly meets the EPA
time-temperature criterion and consistently produces a
compost of adequate dryness. From May through June
1988, compost production was 0.88 m3/wt (28.5 cf/wton)
of sludge cake processed or 0.39 wt product/wt (0.43
wton product/wton) of sludge cake processed.
During November 1987, three samples were tested for
Salmonella and phytotoxicity. None of the samples tested
positive for Salmonella. Two of the samples passed the
germination test for phytotoxicity, but in the third sample,
the germination rate was only 28 percent of the control
sample. This failure was attributed to the presence of
ammonia in the compost due to a short detention time (7
days) at the time the sample was taken.
The composting product meets the State of Florida crite-
ria for Class I sludge. The constituents, their criteria, and
the amount of each component contained in the Sarasota
compost are shown in Table 12.2. The compost can be
applied legally on sod farms, pasturelands, forests, high-
way shoulders, plant nurseries, land under reclamation,
playgrounds, parks, golf courses, lawns, other unre-
stricted public access areas, and agricultural soils includ-
ing those growing food crops (except root crops, leafy
vegetables, tobacco, and vegetables to be eaten raw). Up
to 198,000 kg/m2 (500 Ib/acre) total nitrogen of the com-
post can be applied annually.
72.5 Operations
12.5.1 Staffing
The facility employs four full-time, permanent staff: one
supervisor and three operators. The supervisor has two
associate degrees, one in electricity and one in electron-
ics, and is experienced with instrumentation and con-
veyor systems, but has no wastewater treatment
experience. One of the operators at the Sarasota facility
has a Class A (highest) WWTP license. The other two
operators have no prior wastewater treatment
experience.
144
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Figure 12.9 Temperature Profiles Averages, May to June 1988
30
12
20
40
TEMPERATURE ("C)"
50 55
60
70
SECOND REACTOR
9 a
a o
FIRST REACTOR
BOTTOM OF COMPOST BED
LEGEND
• WEST SECTION
O CENTER SECTION
D EAST SECTION
("C X 1.8) + 32 = °F
FT X .3048 = H
145
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Table 12.1 City of Sarasota Compost Plant Production Calculations Spread Sheet
Date: 6/1/88
Mix Calculations
Target SolkJs 37.9
Studgelb/cf 57.5-
Sludge Dry Solids 14.7
Recycle Ib/cf 29.7
Recycle Dry Solids 50.4
Recycle Volume 9416
Sawdust Ib/cf 18
Sawdust Dry Solids 90
Sawdust Volume 205
Actual Run Time 4.3
Volume Density Wetlbs°
Sawdust 205 18 3,690
Recycle 941 29.7 27,948
Sludge 406 57.5 23,345
Cal. Mix Solids 37.9
Loading Summary
o/o D.S.
90
50.4
14.7
Drylbs
3,321
14,086
3,432
Recycle
Volume Loaded cf 4,046
Density Ib/cf 29.7
Wet Tons 60.4
% Dry Solids 50.4
Dry Tons 30.3
Actual Run Time 4.3
Transferring Calculations
Bio to Cure
Recycle Volume
Recycle Density
Recycle Dry Solids
Actual Run Time
Transferring Summary
Bio to Cure
Cornp Transferred cf
Comp Ibs/cf
Comp Wet Tons
Comp Dry Solids
Comp Dry Tons
Actual Run Time
Sawdust
882
18
7.9
90
7.1
941
29.7
50.4
1.4
1,317
29.7
19.6
50.4
9.9
1.4
Sludge Input
1,746 6,674
57.5 35.4
50.2 118.2
14.7 37.9
•» A A A Q
7.4 44.8
Cure to Storage
Recycle Volume
Recycle Density
Recycle Dry Solids
Actual Run Time
Cure to Storage
Comp transferred cf
Comp Ibs/cf
Comp Wet Tons
Comp Dry Solids
Comp Dry Tons
Actual Run Time
941.0
29.7
67.7
1.0
941
29.7
14.0
67.7
9.5
1
•Ib/cf x 16.02=kg/ma.
fPx 0.02832=m3.
«!bsx 0.4536 = kg.
Table 12.2 Compost Constituents (mg/kg)
Sarasota
Component Compost1
Florida Criteria for Grade
I
Total Nitrogen, N
Total Phosphorus, P
Potassium
Cadmium
Copper
Lead
Nickel
Zinc
Chromium
2.2%"
0,88%"
0.15%"
1.7%"
210"
20"
6.0"
281"
73d
C
<30
£900
< 1,000
<100
< 1,800
—
30-100
900-3,000
1,000-1,500
100-500
1,800-10,000
~~
—
>100
> 3,000
> 1,500
>500
> 10,000
•Lab report dated 3/28/88 of sample composited from grab samples taken
2/23,2/25,3/1, and 3/3/88.
^Percent dry weight.
•— No criteria is established for these components.
*mg/kg dry weight.
146
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Figure 12.10 Sarasota, Florida, Materials Flow Average Daily Quantities, May to June 1988
i
•L.
o
to
CO
v? CO
i I
O CO CM
• ID CO
r->—'ifi
co <
. <
CO •
x
i—i
ac
CM
hs
O
en co
147
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During plant startup, there were high demands on the
operating personnel that resulted in high staff stress and
turnover. These demands were caused by Sarasota's
attempts to start up, train, debug, and process all at the
same time without a complete control system.
Smith and Gillespie subcontracted with Envirotech Oper-
ating Services, who subcontracted with Purac, to provide
startup activities and training. Purac conducted a 40-hr
course on composting principles and equipment opera-
tion, which was then repeated because of the high turn-
over of the initial plant staff. Manuals and software, which
are used to support continued operations and mainte-
nance proficiency, were also produced. A continuing
training program has not been established, however.
The WWTP crews provide maintenance and repair serv-
ices at rates equivalent to 50 hours per week of preven-
tive maintenance, 40 hours per week of electrical repair,
and 40 hours per week of mechanical repair. The city's
mechanical and electrical personnel must be reoriented
somewhat every time they do a job on site. In addition,
the maintenance and repair service needs of the com-
post plant compete with those of the WWTP for limited
personnel.
12.5.2 Marketing and Distribution
A marketing study, conducted prior to the procurement
process as part of the city's 1982 sludge management
study, concluded that there was a market for pellets but
not for compost. Later, the city came to believe that a
compost market could be developed if sufficient high-
quality product was available for use in a wide range of
legal applications. The city also reasoned that a market
could be created easily because Florida has such sandy
soils. This outlet has not yet been exploited, however, due
to the inconsistency of the product during startup.
At the time of the site visit, a local rancher was taking the
compost at no charge. In addition, the compost was
being hauled away for land-spreading. Prior to the site
visit, an RFP was issued for compost-hauling services;
one bid offered $3.3/m3 ($2.50/cy), but at the time of the
visit a contract had not been issued.
12.5.3 Operating Costs
A budget was prepared for fiscal year 1987-1988 (October
through September) prior to startup (see Table 12.3). The
cost to process sludge per dry ton at the Sarasota facility
is$419/dt($380/dton).
The amount of electricity used by the compost plant is
integrated in the WWTP bill. On the basis of motor horse-
power, power usage at the compost plant is estimated at
15 percent of the total electricity bill.
Sawdust is bought from a trucker of wood waste, with
whom a 1-year, fixed-price $15.0/m3 ($11.50/cy) contract
is in place. The sawdust is more expensive than
expected.
Sludge hauling costs have been $9.80/m3 ($7.50/cy).
These costs are higher than anticipated, partly because
the facility expected to sell the compost and therefore
avoid compost disposal costs.
72.6 Update
Sarasota received a permit to process and market com-
post in September 1988. The actual amounts of materials
processed and operating costs are shown in Tables 12.4
and 12.5.
Table 12.3 Dewaterlng and Composting Operations and Maintenance Costs ($1,000)
Labor (2 positions)
Operations
Maintenance11
Power
Materials (mainly
sawdust)
Mechanical and
Electrical Supplies
and Services
Chemical Supplies
(Incl. polymer)
Building and General
Supplies
Engineering and
Architectural
Other
Compost Disposal
TOTAL
Estimated"
87/88
59.8
99.5
56.0
100.7
23.3
44.8
7.6
9.5
0
100.9
502.1
% of
Estimated
11.9
19.8
11.2
20.1
4.6
8.9
1.5
1.9
0
20.1
Budget
88/89
89.6
102.7
56.0
210.0
21.9
67.4
9.4
0
20.8
20.0
597.8
°/o of
Budget
15.0
17.2
9.4
35.1
3.7
11.3
1.6
0
3.5
3.3
'Earlier O&M cost estimates not available; this estimate prepared before startup.
Includes maintenance and repair provided by WWTP personnel.
148
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Table 12.4 Materials Processed
(October 1,1987 through September 30,1988)
Material
Sludge
Amendment
Recycle
Compost Product
Amount
Processed8
1,319'
8,232"
20,411"
10,076"0
Percent
Average
14.8
85.4
59.6
56.0
Solids
Range
13-17
60-90
50-68
50-62
Mix
Ratio
1
0.75
2.5
aTotal dry tons (dton x 0.9078 = dt).
"Cubic feet (ft3 x 0.02832 = m3).
"None of the product was sold.
A two-stage, packed tower wet scrubber to treat 2,830 U
sec (6,000 cfm) of reactor exhaust became operational in
March 1989. Sulfuric acid is used in the first stage and
hypochlorite in the second. The wet scrubber is preceded
by the old ozone contact chamber, which has been
converted to a wet scrubber to remove ammonia. The
chamber is fitted with 22 nozzles, which spray 11 L/sec
(250,000 gal/day) of chlorinated effluent from the WWTP
In December 1988, a 0.9 m by 1.8 m (3 ft by 6 ft) airtight
door and frame assembly, leading to the outside from the
reactor's pressurized tunnel, was "blown out" because
the frame anchor failed. At the time of the blowout, the
operators were in the process of "turning over" the reac-
tor, which had gone anaerobic due to a mix that was too
wet (28 percent solids). When about one-third of the
reactor was empty, the operators closed the doors and
turned on all the air supply blowers. It is assumed the
pressure rose about 34 kPa (5 psi).
Table 12.5 Operating Costs
(October 1,1987 through September 30,1988)
Item
Labor (operations)
Labor (maintenance)
Power and Fuel
Amendment
Maintenance materials
Other
TOTAL
Revenues
Product disposal costs
Amount
$ 59,875
99,459
47,592
100,746
30,655
9,782
$348,109
0
100,927
% of Total
17
29
14
29
9
3
"Includes 35% fringes.
"Cost of city water and chemicals.
"Breakdown of these costs not given.
To prevent future over-pressurization, the city has estab-
lished operator protocols that alert operators to potential
problem situations and help them avoid a recurrence.
Also, the plant is currently using a volumetric mix ratio of
1/1/1 (sludge to amendment to recycle) to guarantee a
drier mix. They recommend that future installations
include a pressure alarm or sacrificial blowoff
diaphragm.
149
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Chapter 13
Schenectady, New York
13.11ntroduction
The city of Schenectady owns an in-vessel composting
facility that was supplied by American Bio Tech (ABT).
The facility uses five rectangular vertical reactors, with a
15 dt/day design capacity and a 9 to 17 dt/day (10 to 19
dton/day) range.
The composting plant is located at the existing wastewa-
ter treatment plant (WWTP) on a 4,450-m2 (1.1-acre) site
at the edge of a broad river valley. A high ridge and a
mixed residential/commercial neighborhood border the
facility on the south side of the plant, and generally open
and flat terrain and a county airport lie across the river on
the north side. The nearest neighbor is 150 m (500 ft)
away.
The site visit took place on November 16 and 17,1988. All
information presented in this study is representative of
the period of July 1987 through January 1988, the only
time that the plant processed material through the entire
system prior to the site visit (see Section 13.2.3).
13.2 History of the Plant
13.2.1 Procurement and Construction
In response to sludge disposal problems, the City of
Schenectady hired O'Brien and Gere Engineers to inves-
tigate disposal alternatives and prepare a sludge man-
agement facilities plan. Completed in November 1983,
the plan reviewed many alternatives that were similar in
cost. In-vessel composting was the favored alternative
based on the results of a sensitivity analysis of man-
power and energy costs, and in-vessel's ability to meet
major criteria as follows:
• Adequately contain odors
• Provide aeration at a relatively low cost
• Provide proper air distribution and water removal
• Handle upset conditions
• Operate with flexibility and ease of routine
maintenance
Because site restrictions required installation of a
vertical-style reactor, city officials investigated various
vertical-reactor systems and visited existing plants in the
United States and in Europe. Finally, they prequalified
three systems: two, Taulman and Purac, on the basis of
experience; and one, ABT on the basis of a successful
pilot test conducted at the Schenectady facility during the
summer of 1984.
With extensive input from the three system suppliers,
O'Brien and Gere developed plans and specifications for
a conceptual system, after which a two-step bidding pro-
cess took place. The first bid was for the design of the
equipment, including the reactors, infeed and outfeed
devices, mixer, conveyance system, sludge and amend-
ment storage facilities, and aeration system. The second
bid was for the construction and installation of the equip-
ment. In May 1985, ABT was awarded the contract,
based on its low equipment bid of $975,000.
The construction and installation contract was bid and
executed according to New York State regulations, with
the city providing all funding for the project. As part of the
contract, ABT had to provide 2-year extended guarantees
for the outfeed devices and the in-vessel piping; all other
equipment was guaranteed for 1 year.
The city and consulting engineer worked together very
closely during the procurement process and the city felt
well represented throughout the process. The engineer
and ABT also provided construction administration and
inspection services to the city. At the end of the procure-
ment process, the plant operator felt prepared to handle
any technical and administrative problems that might
arise.
13.2.2 Capital Costs
The December 1985 construction cost estimate was
$4.368 million, not including $550,000 in improvements
that were made to the WWTP Actual 1987 construction
costs, however, were $6.2 million, including engineering,
contract administration, construction inspection serv-
ices, and change order costs. In addition, costs for the
postconstruction modifications (see below) were esti-
mated to be $470,000.
13.2.3 Operating History and Current Status
The facility started operating on July 8, 1987, and con-
ducted an acceptance test 3 months later. During the test,
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the plant operated using recycle in the mix, and the
compost produced was examined for 30 consecutive
days. To pass, the product had to meet the specifications
set forth for the compost by the State (see
Section 13.4.3). The compost passed the test in terms of
both low moisture content and metals concentrations.
During the acceptance test, sludge loading varied from
12 to 23 dt/day (13 to 25 dton/day).
Shortly after passing the acceptance test, the facility
experienced a number of setbacks. In November 1987,
one of the outfeed devices failed. Although the plant
continued to operate, it did so at a reduced throughput
(see Section 4.3.4.4). In January 1988, three fires
occurred in the reactor system, one in each of the two
first-step reactors on January 12 and 19, and one in a
second-step reactor, also on January 19. All operations
were halted and the first- and second-step reactors were
emptied. Since that date the sludge from the WWTP has
been incinerated.
Due to an unrelated incident, the plant has also been
undergoing modifications since the spring of 1988 to
repair structural problems in the "conditioning" reactor,
which resulted from mechanical difficulties in the outfeed
device.
In addition to damage caused by the fires and the
mechanical difficulties, odor problems and odor treat-
ment modifications have extended the plant shutdown
period.
At the time of the site visit, the plant was not operating,
except for the "conditioning" reactor which was being
run to test reactor structural changes, to test operations
using mix without recycle, and to evaluate odor produc-
tion and treatment. Other parts of the system were func-
tional, but the operator was waiting until the odor control
modifications were completed before resuming opera-
tions. Other planned modifications include installing
infrared heaters in various parts of the main process
building to provide added heat and upgrading the interior
ventilation capacity.
73.3 Description of the Plant
13.3.1 Systems Overview
The Schenectady facility contains one main process
building with an administrative wing, an amendment silo,
an odor scrubber, and a transformer; it also has covered
and uncovered curing/storage yards. The main building
is an unheated three-story steel framework covered with
uninsulated fiberglass, with the bottom of the reactors,
the outfeed devices, and conveyors situated on the first
floor. The aeration blowers are located in hallways around
the midlevel of the reactors, and the top level of the
building provides access to the top of the reactors and
the loading mechanism. The dewatering building is adja-
cent to the main building.
Figure 13.1 is a generalized schematic of the materials
handling train. Amendment and sludge are placed on the
same belt conveyor and carried to the accumulator and
mixer. If recycle is used in the mix, it is carried to the
accumulator and mixer by an outfeed conveyor belt from
the reactors.
The mix is carried to the top of the reactor system. There
are five reactors in the system: one "conditioning" reac-
tor, two first-step reactors, and two second-step reactors.
The two first-step and the two second-step reactors are
adjacent to each other (see Figure 13.1). The "condition-
ing" reactor is located between the first- and second-step
reactors. An outfeed device '(screw auger) located at the
bottom of the reactors moves the compost onto a series
of belt conveyors, which carry the compost to an outdoor
curing/storage area.
The reactors are aerated through a series of air lances
that hang from the top of the reactors. Air is alternately
blown into and drawn out of the composting mass by
several supply and exhaust blowers. The process air is
then directed into a single stage scrubber.
13.3.2 Feed and Mix Characteristics
A mixture of 60 percent waste-activated sludge (WAS)
and 40 percent primary sludge is dewatered by two belt
filter presses. Typically, 50 kg of polymer solution per dry
ton of of sludge (100 Ib/dton) are used in the dewatering
process. The plant is designed to handle 25 percent
solids in the sludge; for 3 years prior to startup, total
solids in the sludge averaged 25 percent with a range of
20 to 28 percent. During the acceptance test, sludge
solids averaged 23 percent and volatile suspended solids
were approximately 73 percent.
The amendment specifications call for roughcut green
pine sawdust with an average particle size of 0.003 m
(0.125 in.), containing no more than 50 percent moisture
and an average total solids of 55 percent. Despite the
specifications, after the acceptance test, the plant
received sawdust that contained a greater amount of fine
powder hardwood material. The resulting compost mix
was very pasty and hardened when dried. This problem
was alleviated when the hardwood sawdust was no
longer used.
The bulk weight of the recycle is approximately 504 kg/m3
(850 Ib/cy). At the time of the'site visit, recycle was not
used in the tests performed in the conditioning reactor
because the recycle acted as a binding agent and
reduced the friability and porosity of the mix.
The mix without recycle contains 35 to 39 percent total
solids and has a good consistency when pine sawdust is
used as amendment. Contract specifications call for 35
percent dry solids in the mix.
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Figure 13.1 Schenectady, New York, Composting Process Train and Materials Handling System
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13.3.3 Materials Handling
Figure 13.2 is a schematic of the materials handling sys-
tem, minus the conditioning reactor. The system can
operate in either the automatic or manual mode. In the
automatic mode, all devices in the system are interlocked
through a central computer, and local controls are avail-
able. The interlocks are not designed to work when the
system is running in the manual mode. One problem with
the automatic system is the lack of an audible signal to
indicate when the equipment is about to start.
Belt-washing devices exist throughout the conveyance
systems to periodically clean the belts, but they are sel-
dom used because they are not needed. Spillage from
the belts has not been a major problem.
The belt conveyors also are equipped with scrapers to
push materials off the belts. Because the scrapers are
surrounded by other pieces of machinery, access to them
is limited, so they are difficult to adjust. The operators
plan to retrofit the belts with a different type of scraper,
thereby making them more accessible.
The sludge is delivered to two storage bins from the
adjacent dewatering building via a covered, exterior flat
belt conveyor. Each bin is 4.9 by 2.0 by 4.3 m (16 by 6 by
14 ft) with a material storage depth of 3.7 m (12 ft).
Each sludge storage bin is equipped with a live-bottom
discharge screw conveyor with six 0.25-m (10-in.) diame-
ter screws. The bins have removable covers and three
0.15 m (6-in.) drains that are vented to the odor control
system (see Section 13.3.7). The mixer feed system can
remove material from the storage bins faster than it can
toad them; the system is equipped with an alarm that
indicates when the sludge bins are full.
Slide gates were installed on the sludge bins to prevent
sludge from oozing out of the bins where the screws from
the screw conveyor penetrate the bin wall. The slide
gates remain closed when the screws are not operating.
The sawdust receiving bin is located near the sawdust
silo. The bin is sunk into the ground so that trucks can
dump material directly into it. The bin is covered by a
stationary grate and a movable solid door. Fugitive dust is
not a problem. A single 0.46-m (18-in.) screw conveyor
moves the sawdust out of the bin onto a belt conveyor.
The sawdust is fed from the belt conveyor to the amend-
ment silo via a vertical bucket elevator with a capacity of
22,700 kg/hr (50,000 Ib/hr). The silo is 8.0 m (26 ft) in
diameter and 12 m (40 ft) high with a 370 m3 (13,000 cf)
minimum capacity. In the base of the silo, a ground-level
work space with 1.9 m (76 in.) of head room contains the
outfeed device, screw conveyor, motors, and other
mechanical equipment.
The amendment silo has a level sensor to indicate how
much material is in the silo, but no motion detector or
alarm. Thus, the operator cannot detect when the saw-
dust feed stops, unless he or she observes the feed belt
directly. The operators plan to install a motion detector.
The amendment silo outfeed device is a 0.3-m (12-in.)
diameter screw conveyor. A rotary disk screen with two
rotating drums and intermeshing teeth is located at the
end of the screw conveyor. The disk removes large 0.02-
to 0.07-m (1-to 3-in.) diameter chunks of wood, stones,
and other trash before the sawdust is deposited onto the
amendment infeed conveyor. This infeed conveyor,
which takes the sawdust into the main process building,
is a flat belt conveyor with a capacity of 0.009 m3/s (1,150
cf/hr).
There is no surge storage for the recycle material, so
recycle is fed to the mixer directly from the bottom of the
reactor. The reactor has a variable speed outfeed device
that can be adjusted by the computer to provide the
appropriate rate of recycle.
The first step in the mixer infeed system is deposition of
sawdust from the amendment infeed conveyor onto the
mixer infeed conveyor, which has a 0.016 m3/s (2,000 cf/
hr) capacity. Next, sludge from the storage bins is depos-
ited onto the same mixer infeed conveyor as the
amendment. The sawdust/sludge combination is added
to the recycle in the accumulator, which has a dual
reversible screw (0.3 m [12-in.] diameter, 32 rpm) with a
capacity of 0.02 m3/s (2,500 cf/hr). From the accumulator,
materials can go to the mixer and then to the reactors, or
they can bypass the mixer and go directly to the top of the
second-step reactor. The mix ratio, based on volume, is
regulated by adjusting the speeds of the discharge
screws of the storage silo and bins.
The mixer is a plow-blade type with a maximum working
volume of 1.2 m3 (42 cf). Running at a constant speed of
1,775 rpm, it can process 71 m3 (2,500 cf) of material per
hour. The mix remains in the mixer for approximately 1
minute, then it is removed by a 0.4-m (16-in.) diameter
screw conveyor.
On several occasions, the mixer at the Schenectady plant
became overloaded and jammed. The mixer discharge
port has been enlarged, but it is not known if this will
solve the problem because the mixer has not been tested
to capacity since the adjustment.
Fog forms in the mixing room because it is not heated
and there is a large amount of moisture in the air.
13.3.4 Reactors
13.3.4.1 Reactor Feed Systems
The mixer outfeed screw deposits material on the reactor
elevator, which is a vertical drag chain conveyor with 0.2-
by 0.33-m (8- by 13-in.) double-leg flights. Thus far, the
drag chain has worked well; flights have not been modi-
fied, and the chain has not jammed.
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Figure 13.2 Schenectady, New York, Materials Handling Schematic
155
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The reactor infeed transfer conveyor is a flat belt con-
veyor with a 0.02 m3/s (2,500 cf/hour) capacity. A rail-
mounted traveling bridge with a belt conveyor plow
removes material from the reactor infeed belt conveyor
and deposits it onto a pan conveyor, which then distrib-
utes it across the width of the reactor.
The materials-handling system is designed so that when
recycle is moved to the mixer from one of the reactors, the
operator cannot simultaneously transfer material from
the first-step reactor to the second-step reactor, or dis-
charge material from the system (see Figure 13.2).
13.3.4.2 Configuration
The "conditioning" reactor is 283 m3 (10,000 cf) in vol-
ume. The other reactors are each 496 m3 (17,500 cf) in
volume with an average depth of 7.5 m (26 ft) when
completely filled (see Figure 13.3). The conditioning
reactor is 283 m3 (10,000 cf) in volume.
In the original operating plan, the mix was to be proc-
essed in the conditioning reactor to adjust its characteris-
tics prior to longer-term composting. However, the reactor
was not actually operated in this manner because condi-
tioning of the mix was unnecessary to produce an
acceptable compost product. At the time of the site visit,
the conditioning reactor was used as a test reactor, and
there are plans to use it as a surge bin for overnight
storage of excess mix.
All reactor walls are made of corrugated fiberglass and
hang above a concrete base slab. The walls of the reac-
tors are supported on an exterior steel frame. A 0.9-m (3-
ft) space between the bottom of the reactor walls and the
concrete floor forms an outfeed device gallery that allows
the outfeed device to pass completely through the reac-
tor space. Steel beams form a reactor "floor" above the
outfeed device. This design allows easy access to all
parts of the vessel.
Fog is created in the headspace when the reactors are
loaded and in the outfeed device gallery when the reac-
tors are unloaded. Due to inadequate ventilation, the
headspace is damp, causing the wheels of the traveling
bridge to slip on the rails. To improve traction, a gritty
material is applied to the rails, but this has caused the
rubber drive tires supporting the traveling bridge to wear
out more quickly than expected.
The conditioning reactor has had several structural prob-
lems. The first occurred at a bottom corner of the reactor
when a weld connecting a beam to a perpendicular cross
beam broke, causing it to fall onto the outfeed screw. The
corner of the reactor was rebuilt and a reinforcing plate
was added As a precaution, all of the corners in all of the
reactors were rebuilt in the same manner.
The second structural problem was caused by compost
that was pushed or "plowed" ahead of the outfeed
device as it traversed the reactor (see Section 13.3.4).
One of the bottom beams of the conditioning reactor bent
outward. The plowed compost also caused the auger of
the outfeed device to rise up off the bottom of the reactor.
Because of the close tolerances between the top of the
auger and the bottom of the cross beam, the auger's
teeth cut notches into the lower edge of the beam before
the operators could stop it. The beam has since been
replaced and slightly raised, and the same modifications
were made to the equivalent beams in the other reactors.
The outfeed device is now run at a traverse speed slower
than the one recommended by the manufacturer to mini-
mize plowing of the compost (see Section 13.3.4.4).
There have been three fires in the reactors since the start
of operation. Because there were no smoke detectors or
fire alarms and the temperatures measured in the reac-
tors were not unusual, the fires were not discovered until
the operators noticed smoke in the building.
To extinguish the first fire, firefighters poured water into
the reactor. This created compost mud, which took opera-
tors 14 hours to unload from the reactor. During the sec-
ond and third fires, firefighters set up fans and misting
hoses to clear the smoke and contain the fire within the
reactor.
Damage from all fires was relatively minor: the fiberglass
reactor body was scorched and some of the air lances
melted. The cause of the fires and melted air lances is not
known. One scenario is that CPVC inserts in the lances
caught fire due to air from the fans, thereby setting up a
localized "blow-torch" effect hot enough to melt steel.
The compost probably ignited prior to the aeration pipes,
because the ignition temperature of the excessively dried
compost 93°C (200° F) is well below the melting tempera-
ture of CPVC 260 °C (500° F).
Plant operators believe the cause of all three fires may
relate to a contingency operating policy put into place to
reduce the operating throughput rate, increase the com-
post detention time, and ease the burden for compost
curing/storage space, which had decreased. Since the
city did not have a New York State permit to sell compost
at that time and the facility was running out of space to
store compost, the city operated the plant at a reduced
capacity. The reduced throughput rate raised the total
reactor detention time from 60 to 90 days. As a result of
the longer detention time, the contents of the second-step
reactor dried to 85 to 90 percent total solids, and sponta-
neous combustion occurred. Operating procedures were
changed so that under lower throughput conditions, the
reactors are not completely filled. This procedure pre-
vents overly long detention times.
There is no built-in, automatic fire detection or suppres-
sion system. The operator purchased portable fire extin-
guishers after facility construction was completed. Two
fire hydrants are provided outside, one near the process
building and one near the curing/storage yard. No modifi-
cations for fire prevention or extinction are planned.
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Figure 13.3 American Bio Tech Reactor at Schenectady Plant
AIR
SUPPLY
BLOWERS
DISCHARGE.
SCREW
AIR-SUPPLY
MANIFOLD
FEED
CONVEYOR
EVACUATION
BLOWERS
TO ODOR
CONTROL
AIRFLOW
DIRECTION
AIR-LANCE
-DISCHARGE
CONVEYOR
157
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73.3.4.3 Reactor Aeration System
The aeration systems in all the reactors are identical. The
first- and second-step reactor systems use three large
low-pressure centrifugal industrial blowers as air supply
blowers (0.47 to 1.5 m3/s [1,000 to 3,200 cfm] at 0.3 m [12-
in.] water column [W.C.]): one for the first-step reactors,
one for the second-step reactors, and one as a standby
unit A smaller blower (0.47 m3/s [1,000 cfm] at 0.3 m [12-
in.j W.C.) aerates the conditioning reactor.
In addition to these blowers, three large low-pressure
centrifugal industrial blowers exhaust air from the first-
and second-step reactors: one for the first-step reactors,
one for the second-step reactors, and again one for
standby. Each large exhaust blower draws 1.9 m3/s
(4,000 cfm) at 0.5 m (20-in.) W.C. with a discharge pres-
sure of 0.35 m (14-in.) W.C. The conditioning reactor is
exhausted by one smaller blower that draws 0.6 m3/s
(1,250 cfm) at 0.5 m (20-in.) W.C. Control dampers are
attached to all of the supply and exhaust blowers, and
flow rates are adjusted by means of a butterfly valve on
the suction side of the blowers.
Air supply pipes in the reactors are composed of vertical
air lances made from stainless steel well-screen cylin-
ders. The pipe inserts are CPVC pipe. The air lances
hang from a manifold located at the top of the reactors
(see Figure 13.4). There are six header channels in the
first- and second-step reactors with eight air lances per
header section. In the conditioning reactor, there are
three headers with eight air lances per header.
Outfeed Screw and Air Lances in Empty Reactor
However, the system is not designed to measure airflows
through the individual headers or lances. At the time of
the site visit, the airflows had not been adjusted.
The practice of operating reactors at less than capacity at
low sludge throughput rates leaves some of the air distri-
bution holes in the lances exposed. Because the plant
has no method to measure airflows through the distribu-
tion system, it is not known if short-circuiting occurs.
The headers are connected to the exhaust blowers via
steel ducting. From the blowers, the air is channeled to an
air-to-air heat exchanger, which cools the exhaust air and
heats the incoming process air. The heat exchanger is
sized to heat incoming air from 1 ° to85°F(34° to 185°F).
The actual observed temperature increase is from 0° to
500C(0°to122°F).
13.3.4.4 Outfeed Device
On the floor of the processing building are two traveling
outfeed devices consisting of a screw auger, drive car-
riage, carriage rack and rail, cable festoon system, and
controls. The augers run on two metal channel tracks and
are pushed through the compost by gears located at the
support carriages (see Figure 13.5). Each auger has a
variable-rotation speed of up to 15 rpm. Two thousand
bolts and bushings are installed across the open sides of
the channels, spaced to accommodate the gears' teeth.
Both outfeed devices can operate under all five reactors,
but only one device can be operated at a time. The other
device is for standby use.
Figure 13.5 Drive End of Outfeed Screw at the Bottom
of the Reactor
The header channels operate in alternating supply and
exhaust aeration modes (i.e., one head is supply; the next
head is exhaust, etc.). The operating mode of the head-
ers is reversed every 20 minutes.
The airflow to individual headers cannot be adjusted in
the field. Airflows can be adjusted and equalized by
inserting air profile tubes in the air lances. The air distri-
bution system was designed to create uniform distribu-
tion of flows and pressures throughout the lances.
When operated for recycle, the outfeed device typically
discharges the material at a rate of 0.004 to 0.006 m3/s
(480 to 760 cf/hr), based on 609 to 673 kg/m3 (38 to 42 Ib/
cf) of product. The reactor outfeed conveyor has a capac-
ity of 0.02 m3/s (2,500 cf/hr).
Although wear on the outfeed device has been nominal
during its short operating period, the device has had two
major problems. First, the pins connecting the screw to
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the reduction gears that drive the nonmotorized support
carriage failed. Consequently, although the screw kept
turning, the nonmotorized support stopped advancing
through the compost and "fell behind" the motorized end
of the screw. Eventually, a 0.76 (2.5-ft) skew developed in
the screw and the screw stopped. Within several days,
the screw had been straightened enough to remove it
from the reactor. This did not require the reactor to be
emptied.
The second outfeed problem occurred when the channel
bolts on the outfeed device broke because the grade of
steel used was not strong enough (see Figure 13.6). The
bolts were being replaced during the site visit.
Figure 13.6 Outfeed Drive Tracks Where the Channel Bolts
Were Breaking
13.3.4.5 Leachate/Condensate
Condensate from the air is collected in the heat
exchanger and drained to the WWTP The condensate
from the reactor and process building is sent to the
WWTP aeration tanks.
At the bottom of each reactor is a shallow basin filled with
gravel, where leachate from the compost originally was
collected, drained away, and pumped to the WWTP head-
works. The original sump pump was undersized, how-
ever, and limited the amount of leachate that could be
drained from the compost to the sump. This problem
restricted the use of belt washing devices and the
amount of water that could be drained frorn the odor
scrubber. To overcome this limitation, at the time of the
site visit, only wastewater drainage from the odor control
scrubber (see Section 13.3.7) drained into the sump.
13.3.5 Exterior Curing/Storage
The curing/storage area is a 15 m by 29 m (50 ft by 96 ft)
paved area covered by a pole structure with three open
sides. Material is stored without aeration in approximately
2.7-m (9-ft) high piles. Because this area is surrounded by
lawns on three sides, only one side of the structure can
be approached with a vehicle. This limits the operator's
ability to maneuver the material and to choose the age of
the material to be moved off site, because the last prod-
uct deposited in the structure must be the first product
removed. There are plans to correct this situation.
The curing/storage area is reportedly designed to hold 3
months' of material. Material is also being stored in an
adjacent unpaved area.
All runoff from the paved curing/storage area is collected
and returned to the WWTP
13.3.6 Nonprocess Air Handling
The sawdust silo is equipped with a separate air supply
system to promote sawdust drying. The air is delivered
with the same kind of lance system used in the reactors.
The supply blower delivers 0.71 m3/s (1,500 cfm) at 0.3 m
(12-in.) W.C. and the exhaust blower draws 0.88 m3/s
(1,875 cfm) at 0.3 m (12-in.) W.C. This system dries the
sawdust to 45 to 60 percent solids, increasing the solids
content of the sawdust by approximately 5 percent.
Two roof exhaust fans are provided in the mixing room;
each removes air at a rate of 0.272 m3/s (576 cfm). To
prevent the discharge of excess odors to the outside
atmosphere, the use of these fans was discontinued.
Finally, one fan in the reactor headspace removes a total
of 2.8 m3/s (6,000 cfm) of air to the odor control scrubber.
Although this flow rate is equivalent to 1.8 air changes per
hour, the fans are not large enough to satisfactorily
reduce the fogging and odors generated in the head-
space area. The proposed modifications to the odor
treatment system (see Section 13.3.7) recommend that
this flow rate be doubled to 5.7 m3/s (12,000 cfm).
13.3.7 Odor
13.3.7.1 Sources and Characterization
The existing WWTP incinerator began operation about
1980. It was not well maintained and sludge was often
pumped to an onsite lagoon when the incinerator was not
operating, resulting in complaints from neighbors. When
the incinerator operated on a regular basis, the plant only
received odor complaints five to six times over a 5-year
period.
After the composting plant started operating, there were
17 odor "incidents" and many more complaints until com-
posting was stopped in January 1988. When the condi-
tioning reactor was started in the summer of 1988, the
plant received only one complaint until November of that
year.
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North and northeast winds carry plant odors directly to
the neighbors, and odor complaints have generally been
limited to the immediate neighborhood. Although experi-
ence is limited, odors appear to be worse in summer
when neighbors complain of a "compost" smell (rather
than ammonia).
The plant manager investigates and responds to most
odor complaints. According to his findings, there is some
question as to what the neighbors are actually smelling,
although occasionally, the odors have been clearly linked
with the composting plant. Regardless, the city council
made a policy decision to satisfactorily address the odor
concerns of the neighbors.
13.3.7.2 Odor Treatment System
The odor treatment system is a single-stage vertical wet
scrubber 1.5 by 3.0 m, 7.1 m3 (5 by 10 ft, 250 cf) designed
primarily to remove ammonia from the process air. The
scrubber was sized for a flow rate of 6.6 to 11 nWs
(14,000 to 23,000 cfm), with a calculated scrubber deten-
tion time of 0.65 to 1.1 seconds.
Sulfuric acid is used to keep the scrubbing solution pH
between 5 and 6. This solution is recirculated, and
"DeAmine" (a proprietary odor control chemical) is
injected into the airstream before it reaches the scrubber.
The plantfe ability to replenish scrubber water (i.e.,
replace the scrubber water with fresh water and chemi-
cals) is limited by the undersized sump pump (see Sec-
tion 13.3.4.5).
Air exits the top of the scrubber, which is located approxi-
mately 1.5 m (5 ft) from the side of the reactor building
and approximately 4.5 m (15 ft) below the top of the
building. This arrangement limits the dispersion of the
discharge air. Based on the ineffectiveness of the original
odor control system, a new odor control system for pro-
cess air was being designed at the time of the site visit,
but exact specifications had not been decided. The new
system will be two-staged and sized to handle 11.0 m3/s
(24,000 cfm) of process air, mixing room air, and air from
the reactor headspace. The existing scrubber will be
used to cool the process air before it is fed into whatever
new control devices are installed.
There is currently no odor control for the mixing room air,
although the system just described will include provi-
sions to control the odors from this area. The operations
room in the process building has a separate air condition-
ing system. Odors (and noise) in this area are reportedly
"acceptable."
Potassium permanganate is added to the sludge before
dewatering to decrease odors.
13.3.7.3 Control Studies and Modifications
Due to complaints received since the start of composting
operations, the city hired TRC Environmental Consult-
ants to conduct an odor control study. In the study, the air
flowing both to and from the existing odor scrubber was
analyzed using dynamic olfactometry and gas
chromatography/mass spectrometry (GC-MS). The tests
indicated that the scrubber was ineffective at removing
odors when using either water or sodium hypochlorite
solutions.
A number of organic compounds were identified in the
airstream. In this stage of the testing, a person familiar
with the odors produced at the plant sniffed the various
compounds alongside the mass spectrometer samples
collected from the plant, as they were eluted from the gas
chromatographic column. In this way, the most unpleas-
ant and most intense odor could be correlated with a
particular compound. Dimethyl disulfide was identified as
the "worst" compound.
To reduce these odors, TRC recommended increasing
the oxidation capabilities of the existing scrubber by
ceasing recirculation of the scrubber liquid and by either
increasing the hypochlorite dose or switching to a
stronger oxidant such as potassium permanganate.
Ammonia is the only compound measured regularly (sev-
eral times per week) at the plant and, typically, the scrub-
ber is found to remove 50 percent of the ammonia from
the airstream. The week before the site visit, with only the
conditioning reactor in operation, the ammonia concen-
tration was 12 ppm in the air flowing in the scrubber and 6
ppm in the air discharged from the system. "Normal"
concentrations in the process air are 20 to 30 ppm. Slight
increases in concentrations were observed just after
reversal of the aeration valves. Some ammonia was also
present in the blower room on the middle level of the
reactor building..
The plant experienced greater odor problems when using
very dry and powdery recycle in the mix (and either
hardwood or pine sawdust). It is not known whether this
was a coincidence. When the plant starts up again, the
operator intends to compost the sludge both with and
without recycle to observe the effects on odor
production.
13.3.8 Support Facilities
The administrative wing of the main process building
supplements the WWTP facility. This area contains the
supervisor's office, a spare office, a reception area,
restrooms, and a conference room. Showers and lockers
are also located in the process building as well as in other
locations in the WWTP The composting facility uses the
WWTP laboratory.
13.4 Monitoring and Performance
13.4.1 Reactor Control Strategy
The existing control system does not include automatic
control of blowers. Instead, based on reactor tempera-
ture, the operator manually controls the aeration system
rate, which is normally kept at full capacity because of the
high reactor temperatures.
160
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13.4.1,1 Temperature Monitoring and Control
The operator would like to keep temperatures in the 55°
to 70°C (131o to 158°F) range. The upper limit of 70°C
(158o F) was chosen for two reasons: first, more odors
appear to be produced in reactors that are operated at
temperatures above 70°C (158o F) than those operated
below 70°C (158o F); and second, the operator is con-
cerned about the possibility of spontaneous combustion
of the compost.
The facility must meet the Federal EPA regulatory
requirements for maintaining a compost temperature of
at least 55°C (131 °F) for 3 days, but it had difficulty
getting its reactor temperatures down into the desirable
range set by the facility. At the time of the site visit, the
bottom of the conditioning reactor was 65° to 75°C (149°
to 167°F). Temperatures in the first-step reactors were
generally greater than 70°C (158°F) and appeared to be
relatively uniform throughout the reactor (i.e., there was
no defined profile of hot or cool zones). Second-step
reactors exhibited the same temperature range.
Temperature is monitored by three temperature probes
mounted on the centerline of each reactor. Each probe
has thermocouples to measure temperatures at three
different depths. The facility's original temperature
probes had calibration problems and new probes were
being installed during the site visit.
13.4.1.2 Other Tests and Equipment
To monitor plant performance, heavy metals in the sludge
are measured on a monthly basis; heavy metals in the
compost must be measured more frequently before the
compost can be sold. Currently, the facility measures
heavy metals each week in the compost from composite
samples (see Table 13.1).
Table 13.1 Sludge and Compost Constituents (Average Values and Ranges)
Constituent
Cadmium, Cd
(mg/kg) -
Chromium, Cr
(mg/kg)
Copper, Cu
(mg/kg)
Mercury, Hg
(mg/kg)
Nickel, Ni
(mg/kg)
Lead, Pb
(mg/kg)
Zinc, Zn
(mg/kg)
Polychlorinated
biphenyls, PCB
(mg/kg)
Percent solids
PH
Fecal coliform/g
Sludge3
8.4
(7.2-9.6)
553
(450-650)
733
(740-870)
0.47
(0.3-0.78)
35
(28-46)
111
(95-140)
2,173
(600-5,000)
0
—
19.7
(18.22)
—
— •
—
—
Mix"
2.3
(2.3- < 4)
15
(12-18)
87
(57-117)
0.12
(0.1-0.14)
6
(5.7-6.3)
20
(18-22)
100
(100)
—
—
—
—
—
—
Compost to
Storage'
3.4
(2.7-4.0)
44
(37-50)
155
(130-180)
0.12
(0.12)
8
(7.5-8.5)
50
(41-58)
155
(136-173)
0.25
(0.2-0.3)
—
—
—
—
Finished
Product"
4.9
(3.5-6.2)
36
(23-42)
141
(57-180)
0.42
(< 0.03-0.6)
9.4
(4.8-13)
42.1
(29-50)
141
(84-180)
0.5
68.0
(66-72)
7.2=
(7.0-7.45)
5.2X10=
(0.24-930) X 10=
NY State
Standard
for Case 1
10
1,000
1,000
10
200
250
2,500
e-
•^
"Five samples, 8/88-10/88.
"Two samples, 9/87-10/87.
"Two samples, 12/87.
"Seven samples, 9/87-11/87.
•— = No standard.
'Four samples, 9/87-11/87.
"Five samples, 9/87-11/87.
161
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To determine the percent solids in the compost mix,
measurements are taken with a heat lamp several times
per shift (or loading period). One sample per load is sent
to the laboratory for solids determination using the stan-
dard oven-drying technique.
Mix solids are tested again twice per loading period using
an analytical microwave technique, which gives results in
15 minutes. In the operator^ experience, the results from
the microwave technique correlate well with the conven-
tional oven-drying technique. During operations, the mix
is controlled "by eye," and is then verified by the micro-
wave test.
13.4.1.3 Data Management
To verify that the facility is meeting time-temperature and
detention-time criteria, the facility must submit a monthly
summary of operating data to the State.
The software of the computer data acquisition system at
the Schenectady facility was upgraded so that it could
record a greater number of measurements, in addition to
simply controlling operations. The aeration and tempera-
ture data are now recorded on a computer every 4 hours.
Once per month, these data are printed in a spreadsheet
format.
13.4.2 Mass Balance and Reactor Detention Time
Due to a lack of data, a mass balance or materials flow
diagram is not included in this case study.
Contract specifications call for a plug-flow detention time
of 28 days under average conditions using recycle in the
mix. The operator calculated that the actual plug-flow
detention time ranges from 18 to 21 days, based on the
average input and output volumes when operating at 17
dt/day (19 dton/day).
13.4.3 Product Quality
Compost is regulated under the New York State sludge
land application regulations. In New York, compost is
classified as a Class I product and can be land-applied
except where food crops are grown. Under New York
State regulations, the user must be notified of the prod-
uctfe origin and characteristics via package labeling.
The design specifications call for a product that is essen-
tially pathogen-free, based on normal reactor operations
at a minimum temperature of 55°C (131 °F) maintained
for a minimum of 3 days. The product must also have the
following characteristics:
• Friable texture
• No objectionable odor of sludge or ammonia
• Complete inactivation of weed seed
• pH between 5.5 and 8.0
• Product moisture of no more than 55 percent
Typical constituent concentrations are shown in
Table 13.1. The product does reheat when it is piled in the
curing/storage yard. Even after a year, the center of a pile
produces steam and is slightly odorous when disturbed.
The operator aims for a solids moisture content of 55 to
65 percent. In his opinion, the potential materials han-
dling problems (stickiness, etc.) and the greater product
weights of compost that contain less than 50 percent
solids decrease the attractiveness of the material to
users. Conversely, the operator's experience has been
that product with a solids content greater than 65 percent
is very dusty and has the potential to cause fires.
73.5 Operations
13.5.1 Staffing
The facility operates 5 days per week, with the hours
determined by the length of time needed to load the
reactors. Reactor loading time is based on two factors:
• The time needed to dewater the day's sludge. (The
capacity to dewater one day's production of sludge in 8
hours is not usually a limiting factor.)
• The time needed to produce recycle or to clear space
in the reactor. (The outfeed device traverse speed is
limited to minimize plowing of the compost in front of
the auger.)
The plant staff consists of one shift supervisor and two
operators, one on permanent assignment and one who
rotates in from the WWTP staff to perform housekeeping.
One front-end loader operator is provided by the WWTP
on an as-needed basis, usually 1 day per week,, and
maintenance crews are also provided by the WWTP staff
as needed. The WWTP plant supervisor oversees the
compost plant as well.
The WWTP supervisor has a B.S. degree in environmen-
tal microbiology; 7 years of laboratory experience; 10
years of WWTP experience; and a Grade la (highest)
operator certification. All operators are certified Grade
lla. No one on the staff has had prior composting
experience.
No formal training on composting principles or process
operations has been provided. On-the-job training in
equipment operation was provided by ABT, who was on
site for training purposes during July through September
1987. The foreman of the WWTP was also trained as a
backup composting plant shift supervisor. A preventive
maintenance program is in place.
162
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13.5.2 Marketing and Distribution
The city does not want to market the product by itself, but
a long-term marketing plan is not in effect. Although local
demand for the product and its marketability have not yet
been determined, to date of the site visit, all compost
produced had been sold to local individuals for $4/m3 ($37
cy). The city was evaluating four different marketing pro-
posals from different commercial bidders, as follows:
• Guarantee to take all compost produced for $0.65/m3
($0.50/cy)
• No guarantee but will pay $1.3/m3 ($1/cy) for all com-
post sold
• No guarantee but will pay $4.0/m3 ($3/cy) for all com-
post sold
• No guarantee but will split revenues with the city for all
compost sold
Final users are expected to be nurseries and
landscapers.
13.5.3 Operating Costs
Due to the lack of operating experience, it was not possi-
ble to estimate operations and maintenance costs. As
discussed, the odor control system upgrades, reactor
structural modifications, and the outfeed device modifi-
cations all increased the costs of operating the facility; in
addition, the incinerator had to keep operating.
73.6 Update
As of June 1989, the plant was still not operating pending
completion of construction of the new odor treatment
system. The three-stage, packed tower wet scrubber is
expected to go on-line by the end of September 1989.
Pilot testing for the odor treatment system was performed
in January using the conditioning reactor. Some odor
sampling and testing, including "sniff" panels was also
done.
163
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APPENDIX A
165
-------
Glossary
Aerobic—A condition in which free molecular oxygen is
available.
Amendment — An organic material added to the feed
mix to increase air voids for proper aeration and increase
the quantity of degradable organics in the mixture.
Anerobic — A condition in which no free molecular oxy-
gen is available.
Back Pressure — The pressure required to force air
from the discharge side of a blower through an aeration
distribution system and a compost pile.
Belt Filter Press—A mechanical dewatering device that
uses a combination of gravity and pressure filtration to
remove water from sludge.
Biofilter — A compost or soil bed through which gases
are passed used for odor control purposes.
Blower Agent — A material of sufficient size to provide
structural support and maintain airspace when added to
wet sludge.
Chokepoint — A point in a system at which the flow of
materials is constricted or reduced.
Clarifiers — A tank through which wastewater is passed
to remove settleable solids.
Compost — A stabilized product consisting largely of
decayed organic matter that can be applied to land with-
out adverse environmental effects.
Composting—The biological decomposition and stabi-
lization of organic substrates under conditions that allow
development of thermophilic temperatures as a result of
biologically produced heat. The final product is suffi-
ciently stable for storage and application to land without
adverse environmental effects.
Curing — Phases of the composting process in which
degradation of refractory organics takes place. Curing is
differentiated from the early stages of the composting
process by a slower overall rate of biological activity.
Dewatered Sludge — A sludge that has had some of its
water content removed by physical and chemical means.
A mixture of primary and waste-activated municipal
sludge that has been thickened and dewatered may have
a solids content ranging from 15 to 40 percent depending
upon the thickening, conditioning, and dewatering tech-
nologies employed.
Dewatering — A physical/chemical process that
removes part of the water in sludge.
Diffuser — A porous pipe used to introduce air into a
compost pile or reactor.
Energy Balance — The accounting of all exchanges and
transfers of energy within a defined system in accord-
ance with the principle that energy is neither created or
destroyed.
Extractoveyor — A device used to remove compost
from Paygro reactors for the purposes of mixing or con-
veying to another part of the compost system.
Fan — A device used to move air or gas at low pressures.
At a compost facility, fans are generally used for building
ventilation.
Feed — The input to the compost process.
First Reactor — The first reactor in series in a compost
system. The first reactor may be the only reactor in the
compost system or it may be followed by other reactors.
Gravity Thickener — A tank used to thicken sludge by
gravity settling.
Headloss — The loss of hydraulic pressure as a fluid or
gas moves through a conduit or porous medium.
Live-Bottom Bin — A storage bin with one or more
screw conveyors located at the bottom for the purposes
of extracting material from the bottom of the bin.
Mass Balance — The accounting of all exchanges and
transfers of mass within a defined system in accordance
with the principle that mass is neither created or
destroyed.
Mix — The combination of sludge, amendment, and recy-
cle that is fed to the composting process.
Negative Aeration — An aeration system which draws
air from the atmosphere through compost by creating a
vacuum at the base of the compost pile.
Nonprocess Air — Air collected from within a solids
handling system that has not been used for the aeration
of compost.
Pathogens — Disease causing organisms.
Phytotoxicity — A toxic or poisonous quality injurious to
plants.
Plenum — An enclosed space in which the pressure of
the air within the closed space is greater than atmo-
spheric pressure. Plenums are used to convey air or gas
streams throughout a treatment facility.
Plow-Blade Mixer — A device for mixing solid materials.
Mixing is accomplished by several plowshare-like mixing
tools fastened to the ends of short arms, which are per-
pendicularly attached to a rotating shaft. The device is
enclosed in a metal housing. Although dual shaft units
166
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are also manufactured, all of the units observed at the
compost facilities surveyed were single-shaft devices.
Plug-Flow Detention Time — The time required for
materials that enter a plug-flow reactor (including any
recycle stream) to pass through the reactor.
Plug-Flow Reactor — A reactor in which all materials
that enter the reactor at the same time flow through with
the same velocity and leave at the same time.
Porosity — The ratio of void volume to total volume.
Positive Aeration — An aeration system which forces
positive pressure air through compost to the
atmosphere.
Primary Sludge — Sludge from a primary clarifier. This
sludge generally consists of settleabie solids and tank
skimmings. Primary sludge produced by a municipal
wastewater facility typically contains degradable organ-
ics and some inerts.
Process Air — Air that has passed through composting
material (i.e., from the active component of the compost
process; not biofilters). Process air may be collected in a
headspace above the pile of compost, collected by a
diff user system below the pile of compost, or collected by
an exhaust system within the pile of compost.
Pugmill Mixer — A device for mixing solid materials.
Mixing is accomplished by dual rotary shafts with per-
pendicular paddles or blades, which intermesh when the
shafts are turning. The shafts are enclosed in a metal
housing.
Recycle — Compost product that is returned to the
beginning of the compost process and incorporated into
the feed mix.
Scrubber — An apparatus for removing impurities from
gases, usually by means of a chemical solution.
Second Reactor — The second reactor in a compost
system. A compost system may or may not have a sec-
ond reactor.
Septic — Anaerobic.
Sludge—The solid and semisolid residuals and concen-
trated pollutants removed by wastewater treatment
processes.
Solids Retention Time — The average time that a typi-
cal solids particle remains within a reactor (including
recycle). Because some portion of the reactor product is
generally recycled back to the reactor feed, the solids
retention time is generally longer than the plug-flow reten-
tion time.
167
-------
-------
APPENDIX B
169
-------
Unit
°C
kg
kg/m3
kg/m*
kg/s
km
kJ/kg
kJ/kg
kg/m3
L
m
m
m
m3
m3
m*
m*
m3/s
m'/m3 x s
mVs
mVs
m3/t
Pa
t
Conversion Factors
Multiply by
1.8 (°C + 32)
2.205
6.243 X10-2
5.448 x 1&4
132.2
0.6214
0.430
126.0
1.686
0.2642
3.281
39.37
1.094
35.31
1.308
10.76
2.471 X 104
2,119
5.999 x 104
1.271 X 105
22.83
32.15
1.45 X10"4
1.102
To get
°F
Ib
Ib/cf
Ib/acre
Ib/min
mile
Btu/lb
kWh/lb
Ib/cy
gal
ft
in.
yd
cf
cy
ft2
acre
cfm
cfm/1,000cf
cfh
mgd
cf/ton
psi
ton (short)
170
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APPENDIX C
171
-------
Examples of Technical Information
Required of System Suppliers and System Selection Criteria
In this appendix are lists of information requirements and
selection criteria used in four different in-vessel projects.
These lists provide examples of the kinds of issues oth-
ers have considered important in choosing an in-vessel
system. Each public agency must to develop its own list
to reflect the purposes and constraints of its project.
City A (1988)
The procurement process at City A includes three
phases. In Phase 1, the City's consulting Engineer will
formally solicit information from equipment suppliers and
prepare system layouts and life-cycle costs from the sup-
pliers' submittals. From this information, and using the
criteria listed below, the individual suppliers will be cate-
gorized as Preferred (one vendor only), Acceptable, or
Unacceptable. In Phase 2, the Engineer will prepare
plans and specifications based on the preferred technol-
ogy. The Engineer will also prepare cost estimates for the
redesign of the facility to accommodate each of the
acceptable technologies. In Phase 3, construction bids
will be solicited. The final selection will be based on the
lowest bid. Bids from acceptable technologies (other
than the preferred one) must include the Engineer's rede-
sign costs.
Technology Selection Criteria
(Weighting factors shown in parentheses are those used
by City A and are not EPA recommendations.)
A. Cost (Group Weighting Factor 30)
1. Initial Construction Cost
Note: After completion of the evaluation process,
but before selection of the preferred system, the
City will request a guaranteed price on all equip-
ment provided by the Supplier.
The Engineer will determine the overall capital
costs of the facility based on information submit-
ted by the supplier and with costs developed by
the Engineer. The Supplier should provide the
following information:
» Costs for each item of equipment that will be
supplied and/or installed.
» Information for all other equipment required
so that Engineer may develop these costs.
• Information (including construction materi-
als) on all buildings and other facilities
required so that the Engineer may develop
these costs.
« Reference all costs to July, 1990, the esti-
mated bid date.
2. Annual Operation and Maintenance Cost
The Engineer will determine the annual opera-
tion and maintenance costs based on an aver-
age sludge production of 9 dt/day (10 dton/day)
using the following information provided by the
Supplier:
• Indicate the number of operators for opera-
tion and maintenance of the facility.
• Provide annual costs for spare parts.
• Provide an estimate of the proposed amend-
ment usage based on the average month
mass balance.
• Provide the average daily energy require-
ment in kilowatt-hours per day. List motor
sizes and operation time requirements for
each piece of equipment provided. Include
conveyors, blowers, fans, mixers and reactor
equipment. Include heating and lighting
requirements.
• Determine the peak power demand in kilo-
watts. Identify equipment operation
assumptions.
3. Present Worth
» The Engineer will use a 20-year life cycle,
and a discount rate of 8.75%.
B. Operations and Maintenance Considerations (20)
1. Number of Operators Required for Operation
and Maintenance (2)
This criterion will address the likelihood that the
City will be able to hire and train qualified opera-
tors for the facility. (The cost aspect of the num-
ber of operators is considered under O&M cost
criterion.)
2. Frequency and Description of Major Mainte-
nance Tasks (2)
Provide a description.
3. Special Skills or Training Required for Operation
and Maintenance (2)
Identify contract maintenance requirements or
training required for the City's operation and
maintenance staff.
172
-------
4. Adequacy of Area for Equipment Access and
Removal (3)
Provide dimensions and descriptions.
5. Method of Adjustment to Loading, Mix Charac-
teristics, Solids Content or Temperature (1)
Include information on how the feed to the mixer
is adjusted (weight or volume basis), and use of
variable speed drives.
6. Simplicity of Instrumentation and Control Sys-
tem (2)
Describe reliability of systems and level of skill
required for operation.
7. Simplicity of Equipment Systems (4)
Describe reliability of systems and level of skill
required for operation.
8. Operator Environment (4)
Describe the operator environment for normal
operation and routine maintenance. Submit
assurances that safety, environmental and
health conditions have been addressed, and
that these conditions meet State and Federal
standards.
C. Process and Performance Considerations (25)
1. Ability to Meet or Exceed Project Performance
Requirements (7)
Project performance requirements are shown in
separate design criteria and a mass balance.
The supplier must provide backup design calcu-
lations that show how each of the requirements
are to be met or exceeded.
2. Redundancy Provided for Critical System Com-
ponents and Standby Equipment (2)
Identify the capacity and the number of backup
equipment units used for short-term or long-term
requirements.
3. Quality of Materials of Construction (2)
Describe the materials used for construction and
the rationale for their use. Include the materials
and type of construction for the composting
buildings, even if they are not provided by your
company.
4. Proposed Hours of Mixing and Materials Trans-
fer(1)
Provide an operating schedule in bar chart (time
line) form.
5. Flexibility of Operation/Plan for Process Upset
(2)
Identify possible upset conditions and remedial
actions.
6. Aeration System Design Approach — Normal
and Upset Conditions (2)
Provide design approach to meet peak, average
and upset conditions.
7. Characteristics of Process Air (4)
Include a description of the air collection 'points
and the chemical characteristics, volume, pres-
sure, humidity and temperature of the process
air. Describe a recommended method for odor
control. (Engineer will design the odor control
system.)
8. Characteristics of Nonprocess/Ventilation Air
d)
Include a description of the air collection points
and the chemical characteristics, volume, pres-
sure, humidity, particulate content, and tempera-
ture of the nonprocess/ventilation air.
9. Building and Site Layout/Total Area Require-
ments (2)
Show the arrangement of all facilities and equip-
ment required, including those facilities not pro-
vided by your company. Consolidation of
operation and visual screening of operation from
neighbors are important considerations.
10. Access Arrangements (1)
Show pedestrian and vehicle access on a site
plan.
11. Utility Relocation Requirements (1)
Provide a utility relocation plan.
D. System Experience and Reliability (25)
1. Provide capacity, configuration, and required
modifications and costs for each example
project.
2. Service Record of Major Equipment/Mechanical
Reliability (5)
For major proprietary equipment items, list facili-
ties with identical or similar equipment, number
173
-------
of years of service, performance, and major
equipment modifications/improvements.
3. Operating Records of Representative Projects/
Performance Test Results (6)
Include data where available.
4. Company Qualifications/Staff Qualifications (3)
Provide company organization and total staff,
and an indication of who will likely work on this
project.
5. Service Capability of Company (5)
Provide equipment and performance warranty.
Indicate length, coverage and conditions, and
extended service capability.
6. Financial Capability (4)
The City intends to require a labor and materials
bond and a performance bond for the construc-
tion contract. Suppliers are requested to furnish
certification that such bonds have been issued
on other projects and can be issued for this
project.
City B (1988)
Evaluation Criteria (Not in order of importance)
1. Construction Costs — From take-off of proposal
submittals using standard cost estimating
documents.
2. Life Cycle O&M Costs — Based on information
submitted in proposals on labor, power, sawdust,
and maintenance costs.
3. Reliability of Equipment and Process
a. Required Downtime
b. Track Record of Proposed in/out
Feed Devices
c. Conveyance Systems
d. Process Reliability
e. Equipment Complexity
4. Marketing Program
5. Flexibility of Equipment and Process
a. Complexity of Equipment
b. Control of Process
6. Efficiency of Site Layout
a. Aesthetics
b. Land Requirement
c. Logistics of Layout
7. Ease of Operations and Maintenance
a. Ease of Access
b. Control of System
c. Ease of Upset Recovery
8. Qualifications, Experience, and Responsibility of
Bidder
a. Qualifications
b. Experience
c. Responsibility of Bidder
9. Ability to Meet Needs of the Project
a. 45% Solids In Compost Product
b. Odor Control Provisions
c. Schedule
City C (1987)
City C solicited proposals for private sludge composting
services. Prospective bidders were requested to submit
financial as well as technical proposals. Documents
required with the proposal:
1. Signed bid proposal filled out as required and
enumerated in the Proposal Section of the Pro-
ject Manual.
2. Cash, Certified Check, or Bid Bond in the
amount of 5 Percent of the Bid Price as bid
security.
3. Corporate Certificate duly authorized including
Source of Authority.
4. Written Affirmative Action Program as specified
in the Project Manual.
5. Bid Letter from approved Surety Company
agreeing to furnish a Performance Bond and a
Labor & Materials Bond in the event of an
award.
6. Contractor's Certification.
7. Subcontractor's Certification (if any).
8. Certificate of Insurance.
9. Any Power of Attorney or other Certification, if
requested.
10. Minority Business Utilization Certification.
11. Description of the Proposed Disposal and/or
Reuse Facilities, if applicable, to be provided by
the bidder.
Selection Criteria
A. Completeness of Proposal
174
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1. Has all required information been supplied?
2. Bid letter supplied?
3. Any affirmative action plan or program
submitted?
B. Experience
1. Performance and Accountability
a. Experience level of the firm in completing
and operating projects of a similar nature
and scope to that detailed in this Request for
Proposal.
b. References from past or current contracts.
c. Established business office and staff.
d. Previous Municipal Contracts.
e. Defaults within past 5 years.
2. Operations
Two years minimum experience in sewage
sludge composting.
3. Personnel
a. Experience qualifications of the personnel
to be assigned to the project.
b. General performance.
c. Training and/or experience.
d. Experience on similar projects.
4. Experience Rating
a. Prior systems, materials and equipment in
relations to the City's requirements.
b. Size of prior similar contracts.
c. Size of other types of work contracts in rela-
tion to this project.
C. Financial Strength
1. Financial stability of the proposing firm and the
adequacy in the aggregate of all insurance,
indemnities, and other protections and security
offered the City to protect the City from liability
and expenses, other than the Contract price, that
may arise or be incurred in connection with the
contract or the construction, if any, and the con-
tinued operation of the project.
2. Solvency of business as evidenced by financial
statements.
3. Ability to finance new equipment through bank,
equipment manufacturer or vendor, and/or letter
of credit.
4. Has firm been engaged in performing similar
services under the same name and organization
for a minimum of 1 year? (For joint ventures,
each firm shall meet same requirement.)
D. Business Proposal
1. Ability of the proposing firm to provide the ser-
vice for the entire proposed contract period.
2. The adequacy of the project development plan,
scheduling and ability to complete the tasks
within the project time frame.
3. Has the proposer indicated his acceptance of
the City's sludge within a reasonable period of
time?
4. The tipping fee or purchase price per wet ton.
City D (1985)
City D solicited proposals for private sludge composting
services. The City was particularly interested in avoiding
any technology which had not operated successfully on a
commercial scale. Accordingly, the information require-
ments for each respondent to the Request for Qualifica-
tions were to be based on an existing sludge composting
plant designed, constructed, owned or operated by the
respondent or using the respondent's licensed technol-
ogy. This is the so-called "Reference Plant". Reference
plants could be either domestic or foreign. The primary
stipulation was that the Reference Plant must have been
operative commercially for at least 1 year.
Besides the technological information requirements
detailed below, the City requested information on market-
ing plans, the organization of the project, and the finan-
cial resources of each respondent. The relative weights
used in making the final choice were:
Technical Information 2
Compost Marketing Information 2
Organizational Information 3
Financial Information 3
Technical Information Requirements
1. Reference Sludge Composting Plant — Mini-
mum Criteria
The Reference Plant must meet the following
criteria:
a. The Reference Plant must be equipped with
control devices which operate effectively in
order to alleviate potential environmental
problems. These must include means for
removal or containment of particulate mat-
175
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ter, elimination of pathogens, and suppres-
sion or containment of odoriferous com-
pounds. In addition, any liquid effluents must
be safely disposed.
b. The Reference Plant must provide for the on-
site storage, mixing and processing of mate-
rials (sludge, bulking agents and recycle
compost) in such a manner that the require-
ments of paragraph 1.a are met.
c. The Reference Plant must include integral
appurtenances for composting and subse-
quent curing at the same time.
d. The processing line(s) must be equipped
with automated process controls which
allow for the optimization of the plant's oper-
ating parameters. Typically, these include
oxygen, carbon dioxide and temperature
monitors.
e. The Reference Plant must perform in a
utility-like capacity, providing dependable
sludge disposal services to an affiliated
wastewater treatment plant and delivering
quality compost for export and sale on a
continuous basis.
f. The Reference Plant must have been in con-
tinuous operation for at least one (1) year
under the direction of a licensed, profes-
sional wastewater treatment plant operator,
or an individual with similar documented
qualifications.
2. Reference Plant — Description
Provide a short written description of the Refer-
ence Plant which includes:
a. Name and location of plant.
b. Owner of plant.
c. Operator of plant.
d. A block diagram showing the principal par-
ties responsible for the Reference Plant and
the contractual relationships between the
principal parties.
e. Technical description:
— Give the name of Reference Plant's
designer.
— Give the name of Reference Plant's
builder.
— Give the name of the regulatory agency
which approved permits.
— Briefly describe the facilities used for
materials storage (i.e. sludge, bulking
agent and recycle compost), giving stor-
age capacity in tons and the design bulk
weights (in storage) in Ib/cu. ft.
— Briefly describe the mixing equipment
in terms of its purpose, type, and
throughput capacities in maximum con-
tinuous rating (MCR), wet short tons per
hour (WSTPH). The MCR capacities
must be stated separately for each com-
ponent and the total mix.
— State number of independent sludge
processing and containment vessels
and/or buildings used together with
their MCR capacities of sludge through-
put in WSTPH for a given solids concen-
tration (in % of TS). Each independent
processing line is meant to comprise all
equipment necessary to process sludge
after mixing into a finished compost
product, including any curing and
screening steps.
— State the number and type of major
materials-handling equipment associ-
ated with each line, including front-end
loaders, bulldozers, conveyors, mixers,
screens, extractors, and the like, giving
their unit capacities in WSTPH or cu. yd.
per hour, as appropriate.
— State the type of air pollution control
device(s) associated with each line and
provide actual test information which
quantifies the controlled emission rate.
Reference the test methods used.
— State the disposal method and/or treat-
ment method associated with each line
for the treatment of leachates and/or
run-off and provide actual test informa-
tion which quantifies the following efflu-
ent parameters: BOD (mg/L), COD
(mg/L) and SS (mg/L).
— Describe briefly the extraction, trans-
port, and storage system used for each
processing line with special emphasis
on the measures taken to avoid materi-
als blockage.
— Describe briefly the type of energy
recovery system used, if available, in
terms of the millions of Btu per hour
recovered at MCR and identify the type
of heat transfer fluid involved, together
with its design pressure (psig) and tem-
perature at the reactor outlet on the gas
side and at the heat exchanger outlet on
the liquid or vapor side.
f. Drawings/Figures
— Site layout plan showing main buildings,
176
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traffic pattern(s) and access from major
highway.
— Area map which relates the composting
facility to the wastewater treatment plant
which supplies the sludge.
— Main processing area/building layout
plan showing handling and storage sys-
tems for sludge and bulking agents, mix-
ing equipment, metering equipment,
independent processing lines, air and
water pollution control equipment, and
finished compost removal screening
and storage systems.
— Main process area/building cross-
section showing each elevation with
installed equipment.
— A cross-section of one sludge process-
ing line including the composting and
curing steps together with the finished
compost discharging mechanism.
'— One complete mass balance. This mass
balance must be subdivided into its gas-
eous, liquid and solid components. Sup-
porting calculation's should be
attached.
— One complete energy balance. This
energy balance must be subdivided into
its electrical, thermal and chemical
components. Supporting calculations
should be attached.
g. Operating Schedule and Staffing Plan
— Briefly describe the normal operating
schedule of the Reference Plant (hours
per day, days per week, and days per
year). State the normal operating mode
(in terms of number of processing lines
operating at the same time, scheduled
maintenance periods, number of shifts,
etc.).
— Submit a staffing plan showing the num-
ber of operating and administrative per-
sonnel employed during each shift and
their functions.
h. Sludge Supplier (for the Reference Plant)
— Give name of sludge suppliers) and
how the sludge is delivered.
— Describe the type of sludge delivered
(raw or digested, primary, secondary,
tertiary and/or mixed). Give solids con-
centration, volatile solids concentration,
and heavy metals content (cadmium,
copper, lead, mercury, nickel and zinc).
Other pertinent parameters such as
iron, nitrogen, phosphorus, potassium
and the pH factor should be included as
well.
i. Compost User(s)
— Describe compost user(s) and how the
compost is used.
— Describe the type of compost sold
(semi-finished or finished compost, soil
conditioner, blended with additives, forti-
fied with chemicals, etc.). State the
mesh size which the average particle
will pass, together with average mois-
ture content and representative bulk
weights in lb./cu. ft.
— One laboratory report either from the
regulatory agency, or from the compost
user, or from an independent testing lab-
oratory which characterizes the physi-
cal, chemical microbiological and
organoleptic properties of the compost
typically produced by the composting
plant and which certifies the acceptabil-
ity of the compost for its intended
use(s).
— Describe the form and major terms and/
or conditions of the compost sales con-
tract between the compost plant and
compost user(s).
6 US. GOVERNMENT PHNT1NG OFFICE: 1860-748-159/B0451
177
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