SEPA
United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
EPA-600/2-78-057
June 1978
Research and Development
A Study of
Forced Aeration
Composting of
Wastewater Sludges
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development. US Environmental
Protection Agency, have been grouped into nine series These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1 Environmental Health Effects Research
2 Environmental Protection Technology
3 Ecological Research
4 Environmental Monitoring
5. Socioeconomic Environmental Studies
6 Scientific and Technical Assessment Reports (STAR)
7 Interagency Energy-Environment Research and Development
8 "Special" Reports
9 Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161
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EPA-600/2-78-057
June 1978
A STUDY OF
FORCED AERATION COMPOSTING
OF HASTEHATER SLUDGE
by
William F. Ettlich
Anne E. Lewis
Culp/Hesner/Culp
Clean Water Consultants
El Dorado Hills, California 95630
Contract No. 68-03-2186
Project Officer
Francis L. Evans, III
Task Officer
Gerald Stern
Hastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publi-
cation. Approval does not signify that the contents necessarily reflect
the views and policies of the U.S.' Environmental Protection Agency, nor
does mention of trade names or commercial products constitute endorsement
or recommendation for use.
ii
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FOREWORD
The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health
and welfare of the American people. Noxious air, foul water, and spoiled
land are tragic testimony to the deterioration of our natural environment.
The complexity of that environment and the interplay between its components
require a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem solu-
tion, and it involves defining the problem, measuring its impact, and search-
ing for solutions. The Municipal Environmental Research Laboratory develops
new and improved technology and systems for the prevention, treatment, and
management of wastewater and solid and hazardous waste pollutant discharges
from municipal and community sources, for the preservation and treatment of
public drinking water supplies, and to minimize the adverse economic, social,
health, and aesthetic effects of pollution. This publication is one of the
products of that research; a most vital communications link between the
researcher and the user community.
Wastewater sludge can be converted by composting into a soil condition-
er as one method of recycling resources. Wastewater sludge has been com-
posted on a regular schedule at Beltsville, Maryland, and Bangor, Maine,
using forced aeration technology developed by the U.S. Department of Agri-
culture, Agricultural Research Service, Beltsville, Maryland. Both of these
operations have received grant funds from the U.S. Environmental Protection
Agency.
The results of an intensive study of these two operations are presented
in this report.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
iii
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ABSTRACT
This study was initiated with the overall objective of developing an
independent assessment of the forced aeration wastewater sludge composting
method as practiced at Beltsville, Maryland, and Bangor, Maine. A number of
visits were made to both sites to observe operations under all weather con-
ditions and to gather data. The analysis developed herein is based on in-
formation obtained during the site visits, information provided by various
agencies and individuals, and independent observations and calculations.
Results of the study indicate that forced aeration wastewater sludge
composting can be carried out in a satisfactory manner under nearly all
weather conditions, including severe New England winters. A number of prob-
lems and potential problems are identified along with possible solutions.
Costs per unit of sludge processed are very dependent on the size of the
operation and the methods used. Several possibilities are explored for re-
ducing costs by modifying operations.
This report was submitted in partial fulfillment of Contract No.
68-03-2186 by Gulp/Wesner/Culp - Clean Water Consultants under the sponsor-
ship of the U.S. Environmental Protection Agency. This report covers the
period June 1, 1976 to July 1, 1977, and work was completed as of July 1,
1977.
iv
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CONTENTS
Foreword ill
Abstract iv
Figures vi
Tables vii
Acknowledgments viii
1. Introduction 1
2. Conclusions and Recommendations 2
3. Operations 4
General description 4
Site visits 5
Materials handling aspects 5
Monitoring 16
Site considerations 20
Weather 20
Nuisances 25
4. Considerations for Future Operations 26
5. Compost Distribution 29
6. Costs 31
7. Pathogens 34
8. Results and Discussion 43
References 47
List of Metric Conversions . . 49
Appendix: Forced aeration composting and applications 50
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FIGURES
Number Pa9e
1 Beltsville composting site layout 7
2 Bangor composting site layout . 13
3 Total coliform densities 36
4 Fecal coliform densities 37
5 Salmonella densities 38
6 Relationship between disease risk and salmonella,
coliforms, and fecal coliforms 42
vi
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TABLES
Number pa
1 Site Visits and Weather ................... 6
2 Beltsville Equipment .... ...... .......... 10
3 Beltsville Materials Requirements For 15,000 Wet
Ton Annual Sludge Input .................. H
4 Approximate Materials Output, Beltsville Type
Operation. . .
5 Estimated Annual Operations Requirements, Bangor ...... 17
6 Bangor Equipment ...................... 18
7 Bangor Materials Requirements For 2,170 Wet
Ton Annual Sludge Input ..... ............. 18
8 Approximate Materials Output, Bangor Type
Operation ......................... 19
9 Beltsville Actual and Projected Costs ......... ... 32
10 Bangor Composting Costs , 1975-1976 ............. 33
11 Sources of Pathogen Data ............... • • • 35
12 Summary of Bacteria Data .................. 35
13 Relation of Dosage of Salmonella typhi to Disease. ..... 40
14 Dose of Various Species and Strains of Salmonella That
Caused Disease in Human Volunteers ............ 41
vii
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ACKNOWLEDGMENTS
Culp/Wesner/Culp - Clean Water Consultants gratefully acknowledge the
cooperation of many agencies in providing information and data for this re-
port.
The Maryland Environmental Service and United States Department of
Agriculture, Agricultural Research Service, provided data related to the
Beltsville operation and unlimited access to the site. The cities of Bangor,
Maine, and Durham, New Hampshire,provided complete data related to their
operations in addition to site visits.
The personnel of the Ultimate Disposal Section, Wastewater Research
Division, US EPA, MERL, provided much of the coordination required between
agencies and performed the technical review of the final report.
viii
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'SECTION 1
•INTRODUCTION
The objective of this study is to report on the major operational
'aspects of the forced-aeration wastewater sludge composting method as prac-
ticed at Beltsville, Maryland (Beltsville), and Bangor, Maine (Bangor).
Beltsville has been supported from December, 1975, through December, 1977,
as follows:
Blue Plains Participants
EPA Region III
U. S. Dept. of Agriculture
EPA Office of Research and
Deveopment
Total
Bangor has received grant funding from the EPA Office of Solid Waste
Management Programs. The forced-aeration composting process is a develop-
ment of the Agricultural Research Staff at Beltsville, Maryland.
The principal sources of information and data were the actual opera-
tions at Beltsville and Bangor with some support information from the
operation at Durham, New Hampshire, and the windrow composting operation at
the County Sanitation Districts of Los Angeles County Joint Wastewater
Treatment Plant at Carson, California.
This report represents an independent assessment of the forced-aeration
technique for composting raw wastewater sludge based on evaluation of avail-
able data and on-site observations.
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SECTION 2
CONCLUSIONS AND RECOMMENDATIONS
CONCLUSIONS
The forced aeration method of wastewater sludge composting is a bio-
logical stabilization process, but in terms of actual work is basically a
materials handling operation. All operations to date use mobile equipment
with little demonstration of fixed materials handling equipment. The use
of mobile equipment does provide for flexible and satisfactory materials
handling, but use of fixed equipment might contribute to decreased opera-
tional costs for some parts of the operation. Presently, operation and
maintenance costs vary from $86 to $150 per dry ton of wastewater solids
processed and depend on the installation size. Removal of costs related to
research and optimization of operations would reduce sludge composting costs
to $50 to $80 per dry ton of wastewater solids.
Composting has been accomplished outdoors in all types of weather on a
continuous basis at both Beltsville and Bangor. Heavy rain and extreme cold
have been minor problems and require more careful operation, but have not
interrupted composting operations.
Screening is greatly affected by weather. Screening cannot be accom-
plished outdoors in rainy or very cold weather. Therefore, screening must
be accomplished during favorable weather or the operation must be weather
proofed. Screening can be a limiting factor for a composting operation and
careful consideration must be given to the type of equipment, the average
daily screening capacity, the amount of material to be screened, moisture
content of the material to be screened, and the number of days during which
screening can take place.
Careful consideration must be given to the type and availability of
bulking agent to be used. Bulking agents can be a major operational cost
and may be difficult to obtain in certain areas of the country. There are
several potential techniques for conservation and reuse of bulking agent.
The most promising is to use compost from a pile being torn down as bulking
agent for new sludge.
The operations at Beltsville and Bangor are carried out with minimal
nuisances. Some odors are generated at both sites, but public complaints
have been nil and operations are never shut down; in fact, it is impossible
to "shut down" operations because composting and curing are continuous pro-
cesses. Operations can be stopped by hauling the materials to a landfill
or disposal site.
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Waterproofing of some of the normal unit operations is desirable. Com-
posting has been carried out at Beltsville for several years on an outdoor
gravel base. The recent installation of hard surfacing at Beltsville has
improved the operations and should be considered for all new installations.
The installation of covered areas for mixing, curing, and screening should
be considered for areas with heavy and prolonged rainfall.
Sufficient pathogen and indicator organisms data are available to
develop some generalized conclusions. Based on these data and additional
calculations, the risk of disease outbreak in adult humans from properly
operated wastewater sludge composting appear to be very slight considering
the tolerance level to pathogens exhibited by adult humans. Some risk may
exist, however, because there is no way to assure complete kill of all
pathogenic or parasitic organisms in outdoor composting unless the compost
is sterilized. Generally, the use of wastewater sludge based compost will
be subject to regulatory agency requirements in most states.
RECOMMENDATIONS
Operational methods should be investigated to minimize materials hand-
ling, use of bulking agent, and screening. Several methods which offer
potential savings in these operations are suggested herein. One major
method is reuse of unscreened compost as bulking agent through one or more
cycles. Some of these methods have been tested on a limited scale, but
should be tested and refined further. Use of these methods should reduce
operation costs and also provide some operation benefits such as conserva-
tion of heat during cold weather operations.
Careful plans should be developed for marketing or disposing of the
compost product. Some revenue may be realized from selling the compost, but
the return will not cover the total costs of producing the compost. Methods
are discussed for reducing compost production if desired, or necessary,
because of disposal limitations.
It is difficult to make recommendations related to pathogenic organisms
because any conclusions or illustrations developed herein are only opinions.
Some pathogenic organisms will likely be present in the compost unless it
is completely sterilized. Even with sterilization, reinfestation by certain
pathogenic bacteria is possible. The possibility of disease outbreak re-
lated to the use of properly developed wastewater sludge compost is very
small because humans do exhibit a tolerance to fairly high levels of patho-
genic organism infection before actually exhibiting signs of the disease.
It is recommended that composting operations be carried out to assure inter-
ior pile temperatures greater than 60°C for several days and that a 30 day
curing period be utilized following composting. These steps will assure
the highest possible pathogen kill. Other steps to minimize the ingestion
of compost by humans such as use of compost on non food crop applications
will reduce any potential of disease outbreak.
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SECTION 3
OPERATIONS
GENERAL DESCRIPTION
The Beltsville composting site has been used for several years for the
research and demonstration project. The composting operations are carried
out by Maryland Environmental Service (MES) personnel. The research acti-
vities are carried out by the U.S. Department of Agriculture, Agriculture
Research Service (ARS). Even though research activities are being carried
out at Beltsville, the basic sludge composting operations are similar to
what may be carried out by any municipality. In 1976, a total of 14,459
wet tons of undigested wastewater sludge conditioned with lime and ferric
sulfate and dewatered (approximately 23% solids) from the Washington, D. C.
Blue Plains plant was composted at Beltsville using the extended pile forced-
aeration method. This method is described in Appendix A. Operations were
carried out on a regular schedule as would be required by a municipal sludge
processing facility. In the first part of 1976, sludge deliveries were made
7 days a week while, during the last part of 1976, deliveries were made 5
days a week with only partial staffing on weekends. Sludge deliveries
averaged approximately 300 wet tons per week (approximately 60 wet tons per
day on a 5 day per week basis). Sludge delivery was accepted every working
day except when Blue Plains could not provide sludge or the delivery trucks
could not operate. This delivery schedule represents operation typical for
a municipality serving the equivalent of 140,000 to 160,000 persons. Sludge
deliveries have been as high as 100 wet tons per day for several days at a
time at Beltsville.
The analysis of operations at Beltsville provides guidance for planning
a regular municipal wastewater sludge composting facility. This report will
focus on the regular operations at Beltsville and will not emphasize the
research operations except where specific data such as pathogen organisms
densities provide applicable information. ^Sh& Beltsville operations
provide useful information for municipal sludge composting during various
weather conditions including cold, snow, rain, inversion conditions, and
fair weather.
Forced-aeration wastewater sludge composting also has been carried out
by the municipality of Bangor for over two years, through two winters, out-
doors and in very severe weather conditions. In 1976, the City composted
approximately 1,944 cu yd (332 dry tons) of 23 percent solids, dewatered
raw (primary) sludge at the City owned site at the Bangor International
Airport. Total Bangor sludge production in 1976 was 3,033 cu yd (525 dry
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tons). The sludge not composted (1089 cu yd) was hauled to a landfill.
This report is based on observations of the Beltsville and Bangor muni-
cipal sludge composting operations with data input from several on-site
visits and information provided by MES, ARS, the Town of Bangor, and USEPA.
SITE VISITS
To develop an independent and realistic assessment of the Beltsville
and Bangor operations/ considerable on-site observations were made at both
sites under various weather conditions as shown in Table 1. Information
was also obtained from discussions with operating, supervisory, and admin-
istrative personnel. Cooperation of all operating agency personnel was
outstanding.
MATERIALS HANDLING ASPECTS
Optimization of materials handling is of primary importance for suc-
cessful municipal sludge forced air composting operations. The volume of
materials handled varies almost directly with the input sludge quantity.
The type and characteristics of materials handled varies from site to site
depending on type of sludge, type of bulking agent, and weather conditions.
The optimum or most cost effective materials handling methods will vary
significantly depending on the size and how the operation is carried out.
Beltsville and Bangor provide information primarily on the use of mobile
equipment for materials handling because neither operation was using fixed
equipment at the time of this study.
Beltsville
An approximate scale layout of the Beltsville operations site is shown
in Figure 1.
Undigested sludge cake (approximately 23 percent solids) from the
Washington, D. C. Blue Plains wastewater treatment plant is delivered to
Beltsville in 10 wet ton loads by conventional 3 axle dump trucks. Two
trucks are used. Each truck makes 3 trips for 60 wet ton per day delivery.
Delivery generally begins early in the morning and continues into early
afternoon. Prior to the sludge delivery, pads of bulking agent are prepared
in the mixing area. These pads are 9 to 10 feet wide, 1 to 2 ft deep and
as long as required for the bulking agent to sludge ratio mix for one truck-
load of sludge. The sludge trucks arrive on site, are weighed, and then
dump the sludge onto the bulking agent pad. A front loader is used to
spread the sludge evenly over the top of the bulking agent. The Terex
composter then moves along one side of the sludge - bulking agent pad and
then along the other side, mixing the materials toward the middle to form a
partially mixed windrow of triangular cross section. The Roto-Shredder then
passes through the windrow, turns around, and passes back through the win-
drow. This operation mixes the sludge and bulking agent to form a relative-
ly homogeneous mixture. The Terex composter and Roto-Shredder are both used
because they are available at Beltsville. Satisfactory mixing can be
accomplished using either machine alone. Mixing can also be accomplished
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TABLE 1. SITE VISITS AND WEATHER
Site
Beltsville
Beltsville
Beltsville
Beltsville
Bangor
Bangor
Date
9-27-76
9-28-76
9-29-76
1-26-77
1-27-77
1-28-77
4-4-77
4-5-77
4-6-77
4-7-77
6-21-77
6-22-77
9-22-76
9-23-76
2-23-77
2-24-77
2-25-77
Weather
Fair
Fair
Fair
Cold, Snow on ground,
some inversion , no
wind in early mornings
Heavy rain, windy
Rain f
Fair, windy
Cloudy, windy
Fair
Fair
Fair
Fair
Fair
Cloudy
Freezing rain, snow
Range of
temperature, °G
10 to 15
10 to 15
-9 to 2
-9 to 4
-9 to 7
4 to 13
4 to 13
2 to 13
1 to 13
21 to 27
21 to 27
2 to 18
2 to 18
-9 to -1
-9 to -2
-4 to -1
Bangor
6-1-77
Clear
21 to 27
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SITE
ENTRANCE
SCREENED
COMPOST
STORAGE
COVERED
MAINTEN-
ANCE
SHOP
lULKING
AGENT
STORAGE
ADMINISTRA
TION AND
APPROXIMATE SCALE
1" = 200'
Figure 1. Beltsville composting site layout.
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using only the front loader, but is more time consuming to produce a good
mix. It is felt that a good mixture, free of large sludge balls, is impor-
tant for good composting and producing a high quality product.
Beltsville has been using the extended forced-aeration pile composting
configuration. Material is added to the pile each day and aeration pipes
are spaced about every 8 feet on center. The composting pad for each days
sludge-bulking agent mixture is prepared by laying out the aeration piping
on the asphalt composting pad and covering this pipe with a 12 in. layer of
wood chips using a front loader. The sludge-bulking agent mixture is then
placed on the wood chip base using a front loader. The mixture is piled to
a height of about 8 ft. The top and ends of the pile are then capped with
an 18 in. layer of unscreened compost or a 12 in. layer of screened compost.
At the end of each days operation the side of the pile (which will be added
to the next day) is covered with a thin layer of compost. The blower is
connected to the piping and a 5 cu yd pile of screened compost is placed
over the end of the blower discharge piping for deodorizing the discharged
air. The blower pulls air from the composting pile and discharges to the
deodorizing pile. The blower is generally operated on an on-off cycle con-
trolled by a timer. The duration of the on and off cycles is adjusted to
suit the particular operating conditions.
Generally, compost is removed from the opposite end of the extended
pile each day or every other day after 21 days of composting. This part of
the pile is torn down using a front loader and is moved to the curing pile.
At Beltsville, the aeration pipe is a light weight plastic considered ex-
pendable and therefore, is moved with the compost. The compost stays in
the curing pile for at least 30 days, usually for a much longer period,
awaiting screening or off-site use. The unscreened compost can be stored
for long periods depending on the needs of the particular operation and
the screening and compost distribution operations.
Processing of the dewatered raw sludge cake and formation of the com-
post pile must be carried out on a regular basis consistent with raw sludge
delivery. Raw sludge is mixed with bulking agent and processed into a
compost pile promptly as it is received on site to avoid odors and health
hazards. Raw sludge is not stored on site. Should site conditions or
weather shut down operations (there are practically no instances of this
occurring at Beltsville) sludge would not be delivered but instead be
either stored at the Blue Plains plant or diverted to other disposal sites.
Screening is used to separate the wood chips from the compost: (1) so
that a portion of the chips can be reused, (2) to control the maximum parti-
cle size of the compost, and (3) to improve the carbon-nitrogen ratio of
the finished compost. Most screening is performed using a 5/8-inch mesh
rotary drum screen. The drum is 7 feet in diameter and about 14 feet long.
The tilt of the drum is adjustable as is the feed rate. Screening can be
scheduled independent of other operations because of unscreened compost
storage availability. In practice, however, enough compost is screened to
meet: (1) any on-site needs, (2) the demand from users, and (3) to provide
room in the unscreened storage area for current production. At Beltsville,
compost is screened at all times when the temperature is above freezing and
8
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rain is not falling. Peak hourly capability of the drum screen with fairly
dry material is about 50 cu yd. However, under actual conditions of start-
up, shutdown, cleanup, and typical breakdowns, the input drum screen capa-
bility is about 150 to 250 cu yd per day. The screen is mounted on wheels
and can be moved with the front loader for cleanup. Moving the drum screen
permits use of the front loader for cleanup rather than having to clean by
hand.
Other site operations include regular cleanup of work areas, receipt
and storage of bulking agent, loading and measurement of finished compost
for users, and equipment maintenance.
The staff at Beltsville consists of 8 people; 2 administrative and 6
operating. This number is more than actually needed for normal operations.
The additional personnel are used for special operations and to support
the research demonstration program. The operating staff is highly trained,
each member is qualified on each piece of equipment, and the staff is able
to perform all preventative maintenance and much of the repair work. Per-
sonnel and equipment are available for the composting operation full time.
A list of equipment is shown in Table 2.
The approximate materials quantities used in the Beltsville operation
are shown in Table 3. These quantities are based on an annual undigested
sludge cake (approximately 23 percent solids) input of 15,000 wet tons, a
ratio of two and one half parts wood chip bulking agent to one part sludge
cake by volume, and 5/8-inch screening of all compost for a wood chip recov-
ery and recycle of 70 percent. Output or production quantities are shown
for both a 12 in screened compost pile cover and an 18 in unscreened com-
post pile cover in Table 4. The unscreened compost production is based on
15 percent reduction in sludge volume during composting and curing. This
example serves only to illustrate general concepts because the materials
loss through composting and curing are estimated and have not been precisely
documented at existing operations.
Bangor
A typical operations layout of the Bangor site is shown in Figure 2.
The layout is to approximate scale.
Undigested raw lime conditioned sludge cake (approximately 25 percent
solids) is delivered from the City of Bangor primary wastewater treatment
plant in 5 to 7 cu yd containers by a single lift and carry type truck.
The sludge is delivered to the composting site located at the Bangor Inter-
national Airport approximately 3 miles from the treatment plant. Generally,
the raw sludge is dewatered, delivered, and composted once a week; on
occasions twice a week, usually the dewatering operation is started the day
before so that all available sludge containers are filled the morning com-
posting is to commence. Sludge hauling to the site begins early on the
morning of composting. An operator and 4 cu yd front loader is available
early at the site on the day of composting. As the containers of sludge
are delivered to the site they are dumped on a previously prepared bed of
bulking agent in the mixing area, mixed immediately with the front loader,
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TABLE 2. BELTSVILLE EQUIPMENT
2 Terex Rubber Tired Front Loaders, 4.5 cu yd
2 Dump Trucks, 10 Ton, 3 Axle
2 Flat Bed Trucks, 1.5 ton
2 Pickups
2 Rubber Tired Farm Tractors, one with loader
1 Rotary Screen with power unit
1 Sweco Screen, fixed (new)
1 Mobile Rotary screen, small (not used)
1 Terex Composter
1 Imco Roto-Shreader
1 Large conveyor with engine drive (not used)
1 Fixed Toledo Truck Scale
1 Mobile Office
1 Storage Building
1 Lot small equipment and hand tools
10
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TABLE 3. BELTSVILLE MATERIALS REQUIREMENT FOR
15,000 WET TON ANNUAL SLUDGE INPUT
Limed raw sludge, wet tons 15,000
Solids, percent 23
cu yd 20,700
Density, Lb/cu yd 1,450
dry tons 3,450
Extended pile construction
Sludge, cu yd 20,700
Bulking agent
mixing w/sludge, cu yd 51,750
pile base, cu yd 8,100
Pile cover
screened compost, cu yd 12,000 (12 in.cover)
unscreened compost, cu yd 18,000 (18 in.cover)
Individual pile construction
Sludge, cu yd 20,700
Bulking agent
mixing w/sludge, cu yd 51,750
pile base, cu yd 17,000
Pile cover
screened compost, cu yd 24,000 (12 in.cover)
unscreened compost, cu yd 36,000 (18 in. cover)
11
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TABLE 4. APPROXIMATE MATERIALS OUTPUT, BELTSVILLE TYPE OPERATION
**
Case
Extended pile with:
screened compost cover
unscreened compost cover
Net
unscreened
compost
production,
cu yd
82,260
87,660
Net
screened
compost ^
production,
cu yd
39,170
35,210
Bulking agent, cu
Total
required Recycled
59,850 43,100
59,850 52,450
yd
*
Net used
(new make-up)
16,750
7,400
Individual piles with:
screened compost cover
unscreened compost cover
101,070 51,570
111,355 43,351
68,750
68,750
49,500
68,000
19,250
750
* Does not reflect actual Beltsville operations because only a portion of the Beltsville
compost is screened.
** Materials input shown in Table 3.
Assumptions:
1. 15 percent reduction in sludge volume during composting and curing.
2. 10 percent of the bulking agent lost in composting and curing cycle.
3. 20 percent of the bulking agent lost in screening (passing through screen into finished
compost).
4. 15,000 wet ton annual sludge input.
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BULKING
AGENT
STORAGE
COMPOSTING
PILE
(4 or 5 PILES)
SCREENED
COMPOST
MIX-
ING
AREA
PAVED AREA
UNSCREENED
'COMPOST
STORAGE
• SCREENING
APPROXIMATE SCALE
1" = 200'
Figure 2. Bangor composting site layout.
13
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and the compost pile is formed over a previously prepared base. Generally,
one composting pile is constructed per week and typically consists of
approximately 40 to 60 cu yd of raw digested sludge cake mixed in 1:3 ratio
with about 120 to 180 cu yd of bulking agent. Approximately 6 to 8 trips
must be made to the site to deliver the sludge cake. The mixing and pile
construction requires a 10 hour day. Materials are measured by counting
loader bucket loads.
Bark is currently used as a bulking agent. Most composting piles are
mixed in a 3:1 ratio of bulking agent to sludge cake. The bark consists of
a wide range of particle sizes from very fine to fireplace size logs. When
the bark moisture content is less than 50 percent it is satisfactory for
composting. Moisture is a problem during rainy weather because the bark is
stored outside. During winter, there is little rain and the bark is quite
satisfactory. A more uniform sized bulking agent, similar to chips, would
probably help the composting operation.
The base for the compost pile is prepared using 7 ft lengths of per-
forated schedule 40 steel pipe joined together by short pieces of plastic
pipe. The City found that the short lengths of steel pipe can be removed
from the pile without significant damage and reused many times. Longer
pipes were used previously, but were easily bent when pulled from the pile.
The City has been experimenting with many different pile configurations.
Currently, no base material is used; the sludge-bulking agent mixture is
placed directly on the pad and aeration pipes. The City has also satisfac-
torily used unscreened compost as the bulking agent in a number of piles.
Unscreened compost has been used up to three cycles as bulking agent with
good results and thereby dramatically reduced requirements for new bulking
material. The City plans further tests using unscreened compost as bulking
agent. The reuse of unscreened compost may increase the nutrient content
of the finished product; however, this needs to be confirmed.
The compost piles are constructed as high as the front loader can
reach and capped with 1 to 2 ft of unscreened compost. The finished pile
is 10 to 12 ft high. The blower is then hooked up and generally operated
on an on-off cycle drawing air from the pile. During cold weather, the
warm exhaust air from an older composting pile is piped into a new pile for
a few days to rapidly increase the temperature of the new compost pile.
The exhaust air from an older composting pile is very wet and will cause
high moisture levels in the new pile receiving the air if used more than
the first several days. The City has also purchased a ram air heater to
provide initial heat to the piles.
The City found that during cold weather all available heat must be
conserved to help the piles come up to temperature. Reuse of unscreened
compost provides a warm bulking agent. The interiors of the bark storage
piles are also sources of warm materials for mixing. Generally, if the
sludge-bulking agent mixture can be kept about 4°C the pile will perform
much better than if the mixture falls below 4°C. Recycle of warm exhaust
air from an older composting pile into the new pile is also helpful for the
first few days.
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The piles are composted at least 21 days. Temperature and oxygen
levels are monitored every 2 to 5 days during the composting cycle. The
blower operating cycle is adjusted according to the performance of the
pile. The aeration pipes, blowers, and moisture traps are checked for
freezing during cold weather because of the large amount of moisture drawn
through the aeration piping.
At the end of the composting cycle the pile is torn down, usually at
the time another pile is constructed. The material removed from the pile
is either used as the bulking agent for the new pile or is transferred to
curing. As the pile is torn down the 7 ft lengths of aeration pipe are
salvaged, cleaned out, and piled to the side for reuse.
Raw sludge is not stored at the compost site, but is stored at the
plant. Generally, operations are scheduled so that sludge is dewatered
and a compost pile constructed once a week. The exact day of pile construc-
tion is varied depending on weather conditions. The City has been able to
compost nearly all of the sludge production simply by picking a good day
each week for compost pile construction. During 1976, about 1,000 cu yd
(700 wet tons) of dewatered sludge was hauled to landfill rather than com-
posted, becAjise composting at the time was being done by public works
personnel, not treatment plant personnel. At times, public works personnel
were unavailable to compost the sludge. Treatment plant personnel feel
that they will seldom have to haul sludge to landfill. Liquid sludge is
stored in the thickeners until dewatered. However, some additional liquid
sludge storage capability would be helpful because of the cyclic nature of
the dewatering operations. The plant is able to operate by storing sludge
in the thickeners.
The City uses a Lindig rotary drum screen to screen compost prior to
distribution. The drum is presently fitted with a 1-in. mesh screen. City
personnel are planning to construct a 5/8-in. screen assembly so that either
size material can be produced. Tests performed by the City indicate that
the screen is capable of handling about 20 to 25 cu yd (900 Ib per cu yd)
of feed per hour under the best conditions. The screen is fed with a front
loader. One loader operator and a laborer are required during operation.
The City performed screening during late summer and fall of 1976, but has
not been able to screen during winter 1976-77. Most of the unscreened com-
post production is stored in curing piles and about 3,000 cu yd is now on
hand awaiting screening. It is estimated that the screen can process about
100 to 150 cu yd of feed per day based on typical operations and that 20 to
30 days of screening will be required to process the present 3,000 cu yd
stock.
Currently, operations at Bangor are being carried out by treatment
plant personnel under the direction of the treatment plant superintendent.
There are no full time composting personnel because of the cyclical nature
of the operations. Approximately 11 man-hours per week are required at
the site, primarily for loader operation. In addition, approximately 9
man-hours per week are required for a truck driver to deliver sludge to the
site. Sampling and monitoring for temperature and oxygen content requires
10 hours per week not including the pathogen and heavy metals monitoring
15
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which is performed under contract by the University of Maine. The super-
vision and administration requirements are about 15 man-hours per week.
Annual equipment and labor requirements are shown in Table 5.
The equipment used for composting operations is shown in Table 6.
This equipment is provided by the City motor pool and is available "for com-
posting when needed.
The approximate materials quantities based on 1976 sludge volume for
the Bangor operation are shown in Table 7. These quantities are based on
an annual sludge input of 2,170 wet tons and a mixture of 3 parts bulking
agent to one part sludge. The calculated materials production is shown in
Table 8 for three assumed cases. These production outputs assume a 15
percent reduction in sludge volume during composting and curing and one
inch screening of compost before distribution. Recovery of bark is esti-
mated at 70 percent by either screening an/or recycle of the unscreened
compost as bulking agent. Table 8 serves only to provide an approximate
indication of materials quantities.
MONITORING
Regular monitoring of operations can provide a good indication of com-
posting effectiveness. The regular operations (not the research operations)
at Beltsville are so routine and dependable that piles are monitored only
occasionally.
The piles at Bangor are monitored regularly for interior temperatures
and oxygen levels. Generally, this monitoring is performed every 2 to 5
days at several points in each pile. These data are used to judge the pile
performance and to set the blower operating cycle. At the end of the com-
posting period each pile is sampled and tested for moisture content and
pathogens. In addition, random sampling and pathogen testing is performed
on raw sludge, curing, and finished compost. Some heavy metals and nutri-
ent testing is also performed.
Experiences at Durham and Bangor tend to indicate that regular moni-
toring of pile temperature can provide a direct indication of pile compost-
ing performance and can help to pinpoint problems. For instance, the
initial rate of temperature increase is a good indicator of the composting
efficiency to be expected. Generally, the temperature of a poorly con-
structed pile (too wet or not well mixed) will rise slowly and in many
cases will not reach proper temperatures. At Durham, such piles have been
torn down and rebuilt with vastly improved performance the second time.
Durham also found it desirable to reverse air flow part way through the
composting process and the time of reversal was determined from temperature
monitoring.
Additional regular monitoring can be performed to include oxygen levels
within the pile, pathogens, heavy metals and nutrients. For a small oper-
ation, at least regular temperature monitoring should be performed. It
has been found useful to plot the composting temperatures on a regular
basis (daily during the first few days as the pile comes up to temperature)
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TABLE 5. ESTIMATED ANNUAL OPERATIONS REQUIREMENTS, BANGOR
Labor, Equipment,
Operation hours hours
Composting, labor 572
front loader 468
Sludge hauling, labor 468
truck 468
Monitoring, labor 520
pickup 520
Administration, labor 780
Screening (8,000 cu yd), labor 1,040
screen 520
front loader 520
Maintenance, labor 100
This Table is based on information provided by the City of Bangor
personnel.
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TABLE 6. BANGOR EQUIPMENT
1 Case W24B Rubber Tired Front Loader, 4 cu yd
1 Rubber Tired Front Loader, 1.5 cu yd
1 Truck, Sludge container hauling
1 Mobile screen, Lindig
Small tools as required
Miscellaneous vehicles as needed from motor pool
TABLE 7. BANGOR MATERIALS REQUIREMENTS FOR
2,170 WET TON ANNUAL SLUDGE INPUT
Limed raw sludge, wet tons 2,170
solids, percent 23
cu yd 3,000
Density, Ib/cu yd 1,450
dry tons 500
Static pile construction
sludge, cu yd 3,000
bulking agent, cu yd 9,000
pile cover, cu yd 1,560
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TABLE 8. APPROXIMATE MATERIALS OUTPUT, BANGOR TYPE OPERATION.
Case*
Net
unscreened
compost
production,
cu yd
Net
screened
compost
production ,
cu yd
Total
regjd
Bulking agent,
Recycled
cu yd
Net used
(new make-up)
New or recycled
bulking agent
used in pile
construction 10,920
Unscreened compost
from pile used
as bulking agent
for one cycle 6,240
Unscreened compost
from pile used
as bulking agent
for two cycles 4,654
4,160
3,198
2,785
9,360
4,680
3,114
6,760
3,042
1,868
2,600
1,638
1,246
* Materials input shown in Table 7.
Assumptions;
1. 15 percent reduction in sludge volume during composting and curing.
2. 10 percent of the bulking agent lost in each composting and curing cycle.
3. 20 percent of the bulking agent lost in screening (passing through screen into
finished compost).
4. 2,170 wet ton annual sludge input.
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during the composting period along with daily precipitation and air temper-
ature. This information provides a profile of each pile. It may also be
desirable to monitor a temperature near the outside of the pile as a check
on minimum temperatures and the effectiveness of the pile cover.
Bangor and Durham have been able to provide pathogen monitoring through
a cooperative agreement with a local university.
SITE CONSIDERATIONS
The site location should be selected with great care considering trans-
portation/ adjacent land use, drainage, winds and similar factors. It is
desirable to provide a buffer strip around the site, preferably one that
will also block the view. Slight odors are generated at times and, al-
though practically no complaints are received at operating facilities, this
factor should be considered in site selection.
The basic requirement for almost any geographical location is to con-
struct a site that is usable by heavy equipment on a year around basis.
The site should be well drained and in most cases this drainage should be
collected to a holding pond. Suitable facilities should be provided for
administration, testing, equipment storage, and maintenance. Access should
be provided to all working areas and this access should be suitable for
heavy equipment. The minimum construction for this access should be gravel,
but paving is better.
All working areas should be heavily gravelled as a minimum. It has
been demonstrated at Beltsville that regular year around operations can be
carried out on crushed stone, but drainage, cleanup, and other working con-
ditions are less than optimum. Also, it is impossible to keep the gravel
out of the materials and some heavy metals can be leached from some types
of gravel. All working areas should be well drained because runoff from
the composting process is significant. Removal of runoff will help to keep
the materials as dry as possible and minimize odors and nuisance problems.
Properly designed hard surfaced work areas will provide more satisfac-
tory operations, better drainage, and will facilitate cleanup. The compost-
ing area should have highest priority for paving, however, the screening
area is also important. If necessary, the bulking agent and compost storage
areas can be operated satisfactorily without paving. It is recommended
that reinforced concrete surfaces be provided, however, asphaltic cement can
be used. The asphaltic cement is more prone to damage from equipment be-
cause it tends to soften during hot weather and from heat under the compost-
ing piles.
WEATHER
Weather can have a significant effect on composting operations and must
be taken into consideration both in design and in carrying out operations.
It is possible to carry out composting operations under nearly all weather
conditions as demonstrated at Beltsville, Bangor, and Durham, but experience
has provided many useful observations. Significant adverse weather
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conditions are snow, rain, cold (below freezing), inversion conditions, and
high winds. The effects of each of these conditions on composting will be
discussed along with methods for mitigating the adverse effects.
Cold Weather Operations
Generally, cold weather above freezing has little effect on composting
operations, therefore, this section will relate to locations with extended
periods of subfreezing weather. Conditions where temperatures dip below
freezing occasionally or at night, but return to above freezing during the
day are of little concern.
While extremely cold temperatures do have some adverse effect on the
compost pile process performance; Beltsville, Bangor, and Durham have demon-
strated that the composting process can perform well in very cold climates.
Extra care is needed in mixing, pile construction, and covering to assure
optimum performance. Most cold weather problems relate to materials hand-
ling and equipment. Equipment will be harder to start in cold weather
(especially diesel engines), but this can be mitigated by installing block
heaters and storing equipment inside. Equipment operator stations should
be enclosed and heated for cold weather operations. Equipment service and
maintenance is more difficult and suitable arrangements should be made for
carrying on regular and emergency maintenance.
Materials handling problems relate primarily to freezing of these
materials. Generally, because of the heat generated, freezing of the com-
posting piles, unscreened compost, or screened compost even in very cold
weather has not been significant. Freezing of the bulking agent can be a
problem. The wood chip bulking agent used at Beltsville does not freeze
into large chunks because the moisture drains well from the material. Re-
cycled wood chips do freeze to some extent because of the additional mois-
ture and smaller particle size. Bulking agents which hold moisture will
tend to freeze, however, the bark at Bangor only freezes on the outside,
and heat is generated inside the piles.
Screening when temperatures are below freezing is a problem. The
mechanical parts of the screen freeze and a residue freezes on the screen.
Screening is not undertaken when it is below freezing at Beltsville and
Bangor. In other cold locations it would be possible to stockpile unscreen-
ed compost during freezing weather and only screen when the temperature is
above freezing. The other alternative would be to screen in a heated space
or screen during freezing weather and try to overcome the problems. Ade-
quate screening capacity must be available so that all required screening
can be performed during the available suitable weather for a particular
location.
The extremely cold weather in Bangor has a noticeable effect on the com-
posting process. It is difficult to pinpoint the exact cause of the prob-
lems. City personnel report that if the sludge-bulking agent mixture can
be kept above 4°C the pile has a much better chance of performing well than
if the mixture temperature drops below 4°C. If wet bulking agent is used or
if mixing takes place in rain the resulting compost pile may not perform well.
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Bangor personnel are testing various methods of improving cold weather
operation such as reusing unscreened compost as bulking agent (use of heat
in the compost), piping the warm air discharge from an older composting pile
into the new pile initially, and using a heater to warm the air blown into
a new pile. It is too early to develop conclusions except that
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considered in planning plant operations and designing facilities for sludge
storage at the plant. Generally, however, the effects of snow on the pro-
cess are minimal.
Inversion Conditions
The composting operations do generate mild odors at times and may be
most noticeable during inversion conditions with light winds. Careful
operations will minimize odor generation. Slight odors were noticed at
Beltsville under inversion conditions, but the problem was very minor and
complaints are nil.
Odors were also noticeable at the Bangor composting site. These odors
might be considered unacceptable to some people visiting the actual site.
Apparently, these odors are dispersed rapidly away from the site because
outside complaints have not been received. The site is about 2,000 feet
from the Bangor International Airport terminal building and about a like
distance from a major shopping center.
The normal operations at Beltsville and Bangor have never been shut
down due to odor generation. There is no easy way to shut down a composting
operation in any case because it is a continuing process involving large
quantities of "active" materials. The best safeguard is to conduct careful
operations, but odors will occasionally be generated. Beltsville does have
both mobile and fixed deodorizing equipment which is used infrequently.
High Winds
It is possible that areas with significant prevailing winds may have
problems with blowing of materials, especially with light bulking agents,
and dust. The best mitigation would be careful site selection and masking
of the site with wind barriers such as trees. In most areas, materials
handling operations can be suspended during occasional periods of high wind.
Dust can be controlled by careful site cleanup and the use of water.
Summary of site Considerations
Operations downtime at Beltsville was minimal during 1976. Regular
sludge deliveries have been accepted on schedule except when the Blue Plains
Plant was unable to provide sludge or trucking was impossible. During the
tsevere weather of January 1977 sludge was accepted five days a week on
schedule. Downtime at Bangor in 1976 was slight and due mostly to schedul-
ing of public works department personnel. Downtime should be reduced even
more now that the treatment plant is providing the personnel. No on-site
sludge storage is available at either composting site and it is recommended
that such storage be limited to the wastewater treatment facility. As far
as can be determined no nuisance complaints were received as a result of
normal Beltsville and Bangor operations.
The essential function of compost pile formation has been performed
on a regular schedule with little disruption. Other operations with more
flexibility such as screening have been performed when conditions are
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favorable, but still have met the needs of the overall operation. Waste-
water sludge composting has been demonstrated on a regular schedule for over
two years at each site completely outside.
The hard surfaced composting pad at Beltsville will not reduce "down-
time" or decrease the costs of operation. The primary advantages relate to
more satisfactory and orderly working conditions. Drainage is much more
satisfactory and cleanup is much easier and more complete. Water drainage
from the composting pile is probably much more satisfactory, but the econo-
mic effect is almost impossible to calculate and probably quite small.
A portion of the new Beltsville covered area will be reserved for
equipment maintenance. The balance of the covered area will be used for
composting operations; primarily research and demonstration. The covered
area will not reduce operational "downtime" but will provide additional data
related to composting without the effects of precipitation. It is antici-
pated that the covered area will have little direct effect on normal
operations. The cost of the Beltsville covered area is approximately $10
per sq ft including the concrete slab.
The primary adverse weather effects at operating installations are
summarized as follows.
Cold
- equipment starting and operation are more difficult.
- equipment service and maintenance requires appropriate facilities.
- equipment cabs must be enclosed and heated.
- freezing of bulking agent may be a problem.
- screening is difficult to carry out.
- composting pile initial heat generation may be slow in extreme cold.
Rain
- site and working area drainage is important.
- runoff collection and treatment must be considered.
- moisture absorption during sludge-bulking agent mixing and pile
formation must be minimized.
- moisture absorption by unscreened compost prior to screening can be
a problem.
- screening is difficult to carry out.
Snow
- generally, similar to rain, but less severe.
- visibility problem.
- site clearing and access is necessary.
Inversion Conditions
- little documented problem under this condition.
High Winds
- blowing materials and dust can be problems.
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NUISANCES
The potential nuisances of a raw sludge, forced aeration composting
operation are:
1. Sludge hauling
2. Visual objections
3. Odor generation
4. Vermin propagation or attraction
5. Noise
No known nuisance complaints have been traceable to raw sludge cake
hauling at Beltsville or Bangor. It is desirable to locate the composting
site as close as possible to the treatment facility thus further reducing
any potential for nuisance problems to develop.
A composting site is a potential visual nuisance because of the large
piles of materials and heavy equipment. The Bangor site is in plain view
to people using the Bangor International Airport, but complaints have not
been teceived. A better approach is to mask the site from view by careful
selection or landscaping if possible.
Odor generation is potentially of great concern because some mild odors
are generated at composting sites; yet, no complaints have been received at
Beltsville or Bangor. odors have been noticeable at the Bangor site, but
are not noticeable off the site. The best solution is to use care in
selection of the site and, perhaps, have mobile deodorizing equipment avail-
able for use.
No rodent, fly, or other vermin problems have been reported at Belts-
ville. Bangor did report some fly problems in 1976 and 1977. During the
1976-1977 winter, mice have been nesting in the curing piles and it is not
known if this will be a problem. To date, the reported and observed vermin
problems seem to be minor to non-existent.
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SECTION 4
CONSIDERATIONS FOR FUTURE OPERATIONS
GENERAL
Actual forced aeration composting operations to date have demonstrated
the ability to process undigested wastewater sludge cake into a useful com-
post product with minimum on-site nuisance and essentially no public com-
plaint. It is probable that further modifications and improvements to the
process will provide additional operational advantages and reduction in
cost.
A number of these possibilities are outlined in this section, but it
is emphasized that process modifications should be tested under carefully
controlled conditions to prevent the possibility of on-site and off-site
nuisances and public complaints in the event that some are unsuccessful.
The type of operation most advantageous in a given situation will depend on
the goals of the operation, the market for compost, available bulking
agents, climate, and similar considerations.
MATERIALS HANDLING
This is probably the most significant factor in the composting opera-
tion; it is a materials handling operation. A study at Durham, New Hamp-
shire (1) investigated the use of fixed materials handling equipment as
compared to the mobile equipment now used almost exclusively. The study
concluded that certain steps in the operation could be mechanized to advan-
tage, specifically, the sludge-bulking agent mixing and movement of some of
the materials using conveyor belts. The design for Camden, New Jersey is
reportedly based on extensive use of mechanized fixed materials handling
equipment.
Mechanization using fixed equipment has not been demonstrated, except
that conveyors are presently used to move materials within the screening
process. The present practice of using mobile equipment provides greater
operational flexibility, but may increase some other operational costs such
as labor. The flexibility required for a composting operation should re-
ceive careful consideration, particularly when planning the materials
handling operations.
There is a need to realistically evaluate various modes of operation
and the effect on materials quantities to be handled in order to minimize
the costs of materials handling.
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BULKING AGENT
The bulking agent is a prime consideration in the composting process
because it can be a major cost item and may be difficult to obtain in cer-
tain parts of the country. The exact considerations relating to bulking
agent are site specific, but, it is possible to develop some general con-
siderations.
It is important to plan for the bulking agent in the early stages of a
wastewater sludge composting project to assure a supply of suitable mater-
ial and for the materials handling equipment. Thereafter, in most cases,
it is desireable to minimize the need for additional material because of the
cost.
It is possible to recover approximately 70 to 80 percent of wood pro-
duct bulking agent used in construction of a pile by screening after comple-
tion of the composting or curing process. This recovered bulking agent can
be reused and removal of this material from the compost helps to increase
the volumetric nutrient content of the final compost product.
It also may be possible to use the compost from a pile being torn down
as the bulking agent for a new pile. This is illustrated in Table 8 and
has been demonstrated in practice at Bangor with satisfactory results. In
fact, Bangor has reused compost through two additional compost cycles (in
addition to the original pile) as bulking agent with satisfactory results.
As illustrated in Table 8, this procedure reduces both the bulking agent
and screening requirements significantly along with other key process mater-
ials handling requirements. This can be significant for operations trying
to minimize costs and illustrates the significant effect that can be obtain-
ed by varying operational procedures.
A number of other materials have been used as a bulking agent such as
shredded tires, peanut hulls, cubed solid waste, and licorice root.
SCREENING
Screening is one of the more difficult and time consuming operations
related to composting. Ideally, it would be desireable to eliminate
screening if a market could be found for coarse compost and/or if an inex-
pensive bulking agent could be found that would essentially disintrigrate
during composting and curing. Elimination of screening would, however,
increase the requirement for new bulking agent. Practically, one solution
is to develop a good balance in operations so that screening is limited to
a manageable level of effort.
The best experience to date has been with horizontal, rotary type
screens with a screen opening of 1/4 to 1 in. An effective drum screen
should be about 10 to 15 ft long to provide adequate tumbling action to
separate the fine and coarse particles. Shorter screens have been used,
but particle separation is not complete and the recovered bulking agent
contains fines. Feed rates must also be controlled to achieve good perfor-
mance. Moisture content is a significant factor, but the horizontal screen
27
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has demonstrated an ability to operate even with relatively wet compost (up
to 50 or 55 percent moisture). Vibratory screens have been satisfactory for
moisture contents up to 45 percent.
With careful planning of operations it may be possible to reduce screen-
ing requirements significantly as illustrated in Table 8. Unless a covered
area is available, screening can be carried out only in good weather and
then the daily production must be carefully evaluated because of downtime,
maintenance, and other factors. Screening capacity could be a limiting
factor unless the capacity is carefully selected in relation to volume of
materials to be screened. Screening typically requires one man plus a
front loader and operator.
It is also possible to reduce screening by marketing some compost un-
screened, but this will increase bulking agent requirements. It may also
be necessary to market some compost unscreened if space is not adequate to
store materials during times when screening cannot take place.
SITE WEATHERPROOFING
A permanent composting site should have certain basic amenities. The
first would be a hard surfaced working area (preferably reinforced concrete)
for mixing and composting. Other areas such as roads, materials storage
and the screening area should be covered with a thick layer of crushed
stone as a minimum though paving would be more satisfactory. Mixing, com-
posting, and curing areas should be well drained to adequate runoff
collection and storage facilities.
Beltsville and Bangor have demonstrated that covered areas are not
necessary to carry out continuous operations. In some climates, covered
areas would be helpful for: (1) protection of bulking agent, (2) protection
of curing compost, and (3) protection of the screening area from rain.
Covered composting pads would possibly contribute to more satisfactory
composting in very wet or cold climates, but there are no data available
to give a quantitive comparison at this time.
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SECTION 5
COMPOST DISTRIBUTION
GENERAL
Compost has been distributed from the Beltsville operation for several
years. Reportedly, all of the production has been distributed in the past
without satisfying the demand. Most of the compost is provided free of
charge to various public agencies who pick the material up at the Beltsville
site. Some compost has been sold for $1.00 per cu yd (approximate weight
850 Ib per cu yd) to private companies (not individuals). Part of the com-
post is screened prior to distribution, and some compost is also distribu-
ted unscreened.
Compost from the Bangor operation has been stock piled. A marketing
program began in Spring, 1977. The policy is to market the compost for
approximately $1.00 per sack (approximately 60 to 80 Ib), $5.00 per cu yd in
bulk, or $10 per bulk pickup load. The compost is picked up at the site by
the user and is loaded by City personnel. The City is also considering
promotional efforts through other outlets such as supermarkets and nurseries
in addition to sale to other City Departments such as Parks.
USES
There are a number of identified end use options for wastewater sludge
compost:
1. Base for potting mixes.
2. Bedding mix for flowers.
3. Planting mix for trees and shrubs.
4. Base for producing "on-site" topsoil.
5. Surface mulch (although this is not recommended by all
authorities).
6. Base for reclaiming mined or other unproductive areas or soils.
7. Land reclamation.
8. Landfill.
DEMAND
No formal studies have been undertaken in Bangor to determine potential
demand for the compost. The City has nevertheless decided to sell the com-
post and has disposed of approximately 80 to 100 cu yd in 1977 through the
month of May, compared to annual production in excess of 4,000 cu yd. it
29
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is very difficult to determine the total volume that can be disposed of in
the Bangor area. The season is very short and there is very little poten-
tial demand in the winter. Adequate space is available for storage of
large quantities of compost during the winter or from year to year. Until
the market for compost can be developed in the Bangor area, the best
strategy is to minimize compost production through appropriate operational
strategies as illustrated in Table 8. It is likely that all of the Bangor
compost could be landfilled at the present sludge disposal site, but this
method of disposal does not meet the City goal of realizing some revenue
from the disposal of the compost.
A comprehensive study^ was recently completed which projected compost
demand in the Washington, D. C. Standard Metropolitan Statistical Area
(SMSA) and within a 100 mile radius of Washington, D. C. Estimates of
demand, considering certain restrictions on use, ranged between 5,600 and
161,000 tons of compost per year within a 100 mile radius of Washington,
D.C. Although this is a broad range, it indicates a potential market for
the forseeable production from the Beltsville site. Production at Belts-
ville would be approximately 20,000 tons of compost per year if all the
compost were screened, through actual production is somewhat higher because
some compost is not screened.
Other wastewater agencies within the Washington, D. C. area are con-
sidering composting and this would increase the quantity of compost
available for distribution.
30
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SECTION 6
COSTS
Composting costs are documented for both Beltsville and Bangor. The
Beltsville costs must be considered carefully because they include allow-
ances for various research activities and the equipment and site may be
capable of handling 2 to 3 times more sludge than is presently processed.
a"1™* fn^nCial aspects of <** Beltsville operations are shown in Table
9. The 1976 actual and projected 1977-1978 costs were developed from in-
formation provided by MES for the total operation including research; but
do not include the amortization of equipment or site costs. The off-site
administrative cost item is basically unrelated to site operations. The
on-site labor cost includes site supervision. Sludge hauling cost is for
contract transport of sludge cake from the Blue Plains Plant to the compost-
ing site. These hauling costs are site specific for the Beltsville opera-
tion. Costs for bulking agent may not reflect actual usage because chips
are purchased when prices are favorable and large quantities can be stock-
piled. Screening in 1976 was performed by outside contract, however, all
screening is now performed using on-site labor. The projected Beltsville
costs for 1977-1978 were modified to show two different sludge input rates
and research related costs were removed. Individual breakdown of costs for
various processes such as mixing, pile construction, and screening are not
available from Beltsville. However, cost estimates were prepared for an
Smltr ^ **5 Beltsville based on time and motion study at Belts-
ville^. This study estimated that total costs, including capital amorti-
zation, are approximately $51 per dry ton of sludge cake solids for an
operation processing 10 dry tons of sludge solids per day. Total capital
costs for site improvement and equipment were estimated at $376,000. Esti-
mated costs for 50 dry tons of sludge solids per day were $36 per dry ton,
including amortization of approximately $1,500,000 of capital costs. Costs
may be
Bangor operation costs for 1975 <4> are shown in Table 10. The capital
investment was very low because only minor site work was required. Equip-
includH, P001' totization of equipment is
included in the hourly equipment charge under "Operations". Cost informa-
tion is not available for 1976. An estimated breakdown of labor and
equipment requirements by operation is shown in Table 5.
i H Pr!o!Snary f timates at Durham indicate a cost of $16.00 per cu yd of
sludge ($107 per dry ton solids for 20 percent solids sludge) <5? for J^_
posting an estimated 200 to 250 dry tons of sludge per year excluding
31
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TABLE 9. BELTSVILLE ACTUAL AND PROJECTED COSTS
CJ
to
Estimated Costs**
Off-site
Admin. , of f -site
On site operations
Telephone & travel
Utilities
Fuel & oil
Sludge hauling
Labor incl. fringes
Misc. contract services
Wood chips
Supplies & materials
Equipment insurance
Total
Total excluding off-site
Dry tons sludge per year
(23 percent solids)
Annual cost, $/dry ton
sludge solids
Projected
Oct. 1977-Sept. 1978
$ 60,000
1,300
2,211
10,500
132,000
125,750
27,540
144,000
22,250
4,000
$529,551
$469,551
3,450
$136
Actual
1976
$ 46,501
3,971
426
13,036
120,000
152,919
112,942*
73,145
32,176
3,955
$559,071
$512,570
3,450
$149
18,200
wet tons/yr****
-
1,300
2,211
10,500
-
80,000
27,540
144,000***
22,250
4,000
$291,801
4,200
$ 69
45,500
wet tons/yr
-
1,300
3,000
25,000
-
125,750
37,000
350,000***
35,000
4,000
$581,050
10,500
$ 55
* Includes screening performed by outside contract, screening now performed on site by MES personnel.
** Excluding requirements of research work.
*** Assume 50 percent of compost marketed unscreened and 70 percent recovery of bulking agent after
screening finished compost.
**** Present rate of 350 wet tons of sludge per week. Reported capacity of site is 875 wet tons of
sludge per week.
-------
TABLE 10. BANGOR COMPOSTING COSTS, 1975-1976(4)
Capital amortization
6 percent, 5 years $ 3.00*
Operations 43.00
Bulking agent 37.00
Analysis 3.QQ
$ 86.00
*Costs per dry ton of sludge for a total of 525 dry tons
per year.
capital amortization.
The major cost items at present composting operations are labor and
bulking agent which can each be 30 to 45 percent of the total annual cost
of composting. Operation and maintenance of the mobile equipment at
Beltsville, excluding capital amortization and direct labor, is 10 percent
or less of the total annual cost. Amortization of the purchase cost of the
equipment at Beltsville (approximately $400,000} over six years a?7 pfrcent
would be approximately $84,000 per year or about 13 to 23 percent of total
annual costs shown in Table 9 depending on the case. ^ercent or tocai
33
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SECTION 7
PATHOGENS
Data are available from several sources (Table 11) that indicate path-
ogen and/or indicator organism density in composted wastewater sludge at
various stages of the process. The total and fecal coliform, and salmonella
data are summarized in Table 12.
Sufficient data are available from Bangor to plot cumulative probabil-
ity curves of bacterial densities in sludge, compost/ and compost stockpile.
These curves (Figures 3, 4, and 5) are plotted to a lower density of one log
(base 10) cells per gram which also include all densities below this level.
Although data vary widely, they are consistent in that some measureable
level of salmonella, fecal coliform, and total coliform can be present in
composted wastewater sludge. In most cases, especially in the static pile
composting method, salmonella densities were below detectable levels,
and when detected were less than one log cells per gram. No salmonella have
been detected in compost samples at Bangor in over a year, although salmon-
ella were detected in compost samples in early 1976. Total and fecal coli-
form densities up to 5 or 6 log cells per gram have been detected, but
typically the levels are below 3 or 4 log cells per gram in the finished
compost.
A study has reported on survival of virus through the windrow compost-
ing process, particularly the F2 bacterial virus(7K This work indicates
reductions in densities to less than one or two log PFU per gram with win-
drow composting and stockpile curing. It is expected that greater reduc-
tions in virus concentrations would be observed using the static pile
method because higher temperatures are developed.
Data indicate that maximum temperatures achieved by static pile com-
posting even during very cold weather are generally greater than 60°C for
at least a day and usually for several days. Exposure to temperatures of
60°C for a period of a day or two will inactivate most pathogens. Temper-
atures are not uniform throughout the pile and temperatures near the outside
of the pile may be significantly below 60°C and not great enough to inacti-
vate all pathogens. Tests have shown that indicator organism densities vary
within the pile. It is possible to inactivate virus, bacteria, protozoa,
and helminth by subjecting all parts of the composting mass to lethal tem-
peratures. With present procedures it is not possible to assure total
inactivation using outdoor composting methods because organisms may survive
near the surface of the piles while total inactivation may be achieved
34
-------
TABLE 11. SOURCES OF PATHOGEN DATA
Location
Type of composition
Reference
Beltsville, MD
Bangor, ME
General
Windrow and Static Pile
Static Pile
Windrow and Enclosed
(6)
Plant Records
(7)
TABLE 12. SUMMARY OF BACTERIA DATA
Location
Range of reported densities
cells/gram of compost
Salmonella Total coliform Fecal coliform
Beltsville, MD6
Highest
-------
2
(9
2
IU
o
o
— SUMMER COMPOST
10 20 30 40 SO 60 70 30 90 95 98 99
PERCENT OF MEASUREMENTS BELOW INDICATED LEVEL
Developed from data from Bangor, Maine
Figure 3. Total coliform densities.
36
-------
(9
3
UJ
u
o
3
5 10 20 30 40 50
60 70 80 90 95 98 99
PERCENT OF MEASUREMENTS BELOW INDICATED LEVEL
Developed from data from Bangor, Maine
Figure 4. Fecal coliform densities.
37
-------
(9
3
UJ
O 4
(9
O
Anr
ual o
:i impost
suiimer a: id wint
reflects
»r percen
he
lie
WINTER COMPOST
ANNUAL COMPOST
combined
levals
aver
10
20 30 40 50 60 70 80
90 95 98 99
PERCENT OF MEASUREMENTS BELOW INDICATED LEVEL.
Developed from data from Bangor, Maine
Figure 5. Salmonella densities.
38
-------
within the pile. Therefore, it must be assumed that pathogenic organisms
will survive the composting and curing processes unless some means of posi-
tive sterilization is used. Although the measurement of indicator organism
density in the compost may not be a reliable method to determine actual
levels of pathogenic organisms it is possible to apply historic ratios of
pathogens to indicators to develop some generalizations of the risk of
illness.
Hornick, et al'9' recently published dose - response data for Salmon-
ella typhi bacteria. These data indicate that the dose required to cause
typhoid fever in humans was quite high (see Table 13). Earlier work^10^11*
(12) has also shown that large doses of other species of Salmonella are re-
quired to cause significant human disease (see Table 14).
Methods have been developed for estimating the disease risk based on
the indicator organism density in drinking water<13)(14)(15). while these
methods have been developed for ingestion of drinking water, they can be
applicable to ingestion of sludge. The major difference is that normally
only small amounts of sludge or compost would be ingested as compared to
water. Data from Mechalas, et al(15) is shown in Figure 6, and provides the
basis for the illustration that follows. For purposes of illustration,
assume that a compost marketed to the public contains a total coliform count
of 10 organisms per gram of compost and that one gram of the compost is
ingested by a human adult. The human would have ingested 103 indicator
organisms. One gram of compost would be approximately 2.0 ml in volume at
850 Ib/cu yd.
According to Figure 6 the risk of illness would be approximately one
in 1,000,000 by extrapolation of the curve (approximately 5 x 104 MPN/100
ml total coliform). Based on earlier work by Kerr and Butterfieldd3) in
estimating relationships between numbers of indicator organisms and disease
organisms in wastewater, a rate of, say, 100 salmonella of all types per
1,000,000 coliforms would be very high. Therefore, for the 103 coliforms
ingested it is quite likely that less than one salmonella would be ingested.
Typical coliform to virus ratios would be 50,000 to 100,000:1(8) and, there-
fore, for 103 coliforms less than one virus would be ingested. It has been
shown for both virus and salmonella that significant numbers must be in-
gested to cause disease in humans; typically over 103 salmonella (of most
types). Small doses of virus typically produce infection but not disease
in humans.
This logic indicates a very low risk in using properly composted
wastewater sludge even though it is not possible to assure complete kill of
pathogenic organisms using outdoor composting techniques. At the level of
coliform organisms found in typical wastewater sludge compost and assuming
normal disease rates within the community, it is highly unlikely that
enough of this compost would be ingested to produce disease in adult humans.
Infants may be more susceptible. However, the danger of disease outbreak
is very small considering the volume of compost that would normally be in-
gested. Also infants are less likely to be exposed than adults.
39
-------
TABLE 13. RELATION OF DOSAGE OF SALMONELLA TYPHI
TO DISEASE
(From Hornick et al.) *9'
No. of viable Total volunteers Number
Salmonella typhi challenged with disease (percent)
g
ioy
O
10b
•7
107
c
105
•3
103
42
9
32
116
14
40
8
16
32
0
(95)
(89)
(50)
(28)
(0)
Use of wastewater sludge compost does entail some disease risk, but
for typical compost with total coliform counts of 10^ to 10 cells per
gram, the chance of disease outbreak in humans is probably less than 1 in
1,000,000. Should an area be subject to outbreak of a particular disease
then the risk of infection from compost would be higher because of the pre-
sence of additional pathogenic organisms.
A potential problem recently observed at the Beltsville composting
operation is the possible presence of secondry pathogens, specifically
fungi. In the composting process fungal growths can proliferate, particu-
larly at too low a composting temperature in some part of the pile, or by
reinfection. In fact, much of the action of composting is probably due to
fungal organisms. Some fungi, for example Aspergillus fumigatus, can pro-
duce toxic effects in susceptible individuals. Excessive concern is not
warranted, but it is advisable that individuals with lung ailments or
asthma not be exposed in the composting plant. Dusty conditions should be
avoided, or dusk masks worn when dusty conditions cannot be avoided.
40
-------
TABLE 14. DOSE OF VARIOUS SPECIES AND STRAINS OF SALMONELLA
THAT CAUSED DISEASE IN HUMAN VOLUNTEERS
(From McCullough and Eisele) (1°)
Salmonella Dose at which 50% or
species/strain more develop clinical disease
S. meleagridis 1 50,000,000
S. meleagridis 11 41,000,000
S. meleagridis 111 10,000,000
S. anatum 1 860,000
S. anatum 11 67,000,000
S. anatum 111 4,700,000
S. newport . 1,350,000
S- derby 15,000,000
S. bareilly 1,700,000
S. pullorum 1 1,795,000,000
S. pullorum 11 163,000,000
S. pullorum 111 1,295,000,000
S. pullorum IV 1,280,000,000
41
-------
0.010
0.001
0.0001
10'
,-1
103
4.9x10*
I
10°
I
104
I
6.8 xlO3
I
01 102
SALMONELLA/LITER
I I
10a
106
COLIFORM.MPN/lOOml
6.Bx104 7.0 xlO5
FECAL COLIFORM, MPN/100 ml
103
10?
J
106
Figure 6. Relationship between disease risk and salmonella,
coliforms, and fecal coliforms (after Mechalas et al.).
42
-------
SECTION 8
RESULTS AND DISCUSSION
Operations at both Beltsville and Bangor have demonstrated that com-
posting operations ca'n be conducted outdoors on a year around basis even
in extremely cold climates. Both sites have been in use for static pile
composting for over one year on a continuous basis. Operations were ob-
served at both sites during all types of weather as shown in Table 1 for
the purpose of analyzing various aspects of the operations under all weather
conditions. The materials handling operations can be conducted outdoors
using readily available mobile equipment under all weather conditions en-
countered at both sites by proper scheduling of work. It may be possible
to use fixed materials handling equipment for portions of the materials
handling operations, but this has not been demonstrated on a full scale
operation.
Both operations are continuous on a year around basis; Beltsville re-
ceives limed sludge cake on a 5 day a week schedule and Bangor once a week
and, occasionally, twice a week. Sludge is not stored at the composting
sites, but is brought to the site when it can be handled and formed immed-
iately into compost piles. The open operations are vulnerable to precipi-
tation during the sludge-bulking agent mixing process, but once formed into
piles the precipitation has little effect. Mixing can be accomplished in
the rain at Beltsville because they have the equipment to accomplish the
operation quickly. Bangor, on the other hand, can not mix during rain and,
therefore, they schedule mixing on favorable days. Sludge can be stored at
the plant to allow this flexibility in scheduling.
The major impacts of weather on outdoor composting operations are
summarized as follows:
1. Rain
- Screening can not be conducted in the rain without cover.
- Moisture absorption by unscreened compost stored outdoors
may make screening difficult or impossible during and follow-
ing periods of rain.
- It may not be possible to conduct sludge-bulking agent mixing
during rainy weather unless the mixing can be accomplished
rapidly.
- Some bulking agents may absorb excessive moisture during rainy
weather when stored outdoors.
43
-------
2. Extreme Cold
- It is necessary to conserve heat as much as possible in order to
maintain optimum composting performance. Bangor has developed
several procedures such as using warm compost from an active
pile being torn down as bulking agent for mixing and construc-
tion of a new pile and/or piping hot exhaust air from an active
pile into a new pile for the first few days and using relatively
heavy covers of compost over the piles as insulation.
- Equipment cabs must be heated and covered maintenance areas must
be provided. It is advantageous to store equipment inside when
not being used.
3. Snow
- Snow has not been a serious problem except for impeded visibil-
ity and need for site clearing.
4. High Winds
- High winds can cause materials to blow and can cause dust con-
ditions.
5. Inversion Conditions
- Inversion conditions have not caused problems at either site
during operations.
Weather conditions have not caused "down time" at either site and, in
fact it is not possible to "turn off" composting operations. Both sites
schedule operations on a regular year-round-basis. Certain site improvements
are necessary or can facilitate easier operations, but may not decrease
costs significantly. The site improvements considered mandatory are heavy
crushed stone cover so that heavy equipment can operate year around. A
crushed stone surface has many limitations, but can be used as demonstrated
at Beltsville. More desireable and almost in the mandatory category would
be hard surfacing (preferably reinforced concrete) for access roads and the
mixing and composting areas. Of lesser importance would be paving of
screening and storage areas. Covered areas for mixing, screening, and un-
screened compost storage would be helpful in areas with high precipitation.
Covered composting areas may be helpful, but are not considered necessary
in most climates.
A covered area under construction at Beltsville will be used for re-
search studies. The normal work areas will not be covered. A.paved mixing
and composting pad was constructed and placed in service for regular oper-
ations. While this pad is a significant improvement over the crushed stone
surface it was not installed because of expected operational cost savings.
The advantages of the pad are less stone pickup in the compost, better
drainage and easier cleanup.
There have been no significant nuisance complaints at either site.
Some odors are noticeable at both sites. Beltsville has an odor control
system, but is is seldom used. Paved work areas would help reduce odors on
site because the compost and sludge can work down into the crushed stone.
Operations have never been shut down at either site during static pile
44
-------
composting because of odor generation. Some fly problems have been noted at
Bangor, but only around composting piles. Good site drainage and housekeep-
ing will minimize fly problems. Both sites have occasional problems with
blowing materials and dust, but just on the sites. Beltsville uses a water
wagon to help reduce dust from equipment operations during dry weather.
Paved work areas also would help reduce the dust problem.
Various weather conditions have had no documented effect on manpower
or on the material quantities processed at either site. Some operations,
such as screening, have to be curtailed during rain but equipment is sized
so that screening can be accomplished during good weather. At Bangor, man-
power is scheduled as needed, but at Beltsville the same level of manpower
is available year around.
It is significant to note that savings can be made in manpower and
materials handling by developing an optimum operation for a particular site
and situation. This is illustrated in Table 8. By changing the bulking
agent to unscreened compost the amount of screening and the volume of
make-up bulking agent can be reduced significantly. This is only one exam-
ple of possible savings for a cost conscious municipal operation. It is
anticipated that other significant cost savings can be made by varying
operations and, in fact, the potential savings are greater than those that
may be realized by weatherproofing the site. Additional study is needed to
find ways to reduce composting operation costs.
The product quality is affected very little by weather at a well run
wastewater sludge compost operation. Available pathogen data do indicate
some minor improved kills in summer above the high kills generally achieved.
No data are available on seasonal volatile solids reduction. Color is a
function of moisture content and is not a reliable indicator of process
efficiency or performance. Odor is not affected by season or weather con-
dition as best can be determined.
Pathogens and indicator organisms may be present in finished compost
and there is no solution to this problem except complete sterilization.
The outside of the piles may not reach lethal killing temperatures and
therefore, some organisms can survive. A risk analysis based on data from
Beltsville, Bangor, and published data indicates that the health hazard to
adult humans is very slight. The methods used herein can be applied to
specific situations for estimating risk to public health.
If the composting site is well laid out with specific separated areas
for mixing,, composting, screening and storage, and if proper procedures are
used for mixing, composting, and curing, it is extremely unlikely that raw
sludge or improperly composted sludge can become mixed with finished com-
post and delivered to users. It is possible to use pathogen or indicator
organism analysis as a basis for quality control. However, it is felt
that this type of sampling and testing is not convenient and can be costly
for most smaller municipalities. This type of testing can be used for spot
checks. Probably the most-useful control is regular temperature monitoring
of the piles. Temperature monitoring is simple, quick and provides a re-
liable indicator of day to day pile performance so that necessary process
45
-------
changes can be made quickly. Daily monitoring is desirable, but perhaps
monitoring every few days will be more practical for most operations. Based
on past operations at Beltsville and Bangor, if inside pile temperatures stay
above 60°C for a period of several days, fecal and total coliform should be
less than 104 cells per gram 90 percent of the time and below 106 cells per
gram 100 percent of the time, and salmonella should be below detectable
levels 90 percent of the time and below 104 cells per gram 100 percent of
the time. Consistent readings of total and fecal coliform above 105 or 106
cells per gram probably indicates poor operations as would salmonella above
10^ or 104 cells per gram.
46
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References
1. Hoyle, Tanner & Associates, Inc. Report on Sludge Composting at the
Proposed Wastewater Treatment Plant, Durham, New Hampshire. EPA/WS
& PCC Project No. C330161-02 Amendment No. 1. August 25, 1976.
2. Urban Services Group, Final Report - Markets Study for Composted
Sewage Sludge in the Metropolitan Washington Area, Metropolitan
Washington Council of Governments. January 17, 1977.
3. Colacicco, D., Epstein, E., Willson, G. B., Parr, J. F., Christensen,
L. A. Costs of Sludge Composting.
4. Joseph, J. T. Feasibility of Composting Raw Sewage Sludge by High Rate
Suction Aeration Techniques. EPA Solid Waste Demonstration Grant
S-803828-01, April 1976.
5. Crombie, George and Barter, Robert D. Application and Economics of
Forced Aeration Composting of Sewage Sludge.
6. Epstein, E., Willson, G. B., Burge, W. D., Mullen, D. C., and Enkiri,
N. K. A Forced Aeration System for Composting Wastewater Sludge.
Jour. WPCF. Vol. 48, No. 4, April 1976. Page 688.
7. Kawata, Dr. K., Cramer, William N., Burge, W. D. Composting Destroys
Pathogens. Water and Sewage Works, April 1977. Page 76.
8. Geldreich, E. E. and Clarke, N. A., "The Coliform Test: A Criterion
for Viral Safety of Water." Proceedings of the 13th Water Quality
Conference, Urbana, 111., 103-113 (1971).
9. Hornick, R. B., et al., "Typhoid Fever: Pathogenesis and Immunologic
. Control." New Eng. J. of Med., 283, 686 (1970).
10. Nccullough, N. B., and Eisele, W., "Experimental Human Salmonellosis I.
Pathogenic!ty of Strains of S. Meleagridis and S. Anatum Obtained from
Spray-Dried Whole Egg." J. Infectious Disease, 88, 278 (1951).
11. McCullough, N. B., and Eisele, W., "Experimental Human Salmonellosis
III. Pathogenicity of Strains of Salmonella Newport, S. Derby and
S. Bareilly Obtained from Spray-Dried Whole Eggs." J. Infectious
Disease, 89, 209 (1951).
47
-------
12. McCullough, N. B., and Eisele, W. , "Experimental Human Salmonellosis
IV. Pathogenicity of Strains of Salmonella Pullorum Obtained from
Spray-Dried Whole Eggs." J. Infectious Disease, 89, 259 (1951).
13. Kerr, R. W., and Butterfield, C. T., "Notes on the Relationship Bet-
ween Coliforms and Enteric Pathogens." Pub. Health Rep., 58, 589
(1943) .
14. Fair, G. M. , Geyer, J. C. and Okun, D. A. Elements of Water Supply
and Wastewater Disposal, 2nd Ed., John Wiley & Sons, New York, 1971,
512.
15. Mechelas, J. M., et al., "An Investigation into Recreational Water
Quality." Water Quality Data Book, Vol. 4. EPA 18040 DAZ 04/72.
U.S. Printing Office, Washington, D. C. (1972).
48
-------
LIST OF METRIC CONVERSIONS
English Unit
acre
bushels
cu ft
.cu yd
op
ft
in
Ib
mile
sq ft
ton (short)
Multiplier
0.405
0.035
28.32
0.765
0.555{°F-32)
0.3048
2.54
0.454
1.61
0.0929
0.907
Metric Unit
ha
cu m
1
cu m
°C
m
cm
kg
km
sq m
ton (metric)
49
-------
APPENDIX
FORCED AERATION COMPOSTING AND APPLICATIONS
Composting is the biological oxidation of organic matter by aerobic
thermophilic organisms, or anerobic, mesophilic organisms. The forced
aeration method of composting is an aerobic, thermophilic process as pre-
sently practiced. Aerobic conditions are maintained in the pile by forced
ventilation, compared to mechanical turning in the windrow process. The
forced aeration method is significant because need for sludge digestion is
eliminated and raw (undigested) sludge cake can be used without causing
odor problems. In addition, the piles remain fixed during the composting
period, higher and more uniform composting temperatures are developed and
maintained, less land is required, and, generally, more conventional and
less costly machinery is required as compared to windrow composting. The
forced mechanical aeration allows operation personnel to effect some pro-
cess control by adjusting the aeration operating cycle.
Raw limed dewatered wastewater sludge (approximately 20-25 percent
solids) is mixed with a bulking agent prior to formation of the pile. The
ratio is not necessarily fixed and, commonly, is two to three parts bulking
agent to one part sludge cake by volume. The purpose of the bulking agent
is to increase the porosity of the sludge cake to assure aerobic conditions
during composting and to reduce the moisture content of the mix to an
acceptable level (50 to 60 percent). Bark and wood chips have been used
successfully as bulking agents, but other materials may also be suitable.
Complete mixing of the bulking agent and sludge is essential.
A base is prepared for the pile consisting of a perforated aeration
header and approximately 12 in of bulking agent or unscreened compost.
The base plan dimensions are the same as the bottom of the finished pile,
typically 15 to 24 ft wide and 40 to 50 ft long, although the dimensions
can be changed to match the requirements of the specific situation. The
base is important in providing proper air distribution in the pile.
The sludge-bulking agent mixture is piled on this base to a typical
height of 10 ft forming a triangular cross section. The pile is capped
on all exposed surfaces with a 12 to 18 in layer of screened or unscreened
compost which insulates the active pile, limits the precipitation which
penetrates into the pile, and absorbs odors. This pile configuration can
be constructed using commercially available front loaders and contains
approximately 50 cu yd of dewatered sludge and 150 cu yd of bulking agent
in addition to the base and cap materials. Other size piles can be con-
structed.
50
-------
Additional individual piles are normally constructed as sludge is
available or the initial pile can be "extended" by placing a new pile
immediately adjacent to one side of the last pile forming a wider continu-
ous pile. These configurations are shown in Figure A.
The aeration header is attached to the suction side of a blower capable
of producing an air flow of 200 to 300 cu ft per minute (for the size pile
described). The blower discharge is piped into a smaller pile of un-
screened compost which effectively deodorizes the gases drawn from the
composting pile. Water condenses in the blower suction piping and some
means must be provided to remove this water either by careful grading of
the piping or installation of a moisture drain or trap.
A seven week cycle consisting of composting and curing has been used
sucessfully. The pile is operated for the first three weeks (or more if
necessary due to weather) with the blower providing the forced aeration.
Interior pile temperatures and oxygen levels should be monitored regularly.
Oxygen levels should be in the range of 5 to 15 percent and interior temper-
atures should rise to and remain above 60° centrigrade during a major por-
tion of the composting period. The lowest temperatures are generally near
the outside of the pile. The blower can be operated continuously or on
various on-off cycles during composting to maintain optimum conditions.
Operating installations have demonstrated that pile temperatures can be
maintained above 60° centigrade for a week or more under severe New England
winter weather conditions if proper composting techniques are used.
Upon completion of composting, the material in the pile is moved to a
stockpile for curing and storage prior to distribution. The curing period
is typically 4 weeks and is an added safety measure prior to distribution
because elevated temperatures are maintained during curing. In most cases
the compost is screened either prior to or after curing to control the
maximum particle size of the compost product and to recover a portion of
the bulking agent. The cost of the bulking agent can be substantial there-
fore, recovery and reuse may be very important to the overall economics.
It has been shown that wood and bark chip bulking agents can be reused a
number of times, however these materials do deteriorate and some passes
through the screening becoming a part of the final compost product, so a
portion is lost during each composting cycle. Reportedly, 20 to 30 percent
is lost during each cycle, but this depends somewhat on the mesh size of
the screening.
The foregoing is only a general description of the process. Continual
modifications and improvements are being made and it is expected that this
process of change will continue.
Two installations have been practicing forced aeration composting for
two years or more and are described as illustrations of the process. The
third location has practiced forced aeration composting for a year and is
planning a new installation which should be under construction soon.
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AIR IN,
DEODORIZED
EXHAUST
AIR
GENERAL LAYOUT
BULKING AGENT AND
SLUDGE MIXTURE
COMPOST COVER
UNSCREENED COMPOST
OR BULKING AGENT
PERFORATED
PIPE
15 TO 24 FT
CROSS SECTION
SCREENED
COMPOST
(5 cu yd)
SUBSEQUENT PILES
FOR EXTENDED PILE
METHOD
Figure A. Typical forced aeration compost pile for 50 cu yd of
dewatered sludge.
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BELTSVILLE, MARYLAND
The MES and ARS have jointly operated a demonstration and research
wastewater sludge composting facility at Beltsville for several years.
Initial work centered on windrow composting, but this process was found to
be unsuitable for composting raw wastewater sludge because of odor problems.
This led to the development of the forced aeration method in 1973. Subse-
quent demonstrations showed that raw wastewater sludge can be composted
by this method without producing offensive odors in addition to producing
higher composting temperatures. This method requires less space and
eliminates the requirement for regular turning of the compost as compared
to windrow composting. Work is continuing in an effort to further improve
this method and develop additional performance data. Approximately 350
wet tons per week of raw dewatered wastewater sludge is composted at the
10 acre Beltsville site.
The site w.hich was previously used for windrow composting, is being
converted to an efficient layout for forced aeration composting. Paved
composting pad areas, both covered and uncovered, are being constructed
in addition to covered storage and working areas. It is estimated by
ARS personnel that at least 150 wet tons per day of sludge could be com-
posted on this site using the extended forced aeration method.
BANGOR, MAINE
The City of Bangor, Maine, population 38,000 has been composting limed
raw primary sludge cake since mid 1975 using the forced aeration method as
developed by USDA.
The City generates approximately 40 wet tons (50 cu yd) of sludge each
week (approximately 20 to 25 percent solids). The sludge is dewatered and
delivered to the compost site once a week from the wastewater treatment
plant. A hydraulically operated, rear loading, lift and carry type
vehicle is used to haul the dewatered sludge to the compost site in open
top containers. The compost site is located 3 miles from the wastewater
treatment plant and 2,000 ft from the Bangor International Airport terminal,
on approximately 66,000 sq ft of abandoned concrete taxiway. Shreaded bark
is used as a bulking agent. A front end loader is used to move the bulking
agent, mix the materials, form the piles, and move the compost.
After composting is complete, the product is stored and it is planned
for use by the City for applications such as land reclamation, ornamental
plantings, parks, highway plantings, and similar soil conditioning appli-
cations.
DURHAM, NEW HAMPSHIRE
The Town of Durham composted approximately 40 to 50 wet tons per week
of limed raw dewatered primary sewage sludge (approximately 20 percent
solids) from early 1975 to mid 1976 using the forced aeration method as
developed by USDA. The purpose of the work at Durham was to determine that
adequate composting temperatures could be attained in a cold northern
53
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climate and to ascertain the costs of operation. The work was funded in
part by the State of New Hampshire Department of Public Health.
As the result of a favorable test program, forced aeration composting
is being incorporated into plans for expansion of the wastewater treatment
facility to secondary treatment. Many of the labor intensive and critical
operations such as sludge - bulking agent mixing will be mechanized to
reduce operating costs and enhance process performance.
Durham officials concluded that forced aeration composting produced a
safe and usable product in a cold northern climate and that the system
design being developed is cost effective compared with other disposal
methods. These conclusions are based on results of the actual full scale
test program during 1975 and 1976 and an engineering predesign study.
54
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
i. REPORT NO.
EPA-600/2-78-057
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE ANDSUBTITLE
5. REPORT DATE
A Study of Forced Aeration Composting of
Wastewater Sludge
June 1978 (Issuing Date]
6. PERFORMING ORGANIZATION CODE
7. AUTHORIS)
William F. Ettlich and
Anne E. Lewis
8. PERFORMING ORGANIZATION REPORT NO.
3. PERFORMING ORGANIZATION NAME AND ADDRESS
Gulp/Wesner/Culp
Clean Water Consultants
P. 0. Box 40
El Dorado Hills, CA 95630
10. PROGRAM ELEMENT NO.
1BC611, AP C611B, SOS #1,
fl
*7
11. CONTRACT/GRANT NO.
68-03-2186 (Task 10)
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory--Cin.,OH
Office of Research and Development
U. S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Francis L. Evans, III 513/684-7610
16. ABSTRACT
The overall study objective was to make an independent assessment of the forced
aeration wastewater sludge composting method as practiced at Beltsville, Maryland
and Bangor, Maine. A number of visits were made to both sites to observe operations
under all weather conditions and to gather data. The analyses developed are based
on information obtained during the site visits, information provided by various
agencies and individuals, and independent observations and calculations.
Results of the study indicate that forced aeration sludge composting can be
carried out in a satisfactory manner under nearly all weather conditions including
severe New England winters. A number of problems and potential problems are
identified along with possible solutions. Costs are very dependent on the size
of the operation and the methods used. Several possibilities are explored for
reducing costs.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
COSATI Field/Group
Composts, Sludge, Sludge disposal,
Sludge drying, Sludge digestion,
Microorganism control (sewage),
Aerobic processes, Distribution,
Cost estimates, Odor control,
Aerobic bacteria, Sewage treatment
Wastewater sludge,
Sludge forced aeration,
Compost distribution
13B
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
63
20. SECURITY CLASS (This page)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
55
U. S. GOVERNMENT PRINTING OFFICE: 1978-757-140/1340 Region No. 5-11
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