vvEPA
United States
Environmental Protection
Agency
Office of Water Program
Operations (WH-547)
Washington DC 20460
June 1981
430/9-81-011
Composting Processes
to Stabilize and Disinfect
Municipal Sewage Sludge
• »-•
• ••
„• • w •«
MCD 79
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EPA 430/9-81-011
JULY 1981
TECHNICAL BULLETIN:
OTOSTING PROCESSES TO STABILIZE AND DISINFECT
MUNICIPAL SEWAGE SLUDGES
U,S, ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF WATER PROGRAM OPERATIONS
WASHINGTON, D,C, 20460
as. E™-™
Fusion ^.Li"? ?,.„„
'
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FOREWORD
This Bulletin was written to provide guidance for the design and
operation of sewage sludge composting facilities. It also will serve as
an aid in their effective establishment and will help alleviate problems
that may arise during everyday normal operation. The guidance consists
primarily of recommended operational procedures and performance levels
related to the composting facilities. The performance levels, recommended
within this Bulletin, are flexible to make allowances for innovatl-^ :,r-
composting system designs. The recommendations will also help assure
that adequate sludge stabilization and disinfection (pathogen reducMo'}
are achieved.
The Agency's main interests lie in the cost-effective design and
operation of composting facilities, the adequate reduction of pathogens
in the finished compost, and the assurance of aesthetically acceptable
and safe working conditions for site employees and nearby residences.
To this end, the guidance given in this Bulletin should minimize possible
adverse operational, aesthetic and health constraints that are often
encountered.
Another very important Agency goal is the proper end-use of the
composted sludge product. This guidance will be provided as part of a
comprehensive sludge regulation now being prepared by EPA under section
405 of the Clean Water Act. This comprehensive regulation will also be
supported by a health assessment. Neither the guidance for proper end-
use nor the health assessment is contained in this document.
Aerobic composting is being considered by a number of communities
as a method for stabilizing and disinfecting municipal sewage sludge.
Two basic methods (windrow and static aerated pile) have been developed
and used sufficiently in the United States and Canada, so that they can
be relied upon as cost-effective and environmentally acceptable options
for processing sludge. A review of eight of these composting facilities
supplied the majority of information that is contained in this Technical
Bulletin.
While the Bulletin stresses the static aerated pile and windrow
methods of composting, it is not intended to indicate an Agency preference
for these methods. These two methods have received the most attention
because they are the methods that have been operationally proven in the
United States in both small and large facilities.
This Technical Bulletin is being issued for immediate use within
the construction grants program. It has undergone a number of reviews
during the past two years of its development. Recognizing that new
developments are rapidly occuring, the guidance and requirements contained
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in this Bulletin are subject to revision or amendment. Your further
comments are welcome. These comments and additional information from future
successful projects will be considered in making any possible revisions.
Any comments pertaining to this Bulletin should be sent to John M. Walker,
(WH-547), Office of Water Program Operations, U.S. Environmental
Protection Agency, Washington, D.C. 20460.
Admini
Opera
n'gest
ator for
'ns (WH-54'
Deputy Assistant
ter Program
n
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DISCUSSION OF REVIEW COMMENTS
The Agency would like to acknowledge the compost plant operators,
municipal officials, researchers, engineers and others who willingly
discussed their composting program. The information they supplied
formed the basis for this document and their assistance is greatly
appreciated.
On several occasions in the development of this Technical Bulletin,
draft copies were sent out for review to the above mentioned individuals;
Federal and State agencies; and other composting facilities, researchers
and consultants. The written comments received were in strong support
of the document and offered many constructive comments for its improvement.
The comments were greatly appreciated and have been carefully considered
in revising and producing this final document. We believe that the
quality of the document has been significantly improved as a result of
these comments.
Many comments have been accepted in the revision of this Technical
Bulletin. These comments were incorporated in the text and were used to
improve the accuracy and point out limitations of the document. For
example, they helped clarify what aspects of a composting operation are
eligible for construction grants funding; describe provisions for handling
condensate runoff; show the distinction between composted sludge only
and a composted sludge/wood chip blend when referring to unscreened
compost as a bulking agent; provide further discussion on the reasons
for curing the compost; add that drying compost, by spreading it out and
disking, facilitates screening; explain how unscreened compost could
reduce odors when used as a bulking agent; clarify that separation
efficiency during screening, along with the screen's ability to handle
high moisture compost, is of major importance when describing the separation
of bulking agent from composted sludge; and emphasize the relationships
among odor production and the moisture contents of sludge and bulking
agent.
The following comments were also used to improve the document;
however, they need further discussion since they were not totally accepted.
Several commentors felt the title should directly state that the
document applies to the windrow and aerated pile methods. However,
since the document provides guidance for all types of methods, we have
declined to make the title change. We decided to give the windrow and
aerated pile greater discussion because of their proven operational
capabilities in this country. Many of the same principles apply to
other methods, such as invessel systems, which are rapidly becoming more
simple, reliable and flexible due to technological improvements. Also,
in the future, high rate-positive aeration systems may offer the potential
for faster composting with a dryer end product.
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Other commentors suggested that more distinction w*s needed in the
Bulletin to identify when discussions related to the windrow, aerated
pile, or both methods. Additional efforts were made to provide these
distinctions. More frequent distinction were not made since both these
methods are similar in many respects.
One commentor suggested that oxygen monitoring is not necessary for
non-aerated windrow operations. The commentor based his belief on the
fact that oxygen concentrations usually drop to extremely low levels
soon after turning the windrow and that little could be done with oxygen
monitoring information. The Agency has not yet resolved the question of
monitoring oxygen levels in non-aerated windrows, but strongly agrees
that superfluous monitoring is an unneeded burden for municipalities.
Presently, the Agency believes that oxygen monitoring for non-aerated
windrows is a desirable operator's tool 1) initially, for new and modified
operations, to get an idea of the oxygen range for proper composting
under the facilities' specific conditions, and 2) when operational
problems are suspected or occur.
The previous commentor also Felt that the Bulletin should address
the merits of using activated carbon and wet scrubbers for odor control,
since their studies indicated they were more efficient than compost
scrubber piles for control!ing odors. The use of some form of odor
scrubbing system is necessary. If the composting facility is located
close to nearby residences and if odor production is the major stumbling
block in gaining public acceptance for the site, the use of activated
carbon and wet scrubbers may possibly be advantageous; however, recommending
these scrubbers would probably be premature because activated carbon and
wet scrubbing systems are relatively expensive and their reliability and
cost-effectiveness have not been demonstrated in large-scale sludge
composting operations.
Some commentors wanted a discussion on regrowth of Salmonella in
finished compost. While there is some uncertainty regarding the potential
health risk from Salmonella regrowth, present available data has not
indicated a Salmonella health problem from the use of composted sludge.
One commentor asked thai the 15 days above 55°C requirement listed
in the Bulletin for windrow composting be changed. They supplied results
of a series of operational windrow composting trials in support of their
request During these trials, in which anaerobically digested sludge
was composted, they closely monitored temperatures, pathogen reduction,
and oxygen and moisture content. Their results showed that it was very
difficult to maintain windrow compost temperatures above 55 C for 15
days as the Criteria for the Classification of Solid Waste Disposal
Facilities and Practices (28) currently require.
Findings of this commentor also suggested that 7 days above 55°C in
windrows with a total windrow composting time of 30 days, produced
pathogen reductions equivalent to those obtained composting for a total
of 30 days, either with 15 days above 55°C in the windrow or 3 days
above 55°C in static aerated piles. Presently, the Criteria (28) allows
IV
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requirements, if the composting facility can
acMevPd wh ^ t0JeVels ^"^alent to the reduction
achieved with the approved time and temperature requirements The
' be a Very d1ff1™lt task for a
_ The composting time/ temperature provisions of the Criteria are
interim final and as such are legally binding. EPA will consider t
commentor's data and other data on pathogen "reduction by composing
during development of the previously mentioned comprehensive sludge
regulation.
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TABLE OF CONTENTS
Page No.
Foreword i
Discussion of Review Comments iii
List of Tables vii
List of Figures viit
I. Purpose 1
II. Use of the Performance Recommendations 1
III. Current Status of Construction Grants Funding 1
IV. Performance Recommendations 2
A. Time/Temperature Requirements 2
B. Monitoring Recommendations 5
V. Background on Present Practice in the United 8
States
A. General Procedure 8
B. Problems Encountered 8
VI. Detailed Guidance for Effective Operation of 10
Composting Systems
A. Dewatering 10
B. Bulking Agent 12
C. Mixing, Configuration, Aeration and Drainage 15
(1) Mixing 15
(2) Configuration 18
(a) Conical Pile/Windrow . 18
(b) Aerated Pile/Extended Aerated Pile 20
(3) Aeration 20
(4) Drainage 24
D. Pad 25
E. Climatic Impacts 25
F. Odor Control 27
G. Screening and Conveying 28
H. Curing (stockpiling) 30
I. Disposition/Use • 32
J. Employee Safety 33
VII. References 35
VIII. Appendices A-l
A. Concepts and Background of Sludge Composting A-!
B. Health Aspects of Sludge Composting B-l
C. Related Publications C-l
VI
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LIST OF TABLES
Table No. Title Page
1 Physical/Chemical Conditions for 3
Sewage Sludge Composting by Static
Aerated Pile and Windrow Techniques
2 Suggested Parameters and Time Sequences 6
for Monitoring
3 Selected Characteristics of Sewage n
Sludge at Studied Composting Facilities
4 Ratio by Volume of Bulking Agent to 14
Sewage Sludge for Composting Based on
Moisture Content, Aeration, and Previous
Stabilization of Sludge
5 Characteristics of Materials Used as 15
Bulking Agents
6 Comparison of Mixing Equipment 17
7 Densities of Various Sludge Composting 17
Materials
8 Mode and Duration of Aeration at Sewage 22
Sludge Composting Facilities
9 Paving Materials for Composting Pads
26
10 Sludge Compost Screening Facilities 3]
for Recovery of Bulking Agent
VII
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LIST OF FIGURES
Figure No. Title Page
1 Monitoring Locations for Temperature 7
and Oxygen Probes Within the Composting
Pile
2 Effect of Solids Content on the Ratio 13
of Wood Chips to Sludge by Volume
3 Configuration of Mechanically Turned 19
Windrow Piles
4 Configuration of Aerated Piles 21
viii
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I. PURPOSE
This Technical Bulletin has been prepared to assist Environmental
Protection Agency (EPA) Regional Offices and delegated State Agencies
in reviewing and evaluating grant applications for the planning,
design and construction of municipal sewage sludge composting
facilities. All municipal sludge management projects involving
such processes will be evaluated in consideration of the recommended
design principles and operational procedures contained in this
Bulletin. In addition, the operational and design information
provided should be of interest to engineers, designers, planners,
and others as an aid in the development of new composting facilities
and to improve the operation of existing facilities. Use of this
information should help enhance the cost-effectiveness and reliability
of composting alternatives.
II. USE OF THE PERFORMANCE RECOMMENDATIONS
Decisions for Federal financial assistance from EPA will be
based on the ability of municipal sewage sludge composting facilities
to achieve the recommended performance levels contained in this
Technical Bulletin. Facilities which differ in their operational
procedures and in their ability to meet the recommended performance
levels given in this Bulletin will be considered if assurance can
be given to the EPA Regional Administrator that comparable satisfactory
performance will be achieved.
This Bulletin is not meant to preclude innovation in composting
practices. EPA's policy is to accept and encourage the use of
innovative Federal funding as described under section 202(a)(2) of
the Clean Water Act of 1977 (PL 95-217). Therefore, EPA Regional
Administrators will give equal consideration to innovative technologies
that are not included in this Bulletin. To ensure cost-effectiveness
and reliability, innovations should be carefully tested before
being adopted on a large scale.
III. CURRENT STATUS OF CONSTRUCTION GRANTS FUNDING
Sewage sludge composting is an acceptable method for stabilizing
and decreasing the pathogen content of sewage sludge. Composting
has been defined as an alternative sludge management system (40
CFR, Part 35, Subpart E) and, thereby, is eligible for 85 percent
funding for facility planning, design and construction under the
innovative/alternative technology provisions of the Construction
Grants Program (section 35.908b, 35.930-5b). Meeting the definition
of an "alternative technology", sludge composting has at least a 15
percent advantage in the cost-effectiveness comparisons with the
least-cost conventional technologies, and is eligible for 100
percent Federal funding for rehabilitation or replacement if the
system fails or breaks down during its first two years of operation.
The land needed for composting, curing and temporary storage of
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sewage sludge compost is also grant eligible (Preamble: land
eligibility, Federal Register, September 27, 1978, 44035; 40 CFR,
section 35.905 "Treatment Works".} Continuation'of these incentives
beyond September 30, 1981, will require amendment of the Clean
Water Act. Amendments to the Clean Water Act to extend the innovative
and alternative technology program are presently being considered
by Congress.
IV. PERFORMANCE RECOMMENDATIONS
Various composting processes have been developed for the
stabilization and disinfection of sewage sludge (see Appendix A)
These sludge composting systems include the static aerated pile
and windrow configurations;, and automated invessel systems. The
aerated pile and the windrow configuration have been used for large-
scale operations in the United States. Invessel systems have not
been used for large-scale sludge composting in this country; however,
such systems have been developed and are being used in some European
countries. For these reasons, this Bulletin focuses mainly on the
aerated pile and windrow methods.
A. Time/Temperature Requirements
In order to minimize possible adverse health impacts that
might result from the utilization of composted municipal sludge,
adequate stabilization and disinfection via the composting process
must be assured. Following the recommended performance levels
given in this Bulletin will provide this assurance.
The pathogen content in municipal sewage sludge is reduced by
the elevated temperatures that are attained during composting.
There are two sets of time/temperature combinations in the Criteria
for the Classification of Solid Waste Disposal Facilities and
Practices (28) that are required for "significant" and "further"
reduction of pathogens. These temperatures should be maintained
for the specified length of time to ensure adequate pathogen destruction
for the appropriate end-use. The two necessary time and temperature
conditions for aerobic sewage sludge composting are summarized in
Table 1.
A "significant" reduction of pathogens will be obtained if the
temperature is at least 40°C for 5 days and exceeds 55°C for 4
hours (during those 5 days). However, the end-use of a compost
that has only undergone "significant" reduction of pathogens is
restricted by required waiting periods and access restrictions. In
contrast, if the compost meets the requirements for the "further"
reduction of pathogens, then waiting periods and access restrictions
should not be necessary.
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Table 1. PHYSICAL/CHEMICAL CONDITIONS FOR SEWAGE SLUDGE COMPOSTING
BY STATIC AERATED PILE AND WINDROW TECHNIQUES (25)
Condition of Sludge/
Bulking Agent Mixture
Levels of Pathogen Reduction
Requirement
I/
Time/Temperature —
"significant"
40°C/5 days with
4 hrs at 55°C or
higher (both
methods)
Conditions Desirable to Achieve Requirement
Moisture
2/
Oxygen -
PH^/
Carbon/Nitrogen (initial) -
Volatile Solids (initial) -/
Time Composting
Time Curing (Stockpiling)
"further"
55°C/3 days (aerated pi la)
55°C/5 days/5 turnings
(windrow)
40-65%
5-15%
5-11
10-30
35% or greater
21 days, minimum
21 days, minimum
I/ Interim Final temperature requirements (28) for sludge composting pro-
~ cesses which "significantly" and "further" reduce pathogens.
2/ Readings somewhat less than 5% would be acceptable during the first
~~ 10 days of composting especially for non-aerated windrows, and readings
above 15% are suitable where high aeration rates are used along with
temperature sensitive switches to increase moisture removal.
3/ A mixture of sludge and bulking agent will compost, even if the pH
~ is 12, provided the lime level is sufficiently low, so that the pH
is not at 12 more than initially.
4/ Carbon/Nitrogen (C/N) ratio may be misleading if the carbon is in
~ a form making it less available. The carbon contained in the coarse
wood chips is not highly reactive and the effective carbon/nitrogen
ratio would be lower than for the same wood chips ground into sawdust.
5/ If the volatile solids content of the sludge is less than about 35%,
~ then a bulking agent that provides a source of available carbon may
be needed to provide optimum composting conditions.
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The following conditions are needed for a "further" reduction
of pathogens. For the aerated pile technique, the temperature must
be at least 55°C continuously, for 3 days, in the coolest part of
the pile (beneath the cover blanket of previously composted sludge).
For the windrow technique, the temperature should be at least 55°C
in the center of the windrow for at least 15 of the total 21-30 day
composting period. Also, there should be a minimum of five turnings
of the windrow, distributed throughout the high temperature period.
Note that the temperature may temporarily decrease Immediately
after turning the windrow.
These temperature requirements for pathogen control have
been included as interim final in the Criteria (28) and therefore,
are legally binding, although subject to change.
Where mobile field composting machines are used, the sludge
and bulking agent should be mixed from one to three times to provide
a good initial mix. Other periodic turnings of the windrows are
necessary for providing aeration, maintaining high temperatures -; ^,d
lowering moisture content. These turnings should be scheduled
according to the need that is indicated by monitoring and weather
conditions. Since the aerated pile is usually not remixed after
establishing the configuration, a good uniform initial mix is
particularly important.
Other important factors that should be considered in temperature
achievement are also listed in Table 1. Ranges for moisture,
oxygen and organic content have been included because these criteria
are generally necessary for attainment of these temperatures with
optimal performance. To consistently provide high temperatures for
pathogen reduction, the proper ranges for moisture, pH and oxygen
must be maintained. This requires a facility to be flexible enough
to adapt to continually changing sludge and bulking agent character-
istics. For example, if a facility doesn't have a flexible design,
a change in sludge moisture content could cause anaerobic conditions
in the compost pile. Hence, the temperature requirements would not
be met.
In general, an active composting period of about 21 days is
needed to achieve stabilization of the composting sludge when using
either the static pile or windrow method. A 3-4 week period for
curing the compost in stockpiles, following the active composting
period (either before or after screening), helps ensure completeness
of stabilization (reduction of possible odor production) and pathogen
reduction.
Other systems for composting municipal sewage sludge should
meet the time and temperature requirements and/or otherwise provide
for adequate stabilization to be considered as suitable technology.
Sludge composting is defined as an alternative technology and may
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also be considered innovative if it meets the additional requirements
in Appendix E of the Construction Grants Regulations. A pilot
study is one useful means of demonstrating the suitability of a
successfully in the United States (i.e., meets the conditions
listed in Table 1),
If a composting design is based on the aerated pile or windrow
technique, the need for a pilot study is greatly reduced because
these processes have been proven to be efficient methods for sludge
stabilization and disinfection, Basic operational and design
parameters are given for these two methods in Section V.
B. Monitoring Recommendations
A proper monitoring program should include at a minimum
frequent checks on temperature and moisture content and periodic
checks on oxygen content. Monitoring will provide information on
the progress of each compost pile or windrow and will serve as an
indicator that the composting process has proceeded correctly. To
obtain monitoring information a facility would need equipment for
moisture analysis (laboratory analysis or resistance probe); a
temperature indicator with at least a 6 foot probe and a scale
reading from 0° to 100°C (32° to 212°F); and a portable 0 to 25%,
dry gas oxygen analyzer. In addition to these measurements, the
monitoring of heavy metals and pathogens in the sludge and/or
finished compost is necessary to insure good quality compost products.
A suggested monitoring program is provided in Table 2.
The following is a suggested protocol for locating the temperature
and oxygen monitoring probes (see Figure 1 for location of the
probes in static piles). Windrows should be probed to measure the
temperature in the center of the row, at several points along the
row.
The newly created segments of the windrows should be monitored
as units for several days until proper composting conditions are
assured. Monitoring for oxygen is more important when beginning a
totally new composting facility, when changing the operation of the
system, or when encountering compost processing problems. As
operations become more routine, oxygen measurements may only be
necessary on a monthly basis, as a .check on operations'. Monitoring
for oxygen in non-aerated windrows can be misleading. Oxygen
levels near zero are often observed within one hour after turning,
but the composting process is still proceeding properly.
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Table 2. SUGGESTED PARAMETERS AND TIME SEQUENCES FOR MONITORING (36
Parameter
Moisture content
Temperature
2.1
Oxygen -
Pathogen survival
Heavy metals -
Process odors
Blower operations
3/
pH of sludge -
I/
Size of operation in tons per
week (dry solids)
25
25 to 250
250
monthly
daily
optional
weekly
daily
monthly
daily
daily
weekly
daily
daily
monthly
as required by local regulations
as required by local regulations
daily da11y
daily daily
monthly monthly
from sewage plant operator.
-srs ss
frequency for measuring oxygen.
3/ eood co-unlcatlons -,d be -Intalned
plant^perators,
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/MONITORING
POINTS
MONITORING
POINTS
EXTENDED PILES (CELLS)
INITIAL PILE (CELL)
FIGURE 1. MONITORING LOCATIONS FOR TEMPERATURE AND OXYGEN PROBES WITHIN
THE STATIC AERATED COMPOSTING PILE. PILE SHOULD BE MONITORED DAILY
FOR TEMPERATURE AND LESS FREQUENTLY FOR OXYGEN. OXYGEN SHOULD BE
MEASURED AT LOCATION B ONLY.
MEASUREMENTS ARE TAKEN AT THE OPPOSITE ENDS OF EACH CELL NEAR THE
BASE MIDPOINT AT:
A-4' ABOVE GROUND AND APPROXIMATELY 2' HORIZONTALLY INTO THE PILE.
(INTERFACE OF BLANKET MATERIAL AND SsJjDGE CHIP MIXTURE.)
B- 4' ABOVE GROUND AND APPROXIMATELY 4' HORIZONTALLY INTO THE PILE.
C- 2' ABOVE GROUND AND 2.5' HORIZONTALLY INTO THE PILE.
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V. BACKGROUND ON PRESENT PRACTICE IN THE UNITED STATES AND CANADA
Several municipal sewage sludge composting facilities have
been operating successfully at different locations throughout the
United States and Canada. A total of eight facilities (including
both large and small) have been studied in detail, and the results
have been used as a basis for the guidelines contained in this
Bulletin.
The sizes &f these facilities vary greatly. The smallest
processed a little over one dry ton of sludge per day and the
largest operation composted over 100 dry tons per day, (i.e.,
currently about 200 dry tons of sludge per day are being composted
in Washington, D.C. area facilities). Although the size of these
facilities varies widely, there are some general procedures that
each facility follows. Their operating procedures have been fourd
to be the most reliable and economical. These procedures, alcrij
with problems encountered and an overall recommendation for flexibility,
are summarized below. In addition, further information on the
concepts and background of composting is contained in Appendix A,
A. General Operating Procedures
A majority of the facilities, studied are located at or near
the municipal wastewater treatment plant. This greatly reduces the
cost for transportation. The smaller facilities may only work with
fresh sludge one or two days per week while larger facilities may
operate with fresh sludge two shifts per day, 6 or 7 days per week.
Several of the facilities contracted for the actual composting
operation.
Two of the facilities studied offset part of their operating
expenses by selling the finished product for a nominal fee. Others
were using sludge as cover material for municipal landfills. A new
regulation is being drafted by the Agency that will govern the
distribution and marketing of composted sludge (30).
B. Problems Encountered
Various problems have arisen in the development of effective
composting practices for the stabilization of sewage sludge.
Factors unfavorable to aerobic biological systems, such as too
much or too little moisture or very high or low pH, will inhibit
composting processes. Moisture content is one of the most important
factors in a composting operation. Excess moisture can cause many
problems, such as anaerobic conditions, malodors, clogging of
screening mechanism, and difficulties in handling the finished
Product Fortunate y, there are a wide range of conditions under
wh?c composting can'occur. However, the effectiveness and efficiency
of composting practices are much less reliable at the extreme ends
of these ranges.
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Two universally encountered problems include difficulties in
controlling odors and in the recovery, when needed, of bulking
materials by screening. High moisture content of the mixture is
the major cause of both problems and the usual cause of low temperatures,
The control of odor during the composting process is probably the
most important factor in gaining public acceptance for a composting
facility. Excessive odors from a compost pile or windrow may
indicate anaerobic conditions. Composting progresses very slowly
under anaerobic conditions and produces malodorous compounds.
Anaerobic conditions can result from an improper mixing ratio
between sludge and bulking material, excessive moisture content of
the mixture, malfunctions in the aeration system or an unclean,
muddy composting pad. Hence, odor control is essential not only
from an aesthetic point of view, but also from an operational
standpoint.
A number of operating facilities have experienced difficulty
when screening bulking agents from finished compost because the
material placed on the screen was too wet. Compost material with
a high moisture content tends to block the mesh openings and
impede the separation of finer compost from coarser bulking agent.
Once the screens are clogged, they usually have to be cleaned
manually. This tends to be a very expensive and time consuming
process. Therefore, screening has often been delayed until the
weather permitted the screening of a relatively dry compost material.
Periods of cold or wet weather can make screening difficult and can
lead to a shortage of recycled bulking material.
New equipment has enabled more efficient screening of a
relatively high moisture content compost (as high as 50 percent),
with relatively few slowdowns resulting from blockage. A discussion
of these screens, along with other possible solutions to this
problem, are included in the detailed guidance section.
Other problems encountered have been from the establishment of
inappropriate pads for the composting operation, poor site and
equipment cleanliness, improper blending of bulking agents with the
sludge, and the failure to promptly correct imbalances in moisture
and aeration. Correcting these problems is essential for maintaining
aerobic conditions, proper composting temperature and odor control.
Careful management and planning can.help minimize these and other
common problems associated with municipal sewage sludge composting.
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VI. DETAILED GUIDANCE FOR EFFECTIVE OPERATION OF COMPOSTING SYSTEMS
The following ten features (subsections A-J) are very important
in developing efficient composting projects.
A. Dewatering
It is important that ciewatering activities be carefully
coordinated with composting activities. If the content of de-
watering chemicals exceeds about 50% (dry weight basis) or if the
volatile solids content is less than 35% in the resultant sewage
sludge, then composting may be difficult without extra careful
selection of bulking materials to provide an additional source of
carbon. For the most part, ths addition of various chemical agents
to improve sludge dewatering has not had an adverse impact uoir,
the composting of the dewatered sludge, except in extreme cases.
Sludges dewatered by a number of techniques (Table 3) have been
successfully composted.
If the lime content is such that the pH of the sludge remains
near 12, the material will either not compost and/or microbial
activity and temperature rise will be appreciably delayed. Furthermore,
the time required for adequate composting will be lengthened. Most
sludges, however, including those with an initial high or low pH,
will rapidly equilibrate to pH levels between 6 and 8. Also, wood
products that are used as bulking agents will usually contribute to
lowering the pH to near neutral.
The capacity and operational reliability of available dewatering
equipment must be adequate to prevent the backlogging of sludge.
Backlogging often results in sludges turning septic before they are
dewatered. Malodor would then be likely for the initial day or two
of composting when a batch of this septic sludge is mixed with
bulking agents.
In one plant,odorous septic tank pumpings that were added to
the locally produced sludge, prior to dewatering, led to odor
production at the composting facility. This facility alleviated
these odor problems by lime treatment of the septic tank pumpings
(adjusting the pH temporarily to near 12), before mixing with the
liquid sludge for dewatering.
It is also important to consider the physical nature of the
dewatered sludge when planning a particular composting project.
Wetter and finer sludges will require an increased amount of bulking
agent. There may also be an increased need for bulking agent
coarseness with very fine or liquid sludge. Increased sludge
fineness may be due to an upgrading of treatment systems to secondary/
tertiary treatment or to improvements made in the dewatering process.
This change in physical properties of the sludge, if unaccounted
for, will result in inadequate penetration of air, a predominance
10
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Table 3. SELECTED CHARACTERISTICS OF SEWAGE SLUDGES AT
STUDIED COMPOSTING FACILITIES (33)
City
Aerated Pile
H
B
C
A
E
D
F
G
Sludge -1
Raw
P+S+CP
P+S+CP
P
P
P
P
Anaerobically Digested
P
AWT
Dewatering
Mode
Vacuum Filter
Vacuum Filter
Belt Press
Vacuum Filter
Coil Filter (CF)
Centrifuge + CF
Vacuum Filter
Solid Bowl/
Basket Centrifuges
Filter Press
Solids
Content
% VS -1
%
17-24 55
17-24 55
24-28 77
22-28 60
20
20-25 50
23 50
40 40
Sludge
Composition
Lime
%
25
25
0
5
10
7
0
35
Fed .,
% 6
8
8
0
0
5
8
0
5
]_/ P = primary; S = secondary; CP = chemical precipitation of phosphate; AWT = P+S+CP + Ion
exchange + Filtration + C adsorption
2/ %VS = percent of the total solids present that are volatile.
-------
of anaerobic over aerobic microorganisms, and hence, malodor.
If composting of only part of the sludge at a locality is planned,
it could be advantageous to keep separate the more easily compost.able
sludge (e.g., primary from secondary sludge).
A very dry filter press sludge cake (e.g., 35 percent solids
content) may require little or no bulking agent, compared with
wetter vacuum filtered sludges (e.g., 20 percent solids content)
which may require bulking agents, such as wood chips, at the rate
of 2 to 2.5 parts bulking agent to 1 part sludge on a volume basis
(Figure 2). A dry filter press sludge cake, however, may require
some form of crushing before composting to assure thorough mixing
and adequate air penetration (17).
The dewatenng system chosen also has an important impact upon
the usefulness of the final composted sludge product. For example,
the use of lime as a dewatering agent may cause excessive lime
accumulation in the composted sludge, which may result in a higher
sludge pH (high sludge pH can cause ammonia release which in turn
can cause corrosion, odor and possibly health problems in indoor
facilities). Such a high lime composted sludge has caused reduced
growth of some species of plants when they were grown in compost
amended soils (7). A second potential problem when using composted
sludge to support the growth of plants can arise from the use of
high concentrations of ferrous sulfate and/or ferric chloride for
dewatering. The salt contained in both the relatively insoluble
ferrous sulfate and the soluble ferric chloride can cause phytotoxicity
in some species of plants. The soluble ferric chloride, however,
can be rather readily leached out. Still another example of a
potential problem is dewatering chemicals which may contain excessive
levels of unwanted heavy metal contaminants, such as cadmium (2).
B. Bulking Agent
The use of a bulking agent, such as wood chips, compost,
straw, etc., has been found to be essential for proper adjustment
of the sludge moisture content, to provide an additional source of
carbon, and/or to improve the porosity that allows an adequate and
uniform penetration of air (3, 35). Various mixture rates of
different bulking agents and sewage sludge have been successfully
used in presently operating systems. Table 4 illustrates the
general relationships between sludge moisture content and bulking
agent.
Note, however, that composting operations may require adjustment
in the blend and/or type of bulking agent, due to climatic conditions,
moisture content and/or fineness of the sludge, the configuration
for composting and the availability of bulking agents. If recycled
compost (screened or unscreened) is used as a bulking agent, the
bulking agent to sludge ratio usually has to be increased to compensate
for the increased wetness of the recycled bulking material.
12
-------
NOTE:
THIS CURVE IS SITE-SPECIFIC
FOR ONE COMPOST OPERATION.
THIS CURVE WILL SHIFT DEPEN-
DING ON THE RELATIVE VOLI-
TILITY AND SOLIDS CONTENT
OF THE WOOD CHIPS AND KLUDGE.
UJ
§
UJ
CJ
O
D
_J
to
ill
O
Q.
I
O
O
O
O
g
<
cc
O
Z
X
S
I
10
20 30
PERCENT SOLIDS IN SLUDGE
40
50
FIGURE 2. EFFECT OF SOLIDS CONTENT ON THE RATIO
OF WOOD CHIPS TO SLUDGE BY VOLUME (3).
13
-------
TABLE 4. RATIO BY VOLUME OF BULKING AGENT TO SEWAGE
SLUDGE FOR COMPOSTING BASED ON MOISTURE CONTENT,
AERATION, AND PREVIOUS STABILIZATION OF SLUDGE (33)
City
Aerated Pile
H
B
C
A
E
D
Windrow
F
G
Solids Content
% Raw
17-24
17-24
24-28
22-28
20
20-25
Anaerobically Digested
23
40
Bulking Agent/Sludge -'
Wood Chips -f
2:1
2-2.5:1
2:1
3:1
3:1
2.5-3:1
Compost
1:1
1:1
I/ The bulking agent to sludge ratios should be increased as the
wetness and fineness of the sludge or bulking agent increase.
2/ City A uses shredded bark instead of woodchips. Also, City A
alternates using fresh or screened shredded bark with unscreened
compost as the bulking agent. City B recycles unscreened compost
as the total or part of the bulking agent through many cycles.
14
-------
Most sewage sludge composting facilities in the United States
have used predominantly only one bulking agent, e.g., wood chips or
compost. Systems designed specifically for the use of one externally
supplied bulking agent may find that adequate supplies are not
always available or that the cost for the bulking agent may fluctuate
widely. Some systems have tried using the unscreened composted
wood chip/sludge blend as a substitute bulking agent in order to
help cut the cost of operation. This option has worked for flexibly
designed and operated facilities which have the equipment to handle
substitute bulking agents and make other necessary adjustments,
such as changing the bulking agent to sludge ratio.
Other bulking materials, such as peanut hulls, leaves, refme,
and shredded rubber tires, have been used successfully on a small
scale. A rating of the suitability of these materials as bulking
agents is given in Table 5. Note that certain of these bulking
agents have the added benefit of reducing the potential for odor
production.
Equations have been derived as a guidance for determining the
proper blend of bulking agent to sludge (9, 27). The simplest
equation relates the mass of water in the initial compost mixture,
to the mass of organic matter degraded under composting conditions.
If the ratio is 10 or less, composting should be adequate.
C. Mixing, Configuration, Aeration and Drainage
1. Mixing
A variety of equipment has been used for mixing. Most of the
successful operations have used various forms of mobile mixing
equipment (Table 6). Mobile equipment assures flexibility in
operation and ease of replacement or repair. The mixer chosen
should be able to handle a wide range of 1) textures and types of
sludges and 2) moisture contents and densities of bulking materials.
Densities of some types of sludges and bulking agents are
given in Table 7. Processing equipment must be able to handle the
density, consistency, and weight of the sludge and bulking agent.
Machinery designed to handle animal manures will often not be
suitable for sewage sludge composting unless suitably modified.
This is because manures generally are lighter in weight than sewage
sludge, due to the presence of bedding material. Also, the design of
facilities for composting animal manures is different, since there
is a much greater reduction in the original volume of animal manure
than of sewage sludge during composting.
Some facilities have tried stationary mixers in the hope of
reducing labor requirements. These fixed mechanical mixers have
had difficulty in handling variations in the quality and quantity
of sludge and bulking agents. These problems have resulted when
15
-------
Table 5. CHARACTERISTICS OF MATERIALS USED AS BULKING AGENTS
BULKING AGENT ^
Woods Chips
Unscreened Compost
Screened Compost
Wood Shavings
Saw Dust
Peanut Hulls
Corn Cobs
Leaves
Garbage
Cotton Gin Trash
Sugarcane Bagasse
Rice Hulls
Cereal Straws
Shredded Bark
Corn Stover
Fly Ash
Shredded Rubber
Tires
Pelleted Refuse
Derived Fuel
1
Effect on
Porosity of
Mixture
good 2/
fair-good —
fair
good
fair
good
good
fair
fair-good
fair
good
fair
good
good
fair-good
poor
good
good
Source of
Available
Carbon
fair
fair
fair
fair
fair
fair
fair
fair
fair
fair
good
fair
fair
fair
good
none
none
fair
RATING
Recoverability
good
good
good
fair-good
none
poor-fair
good
none
fair
none
none
none
none
good
fair
none
good
fair-good
Overall
Suitability
good
fair-good
fair-good
good
fair-good
good
good
fair-good
fair
fair
fair
fair-good
fair-good
good
fair-good
very poor
poor-fair
good
Ability to
Control
Odors
2i slight
-J, moderate
— ' moderate
slight
slight
slight
slight
slight
slight
slight
slight
. slight
slight
slight
slight
none
none
slight
V Availability of some bulking agents may be effected by seasonal changes (i.e., leaves, stover, etc.)
Some bulking agents that perform poorly alone may perform well when mixed with other bulking agents.
Some bulking agents, such as unscreened compost help reduce odor production which is especially
important in windrow composting. Unscreened compost can be used as the sole bulking agent. However,
a coarse and/or drier bulking agent may be needed as a supplement to ensure adequate aeration.
2/ Assumes the unscreened material contains a coarse fraction, si'cn as wood chips.
3/ Depends upon degree of wetness and fineness of fresh sludge.
-------
Table 6. COMPARISON OF MIXING EQUIPMENT (33)
Equipment Suitability
Mobile Rotary Drum Good
Mobile Rotary Belt Fair to Good
Rototiller (tractor) Good
Front End Loader Poor to Fair
Pug Mill Fair to Good
Table 7. DENSITIES OF VARIOUS SLUDGE COMPOSTING MATERIALS (14,27,34)
Material Density, Ib/cu. yd.-/
Digested sludge 1,500 to 1,750
Raw sludge 1,300 to 1,700
New wood chips 445 to 560
Recycled wood chips 590 to 620
2/
Finished screened compost - 930 to 1,350
V 1 Ib/cu yd = 0.6 kg/m3
2/ Varies significantly with % total solids and bulking agent used.
17
-------
unexpected conditions have occured (e.g., the unavailability of a
primary bulking agent and/or a change in the density of the sludge
and bulking agent to be mixed). There has not been sufficient
experience with these mixers, and their associated feeder hoppers
and conveyors, to determine the extent of real labor savings,
flexibility and reliability.
Poor composting and odor problems frequently occur when the
sludge and bulking agent are mixed in the rain (21). If the
sludge/bulking agent mixture becomes very wet, air penetration may
likely be impeded. Temporary storage facilities for sludge,
alternate methods of disposal, or adequate cover over the mixing
area during periods of rain should be provided to avoid these
problems.
2. Configuration
Two general configurations have evolved for large-scale
outdoor composting of sewage sludge. These are the conical pile/
windrow and the aerated static pile/extended aerated static pile
configurations.
(a) Conical Pile/Windrow
The conical pile and the window configurations normally
depend upon natural, convective forces to pull air in through the
side and bottom and up through the pile, as well as aeration by
periodic mixing. One facility has also added the capability of
forced aeration for the windrow configuration. The general considera-
tions for forced aeration that are discussed in the aeration section
of this Bulletin, apply to the forced aeration variation.
Conical piles are generally limited in use to small sized
operations. In this configuration, mixing and turning may be
accomplished with a rototiller or front-end loader. To ensure a
sufficient mass for self heating, the conical pile should be from
4-6 feet high by 6-14 feet at the base. However, excessive heights
of the conical pile may inhibit proper aeration.
Windrows, rather than conical piles, are generally used by
larger installations. Mixing of the sludge and bulking agent is
usually accomplished with a mobile unit especially designed for
mixing and turning compost. The windrows should be from 3-6 feet
high by 6-14 feet at the base. Generally, the most desirable
dimensions are directly related to the porosity of bulking agent/
sludge mixture. There is no blanket of previously composted sludge
either underneath or covering the windrow, (refer to Figure 3)
since the entire mass is turned and mixed periodically during the
composting period. Also, drying may be more readily accomplished
due to increased exposure to sunlight and air; thus, use_of recycled
compost as the sole bulking agent is more easily accomplished.
18
-------
A. CONICAL PILE
SLUDGE & BULKING
AGENT
4-6"
B. WINDROW
SLUDGE & BULKING AGENT
FIGURE 3. CONFIGURATION OF MECHANICALLY TURNED WINDROW PILES
NOTE: WITH FORCED AERATION THESE PILES AND WINDROWS
COULD BE HIGHER AND WIDER AT THE BASE
-------
(b) Aerated Static Pile/Extended Aerated Static Pile
The aerated static pile/extended aerated static pile configurations
depend upon forced aeration in which the surrounding air is drawn
down through the pile by means of air pipes and blowers. The air
pipes lie underneath the composting material in a porous base, such
as wood chips, and are connected to the blowers. In this configuration
the compost material (the sludge thoroughly mixed with bulking
agent) is piled upon the bed of porous base material containing the
aeration pipes. The sludge/bulking agent mixture is then covered
with a blanket of screened or unscreened composted material, and
the pile is not mixed again unless the pile does not adequately
compost. A total pile height of not more than 12 feet has helped
avoid difficulties with aeration, but usually, pile height is
limited by front-end loader capabilities. The aerated static pile
configuration is diagrammed in Figure 4-A. The extended configuration
reduces the need for cover material by as much as 70% and the need
for composting pad space by 50% (shown in Figure 4-B).
The porous base which surrounds the aeration pipes normally
consists of wood chips or unscreened compost and is approximately
12-18 inches thick. The base also has an added benefit of soaking
up excess moisture. A blanket of unscreened composted sewage
sludge is used to insulate the pile for two reasons: 1) so that
the coldest part of the pile attains a temperature of 55°C, for a
minimum of 3 days, and 2) so that malodorous volatile compounds
that may be released from the pile are screened out. Screened or
unscreened compost can be used as a blanket material. However,
using unscreened compost is usually more feasible. The use of wood
chips as a blanket material is not recommended, since the coarse
texture and loose structure of wood chips contributes to heat loss,
fly and odor problems.
A !6-to-20 inch thick blanket of unscreened compost (or 8 to
12 inches of screened compost) have been found to be sufficient.
The thickness of the cover blanket can be usefully increased during
cold weather to enhance composting conditions. When the extended
pile configuration is used, only a 2-to-3 inch layer of screened or
unscreened composted material is applied on the side to which the
next layer will be added. This thin layer serves as a temporary
overnight screen for trapping odors'and retaining heat.
3. Aeration
Forced aeration must be provided in the aerated pile configuration
to achieve the oxygen, temperature and moisture levels necessary
for complete composting. The form and duration of air flow used by
the facilities studied is shown in Table 8. As originally developed,
air is pulled down into the pile from a vacuum created by a small
radial vane fan which is in line with the porous and non-porous
plastic tubing (Figure 4B). The non-porous tubing ends in an odor
20
-------
COMPOSTING WITH FORCED AERATION
SCREENED
COMPOST
WOODCHIPS
AND SLUDGE
POROUS
BASE:
WOOD CHIPS OR
COMPOST
NON-PERFORATED PIPE
EXCEPT FOR WATER
CONDENSATE DRAIN
HOLES
FILTER PILE
SCREENED COMPOST
SCREENED
COMPOST
WOODCHIPS
AND SLUDGE
FILTER PILE
SCREENED COMPOST
POROUS
BASE: WOODCHIPS
OR COMPOST
NON-PERFORATED PiPE
EXCEPT FOR WATER
CONDENSATE DRAIN
HOLES
B. EXTENDED AERATED PILE
FIGURE 4. CONFIGURATION OF AERATED PILES SHOWING CONSTRUCTION OF
PILE AND THE ARRANGEMENT OF AERATION PIPE. (28)
-------
Table 8. MODE AND DURATION OF AERATION AT SEWAGE SLUDGE COMPOSTING FACILITIES (33)
no
City
Aerated Pile
H
B
C
A
E
D
Windrow
F
G
Blower
power
HP
0.33
2.0
40. -f
0.33
0.33
0.33
0.0
20. y
No. Piles/
blower
1
1
Total Site
1
2
2
None
3
Total
Wet tons Air Flow Ti
Sludge/pile Vacuum Positive Co
Minutes Minutes
120 12 every 20
270 15 every 30
30 every 60
50 Cont. 1 day-5
every 10 3/
50 10 days-5 11 days-5
every 30 every 30
125 14 Days-4 14 -Days-4
every 30 every 30
15 every 60
Scheduled
me for ~.
imposting -
Days
21
21
21
21
21
28
30
30
J_/ Centralized systems
2/ Scheduled times for aeration and composting are shown. These are adjusted according to composting
conditions, e.g., total composting time is often extended during cold or wet weather conditions. '
3/ Essentially set individually for each pile depending upon operator perception of sludge and ambient
climate conditions
-------
scrubbing pile of screened or unscreened compost (27, 36). The air
flow schedules, i.e., the form and duration of flow, have been set
based upon maintaining favorable temperature and oxygen levels in
the pile and to drive-off excess moisture (27, 35). The total time
required to achieve complete composting is adjusted to compensate
for ambient weather conditions relative to air temperatures and
moisture levels. For example, during cold climatic conditions, the
required composting time may be extended one to three weeks. Also,
it has been found helpful in these areas, when the sludge/bulking agent
mixture temperature drops below 10°C, to blow warm air from an
active composting pile into a newly constructed pile. This spesds
up the initiation of the composting process (21). Continuation of
this warm air transfer, after the initiation of the composting
process, however, results in excessive moisture accumulation.
As an alternative, forced aeration may be provided to a
windrow or conical pile configuration. This would provide assurance
for attaining an adequate oxygen level, might lessen the need for
coarse bulking agent, and should permit greater heights and widths
of the piles and windrows.
The perforated aeration pipes in the base of a pile should not
be closer than approximately 8 feet from the end or the edge of an
aerated pile to prevent channeling of air (i.e., short circuiting
of air through the porous base). In addition, short circuiting can
be minimized by using a less porous base (e.g., screened or unscreened
compost) and/or by surrounding the base with a less porous blanket
material (e.g., screened or unscreened compost). The aeration pipe
should be from 4-6 inches in diameter. Aeration should be timed at
the rate of approximately 500 cubic feet per hour per ton (cfh/ton)
of dry sludge for no more than half of every hour, and the time
off should not exceed 20 minutes. However, aeration rates varying
from 200 to 1,200 cfh/dry ton have been tried without appreciable
adverse impact (35).
A pile of screened composted sludge can be used to screen out
the odorous gases which have been pulled through the pile by the
blower. Approximately one cubic yard of screened composted sludge
for every 2.5 dry tons of sludge in the static pile can accomplish
this purpose. If the moisture content in the odor screening pile
becomes too high from filtering moist air (65-75%), it will not
adequately remove odors from the exhausting air. Hence, odor
scrubbing compost piles should be replaced when the moisture levels
reach 65-75 percent.
As shown in Table 8, two of the composting facilities periodically
used positive pressure airflow during the latter part of the
composting cycle. This method forces air up through the composting
pile. This practice has provided these locations with not only
more favorable composting conditions and odor control, but also a
more uniform,, low moisture compost. The lower moisture content in
23
-------
the compost greatly facilitates screening of the finished product
and reduces the production of odor in curing piles.
/o ATaltern?tive method for aeration is also now being tested
(5). This method is based upon data which indicate that the optimum
temperature range for biological oxidation and drying is 45-60°C
This temperature is maintained using a positive flow of air with'an
aeration capacity of approximately 10,000 cfh/ton of dry sludge
The air flow is activated with a temperature sensitive switch that
is centrally located, well within the pile. This switch is set to
start aeration when the temperature rises to about 50°C and to stop
aeration when the temperature drops to about 45°C.
The positive rather than negative flow of air apparently nridy
eliminate the need for an odor scrubber pile, relying on the improved
aeration and cover blanket for odor control.
The finished compost produced in the positive forced air
temperature control mode is drier and moisture is more uniformly
distributed. Thereby, greater use of the compost as a bulkinq
agent is possible and screening is greatly facilitated. Additional
studies of this mode will determine how well odors are control led
and whether or not the pathogen kill obtained is equivalent to that
under the negative aeration, higher temperature rise mode. Included
in the testing program will be a disinfection period with temperatures
maintained at 55°C or above to kill pathogens. This disinfection
period will be tried both at the beginning (before high aeration)
and near the end (after high aeration).
4. Drainage
In negative flow aeration systems, water vapor will condense
in the cooler sections of the aeration pipes. Adequate drainage
must be provided to remove this condensate from the aeration system.
One simple method for removing the water is to place small holes
in that portion of the aeration system pipeline where the condensate
would normally gather (portion with lowest elevation). These holes
should not be very large (about 3/8 inch diameter on the 4 and 5
inch diameter pipes) to avoid causing an air bypass that would
result in a drop in vacuum pressure. It is estimated that approximately
6-20 gallons per day of condensate -will accumulate during dry
weather from a pile containing 50 cubic yards of composting sludge.
This liquid must be collected. It then may be discharged through
pipes into a sewer system or held onsite, e.g., in an aerobic pond
for later recycling or disposal.
A few composting facilities have chosen to use permanent air
ducts, which are set in concrete beneath the composting pad surface,
rather than disposable pipe. A centralized blower system has also
been used for connection to these permanent air ducts. These
systems have experienced difficulties with keeping the airflow
24
-------
passageway clear, because condensate water and organic materials
have accumulated in the ducts and corrosion has clogged perforations.
Thus full air flow is prohibited pending a clean-up or overhauling
of the aeration system. The problem of clogging should be resolved
with improved permanent aeration pipe design. In addition, these
systems have limited capability for providing separate aeration
schedules to specific piles, because of the large central blower,
unless control values are provided in the air piping system.
D. Pad
A paved pad is very important to the successful operatic.' of
a composting facility, especially in a humid climate. Table 9 lists
various types of paving materials that have been used with notes
on their suitability. Concrete as a pad material is most suitable
while compacted dirt is least suitable, especially in humid climates.
A crushed rock pad is marginal to unsuitable.
Some of the locations studied had experienced difficulties
with the composting pad due to the high demands placed on the pad
material by the movement of heavy equipment and the high temperatures
attained by composting. Many of the systems were started as temporary
sites using gravel or asphalt pads to reduce capital expenses. The
movement of heavy equipment on these sites to handle the initially
wet sticky sludges has led to rapid deterioration of pads, and
muddy and dirty conditions. This in turn has led to equipment
immobility and breakdown, dust, and odor problems.
Certain forms of asphalt appear to be suitable (e.g., rapid
cure, coarse aggregate asphalt). However, long-term composting
operations, which produce a growth medium for mushrooms in tne
humid northeastern United States, have chosen concrete over asphalt
because of asphalt's tendency to decompose and soften. Where
static pile composting is practiced upon an 8-12 inch wood chip base,
temperatures on the asphalt may not be so great; therefore, asphalt
may be more suitable as a pad for static pile than windrow composting.
There is, however, an initial 6 month period wnen the aspalt pad,
especially fine aggregate asphalt, will have soft spots. In contrast,
concrete is suitable soon after pouring.
E. Climatic Impacts
Outdoor composting has been successfully operated at the
facilities surveyed in air temperatures down to -4U I; (XU-
Compo t ng of sludge has been accomplished in the rain or snow.
While mixing of the sludge with bulking agent can occur in the
rain it s most desirable to have sufficient temporary sludge
storage capacity so that mixing can be delayed until fair weather.
If slSdge and bulking agents are mixed in the rain, more bulking
agent will, be required to compensate for extra water.
25
-------
Table 9. PAVING MATERIALS
Material Suitability
Crushed Rock Marginally suitable. Requires considerable
maintenance, hard to clean, serpentene rock
has proved to increase nickel in compost.
Often has soft spots for 6 months until
pad harden.
F1y Ash Marginally suitable. Dusty, hard to clean.
When wet will become very muddy.
Asphalt Suitable with reservation - Can be kept clean.
Asphalt may soften under temperatures produced
during composting. Mushroom composters report
that asphalt itself may compost.
Concrete Most suitable - In use 50 years for mushroom
composting. Easy to clean. Will not compost.
Dirt Marginally suitable for dry climate - Can be
dusty. Can not clean. Messy in wet periods.
26
-------
Under very cold ambient conditions, it has been found to be
helpful to 1) blow warm air from an existing composting pile into a
newly created pile until temperatures reach 20 or 30°C, 2) increase
the thickness of the insulating cover blanket of previously composted
sludge, and/or 3) operate at a slow rate of aeration. Once a
suitable aerated static pile or windrow has been constructed, rain,
snow orrery cold weather will not adversely affect outdoor sludge
composting operations. However, in windrow composting, after a
rainfall, allow the surface to dry before turning. Since the rain
typically penetrates only the outer surface, drying avoids incorporation
of excessive moisture into the windrow mass.
F. Odor Control
The control of odors is one of the most essential steps in
maintaining a successful sewage sludge composting operation.
Practices that help to minimize odor problems include:
0 Having an adequate buffer zone between facility and
nearby residents.
Handling of sludge and opening composting piles only when
the meteorological conditions are favorable. Unfavorable
conditions are during periods of low wind speed for odor
dispersion, excessive precipitation and temperature
inversion.
0 Monitoring to assure proper temperature, moisture and
oxygen levels. Mai odor is most often associated with
excessive moisture levels.
0 Prompt adjustments of bulking agent/sludge mixture whenever
sludge or bulking agent is wetter than usual due to wet
weather or changes in sludge processing and dewatering.
In this manner, the moisture level should be maintained
within the range to provide for proper gas exchange.
0 Application of a blanket of previously prepared compost
over a new aerated pile to screen out malodorous volatile
gases arising from the pile.
0 Use of a low moisture, screened compost in a separate
pile for screening out odors from air that is drawn
through a static pile by negative aeration.
0 Adequate cleaning of the composting area and equipment.
0 Assuring good drainage of surface and condensate water
from the composting facility with no puddling of liquids.
27
-------
0 Providing for proper handling and treatment of the
drainage water (e.g., in a sewage or an aerobic pond
before sprinkling on land). In cases where the composting
site is adjacent to the wastewater treatment plant, it is
usually expedient to run the drainage back to the head of
the plant.
0 Providing adequate alternative disposal in landfills or
trenches for an occassional bad batch of composted sludge,
or provisions for prompt remixing to reestablish composting
conditions.
0 Regular turning of composting materials in windrow
configurations. Turnings should provide aeration, not
over cool, and turn the outside surface inward for high
temperature decomposition.
0 Promptness of all operations.
0 Anaerobic digestion of sludge substantially reduces the
potential for odor so that windrow composting is generally
possible as a stabilization process without odor scruobing
and/or forced aeration. Anaerobic digestion is generally
not needed to prevent odors when composting in aerated
static pile configurations where compost blanketing and
air scrubbing are practiced.
0 Recycling cured compost as part of the bulking agent also
reduces the production of odor in windrows and aerated
piles. This is because fine textured, cured compost can
internally coat and scrub odorous surfaces better than
coarse wood chips.
0 Positive aeration at high rates that is temperature
controlled may provide for control of odors in blanketed
aerated piles without the need for odor scrubber piles.
0 A limited large-scale demonstration of sludge composting
has been run in aerated trenches in a building. The
study suggests that sludge composting can take place
without the need for compost blankets or scrubber piles
to control odors. Additional experience will be needed
to verify this suggestion and to determine whether
scrubbing of the air leaving the building would be needed
in controlling odor.
6. Screening and Conveying
Aside from controlling odors, one of the biggest problems
in sewage sludge composting is the inability to consistently
and adequately screen compost to recover bulking materials.
28
-------
Screening of compost produced with bulking materials such as wood
chips is important for at least three reasons. First, these materials
can be one of the most expensive components of a sewage sludge
composting system. Proper recovery will permit their reuse as
bulking agents with only a minimum make-up with new wood chips.
Secondly, a screened composted sludge with its relatively low
content of cellulose bulking agent is often easier to utilize and
provides a more valuable product for agricultural and horticultural
use. However, unscreened sewage sludge compost can be of value for
use as a mulch. Third, screening reduces the volume of the finished
product that requires transportation to the site where it wi1! V
used.
Many types of screens will work in separating the wood
chips or other bulking agents from the composted sludge when tht
moisture content of the composted sludge and bulking agent is less
than 45%. Unfortunately, the moisture content of the finished
sludge compost is often about 55%, causing frequent plugging of
most types of screens. Increasing the mesh sizes of the screens
used may help overcome this problem; although it results in a
substantial loss of the finer portion of the bulking material.
Additional drying of compost may be needed to facilitate more
effective screening. If the increase in screening effectiveness
outweighs the labor costs involved in additional drying, compost
could be spread out temporarily to dry before screening.
Recent developments in screening technology have enabled the
efficient screening of wet compost that has a moisture content of
up to 50 percent. Modern screens may incorporate a flexing or
vibrating motion, along with multiple screens of varying sizes to
reduce clogging. The screens may be made of various materials,
such as wire, plastic, stainless steel, or rubber.
One recently developed screening mechanism, which employs
a vibrating rubber screen with a 7/8 inch mesh opening over a
second screen with a 1/4 inch mesh opening, has resulted in excellent
separation of the composted sludge from the wood-chip bulking
agent. An estimated 85 to 90 percent of the wood chips are recovered
with this screen. The capacity for screening the finished compost
has been estimated at 1/2 - 1 cubic yards per minute.
Another screen that successfully processes wet compost uses a
flexing motion. This unit constantly flexes a rubber screen with
1/2-inch mesh that grades down to 1/4-inch mesh. This flexing
action permits self-cleaning of the screen when compost that is
greater than 50 percent moisture is processed. Temporarily stopping
the flow of compost into the screen facilitates self-cleaning via
the flexing action, in lieu of totally ceasing operations for
manual cleaning.
29
-------
u<;Pri ™na ^reenS' a1°ng With many other tyPes' have been
the Unite? LCtir ny 'V56 i'-the maj°r comPosti"9 facilities in
°P6ratl0nal "formation on these screens is
cntaned inabi 1
A maximal percentage recovery of wood chips through screenina
may not always be desirable or economical. An unscreened sufficiently
dry compost, such as obtained from positive rather than negative
aeration, could possibly be used more economically as a bulking
agent than the screened recycled wood chips.
As a second example, a lower percentage recovery which
results in a final product with a larger volume may be desirable if
net revenues are increased from its sale. One composting location
has completed an interesting study on the cost of screening (24)
They have found that their optimal percent recovery of bulkini
agent depends upon the cost of wood chips versus the cost of screening.
Two important features of any screening system are: 1) capacity
rnl 2V*parat ^n effectiveness. Capacity is simply the volume of
composted material at a certain moisture content that can be separated
con? 9iVe? cn5? ^^ (e:9" 2'° cubic ^ds/hr at a moisture
content of 5CU). Separation effectiveness is a measure of a screen's
ability to closely separate the bulking agent from the fine compost.
The most efficient screen would sharply separate the unscreened
compost so that the smallest particle of the bulking agent would be
just larger than the largest particle in the final compost. In
actuality, screens do not produce such a sharp separation (10).
T.hi T?n ratl!!g ^ revl6W Of the effectiveness of screens given in
I able 10 was based upon wood chip recovery and did not really
address separation effectiveness. We refer the reader to Higgins,
et.al. (10) for the results of a detailed study on separation
effectiveness of screens.
H. Curing (Stockpiling)
An active composting period of approximately 21 days is
needed to achieve stabilization of the composting sludge when usinq
either the static pile or windrow method. As an additional safeguard
a 3-4 week period for curing the compost, following the active
composting period (either before or -after screening), helps ensure
completeness of stabilization (reduction of possible odor production)
and pathogen reduction. However, if the compost is too wet and has
a high volatile solids content, it should be composted again to
avoid anaerobic conditions and, hence, malodors. The size of
curing piles will usually depend on the capabilities of available
front end loaders; although, factors such as texture and density of
compost should be accounted for to avoid compaction. Also the
curing '
30
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Table 10. SLUDGE COMPOST SCREENING FACILITIES FOR RECOVERY OF BULKING AGENT (33)
City
Aerated Pile
H
B
C
A
E
D
Form
Rotating Cylinder
Flexing
Harmonic
Vibrating
Vibrat.ing Drum
Rotating
Cyclinder
2-Stage
Vibrating
Screen
Fabric
Stainless
Plastic
Wire
Steel
Steel
Rubber
Moi
sture
Mesh Limit
Inch
3/8
1/2-1/4 -1
1/2
3/4
7/8 over 1/4
%
45
50
60
50
45
Ease of
Cleaning
3
1
4
3
2
3
, .Flow Bulking Agent
-Through Recovery
Cu yd/mi n
1
1-2
0.5
1
1
0.5-1
%
65
80
60
50
45
85
]_/ Index of cleaning ease: 1 easiest, 5 most difficult.
2j Compost first falls on that part of the screen with a larger mesh. The material retained on
the screen is shaken down toward the second half of the screen with the smaller mesh.
-------
pile should be rounded on top, so that water buildup can be minimized,
I. Disposition/Use
A great variety of beneficial uses are potentially available
for properly composted sludge. The application of sludge compost
to marginal soils at rates that supply the fertilizer (e.g.,
nitrogen and phosphorus) requirements of crops can produce signi-
ficantly higher crop yields than commercial fertilizers applied
alone, especially for soils with low organic matter. Higher yields
are attributed to an improvement in soil physical properties by the
compost. Sludge compost is known to enhance aggregation, increase
soil aeration, lower bulk density, lessen surface crusting, and
improve permeability, infiltration, and/or water retention. Sludge
compost added to sandy soils will increase the moisture available
to plants and will reduce the need for irrigation. In heavy-
textured clay soils the added organic matter will increase the
soil's permeability to water and air, and increase water infiltration
into the profile; thereby, surface water runoff is minimized and
water storage capacity is increased. Addition of sludge compost to
clay soils has also been shown to reduce compaction (i.e., lower
the bulk density) and to increase root development (12).
The USDA concluded that sludge compost, like composted manure,
is hygienically and environmentally safe if used and stored properly.
They advised that compost not be stored near children's play areas,
surface water, wells and other water supplies. They also recommend
that all fruits and vegetables should be washed, before they are
consumed, to remove any residual compost that may be present (12).
Composted sludge has proven to be an acceptable substitute for
peat as a soil conditioner. A commercial potting mixture of one-
part composted sludge, one-part sand or soil and one-part vermiculite
has been shown to be an excellent medium for growing and propagating
ornamental plants (6, 12).
Composted sludge has been used with great success in the
reclamation and revegetation of strip mined areas, acid mine
spoils and other barren or disturbed poor soils. Large scale field
demonstrations in the coal mining areas of central Pennsylvania
have enabled Philadelphia to proceed with mine spoil reclamation
and revegetation as one of their primary sludge disposal options
(23).
Sludge compost is an excellent soil amendment for use in tree
nurseries. An application of 100 dry tons per acre to a sandy
nursery soil has resulted in taller, more densely rooted and more
winter-hardy deciduous and conifer tree seedlings. This improvement
is noted when compared with conventional green manure/chemical
fertilizer practices (6, 8).
32
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A one-or two-inch layer of composted sludge placed over the
surface of soil has successfully produced high grade sod on irrigated
commercial turf farms in 6 to 7 months rather than the normal 12 to
18 months (12). Composted sludge also has been used successfully
as a top dressing for established lawns and for amending soils used
for growing flowers and vegetables (6, 12, 20). USDA has identified
composted sludges that when added to soils have resulted in very
little increase in heavy metal content (including cadmium) in the
compost grown food crops, compared with control plants grown in
soils that were not treated with composted sludge (11).
The contents of heavy metals and other toxic organic compounds
present in the sludge may limit the usefulness of the compost
product. Guidance and regulations are currently being developed to
govern the distribution and marketing to the public of composted
sludge derived products. A preproposal draft regulation was circulated
for comment on May 6, 1980 (30). The Criteria for Solid Waste
Disposal Facilities (28), the Sludge Technical Bulletin (25) and
other recent publications (29, 31) provide guidance that is applicable
to the utilization of composted sludge for both agricultural arid
non-agricultural uses. Note compost that has only met the requirements
for a "significant" reduction of pathogens has greater limitations
on the end use than compost that has received "further" reduction
of pathogens (see p. 3).
J. Employee Safety (33)
The following recommendations and provisions are advisable to
ensure the health and safety of employees:
1) Rules pertaining to personal cleanliness should be posted in
appropriate areas. For example, the following items should be
emphasized:
(a) Wash hands before eating, drinking, and smoking.
(b) Wash hands before returning home after work.
(c) Avoid storing food in close proximity to sludge or
compost samples taken for analysis.
2) Showers and lockers should be provided at the composting
facility.
3) The municipality should provide onsite clothing, e.g., coveralls
and safety shoes for all workers.
4) Workers should change from this onsite clothing to street
clothes at the end of each day. The onsite clothing should
not be worn home.
33
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5) As necessary, the municipality should provide for cleaning the
onsite clothing.
6) Site employees should wear hard hats, if appropriate for the
tasks they are performing.
7) During periods of dry weather, the area should be sprinkled
periodically to minimize worker's inhalation of dust. During
such dusty conditions, workers should be encouraged to wear
face masks.
Recent studies have not indicated that Asperqillus fumigates
poses a significant health threat. A possible exception would"be
susceptible individuals working at the compost facility (16, 19,
22). A susceptible individual would be a person weakened by a
primary infection or by some physical disturbance such as surgery
or immunosuppresive therapy, (see Appendix B). Nevertheless, it
might be advisable to locate the composting facility away from
large populations of susceptible individuals (i.e., hospitals or
nursing homes) to minimize any possible adverse impacts. The
minimum separation distance will, however, depend on topography,
climate, and velocity and direction of wind. Milner, et.al. (18)
has presented a methodology for determining siting to minimize
Asperqillus dispersal. In addition, compost site workers should be
selected from the large majority of people who are not sensitive to
A. fumigatus or other respiratory allergens. Finally, increased
use of finished compost as a bulking agent, with fewer woodchips,
has resulted in a decreased level of A. Fumigatus (19).
34
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VII. REFERENCES
(1) Burge, W. D. Colacicco, and W. Cramer. Control of Pathogens
During Sewage Sludge Composting. 1979. In Proc. National Conference
on Municipal and Industrial Sludge Composting, New Carrollton, MD.
Information Transfer, Silver Spring, MD, pp. 105-111.
(2) Daniels, S.L. and E.S. Conyers. 1975. Land Disposal of Chemically
Treated Waters and Sludges. In Proc. 2nd National Conference on
Complete Water Reuse. Chicago, IL, pp. 697-704.
(3) Epstein, E. 1979. Bulking Materials. In Proc. National Conference
on Municipal and Industrial Sludge Composting, New Carrollton, MD.
Information Transfer, Silver Spring, MD, pp. 30-34.
(4) Eralp, A. (ed.). 1981. Proc. on National Conference on Enclosed
Municipal Sludge Composting"! Cincinnati, Ohio (unpublished).
(5) Finstein, M.S., J. Cirello, S.T. MacGregor, F.C. Miller, and K.M.
Psarianos. 1980. Sludge Composting and Utilization: Rational
Approach to Process Control. Final Report. Rutgers University,
New Brunswick, NJ, 211 pages.
(6) Gouin, F.R., Chairman. 1979. American Society Hort Science Working
Group Indexed Bibliography of Municipal and Industrial Waste
Utilization in Horticulture (341 references).
(7) Gouin, F. 1981. Utilization of Compost on Nursery Crops. Hort.
Science (in press).
(8) Gouin, F.R. and J.M. Walker. 1977. Deciduous Tree Seedling
Response to Nursery Soil Amended with Compost Sewage Sludge.
Hort. Science 12:45-47.
(9) Haug, R.T., and L.A. Haug. 1977, 1978. Sludge Composting: A
Discussion of Engineering Principles (Parts 1 and 2). Compost
Science 1816:6-11 and 19*1:10-14.
(10) Higgins, A.J., V. Kasper, Jr., D.A. Derr, M.E. Singley and A.
Singh. 1981. Evaluation of Screens for Sludge Composting.
Biocycle 22#3, pp. 22-26.
(11) Hornick, S.B., J.J. Murray and R.L. Chaney. 1979. Overview on
Utilization of Composted Municipal Sludges. In Proc. of National
Conference on Municipal and Industrial Sludge Composting, New
Carroll ton. MD. Information Transfer, Silver Spring MD, pp. 15-22.
35
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(12) Hornick, S.B., J.J. Murray, R.L. Chaney, L.J. Sikora, J.F. Parr,
W.D. Burge, G.B. Willson, and C.F. Tester. 1979. Use of Sewage
Sludge Compost for Soil Improvement and Plant Growth. USDA, Science
and Education Administration. Agricultural Reviews and Manuals,
ARM-NE-6. 10 pages.
(13) Horvath, R. 1978. Operating and Design Criteria for Windrow
Composting of Sludge. In Proc. National Conference on Municipal
and Industrial Sludge Composting. Silver SprinqT MD. Information
Transfer, Silver Spring, MD, pp. 88-95.
(14) lacoboni, M.D., T.J. LeBrum and J.R. Livingston. 1980. Deep
Windrow Composting of Dewatered Sewage Sludge. In Proc. National
Conference on Municipal and Industrial Sludge Composting, Philadelphia,
PA. Information Transfer, Silver Spring, MD, pp. 88-108.
(15) LeBrum, T.J., M.D. lacoboni and J.R. Livingston. 1980. Overview
of Compost Research. In Proc. National Conference on Municipal
and Industrial Sludge Composting, Philadelphia. PA. Information
Transfer, Silver Spring, MD, pp. 109-116.
(16) Marsh, P.B., P.O. Millner, and J.M. Kla. 1979. A Guide to the
Recent Literature on Aspergillosis as Caused by Aspergillus
fumigatus. USDA, SEA, Agric. Reviews and Manuals, ARM-NE-5. 28
pages.
(17) Memoli, M.A. 1979. Impact of Filter Press Dewatering on Nassau
County Composting. In Proc. National Conference on Municipal and
Industrial Sludge Composting, New Carrollton, MD. Information
Transfer, Silver Spring, MD, pp. 24-29.
(18) Millner, P.O., D.A. Bassett, and P.B. Marsh. 1980. Dispersal of
Aspergillus fumigatus from Sewage Sludge Compost Piles Subjected to
Mechanical Agitation in Open Air. Appl. Environ. Microbiol. 39:1000.
(19) Millner, P.O., P.B. Marsh, R.B. Snowden and J.F. Parr. 1977.
Occurrence of Aspergillus fumigatus during Composting of Sewage
Sludge. Appl. Environ. Microbiol. 34:765-772.
(20) Murray, J.J. 1978. Use of Composted Sewage Sludge in Turf Grass
Production. In Proc. of Waste Water Conference. Chicago, IL.
Sponsor - American Society Golf Course Architects.
(21) Olver, W.M. Jr. 1974. Static Pile Composting of Municipal Sewage
Sludge: The Process as Conducted at Bangor, Maine. US EPA Draft
Report, Office of Solid Waste, Washington, D.C.
(22) Olver, W.M. Jr. 1979. The life and Times of A. fumigatus Compost
Science. 20#2:36-40.
36
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(23) Sopper, W.E. and S.M. Kerr. 1979. Utilization of Municipal Sewage
Effluent and Sludge on Forest and Disturbed Land. The Pennsylvania
State University Press, University Park, PA, 537 pages.
(24) Gruber, E.K. and Brown, C.D. 1979. Bulking Agent Recovery from
Philadelphia Compost. In Proc. National Conference on Industrial
and Municipal Sludge Composting, New Carroll ton, MD. Information
Transfer, Silver Spring, MD, pp. 136-143.
(25) U.S.E.P.A. 1977. Municipal Sludge Management: Environmental
Factors. Office of Water Program Operations, Washington, D.C. EPA
430/9-77-004, MCD-28.
(26) U.S.E.P.A., 1978. Sewage Sludge Composting. In Sludge Treatment
and Disposal. Environmental Research Information Center, Cincinnati,
OH. EPA 625/4-78-012, 2:35-55.
(27) U.S.E.P.A. 1979. Composting. In Process Design Manual for Sludge
Treatment and Disposal. Environmental Research Information Center,
Cincinnati, OH. EPA 625/1-79-011, Chapter 12.
(28) U.S.E.P.A. 1979. Criteria for Classification of Solid Waste Disposal
Facilities and Practices, 40 CFR Part 257. In Federal Register,
September 13, 53438-53468.
(29) U.S.E.P.A. 1980. A Guide to Regulations and Guidance for the
Utilization and Disposal of Municipal Sludge. Office of Water
Program Operations, Washington, D.C. EPA 430/9-80-015, MCD-72,
48 pages.
(30) U.S.E.P.A. 1980. Distribution and Marketing of Sewage Sludge
Products, Preproposal Draft Regulation. Office of Water and Waste
Management, Washington, D.C., May 6.
(31) U.S.E.P.A., F.D.A. and U.S.D.A. 1981. Land Application of Municipal
Sewage Sludge for the Production of Fruits and Vegetables: A
Statement of Federal Policy and Guidance. EPA Office of Solid
Waste, Washington, D.C. SW-905, 21 pages.
(32) Using Municipal and Agricultural Waste for the Production of Horticul-
tural Crops. 1980. Proceedings of a Symposium. Hort. Science.
15(a):161-178.
(33) Walker, J.M., M.S. Winsten, J.E. Hall. 1979. A Critical Review of
the Performance of Sewage Sludge Composting Operations, In Proc.
of National Conference on Municipal and Industrial Sludge Composting,
Philadelphia, PA. Information Transfer, Silver Spring, MD,
pp. 5-14.
37
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(34) Willson, G.B. 1977. Equipment for Composting Sewage Sludge in
Windrows and in Piles. In Proc. National Conference on Composting
Municipal Residues and Sludges. Silver Spring, MD . Information
Transfer, Silver Spring, MD, pp. 56-60.
(35) Willson, G.B., J.F. Parr, and D.C. Casey. 1979. Basic Design
Information on Aeration Requirements for Pile Composting. In
Proc. of National Conference on Municipal and Industrial Sludge
Composting, New Carroll ton, MD. Information Transfer, Silver
Spring, MD, pp. 88-99.
(36) Willson, G.B., J.F. Parr, E. Epstein, P.B. Marsh, R.L. Chaney, D.
Colacicco, W.D. Burge, L.J. Sikora, C.F. Tester, and S. Hornick.
1980. Manual for Composting Sewage Sludge by the Beltsville Aerated
Pile Method. EPA 600/8-80-002, 65 pages.
38
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VIII APPENDICES
Appendix A
Concepts and Background of Sludge Composting
Sewage sludge composting is an aerobic, microbiological decay
process. Under favorable environmental conditions (i.e., correct values
for moisture, pH, oxygen levels, etc.), microorganisms contained in the
sludge will begin to break down (decompose) the large organic molecules
also present in sewage sludge. Using the organic matter as a substrate,
the microorganisms raise their metabolic rates and hence, the surrounding
temperatures. Elevated temperatures in the compost mixture, as well as
microbial antagonism are primarily responsible for the elimination of
pathogenic organisms. The resulting material, after completion of the
process, is an earthy, humus-like material which can be used as a soil
conditioner.
Composting can effectively transform an undesirable municipal
organic waste into an agriculturally and horticulturally useful product
that is easy to handle and readily accepted by the public. Several
composting methods have been extensively researched and tested, and have
been found to be reliable for stabilizing and minimizing the pathogen
content in sewage sludge and septage. The two most widely used and
accepted methods of sewage sludge composting in the United States are
the static aerated pile method developed by the U.S. Department of
Agriculture at Beltsville, Maryland, and the windrow method.
The two methods basically utilize the same biological processes but
the operational procedures for each method differ slightly. The two
methods differ mainly by the way oxygen (air) is supplied throughout all
parts of the pile. The windrow method relies on natural convective
forces to pull air into the pile along with periodic mechanical mixing.
The mixing is necessary for reaeration of the pile, to bring wetter
material from the inside to the surface for drying, and to ensure r,e,::
all material will be in the interior of the pile long enough for the
high temperatures to destroy pathogens. The static aerated piles ^v-er,
is mixed only once. A perforated pipe aeration system with vacuun.
pumping (negative aeration) is used to ensure adequate oxygen supply,
and a "blanket" of finished compost aids in achieving maximum temperatures
for pathogen elimination. In some installations, a combination of
forced aeration and periodic mechanical mixing has been used successfully
with windrow systems.
Some windrow and static pile composting systems have replaced
manual labor with automated machines (i.e., mixers, conveyors, feeder
hoppers). This replacement has the advantage of lower operating cost
(due to the high cost of labor) and the possible added control of odors
(many automated systems are enclosed). However, automation often has
the disadvantage of greater complexity and the inability to adjust
A-l
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readily to constantly changing variations in operational procedures and
quality or quantity of materials (e.g., variation in bulking material,
sludge type, or moisture content).
There were several attempts at the development and introduction of
refuse composting, before sludge composting, in the United States.
These ventures failed, in part, due to the necessity of selling the
composted product at a profit to make the system pay for itself. The
sale of composted sewage sludge and/or solid waste has not made a profit
for municipalities, and should not be made part of any sludge composting
plan, other than to reduce expenses. While the compost product has
value as a soil conditioner and fertilizer, its commodity value has not
been sufficient to cover the processing and handling costs. It is
important to note, however, that the potential value of composted sludge
or refuse may be reduced, or its use may be limited, by excessive levels
of heavy metals or other contaminants.
In some European composting operations, confined (invessel) systems
have been developed and operated for sewage sludge or sludge and solid
waste composting. A few such confined systems have been developed in
the United States for the composting of animal wastes, solid wastes
and/or a combination of sewage sludge and solid wastes.
Future improvements in materials handling technology will increase
the feasibility and desirability of invessel and/or mechanized composting
of sludge A recent conference in Cincinnati revealed major advances in
invessel design. Many of the systems featured simplicity of design and
flexability of operations which are necessary for cost-effective and
reliable composting operations (4). Sewage sludge composting systems_
with greater mechanization and complexity will probably be developed in
the future. The flexibility, reliability and costs of these more complex
systems should be adequately demonstrated in the United States before
adoption for full scale use.
A-2
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Appendix B
Health Aspects of Sludge Composting (1,16,19)
Workers at sludge composting facilities encounter disease risks: 1)
from the pathogens normally present in sewage sludges, and 2) from fungi
and actinomycetes that grow during composting. The former are often
referred to as primary pathogens because they can initiate an infection in
an apparently healthly individual. The latter are referred to as secondary
pathogens because they usually infect only people with debilitated immune
systems, such as those weakened by a primary infection, surgery or i;nmuiio-
suppressive therapy. Densities of secondary pathogens generally are-
increased during composting. The growth of secondary pathogens is not
peculiar to composting sewage sludge but occurs also in many farm and
garden operations, such as during the composting of leaves or other
materials. Examples of primary and secondary pathogens are listed in
Table B-l.
Studies to define the risk of infection by primary pathogens to
people working with sewage wastes are not as extensive as might be desired,
but available data indicate that the risk is low. The predominant routes
of infection from the waste material are through the mouth and inhalation.
Prevention of infection involves such precautions as thorough washing of
the hands before eating to prevent ingestion of the pathogens. The
exposure of workers to primary pathogens in a composting operation is
limited to the pile building operation because the temperatures reached in
the next processing step (composting) reduce primary pathogen densities to
insignificant levels. The initial mixing operation presents little
hazard, because the high moisture levels limit particle migration.
Medical difficulty from secondary pathogens may result from inhalation
of air containing a high density of spores. The probability that indivi-
duals in good health will be infected by secondary pathogens encountered
in composting is very small. However, people who are predisposed because
of such conditions as diabetes, asthma, emphysema, or tuberculosis, or who
may be taking such medication as corticosteroids, broad-spectrum anti-
biotics, or immunosuppressive drugs may be more susceptible to infection.
The help of local medical authorities should be obtained in compiling
a medical history questionnaire for work applicants so that predisposed
people are screened out. Individuals who are "atopic", i.e., prone to
severe allergies, should also be excluded from employment at composting
facilities. Moreover, a complete physical examination is recommended,
plus inoculations for typhoid, tetanus, and polio.
B-l
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Table B-l. EXAMPLES OF PATHOGENS FOUND IN OR GENERATED DURING COMPOSTING
OF SEWAGE SLUDGE, TOGETHER WITH HUMAN DISEASES ASSOCIATED
WITH THESE PATHOGENS.
PRIMARY PATHOGENS
GROUP
Bacteria
Protozoa
Helminths
Viruses
EXAMPLE
Salmonella enteritidis
Entamoeba histolytica
Ascaris lumbricoides
Hepatitis virus
DISEASE
Salmonellosis
(food poisoning)
Amoebic dysentery
(bloody diarrhea)
Ascariasis
(worms infecting the
intestines)
Infectious hepatitis
(jaundice)
SECONDARY PATHOGENS
Fungi Aspergillus fumigatus
Actinomycetes
Micromonospora spp
Aspergillosis
(growth in lungs
and other organs)
Farmer's lung
(allergic response
in lung tissue)
B-2
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Appendix C
Related Publications
Colacicco, D.E. Epstein, 6.B. Willson, J.F. Parr and L.A. Christiansen.
1977. Costs of Sludge Composting. USDA, Agricultural Research Service,
ARS-NE-79. 18 pages.
Haug, T.R. 1980. Compost Engineering Principles and Practice. Ann
Arbor Science Publishers, Inc., Ann Arbor, MI, 655 pages.
Leighton, G.M., R.D. Harter and G.R. Cranbie. Sewage Sludge Composting
in Small Towns. Agricultural Experiment Station, University of New
Hampshire, No. Pub. Date.
Mosher, D., R.K. Anderson. 1977. Composting Sewage Sludge by High-Rate
Suction Aeration Techniques. U.S.E.P.A. Office of Solid Waste, Washington,
DC, SW-614d. 50 pages.
Parr, J.F., Ph.D., G.B. Willson, R.L. Chaney, Ph.D., L.J. Sikora, C.F.
Tester. 1978. Effect of Certain Chemical and Physical Factors on the
Composting Process and Product Quality. In Proc. National Conference
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