United States
Environmental Protection
Agency
Technology Transfer
EPA/625/4-85/014
«>EPA Seminar Publication
Composting of
Municipal Wastewater Sludges
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TECHNOLOGY TRANSFER
EPA 625/4-85-014
Seminar Publication
Composting of
Municipal Wastewater Sludges
August 1985
This document was published by:
U.S. Environmental Protection Agency
Center for Environmental Research Information
Office of Research and Development
Cincinnati, OH 45268
U.S. Environmental Protection Agency
Region 5, Library O-i •'• -•;.
77 West Jackson 1. v . ;•, -^ hoor
Chicago, IL 60604<-~-./J
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This seminar publication is wholly based on presentations made at
U.S. Environmental Agency (EPA) Technology Transfer seminars on
Sludge Composting and Improved Incinerator Performance in
Columbus, Ohio; Philadelphia, Pennsylvania; Dallas, Texas; San
Francisco, California; Atlanta, Georgia; and Boston, Massachusetts,
from July to December 1984. This publication addresses only the
composting portion of the seminars. The presenters were:
Dr. Joel E. Alpert, E&A Environmental Consultants, Inc.,
Stoughton, Massachusetts (Chapter 1).
Joel Thompson, Maryland Environmental Services, Annapolis,
Maryland (Chapter 2).1
Ross Caballero, Sanitation Districts of Los Angeles County,
Carson, California (Chapter 3).
Ron Albrecht, Recovery Associates, Inc., Annapolis, Maryland
(Chapter 4).
Jan Connery of Eastern Research Group, Inc., Arlington,
Massachusetts, prepared the text of this document based on the
speakers' transcripts and slides. John Walker (EPA Office of
Municipal Pollution Control, Washington, DC) and Orville Macomber
(EPA Center for Environmental Research Information, Cincinnati,
Ohio) provided substantive guidance and review. Jerry Goldstein
(Biocycle Journal of Waste Recycling), Elliot Lomnitz (EPA Office of
Water Regulations and Standards), Jack O'Brien (American Bio-
reactor Corporation), Charles S. Spooner (EPA Office of Water),
Howard Wall (EPA Water Engineering Research Laboratory, Cincin-
nati), and George Willson (U.S. Department of Agriculture) also
reviewed the document. Their contributions are gratefully
acknowledged.
This report has been reviewed by the U.S. Environmental Protection
Agency and approved for publication. The process alternatives, trade
names, or commercial products are only examples and are not
endorsed or recommended by the U.S. Environmental Protection
Agency. Other alternatives may exist or may be developed. In addi-
tion, the information in this document does not necessarily reflect
the policy of the Agency, and no official endorsement should be
inferred.
'Joel Thompson is currently employed by the Washington Suburban
Sanitary Commission, Hyattsville, Maryland.
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Contents
Preface iv
1. Composting Facility Design
Introduction 1
Public Relations 4
Aesthetic and Environmental Considerations 5
Marketing and Distribution 5
Economics 6
Design Considerations 8
Site Specifications 14
Equipment 15
Monitoring 20
Public Health Considerations 20
References 24
2. Experiences at Static Pile Composting Operations
Background 25
Western Branch Site 25
The Dickerson Site 26
Site II 30
Marketing 35
References 37
3. Experience at a Windrow Composting Facility:
Los Angeles County Site
Description of the Los Angeles County Sewage System 38
Description of the Joint Water Pollution Control Plant 38
Historical Perspective of Sludge Disposal at JWPCP 39
Current Composting Operation 42
Research into Other Composting Processes 44
Research into Factors Affecting Windrow Composting 46
Important Requirements and Objectives 49
Public Relations 56
Marketing of Compost Products 56
Costs 57
References 58
4. In-vessel Composting
Introduction 59
Types of In-vessel Composting Systems 60
Performance 64
5. Federal and State Regulation
Current Federal Regulation 66
Future Federal Regulation 67
State Regulation 67
List of Abbreviations 68
ill
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Preface
Sludge management is a major problem for many
municipalities. Ever increasing quantities of sludge are being
generated as municipalities begin to comply with the
wastewater treatment requirements of the Clean Water Act
and as advances are made in wastewater treatment. In addi-
tion, some municipalities must seek new disposal alternatives
as former sludge disposal technologies become environmen-
tally unacceptable or too expensive.
The Clean Water Act of 1977, as amended, encourages the
use of "innovative and alternative" technologies for sludge
management. These include technologies that reduce costs,
conserve or recover energy, reclaim or reuse water, recycle
wastewater, improve operational reliability, or improve the
management of toxic substances. In response to a need for
information concerning innovative and alternative technolo-
gies for sludge management, the U.S. Environmental Protec-
tion Agency (EPA) sponsored six regional seminars on
Sludge Composting and Improved Incinerator Performance in
Columbus, Ohio; Philadelphia, Pennsylvania; Dallas, Texas;
San Francisco, California; Atlanta, Georgia; and Boston,
Massachusetts, in 1984. This publication is based on the
sludge composting presentations made at those seminars.
Composting is a natural microbiological process that de-
grades sludge to a stable humus-like material that can be re-
cycled to the land for use as a soil conditioner and low-grade
fertilizer. Composting can have advantages over other sludge
management alternatives, including lower energy require-
ments and capital investment than incineration; a more
manageable product than land application; and a more pro-
ductive, beneficial use of sludge than landfilling or ocean
disposal. The process design is flexible and adaptable to a
wide range of situations. It can be rapidly implemented and
readily adapted to changes in sludge volume. Key features
for successful sludge composting include odor control,
operating flexibility, serviceability, product quality, marketing
of final product, back-up arrangements for sludge disposal
and utilization, and good community relations.
Because composting prepares sludge for use as a resource
rather than as a waste and because it conserves energy, it
qualifies as an alternative technology under the Federal Con-
struction Grants Program. Certain composting methods or
components may also qualify as innovative technologies if
they have not been fully proved over time or are being
employed for the first time in a state or locality. As either an
innovative or alternative technology, a composting system
can qualify for a higher percentage of Federal funding for
eligible cost items than a system using standard technology.
This seminar publication provides practical information on
current methods of composting municipal wastewater
sludges. It is intended for government and private sector indi-
viduals involved in the planning, design, and operation of
municipal sludge treatment and disposal systems. Chapter 1
presents general principles of the composting process and
system design, including windrow, static pile, and in-vessel
systems; public relations; aesthetic considerations; marketing
and distribution of compost; economics; design and site
layout considerations; monitoring; equipment selection; and
public health considerations. Chapters 2 and 3 discuss in
depth the experiences at the Dickerson, Western Branch, and
Site II static pile composting operations in Maryland, and at
the windrow operation in Los Angeles County. In-vessel com-
posting is reviewed in Chapter 4. Chapter 5 discusses current
and proposed regulations and guidelines that pertain to
sludge composting.
This publication is not a design manual nor does it include all
the latest knowledge about composting; additional sources
should be consulted for more detailed information and design
criteria and for the most recent information in this rapidly
developing field. In addition, state and local authorities
should be contacted for regulations and good management
practices applicable to local areas.
IV
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1. Composting Facility Design
Introduction
Composting is one of several approaches to the manage-
ment of municipal wastewater sludge. It is a biological
process that converts sludge into a stable humus that can
be applied to the land as a soil conditioner and low-grade
fertilizer. During the past decade, an increasing number of
municipalities have begun to compost their sludge. Approxi-
mately 115 sludge composting facilities are currently opera-
tional in the United States.
This chapter presents general principles for the design of
municipal wastewater sludge composting facilities.
Although these basic principles apply to all composting
systems, there are many ways to compost, and each sys-
tem must take into consideration the site-specific conditions.
Definitions
For this presentation, composting is defined as a method of
solid waste treatment in which the organic component of
the solid waste stream is biologically decomposed under
controlled aerobic conditions to a state in which it can be
easily and safely handled, stored, and applied to the land
without adversely affecting the environment. Thus, com-
posting is a controlled or engineered biological system.
Composting systems are generally divided into three cate-
gories: windrow, static pile, and in-vessel. In the windrow
approach, a sludge/bulking agent mixture is composted in
long rows (or windrows) that are aerated by convective air
movement and diffusion and are turned periodically by
mechanical means to expose the organic matter to ambient
oxygen. In the static pile (or forced-aeration) approach,
piles of a sludge/bulking agent mixture are aerated using a
forced-aeration system installed under the piles to maintain
a minimum oxygen level throughout the compost mass.
In-vessel composting (also known as mechanical or en-
closed reactor composting) takes place in partially or
completely enclosed containers in which environmental con-
ditions can be controlled. In-vessel systems may incorporate
the features of windrow and/or static pile methods of com-
posting. An estimated 90 percent of the operational facilities
in the United States use static pile composting; the re-
mainder use windrow methods. A few in-vessel composting
systems are just starting up, and several others are under
design or construction.
Compost is defined as the end product of the composting
process. It is a stable humus-like substance with valuable
properties as a soil conditioner. It also contains several
macro- and micronutrients that are favorable to plant growth,
although it is generally not high enough in nitrogen to be
considered a fertilizer. The product is not completely stabi-
lized, i.e., with 100 percent of the organic matter degraded.
Rather it is stabilized sufficiently that the potential for odor
generation is reduced to the point where the product can be
readily stored and marketed. Heat produced during decom-
position destroys many human pathogens, including many
that survive other treatment methods.
Process Flow
Figure 1.1 shows the basic steps, or process flow, that
apply to all types of compost systems. The two compo-
nents of composting are the sludge and a bulking agent,
which can consist of many different types of materials,
including recycled compost, wood chips, sawdust, and
shredded rubber tires. The bulking agent (except tires)
serves as a carbon source during composting. It also in-
creases the porosity and thus the surface area of the sludge
that will be exposed to oxygen during composting, and it
decreases the moisture level of the composting material.
The first step in composting consists of mechanically mixing
the bulking agent with sludge to create a sludge/bulking
agent mix that optimizes the porosity, carbon content, and
moisture level for composting.
The composting process takes place after mixing, and it
generally requires 3 to 4 weeks to complete. During that
time, the mix is aerated and biological processes decom-
pose the sludge and generate high temperatures (above
55°C)1 that destroy pathogens. The oxygen required to fuel
the biological processes can be supplied in two ways:
• By mechanically turning the mixture so that the sludge
is periodically exposed to oxygen in the atmosphere.
Convective air movement and diffusion move oxygen
into the windrow; turning increases porosity for air
movement and helps to distribute anaerobic areas (if
they exist) to the aerobic zone.
• By using a blower to force or draw air through the mix
— a process known as forced aeration.
After composting, the material is usually cured for about
30 days. During this phase, further decomposition, stabiliza-
tion, pathogen destruction, and degassing take place, which
help to make the compost more marketable. After curing,
some systems have a drying stage that can vary from several
days to several months. This drying step is sometimes
necessary if compost is screened to recover the bulking agent
for recycling into the next batch. Compost that is not dry
tends to stick together in balls and does not screen well.
'In this document, measurements of temperature are provided in °C. All
other measurements are in English units with the metric equivalent in
parentheses. This format reflects the way in which data were presented by
the speakers.
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COMPOSTING FACILITY DESIGN
Windrows and static piles at Beltsville, Maryland.
Drying may also be necessary if the bulking agent will be re-
cycled in the windrow process.
Compost may then be stored and marketed or it may go
through an additional screening step to produce a finer
product. Screening is standard in most static pile systems
because wood chips, which are relatively large compared to
compost particles, are generally used as the bulking agent.
Screening is often not necessary in windrow and in-vessel
systems, however, because the bulking agent is usually
sawdust, rice hulls, or some other small-particle organic
material. The need for screening depends on the size and
cost of bulking agent and the desired particle size of the
finished compost.
Windrow Systems
In windrow systems, the sludge/bulking agent mixture is
aerated by mechanically turning over the piles using a
machine such as a front-end loader or specially designed
equipment like the Cobey, SCARAB I, or Brown Bear (see
Equipment section). The piles are turned frequently (e.g.,
daily) at first when the system has a high oxygen demand,
and then about three times per week thereafter. The com-
posted sludge is usually stockpiled for curirjg before dis-
tribution. The advantages and disadvantages of windrow
composting are presented in Tables 1.1 and 1.2.
Static Pile Systems
Figure 1.2 is a schematic illustration of a static pile system.
In static pile composting, the aeration system consists of a
series of perforated pipes running underneath each pile and
connected to a pump that draws or blows air through the
piles. Many static pile facilities use 1- to 5-horsepower (hp)
blowers that are cycled on and off. The pipes are covered
with a layer of wood chips or other bulking agent that acts
as a manifold to provide uniform aeration. The compost/
bulking agent mixture is placed on top of this protective pad
Turning
Curing
Figure 1.1. Composting Process Flow
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COMPOSTING FACILITY DESIGN
In-vessel composting, Portland, Oregon,
to form a pile. The piles are then covered with screened or
unscreened finished compost for insulation to help maintain
uniform temperatures throughout the mass during com-
posting and to provide more uniform aeration. Screened
compost provides better insulation than unscreened com-
post because it is less porous. Several facilities use
screened compost on the ends of the pile (the area farthest
from the aeration pipe) and unscreened compost in the
middle to create more uniform aeration throughout the
system.
A static pile system should be constructed on an impervious
surface to prevent migration of the leachate and/or conden-
sate to the groundwater and to facilitate equipment opera-
tion. The static pile system is easy to implement. A pilot
facility can be set up and made operational within 24 hours.
COMPOSTING EXTENDED
PILES WITH FORCED
AERATION
Figure 1.2. Schematic Diagram of Extended Aerated Pile
The advantages and disadvantages of static pile composting
are presented in Tables 1.1 and 1.2.
In-Vessel Systems
In-vessel systems (also known as mechanical or enclosed
reactor systems) are essentially just a different way of
packaging the windrow and static pile systems. They are
more space efficient than static pile or windrow systems
because the materials can be piled higher. At present, very
few in-vessel systems have begun operation in the United
States, however, several facilities are in construction,
design, or negotiation (see Table 4.1, Chapter 4). The
advantages and disadvantages of in-vessel systems are
presented in Tables 1.1 and 1.2.
Table 1.1 Advantages of the Three Composting Systems
Windrow • Rapid drying of the compost because moisture is
Systems released as the piles are turned over.
• Drier compost material, which results in easier
separation of bulking agent from the compost
during screening and relatively high rates of
recovery for bulking materials when this function
is practiced.
• The capacity to handle a high volume of material.
• Good product stabilization.
• Relatively low capital investment: materials
required are a pad for the piles, a windrow
machine and, generally, a front-end loader.
Static Pile • Low capital costs. The capital equipment required
Systems consists of a paved surface, some front-end
loaders, a screen, relatively inexpensive blowers,
and a water trap. (Capital costs will be higher if
additional facilities such as roofs or buildings are
included, but these are not essential items.)
• A high degree of pathogen destruction. The insu-
lation over the pile and uniform aeration through-
out the pile help maintain pile temperatures that
destroy pathogens.
• Better odor control than windrow composting.
The pile is kept aerobic at all times. Also with
blowers in the suction mode, the odors can be
treated as a point source.
• Good product stabilization. Oxygen and
temperature can be maintained at optimum levels.
In-vessel • Space efficiency.
Systems • Better process control than outdoor operations.
• Protection from adverse climatic conditions.
• Good odor control should be possible.
• Potential heat recovery depending on system
design.
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COMPOSTING FACILITY DESIGN
Table 1.2 Disadvantages of the Three Composting Systems
Windrow • It is not space efficient. Substantial space is re-
Systems quired between windrows, and the windrows
themselves cannot be piled very high. Maximum
dimensions for a windrow are a height of approx-
imately 4.5 to 5 feet (1.4 to 1.5 meters) and a
width of 12 feet (3.7 meters). Some prototype
machines are being developed to build larger piles.
• Although capital investment is relatively low, the
equipment maintenance costs are significant.
• Since the thoroughness of aeration depends on
the skill and diligence of the equipment operator,
windrow composting requires more monitoring
than static pile composting to ensure that aeration
and temperature rise have been adequate. Inade-
quate aeration can result in low, nonuniform tem-
peratures, pathogen survival, and production of
odors.
• Odors tend to be released whenever the piles are
turned, which can be a major public relations
handicap for sites located near residential areas,
particularly when raw and unstabilized sludges are
being composted.
• Unless covered, windrow systems cannot operate
under adverse climatic conditions such as rain.
Operations such as mixing and turning must gene-
rally cease during rainy spells, which is a problem
for most systems where new sludge is generated
daily. Furthermore, rainy conditions decrease the
temperatures of the windrows and the efficiency
of the curing process. This problem can be
circumvented by constructing roofs over piles and
mixing areas; however, this solution considerably
increases capital costs.
• It requires a larger volume of bulking agent than
in-vessel systems, depending on the water content
of the sludge.
Static Pile • Greater land requirements than in-vessel systems.
Systems • Operations affected by climatic variability. Rain
may hamper, but does not preclude, operations
and may result in a less uniform product. Facilities
can be covered to protect the piles from precipita-
tion. Cold weather does not affect the system;
however, cold weather composting may require
certain operational modifications, and the low
temperatures may be more difficult for the
operator.
In-vessel • Potentially higher capital costs. Equipment costs
Systems include the costs of design, manufacturing, and
marketing the systems. These costs may be par-
ticularly high at present because very few systems
have been sold relative to the investment that has
been made in their design and manufacture.
• Lack of operating data, particularly for large
systems. Most enclosed reactor systems that are
currently operating (in Europe) accommodate a
relatively small volume of compost compared to
static pile and windrow systems. A larger facility is
now starting up in Portland, Oregon; its degree of
success will be demonstrated in time.
• Reliance on specialized mechanical systems, re-
sulting in process delays and higher maintenance
costs during breakdowns.
• Potential for incomplete stabilization. Some
systems propose only a few days for the biological
processes, which may not be enough time to en-
sure a stable, odor-free product with acceptable
pathogen destruction.
• Compaction of compost mix in systems that do
not provide for mixing in-vessel can result in
inadequate air flow and an unstable end product.
• Less flexibility in operational mode than with other
systems.
Comparison of Composting and
Other Sludge Management Options
Composting offers several advantages over the four other
major sludge management alternatives: land application,
landfilling, incineration, and ocean disposal. Unlike incinera-
tion, landfilling, and ocean disposal, composting offers a
means of recovering a resource and productively using the
sludge. The compost product is stable and odor resistant
compared to sludge that is directly applied to land. Compost
can be stored conveniently and is easily spread on the land
using many different spreading systems. Sludge, by compari-
son, is difficult to handle and store.
Compared to incineration, composting uses very little exter-
nal energy and the process can be initiated rapidly — in as
little as a month — with relatively low capital investment.
Process design can be flexible and adaptable to a wide range
of situations, including changes in the nature and processing
of wastewater at the treatment plant and resultant varying
solids production. Also, depending on the system chosen,
the sludge may not need to be digested or otherwise stabi-
lized before composting.
Public Relations
As experience at the Dickerson, Maryland, site (see Chap-
ter 2) illustrates, good public relations are critical to the suc-
cess of a compost facility. In general, the earlier the public
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COMPOSTING FACILITY DESIGN
is involved in a project, the greater are its chances of
success. One vehicle for public involvement is a citizens ad-
visory committee. The committee can meet with site repre-
sentatives, designers, and local officials on a regular or
as-needed basis to air local concerns, and it can serve as
the focal point for distributing information to the local com-
munity. Where possible, the committee should become in-
volved in decision making, for example, determining the
architectural treatments and landscaping. An open house or
other opportunities for the public to visit a site will also help
allay concerns.
Aesthetic and Environmental Considerations
The four major aesthetic concerns expressed by local
communities are odor generation, noise, sightliness, and the
influence of traffic patterns on transportation requirements.
These considerations can greatly influence public opinion
about a compost site during siting, design, and operation.
Inadequate attention to them can delay or cancel a project
and can hamper operations at existing facilities. These con-
cerns should be addressed during the siting phase of a proj-
ect to help ensure that public relations problems will not
occur or at least will be controllable after the facility has
been sited. Some constructive approaches to alleviating
aesthetic concerns have included: constructing low build-
ings and obscuring them behind a barrier of trees (the ap-
proach taken at Cape May, New Jersey); scheduling sludge
deliveries to avoid rush hour traffic and evening hours; and
fining drivers for sludge spills during transportation. The
potential for runoff, groundwater contamination, noise, and
disruption during construction and operation must also be
addressed in the work plan for a facility.
Marketing and Distribution
Distribution is the final step in all composting programs but
one of the first that should be considered when designing a
system. The compost may be sold or given to retailers, or it
may be distributed directly to bulk and/or individual users.
In a few cases, compost is used as landfill cover. Whatever
approach is taken, the use or disposal capacity must equal
or exceed the compost supply to prevent a large accumula-
tion of compost on site. Many municipalities hire specialists
to investigate and develop markets for their compost.
The first step in setting up a distribution program is to
conduct a marketing study to identify the market for the
compost. Major potential users for compost are listed in
Table 1.3.
In most localities, private homeowners will constitute only a
small fraction of the market. Most sales will go to bulk
Table 1.3 Major Compost Users
Private Residential
Private Food
Private Nonfood
Public Agencies
Land Reclamation
Landfill-Compost Disposal
Garden Applications for Food
Nonfood Applications
Field Crops for Food and Feed
Garden Crops for Food and Feed
Fruit Trees
Greenhouses
Nurseries
Golf Courses
Landscape Contractors
Turf grass Farmers
Industrial Park Grounds
Cemeteries
Fertilizer Companies
Public Parks
Playgrounds
Roadsides and Median Strips
Military Installations
Public Grounds
Landfill Cover
Strip-Mined Lands
Sand and Gravel Pits
users. Some compost facilities market exclusively to bulk
users, finding that the added expense of producing and
packaging a separate product for individual use is not
justified by the market size. Although application of lime-
stabilized and digested sludge is widely accepted by the
agricultural community, agricultural uses of compost have
tended to be minimal due to such factors as the cost of the
material and the low availability of nitrogen from compost in
the first crop season.
When potential users have been identified, factors such as
distance to the markets, transportation costs, and the costs
of competing products (e.g., topsoil, peat moss, potting
soil, manure, and mulch) must be analyzed to determine the
viability of the market. A decision must be made as to
whether the municipality will market the compost directly or
will distribute it to retailers who will be responsible for
marketing and sales. Unit prices for compost generally
range from $8 to $29 per ton ($9 to $32 per metric ton [mt]).
Although this income does not make the composting opera-
tion self supporting, income from sales can help offset
operational costs. Many facilities have chosen to market the
compost at the wholesale rather than the retail level.
To successfully distribute compost, a demand for the product
must be created through a comprehensive marketing pro-
gram. Potential users should be familiarized with the material
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COMPOSTING FACILITY DESIGN
and its uses through such means as distribution of informa-
tional materials; participation in county fairs, nursery shows,
and flower shows; and educational programs at local agricul-
ture schools. Opportunities for the public to see and touch
the material will help alleviate any negative preconceptions
due to its association with sludge. To establish user con-
fidence, it may be prudent to supply the compost to new
users at no cost so that they can gain experience with the
material. An ongoing citizens participation program (see
Public Relations) can also enhance product marketing and
acceptance.
Attention must be paid to product image. The compost facil-
ity should be designed to generate the type of product most
suitable for the identified users. If necessary, compost quality
can be enhanced by screening, addition of nutrients, and
mixing with other materials to create products such as
mulches and potting soils. A product name and logo should
be developed. Product standards and quality controls should
be instituted so that the public can be assured that they are
receiving a high quality product. Signs with the product
name and logo should be displayed in retail outlets to adver-
tise the product.
Guidelines for use should be prepared. One user question
that may need to be resolved involves identifying a means of
spreading the product, since much standard agricultural
equipment will not spread compost. Application rates should
be specified. User education may help to reduce seasonal
fluctuations in compost demand that can lead to accumula-
tion of materials, creating odor and storage problems. For
example, many users may apply compost in the spring, when
fall application would be more productive. Increasing fall de-
mand is particularly important in states where cold weather
limits much of the outdoor demand for compost. In such
cases the winter compost must be stockpiled until spring, so
it makes sense to distribute as much compost as possible
before winter to miminize storage requirements. Alternatively,
users with offsite storage capacity should be identified.
Alternative options such as landfilling or municipal use should
be identified to handle unexpected problems during compost-
ing, reductions in demand, and/or batches of compost that
do not meet the marketing standards.
The long-term market potential should also be considered.
For example, is the demand from the identified user groups
expected to increase, decrease, or remain stable in the
future? Is it likely that new user groups may develop? Will
more distant markets develop over time as the local com-
munities become familiar with the product?
Although marketing studies are essential to determine the
marketability of the material, their predictions are only
estimates. The marketability of the product will be deter-
mined only by an actual distribution program. The program
should be monitored carefully and modified if necessary to
ensure that distribution keeps pace with supply.
Economics
Composting involves the capital costs of equipment, site
development and construction, and operation and mainte-
nance (O&M) costs for labor, bulking agent, energy,
materials, and supplies. Figures 1.3 and 1.4 show the pro-
jected capital and O&M costs in 1984 for static pile facilities
as a function of sludge solids content. Figures 1.5 and 1.6
show how estimated capital and O&M costs in 1978 for an
in-vessel composting system vary with the sludge solids
content. (The B.A.V. system is an in-vessel system
operating in the Federal Republic of Germany.) These
figures provide examples of approximate composting costs
for illustrative purposes only.
As Figures 1.3 and 1.4 illustrate, both capital and O&M
costs decrease dramatically with increasing solids content,
making dewatering an important aspect of the composting
system. Both capital and O&M costs of dewatering should
therefore be balanced against similar costs for composting
wetter versus drier sludges.
Tables 1.4 through 1.6 provide actual economic and oper-
ating data for small (<1 dry ton/day [<0.9 dry mt/day]),
medium (2 to 9 dry tons/day [1.8 to 8.2 dry mt/day]), and
SOURCE: Reference (1)
SO DRY TONS
5 DRY TONS
Sf
SLUDGE SOLIDS CONTENT (%)
Figure 1.3. Capital Cost for 5, 10, 25, and 50 Dry Ton/Day
Static Pile Compost Facilities as a Function of Sludge
Solids Content
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COMPOSTING FACILITY DESIGN
Table 1.4 Economic and Operating Characteristics of Small Static Pile Sludge Composting Facilities (<1 Dry Ton/Day)a
Facility
Swampscott, MA
Old Town ,ME
Gardner, ME
Durham, NH
Average
Sludge
Composted6
(Dry Tons/ Day)
0.3
0.3
0.9
1.0
Solids
Content
of Sludge
(%)
25
11
20
15
O&M
Costs0
($/Dry Ton
of Sludge)
423.08
408.40
78.28
Marketing
Compensation
($/Dry Ton
of Sludge)
21.45
43.00
Capital
Costs
($/Dry Ton
of Sludge)
146,666
916,666
660,000
Person-hours
Per Day
Per Dry Ton
Sludge
5.8
16.3
8.3
4.4
Dry Tons
Per Hectare
Per Day of
Site Space
2.23
0.49
1.11
2.86
SOURCE: Reference (3).
3Measurements are given in English tons. One English ton =0.907 mt.
bBased on a 5-day week.
C8ased on values provided by the authority; may include dewatering and transportation.
large (>10 dry tons/day [ >9 dry mt/day]) static pile and
windrow facilities. As these tables demonstrate, both capital
and O&M costs vary substantially from one facility to another
within each category. These variations are due to several fac-
tors such as method of cost accounting used, inclusion of
dewatering costs, inclusion of transportation costs to offsite
locations, amortization, and local considerations. Due to
these variations, it is essential that each municipality perform
its own cost estimate.
Another important economic factor to consider is replace-
ment costs. Regardless of the method of composting em-
ployed, equipment has a useful life and must be replaced
periodically. For example. Table 3.5 in Chapter 3 lists life
expectancies for equipment at a large windrow facility.
SOURCE: Reference (1)
SO DRY TONS
10 DRY TONS
5 DRY TONS
O//
8 20 22 24 26
SLUDGE SOLIDS CONTENT (%)
Figure 1.4. Annual Operation and Maintenance Cost for 5, 10,
25, and 50 Dry Ton/Day Static Pile Compost Facilities
as a Function of Sludge Solids Content
30
120
<°
o-i 10
SOURCE: Reference (2)
50
100 150
DRY TONS/DAY
200
250
The B.A.V. system is an m-vessel system operating in the Federal
Republic of Germany. The data shown are estimated costs for
hypothetical facilities that are larger than those currently operating.
Figure 1.5. Capital Costs for B.A.V. Composting System
u.
o
U)
o
SOURCE: Reference (2)
2 -
35°A
"souos
50
100 150
DRY TONS/DAY
200
250
The B.A.V. system is an in-vessel system operating in the Federal
Republic of Germany. The data shown are estimated costs for
hypothetical facilities that are larger than those currently operating.
Figure 1.6. Estimated Annual O&M Costs for B.A.V.
Composting System
-------
COMPOSTING FACILITY DESIGN
Table 1.5 Economic and Operating Characteristics of Medium Sludge Composting Facilities (2-9 Dry Tons/Day)a'b
Facility
Bangor, ME
Morganton, NC
Occoquan, VA
S. Portland, ME
Stratford, CT
Middletown, NJ
W. Warwick, Rl
Hampton Roads, VA
Merrimack, NH
Average
Sludge
Composted0
(Dry Tons/ Day)
2.1
3.2
3.6
3.9
4.5
4.5
5.0
6.8
8.0
Solids
Content
of Sludge
(%)
23
14
19
14
20
21
14
16
21
O&M
Costs"
($/Dry Ton
of Sludge)
31.26
75.22
44.70
157.42
—
169.22
37.77
140.04
101.73
Marketing
Compensation
($/Dry Ton
of Sludge)
35.75
32.18
0
25.00
_
0
0
43.00
7.15
Capital
Costs
($/Dry Ton
of Sludge)
7,174
33,916
412,500
639,534
—
946,000
—
322,666
22,988
Person-hours
Per Day
Per Dry Ton
Sludge
1.1
7.4
2.8
4.1
10.6
5.3
2.2
7.0
3.0
Dry Tons
Per Hectare
Per Day of
Site Space
2.1
5.3
4.4
3.7
11.1
5.9
3.1
4.2
9.8
SOURCE: Reference (3).
aAII facilities are static pile except Occoquan, VA, which is a windrow facility.
bMeasurements are given in English tons. One English ton =0.907 mt.
°Based on a 5-day week.
dBased on values provided by the authority; may include dewatering and transportation.
Table 1.6 Economic and Operating Characteristics of Large Static Pile Sludge Composting Facilities ( >10 Dry Tons/Day)3
Facility
Portland, ME
Camden, NJ
Windsor, Canada
Blue Plains, DC
Philadelphia, PA
Average
Sludge
Compostedb
(Dry Tons /Day)
10
24
24
68
130
Solids
Content
of Sludge
(%)
18
25
28
18
20
O&M
Costs'
($/Dry Ton
of Sludge)
—
88.21
53.57
174.16
192.31
Marketing
Compensation
($/Dry Ton
of Sludge)
21.45
0
0
14.30
7.15
Capital
Costs
($/Dry Ton
of Sludge)
198,198
66,707
33,333
72,752
11,538
Person-hours
Per Day
Per Dry Ton
Sludge
5.9
3.2
1.3
1.7
1.7
Dry Tons
Per Hectare
Per Day of
Site Space
13.8
6.4
9.9
28.0
21.4
SOURCE: Reference (3).
Measurements are given in English tons. One English ton =0.907 mt.
bBased on a 5-day week.
cBased on values provided by the authority; may include dewatering and transportation.
A lifecycle cost analysis can be a useful tool in evaluating
economic alternatives. This analysis considers capital costs,
O&M costs, and replacement costs over a 20-year period.
Design Considerations
Within each of the three basic approaches to composting —
static pile, windrow, and in-vessel — there are many dif-
ferent choices of design and operation. Some choices will
be more appropriate than others depending on the particular
situation. A good system should be reliable, consistent,
flexible, and free of malodors, and it should meet the expec-
tations of the municipality.
An essential step in the decision-making process is to per-
form a site-specific feasibility and pilot study. This study is
normally performed by an engineering consulting firm.
However, it is critical that the municipality play an active
role in determining the type of system for their community.
The involvement of the municipality is particularly important
because composting is a living process that requires atten-
tion to keep the system running properly. The municipality
will ultimately operate the system and therefore must com-
pletely understand the operational requirements. When an
engineering consultant is evaluating and recommending op-
tions, the municipality should ask questions and make sure
they understand all the data.
-------
COMPOSTING FACILITY DESIGN
Frequently, municipalities will get locked into one particular
system based on the advice of a consultant or vendor and
will not adequately consider the other options open to
them. In such cases, feasibility studies amount to justifying
a predetermined choice, rather than equally assessing the
possible choices to make the best decision. The desire to
justify a predetermined choice can result in inadequate
attention being paid to aspects of the technology that may
be difficult to implement. The ultimate result can be a
system that creates frequent problems and is more expen-
sive to operate than was originally expected.
Although compost systems can be rapidly implemented,
rapid startup should be avoided. Design parameters should
be carefully defined and tested through pilot studies if
possible for each individual facility. Facilities should be
designed with sufficient flexibility to accomodate unex-
pected changes in key parameters (see Process and Opera-
tional Specifications in this chapter). Inadequate attention
to design can create problems in the long term.
Process Parameters
Several process parameters are critical to composting. One
of these is the moisture level in the sludge/bulking agent
mixture, which is a function of the individual moisture con-
tent of the sludge and bulking agent and the relative propor-
tions of these materials in the mix. To ensure adequate
composting, the sludge/bulking agent mixture should have
a moisture level no greater than 60 percent for static pile
and windrow composting and no greater than 65 percent
for in-vessel composting. One purpose of the bulking agent
is to reduce the moisture of the sludge, which contains 70
to 87 percent water (13 to 30 percent solids). Climate and
precipitation are important factors that can affect moisture
levels in static pile and windrow systems.
Another key parameter is the carbon-to-nitrogen (C:N) ratio
of the sludge/bulking agent mix. Many references recom-
mend a C:N ratio in the range of 20:1 to 30:1 as optimal for
composting. One danger with applying C:N ratios in the
design of composting systems is the way in which the car-
bon component is measured. Carbon is generally measured
by burning off all the volatile matter in an oven at 450°C.
This process measures the total volatile carbon content.
However, not all the volatile carbon is biologically available
to the organisms that compost the sludge. As a result, even
when the measured C:N ratio appears favorable, enough
easily degradable carbon may not be available for com-
posting. The volatile solids content of the composting mix
should be greater than 50 percent.
The pH of the sludge/bulking agent mixture should gene-
rally be in the range of 6 to 9. Lime-stabilized sludge has
been composted at pH 12; however, it may take longer for
the composting process to achieve the temperatures re-
quired to destroy pathogens. The bulking agent may have a
significant impact on the pH of the mix. For example,
sawdust may be acidic. The composting process tends to
neutralize, so compost has a relatively neutral pH (approx-
imately 6.8), regardless of the sludge pH.
The type of sludge composted affects the process. Both
raw and digested sludge can be successfully composted,
however, raw sludge has a greater potential for odors,
which can be a problem, particularly with windrow systems.
Raw sludges generally have more energy available and will
therefore degrade more readily and have a higher oxygen
demand. They will achieve higher temperatures faster;
however, unless sufficient air is applied to satisfy the oxy-
gen demand and cause evaporative cooling, excessively
high temperatures and odors will result.
The level of oxygen in the pile during composting affects
the process. An atmosphere containing 5 to 15 percent
oxygen is required throughout the pile to ensure aerobic
conditions.
Process and Operational Specifications
Process specifications include such factors as bulking
agent, desired moisture content of the sludge/bulking
agent mix, feed rate of sludge and bulking agent into the
system, retention time of materials at each of the various
phases of composting, and energy requirements. Opera-
tional parameters include number of workers and electricity
and fuel use.
One of the most common operational problems at a com-
post facility is variation in the sludge and bulking agent;
for example, changes in solids concentrations, volatiles
content, and pH. These types of problems are generally
handled by changing the mixing ratios and aeration rates. In
establishing process parameters, it is therefore important to
consider contingencies that may be caused by unanticipated
changes in conditions. For example, what happens if the
sludge solids content is 18 percent yet the facility was
designed for 22 percent sludge solids content? What if the
bulking agent becomes unavailable? What if sludge genera-
tion exceeds expectations for brief or extended periods of
time? What kind of backup requirements are necessary? For
each parameter, the range of possible values under various
conditions should be determined.
-------
COMPOSTING FACILITY DESIGN
Mass Balance
Mass balance is a critical step in the design of a compost
facility. It establishes how much of each material (sludge,
bulking agent, wood chips, etc.) is used during each phase
of the operation and its fate during the process. This infor-
mation is essential for determining design parameters such
as ratio of sludge to bulking agent, size of the compost pad,
equipment and storage needs, etc. An individual mass
balance sheet should be calculated for every facility since
the key parameters vary from one facility to another.
Figure 1.7 is an example of a mass balance sheet. On the
left is listed each of the components involved in compost-
ing: sludge, the bulking agent, compost, mix, etc. For each
of these substances, the following parameters must be
measured or calculated:
• Total volume (in cubic yards [yd3] or cubic meters [m3]).
• Total wet weight (wet pounds [Ib] or wet kilograms [kg]).
• Total solids content (dry Ib or dry kg) calculated by
multiplying percent solids of the material by total wet
weight.
• Volatile solids content (dry Ib or dry kg). Volatile solids
are an indication of the amount of energy in the mix-
ture that is available to the organisms. They affect
Table 1.7 Typical Bulk Density of Various Composting Process
Components
Material
Digested Sludge (20% solids)
Raw Sludge (20% solids)
New Wood Chips (60% solids)
Used Wood Chips (55% solids)
Unscreened Compost
Screened Compost
(%-inch [1-centimeter (cm)] mesh)
Density
(Ib/yd'l (kg/m3)
1,800 1,070
1,400-1,650 830-980
550 325
800 475
1,000 595
1,150 685
SOURCE: Reference (1).
moisture reduction and temperatures during com-
posting.
Water content (Ib or kg).
Bulk density (wet Ib/yd3 or wet kg/m3). This parameter
can be calculated if the wet weight and volume of the
components are known. Table 1.7 provides bulk densi-
ties of various process components.
Percent water content.
Percent volatile solids content.
MIXTURE:
RFHOVFRY:
SLUDGE
COMPOST
MIX
COMPOSTING LOSS
UNSCREENED COMPOST
DRYING LOSS
DRIED COMPOST
CHIPS, SCREENED
COMPOST, SCREENED
TOTAL
(yd3)
TOTAL
(Ib)
TS
(Ib)
VS
(Ib)
WATER
(Ib)
B.D.
(Ib/yd3)
WATER
(%)
VS
(%)
Figure 1.7. Materials Balance
10
-------
COMPOSTING FACILITY DESIGN
The first step in performing a mass balance is to fill in the
information for the sludge and bulking agent. This informa-
tion can then be summed to determine the parameters for
the compost mix. The percent solids of the mix is a key
parameter. It must be at least 40 percent to ensure adequate
composting in windrow or static pile systems. A wetter mix-
ture is likely to cause problems. (In-vessel systems have
higher horsepower blowers and therefore may be able to
successfully compost a mixture with a slightly lower solids
content, but this conjecture has not yet been adequately
demonstrated.)
For a 20 percent solids sludge, the volume ratio necessary to
achieve a mix of at least 40 percent solids is approximately
2.5 to 2.7 parts bulking agent to 1 part sludge depending on
the solids content of the bulking agent (see Figure 1.8).
(Volume measurements are used instead of mass because
materials are generally measured with a front-end loader or
truck of known volume.) In this case, wood chips at 65 per-
cent solids are used as the bulking agent. If the water con-
tent of the sludge is too high, this ratio must increased until
the moisture content is 60 percent or less. With wetter
sludges, a ratio as high as 5:1 or 6:1 may be necessary to
achieve a mixture with a 40 percent solids content. Since this
is a substantial amount of material, some facilities with par-
ticularly wet sludges have tried to reduce costs by operating
using a lower ratio mixture with less than 40 percent solids in
the mix, but this procedure compromises the effectiveness of
the composting process and invariably causes problems.
In some cases, sludges may not be quite as dry as engi-
neering data suggest. For example, a sludge that is purported
to have a 20 percent solids content may in fact have only an
18 percent solids content most of the time. For this reason, it
is prudent to conduct a mass balance using a slightly lower
sludge solids content to anticipate the types of problems that
may arise.
Figure 1.9 and Table 1.8 illustrate the application of mass
balance to an extended static pile compost facility that
handles 10 dry tons (9 dry mt) of sludge per day. The various
components that must be considered in a mass balance are
labeled Item 1 through 8. Item 1 represents sludge; Items 2
and 3 represent the bulking agent (in this case new and
recycled wood chips, respectively). Each of these three com-
ponents has a different moisture content and bulk density.
This information is used to calculate the parameters for the
resulting mix (Item 4). In this example, the sludge (Item 1)
has a solids content of 20 percent, and the wood chips
(Items 2 and 3) have a solids content of 70 percent. This
solids content input results in a mixture (Item 4) with a
41 percent solids content1, which is favorable for
composting.
oc
x
5
O
SOURCE: Reference (4)
20
PERCENT SLUDGE CAKE SOLIDS
40
Figure 1.8. Relationship of Volumetric Mixing Ratio to Percent
Sludge Solids to Achieve a 40% Solids Mixture Using
Wood Chips at 65% Solids
The mass balance calculation of Items 1 through 6, which are
all materials that make up the compost pile, can be com-
pleted without pilot studies during the design of a compost
system. The volume and characteristics of the composted
material before screening (Item 7) and the screened compost
(Item 8) must be determined through pilot studies because
material is lost during the composting, curing, and drying
processes. Volatile solids are lost during composting. In the
example in Figure 1.9, the volatile solids content of the
sludge/bulking agent mix is reduced from 80 percent before
composting to 65 percent after composting and 45 percent
after screening. Water is lost throughout the various phases
of the composting process through evaporation, leaching,
and condensation. The loss of water decreases bulk density
since the water in the pores is replaced by air, which is con-
siderably lighter. However this decrease in bulk density is off-
set by the tendency for bulk density to be increased as the
material is decomposed into finer particles, reducing the
available porosity. Drying causes further water loss. In Figure
1.9, drying and screening reduce compost moisture content
by 5 percent.
The volumes of the different materials indicate the amount of
material that must be moved around the facility during the
various phases of composting. This volume will influence
equipment selection and hence the overall cost of operations.
In this example, there are 87.5 yd3 (67 m3) of sludge; 52.4 yd3
'[(70 x 20%) + <13.1 x 70%) + (39.4 x 70%)] /122.5
11
-------
COMPOSTING FACILITY DESIGN
UNSCREENED
COMPOST
COVER
PILE COMPOSTING
21 DAYS RETENTION
SOURCE: Reference (5)
DRYING
(IF REQUIRED)
6 DAYS RETENTION
RECYCLED
WOOD CHIPS
COMPOST CURING
AND STORAGE
60 DAYS CAPACITY
UNDIGESTED
DEWATERED
SLUDGE
Figure 1.9. Process Flow Diagram for Extended Pile Compost Facility
(10 Million Gallon per Day [MGD] Activated Sludge, 5-Day/Week Operation)
Table 1.8 Materials Balance for Process Flow Diagram (Figure 1.9)
Weight
Component
Undigested Dewatered
Sludge
New Wood Chips
Recycled Wood Chips
Mix
Wood Chip Pad
Unscreened Compost
Cover
21 -Day Compost Pile
Screened Compost
Wet
Tons
70
13
39
123
9
19
90
32
(Wet
Metric
Tons)
(64)
(12)
(35)
(112)
(8)
(17)
(82)
(29)
Densitv
Percent
Solids
20
70
70
41
70
65
65
60
Ib/yd3
1,600
500
600
900
500
725
725
975
(kg/m3)
(950)
(295)
(355)
(535)
(295)
(430)
(430)
(560)
Volume
yd3
88
52
131
271a
34
51
248
66
(m3)
(67)
(40)
(100)
(207)
(26)
(39)
(190)
(51)
Percent
Volatile
Solids
75
90
80
80
90
65
65
45
SOURCE: Reference (5).
"Although this hypothetical mix volume is presented here as the sum of the volumes of the component parts (dewatered sludge, new wood chips and
recycled chips), in reality the mix volume will be significantly less than the sum of the parts.
12
-------
COMPOSTING FACILITY DESIGN
(40 m3) of new wood chips; and 131.3 yd3 (100 m3) of re-
cycled wood chips. A volume of 248.3 yd3 (190 m3) of com-
post must be screened every day. Screening reduces the
original sludge volume by about 26 percent to 65.6 yd3
(50 m3) of finished compost plus the recycled bulking agent.
The importance of conducting a mass balance during the
design phase of composting cannot be overemphasized. It
influences all aspects of the operation. One of the most
frequent problems encountered in composting facilities is
excess moisture. For example, moisture from compost piles
may become trapped in an enclosed composting facility (see
photo), causing corrosion and electrical short circuits. Mass
balance calculations will help prevent these problems by
making designers aware of the amount of moisture that
must be vented or collected during the composting process.
Compost Product
One of the first steps in designing a facility is to identify the
compost users and determine the kind of product they will
need. Should the product be bagged? Will it be used by
homeowners, for land reclamation, nurseries, sod farming,
etc.? The answers to these types of questions will help
determine which characteristics of the compost will be
important goals to be factored into design.
Texture, for example, can affect the marketability of the
material. Homeowners may prefer a finer product over one
that contains clearly identifiable pieces of bulking material.
On the other hand, the reduced porosity in a fine product
may make it less suitable as a soil conditioner for agri-
cultural or horticultural use. Coarsely textured material can
be marketed as a mulch. Moisture content is also important.
A wet product is difficult and messy to handle. Too dry a
Enclosed composting building, Portland, Maine.
Table 1.9 Typical Static Pile Bulking Materials
Primary Bulking Materials
Bark Refuse
Corn Cobs Rice Hulls
Leaves Sawdust
Paper Straw
Peanut Hulls Wood Chips
Secondary Bulking Materials9
Compost Fly Ash
Dried Sludge
aUsed as a supplement to a primary bulking agent. Cannot be used alone.
product creates dust, which is considered a nuisance by
many users.
The metals content may limit its use to nonfood-chain appli-
cations. At high levels, the presence of salts and metals
may affect plant growth (see Public Health Considerations
in this chapter). High levels of metals or salts may require
careful monitoring of the product before distribution. The
content and chemical forms of nitrogen, potassium, phos-
phorous, and trace nutrients, as well as the general growth
characteristics of the compost, will affect its value as a ferti-
lizer and soil conditioner. Another important characteristic
that may affect use is pH. Color and odor may affect its
marketability. Achieving the desired characteristics may
require process modifications.
The size of the user market should be estimated. If it is not
sufficient to absorb the predicted compost production, addi-
tional users must be identified. The additional user groups
may require a different type of product.
Having defined the desired characteristics, the next major
question to ask is: How much will it cost to achieve these
characteristics? For example, a particular type of screen
may be necessary to achieve a desired texture. Is it worth
the additional cost, or should the product use be redefined?
Bulking Agent
The bulking agent is a critical parameter in any composting
operation. Several different bulking agents have been suc-
cessfully used in composting operations, including wood
chips, sawdust, shredded tires, leaves, and chipped brush
(see Table 1.9). The bulking agent affects the process and
the quality of the product. Bulking agent characteristics
such as moisture content must be factored into a mass
balance calculation to evaluate the feasibility of the opera-
tion. Bulking agent characteristics will also affect the
screening requirements.
13
-------
COMPOSTING FACILITY DESIGN
The bulking agent must be available locally at an affordable
cost in sufficient quantities to meet the needs of the com-
post facility. The price of the bulking agent is often tied to
the transportation cost. Provisions at a compost site for
handling full truckloads of bulking agent can greatly reduce
the overall cost.
The availability of the bulking agent in the future, and the
ability of suppliers to meet increased needs that may occur
if the sludge quantities increase, should be determined. An
alternative bulking agent supply or an alternative bulking
agent should be identified in case of any problems.
Site Specifications
Size
A composting facility must have sufficient area to accom-
modate:
• Bulking agent storage.
• Mixing.
• Composting pad.
• Curing.
• Screening.
• Product storage.
• Contingency, such as unexpected surges in sludge
volumes.
• Materials handling.
Figure 1.10 shows how site area requirements for an
in-vessel facility increase with increasing moisture content
of sludge. (The B.A.V. System is a type of in-vessel system
currently operating in Germany.)
In addition, the future capacity of the site must be con-
sidered. How much and how rapidly will the community's
sludge generation increase in the future? Will the site be
expected to handle some or all of this increase? Should
composting be performed at one large site or at two or
more smaller plants? Should the site be temporary or
permanent?
Layout
The layout of a composting site must respond to considera-
tions such as materials flow, traffic patterns, leachate and
runoff, and public relations. Key questions include:
• What is the access to the area? Will a road have to be
built?
• Can runoff be diverted directly into an existing sewer
system or does a separate pond have to be built?
30
£
z
w 20
cc
< 10
ui
CC
SOURCE: Reference (2)
50
100 150
DRY TONS/DAY
200
250
The data shown are estimated area requirements for hypothetical
facilities that are much larger than the B.A.V. in-vessel facilities
operating in the Federal Republic of Germany.
Figure 1.10. Estimated Area Requirements for B.A.V. Composting
System
• Is a buffer zone necessary around the site from either a
regulatory requirement or public relations (noise, odor,
or visual) standpoint?
• Can the site be located next to the wastewater treat-
ment plant? Would local residents object to having
sludge trucked through their communities? What are
the costs of transporting the sludge?
Leachate, Condensate, and Runoff
Leachate and precipitation runoff controls are major con-
siderations in site layout. There are three basic sources of
moisture at a compost site:
• Condensate — moisture in the air that is pulled through
the pile.
• Leachate — liquid that drains from the compost mix.
• Runoff — precipitation that reaches the pad directly
without going through a compost pile.
The amount of condensate generated during composting is
a function of the moisture content of sludge and ambient
conditions. Generally, approximately 10 to 30 gallons of con-
densate are generated per dry ton per day (38 to 114 liters/
mt/day). The amount of leachate generated is similar; most
precipitation that falls on a compost pile is absorbed or
evaporated. The biological and chemical oxygen demands
(BOD and COD) of the leachate are fairly high (Table 1.10),
but have little impact on the treatment process because of
the small quantities of leachate generated. Pooling of
14
-------
COMPOSTING FACILITY DESIGN
Table 1.10 Analysis of Condensate, Leachate and Runoff From a Static Pile Composting Operation (mg/liter)
Source
Condensate
Leachate
Runoff b
Biological
Oxygen
Demand
(BOD)
2,000
2,070
91
Chemical
Oxygen
Demand
(COD)
4,050
12,400
613
Phosphate
Phosphorus
(PO,-P)
1.87
2.13
0.31
Nitrate
Nitrogen
(NO,-N)
0.73
0.46
0.16
Organic
Nitrogen
139
655
58
Ammonia
Nitrogen
(NH3-N)
1,140
905
115
Total
Kjeldhal
Nitrogen
(TKN)
1,279
1,560
173
Alkalinity3
4,030
2,930
361
PH
7.7
7.7
8.2
SOURCE: Referenced).
Expressed as mg/liter CaC03
bRunoff characteristics are a function of rainfall rate and volume.
leachate and runoff is also of concern and can create both
odor problems and the potential for ice formation during
winter months, which is dangerous for heavy equipment
operation.
In static pile and windrow systems, leachate and conden-
sate can be controlled by installing collection devices
underneath the piles and connected to a system consisting
of condensate traps, leachate pumps, and a collection
pond. To prevent pooling of runoff, the compost pad should
have a slope of at least 2 percent. Another approach to
runoff control is to construct a roof over part or all of the
compost operation. Several U.S. facilities are performing
the mixing and screening steps under roofs, as these steps
expose a large portion of the compost surface area to the
air and are therefore most sensitive to precipitation. Drying
operations are also candidates for roofing protection. Static,
forced-aeration compost piles are much less sensitive to
moisture since only the surface, which is a layer of finished
compost, is directly exposed to precipitation.
Odor Considerations
Odor control is the single most important factor that deter-
mines whether a composting facility will be permitted to
operate. Odors are primarily generated by the sewage
sludge rather than the compost mix. They tend to be
created in facilities where sludge piles are left exposed to
the atmosphere before mixing and also where composting is
incomplete, for example due to inadequate aeration. The
best form of odor control is to minimize odor generation by
maintaining the correct oxygen supply and temperatures
during composting and by keeping the compost site clean
and orderly. (Although these measures will not totally
eliminate odors from negatively aerated static piles, they
can reduce odors to very low levels.) In addition, one or
more odor control approaches should be instituted.
Compost is a good natural scrubber for odors produced by
sulfur compounds. Several facilities have controlled odors
by constructing a small pile of recycled compost over the
manifold of a negative (suction) aeration system. One prob-
lem with this approach is that the small piles can rapidly
become saturated, particularly in humid weather. When this
occurs, the piles must be changed frequently, which in-
creases the maintenance cost. To alleviate this problem,
several facilities have enclosed their scrubber piles. Some
compost facilities have treated air from the negative aera-
tion system with a wet scrubber that dissolves the odorous
compounds. Another approach is to oxidize the odor-form-
ing compounds in the condensate and leachate or to scrub
the gases through carbon-type filters. However, activated
carbon tends to clog rapidly and is effective for only a
relatively short period of time. It also requires substantial
maintenance and is fairly expensive.
Equipment
The composting process requires equipment to mix the
sludge and bulking agent, manipulate piles, aerate piles, and
screen compost. Table 1.11 lists some of the main equip-
ment options for these processes.
Mixing Equipment
Mixing the sludge and bulking agent is an essential step in
all composting operations. The goal of mixing is to create
uniform porosity and coating of the bulking material with
sludge throughout the mixture. Nonuniform porosity will
short-circuit the air flow, resulting in nonuniform com-
posting and a poorer quality product. The other goal of mix-
ing is to spread the sludge evenly over the bulking agent.
Composting takes place at the surface where there is con-
tact with oxygen, so the process will be enhanced if the
surface area of the sludge can be increased by contact with
the bulking agent.
15
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COMPOSTING FACILITY DESIGN
Table 1.11 Composting Equipment
Mixing
Compost Machines
Front-end Loaders
Mixing Boxes
Pugmills
Rototillers
Aeration
Stationary Piping
Moveable Piping
Flexible
Nonflexible
Blowers
Multiple
Single
Positive Pressure
Negative Pressure
Both Positive and
Negative Pressure
Pile Manipulation
Conveyors
Flails
Front-end Loaders
Outfeed Devices
Windrow Machines
Screening
Flexing Screen
Rotary
Trommel
Vibratory Deck
Brown Bear machine used for mixing sludge and bulking agent at
St. Paul, Minneosta compost site.
Pugmill mixing devices are used at several compost sites.
Sludge and the bulking agent are fed in from hoppers. Pad-
dle wheels inside the device mix the materials, and the mix
is then dropped onto a conveyor. This is a highly reliable
device that produces a good mix. It is relatively small —
approximately 12 feet (4 meters) long and 1.5 to 2 feet (0.5
to 0.6 meters) wide. In several of these devices, the feed
rate and thus the sludge/bulking agent mix ratio can be
readily varied. One problem with the pugmill system has
been adequate feeding from the wood chip hoppers. This
problem has been experienced by several facilities, however,
it has been largely resolved.
Other types of mixing devices used at several composting
facilities are machines such as the Brown Bear, the Cobey
composter, and the SCARAB I. To use the Brown Bear,
sludge and wood chips must be laid down in alternating
layers. An auger on the Brown Bear then turns over and
mixes these layers. The advantages of this device are that it
can be implemented quickly and can handle large volumes
of material. However, the system requires substantial space,
it is messy, and there is significant potential for odor genera-
tion because sludge is often left on a pad for some time
before being mixed. There is also the danger of an uneven
mix if the original layers of sludge and wood chips are not
uniform. The operator can correct this problem by using a
front-end loader to redistribute the materials, but this may
not happen if the operator is under pressure to complete the
job within a certain period of time.
A third type of mixing device is known as a mixing box. The
sludge and bulking agent are weighed by a built-in electric
scale and mixed using augers enclosed in a metal cylinder. All
these mixing boxes have outfeed devices, and some have ex-
tender conveyors on the side. These devices serve the dual
function of mixing the material and building the piles. They
provide a very uniform material within approximately 3 min-
utes. Originally designed for mixing agricultural feed, which is
a much lighter material than sludge, some devices have been
more successful than others at mixing compost. Problems
have included insufficient power and capacity. However,
several systems have operated successfully at compost sites.
Middletown, New Jersey, and Windsor, Ontario, are two
facilities that currently use these devices. One advantage of
the mixing box is that it is not subject to outdoor conditions.
Most compost facilities — particularly smaller facilities — use
a front-end loader for mixing. This is one of the least expen-
sive mixing options since a front-end loader is already re-
quired for other parts of the composting operation. Since the
quality of the mix depends to a large extent on the amount of
time spent by the operator, front-end loaders are less fool-
proof than other mixing systems.
The composting facility in Hampton Roads, Virginia, uses an
augmented front-end loader approach. This facility con-
structs a layer of wood chips, and then uses a truck to place
a layer of sludge on top. The sludge and wood chips are
mixed with a front-end loader for 20 to 30 minutes, which
provides a mixture with about 44 percent porosity. This
material is then put into a manure spreader, which increases
the porosity to 60 percent. This final step increases the
materials handling requirements but provides the significant
benefits of reducing blower head loss and improving compost
16
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COMPOSTING FACILITY DESIGN
Agricultural rototiller at Hampton Road, Virginia compost site.
quality. As with the other outdoor mixing approaches, this
method is subject to odor and space problems.
A final type of mixing equipment is the agricultural rototiller.
This device provides an adequate mix for composting, but
not as complete a mix as the pugmill, the manure spreader,
or mixing box. The Windsor, Ontario, facility used rototillers
for several years but has recently converted to mixing boxes.
The advantages of rototillers are that they are readily
available and provide a fairly uniform mix.
The cost of the various mixing systems depends on many
factors. Even within a single approach, costs can vary
significantly depending on auxiliary features such as bin size
and the length of conveyor systems. More expensive sys-
tems, such as the compost machines, can be cost effective
for large facilities where the machines would be used con-
tinuously. The potential for schedule disruptions due to
equipment failure should also be considered. The simplest
machines — the front-end loaders and rototillers — can be
readily replaced in the event of equipment failure, however,
they do not routinely provide as good a mix as other devices.
Pile Manipulation Equipment
Pile manipulation equipment is used to move materials
within a site. A major requirement of manipulation equip-
ment at a compost site is that it preserve the porosity of the
material. Pile manipulation devices used at compost sites
include front-end loaders, conveyors, flails, outfeed devices,
and windrow machines.
Aeration Systems
Aeration systems are a component of static pile and
in-vessel composting systems. Aeration for the different
in-vessel systems is discussed in Chapter 4. Static pile aera-
tion has two basic components: conduits (e.g., pipe) to
conduct the air flow, and blowers. The conduits may be sta-
tionary or moveable and the materials may be reusable or
disposable.
Stationary Systems
In stationary, or enclosed piping aeration systems, the aera-
tion channel is a square- or V-shaped channel formed into
the concrete composting pad. Piping is placed in the chan-
nel and covered with wood chips to provide a manifold
effect. A metal grate is then placed over the channel to
cover and protect the pipe. Some stationary systems use no
pipes and rely on the channel to control the air flow.
Stationary systems must have provisions for drainage to
prevent precipitation and condensation from accumulating
in the channels. Also, the channels must be narrow enough
and the protective metal grating strong enough to support
heavy equipment.
The advantages of a stationary system are: reduced time
required for pile set up; reduced operational costs because
less pipe and fewer wood chips are required; and decreased
possibility of crushed pipe. One major disadvantage of sta-
tionary versus moveable systems is the higher capital cost of
constructing concrete channels rather than an asphalt pad.
There is also significant potential for pipe clogging, which
reduces the air flow. Therefore, stationary systems require
periodic maintenance and checking for excessive head loss.
Pile design flexibility and aeration modifications are much
more limited with stationary systems. The configuration can-
not be changed in response to research findings or opera-
tional experience.
The composting facility in Denver, Colorado, is an example of
a stationary pipeless system. The channels are approximately
1.5 feet (0.5 meters) deep and 15 inches (38 cm) wide, with a
shoulder to hold a metal grate. Air is blown through the
channel and flows through holes that are spaced about every
3 or 4 inches (8 to 10 cm) on both sides of the protective
metal grate. A fabric cloth topped with a protective layer of
sand covers the metal grate and provides the manifold effect.
This system experienced some clogging and head loss prob-
lems, so a plastic covering is being evaluated.
Moveable Systems
A moveable system consists simply of piping laid on the
ground surface. It is far more widely used than stationary
systems. The piping can be flexible plastic, rigid plastic, or
17
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COMPOSTING FACILITY DESIGN
steel. Flexible piping is considered disposable and is dis-
carded after each composting period (approximately 21 days).
Moveable systems have many advantages: virtually no main-
tenance; low capital costs; design flexibility; and ease of
modification. For example, if a facility determines that more
air should be drawn through the pile without increasing elec-
trical use, moveable pipes can be mo\/ed closer together. Sta-
tionary systems do not offer this flexibility. The flexible piping
is expensive, however — up to 40 cents a foot ($1.30 per
meter) — which substantially elevates operational costs since
new piping must be purchased frequently. It is also sus-
ceptible to crushing. The reusable rigid steel or plastic pipe is
generally maintenance free and is fairly resistant to being
crushed. It provides the same pile design flexibility as flexible
piping, however, the capital costs are higher and provisions
must be made for recovering and storing the pipe between
uses. Pulling rigid pipe out of the pile requires manpower and
space to maneuver and can be a hazard to personnel.
Blowers
Single or multiple blowers can be used at a composting facil-
ity. Multiple blowers offer flexibility, since the aeration rate
can be varied over the compost period and from one pile to
another. Single blowers have higher O&M costs and much
less flexibility, which increases the potential for problems.
Aeration can be positive or negative or a combination of
both. Negative, or suction, aeration was used in the original
forced-aeration composting system at Beltsville, Maryland.
One advantage of this approach is that any odors are fun-
neled into a point source and can therefore be more readily
controlled. Experience with compost systems suggests that
an interchangeable negative/positive mode may be the best
approach. Generally, a suction mode should be .maintained
for the first 8 to 14 days of composting until the odors
generated during the initial composting phase have dimin-
ished and temperatures that meet Processes to Further
Reduce Pathogens (PFRP) requirements (see Primary
Pathogens in this chapter) have been achieved. Then the
blowers should be reversed to provide positive aeration,
which will accelerate the drying of the compost.
From a biological point of view, the microorganisms need
only about 5 grams (gm) of oxygen per gm of dry solids to
stabilize the material. Drying, however, requires approxi-
mately 9 to 10 times more air. Most of the initial work at
Beltsville, Maryland, was designed in terms of maximizing
temperatures for pathogen destruction at the expense of dry-
ing potential. The parameters developed at Beltsville were
published in several EPA guidance documents (including
reference [5]), and have served as the design basis for many
subsequent facilities, which are consequently underdesigned
Multiple blower system at compost site in Blue Plains, DC.
in terms of aeration capacity for optimum drying capability,
odor control, and stabilization.
Experience during the past few years suggests that whatever
electrical power is theoretically needed to maintain the
ecessary oxygen supply during composting should be quad-
rupled to ensure that larger blowers can be accommodated
later if necessary. It is much easier to install excess electrical
power during construction than to try to increase capacity
after a facility becomes operational. The O&M costs are the
same whether or not the excess power is used. Installing
excess capacity is therefore generally recommended.
Calculating the aeration rates and electrical power supply
needed for a facillt/ depends on the distribution system, head
loss, blower size, etc., and will not be covered in this publica-
tion. This information is available in literature from blower
manufacturers. In Windsor, Ontario, a 1-hp blower is used to
aerate 120 to 140 wet tons (109 to 127 wet mt) of 26 to
30 percent solids sludge cake. Research at Camden, New
Jersey, suggests that about 1,500 cubic feet (ft3) of air per
hour be supplied per dry ton of sludge (47 m3/dry mt/hr) [5].
Newer findings and experience suggest that more air may be
advisable.
Screening Equipment
One piece of equipment that is essential to static pile com-
posting is a screening device to separate the bulking agent
from the composted material before marketing. Screening
has several advantages: it enables a portion of the bulking
agent to be recovered and recycled; it reduces the volume
of material to be marketed; it produces a better quality
product; and it enables the coarseness of the material to be
varied by using screens of different sizes. Screening tends
18
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COMPOSTING FACILITY DESIGN
Table 1.12 Suggested Parameters and Sequence of Sampling for a
Municipal Sludge Composting Operation
Media
SLUDGE
Screen used at Beltsville, Maryland site.
to be a very difficult process because it is very sensitive to
the moisture content of the composted material; too high a
level of moisture makes screening difficult. This can be a
major problem with static pile composting.
Several different types of screening devices are available,
including vibratory decks, rotary screens, and trommels. In
evaluating the various alternatives, the following factors
should be considered:
• Ability to change screen size and produce compost
with different textures.
• Ability to screen compost with different moisture con-
tents.
• Separation efficiency.
• Ability to handle compost with different bulking agents.
• Ease of cleaning.
Equipment Selection
Some key questions to consider when determining the
equipment needs for a system include:
• How reliable is the equipment?
• How rapidly can the equipment be repaired in event of
failure? What is the availability of parts? Can repairs be
made locally? Can they be made at the facility? Are
special tools required to perform repairs?
• What kind of backup equipment is needed?
• Has the equipment been used successfully at compost
facilities? For example, can the hydraulic system of the
front-end loader tolerate the sludge material getting
behind the hydraulic hoses without breaking them?
SLUDGE AND
BULKING
MATERIAL
COMPOST
a. Process
COMPOST
b. Product
SITE
Parameter
Frequency
Moisture Content
Heavy Metals and
Toxic Organics
Pathogens
PH
Moisture Content
Temperature
Oxygen
Odors
Blowers
Heavy Metals
Soluble Salts
Plant Nutrients
Pathogens
Meteorological
Leachate and
Runoff
Initial and Periodic
(Monthly) Depending
on WWTPa Process
Changes
Initially to Establish
Range: Periodic
Sampling Thereafter
As Required by Local
and State Regulations
Initially and as WWTP
Process Changes
Initially to Adjust Bulking
Material Ratio:
Periodic as Related to
Sludge Consistency
Daily Until 3 to 5 Consec-
utive Days Above 55°C
are Recorded in the
"Toes" of the Pile.
Weekly Thereafter
Initially to Set Blower
Operations; Periodic
Depending on the
Temperature Changes
in the Pile
Daily to Identify and
Correct Any Odor
Problems from
Scrubber Piles,
Blower Operations,
or Improper Cover
Daily
Initially and as Required
by Authorities
Initially and as Changes
Occur in the WWTP
Operation
Initially — Establish
Range
As Required by Health
Authorities
Establish Norms for Use
in Design
initially for Design
Purposes
SOURCE: Referenced).
aWWTP=Wastewater Treatment Plant.
19
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COMPOSTING FACILITY DESIGN
• How versatile is the equipment under different operating
conditions? For example, if a screen is being selected,
can the equipment successfully screen compost over a
range of moisture levels? Can it successfully screen
compost with different bulking agents? Can it produce
compost with different textures?
• What kind of power needs does the equipment have?
Would this power be sufficient to produce the desired
results under all conditions?
• What is the expected lifetime of the equipment? What
are the maintenance requirements and costs? Will these
increase over time?
One important way to get information about equipment is to
talk to and visit equipment operators at other compost sites.
Equipment vendors should be able to provide names of other
clients who can serve as references. In evaluating information
from this source, it is important to be aware how the facility
being designed differs from the reference facility. Any dif-
ferences may cause the equipment to perform better or
worse at the new facility than at the reference facility.
Monitoring
Monitoring the sludge, bulking agent, compost process and
product is essential from an operational, marketing, and
public relations standpoint. Table 1.12 lists suggested
parameters for monitoring. The section on Primary Patho-
gens in this chapter describes time-temperature and indi-
cator-organism monitoring for pathogens.
Public Health Considerations
The three major public health concerns from a regulatory
viewpoint are primary pathogens and heavy metals in the
compost product, and secondary pathogens (including
Aspergillus fumigatus) at the compost site.
Primary Pathogens
Primary pathogens can infect most individuals, regardless of
their state of health. The primary pathogens that may be
found in sludge are viruses, bacteria, protozoa, and para-
sites such as helminth worms. One important concept con-
cerning the survival of these pathogens is the D-value. A
D-value is the time it takes to achieve a log,0 destruction of
a specific organism at a specific temperature. Table 1.13
gives D-values for five organisms that may be found in
sludge. According to these data which were obtained under
laboratory conditions, it would take 7.5 minutes at 60°C to
achieve a log,,, destruction of Salmonella.
Table 1.13 D Values for Destruction of Various Microorganisms by
Heat
Organism
D-Value
(Minutes at 60°C)
Adenovirus
Ascaris Ova
Poliovirus
Staphylococci
Salmonella
0.15
1.3
1.5
3.3
7.5
SOURCE: Reference (6).
Table 1.14 Temperatures Attained During 21 Days of Static Pile
Composting of Raw Dewatered Sludge3
Bulking
Material
Wood Chips
Paper Cubes
Auto Salvage
100% Leaves
60% Leaves
40% Wood Chips
Peanut Hulls
Shredded Tiresb
Ratio of
Bulking
Material
to Sludge
(Dry Wt.)
1.5:1
1:1
1:1
2.5:1
2.5:1
2:1
1.8:1:1.5
Avg.
Max.
Daily
Temp.
°C
80
74
74
77
77
77
77
Max.
Temp.
°C
82
80
80
77
84
81
81
No. of Days
Minimum
Temp.
Exceeded
60° C
6.5
8
3.5
9
10
8
14
SOURCE: Reference (4).
3These temperatures were achieved during early developmental research
for sludge composting. It is now realized that temperatures this high are
not optimal for composting. Ideally, temperatures should not exceed 60°C.
bThe ratio for shredded tires includes compost added to reduce the
moisture content of the sludge.
Table 1.14 shows temperatures attained during some early
experiments at the composting facility in Beltsville, Mary-
land, using a variety of bulking agents. Temperatures were
recorded over 21 days of composting. The lowest average
maximum daily temperature was 74°C, and the number of
days during which the minimum temperature exceeded 60°C
was never less than 3.5. A comparison of these data with
the D-values suggests that significant pathogen reduction
probably took place during these composting trials. Tests by
Surge et al. [7] showed that Salmonella, fecal coliform, and
total coliform were virtually destroyed after 10 days of com-
posting (Figure 1.11). Data from Burge et al. [7] indicate
that virus populations are reduced during both windrow and
static pile composting (Figure 1.12). Burge and Cramer [8]
looked at pathogen destruction during windrow composting
as a function of depth within the windrow (Figure 1.13).
20
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COMPOSTING FACILITY DESIGN
C5
C/3
UJ
O
O
O
DC
D
GO
SOURCE: Reference (7)
Total Coliforms
10
DAYS
15
20
Figure 1.11. Destruction of Salmonellae, Fecal Coliforms, and
Total Coliforms During Forced Aeration Composting
of Raw Sludge
They found good destruction of Salmonella at all pile
depths after 15 days. The destruction of total coliform and
fecal coliform was good at depths of 80 to 100 cm after
about 15 days; however, it was not as good at depths of 20
to 40 cm from the pile surface, and at 0 to 20 cm, it was
poor. One reason for the relationship between pile depth
and pathogen destruction in windrows is the lack of an
insulating cover layer on the windrow pile; this makes the
surface layer of windrows sensitive to ambient temperatures.
Based on these and other similar data, the EPA issued the 40
CFR Part 257 regulations, which define Processes to Signifi-
cantly Reduce Pathogens (PSRP) and Processes to Further
Reduce Pathogens (PFRP) that must be followed in com-
posting systems. Static pile, windrow, and in-vessel compost-
ing meet PSRP criteria if the pile achieves temperatures of
SOURCE: Reference (7)
BLANKET
X
10
DAYS
15
20
A shows pile cross-section with approximate location of sampling
sites. PFU/G = plaque-forming units.
Figure 1.12 Destruction of f2 Bacterial Virus During Forced
Aeration Composting of Raw Sludge
40°C for 5 days, with temperatures greater than 55°C for
4 hours. Under these conditions, pathogens are considered
to be significantly reduced. For most compost uses, however,
another, more stringent level of control is mandated. This
level is PFRP. To meet PFRP criteria, in-vessel and static pile
systems must achieve temperatures of 55°C for 3 days. The
products of PFRP processes are considered safe for unre-
stricted direct contact use. Because operational data have
shown that pathogen destruction varies with pile depth in
windrows, the PFRP criteria for windrow composting are
more stringent. They require that windrows maintain a tem-
perature of 55°C for 15 days and that the piles be turned over
at least five times.
Although not required by EPA regulation, compost at some
locations is monitored for levels of indicator organisms that
are present in large numbers before composting and that are
more resistant to destruction than most other similar patho-
gens. Total and fecal coliform and Salmonellae are often used
as indicators of bacterial survival. Eggs of Ascaris lumbri-
21
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COMPOSTING FACILITY DESIGN
SOURCE: Reference (8)
TOTAL COLIFORMS
0—20 cm
40
SALMONELLAE
10
20 30
DAYS
40
Figure 1.13. Pathogen Survival at Three Depths During Windrow
Composting
coides, one of the most resistant pathogens, are used as indi-
cators of parasite survival. Tests for these organisms are
relatively simple and rapid. If indicator organisms have been
sufficiently reduced, it is assumed that all pathogens have
been adequately destroyed. Even with these results, curing
for 30 days can be an additional safeguard to ensure ade-
quate pathogen destruction.
Secondary Pathogens
Aspergillus fumigatus is a common fungus that tends to
proliferate in the warm environments, including rotting
leaves and compost piles. This fungus — a secondary path-
ogen — can cause a respiratory condition, called asper-
gillosis, in susceptible individuals, such as the immune-
suppressed, the elderly, and individuals with asthma or
other respiratory ailments. Aspergillus spores are ubiqui-
tous. They are found in all natural environments. A few
years ago, when there were few data on how composting
operations affected the ambient atmospheric levels of
Aspergillus spores, local communities expressed concern
about whether composting would lead to an increased inci-
dence of aspergillosis because of the propensity of Asper-
gillus for growing on the wood products that serve as
bulking agents for most composting operations. Two recent
[9,10] studies have shown that composting does temporarily
elevate spore levels at the compost site during certain
operations such as mixing, but it does not significantly in-
crease levels in the local community. Concerns about the
health of site personnel can be addressed by screening out
susceptible personnel, using air-conditioned enclosed cabs,
and/or equipping potentially susceptible operators with
masks.
Heavy Metals
Heavy metals are a public health concern because of their
potential to (1) enter the food chain if food crops or
livestock feed are grown on compost-amended soil or if
cattle forage in a compost-amended area; and (2) harm chil-
dren who may consume nonfood substances such as soil or
compost. Many heavy metals are not usually present in
large enough concentrations to pose a threat, are bound by
the soil system in highly insoluble forms so that they are
unavailable to plants, or are not taken up by plants in appre-
ciable amounts. Cadmium has been the metal of greatest
concern associated with land application of sludge and
sludge products since it can be taken up in the edible
portions of plants in significant concentrations. If a sludge
contains heavy metals, public health concerns about the
compost quality can be addressed by crop management,
e.g., by not growing food-chain crops on compost-
amended soil, or by growing only crops that are known not
to take up the metals of concern, or by controlling pH (high
pH reduces metal uptake).
The acceptable heavy metals content of sludge compost
from a public health standpoint depends, therefore, on how
the sludge will be used. Several states regulate the heavy
metals content of sludge compost. The projected metals
content of sludge compost is an important factor to deter-
mine during the planning phase of a compost system as it
may affect the marketability of the compost.
Since composting is a biological process, it does not elimi-
nate metals. Any metals in the sludge will be present in the
compost in virtually the same quantities. Composting may,
however, dilute the concentrations of the metals to some
extent, depending on the type and proportion of bulking
agent. Table 1.15 provides examples of sludge heavy metal
content and Table 1.16 shows some examples of the dilu-
tion effect. Using wood chips as a bulking agent and
assuming a 75 percent recovery of the bulking agent, the
concentration of metals is reduced by about 25 percent. A
dilution factor of 25 percent is a reasonable assumption for
most composting operations that use wood chips as the
bulking agent.
22
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COMPOSTING FACILITY DESIGN
Table 1.15 Total Elemental Composition of Sewage Sludges from
a Number of Municipalities in the United States3
Component
Organic C
Inorganic C
Total N
NH4+-N
NOj-N
Total P
Inorganic P
Total S
Ca
Fe
Al
Na
K
Mg
Zn
Cu
Ni
Cr
Mn
Cd
Pb
Hg
Co
Mo
Ba
As
B
Minimum
6.5
0.3
0.1
0.1
0.1
0.1
0.1
0.6
0.10
0.10
0.10
0.01
0.02
0.03
101
84
2
10
18
3
13
1
1
5
21
6
4
Concentration*1
Maximum
%
48.0
54.3
17.6
6.7
0.5
14.3
2.4
1.5
25.0
15.3
13.5
3.1
2.6
2.0
ppm
27,800
10,400
3,515
99,000
7,100
3,410
19,730
10,600
18
39
8,980
230
757
Median
30.4
1.4
3.3
1.0
0.1
2.3
1.6
1.1
3.9
1.1
0.4
0.2
0.3
0.4
1,740
850
82
890
260
16
500
5
4
30
162
10
33
SOURCE: Referenced!).
aData compiled from over 200 samples from eight states.
bValues expressed on 110"C weight basis.
The dilution factor should be taken into account when
determining whether the compost product will meet state
regulations for heavy metals content. For example, if a
sludge contains 15 parts per million (ppm) cadmium and the
state limit for cadmium in sludge compost is 12 ppm, a
system can probably be designed to produce compost that
will meet the state regulation. If the heavy metals content of
the sludge is too high, however, either the metals content
must be reduced through source controls or an alternative
method of sludge management must be considered.
Table 1.16 Composition of Raw and Digested Sludge and Their
Respective Composts3
Component
pH
Water (%)
Organic carbon (%)
Total N (%)
NhC-N (ppm)
P (%)
K (%)
Ca (%)
Zn (ppm)
Cu (ppm)
Cd (ppm)
Ni (ppm)
Pb (ppm)
PCBsb (ppm)
BHCC (ppm)
DDE" (ppm)
DDT (ppm)
Raw
Sludge
9.5
78
31
3.8
1,540
1.5
0.2
1.4
980
420
10
85
425
0.24
1.22
0.01
0.06
Raw
Sludge
Compost
6.8
35
23
1.6
235
1.0
0.2
1.4
770
300
8
55
290
0.17
0.10
0.01
0.02
Digested
Sludge
6.5
76
24
2.3
1,210
2.2
0.2
2.0
1,760
725
19
—
575
0.24
0.13
—
—
Digested
Sludge
Compost
6.8
35
13
0.9
190
1.0
0.1
2.0
1,000
250
9
—
320
0.25
0.05
0.008
0.06
SOURCE: Reference (12).
"Sludge from the Blue Plains Wastewater Treatment Plant, Washington,
D.C. Compost produced at the U.S. Department of Agriculture
composting facility, Beltsville, Maryland.
bPolychlorinated biphenyls as Arochlor 1254.
The gamma isomer of benzene hexachloride is also called lindane.
dDDE results from the dehydrochlorination of DDT.
23
-------
COMPOSTING FACILITY DESIGN
References
(1) E&A Environmental Consultants, Inc., Stoughton, Massachu-
setts. Internal data.
(2) Camp, Dresser & McKee. 1978. New York City Sludge
Management Plan. Stage 1. Draft Technical Report. Volume 1.
pp. 3-124.
(3) Alpert, J.E. and P. Johnson. 1983. Cost analysis of static pile
composting in the United States. Presented at the Conference
on Composting of Solid Wastes and Slurries. Leeds, England.
(4) Epstein, E. 1980. Bulking materials, pp. 30-35. In: Proc.
Municipal and Industrial Sludge Composting. Information
Transfer Inc., Silver Spring, Maryland.
(5) Singley, M.E.; A.J. Higgins; and M. Frankin-Rosengaus.
1982. Sludge Composting and Utilization: A Design and Oper-
ating Manual. New Jersey Agricultural Experiment Station,
Cook County, Rutgers University, New Brunswick, New
Jersey 08903.
(6) Sivinski, H.D. 1975. Progress Report. Waste Resources
Utilization Program, Waste Management and Environmental
Programs Department 5440, Sandia Laboratories, Albu-
querque, New Mexico. SAND75-0436, p. 35.
(7) Surge, W.D.; W.N. Cramer; and E. Epstein. 1978. Destruction
of pathogens in sewage sludge by composting. Transactions
oftheASAE 21(31:510-514. American Society of Agricultural
Engineers, St. Joseph, Michigan.
(8) Surge, W.D. and W.N. Cramer. 1974. Destruction of patho-
gens by composting sewage sludge. Progress Report —
August 1, 1973 to April 1, 1974. Joint Project: Maryland Envi-
ronmental Service and Water Resources Management Admin-
istration, District of Columbia.
(9) Milner, 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(61:765-772.
(10) ERCO/Energy Resources Co. Inc. 1980. Monitoring of
Aspergillus fumigatus Associated with Municipal Sewage
Sludge Composting Operations in the State of Maine. Final
Report. Energy Resources Co. Inc., Cambridge, Massachu-
setts.
(11) Sommers, L.E. 1977. Chemical composition of sewage
sludges and analysis of their potential use as fertilizers.
J. Environ, dual. 6:225-232.
(12) Parr, J.F.; E. Epstein; and G.B. Willson. 1978. Composting
sewage sludge for land application. Agr/'c. Environ. 4:123-137.
24
-------
2. Experiences at Static Pile
Composting Operations
Background
This chapter discusses experiences at three static pile com-
posting facilities (Figure 2.1): the Western Branch site in
Prince Georges County, Maryland; the Dickerson site in
Montgomery County, Maryland; and Site If in Montgomery
County, Maryland.
Sewage from the District of Columbia, and parts of sewage
from Montgomery and Prince Georges County in Maryland
and Fairfax County in Virginia, are sewered into the Blue
Plains wastewater treatment plant, a regional facility located
in and operated by Washington, D.C. In 1982, the regional
facility produced approximately 1,750 wet tons (1,590 wet
mt) of sludge per day: approximately 300 wet tons (270 wet
mt) from Fairfax, 400 wet tons (360 wet mt) from Mont-
gomery, 350 wet tons (320 wet mt) from Prince Georges
County, and 700 wet tons (640 wet mt) from the District of
Columbia. These four jurisdictions were thus jointly respon-
sible for utilizing.or disposing of sludge from the Blue Plains
facility. In 1974, these jurisdictions signed a Regional Sludge
Agreement to the effect that each jurisdiction would handle
its share of the sludge.
Western Branch Site
Water, wastewater, and sludge from Montgomery and
Prince Georges Counties are managed by the Washington
Suburban Sanitary Commission (WSSC). The WSSC ini-
tially utilized burial in trenches (entrenchment) as an interim
disposal method for sludge from these two counties;
however, this approach met with mounting opposition, had
very expensive land requirements, was subject to permit
restrictions that limited future uses, and ultimately required
expensive land reclamation efforts. In 1980, the Maryland
Health Department discontinued the permitting of sludge
trenching. Faced with the need to find an alternative sludge
use or disposal method, Prince Georges County decided to
conduct static pile composting at a site next to the Western
Branch Wastewater Treatment Plant at the eastern border of
the county.
Design
The Western Branch site was designed and constructed in
1980 by the Maryland Environmental Service (MES) based
on design parameters developed by the U.S. Department of
Agriculture's Research Center at Beltsville, Maryland [1]. At
the time, these parameters represented state-of-the-art
knowledge about static pile composting. Designed to
handle up to 1,000 wet tons (910 wet mt) of sludge per day,
the site was equipped with four aeration pads, each 600 to
700 feet (180 to 210 meters) long and approximately 120
feet (40 meters) wide. An additional 30 acres (12 hectares
[ha]) of paved area were provided for storage, screening,
and drying. The aeration system consisted of Va-hp blowers
spaced 24 feet (7 meters) apart and connected to 4-inch
(10-cm) plastic perforated pipe. Each blower aerated 30 dry
tons (27 dry mt) of sludge. Ponds were constructed to
receive runoff from the site.
Public Relations
The Western Branch site had a minimal public relations
program. In fact, many local residents were not even aware
that a compost facility was being constructed. Concerns
about the local impact of such a facility were minimized
because of its location next to a wastewater treatment
plant, and no significant public opposition was experienced
during construction.
0 DICKERSON
MONTGOMERY COUNTY
PRINCE
GEORGES
COUNTY
The Fairfax County compost facility is not covered in this
paper.
Figure 2.1. Active and Inactive Composting Sites in the Region
Served by the Blue Plains, D.C. Wastewater
Treatment Plant
25
-------
STATIC PILE COMPOSTING OPERATIONS
Operations
In January, 1981, the Western Branch site was forced to
start operation prematurely, when only the four composting
pads — about one-third of the total site area — were
complete. Despite these limitations, the site operated for
four months, composting 350 wet tons (320 wet mt) of
sludge per day with few problems or complaints. However,
the tonnage to the site tripled to approximately 1,000 wet
tons (910 wet mt) per day when the Maryland Health
Department banned sludge landspreading at several
locations in the county due to public opposition.
Although the site had been designed to handle that capac-
ity, construction was still only partially complete, and the
operations suffered as a result. Operations were routinely
continued into the night in an attempt to handle the large
volume of sludge, resulting in diminished quality control and
errors such as inadequate mixing and inadequate aeration
due to damaged system components. As a result, the facil-
ity began to generate odors, and local residents formed a
committee that forced the facility to close until construction
was complete. Once construction was completed, the
facility reopened to handle 350 wet tons (320 wet mt) of
sludge per day. Public sentiment, however, was so negative
that the facility was soon closed permanently. One
repercussion of that experience is that the committee is
now working to close the adjacent wastewater treatment
plant.
The Dickerson Site
In 1977, Montgomery County decided to utilize static pile
composting as its sludge utilization/disposal method. They
designated a site in the eastern part of the county near the
Prince Georges County border as the location for their per-
manent "Site II" composting facility. However, fervent
opposition by local residents in this densely populated area
resulted in construction delays. Despite a 1978 court order
requiring the Washington Suburban Sanitary Commission to
build and operate Site II, construction was delayed for two
more years until 1980, when the State Health Department
decided to discontinue the permit for the sludge trenching
sites. At this point, Montgomery County selected a site near
Dickerson, at the western end of Montgomery County, as
an "interim" facility.
To enhance public relations, the county formed a Citizens
Advisory Committee to serve as a vehicle for disseminating
information about the facility and as a forum for airing and
developing responses to public concerns. This approach
was fairly expensive initially, however, it precluded many
potential public relations problems later. Nevertheless, when
the site was ready for operation in January 1981, a citizens'
US$c
j^^J^I ^scfi
Figure 2.2. Dickerson Compost Facility, Montgomery County, MD
group initiated adjudicatory hearings to delay operations. To
end the hearing process, a stipulation was signed that re-
quired Dickerson to close when Site II opened. At this point
composting commenced at the Dickerson facility and
trenching was discontinued.
Design
The Dickerson site is very similar to the Western Branch
site. It was designed and constructed in 1980 by the MES
based on the design parameters developed at Beltsville.
Figure 2.2 shows the site layout. Like Western Branch,
Dickerson contained four aeration pads, approximately 700
by 120 feet (215 by 35 meters) in area; an additional 30
acres (75 ha) of paved area for storage, screening, and
drying; a series of '/3-hp blowers spaced 24 feet (7 meters)
apart and connected to 4-inch (10-cm) pipes; and runoff
ponds.
Public Relations
The experience at the Dickerson site was essentially the
opposite of that at Western Branch. The public was totally
aware of the site from the beginning. The facility did have
some startup problems that resulted in odor production.
The public, who were essentially waiting for the facility to
26
-------
STATIC PILE COMPOSTING OPERATIONS
make a mistake, reacted immediately through the already-
established channel of the Citizens Advisory Committee.
These problems were immediately and successfully ad-
dressed, and the site continued to operate to the satis-
faction of most of its neighbors.
Process Modifications
Although Dickerson was only a temporary site, it provided
an opportunity to refine the Beltsville process. Continual
efforts were made to improve the process throughout the
site life. In this sense, Dickerson was essentially an
extension of the Beltsville research.
Mixing Procedures
One of the startup problems was inadequate mixing. To
combat this problem, site personnel experimented with
various ways of using a Cobey composter and a front-end
loader until they discovered a standard procedure that
would provide consistent uniform mixing. These methods
were then strictly followed. Another change instituted to
ensure adequate mixing was increased quality control. The
foremen and loader operators were trained so that they
could inspect each mix and determine whether the mix was
thorough and at a proper moisture content. This visual
inspection was double-checked by sampling and laboratory
testing.
Sludge: Wood Chip Ratio
Another problem that resulted in odor generation was the
high moisture content of the mix. Ideally, the sludge:wood
chip mixture should have a moisture content of no greater
than 60 percent to facilitate mixing and to provide the
proper environment for composting. The initial mixing ratio
used at Dickerson was 2.5 to 3 yd3 of wood chips per wet
ton of sludge (2.1 to 2.5 m3 per wet mt of sludge), based on
the experience of MES operations at the pilot plant in
Beltsville, Maryland, with a higher solids content sludge.
(This ratio is in mixed units, i.e, yd3 [volume] for wood
chips and wet tons [weight] for sludge, because these are
the quantities in which wood chips and sludge are de-
livered.) This ratio resulted in a mixture with a 70 percent
moisture content, which created anaerobic conditions and
resultant odors. The excessive moisture was a result of the
decreased sludge solids (13 to 18 percent at Dickerson
versus 20 percent at Beltsville) and the increased moisture
content of the wood chips (45 percent at Dickerson com-
pared with 40 percent at Beltsville).
In response to this problem, the Dickerson facility increased
the wood chip:sludge ratio to approximately 4.5 to 5 yd3 of
wood chips per wet ton of sludge (3.8 to 4.2 m3 of wood
frr
55'1 6.D:1
Figure 2.3. Sludge Moisture vs Bulking Ratio
45% Wood Chip Moisture
Wood Chip Sludge Roti
4251
Figure 2.4. Sludge Moisture vs Bulking Ratio
40% Wood Chip Moisture
chips per wet mt) and varied it as needed to maintain a mix
moisture content in the 60 to 65 percent range.
Figures 2.3 and 2.4 illustrate the wood chip: sludge ratios
required to generate mixes with 60 and 65 percent moisture
content for sludges of various moisture contents. Comparing
Figures 2.3 and 2.4 shows the dramatic difference that a
5 percent change in wood chip moisture content can have on
27
-------
STATIC PILE COMPOSTING OPERATIONS
the ratio necessary to achieve a desired mix moisture
content.
Leachate and Odor Control
Two additional sources of odors at Dickerson were the
leachate and condensate generated during composting.
Leachate generation, in particular, can be substantial at a
compost facility. Figure 2.5 shows the amount of leachate
and condensate generated by a single compost pile con-
taining 29 dry tons (26 dry mt) of sludge. The collected
leachate and condensate peaks at almost 900 gallons (gal)
(3,400 liters) per day, which is equivalent to 30 gal per dry ton
sludge per day (113 liters per dry mt of sludge per day)
approximately on the sixth day of composting.
The Dickerson site was originally designed so that all leachate
and condensate would drain into channels where it would
flow to open ponds and be pumped out and transported to
the nearest wastewater treatment plant. (Due to the rural
location of the site, a sewer line to the plant was not
feasible.) However, as this high strength liquid accumulated
in the ponds before pumping, it became a significant odor
source. The solution to this problem was to install sump
pumps to intercept the liquid as it flowed to the ponds and
pump it into waiting tank trucks, which then hauled it to the
nearest wastewater treatment plant. With this system, liquid
reached the ponds only during periods of peak stormwater
flow when the capacity of the sumps was exceeded. This
liquid tended to be dilute, with little or no odor.
Stringent housecleaning procedures were also instituted to
eliminate any potential for pooling of liquid on the compost
pad. Road sweepers and water trucks were used to keep the
pad clean. These combined measures basically eliminated the
odor problem. Good housekeeping practices had the added
benefit of improving public relations. Complainants were
invited to visit the site. Its tidy appearance generally mollified
their concerns.
The odor control system for the piles originally consisted of
piles of screened compost to filter the air that had been
drawn through the piles. However, these piles tended to
saturate rapidly due to the large amount of moisture in air
exiting the system. In response to this problem, a barrel-
shaped device that functioned as a heat exchanger and water
trap was installed between the blower and the filter pile. As
air passed through the device, its velocity decreased, allow-
ing it to cool; the moisture in the air condensed on the inside
surface of the barrel. When the blower was switched off, the
condensate drained through a check valve into the channels
where it was then pumped into tank trucks. This system was
able to collect up to about 30 gal (115 liters) per day of
condensate (Figure 2.6). Many of the odorous compounds
1000
80C
60C
£ MX)
200
SOURCE: Unpublished data,
Maryland Environmental Service.
10
Time (days)
15
20
Figure 2.5. Volume of Condensate and Leachate Generated by a
Single Compost Pile at the Dickerson Site Containing
29 Dry Tons of Sludge
50
SOURCE: Unpublished data,
Maryland Environmental Service.
20 -
10 -
—r—
10
—I—
1 5
20
Time (days)
Figure 2.6. Volume of Condensate Collected from a 29-Dry-Ton
(of Sludge) Compost Pile by a Two-barrel Condensate
Trap System
were dissolved in the condensate and never entered the
atmosphere. The dehydrated air then passed through the
screened compost filter piles, which retained their odor-
trapping ability for a much longer period of time.
Other problems encountered early in the operation included
poor positioning of the intake pipes from the compost piles
to the blowers, which impaired drainage (see photo). This
impaired drainage caused condensate to collect in the pipes
and block the air flow, again resulting in anaerobic
conditions in the pile. The pipes were repositioned to
28
-------
STATIC PILE COMPOSTING OPERATIONS
Sharp dips in the aeration pipes to and from the blower impaired
air flow at Dickerson.
Rigid pipes with a drain hole between the pipe and blower helped
solve this problem.
correct this problem (see photo). Also, when the plastic
pipes became warm, they tended to sag and dam the
drainage channels, creating many small pools of leachate
and condensate on the site, which contributed significantly
to the odor problem. The solution to this odor source was
to install rigid piping with drain holes that allowed the pipes
to drain when the blowers turned off. This kept the pipes
free of liquid and maximized air flow.
Increasing Air Flow
The Dickerson site was originally designed with 1/s-hp
blowers based on the Beltsville model with no adjustment for
larger pile size. Each blower was connected to three 4-inch
(10-cm) lateral pipes via a 4-inch (10-cm) manifold under-
_
fM
x •
01
-C '
C
^ •
o
liJ
•
< .
o
1-
-09
•08
•07
•06
•05
•o;
•03
•02
•01
I —
—
•09
•08
•07
•06
•05
•Oi,
-03
•02
-01
123 123 123
V3HP V3HP. IMP
(."-MANIFOLD 6"- MANIFOLD 6* MANIFOLD
Figure 2.7. Air Flow Distribution between Laterals as a Function
of Manifold Size
neath the pile. However, the relatively low oxygen levels in
the piles indicated that this system was supplying inadequate
aeration to maintain aerobic conditions throughout the pile,
so the Dickerson staff began to investigate ways to increase
the system's effectiveness. One inexpensive improvement
was to replace the 4-inch (10-cm) manifold with a 6-inch
(15-cm) manifold. As Figure 2.7 illustrates, the smaller size of
the 4-inch (10-cm) manifold tended to cause most of the air
to flow through the inlet pipe closest to the blower. Installa-
tion of the 6-inch (15-cm) manifold resulted in higher airflow
and more even distribution between lateral pipes (laterals).
However, the airflow was still insufficient, so the 1/3-hp
blowers were replaced with 1-hp blowers, which are the
largest blowers that can be operated from the available
110-volt system and that could therefore be installed without
requiring expensive electrical power modifications. The 1-hp
blowers along with the larger manifold improved both air flow
and distribution, resulting in more thorough and consistent
pile aeration and decreased odors.
Another modification to improve air flow was to reorganize
the various materials within the pile to take advantage of their
different resistances to air flow. Figure 2.8 shows the
pressure drop per meter in vertical columns of different
composting materials. New wood chips are the least resistant
to air flow, followed by used wood chips. Unscreened
compost is more resistant, but generally not as resistant as
the sludge/wood chips mixture before composting. Screened
compost is the most resistant; however, the resistance of
both screened and unscreened compost can vary depending
on moisture content.
29
-------
STATIC PILE COMPOSTING OPERATIONS
Initially, the entire bed of each compost pile was constructed
of new wood chips because they are the least resistant to air
flow and produce a manifold effect under the pile. However,
this layout short-circuited the air flow through the bed, and
most of the air came out around the toe of the pile. To
prevent this short-circuiting, the beds were constructed with
a 20-foot (6-meter) wide layer of unscreened compost around
the circumference with new wood chips in the center
(Figure 2.9). The increased resistance of the unscreened
compost helped to force more of the air down through the
pile. Thus, the aeration through the sludge mix was increased
at essentially no increased cost.
Oxygen and Temperature Monitoring
In static pile composting, temperature should be monitored
at the coolest part of the pile, i.e., the toe, due to a lesser
degree of insulation there. A temperature of 55°C for three
days at the lowest point in the pile indicates that tempera-
tures throughout the pile were high enough to provide for
pathogen destruction. Oxygen levels, by contrast, are
generally inversely proportional to temperature and should
therefore be measured at their low point, which is usually at
the center of the pile where temperatures are the highest.
When the Dickerson site opened, both oxygen and tem-
perature were monitored at the ends of the pile within reach
of the 6-foot (2-meter) probes (Figure 2.9). Both the oxygen
and temperature measuring techniques were subsequently
changed. Oxygen was measured through polyethylene tub-
ing, which was placed in the center of the pile during pile
construction. Temperature was measured with thermocouple
wires, which were able to locate the point of measurement
more accurately and provide much more rapid measurements
than probes. These monitoring changes increased the overall
efficiency of the operation and provided much more useful
data on the composting process.
c
'E
o
>-
O
O
60
30
10
NEW
WOOD
CHIPS
USED
WOOD
CHIPS
MIX
SOURCE: Reference (2)
SCREENED
COMPOST,
30%
UNSCREENED
COMPOST
SCREENED
COMPOST
26%
.003 .01 .03 .10 .3
PRESSURE DROP PER METER DEPTH, kPa
Figure 2.8. Pressure Drop per Meter Depth in Vertical Columns of
Different Composting Materials
Inset shows monitoring points.
IIICITIIIMI SECTIII
in ii inn
Figure 2.9. Improved Pile Construction at Dickerson
Site II
After 2 years of operation, the Dickerson site was closed on
December 31, 1982, in accordance with the agreement with
the Citizens Advisory Committee. Site II was nearly com-
plete by that time. Figure 2.10 shows the layout of Site II.
Description
The site encompasses approximately 20 acres (8 ha) —
about half the size of the Dickerson and Western Branch
facilities. Haul trucks enter the site and unload sludge in the
mixing building and wood chips in the wood chip storage
facility. Wood chips are mixed with sludge using a Roto-
shredder and front-end loaders. The mix is then transported
to the two compost pads by dump trucks and formed into
piles using a front-end loader. Pile beds are constructed of
wood chips in the center and unscreened compost around
the circumference. Each pad is about 600 feet (180 meters)
long and 110 feet (35 meters) wide.
The aeration system consists of 6-inch (15-cm) flexible pipes
that are placed under the compost piles. The pipes are
spaced 5 feet (1.5 meters) apart and are connected to
15-horsepower blowers that line the compost pads. A total
of 56 blowers are used. The blowers are operated in the
suction mode and discharge into an underground header
system, which consists of two 30-inch (76-cm) manifolds
attached to a 36-inch (0.9-meter) header that discharges to
30
-------
STATIC PILE COMPOSTING OPERATIONS
Site II aerial view.
a central odor filter pile. Water spray nozzles are located in
the crown of the header for cooling and to scrub air for
odor control. Fifteen-horsepower blowers, as opposed to
the 1-hp blowers at Dickerson, were necessary to supply
the increased aeration required by the system. The need for
greater aeration was due to the central filter pile arrange-
ment. By having all the blowers discharge into one filter
pile, substantial back pressure — up to 28 inches (71 cm) of
water — is created, and this back pressure increases the
power requirements for aeration.
Leachate and condensate flow directly into channels where
they are shunted to underground pipes that discharge di-
rectly into the local sewer system. Storm water collects in
the runoff control pond (see photo) and is metered into the
sewer system in offpeak hours. The control pond is made of
8-inch (20-cm) concrete to enable a loader to drive into the
GENERAL F1MPOIE (UILDINO
WAITS WATER FLOW
yONiTOitma «TRUCTURE
TER PILE
POITINO F>AO( ^
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UIXINQ/ICREENINO
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ODOR FILTER PILE FOR
MIXINa/ICREENINO FACILITY
Figure 2,10. Site II Layout
31
-------
STATIC PILE COMPOSTING OPERATIONS
Runoff control pond at Site II, constructed of 8-inch (20-cm) con-
crete to enable loaders to drive into the pond and clean out any
sediment. Runoff that collects in the pond is metered into the
sewer.
pond and clean out sediment that may collect there,
thereby eliminating the potential for odors.
At Site II, the sludge/wood chip mixture is composted for
21 days. The piles are then torn down and the composted
mix is deposited in a hopper that discharges onto conveyor
belts. These belts transport the compost to the three Liwell
screeners inside a totally enclosed and heated screening
facility. Each screener is fitted with a hood to control dust.
During screening, air is exhausted through the hoods by
three 60-horsepower blowers to an outside filter pile
arrangement. The air control system was created in response
to public concerns over Aspergillus fumigatus and was
mandated by court decrees.
Wood chips recovered from the screening process are re-
turned to the mixing area, and the screened compost is taken
to the uncovered curing area (labeled "compost storage
area" in Figure 2.10), where it is cured for 30 days and then
distributed for use.
Other on-site facilities include a laboratory, truck scales and
wash area, and an administration building.
The Site II facility is unusual in that the composted mixture is
screened before, rather than after, curing. One advantage of
this approach is that a greater proportion of the wood chips
can be recovered and recycled because they are less decom-
posed than cured wood chips. This recycling helps to reduce
bulking agent costs. Screening before curing also reduces
the space required for curing, since screened compost is only
20 percent of the volume of unscreened compost. The
reduced land requirements can significantly reduce capital
costs.
At Dickerson, attempts to cure screened 21-day-old compost
without aeration had resulted in highly odorous material. The
original Site II design did not include power to the curing
pads; however, in light of the Dickerson data, the Site II
curing pads were retrofitted with 1-hp blowers. Subsequent
experiments indicated that the oxygen demands during
curing were much lower than those during composting and
could be supplied by a 1-hp blower in the positive pressure
mode. As a result, the curing piles are now aerated continu-
ously at this low level until the compost is marketed. The
piles are monitored for oxygen to ensure that adequate
aeration is supplied.
Public Relations
The public relations program for Site II was extensive. The
site had been opposed from the concept stage, and the
experience at Western Branch had provided a valuable
lesson about the critical importance of dialogue with the
local citizens. One of the first public relations efforts was to
create and mail a brochure to approximately 60,000 local
residents within a one-mile (1.6 kilometer [km]) radius of the
site. The brochure explained the purpose of the site and
how it functioned, announced the site opening, and invited
local residents to visit.
In January 1983, representatives from each local community
met with site officials and formed the Citizens Liaison
Committee (CLC). The CLC met biweekly and was kept
continually informed about the status of the project by site
officials. Long before any sludge arrived, CLC representa-
tives were given a tour of the site and a description of
operations. An open house was organized and all local
citizens were invited to attend. Events included a ribbon
cutting, speeches by local politicians, site tours, and
giveaways of compost samples. The open house helped to
communicate the attitude that the facility was something to
be proud of and that it had nothing to hide. The event was
very successful, generated many positive comments, and
left most visitors very impressed. Since startup, the CLC
has continued to meet, although less frequently, to air any
concerns that may arise. Local residents are encouraged to
visit the site, and they do so frequently.
Another component of the public relations program was
monitoring of Aspergillus fumigatus in response to citizens'
concerns. High levels of the fungus were found on site,
however, levels at the site border and offsite were negligible
compared to background levels. There was some initial
concern about the health of site workers. However, no
32
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STATIC PILE COMPOSTING OPERATIONS
incidents of respiratory problems associated with com-
posting were reported, so it was decided that no extra-
ordinary measures were necessary to protect workers.
Further discussion on public relations at Site II can be found
in Reference [3].
Process Modifications
Three process modifications — concerning the aeration
system, odor control, and compost moisture reduction —
are discussed in this section. Information on additional
modifications can be found in Reference [4].
Aeration System Modifications
On April 25, 1983, enough sludge was received at Site II to
construct five test piles to determine the most appropriate
aeration rates and timing sequence. Tests were run to
measure aeration rates, back pressures, pile temperatures,
and oxygen levels. These tests indicated that back pressure
in the manifold varied considerably as the various blowers
that discharged into it were turned on and off. The variation
in back pressure in turn affected aeration rates. The solution
to this problem was to install a temperature controller that
caused the blower to stay on until the temperature in the
pile center fell below a specific temperature. The aeration
rates were thus made independent of the back pressure. It
was also hoped that temperature optimization would result
in more thorough drying and a more stable material after
21 days. Further tests were run with temperature controllers
in place, and on the basis of these results, temperature con-
trollers were permanently installed at Site II.
The temperature controller was wired in parallel with a timer
that maintains minimum aeration based on a cycle of 5 min-
utes on, 15 minutes off. Whenever the pile temperature
exceeds the set level, the temperature controller auto-
matically extends the on cycle by up to 15 minutes. Thus, if
necessary, the controller can cause the blower to run
continuously.
The 5-minute-on, 15-minute-off cycle was based on data
like those in Figure 2.11. These oxygen measurements were
taken around the fourth day of composting when the initial
peak of microbial activity is expected. The blowers were
turned off at time = 0. In less than 15 minutes, the oxygen
levels had fallen below the critical 5 percent level; within
35 minutes they were close to zero. The blowers were
turned back on after 38 minutes, and after 5 minutes aera-
tion the pile oxygen levels were satisfactory. Similar
measurements on curing piles indicated that oxygen levels
could drop from 20 to 0 percent within 2 hours. For this
reason, aeration systems were installed for curing, and a
CFH/DT = ft3/hr/dryton
SOURCE: Unpublished data,
Washington Suburban
Sanitary Commission.
BLOWER OFF .@ TIME = 0
BLOWER ON ® TIME = 38 MIN.
I I I
5 10 15 20 25 30 35 40 45
TIME (MINUTES)
Figure 2.11. Oxygen Depletion in a Compost Pile with Blower
Turned Off and On
Figure 2.13 shows location
of the two points.
p ».
f
s »-
SOURCE: Unpublished data,
Washington Suburban
Sanitary Commission.
- POINT MO a
- POINT NO •
Figure 2.12. Temperature Differences at Two Points in a Site
Compost Pile with Uniform Pipe Layout
backup generator was installed to prevent anaerobic condi-
tions and resultant odors in the event of power failure.
Another startup problem was the differential in air flow from
one end of the pipe to the other due in large part to the
15-horsepower blowers. This air flow differential resulted in
huge differences in temperature along the length of the pile
(Figure 2.12). To solve this problem, Site II engineers used a
33
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STATIC PILE COMPOSTING OPERATIONS
computer program and trial-and-error to design a piping sys-
tem that would counteract this effect. To even out the air
flow, the number of perforations were varied along the length
of the pipe. Several different variations were tested; the
optimal system is illustrated in Figure 2.13. The first 16 feet
(5 meters) of perforated pipe out of the blower contain 0.44
square inches of open area per linear foot (9.2 square cm per
meter [cm2/m]); the next 16 feet (5 meters) contain double
that amount — 0.88 square inches of open area per linear
foot (18.4 cm2/m); the next 31 feet (9 meters) contain
1.7 square inches per linear foot (36 cm2/m); and, in the
remainder of the pipe, the open area is more than doubled to
4.3 square inches per linear foot (90 cm2/m). Figure 2.14
shows the more uniform temperatures within the pile that
resulted from improved air flow with the modified pipe
layout.
Odor Control
One major incentive for perfecting the Site II aeration system
was to minimize odor generation. A study on odor [5] funded
by the Washington Suburban Sanitary Commission identified
five compounds that were principally responsible for odor in
sewage sludge: hydrogen sulfide, methyl mercaptan, methyl
sulfide, dimethyl sulfide, and sulfur dioxide. These are all
sulfur compounds with relatively low odor thresholds. Their
concentration in a compost pile tends to increase as the
temperature rises (Figure 2.15). (Ammonia is also an odor
source at composting sites, but has a much higher threshold
and dissipates more rapidly.) Thus the major approach to
odor control at Site II was to control pile conditions so as not
to generate odorous compounds.
Data from Site II indicate that the odor filter piles at the site
are essentially cosmetic. A comparison of the concentrations
of odorous compounds in the manifold and in gases exiting
the filter pile shows that virtually no scrubbing has taken
place. These findings concur with laboratory data [6] that
show that compost is not an effective odor scrubber at high
moisture levels.
Site II engineers have been investigating possible backup
odor control systems for use in case of odor generation due
to temporary failure of some part of the composting system.
The initial backup system investigated was a water-based
system using spray nozzles fitted in the crown of the 36-inch
(91-cm) header. This system was ineffective. Currently,
systems that use chemical oxidizers are being investigated.
The first tests with high concentrations of hydrogen peroxide
introduced into the manifold system through one of the
blowers did result in a detectable reduction of odors from the
filter pile. The next step will be to see if atomizing the
oxidizer will result in even greater odor reduction.
Figure 2.13. Open Area per Linear Foot in Modified Aeration Pipe
Along the Length of the Compost Pile at Site II
SOURCE: Unpublished data,
Washington Suburban
Sanitary Commission.
Hi;; |H | Figure 2.13 shows location of the three points.
Figure 2.14. Temperature Differences at Three Points in a Site II
Compost Pile with Modified Pipe Layout
SOURCE: Unpublished data, Washington Suburban Sanitary
Commission.
Figure 2.15. Maximum Pile Temperature, Minimum Pile Oxygen
Level, and Dimethyl Disulfide Concentration in
Exhaust as a Function of Time
34
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STATIC PILE COMPOSTING OPERATIONS
Table 2.1 Effect of Positive and Negative Aeration on Initial
(Day 0) and Final (Day 21) Moisture Content
of a Compost Pile
Positive
Pressure
Average
Negative
Pressure
Average
Initial
Moisture
Content (%)
62.6
64.8
64.4
64.2
64.0
62.6
62.7
62.4
61.3
62.3
Final
Moisture
Content (%)
54.2
56.4
59.5
55.2
56.3
54.2
58.0
54.0
53.7
55.0
SOURCE: Unpublished data, Washington Suburban Sanitary Commission.
Table 2.2 Comparison of Moisture Content of Compost Piles with
Two Sludges of Different Volatile Solids Content
Initial (Day 0)
Moisture
Content (%}
Blue Plains
(30 to 40% volatile solids)
Average
Western Branch
(70% volatile solids)
Average
64.7
64.3
65.9
65.1
63.4
64.7
62.9
60.8
61.9
Final (Day 21)
Moisture
Content (%)
58.3
57.5
59.3
57.2
56.2
57.7
51.4
53.1
52.3
SOURCE: Unpublished data, Washington Suburban Sanitary Commission.
Compost Moisture Reduction
To successfully screen compost, its moisture content should
not exceed 50 percent. Site II compost had a moisture
content of over 50 percent, which necessitated a drying step
before screening. Several experiments were tried to reduce
the moisture content. Table 2.1 shows the results of tests to
compare the effect of positive and negative aeration on
moisture content. One study [7] indicates that positive-
pressure aeration provides a greater reduction in moisture
content than negative aeration, however, at Site II the
difference was not substantial.
Another experiment involved tests to determine the effect of
sludge volatile solids content on compost moisture levels.
3500i
3000
§
J 2500
fe
£
|2000
Q
& 1500
<
1000
500
BLUE PLAINS
Jill I I I I I I I I
12345
I I I I I
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
TIME (DAYS)
Figure 2.16. Comparison of Aeration Rates Demanded by the Blue
Plains and Western Branch Sludge
Using temperature-controlled aeration, Blue Plains sludge,
which had a volatile solids content of 30 to 40 percent was
composted alongside Western Branch Wastewater Treatment
Plant sludge, which had a 70 percent volatile solids content.
Figure 2.16 compares aeration rates in the two piles. The
Western Branch sludge had a much higher aeration demand
than the Blue Plains sludge. The total air flow through the
Western Branch sludge was 20.1 million ft3 (563,000 m3),
compared to 16.7 million ft3 (468,000 m3) through the Blue
Plains sludge. However, these higher rates did not translate
into significantly lower moisture content (Table 2.2).
Another approach to moisture reduction has involved
restacking the piles. Each time a pile is torn down and
stacked up, moisture is released, largely in the form of
steam. Experiments at Site II indicate that tearing the piles
down and then restacking them for an additional 2 or 3 days
reduces moisture by about 7 percent.
Another modification being considered is to cover more of
the pad area. The mixing process is also being analyzed in an
effort to improve air distribution, odor control, and moisture
control. Preliminary tests indicate that pugmill mixers may
provide a better mix than rotoshredder mixers.
Marketing
Marketing of sludge compost was first initiated by Maryland
Environmental Services (MES) in 1981. The products being
marketed at that time were Dickerson and Western Branch
compost. Since the MES, as a statewide utility, was pre-
35
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STATIC PILE COMPOSTING OPERATIONS
eluded under state law from acting as a retailer, it began its
marketing efforts by approaching private fertilizer marketing
firms. These firms showed no interest in serving as compost
retailers, so the MES created a team consisting of a
consultant, an agronomist, and a dispatcher to develop a
marketing program. One of the first steps in the program
was to develop a product name — "COMPRO" — and an
associated logo (Figure 2.17).
A general distribution permit was obtained from the Mary-
land Department of Health and Mental Hygiene. This permit
allowed the compost to be distributed for all uses except on
food chain crops. The compost could be used on food
chain crops providing an individual permit was first obtained
from the same agency. The distribution permit requires that
the levels of various heavy metals and organic chemicals in
the compost be below specified limits, which are based on
the U.S. Department of Agriculture's Agriculture Informa-
tion Bulletin publication [8]. Levels of heavy metals in
COMPRO are generally very low because the Blue Plains
Treatment Plant serves a primarily residential community
with few industrial dischargers. Site II periodically monitors
the compost for these substances and supplies the data to
the Maryland Health Department. The compost has never
exceeded these limits. In fact, the record has been so good
that the Health Department has reduced the frequency of
testing for organic chemicals.
COMPRO is sold for $4 per yd3 ($5.20 per m3) at the produc-
tion site. Transportation and handling costs increase the retail
price of bulk material to $15 to $30 per yd3 ($19 to $38
per m3).
Distribution has not been a problem at Site II. Demand gen-
erally exceeds supply. Figure 2.18 shows the volume
distribution of 1983 COMPRO sales by user category. At that
time, virtually all sales were to bulk users because COMPRO
was not available in bag form. The primary users are land-
scapers and contractors who account for 40 percent of the
total sales. Institutions such as universities, schools, and
parks use about 25 percent of the material. A network of 50
to 100 retail outlets in Maryland, northern Virginia, and the
District of Columbia handles 23 percent. The remainder is
sold to nurseries, golf courses, and topsoil dealers. The price
is too high for the agricultural market. In the spring of 1984
packaged COMPRO became available, enabling distribution
at the retail level to individuals and small-scale users. Bagging
is performed by three different concerns. Bagged compost is
sold to retailers in bulk quantities. Individuals cannot buy
compost at the site as it was felt this would create traffic
problems and would put Site II in competition with their
retailers.
AGRICULTURAL COMPOST
Figure 2.17. Logo for Maryland Environmental Services Compost
Product
GOLF COURSES
1.3%
NURSERIES
2%
TOPSOIL DEALER
8.4%
LANDSCAPERS
a
CONTRACTORS
Figure 2.18. Volume Distribution 1983 Compro Sales by User
Category
36
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STATIC PILE COMPOSTING OPERATIONS
References
(1) Willson, G.B.; J.F. Parr; E. Epstein; P.B. Marsh;
R.L. Chaney; D. Colacicco; W.D. Burge; L.J. Sikora;
L.F. Tester; and S.B. Hornick. 1980. Manual for Composting
Sewage Sludge by the Beltsville Aerated Pile Method. Joint
USDA-EPA Publication, U.S. Department of Agriculture,
Beltsville, Maryland. EPA 600/8-80-0220.
(2) Willson, G.B.; J.F. Parr; and D.C. Casey. 1979. Basic design
information on aeration requirements for pile composting. In:
Proceedings Natl. Conf. on Municipal and Industrial Sludge
Composting. Information Transfer, Inc., Silver Spring,
Maryland.
(3) Yeaman, B. and J. Walker. 1985. Sludge composting —
learning from experience. Part III. Community relations and
summary. Operations Forum 2(6). In Press.
(4) Yeaman, B. and J. Walker. 1985. Sludge composting —
learning from experience. Part I. Montgomery County's Site
11. Operations Forum 2(4): 11 -14.
(5) Washington Suburban Sanitary Commission. In-house report.
Unpublished.
(6) U.S. Department of Agriculture. Beltsville, Maryland.
Unpublished data.
(7) Finstein, M.S.; F.C. Miller; P.F. Strom; ST. MacGregor; and
K.M. Psarianos. 1983. Composting ecosystem management
for waste treatment. Bio /Technology 1(4). June 1983.
(8) S.B. Hornick; L.J. Sikora; S.B. Sterrett; J.J. Murray;
P.O. Millner; W.D. Burge; D. Colacicco; J.F. Parr;
R.L. Chaney; and G.B. Willson. 1984. Utilization of Sewage
Sludge Compost as a Soil Conditioner and Fertilizer for Plant
Growth. U.S. Department of Agriculture, Agriculture Infor-
mation Bulletin No. 464. Available from U.S. Government
Printing Office, Washington, D.C. 20402.
37
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3. Experience at a Windrow Composting
Facility: Los Angeles County Site
This chapter presents the experiences of composting opera-
tions at the Sanitation Districts of Los Angeles County's
Joint Water Pollution Control Plant (JWPCP) in Carson,
California. Windrow composting has been performed at this
site since the early 1970s. Currently the plant composts
120 dry tons (109 dry metric tons) of sludge per day.
Description of the Los Angeles County
Sewage System
The Sanitation Districts of Los Angeles County (Figure 3.1)
has provided municipal wastewater collection, treatment,
and disposal services for most of urban Los Angeles County
since 1928. The system is exceptionally large, serving
slightly under 4 million people and treating approximately
475 million gal (1.8 billion liters) of wastewater per day. It
consists of a main wastewater treatment plant — the
JWPCP in Carson, California — and five upstream water
reclamation plants that provide hydraulic relief to the
sewage system and high quality reclaimed water for poten-
tial reuse applications. These five plants have a combined
treatment capacity of 150 million gal (570 million liters) per
day. They generate raw (nondigested) primary sludges,
waste-activated sludges, and filter backwash sludges, which
are returned to the sewer system for conveyance to the
JWPCP solids processing facility.
Description of the Joint Water Pollution
Control Plant
The main wastewater treatment plant — the JWPCP —
treats approximately 350 million gal (1.3 billion liters) per
day of municipal wastewater. Current treatment processes
include advanced primary treatment for all wastewater flow.
In this process, anionic polymers are added to the waste-
water to improve the settling characteristics in the primary
sedimentation tanks. The JWPCP also further treats some
of this wastewater using a pure oxygen-activated sludge
secondary treatment system. The JWPCP has the capacity
to treat 200 million gal (760 million liters) per day with
secondary treatment but is not scheduled to reach that
capacity until the summer of 1988 due to the current
inability to dispose of all sludge that could be produced.
Until that time, approximately 125 million gal (470 million
liters) per day of secondary treatment will be employed. The
JWPCP has applied to the EPA for an exemption from full
secondary treatment requirements through the Section
301 (h) provisions of the Clean Water Act. Sludges gene-
rated at the plant include primary sludges from the primary
tanks and waste-activated sludges from the secondary
treatment plant.
Figure 3.2 shows an overall mass balance of the solids
disposal plan of the JWPCP in 1984. About 660 dry tons
(600 dry mt) of solids arrive at the plant each day. These
solids are removed from the wastewater and disposed of
through:
• Anaerobic digestion and burning of digester gas.
• Direct landfilling of sludge cake.
• Composting, which accounts for about 19 percent of the
solids (solids are reduced to 15 percent during com-
posting due to loss of 4 percent solids in the form of
carbon dioxide and water).
• Ocean discharge using a 2-mile (3.2-km) ocean outfall
diffuser system. The ocean discharge of solids is in full
compliance with Federal NPDES permit provisions and
requirements of the State of California for ocean
discharge.
Figure 3.1. Sanitation Districts of Los Angeles County
Joint Outfall System
V A BEST.
28 OTTO 4%
KELLOGG
102 DTPO 15%
Figure 3.2. Solids Disposal at the JWPCP
38
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WINDROW COMPOSTING FACILITY
Digestion takes place in 37 mesophilic anaerobic digesters,
which have a detention time of approximately 17 days. The
digesters produce approximately 7 million ft3 (200,000 m3) per
day of gas, which is used on site to power effluent pumps or
to generate electricity.
Solids from the digesters are dewatered using 44 basket
centrifuges, which produce sludge cakes of approximately
22 to 23 percent solids. The JWPCP also uses 19 newer, low-
speed state-of-the-art scroll centrifuges, which are capable of
producing a sludge cake of 25 to 26 percent solids. All
sludges from the centrifuges are discharged onto conveyor
belts and carried to a storage facility, which consists of
12 storage silos, having a total storage capacity of 6,600 wet
tons (6,000 wet mt).
The dewatering operation occurs 24 hours a day, 7 days a
week. However, sludge processing can only occur during
daylight hours, 6 days per week. During those periods when
the sludge cannot be processed, the JWPCP stores it in the
12 silos. During hours when the sludge is handled, the solids
are removed from the storage silos and loaded into trucks for
conveyance either to a landfill where the solids are codis-
posed with municipal refuse, or to an adjacent field on the
plant site where the solids are composted. The final product
of the composting process is sold to a fertilizer manufac-
turing company, the Kellogg Supply Company.
Composting operations are severely affected by rainfall
during winter months due to reduced drying rates and the
tendency of young and new windrows to slump and collapse.
Since the ability to landfill sludge is also limited during rainy
periods, EPA has authorized the JWPCP, through July 1988,
to discharge sludge to the ocean during extreme wet periods
when the storage silos are filled to maximum capacity. The
JWPCP plans to build a new sludge incineration facility and
to increase the storage silo capacity by 50 percent, thus
providing the capability for onsite disposal of all sludge,
including that generated by 200 million gal (760 million liters)
per day of secondary treatment, regardless of weather condi-
tions. These new facilities are planned to be operational by
July 1988.
Historical Perspective of Sludge Disposal
at JWPCP
Air-dried Lagoon Sludge (1928-1950s)
In 1928, when the JWPCP was constructed, essentially no
residential development surrounded the plant, so odors
from the plant did not create a nuisance. From 1928
through the mid-1950s, sludge was merely air dried in
lagoons at the site. By the early 1950s, the plant had
expanded considerably, and residential development
encroached on the site. At that time, the plant's entire east
side consisted of sludge lagoon drying basins that produced
considerable odor emissions because of the anaerobic
nature of the sludge. This area also attracted large quanti-
ties of insects. By the late 1950s, sludge lagoon air drying
was no longer tolerable to the surrounding community.
Air-dried Centrifuged Sludge (Mid-1950s to Late 1960s)
In the mid-1950s, the Sanitation Districts began using
centrifuges to dewater sludge and return centrate to the
treatment plant. The centrifuges produced a sludge cake
that was fairly dry — approximately 35 percent solids.
However, only about 30 percent of the solids that entered
the centrifuges were actually captured. The other 70 per-
cent remained in the centrate and were discharged along
with the plant's primary effluent into the Pacific Ocean. The
sludge cakes produced by these centrifuges were statically
air dried. They were significantly less odorous and initially
required less land during drying than the lagooned sludge.
By the mid-1960s, areas very close to the plant had been
developed for residential use. By this time, the JWPCP had
removed the sludge lagoon beds from service; however, by
the late 1960s, even static air drying was no longer accept-
able to the community. The population of the JWPCP
tributary area had increased by 40 percent between the
mid-1950s and the mid-1960s, resulting in increased
quantities of sludge cake being placed in deeper piles on
larger areas of land. The interior of these piles did not dry,
but remained wet and anaerobic for the 1 or 2 years they
remained on the field. When the piles were picked up with a
front-end loader, significant amounts of hydrogen sulfide
were released.
Sludge Marketing (1928 to Present)
From about 1928 to the present, the responsibility for
ultimate disposal or utilization of the dried solids from the
lagoons, the drying beds, and the present compost field
belonged to the Kellogg Supply Company, which has
always leased property from the JWPCP on a portion of the
site. Up to the mid-1950s, Kellogg sold all the dried sludge
material to citrus farmers located in Los Angeles and
Orange Counties. These were bulk sales; there were no
sales to the residential community. However, in the late
1950s and early 1960s, the citrus area shrank due to
expanded residential development in Los Angeles County.
Costs to haul the material to the residual citrus groves in
distant areas became prohibitive, so starting in the late
1950s, Kellogg began to market the dry sludge material in
bags to the residential homeowner market through local
nursery supply stores. Today, approximately 90 percent of
39
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WINDROW COMPOSTING FACILITY
Kellogg's sales are to chain store garden shops and
independent nurseries that retail the products.
Compost Pilot Studies (Late 1960s)
By the late 1960s, there was considerable debate about the
future of the sludge drying and fertilizer manufacturing
process due to the odor problem and to the Kellogg prod-
ucts' limited success in competing against cheap, synthetic
chemical fertilizers. This competitive disadvantage in turn
contributed to the growing piles of drying sludge. The
Sanitation Districts' staff debated how the solids should be
disposed of in the future, and serious consideration was
given to the suggestion that most of the sludge be codis-
posed with municipal solid waste in a municipal landfill,
thereby eliminating the problems posed by the site's
proximity to the residential area.
It was also suggested that small-scale composting experi-
ments be conducted to determine whether composting could
solve the odor problem. Initially there was much skepticism
regarding this idea due to the belief that odors would
increase as the rows of anaerobic, drying sludge cake were
turned. But, in fact, experiments showed that odor emissions
were significantly reduced compared to static air drying,
providing that (1) the composting material had an initial solids
content of 40 percent (this percentage was achieved by
recycling a certain amount of dry sludge and mixing it in with
the wet cake) and (2) the mixture was turned daily by a
rototiller.
Compared to the current composting process, initial com-
posting methods used at the JWPCP were quite primitive. To
construct windrows, a layer of dried sludge was spread,
using an earth mover, on the field along the axis of the
proposed windrow, and a layer of sludge cake was dumped
from end dump trucks on top of the dried sludge cake. A
Petibone speed mixer (a large rototiller pulled by a tractor)
was then employed to mix together the dry and wet mate-
rials. Next, a road grader was used to split the mixed material
into several small windrows, which were turned for several
weeks, using the speed mixer, until they dried. The small
windrows measured only about 1.5 feet (0.5 meters) high,
8 feet (2.4 meters) wide at the base, and 400 to 500 feet
(120 to 150 meters) long. This method of windrow
construction was very slow and led to pilot studies to
evaluate different types of composting equipment and to
determine which were best suited for composting wastewater
sludge.
During the initial pilot studies, the Sanitation Districts used a
Terex-Cobey composting machine to build compost piles.
This machine picks up a row of sludge material and deposits
it a few feet from the original spot. However, because the
JWPCP sludges are heavy, the machine was unsuitable and
had a tendency to make wide, sweeping arcs rather than
straight rows. The JWPCP evaluated several other machines
and found the Cobey Roto-Shredder to be very suitable for
the type of windrow composting that the JWPCP con-
ducted. Steam released from the windrows after turning
indicated a high temperature within the windrow — evidence
of good decomposition, something not achieved with the
windrows built with the Petibone speed mixer.
Shortly after it was put into service, the Cobey Roto-
Shredder produced smooth, level conditions on the new,
lime-stabilized dirt field. With time, however, a number of
valleys, hills, and ruts developed in the field and, as a result,
windrow sizes varied considerably. During winter conditions,
the field was wet and equipment dug holes in the field.
These holes affected the ability of the machine to produce
level, straight windrows.
First Full-scale Composting (1972-1977)
From about 1972 through 1977, the JWPCP handled 225
wet tons (205 wet mt) per day of 35 percent solids sludge
produced by the old centrifuges. Approximately 25 percent
of the final product was recycled to bring the initial starting
mixture up to 40 percent solids, the value necessary to
ensure adequate porosity, and thus aeration, throughout the
composting pile.
The operation worked very successfully as evidenced by
monitoring data from the early to mid-1970s, which showed
good temperature elevations within the windrows, good
drying rates, and a reduction of volatile solids indicating
that biological reactions were occurring. The JWPCP did
not monitor frequently for pathogens because they were nol
as great a concern as they are today; however, the
monitoring that was performed showed fairly good patho-
gen reduction, particularly of Salmonellae and total
coliform.
Briefly Expanded Operations (1977)
In the early 1970s, the sludge composted at the JWPCP
represented approximately only 30 percent of the solids
from the sludge stream. The other 70 percent that could nol
be removed by the centrifuges was returned to the plant for
ocean disposal. In response to orders by the U.S. EPA and
the State of California to cease ocean disposal of sludge,
the Sanitation Districts designed an advanced sludge de-
watering station that would, in theory, produce a 23 percenl
solids sludge with a 7-fold increase in sludge wet weight.
Based on the success of the then-existing composting
40
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WINDROW COMPOSTING FACILITY
operation with the smaller amount of sludge, the JWPCP
decided to handle the entire increased sludge volume
through composting. The JWPCP expanded their com-
posting field from approximately 10 acres (4 ha) to 40 acres
(16 ha).
The new centrifuges came on line in mid- to late-1977, and
increased sludge generation began to increase from 225 wet
tons (205 wet mt) per day up to the anticipated 1,600 wet
tons (1,455 wet mt) per day. However, the sludge cake,
which was supposed to be 23 percent solids, was frequently
17 to 19 percent solids. With that much water in the cake,
there was simply not enough dried compost to bring the
material up to 40 percent solids. Consequently, the mixture of
materials applied to the compost field was well below
40 percent solids, and anaerobic zones developed within the
windrows. Odors were emitted and local residents filed such
severe odor complaints, that, after only a few months of
operation, when the sludge production reached approxi-
mately 700 wet tons (636 wet mt) per day, the JWPCP
decided to reduce the volume of sludge composted to
whatever level was necessary to eliminate complaints, and to
landfill the remaining sludge. The maximum acceptable
sludge volume for composting was determined, on a trial-
and-error basis over the next several years, to be 500 wet
tons (455 wet mt) per day.
When the large-scale composting operation first began in
1977, the JWPCP experienced some equipment startup prob-
lems. They tried to use an Athey force-feed loader in con-
junction with a specially designed truck to efficiently remove
the dried compost from the field once the composting cycle
was complete. The plan was for the operator of the Athey
loader to drive down the windrow, pick up the dry material,
and place it on its conveyor, which was covered to minimize
dust generation. The material would rise up the conveyor and
fall off the end into the back of a specially designed, en-
closed truck, which would follow the force-feed loader down
the windrow. The driver of the enclosed truck would back up
the equipment to remain coordinated with the end of the
force-feed loader. After only a few days of operation,
however, it was apparent that the two equipment operators
could not coordinate their equipment well enough, and the
JWPCP abandoned the specially designed equipment in favor
of a front-end loader, which was found to be more versatile.
Another equipment problem experienced during startup was
with the Flow-Boy truck trailers that were initially used to
build windrows. These are trailers on which the sludge-
recycle mixture is dropped into a hopper and conveyed onto
the field in windrows using a chain-driven conveyor system.
These trailers were not designed for the heavy loads
imposed on them 10 hours a day, 6 days a week; one trailer
collapsed under the weight of the material it was hauling.
Another problem was that the movable flight configuration
discharged the material out the back of the truck very
slowly, so that the cake-recycle mixture would sometimes
bridge over the conveyor system, preventing discharge. The
JWPCP replaced the Flow-Boy trailers with a 42-yd3 (33-m3)
end dump truck, and a 42-yd3 (33-m3) horizontal-ram pusher
truck. Both these trucks are twice as productive as the
Flow-Boy trailers.
Process Changes (1980)
From about 1977 through 1980, the revised composting
operation, scaled down to minimize odor emissions,
appeared to be running smoothly. But in 1980, the JWPCP
discovered high levels of coliform and pathogens in the final
compost product. Previous data indicated that this was a
new phenomena and had not been a longstanding problem.
The problem was apparently largely due to a reduction in
sewage discharges by several large paper manufacturing
companies. This reduction was the result of an industrial
waste user fee system, established in the 1970s, that
required industry to pay for using the sewage system based
on discharge volume and strength (i.e., the amount of
organic material and suspended solids in the wastewater) of
the wastewater. Many paper companies with large, high-
strength discharges experienced very high user charges. It
became more economical for the paper companies to pre-
treat their wastewater and either landfill the sludge or reuse
the paper fibers.
Paper solids are rich in cellulose, which is readily
biodegraded in an aerobic composting environment, pro-
ducing high temperatures that destroy pathogens. The
reduction in paper mill wastewater thus reduced the fuel
(volatile solids) for composting, and the windrows were
unable to achieve adequate temperatures for pathogen
destruction. The problem was further increased by the large
surface-to-volume ratio of the windrows, which maximized
heat loss through the surface.
In addition to the reduction in fuel from the decreased paper
solids, the sludge cake had a lower concentration of fibrous
materials than before, and the compost tended to form balls
or clumps. The grinding action of the Cobey composter's
rotating drum, which rotated at approximately 250 revolu-
tions per minute (rpm), was inadequate for breaking up
these clumps. The resulting clumps in the final compost
appeared anaerobic in the center, suggesting they had not
been adequately composted.
41
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WINDROW COMPOSTING FACILITY
The solution to this problem was to eliminate the clumping
phenomena and to build larger windrows with lower
surface-to-volume ratios that help to conserve heat and
maintain high temperatures. The solution was achieved with
a new composting machine — the SCARAB I — which was
capable of building much larger windrows than the Cobey
Roto-Shredder. The 600-rpm speed of the SCARAB'S drum
— more than twice as fast as the Cobey — was effective in
breaking up the clumps. To make the machine work on the
heavy sludges at the plant, the JWPCP converted the
machine from hydraulic drive to a belt-drive arrangement.
Winter 1984 Experience
From 1980 until the winter of 1983-84, the process worked
successfully. However, during a cold spell, with
temperatures of 4.5°C in the winter of 1983-84,
temperatures within the windrows fell below the 55°C
recommended by EPA to ensure pathogen destruction. The
cause of this problem appeared to be the significantly
increased digester solids detention time, which resulted
from placing in service four new, very large anaerobic
digesters in the fall of 1983. The increased detention time
resulted in increased gas generation during digestion that
reduced the fuel value of the resulting sludge for com-
posting. The sludge had decreased volatility, indicating that
the extended digestion process had removed some of the
readily biodegradable material that had been available
previously for composting.
The solution to this problem was to enhance heat conserva-
tion by constructing even larger windrows with reduced
surface-to-volume ratios. This type of construction required
a very large experimental composter. Two years of prior
research at the JWPCP had demonstrated that this device
could construct much larger windrows than any other com-
poster, but that it worked well only on a paved field.
Coincidentally, the Sanitation Districts had just finished
paving a 25-acre (10-ha) composting site. These circum-
stances enabled the JWPCP to make a major revision to its
composting operation in March 1984.
Current Composting Operation
Figure 3.3 shows a block diagram of the windrow com-
posting operation. Dewatered, weighed sludge cake from
the storage silos is loaded into trucks. The trucks then travel
to the amendment sawdust stockpile area and pick up the
appropriate proportion of bulking agent — sawdust, rice
hulls, and/or recycled compost. The ratio of sawdust to
sludge is varied depending on the needs of the final
distributor — the Kellogg Supply Company. Approximately
10,000 yd3 (7,600 m3) of sawdust are on site at any one
AMENDMENTS
IRECYCLE. RICE
HULLS. SAWDUST!
STEP 1
ROWS PUSHED TOGETHER
WITH FRONT-END LOADER
I
STEP 2
..STEP 3
ROWS MIXED
WITH COMPOSTER
..STEP 4
WINDROW COMPLETED:
READY FOR COMPOSTING
Figure 3.3. Construction of Windrows
Figure 3.4. Plan of Compost Field
time. The sludge and sawdust are dumped in the field,
formed into rows by front-end loaders, and mixed by the
composter. Active composting then proceeds over a period
ranging from 41/z weeks in the summer to 13 weeks in the
winter, after which the compost is removed from the field.
Figure 3.4 is a schematic of the 25-acre (10-ha) paved field.
Note the drainage ditch on the south side and berms on the
42
-------
WINDROW COMPOSTING FACILITY
CONSTRUCT 6
SMALL WINDROWS
Weak 3-6
REMOVE COMPOST
CLEAR FIELD
Week 7-8
Week 9
(Note. The procedure was recently revised to eliminate the intermediate wind-
row step. Now, six small windrows are maintained until week 7-8, when they
are combined into one large windrow.)
Figure 3.5. Typical Compost Cell Operation
Figure 3.6. Asphalt Compost Field Design
east and south. There are nine separate cells on the site,
each with room for six separate windrows. Each cell goes
through a typical process as illustrated in Figure 3.5, which
shows a 9-week cycle. At the start of a particular cycle, the
cell is empty. During the first week, six small windrows are
constructed, one per day. Each windrow holds up to 500 to
525 wet tons (455 to 477 wet mt) of sludge cake and the
appropriate volume of amendment and is about 800 to
820 feet (244 to 250 meters) long. During cold winter
months, the amount of sludge in each windrow is reduced
to approximately 300 wet tons (270 wet mt) to reflect the
reduced drying rates during winter. During the first week
and throughout the second week the windrows are turned
daily with the smaller SCARAB I machine to promote rapid
evaporation and to increase the porosity of the material.
At the end of the second week, two of the windrows are
split; half the material is put into each adjacent windrow,
resulting in four intermediate-sized windrows on the field
and two blank spaces. For the next 4 weeks (weeks 3
through 6) the intermediate-sized windrows are turned with
the smaller SCARAB I machine about three times per week.
It is during this period that active composting takes place.
Internal windrow temperatures are about 54° to 64°C and
organic solids are actively degraded.
At the end of the sixth week, the four intermediate-sized
windrows are combined into a very large windrow and
turned by the large windrow composting machine at least
five times during the next 2 weeks. Very high temperatures
are achieved during this stage because of the very low
surface-to-volume ratio and the resulting conservation of
heat. This procedure satisfies EPA's requirement for patho-
gen destruction during windrow composting (i.e., main-
taining temperatures of 55°C for 15 days, during which time
the material is turned five times). It is not uncommon to
observe temperatures as high as 71 °C being maintained for
7 or more days during this 2-week period.
At the end of the eighth week, the composting cycle is
complete, and personnel from Kellogg remove the material
from the field and transport it to their site for subsequent
processing and bagging.
In recent months, the above procedures have been modified
by building two large windrows in each cell during the final
2-week period. Each of these two windrows consists of
material from three of the original six small windrows
constructed in that cell. This modification was necessary to
preserve the large composter, which broke down frequently
when attempting to turn the massive windrows. The large
composter is a one-of-a-kind machine, and no spare or
backup machine is available.
Compost Field
Figure 3.6 shows the current design of the compost field at
the JWPCP. The field consists of a 4-inch (10-cm) layer of
asphalt (2 inches [5 cm] of type C asphalt atop 2 inches
[5 cm] of type B asphalt), which overlies a 10-inch (26-cm)
base of crushed aggregate. The field has a 1 percent slope.
Precipitation drains into a ditch where it is shunted to a
storage pond. The drainage ditch, in conjunction with the
pond, has the theoretical capacity to handle the water from
43
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WINDROW COMPOSTING FACILITY
a 50-year rainstorm without any overflow (backup) onto the
field. In reality, since the JWPCP can discharge some of the
pond contents into the sewer system at certain hours every
day, the drainage system has essentially unlimited capacity.
All stormwater runoff from the compost field is returned to
the influent sewers of the JWPCP.
The compost field had originally been covered with lime-
stabilized dirt. However, this surface had a tendency to
become slippery and to develop holes in wet weather. The
field was resurfaced with asphalt a few years ago based on
tests that showed that operations during rainy spells were
far more productive on an asphalt surface than on dirt.
After a couple of years of operation, however, a substantial
amount of water had accumulated under the field. The
sludge had apparently caused the asphalt field to crack,
enabling water to penetrate upslope. According to the
Asphalt Institute, this effect commonly occurs on rural
roads near cattle crossings where cattle manure accumu-
lates. Based on their recommendations, the JWPCP applied
an oil slurry sealer atop the asphalt.
Sludge Conveyor System
The conveyor belts that transport sludge from the centri-
fuges to the 12 storage silos and to the loading stations are
over 1 mile (1.6 km) in length. The system is controlled by
one operator from a central control panel. Level indicators
show the amount of sludge in each silo. The operator con-
trols the amount of sludge withdrawn from each silo.
Although the conveyors are very reliable, they do have a
tendency to spill and require constant cleanup and mainte-
nance. One year ago, the JWPCP investigated an alterna-
tive sludge conveying system — a piston pump, which uses
a 40-hp hydraulic mover to drive sludge through an
enclosed pipe. This system has several advantages over the
conveyor system: substantially reduced spillage; no
limitations due to incline; and the ability to transport sludge
around 90° bends. These capabilities would enable buildings
at a facility to be spaced more closely together, thus the
system looks extremely promising for large facilities with
space limitations.
Research into Other Composting Processes
Forced-aeration and In-vessel Systems
As part of their research over the years to improve the
on-site composting process, the JWPCP has examined
three alternative composting methods: aerated static pile
composting, an in-vessel system, and a partially enclosed
forced-aeration system.
Static pile systems traditionally use large wood chips as a
bulking agent, which must be screened out and recycled at
the end of each composting cycle. In an effort to eliminate
the screening step, which was considered to be too dusty,
the JWPCP experimented using sawdust or dried compost
as a bulking agent during forced aeration. The experiments
showed that these materials did not provide sufficient
porosity for composting. Their weight compressed the pile
and sealed off all air passages; consequently, black
anaerobic zones developed, particularly at the pile bottoms.
The JWPCP also examined the Fermentechnik unit — a
totally enclosed mechanical composter from West Germany.
The unit uses auger screws and paddles to convey and
churn the mixture. All composting is done within a vessel,
which reduces the possibility for dust. Air is blown into the
vessel and exits through odor scrubbers. An auxiliary piece
of equipment can be used to dry the final sludge and
produce pellets. The JWPCP evaluated the pelletization
process as a means of reducing dust emissions during
sludge transport.
Finally, the JWPCP examined a partially enclosed, forced-
aeration system in which the material is composted in
thermally insulated, enclosed bins. Air headers set in a
concrete pad provide forced aeration. The material is turned
periodically with a front-end loader to enhance aeration.
The three experimental systems were compared to the
existing windrow compost process (Figure 3.7). All three
systems provided excellent temperature elevations enabling
pathogen destruction, but the existing windrow composting
process had far superior drying characteristics, producing
the driest material in the shortest time (Figures 3.8 and 3.9).
65
60
e
55
50 -
WINDROW
AERATED STATIC PILE
MECHANICAL COMPOSTER (NON-ENCLOSED)
MECHANICAL COMPOSTER (ENCLOSED)
10 15 20
NUMBER OF DAYS
Figure 3.7. Duration of Elevated Temperatures
44
-------
WINDROW COMPOSTING FACILITY
A START OF CTCLE
B END OF CYCLE
no
WHOM* AEHATtO MKHMNCM. MECHANICAL
STATIC-PILE COttPMTjER COMPOtTBI
Figure 3.10. Aeration System
Figure 3.8. Drying Characteristics
40
0 -
M DATS
7 DAYS
AERATED MECHANICAL MECHANICAL
STATIC-PILE COMPOSTER COMPOSTER
(NON-ENCLOSED) (ENCLOSED)
LONGITUOWAL SEC7WN
Figure 3.9. Drying Rates for Various Composting Schemes
Combined Forced-aeration/Windrow System
Based on these results, the JWPCP decided to continue
with windrow composting. However, for the past 18 months
they have been experimenting with a combined forced-
aeration/windrow approach to improve their existing
process. Figure 3.10 shows a schematic of one such system
with an aeration trough beneath the windrow. The aeration
trough is shown in Figure 3.11. It consists of a U-shaped
steel plate sealed with rubber gaskets and drilled to create
orifices. An expanded metal grate is placed around the
orifices and covered with large redwood bark chips. Aera-
CROSS SECTION
Figure 3.11. Aeration Trough
tion is provided by a 1,000-ft3/minute (28-m3/minute)
blower that can be operated in a negative or positive mode.
An odor scrubber and water sprays are used to reduce any
odors generated during negative pressure aeration. The
aeration trough also serves as a drainage channel for
leachate and condensate. Air distribution is monitored
throughout the aeration trough, and thermocouples monitor
the temperature of the windrows.
In one experiment with this system, a large windrow
containing approximately 60 percent by volume sawdust
was constructed. Half the windrow was constructed over
the experimental forced-aeration system; the other half was
not aerated. All other aspects of the piles, including the
turning schedule, were identical. Internal temperatures in
the forced-aeration section rose much more rapidly than
45
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WINDROW COMPOSTING FACILITY
.FORCED AERATED ZONE
' NATURALLY AERATED ZONE
4 8 12 18 20 24 28 32 38 40 44 48
Days Into Cycle
Figure 3.12. Effect of Forced Aeration on Internal Windrow
Temperatures (Sawdust Rich)
BLOWER SUCTION
23456
Depth Mow Surface, ft.
Figure 3.13. Temperature vs. Windrow Depth
those in the naturally aerated zone, but then dropped below
55°C after approximately 3 weeks (Figure 3.12).
In another experiment with the forced-aeration system, the
JWPCP manipulated the location of the hot zone by alter-
nating negative and positive pressure aeration. The results
of this experiment are provided in Figure 3.13. During the
first day, the blower was operated in a negative pressure or
suction mode. The suction drew the heat generated within
the pile downwards so that temperature increased with
increasing pile depth, with the hottest temperature found at
the base of the pile. (By contrast, a windrow pile that is not
mechanically aerated is normally cool at the base due to
heat loss to the ground.) The following day, the blower was
reversed, sending compressed air into the bottom of the pile
and out through the surface. This process concentrated the
hot zone in the pile center and warmed the surface. Further
research is being conducted to investigate the efficacy of
60
M
50
45
40
TURNING FREQUENCY: 2 TIMES / WEEK
3 TIMES / WEEK
S TIMES/WEEK
10
15 20 25
DAYS INTO CYCLE
30
40
Figure 3.14. Frequency of Windrow Turning vs. Drying Rate
40
30
10
15 20 25
Days Into Cycle
30 35 40
Figure 3.15. Effect of Frequency of Windrow Turning on Volatile
Solids Destruction
forced aeration for generating and maintaining high
temperatures at all points in the pile. Based on this
information, the JWPCP will decide whether to install
partial or full aeration at the facility.
Research into Factors Affecting Windrow
Composting
Turning Frequency
Many factors affect windrow composting. One factor is the
frequency of turning. In a windrow, all evaporation occurs
at the surface. Thus, the more frequently a windrow is
turned, the faster it dries because wet material from the
46
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WINDROW COMPOSTING FACILITY
70
60
50
40
30
TURNING FREQUENCY:
2 TIMES/WEEK
3 TIMES/WEEK
5 TIMES /WEEK
10 15 20 25 30
DAYS INTO CYCLE
35
40
Figure 3.16. Frequency of Windrow Turning vs.
Internal Temperature
T
MNLNWr
4WFT.
IXO'C
\wc
Figure 3.17. Frequency of Windrow Turning vs. Zones of
Internal Temperatures
^Immediately After Turning
Before Turning
4 6 8 10 12
Days Into Cycle
14
Figure 3.18. Effect of Windrow Turning on Oxygen Demand
center is brought to the surface where the evaporation rate
is rapid (Figure 3.14). Volatile solids destruction also occurs
more rapidly when a windrow is turned frequently due to
the resulting increased aeration (Figure 3.15).
Internal windrow temperatures are also affected by the
turning rate (Figure 3.16). Turning breaks up the hot zones
in compost piles by bringing cooler surface material into the
pile center and hot central material to the pile surface. The
more frequently the windrow is turned, the more difficult it
is to achieve and maintain a high internal temperature. In
one experiment, when the windrow was turned five times
per week, a temperature of 55°C was achieved, but was not
maintained for as long a period as when the windrow was
turned only two or three times per week.
Figure 3.17 shows how the size of the hot zone in a com-
post pile decreases as the turning rate increases. Note that
the hot zone tends to be skewed slightly toward the direc-
tion of sunlight.
In one study, the oxygen demand of the sludge material was
calculated by operating the aeration system in the suction
mode and monitoring the oxygen depletion in the air stream
as it passed through the windrow. The study showed that
turning stimulates the biodegradation, probably by exposing
fresh organic material surfaces to the microorganisms.
Oxygen demand readings taken immediately before turning
were lower than similar readings taken within minutes after
turning (Figure 3.18). This effect occurs throughout the
compost cycle life.
The objective in turning is therefore to strike a balance
between enough turning to ensure adequate aeration in all
sections of the pile, yet not so much turning that heat is
lost and high temperatures are not maintained for an
adequate period of time.
47
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WINDROW COMPOSTING FACILITY
Effects of Bulking Agents on
Pile Temperature and Odor Emissions
The JWPCP uses three bulking agents in their composting
operations: dry compost, sawdust, and rice hulls. Dry
compost was used exclusively until the late 1970s. At that
time, the JWPCP began to add sawdust or rice hulls at the
start of the compost cycle to produce different products for
the Kellogg Company. Currently, several different blends of
bulking agent are used. Figure 3.19 shows the carbon:nitro-
gen ratios of various bulking agents and sludge/bulking
agent combinations. Figure 3.20 shows that the internal
windrow temperatures vary with the type of amendment
used. Rice hulls and sawdust sustain higher temperatures
for a longer period of time than dry compost because of
their higher carbon and nutrient levels. The type of bulking
agent also has a substantial effect on odor emissions
(Figure 3.21).
Effects of Windrow Size and
Aeration on Drying Rates
The JWPCP conducted research to examine the effect of
windrow size on drying rates in naturally aerated (turned)
and forced-aeration windrows (Figures 3.22 and 3.23).
Small, naturally aerated windrows dried fastest and
achieved the required temperature of 55°C in the shortest
period of time. The large, forced-aeration windrow dried
faster than the large windrow that was aerated by turning.
It achieved almost as high temperatures as the small wind-
row, whereas the large, turned windrow took a considerable
amount of time to reach the required 55°C temperature.
s.
400
300-
200-
100
0J
Optimum Value
Rice Sawdust Rice Sawdust/ Recycled
Hulls 'Hulls/ Sludge Compost/
Sludge Mixture Sludge
Mixture Mixture
Figure 3.19. C:N Ratios for Various Agents and Mixes
y
1015202S303S4045SO
DAYS INTO CYCLE
Figure 3.20. Type of Amendment vs. Windrow Internal Temperature
700 i
= 0100
45%
Recycle
39%
Recycle
16%
Rice Hulls
Amendments
71%
Rice Hulls
Figure 3.21. Windrow Surface Odor Emissions
40 60 00
Figure 3.22. Drying Rates of Three Windrow Types
48
-------
WINDROW COMPOSTING FACILITY
SMALL,
NATURAL
AERATED
LARGE,
FORCE-
AERATED
LARGE,
IATURAL
AERATED
10 20 30 40
Days Into Cycle
50
60
651
UNCOVERED
15
20 25
Days Into Cycle
30
35
Figure 3.23. Internal Temperatures of Three Windrow Types
Figure 3.25. Effect of Sunlight on Windrow Internal Temperatures
60i
I55"1
"o
CO
"50-
o
45
UNCOVERED
COVERED
10 15 20 26
Days Into Cycle
30
36
Figure 3.24. Effect of Sunlight on Drying Rate
1.5-1
1.0
.UNCOVERED
10 15 20 25
Days Into Cycle
30
35
Figure 3.26. Effect of Sunlight on Windrow Surface
Evaporation Rate
Benefits of Sunlight
Visitors to the plant frequently ask why the JWPCP does
not place the entire operation indoors in a huge aircraft or
factory-type building since rain in the winter and odor
emissions in summer limit the amount of composting that
can take place. One reason that JWPCP does not compost
indoors is that sunlight substantially benefits the com-
posting process. In one experiment, the JWPCP covered a
20-foot (6-meter) windrow with black plastic on a wooden
frame. The plastic shielded the windrow from the sun but
permitted sufficient air penetration. The device was
removed only to enable turning. Another, similar windrow
was left uncovered and turned with the same frequency.
This experiment showed that the uncovered area, which
was exposed to sunlight, dried considerably faster
(Figure 3.24), had higher internal temperatures (Figure
3.25), and had a consistently higher surface evaporation
rate (Figure 3.26). Based on these findings, the JWPCP
determined that placing the entire process indoors would
not be beneficial since the number of sunny days far exceed
the number of rainy days in the Los Angeles climate even
during the winter months.
Important Requirements and Objectives
As a municipal agency, the JWPCP must ensure that both
its process and products are acceptable to the public. This
task involves continual attention to several key parameters
including:
• Odor monitoring and control.
• Pathogen destruction.
• Heavy metals content of compost products.
• Public relations.
49
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WINDROW COMPOSTING FACILITY
Odor Monitoring and Control
Odors are perhaps the greatest constraint to composting at
the JWPCP. During the afternoon, prevailing winds carry
odors into the heavily populated residential community
southeast of the site. Odor complaints are received when-
ever the amount of sludge composted exceeds the capacity
of existing wind conditions to disperse and dilute the odors.
This is particularly a problem in the summer and is magni-
fied by the fact that people keep their windows open and
are outdoors more at that time of year.
Odor generation at the JWPCP is routinely monitored based
on the American Society for Testing and Materials
D 1391-57 Odor Panel Evaluation Technique, which involves
sniffing of ambient air samples by an odor panel. Samples
of ambient air are taken by placing a 1-foot (0.3-meter)
square by 0.5-foot (0.15-meter) deep box on the windrow
surface. A vacuum pump draws ambient air into an inlet
hose on the side of the box and through an activated car-
bon filter to remove any odors. The air entering the box is
thus odor free. Inside the sampling box several baffles force
the air to travel a serpentine path along the windrow
surface. The air and any odorous compounds that have
been picked up then travel through a hose into a bag,
which is sealed and returned to a laboratory where a panel
of citizen volunteers sniff the odor in this bag. If they
cannot detect an odor, the sample is considered to be odor
free. If the panel does detect an odor, then an additional
sample is taken from the bag and diluted with clean air until
the sample is odor free. The number of dilutions required
indicates the strength of the odor. The quality of the odor
(i.e., whether it smells pleasant or unpleasant) is not
considered. This highly subjective test requires many data
measurements to develop reliable information on the odor
emissions from a particular operation.
Data from several years of odor panel evaluations have
shown that 83 percent of the total odor produced during the
life of a windrow is emitted between turnings, and the
remaining 17 percent is emitted during and immediately
after turning (Figure 3.27). Among those odors which occur
between turnings, the most repulsive odors occur in the
initial days of the cycle (Figure 3.28). After about 10 or 12
days, surface odor emissions drop to a fairly low, constant
background level. During turning, odor generation is rapidly
elevated, but returns to baseline levels within a few minutes
of completion of turning (Figure 3.29). This phenomenon of
peak, intense odor emissions that die off very rapidly is the
same regardless of when during the compost cycle turning
is performed.
Tests with a forced-aeration windrow show that the rate of
odor emissions is a function of surface temperature. In the
positive-pressure aeration mode, the temperature at the
OU = odor unit
Figure 3.27. Sources of Windrow Surface Odor Emissions
15 20 25
Days Into Cycle
30 35 40
Figure 3.28. Average Windrow Surface Odor Emissions
-
E
UJ
25
20-
15
O | 10
0 10 20 30 40 50 60 70 80 90 100
Minutes After Turning
Figure 3.29. Surface Odor Emissions After Windrow Turning
windrow surface increases as air travels through the hot
zone on its way to the surface. Odor increases dramatically
with increasing surface temperature in windrows containing
either sawdust or dried compost as a bulking agent
(Figure 3.30).
50
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WINDROW COMPOSTING FACILITY
35
I 30
o d20
0|
ID 3 15
10
60% SAWDUST
25 30 35 40 45
Surface Temperature, °C
50
Table 3.1 Scrubber Performance
Figure 3.30. Windrow Surface Temperature vs. Surface Odor
Emissions
BLOWER EXHAUST
Wc^Huto
Figure 3.31 Odor Emissions from Forced Aerated Windrows
In the suction or negative-pressure mode, odors are emitted
by the exhaust system. Up to 90 percent removal can be
achieved with some odor-scrubbing media; however, even
at this reduced level, exhaust odors generally equal or
exceed surface odors (Figure 3.31).
JWPCP researchers have experimented with several
approaches and devices for odor control. One relatively
simple approach is the use of water trucks to control odors
associated with dust particles. Another approach is the use
of a packed tower odor scrubber that can use different
scrubbing media: water, permanganate, sulfuric acid, and a
1 percent bleach solution (Figure 3.32). The effectiveness of
all these media has been fairly low, ranging from about
43 percent with straight water to 61 percent with permanga-
nate (Table 3.1).
Scrubbing
Agent
Water
KMN04(0.5lb/gal)
H2S04(0.1N)
NaOCI(1%)
Number
of Runs
13
8
4
8
Average Odor
Removal (%)
43
61
57
45
MBHHtt
Figure 3.32. Odor Scrubber Assembly
Figure 3.33 shows the results of experiments with different
odor-scrubbing devices. The simplest scrubber — a bed of
dry compost — was quite ineffective. Wet scrubbers re-
moved 43 percent of odors; activated carbon removed
90 percent; and a wet scrubber with an activated carbon
unit gave the best overall performance with 95 percent
removal. Unfortunately, the more effective devices are
extremely costly.
Over a period of years, the JWPCP has been approached by
a number of companies that claim to have novel odor-
reduction approaches, such as specialized enzymes or
microorganisms, and the extract of the Yucca plant. All
51
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WINDROW COMPOSTING FACILITY
these claims have turned out to be false or prohibitively
expensive.
Pathogen Destruction
Another very important requirement that the JWPCP, as the
operator of a compost facility, must meet is to ensure that
neither the process nor the product poses a threat to human
health.
Aspergillus monitoring at the JWPCP has shown no evi-
dence of either Aspergillus growth during composting or
elevated levels of Aspergillus around the plant.
The JWPCP routinely monitors compost for total and fecal
coliform, Salmonellae, viable Ascaris ova, and viruses. One
important component of monitoring is the standard to
which the monitoring data are compared. EPA's time-
temperature monitoring requirements (see section on Patho-
gens in Chapter 1) set no standard for actual pathogen
levels. The JWPCP has developed its own standard for
three types of pathogenic organisms based on their research
and a review of the literature (Table 3.2). Ingestion of a
quarter teaspoon of compost (believed to be an unlikely
event) containing organisms at these levels is estimated to
increase a person's risk of disease by about 1 in 200 to 1 in
1,000. Thus, these standards have a wide margin of safety.
These standards do not recognize the very restrictive level
of disinfection that certain health experts have advocated
for Salmonellae. Some of these experts have suggested that
no Salmonellae should exist in any compost released to the
public because of the possibility that Salmonellae could
regrow. The JWPCP has performed research indicating that
Salmonellae regrowth does occur occasionally, but very
infrequently, at least with the products currently produced
from the JWPCP compost. Based on these results, the
JWPCP has rejected for the present time the very restrictive
"no Salmonellae standard" and chosen the 1 MPN (most
probable number)/dry gram of compost (MPN/gm) as a
reasonable alternative. Should the cause of regrowth
become better understood, then the composting process
will be changed, if necessary, to ensure that these
conditions are avoided.
JWPCP researchers have conducted several studies over
the years to examine the relationship between the survival
of the various indicator organisms. Figure 3.34 illustrates the
relationship between Salmonellae and viable Ascaris ova at
the end of the compost cycle based on data from 142 sam-
ples. In the earlier years, the JWPCP had used Salmonellae
as the overall indicator of pathogen destruction; however,
Figure 3.34 illustrates that Salmonellae are not a good
DRY WET
COMPOST SCRUBBER
ACTIVATED WET
SCRUBBER
CARBON
ACTIVATED
CARBON
Figure 3.33. Odor Control Study
N - 142
10-' 10* 10' 10" 10"
SALMONELLAE, MPN / QM
10*
Figure 3.34. Salmonellae as Indicator of Viable Ascaris Ova
Table 3.2 Maximum Population Densities Proposed by JWPCP for
Pathogens in Compost
Salmonellae
Viable Ascaris Ova
Virus
1 MPN/GM
0.5 OVA/GM
0.1 PFU/GM
52
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WINDROW COMPOSTING FACILITY
a
i
1.0
.8
.8
.7
.6
.5
.4
.3
a.
.1
o
10" 10" 10' 10* 10» 10" 10" 10* 10' 10* 10*
TOTAL COUFORM MPN / QM
Figure 3.35. Total Coliform as Indicator of Salmonellae
10"
10-
TOTAL COUFORM MPN / QM
Figure 3.36. Total Coliform as Indicator of Viable Ascaris Ova
1.0
.9
.8
.7
.6
.5
.4
.3
3.
.1
0.0
N - 307
10" 10« 10' 10* 10« 10* 10« 10*
TOTAL COUFORM, MPN / QM
Vf 10»
Figure 3.37. Total Coliform as Indicator of Pathogens
indicator organism since, at levels of Salmonellae greater
than 102 MPN/gm, the probability that there may be greater
than 0.5 viable Ascaris ova per gram actually decreases. At
levels of less than 102 MPN/gm Salmonellae, there is good
correlation between Ascaris and Salmonellae survival.
However, even when the Salmonellae are reduced to detec-
tion limits, there is still a relatively high probability
(15 percent) that viable Ascaris ova will remain.
In another study, based on analysis of 306 finished compost
samples over a 2-year period, the JWPCP examined total
coliform as an indicator that Salmonellae levels were above
the proposed standard of 1 MPN/gm. The data, shown in
Figure 3.35, suggest that total coliform is an excellent
indicator of Salmonellae survival.
Figure 3.36 shows total coliform versus the probability of
Ascaris ova survival based on 143 samples. The data
correlate well at low levels of total coliform, but not at high
levels. However, they do show that when all coliform are
killed, the probability of viable Ascaris ova surviving is
extremely low.
The JWPCP then looked at all their data for 307 samples
collected over the last few years to examine the relationship
between coliform die-off and the destruction of Salmon-
ellae, viable Ascaris ova, and viruses to the "safe" levels
given in Table 3.2.
Figure 3.37 shows the probability that one or more of these
three standards was exceeded as a function of total coli-
form survival at the end of the compost cycle. The data
show that when a significant number of coliform survived,
there was 100 percent certainty that at least one pathogen
standard was exceeded, and therefore the material was not
adequately disinfected. But as the total coliform levels were
reduced to essentially detection level limits, the probability
that any of those standards were violated was only about
2 percent.
These data indicated to the JWPCP that total coliform is a
good indicator of overall disinfection, and use of this indi-
cator would allow the compost facility operator to decide
whether the material was ready to leave the facility or
whether it must remain on the field for a few more days.
Total coliform tests are also practical since they are simple
to run and provide results in 1 or 2 days. The JWPCP in-
tends to propose total coliform monitoring as an alternative
to the time-temperature approach. This proposal is sup-
ported by a previous study [1] based on an extensive litera-
ture search, which recommended that the reduction of total
coliform to a median of 10 MPN/gm be used as an indi-
cator of adequate disinfection.
53
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WINDROW COMPOSTING FACILITY
Figure 3.38 shows the probability of reducing the Salmon-
ellae pathogens to 0.2 MPN/gm, — the detection level limit
— as a function of time at temperatures exceeding 55°C
and 50°C. The data show that Salmonellae can be reduced
to very low levels at 50°C by extending the composting
period slightly. Similarly, a standard for coliform of
10 MPN/gm can be achieved at 50°C by extending the
composting period slightly (Figure 3.39).
Figures 3.40 and 3.41 summarize the JWPCP research data
concerning coliform die-off as a function of days in the
composting cycle. These figures show that at an internal
windrow temperature of 50°C, the proposed standard for
coliform of 10 MPIM/gm can be met after 7 to 25 days of
composting. At 55°C, the required period of time is much
less — about 17 days.
Another related area of research has been to investigate the
conditions that create the temperatures necessary for
pathogen destruction. These conditions include the cross-
sectional area of the windrow, turning frequency, and type
of bulking agent.
Figure 3.42 illustrates internal temperature as a function of
cross section in an irregularly shaped windrow approxi-
mately 20 days into a cycle. The data clearly demonstrate
that as cross-sectional area increases, the surface-to-volume
ratio decreases and the internal temperature increases.
The JWPCP is currently using an intermediate-sized wind-
row with a cross-sectional area of about 32 square feet (ft2)
(3 square meters [m2]), and a large windrow with a cross-
sectional area of about 90 ft2 (8 m2) during the last 2 weeks
of the cycle. Routine temperature measurements of up to
71°C, 2 feet (0.6 meters) under the surface of the large
windrows, demonstrate that the present operation achieves
a very high level of pathogen destruction.
Heavy Metal Content of Sludge Products
Another important constraint at the JWPCP is to ensure
that compost does not exceed the recommended levels for
metals (primarily cadmium and lead) and polychlorinated
biphenyls (PCBs), a toxic organic constituent. The heavy
metal that has received the most attention is cadmium
because of its ability to be taken up into the edible portions
of certain leafy vegetables. The State of California recom-
mends two levels for cadmium in soil amendment products:
25 milligrams (mg)/kg for direct application to food-chain
crops, and 50 mg/kg for the unrestricted use of sludge
products marketed to the public.
Figure 3.43 shows the cadmium content of three main prod-
ucts marketed by Kellogg during 1983 and 1984: a soil
4 e B 10
CONSECUTIVE DAYS
Figure 3.38. Salmonellae Inactivation by Windrow Composting
0 2 4 6 8 10 12 14 18 18 20 22 24
CONSECUTIVE DAYS
Figure 3.39. Total Coliform Inactivation by Windrow Composting
25
Figure 3.40. Coliforms Remaining After Exposure to Internal
Temperatures >50°C
54
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WINDROW COMPOSTING FACILITY
25
Figure 3.41. Conforms Remaining After Exposure to Internal
Temperatures > 55°C
150
140
90
5 1O ^fs 20 2S 30 35
CROSS-SECTIONAL AREA (ft2)
Figure 3.42. Windrow Temperature vs. Cross-sectional Area
o
100
\ 80
n
=. 60-
| 40
20-
Unrestricted Use
1983 1984
(Est.)
Soil
Conditioner
1983 1984
(Est)
Vegetable
Garden
1983 1984
(Est)
Lawn-
Landscaping
Figure 3.43. Cadmium Content of Soil Amendment Products
conditioner; a product specifically intended for vegetable
gardens; and a product marketed as a lawn/landscaping
product. The soil conditioner amendment product exceeded
the recommended limit of 50 mg/kg. The vegetable garden
product, because it contains 75 percent by volume rice
hulls, was well below the 50 mg/kg limit and just in
compliance with the recommended level for food chain
crops. The lawn and landscaping product was well below
the recommended limit of 50.mg/kg, but was above the
recommended limit for direct application to food-chain
crops.
The 1984 values are higher because the full sludge de-
watering achieved in 1984 captured a much higher propor-
tion of the very fine sludge particles with which heavy
metals are associated. In contrast, no fine sludge particles
were ever captured by the old centrifuges used before 1977.
Records show that all the JWPCP sludge products, includ-
ing soil conditioners, contained only about 25 mg/kg
cadmium before 1977.
The 50 mg/kg standard is a highly conservative standard
designed to protect public health in a worst-case scenario:
an individual who amends soil with compost annually, who
lives in an area with naturally acidic soils that promote metal
uptake by vegetables, and who consumes homegrown veg-
etables daily for 50 years. This standard is certainly
conservative in southern California and Arizona where
Kellogg's products are marketed, since these areas have
alkaline soils that minimize metals uptake. Thus, the
JWPCP feels that current levels of cadmium in its products
are well within safe levels for the protection of public
health. In addition, cadmium levels are expected to de-
crease significantly when the EPA categorical industrial
pretreatment program is implemented.
Although lead is not taken up by plants, it is a potential
health concern because of the propensity of some small
children to consume nonfood substances, including soil.
Currently, California recommends that lead levels in soil
amendment products not exceed 500 mg/kg. Figure 3.44
shows that lead levels of all three soil amendment products
were well within this standard in 1983. Additional metals
picked up by full dewatering capabilites were estimated to
bring one product up to the limit in 1984.
Although the JWPCP has never monitored PCBs in bagged
products due to the difficulty of the testing procedure, they
did examine the straight compost before dilution with
sawdust or rice hulls. The undiluted compost was well
within the two California recommended standards
(Figure 3.45), and the diluted products would be expected
to have an even lower level of PCBs.
55
-------
WINDROW COMPOSTING FACILITY
700
600
5500
1400
-a'300
ra
» 200
100
0
Food Cham Crops
& Unrestricted Use
1983 1984
(Est.)
Soil
Conditioner
1983 1984
(Est)
Vegetable
Garden
1983 1984
(Est)
Lawn-
Landscaping
7
I!
82
0
-m^m^m
Soil
Conditioner
(1984
Estimate)
Food _ChaiJi Crops
Unrestricted Use
Vegetable Lawn-
Garden Landscaping
(1984 (1984
Estimate) Estimate)
Finished
Recycle
Compost
(1982-83
Data)
Figure 3.44. Lead Content of Soil Amendment Products
Figure 3.45. PCB Content of Soil Amendment Products
Public Relations
Another objective the JWPCP must meet as an operating
compost facility is good public relations. The JWPCP
experienced severe public relations problems in 1977
because of odors that were generated when the composting
operation was expanded to accomodate the increased
volume of wet sludge cake. As a result, a Citizens Advisory
Committee was created to work with the JWPCP to identify
conditions that lead to odor emissions and to evaluate the
effectiveness of the JWPCP's attempts to correct these
situations.
Various subcommittees were organized in the surrounding
communities. These subcommittees meet individually and
elect representatives to the central Citizens Advisory
Committee. The central committee meets with the JWPCP
on a regular basis to provide feedback on the JWPCP's
performance. The JWPCP makes special efforts to ensure
that the concerns of the Advisory Committee are addressed
in a timely manner and that all complaints are adequately
followed up.
As part of the public relations program, a series of meetings
and Saturday morning tours were organized to acquaint
local citizens with the composting operation. The tours
were very successful and have been continued on^an annual
basis. The plant's weekend staff were trained in how to
receive irate complaints from the community. This training
included how to avoid taking a complaint as a personal
insult, how to record the information in a courteous
manner, and how to ensure that the person making the
complaint will feel that he or she has reached the proper
person and that something will be done about the complaint
as soon as possible.
The overall program has greatly enhanced relations between
the sanitation districts and the surrounding community.
Marketing of Compost Products
The Kellogg Supply Company has been marketing sewage
sludge products from the JWPCP facility since 1928. They
process the compost received from the JWPCP by screen-
ing it to remove large particles (greater than % inch [1 cm])
and then bagging it.
Dozens of different products are formulated from the com-
posted sludge. Kellogg's four largest products in terms of
volume are:
• Nitrohumus, which is a soil amendment consisting of
90 percent sludge and 10 percent sawdust.
• Topper, which is used for top dressing of new lawns
and for gardens; it consists of 60 percent sawdust and
40 percent sludge.
• Amend, which is recommended for vegetable gardens;
it consists of 75 percent rice hulls and 25 percent
sludge cake. Rice hulls are rich in potash, a necessary
nutrient for good vegetable growth.
• Gromulch, which is very similar to Topper but contains
some proprietary ingredients. It is basically composed
of 60 percent sawdust and 40 percent sludge cake.
The materials are primarily marketed in 2-ft3 bags to home
users through 2,000 retail stores in southern, central, and a
portion of northern California; throughout Arizona; and in
the Las Vegas area of Nevada. Kellogg does not advertise
directly to the public. Instead, the firm relies on the nursery
supply sales force to recommend the product to local cus-
tomers. One reason for Kellogg's success in this approach is
that all Kellogg sales personnel are previous owners or
managers of nursery supply stores and know the business
intimately. Also, the Kellogg products work well. They are
low in leachable salt content, ammonia, and chlorides. It is
almost impossible to ruin a garden by overuse. The nurse-
ries know their customers will not be dissatisfied.
56
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WINDROW COMPOSTING FACILITY
Another factor in Kellogg's success has been the diversity
of its product lines. In addition to the soil amendment
products, they also sell steer manure, sawdust, redwood
chips, lime, sulphur, and numerous blends of fertilizers and
herbicide products. Thus nurseries need only make one tele-
phone call to Kellogg to obtain everything they need.
Kellogg maintains a large inventory of products and main-
tains contracts with a large number of private haulers to
offer rapid delivery service — as little as 1-day delivery
throughout southern California.
Kellogg also advertises and promotes their products as top-
of-the-line materials. Their recommended selling prices are
far above all competing fertilizer products. The approach
has been very successful. Although Kellogg does not cur-
rently sell directly to the public, their products are used so
frequently throughout southern California that they have
become identified with gardening activity. The Kellogg
Supply Company currently employs about 75 people and
has annual sales of about $13 million, $9 million of which
comes from sewage-sludge-based products from the
JWPCP.
The volume of Kellogg's product line is limited by the
amount of material they can obtain from the JWPCP. In an
effort to generate greater revenues, Kellogg has been inves-
tigating the possibility of selling the sludge-based products
in much smaller bags at higher prices in supermarkets and
grocery stores.
Costs
The actual costs of composting at the JWPCP in 1984 are
summarized in Table 3.3. Equipment is the single largest
cost component, followed by the wages (including fringe
benefits) of the eight operators and one foreman. Total
direct operational costs are about $3,000 per operating day
(6 operating days per week). In 1984, an average of 104 dry
tons (94 dry mt) of sludge cake were composted per oper-
ating day, a lower level of productivity than in previous
years due to unusually severe odor complaints received in
the summer of 1984 and to the slowdown in operations as
personnel learned the new composting operation that in-
volved building windrows of three different sizes. Thus, the
direct composting cost averaged $28.70 per dry ton
($31.64 per dry mt).
In addition to these direct costs, significant indirect costs
were incurred for management, administrative, and clerical
support services such as personnel timekeeping, purchas-
ing, labor relations, and handling community complaints
regarding odors. When these indirect costs are included,
Table 3.3 1984 Composting Costs
Item
$/Operating Day
Direct Costs
Equipment
Capital Recovery
Fuel
Maintenance
Wages
Indirect Costs
Total Costs
1,036
473
512
964
1,113
4,098
$39.40 Per Dry Ton
the composting unit costs for 1984 averaged $39.40 per dry
ton ($43.44 per dry mt) of sludge composted. The JWPCP
received approximately $4 per dry ton ($4.41 per dry mt)
from the fertilizer manufacturer for the product sale, so the
JWPCP's bottom-line cost was approximately $35.00 per
dry ton ($38.59 per dry mt) in 1984. This cost is much lower
than the cost of approximately $53.00 per dry ton ($58.43
per dry mt) for landfilling. The JWPCP spends about
$300,000 per year in research to improve the composting
operation. This cost was not included in the above unit
costs.
It should be noted that from late 1984 through mid-1985
operating costs have decreased and productivity has in-
creased, reflecting the experience the operators have gained
with the revised composting process. Projecting these latest
7 months of cost and productivity data to an annual average
suggests that 120 dry tons (108 dry mt) of sludge will be
composted per operating day at a unit cost of $34 per dry
ton ($37 per dry mt) of sludge composted. After credit for
sale of the product, the bottom line cost is expected to
average $30 per dry ton ($33 per dry mt) of sludge
composted.
The JWPCP vehicle fleet includes: three tractor trailers
devoted exclusively to composting; other tractor trailers for
hauling material to the landfill; three large composters and
one very large composter; two front-end loaders; and two
water trucks. The original composting vehicle fleet cost
$1.4 million (Table 3.4). Equipment costs are calculated by
amortizing costs over the expected equipment lifetime
(Table 3.5). Composters have only a 5-year life because the
very heavy wear results in a need for major repairs after
5 years.
57
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WINDROW COMPOSTING FACILITY
Table 3.4 Composting Equipment
Equipment Cost Each No.
Tractor-trailer
Large Composter
Very Large Composter
Front-end Loader
Water Truck
$95,000
$125,000
$275,000
$200,000
$50,000
3
3
1
2
2
Total Costs $1,435,000
Table 3.5 Expected Life of Composting Equipment
Equipment Years
Tractor-trailer 10
Large Composter 5
Very Large Composter 5
Front-end Loader 10
Water Truck 15
References
(1) LA/OMA Study Group. 1980. Proposed Sludge Management
Program for the Los Angeles/Orange County Metropolitan
Area (LA/OMA). Draft Environmental Impact Statement/
Environmental Impact Report (p. VI-72). U.S. EPA, Region IX,
San Francisco, California. April 1980.
58
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4. In-vessel Composting
Introduction
In-vessel composting is the biologic stabilization of sludge
under controlled aerobic conditions in a closed vessel or an
enclosed structure. The structure of the in-vessel system
may take many forms (e.g., circular or rectangular towers,
horizontal tunnels, bin or box-type vessels, or various struc-
tures and configurations within a building). As of early
1985, 4 in-vessel facilities have started up or are operational
in the United States, 5 facilities are under construction; and
approximately 14 facilities are in the design or negotiation
stage (Table 4.1).
The basic steps in in-vessel composting are identical to
those in windrow and static pile systems: mixing of sludge
with a bulking agent, aeration to promote the biological
processes that decompose the material and create 50° to
70°C temperatures that destroy pathogens, and curing to
allow further stabilization and to destroy pathogens. The
essential difference between in-vessel and other composting
systems is that the in-vessel processes are highly mech-
anized and take place within one or more confining struc-
tures. In-vessel systems generally require shorter processing
times than static pile and windrow systems because of such
factors as better process control, use of sawdust as a bulk-
ing agent and, in some dynamic systems, good moisture
release through mixing and refluffing.
Figure 4.1 compares the process flow of one type of in-
vessel system with a static pile system. In this in-vessel
system, the compost and bulking agent (usually sawdust
and recycled compost) are mixed in a fixed mixer and then
transported by a conveyor system to the reactor, where the
mix is composted for about 14 days. The compost is then
cured in another reactor for approximately 20 days, during
which time composting continues at a slower rate. In cold
climates, or for particular compost uses, these retention
times may be extended to ensure that the product is suit-
able for the intended use. For some purposes, the compost
can be distributed immediately after curing, and for others,
additional curing may be required. Most in-vessel systems
that use sawdust as the bulking agent do not require
screening of the final product. Frequently, compost is
recycled and incorporated into the sludge/bulking agent
mix. This recycling increases the residue time of the mixture
and reduces the need for new bulking material.
Most of the process parameters for in-vessel composting
are identical to those for static pile and windrow processes.
For ideal processing, the sludge should contain at least
25 percent volatile organic material, and the sludge/bulking
agent mixture should have a moisture content from 50 per-
cent to not more than 65 percent, a carbon-to-nitrogen ratio
Table 4.1 In-Vessel Facilities in the United States, May 1985
Startup or Operational
Cape May, New Jersey
Columbus, Ohio
Portland, Oregon
Wilmington, Delaware
Under Construction
Akron, Ohio
Clinton County, New York
East Richland County, South Carolina
Lancaster, Pennsylvania
Sarasota, Florida
In Design or Negotiation
Baltimore, Maryland
Charlotte, North Carolina
Clayton County, Georgia
Cobb County, Georgia
Endicott, New York
Fort Lauderdale, Florida
Hamilton, Ohio
Henrico County, Virginia
Hickory, North Carolina
Jackson, Mississippi
Juneau, Alaska
Montgomery County, Ohio
Newberg, Oregon
Schenectady, New York
Washington, D.C.
SLUDGE
s_
MIXING
AERATED PILE
2 1 DAYS
CURING
30 DAYS
MARKET
WOOD CHIPS
SLUDGE
-RECYCLED WOOD CHIPS -
Static Pile System
MARKET
RECYCLED COMPOST-
In-vessel System
Figure 4.1. Comparison of Process of Static Pile System and
One Type of In-vessel System
59
-------
IN-VESSEL COMPOSTING
in the range of 20:1 to 30:1, and a pH between 5 and 8.
(Mixtures with an initial pH of 11 or 12 can be successfully
composted but may require a few more days to achieve the
necessary temperature rise.) However, one major difference
is that in-vessel systems generally use a higher ratio of
sludge to bulking agent than do windrow and static pile
systems. Use of this ratio produces less volume of mix per
unit of sludge. The higher ratio is possible due to the
greater surface area of sawdust (the bulking agent most
commonly used in an in-vessel system) and the enhanced
process and odor control of in-vessel systems. According to
manufacturers of in-vessel systems, volumetric sludge:bulk-
ing agent ratios typically range from 1:0.3 to 1:1, depending
on the type of sludge, bulking agent, and moisture content.
However, some in-vessel systems have had to use a greater
proportion of bulking agent than originally claimed due to
greater moisture content of sludge and bulking agents,
difficulties in moving a wet mixture through the system, and
difficulties in moving air through the wet mix.
In-vessel systems offer several advantages over open static
pile and windrow systems:
• Favorable space requirements. In-vessel systems
require less land area than open systems.
• Shorter processing time. In-vessel systems generally
require about 14 days for composting and 20 days for
curing, compared to 14 to 21 days of composting and
30 days of curing in open systems. (However, the
adequacy of pathogen destruction is still unknown for
the shorter composting time.)
• Lower labor costs. In-vessel systems rely heavily on
electric-powered, labor-saving equipment.
• Protection from the effects of weather and climate.
Material processed in an in-vessel system is protected
from precipitation. The system can be insulated for
cold weather.
• Better odor control. Process off gases are more easily
contained and treated in an in-vessel system.
• Reduced nuisance potential. Site cleanliness and dust
control are easily managed.
• Enhanced process control.
• Better public image. An enclosed "compost factory" is
more acceptable to the general public than an open
system.
The disadvantages of in-vessel systems are potentially
higher capital costs and the lack of operating data on the
systems. In particular, the cost of operating the aeration
system and the adequacy of pathogen control have not
been evaluated on any system operating in the United
States. Other potential disadvantages could be higher
maintenance costs and less flexibility in adapting to changes
in the quality of sludge and bulking agents, and to changes
in sludge production rates. Also, some in-vessel systems
have had problems with moisture removal.
Types of In-vessel Composting Systems
In-vessel systems can be divided into two major categories:
plug flow and dynamic. In plug flow systems, the relation-
ship between particles in the composting mass stays the
same throughout the process, and the system operates on
the basis of a first-in, first-out principle. In a dynamic
system, the composting material is mechanically mixed
during the processing. In-vessel systems can be further
categorized based on the geometric shape of the vessels or
containers used.
• Plug flow:
Cylindrical reactors
Rectangular reactors
Tunnel reactors
• Dynamic:
Rectangular tanks
Circular tanks
Cylindrical Reactors
Plug flow cylindrical systems of a small size have been
operating successfully in Europe for many years. A number
are currently under construction in the United States, and
one in Portland, Oregon, is operational. Several other U.S.
facilities are in the planning stages.
In a typical cylindrical system (Figure 4.2), sludge, recycled
compost, and a bulking agent are blended together in fixed
mixers and fed into the top of a silo in such a way that the
fresh mix is distributed over the top of the existing mix
within the reactor. Compost is removed from the bottom of
the reactor using a rotary screw. Removing material from
the bottom causes the mix within the silo to descend, cre-
ating space for new mix to be added at the top. Detention
time within the reactor is typically 14 days. The compost
mix is aerated by a pipe manifold system that forces air into
a plenum in the bottom of the reactor and up through the
mix (see photo). Off gases are removed from the top of the
reactor and treated for odors.
The cylindrical system includes several process controls.
Temperatures in the reactor are measured continuously at
different elevations, and the oxygen or carbon dioxide
content of the off gas can be continuously monitored. The
air flow is adjusted by a microprocessor based on analysis
of the temperature and off gases. Process control is geared
60
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IN-VESSEL COMPOSTING
CO,
MIXER
INFEED
COMPOSTING
MIX
J
JJJJ ' ' -J J
' " *" *" " " r "
AIR^ .
\
* OUTFEED
Figure 4.2; Cylinder Tower Reactor
toward maintaining the hottest zone near the top of the
cylindrical tank to aid in better moisture release.
A complete cylindrical composting system consists of a
storage silo, one or more reactors in which composting
takes place, and curing reactors (see photo). Additional
equipment includes mixers, blowers, controls, and materials
handling equipment. Reactors are typically 31.5 feet
(9.6 meters) high with capacities from 3,500 ft3 (100 m3) to
14,000 ft3 (400m3).
Rectangular Reactors
Rectangular reactors were developed to provide large
capacity within a single reactor and better process control.
The aeration pipe at the bottom of the cylindrical reactor at the
Portland, Oregon, facility. This system distributes air uniformly
throughout the bottom of the silo. The air flow cannot be varied
with respect to different areas within the silo. Only the total air
flow can be varied.
A cylinder composting facility in the Federal Republic of Germany.
Note the insulation around the tower.
In rectangular reactor systems (see Figure 4.3), premixed
material is delivered to the top of the reactor by conveyor
belts that evenly distribute the mix over the top of the
composting mix already contained within the reactor.
Compost is removed at the bottom of the reactor by a
screw that runs on tracks along the bottom of the reactor.
The length of a reactor is not limited, and capacity can be
increased by increasing the length of the reactor. The screw
extractor mechanism can be easily moved from under the
composting mixture for maintenance and repair.
The mix is aerated by forcing air in at the bottonrof the
reactor through an air distribution system that can be
61
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IN-VESSEL COMPOSTING
Demonstration tunnel reactor in Alabama showing sludge infeed
to a hopper; hydraulic ram and door; pump arrangement; and
manifolds used to distribute and collect air.
controlled to change the air flow in different sections of the
reactor. The nozzles are protected by a layer of granular
material such as graded aggregate. Air is forced up through
the composting material and removed by an exhaust mani-
fold. The exhaust manifold is located just below the surface
of the incoming mix. Heat from the active composting
material raises the temperature of the incoming mix. The air
exhaust system operates under negative pressure to ensure
that no odors leak from the reactor and to help remove
moisture from the fresh mix. Temperature, carbon dioxide
content, and air pressure are monitored. The air flow is
adjusted by a microprocessor based on the monitoring data.
Detention time within the base reactor is 20 to 30 days. The
material can be cured either (1) in the same reactor by in-
creasing the detention time, (2) in a second reactor, or (3) in
aerated storage piles.
A rectangular reactor is currently under construction in
Cape May, New Jersey. The facility includes a bulking agent
receiving area, a sludge storage area, a mixing building, a
reactor building, and a materials finishing area where the
compost can be prepared for distribution. The reactor,
which is approximately 30 feet (9 meters) high and 26 feet
(8 meters) wide, is designed and operated to maintain the
hottest (55° to 60°C) zone in the top half. The reactor is
divided into sections sized to accommodate peak sludge
production during summer months in this ocean resort
community and low sludge production during the winter.
Another type of rectangular reactor system currently being
marketed in-the United States uses air lances suspended in
the composting mixture to provide aeration. Lances sup-
MATERIAL FEED
CONVEYOR x
AIR
REMOVAL
SYSTEM -
MATERIAL
REMOVAL .
SYSTEM
-.
1
.^INFLOW
/ TRAVERSE
/ CONVEYOR
?JL^ cp /
»
Hi U U U ll
^ COMPOSTING
MIX-^
GRADED STONE ^ A
fc-1
in
u-
OFF
"" GASES
F MATERIAL
EXTRACTION
MFflHANISM
^-AIR INPUT
R SYSTEM
Figure 4.3. Rectangular Reactor
=-
/— INFEED
( '
V
SSS/S/ Vv^NVv
~^
OUTFEED
O ' ' o
AIR AIR
DISTRIBUTION REMOVAL
Figure 4.4. Tunnel Reactor
ply-ng positive-pressure aeration are alternated with suction
lances to reduce the power required to move the air and to
prevent anaerobic areas by distributing the air throughout
the mass.
Tunnel Reactors
In tunnel systems, a typical reactor would consist of a
12-foot (4-meter) high by 18-foot (5.5-meter) wide rectangu-
lar box (see Figure 4.4 and photo). The length of the reactor
can vary depending on the desired capacity. The sludge/
bulking agent mix is loaded into the charging end of the
62
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IN-VESSEL COMPOSTING
PLENUM''
VAIR
Figure 4.5. Cross-section of a Tank Dynamic System
tunnel by conveyor. The material is loaded into the reactor
by hydraulic rams to a uniform density to enhance even air
distribution. The entire contents are then moved horizon-
tally toward the discharge end of the reactor. As material is
loaded into the reactor, a corresponding finished charge of
material is displaced down a discharge chute to a conveyor.
Composting takes place within the tunnel, with a total
detention time of about 14 days. At the end of the tunnel,
the material is discharged by gravity to a conveyor.
Additional curing may be required after discharge from the
reactor. All moving parts are located outside the tunnel, so
maintenance can be performed without interrupting the
process.
The aeration system consists of two air manifolds — one
positive and one negative — that run along the base of the
reactor parallel to the direction of material flow. The
manifolds feed a network of floor-mounted air headers that
are controlled by modulating valves interlocked with tem-
perature sensors in four zones. Air is distributed through a
series of openings in the floor. The aeration rate and
location can be adjusted to meet specific requirements. The
process is automatically controlled by adjusting the air flow
based on temperature. Data indicate that the process
provides uniform aerobic conditions.
Four tunnel reactors for composting solid waste are oper-
ating in Europe. The only tunnel reactor in the United
States is a small demonstration reactor that is currently
operational in Alabama.
Rectangular Tanks
Rectangular tanks are dynamic composting bins in which
premixed (usually with a pugmill) sludge and bulking agent
are aerated and periodically mixed, moved, and fluffed. The
dimensions of the bin are typically 10 feet (3 meters) deep
and 20 feet (6 meters) wide. Length depends on the volume
of the sludge to be composted. The bottom of the bin
contains a sealed plenum, which is divided into compart-
ments so that the air flow can be varied according to the
processing needs of particular mixes. The mix is supported
above the plenum by a perforated steel plate covered with
coarse limestone pieces. Individually controlled blowers
provide a negative or positive flow of air to each
compartment. Because of the porosity of the mix and the
depth of the bed, less power is required for aeration, which
makes this system less energy intensive than some other
in-vessel systems.
The sludge/bulking agent material is delivered to the bin by
a central conveyor that discharges onto a second conveyor,
which fills the bin using an automatic leveling device.
Within the bin, the material is periodically remixed and
removed by a device called an Extractoveyor, consisting of a
rotary breaker, which penetrates, remixes, and fluffs the
mass; a chain and flight conveyor, which lifts the material
from the bottom of the reactor bin; and a trailing conveyor,
which transfers the material onto a central conveyor. Figure
4.5 shows a schematic drawing of the tank dynamic system
with an Extractoveyer.
Material within the bin is mixed and moved at least twice
during the composting period. This process creates a fluffier
mixture than that produced by static in-vessel systems, and
releases moisture. The system also mixes material from the
top and sides into the interior. The remixing also exposes all
the material to the higher temperatures in the middle of the
mass and to ensure pathogen destruction. By using the
trailing conveyor, the Extractoveyor can move material onto
the central conveyor for relocation within the bins or move-
ment outside the bins.
With this system, material can be composted in about
14 days. In extremely cold climates, 21 days may be
required. According to the manufacturer, curing is not
required. The system is controlled by monitoring tempera-
ture, moisture, and air flow data.
A dynamic composting system in South Charlestown, Ohio,
has been used for over 10 years to compost feedlot manure
and bark. Part of this system was used successfully to
compost sludge. Based on the results of these studies,
similar systems are under construction at several locations
around the United States and are expected to be opera-
tional soon.
Circular Tanks
The dynamic circular composting tank consists of a
completely enclosed circular reactor 20 to 120 feet (6 to
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IN-VESSEL COMPOSTING
36 meters) in diameter and 6 to 10 feet (1.8 to 3 meters)
high. Sludge and bulking agent are fed into the center of
the tank at the top by an overhead conveyor (Figure 4.6).
Mixing normally takes place outside the reactor, but also
can occur during input. The mix is moved by a conveyor
built on a radial arm. The conveyor transports the material
to the perimeter of the tank and places it evenly beside the
wall.
The mixture is agitated and relocated by a series of augers
mounted on a radial arm. The augers are staggered at a
skew angle and rotate on a track atop the perimeter of the
tank. As the augers rotate, material moves in a serpentine
flow pattern toward the center, where it is discharged into
an opening and removed by conveyor belt. The aeration
system is located in the floor of the tank, and the air dif-
fusers are covered with a layer of aggregate. According to
the manufacturer, retention time in the tank is 10 days and
curing is not required. However, it is very likely that external
stockpiling (curing) would be necessary for some uses.
Process control is based on temperatures achieved in the
mix and the permeability of the composting mix as meas-
ured by the head loss of the blowers. The material can be
remixed or fluffed by the augers, and air flows can be
controlled.
Performance
Compost Quality and Quantity
The characteristics of the input sludge and bulking agents
directly affect the characteristics of the finished compost.
During composting, nitrogen and carbon are used for micro-
bial growth, which reduces the volatile solids content of the
sludge by 40 to 55 percent. To ensure pathogen destruction,
a temperature of at least 55°C must be maintained within
each part of the composting mass for 3 days. Complete
destruction of weed seeds may require a higher temperature
for a few hours. As in all composting systems, levels of
trace organics and metals will be reduced in the final
product by the dilution effect of the bulking agent.
The volume and mass reduction achieved by an in-vessel
system depends on the bulking agent and on the solids
content of the sludge. Generally, the compost produced will
be approximately 30 percent lower in volume than the initial
mix, and the compost will be about half as dense as the
incoming sludge.
AUGERS
MATERIAL
REMOVAL
AIR MANIFOLD
Figure 4.6. Circular Dynamic Reactor
operational as of early 1985. In-vessel systems (mostly
cylindrical systems) have, however, been operating in
Europe for over a decade. The earliest European cylindrical
systems experienced problems with aeration and mechani-
cal deficiencies, particularly with the material extraction
systems. Both types of problems caused the systems to be
filled to less than capacity. Much effort was spent
redesigning and modifying the equipment, and recent
systems have proved more reliable. Performance has also
improved with increasing operational experience. One
problem that has been experienced at some facilities is
condensation of moisture on the inside of the vessel,
particularly during cold weather. Condensatfon prevents
moisture release and inhibits the composting process.
Insulation of in-vessel facilities in cold climates and
installation of adequate aeration systems are important
factors for all facilities.
Programmed maintenance of in-vessel systems generally
requires about 1 week of downtime per year. However, it
may take several weeks for the biological system to return
to steady state. It is therefore important to incorporate
provisions for handling maintenance and system loading
variations into system design. Many European suppliers
provide their customers with a maintenance service contract
in which the supplier refurbishes the system once a year.
Reliability and Availability
Experience with in-vessel systems in the United States is
limited. Five systems are under construction, and four are
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IN-VESSEL COMPOSTING
Sludge received during maintenance periods can be handled
by storage, converting cure reactors to main reactors,
curing in open piles, sharing capacity with other facilities, or
using alternative methods of disposal.
The recent increase in use of in-vessel systems reflects the
technical communities' growing acceptance of the process,
which seems to offer several important aesthetic, process
control, and space-saving features. However, it is important
to continually evaluate and improve these various forms of
in-vessel systems as they come on line. Key areas for
further process enhancement include optimization of tem-
perature and oxygen levels to control odor production,
decompose volatile organic matter, remove excess moisture,
and destroy pathogens.
65
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5. Federal and State Regulation
The following information provides general guidance on
Federal and state regulation. The specific requirements
concerning the composting process and product use must
be determined by contacting the appropriate state and local
authorities.
Current Federal Regulation
At present (May. 1985), no Federal regulations deal specifi-
cally with the distribution and marketing of sludge products.
Until more specific regulations are promulgated, distribution
and marketing of sludge compost are covered under the
same regulations that apply to land application, i.e., 40 CFR
Part 257: Criteria for Classification of Solid Waste Disposal
Facilities and Practices. These criteria include requirements
to protect the environment and public health. Requirements
for sludge and sludge products containing high levels of
metals or PCBs are found in 40 CFR Part 761 and in haz-
ardous waste rules under the Resource Conservation and
Recovery Act (RCRA).
Environmental Criteria
Under 40 CFR Part 257, land application of sludge and
sludge products is considered to be a form of solid waste
disposal. Any "facility" or land where sludge or sludge
products is applied must comply with the following
requirements:
• Floodplains. Application sites may be located in a
floodplain; however, they must not "restrict the flow of
the base flood, reduce the temporary water storage ca-
pacity of the floodplain, or result in washout of solid
waste, so as to pose a hazard to human life, wildlife, or
land or water resources." (A base flood is a 100-year
flood.)
• Surface Waters. Discharges from the application site
that would violate Sections 402, 404, or 208 of the
Clean Water Act are prohibited.
• Groundwater. Application sites must not "contaminate
an underground drinking water source beyond the solid
waste boundary." (EPA is currently examining more
stringent controls on land application to protect par-
ticularly vulnerable and valuable groundwater resources
such as irreplaceable aquifers.)
In general, application of sludge compost at reasonable
rates in accordance with state or U.S. Department of Agri-
culture (USDA) guidelines would not be expected to cause
problems within floodplains or to pollute groundwater or
surface waters.
Public Health Criteria
To protect public health. Federal regulations define require-
ments and limitations for application of sludge and sludge
products that contain pathogens, cadmium and other
metals, and PCBs.
Pathogens
Federal regulations in 40 CFR Part 257 define two levels of
pathogen reduction in sludge — Processes to Significantly
Reduce Pathogens (PSRP) and Processes to Further Re-
duce Pathogens (PFRP) — with different use restrictions for
each resultant product. To "significantly" reduce pathogens
through composting, pile temperatures during static pile,
windrow, or in-vessel composting must be maintained at
40°C for at least 5 days, with a temperature exceeding 55°C
for at least 4 hours of that period. The resulting product can
be applied to land; however, public access to the land must
be controlled for at least 12 months, and grazing by animals
whose products are consumed by humans must be pre-
vented for at least 1 month. In addition, food-chain crops
for direct human consumption cannot be grown on the land
for 18 months after application if the edible portion of the
crop might contact the sludge.
To achieve the next level of pathogen reduction (PRFP), the
composting process must meet the following conditions:
• In-vessel composting and aerated static pile com-
posting must maintain internal temperatures at 55°C or
greater for 3 days.
• Windrow composting must maintain pile temperatures
of 55°C or greater for at least 15 days, with a minimum
of five turnings during this period.
Compost resulting from these processes can be applied to
land used for food-chain crops, and crops can be grown
immediately after application.
Cadmium
The total cadmium in compost applied to a site used for
growing tobacco, root crops, and leafy vegetables must not
exceed 0.5 kg/ha/year. Cumulative annual cadmium ap-
plication to sites growing other crops is limited to
1.25 kg/ha/year until 1987, when the limit will drop to
0.5 kg/ha/year. The cumulative application of cadmium
over the lifetime of the site is also limited to a total of 5 to
20 kg/ha depending on soil pH and soil cation exchange
capacity. In general, soil pH must be 6.5 or greater at the
time of planting, and EPA recommends that pH be perma-
nently maintained at or above 6.2. Alternatively, if the crop
is exclusively used for animal feed, cadmium applications
need not be limited, but pH must be consistently main-
tained at or above 6.5; the facility must have an operating
66
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FEDERAL AND STATE REGULATION
plan that shows how the animal feed will be distributed, to
preclude ingestion by humans; and future property owners
must be notified by a stipulation in the land record or
property deed that states that the property received high
concentrations of cadmium and that food-chain crops
should not be grown.
Other Metals
Sludges and sludge products that contain high
concentrations of metals and thus qualify as hazardous
wastes are controlled under provisions of RCRA. Thus far,
very few municipal sludges have been classified as haz-
ardous wastes.
Polychlorinated Biphenyls (PCBs)
Compost containing greater than 10 mg/kg but not more
than 50 mg/kg of PCBs must ordinarily be incorporated into
the soil when applied to land used for producing animal
feed, including pasture crops for animals raised for milk.
Compost containing greater than 50 mg/kg of PCBs must
be treated under the strict requirements of 40 CFR Part
761.60, which allows only incineration (in compliance with
Part 761.70) or disposal in a chemical waste landfill (defined
under Part 761.65). These requirements are separate from
hazardous waste requirements specified under RCRA. Sub-
stitute methods of disposal may be approved by EPA
Regional Offices.
Future Federal Regulation
In I983 the EPA formed a Sludge Task Force to examine
current regulations and guidance and to explore improve-
ments and alternative approaches. The Task Force recom-
mended that a comprehensive technical regulatory program
be developed under the legislative authority of Section 405
of the Clean Water Act, which requires EPA to develop
regulations that:
• Identify sludge use and disposal options.
• Specify factors to be taken into account in determining
the measures and practices applicable for each use or
disposal (including costs).
• Identify concentrations of pollutants that interfere with
each use or disposal.
In addition, the Task Force issued a policy statement,
signed by the EPA Administrator in May 1984, that provides
general guidelines for future Federal and state regulations.
This statement promotes "sludge management practices
that provide for the beneficial use of sludge while main-
taining or improving environmental quality and protecting
public health." It also recognizes, to a greater extent than
before, the site-specific nature of sludge management, and
requires states to take a prominent role in managing sludge,
provided that local management options are consistent with
state and Federal regulations.
Based on the Task Force recommendations, new Federal
regulations that cover all the different sludge use and dis-
posal options (distribution and marketing, land application,
incineration, landfilling, and ocean disposal) are expected to
be issued in I986. Developing these regulations will require a
comparison of risks, benefits, and costs across several
media. The health and multimedia environmental effects of
contaminants in sludge will be profiled to determine which
substances need to be regulated and the appropriate limits.
In addition, EPA plans to establish management practices
that will protect human health and the environment. These
practices will take into account site-specific factors such as
soil conditions and climatic variability. The new regulations
will also specify requirements for compliance monitoring.
State Regulation
As of December 1984, ten states regulate sludge distribution
and marketing: Florida, Illinois, Kentucky, Maryland, North
Carolina, Ohio {in draft), Pennsylvania, Rhode Island, South
Dakota, and Wyoming. Tennessee and New Hampshire pro-
vide guidelines. Several states also regulate sludge land
application. Some state regulations exceed Federal stan-
dards; therefore, it is particularly important when
considering composting to check with the appropriate state
agency.
Federal regulations concerning the responsibility of states in
sludge management are expected to be proposed sometime
in I985. They will include a requirement that each state
describe its plan for managing sludge and ensure that the
plan implements Federal requirements. Each state plan
would describe the site-specific evaluation process and
provide for compliance monitoring and enforcement, con-
tingency planning, and assurance that facilities have
adequate capacity to manage the sludge. The Federal regu-
lations are expected to encourage technical assistance and
research and development by the states to the extent
possible.
67
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Abbreviations
°C = degrees Celsius
CLC = Citizens Liaison Committee
cm = centimeters
cm2/m = square centimeters per meter
C:N = carbon-to-nitrogen
ft2 = square feet
ft3 = cubic feet
gal = gallon
gm = grams
ha = hectare
hp = horsepower
JWPCP = Joint Water Pollution Control Plant
kg = kilogram
km = kilometer
Ib = pounds
m2 = square meters
m3 = cubic meters
MES = Maryland Environmental Services
mg = milligrams
mgd = million gallons per day
MPN = most probable number
mt = metric ton
O&M = operation and maintenance
OU = odor unit
PCBs = polychlorinated biphenyls
PFRP = Processes to Further Reduce Pathogens
PFU = plaque-forming units
ppm = parts per million
PSRP = Processes to Significantly Reduce Pathogens
RCRA = Resource Conservation and Recovery Act
rpm = revolutions per minute
yd3 = cubic yards
68
U. S. GOVERNMENT PRINTING OFFICE: 1985/559-111/20630
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