SVl-614d
JSTINO SEWAGE SLUDGE
by high-rate suction
aeration techniques
*J>*. jr. *•*.•-
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COMPOSTING SEWAGE SLUDGE
by high-rate suction
aeration techniques
THE PROCESS AS CONDUCTED AT BANGOR, MAINE,
AND SOME GUIDES OF GENERAL APPLICABILITY
This interim report (SW-614d) describes work performed
under demonstration grant No. 803828
to the City of Bangor,
and was written by DALE MOSHER and
R. KENT ANDERSON
ibrary
210 South DvBcrbcrn Street
Ci.icsgo, Illinois 60604
U.S. ENVIRONMENTAL PROTECTION AGENCY
1977
For sale by the Superintendent of Documents, U.S. Government
Printing Office, Washington, D.C. 20402
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contents
INTRODUCTION
PART I: THE OPERATION AT BANGOR
THE DIMENSIONS 1
PREPARATIONS FOR COMPOSTING 4
COMPOSTING 15
MONITORING AND ANALYSIS 24
ECONOMIC ANALYSIS 34
PART II: POINTERS FOR OTHER MUNICIPALITIES
SITE, EQUIPMENT, AND MATERIALS REQUIREMENTS 40
SETTING UP AND OPERATING 44
FIGURES
1 SEWAGE TREATMENT PLANT AT BANGOR, MAINE 2
2 COMPOSTING SITE LOCATION 5
3 COMPOSTING SITE LAYOUT 7
4 PLAN VIEWS OF COMPOSTING SYSTEM LAYOUT 11
5 ASSEMBLING THE AERATION LINES ON THE COMPOST PAD 12
6 SELF-DRAINING WATER TRAP 13
7 A WATER TRAP IN PLACE 14
8 BED OF BARK Is LAID AND SLUDGE Is DUMPED ONTO IT 17
9 LOADER MIXES SLUDGE AND BARK AT MIXING AREA 18
10 SLUDGE-BARK MIXTURE Is PLACED ON PREVIOUSLY PREPARED
COMPOSTING PAD 19
11 REMOVAL OF AERATION PIPE PRECEDES PILE TAKEDOWN 22
12 ROTARY SCREEN Is USED TO SEPARATE BARK FROM COMPOST 23
13 PILE MONITORING LOCATIONS USED IN EARLY PHASE OF PROJECT 27
14 CURRENT PILE MONITORING LOCATIONS 28
15 CONCRETE TRENCH AERATION SYSTEM 46
TABLES
1 SLUDGE AND COMPOST ANALYSIS FOR SELECTED CONSTITUENTS 3
2 EQUIPMENT AND MATERIALS USED FOR COMPOST PILE 9
3 TEMPERATURE DATA FROM Two PLANES OF THE COMPOST PILES OVER
A 9-DAY PERIOD 28
4 NUMBER OF DAYS REQUIRED FOR AVERAGE PILE TEMPERATURE TO REACH
55°C FOR AAMBIENT TEMPERATURES ABOVE AND BELOW FREEZING 30
5 COLIFORM AND SALMONELLA IN SAMPLES OF SLUDGE AND COMPOST 30
6 FECAL COLIFORM FOR SELECTED COMPOST SAMPLES 32
7 CAPITAL COSTS FOR COMPOSTING—CITY OF BANGOR 36
8 STARTUP COSTS FOR COMPOSTING—CITY OF BANGOR 37
9 ACTUAL COSTS OF COMPOSTING SEWAGE SLUDGE AT DIFFERENT STAGES
OF THE PROGRAM
10 ANTICIPATED OPERATING COSTS FOR COMPOSTING DURING 1977 AT
BANGOR
ACKNOWLEDGEMENTS
The author extends special thanks to Ralph Mishou, Superintendent of the
Bangor Wastewater Treatment Plant and project director of theBangor composting
demonstration and to Walter Grant, Soil Scientist, U.S. Department of Agriculture -
Agricultural Research Service, Orono, Maine, for the time and effort they have
contributed to make this project a success.
U,S. Environmental Protection Agency
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COMPOSTING SEWAGE SLUDGE
by high-rate suction
aeration techniques
THE PROCESS AS CONDUCTED AT BANGOR, MAINE,
AND SOME GUIDES OF GENERAL APPLICABILITY
THE ULTIMATE DISPOSITION OF SEWAGE SLUDGE is a
major problem facing many municipalities today.
There are many ways in which sludge can be processed
prior to ultimate land disposal. These most commonly
include incineration and anaerobic digestion.
This report describes another method, a composting
process developed specifically for sewage sludge, and its
full-scale implementation at Bangor, Maine.
Composting is not a disposal method, just as
incineration and anaerobic digestion are not disposal
methods. All are simply processing techniques. After
any of them, one must still dispose of a final product.
However, the predisposal processing technique used
will not only dictate the nature of the ultimate disposal
method but may well determine its success.
The U.S. Environmental Protection Agency (EPA)
has been investigating economic and environmentally
sound methods of sewage sludge disposal with that
point in mind. One project being conducted by the U.S.
Department of Agriculture (USDA) with EPA funding
led to the adaptation of solid waste composting
procedures to sewage sludge only. While this process
has been successful with even raw sludges, the USDA
in
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work has not resolved many questions on system
operation and maintenance, cost, and climatic impacts.
Consequently EPA initiated a demonstration project
atBangor, Maine, to obtain answers to these questions.
The method developed by USDA is similar to the
Baden process developed in West Germany for mu-
nicipal solid waste and sewage sludge. It consists
simply of (1) mixing sewage sludge with a suitable
bulking agent, (2) placing the mixture over two parallel
aeration pipes and (3) drawing air through the pile with
a blower regulated to provide optimum aerobic corn-
posting conditions. The exhaust air is discharged into a
pile of bulking material for odor control.
This process produces a product which is (1) easily
stored until needed and (2) odor-free. Properly con-
ducted composting has been found to reduce fecal
coliform organisms to insignificant levels.
The City of Bangor had been dumping its raw
primary sludge over the embankment of an older
portion of the city dump. City officials recognized that
this method of sludge disposal was unacceptable and
that pending State regulations would prohibit this
practice. The city therefore requested EPA's assistance
in finding a suitable solution to its sludge disposal
problem.
After a review of various options, a preliminary
assessment of the composting process indicated that
composting might be the cheapest alternative, was not
capital intensive, and could be placed in operation in a
short period of time. As a result, the city applied to EPA
for a grant to demonstrate the forced aeration corn-
posting process.
The principal objectives of the demonstration grant
were to (I) demonstrate the technical feasibility of
forced aeration composting under cold climatic con-
ditions, (2) evaluate the costs of such a system, (3)
evaluate operational, maintenance and personnel
requirements, and (4) evaluate potential local uses of
the compost product. These objectives were to be met by
the following qualitative and quantitative evaluations:
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(1) evaluate different ratios of bulking agent to sewage
sludge to determine the optimum ratio; (2) determine
the degree of pathogen reduction; (3) make a subjective
evaluation of odors; (4) evaluate public reaction to the
composting operation; (5) determine the optimum
application rates for use of compost in city projects such
as recreation area development; and (6) determine the
value of compost as a replacement for loam, peat and
fertilizer to the city.
Composting got under way at Bangor on an opera-
tional scale in August 1975 and has since continued full
scale as the standard procedure there.
Studies are continuing and much more complete data
are being obtained on the on-going operation as further
improvements are made in it. In particular, much fuller
cost data will become available as variables including
drying, screening, the optimum level for bark recovery,
and the relative usefulness and marketability of
different finenesses of the compost product are further
examined.
However, the technical feasibility of the process has
been established and the operating procedures have
been developed to the point where they are ready for
testing and practical application by other munici-
palities if desired. Prior to beginning such an operation,
economics should be carefully evaluated. This interim
report is therefore being issued because of the wide
public interest in the project.
Part I covers specifically the experience at Bangor.
Part II is extrapolated from the Bangor experience, and
provides more general guidance to other communities in
establishing their own composting projects.
Where space for land disposal of sludge is a limiting
factor, composting obviously does not of itself solve the
problem, since the final volume is somewhat greater
than the original. However, composting may indirectly
offer major relief on the matter of disposal space—
through providing a product of far wider usefulness
than sludge.
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It will be noted that composting at Bangor also
sharply reduced the heavy metals content in the
finished compost over that which had been in the
unprocessed sludge. Where the metals are a critical
factor, composting will offer no final solution to the
problem of ultimate accumulation of heavy metals
residues in the soil when the disposal method requires
repeated applications of material to the same land.
However, substitution of compost for sludge could here
again significantly alleviate the problem, since a
greater number of applications of material could be
made before the critical residue level in the soil is
reached. Composting could make a contribution of
considerably larger scope towards the resolution of the
metals problem in another way, however— again indi-
rectly—because composting greatly increases the
number of potential uses for the product to be disposed:
from a material limited strictly to use on farming land,
as sludge now is, composting expands the potential
avenues for use to such areas as plant nurseries, home
lawns and flower gardens.
With these pluses, aerobic composting offers a
promising option as a predisposal treatment method
which eliminates the problems of pathogenic orga-
nisms and odor when reasonably careful operating
practices are observed.
The product is much more manageable than sludge.
And from the standpoint of aesthetics—which becomes
crucial when sites for ultimate disposal must be found—
by comparison with sludge disposal, the disposal of
compost should be far more acceptable to the public.
The EPA studies at Bangor are expected to be
essentially complete by May 1978. Once all the data are
compiled and analyzed, a final report will be issued on
the project.
VI
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part I
the operation at bangor
THE DIMENSIONS
The City of Bangor, Maine, has a permanent year-
round population of 38,900 residents, of whom 32,000
are served by the sewage treatment plant. The 32,000
provide a flow of about 3 million gallons per day
(mgd)-—or 11.4 million liters per day (mid)—to the
plant. Commercial and industrial establishments
produce an additional 1 mgd and 0.7 mgd respectively,
for a base flow of about 4.7 mgd. Infiltration and storm-
water runoff contribute an additional 2.3 mgd, making
the average total flow about 7 mgd (26 mid).
This flow receives primary treatment consisting of
passage through bar screens, settling in primary
sedimentation tanks, and prechlorination. Sludge is
pumped from the primary sedimentation tanks through
a hydrocyclone for grit removal. The sludge then flows
by gravity to two sludge thickeners. When the
thickeners are full as indicated by the torque on the
rotating bar, the sludge is pumped to a conditioning
tank where lime and a polymer are added and the
mixture is dewatered by vacuum filtration (Figure 1).
SLUDGE : QUANTITY AND CHARACTERISTICS
The treatment plant currently produces about 3,000
cubic yards or 2,500 tons per year (2,300 cubic meters or
2,260 metric tons per year) of vacuum-filtered sludge
with an average solids content of 20 percent. The
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FIGURE 1. SEWAGE TREATMENT PLANT AT BANGOR,
MAINE
The plant provides primary treatment followed by vacuum nitration, producing a
sludge with average solids content of 20%. The vacuum filters are operated about 70
times per year, producing 40 to 60 cubic yards of sludge each time. Annual output
totals about 3,000 cubic yards or 2,500 tons of dewatered sludge.
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vacuum filters are operated about 70 times per year,
producing 40 to 60 cubic yards (30 to 45 cubic meters) of
sludge each time.
The chemical characteristics of Bangor's sludge and
of the compost produced from it are analyzed for
selected constituents (Table 1). Bangor sludge would be
generally classified as a very good sludge in that it is
very low in heavy metals, and the compost would be
considered acceptable for land disposal.
No effect of metals on composting action itself is
known to occur. However, the metals content of the
compost product must be known in order to adequately
determine acceptable uses for the compost product. It
will be noted that some dilution of trace metals occurs as
the result of composting. This can be of benefit in the
event that the metals content of the raw sludge itself
does not meet standards established by regulatory
agencies for utilization of sewage sludge.
TABLE 1. SLUDGE AND COMPOST ANALYSIS
FOR SELECTED CONSTITUENTS
(Dry Weight Basis - Milligrams/Kilogram)
Constituent
Total sulfide
Total phosphorus
Total chloride
Total nitrogen
Cadmium
Copper
Chromium
Mercury
Nickel
Lead
Zinc
Iron
Arsenic
Manganesef
Potassiumf
Calciumt
Magnesium!
Sodiumt
Pile A*
Sludge
mg/kg
121.8
3002.2
661.8
19350.0
4.78
277.7
28.6
93.12
22.6
408.0
453.0
7550.0
0.394
110.7
1015.0
14429.0
2198.0
234.9
Screened
compost
mg/kg
0.5
1010.7
694.4
8620.0
0.67
83.9
17.0
1.46
25.2
118.1
153.7
4173.0
0.260
779.7
1946.0
23689.0
3602.0
698.0
Pile B*
Sludge
mg/kg
192.7
2052.6
718.2
10710.0
0.92
167.3
33.4
19.02
34.8
274.2
282.0
13708.0
1.738
295.0
1725.0
12986.0
4525.0
373.9
Screened
compost
mg/kg
0.5
787.3
762.3
6850.0
0.58
32.2
10.0
0.97
19.2
64.2
98.3
3041.0
0.208
616.0
1683.0
18163.0
3066.0
274.9
*The sludge used in Pile A and that used in PileB came from separate batches
processed through the sewage treatment plant more than 6 months apart.
•(•Increases noted in some cases are due to the bark in the compost.
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PREPARATIONS FOR COMPOSTING
ASSEMBLING THE TOOLS
This chapter describes in detail the composting site
and all other elements which it was necessary to bring
together and put in readiness in order to undertake
composting operations as currently conducted by the
City of Bangor.
There have been significant improvements in the
materials and equipment used and operational
procedures during the first 12 months of composting.
Other changes are currently being evaluated which it is
hoped will further reduce the cost of composting.
This discussion, however, is limited generally to a
description of the facility, equipment, materials and
practices currently employed at Bangor. The chapter on
Composting describes their actual use in the com-
posting operation.
SITE SELECTION AND PREPARATION
The composting site selected by the City of Bangor is
located about 3 miles from the sewage treatment plant.
This temporary location was necessary because space
available at the sewage treatment plant was not
adequate.
In selecting the site for the composting operation, the
city evaluated several possibilities on the basis of the
following criteria: (1) accessibility; (2) proximity to
residential, commercial and industrial areas; (3)
distance from the treatment plant; (4) required area (1.7
acres); (5) availability of electrical power; and (6)
ability to control surface water runoff.
The site ultimately chosen is located on an abandoned
taxiway at Bangor International Airport (Figure 2).
Accessibility is provided by a paved road leading to
the mixing area at the western end of the site. Sludge is
normally delivered to the site via Union Street and
Griffin Road. Because these roads serve residential,
commercial and industrial areas, they are kept clear of
snow during winter months. Thus the city does not have
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Experimental
Drying Area
Westgate
Mall
Pierce
o /:/ St.
Pump Station
Airport Terminal Bldg.
Scale: 1 cm = 120m.
FIGURE 2. COMPOSTING SITE LOCATION
Shy of space at its sew age treatment plant site, B angor hadtofinda separate site for
composting. It needed about 1.7 acres to handle the volume of sludge produced.
Other criteria were accessibility; location relative to residential, commercial and
industrial areas; distance; availability of electrical power; and ability to control
surface water runoff. The site chosen was an abandoned taxiway at Bangor
International Airport, about 3 miles from the sewage treatment plant.
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to make extra effort to provide access to the composting
site during winter months.
The site is about 2,000 feet from residential, com-
mercial and industrial areas. The compost operation,
while in plain view of the public, is not located in
anyone's "back yard."
The total area required for composting 3,000 cubic
yards per year of dewatered sludge at 20 percent solids is
about 1.7 acres. This area is divided into three sections:
storage of bulking agents, 0.2 acres (3 sq. ft./annual cu.
yd. of sludge); mixing and composting, 0.5 acres (7.7 sq.
ft./annual cu. yd. of sludge); screening operations and
screened and unscreened compost storage, 1.0 acre (1.5
sq. ft./annual cu. yd. of sludge). An additional 1.2 acres
(19 sq. ft./annual cu. yd. of sludge) is used for
experimentation with drying unscreened compost and
reusing it directly as a bulking agent.
Electrical power was available within 500 feet of the
composting site. This circumstance minimized efforts
and expenditures required in obtaining electrical power
to operate the aeration fans.
The ability to control surface water from the mixing
and active composting site was perhaps the single most
important factor in site selection for Bangor. There are
two sources of water which must be controlled: runoff
from precipitation, and condensation which forms in
the aeration pipes leading from the piles to the blowers.
Control of these sources is necessary because of the
probability of their being contaminated by organic and
inorganic chemicals and pathogenic organisms.
Control of these waters is provided by a drainage ditch
leading to the sanitary sewer line which transverses
this site downslope from the mixing and active
composting area.
Satisfaction of the factors listed provided the City of
Bangor with a composting site requiring minimal
preparation costs. Those costs were limited to providing
electrical power, establishing the drainage collection
ditch, and fencing the area. A detailed site plan was
prepared that depicts the location of all facilities
pertinent to the composting operation (Figure 3).
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FIGURE 3. COMPOSTING SITE LAYOUT
The flow of materials as composting operations proceed is from left to right above.
For obvious reasons, Bangor spotted its mixing area for sludge and bark closest to
the delivery road (Griffin Road, at left) for materials, and set up its bark storage
adjacent to both the mixing area and compost pile locations (A-F). Composted
product is moved from piles to upper right area to await screening and storage at
lower right, from where it is readily hauled away via Illinois Road at right. Drainage
ditch (top and bottom left) was dug to collect surface water runoff; delivers it direct
into previously existing sanitary sewer line (center).
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EQUIPMENT AND MATERIALS
The equipment and materials used for composting are
generally readily obtainable. Of the equipment and
materials needed (Table 2), only major items will be
elaborated upon here.
The city currently uses a front-end loader equipped
with a 3-cubic yard bucket for mixing of bark and sludge
and transporting bark and compost within the site as
necessary. During the first 6 months of operation,
loaders used were those already in the city's motor pool
inventory. These loaders, with smaller buckets than the
one now used, were found to be less efficient than a
larger unit would be. As a result the city purchased the
larger loader. This loader is used for other purposes also,
and an hourly rental charge is assessed when it is used
at the compost site.
The shredder/screener used for composting was
purchased by the city's motor pool. Unlike the front end
loader, this piece of equipment is used exclusively for
the composting operation. The shredder portion of the
screening device has been removed, however, and no
shredding is done, as it is more economical to separate
and recover as much of the bark as possible for reuse as
bulking agent. The screen is a self-cleaning rotary
screen, 3 feet in diameter and 6 feet long. The screen
opening currently used is 1 inch. However ^-inch arid
V^-inch opening replacement screens are available for
recovering greater quantities of bulking materials.
The blowers used to provide aeration are rated at 335
cubic feet per minute when operating under 4 inches of
water pressure. The on-off time of blowers is controlled
by a timer with 2-minute intervals.
The bulking agent used by the City of Bangor is bark
waste obtained from a pulp and paper facility of the
Diamond International Corporation. The bark is
provided to the city free of charge but is hauled 8 miles at
city expense. The purpose of the bark is to provide voids
for air movement and reduce moisture content so as to
provide a favorable environment for biological activity.
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TABLE 2. EQUIPMENT AND MATERIALS USED FOR
COMPOST PILE OF 50 CUBIC YARDS SLUDGE AND 150
CUBIC YARDS BULKING AGENT*
Item
General specification
Purpose
Pipe
100 ft. of 4-in. diameter carbon
steel pipe—80 ft.
perforated
2 30° 4-in. elbows per pile
4-in. flexible plastic pipe
Blower
Oxygen analyzer
Temperature
indicator
and probe
115 AC, 335 CFM 4-in. pipe
(Dayton Model 7C504 or similar)
Portable 0 to 25% dry gas
oxygen analyzer
Thermistor type with at least
a 3-ft probe and scale from
0 to 100° C
Aeration pipe
Blower connections
Connecting aeration
pipe to blower and
blower to deodorizing
pile
Aeration fan
Oxygen measurement
Temperature measurement
Timer with tripper 2-min. intervals on or off
Front-end loader 2.5 to 3.0 cu yd bucketf
Screen
Bulking agent
Self-cleaning rotary drum
3 ft diam x 6 ft long
with'/i V'2, or 1 in. openings
Ability to reduce moisture
content of mixed sludge and
bulking agent to below 60
percent and provide
adequate voids for
air flow
Control blower
operation
All materials handling
relative to composting
including mixing of bulking
agent and sludges
Separating reusable
bark from compost
products
Create optimal
condition for
aerobic composting
*38 cubic meters sludge and 115 cubic meters bulking agent.
tA larger bucket would be more cost effective if used only at the compost site or
for composting and other low-density materials only. Bangor uses the loader for
other types of operations and materials also.
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PILE AERATION EQUIPMENT SETUP
In the major remaining step prior to the mixing of
bark and sludge, the compost pad was prepared. Pad
preparation consists of connecting aeration pipe, water
traps, blowers and deodorizing piles (Figure 4). The
aeration pipe was a 4-inch diameter steel pipe with I-
inch holes at 1-foot intervals on opposite sides.
Two parallel 40-foot aeration lines made up from 7-
foot lengths of pipe were placed on the compost pad
(Figure 5). Short lengths of pipe facilitate pile take-
down. The lengths of aeration pipe were connected with
plastic connectors. The connectors were 10-inch lengths
of plastic flexible tube. These were cut down one side to
facilitate placement over the ends of the pipes being
coupled together. Coffee cans were placed over the open
ends of the aeration lines to keep out the sludge-bark
mixture.
Flexible plastic pipe was used to connect the aeration
pipe to the water trap and blower. In the early stages of
the project a layer of unscreened compost was used on
the pad both under and on the aeration lines. This
measure was discontinued later. The aeration system
was covered with a layer of unscreened compost to
prevent the wetter, freshly mixed bark and sludge from
plugging the aeration holes. The unscreened compost,
while partially beneficial, did not completely prevent
the material from entering the pipe and its use was
discontinued because of the large quantities of fresh
bark which it required.
A water trap was provided at each pile. The water trap
is a 55-gallon steel drum equipped with an automatic
draining device. All materials used in construction of
the water traps were essentially scrap or used parts
(Figure 6 and 7). The length of the drain tube must be
slightly longer than the maximum head of water pulled
by the blower, to prevent air from entering through the
drain. For the blower used atBangor, this length is 10
inches.
The quantity of water discharged from the water trap
10
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FIGURE 5. ASSEMBLING THE AERATION LINES ON THE
COMPOST PAD
Two parallel 40-foot aeration lines are made up on the pad from 7 ft lengths of 4 in.
diam. perforated carbon steel pipe connected by 10-in. plastic connectors. (Short
lengths of pipe facilitate pile takedown.) Flexible solid plastic pipe is used (right) to
connect aeration lines to water trap and blower. In early stages of the project, when
this photo was made, a layer of unscreened compost was used on the pad beneath
aeration lines, in addition to a layer placed over them, to prevent plugging of lines by
the sludge-bark mixture.
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5 S-L: o>s£
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i- Z U. HO. 5
FIGURE 6. SELF-DRAINING WATER TRAP
The water trap is a 55-gallon steel drum equipped with an automatic draining
device. The length of the drain tube must be slightly longer than the maximum head
of water pulled by the blower, to prevent air from entering the drain. For the blower
used at Bangor, this length is 10 inches. All materials used in construction of the
traps were essentially scrap or used parts.
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FIGURE 7. A WATER TRAP IN PLACE
The quantity of water discharged from the trap is affected by the length of flexible
pipe from pile to trap, the temperature difference between ambient air and exhaust
gas, and the moisture content of the exhaust gas. Placing water trap and blower as
close as possible to the composting pile will minimize amount of condensate to be
dealt with. Discharge air from the blower is still normally saturated; however, this
causes no problem.
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was affected by the length of flexible pipe from the pile
to the trap, the temperature difference between ambient
air and exhaust gas, and the moisture content of the
exhaust gas. Placing the water trap and blower as close
as possible to the composting pile will minimize the
quantity of condensate to be dealt with. The discharge
air from the blower is still normally saturated; however,
this does not cause any problem.
DEODORIZING PILES
Deodorizing piles were made with about 5 cubic yards
of bark to scrub odors from the exhaust air. It was found
that over time, the warm moist air caused settlement of
the deodorizing pile, and this settling eventually
restricted air flow. After this development was noted,
the deodorizing piles were disconnected in September
1976. Because previous checking of exhaust air suggest-
ed that deodorizing piles were unnecessary, at least
during cool weather, they were not replaced. The
operation since that time has still not experienced any
significant odor problems.
COMPOSTING
This chapter describes the composting process itself
as carried out at Bangor, beginning with the mixing of
the sewage sludge and bark and proceeding through
construction and active biological "working" of the pile,
on through pile take-down to storage with further
"curing" of the product on the premises of the compost-
ing facility prior to ultimate disposition elsewhere.
The expected effect of cold temperatures in slowing
down the biological activity, and measures developed to
counter this effect, are discussed.
The step-by-step description of the composting
process which follows, deals with the subject essentially
from the standpoint of the managing of a single pile.
However, at any given time at Bangor, at least three
piles were composting.
This fact has practical, economic importance for an
on-going operation: the sequential handling of multiple
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piles at different stages of maturity makes for econo-
mies of time and motion in terms of optimum use of
l«l">r>r «nH pmiinmpnf;
labor and equipment.
MIXING
On normal mixing days approximately 150 cubic
yards of bark and 50 cubic yards of sludge are combined.
This ratio of 3 parts bulking agent to 1 part sludge by
volume has been found by experience to result iri
adequate air movement within the pile. This ratio also
normally results in an initial moisture content of not
more than 60 percent, which is the maximum for a good
composting operation.
The sludge was delivered to the site in either 5- or 10-
cubic yard containers. Prior to receiving sludge at the
site, the loader operator laid down a bed containing
about 10 cubic yards of bark on the mixing area
(separate from the compost pad). The sludge was
dumped onto this bark bed (Figure 8).
The loader was then used to lift and dump the sludge
and bark in a rolling motion (Figure 9). As the material
was mixed, bark was added as necessary to achieve the
desired bark-to-sludge ratio of 3:1 by volume. This
general ratio was arrived at by the operator's knowing
the number of cubic yards of sludge delivered (each
truckload was either 5 or 10 cubic yards) and the number
of loader buckets (each 3 cubic yards) of bark.
BUILDING THE PILE AND COMPOSTING
As each batch was mixed, it was placed on the
previously prepared composting pad (Figure 10). After
all sludge had been delivered, mixed and placed on the
compost pad, the pile was covered with a blanket of
unscreened compost 1 foot thick. This blanket provided
insulation so that the outer edges of the pile would reach
the desired temperatures.
The blowers were then turned on and composting
normally was conducted for 3 weeks. Measurements of
temperature and oxygen were made 3 days per week.
16
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FIGURE 8. BED OF BARK Is LAID AND SLUDGE Is
DUMPED ONTO IT
On normal mixing days at Bangor about 150 cu. yd. of bark and 50 cu. yd. of sludge
are combined. These volumes make one pile. Sludge is delivered to the site in 5 cu. yd.
or 10 cu. yd. containers. In the final step preliminary to mixing, shortly before the
sludge arrives, the loader operator lays down a bed of about 10 cu. yd. of bark on the
mixing area near the compost pad. The sludge is dumped onto this bark bed for
mixing.
17
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FIGURE 9. LOADER MIXES SLUDGE AND BARK AT MIXING
AREA
The loader lifts and dumps sludge and bulking agent in a rolling motion to mix
them. As more sludge arrives at the site and mixing proceeds, bark is added as
necessary to achieve the desired bark-to-sludge ratio of 3:1 by volume. Operator
achieves and maintains this general ratio by knowing the number of cubic yards of
sludge delivered (each truckload is either 5 or 10 cu. yd.) and the number of loader
buckets (each 3 cu. yd.) of bark. The 3:1 ratio has been found to result in adequate air
movement within the composting pile; it also normally results in initial moisture
content of not more than 60%—the maximum for a good composting operation.
18
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FIGURE 10. SLUDGE-BARK MIXTURE Is PLACED ON
PREVIOUSLY PREPARED COMPOSTING PAD
As each batch is mixed, the loader places it on the pad. After all sludge is delivered,
mixed with bark and placed on the pad, the pile is covered with a blanket of
unscreened compost 1 ft thick. This blanket provides insulation so that outer edges
of the composting pile itself will reach desired temperatures. During winter months
when ambient temperatures are very low, the cover thickness may need to be
increased.
19
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These data were used to determine the blower on-off
time. (The chapter on monitoring and analysis presents
data.)
A time-temperature combination in which tempera-
tures in the pile stayed at 55 degrees Centigrade or
higher for at least 2 days should be sufficient to kill most
bacterial and viral pathogens.
COUNTERING COLD WEATHER EFFECTS
It was noted, predictably, that the time required for
average pile temperatures to reach 55° C. was greater
when ambient temperature were very low. It became
obvious from the data that during winter months cold
temperatures retard biological activity. The reason is
that after mixing, the temperatures of the bark and
sludge are very close to ambient. The initial low
temperature of the compost material, as well as the
colder air being used to aerate the piles, increases the
time needed for the piles to achieve a given temperature.
During winter months it has been found that operating
the blower enough to maintain oxygen at a minimum of
5 percent resulted in cooling of the outer edges of the
pile. During the winter of 1975-76 this problem was alle-
viated by increasing the thickness of the cover material
from 1 foot to 2 feet.
Several alternatives to this procedure are being
investigated. One is reversing air flow after internal pile
temperatures (3 feet from the surface) have achieved 55°
C. A variation on this would be to reverse the air flow
and use the hot exhaust from an adj acent pile as input to
the new pile so as to avoid cooling the center of the pile.
Another alternative is to turn the blower completely off
after internal pile temperatures have reached about
65° C.
A different approach embodies efforts to speed up the
initial achievement of 55° C. throughout the pile. One
such method being tried involves taking some of the hot
compost from a pile being taken down and placing the
hot material within the pile being constructed, to
20
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generate initial heat. Use of the combination of reverse
flow and hot exhaust air from an adjoining pile to
furnish initial heat would provide heat more uniformly
throughout a new pile.
With the latter method there is some possibility that
obnoxious odors could be produced, but based on current
experience, this is not expected to occur. In the use of
this method, however, care must be taken that the
exhaust air from the adjoining pile carries adequate
oxygen to the next pile to keep it aerobic.
It should be noted that the composting operation has
significantly improved since the project was initiated.
The improvements were made for both technical and
economic benefit.
PILE TAKEDOWN
The composting pile was taken down at the end of 3
weeks under normal operating conditions if tempera-
tures in the pile had been at 55° C. or higher for at least 2
days. The unscreened compost was placed in stockpiles
to await screening. Stockpiling of the compost also
allows additional time for further composting action to
continue.
Pile takedown was normally accomplished while
another pile was being built. This was possible at
Bang or because mixing time per load of sludge was
about 15 minutes less than the time required to deliver a
load to the site. While waiting for sludge delivery, the
loader was used to take down a pile. Chains attached to
the aeration pipe were connected to the loader, which
pulled the pipe from the composted material (Figure 11).
After switching to 7-foot links of aeration pipe rather
than the original 21-foot links, this was no longer
needed. After the first section of aeration pipe was
removed, the loader transferred the unscreened compost
to the storage area.
SCREENING
A rotary drum screen is used by the city to recover
bark from the compost (Figure 12). The city obtained its
21
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FIGURE 11. REMOVAL OF AERATION PIPE PRECEDES
PILE TAKEDOWN
The composting pile was taken down at the end of 3 weeks under normal operating
conditions if temperatures in the pile had been at 55° C or above for at least 2 days.
Chain attached to a hole cut in end of aeration pipe was connected to front end
loader, which pulled the pipe from the pile. After the first section of aeration pipe was
removed, the loader transferred the compost to storage area. Pile takedown was
normally accomplished at B angor while a new pile was being built. By using shorter
lengths of pipe under the current operation, a chain is no longer needed to remove
the pipe.
22
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FIGURE 12. ROTARY SCREEN Is USED TO SEPARATE BARK
FROM COMPOST
A self-cleaning rotary drum screen 3 ft in diam. x 6 ft long is used at Bangor to
recover bark and at the same time produce a more manageable compost product
with wider potential usefulness because of the improved carbon-to-nitrogen ratio
and greater fineness of the material ultimately to be disposed. The size of screen
openings used depends on the moisture content of the composted material. With 50%
or greater moisture, a 1-in. screen opening was necessary at Bangor. This yielded a
compost suitable for most uses where the material would be incorporated into
existing soil. The hopper on the right is filled with unscreened compost by a front-
end loader. A belt conveyer moves the material up to the rotating drum. The finer
compost falls through the screen while the bark comes out the end of the drum.
23
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rotary screen in August 1976. Thus, little material had
been screened by the time information was assembled
for this report.
The size of screen opening used depended on the
moisture content of the composted material. For Bangor
it has been found that with a moisture content of 50
percent or greater, a 1-inch screen opening is necessary.
If drying of the composted material were done, then a Vfe-
inch screen would function adequately, and this would
make possible the recovery of a greater quantity of bark.
The economics of greater bark recovery versus drying
has not been fully evaluated. The compost product
produced by the 1 -inch screen was, however, suitable for
most uses where the material would be incorporated
into existing soil. Regardless of the screen size used, the
screened compost has a high carbon-to-nitrogen ratio
and for many potential uses, additional nitrogen would
need to be applied to the soil.
Additional marketability and/or market value of
finer material could offset the added cost of drying.
Based on current available data, the cost of screening
with 1-inch openings without drying was $2.10 per cubic
yard of sludge processed.
MONITORING AND ANALYSIS
Monitoring is basic to the success of the entire
composting process.
Once a pile is built for processing, conversion of the
sludge to compost depends totally on biological activity
within the pile. This activity must proceed under
aerobic conditions (with adequate air supply) to ensure
destruction of pathogenic organisms and to eliminate
odors. Thus it is crucial to know at any given time how
the process is proceeding.
Two tests—both fortunately relatively simple to
perform— give the basic clues necessary. These are tests
to determine temperatures and oxygen content within
the pile. Data must be obtained to demonstrate
pathogen destruction. Monitoring of other factors is
24
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also essential. But pile temperatures and oxygen must
be read most often.
B ecause of the nature of the raw materials involved in
the process, variations in conditions will always occur
from pile to pile, and within piles. Temperature of the
ambient air is a dominant factor. Pile density varied as
a result of variable moisture content of the bark and
sludge used at Bangor. Thus the air flow varied from
pile to pile.
The potential for such changes on the scene of
operations gives added emphasis to the need for good
monitoring. Adjustments in operating conditions are
often necessary after a pile is built and the composting
process is under way. Keeping tab on temperatures and
oxygen in the pile enables the operator to make
adjustments which can markedly improve the efficien-
cy of the operation en route.
Permanent improvements in the process have been
made at Bangor based on analysis of the data, and other
potential improvements are being evaluated there.
Since monitoring is the key, the efficiency of the
monitoring procedure itself and its cost become impor-
tant.
This chapter describes the monitoring conducted at
Bangor; changes made with experience to cut monitor-
ing time and cost; the data collected; and how these
data were analyzed and the results put to use to improve
the overall composting process.
FOR THE RECORD: THE OBVIOUS
Knowledge of the nature and composition of the
sludge to be handled was a prerequisite for the entire
project (Table 1).
For completeness of records for future reference as
well as for current use, the following information was
recorded at the outset for each pile: (1) type of bulking
material used (bark and/or unscreened compost), (2)
mixing ratio, and (3) ambient air temperature.
PILE TEMPERATURE AND OXYGEN
Pile temperature and oxygen data provided the
25
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handle for "management" of the composting operation.
They were used directly to control the blower opera-
tion— to regulate blower on-off time to achieve thermo-
phillic aerobic composting.
At the onset of the project, temperature and oxygen
measurements were made at 16 locations throughout
each pile 5 days per week (Figure 13). This proved to be
very time-consuming because it required about 1 hour
per day per pile, and since there were three piles
composting at one time, monitoring took about 3 hours
per day or 15 hours per week.
At an average labor rate of $4.00 per hour, monitoring
thus was costing about $80 per pile for a pile containing
50 cubic yards of sludge. This translated to about $1,60
for each cubic yard of sludge composted. After the first 3
or 4 months it was decided that, for temperature
measurements, reading 3 days per week at the 16
locations would be sufficient. Monitoring costs, how-
ever, were still excessive, and the data were again
examined to see if further reduction could be made.
It was determined that the lowest temperatures were
occurring at the ends of the piles. It was also noted that
temperature variation within any plane surface was
minimal. Selected data from three locations each in the
east and west end planes of two compost piles over a 10-
day period provided an example of uniformity of
temperature within a plane (Table 3). Therefore it was
decided that a total of four readings, one location
representing each end and side of the pile would be
sufficient for temperature measurements (Figure 14).
The depth at which temperatures are read is equal to the
cover thickness. For a 1 -foot cover, readings are taken at
1-foot depth.
Oxygen data, however, do not always display the
same degree of consistency. Oxygen readings of 3 and
15 percent have been obtained within 6 inches laterally
at the same depth. This is believed to be due to
nonhomogeneity of air flow within the compost pile, the
result of different densities of material from one location
to another. Furthermore, the sampling tip of the oxygen
26
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FIGURE 13. PILE MONITORING LOCATIONS : SI:D IN
EARLY PHASE OF PROJECT
Initially temperature and oxygen readings were made at Bangor ~i da\-; per week at
16 locations throughout each compost pile as shown here This procedure required
about 1 hour per day per pile. Since there were 3 piles composting at one time.
monitoring thus took some3 hours per day or 15 hours per week. Later, the frequency
was reduced to 3 days per week at the same number of locations, but monitoring
costs were still excessive and a further examination of the data showed that the
number of monitoring locations could be reduced.
27
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TABLE 3. TEMPERATURE DATA FROM Two PLANES OF
THE COMPOST PILES OVER A NINE -DAY PERIOD
__~
Day
Plane Location 23459
Temperature CC
1
2
11
12
13
14
15
16
48
56
78
59
62
66
56
67
50
61
62
68
71
72
77
6(1
65
67
70
74
78
60
67
67
74
76
72
67
75
77
Plane
Location
Pile 13
Day
2 3
9
Temperature °C
1
2
11
12
13
14
15
16
11
11
22
11
12
12
11
12
22
11
12
12
54
53
54
50
51
51
62
60
58
54
55
54
8, 9, 10
Compost Pile
15,16
12,13,14 6,7 A' 1,2,3
Side View
A - A'
Cross Section
R — R7
FIGURE 14. CURRENT PILE MONITORING LOCATIONS
Once it was determined that the lowest temperatures within compost piles occurred
at the ends and that temperature variations within any plane were minimal, it was
decided that monitoring locations at each end and side of a pile would provide an
adequate temperature check. Only the four locations shown are now used routinely
for both temperature and oxygen monitoring. However, when low oxygen readings
are obtained, the operator must test additional locations to establish the real
pattern. New operators should check more than the minimum numbers of locations
for both temperature and oxygen for some months until they get a feel for the
process.
28
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probe is only 2 inches long; thus it is entirely possible
that low oxygen readings obtained adjacent to high
oxygen readings could be due to placing the probe into a
"glob" of sludge. It was, however, decided that the four
locations for temperature measurement would be used
for oxygen measurement as well. Then when low
oxygen readings (less than 5 percent O2) are obtained,
the operator must take additional oxygen readings in
different locations to determine if the low reading is true
of the pile in general or only the specific location.
The reduction in number of sampling points to four
has reduced monitoring cost from $1.60 to $0.24 per
cubic yard ($2.10 to $0.32 per cubic meter) of sludge and
has proved to be satisfactory for both temperature and
oxygen.
More recently it has been discovered that temperature
increased with increasing depth in the pile. This point is
still under investigation. Aspects of the response to such
conditions are discussed in the Composting chapter
and briefly summarized along with the presentation of
the actual data in the section which follows this one.
TEMPERATURE RELATIONSHIPS OBSERVED
The time required for average pile temperatures to
reach 55° C. increased during colder weather (Table 4).
As noted earlier, the effect of cold in winter was dual: the
colder air used to aerate the piles retarded biological
activity. The initial low temperature of the compost
mixture itself was also an inhibiting factor. For
example, ambient temperature was minus 4° C. when
pile 15 was being mixed. Pile temperatures recorded
immediately afterwards were between 1° and 2° C.
There was considerable variability in the maximum
temperature of the coolest location from pile to pile. It is
presumed that this was the result of poor mixing,
improper blower operation, and related factors. Varia-
bility was greater during subzero temperatures, sug-
gesting that cold weather accentuates this problem. (As
already observed, running the blower enough to
maintain at least 5 percent oxygen in the pile during
29
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TABLE 4. . NUMBER OF DAYS REQUIRED FOR AVERAGE
PILE TEMPERATURE TO REACH 55°C FOR AMBIENT
TEMPERATURES ABOVE AND BELOW FREEZING
Pile
1
2
3
4
5
6
13
14
15
16
17
18
Average ambient
temperature °C
17
17
9
17
12
10
-10
-17
-4
-5
-5
0
Da>sto55°C
2
2
2
4
6
3
8
11
18
14
9
7
TABLE 5. COLIFORM AND SALMONELLA IN SAMPLES OF
SLUDGE AND COMPOST
Pile
31
33
34
35
38
Sludge
Compost
Sludge
Compost
Sludge
Compost
Sludge
Compost
Sludge
Compost
Total
col iform
MPN*
430
15
240 x 106
.36
2400
240
240
.3
93
430
Fecal
coliforrn.
MPN*
430
36
2400
,3
.3
.3
.91
.3
.3
.3
Salmonella
MPN*
.3
,3
.3
.3
.3
.3
.3
.3
.3
.3
*MPN = Most Probable Number of organisms/gram
30
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winter months resulted in cooling of the outer edges of
the pile.) Temperature variability decreases as tempera-
tures in the pile approach 70° C (Table 3).
Various measures used or being studied to reduce the
time required to achieve 55° C. throughout the pile
during winter operations include increasing the thick-
ness of the unscreened compost cover; reversing the air
flow after temperatures 3 feet inside the pile have
reached 55° C.; reversing the flow but using hot exhaust
from another pile as input to the new pile; cutting off the
blower entirely after internal temperatures reach about
65° C.; placing inside the pile being constructed some
hot compost from a pile being taken down. These
alternatives are covered more fully in the chapter on
Composting.
BACTERIAL DESTRUCTION
Many piles have been sampled for bacteria at the end
of their composting to verify that adequate pathogen
reduction has been achieved.
Bacterial analysis of sludge and compost was limited
to total coliform, fecal coliform and salmonella. Data
were obtained on selected samples. During the first 6
months, the data collected were largely from compost
samples only. Later, data were obtained from both
sludge and composted sludge to show the degree of
pathogen reduction achieved (Table 5). As is apparent
from these data, Bangor's sewage sludge does not have
high bacterial counts.
This is because the sludge is limed to a pH of 11 or 12
prior to vacuum filtration. Fecal coliform does, however,
show a further sharp decrease upon composting.
Liming to pH 11 or 12 was standard operating procedure
at the sewage treatment plant to prevent odors during
vacuum filtration. As previously discussed, sludge
flowed from the primary settling tanks to the sludge
thickeners where it had a residence time of up to 7 days.
This resulted in anaerobic conditions and the produc-
tion of odors which were quite noticeable during
vacuum filtration.
31
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Fecal coliform counts are made on most completed
piles. For piles analyzed from pile 25 through pile 38, it
appears that fecal coliform counts are lower and more
consistent from pile 33 on (Table 6). Other data which
could possibly explain this change were examined in an
effort to account for it. The only factor which showed a
relationship was the number of days required for
average pile temperatures to reach 55° C. This time was
4 days for piles 25,31 and 32. Beginning with pile 33 the
time needed to reach 55° C. was about 3 days.
Basically the improvement can only be attributed to
general changes which had been made in the blower
operation, aeration system and deodorizing piles. In the
most recent compost piles, internal temperatures of 78°
to 80° C. have been attained routinely. This condition
has apparently resulted in establishing a natural air
flow induced by hot air escaping through the top of the
pile and drawing fresh air into the pile from the sides.
As a result, oxygen has been maintained at 10 percent or
higher even with the blowers off in some piles.
OXYGEN CONTENT IN RELATION TO ODOR CONTROL,
As previously mentioned, oxygen content is related to
odor production. Because odor cannot be quantified,
however, it is not possible to determine a precise
TABLE 6. FECAL COLIFORM FOR SELECTED COMPOST
SAMPLES
Pile Fecal coliform
25
31
32
33
34
35
36
38
MPN*
160
3.6
39
.3
.3
.3
.3
.3
*MPN = Most Probable Number of organisms/gram
32
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relationship between oxygen content and level or
strength of obnoxious odors.
Strong obnoxious odors have never been experienced
during the composting operation itself at Bangor
regardless of the compost pile oxygen content. Piles
which have had less than 5 percent oxygen during the
composting period have produced strong obnoxious
odors only while the pile was being taken down. When
oxygen content had been between 5 and 10 percent
during the composting period, some odor occurred
during pile takedown. In both sets of circumstances,
however, the odor dissipated within the boundaries of
the compost site. The strong objectionable odors have
occurred only three times. These occurrences all
resulted from either mixing with bulking materials
which were too wet or mixing during heavy rains. These
conditions resulted in compaction of the compost piles,
restricting air flow to the point where continuous blower
operation was required to maintain 2 to 3 percent
oxygen. Temperatures in such piles were generally in
the range of 50° to 70° C.
In the case of the separate bark piles used for
deodorizing, the unnoticed compaction occurring over
time resulted in restriction of air flow such that
maintaining oxygen above 10 percent in the compost
piles became increasingly difficult. This development
was discovered in early September 1976, and corrected
by replacing all deodorizing piles. As mentioned
previously, 55° C. can now be achieved in the compost-
ing piles in 1 to 3 days, and these piles will apparently
become self-aerating after 10 to 15 days.
Where oxygen was maintained at or above 10 percent
for at least the last few days, the odor during pile take-
down has generally been that of wet bark.
OVERVIEW
The scope of the project involved data collection for
compost management and pathogen destruction. The
readings taken have been useful for these purposes.
They do not provide sufficient data yet for statistical
33
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analysis. Nevertheless, several relationships of practi-
cal significance to the composting operation have been
established.
ECONOMIC ANALYSIS
The economic analysis presented here is based on
carefully documented expenditures for the period of
August 1975 through November 1976. Expenditures
during this period have been placed in two categories:
normal operating costs and development costs. The
latter include costs incurred for special testing for
pathogens and trying different materials or techniques.
This analysis is of the normal operating costs incurred
with currently used equipment, materials and tech-
niques. Alternative equipment, materials and tech-
niques, which may further lower composting costs, are
still being examined. The current cost for Bangor is
$11.41 per cubic yard ($14.91 per cubic meter) of sludge
based on costs for September through November 1976.
This cost is pertinent to conditions at Bangor, Maine,
for composting 3,000 cubic yards (2,300 cubic meters) of
raw vacuum-filtered sewage sludge of 20 percent
average solids content per year. Labor and equipment
hours are discussed as an aid to other municipalities in
estimating their costs. In development of a cost figure
for composting, a number of assumptions had to be
made for one factor—screening—because screening
had taken place for only 6 weeks prior to the assembling
of data for this report. Costs of screening are based on
one 4-hour test. More complete data are being obtained.
Another area where it was necessary to make
assumptions was in taking credit for reclaimed bulking
agent (bark) and compost product after screening.
These assumptions are discussed in greater detail later.
Costs for normal composting operations are broken
down into three areas: capital costs, startup costs and
operating costs. Each is discussed separately.
CAPITAL COSTS
Capital cost items have been broken down into two
34
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areas: major items and minor items. The major items-
land, front-end loader, and screen— are discussed here.
The minor items— aeration pipe, blowers, and monitor-
ing equipment—have been classified as startup costs
and are discussed in the next section. This separation
has been made here for clarity because of the budgeting
method used at the Bangor composting operation. That
method does not separate capital and operating costs,
but carries everything as operating costs. It does,
however, use one device (rental) for charging off major
capital items and a different device for charging off the
minor capital items—writing off the entire cost of the
latter in their first year.
Using its airport property for the composting site, the
city of course did not incur major capital expenses for
land acquisition and preparation. If land and prepara-
tion costs had, for example, been $40,000 for a 2-acre
site, these costs would have been $1.31 per cubic yard or
$6.88 per dry ton of sludge, and the cost of the process
would have been increased by identical amounts.
Actual capital costs were incurred for the loader and
screen. The cost of each, however, is passed on to the
user (the composting operation) as a rental fee (Table 7).
STARTUP COSTS
Startup cost items are items which have a projected
life of at least 5 years but cost less than $2,000 (Table 8).
These costs were borne by the city in their entirety in the
first year of composting. Replacement and upkeep cost
is estimated to be approximately $400 per year.
If startup costs had been capitalized over 5 years (the
anticipated replacement time) at 7.5 percent, the annual
cost would be $1,300 capital costs plus about $400
maintenance costs. As a result of their not being
capitalized, the first-year cost of sludge processing runs
about $1.30 per cubic yard higher than the annual cost
for the following years.
OPERATING COSTS
As mentioned, all capital costs associated with
35
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TABLE 7. CAPITAL COSTS FOR COMPOSTING
CITY OF BANGOR
Item
Description
Cost
Land acquisition
and preparation
Front-end loader
Screen
Area required is approximately
21 sq. ft. per annual cubic
yard of sludge at 20% solids.
Bangor used a section of
abandoned taxiway at Bangor
International Airport
Case W-24 front end loader
purchased by city's motor
pool. First priority is
composting. This equipment
rented to using department at
$10/hour to cover capital and
operating costs
A rotary screen purchased by
motor pool—rental charge to
composting (sole user) is
currently estimated at $8.00/hour
to cover capital and operating
expenses.
Purchase
Price
Annual
Cost*
No costs were incurred
$40,000 $5,702
$15,000 $2,136
*Annual cost computed on basis of 7.5% interest over 10 years
composting appear as operating costs. The cost infor-
mation presented here consists of actual costs, initial
and present and estimated costs based on optimum
operating efficiency.
The costs of composting decreased steadily over time
(Table 9). These costs include all process steps. Costs
went down from approximately $27 per cubic yard of
sludge to $10 per cubic yard as the project progressed.
Increased quantities of sludge composted and im-
proved operating efficiencies brought about this im-
provement. During the initial 12-month period, only 46
percent of sludge produced was composted, and many
piles were built containing less than 25 cubic yards of
sludge. All bulking agent used during this period was
fresh bark except for a few "experimental" piles for
which unscreened compost was used as the bulking
agent.
36
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TABLE 8. STARTUP COSTS FOR COMPOSTING
CITY OF BANGOR
Item
Description
Cost*
Drainage
750 linear feet of open drain
were constructed to collect
all water emanating from mixing
and active compost areas
750
Electrical hookup
Aeration pipe
Temperature
indicator
and probe
Oxygen analyzer
and probe
Plastic pipe
Blowers
Building
materials
300 ft of electrical wire
and a utility pole
500 ft of 4 in. standard wall
steel pipe for aeration
0 to 100° C
0 to 25% oxygen
250 ft of 4 in. flexible pipe
Three 115 V AC, 335 CFM
(Dayton model #7C504
or similar)
Housing for blowers
$ 400
$1,360
$ 700
$1,000
$ 250
$ 230
$ 750
TOTALf $5,440
*Cost includes installation where appropriate.
flf these costs had been amortized for 5 years at 7.5 percent interest, the cost per
year would have been about $1,300. Upkeep and replacement cost of the above items
for the ensuing year was estimated at $400.
TABLE 9. ACTUAL COSTS OF COMPOSTING SEWAGE SLUDGE
AT DIFFERENT STAGES OF THE PROGRAM
Sludge
processed
Period cu. yd.
Aug75-Aug76
Sept 76
Oct76
Nov76
Sept - Nov 76 Avg.
1,384
229
207
241
Totals costs* Unit costs/ cu. yd. of sludge
$/ cu. yd. of sludge Operating Bulking agent Screening
$27.06
$12.97
$12.25
$ 9.21
$11.41
$13.37
$ 7.87
$ 7.15
$ 4.11
$ 6.31
$9.00
$3.00
$3.00
$3.00
$3.00
$4.69
$2.10
$2.10
$2.10
$2.10
*Includes actual capital costs as shown in Table 7, startup costs as shown in
Table 8, and operating costs.
37
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Based on these experimental piles and improved
operator efficiency, the city, in August 1976, modified
the composting operation as follows: (1) all bark would
be used as bulking agent twice prior to screening, thus
reducing the cost of bulking agent by 50 percent; (2) all
sludge produced would be composted, thus all piles
contain about 50 cubic yards of sludge each; (3) the
building of one compost pile and the taking down of
another compost pile would be done on the same day as
much as possible.
The net effect of these changes has been reduction of
bulking agent costs by 66 percent and of operating costs
by 40 percent, based on average costs for the initial and
current periods of operation.
Based on the assumption that the procedures as
modified will be implemented at 100 percent efficiency,
the future annual cost for composting at Bangor is
projected to be $28,655, or $9.55 per cubic yard or $50 per
dry ton ($12.49 per cubic meter or $65 per dry metric ton)
of dewatered sludge (Table 10).
NET COST
The composting costs just presented do not take into
account any credits for reclaimed bulking material after
screening or for the value of the compost product. When
these credits are taken, final cost figures will be lower.
An accurate net cost cannot be developed at this time
because the balance of materials after composting and
screening is not known. The value of the compost
product has been set at $3.00 per cubic yard ($4.00 per
cubic meter) as a replacement for loam in city projects.
The net costs presented here are based on the following:
(1) the volume of material is reduced by 10 percent
during the composting process (assumed), and further
reduction may occur in the stockpile; (2) the ratio of
reclaimed bulking agent to compost product using a 1-
inch screen is 1:3.7 (estimated on the basis of 4 hours of
testing); (3) all bulking agent is used twice prior to
screening.
Under those conditions Bangor should produce 4,050
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TABLE 10. ANTICIPATED OPERATING COSTS FOR
COMPOSTING DURING 1977 AT BANGOR
Operation
Hours required
Costs in dollars
Labor Equipment Total
Mixing and
stockpiling1
Monitoring2
Screening1
Bulking agent4
Miscellaneous5
Cost totals
600
280
312
300
870
2,526
1,000
1,248
2,400
3,480
$10,654
6,000
3,900
5,655
$15,555
8,526
1,000
1,248
6,300
9,135
2,446
$28,655
'Labor required in setting up the aeration system for each pile.
2Monitoring requires approximately 1 hour per day and is currently done 3 days
per week.
3These are estimates based on limited practice. The screen has a rated capacity
of 35 cu. yd./hour. It was operated at 20 to 30 cu. yd./hour for a 4-hour period.
Projections here are based on a rate of 20 cu. yd./hour.
""Transportation costs for obtaining 4,500 cubic yards of bark for bulking agent
using city-owned and -operated dump truck with a 6-8 cubic yard capacity.
5Repair or replacement of blowers, aeration pipe, etc. and overhead of $1,240.
cubic yards (3,100 cubic meters) of unscreened compost
per year. After screening, this will be split into 862 cubic
yards (660 cubic meters) of reclaimed bulking agent
valued at $1.00 per cubic yard ($1.30 per cubic meter)
and 3,188 cubic yards (2,440 cubic meters) of compost
product valued at $3.00 per cubic yard ($4.00 per cubic
meter). Total credits at the end of the year are then
$10,426 or $3.47 per cubic yard ($4.54 per cubic meter) of
sludge.
If the assumptions listed are accurate, then based on
current operating costs, Bangor's net cost for compost-
ing was $6.08 per cubic yard or $31.82 per dry ton ($7.95
per cubic meter or $41.62 per dry metric ton) of sludge.
39
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part II
pointers
for other municipalities
SITE, EQUIPMENT, AND MATERIALS REQUIREMENTS
This chapter and the next constitute an attempt to
distill what has been learned of the process of convert-
ing sewage sludge to compost, as now routinely
conducted full scale atBangor, Maine, and to condense
the essence of the Bangor experience into a sequence of
practical pointers that can be generally applied else-
where. The step-by-step discussion is meant to provide
guidance for other municipalities interested in esta-
blishing a similar process. For convenience, the subject
is sorted by chapters into the preparatory and actual
operating stages.
On some points it was not possible to generalize, and
therefore specific methods, criteria, or equipment used
at Bangor are given. This does not imply that other
methods, criteria, or equipment will not work as well.
But this approach does serve as a reasonable starting
point. As was found at Bangor, periodic modifications
should be made to achieve greater efficiency in compost-
ing.
It must be kept in mind that composting is an
alternative method of stabilization prior to utilization.
At present it appears that the method can be competi-
tive on the basis of cost with other options. Composting
should not necessarily be viewed, however, as a break-
even or profit-making operation.
40
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SITE REQUIREMENTS
LOCATION. The composting operation should be
located at the sewage treatment plant if at all possible.
This will eliminate sludge transportation costs and
reduce costs associated with the collection and treat-
ment of condensate and surface water runoff at the site.
SIZE . The size of the site will depend on the quantity
of sludge produced and how fast the compost product is
removed for utilization. Each compost pile will occupy
an area approximately 20 x 60 ft Bangor has found that
an additional minimum of 10 ft between piles or other
obstructions is necessary for loader maneuvering
during pile construction and takedown. Therefore,
approximately 2,000 sq. ft. are required per pile for 21
days. In addition, a storage area is needed for bulking
material, unscreened compost, and screend compost.
Sufficient area should be available to store 3 months'
bulking agent and up to 6 months' accumulation of
unscreened and screened compost. Additional area is
required for mixing. This can be as little as 2,000 sq. ft. if
mixing is done in batches of up to 10 cu. yd. of sludge.
The area required for screening is minimal, being only
that necessary for the screen and for maneuvering of
the loader used to fill the screen hopper. Bangor uses
about 1,000 sq. ft. for screening operations for the entire
composting of 3,000 cu. yd. of sludge per year. Bangor
has a composting site of 65,000 sq. ft.
CONTROL OF DRAINAGE WATER. Surface water runoff
from the mixing and active composting pile areas must
be collected because of possible contamination with
pathogenic organisms and organic and inorganic
pollutants. The quantities of surface water requiring
collection and treatment will depend on local climatic
conditions and size of affected area. In addition to
surface water, condensate must be collected and treated.
Condensate forms in the aeration system from the
compost pile to the deodorizing pile. The condensate
may also contain some leachate which can enter the
aeration system. Quantities of condensate up to 20
gallons per day per pile have been obtained.
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If the site is at the treatment plant or near a sewer line,
both sources of water can easily be returned to the
treatment plant. At remote compost locations, it may be
necessary to collect such water in a holding pond.
SITE SURFACE. The surface of the compost site
should be of an all-weather type. Asphalt or concrete
surfaces are preferred; however, hard-packed gravel
will suffice. Where a gravel pad is used, it may be
necessary to provide an underdrain to prevent ground-
water contamination. Furthermore, a gravel surface
will most likely require periodic maintenance.
OTHER SITE CONSIDERATIONS. Other factors which
must be considered at any site, but especially sites
located away from the sewage treatment plant, include
the following: availability of electrical power, availabil-
ity of sanitary facilities, and ability to secure emergency
services.
EQUIPMENT AND MATERIALS
Equipment and materials listed here are sufficient for
composting up to 75 cu. yd. (60 cu. m.) of 20 percent
solids sewage sludge per day on a 5-day/week basis.
Where sludge production exceeds this quantity, other
system designs currently being developed by USDA at
Beltsville, Maryland, should be used for economy.
ESSENTIALS REGARDLESS OF NUMBER OF PILES. The
equipment itemized here is required for composting
regardless of the number of piles constructed per week.
FRONT-END LOADER. This loader must be of suffi-
cient capacity to construct a pile 8 to 10 ft high and
about 20 ft wide at the base without running upon the
pile. Running upon the pile would compact the pile to
the extent that adequate aeration cannot be obtained.
The materials handled, bulking agent and sludges,
have densities of 1,000 to 1,700 Ibs./cu. yd.
The bucket size should match the loader's capacity.
Bangor used a Case W24B articulating loader equipped
with a 3-cu. yd. bucket. This machine was capable of
handling compost with a 5-cu. yd. bucket. O bviously the
larger bucket would reduce the time required to move
42
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material from one location to another. Bangor's
particular circumstances were such that this reduction
in time was without economic benefit. Also, aside from
the composting operation, the loader was used to load
sand onto trucks periodically. The sand which would fill
the larger bucket would be too heavy for the loader
equipped with the larger bucket.
OXYGEN ANALYZER AND PROBE . The oxygen analyz-
er should be a portable dry gas analyzer with a 0 to 25%
percent oxygen range. The probe, 5 ft in length, should
be made of durable material such as stainless steel. Inlet
holes about 1/16 inch in diameter should be located
about 2 inches back from a pointed tip. Plastic tubing
can be used to connect the probe to the analyzer. A small
hand vacuum pump is required to draw air into the
analyzer.
TEMPERATURE INDICATOR AND PROBE . There are two
types of temperature measuring equipment which can
be used. Both should have a range of 0 to 100° C. A
portable battery-operated temperature indicator with a
thermistor probe provides a single piece of equipment
which can be used for all required temperature measure-
ments. The probe length should be at least 3 feet; a 5 foot
probe length is preferred.
Dial-type thermometers can also be used. A probe
length of 3 feet is recommended to allow for taking
readings at varying depths. One dial-type thermometer
or one portable thermistor temperature measuring
device is sufficient for any number of composting piles.
SCREEN. The screen is used to remove bulking
material from the composted material. This recovers
bulking agent which can be reused, and produces a finer
compost product. Where the bulking agent is very fine,
screening may not be necessary.
ESSENTIALS FOR ONE PILE. The equipment and
materials described next are necessary for the compost-
ing of one pile of sludge and bulking material contain-
ing about 50 cu. yd. of sludge and 100 to 150 cu. yd. of
bulking agent.
Pipe— 4 -in. steel pipe in 7 or 10 ft lengths; 80 ft must be
43
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perforated (two 35- to 40-ft aeration lines are required).
Perforations should be 1 in. in diameter every 1 ft on
opposite sides of the pipes. An additional 10 to 20 ft of
nonperforated steel pipe, 2 elbows, 1 wye and several 10
in. lengths of plastic 4 in. diameter flexible tube slit
down one side are required to complete the aeration
system hookup. The elbows and wye should be such that
the parallel aeration lines are 7 to 8 ft apart.
Flexible Pipe—4-in. flexible plastic pipe has worked
well in making connections between aeration pipe,
water traps, blowers and deodorizing piles. The length
will depend on distance to the blower, etc., but should be
minimized.
Blowers—The blowers used by Bangor are rated at
335 CFM under 4 in. static pressure (Dayton model
7C504 or similar). Each blower is powered by a 115 V
AC, 1/3 HP motor.
Timer—A timer with 2-minute intervals provides
satisfactory control of blower operations.
Shelter—A small accessory building to house blow-
ers, timers and electrical connections has been found
beneficial at Bangor to shelter the equipment.
Water Traps—A water trap for each compost pile is
necessary to collect condensate in such a manner that
the blower does not have to draw against a head of
water.
Miscellaneous Items—Duct tape, caulking com-
pound, and hose clamps as needed to seal blower arid
aeration pipe connections.
Bulking Agent—Bark waste from a pulp and paper
plant is used by Bangor. Many materials which can
absorb moisture and provide voids for air movement
would make suitable bulking agents. For bark waste a
mixing ratio of 3 parts bark to 1 part sludge by volume is
required if the bark is at 60 percent moisture. The
mixing ratio can be decreased if the bark is drier. For
other materials the mixing ratio will have to be
determined.
SETTING UP AND OPERATING
In preparation for composting, adequate planning
44
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must be given to pad layout and equipment setup to
minimize land requirements and travel time. Figures 3
and 4 show the general site layout and compost pile
design used at Bangor. The compost pile design will be
similar for any community using this system. However,
even here some variation is possible as needed to fit
local conditions.
A description of the permanent and nonpermanent
equipment setup for an individual compost pile follows.
THE AERATION SYSTEM AND ACCESSORIES
AERATION PIPE. The pipe is laid on the ground
surface in two parallel 35- to 40-ft lines. A bark pad
under the compost pile as well as the layer of coarse bark
on top of the aeration system are no longer considered
necessary in Bangor.
Other than steel pipe can be used for aeration, such as
plastic drain pipe. Where a community is only evaluat-
ing the process, the plastic pipe is recommended
because of lower initial cost.
A more permanent aeration system providing greater
economy can be obtained by use of a trench into which
the aeration pipe is laid (Figure 15). The aeration pipe is
connected by solid pipes to the water trap.
WATER TRAP . The water trap should be regarded as a
permanent installation. As the hot moist air leaves the
pile, water begins to condense in the portion of aeration
pipe exposed to ambient temperatures. The quantity of
condensate produced will depend chiefly on ambient
temperatures and length of aeration piping exposed.
Under variable conditions at Bangor the condensate
has been found to vary from 6 to 20 gals, per day per pile.
The amount can be minimized by keeping the length of
pipes from the compost pile to the water trap as short as
possible and by wrapping all exposed pipes with
insulation.
BLOWER. The location of the blower is critical in
areas where freezing temperatures occur. Bangor has a
blower located about 2 ft above and within 5 ft laterally
of the water traps (Figure 4). This location is necessary
45
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Top View
Plane View
Sample
Locations
Compost
Pile
'"///i
Blower
| Water
Deodorizing Trap
Pile
Aeration
Pipe
Cover
Material
Ground
Surface
FIGURE 15. CONCRETE TRENCH AERATION SYSTEM
When a municipality is committed to the compost process, a more permanent
aeration system providing greater economy than the surface pad arrangement is
obtained with use of a trench into which the aeration pipe can be laid below ground
level. Concrete is advised for the permanent trench lining.
46
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to prevent entry of condensate into the blower which
could then freeze during periods when the blower is off.
This problem occurred during the winter of 1975-76,
causing several blower motors to burn out. A small
building is provided at Bangor to house the blowers as
well as all electrical connections.
At present it is not known whether this arrangement
will eliminate the problem. Other solutions include
placing a small drain hole in the bottom of the blower
housing to prevent a buildup of condensate, or wrap-
ping the blower housing with insulating tape.
SCRUBBER PILE. Exhaust air from the blower is
passed through a scrubber pile constructed of about 5
cu. yd. of bulking agent to remove odors which may be
present. Moisture in the exhaust air will eventually
cause the scrubber pile to compact and restrict air flow.
Based on experience at Bangor, it is recommended that
scrubber piles be replaced every 2 months.
OPERATING: FROM PILE CONSTRUCTION TO STORAGE
Sludge quantities and time in which deliveries are
made will vary depending on site location and dewater-
ing equipment capabilities. However, all sludge should
be mixed and made into a pile on the day of delivery so
as to minimize any fly or odor problems. Beginning
shortly before the first load is to arrive, the following
sequence (based on delivery of sludge in 10 cu. yd. loads)
is performed for each pile in carrying out the actual
process of converting sludge to compost.
Step 1—Mixing
The loader operator spreads 10 to 15 cu. yd. of bulking
agent in a 6 to 8 in. layer onto which sludge is dumped
(Figure 8). The loader then begins rolling the material
over with the bucket (Figure 9). Bulking material is
periodically added to achieve the desired mixing ratio.
Mixing will take about 30 min. for a batch consisting of
10 cu. yd. of sludge and 30 cu. yd. of bulking agent.
Step 2—Building
When the material in Step 1 is thoroughly mixed
(determined visually), it is ready to be placed on the
47
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aeration pipe. Starting at the blower end of the pipe, the
loader operator places the material on the aeration
system (Figure 10). Steps 1 and 2 are repeated until all
sludge has been mixed and placed on the aeration pipe.
The pile should then be about 8 ft high at the center, 20 ft
wide at the base and 50 ft long at the base.
Step 3 —Cover Material
A cover of unscreened compost should be applied.
Bulking agent can be used as the cover material for
initial piles; however, unscreened compost is the most
economical cover material. The cover provides needed
insulation to ensure that temperatures of all new
material in the pile reach at least 55° C. Cover material
thickness, therefore, depends on ambient temperatures.
During periods when ambient is below 0° C., 2 ft of cover
is required. For a pile of the dimensions listed, each 1 ft
of cover requires about 50 cu. yd. of cover material. In
placing both material for composting and cover, care
should be taken not to run the loader's tires onto the pile.
Step 4—Blower Operation
This is one of the two most critical factors (proper
mixing is the other), as blower operation (time on-off)
affects both temperature and oxygen content of a
composting pile. In areas where extremely cold winter
temperatures are experienced, blower operation is
extremely critical.
The blower on-off cycle time depends upon the
velocity of air movement, distance from the outer edge
of the pile to the aeration pipe, and the oxygen
consumption rates. For composting to be aerobic, some
oxygen must be present throughout the pile. It is
desirable to have at least 5 percent oxygen. When
ambient air temperatures are very low, the provision of
sufficient air flow to maintain oxygen at 5 percent near
the aeration pipes may result in excessive cooling of the
outer edges of the pile such that 55° C. is not achieved at
this time.
Based on one year's experience atBangor, it has been
found that pile temperature data provide the best
information for blower control, and can be augmented
by oxygen data where necessary. When the ambient
48
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temperature is above 10° C., adequate aeration and
temperature buildup have been attained with the
blowers timed for 4 min. on and 8 min. off. This time
cycle seldom requires changing during warm weather.
During cooler times of the year when ambient tempera-
tures are less than 10° C., two problems develop which
require careful management to overcome. These are the
cited delayed increases in pile temperature and exces-
sive cooling.
Delayed temperature increase results when the sludge
mixture cools during mixing to less than 4° C. At such
temperatures biological activity is greatly reduced and
thus little heat is generated. Once the pile temperature
reaches about 10° C., the increase in temperature
proceeds normally. At Bangor, piles have taken from 5
to 15 days to reach 10° C. This time lag can be overcome
by using hot unscreened compost as bulking agent
and/or "seeding" a pile with hot unscreened compost as
it is being built. The latter method consists simply of
placing a few cubic yards of hot unscreened compost
within a pile being built and, once it is built, covering it
with hot unscreened compost. The blower should be left
off or set for 4 min. on, 26 min. off, until pile
temperatures reach 10° C.
The second problem, excessive cooling, can be
avoided by increasing the thickness of the cover
material. A 2-ft cover works well most of the time. This
may need to be increased to 3 ft during extremely cold
weather.
At present, no better guidance can be given. However,
it is worthy of note that after composting more than
2,000 cu. yd. of sludge in 45 piles, Bangor has had only
two piles which were failures. Both of these occurred
under "experimental" conditions and were predicted to
fail after the first few days. No serious odor problems
developed as a result of these failures.
Step 5—Monitoring
Monitoring of the compost process for temperature
and oxygen is required to ensure that adequate
temperatures are attained in all portions of the pile.
It is suggested that eight locations per pile be
49
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monitored for both temperature and oxygen for a least
the first 6 months. Based on experience at Bangor, the
monitoring locations shown in Figure 14 are recom-
mended with measurements taken just beneath the
cover and at depth of 3 ft. While this monitoring
program is more rigorous than that currently used at
Bangor, it is neccessary initially in order for the
compost operation personnel to develop a thorough
understanding of how the process changes in relation to
pile conditions, weather and other factors.
After the first 6 months, most operators should be able
to obtain adequate control using only the shallow
locations for routine measurement and using the deeper
locations or other locations only for troubleshooting as
needed. Moreover, as the operator gains experience he
will find that oxygen measurements are valuable only
as a diagnostic measurement in troubleshooting prob-
lem piles.
In use of the temperature and oxygen measuring
equipment, careful attention must be given to carrying
out the manufacturers' recommended procedures for
calibration prior to each day's use.
Step 6—Stockpiling
The composting process will take 15 to 21 days
depending on pile characteristics and climatic condi-
tions. At the end of the composting period the pile is
removed from the compost pad and placed in storage to
await further processing if necessary. Stockpiles should
be made as high as possible to conserve space. The
transfer consists simply of using the loader to move the
composted material from the compost pad to the
stockpile.
The composted material should be cured in the
stockpile for at least 30 days. If the nature of the bulking
agent is such that screening is required, the compost
should stay in storage for about 6 months to allow the
material to dry out for more efficient screening. If space
is not available, an area will need to be provided for
drying the compost sufficiently to be screened. The
upper limit of moisture will depend on the bulking agent
and the screen being used.
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