f'« •
composting dewatered
sewage sludge
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LIBRARY OF CONGRESS CATALOG CARD No. 73-603142
Public Health Service Publication No. 1936
For sale by the Superintendent of Documents
U.S. Government Printing Office
Washington, B.C. 20402
Price 45 cents
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foreword
SOMETIMES IT is SAID that solid wastes are all those wastes that
don't go up the chimney or down the sewer. One exception to
this rule-of-thumb definition is that the waterborne wastes in sewers
eventually are treated to effect removal of the solids. Thus, the final
disposal of sewage sludge becomes a concern in solid waste
management.
Among the methods used for disposing of sewage sludge are:
anaerobic digestion, incineration, burial, and application to farm-
land. All of these methods depend upon the concentration of the
sewage sludge solids for the best operation, and, except for anaerobic
digestion, this usually includes dewatering the sludge. Except for
incineration, these dewatering systems produce a sludge that still
contains 70 percent moisture. Further dewatering of the sludge to
reduce its volume and weight is desirable to facilitate its disposal.
This study demonstrates the technical feasibility of composting to
effectively treat dewatered sewage sludge alone to produce a stable
hygienic material. The final compost had a quality similar to soil
conditioner, the nitrogen-phosphorus-potassium ratio was the same
as in cattle manure, and the material was found to be free of viable
plant seeds and pathogens. Thus, the field of solid waste manage-
ment is presented with another engineering alternative, which
includes the possibility of a final disposal for sewage sludge with a
concomitant transformation of waste solids into useful solids.
—RICHARD D. VAUGHAN, Director
Bureau o] Solid Waste Management.
Hi
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contents
INTRODUCTION, 1
SCOPE OF WORK, 1
PROCESS DESCRIPTION, 2
DESCRIPTION OF THE PILOT PLANT, 2
TEST TECHNIQUES, 4
COMPOSTING PROCESS, 5
SURVIVAL OF PATHOGENIC ORGANISMS, 23
DESIGN CRITERIA, 26
CONCLUSIONS, 27
ACKNOWLEDGMENTS, 27
REFERENCES, 27
FIGURES
The Process for Composting of Dewatered Sewage Sludge, vi
Composter Pilot Plant, 3
Effect of Moisture on Compost Particle Size, 6
Effect of Retention Time on Percent Solids Reduction, 8
Effect of Retention Time on Percent Water Reduction, 9
Effect of Retention Time on Percent Weight Reduction, 10
Effect of Retention Time on Percent Volume Reduction, 12
Dewatered Sewage Sludge Composting—Composting Material Balance, 13
Effect of Intermittent Feeding on Compost Bed Temperature, 14
Efficiency of Oxygen Uptake Rate, 15
Effect of Air Flow Rate on Compost Bed Temperature, 16
Effect of Air Flow Rate on Oxygen Concentration, 17
Effect of Air Flow Rate on Carbon Dioxide Concentration, 18
Gas Chromatograph Separation of Composter Off Gases, 19
Time-Temperature Relationship in Compost Curing Process, 21
Assay for Indicator Organisms in Composted Solid Human Wastes, 24
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TABLES
Sources and Quantities of Solids to be Treated at 1-mgd Sewage Treatment Plant, 26
Design Criteria for Composting Plant Handling Solids from a 1-mgd Sewage Treatment Plant, 26
abstract
Stabilization and final disposal of sewage sludge is of primary
concern in waste treatment. The purpose of this study was to de-
termine the feasibility of composting dewatered sewage sludge,
to establish basic design criteria, and to determine the quality of
the final compost.
The pilot-plant study was made with the use of a 40-cu-ft mechan-
ical composter designed by The Eimco Corporation. Provisions were
made to mix and aerate the compost at all times. A pilot, vacuum,
continuous belt-type filter was used to dewater sewage sludge before
it was fed into the composter.
The following general conclusions were established based on
results of the pilot-plant operation.
1. Composting of dewatered sewage sludge is technically feasible.
The data indicated volume and weight reductions of 60 to 85 percent.
2. Mixing and aerating on a continuous basis were required to ensure
a high rate of treatment and to maintain aerobic conditions.
3. Recycling the compost was beneficial in adjusting moisture and
in seeding the filter cake feed to the composter.
4. The final compost had a fertilizer value (nitrogen-phosphorus-
potassium) about the same as cattle manure and was found free of
viable plant seeds and indicator pathogens.
5. Chemicals used for conditioning of sewage sludge before dewater-
ing affected process capacity and final product characteristics.
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LIME
STORAGE
METERING
PUMP
SLUDGE
SOURCE
SLUDGE
STORAGE
FeCb
STORAGE
METERING
PUMP
D FLOCCULATOR
(MIX TANK)
-2
SLUDGE
PUMP
EIMCOBELT
VACUUM FILTER
COMPOST
STORAGE
1
:AKE
i
\
-^
R
E
C
Y
C
E
r— +~ OFF GASES
MECHANICAL
COMPOSTER
^ — AIR
RECYCLE AND CAKE
FILTRATE
CONVEYORS
FIGURE 1. The process for composting of dewatered sewage sludge.
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composting dewatered sewage sludge
TREATMENT AND FINAL DISPOSAL of Sewage
sludge represent a major cost in building
sewage treatment plant facilities. Two major
methods presently used for treatment and dis-
posal of sewage sludges are anaerobic digestion
and incineration. Other methods such as burial
of dewatered sludge and application of un-de-
watered sludge to farmland have severe limita-
tions. When the problems involved with these
presently used disposal methods are considered,
a reduction of volume and weight would be very
beneficial. Composting dewatered sewage sludge
offers this possibility.
Composting of organic matter such as garbage
has been practiced for many years. Composting
can be defined as decomposition of organic waste
by aerobic thermophilic organisms to produce a
stable humus-like material. The byproducts of
this treatment process are carbon dioxide, wa-
ter, and heat. The compost produced can be
used effectively as a soil conditioner with a fer-
tilizer value about the same as cattle manure,
or as innocuous, odor-free landfill that does not
need additional cover material.
Composting of refuse has had many problems.
Some of these problems are associated with the
costs of separating and grinding large volumes
of refuse and obtaining a market for the final
product to offset some of the costs of disposal.
The treatment of a combination of refuse and
sewage sludge by composting is presently being
studied and used.1'2 The addition of sewage
sludge to refuse composting is said to accelerate
the decomposition and improve the final com-
post.3 It is commonly accepted that dewatered
sludge (containing 70% moisture) from a com-
munity represents only about 15 percent by
weight of the total solids disposal problem. The
work completed in this study illustrates the
ability of composting to effectively treat de-
watered sewage sludge alone to produce a stable,
hygienic material.
SCOPE OF WORK
Composting of sewage sludge alone is a new
approach to a very old problem. Preliminary
work indicated that weight, volume, and solids
reduction by this treatment method appeared
very attractive. On the basis of favorable results
from preliminary work, a project proposal was
prepared. This project was funded by the
Bureau of Solid Waste Management as a re-
search contract. The test program was designed
to obtain the following basic information con-
cerning the composting of a combination of
primary and secondary dewatered sewage
sludges: (1) effect of mixing; (2) effect of mois-
ture content; (3) effect of recycling; (4) process
capacity; (5) air requirements; (6) effect of
chemicals used for conditioning the sludge for
dewatering; (7) composition of off-gases; (8)
destruction of pathogenic bacteria, fungi, nema-
todes, and viruses; and (9) chemical and physi-
cal composition of the final compost.
All of the 16-month study, except that related
to the destruction of pathogenic organisms, was
performed by The Eimco Corporation Sanitary
Engineering Research and Development staff
under Director A. A. Kalinske. The work on the
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destruction of pathogenic organisms was per-
formed at the University of Utah, School of
Medicine, Department of Microbiology, under
the direction of Dr. Bill Wiley.
PROCESS DESCRIPTION
Composting, as denned earlier, is an aerobic
thermophilic biological process. As in any
aerobic biological process, an adequate method
for providing oxygen and thorough mixing are
required to obtain efficient odor-free treatment.
Composting of sewage sludge, as studied here,
necessitated three phases of operation: dewater-
ing the sewage sludge, mechanical composting,
and final curing. Each of these phases is de-
scribed with reference to a flow diagram (Figure
1), and the dewatered sewage sludge was fed to
the composter as indicated in the Feeding
Schedule.
Dewatering the Sewage Sludge
The sludge used during this study was a com-
bination of primary and two-stage trickling-
filter secondary sludge from the Salt Lake City
Sewage Treatment Plant. The sludge was ob-
tained each day and stored in a 350-gal storage
tank. Before vacuum filtration, the sludge was
conditioned with ferric chloride and lime or a
polymer to aid filtration. The sludge was then
dewatered on a vacuum filter and the cake was
ready for feeding to the composter.
Feeding Schedule
Dewatered sewage-sludge filter cake was fed
into the composter 8 to 10 hr per day, 6 days per
week. This schedule was chosen to approximate
the feeding schedule that would probably be
encountered in most small-size sewage treat-
ment plants. Most smaller plants filter during
only part of each day and accumulate sludge the
remaining time. Six days of operating data were
averaged to present results on a weekly basis.
No feeding or data collection were made on
Sundays.
Mechnical Composting
The dewatered sludge was then mixed with
recycled compost from the composter to adjust
the moisture content and to thoroughly seed
the incoming sludge with aerobic thermophilic
microorganisms. This mixing was accomplished
in a screw conveyor used to feed the composter.
The combination of sludge filter cake and re-
cycled compost then entered the mechanical
composter where it was continually mixed (by
paddle mixers) and aerated for about 20 days
or more depending on the final volume of com-
post removed from the composter. As the result
of biological action during the composting
period, the temperature of the mixed bed av-
eraged about 140 F.
Final Curing
Each day the final product from the com-
poster was stored in an individual pile, and it
was then observed for 2 weeks.
DESCRIPTION OF THE PILOT PLANT
The pilot plant was located at the Salt Lake
City Sewage Treatment Plant inside the digester
heat-exchanger building (Figure 2). A pipeline
was connected to the plant's main sludge line,
which went from the primary clarifiers to the
digesters. Sludge was drawn from this line by
line pressure to fill a 350-gal storage tank. This
storage capacity represents about 500 Ib of filter
cake having 70 percent moisture.
The sludge was then continuously pumped
with a variable-speed Moyno* pump from the
storage tank to a flocculator. The pumping rate
was varied according to the capacity of the vac-
uum filter to dewater the sludge.
The flocculator consisted of a 20-gal baffled
tank having a variable-speed, 6-in.-diameter
propeller. Chemical conditioners (lime and fer-
ric chloride or polymer) were fed into the floc-
*Mention of commercial products throughout this
report does not imply endorsement by the U.S. Public
Health Service.
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culator by metering pumps. The ferric chloride
and lime were added to the sludge to give a con-
tent of 4.5 and 15.5 percent, respectively (based
on sludge dry solids). Polymer was used in later
tests and was added to the sludge to give a
content of 1.6 percent on the same basis. The
conditioned sludge then overflowed into the
vacuum filter where it was dewatered.
The vacuum filter consisted of a vacuum
pump, filtrate receivers and pumps, and the fil-
ter itself. The filter drum was 3 ft in diameter
and 1 ft wide, with an effective filtration area of
9.4 sq ft. It was operated at a 30 percent drum
submergence and at an average drum speed of
8 mpr (minutes per revolution). During cake
formation, the vacuum was maintained at 12
to 15 in. of mercury and during drying, at 22 to
23 in. of mercury. The filter cake was discharged
into a screw conveyor that moved the cake to a
collection drum. At 30-min intervals, a portion
of the collected filter cake was weighed and
dumped into a second screw conveyor. At the
same time, a portion of composted sludge was
also weighed, recycled, and mixed with the filter
cake with the use of the same screw conveyor.
The mechanical composter consisted of an
open-top vessel having outside dimensions of
4 by 4 by 4 ft (40-cu-ft effective volume), a
porous carborundum bottom, and four 20-in.-
diameter paddle mixers rotating at 9 rpm.
The inside of the composter was baffled and
shaped to minimize cake packing to the sides
and corners and to prevent short-circuiting. Steel
fingers were attached to the inside walls to make
the mixers self-cleaning. A double lobe-type
blower supplied air to the composter; an orifice-
manometer system employing water in the
manometer measured the air; and a mercury
manometer measured line pressure.
The operating level of the composter was
maintained at a selected reference level. Each
morning, before feeding, compost was removed,
weighed, and placed in a storage pile for observa-
tion. The amount removed each day was deter-
mined by reducing the compost bed level to the
reference level. If the initial bed level was below
FIGURE 2. Compost pilot plant.
355-447 O—70-
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the reference level, no compost was removed.
The effluent compost was stored in an outdoor
area, and each area was marked and dated for
further observations.
TEST TECHNIQUES
Sampling and data collection for the following
tests were conducted twice daily except for com-
post temperature and power readings. Compost
temperature data were taken 4 to 10 times daily,
and power data were measured once each week.
Samples of compost were taken at the outflow
of the composter, whereas filter cake samples
were taken directly from the filter discharge.
Both types were discrete (grab) samples. The
application of each test technique to the tested
materials was checked with known standards for
both feasibility and accuracy.
Temperature
Temperature of the compost was measured
with a Weston dial-type, all-metal thermometer,
0 to 220 F range; the stem was penetrated 6 in.
into the compost bed for 5 min.
Volatile Solids
Volatile solids analyses were carried out by the
procedure given in Standard Methods for the
Analysis of Water and Waste Water * with one
modification: ignition time of the sample was
extended to a minimum of 4 hr.
Measurements for pH
Measurements were made for pH with
Hydrion indicator paper in the appropriate
range for each application.
Moisture
Percent moisture by weight determinations
were made with an Ohaus moisture determina-
tion balance.
Bulk Density
Bulk density was measured with an accurately
measured, 5-gal bucket that was filled with
material and weighed. Bulk density was then
calculated from the weight of the material
divided by the volume of the bucket and ex-
pressed as pounds per cubic foot.
Total Weight and Volume
All materials fed, recycled, or wasted were
weighed in a tared 5-gal bucket and recorded to
determine total weight of each. Total volume of
each was determined by dividing the total
weight by its respective average bulk density.
Air Flow
Air flow was measured with a 1.764-in.-di-
ameter orifice in a 3-in.-diameter pipe. Pressure
differential across the orifice was measured with
a water manometer. A mercury manometer
downstream from the orifice indicated line pres-
sure. All readings were corrected to 60 F and 1
atmosphere and expressed as standard cubic
feet per-minute (scfm).
Total Kjeldahl Nitrogen
Total Kjeldahl nitrogen was determined with
a Lavconco Micro Kjeldahl digester and Micro
Kjeldahl distillation unit. Standard procedures
for the analysis of water and waste water4 were
followed, but micro amounts, as described in
Scott's Standard Methods of Chemical Analysis,
were used.6
Phosphorus
Phosphorus was calculated from determina-
tions made for total phosphate following pro-
cedures given in Standard Methods for the
Analysis.of Water and Waste Water for the
aminonaptholsulfonic acid method.* Results
were expressed as phosphorus pentoxide.
Gas Analysis
A 10-ml disposable syringe was employed to'
take gas samples from the compost mass and
from off gases. The syringe was flushed sev-
eral times in the area from which the gas
sample was to be taken to remove any resid-
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ual gases. The sample was then pulled into
the syringe and immediately sealed by push-
ing the needle into a rubber stopper while
ejecting a small portion of the sample col-
lected. This technique avoided contamina-
tion by atmospheric air. The same technique
was used to inject the sample into the gas
chromatograph. The gas samples were then
analyzed for oxygen, nitrogen, and carbon di-
oxide on a volumetric basis with a Varian Aero-
graph model 90-P3 gas chromatograph with a
two-column separation technique employing
dry, purified helium as the carrier gas. The first
column, which consisted of 30 percent bis-2-2-
(methoxyethoxy) ethyl ether on 45/60 Chro-
masorb P, separated carbon dioxide and hydro-
gen sulfide from the other components. The
second column, which consisted of 10 percent
5A and 90 percent 13X molecular sieve carbon-
ate material, separated the remaining compo-
nents into oxygen, nitrogen, methane, and
carbon monoxide. All values recorded were cor-
rected to take into account the inert gases pres-
ent (mainly argon) but not detected.
Power
A Weston Analyzer was used to observe the
power required for mixing the compost; this
was recorded as kilowatts.
COMPOSTING PROCESS
Many factors played important parts in the
treatment process studied. The effect of each of
these factors on the overall results obtained was
considered.
Mixing
Because of the high organic content of the
dewatered sludge (requiring much oxygen) and
its tendency to pack, continuous mixing was
necessary. The major problem was to find a
mixer so designed that it would not plug but
would tend to lift and loosen the material. The
ribbon augers that were tried initially clogged
rapidly. Several modifications of these augers
were tried without success. Finally, paddles,
slightly angled so that lifting would occur, were
attached to the drive shaft. Mixing appeared
satisfactory, and later testing proved it to be
good.
Because the compost had a tendency to pack,
dead areas and clearances between the mixer
and side walls and bottom had to be minimized.
This was done by shaping the internal part of
the composter as closely as possible to that of
the mixers.
Wearing of the mixer paddles indicated that
the abrasive qualities of the compost material
were significant. A great deal of wear occurred
on the mild steel paddles during a 3-month run.
The environment in the compost bed was also
conducive to corrosion; moisture, heat, and
oxygen promoted corrosion and aggravated the
wearing problem.
In composting garbage and refuse, mixing and
aeration are used intermittently. In this study,
both mixing and aeration were continuous. Tests
were made to determine if continuous mixing
was required. Observations made when aeration
alone was employed showed that the compost
bed temperature began to drop within one-half
hr. after mixing was stopped. It was believed
that continuous mixing was required to keep the
bed loose and to prevent channeling of air.
Several tests were made to determine the ex-
tent of mixing that occurred in the composter.
In two separate tests, plastic cubes and cylinders
(approximately 250) were placed in the feed to
the composter. These materials were dispersed
throughout the compost bed within 2 hr. This
rapid dispersion was not due entirely to agita-
tor mixing, but was also related to the amount
of compost recycled. The higher the recycle vol-
ume, the more completely mixed was the com-
post bed. As described later, a high recycle
volume was very beneficial to the treatment
process.
Moisture Content
Moisture content of the filter cake and the
compost was determined daily during this study.
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The moisture content of the dewatered sewage
sludge depended upon the discharge character-
istics of the vacuum filter, the raw sewage sludge
characteristics, and the chemical conditioners
used. The moisture content of the compost de-
pended upon the filter cake moisture, retention
time within the composter, and the rate of the
aeration and mixing.
When ferric chloride and lime were added
for conditioning at values of 4.5 and 15.5 per-
cent, respectively (based on sludge dry solids),
the filter cake had a moisture content of 68 to
74 percent and the material was easily handled
and conveyed. Filter cake conditioned with
Rohm and Haas C-7 to give a content of 1.6
percent (based on sludge dry solids) had an
average moisture content of 77 percent. A mois-
ture content of 70 percent can be realized with
polymers, but this would require a more com-
plex application technique.
Moisture content in the composter ranged
from 20 to 35 percent with an average of 26 per-
cent. The moisture content, as stated before,
depended on many variables, all of which may be
controlled to some degree. The amount of filter
cake and its moisture content determined the
quantity of moisture entering the composter
1/2
7/16
3/8
5/16
1/4
3/16
1/8
1/16
a
«5
a
5
a
&
£
T5
a.
a.
Conditioning Chemicals
O FeCU and Lime
A Polymer
10 15 20 25 30
Percent Moisture By Weight
35
40
45
50
FIGURE 3. Effect of moisture on compost particle size.
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daily. Retention time and rate of aeration af-
fected the degree of evaporative drying.
Moisture content of the feed to the composter
depended on filter-cake and recycled-compost
moisture and their relative quantities. Based on
filter-cake moisture of 72 percent, recycled-com-
post moisture of 26 percent, and a recycle ratio
of 1.5, an input moisture of 45 percent could be
expected.
Moisture content of the recycled compost and
filter cake was a major process limitation. Mois-
ture content controlled particle size of the com-
post (Figure 3). This factor greatly affected
process efficiency. Effective mixing within the
composter was achieved only when the particle
size of the compost was less than ys inch.
Mixing of recycled compost and filter cake was
also affected in the same manner. Recycled com-
post of low moisture content (26 percent) and
small particle size (% inch) when com-
bined with filter cake (72 percent moisture)
produced a feed material easily mixed in the
composter. If the recycled compost were insuffi-
cient and contained a high moisture content, the
combination of recycled compost and filter cake
would result in a sticky mass difficult to mix in
the composter.
The biological action of the microorganisms
present in the composter, as indicated by tem-
perature, was affected by moisture. Temperature
was greatest at moisture contents of 20 to> 30
percent and near nonexistent at moistures above
40 percent or below 15 percent.
The extremes of moisture content, high and
low, created physical problems related to the
operation of the process. If the average moisture
content of the compost mass exceeded 35 per-
cent, the compost mass would begin to ball and
clump together. This, of course, produced a mix-
ing and aerating problem. Moisture content of
the compost mass of 15 percent or less created a
dusting problem because the forced aeration
caused fine particles of compost to exhaust from
the composter.
Recycling
Recycling of compost has several positive ef-
fects on this treatment process. The first, and
probably most important, is the effect of the
composter feed on the moisture content. If re-
cycling were not used, the moisture content of
the composter feed would be that of the sludge
filter cake, and mixing problems (balling and
packing) would develop. As was pointed out
earlier, a feed moisture content of 45 percent'
or less can be obtained using a 1.5 recycle ratio
when the filter cake has 72 percent moisture.
When feed to the composter was consistently
over 50 percent moisture, mixing problems soon
developed.
Another benefit of recycling is that the feed to
the composter is thoroughly seeded with aerobic
thermophilic microorganisms, and the biologi-
cal activity is rapidly started, e.g., a 45 percent
total solids reduction at 1.5 recycle ratio com-
pared with an 18 percent total solids reduction
at 1.0 recycle ratio, both at a 7-day retention
time (based on the volume of dewatered sludge
fed daily) (Figure 4). This indicates, then, that
recycling compost, which seeds the filter cake
and mixes the composter contents, produces
beneficial effects.
The sludge filter cake conditioned with poly-
mer indicated the same results. At a 2.0 recycle
ratio, total solids were reduced 45 percent, with
a 10-day retention time; at a 1.5 recycle ratio,
they were reduced 43 percent, but with a 13-day
retention time. The polymer-conditioned filter
cake required more recycle than did the filter
cake conditioned with lime and ferric chloride.
This was because the polymer-conditioned filter
cake contained 5 to 7 percent more moisture.
Recycling also increased the temperature of
the feed from 10 to 20 F above that of the filter
cake when a recycle ratio of 1.0 to 2.0 was used.
This was believed to have a beneficial effect on
the rapid response of the process.
When the sludge was conditioned with lime
and ferric chloride, the filter cake pH averaged
11.0. The pH of the compost averaged 6.5, and
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70
60
50
c
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3
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EC
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0)
o
a>
a.
30
20
10
i r
"i 1 1 r
T r
Initial Percent Volatile Solids
Conditioning Chemicals
O FeCI3 and Lime
_A Polymer
Air Flow Rate : 10 scfm
1.5:1 Recycle
Ratio
2:1 Recycle
Ratio
1.5:1 Recycle-*^
Ratio
1:1 Recycle Ratio
0 5 10
Calculated Retention Time, Days
FIGURE 4. Effect of retention time on percent solids reduction.
15
therefore, it was probably beneficial to adjust
the feed pH to be more compatible with the
treatment pH by adding recycled compost to
the feed.
Process Capacity
The ability of composting to stabilize the
dewatered sludge and reduce its volume and
weight was of major interest in the test pro-
gram. The process consisted of evaporation
(drying) and biological destruction of vola-
tile solids by thermophilic microorganisms.
Drying, which was explained under the dis-
cussion of mixing, is required not only for ef-
fective mixing, but because biological treatment
is very slow to nonexistent at moisture contents
above about 40 percent. Drying occurs in this
process at a very rapid rate because of the high
temperatures (140F), continuous mixing, and
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aeration. The moisture content of the dewatered
sludge, which averaged 72 percent, was reduced
to an average of 26 percent in the final compost.
This represents a total average weight reduction
of 62 percent and a moisture reduction of 87 per-
cent by weight by drying alone. The percent
water reduction is limited to 98 percent (15%
final moisture content) by the dusting problem
stated earlier. The volume reduction from the
drying process because of shrinkage would prob-
ably account for more than 75 percent of the
total volume reduction.
100
95
1 I • I •
Upper Limit Due To Dusting
90
c 85
o
"o
•a
jjj 80
TO
-------
10
100
95
90
85
o
80
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w 75
CD
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Q_
70
65
60
55
T I I I I I I I T
T T"
Upper Limit Based on Solids and Moisture Reduction Limits
Conditioning
Chemicals
O FeCI3 and Lime
• FeCI3 and Lime
A Polymer
Air Flow Rate : 10 scfm
Recycle
Ratio
1:1
1.5:1
1.5:1
I t I I I I I I I I I I I I
5 10 15
Calculated Retention Time, Days
FIGURE 6. Effect of retention time on percent weight reduction.
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11
Destruction of volatile solids by biological ac-
tion of thermophilic microorganisms caused a
reduction of both weight and volume, but more
important, stabilized the dewatered sludge and
made it hygienic and free of viable plant seeds.
This part of the treatment process is most im-
portant from the standpoint of final disposal.
The final compost is relatively stable organi-
cally, is hygienic, and can be used as a soil con-
ditioner or as land fill.
The intensity of the biological process was
indicated by the high compost-bed temperature
maintained with continuous mixing and aera-
tion, the change in pH from 11.0 to 6.5, and the
reduction of solids. An average total-solids (in-
erts included) reduction of 30 percent and an
average volatile-solids reduction of nearly 50
percent occurred in the process. Solids reduction
is limited to 61 percent, which is the volatile-
solids content of the feed.
Again, the amount of volume reduction that
results from biological action is not known. The
average total volume reduction was 73 percent.
All of the above average data were based on a
retention time of 7 days. To obtain the calculated
retention time, divide the total active volume of
the composter by the volume of unpacked de-
watered sewage sludge fed daily. The actual
retention time in the composter is much greater
than the time indicated in the above calculations
because of the gross reduction of volume oc-
curring during the process. Whereas the calcu-
lated retention time for the above averages was
7 days, the actual average retention time was
closer to 26 days. The relation of calculated
retention time to the moisture, weight, and vol-
ume reduction is based on a comparison of filter
cake with compost (Figures 5 to 7).
A flow diagram was made showing the fate of
100 Ib of dewatered sewage sludge fed to the
composter, with the amount of feed and products
and their respective compositions (Figure 8).
Another factor influencing the rate of the com-
posting process is the effect of the feed schedule.
Intermittent feeding affected the compost-bed
temperature; the bed temperature dropped
355-447 O—70 3
about 15 F during a feed period of 10 hr (Figure
9). It is assumed that continuous feeding would
be beneficial to the treatment process by provid-
ing more uniform operating conditions. Con-
tinuous feed may significantly reduce the cal-
culated retention time by allowing an increase
in the amount of feed while still producing the
results observed during intermittent feeding
used during this study.
Required Air
Forced aeration was applied to the compost
bed (See, Pilot Plant Description), and flow
rates were measured (See, Test Techniques).
Aeration of the compost bed served two main
functions. The first function was to provide a
means for rapid evaporation of excess moisture
fed to the composter. The large volume of air
required for drying was shown to be well in
excess of the air required to maintain aerobic
conditions, the second function of aeration. This
point was demonstrated (Figure 5); the treat-
ment process required a minimum 4-day cal-
culated retention time before solids destruction
occurred. If feed to the composter had been pre-
dried, probably less retention time would have
been required.
From the large reduction in moisture (80 to
90% by weight) during the process and the
retention time required for solids destruction,
composting of dewatered sewage sludge is ob-
viously first a process of evaporation and sec-
ond, of biological destruction of volatile solids.
A straight line plot indicated that the effi-
ciency of oxygen uptake in relation to air ap-
plied was not rate dependent on the oxygen
concentration present in the range of air-flow
rates used during this study (Figure 10).
An air-flow rate of 10 scfm was used during
most of the study because it was sufficient for
drying and did not excessively cool the bed. This
rate represents a flow of 0.25 scfm per cut ft
(based on 40-cu-ft volume) or 0.75 scfm per sq
ft (based on 13.3-sq-ft area) to the compost bed.
Forced aeration had a cooling effect on the
compost bed (Figure 11). The temperature of
-------
12
100
95
90
85
o
"5
•o
EC
80
I 75
"c
0)
o
70
65
60
55
Upper Limit Based on Solids and Moisture Reduction Limits
Conditioning Chemicals
O FeCI3 and Lime
• FeCI3 and Lime
A Polymer
Air Flow Rate : 10 scfm
Recycle
Ratio
1:1
1.5:1
1.5:1
5 10
Calculated Retention Time, Days
15
FIGURE 7. Effect of retention time on percent volume reduction.
-------
13
RAW DEWATERED
SEWAGE SLUDGE
100 Ib Total
72 Ib Water
28 Ib Solids
17 Ib Volatile
11 Ib Inert
45 Ib/cuft
2.2 cu ft
WATER EVAPORATED
62 Ib
87% Reduction
VOLATILE SOLIDS
DESTRUCTION
8 Ib
30% Solids reduction
47% Volatile
solids reduction
FINAL COMPOST
30 Ib Total
10 Ib Water
20 Ib Solids
9 Ib Volatile
11 Ib Inert
48 Ib/cu ft
0.6 cu ft
FIGURE 8. Dewatered sewage sludge composting—material balance.
the compost bed was recorded at four positions
and three levels. (The position numbers are
shown in a top view of the composter, and the
levels noted indicate the distance from the
bottom of the compost bed. A reading at the
6-in. level would be 6 in. upward from the bot-
tom.) The three individual plots show that air
rates up to 10.0 scfm did not lower the compost-
bed temperature significantly. An average tem-
perature decrease of 5 F was noted for air-flow
rates between 5.3 and 10.0 scfm, and an addi-
tional 5F decrease was observed for air-flow
rates between 10.0 and 11.4 scfm.
The concentrations of oxygen and carbon di-
oxide in volume percent as a function of com-
post-bed depth are illustrated (Figures 12 and
13, respectively). These samples were taken at
the same sampling points as the temperature
data discussed above. Even at low air-flow rates
of 3 and 5 scfm, more than 18 percent oxygen by
volume was present at all times (air usually
contains 21 percent oxygen by volume) (Fig-
ure 12). This again points to the fact that at the
air-flow rates employed in this study, the oxygen
concentration present was more than adequate
at all times to maintain aerobic conditions. Also,
as one would expect, the concentration of oxygen
present in the air flowing through the bed was
greatest at the highest air rate.
The concentration of carbon dioxide present
in the air passing through the compost bed at
varying air-flow rates is shown in Figure 13. The
carbon dioxide concentration at an air-flow rate
of 5.3 scfm was greater than that at 3.4 scfm.
The explanation may be that the biological
activity, as indicated by bed temperature, was
greater in the compost bed during the run at 5.3
scfm and, hence, presumably more carbon
dioxide was produced.
According to the oxygen used, the absolute
values of carbon dioxide produced should be
almost twice the concentrations shown in Figure
14. It was assumed that part of the carbon
dioxide reacted with the excess lime present in
the dewatered sewage sludge. Therefore, al-
though the values recorded are not absolute,
they do give an indication of process activity.
-------
14
The lowest oxygen and highest carbon dioxide
concentrations existed in the center of the com-
post bed (Figures 13 and 14). As fresh air fed to
the bottom of the compost bed moves upward,
oxygen is depleted and carbon dioxide is added.
At a point somewhere between 18 and 30 in. (36-
in. total bed depth) above the bottom of the
compost bed, the effect of air diffusing down-
ward from mechanical mixing occurred. From
this point upward, the concentration of oxygen
increased whereas the concentration of carbon
dioxide decreased.
Influence o/ Conditioning Chemicals Used in
Vacuum Filtration
Conditioning chemicals used before vacuum
nitration or any liquid-solids separation process
may be classified into two main categories: inor-
ganic chemicals, such as ferric chloride and lime,
and polymeric organic chemicals. This variable
160
150
u.
o
gT
_3
"S
Q.
E
0)
m
*•
in
o
Q.
I 140
O
130
Daily Duration of
Composter Feeding
Composter Top View
(Position Numbers Shown)
BASIS:
300 Lbs./Day Feed
10.0 scfm Air Flow Rate
1:1 Recycle Ratio
Position Number
3
4
J—L
J L
J I I—l—L
_L
6AM
12 Noon
6PM
Time
12 Midnight
6AM
FIGURE 9. Effect of intermittent feeding on compost bed temperature.
-------
0.16.
0.14
0.12
«
1.
0.10
•o
o
Q.
•o
0)
in
•a
o
Q.
0.08
0.06
0.04
0.02
5 6
Air Flow Rate, scfm
10
FIGUHE 10. Efficiency of oxygen uptake rate. Numbers with data points indicate number of
observations used to average each point.
11
-------
16
160
150
140
130
120
110
100
170
CL?
— 160
LU
DC
D 150
I—
<
DC 140
Q_
^ 130
LU
n 12°
LJ
LU
CO 110
I-
co
o 100
Q_
^ 170
0
CJ
160
150
140
130
120
110
100
1 1
AIR FLOW RATE: 5.3 scfm
~ — — _ _
-^ --
1234
AIR FLOW RATE: 10.0 scfm
— — ____ .— •*
.
I i
234
i i
AIR FLOW RATE: 11.4 scfm
-
"™ "•• *•— _ _,_
~~~~"~^ — __^— ' — "~^~~
TEMPERATURE RECORDING
POINTS
Position Number from
Top View of Composter
EFFLUENT & 3
FEED, T^ 2
DISTANCE ABOVE BOTTOM
OF COMPOST BED:
R jn
18 in.
O A in
2 3
POSITION NUMBER
FIGURE 11. Effect of air flow rate on compost bed temperature.
-------
0)
23
22
21
20
a>
19
18 _
17
Air Flow Rate
10.0 scfm
5.3 scfm
3.4 scfm
\
V
\
\
\
\
\
6 12 18 24
Distance Above The Bottom Of The Compost Bed, Inches
30
36
FIGURE 12. Effect of air flow rate on oxygen concentration. The total depth of the bed was 36 in.
-------
00
Air Flow Rate
10.0 scfm
— 5.3 scfm
3.4 scfm
O
U
o
Q.
0)
_3
O
1 .
Distance Above The Bottom Of The Compost Bed, Inches
FIGURE 13. Effect of air flow rate on carbon dioxide concentration. The total depth of the bed was 36 in.
-------
80,
70
60
50
_o
tj
« 40
wj
O
CO
§ 30
o
20 _
10 .
(0
o
a.
o
Varian Aerograph Model 90-P3
Columns — 20' x Va " Bis—2-2 (Methoxyethoxy)
ethyl ether 30% on 45/60 Chrom-
asorb P, 20' x % " 10% 5A and 90%
13X molecular sieve carbonate.
Temperatures — Column: 48°C Detector: 80°C
Injector: 37°C Collector: 67°C
Carrier Gas — Helium at 33 ml./min.
Detector — 20Q ma.
co
CM
J V
8 10
Separation Time, Minutes
12
14
16
18
FIGURE 14. Gas chromatograph separation of composter off gases (below).
-------
20
in the composting of dewatered sewage sludge
was studied to observe any differences in the
process attributable to the conditioning chemi-
cals. There has been a trend toward the use of
polymers as sludge conditioners because they are
easy to handle.
Different sludge conditioning agents produce
filter cakes with differing physical properties.
Sewage sludge conditioned with ferric chloride
and lime produced a very firm filter cake that
discharged from the filter medium easily. The
polymers, however, produced a filter cake that
was sticky and difficult to discharge from the
filter medium. The filter cake conditioned with
polymer also contained about 5 percent more
moisture. Although an increase in moisture may
account for some difficulty in discharge of the
filter cake from the filter medium, it would not
account for it becoming sticky.
The above change in physical characteristics
caused by the use of polymers resulted in many
handling and mixing problems. Although the
firm filter cake produced by ferric chloride and
lime conditioning was easily conveyed by an
inclined screw conveyor, polymer-conditioned
filter cake produced a sticky, "clumpy" mass
that was both difficult to convey and handle.
The slight increase in moisture in the polymer
filter cake may account for a portion of this
problem, but does not totally explain it. A simi-
lar problem was also encountered in mixing the
recycled compost material with the filter cake.
The filter cake produced with ferric chloride and
lime, with its firm consistency, mixed easily with
the recycled compost and produced a satisfac-
tory feed to the composter unit. This easy
mixing facilitated a well-seeded feed to the com-
poster. Mixing the sticky, polymer-conditioned
filter cake and recycled compost was difficult and
produced a "clumpy" mass to be fed into the
composter. This "clumpy" mass was then also
difficult to mix in the composter.
The sludge-conditioning agents used also
changed other physical properties of the com-
post bed. The filter cake conditioned with ferric
chloride and lime resulted in a compost bed re-
sembling sawdust in consistency. It mixed easily
and posed no handling problems. The compost
bed of dewatered sewage sludge conditioned
with polymers had a particle size of gravel (1
in. or less). One result of this increase in particle
size when polymers were used was that the air-
flow rate through the compost bed was necessar-
ily reduced from 10 to 3 scfm. When the same
air-flow rate was used with polymer as was used
with ferric chloride and lime conditioning, the
compost-bed temperature decreased by an av-
erage of 10 F. The change in particle size of the
compost bed can only be accounted for by the
change in chemicals used for conditioning be-
cause the compost bed contained the same or
less moisture with the polymer than with ferric
chloride and lime. This problem may be related
to the bonding characteristics of the polymer as
opposed to the powTdery consistency of the in-
organic chemicals used.
The final product and its disposal are also in-
fluenced by the conditioning chemicals. As
stated before, the compost resulting from ferric
chloride and lime filter cake was similar to saw-
dust in consistency. This material could easily
be conveyed and disposed of in a variety of ways.
The gravel-size material resulting from compost-
ing filter cake conditioned with polymers may
have to be ground before final use or disposal.
Other than these physical characteristics, the
other properties o-f the final product were about
the same.
Off Gases
Off-gas samples were taken directly above the
surface of the compost bed and analyzed (See
Test Techniques). A typical gas chromatograph
plot illustrates that the gas sample was split
into well defined peaks and was easily meas-
ured (Figure 14).
The composition of gases passing through the
compost bed and the off-gas concentrations of
oxygen and carbon dioxide changed in volume
percent (Figures 12 and 13). Oxygen concen-
tration, even at low air-flow rates, did not drop
-------
below 20 percent at the compost-bed surface
(Figure 12). Carbon dioxide concentration did
not exceed 0.1 percent at the compost-bed
surface (Figure 13). The values of carbon
dioxide and oxygen concentration in the off
gas should not be of concern.
Ammonia was detected during the time filter
21
cake (conditioned with ferric chloride and lime)
was fed to the composter. The strength of am-
monia odor was in direct proportion to the rate
the filter cake was fed. The ammonia quickly
dissipated and created no problem. Ammonia
was not detectable 2 hr after feeding was
discontinued. A quantitative value of the am-
130
120
1 1 —
/^>
/
/
/
^ /
1 1 1 1
Initial Conditions
\ 25% Solids Reduction
\ 32% Moisture
\ 4% Solids Reduction
70
60
50
I ...
0246
i
8
i
10
12 1<
Curing Time, Days
15. Time-temperature relationship in compost curing process.
-------
22
monia concentration was not measured with the
gas chromatograph because the equipment
would need to have been changed for ammonia
analysis and other gas analyses could not then
have been determined. The applied air was be-
lieved to initially strip the ammonia (present
because the pH was high when lime was used)
from the compost. Although the gas chromato-
graph system was capable of detecting hydrogen
sulfide, methane, and carbon monoxide, none of
these gases was detected. These gases could have
been present in trace amounts, but they were not
detected by this analytical technique or by
their odor.
The odor of the combined off gases resembled
the odor of silage: a sweet, musty odor that
might be encountered in a barn or a basement,
not objectionable in any way.
Chemical and Physical Properties of Final
Product
The chemical and physical properties of the
finished compost varied slightly with retention
time. The following values (except for potassium
and nitrate) are averages of 20 separate
analyses:
Total Kjeldahl nitrogen (% N by
weight) ....................
Total nitrate ( % NOj by weight) . .
Total phosphorus (% P205 by
weight) .....................
Total potassium (% K20 by
weight) .....................
Bulk density (Ib/cu ft) ..........
Moisture (% by weight) .......
Color .................... Dark
Odor .........................
2. 21
<0. 01
2. 16
0. 27
6-6. 5
48. 2
20-35
brown
Musty
The "fertilizer value" (nitrogen and phos-
phorus) of the finished compost is low compared
with commercial fertilizers, but is about equal
that of cattle manure. More important is the
ability of the finished compost to condition the
soil to absorb and retain water. The general ap-
pearance is that of a rich humus such as peat
moss.
The value of the finished compost as a soil
conditioner was demonstrated by spreading 12
to 15 Ib of compost per sq yd on a private lawn.
The soil on which the finished compost was
applied was sandy, low in humus, and low in
natural fertilizer content. This compost added
humus and did not cause any noticeable dele-
terious effects; the grass (lawn) responded by
turning a rich, dark-green color and growing
profusely. The added compost also aided soil
aeration by preventing the soil from packing.
All of the above characteristics have value and
indicate that the finished compost used in this
case was effective as a soil conditioner.
The moisture of the finished compost de-
pended on retention time in the composter and
reflected the degree of stabilization accom-
plished. The relation of compost temperature
to number of curing days has been plotted for
compost with moisture contents of 23 and 32
percent (Figure 15).
The temperature of the low-moisture (23%)
finished compost decreased to ambient tempera-
ture almost immediately after being removed
from the composter and placed in open piles.
This indicates the completeness the overall proc-
ess achieved while the compost was in the com-
poster (25% solids reduction). After being re-
moved from the composter, the high-moisture
(32%) finished compost maintained its temper-
ature for 5 to 6 days before decreasing to am-
bient temperature. This also reflected the degree
of stabilization (4% solids reduction). These
results indicate that the moisture of the finished
product reflects the completeness of the overall
treatment process and that both volatile-solids
destruction and drying vary with retention time.
A sample of the finished compost was culti-
vated for 1 month under controlled laboratory
conditions to determine if viable plant seeds
were present. The sample was watered daily
with distilled water and left in a warm, lighted
area. After 1 month, the sample was examined to
-------
determine if seeds, possibly present in the sew-
age sludge initially, were viable after compost-
ing. No seeds germinated; the compost retained
water well and did not pack or lump. The odor
was musty but not objectionable. This test, al-
though conducted only once, added valuable in-
formation about the characteristics of the
finished compost.
Particle size of the final product also varied
with the retention time because size was de-
termined to some extent by moisture content.
Moisture contents of the finished compost (be-
tween 20 to 28%) resulted in a fine-textured
material similar to sawdust with a bulk density
of about 48 Ib per cu ft.
The pH level of the end product was con-
sistently between 6.0 and 6.5. This level should
pose no problem in handling, final use, or
disposal.
The concentration and type of organisms re-
maining in the finished compost would make it a
hygienically safe soil conditioner. Pathogenic
organisms initially present and indicator or-
ganisms added in the pathogenic study were
destroyed in a maximum of 3 days. The excellent
destruction of pathogens was probably due to the
more or less uniformly high temperature main-
tained and the thorough mixing of the compost
bed. The thermophilic organisms, none of which
were pathogenic, should pose no problem at
ambient temperatures.
In general, the physical and chemical prop-
erties of the finished compost would make it a
safe soil conditioner with some "fertilizer value."
SURVIVAL OF PATHOGENIC ORGANISMS
Because of the possible uses of the finished
compost, a very important phase of this study
was the survival of pathogenic organisms
through the treatment process. Studies0"8 of
the destruction of pathogens in composted gar-
bage and a combination of sewage sludge and
garbage indicate that destruction occurs pri-
marily as a result of two actions: thermal kill by
high temperature and the effect of antibiotic
action. This study was made to determine the
23
destruction of pathogenic type organisms associ-
ated with sewage and sewage sludges. Basically,
the organisms chosen were those that simulate
actual conditions and that also cause infection
when entering the body via the gastrointesti-
nal route. Because Clostridium and Staphy-
lococcus aureus do not meet these criteria, they
were not used. The choice was also based on the
anticipated problems of detection in the
filter cake and final compost, their possible
presence in sewage and sewage sludges,
and the various types (bacteria, virus,
etc.) that could be present. The four pathogenic-
type organisms studied were Salmonella new-
port, a bacterium; Candida albicans, a fungus;
Ascaris lumbricoides, a nematode (metazoan
parasite ova); and poliovirus Type I, a virus.
A flow diagram of the sequence of testing and
method of microbial analyses is shown (Figure
16). The initial work involved establishing tech-
niques for detecting each of the above organisms
in filter cake and the final compost. This caused
problems because of the many other organisms
present and the development of a proper sam-
pling technique. Although S. paratyphi was first
studied, the sensitivity of the technique for its
detection was not satisfactory and another or-
ganism, S. newport, was used. In general, much
study was made to gain confidence in detecting
techniques that were to be used during insertion
studies.
Alcaligenes species
The destruction of microorganisms in the
composter was significant. Tests, made with nu-
trient agar incubated at 37 C, indicate that total
organisms were reduced from 2.lXlOT to 1.1
X103, or 99.99 percent kill. The count of gram
negative enteric bacteria, employing MacCon-
key plates for detection, showed 5.6X105
organisms per g in the filter cake and zero or-
ganisms in the final compost. The predominant
organisms in both the filter cake and final com-
post were Alcaligenes species. These are non-
pathogenic, alkali-producing bacteria associated
with the breakdown of proteins in sewage. Inter-
estingly, tests indicated a 50-fold increase in
-------
to
VIRUS
POLIOVIRUS TYPE I
BACTERIUM
SALMONELLA
NEWPORT
FUNGUS
CANDIDA ALBICANS
METAZOAN
PARASITE
ASCARIS
LUMBRICOIDES
COMPOST FROM COMPOSTER
10-g SAMPLE
HOMOGENIZE IN 10ml
PHYSIOLOGICAL SALINE
\
TREAT WITH ETHER,
I
TREAT WITH NITROGEN
GAS
\
CONCENTRATE IN TWO-
PHASE POLYMER
SYSTEM
I
INOCULATE AMNION (FL)
CELL TISSUE CULTURE
TUBES
{
INCUBATE 48 hr
EXAMINE FOR CYTOPATHO-
GENIC EFFECT (CPE)
IDENTIFY CPE BY
SPECIFIC NEUTRALIZATION
WITH TYPE I POLIO-
VIRUS ANTISERUM
REPORT AS POSITIVE
ISOLATIONS/10 g
1-g SAMPLE
ADD DIRECTLY TO
9 ml GRAM NEGATIVE BROTH
INCUBATE 41.5 C 6 hr
\
STREAK SELENITE
BRILLIANT GREEN PLATE
INCUBATE 37 C 24 hr
I
INOCULATE SUSPECTED
SALMONELLA COLONIES
TO TRIPLE SUGAR IRON (TSI)
SLANTS
\
TYPE POSITIVE TSI
REACTIONS SEROLOGICALLY
WITH GROUP C2 ANTISERUM
\
CONFIRM POSITIVE GROUP
C, AGGLUTINATIONS IN
UREA, DULCITOL, LYSINE
DECARBOXYLASE AGAR
AND LACTOSE
\
REPORT AS POSITIVE
ISOLATIONS/g
1-g SAMPLE
ADD DIRECTLY TO
15 ml MOLTEN PAGANO-
LEVINE AGAR
INCUBATE 25 C 48 hr
\
CONFIRM SUSPECTED
COLONIES OF C. ALBICANS
BY PRODUCTION OF
CHLAMYDOSPORES ON RICE
EXTRACT AGAR AT 25 C
I
REPORT AS
ORGANISMS/g
1-g SAMPLE
EMULSIFY IN
25 ml PHYSIOLOGICAL
SALINE
INCUBATE 21 days 25 C
I
CARRY OUT ZINC
SULFATE CONCENTRATION
I
MICROSCOPIC EXAMINATION
FOR VIABLE ASCARIS OVA
I
REPORT AS
OVA/g
FIGURE 16. Assay for indicator organisms in composted solid human wastes.
-------
25
thermophilic organisms from 2X10* organism
per g in the filter cake to 1.5X 10° organisms per
g in the final compost.
Laboratory and insertion tests were made
with the use of each of the microorganisms in-
dicated in Figure 16. Aseptic sampling tech-
niques were used throughout these studies of
pathogens. The results for each are discussed
separately.
Salmonella newport
These pathogenic bacteria are found in sew-
age. When laboratory tests were made to de-
termine the thermal death point (TDP) (time
is a constant and temperature is varied) and the
thermal death time (TDT) (temperature is a
constant and time is varied) of S. newport, the
TDT was 30 min at 60 C and the TDP was 65 C
in 30 min. These results indicated that S. new-
port is susceptible to kill at temperatures exist-
ing during composting treatment (60 C to 140
F). This organism was not isolated in either the
filter cake or final compost before inoculation.
Insertion studies involved 117 microbial anal-
yses after the compost in the composter (40 ft3
volume) was inoculated with 2.7X101- orga-
nisms. Samples of the compost were removed
from the inlet and outlet portions of the com-
poster at given time intervals. One hour after
inoculation the tests indicated S. newport was
present at the inlet zone and after 4 hr, at the
outlet. No S. newport was found in the inlet or
outlet zones 25 hr after inoculation. Sampling-
continued for 10 days with no further isolation
of any organisms, not even coliform,°~12 on the
selective medium or on selenite brilliant green
agar.
Candida albicans
The je pathogenic yeasts survived well at 37 C
in both water and sewage. The selective medium
used to isolate this organism was Pagano-Levin
medium.13' " Again, no organisms were isolated
from the filter cake or final compost before in-
oculation. The TDP was established at 80 C for
30 min, and the TDT was 60 min at 70 C. This
also showed susceptibility to kill at composting
temperatures.
Insertion studies employing 2.0X10° orga-
nisms mixed into the composter feed were tried.
One hour after the compost had been inoculated
with the microorganism, analysis of samples
taken at the composter inlet showed isolation
of C. albicans too numerous to count. At least
one organism per gram was isolated after 28
hr at the outlet from the composter. No further
isolation occurred during the next 3 days of
sampling involving 30 additional analyses.
Ascaris lumbricoides
These pathogenic nematodes (small worms)
are found in the intestines of hogs. The ova used
during this study were taken from hog intestines
obtained from a local abattoir. After the whole
worm was removed from the hog intestines,
their uteri were removed, homogenized, placed
in a saline solution, and refrigerated for future
use.15 Laboratory tests indicated a TDP of 60 C
for 60 min. No tests were made on the TDT.
Insertion studies involving the inoculation of
1.4X108 organisms into the composter were
performed. The results of these tests showed
that viable ova were not present 4 hr after in-
oculation. Ova were found 1 hr after inoculation
at the inlet and up to 76 hr at the outlet of the
composter; after 76 hr, ova were not detected.
The ova began to disintegrate 24 hr after inocu-
lation ; this accounts for their total absence after
76 hr. A total of 110 microbial analyses were
made.
Poliovirus Type I
This virus was the most susceptible to high-
heat kill; its TDP was 50 C for 30 min, and its
TDT was 5 min at 60 C.
The separation technique used to isolate this
virus involved a two-phase polymer system to
concentrate the samples, followed by inocula-
tion into cultures of FL cells and examination
for cytopathogenic effects. The final test con-
sisted of treating suspected samples with anti-
poliovirus Type I antiserum and observing the
results.
-------
26
TABLE 1
SOURCES AND QUANTITIES OF SOLIDS TO BE TREATED AT 1-MGD SEWAGE TREATMENT PLANT
Item
Raw waste:
SS* (ing/liter) -
BOD5 King/liter) - -
Primary treatment:
65 percent SS removal (mg/liter)
Secondary treatment:
85 percent removal of BODs from primary effluent (mg/liter) -
BODs removed converted to solids (' — '50 percent) (mg/liter)
Total dry solids removed (mg/liter)
Total dry solids in 1 mgd (Ib)
Total wet solids (• — '70 percent moisture) in 1 mgd (Ib) J
Total wet solids (45 Ib/cu ft) in 1 mgd (cu ft)
Primary Primary and
treatment secondary
treatment
250
200
160
80
160
1,340
5, 100
114
250
200
160
80
100
50
210
1,750
6,700
149
*Suspended solids.
fBiochemical oxygen demand, 5 day.
JFigures adjusted for the addition of 15 percent conditioning chemicals on a dry solids basis.
Initial analysis of the sewage sludge and com-
post revealed no virus of this strain was present.
In the insertion studies, 2 x 107 organisms were
mixed into the composter feed. All samples
taken at the inlet and at the outlet of the com-
poster were negative. This is reasonable since the
TDT of the virus is only 5 min at 60 C.
DESIGN CRITERIA
Application of the composting process requires
that certain basic criteria be known. In general,
the results of this study established these basic
design parameters, and these parameters struc-
tured the design of a compost process for a
primary and secondary sewage treatment plant
with a sewage flow of 1 mgd. When the sources
and quantities of the sludges to be treated were
listed, it was noted that the secondary portion
of treatment adds about 30 percent more sludge
to be processed (Table 1). Although the test
work involved only the combination of primary
and secondary sludges, there is no reason to be-
lieve that the process will not work just as well
on primary sludge alone. Based on the results ob-
tained from the operation of the pilot plant, a
calculated retention time of 7 days (actual reten-
tion time about 26 days), an air-flow rate of 0.25
scfm per cu ft, and a mixing horsepower of
about 0.05 hp per cu ft (based on paddle mixing,
which is not necessarily recommended) is
suggested.
The size of the equipment required to process
the volume of sludges for the 1-mgd plant
(Table 1) varies according to the treatment
phase (Table 2).
TABLE 2
DESIGN CRITERIA FOR COMPOSTING PLANT HANDLING
SOLIDS FROM A 1-MGD SEWAGE TREATMENT PLANT
Item
Composter:
Volume (cu ft)
Depth (ft)
Mixing using paddles, (hp)
Air required:
Volume (scfm)
Pressure (psig)
Horsepower
Primary Primary and
treatment secondary
treatment
800
3
~40
200
1
~2
1,000
3
—50
250
1
r^S)
In these design criteria, the compost depth is
set at 3 ft. Although this parameter was not
studied, it was believed that lesser depths would
begin to cause increased cooling and greater
depths would require increased power for mix-
ing. Future testing should include a study of
depth if this type of mixing is used.
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27
The type of mixing should also be studied in
future tests. A better method than the type
tested could be more effective. Mixing appeared
to be a significant parameter in the process
efficiency.
The above design does not include the ap-
purtenant facilities required to feed, recycle,
mix, and discharge the final product.
The operation of the pilot plant indicated
that the entire process could be nearly auto-
mated, with a minimum of manual operation
required. Temperature and dissolved oxygen
probes could be used to control the rate of mix-
ing and air-flow rate. Moisture sensors in the
compost bed and feed could be used to control
recycle and feed rate, and a bed level control
could be used to control the discharge of final
product.
The types of equipment for materials han-
dling would necessarily be designed on the basis
of volume, consistency, and location require-
ments of the composter layout.
CONCLUSIONS
1. The process was capable of reducing mois-
ture, volume, weight, and solids by an average
of 87, 73, 73, and 30 percent, respectively.
2. Continuous mixing and aeration of the
compost bed was required to ensure good odor-
free treatment.
3. The air requirement for drying was critical
and well in excess of the air required to maintain
aerobic conditions.
4. Recycling was not only beneficial, but nec-
essary; it facilitated moisture adjustment of the
feed and microbial (aerobic thermophilic)
seeding.
5. The final compost had a nitrogen and
phosphorus content of a little over 2 percent
each, expressed as N and P2O5; this compost
worked well as a soil conditioner and was found
to be free of indicator pathogens and viable
plant seeds.
6. The study of pathogens indicated that all
of the test organisms (Salmonella newport, Can-
dida albicans, Ascaris ova, and poliovirus Type
I) inserted in the compost were killed in less
than 3 days' retention in the composter.
7. The chemicals used had a dramatic effect
on the process. Sludge conditioned with lime and
ferric chloride produced compost that mixed and
handled better than sludge conditioned with
polymer.
8. Moisture content of the feed and the com-
post bed were critical for proper treatment. The
optimum moisture content in the compost bed
was 25 to 30 percent by weight.
ACKNOWLEDGMENTS
The project was financed as research contract
PH 86-67-103 by the Public Health Service of
the U.S. Department of Health, Education, and
Welfare. The Project Officer representing the
Bureau of Solid Waste Management was E. P.
Floyd.
The City of Salt Lake provided the research
site at the Salt Lake City Water Reclamation
Plant. Mr. W. Walters, plant superintendent,
cooperated in the work.
Mr. S. Westerberg and Mr. J. Crookston han-
dled the microbiology study under the direction
of Dr. B. Wiley at the University of Utah. Dr.
Wiley also aided the authors in preparing the
microbiology portion of this report.
REFERENCES
1. WILEY, J. S., F. E. GARTRELL, and H. G. SMITH. Concept and
design of the joint U.S. Public Health Service-Tennes-
see Valley Authority Composting Project, Johnson City,
Tennessee. Cincinnati, U.S. Department of Health Edu-
tion, and Welfare, 1968.14 p.
(Continued)
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28
2. OLDS, J. Houston compost plant—second year report. Compost
Science, 9(1): 18-19, Spring 1968.
3. WILEY, J. S. A discussion of composting of refuse with sewage
sludge. In American Public Works Association Yearbook.
Chicago, American Public Works Association, 1966. p.
198-201, 207-208.
4. AMERICAN PUBLIC HEALTH ASSOCIATION, AMERICAN WATER
WORKS ASSOCIATION, and WATER POLLUTION CONTROL
FEDERATION. Standard methods for the examination of
water and wastewater. 12th ed. New York, American
Public Health Association, Inc., 1965. 769 p.
5. SCOTT, W. W. Standard methods of chemical analysis. 5th ed.
2 v. New York, Van Norstrand Co., Inc., 1939.
6. KNOLL, K. H. Compost preparation from the hygienic view-
point. In International Congress on Disposal and Utili-
zation of Town Refuse. [Britain], Schevenigen, 1959.
mimeo. p. 12.
7. GOLUEKE, C. G., and H. B. GOTAAS. Public health aspects of
waste disposal by composting. American Journal of
Public Health, 44(3): 339-348, Mar. 1954.
8. GOTAAS, H. B. Composting; sanitary disposal and reclamation
of organic wastes. WHO Monograph Series No. 31.
Geneva, World Health Organization, 1956. 205 p.
9. HAJNA, A. A., and S. R. DAMON. New enrichment and plating
media for the isolation of Salmonella and Shigella orga-
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10. KENNER, B. A., and P. W. KABLER. Isolation of members of the
genus Salmonella. Applied Microbiology, 5(5) :305>-307,
Sept. 1957.
11. SPINO, D. F. Elevated temperature technique for the isolation
of Salmonella from streams. Applied Microbiology,
14(4):525-528, July 1966.
12. EDWARDS, P. R., and W. H. EWING. Identification of enter-
obacteriaceae, 2d ed. Minneapolis, Burgess Publishing
Company, 1962. 258 p.
13. STEDHAM, M. A., D. C. KELLEY, and E. H. COLES. Modified
Pagano-Levin medium to isolate Candida species.
Applied Microbiology, 14(4):591-596, July 1966.
14. TASCHDJIAN, C. L. A simply prepared identification medium
for Candida albicans. Mycologia, 45(3): 474-475, May-
June, 1953.
15. FAUST, E. C., and P. F. RUSSELL. Clinical parasitology. 6th ed.
New York, Lea & Febiger Publishing Co., 1957. p. 951-
952.
US. GOVERNMENT PRINTING OFFICE: 1970 O—355-447
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