United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
^"'
Research and Development
EPA/600/S2-85/103 Jan. 1986
&ER& Project Summary
Chemical and Microbial
Aspects of Sludge
Composting and Land
Application
L J. Sikora, P. D. Millner, and W. D. Burge
A series of six studies was under-
taken to study various chemical and mi-
crobial aspects of the composting and
land application of sewage sludge. Am-'
monia, which is generated in sludge by
ammonifying microorganisms, was
shown to be virucidal. In the pH range
of 7 to 9, NH3, but not OH~ or NH4+, was
effective. Temperature effects were re-
lated to a single NH3 concentration.
Sludge and sludge compost with high
concentrations of heavy metals and
chlorinated organic materials appeared
to have no inhibitory effect on soil en-
zyme activity, probably because of the
age or the highly stabilized nature of
the sludge and sludge compost.
Sludges were applied once (100
megagrams [Mg]/ha) or annually (20
Mg/ha) for 5 years to field plots.
N movement down the soil profile oc-
curred in the sludge-amended plots, es-
pecially in all of the split plots that re-
ceived additional fertilizer. Phosphorus
movement occurred only down to the
60-cm depth, or four times the depth of
incorporation in the sludge-amended
plots.
Laboratory studies conducted to test
methods for improving the efficiency of
composting indicated that the most ef-
ficient system was the temperature-
demand aeration system. This method
resulted in twice as much drying and
yielded 2.5 times as much CO2 as con-
stant aeration composting.
Results of studies have shown that
Aspergillus fumigatus (AF) and ther-
mophilic actinomycetes (TA) are
present in air at very low levels in most
non-agricultural sites, even when or-
ganic matter is present and tempera-
tures are occasionally higher than am-
bient. Compost and moldy agricultural
substrates are the biggest reservoirs
and sources of AF and TA. The toler-
ance of AF to broad ranges of tempera-
ture, CO2, and water content limit the
benefits of modifying the composting
process by these parameters to inhibit
this microorganism.
This Project Summary was devel-
oped by EPA's Water Engineering Re-
search Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Applying sewage sludge or sludge
compost to land can affect several
chemical and microbiological factors in
soils. The benefits of this practice,
which include the addition of nutrients,
trace metals, and organic matter, can be
offset by excess additions of phytotoxic
metals, nitrogen, and organic chemi-
cals. The need to differentiate between
proper and excessive sludge additions
to soil is imperative to evaluate properly
the benefits of each sludge product.
Certain sludges have excessive
amounts of toxic materials and should
not be applied to land. Other sludges
may have elevated amounts, but they
have no apparent phytotoxic conse-
quences when added to soil at low
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rates. The long-term effects on soil pro-
ductivity, surface water, and ground-
water need to be elevated along with
the short-term, first-year effects.
This report discusses the effect of NH3
on the survival of viruses and compares
NH3 with halogens for virucidal proper-
ties. The availability of sludge N and P
to plants and the movement of these
nutrients through the soil profile over
the long term are also discussed. Soil
enzymes, the key to successful bio-
chemical transformations in soil, were
monitored to determine the effects of
large or repeated sludge applications.
The data presented here provide a bet-
ter understanding of the long-term ef-
fects of such applications.
Composting stabilizes sewage
sludges and transforms them into an
easily spread and stored material. The
composting process was analyzed ex-
tensively using a self-heating laboratory
composter, and the effects of tempera-
ture and aeration on the efficiency of the
composting process were determined.
These investigations improved our un-
derstanding of the optimum compost-
ing process for producing material for
soil amendments.
Composting is a thermophilic process
and results in a product containing nu-
merous thermophilic organisms that
have various public health implications.
This study provides data on the num-
bers, types, and potential health impli-
cations of thermophilic organisms pro-
duced during the composting of
sewage sludge, the movement of com-
post and other self-heating materials,
and the eventual distribution of these
materials to users.
Kinetics and Thermodynamics
of Viral Inactivation by
Ammonia
Ammonia has been shown to be viru-
cidal in sludge and in NH4CI solutions.
Our studies examined the kinetics and
influence of temperature on the inacti-
vation of f2 bacteriophage and po-
liovirus 1 by NH3. At pH values from 6.5
to 9.5 and NH3 concentrations from 50
to 800 mg/L, the inactivation of both
viruses was pseudo-first-order. The
OH~ had no measurable effect on the
viruses, and the virucidal effect of NH4+
was insignificant compared with that of
NH3. The bacteriophage f2 was approxi-
mately 4.5 times more resistant to the
effects of NH3 than was poliovirus.
A general rate equation was derived
for virus inactivation as influenced by
NH3 concentration. Although NH3 is a
weak disinfectant, it may be a practical
virucide. Temperature strongly influ-
enced inactivation rate. Poliovirus was
inactivated at a greater rate than f2, but
the change in the rate of inactivation
with increasing temperature in the
range of approximately 10° to 40°C was
greater for poliovirus. At higher temper-
atures, the rate of change was greater
for poliovirus. Arrhenius plots of the
data were biphasic, indicating that two
inactivation processes were occurring
at 300 mg/L NH3—one for the low-
temperature range and another for the
high-temperature range (Figure 1).
However, the magnitudes of the ther-
modynamic variables for f2 were low
enough, as calculated for the low (10° to
35°C) and high (35° to 60°C) phases, that
inactivation could have occurred by
breakage of nucleic acid chains. For po-
liovirus, the sizes indicated possible in-
volvement of nucleic acid at the low
range (10° to 40°C) but some unkown
mechanism for the high range (40° and
50°C). The study concluded that inacti-
vation by IMH3 could play an important
role in the destruction of viral patho-
gens in sludge.
Effects of Sludge and Compost
on Soil Enzymatic Activity
Soil enzymes are important factors in
plant nutrient mineralization. They may
4
3
2
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-2
-3-
-4-
-5-
Figure 1.
be temporarily or permanently im-
paired by soil amendments that contain
excessive amounts of heavy metals or
organic chemicals. A study was initiated
to quantitate and compare the effects of
various sewage sludges and sludge
composts on important soil enzymes
such as dehydrogenase, urease, and al-
kaline phosphatase.
A high-metal, high-organic chemical
sewage sludge and its composted coun-
terpart were tested to determine their
effects on soil enzymatic activity. Dehy-
drogenase activity and rate of C02 evo-
lution of the amended soils were closely
correlated with respect to amendment
type and rate, and no inhibition was ap-
parent. Phosphatase activity of all
amendments was correlated to C02
evolution. Net urease activity was corre-
lated to amendment type (degree of sta-
bilization) and rate, and it showed no
apparent inhibition.
The relative level of heavy metals or
organic chemicals therefore may not be
a suitable indicator of the biological ef-
fects of sewage sludge and sewage
sludge composts. Several studies have
indicated that inorganic metal salts
have inhibiting effects on enzymes,
some at very low concentrations. These
data indicate that, through time, metals
are bound or chelated with organic mat-
ter and that organic chemicals are par-
tially degraded or neutralized, resulting
Q
3.0
3.1
3.2 3.3
1000/T
3.4
3.5
The relationship between the inactivation rate constant (kj and temperature
(° Kelvin) for f2 and poliovirus 1 (Chat).
-------
in little or no inhibitory effect on biolog-
ical processes such as enzyme activity.
Effects of Sewage Sludge
Applications on Nitrogen
Mineralization and Phosphorus
Mobility in Soils
Sewage sludges contain a wide range
of essential plant nutrients. Recycling
these nutrients on agricultural land can
augment commercial fertilizer sources
and thereby conserve the energy used
in their production.
In 1978, the W-124 Regional Commit-
tee formulated a cooperative, 5-year re-
search project on the land application of
sewage sludge to answer the questions
concerning the effects of soil type and
climate on N mineralization and P mo-
bility.
The field data collected at Beltsville,
Maryland, generally indicated that a 20-
Mg/ha annual application rate resulted
in yields similar to those from the rec-
ommended fertilizer application rate,
but that slightly more N was taken up by
the plants growing on plots receiving
sludge. The 20-Mg/ha annual applica-
tion resulted in a greater cumulative
yield and N uptake than the single 100-
Mg/ha application added to the plots in
1978. The application of N on the split
plots in 1981 resulted in larger, but not
significant, yield increases and N uptake
increases in most instances. During the
first and second years, phosphorus up-
take was greater in the 100-Mg/ha plots
than in the 20-Mg/ha plots. The uptake
for the third through the fifth years was
the same for each treatment.
Soil samples were taken with depth
after the harvest in 1983 and compared
with soil samples taken from the plots in
1978 before sludge application. Migra-
tion of N was detected to the deepest
sampling depth, which varied from 60
to 180 cm in all N~ and sludge-fertilized
plots. Phosphorus migration down to 60
cm was found in plots receiving sludge
application.
The movement of P does not pose a
problem and may even be beneficial.
The depth of P movement is within the
root zone and below the level of incor-
poration. Plants should therefore bene-
fit from the deeper soil horizons con-
taining P.
Effect of Temperature and
Aeration on the Composting
Process
Composting is a process that is self-
starting, self-heating, and in certain cir-
cumstances, self-limiting. In general,
the variables that control or affect com-
posting are the quality of the starting
materials, moisture, aeration, pH, and
temperature.
In the process of removing water
from composts, heat is concomitantly
removed. Vaporization is the dominant
heat removal mechanism, removing
nearly nine times more heat than con-
vection. Evaporation can be accelerated
by turning piles or by increasing the aer-
ation rate. Thus the control of tempera-
ture is best accomplished through con-
trol of vaporization or moisture
removal.
A laboratory composter that uses
thermistors as temperature-sensing
devices and an aeration system, which
permits diffusion of air into the mass,
was designed for testing parameters to
improve the composting system. Raw,
highly limed filter cake sludge from the
Blue Plains Wastewater Treatment plant
(Table 1) was mixed with woodchips in
a ratio of 1:1.8 (v/v) and composted.
Aeration was controlled either at preset
airflow rates or according to the de-
mand necessary to hold the tempera-
ture desired. One study limited the tem-
perature of the insulating water bath to
55°C, thereby preventing the compost-
ing organic material from reaching
60°C. A second study aerated the
sludge-woodchip mixture at 900 or 1800
m3/h per Mg and compared tempera-
tures, moisture, volatile solids, and C02
loss. A third composter study directly
compared composting of a sludge-
woodchip mixture under constant aera-
tion with that of a temperature-demand
system.
Carbon dioxide data indicated that
the water-bath-controlled, 55°C com-
poster produced 50% more C02 than
the uncontrolled composter that
reached 80°C, probably by having a
more diverse population to degrade a
greater variety of compounds in the
mixture (Table 1). The data obtained
from the study at two aeration rates in-
dicated that continuous rapid aeration
did reduce peak temperatures, but it
also reduced the duration of the com-
posting process. In the temperature-
demand study, 2.5 times more sub-
strate decomposed than in the constant
aeration composter. The final moisture
content of the compost from the
temperature-demand study was 33%.
Constant, high aeration did reduce
peak temperatures, but it did not result
in the most efficient composting sys-
tem. Increasing and decreasing the aer-
ation rates to keep temperatures be-
tween 50° and 55°C allowed the
maximum thermophilic population to
remain active for extended periods.
The microbial data indicated that high
temperatures (>65°C) decreased both
total number and diversity of popula-
tion, which resulted in a substantial de-
cline in C02-C evolution or stabilization.
The disappearance of fungi from the
high-temperature composts and the
significant difference in CO2 evolution
underlined the importance of fungi in
the stabilization process and the neces-
Table 1. Carbon, Nitrogen, and Moisture Losses During Composting of Raw, Limed Sewage
Sludge and Woodchip Mixtures under Various Aeration and Temperature Condi-
tions
Material Losses (%)
Aeration-Temperature
Condition
Bath controlled at 55°C
versus
900 m3/h per Mg
constant aeration
900 m3/h per Mg
constant aeration
versus
1800 m3/h per Mg
constant aeration
Wet
Weight
32.9
29.0
29.4
29.4
Dry
Weight
6.6
0.71-
6.3
6.3
Total
N
2.8
4.6
4.1
3.9
corc*
6.9
3.1
1.8
3.1
Final %
Moisture
38.1
37.7
41.3
39.8
900 m3/h per Mg
constant aeration
versus
Temperature, demand
aeration
22.1
45.3
5.3
12.2
10.6
21.7
2.4
5.9
49.3
33.0
'Grams of COZ-C lost per 100 g dry initial mixture.
*Error was due to inaccurate moisture determination.
-------
sity of a successful compost operation
for controlling temperatures as much as
possible.
Nitrogen loss from organic material
composting has not been studied exten-
sively, and from the data collected in
these studies, the amount of N lost was
proportional to condensate loss. Thus
increased aeration leads to increased
loss of nitrogen, but if the N loss is ac-
companied by an increased loss of
volatile solids (as in the temperature-
demand study), the N content of the
final product changes only slightly.
Nutritional and Non-Nutritional
Factors in the Growth of As-
pergillus fumigatus (AF) and
Natural Sources of Airborne AF
and Thermophilic Actino-
mycetes
The potential public health risk asso-
ciated with aerosols of Aspergillus fu-
migatus (AF) that cound be inhaled at
sludge-composting sites has been a
concern to those who are responsible
for the planning and operation of com-
post facilities. Information about the
ambient levels of AF spores in air could
be used to help evaluate the potential
impact of the aerosols on the public
health in adjacent areas.
The work reported here was under-
taken to increase the data base on the
quantitative differences in airborne lev-
els of AF under environmental circum-
stances that affect the growth of the fun-
gus. An extensive literature survey was
made to determine the present knowl-
edge of the occurrence of AF in the air.
Then a series of air samples was col-
lected from various locations (including
some suspected natural sources) and
analyzed for viable AF.
Studies were also made of the nutri-
tional and non-nutritional factors in
sewage sludge composting that affect
the growth of AF. This part of the work
aimed to determine whether or not
there was a basis for managing the
composting process to produce a mini-
mum of AF growth and aerosols.
Finally, to answer concerns about the
exposure of compost-site workers to
farmer's lung antigens (thermophilic
actinomycetes, or TA), air levels of
these microbes were determined con-
currently with those made for AF in the
different ambient environments.
Air Sampling for Fungi and TA
Selected for study were habitats in
which environmental conditions were
conducive to the growth of AF and TA.
Air samples were obtained by using An-
dersen six-stage, viable (microbial) par-
ticle samplers (Andersen 2000, Inc., At-
lanta, Georgia).* Twenty-one outdoor
and indoor suspected sources and three
unsuspected (reference) sources were
sampled.
Nutritional and Non-Nutritional
Factors Affecting the Growth
ofAF
An investigation was made of the tol-
erance limits on factors that affect the
germination, growth, and sporulation of
AF. The object was to suggest practical
modes of imposing environmental
stress on the organism in the compost-
ing situation.
Natural Airspora of AF
The airspora levels of the different
sampling sites during each season ap-
pear in Table 2. During winter, the air-
*Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
spora in outdoor locations was gener-
ally lower than that during other
seasons. AF levels at the reference sites
were also lower in winter than in sum-
mer, and they were never greater than
12 colony-forming units (cfu)/m3.
Substrate Studies
Several types of wood common to the
mid-Atlantic region of the United
States, as well as oak leaves, paper pel-
lets and cubes, and peanut hulls are ex-
cellent substrates for AF growth and
sporulation when sufficient moisture
and temperature are provided.
Temperature Studies
All of the seven isolates grew poorly
at 55°C and very slowly at 59°C. At 45°C,
the growth rate was 6 to 8 times greater
than that at 50°C.
Gaseous Environment and pH
Studies
In chambers containing 0.5% 02 and
23% CO2, AF spores germinated and
mycelial extension was equivalent to
that observed on the control plates incu-
Table 2. Natural Airspora of Aspergillus fumigatus, 1979-80
Seasonal Counts (colony-forming units/m3)
Site
Fall
Winter
Spring
Summer
Lawn:
During mowing
With mulch
Under trees
Of hospital
Of park
Wood area:
Arboretum
Nature trail
Roadside
Agricultural:
Corn field
Barn
Barnyard
Poultry house
Mushroom house
Brush pile
Refuse:
Municipal dump
Supermarket dumpster
Greenhouse:
Potting room
Low humidity
High humidity
Library stacks
Attic
Zoo - birdhouse
Boiler room
Reference sites:
School playground
University parking lot
Shopping center
1
75
3
2
8
4
56
1
7
2,070
44
21
88,700
7
6
2
868
NS*
NS
171
NS
5
30
6
7
77
5
2
0
0
4
1
0
5
0
105
0
93
740,000
1
2
0
1,350
11
0
0
1
0
38
1
1
7
2
6
5
0
24
6
10
2
0
352
35
2,060
580,000
25
0
0
1,070
312
152
0
1,160
42
1
12
12
7
0
686
4
0
2
136
8
3
4
5,550
4
6
67, 100
5
5
12
9,810
1
2
0
125
2
1
9
9
3
*NS = not sampled.
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bated in air at ambient temperature.
However, only a very limited number of
1 conidiophores and spores were pro-
duced. Rapid growth and sporulation
occur at pH 6, 7, and 8, but decreased
growth rate and sporulation are evident
at pH 9 and 10.
Osmotic Potential
Decreased growth rate occurred at all
temperatures when the osmotic poten-
tial of the growth medium was less than
-40 x 102 kPa.
Natural Airspora of TA
Most outdoor and indoor locations
had fewer than 10 TA cfu/m3. Excep-
tions included the mushroom house,
barn, barnyard, and poultry house. TA
from other locations belonged to the
genus Streptomyces, and an identifica-
tion scheme for the group was devised
based on test results from the type spe-
cies and 55 compost and 35 natural air-
spora strains.
Results of these studies show that AF
and TA are present in air at very low
levels in most nonagricultural sites,
even if organic matter and occasionally
higher-than-ambient temperatures are
present also. Thus the natural sources
for AF and TA are very limited in terms
of abundant production of spores. Few
spores are airborne from the natural
sources, even during mechanical distur-
bances such as mowing. High levels of
AF and TA in air are associated with
heavily colonized substrates. Compost
and moldy agricultural substrates are
the biggest reservoirs and sources of AF
and TA.
Practical alterations of the sewage-
sludge composting process to reduce
AF growth and dispersal from compost
sites should presently focus on the use
of noncellulosic bulking agents. Such
agents would substantially reduce the
inoculum levels entering the process.
The tolerance of AF to broad ranges of
temperature, CC>2, and water content
limit the benefits of modifying the com-
posting process by these parameters to
inhibit this microbe.
Conclusions
Ammonia was shown to be virucidal
in sludge and in NH4C1 solutions. Tem-
perature effects on NH3 activity were
biphasic, indicating that two inactiva-
tion processes were occurring—one for
the low-temperature range and another
for the high-temperature range. The f2
virus was 5.4 times more affected by
NH3 than was poliovirus 1. As a viru-
cide, NH3 is extremely weak compared
with chlorine. Because of its high reac-
tivity with organic material, chlorine is
required in high dosages to disinfect
sludge. These translate into high chlo-
ride levels in the final sludge. The use of
NH3 to disinfect wastewater treatment
plant effluents is precluded by the slow-
ness of its reaction; but in sludge, NH3
may indeed be a practical virucide.
The effects of two types of sludges
and their corresponding sludge com-
posts on soil enzyme activities indicated
that the stabilization level of the sludge
correlated negatively with enzyme ac-
tivity. One sludge that was stored in a
lagoon for several years and contained
high concentrations of heavy metals
and chlorinated organic materials
showed no significant inhibitory effect
on soil enzyme activity. The reason was
probably that the sludge and the corre-
sponding sludge compost were highly
stabilized, with the toxic components
either degraded or bound to organic
matter.
Soil profile data indicated that when a
digested sewage sludge was applied
once at 100 Mg/ha or annually at 20 Mg/
ha for 5 years, mineralized N exceeding
plant needs was found below the root
zone. The addition of fertilizer N to split
plots increased the level of mineralized
N in the deep soil profile. Phosphorus in
the sewage sludge also migrated down
the soil profile, but not to the same
depths as N. The 20-Mg/ha application
rate exceeded the nutrient needs of bar-
ley.
A self-heating laboratory composter
was designed, built, and used success-
fully in studies of test methods for im-
proving the efficiency of composting.
The most efficient composting system
tested was one that adjusted aeration to
hold temperatures near or below 55°C.
This system produced 2.5 times more
decomposition and 2 times more mois-
ture loss from the compost than did a
compost system using a constant rate
of aeration. The loss of N from the com-
post was directly related to the effi-
ciency of removing moisture.
AF and TA are present in air at very
low levels at most nonagricultural sites,
even if organic matter and occasionally
higher-than-ambient temperatures are
also present. Thus the natural sources
for AF and TA are very limited in terms
of abundant production of spores. Com-
post and moldy agricultural substrates
are the biggest reservoirs and sources
of these organisms. The tolerance of AF
to broad ranges of temperature, C02,
and water content limits the benefits of
modifying the composting process by
these parameters to inhibit the microbe.
Recommendations
Ammonium was shown to be viruci-
dal tof2 bacteriophage and poliovirus 1.
The effect of NH3 on the inactivation of
several human pathogens needs to be
evaluated before recommendations can
be made on the use of NH3 for reducing
pathogen numbers in sewage sludge.
Caution should be exercised in evalu-
ating the effects of heavy metals and
chlorinated organic materials in sewage
sludge on soil biochemical properties.
Factors such as age of the sludge and
the sludge treatment process should be
considered when evaluating the effects
of chemicals whose salts are known to
be toxic to biological processes.
The application of sewage sludge to
agricultural land should be coordinated
with the crop's fertilizer need. Nitrogen
mineralization rates of the sludge and
soil should be determined, and applica-
tion rates should be adjusted accord-
ingly. Application rates based on P min-
eralization and movement should be
considered in soils such as sands that
do not fix appreciable amounts of P.
A composting system should be de-
signed to be as efficient as possible.
That is, it should result in a stabilized,
dry product in as short a period of time
as possible. Controlling temperatures
so that they do not exceed 55° to 60°C by
adjusting aeration provides a highly ef-
ficient composting system. However,
destruction of pathogens under such a
system needs to be evaluated in labora-
tory and field trials.
The sewage sludge composting proc-
ess should be altered to use noncellu-
losic bulking agents to reduce AF
growth and dispersal from compost
sites. Such agents would substantially
reduce the inoculum levels entering the
process.
The full report was submitted in fulfill-
ment of Interagency Agreement No.
AD-12-F-2-534 by the Agricultural Re-
search Service, U.S. Department of
Agriculture under the sponsorship of
the U.S. Environmental Protection
Agency.
U. S. GOVERNMENT PRINTING OFFICE: 1986/646-116/20761
-------
L J. Sikora, P. D. Millner. and W. D. Burge are with Agricultural Research Service,
U.S. Department of Agriculture, Beltsville, MD 20705.
Kenneth Dot son is the EPA Project Officer (see below).
The complete report, entitled "Chemical and Microbial Aspects of Sludge
Composting and Land Application," (Order No. PB 85-243 186/AS; Cost:
$16.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-85/103
0000329 PS
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