United States
Environmental Protection
Agency
Robert S. Kerr Environmental Research "
Laboratory
Ada OK 74820
Research and Development
EPA-600/S2-83-024 May 1983
&ERA Project Summary
Resource Conservation and
Utilization in Animal Waste
Management
Raymond C. Loehr, John H. Martin, Jr., and Thomas E. Pilbeam
This study critically evaluated poten-
tial resource conservation and utili-
zation opportunities that could be part
of manure management systems and
thereby reduce the pollution potential
of animal manures. This work was
accomplished by a detailed evaluation
of literature and by laboratory studies.
The following areas were investigated:
(a) manures as a component of animal
feeds, (b) conservation of plant nutri-
ents in manures, (c) enhancement of
manure nutritive value, and (d) energy
production.
When manures are considered as
foodstuffs, they are best compared to
corn silage and forages rather than to
energy or protein feeds. Such utiliza-
tion of manures is feasible only when
they constitute a small fraction, less
than 20%, of an animal ration. Broiler
litter can be feasible at higherfractions.
The amino acid content of animal
manures is enhanced by short-term
(less than 7-day retention time) aero-
bic stabilization. The essential amino
acid concentration increased as much
as 36% and constituted a greater per-
cent of the total amino acids, as a result
of the aerobic treatment
Chemical stabilization and conserva-
tion of the ammonia in manure occurs
primarily as a function of decreased pH
rather than the type of chemical used.
Air stripping of ammonia followed by
capture in an acid solution appears to
be an effective way of conserving
manurial ammonia.
Some form of moisture loss is a pre-
requ isite for any thermochemical ener-
gy production process using manures.
With thermochemical processes, the
monetary value of plant nutrients that
are lost is an opportunity cost that
must be considered when energy con-
version processes are evaluated. The
economic feasibility of biogas produc-
tion depends upon the energy source
that is replaced, and the quantity of
biogas that is utilized. The digester
effluent does not have value as an
animal feedstuff.
This Project Summary was developed
by EPA's Robert S. Kerr Environmental
Research Laboratory, Ada, OK to an-
nounce key findings of the research
project that is fully documented in
three separate reports (see Project
Report ordering information at back).
Introduction
Through the 1950's, animal production
units were relatively small, large in num-
ber, located in relative isolation, and the
source of few identifiable environmental
problems. Developments over the past
two decades have changed this situation.
The number of animal production units—
farms and feedlots—has declined dramati-
cally, resulting in fewer but much larger
operations and more animals per produc-
tion unit In turn, this has resulted in an
increased amount of manure that must be
handled and disposed of in a manner that
does not cause air and water pollution
problems.
Unfortunately, improper handling and
disposal has occurred and has contributed
to water quality concerns such as fish kills
and increased eutrophication, and to air
quality concerns such as odors. As a
result technical approaches to negate or
minimize such environmental problems
have been developed. These approaches
-------
have included systems to control feedlot
and barnyard runoff, aeration units to
control odors and stabilize the manure,
drying systems for odor control and poten-
tial product use, ensiling of manure for use
as a feedstuff, anaerobic units for storage,
stabilization and possible methane gener-
ation, and soil injection systems to reduce
odors. Many of these approachest have
been successful, have reduced the ob-
vious environmental problems associated
with manure management and have been
integrated into existing animal production
units.
Such animal manure management sys-
tems developed in the past two decades in
an era of apparent resource and energy
abundance, did not always consider energy
efficiency or resource conservation. The
usual aim of manure management was
minimum treatment and disposal, rather
than resource conservation and utilization.
The increasing concerns about energy
limitations, adequate food, and environ-
mental pollution have emphasized the
inadequacies of such a minimum treatment
and disposal approach. The idea of a
"finite earth" has brought attention to
manure management methods that will
minimize environmental problems and
conserve resources. Today's treatment
and disposal methods stress conservation
of nutrients and energy in manures used
as fertilizer, feed, and fuel.
Many existing manure management sys-
tems have resource conservation "possi-
bilities" (Figure 1). The objectives of
resource conservation and environmental
protection can be complimentary. For
example, effective conservation and utili-
zation of manurial nitrogen for crop pro-
duction will reduce the quantity of this
nutrient lost to the environment In the
past, with few exceptions, the resource
conservation aspects of specific manure
management processes have not been
critically evaluated.
The objective of the project described
herein was to identify and evaluate poten-
tial resource conservation and utilization
methods which could be adapted to manure
management systems. This objective was
accomplished by a detailed evaluation of
information in the literature and by labora-
tory research on possibilities that appeared
to deserve greater attention. The following
specific possibilities were evaluated: (a)
use of as-collected and processed manures
as a component of animal feed, (b) conser-
vation of plant nutrients in manures, (c)
energy production from manures, and (d)
microbial enhancement of manures to
increase the protein content
Manure
Management
Possibilities
Solids
Separation
Storage and
Stabilization
(Aerobic and
Anaerobic
Land
Application
Crops
(Proper Application
of Residues
Resource Conservation
and Utilization
Possibilities
Animal Feed
Bedding (Animals)
Horticultural Use
Energy Production
Nitrogen Conservation
Nutrient Enhancement
for Animal Feed
Separate Liquid for
Flushing and Other Use
• Nutrient Utilization
for Forage and Grains
Figure 1. Resource conservation and utilization possibilities associated with animal pro-
duction and manure management.
Manures as Foodstuffs
The use of animal manures as feedstuffs
has the potential to reduce feed costs and
to provide a partial solution to manure
management and environmental problems.
During the past forty years, a number of
nutritional and economic studies have
evaluated the possible use of manures as
feedstuffs. These studies evolved from
the detection of "unknown growth factors"
to identification of the nutrient content and
nutritional value of specific manures.
Although a substantial data base exists as
a result of these studies, the data are not
consistent. As a result, neither the nutri-
tional and economic value of manures as
feedstuffs, nor the alleviation of water and
air pollution problems that might result
from the use of manures as feedstuffs is
clear, and the realization of benefits from
use of manures as feedstuffs remains
elusive.
In this study, available information on
the nutrient characteristics of manures
and their utilization as feedstuffs was
assembled and critically reviewed. The
evaluation focused on dairy cattle, beef
cattle, and caged laying hen manures and
on broiler litter, since these manures rep-
resent approximately 85% of the econom-
ically recoverable manure produced an-
nually in the U.S. Sheep manure was not
evaluated, although results from digestibil-
ity trails utilizing sheep as a species fed
manures were included in the evaluation.
The value of manure as feedstuffs was
determined by an assessment of: (a) nu-
trient characteristics of animal manures to
determine if they should be classified as
protein, energy, or forage substitutes, (b)
animal performance in reported feeding
trails, (c) economic benefits that might
result from reduced feed costs and
increased revenue from sale of meat
-------
or eggs, and (d) potential pollution control
benefits. The general types of manures
that were evaluated included solid or semi-
solid manure that was as-collected, dried,
composted or ensiled, and liquid manure
that was aerobically or anaerobically stabi-
lized.
All of the studies on feeding trials that
were located in the literature were not util-
ized in the evaluation. The following
criteria were used to select studies appro-
priate for detailed evaluation: (a) an accu-
rate description of the experimental design
was stated, (b) a positive control group
was utilized, (c) the feedstuffs used in the
rations were conventional and the percent-
ages utilized were reported, and (d) suf-
ficient animal performance data were re-
ported to enable a nutritive evaluation. The
available data were analyzed to identify the
"optimum" and "maximum" nutritional
level at which the manure could be included
in the feed ration. The maximum level was
defined as the maximum percent of manure
that could be included in a feed ration
without adversely affecting the animal
performance as compared to controls. The
optimum level was defined as the percent
of manure in a ration that would provide
the highest level of animal performance as
compared to controls. The optimum level
of manure was less than the maximum
level in a feed ration. In many studies only
the maximum level was able to be defined.
Nutrient Evaluation
Evaluation of the nutrient composition
of manures indicated that they are: (a)
more comparable to corn silage and typical
forages (alfalfa timothy and bermudagrass
hay) for ruminants than they are to energy
or protein feedstuffs, and (b) a source of
amino acids and minerals for laying hens.
The estimated economic value of these
manures, based on their nutrient content
was positive when used to replace corn
silage and forages in ruminant rations and
was highest when dried poultry wastes
(DPW) were used to replace a portion of
the meat and bone meal in diets of laying
hens.
Animal Evaluation
The results of feeding trials indicated
that while animal manures have nutritive
value as a feedstuff, the method of pre-
paring or processing the manures as feed
constituents (drying, composting, ensiling,
etc.) influences their value. Table 1 sum-
marizes maximum and optimum levels of
manures incorporated into rations for laying
t>ens and ruminants. Generally, the maxi-
ium level of manures incorporated into
"animal feed rations is less than 20%.
Table 1, Maximum and Optimum Levels of Incorporating Manures in Laying Hen and Ruminant
Rations Based on Animal Performance
Type of Manure
Species Fed
Maximum Level
of Incorporation into
Rations {%)
Optimum Level
of Incorporation into
Rations (%)
Dried Poultry Manure
Broiler Litter
-as-collected
-dried
-ensiled
-composted
Laying Hen
Steers
Heifers
Dairy Cows
Steers
Steers
Steers
Heifers
Beef Heifers
Brood Cows
Beef Cattle Manure
-as-collected Steers
-dried Steers
-ensiled compared to
-corn silage Steers
-corn grain Ruminants
14-20
5
*
10-12
18-22
11-16
25-52
1-10
75
80
0
0
16-24
10-12.5
LT5
*
LT11
LT18
10-30
LT10
* Unable to be determined from existing data.
LT-less than.
Broiler litter is an exception and can be
incorporated at higher levels without ad-
versely affecting animal performance.
Economic Assessment
The economic value of manures used as
feedstuffsissummarizedinTable2. Dried
poultry manure, broiler litter, and possibly
aerobically processed swine and laying
hen manure have an economic value as
feedstuffs that equals or exceeds their
value as a fertilizer. The economic value of
beef cattle manure and anaerobically pro-
cessed manures when used as a feedstuff
is less than their value as a fertilizer and
frequently results in poor animal perform-
ance.
Conclusions
When animal manures (DPW, broiler
litter, dairy cow and beef cattle manure)
are used as a feedstuff, they are most
comparable to corn silage and forages,
such as alfalfa, timothy and bermudagrass
Table 2.
Economic Assessment of Using Manures as a Feedstuff Based Upon Animal
Performance
Type of Manure
Species Fed
Economic Value as a Feedstuff
Dried Poultry Manure
Broiler Litter
-as-collected, dried
ensiled, composted
Beef Cattle Manure
-as-collected and
dried
-ensiled
Processed Manures
-aerobically
-anaerobically
Laying hens, steers,
dairy cows, heifers
Steers, heifers,
brood cows
Ruminants I'steers)
Swine, laying hens
Steers
Far exceeds its value as a fertilizer; has value
as a substitute for meat and bone meal and
also for silages and forages when used at
optimum level; positive but less than at
optimum level when used at the maximum
level
Positive; more than its value as a fertilizer and
comparable to the value of corn silage and
forages
Negative; adverse animal performance
Negative; unable to compete with low cost of
forages or corn silage, may have positive
value when used at low levels as a substitute
for grain corn
Possibly positive, requires further study
Negative, poor animal performance
-------
hays, and not to energy or protein feeds.
DPW used as a feedstuff for laying hens is
an exception, and is best described as a
source of minerals and amino acids.
The economic value of DPW and broiler
litter as feedstuffs is greater than their
value as a fertilizer. The value of beef cattle
manure and anaerobically digested ma-
nures as a fertilizer is greater than their
value as feedstuffs. Available data indicate
that their use as a feedstuff can impair
animal performance.
Utilization of animal manures as feed-
stuffs is nutritionally and economically
feasible only when such manures consti-
tute a relatively small fraction of the ration,
typically less than 20%. Broiler litter,
however, is feasible at higher levels. The
nature of manure management prior to
utilization as a feedstuff affects its nutri-
tional and economic value.
The utilization of manures as feedstuffs
does not appear to be a management
practice that will reduce potential water
and air pollution problems caused by im-
proper handling and disposal of such
manures. Only a low level of such manures
will be incorporated into animal rations,
and the potential pollution abatement im-
pact will be minimal.
Enhancement of the Nutritive Value
Introduction
Previous research results have suggested
that the nutritive value of animal manures,
particularly the ammo acid content, can be
enhanced during aerobic stabilization.
Batch and continuous flow laboratory ex-
periments were undertaken to determine
the nature of the amino acid transforma-
tions that occur as animal manures are
aerobically stabilized, and to identify aera-
tion system operating parameters that
may be utilized to maximize ammo acid
content and quality.
Fresh manure collected from caged White
Leghorn laying hens was used as the
substrate for both studies. Continually
mixed and aerated four-liter capacity re-
actors were used. A measurable dissolved
oxygen concentration (0.5 mg/liter) was
maintained in the units at all times. The
studies were conducted at ambient labora-
tory temperatures, 22 ± 2°C. For each
batch study, 2.7 liters of slurried poultry
manure was combined with 0.3 liters of
mixed liquor from an oxidation ditch stabi-
lizing caged laying hen manure. The latter
served as a source of an active, adapted
microbial population. The initial mixed
liquor total solids concentrations in all
units was 30 g/liter (3%). Samples were
obtained for analysis daily for the first four
days and at less frequent intervals later in
the 10- to 1 5-day experiments.
The continuous flow reactors were op-
erated at retention times of 3, 5, 7, and 10
days without solids recycle; therefore, the
hydraulic and solids retention times were
the same. The influent total solids concen-
tration for all continuous reactors was 30
g/liter.
Batch Studies
An example of the data from one batch
study is presented in Table 3. In each
study, the total amino acid concentration
increased slightly during the initial stages
of aeration but decreased thereafter. The
essential amino acids increased consider-
ably, as much as 36%, in the first few days
of aeration and were not less than the
initial concentration until after about seven
days of aeration.
Of equal interest is the fact that after
initial increases occurred, essential amino
acids as a percentage of total ammo acids
and mixed liquor volatile solids did not
decrease but rather remained relatively
constant with time.
Continuous Flow Studies
Results from the continuous flow studies
(Table 4) also showed that both the total
and essential amino acid content of poultry
manure can be increased by aeration with
the essential amino acid content increased
significantly. The retention time at which
increases were maximum was short, 3
days or less, which agrees with the results
of the batch studies. As retention time
increased, both total and essential amino
acid concentrations decreased.
Conclusions
These studies have shown that short-
term aerobic stabilization has the potential
to increase the amino acid content of
laying hen manure. Both the quantity of
amino acids and the quality, expressed in
terms of essential amino acids as a per-
centage of total amino acids, is increased
as compared to freshly excreted laying hen
manure (Table 5). On a dry-matter basis
(amino acids as a percent of dry matter),
the amino acid profiles for aerobically
stabilized laying hen manure and soybean
meal are comparable (Table 5).
Table 3. Changes in Total and Essential Amino Acid Concentrations During Aerobic Stabilization of Poultry Manure - Batch Study I
Total Amino Acids
Essential Amino Acids*
Time of Aeration
days
0
1
2
2.5
3
4
7
10
Fraction of
Initial
Concentration t
1.0
0.91
1.0
1.06
0.98
0.95
0.76
0.69
% of Mixed
Liquor Volatile
Solids
16.14
16.36
20.51
23.81
22.13
24.65
22.64
23.27
Fraction of
Initial
Concentration t
1.0
1.09
1.28
1.36
1.25
1.20
0.95
0.87
% of Total
Amino Acids
38
44
48
48
48
48
47
47
% of Mixed Liquor
Volatile Solids
6.11
7.29
9.90
11.53
10.69
11.90
10.68
11.05
* Essential for poultry.
t Concentration at aeration time t/'concentration at time zero.
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Table 4. Changes in Total and Essential Am/no Acid Concentrations During Aerobic Stabilization of Poultry Manure Under Continuous Flow Conditions
Total Am/no Acids Essential Amino Acids*
Retention
Time, Days
3
5
7
10
Fraction of
Initial
Concentration t
1.37
1.08
0.99
0.44
% of Mixed
Liquor Volatile
Solids
32.71
25.27
20.94
9.16
Fraction of
Initial
Concentration f
1.72
1.27
1.22
0.52
% of Total
Amino Acids
45.1
42.4
44.6
43.2
% of Mixed
Liquor Volatile
Solids
14.8
10.72
9.34
3.96
* Essential for poultry.
f Concentration in mixed liquor/concentration in freshly excreted manure.
Table 5. Comparison of the Amino Acid Profiles of Ground Corn, Soybean Meal, and Aerobically Stabilized Laying Hen Manure, % of Total Amino Acids
Laying Hen Manure
Arginine*
Glycine
Histidine*
Leucine*
Isoleucine*
Lysine*
Methionine*
Phenylalanine*
Tyrosine
Valine*
Alanine
Proline
Glutamic acid
Serine
Threonine*
Aspartic acid
Ground
Corn
4.8
0.0
2.4
10.9
4.8
2.4
1.2
4.8
4.8
3.6
9.6
10.9
33.7
1.2
3.6
2.8
Soybean
Meal (49%)
6.8
4.9
2.6
7.7
4.9
6.1
1.2
5.0
3.1
4.9
4.9
5.8
18.6
5.2
3.9
13.0
Batch
(Day 2.5)
5.7
6.6
2.1
9.0
6.5
6.2
1.9
5.0
3.6
6.9
7.8
4.7
13.2
4.4
5.1
11.2
Continuous Flow
(3-day HPT)
6.1
8.2
1.7
7.8
5.7
5.4
2.6
4.1
3.0
6.4
10.2
4.5
13.0
4.6
5.0
11.4
As Excreted
4.0
25.2
1.8
6.2
6.0
4.6
1.6
3.2
2.4
4.6
7.0
4.6
11.4
4.0
4.0
9.3
Total amino acids,
% dry matter basis
Essential amino acids,
% of total
9.47
38.9
56.23
43.1
17.32
48.4
20.76
45.1
15.11
36.1
* Amino acids essential for poultry.
Estimated Losses
Historically, livestock and poultry manures
have been important by products of animal
production and have been applied to crop-
land for centuries in order to utilize their
nutrient content In contrast the extensive
use of inorganic fertilizers has occurred
only in the past 30 years. As recently as
1 950, the combined consumption of nitro-
gen, P205, and K2O in the U.S. was only
2.6 million metric tonnes annually. By
1973, the combined consumption increased
to 1 7.5 million metric tonnes annually,
with nitrogen representing almost 50% of
the consumption. During this period the
cost of inorganic fertilizer nitrogen de-
creased substantially. As a result manures
were not as widely returned to cropland
Conservation of Plant Nutrients
since their transport and distribution costs
exceeded their monetary return. As in-
creased costs for energy are translated
into higher prices for inorganic fertilizers,
interest in the use of manures as fertilizers
has been renewed.
Only between 40 and 50% of the man-
urial plant nutrients annually are collec-
tible and thus potentially available for
utilization. About 50% of the collectible
nitrogen and 10% of the collectible phos-
phorus and potassium in manures is lost
during collection, storage, and disposal of
the manure. This latter fraction of the
collectible nutrients offers the greatest
potential for conservation, recovery, and
use. This quantity is not insignificant. The
quantity of potentially recoverable man-
urial nitrogen in the U.S. is about 1.3
million tonnes annually, represents about
1 7% of the fertilizer nitrogen consumed
annually in the U.S., and represents a
monetary loss of close to $800 million.
The nitrogen losses associated with
various storage and stabilization alterna-
tives for animal manures are noted in
Table6. The variability of manurial nitrogen
losses for specific systems suggests that
management is an important variable af-
fecting these losses. As ammonia is the
principal form in which manurial nitrogen is
lost, laboratory studies were conducted to
evaluate various alternatives for reducing
volatilization of manurial ammonia. The
option of air-stripping of manurial ammonia
followed by recovery also was, examined.
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Table 6. Nitrogen Losses Associated with
Various Animal Waste
Management Alternatives
Observed Nitrogen
Losses, %
Storage Systems
1. Stacking (dairy manure)
2. Liquid manure slurries
3. In-house drying
(laying hen manure)
Aerated Systems
1. Aerated lagoons
2. Oxidation ditches
3. Liquid composting
Anaerobic Lagoons
Anaerobic Digestion
Composting
Pyrolysis
5-29
1-60
22-75
5-65
25-81
13-43
25-62
0-21
3-37
64-89
Chemical Stabilization
Studies earlier in this century suggested
that the addition of chemicals such as
calcium sulfate (gypsum) and calcium
phosphate as well as phosphoric and sul-
furic acids to manures could reduce am-
monia volatilization losses. The objective
of calcium sulfate and calcium phosphate
additions was to form ammonium sulfate
or phosphate which are less soluble than
ammonium carbonate or hydroxide. In
some studies, superphosphate fertilizer
material which contains gypsum and cal-
cium phosphate as principal ingredients
was utilized. The results of these early
studies were highly variable. To reevalu-
ate the potential of this approach, a series
of laboratory experiments using the noted
compounds were conducted with anaero-
bically digested dairy cow manure which
had a high concentration of ammonia
nitrogen.
The effectiveness of the noted chemicals
to stabilize manurial ammonia was evalu-
ated by comparing the quantities of am-
monia nitrogen that could be removed
from treated and untreated manure sam-
ples by drying and air-stripping. Drying
studies were conducted using 100 ml
samples which were dried at 103°C for 24
hours. Dried samples were reconstituted
with distilled water for subsequent analysis.
For the air-stripping trials the following
conditions were standard: sample volume,
one liter; airflow rate, 425 standard liters
per hour (SLH); duration, 24 hours, and
total solids concentration, 20 g/liter. The
nitrogen loss for each sample was deter-
mined by the difference between initial
and final total Kjeldahl nitrogen (TKN) and
ammonia nitrogen concentrations. Initial
and final pH values also were measured.
Results of drying studies in which phos-
phoric and sulfuric acids were added to
digested dairy cow manure showed that
pH reduction can substantially reduce ni-
trogen losses. Both acids were added to
manure samples to obtain initial pH levels
of 7.0,6.5, andG.O. Nitrogen losses of the
control (pH 7.6) and pH 7.0 samples
approaches 100%. Reducing pH to 6.5
reduced losses only slightly. However, by
reducing the pH of the samples to 6.0
before drying, nitrogen losses were re-
duced to 60 and 52% respectively.
Results of air-stripping studies also
showed that using acid to reduce manurial
pH was effective in reducing nitrogen
losses. The TKN and ammonia losses
were lowest for the samples with the
lowest initial and final pH. In all instances,
pH increased during aeration. Illustrative
data from experiments in which acids
were used for pH adjustment are noted in
Table 7.
Also air-stripping studies were con-
ducted to assess the effectiveness of
calcium sulfate and calcium phosphate
additions to reduce manurial nitrogen losses.
These compounds were added at rates of
50, 100, and 200% of the stoichiometric
requirements for converting the ammonia
nitrogen in the sample into sulfate or
phosphate salts. Results of these studies
showed that additions of these chemicals
have almost no effect on reducing nitrogen
losses. Due to low solubility of superphos-
phate fertilizers, studies involving these
chemicals were not pursued.
Aeration
The effect of aeration rate on ammonia
nitrogen losses was evaluated, using rates
of 142, 283, 425, and 566 SLM/liter of
mixture. The TKN losses did not vary
significantly with these rates ranging from
40 to 46% for the digested dairy cow
manure, and from 48 to 54% for the
anaerobic laying hen manure. However,
there appeared to be a linear increase in
the ammonia nitrogen loss as the airflow
rate increased. The ammonia losses for
the two manures increased from 59 to
90% and from 51 to 91 % as the airflow
rate increased from 142 to 566 SLH/liter.
The ammonia losses also increased as the
aeration period increased from 24 to 96
hours.
The feasibility of air-stripping of ammonia
without pH control followed by recovery of
the stripped ammonia in an acid solution
(0.1 N H2 804) was evaluated in another
set of experiments. These studies were
conducted in completely mixed units.
Compressed air was used to strip the
ammonia from the dairy and poultry man-
ures. In each unit, the pH increased,
undoubtedly as a result of stripping of
carbon dioxide. The pH increase occurred
within the first hour and remained relatively
constant at a pH of 8.4 or 8.5 thereafter. In
the experiments, almost all of the stripped
ammonia could be recaptured indicating
that even if ammonia volatilization does
occur, the gaseous ammonia can be cap-
tured if the off gases or ventilation air are
passed through an acid media.
Conclusions
Results from laboratory studies indicate
that the ammonia volatilization losses are
more a function of the pH of a manure
mixture and therefore the quantity of free
ammonia able to be lost than of the type of
chemical used to inhibit the loss or to
complex the ammonia. Therefore, manure
management approaches and chemical
additions that maintain a low pH (less than
6.5) will minimize ammonia losses. The
use of superphosphates was not feasible
due to their minimal solubility in water and
manure slurries.
Treatment
Sulfuric Acid
Control
pH7.0
pH6.5
pH6.0
Phosphoric Acid
Control
pH7.0
pH6.5
pH6.0
A"
Initial
8.0
7.0
6.5
6.0
7.9
7.0
6.5
6.0
T
Final
8.6
8.6
7.5
7.2
8.7
8.6
8.4
6.8
TKN
Loss, %
30.0
28.3
11.3
2.7
29.3
25.6
9.5
0.8
NH3-N
Loss, %
66.3
60.7
24.2
5.8
63.8
55.8
20.7
1.8
"Air-stripped for 24 hours at air flow rate of 425 liters of air/'hr/'liter of slurry.
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Introduction
The quantity of collectible manure that
is potentially available as feedstocks for
energy production is worthy of considera-
tion. Assuming an average heating value
of 1 7.2 GJ/tonne of manurial solids, the
potential energy of the collectible manure
in the U.S. is about 7.2 x 108 GJ/year.
This is equivalent to 1.1 x 108 barrels of
crude oil/year.
Several options are possible for convert-
ing manures into usable forms of energy,
i.e., thermochemical processes such as
direct combustion and pyrolysis, and an-
aerobic digestion, a biological process.
Thermochemical
All thermochemical conversion processes
have three basic components: drying, ther-
mal decomposition, and recovery and utili-
zation of the resultant energy. The first
two components are endothermic and
require close attention when evaluating
net energy production from a thermochem-
ical process. Before temperatures can be
reached at which thermal decomposition
will occur, reduction of moisture, such as
by evaporation, is necessary. Therefore, the
manure moisture content used in thermo-
chemical processes is critical.
The moisture content of manure must
be below 50% on a wet basis (WB) for any
appreciable quantities of recoverable energy
to be produced. For pyrolysis to occur, it
has been estimated that about 0.9 MJ/kg
of total solids is required in addition to
energy inputs for evaporation of moisture.
As produced, livestock and poultry manures
range in moisture content from 7 5% (WB)
for broilers and laying hens to 91 % (WB)
for swine. Thus, some form of moisture
loss is a prerequisite to thermochemical
energy conversion processes using manures
as feedstocks.
One of the less desirable aspects of
using animal manures for thermochemi-
cal processes is the loss of primary plant
nutrients. As noted earlier, considerable
losses of nitrogen can occur in drying
processes. In addition, combustion will
destroy a portion of the other nutrients as
well as all of the organics. When the
option of using manures as fertilizer ma-
terials is available, the monetary value of
the plant nutrients lost is an opportunity
cost that must be considered in evaluating
energy conversion processes. Thermo-
chemical processes are, at best, only mar-
ginally attractive economical energy con-
version processes for manures. When the
opportunity costs associated with plant
Energy
nutrient losses are included, even the
economics of direct combustion of manure
are not attractive.
Anaerobic Digestion
Using anaerobic digestion to produce
biogas (methane and carbon dioxide)'from
livestock and poultry manures has received
considerable attention and evaluation. As
a result, the technical feasibility of using
animal manures for biogas production has
become firmly established, and system
design and operating parameters have
been delineated and refined. The econom-
ic feasibility remains unclear, the greatest
uncertainty being effective gas utilization.
For anaerobic digestion to be econom-
ically attractive, revenue generated by bio-
gas sale or onsite utilization must provide
a suitable rate of return when compared to
capital, management inputs, or other op-
tions. The amount of revenue generated
by biogas utilization is dependent on the
conventional energy source replaced and
the quantity of biogas utilized.
Methane and carbon dioxide are the
principal constituents of biogas which also
contains small amounts of hydrogen, hy-
drogen sulfide, nitrogen, ammonia, and
water vapor. The composition of biogas is
about 50 to 70% methane and 30 to 50%
carbon dioxide. Biogas, assuming 60%
methane and 40% carbon dioxide, has an
energy density of 22.23 MJ/m3, which is
less than liquid fuels such as liquified
petroleum gas. Even with carbon dioxide
removal and compression to 4054 KP,
absolute, the resultant energy density of
1482 MJ/m3 still is not adequate to
realistically consider biogas as a potential
fuel for trucks, tractors, etc. Only liquified
methane has an energy density approach-
ing conventional liquid fuels. Liquification
of methane is energy-intensive requiring
over 30% of the energy available in the
methane.
Thus, available biogas utilization options
are limited to sale as synthetic natural gas
(SNG) or onsite utilization as a boiler fuel
or to generate electricity. The potential for
marketing manurial biogas as SNG appears
limited due to gas purification and com-
pression requirements. When biogas is
used as a boiler fuel, it has a variable
monetary value dependent on the conven-
tional fuel replaced. For example, biogas
used in place of No. 2 fuel oil has a value of
$6.68/GJ and of $2.77/GJ when used in
place of anthracite coal.
Frequently, onsite generation of elec-
tricity is suggested as a biogas utilization
alternative because on-farm demand for
electricity is relatively constant and excess
electricity has the potential to be sold.
Generation of electricity using biogas also
appears attractive because internal com-
bustion engine waste heat may be used to
satisfy digester heating requirements with
the result that total biogas production is
available for utilization.
The thermal efficiency of converting
biogas to electricity is only about 14.5%.
Thus, the value of biogas used to produce
electricity having a unit cost of $ 12.06/GJ
is only $1.75/GJ. Substantial levels of
waste heat recovery and utilization are
necessary to offset the low thermal ef-
ficiency of converting biogas to electricity.
Most cost analyses of producing biogas
from animal manures show that the value
of biogas produced is not adequate to
offset production costs. Several investi-
gators have suggested that producing
biogas from animal manures is econom-
ically feasible if the digester effluent has
value as a feedstuff or a source of plant
nutrients. Comparison of the feedstuff
and plant nutrient composition of manures
before and after anaerobic digestion has
shown that the value of manure as feed-
stuffs or plant materials is at best un-
changed. Therefore, the opportunity cost
associated with using manure as a feed-
stock for biogas production as an alterna-
tive to direct use as a feedstuff or a
fertilizer material should be included in
these cost analyses if the effluent is con-
sidered to have any monetary value.
Conclusions
1) Thermochemical processes are gen-
erally not applicable for converting ma-
nures into usable forms of energy due to
the high initial moisture content of the
manures. Exceptions are feedlot ma-
nures produced in arid and semi-arid
climates, and broiler litter. When op-
portunities to use manures as fertilizer
materials are available, the opportunity
cost associated with plant nutrient
losses that result from the thermo-
chemcial processes offset much of the
value of the energy produced.
2) The attractiveness of anaerobic diges-
tion of animal manures is limited by
biogas utilization alternatives. On-farm
energy demands are primarily for liquid
fuels. Thus, onsite biogas utilization
alternatives which will provide the
highest rate of return to invested capital
and labor are limited. Monetary returns
from sale of biogas as synthetic natural
U. S. GOVERNMENT PRINTING OFFICE: 1983/659-095/1949
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gas or electricity to public utilities are
significantly lower than those from
onsite utilization. Using anaerobic di-
gestion to produce biogas from animal
manures can be attractive, however, in
situations where an onsite operation
provides a constant and substantial
energy demand.
R. C. Loehr, J. H. Martin, Jr., and T. E. Pilbeam are with Department of Agricultural
Engineering, Cornell University, Ithaca, NY 14853.
Lynn R. Shuyler is the EPA Project Officer (see below).
The complete report consists of three volumes entitled "Resource Conservation
and Utilization in Animal Waste Management:" (Set Order No. PB 83-190 264;
Cost: $41.00, subject to change).
"Volume I. Utilization of Animal Manures on Feedstuffs for Livestock and
Poultry,"(Order No. PB 83-190 272; Cost: $26.50, subject to change).
"Volume II. Use of Aerobic Stabilization to Enhance the Value of Animal
Manure as Feedstuff s," (Order No. PB 83-190 280; Cost: $10.00, subject to
change).
"Volume III. Utilization of Animal Manures as Feedstocks for Energy
Production," (Order No. PB 83-190 298; Cost: $ 11.50, subject to change).
The above reports will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
P. O. Box 1198
Ada, OK 74820
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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