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                           D. P. Chynoweth*, A. C. Wilkie**, and J. M. Owens*
                            *Dept. of Agricultural and Biological Engineering
                                  **Soil and Water Science Department
                                 University of Florida, Gainesville, Florida

                                     Written for Presentation at the
                                1998 ASAE Annual International Meeting
                                          Sponsored by ASAE

                                            Orlando, Florida
                                            July 11-16, 1998

The swine industry is growing rapidly along with the world human population. The trend is toward more
concentrated piggeries with numbers in herds in the thousands. Associated with these increased herds
are large quantities of wastes, including organic matter, inorganic nutrients, and gaseous emissions. The
trend in swine waste management is toward treatment of these wastes to minimize negative impact on
the health and comfort of workers and animals and on the atmosphere, water, and soil environments.
This review discusses the present and future role of anaerobic processes in piggery waste treatment with
emphasis on reactor design, operating and performance parameters, and effluent processing.

Piggery wastes, swine wastes, anaerobic treatment, anaerobic digestion, biogas
            D. P. Chynoweth*, A. C. Wilkie**, and J. M. Owens*
             *Dept. of Agricultural and Biological Engineering
                   **Soil and Water Science Department
                  University of Florida, Gainesville, Florida

           A paper presented at the pre-conference session:

Management of Feed Resources and Animal Waste for Sustainable Animal
           Production in Asia-Pacific Region Beyond 2000
           Eighth World Conference on Animal Production

                         June 28 - July 4, 1998

                              Seoul, Korea

The swine industry is growing rapidly along with the world human population. The trend is toward more
concentrated piggeries with numbers of herds in the thousands. Associated with these increased herds
are large quantities of wastes, including organic matter, inorganic nutrients, and gaseous emissions. The
trend in swine waste management is toward treatment of these wastes to minimize negative impact on
the health and comfort of workers and animals and on the atmosphere, water, and soil environments.
Treatment of these wastes has traditionally involved land application, lagoons, oxidation ditches, and
conventional batch and continuously stirred reactor designs. More sophisticated treatment systems are
being implemented, involving advanced anaerobic digester designs, integrated with solids separation,
aerobic polishing of digester effluents, and biological nutrient removal. This review discusses the present
and future role of anaerobic processes in piggery waste treatment with emphasis on reactor design,
operating and performance parameters, and effluent processing.


The swine industry is growing rapidly as more people in developing countries can afford and acquire a
taste for more meat in their diet, including pork. In the past 16 years, the annual global production of
swine has increased from 790 million to 926 million with most of that increase occurring in China, India,
and other emerging countries (FAO, 1990, 1991, 1995, 1996). In past years, piggeries were small
(hundreds or less animals) and wastes were disposed of on the same land used to grow the feed, serving
as fertilizer and soil conditioner. The increased demand for pork has resulted in establishment of larger
centralized piggeries with herds frequently exceeding 1,000 and sometimes more than 10,000 head
(Hatfield et. al, 1998). Wastes from these facilities exceed the capacity for direct land disposal without
severe environmental impacts, including odor, attraction of rodents, insects and other pests, and release
of animal pathogens, atmospheric methane and ammonia, nitrogen, phosphorus, and other nutrients into
ground and surface waters.

The characteristics of swine wastes vary with a number of factors, including the age and diet of the pigs,
type of housing or confinement, and waste removal and pre-processing (Day & Funk, 1998; USDA, 1992;
Zhang & Felmann, 1997). The wastes are either scraped or hydraulically flushed into a holding basin,
after which they are treated directly or after solids separation. Commonly used systems for removal of
organic matter include aerobic and anaerobic lagoons, oxidation ditches, and anaerobic digestion. More
advanced systems include tertiary treatment operations, such as oxidation ponds, aquatic plants,
wetlands, and denitrification units.

This review addresses the current and future role of anaerobic processes for swine waste management.
Anaerobic digestion has been applied in a variety of forms and scales to stabilize the organic matter in
these wastes. This process results in effective organic matter and pathogen reduction with production of
a useful fuel and compost. Properly operated anaerobic digesters result in reduction of odors associated
with these wastes (Wilkie, 1998). Anaerobic treatment may also play an important role in nutrient
removal. Following aerobic oxidation of ammonia to nitrate, nitrogen may be removed by anaerobic
denitrification. Phosphorus removal may also be enhanced by anaerobic pretreatment, which results in
formation of organic acids that enhance phosphorus uptake in aerobic processes. Algae ponds and
wetlands have also been applied for effluent polishing and nutrient removal.

The review summarizes the characteristics of piggery wastes, the role of anaerobic treatment of organic
matter, the role of anaerobic digestion in reduction of pathogens and gaseous emissions, and anaerobic
treatment of nitrogen and phosphorus. It also discusses integrated treatment systems that use anaerobic
processes and future trends in utilization of anaerobic treatment of piggery wastes.


Global Production

Worldwide swine populations by region and select countries for the periods of 1979-81, 1989-91, and
1996 are presented in Table 1. The number of pigs in the world are about one billion, or one

                Table 1. World Population of Swine (1000 Head; FAO, 1990,
                1991, 1995, 1996 1995, 1996)

                        REGIONS                1979-81      1989-91      1996
                          World                779,506      854,213     927,354
                           Asia                368,702      433,194     535,844
                         Europe                173,384      182,930     166,963
                 North and Cent. America        97,327       87,012     96,644
                     South America              51,722       52,378     56,122
                          Africa                10,155       16,522     21,652

                          China                313,660      360,247     452,198
                           USA                 64,045       54,557      60,190
                          Brazil               34,102       33,643      35,350
                        Germany                34,468       33,350      24,698
                         Mexico                16,895       15,715      18,000
                        Viet Nam                  --        12,225      17,200
                         France                11,472       12,233      14,523
                         Canada                 9,709       10,505      12,043
                           India                9,433       11,193      11,900
                        Denmark                 9,669        9,390      10,709
                          Korea                   --         8,007      10,300
                          Japan                 9,851       11,673      10,200
                       Philippines              7,712        7,968       8,941
                            Italy               8,885        9,150       7,984

per six persons. By region, the largest numbers are in Asia, followed by North and Central America,
South America, and Africa. By country, the largest numbers are in China followed by the U.S., Brazil, and
Germany. The most significant increases in numbers have occurred in certain emerging countries such
as China, Vietnam, Korea, and India while numbers have decreased in several developed countries,
including the U.S., Germany, and Italy. These trends may be attributed to a number of factors, such as
improved economies resulting in higher meat consumption and export by emerging countries, and
reduced meat consumption and environmental regulations in developed countries. Estimates of global
production of swine wastes are presented in Table 2.

Future Trends in Wastes Production

The numbers of swine and associated quantities of wastes are likely to increase greatly over the next
several decades due to the projected increase in human population and the trend of developing countries,
with the highest rates of population increase, to shift to diets with a higher meat content. This will be
offset to some extent by reduced red meat (including pork) consumption by the more developed
countries. For example, in the U.S., the market share of red meat has decreased from 74% in 1970 to

59% in 1994 (Zhang & Felmann, 1997). The projected world human population increase of 27% by 2020
(US, 1997) should result in at least the same increase in numbers of swine.

        Table 2. World Swine Waste Production by Region (Safley et al., 1992)

                     Region                   Total Manure,       Volatile Solids,
                                               Mt/day (wet)           Mt/day
              Asia and Far East                 1,663,466            168,010
               Eastern Europe                    788,722              77,641
               Western Europe                    570,932              57,664
                North America                    345,490              34,960
                Latin America                    320,415              32,362
                    Africa                        52,519               5,286
                   Oceania                        24,079               2,432
         Near East and Mediterranean               830                   84
                     Total                      3,746,273            378,439

Swine Production Facilities

The trend in developed and developing countries is to change from small piggeries, where the feed is
grown and wastes are land applied locally, to fewer piggeries with greater numbers per facility and import
of feed. This results in large quantities of wastes which must now be treated in order to prevent major
environmental impact. Also, odors from larger facilities are objectionable to nearby communities. Swine
operations are often separated into feeder pig producers (up to 18 kg; 60 days) and feeder pig finishing
(98 kg or larger; 150-160 days). The traditional operators raise a pig from birth to death (farrow to finish).
The trend in swine housing is confinement in open feedlots or slanted floor units (Day & Funk, 1998;
Zhang & Felmann, 1997). Manure is collected by scraping and land applied (with or without prior
treatment), or hydraulically flushed from slanted or slatted floor housing where the diluted waste is stored
under the house or transported to storage tanks, lagoons, or other waste treatment systems. In some
cases, treated wastewater is reused for flushing. Table 3 indicates that liquid flush systems prevail in
developed countries, while dry storage and drylot systems are more common in emerging countries.

Physical and Chemical Characteristics

Typical values for swine waste characteristics as excreted and as collected in storage tanks and lagoons
and of runoff water and sludges from feedlots are presented in Tables 4, 5, 6, and 7 (Day & Funk, 1998;
USDA, 1992; Zhang & Felmann, 1997). These data are useful in predicting environmental impact and
designing systems for waste treatment.

Environmental Impact

In past years, swine herds were small and wastes could be applied to land used to produce feed and
other crops. In contemporary concentrated piggeries with large herds, wastes may exceed the carrying
capacity of local ecosystems and are a potential cause of a number of pollution and health problems
related to their organic matter, nutrients, pathogens, odors, dust, and airborne microorganisms (Zhang
and Felmann, 1997).

Table 3. Global Swine Waste Management System Usage (percent) (Safley et al., 1992)

            Region                     Anaerobic     Liquid        Daily   Dry Storage    Other
                                                            a                                    b
                                        Lagoons     Systems       Spread    and Drylot   Systems
               Asia                         1          38           1           53           0
        Eastern Europe                      8          39           0           52           1
        Western Europe                      0          77           0           23           6
        North America                      25          50           0           18           6
         Latin America                      0           8           2           51          40
              Africa                        0           7           0           93           0
             Oceania                       55           0           0            0          28
 Near East and Mediterranean                0          32           0           68           0
        Global Average                      5          42           1           45           5
 Includes liquid/slurry and pit storage
 Includes deep pit stacks, litter, and other

               Table 4. Typical Body Mass and Waste Production and
               Characteristics per day per 1000kg of Swine (Day & Funk, 1998)

                               Parameter           Mean      Std. Dev.
                            Live Weight, kg          61          --
                           Total Manure, kg          84         24
                               Urine, kg             39         4.8
                             Density, kg/m          990         24
                            Total Solids, kg         11         6.3
                           Volatile Solids. kg      8.5        0.66
                               BOD5, kg             3.1        0.72
                                COD, kg             8.4         3.7
                                   pH               7.5        0.57
                                TKN, kg*           0.52        0.21
                            Ammonia-N, kg*         0.29        0.10
                              Total P, kg          0.18        0.10
                              Ortho-P, kg          0.12          --

Table 5. Production and Characteristics of Fresh Manure by Pigs (Zhang & Felmann, 1997)

   Parameter         Nursery      Growing             Gestation
                                                 Finishing        Sow and            Boar
                                                        Sow        Litter
   Size, kg         15.9       29.5        68.1          125        170               159
Manure, kg/day       1.0        1.9         4.4           4.0       14.9              5.0
  TS, kg/day        0.091      0.18        0.41          0.36       1.36             0.45
  VS, kg/day        0.077      0.14        0.33          0.30       1.09             0.420
    BOD5            0.032     0.059        0.14          0.12       0.45             0.16
  N, kg/day         0.007     0.013       0.031         0.028       0.10             0.035
 P2O5, kg/day       0.005     0.010       0.023         0.022      0.078             0.027
 K2O, kg/day        0.005      0.11       0.024         0.022      0.082             0.028
              Table 6. Swine Waste Characteristics From Storage Tanks
              Under Slats (USDA, 1992)

                 Component        Farrow      Nursery      Finish     Breeding
                 Moisture, %       96.5        96.0         91.0        97.0
                 TS, % w.b.        3.50        4.00         9.00        3.00
                 VS, % w.b.        2.28        2.79         6.74        1.80
                 FS, % w.b.        1.22        1.71         2.26        1.20
                    N, g/L          3.6         4.8          6.3         3.0
                 NH4-N, g/L         2.8         4.0           -           -
                    P, g/L          1.8         1.6          2.7         1.2
                    K, g/L          2.8         1.6          2.2         2.1
                  C:N Ratio          4           3            6           3

Table 7. Swine Waste Characteristics From Storage/Treatment Facilities (USDA, 1992)

                 Anaerobic Lagoon                             Feedlot*
 Component             Sludge            Supernatant      Settled Sludge        Runoff Water
 Moisture, %            92.4                  99.8              88.8                98.5
  TS, % w.b.            7.60                  0.25              11.2                 1.5
  VS, % w.b.            4.68                  0.12             90.7**                 --
  FS, % w.b.            2.92                  0.13             21.3**                 --
  BOD5, g/L               --                  0.40               --                   --
   COD, g/L             64.6                   1.2               --                   --
    N, g/L               3.0                  0.35             5.6**               2.0**
  NH4-N, g/L            0.76                  0.22             4.5**               1.2**
    P, g/L               2.7                  0.13             2.2**               0.38**
    K, g/L               7.6                  0.38             10.0**              1.10**
  C:N Ratio               8                     2                --                   --
 *Semi-humid climate (76 cm annual rainfall); annual sludge removal
**kg/d/1000kg animal weight

Organic matter is concentrated and undergoes anaerobic decomposition producing odors related to
hydrogen sulfide, ammonia, volatile acids, and other compounds. The highly biodegradable organic
matter also attracts pests, including insects and rodents. Organic matter may also cause oxygen
depletion in surface waters and other undesired effects related to color, turbidity, and taste and odor.
When organic matter undergoes decomposition under highly anaerobic conditions, methane (a major
greenhouse gas) is released into the atmosphere (Safley et al., 1992; USEPA, 1993).

Nutrients each have their own impacts on surface and ground water. Nitrogen may be released as
ammonia into the atmosphere, where it acts as a greenhouse gas and contributes to acid rain in its
oxidized form. Ammonia may also react with nitrate in the atmosphere to form ammonium nitrate
particles which contribute to smog and health problems. High ammonia levels in swine houses may also
cause eye irritation, respiratory problems, and illness in workers and animals. The recommended
maximum acceptable level for human and animal occupancy is 10 ppm (Morrison et al., 1991).

In surface waters, nitrogen in the form of ammonia or nitrates causes blooms of algae and aquatic plants
which contribute to eutrophication and their decomposition may lead to anaerobic conditions. These
blooms, caused also by phosphorus, may consist of highly toxic algae (Pfiesteria) in brackish waters and
have been implicated in kills of fish and other aquatic life, and as a cause of adverse health effects on
humans and animals (Lusk, 1998). Ammonia is toxic to life in surface waters (Zhang & Felmann, 1997).
Concentrations as low as 0.08 mg/L have been shown to cause trout kills. Runoff from swine raising

operations and manure-fertilized fields commonly contains 200-200 mg/L ammonia which is well above
the recommended USEPA standard of 0.02 mg/L (USDA, 1992). Nitrates in groundwater may cause
significant health problems in human and animal development leading to methemoglobinemia, a disease
causing oxygen starvation of developing tissues and possible death. The USEPA drinking water standard
for nitrate-N is 10 mg/L (USDA, 1992). At elevated levels, nitrates are also toxic to fish and other aquatic

Sulfides are generated from degradation of protein and other sulfur-containing compounds in swine
wastes. These may be toxic to aquatic organisms and cause odors and toxicity in swine-housing
(Donham, 1991; Morrison et al., 1991). Hydrogen sulfide, detectable as an odor at concentrations as low
as 0.005 ppm, causes loss of appetite, vomiting, and nausea at 50-500 ppm, and is lethal at 1000 ppm.

Swine wastes contain pathogens and coliform bacteria and other microbial indicators of fecal pollution
(CAST, 1996; Zhang & Felmann, 1997). Although the pathogens are mainly host specific, certain
diseases such as salmonellosis, Q fever, Newcastle disease, histoplasmosis, cryptosporidiosis, and
gardiasis may be transmitted by swine waste. Fecal indicator organisms originating from swine wastes
make it impossible to distinguish the presence of animal or human fecal pollution. Pathogens in the
wastewater may also result in cross-infection of the swine.

Dust and bioaerosols from animal feed and manure may cause health problems in animals and workers if
not controlled by ventilation and other means (Morrison et al., 1991; Zhang & Felmann, 1997). They may
cause infectious and respiratory diseases, reduced immune response, allergies, and discomfort.

Odors are a major environmental problem with large piggeries (Davidson et al., 1995; Wilkie et al.,
1995a). These are caused by numerous volatile compounds such as ammonia, amines, volatile fatty
acids, mercaptans, carbonyls, phenols, and indoles. Odor threshold concentrations are very low and are
a major factor limiting location of these facilities.

Laws, Regulations, Policy

In response to the increased environmental impact of intensive rearing facilities for swine and other
livestock, several laws, regulations, and policies designed to protect public health and the environment
are being called into enforcement as they apply to these industries in the U.S. (Weitman, 1995).

Control of methane, ammonia, and dust emissions may fall under the jurisdiction of agencies charged
with enforcing the Clean Air Act which addresses the air quality of the nation.

The Clean Water Act regulates animal feeding operations considered to be point sources of pollution by
requiring permits issued by the National Pollution Discharge Elimination System (NDPES). Whether a
piggery or other operation falls under the jurisdiction of this Act depends on the number of animal units
(1000 animal units equals 2,500 swine), type of confinement, days of operation, and nature of the water
receiving discharge. For example, Category 1 includes operations with over 1,000 animal units; confines,
feeds, or maintains animals for a total of 45 days or more in a year; and does not sustain any crops,
vegetative forage, or harvest residues. The NDPES permit for this category stipulates that there must be
a storage facility to contain all of the manure plus processing water and runoff from a 25-year, 24-hour
storm event. Monitoring is required at least once per year and a plan for nutrient management must be
approved and implemented. Smaller facilities must submit plans based on Best Available Technology
(BAT) that is economically achievable and Best Conventional Pollutant Control Technology (BCPCT)
based on professional judgment. The Clean Water Act also regulates non-point source pollution by
requiring states to devise a comprehensive plan that addresses contributors. This involves voluntary
adoption of Best Management Practices (BMPs) which are encouraged by educational programs,
training, financial and technical assistance, and demonstration projects.

The Coastal Zone Act Reauthorization Amendments (CZARA) of 1990 regulates animal operations in

35 states which have coastal area watersheds. These facilities are regulated exclusive of those under
the Clean Water Act. These regulations require storage and treatment of wastewater and stormwater,
waste treatment, and nutrient management. Existing facilities in the U.S. under these regulations for
swine have 100-200 head. Odor problems are becoming more prevalent due to the increased scale of
intensive livestock operations and urban encroachment into rural areas; these problems are subject to the
common laws of nuisance. Farmers are still protected to some extent from these complaints by the Right-
to-Farm laws. Federal and state incentives are alternatives to regulation. For example, the
Environmental Quality Incentives Program (EQIP) provides cost sharing of up to 75% of the costs of
pollution prevention practices.

Ultimately, the tax payer, consumer, and producer must pay the price for maintenance of environmental
quality. The cost of environmental pollution is difficult to assess as the effects are usually indirect and
long-term. Whatever the case, human activities, including animal production must strive for sustainablity
which will cost more.


Anaerobic digestion may be defined as the engineered methanogenic anaerobic decomposition of
organic matter. This process, occurring naturally in anaerobic environments such as sediments, soils,
and animal guts, involves a mixed consortium of different species of anaerobic microoganisms that
function in concert to degrade organic matter and complete the carbon cycle for a large fraction of organic
matter (Chynoweth, 1996). Non-methanogenic populations depolymerize organic polymers and ferment
them to acetate (sometimes via other acids and fermentation products), hydrogen, and carbon dioxide.
Different methanogenic bacteria convert acetic acid, hydrogen, and carbon dioxide to methane (Boone et
al., 1993; Smith & Frank, 1988). Most, but not all organic matter can be decomposed by this fermentation
without chemical or physical pretreatment. Lignin is the major natural compound that is refractory to
anaerobic decomposition. Other organics, such as cellulose, may be resistant to degradation when
complexed tightly with lignin (e.g., in pine wood) or contained in biomass that contains methanogenic
inhibitors (e.g., in eucalyptus wood).

Anaerobic digestion has been applied for decades for treatment of domestic sludges, animal wastes,
industrial wastes (McCarty, 1992), and more recently for the organic fraction of municipal solid wastes
(Chynoweth & Isaacson, 1987). It has also been the subject of research for production of substitute
natural gas (SNG) from wastes, energy crops, including terrestrial herbaceous and woody energy crops,
and aquatic (freshwater and marine) energy crops (Chynoweth & Pullammanappallil, 1996; Legrand,
1993; Smith et al., 1988; Smith et al., 1992). Whatever the application, anaerobic digestion produces a
useful energy form (methane) and a stabilized residue that can be subsequently applied to land as a soil
amendment. Currently, its most common applications are treatment of domestic sludges, industrial
wastes, and animal wastes. Its wider use has been hampered by the low cost of fossil-based energy,
limited regulations on waste processing, a history of process instability, and greater knowledge and
popularity of aerobic processes. However, the climate is changing for this technology with incentives to
replace fossil fuels with renewable “greener” energy forms and stricter regulations on management of
organic wastes that will require more costly in-vessel systems compared to land application, landfilling, or
crude open lagoons.

A typical swine waste treatment system is shown in Figure 1. The wastes are transported directly or after

        Figure 1. Illustration of Collection and Management Options for Piggery Wastes

concentration into a digestion vessel which may vary in design from a lagoon to a mixed,
non-mixed plug-flow, or attached film reactor. The operating temperature may be ambient, mesophilic
    o                      o
(35 C), or thermophilic (55 C). The effluent is stored and land applied on a seasonal basis. The
supernatant may be further treated for nutrient removal prior to discharge into receiving waters. The
biogas is used either directly for heating or for operation of internal combustion engines to run equipment
or generate electricity.

Feed Characteristics

Design and operating parameters of anaerobic treatment systems are largely dependent upon the influent
total solids (TS) concentration (Chynoweth, 1987; Chynoweth & Isaacson, 1987). For swine wastes, the
excreted concentration is about 10% and becomes diluted with urine and further diluted with flush water
(in liquid systems), or concentrated when bedding is used in dry storage systems (Day & Funk, 1998;
Sweeten et al., 1981; Zhang & Felmann, 1997). A typical concentration for tanks under slats is 3-4%.
For a specific organic loading rate, the hydraulic retention time (HRT) of conventional stirred tank reactor

(CSTR) anaerobic digesters increases inversely with the total solids concentration. At very low
concentrations (<1-2% TS), attached-film reactors can be used to substantially reduce the HRT by
concentrating microorganisms on media to prevent their washout (Wilkie & Colleran, 1989). The
characteristics of wastes are influenced more by dilution, storage, and separation practices rather than by
diet or other factors (Hatfield et al., 1998).


Wastes may also be separated into solids and liquid fractions by various techniques, including
sedimentation, mechanical screening, centrifugation, and pressing. Separation not only segregates the
wastes for optimization of digester design, but also facilitates manure handling with pumps, pipelines, and
sprayers. As discussed below, it also provides for improved odor control. For digestion, the solids
fraction can be treated in a conventional CSTR or plug-flow design and the liquid in a lagoon or a low-
HRT attached-film design.

Day (1998) and Zhang (1997) reviewed the design, performance, and economics of a variety of
separations techniques. In general, the capital and operating costs are high and must be justified by the
overall goals and economics of the piggery operation. The vibrating screen separator has been shown to
be effective for separating flushed swine waste into solid and liquid fractions. Detailed mass balances are
presented by Holmberg (1982) for the effect of screen mesh size and flow rate on separation of organic
matter and nutrients. Analysis of the effect of an optimum screening regime (60 mesh and flow rates of
457-685 L/min) by Hill (1984), indicated that about 44% of the biodegradable organic matter and 64% of
the total nitrogen passes through the screen. A model for predicting vibrating screen performance is
presented by Ngoddy (1974) .


Ultimate biodegradability under anaerobic conditions is determined by long-term incubations and is
measured in terms of methane yield and reduction in organic matter. Reported ultimate biodegradability
(Bo) is useful in kinetic analyses and has been reported in the range of 0.32 to 0.48 m CH4/kg VS
(Andreadakis, 1992; Hashimoto, 1984; Hill & Bolte, 1984; Iannotti et al., 1979; Safley & Westerman,
1990). This corresponds to volatile solids (VS) reductions in the range of 40-60%. These parameters are
largely dependent upon the composition and digestibility of the feed ration. In full-scale digesters, the
ultimate digestibility is seldom achieved because of retention time limitations. Iannotti (1979) reported a
detailed analysis of digester feed and effluent properties including organic components (Table 8).
Carbohydrates are the major components followed by proteins, lipids, and lignin. The waste composition
would be significantly influenced by the animal diet. Lignin is not only refractory to anaerobic
degradation, but also reduces availability of other components, especially cellulose.

(feed blends and effect of diet blends)

Blending of swine wastes with other organic wastes may be attractive, especially with high-solids
feedstocks. Swine wastes provide excess nutrients and high-solids feedstocks serve as bulking agents,
increasing the solids content of the blends. Studies have evaluated swine waste blended with wheat

Table 8. Digestibility of Organic Matter During Anaerobic Digestion of Piggery Wastes (Iannotti et
al., 1979) (Finishing hogs fed 14% protein ration with corn or milo; mesophilic digester with
retention time of 15 days).

                            Component             Influent    % Destroyed
                                TS, %                6.9          52
                              VS, %TS               82.6          60
                              COD, g/L              73.8          58
                             total N, g/L            3.9          --
                            protein, %TS            19.3          47
                         hemicellulose, %TS         20.1          65
                           cellulose, %TS           12.4          64
                                lipids              14.8          69
                                starch               1.6          94
                                lignin               4.4           3

straw (Llabres-Luengo & Mata-Alvarez, 1987), corn stover (Fujita et al., 1980), algae and water hyacinth
(Campos & d'Almeida Duarte, 1992), and sewage sludge (Wong, 1990). Wong (1989) investigated a
variety of agro-industrial additives, including cardboard, newspaper, sawdust, and sugarcane wastes.
Mixtures of swine waste with sawdust or cardboard gave the highest methane yields. Residues from
sugarcane blends with pig wastes exhibited the highest fertilizer value. Wastes from pigs fed different
sources of fiber (oat hulls, maize hulls, lupin hulls, maize cobs, soya bean hulls, pea hulls, wheat bran,
lucerne stems, and lucerne leaves) exhibited different extents and rates of biodegradability during
anaerobic digestion (Stanogias et al., 1985) . Volatile solids destruction ranged from 45% for wastes from
the lucerne leaf diet to 80.4% from the maize hull diet.

Reactor Designs

The goals in selecting an appropriate anaerobic digester design are to maximize volatile solids (VS)
conversion and associated methane yields, increase conversion rates and process stability, decrease
process energy requirements, and ultimately achieve a reliable system with the lowest possible
installation and operating costs. Odor control may also be a primary concern. No single reactor design is
suitable for all applications in treatment of piggery wastes. Major factors influencing selection include:

           chemical characteristics of feed
           concentration of feed biodegradable matter
           concentration of feed particulate solids
           density of raw and digested feed
           scale of application
           continuity of feed availability
           desired products
           site

The designs most commonly used for treatment of swine wastes are lagoon, batch, fed-batch,
completely-mixed, and plug-flow. Several new high-rate designs have been developed to retain solids
and microorganisms and are particularly suitable for treatment of dilute wastes from flush systems or
liquid fractions of separated wastes (Wilkie & Colleran, 1989). The principles of these various reactor
designs are discussed below, and operating and performance data for several different reactor
configurations are summarized in Table 9.

Table 9. Operating and performance data for different digester designs (CSTR)

       Reference             (Hashimoto,   (Hashimoto,    (Stevens &      (Mills, 1977)   (Fischer et al.,   (Zhang et al.,   (Iannotti et al.,
                                1983)         1983)      Schulte, 1979)                        1979)            1990)              1979)
   Operational Data
    Reactor type               CSTR          CSTR            CSTR            CSTR             CSTR              CSTR               CSTR

      Volume, m                0.004          0.004          2.500           13.500           140.000          199.000             0.420
       Temp, C                   55             35           22.5              35                35              35                  35
     Type of waste             whole          whole          whole                        flushed farrow-       whole            finishing
   Infl. TS, % w.b.            6.36%         6.36%           5.48%           4.30%                              3.00%             6.88%
   Infl. VS, % w.b.            5.04%         5.04%           3.57%                            1.50%             2.38%             5.69%
   Infl. COD, g L              52.10         52.10           74.30           74.30                              41.35              0.07
                -3 -1
  OLR, kg VS m d               10.08         10.08            1.80                             1.30              1.70              3.78
                 -3 -1
 OLR, kg COD m d                                                             7.43
         HRT, d                 5.00          5.00           20.00           10.00                               14.00             15.00

  Performance Data
        3             -1
 MY, m kg VS added              0.31          0.26           0.29
      3                 -1
MY, m kg COD added                                           0.14
            3   -3 -1
    MPR, m m d                  3.12          2.69           0.52                              0.79              0.57               0.52
     VS redn., %                50.2          43.7           22.4                              60.0              66.0               60.0
    COD redn., %                                             35.7             51.0                               73.0               58.0
     Effl. N, g L               3.34          3.29           3.66             2.24             0.38              2.25               3.97
     Effl. P, g L
 Gas Quality, % CH4             61.1          64.2           63.0                              60.0              64.0               59.0

Table 9. Operating and performance data for different digester designs (CSTR and two-phase)

       Reference             (van Velsen,   (van Velsen et al., (Petersen, 1982)   (Cavallero &   (Maekawa et al.,
                                1977)             1979)                            Genon, 1984)       1995)
  Operational Data
    Reactor type                CSTR              CSTR              CSTR             CSTR           two-phase
     Volume, m                  0.240             6.000             40.000            0.050           1.300
      Temp, C                     32                30                37                35              38
    Type of waste               whole             whole             whole          supernatant     whole diluted
   Infl. TS, % w.b.              7.50              5.83                               2.30             4.00
   Infl. VS, % w.b.              5.40              4.39              5.53             1.38
   Infl. COD, g L               80.30             51.75                               32.09
                -3 -1
  OLR, kg VS m d                 4.50              2.64              2.12                              4.74
                  -3 -1
 OLR, kg COD m d                 6.70                                                    2.57
         HRT, d                 12.00                                27.00               12.50         6.75

  Performance Data
        3             -1
 MY, m kg VS added                                0.35               0.29
      3                 -1
MY, m kg COD added
            3   -3 -1
    MPR, m m d                   0.90                                0.61                0.72          1.41
     VS redn., %                 38.3                                46.0                              41.2
    COD redn., %                 40.3                                                    34.2
     Effl. N, g L                2.32
     Effl. P, g L
 Gas Quality, % CH4              76.3                                63.3                78.5          60.1

Table 9. Operating and performance data for different digester designs (attached-film)

       Reference             (Chou et al., 1997) (Bolte et al., 1986) (Bolte et al., 1986)    (Nordstedt &   (Hill & Bolte,    (Hill & Bolte,
                                                                                             Thomas, 1985)       1988)             1986)
    Reactor type        AF upflow immobile AF, nylon mesh & AF, nylon mesh &      AF, oak wood     AF, polyester felt         AF, nylon mesh
                               cells       polyurethane foam polyurethane foam        blocks
     Volume, m                 0.014             0.005             0.005              0.005              0.300                      0.300
      Temp, C                    37                35                55                31.1                35                         35
    Type of waste        screened, diluted flushed, screened flushed, screened settled supernatent flushed screened           flushed screened
   Infl. TS, % w.b.                               1.33              1.42               2.02               1.88                       1.89
   Infl. VS, % w.b.                               1.03              1.13               1.43               1.50                       1.56
   Infl. COD, g L               7.50             16.11             17.80              33.47              19.69                      21.10
                -3 -1
  OLR, kg VS m d                                  3.44             11.34               7.22               7.50                       7.80
                  -3 -1
 OLR, kg COD m d                7.50
         HRT, d                 1.00              3.00              1.00               2.00               2.00                     2.00

        3             -1
 MY, m kg VS added                                       0.30                   0.22             0.23            0.37              0.27
      3                 -1
MY, m kg COD added                  0.12                                                                         0.29              0.20
            3   -3 -1
    MPR, m m d                      0.90                 1.03                   2.43             1.68            2.80              2.15
     VS redn., %                                         46.5                   40.6             38.4            51.0              42.9
    COD redn., %                    61.0                 49.7                   34.5             51.7            46.2              42.1
     Effl. N, g L                                        0.97                   0.98                             0.87              0.82
     Effl. P, g L
 Gas Quality, % CH4                 77.0                 71.0                   67.7             79.2            62.5              60.5

Table 9. Operating and performance data for different digester designs(attached-film)

    Reference             (Wilkie &         (Wilkie &       (Sorlini et al.,   (Sorlini et al.,   (Sorlini et al.,   (Hasheider &
                        Colleran, 1986)   Colleran, 1986)       1990)              1990)              1990)          Sievers, 1983)
  Reactor type          AF, polypropylene AF, polypropylene AF, wood chips        AF, PVC         AF, expanded        AF, limestone
                              rings               rings                                                clay
   Volume, m                  2.800              2.800          0.015              0.015              0.015               0.003
    Temp, C                     25                  35            30                 30                 30                  35
  Type of waste              settled      settled supernatent   whole              whole              whole          screened diluted
 Infl. TS, % w.b.              1.33               1.45           0.62               0.62               0.62
 Infl. VS, % w.b.              0.87               0.96           0.40               0.40               0.40               0.60
 Infl. COD, g L               25.20              29.60           5.73               5.73               5.73
              -3 -1
OLR, kg VS m d                                                                                                            6.00
OLR, kg COD m                 8.40             9.90              0.73               1.18               0.54
       HRT, d                 3.00             3.00              7.89               4.84              10.71               1.00

   MY, m kg VS                                                   0.21               0.14               0.03               0.23
  MY, m kg COD                0.17             0.22
            3  -3 -1
  MPR, m m d                  1.47             2.18              0.21               0.21               0.21               0.90
   VS redn., %                                                   60.0               64.7               13.5               41.9
  COD redn., %                52.0             60.0
   Effl. N, g L                                                                                                           0.41
   Effl. P, g L
 Gas Quality, %               87.0             87.0              72.0               77.0               68.0               71.0

Table 9. Operating and performance data for different digester designs(attached-film and UASB)

            Reference               (Ng & Chin, 1988)     (Wilkie & Colleran,   (Lo et al., 1994)   (Foresti & de      (Owens, 1988)
                                                                 1984)                              Oliveira, 1995)
         Reactor type              AF, activated carbon         AF, clay             UASB                UASB                UASB
          Volume, m                        0.004                 0.180               0.015               0.011               0.002
           Temp, C                           na                    33                  25                  25                 20.7
         Type of waste                     whole          settled supernatent   screened diluted    screened diluted   flushed screened
        Infl. TS, % w.b.                    0.40                  1.51
        Infl. VS, % w.b.
        Infl. COD, g L                     7.80                 30.05                12.00                 3.73             8.84
                     -3 -1
       OLR, kg VS m d
                       -3 -1
      OLR, kg COD m d                     14.20                  5.00                3.58                  4.50             4.40
              HRT, d                       0.75                  6.00                3.28                  0.83             2.00

             3             -1
      MY, m kg VS added                                                                                                     0.05
           3                 -1
     MY, m kg COD added                                          0.39
                 3   -3 -1
         MPR, m m d                        0.46                  1.93                0.71                                   0.13
          VS redn., %                                                                57.0                                   44.4
         COD redn., %                      78.0                  73.3                                      87.0             53.6
          Effl. N, g L                     0.23
          Effl. P, g L
      Gas Quality, % CH4                   84.3                  87.0                67.0                                   80.0

Table 9. Operating and performance data for different digester designs (plug-flow, baffle-flow, lagoons)

    Reference            [Yang, 1985a #89] (Floyd & Hawkes,    (Boopathy &      (Yang & Chou,        (Chandler et al.,   (Safley &
                                                 1986)        Sievers, 1991)        1985)            1983)               Westerman, 1988)

  Reactor type             plug-flow with      Tubular            ABR                 ABR            Anaerobic Lagoon    Anaerobic Lagoon
   Volume, m                  11.500            0.013             0.010               0.020                19,000
    Temp, C                      26               30                35                  30                 ambient
  Type of waste            whole diluted        whole             whole        settled supernatent         flushed
 Infl. TS, % w.b.                                5.30              5.17                                       0.7
 Infl. VS, % w.b.              0.79              3.88              3.86               0.09
 Infl. COD, g L                                                   58.50               1.77
               -3 -1
OLR, kg VS m d                 5.27                                4.00               1.75                   0.11              0.16
OLR, kg COD m                                                                         3.53
       HRT, d                  1.50             10.00             15.00               0.50                  53-60

   MY, m kg VS             0.14               0.25                0.50              0.23
  MY, m kg COD             0.07                                   0.04              0.11
            3  -3 -1
  MPR, m m d               0.71               0.96                2.01                                    0.04-0.05        0.03 (biogas)
   VS redn., %                                88.7                61.0                                       75
  COD redn., %                                                    62.0              28.8
   Effl. N, g L                                                   1.10              0.14                     0.24
   Effl. P, g L
 Gas Quality, %            67.5               63.0                62.0                                       69
Table 9. Operating and performance data for different digester designs (sequencing batch reactors)

       Reference              (Zhang et al.,     (Zhang et al.,    (Masse et al., 1993)   (Hill et al., 1985)
                                 1997)              1997)
    Reactor type                  SBR                SBR                  SBR                   SBR
     Volume, m                    0.012              0.012                0.025                0.454
      Temp, C                       25                 25                   20                   35
    Type of waste            screened diluted   screened diluted        screened           whole, scraped
   Infl. TS, % w.b.                                                      4.80%                12.77%
   Infl. VS, % w.b.              0.90%              3.30%                3.00%                10.82%
   Infl. COD, g L                                                         84.00
                -3 -1
  OLR, kg VS m d                  4.50               5.50                                        0.02
                  -3 -1
 OLR, kg COD m d                                                            1.20
         HRT, d                   2.00               6.00                   78.00               60.00

        3             -1
 MY, m kg VS added                0.24               0.23                   0.76                 0.33
      3                 -1
MY, m kg COD added
            3   -3 -1
    MPR, m m d                    1.08               1.28
     VS redn., %                 39.0%              40.0%                56.0%                 46.4%
    COD redn., %                                                         73.0%
     Effl. N, g L                 0.89               2.47
     Effl. P, g L
 Gas Quality, % CH4              72.0%              61.0%                63.0%                 59.8%

(anaerobic lagoon)

An anaerobic lagoon is a deep pond (~5 meters) with steep sides which is not aerated and operates
largely under anaerobic conditions. The surface may be covered (Chandler et al., 1983; Safley &
Westerman, 1989) to facilitate biogas collection and odor reduction. Lagoons operate at ambient
temperature, receive dilute swine waste slurries, and serve as reactors for treatment and reservoirs for
                                                                         3    3                  3    2
storage. Biogas production rate from a swine waste lagoon was 0.05 m /m per day and 0.13 m /m per
day. Production of biogas was found to be a function of VS loading and lagoon temperature (Cullimore et
al., 1985; Safley & Westerman, 1989; Safley & Westerman, 1988). Lagoons may be inexpensive, but
require large areas and are often associated with odors.


Batch reactors consisting of large circular or rectangular tanks are fed waste along with an inoculum, and
degradation is permitted to startup and proceed to completion. These reactors are often unstable and
require careful attention to the inoculum-to-feed ratio; VS conversion is erratic.

(completely mixed)

The continuously-stirred tank reactor (CSTR) is the most common design used in wastewater and farm
applications treating feeds with >3% solids. This design is usually heated, mixed constantly, and usually
fed intermittently rather than continuously. The major disadvantage is the loss of inoculum and
undigested solids at high loading rates.

(anaerobic contact)

This design employs a CSTR followed by a settling operation to concentrate washed out microorganisms
and undigested solids for recycle back to the CSTR. This results in increased solids retention time (SRT)
and reduced digester volume and is used for dilute wastewater applications.


The plug-flow reactor is non-mixed and the feed passes through a trench or cylinder (Floyd & Hawkes,
1986; Gorecki, 1993). These systems are often covered with a balloon plastic cover (Taiwan, 1993). The
SRT may also be increased using sludge recycle (Yang and Nagano, 1985). Best performance was
obtained at an HRT of 2 days and SRT of 3.25 days. In the baffled modification of this design, internal
baffles facilitate mixing and result in extended retention of microorganisms and solids (Boopathy &
Sievers, 1991; Yang & Chou, 1985; Yang & Moengangongo, 1987). These designs result in more
conversion and higher conversion rates than CSTR or batch reactors.

(continuously expanding/sequential batch)(fed-batch)

Some batch reactors, such as the continuously expanding (Hill et al., 1985) or anaerobic sequential batch
(Dague et al., 1992; Zhang et al., 1997), are intermittently fed, allowing the solids to settle, and
supernatant is withdrawn between feeding intervals. Solids are also removed on an intermittent basis.
These reactors may or may not be heated (Masse et al., 1993). They promote longer solids than liquid
retention times and substantially improve process kinetics over batch and CSTR designs.


Several designs use various inert media for attachment of bacteria, forming biofilms, and thus preventing
their washout at high hydraulic loadings (Wilkie & Colleran, 1989). These designs are applicable for
feedstocks with low suspended solids (<1-2%). Such high-rate systems have permitted reduction of
hydraulic retention times to a few days or hours. In the case of anaerobic filters, the packing media may

include stones, brick, plastic, tubes, etc. Flow direction may be up or down. Other designs (fluidized or
expanded bed) use fine particles such as sand or silica. The waste stream is often recycled to maintain
an expanded or fluidized state of the biofilm coated medium. One of the most popular designs, the
upflow anaerobic sludge blanket (UASB), takes advantage of the formation of granules consisting of
dense consortia of microorganisms which are formed under carefully controlled conditions. Specialized
gas/floc separators are employed to prevent washout of these granules.

Lo (1994) showed that performance of UASB digesters treating screened swine waste could be improved
by incorporation of a rope matrix for attachment of microorganisms in the mid-section of the reactor.
Chou (1997) immobilized inoculum in cellulose triacetate for use in a packed bed reactor. Very low
hydraulic retention times were achieved in this system without sacrifice in performance.

Other successful media for microbial attachment have included nylon cuboid and polyurethane foam
(Bolte et al., 1986; Bossier et al., 1986), wood blocks (Nordstedt & Thomas, 1985), nylon mesh (Hill &
Bolte, 1986), polypropylene cascade mini-rings(Wilkie & Colleran, 1986), limestone (Hasheider & Sievers,
1983), sand and activated carbon (Ng & Chin, 1988). Wilkie and Colleran (1984) compared several
media including clay, coral, mussel shell, and plastic pall-ring support materials, finding similar
performance in upflow filters.

(high solids)

Swine wastes may be mixed with bedding or other wastes, such as the organic fraction of municipal solid
wastes, to produce high-solids feedstocks exceeding 20% total solids. Several reactor designs have
been recently developed to accommodate high-solids feeds without dilution. One group includes mixed
digesters operated at solids concentrations as high as 35%. These include vertical reactors, with feed
mixing with inoculum only during feeding, or intermittent mixing of reactor contents during operation as
described by Chynoweth (1996). Some designs have horizontal reactors mixed by slow rpm mixers.
The sequential batch anaerobic composting design (Chynoweth et al., 1991) uses batch leachbed
reactors which are started up by interacting leachate between new and mature reactor solids beds to
inoculate the new batch and convey fatty acids to a mature reactor during startup. Others interact
leachbeds with methane-phase digesters during startup and operation. Advantages of high-solids
systems include reduced odors, easier nutrient management, and reduced reactor size.


Several designs involve one or more stages (usually two) where depolymerization and fermentation to
organic acids occur in the first stage and degradation of acids and methanogenesis is accomplished in
the second stage, in conventional or high-rate attached-film digesters. Actually in most piggeries, this first
acid-forming stage occurs fortuitously during waste storage prior to treatment in anaerobic digesters.
There are three major advantages to multi-phase designs (Chynoweth & Pullammanappallil, 1996). The
first involves improved stability. In a single, combined-phase digester, overloading and inhibition result in
accumulation of volatile organic acids for which resident populations are not present in sufficient numbers
to metabolize. Enrichment for these organisms can take several weeks. In a two-phase system,
formation of acids is encouraged in the first, or acid phase; therefore, the second methane phase is
constantly receiving acids to encourage high populations of acid-utilizing organisms. In other words, the
acid-phase is an intentionally imbalanced digester which is resistant to further imbalances resulting from
overloading or inhibitors. The second advantage is that the slow-growing populations of microorganisms
(acid utilizers and methanogens) can be concentrated in biofilms, thus permitting short retention times for
the second-phase reactor. This reduces the overall reactor volume requirement, including both stages.
The third advantage is that most of the biogas is produced in the methane-phase digester and the
methane content of this gas is higher because of the prior release of much of the carbon dioxide in the

acid phase. This advantage facilitates biogas utilization by localizing its production and increasing its
methane content.

Yang (1995) proposed a three-stage digester design for undiluted pig wastes (TS 8-10%). With an
organic loading rate of 2.95 g VS/L per day, a methane yield of 0.42 L/g VS and VS reduction of 69.9%
was achieved. Tseng (1992) observed improved digestion of swine wastes employing the first stage
reactor to perform solids sedimentation and acidification, and a second tank to perform methanogenesis.

Operating Parameters

(loading rate)

The most meaningful parameter for describing the feed rate is loading rate which is the feed
concentration divided by the HRT (Chynoweth & Pullammanappallil, 1996). Loading rate is expressed as
weight of organic matter (VS or COD) per culture or bed volume per day (e.g., kg VS/m /day). This
parameter (corrected for head space) describes the reactor volume needed for a particular feed rate.
Other parameters, such as solids concentration and retention time (hydraulic or solids), are misleading
and do not provide a valid basis for comparison of digester costs.

Aside from influencing digester size, solids concentration has a significant effect on digester design and
performance, and on materials handling. Feeds with low concentrations of suspended solids (<1-2%) can
be digested in high-rate attached-film reactors described above. The conventional method is to use
lagoons with low loading rates or conventional digesters following solids separation. Feedstocks with
medium solids concentrations (3-10%) require high hydraulic retention times (>15 days for mesophilic
temperature) or some mechanism for retaining suspended solids, such as solids recycle or concentration
of solids within the reactor, as in the sequential batch, upflow sludge blanket, or baffle-flow designs. In
the case of feed blends (discussed below), feed solids may exceed 10%, allowing for the use of high-
solids designs with reactor solids concentrations up to 35%. Advantages of these designs include higher
loading rates, lower heating energy requirements, and less water as a waste product (Wujick & Jewell,
1980). It has also been shown that cellulolytic enzyme activity per unit reactor volume is higher in high-
solids systems (Rivard et al., 1994). High solids systems have a unique set of advantages and limitations
with respect to materials handling related to feed addition, mixing, and effluent removal.


Effective digester startup is dependent upon the quality and quantity of inoculum (Chynoweth &
Pullammanappallil, 1996). In conventional CSTR digesters, the inoculum-to-feed ratio (VS basis) is
typically greater than 10. In designs where washout of critical organisms is a concern, suspended solids
in the effluent may be settled and recycled. With batch and plug-flow designs, inoculum is obtained from
previous runs or by effluent recycle. Baffled systems trap inoculum throughout the reactor and inoculate
the feed as it passes through. Attached-film reactors often take months to fully start up but have the
advantage of inoculum retention during the course of operation. Under-inoculation of a digester results in
imbalanced performance due to the more rapid growth of acid formers than methane formers leading to
accumulation of organic acids and consequent pH reduction.

Biological methanogenesis has been reported at temperatures ranging from 2 C (in marine sediments) to
over 100 C (in geothermal areas)(Zinder, 1993). Most applications of this fermentation have been
                                           o                       o                           o
performed under either ambient (15 to 25 C), mesophilic (30 to 40 C), or thermophilic (50 to 60 C)
temperatures. In general, the overall process kinetics double for every 10 C increase in operating
temperature up to some critical temperature (about 60 C) above which a rapid dropoff in microbial
activityoccurs (Harmon et al., 1993). The populations operating in the thermophilic range are genetically
unique (Zinder, 1993), do not survive at lower temperatures, and are more sensitive to temperature
fluctuations outside of their optimum range. Digesters with lower temperatures are more stable and
require less process energy, but require larger volumes. Thermophilic digesters have lower volume

requirements but have higher energy requirements and are less stable. Ammonia is also more toxic in
these digesters because the more toxic free ammonia is favored (Hansen et al., 1998).

Typically, most digesters are operated at mesophilic or ambient temperatures. Several researchers have
investigated psychrophilic anaerobic digestion of swine wastes (Safley & Westerman, 1990; Stevens &
Schulte, 1979; van Velsen et al., 1979; Zeeman et al., 1988). Digestion proceeds at temperatures as low
as 10 C, requires longer retention times, and requires a low-temperature inoculum for effective startup.
Mesophilic operation seems to be the most preferred because of the possibility for control of temperature
fluctuations (not possible for ambient temperature operation) and the higher energy costs for thermophilic
digestion. Thermophilic operation is practiced in circumstances when the reduced reactor sizes and the
effective pathogen kill justify higher energy requirements and extra effort to ensure stable performance.


Nitrogen and phosphorus are the major nutrients required for anaerobic digestion. These elements are
building blocks for cell synthesis and are directly related to microbial growth requirements in anaerobic
digesters. An average empirical formula for an anaerobic bacterium is C5H7O2NP0.06 (Speece, 1997).
Thus, the nitrogen and phosphorus requirements for cell growth are 12% and 2%, respectively, of the
volatile solids converted to cell biomass (about 10% of the total volatile solids converted); this would be
equivalent to 1.2% and 0.024% of the biodegradable volatile solids, respectively, for nitrogen and

Previous studies have identified critical feedstock C/N ratios of 15 for seaweed (Chynoweth et al., 1987)
and 15-19 for swine waste (Sievers & Brune, 1978), above which nitrogen was limiting. In fact, nutrient
limitations are better related to concentrations; e.g., a value of 700 mg/L was recently reported for the
optimum NH4-N concentration in high-solids anaerobic digestion of the organic fraction of municipal solid
waste (Kayhanian, 1994). Nutrients may also be concentrated by certain design and operating practices.
For example, designs that concentrate solids (Chynoweth, 1987) or reuse supernatant or leachate from
process effluent (Chen & Chynoweth, 1990; Chynoweth et al., 1991), concentrate nutrients extracted from
the feedstock. Ammonia is also an important contributor to the buffering capacity in digesters but may
also be toxic to processing in high solids digesters. Ammonia toxicity was exhibited from feeds that had
normal C/N ratios because ammonia became concentrated in the supernatant as digestion proceeded
(Jewell et al., 1993).

Other nutrients needed in intermediate concentrations, include sodium, potassium, calcium, magnesium,
chlorine, and sulfur. Requirements for several micronutrients have also been identified, including iron,
copper, manganese, zinc, molybdenum, cobalt, nickel, selenium, and vanadium (Speece, 1997; Wilkie et
al., 1986). Available forms of these nutrients may be limiting because of their ease of precipitation and
removal by reactions with phosphate and sulfide. Limitations of these micronutrients have been
demonstrated in reactors where the analytical procedures failed to distinguish between available and
sequestered forms (Jewell et al., 1993).


Mixing is traditionally thought to be required for optimized digestion to enhance interaction between feed
and cells (inoculation) and remove inhibitory metabolic products from the cells. Mixing is also practiced to
break up floating scum and foam layers which are typical with some feedstocks, such as domestic sludge.
Therefore, conventional digesters include mixing, which is accomplished by mechanical stirring, liquid
recycle, or gas recycle. Mixed digesters have been referred to as “microbial torture chambers” based on
research observations (J.G. Ferry, personal communication) that metabolism of certain compounds (e.g.,
benzoate) is inhibited by mixing and efficient consortia function well in UASB digesters employing
granules or other biofilms (Switzenbaum, 1991). One explanation for this inhibition is that microbial
consortia existing in clumps are disrupted from that optimum arrangement by mixing. Chynoweth (1987)
also demonstrated that a nonmixed solids-concentrating reactor design exhibited more rapid kinetics,

lower nutrient requirements, and greater stability than a CSTR design. This improved performance was
attributed to reduction in washout of solids and critical organisms. The practice of mixing in swine waste
digesters varies depending on the design and was previously addressed under the discussion of reactor
options. However, the energy requirement related to mixing can require as much as 14 percent of the
methane energy product in conventional low solids designs (see below under energy requirements).


Biomethanogenesis is sensitive to several groups of inhibitors, including alternate electron acceptors
(oxygen, nitrate, and sulfate), sulfides, heavy metals, halogenated hydrocarbons, volatile organic acids,
ammonia, and cations (Speece, 1996). The toxic effect of an inhibitory compound depends upon its
concentration and the ability of the bacteria to acclimate to its effects. The inhibitory concentration
depends upon different variables, including pH, HRT, temperature, and the ratio of the toxic substance
concentration to the bacterial mass concentration. Antagonistic and synergistic effects are also common.
Methanogenic populations are usually influenced by dramatic changes in their environment, but can be
acclimated to otherwise toxic concentrations of many compounds.

Inhibition in anaerobic digesters is reflected by accumulation of volatile acids (related to overloading or
toxic feed components), high ammonia levels (related to nitrogenous feeds), or toxic feed components.
Normal and inhibited turnover rates of volatile acids are discussed by Winter (1984). When the
concentration of total volatile organic acids is in the range of 2,000 mg/L or higher, the onset of imbalance
is indicated. Digesters, e.g., acid-phase systems, can acclimate to concentrations as high as 10,000
mg/L. This tolerance is related to alkalinity levels which are influenced by ammonia and bicarbonate.

Swine wastes have a high nitrogen content, resulting in high ammonia concentrations during anaerobic
digestion. Concentrations in the range of 3,000 mg/L or higher have traditionally been thought to be
inhibitory to anaerobic digestion (Braun et al., 1981), especially in combination with high pH that favors
the volatile NH3 form. Some researchers have found that acclimated digester cultures can function
normally at much higher concentrations, even as high as 6,000 mg/L (Hansen et al., 1998; van Velsen,
1977). Cintoli (1995) evaluated use of zeolite to reduce ammonia in piggery wastes to sub-inhibitory
levels (1500 to 300 mg/L) prior to treatment in a UASB digester.

Swine and other animals are fed antibiotics and other drugs to prevent infectious diseases and promote
growth. In general, results have shown that some antibiotics (e.g., monensin and chlorotetracycline)
inhibit digestion (Varel & Hashimoto, 1981; Varel & Hashimoto, 1982) but acclimation is possible, and
others (e.g. arsanilic acid, roxarsone, and avilamycin) either stimulated digestion or had no observable
effect (Brumm & and Sutton, 1979; Brumm et al., 1980; Brumm et al., 1979; Brumm et al., 1977; Sutton et
al., 1989). Camprubi (1988) evaluated different concentrations of antibiotics added to swine waste,
including furazolidone, chloramphenicol, chlorotetracycline tylosin, erythromycin, carbadox, and copper
sulfate. Chloramphenicol was the most potent inhibitor of anaerobic digestion.

Performance Parameters

(Gas and Methane Yields, Rates, and Reduction in Organic Matter)

Total biogas and methane production, when related to organic matter, are directly influenced by the
extent and rate of conversion. Biogas yields are related to organic matter fed which is expressed as
volatile solids (VS) or chemical oxygen demand (COD). These data are typically reported as gas volume
(m ) per weight (kg) VS or COD added. Methane yield is preferred over gas yield because pH changes in
the reactor can cause changes in release or uptake of carbon dioxide that are unrelated to degradation.
Use of VS permits calculation of a materials balance between the feed, effluent solids, and gas. Use of
COD allows for calculation of an oxidation-reduction balance between the feeds and products. In the
context of materials balances, the reduction in organic matter may be calculated as reduction in VS or
COD. A typical methane yield for the organic fraction of swine waste is 0.3 m /kg VS (Table 9) which

corresponds to a volatile solids reduction of 50%.

Methane production rate is a measure of process kinetics and is determined as volume of methane per
                             3 3                                                          3
volume of reactor per day (m /m /day). This parameter is the product of loading rate (kg/m /day) and
methane yield (m /kg VS added). Values for piggery waste digestion have been reported in the range of
             3  3
0.04 to 3.0 m /m /day.

Methane content of the biogas is also a good indicator of stability. Under normal circumstances, this
value is a function of the H/C ratio of the biodegradable fraction and is normally in the range of 50-60%
(Owens & Chynoweth, 1993). Since lowered methanogenic activity is the key factor leading to
imbalance, a reduction of methane gas content is a key performance parameter and has been employed
as an on-line control parameter (Chynoweth et al., 1994). The biochemical methane potential (BMP)
assay (Chynoweth et al., 1993; Owen et al., 1979; Owens & Chynoweth, 1993) is useful for estimating the
ultimate methane yield and relative conversion rates of feed samples, specific feed components, and
remaining biodegradable matter in process residues. This assay may also be used to determine toxicity
of feed components. In general, the test is conducted with miniature digesters (200 mL) which are
optimized for conversion in terms of inoculum, feed concentration, nutrients, and buffer. These miniature
batch digesters are incubated until no further gas production is observed. Measurements include gas
production and composition of influent and effluent organic matter.

(Organic Acids, pH, and Alkalinity)

Organic acids, pH, and alkalinity are related parameters that influence digester performance (McCarty,
1964; WPCF, 1987). Under conditions of overloading and the presence of inhibitors, methanogenic
activity cannot remove hydrogen and organic acids as fast as they are produced. The result is
accumulation of acids, depletion of buffer, and depression of pH. If uncorrected via pH control and
reduction in feeding, pH will drop to levels which stop the fermentation. Independent of pH, extremely
high volatile acid levels (>10,000 mg/L) also inhibit performance. The major alkalis contributing to
alkalinity are ammonia and bicarbonate. A normal healthy volatile acid-to-alkalinity ratio is 0.1. Increases
to ratios of 0.5 indicate the onset of failure and a ratio of 1.0 or greater is associated with total failure.
The most common chemicals for pH control are lime and sodium bicarbonate. Lime produces calcium
bicarbonate up to the point of solubility of 1,000 mg/L. Sodium bicarbonate adds directly to the
bicarbonate alkalinity without reaction with carbon dioxide. However, precautions must be taken not to
add this chemical to a level of sodium toxicity (>3500 mg/L). The alkalinity needed to neutralize volatile
acids (VFA) is calculated by multiplying 0.833 times VFA concentration (mg/L as acetic acid) (WPCF,

Certain volatile fatty acids are particularly associated with the onset of digester failure, including propionic
and higher numbered acids (Gourdon & Vermande, 1987; Hill, 1988; Wilkie and Smith, 1989).
Accumulation of these acids results from a backup of hydrogen (or electron) flow and their formation as
an alternative to methanogenesis for hydrogen utilization. Useful parameters based on this principle are
the ratio of these acids to acetic acid (Hill et al., 1987) and the concentrations of iso-volatile acids (Hill &
Holmbert, 1988).


Numerous models have been developed to provide a theoretical understanding of microbial populations
and their interactions with the physical and chemical environment. Models use mathematical expressions
to describe the interactions between various microbial populations involved in the process, including
substrate utilization rates, microbial growth rates, product formation rates, and physico-chemical

equilibrium relationships. More simplified anaerobic digestion models can be used for optimizing process
design and operation and for process control. These usually incorporate four major steps, including
depolymerization and solubilization, acidogenesis, methanogenesis, and inhibition.

The following equations based on the Contois Model have been used to describe the kinetics of
anaerobic digestion of swine waste at steady state (Chen, 1983; Hashimoto, 1984):

                                K       
                 B  Bo 1                                                                (1)
                            m  1  K 

                                    K       
                   Bo L1                                                              (2)
                                m  1  K 

                 = methane yield, m /kg VS
                Bo = ultimate methane yield at infinite retention time, m /kg VS
                K = kinetic parameter (inversely related to digester performance;
                         values <0.6 indicate stability)
                m = maximum specific growth rate, d

                v = volumetric methane production rate, m CH4/ m digester
                                                             3        3

                         volume per day
                L = volumetric loading rate, kg VS/m digester volume per day

Reported values for ultimate anaerobic biodegradability (Bo) of swine manure in the range of 0.32 - 0.48
m CH4/kg VS were summarized from the literature by Chen (1983). Hashimoto (1984) evaluated the
effect of temperature and influent substrate concentration on a kinetic constant (K) which is an indicator of
digester stability. Temperature had no effect on (K) for influent substrate concentrations between 33.4
and 61.8 g/L. The constant (K) increased exponentially with influent substrate concentration. Chen
(1983) further summarized (K) values from the literature which indicated that inhibition of digestion in
CSTR digesters can be expected at feed volatile VS concentrations exceeding 6%.

Hill (1996) presented a model showing that the ratio of TKN-N in influent-to-effluent is a good indicator of
process steady state in livestock anaerobic digestion. Andreadakis (1992) summarized operation,
performance, and kinetic parameters from the literature. Other models applicable to wastes (including
swine wastes) have design and operating parameters (Hill, 1983), inhibition (Hill et al., 1983), and
economics optimization of design and operation (Hill, 1984; Hill, 1985). Most of these models are limited
to CSTR digesters and would have to be modified to accommodate other designs such as batch,
sequential batch, plug flow, and attached-film digesters.

Pathogen Reduction

Destruction of human, animal, and plant pathogens during treatment of organic wastes (including swine
wastes) is a major concern for any subsequent use of the effluent, such as for land application, recycle, or
discharge. Diseases associated with animal manure include bacterial, ricksettial, viral, fungal, and
parasitic infections. Most studies have shown that anaerobic digestion results in reduction in numbers of
pathogens. This has specifically been shown for swine waste pathogens and indicator organisms in
mesophilic digestion (Duarte et al., 1992). Bendixen (1994) has shown that most pathogens were killed
                                  o                                       o
under thermophilic conditions (55 C) and that mesophilic temperatures (35 C) did not result in effective
reduction. Engeli (1993) has shown that plant pathogens not killed by aerobic treatment were
significantly reduced by thermophilic anaerobic digestion.

Use of Digester Effluent for Nutrients

Digester effluent has been traditionally used as a soil conditioner or good source of inorganic nutrients
(Zhang and Felmann, 1997; Day and Funk, 1998). Prior to such use, effluents should be “cured” or aged
to free them of reduced products such as organic acids and reduced inorganic compounds such as
ammonia and hydrogen sulfide. These reduced compounds may be toxic to some plant forms. In
locations where high concentrations of animals are raised, the capacity of the wastes in terms of
inorganic nutrients may exceed the fertilizer needs and local disposal is not possible on a sustained basis
without contamination of surface and ground waters.

Anaerobic Treatment Of Nutrients

A discussion of anaerobic treatment of swine wastes would not be complete without mentioning its role in
biological nutrient removal. Per 1000 head in a finishing piggery, 18.6 tons per year (tpy) of nitrogen are
ingested and 11.5 tpy excreted; the intake for phosphorus is 4.7 tpy, 4.5 tpy of which is excreted
(Kephart, 1996). Anaerobic processes may play an important role in removal of nitrogen and phosphorus
(Ekama & Wentzel, 1997). For nitrogen removal, ammonia nitrogen resulting from metabolism of
nitrogenous organic compounds must be oxidized aerobically after which nitrogen may be removed
anaerobically by denitrification. This is accomplished by recycle of aerobic effluent back through an
anaerobic denitrification process. Biological removal of phosphorus is sometimes accomplished by the
use of anaerobic prefermenters which produce volatile acids which then enhance uptake of phosphorus
by bacteria in subsequent aerobic operations. This may be accomplished in a sequencing batch reactor
that is alternated between aerobic and anaerobic conditions (Bortone et al., 1992). Algae, aquatic plants,
and wetlands may also be used for nutrient removal from digester effluents (Lincoln & Earle, 1990; Yang
& H., 1994; Zhang & Felmann, 1997).

Gaseous Emissions

Gaseous emissions from piggery wastes are of concern because of their potential health hazards to the
animals and farm workers, their contribution to greenhouses gases related to global warming, and as the
cause of odors which are objectionable to piggery workers and nearby residents. Over 75 compounds
(including ammonia, hydrogen sulfide, volatile organic acids, amines, mercaptans, and heterocyclic
nitrogen compounds) have been identified in animal waste emissions which contribute to odors and are
largely a result of partial anaerobic decomposition of these wastes (Barth & Melvin, 1984). Under totally
aerobic conditions, these compounds would not form as they would be converted to carbon dioxide,
water, and oxides of sulfur and nitrogen, all of which are odorless. Under conditions of high
concentrations of organic matter (such as animal wastes), oxygen is depleted and an imbalanced
anaerobic decomposition occurs giving rise to these products. Many uncontrolled environments where
manure is accumulated (manure piles, collection pits or tanks, etc.) have imbalanced anaerobic
decomposition (Wilkie et al. 1995b). In a balanced methanogenic decomposition, the gases are limited
primarily to methane, carbon dioxide, hydrogen sulfide, and ammonia. Levels of hydrogen sulfide and
ammonia are high for swine wastes, compared to other animal wastes, because of the high protein

As discussed above, ammonia and hydrogen sulfide cause discomfort and represent health risks to
confined animals and workers.

Methane is produced during the anaerobic decay of organic matter in manure. Worldwide emissions from
this source range from 10 to 18 teragrams (Tg) per year, or 2 to 5% of global anthropogenic
emissions(Safley et al., 1992; USEPA, 1993). Swine account for 40% of the emissions from animal
wastes. No information could be found on the contribution of swine-waste ammonia emissions to
atmospheric nitrous oxides.

The influence of anaerobic digestion on odors from swine and other animal wastes can be significant
(Powers et al., 1997; Wilkie, 1998). Anaerobic treatment is conducted in closed vessels and under
conditions that lead to a balanced decomposition. Thus, volatile acids and other reduced compounds
found under imbalanced conditions do not accumulate. Organic matter that would lead to production of
odors is further decomposed leaving a stable residue that can be applied to fields without generating an
odor nuisance. Although the biogas may be odorous due to hydrogen sulfide, the gas is usually enclosed
until it is burned or treated for hydrogen sulfide removal prior to use. The problem with most aerobic
processes treating animal wastes (e.g. composting and oxidation ditches), is that oxygen becomes
limiting and the processes become partially anaerobic leading to volatile gases that are transported to the
atmosphere by the aeration process. Other methods to control odors include, solids separation, aeration
of anaerobic lagoon surfaces, and ventilation of housing gases through biofilters.

Commercial Systems and Economics

Sweeten (1981) reviewed the technical and economic considerations for systems for production of
methane from swine manure. Using 1980 prices, their construction costs ranged from $22-$36 per 68 kg
hog. This is equivalent to $214-$357 per m digester volume. They concluded that treatment of
concentrated wastes (8-10% TS) would be more economical than treatment of diluted flush wastes (2-3%
TS). Chandler (1983) reported that a $89,000 75kw covered lagoon effectively treated wastes from a
1,000 sow (farrow to finish) piggery with an internal return of 34% and a payback period of 3 years.

Yang (1995) used lab-scale data to determine the cost of three-stage anaerobic digestion of undiluted pig
waste. The cost in Hawaii for treatment with this system was $3.73 per head per year for a 300-herd farm
with a profit of $3.01 per head for a 1,000-herd farm.

Oleszkiewicz (1983) compared nine full-scale waste treatment schemes for treatment of large-herd
piggeries (>10,000 animals) with water-flushed slurry wastes in a developed country. It was found that
some of the systems practiced, including extended aeration, chemical coagulation, series of lagoons, and
systems featuring land disposal, are not cost effective. Systems with high-rate anaerobic and aerobic unit
operations could treat the wastes more effectively at one-third to one/half the cost of more traditional
systems. Anaerobic digestion in cost-effective schemes was used for secondary treatment of wastewater
and settled sludges and for denitrification. Aerobic treatment was used to polish digester effluent and for
biological nutrient removal. Durand (1988) presented the results of an economic system model for
anaerobic treatment of confined swine wastes. The best configuration, of 12 evaluated, included flushing
manure collection, thermophilic anaerobic digestion with effluent heat recovery, and direct combustion of
gas. The economics were most sensitive to digester size, energy price, and efficiency of energy

Table 10 summarizes data from several commercial swine waste digester systems in the U.S., including
digester type and volume, gas production, electricity production (if applicable), and capital costs. A
software package is available from a U.S. government program (AgSTAR) that facilitates determination of
design and economics of different animal waste treatment systems based on anaerobic processes.

Table 10. Summary of Economics of Anaerobic Digestion Treatment Systems for Swine Wastes

  Herd        Feed       Digester     Dig.      Gas         Elect.        Cost        Country       Ref.
  Size        Type        Type        Vol.,    Prodn.      Prodn.,
                                       m                   annual
  1,150       flush       non-        207                               $20,000         US         (Lusk,
                          mixed                                        (materials);                1998)

 16,500        flush     covered     29,400      1,960     700,000k       $220,000        US         (Lusk,
                          lagoon                 m /d          wh                                    1998)
 13,000     flush plus   plug-flow    1,302      1,680     1.0 million    $525,000        US         (Lusk,
               dairy                                          kwh                                    1998)
 11,500        flush      covered    26,180        980      600,000       $191,500        US         (Lusk,
                           lagoon               (1960 in    possible                                 1998)
  3,000        flush      covered    10,892        336        175         $85,000         US         (Lusk,
                           lagoon                          estimated                                 1998)
    ?          flush      covered    30,000     40,000      625,000       $232,500        US         (Lusk,
                           lagoon                                                                    1998)
  1,000      scraped        three-     100        131                     $67,558       Hawaii     (Yang &
              (TS 8-        stage                                                                  Kuroshim
               10%)         batch                                                                   a 1995)
                            plus 2
  3,200      scraped        CSTR        88        197       supplies      $62,375         US        (Fischer
                                                             all farm                                et al.,
                                                             energy                                  1979)

The economics of swine waste treatment systems are highly site specific and dependent upon several
factors, including land and labor costs, effluent discharge regulations, and energy prices.


With over eight million hogs raised in Taiwan, the Taiwan Livestock Research Institute(Taiwan, 1993)
developed a standardized waste treatment system that is used in over 90% of the piggeries. This system
involves a complex combination of solids/liquid separation, composting, activated sludge, and anaerobic
digestion operations. Redmud plastic-covered plug-flow digesters are employed to treat the overflow
dilute fraction. The effluent is polished by activated sludge treatment. Solids are treated by composting.
Chou (1995) evaluated automated control of this system. In Denmark, animal wastes are blended with
the organic fraction of solid and industrial wastes for anaerobic digestion. The biogas is utilized primarily
for heating and generation of electricity (Tafdrup, 1995).

Effluent Processing

Fong and Yuen (1986) reported on a lab-scale process for concentrating piggery digester effluent as a
potential animal feed. The effluent contained 14% protein. Yang (1994) evaluated a combined fixed-film
and aquatic plant system for treatment of diluted piggery waste digester effluent. The system effected
90% COD reduction, 95% reduction of TKN-N, and 99% suspended solids reduction at an estimated cost
$2.95 per pig per year for a farm with 1000 head. Camarero (1996) investigated final treatments for
effluents from piggery digesters. Coarser fractions separated by flocculation were further digested as
high-solids feeds; finer fractions were treated by aerobic digestion and chemical oxidation. De la Noue
(1989), Lincoln et al. (1993) and Lincoln et al. (1996) investigated nutrient removal using various

microalgae. Chlorella achieved the highest removal rates but Phormidium and Spirulina would be easier
to harvest. Effective removal of nutrients by Phormidium was confirmed in laboratory experiments
(Canizares-Villanueva et al., 1994; Lincoln & Earle, 1990)

Biogas Utilization

Figure 2 shows various options for utilization of biogas on farms (Ross & Drake, 1996)). The most
efficient use of biogas is direct combustion for heat. Commonly, biogas is used directly with minimal
cleanup (hydrogen sulfide and moisture removal) for electrical generation. The heat from generator
engines may be captured with heat exchangers for digester heating or other uses. Biogas as produced is
typically 60% methane and 40% carbon dioxide, but the methane content can be as high as 80% in
attached-film digesters. The heating value is about 14.8-17.8 kJ/m . The hydrogen sulfide is typically
1%. Iron sponges or iron-impregnated wood chips are often used for hydrogen sulfide removal.
Upgrading biogas by removal of carbon dioxide is possible, and necessary for uses requiring
compression, but is not economic for the small quantities generated by piggeries. Ross (1996) presents
a detailed discussion of properties, conversion, handling and storage, instrumentation and controls,
health, safety, and environmental considerations, and economics related to biogas use.
Net Energy Considerations

Process energy requirements should influence design and operating conditions selected for treatment of
swine wastes and other feedstocks in digesters. For CSTR digesters, the components in order of their
relative importance are feed heating, reactor heat losses, and mixing (Chen, 1983; Chen & Hashimoto,
1981; Srivastava, 1987). The total requirements can range from about 10% to over 100% of the methane
product energy depending upon the design, feed solids concentration, loading rate, mixing regime,
operating temperature, and ambient temperature. Since feed heating is the major requirement, the
requirements are high for colder climates and higher digester operating temperatures. There is a strong
incentive for ambient temperature operation for high-rate digesters receiving dilute waste streams. This is
related to the high energy requirements to heat the dilute feed and the rapid kinetics of these designs at
low temperatures. On the other hand, (Legrand, 1993) has calculated that high-solids digesters may be
self-heating in tropical and sub-tropical climates.


Several demographic trends will influence swine waste management into the twenty-first century:

   increase in human population
   increase in swine meat consumption in developing countries
   decrease in swine meat consumption in developed countries
   centralization of swine production with herds in the tens of thousands

   stricter local and global environmental regulations with respect to gaseous, liquid, and solids
   public health regulations with respect to animal and worker comfort and health

                    Biogas Utilization on Farms
                                   Anaerobic Digester

                                      Biogas Store
                                         (low pressure)

           Scrub H2S                                           Scrub H2S + C02

         low pressure storage                             high pressure storage

           Heating:                    Stationary engine Transport engine
         Process heating               Mechanical power
          (hot water,steam,etc.)
                                       Electrical power
         Space heating
         Domestic heating
         Domestic cooking

                          Figure 2. Biogas Utilization Options

In developed nations like the United States and the European Union, piggeries will be treated like other
industries, with emphasis on clean sustainable animal raising operations. Wastes will be rapidly removed
from their site of production to minimize effects on animals and workers and treated with much the same
objectives as for human and industrial wastes, i.e., removal of solids, organic matter, nutrients, and
pathogens. The trend continues to be toward flushed systems which will be followed by combinations of

operations for separation, anaerobic and aerobic organic matter reduction, nutrient removal, and
disinfection. Anaerobic treatment should play an important role in future swine waste management for
treatment of organics with its minimum biological sludge production, production of a useful methane fuel,
emerging developments for biological removal of nitrogen and phosphorus, and its capacity to reduce
pathogens. There are strong arguments in favor of minimizing water use in management of these
wastes, and in fact to further increase solids by use of straw and other high-solids wastes for bedding.
High-solids management not only reduces required reactor sizes, but also odors and water requirements.

In developing countries, the trends will be the same, but piggeries will be smaller on the average and
emphasis on environmental pollution control will be slower to develop. High level of treatment of piggery
wastes will probably coincide with the emergence of effective treatment of human wastes.

In general, it makes the most sense to grow swine in the vicinity of their feed production. This would
facilitate a sustainable cycle for nutrient management. A modern approach to evaluation of systems
involving anaerobic digestion of wastes from swine, or any feedstock, is life cycle assessment (LCA).
LCA (IEA, 1997) involves the systematic identification of all materials, energy, and economic inputs and
outputs of a system from “cradle to grave”, i.e., from the extraction of raw materials from the environment
to their eventual assimilation back into the environment. The flows are assessed in terms of their
potential to contribute to specific environmental impacts. For swine waste systems, this would involve
assessment of feed production and processing, waste collection and storage, waste transport, air
emissions, water emissions, net energy consumption, compost, and wastes.


The authors acknowledge several colleagues who promptly responded by sending recent reviews and
papers on the subject matter, including R. Zhang, P. Lusk, and C.Y. Chou. They also gratefully
acknowledge Gloria Chynoweth, who located references and entered them into a database.


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