Digestion Summary Definition and objectives Digestion is a process that effectively pre- and produce methane for energy genera- serves carcass materials under acidic condi- tion. tions (using lactic or phosphoric acid) or uses The ultimate goal of carcass digestion fermentative bacteria to convert the materials processes is either to preserve carcass materi- to a mixture of primarily methane, carbon als under acidic conditions or to convert them dioxide and water. to valuable products without creating health The objectives of digestion methods are hazards or negative environmental impacts. to: Three processes are used widely to digest • Provide long-term storage for animal carcasses: lactic acid fermentation, phosphor- carcasses using acid preservation. ic acid preservation and carcass biogas pro- • Prevent the growth of disease-causing duction. Some organic acids, such as acetic, microorganisms. formic and propionic acids, are used to simply • Anaerobically digest animal carcasses preserve the carcasses.
261261 Digestion Summary
Table 1. Methods considerations for the digestion of contaminated animals. Consideration Lactic acid fermentation Phosphoric acid Carcass biogas production preservation Transportation concerns No Yes Yes Agents inactivated Viruses and bacteria Viruses and bacteria Viruses and bacteria (except TSE4) (except TSE4) (Except TSE4) Disposal capacity1 Low Low Low Potential for Low Low Medium environmental impact Regulatory restrictions2 Low Low Medium Cost3 Medium Low High Availability of resources Low Low Low Procedure speed Medium High Low 1 Animal carcasses (tons): Low = < 100 t; Medium = 100–300 t; High = > 300 t 2 The stringency of restrictions imposed by federal, state and local agencies 3 Cost estimate (per ton): Low = < $200; Medium = $200–800; High = > $800 4 TSE = transmissible spongiform encephalopathy
(Cutoff points may vary, depending on such factors as transportation, carcass load, animals affected, disposal facility and level of security.)
262 The carcasses of several kinds of ani- acid preservation are called minor digestion or mals—cattle, swine, poultry, sheep, goats, fish stabilization processes. They cause little no- and wild birds—can be treated in the lactic ticeable change in protein structure, whereas acid fermentation, phosphoric acid preserva- carcass biogas production changes the protein tion and carcass biogas production systems. materials considerably. However, none of these options can inactivate Lactic acid fermentation and phosphoric abnormal proteins (prions). acid preservation not only destroy or inacti- In the lactic acid fermentation process, vate most disease-causing microorganisms, lactic acid bacteria are added to ground car- but also create an acidic pH that pickles casses mixed with fermentable carbohydrates the carcass materials, enabling them to be to produce lactic acid under anaerobic condi- preserved safely for up to 4 months if they tions. These bacteria may produce volatile remain immersed at the proper chemical con- acids, hydrogen peroxide and antibiotic-like centrations. compounds that inhibit many bacterial and Carcass pickling is used for decontamina- viral pathogens. tion and long-term storage of dead poultry. In the phosphoric acid preservation Most rendering companies accept carcasses process, phosphoric acid is added directly pickled in acid because they are ready for to ground or small pieces of carcasses. The cooking and meal production. phosphoric acid disrupts the membrane func- Compared to cold storage, lactic acid tions of the microorganisms, reducing their fermentation costs less to preserve ground and disease-causing activity. homogenized poultry carcasses and transport Lactic acid fermentation and phosphoric them to rendering facilities. However, in lactic
263263 Digestion Summary
acid fermentation, the costs of the additives tain a composition of 63 to 67 percent water, cannot be recovered with any feed ingredient 11 to 14 percent protein, 13 to 14 percent fat, produced. In contrast, the cost of the added and 2 to 3 percent ash, which is similar to the phosphoric acid in the phosphoric acid preser- composition of the original materials. vation process can be recovered as a nutrition- At concentrations of more than 3 percent al phosphorus source in the feed ingredients lactic acid or 6 percent phosphoric acid, many produced from the materials preserved. pathogens such as Salmonella spp., Campylo- Lactic acid fermentation and phosphoric bacter jejuni, fecal coliforms and streptococci acid preservation eliminate the need for are destroyed in poultry offal and carcasses. renderers to pick up the carcasses every day; Lactic acid also reduces the amount of fungi they reduce the biosecurity risks and costs by in broiler carcasses and offal. reducing the number of farm visits. Transport- Some factors make it difficult and expen- ing acid-preserved carcass materials has less sive to control the biological process in lactic potential to transmit disease than does trans- acid fermentation and carcass biogas produc- porting “fresh” carcasses. tion. For example, carcasses have higher nitro- After 30 days at 80 °F, lactic acid fermen- gen content than do most wastes, which results tation of poultry carcasses produces about 4 to in high ammonia concentrations that can 5 percent lactic acid, 0.2 percent acetic acid, inhibit the anaerobic digestion of the carcass 0.2 percent ethanol and 0.2 to 0.3 percent am- wastes. Under controlled conditions, fermenta- monia-nitrogen. The treated materials main- tion failures occur 10 percent of the time.
264 For carcass biogas production, the op- materials. The remaining materials, generally erational cost of using mesophilic organisms amino and fatty acids, can be used for com- (those that are active at 95 to 100 °F) is less posting. than that for thermophilic organisms (those When treated by an anaerobic digester, the active at 131 °F). Mesophilic organisms sludge or semisolid biowaste such as ground require 15 to 30 days of retention time for carcasses mixed with manure can yield 8 to pathogen inactivation; thermophilic organisms 11 cubic feet of methane per pound (0.5 to require 12 to 14 days. 0.67 cubic meter per kilogram) of volatile The thermophilic fermentors used in solids removed by the process. carcass biogas production are better than the Carcass biogas production is a multi-step mesophilic fermentors at reducing to accept- process (Fig. 1): able levels the coliform bacteria, insect eggs 1. Hydrolysis: The biopolymers (car- and internal parasites in the carcass material. bohydrates, fats and proteins) of the However, they may not destroy some patho- animal matter are broken down into gens or temperature-resistant bacteria such as smaller, soluble molecules. Bacillus cereus associated with carcasses. This 2. Fermentation: The products of Step 1 is why additional heat treatment is required to are converted into organic acids (main- fully inactivate the pathogenic agents that can ly acetic), volatile fatty acids, carbon survive carcass biogas production. dioxide and hydrogen. Carcass biogas production considerably 3. Acetogenesis: The volatile fatty acids reduces the chemical and biological oxygen are converted to acetic acid, carbon demand, total solids and volatile solids of the dioxide and hydrogen.
265265 Digestion Summary
4. Methanogenesis: The acetate and toxic to methanogenic bacteria) in the biodi- ethanol compounds are converted to gester. methane and carbon dioxide. Because domestic livestock and poul- Several groups of bacteria perform each try carcasses are composed of more than 50 of these steps in carcass biogas production. percent water, it is easier to use wet digestion, Some of these microorganisms (such as intes- which has a higher efficiency than does dry tinal anaerobic lactic-acid-forming bacteria) digestion. are naturally available in manure and in the Carcass biogas production systems are intestines of poultry and cattle. This is why available in batch or continuous digesters. adding manure to the carcasses speeds the Three types of batch systems—single-stage, fermentation process and enriches the ratio of sequential-batch and hybrid-batch—are used carbon to nitrogen to more than 20:1. for biogas production. The pH of the digester should range from In the single-stage system, a pump recir- 6.8 to 7.5. Because the byproducts of the fat culates and mixes its contents from the bot- degradation inhibit the methanogenic activity tom to the top of the digester, and fermenta- (because the pH is lowered), calcium carbon- tion is allowed to continue until production of ate and calcium hydroxide may need to be the gas stops. Once the digestion is completed added to maintain a near neutral pH and to (no more gas is produced), the effluent is precipitate long-chain fatty acids (which are removed and a new process is started.
266 Figure 1. Anaerobic digestion pathway (Erickson et al., 2004).
1 Hydrolosis Complex organic matter 2 Fermentation (carbohydrates, proteins, fats) 3 Acetogenesis 1 4 Methanogenesis Soluble organic molecules (sugars, amino acids, fatty acids)
2
Volatile fatty acids
3 Hydrogen, Acetic acid carbon dioxide
4 Methane, 4 carbon dioxide
267267 Digestion Summary
A sequential-batch system uses two or reactor coupled with a UASB reactor. In this more reactors. The sludge from the first reac- reactor, methanogenesis takes place and treats tor contains high levels of organic acids and is the liquid effluents with high levels of organic injected into the second reactor. The leachate acids at high loading rates (Fig. 2). from the second reactor—after the pH is ad- In a continuous digester, the organic mate- justed with lime or calcium carbonate—is in- rial is constantly or regularly fed and moved jected into the first digester. Methane produc- through the digester. It produces biogas with- tion occurs efficiently in the second reactor out the interruptions of loading the material because its sludge contains little or no acid. and unloading the effluent. A continuous sys- The third process is a hybrid-batch or tem may be better suited for large-scale opera- up-flow anaerobic sludge blanket (UASB). It tions; however, the input of carcass materials is similar to the multistage system with two should be continuous and have a consistent reactors. The system comprises a simple batch composition.
268 Figure 2. The schematic view and flow of materials in three types of batch reactors, includ- ing single-stage, sequential-batch and hybrid-batch (up-flow anaerobic sludge blanket digester, or UASB). (Courtesy of Erickson et al., 2004)
A. Single-stage B. Sequential-batch C. Hybrid-batch UASB
New Mature Old UASB
269269 Regulatory Synopsis Digestion Coordination and jurisdictional considerations The decision on whether to use digestion command structure must consider the added as a carcass disposal option should be made problem of transportation safety and contami- jointly by the members of the incident com- nation of property. mand structure established by the authorities Digestion should be undertaken only with in the state or local area. explicit approval by institutions and agencies Local authorities must have an inter-coun- that are competent in making determinations ty memorandum of understanding in place so about protecting the environment. that the carcasses can easily be transported to States have ranked preferred methods for another county that has a digestion facility. carcass disposal, and the incident command If the carcasses are to be transported structure must exhaust the preferred options outside the county for digestion, the incident before undertaking digestion activities.
270 Potential pollution and other property-damage considerations The exercise of police power gives gov- gestate,” that is disposed off on land. The ernmental entities and agencies wide discre- sludge may contain pollutants that could tion in making decisions about carcass dis- contaminate water resources from runoff. posal to protect the public health. However, • Odor the exercise of this power does not shield the • Microbiological and toxic risk of gases governmental entities against nuisance actions for people if the proper precautions are not taken. In the If these problems occur because of gov- case of digestion, private firms engaged in ernmental action, they could trigger nuisance digestion could face legal challenges. or other kinds of lawsuits. Sovereign immu- Critical problems associated with diges- nity may not be a defense to such an action. tion include: If a private firm triggers pathogen spread, • Spread of pathogens the firm may be subject to both civil and • Transportation criminal actions. Water contamination and • Waste treatment and disposal to prevent odors may be a basis for a nuisance action. spread of pathogens Digester size constraints may require the • Water contamination because of runoff use of inter-jurisdictional (among counties) from land application. One product from agreements to transport large quantities of the digestion process is sludge, or “di- animals to other sites.
271271 Regulatory Synopsis Digestion
The decision to use digestion must be command structure because injury to people made jointly by the members of the appro- or property could trigger lawsuits similar to priate technical group within the incident those based on biosecurity breach.
272 Planning Digestion Planning considerations Evaluate in advance the advantages and The volume of carcasses in lactic acid fer- disadvantages of batch versus continuous di- mentation can increase by 33 percent, mainly gesters. Although a batch digester is easier and because of fermentation and carbon dioxide less expensive to build than is a continuous production. digester, a batch digester produces less gas, has Plan to store the products of lactic acid a lower loading rate and carries more risk of fermentation and phosphoric acid preserva- explosion while the reactor is being emptied. tion in sealed, vented containers. The odor of A potential disadvantage of continuous the final product is similar to that of ferment- systems is that the bacterial flora may become ed meats. acclimated to inhibitors such as ammonia and Although the phosphoric acid preserva- retard the production of biogas. tion and carcass biogas production of farm To increase the fermentation capacity, plan carcasses are essentially odor-free processes to use several batch digesters and alternate and are publicly acceptable, the lactic acid the loading and emptying processes. In such a fermentation of farm carcasses produces vola- system, the organic material is loaded into the tile and odorous compounds (such as carbon fermentation tank and digested for the des- dioxide, ammonia and organic compounds) ignated retention period. Then the effluent is and should be vented to prevent unpleasant removed and the process is restarted (Fig. 3). consequences.
273273 Planning Digestion
Figure 3. Operation of the anaerobic Wastewater Influent sequencing batch reactor. (Courtesy of Fill Erickson et al., 2004)
Add wastewater React
Reaction period Settle
Clarify Draw
°° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° °° °° °° °° °° °° °° °° °° °° °° °° °° °° °° °° Remove treated wastewater Idle
°° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° °° °° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° °° Waste sludge
274 Plan to use a mobile or portable unit wider range of organic wastes and perform containing pre-breaking, grinding and pump- better than those from other sources, such as ing equipment at the carcass disposal site to the biodigesters of milk processing plants. reduce the size of the ground materials and to For long-term carcass biogas production easily transfer them to the fermentation and (more than 5 years), consider using a non-cor- storage tanks. rosive and acid-resistant fermentation tank, A mobile power generator will be needed such as a stainless steel tank. during emergency situations because natural Consult with construction and design disasters such as flood, tornado, thunderstorm engineers and obtain the required standards, and heat stress may interrupt power in needed including the technical dimensions, to use areas. concrete tanks for short-term carcass biogas For carcass biogas production, plan to use production. Although concrete tanks are often a vertical, cylindrical tank with a conical bot- built partially below ground for better sup- tom to improve mixing and sludge removal. port, they should be strong enough to bear the Use a properly sized conveying (piping) weight and pressures (vertical and lateral) of system to prevent clogging and special solid mixed semi-liquid materials. handling pumps to transfer thick sludge. For efficient digestion and biogas pro- Plan to use sludge from another biogas duction, plan to maintain the digester at the installation to start up a new carcass biogas proper thermophilic temperatures (130 to 140 production system. Microorganisms in mu- °F) with properly designed heat exchange, in- nicipal wastewater sludge can biodegrade a sulation, mixing and sludge removal systems.
275275 Planning Digestion
To prevent explosions, provide gas col- cow carcasses (1,540 pounds per cow), or lection and pressure regulation equipment, 1.54 million pounds, is 7 million cubic feet, including safety devices. with a loading rate of 0.05 pounds per cubic Provide electricity (for grinding, mixing, foot per day of volatile solids. pumping, and separation) and water for the Plan to prevent high ammonium concen- biodigester. Conserve water by reusing it. trations in the digester by increasing the ratio Propane may be needed as a fuel to start up of carbon to nitrogen to between 20 and 40. and supply heat to the digester before enough Ammonium inhibits the biodegradation of biogas is produced. carbon sources at concentrations above 0.187 Plan to feed the carcasses to the active pound per cubic foot (3 grams per liter) of the digester before its temperature drops to below digesting materials. 130 °F. This will prevent any delays associ- The crew will need to be protected from ated with the digestion start-up process. microbiological contamination and toxic Determine the volume of the digester by gases such as hydrogen sulfide produced by using 1 pound of carcass per 4.4 cubic feet per carcass biogas production. day (3.6 kilograms per cubic meter per day), Consider also the issues related to han- assuming that the carcasses have 0.23 pound of dling, packing, storing and conveying the volatile solids per pound of carcass material. carcasses to the digestion facility as described For example, the volume of the digester in the “General Considerations” chapter of needed for anaerobic fermentation of 1,000 this guide.
276 Procedures Digestion Lactic acid fermentation or phosphoric acid preservation Secure the area for carcass preparation and Figure 4. Free-standing polyethylene processing from predators and vermin. horizontal bulk storage tank used to store Use appropriate containers, such as high- lactic or phosphoric acid. (Courtesy of United density polyethylene (Fig. 4) for the lactic acid States Plastic Corporation, Lima, OH) fermentation and phosphoric acid preservation tanks. After grinding the carcasses, add them to the fermentation (lactic acid fermentation) or preservation (phosphoric acid preservation) tank. For lactic acid fermentation, mix the ground carcasses with an organic compound that includes: • A fermentable carbohydrate such as glu- cose, sucrose or lactose at a ratio of 10 percent by weight • Whey, at 17 percent by weight • Molasses or condensed brewer’s solu- Grind the fresh carcasses alone or with one bles, at 20 percent by weight of the above-mentioned materials to particle • And/or finely ground corn, at 20 to 24 sizes of less than 1 inch for phosphoric acid percent by weight preservation or for lactic acid fermentation.
277277 Procedures Digestion
Grinding the mixed materials not only pro- drates) to be converted into lactic acid. motes the homogenization of the phosphoric Check the pH of the lactic acid fermenta- acid and the ground carcass material, but it tion tank 24 hours after start-up. Under proper also helps speeds the fermentation process and working conditions, the pH of the mixed ma- disperse and mix the intestinal anaerobic lac- terial should change from about 6 to less than tic-acid-forming bacteria (Fig. 5). Do not add 5. Within 7 to 10 days of fermentation, the pH decomposed carcasses to a lactic acid fermen- of the ground carcasses mixed with lactic acid tation or phosphoric acid preservation process. bacteria decreases to below 4.5. At this pH A proper pH may not be achieved for the level, the lactic acid bacteria quickly grow to mixed materials, resulting in further spoilage. concentrations that result in the preservation Use starter cultures such as Lactobacillus of the carcass material. species to speed up the fermentation process Pump the products of the lactic acid fer- and provide a margin of safety under less than mentation from the fermentation tank into a ideal conditions. Such conditions may include storage vessel. poor mixing conditions, low fermentation Add an amount of feed-grade phosphoric temperatures (less than 70 °F) and/or larger acid to make the final mixture contain 6 per- particle sizes of the mixed materials. cent of this acid. For pickling, ground car- The temperature in the lactic acid fermen- casses may be dipped in either acetic acid or tation tank should be maintained at 70 to 80 propionic acid at a concentration of 10 or 3.8 °F to allow the sugars (fermentable carbohy- percent concentration, respectively.
278 Figure 5. Views of a mobile grinder used to reduce the size of carcasses. (Courtesy of Haarslev , Bogensevej 85, DK-5471 Søndersø, Denmark)
279279 Procedures Digestion Carcass biogas production Secure the area for carcass preparation and Use a mixer or circulation pump to uni- processing from predators and insects. formly distribute the heat and bacteria by To speed the heat transfer rate and pro- displacing or recirculating the gases collected vide more surface area for fermentation, grind at the top of the fermentation tank. A recir- the carcasses to an average particle size of culation pump is more expensive but more less than 2 inches. Large pieces of bone can efficient than is a mixer (Fig. 6). damage the circulation and sludge-removal In addition to fresh carcasses, the products pumps. of rendering (if not dried) and alkaline hydro- For wet digestion, mix the ground carcass- lysis can be used as input materials for biogas es with water and sludge from another biogas production. installation or from a municipal wastewater Incorporate manure or municipal waste- facility to achieve a concentration of total water sludge into the ground carcasses to solid contents ranging from 10 to 15 percent. achieve a ratio of carbon to nitrogen ranging Use a fermentation tank with a floating lid from 20 to 40 and a biodegradable volatile to accommodate gas expansion with pressure solids content of 60 percent of total solids. control. This type of system is more expensive This mixture will have a culture of beneficial and difficult to manage than is a conventional bacteria and the capability to biodegrade a tank with non-floatable cover or lid. wider range of compounds and wastes. To in-
280 oculate the new system, feed the reactor with is removed and the process is restarted. Make sludge from another installation. sure to dry the sludge and store it before dis- Once digestion is complete, the effluent posal as a fertilizer.
Figure 6. The flow diagram of a wet system.(Erickson et al., 2004)
Oil Electricity
Biogas Dual fuel engine Generator
Digester
Heat Heat exchanger exchanger Gasometer
Digester
Carcass in
Sludge out
281281 Safety Digestion
Table 2. Guidelines for the use of personal protective equipment for digestion operations. Nature Mask/respiratora,b Protective Eye protectiona Glovesa Head/foot of work clothinga protection Zoonotic Non-zoonotic agent agent Direct Disposable None Impermeable Full facepiece Gloves: Heavy Feet: For handling of particulate recommended to liquids; respirator or duty (15–18 workers contaminated respirator unless for foot- may vary, indirectly vented mil) chemical handling material (N95, N99 and-mouth depending goggles; contact resistant gloves carcasses, steel- or N100); disease upon the heat lenses should not that can be toe/steel shank half or full situation be worn under disinfected or waterproof facepiece goggles or safety disposed of boots; for glasses; consider others, steel-toe prescription safety work shoes or goggles; face shield boots unless wearing Head: Hard hat a full facepiece respirator No direct As directed As directed As directed Safety eyewear As directed by As directed handling of by the facility by the facility by the facility the facility safety by the facility contaminated safety officer safety officer safety officer officer safety officer material a For a list of vendors recommended by OSHA, visit www.safetyequipment.org. b For information about a full respiratory protection program, visit www.osha.gov/SLTC/repiratoryprotection/index. c Regulations governing the use of personal protective equipment in hazardous waste operations can be found at 29 CFR 1910.134 and 29 CFR 1910.156 and are summarized in the Safety section of the “General Considerations” chapter of this manual.
282 Diseases of concern For digestion methods, the diseases of monia, foot-and-mouth disease, glanders, concern include those caused by viruses, bac- Japanese encephalitis, Q fever, Rift Valley teria and prions. fever, rinderpest, tularemia and vesicular Viruses and non-spore-forming bac- stomatitis. teria: As with other methods, the periods Spore-forming bacteria: Spore-forming of greatest risk will be during transport and bacteria are temperature susceptible. If not disposal of the contaminated material. Non- destroyed, they will persist in the environment spore-forming bacteria and viral diseases are for long periods. If immediate incineration of generally destroyed by anaerobic digestion at these carcasses is not possible, the carcasses 131 °F (55 °C); however, all pathogens vary must remain intact to prevent the spread of in the amount of time it takes to be deacti- spores into the external environment. vated effectively. For these reasons, digest all It is recommended that anthrax-infected non-spore-forming bacteria and viruses for 8 carcasses be incinerated or deactivated by days to ensure complete deactivation. alkaline hydrolysis. Anaerobic digestion is Diseases for which digestion methods not recommended for spore-forming bacteria are appropriate include African swine fever, unless a high-heat treatment will be conducted highly pathogenic avian influenza, brucellosis after the digestion process. (melitensis, abortus, suis and canis), classical Diseases of concern include anthrax. swine fever, contagious bovine pleuropneu- Prions: Prions are temperature resistant.
283283 Safety Digestion
Exposure to extremely high temperatures Anaerobic digestion is not an effective (more than 1,830 °F, or 1,000 °C) for at least means for destroying TSE-infected carcasses, 15 minutes is necessary to destroy prion-in- and therefore should not be performed on fected carcasses. If they are not heat inacti- them. TSE diseases include bovine spongi- vated, the prions will persist in ash or soil for form encephalopathy, chronic wasting disease a considerable period. and scrapie.
Site safety Heat stress: See guidelines on heat stress cal burns are exceptionally hazardous and in the Safety section of the “General Consid- can cause irreparable damage to the eyes erations” chapter of this guide. within seconds if not removed using copious First aid: Make first aid available to em- amounts of water for at least 15 minutes. ployees at all times. Workers exposed to any amount of sodium Safety observers: Use a safety observer hydroxide in their eyes should use an eyewash who has the authority to stop and correct un- station and report to the nearest emergency safe conditions or operations. room. Chemical hazards: Provide safety show- Ventilation: Although digestion tanks use ers and emergency eyewash stations within enclosed pressure vessels, the area surround- 20 feet of each digestion tank. Caustic chemi- ing the vessel should be ventilated adequately.
284 Biosecurity Digestion
Facilities that accept contaminated ma- vehicles leave the disposal site. See additional terials may be fixed-site facilities located on material in the Safety section of the “General heavily trafficked public or private property, Considerations” chapter of this guide. such as university campuses. Movement of Release of digested material must be non-zoonotic-contaminated plant or animal coordinated with local and state public health, material onto these sites should be very care- environmental quality and land-use authori- fully planned. ties. A public relations plan should already Although transporting carcasses contami- be in place before disposing of any digested nated with non-zoonotic material does not material in a public sewer system or on land, present a health hazard to the public, a signifi- and it must be performed fully in conjunction cant effort must go into public awareness and with state and local authorities. public relations activities well before any car- Because the digested material may contain casses are moved to the site. Do not use such viable pathogens, have this material tested facilities to dispose of carcasses contaminated before disposal or reuse. with zoonotic agents or transmissible spongi- Direct contact may be possible by worker form encephalopathies (TSEs). exposure to dust or biosolids that have been Decontamination of vehicles and any applied to crops in the field. contaminated personnel must occur before the
285285 Environment Digestion Groundwater pollution Close coordination with state and local Although direct release of digested sludge health and public works authorities is essential into surface waters is not recommended and before the release of any digested materials. may be illegal in some jurisdictions, runoff No notable groundwater pollution should from land-applied digested material does have be present if all procedures are followed the potential to be released or migrate to sur- correctly. If necessary, groundwater may be face water bodies, particularly if the land on checked using a groundwater monitoring which the sludge is being applied is immedi- program. ately next to such water bodies. This situation Digested materials should be tested for is analogous to the movement of constituents disease-causing organisms before they are of land-applied manure migrating to surface released onto land or into bodies of water. waters.
286 Soil pollution No soil pollution concerns are associ- process. This can be ground and disposed of ated with digestion processes unless through in landfills as solid waste according to state uncontrolled disposal. and local solid waste regulations. Landfill disposal: Some tissue, such as All waste must be tested before movement bone and teeth, may remain after the digestion or transfer to landfills or other disposal sites.
Air pollution No notable emissions are associated with Concerns are limited to the on-site work- digestion methods of disposal. The biogas ers, who will need personal protective equip- generated from the digestion of carcasses and ment to minimize their exposure to airborne manure will be composed of mainly carbon or aerosolized agents. Hydrogen sulfide may dioxide (about 40 percent) and methane (about pose an immediate exposure risk to on-site 60 percent) and trace amounts of hydrogen personnel; it does not pose a risk to the public sulfide, nitrogen, oxygen, hydrogen and other or the environment. compounds such as methyl mercaptans.
287287 Cost Digestion
The cost breakdown for anaerobic diges- Figure 7. Components of direct and tion destruction follows the general specifi- indirect costs for digestion methods. cations in the Cost section of the “General Considerations” chapter of this manual. The direct fixed cost depends on facility’s capacity (Table 3). The direct cost estimates vary greatly, depending on the ability to reduce energy expenditures by harvesting electricity and generating heat during the anaerobic digestion process (Table 4). Maintenance Interest rate Environmental For indirect cost items, see the Cost sec- and repair impacts tion of the “General Considerations” chapter of this guide. Spreading cost Depreciation Others
Management
Transportation
288 Table 3. Initial investment and annual direct fixed cost estimates of anaerobic digestion with an annual capacity of 637,000 pounds, or 850 cows per year. Item Investment Depreciation Interest rate Annual cost (6%) Digester $350,000 $17,500 $21,000 $38,500 Electrical and heating $235,000 $11,750 $14,100 $25,850 system Solids and liquids $89,000 $4,450 $5,340 $9,790 separation Liquid storage $315,000 $15,750 $18,900 $34,650 Others $43,800 $2,190 $2,628 $4,818 Total $1,032,800 $51,640 $61,968 $113,608 Source: Wright, P., and S. Inglis (2003). An Economic Comparison of Two Anaerobic Digestion Systems on Dairy Farms. ASAE Annual International Meeting, Las Vegas, NV, July 27–30, 2003. Note: The life expectancy of the investment is assumed to be 20 years.
289289 Cost Digestion
Table 3. Estimates per carcass of direct variable cost items of anaerobic digestion. Others Weaned Preweaned Cattle Calves (sheep, hogs hogs lambs, goats) Estimated average direct variable cost per carcass Maintenance1, repairs, insurance $34.85 $12.36 $6.18 $0.28 $3.58 Spreading $68.24 $24.20 $12.10 $0.55 $7.01 Management $7.49 $2.66 $1.33 $0.06 $0.77 Benefit from electricity savings -$49.88 -$17.69 -$8.85 -$0.40 -$5.12 Benefit from heat savings on farm -$7.06 -$2.50 -$1.25 -$0.06 -$0.72
Average direct variable cost per carcass Excluding benefits from energy savings $110.58 $39.22 $19.61 $0.89 $11.36 Including benefits from energy savings $53.64 $19.03 $9.51 $0.43 $5.52 a The maintenance cost per herd is calculated at $29,619/850 lb. Source: Wright, P., and S. Inglis (2003). An Economic Comparison of Two Anaerobic Digestion Systems on Dairy Farms. ASAE Annual International Meeting, Las Vegas, NV, July 27–30, 2003.
290 Besides equipment, management and Table 5. Estimates per ton of direct spreading costs, the disposal cost includes variable cost items of anaerobic digestion for a transportation cost, which depends on the cattle, calves, weaned hogs, preweaned hogs distance that the carcasses are moved. and others (sheep, lambs and goats).
Consideration Cost Maintenancea, repairs, insurance $92.93 Spreading $181.97 Management $19.97 Benefit from electricity savings -$133.01 Benefit from heat savings on farm -$18.83
Average direct variable cost per ton Excluding benefits from energy $294.87 savings Including benefits from energy $143.03 savings a The maintenance cost per herd is calculated by $29,619/850 lb. Source: Wright, P., and S. Inglis (2003). An Economic Comparison of Two Anaerobic Digestion Systems on Dairy Farms. ASAE Annual International Meeting, Las Vegas, NV, July 27–30, 2003.
291291 Cost Digestion
Figure 8. Formulas to estimate the direct variable cost relating to anaerobic digestion de- construction.
Direct variable cost (DVC), excluding benefits from energy savings:
• By number of carcasses:
DVC = 110.58Qcattle + 39.22Qcalves + 19.61Qweaned hogs + 0.89Qpreweaned hogs + 11.36Qothers
Where Qi is the total number of carcasses of animal category i.
• By weight:
DVC = 294.87(Wcattle + Wcalves + Wweaned hogs + Wpreweaned hogs + Wothers ) Where W is the total weight in tons of animal category i. i (Figure continued on next page)
292 Figure 8. (Continued)
Direct variable cost (DVC), including energy savings and sale:
• By number of carcasses:
DVC = 53.64Qcattle + 19.03Qcalves + 9.51Qweaned hogs + 0.43Qpreweaned hogs + 5.52Qothers
Where Qi is the total number of carcasses of animal category i.
• By weight:
DVC = 143.03(Wcattle + Wcalves + Wweaned hogs + Wpreweaned hogs + Wothers ) Where W is the total weight in tons of animal category i. i
293293 Emerging Summary Definition and objectives Emerging methods for disposal of con- pathogens that do not threaten public health. taminated biomaterials include new evolving An example of these methods is crop rotation. disposal technologies, nontraditional disposal The objectives of these emerging methods methods and alternative disposal methods. may include: Evolving disposal technologies use • To quickly and safely dispose of large heat or irradiation processes to inactivate the numbers of animal carcasses before they disease-causing organisms associated with decay or deteriorate dead animals. In some cases, the carcasses • To prevent environmental contamination are converted to inert end products. Evolving and reduce public health hazards during disposal technologies include gasification, animal disposal plasma technology, thermal depolymerization, • To eliminate the extensive amount of dehydration and extrusion. land needed by some conventional Nontraditional disposal methods include carcass disposal methods; however, ocean disposal and the feeding of carcasses to an even larger area of ocean might be exotic animals such as alligators. These meth- needed ods impose minimal harm to public health and • To use the most cost-effective means to the environment. eliminate or reduce the populations of Alternative disposal methods can be plant pathogens in the field used to dispose of plants contaminated with For all of these methods, no specific
294 research information is available on whether cephalopathies from carcasses contaminated they can destroy transmissible spongiform en- with them.
Evolving disposal technologies: General description Evolving disposal technologies include as air-curtain burning, open-air burning gasification, plasma technology, carcass and composting thermal depolymerization, dehydration and • Allow for better control of operating extrusion. Because of their heat generation or parameters such as temperature, mois- irradiation processes, most evolving disposal ture content, pH and particle size, which technologies can inactivate microbial cells and results in a more uniform product viral particles, including airborne pathogens. A disadvantage of most evolving disposal Most of these technologies offer several technologies for animal disposal is that they advantages. They: have low throughput and are not economically • Can be set up as mobile units that can feasible for disposing of large numbers of ani- be moved quickly to the disaster area mal carcasses. However, these methods can be • Generate no leachate from the carcasses modified for higher throughput. and prevent the contamination of soil, For most evolving disposal technologies, groundwater and surface water the carcasses are fed as shredded or ground • Emit fewer toxic gases and odors than material. This preprocessing requires stricter do some conventional methods, such biosecurity measures than for intact carcasses.
295295 Emerging Summary Gasification: Description In gasification operations, animal carcasses the gasification reactions; the rest is combusted. are slowly heated and converted into a produc- If practical, waste heat boilers or electri- er gas that contains methane, hydrogen, carbon cal generating equipment such as a Sterling dioxide and carbon monoxide. Some of the engine can be used to generate heat, steam or producer gas is burned to supply the heat for electricity produced by the operation (Fig. 1).
Figure 1. Schematic of a batch-size carcass gasifier. (Courtesy of Brookes, BGP Inc., Raleigh, NC)
Burner Flue
Primary chamber Feedstock
Secondary chamber 800 °C
296 Carcass gasification occurs at a low oxy- Batch systems have limited throughput gen content to prevent burning and at temper- for carcass gasification; continuous gasifying atures of 1,110 to 1,900 °F (600 to 1,000 °C). systems can accept higher throughput (Fig. 2). Shredded carcasses are mixed with bio- Continuous carcass gasifiers use less fuel and waste sources, such as manure, that contain a have a better fuel efficiency than do batch high ratio of carbon to nitrogen. Alone, ani- systems. mal carcasses have a low carbon-to-nitrogen The carcass gasification throughput is more ratio (less than 5), which is not suitable for than 10 tons per day in Scotland (Fig. 3); in the proper gasification. United States, the throughput for equipment The first stage of gasification requires an under development is about 25 tons per day. auxiliary fuel such as propane. The amount The amount of time required to gasify of fuel consumed depends on the processing carcasses depends on the gasification capacity, technique and the fat and moisture content of the technique and the nature of carcass mate- the carcass. rials. Converting carcasses to gas may take 4 If a new batch of carcasses follows the to 12 hours, with a resulting ash and char of previous batch at a temperature higher than about 5 to 15 percent by volume. 1,500 °F (800 °C) or lower than 740 °F (400 The disposal of ash is similar to the pro- °C), it is called a hot start or warm start, re- cedures followed after carcass incineration in spectively (Table 1). fixed-facility incinerators.
297297 Emerging Summary
Table 1. Gasification efficiency factors. Feed stock type Set temperature DMa processed Propane, ft3 Run time Propane (°C) (kg) (min) (gal/kg DMa ) Fecesws 800 31.0 144 225 0.126 Feceshs 800 31.7 65 150 0.058 Chicken littercs 750 31.1 294 368 0.261 Chicken litterws 870 31.8 114 240 0.099 Pig carcassesws 800 21.1 ~600 315 0.780 Pig carcasseshs 800 21.1 ~200 130 0.261 Poultry carcasseshs 870 28.0 125 286 0.124 a = dry matter ws = warm start cs = cold start hs = hot start
298 Figure 2. Schematic of a continuous-carcass, feed-style gasifier. (Courtesy of Brookes, BGP Inc., Raleigh, NC)
1 1 3 10
2 3
4 1 5 6 7 8 11 9 12 1 4
1 Feeding hopper for sludge-like waste (possibly macerated carcasses) 8 Unloading zone for remaining ash 2 Continuous-feed auger zone for conveying material in the drying zone 9 Fuel-heating chamber 3 Drying zone for carcass materials 10 Exhaust emission channel 4 Feeder zone for conveying the warm and dehydrated carcasses by the 11 Insulating materials second auger to primary chamber for gasification 12 Brick walls 5 Primary chamber for gasification 13 Air filter 6 Feeder zone for conveying gasification 14 Discharge ash auger 7 Carbon cycle or carbon chamber of carcass gasification
299299 Emerging Summary
Figure 3. Views of carcass gasifying system used in Scotland for the bovine spongiform encephalopathy crisis. (Photos courtesy of Brookes, BGP Engineer, presented in the North Carolina Disposal Roundtable, Farm Bureau Federation of Raleigh, NC, March 30, 2006)
300 Plasma technology: Description Plasma technology fluidizes, or converts is a good raw material for manufacturing vari- into a fluid, the inorganic portion and heat- ous forms of brick and tiles or the concrete resistant material of animal carcasses at very filler used in insulation, roadbed construction high temperatures (up to 7,000 °C) after its and composition roofing. No special waste organic portion is converted to vapor at 200 to disposal is required.
600 °C (with no added oxygen2) and converted Although no operating systems are avail- to gas at 600 to 1,000 °C (with limited oxy- able, a mobile system could process 6 to 8 gen). The resulting molten slag (Fig. 4) is col- tons of small, intact carcasses (up to 100 lected in a separate container, where it cools pounds) per day. A plasma reactor eliminates into glasslike material (Fig. 5). the need to shred the carcasses and improves Plasma technology is the application of worker safety. artificial lightning to gasify organic materials A plasma technology unit has the potential (biogas generation) and melt inorganic mate- to convert temporarily stored animal carcasses, rials, including animal carcasses. such as those that have been buried or com- The gases emitted in this method can be posted, into inert materials of lower volume used to produce methanol. The final rock-like than the original mixture of soil/organic matter residue is highly resistant to leaching. Also, it and carcasses.
301301 Emerging Summary
Figure 4. An overall view of shredding, feeding, organic gasifying, melting and solidifying of inorganic materials using carcass plasma technology. (Courtesy of Dr. Lou Circeo, Georgia Tech Research Institute, Atlanta, GA)
Hopper
Large carcass Gates shredder
Gas treatment Plunger
Plasma torch
Molten slag
Molten earth
302 Figure 5. Schematic diagram of plasma technology for disposal of animal carcasses. (Courtesy of Kent Munden, USDA-APHIS, Clifton, TX)
Plasma-fired pit burner
Plasma torch Pit cover Animal carcasses Soil layers (1–2.5 MW) (as required) (20–30 tons)
Fire box Soil layer –1 Pipe 9’
Additional Gas treatment Soil layer air (20–30 tons) (as required) 30’ 9’
303303 Emerging Summary Thermal depolymerization: Description Thermal depolymerization technology is conducted at the molecular level and can can treat ground carcasses under high pres- effectively destroy pathogens, it does not in- sure (600 pounds per square inch, or about 40 activate abnormal proteins such as prions. bars) and high temperatures (steam heating at A thermal depolymerization plant has about 480 °F or about 250 °C) in the presence been built in Carthage, MO, to digest 200 tons of carbon monoxide to create useful organic of turkey processing waste per day. products such as biofuels. This technology is expensive and requires After 15 minutes of carcass depolymeriza- highly skilled personnel to operate the system. tion, the reactor pressure is released rapidly The raw materials and residuals are kept to evaporate most of the water and separate in sealed containers before and during pro- the liquid crude hydrocarbons from the solid cessing. The residuals, generally minerals, can minerals. be land-applied as fertilizer. Although depolymerization of carcasses
304 Dehydration and extrusion: Description In the dehydration and extrusion process, friction heat, shearing and pressure. Extruder superheated air moves the particles of ground screws force the material to pass through a se- carcasses into a hot channel to evaporate ries of hot channels where, within 30 seconds, and reduce their moisture. The materials are the temperature rises from 240 to 2,800 °F conveyed to an extruder barrel, where they (115 to 1,550 °C) and the pressure rises from are blended, cooked, sheared, kneaded and 294 to 600 pounds per square inch (20 to 40 formed into a plastic-like material that is con- bar). The product expands and loses 12 to 15 verted into dried animal feed. percent of its moisture content because the Ground carcasses such as swine are dehy- high pressure drops suddenly to atmospheric drated in a fluidized bed dryer or a flash dryer pressure as the product leaves the extruder. (high temperature and short exposure) at 212 This new technology may emerge as an °F (100 °C) and mixed with an organic carrier alternative to the rendering of dead animals, such as finely ground soybean. This process because the final product has more nutritional reduces the moisture content of the mixed feed value than does the carcass meal pro- materials to about 30 percent and facilitates duced in rendering plants. the extrusion process. The amount of each Dehydration of dead pigs is more efficient specific carrier depends on the moisture and than that of poultry or cattle because swine fat contents of the dead animals. have more fat and less water. The mixed materials are conveyed Flash dehydration of ground carcasses through an extruder channel and subjected to causes little damage to protein quality, and the
305305 Emerging Summary
dried animal feed has superior protein digest- all bacteria, molds, viruses and spores in the ibility. If the product must be sterilized, the carcasses. However, this process should not meal can be dehydrated further to about 10 be used for disposal of carcasses contami- percent moisture and subjected to extrusion nated with transmissible spongiform encepha- processing. lopathies. Because of the high processing tempera- After dehydration and extrusion of ground tures and pressure, the extrusion of ground carcasses, the feed can be processed further to and dehydrated carcasses readily inactivates separate the fat from protein.
Nontraditional disposal methods: Description Nontraditional disposal methods include ment, the carcasses can be disposed of more dumping carcasses into the ocean and feeding quickly if they are preprocessed (for example, carcasses to exotic animals. by grinding them) and if adequate storage is provided. General description The U.S. Environmental Protection Agen- Although intact carcasses can be fed to cy requires that only carcasses used in medi- exotic animals without significant pre-treat- cal research can be disposed of in the ocean.
306 Ocean disposal: Description In the ocean disposal method, carcasses are (40 CFR Parts 227 & 228) and is placed placed deep in the ocean to prevent flotation and in an approved ocean-disposal site. allowed to gradually disintegrate. Ocean is de- • The materials undergo a series of tests and fined as the waters lying seaward of the baseline evaluations to determine whether they meet from which the territorial sea is measured. the EPA’s ocean-dumping criteria. These To transport carcasses for the purpose of criteria include consideration of the effects ocean dumping, you must obtain a permit from of disease-causing organisms, hazards to the Environmental Protection Agency or from navigation and dangers to shorelines and the Army Corps of Engineers (in the case of beaches as a result of the dumping. dredged material), according to the Marine The only materials that are routinely per- Protection, Research and Sanctuaries Act mitted for ocean disposal are dredged material (MPRSA, 33 USC § 1401 et seq.). and fish wastes. These and other materials may Ocean-dumping permits require that: be dumped in an emergency. • The dumping does not unreasonably de- In other cases, an emergency permit is re- grade or endanger human health, welfare quired and may be issued only if the emergency or amenities or the marine environment, poses an unacceptable risk relating to human ecological systems or economic potenti- health and there is no other feasible solution. alities. No permits, even emergency permits, may • The material meets the extensive ocean- be issued for radiological, chemical or biologi- dumping criteria established by the EPA cal warfare agents.
307307 Emerging Summary
Other considerations would not be allowed under an ocean dump- Consider dumping infected and non- ing permit. diseased carcasses in the ocean only in an Roll-off dumpsters may be used to trans- emergency situation when there is no feasible port the carcasses from the farms if the car- alternative. casses are packed with inert, nontoxic materi- When the EPA issues an emergency als to prevent them from floating. ocean-dumping permit, the agency determines The number of personnel needed for the location and method of dumping as a con- ocean disposal has not been determined, but it dition of the permit. may be a major issue in an emergency animal Avoid dumping animal carcasses near the disposal event. shore because they may attract scavengers and Research has not been conducted on what cause health problems for the people living or happens to the disease microorganisms in this recreating near the ocean. new carcass disposal method. It is not clear Dumping large amounts of animal car- whether the pathogens could be re-introduced casses in one location (overdosing) may result to farm animals through fish meal in the feed in floating debris and “dead zones,” which or spread to marine animals and seabirds.
308 Refeeding: Description Refeeding methods use whole or shredded Rendered-animal carcasses are usually farm carcasses as feed for exotic animals and fed at fur farms; however, in some states such at fur and trout farms. as Minnesota, non-rendered animal carcasses According to the National Contract Poul- can be fed to fur animals if these requirements try Growers Association, alligator farming are met: operations have become a viable option for • A permit to feed these animal carcasses disposing of hundreds of thousands of chick- is obtained. ens that die before they reach the processing • The carcasses, facilities and equipment plant. meet the specifications of the Minnesota If a large number of carcasses were caused Board of Animal Health (MBAH, 2003) by a natural disaster, they would need to be for fur-farm consumption, and the farm preprocessed to inhibit decomposition and is in a sanitary condition. then stored in sealed containers or frozen until • Animal carcasses may be fed to fur- they were consumed. bearing animals if their products do not In some states, such as Louisiana, raw reenter the food chain. poultry carcasses cannot be fed to hogs or alli- • The fur-farm owner accepts the risk gators unless the carcasses are first cooked or of diseases being transmitted from the rendered. carcasses to the fur animals.
309309 Emerging Summary Alternative disposal methods: General description Alternative disposal methods focus mainly are planted in the contaminated field. If the on crop rotation to naturally dispose of con- crops are rotated properly, many pathogens taminated plant materials. If the plant disease either die out or their concentrations are is not considered an immediate threat and reduced sharply. For this reason, it is recom- if the pathogen does not pose an epidemic mended that you consult with county Exten- concern, the disease spread can be limited by sion specialists to select an appropriate crop. appropriate quarantines and disease monitor- Soil-borne pathogens that typically infect ing. plants of one or a few species can be reduced Crop rotation is a natural but powerful significantly by planting, for 3 to 4 years, tool for eliminating non-threatening patho- crops that belong to species or families not gens in the field. Crop rotation is often used to attacked by the particular pathogen. reduce plant pathogen populations in agricul- Crop rotation can effectively eliminate ture. At the end of the growing season, the pathogens that are typically considered soil contaminated crops will degrade naturally in invaders; these survive only in living plants or the field. in the plant residue that persists in the soil. In Usually in the next growing season, crops the U.S. Midwest, maize and soybean crops that do not serve as hosts for the pathogens are rotated to manage plant pathogens.
310