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