Rendering is another method of disposing of animal waste. This involves converting animal carcasses/waste into three finished products, carcass meal (protein-containing solids), melted fat/tallow and water. This is done using mechanical processes such as grinding, mixing, pressing, settling and separating, thermal processes such as cooking, evaporation and drying, and chemical processes such as solvent extraction (NABC, 2004). The rendering process simultaneously dries out the material and separates the fat from the bone and proteins. The resulting grease can be used as a low-cost raw material for the production of grease, animal feed, soap, candles and biodiesel, and tallow is an important raw material in the steel rolling industry, providing the lubrication needed to compact steel sheets. The protein flour produced can be used as animal feed. Thus, the products offer the slaughterhouse significant additional income. However, due to BSE-related problems, feeding cattle with meat-and-bone meal is currently banned in industrialized countries. As a result, rendering plants do not play as important a role in the disposal of animal waste as in the past. Composting is an aerobic process in which organic matter is degraded by the activities of successive groups of microorganisms (Dees and Ghiorse, 2001). Substrates vary and can include different types of organic and inorganic waste, sewage sludge, pig, cattle or poultry manure, garden waste and municipal solid waste. Many authors have reported the positive effects of composts on arable soils (Ibekwe et al., 2001; Bailey and Lazarovits, 2003).
In most developing countries, there is no organized strategy for the disposal of solid and liquid wastes produced in slaughterhouses. Solid offal is collected and disposed of in landfills or open areas, while liquid waste is discharged into municipal sewage systems or water bodies, endangering public health and terrestrial and aquatic life. Slaughterhouse wastewater is known to cause an increase in BOD, COD, total solids, pH, temperature and turbidity, and even lead to deoxygenation of water bodies. Slaughterhouse waste is mainly contaminated with organic pollutants. The COD range of typical slaughter wastewater is between 4,400 and 18,000 mg L − 1. Satyanarayan et al. (2005) reported 375,000 mg L – 1 COD in slaughterhouse blood. Wastewater from pig or cattle slaughterhouses includes blood as the main solubility with a COD of 260,000 mg L−1, total nitrogen Kjeldahl (TKN) 31,700 mg L−1 and a TSS of 196,000 mg L−1 (Palatsi et al., 2011). The authors also recorded the following parameters for rumen waste: 152,000 mg kg−1 COD, 1,320 mg kg−1 TKN and 116,000 mg kg−1 TS. Roy et al. (2013) received 69,558 mg L − 1 BOD5, 268,571 mg L − 1 COD and 12,414 mg L − 1 TKN of bovine blood. In addition, Zhang et al.
(2016) characterized ruminal fluid from cattle slaughterhouses with TKN 412 mg L−1, volatile solids (VS) 6,000 mg L−1 and TS 10,000 mg L−1. Garcia et al. (2016) reported that industrial poultry wastewater contained 109,000 mg L − 1 BOD, 266,000 mg L − 1 COD, 26,800 mg L − 1 TKN and 74,400 mg L − 1 TSS. Table 26.3 provides further details on slaughterhouse pollutants. Composting is an alternative to the disposal of slaughterhouse waste. The process has several advantages, including reducing pollution, producing a valuable by-product, and destroying the majority of pathogens (NABC, 2004). However, the successful conversion of this waste into high-quality compost requires close control. If the final product is produced under strict management, it is not expected to pose a risk to human and animal health (Gale, 2004). However, some pathogens cannot be destroyed by composting, such as prions and spore-forming bacteria. New risks have emerged from the implementation of the European Union (EU) Landfill Directive (EU, 1999) for more environmentally friendly waste disposal methods. There is evidence that some pathogens and pests can survive composting or other waste treatment processes, sometimes through inadequate methods or failures in the process (Noble and Roberts, 2004).
In accordance with European Union Regulation (EC) No 1774/2002 on animal by-products (ABP), animal by-products (TNPs) are divided into three classes. Category 2 and 3 TNCs can be processed by licensed composting plants and by AD in approved biogas plants, unlike Category 1 TNCs. The guidelines do not provide for allowances for HA. Most slaughterhouse waste falls into categories 2 and 3. AH represents a relatively new technology for the disposal of animal slaughter equipment and other infectious waste (Kaye, 2003; NABC, 2004). The process uses sodium or potassium hydroxide to catalyze the hydrolysis of biological materials (proteins, nucleic acids, carbohydrates, fats, etc.) into a sterile aqueous solution composed of small peptides, amino acids, sugars and soaps. To speed up hydrolysis, the process is usually carried out with pressure and temperature. To inactivate microbial pathogens, carcasses should be heated to 100 °C and pressurized to 103 kpa for 3 h. To destroy prion-containing material, carcasses should be heated to 150 °C and pressurized to 486 kpa for 6 to 8 h (ssl-edss.tamu.edu/disposal/handbook/04_Alkaline.pdf). Biological pre-treatment of lignocellulosic biomass such as cellulases and cellulase-producing microorganisms improves hydrolysis and promotes fermentable sugar production and improved biogas production (Elliott and Mahmood, 2007; Kumar et al., 2009). Many scientists have discussed coding as another strategy that can be used with various pre-treatment methods to improve the biodegradation of IPP bio-sludge.
PPI sludge can be digested efficiently with more biodegradable substrates such as municipal sludge, food waste, dairy cattle waste, rice straw, pig slaughterhouse waste, grease trap waste, grass silage, and pulp mill organic sludge (Borowski and Kubacki, 2015; Hagelqvist, 2013a,b; Mussoline et al., 2013; Trulli and Torretta, 2015; Yalcinkaya and Malina, 2015; Chakraborty et al., 2017) (Table 3). Lin et al. (2011) used codigestion of pulp mill sludge with monosodium glutamate (MSG) waste from nitrogen source to maintain the C/N ratio during AD. The management of animal carcasses was and still is a concern in animal production, in slaughterhouses, farms and other institutions in which animals are involved. Throughout history, burials and, to a lesser extent, cremation have been the most commonly used methods of eliminating mortality on farms (Gwyther et al. 2011). However, EU Regulation (EC) No 1774/2002 on animal by-products (Anon, 2002) does not allow these practices to be practised within the EU and limits disposal routes to incineration (inside or outside the farm), plastering, ha at high temperature/pressure, disposal on maggot farms or by approved waste collectors (Anon, 2002). The prevention of disposal by burial and burning was justified by the perceived risk of incomplete destruction of pathogens by deaths during these processes and thus the entry of infectious agents into the food chain (Anon, 2002). Similar risks are associated with the slaughter of animals. Unlike incineration, properly functioning incinerators pose fewer pollution problems. In addition, bacteria (including spores) and viruses are not expected to survive the combustion process (NABC, 2004). However, there is concern that transmissible spongiform encaphalopathies (TSEs) such as BSE (bovine spongiform encephalopathy) may survive combustion processes if they are not performed at a sufficiently high temperature (NABC, 2004).