Anaerobic Digestion of Fatty Slaughterhouse Waste Optimising the Digestion Process

The continued growth of world population and food production leads to a continued increase in organic waste, responsible for many environmental and health problems when disposed in the wrong way. In Makkah city, huge amount of raw slaughterhouse waste from the Hajj sacrifice is generated and disposed in landfill without any treatment each year during Hajj. The huge number of pilgrims and a slaughterhouse waste generated from sacrificed animal (2.5 million animals in 2014) results in an environmental and health problems from the landfill area. One of the most applicable and effective methods to treat organic waste materials is anaerobic digestion (AD). In AD, bioactivity of four different groups of microorganisms in sequential metabolic steps under oxygen free conditions is responsible for breaking down complex organic waste into the simplest compounds and producing energy in form of biogas (a mixture of methane and carbon dioxide). However, the process can collapse when the organic waste contains high fatty materials (such as slaughterhouse waste) as long chain fatty acids (LCFAs) produce a strong inhibitory effect on some AD microorganisms. In a batch system, adding a small amount of fat to a large volume of anaerobic seeding sludge in order to provide sufficient amount of anaerobic inoculum could work effectively. However, this will lead to the disadvantage of wasting most of the reactor volume and therefore reducing the capacity to receive fresh organic waste. This research looks into the possibility of optimising the digestion process in order to achieve a good anaerobic digestion process for high fat concentration substrate with a minimum amount of seeding sludge in the reactor. Different strategies were studied to achieve this goal. These included the use of different fat concentrations in co-digestion with vegetable waste, different I/S (inoculum to substrate) ratios vs/vs, different operating conditions of single stage reactors (mesophilic CSTR, thermophilic CSTR, and mesophilic up-flow), and study of different multi stage reactor sets (two, three, and four stages) in order to optimise the digestion process.