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.