Anaerobic digestion is the process of organic molecules being broken down in the absence of oxygen by various bacteria to simpler compounds with a resultant production of several common gases throughout the process. As anaerobic digestion takes place in the presence of biodegradable material, it has been known for some time that the process is capable of breaking down organic municipal waste. Towards the end of the process the main gas being produced is methane which allows the possibility of methane capture and subsequent use as a biofuel which can be used to generate power reducing the dependence upon fossil fuels. The use of anaerobic digestion as a solution to waste disposal can be implemented from existing landfill sites which account for a high proportion of existing total methane release or as newly built reactors designed to maximise methane yields.
The process begins with the seperation of organic and non-organic municipal waste. The waste initially requires shredding, water addition, screening and pasteurisation before the waste is then biologically treated (1).
There are four main biological stages associated with the process of anaerobic digestion, these are:
· Hydrylosis - Break down of proteins carbohydrates and fats by enzymes produced by hydrolytic bacteria, the products of this first process are sugars, fatty acids and amino acids
· Acidogenesis - Decomposition and fermentation of sugars by acidogens produces carbon dioxide, hydrogen, ammonia and organic acids, gas levels are at about 80% carbon dioxide 20% hydrogen
· Acetogenesis - Digestion of remaining molecules by acetogens to produce acetic acid, hydrogen and carbon diozide
· Methanogenesis - Utilization of the remainaing products by methanogens to produce a mixture of methane (60%) and carbon dioxide (40%)