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This is the treatment of organic wastes of solid, liquid or slurry consistency in the absence of molecular oxygen (Ward et al., 2008). The ability of this process to produce biogas as methane has helped to improve its appeal and economic acceptability, particularly since the energy crises of the 1970s, and increasing concern over greenhouse gas emission and global warming. Mass reduction, methane production and improved dewatering are considered to be some of the principal attractions of AD (Ferrer et al., 2008). On the other hand, slow digestion leading to long retention times is a draw back, particularly at mesophilic temperatures. It has been in use since the middle or late nineteenth century, but its application in wastewater treatment did not grow rapidly, due to a lack of fundamental understanding of the process (Wheatley, 1990). It is currently the principal method for treatment of waste sludge, particularly following the invention and success of the up-flow anaerobic sludge blanket

(UASB) (Lettinga, 1995; Lettinga et al., 1980), and variants thereof. It has consequently been extensively studied (Verstreate et al., 1996; Luostarinen et al., 2009), and is in use in many parts of the world, for the treatment of a variety of wastes and effluents including those from food, fermentation, brewery, beverage and paper pulp industries, in addition to domestic, agricultural and municipal wastes and waste water (Ortega et al., 2008; Marcias-Corral et al., 2008; Forster-Carneiro et al., 2008abc; Fezzani and Cheikh 2008; Alvarado-Lassman et al., 2008; Yilmaz et al., 2008; Zupancic et al., 2007), and farm animal house slurries, effluents and wastes (Tricase and Lombardi 2008; Cantrell et al., 2008) as well as slaughter house and meat process wastes (Luostarinen et al., 2009; Buend et al., 2008; Cuetos et al., 2008;).

Continuing research effort has resulted in improved understanding and optimisation of anaerobic digestion and methanogenesis (Cantrell et al., 2008; Forster-Carneiro et al., 2008abc). A number of AD plants currently operate worldwide as means of (centralised) waste treatment and biogas production (Ahring, 1995). Also, a number of small scale household/ farm digesters are being built for dual purpose of gas generation and waste treatment in many developing, warm climate countries. It is estimated that over 5 million cottage digesters operate producing methane and treating wastes in China and India (Poh and Chong 2009; Lansing et al., 2008; Zhao et al., 2008; Coombs, 1994). The use of AD offers a number of other advantages including low cost, low technology, ease of scaling up/ down; considerable stability and ease of start up. It is not energy intensive (if it is operated at mesophilic temperatures), and since anaerobes conserve less energy than aerobes, AD results in less sludge (Schink, 1997). It also achieves reliable waste stabilisation, if it is recognised that AD cannot be used alone for complete waste treatment (Verstreate et al., 1996).

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