The feasibility of anaerobic digestion application comes through enhancing the efficiency of anaerobic digesters. In the case of solid feedstocks, this task is challenging, since the rate limiting step has been recognised to be the disintegration and the hydrolysis of the particulate organic matter. Some of the studied methods for enhancing the biogas production are:

• Pretreatment methods: They are applicable mainly when high solid feedstock is involved. In general, pretreatment methods can be divided into three main types according to the means used for altering its structural features: mechanical, physicochemical and biological. Mechanical pretreatment is almost always applied before any other kind of pretreatment, and actually refers to milling, through which reduction of particle size of solids is achieved. The reduction in particle size leads to an increase of available specific surface. Both physicochemical and biological pretreatment methods may enhance biodegradability, but physicochemical methods yield in general higher efficiencies. During physicochemical pretreatment, the feedstock is exposed to acid, alkaline or oxidative conditions, at ambient or high temperature. The use of high temperatures without the addition of some chemical agent, called thermal pretreatment, can also be used. Combinations of two or more physical and chemical pretreatment methods are also possible, such as acid-catalysed steam explosion, ammonia fiber explosion (AFEX) and CO2 explosion.

For lignocellulosic feedstocks, steam pretreatment, lime pretreatment, liquid hot water and ammonia based pretreatments seem to have high potential (Hendriks and Zeeman, 2009). The main effect of these methods is to dissolve the hemicellulose and alter the lignin structure, improving the accessibility of the cellulose to hydrolytic enzymes. In the case of municipal activated sludge, the goal of pretreatment is to rupture the cell wall and to facilitate the release of intracellular matter in the aqueous phase for subsequent degradation and enhance dewaterability. Various pretreatment methods have also been studied (Weemaes and Verstraete, 1998). Ultrasonic pretreatment seems to be promising, since full-scale studies have showed an improvement in sludge dewaterability (Khanal et al, 2007).

• Use of additives (Yadvika et al., 2004):

— The addition of powdered leaves, crop residues, etc. seem to increase the biogas production; the additives create a more favourable environment for the microorganisms and offer sites for the substrate local concentration through adsorption which seem to have a positive impact on biogas production (Chandra and Gupta, 1997; Dar and Tandon, 1987; Somayaji and Khanna, 1994; Babu et al., 1994).

— The addition of microbial strains (such as cellulolytic bacteria and fungi or cell lysate) increases the substrate digestibility (Tirumale and Nand, 1994; Attar et al., 1998; Geeta et al., 1994; Dohanyos et al., 1997).

— The addition of inorganic elements, adsorbents or chelating agents seems to help through various ways, by: (1) increasing the density of bacterial flocs (Shimizu, 1992), (2) contributing to the formation of vital metal — containing enzymes (Geeta et al., 1990), (3) solubilising trace elements via combining a chelating agent with a metal (Gaddy, 1994), and (4) increasing stability via adsorption (Patel et al., 1992; Patel and Madamwar, 1994).