Anaerobic bacteria can degrade a variety of organic compounds (carbohydrates, proteins, lipids, etc.). The methane content of the biogas mixture depends on the oxidative state of carbon in the compounds present in the feedstock; the more reduced the carbon is, the higher the content of biogas in methane is (Gujer and Zehnder, 1983). The feedstock should also be balanced with respect to the ratio of carbon and nitrogen (C:N = 20:30), since the microorganisms use carbon and nitrogen at this ratio range. Quite often, organic feedstocks contain these nutrients at lower or higher ratios. In such cases, the codigestion of selected feedstocks can adjust the required balance (‘diet’) and enhance biogas production, e. g. codigestion of sewage sludge with agricultural wastes or municipal solid wastes (Alatriste — Mondragon et al., 2006) as well as cattle manure with municipal solid wastes (Hartmann and Ahring, 2005). Apart from C and N, other elements present at trace concentrations are also crucial to the growth of anaerobic microorganisms. For example, Ni (involved in the synthesis of coenzyme F430), Fe (constituent of electron carriers), Mg (stabilising the cellular membranes), Ca (stabilising the cellular wall and contributing to the thermal stability of the endospores), Co (component of the vitamin B12), Zn (constituent of several enzymes). etc. If these trace elements are not contained in the feedstock, they should be supplied since their absence is correlated with decrease in efficiency (Zandvoort et al., 2006).