2.3. Supply of dung and gas production

Biogas, consisting of 60% methane and 40% carbon dioxide, is a high-grade source of energy. However, the calorific value is lower in comparison to bottled gas. One m³ of biogas only replaces about 0.4 kg bottled gas. One kilogram of cattle dung produces about 40 litres of biogas and one kilogram of pig dung about 60 litres of biogas per day. To produce 1 m³ biogas, 25 kg cattle dung or 17 kg pig dung are necessary. The amount of dung occurring is approx. proportional to the live weight of the animals. Random measurements of the amount of dung occurring per animal are essential to define the gas potential on the farm since the biomass produced per animal varies greatly from region to region. A zebu cow in Orissa/India provides e.g. about 5 kg dung per day; a Javanese zebu cow crossed with a Friesian for high milk output, in contrast, produces 15 kg per day. The amount of dung must be known to allow the energy potential occurring daily in the housing to be estimated. Where animals are put out to pasture during the day and only housed at night, experience shows that only about half of the total amount of dung occurs in the housing and thus is available for the biogas plant. Livestock systems, where animals are only out to pasture, cannot be considered for biogas plants as the collecting of the dung dropped is too labour-intensive. This would also mean depriving the pasture of the dung.

A biogas programme can only be considered for pig farming if the animals are kept in sties with concrete floors. It is more difficult to determine the quantity of dung than in cow sheds. The changing numbers also make rapid estimation of the substrate quantities more difficult. However, the number of brood sows often provides the basis for an adequate estimation.. In this respect however it is important to know whether piglets or pigs for slaughter are being reared. Pig breeders often normally know what quantities of dung occur daily or annually. As a rough idea, it can be assumed that each pig with a live weight of over 50 kg produces about 2.5 kg dung. The piglets are then neglected when counting, or they are counted in proportion to their weight. Established values in literature which mostly comes from industrialised nations only rarely apply to developing countries.