Comparaison with existing systems

The current process of biomethanisation, which uses feedstocks like: cattle dung, human faeces, distillery effluents etc is highly inefficient because the nutritionally available calories and nutritive value of these substances is quite low, as the calories present in the food have already been used by the cattle, humans or fermentation processes.

Common sense says that the energy output of a system must matched with the input.

Methane has a calorific value of 11000 kcal/kg. To obtain such a high energy output, the methanisation process should receive a correspondingly high calorific input.

Nowadays, municipal solid waste (MSW) is also being used as a source of methane. Food rests in the MSW have a relatively high caloric content, but in the current process, called the two phase fermentation system, the waste is first subject to an aerobic fermentation in order to reduce its bulk, and only then, having very few calories left in it, being fed to the anaerobic methane digesters.

Such traditional biogas production systems operating on human or animal faeces or with the two-phase fermentation system produce approximatively 10 kg of methane per tonne of feedstock in a process completed in 40 days. The duration can be shortened using thermophilic bacterias but the input to output ratio remains unchanged.

Further, the use of cattle dung as the feedstock for methanisation is a main factor limiting the widespread use of domestic methane plants in India for the supply for household fuel: -the present domestic biogas plant requires 40kg cattle dung per day, ( from 6 to 8 heads of cattle) -because the dung has a retention time of 40 days, the size of the plant is large -the servicing of the plant requires the daily mixing of the dung with water to make it feedstock, filling it into the biogas plant and the disposal of about 80 to 100 liters of effluent slurry, which is a source of complain from the users. Restrictions of space, money, insufficient animals to feed the plant and maintenance problems stop many farmers and households from this technology.