Figure 1: Biogas digester
Design of the digester
The digestion liquor (waste that is diluted) should be in the range of 5 to 12 percent of total solids (TS), and most favorable TS value of slurry for fermentation is 8%. A rule of thumb for diluting cattle waste is 2.5 parts water for every one part of relatively dry waste or one part water for every one part of fresh manure. The standard value of gas production yield from cow dung is considered as0.037m3/kg. The model plant is primarily designed for0.5 m3 of total gas production from cow dung. This means that about 13.5 kg of cow dung is needed. The TS content of cow dung is 17%. Therefore, amount of TS of 13.5 kg of cow dung is 13.5 × 0.17=2.3 kg. Now amount of slurry (fermentable materials plus water) is found from the ratio of amount of TS to the TS of slurry. If the fermentation materials entering the digester are calculated by 8% of TS concentration, the quantity of input mixed materials of digester is 2.3/0.08=28.75 kg. That means 28.75-13.5 = 15.25 kg of water is to be added. This amount of water is consistent to that described in the thumb rule for the dilution of cattle waste.