The sample pH was measured with a digital pH metre (HANNA, HI 98204). The carbon and nitrogen content of the poultry litter, poultry droppings and cow dung were determined with a C–H–N elemental analyser. Total solids (TS) were determined by incubating a sample at 104 °C until no further weight change was evident, and volatile solids (VS) were measured by the loss on ignition of the dried sample at 550 °C. The composition of the gas was measured using an Orsat gas analyser. COD was determined by a chemical method, and the calorific value was determined in a bomb calorimeter. The total gas production was measured via the water displacement method at an interval of 24 h. The pH, VS reduction and COD reduction were measured every 10 days throughout the experiment. Each experiment was conducted at a temperature of 32 ± 3° C for 50 days.
3. Results and discussion
Various parameters of the raw materials are shown in Table 1. Table 1 shows that the C/N ratio of poultry litter is 7.5, which is quite low for optimum biogas generation [8], and it can be increased by mixing with cow dung because cow dung has a high C/N ratio. When a substrate with a low C/N ratio is mixed with a substrate that has a high C/N ratio, better performance is achieved [7]. The pH of the poultry litter is 8.5. The buffering capacity of cow dung is very good, and it also acts as the source of various microorganisms required for anaerobic digestion [9].