Effect of substrate dilution ratios (SDRs) on biogas and bioslurry production from anaerobic digestion of pig faeces under field conditions

Anaerobic digestion is considered the most efficient method for production of renewable energy (biogas) from different categories of biomass such as excreta of livestock including piggery. However, biogas production from pig faeces is very low and inefficient. Therefore, the present study was conducted to maximise biogas production and to explore the bioslurry (digestate) utilisation as a natural fertiliser by investigating the potential of different substrate dilution ratios (SDRs) (hence different organic loading rates, gVS/L) of pig faeces: water, i.e. 1:8 (9.29, T-0), 1:6 (12.36, T-1), 1:4 (18.50, T-2), and 1:2 (37.17, T-3) under field conditions for its easy out scaling. Different parameters, i.e. quantity, quality, VS reduction, hydraulic retention time (HRT), calorific value, manurial value and germination potential of digestate, were studied. The current investigation revealed that biogas production was increased by decreasing the SDR, and the highest (P < 0.01) biogas production (0.05 m(3)/kg fresh faeces) as well as the steepest VS reduction (21%) was achieved by 1:2 SDR as compared to others. The results of the digestate germination trial showed that relative seed germination in the crude slurry was significantly greater (P < 0.01) with increased SDR. From the results, it is recommended that SDR of 1:2 (TS nearly 7.4%) may be used to produce 1 m(3) of biogas from 20 kg of pig faeces (TS nearly 22%) for enhanced biogas production, and digestate obtained as a by-product can enhance the soil fertility in diluted form.