Effect of substrates and granules/inocula sizes to biogas production in anaerobic batch digestion

Abstract

Biogas is a clean, renewable energy produced by anaerobic digestion. This study aimed to evaluate the bio-methane potential of different types of agro-industrial wastewaters and sizes of granules or inocula for biogas production. The granules (CS: from a cassava factory, SS: from a sea food factory, and PS: from a palm oil mill) and wastewaters were collected from three agro-industrial factories: a cassava factory, a seafood factory and a palm oil mill (initial CODs were 18,800, 4,200 and 100,000 mg/L, respectively). Batch systems used for anaerobic digestion were operated at ambient temperature (28-30°C) for 45 days. Modified Gompertz equation was used to compare the data obtained from the experiments. The results showed that wastewater from a cassava factory was the most suitable substrate and gave the highest BMP when used with only granules from its own source (CS) which had the size range of 1.5-1.7 mm. Among three substrate sources, wastewater from seafood factory had the highest nitrogen content (or lowest COD:N ratio) thus, represented the most imbalance nutrient source. In this case, mix-granules (SS+CS) gave higher BMP than that of mix-granules (SS+PS) and the seed from its own source (SS). Palm oil mill effluent did not match COD:N:P ratio criterion for good substrate in anaerobic digestion and had too high COD level which caused substrate inhibition. In this case, the mix-granules (PS+CS) gave highest BMP. In general, larger granule size and the nutrient balance could improve the efficiency of anaerobic digestion and hence increase the biogas production rate. These were indicated by the kinetic parameters such as the maximum methane production rate (Rm, mL/d) and the methane production potential (P, mL).