Influence of inoculum to substrate ratios (ISRs) on the performance of anaerobic digestion of algal residues

With increasing concerns of microalgal-biodiesel, algal residues after lipid extraction are raising great attention for energy production. A batch test of 15 days under mesophilic condition was conducted to evaluate the effects of inoculum to substrate ratios (ISRs) on the methane production by anaerobic digestion of Chlorella sp. residue. The stability and progress of the reaction from algal residue to methane were monitored by measuring the pH, volatile fatty acids (VFAs), total ammoniacal nitrogen (TAN), methane volume on a daily basis. The results indicated that the values obtained were 26.6, 191.6, 195.6 and 210.6 ml CH₄/g volatile solid (VS) for ISRs of 1:2, 1:1, 2:1 and 3:1. The methane production was significantly decreased as the ISR was lower than 1:1, which was resulting from the poor methanogenesis inhibited by NH₄ ⁺-N. It would be of great importance that determination of ISRs might provide useful information on how to initialize a batch digester with algal residue as material.