Optimization of biomethane production from anaerobic Co-digestion of microalgae and septic tank sludge

The objective of this study was to analyze the effects of the total organic concertation and the proportion of microalgae to optimize biomethane production during anaerobic co-digestion of microalgae and septic tank sludge. To achieve this goal, a statistical tool, central composite design, was applied to design the co-digestion experiments. Using multiple variance analysis, the experimental data were analyzed, and the results were presented by the response surface methodology, which illustrated that peak methane production could be achieved when the total volatile solids concentration was in the range of 16–20 g/L and the proportion of microalgal biomass ranged from 26 to 47%. An inhibitory effect was observed from volatile fatty acids accumulation when the total volatile solids concentration was supplied as high as 35 g/L. Organic nitrogen mineralization took place in all groups, ranging from 23 to 62%. There was no direct evidence showing that produced ammonium ion nitrogen or the free ammonia nitrogen had inhibitive effects on methanogenic activity.