Oyster shell waste as potential co-substrate for enhancing methanogenesis of starch wastewater at low inoculation ratio

This study evaluated the effect of oyster shells on the methanogenesis of starch wastewater subjected to over-acidification (pH < 4.5) at low inoculum/substrate ratios, and revealed that oyster shells could be used as co-substrates for methane production. The methane yield was improved by approximate 86-folds with optimal dose when compared with that in control. Oyster shells conditioning synchronously improved the acidogenesis and hydrogenotrophic methanogenesis steps, resulting in high methane production. These improvements were attributed to the fact that the oyster shells served as the neutralizing reagent and buffered the sharp pH drop. Carbon dioxide was also released during this process, which was subsequently converted into methane and contributed 17% of the total methane yield. Furthermore, some spheroid and rod microcolonies were observed on the surfaces of the oyster shells. Along with the remarkable enrichment of acetotrophic and methylotrophic methanogens, these microbes benefitted the successful methanogenesis of starch wastewater.