Synergistic co-digestion of wastewater grown algae-bacteria polyculture biomass and cellulose to optimize carbon-to-nitrogen ratio and application of kinetic models to predict anaerobic digestion energy balance

This study investigated enhancing methane production from algal-bacteria biomass by adjusting the C/N ratio through co-digestion with a nitrogen-poor co-substrate - cellulose. A biomethane potential test was used to determine cumulative biogas and methane production for pure and co-digested substrates. Four kinetic models were evaluated for their accuracy describing experimental data. These models were used to estimate the total energy output and net energy ratio (NER) for a scaled AD system. Increasing the algal C/N ratio from 5.7 to 20-30 (optimal algae: cellulose feedstock ratios of 35%:65% and 20%:80%) improved the ultimate methane yield by > 10% and the first ten days production by > 100%. The modified Gompertz kinetic model demonstrated highest accuracy, predicting that co-digestion improved methane production by reducing the time-lag by similar to 50% and increasing rate by similar to 35%. The synergistic effects increase the AD system energy efficiency and NER by 30-45%, suggesting potential for substantial enhancements from co-digestion at scale.