PNNL

Abstract

Anaerobic digestion of feedstocks with imbalanced C/N ratios leads to reduced conversion rates and methane yield. The current study investigated enhancing methane production and, therefore, energy output and efficiency, from digesting wastewater grown algal-bacteria polyculture biomass by adjusting the C/N ratio through co-digestion with a nitrogen-poor material – cellulose. Four kinetic models, including 1st-order, pseudo-parallel 1st-order, modified Gompertz, and transference function equations, were evaluated for their ability to describe the biogas and methane production from mono-digestion and co-digestion of algal and cellulose biomass. Our data show that increasing the algal biomass C/N ratio from its original value of 5.7 to optimal values of 20-30 by co-digesting with cellulosic biomass (optimal algae to cellulose ratios of 35%:65% and 20%:80%) improves the ultimate methane production by more than 10% compared to mono-digestion. Importantly, the synergistic effects from co-digestion were most significant during the initial phase of fermentation with the cumulative methane yield for the first ten days increasing up to 100%. Further, the modified Gompertz kinetic model most accurately predicted that co-digestion improves the process by shortening the time-lag for methane generation by nearly 50% and by increasing the methane production rate by nearly 35%. The synergy from co-digestion led to boosting the total energy output (combined heat and electricity) and net energy ratio by 30-40% and 20%, respectively. Implementation of a co-digestion approach allows for downsizing volumes for new anaerobic digesters or enhancing methane production in existing digesters.