Effect of solids retention time on wastewater treatment and methane recovery by oxygenic photogranules

Abstract

Oxygenic photogranules consisting of bacteria and microalgae offer a feasible wastewater treatment technology owing to their good settleability and energy efficiency, especially through self-aeration. Anaerobic co-digestion of sewage sludge and grown photogranules is promising for more energy recovery, but the effect of solids retention time and inorganic carbon loads on photogranules for methane recovery is uncertain. Laboratory-scale photogranules and conventional activated sludge reactors were operated in a continuous mode under different solids retention times and inorganic carbon and nutrient loads. Anaerobic digestion of waste photogranules and waste activated sludge yielded approximately 0.3 NL/g-volatile-solids-added of biogas at 19–29 d of solids retention time of biological wastewater treatments. Biogas yield increased at 15 d of solids retention time, but wastewater treatment performance was deteriorated. The waste biomass generation rate relative to total organic carbon in the influent was 28 % for the activated sludge, 69 % for photogranules, and 80 % for photogranules with addition of inorganic carbon. Biological wastewater treatment by photogranules demonstrated potential to achieve a 2.9-fold increase in waste biomass yield compared to that of activated sludge with the addition of inorganic carbon. The present study is limited to stable laboratory conditions of synthetic wastewater and temperature but offers technical insights on developing photogranule systems with anaerobic digestion as efficient wastewater treatment and bioenergy recovery.