Co-digestion of municipal wastewater and microalgae biomass in an upflow anaerobic sludge blanket reactor

Upflow anaerobic sludge blanket reactors associated in series with photobioreactors offers an efficient and low cost solution for the wastewater treatment in small communities. However, there is little information about the performance of such anaerobic process fed with microalgae biomass from high rate algal ponds. This study aimed to evaluate the performance of a 3.78 m3 working volume pilot scale upflow anaerobic sludge blanket reactor during anaerobic co-digestion of municipal wastewater and microalgae biomass from high rate algal ponds (13.7 m3 each) during 365 days. To establish a basis for comparison, in the first research phase the anaerobic process was fed exclusively with municipal wastewater and, in the second phase, with a mixture of municipal wastewater and microalgae biomass. Similar organic and hydraulic loading rates were provided to the process in both research phases. The results showed a great removal of organic matter (COD) and suspended solids (TSS) in anaerobic process during the first phase (61% COD and 74% TSS) and second phase (63% COD and 74% TSS), the latter in anaerobic co-digestion. No sign of toxicity due to excess of ammoniacal, nitrogen or sulfur compounds was found. However, COD mass balance showed an increase of 54% on COD sludge retained and indicated that most of the microalgae biomass was not converted into methane. It was also observed a significant decrease in biogas production and a washout of microalgae cells to the upflow anaerobic sludge blanket reactor effluent in the second phase. The presence of intact microalgae cells retained in the sludge at all levels of this process also evidenced the incomplete anaerobic digestion of the microalgae biomass. These findings prove that an auxiliary pretreatment of the microalgae biomass is essential to improve its conversion into methane in the upflow anaerobic sludge blanket reactor. •No increase in biogas production was detected during anaerobic co-digestion of municipal wastewater and microalgae biomass.•Most of the organic matter from the microalgae biomass was concentrated in the anaerobic sludge.•There was no indication of toxicity due to excess ammoniacal nitrogen or sulfur.•Hydrolytic processes are mandatory to treat the harvested microalgae biomass.