Recent reports on domestic wastewater treatment using microalgae cultivation: Towards a circular economy

Abstract

The conventional wastewater treatment process (WWTP) is under continuous improvement, mainly to reduce the final pollutant contents in the effluent. The circular economy concept is driving this industry towards the recovery of the carbon (C) and nutrient (nitrogen – N and phosphorus – P) loads as valuable products, also to answer issues related to their depletion in nature. In this context, microalgae cultivation on wastewater has shown great promise in both water purification and the recovery of nutrients through additional biomass production. The further direction of the algal biomass to anaerobic co-digestion with wastewater sludge promises biogas yield improvements directing more of the carbon to energy recovery. Recent 100+ publications are here reviewed with the objective of compiling and comparing data on the cultivation of microalgae on domestic wastewater drawn from different stages of the conventional WWTP. A wide range of reactor types and scales, microalgae and bacteria inocula, and operational conditions are included, focusing on carbon and nutrient removal performance and the extent of their recovery in the produced biomass. However, most studies do not provide enough data tracking N, P, and C contents to allow the determination of their distribution among the possible system outputs. Still, some studies quantify the importance of removal mechanisms such as volatilization and precipitation, as well as microalgae uptake. Tentative mass balances on the three keys elements are presented, using the data disclosed in these few studies, highlighting the usefulness of these calculations and the need for their inclusion in future studies.