Cultivation of Microalgae in Municipal Wastewater and Conversion by Hydrothermal Carbonization: A Review

The idea of growing microalgae in wastewaters emerges from the idea of resource conservation and the recovery of nutrients. In fact, microalgae are able to take up nitrogen, phosphorus and carbon from wastewaters, even adsorb metals, and in many cases, can be co‐cultivated with various bacteria that are prevailing in municipal wastewater treatment plants. The cultivation of microalgae in municipal wastewater has been known for about half a century and investigated accordingly. Despite this long history, there are still many questions to answer before this technology will be ready for implementation in large‐scale projects. In this review, recent developments are presented. One crucial point in developing a viable process out of wastewater grown algae is the downstream processing of the accumulated algal biomass. The authors decided to focus on hydrothermal carbonization (HTC) as a processing strategy. HTC uses wet biomass and relatively mild process conditions to produce an energy‐rich biochar and a liquid fraction that can be further processed to higher‐value substances. The latest findings in the carbonization of microalgae are highlighted in the second part of this article. Energy conversion of microalgal biomass is a persistent hot topic in process engineering. Hydrothermal carbonization (HTC) is an interesting method as it easily converts wet biomass into energy‐rich HTC‐char and a promising liquid fraction. To save cost and space for biomass production, microalgae cultivation in municipal wastewaters is proposed – while wastewater treatment just happens along the way.