Substantial energy is reserved in waste activated sludge (WAS) organics but much of it is difficult to recover because the solid organics require long time to solubilize. In this work we introduced the new approach of recovering WAS organics into the biomass of phagotrophic algae. Phagotrophic algae have the unique ability to grow by ingesting insoluble organic particles including microbial cells. This phagotrophic ability renders the solubilization of WAS organics unnecessary and makes this approach remarkably fast. The approach consists of two stages: a short anaerobic digestion treatment followed by the algal growth on treated WAS. The short anaerobic digestion was exploited to release discrete bacteria from WAS flocs. Phagotrophic algae could then grow rapidly with the released bacteria as well as the solubilized nutrients in the treated WAS. The results showed that WAS organics could be quickly consumed by phagotrophic algae. Among all studied conditions the highest WAS volatile solids (VS) reduction was achieved with 72 h anaerobic digestion and 24 h algal growth. In this optimal process, 28% of WAS VS was reduced, and 41% and 20% of the reduced VS were converted into algal biomass and lipids, respectively. In comparison, only 18% WAS VS were reduced after the same time of aerobic digestion without algae addition. Through this approach, the amount of WAS organics requiring further treatment for final disposal is significantly reduced. With the production of significant amounts of algal biomass and lipids, WAS treatment is expected to be more economical and sustainable in material recycling.
Keywords: Lipid production; Phagotrophic algae; Waste activated sludge treatment; Waste recovery.
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