Mixotrophic algal system for centrate treatment and resource recovery

In traditional activated sludge-based wastewater treatment plants equipped with anaerobic digesters, the nutrient-rich centrate from the digester is typically recirculated to the headworks of the plant exerting a parasitic load. As an alternative, mixotrophic algal cultivation in a blend of the centrate and primary effluent is proposed to yield energy- and nutrient-rich biomass from which, liquid biofuel and fertilizers could be recovered. Core components in the proposed sewage treatment and resource recovery (STaRR) system include i) algal sewage treatment/biomass cultivation; ii) hydrothermal liquefaction of algal biomass for energy recovery, and iii) recovery of struvite and concentrated ammonia from the hydrothermal liquefaction byproducts. Results from a 700 l pilot-scale STaRR system run with four different blends of centrate and primary effluent are presented to validate the proposed approach. A 20: 80 centrate-to-primary effluent blend enabled discharge standards for ammoniacal‑nitrogen, phosphate phosphorus, and biochemical oxygen demand to be met in <7 days; volumetric removal rates at this blending ratio were 4.0 mg/l-d, 0.70 mg/l-d and, 10.50 mg/l-d, respectively. Hydrothermal liquefaction of the resulting biomass (20% solids content, 350 °C, and 60 min holding time) yielded 77.5 g light bio-crude oil and 108.0 g heavy bio-crude oil per kg of dry algae. It is estimated that the STaRR system can produce 6.75 kg of struvite and 3.42 kg of concentrated ammoniacal nitrogen per 100 m³ of blended wastewater.