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.