Recovery of struvite from hydrothermally processed algal biomass cultivated in urban wastewaters

Abstract A mixotrophic algal wastewater treatment system capable of reducing biochemical oxygen demand (BOD), ammoniacal nitrogen (NH4+-N), and phosphate (PO43−-P) in primary effluent in a single step has been demonstrated previously. Downstream processing of the resulting algal biomass by hydrothermal liquefaction (HTL) to recover its energy-content as biocrude oil has also been demonstrated. The current study evaluated the feasibility of recovering struvite from the two byproducts of HTL– a solid residue, rich in PO43–P; and an aqueous phase, rich in NH4+–N, for use as fertilizer. Results of this feasibility study and a preliminary mass balance model show that the proposed pathway can potentially yield ~2.5 kg struvite per 100 m3 of wastewater treated, recovering nearly 73% of PO43–P and 6% of NH4+–N in the primary effluent. Chemical and microscopic analysis of the wastewater-derived struvite revealed its composition to be close to that of pure struvite, with more than 12% P-content by weight. Scanning electron microscopy images depicted uniformly distributed sizes of irregular crystals in the wastewater-derived struvite. Low calcium content of this struvite indicated minimum amorphous calcium phosphate (ACP) impurities. Trace metal analysis showed that heavy metal levels in the recovered struvite were below current guidelines. This study confirmed the feasibility of recovering high-quality struvite from wastewaters, warranting further process optimization and characterization of the recovered struvite.