Struvite phosphorus recovery from aerobically digested municipal wastewater

Abstract

Small, municipal wastewater treatment plants (WWTPs) that use aerobic digestion treat approximately 40% of the discharged wastewater in the USA, and yet they are an overlooked source of recoverable P. There are no known reports of small, aerobicWWTPs recovering P through struvite (MgNH4PO4·6H2O) precipitation for repurposing as amineral fertilizer, even though some largeWWTPs with anaerobic digestion are. Four smallWWTPs in north Florida, USA, with treatment capacities from 371 to 2650 m3 wastewater d-1 and incoming P loads from 2 to 14 kg d-1 were investigated for their potential to produce struvite from digester filtrates. A chemical equilibrium model was used to predict the feasibility of struvite production and the results compared with actualWWTP filtrate measurements. Filtrates from aerobic digesters were able to form struvite if solution pH was increased by ≤1 pH unit. Depending on the WWTP, P recovery in filtrates through struvite precipitation ranged from 27-57% by mass at pH 8.5, via NaOH additions or air sparging. Increasing filtrateMg concentrations improved P recovery up to 97%. Based upon these results, smallWWTPs using aerobic digestion will be able to lower their P waste output through recovery as struvite fertilizer.