Towards a Sustainable P Future: Assessing Small WWTP As an Overlooked P Recovery Source.

While human demand for phosphorus (P) continues to increase, economically available P₂O₅ may be exhausted within this century. Municipal wastewater treatment plants (WWTPs) may be an attractive source of renewable P. P recovery from small WWTPs may provide a useful renewable P fertilizer source; however, little research has been conducted on these aerobic systems. Small WWTPs with aerobic digestion of solids were studied as a potential renewable source of P. Struvite recovery from four small WWTP in north Florida with treatment capacities from 371 to 2,650 m³/d and incoming P loads from 2 to 14 kg/d was investigated. A chemical equilibrium model was developed with Visual MINTEQ Version 3.0 to predict the feasibility of struvite production from aerobically digested sludge filtrate. The model results confirm the potential for aerobic digesters at small WWTPs to form struvite with the use of minor pH modifications. Struvite is forecast to be the dominant mineral solid, with P recovery efficiencies from 27–60%. Struvite was formed from aerobically digested filtrate by adjusting the pH of wastewater filtrate from an average of 7.0 to 8.5 with a base (NaOH) and air sparging. Struvite formation in the air sparged experiments showed a 2–14% increase in yield over the wastewater filtrate in the NaOH experiment. The XRD analysis showed that no other phase was present and the struvite formed was pure. This paper estimates that the four small WWTPs in the study can produce 51–4,724 kg of P/year. The researchers also calculated that the combined small WWTPs in the United States represent 40% of the discharged treated wastewater and are capable of producing 1.2 million MT of P₂O₅ equivalents per year. The recovery of P from small WWTP could meet 4% of the demand for P in the United States.