Study of P recovery from urban WWTP sludges by organic phosphorus extraction through enzymatic hydrolysis with phytases

An important part of the organic P present in wastewater is associated with the phytic acid molecule. This paper experimentally evaluated the ability of hydrolytic enzymes, specifically phytases, to degrade organic phosphorus-containing compounds to release inorganic phosphorus in the mixed sludge from a WWTP (wastewater treatment plant). Results indicated that a dose of 25 g enzyme·kg −1 sludge (w.w.) showed the highest phosphate release (46 mg P·L −1 ). Increasing the enzyme dose from this value did not significantly improve the net release of phosphate. These results served as a reference for exploring, by simulation, the implementation of different scenarios for P extraction and recovery combining two elutriation modes (gravity and dynamic thickener) with and without enzyme addition in the Calahorra WWTP (Spain). The combination of enzyme addition to the mixed sludge with further mixed sludge elutriation in a gravity thickener showed the highest P recovery as struvite (15.3 kg P·d −1 ). However, the high cost of the enzyme remains a limiting factor for its implementation. Nonetheless, the elutriation in a gravity thickener without enzyme addition also showed a high P recovery as struvite (12.1 kg P·d −1 , which represents 16% of P entering the WWTP) despite not showing the best economic ratios (a revenue of 0.022 €·m −3 of treated water).