Coupling enhanced biological phosphorus removal (EBPR) and magnesium ammonium phosphate (MAP) through ozonated sludge for nutrient recovery

Sludge ozonation was investigated in the laboratory as a means of nutrient recovery in the context of an activated sludge (AS) wastewater treatment plant (WWTP) performing enhanced biological phosphorus removal (EBPR). Experiments for different ozone doses were carried out and the resulting supernatants were fed as carbon source to induce anaerobic phosphorus release (APR) in batch tests. Volatile fatty acid (VFA) exhaustion -rather than production- and nitrate accumulation were observed with increasing dose when ozonating continuously, and deemed as the cause for the low APR kinetic rate observed when using ozonated supernatant as substrate: 78±6 and 100±8 mgP/gCODPAO/d for ozonation doses of 86±7 and 118±9 mgO3/gVSS respectively. In contrast, no significant amounts of nitrate accumulated when ozonating intermittently in 60-minute steps, up to 6 hours. Calculations based on analysis of experimental samples showed struvite (magnesium ammonium phosphate or MAP) as the solid phase closest to saturation, although precipitation would require mixing the anaerobic reactor's effluent supernatant with an ammonium-rich stream and/or chemical addition of magnesium at pH around 9.3