Integrated Application of Struvite Precipitation and Biological Treatment in Treating Autothermal Thermophilic Aerobic Digestion Supernatant Liquid

This work dealt with the integrated application of struvite precipitation and biological treatment of the supernatant liquor after autothermal thermophilic aerobic digestion (ATAD). To the best of our knowledge, no feasibility study of NH4-N and PO4-P removal from ATAD supernatant by struvite precipitation has been performed before. Furthermore, it was worthwhile and interesting to evaluate the biological treatability of combined wastewater, in which the struvite supernatant was mixed with sewage wastewater. Batch experiments were conducted to examine the effects of pH, dosage of magnesium and orthophosphate, and the addition of preformed struvite deposits as a seeding material on the removal of NH4-N and PO4-P from the ATAD supernatant of sewage sludge. The struvite precipitation experiments showed that pH was an important factor in the simultaneous removal of NH4-N and PO4-P. It was noticeable that the high concentration of heavy metals in ATAD supernatant liquid could significantly inhibit NH4-N removal during struvite precipitation. An overdose of orthophosphate outstandingly enhanced NH4-N removal, but decreased PO4-P removal. When the preformed struvite was added (5 g/L) as a seeding material, the settled sludge volume decreased to less than half. It was revealed that the optimal volume ratio of the struvite supernatant to sewage wastewater should be up to 1:100 maximum (the average influent NH4-N loading rate of 0.135 kg NH4-N/m(3) per day) to prevent inhibition of nitrification when the combined wastewater is applied to biological systems.