Disinhibition of the ammonium nitrogen in autothermal thermophilic aerobic digestion for sewage sludge by chemical precipitation

Magnesium ammonium phosphate (MAP) precipitation was introduced to remove ammonium nitrogen (NH4+-N) in autothermal thermophilic aerobic digestion (ATAD) in this study by addition of MgCl2·6H2O and NaH2PO4·2H2O. The results showed that the lowest NH4+-N concentration was found in the D2 digester after 2nd day dosing treatment and 38.12% of VS removal efficiency was obtained after 15 days ATAD treatment. Sludge stabilization was achieved in the D2 digester 6 days earlier than the non-dosing digester when 8.7 g/L MgCl2·6H2O and 6.7 g/L NaH2PO4·2H2O were added into the digester. Furthermore, the highest VS removal efficiency of 40.03% was observed after 21 days digestion in D2 digesters. Therefore, MAP precipitation was an effective method for the ammonium nitrogen disinhibition when 8.7 g/L MgCl2·6H2O and 6.7 g/L NaH2PO4·2H2O were added into on the 2nd day after the digester startup.