Performance and mechanism of urea hydrolysis in partial nitritation system based on SBR

Urea hydrolysis in partial nitritation process forming nitrite and ammonia is advantageous to subsequent treatment with ANAMMOX for total nitrogen removal. In this study, stable partial nitritation for urea wastewater with urea increasing from 250 to 2000 mg L^-1 were achieved in an aerobic SBR. Urea removal efficiency and nitrite accumulation percentage both kept above 98%, with nitrite production rate about 0.985 kg N·m^-3·d^-1. Urea hydrolysis mechanism in this aerobic system was described as, (1) massive urea in the bulk was absorbed into cell, (2) urea was hydrolyzed by intracellular urease inside cell, (3) produced ammonia then slowly diffused into the bulk through membrane, which is later converted by ammonia-oxidizing bacteria (AOB) into nitrite. Due to this mechanism, the activity of AOB could not be inhibited by high FA (free ammonia) value under high urea concentration condition while nitrite-oxidizing bacteria (NOB) remained to be inhibited. An uncultured genus belonging to poorly characterized phylum Gemmatimonadetes was found enriched in this process and became dominant genus. This genus was speculated to have same energy pathway like ureaplasma, by absorbing excessive urea from environment and utilize urea hydrolysis to generate energy. So it was believed to be responsible for urea hydrolysis mechanism mentioned above. This SBR showed stable partial nitritation and high urea removal efficiency for treating urea wastewater, which was obviously feasible as the pretreatment process for subsequent ANAMMOX.