Upgrading Sewage Treatment Plants for Higher Performance by Optimising Reject Water Treatment

Abstract

Separate treatment of the nitrogen rich reject water improves the COD/N ratio in the mainstream activated sludge process and consequently improves the efficiency of the biological nitrogen removal in sewage treatment plants. The composition of the reject water is suitable for the use of new biological processes like nitritation/denitritation. Primary sludge can be successfully used as the carbon source for the denitritation, which can be complete and controlled by the primary sludge / reject water ratio. Treating the reject water with the extremely low CODsoluble/N-NH 4 + ratio of 0.64, without an external carbon addition (considering the primary sludge as an internal source), at the volumetric loading rate up to 0.32 kgN/m3, the average nitrogen removal efficiency above 70% was achieved.