Full-scale sludge liquor treatment for ammonium reduction with low carbon dosage

The separate treatment of sludge liquor, produced by dewatering anaerobic digested sludge at wastewater treatment plants, gives rise to extreme environments for nitrogen removal. A full-scale sequencing batch reactor was operated with the aim of introducing and studying denitritation as a supplement to nitritation in order to reduce operating costs. Since the main plant only has problems with ammonium reduction capacity, the initial strategy was to have sufficient ammonium reduction with optimal alkalinity production by denitrifiers, i.e. low carbon dosage and minimum alkalinity and residual oxidized ammonium in the effluent. This strategy led to an unbalanced and sensitive process because the denitrifiers were often inhibited. High dissolved oxygen (DO) readings and no decrease in oxidation-reduction potential (ORP) during anoxic phases with simultaneous ethanol dosage indicated inhibition of denitrifiers, probably by the intermediate product NO, which may have interfered with the DO sensor. Nitric oxide production was believed to be favoured in the beginning of the anoxic phase as a result of low pH and high nitrite concentration. A stable nitritation-denitritation process could be achieved when the aerobic hydraulic retention time (HRT) was decreased to the same length as the anoxic HRT, which resulted in increased unused alkalinity.