Nitritation and denitritation in sludge liquor treatment

Sammanfattning: Enhanced biological nitrogen removal at municipal wastewater treatment plants is a common procedure to prevent eutrophication in water bodies. Increased load, stricter effluent regulations, and demands for reduced resource and energy consumption present new challenges for the wastewater treatment plants. One option when upgrading a wastewater treatment plant is to treat the sludge liquor produced when dewatering digested sludge separately. The aim of this thesis was to analyse and evaluate the operational aspects of two nitrite accumulated systems, nitritation and nitritation-denitritation, in a full-scale sequencing batch reactor at Sjölunda Wastewater Treatment Plant in Malmö, Sweden. Other aims were to quantify nitrogen oxides emissions in the off-gas from the SBR, as well as to evaluate the dynamics of these emissions. Finally, to get an overall view of the chosen processes and configuration, another part of the aim was to compare these to other methods for sludge liquor treatment in terms of operational, financial and environmental aspects. High ammonium removal and removal rates are possible in a nitritation system treating sludge liquor at a constant low pH (6.8) and at high nitrite concentrations (around 800 mg NO2--N/L), i.e. in conditions with high free nitrous acid concentrations. Conversely, denitritation was much more sensitive to free nitrous acid than nitritation, making it hard to operate the denitritation process only for alkalinity production. However, the chemical costs were higher for the nitritation system. Problems retaining nitrite accumulation were not observed in either of the systems. The nitrogen oxides emissions from the nitritation system were measured during normal operation. Emissions of nitrous oxide, nitric oxide and nitrogen dioxide were found to be 3.8%, 0.06% and 0.01% of the ammonium nitrogen load. A significant relationship was found between the length of the anoxic period and the nitrous oxide emissions. The nitric oxide emissions were dependent on the amount of ammonium oxidised. However, future studies have to reveal the responsible processes for these emissions.