Preferable Operational Range of External Selector to Attain Partial Nitritation in the Main Aeration Tank

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To attain mainstream nitritation in the aeration tank receiving low-strength wastewater, an external selector to reduce nitrite-oxidising organism biomass was investigated. In the selector, the highly thickened microorganisms transferred from the aeration tank via a mechanical solid-liquid separation were exposed to a poisonous compound and returned to the aeration tank, where a part of the nitrifiers were killed. Using a mixture with 1,000 mg-N L⁻¹ of nitrite and 1,000 mg-N L⁻¹ of ammonium, about 60% of nitritation based on the effluent nitrogen was successfully maintained in the lab-scale system for more than 150 days without significant accumulation of nitrate. The poisonous compound also led to the decay of ammonium-oxidising organism to a certain extent. However, by optimising recycle rate to the selector, solids retention time of the selector, and excess sludge withdrawal rate, selective wash-out of nitrite-oxidising organism was possible. From the experimental results, the preferable operational range of the selector was explored in contour plots using the modified IWA-ASM1 model. The model indicated the preferable external recycle rate and solids retention time of the selector were about 0.2–0.4 d⁻¹ and 1–2 d, respectively, when the aeration tank was operated with excess sludge withdrawal of 0.1–0.17 d⁻¹.