At Sjölunda WWTP, a full-scale SBR for treatment of mesophilic sludge digester liquor has been operated almost a year with stable nitrite accumulation. Only nitritation of the sludge liquor is needed since the oxidized ammonium is denitrified in the first anoxic zone of the high-loaded activated sludge in the main plant. The process strategy was to have an ammonium set-point to end the aeration, a low DO concentration and a low pH set-point. An increase of pH set-point from 6.8 to 7.2 increased loss of alkalinity in the effluent and increased sodium hydroxide dosing. An increase of DO set-point from 1.1 mg O2 L−1 to 1.3 mg O2 L−1 markedly increased ammonia reduction rates and only slightly increased nitrate production. Today, an introduction of denitritation in the SBR will be a more cost-effective treatment of sludge liquor at Sjölunda WWTP. However, the choice of operation with only nitritation or nitritation/denitritation in sludge liquor treatment should always include a consideration of chemical costs and treatment capacity of both main plant and side-stream plant.

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