Sludge liquor from an anaerobic sludge digester with an average N–NH4 concentration of γ = 1185 mg L–1 was treated in a pilot-scale SBR (sequencing batch reactor) system. The returned activated sludge of a WWTP was used as inoculum. The average efficiency of N–NH4 removal was over η = 90 %. Concentrations of N–NH4 in theeffluent were typically below 10 mg L–1. The maximal achieved nitrification rate was rN = 9.1 mg g–1 h–1 (relative to MLVSS). Wastewater of methyl ester wash arising during biodiesel production was used as an external carbon source for denitrification. A dosage of 3.5 – 4.5 g of COD per 1 g of nitrogen available for denitrification was found optimal. Typical effluent N–NO3 concentration was about γ = 25 mg L–1 and maximal achieved denitrification rate was rD = 14.5 mg g–1 h–1. Operation of the SBR was stable at a HRT of τ = 4 – 5 days.
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