A 2-year pilot study was conducted by the City of Malmo, Sweden, to determine the maximum capacity of existing trickling filters when converted from carbonaceous duty to nitrification duty. Operating variables examined included a comparison of alternating two-stage to single-stage operation, flushing intensity, and predator control techniques. Distributor speed control had only a small effect on nitrification efficiency, and motorized distributors are not required in this application. Two-stage operation in an alternating mode provided for higher nitrification rates and lower effluent ammonia values than in single-stage operation. A nitrification model was used to analyze reaction rate data. The analysis showed that alternating two-stage operation mitigated the suppressing effect of influent suspended solids, resulting in enhanced nitrification rates. The filter macrofauna were dominated by worms rather than filter flies, so that regular filter flooding did not enhance nitrification rates.