The performance and economics of four recently constructed or modified BNR municipal wastewater treatment plants located in the Chesapeake Bay Watershed, USA were evaluated, and compared to a treatment plant implementing chemical phosphorus removal and complete nitrification. Phosphorus removal has been very reliable to effluent concentrations below 0.5 mg/L without chemical addition or effluent filtration at BNR plants that have been operating for more than two years. Significant variation was observed in the wastewater characteristics, and this has affected biological phosphorus removal. Chemical precipitation effluent TP concentrations have averaged less than 0.1 mg/L. The small BNR plant was clearly the most costly to operate per 1000 m(3)/d of flow, which illustrates economy of scale. The chemical precipitation plant was generally more expensive to operate than the large BNR plants.