AbstractA new design in a high rate nitrification system has been tested at the laboratory scale. Ammonia removal rates per unit of filter volume at influent ammonia levels of 0.02 mg NH3‐N were observed to be 5 to 10 times more rapid than rates observed in conventional field‐scale nitrification filters. At these levels 95 to 99% ammonia removal has been achieved with filter detention times as short as 3 minutes. The data are compared to data for various field units and the reasons for the observed high removal rates are discussed. The possibility of retaining these high efficiencies in scaled‐up filters is also disc