One autotrophic and two heterotrophic chemostats were operated during the study period. The autotrophic unit was operated using an inorganic synthetic ammonium medium to provide a culture of nitrifying organism and to study cell yield. The two heterotrophic units were operated with a feed containing 50 and 100 mg/l COD respectively along with the inorganic ammonium medium to determine the effects of heterotrophic activity on the nitrification process. The parameters monitored included mixed liquor suspended solids, suspended solids concentration in the effluent and influent; effluent ammonia and nitrate nitrogen concentration. The culture from the autotrophic unit was used as inoculum of nitrifying organisms for the other phases of the study. These phases of the study included determination of the effect of ammonium concentration, temperature and pH on nitrification. Temperature was found to be the main controlling factor for nitrification. An equation for the sludge age required at operating temperature was developed and a graph was prepared to determine the MLSS required, for a given influent BOD and the determined sludge age, that will assure nitrification on yearly basis. Changes in the pH within the range of 7 to 9 were found to have no significant influence on the nitrification rate. The nitrification rate decreased at pH outside this range. It was found that the presence of heterotrophic activity had no effect on the nitrification. In fact the presence of heterotrophic biomass was found desirable because of the improvement in the effluent quality. / Master of Science