This study investigates the effects of phosphorus supplementation on the formation potential of total trihalomethanes (TTHMfp) and five species of haloacetic acids (HAA5fp) during exposure to clearwell disinfection contact times. In addition, the study investigates the effects of phosphorus supplementation on the dissolved oxygen, organic carbon and nitrogen removal along with biofilm coverage of the filter media and biomass viability of the attached biofilm. The uptake of total phosphorus in the P enhanced filter did not correspond to the consumption of readily assimilated nitrogen or the consumption of soluble carbon. As such, the dissolved organic carbon reduction in the biologically active filters was shown to not be phosphorus nutrient limited. The clearwell TTHMfp was shown to be reduced in all filters across all measured biological filtration times in the control and P enhanced filters. The HAA5fp increased with phosphorus-supplemented operation at specific filtration cycle times as compared to non-phosphorus-supplemented operation, indicating the potential for production of HAA5 with phosphorus supplementation. Enhanced biofilm coverage of the anthracite and sand media was observed during phosphorus supplementation. In addition, increased viability of the cells embedded in the biofilm was observed in the sand media at depth during phosphorus-supplemented operation.