Unintentional, indirect wastewater reuse is occurring as upstream wastewater treatment plant (WWTP) discharges impact downstream drinking water treatment plants. As part of a project to evaluate the contribution of wastewater to disinfection by-product (DBP) formation in drinking water, one of the objectives was to compare different WWTP processes for the control of dissolved organic carbon and nitrogen (DOC and DON), and DBP precursors in effluent organic matter (EfOM). Nitrification plays a key role in determining the quality of EfOM in terms of traditional wastewater parameters, natural organic matter character, and DBP-related parameters. Some nitrification strongly altered the levels of total Kjeldahl nitrogen (TKN), ammonia (NH3-N), DON, carbonaceous biochemical oxygen demand (CBOD), DOC, ultraviolet absorbance (UVA), and chemical oxygen demand. Good nitrification (NH3-N <2 mg/L) augmented the changes for TKN, NH3-N, and CBOD; reduced the concentration of biodegradable organic carbon (BDOC); and dramatically changed the ratio of BDOC/DOC. During wastewater treatment, some of the particulate and DON was transformed to biomass, NH3-N, nitrite, nitrate, and/or nitrogen gas, whereas a portion of the DOC was transformed to biomass, carbon dioxide, methane, and/or other gases. Although nitrification reduced the level of UVA, it resulted in an increase in specific UVA (SUVA). This was probably due to preferential removal of the less UV-absorbing (non- humic) portion of the DOC during biological treatment.