In this study, the sufficiency of flocculation and adsorption with reduced doses of ferric chloride and powdered activated carbon (PAC) as pretreatment to ultrafiltration. (UF) was investigated. A 50 mg/L dose of FeCl3 and 0.5 g/L of PAC removed a majority of organic matter (OM; 88%), thus reducing the organic loading on UF that was used as posttreatment. Although flocculation with lower doses of FeCl3 (10 mg/L) followed by PAC adsorption of 0.5 g/L and UF removed the same amount of organics (similar to 88% OM), the majority of the OM removal was by the posttreatment of UF rather than by pretreatment, resulting in a significant decline of flux in UF. A detailed relative molecular mass (RMM) distribution analysis was made with flocculation, adsorption, and UF permeates. PAC adsorption decreased the majority of the relative intensity of smaller RMM of OM from the preflocculated water. Flocculation with <40 mg/L FeCl3 was not sufficient to remove the relative intensity of the large RMM even after a postadsorption. The detailed analysis on weight-averaged RMM (M,,,) indicated that the M,, values of OM in the wastewater and in the flocculated effluent were 29,800 daltons (initial), >25,000 (after flocculation with <= 40 mg/L FeCl3), and <1000 (afterflocculation with >= 50 mg/L FeCl3). (c) 2005 American Institute of Chemical Engineers AIChE J, 52: 207-216, 2006.