The possibility of the existence of an iron-organic interaction in Harwood's Mill Reservoir contributing to a problem with floe formation after chlorinating filter-applied water was investigated. Shortened filtration-cycle times resulted when the filter-applied water contained the floc. The effects of varying pH, temperature, alum dosage, and oxidant addition on organic and meta.ls removals were examined with jar tests. Ultrafiltration analyses were performed to determine with which molecular size range of organic matter the iron may have been associated. Particle-size analysis was used to further examine the chlorination phenomenon. The low iron concentrations in the raw water were removed easily under any experimental condition. Organic removal, however, was optimized by alum coagulation ( 50 mg/L) at pH 5. 5 and a preoxidant dose of 2 mg/L. Improvements in organics removal over that of the WTP suggested that poor organic removal contributed to the floe-formation problem. / M.S.