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The University of Massachusetts, Amherst and the South Central Connecticut Regional Water Authority have been conducting pilot studies at the West River Treatment Plant CWRTP) since 1989 to investigate the use of ozone and peroxone in in-line fIltration. Many studies in the past, both pilot and full-scale, have shown improved perfonnance on inclusion of pre-ozonation in to the treatment scheme. Peroxone has been found to be more effective than ozone in oxidizing taste and odor compounds. This study investigated the effects of the use of ozone and peroxone on fIlter perfonnance, and particle and manganese removals. Pilot run experiments were conducted in the summer of 1990 to compare ozone with no pre-oxidant, and ozone with peroxone. Treatment was evaluated in tenns of effluent turbidity and UV absorbance, headloss development in the fIlter, particle counts, and DOC and manganese analyses. Results from these experiments showed that the perfonnance of ozone and peroxone was very similar in tenns of fllter perfonnance and particle removal. Both ozone and peroxone trains gave effluent DOC levels similar to the no pre-oxidant train and the WRTP. The ozone train was able to achieve significantly greater removals of manganese compared to the peroxone train. However, the WRTP, which adds potassium pennanganate specifically for the oxidation of manganese, was achieving manganese removals far in excess of the pilot trains. Studies were conducted to investigate the large difference in perfonnance of the two pilot trains compared to the WRTP in removing manganese. A laboratory column set up with WRTP fllter media showed that low levels of manganese in the WRTP fllter effluent was due to direct oxidation by pennanganate rather than adsorption of MnCm by oxide coatings on fllter media during flltration. The oxidation of reduced manganese by ozone was also studied. Ozone doses between 1 and 2 mg/L oxidized a significant fraction of reduced manganese to colloidal oxides, which formed particulates after coagulant addition and rapid mixing. These particulates were then removed by mtration. At pH less than 7.0, increasing the dose showed some additional oxidation. However, at higher pHs of 7.5 and 8.0, manganese oxidation was largely unaffected by the ozone dose. This may indicate that when the molecular ozone reaction pathway is dominant, ozone is able to oxidize manganese due its selective nature of reactions. However, when the free radical pathway is dominant, the free radicals may be quickly consumed in competing reactions before reacting with manganese. iv