Effect of Natural Organic Matter on Lime Softening and Removal of Bromide and Dissolved Organic Matter Using Ion Exchange: Two Applications of Magnetic Ion Exchange
Creators: name: Hsu, Susan
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 2008

description: This report consists of two studies investigating applications of a magnetic ion exchange (MIEX)resin designed for the removal of natural organic material (NOM) in drinking water treatment. The first project explores the effects of NOM on lime softening. Several model waters with various dissolved organic carbon (DOC) concentrations were lime-softened in batch treatment experiments to determine the effects of DOC on calcium removal. Water with 8 mg/L of DOC showed reduced calcium removal compared to waters with 0 and 1.5 mg/L of DOC. Waters containing DOC exhibited high degrees of oversaturation, providing evidence that NOM interferes with lime softening by inhibiting calcium carbonate nucleation and crystal growth. The model water with 8 mg/L DOC was also treated with the MIEX resin prior to softening and the DOC concentration was reduced to 35 mg/L. The MIEX-treated water showed an increase in calcium removal by batch lime-softening tests compared to the untreated water. Raw and MIEX-treated waters having DOC concentrations of 7.7 and 21 mg/L, respectively, were obtained from the Village of Palm Springs, FL, a utility that recently implemented the MIEX process for color removal and control of disinfection byproduct (DBF) formation. Softening of the Palm Springs waters showed similar results as for the model waters; hardness removal was slightly greater for the MIEX-treated water. Analysis of raw and finished water quality at the Palm Springs water treatment plant showed improved hardness removal after implementation of the MIEX process on a full-scale. This reduction in residual hardness was seen even when lower lime doses were used. These findings provide evidence that NOM inhibits calcium carbonate precipitation and interferes with lime softening and that DOC removal by the MIEX resin alleviates this inhibitory effect. The second project explored the effects of water quality, process configuration and resin characteristics on bromide and DOC removal through ion exchange. Bromide and DOC are precursors for the formation of DBFs when drinking water is chlorinated. Simulated natural waters containing different amounts of NOM, bicarbonate, chloride and bromide were batch-treated with MIEX resin to evaluate the effectiveness of the resin for bromide removal. Alkalinity, DOC and chloride concentrations were both shown to inhibit bromide removal in waters with bromide concentrations of 100 and 300 μg/L. Model waters were also treated using a two-stage, batch MIEX process. Two-stage treatment resulted only in a slight improvement in bromide removal because of the high chloride concentrations in the model waters. Chloride was found to be the dominant competing anion limiting bromide removal which poses problems because chloride is typically found at 330 times greater than bromide concentrations in most natural waters. The effectiveness of polystyrene and polyacrylic resins on bromide and DOC removal was compared by treating water with MIEX, Ionac A-641 and Amberlite IRA910 resins. The two polystyrene resins, A-641 and IRA 910, were seen to be more effective for bromide removal while the MIEX polyacrylic resin was more effective at removing DOC.