IXOM Watercare

Laboratory Study of Conventional Alum Treatment versus MIEX® Treatment for Removal of Natural Organic Matter

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In recent years, considerable effort has been made in drinking water treatment research to develop methods with improved natural organic matter (NOM) removal from water. Reduction in the level of NOM before disinfection can minimise the formation of disinfection by-products and reduce the residual required to control bacterial regrowth in the distribution system. This contributes to a higher quality water that is delivered to the consumers.

This paper describes a laboratory study using (a) conventional alum treatment (without pH control), (b) enhanced alum treatment (controlled at pH 6), (c) magnetic ion-exchange resin (MIEX?*) and (d) alum and MIEX? combined treatment. The comparison focussed on the removal of dissolved organic carbon (DOC) and treated water quality, particularly the formation of disinfection by-products and bacterial regrowth potential. The impact on disinfectant stability was determined by monitoring chlorine decay over a period of 72 hours with periodic samples taken for disinfection by-product formation analysis and bacterial regrowth potential (BRP) measurement. The character of the residual organic compounds was examined using specific UV absorbance (SUVA) and high performance size exclusion chromatography (HPSEC) for molecular weight distribution.

Two reservoir waters, Hope Valley and Myponga, were selected based on the differences in the character of their dissolved organic matter. The SUVA values were 2.4 m-1mg-1L and 3.5 m-1mg-1L for Hope Valley and Myponga, respectively. The higher SUVA values for Myponga water indicate that the DOC is composed largely of aquatic humics and a relatively higher content of hydrophobic and aromatic components.

Removal of DOC under optimised treatment conditions indicated alum and MIEX® combined (Option d) was very similar to MIEX® alone (Option c) and much better than conventional or enhanced coagulation with alum. Combined treatment (alum and MIEX®) removed 2.3 and 1.4 times the DOC removed by enhanced coagulation with alum from Hope Valley and Myponga, respectively. The DOC remaining after each treatment strategy was quite different in character. The HPSEC plots of the treated waters show that with alum treatment (enhanced and conventional) all UV absorbing compounds greater than 2000 AMW were removed. Treatment with MIEX® alone resulted in a much greater removal of compounds which were less than 2000 apparent molecular weight (AMW) compared with the alum treatment. However unlike alum treatment there were compounds greater than 2000 AMW remaining after MIEX® treatment. Combining the alum with MIEX® resulted in a significant reduction of UV absorbing compounds above and below 2000 AMW. The character of the dissolved organic matter of the treated water was the same regardless of whether MIEX® was dosed prior to or after alum.

Including MIEX® in the treatment stream reduced the chlorine decay and trihalomethane (THM) formation. Using MIEX® alone or combined with alum the amount of chlorine consumed was 50 percent and 80 percent of that obtained with conventional alum treatment (after 60 minutes). The ability of the water to support bacterial growth as measured by BRP was the lowest after MIEX® treatment (option c) compared with the three other treatments (Options a, b & d). In summary, laboratory tests show that including MIEX® in the treatment process can improve DOC removal, resulting in lower chlorine decay and THM formation.

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