The impact of recalcitrant organic character on disinfection stability, trihalomethane formation and bacterial regrowth: An evaluation of magnetic ion exchange resin (MIEX®) and alum coagulation

Abstract

Two South Australian reservoir waters, Hope Valley and Myponga, were selected in this study based on the differences in the character of their organic matter. Four treatment options: (a) alum coagulation without pH adjustment; (b) alum coagulation at pH 6; (c) magnetic ion-exchange (MIEX^®) resin; and (d) combined alum/MIEX^® treatment, were used to compare the removal of dissolved organic carbon (DOC) and treated water quality, particularly the formation of disinfection by-products and bacterial regrowth potential. Improved DOC removal was achieved with the inclusion of MIEX^® treatment in the process. Removal of DOC under optimised treatment conditions indicated combined alum and MIEX^® treatment was very similar to MIEX^® alone but much better than conventional and enhanced coagulation with alum. Combined treatment (alum and MIEX^®) removed 2.3 and 1.4 times the DOC of enhanced coagulation with alum from Hope Valley and Myponga respectively. The DOC remaining after each treatment strategy was different in character. The molecular weight distribution results indicated that MIEX^® treatment removed a broad range of compounds, whilst alum treatment targeted the removal of high molecular weight compounds. In addition, the DOC remaining after MIEX^® treatment consisted of compounds with lower specific UV absorbance (SUVA). Including MIEX^® in the treatment stream provided better DOC and bromide removal thus reducing chlorine decay and trihalomethane (THM) formation. The ability of the water to support bacterial growth as measured by bacterial regrowth potential (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 treated water quality.