The impact of alum coagulation on the character, biodegradability and disinfection by-product formation potential of reservoir natural organic matter (NOM) fractions

Abstract

Natural Organic Matter (NOM) from Myponga Reservoir, South Australia, was separated into four organic fractions based on their hydrophobic and hydrophilic properties using a sequence of non-ionic and ionic resins. NOM fractions were isolated for the purpose of determining the impact of alum coagulation on removal of these fractions in conventional water treatment, and their potential as precursors in the formation of disinfection by-products (DBP) and in supporting microbial growth. The NOM comprised VHA (very hydrophobic acids), SHA (slightly hydrophobic acids), CHA (charged hydrophilics) and NEU (neutral hydrophilics) fractions. These fractions were then jar tested with alum using low (50 mg/L), operational (100 mg/L) and very high (200 mg/L) doses to assess the removal capacities for these fractions in a conventional treatment plant. High-performance size exclusion chromatography-UV-DOC (HPSEC-UV-DOC) revealed that alum removed more of the hydrophobic and higher molecular weight components of NOM, but less of the NEU fraction and lower molecular weight components of NOM. Determination of biodegradable dissolved organic carbon (BDOC) indicated that the NEU fraction had the highest biodegradability, followed by the CHA, SHA and VHA fractions. The VHA fraction had the highest total-trihalomethane formation potential (t-THMFP), followed by NEU, SHA and CHA. The NOM not removed by alum coagulation had the potential to support microbial growth (NEU fraction), and disinfection by-product (DBP) formation (VHA and NEU fractions). To obtain treated water with lower overall residual NOM, other treatment methods would need to be applied in addition to alum coagulation in order to reduce the concentration of the neutral fraction.