Understanding the behaviour of molecular weight fractions of natural organic matter to improve water treatment processes

Abstract

Water utilities have experienced increasing pressure to minimise the formation of disinfection by-products (DBPs), as reflected in the increasingly stringent regulations and guidelines for the concentrations of DBPs in drinking water. Understanding the disinfection characteristics and molecular weight (MW) distribution of natural organic matter (NOM) will assist in the optimisation of drinking water treatment processes to minimise the formation of DBPs. This study investigated the disinfection behaviour of MW fractions of NOM isolated from a Western Australian source water. The NOM was fractionated and separated using preparative size exclusion chromatography (SEC) and the fractions were chlorinated in the presence of bromide ion. The larger MW fractions of NOM were found to produce the highest concentrations of DBPs (trihalomethanes, haloacetic acids, haloacetonitriles, haloketones, and haloaldehydes), with the low MW fractions still producing significant amounts of these DBPs. The results also showed a trend of an increasing proportion of brominated DBPs with decreasing MW and aromatic character. Considering that the smaller MW fractions of NOM produce significant amounts of DBPs, with a higher relative contribution from brominated DBPs, water treatment processes need to be optimised for either bromide removal or the removal of aliphatic, small MW fractions of NOM, in order to meet DBP guidelines and regulations.

Chlorine addition prior to granular activated carbon contactors improves trihalomethane control

“…Low MW NOM is usually associated with the hydrophilic fraction, constituted of aliphatic ketones and alcohols, and shows lower UV 254 absorbance and higher reactivity with aqueous chlorine (HOCl/OCl − ) and bromine (HOBr/OBr − ) species (Chowdhury, Champagne, & McLellan, 2010;Liang & Singer, 2003). Low MW or hydrophobic NOM reacts at a higher rate with aqueous bromine than chlorine species and favors the formation of brominated DBPs (Farré et al, 2013;Kristiana, Allpike, Joll, Heitz, & Trolio, 2010;Liang & Singer, 2003). Hydrophobic NOM has higher MW and more phenolic groups than hydrophilic NOM fractions and exhibits higher specific UV 254 absorbance (Chowdhury et al, 2010).…”

N-nitrosodimethylamine and trihalomethane formation and minimisation in Southeast Queensland drinking water

“…When DOC concentration is lowered by coagulation, Br − competes more effectively for oxidant and therefore forms a higher concentration of BrO − . Although BrO − can react directly with DOC to form organobromine by-products including Br-THMs, ClO − has a greater oxidation potential than BrO − , while BrO − has a greater rate of electrophilic substitution than ClO − (Kristiana et al 2010). That is, Br-THM formation is maximised by ClO − oxidation of DOC to yield CHCl 3 , followed by sequential electrophilic substitution by BrO − to form the Br-THMs.…”

High-performance size exclusion chromatography with a multi-wavelength absorbance detector study on dissolved organic matter characterisation along a water distribution system

“…A decrease in disinfectant residual has also been suggested to be associated with interaction between disinfectant and DOM (Wilczak et al, 2003;Kristiana et al, 2013). The properties of DOM, particularly that of molecular weight, have been demonstrated to strongly affect natural organic matter reactivity in natural systems as well as different stages of the treatment process Korshin et al, 2009;Kristiana et al, 2010). Hence, an effective determination of the molecular weight distribution of DOM is critical not only for improving the water treatment process in terms of removal but also for understanding the disinfection process downstream in the distribution system.…”

“…These previous studies have demonstrated that the low molecular weight DOM compounds are recalcitrant to removal by conventional coagulation treatment. Some studies have also indicated that these low molecular weight fractions remaining in treated water are particularly associated with water quality degradation in distribution systems (Kristiana et al, 2010(Kristiana et al, , 2013. In most cases, such measurements have generally focused on aromatic components by detection at a single wavelength in the UV range of 250-280 nm.…”