Characterising natural organic matter in surface waters and the minimisation of disinfection by-product formation

Abstract

Modern water treatment processes are necessary to create an adequate and continuous supply of water that meets regulatory standards. The presence of natural organic matter (NOM) in water courses impacts negatively upon aesthetic and chemical standards and as such requires removal during water treatment processes. Variable structural composition and sources of NOM denote that high NOM removal efficiencies are rarely achieved at conventional water treatment works (WTW). Poor removal of NOM can result in biofilm re-growth in distribution systems and the formation of potentially carcinogenic disinfectant by-products (DBP) such as trihalomethanes (THM) and haloacetic acids (HAA), formed when residual NOM reacts with disinfectants such as chlorine.

NOM characterisation methods were used to investigate NOM composition at sixteen surface water sites operated by Severn Trent Water Ltd, to establish potential links between NOM character and the formation of potential carcinogeous DBP, and assess potential DOC removal using current and low pH coagulation. Comparisons were made between existing NOM characterisation methods and the identification of limitations. HPSEC and fluorescence EEM spectroscopy were found to be reliable and practical measures of NOM character and treatability. Statistical analysis techniques such as discriminant analysis and principal component analysis proved essential analysis tools for large data sets, identifying sites with similar raw characteristics and highlighted relationships with DBP precursors.

The suitability of carbon isotopes analysis and environmental nanoparticles analysis as two novel NOM characterisation methods were also investigated and compared with existing methods. Carbon isotope analysis documented an input of heavier 13C signatures and a decreased percentage modern carbon 14C. Possible causes for this were the addition of GAC fines, fractionation of 12/13C through treatment processes or through microbial growth on the GAC column. Finally, a detailed assessment of current coagulation potential for increased NOM removal and the potential for a reduction in DBP formation during differing NOM composition profiles was investigated with an economic assessment for a river abstraction WTW with rapidly changing NOM character. Low pH coagulation was found to substantially increase potential DOC removal and limit THM, TTHMFP and HAAFP formation and identified the need for process optimisation on WTW before treatment alternatives need are considered.