Natural organic matter character and reactivity: assessing seasonal variation in a moorland water

Natural organic matter (NOM) is described as an intricate mixture of organic compounds that occurs universally in ground and surface waters. After treatment for potable use, there is NOM remaining in the water that reacts with the chlorine used for disinfection to form disinfection by-products (DBPs). Some of the DBPs, trihalomethanes (THMs) are regulated. Several water treatment works (WTWs) in the Yorkshire Water and United Utilities (previously North West Water) region in England have recently experienced difficulty in meeting THM limits (100 µg L-1) in their finished drinking water at certain times of the year.

An investigation into how NOM changes seasonally, pragmatic methods of NOM analysis and its reactivity with chlorine was undertaken. By separating the NOM using adsorbent resins into fractions, it was possible to gain an insight into the seasonality of NOM. It was observed that a particular, difficult to remove fraction was always more reactive with respect to THM formation in autumn.

Some of the methods proposed in the literature were used here with varying successes. It was found that High Performance Size Exclusion Chromatographic methods were most useful to the WTW operators for optimising treatment processes.

It is known that the formation of DBPs is very complex. An attempt was made to predict the reactivity of a raw water in terms of THM-FP by looking at the NOM makeup. However, it was found that the fluorescence spectra combined with the fluorescence index of raw water and chlorinated samples gave more insight into the reactivity of the raw water at a particular time than knowing the fraction distribution. The use of fluorescence as a tool for understanding chlorine-NOM reactions is promising.