NOM-removal by the SIX®-process

Some waters can have elevated concentrations of dissolved organic carbon (DOC), especially sources like surface waters that are under the influence of secondary effluent, recreation, heavy population, farming and industry. In a number of locations in North-West Europe, for example the United Kingdom and Scandinavia, DOC levels are increasing over time most likely due to climate change effects and changes in land use. For these types of water, ion exchange (IX) is of interest as a pre-treatment because the removal of colour and DOC by IX will increase the efficiency of all downstream processes including; coagulation, membrane filtration, advanced oxidation processes (AOP) and granular activated carbon filtration (GAC). It will also lead to improved water quality (i.e. less by-product formation) and most likely improvements in biostability within the distribution network. Surface waters also contain suspended and colloidal matter making it nearly impossible to use standard state-ofthe-art, fixed bed IX columns. This is because these beds will foul quickly (i.e., head loss build-up) with the suspended matter. When this happens, the IX bed starts to function as a filtration bed rather than as an adsorption media. The newly developed suspended ion exchange process SIX (suspended ion exchange, PWN Technologies, Netherlands) presents an advanced solution for a world-wide challenge: how to remove natural organic matter (NOM/DOC) as a first step in surface water treatment to improve the efficiency of downstream processes and water quality. Besides the possibility to treat water that contains suspended matter another advantage is that the process has advanced to an economically and technically feasible process, requiring low contact times and small resin inventories and with a large tolerance for flow fluctuations. Depending on the water source, adding a relatively low concentration of coagulant after IX removes even greater quantities of DOC, especially in the fraction of the high molecular weight organic carbon (whilst the IX primarily removed the humic and fulvic organic fractions). The most important advancement is that almost any commercially available resin can be used, creating the desired flexibility in resin suppliers for water supply companies. This paper describes the process and its advantages and disadvantages compared to conventional technologies. NOMcharacterisation with size exclusion chromatography, liquid chromatography – organic carbon detection (SEC/LC-OCD) before and after this process showed the outstanding performance of the process, especially on water types which contain high colour/DOC-concentrations and low total dissolved solids, which are typical of the majority of the surface waters in the North-West of Europe.