Safe drinking water in a changing environment : Membrane filtration in a Swedish context

Abstract: Surface water is vital for Swedish drinking water supply. In the past decades, a trend of increased total organic carbon (TOC) concentrations has led to higher consumption of coagulants in drinking water treatment, and has pushed the levels in the outgoing water closer to the allowed levels of TOC. Also, two occasions of cryptosporidium outbreaks in northern Sweden have stressed the importance of reliable microbiological barriers.Ultrafilters and tighter membranes can, due to size exclusion, produce a safe water by reducing the occurrence of parasites, bacteria and virus 10,000-fold or more. Combined with a coagulation pretreatment, ultrafiltration has the additional benefit of removing natural organic matter (NOM), whereas nanofiltration can remove NOM without coagulant. This dissertation presents results for NOM removal by ultrafiltration and nanofiltration from several raw water sources. The results were collected in mobile pilot plants at several locations. The coagulation and ultrafiltration process could achieve similar NOM removal as current chemical treatment, to a similar cost, and with a lower environmental impact. Hollow fiber nanofiltration achieved advanced NOM removal, reducing TOC with around 90 % and UV-absorbing species at 254 nm of up to 97 %. Thus, it is selective to aromatic NOM, similar to conventional treatment. The cost for the operation of a treatment process would increase if coagulation/sedimentation would be replaced with a nanofiltration step, but the environmental impact would decrease substantially. The NOM removal was studied by the aid of fluorescence spectrometry. Fluorescence can be related to characteristics of NOM, which has been implemented in this study. The application of fluorescence as a monitoring method has been evaluated through indices and other fluorescence derived parameters. Some of these, e.g. fluorescence index, have showed significant correlations to treatment efficiencies. Similar to TOC and UV-absorbance, significant changes in the nanofilter permeate was possible to relate to integrity breaches, and these NOM related parameters have shown potential for integrity monitoring.With such advanced NOM removal and advanced monitoring techniques, membrane filtration has a promising future in Swedish drinking water treatment. It decreases the risk for waterborne pathogens. Specifically, nanofiltration can lead to lower risks for disinfection by-products and regrowth in the distribution system.