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Since 2009, UV-irradiation has been the dominant method of primary water disinfection in Norway, applied at close to 1000 facilities. Prior to UV treatment, one out of three primary water treatment processes are being used: (i) Enhanced coagulation (EC), (ii) Nanofiltration (NF), or (iii) Ozone and biofiltration (OBF). With natural organic matter (NOM) laden source waters and chlorine-free distribution systems, safeguarding of high NOM removal efficiency and biostability are important issues in Norwegian water supplies. The effects of water treatment technology and UV disinfection on NOM, NOM fraction removal and biostability, measured as biodegradable organic carbon (BDOC) were investigated at a number of full-scale WTPs in Norway. Samples of untreated raw water, from different treatment steps and from the distribution system were analysed using rapid NOM fractionation and columns-in-series BDOC techniques. In addition, the formation of biofilm and deposits on pieces of PE pipes was studied in continuous biofilm monitors (BFMs) fed with untreated raw water and treated (distributed) water. The results show significant differences among the treatment technologies: EC or NF treatments were capable of reducing NOM (DOC) effectively and maintaining BDOC levels close to zero after treatment. For the investigated OBF-facilities the results show that the hydrophobic NOM fractions were transformed by ozone to hydrophilic and more biodegradable fractions, thereby increasing the BDOC significantly, thus increasing the potentials for biological regrowth and elevated heterotrophic plate counts in the distribution systems. For all facilities investigated, BDOC levels were found to correlate well (R2>0.85) with the hydrophilic NOM content. Furthermore, the results indicate that with the UV-disinfectant doses normally applied (40 mJ/cm2), no significant effects were identified on NOM fractions or BDOC levels. The results from the biofilm monitors (BFMs) show that the amounts of TOC and ATP correlate well with the amount of metals (e.g. Fe) in the deposits, thus indicating that adsorption of NOM and microorganisms to metal precipitates on the pipe wall is an important deposition mechanism.

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