Fractionation of NOM in water from the treatment process at Ringsjöverket

Sammanfattning: During the last decades, increased concentrations of natural organic matter (NOM) and colour of surface waters have been reported from northern Europe and North America. Since surface waters are widely used for drinking water production this development has been problematic for drinking water producers. Even though NOM in itself is not harmful, it can cause production of undesirable disinfection by-products, decrease efficiency of UV-treatment and enhance biological growth within distribution systems. Moreover, drinking water treatment plants that use chemical precipitation has been forced to increase their coagulant dosages. This in order to obtain sufficient removal of NOM. In addition to increased concentrations of NOM, the colour of surface waters and UV absorbance have been shown to increase more than DOC values. This is not only an indication of increasing NOM-concentrations but also of that the characteristics of the NOM in surface waters are changing. In order to be able to optimise and improve the performance of the applied processes, it is essential not only to investigate different options of treatment techniques, but also to analyse the characteristics of the NOM in the water. In this master thesis, the characteristics of water from the process at Ringsjöverket were investigated. Ringsjöverket is a drinking water treatment plant located in Scania and owned by Sydvatten AB. In order to characterise NOM based on hydrophobicity the rapid fractionation method was used. Applying this method, water was fractionated into very hydrophobic acids (VHA), slightly hydrophobic acids (SHA), charged hydrophilic acids (CHA) and neutral hydrophilic material (NEU). After fractionation the quantity of each fraction was determined by dissolved organic carbon (DOC) measurements. This study shows that the raw water from lake Bolmen has a hydrophobic character. The dominating fraction in the raw water, VHA, constitutes together with the SHA, 85% of all DOC in the raw water. Chemical precipitation and the following rapid sand filtration was shown to be effective in separating VHA, SHA and CHA. However, these treatment processes showed no ability in separating the NEU fraction. The subsequent treatment in the slow sand filters showed no significant removal of any of the NOM fractions. Since 2017, two pilots, one ultra-filtration pilot and one infiltration sand filter pilot, have been running at Ringsjöverket. The purpose is to investigate future options for treatment techniques. However, compared to the more conventional treatment including chemical precipitation and rapid sand filtration, the chemical precipitation followed by ultra-filtration turned out to be less effective in separating NOM. However, small differences in separation of each fraction could be distinguished. The NEU fraction of NOM, which appeared to be unaffected during all other treatment steps, was separated in the ultra-filtration membrane. The sand filter pilot showed no ability of separating any of the fractions of NOM. On the contrary, increased concentrations of the fraction VHA after the sand filter indicated leaching of NOM from the sand filter. However, there are some uncertainties related to the result from the pilots. Therefore, more fractionation of this water is needed for a reliable result.