Abstract
Drinking water treatment plants (DWTPs) designed to remove natural organic matter (NOM) are challenged as concentrations of NOM in raw waters are increasing. Here, we assess seasonal differences in NOM quality and quantity, from raw waters to the distribution network, at three large DWTPs in Oslo, Stockholm and Helsinki. Samples, collected during stable stratification in both winter and summer and during the autumnal turnover, were analysed for NOM concentrations and composition. The NOM was characterized by common routine parameters, size and content (TFF, LC-OCD, fluorescence) and biodegradability. The NOM concentration decreased to 2.5 mg/L (55%), 4.0 mg/L (48%) and 5.7 mg/L (76%) at the respective DWTPs in Oslo, Stockholm and Helsinki. The NOM in raw waters were predominantly in the largest size fraction (>50 kDa), in particular from Oslo. High MW fractions >50 kDa and humics remained the largest fractions with minimum 30% and maximum 80% of the total NOM. The BDOC in treated water <0.3 mg/L and the conditions in the distribution network imply low probability for bacteria regrowth.The multi-step treatment consisting of coagulation/flocculation, sedimentation, rapid sand filtration, ozonation and biological activated carbon filtration (BAC) was most effective in removing NOM. Coagulation/flocculation followed by sedimentation and sand filtration were critical, especially for the removal of biopolymers and humics, and somewhat for building blocks. The sand filtration provided up to 25% additional removal of biopolymers and below 7% removal of other fractions. The ozonation and BAC was more effective and removed 11% of biopolymers, and about 35% of building blocks and LMW neutrals.
Highlights
In Fennoscandia, surface waters are the main sources for drinking water production
This study investigated three drinking water reservoirs located in Fennoscandia, namely Lake Maridalen (Berge et al, 2011), Lake Mälaren (Johansson et al, 2010; Köhler et al, 2013) and Lake Päijänne (Castrén et al, 2014; Forsius et al, 2017), (Fig. 1a), and three full-scale drinking water treatment plant (DWTP) that provided drinking water to capitals in Norway (Oslo), Sweden (Stockholm) and Finland (Helsinki) (Fig. 1b)
Seasonal differences in natural organic matter (NOM) fractions removal throughout the year were more pronounced for DWTPs treating water from the lakes with the shorter residence time
Summary
In recent decades increasing concentrations of dissolved natural organic matter (NOM) have been observed (de Wit et al, 2007, 2016). The primary drivers of browning surface waters typically considered are reduced sulfate deposition (Monteith et al, 2007) and climate change (de Wit et al, 2016), suggesting that current browning trends will continue into the future. Water colour has shown larger increases than dissolved organic carbon (DOC) concentrations (Haaland et al, 2010), suggesting a change in NOM composition from lower to higher molecular weight and increasing aromaticity. Removal of increasing concentrations of NOM to reach compliance with drinking water quality regulations is a challenge for many drinking water treatment plants (DWTPs) in Fennoscandia (Ledesma et al, 2012). For many DWTPs that waterworks have had sufficient raw water qualities
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