In Latvia, as in other countries of the Boreal region, raw water sources contain elevated concentration of natural organic matter (NOM). Originally the conventional water treatment plants (WTP) were mainly designed for removal of turbidity hence the optimization of water treatment processes for more effective removal of NOM is often needed nowadays. In this paper a rapid fractionation (RF) technique of NOM developed by Chow et al. was applied to better understand the removal mechanisms of NOM during chemical precipitation and ozonation-biofiltration processes at WTP in Riga, Latvia. Results showed that NOM removal, determined as dissolved organic carbon (DOC) constituted from 15.34±3.84 mg/L in raw water to 5.33±1.45 mg/L in drinking water and was mainly due to decrease of very hydrophobic acids (VHA) during coagulation and precipitation processes. Other NOM fractions viz. slightly hydrophobic acids (SHA) and hydrophilic charged acids (CHA) were only slightly removed. During ozonation the fraction of CHA were transformed into hydrophilic neutral (NEU) fractions and removed by biologically activated carbon (BAC) filters. However, this trend was not observed during the cold season. In conclusion, coagulation and precipitation mainly removed humic substances, as indicated by VHA removal, whereas ozonation-BAC filtration was not effective since biologically degradable fractions of NOM were either transformed into other substances (from NEU to VHA) or were not removed (in cold season) due to insufficient biological activity or contact time in BAC filters.

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