NOM increase in Northern European Source Waters: Impacts on coagulation/contact filtration processes

Summary

A significant increase in the content of natural organic matter (NOM) has been observed in several surface water sources in Norway during the past 10-12 years. Similar observations are reported also from test sites in Europe and North America. Water works in the southern part of Norway have experienced a doubling and in some cases even a tripling of source water color levels during the last decade. Other important characteristics of NOM, e.g. the specific UV absorption (SUVA), have changed as well. The change in raw water quality increases the required NOM removal capacity in water treatment, and significantly affects treatment process selection, design and operation. However, the operational impacts on water treatment processes are poorly quantified. This paper addresses the observed NOM increase in parts of Europe and North America. Although the reasons are not fully understood or identified, possible causes are presented and discussed. Factors like climate change or variability, especially alteration in precipitation patterns and land-use, as well as reductions in anthropogenic sulfur loadings seem relevant. From pilot testing on relevant raw waters, major operational impacts of increasing NOM and SUVA levels on coagulation-contact filtration processes are identified and quantified. As an illustration, an increase in raw water color from 20 to 35 mg Pt/L, increased the required coagulant dose, sludge production, number of backwashes per day and residual TOC by 64 %, 64 %, 87 %, and 26 %, respectively. In addition, hydraulic capacity and filter run time decreased by 10 % and 47 %, respectively.