Partial renewal of granular activated carbon filters for improved drinking water treatment

Licentiate thesis, 2017

Drinking water is widely collected from surface water sources. In these water sources, both the quantity and quality of natural organic matter (NOM) have been affected around the world during the last decades, especially in Northern Europe and North America. This increasing NOM and its composition change challenge the drinking water treatment plants (DWTPs) due to e.g. increased coagulant demand, and because NOMs constituents act as precursors for potentially harmful disinfection by-products. Many DWTPs employing conventional treatment are currently struggling to maintain sufficient NOM removal, and are facing significant investments to upgrade existing treatment processes. In this thesis, a modification strategy to improve NOM removal by existing biological activated carbon (BAC) filters was tested. Analytical techniques like dissolved organic carbon, spectroscopic methods (absorbance and fluorescence) were used to monitor the performance of the modified filters in comparison to reference filters. In the second phase of the study, the modification strategy was employed in a different DWTP with different source water and coagulation treatment in order to validate the effectiveness of the proposed strategy under diverse conditions. Results show that replenishment of about 10% activated carbon media with new carbon media in BAC filters resulted in improved performance. The modified biofilters showed improved organic matter removal lasting for 10-20 days, depending on surface loading. In addition to improving the adsorption of humic-like NOM fractions, biological removal by the saturated filter media was enhanced. A subsequent validation study showed that improvement of biodegradation and adsorption mechanisms occurred in different DWTPs regardless of differences in NOM composition and coagulation processes prior to the BAC filters.