Evaluation of Water Treatment Efficiency of Full-Scale Slow Sand Filtration Process: Comparison of Summer and Winter Season

Abstract

Objectives : The objective of this study is to evaluate the feasibility of sustainable operation in a conventional full-scale slow sand filter process.Methods : A slow sand filtration process was operated in small scale water treatment plants for valley water. The water quality such as particle matter, dissolved organic matter, the number of bacteria, and disinfection by-product (DBP) were monitored at four designated points once a week and was compared during summer (July-August) and winter season (December-January).Results and Discussion : Although the number of particle matter in summer was higher than in winter by 39% (2.91×10⁴ /mL), particle matter in slow sand filter process was removed by 99% regardless of season (temperature). The removal efficiency of dissolved organic carbon (DOC) and UV₂₅₄ was higher in summer than in winter by 15% and 21%, respectively. In addition, concerning the organic fraction, higher molecular weight led to higher removal efficiency in the following sequence: BP (biopolymer, MW: 20,000 g/mol) > HS(humic substances, MW : 1,000~20,000 g/mol) > BB(building blocks, MW : 300~500 g/mol) > LMWs(low molecular weights, MW : 350 g/mol. The concentration of trihalomethanes (THMs) and haloacetic acids (HAAs), indicator of DBP, were detected at 12.2 and 9.4 μg/L in summer and 8.1 and 6.3 μg/L in winter, respectively, which would be considered very low concentration related to the drinking water regulation of 100 μg/L. Finally, the active bacteria was removed up to 93 and 92% in summer and winter, respectively.Conclusion : The feasibility of a slow filtration process was evaluated to compare particle and dissolved matter in summer and winter operation. Despite a 39% increase in particle matter in summer, the removal efficiency was maintained at 98% in summer and winter. Due to the increased activation of biofilm in summer, the removal of DOC and UV₂₅₄ was higher in summer than winter by 15 and 21%, respectively. The significant low concentration of THMs and HAAs, regardless of season, would be negligible. In addition, achieving over 92% removal of activated bacteria secured the biological safety. These stable operations ensure that slow filtration process is effective in controlling of water quality.