Ultrafiltration membranes in managed aquifer recharge systems

The natural organic matter (NOM) and color content of surface waters are increasingly becoming an issue for artificial groundwater recharge plants. Water from Lake Bolmen, in southern Sweden, had in 2017 an annual average NOM and color content of 8.6 mg/L total organic carbon (TOC) and 57 mg Pt/L respectively, and values ranging from 7.8 to 9.6 mg/L TOC and 50-70 mg Pt/L. Since water from Lake Bolmen will be used at Vomb Water Works, an artificial groundwater recharge plant, the high NOM-content of Lake Bolmen must be reduced prior to aquifer recharge. From experiences of full-scale operations of chemical flocculation, lamella sedimentation and rapid sand filtration using ferric chloride, three different pre-treatment methods were proposed; conventional precipitation, stand-alone direct precipitation before ultrafiltration (UF), and conventional precipitation with ultrafiltration after lamella sedimentation. In this study, a hollow fiber membrane (MWCO of 150 kDa) was used in different configurations during a 15 months pilot trial. The results showed the possibility to reduce NOM equal to conventional precipitation when a stable net-flux of 40 and 70 L/(m2·h) was used for direct precipitation before UF and conventional precipitation with UF, respectively. This paper presents these treatment methods and evaluates their viability as full-scale treatment steps.