Advanced wastewater treatment by nanofiltration and activated carbon for high quality water reuse

Hybrid processes combining activated carbon and nanofiltration have been studied to identify the optimum solution for advanced wastewater treatment in high quality water reclamation and reuse. With a focus on the removal of bulk and trace organic compounds the investigation identified three promising process combinations, namely powdered activated carbon followed by nanofiltration (PAC/NF), granular activated carbon followed by nanofiltration (GAC/NF) and nanofiltration followed by granular activated carbon (NF/GAC). The removal potential was examined in lab and pilot scale for a range of refractory pharmaceuticals and industrial chemicals typically detected in effluent in trace concentrations ranging from ng/L to ìg/L. Fluorescence excitation emission spectroscopy was employed for the investigation of the fate of effluent organic matter. The optimum strategies for operation of the hybrid processes were determined in pilot scale. The experiments were conducted at the Wastewater Treatment Plant Aachen Soers providing an effluent of high quality with low dissolved organic carbon (DOC) concentrations of about 5 mg/L. In comparison to a single stage NF or a single adsorption stage, all three hybrid processes provide a superior product quality with DOC concentrations clearly below 0.5 mg/L and organic micropollutant concentrations close or below the limit of quantification. If a high degree of salinity removal is not required, nanofiltration-activated carbon hybrid processes can be regarded as a reliable and economic alternative to dual membrane processes (ultrafiltration and reverse osmosis). They are well suited for high-grade water reuse applications. Due to lower filtration pressures the hybrid processes feature lower energy consumption and produce less problematic concentrates mainly consisting of organics and multivalent ions which can be precipitated. They feature also some advantages in indirect potable reuse applications such as managed aquifer recharge since the salt content of the product water is closer to natural conditions. The optimum combination of nanofiltration and activated carbon depends on local boundary conditions such as size of plant, raw water characteristics and plant location. In the light of growing water scarcity and increasing concerns about organic micropollutants activated carbon treatment in combination with nanofiltration has the potential to be applied in an increasing number of cases