Advanced Treatment Combination for Groundwater Recharge of Municipal Wastewater by Nanofiltration and Ozonation

This paper presents the results of research undertaken on an advanced treatment combination for polishing municipal wastewater with the purpose of a safe groundwater recharge. The results of a former study of DWQC initiated this research. It is envisaged that tertiary effluent is nanofiltrated to reject dissolved organic carbon (DOC) and adsorbable organic halogens (AOX) to concentrations less than 2–3 mg DOC/L respectively <2 μg AOX/L. The brine will be given back in a recycling process to the sewage treatment plant after passing an oxidation step. To avoid rising scaling potentials and other negative impacts due to increasing salinity, the rejection characteristics of several NF-membranes were investigated. They show a strong dependence between DOC and sulfate removal. Biofouling on the membrane surface (Desal DK5) can be controlled by higher cross-flow velocities (CFV) of about 1 m/s, however, a suitable pre-treatment like slow sand filtration is required. High water conversion factors result in moderately higher biofouling. This shows that water quality is the main factor responsible for fouling and not the concentration of constituents. Ozonation experiments with the concentrate confirmed an enhanced biodegradability of refractory DOC. At a specific ozone consumption 1,7 mg O₃/mg DOC₀ the DOC reduction by micro-organisms (aerobic biotest) reaches its maximum after 14 days of biodegradation with a total reduction of 60%.