Coupling Riverbank Filtration with Reverse Osmosis May Favor Short Distances between Wells and Riverbanks at RBF Sites on the River Danube in Hungary

Abstract

Bank filtration and other managed aquifer recharge techniques have extensive applicationin drinking water production throughout the world. Although the quality of surface water improvesduring these natural processes, residence time in the aquifer and length of the flow paths arecritical factors. A wide range of data is available on the physical–chemical processes and hydraulicconditions, but there is limited knowledge about the top layer of the porous media. An investigationwas conducted on the hydraulic behavior and on the change of microbiological indicator parametersin the filter cake. The purpose of the experiment was to: (1) investigate if the reverse osmosis issustainable when fed with only slow filtered water, and (2) show that a short travel distance canprovide extensive pathogen removal and beneficial conditions for the reverse osmosis. A slow sandfilter was operated over a one-year long period while changes in head loss and microbiologicalparameters were being monitored. Head loss and membrane permeability were monitored between3 November 2016 and 24 October 2018 and microbiological sampling was performed from 19 July2017 to 6 November 2018. The filtered water was fed to a reverse osmosis (RO) filter as the waterabove the sand filter had been spiked with dissolved iron. Results show that even a thin biofilm cakeof 1–3 mm thickness can result in a significant (10–100%) reduction in microbiological activity in theinfiltrate, while favorable short retention times and oxic conditions are maintained. Avoiding anoxicconditions, subsequent iron and manganese dissolution and precipitation is beneficial for membraneprocesses. Building on these results, it can be stated that when reverse osmosis is directly fed withslow filtered or bank filtered water, (1) a short distance from the surface water body is requiredto avoid dissolved iron and manganese from entering the groundwater and (2) proper pathogenrejection can be achieved even over short distances.