Bank Filtration as Natural Filtration
When wells are placed close to a surface water source (such as a lake or a river) and pumped, a portion of the surface water is induced to flow to the well. As the water travels from the river to the well through the riverbed sediments and underlying aquifer material, suspended and dissolved contaminants of surface water are “naturally” filtered out using a combination of physical, chemical, and biological processes. If the surface water is a river, the system is called riverbank filtration (RBF). If a lake serves as source water, the system is a lakebank filtration. These natural filtration systems have been operating for more than 100 years in Europe and for over half a century in the United States, providing safe drinking water to communities. For the RBF systems to work effectively, there must be a hydraulic connection between the river and the alluvial aquifer where the wells are located. Unclogged river bottoms are ideal for RBF operations. RBF systems are known to remove turbidity, microbes, and chemicals present in surface water and the removal efficiency is a function of well location, pumping rate, source water quality, etc. A fraction of dissolved organic carbon (DOC) is also removed which helps in reducing the formation potential of disinfection byproducts during chlorination of the filtrate from RBF systems. RBF systems can be adapted to a given site using engineering judgment. Use of inflatable dams to raise water levels in rivers in low-flow periods can augment well yields. Similarly, diverting a part of the water from the river to an infiltration basin and strategically placing wells between the river and the infiltration basin can enhance yield. Over the years, several improvements to the design and construction of the RBF systems have taken place. Use of these methods at future sites can improve the efficiency of RBF. Besides siting issues, periodic maintenance and early-warning systems to monitor river water quality are needed for sustainable operation of RBF systems. RBF has one of the best potentials to be used as a natural filtration system in populated riparian communities in developing countries. This chapter presents some of the advantages and limitations of using RBF for water treatment. Design, cost, maintenance, and future research needs are presented.