Bank Filtration for Water Treatment

Dr. Ray has recently been working on studies examining the performance of bank filtration for drinking water treatment. The study site is located at a town 30 miles north of Peoria on the western bank of the Illinois River in Illinois.

Concept of Bank Filtration

The bank filtration technique takes advantage of existing geologic formations adjacent to water bodies to filter drinking water. Wells are dug in fine, sandy sediments next to water bodies and water is extracted from these wells. Water in the water bodies filters through the sediments removing contaminants. The water obtained is often of much higher quality than the raw surface water. The removal of contaminants in surface water is important because when the water is chlorinated these contaminants can be converted to harmful chemicals referred to as disinfection byproducts (DBPs). Chlorine must be added to most drinking water supplies to kill pathogens. The emphasis on controlling DBPs began when research showed that trihalomethanes (THMs) were produced by the reaction of chlorine with aquatic humic substances during the chlorination of drinking water. THMs include some known and suspected carcinogens (chloroform and dichloroacetic acid). National surveys by USEPA indicated the presence of THMs in virtually every chlorinated water supply tested. Research was initiated in the 1970's, and continues today, into identification of significant DBPs and methods to avoid their formation in the drinking water. Bank filtration wells can either be horizontal or vertical depending upon the hydrogeologic setting, required production rate, and the utility's preference. Shallow alluvial deposits and a higher rate of pumping from a given location often favor horizontal wells, sometimes called Ranney wells or laterals. The laterals of the collector wells can all be directed towards the river or distributed in all directions. Figure 1 provides a schematic diagram of a bank filtration well.

In Europe, particularly in Germany, bank filtration as a water treatment process has been applied since the 1870's and the first research project on bank filtration started in 1898. In the United States there have been bank filtration wells operating for nearly half a century. Since Sandoz Pharmaceutical's accidental spill of herbicides into the Rhine River in 1986, a rigorous 3-year study involving 6 water works, five research institutes, and several universities was conducted by Prof. Heinrich Sontheimer at the University of Karlsruhe, Germany. The study focused on hydrology, removal of organic compounds that included natural organic matter (NOM) and DBP pre-cursors, removal of particles, and the simulation of ground water passing through test filters.

Site suitability for bank filtration

Geohydrologic conditions impact the effectiveness of bank filtration. The permeability of the sediment affects seepage velocity and often internal clogging is associated with sediments having low hydraulic conductivity and small vertical gradients. Sediments that have excessively high conductivity will not be efficient in removing contaminants.

Removal of DBPs and DBP precursors

Chlorine must be added to most drinking water supplies to keep them free of pathogens. Chlorination of water that contains certain naturally occurring organic compounds produces certain harmful chemicals referred to as disinfection byproducts (DBPs). The emphasis on controlling DBPs began in earnest when research showed that trihalomethanes (THMs) were produced by the reaction of chlorine with aquatic humic substances during the chlorination of drinking water. National surveys by USEPA indicated the presence of THM in virtually every chlorinated water supply tested. Considerable research was initiated in the 1970's, and continues today, into identification of significant DBPs and methods to avoid their formation in finished water. Methods for avoiding the formation of DBPs fall into several categories including the removal of the DBP precursors prior to disinfection, removal of DBPs after formation, and use of alternative disinfectants that do not promote DBP formation. Removal of DBPs after their formation has been extensively studied, using techniques such as oxidation, sorption, and aeration. Although many of these techniques are effective, they are not economical. Alternative disinfection method such as ozonation and UV irradiation have problems. Ozone and its decomposition product, hydroxyl radical, react with most organic matter. Aldehydes are known by-products of ozonation, and hydroxyl radical reactions. In addition bromate is formed during the ozonation of bromide-containing waters, through both ozone- and hydroxyl radical-mediated pathways. Bromate formation currently represents a fairly serious barrier to the more widespread adoption of ozone as an alternative to chlorine in the U.S. Chlorine dioxide is not as good a disinfectant as chlorine or ozone, and may produce chlorite.

Chlorine therefore remains the most common disinfectant for drinking water supplies, despite the DBP problem. Currently, THMs, halogenated acetic acids (HAAs), acetonitriles, chloral hydrate, and cyanogen chloride receive most attention as DBPs in chlorinated drinking water supplies. As a result of the deficiencies of alternative disinfectants, DBP precursor removal is considered the most promising solution and considerable research is currently being conducted to understand and improve removal by pre-treatment at the plant.

Numerous studies have proven bank filtration's ability to remove DBP precursors, microbes, particles, some pesticides and trace organics. In addition to proposing maximum contaminant levels (MCLs) for total THMs and HAAs, the U.S. EPA's proposed Disinfection/Disinfection By-Product Rule, also focuses on DBP precursor removal. To be in compliance with the rule (finalized on December 16, 1998) conventional water treatment facilities treating surface water or ground water under the influence of surface water must achieve a specified percentage removal during pre-treatment, or have a total organic carbon (TOC) residual of <2 mg/L, prior to disinfection. Bank filtration facilities, with their pre-filtered feed water, will have an advantage over conventional treatment plants in meeting these new EPAs requirements.