Radionuclides in Drinking Water
Coagulation/FiltrationCoagulation and filtration is one of the most common water treatment techniques used by larger water systems, used for removing particulates and turbidity from surface water. A coagulant (typically either iron or aluminum salts with polymeric materials) is added and mixed with the influent water. The larger particles formed by coagulation are then removed from the water by filtration (typically sand, anthracite coal, or a combination of the two).
Coagulation/filtration process has been identified by EPA as a “best available technology” for uranium. It may remove up to 90 percent of uranium at pH 10 and can also remove arsenic, iron, and manganese. It may be an attractive option for systems that already have a filtration process in place.
Coagulation/filtration is generally not effective for radium removal, and uranium removal efficiency will depend on water quality parameters, especially pH. While uranium removal is more efficient at a higher pH, turbidity removal is not. At pH levels typically used in treatment plants, removal efficiencies are generally between 50 and 80 percent. For systems that do not have existing filtration, the capital costs and advanced operator skill level required may make the process unattractive.
Choosing the most suitable coagulant for a system requires an understanding of source water characteristics, especially pH. Coagulation/filtration will remove uranium at pH 10, but at this pH, removal of arsenic is not practical. Alum is not very effective and a significant amount of ferric is required. This technology will likely be considered only for surface water systems and there are very few surface water supplies that have uranium. The choice of coagulants will affect the characteristics of the residuals produced during treatment. Using this technology will require having a highly skilled system operator.
Treatment residuals generated by coagulation/filtration will include backwash water, sludge, and aged/ineffective filtration media. Liquid disposal options may include discharge to a wastewater treatment plant or disposal to an underground injection well. Direct discharge may be possible if the backwash water can be blended to significantly reduce radionuclide concentrations and total dissolved solids. Aged/ineffective media must be disposed of in an appropriate class of landfill. Refer to the disposal section of the Web site for more detailed information. Disposal Issues.