TENORM: Drinking Water Treatment Residuals
All rocks and soils contain some trace amounts of Naturally Occurring Radioactive Materials Materials found in nature that emit ionizing radiation that have not been moved or concentrated by human activity.(NORM). When a drinking water source, either surface or groundwater, comes in contact with NORM-bearing rocks and soils, radionuclides can accumulate in the source water. The most frequently-occurring radionuclides (and their decay productsThe atoms formed and the energy and particles emitted as radioactive material decays to reach a stable form.) found in source water include:
Radium breaks down to form the radioactive gas, radon. Because uranium, radium and radon can dissolve in water, these radionuclides can be present in source water. The likelihood that source water in a certain area will contain radionuclides depends on soil and rock conditions in that area.
EPA has specific regulations under the Safe Drinking Water Act (SDWA) that limit the amount of radioactivity allowed in community water systems.
As water is treated to remove impurities, radionuclides may collect as sediment and sludge and also build up in filters, tanks and pipes at treatment plants. Because the NORM is concentrated due to human activity, it is classified as Technologically Enhanced Naturally Occurring Radioactive Material (TENORM). Most of this waste is disposed in landfills and storage ponds, or is land-applied.
Radionuclides in Drinking Water
Across the United States, there are varying amounts of NORM found in drinking water. Community water systems are required to test for and report levels of radionuclides in drinking water to ensure that consumers do not drink water that exceeds standards set by the Environmental Protection Agency. These standards are called Maximum Contaminant Levels (MCLs) and are based on chronic exposure from drinking two liters of water a day over seventy years. For more information about MCLs and radionuclides in drinking water, visit the Radionuclide Rule page from the Office of Water on EPA.gov.
While EPA sets standards for large and small entity drinking water treatment plants, we do not set standards for private drinking wells. The limits that EPA sets for community drinking water systems under the SDWA can be used as guidelines for drinking water wells.
The concentration and distribution of radionuclides in drinking water varies from one area to another depending on the following variables:
- Geological location.
- Surface water and groundwater chemistry.
- Radionuclide solubility and half-life.
- Water withdrawal rates.
In order to mitigate the amount of radionuclides in the water that people drink, water treatment plants use a variety of different methods to remove or dilute the amount of radionuclides to a level that meets the MCLs. There are a variety of water treatment processes that remove radioactivity from community drinking water systems.
Water Treatment Processes
At large water treatment plants, aluminum sulfate (“alum”) is added to water and forms a gel which bonds to the particles (including radionuclides) found in the source water. This substance gradually collects at the bottom of the treatment tanks and is referred to as alum sludge. Alum sludge is removed by sedimentation and filtration (through the use of sand, filters or membranes).
Similar to alum treatment, lime softening is used on small and large water supply systems to soften water with the addition of calcium hydroxide. Calcium hydroxide raises the pH of water, causing calcium and magnesium to settle out and form a solid sludge. Eighty to 90 percent of the radium in the water becomes trapped in the sludge, which is removed by sedimentation and filtration.
Ion-exchange beads are used on smaller water supply systems to soften water by replacing Calcium ions (Ca2+) and Magnesium ions (Mg2+) ions with Sodium ions (Na+). In this process, about 95 percent of the radium is also removed. Ion-exchange is also used for the removal of uranium. The beads are usually backwashed and reused. Radionuclide content eventually builds up in the beads after prolonged usage and are then disposed.
Reverse osmosis is a pressure-driven membrane separation process used in smaller water supply systems. Water is forced through a membrane with small pores at a very high pressure. Any molecules larger than the pore openings are collected and separated from the stream. Treatment residuals generated by reverse osmosis may include liquid concentrate (reject water) and spent/used membranes. Disposition of reject water and membranes are handled at the state level.
Disposal, Reuse and Waste
Not all treatment facilities produce TENORM wastes. The volumes and concentrations of TENORM in the residuals vary according to the amount of radionuclides in the water and the treatment methods employed.
However, thousands of metric tons (MT) of TENORM wastes are generated each year by U.S. drinking water treatment facilities. Most of the contaminated waste is sludge, spent filters, ion exchange beads and spent/used membranes from reverse osmosis.
A variety of methods are used to dispose of drinking water treatment residuals, including storage ponds, landfills, land spreading/soil conditioning and sewer systems.
The majority of alum sludge generated from large community drinking water treatment systems is disposed of in storage ponds. Any radium or uranium present in the sludge will settle in sediment at the bottom, which may have to be periodically dredged and properly managed.
Alum sludge dredged from storage ponds can be disposed of in solid waste landfills or monofills (an impoundment that is permitted to accept only one waste stream). Contaminated materials are typically covered and compacted on a daily basis. In order to accept these materials, landfill permits require that steps are taken to prevent radon emissions and radionuclides from leaching into the groundwater. Landfill permits are issued locally and regulated at the state level.
Land Spreading/Soil Conditioning
Some alum sludge dredged from storage ponds is land applied for beneficial reuse (i.e. to improve soil conditions or to fertilize the soil). The sludge is plowed directly into the soil to limit water runoff and to ensure proper sanitation. Land spreading requirements vary by state.
Sludge and other wastes from some drinking water treatment processes can be released into sanitary sewers and are ultimately treated at permitted waste water treatment facilities. These permits are issued locally and regulated at the state level.
EPA provides guidance on managing water treatment wastes and addresses the following topics:
- Technologies for treating and removing radionuclides from drinking water.
- General principles of radioactivityThe emission of ionizing radiation released by a source in a given time period. The units used to measure radioactivity are becquerel (Bq) and curie (Ci)..
- Recommendations for occupational safety of workers at the Community Water Systems (CWS) including guidance for radiation protection and protection from radon.
- Regulatory requirements governing the radioactive wastes generated by the CWS.
To learn more about EPA drinking water treatment waste management guidance, see, A Regulators' Guide to the Management of Radioactive Residuals from Drinking Water Technologies.