TENORM: Drinking Water Treatment Wastes
Certain rock types naturally contain radioactive elements referred to as NORM (Naturally Occurring Radioactive Materials
Naturally Occurring Radioactive MaterialsMaterials found in nature that emit ionizing radiation that have not been moved or concentrated by human activity.). When a source of drinking water comes in contact with NORM-bearing rocks, radionuclides may accumulate in the water to levels of concern. The predominant radionuclides found in water include:
- Radium.
- Thorium.
- Uranium.
- Their decay productsdecay productsThe atoms formed and the energy and particles emitted as radioactive material decays to reach a stable form..
As water is treated to remove impurities, radionuclides may collect and eventually build up in filters, tanks and pipes at treatment plants. The small amounts of NORM present in the source water may concentrate in sediment or sludge. 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 lagoons, or is land-applied.
Learn more about the Safe Drinking Water Act.
Most drinking water treatment sludges from conventional treatment contain radium (Ra-226) levels comparable to typical concentrations in soils. However, some water supply systems, primarily those relying on ground water sources, may generate sludge with much higher Ra-226 levels. Some water treatment systems generate sludges with higher Ra-226 levels because they are more effective than others in removing naturally occurring radionuclides from the water.
Radiation Levels
The concentration and distribution of radionuclides in drinking water varies from one area to another depending on the following variables:
- Geological location.
- Ground water pH.
- Redox potential.
- Radionuclide solubility and half-life.
- Water withdrawal rates.
- Amount of sludge produced per unit volume of water processed.
Radium levels are typically found in higher concentrations in ground water than in surface water. Concentrations in ground water typically range from 0.5 - 25 picocuries per liter (pCi/L) compared to 0.01 - 1 pCi/L for surface water. Radium concentrations as high as 200 pCi/L have been reported, although this is rare (levels are seldom seen above 50 pCi/L).
Radium-226 average concentration in community drinking water supplies is estimated to range from 0.3 - 0.8 pCi/L. Learn more about community drinking water supplies.
Uranium activity as high as 652 pCi/L has been observed in both surface and ground water samples. The average uranium concentrations in surface and ground water are believed to be about one and three pCi/L, respectively. The population weighted average uranium concentration in community drinking water supplies is estimated to range from 0.3 - 2 pCi/L.
The presence of radon in ground water is known to vary significantly, with the concentration being over six orders of magnitude different from that of Ra-226. The geometric mean of ground water radon concentrations is nearly 1,000 pCi/L. For radon, the population weighted average is believed to range from 194 - 780 pCi/L.
Thousands of metric tons (MT) of TENORM wastes are generated each year by U.S. water treatment facilities. This is equivalent to 600 MT of TENORM per utility. Most of the contaminated waste is filter sludge and the remainder is ion exchange resins and charcoal.
Not all treatment facilities produce TENORM wastes. The degree of contamination varies according to the natural abundance of radionuclides in the water and the treatment methods employed.
Removal
There are a variety of water treatment processes that remove radioactivity from community drinking water systems. Two common processes are lime softening, and ion-exchange and activated charcoal.
Lime softening
Lime softening is used on large water supply systems to soften water by 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. This process typically produces about four cubic yards (3.1 m3) of dewatered sludge per million gallons of processed water.
Ion-Exchange and Activated Charcoal
Ion-exchange resins are used on smaller water supply systems to soften water by replacing Ca2+ and Mg2+ ions with Na+ ions. In this process, about 95 percent of the radium is also removed. However, the resins are usually back washed for reuse rather than being disposed. The backwash water, which contains radium, is typically discharged to storm sewers, underground injection wells or septic tanks, or is back washed to another ion-exchange column for the selective removal of radium. Radionuclide content eventually builds up in the resin after prolonged usage.
Learn more about radionuclides in drinking water.
Disposal and Reuse
A variety of methods are employed for disposing of drinking water treatment sludge, including lagoons, landfills, sewer systems, deep well injection and land spreading/soil conditioning.
Lagoons
Approximately 42 percent of sludge is disposed of in lagoons. Any radium present in the sludge will settle in bottom sediments, which may have to be periodically dredged and properly disposed of.
Landfills
Approximately 30 percent of generated sludge is disposed of in landfills. Contaminated materials are typically covered and compacted on a daily basis. Features such as clay layers are emplaced above and below the buried waste to prevent radon emissions and radionuclides from leaching into the ground water.
Sewer Systems
Approximately five percent of sludge is discharged into sewer systems. Sludge and other wastes released into sanitary sewers are ultimately treated at sewage treatment facilities. Some of the discharged sludge may wind up in storm sewers. Storm sewers are typically routed to natural bodies of water, where sludge may accumulate in aquatic sediments.
Deep-Well Injection
Deep-well injection involves the pumping of sludge into a stable geologic formation. Deep-well injection is not commonly used and is specifically prohibited in the states of Wisconsin and Illinois. Because of its potential adverse impact on ground water aquifers, EPA uses its authority under the Safe Drinking Water Act to control and discourage this practice.
Land Spreading/Soil Conditioning
About 20 percent of sludge is disposed of by land application 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.
EPA has developed guidance on managing drinking water treatment waste and is conducting a study to learn more about the levels of radioactivity in these wastes.
A Regulators' Guide to the Management of Radioactive Residuals from Drinking Water Technologies (PDF) (16 pp, 637K About PDF)
This report includes guidance on managing water treatment wastes and addresses:
- Technologies for treating and removing radionuclides from drinking water.
- General principles of radioactivityradioactivityThe emission of ionizing radiation released by a source in a given time period. The units used to measure radioactivity are curie (Ci) and becquerel (Bq)..
- 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 CWSs.