Measurements and dose consequences of tritium in municipal sewage sludge
Extended executive summary
The Canadian Nuclear Safety Commission (CNSC) regulates and monitors environmental releases of tritium from human activities, in order to protect the health and safety of the public and the environment.
In 2013, as requested by the Commission, CNSC staff measured the concentrations of tritium in sewage sludge in various municipalities in Ontario (11 in total). This request was in response to concerns raised in 2011 by members of the public during the Commission meeting for SRB Technologies (Canada) Incorporated’s (SRB) Annual Status Report on the Safety Performance of the Facility.
The tritium concentrations in sewage sludge and liquid effluent were below the analytical detection limit (i.e., the minimum concentration that can be detected by instrumentation) for all the wastewater treatment plants (WWTPs) sampled, except those in Peterborough and Pembroke.
Using the finding from the Pembroke WWTP, the doses of radiation from the measured concentration of tritium in sewage sludge were calculated for two representative persons – a worker at a WWTP involved in sewage sludge loading, and a worker at a municipal landfill who is involved in applying landfill cover to waste. The estimated annual effective doses were well below both the annual public dose limit of 1 millisievert (mSv) and the doses known to cause health effects. The dose from tritium in sewage sludge also represents a small fraction of the natural background radiation. As a result, the report concludes that there is no impact on public health.
What is tritium?
Tritium is a radioactive isotope of hydrogen. It emits low-energy beta radiation that can be absorbed by paper, plastic, glass or metal. Tritium can pose a health risk if a person ingests it by drinking water or by consuming food, or if it is inhaled or absorbed through the skin in large quantities.
Tritium is formed naturally in the upper atmosphere from the interaction of gases and cosmic radiation. Tritium is also produced as a by-product of the operation of nuclear reactors, such as the CANada Deuterium Uranium (CANDU) reactors, as well as the operation of some research reactors. Some of the tritium produced by nuclear and research reactors is recovered and used by processing facilities for the manufacture of non-electrical self-luminescent lights and paints (e.g., those used in exit signs, airport runway lights, watch dials and gunsights).
Measurements of tritium in municipal sewage sludge
Sewage sludge is the semi-solid material that is produced after sewage has been processed. Tritium occurs in the atmosphere in the same chemical forms as hydrogen. Tritium thus easily forms water molecules and is found wherever water is present, including precipitation, surface water, groundwater and sewage.
In 2013, CNSC staff measured the concentration of tritium in sewage sludge in 11 municipalities in Ontario. The treatment processes at the WWTPs sampled vary from municipality to municipality. In general, the incoming sewage is processed to produce a semi-solid material (i.e., biosolid or sewage sludge) and a liquid effluent. To get a complete picture of the tritium levels in sewage after it has been processed, CNSC staff measured the tritium concentrations in both liquid effluent and sewage sludge.
The tritium concentrations in sewage sludge and liquid effluent samples were below the analytical detection limit for the samples obtained at all the Ontario WWTPs except for those in Peterborough and Pembroke. In 2013, both Shield Source Incorporated, located in Peterborough, and SRB, located in Pembroke, were in operation, manufacturing self-luminous safety signs using tritium.
Dose consequences of tritium in municipal sewage sludge
The annual effective dose was calculated using the measured values for tritium in sewage sludge from the Pembroke WWTP. Based on a literature review of available radiation dose assessment models, the two representative persons described above were considered. These representative persons were considered as members of the public as opposed to nuclear energy workers. Exposure to members of the public due to ongoing sewage sludge disposal was determined to be negligible since members of the public would have limited access to these facilities. This approach is consistent with other studies in the literature.
The estimated doses for both representative persons were several orders of magnitude (i.e., 10,000 to 5,000,000 times) below the regulatory annual public dose limit of 1 mSv, well below doses known to cause health effects.
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