NRPB-W32: Non-technical summary

Non-technical summary

Waste water treatment works receive small amounts of radioactivity that have been discharged under regulation from hospitals and research laboratories. These treatment works produce water and solid sludge, and the radioactivity becomes distributed between these two materials. The water is released into rivers or the sea, and the sludge may either be disposed of, for example by burning it, or be used as a fertiliser on agricultural land. We need to be able to evaluate the effects of using the sludge as a fertiliser in terms of how much radioactivity is transferred to the food that is produced. To do this we have to have information on the radioactive isotopes involved, how much of the radioactivity remains with the sludge, the types of crop that can be grown on the land after sludge has been applied to it, how long we have to wait before we can grow and harvest these crops and how much of the radioactivity will be taken up. The information on these different subjects has been reviewed by NRPB, with some financial support from the Food Standards Agency.

The Environment Agency regulates the discharge of radioactivity by hospitals and research laboratories and publishes information on the radioactive isotopes involved. From this, we can identify those that are most commonly used. Some of these isotopes have radioactive half-lives of only a few hours or days. The time delay between the discharge of the radioactivity and the harvesting of the crops is therefore important. The time taken to produce sludge was obtained from the operators of treatment works, and there is a voluntary code, the Safe Sludge Matrix, that sets out the delays that are needed before different types of crop can be harvested. We used this information to produce a list of 18 radioisotopes that required further study.

We reviewed published information on how much of each radioisotope would be retained by the sludge. Advice was also obtained from an expert in the water industry. For some radioisotopes, good information was already available, but for others data were scarce and more research is needed. We also studied the published information on uptake of radioisotopes by crops grown on treated land. Very little specific information could be found, and there was none on the radioisotopes of interest in this study. However, some useful general points were found. For example, several of the radioisotopes of interest can form compounds that can be lost from the treated soil into the atmosphere before crops can take them up. In addition, the amount that can be taken up is likely to change as the sludge breaks down in the soil. Finally, when we want to predict what the effects of a future discharge will be, it is important to take account of the Safe Sludge Matrix so that only the appropriate crop types are included.

Measurements available so far indicate that it should be possible to study the distribution of radioisotopes between sludge and water in an operational waste water treatment works. However, field studies of uptake from treated land into crops are unlikely to be feasible because the amounts expected in the plants would be too small to be detectable.

Last reviewed: 1 September 2009