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Residues from drinking-water treatment
- In water treatment facilities, radionuclides previously dissolved in untreated water can accumulate undesirably in treatment residues.
- Residues with enhanced radionuclide content arise primarily in two of the typical treatment processes: during manganese and iron removal and during deacidification.
- For waste slurry from manganese and/or iron removal and from neutralizing, an inadmissibly high radiation exposure is not to be expected from the currently practised recycling and disposal methods, even where there is a high radionuclide content.
In water treatment facilities, radionuclides previously dissolved in untreated water can accumulate undesirably in treatment residues. Depending on the treatment process and on the untreated water composition, residues can arise with radionuclide content (specific activity) that exceeds the natural background content of soil and rocks many times over. Residues with an enhanced content of natural radionuclides arise primarily in treatment processes for the use of groundwater for drinking water purposes.
Residue type and specific activity
Often, groundwater must first be treated before it can be used as drinking water. Residues with increased radionuclide content arise primarily in two of the typical treatment processes:
During the removal of manganese and iron, dissolved manganese and iron are removed from the untreated water by applying oxygen. This encourages the formation of iron oxide and manganese oxide, which are hardly soluble.
As these oxides have very reactive surfaces, dissolved heavy metals and therefore also radionuclides can accumulate on them. The arising oxides are then removed from the water using sand or gravel filters. The filters are purged regularly to aware them from clogging. The accumulated radionuclides pass with the oxides into the slurry that is created by the rinsing process.
As iron oxide and manganese oxide sometimes remain on the filter gravel, radionuclides can also accumulate there. Because the filter gravels are used for many years, these residues arise only rarely in comparison to the slurries.
The specific activity for the reference nuclides radium-226 and radium-228 is less than 0.5 becquerels per gram to 20 becquerels per gram in filter gravel or slurries; in exceptional cases up to 50 becquerels per gram.
To avoid corrosion damage to the pipeline network from aggressive acids, the pH-level of untreated water is raised during neutralizing. A typical procedure for this is a chemical reaction using a limestone bed filter.
The pH-level has a great influence on the mobility of heavy metals. Many heavy metals, such as lead, are more mobile at lower pH-levels ("acid water") than at higher pH-levels. If the pH-level increases, heavy metals and radionuclides precipitates on surfaces (for example on particles).
Limestone bed filters must likewise be backflushed in order to remain functional. This can also result in residues containing radionuclides. The specific activity for the reference nuclide lead-210 is less than 0.5 becquerels per gram to 10 becquerels per gram for this slurry; in exceptional cases up to 20 becquerels per gram.
The processes described here can also be used in combination. Accordingly then, the nuclides radium-226, radium-228 and lead-210 can occur in the slurries. As far as is known, the specific activity for these nuclides is less than 0.5 becquerels per gram to 10 becquerels per gram for these slurries; in exceptional cases up to 20 becquerels per gram.
Less widespread to date is the targeted removal of uranium. In a few waterworks, the uranium concentration is higher than the limit value of 10 micrograms per litre set out in the new drinking water ordinance. In order to adhere to this limit value, special absorbent resins are used. Once used, these are loaded with uranium and can show specific activities of several hundred becquerels per gram for uranium-238 and/or uranium-234.
Disposal of the residues
According to legal waste restrictions, the recycling of residues takes precedence over disposal. To date, around one third of backflushing slurry has been disposed of while the majority has been recycled.
Depending on the chemical composition, it is possible according to the technical rules on Fact Sheet W-221-3 from the German Technical and Scientific Association for Gas and Water (DVGW) to recycle residues
- in the cement and brick industry,
- in the manufacturing of plant granulate,
- in road construction,
- as a precipitant in waste water works and
- in agriculture and forestry (only neutralizing slurry).
Water suppliers have also implemented this in the past.
Filter gravels are used for several years or even decades in water works. Exchanging them usually takes place only when renovation work is carried out on water works. Information on the quantities of residues recycled or disposed of is not published and is also not available to the DVGW. Individual cases are known where the gravel has been used for the operational start-up of new filter systems in other water works or in road construction.
Currently, exchanger resins are usually regenerated by chemically removing the uranium from the absorbing resins. The resins can then be reused in drinking water treatment.
Indeed, absorbing resins could be recycled thermally in conventional waste incineration plant but the high uranium content precludes this. Disposal of the exchanger resins in landfills of classes 0 to 3 is not possible due to the high calorific value. The resins can therefore only be disposed of underground or in special waste incineration plant. Where there is lower uranium content, thermal recycling in conventional waste incineration plant is easier to implement. An alternative is to load the resins partly, this means to shorten the use of the resin before exchanging them.
There is comprehensive data on the radionuclide content of slurries from manganese and/or iron removal and also from neutralizing, as well as information on recycling and/or disposal. An inadmissibly high radiation exposure is not to be expected from the currently practised recycling and disposal options, even where there is a high radionuclide content.
There is little information available on quantity, radionuclide content and disposal practices of filter gravel and absorbing resins. Although filter gravel is seldom exchanged, several hundred tons of residue can be generated when this does take place. When exchanging filter gravel with high specific activities, the BfS estimates that an exceedance of the dose guideline of 1 millisievert per year cannot be ruled out in individual cases. In these cases, individual testing is recommended and if necessary limiting requirements can be set by the responsible state authority.
State of 2017.07.18