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Residues from drinking-water treatment

  • Residues from certain processes of groundwater treatment for drinking water purposes can exhibit enhanced contents of radionuclides in comparison to the natural background level of soils.
  • In unfavourable circumstances, the handling of these residues can lead to an increased level of radiation exposure for workers (inside the waterworks as well as in the course of recycling or disposal).
  • This article describes the origin of these residues and highlights which exposure pathways can lead to the increased radiation exposure of workers.

Naturally occurring radionuclides in groundwater

Radionuclides from the natural decay chains of uranium-238, uranium-235 and thorium-232 are present in trace quantities in all rocks. If groundwater comes into contact with these rocks, a small proportion of the radionuclides dissolve from the rock and thus enter the groundwater.

The activity concentration is highly dependent on the type of rock and its chemical composition, as confirmed in the study "Radiation exposure from natural radionuclides in drinking water in the Federal Republic of Germany" (in German), which was conducted by the Federal Office for Radiation Protection (BfS). The table below shows an extract of the results:

Median values (Med) and maximum values (Max) of the activity concentrations of frequently occurring natural radionuclides in untreated groundwater in millibecquerel per litre with respect to the rock type of the aquifer ("Radiation exposure from natural radionuclides in drinking water in the Federal Republic of Germany")
Rock typeU-238Ra-226Pb-210Po-210Ra-228
MedMaxMedMaxMedMaxMedMaxMedMax
basalt<0.745.71.02.81.9200.995.6<0.815.4
gneiss<0.74152.27.07.8291.88.34.37.8
granite1.25312989.5702.0191129
limestone6.02105.91603.2231.3185.4110
sand3.61207.1362.1181.3196.746
shale1.1972.6272.1191.89.43.726
sandstone17590123803.6312.96309.3210
other rock2.56208.03004.12702.04107.5130

The activity concentration of radionuclides in untreated groundwater also depends on:

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Residue types of water treatment processing

In order to comply with the requirements of the Drinking Water Ordinance, groundwater must be treated to drinking-water quality where appropriate. Depending on the chemical composition of the water, different water treatment processes are used to remove undesirable substances. According to a definition [1] issued by the German Technical and Scientific Association for Gas and Water (DVGW), the resulting residues are primarily:

  • iron-containing slurries
  • lime-containing slurries
  • slurries from flocculation
  • activated carbon
  • screenings

Further residues can arise during the replacement of filter material (e.g. filter sand/filter gravel) or special absorbing resins, as well as during plant dismantling (e.g. precipitates in tubing).

Until now, the specific removal of radionuclides during water treatment has not normally been carried out. Nevertheless, radionuclides can accumulate in some of the residues to a level exceeding that naturally found in soils and rocks. Surface water has a lower concentration of natural radionuclides than groundwater, and so residues with enhanced contents of radionuclides are to be expected during the processing of groundwater in particular.

Median values (Med) and maximum values (Max) of the activity concentration of frequently occurring natural radionuclides in surface water and groundwater in millibecquerel per litre ("Radiation exposure from natural radionuclides in drinking water in the Federal Republic of Germany")
Type of waterU-238Ra-226Pb-210Po-210Ra-228
MedMaxMedMaxMedMaxMedMaxMedMax
groundwater5.46208.43802.7821.66307.8210
surface water1.3394.3322.2291.6194.242

With regard to residues from the processing of groundwater into drinking water, enhanced contents of radionuclides are known to occur in:

  • iron slurries and lime slurries from neutralisation
  • absorber resins, activated carbon and filter gravel from iron/manganese removal

There are also reports of incrustations containing radionuclides in thermal water sources (springs).

Enhanced contents of radionuclides are not to be expected either in flocculent slurries, which arise primarily during the treatment of surface water, or in screenings.

In its Fact Sheet W256[2], the German Technical and Scientific Association for Gas and Water (Deutscher Verein des Gas- und Wasserfaches, DVGW) offers further information on the occurence, the re-use and the disposal of radionuclide bearing residues from waterworks.

Iron slurriesShow / Hide

Dissolved iron and manganese, which can occur in groundwater with a low redox potential (“reduced groundwater”), are removed during the processing of untreated water into drinking water. The addition of an oxidising agent (air or ozone, for example) results in the oxidation of soluble iron(II) and manganese(II) ions and the precipitation of iron(III) and manganese(IV) oxides or hydroxides.

As these oxides have very reactive surfaces, dissolved heavy metals and therefore also radionuclides (above all radium-226 and radium-228) can accumulate on them. Sand or gravel filters remove oxides and hydroxides from the water, and the filters are purged regularly to prevent them from clogging up. This flushing results in thin slurries that primarily contain the iron oxides, whereas the manganese dioxides mostly remain on the filter gravel. This treatment process is also used for the removal of arsenic.

The specific activity of radium-226 and radium-228 in iron slurries ranges from less than 0.2 to 10 becquerel per gram and, in a few exceptions, up to 50 becquerel per gram. In Germany, the annual formation of up to 6,500 tons of iron slurries with an enhanced content of radionuclides is estimated.

Lime slurries from neutralisationShow / Hide

As water with a low pH can cause corrosion damage in the water distribution system, the pH is adjusted to a value between neutral and slightly basic during water treatment process.

At some waterworks, aeration or degassing is used to remove the dissolved carbon dioxide. In oxygen-poor groundwater, this can result in the precipitation of iron or manganese oxides, which must then be filtered out of the water (cf. iron slurries). It is also possible to raise the pH by passing the water through filters consisting of limestone or dolomite. Regular backflushing is required to prevent the filter from clogging. The slurries may contain fine particles of the filter medium and somethimes iron and manganese precipitates as well [1]. The higher pH leads to reduced mobility of heavy metals. This affects natural radionuclides – especially lead-210 and polonium-210 as well as, to a lesser extent, radium-226 and radium-228.

The specific activity of lead-210 in lime slurries ranges from less than 0.2 to 5 becquerels per gram and, in a few exceptions, up to 20 becquerels per gram. Up to 500 tons of these slurries arise across Germany each year.

Absorber materialsShow / Hide

Previously, drinking-water treatment rarely involved the specific removal of radionuclides (especially uranium). Since 2016, however, the Drinking Water Ordinance established limit values for natural radionuclides. At certain water suppliers, this may result in the need for new treatment processes to remove radionuclides. So far, however, additional treatment processes are only known for the removal of uranium.

One highly effective way of removing uranium from water is to use ion-specific absorbing resins. If the resins are heavily loaded, specific activities of several hundred becquerel per gram of wet weight can occur for uranium-238 and uranium-234. In order to simplify the reuse or disposal of the residues, the resins are exchanged once they are only partly loaded – that is, after shorter times in service. Depending on how the process is managed, this may lead to a lower specific activity (10 or 50 becquerel per gram respectively). The annual quantity of residues with an enhanced content of radionuclides is estimated as less than 100 tons.

Another absorber material is activated carbon, but water suppliers rarely use filters made purely of this material. To complement the processes of manganese removal, iron removal or neutralisation, some waterworks use activated carbon in multi-layer filter units. The specific activity for radium-226 and uranium-238 in used activated carbon ranges from less than 0.2 to 10 becquerel per gram. Annually, less than 1,000 tons are expected to arise as residues with an enhanced content of radionuclides.

PrecipitatesShow / Hide

Precipitates ("incrustations", also referred to as scales) can occur in the tubing of waterworks. There is very little literature available on the specific activities concentrations of incrustations in pipes from waterworks. For thermal water sources (springs), specific activities are reported to be in the order of 10 becquerel per gram for radium-226 or radium-228. Comparable precipitates can also arise in drinking-water treatment plants – depending on pressure, temperature and geochemical conditions.

However, no data about the radiological inventory of these residues in water treatment facilities are available. The quantity of contaminated plant components arising across Germany is estimated at less than 500 tons per year

Filter gravelShow / Hide

At waterworks, filter gravel remains in operation over many years or even decades before it is replaced. As the operating time increases, the precipitation of manganese leads to a growing layer of manganese dioxide around the filter grains (see "Iron slurries"). Manganese dioxide is highly effective at taking up ("sorbing") heavy metals and therefore also radionuclides. Over the years, therefore, the specific activity of radium-226 and radium-228 in filter gravel/filter sand may be higher than in the mainly iron-containing slurries from backflushing. These radionuclides are to be found almost entirely in the coatings that have grown on the gravel.

Until now, however, very little research has been carried out into these residues. From the available data, it is possible to deduce an enhanced content of radionuclides by up to a factor of five relative to the slurry in individual cases. The activity concentrations from previous analyses range from less than 0.2 to 50 becquerel per gram for radium-226 and radium-228. The accrued quantity of residues with an enhanced content of radionuclides can only be estimated at several tens of thousands of tonnes. Given the long times in service, filter gravels only occasionally arise as residues in individual waterworks.


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Disposal or recycling

Pursuant to the German Recycling Act (KrWG), recycling is preferable to disposal. However, recycling does not take precedence in the event of a danger to humans or the environment. Of the above residues, the following can in principle be recycled:

  • iron slurries
  • lime slurries from neutralisation
  • filter gravel

In its Fact Sheet W221-3[1], the German Technical and Scientific Association for Gas and Water (Deutscher Verein des Gas- und Wasserfaches, DVGW) recommends various ways of recycling these residues:

  • Iron slurries are used in environmental engineering to reduce the level of phosphate and hydrogen sulfide. In addition, they are used as a secondary raw material in the brick and cement industries as well as in the production of plant granulate. In the past, some 35% of iron slurries were disposed of to landfill; this proportion is predicted to fall in the future as a result of waste legislation.
  • In agriculture and forestry, lime slurries from neutralisation are applied to land to improve (“ameliorate”) the pH in the soil. It is also conceivable to recycle these residues – for example, in the production of lime and cement or the production of artificial soil substrates.
  • As filter gravel from iron and manganese removal remains in use at the waterworks over a period of several years to decades, it only occasionally arises as a residue at the water distribution companies. Therefore, no fixed disposal routes have yet gained general acceptance for these residues. There are examples in which the gravel is used in road contruction or for the commissioning of new filter units at other waterworks. It could also be reused in landscaping. Information on the quantity of recycled or landfilled residues has not been published and is not available to the DVGW.

As yet, there is no known recycling option for incrustations, whereas activated carbon and absorbing resins, in principle, are suitable for thermal recycling due to their high carbon content.

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Legal framework

At the beginning of 2014, the European Atomic Energy Community (EURATOM) published the European basic safety standards directive on radiation protection. In this directive, residues from groundwater filtration facilities are classified as an industrial sector involving naturally occurring radioactive materials. Euratom Member States had to implement these regulations into national law.

Radiation Protection Act and Radiation Protection Ordinance

In Germany, this implementation was achieved in 2017 with the Radiation Protection Act. To supplement this, the Radiation Protection Ordinance was revised in 2018. Both legal regulations have been in force since 31 December 2018.

In Annex 1 to the Radiation Protection Act, filter gravel, filter sand and granular activated carbon are included in the list of residues to be considered for the first time and are therefore subject to the regulations of the Radiation Protection Act.

Further guidelines

Insofar as residues from waterworks are reused in building products, it is also necessary to adhere to the guidelines of the European recommendation on natural radioactivity in building materials, which states that standard building materials should not lead to enhanced radiation exposure of the general public. The Radiation Protection Act also lays down regulations for building products, and these regulations also entered into force on 31 December 2018.

Furthermore, it is necessary to check whether the planned mode of recycling or disposal is permissible according to waste legislation. Especially in the case of reuse in landscaping or road building, the requirements for the material recycling of mineral residues/waste, as set out by the Federal/Laender Working Group on Waste (LAGA), must be adhered to in relation to the leaching behaviour of mineral residues.

For the transport of materials, the European Agreement concerning the International Carriage of Dangerous Goods by Road (ADR) must be adhered to.

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Exposure pathways and exposure scenorios

Depending on the quantity, the radionuclide content in the residue, and the chosen mode of disposal or recycling, employees – in waterworks or at waste disposal or recycling companies – may obtain an enhanced radiation exposure when handling residues from waterworks. Based on a dose estimation, it is possible to determine whether there is a risk of significantly enhanced radiation exposure for employees (more than 1 millisievert per year in addition to the natural background radioactivity). A dose estimation should take account of the following situations:

  • time spent by employees in rooms in which residues are stored
  • handling of residues during storage, recycling, transport or disposal
  • maintenance and/or cleaning of operating equipment

From a radiation protection perspective, the following exposure pathways must be considered here:

Furthermore, if residues are sent to landfill or reused in road building, and especially in landscaping, radionuclides may be dissolved in seepage water and carried into the groundwater. The use of this groundwater can potentially lead to an additional exposure pathway for the general public. Therefore, the use of affected groundwater from a private well for drinking water purposes or for irrigation must be taken into account in dose estimations.

Estimation of radiation exposure for employees at waterworks, wasta disposal companies and recyclers

In various studies, the radiation exposure of employees at waterworks, disposal companies and recyclers has been estimated in relation to the handling of iron, manganese and lime slurries (Ermittlung von Arbeitsfeldern mit erhöhter Exposition durch natürliche Radionuklide und überwachungsbedürftige Rückstände – Rückstände aus der Trinkwasseraufbereitung, Teil I und Teil II [“Identification of working environments with enhanced exposure to natural radionuclides and residues requiring surveillance – Residues from drinking-water treatment, Part I and Part II”, in German]). The results showed that, even with the least favourable assumptions, the dose reference value for the general population of 1 millisievert per year is unlikely to be exceeded. Likewise, the previously published activity contents for activated carbon and incrustations from waterworks do not point to increased radiation exposure of the general population.

Based on current knowledge, the exceedance of the dose reference value cannot be ruled out completely in unfavourable conditions during the disposal or recycling of filter gravel from manganese/iron removal and of heavily loaded absorber resins resulting from the specific removal of uranium. It is advisable to consider such situations on a case-by-case basis. If, following this assessment, the dose reference value is actually exceeded, it is essential to clarify which dose-reduction measures can be introduced with a reasonable amount of effort in these situations. For example, these measures include wearing personal protective equipment or searching for alternative disposal routes.

Calculation guidelines

The BfS is currently drawing up calculation guidelines that allow an estimation to be made of the effective dose for employees and members of the public due to exposure to naturally occurring radioactive materials (Calculation Guide NORM). Until such time as this calculation guideline is completed, the BfS offers recommendations for a simplified estimation of the radiation exposure of employees and members of the public.

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References

[1] DVGW (2000): Rückstände und Nebenprodukte aus Wasseraufbereitungsanlagen; Teil 3: Vermeidung, Vermarktung und Verwertung. DVGW-Arbeitsblatt W221-3 (in German)

[2] DVGW (2020): Radionuklidhaltige Rückstände aus der Aufbereitung von Grundwasser – Bewertung und Entsorgung“ DVGW-Arbeitsblatt W256 (in German)

State of 2024.04.17

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