Can activated carbon filtration of groundwater cause radiation safety problems?

Abstract

Activated carbon filtration is commonly used to remove groundwater contaminants in water utilities. During its use, also groundwater radon is adsorbed on activated carbon where it radioactively decays to Pb-210 within a few days. The intermediate stages of radioactive decay produce gamma radiation outside the filter, which can be harmful to health. In addition, EU legislation sets a clearance level of 1 kBq/kg for the Pb-210 concentration of waste, above which disposal of carbon waste can be complicated, depending on national legislation. This study focused on activated carbon filters from two groundwater plants. Measurements were made over the service lifetime of the activated carbon and included measurements of adsorption of radon from water to carbon, radiation dose rates outside the filters and Pb-210 concentrations in the carbon. The study observed a rapid decrease in the radon adsorption rate of activated carbon during the first year of use. This phenomenon has a significant effect on the concentration of Pb-210 in the activated carbon waste at the end of service life. The calculated forecast model clearly showed that the risk of exceeding the clearance level of Pb-210 increases if the radon concentration in the raw water exceeds 100 Bq/L.

Natural radioactivity of residues from groundwater treatment facilities in Finland

“…Even U-238 and Pa-234m show disequilibrium in some samples. The radiation safety of GAC filters has been investigated previously due to the gamma radiation emitted by radon progeny and including the accumulation Pb-210 in the GAC filters [29]. Here, we report also the other natural radionuclides in the same GAC filter samples (table 4).…”

“…Soil samples of backwash discharge-affected soil (N = 5) and control soil samples (N = 5) were taken if the facility had previously discharged backwash water into the soil. Data for GAC filter samples from which Pb-210 was reported in [29] are present here for the other radionuclides as mean values for each facility.…”

“…efficiency drops, or the water quality does not comply anymore with regulations. For GAC-filters, the time of use before the change effects the amount of accumulated Pb-210 [29].…”

“…Filters that are in use under a water layer or inside containers do not cause internal radiation exposure to workers at GTF's through inhalation or ingestion during normal operation. In GAC-filters the short-lived radon daughters may lead to elevated external dose rates close to the filter [29], but the occupancy of GTF workers very close to filter materials is low for GAC-filters [29], and also for backwash sludges [15]. The filters and associated backwash sludges may cause exposure or workers mainly when handled for removal.…”

“…Compared to the reference levels, the main cause of worker exposure to natural radiation at GTF's is indoor radon [27,28]. It is also known that granulated activated carbon (GAC) filters can accumulate short lived radon progeny during the operation of a GTF at a level that can be of concern for the radiation exposure of workers [29]. There is a fair amount of information about drinking water, indoor radon and GAC filters at GTF's in Finland, but there is little data about the long-lived natural radionuclides from the solid residues formed at GTF's.…”