Radiation impact of ashes from the combustion of bottom sediments in a municipal sewage treatment plant, Czajka, located in Warsaw, Poland

The “Czajka” Sewage Treatment Plant (Fig. 1) processes many liquid pollutants generated in the Warsaw agglomeration. The incineration plant, located in the “Czajka” plant, is one of the largest facilities of this type in Poland. Approximately 500 tons of sewage sludge is produced daily in the “Czajka” plant. The combustion process carried out in the incineration plant reduces their amount by 10 times. However, the annual amount of waste is so large that there is an urgent need to solve the problem of its disposal.

Fig. 1.

The sewage sludge incineration system of the incineration plant consists of two independent technological lines that operate in a continuous mode. Each of them is equipped with a separate furnace with a recuperator, a heat recovery boiler, and a three-stage purification system. The installation has a capacity of 24.5 t/h of wet sludge, and the incineration furnaces present use the Pyrofluid® fluidized bed technology that can burn 150 tons of dry sludge in 24 h. To manage and use the ashes produced during combustion, e.g., in agriculture as fertilizers or in construction as an additive in cement production, they should be tested, among others, for the content of radionuclides, and their levels should be assessed.

Thus, a series of fly ashes collected from the “Czajka” plant was subjected to intensive radiometric tests to accurately estimate the levels of radioactivity.

In Figs. 2–4, a part of the γ-spectra of a representative sample of ashes is presented.

The survey included a measurement error (0.05–3.4%), an error in determining the ashes composition (1%), and a mathematical energy-efficiency calibration error (0.1%) [7]. The measurement time and weighing error can be neglected. Hence, the resultant uncertainty is 3.5%.

The calculated IAFEPE values are as follows: IAFEPE_{Marinelli_beaker} = 18.015 keV and IAFEPE_{Simplified_beaker} = 18.325 keV. It means that both sample geometries are comparably effective. The deterioration in the recording efficiency of the photons emitted from the simplified beaker compared to the Marinelli beaker is compensated by the increase in the mass of the sample.

Table 2 displays the specific activity of the radionuclides found in the samples. They are part of three natural decay sequences. In addition to those, we identified ⁷Be, ⁴⁰K, and ¹³⁷Cs. ⁴⁰K is a naturally occurring radionuclide, ⁷Be is cosmogenic, and ¹³⁷Cs is an artificial fission product. ¹³⁷Cs was found in the recent global fallout due to nuclear tests and accidents at nuclear reactors like Windscale, Chernobyl, and Fukushima. The activities of ¹³⁷Cs and ⁴⁰K in environmental samples monitored by Central Laboratory for Radiological Protection (CLRP) are approximately at the same levels [8]. ⁷Be is produced in the solar system relatively recently and through spallation in the Earth’s atmosphere. It happens on Earth due to precipitation in the form of both rain and dry fallout. Due to the unknown time that passed between the formation and its appearance in the food chain, the found radioactivity of ⁷Be cannot be compared with its level in the atmosphere, surface waters, etc.

In the measured samples, we observed slightly higher levels of ²²⁶Ra and ²²⁸Ac in comparison with soil samples [8]. However, it is not advisable to compare the radioactivity of the ashes with that of any of the environmental samples. It is because the material from which the ashes are obtained undergo multiple metabolic, chemical, and physical transformations in food chains.

The specific activities of the samples are very similar after their long-term storage. However, this is not the case with ⁷Be, which simply disintegrates due to its relatively short T_1/2. The distribution of specific activity for radionuclides is very wide, ranging from a few becquerels for ²³⁵U to tenths of a megabecquerel for ^234mPa. The reason for the relatively high ^234mPa concentrations is unknown. However, its short T_1/2 makes the potential radiation hazard negligible.

A large variability in the content of each radionuclide was also observed. The monthly distribution of radioactivity was chaotic and does not reflect a seasonal dependence. Thus, we operate with mean values and the SD.

The ashes from the “Czajka” plant cannot be considered as environmental samples. The elements including those belonging to natural decay chains enter various food chains many times. Thus, their concentrations are unknown and difficult for interpretation. The concentrations of most radionuclides are different that those usually observed in environmental samples [8]. Their specific activity changes from a few parts of Bq/kg to tenths of MBq/kg. The expected dispersion of activity between the samples was up to several dozen percentage. No seasonal changes were observed across the entire series of measurements.

Fly ashes from the “Czajka” incinerator can be added to building materials or used as artificial fertilizers without causing a significant increase in radiation exposure.

Thus, one can assume the option that radiation hazards due to the utilization of ashes because of burning bottom sediments from sewage treatment plants can be neglected.

Neither the Marinelli beaker nor the simplified beaker ensured the tightness of the sample. In this way, part of the radon radionuclides could have been released from the containers. Thus, the radiative equilibrium was not entirely preserved.