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.
Schematic of the “Czajka” sewage treatment plant located in Warsaw, Poland.
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.
Totally, 17 monthly samples of fly ash, weighing ≈1 kg, were collected at the end of the thermal disposal line in the period from August 2022 to April 2023. They were homogenized by quartering, dried at 80°C for 6 h, and stored in closed polypropylene containers. Then laboratory samples weighing ≈900 g and having a density 0.77 ± 0.04 g·cm−3 were prepared for future testing.
Energy dispersive X-ray spectroscopy (EDX) provided the average chemical composition of the tested fly ashes (Table 1). Chemical composition plays an important role during energy-efficiency calculations due to self-attenuation in the sample bulk.
Average content in mass percentage determined by the EDX technique
Element | Average content in mass percentage |
---|---|
C, carbon | 1.86 |
O, oxygen | 33.99 |
Na, sodium | 0.40 |
Mg, magnesium | 1.60 |
Al, aluminum | 13.15 |
Si, silicon | 13.36 |
P, phosphorus | 9.18 |
S, sulfur | 0.92 |
K, potassium | 1.33 |
Ca, calcium | 13.93 |
Ti, titanium | 0.70 |
Mn, manganese | 0.25 |
Fe, iron | 8.13 |
Cu, copper | 0.43 |
Zn, zinc | 0.75 |
The uncertainty of determining the content of the individual elements using the EDX method is smaller than 1%.
Radioactive equilibrium refers to the state in which the activity of a radioactive parent nuclide and that of its radioactive daughter nuclide become equal. Radioactive equilibrium is named secular equilibrium and occurs in a radioactive decay chain only if the half-life (
Let us assume that two radioactive elements compose a chain, and the first one is a parent and the second one, also radioactive, is a progeny radionuclide. Their activity varies over time in accordance with the law of decay. The activity of the progeny radionuclide
Determination of 238U (
Interference of radionuclides FEAPs: 232Th and 234Th, 212Pb and 214Pb, 212Pb and 228Ac based on current results.
235U (
Interference of 235U and 226Ra FEAPs based on current results.
The determination of 210Pb radioactivity is done directly via the γ-peak at the comparatively low energy of 46.54 keV. The determination of 228Th radioactivity from the radionuclides 212Pb and 208Tl is utilized. For the analysis, the γ-peak of 212Pb at 238.63 keV is predominantly recommended. 208Tl radioactivity is usually easy to obtain. Radionuclides like 7B, 137Cs, and 40K do not belong to the natural decay series. The initial two radionuclides can be identified using the software that typically comes with a modern γ-spectrometer. During the determination of 40K radioactivity, it is considered that its main spectrometric line of 1460.82 keV interferes with the line of 1459.14 keV related to 228Ac disintegration (Fig. 4). Based on 228Ac activity, one can calculate the contribution to the peak area at 1460.82 keV coming from this radionuclide. The rest of the peak area is related to 40K activity [4].
Interference of 40K and 228Ac FEAPs based on current results.
Photons were detected using an high purity germanium (HPGe) detector (coaxial, p-type, relative efficiency 33.8%, full width at half maximum (FWHM) (
The Laboratory Sourceless Calibration Software (LabSOCS®) was utilized for the numerical modeling of the photon transit within the sample and detector volume and for the mathematical energy-efficiency calibration. Geometry Composer® was utilized to describe the measurement geometry. Next, a logarithmic polynomial up to the fifth degree interpolated the pair consisting of photon energy and the corresponding registration efficiency (see Eq. (2)).
For the validation of the radiometry in terms of its effectiveness, the analytical tool integrated absolute full energy peak efficiency (IAFEPE) integral of the efficiency function
The first series of measurements was completed using a simplified Marinelli beaker. The ashes were placed in the container up to 1 cm from the lid. The mean mass of the ashes was ≈480 g. The cut view of the measurement geometry is shown in Fig. 5. It also served as the input for the MCNP calculations, leading to mathematical energy-efficiency calibration.
The measurement geometries adopted for both series of measurements. Simplified beaker (on the left) and Marinelli beaker. Own drawing based on the Geometry Composer screen.
After completing the first series of measurements, the ash was withdrawn to a simplified beaker made of PVC, having a cone shape, and connected to the remaining part of the ash (Fig. 5). The mass of ashes increased to ≈660 g. Then, the beaker was suspended for 3 months, and the second series of measurements was completed. The reason for such a procedure to take place was due to the expectation of achieving a state of radioactive equilibrium for progeny radionuclides with a
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: IAFEPEMarinelli_beaker = 18.015 keV and IAFEPESimplified_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.
The mean value of the specific activity and its standard deviation (SD) for radionuclides detected in samples of ashes from the “Czajka” plant
Radionuclide | Mean specific activity ± SD (Bq/kg) |
---|---|
137Cs | 5 ± 2 |
7Be | 479 ± 98 |
40K | 417 ± 442 |
Uranium–radium series | |
234mPa | 149 881 ± 97 150 |
210Bi | 2 762 ± 2 310 |
234Th | 446 ± 395 |
214Bi | 4 670 ± 2 762 |
210Pb | 286 ± 228 |
226Ra | 41 ± 30 |
Thorium series | |
228Ac | 382 ± 239 |
212Bi | 960 ± 566 |
212Pb | 92 ± 44 |
208Tl | 15 ± 5 |
Uranium–actinium series | |
235U | 5 ± 3 |
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
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.