Three Sicilian wastewater treatment plants were monitored to assess the occurrence and the behaviour of radionuclides. Two sampling campaigns (screening and long-term) were carried out during which liquid and solid samples have been analysed. It was found that 131I mostly occurred in the samples analysed during the screening campaign (43% of the analysed samples contained 131I). High 131I specific activity was found in the mixed liquor, recycled sludge and dehydrated sludge samples. This finding was mainly due to the tendency of 131I to be associated with solid particles. During the long-term sampling campaign an influence of the sludge retention time (SRT) on the 131I behaviour was found. Increasing the SRT and consequently decreasing the fraction of active organic biomass inside the system, the specific activity of 131I decreases.

INTRODUCTION

Over the past decade, the attention on the presence of contaminants of emerging concern (CEC) in wastewater and surface water has considerably increased (Loos et al. 2013). Several groups of compounds are defined as CECs: personal care products, endocrine disruptors, illicit drugs, surfactants, pharmaceuticals, gasoline additives and radionuclides (Loos et al. 2013). Although these CECs are not commonly monitored in the environment, they can cause known or suspected adverse ecological and/(or) human health effects (US-EPA 2013). Thus, the dynamics and fate of CECs are of particular interest for environmental protection. Wastewater treatment plants (WWTPs) can have an important role in the protection of the environment from pollution of CECs. Several studies on the occurrence and behaviour in urban wastewater of different CECs such as endocrine disruptors pollutants, xenobiotic organic chemicals and other industrial chemicals have been performed in the literature (Martinez Bueno et al. 2012; Verlicchi et al. 2012; Loos et al. 2013; Secondes et al. 2014). Nevertheless, the knowledge of radionuclides is still limited. Indeed, only a few studies have been carried out in order to measure radioactivity of liquid and solid samples from WWTPs (Camacho et al. 2012; Montaña et al. 2013). The knowledge acquired demonstrates that the urban wastewater can contain natural or anthropogenic radionuclides. Radionuclides such as 238U, 232Th and 40K are naturally contained in surface and ground water used for drinking and industrial processes (Camacho et al. 2012) and consequently in wastewater. The anthropogenic source is mainly due to the release of the nuclear industry. However, the radioactive effluents produced at medical facilities also have to be taken into account. Indeed, in nuclear medicine, several radioisotopes (e.g. 131I, 99mTc, 51Cr, 68Ga and 58Co) are used both for diagnostic and therapeutic purposes (Malta et al. 2013). After radionuclide therapy, patients generate wastewater with an elevated count of radioactivity. To reduce the effects of the human radioactive exposition, as suggested by the European Council Directive 96/29 (Euratom 1996), hospitals are usually equipped with storage tanks for wastewater radioactive decay before release into the sewage. However, inpatient subjects directly discharge into the sewage. Only a few studies in the literature deal with the fate and the behaviour of radionuclides in the WWTPs (Sundell-Bergman et al. 2008; Montaña et al. 2013). Further, the role of the WWTP-operating conditions has been poorly investigated in the literature (Camacho et al. 2012). Sundell-Bergman et al. (2008) demonstrated that the radioactivity of sludge can be used as an indicator for radioactive materials released into the sewage system. The authors have shown that radionuclides in the sludge follow the same behaviour as solids or toxic metals. This statement has particular interest in regard to environmental protection, since no limits in terms of radioactivity are reported in the Italian Directive that regulates the disposal of the sludge generated in WWTPs (Decreto Ministeriale 2010). Indeed, the Italian Directive (Decreto Legislativo 2000) considers only limits on workers and members of the public that may be exposed to significant doses of radiation. Montaña et al. (2013) have investigated the behaviour of radionuclides both in liquid and sludge samples collected from 11 municipal WWTPs in Spain. In some of these WWTPs, the authors found that the activity of effluent liquid samples decreased with respect to the influent liquid samples, thus corroborating that the activity can be transferred to the solid phase. Furthermore, a high amount of 131I was also found in the sludge samples. Camacho et al. (2012) have investigated the behaviour of natural radionuclides in two Spanish WWTPs with different potentiality and employed treatments. They found that gross beta activities were not influenced by the type of treatment; conversely, the gross alpha activities increased from influent to effluent depending on the employed treatment and on the hydraulic retention time (HRT). The above review points out that the issue related to the occurrence of radionuclides in WWTPs has not been investigated much. Indeed, as far as the authors are aware, the knowledge about the fate and the behaviour of radionuclides in the WWTPs is still limited. The objective of this paper is to provide a comprehensive analysis of the behaviour of radionuclides in three Sicilian WWTPs. The analysis was performed by employing two different sampling campaigns: screening and long-term. During each sampling campaign liquid and solid samples were collected at different WWTPs sampling locations. Further, the workers' exposition dose was also measured and estimated. The study presented here contributes to improve the degree of knowledge acquired on the behaviour of radionuclides in WWTPs.

MATERIALS AND METHODS

Sample collection and preparation

Two sampling campaigns have been performed during the study: (i) screening campaign and (ii) long-term campaign. During the screening campaign three WWTPs (namely, WWTP-1, WWTP-2 and WWTP-3) located at the north-western Sicilian coast have been monitored. All WWTPs have the flow scheme shown in Figure 1, mainly differing in their potentiality.

Figure 1

Flow scheme and features of the WWTPs under study with indication of sampling locations.

Figure 1

Flow scheme and features of the WWTPs under study with indication of sampling locations.

The average daily flow, expressed as m3d−1, for WWTP-1, WWTP-2 and WWTP-3 is 153,600, 19,704 and 44,448, respectively. The mixed wastewater (domestic and hospital wastewater and rain water) is first subjected to the primary treatments (screening for solid separation, oil and grease removal); later the secondary treatments, such as activated sludge (AS) processes, are employed. The AS biological nitrogen removal process and the chemical phosphorus process are also implemented in WWTP-2. The sludge withdrawn from the primary and the secondary settlers is treated by means of the aerobic digestion process. The sludge humidity is then reduced through two sequential units: thickener and dewatering unit. During the screening campaign liquid and sludge samples (volume of the sample 1 L) have been collected from the sampling locations as indicated in Figure 1. Specifically, during the screening campaign, samples of grab influent wastewater (location 0), grab mixed liquor in the aerobic tank (location 3), liquid-treated effluent (location 4), recycled sludge (location 5) and the dehydrated sludge (location 10) were collected twice during October 2012. Liquid samples were collected by taking into account the HRT of the system. Results of the screening campaign have been used to design the long-term campaign both in terms of WWTP and radionuclide to be selected for further investigation. During the long-term campaign samples were withdrawn every 15 days (from October 2012 to April 2013). During both screening and long-term campaigns the concentration of total suspended solids (TSS) and volatile suspended solids (VSS) of each sample was analysed using Standard Methods (APHA 1998).

Test methods to measure the activities

The qualitative and quantitative determination of the gamma-emitting radionuclides has been performed by using the laboratory gamma ray spectrometry technique. In particular, a high resolution germanium semiconductor detector, having coaxial geometry, has been used (Bruzzi et al. 1990; Knoll 2000). The activity (Ai) of the generic radioisotope i, expressed in Bequerel (Bq), has been computed according to Equation (1), where C represents the area of the photoelectric peak belonging to the radioisotope of interest, T is the time required for the measurement that is corrected on the basis of the radioisotope decay, I represents the number of gamma rays emitted during each isotope disintegration (intensity) and ε is the detector's nominal efficiency and represents the fraction of the photons emitted from a source which generate the detector's photoelectric effect. 
formula
1
The general expression of T is reported in Equation (2), where Tc (Tc = 80,000 s) and λ represent the counting time and the disintegration constant, respectively. 
formula
2
It is important to specify that the detector used has a nominal relative efficiency, ε, and a resolution value of 35% and 1.8 keV, respectively. The detector's efficiency depends on the energy of the photoelectric peak. When the analysed sample weight is known the specific activity is evaluated. The specific activity of the generic radionuclide can be expressed as , in case the dry weight of the sample is considered, or as , in case the wet weight of the sample is taken into account. Further, GammaVision® software was used to acquire and subsequently elaborate the information provided by the gamma spectra. In this study, the time between sample collection and measurement ranges between 0 and 4 days.

Evaluation and measure of the radiation doses

During the long-term campaign the exposition dose for workers has also been estimated and measured. The estimation has been performed by using the MCNP4C2 Monte Carlo code (Briesmeister 2000). The code is able to simulate the transport of the radiation emitted by a source and allows to estimate the number of radiation that reaches a given volume and the amount of energy deposited per unit mass of the volume. The ratio between the total energy deposited on the volume and the total mass represents the absorbed radiation dose. The code has been applied by considering analysis of two scenarios. In both scenarios 1,000 h in a year of exposition time was considered:

  • scenario 1: the worker is exposed to the storage tank of the dehydrated sludge that is at the 50% of its volume;

  • scenario 2: the worker transits through the secondary settler and the aerobic tank.

The exposition dose for workers has also been measured close to the storage tank of the dehydrated sludge and to the area between the secondary settler and the aerobic tank. The measurement has been performed by using the organic scintillator probe AUTOMESS 6150 AD-b/H.

RESULTS AND DISCUSSION

Screening campaign

131I, 137Cs, 111In, 99Mo-99mTc and 67Ga have been detected during the screening campaign. In terms of 137Cs, 111In, 99Mo-99mTc and 67Ga, 94% of the analysed samples showed that the specific activity was lower than the instrument's minimum detectable activity. Thus, for the sake of conciseness detailed results of only 131I, expressed as , are summarized in Table 1.

Table 1

Summary of the results of the screening sampling campaign in terms of 131I for each monitored WWTP and sampling section locations (0, 3, 4, 5 and 10)

    131I
 
Plant Sampling Sampling section Activity (Bq kgww−1Uncertainty (%) MDA (Bq kgww−1
WWTP-1 n.d. – 0.08 
0.42 22 0.13 
n.d. – 0.16 
1.10 13 0.13 
10 9.89 10 0.34 
5.01 10 0.07 
n.d. – 0.12 
0.90 20 0.16 
0.80 19 0.22 
10 11.57 12 0.35 
WWTP-2 n.d. – 0.07 
n.d. – 0.30 
n.d. – 0.36 
n.d. – 0.30 
10 1.49 27 0.56 
0.63 15 0.06 
1.45 0.02 
n.d. – 0.12 
n.d. – 0.11 
10 2.45 18 0.22 
WWTP-3 n.d. – 0.08 
0.27 19 0.08 
n.d. – 0.08 
n.d. – 0.06 
10 n.d. – 0.13 
n.d. – 0.06 
n.d. – 0.07 
n.d. – 0.90 
n.d. – 0.19 
10 1.55 12 0.07 
    131I
 
Plant Sampling Sampling section Activity (Bq kgww−1Uncertainty (%) MDA (Bq kgww−1
WWTP-1 n.d. – 0.08 
0.42 22 0.13 
n.d. – 0.16 
1.10 13 0.13 
10 9.89 10 0.34 
5.01 10 0.07 
n.d. – 0.12 
0.90 20 0.16 
0.80 19 0.22 
10 11.57 12 0.35 
WWTP-2 n.d. – 0.07 
n.d. – 0.30 
n.d. – 0.36 
n.d. – 0.30 
10 1.49 27 0.56 
0.63 15 0.06 
1.45 0.02 
n.d. – 0.12 
n.d. – 0.11 
10 2.45 18 0.22 
WWTP-3 n.d. – 0.08 
0.27 19 0.08 
n.d. – 0.08 
n.d. – 0.06 
10 n.d. – 0.13 
n.d. – 0.06 
n.d. – 0.07 
n.d. – 0.90 
n.d. – 0.19 
10 1.55 12 0.07 

MDA: minimum detectable activity, n.d.: not detected.

The greater part of the liquid influent samples (location 0) has a very low or undetectable specific activity. This result seems to contrast with the high observed specific activity in the dehydrated sludge samples (location 10). However, this result suggests that a sort of ‘activity accumulation’ can occur over time inside the system. A high occurrence of 131I (having a half-time of 8.04 days) was found in the analysed samples. Indeed, 43% of the analysed samples had a detectable 131I specific activity. This result is likely due to the medical use of 131I. Indeed, 131I is widely used in medical practice, such as therapeutic treatments of hyperthyroidism and the treatment of thyroid cancer. Regarding the behaviour of 131I, an increasing trend from the liquid influent (location 0) to the solid effluent (location 10) samples was found. Such a result, emphasized for the WWTP-1, is in agreement with the results obtained by Gäfvert et al. (2002). Indeed, the authors have shown that the activity of water samples decreases with the removal of particles, thus showing that the activity is mainly associated with particles of the samples (Montaña et al. 2013).

Long-term campaign

Results of the screening campaign have suggested that the behaviour of 131I merited further investigation. Therefore, the long-term campaign was designed on the basis of the findings of the screening campaign. During the long-term campaign, only the WWTP-1 was monitored considering the same sampling locations as reported in Figure 1. The choice of the WWTP-1 was mainly due to the fact that the increasing trend of 131I from influent to the solid samples was more evident in WWTP-1 than the other plants.  Figure 2 summarizes the results and the uncertainty bars of the long-term campaign. More specifically, Figure 2(a) shows the 131I specific activity of the influent and effluent samples (location 0 and 4, respectively). The results related to the mixed liquor, recycled sludge and dehydrated sludge (locations 3, 5 and 10, respectively) are shown in Figure 2(b).

Figure 2

131I activity expressed as for liquid samples (locations 0 and 4) (a) and for solid samples (locations 3, 5 and 10) (b).

Figure 2

131I activity expressed as for liquid samples (locations 0 and 4) (a) and for solid samples (locations 3, 5 and 10) (b).

By analysing Figure 2(a) one can observe that, except for the 11th sampling, a very low specific activity of 131I was measured in the influent grab samples (location 0). The specific activity of the influent grab samples was on average (computed excluding the null values) equal to . The pick value obtained for the 11th sampling () is likely due to the influence of hospital wastewater discharging. With this regard, no particular analysis has been performed due to the lack of data related to the hospital's wastewater flow rate. From Figure 2(a) one can also observe that in the effluent wastewater (location 4) a very low 131I specific activity was found (between 0 and ). Further, no noticeable influence of the physical and biological processes has been obtained for the liquid samples (Figure 2(a)). The specific activities of 131I reported in solid samples (Figure 2(b)) are much higher than the values measured for the liquid samples (Figure 2(a)). Indeed, the maximum value () of 131I measured in the ninth sample of dehydrated sludge is almost 30 times the maximum value of the influent sample (Figure 2(b)). Such a result suggests that inside the system a sort of ‘activity accumulation’ in the particles takes place, corroborating the fact that radionuclides have the same behaviour as solids or toxic metals inside natural or biological systems (Sundell-Bergman et al. 2008). Data reported in Figure 2(b) show that, during the first seven samplings, 131I specific activity for the three locations is on average higher than the last 6 days. This result has been linked, especially for the locations 3 and 5, to the different operating conditions of WWTP-1. Indeed, as reported in Table 2 during the first seven samplings, the total sludge retention time (SRT) of WWTP-1 was approximately 25 days, and the TSS concentration (TSSav1–7) in the mixed liquor and recycled sludge was on average equal to 4.1 g L−1 and 10.8 g L−1, respectively (Table 2). Conversely, during the last six samplings the SRT was equal to 40 days and the average TSS concentration (TSSav8–13) in the mixed liquor and recycled sludge was equal to 6.3 g L−1 and 11.2 g L−1, respectively (Table 2). Further, the two different operating conditions influenced both the ratio between the active organic particles (evaluated as VSS) and TSS inside the system (higher for the first seven samplings) and the time available to the influent radionuclide to decay (higher for the last six samplings) (Table 2).

Table 2

TSS and VSS average concentration during the first seven samplings (TSSav1–7 and VSSav1–7, respectively) and during the last six samplings (TSSav8–13 and VSSav8–13, respectively) in the mixed liquor (location 3) and recycled sludge (location 5)

  SRT (d)
 
 25
 
40
 
Sampling location TSSav1–7 (g L−1VSSav1–7 (g L−1TSSav8–13 (g L−1VSSav8–13 (g L−1
4.1 2.9 6.3 2.1 
10.8 7.6 11.2 3.9 
  SRT (d)
 
 25
 
40
 
Sampling location TSSav1–7 (g L−1VSSav1–7 (g L−1TSSav8–13 (g L−1VSSav8–13 (g L−1
4.1 2.9 6.3 2.1 
10.8 7.6 11.2 3.9 

Thus, different values of TSS and VSS concentrations and SRT define the higher or lower specific activity of the mixed liquor samples in comparison with the recycled sludge ones. To estimate the weight of the SRT, TSS and VSS concentration in influencing the radionuclide behaviour, a more extensive database is required. However, results obtained in this study suggest that the 131I specific activity decreases with increasing SRT. The dependence between SRT and 131I specific activity is quite complex and can be related to the following reasons. (i) With increasing SRT, even though the TSS concentration increases, the VSS concentration decreases because a progressive accumulation of inert solids occurs. Thus, the affinity discussed in the literature of the radionuclide to be mainly associated with the organic particles (Sundell-Bergman et al. 2008; Montaña et al. 2013) is reduced. (ii) With increasing SRT the time available to the influent radionuclide to decay increases. By analysing Figure 2(b) one can also observe that the 131I specific activity of the dehydrated sludge samples (location 10) is higher than the other two samples (mixed liquor and recycled sludge, locations 3 and 5, respectively). This finding may mainly be due to the affinity of radionuclide to be mainly associated with the organic particles (Sundell-Bergman et al. 2008; Montaña et al. 2013). Indeed, 1 L of dehydrated sludge (location 10) contains, on average, 313 g of dry solids; among these solids about 30% (94 gVSS) have organic nature. Conversely, 1 L of mixed liquor and recycled sludge samples, due to the high water content (humidity almost equal to 99%), contains on average 2.2 gVSS and 5.4 gVSS, respectively. Further, the average 131I specific activity of the dehydrated sludge during the first seven samplings is higher than the last six samplings. The high 131I specific activity in the dehydrated sludge is extremely important in terms of protection of workers and the environment from radiation. Indeed, the dehydrated sludge is relieved discontinuously from the WWTP and is collected inside an open tank. Thus, the workers are continuously submitted to the radiation of the dehydrated sludge. Furthermore, the detention time of the dehydrated sludge inside the collecting tank may be insufficient to achieve the natural decay of radionuclides, thus transferring the radiation into the landfill during the sludge disposal. To provide a first attempt to explain the role of TSS in the 131I variation, the measured values of the 131I specific activity of the mixed liquor and of the recycled sludge samples (locations 3 and 5, respectively) have been linearly correlated with the measured TSS concentration of the same samples. When increasing the TSS concentration a decrease of the 131I specific activity takes place both for mixed liquor (location 3) and recycled sludge (location 5) (Figure 3).

Figure 3

Linear correlation between TSS concentration and 131I specific activity (expressed as ) in the sampling locations 3 (a) and 5 (b).

Figure 3

Linear correlation between TSS concentration and 131I specific activity (expressed as ) in the sampling locations 3 (a) and 5 (b).

The linear regression coefficients (R2) obtained for the mixed liquor (location 3) and the recycled sludge (location 5) are 0.75 and 0.78, respectively. The negative linear correlation between the 131I activity and the TSS concentration could be due to the fact that with increasing TSS, the fraction of VSS decreases, thus limiting the affinity of 131I in the organic matter (Table 2). Regarding the role of the time available to the influent radionuclide to decay, results obtained for the dehydrated sludge suggest that the negative influence of this time (by increasing this time the 131I specific activity decreases) could be considered negligible when a very high content of VSS occurs.

Radiation doses

The estimation of the radiation dose has been performed only for the WWTP-1. The radiation dose for scenario 1 and scenario 2 has been performed by using the maximum values of the 131I activity measured in the mixed liquor () and in the dehydrated sludge (), respectively. Radiation dose of 3.9 nSvh−1 and 0.4 nSvh−1 was estimated for scenario 1 and scenario 2, respectively. These values are lower than the limit threshold established by Italian law (D.L. 241/200) (namely > 1 mSv year−1) for defining a worker as exposed to radiological risk. Thus, for the two analysed scenarios the worker can be defined as not having been exposed to radiological risk. These findings have also been corroborated by the low measured value for the radiation dose (ranges between 35 and 85 nSvh−1 with an average value of 56 nSvh−1).

CONCLUSIONS

The occurrence and the behaviour of radionuclides inside WWTPs have been analysed. Results highlighted that 43% of the analysed samples contained 131I. 131I was mainly contained in sludge samples due to the tendency of activity to be mainly associated with particles. The tendency of 131I to be associated with particles was influenced by the SRT of the system and consequently by the VSS content inside the system. In this regard, with increasing SRT, even though the TSS concentration increases, the average VSS concentration decreases and the reduction of the 131I specific activity took place. A satisfactory linear correlation between TSS concentration and 131I specific activity was found both for the mixed liquor and recycled sludge samples (with increasing TSS concentration a decrease of the 131I takes place). Further, it was found that the role of the time available to the influent radionuclide to decay could be considered negligible when a very high content of VSS takes place.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the engineers Cristina Musmarra and Valentina Pipitone for their support with this work. This study was financially supported by the Italian Ministry of Education, University and Research with the Research project of national interest PRIN 2010–2011 (D.M. 1152/ric 27/12/2011, prot. 2010 WLNFYZ) entitled ‘Emerging contaminants in air, soil, and water: from source to the marine environment’.

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