Regulatory approach for dilute and disperse or concentrate and store of the patient’s excreta after iodine therapy

$A\left(T\right)=\frac{P}{\lambda }=1.44\times A\times \frac{T_{\frac{1}{2}}}{t}\text{ (2)}$

In the presence of continuous introduction of radionuclides whether into storage tanks or to the environment, the evaluation point can be indicated by the ratio of release rate and half-life of released radionuclide from a single source. If the duration of the discharge is greater than half-life, the accumulated activity will decrease and versa visa. This amount will be diluted with flushing and washing water volume (V). So, the activity concentration A_V is:

$A_V=\frac{A(T)}{V}\text{ (3)}$

In a medical facility that has more than one iodine therapy room discharging the Patient’s Excreta to decay tanks. The accumulated activity will be multiplied by the number (n) of rooms. In addition to excreting only 85% off (A_admin) over hospitalization period. Over time, double amount of administrated radioactivity will be accumulated in decay tanks as a result of the patient stay for 5 days [A(T)=1.44 × 1.6 × 0.85 ×A_admin = 1.96 A_admin] . Subsequently, double of the exposure to involved workers. The resulted exposure actually is depending on the amount of diluted water which is most likely insufficient to ensure not exposed the workers. The dose rate at distance (d) from the stored activity concentration in tanks is:

$D=\frac{\pi A_V\Gamma (1-e^{-\mu x})}{\mu }\times \ln \left[\frac{r^2+d^2}{d^2}\right]\text{ (4)}$

On the other hand, if this amount is discharged directly to the sewer system, it will be mixed with the other liquid effluent or sewer stream. Eventually, the effluent reaches the environment or to the treatment facility with an additional substantial dilution that would reduce the concentrations of the radionuclides significantly even reach the acceptable levels if combined with sufficient dilution depending on the adequacy of the sewer system.

When there is (n) number of iodine patient rooms directly discharging to the same effluent or to the environment. The production rate would be subject to dilution process in several points along its pathway until reach final destination starting from toilet flushing point, through medical facility effluent, to other mixing and dilution points within sewage system along its journey. The fate of ¹³¹I discharged to the sewers has been modeled and quantified by Punt, et al. [4]. Approximately one-third of the iodine activity will be retained in the sewage system during flow periods and therefore, additional reduction on the discharged amount would be experienced too where only (0.66) of discharged radioactive iodine (i.e. A_discharge = 0.85 A_admin) reach the final destination. In this case, the final activity concentration (A_final) would be accumulated over time in a treatment facility or environment once mixing with additional (V_Sewage) would be:

$A_{final}=1.44\times \frac{2}{3}\times \frac{T_{\frac{1}{2}}}{t}\times n\times \frac{A_{discharge}}{V_{Sewage}}\text{ (5)}$

In presence of an adequate sewage system, the (V_Sewage) almost would be quite sufficient to achieve the safe discharge requirements. The adequacy of the sewage system whether in the main discharge point at the facility in a treatment facility or to the environment can be examined easily by using equation (5).

Discussion

In order to test the provided model. if the average of the administered activity from a single treatment is about 7.5GBq in the conservative scenario for continuous process repeated every 5 days, the accumulated activity in the delay tank after a while will be about 15GBq. The released amount of activity was already diluted with 250 liters of liquid from flushing and washing for each patient. The activity concentration (AV) will be constant (about 25MBq per liter) over time. In order to calculate the possible exposure, equation (4) may be used. In this case, additional information about tank specifications, engineering design, and liquid characteristics is required. Alternatively, the volume can be divided into several segments and thereafter calculate the dose resulting from each segment or by approximation to point source at the point of interest where the representative person could be existing. I do so, the calculated dose rate at one meter from point sources equals 1.15mSv/h in the delay tank adjacent area in the worst scenario. Also, the occupationally exposed workers may be existing closer to the sources, hence receiving more exposure . Moreover, the additional costs of providing, installing, and maintaining delay tanks, as well as, establishing a proper acceptable control and measures. It has been estimated that with no additional dose reduction intervention, the annual increase in the radiation dose to facility maintenance works up to 1.5mSv. Also, it was extrapolated that £5 million had been necessary to save 1 mSv of dose [2].

On the opposite, in case of direct discharging. The final activity concentration (A_final) if we assumed that there is one source introducing the radionuclides from single and repeated treatment procedures over time. The accumulated activity would be about 10GBq at any time during the year from single procedures. This amount will be diluted with (V_Sewage) which almost equals hundreds or thousands of cubic meters over the production duration (i.e.5 days). If sewage volume is in order of hundreds of cubic meters during this period, the activity concentration in the final destination will be within a range less than 100 Bq per gram when applying equation (5). This activity concentration surely meets the clearance and exempted levels. If the sewage volume increased, the number of production sources will be increased too. Also, if these conditions have been met at the facility itself, then the clearance and exempted levels will meet at the facility discharge point. Nevertheless, in order to estimate the volume of the effluent that would achieve the exemption criteria, the radiation risk is sufficiently low as not to warrant any further control and has no regulatory concern. Equation (5) can be used as follows :

$A_{final}=1.44\times \frac{2}{3}\times \frac{T_{\frac{1}{2}}}{t}\times n\times \frac{A_{discharge}}{V_{Sewage}}\le Exemption level$

The exempted level or clearance level may be in terms of volume (ml, L) or in terms of weight (g, kg). it is important to carefully obtain sufficient information on effluent characteristics such as density (ρ). Usually, the density of wet wastewater is about (1.0 to 1.03g/cm3) and may reach up to (1.2 to 1.4g/ cm3) for dry or heavy sludge.

For example, Patient excreta are exempt from sewer discharge regulations in the United States. Several studies have measured I-131 in sewage sludge. The results of several investigations indicate that most of the I-131 entering Water Pollution Control Plants (WPCPs) are discharged in the effluent eventually. Therefore, these studies suggest that the occurrence of I-131 in sewage may be common. Paula S, et al. [5] measured the Iodine-131 concentrations in sewage sludge collected at few WPCPs. The I-131 concentrations in the collected sample ranged from 0.027 to 0.41Bq/g in WPCP with an average flow rate of 4.9ML per day, and 0.139 to 0.214Bq/g in another WPCP with an average flow rate of 3.2ML per day, while ranging from 20-149Bq/g in another one with a flow capacity of 6.8ML per day subject to seasons fluctuations [5]. Also, other studies reported that the specific activities of I-131 in sewage sludge are generally ranged from 0.004 to 2.5Bq per g in dray sample [5]. If these flow rates are applied for the production rate theoretical model with the average administered activity of 5.5GBq of I-131, for 5 hospitalization days and assuming continuous treatments. The calculated activity concentrations would be about 0.3-0.5Bq per ml. with consideration of the density of dry sludge. These calculated activity concentrations are within the majority of the reported and published data. However, this proposed model is based on theoretical assumptions, not an empirical ones. Therefore, further field investigations and tests are still recommended.

Additionally, an analysis of possible exposure pathways indicated that negligible human exposure resulted from the Iodine-131 released [6]. Radiological Protection Institute of Ireland (RPII) report [7-14] concluded that the direct discharge to sewer does not lead to any significant exposure, has no additional cost burden or other identifiable disbenefits. Hospital plumbers may receive elevated exposure, while the annual dose for Sewer workers, Sewage Treatment workers, and others was less than 10μSv. These conclusions are supporting that the dilute and disperse approach is desirable for managing the patient excreta since no occupational exposure is raised to workers in the sewer system or treatment plant.

Conclusion

The production rate term is a simple model that can be used to estimate the discharged activity to the final effluent destination as a fraction of the administered activity. The estimated discharged activity and its relevant resulted exposure are mainly depending on the dilution volume. Also, it is easy to obtain and determine the adequacy of the sewage system that can achieve exempted criteria, ensure no radiation risk, and warrant no regulatory concern. Furthermore, it can apply and manage of discharge of low-level liquid radioactive waste generated in other medical activities, educational, research, and Industrial Facilities. In the presence of appropriate sewage infrastructure, it would be better to dilute and disperse the waste activity in a continuous sewage system rather than concentrate and store for decay in order to avoid unnecessary occupational exposure to workers at the facility during the storage period. In addition to reducing the potential of occupational exposure due to contamination also. Moreover, achieve the best cost-benefit balance. The dilute and disperse approach is desirable for managing the patient excreta containing radionuclides since no occupational exposure is raised to workers in the sewer system or treatment plant at no additional costs. In addition, consentient with ICRP, IAEA recommendation and RPII conclusions where they recommended that do not require to urine to be stored and no need to provide and install delay tank.