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Medically-derived 131I in municipal sewage effluent
w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 6 6 3 e5 6 7 1
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Medically-derived
131
I in municipal sewage effluent
Paula S. Rose*, R. Lawrence Swanson, J. Kirk Cochran Marine Sciences Research Center, School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794-5000, USA
article info
abstract
Article history:
This work presents 131I (t½ ¼ 8.04 d) concentrations in sewage effluent from the Stony Brook
Received 30 March 2012
Water Pollution Control Plant (WPCP), a small plant serving a regional thyroid cancer
Received in revised form
treatment facility in Stony Brook, NY, USA. The concentrations detected in sewage effluent
27 June 2012
ranged from 1.8 0.3 to 227 2 Bq L1. The primary source of
Accepted 23 July 2012
cancer inpatients treated at the Stony Brook University Medical Center. Based on several
Available online 2 August 2012
time series measurements following known inpatient treatments, the mean sewage half-
131
I is excreta from thyroid
life (Ts) of iodine is 3 d in this plant. The Ts, analogous to a radioactive half-life, Keywords:
describes the time it takes for half of a wastewater component to be removed from
Iodine
a WPCP. Flow recycling, or activated sludge, used to maintain bacterial populations
131
necessary for sewage treatment causes iodine to remain in this plant far longer than its
Sewage
hydraulic retention time. The experimental results suggest that most
Sewage effluent
Stony Brook WPCP leaves in sewage effluent, not in sewage sludge. Patient treatments can
I
131
I entering the
Wastewater
result in continuous discharges of 131I to surface waters where it can be used as a tracer of
Medical radioisotopes
sewage-derived material and to understand the behavior of
Nuclear medicine
1.
Introduction
Iodine-131 is released from nuclear power plants, during nuclear weapons tests, nuclear fuel reprocessing and weapons production. Medical use is perhaps the more widespread source of 131I to the environment. It is the most widely used radiopharmaceutical in nuclear medicine for therapeutic purposes, primarily to treat hyperthyroidism and thyroid cancer. In developed countries, the average number of hyperthyroidism treatments is 150 per million people and 38 thyroid cancer treatments per million people; however, the latter represents a greater potential source to water pollution control plants (WPCPs). The standard protocol for treating thyroid cancer is removal of the whole thyroid gland followed by administration of 131I to destroy any remaining tissue or cells. Thyroid cancer patients are typically given 4000e8000 MBq compared to 100e1000 MBq of 131I for
131
I in aquatic environments.
ª 2012 Elsevier Ltd. All rights reserved.
hyperthyroid treatments and most of the initial dose is eliminated from the body in urine (ICRP, 2004; UNSCEAR, 2000). In the United States, patient excreta are exempt from sewer discharge regulations and are therefore released directly into sewer systems (Martin and Fenner, 1997). Iodine-131 has been measured in aquatic environments receiving sewage effluent discharges, yet few published data exist for the radioisotope in sewage effluent (Chang et al., 2011; Erlandsson et al., 1989; Fischer et al., 2009; Kleinschmidt, 2009; Puhakainen, 1998; Rose, 2003; Smith et al., 2008; Sodd et al., 1975); most work has focused on sewage sludge. The results of several investigations indicate greater than 75% of 131I entering WPCPs leaves in the effluent (Barci-Funel et al., 1993; Dalmasso et al., 1997; Erlandsson et al., 1989, 1983; Erlandsson and Mattsson, 1978; Martin and Fenner, 1997; Prichard et al., 1981; Puhakainen, 1998; Stetar et al., 1993). It is not surprising then that medically-derived
* Corresponding author. Present address: US Naval Research Laboratory, NRC Postdoctoral Research Associate, Marine Biogeochemistry, Code 6114, 4555 Overlook Avenue SW, Washington, DC 20375, USA. Tel.: þ1 202 767 0787. E-mail address: [email protected] (P.S. Rose). 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2012.07.045
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131
I is readily measurable in the environment (Fischer et al., 2009; Howe and Hunt, 1984; Howe and Lloyd, 1986; Marsh et al., 1988; Puhakainen, 1998; Rose, 2003; Smith et al., 2008; Sodd et al., 1975; Waller and Cole, 1999). While the occurrence and concentrations of 131I in sewage effluent have been poorly characterized, these studies suggest discharges of the radioisotope may be widespread and therefore useful as a tracer in receiving waters to study biogeochemical processes occurring on the time scale of approximately one month. Distributions of naturally occurring iodine have been wellstudied and indicate that iodine is biologically cycled and remineralized similar to other nutrient elements (Ullman and Aller, 1980, 1983, 1985; Upstill-Goddard and Elderfield, 1988). The nutrient-like behavior of iodine suggests that medicallyderived 131I may be used to study the short-term fate of wastewater nitrogen in aquatic systems. Wastewater discharges of 131I would also be useful to investigate the natural cycling of iodine in receiving waters. More specifically, the rates and mechanisms governing the transformations of natural iodine in aquatic systems are not well-known. Additionally, Smith et al. (2008) proposed 131I as a wastewaterspecific particle tracer and suggested its use as a tracer for short-term sediment dynamics. The objective of this study was to determine the occurrence and concentrations of 131I in sewage effluent at a relatively small WPCP with known inputs of the radioisotope from thyroid cancer inpatient treatments.
2.
Methods
2.1.
Study site
The Stony Brook WPCP, Suffolk County Sewer District #21, is a tertiary treatment facility located on the campus of Stony Brook University, Stony Brook, NY, USA. The plant’s service area includes the campus, the Stony Brook University Medical Center and a small number of private homes. The estimated size of the population served is 20,000. At the time of sampling, tertiary treatment was achieved via an oxidation ditch with both activated sludge and mixed liquor returns. Sodium hypochlorite was added to sewage effluent for disinfection prior to discharge from the plant. Travel time from the plant to its outfall in Port Jefferson Harbor, NY is 4e6 h. The oxidation ditch no longer receives mixed liquor returns and sewage effluent is disinfected using ultraviolet irradiation at the Port Jefferson WPCP (Suffolk County Sewer District #1) just prior to its discharge into Port Jefferson Harbor (Rose, 2011). The design capacity of the plant is approximately 9.5 106 liters per day (MLD) or 2.5 106 gallons per day (MGD). Average flow is approximately 6.8 MLD (1.8 MGD). Average daily maximum and minimum flows are approximately 7.6 MLD (2.0 MGD) and 3.8 MLD (1.0 MGD), respectively. During the summer months and weekends, when school is not in full session, average flows decrease as much as 20%. The average daily maximum to minimum ratio of daily flow is approximately 2, but maximum flow in a given day can exceed 4 times the minimum daily flow. Minimum flows generally occur between 7 and 8 AM, after which there is a rapid increase in flow until it peaks around 12 PM. Flow then decreases slowly until about 2 AM, then more rapidly until about 7 AM (Rose, 2011).
The hydraulic retention time of sewage in the Stony Brook WPCP is approximately 24e36 h, depending on the University schedule as mentioned above. The mean cell residence time, or the residence time of organic matter in the system, is about 36 d (Rose, 2011).
2.2.
Sample collection and preparation
Sewage effluent was collected as a grab sample from the final effluent stream before discharge from the plant using a 1 L high density polyethylene bottle. More than half of the samples were analyzed with no further treatment. The remaining samples were vacuum filtered through a 47 mm 0.7 mm glass fiber filter. Aliquots of the filtrate were retained and analyzed with no further treatment. Sample volumes analyzed were 150 mL and 170 mL. Straight-side polypropylene jars (64 mm height; 64 mm diameter) were used for counting. Pre-weighed filters that were retained for g-ray spectrometry determinations were dried at 40 C overnight and re-weighed before analysis. More than 80% of the samples were collected between 12 PM and 3 PM. Sampling replicates were also collected at the Stony Brook WPCP on six additional days.
2.3.
Determination of
131
I
The activity of 131I was determined by g-ray spectrometry (364.5 keV peak; branching ratio ¼ 0.812) using Canberra low energy germanium detectors. Generally, samples were counted for one day. Due to the relatively short half-life of 131I, activities were corrected to account for decay during data acquisition as described in Hoffman and Van Camerik (1967). All concentrations of 131I are reported for time of collection and with a 1s counting error. The counting efficiency for each geometry and each detector used in this investigation was determined using a certified 131I standard solution (Eckert & Ziegler Isotope Products, Valencia, CA). For the 150 mL and 170 mL sewage effluent samples, deionized water was spiked with the 131I standard solution and counted three times on each detector to determine the counting efficiency of these geometries (3 replicates per detector). The 131 I standard solution was applied to three filters with a pipette, using enough liquid to wet the filter entirely. Each filter was counted three times on each detector to determine the counting efficiency of the suspended solids samples (9 replicates per detector). In each case, the mean counting efficiency was used to calculate sample activities. Multi-day continuum background counts were determined for each detector with several 131I-free sewage effluent samples. The mean background count for each detector was used to determine limit of detection (LD) as described by Currie (1968). The detection limits were 1.7 Bq L1 for sewage effluent samples and 50 Bq g1 for suspended solids.
3.
Results
3.1.
Unfiltered effluent
Iodine-131 concentrations detected in unfiltered sewage effluent collected on 77 different days between June 2006 and March 2009
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ranged from 1.8 0.3 to 217 1 Bq L1 (median ¼ 11 Bq L1); nine samples < LD (Table 1). Percent differences among 131I concentrations in replicate samples were 0.9e7%. The variation among the replicates was within the counting error.
3.2.
4.
Filtered effluent
Iodine-131 concentrations detected in filtered sewage effluent collected on 46 different days between January 2007 and March 2009 ranged from 2.5 0.3 to 227 2 Bq L1 (median ¼ 18 Bq L1); three samples < LD (Table 2).
3.3.
The 131I and suspended solids concentrations are shown in Table 3 for samples with activities > LD.
Suspended solids
Iodine-131 concentrations detected in suspended solids > 0.7 mm in sewage effluent collected on 36 different days between March 2007 and March 2009 ranged from 61 12 to 2801 32 Bq g1 (median ¼ 304 Bq g1); seven samples < LD.
Discussion
Previous studies have reported values up to 32.2 Bq L1 of 131I in sewage effluent from WPCPs ranging in size from 30 to 645 MLD (8e170 MGD) (Erlandsson et al., 1989; Fischer et al., 2009; Puhakainen, 1998; Smith et al., 2008; Sodd et al., 1975). The highest concentrations measured in sewage effluent from the Stony Brook WPCP were 217 1 Bq L1 following five inpatient treatments (unfiltered sewage effluent, Table 1) and 227 2 Bq L1 following four inpatient treatments (filtered sewage effluent, Table 2). Dates of inpatient treatments at the Stony Brook University Medical Center are listed in Table 4. It should be noted that while the suspended solids have relatively high specific activities of 131I, the contribution of the
Table 1 e Iodine-131 concentrations in unfiltered effluent samples collected from the Stony Brook WPCP. Sample volume [ 0.17 L.
131
I (Bq L1)
48.4 0.8 21.8 0.6 93 1 65 1 60 1 21.4 0.6 17.6 0.5 16.1 0.5 9.6 0.3 9.0 0.4 11.9 0.5 8.8 0.3 5.5 0.4 2.0 0.3 4.0 0.3
Sample collection date & time 8/30/06 1:46 PM 8/31/06 2:35 PM 9/14/06 2:22 PM 9/15/06 2:34 PM 9/18/06 2:25 PM 9/20/06 1:44 PM 9/22/06 12:41 PM 9/29/06 1:31 PM 10/19/06 2:05 PM 12/6/06 2:08 PM 12/7/06 1:37 PM 12/8/06 12:30 PM 12/11/06 12:43 PM 12/12/06 2:22 PM 12/13/06 1:00 PM 12/14/06 1:47 PM 12/15/06 12:29 PM 12/19/06 2:05 PM 12/20/06 11:22 AM 1/3/07 1:48 PM 1/5/07 12:42 PM 1/8/07 12:00 PM 1/9/07 1:35 PM 1/11/07 12:00 PM 1/15/07 2:18 PM 1/16/07 1:00 PM 1/23/07 12:53 PM 1/24/07 12:35 PM 1/25/07 1:25 PM 1/26/07 10:05 AM 1/28/09 12:00 PMa 3/14/09 1:07 PMa 3/15/09 1:31 PMa 3/16/09 12:36 PMa 3/17/09 1:30 PMa 3/18/09 12:30 PMa 3/19/09 12:34 PMa 3/20/09 1:00 PMa
I (Bq L1)
131
46.8 0.8 30.5 0.4 8.8 0.2 108 3 23.1 0.2 14.0 0.4 7.2 0.2 187 1 83.2 0.6 10.6 0.2 8.5 0.2 6.7 0.1 10.9 0.2 7.9 0.2 6.2 0.2 4.9 0.2 4.3 0.1 7.1 0.2 7.3 0.2
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Table 2 e Iodine-131 concentrations in filtered effluent samples collected from the Stony Brook WPCP. Sample volume [ 0.15 L.
I (Bq L1)
131
Sample collection date & time
5.6 0.3 34.1 0.5 24.8 0.9 5.0 0.3 4.0 0.3 43.0 0.8 42 1 8.4 0.4
3/29/07 11:35 AM 5/23/07 11:46 AM 5/24/07 10:33 AM 5/25/07 1:31 PM 5/30/07 12:12 PM 5/31/07 1:00 PM 6/4/07 11:39 AM 6/5/07 2:09 PM 6/6/07 12:59 PM 6/7/07 2:18 PM 6/8/07 2:25 PM 6/11/07 1:33 PM 6/12/07 2:33 PM 6/13/07 1:00 PM 6/14/07 2:30 PM 6/15/07 2:30 PM 6/19/07 2:31 PM 6/20/07 12:58 PM 6/21/07 2:15 PM 6/22/07 1:13 PM 6/28/07 1:55 PM 7/12/07 1:35 PM 7/14/07 1:00 PM
I (Bq L1)
131
136 2 102 1 85 1 42.5 0.5 18.0 0.6 12.1 0.3 4.6 0.3 2.5 0.3 17.7 0.4 24.4 0.3 13.6 0.6 3.1 0.3 2.5 0.3 27.8 0.8 16.4 0.7 55.3 0.9 4.6 0.3 2.9 0.5 14.5 0.8 11.7 0.6 3.6 0.3 97 1 115 1
a Sample volume ¼ 0.17 L.
suspended solids to the total activity of the unfiltered effluent is minimal because the suspended solids concentrations are low (Table 3). Additionally, no settling of solids was observed during counting and therefore homogeneity of the sample was maintained during counting. The following discussion assumes there is no difference between the concentrations of 131 I measured in whole effluent and the filtered effluent collected at the Stony Brook WPCP. These concentrations can be primarily attributed to the frequency of thyroid cancer inpatient treatments performed at the University Medical Center, the size of the plant and sewage half-life. There are approximately 60 inpatient treatments per year at the University Medical Center with about an equal number of outpatients. While outpatients treated at the University Medical Center and other facilities are possible sources to the plant, they are not a significant source to the Stony Brook WPCP. Outpatients at the University Medical Center leave the hospital following administration of the 131I. Frequent or large inputs from persons treated at another facility are unlikely considering the size and composition of the population, which consists mainly of University students and employees. The University Medical Center is a regional thyroid cancer treatment facility. These treatments require specialized rooms and personnel, and therefore are not performed at all hospitals. Medical use is the only known source of 131I to this system (Rose, 2011). Typically, thyroid cancer inpatients remain in the hospital 24e30 h following treatment. Length of hospital stay is dependent on the time it takes for the patient’s external dose rate, measured at 1 m, to decrease to less than 10 mrem h1 from 20 to 30 mrem h1 after administration of the dose (Rose, 2011). This is consistent with the work of Driver and Packer (2001) who found that 55% of the initial activity administered to thyroid cancer patients is excreted in the first 24 h.
The data presented here clearly indicate that 131I is discharged from the plant for many days following a patient treatment. Larsen et al. (1995) found elevated 131I concentrations in digested sewage sludge at the Oak Ridge WPCP (Oak Ridge, TN) for more than one month following a single patient treatment. Previous work in the Stony Brook WPCP indicated that 131I can be detected in sewage sludge for at least two weeks after known inputs (Rose, 2003). Retention of sewage sludge is dependent on plant design and sludge removal practices. The recycling of biomass within a sewage treatment plant, or activated sludge, helps maintain a standing stock of bacteria necessary for sewage treatment. Therefore, organic matter may remain in a WPCP for a few weeks due to this recycling of solids. The residence time of organic matter in a system is well-known and described by the mean cell residence time. Martin and Fenner (1997) examined 131I concentrations in primary sewage sludge at the Ann Arbor WPCP (Ann Arbor, MI) following an input of 131I from a thyroid cancer treatment. They determined the effective sewer half-life for 131 I in that plant to be 1.6 d. In the same way, the concentrations of 131I in sewage effluent can be used to determine its sewage half-life. The term sewage half-life refers to the half-life of a wastewater constituent in a WPCP and is used to distinguish it from the sewer half-life described by Martin and Fenner (1997) derived from primary sludge data. The concentration of a wastewater constituent over time with no further input, follows an exponential decay: Ct ¼ C0 elt
(1)
where Ct is the concentration at time t, C0 is the initial concentration and l is the decay constant. The natural log of the 131I concentration in sewage effluent plotted versus time
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Table 3 e Iodine-131 concentrations in suspended solids > 0.7 mm in sewage effluent samples collected from the Stony Brook WPCP. Corresponding 131I values for the filtered sewage effluent. The mean contribution of 131I associated with the suspended solids to the total activity in an unfiltered effluent sample is 3%. Sample collection date & time 3/10/07 11:15 AM 3/11/07 10:42 AM 3/12/07 1:07 PM 3/13/07 12:48 PM 3/14/07 12:43 PM 3/15/07 12:46 PM 3/16/07 12:41 PM 3/20/07 11:12 AM 3/21/07 1:12 PM 3/23/07 12:42 PM 3/27/07 12:43 PM 3/28/07 1:12 PM 5/23/07 11:46 AM 5/24/07 10:33 AM 5/25/07 1:35 PM 5/30/07 12:12 PM 5/31/07 1:00 PM 6/4/07 11:39 AM 6/5/07 2:09 PM 6/6/07 12:59 PM 6/7/07 2:18 PM 6/8/07 2:25 PM 6/11/07 1:33 PM 6/13/07 1:00 PM 6/14/07 2:30 PM 6/15/07 2:30 PM 6/19/07 2:31 PM 7/12/07 1:35 PM 7/14/07 1:00 PM
ISolids (Bq g1)
131
131
651 17 341 12 221 8 295 11 312 8 2801 32 1289 19 91 5 80 4 227 6 76 6 779 11 1797 34 353 5 916 19 345 7 383 10 153 6 61 12 126 10 260 18 164 17 72 9 369 30 203 35 722 55 154 21 1282 27 1157 28
51.6 0.9 37.5 0.7 24.2 0.6 29.7 0.8 23.3 0.6 227 2 175 2 8.6 0.4 4.6 0.3 32.9 0.5 5.1 0.4 93 1 102 1 85 1 42.5 0.5 18.0 0.6 12.1 0.3 4.6 0.3 2.5 0.3 17.7 0.4 24.4 0.3 13.6 0.6 3.1 0.3 27.8 0.8 16.4 0.7 55.3 0.9 4.6 0.3 97 1 115 1
results in a line with slope l from which the sewage half-life of 131 I (TI-131) can be derived: l¼
ln 2 TI-131
IEffluent (Bq L1)
(2)
In this study, there were twelve time series with sufficient data following an inpatient treatment, in which there was no further known input of 131I. The TI-131 determined from these plots ranged from 0.75 to 3.47 d (mean ¼ 2.1 0.7 d, Fig. 1). This is reasonable if we consider that these data indicate that 131I can be measured in sewage effluent for two weeks following a patient treatment. The TI-131 represents removal of 131I from the plant in sewage effluent discharges, losses to sewage sludge as well as through radioactive decay. The sewage half-life of non-radioactive iodine (Ts) can be estimated by accounting for radioactive decay. For each time series, the 131I concentrations were decay-corrected using the relationship in Equation (1) and the known decay constant for 131 I, where C0 ¼ the concentration of 131I measured on the first day of the time series and t ¼ time elapsed since the first day of the time series. Decay-corrected data plotted as described above for each time series are shown in Fig. 2. The range of values for Ts is between 0.86 and 6.20 d (mean ¼ 3 1 d, Fig. 2). The time series beginning June 14, July 27 and May 23 are the longest time series containing the most data. The Ts values for these three time series are in good agreement: 3.79, 3.33 and 4.01 d, respectively. Construction in the Stony Brook plant likely affected the flow in March 2009 and may account for the
Suspended solids > 0.7 mm (103 g L1) 1.9 2.6 2.9 2.5 2.4 2.4 4.6 3.3 3.2 3.2 2.3 1.9 1.2 4.4 1.6 2.3 2.6 2.7 2.0 1.5 0.8 1.3 1.5 1.1 1.1 0.9 1.1 0.8 1.4
comparatively short Ts for that sampling period (Rose, 2011). The time series beginning June 21, 2007 results in a relatively long Ts (6.2 d) that does not seem reasonable. Rather, it is possible that there was an input of 131I between June 22 and June 28, the dates of the second and third samples taken in the time series. Daily fluctuations in flow are likely to be the primary source of variation in these estimates. The behavior of iodine in the Stony Brook WPCP is consistent with a dissolved constituent that is not retained in the plant or lost to sewage sludge to a significant extent. The theoretical removal of a non-reactive, dissolved species is described by Equation (1), where l is the ratio of the plant through flow to the total volume, which includes return flows. In the Stony Brook WPCP, the through flow is 6.8 MLD (1.8 MGD) and the total flow out of the oxidation ditch is 21.9 MLD (5.8 MGD, Fig. 3). Using these values, the theoretical l is 0.3 d1. The mean l for iodine in the Stony Brook WPCP is also 0.3 d1 (Fig. 2). The agreement between these values suggests that most 131I entering the plant leaves in the effluent. Iodine-131 was not measured in sewage sludge as part of this work, but was investigated previously (Rose, 2003). In that study, the highest 131I concentration in sewage sludge was 148 Bq g1 dry weight. In the present study, most 131I concentrations in the effluent suspended solids are at least twice that value. We do not believe that the 131I concentrations in the effluent suspended solids are representative of sewage sludge. Iodine is likely associated with particulate organic matter (Bors et al., 1991; Brewer et al., 1980; Calvert
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Table 4 e Thyroid cancer inpatient treatment dates at the Stony Brook University Medical Center between June 2006 and July 2007 and JanuaryeMarch 2009. Approximate total 131I activity administered each month (Rose, 2011). Year
Month
Inpatient treatment date
Approximate total
a
131
I administered (GBq)
2006
June July August September October November December
6, 8, 9, 13, 23 18, 20, 25, 26 7, 9,a 10, 11, 15, 16, 26 6, 8,a 14, 15, 27, 28 5, 6, 11, 18, 20, 24, 25, 31 1, 2, 14, 17 1, 8, 12, 15, 20, 28
38.5 26.5 43.7 43.5 38.9 22.8 31.8
2007
January February March April May June July
6, 24, 25, 31 7 9,a 12, 14, 21, 27, 28, 29, 30a 13, 17 2, 15, 17, 18, 21, 22 5, 12, 14, 20, 28, 29 2, 5, 6, 11, 12, 13, 18, 25
22.9 7.4 55.5 13.0 35.2 37.9 45.3
2009
January February March
14, 15, 23 20, 25, 27 13, 18
11.1 13.9 13.9
a Indicates two inpatient treatments.
specific activity of 131I in sewage sludge relative to suspended solids. Thus, a mass balance using suspended solids and effluent concentrations is not appropriate. Furthermore, the frequency of patient treatments and the time it takes for 131I to
et al., 1993; Francois, 1987; Sheppard and Thibault, 1992; Ullman and Aller, 1980; Upstill-Goddard and Elderfield, 1988; Whitehead, 1973a,b) and diluted by denser, mineral phases (e.g., sand) in sewage sludge, which would decrease the 5.0 4.0 ln [ 131I ]
3.0
June 14 - 23, 2006 TI-131 = 2.64 ± 0.02 d
4.0
June 26 - 30, 2006 TI-131 = 2.15 ± 0.08 d
2.0
4.0
TI-131 = 2.11 ± 0.05 d
3.0
1.0 y = -0.263 ± 0.002 x + 4.510
0 6.0
2
4
6
8
y = -0.323 ± 0.013 x + 2.493
0.0 10
0 5.0
August 17 - 25, 2006
1
2
3
4
0
1
5.0
August 29 - 31, 2006
2
3
4
0.0 5
TI-131 = 1.98 ± 0.03 d
February 1 - 7, 2007
3.0
TI-131 = 1.54 ± 0.01 d
4.0
3.0 y = -0.350 ± 0.004 x + 5.187
1.0 0
2
4
6
8
y = -0.450 ± 0.003 x + 4.289
2.0 10
June 6 - 12, 2007 TI-131 = 2.70 ± 0.13 d
0 5.0
1
2
TI-131 = 1.75 ± 0.04 d
3.0
2.0
2.0
0 4.0
June 15 - 20, 2007
4.0
TI-131 = 1.12 ± 0.04 d
1
2
3
4
5
0.0 6
6
8 10 12
May 23 - June 5, 2007 TI-131 = 2.59 ± 0.03 d
y = -0.267 ± 0.003 x + 4.530
0 2 4 6 8 10 12 14 6.0
June 21 - 28, 2007
March 14 - 18, 2009 TI-131 = 0.75 ± 0.01 d
TI-131 = 3.47 ± 0.15 d
3.0
4.0
3.0 2.0
2.0 2.0
1.0 0.0
4
1.0 y = -0.396 ± 0.009 x + 3.506
0.0
3
2
4.0
3.0
1.0
2.0
y = -0.328 ± 0.007 x + 3.733
0 5.0
4.0
4.0
3.0
y = -0.344 ± 0.008 x + 3.196
1.0 5
5.0
4.0
2.0
2.0
1.0
1.0
ln [ 131I ]
TI-131 = 2.02 ± 0.04 d
3.0
July 27 - August 7, 2006
3.0 2.0
ln [ 131I ]
5.0
July 21 - 25, 2006
2.0 1.0
1.0 y = -0.257 ± 0.012 x + 3.300
0 1 2 3 4 5 6 7 Time (d)
y = -0.617 ± 0.020 x + 4.000
0.0 0
1
2
3
4
Time (d)
5
y = -0.200 ± 0.009 x + 2.700
0.0 6
0
2
4 Time (d)
6
0.0 8
y = -0.918 ± 0.013 x + 4.139
0
1
2
3
4
Time (d)
Fig. 1 e Weighted error least squares regression of the natural log of 131I concentrations in sewage effluent versus time in the Stony Brook WPCP for twelve sampling periods between inpatient treatments. TI-131 represents the sewage half-life of 131 I for each time series (mean TI-131 [ 2.1 ± 0.7 d).
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5.0
3.0
June 14 - 23, 2006
5.0
July 21 - 25, 2006
Ts = 3.13 ± 0.12 d
Ts = 3.79 ± 0.02 d
ln [ 131I ]
4.0
June 26 - 30, 2006
Ts = 2.79 ± 0.05 d
4.0
2.0
3.0
3.0
1.0
2.0
July 27 - August 7, 2006
4.0
Ts = 3.33 ± 0.06 d
3.0 2.0 1.0
y = -0.183 ± 0.001 x + 4.524
y = -0.222 ± 0.008 x + 2.455
2.0 0
2
4
6
8
10
0 5.0
August 17 - 25, 2006
5.0
Ts = 2.61 ± 0.02 d
1
2
3
4
5.0
1
Ts = 1.84 ± 0.01 d
2
3
4
5
0
2
5.0
February 1 - 7, 2007
4.0
6
8 10 12
May 23 - June 5, 2007
4.0
Ts = 2.25 ± 0.04 d
4
Ts = 4.01 ± 0.03 d
3.0 3.0
3.0
2.0
3.0 y = -0.266 ± 0.002 x + 5.218
y = -0.376 ± 0.002 x + 4.335
1.0 2
4
6
8
10
June 6 - 12, 2007 Ts = 2.69 ± 0.07 d
3.0
0 5.0
1
2
4.0
Ts = 1.31 ± 0.03 d
y = -0.173 ± 0.001 x + 4.544
1.0 0
3
June 15 - 20, 2007
4.0
y = -0.309 ± 0.005 x + 3.528
0.0
2.0 0
4.0
2.0
1.0
2.0
ln [ 131I ]
0.0 0
5
August 29 - 31, 2006
4.0
4.0
y = -0.208 ± 0.003 x + 3.626
1.0
0.0
6.0
ln [ 131I ]
y = -0.248 ± 0.004 x + 3.175
1
2
3
4
5
6
0 2 4 6 8 10 12 14 6.0
June 21 - 28, 2007
March 14 - 18, 2009
Ts = 6.20 ± 0.17 d
3.0
Ts = 0.86 ± 0.01 d
4.0
3.0 2.0 2.0
2.0
1.0
y = -0.257 ± 0.007 x + 3.378
0 1 2 3 4 5 6 7 Time (d)
2.0
1.0
1.0 y = -0.529 ± 0.012 x + 4.012
0.0 0
1
2
3
4
5
Time (d)
y = -0.112 ± 0.003 x + 2.665
0.0 6
0
2
4
6
0.0 8
Time (d)
y = -0.808 ± 0.010 x + 4.121
0
1
2
3
4
Time (d) 131
Fig. 2 e Weighted error least squares regression of the natural log of decay-corrected I concentrations in sewage effluent versus time in the Stony Brook WPCP for twelve sampling periods between inpatient treatments. Ts represents the sewage half-life for each time series (mean Ts [ 3 ± 1 d).
Fig. 3 e Flow schematic of the Stony Brook WPCP. The flow values represent average flows, rather than design flows. Average flows for the activated sludge and mixed liquor returns were determined in a June 2006 survey of the plant and represent the most accurate data available (Rose, 2011). Sludge removal processes are not included in this diagram.
be removed from the Stony Brook WPCP preclude any reasonable estimate of total discharges leaving in the effluent from a single patient treatment using this data set.
5.
Conclusions
Iodine-131concentrations in sewage effluent from the Stony Brook WPCP are a function of thyroid cancer inpatient treatments at the University Medical Center as well as the size and
design of the plant. Activated sludge and mixed liquor returns cause 131I to remain in the plant significantly longer than the 24-h hydraulic retention time of this facility. Sewage half-life describes the time it takes to decrease the concentration of a wastewater constituent by 50%. In the Stony Brook WPCP, it requires five sewage half-lives (w15 days) to remove most of the iodine. The mean decay constant determined experimentally for iodine (l ¼ 0.3 d-1, Fig. 2) is the same as the theoretical decay constant for a non-reactive, dissolved wastewater constituent and suggests that most of the 131I
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w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 6 6 3 e5 6 7 1
entering the plant leaves in the effluent. As a result, concentrations up to 227 Bq L1 were measured following a series of inpatient treatments. This concentration is seven times greater than the highest value reported in the literature (32.2 Bq L1, Puhakainen, 1998). This work demonstrates that medical use of 131I can result in relatively continuous discharges of this radioisotope in sewage effluent. Considering the annual number of therapeutic treatments per capita (average ¼ 188 per million people), WPCPs serving major population centers are likely to be continuous sources of 131I to the environment. Our field study showed that medically-derived 131I can be used to investigate the fate of wastewater constituents in surface waters (Rose, 2011). Sewage effluent discharges of 131I can also be used to further understand the aquatic biogeochemistry of naturally occurring iodine and the behavior radioactive iodine in the context of accidental releases. However, data from more WPCPs are needed to determine the extent to which 131I can be used as a tracer of biogeochemical processes in receiving waters.
Acknowledgments We thank Milton Cruz and Eugene Brewer at Suffolk County Department of Public Works for help with sample collection and providing information about the SBWPCP. Joseph Daly at Stony Brook University, Environmental Health and Safety provided us with patient treatment data and information regarding the medical use of 131I. We also thank David Hirschberg, Christina Heilbrun and Alisha Renfro for assistance in the laboratory at Stony Brook University.
references
Barci-Funel, G., Dalmasso, J., Magne, J., Ardisson, G., 1993. Simultaneous detection of short-lived 201Tl, 99mTc and 131I isotopes in sewage sludge using low-energy photon spectrometry. Science of the Total Environment 130, 37e42. Bors, J., Erten, H., Martens, R., 1991. Sorption studies of radioiodine on soils with special references to soil microbial biomass. Radiochimica Acta 52 (3), 317e325. Brewer, P.G., Nozaki, Y., Spencer, D.W., Fleer, A.P., 1980. Sediment trap experiments in the deep North Atlantic e isotopic and elemental fluxes. Journal of Marine Research 38 (4), 703e728. Calvert, S.E., Pedersen, T.F., Thunell, R.C., 1993. Geochemistry of the surface sediments of the Sulu and South China Seas. Marine Geology 114 (3e4), 207e231. Chang, B.U., Choi, S.W., Song, M.H., Lee, J.S., Kim, Y., 2011. Medically used radionulides (131I, 99mTc) in the urban sewage system: the case of the Daejeon metropolitan city, Korea. Radiation Protection Dosimetry 146 (1e3), 318e321. Currie, L.A., 1968. Limits for qualitative detection and quantitative determination e application to radiochemistry. Analytical Chemistry 40 (3), 586e593. Dalmasso, J., Barci-Funel, G., Magne, J., Barci, V., Ardisson, G., 1997. Study of the transfer of the medically used radionuclides in sewage systems. Radiochimica Acta 78, 167e171. Driver, I., Packer, S., 2001. Radioactive waste discharge quantities for patients undergoing radioactive iodine therapy for thyroid carcinoma. Nuclear Medicine Communications 22 (10), 1129e1132.
Erlandsson, B., Mattsson, S., 1978. Medically used radionuclides in sewage sludge. Water Air and Soil Pollution 9 (2), 199e206. Erlandsson, B., Bjurman, B., Mattsson, S., 1989. Calculation of radionuclide ground deposition by means of measurements on sewage sludge. Water Air and Soil Pollution 45 (3e4), 329e344. Erlandsson, B., Ingemansson, T., Mattsson, S., 1983. Comparative studies of radionuclides from global fallout and local sources in ground-level air and sewage sludge. Water Air and Soil Pollution 20 (3), 331e346. Fischer, H.W., Ulbrich, S., Pittauerova, D., Hettwig, B., 2009. Medical radioisotopes in the environment e following the pathway from patient to river sediment. Journal of Environmental Radioactivity 100 (12), 1079e1085. Francois, R., 1987. The influence of humic substances on the geochemistry of iodine in nearshore and hemipelagic marine sediments. Geochimica et Cosmochimica Acta 51 (9), 2417e2427. Hoffman, B.W., Van Camerik, S.B., 1967. A table and method for determining true time representing a count rate observed in radionuclear counting. Analytical Chemistry 39 (10), 1198e1199. Howe, J.R., Hunt, A.E., 1984. Swan thyroid glands and river algae as indicators of 125I and 131I in the River Trent and its tributaries. Science of the Total Environment 35 (3), 387e401. Howe, J.R., Lloyd, M.K., 1986. Radioiodine in thyroid glands of swans, farm animals and humans, also in algae and river water from the Thames Valley, England. Science of the Total Environment 48 (1e2), 13e31. ICRP, 2004. Release of Patients after Therapy with Unsealed Radionuclides, vol. 94. ICRP Publication. International Commission on Radiological Protection. Kleinschmidt, R., 2009. Uptake and depuration of 131I by the macroalgae Catenella nipae e potential use as an environmental monitor for radiopharmaceutical waste. Marine Pollution Bulletin 58 (10), 1539e1543. Larsen, I.L., Stetar, E.A., Glass, K.D., July/August 1995. In-house screening for radioactive sludge at a municipal wastewater treatment plant. Radiation Protection Management, 29e38. Marsh, K.V., Buddemeier, R.W., Wood, W., Smith, C., 1988. Radioiodine in kelp from Western Australia. Journal of Radioanalytical and Nuclear Chemistry 123 (1), 199e213. Martin, J.E., Fenner, F.D., 1997. Radioactivity in municipal sewage and sludge. Public Health Reports 112 (4), 308e316. Prichard, H.M., Gesell, T.F., Davis, E., 1981. 131I levels in sludge and treated municipal wastewaters near a large medical complex. American Journal of Public Health 71 (1), 47e52. Puhakainen, M., 1998. Detection of radionuclides in sewage water and sludge. Radiochemistry 40 (6), 529e533. Rose, P.S., 2003. Medical radioisotopes in municipal sewage and sewage sludge. Master’s thesis, Stony Brook University, Stony Brook, NY, 63 pp. Rose, P.S., 2011. Medically-derived 131I as a tracer in aquatic environments. Ph.D. dissertation, Stony Brook University, Stony Brook, NY, 197 pp. Sheppard, M.I., Thibault, D.H., 1992. Chemical behaviour of iodine in organic and mineral soils. Applied Geochemistry 7 (3), 265e272. Smith, J.P., Oktay, S.D., Kada, J., Olsen, C.R., 2008. Iodine-131: a potential short-lived, wastewater-specific particle tracer in an urbanized estuarine system. Environmental Science & Technology 42 (15), 5435e5440. Sodd, V.J., Velten, R.J., Saenger, E.L., 1975. Concentrations of medically useful radionuclides, Technetium-99m and Iodine131 at a large metropolitan waste-water treatment plant. Health Physics 28 (4), 355e359. Stetar, E.A., Boston, H.L., Larsen, I.L., Mobley, M.H., 1993. The removal of radioactive cobalt, cesium, and iodine in a conventional municipal wastewater treatment plant. Water Environment Research 65 (5), 630e639. Ullman, W.J., Aller, R.C., 1980. Dissolved iodine flux from estuarine sediments and implications for the enrichment of
w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 6 6 3 e5 6 7 1
iodine at the sediment water interface. Geochimica et Cosmochimica Acta 44 (8), 1177e1184. Ullman, W.J., Aller, R.C., 1983. Rates of iodine remineralization in terrigenous near-shore sediments. Geochimica et Cosmochimica Acta 47 (8), 1423e1432. Ullman, W.J., Aller, R.C., 1985. The geochemistry of iodine in nearshore carbonate sediments. Geochimica et Cosmochimica Acta 49 (4), 967e978. UNSCEAR, 2000. UNSCEAR 2000 report to the general assembly with scientific annexes. Sources, United Nations Scientific Committee on the Effects of Atomic Radiation, vol. I.
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Upstill-Goddard, R.C., Elderfield, H., 1988. The role of diagenesis in the estuarine budgets of iodine and bromine. Continental Shelf Research 8 (4), 405e430. Waller, E.J., Cole, D., 1999. An environmental radionuclide baseline study near three Canadian naval ports. Health Physics 77 (1), 37e42. Whitehead, D.C., 1973a. Sorption of iodide by soils as influenced by equilibrium conditions and soil properties. Journal of the Science of Food and Agriculture 24 (5), 547e556. Whitehead, D.C., 1973b. Studies on iodine in British soils. Journal of Soil Science 24 (2), 260e270.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Medically-derived
131
I in municipal sewage effluent
Paula S. Rose*, R. Lawrence Swanson, J. Kirk Cochran Marine Sciences Research Center, School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794-5000, USA
article info
abstract
Article history:
This work presents 131I (t½ ¼ 8.04 d) concentrations in sewage effluent from the Stony Brook
Received 30 March 2012
Water Pollution Control Plant (WPCP), a small plant serving a regional thyroid cancer
Received in revised form
treatment facility in Stony Brook, NY, USA. The concentrations detected in sewage effluent
27 June 2012
ranged from 1.8 0.3 to 227 2 Bq L1. The primary source of
Accepted 23 July 2012
cancer inpatients treated at the Stony Brook University Medical Center. Based on several
Available online 2 August 2012
time series measurements following known inpatient treatments, the mean sewage half-
131
I is excreta from thyroid
life (Ts) of iodine is 3 d in this plant. The Ts, analogous to a radioactive half-life, Keywords:
describes the time it takes for half of a wastewater component to be removed from
Iodine
a WPCP. Flow recycling, or activated sludge, used to maintain bacterial populations
131
necessary for sewage treatment causes iodine to remain in this plant far longer than its
Sewage
hydraulic retention time. The experimental results suggest that most
Sewage effluent
Stony Brook WPCP leaves in sewage effluent, not in sewage sludge. Patient treatments can
I
131
I entering the
Wastewater
result in continuous discharges of 131I to surface waters where it can be used as a tracer of
Medical radioisotopes
sewage-derived material and to understand the behavior of
Nuclear medicine
1.
Introduction
Iodine-131 is released from nuclear power plants, during nuclear weapons tests, nuclear fuel reprocessing and weapons production. Medical use is perhaps the more widespread source of 131I to the environment. It is the most widely used radiopharmaceutical in nuclear medicine for therapeutic purposes, primarily to treat hyperthyroidism and thyroid cancer. In developed countries, the average number of hyperthyroidism treatments is 150 per million people and 38 thyroid cancer treatments per million people; however, the latter represents a greater potential source to water pollution control plants (WPCPs). The standard protocol for treating thyroid cancer is removal of the whole thyroid gland followed by administration of 131I to destroy any remaining tissue or cells. Thyroid cancer patients are typically given 4000e8000 MBq compared to 100e1000 MBq of 131I for
131
I in aquatic environments.
ª 2012 Elsevier Ltd. All rights reserved.
hyperthyroid treatments and most of the initial dose is eliminated from the body in urine (ICRP, 2004; UNSCEAR, 2000). In the United States, patient excreta are exempt from sewer discharge regulations and are therefore released directly into sewer systems (Martin and Fenner, 1997). Iodine-131 has been measured in aquatic environments receiving sewage effluent discharges, yet few published data exist for the radioisotope in sewage effluent (Chang et al., 2011; Erlandsson et al., 1989; Fischer et al., 2009; Kleinschmidt, 2009; Puhakainen, 1998; Rose, 2003; Smith et al., 2008; Sodd et al., 1975); most work has focused on sewage sludge. The results of several investigations indicate greater than 75% of 131I entering WPCPs leaves in the effluent (Barci-Funel et al., 1993; Dalmasso et al., 1997; Erlandsson et al., 1989, 1983; Erlandsson and Mattsson, 1978; Martin and Fenner, 1997; Prichard et al., 1981; Puhakainen, 1998; Stetar et al., 1993). It is not surprising then that medically-derived
* Corresponding author. Present address: US Naval Research Laboratory, NRC Postdoctoral Research Associate, Marine Biogeochemistry, Code 6114, 4555 Overlook Avenue SW, Washington, DC 20375, USA. Tel.: þ1 202 767 0787. E-mail address: [email protected] (P.S. Rose). 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2012.07.045
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w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 6 6 3 e5 6 7 1
131
I is readily measurable in the environment (Fischer et al., 2009; Howe and Hunt, 1984; Howe and Lloyd, 1986; Marsh et al., 1988; Puhakainen, 1998; Rose, 2003; Smith et al., 2008; Sodd et al., 1975; Waller and Cole, 1999). While the occurrence and concentrations of 131I in sewage effluent have been poorly characterized, these studies suggest discharges of the radioisotope may be widespread and therefore useful as a tracer in receiving waters to study biogeochemical processes occurring on the time scale of approximately one month. Distributions of naturally occurring iodine have been wellstudied and indicate that iodine is biologically cycled and remineralized similar to other nutrient elements (Ullman and Aller, 1980, 1983, 1985; Upstill-Goddard and Elderfield, 1988). The nutrient-like behavior of iodine suggests that medicallyderived 131I may be used to study the short-term fate of wastewater nitrogen in aquatic systems. Wastewater discharges of 131I would also be useful to investigate the natural cycling of iodine in receiving waters. More specifically, the rates and mechanisms governing the transformations of natural iodine in aquatic systems are not well-known. Additionally, Smith et al. (2008) proposed 131I as a wastewaterspecific particle tracer and suggested its use as a tracer for short-term sediment dynamics. The objective of this study was to determine the occurrence and concentrations of 131I in sewage effluent at a relatively small WPCP with known inputs of the radioisotope from thyroid cancer inpatient treatments.
2.
Methods
2.1.
Study site
The Stony Brook WPCP, Suffolk County Sewer District #21, is a tertiary treatment facility located on the campus of Stony Brook University, Stony Brook, NY, USA. The plant’s service area includes the campus, the Stony Brook University Medical Center and a small number of private homes. The estimated size of the population served is 20,000. At the time of sampling, tertiary treatment was achieved via an oxidation ditch with both activated sludge and mixed liquor returns. Sodium hypochlorite was added to sewage effluent for disinfection prior to discharge from the plant. Travel time from the plant to its outfall in Port Jefferson Harbor, NY is 4e6 h. The oxidation ditch no longer receives mixed liquor returns and sewage effluent is disinfected using ultraviolet irradiation at the Port Jefferson WPCP (Suffolk County Sewer District #1) just prior to its discharge into Port Jefferson Harbor (Rose, 2011). The design capacity of the plant is approximately 9.5 106 liters per day (MLD) or 2.5 106 gallons per day (MGD). Average flow is approximately 6.8 MLD (1.8 MGD). Average daily maximum and minimum flows are approximately 7.6 MLD (2.0 MGD) and 3.8 MLD (1.0 MGD), respectively. During the summer months and weekends, when school is not in full session, average flows decrease as much as 20%. The average daily maximum to minimum ratio of daily flow is approximately 2, but maximum flow in a given day can exceed 4 times the minimum daily flow. Minimum flows generally occur between 7 and 8 AM, after which there is a rapid increase in flow until it peaks around 12 PM. Flow then decreases slowly until about 2 AM, then more rapidly until about 7 AM (Rose, 2011).
The hydraulic retention time of sewage in the Stony Brook WPCP is approximately 24e36 h, depending on the University schedule as mentioned above. The mean cell residence time, or the residence time of organic matter in the system, is about 36 d (Rose, 2011).
2.2.
Sample collection and preparation
Sewage effluent was collected as a grab sample from the final effluent stream before discharge from the plant using a 1 L high density polyethylene bottle. More than half of the samples were analyzed with no further treatment. The remaining samples were vacuum filtered through a 47 mm 0.7 mm glass fiber filter. Aliquots of the filtrate were retained and analyzed with no further treatment. Sample volumes analyzed were 150 mL and 170 mL. Straight-side polypropylene jars (64 mm height; 64 mm diameter) were used for counting. Pre-weighed filters that were retained for g-ray spectrometry determinations were dried at 40 C overnight and re-weighed before analysis. More than 80% of the samples were collected between 12 PM and 3 PM. Sampling replicates were also collected at the Stony Brook WPCP on six additional days.
2.3.
Determination of
131
I
The activity of 131I was determined by g-ray spectrometry (364.5 keV peak; branching ratio ¼ 0.812) using Canberra low energy germanium detectors. Generally, samples were counted for one day. Due to the relatively short half-life of 131I, activities were corrected to account for decay during data acquisition as described in Hoffman and Van Camerik (1967). All concentrations of 131I are reported for time of collection and with a 1s counting error. The counting efficiency for each geometry and each detector used in this investigation was determined using a certified 131I standard solution (Eckert & Ziegler Isotope Products, Valencia, CA). For the 150 mL and 170 mL sewage effluent samples, deionized water was spiked with the 131I standard solution and counted three times on each detector to determine the counting efficiency of these geometries (3 replicates per detector). The 131 I standard solution was applied to three filters with a pipette, using enough liquid to wet the filter entirely. Each filter was counted three times on each detector to determine the counting efficiency of the suspended solids samples (9 replicates per detector). In each case, the mean counting efficiency was used to calculate sample activities. Multi-day continuum background counts were determined for each detector with several 131I-free sewage effluent samples. The mean background count for each detector was used to determine limit of detection (LD) as described by Currie (1968). The detection limits were 1.7 Bq L1 for sewage effluent samples and 50 Bq g1 for suspended solids.
3.
Results
3.1.
Unfiltered effluent
Iodine-131 concentrations detected in unfiltered sewage effluent collected on 77 different days between June 2006 and March 2009
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w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 6 6 3 e5 6 7 1
ranged from 1.8 0.3 to 217 1 Bq L1 (median ¼ 11 Bq L1); nine samples < LD (Table 1). Percent differences among 131I concentrations in replicate samples were 0.9e7%. The variation among the replicates was within the counting error.
3.2.
4.
Filtered effluent
Iodine-131 concentrations detected in filtered sewage effluent collected on 46 different days between January 2007 and March 2009 ranged from 2.5 0.3 to 227 2 Bq L1 (median ¼ 18 Bq L1); three samples < LD (Table 2).
3.3.
The 131I and suspended solids concentrations are shown in Table 3 for samples with activities > LD.
Suspended solids
Iodine-131 concentrations detected in suspended solids > 0.7 mm in sewage effluent collected on 36 different days between March 2007 and March 2009 ranged from 61 12 to 2801 32 Bq g1 (median ¼ 304 Bq g1); seven samples < LD.
Discussion
Previous studies have reported values up to 32.2 Bq L1 of 131I in sewage effluent from WPCPs ranging in size from 30 to 645 MLD (8e170 MGD) (Erlandsson et al., 1989; Fischer et al., 2009; Puhakainen, 1998; Smith et al., 2008; Sodd et al., 1975). The highest concentrations measured in sewage effluent from the Stony Brook WPCP were 217 1 Bq L1 following five inpatient treatments (unfiltered sewage effluent, Table 1) and 227 2 Bq L1 following four inpatient treatments (filtered sewage effluent, Table 2). Dates of inpatient treatments at the Stony Brook University Medical Center are listed in Table 4. It should be noted that while the suspended solids have relatively high specific activities of 131I, the contribution of the
Table 1 e Iodine-131 concentrations in unfiltered effluent samples collected from the Stony Brook WPCP. Sample volume [ 0.17 L.
131
I (Bq L1)
48.4 0.8 21.8 0.6 93 1 65 1 60 1 21.4 0.6 17.6 0.5 16.1 0.5 9.6 0.3 9.0 0.4 11.9 0.5 8.8 0.3 5.5 0.4 2.0 0.3 4.0 0.3
Sample collection date & time 8/30/06 1:46 PM 8/31/06 2:35 PM 9/14/06 2:22 PM 9/15/06 2:34 PM 9/18/06 2:25 PM 9/20/06 1:44 PM 9/22/06 12:41 PM 9/29/06 1:31 PM 10/19/06 2:05 PM 12/6/06 2:08 PM 12/7/06 1:37 PM 12/8/06 12:30 PM 12/11/06 12:43 PM 12/12/06 2:22 PM 12/13/06 1:00 PM 12/14/06 1:47 PM 12/15/06 12:29 PM 12/19/06 2:05 PM 12/20/06 11:22 AM 1/3/07 1:48 PM 1/5/07 12:42 PM 1/8/07 12:00 PM 1/9/07 1:35 PM 1/11/07 12:00 PM 1/15/07 2:18 PM 1/16/07 1:00 PM 1/23/07 12:53 PM 1/24/07 12:35 PM 1/25/07 1:25 PM 1/26/07 10:05 AM 1/28/09 12:00 PMa 3/14/09 1:07 PMa 3/15/09 1:31 PMa 3/16/09 12:36 PMa 3/17/09 1:30 PMa 3/18/09 12:30 PMa 3/19/09 12:34 PMa 3/20/09 1:00 PMa
I (Bq L1)
131
46.8 0.8 30.5 0.4 8.8 0.2 108 3 23.1 0.2 14.0 0.4 7.2 0.2 187 1 83.2 0.6 10.6 0.2 8.5 0.2 6.7 0.1 10.9 0.2 7.9 0.2 6.2 0.2 4.9 0.2 4.3 0.1 7.1 0.2 7.3 0.2
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Table 2 e Iodine-131 concentrations in filtered effluent samples collected from the Stony Brook WPCP. Sample volume [ 0.15 L.
I (Bq L1)
131
Sample collection date & time
5.6 0.3 34.1 0.5 24.8 0.9 5.0 0.3 4.0 0.3 43.0 0.8 42 1 8.4 0.4
3/29/07 11:35 AM 5/23/07 11:46 AM 5/24/07 10:33 AM 5/25/07 1:31 PM 5/30/07 12:12 PM 5/31/07 1:00 PM 6/4/07 11:39 AM 6/5/07 2:09 PM 6/6/07 12:59 PM 6/7/07 2:18 PM 6/8/07 2:25 PM 6/11/07 1:33 PM 6/12/07 2:33 PM 6/13/07 1:00 PM 6/14/07 2:30 PM 6/15/07 2:30 PM 6/19/07 2:31 PM 6/20/07 12:58 PM 6/21/07 2:15 PM 6/22/07 1:13 PM 6/28/07 1:55 PM 7/12/07 1:35 PM 7/14/07 1:00 PM
I (Bq L1)
131
136 2 102 1 85 1 42.5 0.5 18.0 0.6 12.1 0.3 4.6 0.3 2.5 0.3 17.7 0.4 24.4 0.3 13.6 0.6 3.1 0.3 2.5 0.3 27.8 0.8 16.4 0.7 55.3 0.9 4.6 0.3 2.9 0.5 14.5 0.8 11.7 0.6 3.6 0.3 97 1 115 1
a Sample volume ¼ 0.17 L.
suspended solids to the total activity of the unfiltered effluent is minimal because the suspended solids concentrations are low (Table 3). Additionally, no settling of solids was observed during counting and therefore homogeneity of the sample was maintained during counting. The following discussion assumes there is no difference between the concentrations of 131 I measured in whole effluent and the filtered effluent collected at the Stony Brook WPCP. These concentrations can be primarily attributed to the frequency of thyroid cancer inpatient treatments performed at the University Medical Center, the size of the plant and sewage half-life. There are approximately 60 inpatient treatments per year at the University Medical Center with about an equal number of outpatients. While outpatients treated at the University Medical Center and other facilities are possible sources to the plant, they are not a significant source to the Stony Brook WPCP. Outpatients at the University Medical Center leave the hospital following administration of the 131I. Frequent or large inputs from persons treated at another facility are unlikely considering the size and composition of the population, which consists mainly of University students and employees. The University Medical Center is a regional thyroid cancer treatment facility. These treatments require specialized rooms and personnel, and therefore are not performed at all hospitals. Medical use is the only known source of 131I to this system (Rose, 2011). Typically, thyroid cancer inpatients remain in the hospital 24e30 h following treatment. Length of hospital stay is dependent on the time it takes for the patient’s external dose rate, measured at 1 m, to decrease to less than 10 mrem h1 from 20 to 30 mrem h1 after administration of the dose (Rose, 2011). This is consistent with the work of Driver and Packer (2001) who found that 55% of the initial activity administered to thyroid cancer patients is excreted in the first 24 h.
The data presented here clearly indicate that 131I is discharged from the plant for many days following a patient treatment. Larsen et al. (1995) found elevated 131I concentrations in digested sewage sludge at the Oak Ridge WPCP (Oak Ridge, TN) for more than one month following a single patient treatment. Previous work in the Stony Brook WPCP indicated that 131I can be detected in sewage sludge for at least two weeks after known inputs (Rose, 2003). Retention of sewage sludge is dependent on plant design and sludge removal practices. The recycling of biomass within a sewage treatment plant, or activated sludge, helps maintain a standing stock of bacteria necessary for sewage treatment. Therefore, organic matter may remain in a WPCP for a few weeks due to this recycling of solids. The residence time of organic matter in a system is well-known and described by the mean cell residence time. Martin and Fenner (1997) examined 131I concentrations in primary sewage sludge at the Ann Arbor WPCP (Ann Arbor, MI) following an input of 131I from a thyroid cancer treatment. They determined the effective sewer half-life for 131 I in that plant to be 1.6 d. In the same way, the concentrations of 131I in sewage effluent can be used to determine its sewage half-life. The term sewage half-life refers to the half-life of a wastewater constituent in a WPCP and is used to distinguish it from the sewer half-life described by Martin and Fenner (1997) derived from primary sludge data. The concentration of a wastewater constituent over time with no further input, follows an exponential decay: Ct ¼ C0 elt
(1)
where Ct is the concentration at time t, C0 is the initial concentration and l is the decay constant. The natural log of the 131I concentration in sewage effluent plotted versus time
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Table 3 e Iodine-131 concentrations in suspended solids > 0.7 mm in sewage effluent samples collected from the Stony Brook WPCP. Corresponding 131I values for the filtered sewage effluent. The mean contribution of 131I associated with the suspended solids to the total activity in an unfiltered effluent sample is 3%. Sample collection date & time 3/10/07 11:15 AM 3/11/07 10:42 AM 3/12/07 1:07 PM 3/13/07 12:48 PM 3/14/07 12:43 PM 3/15/07 12:46 PM 3/16/07 12:41 PM 3/20/07 11:12 AM 3/21/07 1:12 PM 3/23/07 12:42 PM 3/27/07 12:43 PM 3/28/07 1:12 PM 5/23/07 11:46 AM 5/24/07 10:33 AM 5/25/07 1:35 PM 5/30/07 12:12 PM 5/31/07 1:00 PM 6/4/07 11:39 AM 6/5/07 2:09 PM 6/6/07 12:59 PM 6/7/07 2:18 PM 6/8/07 2:25 PM 6/11/07 1:33 PM 6/13/07 1:00 PM 6/14/07 2:30 PM 6/15/07 2:30 PM 6/19/07 2:31 PM 7/12/07 1:35 PM 7/14/07 1:00 PM
ISolids (Bq g1)
131
131
651 17 341 12 221 8 295 11 312 8 2801 32 1289 19 91 5 80 4 227 6 76 6 779 11 1797 34 353 5 916 19 345 7 383 10 153 6 61 12 126 10 260 18 164 17 72 9 369 30 203 35 722 55 154 21 1282 27 1157 28
51.6 0.9 37.5 0.7 24.2 0.6 29.7 0.8 23.3 0.6 227 2 175 2 8.6 0.4 4.6 0.3 32.9 0.5 5.1 0.4 93 1 102 1 85 1 42.5 0.5 18.0 0.6 12.1 0.3 4.6 0.3 2.5 0.3 17.7 0.4 24.4 0.3 13.6 0.6 3.1 0.3 27.8 0.8 16.4 0.7 55.3 0.9 4.6 0.3 97 1 115 1
results in a line with slope l from which the sewage half-life of 131 I (TI-131) can be derived: l¼
ln 2 TI-131
IEffluent (Bq L1)
(2)
In this study, there were twelve time series with sufficient data following an inpatient treatment, in which there was no further known input of 131I. The TI-131 determined from these plots ranged from 0.75 to 3.47 d (mean ¼ 2.1 0.7 d, Fig. 1). This is reasonable if we consider that these data indicate that 131I can be measured in sewage effluent for two weeks following a patient treatment. The TI-131 represents removal of 131I from the plant in sewage effluent discharges, losses to sewage sludge as well as through radioactive decay. The sewage half-life of non-radioactive iodine (Ts) can be estimated by accounting for radioactive decay. For each time series, the 131I concentrations were decay-corrected using the relationship in Equation (1) and the known decay constant for 131 I, where C0 ¼ the concentration of 131I measured on the first day of the time series and t ¼ time elapsed since the first day of the time series. Decay-corrected data plotted as described above for each time series are shown in Fig. 2. The range of values for Ts is between 0.86 and 6.20 d (mean ¼ 3 1 d, Fig. 2). The time series beginning June 14, July 27 and May 23 are the longest time series containing the most data. The Ts values for these three time series are in good agreement: 3.79, 3.33 and 4.01 d, respectively. Construction in the Stony Brook plant likely affected the flow in March 2009 and may account for the
Suspended solids > 0.7 mm (103 g L1) 1.9 2.6 2.9 2.5 2.4 2.4 4.6 3.3 3.2 3.2 2.3 1.9 1.2 4.4 1.6 2.3 2.6 2.7 2.0 1.5 0.8 1.3 1.5 1.1 1.1 0.9 1.1 0.8 1.4
comparatively short Ts for that sampling period (Rose, 2011). The time series beginning June 21, 2007 results in a relatively long Ts (6.2 d) that does not seem reasonable. Rather, it is possible that there was an input of 131I between June 22 and June 28, the dates of the second and third samples taken in the time series. Daily fluctuations in flow are likely to be the primary source of variation in these estimates. The behavior of iodine in the Stony Brook WPCP is consistent with a dissolved constituent that is not retained in the plant or lost to sewage sludge to a significant extent. The theoretical removal of a non-reactive, dissolved species is described by Equation (1), where l is the ratio of the plant through flow to the total volume, which includes return flows. In the Stony Brook WPCP, the through flow is 6.8 MLD (1.8 MGD) and the total flow out of the oxidation ditch is 21.9 MLD (5.8 MGD, Fig. 3). Using these values, the theoretical l is 0.3 d1. The mean l for iodine in the Stony Brook WPCP is also 0.3 d1 (Fig. 2). The agreement between these values suggests that most 131I entering the plant leaves in the effluent. Iodine-131 was not measured in sewage sludge as part of this work, but was investigated previously (Rose, 2003). In that study, the highest 131I concentration in sewage sludge was 148 Bq g1 dry weight. In the present study, most 131I concentrations in the effluent suspended solids are at least twice that value. We do not believe that the 131I concentrations in the effluent suspended solids are representative of sewage sludge. Iodine is likely associated with particulate organic matter (Bors et al., 1991; Brewer et al., 1980; Calvert
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Table 4 e Thyroid cancer inpatient treatment dates at the Stony Brook University Medical Center between June 2006 and July 2007 and JanuaryeMarch 2009. Approximate total 131I activity administered each month (Rose, 2011). Year
Month
Inpatient treatment date
Approximate total
a
131
I administered (GBq)
2006
June July August September October November December
6, 8, 9, 13, 23 18, 20, 25, 26 7, 9,a 10, 11, 15, 16, 26 6, 8,a 14, 15, 27, 28 5, 6, 11, 18, 20, 24, 25, 31 1, 2, 14, 17 1, 8, 12, 15, 20, 28
38.5 26.5 43.7 43.5 38.9 22.8 31.8
2007
January February March April May June July
6, 24, 25, 31 7 9,a 12, 14, 21, 27, 28, 29, 30a 13, 17 2, 15, 17, 18, 21, 22 5, 12, 14, 20, 28, 29 2, 5, 6, 11, 12, 13, 18, 25
22.9 7.4 55.5 13.0 35.2 37.9 45.3
2009
January February March
14, 15, 23 20, 25, 27 13, 18
11.1 13.9 13.9
a Indicates two inpatient treatments.
specific activity of 131I in sewage sludge relative to suspended solids. Thus, a mass balance using suspended solids and effluent concentrations is not appropriate. Furthermore, the frequency of patient treatments and the time it takes for 131I to
et al., 1993; Francois, 1987; Sheppard and Thibault, 1992; Ullman and Aller, 1980; Upstill-Goddard and Elderfield, 1988; Whitehead, 1973a,b) and diluted by denser, mineral phases (e.g., sand) in sewage sludge, which would decrease the 5.0 4.0 ln [ 131I ]
3.0
June 14 - 23, 2006 TI-131 = 2.64 ± 0.02 d
4.0
June 26 - 30, 2006 TI-131 = 2.15 ± 0.08 d
2.0
4.0
TI-131 = 2.11 ± 0.05 d
3.0
1.0 y = -0.263 ± 0.002 x + 4.510
0 6.0
2
4
6
8
y = -0.323 ± 0.013 x + 2.493
0.0 10
0 5.0
August 17 - 25, 2006
1
2
3
4
0
1
5.0
August 29 - 31, 2006
2
3
4
0.0 5
TI-131 = 1.98 ± 0.03 d
February 1 - 7, 2007
3.0
TI-131 = 1.54 ± 0.01 d
4.0
3.0 y = -0.350 ± 0.004 x + 5.187
1.0 0
2
4
6
8
y = -0.450 ± 0.003 x + 4.289
2.0 10
June 6 - 12, 2007 TI-131 = 2.70 ± 0.13 d
0 5.0
1
2
TI-131 = 1.75 ± 0.04 d
3.0
2.0
2.0
0 4.0
June 15 - 20, 2007
4.0
TI-131 = 1.12 ± 0.04 d
1
2
3
4
5
0.0 6
6
8 10 12
May 23 - June 5, 2007 TI-131 = 2.59 ± 0.03 d
y = -0.267 ± 0.003 x + 4.530
0 2 4 6 8 10 12 14 6.0
June 21 - 28, 2007
March 14 - 18, 2009 TI-131 = 0.75 ± 0.01 d
TI-131 = 3.47 ± 0.15 d
3.0
4.0
3.0 2.0
2.0 2.0
1.0 0.0
4
1.0 y = -0.396 ± 0.009 x + 3.506
0.0
3
2
4.0
3.0
1.0
2.0
y = -0.328 ± 0.007 x + 3.733
0 5.0
4.0
4.0
3.0
y = -0.344 ± 0.008 x + 3.196
1.0 5
5.0
4.0
2.0
2.0
1.0
1.0
ln [ 131I ]
TI-131 = 2.02 ± 0.04 d
3.0
July 27 - August 7, 2006
3.0 2.0
ln [ 131I ]
5.0
July 21 - 25, 2006
2.0 1.0
1.0 y = -0.257 ± 0.012 x + 3.300
0 1 2 3 4 5 6 7 Time (d)
y = -0.617 ± 0.020 x + 4.000
0.0 0
1
2
3
4
Time (d)
5
y = -0.200 ± 0.009 x + 2.700
0.0 6
0
2
4 Time (d)
6
0.0 8
y = -0.918 ± 0.013 x + 4.139
0
1
2
3
4
Time (d)
Fig. 1 e Weighted error least squares regression of the natural log of 131I concentrations in sewage effluent versus time in the Stony Brook WPCP for twelve sampling periods between inpatient treatments. TI-131 represents the sewage half-life of 131 I for each time series (mean TI-131 [ 2.1 ± 0.7 d).
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5.0
3.0
June 14 - 23, 2006
5.0
July 21 - 25, 2006
Ts = 3.13 ± 0.12 d
Ts = 3.79 ± 0.02 d
ln [ 131I ]
4.0
June 26 - 30, 2006
Ts = 2.79 ± 0.05 d
4.0
2.0
3.0
3.0
1.0
2.0
July 27 - August 7, 2006
4.0
Ts = 3.33 ± 0.06 d
3.0 2.0 1.0
y = -0.183 ± 0.001 x + 4.524
y = -0.222 ± 0.008 x + 2.455
2.0 0
2
4
6
8
10
0 5.0
August 17 - 25, 2006
5.0
Ts = 2.61 ± 0.02 d
1
2
3
4
5.0
1
Ts = 1.84 ± 0.01 d
2
3
4
5
0
2
5.0
February 1 - 7, 2007
4.0
6
8 10 12
May 23 - June 5, 2007
4.0
Ts = 2.25 ± 0.04 d
4
Ts = 4.01 ± 0.03 d
3.0 3.0
3.0
2.0
3.0 y = -0.266 ± 0.002 x + 5.218
y = -0.376 ± 0.002 x + 4.335
1.0 2
4
6
8
10
June 6 - 12, 2007 Ts = 2.69 ± 0.07 d
3.0
0 5.0
1
2
4.0
Ts = 1.31 ± 0.03 d
y = -0.173 ± 0.001 x + 4.544
1.0 0
3
June 15 - 20, 2007
4.0
y = -0.309 ± 0.005 x + 3.528
0.0
2.0 0
4.0
2.0
1.0
2.0
ln [ 131I ]
0.0 0
5
August 29 - 31, 2006
4.0
4.0
y = -0.208 ± 0.003 x + 3.626
1.0
0.0
6.0
ln [ 131I ]
y = -0.248 ± 0.004 x + 3.175
1
2
3
4
5
6
0 2 4 6 8 10 12 14 6.0
June 21 - 28, 2007
March 14 - 18, 2009
Ts = 6.20 ± 0.17 d
3.0
Ts = 0.86 ± 0.01 d
4.0
3.0 2.0 2.0
2.0
1.0
y = -0.257 ± 0.007 x + 3.378
0 1 2 3 4 5 6 7 Time (d)
2.0
1.0
1.0 y = -0.529 ± 0.012 x + 4.012
0.0 0
1
2
3
4
5
Time (d)
y = -0.112 ± 0.003 x + 2.665
0.0 6
0
2
4
6
0.0 8
Time (d)
y = -0.808 ± 0.010 x + 4.121
0
1
2
3
4
Time (d) 131
Fig. 2 e Weighted error least squares regression of the natural log of decay-corrected I concentrations in sewage effluent versus time in the Stony Brook WPCP for twelve sampling periods between inpatient treatments. Ts represents the sewage half-life for each time series (mean Ts [ 3 ± 1 d).
Fig. 3 e Flow schematic of the Stony Brook WPCP. The flow values represent average flows, rather than design flows. Average flows for the activated sludge and mixed liquor returns were determined in a June 2006 survey of the plant and represent the most accurate data available (Rose, 2011). Sludge removal processes are not included in this diagram.
be removed from the Stony Brook WPCP preclude any reasonable estimate of total discharges leaving in the effluent from a single patient treatment using this data set.
5.
Conclusions
Iodine-131concentrations in sewage effluent from the Stony Brook WPCP are a function of thyroid cancer inpatient treatments at the University Medical Center as well as the size and
design of the plant. Activated sludge and mixed liquor returns cause 131I to remain in the plant significantly longer than the 24-h hydraulic retention time of this facility. Sewage half-life describes the time it takes to decrease the concentration of a wastewater constituent by 50%. In the Stony Brook WPCP, it requires five sewage half-lives (w15 days) to remove most of the iodine. The mean decay constant determined experimentally for iodine (l ¼ 0.3 d-1, Fig. 2) is the same as the theoretical decay constant for a non-reactive, dissolved wastewater constituent and suggests that most of the 131I
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entering the plant leaves in the effluent. As a result, concentrations up to 227 Bq L1 were measured following a series of inpatient treatments. This concentration is seven times greater than the highest value reported in the literature (32.2 Bq L1, Puhakainen, 1998). This work demonstrates that medical use of 131I can result in relatively continuous discharges of this radioisotope in sewage effluent. Considering the annual number of therapeutic treatments per capita (average ¼ 188 per million people), WPCPs serving major population centers are likely to be continuous sources of 131I to the environment. Our field study showed that medically-derived 131I can be used to investigate the fate of wastewater constituents in surface waters (Rose, 2011). Sewage effluent discharges of 131I can also be used to further understand the aquatic biogeochemistry of naturally occurring iodine and the behavior radioactive iodine in the context of accidental releases. However, data from more WPCPs are needed to determine the extent to which 131I can be used as a tracer of biogeochemical processes in receiving waters.
Acknowledgments We thank Milton Cruz and Eugene Brewer at Suffolk County Department of Public Works for help with sample collection and providing information about the SBWPCP. Joseph Daly at Stony Brook University, Environmental Health and Safety provided us with patient treatment data and information regarding the medical use of 131I. We also thank David Hirschberg, Christina Heilbrun and Alisha Renfro for assistance in the laboratory at Stony Brook University.
references
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