erlandsson1978

MEDICALLY USED RADIONUCLIDES
IN S E W A G E S L U D G E
B. ERLANDSSON* and S. MATTSSON
Technical Department, Personnel Health Care, City of MalmO, S-220 02 MalmO, Sweden.
and
Department of Radiation Physics, MalmO General Hospital, S-214 O1MalmO, Sweden
(Received 17 May, 1977; revised 25 August, 1977)
Abstract. The concentration of medically used radionuclides has been studied in sludge from the sewage
treatment plant serving the borough of Malta0. In this area all nuclear medicine procedures are carried out
in one hospital and almost all patients live in the borough. Therefore, the input of medically used radionuclides into the sewage system can be estimated with good accuracy. Samples of digested sludge have been
tal(en once or twice a week during half a year.
Iodine-131 (physical half life (T) = R.05 d) was detected in all samples. The 13q-activity concentration due
to medical use varied between (0.034-0.01) and (0.124-0.02) nCi kg -1. The ratio between the total output of
13tI via the sludge and an adherent input of the radionuclide into the sewage system was determined to
(2.6 + 0.6) × 10-3, which is equivalent to a ratio of (2 + 1) × 10-2 for stable I.
Occasionally measurable activities of 198Au (T= 2.7 d) and 201T1(T= 3.1 d) have been found.
The radioactivity concentration of medically used radionuclides in the sludge is low and constitutes no
health problems for the persons involved. The sludge however has proved to be a very sensitive and suitable
integrator of radioactive material released from a large urban area.
1. Introduction
The use of radionuclides in medicine leads to the release of radioactivity in the wastes
from different hospital laboratories and from the excreta of patients inside and
outside the hospital.
The need to control excreta from patients who have received therapeutic activities
of radionuclides, e.g., 10 to 100 mCi of 13xI, has long been recognized, but very few
direct measurements in sewage systems have been made (Moss, 1973; Sodd et al.,
1975).
During the last years, there has been a large increase in the number of patients
who have been investigated with the widening range of radiopharmaceuticals clinically
available. The activities used can be of the order of 15 mCi 99Tcm for brain and
sceletal scintigraphy as well as for circulation studies, 2 to 3 mCi 99Tcm for liver and
lung scintigraphy, and 2 mCi 2°1T1for myocard studies, etc.
The concentration of various medically used radionuclides in the sewage system
and in the recipient has hitherto mainly been estimated according to very rough
theoretical models, and there is a growing need to control such estimates by
measurements.
The borough of Malm0 has unique features for such a study. All nuclear medicine
*Present address: Department of Nuclear Physics, University of Lund and Lund Institute of Technology,
S-223 62 Lund, Sweden.
Water, Air, and Soil Pollution 9 (1978) 199-206. All Rights Reserved
Copyright © 1978 by D. ReideI Publishing Company, Dordrecht, Holland
200
B.ERLANDSSONANDS. MATTSSON
procedures are carried out in one hospital and almost all patients live in the borough,
which has one main sewage treatment plant. Thus the excreta from the patients
reach the same treatment plant both when they are in hospital and in their homes.
Thus we have good information about the input of medically used radionuclides in
the Sewage system.
2. Description of the Sewage System
The sewage from about 225 000 of the 250 000 inhabitants of the borough of MalmO
is treated at SjOlunda in the northern part of MalmO at the Sound (Figure 1), and
only 10% of the sewage, from about 25 000 inhabitants, is treated at a smaller plant.
To the SjOlunda plant is also connected the sewage system of the boroughs of
Lomma and BurlOv, and parts of Staffanstorp (totally 25 000 inhabitants).
The total serviced area is 134 km 2, but only parts of this area are overbuilt and
certain parts are serviced by a duplicate system in which rainwater passes directly
into the sea without passing the sewage treatment plant. The overbuilt area from
which the rainwater passes through the SjOlunda plant is therefore only about
28 km 2.
The sewage treatment plant is constructed to give partial biological cleaning. The
Fig. 1.
M a p over M a l m 0 with neighbourhood and the collecting area of Sj01unda sewage treatment plant.
201
M E D I C A L L Y U S E D R A D I O N U C L I D E S IN SEWAGE S L U D G E
SEWAGE WATER
SEDIMENTATION
SLUDGE ~3] SLUDGE
SLUDGE~ SLUDGE
0.5-1 d,m~RESERVOIRS 4-7 °/odin. DIGESTER
~/o
/
/
WATER
/
/
/
BC
O
ILL
IG
AL
EO
AG
NN
IC
WATER
DIGESTEDsLuDGE
l =4%d'm'
//SLUDGE
I
PRESS
DIGESTED
SLUDGE
=z,
°/o
" d.m.1
ESERVOIR
DIGESTED SLUDGE
2 0 - 2 5 % d.m.
d.m. = dry m a t e r i a l
Fig,2. Block diagram of the Sj01undasewagetreatment plant.
main part of the sewage passes through the plant in about 5 h. Figure 2 shows
schematically how the sewage water is processed. During a month about 3.2 × 106 m 3
of the waste water reaches the plant. The sewage sludge is pumped from the sedimentation reservoir (1) to a reservoir (2) where some of the water is separated and then to
the sludge digesters. These consist of two parallel blocks of chambers (3) with a total
volume of 11 300 m 3. When the volume of digested sludge increases, part of it is
pumped to a tank with a volume of 5 000 m 3 (4) and from this tank, part of the
sludge is removed periodically. This sludge is compressed (5) so that the content of
dry substance increases to 20 to 25%, with a mean value of 23%, and is then
transported to different users. A mean volume of 1 320 m 3 of digested sludge leaves
the plant every month.
In this work we have studied the radioactivity concentration in this final f o r m of
digested sludge.
3. Radionuclide Sources
The most important source for radionuclides found in the Sj 01unda sewage treatment
plant is together with artificial- and natural fall-out the effluent from Malm0
General Hospital.
The activity of various radionuclides, which during one year are administered to
patients for radiotherapy and diagnostic procedures, is given in Table I. The radionuclides 90y and 198Au are normally injected into body cavities to produce a local
202
B. ERLANDSSON AND S. MATTSSON
TABLE I
Activity of radionuclides which were administered to patients during one year (1976)
For radiotherapy
Radionuclide
For diagnostics (in vivo procedures)
T
Activity per year
Radionuclide
T
Activity per year
32p
14.3 d
0.04 Ci (1 GBq)
85Sr
65.1 d
0.03 Ci (1 GBq)
9oy
1311
198Au
2.67 d
8.05 d
2.70 d
0.06 Ci (2 GBq)
1.3 Ci (0.05 TBq)
0.3 Ci (0.01 TBq)
99Tcm*
131I
133Xe
6.03 h
8.05 d
5.27 d
32 Ci (1.2 TBq)
0.03 Ci (1 GBq)
3 Ci (0.1 TBq)
2°iT1
73.5 h
0.2 Ci (7 GBq)
*As radionuclide impurities 0.008 Ci 188Re (16.8 h), 0.001 Ci 99Mo (66.7 h), 0.003 Ci 186Re (88.9 h), etc.
For in vitro diagnostics: 0.4 Ci 125I (60.2 d).
irradiation and the leakage from these cavities is normally very small. Therefore
these radionuclides are expected to be found in measurable amounts in the sewage
water only in connection with accidents and post-mortem examinations of treated
patients.
The medically used radionuclide which is supposedto be easily detected in the
present study is 1311.Normally about two thirds of the orally given activity is excreted
via the urine of the treated patients in the first 24 h and smaller fractions subsequently.
Concerning the radionuclides used for nuclear diagnostics 99Tcm and 2°1T1 might
be found in various parts of the sewage system. Because of the short physical halflife of 99TCm, this radionuclide is not supposed to appear in detectable concentrations
in the digested sewage sludge.
4. Sample Collection and Measurement
Samples of digested sludge were collected once a week from the beginning of July
1976 to the end of January 1977. From mid-October to mid-December the sludge
was collected twice a week.
The sludge was packed in a 2 000 ml cylindrical plastic can (~b= 116 mm, height
= 190 mm), which was filled up. The can was placed with its central axis perpendicular to the central axis of a 46 cm 3 Ge(Li)-detector. The distance between the
center of the sample and the detector end window was 9 cm. The Ge(Li)-spectrometer
provided a good energy resolution (FWHM = 1.99 keV at 1.33 MeV). The detector
and the sample were housed in a lead cave with 8 cm thick walls.
The detection efficiency for photons of various energies was determined from
measurements on water solutions containing known activities of 22Na, 57Co (The
Radiochemical Centre, Amersham, England), and 152Eu (Laboratoire de Metrologie
des Rayonnements Ionisants, Gif-sur-Yvette, France) in the same plastic cans as the
sludge samples. The counting time was normally 54 000 s (15 h) or 80 000 s ( ~, 22.2 h)
MEDICALLYUSEDRADIONUCLIDESIN SEWAGESLUDGE
203
per sample. The minimum detectable activity (ICRU, 1972) of 131Iin a typical sewage
sludge sample was with these counting times about 28 pCi and 23 pCi respectively.
5. Result and Discussion
5.1. IODINE-131
Figure 3 shows the 1311activity concentration (nCi kg "1) in the digested sludge ("~ 23 %
dry substance) together with the estimated release of 131I (mCi) into the sewage
system. For each treated patient the release has been calculated from information on
activity administered and from earlier determined individual values on the urinary
excretion of 131I.
At the beginning of December 1976 minor activities from patients treated at the
university hospital in Lund were released into the system. This has been accounted
for. The curve shows two peaks which can be connected with known release of 131I
into the system. The first one occurs around 20 August arising from a single input of
about 36 mCi during the first days of the month. The maximum activity of 131I is
measured about 2 to 3 weeks after the release. The other peak around 26 October
[(0.42+0.01)nCi kg -1] is probably due to a nuclear weapon test in the atmosphere
131I-coneentration in digested sludge
nCi.kg -1
0.5
I
r
I
I
"China 09 26 .
I
.
.
.
TT
I
I
China 11 17"
I
0.4
0.3
131I-activity
in e x c r e t a from
rnCi
(staptes)
patients
~ 30
0.2
20
0.1
10
0
August
1976
Fig. 3.
September October
November December
January
1977
131I-activity concentration in digested sludge (left ordinate) and estimated daily release of 131I
from patients (right ordinate) during the period August 1976 to January 1977.
B. ERLANDSSONAND S. MATTSSON
204
over the western parts of the People's Republic of China on 26 September, 1976.
Short lived fission products were first registered in surface air over Sweden during
the week 4-11 October (DeGeer, 1976). A third bump occured in December. Because
no increase of other fission products than 131I is seen from another Chinese nuclear
weapon test on 17 November, (Erlandsson and Mattsson, 1978), the 131I-peak in
December might be explained by the release of 1311caused by medical use. For 131Iit
is now possible to calculate the ratio between the output to the digested sludge and
the input to the sewage system. The output data have been compared with input data
18 d earlier to account for the dwelling time in the sludge digester. The results are
presented in Table II.
The second part of this table refers to the bomb fall-out. In situ deposition
measurements at Barseb~ick 20 km north of Malmt~ on 1976-10-29 gave the value
(1.1+0.2)nCi m -2, (Finck et al., 1977). With a collecting area of 28 km 2 and
complete run off this gives an input of 208 mCi (this activity value refer to 1976-10-07).
When the output was calculated, a constant 'hospital' background of 0.06 kg -1 was
subtracted. The background was estimated using the input from the General
Hospital during the period 1976-09-20 to 1976-10-20, the output of digested sludge
and an output-input factor of 2.6 × 10-3. This gives a factor of 1.6 × 10 -3 between
output and input of 1311from the China bomb which is in good agreement with the
mean value for the release from the hospital, which is (2.6+0.6) × 10 -3. The mass
ratio of outcoming sludge and the incoming sewage water to Sj01unda is 4.1 × 10-4.
Thus the ratio between 131I concentration in the digested sludge and in the input
sewage water is about 6.
If we assume that all I behaves in the same way, the data for 1311can be used to
estimate the situation for other I isotopes by correcting for the different physical
half-life and thus the different residence time in the sludge digesters. If 1251is released
into the sewage system 1.5 % of the activity will leave the sewage plant via the sludge,
TABLE I1
The ratio between output and inl~ut (18 days earlier) of 1311 in the Sj01unda sewage
treatment plant
Time period
Input (mCi)
Release from the hospital:
760715-760915
760802-761003
87.0
761112~770101
761130-770118
O u t p u t (mCi)
Output/input
2.0 x 10 -3
0.174
47.1
3.1 × 10 -3
0.145
M e a n value 2.6 x 10 -3
B o m b fallout
761005-761029
761017-761119
224
0.365
1.6)< 10 -3
MEDICALLY USED RADIONUCLIDES IN SEWAGE SLUDGE
205
which has its maximum 125I concentration after around 20 days. If 129I or stable I is
released, about 2°7o will be found in the sludge with maximum concentration after
around 28 days. The ratio between 1291 or stable I concentration in the digested
sludge and in the input sewage water is about 50.
In the work by Sodd et al. (1975) there was a remarkable discrepancy between the
activity administered to the patients (10 to 20 mCi per week) and the activity found
in the effluent from the sewage treatment plant (93 mCi per week). This discrepancy
was explained as arising from outlets from other radionuclide users in the area. This
explanation would call for a release 4 to 5 times greater than the medical. In view of
our results a possible explanation could be that this increased activity is partly due to
fall-out from a nuclear weapon test over the People's Republic of China on 27 June,
1973.
5.2. OTHER RADIONUCLIDES
The presence of 6ther medically used radionuclides than 131I has been detected only
occasionally.
Normally, the activity concentration of 19SAuwas below the minimum detectable,
which is 0.014 nCi kg"x(counting time: 8 × 104 s). On 27 August the sludge contained
(0.069 + 0.005) nCi kg-1, and one week later on 3 September (0.030 + 0.004) nCi kg-1.
After one more week 198Auwas not detectable.
Our result indicates that there was a release of 198Au around 15 August. The most
likely explanation to this release is a post-mortem examination on 18 August of a
patient, who 7 days earlier had received 125 mCi of 198Au-colloid for therapy. Upon
examination the body contained about 20 mCi of 19SAu. As the leakage of the radionuclide at the autopsy is very difficult to estimate, no sure output/input ratio can be
given in this case.
In one sample of digested sludge, from 17 January, 1977, measurable content of
2°lT1 (0.19 nCi kg-1) was registered. Both the 165 keV, and the Hg X-ray peaks
appeared in the gamma-spectrum. Normally the 2°iT1 activity concentration was
below the minimum detectable which is 0.15 nCi kg-1.
As a mean around 0.7 mCi of SSSr is administered to patients every week. This
gives an estimated weekly release of about 0.2 mCi to the sewage system. During
certain periods much ihigher counting rates in the 514 keV peak than normally found
have been recorded. This is difficult to explain because the input activity of 85Sr per
week is almost constant which should cause a constant activity concentration in the
sludge. An explanation is that other radionuclides also emitting 514 keV gamma-rays
tempo r a r ily disturb the 85Sr measurements. Further investigations along this line are
in progress (Erlandsson and Mattsson, 1978).
6. S u m m a r y and C o n c l u s i o n
Iodine-131 was detected in all samples of digested sludge collected in Malm0 sewage
treatment plant. Due to the medical use of the radionuclide the activity concentration
206
B. ERLANDSSON AND S. MATTSSON
in the sludge varied between 0.03 and 0.12 nCi kg-1. A single 200 kton nuclear
weapon test in China gave a maximum value of 0.42 nCi kg "1 about 40 days after the
test.
The ratio between the total output of 131I via the sludge and the input of 131I into
the sewage system is (2.6+0.6) × 10-3, which is equivalent to a ratio of ( 2 + 1) × 10-2
for stable I.
The 131I-content in the sludge due to medical use of the radionuclide is low and does
not introduce any radiation protection problems for the persons working at the sewage
treatment plant.
At present the urine from patients receiving 100 mCi or more of 131I is stored for
around 25 days in the hospital. Because the hospital staff has to handle the urine bags
this storage will probably give a higher collective dose than if it was flushed into the
sewage immediately. Therefore it is proposed that all urine from therapy patients is
flushed into the sewage system without any storage.
The small amount of other medically-used radionuclides found in the sludge is
insignificant. This work however shows that digested sludge from a sewage treatment
plant is a very sensitive and suitable integrator of reasonably longlived radioactivity
released from a large urban area.
Acknowledgments
The authors acknowledge T. Bramst~ng, C. Pettersson, and B. Nosslin, MalmO
General Hospital, for valuable information, fruitful discussions and support. They
also appreciate the valuable information about the sewage system and the SjOlunda
sewage treatment plant given by P. Ansner, L. Linde and A. Zimmergren.
References
DeGeer, L-E.: 1976, Private communication. The Research Institute of National Defence, Sundyberg,
Sweden.
Erlandsson, B. and Mattsson, S.: 1978, to be published.
Finck, R., Lid6n, K., and Persson, R. B. R.: 1977, Paper presented at the IRPA IV Int. Congress, Paris,
24-30 April, 1977.
International Commission on Radiation Units and Measurements: 1972, Measurement of Low-Level
Radioactivity, Report 22, ICRU, Washington, D.C., U.S.A.
Moss, C. E.: 1973, Health Phys. 25, 197.
Sodd, V. J., Velten, R. J., and Saenger, E. L.: 1975, Health Phys. 28, 355.