Radionuclides removal

Membranes and Membrane Processes
in Environmental Protection
Monographs of the Environmental Engineering Committee
Polish Academy of Sciences
2014, vol. 119, 289-297
ISBN 978-83-63714-18-5
THE EFFECTIVENESS OF RADIONUCLIDES REMOVAL FROM
GEOTHERMAL WATERS USING DOUBLE BWRO SYSTEM
Barbara TOMASZEWSKA1*, Michał BODZEK2,3
Abstract: The paper presents the result of investigating in the field of radionuclides removal
from geothermal waters using double brackish water reverse osmosis system connected in
series. The determination of the total  and  activity levels, concentrations of tritium,
radon (222Rn), uranium (238U, 234U) and radium (226Ra,224Ra) isotopes and annual
radiological doses were calculated for all the geothermal and treated waters investigated
and for different human age groups. The rejection rate of the isotopes tested ranged from
70.7% to 99.6%. The highest rejection rate, between 96.5% and 99.6% (with an average of
98.5%), was recorded for 226Ra. No relationship was identified between the rejection rate
and the water mineralisation level. In terms of gross and  activity, the average rejection
rates equalled 89.9% and 85.4%, respectively.
Keywords: radionuclides, gross , gross effective dose equivalent, geothermal water.
INTRODUCTION
Desalination of geothermal waters is being considered in many parts of the
world, in particular in arid areas, mainly for irrigation purposes, in order to reduce
the negative impacts of saline geothermal waters discharged to water bodies and
surrounding agricultural areas and also as a possible solution leading to the
decentralisation of drinking water supply [1-6]. In Poland, waters from three
different geothermal areas were tested, i.e. the Podhale basin (GT-1), Polish
Lowlands (GT-2) and Western Carpathian Mountains (GT-3) using a integrated
process combining ultrafiltration and two independent stages of reverse osmosis
with low-pressure BWRO membranes (RO-1 and RO-2) connected in series.
1
Mineral and Energy Economy Research Institute of the Polish Academy of Sciences, Wybickiego 7,
31-261 Kraków, Poland
2
Silesian University of Technology, Institute of Water and Wastewater Engineering, Konarskiego 18,
44-100 Gliwice, Poland
3
Institute of Environmental Engineering of the Polish Academy of Sciences, M. Curie-Skłodowskiej 34,
41-819 Zabrze, Poland
* corresponding author: [email protected]
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Tomaszewska B., Bodzek M.
Radioactive substances in geothermal water are principally of natural origin,
from the dissolution of gases and rock minerals and as a result of recoil nuclides.
Naturally occurring radionuclides are isotopes of uranium and thorium as well as
their daughter products and some of them are  emitters, thus they are dangerous
when ingested [7-11].
The United Nations Scientific Committee on the Effects of Atomic Radiation
[12] has estimated that the global average annual dose per person from all sources
of radiation in the environment is approximately 3.0 mSv/year. The dose of 1Sv is
equivalent to absorbing 1 Joule (1 J) of energy per 1 kg body mass, adjusted for the
type of radiation and taking into account the fact that different isotopes have
different persistence times in the human body and therefore cause different
biological effects [13]. To deal with the rather large unit of Sv, a thousandth of an
Sv, or mSv, has been used in practice. Of this, 80% (2.4 mSv) is due to naturally
occurring sources of radiation, 19.6% (almost 0.6 mSv) is due to the use of
radiation for medical diagnosis and the remaining 0.4% (around 0.01 mSv) is due to
other sources of human-made radiation [12].
Membrane technologies, including especially reverse osmosis, are deemed
efficient in eliminating dissolved radionuclides from water. A comprehensive
review of the removal of dissolved radionuclides by water treatment processes is
presented in Table 1 [12]. As shown in Table 1 reverse osmosis has more than 70%
effectiveness in removing natural radioactive elements from water.
Table 1. Treatment performance for some common radionuclides (after [12]).
Element
Strontium
Iodine
Caesium
Radium
Uranium
Plutonium
Americum
Tritium
Sand
ActivaPrecipitaIon
Reverse
filtrated
tion
exchaosmosis
tion
carbon
softening
nge
10-40%
10-40%
<10%
>70%
40-70%
>70%
10-40%
10-40% 40-70%
<10%
40-70%
>70%
10-40%
10-40%
<10%
10-40%
40-70%
>70%
10-40%
40-70% 10-40%
>70%
>70%
>70%
>70%
<10%
10-40%
>70%
>70%
>70%
>70%
10-40% 40-70%
<10%
>70%
>70%
>70%
10-40% 40-70%
<10%
>70%
>70%
Not possible to remove (some removal by aeration of water, not
quantified)
Coagulation
The work presented was designed to perform a comprehensive study of natural
radioactivity in the geothermal waters tested (representing different types of waters
and different degrees of mineralization) and in product waters. The study comprised
in particular the determination of the total  activity and total  activity,
concentrations of tritium, radon (222Rn), uranium (238U, 234U) and radium
(226Ra, 224Ra) isotopes and calculating the radiological dose rates resulting from
consumption of the waters investigated. The results have been used to assess the
The effectiveness of radionuclides removal from geothermal waters...
291
efficacy of a desalination system based on low-pressure BWRO modules in the
removal of isotopes determining global /radiation.
MATERIALS AND METHODS
The chemical composition of the waters tested are shown in Table 2.
Geothermal water cooled down in heat exchangers was fed into a desalination plant
in a closed circuit [14].
Table 2. The chemical composition of the geothermal water tested.
Parameters
TDS, mg/L
Temperature, °C
Conductivity, mS/cm
Na, mg/L
K, mg/L
Ca, mg/L
Mg, mg/L
Cl, mg/L
SO4, mg/L
B, mg/L
SiO2, mg/L
Tritium, TU/L*
Gross , mBq/L
Gross , mBq/L
Uranium 238U, mBq/L
Uranium 234U, mBq/L
Radium 226Ra, mBq/L
Radium 228Ra, mBq/L
Radon 222Rn, Bq/L
GT-1
2561.8
30.0
3.550
466.8
45.2
196
42.7
536.0
938.2
8.98
42.73
0.00.4
620150
1,150340
3.70,3
5.90,3
51460
15755
5.40.4
GT-2
6556.0
30.0
10.960
2297
27.2
146.8
26.2
3574
193.7
2.53
35.78
0.70.4
30060
370110
2.50.3
2.20.3
27210
24238
5.00.6
GT-3
24447.0
22.0
35.500
9492
83.1
71.24
36.56
12815
<3
96.73
11.63
0.10.4
910180
1,250380
10.61.0
9.40.9
61514
60745
0.70.3
*
TU - Tritium Units where 1 TU is defined as the ratio of 1 tritium atom in
1018 hydrogen atoms.
The geothermal water desalination procedure was developed using the results of
boron retention level testing as a function of pH and as a function of the water
recovery level, as well as using membrane-scaling projections [14-16]. The water
pre-treatment facility includes a mechanical filter, an iron removal stage and an
ultrafiltration module (UFC M5, X-Flow). The UF system was used to remove
microsuspensions, colloids and bacteria. After pre-treatment the water was fed to
the first reverse osmosis stage (RO-1), which was equipped with spiral wound
292
Tomaszewska B., Bodzek M.
DOW FILMTEC BW30HR–440i polyamide thin-film composite membranes. To
control the high level of feed water hardness its pH was brought down to 5. To
bring the boron concentration below its maximum level for drinking water (1 mg/L)
the pH of permeate was corrected to 10 after RO-1 and put to further filtration at the
second reverse osmosis stage (RO-2) [14]. Both reverse osmosis stages worked
independently and were connected in series. Desalination tests were performed at
a transmembrane pressure of 1.1 MPa at both RO-1 and RO-2 membrane
configurations. The pilot desalination tests were performed at a semi-production
scale (ca. 0.5-1.0 m3/h of desalinated water production) during an eleven hour
period.
Measurements of total activity of the radioactive  and  nuclides and the
concentration of radon (222Rn), uranium (238U, 234U) and radium (226Ra, 228Ra)
isotopes were performed at the Environmental Physics Group of the Faculty of
Physics and Applied Computer Science, University of Science and Technology in
Krakow. An Alpha Spectrometer Model 740l with a semi-conductor detector was
used as well as an alpha/beta detector with a Wallac Guardian 1414 liquid
scintillation counter. The same laboratory performed tests for tritium on an
electrolyte-enriched sample using the liquid scintillation counting method and
a Packard Model 2500 P TR/AB device.
The annual effective dose equivalents were assessed using IAEA standards
[17], for adults, children and infants of lactation age that drink 730, 350 and 250 L
of water per year, respectively. It was assumed that radionuclide values below the
lower limit of detection are equal to that limit (<DL = DL).
RESULTS AND DISCUSSION
The gross  activity for the waters analysed have an average of 403 mBq/L and
ranges from 30060 – 910180 mBq/L (Table 2). The highest  activity was found
in the geothermal waters from well GT-3 with a mineral content of 24.4 g/L. The
water is linked to fleisch formations of the Western Carpathian Mountains
(siltstones, mudstones and sandstones) and is extracted from a depth of 1200 m. The
same well also yielded the highest concentration of isotopes of uranium (238U and
234
U at 10.61.0 mBq/L and 9.40.9 mBq/L) and radium (226Ra and 228Ra at
61514 mqB/L and 60745 mqB/L). The radium isotope analysis from well GT-1
produced β-active 228Ra (of the series 232Th) at 157±55 mBq/L and α-active 226Ra
(of the series 238U) at 514 ± 60 mBq/L. The total β activity in the water from well
GT-1 is similar to that from GT-3.
The gross β activity for the waters analysed has an average value of 923 mBq/L
and ranges between 370±110 mBq/L (in well GT-2) and 1250 mBq/L (in well
GT-3). As shown in Table 2, the lowest gross  and  activities were also
determined in the water taken from well GT-2 extracted from interspersed sandy,
sandy-marly and sandy-mudstone layers of a Lower Cretaceous reservoir. The
The effectiveness of radionuclides removal from geothermal waters...
293
Cretaceous formations of the Polish Lowlands exhibit high hydraulic permeability
and porosity of the reservoir, which means that the capacity of hydrogeothermal
wells discharges tends to be high (from 25 to more than 200 m3/h). Samples of
geothermal waters had near-zero tritium levels, within the error margin, which
means that these waters came from infiltration which occurred before 1952 when
atmospheric tests of thermonuclear weapons began.
In accordance with the guidelines of the EU Directive [18] and with national
standards [19] the permissible radionuclide dose in drinking water equals 0.1 mSv.
This estimate explicitly excludes tritium, potassium 40K, radon and its decay
products.
Committed unit doses account for the varied effect of ionising radiation on the
human body depending on the isotope and the consumer’s age. Radium isotopes
have higher effects on age groups of <1 year and 12-17 years than on other groups.
This is how potential consumers are protected at a time of intensive bone tissue
growth. Annual effective dose equivalents shown in Table 3 were calculated for
adults (>17 ya) assuming a daily water intake of two litres. This means that the
annual effective dose equivalents from the absorption of natural isotopes in the
geothermal waters tested are greater than the permissible level (0.1 mSv) [8].
The efficiency of the water treatment system on natural radionuclides was
assessed by testing the water from the second RO stage. Table 3 contains results of
radiological testing of treated geothermal waters. In all the samples tested, tritium
concentration was zero within the accuracy of measurement. The permissible
tritium level is 100 Bq/L, which corresponds to 850 TU/L. Therefore these waters
constitute no tritium-related radiological hazard to potential consumers [8].
The radioactive content in the product water of the RO in question is
considerably lower than in the feed water. As shown in Table 3 the nuclide
concentration did not affect the drinking water standards and the rejection rate
ranged from 70.7 to 99.6%. The results were compatible with the earlier WHO
reports [13], as illustrated in Table 1.
The highest rejection rates, ranging from 96.5% to 99.6% (98.5% on average)
were measured for the 226Ra isotope. The other radium isotope, 228 Ra, ranged from
70.7% rejection rate in water from well GT-3 to 98.3% in water from well GT-1.
No dependency was identified between the rejection rates and the mineralisation
levels. Despite the fact that desalination of water from well GT-3 was not
successful, as the installation capacity dropped from 5·106 m3/m2s to 0.35·106
m3/m2s within one hour, the rejection rates on isotopes 238U, 234U, 226Ra and 228Ra
were high at 95.2%, 94.6 %, 99.6% and 98.3%. The rejection rate of the gross
and  activity was 89.9% and 85.4%.
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Tomaszewska B., Bodzek M.
Table 3. Gross activity, gross  activity, annual effective dose equivalent and contents of
tritium, uranium and radium isotopes in product water.
Element
Tritium , TU/L
GT-1
GT-2
GT-3
0.0 ± 0.3
0.0 ± 0.4
0.3 ± 0.4
≤50
≤100
≤0.50
≤50
≤100
≤0.50
Standard
100 Bq/L
(850 TU/L)
5003)
1,0003)
10,0003)
≤0.50
≤0.50
1,0003)
≤2.0
≤2.0
1,0003)
≤10.0
≤10.0
1003)
0.005
0.005
0.12)
≤50
Gross , mBq/L
≤100
Gross , mBq/L
238
Uranium
U
≤0.50
concentrations,
234
U
≤0.50
mBq/L
226
Radium
Ra
18.0 ± 2
concentrations,
228
Ra ≤47.0 ± 5
mBq/L
Annual
effective
dose
0.028
equivalent, mSv/year 1)
1)
Annual effective dose equivalent for adults who daily are drinking water in average about
2 liters; 2) concentration adopted from [19], 3) concentration adopted from [13].
The annual effective dose equivalents of natural radioactivity absorbed by
adults (>17 ya) by consuming the waters tested, assuming the daily water intake of
two litres per person, are considerably lower than the permissible level (0.1 mSv).
Based on the activity of isotopes 238U, 234U, 226Ra and 228Ra, measured in
desalinated water, their annual dose in people above 17 y.a. would equal:
0.028 mSv in GT-1, or 28% of the permissible level, and 0.005 mSv in GT-2 and
GT-3, or 5% of the permissible level. Assuming an annual infant (<1 ya) water
intake of 250 litres the annual effective dose equivalent absorbed from uranium and
radium in desalinated water from GT-1 would equal 0.37 mSv, or 3-4 times more
than the acceptable level for drinking waters (Table 3). As mentioned above, the
effective unit equivalent doses take into account the varied effect of the ionising
radiation on the human body depending on the isotope and the consumer’s age. This
is one of the reasons why the committed unit dose has been found to be exceeded in
this age group [8]. The other reason is the water consumption, which is markedly
lower in infants when compared to other age groups.
CONCLUSIONS
Membrane techniques in general, and reverse osmosis in particular, are judged
effective in removing dissolved radionuclides from water.
In the RO system tested the concentration of radioactive substances in product
water was much lower than in the feed water. The rejection rate ranged from 70.7%
to 99.6%. The highest rejection rates of 96.5%-99.6% were achieved for the radium
The effectiveness of radionuclides removal from geothermal waters...
295
226
Ra isotope while in 228 Ra it ranged from 70.7% to 98.3%. The research identified
no dependency between radionuclide rejection and mineral content. In terms of the
gross and  activity, the rejection rates were 89.9% and 85.4%.
The committed unit doses take into account the varied effect of the ionizing
radiation on the human body depending on the isotope and the consumer’s age.
Radium isotopes have higher effects for age groups of <1 year and 12-17 years than
in other groups. This is how potential consumers are protected at a time of intensive
bone tissue growth. The calculated annual effective dose equivalents from the
absorption of natural isotopes in adults and children were much below the
permissible level, i.e. 0.1 mSv.
Assuming, however, an annual infant (<1 ya) water intake of 250 litres and the
higher effective dose equivalents typical for this age group, the annual effective
dose equivalent from radionuclides was 3-4 times more than the acceptable level.
The study has, therefore, demonstrated that from the radiological standpoint
desalinated geothermal waters may be used as drinking water for children and
adults, but not for infants.
ACKNOWLEDGEMENTS
This work was financed by the Polish Minister of Science and Higher
Education, grant No N R09 0003 04, during 2008-2012.
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EFEKTYWNOŚĆ USUWANIA READIONUKLIDÓW Z WÓD
TERMALNYCH PRZY WYKORZYSTANIU PODWÓJNEGO
SYSTEMU BWRO
Barbara TOMASZEWSKA, Michał BODZEK
Streszczenie: W pracy przedstawiono wyniki badań na polu usuwania radionuklidów z wód
termalnych przy wykorzystaniu dwóch stopni odwróconej osmozy dla wód słonawych,
połączonych szeregowo. Ustalono poziom całkowitej aktywności  i , stężenia izotopów
trytu, radonu (222Rn), uranu (238U, 234U) i radu (226Ra, 224Ra). Obliczono roczną dawkę
radionuklidów dla badanych wód termalnych i wód uzdatnionych, przy uwzględnieniu
różnych grup wiekowych. Stopień odrzucenia badanych wskaźników wyniósł od 70,7 do
99,6%. Najwyższy stopień odrzucenia od 96,5 do 99,6% (średnio 98,5%) uzyskano dla
izotopu 226Ra. Nie stwierdzono zależności stopnia odrzucenia od mineralizacji wody. Biorąc
pod uwagę całkowitą aktywność i  stopień odrzucenia wyniósł średnio 89,9% i 85,4%.
Słowa kluczowe: radionuklidy, całkowita aktywność  i całkowita dawka skuteczna,
woda termalna.