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Treatment of the wastewater containing low-level 241Am using flocculation-microfiltration process
Separation and Purification Technology 40 (2004) 183–189
Treatment of the wastewater containing low-level 241Am using flocculation-microfiltration process Gao Yong a , Zhao Jun b , Zhang Guanghui a , Zhang Dong b , Cheng Weiwen a , Yuan Guoqi b , Liu Xuejun b , Ma Bangzhong b , Zeng Junhui b , Gu Ping a,∗ a
b
School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China Institute of Nuclear Physics and Chemistry, Chinese Academy of Engineering Physics, Mianyang 621900, China Received in revised form 15 February 2004; accepted 28 February 2004
Abstract This paper studies the treatment of low-level radioactive wastewater containing 241 Am by a combined flocculation-microfiltration (FMF) process. The average concentration ratio of the treated wastewater to the discharged sludge was 189.8 in the cold test. After operating for 4 months, the fouled membrane was cleaned and cleaning effects of physical and chemical methods were compared. In the hot test, the pH was adjusted with nitric acid and 20 mg/l potassium permanganate was added to remove surfactants from the feed during pretreatment. The pretreated wastewater was pumped to the membrane reactor, where the pH was adjusted to above 8 and the dosage of Fe3+ was 30 mg/l. The mixed liquor was then filtered through the submerged microfiltration (MF) membrane. When the radioactivity of 241 Am in the feed was 809.2 Bq/l, the effluent was below 1.0 Bq/l. The removal of 241 Am was higher than 99.9%. The decontamination factor (DF) is more than 1651.4. The results showed that the FMF process has significant potential in treating low-level radioactive wastewater containing 241 Am. © 2004 Elsevier B.V. All rights reserved. Keywords: Radioactive wastewater; 241 Am; Flocculation-microfiltration (FMF); Membrane fouling; Decontamination factor (DF)
1. Introduction 241 Am
is a virulently radioactive nuclide in the periodic table of elements whose half-life is 432.9 years. It is primarily used to produce density detectors, as an excitation source of fluorescence analysis and neutron sources. 241 Am can emit ␣ and  rays during decay. The ␣ rays can do a great harm to the environment and the health of human being [1]. The highest allowable level of the total ␣ radioactivity in wastewater before discharge compelled by Integrated Wastewater Discharge Standard (GB8978-1996) is 1.0 Bq/l [2]. Radioactive wastes may bring potential impact to the environment through different transport ways [3–5]. They cannot be decomposed or biodegraded by physical, chemical or biological methods. The radioactivity of the wastes gradually weakens or dies out only by natural decay. High-level radioactive waste is loaded in sealed containers that are ∗ Corresponding author. Tel.: +86-22-27405059; fax: +86-22-27405059. E-mail address: [email protected] (G. Ping).
1383-5866/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2004.02.009
buried deeply under ground. Medium- or low-level radioactive waste is frequently concentrated to a small volume and then is solidified. However, a large quantity of wastes whose radioactivity meets the discharge standard can be discharged directly to the environment. Traditional treatment methods include flocculation-precipitation, ion exchange, distillation [6], and other methods [7–9]. The effluent quality was improved greatly when membrane separation was used in the treatment of radioactive wastewater [10]. Methods of membrane separation used to treat low-level radioactive wastewater, include liquid membrane (LM) [11,12], membrane distillation (MD) [13–15], electrochemical ion exchange (EIX) [16], polymer-ultrafiltration (PUF) [17–19], microfiltration– ultrafiltration (MF–UF), ultrafiltration–nanofiltration (UF– NF), ultrafiltration–reverse osmosis (UF–RO), etc. The processes mentioned above have high efficiency in treating lowlevel radioactive wastewater; however they have some shortcomings also, such as high operating pressure or energy consumption, frequent maintenance, etc. Compared with the above processes of membrane separation, the flocculationmicrofiltration (FMF) process developed in this paper has
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the advantage of being a simple process, automatic and safe operation, low operating pressure and energy consumption. In this study, the FMF process was used to treat low-level radioactive wastewater containing 241 Am and detergents (surfactants) from the laundry and washing floor of retired nuclear facilities. The radioactivity of 241 Am in the wastewater was about 103 –104 Bq/l, and must be decontaminated before being discharged. According to a report [20], when the concentration of 241 Am in the wastewater is 308.4 mg/l, the radioactivity is 1 Ci/l (3.7 × 1010 Bq/l), i.e., 1 mg/l 241 Am wastewater has radioactivity of 1.2×108 Bq/l. Therefore, the concentration of 241 Am in the wastewater was about 10−4 mg/l. The primary removal mechanism of 241 Am is that the solubility of americium hydroxide is low and americium hydroxide will precipitate out of the solution when alkali is added. The principle is as follows [21] Am3+ + 3OH− → Am(OH)3 ↓ Ksp = [Am3+ ][OH− ]2.4 = (3.4 ± 0.3) × 10−18 However, the solubility product constant (Ksp ) of americium hydroxide is not low enough to precipitate 241 Am to meet the discharge standard. It is known that flocs formed by ferric hydroxide are of strong adsorption capacity, and may be used as adsorbents to adsorb residual Am3+ [22]. Ferric chloride and sodium hydroxide were dosed simutlaneously into the feed to form a large amount of flocs in the reactor, prior to the FMF, to improve the removal of 241 Am.
2. Experimental A schematic of the FMF process is shown in Fig. 1. In the tests, the membrane reactor was aerated continuously to induce mixing. Effluent was removed intermittently, controlled by a programmable logic controller (PLC). The feed was pumped into the membrane reactor from the wastewater tank. Ferric chloride and sodium hydroxide were simultaneously added by dosing pumps. The mixing, precipitation, and microfiltration (MF) were completed in the membrane reactor. Liquid level probes (including a high level detector and a low one) were placed in the reactor. Pumps and automated valves were interconnected via the PLC with the liquid level probes. At low liquid level, the pump was started and the discharge valve was shut off, this was reversed at high liquid level. The total volume of the reactor is 11.8 l and the hydraulic residence time (HRT) was 1.49 h. The volume ratio of supplied air for aeration to treated wastewater was 13:1. The reactor was operated automatically. The valves for sludge discharge and the air inlet were set at the bottom of the reactor. The characteristics of the MF membrane module are shown in Table 1. Transmembrane pressure was provided by the water level difference between the level of the membrane reactor and that of the effluent tank. Considering safety, reliability, and convenience, the test was divided into two steps: the cold test and the hot test. In the cold test, the radioactive wastewater was replaced by tap water. It is very low in concentration of 241 Am, and does not
Fig. 1. Schematic of the FMF process. The part in the dash dot line is for pretreatment in the hot test, with no pretreatment in the cold test. Pump A: dosing KMnO4 solution; pump B: dosing HNO3 ; pump C: dosing NaOH solution; pump D: dosing FeCl3 solution. Table 1 The MF membrane module Type Material Manufacture
Hollow fiber Polyvinylidene fluoride (PVDF) Motian membrane engineering and technology Co., China
Nominal pore size (m)
0.22
a
Surface area (m2 ) Fiber length (m) Outer diameter of fibers (mm) Inner diameter of fibers (mm) Maximum specific fluxa (l/m2 ·h·m)
The maximum specific flux of the membrane refers to the maximum flux of clean water under transmembrane pressure of 1.0 m water head.
0.5 1.0 0.8 0.5 37.5
G. Yong et al. / Separation and Purification Technology 40 (2004) 183–189
cause significant membrane fouling, so tap water was used to observe the fouling characteristics of the membrane with ferric chloride as flocculants. The cold test was operated in 4 runs, and the membrane was cleaned by physical cleaning (discharging sludge, and aerating for 20 h with no feed and no effluent) after each run. The hot test used the same process as the cold test, but was operated with the site radioactive wastewater to test the actual removal of 241 Am. The radioactive wastewater was pretreated to decompose surfactants by oxidation using potassium permanganate at a pH of 2–4. Nitric acid was used to adjust the pH and the dosage of permanganate was 20 mg/l (dash dot line showed in Fig. 1). Turbidity measurements of the effluent were made using a photoelectric turbidimeter (Model GDS-3B). Effluent flows were measured by a liquid flowmeter (Model LZB). The pH measurements were made on a precise pH-meter (Model PHS-3C). Total ferric content was measured by the ferroin-spectrophotometric method with an ultraviolet/visible spectrophotometer (Model TU-1800). Measurements of calcium and magnesium ions were made by the titration method of ethylene-diamine tetraacetic acid disodium salt (EDTA). The suspended solids (SS) in the membrane reactor were measured by weight methods with an electronic balance (Model AB104-S). The ␣ radioactivity of 241 Am was measured by a high purity germanium -spectrometer (Model GEM35190). All the chemical agents used in the test were analytical reagents.
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3. Results and discussion 3.1. Cold test In the cold test, ferric chloride was used as the flocculant and adsorbent after reaction with alkali. Sodium hydroxide was used to adjust the pH value of the feed. Ferric hydroxide particles are the sole solid phase in the mixed liquor. The purpose of this part of the test was to observe fine particles separation by the membrane and membrane fouling caused by the flocs. Effluent turbidity and the maximum flow rate (MFR) at a constant transmembrane pressure were measured. Zero or low turbidity means good removal of fine particles in the reactor. In practice, the abandoned membrane must be treated as radioactive waste, and the disposal cost is dependent on the membrane performance. The measurement of flow rate and the effects of membrane cleaning were used to determine the feasibility of this process for use in radioactive applications. The concentration of SS of the mixed liquor in the reactor increased with the operation time. When the concentration of SS increased to a critical value, the flow decreased rapidly, as shown in Fig. 3. When the flow rate decreased below the design value (12 l/h), the sludge in the reactor was discharged and the test was stopped. The cold test was operated in 4 runs with an operation time of 1025.3 h (96 days, 7–9 h per day). The volume of the treated wastewater was 5177.6 l and
Table 2 Cold test data Operating parameters
Run number
Operation time (d) Design flow rate (l/h) Dosage of Fe3+ (mg/l) pH of mixed liquor Volume of the treated wastewater (l) SS in the reactor at the end of each run (g/l) Concentration ratioa
2
3
4
35 12 27–65 8–11 1820.8 26.9 267
14 12 65 8–11 841.6 20.3 123
19 12 65 8–11 867.2 19.9 127
28 12 30 8–11 1648 22.1 242
The concentration ratio is the ratio of the total volume of treated wastewater to that of discharged sludge.
Run 1
Turbidity(NTU)
a
1
Run 2
Run 4
Run 3
4 3. 5 3 2. 5 2 1. 5 1 0. 5 0 0
20
40
60 Run Time(d)
Fig. 2. Turbidity of effluent.
80
100
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the volume of discharged sludge was 27.28 l, so the average concentration ratio of water to sludge was 189.8. The results of the 4 runs in the cold test are shown in Table 2. 3.1.1. Effluent turbidity The effluent was colorless and transparent. Most of effluent turbidity varied between 0 and 1 NTU (shown in Fig. 2). The average value was 0.3 NTU. Fig. 2 showed that effluent turbidity was slightly higher at the beginning of each run. The reason is that the removal of solids relies on the physical separation of membrane and the sludge cake on the membrane surface, however the cake layer is not thick enough at the beginning of each run. Turbidity of effluent rose to 3.8 NTU suddenly on the 43 day due to breakage of the membrane in the membrane reactor. The SS in the membrane reactor passed through the breakage of the membrane and deteriorated the effluent, but the turbidity dropped to below 1.0 NTU after one day’s operation. This indicated the sludge would block the leak gradually and the membrane would recover automatically. After a period of operation, the thickened cake layer played an important role as filtration media. The effluent quality became better during each run. Actinide concentrations are linked with fine particles [22]. As a consequence, SS removed completely by microfiltration provides a solid basis for the hot test.
brane surface. The particles separated by the membrane also increased the sludge concentration in the membrane reactor. A concentration gradient of sludge occurred between the membrane surface and the bulk of mixed liquor, which brought more resistance in the boundary layer. These factors increased the cake resistance and decreased the MFR [23]. The relationship between the MFR and the concentration of SS in the reactor is shown in Fig. 3. As the concentration of SS reached a critical value, the flow rate decreased rapidly. The critical values of the 4 runs were about 25, 18, 17, and 16 g/l, respectively, (shown in Fig. 3). In the first run, the flow rate remained around the MFR for a long time. In the other 3 runs, the initial MFR was slightly lower than that of run 1. The results showed that irreversible fouling of the membrane had token place and the MFR cannot recover to the original value even if the sludge was discharged and the membrane was cleaned with physical methods. Particles deposited on the membrane surface or in the pore channel could not be removed completely by physical cleaning. The effluent flow rate slightly decreased with the operation time, but did not decrease rapidly under the design flow rate (the low limit of the MFR to meet design capacity of the membrane reactor) until the concentration of SS reached the critical value, which was related to the degree of membrane fouling.
3.1.2. Maximum flow rate Membrane permeability is defined here as the effluent maximum flow rate at the constant transmembrane pressure (1.6 m water head, the water level difference between the high level of the membrane reactor and that of the effluent tank). The MFR decreased with operation time in each run due to the SS concentration increasing and membrane fouling. The initial MFRs in the 4 runs were 30, 29.6, 28.3, and 25 l/h respectively. The results indicated that irreversible membrane fouling occurs in different degrees after each run’s operation. With the increasing concentration of the SS in the reactor, more and more tiny particles and inorganic ions were attached to the membrane surface, and deposited in the membrane pores. This blocked the permeate channels, and formed a sludge cake layer on the mem-
3.1.3. Membrane cleaning When the concentration of SS increased to a critical value, the membrane MFR dropped below the design value, and the sludge in the reactor was discharged and membrane cleaning was initiated. In the cold tests, the sludge cake on the membrane surface appeared brown–yellow and loose; the hollow fibers did not adhere to each other. After physical cleaning (discharging sludge, and aeration for 20 h without feed and effluent), there was no residual sludge on the membrane surface. The flow rate after physical cleaning was slight lower than that of the new membrane at the end of run 1, 2, and 3, however much lower at the end of run 4, because the most of the membrane fouling was irreversible after long time operation, and could not be resolved thoroughly by physical cleaning, as shown in Table 3.
50 Maximum flow(l/h)
First run 40 30
Second run
20
Third run
10
Fourth run
0 0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
Concentration of SS in the reactor (g/l)
Fig. 3. Relationship of MFR and concentration of SS in the reactor.
G. Yong et al. / Separation and Purification Technology 40 (2004) 183–189 Table 3 MFR, MSF (Maximum specific flux) and percentage of MSF recovered by membrane cleaning during 4 runs Runs of test
MFR (l/h)
MSF (l/m2 ·h· m)
Before the test
30.0
37.5
10.0 29.44
12.27 36.81
98.16
7.2 28.3
8.83 34.72
92.59
9.5 25.0
11.66 30.67
83.32
9.8 13.8 27.5
12.3 17.3 34.4
46.13 91.73
Percentage of MSF recovered by membrane cleaning (%)
Run 1 End of run 1 After physical cleaning Run 2 End of run 2 After physical cleaning Run 3 End of run 3 After physical cleaning Run 4 End of run 4 After physical cleaning After chemical cleaning
The specific flux of the membrane (MSF) is defined as the MFR per 1.0 m2 area of membrane under 1.0 m water head transmembrane pressure. When the MFR cannot be recovered by physical cleaning, chemical cleaning can be adopted and hydrochloric acid was used as the cleaning agent. The fouled membrane was soaked in 0.4% hydrochloric acid for 10 h, and then rinsed with tap water. After chemical cleaning, the specific flux of the membrane was 91.7% of the new membrane. Most of the foulants in or on the membrane were transferred into the acidic waste. The results of membrane cleaning during 4 runs are listed in Table 3. No broken membrane fiber was observed after the chemical cleaning and effluent turbidity kept normal. In the acidic waste from chemical cleaning, the concentration of Fe3+ was 2.7 mg/l while the total of Ca2+ and Mg2+ was 1.4 mg/l. Assuming that the inorganic ions are distributed evenly on the membrane surface, the adsorption ratio of Fe3+ is 6.3 mg/m2 and that of the total of Ca2+ and Mg2+ is 3.3 mg/m2 . The membrane fouling appeared to be mainly caused by inorganic compounds containing ferric ion. Most of the MFR could be recovered after physical cleaning (especially at the ends of the former 3 runs, above 80% Table 4 Removal of Batch No.
1 2 3 4
241 Am
187
of the MFR recovered, as shown in Table 3). Physical cleaning could be done in the reactor by discharging sludge and aeration. It is operated easily and no additional wastewater is discharged. Physical cleaning is suitable for the treatment of the radioactive wastewater. Even the chemical cleaning is more efficient to remove foulants from the membrane than physical cleaning, however, the cleaning process will produce some wastewater with low pH value. The wastewater may be with significant radioactivity. Replacing the fouled membrane or cleaning it chemically, should be decided by technical–economical analysis. 3.2. Hot test The hot test was conducted on the same basis of the cold test, and the operating parameters of the hot test were the same. The radioactive wastewater containing 241 Am was taken from a wastewater tank on site. The pH value of the wastewater was 6.8 and the radioactivity of 241 Am was 103 –104 Bq/l depending on the sampling points. There were some surfactants in the radioactive wastewater that originate from the laundry. The preliminary test indicated that the main component of the surfactants was linear alkyl sulphonate with a concentration of about 1.7 × 10−5 mol/l, far lower than the critical micelle concentration (cmc), 0.01 mol/l) [24]. So the surfactants cannot form micelles, dissolve in the radioactive wastewater. The anionic surfactants with negative charges may adsorb some Am3+ in the radioactive wastewater [25], make it difficult to form Am(OH)3 polymers with OH− . This part of Am3+ cannot be adsorb to the Fe(OH)3 particles and so cannot be rejected by microfiltration. Thus the surfactants may have a negative influence on the removal of 241 Am. 3.2.1. No pretreatment to remove the surfactant in the radioactive wastewater Without pretreatment of the radioactive wastewater, the hot test was operated for 308.9 h, and the treated radioactive wastewater volume was 1560 l. The radioactivity of 241 Am in the effluent was 93.7–546.4 Bq/l and the decontamination factor (DF, ratio of the radioactivity of the influent to that of the effluent) was only 1.48–12.6, as shown in Table 4. Five batches were processed, each of different feed activity, and each processed at a different final pH.
by FMF process without pretreatment
pH
6.8 6.8 6.8 6.8
Characteristics of the radioactive wastewater
Effluent of FMF
DF
Radioactivity (Bq/l)
pH
Radioactivity (Bq/l)
Radioactivity removal (%)
1500 1270 1180 1260
12 8 9 9.5
379.3 162.2 93.7 377.9
74.7 87.2 92.1 70.0
3.95 7.83 12.6 3.33
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Table 5 Removal of
241 Am
by FMF process combining with recycling or ultrafiltration without pretreatment
Batch No.
Reprocessing methods
pH
Reprocessing effluent Radioactivity (Bq/l)
Radioactivity removal (%)
1 2 3 4 5
Recycling treatment Recycling treatment Recycling treatment Recycling treatment Ultrafiltration
12.0 11.5 9.5 9.5 9.5
348.8 128.1 83.2 488.0 316.6
8.0 21.0 11.2 10.7 16.2
The effluent of FMF (without pretreatment) was treated further by recycling back to feed tank or UF (molecular weight cut-off 6000, surface area of membrane 2 m2 ), but the radioactivity of 241 Am in the effluent was little changed and the DF only increased by 1.09–1.27. The results are shown in Table 5. The residual radioactivity of the final effluent was 83.2–488.0 Bq/l. The total DF was very small (1.66–14.2). The results showed that there might be some particles of americium that could not be congregated together or adsorbed by flocs. Their sizes were smaller than the pore size of ultrafiltration membranes, so the particles of americium could pass through the membrane, which caused the high radioactivity of the effluent. This phenomenon was probably caused by the surfactant in the feed wastewater. 3.2.2. Pretreatment to remove the surfactant in the radioactive wastewater The radioactive wastewater was pretreated to remove the surfactant by oxidation at a pH value of 2–4. The dosage of potassium permanganate was 20 mg/l and the mixed liquor was stirred for 10 min. The membrane reactor was continuously operated for 80 h and the volume of the treated radioactive wastewater was 280 l. The results are shown in Table 6. The results of the hot test indicated that 241 Am in the wastewater could be reduced to a level 0.49 Bq/l, lower than the discharge standard (1.0 Bq/l). Effluent turbidity was in accordance with that of the cold test. In addition, when the pH was varied in the range of 7.8–13.5, the removal of 241 Am was not influenced significantly. Table 6 Removal of Batch No.
1 2 3 4 5 6 7 a b
241 Am
DF
1.09 1.27 1.13 1.12 1.19
FMF+ reprocessing Radioactivity removal (%)
DF
76.7 89.9 92.9 39.7 74.9
4.30 9.91 14.2 1.66 3.98
A comparison of Tables 4–6 shows that the surfactant in the radioactive wastewater has a great effect on the removal of 241 Am with the FMF process. If the surfactant in the radioactive wastewater was not removed, even for the combined FMF–UF process, the radioactivity of 241 Am in the effluent was still 316.6 Bq/l and DF was only 3.98. However, if the surfactant was oxidized by potassium permanganate in the pretreatment, the radioactivity of 241 Am in the effluent could decrease to 0.49 Bq/l by the FMF process. The removal of 241 Am was higher than 99.9% and the DF was more than 1651.4. The surfactant in the wastewater must be removed prior to the FMF process. 3.2.3. Maximum flow rate At the end of the second stage of the hot tests (without pretreatment and with pretreatment), total volume of the treated radioactive wastewater was 1840 l, which was close to that of the first run of the cold test (1820.8 l), the dosage of Fe3+ was the same, and no discharge of the sludge. Though the MnO2 precipitate was formed in the hot test with pretreatment, the weight of the MnO2 precipitate was about 10% of that of Fe(OH)3 precipitate, and the treated radioactive wastewater volume was 280 l during the hot test with pretreatment. The contribution of MnO2 formed in the oxidation to SS was small. The MFR declined rapidly below the design value at the end of the hot test. Therefore, it could be deduced that the relationship between the SS concentration in the reactor and MFR of the hot test was similar to that of the cold test.
by FMF process with pretreatment Radioactive wastewater
Effluent of FMF process
pH
Radioactivity (Bq/l)
pH
Radioactivity (Bq/l)
6.8 6.8 6.8 6.8 6.8 6.8 6.8
809.2 809.2 1260 11900b 1260 1260 1260
12.0 10.0 7.8 13.5 11.0 12.5 13.0
0.33 0.49a 0.45a 0.25a 0.34a 0.33a 0.34a
The minimum detection limit of ␥-spectrometer. The sample was from the lowest position of the wastewater tank.
Radioactivity removal (%)
DF
99.959 99.939 99.964 99.998 99.973 99.974 99.973
2452.1 1651.4 2800.0 47600.0 3705.9 3818.2 3705.9
G. Yong et al. / Separation and Purification Technology 40 (2004) 183–189
4. Conclusions The FMF process is a combination of flocculation and microfiltration. All of the reactions are conducted in an integrated reactor. The HRT is less than 1.5 h and the space of the reactor is very small. The volume ratio of supplied air to treated wastewater is 13:1. It can be operated automatically. The operating pressure or energy consumption is very low. The average volume concentration ratio of the treated wastewater to the discharged sludge was 189.8. The volume of the final sludge is small. Membrane fouling can be minimized effectively by aeration in the reactor. After operating for 4 months, the specific flux of the membrane varied from 37.5 l/m2 h m (initial) to 34.38 l/m2 h m (after chemical cleaning). The physical cleaning can recover the most of the flux while no broken membrane fiber was observed and no additional wastewater was discharged. So the physical cleaning is suitable for the treatment of the radioactive wastewater. Surfactants in the radioactive wastewater have a great influence on the removal of 241 Am. Without pretreatment, even the combined FMF–UF process was used, the radioactivity of 241 Am in the effluent was as high as 316.6 Bq/l and DF was only 3.98. However, after pretreatment with potassium permanganate, the FMF process can achieve a significant removal of 241 Am. Effluent can be produced below the discharge standard (1.0 Bq/l). The removal of 241 Am was higher than 99.9% and DF was more than 1651.4. As a result, FMF process is feasible for the treatment of the low-level radioactive wastewater containing 241 Am, provided fouling of the membrane is rectified by an effective cleaning system.
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Treatment of the wastewater containing low-level 241Am using flocculation-microfiltration process Gao Yong a , Zhao Jun b , Zhang Guanghui a , Zhang Dong b , Cheng Weiwen a , Yuan Guoqi b , Liu Xuejun b , Ma Bangzhong b , Zeng Junhui b , Gu Ping a,∗ a
b
School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China Institute of Nuclear Physics and Chemistry, Chinese Academy of Engineering Physics, Mianyang 621900, China Received in revised form 15 February 2004; accepted 28 February 2004
Abstract This paper studies the treatment of low-level radioactive wastewater containing 241 Am by a combined flocculation-microfiltration (FMF) process. The average concentration ratio of the treated wastewater to the discharged sludge was 189.8 in the cold test. After operating for 4 months, the fouled membrane was cleaned and cleaning effects of physical and chemical methods were compared. In the hot test, the pH was adjusted with nitric acid and 20 mg/l potassium permanganate was added to remove surfactants from the feed during pretreatment. The pretreated wastewater was pumped to the membrane reactor, where the pH was adjusted to above 8 and the dosage of Fe3+ was 30 mg/l. The mixed liquor was then filtered through the submerged microfiltration (MF) membrane. When the radioactivity of 241 Am in the feed was 809.2 Bq/l, the effluent was below 1.0 Bq/l. The removal of 241 Am was higher than 99.9%. The decontamination factor (DF) is more than 1651.4. The results showed that the FMF process has significant potential in treating low-level radioactive wastewater containing 241 Am. © 2004 Elsevier B.V. All rights reserved. Keywords: Radioactive wastewater; 241 Am; Flocculation-microfiltration (FMF); Membrane fouling; Decontamination factor (DF)
1. Introduction 241 Am
is a virulently radioactive nuclide in the periodic table of elements whose half-life is 432.9 years. It is primarily used to produce density detectors, as an excitation source of fluorescence analysis and neutron sources. 241 Am can emit ␣ and  rays during decay. The ␣ rays can do a great harm to the environment and the health of human being [1]. The highest allowable level of the total ␣ radioactivity in wastewater before discharge compelled by Integrated Wastewater Discharge Standard (GB8978-1996) is 1.0 Bq/l [2]. Radioactive wastes may bring potential impact to the environment through different transport ways [3–5]. They cannot be decomposed or biodegraded by physical, chemical or biological methods. The radioactivity of the wastes gradually weakens or dies out only by natural decay. High-level radioactive waste is loaded in sealed containers that are ∗ Corresponding author. Tel.: +86-22-27405059; fax: +86-22-27405059. E-mail address: [email protected] (G. Ping).
1383-5866/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2004.02.009
buried deeply under ground. Medium- or low-level radioactive waste is frequently concentrated to a small volume and then is solidified. However, a large quantity of wastes whose radioactivity meets the discharge standard can be discharged directly to the environment. Traditional treatment methods include flocculation-precipitation, ion exchange, distillation [6], and other methods [7–9]. The effluent quality was improved greatly when membrane separation was used in the treatment of radioactive wastewater [10]. Methods of membrane separation used to treat low-level radioactive wastewater, include liquid membrane (LM) [11,12], membrane distillation (MD) [13–15], electrochemical ion exchange (EIX) [16], polymer-ultrafiltration (PUF) [17–19], microfiltration– ultrafiltration (MF–UF), ultrafiltration–nanofiltration (UF– NF), ultrafiltration–reverse osmosis (UF–RO), etc. The processes mentioned above have high efficiency in treating lowlevel radioactive wastewater; however they have some shortcomings also, such as high operating pressure or energy consumption, frequent maintenance, etc. Compared with the above processes of membrane separation, the flocculationmicrofiltration (FMF) process developed in this paper has
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the advantage of being a simple process, automatic and safe operation, low operating pressure and energy consumption. In this study, the FMF process was used to treat low-level radioactive wastewater containing 241 Am and detergents (surfactants) from the laundry and washing floor of retired nuclear facilities. The radioactivity of 241 Am in the wastewater was about 103 –104 Bq/l, and must be decontaminated before being discharged. According to a report [20], when the concentration of 241 Am in the wastewater is 308.4 mg/l, the radioactivity is 1 Ci/l (3.7 × 1010 Bq/l), i.e., 1 mg/l 241 Am wastewater has radioactivity of 1.2×108 Bq/l. Therefore, the concentration of 241 Am in the wastewater was about 10−4 mg/l. The primary removal mechanism of 241 Am is that the solubility of americium hydroxide is low and americium hydroxide will precipitate out of the solution when alkali is added. The principle is as follows [21] Am3+ + 3OH− → Am(OH)3 ↓ Ksp = [Am3+ ][OH− ]2.4 = (3.4 ± 0.3) × 10−18 However, the solubility product constant (Ksp ) of americium hydroxide is not low enough to precipitate 241 Am to meet the discharge standard. It is known that flocs formed by ferric hydroxide are of strong adsorption capacity, and may be used as adsorbents to adsorb residual Am3+ [22]. Ferric chloride and sodium hydroxide were dosed simutlaneously into the feed to form a large amount of flocs in the reactor, prior to the FMF, to improve the removal of 241 Am.
2. Experimental A schematic of the FMF process is shown in Fig. 1. In the tests, the membrane reactor was aerated continuously to induce mixing. Effluent was removed intermittently, controlled by a programmable logic controller (PLC). The feed was pumped into the membrane reactor from the wastewater tank. Ferric chloride and sodium hydroxide were simultaneously added by dosing pumps. The mixing, precipitation, and microfiltration (MF) were completed in the membrane reactor. Liquid level probes (including a high level detector and a low one) were placed in the reactor. Pumps and automated valves were interconnected via the PLC with the liquid level probes. At low liquid level, the pump was started and the discharge valve was shut off, this was reversed at high liquid level. The total volume of the reactor is 11.8 l and the hydraulic residence time (HRT) was 1.49 h. The volume ratio of supplied air for aeration to treated wastewater was 13:1. The reactor was operated automatically. The valves for sludge discharge and the air inlet were set at the bottom of the reactor. The characteristics of the MF membrane module are shown in Table 1. Transmembrane pressure was provided by the water level difference between the level of the membrane reactor and that of the effluent tank. Considering safety, reliability, and convenience, the test was divided into two steps: the cold test and the hot test. In the cold test, the radioactive wastewater was replaced by tap water. It is very low in concentration of 241 Am, and does not
Fig. 1. Schematic of the FMF process. The part in the dash dot line is for pretreatment in the hot test, with no pretreatment in the cold test. Pump A: dosing KMnO4 solution; pump B: dosing HNO3 ; pump C: dosing NaOH solution; pump D: dosing FeCl3 solution. Table 1 The MF membrane module Type Material Manufacture
Hollow fiber Polyvinylidene fluoride (PVDF) Motian membrane engineering and technology Co., China
Nominal pore size (m)
0.22
a
Surface area (m2 ) Fiber length (m) Outer diameter of fibers (mm) Inner diameter of fibers (mm) Maximum specific fluxa (l/m2 ·h·m)
The maximum specific flux of the membrane refers to the maximum flux of clean water under transmembrane pressure of 1.0 m water head.
0.5 1.0 0.8 0.5 37.5
G. Yong et al. / Separation and Purification Technology 40 (2004) 183–189
cause significant membrane fouling, so tap water was used to observe the fouling characteristics of the membrane with ferric chloride as flocculants. The cold test was operated in 4 runs, and the membrane was cleaned by physical cleaning (discharging sludge, and aerating for 20 h with no feed and no effluent) after each run. The hot test used the same process as the cold test, but was operated with the site radioactive wastewater to test the actual removal of 241 Am. The radioactive wastewater was pretreated to decompose surfactants by oxidation using potassium permanganate at a pH of 2–4. Nitric acid was used to adjust the pH and the dosage of permanganate was 20 mg/l (dash dot line showed in Fig. 1). Turbidity measurements of the effluent were made using a photoelectric turbidimeter (Model GDS-3B). Effluent flows were measured by a liquid flowmeter (Model LZB). The pH measurements were made on a precise pH-meter (Model PHS-3C). Total ferric content was measured by the ferroin-spectrophotometric method with an ultraviolet/visible spectrophotometer (Model TU-1800). Measurements of calcium and magnesium ions were made by the titration method of ethylene-diamine tetraacetic acid disodium salt (EDTA). The suspended solids (SS) in the membrane reactor were measured by weight methods with an electronic balance (Model AB104-S). The ␣ radioactivity of 241 Am was measured by a high purity germanium -spectrometer (Model GEM35190). All the chemical agents used in the test were analytical reagents.
185
3. Results and discussion 3.1. Cold test In the cold test, ferric chloride was used as the flocculant and adsorbent after reaction with alkali. Sodium hydroxide was used to adjust the pH value of the feed. Ferric hydroxide particles are the sole solid phase in the mixed liquor. The purpose of this part of the test was to observe fine particles separation by the membrane and membrane fouling caused by the flocs. Effluent turbidity and the maximum flow rate (MFR) at a constant transmembrane pressure were measured. Zero or low turbidity means good removal of fine particles in the reactor. In practice, the abandoned membrane must be treated as radioactive waste, and the disposal cost is dependent on the membrane performance. The measurement of flow rate and the effects of membrane cleaning were used to determine the feasibility of this process for use in radioactive applications. The concentration of SS of the mixed liquor in the reactor increased with the operation time. When the concentration of SS increased to a critical value, the flow decreased rapidly, as shown in Fig. 3. When the flow rate decreased below the design value (12 l/h), the sludge in the reactor was discharged and the test was stopped. The cold test was operated in 4 runs with an operation time of 1025.3 h (96 days, 7–9 h per day). The volume of the treated wastewater was 5177.6 l and
Table 2 Cold test data Operating parameters
Run number
Operation time (d) Design flow rate (l/h) Dosage of Fe3+ (mg/l) pH of mixed liquor Volume of the treated wastewater (l) SS in the reactor at the end of each run (g/l) Concentration ratioa
2
3
4
35 12 27–65 8–11 1820.8 26.9 267
14 12 65 8–11 841.6 20.3 123
19 12 65 8–11 867.2 19.9 127
28 12 30 8–11 1648 22.1 242
The concentration ratio is the ratio of the total volume of treated wastewater to that of discharged sludge.
Run 1
Turbidity(NTU)
a
1
Run 2
Run 4
Run 3
4 3. 5 3 2. 5 2 1. 5 1 0. 5 0 0
20
40
60 Run Time(d)
Fig. 2. Turbidity of effluent.
80
100
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the volume of discharged sludge was 27.28 l, so the average concentration ratio of water to sludge was 189.8. The results of the 4 runs in the cold test are shown in Table 2. 3.1.1. Effluent turbidity The effluent was colorless and transparent. Most of effluent turbidity varied between 0 and 1 NTU (shown in Fig. 2). The average value was 0.3 NTU. Fig. 2 showed that effluent turbidity was slightly higher at the beginning of each run. The reason is that the removal of solids relies on the physical separation of membrane and the sludge cake on the membrane surface, however the cake layer is not thick enough at the beginning of each run. Turbidity of effluent rose to 3.8 NTU suddenly on the 43 day due to breakage of the membrane in the membrane reactor. The SS in the membrane reactor passed through the breakage of the membrane and deteriorated the effluent, but the turbidity dropped to below 1.0 NTU after one day’s operation. This indicated the sludge would block the leak gradually and the membrane would recover automatically. After a period of operation, the thickened cake layer played an important role as filtration media. The effluent quality became better during each run. Actinide concentrations are linked with fine particles [22]. As a consequence, SS removed completely by microfiltration provides a solid basis for the hot test.
brane surface. The particles separated by the membrane also increased the sludge concentration in the membrane reactor. A concentration gradient of sludge occurred between the membrane surface and the bulk of mixed liquor, which brought more resistance in the boundary layer. These factors increased the cake resistance and decreased the MFR [23]. The relationship between the MFR and the concentration of SS in the reactor is shown in Fig. 3. As the concentration of SS reached a critical value, the flow rate decreased rapidly. The critical values of the 4 runs were about 25, 18, 17, and 16 g/l, respectively, (shown in Fig. 3). In the first run, the flow rate remained around the MFR for a long time. In the other 3 runs, the initial MFR was slightly lower than that of run 1. The results showed that irreversible fouling of the membrane had token place and the MFR cannot recover to the original value even if the sludge was discharged and the membrane was cleaned with physical methods. Particles deposited on the membrane surface or in the pore channel could not be removed completely by physical cleaning. The effluent flow rate slightly decreased with the operation time, but did not decrease rapidly under the design flow rate (the low limit of the MFR to meet design capacity of the membrane reactor) until the concentration of SS reached the critical value, which was related to the degree of membrane fouling.
3.1.2. Maximum flow rate Membrane permeability is defined here as the effluent maximum flow rate at the constant transmembrane pressure (1.6 m water head, the water level difference between the high level of the membrane reactor and that of the effluent tank). The MFR decreased with operation time in each run due to the SS concentration increasing and membrane fouling. The initial MFRs in the 4 runs were 30, 29.6, 28.3, and 25 l/h respectively. The results indicated that irreversible membrane fouling occurs in different degrees after each run’s operation. With the increasing concentration of the SS in the reactor, more and more tiny particles and inorganic ions were attached to the membrane surface, and deposited in the membrane pores. This blocked the permeate channels, and formed a sludge cake layer on the mem-
3.1.3. Membrane cleaning When the concentration of SS increased to a critical value, the membrane MFR dropped below the design value, and the sludge in the reactor was discharged and membrane cleaning was initiated. In the cold tests, the sludge cake on the membrane surface appeared brown–yellow and loose; the hollow fibers did not adhere to each other. After physical cleaning (discharging sludge, and aeration for 20 h without feed and effluent), there was no residual sludge on the membrane surface. The flow rate after physical cleaning was slight lower than that of the new membrane at the end of run 1, 2, and 3, however much lower at the end of run 4, because the most of the membrane fouling was irreversible after long time operation, and could not be resolved thoroughly by physical cleaning, as shown in Table 3.
50 Maximum flow(l/h)
First run 40 30
Second run
20
Third run
10
Fourth run
0 0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
Concentration of SS in the reactor (g/l)
Fig. 3. Relationship of MFR and concentration of SS in the reactor.
G. Yong et al. / Separation and Purification Technology 40 (2004) 183–189 Table 3 MFR, MSF (Maximum specific flux) and percentage of MSF recovered by membrane cleaning during 4 runs Runs of test
MFR (l/h)
MSF (l/m2 ·h· m)
Before the test
30.0
37.5
10.0 29.44
12.27 36.81
98.16
7.2 28.3
8.83 34.72
92.59
9.5 25.0
11.66 30.67
83.32
9.8 13.8 27.5
12.3 17.3 34.4
46.13 91.73
Percentage of MSF recovered by membrane cleaning (%)
Run 1 End of run 1 After physical cleaning Run 2 End of run 2 After physical cleaning Run 3 End of run 3 After physical cleaning Run 4 End of run 4 After physical cleaning After chemical cleaning
The specific flux of the membrane (MSF) is defined as the MFR per 1.0 m2 area of membrane under 1.0 m water head transmembrane pressure. When the MFR cannot be recovered by physical cleaning, chemical cleaning can be adopted and hydrochloric acid was used as the cleaning agent. The fouled membrane was soaked in 0.4% hydrochloric acid for 10 h, and then rinsed with tap water. After chemical cleaning, the specific flux of the membrane was 91.7% of the new membrane. Most of the foulants in or on the membrane were transferred into the acidic waste. The results of membrane cleaning during 4 runs are listed in Table 3. No broken membrane fiber was observed after the chemical cleaning and effluent turbidity kept normal. In the acidic waste from chemical cleaning, the concentration of Fe3+ was 2.7 mg/l while the total of Ca2+ and Mg2+ was 1.4 mg/l. Assuming that the inorganic ions are distributed evenly on the membrane surface, the adsorption ratio of Fe3+ is 6.3 mg/m2 and that of the total of Ca2+ and Mg2+ is 3.3 mg/m2 . The membrane fouling appeared to be mainly caused by inorganic compounds containing ferric ion. Most of the MFR could be recovered after physical cleaning (especially at the ends of the former 3 runs, above 80% Table 4 Removal of Batch No.
1 2 3 4
241 Am
187
of the MFR recovered, as shown in Table 3). Physical cleaning could be done in the reactor by discharging sludge and aeration. It is operated easily and no additional wastewater is discharged. Physical cleaning is suitable for the treatment of the radioactive wastewater. Even the chemical cleaning is more efficient to remove foulants from the membrane than physical cleaning, however, the cleaning process will produce some wastewater with low pH value. The wastewater may be with significant radioactivity. Replacing the fouled membrane or cleaning it chemically, should be decided by technical–economical analysis. 3.2. Hot test The hot test was conducted on the same basis of the cold test, and the operating parameters of the hot test were the same. The radioactive wastewater containing 241 Am was taken from a wastewater tank on site. The pH value of the wastewater was 6.8 and the radioactivity of 241 Am was 103 –104 Bq/l depending on the sampling points. There were some surfactants in the radioactive wastewater that originate from the laundry. The preliminary test indicated that the main component of the surfactants was linear alkyl sulphonate with a concentration of about 1.7 × 10−5 mol/l, far lower than the critical micelle concentration (cmc), 0.01 mol/l) [24]. So the surfactants cannot form micelles, dissolve in the radioactive wastewater. The anionic surfactants with negative charges may adsorb some Am3+ in the radioactive wastewater [25], make it difficult to form Am(OH)3 polymers with OH− . This part of Am3+ cannot be adsorb to the Fe(OH)3 particles and so cannot be rejected by microfiltration. Thus the surfactants may have a negative influence on the removal of 241 Am. 3.2.1. No pretreatment to remove the surfactant in the radioactive wastewater Without pretreatment of the radioactive wastewater, the hot test was operated for 308.9 h, and the treated radioactive wastewater volume was 1560 l. The radioactivity of 241 Am in the effluent was 93.7–546.4 Bq/l and the decontamination factor (DF, ratio of the radioactivity of the influent to that of the effluent) was only 1.48–12.6, as shown in Table 4. Five batches were processed, each of different feed activity, and each processed at a different final pH.
by FMF process without pretreatment
pH
6.8 6.8 6.8 6.8
Characteristics of the radioactive wastewater
Effluent of FMF
DF
Radioactivity (Bq/l)
pH
Radioactivity (Bq/l)
Radioactivity removal (%)
1500 1270 1180 1260
12 8 9 9.5
379.3 162.2 93.7 377.9
74.7 87.2 92.1 70.0
3.95 7.83 12.6 3.33
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Table 5 Removal of
241 Am
by FMF process combining with recycling or ultrafiltration without pretreatment
Batch No.
Reprocessing methods
pH
Reprocessing effluent Radioactivity (Bq/l)
Radioactivity removal (%)
1 2 3 4 5
Recycling treatment Recycling treatment Recycling treatment Recycling treatment Ultrafiltration
12.0 11.5 9.5 9.5 9.5
348.8 128.1 83.2 488.0 316.6
8.0 21.0 11.2 10.7 16.2
The effluent of FMF (without pretreatment) was treated further by recycling back to feed tank or UF (molecular weight cut-off 6000, surface area of membrane 2 m2 ), but the radioactivity of 241 Am in the effluent was little changed and the DF only increased by 1.09–1.27. The results are shown in Table 5. The residual radioactivity of the final effluent was 83.2–488.0 Bq/l. The total DF was very small (1.66–14.2). The results showed that there might be some particles of americium that could not be congregated together or adsorbed by flocs. Their sizes were smaller than the pore size of ultrafiltration membranes, so the particles of americium could pass through the membrane, which caused the high radioactivity of the effluent. This phenomenon was probably caused by the surfactant in the feed wastewater. 3.2.2. Pretreatment to remove the surfactant in the radioactive wastewater The radioactive wastewater was pretreated to remove the surfactant by oxidation at a pH value of 2–4. The dosage of potassium permanganate was 20 mg/l and the mixed liquor was stirred for 10 min. The membrane reactor was continuously operated for 80 h and the volume of the treated radioactive wastewater was 280 l. The results are shown in Table 6. The results of the hot test indicated that 241 Am in the wastewater could be reduced to a level 0.49 Bq/l, lower than the discharge standard (1.0 Bq/l). Effluent turbidity was in accordance with that of the cold test. In addition, when the pH was varied in the range of 7.8–13.5, the removal of 241 Am was not influenced significantly. Table 6 Removal of Batch No.
1 2 3 4 5 6 7 a b
241 Am
DF
1.09 1.27 1.13 1.12 1.19
FMF+ reprocessing Radioactivity removal (%)
DF
76.7 89.9 92.9 39.7 74.9
4.30 9.91 14.2 1.66 3.98
A comparison of Tables 4–6 shows that the surfactant in the radioactive wastewater has a great effect on the removal of 241 Am with the FMF process. If the surfactant in the radioactive wastewater was not removed, even for the combined FMF–UF process, the radioactivity of 241 Am in the effluent was still 316.6 Bq/l and DF was only 3.98. However, if the surfactant was oxidized by potassium permanganate in the pretreatment, the radioactivity of 241 Am in the effluent could decrease to 0.49 Bq/l by the FMF process. The removal of 241 Am was higher than 99.9% and the DF was more than 1651.4. The surfactant in the wastewater must be removed prior to the FMF process. 3.2.3. Maximum flow rate At the end of the second stage of the hot tests (without pretreatment and with pretreatment), total volume of the treated radioactive wastewater was 1840 l, which was close to that of the first run of the cold test (1820.8 l), the dosage of Fe3+ was the same, and no discharge of the sludge. Though the MnO2 precipitate was formed in the hot test with pretreatment, the weight of the MnO2 precipitate was about 10% of that of Fe(OH)3 precipitate, and the treated radioactive wastewater volume was 280 l during the hot test with pretreatment. The contribution of MnO2 formed in the oxidation to SS was small. The MFR declined rapidly below the design value at the end of the hot test. Therefore, it could be deduced that the relationship between the SS concentration in the reactor and MFR of the hot test was similar to that of the cold test.
by FMF process with pretreatment Radioactive wastewater
Effluent of FMF process
pH
Radioactivity (Bq/l)
pH
Radioactivity (Bq/l)
6.8 6.8 6.8 6.8 6.8 6.8 6.8
809.2 809.2 1260 11900b 1260 1260 1260
12.0 10.0 7.8 13.5 11.0 12.5 13.0
0.33 0.49a 0.45a 0.25a 0.34a 0.33a 0.34a
The minimum detection limit of ␥-spectrometer. The sample was from the lowest position of the wastewater tank.
Radioactivity removal (%)
DF
99.959 99.939 99.964 99.998 99.973 99.974 99.973
2452.1 1651.4 2800.0 47600.0 3705.9 3818.2 3705.9
G. Yong et al. / Separation and Purification Technology 40 (2004) 183–189
4. Conclusions The FMF process is a combination of flocculation and microfiltration. All of the reactions are conducted in an integrated reactor. The HRT is less than 1.5 h and the space of the reactor is very small. The volume ratio of supplied air to treated wastewater is 13:1. It can be operated automatically. The operating pressure or energy consumption is very low. The average volume concentration ratio of the treated wastewater to the discharged sludge was 189.8. The volume of the final sludge is small. Membrane fouling can be minimized effectively by aeration in the reactor. After operating for 4 months, the specific flux of the membrane varied from 37.5 l/m2 h m (initial) to 34.38 l/m2 h m (after chemical cleaning). The physical cleaning can recover the most of the flux while no broken membrane fiber was observed and no additional wastewater was discharged. So the physical cleaning is suitable for the treatment of the radioactive wastewater. Surfactants in the radioactive wastewater have a great influence on the removal of 241 Am. Without pretreatment, even the combined FMF–UF process was used, the radioactivity of 241 Am in the effluent was as high as 316.6 Bq/l and DF was only 3.98. However, after pretreatment with potassium permanganate, the FMF process can achieve a significant removal of 241 Am. Effluent can be produced below the discharge standard (1.0 Bq/l). The removal of 241 Am was higher than 99.9% and DF was more than 1651.4. As a result, FMF process is feasible for the treatment of the low-level radioactive wastewater containing 241 Am, provided fouling of the membrane is rectified by an effective cleaning system.
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