Ultrafiltration-Reverse Osmosis-Electrodialysis Combined Process

Abstract: This paper introduces the treatment of low-level radioactive wastewater discharged from the radiochemistry laboratory using a combined ultrafiltration-reverse osmosis-electrodialysis process. The effects of internal pressure tubular ultrafiltration, hollow fiber reverse osmosis and electrodialysis in wastewater treatment are described, as well as the comparison of the effects of two cleaning methods on the recovery of ultrafiltration membrane flux. The URE process composed of the "three membranes" combined process has a decontamination factor as high as 3.2×103, which provides a new method for the treatment of radioactive wastewater. Keywords: radioactive wastewater, ultrafiltration, reverse osmosis, electrodialysis, combined process I. Introduction Our institute has successfully carried out the treatment of radioactive wastewater using two processes, "four electrodialysis units" and "electrodialysis unit-packed bed electrodialysis unit", since the 1970s. However, it was also found that the effect was not ideal when treating the radioactive wastewater discharged from the radiochemistry laboratory of our institute. The main reason is that the wastewater has a complex composition, especially the organic macromolecules and complexes contained therein, which are difficult to remove by electrodialysis, thus affecting the purification effect [2]. In recent years, we have developed the YM-type sulfonated polysulfone ultrafiltration membrane and conducted exploratory experiments on the treatment of radioactive wastewater using ultrafiltration membranes [3]. We have also studied the method of treating radioactive wastewater using reverse osmosis [4]. Based on this, and combining the advantages of various treatment methods, we proposed a new process for treating low-level radioactive wastewater using a combined ultrafiltration (UF)-reverse osmosis (RO)-electrodialysis (ED) process (referred to as the URE process).

II. Process and Equipment

The YM-type internal pressure tubular ultrafiltration membrane (sulfonated polysulfone ultrafiltration membrane, molecular weight cutoff of 20,000) developed by our institute was used. The membrane area was 1.5 m², and the pure water flux was 250 L/h (pressure 0.25 MPa). The reverse osmosis unit was an HRC-type hollow fiber module developed by the Second Institute of Oceanography, with a membrane area of 40 m² and a pure water flux of 270 L/h (pressure 1.3 MPa). The electrodialysis unit was 400 mm × 800 mm, with one stage and 40 membrane pairs, assembled by our institute. Low-level radioactive wastewater discharged from the radiochemistry laboratory enters a settling tank. After 24 hours of settling and clarification, the supernatant is transferred to an ultrafiltration raw water tank. After ultrafiltration treatment, the permeate enters an intermediate tank. Simultaneously, the reverse osmosis unit and electrodialysis unit are started. The reverse osmosis unit further desalinates and removes contaminants, and the permeate can be directly discharged or flow into a mixed bed for further treatment. Electrodialysis serves as a concentration agent. The final concentrates from ultrafiltration and electrodialysis treatments are reserved for solidification treatment. All three units operate in a circulating manner.

III. Full-Process Cold Test Operation

The cold test operation lasted for a total of 147.5 hours, treating a total of 14 m³ of simulated wastewater. The simulated wastewater was prepared according to the composition of actual radioactive wastewater.

The specific formula is: NaHCO3 60mg/L, NaNO3 146mg/L, NaCl 128mg/L, CaCl2 88mg/L, MgCl2 71mg/L,

Na2SO4 7mg/L, 30% TBP-kerosene 50mg/L, machine oil 50mg/L, and detergent 50mg/L. The cold test operation is described below:

1. Ultrafiltration Unit

In the URE process, UF is used as a pretreatment to remove most of the organic matter and macromolecules to ensure the feed water requirements of RO and improve the concentration effect of ED.

(1) Desalination effect

Unlike ordinary ultrafiltration membranes, sulfonated polysulfone ultrafiltration membranes are charged and therefore have a certain desalination capacity. However, the desalination rate decreases with increasing salt content and decreasing pH value in the raw water (Table 1).

Table 1. Effects of raw water salinity and pH on desalination rate

(2) Factors affecting flux

The composition, concentration, and temperature of the raw water all affect the flux of the microfluidic reactor (UF). The fluxes when the raw water contains no organic matter (i.e., no added oil, detergent, etc.) and when it contains organic matter are 73.87 L/m²h and58.30 L/m²h , respectively. Furthermore, the flux gradually decreases with increasing feed concentration, while the flux gradually increases with increasing feed temperature.

(3) Changes in turbidity and chemical oxygen demand

After ultrafiltration, the turbidity of the wastewater was significantly reduced, ensuring the feed water requirements for reverse osmosis. The decrease in the COD value of the wastewater indicates that most of the organic matter has been removed, making downstream treatment processes easier (Table 2).

Table 2. Changes in turbidity and COD values

(4) Membrane cleaning method test

As the operating time increased, the ultrafiltration flux gradually decreased. The experiment used chemical cleaning, mechanical cleaning with sea balls, and a combination of these methods to clean the filter and restore the flux (Figure 2).

Chemical cleaning can effectively restore flux, but flux decays rapidly upon restarting and generates waste liquid twice. Mechanical cleaning with sea-surface balls, on the other hand, involves simply rotating the ball-washing valve 180 degrees, allowing the sea-surface balls stored inside to enter the membrane along with the feed solution. After scrubbing the membrane surface, the sea-surface balls return to the valve for later use. While the initial flux after cleaning is not as high as with chemical cleaning, it remains stable for a longer period. This method is simple, does not disrupt production, and does not generate waste liquid twice, making it suitable for treating radioactive wastewater.

• Flux after chemical cleaning;

• Flux after chemical cleaning followed by ball washing;

• Flux after ball washing

2. Reverse osmosis unit

In the URE process, RO is used for deep purification. The experiment explored the location of RO in the process and other influencing factors.

(1) The position of reverse osmosis in the URE process

Initially, the URE process was envisioned as UF-RO-ED, where wastewater, after ultrafiltration, would enter reverse osmosis for desalination and concentration by a factor of two, followed by further concentration via electrodialysis. However, experiments revealed that as the salt content of the reverse osmosis feed solution increased due to concentration, the desalination rate decreased, and the salt content of the permeate also increased, burdening the downstream treatment. To better utilize reverse osmosis, the process was changed to UF-ED-RO, where the ultrafiltration-treated feed solution is first desalinated via electrodialysis to reduce the salt content to 500 mg/L, before further desalination via reverse osmosis. After this modification, the reverse osmosis desalination rate stabilized at 85%.

(2) Flux Change

During the initial 40 hours of operation, the RO flux decreased from 141 L/h to 112 L/h (1.3 MPa), but remained relatively stable for the next 100+ hours without further decline. This can be attributed to the significantly reduced fouling of the RO membrane due to the use of UF as a pretreatment method. The initial flux decrease was caused by the membrane compaction effect.

3. Electrodialysis and ion exchange units

Electrodialysis and ion exchange are mainly used for concentration and subsequent deep purification in the URE process, respectively (Tables 3 and 4).

Table 3. Results of cold tests on electrodialysis and ion exchange units

Table 4 Summary of URE process cold test results

*Volume concentration ratio = Feed liquid volume/Concentrated sludge volume

IV. Radioactive Wastewater Treatment Experiment

Based on the full-process cold test operation, a low-level radioactive wastewater treatment experiment was conducted. The low-level radioactive wastewater came from actual wastewater from the institute's radiochemistry laboratory, with a specific radioactivity level of 7.4 kBq/L. The main radionuclides were 90Sr - 90Y and 137Cs, and the salinity was 800 mg/L. To further verify the membrane's ability to remove organic matter, the same organic components as in the cold test were added to the wastewater. The hot test ran for a total of 104.5 hours, treating 7.5 m³ of radioactive wastewater. The effect of the influent concentration of the reverse osmosis unit on desalination and decontamination was further measured during the experiment, and the removal of high-valence ions was also analyzed.

1. The impact of raw water salinity on the decontamination rate of the reverse osmosis unit

Similar to the cold test results, when the raw water has a high salt content, the RO desalination rate decreases, and the decontamination rate also decreases. By starting the ED first, and keeping the salt content of the RO feed liquid at around 500 mg/L, the RO desalination rate can reach over 90%, and the decontamination rate can also be increased to over 95% (Table 5).

Table 5. Effect of raw water salinity on the decontamination rate of the reverse osmosis unit.

2. Removal effect on high-valence ions

The removal rates of Ca2 + and Fe3 + ions in wastewater by UF and RO were determined in the thermal test (Table 6).

The results showed that both UF and RO had higher removal rates for divalent ions than for mixed ions. The removal rate for iron ions with complex and high valence states was close to 100%, indicating that membrane separation is extremely effective in removing complex, high-valence ions.

Removal efficiency of ultrafiltration and reverse osmosis for Ca²⁺ and Fe³⁺

3. Overall stain removal effect

During the full-process hot commissioning, total β was measured using a β-weak radioactivity measuring device, and total γ was measured using an HP-Ge probe S-85 multichannel analyzer system. Samples were taken and measured every 2 hours. The decontamination effect of the URE process and the results of 3 H measurement using thermoluminescence method are shown in Table 7.

The results of the URE process thermal test indicate that radioactivity removal mainly relies on reverse osmosis (total β and total γ removal rates were 95.0% and 93.7%, respectively). This process has no effect on 3H removal. The highest dose accumulation in the table was measured periodically within a fixed area of the ultrafiltration and reverse osmosis units using a β-γ radiation meter. The highest dose during the thermal test never exceeded 7.74 × 10⁻⁶ c /kg, indicating that the ultrafiltration and reverse osmosis units do not cause dose accumulation.

4. Full process evaluation

Based on the results of the cold and hot tests throughout the entire process, the URE process is evaluated as follows:

(1) Ultrafiltration replaces the coagulation and sedimentation process in the original process, reducing the need for solid waste disposal equipment, resulting in a high wastewater volume reduction ratio, stable operation, and ease of operation. Ultrafiltration significantly removes organic matter from wastewater, producing low effluent turbidity, meeting the feed water requirements of reverse osmosis, and improving the purification effect of downstream processes. Mechanical cleaning with sponge balls can appropriately restore the flux, without affecting production or generating secondary wastewater.

Table 7. Decontamination effect of URE process

*The H3 ratio of the raw water is 4.77 × 10⁶ , and the H3 ratio of the most concentrated water is 4.55 × 10⁶ .

(2) Reverse osmosis significantly improves desalination efficiency compared to electrodialysis and packed-bed electrodialysis (Table 8). In practical applications, the installation and operation of reverse osmosis are much simpler than those of electrodialysis or packed-bed electrodialysis. Reverse osmosis can remove not only ions but also complex macromolecules, thus improving purification efficiency. The reverse osmosis unit used in this experiment was a low-pressure type, which experienced a decrease in desalination and decontamination rates as the salt content increased. Future experiments using high-pressure or medium-pressure reverse osmosis units could overcome this weakness and further improve desalination and decontamination capabilities, eliminating the need for subsequent ion exchange units and simplifying the process.

Table 8 Comparison of decontamination effects between electrodialysis and reverse osmosis

(3) A comparison was made between the processes of four electrodialysis units, the electrodialysis-packed bed electrodialysis unit, and the URE process in treating wastewater from the radiochemistry laboratory of this institute. Obviously, the URE process has a higher decontamination capacity (Table 9).

Table 9 Comparison of the decontamination effects of three processes for treating low-level radioactive wastewater

References

1. W R Herald R C Roberts, MLM-2448,2538,2864,2795 (1976-1981)

2. Lou Fule, Water Treatment Technology, 1981, (Supplement): 1

3. Lou Fule, Water Treatment Technology, 1984, (5): 35

4. Lu Xiaofeng, Water Treatment Technology, 1988, (3): 81

Treating the radioactive waste water by UF,RO and ED combined technological process

Lu Xiaofeng  Lou Fule  Mao Weigang

Liang Guoming  Li Guozhen  Liu Guangquan

(Shanghai Institute of Nuclear Research,Academia Sinica)

Abstract

The UF,RO and Ed combined technological process was used to treat the low-level radioactive waste water coming from the radiochemistry laboratory in our institute. This paper referred to the percentage of desalination and decontamination and their function in the technological process. And a comparison was made of the cleaning efficiency using the chemical method and the spongeball method. The URE process was found to have a good decontamination efficiency, its D.F. reaching 3.2×10³.The results of the low-level radioactive waste water.

Key words: radioactive waste water, ultrafiltration, reverse osmosis, electrodialysis, treatment, combined technological process