Ultrafiltration-Reverse Osmosis-Electrodialysis Combined Process

I. Introduction

Since the 1970s, our institute has successfully treated radioactive wastewater using two processes: "four electrodialysis units" and "electrodialysis unit-packed bed electrodialysis unit". However, we have also found that the treatment effect on radioactive wastewater discharged from our institute's radiochemistry laboratory is not ideal. This is mainly because the wastewater has a complex composition, especially containing organic macromolecules and complexes, 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 ultrafiltration membrane treatment of radioactive wastewater [3]. We have also studied the method of reverse osmosis treatment of radioactive wastewater [4]. Based on this, and combining the advantages of various treatment methods, we have proposed a new process for treating low-level radioactive wastewater using a combination of ultrafiltration (UF)-reverse osmosis (RO)-electrodialysis (ED) process (hereinafter referred to as URE process).

II. Process and Equipment

The URE process for treating low-level radioactive wastewater is shown in Figure 1. The YM-type internal pressure tubular ultrafiltration unit (sulfonated polysulfone ultrafiltration membrane, molecular weight cutoff of 20,000) developed by our institute was used, with a membrane area of 1.5 m², a pure water flux of 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, a pure water flux of 270 L/h (pressure 1.3 MPa). The electrodialysis unit was 400 mm × 800 mm, one stage, with 40 membrane pairs, assembled by our institute.

Low-level wastewater discharged from the radiochemistry laboratory entered the settling tank. After 24 hours of settling, the supernatant was transferred to the ultrafiltration raw water tank. After ultrafiltration treatment, the permeate entered the intermediate tank. Simultaneously, the reverse osmosis unit and the electrodialysis unit were started. The reverse osmosis unit further desalinated and decontaminated the permeate, which could be directly discharged or flowed into a mixed bed for further treatment. Electrodialysis served as a concentration agent. The final concentrates from ultrafiltration and electrodialysis were retained for solidification treatment. All three units operated in a circulating manner. III. Full-Process Cold Test Operation

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

specifically: NaHCO3 60 mg/L, NaNO3 146 mg/L, NaCl 128 mg/L, CaCl2 88 mg/L, MgCl2 71 mg/L,

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

1. Ultrafiltration unit

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

⑴Desalination effect

Different from ordinary ultrafiltration membranes, because the sulfonated polysulfone ultrafiltration membrane is charged, it has a certain desalination ability. However, the desalination rate decreases with the increase of salt content in raw water and the decrease of pH value (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 ultrafiltration (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 and 58.30 L/m²h, respectively. Furthermore, the flux gradually decreases with increasing feed concentration, while it gradually increases with increasing feed temperature.

(3) Changes in Turbidity and Chemical Oxygen Demand (COD)

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

Table 2: Changes in Turbidity and COD

(4) Membrane Cleaning Method Experiment

As operating time increased, the ultrafiltration flux gradually decreased. Experiments were conducted using chemical cleaning, mechanical cleaning with sponge balls, and combinations thereof to restore flux.

Chemical cleaning effectively restored flux, but flux decayed rapidly upon restarting, and two waste liquid processes were generated. Mechanical cleaning with sponge balls involved simply rotating the ball-washing valve 180 degrees, allowing the sponge balls stored inside to enter the membrane along with the feed solution. After scrubbing the membrane surface, the sponge balls returned to the valve for later use. While the initial flux after cleaning was not as high as with chemical cleaning, the flux remained stable for a longer period. This method is simple, does not affect production, and does not generate two waste liquid processes, making it suitable for radioactive wastewater treatment.

1. Flux after chemical cleaning; 2. Flux after ball washing following chemical cleaning; 3. Flux after ball washing

2. Reverse Osmosis Unit

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

⑴ Position of Reverse Osmosis in the URE Process

Initially, the URE process was envisioned as: UF-RO-ED. Wastewater, after ultrafiltration, enters the reverse osmosis system for desalination and concentration by a factor of 2, followed by further concentration by electrodialysis. However, the experiment found that when the salt content of the reverse osmosis feed solution increased due to concentration, its desalination rate decreased, and the salt content of the permeate also increased, increasing the burden on the tail-end treatment. To better utilize reverse osmosis, its position was changed to UF-ED-RO. The feed solution after ultrafiltration is first desalinated by electrodialysis to reduce the salt content to 500 mg/L, and then further desalinated by reverse osmosis. After the modification, the desalination rate of reverse osmosis stabilized at 85%.

(2) Flux Changes

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. 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 due to 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: Cold Test Results of Electrodialysis and Ion Exchange Units

Table 4 Summary of URE process cold test results

*Volume Concentration Ratio = Feed Liquid Volume / Concentrated Wastewater 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 feed water 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. Effect of Raw Water Salinity on Reverse Osmosis Unit Decontamination Rate

Similar to the cold test results, when the raw water salinity was high, the RO desalination rate decreased, and the decontamination rate also decreased. By first starting the ED (Enhanced Environmental Deionization) cycle and maintaining the salinity of the RO feed solution at around 500 mg/L, the RO desalination rate can reach over 90%, and the fouling removal rate also increases to over 95% (Table 5).

Table 5: Effect of Raw Water Salinity on Fouling Removal Rate of Reverse Osmosis Unit

2. Removal Efficiency for High-Valence Ions

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

The results showed that both UF and RO achieved higher removal rates for divalent ions than for mixed ions. The removal rate for iron ions, which have more complex and higher valence states, approached 100%, indicating that membrane separation methods are extremely effective in removing complex high-valence ions.

Table 6 Removal Efficiency of Ultrafiltration and Reverse Osmosis for Ca2+ and Fe3+

3. Overall Decontamination Effect

During the overall hot commissioning of the process, 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 3H measurement using thermoluminescence are shown in Table 7.

The results of the URE process hot commissioning show that radioactivity removal mainly relies on reverse osmosis (the decontamination rates of total β and total γ were 95.0% and 93.7%, respectively). This process has no effect on 3H removal. The highest dose accumulation in the table is measured periodically within a fixed area of the ultrafiltration and reverse osmosis units using a β-γ radiation meter. The highest dose during the hot commissioning period 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 of the entire process, the URE process is evaluated as follows:

(1) Ultrafiltration replaces the coagulation and sedimentation 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 for 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 the URE Process

*The 3H ratio of the raw water is 4.77 × 10⁶, and the 3H ratio of the concentrated water is 4.55 × 10⁶.

(2) Reverse osmosis significantly improves desalination 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, and its desalination and decontamination rates decreased as the salinity increased. If high-pressure or medium-pressure reverse osmosis units are selected in future experiments, this weakness can be overcome, and the desalination and decontamination capabilities can be further improved, eliminating the need for a downstream ion exchange unit and simplifying the process.

Table 8 Comparison of Decontamination Effects between Electrodialysis and Reverse Osmosis

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

Table 9 Comparison of Decontamination Effects of Three Processes in 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