Chemical cleaning of water treatment reverse osmosis and nanofiltration systems-Hezhong Environment (Beijing) Co., Ltd. was established in 2003

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Chemical cleaning of water treatment reverse osmosis and nanofiltration systems

Published Time:

2022-11-02

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Chemical Cleaning Standards

Chemical cleaning is necessary when physical flushing fails to restore the performance of the reverse osmosis membrane. This occurs under the following circumstances:

① Water production under standardized conditions decreases by 10-15%;

② The system pressure difference between feed water and concentrate water increases to 1.5 times the initial value;

③ The quality of produced water decreases by 10-15%.

Frequency of Chemical Cleaning

Membrane elements should be cleaned promptly when mild fouling occurs. Severe fouling can affect cleaning efficiency due to factors such as the difficulty of chemical agents penetrating the fouling layer and the difficulty of flushing pollutants out of the membrane. If the performance of the membrane element decreases to 30-50% of its normal value, it is difficult to clean it and restore it to the initial performance of the membrane system.

The cleaning cycle of the membrane depends on the actual fouling conditions on site. The normal cleaning cycle is once every 3-12 months. If cleaning is performed more than once a month, improvements to pretreatment, such as additional investment or redesign of the membrane system, are needed; if the cleaning cycle is once every 1-3 months, focus should be placed on adjusting and optimizing the operating parameters of the existing system. Even if the system has not been fouled for a long time, to better ensure the normal operation of the system, it is generally advisable to consider chemical cleaning once every 6 months.

When inorganic flocculants are used in the pretreatment process, there are often incomplete reactions where inorganic salts do not form filterable flocs. Users should ensure that no excessive flocculant enters the membrane system. Excessive flocculant can be determined using an SDI testing device, for example, 3 μg/piece of iron on the SDI membrane, which should not exceed 5 μg/piece at any time.

In addition to turbidity and SDI measurements, particle counters can also accurately measure whether the RO/NF feed water is qualified. The number of particles larger than 2 μm should be <100/ml.

Selection of Cleaning Agents

The types of pollutants and the changes in system operation during fouling are listed in the table below.

Introduction to Conventional Cleaning Solutions

1 Solution 1 : 2.0% (W) citric acid (C6H8O7) low pH cleaning solution. Highly effective in removing inorganic scale (such as calcium carbonate scale, calcium sulfate, barium sulfate, strontium sulfate scale, etc.), metal oxides or hydroxides (iron, manganese, copper, nickel, zinc, etc.), and inorganic colloids. Commercially available citric acid is a powder solid.

2 Solution 2 : 2.0 (W)% STPP (sodium tripolyphosphate Na5P3O10) and 0.8% (W) Na-EDTA mixed high pH (pH 10) solution. It is specifically used to remove calcium sulfate scale and mild to moderate natural organic pollutants. STPP has the functions of an inorganic chelating agent and detergent. Na-EDTA is an organic chelating cleaning agent with chelating properties, effectively removing divalent and trivalent cations and metal ions. Commercially available STPP and Na-EDTA are powder solids.

3 Solution 3 : 2.0 (W)% STPP (sodium tripolyphosphate Na5P3O10) and 0.25% (W) Na-DDBS [sodium dodecylbenzenesulfonate, C6H5(CH2)12-SO3Na] mixed solution with a pH of 10. This cleaning solution is used to remove severe natural organic matter (NOM) pollution. STPP has the functions of an inorganic chelating agent and detergent, while Na-DDBS is an anionic detergent.

4 Solution 4: 0.5% (W) hydrochloric acid low pH cleaning solution (pH 2.5), mainly used to remove inorganic scale (such as calcium carbonate scale, calcium sulfate, barium sulfate, strontium sulfate scale, etc.), metal oxides/hydroxides (iron, manganese, copper, nickel, aluminum, etc.), and inorganic colloids. This cleaning solution is stronger than Solution 1; hydrochloric acid is a strong acid solution, and relevant precautions must be followed during preparation.

5 Solution 5: 1.0% (W) sodium dithionite (Na2S2O4) low pH cleaning solution (pH=4-6). It is used to remove metal oxides and hydroxides, and can remove calcium sulfate, barium sulfate, and strontium sulfate scale to a certain extent. Sodium dithionite is a strong reducing agent, and this solution has a strong odor and requires adequate ventilation. Sodium dithionite is a powder solid.

6 Solution 6: 0.1% sodium hydroxide and 0.03% (W) SDS (sodium dodecyl sulfate) high pH mixed solution (pH 11.5). It is used to remove natural organic pollutants, inorganic/organic colloid mixed pollutants, and microbial (bacteria, algae, mold, fungi) pollution. SDS is an anionic surfactant-type detergent that produces some foam. Please note that when using this cleaning solution, do not exceed the maximum pH and temperature limits in Table 2 。

7 Solution 7 : 0.1% (W) sodium hydroxide high pH cleaning solution (pH 11.5). Used to remove polysilicic acid scale; this is a relatively strong alkaline solution. Please note that when using this cleaning solution, do not exceed the maximum pH and temperature limits in Table 2.

Please follow the precautions for the relevant chemicals when preparing the agents and take necessary protective measures. 。

The pH range during chemical cleaning is limited by the temperature of the cleaning solution. Table 1 gives the limited pH range for cleaning different membrane elements at different temperatures. Exceeding this range may cause irreversible damage to the membrane elements.

Table 2: Cleaning Solution pH Relationship between value and cleaning water temperature

During the cleaning process, pollutants consume cleaning chemicals, causing the pH value to change and reducing the cleaning ability of the chemicals. Therefore, it is necessary to monitor the pH value changes at any time and adjust the pH value in time. Generally, when the measured pH value deviates from the set pH value by more than 0.5, chemicals need to be added.

Online Chemical Cleaning

01 Chemical Cleaning Equipment

Online cleaning is performed with the membrane elements remaining in the pressure vessel, as shown in Figure 3. The cleaning equipment generally includes a cleaning water tank filter, circulation pump, pressure gauge, thermometer, pressure gauge, valves, sampling point, and pipelines. The volume of the cleaning water tank must be sufficient to meet the requirements of the connecting hoses, pipelines, and the water volume for replacing water in the reverse osmosis pressure vessel.

(Figure 1: Schematic diagram of chemical cleaning device

02 Chemical Cleaning System Specifications and Precautions

03 Cleaning System Water Volume

Calculate the volume of the reverse osmosis membrane elements, security filter, and pipelines, estimate the required volume of cleaning solution, and ensure the amount of cleaning solution. The calculation method for the volume of cleaning solution required when cleaning reverse osmosis membrane elements is shown in Table 7. The numbers in the table do not include the security filter, pipelines, and the initial 20% discharge of the required cleaning solution volume.

04 Water Quality for Cleaning

Cleaning water is used to dissolve chemical reagents, so it is required to use reverse osmosis water. If there is no reverse osmosis water, the water used must be free of hardness, free chlorine, and iron.

05 Precautions and Preparations before Cleaning

① Before using the chemicals, carefully read the chemical safety data sheet (MSDS) and instructions provided by the chemical company;

② When operating, wear safety glasses, protective gloves, and work clothes;

③ Calibrate the pH meter before use;

④ Estimate the volume of cleaning solution used;

⑤ Ensure that all cleaning chemicals are completely dissolved and mixed before entering the system;

⑥ The temperature and pH value of the cleaning solution are within the specified range.

06 Cleaning Method

The general chemical cleaning steps are as follows. Specific matters should be determined according to the on-site working conditions and cleaning requirements.

① Rinse the membrane elements. Rinse with reverse osmosis water or ion exchange water for a few minutes.

② Prepare the chemicals. Ensure that the chemicals are completely dissolved.

_{③ Circulation. Replace the water in the membrane elements with a low flow rate, discharge part of the cleaning solution, and then circulate. The circulation flow rate should be within the range shown in Table 8. In case of serious pollution, if a large flow rate circulation is selected, it may cause excessive pressure difference, which may damage the membrane elements (maximum pressure drop of a single membrane element is 0.07 MPa). In this case, the following method is more reliable:}

_{- Circulate for 5 minutes at the beginning, at 1/3 of the set flow rate;}

_{- Circulate for 5-10 minutes at 2/3 of the set flow rate;}

_{- Circulate according to the flow rate requirements after 10 minutes.}

_{The circulation time should be determined according to the specific situation. Generally, it is about 1 hour. During this period, it is necessary to check whether the pH value changes at any time. The pressure should be kept as low as possible, and no water production should occur as much as possible. At the same time, monitor whether the pressure difference changes.}

07 Offline Cleaning

Offline cleaning refers to taking out a membrane element from a large system and performing chemical cleaning using a specific cleaning device. Offline cleaning has three main advantages:

- Targeted testing of membrane elements with problems in the system;

- Testing the cleaning effect of a single membrane element before cleaning a large system;

- Used for small system cleaning, the single membrane cleaning method can increase the cleaning effect.

08 System Disinfection

Reverse osmosis and nanofiltration membrane elements can be disinfected or stored for a long time using the following methods:

① Formaldehyde A 0.1-1.0% formaldehyde solution is suitable for system disinfection or long-term storage. It should be in contact with formaldehyde at least 24 hours after the membrane element is used.

② Glutaraldehyde A 0.1-1.0% glutaraldehyde solution is suitable for system disinfection or long-term storage. It should be in contact with glutaraldehyde at least 24 hours after the membrane element is used.

Note: Using formaldehyde or glutaraldehyde on new membrane elements may reduce water flux by 10-20%, so contact with these disinfectants should be allowed only after the membrane elements have been in use for at least 24 hours. In any case, the first contact with formaldehyde or glutaraldehyde may cause a permanent reduction in water flux, and subsequent use of these disinfectants may cause a temporary reduction in water flux, requiring thorough rinsing of residual disinfectant until the system recovers its original water flux.

③ Isothiazolinone Isothiazolinone is provided by water treatment chemical manufacturers under the brand name Kathon; commercially available solutions contain 1.5% active ingredient. A concentration of 15-25 ppm is recommended for disinfection and long-term storage.

④ DBNPA (Dibromonitropropanamide) DBNPA is an effective bactericide. For shock addition, the active concentration of DBNPA added is 10-30 ppm, circulating the cleaning system membrane system for 30 minutes to 2 hours. When continuously added, a 0.5-1 ppm DBNPA solution should be added. DBNPA is only suitable for disinfection; it is not suitable for long-term storage of membrane elements because it causes membrane element degradation and reduces desalination rate.

⑤ Sodium bisulfite ： Sodium bisulfite inhibits microbial growth. When controlling microbial growth, add 500 ppm of sodium bisulfite solution daily for 30-60 minutes. Use a 1% sodium bisulfite solution for long-term storage.

⑥ Hydrogen peroxide ： Hydrogen peroxide or hydrogen peroxide solutions containing peracetic acid can be used for disinfection. When using hydrogen peroxide for sterilization, it is necessary to ensure that the water used to prepare the solution does not contain transition metals (iron, manganese, etc.) to avoid catalytic oxidation and damage to the membrane elements. The concentration of hydrogen peroxide should not exceed 0.2%, and the temperature should not exceed 25℃. Hydrogen peroxide cannot be used as a protective liquid for long-term shutdown of the membrane.

When the disinfected system is a system for producing drinking water or water for the food industry, a disinfectant that meets the production standards of that industry must be selected.