Details of chemical cleaning of reverse osmosis membrane elements

Analysis and Judgment of Reverse Osmosis Membrane Fouling

In order to improve the cleaning effect, it is necessary to analyze the membrane fouling condition before cleaning to determine the type of pollutants and select targeted cleaning agents according to the specific situation of reverse osmosis membrane fouling and scaling.

1. Analysis Methods

(1) Analyze equipment performance data.

(2) Analyze potential pollutants and scaling components in the feed water.

(3) Analyze the pollutants collected by the membrane filter of the SDI instrument.

(4) Analyze the pollutants of the filter cartridge filter.

(5) Check the condition of the inner surface of the pipeline and both ends of the membrane element.

(6) If necessary, dissect the membrane element for analysis to find pollutants and scaling components.

2. Analysis Methods

In case of severe membrane fouling, the causes can be found and the fault location can be determined by dyeing test, microscopic analysis, Fourier transform infrared spectroscopy (FTIR) analysis, scanning electron microscopy (SEM) analysis, energy dispersive X-ray (EDX) analysis, etc. Some of the above analysis and identification methods require sacrificing the membrane element, while others require specialized instruments and equipment, which are expensive. In practical applications, the following simple and easy-to-use analysis methods are often used.

(1) Visual Inspection

When it is determined that the system has been fouled and chemical cleaning needs to be implemented, it is best to open the pressure vessel end plate first and directly observe the accumulation of pollutants in the gap between the pressure vessel end plate and the membrane element. Generally, the type of pollutant can be basically determined by direct observation, and then the corresponding cleaning scheme can be determined.

A. Observation of Front-End Fouling

Pretreatment filter material (sand, activated carbon) leakage, colloidal fouling, organic fouling, and biological fouling are most serious at the front end, and particulate matter and mucus-like fouling can be observed from the inlet of the front-end membrane element. When biological fouling occurs, a fishy smell and mucus-like substance will be found. Burning the scraped biological sludge (mucus) will produce a burnt smell of protein.

B. Observation of End-Point Fouling

Inorganic salt scaling is most serious at the end of the system, and rough powdery substances can be felt at the end of the end membrane element. When dissolved with hydrochloric acid (pH 3~4), gas is emitted, indicating that the precipitate is most likely CaCO3. Sulfate scale and silica scale are difficult to dissolve even at very low pH. If the scale is soluble in 0.1 mol/L HF solution, it may be silica scale.

(2) Weighing

Pollutants adhere to the inlet flow channel of the fouled membrane element, increasing its overall weight. After removing the membrane element, place it upright, drain the water, and weigh it. Compare it with the reference weight of the membrane element. The extra weight is the weight of the attached pollutants.

(3) Judgment Based on Reference Pollution Characteristics

In addition, the analysis and judgment can also be based on the pollution characteristics of reverse osmosis membrane pollutants.

Chemical Cleaning of Reverse Osmosis Membranes

1. Cleaning Conditions

The membrane element needs to be cleaned in time when the following situations occur:

(1) Standardized water production decreases by 10%;

(2) The pressure difference between the feed water and the concentrate water increases by 15%;

(3) The standardized salt passage rate increases by 10%~15%;

(4) The pressure difference in each section of the system increases significantly.

Generally, the operation record obtained in the first 48 hours of equipment operation is used as the standardized comparison data.

2. Cleaning Equipment

(1) Cleaning Tank

It plays a mixing and circulation role. It requires corrosion resistance. The materials can be selected from fiberglass reinforced plastics, polyvinyl chloride plastics, or steel tanks lined with rubber, etc. The cleaning water tank should be equipped with a thermometer. The size of the cleaning tank is roughly the sum of the volume of the empty pressure vessel and the volume of the cleaning liquid circulation pipeline. Generally, a 20% margin should be considered.

(2) Cleaning Pump

It should be corrosion-resistant, such as a fiberglass reinforced plastics pump. The pressure it provides should be able to overcome the pressure drop of the security filter, the pressure drop of the membrane module, and the pipeline resistance loss, etc. Generally, a pressure of 0.3~0.5 MPa is selected. The material of the water pump must be at least 316 stainless steel or non-metallic polyester composite material.

(3) Others

The cleaning system should be equipped with necessary valves, flow meters, and pressure gauges to control the cleaning flow rate. The connecting pipelines can be either hard pipes or soft pipes, and should be resistant to acid and alkali corrosion.

3. Cleaning Agents

The chemical cleaning methods for membrane separation systems mainly include acid methods and alkali methods. The specific cleaning method depends on the nature of the pollutants in the membrane system.

4. Cleaning Steps

(1) Cleaning a Single-Stage System

A. System Flushing

Flush the reverse osmosis membrane components and system pipelines with reverse osmosis product water (pre-treated water or filtered water that meets the reverse osmosis feed water standard can also be used).

B. Prepare Cleaning Solution

Cleaning solution is generally prepared using reverse osmosis product water. The cleaning agent should be fully dissolved and mixed evenly, then adjust the pH (generally use ammonia water to adjust for acid washing, and hydrochloric acid for alkali washing), and repeatedly confirm whether the cleaning temperature is appropriate.

C. Low Flow Rate Input of Cleaning Solution

The cleaning solution should displace the original water within the components at the lowest possible pressure, only needing enough pressure to compensate for the pressure loss from feed water to concentrate water, i.e., the pressure must be low enough to avoid significant permeate production. Low-pressure displacement minimizes the redeposition of fouling on the membrane surface. Depending on the situation, part of the cleaning solution that initially returns should be discharged to prevent dilution of the cleaning solution.

D. Circulation

After the original water is displaced, circulate the cleaning solution back to the cleaning water tank and maintain a constant cleaning solution temperature. The circulation time is generally 60 minutes. Treat the returned cleaning solution as needed. If the returned cleaning solution is significantly discolored or turbid, it should be discharged and the cleaning solution should be re-prepared; if the pH of the returned cleaning solution changes by more than 0.5, it is best to adjust the pH, and replace the cleaning solution if necessary.

E. Soaking

Stop the cleaning pump and allow the membrane elements to fully soak in the cleaning solution. Generally, soaking for 1 hour is sufficient; however, for stubborn contaminants, the soaking time needs to be extended to 10-15 hours or overnight.

F. High Flow Rate Circulation

High flow rate circulation for 30-60 minutes. High flow rate can flush away the contaminants cleaned by the cleaning solution. Under high flow rate conditions, excessive pressure drop may occur. The maximum allowable pressure drop for a single unit is 0.1 MPa, and the maximum allowable pressure drop for a multi-unit pressure vessel is 0.35 MPa, whichever is exceeded first.

G. Rinsing

Rinse the cleaning solution in the system with reverse osmosis product water or qualified pre-treated effluent. The minimum rinsing temperature is 20℃, and the rinsing time is approximately 1 hour.

(2) Cleaning Multi-Stage Systems

For multi-stage reverse osmosis systems, cleaning should generally be performed section by section, with the cleaning water flow direction being the same as the operating direction. Generally, the cleaning solution does not need to be drained into the ditch, and can be directly circulated; when the pollution is relatively minor, multiple sections can be cleaned together; when the components are severely polluted, the cleaning solution can be drained into the ditch for the first few minutes, and then circulated. The circulation time for cleaning each section is usually 1.5 hours, and should be extended in case of severe pollution. After cleaning, rinse the reverse osmosis system with reverse osmosis effluent for no less than 30 minutes. When the membrane is severely fouled, the solution used to clean the first section should not be used to clean the second section, and a new cleaning solution should be prepared. To improve the cleaning effect, the cleaning solution can be allowed to soak the membrane elements, but the time should not exceed 24 hours.

Cleaning Key Points

(1) High Flow Rate

The cleaning flow rate should be higher than the normal operating flow rate, generally 1.2 times the normal operating concentrate flow rate.

(2) Low Pressure

The cleaning pressure should be as low as possible, and it is recommended to control it below 0.3 MPa. If it is difficult to meet the flow rate requirements below 0.3 MPa, the feed water pressure should be controlled as much as possible, with no permeate production as the standard. The feed water pressure should generally not exceed 0.4 MPa.

Precautions During Cleaning

(1) Cautious Selection of Cleaning Agents

The cleaning agents used should be carefully selected, and the instructions and process conditions for the agents should be strictly followed during cleaning. The temperature and pH changes of the cleaning solution should be carefully observed during cleaning. Before cleaning large systems, it is recommended to remove one membrane element from the system to be cleaned and conduct a single membrane element cleaning effect test to confirm the cleaning effect before implementing the cleaning of the entire system.

(2) Cleanliness of the Cleaning Solution

The cleanliness of the cleaning solution directly affects the cleaning effect. Cleaning solutions containing a large amount of impurities and particulate matter will cause secondary pollution to the membrane system and cause certain mechanical damage to the membrane surface. The amount of small molecule impurities in the cleaning solution used for backwashing should be as small as possible. The ideal cleaning solvent is the product water of the membrane system, but qualified pre-treated effluent can also be used to prepare the cleaning solution.

(3) Flow Direction of the Cleaning Solution

Generally, the flow direction of the cleaning solution should be the same as the water flow direction during normal operation of the system to prevent the occurrence of the "telescope" phenomenon in the components, because the thrust ring in the pressure vessel is only installed at the concentrate end of the pressure vessel. Reverse cleaning is only applicable to components with severe blockage at the feed end, and pressure cannot be applied in the reverse direction on the permeate side of the membrane to avoid mechanical damage to the membrane itself. Here, reverse cleaning refers to pumping the cleaning solution into the concentrate end of the membrane component and performing internal circulation in the component on the membrane outer side, allowing the cleaning solution to flow through the membrane surface, forming a certain scouring force on the membrane surface at an appropriate flow rate to remove and discharge the contaminants inside the system and on the membrane surface. In the membrane cleaning (the reverse osmosis membrane component is composed of a three-stage, single-stage structure, and the arrangement is 10:5:3) that the author participated in, the first membrane in the first stage is usually reversed and placed at the end of the first stage (note that the concentrate seal ring is also moved to the other end accordingly), and then cleaned according to the normal cleaning procedure.

(4) Temperature of the Cleaning Solution

The temperature of the cleaning solution has a significant impact on the cleaning effect. Within a reasonable temperature range, increasing the cleaning solution temperature as much as possible can more effectively restore the original performance of the membrane. This is because: higher cleaning temperatures can increase the solubility and washing ability of the cleaning solution; in addition, cleaning solutions above the normal operating temperature help expand the membrane micropores, promoting the discharge of contaminants within the micropores.

(5) Cleaning Sequence

Chemical cleaning of membrane separation systems mostly uses an alternating acid and alkali method, but it is important to note that acid and alkali cleaning cannot be performed consecutively. A reasonable cleaning sequence is: acid washing → water washing to neutral → alkali washing → water washing to neutral or alkali washing → water washing to neutral → acid washing → water washing to neutral.