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 feed water and concentrate water increases by 15%;

(3) Standardized salt passage 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, requires corrosion resistance, and the material can be selected from fiberglass reinforced plastic, polyvinyl chloride plastic, 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, and generally 20% margin should be considered.

(2) Cleaning Pump

It should be corrosion-resistant, such as a fiberglass reinforced plastic 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 selecting a pressure of 0.3~0.5 MPa. 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 of 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

Rinse 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 using ammonia water for acid washing and hydrochloric acid for alkali washing), and repeatedly confirm whether the cleaning temperature is appropriate.

C. Low Flow Rate Cleaning Solution Input

The cleaning solution should displace the original water within the components at the lowest possible pressure, which only needs to be sufficient 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 a new cleaning solution 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 components 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 water. 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 sewer, 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 sewer 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 permeate, for no less than 30 minutes. When membrane fouling is severe, 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 components, 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 the flow rate requirement is difficult to achieve 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) Careful Selection of Cleaning Agents

The cleaning agents used should be carefully selected, and the instructions and process conditions for the use of the chemicals should be strictly followed during cleaning, and the temperature and pH changes of the cleaning solution during cleaning should be carefully observed. Before cleaning large systems, it is recommended to remove one membrane component from the system to be cleaned and conduct a single membrane component 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 membrane system's permeate, but qualified pre-treated water can also be used to prepare the cleaning solution.

(3) Flow Direction of the Cleaning Solution

Generally, the flow direction of the cleaning solution is required to be the same as the water flow direction during normal operation of the system, to prevent the components from exhibiting a "telescope" phenomenon, 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, performing internal circulation in the component on the outside of the membrane, allowing the cleaning solution to flow through the membrane surface, and 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, first-level system, arranged as 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 at temperatures higher than 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.