Understanding Chemical Cleaning of Reverse Osmosis Membrane in a Single Article

Why do we need chemical cleaning?

There are various substances in the feedwater of water treatment systems that can cause contamination of the surface of reverse osmosis membranes, such as hydrated metal oxides, calcium precipitates, organic matter and microorganisms. After a long period of operation or insufficient pre-treatment, fouling may form on the membrane surface, which can affect the system's production capacity and even cause irreversible damage to the membrane elements. Therefore, chemical cleaning of reverse osmosis membranes is an indispensable part of the production process.

What factors can cause fouling in membrane systems?

• Imperfect or malfunctioning pre-treatment system

• Inappropriate selection of system materials (such as pumps and pipelines)

• Malfunctioning pre-treatment dosing system

• Inadequate or insufficient flushing after system shutdown

• Long-term accumulation of membrane surface deposits (such as barium and silicon scale)

• Changes in feedwater composition or other conditions

• Microbial contamination of feedwater

• Non-compliant quality of supporting agents

Fouling on the membrane surface will accelerate the decline in system performance, such as reduced water production flow and lowered salt rejection rate. Another negative effect of fouling is an increase in the standardized pressure difference between the feedwater and the concentrate. As long as effective measures are taken in a timely manner, the performance of the membrane system can be effectively restored to its maximum. However, if cleaning is delayed for too long, it will be difficult to completely remove pollutants from the membrane surface.

For specific pollutants, only by adopting corresponding cleaning methods and chemicals can good results be achieved. Choosing the wrong cleaning chemicals and methods can sometimes worsen membrane system pollution.

How to determine when cleaning is necessary?

In normal operation, the membrane sheets inside the reverse osmosis element can be contaminated by inorganic salt scale, microorganisms, colloidal particles and insoluble organic matter. During operation, these pollutants deposit on the surface of the membrane, causing a decrease in the standardized water production flow and the system's salt rejection rate, or both.

When any of the following conditions occur, the membrane element needs to be cleaned:

• A decrease in water production by more than 10%

• A 15% increase in the standardized pressure difference between the feedwater and the concentrate

• A 5% increase in the standardized salt rejection rate

The above standards (criteria) for comparison are derived from the performance of the system after the initial 48 hours of operation.

Chemicals for cleaning reverse osmosis membranes

The chemical cleaning of membrane systems is mainly divided into acid cleaning and alkaline cleaning. Acid cleaning agents are used to remove inorganic pollutants including iron pollution, while alkaline cleaning agents are used to clean organic pollutants including microorganisms. When preparing cleaning agents, it is best to use the system's produced water to prepare the cleaning solution. Of course, in many cases, qualified pre-treated effluent can also be used to prepare the cleaning solution. The raw water may have a large buffering capacity, and more cleaning agents may need to be consumed to achieve the specified pH values. The pH of acid cleaning is about 2, while for alkaline cleaning, the pH is about 12.

The process of cleaning reverse osmosis membranes

Cleaning a single-stage system

The membrane elements should be cleaned using the following six steps:

Prepare cleaning solution→input cleaning solution at low flow rate→circulate→soak→circulate at high flow rate→flush

During acid cleaning, the change in pH value of the cleaning solution should be checked at any time. When the acid dissolves inorganic salt deposits, if the increase in pH exceeds 0.5 units, acid should be added to the cleaning box. The total circulation time of the acid cleaning solution should not exceed 20 minutes. After this time, the cleaning solution may be saturated with inorganic salts that have been washed out and the pollutants may precipitate on the membrane surface again. At this time, the first cleaning solution in the membrane system and the cleaning system should be drained using qualified pre-treated effluent, and the second acid cleaning operation should be performed using a new cleaning solution. If the system must be shut down for more than 24 hours, the elements should be stored in a 1% (by weight) solution of sodium bisulfite. Before cleaning a large-scale system, it is advisable to remove a membrane element from the system to evaluate the cleaning effect of a single element.

Cleaning a multi-stage system

In the rinse and soak steps of a multi-stage system, all stages of the system can be cleaned simultaneously. However, for high-flow circulation, it must be done in stages to ensure that the circulation flow rate is not too low for the first stage or too high for the last stage. This can be achieved by using one pump to clean each stage separately or by setting different cleaning pumps for each stage's flow rate requirements. The cleaning process is the same as for a single-stage system.

Generally, the system needs to alternate between acid and alkali cleaning and cannot be performed simultaneously. The suggested cleaning sequence is:

Acid cleaning→Flush to neutral→Alkaline cleaning→Flush to neutral→Acid cleaning→Flush to neutral

Safety precautions for cleaning

• When using any cleaning chemicals, you must follow approved safety procedures. For details on the safety, usage and discharge disposal of chemicals, consult the chemical manufacturer.

• When preparing the cleaning solution, make sure that all cleaning chemicals are well dissolved and mixed before entering the element cycle.

• After cleaning the chemical agents and membrane components in circulation, high-quality water without residual chlorine and other oxidizing agents should be used to rinse the membrane components (minimum temperature > 20ºC). Produced water from the membrane system is recommended. If there is no corrosion problem with the pipeline, dechlorinated drinking water and pre-treated feed water can also be used. Before returning to normal operating pressure and flow rate, it is necessary to start flushing with a large amount of cleaning solution at low flow and pressure. In addition, the cleaning solution will also enter the produced water side during the cleaning process, so the produced water must be discharged for at least 10 minutes or until the produced water is clear after the system is started normally.

• During the cleaning solution circulation period, the temperature should not exceed 50ºC at pH 2~10, 35ºC at pH 1~11, and 30ºC at pH 1~12.

For components with a diameter greater than 6 inches, the flow direction of the cleaning solution must be the same as the normal operating direction to prevent the components from producing the "telescope" phenomenon, because the thrust ring inside the pressure vessel is only installed at the concentrate end of the pressure vessel.