Key Chemicals for Cleaning Reverse Osmosis (RO) Membranes and How to Use Them

Reverse osmosis (RO) membranes are the heart of desalination and water treatment systems. The efficiency of the entire system depends on the integrity and permeability of these membranes. Over time, the membranes are subject to natural contamination processes resulting from the accumulation of salts, minerals, and organic matter. Therefore, understanding the mechanisms of chemical RO membrane cleaning and preventative maintenance is essential for every system operator. This guide aims to highlight the most important chemicals used. The technical parameters that necessitate intervention, and best practices for membrane storage and maintenance to ensure sustained performance and minimize operating costs.

Indications for Cleaning

Cleaning RO membranes is essential for maintaining system efficiency and extending membrane life. The cleaning process should be initiated when the operator observes a 10% to 15% decrease in the product water flow (after correcting standard conditions) or when a 10% to 15% increase in the differential pressure between the feed water and the concentrated water is observed. A decrease in the quality of the produced water and an increase in its salinity are strong indicators of sludge buildup requiring chemical intervention.

Steps of the Chemical Cleaning Process

The cleaning process begins by preparing a cleaning solution appropriate for the type of accumulated sludge. The solution is then pumped at low pressure and a high flow rate to dislodge suspended contaminants. Next, the solution is circulated within the system for a specific period to ensure the chemicals react with and break down the sludge. In cases of severe contamination, it is recommended to soak the membranes in the solution for a longer period to achieve optimal results. The process concludes with a thorough flush of the system using high-quality water (such as product water) to remove any remaining detergent before resuming normal operation.

Types of Sludge and Materials Used

The chemicals used vary depending on the nature of the contaminants. Acidic agents are used to remove mineral scale, such as calcium carbonate, while alkaline detergents are used to remove organic matter, oils, and biological growth (bacteria). It is crucial to follow the manufacturer’s instructions regarding pH and temperature during cleaning to avoid damaging the delicate membrane structure.

Preventive Maintenance Guidelines

Membrane protection involves more than just periodic cleaning; it also includes ongoing preventive maintenance. The efficiency of pretreatment units, such as sand and carbon filters, must be ensured to guarantee relatively clean water enters the RO system. Regular use of antiscalants is recommended to prevent scale buildup, along with regular flushing to remove suspended solids before they adhere to the membrane surface.

The Importance of Timing and Accuracy

Delaying membrane cleaning when signs of contamination appear can lead to irreversible contamination. In this case, the membrane pores become completely clogged, rendering chemical cleaning ineffective and forcing the user to replace the entire membrane at a costly expense. Therefore, careful monitoring and recording of daily operating data is key to determining the optimal cleaning time and ensuring the system’s continued operation at peak efficiency.

Cleaning Standards

Microorganisms, colloidal solids, and other impurities cannot be completely removed from water during the pretreatment process. Consequently, reverse osmosis membranes become contaminated over time, leading to reduced performance of the membrane elements. Clean the membrane elements when the following cleaning criteria are met:

• Filter flow rate decreases by 10% since the last cleaning;
• Salt throughput increases by 5%;
• Pressure drop between the feed and concentrate solutions increases by 15% before the reverse osmosis system shuts down.
• Membrane cleaning is also part of the daily maintenance of the reverse osmosis system.

Contamination

Visual Inspection

• Check the integrity of the membrane element’s outer casing.
• Check for sediment or other foreign material adhering to the membrane’s outer casing at the feed end.
• Check for large particles, activated carbon granules, or other objects on the membrane’s outer casing at the feed end.
• Check if the feed tray or filter tray at the concentrate end has been washed or if there is any sediment present.

Weight Comparison

Remove the membrane element and place it vertically. After draining the water, weigh the membrane element and compare its weight to that of an unused membrane element to determine the type of contaminants present.

Read also: Technical Criteria for Selecting Reverse Osmosis (RO) Membranes in Industrial Applications

Membrane Cleaning

Inorganic Salt Deposits

This problem manifests as a significant decrease in the membrane’s ability to expel salts. Along with an increase in the pressure difference between the feed water and the concentrated water, and a slight decrease in system throughput. To address this issue, a 2% citric acid solution at 40°C with a pH adjusted to 4.0 using ammonia is used. Alternatively, a 0.5% hydrochloric acid solution (pH 2-3) can be used to clean the membranes.

Metal Oxides and Hydroxides (such as Iron, Nickel, and Copper)

These contaminants cause a rapid and sharp decrease in salt expulsion and overall system throughput, along with a rapid and significant increase in the pressure difference. The solution is to use the same procedure as for inorganic salt precipitation: a 2% citric acid solution (pH 4.0) at 40°C, or a 0.5% hydrochloric acid solution.

Mixed colloids (iron, organic matter, and silicates)

This problem manifests as a slight, gradual decrease in salt ejection and system pressure. Along with a gradual increase in pressure differential. To address this, a 2% sodium tripolyphosphate (STPP) solution is used at 40°C. The pH was adjusted to 10.0 using sulfuric acid. Sometimes. A sodium hydroxide (NaOH) solution is used, provided the pH is below 10.

Calcium sulfate deposits

These deposits lead to a significant decrease in salt displacement. With a slight to moderate increase in differential pressure and a slight decrease in productivity. The solution is to use a 0.8% EDTA solution. The option of adding a 0.25% sodium dodecylbenzenesulfonate solution if needed. The pH should be adjusted to 10.0 using a 2% STPP solution at 40°C, or a NaOH solution (at a pH below 10).

Organic deposits

These deposits may cause a decrease in salt displacement, leading to a gradual increase in differential pressure and a gradual decrease in system productivity. It is treated using a 0.8% EDTA solution (with the addition of 0.25% sodium dodecylbenzenesulfonate if necessary). The pH is adjusted to 10.0 using a 2% STPP solution at 40°C.

Bacterial and Biological Contamination

This type of contamination leads to a very significant increase in pressure differential and a sharp decrease in system throughput, potentially affecting the quality of salt removal. To address this issue, a 0.8% EDTA solution and a 2% STPP solution are used at 40°C (pH 10.0). Or a 0.1% sodium hydroxide (NaOH) solution is used. Adjusting the pH to 11.5 with a 0.03% sodium dodecylbenzenesulfonate solution.

See also: Membrane Performance Measurements in Reverse Osmosis Systems

Precautions

Membrane element cleaning must be performed by qualified technicians. Chemical cleaning is also an option. Appropriate protective measures must be taken to prevent chemicals from harming workers. Such as wearing safety goggles, gloves, and work clothes. This is especially important when using highly corrosive chemicals. In case of accidental contact, please dispose of them according to the Material Safety Data Sheets (MSDS) provided by the chemical suppliers. The use of any chemical that could damage the membrane elements during cleaning is prohibited.

Reverse Osmosis Membrane Maintenance

Pre-Installation Storage

Upon leaving the factory, the membrane elements must be placed in a sealed bag containing a storage solution. During use and transport, the elements must be stored in a cool place. Dry place at a temperature between 20°C and 35°C. The elements must be protected from direct sunlight, and contact with oxidizing gases must be avoided.

Post-Installation Storage

Short-Term Storage

This type of storage is suitable for periods when the reverse osmosis system must be shut down for more than 5 days but less than 30 days. That the reverse osmosis elements remain in place. Store the components as follows:

• Rinse the reverse osmosis system with feed water, simultaneously venting any residual gases from the system;
• Operate the system for one to two hours daily. Close the relevant valves to prevent gases from entering the system.

Long-term storage

This storage method is suitable for situations where the reverse osmosis system must remain out of service for more than 30 days while the reverse osmosis components remain in place. Store the components as follows:

• Rinse the reverse osmosis system with feed water, simultaneously venting any residual gases from the system;
• Fill the reverse osmosis system with a biocidal solution prepared from filtered water and 1% sodium hydrogen sulfite, ensuring it is completely filled.
• Repeat step 2 with a fresh solution every 30 days if the temperature is below 27°C, or every 15 days if the temperature is above 27°C;

When the reverse osmosis system is ready to be restarted, flush it for one hour with low-pressure feed water. Before restarting the reverse osmosis system, check for any biocidal residue in the filtered water.