Preliminary water treatment for optimal use of membrane in reverse osmosis

Water pretreatment by reverse osmosis

Pretreatment is performed to optimize the use of membranes and increase the operating time of the membrane, and during this operation, the water is prepared to pass through the membrane. These operations include:

a) Separation of suspended solid particles in the incoming water:

Suspended and colloidal substances in the water cause membrane clogging. Membrane clogging is determined by the clogging index, and it is necessary that the clogging index of water passing through a specific membrane is below the permissible limit set for the membrane.

For this purpose, water is passed through 5 to 10-micron cartridge filters.

b) Removal of microorganisms:

Microorganisms, such as bacteria, can cause membrane clogging. In some cases, enzymes secreted by microorganisms have even had a destructive effect on the membranes. For this purpose, the feed water must be disinfected by ultraviolet radiation or chlorine before passing through the membrane. If chlorine is used to prevent the useful life of the membranes from being reduced, excess chlorine must be removed. This is done by filtering the activated carbon bed and sulfites. Cellulose acetate membranes are highly susceptible to microorganisms.

c) Adjusting the pH of the water:

It is essential to adjust the pH of the water to increase the life of the membrane and prevent the precipitation of some salts. The useful life of the membranes is greatly reduced at inappropriate pH due to hydrolysis. The appropriate pH for polyamide membranes is between 4 and 11, while for cellulose acetate membranes it is between 4.5 and 6.5. Therefore, the operation of cellulose acetate membranes requires great care.

d) Temperature regulation:

To protect the membrane, the temperature of the feed water must be controlled at different times within the range of 20 to 25 degrees Celsius. The maximum permissible temperature for polyamide membranes is 35 degrees and for cellulose acetate membranes is 30 degrees Celsius. High temperatures at inappropriate pH increase the rate of membrane hydrolysis.

e) Control of fouling agents:

Oxidizing agents such as oxides and hydroxides of iron, manganese and silica can cause problems by fouling the membrane during the desalination process and thus reduce the efficiency of the membrane. Hexametaphosphate compound can be used to control these fouling agents.

Increase feed water pressure

After preliminary treatment, the feed water is compressed by a pump to have the potential to pass through the membrane. The maximum allowable pressure for polyamide membranes is 28 atmospheres and for cellulose acetate membranes is 60 atmospheres depending on the equipment used. A control valve is used to regulate the pressure. For waters with high T.D.S. that require high pressure, it is necessary to select a cellulose membrane.

Components of the Reverse Osmosis System

The membrane is the main component in the treatment, but in the reverse osmosis system, there are the following devices and equipment:

• Solution preparation tanks and phosphate acid injectors: These devices are used to prepare acid with the appropriate concentration and inject it into the feed water to adjust the pH of the water and to supply and inject phosphate into the water to prevent the precipitation of calcium and magnesium salts.
• Cartridge and activated carbon filters: These devices are used to increase the life of the membrane by introducing water free of suspended solids and removing residual chlorine from water disinfection.
• High-pressure pump: The pump is needed to neutralize the osmotic pressure, compensate for the pressure drop caused by the membrane and the equipment in the path.
• Frames (modules): Membranes are extremely sensitive and vulnerable. Due to the high pressure of the reverse osmosis system, frames are used. Frames are strong metal or polymer containers that house the membrane. The frames house the membrane surface in a low volume. The modules are responsible for conducting the fluid.

Features of a high-performance module

Improper module design reduces the performance of membranes. A properly designed module should have the following features.

• Providing the necessary mechanical strength
• Covering a high surface area of ​​the membrane
• Proper control of fluid movement
• Ease of physical and chemical cleaning of the membrane
• Ease of maintenance

Types of osmosis membranes

Industrial osmosis membranes are made of various polymer materials and are diverse in terms of geometric shape and materials used. The most important membranes are:

Tubular membranes

These membranes are mostly made of polycellulose acetate and are in the form of tubes with a diameter of 20 to 30 mm and a length of 200 to 250 cm. About 20 of these membranes are placed inside a steel frame.

Spiral-Wound membrane

This type of membrane is also made of a 100 micron thick polycellulose acetate compound. Two or three layers of these membranes, with a plastic mesh between them, are wrapped around a mesh polymer tube or mesh steel tube and the assembly is placed inside the frame.

Hollow fiber Membrane

These membranes are in the form of hollow plastic fibers made of polyamide. A bundle of these fibers, consisting of thousands of strands, is placed inside a frame. Feed water is passed over the membrane and the purified water seeps into the longitudinal cavity of the fibers. The collection of seeped water is directed outside the frame.

Washing the membrane of the reverse osmosis purification device

With preliminary water purification to increase the operating time of the membrane, after a while, colloidal particles, sediments, microbial contaminants and water-soluble salts eventually clog the membrane due to the passage of water over the membrane surface, which requires periodic cleaning of the reverse osmosis membrane (CIP). Abrizan Company has produced specialized membrane cleaning solutions and supplied them to industries to remove these contaminants.

Reverse osmosis precision instruments and measuring devices

Automatic control devices for pressure, temperature, pH, electrical conductivity, manometers and flow meters are other devices used in a reverse osmosis system.