Water treatment is a crucial component of the water economy in every country. In recent years, there has been rapid development in membrane techniques that aid in the separation of pollutants at the molecular or ionic level. A membrane acts as a filter that retains undesirable components from the filtered mixture. Further testing and development of this technology have the potential to bring about a breakthrough and significantly enhance the quality of water in our water reservoirs.
What are the different membrane techniques available?
Microfiltration
Microfiltration is utilized in various processes, such as in the wine-making industry. Pall Corporation supplies microfiltration membranes that effectively remove solids and yeast while preserving the desired ingredients and taste. It separates particles with a diameter of 10–50 μm (micrometers) from the solvent and particles smaller than 10 micrometers in the solution. The process relies solely on a force mechanism. The microporous membranes used have pore diameters ranging from 10 μm to 50 μm. They are commonly employed in the industrial sector or laboratories to separate aqueous solutions of salts, sugars, or certain proteins. Microfiltration membranes are made of organic or inorganic polymers (ceramic products, glass, or metal) and are produced using methods such as phase inversion, stretching of polymer films, sintering, modeling, and bombardment of polymer films in a nuclear reactor.
Ultrafiltration
Ultrafiltration is employed for solutions concentration, compound fractioning, and clarification. Ultrafiltration membranes find applications in the food industry, textile industry, pharmaceutical industry, metallurgical industry, and environmental protection for water and sewage treatment and preparation of water for the reverse osmosis (RO) process. The “WaterHealth” program implemented by WaterHealth International utilizes ultrafiltration membranes to produce clean drinking water for rural communities in India and other developing countries.
This method involves the physical sieving of particles of dissolved or colloidal substances through membranes with an appropriate pore size (1–100 nanometers). Asymmetrical porous membranes have a thickness of approximately 150 micrometers, and the driving force of the membrane is pressure between 0.1–1 MPa.
Nanofiltratrion
Nanofiltration exploits the difference in pressure on both sides of the membrane, possessing properties between reverse osmosis and ultrafiltration. The membranes used in this technique are asymmetrically porous and made of polymers, approximately 150 micrometers thick, with a pore diameter of around 1 nanometer (nm).
This process aids in removing organic compounds with a molecular mass higher than 200–300 kDa (kilodalton) as well as salts. Nanofiltration finds application in groundwater and surface water treatment, lactose and whey protein recovery along with simultaneous desalination, removal of bacteria, calcium salts, sulphates, and magnesium from drinking water and process water. It is also utilized in the textile industry to reclaim dyes from dyeing processes. Nanofiltration is highly effective in removing colloidal compounds, micro-organisms, and turbidity, but its main drawback is its high cost.
Reverse osmosis
The first application of reverse osmosis was for seawater desalination. The process was introduced to the industry in the 1960s with the development of high-performance asymmetric membranes. Its functioning is based on the natural osmosis process and is used to separate low-molecular-weight compounds from the solvent. To ensure the correct progression of the entire process, higher transmembrane pressures are required compared to those used in ultrafiltration and microfiltration, as low-molecular-weight compounds have a higher osmotic pressure. Reverse osmosis is particularly employed for phosphoric acid recovery, softening boiler feedwater, concentrating solvent-containing sewage, and preparing ultrapure water.
In Singapore, there is a seawater desalination plant that utilizes reverse osmosis technology to produce clean drinking water, capable of producing 137,000 m3 of fresh drinking water per day.
Advantages and weaknesses of pressure membrane processes
The most significant advantage of using membranes is the reduced electricity consumption due to their ability to function at the molecular level. Another advantage is the full automation of the process and ease of operation. Membranes are resistant to high temperatures and can function across the full pH range. They are also easy to learn and operate, allowing employers to save money on hiring a qualified workforce. The primary weakness of this technology is fouling, which refers to the contamination of the membrane, resulting in increased electricity consumption, the need for more frequent cleaning, and a shortened service life of the membranes.
