Selecting the appropriate membrane for a water treatment plant
Selecting the appropriate membrane for a water treatment plant is a highly sensitive task. Choosing the right membrane for each application is vital, and experts can assist engineers in making the correct decision. Membranes are used in numerous applications, and at CareWater, we can provide membrane filtration units as a standard design, which requires minimal testing. In other cases, design engineers may need to comprehensively test the process on-site. We have extensive experience in water treatment systems at CareWater to provide solutions for filtration problems across a wide range of industrial sectors.
What is the nature of the water you want to filter?
The nature of the water is one of the key factors in determining the best membrane for use. Knowing the content of dissolved solids, the molecular weight of substances, and the nature and load of any suspended matter will guide the engineer toward the proper membrane configuration and engineering. pH and temperature of the feed stream are also important in making the final decision.
Decide on the type of filtration you need
The filtration spectrum starts at the molecular level with reverse osmosis (RO), providing the finest degree of separation, followed by nanofiltration, ultrafiltration, and microfiltration. Between these, these processes separate particles that vary in size from a few angstroms (10-10 meters) to a few microns (10-6 meters).
Internal pressures range from 30-80 bar in high-pressure systems to 1-5 bar in low-pressure microfiltration units. Filtration relies on this pressure to drive the liquid through a physical barrier (the membrane). This results in the separation of suspended and dissolved materials in the feed into the desired concentrated final permeate. Through proper configuration selection, particles of specific sizes can be isolated or allowed to pass through the membrane, depending on the membrane type.
Reverse osmosis uses a tight membrane that retains most dissolved species such as particles and salts. Pressure in these systems must exceed osmotic pressure to push the liquid fraction through the semi-permeable membrane. The food industry often uses this membrane system to treat fruit juices, tea, coffee, and sugar solutions. It is also widely used in waste stream treatment.
Nanofiltration acts as a bridge between reverse osmosis and ultrafiltration; often used in desalination and concentration duties in the textile industry. Ultrafiltration finds widespread industrial applications, suited to detail stream refinements from dye factories and pulp mills to clarifying fruit juices. Moving towards microporous filtration, ceramic membranes are often used to provide a wide range of pore sizes in the food and beverage, pharmaceutical, and chemical industries, as well as for wastewater separation.
Choose the membrane according to the materials
Membrane filtration technology has evolved both in membrane packing methods and in the type of materials used. The result is a wide range of unit configurations and membrane forms that suit a variety of applications. Polymer membranes constitute the majority of membranes currently in use. Several different polymers are used to match the required molecular weight cutoffs or to achieve the desired resistance to fouling or performance when in contact with a specific process liquid.
Common polymers include polysulfone and polyethersulfone, both used for a full range of ultrafiltration membrane applications. Polyvinylidene fluoride (PVDF) is often used for open ultrafiltration membranes. That polyamide serves as a thin film layer in nanofiltration and reverse osmosis membranes. Cellulose acetate is the first polymer widely used for membranes. Still finds use in some applications where superior fouling properties are exhibited. However, its use is limited due to its tendency to discolor in alkaline conditions. Membranes can be formed into configurations of tubular, spiral-wound, flat sheets, or hollow fibers.
