An overview of membrane separation processes
Membrane separation is an intricate technology that separates materials through minuscule gaps or pores with the help of pressure. This process involves selective filtration and has a wide range of applications, especially in water purification.
Depending on the pore size and the pressure applied, membrane separation may be classified as the following:
- Microfiltration: Membranes used in microfiltration have the largest pore size measuring 0.1 to 10 μm. The pressure used for separation is usually on the lower side. Microfiltration is usually one of the first steps of filtration and it removes bacteria.
- Ultrafiltration: The pore size of the membrane used in ultrafiltration is 1 to 100nm. This size makes it a good option for membrane separation in many industries such as food & beverages, and the automotive industry.
- Nanofiltration: Nanofiltration makes use of both charge and size for separation. They have a smaller pore size of 0.1 to 10 nm. These membranes can retain uncharged solutes and particles of lower molecular weight. They can also retain charged molecules (polyvalent ions and large monovalent ions) except small monovalent ions. Nanofiltration is commonly used to remove hardness in water and wastewater management.
- Reverse Osmosis (RO): RO membranes are the smallest of them all in the membrane separation process. They can separate even monovalent ions with the utmost ease. But, RO is not just about size-based separation. It also includes diffusion and high pressure utilization for efficient filtration. RO is mainly used in the preparation of beverage concentrates and drinking water.
Purification and recovery of products or water
Preparation of drinking water involves the removal of various contaminants, some of which are at a nano scale. Multiple membranes are necessary to get rid of salts, particles, bacteria, viruses, and other contaminants. These include all the aforementioned processes. Each membrane is responsible for the separation of certain particles:
- Forward osmosis or osmosis: This is the most basic form of water purification, which relies on the principle of osmosis wherein water with a lower concentration of dissolved minerals moves to water with a high concentration and dilutes it. A semi-permeable membrane separates the 2 concentrations.
- Micro filtration and ultra filtration: These membranes remove suspended solids and work just like traditional media filters and sand. In the water purification process, they remove bacteria and a few larger viruses.
- Reverse osmosis: It uses a semi-permeable membrane and high pressure to force pure water through. It can retain dissolved solids, heavy metals, calcium, and some concentrating salts. This is why RO is also popularly used for desalination of water and purifying brackish water to make it potable.
- Nano filtration or membrane softening: While the mechanism of nano filtration is similar to RO, it uses a looser membrane that allows smaller salts to pass through. However, it can take care of the hardness of water and organic particles with very high efficiency.
Challenges of membrane filtration for drinking water
While all the processes explained here are efficient and reliable, there are a few downsides as well:
• Water wastage – A large amount of water is wasted in the typical water purification process
• Removal of essential minerals – Water, in its natural form, is full of healthy minerals that are removed during membrane separation
• Degradation over time – Membranes perform well only when they are maintained well, which may not be possible for the end consumer. Biofilm formation is a common problem in this regard, which leads to fouling, more energy usage, and lower effectiveness.
Permionics – An innovative solution
At Permionics, we are constantly researching and developing effective technologies to minimize the challenges and improve the efficacy of water treatment. Our membranes require less pre-treatment and minimize operational problems. We enable the recovery of at least 80 to 95% of the wastewater treated for effluents in various industries.
For more information, we invite you to visit our website or get in touch with us.