Membranes emerged as a visible means of water purification in the 1960s with the development of high-performance synthetic membranes. Advanced membranes are introduced for water treatment that made from new materials and employed in various configuration. The Membranes became a viable alternative to evaporation based technologies in the water treatment market. Over the years, purified water standards have become more stringent, and many new applications have appeared. However, membranes have risen to the challenge and continue to perform efficiently and effectively. RO membranes made their commercial debut when Gulf General Atomics and Aerojet General employed the Loeb-Sourirajan cellulose acetate (CA) membranes in spiral wound modules to purify water.
Types of RO Membrane
Types of RO membranes are discussed below:
1: Cellulose-Based Membranes
The initial leader in the RO membrane was CA membrane. The CA membranes were asymmetric and exhibited NaCI rejection values of approximately 99.5% using a feed solution of 52,500 mg/L NaCI and flux values from 5 to 11 gallons per square foot per day. CA membranes were made from acetylated cellulose. Cellulose is a naturally occurring polymer found in plants such as cotton. It is a rod-like material that is not flexible that renders CA membranes mechanically robust. CA membranes provide several advantages to RO than other membranes which are available in the market. They are easy to make and relatively excellent mechanical properties. They are resistant to attack by chlorine.
2: Thin Film Composite Membranes
Cellulose acetate membranes were the dominant choice for RO membranes until the advent of thin film composite (TFC) RO membranes in 1972. Most TFC membranes are made with a porous, highly permeable support such as polysulfone, which is coated with a cross linked aromatic polyamide thin film. The coating provides the salt rejection properties of the membrane. TFC membranes offer more advantages over CA membranes. For this, they can also reject some low molecular weight organics. They are also stable over a larger PH range and at higher temperatures than CA. The one drawback of TFC membranes is their sensitivity to chlorine. They are highly risk-able that can be attacked by chlorine.
3: Membrane Fouling
A significant problem that was commonly found in CA and TFC membranes are fouling. In general, fouling occurs either on the surface of a membrane or within its pores that causes a decrease in flux. There are four major types of fouling named as: biofouling, scaling, organic and colloidal. Fouling can be controlled to some extent by adding disinfectants, anti-scaling agents, and other pre-treatment steps. However, these are not remedies to the problem, and fouling remains a key area in definite need of improvement of RO membranes.
Difference between MF and UF membranes
Five major differences between Micro Filters and Ultra Filters membranes are:
1: Separation scale
UF membrane pores are smaller as compared to the MF membrane. The smaller (tighter) MF membrane pore sizes occasionally overlap the larger (more open) UF pores. However, UF pores are generally smaller than MF pores.
2: Separation mechanism
UF membranes have a skin layer on the structure of membrane surface. This layer contains the pores that affect the rated separation or rejection of substances larger than the membrane pores. This creates a concentration polarization at, or near, the membrane surface. With MF membranes, however, the concentration polarization phenomenon does not occur as the smaller substances permeate the membrane.
3: Operating pressure
Microfilters are usually operated below 200 kpa. Ultrafilters operate up to 300 kpa.
4: Thickness and Strength of membrane
UF membranes have a wall thickness. The wall thickness of MF hollow fiber membrane manufactured with elongation process but have similar strength properties as UF hollow fibers.
5: Surface Features
MF pores are clearly discernible, while the pores on the UF surface are not.
What kind of membrane is used in reverse osmosis?
TFC membranes are commonly classified as nanofiltration (NF) and reverse osmosis (RO) membranes. Both are made up of a thin polyamide layer deposited on top of a polyethersulfone or polysulfone on top of a nonwoven fabric support sheet. The three-layer configuration gives the desired properties of high rejection and is chosen primarily for its permeability to water and relative impermeability to various dissolved impurities including salt ions and other small, unfilterable molecules.
A filtration membrane’s performance is rated by selectivity, chemical resistance, operational pressure differential, and the pure water flow rate per unit area. Due to emphasis on flow rate, a membrane is manufactured as thin as possible. These thin layers introduce defects that may affect selectivity, so system design usually trades off the desired flow rate against both selectivity and operational pressure.
What is the RO membrane pore size?
The pore size of the membrane gives an indication of the median or mean size of the pores on a membrane surface. The pore size of a membrane can range from 1000 to 0.0001 microns, encompassing the four main types of membrane, microfiltration (MF), Ultrafiltration (UF), Nanofiltrations (NF) and Reverse Osmosis (RO). RO has a pore size range of 0.0001-0.001. It is by far the finest separation material available to industry. It is used on large scale desalination and purification of water as it filters out everything but water molecules, with pore sizes approaching the radius of some atoms in many cases. The pore size of RO indicates that it is the only membrane that can reliably filter out salt and metallic ions from water.