I. Introduction to Reverse Osmosis Membrane
RO stands for Reverse Osmosis membrane. Normally, water flows from low concentration to high concentration. However, under pressure, the flow reverses-from high concentration to low concentration. This is known as the principle of reverse osmosis.
The pore size of RO membranes is about 0.0001 microns-around 1/5,000 the size of bacteria or viruses-so it allows only water molecules and some beneficial mineral ions to pass through. Other impurities and heavy metals are discharged through the wastewater outlet. RO membranes are widely used in seawater desalination and space waste water recycling, and are therefore often referred to as high-tech artificial kidneys.
RO membranes are synthetic semi-permeable membranes designed to mimic biological membranes. They're commonly made from polymer materials like cellulose acetate, aromatic polyhydrazide, or aromatic polyamide. The pores on the membrane surface typically range from 0.5 to 10 nm, and their permeability depends on the membrane's chemical structure. Some materials reject salts well but have slower water permeability, while others with more hydrophilic groups allow faster water flow.
1. Principle of Reverse Osmosis
To understand RO, we first need to know what "osmosis" is: when two solutions with different salt concentrations are separated by a semi-permeable membrane, water naturally flows from the side with lower salt concentration to the side with higher salt concentration. The salts themselves do not pass through. This process continues until equilibrium is reached and is driven by osmotic pressure.
If pressure greater than the osmotic pressure is applied to the high-salinity side, the water flow can be reversed-this is reverse osmosis. In this way, water molecules are forced through the membrane to the other side, leaving salts and impurities behind, effectively purifying the water.
2. Origin of RO Technology
In the 1950s, American scientist Dr. S. Sourirajan discovered a thin membrane in the body of a seagull, which could separate freshwater from seawater inside the bird's body using pressure. This concept laid the foundation for RO theory.
In 1953, the University of Florida applied this technology to desalination equipment. In 1960, the U.S. government funded research at UCLA Medical School, led by Dr. Sidney Lode and Dr. Sourirajan, to develop RO membranes for space programs. The aim was to reduce the need to carry large amounts of water into space. Over time, more researchers became involved, greatly advancing the quality and quantity of RO membrane technology and solving key water purification challenges for humanity.
II. Introduction to Ultrafiltration Membrane
Ultrafiltration (UF) membranes have uniform pore sizes ranging from 0.001 to 0.02 microns. Under pressure, these membranes filter out molecules larger than the pore size, effectively separating particles with molecular weights over 500 Daltons or sizes greater than 10 nm. UF membranes were one of the first polymer separation membranes developed and have been used industrially since the 1960s.
UF filtration relies on pressure differences to separate solutes and concentrate substances. These membranes are usually made from cellulose acetate or similar polymer materials. They are especially useful for separating colloidal suspensions that are difficult to handle with other methods. UF membrane applications continue to expand.
Pressure-driven membrane filtration includes three main types:
Microfiltration (MF): 0.02–10 μm
Ultrafiltration (UF): 0.001–0.02 μm
Reverse Osmosis (RO): 0.0001–0.001 μm
III. Features of RO Membranes
High desalination rate even at high flow rates
Strong mechanical strength and long service life
Effective performance under low operating pressure
Resistant to chemical and biochemical reactions
Minimal impact from pH, temperature, and other factors
Easy-to-source raw materials, simple manufacturing, and low cost
IV. Features of UF Membranes
No phase change during the UF process; stable operation at room temperature
Compact equipment design, small footprint, and easy to operate
Simple separation process with a high level of automation
Capable of separating substances based on molecular weight
Broad adaptability to different water qualities and a wide range of applications
V. Applications of RO Membranes
RO membranes are used across various industries, including:
Power generation
Petrochemical
Steel manufacturing
Electronics
Pharmaceuticals
Food and beverage
Municipal water treatment and environmental protection
They play key roles in:
Desalinating seawater and brackish water
Producing boiler feedwater
Creating industrial pure water and ultra-pure water for electronics
Making drinking water
Wastewater treatment
Specialized separation and purification processes
