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Osmosis is a common phenomenon in nature. For example, if a cucumber is placed in salt water, it will shrink due to water loss. The process of water molecules from the cucumber entering the salt water solution is osmosis. As shown in Figure 1, if a pool of water is divided into two parts by a thin membrane that only water molecules can pass through, and pure water and salt water are poured into each side of the membrane to the same height, after a period of time, it can be observed that the level of pure water decreases while the level of salt water increases. We call the phenomenon of water molecules migrating through this membrane into the salt water osmosis. The rise in the salt water level is not endless; it will reach an equilibrium point at a certain height. The pressure represented by the difference in liquid levels at the two ends of the membrane is called osmotic pressure. The magnitude of osmotic pressure is directly related to the concentration of the salt water.
After the above device reaches equilibrium, if a certain pressure is applied to the surface of the salt water side, water molecules will migrate from the salt water side to the pure water side. This phenomenon of liquid molecules migrating from a dilute solution to a concentrated solution under pressure is called reverse osmosis. If brine is added to one end of the above-mentioned device and a pressure exceeding the osmotic pressure of the brine is applied to that end, we can obtain pure water at the other end. This is the principle of reverse osmosis water purification.
The key to producing pure water with reverse osmosis equipment lies in two things: a selective membrane, called a semi-permeable membrane, and a certain pressure. Simply put, the reverse osmosis semi-permeable membrane has numerous pores, the size of which is comparable to the size of water molecules. Since bacteria, viruses, most organic pollutants, and hydrated ions are much larger than water molecules, they cannot pass through the reverse osmosis semi-permeable membrane and are separated from the water that does pass through. Among the many impurities in water, dissolved salts are the most difficult to remove. Therefore, the water purification effect of reverse osmosis is often determined by the desalination rate. The desalination rate of reverse osmosis mainly depends on the selectivity of the reverse osmosis semi-permeable membrane. Currently, high-selectivity reverse osmosis membrane elements can achieve a desalination rate of up to 99.7%.
Advantages of Reverse Osmosis:
Continuous operation, stable product water quality
No need for acid/alkali regeneration
No downtime due to regeneration
Saves backwashing and cleaning water
Produces ultrapure water with high yield (up to 95%)
No regeneration wastewater, no wastewater treatment facilities required
No need for acid/alkali storage and dilution/transport facilities
Reduces workshop floor space
Safe and reliable operation, avoiding worker contact with acids/alkalis
Reduces operating and maintenance costs
Simple installation, low installation cost
Weaknesses and Solutions of Reverse Osmosis
The system desalination rate of reverse osmosis equipment is generally 98-99%. This desalination rate is sufficient in most cases. However, in the electronics industry, ultra-high pressure boiler feedwater, and some pharmaceutical industries, the requirements for pure water may be higher. In these cases, single-stage reverse osmosis equipment cannot meet the requirements. The following methods can be used to further purify reverse osmosis water to meet requirements:
1: Two-stage reverse osmosis: Perform a second reverse osmosis treatment on the single-stage reverse osmosis water to improve its purity.
2: Combining reverse osmosis with EDI: This allows for the production of ultrapure water with a smaller plant and lower operating costs.
3: Combining reverse osmosis with ion exchange: This reduces the required plant area and lowers operating costs.
