As an advanced technology, the reverse osmosis membrane method can remove inorganic ions, bacteria, viruses, organic matter and colloids from raw water to produce high-quality pure water. It is widely applied in brackish water desalination, drinking water treatment, high-hardness water sources, power plants, electronics, pharmaceuticals, metal processing pure water, food, beverages, bottled pure water, hospitals, laboratory dedicated water and many other fields. According to different industrial requirements for water conductivity and resistivity, reverse osmosis equipment on the market is divided into single-stage reverse osmosis and double-stage reverse osmosis. What are the differences between the two?
Single-stage reverse osmosis means raw water passes through the pretreatment system and precision filter, then is delivered to the reverse osmosis unit by a high-pressure pump. It is mainly used in scenarios with good raw water quality or relatively low requirements for effluent quality.
Different from single-stage reverse osmosis, in double-stage reverse osmosis, raw water is pretreated first. The water produced by the first-stage reverse osmosis flows into an intermediate water tank, then is pumped into the second-stage reverse osmosis for secondary filtration. Therefore, the effluent quality of double-stage reverse osmosis is generally better than that of single-stage type.
Differences in Application Fields
Single-stage reverse osmosis equipment is widely used in: food industry — formula water and production water; beverage industry — formula water, production water and washing water; breeding and planting industry — water for animals and plants.
Double-stage reverse osmosis equipment has broader application scope. Besides food, beverage, breeding and planting industries, it is also suitable for more fields:
Power industry — boiler make-up water and cooling water;
Electronics industry — ultrapure water for semiconductors, IC cleaning water and formula water;
Pharmaceutical industry — process water, preparation water, washing water, water for injection and sterile water production;
Chemical industry — production water and cosmetics manufacturing water;
Drinking water projects — ultrapure water preparation and drinking water purification;
Petrochemical industry — oilfield injection water and advanced treatment of petrochemical wastewater;
Seawater desalination — domestic and production water for islands, coastal water-scarce areas, ships and offshore oilfields;
Environmental protection field — recovery of precious metals and water from electroplating rinsing water to achieve zero or low wastewater discharge.
Differences in Operating System
A single-stage reverse osmosis unit adopts membrane separation technology to remove impurities from water. Double-stage reverse osmosis filters raw water through two reverse osmosis processes to achieve a higher salt rejection rate. Generally, single-stage reverse osmosis is adopted when industrial water quality requirements are moderate, while double-stage reverse osmosis is selected for strict water quality standards.
Differences in Process Flow
Single-stage reverse osmosis: Raw water goes through pretreatment and precision filtration, then is pressurized by a high-pressure pump and sent directly to the reverse osmosis system to produce finished water, also known as one-pass reverse osmosis.
Double-stage reverse osmosis: The product water from the first-stage reverse osmosis enters an intermediate tank, then is pressurized by a secondary high-pressure pump into the second-stage reverse osmosis system for further purification.
Differences in Investment Cost
Due to different process designs and configuration standards, the two types have different prices. The cost of double-stage reverse osmosis is higher than single-stage type. To achieve better effluent quality, double-stage equipment requires upgraded accessories and more supporting components, resulting in higher overall investment. The larger the equipment capacity, the greater the cost gap.
