Reverse osmosis (RO) system is mainly designed to remove all kinds of solvent solids, colloids, and organics. How to select the right membrane element? These following facts should be taken into consideration, including salinity of feed water, rejection rate, good chemical stability, higher anti-pollution quality and good mechanical intensity.
According to the times of raw water passing through the RO membrane, RO device is classified into the 1st pass, 2nd pass and even multilevel pass RO device. 1st pass and 2nd pass RO are generally used, so what the difference between 1st and 2nd seawater RO plant?
1.Different technological process
1st pass RO: raw water? multimedia filter? security filter 5um?security filter 1um?1st pass HP pump?1st RO unit? fresh water tank? fresh water pump? UV sterilizer? water usage point
2nd pass RO: raw water? multimedia filter? security filter 5um?security filter 1um?1st pass HP pump?1st pass RO unit? middle tank? middle pump? security filter 1um?2nd pass HP pump?2nd pass RO unit? fresh water tank? fresh water pump? ultraviolet sterilizer? water usage point.
2. Same water quality requirement
| Water Quality Index of RO Inlet | |
| Item | Index |
| Temperature??? | 5~45 |
| pH | 2~11 |
| Turbidity?NTU) | <1 |
| SDI15 | <5 |
| Residual Chlorine?mg/L) | <0.1 |
| COD?mg/L) | <2 |
| Fe?mg/L) | <0.1 |
| P.S.: all index is for spiral polyamide membrane | |
The outlet from 1st pass RO is the inlet of 2nd pass RO, membrane inlet water quality requirements are the same.
3. Different system recovery rate
The percent of recovery is the percentage of the feed water which becomes product water. To find system recovery take the ratio of permeate flow to feed flow(Feed flow = Permeate flow + Concentrate flow).
For every RO system,
| Recovery rate | Tap water
(TDS?1000ppm) |
brackish water
(TDS?5000ppm) |
Seawater
(TDS?40000ppm) |
| 1st pass RO | ?75% | ?65% | ?40% |
| 2nd pass RO | ?80% | ?75% | ?75% |
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4. Different applications
RO technology is widely used in many applications, including electric power industry( boiler water); food & beverage industry (recipe water, production water and purified drinking water); pharmaceutical industry (process water, water for injection, medicine…); seawater desalination (marine, sea oil area, coastal water-deficient region, etc. ).
When water quality is not required very high, it is no problem to use 1st pass RO system, like agricultural irrigation, domestic water, recycled water reuse etc. When water quality is required very high, it is better to design 2nd pass RO system, for example, pharmaceutical and medical production process water, drinking water purification (bottled water), and food and drinking water are generally designed into multilevel RO system.
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