Bacteria:
You can use chlorine sterilization, but the residual chlorine content can not exceed 0.1mg/L,otherwise the membrane element will be oxidized. The residual chlorine can be removed by sodium metasulfite or activated carbon. In the case of dissolved oxygen, 1kg chlorine needs 3kg sodium metasulfite. The sterilization effect is better when the water temperature is low and the pH is low.
Iron content:
The oxidation rate of iron depends on the iron content, the concentration of dissolved oxygen and pH value in water. The higher the pH value is, the faster the oxidation rate is. Therefore, reducing the pH value can prevent oxidation.
The relationship between the max allowable iron content and oxygen content in feed water and pH value is as follows:
| Oxygen content (mg/L) | PH value | Allowable iron content (mg/L) |
| ＜0.5 | ＜6.0 | 4.0 |
| 0.5-5 | 6-7 | 0.5 |
| 5-10 | ＞7 | 0.05 |
Particulate matter:
Particulate matter larger than 5um is not allowed to enter the high-pressure pump and reverse osmosis components. This must be ensured to avoid damaging the equipment.
SDI and turbidity:
SDI must be less than 5, the smaller the better, and turbidity should be less than 0.2NTU (the max allowable 1NTU).
Oil and grease:
no oil and grease is allowed in water.
Organic matter:
Organic matter in water has the most complex effect on RO membrane, activated carbon filtration can be used to reduce the concentration of organic matter.
SiO2:
it is not allowed to precipitate SiO2 in concentrated water. When it is supersaturated, it may polymerize to form insoluble colloidal silicon or silica gel, and cause scaling.
At 25 ℃, the solubility of amorphous silicon is 100 mg / L, which changes linearly with temperature. It is 0 mg / L at 0 ℃ and increases to 160 mg / L at 40 ℃. At neutral pH, only silica gel is dissolved. In alkaline solution, the solubility of amorphous silicon is higher than that of neutral solution, which is mainly due to the ionization of silicic acid. However, in the presence of aluminum, the solubility may decrease a lot, the reason is that the solubility of aluminum silicate is very small.
The SiO2 content can be reduced by controlling the system recovery rate.Use lime to soften or add more ferric chloride and sodium aluminate in the clarifying tank;It can also be controlled by temperature.
Anti scaling:
CaCO3, CaSO4, BaSO4 and CaF2 must be prevented from scaling
Generally, by reducing the system recovery rate, to avoid exceeding the product of solubility; using ion exchange softening to remove calcium ion; adding acid to remove carbonic acid or bicarbonate ion; adding scale inhibitor.
Water temperature:
Reverse osmosis membrane have certain requirements for water temperature, the inlet water temperature of composite membrane is generally controlled at 0-45 ℃.
Max inlet pressure:
Reverse osmosis membrane assembly has certain requirements for the max inlet pressure.
Max water inflow:
Reverse osmosis membrane has certain requirements for max water inflow, see the design manual for details.
Ratio of concentrated water and permeable water:
Considering the membrane fouling and other factors of the membrane, there are certain requirements for concentrated water and permeable water of each membrane.
