To extend the service life of reverse osmosis and nanofiltration membranes and improve their operating efficiency, effective pretreatment of raw water is necessary. By selecting appropriate pretreatment processes based on the raw water quality and system design requirements, it is possible to reduce membrane fouling, scaling, and membrane degradation, thereby significantly improving system efficiency and maximizing system operating benefits.
Fouling: The deposition and accumulation of organic matter or colloids on the membrane surface.
Scaling: Excessive local concentration of certain salts causes them to precipitate on the membrane surface, such as calcium carbonate, calcium sulfate, and barium sulfate.
Membrane degradation: The performance of membrane elements deteriorates due to contamination by certain substances.
Pretreatment must comprehensively consider the needs of continuous and reliable operation of the entire system. A suitable pretreatment scheme depends on the composition of the raw water and application conditions. Generally, well water is stable and low in pollution, requiring only simple pretreatment; however, groundwater in some areas requires softening. Surface water, directly affected by seasonal changes, is likely rich in microorganisms and colloids, causing corresponding pollution. The required pretreatment is more complex than for well water, with additional pretreatment steps including chlorination, flocculation, clarification, multi-media filtration, dechlorination, and scale inhibition. Industrial and municipal wastewater contains more complex organic and inorganic components, including large amounts of organic matter, inorganic matter, viruses, bacteria, and algae. Some organic matter may even severely affect the RO/NF membrane, causing membrane degradation. Seawater, due to its high salinity, high boron content, and large seasonal variations, requires even more complex pretreatment. Therefore, a detailed pretreatment scheme is essential to ensure the safe operation of the final system, and comprehensive water quality analysis is the most critical basis for establishing a suitable pretreatment scheme and RO/NF system layout design. The required feed water quality indicators for reverse osmosis membranes are shown in Table 4.2.
Table 4.2 Water quality indicators for reverse osmosis membrane feed water
|
|
Index |
Unit |
Allowed values |
Solution |
|
1 |
Turbidity |
NTU |
<1, ideally kept below 0.2. |
Filtration, flocculation and sedimentation, microfiltration, ultrafiltration |
|
2 |
SDI15 |
|
<5, it is recommended to keep it below 3. |
Filtration, flocculation and sedimentation, microfiltration, ultrafiltration |
|
3 |
Particulate matter |
pcs/ml |
Substances larger than 5µm are not allowed to enter the RO system. |
Filtration, flocculation and sedimentation, microfiltration, ultrafiltration |
|
4 |
Microorganism |
pcs/ml |
<1 |
Sterilization, microfiltration, ultrafiltration |
|
5 |
Iron Fe3+ |
ug/L |
When dissolved oxygen is >5 ppm, it should be <50 ppm; the lower the better. |
Oxidation precipitation or filtration |
|
6 |
Manganese |
ug/L |
<50, the lower the better |
Oxidation precipitation and the use of dispersants |
|
7 |
Al |
ug/L |
<50, the lower the better |
Oxidation precipitation or filtration |
|
8 |
Oils and fats |
|
Must not be detected |
Air flotation, adsorption, stripping |
|
9 |
TOC |
ppm |
<5, it is recommended to keep it below 3. |
Activated carbon, filtration, adsorption resin |
|
10 |
COD |
ppm |
<10, the lower the better. |
Activated carbon, filtration, adsorption resin, ultrafiltration, biochemistry |
|
11 |
BOD |
ppm |
<5, it is recommended to keep it below 2. |
Activated carbon, filtration, adsorption resin, ultrafiltration, biochemistry |
|
12 |
H2S |
ppm |
<0.1, the lower the better. |
Catalytic oxidation |
|
13 |
Alcohol |
% |
<10, the lower the better. |
Activated carbon, filtration, adsorption resin, ultrafiltration, biochemistry |
|
14 |
Surfactants |
|
Must not be detected |
Choose anionic surfactants |
|
15 |
Residual chlorine |
ppm |
<0.1 |
Activated carbon adsorption and reducing agent |
|
16 |
Ozone |
ppm |
0 |
Activated carbon adsorption and reducing agent |
|
17 |
CaCO3 |
|
LSI<0 |
Recovery rate, scale inhibitor, ph adjustment |
|
18 |
CaSO4 |
% |
<230 |
Recovery rate, scale inhibitor, ph adjustment |
|
19 |
BaSO4 |
% |
<6000 |
Recovery rate, scale inhibitor, ph adjustment |
|
20 |
SrSO4 |
% |
<800 |
Recovery rate, scale inhibitor, ph adjustment |
|
21 |
CaF2 |
ppm |
Concentration on the concentrate side <1.7 |
Recovery rate, lowering ph value |
|
22 |
CaPO4 |
|
The concentration of the concentrated solution must not exceed its solubility. |
Recovery rate, lowering ph value |
|
23 |
SiO2 |
% |
<100, empirically recommended to keep the influent ppm below 20 ppm. |
Recovery rate, increase ph value, increase temperature |
|
24 |
ph value |
|
3-10, it is recommended to keep it between 6.5-8.3. |
Adjust with acid and alkali, and remove by blowing. |
|
25 |
Temperature |
℃ |
5-45, it is recommended to keep it around 25℃. |
heat exchanger |
|
26 |
Hardness |
ppm |
Unless otherwise specified, if the concentration exceeds 50 ppm, acid should be added to adjust the ph to around 5, and a scale inhibitor should be added. |
Recovery rate, scale inhibitor dispersion, ph adjustment, ion exchange, softening with sodium carbonate/caustic soda/calcium oxide, etc. |
|
27 |
Flocculants/coagulants |
ppm |
Must not be detected |
Adjust the dosage and duration of action. |
