Remineralization of desalinated water
Due to the process of reverse osmosis, desalinated water tend to be very corrosive, and is characterised by:
- Low pH
- Soft or with primarily non-carbonate hardness
- Low alkalinity
Average composition of desalinated water:
|Alk (mg of CaCO3/L)||1,97|
*Tendency of water to dissolve CaCO3.
The purpose of remineralization is generally to produce water with a Langelier index very close to zero that remains stable in contact with the atmosphere. Moreover, it is important to accomplish this with a minimum consumption of CO2 in order to minimize operating cost. In a nutshell, the objectives are:
- Rise water pH to 6,5 to 9,5.
- Rise Alkalinity
- Rise hardness
- To obtain a Langelier index of -0.5 to +0.5.
- Improve water taste
- Improve the health of the consumers
- Protect pipes, tanks, valves every non protected metallic equipment from corrosion
The below table shows the recommended characteristics for the desalinated water with an adequate level of remineralization:
|pH||8.2 ± 0.1|
|Alk (mg of CaCO3/L)||56 ± 3|
|HCO3– (mg/L)||68 ± 3|
|CO2 (mg/L)||0.7 ± 0.1|
|Ca2+ (mg/L)||21 ± 2|
|Langelier (SM2330)||± 0.15|
The following example shows an scenario where a client has a permeate/desalinated water (1) and has some requirements (2) to meet.
|Permeate water||Requirements||After limestone contactors and no pre CO2 dosing
||Adding CO2 before limestone contactors||Adding NaOH after limestone contactors||Units|
|CO2||0,53||0||1,4||0,71||mg CO2 /l|
|Turbidity increase||< 0,25||NTU|
* Tendency of water to dissolve CaCO3.
**Calcium Carbonate Precipitation Potential.
Possible treatment options:
- No treatment: Water stays corrosive.
- Final water quality requirements
- Treatment with limestone contactors and no pre CO2 dosing:
- Trying to optain CCPP=0 is difficult as the water has reached saturation. An alternative option is to add NaOH. However, water with less alkalinity than 50 mg CaCO3/L and 0,6 mg CO2/L is very unstable and will tend towards a negative CCPP.
- The pH raises above 9 in less than 5 minutes
- Calcium only increases to 4,9.
- Treatment with limestone contactors with pre CO2 dosing: Adding CO2 before limestone contactors improves limestone disolution, therefore pH and Calcium required parameters are reached, however, Langelier index and CCPP stays in negative values.
- Treatment with limestone contactors with pre CO2 dosing and post NaoH dosing: Adding NaOH after limestone contactors will increase Langelier index and CCPP and slightly affecting the rest of the values.
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