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Remineralisation 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:
| pH | 5.65 |
| Alk (mg de CaCO3/L) | 1,97 |
| HCO3– (mg/L) | 1,62 |
| CO2 (mg/L) | 8,46 |
| Ca²+ (mg/L) | 1,43 |
| Langelier* (SM2330) | -5,46 |
The purpose of remineralisation 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 remineralisation:
| Parameter | Range |
| 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 |
Example case
The following example shows an scenario where a client has a permeate/desalinated water (Nº 1) and has some requirements (Nº 2) to meet.
| Nº | 1 | 2 | 3 | 4 | 5 | |
| Permeate water | Requirements | After limestone contactors and no pre CO2 dosing |
After limestone contactors and pre CO2 dosing | Adding NaOH after limestone contactors pre CO2 dosing | Units | |
| TDS | 48 | 55 | 101 | 102 | mg/l | |
| pH | 7,68 | 6-9 | 9,50 | 8,00 | 8,30 | |
| Alkalinity | 12,6 | 20,00 | 65,80 | 67,00 | mg CaCO3/l | |
| Calcium | 2 | > 10 | 4,9 | 23,3 | 23,3 | mg/l |
| CO2 | 0,53 | 0 | 1,4 | 0,71 | mg CO2 /l | |
| LSI* | -2,23 | >0 | -0,25 | 0,03 | ||
| CCPP** | -6,9 | >0 | -0,04 | -1,75 | 0,21 | mg CaCO3/l |
| Turbidity increase | < 0,25 | NTU | ||||
| Temperature | 16 | ºC |
* Langelier Saturation Index
**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.
