Experts in water remineralization

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:

pH 5.65
Alk (mg of CaCO3/L) 1,97
HCO3 (mg/L) 1,62
CO2 (mg/L) 8,46
Ca2+ (mg/L) 1,43
Langelier* (SM2330) -5,46

*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:

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

 

Case study

The following example shows an scenario where a client has a permeate/desalinated water (1) and has some requirements (2) to meet.

Water type  1  2 3 4 5
Permeate water Requirements After limestone contactors and no pre CO2 dosing
Adding CO2 before limestone contactors Adding NaOH after limestone contactors 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
Langelier*  -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

* Tendency of water to dissolve CaCO3.

**Calcium Carbonate Precipitation Potential.

Possible treatment options:

  1. No treatment: Water stays corrosive.
  2. Final water quality requirements
  3. 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.
  4. 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.
  5. 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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