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Ro & Water Chemistry

Performance Limitations

  • Water Chemistry Related

  • Membrane Related

  • System Design Related

Design Limiting Items

  • TDS

  • Calcium

            - Sulfate

            - Bicarbonate

  • Barium & Strontium Sulfate

  • Flouride

  • Silica

TDS

  • Product Quality

  • Recovery

  • Operating Pressure

 

                         INORGANIC FOULANTS

DEPOSIT

COMMON

LESS COMMON

PHOSPHATES

CA, MG

AL, FE, SR, BA, F, PB

CARBONATES

CA, MG

SR, BA, PB, CU

SULFATES

CA

SR, BA

SILICATES

AL, TI, K, NA

FE, MN, MG, CA

HYDROXIDES

FE, MG, MN

AL

SULFIDES

FE, MN

 

 

WATER HARDNESS

1.         Total of calcium and magnesium content.

2.         Expressed in Mg/l of calcium carbonate.

                                                1 Grain = 17.1 Mg/l

3.         When hardness is greater than total alkalinity then the excess

                        is called permanent hardness.

                        When harness is less than total alkalinity, all of it is carbonated

            or temporary hardness.

4.         Water softening exchanges calcium and magnesium ions (hardness)

                        with sodium ions (from salt)

 

TERM                                                HARDNESS/GRAINS

Soft Water                                                                  0  -  1

Slightly Hard                                                              1  -  3

Moderately Hard                                                        3  -  7

Hard                                                                            7  -  10

Very Hard                                                                   Over  10

 

NOMOGRAM FOR DETERMINING pH VALUE FROM KNOWN CO2 AND ALKALINITY

LANGELIER SATURATION INDEX (CACO3 SATURATION INDEX)

- Criteria used for determining the need for Calcium Carbonate scale control.

LSI =  pH - pHs

pHs = EPCA + p ALK + C  - From graph

pHs = (9.30 + A + B) - ( C + D)  - From data book

            To  Calculate pHs,  you need to know:

* Calcium Conc. of Reject (CACO3)

* Alkalinity of Reject  (CACO3)

* TDS of Reject

* pH of Reject

If LSI is positive  = Scale forming

If LSI is 0             = Neutral

If LSI is negative = Corrosive Tendencies

SOLUBILITY COMPARISON

 

CaSO4

 

PPM Ca × PPM SO4
AS CaCO3

MG/L AS CaSO4

Pure Water

1.9 M

1874

@ IR = 0.1133

2.8 M

2275

10 MG/L HMP

10 M

4298

 

SRSO4

 

PPM SR × PPM SO4
AS CaCO3

MG/L AS SRSO4

Pure Water

3.1 K

102

@ IR = 0.1133

12 K

201

10 MG/L HMP

24 K

402

 

BASO4

PPM BA × PPM SO4
AS CaCO3

MG/L AS BASO4

Pure Water

0.9

2.2

@ IR = 0.1133

2.9

6.8

10 MG/L HMP

5.8

13.6

 

CAF2

MG/L AS CAF2

Pure Water

16.8

Silica

  • Solubility - Temperature, pH

  • Presence of Particulates

  • Recovery

  • System Flush

 

Impact of Silica in Reverse Osmosis

Silica is present in water in the range of 1-100 mg/l.  At 25°C (or 77°F) and a pH of the solubility of silica in water is about 100 mg/l.  At 30°C (or 86°F) solubility is 120 mg/l whereas at pH of 7 and 150 mg/l at a pH of 5.  This information is given in a graphic from below:

 

Other Insoluble Compounds

 IRON: Air exposure, chlorination

 ALUMINUM HYDROXIDE: Least soluble at A pH of 5.7    Cleaning effective at a pH of 3 to 4.

 HUMIC ACID: High Mol. Wt. organic acids.  Solubility low at pH 5 to 6.  Recommend filtration after pH adjustment.

 SULFUR: pH of less than 6, sulfide in water converted to Hydrogen Sulfide.  Poor rejection.

Remove by degasifier.  Some sulfur formed converted to sulfate by chlorination.  Sulfur fouling irreversible.

 

Membrane Performance With Time 

                        1.         Suspended solids are slowly deposited.

                        2.         Micro-Organism slowly grow and multiply.

                        3.         Scaling may take place.

                        4.         Oxidizing agent (Chlorine) attack the membrane.

                                                Increase in pressure drop

                                                Loss in permeate flow

                                                Increase in permeate TDS

 

 
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