Ro & Water Chemistry
Design Limiting Items
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)
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
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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