Water softening Flashcards
Moderate hard to hard is
Scale from 0-250
91-130 mg/l of CaCO3
Hardness is caused mainly by
The salts of calcium and magnesium, such as bicarbonate, carbonate, sulfate, chloride, and nitrate
Total hardness is commonly measured by
Commonly measured by titration and is described in two ways:
1. Sum of hardness caused by both calcium and magnesium ions expressed (CaCO3)
- The sum of carbonate (temporary) and non carbonate (permanent) hardness
Calcium carbonate (CaCO3) equivalent is
An expression of the connection of a chemical in terms of their equivalent value to calcium carbonate
Carbonate hardness is caused by
alkalinity present in the water up to the total hardness. It’s the total measure of the waters alkalinity
Non carbonate hardness is
That portion of the total hardness in excess of the alkalinity. Requires use of both lime and soda ash to remove
Alkalinity is the capacity… and measures
The capacity of water to neutralize acids.
It’s measure of how much acid must be added to a liquid to lower ph to 4.5. This capacity is caused by the water’s content of bicarbonate, carbonate, hydroxide, and sometimes borate, silicate and phosphate
Two types of lime
- Quick (CaO)- calcium oxide
- Hydrated (Ca(OH)2
You should never use the same conveyor or container for quicklime because
It could explode under certain conditions
Types of alkalinity that can be present at ph values (7)
- Below 4.5 only CO2 present, no alkalinity
- Between 4.5 to 8.3, CO2 and bicarbonate present
- Above 8.3, alkalinity may consist of bicarbonate, carbonate, and hydroxide (no CO2 present)
- When above 8.3 the amount of titrant used to reach ph 8.3 is the phenolphthalein alkalinity
- Between 10.2 to 11.3 carbonate and hydroxide
- At 9.4 calcium carbonate becomes insoluble and precipitates
- At 10.6 magnesium hydroxide becomes insoluble and precipitates
Hardness relationship to alkalinity (2)
- Alkalinity > total hardness (All hardness is in carbonate form)
TH=CH - Alkalinity < total hardness (both carbonate and non carbonate hardness is present)
CH=alkalinity
TH=CH+ NCH
Alum sludge has a tendency to
Harden the soil
What can you use when water can’t be softening to desired level
Soda ash or caustic soda
Hardness in water (2)
- High concentrations of calcium (Ca2+) and magnesium (Mg2+) ions cause water hardness
- Water with more than 100mg/l of calcium carbonate is considered hard
Methods of removing hardness (3)
- Lime softening (chemical precipitation) - solubility level of about 35 mg/l (CaCO3)
- RO (nanofiltration) ( membrane filtration) - 85-90% removal
- Ion exchange (chemical exchange) - basically zero water must be blended
(6) benefits of lime softening
- Removal of Ca and Mg hardness
- Removal of iron, manganese, arsenic and uranium
- Reduction of solids, turbidity, and TOC
- Removal and inactivation of bacteria and viruses due to high ph
- Raise ph and prevent corrosion
- Removal of excess fluoride
(4) chemical precipitation
- Hardness causing ions converted from soluble to insoluble at high ph
- Adding lime increases the hydroxide concentrations, increasing the ph
- Adding lime converts alkalinity from bicarbonate form to carbonate causes calcium to precipitate
- Adding lime the phenolphthalein alkalinity increased to a level where hydroxide becomes present allowing magnesium to precipitate
Following the softening process the (3)
- Ph is high
- Water is supersaturated with excess caustic alkalinity in either hydroxide or carbonate form
- Carbon dioxide can be used to decrease the causticity and scale forming tendency before filtration
Chemical titration with methyl orange (T) (2)
- Methyl orange is used to determine the combination of alkalinity provided by carbonate, bicarbonate, and hydroxide or total alkalinity
- Added until color is absent then the total alkalinity (T) is computed
Chemical titration with phenolphthalein (P) (2)
- P is used to determine the carbonate and hydroxide alkalinity present
- Added to color is absent then hydroxide and carbonate alkalinity (P) then computed
(5) lime softening limitations
- Unable to remove all carbonate and non carbonate hardness
- High degree of operator control
- Color removal may be hindered due to high ph
- Improper ph can cause scaling or corrosion
- Sludge handling and disposal can be costly
Primary coagulants used in lime softening (2)
- Acidic compounds- alum (Al2(SO4)3, Ferris sulfate, ferric sulfate and ferric chloride
- Basic compounds- sodium aluminate
Split treatment
- 80% treated and 20% raw
- Used to reduce eliminate carbon dioxide
Quicklime slaked (2)
Dry powder must be soaked for 15/30 minutes at cold temps
Slaking produces calcium hydroxide
Sodium alum, bentonite, sodium silicate, synthetic canonic are all
Secondary coagulation aids
Recarbonation in lime softening
- Water is supersaturated
- If not recarbed ca and mg will form in filters and piping
