Water treatment

Acid rain

SO2 + H2O → H2SO3

Sedimentation

Water sits in resevoirs so heavy solids settle

Clarification (coagulation)

Al₂(SO₄)₃(aq) + 3Ca(OH)₂(s) → 2Al(OH)₃(s) + 3CaSO₄(aq)

Al(OH)3 is gelatinous and traps fine particles and bacteria

Sterelization

Cl₂ + H₂O ⇌ HOCl + HCl

HOCL kills microbes

Chlorine pros

It is cheap and effective

Chlorine cons

Affects taste and smell, UV light can generate radicals

Air stripping (Cl alternative)

Water comes into contact with a tower with updraft, dissolved gases transfer to air

Air stripping pros

Cheap and reliable to remove VOCs

Air stripping cons

Doesn’t get rid of microbes, causes air pollution

Ozone (Cl alternative)

High voltage is discharged through air O2→ O3

O3 is absorbed in water

Ozone pros and cons

More effective than Cl2 but is expensive and cannot be stored

UV radiation (Cl alternative)

Damages DNA of organisms

UV radiation pros and cons

Leaves no residual and is effective, but it is one time use and microbes can adapt

Why is Cl2 still used?

Residual protection

What is water hardness and where does it come from

Dissolved metal ions (main Ca2+, Fe2+, Mg2+).

Comes from dissolution of minerals like CaCO3

CaCO3 + H2O + CO2 → Ca2+ + 2HCO3

Problems with hard water

1. Scum

Ca+2 + soap → insoluble soap

2. Scale

Heating hard water causes CaCO3 to precipitate

Scale blocks pipes and can cause bioler explosions because its a poor conductor

Water softening by precipitation

1) Boil (removes carbonate hardness)

Ca +2HCO3 → CaCO3 + CO2 + H2O

2) Add Bases (Ca(OH)2), NH3, NaOH) ACID-BASE PRINCIPLE

Ca + 2HCO3 + Ca(OH)2 → 2CaCO3 + 2H2O

3) Add Na2CO3 INSOLUBILITY PRINCIPLE

Ca + Na2CO3 → CaCO3 + 2Na

Water softening by sequestering agents

Chelating ligands wrap around Ca→ making stabke rings and no scum/scale produced

a) Tripolyphosphate TPP 5- tridentate

• Strong chelating agent

• Stable TPP-Ca complexes

• pH 7-11

• Algae blooms

b) EDTA 4- hexadentate

• huge k, very strong, not biodegradable pH 7-11

c) hydrocarboxylicates (citrates)

• weak binding but cheap and biodegradable, pH>12 used in food and detergent

Water softening by ion exchange

Makes ultra pure water.

cation exchange

Ca 2+, Mg2+ exchanged for Na2+ and H+

SO4-, Cl- removed

Gives demineralized water

ZEOLITES