CEE 160L – Introduction to Environmental Engineering and Science Lecture 9 Drinking Water

Typical Characteristics of DW Sources: Groundwater (GW) vs. Surface Water (SW) Groundwater High mineral content Low turbidity Low color Low or no DO High hardness High Fe, Mn Low bacteria Surface water Low mineral content High turbidity Colored DO present Low hardness Taste and odor High Bacteria

Groundwater Treatment Ground water from wells Sedimentatio n basin Sludge Recarbonation To Distribution System Rapid Mix Flocculation Basin Disinfection Storage CO 2 Filter Lime/soda ash softening (removal of hardness)

Hardness Definition: Has high dissolved minerals – calcium bicarbonate and magnesium bicarbonate Effects of hardness 1. Causes soap scum and water spots Ca 2+ + (Soap) -  Ca(Soap) 2 (s) Increases amount of soap required for cleaning 2. Causes scaling on pipes (especially in boilers) Remove scale and biofilms by “pigging” small sections. CaCO 3 and Mg(OH) 2

Soft vs. Hard Water Soft water Soft water –“I can’t rinse the soap off.” –Actually, rinsing is more complete with soft water. Hard water Hard water –Soap precipitate gets on skin and in hair. Therefore, it doesn’t feel slippery. Therefore, it doesn’t feel slippery.

Hardness Values Durham treated water (Brown Water Treatment Plant): 21 mg/L as CaCO 3

Formation of Hardness CH 3 COOH  H + + CH 3 COO- CO 2 + H 2 O  H 2 CO 3  H + +HCO 3 - Subsoil Rock CaSO 4 (s)  Ca 2+ + SO 4 2- CaCO 3(s)  Ca 2+ + CO 3 2- MgCO 3(s)  Mg 2+ +CO 3 2- Precipitation Topsoil Non-carbonate hardness Carbonate hardness

Lime/Soda Softening Process Neutralization of carbonic acid – CO 2 + Ca(OH) 2  CaCO 3(s) + H 2 O Precipitation of Ca carbonate hardness 1 – Ca HCO Ca(OH) 2  2CaCO 3(s) + 2H 2 O Precipitation of Mg carbonate hardness – Mg 2+ +2HCO 3 - +Ca(OH) 2  Mg 2+ +CO CaCO 3(s) + 2H 2 O – Mg 2+ + CO Ca(OH) 2  Mg(OH) 2(s) + CaCO 3(s) 1 the portion of the hardness associated with carbonate or bicarbonate ions K sp (MgCO 3 )= K sp (Mg(OH) 2 )=

Lime/Soda Softening Process (cont’d) Removal of Ca noncarbonate hardness Ca 2+ + Na 2 CO 3  CaCO 3(s) + 2Na + Removal of Mg noncarbonate hardness Mg 2+ + Ca(OH) 2  Mg(OH) 2(s) + Ca 2+ Ca 2+ + Na 2 CO 3  CaCO 3(s) + 2Na +

Ion Exchanger

Recarbonation CO 2 + H 2 O  H 2 CO 3 +CO 3 2-  2HCO 3 - Recarbonation - Because of the lime that was added, the water has a pH over 11. Compressed carbon dioxide gas is bubbled through the water to lower pH and stop the chemical reactions of softening. The high pH also creates a bitter taste in the water. Recarbonation removes this bitterness. (Many places use liquid CO2)

Process Sequence Rapid Mix Ca(OH) 2, Na 2 CO 3 Recarbonation

Coagulation and Flocculation Purpose – To bring small colloids into larger flocs for settling Colloidal particles (  m) Flocs (  m) Coagulant addition Al 3+, Fe 3+ Usually negatively charged

Sedimentation tanks

Filtration – remove suspended solids Purpose –

Series of treatments

Disinfection Purpose – Kill microorganisms in the water Disinfectants – Cl 2 – Hypochlorite salts (NaOCl, Ca(OCl) 2 ) – Chloramines (NH 2 Cl, NHCl 2, NCl 3 ) – Chlorine dioxide (ClO 2 ) – Ozone (O 3 ) – UV radiation Most common in US Very effective against protozoan cysts

Chlorine Reactions in Water Cl 2 (g) + H 2 O HOCl + H + + Cl - – pH dependent – Essentially complete within a few milliseconds HOCl H + + OCl - [HOCl] + [OCl - ] = free available chlorine X more effective than OCl - for killing E. coli

Surface Water Treatment Surface water from supply Rapid Mix Flocculation Basin Sedimentatio n basin Sludge Rapid Sand Filter Disinfection Storage To Distribution System Screen Coagulation and Flocculation (colloid removal)

Colloid Treatment – Low Hardness Typically for Surface Water n Small particles n to 1  m n Usually negatively charged n Particles repel so suspension is stable n Treat by coagulation- flocculation