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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 Lime/soda ash softening (removal of hardness) Sedimentatio n basin Ground water from wells Recarbonation Rapid Flocculation Mix Basin Sludge Disinfection Filter CO2 Storage To Distribution System Hardness • Definition: Has high dissolved minerals – calcium bicarbonate and magnesium bicarbonate • Effects of hardness 1. Causes soap scum and water spots Ca2+ + (Soap)- Ca(Soap)2 (s) • Increases amount of soap required for cleaning 2. Causes scaling on pipes (especially in boilers) CaCO3 and Mg(OH)2 Remove scale and biofilms by “pigging” small sections. Soft vs. Hard Water • Soft water – “I can’t rinse the soap off.” – Actually, rinsing is more complete with soft water. • Hard water – Soap precipitate gets on skin and in hair. • Therefore, it doesn’t feel slippery. Hardness Values Description Soft Moderately hard Hard Very hard Hardness range (mg/L as CaCO3) 0 - 75 75 - 100 100 - 300 > 300 Durham treated water (Brown Water Treatment Plant): 21 mg/L as CaCO3 Formation of Hardness Precipitation Topsoil Subsoil Rock CH3COOH H+ + CH3COOCO2 + H2O H2CO3 H+ +HCO3CaSO4 (s) Ca2+ + SO42CaCO3(s) Ca2+ + CO32MgCO3(s) Mg2+ +CO32- Non-carbonate hardness Carbonate hardness http://www.geo.arizona.edu/geos256/azgeology/bcd/cavewebpg.htm Lime/Soda Softening Process • Neutralization of carbonic acid – CO2 + Ca(OH)2 CaCO3(s) + H2O • Precipitation of Ca carbonate hardness1 – Ca2+ + 2HCO3- + Ca(OH)2 2CaCO3(s) + 2H2O • Precipitation of Mg carbonate hardness – Mg2+ +2HCO3- +Ca(OH)2 Mg2+ +CO32-+ CaCO3(s) + 2H2O – Mg2+ + CO32- + Ca(OH)2 Mg(OH)2(s)+ CaCO3(s) Ksp (MgCO3)= 10-5 Ksp (Mg(OH)2)=10-10.7 1 the portion of the hardness associated with carbonate or bicarbonate ions Lime/Soda Softening Process • Removal of Ca noncarbonate hardness Ca2+ + Na2CO3 CaCO3(s) + 2Na+ • Removal of Mg noncarbonate hardness Mg2+ + Ca(OH)2 Mg(OH)2(s)+ Ca2+ Ca2+ + Na2CO3 CaCO3(s) + 2Na+ (cont’d) Ion Exchanger Recarbonation CO2 + H2O H2CO3 +CO32- 2HCO3Recarbonation 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 Ca(OH)2, Na2CO3 Rapid Mix Recarbonation Coagulation and Flocculation • Purpose – To bring small colloids into larger flocs for settling + Usually negatively charged Coagulant addition Al3+, Fe3+ Colloidal particles (0.001 - 1 m) + + + + + + + + + + ++ ++ + + + ++ + + + ++ + + + ++ ++ ++ + + + + Flocs (1 - 100 m) Sedimentation tanks Filtration – remove suspended solids • Purpose – Series of treatments Disinfection • Purpose – Kill microorganisms in the water • Disinfectants – – – – – – Cl2 Hypochlorite salts (NaOCl, Ca(OCl)2) Most common Chloramines (NH2Cl, NHCl2, NCl3) in US Chlorine dioxide (ClO2) Ozone (O3) UV radiation Very effective against protozoan cysts Chlorine Reactions in Water • Cl2 (g) + H2O HOCl + H+ + Cl- – pH dependent – Essentially complete within a few milliseconds • HOCl H+ + OCl- [HOCl] + [OCl-] = free available chlorine 80-100X more effective than OCl- for killing E. coli Surface Water Treatment Coagulation and Flocculation (colloid removal) Screen Surface water from supply Rapid Flocculation Rapid Mix Basin Sand Filter Disinfection Storage Sedimentatio n basin Sludge To Distribution System Colloid Treatment – Low Hardness Typically for Surface Water Small particles 0.001 to 1 m Usually negatively charged Particles repel so suspension is stable Treat by coagulation- flocculation