Purifying water Flashcards
What does aeration do? (bubble diffusers, fountains)
~ removes dissolved gases like H2S and CH4 via oxidation
~ converts soluble Fe2+ (ferrous) to Fe3+ (ferric)
What is the first step of the addition of slaked lime (Ca(OH)2)?
~ removes soluble iron and magnesium
~ 3Ca(OH)2 + 2Fe3+ –> 2Fe(OH)3 + 3Ca2+
~ Ca(OH)2 + Mg2+ –> Mg(OH)2 + Ca2+
What is the second step of the addition of slaked lime?
~ the added and already present soluble calcium is precipitated via soluble CO2
~ CO2 + H2O = H2CO3 (carbonic acid)
~ H2CO3 + H2O = H3O+ + HCO3-
What’s the third step of the addition of slaked lime/
~ Ca(OH)2 = Ca2+ + 2OH-
~ OH- + HCO3- = CO32- + H2O
~ Ca2+ + CO32- = CaCO3
What happens to the precipitates formed from slaked lime?
~ they are allowed to settle in a primary settling basin
What is turbid water? What is used to treat it?
~ CaCO3 and MgCO3 remaining in suspension
~ a coagulant, alum (Al2(SO4)3*H2O) is used
~ alum is combined with water and slaked lime
~ the gibbsite (AL(OH)3) particles formed are “flash mixed” (aggressively stirred) with the water
What is the effect of flash mixing of gibbsite (Al(OH)3) with water?
~ the mutually-repulsive surface charges of nearby colloidal particles are in some way stripped away, causing agglomeration (coagulation) with the Al(OH)3
~ during subsequent brownian motion, these fluffy precipitates contact and flock together (floculation), which then settle out
What is a sand filter used for?
~ to catch remaining suspended particles
What is an activated charcoal filter used for?
~ to remove organic matter like pesticides
Why is chlorine added?
~ to form hydochlorous acid, which kills bacteria
Why is NaF added?
~ to fight tooth decay
~ F- substitution for OH- in hydroxyapatite teeth makes them more resistant to cavity formation
Why is water made slightly basic?
~ to prohibit corrosion of pibes (Fe, Pb are more soluble under acidic conditions)
Why is orthophosphoric acid (H3PO4) added?
~ to protect the oxide passivation layers that form on Fe and Pb pipes
What is hard water?
~ contains higher conc. of Ca2+ and Mg2+
~ heating causes the precipitation reaction: Ca2+ + 2HCO3- = CaCO3 + H2O + CO2, which leaves deposits
How is hard water softened?
~ via ion exchange using zeolites or ion exhange resins
~ ex: cation exchange resin based on polystyrene: 2Na+ substitute with Ca+; no precipitation reaction occurs with Na+
How do you form deionized water from potable water?
~ water is run thorugh mixed cation and anion exchange resins which exchange for H3O+ and OH-, which in turn combine to form H2O
What is a measure of “reagent grade” deionized water?
~ a low ionic conductance (<5~μmho/cm)
~ city water: 650 μmho/cm
