Monday October 15

welcome back...

Finish Chapter 5 -- Water

• sources of fresh water
  • wells, springs
  • surface waters (rivers, lakes)
  • rain and cisterns (store)
• salty water (oceans, salt lakes)
  • unusable for drinking
  • can desalinate
    • process is expensive

Desalination:

1. distillation

• boil water and evaporate it
  • (costly-- energy requirements)
  • cool and condense the vapor
    • (costly, cooling process)
  • most impurities do not vaporize
  • stream condenses into nearly pure distilled water
    • main impurities: O₂, N₂, CO₂, NH₃
  • costly: boiler gets caked with minerals, needs mechanical cleaning

2. freezing

• when aqueous solution freezes it's pure ice that forms
  • ice with no impurities
  • aqueous = solution with water as the solvent
    • the liquid keeps the impurities (solutes)
      • (applejack -- a way to concentrate alcohol in hard cider)
      • was favored method in colonial days to prepare high alcohol beverages without using a still.
    • desalination by freezing does work, costly (energy costs mainly)

3. Ion Exchange

• water softeners, Britta filters
  • Britta, etc. mostly charcoal treatment
• chemical solids that swap or exchange ions
  • cation exchange: remove Ca²⁺, Mg²⁺, return Na+
    • clears up hard water, but leaves sodium (salty)
    • there are other resins: remove Na+ too, return H+
  • anion resins, swap Cl-, SO₄2- for OH-
• H+ and OH- react, form water
• can completely remove all ionic species
• cost increases for water with higher impurity levels

4. Reverse osmosis

Normal Osmosis

• membrane (thin plastic layer, biological)
• some will transmit water, not other species
  • semipermeable membranes
  • occurs in biology: cell walls
• Osmosis seems to be wrong direction for us
  • water passes through membrane
  • would eventually produce equal salt concentration
  • if you started with pure water/ salt water
    • pure water goes into salt water region
• Osmotic Pressure
• slowly a pressure builds up across membrane
  • water flow stops at this pressure
  • resumes if we decrease the pressure
  • reverses if we increase the pressure

Reverse Osmosis

• add enough pressure to make water move from salty side to pure side
• gradually produce more pure water
  • also produces saltier water
    • best to discard, get more original water, not to use increasing pressures to purify this
• Reverse Osmosis
• purify sea water
• (need relatively high pressures, 25 atm.)
• purify drinking water
• (relatively low pressures)
• rarely use "distilled water' now
• our labs have RO units, stores sell RO water
• RO, like distilled water, water tastes "blah"-- some minerals add taste

The technology exists to convert bad water to good water

• sea water and brackish wells
• hard water (excess mineral content)
• unhealthy water supplies
• sewage
  • It's really practical to recycle water
  • for most of us, it's cheaper to start with fresh water

The issue is generally cost

• good water supply yields safe water at low cost
  • perhaps filter it, treat with chlorine or ozone
  • pump it around in cities
• notice, this is cost for both drinking water, laundry, mixing concrete
• often cheaper to bring in cleaner water from significant distance
  • Romans mastered aqueducts, brought in water hundreds of miles
  • True also in parts of ancient Iraq
• in much of the world, clean safe water is still too costly
• much of the world relies on untreated water, poor separation between sewage and drinking water
  • bacteria: cholera, typhoid
  • parasites:
  • mining and industrial wastes

What's routinely found in wells, surface waters?

• species and amounts
• is it a problem?
• sources
• preserving safe water resources
• treating water
  • treating sewage is part of the picture
  • industrial discharge is part of the picture

Specific Species (examples)

Bacteria, Viruses and Parasites

• relatively easy to kill
• halogen treatment
  • usually with NaOCl ("bleach") or Chlorine gas
  • occasionally with bromine
  • in camping kits-- iodine treatment
• Europe generally favors ozone treatment

Water hardness

• generally water with modest Ca⁺² concentration
  • typically dissolved from rock (limestone, CaCO₃)
  • often some Mg²⁺ (perhaps Fe²⁺, Mn²⁺)
• problem: reacts with soap
  • soap + Ca²⁺ --> insoluble material
  • soap scum (coats sinks, tubs, deposits in laundry)
  • removes soap... need to add extra soap
• solutions to the problem?
  • find "softer" water
    • in NW Ohio, well water is very hard
    • rivers and lakes have less dissolved mineral content
  • add a water softener
  • add chelating agents to soap products
    • (tie up Calcium)
    • borax is such a species (relatively cheap)
    • phosphates were widely used
      • unfortunately, phosphates lead to algae growth in sewage discharge
      • classic case of a "solution" producing an unexpected problem
      • phosphate use in detergents is highly restricted
  • use detergents rather than soaps (less of a problem)
• another problem with hard water
  • evaporation leads to mineral deposits
  • some Wood County wells-- boil water and get a solid film on walls of the pot
  • CaCO₃ is less soluble in hot water
• Water hardness is an economic disadvantage, but not a source of illness

Clear Health Risks

• Nitrate NO₃- and Nitrite NO₂- Ions
  • Some comes from nitrogen fixation in the air
  • Some comes from sewage and animal wastes
  • Some comes from agricultural runoff (nitrogen fertilizers)
  • generally peaks in spring, following a modest rain
• trace amounts seem to be relatively safe
  • at higher concentration, nitrite interferes with blood oxygen
  • serious risk for infants and nursing mothers
• water suppliers must monitor, issue warnings
  • must find ways to reduce levels if too high
• Bowling Green-- this is a periodic problem
  • water source: Maumee River
    • for 5-10 days each spring
  • solution: reservoir, filled at low nitrate levels
    • mixed with river water to keep levels below recommended value
    • removal of nitrate would be difficult and expensive
      • no good specific treatment-- would need to reduce all ions
      • cost effective only for drinking water

Herbicides and Insecticides

• may be harmful at concentrations of a few ppm
• hard to apply to fields and keep out of surface water
• generally medium sized organic molecules (2-10 C atoms +...)

Bad Taste (Spring Algae blooms)

• typically more unpleasant than dangerous
• One effective treatment for both cases is removal with Carbon
  • Activated carbon is good surface adsorbing agent
  • also relatively inexpensive and disposable
• can pass water through a cylinder of charcoal granules
  • alternative: add to a tank, stir and let it settle out
  • (major active species in most gas masks)
• fairly selective for organic molecules
  • ignores water, ions, oxygen, ...
  • (otherwise, would be used up very quickly)

Heavy Metals (Hg, Pb, Cd, As)

• can be very toxic at low concentrations
  • may be naturally present in soil and hence in well water
  • often brought to the surface by mining activity
    • regulated to some extent now in US (not all that well)
    • (Controversy in early 2001 over Arsenic-- lowering safe level in drinking water would making mining more costly.)
    • often problem from century old mine tailings
    • in much of the world, poorly regulated
  • often are by products of industrial processes
    • mercury in Great Lakes, mainly from Chlorine production
    • concentrates in fish
  • Lead has also been a problem in plumbing (lead pipes, solders)
  • may be unable to find better water source in the region
  • can treat, but it is costly (could double cost of delivered water.)