for plant growth and is commonly spread on soil in locations where sufficient land is
available. Another alternative is to chemically gasify the leftover sludge with oxygen
by high-temperature processes similar to those used to gasify coal (see Section 6.7).
Properly designed and operated wetlands for advanced treatment of secondary sewage
effluent are a means of purifying the effluent without consuming additional chemicals
and energy while providing desirable areas for wildlife. It should be kept in mind that
all of these applications require sewage that is relatively free of persistent pollutants,
particularly poorly degradable organic compounds and heavy metals, so control of
materials discharged with sewage is important.
QUESTIONS AND PROBLEMS
- Why does the water molecule have a bent structure?
- What does it mean that the water molecule is a dipole? Why is it a dipole?
- In addition to its polar nature, what is the other major characteristic of the water
molecule and its interaction with other water molecules that largely determines
water’s behavior? - How does water behave with respect to heat? What are the practical implications of
this behavior? - What is shown by the hydrologic cycle
- What happens to a body of water that is not stirred significantly by wind or currents
as it is heated by sunlight during the summer? - What forms the basis of the food chain in an aquatic ecosystem?
- What is meant by biochemical oxygen demand?
- How is photosynthesis involved in the production of mineral calcium carbonate in
water? - What happens to sulfur during the reaction H 2 S + 2O 2 → SO 42 - + 2H
- ?
- How is the solubility of a gas in water related to the partial pressure of the gas in
contact with the water? - Why is rainwater a weak acid?
- Which two important properties of water are due to the presence of HCO 3
-
ion and
Ca2+ ion?- Why is it not completely accurate to represent dissolved calcium ion in water simply
as Ca2+? - What is a metal chelate in water? What is a common kind of naturally occurring
chelating agent?
Chap. 7. Water, The Ultimate Green Solvent 191