Water Pollution 794.11 An industry discharges sufficient quantities of organic wastes to depress the
oxygen sag curve to 2 mgL 5 miles downstream, at which point the dissolved oxygen
begins to increase. The state regulatory agency finds the industry out of compliance
because the minimum dissolved oxygen violates state standards, and wants to impose
a fine. A wastewater treatment plant is too expensive, so the industry looks around
for another solution. A salesman suggests that a special kind of freeze-dried bacteria,
particularly adapted to the plant's organic waste, be dumped into the river at the point of
discharge: the freeze-dried bacteria will come to life, break down the organic material,
and solve the problem. Will this approach work? Why or why not? Draw the dissolved
oxygen sag curve before and after the bacteria are added and explain what happens.
4.12 An industry discharges into a river with a velocity of 0.1 ds, a temperature
of 26"C, and an average depth of 4 m. The ultimate oxygen demand immediately
downstream from the effluent discharge point where the waste has mixed with the river
water (Lo) is 14.5 mgL and the dissolved oxygen is 5.4 ma. Laboratory studies show
that ki = 0.11 day-'. Find the point of critical dissolved oxygen and the minimum
dissolved oxygen. You may solve this problem by writing a computer program or by
using a spreadsheet program.