Environmental Engineering FOURTH EDITION

Water Treatment 15 1

Many water plant operators prefer to maintain a residual of chlorine in the water.
Then if organic matter, like bacteria, enters the distribution system, there is sufficient
chlorine present to eliminate this potential health hazard.
Chlorine may have adverse secondary effects. Chlorine is thought to combine
with trace amounts of organic compounds in the water to produce chlorinated organic
compounds that may be carcinogenic or have other adverse health effects. A recent
risk analysis (Morris 1992) has found a weak positive association between bladder and
rectal cancer and consumption of chlorinated drinking water, indicating that there may
be some risk of carcinogenesis. The authors suggest use of both chlorine and ammonia
as disinfectants in order to reduce the chlorine concentration used. Disinfection by
ozonation, bubbling ozone through the water, also avoids the risk of side effects from
chlorination.
A number of municipalities also add fluorine to drinking water, since fluorine
has been shown to prevent tooth decay in children and young adults. The amount of
fluorine added is so small that it does not participate in the disinfection process.
From the clear well (step 6 in Fig. 7-1) the water is pumped into the distribution
system. This is a closed network of pipes, all under pressure. In most cases, water
is pumped to an elevated storage tank that not only serves to equalize pressures but
provides storage for fires and other emergencies as well.

CONCLUSION

Water treatment is often necessary if surface water supplies, and sometimes ground-
water supplies, are to be available for human use. Because the vast majority of cities
use one water distribution system for households, industries, and fire control, large
quantities of water often must be made available to satisfy the highest use, which is
usually drinking water.
However, does it make sense to produce drinkable water and then use it to irrigate
lawns? Growing demand for water has prompted serious consideration of dual water
supplies: one high-quality supply for drinking and other personal use, and one of lower
quality, perhaps reclaimed from wastewater, for urban irrigation, fire fighting, and
similar applications. Many engineers are convinced that the next major environmental
engineering concern will be the availability and production of water to meet an ever-
increasing demand. The job, therefore, is far from done.

PROBLEMS

7.1 A water treatment system must be able to deliver 15 mgd of water to a city of
150,000 people. Estimate: (1) the diameter of three equally sized mixing basins 10 ft
deep, with a detention time of 2 min, (2) the length, width, and corresponding surface
area of three flocculator basins that are 10 ft deep, (3) the surface area of three settling
basins that are 10 ft deep with 2-h detention periods, and (4) the required area of each
of 15 rapid sand filters rated at 2 gdmin-ft’.