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SEWAGE DISPOSAL 


Wien man i 
Occasionally disease-producing bac- 
teria are present, derived either from 
direct sewage discharge or from the 
washing in of human wastes from the 
shore or through the ground. 
Impure water must be either stored 
or treated before it is safe for drinking. 
‘Treatment may be by slow sand filtration, 
by rapid mechanical filtration after the 
addition of chemicals which produce a 
flocculent filtering layer, or by disinfec- 
tion. 
Purification of a water supply is usu- 
ally followed by a marked drop in the 
typhoid fever death rate; from a purely 
economic standpoint this saving of life far 
outweighs the cost of treatment. 
SEWAGE DISPOSAL 
Cities usually discharge their sewage 
into the nearest large body of water: 
New York, for example, uses its two 
rivers and the bay for this purpose. 
The sewage, sometimes washed 
back and forth for days by the tides, 
causes our waterways to be grossly 
polluted. Such a condition, always a 
menace to health, is especially dangerous 
in fresh-water streams or lakes where 
the polluted water has access to the 
supply of some other city. 
The food supply may also become 
contaminated in this way. 
Such local nuisances and dangers to 
health may be avoided by treating sewage 
before it is finally discharged. To do 
this a city has several alternatives but the 
most effective process involves a number 
of successive treatments. 
In CENTER’ \CASE 2 and m 
CHART § are given instances of epi- 
demics caused by disease-producing 
bacteria in drinking water. 
In CENTER CASE 3 is a model of 
a mechanical filter and in CENTER 
CASE 4 that of a slow sand filter and 
of an apparatus for disinfection with 
liquid chlorine. 
A model of a plant for treating water 
with bleaching powder is exhibited in 
CENTER CASE «. 
The data for a number of cities are 
presented in CHARTS 4 and 6. 
CHARTS in CENTER CASE6 show 
the system of sewers in use by New 
York at present and the system as it wil} 
be when completed, the path of a float 
set adrift in the Hudson River, and 
the extent of pollution at various points. 
In the same case is depicted a scene 
on the water-front showing one dan- 
gerous phase of pollution. 
Another model in CENTER CASE 6 
shows one way in which shellfish be- 
come polluted. 


