462 
RESPIRATION AND EXCRETION 
located on or near the ocean or Great Lakes let their sewage 
run into them. Those that are built on a stream or river 
empty into this small body of water and the town or city 
farther down the stream does the same. Smaller towns that 
are not located upon any body of water have installed fil¬ 
tration plants. These are very effective where the total 
amount of sewage is small. Where does the sewage of your 
city or town go ? 
Stream Purification. — Some recent studies made under 
the direction of the Massachusetts Institute of Technology 
show that there are a number of biological factors that assist 
in the purification of sewage polluted water. These are a 
number of organisms that feed upon the organic matter that 
is suspended in the water. The first living things to be found 
where the water is most polluted are bacteria. As the 
sewage empties into the stream it is rich in food for bacteria 
and they become very numerous. As they feed upon this 
food, it is broken up into simpler chemical bodies and the 
water becomes clearer. A little farther away from the mouth 
of the sewer, are found numerous protozoa that feed upon 
these bacteria and thus tend to remove them from the water; 
while still farther down the stream, are to be found numerous 
tube-bearing worms that in turn feed upon the protozoa. 
If the water is now examined, it is found to be much clearer 
for most of the sediment has either settled to the bottom 
or been destroyed by these several organisms. Such water 
as this is not fit to drink. 
Water Supply. — Plenty of pure water for drinking and 
cooking is indispensable to man. The necessity for disposing 
of the sewage has made this problem increasingly difficult. 
Sewage-polluted water is never entirely safe for drinking 
water. The two must be kept separate. This is the main 
reason why so much money has had to be spent to bring our 
drinking water to the cities and this is the reason why so 
much care is taken to prevent this same drinking water from 
