The Sanitary Engineer and the Public Health. 45 
ment of water that is to be used for drinking purposes is slightly dif- 
ferent from the treatment given sewage, but most of the methods that 
are effective with one are also effective with the other. Many plans have 
been proposed and tried, but so far sand filtration has been found to be 
more effective than any other practical method. Chemical precipita- 
tion, alone or in connection with filtration, has been extensively tried, 
but when used alone it has not been found to be effective, while the sand 
filtration method has. In treating water for the use of cities it is often 
necessary to throw down the heavier particles of silt and other sus- 
pended matter before filtering, so as to avoid clogging the filters, and 
also to reduce the amount of work required of them. The coagulant 
usually employed is sulphate of alumina, though salts of iron and lime 
are also effective, but the lime salts are open to the objection that they 
make the water hard. 
Two methods of filtration are in general use in purifying water sup- 
plies — the one known as the English, the other as the American system, 
the latter being often referred to as mechanical filtration. Omitting 
technicalities, it may be said that the difference lies chiefly in the rate 
of application to the filtering area, and the fact that coagulants are used 
with the process of mechanical filtration, though it is sometimes neces- 
sary to first effect clarification before applying the water to the filter in 
the slow sand filters also. 
In the experiments recently ended at Pittsburg, which were carried on 
for nearly a year in order to determine the best method to be used there, 
the merits of the two systems were carefully studied for the particular 
sources of supply available — namely, the Allegheny and Monongahela 
rivers. It was stated that the slow sand filters would purify from two 
to five million gallons of water per acre per day, while the mechanical 
filters would purify at the rate of 104 million gallons per acre per day, 
though the cost of operating the latter was greater. In connection with 
Mr. Bouscaren^s report on the matter of water purification for Cincin- 
nati, it was stated that mechanical filters could be successfully worked at 
the rate of 125 million gallons per acre per day. It was found at Pitts- 
burg that both methods clarified the water and reduced the number of 
bacteria within the limits of what is required for a potable water. For 
example, the sand filters reduced the number of bacteria from 16,340 to 
153 per cubic centimeter — an average of two sets of tests — while the 
mechanical filters reduced the number from 11,427 to 262 per cubic 
centimeter. The sand filters, therefore, had an efficiency of over 99 
per cent., while the mechanical filters had an efficiency of 97.7 per cent. 
These tests were made on water from the Allegheny river from which, if 
I remember correctly, the water supply was drawn in 1892, when I was 
in Pittsburg. 
