26 
TYPHOID FEVER IX DISTRICT OF COLUHBL^. 
Falls; about 15 per cent enters it at points between 50 and 175 
miles above Great Falls, and 5 per cent, contributed by about 2,200 
of the population, enters at points between 19 and 50 miles above the 
intake. 
Practically no direct pollution of the Potomac takes place within 19 
miles of the intake, but as this portion of the watershed is inhabited, 
the possibility and danger of direct pollution of the river within this 
section can not be is:nored. 
The great bulk of the pollution of the Potomac being distant under- 
goes more or less natural purification, and in addition becomes very 
greatly diluted before it reaches the intake at Great Falls. 
We recommend the enforcement of streams-pollution acts looking 
to the removal of overhanging privies and other sources of contami- 
nation on the Potomac and its tributaries, especially from Harpers 
Ferrv to the intake. 
THE SAND FILTERS. 
In our opinion the sand filtration plant represents a high degree 
of engineering skill and intelligent management. The water is 
greatly improved both bacteriologically and chemically by the storage 
and filtration, as indicated by the figures on page 27 et seg^. 
It was knovm before the filters were installed that slow sand filtra- 
tion alone could not at all times render the Potomac water clear; 
experience has shown this to be correct. Some other treatment, 
such as increased storage capacity, preliminary filtration or the 
occasional use of a coagulant during periods of high turbidity, is 
necessary in order to obtain a clear effluent at all times. 
It is reasonable to expect that continued experience and further 
experiments vfith the special problem in hand will result in still fur- 
ther improvements in the filtered water, especially so far as turbidity 
is concerned. 
MUD IN THE WATER PIPES. 
The su 2 :gestion has been advanced that the large amount of mud 
contained in the pipes of the water system retains the t}’phoid infec- 
tion and thence it is distributed throughout the city. From our 
knowledge of the viability of the typhoid bacillus we think this 
extremely unlikely, and a few experiments with the mud in question 
confirms this opinion. 
