Evening star. [volume] (Washington, D.C.) 1854-1972, 1899-07-15

Action Of the Medieal Soctety.
From 1887 until 1894 the question OF NJ
! tration appeared to have been settled. But
| sometime during the winter of 1S93-9i, at
| meeting of the Sanitary League DR.
Charles Smart of the army, severely ar
| raigned the water supply of Washington, in
| discussing the enormous typhoid fever
! death rate. The Medical Society of the Dis
| trict of Columbia had in 1885 made re-
port in which t was alleged that the Poto-
| mac river supplied a water which in ex
| ce11ence a drinking water ranked fourth
| or fifth among the water supplies of the
country. and DIR. Smarts statement stirred
| the society into action. committee was
| at once appointed to investigate the al
legged prevalence of typhoid fever in the
| District of Columbia, and DIR. Smarts
statements were more than sustained. The
report of that committee which sub
! mitted on June g. 1894, urged among other
things the early completion of the plans
| recommended by Colonel Elliot, in charge
| of the Washington aqueduct and now in
course of execution, which have in view the
| sedimentation of the Potomac water and
ultimately the completion of works for fill
tration, the only proper method of purifica
ton" Since that time the question of nj
| tering the water supply of Washington has
been constantly before the public. and
| demand which has been so often made
| during the past forty-fve years. and which
| is now supported by facts SO clearly dem
onstrated, cannot go unsatisfied much
/ longer
| For the filtration of public water sup
plies two methods are in common use
sand filtration and mechanical filtration.
The former accomplishes the desired
| suit by the passage of the water slowly
through layers of sand and gravel of grad
uated sizes. the filtering medium being
| spread out in extensive beds. The latter
purifies 1t by forcing it rapidly through a
certain amount of sand, after it has been
treated with some chemical, usually alum,
| the sand being contained in large metal
! cylinders.
Sand Filtration.
The ordinary capacity of sand filter is
100000 gallons of water per acre per day.
A sand filter for this city would, therefore,
have to be constructed to permit
| about twenty acres to be constantly in
operation, and certain part of such
| filter is constantly out of use in the process
| of cleaning, it would probably cover at
least twenty-five acres of actual working
| surface. without the sedimentation basins,
which must be used in connection with it.
| In the construction of a sand filter it is
usual to divide 1t into a number of separate
| filter beds, so that accident to one or
more need not stop the entire process. and
| so that the different part can be used and
cleaned in rotation. Each filter bed is
usually about one acre in extent, and is
sentially tight reservoir, suitably under
drained, and containing about six feet of
| fltering material, in layers, and arranged
! in progressive degrees of fineness, beginning
at the bottom with a layer of broken stone
! and with the uppermoSt layer of fine sand.
| In localities where the temperature remains
below the freezing point for considerable
| periods of time, filter beds must be covered
so as to prevent the formation of ice. In
| connection with hand filters sedimentation
| basins are commonly used so as to dispose
| of as much suspended matter as possible
without cloggng the filter with it
Efficiency of Filter.
After a filter has been constructed its
efficiency depends upon the care with which
| it is operated. If water is allowed to flow
| through the sand too rapidly the filter is
| damaged and satisfactory results ere not
| accomplished. On the other hand too low
| a rate Of filtration is unnecessarily expen-
| sive as it increases the area of filter surface
which is necessary to construct. Four
| vertical inches per hour is about the limit
| of speed for filtration. In order that the
rate of filtration m&y be practically uni
form, devices are necessary to regulate the
mead OF water. In any case the water
which Is first run on the filter bed is allow
| ed to run off without being used, as a filter
| bed, which is perfectly new or which has
Just been cleaned is not so efficient as ne
| which has been in use for a few days.
Contrary to what might have been eX-
| pected, tire sand and gravel of which the
| filter is composed do not accomplish alone
| the real work of the filter and in this fact
| lies the necessity for allowing the first
| how of water to waste. During the period
| when such water is running through there
| Is formed on the upper surface of the sand
gelatnouS substance, frequently known
| as the dirt layer" varying in thickness
| from one-half inch to one inch. Eff
| fective filtration does not occur until this
| has been formed, nor will it occur after it
Las been broken But as the quantity of
water which is passed through the filter
increases this Shiny layer becomes thicker
and thicker until the flow of water is so
retarded as to render it necessary to dis
[ continue the use of the filter bed, The
length of time during which a single bed
can be used varies according to the char
acter and quantity of water filtered and
the efficiency of sedimentation. but is or
dinarily from two to six weeks. When a
bed has become SO clogged to be no
longer mt for use it must be cleaned. In
order to do this it is necessary to drain
the water from it and to scrape off by hand
that part of the sand which contains the
Shiny mass or dirt layer" After cleaning
the filter is again ready for use, but when
the process mas been repeated several times
fresh sand must be supplied to restore the
thickness. The dirty sand can be washed
and used for this purpose. 1t must not be
understood that the gelatinous layer which
covers the top of the filter accomplshes all
of the work. While it contains more than
one-half of the total number of bacteria
| removed from the water by the filter bed,
the lower layers must be depended upon to
remove the remainder.
BacterioiOgicnl Supervision Necessary
For the satisfactory operation of the fill
! necessary. Samples of water are collected
from the main from which the water flows
into the filter, and corresponding samples
are taken from the outflow. A filter bed in
good condition should remove from 9S per
cent to 1913 per cent of all bacteria which
find their way into it. If at any time the
analyses show an excessive number of or
ganisms present in the filtered water the
particular filter bed affected is put out of
use and an investigation made to deter
| mine the cause, so that the proper remedy
can be applied.
The Mechanical System.
The mechanical system of filtration is an
attempt to initiate the method of sand fill
tration by producing in the water what We
may call am artificial dirt layer In this
case no filter beds are used, but the water
! is filtered in large tanks. The method most
commonly used is adding a minute quantity
of alum, varying from one-quarter to one
half grain per gallon to the water to be
! filtered, after which it is passed through
| fine sand which is contained in large iron
cylinders. The addition of the alum causes
a white gela tinous precipitate, which is ar
I h a IF l.,