The Washington times., 1894-12-20
FILTERS FOR WATER WORKS.
Reparts-That He Does Not Regard
Them Unneces ssary.

Them Unnecessary
The causes of the discoloration, the health
fulness, and the suggested method of filtration
OF the waters of the Potomac. are three of the
subjects interestingly treated of in the recent
annual report CT Col. George 81. Elliott, en-
gineer in charge of the Washington aqueduct.
The diseoloration, he:state.is caused by the
loosening of the earth by alternate freezing,
and thawing in the winter season, and in the
spring by the waters which feed the river
passing through fields broken up by the plow.
Notwiths landing the appearance ofthe water,
which he describes as offensive to the eye, he
believes that it does not contain germs of
disease and is not unheathful. He agrees
with the gere al impression on this point.
Nevertheless he advises that in the next ap-
preparation for the agueduct a provision De
made for monthly chemical and bacterio-
logical examination OF the water as 1t comes
from the general reservoir.
As TO the healthfulness of the water he
calls attention 10 the reports of DIR. Busey.
president OF the District Medical Society, and
OF eX-Surgeon General Wales. Of the Navy.
who declare the water of the Potomac to be
as heaithful for drinking purposes as that OF
any other river. Col Elliott describes
dlSO the NdflOuS lI1BulbitNC duO ALO oOClNOo
of the Potomac.
The problem of filtration claims a large
share of Col Elliott's attention, and in this
relation he speaks in detail OF the rapid. or
mechanical, and the slow, or natural, pro-
cesses. He notes the fact that the slow pro-
cess is in vogue in. London, Berlin, Hamburg,
and other large cities The mechani al. or
American system, is not advised by Col. Ell
lioit in the case of the Potomac, until the
process has been more thoroughly tested.
The American system he describes as fol-
lows:
The fall in the column of water above the
sand is at the rate of five inches or more a
minute. making a yield of filtered water of
5000 to 1000 galloDs per square foot OF filter
surface per diem. This rapid rate is produced
either by great pressure or head of water
on the sand by which the water is forced
at a rapid rate through 1t, or by frequent
cleansing of the sand for the purpose of re-
moving distractions 10 a rapid passage OF the
water, or by both.
1.-The objection to the first is that assures
or channels are liable to be forced throu ga
the sand through which the water may pass
without any modification, and the oh, ,ectIon
to the second is that clean sand alone does
not make an effective filter against bacteria
contained in the water, This latter objection,
it is claimed, has been overcome by the use
of alum, which is said 20 combine with the
carbonate of lime, existing to a greater or
less degree in all natural waters, and to form
a jelly-like substance (hydrate OF alumina) on
the surface and in the intcrstiees of the sand.
which collects and retains the bacteria con-
tanned in the water until after the stopping
of the inflow, the reverse current is turned on
and the sand is cleaned
Speaking of the European or slow process
he saws that the efficiency OF the filters de-
pends on the Shiny deposit that commences
at once to form on the surface of the sand
and in its interstices, aad which, when in
sufficient quantities, not only collects, but
consumes and destroys the bacteria.
| The depth of sand in the European system
being on an average say three feet, and in
the American system on an average say five
feet, it follows that water in process of filter
! ing is, in the European system, in contact
with the flltering sand about three hours, and
| in the American system about four minutes.
His estimates are based on 50,000,000 gal
! Ions consumed and wasted daily in this city
and the filtration of Is, 10000 600 gallons as the
! approximate volume to be used ten years
hence. The first cost for the former amount
| he places at 5000, ,G67, and of the second
/ amount at 81 ,000. 600. by the natural or slow
/ process. BY the mechanical process the figure:
would be raised to SSS3 600 and 81. .423, 500
respectively.
! The annual cost of maintenance at the rate
/ of $4,000. 600 gallons per day by the natura
filtration system would te S24, 1888, and by