Evening star. [volume] (Washington, D.C.) 1854-1972, 1894-12-19
WATER FILTRATION
Merits of Two Systems Discussed by
Col. Elliot.
ESTIMATED Cost APPLIED LOCALLY
There is No Immediate Need for
Any Filtration Scheme.
on r VA 7 E r 81 P p l. Y

In his annual report, Col. George H. FL
lot, the engineer officer in charge OF the
Washington aqueduct, devotes a chapter
TO an interesting review of the different
methods and advantages of filtration.
Every year, mainly in the latter part Of
winter he says, when heavy rains fall
ing on the watershed of the Potomac end
its tributaries. especially the Shenandoah,
have found the ground loosened up by
alternate thawing and freezing, there are
complaints in the press and elsewhere OF
the tur)idity of the Potomac water, and
the same is true in the spring when the
fields are plowed. They are not very ur-
gent or very prolonged, for the water soon
clears; but as they are sometimes accom-
panied by demands for the filtration Of the
water, 1t may be we'll in this, my last an
nual report before retirement from active
service, to describe in general way the
two systems of filtration now in use, and to
state what would be approximately the
cost of each if applied to the water fur
nished to the District of Columbia.
II should first remark that it seems to be
commonly believed that, although our Po-
toInac water is often so offensive to the
eye as to make it appear to be unfit even for
bathing purposes, IL does not contain germs
of disease and is not unhealthful l think
this belief is well founded
Two Methods of Filtration.
Gel. Elliot refers to reports made by DR..
Busey, president of the District Medical
society, and by ex-Surgeon General Wales
of the navy, to the effect that the water
of the Potomac is as good for drinking
purposes as any river water in the world.
He describes the sources OF the Potomac,
and express es the convection that the peo-
pie of the District of Columbia have little
10 fear from the water OF that river. Al
though all the conditions are favorable TO
the healthfulness Of our Potomac water,
Col. Elliot thinks it would be wise to at
tach to the next appropriation for the
Washington aqueduct a special provision
of law for systematic monthly chemical
and archaeological examination of the
water as it is sent to the city from the dis
tributing reservoir
Col Elliot describes the two PRINCIPAL
systems of filtration. Mechanical. or rapid
filtration, is a bToad term used to denote
those systems of water publication in
which an exceedingly rapid rate of Ultra
lon is made possible by means of mechapi-
cal devices for frequent, quick and thor
ough waShingS of the filtering materials
without removing them from the filter. The
other system, known as natural, or slow
Ultra tion," has been in use for the purid-
cation of public water supplies of Europe
for very long period. The water of Lon
don, Berlin and Hamburg, and other large
cities, is filtered by this system, the most
extensive application of it being at London,
where about 2oO,VOO,0OO gallons are uttered
daily. The system has also been used in
several cities in this country, and is not
patented. The mechanical system which is
known as the American system, accord
ing to Col. Elliot, should in no case be ap-
piled to our Washington water supplies
which would require probably 200 more
filters of the larger size, until it has been
more thoroughly tested.
The American system.
''In the American SYSTEM" says Col. Ell
lot, 1.-The fall in the column of water above
the sand is atthe rate of five inches or more
a minute. making yield of filtered water
of 5000 to 1000 gallons per square foot of
filter surface per diem. This rapid rate is
produced either by a great pressure or
head of water on the sand by which the
water is forced at rapid rate through it,
or by frequent cleansing of the sand for
the purpose of removing Instructions to a
rapid passage of the water, or by both.
The objection to the first is that assures
or channels are liable to be forced through
the sand through which the water may
pass without any modification, and the OBJ
lection 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 to combine with the carbonate of lime,
existing to a greater or less degree in all
natural waters, and to form jelly-hke
substance Chydrate of alumina) on the sur
face and in the interstices of the sand,
which collects and retains the bacteria
contained In the water until, after the
stopping of the inflow, the reverse current
Is turned on and tho sand Is cleaned.
Use of Alum.
1.-THE amount Of alum used, which varies
with the turpidity of the water, is ordi-
narily about three-fifths of grain to
gallon of water. 1t is one of the principal
items of expense in this system. At At
lanta Ca, 7O,032 pounds of alum were
used in filtering the 756, 10000 gallons of
water used by that city in 1889. At this
rate there would be required for or Wash
ington supply of about J0,COOOOO gallons
per diem 800 pounds of alum per diem,
or 800, 800 pounds per annum, the cost of
which, at cents per pound, would be
SIS, 1906
"By some the use of alum is objected to
as dangerous to health. but 1t is claimed
by the owners of the patent and makers of
the filters that no alum passes through the
sand and that none can be found in water
filtered by this system when the process Is
used as directed. Many testimonials to
this effect and of the general excellence of
this system accompany the advertisements
of these filters; but this, 1t may be re-
marked. Is the case with the advertise
ments of all patented articles.
The European System.
In regard to the European, or natural
system of slow filtration. Col. Elliot says
that the efficiency of the filters depends
on the Shiny deposit that commences at
once to form on the surface Of the sand.
and in it's interstices, and which, when In
sufficient quantities, not only collects, but
consumes and destroys the bacteria. when
the deposit forms to such degree, says
Col Elliot, "as to impede too much the
percolation of water. the upper portion OF
the sand is removed and at proper inter-
walls the entire body of sand is replaced
by fresh sand
From time to time, as Is found neces-
sary the water Is drained from the sand
and the filter is allowed to rest. The air
which replaces the water oxidzes any
organic matter that remains in the sand.
in order to protect the water from the
heat of summer and from freezing In WIN
ter filter beds in this climate should be
rooted. When a filter bed is first put In
operation and afterward, after each Of
these changes the water passing through
the filter is allowed to run to waste for
about ten days or two weeks. until it is
found to be chemically and bacteriologi-
cally pure by a chemist and biologist con-
stantly employed at the filtration works.
Rate of Filtration.
1.-The vertical rate of fall Of water in the
ole