Evening star. [volume] (Washington, D.C.) 1854-1972, 1895-07-10

face and in the interstices Of the sand,
which collects and retains the bacteria con-
ping Of the inflow. the reverse current is
TINED on and the sand is cleaned.
The amount of alum used, which varies
with the turbidity of the water, is ordi-
nar1ly about three-fifths of a grain to a
gallon of water. 1t is one of the principal
items Of expense in this system. At At
lanta, Ca. 70.O32 pounds of alum were used
In fltering the 756. 1872, 900 gallons of water
used by that city in 1880. At this rate
there would be required for our Washing
ten supply of about 5O.OO0,OOO gallons per
diem 800 pounds Of alum per diem, or
1905, 800 pounds per annum, the cost of
$3S, LOG.
Objections to Alum.
by some the use of alum is objected to
as dangerous to health. but it is claimed by
tn, owners Of the patent and makers Of
the filters that no alum passes through the
sand and that none can be eound in water
filtered by this system when the process
is used as directed. There is one point in
connection with these filters that I have
not seen mentioned. After the sand has
been cleansed, and the water again passes
downward through the sand, certain
time must elapse before the alumina jelly,
on which the efficiency Of the filter as
against the bacteria depends, is again
formed on the surface and in the inter
stices of the sand and replaces that which
mas been washed away by the reverse cur
rent together with the dirt retained by
the filter. Until this time it is obvious that
the water passing through the filter should
De wasted and not sent into the mains, but,
as far as have observed. This has not been
done In any use of this system for the
publication Of public water supplies.
In my judgment, this system should
In no case be applied to our Washington
water supply. which would require proba-
mr. mn of 4 Ia1 o,.,
est size. before it has been thoroughly test
ed without expense to the United States
or the District of Columbia. This could
be done at cost not exceeding 5000 by
means of a single filter through which is
made to pass, for such length of time as
may be deemed expedient, water filled with
bacteria. cultivated for the purpose, II
necessary. and comparing the results with
the known results of the other system of
hlters The test for color should also be
made at the same time, and the water for
testing should be taken from the conduit
Immediately after one of our heavy spring
freshets.
The Natural Process.
THE slow method has been in use a
long time in the largest European Cities"
continued Col. Elliot, Including Hamburg,
Berlin and London 1t has also been adopt
ed by certain American cities. 1t Is not
patented In this system filter bed is
small basin generally Of an acre or less
in size, with water-tight side walls of
masonry or of earth paved with masonry
The number of beds required III any case
depends on the daily supply of filtered
water required. Above system of drains
are placed several layers of tiltering ma
terial, increasing in fineness to the top.
The lower layer is generally formed of
small stones or broken stone, then, pro-
ceeding upward. there Is coarse gravel,
then fine gravel, then coarse sand, and
lastly, at top the fltering material proper,
which Is fine sand. The aggregate depth
of the layers and the depth of each layer
are not uniform in the different countries
and in the different cities of the same
country, but they vary with the materials
available and the judgment of the engi-
neers. The aggregate thickness of the lay
ers Is usually from six to eight feet and
the thickness of the sand Is from two to
four feet. On this latter Depth the depth
or pressure of water on the sand. and the
degree of cleanness of the sand, mainly
depend the vertical rate of fall of the col
umn of water above the sand, the rate and
the duration Of the percolatien of water
through the sand and the yield Of filtered
water per square foot of filter surface and
on them. in turn, depends the degree of
efficiency of the filter. especially as against
bacteria.
The Slime Does the Work.
THE efficiency Of the filters depends on
the Shiny deposit that commences at once
to form on the surface of the sand and in
its interstices, and which, when in suffi-
cient quantity. not only collects, but con-
sumes and destroys the bacteria. When
the deposit forms to such a degree as to
impede too much the percolation of water
the upper portion Of the sand is removed,
and at proper intervals the entire body of
sand Is replaced by fresh sand. From time
to time. as is found necessary. the water
is drained from the sand and the filter is
allowed to rest. The air which replaces
the water oxidizes any organic matter that
remains In the sand. In order to protect
the water from the heat of summer and
from freezing in winter filter beds in this
climate should be rooted
Prof. Leeds of Stevens Institute makes
this explanation of how the bacteria them
selves purify the water: 1t is easy to see
how the filters remove the dirt and sus-
pended r-atter. but the way in which bac-
terra were eliminated was complete mys-
terry until the last four five years. But
few people had ever seen or examined
bacteria before that period. 1t now has
been shown that the bacteria remove the
bacteria. The bacteria in the waters are
comparatively few of a dangerous charac-
ter. the great bulk of them are our great
est friends. 1t Is through their aid, to-
gether with the oxygen of the air that the
filth in the water is destroyed. They feed
upon 1t and they feed upon each other.
Since that knowledge has been obtained,
the object now Is to cultivate the bacteria.
In order to make the filter bed do Its work
effectively 1t Is necessary that the growth
OF lne DdCteTld Snail De 1dClHCateu until q
filter bed becomes populated with an in
credible number of millions of them. AS
the result of their activity they multiply
themselves in vast numbers, and they form,
at the top of the filter beds and between
particles of sand, sort of jelly or slime
a bacteria jelly-and 1t is by the aid of this
bacteria Jelly that the bacteria in the I'M
filtered water are removed.
when filter bed Is first put in opera
ton 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 bacteriologically pure
by chemist and biologist constantly en-
ployed at the filtration works.
Better Filtration II Slower.
"In the European system there is a yield
Of filtered water of from so to 100 gallons
per square foot of filter surface. The in
terstices in the sand occupying as they
do about one-third of the body of the sand
the downward rate Of percolation through
the sand in the European system is about
1 foot an hour, and in the American sys-
tenn Is about Is feet or more an hour. The
depth of sand in the European system be-
ing on an average, say feet, and in the
American system on average, say 5