The Providence news. (Providence [R.I.]) 1891-1906, 1894-06-22
NATURAL FILTRATION.
A Description Of the Plan Ensinesr
Shedd Has in Mind.

The discussion OF Pawtuxet filtration Is
still on among the commissioners and
other members Of the city government
with tho word "mechanical" left out.
filter, such as was advocated by
City Engineer Shedd at the
meeting of the Common Council
when the resolution was passed to appro-
pirate $2S1 900 for q mechanical filter
plant. The. Morrison Jewell system
npon npS Dsin whan partially e Was decided foasd
through the decision OF the United States
Circuit Court at New York that il in
volved an infringement of patent and
the original intention was necessarily
given up. The main point of similarity
between the natural and mechanical fill
ters Is that both accomplished the same
good results; and the principal difference
that in original cost and subsequent cost
OF maintenance the natural filter Is much
cheaper.
Yesterday the question OF amending the
original resolution by striking out the
word "me echanicap was brought up 10
the Board of Aldermen but laid over for
the next meeting. The plan that the
commissioners have now for consideration
is one submitted by Everson Il Liddel, and
the total cost u estimated at 100000 and
the total annual cost of maintenance at
$4,000, as against S2S1 900 and 10000 for
the MOrrlson-JeWel system.
There are in the proposed natural sys-
tenn six distinct filter beds with an area Of
150x300 feet each. A central wall runs
through with three beds on either side,
and in this wall are located the
three principal conduits running
one above the other. The uppermost one
Is the supply conduit from which
face face Of the beds, tho second s
or efiiuent conduit is on level with the
bottom Of the beds and conducts
the filtered water OF to the
reservoirs, and beneath that Is the
third or suction pipe connecting with the
Worthington engine by which any one or
all OF the beds may be drained dry when
1t is necessary for cleaning or other pur-
poses.
The construction of the filter bed pro-
per is simplicity itself. At the bottom
there is a series Of water channels formed
by bricks placed upon end and eight
inches apart from center. to center;
vertical bricks there is q solid flooring of
bricks laid horizontally, end to end, but
without cement or plaster in the joints.
or this floor is 6-inch layer OF
coarse sand or pebbles large enough
not to get through the crevices in the
brick and over this is the top layer of fine
depth. The water supply % 'is' is kept
uniform so there is always depth Of four
feet over the upper strata Of sand,
and 1t percolates through at the rate
OF three inches an hour, while
the aggregate outflow from the entire
area of the six filters into the efflu-
ent conduit is two feet per second
or practically 10 gallons per day
OF water from which all suspended
matter has been removed as well as 99b
per cent. of the bacteria. The process
of cleaning is required about once a
month and the approved method is to
remove as thin a layer as possible of the
top surface OF the sand, on an average of
about a quarter of an inch. The scraping
is cleaned by being subjected to a power
-fnI fire stream and is then ready Tor use
again.
There is a preliminary cleaning process
through which the water goes before
1t flows on the filter basin. A
fore bay is formed in the river
by a retaining wall, which keeps the
water at greater height than it is in the
filter. In flowing from the basin thus
made to the supply conduit it
passes through a rack and six lines OF
graded screens, the first with a 81 inch
mesh, the second u inch, the third Y
inch, the fourth 1-18 inch, and the last
two with 1-22 inch mesh each.
The total area of the filter beds is 6is
acres while that required for the whole
plant Is between eight and nine acres. III
adopted the plant can be in operation in
another year.
On Wednesday the members of the
Board OF Aldermen are to go to Lawrence.
Mass, and inspect the natural filtration
system there which is practically the
same as that proposed for this city.
ron for Mountain or Seashore,
stock Yourself with Shoes at