New-York tribune. [volume] (New York [N.Y.]) 1866-1924, 1906-01-14

thick II adds to the efficiency of the filter. for II
helps TO retard material and bacteria which
might have passed through the interstices OF the
sand At certain period this blanket or felt
on the surface of the sand. called by the Ger
mans ''schmutzc decke." becomes compact
that it stops filtration and the water must then
be drawn off Then workmen with shovels arr
sent in 10 clean off the dark mud film from the
top or the white sand bed by scruping. They DO
this with ordinary shovels. All sorts of imple-
mcrts have been devised for Scraping off the
filter beds eXpeditiousy, but they are really not
necessary. \ plain shovel in the hands OF =
brisk workman viii do the work about Us ef-
fcctiycly could be desired y score Of men
get inside ana. standing on broad flat heather
mud shoes" they begin carefully scraping off
the top Of the sand bed with its "schmutze
dcckc' everything it The fine edge Of the shove!
| need be inserted only quarter or half an inch
in the sand To scrape it. The brown scum
top is removed in Wheelbarrows. and the top
OF the filter bed is again left clean and white
When the whole surface has been scraQci about
half an inch deep in this fashion. every par,
ticle OF mud being removed in the course Of q
f"W hours the filter is again ready for use
Skilled labor Is .ot essential. for the negro
workmcn in Washington learned readily how to
scratch on the thin dark layers Of clay. which
balls a. II is scraped, leaving the white sand
beneath.
The removal OF the top layers after certain
period would of course render the thickness Of
the sand layer too small to filter properly and
new sand must be placed in the filter TO take
the place OF that removed by sCrapuC. but il
is found in practice that two foot sand bed
is efficient. and as it would take twenty four
scrapngs OF a three foot bod To reduce il 10 two
feet. and as scrapirFs are needed only once in
| thirty days. filter bed could two years on
an average without renewal
The sand that has already performed service
for nltering and has been scraped from t1e top
OF the bed in the opinion Of the engineers. has
not been impaired in value for further use. Vt
present this sand which is taken off the beds
Is collected the roof for future washing when
automatic hydraulic cleaning devices are in
| stalled. they will be in n few weeks. In con-
| neotion with these the scraped sand in mo
filters will be elected AUTOMATICALLY to the wash
ing device-an iron chamber-through which
the sand in passing is thoroughly cleanseL Of
PARTICLES of mud and germs by powerful sprays
OF filtered water. The mud flows Off into The
sewers, while the sand is removed to storage
bins for further use.
That the sand filter does remove practically
every trace of bacteria has been thoroughly
proved at the Washington filtration plant.
Weekly records before the operation Of the
filter showed that whereas the Potomac water
supplied To the residents of Washington then
contained from 40 10 80 bacteria to thu. cu..
centimetrc after being filtered it contained
only from IL. TO 10 similar bacteria. The
average turbidity of the water received At
Great Falls for the month of October 81
parts OF suspended matter in Il million Ky the
filter it clarified to average of l.; Tor
the month.