Evening star., 1901-02-19
Other Important Factors.

ora otbar very iu7Dortant factors to be con-
sidered besides the average turbidity or the
weight of suspended matter Most proml-
nent among them are the hneness OF the
suspended matter during noou pefious anu
the duration of periods of tuIbid water
with streams where periods OF turbldity
are of short duration 1t Is OFTEN possible at
such times to close the intake, thereby
avoiding the necessity of treating the most
turbid water The supply IN the meantime
must be maintained from storage reservoirs
We find that for Washington this procedure
would not always be practicable, because
the periods of highly turbid water some
times last much longer than the period dur
ing which the supply could be maintained
from the existing reservoirs, or from reser-
voirs which il is practicable TO build.
1--WE find that the water OF the Potomac
river at Great Fails following heavy rains
IS more turbid than any water which is
successfully purified by slow or sand filters
in this country. Few exact data regarding
the turbidities of European streams are
available but it is certain that none of the
waters filtered by the better known Euro
pean sand filters approach, on an average,
the turbidity of the Potomac water.
by subsistence it is possible to reduce
materially the excessive turbidity of river
waters by passing them through reservoirs.
According to the best evidence now ava1la-
BILE there times when plain subsistence
of this river water for any reasonable pc
Rod of time would not afford an adequate
treatment preparatory to its successful
publication by sand filtration. During some
years this condition would not obtain, While
during others subsistence might be made
quate for periods amounting in the aggre-
gate to ten weeks and on an average to
about one month each year.
1T IS further probable that the bacterial
efficiency Of the process might sometimes
become inadequate, due to the general dis
arrangement of the filters caused by excess
sive turbidity. This condition would be less
important at Washington than it was at
ff1,f. f.;,o,a -trn Dtap of 1Dn tix-
at Cincinnati was found to be SO turb!d that
that 1t could not be adequately purified even
after sedimentaton, by sand filtration. for
about 30 per cent of the time On this
basis we estimate that at Washington the
proportion of the time when sand filters
alone would be Inadequate would amount
to about per cent.
1--WE have based our consideration of the
problem on daily supply of 75,000,G00 gal
Ions of water, which approximues the pres
ent carrying capacity of the aqueduct This
amount is sufficient to supply all legitimate
needs of population much greater than the
present one. Whenever new aqueduct be-
comes necessary the question of purifying
the additional supply of water thus obtained
will be a part of the new problem In our
opinion it is unnecessary now to consider
the publication of more wafer than can be
carried by the existing aqueduct.
Large Waste of Water.
I'm this connection we beg to call your
attention to the large waste Of water in
Washington, and to suggest the advisable
ity of taking measures to limit such waste.
One of the most effective means of doing
this is by the use of meters; and we recon
mend that they be placed on all public
buildings at once, and on private services,
when deemed necessary, as rapidly as prac-
ticable.
THE question of deciding between the two
systems of purfication, with the improve
ments above indicated, is in some respects
In oh
Washington is intermediate between the
water for Pittsburg, where sand filters are
best adapted TO the conditions, and the
water for Cincinnati, where American filters
are preferable If the Potomac water were
more turbid, turbid for longer periods
than the records show, mechanical filters
would unques tionably have the preference
If 1t were less turbid, il turbid periods
were of shorter duration, the advantage
would clearly lie with sand filters.
practical experience with sand filters is
much more extensive and more favorable
than that with mechanical filters. Our
Knowledge of what they will do rests not
alone upon experimental Investigations, but
upon actual use for many years by some
of the largest cities Of the world. The
force Of this statement Is somewhat re-
drced, however. by the fact that the raw
water at Washington Is more turbid than
the raw water at the places where sand
nlters have been generally used.
1T appears from consideration of the
eHldehce al our dlsposal that the average
yearly bacterial efficiency of the two sys-
TENS would be about equal During the
warmer months Of the year this efficiency
would probably be greatest In the case OF
sand filters. When the river water is at or
near the freezing point, the conditions upon
which sand filters depend are less effective,
and the efficiency of mechanical filters
would probably be a little greater. This
deterioration of sand filters would not be
changed by covering them aS
1t appears to depend upon the tempera-
ture of the water while passing the filters,
and this Is not materially influenced by
covers.
Little Difference I'm Cost.
There Will apparently be but little diff
ference In the cost of the two systems.
The cost OF constructing sand filters is
larger than the cost of constructing me-
chanical filters, but the Inter are more
expensive to operate; and when interest
and depreciation charges on the invest
ments are added to the costs of operation,
the difference between the totel costs Is
"lthiIl the HmltS OF accuracy OF the Two
estimates.
After full consideration Of the various
aspects OF the problem we are of the opin-
ION that the long and favorable experience
WIN sand nlters, particularly in the light
of the effect which they have had upon the
health of the communities using them,
should be given greater weight than the
present evidence that American filters are
efficiency. In view of tha fact that there
is no available evidence OF, decided advan-
tage to be gained by adopting the newer
method, we prefer in this case to adhere
to the one supported by iang precedent.
I'm regard to the exact site for filters
the short time at our disposal has not al
lowed US to make the NECESSARY surveys
and detailed estimates OF cost to decide
definitely whether a site near the new How
ard University reservoir sufficient for the
construction Of filters to purify all the wa
ter that can be gotten through the present
aqueduct, or a site beyond the District
limits, and near the line Of the aqueduct
large enough to allow almost indefinite eX-
tension, is to be preferred The latter site
has this advantage: That filters constructed
upon it would be at such an elevation that
the filtered water could be supplied at
greater elevation than thai which would be
available from the Howari1 University res
cit may be found upon further study that
this additional pressure Twill justify the
somewhat greater cost Of filters IN this vi
cinfty. n
Tin consideration Of the full evidence we
recommend the construction OF a complete
system Of slow or sand filters. with such
auxiliary works as may be necessary for
preliminary sedimentation. and the use Of
coagulant for part of the time. There is
no reason to believe that the use Of this
coagulant will in any degree affect the
Of the water"