hasten the restoration of the capacity of a filter, which proved to
be a most useful one. The removal of the scraped sand from a filter
was a matter of a good many hours' work, under the most favorable
conditions. To get the filters quickly into service again, the dirty
sand in a number of them was simply scraped from the surface, heaped
into piles, and left there; then the water was turned in, and the
filter was started again. This was done with some hesitation at
first for fear the presence of the piles of dirty sand might cause
high bacterial counts in the effluents of those filters. No such
effect was observed, however, the counts being entirely normal
throughout. The writer subsequently found the same treatment being
applied as an emergency measure at the Torresdale plant, in
Philadelphia, and, through the courtesy of the Chief Engineer of the
Bureau of Filtration, was furnished with the bacterial counts
through a number of runs made under these conditions, and there,
too, the results were entirely normal.
There was practically no economy in this method, as the sand had
ultimately to be ejected and washed. The piling up of the sand had
the effect of reducing the effective filtering area by a small
percentage, with a corresponding increase in the actual rate of
filtration, but this was of trifling importance. The great benefit
derived from the method was the saving of time in getting a filter
back into service after scraping, and in this respect it was very
valuable.
~Physical Theory of Purification of Water by Slow Sand Filters~.
The first and most natural conception of the action of a sand filter
is that the removal of impurities is effected by a straining action.
This, of course, is perfectly true as far as it relates to a large
part of the visible impurities. Much of this is gross enough to be
intercepted and held at the surface of the sand. This very straining
action is an accumulative one. After a quantity of suspended matter
thus strained out mats itself on the surface of the sand, it in turn
becomes a strainer, even better adapted than the clean sand surface
which supports it for the removal of suspended matter from the
water.
This, however, cannot explain certain features of the purification
of water by a layer of sand. The removal of color, the reduction of
nitrates, and certain other changes in the organic content of the
water have for a long time been recognized as due to a bio-chemical
action carried on by certain bacteria in the sand. Both the
straining action and this bio-chemical action are not all-sufficient
for the explanation of certain phenomena, and it has been
recognized, too, that sedimentation in the pores of the sand played
a large part in the purification process in those cases in which it
was apparent that the biological agencies were not the chief ones.
In the purification of water containing only insignificant
quantities of suspended matter, but a relatively large amount of
unstable organic matter, it will be conceded at once that the chief
factor in the purification is the nitrification produced by the
bacteria in the upper layers of the sand. On the other hand, the
purification by sand filters of a hypothetical water containing no
organic matter, but only finely-divided mineral matter in
suspension, could take place only by the physical deposition of the
particles upon the sand grains. Between these two extremes lie all
classes of water. In all problems of water purification by
filtration through sand, both these factors--biological action and
sedimentation--play their parts, assisting and supplementing each
other, the relative importance of one factor or the other depending
on the place of the particular water in question on the scale
between the two extreme conditions just mentioned.
In Mr. Hazen's paper on "Sedimentation"[1] there is an interesting
development of the theory of the removal of suspended matter by
sedimentation in the pores of a layer of sand. The factors influencing
this removal are the rate of filtration, the effective size of the sand,
and the temperature of the water. For the conditions at the Washington
plant, it may be assumed that the first two of these factors are
constant. The third factor, however, varies through wide limits, and the
observations on the turbidity removal, and on the different phases of
the filter operation of which the turbidity of the water is a factor
under varying temperature conditions, together with the known relations
between hydraulic values and temperatures of water, furnished good
substantiative evidence that this highly-induced sedimentation may be a
considerable factor in the purification of the water as effected at this
plant. This temperature relation, briefly stated, is as follows: For
particles of a size so small that the viscosity of the water is the
controlling factor in determining the velocity of their subsidence in
still water, that velocity will vary directly as (T + 10) / 60, in which
T is the temperature, in degrees, Fahrenheit. That is, when the
temperature of the water is between 70 deg. and 80 deg. Fahr., a
particle will settle with twice the velocity it would have if the water
were near the freezing point.
[Footnote 1: _Transactions_, Am. Soc. C. E., Vol. LIII, p. 59.]
The layer of sand in a slow sand filter may be considered as a very
great number of small sedimentation basins communicating one with
another, not in the manner of basins connected in series, but
rather, as Mr. Hazen has expressed it, as a long series of
compartments connected at one side only with a passageway in which a
current is maintained. In any section of the sand layer there are
areas through which the water passes with a velocity much greater
than its mean velocity through the total area of voids, while there
are other areas in which the velocity is very much less, perhaps in
an almost quiescent state from time to time, greatly favoring the
deposition of particles, but with a gentle intermittent circulation,
displacing the settled or partly-settled water and supplying from
the main currents water containing more suspended matter particles
to be removed. There is thus a considerable percentage of the total
volume of voids in which the water is subjected to very favorable
conditions for sedimentation, almost perfect stillness and an
exceedingly small distance for a particle to settle before it
strikes bottom on the surface of a grain of sand.
If sedimentation were the predominating factor in the purification
of the water, we would then expect to find the following phenomena
in the operation of the filters: A more rapid deposition of a given
amount of sediment under summer temperature conditions than under
winter, as the water passes through the sand, and therefore, for the
former condition of higher temperature:
(a) A greater concentration of this turbidity-producing material
in the top layer of sand, or, in other words, a thinner sand
layer to be removed in scraping if all the dirty sand is
removed;
(b) Because of the greater concentration, a greater rate of
Increase of the loss of head, and consequently shorter periods
of service between scrapings;
(c) A higher limit for turbidity in the water applied to the
filter to produce a given turbidity in the effluent.
~Table 30--Service Periods and Scraping Depths for Runs
Ending In Various Months; Covering Entire Period,
October 1st, 1905, To March 1st, 1907.~
==========+=========+===========+===============+=============
| | Average | Average | Mean
| Number | period of | depth of sand | temperature,
Month. | of | service | removed, in | in degrees,
| filters.| in days. | inches. | Fahrenheit.
----------+---------+-----------+---------------+-------------
January | 13 | 75 | 2.09 | 39
February | 6 | 98 | 2.46 | 37
March | 5 | 130 | 2.66 | 41
April | 8 | 149 | 2.96 | 53
May | 7 | 130 | 2.80 | 67
June | 11 | 124 | 2.35 | 77
July | 17 | 70 | 2.12 | 81
August | 2 | 49 | 1.98 | 80
September | 5 | 73 | 2.48 | 76
October | 37 | 70 | 1.56 | 64
November | 20 | 42 | 0.81 | 49
December | 14 | 57 | 0.94 | 40
==========+=========+===========+===============+=============
The operation of this plant during the first year and a half offered an
excellent opportunity for the study of sedimentation in the sand, and
the data in Table 30 are presented to show that certain of the phenomena
of filter operation observed during this period seem to be fairly
explicable by the physical theory of purification. These data are given
only for the period of operation before the summer of 1907. At that time
the experiments in filter cleaning already described were begun. Before
that time, whenever a filter had been cleaned, all the discolored sand
had been removed, leaving for the following run a new sand surface
substantially in the perfect condition of a newly-constructed filter.
After that time the experimental methods of cleaning, and the new
routine adopted as a result thereof, interfered with the tracing of the
evidence as clearly as during the earlier periods.
[Illustration: ~Figure 14--Periods of Service and Depths of Scraping
for Runs Ending in Various Months Covering Entire Period Oct. 1, 1905,
to Mar. 1 1907.~]
Table 30 and the corresponding diagram, Figure 14, show the general
variations in the length of runs and depth of penetration, with the
seasonal temperature changes. The increase in length of runs and
quantity of sand removed under low temperature conditions is very
marked. There is, however, a secondary maximum which appears, as the
diagram shows, where a minimum for the year would be expected. This
may have been an irregularity occurring this one year, which will
not appear in the average of several years, and caused by some
factor which has escaped observation. A careful analysis of the data
at hand fails to show any explanation for it. It may exist in some
of the little-understood biological actions which have their maximum
effect under warm-water conditions, or it may be due--in some
obscure way--to the liberation of air under the surface of the sand,
accumulating with pressure enough to break the surface at
innumerable points, thereby reducing the loss of head and extending
the period of service. Some evidence was observed pointing to this
explanation, but it was never conclusively proven.
The general effect of temperature changes on the rapidity of removal
of the sediment and its consequent concentration in the sand layer,
however, seems plainly evident.
In corroboration of the third point mentioned in the theoretical
consideration of turbidity removal in the filters, the daily
turbidities of the filtered water have been classified and
summarized for different turbidities in the applied water, and also
for different temperatures. The average turbidities thus obtained
are given in Table 31.
~Table 31--Turbidity in Filtered Water at
Different Temperatures Produced by
Given Turbidity in Applied Water.~
==========+=================================================
Turbidity |
of | ~Temperature, in Degrees, Fahrenheit.~
applied |---------+---------+---------+---------+---------
water. | 40 | 40 - 50 | 50 - 60 | 60 - 70 | 70
----------+---------+---------+---------+---------+---------
20 | 1.8 | 1.3 | 1.2 | 1.5 | 1.7
20-40 | 4.8 | 5.0 | 3.5 | 3.0 | 2.6
40-60 | 7.9 | 6.9 | 5.4 | ... | 3.7
60-80 | 10.7 | 7.7 | ... | ... | 5.4
80-100 | 11.3 | ... | ... | ... | ...
100 | ... | ... | ... | ... | 12.0[1]
==========+=========+=========+=========+=========+=========
[Footnote 1: For an average turbidity = 150. approximately.]
The influence of the temperature of the water on the turbidity of
the effluent is very pronounced. For a temperature of less than 40 deg.
Fahr. (actual average temperature about 35 deg.), the turbidity of the
filtered water for a given turbidity of the applied water is
practically twice as great as for a temperature greater than 70 deg.
(actual average temperature about 75 deg.). This fact fits in very
nicely with the influence of temperature on sedimentation. Referring
again to this temperature relation, as set forth on a previous page,
the hydraulic subsiding value of a particle in water, of a size so
small that viscosity is the controlling factor in its downward
velocity, is approximately twice as great at 75 deg. as at 35 degrees.
We would then expect to find that, in order to obtain a given
turbidity in the filtered water, a raw water may be applied at 75 deg.,
having twice the turbidity of the water applied at 35 deg., to produce
the same turbidity; and further, as the turbidity of the filtered
water, for a given temperature condition, varies quite directly in
proportion to the turbidity in the applied water, it follows that an
applied water of given turbidity will produce an effluent at 35 deg.
with a turbidity twice as great as at 75 degrees. This is quite in
accordance with the facts obtained in actual operation, as indicated
on the diagram, Figure 15.
_Preliminary Treatment of the Water._--The most striking features of
the bacterial results given in Table 4 are, first, the uniformly low
numbers of bacteria in the filtered water during perhaps 8 or 9
months of the year, and the increase in numbers each winter. This is
shown clearly in the analysis of bacterial counts in Table 32.
~Table 32--Classification of Daily Bacterial Counts in
the Filtered-Water Reservoir During the Period,
November 1st, 1905, to February 1st, 1908.~
==========================+==============+======================
Bacterial count between: | No. of days. | Percentage of whole.
--------------------------+--------------+----------------------
0 and 20 per cu. cm. | 291 | 41.0
20 and 40 per " " | 245 | 34.6
40 and 60 per " " | 63 | 8.9
60 and 80 per " " | 30 | 4.2
80 and 100 per " " | 28 | 4.0 92.7
--------------------------+--------------+----------------------
100 and 200 per " " | 29 | 4.1
200 and 300 per " " | 13 | 1.8
300 and 500 per " " | 5 | 0.7
500 and 1000 per " " | 5 | 0.7 7.3
--------------------------+--------------+----------------------
| | 100.0
==========================+==============+======================
The tests for _Bacillus Coli_ in Table 5 show results which
correspond closely to these, with this organism detected only
infrequently, except during the periods of high bacteria, and both
of these are parallel to the turbidity variations in the filtered
water. These variations follow closely the variations in the
turbidity and in the bacterial content of the water applied to the
filters.
By all standards of excellence, the sanitary quality of the water
during the greater part of the time is beyond criticism. In view of
the close parallelism of turbidity and bacterial results in the
applied and in the filtered water, it is entirely logical to
conclude that, if the quality of the applied water could be
maintained continually through the winter as good as, or better
than, it is during the summer, then the filtered water would be of
the perfect sanitary quality desired throughout the entire year.
This was all foreseen ten years ago, when Messrs. Hering, Fuller,
and Hazen recommended auxiliary works for preliminary treatment of
the supply, although, as the author states, these works were not
provided for in the original construction. As prejudice against the
use of a coagulant seemed to be at the bottom of the opposition to
the preliminary treatment, a campaign of education bearing on this
point was instituted, in addition to the systematic studies of
different preliminary methods to which the author refers. As a
result of the combined efforts of all those interested in promoting
this improvement, an appropriation was finally made for the work in
1910. The coagulating plant has since been built, and the writer is
informed that coagulation was tried on a working scale a short time
ago during a period of high turbidity. A statement of the results of
this treatment on the purification of the water in the reservoir
system and in the filter plant would be of great interest.
[Illustration: ~Figure 15-- Turbidity in Applied Water.~]
_Hydraulic Replacing of Filter Sand._--The author has adopted a
method of replacing clean sand in the filters which will commend
itself to engineers as containing possibilities of economy in
operation. The first experiments in the development of this method
at the Washington plant were carried out some three years ago, while
the writer was still there. Substantially the same methods were used
then as are described in this paper, but examination of the sand
layer by cutting vertically downward through it after re-sanding in
this manner showed such a persistent tendency toward the segregation
of the coarse material as to hold out rather discouraging promises
of success. The greatest degree of separation seemed to be caused by
the wash of the stream discharging sand on the surface. It was
observed that, near the point where the velocity of the stream was
practically destroyed, there seemed to be a tendency to scour away
the fine sand and leave the coarse material by itself, and pockets
of this kind were found at many points throughout the sand layer.
The author states that, in the recent treatment of the filters by
this method, there has been no apparent tendency for the materials
to separate into different sizes, and it is fortunate if this work
can be done in such a manner as to avoid this separation entirely.
It may be questioned whether a certain amount of segregation of the
materials will make any practical difference in the efficiency of a
filter. In all probability this depends on the degree of the
segregation, the quantity of pollution in the water to be filtered,
the rate of filtration, and the uniformity of methods followed in
the operation, etc. For an applied water as excellent in quality as
that of the Washington City Reservoir during favorable summer
conditions, a considerable degree of segregation might exist without
producing any diminution in efficiency. For a badly polluted water,
however, such as the applied water at this plant during certain
winter periods, or the water of a great many other polluted
supplies, it might be found that even a slight lack of homogeneity
in the sand might make an appreciable difference in the results of
filtration.
As a result of the experiments herein described, however, this
method may be applied at other plants where conditions seem to
warrant it, with a largely increased measure of confidence;
although, as in the case of the adoption of any new or radical
departure, that confidence must not be permitted to foster contempt
of the old and tried methods, but its operation must be watched with
the utmost caution, until long experience shall have demonstrated
its perfect suitability and defined its limitations.
~E. D. Hardy, M. Am. Soc. C. E.~ (by letter).--It was not the
writer's original intention to enter into a discussion of either the
theory of water purification or of the experimental work on sand
handling, but simply to present the main results of operation
largely in tabular form. He is gratified, however, to have these
sides of the question so ably brought out in Mr. Longley's
discussion.
Mr. Hazen referred to the inferior efficiencies of the experimental
filters for rate studies (as shown in Table 20) in the removal of the
_B. Coli_ from the water tested. This inferiority is really less than
the figures in the table would indicate, as the tests for the
experimental filters were presumptive only (as shown by the note at the
foot of Table 20), while those for the main filters were carried through
all the confirmatory steps.
From experiments[1] made by Messrs. Longley and Baton in the
writer's office, it would seem reasonable to assume that about
one-half of the positive results, would have been eliminated had the
confirmatory steps been taken. In other words, the figures showing
the number of positive tests for _B. Coli_ in Table 20 should be
divided by two when comparing them with corresponding ones for the
main filters.
[Footnote 1: Published in the _Journal of Infectious Diseases_,
Vol. 4, No. 3, June, 1907.]
Mr. Knowles seems somewhat apprehensive regarding the methods described
in the paper of restoring the capacity of the filters by raking, and
replacing sand by the hydraulic method, and yet, from Mr. Johnson's
discussion, it would seem that the practice of raking filters between
scrapings had recently been adopted at the Pittsburg plant.
Before the practice of raking was finally adopted as a part of the
routine filter operation, the subject was given a great deal of
thought and study, as may be seen by referring to Mr. Longley's
discussion.
The re-sanding has been done by the hydraulic method, for nearly two
years, and, as far as the writer is able to judge, this method has
been more economical and also more satisfactory in every way than
the old one. As Mr. Hazen states, this does not prove that the
hydraulic method would be as satisfactory for other filter plants
and other grades of sand. The elevated sand bins at the Washington
plant fit in well with this scheme, and save the expense of one
shoveling of the sand; and the low uniformity coefficient of the
sand is favorable in decreasing its tendency to separate into
pockets or strata of coarse and fine sand. The method of washing is
also well adapted to this method of re-sanding, as the sand is made
very clean in its passage through the washers and storage bins. The
hydraulic method of replacing sand tends to make it cleaner still,
because any clay which may be left in the sand is constantly being
carried away over the weir and out of the bed, to the sewer. Sand
replaced by the hydraulic method is much more compact than when
replaced by other methods, and consequently the depth of penetration
of mud in a filter thus re-sanded is less. Careful tests of the
effluents from filters which have been re-sanded by the two methods
have invariably shown the superiority of the hydraulic method.
The experiment of replacing sand by water, referred to by Mr.
Longley, was not considered a success at the time, and the method
was abandoned for about a year. At that time an attempt was made to
complete the re-sanding of a filter which had been nearly completed
by the old method. The precaution of filling the filter with water
was not taken, nor was any special device used for distributing the
sand. When this method was again taken up, various experiments were
tried before the present method was adopted.
Mr. Whipple's remarks on the results from the operation of filters
under winter conditions are very interesting, and, considering his
standing as an authority in such matters, they are worth careful
consideration.
In the operation of the Washington plant, it has always been
noticeable that the results were much poorer in winter than in
summer. In fact, nearly all the unsatisfactory water which has been
delivered to the city mains has been supplied during the winter
months. On the other hand, the typhoid death rate has always been
comparatively low in cold weather. These facts would seem to
indicate that the water supply was not responsible for the typhoid
conditions.