published records of the Albany filter, from 1899 to 1909. These
data include: The numbers of bacteria before and after filtration;
the percentage of bacteria remaining in the effluent; the average
quantity of water filtered, in millions of gallons per day; the
quantities of water filtered between scrapings; the turbidity of the
raw water; the cost of filtration, including capital charges and
cost of operation; and the typhoid death rates of the city per
month. Several points are brought out conspicuously by this diagram.
One is the uniformly low death rate from typhoid throughout the
entire period. The filter was operated from 1899 until the fall of
1907 with raw water taken from what is known as the "Back Channel."
Since then it has been taken from a new intake which extends into
the Hudson River itself. Until the fall of 1908 the preliminary
treatment consisted merely of sedimentation, but since then the
water has received an additional preliminary treatment in mechanical
filters operated without coagulant, along the lines of the
experiments just mentioned. During this time the average rate of
filtration of the sand filter has not changed materially, although
it is said that the maximum rate has been increased since the
preliminary filters were put in service. The study of the
bacteriological analyses shows that the best results were obtained
during 1902, 1903, and 1904. Since then the numbers of bacteria in
both the raw and filtered water have increased. This was especially
noticeable during the winters of 1907 and 1908 when the water was
taken from the new intake. It will be interesting to compare the
results after the preliminary filters have been operated for a long
period to ascertain their normal effect on efficiency and on the
increased yield.
[Illustration: ~Figure 11--Filters at Albany, N. Y. Results of
Operation. 1899-1909. Compiled from data in Annual Reports.~]
Another fact to be drawn from the plotted Albany data is the
increase in the cost of filtration, both in capital charges and in
operation. From 1899 until 1906 the cost of operation, including the
cost of low-lift pumping, was approximately $5 per million gallons
of water filtered; and the total cost of filtration, including
capital charges, was about $10 per million gallons. During the year
ending September 30th, 1909, the cost of operation had increased to
$7.63 per million gallons, and the total cost of filtration to
$15.92 per million gallons, or approximately 50% in three years.
~Table 27--Results of Bacteriological Analyses of Samples of
Water at Peekskill, N. Y., Before and After Filtration.~
~Bacteria per cubic centimeter.~
==============+=======+=======+========+========+========+========+========
| Raw | Clear |Effluent|Effluent|Effluent|Effluent| Tap in
Date. | water.| reser-| No. 1. | No. 2. | No. 3. | No. 4. | city.
| | voir.| | | | |
--------------+-------+-------+--------+--------+--------+--------+--------
1909.
--------------+-------+-------+--------+--------+--------+--------+--------
December 29th | 190 | 100 | ... | ... | ... | ... | ...
--------------+-------+-------+--------+--------+--------+--------+--------
1910.
--------------+-------+-------+--------+--------+--------+--------+--------
February 15th | 135 | 10 | 10 | 30 | 20 | ... | 265
March 31st | 225 | 50 | 25 | 45 | 60 | ... | 35
May 18th | 300 | 29 | 22 | 26 | 35 | 43 | 36
July 6th | 300 | 44 | 9 | 3 | 41 | 10 | 31
August 16th | 60 | 5 | 0 | 4 | 1 | 13 | 15
October 3d | 550 | 14 | 12 | 14 | 38 | ... | ...
November 21st | 315 | 22 | 26 | 17 | 6 | ... | ...
--------------+-------+-------+--------+--------+--------+--------+--------
1911.
--------------+-------+-------+--------+--------+--------+--------+--------
January 25th | 415 | 7 | 8 | 4 | 6 | ... | 7
--------------+-------+-------+--------+--------+--------+--------+--------
Average | 277 | 30 | 14 | 16 | 26 | 22 | 65
==============+=======+=======+========+========+========+========+========
~Table 17~--(_Continued._)
~Tests for~ _B. Coli._
==================+======================
|~Percentage of Samples
Quantity of water |Containing~ _B. Coli_.
tested. +--------+-------------
| Raw. | Filtered.
------------------+--------+-------------
0.1 cu. cm. | 0 | 0
1.0 cu. cm. | 20 | 0
10.0 cu. cm. | 40 | 0
==================+========+=============
As a matter of record, the results of a series of analyses made at
Peekskill, N. Y., during 1910 are presented in Table 27. A sand
filter was constructed for the water supply of this city in 1909,
and put in operation in December. The filter has a capacity of
4,000,000 gal. per day. The supply is taken from Peekskill Creek,
and the water receives about one week's nominal storage before
flowing to the filters. An aerator is used before filtration during
the summer, when algae are likely to develop in the reservoir. The
filter was installed after an epidemic of typhoid which was
apparently caused by an infection of the water supply. Normally, the
water has been little contaminated, but the supply is subject to
accidental contamination at any time, among other possible sources
of infection being the camps of workmen now engaged in constructing
the Catskill Aqueduct for New York City.
~Table 28--Average Results of Chemical Analysis at Peekskill,
N. Y., Made at Intervals of Six Weeks During 1910.~
=====================+=================+================+=================
| ~Parts per | | ~Parts per
| Million.~ | | Million.~
|--------+--------| |--------+--------
| Raw |Filtered| | Raw |Filtered
| water. | water. | | water. | water.
---------------------+--------+--------+----------------+--------+--------
Turbidity | 2. | 0 |Total residue | 70. | 76.00
Color | 25. | 20. |Loss on ignition| 19.00 | 17.00
Nitrogen as albumi- | | |Fixed residue | 50.00 | 59.00
noid ammonia | 0.112 | 0.076 |Iron | 0.17 | 0.13
Nitrogen as free | | |Total hardness | 38.70 | 45.10
ammonia | 0.024 | 0.006 |Alkalinity | 33.90 | 42.60
Nitrogen as nitrites | 0.001 | 0.001 |Incrustants | 4.60 | 4.50
Nitrogen as nitrates | 0.06 | 0.06 |Chlorine | 2.60 | 2.70
=====================+========+========+================+========+========
~F. F. Longley, Assoc. M. Am. Soc. C. E.~ (by letter).--In this
paper the author has presented a mass of data which will be welcomed
by engineers engaged in water purification work, because complete
operating records form a substantial basis for improvement in the
art, and are often the inspiration for interesting discussions and
the exchange of experiences of different observers whose views are
mutually appreciated.
Recent tendencies in filtration engineering have been largely in the
direction of reducing the cost of operation. A comparison of the
operating costs of the earlier American plants of about a decade
ago, with those here presented of the Washington plant, is very
gratifying to those who have been intimately connected with the
latter work. Through perfection in design and reasonable care in
operation, the cost of filter cleaning, which is a very considerable
part of the total cost, has been reduced to an unusually low figure,
without any sacrifice in efficiency, and in the interests of the
public health.
Table 14 shows that, from the first year, there has been a
progressive increase in the total cost of operation per million
gallons filtered, but this has not meant an increase in the annual
total expenditure. The largest percentage of increase in any item
has been in "Care of Grounds and Parking," and covers much-desired
landscape improvements. Aside from this, the principal factor
affecting the table of costs has been the reduction in water
consumption in the District of Columbia. Nothing pertaining to this
reduction has produced any corresponding reduction in the force
required for the maintenance and operation of the filtration plant,
office and laboratory, and pumping station, though probably there
has been some reduction in filter cleaning. Obviously, then, the
total cost per million gallons would increase.
This decrease in consumption has been brought about by the
elimination of waste in the distribution system, which is not in the
same department as the filtration plant, but with regard to which a
word may not be amiss in connection with this discussion.
The Washington Aqueduct was built half a century ago on lines which
at that time were considered extraordinarily generous. Until
recently, therefore, there has been no occasion for concern over the
high rate of consumption. During recent years, however, the use and
waste of water have increased, reaching a climax under unusual
conditions in the winter of 1904-05. The maximum capacity of the
aqueduct system is about 90,000,000 gal. The maximum daily
consumption at the time mentioned arose almost to 100,000,000 gal.,
with the result that, before normal conditions were restored, the
reservoirs of the system were almost depleted.
This had a beneficial effect, as provision was made for an active
campaign for reducing the waste of water, which was known to be very
large. These investigations, using the pitometer, were begun in
July, 1906, and have been pursued continuously since that time, with
most excellent results. Up to January, 1909, leaks aggregating about
12,000,000 gal. per day were detected and eliminated, and about half
the house services had still to be covered by the pitometer bureau.
Although this reduction in waste has brought about an apparent
increase in the cost of filtration, its economical results have been
far-reaching. The causes which brought about this investigation also
resulted in securing an appropriation for the study of the question
of increased supply. The writer was in charge of these studies, and
the most significant conclusion was that, owing to the excellent
results of the efforts for waste restriction, the total consumption
and waste of water in the district during the next few years would
be far enough below the safe working capacity of the existing
aqueduct system to make it entirely safe to postpone the
construction of new works, involving the expenditure of several
million dollars, in spite of the threatening conditions of a few
years ago.
There has been so much controversy over typhoid fever in the
District of Columbia that the writer hesitates to discuss this
subject. Viewing the situation through the perspective of several
years, however, it does not seem to be as hopeless as the criticisms
of four or five years ago would lead one to believe.
In Table 9, showing the typhoid death rates, out of nine years given
prior to 1905-06, when the filters were started in operation, only
one shows an annual death rate as low as the highest one since that
year. Further than this, the annual average typhoid death rate for
the period since that year has been one-third lower than for a
corresponding period before the filters were started.
The exhaustive researches of the Public Health and Marine Hospital
Service into this whole question, covering a period of about four
years, have raised the present filtered water supply of the District
of Columbia above any well-founded criticism. There has long been a
strong and growing feeling that the water supply, before filtration
was introduced, had been blamed for more than its share of the
typhoid, and this is borne out by much evidence that has been
presented from time to time.
It is not an unreasonable conjecture, therefore, that perhaps the
reduction of one-third in the total typhoid death rate may represent
a much larger reduction in that part of the total which was due to
polluted water alone; and that, as the authorities in the District
of Columbia and in certain other cities, particularly in the South,
are now recognizing, the fight against much of the remaining typhoid
must be in the direction of the improvement of milk supplies,
precautions against secondary infection, and attention to a large
number of details surrounding the individual, which may effectively
protect him against the insidious attack of the disease favored by
unknown agencies.
~Experiments in Filter Cleaning~.
The author refers to the difficulty encountered during the first two
summers in keeping the filters cleaned fast enough to maintain the
capacity of the plant. The real seriousness of this may be judged
from the following facts. The average increase in loss of head on
all the filters for the entire year, July 1st, 1906, to July 1st,
1907, was about 0.053 ft. per day. During the 1906 period of low
capacity under discussion, the loss of head on twelve of the filters
increased for a period of eight days at the average rate of 0.45 ft.
per day, or about nine times the normal rate of increase. This
difficulty was caused by the presence of large numbers of
micro-organisms in the applied water. During the first summer (1906)
this fact was not recognized, but the sudden decrease in capacity
was supposed to have been caused by the unusually high and
long-continued turbidity which prevailed during that summer in the
Potomac River, and persisted in the water supplied to the filters
even after about four days of sedimentation in the reservoirs.
During the second summer (1907) the same phenomenon of suddenly and
rapidly increasing losses of head appeared again, but without any
unusual turbidity in the applied water. Investigation, however,
showed the presence of large quantities of organisms, particularly
_melosira_ and _synedra_, in the applied water, and examinations in
subsequent years have shown a periodic recurrence of these forms in
quantities sufficient to cause the trouble mentioned. In June, 1907,
examination showed repeatedly more than 1,000 and 1,500 standard
units of _melosira_ per cu. cm., and one count showed nearly 3,000
standard units.
Several expedients were tried in an effort to restore the rapidly
decreasing capacity of the filters. One of the earlier conjectures
as to the cause of the trouble was that it might be due to the
accumulation of large quantities of air under the surface of the
sand, as air had been observed bubbling up through the sand,
especially in filters which had been in service for some time. The
expedient was tried, therefore, of draining the water out of the
sand and then re-filling the filter in the usual manner from below,
in the hope of driving out the entrained air. Presumably this
treatment got rid of the air, but it did not restore the capacity of
the filter, as the point of maximum resistance was in the surface of
the sand and not below it.
As the author states, raking the filters was tried and found to give
results which were satisfactory enough to meet the emergencies
already referred to. When the filters were first put in operation,
in the fall of 1905, the method of bringing back the capacity of a
filter after the end of a run was to remove all the dirty sand to a
depth determined by the marked discoloration caused by the
penetration of the clay turbidity. This sometimes necessitated the
removal of large quantities of sand at a cleaning, as the turbidity
was exceedingly fine, and penetrated at times to a depth of 3 or 4
in.
With the idea of effecting an economy in the cost of cleaning the
filters, a schedule of experiments was arranged shortly before July
1st, 1907. The general object of the experiments was to determine,
first, the relative costs of all different methods tried; second,
whether the removal of only a thin layer of sand, or the mere
breaking up of the surface of the sand by thorough raking, would
give the filter its proper capacity for the succeeding run; third,
whether the filters under these treatments would maintain a high
standard of quality in the effluents; fourth, whether the continued
application of any less thorough method than the one then in use
might materially affect the future capacity of the filters.
To this end the filters were divided into four groups which, during
a period of about six months, were subjected to treatments as
follows:
Group _A._--Filters scraped deep at the end of each run;
Group _B._--Filters scraped light at the end of each run;
Group _C._--Filters raked at the end of each run, until raking
failed to bring back the proper capacity; then they
were scraped light, and at the end of the next run
the raking was resumed;
Group _D._--Light scrapings and rakings alternate at ends of
runs.
The term "deep scraping" means the removal of practically all the
discolored sand, in accordance with the usual practice prior to the
beginning of these experiments; "light scraping" means the removal
of only a thin surface layer of sand. This depth has usually
averaged about 3/8 in. "Raking" means the thorough breaking up of
the clogged surface of the filter by iron-toothed rakes, to a depth
of about 1 or 2 in.
_Results._--A general summary of the results of these experiments is
given in Table 29, which also shows the relative costs of the
different methods per million gallons of water filtered. A normal
period of 9 months just prior to the beginning of these experiments
shows a labor cost (corresponding to that in Table 29) of $0.29-1/4
per million gallons filtered.
~Table 29--Average Results.~
Columns:
A - Group.
B - Number of filters.
C - Number of days of service.
D - Million gallons filtered.
E - Cost of labor per treatment.
F - Sand removed in cubic yards.
G - Sand removed in cubic yards.
H - Cost of labor.
I - Bacteria per cu. cm. in effluent.
J - Turbidity in effluent.
=========================================+===============+==============
| Per Million | |
Per Run: | Gallons | |
| Filtered | I | J
-----+-----+-----+-------+-------+-------+-------+-------+ |
A | B | C | D | E | F | G | H | |
-----+-----+-----+-------+-------+-------+-------+-------+-------+------
_A_ | 5 | 82 | 221.2 |$68.44 | 215 | 1.11 |$0.309 | 13 | 1
_B_ | 9 | 36 | 101.4 | 29.25 | 84 | 0.83 | 0.288 | 16 | 1
_C_ | 5 | 21 | 60.0 | 10.92 | 24 | 0.40 | 0.182 | 18 | 1
_D_ | 10 | 32 | 86.0 | 20.10 | 46 | 0.54 | 0.234 | 22 | 1
=====+=====+=====+=======+=======+=======+=======+=======+=======+======
_Capacity of Filters._--The capacity of the filters under the
different methods of treatment are shown in a general way in Table
29 for days of service and millions of gallons filtered per run.
This element by itself is decidedly in favor of the deep scrapings,
and least in favor of the repeated rakings.
A clearer conception of the capacities of the filters under these
different conditions may be obtained from the four diagrams, Figure
12, showing, for the four different groups, the average number of
days of service of the successive runs. The diagram for Group _A_
shows that the variations in the period of service of the filters
scraped each time to clean sand follow a more or less definite curve
from year to year. For the period covered by this curve, the
tendency seems to be toward a slight decrease in capacity from year
to year, as shown by the lower average maximum and minimum in the
second year than in the first. Group _B_ shows a sudden decrease in
capacity following the first light scrapings and, since that time, a
low but quite constant capacity. Group _C_ shows a constantly
decreasing capacity with successive rakings. The only significance
attaching to the curve after the first raking is the prohibitively
low capacity indicated, and the ineffectiveness of the measures
taken to restore the capacity after the sixth raking. Group _D_,
after the first raking, shows a prohibitively low and constantly
decreasing capacity. The diagrams for _C_ and _D_ indicate a
dangerous reduction in capacity if long persisted in. The method
followed with Group _C_ may be dismissed with the statement that it
is entirely insufficient, and would be of use only in the rarest
emergencies.
As far as the question of capacity is concerned, these diagrams indicate
that a filter in normal condition may safely be raked once. It is
believed that the constantly decreasing capacity shown in Group _D_ is
not due so much to the rakings as to the small quantities of sand
removed at the alternate scrapings, and therefore it would not be proper
to condemn this method of treatment without a further trial in which
this defect was remedied. This view seems to be supported by the results
of Group _B_. The low but approximately constant capacity there shown
would undoubtedly have been higher if a greater depth of sand had been
removed each time.
[Illustration: ~Figure 12--Average Number of Days of Service of
Successive Runs for Groups _A_, _B_, _C_, and _D_.~]
_Quality of the Effluent._--The averages given in Table 29 show but
little difference in the bacterial contents of the effluents from
the four groups of filters. All are entirely satisfactory, and the
differences in favor of one method or another are small. In looking
for possible differences in the quality of the effluents from the
four groups, it was thought that such differences might be most
apparent at a time when the entire plant was working under the most
adverse conditions. The bacterial counts, therefore, were summarized
for the period from December 23d, 1907, to January 6th, 1908,
inclusive, following a period of high turbidity and high bacteria in
the raw water, with results as follows:
Group....... _A_ _B_ _C_ _D_
Maximum..... 204 178 189 206
Minimum..... 61 45 62 57
Average..... 120 107 104 155
The following is a summary of the turbidity results for a similar
period:
Group....... _A_ _B_ _C_ _D_
Maximum..... 10.8 11.7 8.7 9.3
Minimum..... 6.7 4.7 6.2 5.7
Average..... 8.7 8.3 7.2 7.9
These numbers, though high, do not show any significant differences.
All the averages for each group are less than the lowest maximum,
and all are greater than the highest minimum, and therefore vary
less than do the individual filters, from other causes, within the
different groups.
_Future Capacity of the Filters._--An indication of the dangers
which might affect the future capacity of the filters was shown in
the above discussion of the present capacity. A more effective way
of showing this was obtained by a study of the initial resistances
or losses of head in the four groups. A filter kept in ideal
condition would show no increase in this initial loss of head from
one run to the next. If there is such an increase, it means that at
some future time measures more heroic than ordinarily used would be
necessary to restore the proper capacity.
The average initial losses of head for the different groups are
plotted on the diagram, Figure 13. Group _A_ shows an initial loss of
head, increasing gradually but slightly during more than two years
of service. In Group _B_ the initial loss of head increased in a
manner similar to that in Group _A,_ up to the time of the beginning
of these experiments; after which the increase becomes more rapid.
Groups _C_ and _D_ show conditions generally similar to Group _B_,
with some variations which are self-explanatory.
_Conclusions._--The quality of the effluents from all four groups
was satisfactory, and no consistent difference was apparent in favor
of one or another method of treatment. The method pursued with Group
_C_ was entirely insufficient to maintain the capacity indefinitely.
The methods pursued in Groups _B_ and _D_ were both insufficient,
but would have been more effective if a greater depth of sand had
been removed. The costs of treatment of Groups _B_ and _D_ were less
than for Group _A_. It appears, then, that a treatment which would
be more economical than the old method of Group _A_, and would still
maintain the proper capacity, would be one similar to that of Groups
_B_ or _D_, with the removal of a quantity of sand greater than was
done in the case of these two groups, but less than in the old
method.
[Illustration: ~Figure 13--Average Initial Losses of Head for Groups
_A_, _B_, _C_, and _D_ for Successive Runs.~]
At the time the above results were summarized, it was proposed to
proceed with the filter treatment along the lines just mentioned.
The writer did not have an opportunity to study the subsequent
results, as he was transferred to other work. A statement by the
author of any new facts that may have come to light in this
connection would be of interest.