======================+===================================================
|Nominal rate, in millions of gallons per acre daily:
+------------+------------+------------+------------
| 3 | 5 | 10 | 20
----------------------+------------+------------+------------+------------
Percentage which | | | |
average yield is of | | | |
nominal rate..........| 85 | 80 | 75 | 65
----------------------+------------+------------+------------+------------
Average output per | | | |
acre, in millions of | | | |
gallons per day.......| 2.55 | 4.00 | 7.5 | 13.0
----------------------+------------+------------+------------+------------
Cost of that part of | | | |
filters per acre | | | |
dependent on rate.....| $12,000 | $20,000 | $40,000 | $80,000
----------------------+------------+------------+------------+------------
Cost of that part of | | | |
filters per acre not | | | |
dependent on rate.....| 50,000 | 50,000 | 50,000 | 50,000
----------------------+------------+------------+------------+------------
Total cost of filters | | | |
per acre..............| 60,000 | 70,000 | 90,000 | 130,000
----------------------+------------+------------+------------+------------
Cost per million | | | |
gallons of capacity...| 20,600 | 14,000 | 9,000 | 6,500
----------------------+------------+------------+------------+------------
Cost per million | | | |
gallons of average | | | |
daily output..........| 24,400 | 17,500 | 12,000 | 10,000
----------------------+------------+------------+------------+------------
Capital charges and | | | |
depreciation at 6% on | | | |
cost per million | | | |
gallons...............| 4.00 | 2.87 | 1.97 | 1.64
----------------------+------------+------------+------------+------------
Operating expenses, | | | |
the same at all | | | |
rates.................| 1.00 | 1.00 | 1.00 | 1.00
----------------------+------------+------------+------------+------------
Total cost of | | | |
filtering, excluding | | | |
pumping, storage, and | | | |
all auxiliaries.......| 5.00 | 3.87 | 2.97 | 2.64
----------------------+------------+------------+------------+------------
Relative cost.........| 1.29 | 1.00 | 0.77 | 0.68
======================+============+============+============+============
When the costs of pumping, pure-water reservoirs usually necessary,
etc., are taken into account (which add equally to the cost at all
rates), the cost of filtering will vary less with the rate than is
indicated.
The effect of rate on cost, as calculated in Table 23, and also the
percentages of the bacteria of the raw water found in the effluents
by the author and by Mr. Clark, are shown on Figure 10.
Considering all these results together, and also all the other
evidence known to the writer bearing on this point, it seems clear
that filters are not as sensitive to changes in rate, within
reasonable limits, as has been frequently assumed; but, on the other
hand, there is usually a substantial increase in the percentage of
bacteria passing through a filter with increased rate.
Filters furnish relative, not absolute, protection against
infectious matter in the raw water. The higher the bacterial
efficiency, the more complete is this relative protection.
The cost of filtering does not decrease in inverse ratio to the
rate, but at a much slower rate. This is especially true with rates
of more than 5,000,000 or 6,000,000 gal. per acre daily.
In general, a rate of filtration may rationally be selected at which
the value of the possible danger resulting from an increase in rate
is equal to the saving that may be made in cost by its use. This
point must be a matter of individual judgment. The tendency of the
last few years has been to use higher rates, or, in other words, to
cheapen the process and to tolerate a larger proportion of bacteria
in the effluent. The use of auxiliary processes has been favorable
to this, especially the use of chloride of lime, in connection with
either the raw water or the effluent.
[Illustration: ~Figure 10--Rate Million Gallons Per Acre Daily.~]
By the judicious use of this substance, efficiency may be maintained
while using higher rates than would otherwise have been desirable.
The writer believes that there will be many cases where the added
risk of using too high a rate is not worth the relatively small
saving in cost that accompanies it.
~George A. Johnson, Assoc. M. Am. Soc. C. E~.--This paper contains
information of an exceedingly interesting nature. There is
comparatively little difficulty in obtaining accurate figures on the
cost of construction of water purification works, but, with costs of
operation of such works, it is different. The data available in
published reports and papers are usually more or less fragmentary,
and unexplained local conditions with reference to the character of
the raw water, the cost of labor and supplies, and methods of
apportioning these costs, introduce variables so wide as frequently
to render the published figures almost useless for purposes of
comparison.
Mr. Hardy's paper is noteworthy in that it presents certain
relatively new features of slow sand filter operation which have
been only lightly touched on in water purification literature up to
the present time. These refer particularly to means whereby a filter
may be continued in service without removing a portion of the
surface layer of the filter surface itself when the available head
has become exhausted, and to methods whereby washed sand may be
expeditiously and more economically restored to the filter than has
been the case hitherto.
Sand handling is the most important item of expense in the operation
of a slow sand filter. Quite recently a charge of $1.50 per cu. yd.
for sand scraping, transportation to sand washers, washing, and
restoring to the filter, was not considered exorbitant, but the
improved methods developed during recent years at Washington,
Philadelphia, Albany, and more recently at Pittsburg (at all of
which places hydraulic ejection plays an important part), have
shown the feasibility of reducing this figure by nearly, if not
quite, two-thirds.
The practice observed at Washington of raking over the surface of
the sand layer when the available head becomes exhausted, in order
to avoid the cost and loss of time necessitated by shutting down the
filter and scraping off the surface layer, is unquestionably one of
the most striking advances in slow sand filter operation in recent
years. In rapid sand filter operation, to prolong the period of
service between washings, agitation of the filter surface has been
used to advantage for many years. The full value of surface raking
may not be generally appreciated, but the results which have
followed a trial of this procedure at Washington, Philadelphia, and
Pittsburg have shown that the output of filtered water between
scrapings may be doubled or trebled thereby, with no injury to the
filter itself or to the quality of the filtered water. The cost of
raking over the surface of a 1-acre slow sand filter unit is less
than $10 at all the above-mentioned places, which fact in itself
shows the great saving in money and time effected by periodically
substituting surface raking for scraping. Under ordinary conditions
it has been found that a filter can be raked to advantage at least
twice between scrapings.
In the case of filters thus raked, a deeper penetration of suspended
matter into the sand layer is inevitable, but at Pittsburg, as at
Washington, such penetration does not extend more than about 2 in.
below the filter surface. When the filter is finally scraped, a
deeper layer is removed, of course, but it is clearly more
economical to remove a deep layer at one operation than to remove
separately several thinner layers of an equal total thickness.
The lost-time element is an important one, and at Washington this
was the main reason for trying surface raking. It became necessary
to increase the output of the filters, and the ordinary scraping
consumed so much time that the sand-handling force was increased,
working day and night. The raking expedient introduced at this time
overcame this, and Mr. Hardy states that it is still followed when
the work is at all pressing. The speaker has found at Pittsburg, as
Mr. Hardy has found at Washington, that raking is nearly if not
quite as effective as scraping in restoring the filter capacity.
Eleven years ago the speaker was connected with the preliminary
investigations into the best methods of purifying the Potomac River
water for Washington. It then appeared that while for the greater
part of the time during an average year the Potomac River could be
classed among the clear waters of the East, there were periods when
excessive turbidity made it necessary to consider carefully methods
of preparatory treatment before this water could be filtered
effectively and economically. As Mr. Hardy has said, considerable
prejudice existed against the use of a coagulating chemical, and the
expedient was therefore adopted of giving the water a long period of
sedimentation in order to remove enough of the suspended matter to
allow the clarified water to be treated on slow sand filters. The
expert commission, consisting of Messrs. Hering, Fuller, and Hazen,
recommended the occasional use of a coagulating chemical, but this
recommendation was not carried out.
The Potomac River is somewhat peculiar, in that the turbidity of its
waters, as shown by the results presented in Mr. Hardy's paper,
ranges from 3,000 to practically nothing. The bacterial content also
varies widely, and Mr. Hardy's tables show this variation to be from
76,000 to 325 per cu. cm. Such a water as this requires particularly
careful preparatory treatment. The Dalecarlia Reservoir has a
capacity of something like 2 days' storage, the Georgetown Reservoir
the same, and the McMillan Park Reservoir nearly 3 days, making a
total sedimentation of more than 7 days. Without the use of a
coagulant, it is significant that during a period of five years,
even with 7 days' sedimentation, the average maximum turbidity of
the water delivered to the filters was 106 parts per million, and
the maximum average turbidity in one month was 250 parts per
million. The water filtration engineer can readily understand that
waters as turbid as this cannot be treated economically and
efficiently in slow sand filters. It would appear that coagulating
works might advantageously have been installed at the entrance to
the Dalecarlia Reservoir. If this had been done, and coagulant had
been added to the water at times when it was excessively turbid, a
considerably shorter period of subsequent sedimentation than now
exists would in all probability have rendered the water at all times
amenable to efficient and economical slow sand filter treatment.
The prejudice in Washington against the use of coagulants has also
manifested itself in other localities, but the results which have
been obtained during the past twenty years from rapid sand filters
and from slow sand filters, treating waters previously coagulated
with salts of iron or alumina, have shown how thoroughly
unreasonable were these objections. In this connection it is
interesting to note that there are in the United States more than
350 rapid sand filter plants, and that nearly 12% of the urban
population of Continental United States is being supplied with water
filtered through rapid sand filters, in connection with all of which
a coagulating chemical is used in the preparatory treatment.
~Table 24--Typhoid Fever Death Rates in Cities of the United
States with Populations in 1910 of 100,000, or More.~
Statistics gathered by correspondence and from Reports of
the Bureau of the Census, Department of Commerce and Labor,
Mortality Statistics.
~Note~.--Statistics from Birmingham, Ala., Dayton, Ohio, Fall River,
Mass., Louisville, Ky., Memphis, Tenn., Oakland, Cal., and
Providence, R. I., are not included, as they are
incomplete.
Columns:
A - Average for six years, 1900-05, inclusive.
B - Average for five years, 1906-10, inclusive.
C - Average for 11 years, 1900-11, inclusive.
====================+===============================================
| ~ Typhoid Fever Death Rate
City. | per 100,000 Population~.
+-----+-----+-----+-----+-----+-----+-----+-----
| 1906| 1907| 1908| 1909| 1910| A | B | C
--------------------+-----+-----+-----+-----+-----+-----+-----+-----
Albany, N. Y. | 20 | 20 | 11 | 19 | 15 | 25 | 17 | 21
Atlanta, Ga. | 50 | 64 | 47 | 44 | 43 | 65 | 50 | 58
Baltimore, Md. | 34 | 41 | 31 | 23 | 41 | 36 | 34 | 35
Boston, Mass. | 22 | 10 | 26 | 14 | 11 | 23 | 16 | 20
Bridgeport, Conn. | 10 | 13 | 13 | 13 | 9 | 15 | 12 | 14
Buffalo, N. Y. | 24 | 29 | 21 | 23 | 20 | 29 | 23 | 26
Cambridge, Mass. | 18 | 10 | 10 | 9 | 12 | 18 | 12 | 15
Chicago, Ill. | 18 | 18 | 15 | 12 | 14 | 27 | 16 | 22
Cincinnati, Ohio | 71 | 46 | 19 | 13 | 6 | 54 | 31 | 44
Cleveland, Ohio | 20 | 19 | 13 | 12 | 19 | 51 | 17 | 36
Columbus, Ohio | 45 | 38 | 110 | 17 | 13 | 61 | 45 | 54
Denver, Colo. | 68 | 67 | 58 | 24 | 30 | 37 | 49 | 42
Detroit, Mich. | 22 | 28 | 22 | 19 | 16 | 17 | 22 | 19
Grand Rapids, Mich. | 39 | 30 | 30 | 17 | 27 | 34 | 28 | 31
Indianapolis, Ind. | 39 | 29 | 26 | 22 | 31 | 76 | 30 | 55
Jersey City, N. J. | 20 | 14 | 10 | 8 | 10 | 19 | 12 | 16
Kansas City, Mo. | 38 | 40 | 35 | 23 | 38 | 48 | 35 | 42
Los Angeles, Cal. | 18 | 23 | 19 | 18 | 12 | 35 | 18 | 27
Lowell, Mass. | 7 | 9 | 24 | 11 | 21 | 19 | 14 | 17
Milwaukee, Wis. | 31 | 26 | 17 | 21 | 45 | 19 | 28 | 23
Minneapolis, Minn. | 33 | 26 | 18 | 20 | 58 | 38 | 29 | 34
Nashville, Tenn. | 66 | 85 | 62 | 53 | 48 | 54 | 58 | 56
Newark, N. J. | 18 | 24 | 12 | 11 | 13 | 17 | 16 | 17
New Haven, Conn. | 54 | 30 | 34 | 20 | 17 | 44 | 31 | 38
New York, N. Y. | 15 | 17 | 12 | 12 | 12 | 19 | 14 | 17
New Orleans, La. | 30 | 56 | 31 | 25 | 28 | 40 | 34 | 37
Omaha, Nebr. | 28 | 24 | 22 | 31 | 75 | 20 | 36 | 27
Paterson, N. J. | 4 | 11 | 10 | 5 | 7 | 25 | 7 | 17
Philadelphia, Pa. | 74 | 60 | 36 | 22 | 17 | 47 | 42 | 45
Pittsburg, Pa. | 141 | 135 |53[1]|13[1]|12[1]| 132 | 71 | 104
Richmond, Va. | 44 | 41 | 50 | 24 | 22 | 66 | 36 | 53
Rochester, N. Y. | 17 | 16 | 12 | 9 | 13 | 15 | 13 | 14
St Louis, Mo. | 18 | 16 | 15 | 15 | 14 | 33 | 16 | 25
St Paul, Minn. | 21 | 17 | 12 | 20 | 20 | 14 | 18 | 16
San Francisco, Cal. | ... | 57 | 27 | 17 | 15 | 20 | 29 | 24
Scranton, Pa. | 11 | 76 | 11 | 11 | 14 | 18 | 35 | 26
Syracuse, N. Y. | 10 | 16 | 15 | 12 | 30 | 14 | 17 | 15
Toledo, Ohio | 45 | 36 | 40 | 31 | 32 | 36 | 37 | 36
Worcester, Mass. | 12 | 14 | 10 | 8 | 16 | 17 | 12 | 15
Washington, D. C. | 52 | 36 | 39 | 33 | 23 | 59 | 37 | 49
====================+=====+=====+=====+=====+=====+=====+=====+=====
[Footnote 1: Filtered water section. Allegheny District not
included.]
Attention has repeatedly been called to the fact that the relatively
high typhoid death rate in Washington, since the filter plant was
installed, was a possible indication that the filters were
inefficient. It is true that there has not been the marked reduction
in the typhoid death rate in Washington, following the installation
of the water filtration works, that has been observed in other
cities in America. For the six years prior to the date on which
filtered water was supplied to the citizens of Washington, the
average typhoid fever death rate was 59 per 100,000 population, as
against 37 per 100,000 for the five years following, a reduction of
37 per cent. At Albany, N. Y., where the first modern slow sand
filter was built in 1899, the typhoid death rate has been reduced by
75 per cent. At Cincinnati, Ohio, the average death rate from
typhoid ranged around 50 per 100,000 for years, but since the
installation of the filtration plant it has been reduced to a point
which places that city, with respect to freedom from typhoid fever,
at the head of all the large cities in America; in 1910 the death
rate from typhoid in Cincinnati was 6 per 100,000. Similarly, at
Columbus, Ohio, where the typhoid death rate before the installation
of the filtration plant in 1906 was even higher than at Cincinnati,
it was reduced to less than 13 per 100,000 in 1910, whereas, for the
previous five years, it was 61 per 100,000. Philadelphia, before the
installation of the filtration works, had a typhoid death rate of 60
or more per 100,000, and in 1910 the death rate from this disease
was 17. Pittsburg, at least that part of it now supplied with
filtered water, for years had a typhoid death rate of more than 130
per 100,000, but the present rate is about 12 per 100,000.
~Table 25--Average Monthly Results for the
Period, 1905-1910.~
Columns:
A - Period of sedimentation in days.
B - Turbidity in parts per million.
C - Bacteria per cubic centimeter.
============+=====+=====+=======+=====================
| | | |~Percentage Removed~
Reservoirs.| A | B | C |----------+----------
| | | | Turbidity| Bacteria
------------+-----+-----+-------+----------+----------
River | ... | 106 | 6,400 | ... | ...
Dalecarlia | 2.2 | 50 | 5,000 | 53 | 22
Georgetown | 2.2 | 38 | 3,400 | 24 | 32
McMillan | 2.8 | 26 | 2,000 | 31 | 41
------------+-----+-----+-------+----------+----------
Totals and | | | | |
averages | 7.2 | ... | ... | 75 | 69
============+=====+=====+=======+==========+==========
While it may perhaps seem unreasonable to single out Washington as a
particular sufferer in this respect, it is highly probable that a
large share of the typhoid is still caused by secondary infection,
flies, impure milk, and private and public wells. The speaker
remembers distinctly that ten years ago, when he made an
investigation into the purity of the water of about 100 public wells
in that city, a large number of them showed unmistakable evidence of
being polluted with sewagic matter. Conclusive evidence would be
secured to dispel any doubt as to the sanitary quality of the
filtered product if hypochlorite of lime were added to the filtered
water throughout one year or throughout the typhoid months. It seems
strange to the speaker, that for this, if for no other reason, this
safe and non-injurious germicide has not as yet been used at
Washington, in view of the fact that at the present time it is being
used continuously or intermittently in the treatment of the water
supplies of scores of the most important cities of this country,
among which may be mentioned New York, Philadelphia, Cincinnati,
Pittsburg, St. Louis, and Minneapolis.
~Morris Knowles, M. Am. Soc. C. E~. (by letter).--This description
of the operation of the Washington Filtration Works is timely and of
great interest. It is ten years since the writer, in collaboration
with Charles Gilman Hyde, M. ~Am. Soc~. C. E., presented a similar
record for the Lawrence, Mass., filter. That paper was the first
complete, detailed, and continuous history of the actions and
results obtained for a long period of time with such a purification
works.[1] Since then, the art of filtration has advanced in many
ways, particularly in regard to the methods of cleaning slow sand
filters and in the accompanying processes. It is well, therefore,
again to take account of stock and see really what progress has been
made. Therefore, Mr. Hardy's paper, giving a description of the
operations of a system thoughtfully designed, after long
consideration of the problem, and of operations carried on under
efficient and economical administration, with thorough record of all
details, should furnish a groundwork for the careful consideration
of the question stated above.
The writer, using as a text some of the ideas given in the paper,
but more particularly some of those becoming prevalent elsewhere,
desires to discuss methods and costs of operation, especially in
relation to sand handling; and to offer suggestions looking toward
greater efficiency, as well as economy, in carrying out the standard
and well-tried methods.
_Theory of Slow Sand Filtration._--First, what is the process of slow
sand filtration? The answer to this question involves many factors, some
of which are even yet but imperfectly understood. In the early history
of filtration, at the time of the construction of the London filters,
only the straining capacity of the sand bed, to remove gross particles,
was known. Later, when the organic contents of water had become better
understood, the chemical or oxidizing powers of the process were
recognized as performing an important part. Finally, co-existent with
the discovery of the so-called "germ theory of disease," a study of the
bacterial action of filters resulted in the recognition of its
importance. It is now universally thought that each of these factors
performs its useful function; that the size of the sand, the amount of
organic matter remaining on the surface of the bed, the turbidity of the
applied water, and the bacterial content of the influent, are some of
the things on which depends the determination of the relative importance
of each.
[Footnote 1: _Transactions_, Am. Soc. C. E., Vol. XLVI, p. 258.]
Engineers have been taught to believe, by the German school of
thought, that the film of organic matter on the surface of the sand
plays a very important role in filtration. This _Schmutzdecke_, as
it is called, has been considered so precious that stress has been
placed on treating it with great care. It was not to be wholly
removed at the time of cleaning, and it was not to be walked on, or
indented, or in any other way consolidated or destroyed. In fact, in
some cases, the wasting of the first water after cleaning has been
advocated, for the reason that not a sufficient amount of this
organic film would be left on top of the sand to begin the
filtration process properly immediately after the cleaning.
In late years, however, there has been a tendency to depart from
this fundamental doctrine of slow sand filtration. Various new
processes for cleaning the sand surface have been advocated; some of
these partly destroy and others completely exterminate any semblance
of a bacterial film on the sand bed. These ideas, advanced without
any real and serious discussion of their intrinsic merits, or their
effects on the public health, are not founded on long continuous
records of such results as are necessary to establish confidence in
the final value of any of these methods.
Rapid advances along this line have been made more recently,
notwithstanding the occurrence of notable instances of trouble and
the resultant need of complete repair of filtration beds. Because of
the rough treatment of the sand surface, a penetration of organic
matter and filth into the bed had taken place. This caused deep
clogging, prevented the usual yield of water, and brought about a
lessened bacterial efficiency, due to the attempt to force water
through the filters, and because some organic matter and growths in
the lower part of the bed had furnished a breeding place for more
bacteria.
All these endeavors to reduce the work of cleaning have been
commendable, because scraping and sand handling are the items of
greatest expense in slow sand filter maintenance. Every one has been
desirous of minimizing this cost. However, as the writer will
endeavor to show, it seems that attempts along this line should be
with the idea of doing more economically, as well as efficiently,
the things which one knows will accomplish the proper results,
rather than unwisely to adopt new methods which have not been tried
for a long enough period to determine their effect on the public
health.
_Pittsburg Methods._--When first taking up the problem of design in
Pittsburg, in 1902, the writer had presented to him for
consideration and adoption, a suggestion that a certain method of
cleaning sand filters, which would involve the washing of the sand
in place (similar to that recently tried at the Jerome Park
Experiment Station, New York City), would be advisable and
economical. The decision then made has never been regretted. As this
plan involved such a complete departure from those principles which
had been well tried and had proven successful, it was believed that
it was not safe to adopt such a method on the municipal filtration
works, from which the people were to derive their drinking water.
There is more to be considered in such a problem than mere economy
of operation; the economy of human life, the effect on which
requires far longer than a few months of trial to determine, is a
much more important factor. Believing that no one should depart,
until after a long period of conclusive experimentation, from that
principle which is known to be safe (viz., to take off a small
portion of the clogging surface), the writer studied to determine
more efficient and economical methods of accomplishing this end.