INTERMITTENT FILTRATION.
By intermittent nitration is understood that filtration in which the
filtering material is systematically and adequately ventilated, and
where the water during the course of filtration is brought in contact
with air in the pores of the sand. In continuous filtration, which
alone has been previously considered, the air is driven out of the sand
as completely as possible before the commencement of filtration, and
the sand is kept continuously covered with water until the sand becomes
clogged and a draining, with an incidental aeration, is necessary to
allow the filter to be scraped and again put in service.
In intermittent filtration, on the other hand, water is taken over the
top of the drained sand and settles into it, coming in contact with
the air in the pores of the sand, and passes freely through to the
bottom when the water-level is kept well down. After a limited time the
application of water is stopped, and the filter is allowed to again
drain and become thoroughly aerated preparatory to receiving another
dose of water.
This system of treating water was suggested by the unequalled
purification of sewage effected by a similar treatment. It has been
investigated at the Lawrence Experiment Station, and applied to the
construction of a filter for the city of Lawrence, both of which are
due to the indefatigable energy of Hiram F. Mills, C.E.
In its operation intermittent differs from continuous filtration in
that the straining action is less perfect, because the filters yield
no water while being aerated, and must therefore filter at a greater
velocity when in use to yield the same quantity of water in a given
time, and also on account of the mechanical disturbance which is
almost invariably caused by the application of the water; but, on the
other hand, the oxidizing powers of the filter, or the tendency to
nitrify and destroy the organic matters, are stronger, and in addition,
if the rate is not too high, the bacteria die more rapidly in the
thoroughly aerated sand than is the case with ordinary filters.
It was found at Lawrence in connection with sewage filters that when
nitrification was actively taking place the numbers of bacteria were
much lower than under opposite conditions, and it was thought that
nitrification in itself might cause the death of the bacteria. Later
experiments, however, with pure cultures of bacteria of various kinds
applied to intermittent filters with water to which ammonia and salts
suitable for nitrification were added, showed that bacteria of all
the species tried were able to pass the filter in the presence of
nitrification, producing at least one thousand times as much nitrates
as could result in any case of water-filtration, as freely as was
the case when the ammonia was not added and there was but little
nitrification. These results showed conclusively that nitrification
in itself is not an important factor in bacterial removal, although
nitrification and bacterial purification do to some extent go together;
perhaps in part because the nitrification destroys the food of the
bacteria and so starves them out, but probably much more because the
conditions of aeration, temperature, etc., which favor nitrification
also favor equally, and even in its absence, the death of the bacteria.
The rate at which water must pass through an intermittent filter
is, on account of the intervals of rest, considerably greater than
that required to give a corresponding total yield from a continuous
filter, and its straining effect is reduced to an extent comparable to
this increase in rate; and if other conditions did not come in, the
bacterial efficiency of an intermittent filter would remain below that
of a continuous one.
As a matter of fact the bacterial efficiency has usually been found
to be less with intermittent filters at the Lawrence Experiment
Station, when they have been run at rates such as are commonly used for
continuous filters in Europe, say from one and one half to two million
gallons and upwards per acre daily. With lower rates, and especially
with rather fine materials, the bacterial efficiency is much greater;
but it may be doubted whether it would ever be greater than that of a
continuous filter with the same filtering material and the same total
yield per acre. The number of bacteria coming from the underdrains is
apparently generally less, and with very high summer temperatures much
less, than in continuous filters, and this often gives an apparent
bacterial superiority to the intermittent filters.
The effluents from intermittent often contain less slightly organic
matter than those from continuous filters; but, on the other hand,
hardly any water proposed for a public water-supply has organic matter
enough to be of any sanitary significance whatever, apart from the
living bodies which often accompany it; and if the latter are removed
by straining or otherwise, we can safely disregard the organic matters.
In addition, the water filtered will in a great majority of cases have
enough air dissolved in itself to produce whatever oxidation there is
time for in the few hours required for it to pass the filter, and it is
only at very low rates of filtration that intermittent filters produce
effluents of greater chemical purity than by the ordinary process. The
yellow-brown coloring matter present in so many waters appears to be
quite incapable of rapid nitrification; and where it is to some extent
removed by filtration, the action is dependent upon other and but
imperfectly understood causes which seem to act equally in continuous
and intermittent filters.
The peculiarities of construction involved by this method of filtration
will be best illustrated by a discussion of the Lawrence city filter
designed by Hiram F. Mills, C.E., which is the only filter in existence
upon this plan.[30]
THE LAWRENCE FILTER.
The filter consists of a single bed 2-1/2 acres in area, the bottom of
which is 7 feet below low water in the river, and filled with gravel
and sand to an average depth of 4-1/2 feet. The filter is all in a
single bed instead of being divided into the three or four sections
which would probably have been used for a continuous filter of this
size. The water-tight bottom also was dispensed with, and the gravel
was prevented from sinking into the silt by thin intermediate layers
of graded materials. The saving in cost was considerable; but, on the
other hand, a considerable quantity of ground-water comes up through
the bottom and increases the hardness of the water from 1.5 to 2.6
parts of calcium carbonate in 100,000; and while the water when
compared with many other waters is still extremely soft, the addition
cannot be regarded as desirable. The ground-water also contains iron,
which increases the color of the water above what it would otherwise be.
The underdrains have a frictional resistance ten times as great as
would be desirable for a continuous filter, the idea being to check
extreme rates of filtration in case of unequal flooding, and also to
limit the quantity of water which could be gotten through the filter to
that corresponding to a moderate rate of filtration.
The sand, instead of being all of the same-sized grain, is of two
grades, with effective sizes respectively 0.25 and 0.30 mm., the
coarser sand being placed farthest away from the underdrains, where
its greater distance is intended to balance its reduced frictional
resistance and make all parts filter at an equal rate.
The surface instead of being level is waved, that is, there are ridges
thirty feet apart, sloping evenly to the valleys one foot deep half
way between them, to allow water to be brought on rapidly without
disturbing the sand surface. For the same reason, as well as to secure
equality of distribution, a system of concrete carriers for the raw
water goes to all parts of the filter, reducing the effective filtering
area by 4 or 5 per cent. The filter is scraped as necessary in
sections, the work being performed when the filter is having its daily
rest and aeration. Owing to the difference in frictional resistance
before and after scraping, and to the fact that it is impossible to
scrape the entire area in one day, considerable variations in the rate
of filtration in different parts of the filter must occur. The heavy
frictional resistance of the underdrains when more than the proper
quantity of water passes them tends to correct this tendency especially
for the more remote parts of the filter, but perhaps at the expense of
those near to the main drain.
The filter is not covered as the suggestions in Chapter II would
require, but this is hardly on account of its being an intermittent
filter.
The annual report of the Massachusetts State Board of Health for 1893
states that during the first half of December, 1893, the surface
remained covered, that is, it was used continuously, and after December
16th it was so used when the temperature was below 24°, and was drained
only when the temperature was 24° or above. The days on which the
filter was drained during the remainder of December are not given, but
during January and February, 1894, the filter remained covered 29 days
and was drained 30 days. Bacterial samples were taken on 44 of these
days, 22 days when it was drained and 22 when it was not. The average
number of bacteria on the days when it was not drained was 137 and on
those days when it was drained 252 per cubic centimeter.
From February 24th to March 12th the number of bacteria were unusually
high, averaging 492 per cubic centimeter, or 5.28 per cent of the 9308
applied. During this period the filter was used intermittently; there
was ice upon it, and parts of the surface were scraped under the ice,
and high rates of filtration undoubtedly resulted on the scraped areas.
After March 12th the ice had disappeared and very much better results
were obtained.
While there may be some question as to the direct cause of this
decreased efficiency with continued cold weather and ice, the results
certainly are not such as to show the advisability of building open
filters in the Lawrence climate.
The cost of building the filter in comparison with European filters
was extraordinarily low—only $67,000, or $27,000 per acre of filter
surface. To have constructed open continuous filters of the same area
with water-tight bottoms, divided into sections with separate drains
and regulating apparatus, with the necessary piping, would have cost at
least half as much more, and with the masonry cover which I regard as
most desirable in the Lawrence climate the cost would have been two or
three times the expenditure actually required.
It was no easy matter to secure the consent of the city government to
the expenditure of even the sum used; there was much skepticism as to
the process of filtration in general, and it was said that mechanical
filters could be put in for about the same cost. Insisting upon the
more complete and expensive form might have resulted either in an
indefinite postponement of action, or in the adoption of an inferior
and entirely inadequate process. Still I feel strongly that in the
end the greater expense would have proved an excellent investment in
securing softer water and in the greater facility and security of
operating the filter in winter.
In regard to the effect of the Lawrence filter upon the health of
the city, I can best quote from Mr. Mills’ paper in the Report of
the Massachusetts State Board of Health for 1893, and also published
in the Journal of the New England Water-works Association. Mr. Mills
says: “In the following diagram [Fig. 15] the average number of deaths
from typhoid fever at Lawrence for each month from October to May, in
the preceding five years, are given by the heavy dotted line; and the
number during the past eight months are given by the heavy full line.
“The total number for eight months in past years has been forty-three,
and in the present year seventeen, making a saving of twenty-six. Of
the seventeen who died nine were operatives in the mills, each of whom
was known to have drunk unfiltered canal water, which is used in the
factories at the sinks for washing.
[Illustration: FIG. 15.—TYPHOID FEVER IN LAWRENCE.]
“The finer full line shows the number of those who died month after
month who are not known to have used the poisoned canal water. The
whole number in the eight months is eight.
“It is evident from the previous diagram [not reproduced] that the
numbers above the fine full line, here, follow after those at Lowell in
the usual time, and were undoubtedly caused by the sickness at Lowell;
but we have satisfactory reason to conclude that the disease was not
propagated through the filter but that the germs were conveyed directly
into the canals and to those who drank of the unfiltered canal water.
Among the operatives of one of the large corporations not using the
canal water there was not a case of typhoid fever during this period.
Warnings have been placed in the mills where canal water is used to
prevent the operatives from drinking it.
“We find, then, that the mortality from typhoid fever has, during the
use of the filter, been reduced to 40 per cent of the former mortality,
and that the cases forming nearly one half of this 40 per cent were
undoubtedly due to the continued use of unfiltered river water drawn
from the canals.”
The records of typhoid fever in Lawrence before and after the
introduction of filters are as follows:
DEATHS FROM TYPHOID FEVER IN LAWRENCE, 1888-98.
--------+----------+-------------+------------------------------------
| | | Persons who are known to have been
Years. | Total | Deaths | exposed to infection.
| Number | per +--------------+---------------------
| of | 10,000 | | While living out
| Deaths. | of | By drinking | of town just before
| | Population. | Canal Water. | falling sick in
| | | | Lawrence.
--------+----------+-------------+--------------+---------------------
1888 | 48 | 11.36 | |
1889 | 55 | 12.66 | |
1890 | 60 | 13.44 | |
1891 | 55 | 11.94 | |
1892 | 50 | 10.52 | |
1893 | 39 | 7.96 | |
1894 | 24 | 4.75 | 12 |
1895 | 16 | 3.07 | 9 | 2
1896 | 10 | 1.86 | 2 | 4
1897 | 9 | 1.62 | |
1898 | 8 | 1.39 | 1 |
--------+----------+-------------+--------------+---------------------
Filter put in operation September, 1893.
Average rate before the introduction of filtered water (1888-92) 11.31
Average rate afterward (1894-98) 2.54
These results show a striking reduction in the deaths from typhoid
fever with the introduction of filtered water, which has been most
gratifying in every way.
The more recent history of the underdrains of the Lawrence filter
is particularly instructive. Owing to the absence of a water-tight
bottom to the filter, and its low position, a certain amount of water
constantly entered the filter from the ground below. This water
contained iron in solution as ferrous carbonate. When this water came
in contact with the filtered water in the gravel and underdrains, the
iron was oxidized by the dissolved oxygen carried in the filtered water
and precipitated. This was accompanied by a growth of crenothrix in
the gravel and underdrains, which gradually reduced their carrying
capacity. This reduction in carrying capacity first became apparent
in cold weather when the yield from the filter was less free than
formerly. There was difficulty in maintaining the supply during the
winter of 1896-7 and more difficulty in the following winter.
[Illustration: FIG. 16.—TYPHOID FEVER IN LAWRENCE, 1888 TO 1898.]
The sand of the filter was as capable of filtering the full supply
of water as it ever had been, and the efficiency was as good; but
the underdrains were no longer able to collect the filtered water
and deliver it. As the filtering area was ample for the supply, it
was desired to avoid construction of additional filtering area. The
underdrains were dug up and cleaned during the periods when the filter
was drained. As the filter is all in one bed, the times when the filter
could be allowed to remain drained, and when the work could proceed,
were limited. Great care was taken to leave the work in good condition,
and free from passages, at the end of each day’s work, but the numbers
of bacteria in the effluent nevertheless increased somewhat. Some
weeks afterward the number of cases of typhoid fever in the city
increased. The numbers did not become as high as they had been prior to
the introduction of filtered water, but they were much higher than they
had been since that time, and they pointed strongly to the disturbance
of the underdrains as the cause of the increase.
The numbers of bacteria in the applied water and in the effluent from
the Lawrence filter by months, from the time the filter was put in
operation, compiled from the reports of the State Board of Health, as
far as available, are as follows:
BACTERIA IN WATER APPLIED TO AND EFFLUENT FROM LAWRENCE FILTER.
RAW WATER.
----------------+--------+--------+--------+--------+--------+--------
| 1893. | 1894. | 1895. | 1896. | 1897. | 1898.
----------------+--------+--------+--------+--------+--------+--------
January | | 7,700 | 18,700 | 7,500 | 13,314 | 6,519
February | | 7,600 | 15,040 | 12,600 | 13,113 | 4,653
March | | 6,500 | 20,770 | 5,900 | 12,055 | 3,748
April | | 11,200 | 8,420 | 3,800 | 6,904 | 2,320
| | | | | |
May | | 6,000 | 7,000 | 9,600 | 4,625 | 2,050
June | | 8,300 | 9,000 | 6,400 | 4,650 | 6,775
July | | 2,400 | 10,000 | 3,900 | 6,240 | 2,840
August | | 3,100 | 5,000 | 2,700 | 10,700 | 8,575
| | | | | |
September | 57,500 | 6,500 | 5,000 | 12,300 | 27,300 | 6,100
October | 22,200 | 25,300 | 19,000 | 5,300 | 13,200 | 5,120
November | 10,800 | 16,600 | 8,700 | 5,600 | 6,644 | 4,310
December | 8,100 | 23,800 | 6,700 | 9,695 | 5,581 | 5,200
+--------+--------+--------+--------+--------+--------
Average | 24,650 | 10,417 | 11,111 | 7,108 | 10,360 | 4,850
EFFLUENT.
January | | 129 | 206 | 166 | 91 | 39
February | | 244 | 283 | 315 | 79 | 45
March | | 455 | 405 | 133 | 67 | 34
April | | 281 | 84 | 40 | 47 | 21
| | | | | |
May | | 134 | 68 | 56 | 35 | 48
June | | 110 | 68 | 22 | 56 | 50
July | | 25 | 50 | 39 | 106 | 22
August | | 36 | 38 | 146 | 72 | 28
| | | | | |
September | 6,850 | 42 | 40 | 37 | 98 | 67
October | 1,216 | 116 | 60 | 30 | 33 | 28
November | 161 | 175 | 64 | 37 | 27 | 122
December | 111 | 364 | 84 | 67 | 24 |
+--------+--------+--------+--------+--------+--------
Average | 2,084 | 176 | 121 | 91 | 61 | 46
| | | | | |
Average | | | | | |
efficiency | 91.55 | 98.31 | 98.91 | 98.72 | 99.41 | 98.95
CHEMNITZ WATER-WORKS.
The only other place which I have found where anything approaching
intermittent filtration of water is systematically employed is
Chemnitz, Germany. The method there used bears the same relation to
intermittent filtration as does broad irrigation of sewage to the
corresponding method of sewage treatment; that is, the principles
involved are mainly the same, but a much larger filtering area is used,
and the processes take place at a lower rate and under less close
control.
[Illustration: FIG. 17.—PLAN OF AREA USED FOR INTERMITTENT FILTRATION
AT CHEMNITZ.]
The water-works were built about twenty years ago by placing
thirty-nine wells along the Zwönitz River, connected by siphon pipes,
with a pumping-station which forced the water to an elevated reservoir
near the city (Fig. 17). The wells are built of masonry, 5 or 6 feet in
diameter and 10 or 12 feet deep, and are on the rather low bank of the
river. The material, with the exception of the surface soil, and loam
about 3 feet deep, is a somewhat mixed gravel with an effective size of
probably from 0.25 to 0.50 mm., so that water is able to pass through
it freely. The wells are, on an average, about 120 feet apart, and the
line is seven eighths of a mile long.
It was found that in dry times the ground-water level in the entire
neighborhood was lowered some feet below the level of the river without
either furnishing water enough or stopping the flow of the river below.
The channel of the river was so silted that, notwithstanding the porous
material, the water could not penetrate it to go toward the wells.
A dam was now built across the river near the pumping-station, and
a canal was dug from above the dam, crossing the line of wells and
running parallel to it on the back side for about half a mile. Later a
similar canal was dug back of the remaining upper wells. Owing to the
difference in level in the river above and below, the canals can be
emptied and filled at pleasure. They are built with carefully prepared
sand bottoms, and the sand sides are protected by an open paving, to
allow the percolation of as much water as possible, and the sand is
cleaned by scraping, as is usual with ordinary sand filters, once a
year or oftener.
The yield from the wells was much increased by these canals, but the
water of the river is polluted to an extent which would ordinarily
quite prevent even the thought of its being used for water-supply, and
it was found that the water going into the ground from the canals,
and passing through the always saturated gravel to the wells, without
coming in contact with air at any point, after a time contained iron
and had an objectionable odor.
To avoid this disagreeable result the meadow below the pumping-station
was laid out as an irrigation field (Fig. 16). The water from above the
dam was taken by a canal on the opposite side of the river through a
sedimentation pond (which, however, is not now believed to be necessary
and is not always used), and then under the river by a siphon to a
slightly elevated point on the meadow, from which it is distributed
by a system of open ditches, exactly as in sewage irrigation. The
area irrigated is not exactly defined and varies somewhat from time
to time; the rate of filtration may be roughly estimated at from
100,000 to 150,000 gallons per acre daily, although limited portions
may occasionally get five times these quantities for a single day. The
water passes through the three feet of soil and loam, and afterward
through an average of six feet of drained coarse sand or gravel in
which it meets air, and afterward filters laterally through the
saturated gravel to the wells. The water so obtained is invariably of
good quality in every way, colorless, free from odor and from bacteria.
The surface of the irrigated land is covered with grass and has
fruit-trees (mostly apple) at intervals over its entire area.
This first system of irrigation is entirely by gravity. On account
of natural limits to the land it could not be conveniently extended
at this point, and to secure more area, the higher land above the
pumping-station was being made into an irrigation field in 1894. This
is too high to be flooded by gravity, and will be used only for short
periods in extremely dry weather. The water is elevated the few feet
necessary by a gas-engine on the river-bank. In times of wet weather
enough water is obtained from the wells without irrigation, and the
land is only irrigated when the ground-water level is too low.
During December, January, and February irrigation is usually impossible
on account of temperature, and the canals are then used, keeping them
filled with water so that freezing to the bottom is impossible; but
trouble with bad odors in the filtered water drawn from the wells is
experienced at these times.
The drainage area of the Zwönitz River is only about 44 square miles,
and upon it are a large number of villages and factories, so that the
water is excessively polluted. The water in the wells, however, whether
coming from natural sources, or from irrigation, or from the canals,
has never had as many as 100 bacteria per cubic centimeter, and is
regarded as entirely wholesome.
In extremely dry weather the river, even when it is all used for
irrigation so that hardly any flows away below, cannot be made to
supply the necessary daily quantity of 2,650,000 gallons, and to supply
the deficiency at such times, as well as to avoid the use of the canals
in winter, a storage reservoir holding 95,000,000 gallons has recently
been built on a feeder of the river. This water, which is from an
uninhabited drainage area, is filtered through ordinary continuous
filters and flows to the city by gravity. Owing to the small area of
the watershed it is incapable of supplying more than a fraction of the
water for the city, and will be used to supplement the older works.
This Chemnitz plant is of especial interest as showing the successful
utilization of a river-water so grossly polluted as to be incapable of
treatment by the ordinary methods. Results obtained at the Lawrence
Experiment Station have shown that sewage is incapable of being
purified by continuous filtration, the action of air being essential
for a satisfactory result. With ordinary waters only moderately
polluted this is not so; for they carry enough dissolved air to effect
their own purification. In Chemnitz, however, as shown by the results
with the canals, the pollution is so great that continuous filtration
is inadequate to purify the water, and the intermittent filtration
adopted is the only method likely to yield satisfactory results in such
cases.
Intermittent filtration is now being adopted for purifying brooks
draining certain villages and discharging into the ponds or reservoirs
from which Boston draws its water-supply. The water of Pegan Brook
below Natick has been so filtered since 1893 with most satisfactory
results, and affords almost absolute protection to Boston from any
infection which might otherwise enter the water from that town. A
similar treatment is soon to be given to a brook draining the city of
Marlborough. The sewage from these places is not discharged into the
brooks, but is otherwise provided for, but nevertheless they receive
many polluting matters from the houses and streets upon their banks.
The filtration used resembles in a measure that at Chemnitz, and I am
informed by the engineer, Mr. Desmond FitzGerald, that it was adopted
on account of its convenience for this particular problem, and not
because he attaches any special virtue to the intermittent feature.
APPLICATION OF INTERMITTENT FILTRATION.
In regard to the use of waters as grossly polluted as the Zwönitz,
the tendency is strongly to avoid their use, no matter how complete
the process of purification may be; but in case it should be deemed
necessary to use so impure a water for a public supply, intermittent
filtration is the only process known which would adequately purify
it. And it should be used at comparatively low rates of filtration.
I believe that an attempt to filter the Zwönitz at the rate used for
the Merrimac water at Lawrence, which is by comparison but slightly
polluted, would result disastrously.
The operation in winter must also be considered. Intermittent
filtration of sewage on open fields in Massachusetts winters is only
possible because of the comparatively high temperature of the sewage
(usually 40° to 50°), and would be a dismal failure with sewage at
the freezing-point, the temperature to be expected in river-waters in
winter.
It is impossible to draw a sharp line between those waters which are
so badly polluted as to require intermittent filtration for their
treatment and those which are susceptible to the ordinary continuous
filtration. Examples of river-waters polluted probably beyond the
limits reached in any American waters used for drinking purposes and
successfully filtered with continuous filters are furnished by Altona,
Breslau, and London.
Intermittent filtration may be considered in those cases where it
is proposed to use a water polluted entirely beyond the ordinary
limits, and for waters containing large quantities of decomposable
organic matters and microscopical organisms; but in those cases where
a certain and expeditious removal of mud is desired, and where
waters are only moderately polluted by sewage, but still in their raw
state are unhealthy, it is not apparent that intermittent filtration
has any advantages commensurate with the disadvantages of increased
rate to produce the same total yield and of the increased difficulty
of operation, particularly in winter; and in such cases continuous
filtration is to be preferred.
In the removal of tastes and odors from pond or reservoir waters which
are not muddy, but which are subject to the growths of low forms of
plants, which either by their growth or decomposition impart to the
water disagreeable tastes and odors, intermittent filtration may have a
distinct advantage. In such cases there is often an excess of organic
matter to be disposed of by oxidation, and the additional aeration
secured by intermittent filtration is of substantial assistance in
disposing of these matters.