combinations of two or more to yield different degrees of purification
that will meet varying local requirements. Which of these or what
combination of processes to use according to local requirements is the
all important question for a city to answer. Several cities either have
adopted or are planning to adopt the plan advocated by John A. Giles,
Commissioner of Public Works of Binghamton, New York, to include a
number of the different stages of treatment in the original design so
that when future installation is necessary on account of increased
population, with its increased pollution, or the need for a greater
degree of purification becomes imperative, the addition can be made on
the site already provided for and each unit will fit into the complete
structure at a minimum cost. The consensus of opinion is that a disposal
works can be designed and constructed which will produce an effluent
that will not deteriorate the water into which it is discharged, that
will create no nuisance from odor or from flies and that the cost will
be strictly proportionate to the sanitary and esthetic results achieved.
An approximate idea of the efficiency of the various well known
processes in the removal of bacteria was given by Professor George G.
Whipple, Professor of Sanitary Engineering, Harvard University, before
the New York State Conference of Mayors and Other City Officials:
┌──────────────────────────────┬──────────────────────────────┐
│ Process │Percentage of Bacteria Removed│
├──────────────────────────────┼──────────────────────────────┤
│Fine screens │ 10 to 15 │
│Settling tanks │ 60 to 70 │
│Septic tanks │ 60 to 70 │
│Chemical precipitation │ 80 to 90 │
│Contact filters │ 75 to 85 │
│Percolating filters │ 85 to 95 │
│Intermittent sand filters │ 95 to 99 │
│Broad irrigation │ 95 to 99 │
└──────────────────────────────┴──────────────────────────────┘
Dilution
Comparatively few cities can much longer depend upon large bodies of
water to dilute their untreated sewage. Even those cities located on the
seacoast and on the banks of large rivers and lakes have either provided
some method of treatment, usually one or more of the processes in the
preliminary group, or are planning to do so. New York City which has an
adjacent large body of water into which it discharges its sewage without
treatment of any kind, now finds it necessary to adopt a combination of
processes to eliminate the nuisance the waste is causing. In some places
where dilution is depended upon, the existing nuisances have been caused
by the outlets being extended only to the high water line of the water
course, thus preventing a proper mixture of sewage with a sufficient
volume of water adequately to dilute it. Other difficulties experienced
when untreated or raw sewage is discharged into large volumes of water
in excessive quantities are the formation of deposits of sludge, the
residue after sewage has been allowed to settle, on the banks and the
bottom; turbidity, milkiness and oiliness of the water, bad odors, the
formation of scum upon the water and the destruction of shellfish. To
overcome these difficulties some cities have resorted to dredging,
screening and sedimentation. Others have been compelled to adopt some
more complicated process.
The California State Board of Health in one of its bulletins quotes its
consulting engineer, Charles G. Hyde, as saying that experience has
demonstrated rather definitely that a nuisance will be caused if sewage
is diluted with less than about twenty volumes of water while from forty
to fifty may in some cases be necessary. Weston believes that in
ordinary cases mixtures of sewage and water should be fifty per cent.
saturated with oxygen, and when there is an excessive deposit of sludge
even seventy per cent. of saturation may be insufficient. Herring and
Gregory, in their report on the Albany, New York, system, say: “From
observations made in many rivers it has been found that a flow of well
oxygenated river water of from three to six cubic feet per second is
capable of diluting the sewage from a population of 1,000 to a degree
that will allow oxygen in the river water to oxidize the easily
putrescible organic matter in the sewage and thereby prevent the water
from becoming offensive, provided the velocity of flow is sufficient to
prevent accumulations of sewage sludge on the bottom of the stream.”
Screening
The screening process consists of running the sewage through coarse or
fine screens, either hand cleaned or mechanically operated, to remove
suspended and floating matter. There is almost an unanimity of opinion
now in favor of the use of mechanically operated fine screens. The
efficiency depends largely although not entirely, upon the size of the
mesh or openings through which the sewage passes. Coarse screens, which
are cleaned by hand, will remove from two to ten per cent. of the
suspended matter and fine screens which are mechanically operated will
in some cases remove as much as 25 per cent. Screening will not
materially change the turbidity of the liquid or the greasy appearance
nor will it remove all of the suspended matter.
Experience has shown that the screening process is valuable in
connection with sewage pumping works and inverted siphons, when sewage
is disposed of by dilution and when raw sewage is applied without any
other preliminary treatment to a final process as it prevents the
clogging of machinery and filters.
When the process is used the screenings must ordinarily be disposed of
within twenty-four hours on account of fermentation and decomposition
which sets in quickly. In some cities the deposits are buried and in
others they are burned after having been artificially dried. Robert
Spurr Weston says that it seems unwise to attempt to dispose separately
of two kinds of sludge, namely that removed before and that remaining
after subsidence. “On the other hand,” he continues, “the screening of
the effluent from a settling tank in order to reduce the operative
charges for cleaning sprinklers is an economical practise. Furthermore,
the actual amount of material screened from the effluent is small in
comparison with that removed from unsettled sewage and its subsequent
disposal is not a serious burden.”
Grit Chambers
If a sewage disposal plant is operated in connection with a combined
sewerage system grit chambers are usually necessary for the removal of
sand, gravel and dirt before the sewage passes on for further treatment.
Where a city has a separate system of sewerage grit chambers are held by
some authorities to be unnecessary unless the first wash of the street
after a storm is intercepted and the waste is treated. Gregory has
expressed the belief that the safest plan under ordinary conditions
seems to be to provide a grit chamber. It is generally agreed that the
chambers should be so constructed that the sewage will flow through
slowly enough for the grit to settle out, but fast enough to carry the
organic matter in suspension. To insure proper operation the chamber
must be cleaned out frequently. At the Cleveland Sewage Testing Station
it has been found that velocities ranging from 30 to 60 feet per minute
produce a grit of proper character. The California State Board of Health
has advocated chambers with a capacity such that a net period of storage
of at least three minutes be allowed and a velocity of not less than
five feet per minute.
Straining or Roughing
There are few cities which treat their sewage by the process of
straining and roughing. This consists of removing the suspended matter
by means of rapid straining through beds of coke or sand arranged like
the rapid sand or mechanical water filter. Coke beds, especially in cold
climates, have not been a success. The chief objection to the rapid sand
filter is the wash water which contains much organic or mineral
impurities of the sewage and which requires special treatment which
experience has shown to be difficult and expensive. Difficulty has also
been found in disposing of the sludge deposited upon the filter surface.
Of this process the bulletin of the California State Board of Health
says: “The process is an expensive one at best, both as respects
construction and operation. The effluent from such works can be made
fully equal to, if not better than the effluent of plain sedimentation
basins from a sanitary point of view.” The experience of the Cleveland
Testing Station with these filters was not favorable. The filters when
operated at rates from 30 to 60 gallons per acre per 24 hours removed
from 25 to 40 per cent. of suspended matter and their action was simply
mechanical, there being no increase in the dissolved oxygen content. The
report from the station says that the difficulties encountered in their
operation were sufficient to eliminate the process as a method in itself
or in combination with other processes.
Treatment in Tanks
The treatment of sewage in tanks, either by chemical or biological
processes, has been adopted by many cities, especially as a preliminary
treatment. These processes are known as plain sedimentation, chemical
precipitation and the septic process. Of these the treatment in the
Imhoff tank is the most popular at the present time.
Plain Sedimentation
By allowing the sewage either to flow into properly constructed tanks or
through them at a velocity low enough to allow some of the suspended
matter to separate from the liquid and to be deposited on the bottom
from which the sludge is removed, is another process that has been used
by a number of American and European cities. The first tanks were
constructed so that they could be filled with sewage and then after the
suspended matter had settled the effluent was drawn off. This was known
as the fill and draw plan. Later what is now known as the continuous
flow principle was used. The velocity of the flowing sewage is reduced
sufficiently as it enters and passes through the tank for the suspended
matter to settle. The sludge which collects at the bottom of the tank
must be removed frequently. The results are affected by the quantity and
quality of the sewage, fresh sewage being capable of greater
clarification by sedimentation than stale sewage. The range in storage
period for American sewages is from four to twelve hours and the removal
of suspended matter is from 45 to 75 per cent.
In some cities plain sedimentation has been used in connection with
dilution and in others as an aid to filtration. The chief objection to
the process is the sludge which is extremely offensive and must be
treated separately. It does not dry readily, is difficult to handle and
if allowed to accumulate causes serious nuisance. Because of these
difficulties and the fact that the sludge from the Cameron and Imhoff
tanks can be more easily disposed of the septic process has gradually
forced plain sedimentation into the background.
Colloidal tanks were designed to carry the process of clarification
further than plain sedimentation, but they have not come into general
use. Metcalf and Eddy in their “American Sewerage Practice” say of this
process: “There has been a feeling that while under some conditions a
portion of the colloidal solids could be removed by such devices, the
work accomplished was not likely to be sufficient to offset the expense
of construction and some difficulties in operation.”
The Septic Process
In the septic process the raw sewage is conveyed to tanks, and allowed
to stand until the solids have settled to the bottom and have been
partially destroyed or liquefied by bacterial action. Two types of tanks
are used in the septic process, one known as the Cameron type and the
other as the Emscher or Imhoff tank.
The best constructed Cameron tanks are not less than 8 feet in depth and
are usually large enough to hold about six hours’ maximum flow of
sewage. The desirable time of detention depends upon the character of
the sewage, both as to strength and freshness, strong and stale sewages
demanding a longer period. The tanks are usually built with baffles at
the entrance to retard the current and to deflect the suspended matter
to the bottom which is so constructed that the sludge, after bacterial
action has taken place, can be drawn off from time to time.
H. W. Clark, formerly chemist of the Massachusetts State Board of
Health, has expressed the belief that the rate of flow through a septic
tank should not be greater than that which will cause passage in twelve
hours.
Charles G. Hyde in the California Board of Health Bulletin says that as
a rule the period should not be greater than 24 hours nor less than 12
hours, except possibly with weak or stale sewages. He advocates multiple
units so that the storage periods may be controlled to give optimum
results.
The effluent which is turbid, putrescible and rich in organic matter
cannot be discharged into streams with safety without further treatment,
unless the volume of water is sufficient to complete the purification by
dilution. As the solids settle a scum which forms on top of the tank,
keeps out light and air and produces a condition favorable for the
bacterial activity caused by minute organisms known as anaerobic
bacteria. These germs thrive and functionate best in the absence of
oxygen, and their chief function in sewage treatment is the conversion
of the solid organic matter into a soluble form, somewhat less complex
in chemical composition. The sludge is rotted and when full bacterial
action has taken place is humified. In plain sedimentation the solids
are simply deposited upon the bottom of the tank and are removed
practically unchanged. In the septic tank, however, a part of the solids
after settling are broken down or digested, thus somewhat lessening the
difficulty of disposing of the sludge.
Reports vary widely as to the amount of suspended matter that can be
removed by the septic process. The Iowa State College bulletin says that
the amount of purification does not usually exceed 25 to 40 per cent.
Professor Whipple places the removal between 60 and 70 per cent., and
the State Board of Health of California says it may vary between 35 per
cent. and 85 per cent., averaging perhaps 50 to 60 per cent. H. W. Clark
places the amount at not less than 40 per cent. and adds that it will
vary according to the character of the sewage, the variations being from
30 per cent. with weak sewage to 80 per cent. with strong sewage.
All reports concur that in many cases the Cameron type of tank has
failed to produce efficient results. Among the objections raised by
authorities are the following:
The sludge is not thoroughly digested and is somewhat offensive. The
odor is obnoxious and the effluent is too stale and is treated with
difficulty by oxidation processes. Gilbert J. Fowler, Sanitary Expert of
England, says the defects which have shown themselves are a nuisance
both from the tank effluent and the sludge and an excessive quantity of
suspended solids in the tank effluent. Charles G. Hyde believes a review
of the principles and results of operation appear to justify the
conclusion that “the septic effluents are only less dangerous than crude
sewage to the extent of efficiency of removal of organic matter.”
The Imhoff Tank
In an effort to overcome the defects in the Cameron tank, the Imhoff or
Emscher tank was developed and this now seems to have the preference
among cities making new installations. The tank consists of two
compartments, one above the other. It has a smaller area than the
ordinary septic tank, but is much deeper. The sewage passes at a low
velocity through the upper chamber, which is comparatively shallow and
V-shaped, the sides being sufficiently steep to allow the solids to be
deposited at the bottom of the V which is equipped with slots. Through
these the solids pass into the second chamber below which is much deeper
than the other. The inclined partition wall must be cleaned frequently
with hose or squeegee in such a way as not to clog the slots. The
floating pieces of wood and cork must be skimmed off, but the greater
part of the suspended matter that floats will generally sink after a
time. Dr. Karl Imhoff, the inventor of the tank, advises spraying with a
hose to expedite the sinking. Care must be taken to keep the sides clean
and the sludge in the lower tank below the slot level. If neglected
suspended matters will rise to the surface behind as well as in front of
the scum boards. Dr. Imhoff advises the reversal of the flow of sewage
about every three weeks after skimming off the floating matter when one
sedimentation chamber feeds more than one sludge chamber. The rate of
flow in the upper chamber is sufficiently rapid to prevent any septic
action, yet slow enough to allow much of the suspended matter to settle.
The effluent in a comparatively fresh condition passes out of the tank
for further treatment or for discharge into water courses. It therefore
does not become stale nor does it come in contact with decomposing
sludge, thus eliminating in part the objections advanced by authorities
against the Cameron tank.
In the lower tank the sludge, after passing through the slots is slowly
digested through septic and other actions without any disturbance by the
flow of the liquid sewage, above. Before the tank can deliver good, well
digested sludge—that is, a black alkaline odorless sludge—it must be
inoculated with a proper amount of good sludge, or the raw sludge must
be permitted to “ripen.” Dr. Imhoff has found that even without
inoculation a tank will discharge good sludge from the beginning if ripe
sludge is emptied into the system from cesspools which have been in use
a long time.
In some instances cities have had considerable trouble with acid
decomposition during the ripening period. This produces a sludge of
objectionable odor and one not easily dried. It decomposes very slowly
and may rise in a mass to the surface of the sludge chamber. Various
remedies have been suggested, among them the addition of lime. “I cannot
advise such addition,” Dr. Imhoff has written. “All plants which are
known to me and in which acid decomposition has occurred have sooner or
later adjusted themselves of their own accord.”
When properly inoculated the particles of sludge rise and fall
constantly in the process of giving off the gases. The fresh sludge
particles entering the chamber through the slot are covered so that the
entire mass becomes thoroughly mixed and the untreated sludge in a short
time is inoculated with the proper organisms. The decomposed sludge is
discharged from time to time through pipes leading from the bottom of
the tank to drying beds.
Dr. Imhoff has advocated the discharge of sludge from each sludge
chamber once every two to six weeks, that the optimum of the sludge
level should be about three feet below the slot level and if it is
desired to promote the early incidence of proper decomposition the
sludge should not be allowed to remain quiet at the bottom of the sludge
chamber. He advocates constant stirring and a uniform introduction of
fresh organic matter and the discharge of the decomposed matter. The
scum layer, he says, must be agitated frequently by a jet of water or
otherwise and the sludge at the bottom of the chamber should be agitated
by a water stirring system. As a substitute, he suggests that the whole
body of sludge be pumped out and returned. To determine the elevation of
the sludge surface, he advises lowering into the sludge chamber a very
thin piece of sheet iron one foot square in area held in a horizontal
position. If the level is too high, there will be gas bubbles on the
surface of the settling chamber above the slot or there will be floating
sludge and in extreme cases foaming sludge. As compared with other tank
processes the experience of cities indicates that the Imhoff type has
many advantages. Certain inherent difficulties, however, have been
pointed out in several reports. Gilbert J. Fowler has expressed the
belief that “the comparative short time of settlement means that
variations in the character of the sewage must be quickly reflected in
the character of the tank effluence and that the filters (when they are
used for further treatment) must be called upon rapidly to accommodate
themselves to fluctuating conditions.” He believes that this is not
conducive to the development of the most efficient bacterial activity.
Storm water above moderate dilution, he says, will have to receive
separate treatment and he is of the opinion that ordinary stand-by tanks
will still be necessary for this purpose, the sludge from which will
have to be dealt with. From the results of the operation of an
experimental plant in Worcester, Massachusetts, Matthew Gault,
Superintendent of Sewers, draws these conclusions: “It appears to be
perfectly feasible to treat Worcester sewage by means of Imhoff tanks
and sprinkling filters. The results of experimental treatment of the
effluent from chemical precipitation tanks indicated that the advantages
gained by chemical precipitation as a preliminary treatment were not
commensurate with the cost. The Imhoff tank was quite as efficient in
sludge digestion as experimental septic tanks have been and much more
efficient so far as sedimentation of the sewage is concerned. It was
operated without the production of the offensive odors characteristic of
the septic tank and the sludge itself was disposed of without creating a
nuisance. The effluent from the Imhoff tank was normally as fresh in
appearance and odor as the sewage flowing into the tank.”
The experience of the New Jersey State Board of Health with Imhoff tanks
has been that if properly designed, constructed and operated, they are a
valuable means of sewage clarification. The observation of its engineers
has shown that under these conditions the tanks overcome a great deal of
trouble due to odors and greatly simplify the sludge problem. “However,
their proper operation is a considerable problem,” reads one of its
reports. “And the cost of keeping them in working order is several times
greater than for septic or sedimentation tanks.” In view of the initial
cost of this form of tank as compared with the older single story types
the New Jersey engineers believe that “in cases where the works are far
removed from a populous community, so that the odor problem is not
serious, it is doubtful whether the Imhoff tank has any material
advantage over a properly constructed, well baffled sedimentation tank
of the old type.”
The Cleveland Sewage Testing Station reports that the most consistent
results were obtained from the operation of the Imhoff tank, an average
suspended matter removal of 50 per cent. being secured. A recent city
report says: “In general it may be said that a detention period of
thirty minutes accomplished a removal of suspended matter from 40 to 45
per cent. as compared with a 50 per cent. removal effected by a
detention period of two hours and fifteen minutes.”
In a bulletin of the California Board of Health, Charles G. Hyde sums up
the importance of the septic process thus: “The septic process as
carried out either in the Cameron or Imhoff type, but especially in the
latter, has at present two distinct fields of usefulness; first, it
constitutes an effective means of preparation for any final process
which can be better conducted with a sewage from which the suspended
solids are more or less completely removed; secondly, it may be employed
when disposal by dilution is permissible if the source of unsightly
sludge and scum is removed.” Another advantage may be added, the Imhoff
tank produces a sludge that can be disposed of easily.
Chemical Precipitation