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Oct. 15, 1946.-
w. M. FIAT-r
l 2,409,585
lOriginal Filed April 16, 1942 -
Patented Oct. 15,1946
William M. Piatt, Durham, N. C., assigner to
Paciñc Flush Tank Company, Chicago, Ill., a
corporation of Illinois
Continuation of application Serial No. 439,188,
April 16, 1942. `This application July 21, 1945,
Serial No. 666,412
3 Claims.
(Cl. 210-2)
My invention relates to apparatus and method
for the digestion of sewage sludge, and it has to
do particularly with control of >the removal oi
tank. On the other hand, the partially digested
by anaerobic bacteria. In the digestion tank, the
natant liquor containing least quantities of solids
or undigested solids which are undergoing active
digestion and consequently releasing gases which
supernatant liquor from digestion tanks. ’
tend to rise vertically, will keep the upper por
The present application is a continuation of 5 tion of the digestion tank more or less stirred up.
my copending application Serial No. `439,188 ñled
Depending upon the quantities of raw solids
April 16, 1942.
which are being added, the temperature of the
In the operation of sewage treatment plants,
digestion tank and other uncontrollable factors,
it is quite customary to first introduce raw 'sew
the digestion tank contents are rather unpre
age into a preliminary settling tank wherein set l0 dictable as to the nature of the material which
tleable solid materials are separated from the
may be found at any one point in the tank and
liquid, the liquid being discharged to other por
it is for that reason, as above stated, that the
tions of the plant for further treatment or dis
location and quantity of supernatant liquor which
posal while the solid material or sludge is trans
may be suitable for. withdrawal is highly vari
ferred to a digestion tank where it is acted upon .
able and the selection and removal of the super
presents a diiiicult problem.
tions resulting in the destruction of the organic
The usual method of disposal of supernatant
portions of the sludge which have objectionable
liquor is to return it to the inlet of the sewage
qualities. In the handling ofv ordinary domestic v20 plant for treatment along with the raw sewage,
sewage, between forty and'sixty per cent of the
that is, either before or after the raw sewage
has passed through the primary settling tank.
solid matter in the raw sludge will be destroyed
and released as gas during the digestion period.`
In view of the fact that this supernatant liquid
Raw sludge oi this character as added to the
contains some solids from an anaerobic digester
digester may contain between ninety-four and 25 in which the reactions take place in the absence
ninety-eight per cent water, while the digested
of oxygen, these solids have a very high afñnity
and completely treated sludge, as withdrawn from
,for oxygen and will deplete oxygen in the raw
sewage, thereby causing septic action to take
the digester, will contain from about eighty-eight
to ninety-four per cent water or moisture. In
place which, when it occurs, disturbs the proper
the digestion process there is naturally an in 30 functioning of the entire plant. Consequently, it
is essential that the supernatant liquor sent back
creased concentration of the solids in the di
gested sludge as withdrawn from the digester
into the plant contain as little solid material
and, because of the considerable quantity of sol
from the digester as possible so as to lessen this
solids undergo decomposition, a series of reac
ids which may be destroyed, as above explained,
upsetting effect on the entire sewage treatment
it will be appreciated that there will be large 35 process.
quantities of liquid, usually termed “supernatant
Heretoiore, digestion tanks have taken various
liquor,” which must be withdrawn from the di
gester and disposed of. This liquor will gener
ally collect or separate to form substantially hori
zontal strata or layers of clear liquid and, while
forms and various means have been provided for
supernatant liquor draw-off, all of which, how
ever, are subject to objection from the foregoing
standpoints. For example, digestion tanks have
been employed ‘with either a iixed or a floating
cover and they have been `provided with one or
more pipes at the side of the tank for removing
this stratification may occur at almost any level
in the digestion tank, the clear liquid is usually
found in the upper half of the contents of the
tank although not at the very top.
During the digestion action, those solids which
the supernatant liquor from the digester.
are more completely digested and are therefore
quite important that a multiplicity of pipes be
provided at diiierent elevations along the side
not being acted upon by bacteria and releasing
of the'tank so that the operator may at will
select a point of supernatant liquor draw-ofi
gases, >will tend to settle to the bottom of the
digestion tank. These solids will normally have
most of the organic material digested from them
so that they will contain the inert and heavier
portions of the solids, which will cause them to
settle in the bottom of the tank, and as they set
tle they will release some of the liquid by corn
the use of a floating cover type of tank, it is
from that stratum where, in his opinion, the
best quality of liquor occurs. This same arrange
ment may be employed with a fixed cover or, as
in usual practice, a single point of take-off of
material may be provided, with the material
pacting and concentrating at the bottom oi the 55 merely overflowing from this point whenever sol
Figure I is a vertical, sectional view of one form
of sewage sludge digestion tank embodying my
ids are added to the tank. With these prior ar
rangements, it is necessary to give very careful
attention to manual control of the draw-off, ob
Fig. 2 is a separate fragmental View of one form
of selector or strainer device that may be employed
the material is of a character which will permit
in carrying out my invention; and
the return of the same to the treatment plant.
Fig. 3 is a diagrammatic View of a sewage treat
Even with very careful control, it is not possible _
to always return the best quality of material to .Y Ament system including the digestion tank shown
serving all during the draw-off period whether
the plant for retreatment, thereby resultingr in
Y in Fig. 1.
explained. Moreover, the withdrawal is done peri
odically, rather than continuously throughout the
In the drawing, I have shown my invention ap
plied to sewage treatment system including a di
gestion tank I0 of conventional form having an
digestion process, so that the rate of return dur
_ annular side Wall Ita and a bottom Iii". The tank
an upsetting of the treatment process as-above 10
is provided with a 'so-called iioating cover II
ing such periods is relatively high, thereby tend
ing to upset the treatment process. Also, with 15 having the usual gas-collecting dome I2, gas pipe
housing I2“ and gas discharge pipe I3. Raw,
these prior arrangements, constant attention is
undigested sludge from any suitable source, such
required, thereby tending to increase operating
personnel necessary for proper operation of the
as a primary clarifier 4, is fed to the digester lil
through piping I4 including an inlet pipe Illa, and
the digested sludge is discharged from the bottom
of the tank through a draw-off pipe I5. It will
be understood that, although I have shown my
One of the objects of my invention is to provide
improved sewage digestion means including im
proved supernatant liquor draw-oiî means by
invention as applied to a floating cover type of
which manual control and guesswork are elim
tank, it may be equally well applied to the Well
inated in the selection oi the supernatant liquor to
fixed cover or any other conventional type
be drawn off, and by which all such liquor which 25 known
of sludge digestion tank. Raw sewage may be
is properly clariñed is drawn olf.
supplied to the primary clariñer or settling tank
Another object is to provide means for selec
4 by an inlet 5, and the liquid separated from
tively discharging supernatant liquor from a sew
the solids in the clariñer may be discharged by
age digestion tank.
outlet 6.
Still another object is to provide improved 30 anIn
carrying out my invention, I provide a se
means for automatically and selectively discharg
lector device which is disposed within the digest
ing supernatant liquor from a digestion tank, at
ing material and which is constructed and ar
varying levels therein where the supernatant
ranged to automatically draw off supernatant
liquor is sufñciently clariñed and free of suspended
liquor therefrom at any elevation therein where
solids that its oxygen demand is minimized and
such liquor ordinarily collects and is clariñed or
it may be retreated with raw sewage without up
solids content reduced to a predetermined extent.
setting the normal treatment functions of the
More particularly, I provide a perforated or
sewage plant as a whole.
slotted selector device I6 (Fig. 2) which is
An additional object is to provide means for
vertically within the digestion tank and
automatically selecting and discharging superna 40 mounted
which extends from near the bottom of the tank
tant liquor from a digestion tank, at varying levels
to a point near the top of the material therein.
therein, according to the solids content thereof,
This selector device I6 takes the form of an elon
whereby only that liquid which is clarified to a
gated tubular member having its opposite ends
predetermined extent and is substantially free of
45 blocked or plugged as at Il' and its intermediate
suspended solids is discharged.
portion perforated or slotted to provide annular
A more speciñc object is to provide a digestion
or circumferential series of superimposed elon
tank having therein a vertically disposed selector
gated slots or orifices I8. The particular strain
discharge device which extends substantially
er shown in the drawing (Fig. 2) may take the
throughout such depth of the material in the tank
Where the clearest liquor is most likely to occur, 50 form of a plurality of circumferentially-spaced
vertical supports I9 having a rectangular, Wire
which selector device is perforated or Islotted sub
like member 20 wound spirally thereabout to
stantially throughout its length so as to afford a
the slots I8. The slots I8 may vary from
straining action predetermined in character and
ïlë" to %” in width, and I have found that eX
whereby only clarified liquid substantially free of
55 cellent results may be obtained by using slots
suspended solids is discharged.
about- 1/8” in width. With this arrangement, the
A further object is to provide a sewage treat
orifices or slots I8 extend from near the bottom
ment system for digesting sewage by anaerobic de
Illb of the tank substantially throughout the
composition, in which supernatant liquor evolved
depth of the material therein so that supernatant
in the digestion process is withdrawn from the
digestion tank and introduced into the raw sew 60 liquor sufficiently clarified or free of suspended
solids to pass through the oriñces I8, at any point
age to be treated, the withdrawal being continu
or level within the depth of the digesting mate
ous throughout the process and limited to sub
rial, will be drawn off. The slots I8 could, of
stantially clear liquor whereby the tendency to
course, be located only in the upper half of the
deplete the oxygen in the raw sewage is held at
65 tank where the clarified liquor usually occurs in
a minimum throughout the process.
normal operation. The rectangularly shaped
Still another object is to provide a novel method
slots I8 have squared edges that are highly con
of withdrawing supernatant liquor from a digesn
ducive to an eifective straining action in that
tion tank so that only the clearest liquor is with
solids suspended in the supernatant liquor will
drawn, the withdrawn liquor thus being of a char
acter that it may be returned to the raw sewage 70 initially tend to collect and adhere around the
edges of the slots, thereby reducing the area of
without materially depleting the oxygen content
the same and tending to provide a very effective
straining action. That is to say, the solids that
Other objects and advantages will become ap
have collected around the edges of the slots pro
parentas this description progresses and by ref
75 vide a medium which not only further restricts
erence to the drawing, wherein:
the slots but also provides a medium to which
the upsetting eiiect occasioned by intermittent
solids in the material tending to pass through the
return of poor supernatant liquor, the main units
oi the treatment plant may be designed without
extra capacity which has been deemed
slots will adhere so that the material which ac
tually passes through the slots for draw-ofi pur
poses will be free or practically free of suspended
solids. Consequently, with this arrangement, au
tomatic selection and draw-oli, without any
manual control or guesswork, is provided, the best
in order to take care or" the foregoing
condition. This will naturally result in a con
siderable saving of cost of plant structure in ad
dition to the improved operation and eñiciency of
the ‘treatment plant as a whole.
In addition to
supernatant liquor for return to the raw sewage
for further treatment is discharged, and the like
lihood of upsetting the treatment plant as a whole
limiting the liquor returned to that which is sub
is eliminated. Furthermore, the return of such
liquor to the inlet 5 will be continuous throughout
the digestion process without excessively de
throughout operation of the plant at a relatively
low »rate of flow so that the oxygen demand due
stantially clear, the present invention provides
means for returning the liquor constantly
pleting the oxygen content of the raw sewage. 15 to such return is held at a minimum.
I claim:
It will be understood that, while I prefer to use
i. In structure of the class described, a closed
a 4selector device like that illustrated in Fig. 2,
tank free of obstructions preventing horizontal
other forms of slotted or perforated devices may ì
flow of liquid material in said tank, said tank
be employed. Also, while the slots I8 are prefer
ably of uniform size throughout the tubular 20 being adapted to receive sewage sludge for di
gestion thereof and without mechanical agita
member, they may be varied somewhat, the main
tion, means for passing sludge into said tank to
purpose being to provide a selector device having
be digested, and means for automatically dis
a straining action which, cooperatively with the
charging the supernatant liquor from varying
material in the tank, will eiîect a properly clari
ñed supernatant liquor discharge,
25 levels at which it may accumulate and substan
tially free of suspended solids, including a tubu
The selector device I6 is connected to a draw
lar member disposed vertically in said tank and
ofî pipe 2| which is adapted to return the super
extending from near the top thereof downwardly
natant liquor to the inlet 5, or ii desired to a point
through at least the upper half of said'tank,
beyond the primary settling tank- 4, where it is
added to raw incoming sewage for retreatment 30 said member being provided with elongated slots
of a width varying from île" to 3/8”, said slots
along with the raw sewage in the usual manner.
being located both circumferentially around and
Or, if desired, this draw-oit pipe 2l may lead to
longitudinally of said member whereby super
any other point of final disposal permitted in the
natant liquor at varying levels in said tank which
operation of the sewage plant, as indicated by
the branch pipe 2i). Also, in the use oi a iioating 35 is freed sufficiently of suspended solids to pass
through said slots will pass into said tubular
cover, it is preferably provided with a selector
member, and means including a generally hori
housing Ila which accommodates the selector I6
zontal closed conduit connected to said tubular
as the cover is raised and lowered within certain
member between its ends for discharging the
In the use of a selector device I6 such as de 40 liquor from said tubular member to a point of
scribed, there may be times when the slots I8 may
tend to become clogged to an extent impairing
2. In a structure of the class described, a di
gestion tank for receiving sewage sludge and in
the draw-off capacity of the device. To take
which digestion of said sludge takes place with
care of that condition, I provide means for back
íiushin-g or backwashing the device to free the 45 resultant formation of variably located horizon
tal strata of sludge and supernatant liquor hav
slots that may become clogged. This backwash
ing is accomplished by feeding a washing fluid
ing suspended solids content which vary through
backwardly through the discharge pipe 2l. To
this end, I provide a backwashing conduit 22,
out said several strata, means including a con
duit extending into said tank for admitting
which may normally be closed by a valve 23, and 50 sludge to be digested, means including a conduit
extending from said tank for withdrawing digest
a pump 24 which may be employed for forcing»
ed sludge therefrom, and means for continuously
the backwashing iiuid or liquid through the pipe
and automatically selecting and drawing off su
2l and into the device is. In some instances,
pernatant liquor from those strata, wherever
when a proper head pressure is available, the
located, wherein the solids contents have been
pump 24 may be dispensed with and the fluid fed
reduced to a predetermined extent, said last
backwardly by gravity. The backwashing ñuid
named means including an elongated, hollow se
may be ordinary city water or it may take the
lector member extending in a generally vertical
form of any reasonably clear fluid that may be
direction in said tank, said selector member hav
available in a sewage system. I have found that
excellent results may be obtained by forcing the 60 ing a plurality of elongated slots therein of a
width vvarying from à” to %" and disposed in
backwashing fluid through the device at a pres
_ sure of approximately ñfty pounds per square
a longitudinally extending, closely spaced series
inch, but this pressure may, of course, be varied
without departing from my invention.
I believe that the operation and advantages of
my invention will be well understood from the
foregoing description. By this arrangement one
projecting to beyond the upper and lower limits
of the strata from which supernatant liquor is to
be drawn off, said openings providing inlets into
supernatant liquor will pass from those
in which `the solids content has been re
to said predetermined extent and through
can secure the removal of the best supernatant
the solid material in the remaining strata
liquor from a digestion tank with a minimum of
diiiiculty and a minimum of operating control. 70 will not pass but will build up on the exterior of
said selector member at said openings to form
If the liquor is returned to the inlet of the plant
with the adjacent imperforate portions oi said
for retreatment with raw sewage it will not cause
selector member a substantially enclosed dis
an upsetting of the plant operation as a whole,
thereby insuring the most eiîective use of the
charge conduit leading from the portion or por
actual plant treatment units. By eliminating 75 tions of said selector member into which super
natant liquor is drawn and an imperforate draw
off conduit connected to said selector member at
a predetermined height therein and extending
therefrom and out of said tank.
3. In a method of treating sewage wherein raw
sewage is passed to a primary settling zone and
the settled sludge from said primary settling zone
is passed to a digestion zone wherein the sludge
is digested by anaerobic decomposition during
natant liquor from said digestion zone through
oriñces of a width varying from 11g" to %”
formed in an elongated selector member extend
ing in a generally vertical direction in said diges
tion zone, said oriñces being disposed in a longi
tudinally~extending, closely-spaced series pro
jecting to beyond the upper and lower limits of
the strata from which supernatant liquor is to
be drawn off, whereby supernatant liquor is
which decomposition horizontal strata of sludge 10 drawn into said orifices at said one stratum and
and supernatant liquor are formed, the super
the solids contents of the remaining strata block
natant liquor at one stratum, at least, having a
ofi” the oriñces at those strata, withdrawing the
suspended solids content which will pass through
liquor from said selector member through an im
oriñces of from île” to %” in size, which stratum
perforate draw-off conduit connected to said se
of supernatant liquor is in condition to be with 15 lector member, and delivering such liquor to the
drawn and passed to the raw sewage for retreat
ment therewith, the steps of withdrawing super
raw sewage passing to said settling zone.
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