Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
METHOD AND APPARATUS FOR TREATING ORGANIC WASTEWATER
Principals Upon Thigh the Invention is Based
Parameters for the invention derive from experience
gained in operating a 43 year old activated sludge plant
that handles peak wastewater flows of up to 1440 MGD (million
gallons per day). In 1971, just prior to commencement of a
vigorous program to optimize quality, the treated effluent
for the plant contained an average BOD5 of 23 my/liter,
30 mg/liter suspended solids and 11.8 mg/liter ammonia.
Since then, treatment quality has been steadily improved.
Last year (1981), the treated effluent contained a residual
impurity of only 6 mg/liter BOD5 6 mg/liter suspended
solids and 1.4 mg/liter ammonia.
Lessons derived thereby are as follows:
(1) Maintain a minimum dissolved oxygen (D.O.)
content of 4 mg/liter throughout the treat-
ment system. IIn our invention, pressuriza-
tion enhances oxygen transfer and helps
maintain high levels of D.O.)
(2) Avoid degredation of activated sludge by
minimizing its retention time in the clarifier.
(In our invention, the sludge floc is removed
from the separator/clarifier vessel within
eight minutes.)
(3) Food to micro-organism ration (F:M ratio)
should be kept under 0.2, wherein "food" is
computed as pounds per day of wastewater
BOD5, and "micro-organisms" are computed as
total dry pounds of mixed liquor suspended
solids (MLSS). To stay within that ratio,
TV
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mixed liquor in the 43 year old plant is
maintained at a concentration OL 2500 mg/liter
MLSS during the summer, and upwards of 3500
mg/liter ML~S in the winter when bacterial
action is slow. (Our invention is designed
to safely handle 10,000 mg/liter ~S5)
(4) Provide ample mixing action to enhance
assimilation of the waste by the floc, re-
stricting sedimentation of solids to the
clarifier units. (In our invention, the
location of air diffuser heads, the shape of
the process vessels, the rate of sludge return,
and piping arrangements combine to provide
excellent mixing.)
(5) Prevent excessive turbulence so as to avoid
tearing the fragile bacteria floc. (In our inven-
tion there is a gentle gravity flow between
the three process vessels. The only place
in the system with substantial turbulence
is in the sludge return pump.)
In summation, we learned from the old treatment
plant that: "high quality biological treatment requires a
favorable environment wherein useful organisms proliferate
and undesirable types regress."
Existing Limitations in Treatment Facilities
In theory, the size requirement of an activated
sludge aeration tank is inversely propor~-ional to the con-
centration of a mixed liquorO In other words, if an aeration
tank is designed to operate at a concentration of 5000
a?~3
mg/liter ~L5S instead of 2500 mg/liter, its size can be
reduced by half and still have the same capacity. Due to
certain practical limitations however, it is extremely
difficult to operate at concentrations about 4000 mg/liter.
The problems have been:
(a) Poor Oxygen Transfer E,fficiency.
The oxygen demand of liquor concentrations
higher than 4000 mg/liter MLSS usually exceed
the capacity of the aeration system. The
two exceptions to this limitation have been
pure oxygen systems (which offer good transfer
efficiency but are plagued with mechanical
problems) and pressurized aeration systems
(which tend to be difficult to regulate and
service, as per experience with U.S. Patent
#3,560,376 by R. W. Heil). Our invention
rectifies this problem with a simplified
pressure system that is self regulating and
easy to service.
(b) Vulnerability to Upset:
Conventional clarifiers are easily upset when
they are fed high concentrations of mixed
liquor. The sludge blanket may
rlse too fast and/or density currents may
sweep floc over the effluent weir.
This vulnerability to upset is due to
design compromise wherein three (not alto-
gether compatible) functions are performed
in the clarifier:
1. flocculation of the micro-organisms.
2. separation and removal of the effluent.
3. Concentration and return of the
micro-organisms.
Performances of all these functions within
one compartment necessitates use of a relatively
large surface area and substantial holding
capacity. Above all, the inventory of solids
(micro-organisms) must be managed with great
care. If not retained long enough, the return
sludge will be dilute and thus too voluminous
for the system to handle. However, if the
solids are retained too long, the dissolved
oxygen will be depleted and the micro-
organisms deteriorate.
In our invention, vulnerability to clarifier
upset is resolved by handling each of the
above functions in separate specialized
compartments.
(c) Voluminous Clarifier Sizes:
To function properly, conventional clarifiers
require substantial depth and surface area.
Much of the volume provided is used for
dissipation of density currents and for
transition space between function zones.
The small diameter gravity clarifiers are
especially plagued by poor settling performance
(not enough volume to dissipate density currents).
Large clarifiers perform better but do not
lend themselves to a pressurized treatment
system, being too difficult and too expensive
-to enclose. As a result, most pressurized
aeration systems use an open flotation tank
for separation of sludge and effluent (as
in U.S. Patent #3,444,076 by Sekikawa et al.)
Flotation separators have the advantage of
compactness, low capital cost, quick separaton
and high solids concentration. However,
upon release of a pressurized mixed liquor
tG atmospheric conditions the explosive
release of dissolved gas tends to tear-up
the organic floc and produce a muddy effluent.
Our invention solveR these problems with
a two stage pressurized separa~or/clarifier
unit.
(e) I deq~ate Monitoring of MLSS
To maintain a proper concentration of
activated sludge in the aeration tanks, a
treatment plant operator needs to know his
mixed liquor suspended solids (MLSS). With
this information, he can determine the rate
at which the sludge should be wasted to
maintain a proper concentration. If his
waste rate is too low, the solids concentration
will become excessive and overflow the clarifier.
If too many solids are wasted, treatment quality
declines because of insufficient bacteria
to assimilate the waste.
The operator can draw a sample and
determine the MLSS by laboratory analysis,
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but the procedure is slow and requires
considerable skill and equipment. Alterna-
tively, there are various commercial density
meters that can provide an approximation of
the MLSS on a continuous basis. Most of
them employ some means of penetrating the
MLSS with an energy source (sonic waves, light
beams, gamma rays, etc.) and measuring the
attenuation. Unfortunately, such meters
require frequent calibration, their cost is
high and their reliability is low.
In our invention we utilize the rheological
properties of the sludge to obtain an instant,
close approximation of loss. Unlike exist-
ing density meters, the rheological method
is low in first cost and upkeep.
Rheologically, activated sludge is a
"thixotropic pseudo-plastic". Simply put,
its flow behavior is more like tooth paste
than water. The term "thixotropic" indicates
it stiffens or gels when at rest, but breaks
down and becomes fluid when subjected to
agitation or high shear stress. In our in-
vention, the sludge is kept in constant motion
(offering no opportunity for gel) therefore
thixotropy can be ignored.
As the MLSS concentration of activated
increases, it becomes increasingly resistive
to flow. If the sludge is purnped into a small
tube of fixed length and diameter and
maintained at a constant pressure dif-ferential;
3 ~3
its velocity will be an inverse function of
MLSS concentration. When velocity (ordinate)
is plotted on a graph as a function of MLSS
labscissa), the resulting curve is nearly
horizontal at low concentrations of MLSS,
descending in a downward spiral as the MLSS
increases. At some critical concentration,
flow velocity will cease altogether. If
the SVI (sludge volume index) of the sludye
is relatively constant, the graph is de-
pendably repeatable.
With such a graph, a treatment plant
operator can readily determine his MLSS
simply by reading a magnetic flow meter.
Furthermore he need concern himself with only
two velocity limits, minimum (indicating upper
permissible concentration), and maximum
(indicating the thinnest permissible concen-
tration).
Due to very high dissolved oxygen levels,
our invention provides unusually low and
fairly uniform SVI's (ranging from about 40 to
60). Thus the relationship between rheological
behavior and Russ concentration is dependably
consistent. Furthermore, the only instr~ent
or sensor required for our solids monitor is
a magnetic flow meter, and experience has shown
such meters to be one of the few instruments
used in pollution control that can give
maintenance free performance in a troublesome
fluid like sewage sludge.
8.
Design Obiectives
The object of our inven-tion is to prcvide a treatment
process for organic wastewa~er that is compact (relative to capa-
city), simple to construct and operate, is energy efficient,
requires little maintenance, and achieves tertiary quality.
To accomplish this, all major components of the apparatus
are pressurized equally (up to about 35 psig). For ease of
maintenance all moving parts are placed outside the pressure
vessels, and the interior of the vessels are clear of anything
that can clog or foul with slime growth. An air diffuser head
in each of the aeration vessels provides mixing and promotes
oxygen transfer.
The size of the clarifier unit has been minimized by
providing a unique two tier cyclone separator that utilizes
density currents to affect rapid centrifugal separation. Solids
are drawn from the bottom tas return sludge) and the clear
centrate i5 displaced by gravity into the vortex, where it flows
upwarA into the top tier for final clarification. Detention time
for the sludge is only eight minutes, thereby eliminating the risk
of oxygen depletion. Of key importance, the separator-clarifier
~essel's compact size substantially reduces the cost for pressurized
containment.
Within the entire process, there is only one component
that causes enough turbulence to damage the sludge floc, namely
the return sludge pump. However, due to its location in the
process system, the floc has ample opportunity to recover. It
therefore enters the separator unit in excellent condition,
ideal for effecting rapid separation.
~;~32~
The rate of return sludge in our invention (300%
of the design capacity flow) is unusually high; however use
of a constant high return flow provides a number of advan-
tages:
(1) Aids in maintaining a gentle cyclone action
within the separa-tor unit.
(2) Minimizes sludge detentlon time, thus reducing
oxygen depletion in the clarifier.
(3) Minimizes the amount of sludge thickening
needed to sustain high MLSS concentrations.
(4) Self regulates the thickening and removal
process, thereby eliminating need for costly
sludge blanket control.
(5) Promotes better mixing action.
BRIEF DESCRIPTION OF TOE DRAWINGS
_ .
Figs. 1 and 2 show two alternative means for con-
necting the four process compartments in series.
DETAILED DESCRIPTION
_ .
Referring now to the drawing of -the apparatus, all
standard components shown, such as the valves, pumps, blower,
air compressor assembly, liquid level controller and piping
(which are not described in the claims) are conventional and
their uses and functions are well known in the art. Refer-
ence may also be made to our copending Canadian application
Serial No. 432,733, filed July 19, 1983.
The configurations shown in both Figure #l and
Figure #2 have been sized for a sewage strength of 200 mg/l
BOD5 and a treatment rate of 12,000 gallons per day (gpd).
The mixed liquor concentration will be maintained at about
mg/l SS, for a food to micro-organism (F:M) ratio of about
0.20. Sizing and dimensions are only approximate and could
be scaled upward or downward, depending on the strength and
quantity of sewage to be treated.
lcm/ -10-
l r~3
Referring to Figure 1, organic wastewater that has been
screened and degritted (or macerated) is pumped continuously or
intermittently (as needed) into the first stage aeration compart-
ment (vessel #l) by means of a suitable single stage centrifugal
pump 4, through a suitable flap gate check valve 5 into the top of
air lift or pump 26, where it intermixes with the return sludge
and is discharged into aeration tank #1.
The mixed liquor ~wastewater and return sludge) in
compartment #l is dosed with a continuous stream of air bubbles
8 rising from aerator head 7 located in the bottom of the compart-
ment of vessel #1. The diffused air bubbles 8 become attached
to the sludge floc and bouy it upward to the lnterface of the
vessel #l air dome 9, where the floc is further saturated with
air before sinking downward along the walls of compartment of
vessel #1.
Oxygen transfer to the mixed liquor occurs through
diffusion of air bubbles from aerator head 7 into the liquor and
from a- rolling interface of the liquor with the air dome 9.
Elevated pressure within the aeration compartment (preferably
about 35 pounds per sequare inch gauge) provides an extremely
efficient driving force for transfer of oxygen into the mixed
liquor. The mixed liquor within the compartment of vessel #l
cycles continuously upward above the diffuser head 7, and downward
near the perimeter of the compartment defined by vessel #1,
eventually flowing out (hy gravity) through conduit 10 into the
aeration compartment defined by vessel #2.
As receiving unit for sewage influent, the first stage
aerator compartment (of vessel #l)sustains the highest oxygen
demand. After being partially satiated in the first stage, the
oxygen demand of the mixed liquor tapers off sharply- in the second
11 0
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stage unit (the compartment of vessel #2). Accordingly, we have
provided an adjustable throttle valve 11 (that may be of any
conventional type) on air supply line 34 to air diffuser head 12
in vessel #2. Whereas full air flow to diffuser head 7 is to and
does create a vigorous boil in the cornpartment of vessel #1,
the throttled air supply to diffuser head 12 can be and is ad-
justed to produce only a gentle rolling action that enhances
flocculation yet is sufficient to satiate oxygen demand. When
the sludge floc finally leaves the aeration compartment defined
by vessel #2, it is strong and dense, settles easily, and has
virtually assimilated all the organic waste in the wastewater.
The level of mixed liquor in both the aeration compart-
ments of vessels tl and #2 is controlled by the overflow/transfer
box 13 (located in the compartment of vessel #2), from whence
the fully aerated mixed liquors enters conduit 14 and flows into
the cyclone separator compartment 15 of vessel #3 in tangent
relation to the cylindrical wall of vessel #3 defining compartment
15, and in between same and the annual, tubular cone 20 that is
suitably mounted in vessel #3. The differential head between
liquid levels in vessels #2 and #3 and the discharge level of the
liquor at the outlet of conduit 14 imparts a velocity head to the
liquor flow in cyclone separator compartment 15. The mixed liquor
spins in a discending spiral down to and down along the wall of
the lower or bottom cone of vessel #3, as disclosed in our above
noted copending application. The floc (being heavier than water)
is spun to the outside, the the centrate (consisting of water and
pin floc) is displaced to the center (the vortex of the cyclone).
12.
The heavy sludge floc descends to the bottom of the cone where it
is continuously drawn off into suction line G leading to air lift
pump 2fi.
Centrate from the cyclone vortex operating in compart-
ment 15 flows up trough opening 18 at the bottom of the upper
tubular, annular cone 20 and flows into the settling compartment
19 of vessel #3 (that is separated from separator compartment 15
by cone 20). In the settling compartment 19, the pin floc deposits
out along the sides of the upper cone 20 and slides downward through
the opening l into the lower chamber defined by compartment 15
and is withdrawn with the return sludge into suction line 6. The
resulting effluent spills over the diagrammatically illustrated
V-notch weirs into four convention radial draw-off troughs 21,
and thence into a collector pocket 13, connected to a discharge
pipe 2~, then past valve 24, which is regulated by a conventional
liquid level controller 25, and discharged from vessel #3.
Return sludge from vessel #3 ls drawn through line 6
by an alr lift pump 26 and pumped thereby back into aeration
compartment #1. Pump 26 is designed for a 300% volume of return
sludge (300% of treatment flow capacity). In the specific example
shown on the drawing, the design capacity of the return sludge
pump is (12,~00 gpd) x (300/100) - 36,000 gpd which is 25 gpm
(gallons per minute). The static head against which it pumps is
relatively constant, namely the elevation difference between the
crest of effluent in the weirs 21 in vessel #3 and the top of
air lift 26, plus a small dynamic head loss in pipe 6 pump 26.
Supply air for the aeration bubbler heads 7 and 12 and
air domes 9 of all vessels 1, 2, and 3 is provided by a suitably
rotary compressor 31 suitably connected to manifold 34 and
capable of achieving pressures of up to about 35 psig (the
preferred operating pressure in manifold
13.
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and domes 9 is about 35 p5ig). In addition to a low volume of
fresh supply air from compressor 31 that is thereby provided the
system, there is a high volume of recycle air provided from air
manifold 35 to diffuser heads 7 and 12 via suitable blower 32
ancl the feed lines 34. Suitable slower 32 is suitably connected
between manifold 35 and feed lines 34, as indicated. Because blower
32 draws previously pressurized air from manifold 35, it neecl
develop only enough pressure to overcome the liquid head above
aerator heads 7 and 12, about 4 psig, thus providing a suitable
high volume air movement at a minimum expenditure of energy. The
higher pressure rotary compressor (35 psig) accordingly need sùpply
only enough volume of fresh air at the indicated pressure level
to satisfy oxygen up-take plus a small allowance for wasteage.
Pressure manifold 35 provides for equalization of air
pressure between all three pressure vessels #l, ~2, #3, thereby
creating a common atmosphere at the indicated pressure level within
the process system in which the liquid can move by gravity flow
without necessitating complex valve controls. Spent air in the
system is exhausted through suitable pressure relief valve 36
which also is suitably set to regulate air pressure in the system
at the indicated preferred pressure level.
The second feed line 34 is also connected through a
valve 37 that is similar to valve ll to supply air to the pump
26 to operate the latter as has been indicated.
Optimal mixed liquor concentration for the system
is maintained by means of wasting part of the return sludge
through drain line 29 (-to a holding tank for removal by a
honeywagon, or to a digester unit). Said wasting is accom-
plished by suitably opening off-on valve 30 as needed once
or twice a day to bleed off the surplus sludge.
lcm/ -14-
:~ ~3~
The approximate mixed llquor concentration in the system
is determined instantly by comparing readings of suitable magnetic
flow meter 27 with a previously calibrated chart, wherein there
is an equivalent MLSS in mg/l for each rate of flow.
Whexeas air lift 26 rises against a fixed head (crest
of effluent wier 21 to top of air lift 26), and is energized
by a constant amount of air through valve 37 and line 34, the
volume of sludge lifted (as measured at magnetic flow meter 27)
becomes an inverse function of the return sludge solids in mg/1.
(Simply put, the thicker the solids, the slower the flow).
By taking four or more samples of different return sludge
concentrations, recording their meter readings and obtaining a lab
analysis of the suspended solids concentration, a graph is produced
by plotting the meter readings (ordinate) as a function of suspended
solids (abscissa). The plot points will be interconnected with a
curved line that can be used to extrapolate suspended solids from
future meter readings.
The meter 27 measures only the return sludge suspended
solids, not the mixed liquor suspended solids MLSS. however, there
being a continuous fixed rate of return sludge (300~ of influent
design capacity), the MLSS itself will merely be 3/4's of the
return sludge SS. This calculation is slightly in error when the
system is operating at less than design capacity, but will be
adequate for control purposes.
The sludge wasting should be commenced when the magnetic
flow meter 27 reaches its minimum allowable reading (indicating
maximum allowable concentration of ~LSS). Wasting is continued
until the meter reading rises to the allowable maximum (indicating
minimum allowable concentraton of MLSS), whereupon waste valve 30
is closed completely. The waste system can be either operated
manually by the treatment operator, or automatically with a con-
ventional micro-processor, programmed to respond to light signals
from the magnetic flow meter.
"3~
laste line 29 is on a tangential extension of return
sludge line 6 in order to selectively extract grit and other heavy
particles from the sludge recirculation system. Due to the upend
curve in line 6 into pump 26, the heavy particles continue
tangentially into the waste line draw-off stub 29, displacing any
sludge floc that become trapped therein. Whenever valve 30 is
opened, the captured grit flows out with the waste sludge.
In Figure #1, the four compartments are contained
in three equally pressurized vessels, and in Figure #2 the four
compartments are contained in two equally pressurized vessels. In
Figure l all three vessels are approximately four feet in dia-
meter and ten feet in height.
In Figure #2, the first vessel-(which encloses aeration
compartments #l and #2), has a diameter which is approximately
six feet, and a height which is ten feet. A flat partition 37
divides aeration compartments l and #2 (there being no pressure
differential to resist between the two compartments). Flow from
compartment #l progresses to compartment #2 through a circular
opening 10 in the partition 37.
The second vessel in Figure #2 is identical to the
third vessel in Figure #1, containing both the separator compart-
ment and the clarifier compartment.
The foreqoin~ description and the drawings are given
merely to explain and illustrate the invention and the invention
is not to be limited thereto, except insofar as the appended
claims are so limited, since those skilled in the art who have
the disclosure before them will be able to make modifications
and variations therein without department from the scope of the
invention.
16.
