Apparatus for stabilizing sludge

An apparatus and method for stabilizing sludge such as the sludge produced by municipal waste water treatment plants includes thickening of the sludge to a dry solid content in the range of 3% to 8% before feeding it to at least one comparatively small reactor tank that extends horizontally and has an inlet and outlet adjacent opposite end walls. The system preferably utilizes multiple tanks stacked one above the other and connected in series. The tanks have an in-built mixer that sweeps through the interior of the tank. The mixer is eccentrically mounted so that its mixing members carry the sludge through the uppermost portion of the tank interior and are spaced from the bottom surface of the tank. A sparger is located either in the inlet or in the bottom clearance of the tank to introduce microscopic bubbles of oxygen and ozone into the sludge. A pressure regulating valve controls the flow of the stabilize sludge from the uppermost reactor tank. The pressure regulating valve and the metering pump together maintain a hyperbaric pressure within the tanks.

BACKGROUND OF THE INVENTION 
This invention relates in general to apparatus and methods for the 
stabilization of sludge such as sludge formed during the treatment of 
municipal waste water. More specifically, it relates to an apparatus and 
method where the sludge is intensively mixed with microscopic bubbles of 
oxygen and ozone in one or more reaction tanks each having an in-built 
mixer. 
The disposal of sludge produced by waste water treatment plants is an 
increasingly difficult problem. In recent years the problem is 
intensifying on the supply side as the population grows and there is an 
increased emphasis on the treatment of waste water to meet pollution 
control standards. On the disposal side, landfill sites are becoming less 
available, or available only if the sludge is transported over longer 
distances at an increased cost. Heretofore, a principal disposal technique 
was ocean dumping. However, ocean dumping has been found to have an 
adverse environmental impact and is scheduled to be totally phased out, at 
least in the United States, in the near future. 
Other known disposal techniques for sludge include burning it, particularly 
at large centralized incinerators, and using it as a fertilizer or a mulch 
for agricultural applications. The burning of sludge at present produces a 
variety of air pollution problems, particularly since sludge typically 
contains high concentrations of heavy metals. The heavy metal content and 
the presence of harmful bacteria and other organic matter seriously limits 
the use of sludge for agricultural purposes. In addition, conventional 
treatment methods result in the sludge product, even one where heavy metal 
content is controlled and the sludge is otherwise stabilized, that has a 
high water content and therefore is heavy and expensive to transport. 
To disinfect sludge, it has been known for some time that it may be treated 
with ozone and/or oxygen. For example, D. Thiirumirthi discloses the 
application of ozone for waste water treatment in Water and Sewerage Works 
(1968) at page L-R106. Further, as early as 1971 H. M. Rosen made a 
presentation at the University of Wisconsin relating to the application of 
ozone to the treatment of sludge. It is also known to adjust the pH of the 
sludge to remove heavy metals. 
U.S. Pat. Nos. 3,525,685; 3,772,188; and 4,581,137 to R. N. Edwards 
describe various ways to use oxygen and ozone to treat sewer lines, 
municipal sewerage, and liquid sludge. U.S. Pat. Nos. 4,464,257 and 
4,500,428 to J. M. Lynch disclose a similar system for treating sludge in 
several hyperbaric reaction vessels with the use of intermediate sludge 
thickening devices. 
U.S. Pat. Nos. 3,772,118 and 4,581,137 to Edwards describe apparatus for 
treating municipal sewerage with oxygen and ozone gas in a large 
spherical, stainless steel reactor vessel. The sludge fills the vessel 
approximately halfway. An oxygen diffuser, an improved version of which is 
subject of the '137 patent, diffuses the oxygen and ozone gases into the 
liquid sewerage at the bottom of the reactor vessel. To interact the 
sewerage and the gas, the liquid sewerage is directed by a pump and pipes 
to the upper end of the vessel where it is discharged against a rotating 
commutator that breaks up the sewerage into droplets which are deflected 
downwardly through the upper half of the vessel toward a pool of sewerage 
held in its bottom half. The sewerage interacts with the oxygen and ozone 
gases that fill the upper half of the vessel. The droplet to gas contact 
surface area and reaction time are not conducive to a rapid and complete 
stabilization of the sludge, e.g. in one spraying. The gas also interacts 
with the sewerage through direct diffusion into the sewerage pool because 
the diffuser is submerged within the sewerage at the bottom of the vessel, 
but this additional gas-sludge interaction does not, in practice, 
sufficiently enhance the performance characteristics of the Edwards type 
apparatus to a level that is comparable with that achievable with the 
present invention. 
In U.S. Pat. Nos. 4,464,257 and 4,500,428 to Lynch et al., the sewerage 
interacts with oxygen and ozone gas in much the same manner as in Edwards. 
Lynch et al. use a large closed vessel with a sludge disperser mounted 
within the vessel that sprays watery sludge into an open part of the 
vessel filled with ozone and oxygen or air. A pump and recycling piping 
directs the watery sewerage from the bottom of the vessel to the top where 
it is then directed to the spray disperser. Lynch also teaches adding 
chlorine to enhance oxidation, but at a substantial cost disadvantage. The 
pH is adjusted by adding sulphuric acid to the sludge held in the tank. 
Lynch also teaches an oxygen diffuser located near the bottom of the 
reaction vessel at its interior. It is significant to note that the 
reaction time to achieve stabilization for both Edwards and Lynch is 
comparatively long. In the '137 patent, Edwards mentions a 90 minute cycle 
of reaction. Lynch et al. mention residence times of 15-60 minutes in each 
of two reaction vessels which operate on the sewerage, one after the 
other. 
While both the Edwards and Lynch systems are capable of producing a 
stabilized sewerage end product, the systems are costly, comparatively 
slow, operate in a batch mode, should have an attendant to monitor their 
operation, and require that the sludge or waste sewerage being processed 
be in a liquid state capable of being sprayed within the reaction vessel. 
For municipal waste water treatment sludge, being in a liquid phase 
capable of being sprayed means having a dry solid content of about 1% to 
3%. 
Therefore a principal object to the present convention is to provide a 
highly compact sludge stabilization apparatus and method which has a 
comparatively low capital cost and can be operated fully automatically on 
a continuous basis to produce a treated sludge product that is not 
biologically dangerous and has removed from it potentially hazardous heavy 
metals. 
Another principal object of the present invention is to provide a sludge 
stabilization apparatus and method with the foregoing advantages which can 
treat sludge in a thickened phase with a dry solid content in excess of 
3%. 
Yet another principal advantage of the present invention is to provide a 
sludge stabilization apparatus and method with all the foregoing 
advantages which has a reaction period which is several times faster than 
known prior art apparatus and techniques. 
A further object of the invention is to provide a system for stabilizing 
sludge which results in a comparatively inert, heavy metal-free sludge 
which has a dry solid content of approximately 20% to 40%, with attendant 
lower weight and lower transportation costs. 
A still further object of the present invention is to provide a sludge 
stabilization apparatus and method which can turn normal municipal waste 
sludge into products which can be used as a fuel or for agricultural 
purposes such as fertilizer or mulch. 
Still other objects of the present invention are to provide sludge 
stabilization apparatus and method which utilize reactor tanks that can be 
approximately 1/4 of the size of reactor tanks in prior art systems 
capable of handling equivalent volumes of sludge, which utilize 
conventional piping and which avoid the cost, safety testing, and safety 
hazards of known prior art sludge stabilization systems which pump the 
sewerage at high pressures for spraying. 
SUMMARY OF THE INVENTION 
The sludge stabilization and apparatus and method of the present invention 
operate on thickened sludge having a dry solid content in the range of 
approximately 3% to 8%. This comparatively thick sludge is treated in a 
reactor tank having an in-built mixer, preferably one which is driven to 
rotate by a dedicated gearmotor. Microscopic bubbles of oxygen and ozone 
gas or oxygen-bearing gas and ozone are diffused into the sludge, at an 
inlet to the tank, using a sparger located at the bottom portion of the 
tank, or using a combination of both of these approaches. The mixer is 
mounted eccentrically so that it clears a bottom-mounted sparger while 
also carrying the sludge to the uppermost portions of the interior of the 
reactor tank where the diffused gases would otherwise tend to accumulate. 
The mixer preferably has a shaft that extends parallel to the longitudinal 
axis of a horizontally oriented reactor tank having a generally circular 
cross-section. The mixer also preferably has four equiangularly spaced, 
radially extending plates that are mounted on the shaft. As the shaft is 
rotated, the plates sweep through substantially the entire interior volume 
of the reactor tank, with the exception of a small clearance space at the 
bottom of the tank occupied by the sparger. The mixer plates include 
openings to allow the passage of the sludge and diffused gas contained in 
the sludge to pass therethrough. Rotation of the mixer, typically at 100 
to 200 rpm, produces an intensive mixing of the sludge and the microscopic 
gas bubbles which increases the contact between the bubbles and the sludge 
and reduces the reaction time necessary for stabilization. 
The tank has an inlet which feeds the tank at a lower wall portion near one 
end of the tank and an outlet located at the opposite end of the tank. In 
one form the tanks are connected in parallel to provide an enhanced 
processing capability, but with a shorter reaction time for the stabilizer 
to act on any given portion of the sludge being treated. In another 
perferred form, a plurality of tanks are connected in series and stacked 
vertically, one above the other, with the outlet from one tank feeding 
directly to an inlet of the immediately adjacent overlying tank. If such a 
vertical stack is used, the outlet on the uppermost tank is also located 
at the bottom portion of the tank and a vent is provided at the upper 
region of the upper tank to release accumulated gases. The flow of sludge 
through the reactor tank or tanks of the present invention is controlled 
to maintain a hyperbaric pressure, typically 3 to 8 bars, and preferably 
by providing a metering pump to feed the inlet of the first tank and a 
regulating outlet valve in the final outlet line. 
Viewed broadly as a process, the present invention involves prethickening 
municipal waste water sludge to a solid content in the range of 
approximately 3% to 8% and acidifying the sludge to a pH value which 
results in the removal of heavy metals from the sludge. This acidified and 
prethickening sludge is then fed at a controlled rate to at least one 
reactor vessel where microscopic bubbles of oxygen and ozone gas are 
injected into the sludge and an inbuilt mixer that is eccentrically 
mounted intensively mixes the sludge and gas bubbles. The process also 
includes withdrawing the stabilized sludge from the tank at a controlled 
rate to maintain a hyperbaric pressure within the tank of 3-8 bars. 
Viewed as a system, whether in apparatus or method form, the present 
invention include a first mixer or mixing step which utilizes a polymer 
admixed to the liquid sludge received from a settling tank or flotation 
type clarifier to produce a mixture which is fed to a reducer that removes 
sufficient water from the polymer-treated and mixed sludge to produce a 
prethickened sludge with a dry solid content in the range of approximately 
3% to 8%. The prethickened sludge is then directed to a second mixer or 
mixing step where acid is added to the prethickened sludge to adjust its 
pH level to one which will remove heavy metals from the sludge. A metering 
pump or other flow regulating mean then directs the acidified and 
prethickened sludge to one or more reaction tanks comprising a sludge 
stabilization system constructed and operated as described above. Then 
stabilized sludge is withdrawn from the stabilizer to a second thickening 
apparatus or process step, such as a conventional twin belt press, which 
reduces the water content of the sludge until it has a dry solid content 
in the range of 20% to 40%. This dried and stabilized sludge can then be 
directed from the thickener directly to a truck or other transportation 
container for disposal or further processing. The end product is also 
suitable for use as a fuel, either to be burned directly as briquets or 
for gasification to operate a diesel engine which provide electrical power 
on site or at an off-site location. 
These and other features and objects of the invention will be fully 
understood from the following detailed description of the preferred 
embodiments which should be read in light of the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 1 and 2 show a set of three reaction tanks 10, 10 and 10' arranged 
vertically, one above the other, and connected in series as a 
stabilization reactor 72 to process a flow of waste water sludge which has 
been prethickened to contain approximately 3% to 8% dry solid content. The 
sludge is fed to the lowest tank 10 through an inlet conduit 12 and exits 
the three series-coupled tanks via an outlet conduit 14 associated with 
the uppermost reaction tank 10'. As shown, each reaction tank 10, 10, 10' 
preferably has a cylindrical side wall which produces a generally circular 
cross-section. The longitudinal axis of each tank 10, 10, 10' extends 
generally horizontally from a first end wall 10a to a second end wall 10b. 
Because of this preferred, tube like configuration, the reaction tanks 
will sometimes be referred to hereinbelow as reactor tubes. Each tube 10, 
10, 10' is preferably formed of stainless steel to resist corrosion by the 
acids and other chemicals used to stabilize the sludge. By way of 
illustration, but not of limitation, the tubes extend horizontally 
approximately five feet with a diameter of approximately 20 inches. 
As shown, each reactor tube 10, 10' has an inlet 12 and an outlet 14 formed 
by a flanged pipe welded or otherwise mounted on the side wall and open to 
the interior of the associated reactor tube. The inlets and outlets of 
each tube are located at opposite ends of the tube. As shown, the lowest 
reactor tube 10 has its inlet 12 located at its lower right hand side 
adjacent the side wall 10a, and its outlet 14 is open to the uppermost 
portion of the tube 10 adjacent its left hand side wall 10b. To achieve 
the aforementioned series coupling of the three tubes, the inlet 12 of the 
middle reactor tube 10 is coupled directly to the outlet 14 of the lowest 
tube 10, and the outlet 14 of the middle tube is at its upper right. This 
outlet is in turn coupled directly to the inlet 12 of the upper reactor 
tube 10'. However, the outlet of this upper tube is located adjacent the 
opposite end wall from the inlet, but at the bottom side of the reactor 
tube, not the top. A vent 16 is located at the upper surface of the upper 
tube 10' to release accumulated gases from the stabilization reactor 72. 
A liquid oxygen supply 18 and an ozone generator 20 (FIG. 3) provide flows 
of oxygen gas and ozone gas to each of the three reactor tubes via 
spargers 22 located in each tank and directly over the sludge inlet 12 for 
that tube. With the series-connected arrangement shown in FIGS. 1-3, 
preferably oxygen and ozone are supplied to the lowest tube 10, but only 
oxygen is supplied to the middle tube 10 and the upper tube 10'. 
Alternatively, or in addition to the spargers 22 located within each 
reaction tank, a large porous metallic tube sparger 24 can be located in a 
vertical inlet conduit extension pipe 26 coupled to the inlet 12 of the 
lower reactor tube 10. The spargers provide a supply of microscopic 
bubbles of oxygen and ozone which diffuse into the sludge as it flows 
through each inlet 12 into the tubes 10 and 10'. A metering pump 28 (FIG. 
3) supplies a regulated infeed of prethickened sludge to the lowermost 
inlet 12 via pipe 30 which feeds the vertical inlet extension pipe 26, 
which in turn feeds the first flanged inlet 12 in the series. A pressure 
controlling valve 32 regulates the outflow of stabilized sludge from the 
reactor tube 10' so as to maintain a preselected hyperbaric pressure 
within the reactor tube, preferably in the range of 3-8 bars. A pressure 
transducer 33 that senses the sludge pressure within the tube 10' produces 
a control signal for the valve 32. 
A principal feature of the present invention is an in-built mixer 34 
located within each of the reactor tubes. The mixer in its preferred form 
includes a central rotatable shaft 38 and four equiangularly spaced plates 
40 mounted on the shaft 38 that are each directed radially outwardly from 
the shaft and extend from the shaft to an outer edge 40a. The radial 
dimension of each plate is such that it is closely spaced from the upper 
interior surface 10c of the reactor tube when the plate is in a vertical 
and upwardly extending position. Each plate is preferably formed of a 
plastic material having sufficient structural strength to mix the 
prethickened sludge and formed of a material which is substantially 
impervious to the chemical environment found within the reactor tubes. 
One-quarter inch thick plates of plastic or stainless steel are suitable. 
As shown in FIG. 1, each plate 40 extends to substantially fill the 
interior volume of the associated reactor tube when viewed in vertical 
section taken through the plate when the plate is in its vertical upright 
position. This ensures that there are substantially no dead spaces within 
the reactor tube where the sludge can avoid being mixed by the plates. 
Each plate contains a series of openings 42 which allow the sludge held in 
the container in the reactor tube 10 to pass through the plate with a 
squeezing action which produces a intensive mixing of the sludge and the 
microscopic oxygen and ozone bubbles diffused within the sludge. 
In the preferred form shown, the mixer is mounted within each reactor tube 
for rotational motion about the shaft 38 supported in bearings 44 carried 
in the end walls of the associated reactor tube. Seals 46 also located at 
each end wall block any leakage of the sludge to the bearings or the 
exterior of the reactor tubes. As shown, each mixer is preferably driven 
by a separate, dedicated gearmotor 48 which drives the mixer in a 
continuous rotating motion with a suitable power reduction set by the gear 
ratio between the motor and the mating gear 50 supported on a portion of 
the shaft 38 projecting outwardly from the side wall 10a of each reactor 
tube. The gearmotors 48 preferably operate at 100 to 1,000 rpm and the 
mixer preferably rotates at 100 to 200 rpm. 
As is also best seen in FIGS. 1 and 2, it is significant that the mixer is 
mounted eccentrically within the reactor tube, that is, with the center of 
rotation laterally centered but displaced vertically upward from the 
center line of the reactor tube, as is best seen in FIG. 2. Because the 
interior of the reactor tube is generally circular, and because the mixer 
is generally symmetric with each plate 42 having equal radial dimension 
from the axis of rotation of the mixer, there is a spacing 52 at the 
bottom portion of each reactor tube between the outer edge 40a of the 
plates 40 and the adjacent lower interior surface of the circular side 
wall of the reactor tube when the plates are in a vertical and downwardly 
oriented position. The spacing provides a clearance which allows the 
positioning of the sparger 22 inside the tube without interfering with the 
rotation of the mixer. More importantly, this ensures that the plates 42 
carry the sludge, which substantially completely fills each of the reactor 
tubes, through the uppermost portion 10d of the interior of each reactor 
tube where the oxygen and ozone gases would otherwise tend to accumulate 
and not interact with the sludge. 
FIG. 4 shows three reactor tubes 10 of the same construction as described 
above with respect to FIGS. 1 and 2, but connected in parallel, not in 
series. Like parts have the same reference numbers. This stabilization 
reactor can process a larger volume of sludge in a given amount of time 
than the series arrangement shown in FIGS. 1-3, but the reaction time is 
shorter and therefore it is suitable only where a shorter reaction time 
will nevertheless be sufficient to treat the sludge. It should also be 
noted that the parallel-connected tubes 10 do not need to be in a vertical 
array, as is the case with the series connected arrangement. 
The foregoing apparatus provides sludge stabilization with several quite 
distinct differences from the prior art arrangements described in the 
aforementioned Edwards and Lynch et al. U.S. patents. First, with respect 
to the size of the reaction vessels, the present invention allows the 
reaction vessel to be approximately 1/4 the size of the reaction vessel of 
Edwards or Lynch et al. when used to process what is initially a 
comparable volume of sludge. One reason is that in Edwards and Lynch half 
of the vessel is open to provide a space where the sprayed sludge can 
interact with the oxygen and ozone gases (an accumulated pool of liquid 
sludge occupies the bottom half of the vessel). In contrast, in the 
present invention the sludge fills substantially all of the reaction 
chamber thereby providing, in general, a reduction of 50% in the volume 
reactor vessel. Second, because the present invention is capable of 
operating on a thickened sludge and does not require spraying the sludge 
in a liquid state, there is at least another 50% reduction due to the 
increase of the dry solid content of the sludge, from a typical value of 
1.5% for liquid phase treatment as in Edwards and Lynch et al., to the 
prethickened phase treatment of the present invention where the sludge has 
a solid content of at least 3%. Further, in Edwards and Lynch et al. the 
contact surface between the comparatively large sludge droplet and the 
surrounding oxygen or ozone held in the upper half of the container 
vessels is comparatively small. Consequently the liquid sludge in the 
prior art system must be recycled and resprayed multiple times to provide 
a sufficient reaction time to achieve the necessary stabilization. In 
contrast, applicant uses a intensive, mechanical intermixing to work 
millions of microscopic bubbles of the oxygen and ozone gas through the 
prethickened sludge. This provides a much better contact surface for a 
interaction between the gas and the sludge and a corresponding reduction 
in the reaction time necessary to achieve stabilization. For most 
applications, with the present invention it is possible to achieve a 
stabilization of the sludge with a reaction time of only 15-45 minutes as 
compared to 90 minutes for one of the prior art patents and successive 
reaction periods of 15-60 minutes for each of two stages of reaction of 
the vessels in the Lynch et al. system. 
With reference to FIG. 3, there is depicted in schematic form a complete 
sludge stabilizing and thickening system which takes as an input the 
sludge produced by a conventional municipal waste water treatment 
facility, such as the sludge from a settling tank or a flotation type 
clarifier such as the one manufactured by the Krofta Engineering 
Corporation under the trade designation Supracell, and produces a 
substantially dry, biologically inactive, and metal-free sludge with a dry 
solid content in the range of 20% to 40%. Sludge with this low water 
content is suitable for transportation by a truck 54, as shown, by 
railroad cars, by any conventional refuse container, or for use on site as 
a fuel, whether directly or to be gasified to fuel a diesel engine or the 
like to produce electricity. 
Sludge from the flotation clarifier or primary and secondary settling tanks 
(which has been mixed together in an equalization tank) enters the sludge 
stabilization system through a pipe 56 and is pumped through a regulating 
valve 58 to a mixing tank 60 where a conventional polymer is added as 
indicated at 62. The overflow from the mixer 60, a watery sludge with 
mixed-in polymer, is directed through an overflow 64 to a primary 
thickening apparatus 66 of conventional design. The thickener 66 can, for 
example, be of the screen thickener type or a flotation thickener. In any 
event, the primary thickener 66 reduces the water content of the sludge to 
a desired prethickened value of a dry solid content in the range of 
approximately 3% to 8%. The water removed at the primary thickener 66 is 
preferably partially recycled for use in cleaning showers. 
The prethickened sludge, as indicated by an arrow 68, is then directed into 
a second mixing tank 70 where acid is added, as indicated at 72, to lower 
the pH level of the sludge to remove heavy metals from it. The metering 
pump 28, preferably a positive displacement type pump, pumps the acidified 
and prethickened sludge from the tank 70 into the stabilization reactor 72 
formed by three series-connected reactor tubes 10, 10, and 10' operating 
as described above with respect to FIGS. 1 and 2. The metering pump 28 
presets the flow rate of thickened sludge into the stabilization reactor 
72. A level sensor 74 associated with the second mixing tank 70 produces 
an output signal which controls the inlet flow regulating valve 58. As a 
result, the thickening and pH control processes provided by first and 
second mixing tanks 60 and 70 and the primary thickener 66 can operate 
independently of the need to feed the stabilization reactor 72. 
The stabilization reactor operates in a manner described above. The vent 16 
can release excess gas directly to the atmosphere, or it can be directed 
to a compressor (not shown) and recycled back into the lower tank 10 of 
the stabilization reactor 72. Treated sludge discharged at the outlet 14 
from the upper reactor tube 10' exits through a conduit 76 connected to 
the outlet 14 of the reactor tube 10'. The flow regulating valve 32 is 
installed in the conduit 76. The treated sludge, indicated by the arrows 
68', is then directed to a second thickening device 78 such as a twin belt 
press which reduces the liquid content of the sludge to the desired final 
dry content value of approximately 20% to 40%. The water removed from the 
sludge by the belt press 78, as indicated by the arrow 80, is returned to 
the inlet of the sewerage plant. The final dried and treated sludge 68" 
has a water content roughly equivalent to that of a dry apple. It has a 
BTU value comparable to that of wood, and therefore is a good fuel. 
While the present invention has been described with respect to its 
preferred embodiments, it will be understood that various modifications 
and alterations will occur to those skilled in the art from the foregoing 
detailed descriptions and the accompanying drawings. For example, while 
the stabilization reactor has been described as a unit using three reactor 
tubes of a particular size and configuration, the number of tubes can be 
one, two, three, or more and that the size and configuration of the 
reactor tanks 10, 10' may be changed without departing from the scope of 
the present invention. For example, the cross-sectional configuration of 
the tank can be other than circular provided that sufficient diffusion and 
intensive mixing of the sludge and dissolved microscopic gas bubbles 
occurs to stabilize the sludge. Similarly, while a preferred form of 
rotating mixer has been described, it will be understood that an intensive 
mechanical mixing of the sludge and diffused gas bubbles can be achieved 
using other, although perhaps more costly or less efficient, mixing 
arrangements. For example, the mixer can be formed with sets of spaced 
apart vanes, rather than solid plates with simple circular openings, to 
produce the desired mixing. The mixer could also operate with a screw-type 
impeller, or even operate with a linear or rotating reciprocating motion, 
as opposed to the continuous rotational motion produced by the gearmotors 
48. Also, while the invention has been described in its preferred form 
with a paddle-like rotating mixer which sweeps the sludge through the 
uppermost portions of the tanks interior with a clearance at the bottom, 
it is not absolutely essential that the mixer be in an extremely close, 
wiping relationship with the upper interior surface of the tank, nor is it 
necessarily essential that the mixer have a clearance between its 
operating members and the lower interior surface of the tank, particularly 
if oxygen and ozone are diffused into the sludge via a sparger located in 
the inlet conduit 12, rather than directly inside the tanks 10, 10'. It 
will be understood, also, that while the invention has been described with 
respect to a metering pump and pressure regulating valve controlling the 
flow of sludge and its pressure in the stabilization reactor, there are 
other known arrangements for controlling flow and pressure, although 
perhaps not as efficient and reliable as the arrangement described 
hereinabove. These and other variations and modifications are intended to 
fall within the scope of the appended claims.