Note: Descriptions are shown in the official language in which they were submitted.
1085976
INTRODUCTION
This invention is concerned with the treatment
of aqueous sludges, particularly but not exclusively
sewage sludges.
PRIOR ART
In conventional sewage treatment, raw sewage is
subjected to sedimentation and the sedimented sludge
(called a primary sludge) is separated from the liquor.
The liquor is then subjected to a biological oxidation
treatment such as, for example, in a percolating filter
plant, a high rate percolating filter plant, an activated
sludge plant or by contact stabilisation. The treated
liquor is subjected to sedimentation and the sludge formed
is called a secondary sludge. Usually, a mixture is then
formed of primary and secondary sludge, and the mixture is
either subjected to anaerobic digestion or chemical treat-
ment or is dumped (for example at sea). Final treated
sludges are sometimes lef* in open drying beds or spread
on agricultural land.
Attention has been focused recently on the
possibility of modifying this conventional process by sub-
~ jec*ing the mixture of primary and secondary sludges to
- aerobic digestion. Among the advantages of such a pro-
cedure is the possibility of the sludge mixture becoming
2~ heated (by the exothermic fermentation therein) to fairly
high temperatures (up to 60C) under the fermentation con-
ditions. Heating of sludge to such temperatures kills many
of the normal pathogens, such as salmonella, present therein
and usually the heated sludge can be safely disposed of
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on agricultural land without substantial risk of
spreading infection. Another advantage of aerobic
digestion is that it generally takes a shorter time than
anaerobic digestion.
In the proposals so far made for effecting
aerobic digestion of sludges, air or pure oxygen is
pumped into the sludge and the sludge is vigorously
agitated to disperse the oxygen therein. The sludge is
maintained in a closed vessel, and the oxygen atmosphere
in the vessel above the sludge may be re-circulated
through the sludge. In most of the prior proposals, the
sludge is agitated by means of a paddle stirrer, and the
oxygen introduced into the sludge through a submerged
disperser, of which several types have been used.
Prior art proposals are given in Water Research~
Vol.9, pages 17 to 24 (Pergammon Press, Great Britain,
1975); Water Research, Vol.6, pages 807 to 815 (Pergammon
Press, Great Britain, 1972); British patent nos. 1,325,321,
1,325,322, 17325,323, 1,325,324, 1,325,325 ,and 1,375,386.
Whilst it is possible using these prior proposals,
to effect aerobic digestion, none of the proposals is
entirely satisfactory. Those necessitating a closed vessel
involve the use of expensive equipment, and the use of
paddle stirrers and submerged oxygen dispersers is not a
very efficient way of obtaining thorough and intimate
sludge/oxygen contact throughout the sludge mass being
treated. It will be appreciated that full scale equipment
may well involve a vessel of 250,000 gallon capacity.
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108S976
OBJECTS
It is an object of an aspect of this invention to
provide an improved process and apparatus for effecting
aerobic oxidation of aerobically digestible sludges.
It is an object of an aspect of the invention to
provide such a process in which improved efficiency of
digestion is obtainable.
It is an object of an aspect of the invention to
provide a process for aerobically digesting sludge in which
the operating dangers associated with the mechanical impellers
and stirrers of the prior art are overcome.
It is an object of an aspect of the present invention
to provide a process wherein the oxygen is more thoroughly
dispersed throughout the digesting sludge.
INVENTION
According to one aspect of the present invention,
there is provided a process for the aerobic digestion of
sludge which comprises agitating a mass of an aerobically
digestible sludge in a vessel whilst passing an oxygen-
containing gas into the agitated sludge, wherein the sludgeis agitated by directing one or more jets of
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sludge under pressure into the sludge from one or more
nozzles disposed in the vessel below the surface of the
sludge to distribute the said gas throughout the sludge
mass.
The invention also provides apparatus for the
aerobic digestion of sludge, which comprises a vessel
for holding the sludge, means for introducing an oxygen-
containing gas into the vessel below the level of the
sludge therein in use, and pump means for pumping sludge
to at least one nozzle in the vessel below the level of
the sludge therein in use, the nozzle(s) being so dis-
posed that, in use, the sludge jet(s) therefrom vigor-
ously agitate the sludge and disperse the said gas there- ;
through.
We have found that, in the process of the
invention, by suitable control and positioning of one or
more nozzles, improved agitation of the sludge, and hence
improved dispersion of oxygen throughout the sludge mass,
can be obtained than with the prior used paddle stirrers
and like devices.
, In one arrangement, the or each nozzle is
supplied with sludge under pressure by a pump located
interiorly of the vessel and submerged in the mass of
sludge, the pump drawing sludge feed from the surrounding ~-
mass of sludge in the vessel. Alternatively, the or each
nozzle can be supplied with sludge under pressure by a
pump located exteriorly of the vessel, the pump being
fed with sludge from the mass of sludge in the vessel.
Supply conduits are arranged to carry the pressurised
i:i
sludge from the exterior pump to the nozzlez in the
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vessel below the sludge surface therein. For example,
a pump mounted externally of the vessel can have a
supply line from the pump which enters a slide or bottom
wal] of the vessel below the sludge surface. The sludge
S feed to the exterior pump will normally be from the sludge
in the vessel, but fresh sludge to be treated may be
introduced into the vessel via the feed to the pump.
By using one or more jets of sludge to agitate
the sludge in the vessel, the oxygen can be introduced in
any manner such that it becomes intimately dispersed in
the sludge in the vessel. Thus, one or more dispersers
can be provided. Preferably, these will be located close
to or adjacent the issuing sludge jets so that oxygen
may be entrained thereby and so carried throughout the
sludge in the vessel.
It is, however, preferred to pass at least some
of the oxygen-containing gas into the sluage mass in
admixture with the sludge jets. Thus, for example, the
oxygen can be supplied direct to the pump so that a
mixture of oxygen and sludge is compressed by the pump,
it being probable that at least some of the oxygen would
then dissolve under pressure in the sludge. As the
oxygenated sludge is driven fron the nozzles as a jet
into the surrounding sludge, the release in pressure can
cause any dissolved oxygen to come out of solution in the
form of a host of very small bubbles. The oxygen per-
meates through, and is vigorously mixed with, the sludge
so that the sludge in the vessel is very efficiently
oxygenated.
Whilst the oxygen-containing gas can be supplied
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to the pump itself, it can instead be supplied upstream
or downstream of the pump, in the latter case before the
sludge jet(s) impinge on the sludge mass. Thus, for
example, the gas can be mixed with the sludge in the
noæzles. For this purpose, a nozzle may be used which
includes a chamber having a sludge inlet connected to
the pump and a sludge outlet through which, in use, the
sludge issues as a jet into the vessel, and wherein the
gas-introducing means communicates with the chamber so
that, in use, the gas is mixed with the sludge in the
chamber. Alternatively, the sludge can pass over one or
more oxygen diffusers located in a conduit, before issuing
as a jet into the sludge mass.
For convenience, where (as is usual) two or
more nozzles are used, these nozzles may be mounted on a
common manifold supplied with sludge under pressure from
the pump.
The highly efficient mixing of the oxygen with
the sludge, which can be achieved in the present invention,
leads to a number of substantial advantages. Firstly, it
is possible to arrange the oxygen feed rate and other
relevant parameters, that a substantial proportion of the
oxygen supplied is absorbed by the digesting sewage. It
is then unnecessary to provide a closed vessel for
recirculating the non-absorbed oxygen, although such
recirculation can still be effected if desired. Secondly,
the excellent dispersal of oxygen in the sludge causes the
rate of digestion of the mass of sludge in the vessel to
be high, which means that
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less time is required for treating a given batch of
sludge. It is possible by operating according to this
invention to digest a batch of sludge in five to seven
days (as compared with about 28 days for conventional
anaerobic digestion). Furthermore, the process can be
autothermic, i.e. the spare heat generated by the
exothermic fermentation can be such as to maintain the
digesting sludge at a temperature of 45 to 65C. This
is advantageous because the micro-organisms involved
are thermophilic and react faster at these higher
temperatures. In addition, pathogens in the sludge are
killed by high temperature. The vessel containing the
sludge may, if desired, be thermally insulated to
reduce heat loss.
Another advantage of the very efficient mixing
of the sludge and oxygen is that, overall, less oxygen
can be required. It is preferred in the process of the
invention to use pure oxygen or an oxygen-enriched gas
(containing for example at least 90% by volume oxygen)
20 rather than air, principally because the reaction can be
more efficiently carried out in this way. Thus, for
examply, the non-oxygen components of air do not react
with the sludge but merely pass through the sludge, and
in so doing can effect a significant and undesirable
cooling effect on the sludge.
The sludges which can be treated by the present
invention include all aerobically digestible sewage and
water treatment sludges, and also other similarly diges-
tible sludges such as brewery wastes, abattoir wastes,
yeast extracts, pharmaceutical wastes and vegetable
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108S976 ~: ~
wastes. The sludges are preferably pre-macerated and
screened (to avoid clogging of the pump, nozzles and
conduits).
The aerobic digestive process of the invention
may be operated on a batchwise or continuous basis. In
the latter, portions of the digested sludge may be removed
periodically (e.g. daily) and replaced by untreated sludge.
In order that the invention may be more fully
understood, three embodiments thereof will now be des-
cribed, by way of example only, with reference to theaccompanying drawings, in which:
FIGURE 1 is a schematic vertical sectional view
of one form of apparatus of the invention;
FIGURE 2 is a perspectiv~ schematic view of a
suitable submersible pump for use in the apparatus of -
Figure l;
FIGURE 3 is a vertical sectional schematic view
of a second form of apparatus of the invention;
FIGURE 4 is a vertical sectional schematic view
of a third form of apparatus of the invention;
FIGURE 5 is a plan view of the manifold of
Figures 3 and 4;
FIGURE 6 is a section on the line AA of Figure
5; and
FIGURE 7 is a longitudinal sectional view of a
nozzle of Figure 6.
Referring to Figure l,there is shown a vessel
1 containing sludye 2, the vessel having a cover 3. On
the bottom of the vessel, submerged in the sludge~ Is a
pump 4 (more fully detailed in Figure 2) to which oxygen
is supplied via line 5.
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108S976
In operation, the sludge to be treated is placed
in the vessel. The sludge may be preheated, say to about
30C, to allow the sludge more quickly to reach a high
equilibrium temperature during the process. The cover 3
is placed over the vessel. This serves merely to avoid
undue heat loss by convection. If desired, means can be
provided (not shown) for recirculating oxygen passing
through the sludge.
The pump is energised and oxygen supplied thereto
in line 5. The pump ejects a pressurised jet of oxygenated
sludge from nozzle orifice 5. (Further description of the
pump is given below.) This jet impinges on the sludge in
the vessel 1 and causes it to circulate with turbulence,
thus mixing the oxygen throughout the sludge. The pump and
oxygen supply are continued until the digestion is complete.
The temperature of the sludge increases to about 60C or
above. In order to assist this temperature rise, thermal
insulation can be provided around the vessel. When the
process is operating on a continuous basis the average
residence time of sludge in the vessel may be as little as
4 to 7 days.
It will be appreciated from the preceding general
description, that in the Figure 1 process, the oxygen can
be supplied not to the pump but instead to one or more
diffusers in the sludge mass.
Referring now to Figure 2, one suitable form of
pump comprises a motor in housing 1, and an impeller in
housing 2. Housing 2 is mounted on legs 5 and has a sludge
inlet 4. An oxygen supply line 6 connects to housing 2. A
nozzle 3 is provided for the jet outlet.
In operation, sludge is drawn through inlet 4
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~85976
into impeller housing 2 where it is mixed under pressure
with oxygen (from line 6) and forced out as a jet from
nozzle 3. In the impeller 2, at least some of the oxygen
(or air) dissolves in the sludge. One suitable form of
pump/oxygenator for use in the present invention, is
described in ABS Pumpen British patent application
published under No. 1520536 on August 9, 1978.
The apparatus of Figure 3 is essentially similar
to that of Figure 1, but the pump and nozzle arrangement
is different. In Figure 3, there is shown a vessel 1,
with insulation 2, having a removable access cover 3 with
a vent 4. In the base of the vessel is located a pump 5
with an annular manifold 6 on which are mounted nozzles 7
(this arrangement being more fully described hereinafter).
Electrical supply cables 8 and the oxygen-supply conduit 9
enter through ports in access cover 3.
The apparatus of Figure 4 is essentially similar
(and like numerals indicate like parts to Figure 3~, but
the pump is located exteriorly of vessel 1. Thus, in
Figure 4, pump 5 is connected to manifold 6 by conduit 10
(passing through a side wall of vessel 1), and a standby
pump 5' is provided. Isolating valves 11 and non-return
valves 12 are also included as indicated. A sludge outlet
13 is provided in vessel 1, and connected to the feed port
of each pump as indicated by line 14. Oxygen is fed to
the nozzles 7 by line 15.
The arrangement of manifold 6 and nozzles 7
for Figure 3 is shown in Figures 5 and 6 (in which like
numerals indicate like parts). The manifold 6 is an
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annular tubular member located around the pump 5. The
outlet 20 of the pump 5 connects to the manifold inlet
port 21 to deliver sludge under pressure internally into
the manifold. Six nozzles 7 (in practice more or less
than six can be used depending on circumstances) are
located outwardly around the periphery of the manifold 6
by feed pipes 23 and receive sludge from the manifold.
Oxygen is also fed to each nozzle via a conduit 22.
The construction of each nozzle 7 is illustrated
in Figure 7. The nozzle is conical in shape and defines
an internal chamber 30 formed by conical wall 31 and a
base 32. At the apex of the cone-shape is an outlet
orifice 33 of diameter "D". The base 32 comprises an
annular duct 40 around feed pipe 23, formed by walls 34
and 35 and side walls 36 and 37. Oxygen supply duct 22
opens into annular duct 40. The front wall 34 has a
series of orifices 50 therein to allow oxygen to pass from
duct 40 into chamber 30. (Alternatively, the wall could
be a sintered ceramic or metal plate or diffuser, for
example.) Rear wall 35 is bolted to flange 38 on feed
pipe 23. Side walls 37 lie snugly adjacent wall 31 of
chamber 30. Feed pipe 23 terminates in chamber 30 and the
diameter of the pipe is the same as, or closely similar to,
the diameter "D" of outlet orifice 33.
The arrangement illustrated in Figures 3 and
5 to 7 operates as follows. The pump 5 pumps sludge
under pressure into manifold 6. From manifold 6, the
sludge passes via feed pipes 23 into the nozzle chambers
30. Oxygen is supplied via conduit 22 into the ducts 40
under sufficient pressure for it to pass through orifices
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50 into chamber 30, and there mix with the sludge. On
entering chamber 30, the sludge experiences a reduction
in pressure and after mixing with the oxygen is expelled
through outlet orifice 33 as a jet to impinge on the
surrounding sludge and cause turbulence and mixing of
the oxygen therewith. Sludge in chamber 30 is prevented
from entering gas orifices 50 by the pressure of the
oxygen gas issuing therefrom.
It will be understood that the number of nozzles
used, their orientation in the vessel, their size and the
other operating conditions are variable and will be chosen
according to the particular circumstances prevailing in
any particular case. By way of example, in a 150,000
vessel, we have found that orifice 33 diameter D may
suitably be about 4 cm., with 16 orifice 50 each about
6 mm. diameter.
The arrangement of Figures 4 to 7 operates in
essentially the same manner as that of Figure 3, but the
pump 5 is external of vessel 1 and manifold 7 is supported
by some suitable means in vessel 1. The pump 5 (or 5')
draws sludge from vessel 1 via line 14 (instead of, in
Figure 3, from the immediately surrounding sludge mass).
By way of example only, we have successfully
tested the apparatus of Figures 3 and 5 to 7, using a
150,000 gallon vessel~(but not thermally insulated and
without a complete cover) under the following conditions:
pump having 25 horsepower motor; manifold overall diameter
of 1 metre; six nozzles symmetrically arranged as shown in
Figure 5; sludge velocity in the jets of 15 metres/
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108~;9~6
second; oxygen input (commercial oxygen gas) 2 cubic
metres per minute (expressed at NTP); a mixed sludge
comprising primary and secondary sludge, activated sludge
and septic tank sludge; a nozzle outlet orifice of 4 cm.
diameter; 6 mm. diameter oxygen orifices into nozzle
chamber; 16 oxygen orifices in each nozzle; sludge
solids content (initially) of 4 to 5% solids (treated
sludge output about 2% solids). Excellent aerobic
digestion was obtained and even without any attempt
(under the test conditions) to insulate the vessel ther-
mally and provide a closed cover, temperatures of over
40~C occurred.
It will be appreciated that, in many of the
prior proposals for aerobic digestion of sludge, a
specially constructed vessel is required, particularly
where gas recirculation is essential. In the present
invention, however, a special vessel is unnecessary.
Furthermore, in certain prior proposals, particularly
those involving recirculation, the gases in the vessel
above the sludge surface, contact moving parts of the
equipment, such as stirrer rods. This is a potentially
dangerous situation which is avoidable according to the
present invention.
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