Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a process ror the treat-
ment of liquid carrying biologically-degradable material in
solution and/or suspension and in particular to a process
for the treatment of sewage, i.e. liquid carrying biologically-
degradable waste material including all types of biologically-
degradable domestic and industrial waste material, for example
normal domestic waste and the effluents produced by farms,
food factories and other industries producing such waste.
The processes generally employed in the treatment
of sewage comprise essentially an initial treatment by phy-
sical methods such as screening and degritting to remove
coarse and heavy material followed by a further treatment
using biological methods to remove organic materials. In
so far as the present invention relates to the treatment of
sewage it relates to the further treatment using biological
methods.
According to the present invention we provide a
process for the treatment of liquid carrying biologically-
degradable material in solution and/or suspension wherein
an oxygen-containing gas (as hereinafter defined) is intro-
duced into the said liquid and a culture of microorganisms
is maintained therein, the conditions being such that for
a period at least part of the said liquid is subjected to
a low DOT (as hereinafter defined) and/or at least part of
the said liquid is subjected to a high DOT, whereby the
ratio of carbon dioxide to cellular material produced by
the culture is increased during the process, the period of
time during which any part of the said liquid is subjected
to a low or high DOT being sufficiently short and there
being also a period during which that part of the liquid
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is subjected to a DOT intermediate between a low and high
DOT such that the said microorganisms are not affected in
such a way as to be substantially detrimental tc their
function in the treatment process.
The phrase oxygen-containing gas is to be under-
stood to mean molecular oxygen or any gaseous mixture,
such as air, containing molecular oxygen.
The term DOT (dissolved oxygen tension) is to
be understood to mean the partial pressure of oxygen in
the liquid. See the article by Maclennan and Pirt, J.
Gen. Microbiol., (1966), 45, 286-302, in particular page 290.
- In a high DOT region the DOT is suitably at least
450 millibars and preferably within the range 1000 to 1350
~illibars. It may however be higher for example up to 2000
millibars.
In a low DOT region the DOT is suitably less than
60 millibars, preferably less than 30 millibars and especi-
ally less than 10 millibars, e.g. zero or substantially zero.
In the process o, the invention, if there is an
increase in carbon dioxide production by the culture there
will be a corresponding increase in oxygen utilization.
The period during which any part of the liquid
is subjected to a low DOT is suitably not greater than 5
minutes, preferably not greater than 1 minute and especially
not greater than 30 seconds. The period during which any
part of the liquid is subjected to a high DOT may suitably
be not greater than 10 minutes but is preferably not greater
than 5 minutes and especially not greater than 3 minutes.
Thus the liquid is preferably subjected to a low and/or
high DOT shock or to a series of such shocks, the DOT of
,1
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the 'iq~id when noi subjected to such a shock, e.g. fol-
lowing each shock, being at a level intermediate between
a low and a high DOT. Periods of time during which any
part is subjected to a low or a high DOT should not be
long enough to affect the microorganisms in such a way
as to be substantially detrimental to their function in the
treatment process, e.g. by encouraging the development of
diferent microorganisms which are harmful in the treatment
process or by destroying microorganisms which are useful
in the process to such an extent that effective treatment
of the liquid by the process can no longer be carried out.
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The process of the invention may be effected by inject-
ing an oxygen-containing gas at intervals or by varying the rate of
injection of such gas lnto liquid carrying biologically-degradable
material in a container and thereby causing the DOT to vary with
time and producing low and/or high DOT regions in the liquid.
Preferably however the oxygen-containing gas is injected
into a stream of liquid, thereby causing the DOT within the liquid
to vary along its flow path. The liquid may flow through a series
of connected zones, the gas being injected into or between one or
more of the zones. The method of the invention is particularly
suitable for use where the liquid is circulated, as for example
described in our British Patent No. 1473665, around a system com-
prising a compartment of descending flow (hereinafter referred to
as the downcomer) and a compartment of ascending flow (hereinafter
referred to as the riser) communicating with each other at the
upper and lower ends, an oxygen-containing gas being injected into
the liquid as it passes through the downcomer. When the gas is
injected into flowing liquid there may be recycling, for example
any particular body of liquid may be recycled 10 times, preferably
20 to 40 times.
The process of the invention is particularly useful as a
stage in the biological treatment of sewage i.e. as the aeration
and/or digestion stages of this treatment and throughout the re-
mainder of this specification will be described with reference to
sewage treatment using the system of British Patent No. 1473665.
In the treatment of sewage by the process of the invention
using the system of British Patent No. 1473665, the supply (i.e. the
rate and position of supply) of o~ygen-containing gas to sewage cir-
culating around the system is controlled so that microorganisms pre-
sent in the sewage (mainly bacteria and bacteriophagic organisms -
usually protozoa) are subjected to marked changes in DOT and at
least one low and/or high DOT region as they pass around the system.
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In the system of British Patent No. 1473665 the down-
comer and riser may be of any convenient cross-sectional shape,
e.g. circular or semi-circular. They may be disposed externally
to each other but are preferably disposed within a single structure
(preferably cylindrical) divided internally by a partition or par-
titions or with the downcomer formed by a tube inside the struc-
tural tube, the outer space forming the riser. A wide variety of
geometrical arrangements is possible. The system may comprise a
plurality of risers and/or downcomers, e.g. two downcomers combined
with a single riser all located within the same structure.
Suitable sewage, if necessary after initial treatment,
passes into a basin in which gas-disengagement can occur during
the operation of the method of the invention. The downcomer and
riser extend below the level of the base of the basin. Thus when
the basin is situated at or below ground level the structure con-
taining the riser and downcomer is a shaft tpreferably cylindrical)
extending into the ground. The shaft may extend into the ground
at a position external to the basin but is preferably below it, the
upper ends of the riser and the downcomer opening into the basin.
In some cases the downcomer extends above the level of sewage in
the basin. In such cases however the downcomer extends for a
major proportion of its length below the level of the base of the
basin. In these cases the upper end of the riser opens into the
basin whilst the upper end of the downcomer communicates through
a conduit with sewage in the basin.
Suitably the system extends for at least 40 metres verti-
cally below the level of sewage in the basin, but preferably for
80 metres or more, especially 150-300 metres below. The total ef-
fective cross-sectional area of the riser or risers preferably is
equal to or exceeds that of the downcomer or downcomers. Suitably
the ratio of the total effective cross-sectional area of the riser
or risers to that of the downcomer or downcomers is within the
range 1:1 to 2:1.
.~ ~ - 5
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Any suitable means may be used to circulate sew-
age around the system. Very suitably however, in addition
to controlling the DOT, the injection of the oxygen-con-
taining gas into the system may be used to produce circu-
lation of the liquid around the system.
Suitably the oxygen-containing gas (preferably
air) is injected into both the downcomer and the riser.
Preferably gas injection into the two chambers takes place
at positions of equal hydrostatic pressure. Thus, since
the upper part of the riser will contain a greater propor-
tion of gas bubbles than does the upper part of the down-
comer (which will contain little or substantially no gas),
the position of gas injection into the riser is preferably
slightly lower than that into the downcomer. In practice
however it is satisfactory if gas injection into both
chambers is made at substantially the same distance below
the level of sewage in the basin. The gas to both injection
positions may then be supplied using the same compressor,
the proportions injected into the riser and downcomer
respectively being controlled by valves.
Preferably gas is injected into both chambers
at a position between 0.1 to 0.4 times their total length
below the level of sewage in the basin i.e. 15 to 120 metres
below when the system extends from 150 to 300 metres below
this level. It is preferred that gas injection takes place
at a position more than 20 metres below the level of sewage
in the basin although of course injection can take place
at less than 20 metres below the level.
During start-up of the system all or most of the
oxygen-containing gas is injected into the riser causing
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. .
its upper section to act as an air-lift pump. When the
initial start-up period has elapsed and the sewage is
circulating satisfactorily at a suitable velocity, e.g.
at least 0.8 metre/sec in the downcomer, the proportion
of the gas supplied to the downcomer may be greatly in-
creased, preferably until at least 50% and in some instances
until all of the gas is supplied to the downcomer. Sewage
in the system may then be continuously circulated under
these conditions.
When the method is being operated steadily after
the initial start-up period, gas bubbles injected into the
downcomer are borne rapidly downwards by the circulating
sewage to levels of higher pressure and their size diminishes.
Ultimately in the lower levels of a deeply-sunk apparatus
many of the bubbles will be entirely absorbed into the
sewage. As the sewage rises up the riser the bubbles will
first reappear and then increase in size. Thus by inject-
ing air into the downcomer at some suitable level below
the top level of the system, the riser as a whole will
contain more gas bubbles than the downcomer and the system
will continue to function as an air lift pump even though
all or a major proportion of the gas is being injected
into the downcomer. Indeed once circulation has commenced
and gas bubbles injected into the downcomer are borne
downwardly at a suitable rate, e.g. above 0.8 metre/sec,
the effect of injecting gas into the downcomer will be to
add to the effect of any gas injected into the riser in
driving the circulation through the two chambers.
During treatment sewage will generally circulate
around the system a large number of times, one complete
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circulation generally taking between 2 and 8 minutes depending
upon the dimensions of the system, The total duration of
treatment will depend upon whether it is employed as the
aeration or digestion step. In the former case the period
during which the sewage is circulates will generally be
1/4 to 4 hours for weak sewage but may be longer for stronger
sewage whilst in the latter it will be longer, e.g. 2 to 30 days
depending upon the rate at which sewage is supplied to the
apparatus,
It is envisaged that the method of the invention
may be most conveniently performed with the riser and down-
comer sunk into the ground in a deep shaft having e.g~ a
concrete lining which may form their external wall.
The values of DOT at various points around the
system which are desirable depend upon whether the inven-
tion is being used in the aeration or the digestion steps
of sewage treatment. However the main low DOT region is
preferably at the upper end of the downcomer above the
position at which gas is supplied to that limb. Another
preferred low DOT region is in the riser just below the
position at which the gas is supplied to that limb. The
preferred high DOT region is in the downcomer below the
position at which gas is supplied to that limb. Preferred
values of DOT around the system vary with the following
ranges and the ratejof injection of oxygen-containing gas
into the system when the sewage is circulating satisfacto-
rily is suitably controlled to give DOT values within
these ranges (see also Figure 3 of drawings):
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Upper end of downcomer 30 - 0 millibars de-
(above spargers) creasing downwards
Lower part of downcomer (from 0 - 1000 millibars
position above spargers) increasing downwards
Lower part of riser (to posi- 1000 - 0 millibars de-
tion below sparger if any) creasing upwards
Upper end of riser (from 0 - 30 millibars in-
below sparger if any) creasing upwards
These values of DOT are quoted as examples only.
If desired the DOT may be measured at at least
one position in the system and the results of this measure-
ment or measurements may be used to enable the supply ofoxygen-containing gas to be controlled. DOT measurements
can be made using probes, for example an oxygen electrode
can comprise for instance a membrane covered galvanic
probe, a typical example of which is the Mackereth elec-
trode or a membrane covered amperometric probe such as
the Clerk electrode. Suitably these probes may be posi-
tioned towards the upper ends of the riser and/or down-
comer, especially in the vicinity of the spargers through
which oxygen-containing gas is supplied e.g. within 20
to 50 metres of the spargers usually before the sparger
in the direction of flow. When an oxygen-containing gas
is being supplied to both the downcomer and the riser
the DOT probes are preferably positioned one above the
sparger into the downcomer and the other below the spar-
ger into the riser. When an oxygen-containing gas is
being supplied to the downcomer alone, i.e. substantially
no gas is being supplied to the riser, DOT probes are
positioned at the top of the downcomer and if desired
also at the top of the riser.
If required the downcomer or riser may contain
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auxiliary spargers for dribbling small amounts of oxy-
gen-containing gas into the system if and when necessary.
Suitably an auxiliary sparger is positioned at the top
of the downcomer.
DOT has a profound effect on the selection of
the microorganisms which thrive in the sewage during
treatment. Careful selection of the magnitude of this
factor and its degree of variation around the system causes
selection of a microorganism population ideally suited for
the treatment of sewage. A particular choice enables a
large population of nitrifying bacteria to be selected
resulting in good nitrification of the sewage. The choice
of conditions varies between the aeration and digestion
stages and also differs if it is desired to produce a
floating sludge or a settling sludge.
High values of DOT allow the build up and main-
tainance of high concentrations of microorganisms and cause
some uncoupling of oxidative phosphorylation resulting in
the oxidation of larger amounts of carbon to CO2 and less
to cells thus reducing sludge production. Similarly ex-
posing the microorganisms to brief periods of low DOT also
causes increased CO2 production.
The time taken by microorganisms to react to
changes in DOT varies but is usually less than the cir-
culation time around the system. Some types of response
take place in seconds while other responses involving
feed-back control mechanisms in metabolic pathways take
minutes. Some responses can take days depending upon
the growth rate of the microorganism if this results in
a selection of mutants. Intermediate time responses are
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known where repression or induction of enzyme synthesis
may take hours.
The invention is illustrated by the accompany-
ing drawings wherein:
Figures 1 to 4 are examples of suitable DOT
profiles in systems such as those shown in Figures 5 and 6.
Figures 5 and 6 are sectional diagrams of
systems in which the process of the invention may be per-
formed.
Figures 1 to 4 show four different DOT profiles,
i.e. diagrams illustrating variation of DOT, around
systems such as those shown in Figures 5 and 6. In these
diagrams, DOT profiles in the risers are indicated by
upward-pointing arrows and in the downcomers by downward-
pointing arrows. The magnitude of the DOT is shown by
the horizontal co-ordinates in the figures. Thus changes
in the DOT around the system are shown. Sparger posi-
tions are shown by dotted lines. Oxygen-containing gas
is sparged into the system shown in the figures as follows:
Figure 1: Downcomer at one position
Figure 2: Downcomer at two positions
Figure 3: Downcomer and riser at the same level
Figure 4: Downcomer and riser at the same level
The low DOT regions are in the following positions:
Figure 1: Downcomer above sparger
Figure 2: Upper part of riser
Downcomer above upper sparger
Figure 3: Riser below sparger
Downcomer above sparger
Figure 4: Riser below sparger
The main high DOT regions in all cases are in
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the downcomer below the sparger (i.e. the lower sparger
in Figure 2).
In the apparatus shown in Figure 5 Spargers 16
and 17 are situated in downcomer 14 and riser 15 respec-
tively and are both connected to compressor 18. The
flow of gas to riser 15 and downcomer 1~ is controlled
by valves 19 and 20 respectively. Operation of valves
19 and 20 is controlled by activator 21 which is connected
to flow-velocity measuring device 22 positioned towards
the upper end of downcomer 14. In this apparatus down-
~omer 14 and riser 15 are located in separate shafts sunk
below ground level A-A communicating with each other at
their lower ends via connecting tu~e 12.
When the apparatus shown in Figure 5 is used
as the aeration stage of an activated sludge system sewage,
after initial treatment and possibly also primary settling,
enters basin 13 through a channel (not shown in Figure 5)
opening into the basin at a point near the open upper end
of downcomer 14 and liquid plus activated sludge leaves
the basin through another channel (not shown in Figure 5)
opening out of basin 13 at a point below the liquid level
B-B and located at a distance from the inlet channel, and
passes to a settling tank.
With liquid occupying basin 13 up to the level
B-B, valve 19 open and valve 20 wholly or partially
closed, the system shown in Figure 5 is started up by in-
jecting air from compressor 18 wholly or mainly into the
riser 15. This causes the upper part of riser 15 to
operate as an air-lift pump and sewage begins to circu-
late around the system in the direction shown by the
~ arrows in Figure 5. When the flow rate as measured by
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device 22 reaches a pre-determined minimum value, acti-
vator 21 causes valve 19 to be wholly or partially
closed and valve 20 to be opened. Desirably the opening
of valve 20 and closing of valve 19 takes place in
stages as the velocity of the sewage in downcomer 14
increases. When the system is operating steadily the
total volume of air injected into the system and the
relative proportions in which it is injected into the
riser and the downcomer is controlled so as to produce
a satisfactory DOT profile around the system and to sub-
ject microorganisms circulating around the system to a
low and/or a high DOT region. Control of the air in-
jection may of course be performed manually by operators
but is more conveniently performed automatically using
activator 21 and device 22.
In the apparatus shown in Figure 6, downcomer
14 and riser 15 are located in the same shaft sunk below
ground level A-A, being separated from one another by
partition 23. Communication at the lower ends of down-
comer 14 and riser 15 is via an opening at the lower endof partition 23. The upper ends of partition 23 and of
the outer wall of downcomer 14 are bent over within
basin 13 to give deflectors 25 which produce a suitable
circulation within basin ~3. Otherwise the apparatus
of Figure 6 resembles that of Figure 4 and its mode of
operation is similar. If desired the flow of gas to
riser 15 and downcomer 14 in the apparatus of Figure 6
may be done by any suitable means e.g. that shown in
Figure 5.
