CA1054729A - Treatment of biologically-degradable material - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A process for the treatment of liquid carrying bio-logically degradable material in solution and/or suspension, particularly waste-water treatment, wherein the liquid is subjected to a low DOT (dissolved oxygen tension) and/or to a high DOT for a period thereby increasing the ratio of carbon dioxide to cellular material produced by microorganisms present in the liquid. In particular the liquid is subjected to a series of high and/or low DOT shocks.

Description

lOS4729 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

- 2 -~p 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 ~ - 3 -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.

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.

~,i 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 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 . .
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 lOS47Z9 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):

lOS4729 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 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 ~0547Z9 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 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 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.

Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the treatment of a liquid con-taining biologically-degradable material by microorganisms characterized in that said liquid and microorganisms are subjected to molecular oxygen or a gaseous mixture containing molecular oxygen to a cycle of a high DOT region and a low DOT region, and controlled to such condition that, within said cycle system,said liquid and microorganisms are subjected to a DOT not exceeding 30 millibars at said low DOT region for a period not exceeding 5 minutes and/or said liquid and micro-organisms are subjected to a DOT exceeding 450 millibars at said high DOT region for a period not exceeding 10 minutes.

2. A process according to Claim 1 wherein the oxygen-containing gas is air.

3. A process according to Claim 1 wherein when at least part of the liquid is subjected to a high DOT the high DOT is within the range 1000 to 1350 millibars.

4. A process according to Claim 1 wherein when at least part of the liquid is subjected to a low DOT the low DOT
is less than 30 millibars.

5. A process according to Claim 1 wherein the period during which any part of the liquid is subjected to a low DOT is not greater than 1 minute.

6. A process according to Claim 1 wherein the period during which any part of the liquid is subjected to a high DOT
is not greater than 3 minutes.

7. A process according to Claim 1 wherein the liquid is subjected to a series of high and/or low DOT shocks, the DOT
of the liquid when not subjected to such a shock being at a level intermediate between a low and a high DOT.

8. A process according to Claim l wherein an oxygen-containing gas is injected into a stream of liquid, thereby causing the DOT within the liquid to vary along its flow path.

9. A process according to Claim 1 wherein the liquid is circulated around a system and an oxygen-containing gas is injected into the circulating liquid, any particular body of liquid being circulated around the system between 20 and 40 times.