GB2128175A

GB2128175A - Method and apparatus for treating sewage

Info

Publication number: GB2128175A
Application number: GB08228672A
Authority: GB
Inventors: Michael Ernest Garrett; Peter Jackson
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1982-10-07
Filing date: 1982-10-07
Publication date: 1984-04-26
Also published as: ZA837120B; GB2128175B; BR8305538A; AU1993083A; JPS59130600A; JPH0355199B2

Abstract

In order to prevent bacterial formation of hydrogen sulphide in a gravity sewer, the sewage flowing therethrough is oxygenated. A part of the flow is diverted from the main sewer through a conduit 6 into an oxygenator 10 that includes a well 16 having a pipe or tube 18 inserted therein. The well 16 and pipe or tube 18 define a downward leg 12 and an upward leg 14 for the sewage. Flow of sewage through the oxygenator is made possible or assisted by the introduction of a multiplicity of oxygen bubbles into the bottom of the upward leg 14, which bubbles dissolve as they ascend the upward leg 14. Oxygenated sewage flows from the upward leg into a second conduit 8 and back to the gravity sewer. <IMAGE>

Description

SPECIFICATION Method and apparatus for treating sewage This invention relates to a method and apparatus of treating sewage and in particular to a method and apparatus for preventing the bacterial formation of sulphide, particularly hydrogen sulphide and other malodorous sulphides, in a sewer.

Municipal sewage typically contains bacteria and other micro-organisms that respire oxygen. They normally obtain oxygen for respiration from the dissolved oxygen that is present in the sewage. As the sewage flows along the sewerage system, there is a tendency however for the micro-organisms to deplete the dissolved oxygen content of the sewage at a rate faster than that at which oxygen is dissolved from the air in the head space of the gravity sewer or sewers forming part of the sewerage system. If there is no such dissolved oxygen remaining in the sewage, certain bacteria, known as 'facultative' bacteria obtain oxygen by breaking down dissolved oxygen containing compounds, particularly sulphates, in the sewage with the attendant formation of hydrogen sulphide and other sulphides, thereby rendering the sewage septic.The sulphides formed exist in equilibrium with sulphide and bisulphide ions as a function of pH septic sewage, which is acid, hydrogen sulphide is lost from solution into the atmosphere above the sewage. Hydrogen sulphide gas is oxidised to sulphuric acid by other bacteria which adhere to the moist walls and roof of the roof of the sewer, and the acid so-formed causes rapid corrosion of concrete surfaces in the sewerage system.

Our UK patent specification No. 1452961 discloses a particularly effective method of overcoming the problem of preventing bacterial formation of sulphide. The method involves the injection of (typically) commercially pure oxygen under pressure into the sewer.

For gravity sewers, the aforesaid UK patent specification discloses withdrawing a portion of the sewage from the sewer, pressurising it, oxygenating the pressurised sewage, and the returning the oxygenated sewage to the sewer. Our UK patent specification No.

1 596 296 discloses a modification to this technique in which the sewage withdrawn from the sewer is pumped into a holding tank, above ground, in which it is oxygenated. The sewer is then periodically flushed with oxygenated sewage released from the holding tank.

The aforementioned methods of oxygenating sewage flowing through a gravity sewer require the use of pumps. Pumps need to be of a kind that can handle crude sewage (which typically contains gross bodies such as rags). Suitable pumps are commercially available but are relatively expensive and generally require a power supply to be available.

It is an aim of the present invention to provide a method and apparatus for preventing bacterial formation of hydrogen sulphide in a gravity sewer by oxygenating the sewage without the need to provide a mechanical pump.

According to the present invention there is provided a method for preventing the bacterial formation of hydrogen sulphide in a gravity sewer, comprising the steps of causing crude sewage flowing through the sewer to pass out of the sewer and along a path comprising an elongate downward leg and an elongate upward leg, the downward leg leading the flow to the upward leg; introducing oxygenating gas into the sewage in at least one of the said legs at a position in the flow path remote from the top of said upward leg, whereby the sewage is oxygenated and causing the thus oxygenated sewage to return to the sewer.

The invention also provides apparatus, for association with a gravity sewer, capable of operation to prevent the bacterial formation of hydrogen sulphide in the sewer, which apparatus comprises a first conduit leading from the sewer to an oxygenator defining a flow path comprising elongate upward leg and and elongate downward leg, the downward leg leading to the upward leg, and means for introducing an oxygenating gas into the sewage in at least one of said legs at a position in the flow path remote from the top of said upward leg, and a second conduit leading from the top of the upward leg to the sewer to enable oxygenated sewage to return to the sewer.

The flow of sewage from said sewer to the flow path, therealong and back to the sewer can be arranged to be under gravity and made possible by or assisted by the buoyancy (ie 'gas lift' effect) of bubbles of oxygenating gas in the upward or rising leg of the oxygenator, thus obviating the need to employ a mechanical pump.

By preventing bacterial formation of hydrogen sulphide, it becomes unnecessary to line the sewer pipe with an acid-resistant coating.

The oxygenator is typically located underground. Thus, the apparatus according to the invention need not involve the conveying of sewage to an above-ground oxygenation plant (unlike the apparatus disclosed in our UK patent specification No. 1 596 246).

The said legs of the flow path may be provided in a well whose depth is chosen in accordance with the amount of oxygen that needs to be dissolved, and is typically sufficient to provide a liquid head of at least 9 metres. The well may include an inner pipe, shaft or tube to define the flow path. In such an arrangement the downward leg of the flow path can be defined by the space between the well shaft and the inner pipe, shaft or tube, and the upward leg by the tube, pipe or shaft, or vice versa. Typically, a suitable hole or shaft can be bored in the ground and then a concrete shaft sunk into, or formed in situ in the hole. An inner tube or pipe, of any suitable material, eg ABS plastics, or mild steel, may then be positioned within the shaft.

Preferably, the inner tube or pipe is readily removable from the outer shaft to facilitate periodic scouring of the well to remove grit and the like, and also to facilitate routine cleaning. If desired, the outer shaft may have a bottom so curved as to make possible (with a sufficient liquid velocity) carry-over of the grit from the downward leg to the upward leg without deposition of the grit at the bottom of the well. It is alternatively or additionally possible to provide a preliminary, cleanable, settling vessel intermediate the sewer and the well to remove solids that might block the well.

The oxygenating gas is typically commercially pure oxygen or oxygen-enriched air. It is preferably introduced into the sewage flowing along the flow path through at least one diffuser or other means capable of distributing the gas as a multitude of bubbles. The diffuser may include a porous gas distribution member, typically of sintered material. Such distribution members are generally capable of forming fine gas bubbles (eg bubbles having a diameter in the order of 0.1 mm or less).

There are preferably at least two porous gas distribution members spaced apart from one another such that the respective flows of gas bubbles emerging from the members do not merge into one another near to said distribution members. Such a merger would tend to cause bubbles to coalesce with one another, thereby hindering their dissolution.

The oxygenating gas is preferably introduced into the upward leg at a regio or regions near to the bottom thereof. By releasing a multitude of gas bubbles to the liquid in the upper leg the effectie density of such liquid is reduced. This phenomenon facilitates a maintenance of flow of the crude sewage down one leg and up the other leg of the flow path. Moreover, the head of liquid in the upward leg facilitates the dissolving of the oxygen. Typically, a large proportion (say about 90%) but not all the oxygen will be dissolved. Moreover, some gas already dissolved in the incoming sewage, (eg nitrogen) will be displaced from solution by oxygen dissolving in the sewage.It is preferred that the oxygenation is conducted such that gas flow from the oxygenator to the ullage space of the sewer does not affect the concentration of oxygen in the ullage space to such an extent that an oxygen concentration significantly greater than in the atmosphere is created in any location thereof. It is believed that there will be no such enrichment of the ullage space in the sewer if the oxygen (or oxygen-enriched air) is bubbled into the flow path at a region remote from the top of the upward leg and if there is an adequate head of sewage flowing from the upward leg (eg 9 metres or greater). If however such enrichment does take place additional ventilation of the sewer may be provided or alternatively the enriched region may be kept distinct from the rest of the atmosphere by means of air or water 'curtains'.

Typically, during average dry weather flow, the rate of dissolving oxygen in the sewage passing through the oxygenator is chosen to be sufficient to give a dissolved oxygen concentration of at least 200 ppm. Typically, when the sewage is returned to the sewer (preferably at a region downstream of that from which it is taken) the dissslved oxygen concentration of the sewage in the sewer will thus be increased to, say, 5 to 10 ppm depending on the proportion of the flow through the main sewer pipe that is diverted to the oxygenator. Typically, from a quarter to a third of the flow through the sewer may be diverted. In order to ensure an adequate flow of sewage through the oxygenator during average dry weather conditions, a dam or like member may be provided in the sewer.If the sewage is returned to the sewer at a region downstream of that from which it is taken, the dam is typically positioned intermediate such locations.

The oxygenating gas may readily be supplied under the necessary pressure to overcome the head of liquid acting on the diffuser or diffusers (or other gas distribution devices).

For example, there can be used a conventional insulated storage tank for liquid oxygen fitted with a vaporiser and arranged such that the pressure in the ullage space of the storage tank could be used to pass the liquid oxygen through the vaporiser and thus produce oxygen gas at super-atmospheric pressure.

The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a general schematic plan view of a gravity sewer adopted in accordance with the invention; and Figure 2 is a schematic side view, partly in section, of the sewer shown in Fig. 1.

Referring to the accompanying drawings, a gravity sewer 2 includes a sewer pipe 4 along which crude sewage flows under gravity. Associated with the sewer pipe 4 is a first conduit 6 along with sewage is able to flow from the main sewer pipe 4 to an oxygenator 10. Also associated with the sewer pipe 4 is a second conduit 8 which is able to conduct crude sewage from the oxygenator 10 to the sewer pipe 4. The outlet of the second conduit 8 is located downstream of the inlet to the first conduit 6, the two conduits typically being separated by a distance along the sewer pipe 4 in the range of 2 to 10 metres.

The oxygenator 10 includes a first leg 12 down which the crude sewage flows and on up leg 14, up which sewage flows from the downleg 12 to be discharged into the second conduit 8 for return to the main sewer pipe 4.

The legs 12 and 14 are defined by a well 16 and an inner tube 18. The well 16 is formed by boring a holl in the ground and then lining the hole by inserting therein a concrete pipe or forming such a pipe in situ. The well 16 is provided with suitable inlet and outlet apertures which receive the outlet of the first conduit 6 and the inlet to the second conduit 8. The inner tube or pipe 18 is located coaxially within the well 16 and supported therein by means not shown. The bottom of the pipe 18 is situated about 15 cm above the bottom of the well 16 and the top or outlet of the pipe 18 is situated at the level of the inlet to the second conduit 8. The downleg 12 is defined by the annular space between the wall of the well 16 and the pipe 18 and the upleg by the pipe 18.The outlet of the pipe is typically located at least 8 metres (and generally 9 or more metres) above the inlet to the pipe 18. In operation of the oxygenator, crude sewage flows down the leg 12 and up the leg 14.

The oxygenator 10 also includes four circumferentially spaced apart diffusers 30 having bubble-forming members comprising porous, sintered material. The diffusers 30 are supplied with oxygen from a ring main 28 connected by a pipe 26 to a source 24 of gaseous oxygen. The diffusers 30 are located within the pipe 18 at or near the bottom thereof. In operation, a multitude of fine bubbles (typically, on average, having a diameter of less than 0.1 mm) flow into the sewage flowing up the leg 14 of the oxygenator 10. The multitude of bubbles so introduced into the sewage in the upleg 14 assist the flow of sewage up through this leg. This is because the bubbles appreciably reduce the effective density of the liquid in the upleg 14.

Indeed, as shown in Fig. 2, the liquid level in the upleg 14 may therefore be higher than the liquid level in the downleg 12, although this is not an essential feature of the invention. The oxygen source 24 is located above ground as shown in Fig. 2. It typically comprises a vacuum-insulated vessel holding a volume of commercially pure liquid oxygen under the pressure of gaseous oxygen in the ullage space. There are means (not shown), as is conventional, for conducting liquid under the pressure of the gas in the ullage space to a vaporiser for vaporising the liquid oxygen.

In consequence gas can be supplied from the oxygen source 24 through the pipe 26 (which extends through a cover 20 closing the shaft 16 at ground level) to the diffusers 30 at a pressure able to overcome the head of liquid in the leg 14.

The outlet pipe 18 is provided with a spout 32 extending into the conduit 8. In order to prevent that flow of liquid from the conduit 8 to the oxygenator 10 a baffle 34 is provided at the inlet of the conduit 8 and the spout extends through such baffle 34 into the conduit 8.

If desired, in order to facilitate flow of crude sewage from the main sewer pipe 4 to the conduit 6, a dam 22 may be provided in the main sewer pipe 4 intermediate the inlet to the first conduit 6 and the outlet from the second conduit 8. Typically, the height of the dam 22 is in the order of a quarter or a third of the internal diameter of the sewer pipe 4.

The apparatus shown in Figs. 1 and 2 is located at a region in the sewarage system upstream of where there is a tendency for the sewage to become depleted of dissolved oxygen.

In operation, crude sewage flows under gravity out of the sewer pipe 4 through the first conduit 6 and into the oxygenator 10.

Once the shaft 16 fills with sewage to an appropriate level a flow of sewage down the leg 12 and up the leg 14 is established. Such a flow regimen is necessitated by the introduction of a multitude of fine oxygen bubbles through the diffuser 30 into the sewage at the bottom of the pipe 18 defining the upleg 14.

The apparatus is typically dimensioned such that during average dry weather flow conditions from about a quarter to a third of the flow of crude sewage in the gravity sewer is diverted through the oxygenator 10. Typically, this sewage will have a dissolved oxygen concentration no greater than one or two ppm as it enters the oxygenator 10. Oxygen is bubbled into the sewage ascending the upleg 14 at a rate sufficient to increase its dissolved oxygen concentration to 20 ppm as it leaves the oxygenator 10.

The head of liquid in the upleg 14 provides a pressure which facilitates the dissolving of the oxygen bubbles introduced into the sewage through the diffusers 30. Thus, as the liquid flows up the leg 14 so the concentration of undissolved bubbles therein diminishes. Typically, at least 90% of the oxygen introduced into the sewage dissolves therein.

It is to be appreciated that the sewage entering the oxygenator will contain dissolved gas other than oxygen. Typically, it contains some dissolved nitrogen, it may also contain some dissolved carbon dioxide. The introduction of fine oxygen bubbles into the sewage in the oxygenator 10 displaces some of this dissolved gas from solution. Accordingly, the gas leaving the surface of the liquid in the upleg 14 will be a mixture of oxygen and other gases displaced from the sewage. Some of this gas will tend to flow back into the main sewer pipe 4. Such gas will not raise the concentration of oxygen in the sewer significantly above that in ambient air.

The free fall of sewage through the oxygen ator 10 to the gravity sewer pipe 4 takes place under gravity (through the assistance of the oxygen bubbles in the upleg 14) and therefore no mechanical pump and associated electrical power source is required. Moreover, no such pump is required for supplying the oxygen to the diffusers 30.

The oxygenated sewage returning from the conduit 8 to the gravity sewer 4 is able to increase substantially the cocentration of the dissolved oxygen in the sewer downstream of the region to which the sewage is so returned.

Typically, the dissolved oxygen concentration is increased to 8 to 10 ppm. As the sewage continues to flow along the sewer so the micro-organisms in the sewage and on the sewer wall would tend to deplete the dissolved oxygen concentration, although further oxygen will be dissolved at the surface of the sewage by virtue of a contact between the surface of the sewage in the sewer and the gas (which contains oxygen) in the ullage space of the sewer. If there is a tendency for the net result of these processes to produce sewage containing no dissolved oxygen, a further apparatus in accordance with the invention may be employed downstream of the apparatus depicted in Figs. 1 and 2 of the accompanying drawings or, alternatively, more than one oxygenator can be employed to serve a particular location in a gravity sewer. For example, the total flow through the sewer could be oxygenated by dividing the flow into four streams and passing each through its own oxygenator.

By maintaining a positive dissolved oxygen concentration in the sewage flowing along the sewer, in accordance with the invention, the need to line the sewer pipe with a protective coating is obviated. This is a substantial economic advantage particularly if the gravity sewer pipe is of a large diameter eg. 2.5 ft or greater.

The pipe 18 is preferably removable from the shaft 16 so as to facilitate the cleaning of the wall 16. The pipe 18 may be periodically removed and the bottom of the shaft scoured.

Claims

1. A method for preventing the bacterial formation of hydrogen sulphide in a gravity sewer, comprising the steps of causing crude sewage flowing through the sewer to pass out of the sewer and along a flow path comprising an elongate downward leg and an elongate upward leg, the downward leg leading to the upward leg; introducing oxygenating gas into the sewage in at least one of said legs at a position in the flow path remote from the top of said upward leg, whereby the sewage is oxygenated, and causing the thus oxygenated sewage to return to the sewer.

2. A method as claimed in claim 1, in which the passage of the sewage out of the sewer along the flow path and back to the sewer takes place without the aid of a mechanical pump being made possible or assisted by the buoyancy of a multiplicity of bubbles of oxygenating gas in the sewage ascending the upward leg of the flow path.

3. A method as claimed in claim 1 or claim 3, in which the legs of the flow path are provided in a well, there being an inner pipe shaft or tube in the well, the downward leg being defined by the space between the inner pipe, tube or shaft and the well shaft, and the upward leg being defined by the inner pipe, tube or shaft or vice versa.

4. A method as claimed in claim 3, in which the depth of the well is sufficient to provide a liquid head of at least 9 metres.

5. A method as claimed in any one of the preceding claims, in which the oxygenating gas is introduced into the sewage flowing along the flow path in a multitude of bubbles.

6. A method as claimed in claim 5, in which the bubbles have a diameter of 0.1 mm or less.

7. A method as claimed in claim 5 or claim 6, in which the oxygenating gas is introduced into the upward leg at a region or regions near to the bottom thereof.

8. A method as claimed in any one of the preceding claims, wherein the oxygenation is conducted such that gas flow from the upward leg to the ullage space of the sewer does not affect the concentration of oxygen in the said ullage space to such an extent that an oxygen concentration significantly greater than in the atmosphere is created.

9. A method as claimed in any one of the preceding claims, in which from a quarter to a third of the flow of sewage through the gravity sewer is diverted through the flow path (during average dry flow conditions).

10. A method as claimed in any one of the preceding claims, in which the rate of dissolving oxygen in the sewage passing along the flow path is chosen to give a dissolved oxygen concentration of at least 20 ppm.

11. A method as claimed in any one of the preceding claims, in which the oxygenating gas is commercially pure oxygen.

12. A method for preventing the bacterial formation of hydrogen sulphide in a gravity sewer, substantially as herein described with reference to the accompanying drawings.

13. Apparatus, for association with a gravity sewer, capable of operation to prevent the bacterial formation of hydrogen sulphide in the sewer, which apparatus comprises a first conduit leading from the sewer to an oxygenator defining a flow path comprising an elongate downward leg and an elongate upward leg, the downward leg leading to the upward leg, and means for introducing an oxygenating gas into the sewage in at least one of said legs at a position in the flow path remote from the top of said upward leg, and a second conduit leading from the top of the upward leg to the sewer to enable oxygenated sewage to return to the sewer.

14. Apparatus as claimed in claim 13, including at least one gas diffuser for introducing the oxygenating gas into the sewage at a region or regions of the upward leg near the bottom thereof as a multiplicity of bubbles, whereby passage of the sewage out of the sewer, along the flow path and backto the sewer is able to take place without the aid of a mechanical pump.

15. Apparatus as claimed in claim 14, in which said diffuser includes a porous gas distribution member of sintered material.

16. Apparatus as claimed in any one of claims 13 to 16, in which the legs of the oxygenator are provided in a well, there being an inner pipe, shaft or tube in the well, the downward leg being defined by the space between the inner pipe, tube or shaft and the well shaft, and the upward leg by the inner pipe, tube or shaft or vice versa.

17. Appararus as claimed in any one of claims 13 to 16, in which the legs of the oxygenator are provided in a well, there being an inner pipe, shaft or tube in the well, the downward leg being defined by the space between the inner pipe, tube or shaft and the well shaft, and the upward leg by the inner pipe, tube or shaft, or vice versa.

18. Apparatus as claimed in claim 17, in which the depth of the well is sufficient to provide a liquid head of at least 9 metres.

19. Apparatus as claimed in claim 17 or 18, in which the inner pipe, tube or shaft is removable from the well.

20. Apparatus, for association with a gravity sewer, capable of operation to prevent the bacterial formation of hydrngen#sulphide in the sewer, which apparatus is substantially as described herein with reference to the accompanying drawings.