GB1594963A

GB1594963A - Method and apparatus for treating sewage with oxygen

Info

Publication number: GB1594963A
Application number: GB8504/77A
Authority: GB
Original assignee: BOC International Ltd
Current assignee: BOC Group Ltd
Priority date: 1978-02-28
Filing date: 1978-02-28
Publication date: 1981-08-05

Description

(54) METHOD AND APPARATUS FOR TREATING SEWAGE WITH OXYGEN (71) We, BOC INTERNATIONAL LIM ITED, of Hammersmith House, London W6 9DX, England, a British company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a method and apparatus for the treatment with oxygen of sewage while it is being held in or is flowing through a sewer and is an improvement in, or modification of, the invention disclosed in our United Kingdom patent specification No. 1 452 961.

It is proposed in the above-numbered specification to treat sewage in a sewer by injecting oxygen-rich gas under pressure into the sewage system. More specifically, the gas is injected through a pipe (having orifices therein) into the sewage flowing through the sewer, into the volute of a pump for driving sewage through a rising main sewer or into a sump in which the sewage is contained prior to its being pumped through the sewer.

The present invention resides in a method of treating sewage while it is being held in or is flowing through a sewer, wherein oxygenrich gas is injected into sewage in the sewer through a gas-permeable wall which defines part of the surface area of an impeller of a pump provided to pass sewage through the sewer.

The invention also resides in apparatus for treating sewage while it is being held in or is flowing through a sewer comprising means for injecting oxygen-rich gas into the sewage and wherein such means comprise a gaspermeable wall which defines part of the surface area of an impeller of a pump for passing sewage through the sewer, The invention furthermore resides in a sewer provided with such apparatus.

The term "oxygen-rich gas" is used herein to mean oxygen or a gaseous mixture having a proportion of oxygen higher than that of air, preferably greater than 95% (by volume) oxygen.

An advantage of a method and apparatus according to the invention is that when the pump is running gas can be injected into the sewage under extremely turbulent conditions whereby the gas is readily broken into a fine bubble form facilitating dissolution of the gas in the sewage. By injecting the gas through the impeller, the risk of cavitation damage to the pump is likely to be less than if the gas was injected upstream of the impeller.

Preferably the impeller is hollow to define a chamber which in use is supplied with pressurised air or oxygen-rich gas, the chamber being bounded in part by one or more such gas-permeable walls through which the gas permeates into the sewage passing over the impeller. The chamber may be supplied with the gas via a tubular impeller drive shaft.

The aforesaid wall or walls may be gaspermeable by virtue of porosity or perforation. The pores or holes of such a wall preferably have greatest transverse dimensions of 0.010 inch or less, and typically not greater than 0.005 inch. It is also preferred that each such pore or hole is spaced from its neighbours by a distance substantially greater than the aforesaid transverse dimension, and most preferably by a distance of at least 0.030 inch, e.g. a distance of 0.060 inch or greater, to minimise the likelihood of coalescence of bubbles of gas emerging from the pores or holes. Suitable porous materials from which to make such walls may be sintered non-ferrous materials, e.g. sintered bronze.

When the gas is substantially pure oxygen it is conveniently supplied to the sewage from a reservoir of liquefied oxygen contained in vacuum insulated evaporator, the liquid being evaporated by heat exchange with the ambient atmosphere.

In some methods according to the invention the concentration of dissolved oxygen in the sewage or the biochemical oxygen demand of the sewage may be monitored downstream of the point of gas injection, and the rate of gas injection adjusted in accordance with changes in the monitored value.

Such adjustment may take place automatically using a control system, e.g. an analogue or digital computer, which receives signals from a dissolved oxygen meter and emits signals to actuate means for controlling the rate of flow of gas.

One embodiment of the invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of apparatus for introducing an oxygen-rich gas into sewage flowing through a rising main sewer; and Figure 2 is a schematic view of a sewage pump forming part of the apparatus of Figure 1.

Referring to Figure 1 there is shown a rising main sewer 10 leading from a pumping station 11. A sewage inlet 12 is provided to pass sewage to a sump 13. A centrifugal pump 14 draws the sewage from the sump, pressurises the sewage and delivers it into the sewer conduit 10. A non-return valve 15 is provided in conduit 10 near pump 14 to prevent backflow of sewage into the pump.

Oxygen-rich gas is supplied under pressure through a line 17 and injected into sewage in pump 14. Referring to Figure 2, the pump 14 has a hollow impeller 30 which provides an internal chamber 31 to which pressurised oxygen-rich gas is supplied. The impeller 30 has a hollow drive shaft 32. The oxygen-rich gas is supplied to the chamber 31 from a gas line 17 communicating with the interior of the drive shaft 32 and hence the chamber 31.

Portions of the surface area of the impeller 39 are defined by porous plates 34 set into apertures provided in the body of the impel- ler. The plates 34 form part of the boundary of chamber 31 so that pressurised gas in the chamber permeates through the plates 'into sewage passing over the impeller. The plates are made of a porous non-ferrous material, e.g. sintered bronze, such that the gas enters the sewage in fine bubble form to be broken into even finer bubble form by the turbulence in the sewage passing over the impeller.

In other embodiments of the invention, the plates 34 may be replaced by areas of the impeller which are made gas-permeable by forming in them a multiplicity of fine holes.

The gas, when substantially pure oxygen, may be contained in liquid form in a conventional vacuum insulated evaporator (not shown) whereby the liquid is evaporated by heat exchange with the ambient atmosphere to provide a pressurised supply of gas to line 17.

The flow of gas is controlled by a solenoid operated valve 18 which is operated automatically to adjust the rate of flow through line 17 in accordance with variations in the value of the concentration of dissolved oxygen measured at the outfall 19 of the rising main by a dissolved oxygen meter 20.

In a typical example of a process according to the invention using apparatus of the type illustrated in the drawings, sewage is pumped through the sewer at a flow rate of 100 litres/sec and at a pressure of 10 metres water gauge while substantially pure oxygen is supplied to the impeller chamber 31 at a pressure of 3 Kgs/sq cm, from which chamber oxygen in fine bubble form issues into the sewage at a rate of 10 g/sec. The dissolved oxygen content of sewage downstream of the pump 14 is of the order of 100 ppm.

WHAT WE CLAIM IS: 1. A method of treating sewage while it is being held in or is flowing through a sewer, wherein oxygen-rich gas is injected into sewage in the sewer through a gas permeable wall which defines part of the surface area of an impeller of a pump provided to pass sewage through the sewer.

2. A method according to claim 1 wherein the impeller is hollow to define achamber which is supplied with pressurised air or oxygen-rich gas, the chamber being bounded in part by one or more such gas permeable walls through which the gas permeates into sewage passing over the impeller.

3. A method according to claim 2 wherein said chamber is supplied with the gas via a tubular impeller drive shaft.

4. A method according to any preceding claim wherein the or each such wall is porous or perforate.

5. A method according to claim 4 wherein the pores or holes of the or each such wall have greatest transverse dimensions of 0.010 inch or less.

6. A method according to claim 4 or claim 5 whererin the pores or holes of the or each such wall are spaced apart by distances substantially greater than the greatest transverse dimensions thereof.

7. A method according to any preceding claim wherein the gas is supplied to the sewage from a reservoir of liquefied oxygen.

8. A method according to any preceding claim wherein the dissolved oxygen concentration or biochemical oxygen demand of the sewage is monitored downstream of the point of gas injection and the rate of gas injection is controlled in accordance with changes in the monitored value.

9. Apparatus for treating sewage while it is being held in or is flowing through a sewer

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. with the ambient atmosphere. In some methods according to the invention the concentration of dissolved oxygen in the sewage or the biochemical oxygen demand of the sewage may be monitored downstream of the point of gas injection, and the rate of gas injection adjusted in accordance with changes in the monitored value. Such adjustment may take place automatically using a control system, e.g. an analogue or digital computer, which receives signals from a dissolved oxygen meter and emits signals to actuate means for controlling the rate of flow of gas. One embodiment of the invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of apparatus for introducing an oxygen-rich gas into sewage flowing through a rising main sewer; and Figure 2 is a schematic view of a sewage pump forming part of the apparatus of Figure 1. Referring to Figure 1 there is shown a rising main sewer 10 leading from a pumping station 11. A sewage inlet 12 is provided to pass sewage to a sump 13. A centrifugal pump 14 draws the sewage from the sump, pressurises the sewage and delivers it into the sewer conduit 10. A non-return valve 15 is provided in conduit 10 near pump 14 to prevent backflow of sewage into the pump. Oxygen-rich gas is supplied under pressure through a line 17 and injected into sewage in pump 14. Referring to Figure 2, the pump 14 has a hollow impeller 30 which provides an internal chamber 31 to which pressurised oxygen-rich gas is supplied. The impeller 30 has a hollow drive shaft 32. The oxygen-rich gas is supplied to the chamber 31 from a gas line 17 communicating with the interior of the drive shaft 32 and hence the chamber 31. Portions of the surface area of the impeller 39 are defined by porous plates 34 set into apertures provided in the body of the impel- ler. The plates 34 form part of the boundary of chamber 31 so that pressurised gas in the chamber permeates through the plates 'into sewage passing over the impeller. The plates are made of a porous non-ferrous material, e.g. sintered bronze, such that the gas enters the sewage in fine bubble form to be broken into even finer bubble form by the turbulence in the sewage passing over the impeller. In other embodiments of the invention, the plates 34 may be replaced by areas of the impeller which are made gas-permeable by forming in them a multiplicity of fine holes. The gas, when substantially pure oxygen, may be contained in liquid form in a conventional vacuum insulated evaporator (not shown) whereby the liquid is evaporated by heat exchange with the ambient atmosphere to provide a pressurised supply of gas to line 17. The flow of gas is controlled by a solenoid operated valve 18 which is operated automatically to adjust the rate of flow through line 17 in accordance with variations in the value of the concentration of dissolved oxygen measured at the outfall 19 of the rising main by a dissolved oxygen meter 20. In a typical example of a process according to the invention using apparatus of the type illustrated in the drawings, sewage is pumped through the sewer at a flow rate of 100 litres/sec and at a pressure of 10 metres water gauge while substantially pure oxygen is supplied to the impeller chamber 31 at a pressure of 3 Kgs/sq cm, from which chamber oxygen in fine bubble form issues into the sewage at a rate of 10 g/sec. The dissolved oxygen content of sewage downstream of the pump 14 is of the order of 100 ppm. WHAT WE CLAIM IS:

1. A method of treating sewage while it is being held in or is flowing through a sewer, wherein oxygen-rich gas is injected into sewage in the sewer through a gas permeable wall which defines part of the surface area of an impeller of a pump provided to pass sewage through the sewer.

comprising means to inject air or oxygen rich gas into the sewage and wherein such means comprise a gas permeable wall which defines part of the surface area of an impeller of a pump for passing sewage through the sewer.

10. Apparatus according to claim 9 wherein the impeller is hollow to define a chamber connectable to a source of pressurised air or oxygen-rich gas, the chamber being bounded in part by one or more such gas permeable walls.

11. Apparatus according to claim 10 wherein the pump comprises a tubular impeller drive shaft through which pressurised gas can be supplied to said chamber.

12. Apparatus according to any one of claims 9 to 11 wherein the or each such wall is porous or perforate.

13. Apparatus according to claim 12 wherein the pores or holes of the or each such wall have greatest transverse dimensions of 0.010 inch or less.

14. Apparatus according to claim 12 or claim 13 wherein the pores or holes of the or each such wall are spaced apart by distances substantially greater than the greatest transverse dimensions thereof.

15. Apparatus according to any one of claims 9 to 14 comprising a gas supply in the form of a reservoir of liquid oxygen.

16. Apparatus according to any one of claims 9 to 15 comprising means for monitoring the dissolved oxygen concentration or biochemical oxygen demand of sewage at a selected point in the sewer and means for controlling the rate of gas injection in accordance with changes in the monitored value.

17. A sewer provided with apparatus according to any one of claims 9 to 16.

18. A method of treating sewage substantially as hereinbefore described with reference to the accompanying drawings.

19. Apparatus for treating sewage substantially as hereinbefore described with reference to the accompanying drawings.