GB1593253A - Biological treatment of aqueous waste material with oxygen - Google Patents

Description

(54) BIOLOGICAL TREATMENT OF AQUEOUS WASTE MATERIAL WITH OXYGEN (71) We, BOC Limited, of Hammersmith House, London W6 9DX, England, an English 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 the biological treatment with oxygen of aqueous waste material having a biochemical oxygen demand (BOD).

A known waste water treatment process uses two concentric pipes which extend into a bore made in the ground to a depth of up to 300 feet. This apparatus provides two vertical passages, namely the bore of the inner pipe and the annular space between the pipes, which passages are connected at their lower ends. Sewage is introduced into one of the passages and is circulated down that passage and up the second passage.

Bubbles of air are introduced into the downflowing sewage stream at a position a number of feet below the surface of such stream. Bubbles are carried downwards by the flow of sewage and are gradually dissolved therein as the partial pressure of dissolved oxygen increases at greater depths in the downflowing sewage stream. The sewage then returns to the surface through the second passage. In this passage bubbles of carbon dioxide, and possibly containing nitrogen and oxygen are desorbed from solution as the hydrostatic pressure reduces. The volume of gas desorbed is such that the density of the column of fluid in the second passage is less than that of the column of fluid in the first passage so that circulation of the sewage through the pipes is maintained in a fashion analogous to a gas lift pump.

The invention provides a method for treating aqueous waste material, which method comprises establishing a column of the waste material in a first substantially vertical passage, the two passages communicating with one another at their lower ends and the second passage having an upper portion of wider cross-section than the portion thereblow; introducing further aqueous waste material into the upper end of the first passage; causing there to be different amounts of undissolved gas and thereby different fluid densities in the respective passage such that a flow of liquid down the first passage and up the second passage is maintained; introducing oxygenrich gas into the downflowing liquid at a level below the bottom of the upper portion; and controlling the rate of introduction of oxygen-rich gas such that the gas is dissolved to a large extent in the downflowing liquid, any undissolved gas in the liquid at the bottom of the first passage being in the form of fine bubbles, (typically less than 0.5mm in diameter), and oxygen coming out of solution as the liquid ascends the second passage, and in which method the dimension of the upper portion are such that it causes a reduction in the liquid velocity sufficient for most of the bubbles of oxygen that come out of solution in the second passage to redissolve.

The term "oxygen-rich gas" is used herein to mean oxygen or a gaseous mixture which has an oxygen content greater than that of air. Preferably in a process according to the invention the oxygen-rich gas has an oxygen content of at least 80% by volume and most preferably at least 95 % by volume.

The invention also provides apparatus for performing the above-stated method, which apparatus comprises a pair of substantially vertical passages which are joined at their lower ends, and means for introducing an oxygen-rich gas (as hereinbefore defined) at a controlled rate into one of the passages to a position spaced below the upper end of such passage, the second passage having a portion of increased cross-sectional area adjacent its upper end at a location above the level at which the aforesaid means for introducing oxygen-rich gas are located.

In a preferred embodiment there are provided two vertical pipes one within the other, said passages being provided by the bore of the inner pipe and the space between the pipes respectively. It is also preferred that the pipes are coaxial.

By adopting a process and apparatus according to the invention, the possible loss of oxygen-rich gas from the surface of the up-flowing stream of sewage can be reduced thereby enabling a more economic use of such gas in the above described known waste water treatment process. Not only can the loss of oxygen-rich gas be reduced but also the amount of foaming in the upflowing sewage due to the desorbing carbon dioxide is likely to be reduced so that sludge separation is facilitated enabling clarified water to be withdrawn from the apparatus. It is also though that a further advantage of a process according to the invention is that the likelihood of a discharge from the apparatus proving detrimental to fish life owing to super saturation thereof with nitrogen may be reduced enabling the process to be used successfully on waterways which support fish life.

A specific embodiment of the invention will now be described by way of example and with reference to the accompanying drawing which is a diagrammatic representation of an apparatus for treating aqueous waste material embodying the invention.

Referring to the drawing, a waste water treatment apparatus comprises a pair of coaxial pipes 10 and 11 located in a deep bore provided in the ground. The lower end of the inner pipe 10 is spaced from the base of the bore so that the lower ends of pipes 10 and 11 are in communication to allow a flow of fluid to pass downwardly through pipe 10 and then upwardly through the annular space between the pipes 10 and 11.

Columns of acqueous waste material are established in the two pipes 10 and 11.

These are caused to be different amounts of undissolved gas and thereby different fluid densities in the respective pipes such that a flow of liquid down the pipe 10 and up the pipe 11 is maintained. Sewage sludge is introduced into the upper end of pipe 10 and through an inlet conduit 12 and clarified liquor is run off from the upper end of pipe 11 through an outlet conduit 13. A gas diffuser 14 is located in pipe 10 about one third of the way down such pipe. The diffuser is in the form of a porous cylinder the interior of which is connected by a pipe 15 to a pressurised supply of oxygen-rich gas. The line 15 is provided with a control valve 16 for regulating the amount of gas introduced into the downflowing liquid in pipe 10. An upper end section 17 of the pipe 11 is formed to have an enlarged diameter as compared with the remainder of the pipe for a purpose described below.

In use the sewage sludge is introduced through conduit 12 and circulated down through the bore pipe 10 and then upwardly through the annular space 18 between pipes 10 and 11...

This circulation is maintained by ensuring that the density of the fluid in space 18 is less than the density of the fluid in pipe 10.

This is achieved by producing a greater proportion of bubbles of free gas space in the annular space 18 than in the pipe 10.

Oxygen-rich gas is injected through diffuser 14 and the volume of such gas which is carried downwardly in the flow of liquid through pipe 10, starts to decrease for the reasons that some of the gas goes into solution and that the increasing'pressure reduces the absolute volume of undissolved gas bubbles. The rate of introduction of the gas in controlled such that only a sufficient quantity of the gas which can be consumed within the process is introduced through diffuser 14. At the bottom of pipe 10 the oxygen-rich gas is either completely dissolved and the undissolved gas is in the form of extremely fine bubbles which form on the multitudinous nucleating centres which are available in the mixture of activated sludge in the sewage which biologicially breaks down the sewage under aerobic conditions.

The liquid then flows upwardly through space 18. Any undissolved gas starts to expand and a quantity of dissolved gas which exceeds the solubility limit, which reduces as the hydrostatic pressure reduces, is desorbed. At a level above the level of diffuser 14, the density of fluid in the pipe 10 and space 18 is equal, and the extra buoyancy above this point provided by further gas bubbles in the fluid in the space 18 provides the necessary driving force to maintain the circulation of the fluid through the apparatus.

The enlarged diameter portion 17 of pipe 18 is provided above the aforesaid point at which the density of fluid in pipe 10 and space 18 are equal.

As the upward flow of sewage through space 18 emerges into the larger diameter portion 17 thereof its upward velocity is reduced and provided that the bubbles of undissolved oxygen-rich gas remain small they will tend to adopt the upward velocity of the liquid in this portion rather than an independant buoyancy. In this way oxygen which is actively coming out of solution in the upward flow of the sewage is given the opportunity to redissolve replacing that being consumed in the bacteriological process by the activating sludge.

Therefore by controlling the upward velocity of the gas bubbles by providing the enlarged diamter portion 17 which reduces the upward velocity of sewage, the undissolved gas tends to redissolve in the sewage in place of dissolved gas which has been consumed by the bacteria in the activated sludge. In this way most of the oxygen supplied through diffuser 14 will be consumed within the process with a minimum loss of such gas to the ambient atmosphere thereby making the process economical.

WHAT WE CLAIM IS: 1. A method for treating aqueous waste material, which method comprises establishing a column of the waste material in a first substantially vertical passage and a column of the waste material in a second substantially vertical passage, the two passages communicating with one another at their lower ends and the second passage having an upper portion of wider cross-section than the portion therebelow; introducing further aqueous waste material into the upper end of the first passage; causing there to be different amounts of undissolved gas and thereby different fluid densities in the respective passages such that a flow of liquid down the first passage and up the second passage is maintained; introducing oxygenrich gas (as hereinbefore defined) into the downflowing liquid at a level below the bottom of said upper portion, and controlling the rate of introduction of oxygen-rich gas such that the gas is dissolved to a large extent in the downflowing liquid, any undissolved gas in the liquid at the bottom of the first passage being in the form of fine bubbles, and gas coming out of solution as the liquid ascends the second passage, and in which method the dimensions of the upper portion are such that it causes a reduction in the liquid velocity sufficient for most of the bubbles of oxygen that come out of solution in the second passage to redissolve.

2. A method as claimed in claim 1, in which the oxygen-rich gas contains at least 80%by volume of oxygen.

3. A method as claimed in claim 2, in which the oxygen-rich gas contains at least 95%by volume of oxygen.

4. A method of treating aqueous waste material, substantially as herein described with reference to the accompanying drawings.

5. Apparatus for performing the method claimed in claim 1, which apparatus comprises a pair of substantially vertical passages which are joined at their lower ends, and means for introducing oxygen-rich gas (as hereinbefore defined) at a controlled rate into one of the passages to a position spaced below the upper end of such passage, the second passage having a portion of increased cross-sectional area adjacent its upper end at a location above the level at which the aforesaid means for introducing oxygen-rich gas are located.

6. Apparatus as claimed in claim 5 in which there are two vertical pipes one within the other, the aforesaid passages being respectively provided by the bore of the inner pipe and the annular space between the pipes.

7. Apparatus as claimed in claim 6, in which the pipes are coaxial.

8. Apparatus as claimed in any one of claims 5 to 7, in which the means for introducing oxygen-rich gas includes a diffuser in the form of a porous cylinder.

9. Apparatus for treating waste water, substantially as described herein with reference to, and as shown in, the accompanying drawing.

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

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. Therefore by controlling the upward velocity of the gas bubbles by providing the enlarged diamter portion 17 which reduces the upward velocity of sewage, the undissolved gas tends to redissolve in the sewage in place of dissolved gas which has been consumed by the bacteria in the activated sludge. In this way most of the oxygen supplied through diffuser 14 will be consumed within the process with a minimum loss of such gas to the ambient atmosphere thereby making the process economical. WHAT WE CLAIM IS:

1. A method for treating aqueous waste material, which method comprises establishing a column of the waste material in a first substantially vertical passage and a column of the waste material in a second substantially vertical passage, the two passages communicating with one another at their lower ends and the second passage having an upper portion of wider cross-section than the portion therebelow; introducing further aqueous waste material into the upper end of the first passage; causing there to be different amounts of undissolved gas and thereby different fluid densities in the respective passages such that a flow of liquid down the first passage and up the second passage is maintained; introducing oxygenrich gas (as hereinbefore defined) into the downflowing liquid at a level below the bottom of said upper portion, and controlling the rate of introduction of oxygen-rich gas such that the gas is dissolved to a large extent in the downflowing liquid, any undissolved gas in the liquid at the bottom of the first passage being in the form of fine bubbles, and gas coming out of solution as the liquid ascends the second passage, and in which method the dimensions of the upper portion are such that it causes a reduction in the liquid velocity sufficient for most of the bubbles of oxygen that come out of solution in the second passage to redissolve.

2. A method as claimed in claim 1, in which the oxygen-rich gas contains at least 80%by volume of oxygen.

3. A method as claimed in claim 2, in which the oxygen-rich gas contains at least 95%by volume of oxygen.

4. A method of treating aqueous waste material, substantially as herein described with reference to the accompanying drawings.

5. Apparatus for performing the method claimed in claim 1, which apparatus comprises a pair of substantially vertical passages which are joined at their lower ends, and means for introducing oxygen-rich gas (as hereinbefore defined) at a controlled rate into one of the passages to a position spaced below the upper end of such passage, the second passage having a portion of increased cross-sectional area adjacent its upper end at a location above the level at which the aforesaid means for introducing oxygen-rich gas are located.

6. Apparatus as claimed in claim 5 in which there are two vertical pipes one within the other, the aforesaid passages being respectively provided by the bore of the inner pipe and the annular space between the pipes.

7. Apparatus as claimed in claim 6, in which the pipes are coaxial.

8. Apparatus as claimed in any one of claims 5 to 7, in which the means for introducing oxygen-rich gas includes a diffuser in the form of a porous cylinder.

9. Apparatus for treating waste water, substantially as described herein with reference to, and as shown in, the accompanying drawing.