GB2175891A

GB2175891A - Biological treatment of aqueous liquids

Info

Publication number: GB2175891A
Application number: GB8512777A
Authority: GB
Inventors: Paul Frederick Cooper
Original assignee: Water Research Centre
Current assignee: Water Research Centre
Priority date: 1985-05-21
Filing date: 1985-05-21
Publication date: 1986-12-10
Also published as: GB8512777D0

Abstract

Treatment is carried out by introducing the liquid into a treatment column comprising porous bodies, preferably of reticulated foamed plastic material, supporting microorganisms. The bodies are present in such a quantity, ideally 60-70% by volume of the column, that they are packed together and are thereby restrained from movement. Oxygen containing gas may be added to the column.

Description

SPECIFICATION Biological treatment of aqueous liquids The present invention relates to a method and apparatus for the biological treatment of aqueous liquids. The invention relates particularly, but not exclusively to treatment of liquids by contact with micro-organisms in such processes as biological fermentation and the treatment of aqueous effluents (such as sewage) by an activated sludge process.

It is known to contact an aqueous liquid with micro-organisms located and supported on deformable porous bodies. Such bodies are known which have a substantial voidage within their internal structure and an extensive area of access from the outer surface to the voidage. Examples of these bodies are known which are reticulated polyether pads sized 25mmX25mmX10 or 12.5mm.

In known processes utilising such bodies, the liquid to be treated is passed through an aerated tank containing a large number, e.g.

40 pads per litre, of the bodies. The process is an improvement over conventional activated sludge treatment processes since it is possible to complete the treatment in a smaller tank as a result of the higher biomass concentrations that can be maintained, the waste sludge produced by the process is of much higher concentration, which saves on sludge treatment and disposal costs, and the oxygen passed into the system is used more efficiently because the rate at which it rises through the liquid is reduced by the presence of the bodies and because some of the air enters within the reticulated bodies themselves.

However, the process does have certain disadvantages. It is considered necessary that the bodies are free to move in the liquid and for this purpose it is necessary to control their buoyancy so that they neither sediment nor float to the surface, but rather remain suspended in the liquid. However, it is common for the bodies to accumulate near the top of the tank which causes the biomass within the bodies to be starved of oxygen and to behave anaerobically. Furthermore, as the liquid to be treated flows through the tank, the support bodies are carried downstream and are not therefore evenly distributed throughout the length of the tank. It is also known to treat biologically aqueous liquids by passing them through a column containing a fixed support material for the micro-organisms. Typical supports that have been used are stone, slag, plastics material and baked clay pellets.These materials may comprise as much as 70% by volume of the column.

Such treatment columns suffer however from certain disadvantages. Only the external surfaces are available for colonisation by the micro-organisms which are thus limited in area and less firmly attached. The use of porous bodies provides a much greater surface area for the micro-organisms, and also allows the liquid and oxygen or air to pass through the bodies which ensures highly effective treatment.

It is an object of the present invention to provide a method and apparatus for biological treatment of aqueous liquids which overcomes the above disadvantages and which provides a compact treatment apparatus having a high efficiency.

According to a first aspect of the present invention, there is provided a method of biologically treating an aqueous liquid containing biodegradable material comprising introducing said liquid into a treatment column comprising such a plurality of porous bodies supporting micro-organisms that the bodies are packed together and are restrained from movement.

Preferably the method comprises the additional step of introducing oxygen containing gas into the column.

Advantageously the porous bodies comprise pads of reticulated foamed plastics material, e.g. polyether, and are preferably present in such a quantity that they comprise 60-70% by volume of the column.

This contrasts with known treatment processes in which similar bodies are free to move in the treatment liquid and in which they are present as approximately 30% by volume of the total.

The column may be contained within liquid permeable means, e.g. grids, at its top and bottom ends.

At least a part of the liquid is preferably recirculated through the column, and the method may also comprise the step, carried out periodically, of injecting air under pressure into the column to clean the bodies of excess biomass.

Alternatively or additionally, a proportion of the pads may be removed from the column, subjected to a physical separation process to extract excess biomass, and returned to the column.

According to a second aspect of the present invention there is provided an apparatus for carrying out a method as described above.

Preferably, the apparatus comprises a container, a plurality of porous bodies substantially filling said container, means to introduce oxygen containing gas at or adjacent to the base of the containing, means to introduce liquid to be treated and means to recirculate to said introduction means at least a proportion of the treated liquid.

Embodiments of the present invention will now be more particularly described by way of example only.

The reactor comprises a vertical column within a container. The column consists of reticulated polyether pads 25mmX25mmX12.5 mm, which are packed irregularly into the column. Ideally, they are packed sufficiently tightly that they are unable to move during the treatment process but not so tightly as to be compressed to the point where the pores become closed. Using pads of the above dimensions, the column comprises 80-100 pads per litre which is approximately 60-70% by volume of the column. The pads may be seeded with micro-organisms appropriate to the process to be carried out in the column.

Air under pressure is introduced at the base of the column and passes upwardly between and through the pads. One purpose for which the process may be used is treatment of municipal sewage which, after preliminary treatment, is introduced to the base of the column. The liquid also passes upwardly and is withdrawn at the top of the column and passed to a settlement tank. From the tank, a proportion of the liquid is run-off for further treatment if necessary, while the remainder is recycled to the column. Typically, the total residence time of the liquid, including the recirculation time is 1 hour. The recirculation allows sufficient oxygen to be present for efficient treatment.

The process has been described as having co-current flow of liquid and oxygen containing gas. However, countercurrent flow is equally effective and thus, the liquid may be introduced at the top of the column and flow downwardly to be removed at the base.

As the process proceeds, biomass builds up on the pads and in order for the process to continue beyond a certain point, the pads must be cleaned of excess biomass. The presently preferred method of carrying out the cleaning is to introduce high pressure air togother with water at the base of column. This air scour forces the excess biomass out of the pores and it may be removed from the top of the column for disposal. If it is desired to retain the pads immobile, it is necessary to provide a grid or other means to hold them down during the air scour process. Alternatively, they may be allowed to move, in which case abrasion between adjacent pads may remove further biomass.

In another embodiment of the invention, it is possible to provide a similar apparatus to that described above but with the omission of the introduction of oxygen containing gas. In this case, the column is preferably fed with liquid from the top and the column acts as a biological filter. The pads form a good filtration medium since they have a large surface area.

Such a filter is thus more efficient than conventional filters utilising, e.g. sand, as thefiltration medium.

The two embodiments of the present invention may be used in series as secondary and tertiary treatment stages in the purification of sewage or other effluent.

Claims

1. A method of biologically treating anaqueous liquid containing biodegradable material comprising introducing said liquid into a treatment column comprising such a plurality of porous bodies supporting micro-organisms that the bodies are packed together and are thereby restrained from movement.

2. A method as claimed in claim 1, wherein the porous bodies comprise pads of reticulated foamed plastics material and are present in such a quantity that they comprise 60-70% by volume of the column.

3. A method as claimed in either claim 1 or claim 2, further comprising the additional step of introducing oxygen containing gas into the column.

4. A method as claimed in any one of the preceding claims, wherein at least a part of the liquid is recirculated through the column.

5. A method as claimed in any one of the preceding claims, further comprising the step, carried out periodically, of injecting air under pressure into the column to clean the bodies of excess biomass.

6. A method as claimed in any one of claims 1 to 4, further comprising the steps of periodically removing a proportion of the bodies from the column, subjecting them to a physical separation process to extract excess biomass, and returning them to the column.

7. A method of biologically treating an aqueous liquid containing biodegradable material substantially as described herein.

8. An apparatus for biologically treating an aqueous liquid containing biodegradable matter comprising a container, a plurality of porous bodies substantially filling said container, means to introduce oxygen containing gas at or adjacent to the base of the container, means to introduce liquid to be treated and means to recirculate to said introduction means at least a proportion of the treated liquid.

9. An apparatus as claimed in claim 8, wherein the porous bodies comprise pads of reticulated foamed plastics material and are present in such a quantity that they comprise 60-70% by volume of the column.

10. An apparatus as claimed -in claim 9, wherein said plastics material comprises polyether.

11. An apparatus as claimed in any one of claims 8 to 10, wherein the bodies are retained in the container by liquid permeable means at least at the downstream end of the container.

12. An apparatus for biologically treating an aqueous liquid containing biodegradable matter substantially as described herein.