WO1995006010A1

WO1995006010A1 - Biological aerated filter

Info

Publication number: WO1995006010A1
Application number: PCT/GB1994/001807
Authority: WO
Inventors: David Peter Froud
Original assignee: David Peter Froud
Priority date: 1993-08-25
Filing date: 1994-08-18
Publication date: 1995-03-02
Also published as: GB2294458B; GB2294458A; AU7390094A; GB9600512D0

Abstract

The invention relates to a biological aerated filter. A hollow upright cylindrical tube (1, 20, 40, 60, 80, 90) is provided with an internal packing of solid support media (5, 26, 94) which provides a large surface area for supporting microbial biomass. The tube is provided with an inlet (6, 21, 41, 61, 81, 91) and an outlet (7, 22, 42, 68, 82, 92) for passage of liquid waste to be treated inside the tube. An air diffuser (4, 67, 93) having a flexible multi-perforated membrane (36) is arranged at the base of the tube. In use, a stream of air bubbles is passed upwardly from the air diffuser through liquid in the tube. The membrane of the air diffuser is sealed to the lower part of the tube so as to prevent solids passing around and below the air diffuser.

Description

BIOLOGICAL AERATED FILTER

This invention relates to a biological aerated filter, and the use thereof in the treatment of liquid waste. The invention is particularly but not exclusively concerned with sewage treatment, and means for improving the quality of effluent from a sewage treatment works, reducing the capital cost of such works and reducing the size of the footprint of the plant.

In a conventional sewage treatment plant, crude sewage is first screened to remove trash, wood, large rags etc. It is then passed to a primary sedimentation tank where suspended solids are allowed to gradually settle and are removed in the form of a liquid raw sludge. The raw sludge is passed to a sludge digestion plant and the digested sludge is subsequently dried. Effluent from the primary tank is subjected to a secondary treatment. A commonly used form of secondary treatment is a so-called "biological filter". This is not a filter as such, but is a tank containing solid media to provide a large surface area on which colonies of microorganisms (generally aerobic microorganisms) , form a biomass and are exposed to the liquid effluent and to the surrounding air. The microorganisms break down noxious substances in the effluent. Effluent from the biological filter is passed to a humus tank where settlement is again allowed to take place. Nitrification (conversion of ammonia to nitrates) takes place in the humus tank and may also occur in the biological filter. Settled sludge from the humus tank is returned to the primary sedimentation tank. Effluent from the humus tank is discharged to a water course. Disinfection of the final effluent is increasingly called for, prior to discharge. Such basic processes are described in "An Introduction to Sewage Treatment", published by the Institute of Water Pollution Control, 1987.

A common problem for existing sewage treatment works is that they have become overloaded with the increase in population being served by the works. They have very little land available for expansion, and in any case the engineering works needed for providing additional tanks is very expensive.

Another problem faced by sewage treatment plants is the variability of the incoming crude sewage. The volume and concentration of incoming crude sewage can vary depending on weather conditions and seasonal variations. For example, in holiday resorts, it is desirable to be able to provide increased capacity for sewage works to cope with seasonal increases in the load, without the need for expensive civil engineering work to increase the hydraulic and biological capacity of the works. It is also desirable that the additional capacity required to cope with seasonal load variations can be brought up to optional efficiency quickly.

Biological aerated filters have been developed to improve sewage treatment. A biological aerated filter is a biological filter including means for introducing additional air into the effluent under treatment. It generally comprises a self-supporting matrix, for example corrugated layers of plastic material, such as rigid PVC, which provides a large surface area as a support for microbial biomass. Alternatively, it may contain random fill media, again with a large surface area, which can be plastic mouldings or mineral chips. The matrix is contained in a treatment tank, and air diffusers are provided beneath the matrix, to enable a constant stream of air bubbles to be blown through the liquid waste in and around the solid media. Such biological aerated filters are versatile and rugged, producing good quality effluent and at a lower cost than conventional biological filters.

Although biological aerated filters can be constructed as movable modular units, they are quite expensive to construct and not easily movable. Furthermore, the degree of aeration of the solid media may not be sufficient in regions which are not immediately above the air diffusers. The performance from existing designs is hampered by the physical constraints of the depths of the biological filter, the difficulty of servicing filters in deep tanks or shafts, and the inability to use air for biological degradation efficiently because of wastage due to the need to achieve scouring velocities with the inflowing air.

A more recent design of biological aerated filter makes use of columns or tubes as a housing in which to pack the biological matrix. An air diffuser is fitted in a housing at the base of the column or tube. A disadvantage of such a construction is that "dead" (i.e. non-aerated) regions at the periphery of the matrix can result if the diffuser is of a smaller size than the matrix, and suspended solids can be deposited as sludge around and below the air diffuser. The present invention provides a solution to these problems.

According to the invention, there is provided a biological aerated filter, comprising a hollow upright cylindrical tube, a packing in said tube of solid support media providing a large surface area for supporting microbial biomass, an inlet to and an outlet from the tube for passage of liquid waste to be treated in said tube, and an air diffuser having a flexible multi-perforated membrane at the base of the tube, whereby in use a stream of air bubbles is passed upwardly from the air diffuser through liquid in the tube, characterised in that the membrane of the air diffuser is sealed to the lower part of the tube so as to prevent solids passing around and below the air diffuser.

Preferably, the membrane of the air diffuser is mounted between a lower plate with a hole for air supply and a perforated upper plate, the upper plate being sealed to the base of the tube. A sludge drain may be positioned at the base of the tube. The solid support media may be mounted on a perforated plate which is seated on a circumferential ledge inside the tube near the bottom end.

It is possible for the upper plate to have at least one hole into which the air diffuser membrane can extend in use, and a further hole outside the periphery of the membrane for an inlet or outlet connection to the interior of the tube.

In embodiments where the outlet of the tube is near the upper end, the outlet may be connected to an internal riser tube which extends internally up at least part of the height of the main tube.

The main tube may be joined to a base portion, housing the air diffuser, which is of larger diameter than that of the biological column, thus allowing a diffuser with a larger diameter than the biological column to be used. This ensures that all of the biological media is aerated and that there are no "dead" zones which may cause sludge to be deposited in the biological media.

The main tube may be joined by a circumferential flange to a corresponding flange on a lower, shorter column which can be of equal or larger diameter and which is closed at its base. The shorter column is free of biomass and is used as a housing for the air diffuser, which is fitted between the flanges on the respective columns. The multi-perforated flexible membrane of the air diffuser may be supported on its underside by a disc, containing a hole for air supply preferably fitted with a non-return valve, and optionally on its upper side by a disc with cut outs to allow the membrane to inflate and diffuse air. The non-return valve in the lower disc allows air to pass from the underside of the disc to the upper side where it partly inflates the membrane and then diffuses through the multiple perforations to emerge as a stream of air bubbles. The upper disc restricts the movement of the flexible membrane and provides support to prevent premature rupture. However, it may sometimes not be necessary, depending on the size of the membrane and other factors.

The filter media may be suspended on a perforated support plate within the tube. Alternatively, it may be suspended in a mesh net. The mesh may, for example, have 50 mm square holes, or even larger, depending on the size of the random media used. The mesh is inserted into the tube and hung from the top, or from supports within the tube. This obviates the need for a support plate across the bottom of the tube, and enables all of the base area of the random fill media to be aerated. This arrangement helps to avoid trapping of solids within the tube which can occur on a ledge or lip at the bottom of the tube for mounting a support plate.

Reference is now made to the accompanying drawings, in which:

Figure 1 is a side view of a biological aerated filter according to an embodiment of the invention;

Figure 2 is a side view on enlarged scale of the lower part of the apparatus;

Figure 3 is a plan view of the lower part of the apparatus shown in Figure 2;

Figure 4 is a side view of a biological aerated filter according to another embodiment;

Figures 5A and 5B are respectively plan views and side views of an upper plate forming part of the diffuser assembly;

Figures 6A and 6B are respectively plan views and side views of the membrane forming part of the air diffuser assembly;

Figures 7A and 7B are respectively plan and side views of a lower plate forming part of the air diffuser assembly;

Figure 8 is a side view of a biological aerated filter according to another embodiment of the invention;

Figure 9 is a side view of a biological aerated filter according to a further embodiment;

Figure 10 is a side view of a still further embodiment of the invention; and

Figure 11 is a side view of a further embodiment in which the filter media is supported in large mesh netting.

Referring first to Figures 1, 2 and 3, the apparatus includes a main tube 1 which is hollow and upright. At its lower end, the tube 1 has a circumferential outer flange 2, which mates with a corresponding flange at the upper end of a short lower column 3 of slightly greater diameter than the tube 1. An air diffuser 4 is mounted at the top of the lower column 3. Random filter media 5 is packed inside the tube 1, and supported by a perforated plate (not shown) near the bottom of the tube 1. Alternatively, the filter media may be suspended in a mesh net which may be hung at the top around the upper rim of the tube 1, or from supports within the tube 1. A waste water inlet 6 enters into the tube 1 near its lower end, and a corresponding outlet 7 is arranged near the top of the tube 1. A sludge drain 8 is arranged near the bottom of the tube l. An air inlet 9 passes into the side of the lower column 3.

As shown in more detail in Figure 2, the air diffuser comprises a lower plate with a central hole communicating with the air inlet 9. Above this is arranged an upper plate which is substantially open in its central part, and sandwiched between the two plates is a flexible multi-perforated membrane of rubber, synthetic rubber or similar material. The active diameter of the membrane of the diffuser, i.e. that part which is exposed through the opening in the upper plate, is the same as or slightly greater than the internal diameter of the tube 1. The membrane of the air diffuser is thus effectively sealed to the lower end of the tube 1. This ensures complete aeration of the biological filter media 5 without any dead spots, and also prevents suspended solids from passing around and below the air diffuser.

A slightly different arrangement is shown in Figures 4 to 7. The tube 20 is provided with an inlet 21 at its lower end and an outlet 22 at its upper end. A sludge drain 23 at the lower end, diametrically opposite the inlet 21, is provided with a valve 24. A perforated internal support plate 25 supports an internal packing of biological filter media 26. A further perforated plate 27 is arranged on top of the media. Alternatively, the internal packing may be suspended in a mesh net hung within the tube 20.

The main tube 20 has a circumferential flange 28 at its lower end. This matches a corresponding flange 29 around the upper rim of a short lower column 30, closed at its lower end and of about the same diameter as the main tube 20. The two flanges are attached together by a series of bolts 31. Sandwiched between the two flanges are a lower support plate 32 and an upper plate 33, the construction of which is shown in more detail in Figures 7 and 5, respectively. Each plate has a series of peripheral holes for the bolts 31. The lower plate 32 has a central hole 34 for connection to an air inlet 35 which passes into the lower column 30. The upper plate 33 is substantially open in its central section, having a central disc and four radial arms. The multi-perforated air diffuser membrane 36 is shown in Figures 6A and 6B, and again has peripheral holes for the bolts 31. The membrane 36 is sandwiched between the upper and lower plates 33 and 32. The result of this arrangement is that the active area of the membrane 36, i.e. where the upper surface is exposed through the openings in the plate 33, corresponds to the internal diameter of the main tube 20. Thus, the membrane is effectively sealed to the lower part of the main tube 20. This ensures effective aeration across the entire internal cross section of the biological filter media 26, and solids in suspension in the liquid waste are prevented from passing around and below the air diffuser.

In Figure 8, the main tube 40 has a lower inlet or outlet 41, and an upper outlet or inlet 42. Internally around the base of the tube 40 there is a peripheral ledge 43 with a sloping upper surface. A perforated media support plate 44 is seated on the ledge 43. Biological filter media is packed inside the tube 40 and supported by the support plate 44. The plate 44 simply rests on the ledge 43 and there is no need for permanent fixing. This provides for simple assembly and maintenance of the apparatus. Alternatively, the filter media may be suspended in a mesh net hung within the tube 40, in which case the ledge 43 and plate 44 are not necessary.

At its lower end, the tube 40 has an outer peripheral flange 45 which is bolted to a lower support plate 46 forming part of the air diffuser assembly. The plate 46 has a central hole 47 for connection to an air inlet. The air diffuser membrane rests over the plate 46, and is supported from above by an upper plate which may correspond to the plate 33 shown in Figure 5. The upper plate is held firmly, by the bolts between the flange 45 and plate 46, against the lower part of the ledge 43. The air diffuser is thus sealed to the lower part of the tube 40. This arrangement again ensures complete aeration of the biological filter media, and prevents solid matter suspended in the liquid waste from passing around and below the air diffuser.

Figure 9 shows a modification of the embodiment of Figure 8, in which an inlet or outlet is arranged in the base of the tube. The tube 60 is provided with an inlet or outlet 61 at its upper end. The tube has a lower flange 62. This is bolted to a support plate 63. The support plate 63 has a central hole 64 for connection to an air inlet. A media support plate 65 is mounted inside the tube 60, and provides support for the biological filter media (not necessary, if the media is suspended in a mesh net in the tube 60) . An upper plate 66 is fitted between the flange 62 and lower plate 63. The perforated membrane of the air diffuser 67 is fitted between the plates 63 and 66. The plate 66 has a main opening to expose most of the upper surface of the membrane. There is a side opening in each of the plates 63 and 66 which communicate to form an inlet or outlet 68 to or from the interior of the tube 60. In some embodiments it can be advantageous to have such an inlet or outlet 68 effectively in the base of the tube 60. The air diffuser is still sealed effectively to the lower part of the tube 60 and so achieves the technical effects of the invention as described above.

In Figure 10, the tube 80 is provided with an inlet 81 and outlet 82 both near the top. A sludge drain 83 fitted with a valve 84 communicates with the base of the tube. The support plate for the biological filter media and the arrangement of the air diffuser membrane are as in previous embodiments. An internal riser tube 85 communicates from the bottom of the interior of the main tube 80 and with the outlet 82. The upward passage of bubbles of air from the air diffuser helps to entrain the flow of suspended solids through the riser tube and out from the outlet 82. This assists in the prevention of build-up of sludge in the main tube 80.

In Figure 11, the tube 90 is provided with an inlet or outlet 91 near the top and a corresponding outlet or inlet 92 near the bottom. An air diffuser 93 is arranged at the bottom of the tube as in previous embodiments. Random filter media 94 is suspended in a mesh net 95 in the tube. The net is hung around the top of the tube from support posts 96. In this arrangement, a media support plate and corresponding internal ledges are not necessary, thereby substantially reducing potential build up of deposited solids.

Columns such as those described above can be used singly or in groups, depending on the effluent to be treated. They can be used in different ways in conjunction with conventional sewage works, or on their own as a free-standing and portable effluent treatment apparatus (with suitable screening means) .

Colonies of microorganisms form as biomass on the media and break down noxious elements in the effluent with the assistance of air supplied from the air diffuser. Effluent generally enters from the inlet and passes through the media compartment. The effluent may pass from the bottom to the top or from the top to the bottom depending on the particular arrangement.

The biological aerated filter column may be a permanent fixture, or alternatively it can be provided as a movable modular unit which can be quickly erected on site and used, for example, for emergency response to pollution incidents. In this latter case, the biological treatment can be started within a few hours, which is not possible with currently available apparatus. There are no moving parts within the columns, and additional modules can be fitted as required or removed when not required, e.g. to take into account seasonal variations in load.

The biological aerated filter columns can be installed in trailers or skid-mounted tanks to provide rapid upgrading of sewage works, or to provide emergency capacity. The columns can be used to treat screened crude sewage, settled sewage, top liquors, leachate, silage effluent, effluent from slurry tanks, or humus effluent to effect nitrification.

The invention provides a number of advantages:

1. The treatment apparatus is modular and the modules can generally be manhandled by two men.

2. The apparatus can be arranged in an infinite number of permutations, allowing configuration to produce effluent to meet the consent standard (legally required degree of purity) with minimum cost.

3. The number of settlement columns can be increased to decreased to comply with suspended solids consent levels.

4. If ammonia consent levels are made more strict after a sewage works has been constructed, additional biological aerated filter columns can be added to achieve nitrification.

5. The rate of air diffusion via the membrane diffuser can be matched to the volume of the support matrix so that the process air produces sufficient velocity to achieve scouring of surplus biomass with minimal wasting of process air.

6. Recirculation within the system is possible by varying the air flow and influent flow rates. Tall columns enhance oxygen transfer and increase biochemical oxygen demand (BOD) reduction rates, thus reducing operating costs. The columns can be provided in variable lengths to suit site conditions.

7. The columns can be installed in series or parallel.

8. The columns can be surface mounted, installed in existing void structures or sunk into the ground.

9. Maintenance is simplified, as the columns can be lifted out of deep shafts or tanks, obviating the need for breathing apparatus.

10. Surface mounted units can be suspended or laid on their sides to service the air diffusers.

11. The use of commercially available tubes (for example of PVC or glass reinforced plastic) for the containment of the biological support media reduces the cost of the housing.

Claims

CLAIMS ;

1. A biological aerated filter, comprising a hollow upright cylindrical tube (1, 20, 40, 60, 80, 90) , a packing in said tube of solid support media (5, 26, 94) providing a large surface area for supporting microbial biomass, an inlet (6, 21, 41, 61, 81, 91) to and an outlet (7, 22, 42, 68, 82, 92) from the tube for passage of liquid waste to be treated in said tube, and an air diffuser (4, 67, 93) having a flexible multi-perforated membrane (36) at the base of the tube, whereby in use a stream of air bubbles is passed upwardly from the air diffuser through liquid in the tube, characterised in that the membrane (36) of the air diffuser (4, 67, 93) is sealed to the lower part of the tube (1, 20, 40, 60, 80, 90) so as to prevent solids passing around and below the air diffuser.

2. A biological aerated filter according to claim 1 , in which the membrane (36) is mounted between a lower plate (32, 46, 63) with a hole for air supply, and a perforated upper plate (33, 66), the upper plate being sealed to the base of the tube (1, 20, 40, 60, 80) .

3. A biological aerated filter according to claim 2, in which the upper plate has at least one major hole for exposing the upper surface of the membrane, and a further hole outside the periphery of the membrane for connection to an inlet or outlet (68) communicating with the interior of the tube (60) .

4. A biological aerated filter according to any of claims 1 to 3, in which the solid support media is supported by a perforated plate (44) which is seated on a circumferential ledge (43) inside the tube (40) near the bottom end thereof.

5. A biological aerated filter according to any of Claims 1 to 3, in which the solid support media (94) is suspended in a mesh net (95) inside the tube (90) .

6. A biological aerated filter according to any of claims 1 to 5, including a sludge drain outlet at or near the bottom of the tube.

7. A biological aerated filter according to any of claims 1 to 6, in which the outlet (82) is arranged near the top of the tube (80) and communicates with an internal riser tube (85) which extends from an open lower end, near the lower end of the tube (80) up to a junction with the outlet (82) .