EP0121548A1 - Method and apparatus for treating effluent - Google Patents

Abstract

An effluent treatment method and apparatus comprises mixing the effluent with a gas (11, 18) to obtain a mixture of effluent and gas, as well as passing said mixture through a bed of material packaging (19a) which is completely immersed in said effluent.

Description

METHOD AND APPARATUS FOR TREATING EFFLUENT The present invention relates to a method and apparatus for treating effluent containing organic substances in solution or in suspension.

It is known to cause the biological degradation of an effluent having dissolved organic substances by encouraging biological organisms to grow on a surface over which the effluent is caused to flow. The term biological organisms in this specification is intended to include living organisms such as bacteria, protosa, fungi 0etc.

Two basic methods using this principle are known. The first method (commonly referred to as a Biotower method) is to pack a tower with a packing media (such as pieces of plastics) and to trickle the effluent down the 5tower so that it perculates its way down through the tower. Gas for maintaining the bacterial organisms alive (such as air in the case of aerobic degradation) is caused to flow up the tower and the organisms are nourished by the dissolved organic substances and thereby oσauses its removal. The second method is to use a slowly rotating drum formed of a plastics material to define the surface on which the organisms grow and half submerge the drum in the liquid to be treated. Active organisms accumulate on the surface of the drum and the 5oxygen required for aerobic decomposition of the effluent is supplied by exposure of the surface to air.

The biotowers and rotating biological contactors described above whilst obstensibly acting as aerobic reactors can both suffer from odour problems which indicate that anaerobic conditions prevail in some sections of the reactors.

According to one aspect of the present invention there is provided a method of treating effluent comprising mixing the effluent with a gas to create an effluent/gas mixture and circulating said mixture through a bed of packing material which is totally immersed in said effluent.

According to another aspect of the present invention there is provided apparatus for treating effluent including a housing for containing said effluent, the housing containing a bed of packing material for encouraging growth of biological organisms which is totally immersed in the effluent contained in the housing, means for introducing a gas for maintaining biological growth on the packing, means for mixing the gas and effluent to create an effluent/gas mixture and means for causing circulation of the effluent/gas mixture through the bed of packing material.

Reference is now made to the accompanying drawings in which:

Figure 1 is an axial sectional view of an embodiment according to the present invention; and Figure 2 is a plan view of the apparatus shown in Figure 1.

The apparatus illustrated in the drawings basically comprises a cylindrical housing 10 having a base 14. The housing 10 is filled up to the level indicated as T. .L. with effluent to be treated, the level T.W.L. being determined by the position of outlet 44.

An axially extending cylindrical conduit 18 is mounted on top of a diffuser unit 30 to define an outer annular passage 19. The passage 19 is provided with a perforated floor 21, preferably formed by a mesh of a suitable material.

The conduit 18 is supported within the housing by being attached to one or more cross-supports 24 which is (/are) secured to the sides of the housing 10. The conduit 18 is suspended so that its upper opening 18a is located below the level T.W.L. The annular passage 19 is filled with a conventional packing material 19a such as FLOCOR E (Modular packing) or FLOCOR R (Random packing) up to a level which is below the upper opening 18a of the conduit and so forms a bed of packing material which surrounds the conduit 18.

An apparatus 30 for introducing a gas into the housing is located beneath the conduit 18. The apparatus 30 illustrated is the same as that disclosed in our U.K. Patent 2 069 353 B and reference should be made thereto for a full disclosure of the apparatus 30. The cylindrical wall 11 of apparatus 30 is connected to the lower end of the conduit so that the liquid/gas mixture expelled from apparatus 30 is caused to flow upwardly along the conduit 18. The base 38 of apparatus 30 is conveniently secured to the base 14 by means of welding or bolts.

In use, gas under pressure is introduced down pipe 40 to be expelled from the diffuser 12 which causes an upsurge of liquid/gas mixture upwards along the conduit 18.

Accordingly, liquid is drawn into apparatus 30 from the annular passage 19 i.e. from the base of the packed bed so that circulation of liquid within the housing is created generally in the direction as indicated by the arrows. In this way, liquid containing fine bubbles of introduced gas and also introduced gas dissolved therein is continuously circulated through a totally immersed packed bed. Growth of biological organisms which coat surfaces of the packing media are maintained alive by the introduced gas. Since the packing medium is completely immersed and since the liquid is continuously circulated through the packing

OMPI medium it has been found that a more efficient anaerobic degradation (if the introduced gas is say carbon dioxide or methane) or a more efficient aerobic degradation (if the introduced gas is say oxygen or air) of the liquid to be treated is achieved when compared with either the conventional bio tower or rotating drum systems. For instance, in the aerobic treatment of a strong effluent containing mainly fruit sugar from the washing of dried fruit it is possible to achieve over 90% reduction of the BOD value with a raw effluent strength of over 6000 mg/1 BOD at a treatment rate in excess of 5 kg BOD/m of packing/day in a single reactor. Furthermore, the sludge produced by the process settles readily to a relatively small volume. The process according to the present invention operates on a continuous basis but it has been shown to recover quickly from shock loadings. In the case of the plant described above, effluent is introduced only during the five day production period operated by the factory each week but readily accepts the raw effluent on the Monday after the weekend shutdown without any apparant reduction in performance. The air supply is of course supplied continuously in order to keep the contents aerobic at all times. An important consequence of this is that the plant is essentially odour free.

Raw effluent can be introduced either at the top or at the bottom of the housing and treated effluent is displaced through an outlet pipe 44 at high level. This feature is another significant advantage of the process as it is threfore possible to gravitate from one treatment unit to another placed in series or to a settlement tank placed in series without the need for additional pumping. The rate at which liquid is introduced into the housing determines the length of time the liquid is recirculated within the housing and the 3 loading of the system in terms of kg BOD/m of packing/day (where BOD is the Biochemical Oxygen Demand of the effluent after five days). Accordingly, the size of the housing and the volume of packing material depends on the volume and strength of the effluent to be treated and the degree of treatment required. In this respect it may be convenient on a large installation to have a number of apparatus 30 and conduit 18 units evenly spaced in one large housing to achieve the oxygenation and circulation effect described above for a single unit.

From the foregoing description it will now be appreciated that apparatus 30 in conjunction with conduit

18 performs four basic functions wihin the housing, viz.

(a) it introduces gas into the housing, (b) it mixes the effluent and gas to create an effluent/gas mixture, (σ) it creates a vigorous top to bottom circulation of the contents of the housing thus maintaining solids in suspension and providing the essential food and oxygen supplies required by the biological organisms growing on the surface of the packing ( in the case of an aerobic process) and (d) it evenly blends the incoming untreated effluent with the tank contents to provide a liquor of uniform composition throughout the housing.

It is envisaged that the apparatus 30 may be replaced by independent means which form these functions either singly or in combination. For instance it is envisaged that a mechanical surface aerator mounted at the top of the conduit 18 may be used to act as a low lift pump and to aerate the liquid. Although the above description is based on aerobic treatment, the same system can be used in an enclosed vessel where the turbulence and movement of liquid through the packed bed is caused by the injection of methane gas produced as a biproduct of the anaerobic fermentation process.

Claims

1. A method of treating effluent comprising mixing the effluent with a gas to create an effluent/gas mixture and circulating said mixture through a bed of packing material which is totally immersed in said effluent.

2. Apparatus for treating effluent including a housing for containing said effluent, the housing containing a bed of packing material for encouraging biological growth which is totally immersed in the effluent contained in the housing, means for introducing a gas for maintaining the biological growth alive, means for mixing the gas and effluent to create an effluent/gas mixture and means for causing circulation of the effluent/gas mixture through the bed of packing material.

3. Apparatus according to Claim 2 wherein the bed of packing material extends upwardly from the bottom of the housing and is spaced beneath the level of the effluent contained in the housing, a conduit being provided to define a flow path for effluent so as to enable effluent to be drawn from the bottom region of the bed and fed to the top of the bed.

4. Apparatus according to Claim 3 wherein the means for introducing the gas, for creating the effluent/gas mixture and for circulating the mixture comprises an apparatus according to any claim of U.K. Patent 2 069 353 B which is located so as to communicate with said conduit.

5. Apparatus for treating effluent substantially as described with reference to and as illustrated in any of the accompanying drawings.