GB2130060A - Effluent treatment - Google Patents

Abstract

A method of treating an effluent containing solids together with high counts of bacteria comprises at least partially removing liquid from the effluent and sterilising the remaining material by X-ray or gamma -ray radiation. Filters 1 pass high solids material 11 to a conveyor 13, which delivers the material via a crusher 2 to gamma -ray sterilising means 3. <IMAGE>

Description

SPECIFICATION Effluent treatment The present invention relates to the treatment of effluents containing solids in conjunction with high counts of potentially harmful bacteria, and more particularly (although not exclusively) to the treatment of sewage.

The high bacterial count in sewage renders its disposal a problem. Conventional methods of treating sewage include a first, settling stage resulting in the production of a low solids content (including bacteria), water and a primary sludge containing about 5% solids. Provided that the solids (including bacteria) content of the water is less than a stipulated value (the current U.K.

maximum is 30 ppm), the water may be discharged into a river or other water course. The remaining primary sludge is next subjected to a digestion operation in which the sludge is maintained at 30-350C so that the bacteria multiply and act to break down the fat and grease content from about 30% by weight of solids to approximately 5% on the same basis.

The thus treated sludge still contains a high loading of bacteria but, because of its reduced fat and grease content, may be disposed of in a number of ways, namely (1) by discharge to settling lagoons (to which toxic industrial waste may be added) which are subsequently back filled; and (2) by application under very carefully controlled conditions as a fertiliser to certain types of agricultural land.

Both of options (1) and (2) will generally involve transport of the sludge which, because it has a high water content (greater than 90%) means that much of the fuel costs are in respect of the transport of water.

Option (1) may be environmentally unsatisfactory.

So far as option 2) is concerned, the sludge does indeed have a high fertiliser value. However regulations currently in force in the U.K. require that the land to which the sludge has been applied cannot be used for grazing for at least 6 months, which may obviously be unacceptable. If this regulation is not observed, the animals grazing on the land may be adversely affected by the bacteria. Additionally, forthcoming legislation may ban the application of bacteria containing sewage sludge to the land.

It will therefore be appreciated from the foregoing description that disposal of sludge is expensive and that the sludge cannot currently achieve its potential as a fertiliser because of problems associated with bacteria.

It is also known to incinerate sewage sludges (without prior digestion to reduce grease and fat) and such incineration produces a residue with no bacteria. Such incineration is however extremely expensive and the residue has no fertiliser value.

Furthermore, it is also known to transfer primary sludge (without prior digestion), to ocean going bulk carriers which dump the sludge at sea.

This is however unduly expensive because the bulk of the sludge (greater than 90% by weight) is water and this accounts for the bulk of the transport costs. This problem of high transport costs applies also to options (1) and (2) above.

It is an object of the present invention to provide a process for treating effluents containing solids together with high contents of potentially harmful bacteria which yields a product which may be safely and more cheaply disposed of than hitherto.

According to the present invention there is provided a method of treating an effluent containing solids together with high counts of bacteria, comprising at least partially removing liquid from the effluent and sterilising the remaining material by X-ray or y-ray radiation.

The use of X-ray or y-ray radiation is the method of the invention serves to sterilise the material remaining from the liquid removal step by killing the bacteria present therein. The material is thus rendered safe for disposal or further use as detailed more fully below.

Additionally the weight of the remaining material is of course reduced compared to that of the incoming effluent because of the liquid removal stage, and this has advantages for the costs of transporting the material.

The effluent to be treated will generally be aqueous, and the invention is particularly applicable to the treatment of sewage. Preferably the sewage to be treated is in the form of a sludge and preferably one which has been subjected to a prior digestion treatment to reduce its grease and/or fat content.

Preferably also sufficient liquid is removed in the liquid removal stage to leave a solid material which is passed to the sterilisation stage. If the effluent being treated is a sludge with reduced fat and/or grease content, the resulting solid material may be used as a fertiliser. In this case, reduction in the content grease and/or fat is necessary otherwise they would "sour" the land as they would not have broken down sufficiently quickly by bacterial attack.

The removal of liquid from the effluent may be effected by any suitable means will remove the desired amount of water. Preferably the liquid removal means will produce a solid, e.g a friable material, from the effluent.

Preferably the liquid removal is effected by a filtration method in which the effluent is filtered through a layer of porous elastomeric material, and, once sufficient solid has built up on the elastomer, the flow of effluent is stopped and the elastomer is expanded by means of an expansion fluid so as to remove collected solids and clear the pores of the elastomeric layer.

A suitable filtration apparatus for performing this method is one having a layer of porous elastomeric material mounted to be expandable on the application of fluid pressure to one side thereof, a filtration inlet and a filtration outlet respectively on opposite sides of said layer, and a fluid inlet through which an expansion fluid may be supplied to the opposite side of the layer to which fluid to be filtered is supplied to expand said layer to remove collected solids.

Such a filtration method and apparatus is described more fully in our European Patent Application No. 82 303400.4 (Publication No.

the disclosure of which is herein incorporated.

After the filtration operation, the residual solid material is subjected to sterilisation by X-rays or y-rays.

X-rays have wavelengths ranging from 10-8.5 to 1 O-'O~'m and gamma-rays are of higher energy with wavelengths of 10-10.7 to 1 0.-13.3. Both types of radiation are photon fluxes and are suitable for sterilising the solids because, provided they are of sufficient intensity, they will kill all living organic matter, bacteria etc., in the solids without causing chemical change in the composition of the solids or leaving the solids radioactive. This is in contrast to high energy particle radiation, e.g. neutron beams, which may produce radioactive isotopes in the solids.

Consequently, it is important to ensure that the radiation used in the method of this invention is provided by an X- or gamma-ray source which does not emit any significant amount of particle radiation.

A suitable minimum intensity of X- or gammarays radiation to effect complete sterilisation in the solids is approximately 20,000 i5/hr. The source must be sufficiently intense to provide a sterilising intensity of radiation throughout the solids.

Most preferably gamma-rays are used in the sterilising radiation and may be provided by an isotope source e.g. Soco or 13'Cs which are gamma emitters. Ay minor amount of particle emission, e.g. electron emission, will be absorbed within a very short distance.

The use of an isotopic source for providing gamma-rays has the advantage that it is possible to calculate an initial intensity of source which will continue to be effective for sterilisation for a given time, e.g. several years. In other words, if it is desired that the source be effective for x years, the source used must initially be of such intensity that after decay over x years it will be sufficiently intense to provide a given minimum of intensity of sterilising radiation In the solids.

The invention is further described by way of example only with reference to the accompanying drawings which schematically illustrates one embodiment of apparatus for carrying out the method of the invention.

The apparatus illustrated in Fig. 1 comprises a bank of filtration units 1 arranged in parallel, a cake crushing device 2, and a y-ray sterilisation unit 3.

Each of filtration units 1 is of the type described in our aforesaid European Patent Application and is not described further herein save to say that the units 1 each have a rubber membrane 4 through which filtration takes place and on which filtered solid material collects. The membrane 4 is expansible, as illustrated by dotted lines to remove collected solids.

The y-sterilisation unit 3 comprises an outer casing 5 of shielding material within which is an upper source 6 and a lower sterilisation chamber 7. Between the source 6 and chamber 7 is a y-shut off member 8 for selectively, isolating chamber 7 from source 6. A sliding shield gate 9 is provided for the purpose to be described.

The operation of the apparatus will now be described.

Liquid effluent is supplied along inlet line 10 and passed in parallel through filters 1. The liquid effluent may for example be a previously digested sewage sludge containing less than 10% by weight solids.

The efficiency of filters 1 is such that a solid, friable material 11 containing 4#60% 5096 by weight solids (the remainder being liquid) is obtained.

Filtrate from filters 1 is discharged along line 12.

The filtrate will generally have a sufficiently low solids content that it may be discharged directly into a river.

The solid material 11 is collected on a conveyor 13 and thence to a conveyor 14 to pass beneath crusher 2. The material 11 is subsequently transferred to a conveyor 15.

Material to be sterilised in chamber 7 is supplied therein by conveyor 15. To introduce material 11 to be sterilised into chamber 7, it is provided on belt 15, the shut-off member 8 is positioned to shield chamber 7 from source 6, gate 9 is opened, and conveyor 15 moved to introduce the material into chamber 7. Gate 9 is now closed and shut-off member 8 moved so that the material is irradiated by source 6. The combination of shut off member 8 and gate 9 ensure that personnel are never exposed to the ysource 6. Once irradiation has been completed, the source 6 is once again isolated from chamber 7, gate 9 is opened, and conveyor 15 moved in the reverse direction to remove the solids from chamber 7.

The sterilised solid 1 its may now be collected for either disposal or subsequent use, e.g. as fertiliser or cattle feed in the case where the effluent is a digested sewage sludge.

Although the invention has been described with specific reference to the treatment of sewage sludges, it should be appreciated that the invention is also applicable to the treatment of the liquor which is obtained after the preliminary settling operation in sewage treatment and which is normally discharged into a river. In this case however, the apparatus has to be capable of treating larger volumes of liquid with less recovery of solids.

Claims (10)

Claims

1. A method of treating an effluent containing solids together with high counts of bacteria, comprising at least partially removing liquid from the effluent and sterilising the remaining material by X-ray or y-ray radiation.

2. A method as claimed in claim 1 wherein the effluent is aqueous.

3. A method as claimed in claim 2 wherein the effluent is sewage.

4. A method as claimed in claim 3 wherein the effluent is a sewage sludge.

5. A method as claimed in claim 4 wherein the sewage sludge has previously been treated to reduce its fat and/or grease content.

6. A method as claimed in any one of claims 1 to 5 wherein the liquid removal is effected by a filtration operation in which the effluent is filtered through a layer of porous elastomeric material which is subsequently expanded to remove collected solids.

7. A method as claimed in any one of claims 1 to 6 wherein the liquid removal stage produces a friable material.

8. A method as claimed in any one of claims 1 to 7 wherein the radiation used is y-rays provided by an isotopic source.

9. A method as claimed in claim 8 wherein the isotopic source is BOCo or 137Cs.

10. A method of treating an effluent substantially as hereinbefore described.