GB2233340A

GB2233340A - Treatment of organic slurry

Info

Publication number: GB2233340A
Application number: GB8915074A
Authority: GB
Inventors: Michael John Archer Ling
Original assignee: LING DEVELOPMENTS Ltd
Current assignee: LING DEVELOPMENTS Ltd
Priority date: 1989-06-30
Filing date: 1989-06-30
Publication date: 1991-01-09
Anticipated expiration: 2009-06-30
Also published as: GB8915074D0; GB2233340B

Abstract

In a method for the treatment of organic slurries, such as sewage slurries the slurry is subjected to an anaerobic digestion process at 10 to yield methane gas which is used to produce heat at 13, part of which is used to maintain the anaerobic digestion process. The digested slurry is drawn off and separated at 18 into a liquid fraction and a solid fraction; the liquid fraction is subjected in an aerobic digestion at 20 to reduce the BOD to an environmentally-acceptable value, whereafter the liquid is discharged to a water course. The solids fraction has the moisture content reduced to not more than 20% at 21 and then is combusted in a gasifier 23 to yield producer gas and ash; the producer gas is collected for subsequent burning to produce useful energy. <IMAGE>

Description

TREATMENT OF ORGANIC SLURRY This invention relates to the treatment of organic slurries, such as sewage slurries, and in particular to the treatment of such a slurry to yield end products which are either useful in themselves, or which may be disposed of in environmentally-acceptable manners.

Large volumes of sewage slurry are produced from all modern centres of population, and the disposal of that slurry in environmentally-acceptable manners is becoming an increasingly serious problem. Equally, the disposal of other organic slurries such as animal wastes gives rise to significant problems. The biological oxygen demand (BOD) of the liquid content of the slurry must be reduced to a sufficiently low level to ensure that no harm is caused to flora and fauna when that liquid is discharged to a local water course. The disposal of the solid content, once separated from the liquid content, is more difficult; sometimes it is burnt, sometimes it is subjected to expensive treatments in order to render it inert and possibly suitable for use as a fertiliser, and - particularly in the case of the United Kingdom, which is surrounded by seas - it is taken out to sea and dumped.It is now being realised that the latter action is harmful to the ecology and that the pollution, particularly of the North Sea, rising to an unacceptable extent.

As a result of ever greater pressure being exerted on sewage disposal authorities, considerable efforts are being expended to find ways of disposing of sewage slurry in more acceptable ways. Most of the solutions so far proposed have however involved very high costs not only for the installation of equipment able to deal with the slurry - but also in terms of the day-to-day running costs, which have to be borne by the community which gives rise to the slurry requiring treatment.

Bearing in mind the foregoing, we have now determined that if sewage slurry is initially treated to an anaerobic digestion process, the subsequent disposal of that slurry can be achieved in an environmentallyacceptable manner, but with the added advantage that it is possible to process the solids fraction to obtain an end product which is useful commercially, whereby the running costs for the processing may, at least in part, be offset by the value of that product.

Accordingly, one aspect of this invention provides a method of processing an organic slurry, which method comprises: subjecting the slurry to an anaerobic digestion process within a closed chamber to yield methane gas; extracting from said chamber the methane gas produced as a result of the anaerobic digestion; burning the extracted methane to produce useful heat output; supplying sufficient heat to the anaerobic digestion process to ensure that the process is selfsustaining, at least some of that required heat being obtained from the burning aforesaid of the methane; drawing off digested slurry from said chamber and separating the liquid fraction from the solids fraction; further treating the liquid fraction to render it suitable for discharge to the environment; processing the solids fraction to render it suitable for combustion in a gassifier;; combusting the processed solids fraction in a gassifier to yield producer gas and ash; and collecting the producer gas for subsequent burning to produce further useful energy.

Though the invention is applicable to various kinds of organic slurries, it finds particular utility with sewage slurries and will therefore be described with reference to such slurries. The term "sewage slurry" should accordingly be interpreted broadly, as appropriate.

Anaerobic digestion processes for organic slurries are known, and have been used for example for the treatment of animal sewage, on farms. Such digestion processes yield large quantities of methane gas (socalled 'bio-gas') and in the method of this invention, that gas is collected for burning in air to obtain heat energy. Some of the collected gas advantageously is recirculated occasionally through the chamber in which the anaerobic digestion process takes place, so as to agitate the slurry; typically, a portion of the collected gas may be bubbled through the slurry in the closed chamber for a few minutes every hour.

The burning of the extracted methane most preferably is performed in a so-called combined heat and power (CHP) unit comprising an internal combustion engine driving an electricity generator, in combination with heat exchangers associated with the engine cooling and engine exhaust. If however no electricity is required, then the gas may be burnt for instance in a boiler of conventional design, merely to heat water.

In order to sustain an anaerobic digestion process within the sewage slurry, the temperature in the digestion chamber must be maintained at about 350C.

Heat must therefore be added to the digestion chamber, and in the present invention, this is obtained from the step of burning the methane extracted from the digestion. Preliminary calculations show that in the treatment of typical sewage slurries, only a relatively small fraction of the available heat energy from the methane produced will be required to maintain the temperature at 350C; it is therefore much preferred for the burning of the methane to be performed in a CHP unit capable of generating electricity in addition to heat, whereby that electricity may be employed to run the services which a plant implementing this treatment method will require - for example, the pumps and process controllers.

The method of this invention is preferably performed on a continuous basis. Thus, sewage slurry may be fed into the closed chamber continuously, and digested slurry removed continuously. For this, the chamber must have a sufficient volume having regard to the likely input slurry flow rate. Appropriate monitoring equipment may be provided to ensure that slurry is only drawn off after sufficient anaerobic digestion has occurred - and if necessary, the input slurry may have to be stored for some period of time, for example during a peak in flow rates. In the alternative, the method may be performed as a batch process.

The separation of the digested slurry into its liquid and solids fractions may be performed by any of various known suitable processes. For example, a beltpress separator may be used, or a centrifugal separator.

The liquid fraction may subsequently be treated by any suitable known usual sewage plant treatment process for the aerobic digestion of such liquids; for instance, one such process uses a rotary sprinkler bed. Then, once the BOD of the liquid has been reduced to an acceptable level, that liquid may be discharged to any convenient water course, without harming the ecology.

Depending upon the nature of the liquid/solids separator, the moisture content of the solids fraction may typically be around 30%. Known forms of gassifier require small pieces of solid fuel able to form a bed of carbon, which fuel may not have a moisture content of more than about 20%. It may therefore be necessary to perform a drying process on the solids fraction. In the alternative, a drying agent, such as ash, could be added to the solids fraction, and if the calorific value of the solids fraction also needs to be raised, then the drying agent could be a combustible material, such as finely divided waste products from industrial processes, coal dust, or the like.

After the reduction of the moisture content, the solids fraction may be pelletised, or formed into brickettes, so as to render it suitable for use in a gassifier. Such processes are well known in the fuelmaking art and will not be described further here.

So-called gassifiers are well known for the production of producer gas - a mixture of about 30% carbon monoxide and 70% nitrogen, though other gasses may be present in small quantities. In the gassifier, the fuel is reduced to a bed of carbon at a temperature of above 10000C, a stream of air being passed through the bed with the combustion conditions set so that the oxygen in the air combines with the carbon to form carbon monoxide. Other gasses - such as methane and hydrogen - may also be produced, depending upon the chemical composition of the fuel employed.

A product of the combustion in the gassifier is ash, but this is of a relatively low density and of small volume compared to the original solids fraction; it is consequently relatively easy to dispose of, especially since it is wholly sterile. By contrast, the producer gas may be used for a variety of purposes, though since it is highly toxic in view of the carbon monoxide content, it must nevertheless be treated with care. For example, the collected producer gas may immediately be used in an internal combustion engine for the generation of electricity, or it may be used in a CHP unit for the generation of both heat and power.

This invention extends to a sewage slurry treatment plant whenever adapted and configured to perform the method of this invention, as described above.

The sewage slurry treatment process of the invention as described above will now be described in greater detail, reference being made to the accompanying drawing which shows a treatment plant adapted for the treatment of sewage slurry in accordance with this invention.

Referring to the drawing, there is shown an anaerobic digestion chamber 10 having a sewage slurry inlet 11 and within which appropriate biological conditions are maintained to ensure that an anaerobic digestion process takes place. Apart from ensuring the appropriate bacteria are present, heat is added to the chamber by way of a heat exchanger 12, supplied with heat from an appropriate source during the initial stages of commissioning the plant, but subsequently obtained from a CHP unit 13, to be described below.

Operation of the anaerobic digestion process generates large quantities of methane, which are collected from the upper portion of the chamber 10 and fed by a pump 14 to a storage vessel 15. In order to agitate the digesting slurry, typically for two or three minutes every hour, methane is drawn from the storage vessel 15 and is pumped by pump 16 into a perforate tube 17 within the vessel so as to bubble through the contents.

The CHP unit 13 generates both heat and electricity, using as a fuel the methane from the storage vessel 15. By providing such a storage facility, fluctuations in the methane generation may be damped out, allowing the CHP unit to run continuously with a substantially constant power output. A portion of the heat produced is fed back to the heat exchanger 12 to maintain in the digester an internal temperature of about 350C, and excess heat may be used for space heating, as required. The electricity generated may be used to supply power to the various services required for the operation of the sewage slurry treatment installation, including the operation of the pumps, monitoring equipment, lighting and so on. If there is an excess of electricity, then that (in the United Kingdom) may be sold to the Electricity Generating Board and fed into the National Grid.

Digested slurry is drawn from the digestion chamber 10, appropriate monitoring equipment being provided to ensure that the slurry drawn off has been sufficiently digested. That digested slurry is fed to a separator 18, such as a known form of belt separator, where the solids fraction is separated from the liquid fraction. The latter is pumped by pump 19 to a BOD reduction plant 20, where an aerobic digestion process takes place, so as to lower the BOD of the liquid to an environmentally-acceptable level. From there, the resultant liquid is discharged to any convenient water course.

The solids fraction is fed to a moisture content adjustment plant 21, where the moisture content of the solids fraction is adjusted to be about 20%. This is achieved for example by adding coal dust, which also raises the calorific value of the solids fraction.

The solids fraction is next subjected to a pelletisation step, where the solids are compressed into individual self-supporting pellets suitable for use as a fuel. These pellets may be stored, or may immediately be fed to a known form of so-called gassifier 23, where producer gas is formed. The producer gas is collected from the upper portion of the gassifier and is fed via pipe 24 to a suitable storage vessel, ready for subsequent combustion to produce heat and/or electricity. For example, the producer gas may be burnt in an internal combustion engine driving an electricity generator. The other product of combustion within the gassifier 23 is ash, which may readily be disposed of, since it has a low volume and density as compared to the original solids content, and also is entirely sterile.

Claims

1. A method of processing an organic slurry, which method comprises: subjecting the slurry to an anaerobic digestion process within a closed chamber to yield methane gas; extracting from said chamber the methane gas produced as a result of the anaerobic digestion; burning the extracted methane to produce useful heat output; supplying sufficient heat to the anaerobic digestion process to ensure that the process is selfsustaining, at least some of that required heat being obtained from the burning aforesaid of the methane; drawing off digested slurry from said chamber and separating the liquid fraction from the solids fraction; further treating the liquid fraction to render it suitable for discharge to the environment; processing the solids fraction to render it suitable for combustion in a gassifier;; combusting the processed solids fraction in a gassifier to yield producer gas and ash; and collecting the producer gas for subsequent burning to produce further useful energy.

2. A processing method according to Claim 1, in which some of the methane gas extracted from said chamber is recirculated through the chamber to agitate the slurry therein.

3. A processing method according to Claim 2, in which gas extracted from said chamber is bubbled through the slurry in the closed chamber for a few minutes every hour.

4. A processing method according to any of the preceding Claims, in which the burning of the extracted methane is performed in a combined heat and power (CHP) unit comprising an internal combustion engine driving an electricity generator, in combination with heat exchangers associated with the engine cooling and engine exhaust.

5. A processing method according to any of Claims 1 to 3, in which the burning of the extracted methane is performed in a boiler of conventional design, to heat water.

6. A processing method according to any of the preceding Claims, in which the temperature in the digestion chamber must is maintained at about 350C.

7. A processing method according to any of the preceding Claims, in which the method is performed on a substantially continuous basis.

8. A processing method according to any of the preceding Claims, in which the liquid fraction of the digested slurry is processed by aerobic digestion until the BOD of the liquid has been reduced to an acceptable level, whereafter the liquid is discharged to a convenient water course.

9. A processing method according to any of the preceding Claims, in which the moisture content of the solids faction of the digested slurry is reduced to not more than about 20%.

10. A processing method according to Claim 9, in which a drying agent is added to the solids fraction.

11. A processing method according to Claim 10, in which the drying agent is a combustible material to raise the calorific value of the solids fraction.

12. A processing method according to any of Claims 9 to 11, in which the moisture-reduced solids fraction is pelletised, or formed into brickettes, so as to be rendered suitable for use in a gassifier.

13. A processing method according to any of the preceding Claims, in which the conditions in the gassifier are set to reduce the fuel to a bed of carbon at a temperature of above 1000 0C, whilst a stream of air is passed through the bed whereby the oxygen in the air combines with the carbon to form carbon monoxide.

14. A processing method according to any of the preceding Claims, in which the collected producer gas from the gassifier is used in an internal combustion engine for the generation of electricity, or is used in a CHP unit for the generation of both heat and power.

15. A processing method for organic slurries, substantially as hereinbefore described, with reference to the accompanying drawings.

16. An organic slurry treatment plant whenever configured to perform a processing method according to any of Claims 1 to 14.

17. An organic slurry treatment plant according to Claim 16 and substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.