GB2428670A

GB2428670A - Anaerobic digestion of organic wastes

Info

Publication number: GB2428670A
Application number: GB0615455A
Authority: GB
Inventors: Paul Ditchfield; Barry John Howard
Original assignee: BARRY HOWARD WASTE MAN Ltd
Current assignee: BARRY HOWARD WASTE MAN Ltd
Priority date: 2005-08-04
Filing date: 2006-08-03
Publication date: 2007-02-07
Also published as: GB0516311D0; GB0615455D0; WO2007015098A1

Abstract

A process and corresponding apparatus for the anaerobic digestion of an organic waste comprises a hydrolysis reactor (14) in which the organic waste is subject to hydrolysis before the liquid fraction of the output from the hydrolysis reactor is fed to an anaerobic reactor (19). The organic waste to be treated is mixed with a liquid containing an enzyme prior to hydrolysis. The solid fraction of the output of the hydrolysis reactor is removed by way of a separator (16). Liquid outlet from the anaerobic reactor is returned via a return line (34) after optional heating by a heater (35) to be mixed with the influent flow of the material to be treated. Preferably, the inflow material is macerated in a macerator (14) and emulsified in an emulsifier (11) prior to being fed to the hydrolysis reactor (14).

Description

ANAEROBIC DIGESTION OF ORGANIC WASTES

This invention relates to the anaerobic digestion of organic wastes, and in a preferred embodiment provides a highly effective digestion system which offers a number of substantial advantages as compared with the prior art.

The use of micro-organisms in an anaerobic digestion process for the treatment of organic waste is well known. At its simplest, organic waste and a suitable culture of anaerobic bacteria are contained within a sealed vessel and the bacteria digests the organic material to produce a harmless liquid effluent and a gas mixture containing methane.

Anaerobic Digesters ("ADs") working on a batch principle are highly ineffective and have limited commercial application. Accordingly, continuous flow ADs have been developed which allow a continuous flow of influent material to be supplied to an AD for anaerobic treatment. Background information relation to the development and practice associated with ADs can be found in the book The Microbiology of Anaerobic Digesters by Michael H Geraldi published by Wiley- lnterscience, John Wiley & Sons, Inc in 2003. The basic idea underlying continuous flow ADs is that a colony of appropriate micro-organisms is established on a fill material having a high surface area, and that the material to be treated flows through the fill material so that the organic material contacts the micro-organisms. In order to ensure that the micro-organism colonies on the fill are not disturbed fluid flow rates of approximately lxlO'3m second" are generally considered desirable. However, these flow rates give rise to significant practical problems in conventional continuous flow AD systems.

In particular, in conventional systems influent material is added to the bottom of a closed tank and flows upwardly through an active digestion region in which the micro-organisms are located. The low flow rate required in the active region means that material is added to the bottom of the tank at a relatively slow rate. As a result, sedimentation of solid materials is liable to occur in the bottom of the tank. Not only

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does such sedimentation require periodic shut down of the process to facilitate mechanical removal of the sediment, but the sediment disrupts flow patterns within the tank, increasing the flow velocity in portions of the fill adjacent the infeed and reducing flow velocity at points remote from the infeed. Further, in order to optimise the digestion rate it would be desirable to control exactly the temperature and the pH of the material which is supplied to the digestion zone. This is very difficult in the context of a system in which the lower portion of the tank is partially filled with sediment material.

We have now devised an improved continuous flow AD system in which the difficulties of the prior art outlined above are obviated. Further, the preferred system of the present invention facilitates precise control of the temperature and pH within the active digestion zone thereby optimising AD digesting characteristics. As a result, the preferred embodiment of the present invention is able to achieve a substantially better rate of digestion per unit digester volume than systems of the prior art.

Accordingly, digester systems in accordance with the preferred embodiments of the present invention can be made smaller than those of the prior art and achieve the same throughput capacity, or, if made the same size as systems of the prior art, can achieve a substantially higher throughput capacity.

In accordance with a first aspect of the present invention there is provided a method for the anaerobic digestion of organic waste, the method comprising: mixing organic waste to be treated with a liquid containing an enzyme; treating the resultant mixture in a hydrolysis reactor to induce at least partial hydrolysis of the mixture; and feeding at least a liquid fraction of the treated mixture to an anaerobic reactor for anaerobic digestion, the liquid containing an enzyme being derived from the anaerobic reactor.

The pre-treatment of the organic material by a hydrolysis process induced by enzymes results in a feed to the anaerobic reactor which can be rapidly and efficiently digested to produce an output of combustible gas and a liquid which is acceptable to regulatory authorities for discharge to drain. In practice, the liquid is rich in enzymes and is utilised in the pre-treatment of the organic material in the hydrolysis reactor.

Any surplus liquid is discharged to drain.

In a particularly preferred embodiment of the invention treated material from the hydrolysis reactor is subject to a solid separation process before the liquid fraction is fed to the anaerobic reactor. By this means, at least the bulk of the solid non- digestible material from the organic material to be treated is removed before the remaining material is fed to the anaerobic reactor. This substantially reduces the tendency for sedimentation to takes place in the anaerobic reactor and allows for maximum utilisation of the interior space of the anaerobic reactor.

In a particularly preferred embodiment of the invention the organic material is emulsified prior to feeding to the hydrolysis reactor. Preferably, the liquid containing an enzyme is mixed with the organic material to be treated prior to the emulsification process.

In a particularly preferred embodiment of the invention the organic material to be treated is subject to a maceration process prior to emulsification. The maceration process may be affected with a suitable comminuter device. Preferably, the liquid containing an enzyme is at least in part added to the organic material to be treated subsequent to maceration in order to adjust the liquid content to an optimum level for emulsification and subsequent hydrolysis.

Preferably, the gas produced in the anaerobic reactor is used as fuel for an engine to derive useful work for export. Waste heat derived from the engine, and/or heat derived directly from the gas produced in the anaerobic reactor, may be used to maintain optimum process temperatures throughout the treatment plant. Surplus waste heat may be utilised for other purposes, for example space heating.

In accordance with a second aspect of the present invention there is provided apparatus for carlying out the method in accordance with the present invention.

The invention will be better understood from the following description of a preferred embodiment thereof, given by way of example only, reference being had to the accompanying drawing wherein the single figure illustrates schematically the preferred method of the present invention.

The drawing illustrates an anaerobic digestion plant I for the treatment of an organic material which is supplied through an inlet 2. Typically, the organic material will be a waste material, for example derived from an abattoir or food processing plant. The organic material arriving on the inlet 1 is, if appropriate, screened to remove potentially damaging non-organic material and is then fed by a screw conveyor 3 to a macerator 4 for primaiy maceration. The macerator may, for example, comprise a commercially available comminuter, for example a mono muncher. In the macerator, the organic material to be treated is mixed with a liquid received via an input 5 from an anaerobic digestion plant 6, to be described in more detail hereinafter.

The output of the macerator is fed via a line 7 to a mixing and dilution tank 8.

Further liquid derived from the anaerobic digester 6 is added to the macerated material via a line 9 to optimise the mixture within the tank 8. As will be explained in more detail hereafter, the liquid derived from the anaerobic digestion plant is rich in enzymes and accordingly the material in the tank 8 consists of a slurry of organic material, water and enzymes. The storage temperature and chemical conditions within the tank 8 are optimised to ensure initiation ofsolublisation and pre- hydrolysis of the organic material.

Material from the tank 8 is fed via a line 10 to an emulsifier 11. Additional liquid containing an enzyme, again derived from an anaerobic reactor may be added via a line 12 to the emulsifier. The emulsifier 11 may be a commercially available emulsifier, for example from Silverson machines. The emulsified material from the emulsifier is fed via a line 13 to a hydrolysis reactor 14. The hydrolysis reactor may be a commercially available hydrolysis reactor, for example a BI-IP high rate hydrolyser.

By controlling the particle size of the material fed to the hydrolysis reactor (by appropriate choice and control of the emulsifier 11) and by optimising the temperature and chemical conditions within the hydrolysis reactor 14, substantially complete hydrolysis of the organic material may be obtained in the hydrolysis reactor 14 with the result that the material leaving the hydrolysis reactor 14 via line 15 contains substantially only suspended solids which are not suitable for anaerobic digestion and a solution which is susceptible to anaerobic digestion. Typically, the operating temperature of the hydrolysis reactor may be about 80 C. The outflow mixture from the hydrolysis reactor is fed to a suitable separator 16 to remove the solids. The solids may be removed by any suitable separator and may, for example, be vacuum dried to produce a solid material which may be sold as fertiliser. Since the outflow temperature of the hydrolysis reactor is considerably higher than the temperature required for effective operation of the anaerobic reactor, heat may be recovered from the line 15 to reduce the temperature of the outflow mixture prior to removal of solids by the separator 16. Alternatively, heat may be removed with the solids to reduce the temperature of the liquid leaving the separator 16 relative to the temperature of the mixture arriving at the separator 16.

The liquid fraction of the material from the hydrolysis reactor 14 passes through the separator 16 and is fed by a pump 17 as an input 18 to an anaerobic reactor 19. The anaerobic reactor 19 may be of substantially known design and comprise a fill on which is established a colony of anaerobic bacteria for digestion of the solution supplied through the inlet 18. Because the material supplied through the inlet 18 is substantially only a solution of material susceptible to an anaerobic digestion, the anaerobic reactor may operate in a highly efficient mode to produce rapid anaerobic digestion. In order to optimise the anaerobic reactions the temperature with in the reactor is preferably controlled (e. g. to about 3 8 C) by means of a suitable heater 20 and the pH of the mixture within the reactor is controlled by a suitable pH correction device 21. Heat for the heater 20 may be derived from any suitable source, but in the preferred embodiment is derived from waste heat produced by an engine 22 which is powered using bio-gas produced in the reactor 19 and supplied to the engine via a line 23.

As will be understood by those skilled in the art, within the anaerobic reactor 19 anaerobic digestion of the inflow material is effected by the bacteria colony which produces a bio-gas output which exits the top of the reactor vessel via a line 24. The bio-gas mixture typically comprises carbon dioxide and methane. Liquid effluent from the reactor leaves via a line 25 and passes through a de-gasser 26, the gas output of which flows via line 27 to join the gas exiting the top ofthe reactor via line 24. Liquid from the de-gasser passes through a line 28 to an overflow tank 29.

The liquid in the overflow tank 29 is rich in enzymes but is of a quality permitted for discharge into a drainage system via an overflow line 30. Material from the overflow tank 29is also used via a recycle line 31. Liquid from the line 31 passes through the heater 20 and receives, if necessary, pH correction material from the dosing device 21 prior to passing through a mixer 32 and being returned to the inlet 18 of the anaerobic reactor. As will be understood by those skilled in the art by controlling the recycle flow rate and the inflow rate from the pretreatment equipment the biological processes within the anaerobic reactor may be controlled to ensure the necessary quality of effluent.

Liquid from the overflow tank 29 is also returned by a recycle pump 33 to a line 34 to provide an enzyme rich liquid to the pre-treatment equipment, as described above. It may prove desirable to increase, to a significant extent, the temperature of the liquid supplied on the line 34. Although this liquid will contain a mixture of micro- organisms and enzymes, its use is solely as a diluent and as a source of enzymes to the hydrolysis reactor 14. Accordingly, the liquid can be heated to a temperature at which any micro-organisms within the liquid are destroyed, without reducing the effectiveness of the liquid as a diluent or as a source of enzymes. Heating the liquid in line the 34 to a suitable temperature, for example up to 90 C, provides a convenient means of heating the influent flow to an optimum temperature for both pre- treatment in the tank 8 and hydrolysis within the hydrolysis reactor 14. To this end, a suitable heater 35 may be provided for heating the contents of the line 34. The heater 35 may derive its heat from any suitable source, for example waste heat from the engine 22 or by burning bio-gas from the anaerobic reactor.

The engine 22 which runs at least in part on bio-gas produced within the anaerobic reactor 19 is used to produce useful work which is exported from the process. Waste heat from the engine may also be exported for useful purposes, for example space heating or process heating.

The above described embodiment of the invention produces a highly efficient digestion of organic material. By subjecting the material to a pre-treatinent with an enzyme containing liquid and hydrolysis, followed by separation of the solids from the mixture before it is fed to the anaerobic reactor, a highly efficient anaerobic reaction process may be carried out. As a result, the overall throughput of the plant is substantially higher than that which would previously had been expected from a plant having an anaerobic reactor of the size present in the embodiment described. Further, the overall process is at least energy neutral and, if efficiencies at each stage are optimised can result in the export of useful energy from the plant.

Claims

CLAIMS: 1. A method for the anaerobic digestion of organic waste, the

method comprising: mixing organic waste to be treated with a liquid containing an enzyme; treating the resultant mixture in a hydrolysis reactor to induce at least partial hydrolysis of the mixture; and feeding at least a liquid fraction of the treated mixture to an anaerobic reactor for anaerobic digestion.
2. A method according to Claim 1 in which the liquid containing an enzyme is derived from an anaerobic reactor.
3. A method according to Claim 1 or Claim 2 wherein the material from the hydrolysis reactor is subject to a solid separation process before the liquid faction is fed to the anaerobic reactor.
4. A method according to any preceding claim wherein the organic material is emulsified prior to feeding to the hydrolysis reactor.
5. A method according to Claim 4 wherein at least some of the liquid containing an enzyme is mixed with the organic matter to be treated prior to the emulsification process.
6. A method according to Claim 4 or Claim S wherein the organic material is subject to a maceration process prior to emulsification.
7. A method according to any preceding claim wherein the output from the anaerobic reactor comprises a liquid effluent, and part of said liquid effluent is, after optional heating and pH correction, fed to the inlet of the anaerobic reactor.
8. A method according to Claim 7 wherein a portion of the liquid effluent is, after optional heating, mixed with the inflow of material to be treated.
9. A method according to any preceding claim wherein the method is a continuous flow method.
10. Apparatus for cariying out the method according to any preceding claim comprising a hydrolysis reactor to induce at least partial hydrolysis of organic waste; an anaerobic reactor connected to the hydrolysis reactor for receiving liquid therefrom to subject said liquid to an anaerobic digestion; and means for delivering a liquid containing an enzyme from the output of the anaerobic reactor to the hydrolysis reactor.