GB2478383A

GB2478383A - Treatment of aqueous organic streams by biodigestion and pervaporation

Info

Publication number: GB2478383A
Application number: GB1101735A
Authority: GB
Inventors: Tabarik Faisal Salam; Victoria Melchor Rojas
Original assignee: Cpi Innovation Services Ltd
Current assignee: Cpi Innovation Services Ltd
Priority date: 2010-02-03
Filing date: 2011-02-02
Publication date: 2011-09-07
Also published as: WO2011095805A1; GB201101735D0; GB201001709D0

Abstract

A method for treating aqueous organic waste or by-product streams 12 comprises passing an aqueous organic waste or by-product stream comprising soluble and insoluble solids through a biodigester reactor 18. The stream is also subjected to pervaporation separation using a membrane which permits only water vapour to be transported through the membrane, thereby generating a contaminant free water stream and a stream containing an increased dry matter content as compared to the stream immediately prior to the pervaporation separation. One or more output streams from the biodigester may be held at a temperature of at least 70°C to ensure the stream(s) is/are aseptic. It is preferred that the bio-digester reactor is an anaerobic reactor. In another aspect, a plant for treating aqueous organic waste or by-product streams is claimed. The plant is capable in use of generating output streams consisting of biogas 42, contaminant free water 30 and a digestate stream 44. Aseptic fertilizer products are also claimed which comprise biodigester reactor digestate having a dry matter content of at least 20% by weight. In further aspect, the aseptic fertilizer product also comprises a fertilizer-enhancing additive.

Description

I

METHODS AND PROCESS PLANT FOR THE TREATMENT OF AQUEOUS

ORGANIC WASTE STREAMS

Field of the Invention

The invention relates to methods and process plant for the treatment of aqueous organic waste (and aqueous organic by-product) streams. More particularly, the invention relates to methods and process plant for the treatment of aqueous organic waste (and aqueous organic by-product) streams from food and agricultural processes, for example the aqueous organic waste streams generated from brewing and distilling operations, soft drink manufacturing, food manufacturing processes and agricultural processes such as slurry removal, agricultural crops/plants (including crops/plants grown for the purpose of energy generation) etc.

Background of the Invention

Agricultural and food processes usually generate solid and/or liquid waste streams that have dissolved or suspended organic solids. Such waste poses a significant environmental disposal problem as well as at least potentially significant additional costs associated with environmentally acceptable disposal techniques.

Examples of aqueous organic waste or by-product streams include inter a/ia cheese manufacture, which generates primarily a liquid whey effluent; whisky or other spirit manufacture, which results in a waste liquids pot ale stream, a spent lees stream and the solid by-products of spent grain and yeast streams; meat industries utilising beef, lamb, poultry and swine sources; and food manufacture, which can result in waste dough together with raw or processed meats, blood and fats. Abattoir and tannery processes may also provide suitable aqueous waste streams which require disposal. Such aqueous organic waste streams may also be those defined under Category 2 and 3 waste streams as defined under the Agricultural By-Products Regulations, ABPRO and may include excrement and/or urine of farm animals with or without litter, or guano.

Aqueous organic waste or by-product streams are frequently categorised, when in a liquid or sludge form, by their chemical or biological oxygen demand (COD or BOD), usually expressed in mg/I.

Techniques are known to reduce the COD or BOD demand of such organic waste or by-product streams prior to disposal of the liquid and solid contents of such streams. Such techniques include filtration, (vacuum) distillation, reverse osmosis and bio-digestion, for example aerobic digestion (both wet and dry).

Examples of some of these techniques are described in US 4959237 (Du Pont, juice production), US 5250182 (Zenon, lactic acid), US 6036854 (Shane-Agra, sugars), US 6423236 (Nippon Shokubai, waste water), US 7267774, US 2005/0252858 (Peyton et al. distilleries), US 6368849 (Bioscan, agricultural) and US 5374356 (Pall Corp, waste water).

When aqueous organic waste or by-product streams are processed in bio-digesters that utilise anaerobic processes, some of the COD of the waste is preserved. Anaerobic digestion is often used for sewage processing. Examples of process plants utilising anaerobic digesters include US 6884355 (IN/litsubishi), US 2004/0134853 (IN/filler), US 5451319, US 2006/0266703 (USE) and DE 20122384 U (G Lorenz).

Anaerobic digesters use processes which are based around mesophiic temperature conditions (32- 39°C) and thermophilic temperature conditions (50-58°C). Examples of anaerobic digester systems may be found in publications such as "Anaerobic Digestion of Biomass" by D.P. Chynoweth and R. Isaacson.

Process plant utilising anaerobic digesters may also include vacuum distillation which is commonly utiised in process operations in industry. It has been utilised in food manufacturing over many years and is accepted practice. It has been proposed to use vacuum distillation upstream of an anaerobic digester system for concentrating the dry matter content of sewage sludge (US 5451319, Kobayashi).

In another proposal, vacuum distillation has been incorporated as an integral part of the anaerobic digester design, i.e. as a vacuum retort anaerobic digester vessel (US 6942798, published as US 2004/0134853).

The output of anaerobic digester plants is still classified as waste and has to be treated to produce environmentally-acceptable waste streams. Examples of process plant in which the digestate stream from anaerobic digesters is treated may be found in the aforementioned US 6036854, US 6368849 and US 7267774 (US 2005/0252858).

The outputs from such process plant may be useful product streams such as fertilizer and clean water.

Although the product streams identified as clean water may be suitable for disposal in the sewage system, or even as potable (drinking) water or for re-use as process water, such clean water streams still contain, albeit at relatively low levels, contaminants such as ethanol, acetic acid, lactic acid, ammonium hydroxide etc which are not removable by the techniques previously utiised in such plants. For safe disposal or re-use of such clean water streams, many environmental regulations require levels of organic species of not more than 30 ppm and may be as low as not more than 10 ppm.

However, in other industries such as the brewing and distilling industries, the soft drinks industries and certain food preparation industries, clean water streams that contain residual levels of contaminants are not acceptable as potentially they could affect the quality of the products being manufactured.

Accordingly, such industries have a substantially zero-contaminant requirement for recycled water to ensure such recycled water does not affect the quality of the products being manufactured.

It has also been proposed to generate product streams of alcohols, solvents or water-soluble organic species using pervaporation to remove such products from clean aqueous solutions or to concentrate such solutions by removing some water from them. Examples of processes for generating such product streams may be found in DE 3840576, EP 0164608, EP 0369787, GB 2346095, US 2006/2601 86 and WO 2006/029971.

A further requirement for recycled water streams, whether substantially contaminant free or a clean water stream, is that they are aseptic. The output streams of bio-digesters, especially anaerobic digesters, and, frequently, the input streams to such digesters, are not aseptic. To achieve an aseptic stream, the septic stream has to be subjected to a suitable pasteurising process. One suitable process as defined in PAS 110 (Public Acceptability Standard) requires the septic stream to be subjected to a temperature of 70°C for at least 1 hour (or for less time at higher temperatures).

It is an object of the present invention to provide a water stream associated with a bio-digester process plant that is essentially contaminant free and as such is significantly purer than the current potable water quality standards applicable in the EU Council Directive 98/83/EC, 1998 and the US Clean Water Act.

It is a further object of the present invention to provide a water stream associated with a bio-digester process plant that is essentially contaminant free and aseptic.

As used in this specification, the term "essentially contaminant free" means the water stream contains essentially no contaminants at all. Expressed alternatively, the water stream will have levels of organic species of not more than 5 ppm, more preferably of not more than I ppm, more particularly not more than 0.1 ppm and especially of not more than 0.01 ppm.

It is yet a further object of the present invention to provide both a water stream associated with a bio-digester process plant that is essentially contaminant free and aseptic and an output digestate stream that is essentially aseptic.

Summary of the Invention

According to the present invention, a method for treating aqueous organic waste or by-product streams comprises passing an aqueous organic waste or by-product stream comprising soluble and insoluble solids through a bio-digester reactor, and subjecting said stream to a pervaporation separation using a membrane which permits only water vapour to be transported therethrough to generate therefrom an essentially contaminant free water stream and a stream containing an increased dry matter content as compared to said stream immediately prior to said pervaporation separation.

The term "dry matter content" refers to the amount of both soluble and insoluble solids left after a sample of the stream is subjected to an evaporation process to eliminate water and other volatile components.

The step of subjecting said stream to a pervaporation separation may occur either upstream of the bio-digester reactor or downstream of the bio-digester reactor. In an alternative embodiment, the step of subjecting said stream to a pervaporation separation may occur both upstream and downstream of the bio-digester reactor to generate at least two essentially contaminant free water streams from the aqueous organic waste or by-product stream and at least two streams containing increased dry matter content as compared to said streams immediately prior to said pervaporation separations.

In one embodiment of the invention, the method comprises subjecting said stream to a pervaporation separation downstream of the bio-digester reactor. In a preferred embodiment, the method comprises, prior to subjecting said stream to a pervaporation separation, subjecting said stream as digestate from the bio-digester reactor to a preliminary separation process to generate a primary sub-stream having a reduced dry matter content as compared to said stream, but still containing soluble and insoluble solids, and a secondary sub-stream having an increased dry matter content as compared to said stream, wherein the primary sub-stream is subjected to said pervaporation separation to generate therefrom an essentially contaminant free water stream and a stream containing an increased dry matter content as compared to said stream immediately prior to said pervaporation separation.

Typically, a "wet" bio-digester reactor produces a digestate stream that has between 3% and 12% by wt dry matter content, more especially between 6% and I Q% by wt dry matter content. In this instance, it is preferred the preliminary separation process reduces the dry matter content of said primary sub-stream to less than 3% by wt, more preferably to less than 2% by wt and typically to between 1% and 2% by wt and increases the dry matter content of said secondary sub-stream to more than 20% by wt, more preferably to more than 25% by wt and typically to between 3Q% and 35% by wt.

Alternatively, a "dry" bio-digester reactor typically produces a relatively high dry matter content digestate stream which has typically between 20% and 45% by wt dry matter content, more especially between 25% and 35% by wt dry matter content, and a relatively low dry matter digestate stream which has typically between 5% and 20% by wt dry matter content, more especially between 10% and 15% by wt dry matter content. In this instance, it is the relatively low dry matter digestate stream which constitutes said stream subjected to a preliminary separation process in accordance with the invention to generate primary and secondary sub-streams. It is preferred the preliminary separation process reduces the dry matter content of said primary sub-stream to less than 3% by wt, more preferably to less than 2% by wt and typically to between 1% and 2% by wt and increases the dry matter content of said secondary sub-stream to more than I 5% by wt, more preferably to more than 20% by wt and typically to between 20% and 3Q% by wt. The secondary sub-stream may be combined with the relatively high dry matter content digestate stream emanating from the bio-digester reactor.

Conveniently, the preliminary separation process may he performed using any suitable means for removing liquids from solids-containing streams to produce a sub-stream that has a relatively low dry matter content and a sub-stream that has a relatively high dry matter content. Examples of such techniques include decanter centrifuges, belt filters, screw presses, plate filters etc. In one embodiment of the invention, the method comprises subjecting said primary sub-stream to a pervaporation separation immediately following said preliminary separation process.

In an alternative embodiment of the invention, the method comprises, following said preliminary separation process, subjecting said primary sub-stream to a secondary separation process to further reduce the dry matter content of said primary sub-stream. It is preferred the secondary separation process reduces the dry matter content of said primary sub-stream to less than O.S% by wt, more preferably to less than O.l% by wt and typically to between O.Ol% and O.l% by wt. However, such a primary sub-stream, as part of its dry matter content, will still contain soluble and insoluble solids.

The secondary separation process also produces another secondary sub-stream of relatively high dry matter content. This further secondary sub-stream may be either processed separately or, more conveniently, may be combined with the initial secondary sub-stream.

Conveniently, the secondary separation process may be performed using appropriate combinations of coarse mechanical filters to remove larger solids material and/or micro-filters and/or ultra-filters and/or membrane filters as are required to achieve the desired dry matter content reduction, although such techniques do not eliminate all soluble or insoluble solids in the aqueous streams.

In yet a further alternative embodiment, the primary separation process may be used to reduce the dry matter content of said primary sub-stream to below l% by wt. For example, this may be achieved in the primary separation process by operating a decanter centrifuge such that the throughput of said stream from the bio-digester reactor is balanced against the speed of the centrifuge to improve the cut off of solids and/or using at least two centrifuges in series to improve the cut off of solids. The primary sub-stream output from such an enhanced primary separation process may still be subjected to the secondary separation process, if desired.

The secondary sub-stream or sub-streams may be subjected to further processing to remove contaminants such as ammonia (if present), organic species (if removal is desired) and to reduce further the water content thereof. The dry matter content of the digestate may be increased to between about 2O% and about 95% by wt. When the dry matter content of the digestate is increased to be in the range 2O% to 40 by wt, more preferably to be in the range 2S% to 3S% by wt, the resultant slurry may conveniently used as an easily spreadable enhanced soil conditioner for agricultural land using conventional semi-dry-spreading or dry-spreading equipment. Furthermore, digestates having dry matter contents in the above ranges may be mixed with other appropriate materials to tailor the sub-streams as soil conditioners for specific applications.

In particular, thc prcscnt invention includes an aseptic fertilizer product comprising a bio-digester reactor digestate having a dry matter content of at least 2O% by wt.

More especially, the present invention includes an aseptic fertilizer product having a dry matter content of at least 2O% by wt, said product comprising a bio-digester reactor digestate and a fertilizer-enhancing additive.

Conveniently, the further processing of the secondary sub-stream(s) may be by (vacuum) distillation, reverse osmosis and similar techniques.

The water separated from the secondary sub-stream(s) may be used as process water or disposed to local sewage systems when the presence of low amounts of contaminants may be tolerated.

Alternatively, the water separated from the secondary sub-stream(s) may be subjected to a second pervaporation separation to generate an essentially contaminant free water stream therefrom.

Typically, upstream of a "wet" bio-digester reactor (or "semi-dry" bio-digester reactor), said stream has an initial dry matter content between O.5% and lO% by wt dry matter content, more usually between 1% and 6% by wt dry matter content. It is preferred that prior to said stream entering the bio-digester reactor, the dry matter content of said stream is increased. Preferably, the dry matter content is increased to between 15% and 3Q% by wt, more preferably to between I 8% and 25% by wt.

Typically, upstream of a "dry" bio-digester reactor, said stream has an initial dry matter content between 15% and 45% by wt dry matter content, more usually between 20% and 4Q% by wt dry matter content and more especially between 25% and 35% by wt dry matter content. Feedstock for "dry" bio-digester reactors are typically energy crops and similar biomass materials and, as such, usually do not need any front end processing; they are merely fed into the "dry" bio-digester reactor.

However, should it be necessary to do so, appropriate front end processes as described herein may be used to adjust or manipulate the material content and/or water levels of such feedstock.

The method of the invention may achieve such concentration, ie increase in dry matter content, using conventional techniques such as micro-filtration and ultra-filtration and reverse osmosis.

Alternatively, when the method comprises subjecting said stream to a pervaporation separation upstream of the bio-digester reactor, the pervaporation separation is used to concentrate, ie increase in dry matter content, said stream prior to said stream entering the bio-digester reactor.

The method further comprises, prior to subjecting said stream to the upstream pervaporation separation, treating said stream to remove gross contaminants such as fibres etc. Such gross contaminants may be removed using suitable filters such as belt filters, decanter centrifuges, coarse and micro filtration units and the like as are well understood in the art.

Preferably, the pervaporation separation is performed at a temperature of at least 70°C, more preferably at a temperature of at least 90°C, and more especially at a temperature of at least 100°C.

Preferably, the pervaporation separation is performed at a temperature of not more than 120°C, more especially not more than 110°C.

Preferably, the method comprises subjecting said stream to pervaporation separation at a temperature of at least 70°C for a period sufficient to ensure the essentially contaminant free water stream is aseptic. This may be achieved by managing the throughput of said stream through the pervaporation separation step.

By performing the pervaporation separation at such temperatures, the method according to the invention ensures the final high dry matter content digestate generated by the method is also aseptic.

Additionally, or alternatively, the essentially contaminant free water stream and/or the final high dry matter content digestate generated by the method may be held at a temperature of at least 70°C for a period sufficient to ensure it is/they are aseptic. The additional or alternative heat treatment may be particularly applicable if at least a part of the final high dry matter content digestate generated by the method has not been subjected a pervaporation separation. In that instance, the heat treatment may conveniently be included as part of further processing of the secondary sub-stream or sub-streams, for example by performing ammonia stripping at an elevated temperature of at least 70°C.

The pervaporation separation in the method of the invention utilises a suitable hydrothermally stable polymeric or, preferably, ceramic membrane which permits only water vapour to be transported through the membrane. Particularly useful membranes are ceramic membranes which consist of nano-porous layers on top of a macro-porous support. The pores are large enough to let water molecules pass through and retain any other components that have a larger molecular size such as ethanol.

Typically, such membranes are based on zeolites and amorphous silica layers which provide a molecular sieve functionality having a pore size of about 4 A. Polymeric membranes which may operate under similar conditions to the zeolite and silica based membranes are also being developed.

Particularly useful membranes may be silica pervaporation membranes available from Pervatech; 4 A zeolite pervaporation membranes available from Zeolite Solutions Ltd; Zenon (trade name) zeolite membranes available from GE Water & Process Technologies; and zeolite membranes from P Nanotec.

The method of the invention includes subjecting said stream and the essentially contaminant free water stream(s) post the pervaporation separation step(s) to heat exchange to reduce the temperature of those streams and to recover useful heat for re-use in the process or for use elsewhere and to manage the temperature of those streams for subsequent processing.

The bio-digester reactor may be an aerobic reactor or, more preferably, an anaerobic reactor or a combination of both. A combination of both aerobic and anaerobic reactors may have particular utility in treating feedstock such as high nitrogen contcnt wastc, for instance, slaughterhouse wastewaters; and waste streams with toxic compounds such as PCBs.

When the bio-digester reactor is an anaerobic reactor, the method comprises operating the anaerobic reactor under mesophilic temperature conditions or thermophilic temperature conditions.

The anaerobic reactor may comprise a single reactor unit or, alternatively, at least two reactor units whereby digestate storage may be achieved to enable management of throughput of said stream in the form of digestate in the downstream process steps.

Bio-gas generated by the anaerobic reactor may be utilised in a bio-gas fuelled boiler to generate hot water and/or steam or in a combined heat and power (CHP) unit such as a gas engine or a fuel cell.

When the CHP unit is a gas engine, useful heat may be extracted in the form of hot water, typically at round 85°C, and hot gases, typically at 350°C. The hot gases may be used to heat the water to generate saturated steam or pressurised hot water, typically at 110°C, for use in the processes of the method of the invention or elsewhere. When the CHP unit is a fuel cell, useful heat may be extracted as steam, typically at around 350°C, for use in the processes of the method of the invention or elsewhere.

The present invention also provides process plant by which the method of the invention may be carried out, the various elements of the process plant having been described hereinbefore.

In particular, the present invention provides process plant for treating aqueous organic waste or by-product streams comprising a bio-digester reactor for treating an organic waste or by-product stream comprising soluble and insoluble solids and a pervaporation separation unit comprising a membrane which permits only water vapour to be transported therethrough and from which an essentially contaminant free water stream and a stream containing an increased dry matter content as compared to said stream immediately prior to said pervaporation separation may be generated.

In a preferred embodiment, the present invention also provides process plant for treating aqueous organic waste or by-product streams comprising a bio-digester reactor for treating an organic waste or by-product stream comprising soluble and insoluble solids and a pervaporation separation unit comprising a membrane which permits only water vapour to be transported therethrough and from which an essentially contaminant free water stream and a stream containing an increased dry matter content as compared to said stream immediately prior to said pervaporation separation may be generated, said process plant being capable in use of generating output streams consisting essentially of bio-gas, essentially contaminant free water and a digestate stream. Preferably, the process plant is capable in use of generating output streams consisting essentially of bio-gas, aseptic essentially contaminant free water and an aseptic digestate stream.

Brief Description of the Drawing

The invention will now be described by way of illustration only with reference to the accompanying drawing in which Figure 1 shows a schematic of the process of the invention.

Detailed Description of the Preferred Embodiments

Referring to Figure 1, a process plant 10 for treating aqueous organic waste or by-product streams 12 is shown schematically. As shown, the process plant 10 may have a preliminary treatment unit 14, a water reduction unit 16, a bio-digester reactor 18, a dry matter reduction unit 20, a second water reduction unit 22 and a final processing unit 24.

The bio-digester reactor 18 may be an aerobic reactor or an anaerobic reactor. In some embodiments, a combination of both aerobic and anaerobic reactors may be used, typically the anaerobic reactor being followed by the aerobic reactor. When the bio-digester reactor 18 is an anaerobic reactor, the reactor 18 may be operated under mesophilic temperature conditions or thermophilic temperature conditions as previously discussed. In the illustrated example of Figure 1, the bio-digester reactor 18 is a "wet" anaerobic reactor.

The reactor 18 may comprise a single reactor unit or, alternatively, at least two reactor units to provide storage capacity within the plant 10 to enable management of throughput of said stream in the form of digestate in the downstream process steps.

The aqueous organic waste or by-product streams 12 containing both soluble organic and inorganic components and insoluble organic and inorganic solids may be derived from a variety of sources as previously discussed.

For example the streams 12 may be derived inter alia from brewing operation wastes such as pot ale or spent lees or yeast residues; from a distillery; waste water from food manufacture, eg soup, meat processing etc; a chemical treatment source such as glycerol generated as a bi-product during the manufacture of bio-diesel; or biomass derived from plants and crops. Such streams 12 have the common factor that they are suitable for treatment using a bio-digester reactor.

If COD of the input stream 12 is sufficiently high, typically above about 5000 mg/i, more preferably above about 10000 mg/I and typically at least 20000 mg/I, it may be injected directly into the bio-digester reactor 18 as shown at 26, the units 14 and 16 being omitted from the plant 10. In such circumstances, direct injection of the input stream 12 into the bio-digester reactor 18 will maximise the economic criteria both for building the process plant 10 and for running the process plant 10.

For input streams 12 having a lower COD, the stream 12 is fed as indicated at 28 directly into the unit 16 to be subjected to front-end water content reduction treatment in the unit 16, the unit 14 being omitted from the process plant 10. Such treatment may involve mechanical filtration, for example decanter centrifuges, coarse filtration units etc and, optionally pressure filtration such as ultra filtration and/or reverse osmosis and/or pervaporation separation using a membrane which only permits water vapour to be transported tberethrough. Water recovered from the unit 16 as shown at 30 may be used directly in the process as shown at 32; or returned to be used in the primary food or other processing plant(s); or be subjected to a pervaporation separation using a membrane which only permits water vapour to be transported therethrough within the unit 16 or separately therefrom to generate an essentially contaminant free water stream and a remaining stream having an increased dry matter content as compared to the dry matter content of the stream immediately prior to the pervaporation separation.

Waste streams 12 may also be a relatively solid or low moisture organic waste, for example draff (distillers' grain) from a distillery. Alternatively or additionally, water-insoluble organic materials may be generated in food or chemical processing, for example fats etc, which materials may be suitable for biological degradation. Such waste streams may be mixed with water to form aqueous waste streams suitable for processing in accordance with the invention. The source of water for mixing with such waste streams is typically another aqueous waste stream or recycled recovered water from the process plant 10.

Some such materials, typically Category 2 and 3 waste streams will require thermal treatment, for example being held at 70°C for 1 hour in accordance with PAS 110, to make them aseptic and suitable for further treatment. The heat treatment process initiates and/or promotes hydrolysis of the fats and other materials in such streams. In respect of such feeds, the pre-treatment may include pre-hydrolysis of the fats etc using alkalis and/or acids to enhance degradation in the bio-digester reactor 18. Such treatment may take place in the optional preliminary treatment unit 14, the process plant 10 being configured to provide heat input at 34 into the process stream to ensure the temperature of the process stream is at least 70°C and to extract heat at 36 from the process stream to control the temperature of the stream downstream of the unitl4.

As previously described, water from unit 16 may be recycled to unit 14 as shown at 32 to aid in creating a solubiised aqueous stream which is passed to unit 16.

Waste streams 12 may also be generated from agricultural waste such as cow and pig slurry and straw manure and chicken litter. If required, unit 14 may include coarse filter equipment to remove gross contaminants such as fibrous materials etc. Waste streams 12 comprising biomass materials, such as energy crops, algae, cow and pig slurry/manure and sewage sludge etc, may be mechanically treated in preliminary treatment unit 14 to create conditions conducive to pressure cell disruption whereby subsequent processing of the streams will optimise gas generation in the bio-digester reactor 18.

Again, if required, water from unit 16 may be recycled as shown at 32 to unit 14 to aid in creating a solubilised aqueous stream which is passed to unit 16.

The waste stream 12 is either fed directly into the water reduction unit 16 as shown at 28 or, if pre-treatment is required, is fed from preliminary treatment unit 14 into the water reduction unit 16.

The water reduction unit 16 may conveniently have decanter centrifuges, belt filters, screw presses, plate filters and similar equipment.

In one embodiment of the invention, the water reduction unit 16 may alternatively or additionally have a pervaporation separation unit having a membrane which permits only water vapour to be transported therethrough to generate an essentially contaminant free water stream 30 and a stream having an increased dry matter content as compared to the stream immediately prior to the pervaporation separation unit.

In embodiments in which the unit 16 contains a pervaporation separation unit, the process plant 10 is preferably configured to provide heat input at 38 into the process stream to ensure the temperature of the process stream is at least 70 °C to improve the efficiency of the pervaporation separation unit and to extract heat at 40 from the process stream to control the temperature of the stream entering the bio-digester reactor 18. If required, the throughput of the process stream in the unit 16 is controlled to hold the stream at the elevated temperatures for a sufficient period to provide an aseptic output stream, at least in respect of the an essentially contaminant free water stream at 30. This may be instead of or additional to any heat treatment of the process stream that may occur in unit 14.

If desired, the hot essentially contaminant free water stream 30 may be used in other processes either at about the temperature it exits the unit 16 or at lower or higher temperatures as the other processes demand by cooling or additional heating.

The unit 16 typically increases the dry matter content of the process stream from an initial value between l% and lO% by wt to a dry matter content of between IS% and 30% by wt as previously described.

The process stream passes from the unit 16 into the bio-digester reactor 18.

Bio-gas produced in the reactor 18 passes at 42 to a bio-gas fuelled boiler or a combined heat and power (CHP) unit (not shown) such as a gas engine or a fuel cell. When the CHP unit is a gas engine, useful heat may be extract in the form of hot water, typically at round 85°C, and hot gases, typically at 350°C. The hot gases may be used to heat the water to generate saturated steam or pressurised hot water, typically at 110°C, for use in the processes of the method of the invention or elsewhere. When the CHP unit is a fuel cell, useful heat may be extracted as steam, typically at around 350°C to 550°C, for use in the processes of the method of the invention or elsewhere.

The process stream in the form of a digestate stream 44 passes from the reactor 18 to the dry matter reduction unit 20 in which a preliminary separation process may be performed to generate a primary sub-stream 46 having a reduced dry matter content as compared to said digestate stream 44 and a secondary sub-stream 48 having an increased dry matter content as compared to said digestate stream 44.

Conveniently, the preliminary separation process may be performed using decanter centrifuges, belt filters, screw presses, plate filters and similar techniques.

Typically, the process stream 44 emanating from the reactor 18 has between 3% and l2% by wt dry matter content, more especially between 6% and I O% by wt dry matter content. In the unit 20, the dry matter content of the primary sub-stream 46 is reduced to less than 3% by wt, and more typically is reduced to between l% and 2% by wt.

The primary sub-stream 46 may then be passed directly to the water reduction unit 22 for further processing.

In an alternative embodiment of the invention, the primary sub-stream 46 may be subjected to a secondary separation process in the unit 20 to effect an additional reduction in the dry matter content of the primary sub-stream, typically to reduce the dry matter content of the primary sub-stream 46 to less than O.5% by wt, more especially to less than O.l% by wt.

The further processing in the unit 20 may be performed using appropriate combinations of coarse mechanical filters to remove larger solids material and/or micro-filters and/or ultra-filters and/or membrane filters as are required to achieve the desired dry matter content reduction.

In an alternative embodiment, the process stream 44 may be passed through primary separation process in unit 20 involving decanter centrifuge(s) to achieve a reduction in the dry matter content of the primary sub-stream 46 to below l% by wt. Such a reduction may be achieved by operating a decanter centrifuge such that the throughput of the process stream 44 is balanced against the speed of the centrifuge to improve the cut off of solids and/or using at least two centrifuges in series to improve the cut off of solids. The primary sub-stream 46 emanating from such an enhanced primary separation process may still be subjected to the secondary separation process in unit 20, if desired.

The concentrated slurry forming the secondary sub-stream or streams 48 generated from the separation processes in unit 20 are passed to the final processing unit 24.

As previously described, the primary sub-stream 46 then passes to the water reduction unit 22 for further processing.

The unit 22 contains a pervaporation separation unit having a membrane which permits only water vapour to be transported therethrough and is operated to provide an essentially contaminant free water stream 54 and a secondary sub-stream having an increased dry matter content as compared to the dry matter content of the primary sub-stream 46 that is passed into the pervaporation separation unit. The process plant 10 is configured to provide heat input at 50 into the primary sub-stream 46 to ensure the temperature of the primary sub-stream 46 is at least 70 °C to improve the efficiency of the pervaporation separation unit and to extract heat at 52 from the secondary sub-stream 56 to control the temperature of the secondary sub-stream 56 entering the final processing unit 24. If required, the throughput of the primary sub-stream 46 in the unit 22 is controlled to hold the stream at the elevated temperatures for a sufficient period to provide an aseptic output stream, at least in respect of the an essentially contaminant free water stream 54.

If desired, the hot essentially contaminant free water stream 54 may be used in other processes either at about the temperature it exits the unit 22 or at lower or higher temperatures as the other processes demand by cooling or additional heating.

The secondary sub-stream 56 from the unit 22 is then passed to the final processing unit 24.

The concentrated slurry forming the secondary sub-stream or streams 48 from the unit 20 and the concentrated slurry forming the secondary sub-stream 56 from the unit 22 may be subjected to further processing in the unit 24 either as separate secondary sub-streams or, more preferably, as a combined final digestate stream.

The secondary sub-streams 48, 56 may subjected to further processing in unit 24 to remove contaminants such as ammonia (if present), organic species (if removal is desired) and, if required, to reduce the water content thereof further. The dry matter content of the final digestate stream may be increased to between about 2O% and about 9S% by wt.

Conveniently, the further processing of the process stream in unit 24 may be by (vacuum) distillation, membrane filtration, reverse osmosis and similar techniques.

The water separated from the secondary sub-streams 48, 56 in unit 24 may be used as process water or disposed to local sewage systems as shown at 58 when the presence of low amounts of contaminants may be tolerated. Alternatively, the water separated from the secondary sub-streams 48, 56 in unit 24 may be subjected to a second pervaporation separation process by being recycled to the unit 22 as shown at 60 to generate an essentially contaminant free water stream therefrom.

When the dry matter content of the final digestate stream 62 is increased to be in the range 20% to 40 by wt, more preferably to be in the range 2S% to 3S% by wt, the resultant slurry may conveniently used as an easily spreadable enhanced soil conditioner for agricultural land using conventional dry-spreading equipment. At higher dry matter content levels in the range 40% to 9S% by wt, more typically in the range 40% to 6S% by wt, the final digestate stream 62 may be mixed with other appropriate materials to tailor the final digestate stream 62 into soil conditioners for specific applications. Without limiting the scope of materials that may be added to the final digestate stream 62, typical examples of materials which may be added to it include lime, dololime, blast furnace slag (which contains valuable trace minerals) and similar waste products from a variety of metallurgical and chemical processes. Many of these materials are dry, fine, hygroscopic powders which, when combined with the final digestate stream 62 absorb water from the stream and increase the ease with which it may be dispersed as a relatively dry material.

As described previously, preferably, the pervaporation separation in units 16 and/or 22 is performed at a temperature of at least 70°C, more preferably at a temperature of at least 90°C, and more especially at a temperature of at least 100°C. Preferably, the pervaporation separation is performed at a temperature of not more than 120°C, more especially not more than 110°C.

Preferably, the streams subjected to pervaporation separation are held at a temperature of at least 70°C for a period sufficient to ensure the essentially contaminant free water stream emanating therefrom are aseptic. This may be achieved by managing the throughput of the respective streams through the respective pervaporation separation steps.

Typically, such membranes are based on zeolites and amorphous silica layers which provide a molecular sieve functionality having a pore size of about 4 A. Particularly useful membranes may be silica pervaporation membranes available from Pervatech; 4 A zeolite pervaporation membranes available from Zeolite Solutions Ltd; Zenon (trade name) zeolite membranes available from GE Water & Process Technologies; and zeolite membranes from P Nanotec.

From the foregoing description, it will be appreciated the present invention is capable of removing from waste water soluble and insoluble materials such as; viruses, parasites, other microorganisms (including pathogens), heavy metals, PCBs (polychlorinated biphenyls), pesticides, hormones and antibiotics to provide an essentially contaminant free water stream and a useful digestate product stream. The present invention is not compromised by aggressive cleaning processes such as the use of detergents, pH extremes or elevated temperatures which are known to hamper known solutions.

Claims

Claims 1. A method for treating aqueous organic waste or by-product streams comprising passing an organic waste or by-product stream comprising soluble and insoluble solids through a bio-digester reactor, and subjecting said stream to a pervaporation separation using a membrane which permits only water vapour to be transported therethrough to generate therefrom an essentially contaminant free water stream and a stream containing an increased dry matter content as compared to said stream immediately prior to said pervaporation separation.
2. The method according to claim I wherein the bio-digester reactor is an anaerobic reactor.
3. The method according to claim I or claim 2 wherein the step of subjecting said stream to a pervaporation separation occurs either upstream or downstream of the bio-digester reactor.
4. The method according to claim I or claim 2 wherein the step of subjecting said stream to a pervaporation separation occurs both upstream and downstream of the bio-digester reactor to generate at least two essentially contaminant free water streams from said stream.
5. The method according to any one of the preceding claims comprising, prior to subjecting said stream to pervaporation separation downstream of the bio-digester reactor, subjecting said stream to a separation process to generate a primary sub-stream having a reduced dry matter content as compared to said stream and a secondary sub-stream having an increased dry matter content as compared to said stream, wherein the primary sub-stream is subjected to said pervaporation separation.
6. The method according to claim 5 wherein comprises processing said secondary sub-stream to remove possible contaminants and to reduce the water content thereof.
7. The method according to claim 6 wherein subjecting water separated from said secondary sub-stream by said further processing to pervaporation separation to generate an essentially contaminant free water stream therefrom.
8. The method according to any one of the preceding claims comprising performing pervaporation separation at a temperature of at least 70°C.
9. The method according to any one of the preceding claims comprising holding one or more of the output streams from the bio-digester reactor at a temperature of at least 70°C for a period sufficient to ensure the stream or streams are aseptic.
10. The method according to any one of the preceding claims comprising subjecting output stream or streams and/or the essentially contaminant free water stream(s) post pervaporation separation step(s) to heat exchange to reduce the temperature of those streams and to recover useful heat for re-use in the process or for use elsewhere and to manage the temperature of those streams for subsequent processing.
11. Process plant for treating aqueous organic waste or by-product streams comprising a bio-digester reactor for treating an organic waste or by-product stream comprising soluble and insoluble solids and a pervaporation separation unit comprising a membrane which permits only water vapour to be transported therethrough and from which an essentially contaminant free water stream and a stream containing an increased dry matter content as compared to said stream immediately prior to said pervaporation separation may be generated.
12. Process plant according to claim 11 wherein said pervaporation separation unit is located either upstream or downstream of the bio-digester reactor.
13. Process plant according to claim 11 comprising at least two pervaporation separation units, one said unit being located upstream of the bio-digester reactor and one said unit being located downstream of the bio-digester reactor.
14. Process plant according to any one of claims 11 to 13 wherein the bio-digester reactor is an anaerobic reactor.
15. Process plant according to claim 14 wherein a bio-gas outlet of the anaerobic reactor is connected to a combined heat and power unit..
16. An aseptic fertilizer product comprising a bio-digester reactor digestate having a dry matter content of at least 2O% by wt.
17. An aseptic fertilizer product having a dry matter content of at least 2O% by wt, said product comprising a bio-digester reactor digestate and a fertilizer-enhancing additive.
18. Process plant for treating aqueous organic waste or by-product streams comprising a bio-digester reactor for treating an organic waste or by-product stream comprising soluble and insoluble solids and a pervaporation separation unit comprising a membrane which permits only water vapour to be transported therethrough and from which an essentially contaminant free water stream and a stream containing an increased dry matter content as compared to said stream immediately prior to said pervaporation separation may be generated, said plant being capable in use of generating output streams consisting essentially of bio-gas, essentially contaminant free water and a digestate stream.
19. Process plant according to claim 18 capable in use of generating output streams consisting essentially of bio-gas, aseptic essentially contaminant free water and an aseptic digestate stream.