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Title:
BIODEGRADATION OF ORGANIC COMPOUNDS
Document Type and Number:
WIPO Patent Application WO/2007/085039
Kind Code:
A1
Abstract:
A method for the biodegradation of organic compounds in a Bayer circuit, the method including the steps of: treating a portion of the Bayer circuit to provide a pH of between about 8 and about 12 wherein the portion of the Bayer circuit comprises an alkaliphilic microorganism and an electron acceptor and the microorganism is capable of anaerobic respiration in the presence of the electron acceptor, such that at least a portion of the organic compounds are anaerobically degraded by the microorganism.

Inventors:
TILBURY, Amanda (3 Rendell Avenue, Atwell, Western Australia 6164, AU)
NYEBOER, Hugh (36 Grassbird Loop, Yangebup, Western Australia 6164, AU)
COOLING, David, John (19 Tanjinn Street, Dawesville, Western Australia 6210, AU)
SUMICH, Matthew (23a Ventnor Avenue, Mount Pleasant, Western Australia 6153, AU)
Application Number:
AU2006/000109
Publication Date:
August 02, 2007
Filing Date:
January 30, 2006
Export Citation:
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Assignee:
ALCOA OF AUSTRALIA LIMITED (Cnr Davy And Marmion Streets, Booragoon, Western Australia 6154, AU)
TILBURY, Amanda (3 Rendell Avenue, Atwell, Western Australia 6164, AU)
NYEBOER, Hugh (36 Grassbird Loop, Yangebup, Western Australia 6164, AU)
COOLING, David, John (19 Tanjinn Street, Dawesville, Western Australia 6210, AU)
SUMICH, Matthew (23a Ventnor Avenue, Mount Pleasant, Western Australia 6153, AU)
International Classes:
C01F7/47; C22B3/18
Attorney, Agent or Firm:
WRAY & ASSOCIATES (Level 4, The Quadrant 1 William Stree, Perth Western Australia 6000, AU)
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Claims:

The Claims Defining the Invention are as Follows

1. A method for the biodegradation of organic compounds in a Bayer circuit, the method including the steps of:

treating a portion of the Bayer circuit to provide a pH of between about 8 and about 12 wherein the portion of the Bayer circuit comprises an alkaliphilic microorganism and an electron acceptor and the microorganism is capable of anaerobic respiration in the presence of the electron acceptor,

such that at least a portion of the organic compounds are anaerobically degraded by the microorganism.

2. A method for the biodegradation of organic compounds in a Bayer circuit according to claim 1, wherein the organic compounds are provided in the form of formate, acetate, oxalate, malonate and/or succinate ions.

3. A method for the biodegradation of oxalate in a Bayer process residue, the method including the steps of:

treating a portion of the Bayer circuit to provide a pH of between about 8 and about 12 wherein the portion of the Bayer circuit comprises an alkaliphilic microorganism and an electron acceptor and the microorganism is capable of anaerobic respiration in the presence of the electron acceptor; and

adding oxalate to the portion of the Bayer circuit,

such that at least a portion of the oxalate is anaerobically degraded by the microorganism.

4. A method for the biodegradation of oxalate in a Bayer circuit according to claim 3, wherein the oxalate is provided in the form of plant sodium oxalate cake, recovered from the Bayer process.

5. A method for the control of oxalate in a Bayer circuit, the method including the steps of:

treating a portion of the Bayer circuit to provide a pH of between about 8 and about 12 wherein the portion of the Bayer circuit comprises an alkaliphilic microorganism and an electron acceptor and the microorganism is capable of anaerobic respiration in the presence of the electron acceptor,

adding oxalate to the portion of the Bayer circuit, wherein the oxalate is provided in the form of plant sodium oxalate cake, recovered from the Bayer circuit

such that at least a portion of the oxalate is anaerobically degraded by the microorganism.

6. A method for the biodegradation of organic compounds in a Bayer circuit according to any one of the preceding claims, wherein the portion of the Bayer circuit has a pH of between about 9 and about 10.5.

7. A method for the biodegradation of organic compounds in a Bayer circuit according to any one of the preceding claims, wherein the portion of the

Bayer circuit is carbonated or neutralised with sea water to provide a pH of between about 8 and about 12.

8. A method for the biodegradation of organic compounds in a Bayer circuit according to any one of the preceding claims, wherein the portion of the Bayer circuit is provided in the form of a residue from the Bayer circuit.

9. A method for the biodegradation of organic compounds in a Bayer circuit according to claim 8, wherein the Bayer circuit residue is provided in the form of a residue bed.

10. A method for the biodegradation of organic compounds in a Bayer circuit according to any one of the preceding claims, wherein the portion of the

Bayer circuit comprises more than one species of naturally occurring microorganism .

11. A method for the biodegradation of organic compounds in a Bayer circuit according to any one of the preceding claims, wherein the method comprises the further step of:

adding an electron acceptor to the Bayer circuit.

12. A method for the biodegradation of organic compounds in a Bayer circuit according to any one of the preceding claims, wherein the method comprises the further step of:

adding nutrients to the Bayer circuit.

13. A method for the biodegradation of organic compounds in a Bayer circuit according to claim 12, wherein the method comprises the further step of:

adding trace elements to the Bayer circuit.

14. A method for the biodegradation of organic compounds in a Bayer circuit according to claim 12, wherein the nutrients comprise at least one of nitrogen, phosphorus, magnesium and iron.

15. A method for the biodegradation of organic compounds in a Bayer circuit according to claim 14, wherein the nutrients are selected from the group comprising nitrates, urea, ammonia, phosphoric acid, mono ammonium phosphate, polyphosphates and yeast/meat extracts.

16. A method for the biodegradation of organic compounds in a Bayer circuit according to claim 15, wherein the nitrogen source is a combination of ammonia, nitrate and urea.

17. A method for the biodegradation of organic compounds in a Bayer circuit according to claim 14, wherein the magnesium source is magnesium sulfate.

18. A method for the biodegradation of organic compounds in a Bayer circuit according to claim 14, wherein the iron source is iron sulfate.

19. A method for the biodegradation of organic compounds in a Bayer circuit according to any one of the preceding claims, wherein the electron acceptor is a source of at least one of nitrate (NO 3 ' ), nitrite (NO 2 " ), iron (Fe 3+ ), sulfate (SO 4 2' ), sulfur (S 0 ), or carbon dioxide (CO 2 ).

20. A method for the biodegradation of organic compounds in a Bayer circuit according to claim 19, wherein the electron acceptor is provided in the form of sulfate (SO 4 2" ) or nitrate (NO 3 " ).

21. A method for the biodegradation of organic compounds in a Bayer circuit as hereinbefore described with reference to the accompanying Examples.

22. A method for the biodegradation of oxalate in a Bayer circuit as hereinbefore described with reference to the accompanying Examples.

23. A method for the control of oxalate in a Bayer circuit as hereinbefore described with reference to the accompanying Examples.

Description:

"Biodegradation of Organic Compounds"

Field of the Invention

The present invention relates to a method for the anaerobic biodegradation of organic compounds in a Bayer circuit.

Background Art

The Bayer process is widely used for the production of alumina from aluminium containing ores such as bauxite. The process involves contacting alumina- containing ores with recycled caustic aluminate solutions at elevated temperatures, in a process commonly referred to as digestion.

The sodium aluminate solution so produced also contains insoluble residues from the bauxite ore, and the solids are separated from the solution in a thickener or clarifier. The solids, known as 'red mud', are taken as underflow from the thickeners, then typically washed to recover caustic values and render the mud suitable for disposal. The dense slurry is pumped to drying beds and distributed over a surface to allow the residue to dry atmospherically, with entrained caustic liquor being recovered via an underdrain system. The underdrain caustic liquor is recycled for further use in the Bayer circuit along with spent liquor from other locations in the circuit.

In some regions of the world, a significant amount of organic material accompanies the bauxite, a portion of which is responsible for the presence of a range of organic compounds in the resulting solutions. The presence of organic compounds (also known as Total Organic Carbon or TOC) in Bayer process solutions reduces productivity largely through two effects. Firstly, organic compounds combine with free soda reducing the soda available to dissolve gibbsite and form sodium aluminate in solution. The solubility limit of certain organics ultimately restricts the overall concentration of soda in the liquor circuit. Secondly, the presence of organic compounds reduces the hydrate precipitation rate, due to crystallisation poisoning. Benefits associated with removal of organic

compounds from Bayer process solutions include a reduction in the amount of soda in the alumina product, reduced liquor viscosity and improved hydrate agglomeration. Subsidiary disadvantages associated with organic compounds of Bayer process solutions include reduced boiling point, foaming, liquor and hydrate absorbance and liquor density.

Refineries employ several methods to reduce or control the levels of organic compounds in alkaline solutions in process liquor. These may include specific removal processes, controlled entrainment of liquor to residue disposal and natural loss associated with TOC adsorption onto product hydrate.

A specific organic, sodium oxalate (oxalate), is targeted for removal in refineries. Oxalate forms a substantial component of the overall TOC in process liquor. It builds up in the liquor stream as a result of direct input from bauxite and from the natural degradation of other organics as the liquor is continually recycled through the Bayer circuit.

Current oxalate disposal operations involve conversion of sodium oxalate with slaked lime, to calcium oxalate that is transferred to a residue area as a slurry. A significant amount of the calcium oxalate is naturally reconverted to soluble sodium oxalate and returns to the process with lake water return. The remaining calcium oxalate is incorporated in the mud residue. The amount of sodium oxalate returning to the refinery into the process circuit as a result of reversion back to sodium oxalate has a significant detrimental effect on the process. As a consequence, there is a requirement to permanently remove sodium oxalate from the circuit to enable production rates of alumina to be sustained, until such time that an alternative oxalate disposal / destruction method is implemented.

The invention presented herein was developed to provide an alternative method for the treatment of organic compounds from the Bayer circuit.

The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any

of the material referred to was part of the common general knowledge in Australia as at the priority date of the application.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Throughout the specification, unless the context requires otherwise, the word "solution" or variations such as "solutions", will be understood to encompass slurries, suspensions and other mixtures containing undissolved solids.

Throughout the specification, unless the context requires otherwise, the term 'alkaliphilic' will be understood to encompass a microorganism that can grow in alkaline solutions.

Throughout the specification, unless the context requires otherwise, the acronym TOC will be understood to refer to all organic compounds in Bayer process solutions. 'GC-TOC relates to those biodegradable compounds specifically identified by gas chromatography (hereinafter GC), namely formate, acetate, oxalate, malonate and succinate. The term 'OB-TOC will be understood to refer to biodegradable organic compounds not identified by GC.

Throughout the specification, unless the context requires otherwise, the words formate, acetate, oxalate, malonate and succinate, and will be understood to refer to all anions, organic acids and salts of formate, acetate, oxalate, malonate and succinate.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

- A -

The present invention is not to be limited in scope by the specific embodiments described herein, which are intended for the purpose of exemplification only. Functionally equivalent products, compositions and methods are clearly within the scope of the invention as described herein.

Disclosure of the Invention

In accordance with the present invention, there is provided a method for the biodegradation of organic compounds in a Bayer circuit, the method including the steps of:

treating a portion of the Bayer circuit to provide a pH of between about 8 and about 12 wherein the portion of the Bayer circuit comprises an alkaliphilic microorganism and an electron acceptor and the microorganism is capable of anaerobic respiration in the presence of the electron acceptor,

such that at least a portion of the organic compounds are anaerobically degraded by the microorganism.

It will be appreciated that the portion of the Bayer circuit to be treated will be any portion where the organic compounds are bioavailable to the microorganism.

Preferably, the organic compounds are provided in the form of formate, acetate, oxalate, malonate and/or succinate ions.

Preferably, the portion of the Bayer circuit has a pH of between about 9 and about 10.5.

The Bayer circuit may be treated by any method known in the art to provide a pH of between about 8 and about 12, including carbonation and sea-water neutralisation. Preferably, the portion of the Bayer circuit is carbonated.

Preferably, the portion of the Bayer circuit is provided in the form of a residue from the Bayer circuit. In one form of the invention, the Bayer circuit residue is

provided in the form of a residue bed. In an alternate form of the invention, the Bayer circuit residue is provided in the form of a superthickener, wherein the underflow from the superthickener is passed to the residue bed. It should be appreciated that the superthickener may not provide sufficient residence time for complete degradation of biodegradable organic compounds and that any degradation commenced in the superthickener may continue in the residue bed.

It will be appreciated that the portion of the Bayer circuit may comprise more than one species of naturally occurring microorganism.

In one form of the invention, the method comprises the further step of:

adding an electron acceptor to the Bayer circuit.

In one form of the invention, the method comprises the further step of:

adding nutrients to the Bayer circuit.

Where the method comprises the step of adding nutrients to the Bayer circuit, the method preferably comprises the further step of:

adding trace elements to the Bayer circuit.

It will be appreciated that the portion of the Bayer circuit to be treated will be any portion wherein the nutrients and/or trace elements are bioavailable to the microorganism.

Preferably, the nutrients comprise at least one of nitrogen, phosphorus, magnesium and iron and may be selected from the group comprising nitrates, urea, ammonia, phosphoric acid, mono ammonium phosphate, polyphosphates and yeast/meat extracts. It will be appreciated that the source of nutrients should make them bioavailable to the microorganisms. Where the nutrients comprise nitrogen, the nitrogen source is preferably a combination of ammonia, nitrate and urea. Where the nutrients comprise magnesium, the magnesium source is

preferably magnesium sulfate. Where the nutrients comprise iron, the iron source is preferably iron sulfate.

Preferably, the electron acceptor is a source of at least one of nitrate (NOs " ), nitrite (NO 2 " ), iron (Fe 3+ ), sulfate (SO 4 2" ), sulfur (S 0 ), or carbon dioxide (CO 2 ). It will be appreciated that the source of electron acceptor should make them bioavailable to the microorganisms. More preferably, the electron acceptor is in the form of sulfate or nitrate.

Without being limited by theory, it is believed that the addition of nutrients and electron acceptors affects metabolic pathways under anaerobic conditions by increasing the population of microorganisms which can respire anaerobically. For example, organic compounds may undergo fermentation to produce acetate and salts of other simple organic acids. Without being limited by theory, it is believed that a population of microorganisms capable of anaerobic respiration (for example sulfate-reducing bacteria) is necessary for the mineralisation of these fermentation products and for the mineralisation and/or partial oxidation of other biodegradable organic compounds (e.g. oxalate). By careful selection of nutrients and electron acceptors, microbial populations capable of mineralising organic compounds may be selected for.

The method of the present invention may be used to selectively degrade oxalate ions in preference to acetate ions.

The method of the present invention may be used to control oxalate concentrations within a Bayer process circuit.

Where the portion of the Bayer circuit is provided in the form of a Bayer circuit residue, the method of the present invention is preferably enhanced by increasing the residence time of solutions containing organic compounds in the residue.

In accordance with the present invention, there is provided a method for the biodegradation of oxalate in a Bayer process residue, the method including the steps of:

treating a portion of the Bayer circuit to provide a pH of between about 8 and about 12 wherein the portion of the Bayer circuit comprises an alkaiiphilic microorganism and an electron acceptor and the microorganism is capable of anaerobic respiration in the presence of the electron acceptor; and

adding oxalate to the portion of the Bayer circuit,

such that at least a portion of the oxalate is anaerobically degraded by the microorganism.

Preferably, the portion of the Bayer circuit has a pH of between about 9 and about 10.5.

The Bayer circuit may be treated by any method known in the art to provide a pH of between about 8 and about 12, including carbonation and sea-water neutralisation. Preferably, the portion of the Bayer circuit is carbonated.

Preferably, the portion of the Bayer circuit is provided in the form of a residue from the Bayer circuit. In one form of the invention, the Bayer circuit residue is provided in the form of a residue bed. In an alternate form of the invention, the

Bayer circuit residue is provided in the form of a superthickener, wherein the underflow from the superthickener is passed to the residue bed. It should be appreciated that the superthickener may not provide sufficient residence time for complete degradation of biodegradable organic compounds and that any degradation commenced in the superthickener may continue in the residue bed.

Preferably, the oxalate is provided in the form of plant sodium oxalate cake, recovered from the Bayer process.

It will be appreciated that the portion of the Bayer circuit may comprise more than one species of naturally occurring microorganism.

In one form of the invention, the method comprises the further step of:

adding an electron acceptor to the Bayer circuit.

In one form of the invention, the method comprises the further step of:

adding nutrients to the Bayer circuit.

Where the method comprises the step of adding nutrients to the Bayer circuit, the method preferably comprises the further step of:

adding trace elements to the Bayer circuit.

It will be appreciated that the portion of the Bayer circuit to be treated will be any portion wherein the nutrients and/or trace elements are bioavailable to the microorganism.

Preferably, the nutrients comprise at least one of nitrogen, phosphorus, magnesium and iron and may be selected from the group comprising nitrates, urea, ammonia, phosphoric acid, mono ammonium phosphate, polyphosphates and yeast/meat extracts. It will be appreciated that the source of nutrients should make them bioavailable to the microorganisms. Where the nutrients comprise nitrogen, the nitrogen source is preferably a combination of ammonia, nitrate and urea. Where the nutrients comprise magnesium, the magnesium source is preferably magnesium sulfate. Where the nutrients comprise iron, the iron source is preferably iron sulfate.

Preferably, the electron acceptor is a source of at least one of nitrate (NO 3 " ), nitrite (NO 2 ' ), iron (Fe 3+ ), sulfate (SO 4 2" ), sulfur (S 0 ), or carbon dioxide (CO 2 ). It will be appreciated that the source of electron acceptor should make them bioavailable to the microorganisms. Preferably, the electron acceptor is in the form of sulfate or nitrate.

Without being limited by theory, it is believed that the addition of nutrients and electron acceptors affects metabolic pathways under anaerobic conditions by increasing the population of microorganisms which can respire anaerobically.

For example, organic compounds may undergo fermentation to produce acetate and salts of other simple organic acids. Without being limited by theory, it is believed that a population of microorganisms capable of anaerobic respiration (for example sulfate-reducing bacteria) is necessary for the mineralisation of these fermentation products and for the mineralisation and/or partial oxidation of other biodegradable organic compounds (e.g. oxalate). By careful selection of nutrients and electron acceptors, microbial populations capable of mineralising organic compounds may be selected for.

The method of the present invention may be used to control oxalate concentrations within a Bayer process circuit.

Where the portion of the Bayer circuit is provided in the form of a Bayer circuit residue, the method of the present invention is preferably enhanced by increasing the residence time of solutions containing oxalate in the residue.

In accordance with the present invention, there is provided a method for the control of oxalate in a Bayer circuit, the method including the steps of:

treating a portion of the Bayer circuit to provide a pH of between about 8 and about 12 wherein the portion of the Bayer circuit comprises an alkaliphilic microorganism and an electron acceptor and the microorganism is capable of anaerobic respiration in the presence of the electron acceptor; and

adding oxalate to the portion of the Bayer circuit, wherein the oxalate is provided in the form of plant sodium oxalate cake, recovered from the Bayer circuit,

such that at least a portion of the oxalate is anaerobically degraded by the microorganism.

Preferably, the portion of the Bayer circuit has a pH of between about 9 and about 10.5.

The Bayer circuit may be treated by any method known in the art to provide a pH of between about 8 and about 12, including carbonation and sea-water neutralisation. Preferably, the Bayer circuit is carbonated.

Preferably, the portion of the Bayer circuit is provided in the form of a residue from the Bayer circuit. In one form of the invention, the Bayer circuit residue is provided in the form of a residue bed. In an alternate form of the invention, the

Bayer circuit residue is provided in the form of a superthickener, wherein the underflow from the superthickener is passed to the residue bed. It should be appreciated that the superthickener may not provide sufficient residence time for complete degradation of biodegradable organic compounds and that any degradation commenced in the superthickener may continue in the residue bed.

It will be appreciated that the portion of the Bayer circuit may comprise more than one species of naturally occurring microorganism.

In one form of the invention, the method further comprises the further step of:

adding an electron acceptor to the Bayer circuit.

In one form of the invention, the method further comprises the further step of:

adding nutrients to the Bayer circuit.

Where the method comprises the step of adding nutrients to the Bayer circuit, the method preferably comprises the further step of:

adding trace elements to the Bayer circuit.

It will be appreciated that the portion of the Bayer circuit to be treated will be any portion wherein the nutrients and/or trace elements are bioavailable to the microorganism.

Preferably, the nutrients comprise at least one of nitrogen, phosphorus, magnesium and iron and may be selected from the group comprising nitrates,

urea, ammonia, phosphoric acid, mono ammonium phosphate, polyphosphates and yeast/meat extracts. It will be appreciated that the source of nutrients should make them bioavailable to the microorganisms. Where the nutrients comprise nitrogen, the nitrogen source is preferably a combination of ammonia, nitrate and urea. Where the nutrients comprise magnesium, the magnesium source is preferably magnesium sulfate. Where the nutrients comprise iron, the iron source is preferably iron sulfate.

Preferably, the electron acceptor is a source of at least one of nitrate (NO 3 ' ), nitrite (NO 2 " ), iron (Fe 3+ ), sulfate (SO 4 2" ), sulfur (S 0 ), or carbon dioxide (CO 2 ). It will be appreciated that the source of electron acceptor should make them bioavailable to the microorganisms. Preferably, the electron acceptor is in the form of sulfate or nitrate.

Without being limited by theory, it is believed that the addition of nutrients and electron acceptors affects metabolic pathways under anaerobic conditions by increasing the population of microorganisms which can respire anaerobically. For example, organic compounds may undergo fermentation to produce acetate and salts of other simple organic acids. Without being limited by theory, it is believed that a population of microorganisms capable of anaerobic respiration (for example sulfate-reducing bacteria) is necessary for the mineralisation of these fermentation products and for the mineralisation and/or partial oxidation of other biodegradable organic compounds (e.g. oxalate). By careful selection of nutrients and electron acceptors, microbial populations capable of mineralising organic compounds may be selected for.

The method of the present invention may be used to selectively degrade oxalate ions in preference to acetate ions.

The method of the present invention may be used to control oxalate concentrations within a Bayer process circuit.

Where the portion of the Bayer circuit is provided in the form of a Bayer circuit residue, the method of the present invention is preferably enhanced by increasing the residence time of solutions containing organic compounds in the residue.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to two embodiments thereof, and the accompanying drawings, in which:-

Figure 1a is a schematic flow sheet showing how a method in accordance with a first embodiment of the present invention may be utilised in a Bayer Process circuit;

Figure 1b is a schematic flow sheet showing how a method in accordance with a second embodiment of the present invention may be utilised in a Bayer Process circuit;

Figure 2 is a schematic flow diagram of a bench trial pilot plant;

Figure 3 is a plot of the concentration of organic species in sand column 1 ;

Figure 4 is a plot of the concentration of organic species in sand column 2;

Figure 5 is a plot of the concentration of organic species in sand column 3; and

Figure 6 is a plot of the concentration of organic species in sand column 1.

Best Mode(s) for Carrying Out the Invention

Figure 1a shows a schematic flow sheet of the Bayer process circuit 10 comprising the steps of:

digestion 12 of bauxite 14 in a caustic solution;

liquid-solid separation 16 of the mixture to residue 18 and liquor 20;

filtration 22 of the liquor 20;

cooling of the liquor 20 to cause aluminium hydroxide precipitation 24;

separation of the aluminium hydroxide 24 and liquor 26;

recycling of the spent liquor 26 to digestion 12;

calcination 28 of the aluminium hydroxide 24 to alumina;

carbonation 30 of the residue 18;

drying of the carbonated residue 32 in a residue bed 34; and

recycling of the underdrain liquor 36 from the residue bed 34 to digestion 12.

In accordance with a first embodiment of the present invention best seen in Figure 1a, the residue 18 is carbonated 30 to reduce the pH to about 10.5 and the carbonated residue passed to the residue bed. Naturally occurring microorganisms in the residue then biodegrade organic compounds in the residue. By the careful addition of nutrients and electron acceptors, and by controlling the retention time of the organic compounds in the bed, it is possible to control metabolic pathways.

Figure 1a shows a schematic flow sheet of the Bayer process circuit 10 comprising the steps of:

digestion 12 of bauxite 14 in a caustic solution;

liquid-solid separation 16 of the mixture to residue 18 and liquor 20;

filtration 22 of the liquor 20;

cooling of the liquor 20 to cause aluminium hydroxide precipitation 24;

separation of the aluminium hydroxide 24 and liquor 26;

recycling of the spent liquor 26 to digestion 12;

calcination 28 of the aluminium hydroxide 24 to alumina;

carbonation 30 of the residue 18;

drying of the carbonated residue 32 in a residue bed 34; and

recycling of the underdrain liquor 36 from the residue bed 34 to digestion

12.

In accordance with a second embodiment of the present invention best seen in Figure 1 b, plant oxalate cake 38 as a concentrated slurry of sodium oxalate is added to the carbonation tank along with the residue and the carbonated residue passed to the residue bed.

In a standard Bayer circuit, the residue is known to contain amounts of oxalate, notwithstanding processes to remove the oxalate. In an uncarbonated residue bed, some of the oxalate returns to the Bayer circuit via the residue bed underdrain system. Depending on residence time and moisture, up to 40 % of the oxalate entering the bed may be returned to the circuit, with the balance of the oxalate remaining in the residue. If the residue is carbonated, it is believed that, given sufficient residence time and the availability of appropriate electron acceptor, and under certain circumstances, nutrient addition, all of the oxalate in the residue may be degraded.

In accordance with the second embodiment of the invention, the method may be used to dispose of plant oxalate. Based on experimental information, it is believed that the oxalate concentration in the bed may be increased at least four times and with appropriate holding time and the availability of appropriate electron acceptor, and under certain circumstances, nutrient addition, all of the oxalate in the residue may be degraded.

In order to assess the opportunity for anaerobic biodegradation, a number of leach sand columns were constructed and operated in a continuous mode in an attempt to replicate the conditions in the residue area underdrain layer, where it

was expected that the majority of anaerobic activity would occur. The effects of oxalate concentration and nutrients and trace metal addition were assessed. A flow diagram of a bench trial pilot plant is shown in Figure 2.

Examples

The following examples are intended to assist in the understanding of the reaction parameters of the present invention and serve to more fully illustrate the invention. It must be appreciated that the following description of the examples is not to limit the generality of the description of the invention.

The concentrations of formate, acetate, oxalate, malonate and succinate in influent and effluent liquors were determined using a Hewlett Packard 5890 Gas Chromatograph equipped with a Supelcowax capillary column (60 m x 0.2 mm x 0.2 μm) and flame ionisation detector.

The concentration of Total Organic Carbon (TOC) was determined using an Ol Analytical 1010 Total Organic Carbon Analyzer with a 1051 Vial Multisampler.

Water washed residue area sand was used as a medium for the columns. The sand was washed to remove clay fines to increase flow rate through the column. The silica based sand was sourced from within the boundaries of the Applicant's refinery's residue area in Kwinana, Western Australia and is the same sand that is used for the underdrain system of the residue beds. The particle size breakdown of the sand is shown in Table 1. Table 2 provides comparative information on properties of residue area sand and the sand used in the columns. It should be noted that the values in Table 2 will change in direct relation to changes in underdrain flow or feed rate.

Approximately 150 kg of washed residue area sand and approximately 40 L of effluent liquor from an aerobic treatment plant were used to pack the columns.

The liquor had low pH and was chosen to represent a lakewater containing a natural population of TOC degrading microorganisms, which are preferably

anaerobic and/or facultatively anaerobic. Some typical effluent liquor values are shown in Table 3.

Table 1. Particle size distribution of sand sample.

Liquor Capacities Typical Residue Area Sand Typical Sand Column Sand

Surface Area (m 2 ) 300000 0.07

Sand Layer Depth (m) L 0.8 2.0

Sand Porosity (%) i 30 i 30 j Sand Saturation Level (%) I 50 I I 100

I Volume of Liquor in Sand (kL) I 36000 ] ! 0.0402 jjJnderdrain Flow Rate (kL/hr) I 30.0 I i 0.00012

I Retention Time - Total (day) i 50 14

I Permeation Rates (L/hr/ m 2 ) I 0.10 1.792

I Permeation Velocity (mm/day) I 8.00 143.4

Table 2. Sand properties.

Table 3. Column feed liquor properties.

Alternating portions of sand and effluent were added to the column to ensure a good mix of sand and liquor containing biomass. The sand column was purged with pH 8 carbonated lakewater at a flow rate of 1 L/hr for 48 hours, decreased to 130 mL/hr for 5 days while pH 10.5 test liquor was prepared.

Test liquor was transferred from a storage tank to influent liquor storage tanks located in a cool room. At this time if required under the experimental conditions, a nutrient and trace metal solution was added to the influent storage tanks and

mixecl thoroughly. The temperature of the refrigerated cool room was maintained at 9 0 C to inhibit microbial growth and thus TOC degradation occurring in the influent liquor prior to its application to the sand columns.

To avoid blockages occurring in the sand columns, the influent liquor was filtered through a 5 μm in-line filter to remove any precipitated compounds prior to addition to the sand columns. The influent liquor was added to the top of each sand column using a Grundfos positive displacement dosing pump that could deliver a constant influent liquor flow rate in a range from 0 to 2.5 L/hr. The influent liquor flow rate was set at 120 mL/hr, providing a 14 day residence time in each of the sand columns. (The calculated residence time in the residue sand layer being about 45 days based on sand layer saturation level, underdrain flow rate and sand layer area/depth. However, due to the distribution pattern of the underdrain pipe work, the expected and previously observed residence times would be less than this.) The influent flow rate was monitored periodically and the settings on the pump controller were adjusted accordingly. The columns were housed in a climate controlled room set to maintain the temperature at, typically, 20 0 C; similar to that of the underdrain system.

The effluent discharged from the bottom of the columns was routed back up to within 50 mm of the top of the columns where it then discharged into a larger diameter tube that directed flow to a drain. This ensured that the sand within the columns was fully saturated. An anaerobic water seal at the top of each column was used as a pressure relief system in case the column discharge was blocked or gases built up within the columns. They also prevented air from re-entering the columns, maintaining anaerobic conditions.

Lakewater was carbonated using CO 2 , the precipitated alumina solids allowed to settle overnight and the supernatant pumped into a separate holding tank.

In order to determine the feasibility of disposal of plant sodium oxalate cake via carbonated superthickener underflow, an influent that contained carbonated lakewater and plant oxalate cake was also prepared. The feed tanks in the coolroom were topped up with this liquor.

Plant sodium oxalate cake typically contains 40 % wt / wt sodium oxalate with the remainder consisting of Bayer liquor of a varying composition.

Test liquor with oxalate concentration of 2.5 g/L was chosen to represent a Bayer circuit liquor from a superthickener underflow and test liquor with oxalate concentration of 10 g/L was chosen to represent the underflow liquor with additional oxalate added. The latter test was intended to represent the situation that would be encountered under oxalate disposal conditions.

A nutrient and trace metal solution was added to the influent liquor in the influent liquor storage tanks for a number of the trials. The nutrient target concentration in the influent liquor was based on the assumptions that 5 % of the total biodegradable TOC was used for biomass production, and the ratio of carbon, nitrogen and phosphorus in biomass was CioεNieP-i.

The nutrient and trace metal solution contained Metals 44/Modified MSB solution without the addition of nitriliotriacetic acid, and with the addition of 32 g/L NH 4 CI and 3.7 g/L H 3 PO 4 as a source of N and P, respectively. For low concentration oxalate liquors, the nutrient and trace metal solution was added to the influent feed liquor at a rate of 1 mL/L, giving a final concentration of 0.0319 g/L NH 4 CI (8.42 mg/L as N) and 0.0037 g/L -H 3 PO 4 (1.18 mg/L as P) (see Tables 4 and 5).

Concentrated H 2 SO 4 (a few drops) was added to the distilled water before addition of reagents to prevent precipitation.

All reagents were added to 700 mL dHaO and made up to 1000 mL with dH 2 θ. The pH was adjusted to 6.6-6.8. The modified MSB solution was stored at 4 0 C.

After day 21 of the experiments, the nutrient and trace metal solution was modified by replacing 32 g/L NH 4 CI with 48.1 g/L NH 4 NO 3 , which gave a final concentration of 0.0481 g/L NH 4 NO 3 (16.8 mg/L as total N, 8.42 mg/L as N from nitrate). The use of NH 4 NO 3 ensured that N from the nitrate would be available to the biomass within the sand column. The pKa of NH 3 is 9.3. At pH 10.3, 91% exists as NH 3 and is likely to volatilise before leaching into the sand column. It

was important not to add excess nitrate to the sand columns, as nitrate reduction would occur in preference to sulfate reduction. If nitrate reduction occurs within the sand columns, the NH 3 produced would not volatilise under the hydraulic flow of the column and would most likely still be available as a nitrogen source for sulfate-reducing bacteria further down the sand column. The total amount of nitrate added to the sand column would only support a small proportion of the biodegradable organic compounds in the liquor being oxidised under a nitrate- reducing pathway, so a nitrate-reducing microbial population would not outcompete the growth of sulfate-reducing microorganisms.

Table 4. Concentrations of reagents in 144 Metal solution.

For high concentration oxalate liquors, the nutrient and trace metal solution was added to the influent feed liquor at a rate of 2.3 mL/L, giving a final concentration of 0.1106 g/L NH 4 NO 3 (38.7 mg/L as total N, 19.37 mg/L as N from nitrate) and 0.0085 g/L H 3 PO 4 (2.71 mg/L as P). The final concentrations of metals in both low and high oxalate influent liquors are given in Table 6.

The sand columns were situated in a temperature controlled room at 20 0 C, the temperature chosen to approximate the average temperature which would be expected within the residue area.

Table 6. Concentrations of metals in low and high concentration oxalate influent liquors. During the commissioning phase, the columns were packed with washed sand using 30 L of a lakewater at pH 10.3 which contained a natural population of TOC degrading microorganisms, including facultatively anaerobic microorganisms, which was used as an initial source of biomass for the sand columns. No additional biomass was added, although any source of biomass can be used to enhance the start-up of the process e.g. sludge.

The test conditions for each sand column are presented in Table 7.

A Gastec gas pump and Drager tube were used to determine whether the volatile gases SO 2 , H 2 S and NH 3 were present in the anaerobic column room and/or within each of the sand columns. A Drager tube, a specific colour indicator tube for each of the above gases, was fitted to the gas pump. Volumes of air were drawn into the tube from various points within the anaerobic column room. For the detection of the gases from the sand column effluents, the Drager tube was inserted into the effluent tube of each column and volumes of air were drawn into the tube.

The concentrations of free sulfide (as S 2' ) and total sulfide (S 2" and polysulfides) in the effluents of sand columns 1 , 2 and 3 were determined by sulfide titration.

A reduction in the concentration of sulfate and the presence of sulfides in the effluents is an indication that sulfate reduction has occurred. The concentrations of free sulfide (as S 2" ) and total sulfide (S 2" and polysulfides) in the effluents of sand columns 1 , 2 and 3 were determined by titration. A suitable aliquot of sample, containing 45-300 μg of sulfide, was titrated potentiometrically with 0.002821 M Ag + using an Ag/AgS electrode, preferably with a KNO 3 salt bridge to prevent clogging.

The final titrated solution should have an alkalinity of at least 1 M NaOH. Any autotitrator capable of finding the end point differentially is suitable, such as the Mettler DL-70. The electrode response is slow when the concentration of sulfide is below 1 μg/mL in the titrated solution. Therefore, parameters must be set to allow sufficient time for the particular electrode to respond to each addition of silver. Other halogens do not interfere with quantification of free sulfide but iodide must be absent or allowed for by standard addition if partially oxidised forms of sulfide, such as polysulfides, are titrated. Other compounds which complex with silver or sulfide such as cyanide and mercury may interfere. The concentration of sulfide was determined from 1 mL of 0.02821 M AgNO 3 is equivalent to 1.100 mg Na 2 S or 0.4523 mg S 2" .

To determine the effect of nutrients on the anaerobic degradation of oxalate, carbonated lakewater (a low pH -10-10.5 with an alkalinity in the range of ~20- 30 g/L as Na 2 CO 3 , which contains ~2-2.5 g/L sodium sulfate, and formate, acetate, oxalate, malonate and succinate, and other organic compounds in varying concentrations) was supplied to sand columns 1 and 2 at an influent flow rate of 120 mL/min, in the absence of and with the addition of nutrients respectively. The sand columns were operated under these conditions for 146 days for column 1 and 132 days for column 2 (Tests 1a and 2a of Table 7 and Figures 3 and 4).

Oxalate degradation was not observed in column 1 during the 146 days of operation at 120 mL/min. However, during this time, the concentration of TOC in sand column 1 decreased by an average of 8% and OB-TOC decreased by an average of 11 % (Figure 3).

The reduction in the concentration of sulfate in sand column 1 was very low for the first 14 days, and was followed by a constant trend, fluctuating between a 6- 16 % reduction up until day 115. During this time there were a number of occasions where the percentage reduction in sulfate reached as high as 20-23 %, however these occurred after influent changes, and it is believed they were the result of higher concentrations of sulfate in the influent from new batches of liquor, giving a greater difference in the concentration of sulfate between the influent and the effluent, rather than an actual decrease of sulfate within the sand column. Furthermore, there were no increases in the degradation of TOC or any other measured organic compounds which coincided with these occurrences.

From days 115 to 145 there was an increase in the reduction of sulfate which remained constant at approximately 28%; however there was no concomitant decrease in the concentration of oxalate or additional decrease in the concentration of TOC or OB-TOC. This suggested that a population of sulfate- reducing bacteria were beginning to establish within the sand column.

The concentration of acetate in the effluent from sand column 1 was greater than its concentration in the influent from approximately day 64 onwards. The concentration of acetate in the effluent of sand column 1 increased over time, reaching as high as 23% greater than its concentration in the influent by day 139. This suggested that fermentation of organic compounds was occurring within the sand column while the population of sulfate-reducing bacteria was establishing.

Similarly, oxalate was not degraded after 125 days in sand column 2, during which time the concentrations of TOC and OB-TOC in sand column 2 decreased by an average of 7.5 % and 10 %, respectively. There was no trend in the degradation of TOC and OB-TOC in both sand columns 1 and 2 to suggest that degradation of these compounds were increasing over time (Figure 4).

The reduction in the concentration of sulfate in sand column 2 was constant and fluctuated between 7-18 % over the first 89 days. From days 89 to 125, the percentage reduction of sulfate increased to 32 %, and more than doubled to 68% reduction in one week at day 132. It was during this period that a reduction in the concentration of oxalate and acetate occurred. The concentration of oxalate in sand column 2 decreased by 20 % at day 132, which occurred within a week, suggesting that a sulfate-reducing microbial population had established and that oxalate degradation had begun.

Acetate was also produced in sand column 2 over the first 133 days. The difference in the concentration of acetate between the influent and effluent at days

125 and 132 (Figure 4) are misleading, as the difference in acetate was primarily due to the reduction in acetate in the influent and not production within the sand column. Despite fluctuations within the influent acetate concentrations column 2, acetate was still produced in column 2, indicating that fermentation of organic compounds was occurring.

To determine the effect of residence time on the anaerobic degradation of oxalate, the influent flow rate was halved from 120 to 60 mL/hr on days 147 and 133 for sand columns 1 and 2 respectively, increasing the residence time within each sand column from approximately 14 to 28 days (Tests 1 b and 2b, Table 7).

The increase in the residence time had an immediate effect on the concentration of oxalate in the effluents of both sand columns 1 and 2, with oxalate being completely degraded within 29 days after the change in influent flow rate. There was no difference in the rate of oxalate degradation between columns 1 and 2, which suggested that the residence time had a greater impact on oxalate degradation than nutrient addition. The results also showed that the addition of nutrients were not essential for the degradation of oxalate, but reduced the time required to commence oxalate degradation by increasing the growth rate and establishment of a sulfate-reducing microbial population.

The increase in the concentration of oxalate in the effluent from sand column 2 on day 188 was believed to be a result of a blockage in the influent pump which

occurred approximately two weeks earlier, on day 176. The concentration of oxalate in the influent increased from 2.31 to 2.64 g/L at this time, as a result of cleaning the pump, however, the surge in oxalate concentration in the effluent was believed to be from a disturbance with the biomass, as the concentration of sulfate followed the same trend as that for oxalate. If the increase in the concentration of oxalate in the effluent was a result of an increase in the concentration of oxalate in the influent, then the sulfate concentration would have been expected to remain constant. Furthermore, there was also an increase in the concentration of acetate in the influent at this time, as a result of cleaning the pump.

The second increase in the concentration of oxalate in the effluent from sand column 2 on day 252 was also believed to be due to a disturbance with the biomass, as the concentration of sulfate followed the same trend again.

A possible explanation for these two biomass disturbances is a change in the composition of the influent with new batches of feed liquor, which occurred around the same time as each increase in the concentration of oxalate in the effluent. Not from an increase in the concentration of oxalate in the influent, but from possible temporary "toxic" shocks from new liquor batches, which has been shown in previous aerobic experiments.,.

In column 1 , without additional nutrients, fermentation of organic acids and the production of acetate continued after an increase in residence time (day 147), despite a reduction in the concentration of sulfate and the complete degradation of oxalate. This was believed to be due to the population of sulfate-reducing bacteria not having reached steady state, which is supported by the higher concentrations of sulfate in the effluent of column 1 compared with column 2. It was expected that over time, once the sulfate-reducing population had reached a certain threshold density, that the products of fermentation (acetate) would be also be oxidised.

After the increase in residence time on day 133, the concentration of acetate in sand column 2, with additional nutrients, started to decrease and by day 223, had

reduced by approximately 85 %. At this time, the concentration of sulfate had reduced and oxalate was completely degraded. The only difference between sand columns 1 and 2 was the addition of nutrients to column 2 (Tests 1b and 2b). The results suggested that the nutrients increased the rate of growth of sulfate- reducing bacteria and thus increased the rate at which acetate and other products of fermentation could be degraded.

Nutrient addition may not only increase the rate of degradation of TOC but may also have an effect on the metabolic pathways of organic acid degradation by enhancing the growth rate and establishment of a population of bacteria that can respire anaerobically. Without being limited by theory, it is believed that fermentation of organic compounds occurs, resulting in the production of acetate and other simple organic acids, and once a population of sulfate-reducing bacteria has reached certain threshold population density, the products of fermentation are degraded through anaerobic respiration. The addition of nutrients is believed to enhance the growth rate of microorganisms capable of anaerobic respiration and hence increase the rate of degradation of organic compounds.

The results from both sand columns indicate that the growth of a population of sulfate-reducing bacteria for the anaerobic degradation of organic compounds at pH -10.5 is a slow and gradual process. The degradation of oxalate and acetate and a concomitant reduction in the concentration of sulfate occurred in sand column 2 before sand column 1 , which suggested that the addition of nutrients enhanced the growth and development of a sulfate-reducing microbial population.

The results also showed that, while nutrients may increase the growth rate of a sulfate-reducing microbial population, the addition of nutrients were not essential to achieve degradation of oxalate.

Residence time had a greater impact on oxalate degradation than the addition of nutrients. Without being limited by theory, it is believed that this could have been due to the dilution of a compound which had toxic effects on the growth of biomass, as when the residence time was reduced, degradation of organic compounds (oxalate) was maintained.

Acetate production in sand columns 1 and 2 could not be quantified due to changes in acetate concentration in the influent with fresh batches, and the residence time difference within the sand columns. Despite fluctuations within the influent acetate concentrations, acetate was produced in sand columns 1 and 2, most likely from fermentation reactions. The only other anaerobic process which could result in the production of acetate is acetogenesis, which involves the reduction of CO 2 to acetate by H 2 . Organic acids can also act as electron donors in addition to H 2 . However, acetogenesis generally does not occur in the presence of sulfate-reducing bacteria.

In order to determine the feasibility of disposing plant sodium oxalate cake via carbonated superthickener underflow, the effect of additional oxalate in the influent liquor feed on anaerobic degradation was examined in sand column 3 (Test 3a). Nutrients were added to the high concentration oxalate influent liquor for sand column 3; sand column 1 was run in parallel with high concentration oxalate liquor without the addition of nutrients (Test 1 c).

Oxalate degradation occurred at day 78, after a reduction in the concentration of sulfate occurred (Figure 6). The pH of the influent liquor was found to be pH 9.5, which was most likely the reason for the degradation of oxalate, as it provided a more favourable environment for the growth of sulfate-reducing bacteria. After 112 days, 10 g/L oxalate had almost completely degraded within the sand column and the concentration of sulfate had reduced by 88 %.

On day 175 it was discovered that gas production by anaerobic metabolism in sand column 3 had displaced liquor within the column. The column had to be filled up with 8 litres of liquor, almost % of the total liquor volume of the column. The displacement of the liquor by the volatile gas, which was most likely H 2 S, NH 3 , other sulfide gases or a combination of these, resulted in a decrease in the liquor retention time of sand column 3. It was not known when the liquor displacement occurred and but was most likely the cause of the increase in the concentration of oxalate in the effluent on day 169.

Test 3a showed that 10 g/L oxalate could be completely degraded within the sand column with the addition of nutrients at a flow rate of 120 mL/min. A small amount of acetate appeared to have been produced prior to oxalate degradation, but was not quantified. Once oxalate had completely degraded, the concentration of acetate in the effluent fluctuated and followed the same trend as that of the influent with its concentration generally lower, but it was not degraded. It was possible that either the amount of nutrients provided in the influent was only sufficient for oxalate degradation or that oxalate-degrading bacterium dominated the sand column due to the high concentration of oxalate.

The test also showed that gas production occurred and that the retention time within the column has a great effect on whether oxalate is completely degraded or not.

Previous experiments have shown that the concentration of oxalate has an effect on the growth of biomass and thus the rate of degradation. Higher concentrations of oxalate have been shown to increase the rate of growth of biomass. For example, a 20 g/L solution of oxalate contains much greater than four times the amount of biomass than a 5 g/L solution of oxalate after a set period of time.

In order to compare the effect of nutrient addition on the anaerobic degradation of oxalate in high concentration oxalate liquor, sand column 1 was operated with high concentration oxalate liquor and no additional nutrients, as a continuation from Test 1 b. The flow rate was increased from 60 mL/hr to 120 mL/hr and the concentration of oxalate in the influent liquor was approximately 10.5 g/L. This resulted in the mass of oxalate per unit volume to the sand column increasing by a factor of 8 from the previous test.

High concentration oxalate liquor was fed into the column at day 208. The concentration of oxalate in the effluent increased to 6 g/L by day 221 , where it stayed relatively constant until day 300, where it was discovered that the influent liquor had run out. There were two occurrences within this time that the concentration of oxalate increased in the effluent, and these both coincided with fresh liquor changes. These were most likely due to increases in the concentration

of oxalate in the influent liquor rather than a decrease in the activity of the sulfate- reducing bacteria, as the concentration of sulfate was not largely affected (Figure 6).

A Drager tube was used to determine whether sulfur dioxide, hydrogen sulfide and/or ammonia could be measured in the anaerobic column room or directly from the columns.

None of the gases sulfur dioxide, hydrogen sulfide and/or ammonia could be measured in samples of air from the anaerobic column room, even when multiple volumes of air where drawn into the Drager tube. However, a detectable odour of hydrogen sulfide was present within the column room. Gases, such as H 2 S 1 have very low odour thresholds. Gas samplers, such as the Drager unit, cannot measure the presence of gases at very low levels. The presence of hydrogen sulfide provides further evidence of sulfate reduction.

Ammonia could be detected after multiple samples of air were drawn directly from the effluent tubes from sand columns 2 and 3, but not from sand column 1. Given that the influent liquor to sand columns 2 and 3 contained a nutrient solution as a source of ammonia, this result was to be expected. The only possible source of ammonia is from the nutrient, ammonium nitrate. This could occur directly from ammonium nitrate, or from the anaerobic reduction of nitrate.

Sulfur dioxide was not detected from any sand column, while hydrogen sulfide was detected in all three sand columns after multiple samples of air were drawn directly from the effluent tubes.

The gases could not be quantified since the volume of ambient air that was mixed with emitted gases was not controlled and is highly dependant on the sampling location.

The concentrations of free sulfide (as S 2" ) and total sulfide (S 2" and polysulfides) in the effluents of sand columns 1 , 2 and 3 are shown in Table 8.

The results show that the concentrations of sulfide in the effluents of the anaerobic sand columns are much less than the concentration expected from the reduction of approximately 2.5 g/L sulfate, indicating that the majority of sulfide produced has precipitated as metal sulfides. Furthermore, the effluents contained fine black particulates, which were believed to be metal sulfides.

Table 8. Concentrations of sulfide and total sulfide in the effluents from the anaerobic sand columns.

* The concentration of sulfide expected if the entire concentration of sulfate in the influent (2.5 g/L as Na 2 SO 4 , or 1.69 g/L as SO 4 2" ) was reduced to sulfide.

This observation has important implications on the process in a Bayer circuit, as free sulfide in the underdrain liquor would be oxidised back to sulfate in an aerobic biodegradation plant and return to the process stream. If the free sulfide is removed from the underdrain liquor by precipitation with metals, then it would not be oxidised back to sulfate so the net result of the anaerobic process will include sulfate removal as well as organic removal.

It has been shown that:

• anaerobic processes may be used to completely degrade high concentrations of oxalate (10 g/L). IF anaerobic conditions are provided in a residue bed with a suitable pH, such as a carbonated residue bed, it is possible to biodegrade oxalate through incorporation with carbonated residue disposal.

• the addition of nutrients and retention time affects the time required to establish a sulfate-reducing microbial population and thus the rate of oxalate degradation.

• the addition of nutrients does not appear essential for the complete degradation of oxalate but does increase the rate at which it occurred, believed to be due to an increase in the rate of growth of sulfate-reducing bacteria.

• volatile gases may be produced under anaerobic conditions.

• the production of acetate, believed to be due to the fermentation of organic compounds, occurred prior to sulfate reduction.

• the addition of electron acceptors affects metabolic pathways under anaerobic conditions by increasing the population of microorganisms which can respire anaerobically.

• in a low oxalate concentration liquor, the addition of nutrients allowed the degradation of acetate.