Method for treating spent caustic to recover caustic and sulphur by a bioelectrochemical process

Abstract

The present invention relates to an apparatus and method for bio-assisted treatment of spent caustic obtained from hydrocarbon and gas processing installations. The present invention also relates to method for recovery of caustic and recovery of sulfur from spent caustic. According to present invention, the sulfide removal is about 96% and the sulphur formation and deposition on the electrode lies in range of 72±8%.

Claims

1. A method for treatment of spent caustic and recovery of caustic and sulphur by bioelectrochemical process, the method comprising: (i) treating a spent caustic in an electrochemical reactor comprising two chambers separated by a cation exchange membrane, wherein a first chamber comprises an electrode wrapped with an activated carbon cloth and a second chamber comprises an electrode coated with a noble metal, wherein cations present in the spent caustic are exchanged via the cation exchange membrane from the first chamber to the second chamber causing a pH of about 12-14 in the second chamber and a pH of about 7-9 of the spent caustic in the first chamber; (ii) treating the spent caustic from the first chamber using a biocatalyst for anaerobically oxidizing sulfides to obtain a spent caustic stream having an elemental sulphur or its oxidized form, wherein the biocatalyst comprises one or more microbes selected from a group consisting of Thiobacillus sp., Thiomicrospira sp. and Pseudomonas putida; and (iii) treating the spent caustic stream obtained from step (ii) using an aerobic biocatalyst to obtain a liquid with reduced concentration of sodium hydroxide, sulfides, amines, thiols, sulphur containing compounds, phenols, hydrocarbons, naphthenic acids and their derivatives, wherein the biocatalyst comprises one or more of Pseudomonas putida (MTCC 5385), Pseudomonas aeruginosa IOCX (MTCC 5389), Bacillus substilis (MTCC 5386), Achromobacter xylosoxidan IOC-SC-4 (MTCC 25024), Pseudomonas stutzeri (MTCC 25027), Arthrobacter sp. (MTCC 25028), Bacillus subtilis (MTCC 25026), and Achromobacter xylooxidan (MTCC 25024).

2. The method as claimed in claim 1, wherein the microbes in the biocatalyst of step (ii) are used in an adsorbed form or a free form or immobilized on synthetic plastics, surface-modified carbon nanotubes, poly (tetrafluoroethylene) (PTFE) fibrils, a zeolite, a clay, an anthracite, a porous glass, an activated charcoal, ceramics, an acrylamide, polyurethane, polyvinyl, resins or a natural polymer.

3. The method as claimed in claim 1, wherein the pH in the second chamber is 12.68 and the pH of spent caustic in the first chamber is 7.08 in 12 hours.

4. The method as claimed in claim 1, wherein anaerobically oxidizing sulfides comprises anaerobically oxidizing about 96% of sulfides and wherein obtaining the spent caustic stream having an elemental sulphur comprises obtaining the spent caustic stream having 72±8% of sulphur.

5. A method for treatment of spent caustic and recovery of caustic and sulphur by bioelectrochemical process, the method comprising: (i) treating a spent caustic in an electrochemical reactor comprising two chambers separated by a cation exchange membrane, wherein a first chamber comprises an electrode wrapped with an activated carbon cloth and a biocatalyst present as biofilm on the electrode and a second chamber comprises an electrode coated with a noble metal, wherein cations present in the spent caustic are exchanged via the cation exchange membrane from the first chamber to the second chamber causing a pH of about 12-14 in the second chamber and a pH of about 7-9 of the spent caustic in the first chamber; wherein the biocatalyst present as a biofilm anaerobically oxidizes sulfides to obtain a spent caustic stream having an elemental sulphur or its oxidized form, wherein the biocatalyst comprises one or more microbes selected from a group consisting of Thiobacillus sp., Thiomicrospira sp. and Pseudomonas putida; and (ii) treating the spent caustic stream obtained from step (i) using an aerobic biocatalyst to obtain a liquid with reduced concentration of sodium hydroxide, sulfides, amines, thiols, sulphur containing compounds, phenols, hydrocarbons, naphthenic acids and their derivatives, wherein the biocatalyst comprises one or more of Pseudomonas putida (MTCC 5385), Pseudomonas aeruginosa IOCX (MTCC 5389), Bacillus substilis (MTCC 5386), Achromobacter xylosoxidan IOC-SC-4 (MTCC 25024) Pseudomonas stutzeri (MTCC 25027), Arthrobacter sp. (MTCC 25028), Bacillus subtilis (MTCC 25026), and Achromobacter xylooxidan (MTCC 25024).

6. The method as claimed in claim 5, wherein the pH in the second chamber is 12.68 and the pH of spent caustic in the first chamber is 7.08 in 12 hours.

7. The method as claimed in claim 5, wherein anaerobically oxidizing sulfides comprises anaerobically oxidizing about 96% of sulfides and wherein obtaining the spent caustic stream having an elemental sulphur comprises obtaining the spent caustic stream having 72±8% of sulphur.

8. A system for treatment of spent caustic to recover caustic and sulphur, the system comprising: (i) an apparatus for stage 1 comprising two chambers separated by a cation exchange membrane, a first chamber receiving spent caustic comprises an electrode wrapped with an activated carbon cloth, a reference electrode and connected to a power supply; a second chamber receiving distilled water comprises an electrode coated with a noble metal, a reference electrode and connected to a power supply; the first chamber and the second chamber are maintained at anaerobic condition, wherein cations present in the spent caustic are exchanged via the cation exchange membrane from the first chamber to the second chamber causing a pH of 12-14 in the second chamber and a pH of 7-9 of spent caustic at anode in the first chamber, wherein caustic is regenerated and recovered in the second chamber; (ii) an apparatus for stage 2 comprises a biocatalyst, wherein said apparatus receives the spent caustic stream from first chamber with lowered pH from stage 1 and anaerobically oxidizes sulfides to obtain a spent caustic stream having an elemental sulfur or its oxidized form by using biocatalyst and recovers sulphur, wherein the biocatalyst comprises one or more microbes selected from group consisting of Thiobacillus sp., Thiomicrospira sp. and Pseudomonas putida; and (iii) a reactor for stage 3 comprises aerobic biocatalyst, wherein said apparatus receives spent caustic stream from stage 2 for treatment by the aerobic biocatalyst and discharges completely treated liquid with reduced concentration of sodium hydroxide, sulfides, amines, thiols, sulphur containing compounds, phenols, hydrocarbons, naphthenic acids and their derivatives, wherein the biocatalyst comprises one or more of Pseudomonas putida (MTCC 5385), Pseudomonas aeruginosa IOCX (MTCC 5389), Bacillus substilis (MTCC 5386), Achromobacter xylosoxidan IOC-SC-4 (MTCC 25024) Pseudomonas stutzeri (MTCC 25027), Arthrobacter sp. (MTCC 25028), Bacillus subtilis (MTCC 25026), and Achromobacter xylooxidan (MTCC 25024).

9. The system as claimed in claim 8, wherein stage 2 is combined with stage 1 and the biocatalyst of stage 2 is present as biofilm on the electrode in the first chamber of stage 1, wherein the spent caustic is treated with the biocatalyst for anaerobically oxidizing the sulfides to obtain a spent caustic having an elemental sulfur or its oxidized form and recovering sulphur; and wherein the spent caustic stream is fed to subsequent stage reactor comprising aerobic biocatalyst for treating the spent caustic stream and discharging completely treated liquid with reduced concentration of sodium hydroxide, sulfides, amines, thiols, sulphur containing compounds, phenols, hydrocarbons, naphthenic acids and their derivatives.

10. The system as claimed in claim 8, wherein in stage 1, the electrode in the first chamber comprises graphite rod, graphite plate, carbon brush, carbon paper, graphite felt; and the electrode in the second chamber comprises a carbon based electrode coated with noble metals, preferably graphite electrode wrapped with stainless steel mesh.

11. The system as claimed in claim 8, wherein the apparatus for stage 2 is configured to anaerobically oxidize about 96% of sulfides and recover 72±8% of sulphur.

12. The system as claimed in claim 9, wherein in stage 1, the electrode in the first chamber comprises graphite rod, graphite plate, carbon brush, carbon paper, graphite felt; and the electrode in the second chamber comprises a carbon based electrode coated with noble metals, preferably graphite electrode wrapped with stainless steel mesh.

13. The system as claimed in claim 9, wherein the apparatus for stage 1 is configured to anaerobically oxidize about 96% of sulfides and recover 72±8% of sulphur.

14. The method as claimed in claim 5, wherein the microbes in the biocatalyst of step (i) are used in an adsorbed form or a free form or immobilized on synthetic plastics, surface-modified carbon nanotubes, poly (tetrafluoroethylene) (PTFE) fibrils, a zeolite, a clay, an anthracite, a porous glass, an activated charcoal, ceramics, an acrylamide, polyurethane, polyvinyl, resins or a natural polymer.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) To further clarify advantages and aspects of the invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof, which is illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail with the accompanying drawings in accordance with various embodiments of the invention, wherein:

(2) FIG. 1: Two-reactor configuration where the stage-1 and stage-2 are performed in the same reactor for recovery of caustic and sulfur along with treatment of spent caustic.

(3) FIG. 2: Three-reactor configuration for recovery of caustic and sulfur along with treatment of spent caustic.

(4) FIG. 3: Control experiment used in the present invention.

(5) Furthermore, one or more elements may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the invention so as not to obscure the drawings with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.

DETAILED DESCRIPTION OF THE INVENTION

(6) While the invention is susceptible to various modifications and alternative forms, specific embodiment thereof will be described in detail below. It should be understood, however that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternative falling within the scope of the invention as defined by the appended claims.

(7) Although one or more features and/or elements may be described herein in the context of only a single embodiment, or alternatively in the context of more than one embodiment, or further alternatively in the context of all embodiments, the features and/or elements may instead be provided separately or in any appropriate combination or not at all. Conversely, any features and/or elements described in the context of separate embodiments may alternatively be realized as existing together in the context of a single embodiment.

(8) The present invention relates to a system and method for bio-assisted treatment of spent caustic obtained from hydrocarbon and gas processing installations. The present invention also relates to method for recovery of caustic and recovery of sulfur from spent caustic.

(9) The invention is explained with respect to the drawings accompanying this specification.

(10) Two embodiments of using the inventive method of treating spent caustic are described in FIG. 1 and FIG. 2 along with control experiment as FIG. 3.

(11) In accordance with the present invention, a three stage electro-bio-assisted assisted method of treating spent caustic is disclosed, said method comprising the steps of: (i) treating the spent caustic (4) in an electrochemical reactor comprising of two chambers separated by cation exchange membrane (1), electrode (2) wrapped with activated carbon cloth in first chamber and electrode (3) wrapped with noble metal in the second chamber, cations (8) present in the spent caustic are exchanged via cation exchange membrane (1) from first chamber to the second chamber, pH increases to 12-14 in the second chamber and associated pH drop of spent caustic is 7-9 in the first chamber, regenerating caustic and recovering caustic; (ii) treating the spent caustic stream with lowered pH obtained from step (i) using a biocatalyst (14) for anaerobically oxidizing sulfides and other related compounds to elemental sulfur or its oxidized form; said biocatalyst (14) comprising one or more microbes selected from group comprising Thiobacillus sp., Thiomicrospira sp. and Pseudomonas putida; and (iii) treating the spent caustic stream obtained from step (ii) using an aerobic biocatalyst (18) and obtaining a liquid with reduced concentration of sodium hydroxide, sulfides, amines, thiols, sulphur containing compounds, phenols, hydrocarbons, naphthenic acids and their derivatives; said biocatalyst (18) comprising one or more of Pseudomonas putida (MTCC 5385), Pseudomonas aeruginosa IOCX (MTCC 5389), Bacillus substilis (MTCC 5386), Achromobacter xylosoxidan IOC-SC-4 (MTCC 25024) Pseudomonas stutzeri (MTCC 25027), Arthrobacter sp. (MTCC 25028), Bacillus subtilis (MTCC 25026), Achromobacter xylooxidan (MTCC 25024).

(12) In an embodiment of the present invention, apparatus for stage-1 comprise of two chambers separated by cation exchange membrane (CEM). One chamber is inserted with graphite rod wrapped with activated carbon cloth (ACC) and considered as working chamber where the spent caustic treatment occurs. The electrodes for this chamber can be varied, viz., graphite plate, carbon brush, carbon paper, graphite felt, etc. The other chamber is inserted with graphite electrode wrapped with stainless steel mesh (SS) and considered as counter chamber. The counter chamber may be of any carbon based electrodes coated with noble metals. One chamber having ACC electrode was fed with spent caustic, while the other chamber was fed with distilled water. The spent caustic chamber as well as counter chamber was maintained under anaerobic conditions. Both the chambers are equipped with Ag/AgCl (3M KCL) reference electrode. Both these electrodes will be connected through resistance or through power supply. The bioreactor can be operated at temperature ranging from 20−40° C. under constant applied voltage in the range of 0.1-5 V or current in the range of 5-250 A/m.sup.2 vs Ag/AgCl reference electrode. The current can be provided from any renewable sources like solar or from electrical grid. The bioreactor can be operated in batch mode with a hydraulic retention time (HRT) of 5-48 h or in continuous mode with 2-24 h HRT. In continuous mode, spent caustic will fed to the working chamber at a flow rate of 1-20 ml/h and de-ionized water will fed to cathode at a flow rate of 1-25 ml/h.

(13) In yet another embodiment of the present invention, the bioreactor in stage 2 of 3 stage process can be a suspended or packed column reactor having selective bacteria for sulfide oxidation to sulfur. The outlet from working chamber of stage-1 bioreactor having lowered pH (7-8) will be fed to this bioreactor and operated under anaerobic conditions. The packing material for the bioreactor can be gravel stones, polymeric material, sponge beads, etc. The bioreactor can be operated in batch mode with a HRT of 6-24 h or in continuous mode with 2-18 h HRT. Continuous mode operation can be at a flow rate of 5-50 ml/h. The bioreactor can be operated at temperature ranging from 25-45° C.

(14) FIG. 2 represents three stage method for treatment of spent caustic and recovery of caustic and sulfur. The apparatus for stage-1 comprise of two chambers separated by cation exchange membrane (CEM) 1. One chamber is inserted with graphite rod wrapped with activated carbon cloth (ACC) 2 and considered as working chamber where the spent caustic treatment occurs. The electrodes for this chamber can be varied, viz., graphite plate, carbon brush, carbon paper, graphite felt, etc. The other chamber is inserted with graphite electrode wrapped with stainless steel mesh (SS) 3 and considered as counter chamber. The counter chamber may be of any carbon based electrodes coated with noble metals. One chamber having ACC electrode 1 was fed with spent caustic 4, while the other chamber was fed with distilled water 5. The spent caustic chamber as well as counter chamber was maintained under anaerobic conditions. Both the chambers are equipped with Ag/AgCl (3M KCL) reference electrode 6. Both these electrodes will be connected through resistance or through power supply 7. During stage-1 of treatment, the spent caustic 4 loaded to the working electrode chamber and distilled water to the counter electrode chamber. The cations 8 present in the spent caustic will be exchanged via the CEM 1 to the counter electrode chamber resulting in the lowering of pH 9 to 10. The exchanged cations from working chamber to the counter electrode chamber increase the pH of counter chamber from 11 to 12, regenerating the caustic 13, which can be recovered. However, there will be no biocatalyst present in the working chamber to enable the sulfur recovery in this approach. The effluent from working chamber 16 with lowered pH will be fed to the reactor in stage-2, where the selective biocatalyst 14 will anaerobically oxidize the sulfide to elemental sulfur 15 and recovered. The effluent 17 of stage-2 will be subjected to stage-3, where the aerobic biocatalyst 18 will be used for treating the remaining organic content and the completely treated effluent 19 will be discharged.

(15) In an embodiment of the present invention, a two-reactor configuration is used in accordance with the invention, where the stage-1 and stage-2 are performed in the same reactor for recovery of caustic and sulfur along with treatment of spent caustic.

(16) FIG. 1 represents the two stage method for treatment of spent caustic and recovery of caustic and sulfur. The apparatus for stage-1 comprise of two chambers separated by cation exchange membrane (CEM) 1. One chamber is inserted with graphite rod wrapped with activated carbon cloth (ACC) 2 and considered as working chamber where the spent caustic treatment occurs. The electrodes for this chamber can be varied, viz., graphite plate, carbon brush, carbon paper, graphite felt, etc. The other chamber is inserted with graphite electrode wrapped with stainless steel mesh (SS) 3 and considered as counter chamber. The counter chamber may be of any carbon based electrodes coated with noble metals. One chamber having ACC electrode 1 was fed with spent caustic 4, while the other chamber was fed with distilled water 5. The spent caustic chamber as well as counter chamber was maintained under anaerobic conditions. Both the chambers are equipped with Ag/AgCl (3M KCL) reference electrode 6. Both these electrodes will be connected through resistance or through power supply 7. During stage-1 of treatment, the spent caustic 4 loaded to the working electrode chamber and distilled water to the counter electrode chamber. The cations 8 present in the spent caustic will be exchanged via the CEM 1 to the counter electrode chamber resulting in the lowering of pH 9 to 10. The exchanged cations from working chamber to the counter electrode chamber increase the pH of counter chamber from 11 to 12, regenerating the caustic 13, which can be recovered. Simultaneously, the biocatalyst 14 present in working electrode chamber as biofilm on the electrode and in the suspension will anaerobically oxidize the sulfides present in the spent caustic converting them to elemental sulfur 15, which can be recovered. The effluent from working chamber 16 with low sulfide content and lowered pH will be fed to the reactor in stage-2, where the aerobic biocatalyst 18 will be used for treating the remaining organic content and the completely treated effluent 19 will be discharged.

(17) In yet another embodiment of the present invention, the bioreactor in stage-3 for treating the left over contaminants such as hydrocarbons and phenols will in suspended mode added with aerobic bacteria. The outlet from the sulfur recovery bioreactor will be fed to this bioreactor and operated under aerobic conditions. The bioreactor can be operated in batch mode with a HRT of 2-18 h or in continuous mode with 2-10 h HRT. Continuous mode operation can be at a flow rate of 5-50 ml/h. The bioreactor can be operated at temperature ranging from 25−45° C.

(18) In yet another embodiment of the present invention, the electrochemical treatment will result in regeneration of caustic at cathode and to increase its pH to 12-14 and decrease the pH of anode to 7-9. In accordance with the present invention treatment is done in batch mode as well as continuous mode using continuously stirrer reactor, up-flow reactor and such suitable reactor. In an embodiment of the present invention, the method of treatment of spent caustic can be used for recovery of sodium hydroxide from the spent caustic. In accordance with the present invention, the method of treatment of spent caustic can be used for recovery of elemental sulfur from the spent caustic.

(19) In an embodiment of the present invention, the spent caustic treated in stage-2 is treated using a microbial consortia in stage 3 which resulted in reduced concentration of sulfides, amines, thiols, other sulphur containing compounds, phenols, hydrocarbons, naphthenic acids and their derivatives at least by 90%.

(20) In accordance with the present invention the said microbes which can be used in stage 2 includes, but not limited to, Thiobacillus sp, Thiomicrospira sp, Pseudomonas putida, alone or in combination with each other. The representative species of the biocatalyst (14) are publically available in the depositories and are not claimed by the applicant. All species of these genus will perform the function, however the isolates may be characterized for following features:

(21) Thiobacillus:

(22) Gram staining: Negative

(23) Colony morphology on thiosulphate-gellan gum plate: White to whitish yellow, cloud like shape

(24) Motility: Positive

(25) Growth on glucose, methanol, pyruvate: Negative

(26) Iron oxidation: Negative

(27) Nitrate respiration: Negative

(28) Catalase: Positive

(29) Oxidase: Positive

(30) Thiocynate oxidation: Positive

(31) More 99% homology with following sequences:

(32) TABLE-US-00001 >Thiobacillus sp. 16S ribosomal RNA gene (SEQ ID NO: 1) AGAGTTTGATCCTGGCTCAGATTGAACGCTGGCGGAATGCTTTACACATG CAAGTCGAACGGCAGCACGG GAGCTTGCTCCTGGTGGCGAGAGGCGAACGGGTGAGTAATGCGTCGGAAC GTACCGAGTAATGGGGGATA ACGCAGCGAAAGCTGTGCTAATACCGCATACGCCCCGAGGGGGAAAGCAG GGGATCGCAAGACCTTGCGT TATTCGAGCGGCCGACGTCTGATTAGCTAGTTGGTGGGGTAAAGGCCTAC CAAGGCGACGATCAGTAGCG GGTCTGAGAGGATGATCCGTCACACTGGGACTGAGACACGGCCCAGACTC CTACGGGAGGCAGCAGTGGG GAATTTTGGACAATGGGGGCAACCCTGATCCAGCCATTCCGCGTGAGTGA AGAAGGCCTTCGGGTTGTAA AGCTCTTTCAGAAGGAACGAAACGGTACGCACTAATATTGTGTGCTAATG ACGGTACCGGCAGAAGAAGC ACCGGCTAACTACGTGCCAGCAGCCGCGGTAATACGTAGGGTGCGAGCGT TAATCGGAATTACTGGGCGT AAAGCGTGCGCAGGCGGATTATTAAGCAAGACGTGAAAGCCCCGGGCTTA ACCTGGGAATGGCGTTTTGA ACTGGTAGTCTAGAGTGTGTCAGAGGGGGGTGGAATTCCACGTGTAGCAG TGAAATGCGTAGATATGTGG AGGAACACCAATGGCGAAGGCAGCCCCCTGGGATAACACTGACGCTCATG TACGAAAGCGTGGGTAGCAA GCAGGATTAGATACCCTGGTAGTCCACGCCCTAAACGATGTCAACAGGTT GTTGGGGGAGTGAAATCCCT TAGTAACGAAGCTAACGCGTGAAGCTGACCGCCTGGGGAGTACGGTCGCA AGATTAAAACTCAAAGGAAT TGACGGGGACCCGCACAAGCGGTGGATGATGTGGATTAATTCGATGCAAC GCGAATCACCTTACCTACCC TTGACATGTCCAGAATCCTGCAGAGATGCGGGAGTGCCCGAAAGGGAATT GGAACACAGGTGCTGCATGG GTGTCGTCAGCTCGTGTCGTGAGATGTTGGGTTAAGTCCCGTAACGAGCG CAACCCTTATCATAAGTTGC TACGCAAGGGCACTCTAATGAGACTGCCGGTGACAAACCGGAGGAAGGTG GGGATGACGTCAAGTCCTCA TGGCCCTTATGGGTAGGGCTTCACACGTCATACAATGGTCCGTACAGAGG GTTGCCAAGCCGCGAGGTGG AGCCAATCCCAGAAAGCCGATCGTAGTCCGGATTGTTCTCTGCAACTCGA GAGCATGAAGTCGGAATCGC TAGTAATCGCGGATCAGAATGCCGCGGTGAATACGTTCCCGGGTCTTGTA CACACCGCCCGTCACACCAT GGGAGTGGAATCTGCCAGAAGTAGGTAGCCTAACCGCAAGGAGGGCGCTT ACCACGTTGGGTTTCATGAC TGGGGTGAAGTCGTAACAAGGTAACCT

Example

(33) The example of such microbe are, but not limited to DSM 12475, DSM 5368, DSM 505, DSM 19892, DSM 700, DSM 3134, ATCC 25259, ATCC 23648, ATCC 8158 etc.

(34) Thiomicrospira sp.

(35) Gram staining: Negative

(36) Cells: motile and rod-shaped

(37) Colony morphology on thiosulfate agar, cells produce yellow, smooth, entire colonies

(38) Motility: Positive

(39) Growth on glucose, methanol, pyruvate: Negative

(40) Catalase: Positive

(41) Oxidase: Positive

(42) Thiocynate oxidation: Negative

(43) More 99% homology with following sequences:

(44) TABLE-US-00002 >Thiomicrospira sp. 16S rRNA gene (SEQ ID NO: 2) TCTGGCGGYAGGCTTAACACATGCAAGTCGGACGGAAACGATAGAGAAGC TTGCTTATCTAGGCGTCGAG TGGCGGACGGGTGAGTAACGCGTGGGAATCTACCCTATAGTTGGGGACAA CGTATGGAAACGTACGCTAA AACCGAATATGCTCTACGGAGTAAAGGAGCCCTCTTCTTGAAAGGTTTCG CTATAGGATGAGTCCGCGTA AGATTAGCTAGTTGGTAAGGTAATGGCTTACCAAGGCAACGATCTTTAGC TGGTTTGAGAGGATGATCAG CCACACTGGGACTGAGACACGGCCCAGACTCCTACGGGAGGCAGCAGTGG GGAATATTGCACAATGGACG AAAGTCTGATGCAGCCATACCGCGTGTGTGAAGAAGGCCCGAGGGTTGTA AAGCACATTCAATTGTGAGG AAGATAGWGTAGTTAATACCTGCWTTGTTTGACGTTAACTTTAGAAGAAG CACCGGCTAACTCTGTGCCA TCAGCCGCGGTAATACAGAGGGTGCAAGCGTTATTCGGAATTACTGGGCG TAAAGCGCGCGTAGGCGGAT TATTAAGTCAGTTGTGAAAGCCCTGGGCTCAACCTAGGAACTGCATCTGA TAGTGGTAATCTAGAGTTTA GGAGAGGGAAGTGGAATTCCAGGTGTAGCAGTGAAATGCGTAGATATCTG GAGGAACATCAGTGGCGAAG GCCACTTCCTGGCCTAAAACTGACGCTGAGGTGCGAAAGCGTGGGTAGCG AACGGGATTAGATACCCCGG TAGTCCACGCCGTAAACGATGTCAACTAGTTGTTGGTCCTATTAAAAGGA TTAGTAACGAAGCTAACGCG ATAAGTTGACCGCCTGGGGAGTACGGTCGCAAGATTAAAACTCAAAGGAA TTGACGGGGGCCCGCACAAG CGGTGGAGCATGTGGTTTAATTCGATGCAACGCGAAGAACCTTACCATCC CTTGACATCCTGCGAACTTT CTAGAGATAGATTGGAGCCTTCGGGAACGCAGTGACAGGTGCTGCATGGC TGTCGTCAGCTCGTGTCGTG AGATGTTGGGTTAAGTCCCGCAACGAGCGCAACCCTTATCAAAAGTTGCT AACATTTAGTTGAGAACTGT AATGAGACTGCCGGTGATAAACCGGAGGAAGGTGGGGACGACGTCAAGTC ATCATGGCCCTTATGGGATG AGCTACACACGTGCTACAATGGGGGGTACAAAGAGCTGCCAACTGGCAAC AGTGCGCGAATCTCAAAAAA CCTCTCGTAGTCCGGATCGGAGTCTGCAACTCGACTCCGTGAAGTCGGAA TCGCTAGTAATCGTGGATCA GAATGCCGCGGTGAATACGTTCCCGGGCCTTGTACACACCGCCCGTCACA CCATGGGAGTGGATTGCAAA AGAAGTAGGTAGCTTAACCTTCGGGAGGGCTT

Example

(45) The example of such microbe are, but not limited to ATCC 700954 DSM13453, ATCC 700955, DSM13458, DSM 1534, DSM 12351, DSM 12352, ATCC 35932, ATCC 700877, ATCC 49871, DSM: 12353, DSM: 13229.

(46) Pseudomonas putida

(47) Gram Negative

(48) Catalase Positive

(49) Oxidase Positive

(50) Arginine dihydrolase: Positive

(51) Gelatin: Negative

(52) Urease: Negative

(53) Nicotinate: Negative

(54) TABLE-US-00003 >Pseudomonas putida strain 16S ribosomal RNA gene (SEQ ID NO: 3) TGGCGGACGGGTGAGTAATGCCTAGGAATCTGCCTGGTAGTGGGGGACAA CGTTTCGAAAGGAACGCTAA TACCGCATACGTCCTACGGGAGAAAGCAGGGGACCTTCGGGCCTTGCGCT ATCAGATGAGCCTAGGTCGG ATTAGCTAGTTGGTGAGGTAATGGCTCACCAAGGCGACGATCCGTAACTG GTCTGAGAGGATGATCAGTC ACACTGGAACTGAGACACGGTCCAGACTCCTACGGGAGGCAGCAGTGGGG AATATTGGACAATGGGCGAA AGCCTGATCCAGCCATGCCGCGTGTGTGAAGAAGGTCTTCGGATTGTAAA GCACTTTAAGTTGGGAGGAA GGGCAGTAAGCTAATACCTTGCTGTTTTGACGTTACCGACAGAATAAGCA CCGGCTAACTCTGTGCCAGC AGCCGCGGTAATACAGAGGGTGCAAGCGTTAATCGGAATTACTTTGCGTA AAGCGCGCGTAGGTGGTTCG TTAAGTTGGATGTGAAAGCCCCGGGCTCAACCTGGGAACTGCATCCAAAA CTGGCGAGCTAGAGTACGGT AGAGGGTGGTGGAATTTCCTGTGTAGCGGTGAAATGCGTAGATATAGGAA GGAACACCAGTGGCGAAGGC GACCACCTGGACTGATACTGACACTGAGGTGCGAAAGCGTGGGGAGCAAA CAGGATTAGATACCCTGGTA GTCCACGCCGTAAACGATGTCAACTAGCCGTTGGAATCCTTGAGATTTTA GTGGCGCAGCTAACGCATTA AGTTGACCGCCTGGGGAGTACGGCCGCAAGGTTAAAACTCAAATGAATTG ACGGGGGCCCGCACAAGCGG TGGAGCATGTGGTTTAATTCGAAGCAACGCGAAGAACCTTACCAGGCCTT GACATGCAGAGAACTTTCCA GAGATGGATTGGTGCCTTCGGGAACTCTGACACAGGTGCTGCATGGCTGT CGTCAGCTCGTGTCGTGAGA TGTTGGGTTAAGTCCCGTAACGAGCGCAACCCTTGTCCTTAGTTACCAGC ACGTTATGGTGGGCACTCTA GGGAGACTGCCGGTGACAAACCGGAGGAAGGTGGGGATGACGTCAAGTCA TCATGGCCCTTACGGCCTGG GCTACACACGTGCTACAATGGTCGGTACAGAGGGTTGCCAAGCCGCGAGG TGGAGCTAATCTCACAAAAC CGATCGTAGTCCGGATCGCAGTCTGCAACTCGACTGCGTGAAGTCGGAAT CGCTAGTAATCGCGAATCAG TATGTCGCGGTGAATACGTTCCCGGGCCTTGTACACACCGCCCGTCACAA CCATGGGTAGTGAAA

Example

(55) The example of such microbe are, but not limited to Pseudomonas putida MTCC 5388, Pseudomonas putida MTCC 5387.

(56) The microbes used at stage 2 also can be used in adsorbed form or in free form. The bacteria can be immobilized on synthetic plastics, surface-modified carbon nanotubes, poly (tetrafluoroethylene) (PTFE) fibrils, zeolite, clay, anthracite, porous glass, activated charcoal, ceramics, acrylamide, polyurethane, polyvinyl, resins and natural polymer etc. The advantage of the process based on immobilized biomass include enhancing microbial cell stability, allowing continuous process operation and avoiding the biomass—liquid separation requirement. The immobilization can be done as per the method known in prior art.

(57) In yet another embodiment of the present invention, the method of the stage 2 of treatment of spent caustic use a nutrient system consisting of K.sub.2HPO.sub.4 (2-10 g/l), KH.sub.2PO.sub.4 (2-15 g/l), MgCl.sub.2 (0.1-5 g/l), 0.5-2 ml trace elements, sodium carbonate (1-20 g/l), yeast extract (2-10 g/l), ammonium nitrate (3-7 g/l), citrate (1-20 g/l), Oleic acid (10-1000 ppm), pantothenic acid (2-500 ppm), thiamine (2.5-200 ppm). The trace element solution (gram per liter) comprises nitrilotriacetic acid (1.5), FeSO.sub.4.7H.sub.2 (0.05), MnCl.sub.2.4H.sub.2O (0.015), CoCl.sub.2.6H.sub.2O (0.09), CaCl.sub.2.2H.sub.2O (0.50), ZnCl.sub.2 (0.50), CuCl.sub.2.H.sub.2O (0.03), H.sub.3BO.sub.3 (0.02), Na.sub.2MoO.sub.4 (0.02).

(58) In yet another embodiment of the present invention, bioreactor having electrode pair was used in stage-1 followed by bioreactor s with selective microbial consortia in stage-2 and stage-3.

(59) Another embodiment of the present invention relates to the microbes used in stage 2 which can work in pH range 7-9 and oxidized sulfides to elemental sulphur and sulphate where elemental sulphur form is at least 60%.

(60) In yet another embodiment of the present invention, the spent caustic treated in stage-2 is treated using a microbial consortia in stage 3 which resulted in reduced concentration of sulfides, amines, thiols, other sulphur containing compounds, phenols, hydrocarbons, naphthenic acids and their derivatives at least by 90%.

(61) In yet another embodiment of the present invention the consortia of bacteria used in stage 3 include Pseudomonas putida (MTCC 5385), Pseudomonas aeruginosa IOCX (MTCC 5389), Bacillus substilis (MTCC 5386), Achromobacter xylosoxidan IOC-SC-4 (MTCC 25024) Pseudomonas stutzeri (MTCC 25027), Arthrobacter sp. (MTCC 25028), Bacillus subtilis (MTCC 25026), Achromobacter xylooxidan (MTCC 25024).

(62) TABLE-US-00004 S. No. Name of microbe Short Description 1 Pseudomonas putida Gram-negative, rod-shaped, (MTCC 5385) Catalase: +,Cytochrome C oxidase: +,Lecithinase/alpha: −; Casein hydrolysis: −,D-trehalose: −,Poly-β- hydroxybutyric acid: + Extracellular electron acceptor 2 Pseudomonas Gram-negative, rod-shaped, non- aeruginosa spore-forming elecroactive IOCX bacterium. Oxidase test- (MTCC 5389) Positive, indole negative, methyl red negative, Voges-Proskauer test Positive and citrate positive. 3 Bacillus substilis Gram-positive spore forming motile (MTCC 5386) bacterium, catalase-positive, Oxidase -Positive, Indole- Negative, Citrate Negative, Voges-Proskauer- Positive, Protease- Positive, Gelatinase-Negative, MR (Methyl Red)- Negative, Urease-Negative 4 Achromobacter Ccatalase- and oxidase-positive, xylosoxida IOC-SC-4 motile, Gram-negative rod that (MTCC 25024) oxidizes xylose and glucose. citrate-positive. Urease and indole- negative Electroactive in nature. 5 Pseudomonas stutzeri Gram-negative, rod-shaped, non- (MTCC 25027) spore-forming bacterium. Positive for both the catalase and oxidase tests Electroactive in nature. 6 Arthobacter sp. Irregular-shaped Gram-negative (MTCC 25028) rods changing to Gram-positive coccoid cells on further incubation; aerobic; liquefying gelatine slowly, chemo-organotrophs; catalase positive; oxidase positive. Starch Hydrolysis Test: positive, Casein Hydrolysis Test: positive, Gelatin Hydrolysis Test: negative, DNA Hydrolysis Test: negative Lipid Hydrolysis Test: positive, Methyl Red Test: negative, Voges Proskauer Test: negative, Citrate Test: negative 7 Bacillus subtilis Gram-positive spore forming motile (MTCC 25026) bacterium, catalase-positive, Oxidase -Positive, Indole- Negative, Citrate Negative, Voges-Proskauer- Positive, Protease- Positive, Gelatinase- Positive, MR (Methyl Red)- Negative, Urease-Positive, Casein Hydrolysis- Positive, Extracellular electron acceptor 8 Achromobacter Same microbe as S.No.4 xylooxidan (MTCC 25024)

(63) In yet another embodiment of the present invention, the nutrient formulation used in Stage-3 comprises KH.sub.2PO.sub.4, K.sub.2HPO.sub.4, MgSO4, (NH.sub.4).sub.2SO.sub.4, KNO.sub.3, peptone, yeast extract, trace element and multi vitamin solution.

(64) In an embodiment of the present invention, Stage 1 and Stage 2 are performed in same reactor. In another embodiment of the present invention, Stage 1 and Stage 2 are performed in different reactor. In yet another embodiment of the present invention, Stage 2 and Stage 3 are performed in different reactors. In yet another embodiment of the present invention, the working chamber with spent caustic of stage-1 can be abiotic or can be added with the selective bacteria for sulfide oxidation to sulfur.

(65) Yet another embodiment of the present invention relates to a method where in stage 2 the oxygen concentration remains less than 7 mg/l.

(66) In yet another embodiment of the present invention, the electrochemical treatment will result in regeneration of caustic at cathode and to increase its pH to 12-14 and decrease the pH of anode to 7-9.

(67) FIG. 3 Represents the Control (Experiment):

(68) In control experiment, the apparatus for stage-1 only is used without any biocatalyst and any further stages of treatment. The apparatus is similar to the other two approaches comprising of two chambers separated by cation exchange membrane (CEM) 1. One chamber is inserted with graphite rod wrapped with activated carbon cloth (ACC) 2 and considered as working chamber where the spent caustic treatment occurs. The electrodes for this chamber can be varied, viz., graphite plate, carbon brush, carbon paper, graphite felt, etc. The other chamber is inserted with graphite electrode wrapped with stainless steel mesh (SS) 3 and considered as counter chamber. The counter chamber may be of any carbon based electrodes coated with noble metals. One chamber having ACC electrode 1 was fed with spent caustic 4, while the other chamber was fed with distilled water 5. The spent caustic chamber as well as counter chamber was maintained under anaerobic conditions. Both the chambers are equipped with Ag/AgCl (3M KCL) reference electrode 6. Both these electrodes will be connected through only external resistance but no power supply 7 given. The spent caustic 4 loaded to the working electrode chamber and distilled water 5 to the counter electrode chamber. As there is no potential gradient created, no cations 8 exchange via the CEM 1 to the counter electrode chamber observed, resulting in no pH alteration observed in both working and counter electrode chambers. There is no caustic 13 regeneration and sulfur 15 recovery as well as treatment of spent caustic, as there is no additional treatment stages available in control operation.

(69) Having described the basic aspects of the present invention, the following non-limiting examples illustrate specific embodiment thereof.

Example 1: Recovery of Caustic from the Spent Caustic

(70) The recovery of caustic from spent caustic is done in stage-1 process where, one of the two chambers is inserted with graphite rod wrapped with ACC and fed with spent caustic, while the counter chamber is inserted with graphite electrode wrapped with SS and fed with distilled water. The electrodes were connected to potentiostat and voltage of +2V against Ag/AgCl reference electrode was applied to the electrode in counter chamber and pH of the counter chamber was monitored at regular time intervals. The current generation from the system started increasing with time and in 2h of operation, it reached 15±0.5 A/m2, which sustained afterwards at more or less similar value till 12 h of operation. Within 12 h, the pH of the counter chamber reached to 12.67 and the pH of working chamber reached 12.89 indicating the caustic recovery at the counter chamber. Immediately, the content from counter chamber was replaced with fresh distilled water and the applied potential continued. During this cycle, the current from the cell decreased a bit but sustained at more or less similar value (13±0.84 A/m2) till 4 h of operation followed by a gradual decrement to a lower value by 9.sup.th h of operation (4.5±1.26 A/m2) and remained at the same value thereafter. During this cycle, the pH of counter chamber again started increasing immediately after start up and reached to 12.04 in 12 h and the associated pH drop of spent caustic is 7.94.

(71) TABLE-US-00005 TABLE 1 Change in pH with time in working and counter chambers Cycle 1 Cycle 2 Distilled Distilled Time (h) Spent caustic water Spent caustic water 0.sup.th h 14 6.20 12.89 6.23 1.sup.st h 14 6.92 12.02 7.24 4.sup.th h 13.62 8.91 10.67 9.06 8.sup.th h 13.15 10.68 8.86 11.14 12.sup.th h  12.89 12.67 7.94 12.04

Example 2: Simultaneous Recovery of Caustic and Sulfur

(72) The recovery of caustic along with sulfur was attempted by adding selective sulfide oxidizing bacteria to the spent caustic at working chamber of stage-1 bioreactor. One of the two chambers is inserted with graphite rod wrapped with ACC and fed with spent caustic, while the counter chamber is inserted with graphite electrode wrapped with SS and fed with distilled water. Working chamber was inoculated with selectively enriched sulfide oxidizing bacteria (10% v/v). The electrodes were connected to potentiostat and voltage of electrode in working chamber was maintained around −0.3 V (vs Ag/AgCl) by regulating the potential of electrode in counter chamber against Ag/AgCl reference electrode. The applied potential of electrode in counter chamber was adjusted to +1 V initially to maintain the working chamber at −0.3 V but within 1 h, this has come down to +0.8 V due to the start up of biocatalyst function. This was sustained till the end of operation. Change in pH of the counter chamber and the sulfide content of the spent caustic at working chamber was monitored at regular time intervals. The current generation from the system started increasing with time and in 4 h of operation, it reached 5±0.5 A/m2, which sustained afterwards at more or less similar value till 12 h of operation. Within 12 h, the pH of the counter chamber reached to 13.04 and the pH of working chamber reached 12.12 indicating the caustic recovery at the counter chamber. On the other hand, the sulfide content of the spent caustic decreased by 30% in first 12 h. Immediately, the content from counter chamber was replaced with fresh distilled water and the applied potential continued. No significant change in current was observed till 6 h of operation (4.8±0.92 A/m2) followed by a rapid decrement to lower value within 2 h (2.8±1.14 A/m2) and remained more or less similar till the end of operation. During this cycle, the pH of counter chamber again started increasing immediately after start up and reached to 12.68 in 12 h and the associated pH drop of spent caustic is 7.08. (Table-2). Similarly, the sulfide removal reached to about 96% and the sulfur formed during reaction was deposited on the electrode which was measured to be 72±8%. (Table-3)

(73) TABLE-US-00006 TABLE 2 Change in pH with time in working and counter chambers Cycle 1 Cycle 2 Distilled Distilled Time (h) Spent caustic water Spent caustic water 0.sup.th h 14 6.24 12.12 6.18 1.sup.st h 14 6.52 11.06 7.45 4.sup.th h 14 7.91 10.22 9.46 8.sup.th h 13.15 10.24 8.36 11.54 12.sup.th h  12.12 13.04 7.08 12.68

(74) TABLE-US-00007 TABLE 3 Change in sulfide content (% w/v) with time in working and counter chambers Cycle 1 Cycle 2 Sulfide Sulfide content content Time (h) (% W/v) Removal (%) (% W/v) Removal (%) 0.sup.th h 2.52 0 1.42 0 1.sup.st h 2.48 1.59 1.06 57.93 4.sup.th h 2.04 19.05 0.81 67.85 8.sup.th h 1.86 26.19 0.44 82.54 12.sup.th h  1.42 43.65 0.26 96.50

Example 3: Biological Conversion of Sulfides to Sulfur

(75) The spent caustic treated in stage-1 was fed in a CSTR with air bubbling system reactor and to the reactor nutrient system containing K.sub.2HPO.sub.4 (4 g/l), KH.sub.2PO.sub.4 (4 g/l), MgCl.sub.2 (0.2 g/l), 0.5 g/l of trace elements, sodium carbonate (2 g/l), yeast extract (5 g/l), sodium nitrate (4 g/l), citrate (5-10 g/l), sorbitol ester (5 ppm), Oleic acid (100 ppm), pantothenic acid (20 ppm), thiamine (25 ppm) was added. The reactor is inoculated with Thiobacillus sp. The oxygen concentration of the reactor was maintained to 5 mg/ml initially for 2 hr followed 2 mg/ml level for next two hours. The stirring of the reactor was adjusted at 50 rpm. To prevent the release of volatile compounds from the system, gas phases are continuously recycled. The recycled gas is first passed to a condenser (maintained at 5 degree Celsius) to recover the volatile compounds and metabolites. An control without bacteria was also run under similar conditions. Various sulphur species were analysed according to Chen and Moris 1972 (Environmental Science and Technology Vol 6, No. 6, pp 529-537). The quantitative result showed conversion of more than 70% of sulfides to the elemental sulfur.

Example 4: Removal of the Other Contaminants from Spent Caustic

(76) Treatment of effluent of stage 2 is done in continuously fed CSTR. The spent caustic is fed in the reactor (2 L volume) along with nutrient system consisting of K.sub.2HPO.sub.4 (4 g/l), KH.sub.2PO.sub.4 (4 g/l), MgCl.sub.2 (0.2 g/l), 0.5 g/l of trace elements, sodium carbonate (5 g/l), yeast extract (7 g/l), ammonium nitrate (8 g/l), citrate (8 g/l), sorbitol ester (5 ppm), Oleic acid (230 ppm), pantothenic acid (20 ppm), thiamine (25 ppm). The first reactor (2 L volume) was operated as 40 degree Celsius and incoculated with microbial consortium to obtain the cell count of >10.sup.2 CFU/ml and the spent caustic solution was continuously fed with HRT of 24 hrs with percentage of oxygen saturation level was maintained at 100% with stirring of 500 rpm. To prevent the release of VOC from the system, the gas phases were continuously recycled. The recycled gas first passed a condenser to recover VOC and the fed to the same reactor. A control without microbes was run parallel. Un-treated and treated were analyzed for contaminant level using appropriate analytical tools. The results are shown in table-4.

(77) TABLE-US-00008 TABLE 4 Treatment of spent caustic in continuous mode Content in % After treatment with Control without Contaminant microbial blend microbial blend Mercaptans 0.01 2.86 Phenol 0.0002 0.029 Hydrocarbons 0.0001 0.32 Napthenic acid 0.0002 0.029 Amines 1.2 0.015

(78) While specific language has been used to describe the present subject matter, any limitations arising on account thereto, are not intended. As would be apparent to a person in the art, various working modifications may be made to the method in order to implement the inventive concept as taught herein.

Advantages of Invention

(79) 1) Environmentally benign, faster and efficient method compared to the existing methods 2) Less energy intensive than existing processes. 3) Regeneration of caustic is possible which can be re-used as such or at least can be used for make-up avoiding the fresh requirement in bulk. 4) Recovery of sulfur in eco-friendly manner which can avoid the additional treatment methods and also the sulfur can be marketed or may be re-used. 5) ETP operation made easy, economic and also helps in meeting the stringent regulations 6) Reduction in overall cost of the treatment process.