Claims:1. A process for recovering energy from petroleum refinery waste activated bio-sludge, the process comprising:
(a) subjecting the waste activated bio-sludge to a blend of additives in an aqueous medium under mild hydrothermal conditions for pre-treatment to produce a pre-treated bio-sludge;
(b) adding an organic co-substrate to the pre-treated bio-sludge and mixing to form a mixture;
(c) subjecting the mixture obtained in step (b) to a two-stage anaerobic digestion unit; and
(d) recovering biogas and evaluating leftover slurry;
wherein the blend of additives comprise an oxidizing agent, an alkalizing agent, and a conditioning/precipitating agent.

2. The process as claimed in claim 1, wherein the waste activated bio-sludge comprises a hydrocarbon content ranging from 1-15% (w/w).

3. The process as claimed in claim 1, wherein the oxidizing agent comprises potassium permanganate (KMnO4), the alkalizing agent comprises lime, and the conditioning/precipitating agent comprises ferric chloride (FeCl3).

4. The process as claimed in claim 1, wherein the mild hydrothermal conditions comprise temperature in a range 90-130°C, pressure in a range of 0.25-1 bar, and time in a range of 10-30 minutes.

5. The process as claimed in claim 1, wherein the organic co-substrate comprises kitchen waste or cattle dung.

6. The process as claimed in claim 1, wherein the organic co-substrate is added to the pre-treated bio-sludge at a ratio ranging from 1:10 to 10:1.

7. The process as claimed in claim 1, wherein the two-stage anaerobic digestion unit operates at a temperature ranging from 40-60°C.

8. The process as claimed in claim 1, wherein the two-stage anaerobic digestion unit comprises the following steps:
(a) subjecting the mixture obtained in step (b) as claimed in claim 1 to acidogenic microbial blends and incubating for a period of 5-15 days at a temperature ranging from 40-60°C;
(b) neutralizing pH of the mixture using an alkali; and
(c) adding methanogenic consortia to the mixture and incubating for a period of 3-5 days at a temperature ranging from 40-60°C.

9. The process as claimed in claim 1, wherein the waste activated bio-sludge comprises 0.01-20% total solids.
, Description:FIELD OF THE INVENTION:
[001] The invention relates to a method for the treatment of petroleum refinery waste activated bio-sludge rich in hydrocarbon/bio-sludges from an oil refinery wastewater treatment and/or bio-sludge having recalcitrant, toxic hydrocarbon compounds. The aim is to recover energy and simultaneously handle hydrocarbon rich bio-sludge disposal issues.

BACKGROUND OF THE INVENTION:
[002] Petroleum refinery waste activated bio-sludge is rich in hydrocarbon and is a major by-product of biological wastewater treatment plant of petroleum refinery industry. Such bio-sludges are rich in pathogens, polymeric substances, bacterial cells, recalcitrant organics, polyaromatic hydrocarbons (PAHs) and high concentrations of other toxic substances hazardous to environment. Various petroleum refineries generate huge quantities of hazardous oily sludge rich in recalcitrant hydrocarbons and various organic, non-organic compounds. It has been projected that around 8×104 to 1×107 tons of petroleum hydrocarbons are globally produced per annum (Liu et al., 2015).

[003] As per 1992, US-EPA report, oil refineries produce at least 30,000 tons of oil sludge waste streams per annum per refinery. And in India itself, more than 2.62 lac tons of sludge are produced in a per annum. Of which, at least 80% comes under the EPA hazardous waste numbers F037 and F038 (Dasgupta, Dhruva, 2014). Thus, its management and disposal are challenging problems worldwide as is the most significant solid wastes generated by this industry. Various study reported its recalcitrance due to complex and stable emulsion of various toxic, inorganic, and persistent organic pollutants, petroleum hydrocarbons (PHCs), heavy metals, water, and solid particles (Hu et al., 2013; Roy et al., 2016, 2018). Few even characterized it and reported that the petroleum hydrocarbons (PHCs) comprise a large proportion of compounds in such bio-sludge, which includes alkanes (40-50%), aromatics (28?31%), asphaltenes (8?10%) and resins (7?22.4%) (wt.% of total PHCs) (Van Hamme et al., 2000; Liang et al., 2019). Such recalcitrant hydrocarbons are known to be least responsive to even oxidation and need external treatment for bond breakdown (Callaghan et al., 2009). Thus, such sludges with highly hazardous nature and rich with the recalcitrant hydrocarbons pose threat to environment and are difficult to digest. Its high energy oil content and high calorific values makes it is a valuable fuel and are thus, as a practice subjected to gasification, pyrolysis, and incineration to destroy recalcitrant organics into fuel, pyrolytic gas, or carbon dioxide, and for heat recovery (Wang et al., 2016). This results in generation of toxic gases.

[004] For biological treatment such sludges are subjected to bioremediation using mixture of hydrocarbon degrading aerobic microbes resulting in energy loss and release of greenhouse gases due to microbial metabolism (Hu et al., 2013; Pilli et al., 2015; Roy et al., 2018). Thus, the prior art utilizes energy intensive, higher cost, unsafe conditions for handling such bio-sludges.

[005] Alternatively, anaerobic digestion is a sustainable technique for recovering energy and simultaneously handling sludge disposal issues. It is quite common in treatment of municipal and sewage sludges to generate biogas, however, there are very few studies on anaerobic digestion of refinery waste activated sludge due to strong reducing conditions, anoxic levels, hydrocarbon rich environment, paucity of nutrients, high presence of inhibitory conditions (Roy et al., 2018; Liang et al., 2019). Thus, there is scope for exploration for conversion of such hazardous waste to energy for commercialization purposes. The challenge is that the above-mentioned recalcitrant compounds are not completely biodegraded during anaerobic digestion resulting in lower biogas yields and their accumulation, making it difficult to dispose the digested sludge having high metal and oil content. Therefore, any process/methodology which could degrade such recalcitrant compounds, polymeric substances and break the bacterial cell walls in the bio-sludge will enhance biogas generation and aid safe disposal of digested sludge. The digested sludge can be even used as fertilizer.

[006] Among various refinery sludges, biological sludge is generated during biological treatment of wastewater and obtained from trickling filter and clarifier units. Various conventional treatment methods are used for treatment of bio-sludge including landfill and incineration. The main drawback of such treatment methods include: (a) contamination of soil and groundwater due to toxic elements in the bio-sludge; (b) fugitive toxic emissions from incineration; and (c) ashes resulting from the incineration process generating heavy metals in concentrated form.

[007] Therefore, to overcome the limitations associated with the conventional treatment methods the present invention provides a method for the treatment of petroleum refinery waste activated bio-sludge rich in hydrocarbon/bio-sludge from an oil refinery wastewater treatment and/or bio-sludge having recalcitrant, toxic hydrocarbon compounds. Furthermore, the present invention provides a method to recover energy and simultaneously handle hydrocarbon rich bio-sludge disposal issues.

[008] Various processes for bio-sludge treatment have been disclosed in the prior art. US patent 10099949B2 by Individual discloses a method for making a bio-sludge-based biomass fuel. The invention discloses about the introduction of an oxidizing agent gas like ozone and water into a bio-sludge for facilitating lysis of bacteria contained in a bio-sludge to form a pre-treated bio-sludge mixture and then filtering out the oxidized pre-treated bio-sludge under pressure accompanied by drying the bio-sludge solids. These solids are then mixed with oil sludge to form conventional biofuel with great oil absorbability and relatively high heating value resulting into high heat of combustion. This waste oil sludge is mentioned to be obtained from petrochemical industries, degreasing treatments, cutting oils, etc. Therefore, it only discloses one way of utilizing such oily waste to fuels by combustion. However, it does not mention about waste activated sludge obtained from effluent treatment plant refinery for energy recovery.

[009] EP patent 1141177B1 by Texaco Development Corp. discloses use of bio-sludge having or combined with a supplemental hydrocarbonaceous fuels like coal or oil which then undergo co-gasification (in a partial oxidation reaction) to generate synthesis gas, also referred to as "syngas" as the fuel. In this the reaction, temperatures typically range from 900°C to 2000°C, pressures typically range from 100 to 25,000 kPa, implying very high energy involvement. Therefore, it discloses only about the direct utilization of bio-sludge as fuel by gasification, for production of syngas.

[0010] WO publication 1988004282A1 by Waste Energy Corporation discloses a method for restructuring wastewater treatment sludges and conditioned solid wastes to remove non-biodegradable solids and to convert biodegradable matter into methane fuel gas. The restructuring process includes particulate size reduction, enzyme hydrolysis, alkaline chemical hydrolysis, and removal of non-biodegradable particulate matter, including metallic hydroxides. However, it does not disclose a two-phase anaerobic digestion process using blend of additives.

[0011] JP application 2003181486A by STM Engineering Co., Ltd. relates to a method for purifying wastewater such as bio-sludge. However, it does not disclose a two-phase anaerobic digestion process using blend of additives.

[0012] Various non-patented documents have reported methods for treating sludges from an oil refinery wastewater treatment and/or an oil refinery wastes and/or oily hydrocarbon rich bio-sludges for recovering energy in form of biogas.

[0013] Roy, A. et al. discloses about the petroleum hydrocarbon rich oil refinery sludge harboring anaerobic, fermentative, sulfate-reducing, syntrophic and methanogenic microbial populations. These were characterized for managing high volumes of hazardous oily sludges via bioremediation. This mentions only about the presence of methanogenic populations in such sludges and characterization of microbial communities involved in bioremediation. However, it does not disclose about energy recovery [BMC Microbiol. 18 (2018) 1–22].

[0014] Mountouris, A. et al. discloses biotreatment of refinery oily sludge via biotreatment method of biopiles to achieve high biodegradation efficiencies for organic substances and low leaching levels for heavy metals. Again, the focus is on bioremediating the oil refinery sludge and not energy recovery [Desalin. Water Treat. 33 (2011) 194–201].

[0015] Kriipsalu, M. et al. discloses characterization of oily sludge from a wastewater treatment plant flocculation-flotation unit in a petroleum refinery and its treatment implications. It classified the heterogeneous nature of such sludge comprising of different metals, nonpolar aliphatic hydrocarbons, from the benzene, toluene, ethylbenzene, xylene group, xylenes, polycyclic aromatic hydrocarbons, naphthalene, fluorene, and phenanthrene, etc. It mentions about the importance of pre-treatment before any main treatment which may be physicochemical or biological. The biological treatment related to composting, land farming, bio-piles, burning in boilers as fuel; Co-incineration with non-hazardous waste is compared. However, it did not mention about energy recovery as biogas [J. Mater. Cycles Waste Manag. 10 (2008) 79–86].

[0016] Haak, L. et al. reports about the toxicity and biogas production potential of refinery waste sludge for anaerobic digestion. It reports the high toxicity to anaerobes posed by such hazardous oily sludges. Here, ozonation was used as preliminary treatment to improve sludge solubilization and reduce toxicity. This was reported to improve biogas production in both dissolved air flotation unit (DAF sludge) and waste activated sludge (WAS) obtained from refinery waste. Anaerobic biodegradability assay, bio-methane potential test and anaerobic toxicity assay were evaluated for longer SRTs (30-50 days). However, no evaluation with respect to metal or toxins removal was made for analysis of left-over sludge for further use. The study did not focus on any process development for biogas recovery [Chemosphere. 144 (2016) 1170–1176].

[0017] In another study, J. Liang discloses use of semi-continuous stirred tank reactors (CSTRs) for the hydrolysis and acidification of activated sludge from a petroleum refinery. The focus was on hydrogen and VFA production but not biogas generation. The study characterized different microbial guilds as dominating i.e., Coprothermobacter, Fervidobacterium, Caldisericum and Tepidiphilus with potential to degrade petroleum compounds and generate VFAs [J. Pet. Sci. 16 (2019) 428–438].

[0018] Wang, Q. reported the potential for two-phase anaerobic digestion of oil refinery waste activated sludge and its optimization. It further reported about the microbial community analysis related to the same. The study claims the most effective preliminary treatment as thermal pretreatment under 170°C for sludge solubilization (60 minutes, in a muffle furnace). Higher biogas yields were obtained with two-phase anaerobic digestion in comparison to single phase, where the thermophilic first phase was followed with mesophilic second phase with reduction of initial oil content from 5.3 to 1.2% g/g-TS only, having about 64 different compounds as detected by GC-MS analysis. However, in present study, mild conditions in combination with additive blends were used (instead of highly energy intensive high temperature treatment) resulting in higher removal of toxins and hazardous oily compounds (more than 99%) for generation of bio-fertilizer for further use [Sci. Rep. 6 (2016) 1–10].

[0019] Roy, R. discloses anaerobic digestion for solids reduction and detoxification of refinery waste streams i.e., waste activated sludge and float from the dissolved air flotation (DAF) unit. Pre-treatment of these special DAF units by ozonation exhibited improvement in the digestion of such sludges. This paper studied both DAF and WAS from an oil refinery as substrates for anaerobic digestion in order to reduce the quantity of waste and recovery of biogas as a resource. The process had very high HRT of about 30 days and 60 days without evaluation of disposed sludge [Process Biochem. 51 (2016) 1552–1560].

[0020] Sampson I. E. discloses economic details for 5000 kg per day of petroleum sludge plant capacity and further to process design of anaerobic reactor for the treatment of petroleum sludge. In this the petroleum sludge was heated on a hot plate and dried in an oven to remove waste and subjected to anaerobic digestion by inoculating a single species for solids retention time of Sixteen days. However, no mention of waste activated bio sludge was made with no evaluation of left-over sludge for disposal and use [Int. J. Adv. Acad. Res. 4 (2018) 27–61].

[0021] Yang, Q. discloses the method for the treatment of oily sludge (generated from various oil exploitation and smelting process in petroleum industries). It reports anaerobic co-digestion of oil sludge with corn stover for efficient biogas production. It also evaluates the influence of different raw material ratios and inoculum volumes on the properties of the generated gas at thermophilic conditions with retention times of 30 days. The method resulted in generation of 10.16 mL/g VS with 1:1 co-digestion and a TS of only about 3%. However, it targeted oily sludge from oil exploitation and smelting accompanied with high retention times [Sustainability. 12 (2020) 1861].

[0022] Liang J. et al. relates to two-phase anaerobic digestion of oil refinery waste activated sludge. Comparative studies of single-phase and two-phase anaerobic digestion in terms of organic removal, biogas production and methane concentration were conducted. However, it fails to disclose a two-phase anaerobic digestion process using blend of additives [Petroleum Science volume 16, pages 428–438, 2019].

[0023] From the referred prior arts, it can be seen that various methods have been reported for treating sludges from an oil refinery wastewater treatment. However, these methods face certain issues like most of the methods for treating activated sludge obtained from effluent treatment plant refinery do not disclose energy recovery. Some disclose about the direct utilization of bio-sludge as fuel by gasification, for production of syngas. Most of the prior art is related to processes for the aerobic degradation of compounds in such petroleum refinery waste activated bio-sludge using mixed bacterial culture i.e., bioremediation. This biological process is not accompanied with energy generation. So, any methodology which can result in generation of energy from such wastes is highly useful. Therefore, a process is needed to overcome the limitations of the methods available in the art.

[0024] The present invention thus, discloses methodology for the production of biogas from hydrocarbon rich oily bio-sludge rich in microbial cells, polymeric substances, and recalcitrant compounds. Such bio-sludges require pretreatment to overcome the physicochemical properties of bio-sludges that hinder microbial/enzyme accessibility for biogas production and further providing favorable conditions for microbial growth and activity. The problems in the few available prior arts that are solved by the present invention are viz- feasibility, cost, use of energy intensive processes like pyrolysis and gasification for energy recovery, use of hazardous chemicals/conditions, high SRT, low energy recovery, lower oil degradation (may not be suitable for disposal), higher temperature conditions for the process, sludge volume handling issues as the digested sludge doesn’t meet disposal characteristics, treatment of left-over digested sludge.

[0025] The present invention addresses these limitations as it discloses the following technical advancements over the prior art:
• In the present invention, an optimal blend of additives in aqueous medium is disclosed in combination with mild hydrothermal treatment for the preliminary treatment of petroleum refinery waste activated bio-sludge rich in hydrocarbon.
• The blend of additives comprise an oxidizing agent, alkalizing agent and conditioning/precipitating agent, which along with mild hydrothermal treatment i.e., temperature in a range 90-130°C, pressure in a range of 0.25-1 bar, and time in a range of 10-30 minutes; for the preliminary treatment of hydrocarbon rich oily sludge. This results in energy saving and higher sludge disintegration/lysis.
• The methodology in the present invention discloses that after preliminary treatment the petroleum refinery waste activated bio-sludge is subjected to two phase anaerobic digestion, which is capable at wide range of temperature (40 to 60°C). The preferable sludge retention time for the developed process from 5 to 15 days.
• The present invention further discloses addition of organic wastes to petroleum refinery waste activated bio-sludge rich in hydrocarbon at ratio between 1:10 to 10:1, resulting in enhanced biogas production.
• The microbial enumeration data shows higher growth of biogas producing microbes after the disclosed pretreatment method. Thus, the present invention discloses a pre-treatment methodology resulting in better growth conditions due to enhanced sludge lysis and release of organics and micronutrients. This aids biogas generation at wide range of temperatures.
• There is reduction in oil and grease content; the total petroleum hydrocarbons (TPH) and polyaromatic hydrocarbons (PAHs), toxic metals/sulphates/sulphides and total suspended solids (TSS) after the anaerobic co-digestion of pre-treated bio-sludge and organic wastes at ratio between 1:10 to 10:1. This results in generation of digested sludge which is able to meet up the sludge disposal limit and further, enhances conversion of high energy recalcitrant compounds present in hydrocarbon rich oily sludge to biogas. The addition of organic waste at ratio between 1:10 to 10:1, further decreases the metal content in the digested sludge which made it further suitable for disposal.
• The present invention offers dual advantage of degradation of hazardous oily wastes and generation of biogas.
• The analysis confirms removal of metals like hexavalent chromium and others like sulphate, phenol, and sulphides by microbial activity. This results in digested sludge having lower toxicity which is suitable for disposal/use as fertilizer.

SUMMARY OF THE INVENTION:
[0026] The present invention discloses a process for treatment of petroleum refinery waste activated bio-sludge rich in hydrocarbon with an optimized blend of oxidizing agent, alkalizing agent, conditioning/precipitating agent and microbial blend, which synergistically improves the treatment process and biogas yield at low temperature, low pressure, and reduced time compared to conventional methods. The present invention further provides a process that involves anaerobic co-digestion of petroleum refinery waste activated bio-sludge rich in hydrocarbon with organic wastes resulting in enhanced production of biogas with reduction in hazardous characteristics of digested sludge for easy disposal.

TECHNICAL ADVANTAGES OF THE INVENTION:
[0027] The present invention has the following advantages over the cited prior arts:
(i) Use of blend of additives in optimal concentration with low temperature, low pressure, reduced time/mild hydrothermal conditions for the preliminary treatment of petroleum refinery waste activated bio-sludge rich in hydrocarbon results in better sludge lysis and better growth conditions for biogas producing microbes.
(ii) Process used makes it possible to have the anaerobic digestion process at wide range of temperature (40 to 60°C), for lower SRTs. This enables feasible variation in the range of operating temperatures and energy savings.
(iii) Co-addition of organic wastes to petroleum refinery waste activated bio-sludge rich in hydrocarbon at ratio between 1:10 to 10:1, results in enhanced biogas production, further reduction in hazardous compounds in digested sludge.
(iv) Process used results in reduction of oil and grease content, total petroleum hydrocarbons (???), polyaromatic hydrocarbons (PAH), toxic metals/sulphates/sulphides and total suspended solids (TSS), along with simultaneous recovery of energy as biogas from the petroleum refinery waste activated bio-sludge rich in hydrocarbon. This makes the disposal of digested sludge easier due to reduction in volumes and reduced hazardous compounds meeting disposal limits.

OBJECTIVES OF THE INVENTION:
[0028] It is a primary objective of the present invention to disclose a method for treating petroleum refinery waste activated bio-sludge obtained from effluent treatment plant (ETPs) rich in hydrocarbon, wherein the hydrocarbon content is 1 to 15% (w/w)/bio-sludges from an oil refinery wastewater treatment and/or bio-sludge for recovering energy in form of biogas and use of left-over sludge as bio-fertilizers.

[0029] It is a further objective of the present invention to provide a process for treatment of petroleum refinery waste activated bio-sludge rich in hydrocarbon with an optimized blend of oxidizing agent, alkalizing agent, conditioning/precipitating agent and microbial blend, which synergistically improves the treatment process and biogas yield at low temperature, low pressure and reduced time as compared to conventional methods.

[0030] Yet another objective of the invention is to provide a process that involves anaerobic co-digestion of petroleum refinery waste activated bio-sludge rich in hydrocarbon with organic wastes resulting in enhanced production of biogas with reduction in hazardous characteristics of digested sludge for easy disposal.

[0031] It is another objective of the present invention to reduce oil and grease content, total petroleum hydrocarbons (???), polyaromatic hydrocarbons (PAH), toxic metals/sulphates/sulphides and total suspended solids (TSS), along with simultaneous recovery of energy as biogas from the hydrocarbon rich oily bio-sludge. Besides it provides better growth conditions for biogas producing microbial populations. The digested sludge can be used as a fertilizer or easily disposed, due to reduction in hazardous compounds.

BRIEF DESCRIPTION OF THE DRAWINGS:
Figure 1 illustrates the schematic representation of the process.
Figure 2 illustrates the process flow diagram.
Figure 3 illustrates the graphical representation of the bacterial enumeration data.

ABBREVIATIONS:
PAH: Polyaromatic Hydrocarbons
PHC: Petroleum Hydrocarbons
DAF: Dissolved Air Flotation
WAS: Waste Activated Sludge
CSTR: Continuous Stirred Tank Reactors
TPH: Total Petroleum Hydrocarbons
TS: Total Solids
TSS: Total Suspended Solids
ETP: Effluent Treatment Plant
SRT: Solid Retention Time

DETAILED DESCRIPTION OF THE INVENTION:
[0032] Those skilled in the art will be aware that the present disclosure is subject to variations and modifications other than those specifically described. It is to be understood that the present disclosure includes all such variations and modifications. The disclosure also includes all such steps of the process, features of the system, referred to or indicated in this specification, individually or collectively, and any and all combinations of any or more of such steps or features.

Definitions
[0033] For convenience, before further description of the present disclosure, certain terms employed in the specification, and examples are collected here. These definitions should be read in the light of the remainder of the disclosure and understood as by a person of skill in the art. The terms used herein have their meanings recognized and known to those of skill in the art, however, for convenience and completeness, particular terms and their meanings are set forth below.

[0034] The articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.

[0035] The terms “comprise” and “comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as “consists of only”.

[0036] Throughout this specification, unless the context requires otherwise the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated element or step or group of element or steps but not the exclusion of any other element or step or group of element or steps.

[0037] The term “including” is used to mean “including but not limited to”. “Including” and “including but not limited to” are used interchangeably.

[0038] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the preferred methods, and materials are now described. All publications mentioned herein are incorporated herein by reference.

[0039] The present disclosure is not to be limited in scope by the specific embodiments described herein, which are intended for the purposes of exemplification only. Functionally equivalent products and processes are clearly within the scope of the disclosure, as described herein.

[0040] The present invention relates to a methodology which uses sustainable way of recovering energy from petroleum refinery waste activated bio-sludge rich in hydrocarbon, using blend of oxidizing agent, alkalizing agent, and conditioning/precipitating agent in combination with milder hydrothermal conditions. Further, anaerobic co-digestion with organic wastes results in improved biogas generation and digested sludge with better disposal characteristics.

[0041] In an embodiment of the invention, the present invention discloses a process for recovering energy from petroleum refinery waste activated bio-sludge, the process comprising:
(a) subjecting the petroleum refinery waste activated bio-sludge to a blend of additives in an aqueous medium under mild hydrothermal conditions for pre-treatment to produce a pre-treated bio-sludge;
(b) adding an organic co-substrate to the pre-treated bio-sludge and mixing to form a mixture;
(c) subjecting the mixture obtained in step (b) to a two-stage anaerobic digestion unit; and
(d) recovering biogas and evaluating leftover slurry; wherein the blend of additives comprises an oxidizing agent, an alkalizing agent, and a conditioning/precipitating agent.

[0042] The present invention discloses the use of mild hydrothermal treatment conditions i.e., in range of 90°C to 130°C, 0.25-1 bar, 10-30 minutes in the presence of oxidizing agent KMnO4, alkalizing agent like lime, conditioning/precipitating agent FeCl3 within optimized range. This results in huge energy saving, sludge disintegration/lysis and improved growth conditions for biogas producing microbes. This basically weakens the cell walls and ruptures it under combined effect of additives, heat, and pressure to release the intracellular water as free water or vapors.

[0043] In yet another embodiment, the mild hydrothermal conditions comprise temperature in a range 90-130°C, pressure in a range of 0.25-1 bar, and time in a range of 10-30 minutes.

[0044] In yet another embodiment, the organic co-substrate comprises kitchen waste or cattle dung. The organic co-substrate is added to the pre-treated bio-sludge at a ratio ranging from 1:10 to 10:1.

[0045] In another embodiment, the waste activated bio-sludge comprises a hydrocarbon content ranging from 1-15% (w/w) and 0.01-20% total solids.

[0046] In an embodiment of the invention, the two-stage anaerobic digestion unit comprises the following steps:
(a) subjecting the mixture obtained in step (b) as claimed in claim 1 to acidogenic microbial blends and incubating for a period of 5-15 days at a temperature ranging from 40-60°C;
(b) neutralizing pH of the mixture using an alkali; and
(c) adding methanogenic consortia to the mixture and incubating for a period of 3-5 days at a temperature ranging from 40-60°C. The two-stage anaerobic digestion unit operates at a temperature ranging from 40-60°C, for lower solid retention time (SRT).

[0047] In an embodiment of the invention, the development of such a pretreatment process is the novel step which results in sludge disintegration and improves growth conditions for biogas producing microbes. This makes the anaerobic digestion process works at wider range of temperature (40 to 60°C), for lower SRT.

[0048] In an embodiment of the invention, the inclusion of co-digestion with organic wastes is in the ratio of 1:10 and 10:1, which improves biogas recovery from such recalcitrant hydrocarbon rich bio-sludge and further reduction of oil and grease content, total petroleum hydrocarbons (???), polyaromatic hydrocarbons (PAH), toxic metals and total suspended solids (TSS). The invention thus, discloses simultaneous recovery of energy as biogas from the petroleum refinery waste activated bio-sludge rich in hydrocarbon and solution to the sludge disposal issues due to reduction in hazardous compounds.

[0049] In yet another embodiment, waste activated sludge was obtained from refinery ETP and mixed with aqueous blends of additives having oxidizing agent KMnO4 and alkalizing agent. This was followed by hydrothermal treatment conditions i.e., in range of 90°C to 130°C, 0.25-1 bar, 10-30 minutes. This results in breakdown of recalcitrant compounds along with cellular lysis for easier breakdown. After the above pre-treatment the bio-sludge was mixed with organic co-substrates in ratio of 1:10 to 10:1. The above mix was subjected to two-stage anaerobic digestion, where in acidogenic microbial blends were inoculated and retained for 5 to 15 days, between 40-60°C. This was followed by pH neutralization using alkali (if required). Then under strict anaerobic conditions it was mixed using methanogenic consortia and incubated for 3 to 5 days, between 40-60°C. The energy was recovered as biogas and the left-over slurry was evaluated for use as bio-fertilizer. Figure 1 discloses the schematic representation of the process and Figure 2 discloses the process flow diagram of the process.

[0050] In another embodiment, refinery waste activated sludge was collected from Panipat refinery, Indian oil corp. Ltd, Uttar Pradesh, India (29.4762° N, 76.8809° E) and maintained at 4°C for experimentation. The bio-cake and bio-slurry obtained had following characteristics as tabulated in Table 1.

Parameters Bio-cake and Bio-slurry characteristics
Total solids 10.03%
Volatile solids 78.8%
pH 8.0
Density 1.034 g/c.c.
Oil and grease 9.35%
Metal content 1600 ppm Al, 800 ppm Ca, 1600 ppm Fe, 200 ppm K, 260 ppm Mg, 240 ppm Mn, 280 ppm Na, 190 ppm Zn, Ba < 25ppm, Co < 25ppm, Cu < 25ppm, Li < 25ppm, Mo < 25ppm, Ni < 25ppm, Ti < 25ppm and V < 25ppm.
Other hazard Cr(VI)- 22 ppm, SO4- 310 ppm, Phenol 59 ppm, Sulphide 28 ppm
Hydrocarbon content Paraffins-55 ppm, Monocycloparaffins- 50 ppm, Dicycloparaffins-47 ppm, Tricycloparaffins-41 ppm, Tetracycloparaffins- 13 ppm, Pentacycloparaffins- 1, Hexacycloparaffins- 0 ppm, Heptacycloparaffins- 0 ppm, Saturates- 206 ppm, Alkylbenzenes- 29 ppm, Benzocycloparaffins- 19 ppm, Benzodicycloparaffins- 19 ppm, Naphthalenes- 18 ppm, Acenaphenes, biphenyls- 14 ppm, Acenaphthylenes, fluorenes- 21 ppm, Phenanthrenes - 12 ppm, Pyrenes- 10 ppm, Chrysenes- 3 ppm, Benzopyrenes-2 ppm, Aromatics-147 ppm, Thiophenes-0 ppm, Benzothiophenes- 28 ppm, Dibenzothiophenes- 18 ppm, Naphthobenzothiophenes- 1 ppm, Sulfur Compounds-47 ppm
Table 1: Characteristics of Bio-cake and Bio-slurry

Experimental Conditions:
[0051] Sludge with at least 0.01% total solids was mixed with aqueous media having blend of KMnO4, Lime and FeCl3 each in the range of 0.01 to 0.1 gram per gram of dry weight of the bio-sludge, along with 1ml of trace metal solution per Kg sludge. The trace metal solution comprises: CoCl2.6H2O- 200 ppm; NiCl3-100 ppm; Na2SeO3.5H2O-100 ppm; Na2WoO4.2H2O-100 ppm; (NH4)6MoO24.4H2O-100ppm; H3BO3-50ppm; ZnCl2 50 ppm; CuCl2.2H2O 50ppm; MnCl2.4H2O- 200 ppm; HCl (25%, v/v) 1.0 ml. The water content ranges from 95% to 75%.

[0052] The above set was hydrothermally treated between 90°C to 130°C, 0.25-1 bar, 10-30 minutes, under anaerobic conditions; preferably at 100°C for 25 minutes for cell lysis and breaking of various bonds resulting in pre-treated bio-sludge slurry. To the above pre-treated sludge, organic wastes were added as the co-substrate and mixed properly. The ratio of pre-treated sludge and co-substrate can be varied between 1:10 to 10:1; preferably with co-dung. However, better performance was achieved with increased proportion of co-substrate with the bio-sludge; organic waste with total solids (TS) between 10-25%.

[0053] Anaerobic digestion in two stage elucidated better biogas recovery due to exposure of different bacterial guilds with suitable conditions for acidogenesis and methaogenesis. Therefore, in the present invention, the mixture was inoculated with acidogenic microbial blend and retained for at least 5 to 15 days under strict anaerobic conditions, between 40-60°C; preferably at 55? for 12 days.

[0054] Acidogenic and methanogenic cultures were obtained from ongoing two stage bio-methanation plant. Acidogenic consortia used in the invention were collected from the acidogenic digestion reactor and methanogenic cultures were obtained from methanogenic digestion reactor. Tables 2 and 3 disclose the characteristics of the inoculum.

Parameters Inoculum
Total solid (%) 2.24 ± 0.32
Volatile Solid (%) 94.6 ± 0.05
pH 5.2± 0.22
TOC(g/l) 13.7 ± 2.9
Carbohydrates (g/l) 3.6 ± 0.9
Proteins (g/l) 5.5 ± 1.3
Lipids (g/l) -
Cfu/ml 4.6 × 104
Table 2: Physicochemical characteristics of the Acidogenic culture

Parameters Inoculum
Total solid (%) 3.65 ± 0.19
Volatile Solid (%) 85.1 ± 0.05
pH 7.0 ± 0.20
TOC(g/l) 16.1 ± 3.4
Carbohydrates (g/l) 8.8 ± 1.0
Proteins (g/l) 5.4 ± 1.1
Lipids (g/l) -
Cfu/ml 5.7 × 106
Table 3: Physicochemical characteristics of the Methanogenic culture

[0055] The next step comprises of neutralizing the pH of the above slurry with pH adjuster, if required before carrying out second phase of digestion. The methanogenic inoculum (10%) was added and after 3 to 5 days biogas was recovered, preferably after 3 days. The left-over material can be used as fertilizer. Thus, the present invention further provides a process for the treatment of petroleum refinery waste activated bio-sludge rich in hydrocarbon, producing a biogas and bio-fertilizer. The said process works at wide temperature ranging from 40°C to 60°C.

[0056] In another embodiment, the present invention develops a process making the recalcitrant compounds, polymeric substances, microbial cells present, amenable to digestion recovering energy and simultaneously treating the hazardous waste to non-hazardous form by reduction in oil and grease content, total petroleum hydrocarbons (???), polyaromatic hydrocarbons (PAH), toxic metals and total suspended solids (TSS). This can thus be used as bio-fertilizer.

[0057] In the present invention, the above was achieved by the blend of additives used in combination with mild hydrothermal pretreatment, which was further enhanced due to addition of co-substrate. Further, the process used was safe due to use of mild additives and temperatures. However, better performance was achieved with increased proportion of co-substrate or temperature with the bio-sludge.

[0058] For the enumeration of methanogens Wolfe and Balch medium was used, having per liter: NH4Cl 1g; NaCl 0.6g, NaHCO3 5g; KH2PO4 0.3g; K2HPO4 0.3g; MgCl26H2O 0.16g; CaCl2.2H2O 0.009g; resazurin 0.1% solution, Agar 25g; 1 mL; 10 mL of vitamin solution; 10 mL of methanogens mineral solution. 0.2M L-cysteine HCl and 0.2M Na2S9H2O are added separately, after autoclaving. About 10 ml of filter sterilized vancomycin hydrochloride stock solution containing 100 mg/10 ml of distilled water was also added. Vitamins solution was made with 10 mg of each per liter: p-aminobenzoic acid, nicotinic acid, calcium pantothenate, pyridoxine, riboflavin, thiamine, and 5 mg of each per liter: biotin, folic acid, alpha-lipoic acid, and Vitamin B12. Methanogens minerals solution (g/L) was prepared having tri-sodium nitrilotriacetic acid 1.5g; Fe(NH4)2(SO4)2-0.8g; NaSeO3-0.2g; CoCl26H2O-0.1g; MnSO4H2O-0.1g;Na2MoO42H2O-0.1g; NaWO42H2O-0.1g; ZnSO47H2O-0.1g; NiCl26H2O-0.1g; H3BO3-0.01g; CuSO45H2O-0.01g. Reducing agents solutions were prepared separately i.e., 0.2 M solution of cysteine HCl and 0.2 M of Na2S9H2O under strict anaerobic conditions, in hot distilled water and later flushed with N2 gas. The sludge samples were diluted using the above medium under 85% N2, 10% CO2, and 5% H2 conditions. Serial dilutions were made in butyl rubber stopper sealed tubes for plating and enumeration. For the enumeration of acidogens- medium contained per liter: K2HPO4 0.94 g; KH2PO4 0.28 g; NaCl 1.4 g; KCI 1.6 g; MgSO4.7H20 0.2 g; CaCl2.2H20 0.1 g; NH4Cl 1.0 g; acidogen mineral solution 10 ml; vitamin solution 5 ml; 0.1% (wt./vol) resazurin, yeast extract 1 ml, agar, 25 g; distilled water to make up the volume. Vitamin solution was prepared containing 100 mg of each per liter of distilled water: ascorbic acid, calcium pantothenate, choline chloride, folic acid, i-inositol, niacinamide, nicotinic acid, p-aminobenzoic acid, pyridoxal hydrochloride, pyridoxine hydrochloride, riboflavin, and thiamine hydrochloride and biotin about 10 mg. The medium was prepared under 85% N2, 10% CO2, and 5% H2 conditions. After the medium was autoclaved (15 min at 121°C) and cooled, the following additions were made: 10% NaHCO3 (filter sterilized), 50 mM 2-bromoethanesulfonic acid (filter sterilized) and 1% (wt./vol) L-cysteine hydrochloride and 1% (wt./vol) Na2S.9H20. Bromocresol Green 0.01% (wt./vol) was used to enumerate acid producers which revealed a light-colored zone around them. For total bacterial count brewers anaerobic agar medium was used. Oil extraction was performed using Soxhlet extraction method and hexane as extractant (EPA 9071b). The extracted oil was characterized using high-resolution mass spectrometry. The metal analysis was performed with inductively coupled plasma atomic emission spectroscopy (ICP-AES). For TS, VS standard methods were followed (APHA, 21st Edition, 2005).

EXAMPLES:
[0059] Having described the basic aspects of the present invention, the following non-limiting examples illustrate specific embodiments thereof. Those skilled in the art will appreciate that many modifications may be made in the invention without changing the essence of the invention. The process for the treatment of petroleum refinery waste activated bio-sludge and recovering energy from the same mentioned in the present invention is illustrated by the following examples.

Example-1: Effect of additive blends on biogas output
[0060] Bio-sludge with 10.03% TS was mixed with aqueous media having different blends of additives as tabulated below. The blends of additives were evaluated for energy recovery from the petroleum refinery waste activated bio-sludge rich in hydrocarbon, at mild hydrothermal pretreatment (at 100°C for 25 minutes, 0.5 bar). The performance comparison is disclosed in Table 4 as follows:

S. No Additives used Output Biogas (m3/tonne)
1 Nil 2.04
2 FeCl3 0.5% (w/v) 2.91
3 KMnO4 0.5% (w/v) 3.25
4 Lime 0.5% (w/v) 2.63
5 FeCl3 0.5% (w/v), KMnO4 0.5% (w/v), Lime 0.5% (w/v) 9.75
Table 4: Performance Comparison of Example 1

Example-2: Pretreatment with and without additives at different temperatures
[0061] The performance of pre-treating bio-sludge at different temperatures with and without additives was evaluated in terms of biogas production and is disclosed in Table 5.

S. No Conditions Output Biogas (m3/tonne)
1 Hydrothermal pre-treatment at 100°C, 0.5 bar, 25 mins 2.04
2 Addition of additives (FeCl3, KMnO4, Lime) and Hydrothermal pre-treatment at 100°C, 0.5 bar, 25 mins 9.75
3 Hydrothermal pre-treatment at 150°C, 0.5 bar, 25 mins 5.75
Table 5: Performance of Pre-treating Bio-sludge of Example 2
It was observed that even at lower temperatures in the presence of aqueous blends of additives higher biogas yield was obtained.

Example 3: Process conditions for biogas recovery in single and two staged anaerobic digestion
[0062] Waste activated bio-sludge obtained from refinery was mixed with aqueous media having blend of additives and pre-treated hydrothermally at 100°C for 25 minutes, 0.5 bar. The experiments were conducted in single and two staged anaerobic digestion and details are disclosed in Table 6.

S. No Sample Conditions Output Biogas (m3/tonne)
1 Single stage Anaerobic Digestion 7.18
2 Two-staged stage Anaerobic Digestion 9.75
Table 6: Experiments in Single and Two-staged Anaerobic Digestion

Example 4: Process conditions for enhanced biogas with co-digestion
[0063] Waste activated bio-sludge obtained from refinery was mixed with aqueous media having blend of additives and pre-treated hydrothermally at 100°C for 25 minutes, 0.5 bar. About 15% of total solid co-substrate was mixed with the above pre-treated slurry for anaerobic digestion; with about total of 7.5% solid content. The performance comparison is disclosed in Table 7.

S. No Sample Pre-treatment Output Biogas (m3/tonne)
1 Bio-sludge Only Addition of additives (FeCl3, KMnO4, Lime) and Hydrothermal pre-treatment at 100°C, 0.5 bar, 25 mins 9.75
2 Combination of Bio-sludge (85% v/v) and Kitchen waste- (15% v/v) Addition of additives (FeCl3, KMnO4, Lime) and Hydrothermal pre-treatment at 100°C, 0.5 bar, 25 mins 11.4
3 Combination of Bio-sludge (85% v/v) and Cattle dung- (15% v/v) Addition of additives (FeCl3, KMnO4, Lime) and Hydrothermal pre-treatment at 100°C, 0.5 bar, 25 mins 13.9
Table 7: Performance Comparison of Example 4

Example 5: Effect of different pretreatment and addition of organic co-substrates on biogas producing bacterial population
[0064] It is proposed that the pre-treatment disclosed herewith resulted in improvement in bacterial populations capable of enhancing biogas generation, due to presence of favourable conditions, substrates and absence of inhibitors as elaborated by Table 8. Figure 3 discloses the graphical representation of the bacterial enumeration data.

S. No. Sample Type Acidogens Methanogens Total Bacteria
Sample 1 Bio-sludge without any pretreatment 4.20E+03 0.00E+00 2.40E+04
Sample 2 Bio-sludge Pretreated with Hydrothermal (100°C, 0.5 bar, 25 mins) 1.80E+04 1.30E+03 9.10E+05
Sample 3 Bio-sludge Pretreated with Hydrothermal (100°C, 0.5 bar, 25 mins) in presence of aqueous blend of FeCl3, KMnO4, Lime 8.80E+04 3.30E+04 9.70E+07
Sample 6 Bio-sludge Pretreated with Hydrothermal (100°C, 0.5 bar, 25 mins) in presence of aqueous blend of FeCl3, KMnO4, Lime; and co-digested with kitchen waste 1.10E+04 3.60E+04 5.70E+08
Sample 5 Bio-sludge Pretreated with Hydrothermal (100°C, 0.5 bar, 25 mins) in presence of aqueous blend of FeCl3, KMnO4, Lime; and co-digested with cattle dung 6.10E+04 7.60E+05 3.90E+09
Table 8: Effect of different pre-treatments

Example 6: Recovery of energy from oily bio-sludge with generation of non-hazardous waste capable of use as bio-fertilizer
[0065] To generate performance comparison, under pre-treated (at 100°C for 25 minutes, 0.5 bar) and non-pre-treated conditions, under influence of blend of additives (FeCl3 0.5% w/v, KMnO4 0.5% w/v, Lime 0.5% w/v) and different co-substrates (15% of total solid co-substrate was mixed with the pre-treated slurry) for anaerobic digestion. The comparison is made with respected biogas production, total % of oil and grease removed, % metal removed, effects on removal of different hydrocarbon groups (%), reduction in sulphur compounds (%), sulphates, sulphides, etc. The details are tabulated in Table 9.

S. No. Treatment Output Biogas (m3/tonne) Oil & Grease Reduction (%) Chromium, phenol, sulphide, and sulphate removal (%) Reduction in various hydrocarbon groups
1 Bio-sludge without any pretreatment 0 3.32% Chromium - 2.1%,
Phenol - 5.7%, Sulphide - 1.1%,
Sulphate - 6.4% Negligible
2 Bio-sludge Pretreated with Hydrothermal (100°C, 0.5 bar, 25 mins) only 3.40 26.8% Chromium - 4.1%,
Phenol - 18.9%, Sulphide - 6.8%,
Sulphate - 12.9% Monocycloparaffins by 16%, Dicycloparaffins by 2%, Alkylbenzenes by 48%, Benzocycloparaffins by 37%, Benzodicycloparaffins by 38%, Naphthalenes by 49%, Acenaphenes biphenyls by 13%, Acenaphthylenes fluorenes by 19%, Aromatics by 17.5%, Sulfur Compounds by 3.5%.
3 Bio-sludge Pretreated with Hydrothermal (100°C, 0.5 bar, 25 mins) in presence of aqueous blend of FeCl3, KMnO4, Lime 9.92 81.31% Chromium - 52.1%,
Phenol - 68.1%, Sulphide - 72.1%,
Sulphate - 74.19% Dicycloparaffins by 7%, Tricycloparaffins by 29%, Alkylbenzenes by 47%, Benzocycloparaffins by 67%, Benzodicycloparaffins by 67%, Naphthalenes by 66%, Acenaphenes biphenyls by 51%, Acenaphthylenes fluorenes by 52%, Phenanthrenes by 8%, Aromatics by 34%, Benzothiophenes by 43%, Sulfur Compounds by 10%.
4 Bio-sludge Pretreated with Hydrothermal (100°C, 0.5 bar, 25 mins) in presence of aqueous blend of FeCl3, KMnO4, Lime; and co-digested with kitchen waste (15% v/v) 11.4 96.03% Chromium - 69.9%,
Phenol - 92.7%, Sulphide - 83.3%,
Sulphate - 87.1% Paraffins by 16%, Dicycloparaffins by 31%, Tricycloparaffins by 31%, Alkylbenzenes by 59%, Benzocycloparaffins by 70%, Benzodicycloparaffins by 75%, Naphthalenes by 72%, Acenaphenes biphenyls by 62%, Acenaphthylenes fluorenes by 50%, Aromatics by 38%, Benzothiophenes by 48%, Sulfur Compounds by 14%.
5 Bio-sludge Pretreated with Hydrothermal (100°C, 0.5 bar, 25 mins) in presence of aqueous blend of FeCl3, KMnO4, Lime; and co-digested with cattle dung (15% v/v) 13.9 99.67% Chromium - 81.8%,
Phenol - 97.7%, Sulphide - 85.7%,
Sulphate - 93.54% Dicycloparaffins by 35%, Tricycloparaffins by 34%, Alkylbenzenes by 29%, Benzocycloparaffins by 73%, Benzodicycloparaffins by 77%, Naphthalenes by 73%, Acenaphenes biphenyls by 67%, Acenaphthylenes fluorenes by 54%, Phenanthrenes by 15%, Aromatics by 42%, Benzothiophenes by 59%, Sulfur Compounds by 27%.
Table 9: Performance Comparison, Under Pre-Treated and Non-Pre-Treated Conditions

Example 7:
[0066] The performance of untreated and treated waste activated bio-sludge with respect to energy recovery and removal of hazardous compounds was evaluated. The treated bio-sludge was exposed to hydrothermal pre-treatment in presence of aqueous medium containing FeCl3, KMnO4 and Lime and mixed in 1:1 ratio with organic co-substrate for energy recovery via two-stage anaerobic digestion. Table 10 discloses this evaluation.

S. No. Bio-sludge Type Output Biogas (m3/tonne) Oil & Grease Reduction (%) Chromium removal (%) Phenol removal (%) Sulphide removal (%) Sulphate removal (%)
1 Untreated bio-sludge 0 3.32% 2.1% 5.7% 1.1% 6.4%
2 Pre-treated biosluge having organic co-substrate (in 1:1 ratio) 24.7 99.87% 85.6% 99.7% 87.5% 96.7%
Table 10: Performance Evaluation of Untreated and Pre-Treated Bio-sludge