WO2019102364A1 - High rate anaerobic digestion system for solid organic wastes - Google Patents

Abstract

A system (300) for high-rate, compact and efficient anaerobic digestion (AD) of solid organic waste is provided. It includes a first reactor (106), which pre-treats shredded/grinded material generating pre-treated slurry; a second reactor, wherein (108) independent set of anaerobic microbes and enzymes, hydrolyze pretreated slurry generating hydrolyzed slurry; a plug flow digester (120) which performs high rate bio- methanation on the hydrolyzed liquid obtained from the hydrolyzed slurry to generate biogas. The process disengages SRT, HRT, and substrate retention time. It also allows optimizing the process as per the complexity of the substrate. Simpler and complex substrates can undergo anaerobic degradation within a HRT of 5-13 days with volume (NTP) of biogas (65-70% methane) generated being 3.5-5.5 times the volume of all reactors combined. The overall process operates with negligible demand for fresh water and neutralizing agents, minimal by-product generation and results in 30-50% reduction and in capital and operating costs.

Description

HIGH RATE ANAEROBIC DIGESTION SYSTEM FOR SOLID ORGANIC

WASTES

FIELD OF INVENTION

[0001] The present invention generally relates to the field of waste treatment and energy generation, more particularly to an improved anaerobic digestion system and treatment method for solid organic wastes, soluble organic matter, by-products and residues; with special consideration on treatment of lignocellulosic wastes, for energy production.

BACKGROUND OF THE INVENTION

[0002] Background description can include information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.

[0003] Solid organic wastes such as straw, alcohol stillage, vegetable refuse, energy crops and lignocellulosic crop residues, animal wastes, i.e., manure and others have been, since long, considered a potential resource for the production of methane gas. Substantial amounts of money and effort have been directed towards providing a practical process for utilization of this resource. Typical methods involve anaerobic digestion of the waste within a complex system.

[0004] Most prior art systems involve treating an aqueous suspension of the waste having a solids content of about 10% or greater in a fermentative system which requires heating of at least a part of the system. Such systems also typically require intermittent or continuous mixing. Initial capital investment to develop or acquire such systems is generally prohibitive.

[0005] Anaerobic digestion has been recognized to be able to stabilize sludge and other predominantly organic wastes, and produce usable product gas of varying concentration. Anaerobic digestion is a processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste or to produce fuels. Anaerobic digestion uses a consortium of natural bacteria working synergistically to convert organic waste to carbon dioxide and methane in the absence of oxygen, which involves four steps, namely hydrolysis, acidogenesis, acetogenesis, and methanogenesis, of which hydrolysis is the rate-limiting step for most of the complex organic substrates stated earlier.

[0006] In conventional anaerobic treatment processes, a slurry of organic matter, dissolved or suspended, is treated in a single vessel or anaerobic region utilizing suspended bacteria. However, the anaerobic digestion of organic matter may or may not be completed in single step digestion.

[0007] However, major problems faced by the conventional systems is the rate of production of biogas (also interchangeably referred to as “methane gas” or “methane”), also referred as the rate of degradation of the available volatile fatty acids. The root cause of this being the unavailability of adequate amount of specialized micro- organisms for high rate degradation of available food/biomass and required conditions for their growth beyond a threshold limit. Thus, the efficiency of degradation of biomass and/or the amount of biogas generated per kg of volatile solids fed in the reactor and/or the amount of time required to achieve complete degradation is still not sufficient and efficient enough.

[0008] Thus, a major challenge still remains is the complete utilization of organic waste through anaerobic digestion in a fast-tracked manner to achieve biogas production equivalent to theoretical values. Organic wastes utilized include primary and secondary activated sludge obtained from sewage treatment plants, cattle manure, energy crops, lignocellulosic crop residues, waste vegetable and fruits, household and municipal food wastes, concentrated sanitation wastes, brewery wastes, textile industry waste, food processing industry effluents and others. Special attention on AD of lignocellulosic waste is imparted because it being readily available in abundant amounts and otherwise is openly incinerated in most of the developing countries; leading to huge amounts of green-house gas emissions. This type of biomass/substrate/organic waste is typically a more complex type of organic waste wherein the cellulose contained in the biomass is bonded to and covered by an almost inert layer of lignin which makes the degradable matter difficult to be completely accessed for any chemical or microbial process/ reacti on/ degradati on .

[0009] However, existing processes fail to utilize the complete potential of the organic matter using anaerobic treatment process. Disadvantages of the conventional anaerobic treatment include incomplete digestion of substrates, longer retention/degradation times, less biogas generation per unit volume of digesters, lesser utilization of volatile solids fed. Also huge amounts of energy and chemical demands for pre-treatment of substrates, among others. All these factors lead to escalation in the capital and operational cost of the overall system in turn making the process difficult to be adopted on a wider scale.

[0010] There is a need in the art for high rate, simplified, compact, efficient and cost effective anaerobic digestion system and method for organic wastes: solid and soluble organic wastes, by-products and residues, energy crops and lignocellulosics, industrial organic waste and other such solid organic wastes for energy production.

[0011] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

[0012] In some embodiments, the numbers expressing quantities or dimensions of items, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0013] As used in the description herein and throughout the claims that follow, the meaning of“a,”“an,” and“the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of“in” includes “in” and“on” unless the context clearly dictates otherwise.

[0014] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.

[0015] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all groups used in the appended claims. OBJECTS OF THE INVENTION

[0016] Some of the objects of the present disclosure, which at least one embodiment herein satisfies are as listed herein below.

[0017] It is an object of the present disclosure to provide a high rate, efficient, compact and economical anaerobic digestion system for solid organic wastes.

[0018] It is another object of the present disclosure to provide an improved anaerobic digestion system and method for organic wastes: solid and soluble organic wastes, by-products and residues, energy crops and lignocellulosics, industrial organic wastes for energy production.

[0019] It is yet another object of the present disclosure to provide a high rate process for biogas generation using a combination of techniques for pretreatment, hydrolysis, bacterial intensification in two phase anaerobic digestion generating negligible amount of liquid effluent and without the need of neutralization chemicals.

[0020] It is yet another object of the present invention to provide a system and method for ensuring maximum utilization (>85% of volatile solids) of biodegradable matter and conversion to biogas, in the biomass/substrate/organic waste and reduce the hydraulic retention time (HRT) for the process to 5-13 days.

SUMMARY

[0021] The present invention generally relates to the field of waste treatment and energy generation, more particularly to an improved anaerobic digestion system and method for solid organic wastes, soluble organic matter, by-products and residues; with special consideration on treatment of lignocellulosic wastes, for energy production.

[0022] Problems to be solved in the present invention are that there are various technologies or devices or process available but speed of the conventional processes, time required getting the required gas yield, the volume of reactors employed by conventional processes, the space, electrical input required, the byproducts generated, the capital and operational cost is very high. Whereas the amount of utilized volatile solids, the gas formed per unit volume of reactor is less. Further, the conventional processes cannot uncouple the solid, bio-solids and liquid retention time from each other and treatment takes places at non-optimal process conditions. Optimized hydrolysis methodologies and hydrolysis mechanisms for waste treatment or liquefaction is unavailable in the conventional processes or methods. One major problem with conventional processes is that; the whole process is carried out at specific conditions, irrespective of the microbial activity, they may not necessarily be the best conditions for the different variety of microbes involved in the processes. Hence limiting the ability of certain microbes to perform efficiently. For example, the methanogeic bacteria have an optimum growth condition as: pH: 6.8-7.2, temperature: 35-38C etc. and the hydrolysis and acidogenesis bacteria perform their best at ph: 5.5- 6.5 and temperature 32-35C. Majority of the conventional reactor systems carry out the anaerobic digestion of solid organic waste at one set of conditions, temp: 37C and ph: 7- 7.2 which represent best conditions for methanogenesis phase but not quite the optimum for hydrolytic and acidogenetic microbes. There is a huge difference observed in treatment efficiency or microbial action at conditions deviating from the optimum.

[0023] Therefore, there is a need in the art for high rate, simplified, efficient, compact and cost effective anaerobic digestion system and method for organic wastes degradation: solid and soluble organic wastes, by-products and residues, energy crops and lignocellulosics, industrial organic waste for energy production.

[0024] To solve the above problems, the present invention is achieved by the following solution:

[0025] The current invention addresses this issue by carrying out Hydrolysis, acidogenesis in a different reaction vessel at optimum process conditions. The reactor configuration is defined in such a way that maximum utilization of substrate solids is carried out in the hydrolysis reactor. Avoiding bypass of unreacted solids, appropriate growth conditions for microbes and enzymes, alkalinity, periodic mixing and maintaining appropriate solid to liquid ratio, gas generation and its composition etc. are some of the important factors that have been kept in mind while defining the reaction mixture composition, dosing, recirculation, solid dosing and reactor design.

[0026] An aspect of the present disclosure relates to a system for treatment of organic material in a two-stage anaerobic biochemical reactor configuration with occasional recirculation between the two reactors. The system can include, a pre- treatment reactor, a hydrolysis reactor, and a high rate digester. The first reactor receives the organic material to perform pre-treatment, wherein heat and moisture and pre-treating agents penetrate through the inert layer on substrates and exposes the biodegradable matter for further treatment resulting in formation of a pre-treated slurry material. The first reactor may be employed in case of usage of very tough substrates like lignocellulosic crop residue and bio-energy crops etc. The second reactor can receive the pre-treated slurry material from the first reactor or from the size reduction unit, depending on the substrate type, to automatically execute hydrolysis and acidogenesis of the pre-treated slurry or the size reduced material, using a pre- determined dose of mixture of enzymes and microbes secreted by the set of grown anaerobic microbes in the second reactor, resulting in formation of a hydrolyzed slurry material. The plug flow digester receives a hydrolyzed liquid having dissolved organic acids to anaerobically generate biogas by the action of anaerobic microbes present in the plug flow digester, wherein the hydrolyzed liquid having dissolved organic acids is obtained by passing the hydrolyzed slurry material from the second reactor through one or more material separating techniques.

[0027] In an aspect, the first reactor can be a pretreatment reactor for providing thermal treatment to the received organic material.

[0028] In an aspect, the second reactor can be a plug flow reactor (PFR), and wherein the plug flow reactor (PFR) is a jacketed horizontal plug flow reactor having radially mixing mechanism.

[0029] In an aspect, the anaerobic treatment can be methanogenesis, hydrolysis, acetogenesis, and acidogenesis.

[0030] In an aspect, the organic material received in the first reactor can be a size reduced organic material obtained from a shredder/grinder.

[0031] In an aspect, the organic material received in first reactor can include a particle size between 1 mm - 10 mm.

[0032] In an aspect, the organic material received can be pre-treated with a dilute acid-thermal pretreatment to reduce lignin content in the organic material. [0033] In an aspect, the size reduced substrates can be directly fed to the hydrolysis reactor, bypassing the pre-treatment reactor.

[0034] In an aspect the substrate can be simpler organic substrates(SOS) including primary and secondary activated sludge obtained from sewage treatment plants, cattle manure, vegetable and household food wastes, concentrated sanitation wastes, brewery wastes, textile industry waste, food processing effluents and others [0035] In an aspect, the organic material can be Complex organic substrates

(COS) but not limiting to lignocellulosic crop residues, agro wastes, yard clippings, bioenergy crops etc.

[0036] In an aspect, the first reactor can receive fresh water along with the organic material and the second reactor can receive fresh water along with the pre- treated slurry material.

[0037] In an aspect, the solids separating arrangements can be selected from any or combination of bar screens, vibrating screens, rotary drum screens, and a basket centrifuge.

[0038] In an aspect, the plug flow digester can include a clarifier to maintain a pre-determined concentration of the second set of grown anaerobic microbes in the plug flow digester.

[0039] In an aspect, the plug flow digester can include a solid-liquid-gas separator for efficient separation.

[0040] In an aspect, the first reactor, pre-treatment reactor, may or may not be utilized for simpler substrates.

[0041] In an aspect the first reactor and the second reactor may function in batch mode or in the continuous mode.

[0042] In an aspect, the first set of grown anaerobic enzymes and the second set of grown anaerobic enzymes can be selected from any or combination of cellulase, xylase, amylase, arabinose, glucanase, b-glucosidase enzymes and other such species [0043] An aspect of the present disclosure relates to a method for treating organic material in a two-step anaerobic biochemical reactor configuration with recirculation between the two reactors. The method include the following steps: thermal or chemical pretreatment of size reduced organic material obtained from a shredder, resulting in formation of a pre-treated slurry material at a first reactor of a system; an anaerobic treatment and digestion of the pre-treated slurry material received from the first reactor using a pre-determined dose of enzymes secreted by a first set of grown anaerobic microbes in the second reactor, resulting in formation of a hydrolyzed slurry material at a second reactor and a plug flow digester of the system can receive a hydrolyzed liquid having dissolved organic acids to automatically generate biogas by the action of anaerobic microbes present in the plug flow digester, wherein the hydrolyzed liquid having dissolved organic acids is obtained by passing the hydrolyzed slurry material from the second reactor through one or more material separating techniques to automatically separate unreacted solids, the hydrolyzed liquid having dissolved organic acids and formed gases from the hydrolyzed slurry material, and wherein the plug flow digester comprise of a clarifier or a solid-liquid-gas separator to maintain a pre-determined concentration of the second set of grown anaerobic microbes in the plug flow digester.

[0044] In an aspect, when the proposed anaerobic digester (AD) is used for lignocellulosic waste the process can be completed within a hydraulic retention time of 9-13 days with a methane yield of 245-250ml CH4/gVS fed. A Volatile solids utilization between 70-90% has been achieved in the process with the combination of techniques as discussed. The biogas volume (NTP) generated observed is 3.5-5 times the total reactor volume as per the methodologies displayed in certain examples mentioned in the disclosure.

[0045] In an aspect, the liquid digestate generated may be selectively and occasionally recycled to an extent of 50 to 80% of the fresh water demand for mixture preparation.

[0046] In an aspect, the phase 1 and phase 2 reactors when commissioned to continuous process do not require any neutralizing agents as per the proposed procedures.

[0047] In an aspect, the process according to the present invention when applied for organic food waste or simpler organic waste can help to reduce the hydraulic retention time drastically. As a result anaerobic digestion of food waste can be completed within a retention time of 5-8 days with a methane yield of 280-300ml CH4/gVS fed. The biogas (65-70% methane) volume generated/cu.m was 3.5-5.5 times the total reactors volume.

[0048] In an aspect the proposed process and the respective system enables to achieve a reduction in capital cost by a factor of 50% and reduction in production cost by a minimum of 40% for anaerobic digestion of simpler and complex solid organic wastes.

[0049] Other features of embodiments of the present disclosure will be apparent from accompanying drawings and from detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and together with the description, serve to explain the principles of the present disclosure.

[0051] The diagrams are for illustration only, which thus is not a limitation of the present disclosure.

[0052] FIG. 1 illustrates a block diagram of a proposed improved anaerobic digestion system, in accordance with the embodiments of the present disclosure.

[0053] FIG. 2 illustrates a detailed block diagram of the proposed improved anaerobic digestion system, in accordance with an exemplary embodiment of the present disclosure.

[0054] FIG. 3 illustrates an exemplary flowchart of the proposed improved anaerobic digestion system, in accordance with the embodiments of the present disclosure.

[0055] FIG. 4 illustrates a flowchart for the exemplary accelerated anaerobic digestion system, in accordance with an exemplary embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0056] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims. Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the "invention" may in some cases refer to certain specific embodiments only. In other cases, it will be recognized that references to the "invention" will refer to subject matter recited in one or more, but not necessarily all, of the claims.

[0057] As used in the description herein and throughout the claims that follow, the meaning of“a,”“an,” and“the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of“in” includes “in” and“on” unless the context clearly dictates otherwise.

[0058] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g.,“such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention. Various terms as used herein are shown below.

[0059] To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.

[0060] Various objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like features.

[0061] As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure.

[0062] Unless otherwise defined, 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 invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety for all purposes. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

[0063] The present invention generally relates to the field of waste treatment and energy generation, more particularly to an improved and accelerated anaerobic digestion system and method for organic wastes.

[0064] According to a survey conducted, the amount of waste generated across the world crossed approximately 12 Billion tons per year in 2002, which constituted of 11 Billion tons of Industrial waste and 1.6 Billion tons of Municipal solid waste (MSW). Out of the total Municipal solid waste around 40% is biodegradable organic waste as per another survey conducted by the World Bank. Also, agricultural waste in India alone stands to an average of 600MT annually and the MSW stands to around 48MT annually (year 2011-12).

[0065] There can be many types of raw materials (substrates) from which biogas production is possible the amount of gas and the amount of methane content in biogas is solely dependent on the process that is followed for production and the type of substrate. Main substrates employed include, primary and secondary sludge obtained from sewage treatment plants, cattle manure, energy crops including lignocellulosic crop residues, vegetable and household food wastes, concentrated sanitation wastes, brewery wastes, textile industry waste, food processing industry effluents and others.

[0066] All these wastes have variable amounts of methane production potential and hence would prove to be a profitable way of energy and heat generation with proper techniques and systems employed for the same. This would also help to achieve a carbon neutral cycle in a faster way. For the current innovation a similar type of biodegradable material with lignocellulosic content having immense availability is taken into consideration and the process is developed around the same.

[0067] The composition of every waste varies in terms of carbohydrates, proteins, fats, lignin and other complex compounds which act as the main source for methane production. The concentration and easy availability of these complex compounds in the employed substrates plays an important role in the anaerobic digestion process and efficient degradation of the substrate. Substrates in which the complex compounds can be easily degraded by simple enzymatic and microbial hydrolysis into simple organic acids can be categorized as simple organic substrates (SOS).

[0068] For some substrates like the energy crops or crop residues, the access to cellulose and other complex chemicals is hindered by a complex component, lignin, present in the substrates and hence anaerobic degradation of such complex substrates poses a challenge. A number of pretreatments for the successful exposure of the complex constituents and destruction of lignin are undertaken before anaerobic digestion of such substrates. Acid hydrolysis, Alkali hydrolysis, steam explosion, size reduction, microbial or enzymatic hydrolysis are some methods of pretreatments that are adopted according to the suitability to the substrates.

[0069] For the proposed technique of biogas generation from solid organic substrates, the substrate type can be divided into two different categories; the simple organic substrates (SOS) in which the lignin content is negligible. The others which can be broadly categorized as complex organic substrates (COS) which mainly constitute bioenergy crops, agricultural wastes, lignocellulosics or residues. The presence of high amounts(>5% of dry solids) of lignin in these substrates makes it difficult for these substrates to be hydrolyzed and liquefied easily.

[0070] Anaerobic digestion is a four step process comprising of Hydrolysis,

Acidogenesis, Acetogenesis and Methanogenesis. These stages are carried out by different set of micro-organisms which perform various functions. Different set of micro-organisms survive well and perform best at different environmental/process conditions. One major problem with most of the conventional anaerobic digestion process methodologies is that the whole process is carried out at specific conditions, without keeping in mind the microbial activity and favorable conditions for various microbial groups that perform the four individual functions. Hence limiting the ability of certain microbes to perform efficiently. For example, the methanogeic bacteria perform best in process conditions where pH: 6.8-7.2, temperature of mixture: 35-38C etc. Whereas, the hydrolytic and acidogenetic bacteria perform their best at ph: 5.5-6.5 and temperature 32-35C. Majority of the conventional reactor systems carry out the anaerobic digestion of solid organic waste at one set of conditions, temp: 37C and ph: 7- 7.2 which are favorable for methanogenetic microbes and not the hydrolytic or acedogenetic microbes. There is a huge difference observed in treatment efficiency when such discrepancies prevail.

[0071] The current invention addresses this issue by carrying out Hydrolysis, acidogenesis in a different reaction vessel at process conditions favorable for these processes. The reactor configuration is defined in such a way that maximum utilization of substrate solids is carried out in the hydrolysis reactor (second reactor). Avoiding bypass of unreacted solids, appropriate growth conditions for microbes and enzymes; alkalinity, periodic mixing and maintaining appropriate solid to liquid ratio, gas generation and its composition etc. are some of the important factors that have been kept in mind while defining the reaction mixture composition, dosing, recirculation and solid dosing. [0072] It was observed that at a certain set of condition in the hydrolysis reactor; the gas generated consists predominantly of carbon dioxide and no traces of methane are observed. This is an indication of well-developed conditions predominantly favoring hydrolysis and acedogenesis of solid waste. For which the retention time of hydrolysis of lignocellulosic material such as com stover, rice straw and wheat straw is in the range of 1-6 days. For simpler organic substrates (SOS) such as domestic food waste, primary sludge and food industry waste the retention time of hydrolysis reactor ranges between 12-48 hours.

Pre-treatment and post hydrolysis separation:

[0073] The high rate digester is fed with the liquid separated from the hydrolyzed mixture by using separating techniques like bar screen, vibrating screens, rotary drum screens or a basket centrifuge. The hydrolyzed liquid obtained is rich in Volatile fatty acids (VFA) and has a dissolved COD content ranging from 2000mg/ltr to 60,000mg/ltr. The waste solids obtained after filtering the liquid is subjected to composting to ensure 100% treatment of organic solids. The unreacted substrates may or may not be recycled depending on the strength of the hydrolyzed liquid obtained.

[0074] The present method or process proves helpful in-case of treating organic waste with non-biodegradable impurities in the range of 5-10% of substrate weight, since the filtration mechanism filters out maximum number of solids allowing only liquid to enter into the digester. This keeps away unwanted and un-useful material from entering the digestion reactor and occupying the active reaction volume. This is one of the highlights that distinguished the proposed methodology conventional systems. The proposed advantage makes the process idea for treatment of Organic fraction of Municipal solid waste (OFMSW) obtained post segregation from automated systems and techniques.

[0075] The strength of liquid hydrolysate obtained after solid liquid seperationcompletely depends on the reaction conditions in the hydrolysis reactor; the amount of solids fed/hour, the pretreatment carried out on the material fed, activity of the enzymes and microbes present etc. Simpler organic substrates (SOS) do not require intense pretreatment techniques. Whereas advanced pre-treatment like thermal pretreatment, acid/alkali pretreatment can help to enhance the hydrolysis process for tough and difficult substrates and minimize the retention time in the hydrolysis stage. For agricultural waste containing lignocellulosics, hydrolysis is the rate limiting step and takes up maximum time of the degradation process. The use of advance pretreatment techniques is hence necessary so as to increase the overall rate of the anaerobic digestion process. Physical pretreatment such as size reduction increases the internal surface area and exposes the easily biodegradable material to hydrolyzing microbes and enzymes. Dilute-acid pretreatment helps to enhance the dissolution of complex compounds like cellulose which can be easily consumed by the microbes in hydrolysis stage. Thermal pretreatment can help to rupture the inert lignin wall exposing the cellulose to be consumed by microbes. The alkalinity of the recycled digestate as the pretreatment medium helps to reduce and at large diminish the need of addition of external neutralizing agents. The selectively recycled digestate fraction may vary between 0-0.8 as per the requirement of the process. Part of the digestate may or may not be required to dilute the digester influent. The use of digestate alkalinity against alkali like NaOH and NaHC03 helps to save in the cost of neutralization agents thus contributing to the savings in operation costs. The rate of utilization of volatile solids poses a major challenge in anaerobic digestion of SOS and agricultural wastes. The total residence time required for satisfactory reduction of substrates by anaerobic digestion process ranges between 25-45 days using conventional techniques. The changes in the conditions in the reactor proposed by the current technique, the use of pretreatments coupled with reactor configuration enables to greatly reduce the time required for hydrolysis for the same amount of reduction. The reduction in volatile solids (VS) fed to the reactor ranges between 50-90% at the end of the hydrolysis process. The time required by the hydrolysis process ranges between 12 hours to 6 days. Maximum amount of solids are utilized in case of simple organic substrates (SOS), whereas the hydrolysed solids for agricultural waste may or may not be selectively recycled.

[0076] The conventional processes carry out methanogenesis or the methane generation in the same reactor as the hydrolysis and other stages. These reactors are stirred at regular intervals of time and all the sets of microbes exist in the reactor suspended in the slurry. As the mixing keeps the reaction mixture homogenous a fraction of microbes is washed out of the system with the outgoing material. Thus keeping the microbial population limited. Such equilibrium supports a limited amount of material degradation and thus the conventional processes are slower and can bear lesser material load. In-case of excess organic load, the acid generation process dominates and the reaction mixture turns acidic hindering the growth of microbes. This is one of the biggest limitations of the conventional processes. The proposed technique approaches this limitation with a number of changes in process. Post hydrolysis and solid-liquid separation, the hydrolyzed liquid (VF A/COD rich liquid) is fed to the anaerobic digester. The digester with influent COD ranging from lOkg COD/cu.m to l5kg COD/cu.m. The periodic stirring is omitted and natural setting of microbe flocks and granules is encouraged by change in reactor configuration. A high rate digester with the granulation of sludge, higher settling velocity and addition of necessary in-organic nutrients is advocated. Organic loading of l0-20kg COD/cu.m/day result in a removal efficiency of 89.6% to 91.2% when temperature of reaction mixture is maintained at mesophilic conditions, 35-37C. A gas yield approximately in the range of 0.32-0.36 cu.m CH4/kg COD utilized can be obtained. The biogas composition ranges from 55.3% methane to 71.6% in terms of methane content at various stages of the methanogenesis reactor. This technique of methanation has been previously employed for reduction or treatment of highly concentrated liquid effluents, having high concentration of dissolved impurities, no solids, from textile, food processing, pharmaceutical, hospitality industries. In the proposed technique by engaging hydrolytic liquefaction and then treating the concentrated liquid stream by high rate digestion technique, a decrease in the overall time required for digestion is observed. Whereas devising a process for faster hydrolysis of solids and its liquefaction, with a good efficiency remains the backbone of the proposed technique. The division of the conventional process into two stages and the use of separation techniques gives a good hold on the unbalanced process and helps to broaden the threshold toxicity values adhering to which the conventional reactors functioned.

[0077] Accordingly, the present disclosure provides an accelerated process for biogas generation using a combination of techniques for pretreatment, hydrolysis, bacterial intensification in two phase anaerobic digestion without generating any liquid effluent and without utilizing any neutralization chemicals.

[0078] In an embodiment, the subject matter, in general, relates to anaerobic digestion systems, and in particular, to an accelerated and efficient process for biogas generation by incorporation of a compact two-phase digestion system.

[0079] In an embodiment, a system according to the present disclosure incorporates a two-phase anaerobic digestion process, wherein the process includes a combination of techniques such as e.g. waste pre-treatment, hydrolysis, bacterial intensification, and methogenesis. Notably, the system produces biogas without utilizing neutralization chemicals and generates negligible amount of liquid effluent in a much lower hydraulic retention time.

[0080] In an exemplary embodiment, firstly, an organic dry agricultural waste/biomass that includes lignocelluloses material is reduced in size (preferably having a particle size of less than 3 mm). As a result, a rupture of the lignin cover takes place and the cellulosic material is exposed. The material is then subjected to thermal or acid/alkali pretreatment followed by hydrolysis, solid-liquid separation with re- circulation and anaerobic digestion.

[0081] In an aspect the system and method enables to ensure complete digestion of biodegradable organic matter fed to the system and thereby achieves/provides maximum yield in gas production nearing theoretical values.

[0082] In an aspect, a process as proposed according to the present disclosure has an ability to work at zero liquid discharge rates. The digestate is passed through a solid liquid separation mechanism and the liquid obtained is partially or completely recycled so as to maintain the concentration of nutrient, microbes, enzymes or inhibitory ions. [0083] An aspect of the present disclosure relates to a unique process for biogas generation from solid organic wastes. The process can use a combination of one or more techniques such as physical pretreatment, enzymatic hydrolysis, simple solid-liquid separation, bacterial intensification and anaerobic digestion etc. The process can disengage the retention time of bio-solids, substrate-solids and the liquid medium. The process demonstrates a procedure for anaerobic digestion (AD) of solid organic waste and generating negligible amounts of waste liquid and sludge, while ensuring satisfactory utilization of solid waste. The process runs smoothly without the need of neutralization chemicals dosing at steady state. This technique of biogas generation can be adopted for various types of solid organic wastes including agricultural residues, lignocellulosic wastes, domestic food waste, slaughter house waste, municipal sewage and waste from food industry etc. Providing enzymes and microbes with optimum growth conditions, the process can be a combination of three main stages, pretreatment, hydrolysis and digestion. This result in a faster and efficient rate of degradation at every stage. The digestion of domestic food waste/municipal organic waste by this methodology provides a methane yield of 280-300ml CH4/gVS fed in a retention time of 5-8 days with‘the amount of biogas produced/ltr of total reactor volume to be 3.5-5.5 (60-65% methane)’. Similar results are applicable in case of treatment of agricultural waste. Com stover digestion yields 245-250ml CH4/gVS fed after retention time of 9- 13 days with amount of biogas produced/ltr of total reactor volume to be 3.5-5 (60-65% methane). The main highlight of the process is reduction in the capital cost and the operation cost by a minimum of 40% due to reduction in retention time of the process and bio-digesters without stirring mechanism.

[0084] Referring now to FIG. 1, a block diagram of a proposed improved anaerobic digestion system 100, and FIG. 2 a detailed block diagram of the proposed improved anaerobic digestion system, in accordance with an exemplary embodiment of the present disclosure is provided.

[0085] As shown in FIG. 1, the present invention involves four major stages viz. a size reduction treatment(s) stage 102, a pre-treatment stage 103, a hydrolysis stage 104, an anaerobic digestion stage 106, and the final product is stored in biogas storage 107. A shown in FIG. 2, the proposed improved anaerobic digestion system 200 receives a biomass 202 as an input and the system 200 includes a size reduction stage 204, a first reactor 206 to receive the sized reduces biomass, a second reactor 208 to perform anaerobic digestion phase and generate a biogas 210 at a plug flow digester 209.

[0086] In an embodiment, according to the category of the substrates the treatment methodology of the substrates may vary significantly. The complex substrates require undergoes size reduction followed by chemical/thermal pretreatment and then enzymatic-microbial hydrolysis for efficient and faster conversion of complex compounds into simpler organic acids, C1-C6 carboxylic acids. Whereas, the simpler organic substrates do not require chemical/thermal pre-treatment.

[0087] In the size reduction treatment(s) phase 102, a physical size reduction treatment s) 204 are applied on the biomass 202 received as input. In this phase, the size reduction of dry biomass, preferably having 14-16% moisture content, is done to approximately < 3mm particle size. In an implementation, size reduction pretreatment is adopted by to get a particle size less than 5 mm, this in turn helps to rupture the lignin cover and expose the cellulosic material to a good extent, in lignocellulosic substrates. The size reduction of dry biomass also increases the internal surface area and exposes the biodegradable material to hydrolyzing microbes and enzymes.

[0088] In pre-treatment phase 103, a dilute-acid pretreatment helps to enhance the dissolution of complex compounds like cellulose which can be easily consumed by the microbes in hydrolysis stage. Thermal pretreatment can help to rupture the inert lignin wall exposing the cellulose to be consumed by microbes and enzymes. The alkalinity of the recycled digestate as the pretreatment medium helps to reduce and at large diminish the need of addition of external neutralizing agents. The selectively recycled digestate fraction may vary between 0-0.8 parts of the fed liquid, as per the requirement of the process. Part of the digestate may or may not be required to dilute the digester influent. The use of digestate alkalinity against alkali like NaOH and NaHC03 helps to save in the cost of neutralization agents thus contributing to the savings in operation costs. [0089] In an exemplary implementation, the size reduction for can be achieved by shredding/grinding, the material to a particle size less than 5mm. Once shredded, the material is then mixed in fresh/recycled water in the ratio varying from 1 : 1 to 1 :3 by weight to undergo thermal or chemical pretreatment depending upon the composition of waste stream. A substrate containing lignin more than 5-8% of the total solid weight, is categorized as the complex organic substrate and needs to undergo dilute acid pretreatment. Sulphuric acid 0.5-1% by weight of the total dry solids needs to be mixed with the slurry and is heated to 100-120C for 20-60 minutes with maintaining reaction pressure at 1.2-1.5 bar. The pretreatment takes place in a batch reactor (first reactor 206) with external heating.

[0090] In hydrolysis phase 104, a hydrolysis and acidogenesis of the biomass takes place. Post acid pre-treatment the material is cooled and hydrolyzed after neutralizing (by NaOH or NaHC03) it to ph: 6-6.5 The cooling is done by natural air circulation and periodic mixing. The material is then added to the enzymatic-microbial hydrolysis and further diluted by fresh water to obtain a resultant solid content ranging from 5% to 8%. The hydraulic retention time for mixture for hydrolysis is between 12 hours to 6 days. The temperature in the hydrolysis is maintained at 32-35C. The mixture is stirred occasionally and the second reactor 208 is operated in continuous mode. The hydrolysis reactor or the second reactor is a horizontal plug flow reactor with a height to diameter (H/D) ratio about 5-8 and provided with a radial mixing mechanism. The pH in the reactor is maintained between 5.5-6.5. The outgoing material is fairly hydrolyzed with a reduction in volatile solid content by 50-80%. The ph in the hydrolysis mixture is observed to around 5.5-6.5 at appropriate loading conditions.

[0091] In an exemplary implementation, either of Acid hydrolysis, Alkali hydrolysis, steam explosion, size reduction, microbial or enzymatic hydrolysis methods of the pretreatments can be adopted according to the suitability to the substrates.

[0092] The hydrolyzed mixture having a solid content from 1-4% is filtered through a solid-liquid separating mechanisms. The liquid obtained has a pH ranging in 5.5-6.5 with a COD content of l0,000mg/ltr to 60,000mg/ltr. The liquid obtained is further fed to a high rate digester operated on a continuous mode performing acetogenesis and methanogenesis.

[0093] In anaerobic digestion phase 106, a high rate digester is designed considering the concentration of COD and Volatile fatty acids in the influent. Important components of the digester being the Gas-Solid-liquid separator the digestion chamber and the inorganic sludge collection chamber. The high rate digester is designed to work with the use of granulated sludge having high methane activity for efficient methanation of dissolved COD. The amount of granulated sludge and the activity are dependent on various factors like the up-flow velocity, the COD fed/cu.m/day and the temperature of reaction mixture.

[0094] The digester has a Gas-solid-liquid separator for separating the biosolids, liquid and gas formed (biogas 210) effectively, maintaining the concentration of microbes is accomplished by the use of a solid retaining mechanism at the top. The reactor has a vertical plug flow arrangement, with a height to diameter ratio ranging between 5 to 8. A part of the digestate is wasted in the form of sludge which can be used as organic fertilizer on dewatering. The flow of the influent into the digester is a regulated gravitational flow. The mixing in the digester is induced by the flow of influent. The granulated sludge is in a fluidized state due to the continuously up-flowing influent this leads to higher interfacial contact between the dissolved COD and the granulated sludge resulting to higher rate of degradation. A reduction of 89.6-91.2% of the dissolved COD is observed at the optimum hydraulic retention time of 24-48 hours and a recycle ratio of 0-0.9. A gas yield ranging from 0.32-0.36 cu.m of methane/kg of VFA utilized is obtained. Granules of sludge ranging from l-4mm are present in the digester with a settling velocity in the range 40-60 m/hour which have a higher methane activity per kg of VS S.

[0095] In biogas storage 107, the gas produced (biogas) in the bioreactor is stored.

[0096] Accordingly, the system and method enables to ensure the complete digestion of biodegradable organic matter fed to the system and thereby achieves/provides maximum yield in gas production nearing theoretical values. [0097] In an embodiment, an anaerobic digestion process is two stage high rate processes for the anaerobic digestion of solid organic waste, which uses enzymatic hydrolysis for liquefaction of waste in the first stage and digestion of liquefied waste in the second stage by a high rate anaerobic digester.

[0098] In an embodiment, the hydrolysis can be carried by a predefined dose of enzymes, secreted by anaerobic microbes, added with the waste in each batch.

[0099] In an embodiment, the anaerobic digestion process can be applicable for all types of solid organic wastes, including cooked/uncooked food waste, agricultural wastes including lignocellulosic material, slaughter house waste, municipal organic waste, garden clippings etc.

[00100] In an embodiment, for the purpose of categorization the process defines waste by the percentage of lignin present in the waste. For waste having lignin content (>5% of dry solids) a dilute acid thermal pretreatment can be employed post size reduction and prior to enzymatic/microbial hydrolysis. The particle size of material can be hydrolyzed should be less than 3mm, irrespective of the category of waste. By applying a combination of size reduction, thermal dilute acid pretreatment, enzymatic hydrolysis, separation of concentrated liquid and digestion techniques; the digestion time of lignocellulosic wastes has been reduced dramatically. As a result the whole process can be completed within a retention time of 5-13 days with a methane yield of 245-250ml CH4/gVS fed. By applying a combination of size reduction, enzymatic hydrolysis, thermal dilute acid pretreatment, separation of concentrated liquid and digestion techniques; the digestion time of organic food waste/municipal organic wastes has been reduced dramatically. As a result the whole process can be completed within a retention time of 3-8 days with a methane yield of 280-300ml CH4/gVS fed. A volatile solids utilization in the range 70-90% has been achieved in the process with the combination of techniques as provided in the disclosure. A biogas generated/cu.m of total reactor volume of 3.5-5 for corn Stover is achieved as per the methodologies displayed in certain examples mentioned in the disclosure. A biogas generated/cu.m of total reactor volume of 3.5-5.5 for organic food waste/municipal solid waste is achieved as per the methodologies displayed in certain examples mentioned in the disclosure. A reduction in capital cost by a factor of 50% and the production cost by a minimum of 40% can be achieved by the implementation of methodology for degradation of solid organic wastes. The phase 1 (first reactor) and phase 2 (plug flow digester) reactors when commissioned to steady state continuous process do not require any chemicals or neutralization. The liquid digestate generated may be selectively recycled to an extent of 50 to 80% and fresh water demand for mixture preparation is significantly less.

[00101] Example 1 : Food waste hydrolysis at various concentrations keeping batch time constant:

Wet food waste (moisture content 50-80%) was hydrolyzed in a batch reactor. Ground waste, with particle size <3 mm is mixed with 1000ml of pre-prepared enzymatic solution and fresh water in the weight ratio 1 :4 for sample 1 (Sl), 1 :4.5 (S2), 1 :5 (S3), 1 :5.5 (S4), 1 :6 (S5), the temperature of the batch is maintained at 32-35C with stirring for few minutes at 50-150RPM at the regular interval of l5mins. Initial pH of mixture was observed to be 7.0. The mixture was hydrolyzed at anaerobic conditions in a reactor with an arrangement for gas outlet and storage. The gas generated is stored in a storage balloon. The treatment is carried out for a period of 18 hours. The mixtures Sl and S2 developed a very low pH between 3.5-3.8 whereas S3, S4, S5 had a pH within the range 4.3 to 5.2. The COD content nearby for all the mixtures ranging between 22600mg/ltr to 36200 mg/ltr. The gas produced by mixtures S4 and S5 was the maximum and slightly flammable.

[00102] Example 2: Variation of COD with solids constant and varying time.

Another set of samples of mixed food waste was analyzed for hydrolysis by mixing with enzyme and microbe rich liquid mixed with water. The samples having a waste to liquid ratio (1 :6) was hydrolyzed for different time durations. The time duration was l2(S6), 24(S7), 36(S8), 48(S9), 60(Sl0), 72(S 11) hours. With temperature and mixing as per example 1. It was observed that after a reaction period of 48 hours the change in soluble COD was not significantly high, referring towards the end digestion/hydrolysis. The ph and COD content observed can be as follows:

Example 3: Com stover hydrolysis at various solid loading and COD concentration of Hydrolysate liquid. Agro-waste in the form of com stover was used for experimentation, wherein waste collected from a nearby field was shredded to a particle size below 3mm. The material was mixed with freshwater in the ratio from 1 : 1 to 1 :3. 0.5M sulphuric acid was added in the amount equivalent to 1% the weight of Solid waste, the mixture was stirred well and subjected to thermal treatment for 30-60 minutes at a temperature of 120C with pressure maintained between 1 to 1.2 bars. The mixture was air cooled and then neutralized to ph 6.5 by using neutralizing agents like NaOH and NaHC03, 100 ml of pre-prepared enzymatic and microbial solution was added as an inoculum. The final dry solid content in the mixture was maintained in the range 4 to 10%. The mixture was then hydrolyzed in a plug flow rector with a H:D ratio of 8-10 and maintained at a 32-35C. The retention time of the mixture varies from 3 days to 10 days. The mixture was mixed periodically at 100-150 RPM. The soluble COD generated was in the range 20,000mg/ltr to 30,000mg/ltr. The pH of the mixture ranged from 5.1 to 5.9. The amount of reduction in the total dry solid content can be observed to be 50-70%. Settling techniques for the separation of hydrolyzed material with vertical reactor and high H:D ratio can be employed. Enzymatic hydrolysis of com stover resulted into effective liquefaction of complex compounds like cellulose, hemicellulose, and lipids.

Example 4: Corn Stover hydrolysis of a fixed mixture with varying time frame: Gounded com stover with particle size less than 3mm was mixed with water, pretreated with dilute acid pretreatment as per example 3. The cooled and neutralized solution was mixed with pre-prepared enzyme and microbe rich solution. The dry solids content was maintained in the range 7-10% by dilution with water. Three batch reactors set up in parallel were for a duration of 5 days (S12), 7 days(Sl3) and 10 days(Sl4) each. Temperature for the subjected anaerobic microbes and enzymes was maintained at 35- 37C. The reactors had a configuration equivalent to a horizontal plug flow with diagonally opposite inlet and outlet. A sampling and pH recording was done at regular intervals of time. Neutralizing chemicals were added from the top of the reactor whenever necessary to maintain pH between 5.5-6.5. The reactor was maintained at anaerobic conditions. The gas formed was stored and checked for inflammability.

Example 5:

The hydrolyzed com Stover slurry was filtered using a vibrating screen, mounted with a PP filter cloth having mesh size 120, the filtered liquid was then fed to a high rate digester operated on continuous mode, inoculated with granulated sludge. The digester was fed with a hydrolyzed liquid with COD (5,000mg/ltr); the initial hydraulic retention time of the digester was 5 days and was gradually reduced. The hydraulic retention of the digester was reduced after a specific time span observing the amount of gas generated, pH of digested liquid and the change in COD content of the effluent. The COD loading was gradually increased in a stepwise manner, with initial loading of 3kg COD/cu.m then 5, 9, 12, 15, 18, 20 and 23. The amount of gas generated per day was observed to be proportional to the COD loading, the maximum gas yield formation was observed at the COD loading around 20-22kgCOD/cu.m. Day. The methane yield was observed to range from 0.32 to 0.36 cu.m of methane per Kg COD fed compared to the theoretical yield of 0.35cum/kg. The COD reduction efficiency obtained was between 89 to 91.2% of COD. The pH of influent was regulated between 6.0 to 6.5 whereas the pH of digestate obtained, fluctuated between 7.1 to 7.4. The effluent COD can be observed to be between 500 to 3500mg/ltr. At higher influent COD concentration the, influent can be diluted by the effluent with a fraction of 0-0.8 carefully maintaining the COD loading per day and the hydraulic retention time. The VSS generated can be dewatered and excess effluent liquid can be further treated using a constructed wetland system.

Example 6:

For a lOOOgm (60% moisture) raw food waste obtained from a nearby eatery. The ground waste was mixed with water and enzyme rich pre-prepared solution (lOOml). The mixed slurry was subjected to hydrolysis for a period of 48 hours in a batch reaction system. The dry solids content were maintained at 16.67% and was neutralized with NaOH from time to time to maintain the hydrolysis mixture between 5.5 to 6.5. The slurry was maintained at 32-35C with the help of a water bath and stirred periodically at 10-150 RPM. Post hydrolysis the mixture was filtered and liquid slurry was obtained. The total dissolved COD obtained from the mixture was approximately 320gms. As per empirical values the amount of methane generated is estimated to be 0.l05cu.m CH4, equivalent to 280-300ml CH4/gVS fed. As per the above observations the hydraulic retention time for hydrolysis is 2-3 days and the hydraulic retention time for Digestion to be 3-5 days. Hence the total retention time for the system turns out to be in the range of 5-8 days with the amount of biogas produced/ltr of total reactor volume to be 3.5-5.5 (65-70% methane).

Example 7:

For a 1000 gms of com stover (moisture content 15%, Ash content: 5%) was mixed with water in the weight ratio 1 :3. The mixed slurry underwent a dilute acid pretreatment with 0.5M sulfuric acid. Amount of total acid added was lOgms. The Pretreatment was carried out as in example 3. The pretreated solution was cooled and neutralized with NaOH. lOOml of pre-prepared enzyme rich solution was added and further diluted to get a total solid content of 7%. The mixture was then hydrolyzed for 4 days, with periodical stirring and was neutralized occasionally to maintain the pH between 5.5 to 6.5. The hydrolyzed solution was then filtered, the solution was analyzed for COD content, pH was also recorded. The filtered solids were dried; moisture content, VS content and ash content was recorded. The results indicated a 65-68% reduction in the total solids. The amount of total COD obtained is approximately 560gms, which implied an estimated 0.196 cu.m of CH4 generation, equivalent to 245- 250ml CH4/gVS fed. The hydrolysis was completed in 4-8 days of retention time, whereas the hydraulic retention time for the liquid methanation by high rate AD process was 3-5 days. The total retention time in the range of 7-12 days with amount of biogas can be produced/ltr of total reactor volume to be 3.5-5 (65-70% methane).

[00103] FIG. 3 illustrates an exemplary working of the proposed improved anaerobic digestion system (300), in accordance with the embodiments of the present disclosure.

[00104] For the sake of clarity and understanding the invention in better manner, below reference numerals are provided for various parts/components/element of the proposed improved anaerobic digestion system:

[00105] In an embodiment, raw material storage (102) can store the raw materials (a). There are many types of raw materials (a) (substrates) from which biogas production is possible and the amount of gas and the amount of methane content in biogas is solely dependent on the process that is followed for production and the type of substrate.

[00106] In an embodiment, the raw material (a) can be passed through a shredder

(104). The shredder (104) can shred/ground the raw material (a). The size reduction for the raw material can be achieved by shredding/grinding by using the material to a particle or shredded solids (b). The shredded solid (b) size can be less than 5mm. [00107] In an embodiment, shredded solid (b) can be mixed with fresh/recycled water (c) in the ratio varying from 1 : 1 to 1 :3 by weight to undergo thermal or chemical pretreatment depending in a thermal pretreatment reactor or first reactor (hereinafter interchangeably referred as“first reactor”) (106) upon the composition of waste stream. The first reactor (106) can automatically execute hydrolysis and dissolution of the organic material, resulting in formation of a pre-treated slurry material (d).

[00108] In an embodiment, the jacketed horizontal plug flow reactor or plug flow reactor or a second reactor (hereinafter interchangeably referred as“plug flow reactor” or a“second reactor”) (108) can receive the pre-treated slurry material (d) from the first reactor (106) and automatically execute an anaerobic treatment of the pre-treated slurry material (d) (hereinafter interchangeably referred as“pre-treated slurry”) using a pre- determined dose of enzymes secreted by a first set of grown anaerobic microbes in the second reactor, resulting in formation of a hydrolyzed slurry material (hereinafter interchangeably referred as“hydrolyzed slurry” or“hydrolysed slurry solid”) (e).

[00109] In an embodiment, a substrate containing lignin more than 5-8% of the total solid weight is categorized as the complex organic substrate and needs to undergo dilute acid pretreatment. Sulphuric acid 0.5-1% by weight of the total dry solids needs to be mixed with the slurry and is heated to 100-120C for 20-60 minutes with maintaining reaction pressure at 1.2- 1.5 bar. The pretreatment takes place in a batch reactor with external heating. Post acid pre-treatment the material is hydrolysed by cooling and neutralizing (by NaOH or NaHC03) it to ph: 6-6. The cooling is done by natural air circulation and periodic mixing. The material or pretreated slurry (d) is then added to the enzymatic-microbial hydrolysis and further diluted by fresh water or digestate to a solid content ranging from 5% to 8%. The hydraulic retention time for mixture for hydrolysis is between 12 hours to 6 days. The temperature in the hydrolysis is maintained at 32-35C. The mixture is stirred occasionally and the reactor is operated on a continuous mode. The hydrolysis reactor is a horizontal plug flow reactor (108) with a height to diameter (H/D) ratio about 5-8 and provided with a radial mixing mechanism. The pH in the reactor (108) can be maintained 5.5-6.5. The outgoing material or hydrolysed slurry solid (e) is a fairly hydrolyzed with a reduction in volatile solid content by 50-80%. The ph in the hydrolysis mixture or hydrolysed slurry solid (e) is observed to around 5.5-6.5 at proper loading conditions.

[00110] In an embodiment, the hydrolysed slurry solid (e) can have a solid content from 1-4% as per the extent of solids hydrolyzed, this mixture is filtered through solid-liquid separating mechanisms. The liquid obtained has a pH ranging in 5.5-6.5 with a COD content of l0,000mg/ltr to 60,000gm/ltr. The liquid obtained is further fed to a high rate digester operated on a continuous mode in methanogenic stage.

[00111] In an embodiment, a digester or liquid plug flow digester (120) (hereinafter interchangeably referred as “liquid plug flow digester” or“high rate digester”) can be fed with the liquid separated from the hydrolyzed mixture by using separating techniques like bar screen, vibrating screens (110), rotary drum screens (110) or a basket centrifuge (114). The high rate digester (120) can be designed considering the concentration of COD and Volatile fatty acids in the influent. The major components of the digester (120) being the Gas-Solid-liquid separator, the digestion chamber and the inorganic sludge collection chamber. The high rate digester (120) is designed to work with the use of granulated sludge having high methane activity for efficient methanation of dissolved COD. The amount of granulated sludge and the activity are dependent on various factors like the up flow velocity of the influent, the COD fed/cu.m/day and the temperature of reaction mixture.

[00112] In an embodiment, the digester (120) can receive a hydrolyzed liquid

(also referred to as “Screened liquid”) (f) having dissolved organic acids to automatically generate a biogas (11) using a pre-determined dose of enzymes and microbes secreted by the set of anaerobic microbes present in the digester (120). The screened liquid will contain dissolved organic acid that is obtained by passing the hydrolyzed slurry (e) from the second reactor (108) through one or more material separating techniques.

[00113] In an embodiment, the digester (120) can have a Gas-solid-liquid separator for separating the bio solids, liquid and gas generated effectively; maintaining the concentration of microbes is accomplished by the use of a clarifier at the top. [00114] In an embodiment, the digester (108) can have a vertical plug flow arrangement, with a height to diameter ratio ranging 5 to 8. A part of the digestate (h) can be wasted in the form of sludge which can be used as organic fertilizer on dewatering. The flow of the influent into the digester (120) is a regulated gravitational flow. The mixing in the digester (120) is induced by the flow of influent. The granulated sludge is in a fluidized state due to the continuously up-flowing influent this leads to higher interfacial contact between the dissolved COD and the granulated sludge resulting to higher rate of degradation. A reduction of 89.6-91.2% of the dissolved COD is observed at the optimum hydraulic retention time of 24-48 hours and a recycle ratio of 0-0.8. A biogas gas yield ranging from 0.32-0.36 cu.m of methane/kg of VFA utilized is obtained. Granules of sludge ranging from l-4mm are observed with a settling velocity in the range 40-60 m/hour which have a higher methane activity per kg of VSS. The biogas (11) can be stored in the biogas storage (124).

[00115] In an embodiment, a Gas flow-meter (122) can be post moisture removal. As shown in FIG. 3, sewage pump (116, 116’ and 116”) can be mono-block for pumping liquid slurry.

[00116] FIG. 4 illustrates a flowchart for the exemplary accelerated anaerobic digestion system, in accordance with an exemplary embodiment of the present disclosure.

[00117] At step 402, executing automatically, at a first reactor of a system, thermal and chemical pre-treatment of a size reduced organic material obtained from a shredder, resulting in formation of a pre-treated slurry material.

[00118] At step 404, executing automatically, at a second reactor of the system, hydrolysis and acidogenesis of the pre-treated slurry material received from the first reactor using a pre-determined dose of enzymes secreted by a first set of grown anaerobic microbes in the second reactor, resulting in formation of a hydrolyzed slurry material.

[00119] At step 406, receiving, at a plug flow digester of the system hydrolyzed liquid having immersed organic acids to automatically generate a biogas using a pre determined dose of enzymes secreted by a second set of grown anaerobic microbes in the plug flow digester, wherein the hydrolyzed liquid having dissolved organic acids is obtained by passing the hydrolyzed slurry material from the second reactor thorough solid liquid separating techniques to automatically separate unreacted substrate solids, and the hydrolyzed liquid having dissolved organic acids. The plug flow digester comprise of a solid liquid gas separator to maintain a pre-determined concentration of the second set of grown anaerobic microbial granules in the plug flow digester.

[00120] Moreover, other implementations of the present application will be apparent to those skilled in the art from consideration of the specification and practice of the example implementations disclosed herein. Various aspects and/or components of the described example implementations may be used singly or in any combination. It is intended that the specification and examples be considered as examples, with a true scope and spirit of the application being indicated by the following claims.

Claims

I Claim:

1. A system (300) for treatment of organic material in a two-step anaerobic biochemical reactor configuration with selective recirculation between the two reactors, the system comprising:

a first reactor (106) to receive the organic material (b) to execute a pre-treatment of the organic material, resulting in formation of a pre-treated slurry material;

a second reactor (108) to receive the pre-treated slurry material from the first reactor (106) to automatically perform an hydrolysis and acidogenesis of the pre-treated slurry material using a first set of grown anaerobic microbes in the second reactor (108) and a pre-determined dose of enzymes secreted generated by the first set of grown anaerobic microbes, resulting in formation of a hydrolyzed slurry material; and

a plug flow digester (120) to receive a hydrolyzed liquid having dissolved organic acids/COD to perform an acetogenesis and a methanogenesis to generate biogas by the action of granules of a second set of anaerobic microbes formed in the plug flow digester, wherein the hydrolyzed liquid having dissolved organic acids is obtained by passing the hydrolyzed slurry material from the second reactor through at least one material separating techniques to automatically separate unreacted solids, the hydrolyzed liquid having dissolved organic acids and formed gases from the hydrolyzed slurry material, and wherein the plug flow digester comprise of a solid liquid gas separator to maintain a pre-determined concentration of the second set of anaerobic microbes in the plug flow digester.

2. The system (300) as claimed in claim 1, wherein the first reactor (106) is a pretreatment reactor for providing thermal pre-treatment or a chemical pretreatment to produce the organic material (b) ready for a microbial action, wherein the first reactor (106) is a jacketed continuous stirred tank reactor (CSTR) with operating temperature between 100-120 degree C and operating pressure between 1.0-1.2 bars.

3. The system (300) as claimed in claim 1, wherein the second reactor (108) is a plug flow reactor (PFR), and wherein the plug flow reactor (PFR) is a jacketed horizontal plug flow reactor having radially mixing mechanism (RPM 15-20), wherein the horizontal plug flow reactor operates at a temperature between 55-60 degree C and normal pressure conditions.

4. The system (300) as claimed in claim 1, wherein the plug flow digester (120) performs an anaerobic treatment in the form of the acetogenesis and the methanogenesis with granulated sludge present in a fluidized state, maintained at mesophilic temperature (36-40C) conditions.

5. The system (300) as claimed in claim 1, wherein the organic material (b) received in the first reactor (106) is a size reduced organic material obtained from a shredder/grinder (104).

6. The system (300) as claimed in claim 1, wherein the organic material (b) received in first reactor (106) comprise of a material of particle size between 1 mm - 10 mm, and wherein the organic material received is pre-treated with a dilute acid pretreatment or a thermal pre-treatment to reduce lignin content in the organic material, wherein the dilute acid pretreatment comprises a sulphuric acid 0.5-1% by weight of a total organic material, and is heated to 100-120 degree C for 20-60 minutes with maintaining a reaction pressure at 1.2-1.5 bar.

7. The system (300) as claimed in claim 1, wherein the organic material comprises of organic solid waste including cooked/uncooked food, vegetable and fruit waste, agricultural wastes including lignocellulosic material and bio-energy crops, slaughter house waste, organic fraction of municipal solid waste, garden clippings, food processing industry waste.

8 The system (300) as claimed in claim 1, wherein: the first reactor (106) receives a fresh water (c) and a recycled digestate from the plug flow digester (120), also organic material from the shredder, and

the second reactor (108) receives fresh water (c’), the recycled digestate from the plug flow digester (120) along with the pre-treated slurry material from the first reactor, and

wherein the recycled digestate content can be 50-80% of the total fed liquid content.

9. The system (300) as claimed in claim 1, wherein the material separating arrangements are selected from any or combination of bar screens, vibrating screens, rotary drum screens, and a basket centrifuge (110), and wherein the material separating arrangements automatically separates bio-solids and the hydrolyzed liquid having dissolved organic acids from the hydrolyzed slurry material.

10. The system (300) as claimed in claim 1, wherein the plug flow digester (120) comprise of a solid liquid gas separator to maintain a pre-determined concentration of the second set of grown anaerobic microbes, and enzymes in the plug flow digester.

11. The system (300) as claimed in claim 1, wherein the first set of grown anaerobic microbes and the second set of grown anaerobic microbes are selected from any or combination of cellulase, xylase, amylase, arabinose, glucanase, b-glucosidase enzymes and other such species.

12. The system (300) as claimed in claim 1, wherein the system generates a biogas volume (NTP) 3.5-5.5 times of total reactor volume from the organic material, and wherein the system enables to achieve reduction in a capital cost by a factor of 50% and reduction in the production cost by a minimum of 40% for degradation of the organic material.

13. The system (300) as claimed in claim 1, wherein the digestate is selectively recycled to an extent of 50 to 80% reducing a fresh water demand for mixture preparation. The freshwater demand for the process is significantly less.

14. A method (400) for treating organic material in a two-stage anaerobic biochemical reactor configuration with recirculation, the method comprising:

executing (402) automatically, in the first reactor of the system, thermal or chemical pre-treatment of a size reduced organic material obtained from a shredder or grinder, resulting in formation of a pre-treated slurry material;

executing (404) automatically, in the second reactor of the system, hydrolysis and acidogenesis of the pre-treated slurry received from the first reactor using a first set of grown anaerobic microbes in the second reactor (108) and a pre-determined dose of enzymes secreted by the first set of grown anaerobic microbes, resulting in formation of a hydrolyzed slurry material; and

receiving (406), at a plug flow digester of the system, a hydrolyzed liquid having dissolved organic acids to perform acetogenesis and methanogenesis to generate biogas by the action of granules of second set of anaerobic microbes formed in the plug flow digester, wherein the hydrolyzed liquid having dissolved organic acids is obtained by passing the hydrolyzed slurry material from the second reactor through at least one material separating techniques to automatically separate unreacted solids, the hydrolyzed liquid having dissolved organic acids from the hydrolyzed slurry, wherein the plug flow digester comprises of a solid-liquid-gas separator to maintain a pre- determined concentration of the second set of anaerobic microbes in the plug flow digester.