Description:DESCRIPTION
TITLE: Integrated Process for Enhanced Biogas Production via Co-Digestion with Spent Mushroom Substrate

FIELD OF THE INVENTION
The present invention relates to the field of biomass pretreatment and anaerobic digestion systems for renewable energy generation and wastewater management. Particularly, the present invention relates to pretreatment methods for lignocellulosic agricultural residues such as paddy straw to enhance biodegradability and biogas production efficiency. More particularly, the present invention relates to an integrated hybrid biological treatment system combining pretreated paddy straw with sludge or wastewater substrates in a gas-assisted anaerobic granular bio-reactor configuration to improve methane yield, biomass retention, and process stability under reduced hydraulic retention time conditions. The advantage of the present invention is to enhance biogas production efficiency, improve lignocellulosic degradation, reduce retention time, and provide stable reactor performance through improved substrate accessibility and biomass retention mechanisms. The use of the invention is suitable for agricultural waste management facilities, decentralized domestic wastewater treatment plants, biogas generation units, rural energy recovery systems, and integrated solid-liquid waste treatment installations.

BACKGROUND OF THE INVENTION
One of the prior art number WO2006056819, titled “Method for Increased Production of Biogas”, describes an invention consisting of increasing biogas production by inoculating anaerobic fermentation systems with hydrogen-producing thermophilic microorganisms. The disclosure focuses on improving methane yield through microbial augmentation under thermophilic conditions, relying on the addition of specific microbial strains to intensify biogas generation.

Another prior art no. US20230357683, titled “Anaerobic Process for Production of Methane Rich Biogas”, describes an invention consisting of a multi-stage anaerobic digestion system incorporating pretreatment techniques such as shredding, oxidation, hydrothermal extraction, hydrogenation, and cavitation. The invention emphasizes mechanical and physicochemical pretreatment methods and reactor configuration optimization for improving methane-rich biogas production.

Yet another prior art no. 202441076811, titled “A System for Enhancing Wastewater Treatment Efficiency Through Microbial Consortia Optimization”, discloses an invention consisting of optimizing and managing microbial consortia within wastewater treatment systems to improve treatment efficiency and process stability through controlled microbial enhancement strategies.

Another prior art reported in various non-patent scientific publications discloses anaerobic co-digestion of sewage sludge with agricultural residues for improving methane yield and volatile solids reduction. Such disclosures consist of balancing carbon-to-nitrogen ratios and improving substrate digestibility, often requiring mechanical size reduction, thermal pretreatment, alkaline treatment, steam explosion, or enzymatic hydrolysis of lignocellulosic materials prior to digestion.

Yet another prior art described in non-patent literature relating to spent mushroom substrate primarily discloses its utilization as compost, soil amendment, or animal feed. Although certain studies suggest its potential as an additional feedstock in anaerobic digestion systems, these disclosures do not describe mushroom cultivation as a deliberate biological pretreatment strategy integrated with downstream anaerobic co-digestion of sewage sludge for systematic enhancement of biogas production.

Further, scientific literature relating to white-rot fungi and edible mushroom species discloses partial delignification of lignocellulosic biomass during cultivation. However, such disclosures do not describe a structured and integrated process wherein mushroom cultivation is intentionally employed as a controlled biological pretreatment stage, followed by pulverization and defined co-digestion with sewage sludge under anaerobic conditions for enhanced biogas generation. However, the limitations of the prior art include reliance on energy-intensive mechanical or physicochemical pretreatment methods, dependence on specialized microbial inoculation strategies, and the absence of an integrated biological pretreatment and co-digestion framework specifically designed for lignocellulosic agricultural residues. Existing technologies do not disclose or suggest a systematic process combining biological delignification through mushroom cultivation with subsequent anaerobic co-digestion of sewage sludge for improved biogas production efficiency.

Accordingly, there is a need for a more effective solution that integrates biological pretreatment and anaerobic digestion in a technically coherent and energy-efficient manner. Therefore, the present invention overcomes the drawbacks of the prior art by providing an advanced integrated biological pretreatment and anaerobic co-digestion system that enhances substrate biodegradability and process efficiency while reducing dependence on intensive mechanical or chemical preprocessing.

OBJECTS OF THE INVENTION
1. The principal object of the present invention is to provide an integrated biological pretreatment and co-digestion process for enhanced biogas production from lignocellulosic agricultural residues and sewage sludge. The invention aims to establish a structured process wherein lignocellulosic biomass is biologically modified prior to anaerobic digestion in order to improve substrate digestibility and overall methane generation efficiency.
2. Another object of the present invention is to employ mushroom cultivation as a controlled biological pretreatment stage to achieve partial delignification and structural modification of lignocellulosic agricultural residues, thereby generating spent mushroom substrate with improved biodegradability.
3. Another object of the invention is to optimize the process of biogas production using paddystraw colonized with the mushroom mycelium as the substrate as well as to use the spent mushroom as the substrate for biogas production.
4. Another object of the invention is to eliminate or significantly reduce the need for energy-intensive mechanical, chemical, or thermal pretreatment techniques commonly used in conventional anaerobic digestion systems.
5. Yet another object of the present invention is to utilize the biologically modified spent mushroom substrate in combination with sewage sludge under anaerobic co-digestion conditions with no additional inoculum for enhanced biogas production to optimize carbon-to-nitrogen ratio, enhance volatile solids reduction, increase methane-rich biogas yield, and reduce hydraulic retention time.
6. Yet another object of the present invention is to improve process efficiency through controlled mixing ratios and improved substrate characteristics derived from biological pretreatment.
7. Yet another object of the invention is to convert the biologically pretreated paddy straw into powder form with minimal processing compared to mechanical processing, thereby enhancing the biogas. The powder ensures the homogeneity during the process, followed by controlled mixing and ratios enables the enhanced biogas production with the reduced retention time.
8. A further object of the present invention is to provide a sustainable and circular waste management approach by integrating agricultural residue valorization, mushroom cultivation, renewable energy generation, and production of nutrient-rich digestate suitable for use as biofertilizer.
9. A further object of the present invention is to offer a low-energy, environmentally sustainable, and technically improved alternative to conventional biogas production processes.

SUMMARY OF THE INVENTION

The present invention provides an integrated biological pretreatment and co-digestion process for enhanced biogas production from lignocellulosic agricultural residues and sewage sludge. The process comprises subjecting lignocellulosic biomass to mushroom cultivation to obtain spent mushroom substrate, followed by anaerobic co-digestion of the spent mushroom substrate with sewage sludge under controlled conditions for improved methane-rich biogas generation.

In accordance with one aspect of the invention, lignocellulosic agricultural residues are utilized as a substrate for mushroom cultivation, wherein the fungal growth facilitates partial delignification and structural modification of the biomass. The biological activity of the mushroom species results in the breakdown of lignin components and loosening of the lignocellulosic matrix, thereby enhancing the accessibility of cellulose and hemicellulose fractions for subsequent anaerobic digestion. The resulting spent mushroom substrate exhibits improved biodegradability compared to untreated lignocellulosic biomass.

In accordance with another aspect of the invention, the spent mushroom substrate is processed and co-digested with sewage sludge under anaerobic conditions in defined proportions. The co-digestion optimizes the carbon-to-nitrogen ratio of the feedstock, enhances volatile solids reduction, increases methane yield, and reduces hydraulic retention time. The process eliminates the requirement for intensive mechanical size reduction, chemical pretreatment, or thermal treatment of the lignocellulosic biomass prior to digestion.

The invention thus provides a biologically integrated, low-energy pretreatment and digestion strategy that structurally modifies lignocellulosic biomass prior to anaerobic digestion and enables improved biogas production efficiency. By combining mushroom cultivation and anaerobic co-digestion in a sequential and coordinated process, the invention offers a technically distinct, sustainable, and economically viable solution for renewable energy generation and agricultural residue valorization.

BRIEF DESCRIPTION OF DRAWINGS
The accompanying drawings are provided to enhance the understanding of the present disclosure and form an integral part of this specification. These figures illustrate various aspects of the biogas production using co-digestion process of spent mushroom sludge and sewage sludge. By incorporating these visual representations, the invention’s technical details, biological mechanisms, and practical applications are conveyed with greater clarity and comprehensiveness.

FIG. 1 illustrates a schematic flow diagram of the integrated biological pretreatment and anaerobic co-digestion process for enhanced biogas production in accordance with the present invention. The figure depicts the sequential processing of lignocellulosic agricultural residue through mushroom cultivation to obtain spent mushroom substrate (SMS), followed by processing, mixing with sewage sludge, anaerobic digestion under mesophilic conditions, biogas recovery, and digestate utilization as biofertilizer.

FIG. 2 illustrates the biological pretreatment stage through mushroom cultivation. The figure shows lignocellulosic biomass subjected to mushroom spawn inoculation and controlled incubation conditions, resulting in fungal growth and partial lignin degradation, production of mushroom fruiting bodies, and generation of spent mushroom substrate with improved biodegradability suitable for anaerobic digestion.

FIG. 3 illustrates the anaerobic co-digestion system. The figure shows pulverized spent mushroom substrate and sewage sludge being introduced into a mixing tank in defined proportions to optimize the carbon-to-nitrogen ratio, followed by feeding into an anaerobic digester operating under mesophilic temperature conditions for methane-rich biogas production and digestate discharge.

FIG. 4 illustrates the integrated circular economy model associated with the present invention. The figure depicts the valorization pathway wherein agricultural residue is utilized for mushroom cultivation, the resulting spent mushroom substrate is subjected to anaerobic digestion for biogas generation, and the digestate obtained is applied as biofertilizer for agricultural soil enrichment, thereby completing a sustainable resource cycle.

DETAILED DESCRIPTION OF INVENTION

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.

The present invention will now be described in detail with reference to the accompanying drawings. The drawings are schematic representations intended to illustrate the functional stages of the integrated biological pretreatment and co-digestion process and are not necessarily drawn to scale. The description provided herein enables a person skilled in the art to perform the invention. Various modifications and adaptations may be made without departing from the scope of the invention.

Referring to FIG. 1, an integrated biological pretreatment and anaerobic co-digestion system (100) is illustrated. The system comprises a lignocellulosic agricultural residue input (102), a mushroom cultivation unit (104), a spent mushroom substrate collection unit (106), an optional processing or pulverization unit (108), a mixing tank (110), an anaerobic digester (112) with sewage sludge (105), a biogas collection line (114), and a digestate outlet (116). The invention provides a sequential process wherein lignocellulosic biomass is first biologically modified and subsequently co-digested with sewage sludge for enhanced methane-rich biogas production.

BIOLOGICAL PRETREATMENT STAGE
In one embodiment of the present invention, paddy straw is utilized as the primary lignocellulosic agricultural residue (102). Paddy straw comprises cellulose, hemicellulose, and lignin. Lignin forms a protective and rigid structural barrier that restricts microbial enzyme accessibility during conventional anaerobic digestion.

In another embodiment, the paddy straw is used without intensive mechanical preprocessing prior to mushroom cultivation. Mechanical preprocessing such as chopping or reducing straw length to 2–5 cm is not mandatory. The biological activity of fungal mycelium itself facilitates structural softening and fiber weakening, thereby reducing energy consumption otherwise required for mechanical size reduction.

The lignocellulosic residue is introduced into mushroom cultivation unit (104), as shown in FIG. 2. Edible ligninolytic mushroom species including Pleurotus species, Calocybe species, or combinations thereof are inoculated onto the biomass using mushroom spawn. The inoculated substrate is incubated under controlled environmental conditions conducive to fungal colonization.

During incubation, fungal mycelium penetrates the lignocellulosic matrix and secretes extracellular ligninolytic enzymes including laccases, manganese peroxidases, and lignin peroxidases. These enzymes partially degrade lignin and disrupt cross-linkages between lignin and carbohydrate polymers. The result is structural modification, reduced lignin content, increased porosity, and improved accessibility of cellulose and hemicellulose fractions.

The incubation is continued until mushroom fruiting bodies are formed and harvested. Upon harvesting, the residual biomass constitutes spent mushroom substrate (SMS), which is collected at unit (106). The SMS exhibits enhanced biodegradability relative to untreated biomass.

PROCESSING OF SPENT MUSHROOM SUBSTRATE
In one embodiment, the collected SMS is transferred to processing unit (108) for pulverization or mild mechanical treatment. The SMS may be processed into powder form to facilitate homogeneous mixing with sewage sludge. The processing is minimal and does not involve chemical or thermal pretreatment.

CO-DIGESTION STAGE
Referring to FIG. 3, the pulverized SMS (301) is introduced into mixing tank (302) and combined with sewage sludge (306). The sewage sludge may be obtained from municipal wastewater treatment plants and contains organic matter, nitrogenous compounds, and indigenous microbial populations.

In one embodiment, SMS and sewage sludge are mixed on a volatile solids (VS) basis. The mixing ratio may be selected from 1:1, 3:2, 7:3, or 1:4 (SMS:sewage sludge). The mixture is prepared to optimize carbon-to-nitrogen ratio and microbial stability. The homogenized mixture is introduced into anaerobic digester (303). In one embodiment, digestion is carried out under mesophilic temperature conditions ranging from 30°C to 40°C. The digester may operate in batch, semi-continuous, or continuous mode.

ANAEROBIC DIGESTION MECHANISM
Within anaerobic digester (303), the digestion process proceeds through four coordinated biochemical stages.

HYDROLYSIS
Hydrolytic bacteria secrete extracellular enzymes including cellulases and hemicellulases, which convert cellulose and hemicellulose into soluble sugars. Proteases degrade proteins into amino acids, and lipases convert lipids into fatty acids and glycerol. The prior fungal delignification enhances enzyme accessibility and accelerates hydrolysis.

ACIDOGENESIS
Acidogenic bacteria metabolize soluble sugars and amino acids into volatile fatty acids (VFAs), hydrogen, carbon dioxide, and ammonia. The buffering capacity of sewage sludge stabilizes pH and prevents acid accumulation.

ACETOGENESIS
Acetogenic bacteria convert higher VFAs into acetate, hydrogen, and carbon dioxide. Efficient hydrogen removal by methanogens maintains favorable thermodynamic conditions.

METHANOGENESIS
Methanogenic archaea convert acetate and hydrogen into methane. Methane is generated through acetoclastic and hydrogenotrophic pathways. Enhanced hydrolysis due to biological pretreatment contributes to improved methane production efficiency.

BIOGAS AND DIGESTATE
Biogas generated is withdrawn through gas collection line (304) and may be utilized for heat and power generation. The stabilized digestate is discharged through outlet (305). The digestate may undergo solid-liquid separation, and the fractions may be used as solid or liquid biofertilizer.

PERFORMANCE ENHANCEMENT EMBODIMENT
In one embodiment, co-digestion of pulverized SMS with sewage sludge results in 20–30 % enhancement in biogas production compared to mono-digestion controls of untreated paddy straw or sewage sludge alone. In one embodiment, improved substrate accessibility contributes to reduced hydraulic retention time relative to untreated lignocellulosic digestion.

ALTERNATIVE FEEDSTOCKS
In another embodiment, lignocellulosic substrates including napier grass, wheat straw, barley straw, oat straw, rye straw, corn stover, cotton stalks, sunflower stalks, sugarcane bagasse, rice husks, coconut husks, coconut shells, peanut shells, almond shells, walnut shells, cocoa pods, sugarcane tops, cassava stems, sawdust, bark, wood chips, horticultural waste, and food processing residues may be used.

FEEDSTOCK COMPOSITION EMBODIMENT
In another embodiment, a biogas-producing feedstock composition is provided comprising spent mushroom substrate derived from paddy straw cultivation mixed with sewage sludge, wherein the mixture is adjusted to achieve an optimized carbon-to-nitrogen ratio.

INTEGRATED CIRCULAR ECONOMY EMBODIMENT
Referring to FIG. 4, agricultural residue (401) is utilized for mushroom cultivation producing edible mushrooms (402). The resulting SMS (403) is subjected to anaerobic co-digestion to generate biogas (405). The digestate (406) is processed into biofertilizer and returned to agricultural land.

PROCESS FLOW
the process comprises agricultural residue collection, mushroom inoculation, incubation, mushroom harvesting, SMS collection, optional pulverization, mixing with sewage sludge, anaerobic digestion, biogas recovery, and digestate utilization.
INDUSTRIAL APPLICABILITY
The invention is applicable to municipal wastewater treatment plants, agricultural waste management systems, decentralized biogas units, renewable energy projects, and circular economy initiatives. The invention enables integrated production of edible mushrooms, methane-rich biogas, and biofertilizer from agricultural residues. The present invention thus provides a biologically integrated, energy-efficient, and sustainable process for enhancing biogas production through structured biological pretreatment followed by anaerobic co-digestion.

ADVANTAGES OF THE PRESENT DISCLOSURE
• The present invention provides a biologically integrated pretreatment and co-digestion process that eliminates the need for intensive mechanical preprocessing of lignocellulosic agricultural residues such as paddy straw. Conventional systems require chopping or size reduction prior to digestion; however, the present invention utilizes mushroom-based biological delignification to structurally soften and modify the biomass, thereby reducing energy consumption associated with mechanical preprocessing.
• The invention avoids the use of chemical or thermal pretreatment techniques such as alkaline hydrolysis, steam explosion, or acid treatment, which typically increase operational complexity, capital cost, and environmental risk. The biological pretreatment stage offers a low-energy and environmentally sustainable alternative for improving substrate digestibility.
• The fungal enzymatic action during mushroom cultivation enhances lignin degradation and increases accessibility of cellulose and hemicellulose fractions, thereby improving hydrolysis efficiency during anaerobic digestion. This structural modification contributes to improved microbial conversion in subsequent digestion stages.
• Co-digestion of spent mushroom substrate with sewage sludge improves the carbon-to-nitrogen balance of the feedstock and supports stable microbial activity within the anaerobic digester. The optimized feed characteristics contribute to enhanced biogas generation compared to mono-digestion of untreated biomass or sewage sludge alone.
• The process enables biogas enhancement within the range of 20–30% under optimized mixing conditions and contributes to improved methane-rich biogas production. Improved substrate biodegradability also supports reduction in hydraulic retention time compared to untreated lignocellulosic digestion.
• The invention provides a multi-product valorization pathway by integrating edible mushroom production, renewable biogas generation, and nutrient-rich digestate recovery within a unified process framework. The digestate obtained from anaerobic digestion may be utilized as solid or liquid biofertilizer, thereby contributing to soil fertility enhancement.
• The integrated system supports circular resource utilization by converting agricultural residues into valuable products while reducing open-field burning of paddy straw and minimizing associated environmental pollution. The process is scalable and applicable to municipal wastewater treatment plants, agricultural waste management systems, and renewable energy generation facilities.
, Claims:CLAIMS
I/WE CLAIM:
1. A process for producing methane-rich biogas, the process comprising:
- providing a lignocellulosic agricultural residue without mechanical size reduction and without chemical or thermal pretreatment;
- cultivating ligninolytic mushrooms directly on the lignocellulosic agricultural residue to biologically delignify and structurally modify the lignocellulosic agricultural residue, thereby resulting in substrate with mushroom cultivation as spent mushroom substrate or a substrate without mushroom cultivation;
- processing the spent mushroom substrate/without mushrooming substrate into a pulverized or homogenized form suitable for anaerobic digestion;
- combining the processed spent mushroom substrate /without mushrooming substrate with sewage sludge to form a co-digestion feed mixture; and
- subjecting the co-digestion feed mixture to anaerobic digestion under controlled temperature conditions to produce biogas and digestate without supplement of additional inoculum.
2. The process as claimed in claim 1, wherein the lignocellulosic agricultural residue comprises paddy straw.
3. The process as claimed in claim 1, wherein the ligninolytic mushrooms cultivated on the lignocellulosic agricultural residue are selected from Pleurotus species, Calocybe species, or combinations thereof.
4. The process as claimed in claim 1, wherein the processing of the spent mushroom substrate comprises pulverizing the spent mushroom substrate/without mushrooming substrate into powder form to improve homogeneity of the co-digestion feed mixture.
5. The process as claimed in claim 1, wherein the processed spent mushroom substrate/without mushrooming substrate and the sewage sludge are combined on a volatile solids basis in a ratio selected from 1:1, 3:2, 7:3, or 1:4.
6. The process as claimed in claim 1, wherein the controlled temperature conditions comprise mesophilic temperature conditions ranging from 30°C to 40°C.
7. The process as claimed in claim 1, enhanced biogas yield under is achieved with less energy and less infrastructure
8. The process as claimed in claim 1, wherein the co-digestion feed mixture produces enhanced biogas yield compared to anaerobic digestion of untreated lignocellulosic agricultural residue or sewage sludge alone.