FORM - 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2006
COMPLETE
Specification
(See Section 10 and Rule 13}
BIOGAS PLANT
THERMAX LIMITED
an Indian Company
of Thermax House,
4, Mumbai-Pune Road,
Shivajinagar, Pune - 411 003,
Maharashtra, India
Inventors: a) Venkatraman Kalyanraman; b) Janardhan Bhikaji Bornare; & c) Upendra Shankar Adhyapak
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE
MANNER IN WHICH IT IS TO BE PERFORMED.

FIELD OF INVENTION
The present invention relates to a biogas generation plant.
DEFINITION OF TERM USED IN THE SPECIFICATION
The term "F/M ratio" used in the specification means a measurement of the amount of food to the amount of microorganisms in the digestion tank.
BACKGROUND
Biogas, also referred to as sustainable or renewable energy, is the gas generated by anaerobic digestion of organic matter. Anaerobic digestion involves a series of processes in which microorganisms biologically break down the biodegradable organic matter in the absence of oxygen. This process is commonly used industrially or domestically to manage the organic waste and generate green energy.
In a single-stage anaerobic digestion system, all of the biological reactions occur within a single sealed reactor. Such single-stage anaerobic digestion systems are easy to build and require a small capital investment. However, in such systems, where the acidogenic and methanogenic bacteria grow together, it is difficult to provide favorable conditions for the cultivation of both these bacteria, which require different pH ranges. Hence, the biological reactions in the single-stage digester cannot be easily controlled.
To overcome this drawback of the single-stage anaerobic digester, a two-stage anaerobic digester is used. In the two-stage process, specific bacteria feed on certain organic materials. In the first stage, acidic bacteria break the complex organic molecules into peptides, glycerol, alcohol and simpler sugars. Subsequently, a second type of bacteria converts these simpler compounds into methane. The

methane producing bacteria are particularly influenced by the ambient conditions, thereby slowing or halting the degradation process in the absence of favorable conditions.
A conventional biogas plant typically comprises of two digesters, viz. a pre-digester and a main digester. The organic biodegradable matter is blended in a mixer, with additional water, to obtain organic slurry. This organic slurry is fed to the pre-digester. The digestion process begins with bacterial hydrolysis of the organic matter in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Later, the acetogenic bacteria react to convert the resulting organic acids into acetic acid and additional ammonia, hydrogen and carbon dioxide. The process of hydrolysis, acidogenesis and acetogenesis takes place in the pre-digester. The partially digested slurry from the pre-digester is then fed to the main digester where the methanogens convert these products to biogas i.e. mainly methane and carbon dioxide. The favorable pH range for the methanogens is between 6.5 to 8.5.
A major drawback of the conventional system is that the partially digested slurry which is fed to the main digester from the pre-digester has an acidic pH. The acidic pH creates unfavorable environment for the growth of methanogens in the main digester, eventually reducing the population of the methanogens in the main digester. This results in a sharp decrease (about 60 to 70%) in the biogas generation efficiency of the plant over a period of time. Thus, rendering conventional biogas plant uneconomical with respect to the energy generation costs involved and the amount of energy generated. Several efforts have been made in the past to enhance the efficiency of a biogas plant, some of these disclosures are cited below:

US 20090221054 discloses a biogas system comprising a fermenter having a first fermenting chamber and at least a second fermenting chamber for the fermentation of a fermenting medium, wherein the first fermenting chamber and the second fermenting chamber are separated by a wall and a riser pipe disposed therein transfers the biogas generated in the first chamber to the second chamber. A post-fermenting chamber is provided which further comprises a first and a second fermenting chamber. The 4-chamber biogas system provides a plug-flow to avoid the hydraulic short-circuiting. Also, seeding sludge is fed to the fermenter and the post-fermenting chamber to maintain the F/M ratio.
US20080124775 discloses a method for increasing the production of biogas (methane) in a biomass fermenting system in which the biomass is fermented under anaerobic and thermophilic conditions, wherein the methane production is increased by adding, one or more times, a bacterial culture of thermophilic, anaerobic, acetogenic and hydrogen-producing microorganism.
In the cited prior art, biomass substrate is selected such as to provide a methanogen source. Generally, liquid manure is added for this purpose which originally contains methanogens. The present invention aims at providing a biogas plant and a method thereof which provides enhanced efficiency and steady biogas production and which does not essentially require large amount of a biomass substrate containing methanogens. Also, the present invention provides better quality biogas having higher calorific value at a low production cost.

OBJECTS OF THE INVENTION
An object of the present invention is to provide a biogas plant and a method thereof which provides enhanced efficiency and steady biogas production.
Another object of the present invention is to provide a method for preparing an inoculum which provides rich-bacterial culture.
Yet another object of the present invention is to provide a biogas plant which provides better quality biogas having higher calorific value at a low production cost.
Still another object of the present invention is to provide a biogas plant and a method thereof which does not require large amount of a methanogen-containing biomass substrate.
One more object of the present invention is to provide a biogas plant which prevents hydraulic short-circuiting.
Yet one more object of the present invention is to provide a biogas plant which is largely operated by gravity therefore reducing the pumping costs.
SUMMARY OF THE INVENTION
In accordance with the present invention, is provided a biogas plant comprising:
■ a first digestion chamber adapted to at least partially digest a biodegradable organic slurry;
■ a second digestion chamber provided in operative communication with said first digestion chamber for receiving the partially digested organic slurry,

said second digestion chamber being adapted to anaerobically digest the partially digested organic slurry to produce biogas and sludge;
■ a reservoir for preparing and storing high-yield methanogenic bacterial culture, said reservoir being adapted to provide bacterial culture for maintaining adequate methanogenic bacterial population in said second digestion chamber; and
■ recirculation means operatively connected between said second digestion chamber and said reservoir, said recirculation means adapted to circulate at least a portion of sludge between said second digestion chamber and said reservoir.
Typically, in accordance with the present invention, said reservoir is positioned at a relatively higher level than said second digestion chamber to enable flow of bacterial culture from said reservoir to said second digestion chamber by gravity.
Preferably, in accordance with the present invention, said second digestion chamber comprises at least one post-chamber for collecting the sludge.
Typically, in accordance with the present invention, said first digestion chamber comprises a biodegradable organic slurry inlet and a partially digested organic slurry outlet disposed near the operative top-end of said first digestion chamber.
Preferably, in accordance with the present invention, said second digestion chamber comprises a partially digested organic slurry inlet and a sludge outlet disposed near the operative bottom-end of said second digestion chamber.

Typically, in accordance with the present invention, said recirculation means are provided for conveying at least a portion of sludge from said sludge outlet to said partially digested organic slurry inlet of said second digestion chamber.
Preferably, in accordance with the present invention, a blending device is provided in operative communication with said first digestion chamber to provide the biodegradable organic slurry, said blending device being provided at a relatively higher level than said first digestion chamber and said second digestion chamber.
Typically, in accordance with the present invention, said first digestion chamber is positioned at a relatively higher level than said second digestion chamber.
In accordance with the present invention is provided a method for producing biogas, said method comprising the steps of:
■ partially digesting a biodegradable organic slurry in a first digestion chamber by thermophilic reactions, preferably at a temperature in the range of 45 - 50 °C, to obtain a partially digested organic slurry;
■ conveying the partially digested organic slurry to a second digestion chamber;
■ feeding a methanogenic bacterial culture periodically from a reservoir to said second digestion chamber for maintaining adequate methanogenic population;
■ degrading the partially digested organic slurry anaerobically in said second digestion chamber by means of the high-yield methanogenic bacteria to produce biogas and sludge;
■ collecting the sludge in at least one post-chamber; and

■ accumulating the biogas in a dome-structure located on the operative top-end of said second digestion chamber via a gas outlet pipe.
Typically, in accordance with the present invention, the method includes the step of blending a solid biomass substrate with water to obtain the biodegradable organic slurry.
Preferably, in accordance with the present invention, the method includes the step of feeding hot water in said first digestion chamber.
Typically, in accordance with the present invention, the method includes the step of feeding compressed air in said first digestion chamber.
In accordance with the present invention, the method includes the step of recirculating at least a portion of sludge between said second digestion chamber and said reservoir.
Typically, in accordance with the present invention, the method includes the step of preparing the methanogenic bacterial culture by combining jaggery and rotten banana with sludge and hot water.
Preferably, in accordance with the present invention, the ratio of jaggery, rotten banana, sludge and hot water is 1.5:3:50:150.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will now be described with the help of the accompanying drawings, in which,

FIGURE 1 illustrates a block diagram for a conventional single-stage biogas plant;
FIGURE 2 illustrates a block diagram for a conventional two-stage biogas plant;
FIGURE 3 illustrates a schematic of the biogas plant set-up in accordance with the present invention;
FIGURE 4 illustrates a process flow diagram of the method for generating biogas in accordance with the present invention;
FIGURE 5 illustrates a block diagram for the biogas plant in accordance with the present invention; and
FIGURE 6 illustrates a graphical representation comparing the operation of the conventional two-stage biogas plant and the operation of the biogas plant of the present invention.
DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will now be described with reference to the accompanying drawings which do not limit the scope and ambit of the invention. The description provided is purely by way of example and illustration.
FIGURE 1 illustrates a block diagram for a conventional single-stage biogas plant. The biomass substrate 10, i.e. organic waste matter, in this case solid kitchen waste comprising plant/animal refuse, paper/paper products, cooked/uncooked food. extracted tea powder, waste milk/milk products, egg shells and the like, is received

in a mixer 12. The organic waste matter 10 is made in to organic slurry by crushing in the mixer 12 with water. The organic slurry is then fed to a single-stage digester 14 where all the biological anaerobic reactions occur to generate biogas 16. The waste sludge from the digester 14 is sent to sludge drying bed 18 to reduce the volume before disposal or application as a fertilizer. In a single-stage digester 14, the favorable conditions for the cultivation of the different types of microorganisms cannot be achieved; therefore, the efficiency of such a biogas plant is very low.
FIGURE 2 illustrates a block diagram for a conventional two-stage biogas plant. The organic waste matter 20, in this case solid kitchen waste, is received in a mixer 22. In the mixer 22, the organic waster matter 20 is blended with equal amount of water to obtain organic slurry. The organic slurry is collected in the pre-digester tank 24, The growth of thermophilic bacteria in the pre-digester tank 24 is enhanced by mixing the waste matter 20 with hot water and maintaining the temperature in the range of 45 -50 °C. After partial digestion in the pre-digester 24, the organic slurry is transferred to a main digester tank 26 where the organic matter 20 undergoes anaerobic degradation by a consortium of archaebacteria belonging to Methanococcus group. These bacteria, which are present in nature in the alimentary canal of ruminant animals (cattle), are primarily responsible for producing methane 28 from the cellulosic matter in the slurry. The undigested lignocellulosic and hemicellulosic matter is passed on to sludge drying bed 30 to reduce the volume. After about a month, high quality manure can be dug out from the drying bed 30 which can be used to increase the quality of humus in soil thereby to increase soil fertility. The biogas 28 so generated rises in the main digester tank 26 which comprises a dome structure at the top. As the biogas 28 is generated the dome structure rises. The dome structure reaches a maximum height of up to 4 feei with 25

m of biogas. The biogas 28 is collected through a conduit and subsequently used as fuel. The conduits are provided with drains to eliminate any condensate.
A drawback of the conventional two-stage biogas plant, as shown in FIGURE 2, is that the organic slurry entering the main digester tank 26 from the pre-digester tank 24 is acidic. The acidic pH creates unfavorable conditions for the growth of the methanogens in the main digester 26 thereby diminishing their population in the main digester 26. Also, unlike manure, kitchen waste does not provide any natural source of methanogens to maintain the sufficient methanogenic population in the main digester 26. The decrease in microbial population affects the F/M ratio and thereby creates disturbances which results in unbalance of the system; therefore leading to failure of the process and decrease in the biogas quality and quantity.
The present invention, therefore, envisages a biogas plant and a method thereof to overcome the afore-mentioned drawbacks of the conventional two-stage biogas plant. By using the method of the present invention, enhanced and steady biogas generation can be achieved at a low production cost. Referring to FIGURE 3, 4 & 5, is illustrated the biogas plant and method for producing biogas in accordance with the present invention. The biogas plant of the present invention, generally represented in Figure 3 by numeral 100, comprises a first digestion chamber 112 and a second anaerobic digestion chamber 120 for degrading a biomass substrate by thermophilic reactions to produce biogas. A solid biomass substrate, typically solid kitchen waste comprising raw/cooked food, vegetable/meat refuse, egg shells, milk/paper products, extracted tea powder, and the like, is primarily fed to a blending device 104, at a location illustrated by arrow 106 in Figure 3. The solid biomass is- fed from a hopper (not shown) provided at the top. In the blending device 104, the solid biomass substrate is made into biodegradable organic slurry by mixing

with equal amount of water. The biodegradable organic slurry flows out from an outlet provided at the operative bottom of the blending device 104 and is fed into the first digestion chamber 112 at biodegradable organic slurry inlet 140. The blending device 104 is positioned at a relatively higher level than the first digestion chamber 112 so as to provide the biodegradable organic slurry under gravity.
The first digestion chamber 112 is adapted to digest the biodegradable organic slurry by means of thermophilic bacteria. The first digestion chamber 112 is typically a cylindrical tank which is provided below the ground level. The cylindrical tank has a sufficient internal diameter (ID) and height and is made of brick masonry/concrete having a concrete cover. The first digestion chamber 112 is provided with a first baffle wall 114 which divides the chamber 112 in a first section 144 and a second section 146. The first baffle wall 114 is provided with a first opening 113 at the operative bottom-most region which provides a passage for the organic slurry to pass from the first section 144 to the second section 146. The first opening 113 is in the form of a gap extending along the width of the first baffle wall 114. The first baffle wall 114 prevents hydraulic short-circuiting in the chamber 112. The first section 144 is in operative communication with the biodegradable organic slurry inlet 140 to receive the organic slurry, wherein the biodegradable organic slurry inlet 140 is disposed at the operative top-end of the first digestion chamber 112. The organic slurry flows in a downflow direction in the first section 144 and enters the second section 146 via the first opening 113. This downward flow ensures circulation around the first digestion chamber 112. To promote the growth of thermophiles in the first digestion chamber 112, the solid biomass can be mixed with hot water so as to maintain the temperature of the biodegradable organic slurry in the first digestion chamber 112 at 45 - 50 °C. Heating means 108, typically a solar heater, is provided in operative communication with the first digestion chamber 112 to provide the hot

water. The solar heater comprises: a cold water storage tank, hot water storage tank, solar panels and pipelines interfaced with the first digestion chamber 112 to provide hot water for promoting growth of thermophiles. Insulation is provided on the pipeline and hot water tank to minimize the heat losses.
Compressed air can be introduced in the first digestion chamber 112 by air compressor means 132 to aid the thermophilic reactions. The blending device 104 and the air compressor means 132 are located on a platform above ground level. The partially digested organic slurry is discharged from the first digestion chamber 112 at partially digested organic slurry outlet 142 which is provided towards the operative top-end of the first digestion tank 112 in the second section 146. At least one pre-chamber 115 is provided in operative communication with the second section 146 to receive the partially digested organic slurry. The pre-chamber 115 has a minimum depth of 1 m over the standard water depth (SWD) of the second digestion chamber 120 and is positioned after the first digestion chamber 112 to provide sufficient head. The connecting conduits for carrying the organic slurry are made of reinforced cement concrete (RCC).
The partially digested organic slurry is then received in the second digestion chamber 120 via partially digested organic slurry inlet 152, where the partially digested organic slurry inlet 152 is disposed near the operative bottom-end of the chamber 120. In the second digestion chamber 120 the partially digested organic slurry is further digested by anaerobic degradation by a consortium of archaebacteria of the Methanococcus group which produce methane from the cellulosic material in the organic slurry. The second digestion chamber 120 is designed to hold organic slurry equivalent to about 10 days of daily feeding, i.e. Hydraulic Retention Time (HRT). The second digestion chamber 120 comprises: a cylindrical body, a second

baffle wall 124, a dome-structure 126, partially digested organic slurry inlet 152, a sludge outlet 154, at least one post-chamber 130 and a gas outlet pipe (not shown). The cylindrical body of the second digestion chamber 120 is positioned below ground level and constructed of brick masonry, cement concrete (CC), reinforced cement concrete (RCC), or stone masonry having at least 35 m capacity. Preferably, the first digestion chamber 112 is positioned at a relatively higher level than the second digestion chamber 120.
The second baffle wall 124 divides the second digestion chamber 120 into a third section 148 and a fourth section 150. The third section 148 is provided in communication with the inlet 152 to receive the organic slurry. The second baffle wall 124 is provided with a second opening 156 in the operative top-most region. The second opening 156, which is in the form of gap extending along the width of the second baffle wall 124, prevents hydraulic short-circuiting. The organic slurry flows in an upflow direction in the third section 148 and enters the fourth section 150 through the second opening 156. This upflow ensures circulation through the second digestion chamber 120.
In accordance with the present invention, a methanogen-rich bacterial culture is periodically added to the partially digested organic slurry in the third section 148 of the second digestion chamber 120. The bacterial culture is stored in a reservoir 138 which is provided in operative communication with the second digestion chamber 120 and positioned at a relatively higher level than the chamber 120 to allow the bacterial culture to be fed under gravity. The reservoir 138 is typically made of plastic and located overhead of said second digestion chamber 120. The methanogen population in the second digestion chamber 120 is maintained by adding bacteria] culture prepared in the reservoir 138. The methanogen bacteria, identified as

methanosaeta sp., were isolated from rotten banana for culture propagation in the present invention. The inoculum used in the culture is the sludge generated in the second digestion chamber 120. The sludge from the second digestion chamber 120, discharged at the sludge outlet 154, is mixed with hot water from heating means 108 and combined with jaggery and rotten bananas; the resultant mixture is stored in said reservoir 138 and kept aside for 10 - 15 days to cultivate the methanogen bacteria before adding to said third section 148 of said second digestion chamber 120. Preferably, the ratio of jaggery, rotten banana, sludge and hot water is 1.5:3:50:150. Typically, a fortnightly addition of the methanogen-rich bacterial culture from the reservoir 138 to the third section 148 of the second digestion chamber 120 helps in maintaining the adequate methanogenic population.
The methanogen bacteria react to convert the cellulosic matter in the organic slurry to biogas, mainly methane, and generate sludge as a side-product. The sludge is discharged in the at least one post-chamber 130 via the sludge outlet 154 which is located at the operative bottom-end of the fourth section 150 of the second digestion chamber 120. A sludge tank 134 is provided in communication with the at least one post chamber 130 for collecting the sludge. The at least one post-chamber 130 has a minimum depth of lm above the standard water depth (SWD) of the second digestion chamber 120, with outlets at three different levels to resist the back pressure due to gas hold-up. The sludge tank 134 is provided below ground level and made of bricks masonry. The sludge may be dried and used for further applications or disposed. A recirculation means is provided for conveying at least a portion of the sludge to the partially digested organic slurry inlet 152 from the sludge outlet 154. The recirculation means comprise a pump installed at the outlet 154 of the second digestion chamber 120. The pump is adapted to recycle approximately 100 liters of sludge from the outlet 154 to the inlet 152. The recirculation means are also adapted

to convey at least a portion of the sludge periodically to the reservoir 138 from the sludge outlet 154. A pump adapted to convey approximately 50 liters of sludge from the sludge outlet 154 to the reservoir 138 for mixing with jaggery and rotten bananas is provided in communication with the recirculation means.
The biogas so generated rises in the second digestion chamber 120 and is conveyed through the gas outlet pipe to the dome-structure 126 which is adapted for holding the biogas. The dome-structure 126 is a drum-like structure which is fabricated from mild steel sheets or fiber glass reinforced plastic (FRP). The dome-structure 126 is made to fit like a cap on the top mouth of the second digestion chamber 120, where it is submerged in slurry and rests on the ledge constructed inside the second digestion chamber 120 for this purpose. The dome-structure 126 collects the biogas produced inside the second digestion chamber 120 as the gas rises upwards. To ensure the flow of stored biogas on its own to the point of utilization, the biogas is stored in the dome-structure 126 at just above atmospheric pressure. This pressure is achieved by weight of the dome-structure 126. The movement of the dome-structure 126 is guided by a central guide pipe (not shown). A gas outlet 136 is provided at the operative top of the dome-structure 126.
EXAMPLE 1:
A methanogen-rich bacterial culture was prepared by combining jaggery, rotten banana, hot water, and sludge from second digestion chamber 120.
A composition of 200 L bacterial culture comprises the following;

Ingredient Quantity
Jaggery 1.5 kg
Rotten banana 3 dozens
Sludge from chamber second digestion 50 liters
Hot water 150 liters
Total volume 200 liters
The present invention provides high-yield methanogen bacteria in the organic slurry, thereby improving the performance of the biogas generation process. The quality and quantity of methane is increased by the retention of methanogens in the second digestion chamber 120.
Figure 6 illustrates the biogas production data obtained by conventional two-stage biogas plant (A) and the biogas plant of the present invention (B). Biogas generation data was collected for a feed of 150 kg/day of solid kitchen waste. In the conventional biogas plant (A), frequent addition of cow dung was required. Initially, on addition of cow dung, a gradual increase in biogas production of up to 14m /day on 20th day was seen. Steep decrease in biogas production was observed after the 22nd day, and the biogas production reduced even further to 5 m7day on the 28th day. The same results were obtained when the conventional plant (A) was operated continuously for a month.
On the other hand, an increase in the biogas generation was observed when the biogas plant (B) of the present invention was operated under similar conditions. After the addition of methanogenic bacterial culture, the biogas generation continuously increased up to 15 m /day on the 16th day and remained steady for a month.

TFXHNICAL ADVANTAGES
A biogas plant and a method for producing biogas thereof; as disclosed in the present invention has several technical advantages including but not limited to the realization of:
• the biogas plant of the present invention and the method thereof provides enhanced efficiency and steady biogas production;
• the method of the present invention does not require large amount of a methanogen-containing biomass substrate;
• the present invention provides a method for preparing an inoculum which provides rich-bacterial culture;
• the biogas plant of the present invention provides better quality biogas having higher calorific value at a low production cost;
• the biogas plant of the present invention is adapted to prevent hydraulic short-circuiting; and
• the biogas plant of the present invention is largely operated by gravity therefore reducing the pumping costs.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the invention, unless there is a statement in the specification specific to the contrary.
In view of the wide variety of embodiments to which the principles of the present invention can be applied, it should be understood that the illustrated embodiments

are exemplary only. While considerable emphasis has been placed herein on the particular features of this invention, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principle of the invention. These and other modifications in the nature of the invention or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

We Claim:
1. A biogas plant (100) comprising:
■ a first digestion chamber (112) adapted to at least partially digest a biodegradable organic slurry;
■ a second digestion chamber (120) provided in operative communication with said first digestion chamber (112) for receiving the partially digested organic slurry, said second digestion chamber (120) being adapted to anaerobically digest the partially digested organic slurry to produce biogas and sludge;
■ a reservoir (138) for preparing and storing high-yield methanogenic bacterial culture, said reservoir (138) being adapted to provide bacterial culture for maintaining adequate methanogenic bacterial population in said second digestion chamber (120); and
■ recirculation means operatively connected between said second digestion chamber (120) and said reservoir (138), said recirculation means adapted to circulate at least a portion of sludge between said second digestion chamber (120) and said reservoir (138).

2. The biogas plant as claimed in claim 1, wherein said reservoir (138) is positioned at a relatively higher level than said second digestion chamber (120) to enable flow of bacterial culture from said reservoir (138) to said second digestion chamber (120) by gravity.
3. The biogas plant as claimed in anyone of the preceding claims, wherein said second digestion chamber (120) comprises at least one post-chamber (130) for collecting the sludge.

4. The biogas plant as claimed in claim 1, wherein said first digestion chamber (112) comprises a biodegradable organic slurry inlet (140) and a partially digested organic slurry outlet (142) disposed near the operative top-end of said first digestion chamber (112).
5. The biogas plant as claimed in claim 1, wherein said second digestion chamber (120) comprises a partially digested organic slurry inlet (152) and a sludge outlet (154) disposed near the operative bottom-end of said second digestion chamber (120).
6. The biogas plant as claimed in anyone of the preceding claims, wherein said recirculation means are provided for conveying at least a portion of sludge from said sludge outlet (154) to said partially digested organic slurry inlet (152) of said second digestion chamber (120).
7. The biogas plant as claimed in claim 1, wherein a blending device (104) is provided in operative communication with said first digestion chamber (112) to provide the biodegradable organic slurry, said blending device (104) being provided at a relatively higher level than said first digestion chamber (112) and said second digestion chamber (120).
8. The biogas plant as claimed in anyone of the preceding claims, wherein said first digestion chamber (112) is positioned at a relatively higher level than said second digestion chamber (120).
9. A method for producing biogas, said method comprising the steps of:

■ partially digesting a biodegradable organic slurry in a first digestion chamber by thermophilic reactions, preferably at a temperature in the range of 45 - 50 °C, to obtain a partially digested organic slurry;
■ conveying the partially digested organic slurry to a second digestion chamber;
■ feeding a methanogenic bacterial culture periodically from a reservoir to said second digestion chamber for maintaining adequate methanogenic population;
■ degrading the partially digested organic slurry anaerobically in said second digestion chamber by means of the high-yield methanogenic bacteria to produce biogas and sludge;
■ collecting the sludge in at least one post-chamber; and
■ accumulating the biogas in a dome-structure located on the operative top-end of said second digestion chamber via a gas outlet pipe.
lO.The method as claimed in claim 9, which includes the step of blending a solid biomass substrate with water to obtain the biodegradable organic slurry.
11.The method as claimed in claim 9, which includes the step of feeding hot water in said first digestion chamber.
12.The method as claimed in claim 9, which includes the step of feeding compressed air in said first digestion chamber.
13.The method as claimed in claim 9, which includes the step of recirculating at least a portion of sludge between said second digestion chamber and said reservoir.

14.The method as claimed in anyone of the preceding claims, which includes the step of preparing the methanogenic bacterial culture by combining jaggery and rotten banana with sludge and hot water.
15.The method as claimed in claim 14, wherein preferably the ratio of jaggery, rotten banana, sludge and hot water is 1.5:3:50:150,