WO2008026221A1

WO2008026221A1 - A self mixing anaerobic digester useful for treatment of solid organic waste

Info

Publication number: WO2008026221A1
Application number: PCT/IN2007/000353
Authority: WO
Inventors: Anupoju Gangagni Rao; Jonhy Joseph; Sunkavalli Surya Prakash; Annapuma Jetty; Ponnapalli Nageswara Sarma
Original assignee: Council Of Scientific & Industrial Research; Department Of Biotechnology
Priority date: 2006-08-29
Filing date: 2007-08-20
Publication date: 2008-03-06

Abstract

The present invention relates to a self mixing anaerobic digester (1) useful for the treatment of solid organic waste. The digester comprises: i) at least one bottom (11) and at least one upper (12) reaction chamber hydraulically connected through a central draft pipe (13); ii) a feed tank (3) connected to the inlet pipe (4) of the bottom reaction chamber; iii) a feed preparation system (2); iv) an automatic control valve (7); v) a discharge tank (10) to collect the digested slurry.

Description

"A SELF MIXING ANAEROBIC DIGESTER USEFUL FOR TREATMENT OF SOLID ORGANIC WASTE"

Field of the invention: The present invention relates to a self mixing anaerobic digester useful for the treatment of solid organic waste.

More particularly, the present invention relates to a self mixing anaerobic digester consisting of feed preparation system, feed tank, two reaction chambers, an automatic control valve system and discharge tank, wherein the said digester is useful for anaerobic digestion of solid organic waste.

Background and prior art of the invention:

Successful anaerobic digestion of organic wastes usually requires a mixed culture of bacteria with a complex interdependency, terminating in the production of methane by methanogenic bacteria (Hawkes et al., 1987). The objectives of the digestion process are to reduce the total amount of volatile solids in the sludge and to produce a stabilized sludge for discharge to the environment for further use as organic manure and to produce biogas for utilizing it as non-conventional energy.

The first application of anaerobic biological process to the treatment of organic wastes was the septic tank, invented in 1895. A variety of applications of the anaerobic concept have evolved over the past nearly 100 years. Early applications, beginning in 1918, were to the treatment of sludge from domestic sewage. Much later, beginning in the 1950's, a process that was called "anaerobic contact" and "anaerobic activated sludge" was applied to livestock slaughtering wastewaters. This process made use of a clarifier unit to separate solids from the liquid. Still later, beginning in the 1970's, anaerobic treatment applications included various attached growth (upflow, downflow, and expanded bed) treatment approaches.

The main disadvantage of anaerobic digesters is the long residence time typically required to digest organic waste. Mostly, anaerobic digesters for the treatment of organic solid waste are "batch" or one-stage digesters. The batch digester is a closed or domed vessel within which very large quantities of organic waste is fermented. Anaerobic batch digesters take 40 to 50 days to adequately digest the organic solids (See U.S. Patent No. 5,637,219). Studies were reported on batch type digesters of KVIC and DENABAND HU models for production of biogas from solid organic waste, which are otherwise used for dung digestion (Rajashekhar Reddy et al., 1996). These reactors have drawbacks of high residence time (40 - 50days) and formation of scum. No mixing mechanism is available in this digester. Moreover, these batch type plants are not suitable for the treatment of large quantities of solid organic waste. As a result, many municipal and industrial wastes are processed using aerobic digestion systems or a combination of aerobic with anaerobic systems (See U.S. Patent No. 4,885,094).

Two important design features of the digesters are the rate at which waste can be processed, and the fraction of solids in the waste that can be digested. The loading rate or flow rate determines the residence time in the digester. The residence time required by standard anaerobic digesters (KVIC, Dheenabandhu models) the contents of which are unmixed and unheated is quite long and of the order of 40 to 60 days. Proper mixing achieves optimum anaerobic digester performance. Mixing has been achieved by gas injection, mechanical stirring, and mechanical pumping. But several digesters handling sewage sludge have failed for want of proper mixing and maintenance of mixers (Badrinath, 2000).

Anaerobic digesters include both batch and continuous digesters. A continuous process is usually favored, since the waste is processed continuously, and there is a steady supply of methane. Some anaerobic digesters are considered two- stage digesters, because the processes of hydrolysis and acidification are separated from the processes of methanogenesis. This separation usually produces methane gas with lower levels of impurities (See U.S. Patent No.5637219). Complex, multi-stage digesters were described that spread out the digestive processes into three or more sections (See U.S. Patent No. 4,604,206 and 5,637,219).

In most digesters, bacteria are added to the organic waste, and the temperature is controlled. The bacteria determine the optimum temperature for the digester to operate efficiently. Two common temperature ranges of digesters are mesophilic temperature range (2O⁰C to 45°C) or thermophilic temperature range (50⁰C to 65⁰C). Methane production decreases if the optimal temperature range of the methanogenic bacteria is exceeded. For example, a maximum volume of methane is produced by mesophilic anaerobic bacteria at a temperature of about 35⁰C, and by thermophilic bacteria at a temperature of about 55⁰C. Many digesters also control pH. Methanogenesis is pH dependent, with the optimal pH range of about 6 to 8.

U.S. Patent No. 6,254,775 describes an anaerobic digester system based on an upright vessel with internal matrices for bacteria immobilization. U.S. Patent No.5637219 describes a complex, multi-stage anaerobic digester that is based on an internal rotor assembly that provides for mixing of solids and for heat and mass transfer. The digester is divided by the rotor assembly into at least three or more chambers. Initially, the digester is seeded using a mixed population of anaerobic bacteria. U.S. Patent No. 4,604,206 describes a complex anaerobic digester with four different treatment sections to separate the acid-forming and gas-forming phases of anaerobic digestion and the mesophilic and thermophilic bacteria. In each section is a rotating biological contractor and series of partitions to create zones in which the waste concentration is high and reaction rates are maximized. The digester has multiple internal heaters to control the temperature. The microorganisms in each section are pre-established on fixed media matrices that help prevent microbial movement from one compartment to the next.

U.S. Patent No. 6673243 describes an anaerobic digester, which is a multi- chambered digester that can handle wastewater and sludge in a large volume at a high flow rate. The reactor is based on a sequential series of reaction chambers in a design. After the reaction chambers and just prior to leaving as effluent, a settling chamber is located to reclaim the microbes and remove additional solids.

Closest prior art to our invention is Biogas Induced Mixing Arrangement (BIMA) digester technology, Austria. The main disadvantages of BIMA are

1. Short-circuit of feed slurry in the digester (Due to this effect partially digested feed gets discharged from the digester resulting in less reduction of volatile solids compared to the actual reduction) 2. Top chamber volume is relatively smaller (15-25% of the total volume of the digester) compared to the bottom chamber volume. So, the volume of slurry moving to top chamber of the digester is only 15 -25% of the total slurry available in the digester. Due to this mixing of slurry is not effective.

3. Mechanical devices (booster etc) are used to create differential pressure required to lift the slurry to the top chamber through the draft tube. The very idea of biogas induced mixing arrangement (BIMA) as claimed, is violated by using the booster. 4. The design of the draft pipe connecting upper chamber and lower chamber is such that it can lead to choking.

5. The design of the scum eliminator is not effective.

6. The feed system employs mechanical pump and often not suitable for feed containing abrasive matter. 7. The digested slurry discharge arrangement from the top of the digester employs substantial pipe lengths of large diameter with U-type. Unlike for wastewater systems, the U-might clog for solids slurry.

8. Full-scale application of BIMA digester to abattoir waste (M/s Alkabeer at Hyderabad) resulted in HRT of 30-35 days. This HRT is more or less same as conventional batch digester.

The novelty of the present invention is the reduction of hydraulic retention time and there by reduction of reactor volume without compromising on the methane production and volatile solids reduction by effective mixing of slurry while addressing all the drawbacks in the BIMA digester.

Objects of the invention;

The principal objective of the present invention is to provide a self mixing anaerobic digester useful for the digestion of organic solid waste, wherein the said digester consisting of feed preparation system, feed tank, two reaction chambers, an automatic control valve system and discharge tank.

Another object of the present invention is to provide a process of anaerobic digestion as obtained by using the said self mixing anaerobic digester. Still another objective of the present invention is to provide effective mixing of waste slurry in the digester without using any separate mechanical device. Yet another objective of the present invention is to eliminate the scum formation problem in anaerobic digester.

Still other objective of the present invention is to provide a self mixing anaerobic digester for enhancing the production of biogas per kg of Volatile solid destroyed.

Yet another objective of the present invention is to provide a self mixing anaerobic digester for increasing the solid loading rate (kg solid/ m³ of reactor/ day) and volatile solid destruction rate.

Still another objective of the present invention is to prevent choking and clogging of the anaerobic digester.

Summary of the invention:

The invention provides a self mixing anaerobic digester useful for the digestion of organic solid waste, wherein the said digester consisting of feed preparation system, feed tank, two reaction chambers, an automatic control valve system and discharge tank. The present invention also provides improvement of existing solid loading rate, volatile solid destruction rate and mixing of slurry. The present invention eliminates the short circuit of feed slurry and usage of mechanical devices for gas mixing.

Accordingly, the present invention provides a self mixing anaerobic digester useful for the digestion of organic solid waste, wherein the said digester consisting of the following parts: i. at least one bottom and at least one upper reaction chamber having conical bottom, wherein the chambers being provided with biogas outlets and being hydraulically connected through a central draft pipe; ii. a circular feed tank connected directly to inlet pipe of bottom chamber of the said digester; iϋ. a feed preparation system comprising a mixing vessel and medium speed stirrer, connected with the said feed tank, iv. an automatic control valve system attached to the bottom and upper chambers of the digester and v. a circular/ rectangular discharge tank having conical bottom surface with discharge valve and lid at the bottom of the said valve system to collect the digested slurry.

In an embodiment of the present invention, the height of the said feed tank is adjusted to prevent failure of digester.

In another embodiment of the present invention, the bottom chamber comprising a feed inlet, a gas outlet and a man hole to facilitate emergency discharge and the upper chamber comprising a gas inlet from bottom chamber, a gas out let for utility system and a discharge outlet.

In still another embodiment of the present invention, the volume of the said upper chamber is about two-third of the volume of the said bottom chamber.

In yet another embodiment of the present invention, the organic solid waste is fed to the bottom chamber from feed tank by gravity and is moved to upper chamber through central draft pipe.

Further in an embodiment of the present invention, the said central draft pipe is useful for promoting upward and downward movement of the slurry.

In still another embodiment of the present invention, the length of the said central draft pipe is adjusted such that sufficient gap is provided between bottom of the digester and central draft pipe bottom end to eliminate choking of the draft pipe.

In yet another embodiment of the present invention, the said reaction chambers are adapted to foster the anaerobic microbial digestion of organic waste.

In still an embodiment of the present invention, the said automatic control valve system consisting of sensor, controller and final control element to release the biogas when pressure in the bottom chamber reaches the set value. Further in an embodiment of the present invention, the discharge tank comprising a circular/ rectangular tank having suitable volume depending upon feeding rate to hold the discharge slurry and having supporting structure such as conical bottom with discharge valve and lid to control odor.

In still another embodiment of the present invention, a process of anaerobic digestion as obtained by using the said self mixing anaerobic digester, wherein the said process comprising following steps of: (a) introducing the organic wastes into the feed preparation system of the digester to separate grits from the organic solid waste; (b) preparing homogenized slurry as obtained from step (a) and feeding the slurry through feed tank to the inlet of the bottom reaction chamber of the said digester by gravity; (c) allowing the movement of the waste as obtained from step (b) from the bottom reaction chamber to the upper chamber through central draft pipe in response to increase in biogas pressure in the bottom chamber;

(d) allowing the movement the partially digested slurry from upper chamber to bottom chamber by releasing the biogas as obtained from step (c) using automatic control valve system, in response to increase in biogas pressure in upper chamber;

(e) repeating the steps of (a) to (d) to obtain thorough mixing of organic waste slurry without using any mechanical device; (f) applying sufficient hydraulic residence time to reduce the discharged solid waste as obtained from step (e) from the upper chamber at least up to 50% in total volatile solids as compared to the input volatile solids organic waste;

(g) discharging calculated quantity of digested slurry as obtained from step (f) from the upper chamber to discharge tank through discharge outlet;

(h) releasing and collecting biogas evolved during the anaerobic microbial digestion of organic waste in the above mentioned steps. In yet another embodiment of the present invention, the organic waste used is selected from the group comprising cow dung, poultry litter, ensilaged mango peels and similar wastes.

In still another embodiment of the present invention, the hydraulic residence time is about 5 - 15 days.

Further in another embodiment of the present invention, the discharging of digested slurry to discharge tank is carried out at least once in a day.

In yet another embodiment of the present invention, the said digester is made of material selected from the group comprising PPFRP, HDPE, RCC (Please provide the full names) to control corrosion of said digester.

In still another embodiment of the present invention, the said digester is useful for hydrolysis, acidogenesis, methanogensis of organic waste.

Further in another embodiment of the present invention, the pH for digestion in the said digester is in the range of 4 - 8, so as to function under acidogenic or methanogenic conditions.

In still another embodiment of the present invention, the temperature range for digestion in the digester is 30⁰C - 40⁰C (mesophilic) or 50⁰C - 60⁰C (thermophilic).

Brief description of the drawing:

Figure 1 represents a self-mixing anaerobic digester. The digester consists of following parts: i. a feed preparation system denoted by 2, ϋ. a feed tank denoted by 3, iii. an influent tube denoted by 4, iv. a digester with 2 chambers denoted by 1 , v. a manhole denoted by 5, vi. a bottom chamber gas exit port denoted by 6, vii. an automatic control valve denoted by 7, vϋi. a top chamber gas exit port denoted by 8, ix. a discharge port 9 and a discharge tank denoted by 10. x. The digester comprising a bottom reaction chamber denoted by 11 and xi. The digester comprising a top chamber denoted by 12. xii. A central draft pipe denoted by 13, which connects the top chamber and bottom chamber hydraulically, wherein the central draft pipe length is adjusted such that two-third quantity of bottom slurry is raised to top chamber.

Detailed description of the invention:

Various designs of digesters exist for the processing and treatment of organic solid wastes to produce non-hazardous, and sometimes beneficial, products for release to the environment. Digesters may be designed for use in low technology rural areas or for sophisticated industrial areas. Many types of solid organic wastes (i.e., municipal, industrial, agricultural, and domestic wastes) may be treated by anaerobic digestion.

An unfilled need exists for a simple, inexpensive anaerobic digester that can efficiently treat organic waste of higher solids content at a shorter residence time than the conventional anaerobic digesters.

In the conventional anaerobic digesters of fixed dome or floating dome model, organic solids slurry of known Total Solids (TS) is fed to the reactor from one end and outlet is taken from another end after allowing the slurry to digest for 40 - 50 days in the reactor. We have discovered a simple, reliable, inexpensive, and efficient anaerobic digester for treating organic wastes at a shortened residence time. The present invention is related to the enhancement of anaerobic digestion efficiency of the conventional digester by mixing the reactor contents with biogas generated in the process.

The present invention results in a reactor design that prevents any possible clogging inside the reactor and in the associated lines. Simplified feeding arrangement and digested slurry discharge system reduces the installation cost and operating cost. This also prevents maintenance cost and eliminates possible plant shut down and provides trouble free operation. The self-mixing anaerobic digester (SMAD) in the present invention, consists of one bottom chamber and another upper chamber. Both the chambers are hydraulically connected with central draft pipe. The bottom compartment is essentially a pressure vessel and biogas pressure is alone utilized without any mechanical device like a blower, to lift the slurry to the top chamber through the central draft pipe. Upper chamber is having conical bottom and central draft pipe from the bottom is exactly fixed to the conical bottom of the top chamber. Bottom compartment is provided with a manhole to facilitate emergency discharge of accumulated inorganics that might enter into the reactor along with feed. The length of the central draft pipe ends above the solids discharge outlet of the bottom chamber. Both the chambers are having biogas outlets. The bottom chamber biogas outlet is connected to upper chamber and upper chamber biogas outlet is connected to biogas utility system. The digester employs an actuated butterfly valve to release the gas pressure from the bottom chamber, equalizing the gas pressure across the two chambers. This leads to immediate down flow of the slurry from the top compartment to the lower compartment. Due to this movement of slurry, entire reactor content mixes thoroughly and enhances the overall digestion process. Thorough mixing also eliminates the possibility of short-circuiting of fresh feed.

The volume of the top chamber relative to the volume of the bottom chamber is important, and should be about two-third of the volume of bottom chamber. Volume of the chamber determines the relative residence time for any given flow rate. Organic solids slurry is fed to the bottom chamber with inoculum by gravity feeding system. The digester is under anaerobic conditions. Therefore, biogas is generated in the bottom chamber due to the presence of organic slurry and gets pressurized slowly as the bottom chamber gas exit port 6 is in closed position, where as top chamber is at the pressure equivalent to the biogas holder pressure or atmospheric pressure. Due to this differential pressure between the two chambers, which are hydraulically connected, the slurry in the bottom chamber moves upward through the draft tube to the upper chamber. The differential pressure increases gradually and finally two-thirds slurry is lifted up and occupies the top compartment depending upon the length of central draft pipe extending down in the bottom chamber. When the pressure in the bottom chamber reaches the set value (set value depends on the volume of the reactor), automatic biogas control valve 7 gets opened and the biogas goes to the top chamber. Simultaneously slurry in the top chamber flows back to the bottom chamber. Due to this phenomenon, total slurry in the digester is mixed thoroughly. This mixing enhances the volatile solids destruction rate and biogas production rate. The mixing also reduces the scum formation in the digester. The number of mixing cycles per day depends upon the biogas production potential, set pressure value, temperature, volatile solids loading rate and volatile solids destruction rate. Everyday known amount of slurry is fed to the bottom chamber of the digester and equal quantity of the digested slurry is discharged from the top chamber of the digester to the discharge tank. The production of methane can be estimated by methods known in the art. See Ch. 8, Metcalf & Eddy, Inc. (1991).

In the present invention feeding is done by gravity instead of pumping system. The feed preparation system is designed to screen the inorganic fraction such as grits from the slurry during homogenization. The system is capable of metering the slurry while feeding the reactor. The feed preparation system consists of specially designed mixing vessel with arrangement for separating grits from the organic solid waste and homogenizing slurry. System is complete with provision to add organic solid waste to the mixing vessel and arrangement for feeding calculated quantity of homogenized slurry to the feed tank of self-mixing anaerobic digester. The main feature of the feed preparation system is its design to integrate with slurry digester and addition of homogenized feed to the digester without using a pump.

This feed preparation system is a rectangular/ circular PPFRP / HDPE / RCC construction with suitable capacity depending on feed rate of organic solid waste and fitted with a medium speed stirrer (electrically operated paddle stirrer) to homogenize the slurry. The feed preparation tank has a slanting bottom with bottom discharge to remove the grits. The feed preparation system has a lid and suitable overflow arrangement with damper to discharge the homogenized slurry to the feed tank of digester.

The feed tank height with respect to digester height is very important to prevent the digester failure. Automatic control valve, which should release the biogas at set pressure, may malfunction sometimes and due to this pressure in the bottom chamber increases beyond set pressure. This leads to the digester failure because of overflow of slurry through the draft tube to the top chamber. To prevent this, feed tank height is adjusted such that when the biogas pressure in bottom chamber increases above the set value or slurry level in the top chamber increase above the set level, the slurry overflows from the feed tank.

In the normal course of action, digested slurry from digester is collected in discharge tank. This unit is suitably designed to receive calculated quantity of digested slurry from the digester. Discharge tank is located such that slurry from the same can be sent to the dewatering system/ dying yard by gravity. The discharge tank is basically a rectangular/ circular tank having suitable volume depending upon feeding rate to hold the discharge slurry. The discharge tank has supporting structure, conical bottom with discharge valve and lid to control odour.

11. Organic waste is initially fed to the digester at a low flow rate to achieve a hydraulic residence time (HRT) of approximately 50 days. This is equivalent to a solids loading rate of 2 kg TS/m³-day. After attaining steady state at this HRT, flow rate is increased in a stepwise manner to achieve an HRT of 5-15 days. Steady state in terms of volatile solids destruction (30-50%) and biogas production (0.5-0.7m³/ kg VS destroyed) is obtained at each flow rate before operating the digester at higher flow rate. The steady state operation reveals that digester could be operated at a solid loading rate of 3.7 - 9.7 kg TS/ m³ per day (please confirm) with an HRT of 5-15 days.

The following examples are given by the way of illustration of the present invention and should not be construed to limit the scope of the present invention.

Example 1

Anaerobic digestion of cow dung in conventional digester

Experiments were conducted in the conventional anaerobic digester having capacity of 200 liters. The digester was inoculated with anaerobic sludge obtained from a digester treating municipal sewage sludge. The digester was started by feeding cow dung slurry (10% TS concentration) at the volatile solids (VS) loading rate of 1 kg VS/ m³/day and HRT of 80 days. Subsequently, the VS loading rate was increased and HRT was decreased in a stepwise manner. At each loading rate, during the stabilization, steady VS destruction rate and gas production was ensured. Ultimately, the digester was stabilized and optimized at the VS loading rate of 2 kg VS/ m³.day and HRT of 40 days.

Example 2 Anaerobic digestion of cow dung in self-mixed anaerobic digester

The same experiments were conducted in the self-mixed anaerobic digester having capacity of 200 liters as example 1. The digester was inoculated with anaerobic sludge obtained from a digester treating municipal sewage sludge. The digester was started by feeding cow dung slurry (10% TS concentration) at the volatile solids (VS) loading rate of 1.6 kg VS/ m3.day and HRT of 50 days. Subsequently, the VS loading rate was increased and HRT was decreased in a stepwise manner. At each loading rate, during the stabilization, steady VS destruction rate and gas production was ensured. Ultimately, the digester was stabilized and optimized at the VS loading rate of 5.3 kg VS/ m3.day and HRT of 5 days.

Example 3 Anaerobic digestion of poultry litter in conventional digester

Experiments were conducted in the conventional anaerobic digester having capacity of 200 liters. The digester was inoculated with anaerobic sludge obtained from a digester treating municipal sewage sludge. The digester was started by feeding poultry litter slurry (10% TS concentration) at the volatile solids (VS) loading rate of 0.875 kg VS/ m³.day and HRT of 80 days. Subsequently, the VS loading rate was increased and HRT was decreased in a stepwise manner. At each loading rate, during the stabilization, steady VS destruction rate and gas production was ensured. Ultimately, the digester was stabilized and optimized at the VS loading rate of 1.75 kg VS/ m³.day and HRT of 40 days.

Example 4 Anaerobic digestion of poultry litter in self-mixed anaerobic digester

1.4 The same experiments were conducted in the self-mixed anaerobic digester having capacity of 200 liters as example 3. The digester was inoculated with anaerobic sludge obtained from a digester treating municipal sewage sludge. The digester was started by feeding poultry litter slurry (10% TS concentration) at the volatile solids (VS) loading rate of 1.4 kg VS/ m3.day and HRT of 50 days. Subsequently, the VS loading rate was increased and HRT was decreased in a stepwise manner. At each loading rate, during the stabilization, steady VS destruction rate and gas production was ensured. Ultimately, the digester was stabilized and optimized at the VS loading rate of 4.7 kg VS/ m3.day and HRT of 10 days.

Example 5 Anaerobic digestion of Ensilaged mango peels in conventional digester Experiments were conducted in the conventional anaerobic digester having capacity of 200 liters. The digester was inoculated with anaerobic sludge obtained from a digester treating municipal sewage sludge. The digester was started by feeding Ensilaged mango peels slurry (10% TS concentration) at the volatile solids (VS) loading rate of 1 kg VS/ m³/day and HRT of 80 days. Subsequently, the VS loading rate was increased and HRT was decreased in a stepwise manner. At each loading rate, during the stabilization, steady VS destruction rate and gas production was ensured. Ultimately, the digester was stabilized and optimized at the VS loading rate of 2 kg VS/ m³.day and HRT of 40 days.

Example 6

Anaerobic digestion of Ensilaged mango peels in self-mixed anaerobic digester

The same experiments were conducted in the self-mixed anaerobic digester having capacity of 200 liters as example 5. The digester was inoculated with anaerobic sludge obtained from a digester treating municipal sewage sludge. The digester was started by feeding ensilaged mango peels slurry (10% TS concentration) at the volatile solids (VS) loading rate of 1.6 kg VS/ m3.day and HRT of 50 days. Subsequently, the VS loading rate was increased and HRT was decreased in a stepwise manner. At each loading rate, during the stabilization, steady VS destruction rate and gas production was ensured. Ultimately, the digester was stabilized and optimized at the VS loading rate of 5.3 kg VS/ m3.day and HRT of 15 days.

Example 7

Comparative evaluation of anaerobic digestion in conventional digester and self-mixed anaerobic digester using cow dung as a feed

The anaerobic digestion performed in example 1 and 2 using cow dung as a feed in conventional digester and self-mixed anaerobic digesters were compared to evaluate the benefits of the present invention.

Example 8

Comparative evaluation of anaerobic digestion in conventional digester and self-mixed anaerobic digester using poultry litter as a feed

The anaerobic digestion performed in example 3 and 4 using poultry litter as a feed in conventional digester and self-mixed anaerobic digester were compared to evaluate the benefits of the present invention

Example 9

Comparative evaluation of anaerobic digestion in conventional digester and self-mixed anaerobic digester using Ensilaged mango peels as a feed.

The anaerobic digestion performed in example 5 and 6 using Ensilaged mango peels as a feed in conventional digester and self-mixed anaerobic digester were compared to evaluate the benefits of the present invention

Advantages of the invention:

The advantages of the present invention are:

1. reduction of hydraulic retention time (HRT) from 40 to 15 days, which results in less volume of the digester per ton of organic waste to be treated compared to the conventional digester.

2. increase in solids loading rate from 2.5 to 6.7 kg TS/ m³.day. 3. increase in VS destruction rate from 40% to 50%.

4. scaling up of digester to handle a wide range of waste volumes, from small volumes (e.g., small communities, coastal communities, small industries, seafood process, etc) to high volumes (e.g., large industries and municipal wastes). 5. intermittent mixing of digester slurry by using the biogas pressure.

6. de-alienation of scum formation problem, which is common in conventional digesters.

7. increase in the biogas production and methane content in biogas.

8. elimination of short-circuit of feed in the digester. 9. digester energy-efficiency since gravity feeding instead of pumping and neither internal moving parts nor heating coils are required.

10. elimination of choking and clogging completely.

11. suitable reactor design for a wide variety of organic solid wastes

Claims

1. A self mixing anaerobic digester useful for the digestion of organic solid waste, wherein the said digester consisting of the following parts: i. at least one bottom and at least one upper reaction chamber having conical bottom, wherein the chambers being provided with biogas outlets and are being hydraulically connected through a central draft pipe; ii. a circular feed tank connected directly to inlet pipe of bottom chamber of the said digester; iii. a feed preparation system comprising a mixing vessel and medium speed stirrer, connected with the said feed tank; iv. an automatic control valve system attached to the bottom and upper chambers of the digester and v. a circular/ rectangular discharge tank having conical bottom surface with discharge valve and lid at the bottom of the said valve system to collect the digested slurry.

2. A self mixing anaerobic digester as claimed in claim 1, wherein the height of the said feed tank is adjusted to prevent failure of digester.

3. A self mixing anaerobic digester as claimed in claim 1, wherein the bottom chamber comprising a feed inlet, a gas outlet and a man hole to facilitate emergency discharge and the upper chamber comprising a gas inlet from bottom chamber, a gas out let for utility system and a discharge outlet.

4. A self mixing anaerobic digester as claimed in claim 3, wherein the volume of the said upper chamber is about two-third of the volume of the said bottom chamber.

5. A self mixing anaerobic digester as claimed in claim 3, wherein the organic solid waste is fed to the bottom chamber from feed tank by gravity and is moved to upper chamber through central draft pipe.

6. A self mixing anaerobic digester as claimed in claim 5, wherein the said central draft pipe is useful for promoting upward and downward movement of the slurry.

7. A self mixing anaerobic digester as claimed in claim 5, wherein the length of the said central draft pipe is adjusted such that sufficient gap is provided between bottom of the digester and central draft pipe bottom end to eliminate choking of the draft pipe.

8. A self mixing anaerobic digester as claimed in claim 3, wherein the said reaction chambers are adapted to foster the anaerobic microbial digestion of organic waste.

9. A self mixing anaerobic digester as claimed in claim 1, wherein the said automatic control valve system consisting of sensor, controller and final control element to release the biogas when pressure in the bottom chamber reaches the set value.

10. A self mixing anaerobic digester as claimed in claim 1, wherein the discharge tank comprising a circular/ rectangular tank having suitable volume depending upon feeding rate to hold the discharge slurry and having supporting structure such as conical bottom with discharge valve and lid to control odor.

11. A process of anaerobic digestion as obtained from the said self mixing anaerobic digester as claimed in claim 1, wherein the said process comprising following steps of:

(a) introducing the organic wastes into the feed preparation system of the digester to separate grits from the organic solid waste;

(b) preparing homogenized slurry as obtained from step (a) and feeding the slurry through feed tank to the inlet of the bottom reaction chamber of the said digester by gravity;

(c) allowing the movement of the waste as obtained from step (b) from the bottom reaction chamber to the upper chamber through central draft pipe in response to increase in biogas pressure in the bottom chamber; (d) allowing the movement of the partially digested slurry from upper chamber to bottom chamber by releasing the biogas as obtained from step (c) using automatic control valve system, in response to increase in biogas pressure in upper chamber;

(e) repeating the steps of (a) to (d) to obtain thorough mixing of organic waste slurry without using any mechanical device;

(f) applying sufficient hydraulic residence time to reduce the discharged solid waste as obtained from step (e) from the upper chamber at least up to 50% in total volatile solids as compared to the input volatile solids organic waste; (g) discharging calculated quantity of digested slurry as obtained from step (f) from the upper chamber to discharge tank through discharge outlet; (h) releasing and collecting biogas evolved during the anaerobic microbial digestion of organic waste in the above mentioned steps.

12. A process of anaerobic digestion as claimed in claim 11, wherein the organic waste used is selected from the group comprising cow dung, poultry litter and ensilaged mango peels and similar waste.

13. A process of anaerobic digestion as claimed in claim 11, wherein the hydraulic residence time is about 5 - 15 days.

14. A process of anaerobic digestion as claimed in claim 11, wherein the discharging of digested slurry to discharge tank is carried out at least once in a day.

15. A self mixing anaerobic digester as claimed in claim 1, wherein the said digester is made of material selected from the group comprising PPFRP, HDPE, RCC to control corrosion of said digester.

16. A self mixing anaerobic digester as claimed in claim 1, wherein the said digester is useful for hydrolysis, acidogenesis, methanogensis of organic waste.

17. A self mixing anaerobic digester as claimed in claim 1, wherein the pH for digestion in the said digester is in the range of 4 - 8, so as to function under acidogenic or methanogenic conditions.

18. A self mixing anaerobic digester as claimed in claim 1, wherein the temperature range for digestion in the digester is 30⁰C - 40⁰C (mesophilic) or 5O⁰C - 60⁰C (thermophilic) .

19. A self mixing anaerobic digester as claimed in claim 1, wherein the total solids loading rate is in the range of 3.7 - 9.7 kg TS/ m3 per day.

20. A self mixing anaerobic digester as claimed in claim 1, wherein the volatile solids loading rate is in thee range of 3.5 - 6.5 Kg VS/ m3 per day.