AU2014200009B2

AU2014200009B2 - Anaerobic gas lift reactor for treatment of organic waste

Info

Publication number: AU2014200009B2
Application number: AU2014200009A
Authority: AU
Inventors: Anupoju Gangagni Rao; Yerramsetti Venkata Swamy
Original assignee: Council of Scientific and Industrial Research CSIR
Current assignee: Council of Scientific and Industrial Research CSIR
Priority date: 2013-01-03
Filing date: 2014-01-02
Publication date: 2016-08-04
Anticipated expiration: 2034-01-02
Also published as: AU2014200009A1

Abstract

ANAEROBIC GAS LIFT REACTOR FOR TREATMENT OF ORGANIC Accordingly, the present invention provides a reactor (1) for anaerobic digestion of slurry comprising a tank (2) defining a bottom zone (3), a separation zone and a top zone (4). A slurry inlet mechanism (5) is located at about the bottom zone (3) for feeding the slurry and a gas inlet mechanism (6) is located at about the bottom zone (3) for feeding gas so as to mix the slurry at the bottom zone (3). The reactor comprises a transporting mechanism (7, 8) for assisting in an upward movement of the slurry from the bottom zone (3) to the top zone (4) based on a differential pressure between the zones. The reactor (1) is further provided with a downer (9) operatively connecting the top zone (4) and the bottom zone (3) for assisting in a downward movement of the slurry in the absence of the differential pressure. The reactor (1) is further provided with a gas withdrawal mechanism (10a, 10b, 11) for withdrawing a bio-gas produced within the tank (2). The gas withdrawal mechanism (10a, 10b, 11) is operably connected to the gas inlet mechanism (6) for recirculating at least a part of the bio-gas withdrawn. The reactor (1) is further provided with an automatic pressure sensitive valve mechanism (28) operably connected to the gas withdrawal mechanism (10a, 10b, 11) and/or the gas inlet mechanism (6) so as to maintain a predetermined pressure inside the tank (2).

Description

ANAEROBIC GAS LIFT REACTOR FOR TREATMENT OF ORGANIC

WASTE

Field of invention [0001] This invention relates to a reactor for the anaerobic digestion of slurry comprising organic waste. The highlight of the reactor of the present invention is that it could effectively digest under anaerobic conditions organic solid wastes having low C/N ratio to alienate ammonia inhibition problem in the reactor. Other highlighting features of the reactor of the present invention include attaining a reduction of hydraulic retention time, a reduction in a volume of reactor, a reduction in volatile solids and a reduction in COD, without compromising on methane production.

State of the art in the field [0002] The objectives of the digestion process are to reduce the total amount of volatile solids in the waste and to produce a biogas for utilizing it as renewable energy and discharge the stabilized solid waste for further use as organic manure. 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).

[0003] 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 for 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 (up flow, down flow, and expanded bed) treatment approaches.

[0004] The main disadvantage of anaerobic digesters is the long residence time typically required to digest organic wastes. 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 (U.S. Pat. No. 5,637,219). Studies were reported on batch type digesters of KVIC and DENABANDHU 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. Pat. No. 4,885,094) [0005] 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).

[0006] 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 (U.S. Pat. No. 5,637,219). Complex, multistage digesters were described that spread out the digestive processes into three or more sections (U.S. Pat. Nos. 4,604,206 and 5,637,219).

[0007] In most digesters, a microbial consortium is 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 (20° C. to 45° C.) or thermophilic temperature range (50° C. to 65° C.). 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 35° C., and by thermophilic bacteria at a temperature of about 55° C. Many digesters also control pH. Methanogenesis is pH dependent, with the optimal pH range of about 6 to 8.

[0008] U.S. Pat. No. 6,254,775 describes an anaerobic digester system based on an upright vessel with internal matrices for bacteria immobilization. U.S. Pat. No. 5,637,219 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. Pat. 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 are a rotating biological contactor and a 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.

[0009] U.S. Pat. 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. A settling chamber is provided to reclaim the microbes and remove additional solids after the reaction chambers.

[0010] US Patent No. 4726899 describes a reactor for anaerobic digestion of organic waste material comprising of a container divided into three interconnected chambers situated one below the other, the upper chamber being an inlet chamber, and the lower chamber being a mixing chamber and the intermediate chamber being a settlement chamber. The inlet chamber is having an inlet for material to be treated, a duct from the inlet chamber down into the mixing chamber, at least one transfer duct from the mixing chamber up into the settlement chamber, the duct or ducts extending below the upper extent of that mixing chamber, a gas collection zone at the upper extent of the mixing chamber, at least one recirculation duct from the base of the settlement chamber down into the mixing chamber, a further gas collection zone in the upper extent of the settlement chamber and an outlet from the settlement chamber situated below the further gas collection zone.

[0011] US Patent No. 4302329 describes an installation for the recovery of methane gas from organic waste with a fermentation space, a gas collecting space and a post-fermentation space, as well as with at least one supply line into the fermentation space and one offtake for the excess, fermented liquid from the post-fermentation space, the spaces being heat-insulated and preferably embedded at least partially in the ground, and the post-fermentation space, which preferably is arranged above the fermentation space, being connected in the manner of communicating vessels with the fermentation space, preferably in one structural unit.

[0012] Biogas Induced Mixing Arrangement (BIMA) digester technology, Austria and US Patent No. 4302329 describes an installation for the recovery of methane gas from organic waste. The main disadvantages of US Patent No. 4302329 which describes an installation for the recovery of methane gas from organic waste are; no advantage in terms of biogas production and volatile solid destruction, no reduction of hydraulic residence time of the digester which is the crucial parameter for cost effective design of the digester, no enhancement of solids loading rate which is the prime design factor that decides the volume of the digester. The main disadvantages of BIMA are; 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), design of the scum eliminator is not effective, full-scale application of BIMA digester to abattoir waste (M/s Alkabeer at Hyderabad) resulted in HRT of 30-35 days which is more or less the same as conventional batch digester.

[0013] Solid organic wastes such as poultry litter, piggery manure, sewage sludge, water hyacinth, food waste, goat and sheep manure etc contains high level of organic nitrogen. High amounts of ammonia and ammonium were produced due to anaerobic degradation of protein and amino acids present in these wastes (Kelleher et al., 2002). Ammonium ion, which is an essential nutrient to the microorganisms, can be utilized by methanogenic bacteria. However, the excess of ammonia can inhibit the anaerobic microbial consortia for the production of volatile fatty acids (VFA) and methane. The inhibitory effects of ammonia mainly influence the methanogenesis in anaerobic reactors (Krylova et al., 1997). Studies reported that free ammonia was a more powerful inhibitor of methanogenic activity than ammonium and fraction of unionized ammonia increases with temperature (Lettinga 2004) and it was found to be more toxic than the ionized one because of its capability to penetrate through the cell membrane. Carbon to nitrogen ratio of poultry litter is in the range of 6 - 12 compared to the optimally required ratio in the range of 20 - 30 for biomethanation. Due to this reason, total ammoniacal nitrogen (TAN) generated during anaerobic degradation beyond the limit of 2000 -6000 mg/1 inhibits the process (Gangagni Rao et al., 2008). Therefore, the type of organic wastes which are having high nitrogen content compared to carbon (low C/N ratio) than optimally required needs a different type digester design to address the issue of ammonia inhibition in the biomethanation.

[0014] Despite all of the above, an unfilled need exists to provide an improved reactor for the anaerobic digestion of slurry comprising organic waste.

Objectives of the present invention [0015] It is an object of the present invention to substantially overcome or ameliorate one or more of the above disadvantages, or at least provide a useful alternative.

Summary of invention [0015a] In a first aspect, the present invention provides a reactor for anaerobic digestion of a slurry comprising organic waste, said reactor comprising: a tank defining a bottom zone, a separation zone and a top zone; a slurry inlet mechanism located at about the bottom zone of the tank for feeding the slurry; a gas inlet mechanism located at about the bottom zone of the tank for feeding gas so as to mix the slurry at about the bottom zone; a transporting mechanism assisting in an upward movement of the slurry from the bottom zone to the top zone based on a differential pressure between the zones; a downer operably connecting the top zone and the bottom zone for assisting in a downward movement of the slurry in the absence of the differential pressure; a set of baffles located at about the separation zone for diverting contents of the slurry in the reactor; a gas withdrawal mechanism for withdrawing a bio-gas produced within the tank, the gas withdrawal mechanism being operably connected to the gas inlet mechanism for recirculating at least a part of the bio-gas withdrawn; and an automatic pressure sensitive valve mechanism being operably connected to the gas withdrawal mechanism and/or the gas inlet mechanism so as to maintain a predetermined pressure inside the tank.

[0016] In a second aspect of the present invention provides a method for anaerobic digestion of a slurry comprising organic waste in a reactor, said method comprising: feeding the slurry to a bottom zone of a tank, defining a bottom zone, a separation zone and a top zone; feeding a gas to the bottom zone of the tank so as to mix the slurry at the bottom zone; enabling an upward movement of the slurry from the bottom zone to the top zone based on a differential pressure between the zones by providing a transporting mechanism; enabling a downward movement of the slurry in the absence of the differential pressure by providing a downer operably connecting the top zone and the bottom zone; diverting the contents of slurry in the reactor by baffles located at about the separation zone; withdrawing a bio-gas produced within the tank and recirculating at least a part of the withdrawn bio-gas using an automatic pressure sensitive valve mechanism so as to maintain a predetermined pressure inside the tank.

Brief description of the accompanying drawings [0017] Preferred embodiments of the invention will be described hereinafter, by way of examples only, with reference to the accompanying drawings.

[0018] FIGURE 1 is a sectional view of the reactor in accordance with an embodiment of the present invention.

[0019] Skilled artisans will appreciate that elements in the drawings are illustrated for simplicity and may not have been necessarily been drawn to scale. For example, the dimensions of some of the elements in the drawings may be exaggerated relative to other elements to help to improve understanding of aspects of the present invention. Furthermore, the one or more elements may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the drawings with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.

Detailed description of the invention [0020] Detailed descriptions of the preferred embodiment are provided herein. It is to be understood, that the present invention may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately detailed system, structure or manner.

[0021] While the invention is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the invention as defined by the appended claims.

[0022] The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that one or more devices or sub-systems or elements or structures proceeded by "comprises... a" does not, without more constraints, preclude the existence of other devices or other sub-systems or other elements or other structures or additional devices or additional sub-systems or additional elements or additional structures. Similarly, a method step proceeded by “comprising” or any variation thereof, does not, without more constraints preclude the existence of additional steps or repetitive steps.

[0023] Referring to figure 1, it can be noticed that the reactor (1) in accordance with one embodiment of the present invention comprises: • a tank (2) defining a bottom zone (3), a separation zone and a top zone (4); • a slurry inlet mechanism (5) located at about the bottom zone (3) of the tank (2) for feeding the slurry; • a gas inlet mechanism (6) located at about the bottom zone (3) of the tank for feeding gas so as to mix the slurry at about the bottom zone (3); • an transporting mechanism (7, 8) assisting in an upward movement of the slurry from the bottom zone to the top zone based on a differential pressure between the zones; • a downer (9) operably connecting the top zone and the bottom zone for assisting in a downward movement of the slurry in the absence of the differential pressure; • a gas withdrawal mechanism (10a, 10b, 11) for withdrawing a bio-gas produced within the tank (2), the gas withdrawal mechanism being operably connected to the gas inlet mechanism (6) for recirculating at least a part of the bio-gas withdrawn; and • an automatic pressure sensitive valve mechanism (28) being operably connected to the gas withdrawal mechanism (10a, 10b, 11) and/or the gas inlet mechanism so as to maintain a predetermined pressure inside the tank (2).

[0024] In accordance with another aspect of the invention as illustrated in figure 1, the slurry inlet mechanism (5) is provided with a slurry distribution mechanism (12) for supplying the slurry comprising organic waste in a distributed manner to the bottom zone (3). The slurry inlet mechanism (5) is provided with valve mechanism (not shown) for feeding the slurry to the tank under atmospheric conditions.

[0025] In accordance with yet another aspect of the invention as illustrated in figure 1, the gas inlet mechanism (6) is provided with a gas distribution mechanism (13) for supplying the gas in a distributed manner to the bottom zone (3).

[0026] In accordance with still another aspect of the invention and as illustrated in figure 1, the separation zone comprises a first level separation zone (14) and a second level separation zone (15).

[0027] In accordance with a further aspect of the invention and as illustrated in figure 1, the transporting mechanism (7, 8) comprises a riser (8) operably connecting the bottom zone (3) and the top zone (4) and an upward transporting and separating mechanism (7) located at about the separation zone. The upward transporting and separating mechanism (7) may comprise: • a first stage transportation and separation mechanism (16) located at about the first level separation zone (14) for receiving the upwardly moving slurry, separating the same into a gas phase, a liquid rich phase and a solid rich phase and transporting the liquid rich phase from the first level separation zone (14) to the second level separation zone (15); and • a second stage transportation and separation mechanism (17) located at about the second level separation zone (15) for receiving the upwardly moving liquid rich phase, separating the same into a gas phase, a substantially liquid phase and a solid rich phase and transporting the substantially liquid phase from the second level separation zone (15) to the top zone (4).

[0028] In a preferred aspect of the present invention, each of the first stage transportation and separation mechanism (16) and the second stage transportation and separation mechanism (17) is in the form of an inverted funnel. While the top end (18) of the first stage transportation and separation mechanism (16) is open, the top end (19) of the second stage transportation and separation mechanism (17) is closed using a cap (20), wherein the cap is provided with the gas withdrawal mechanism (11). The gas withdrawal mechanism (11) is operable to withdraw the gas collected in the second stage transport mechanism (17). A top end of the tank is provided with additional gas withdrawal mechanisms (10a, 10b) for withdrawing the gas collected within the tank (2). The gas withdrawal mechanism (10a, 10b, 11) may be connected to a gas collection system (21) and the gas inlet mechanism (6) may be connected to the gas collection system (21) for withdrawing a part of the gas thus collected and feeding the same at the bottom zone (3) so as to mix the slurry. Particularly, the gas withdrawal mechanisms (10a, 10b, 11) are connected to the gas collection system (21) and the gas inlet mechanism (6) through an automatic valve mechanism (28) (electrical or pneumatic). This valve mechanism operates based on set pressure in the reactor (1).

[0029] The tank is further provided with a liquid withdrawal mechanism (22) located at about the top zone (4) for withdrawing a part of the substantially liquid phase. More particularly, a first end of the liquid withdrawal mechanism (22) is operatively placed outside the second stage transportation and separation mechanism (17) for withdrawing a part of the substantially liquid phase. A second end of the liquid withdrawal mechanism (22) is operatively coupled to a U-Tube mechanism (23) for arresting escape of a gas phase while withdrawing the substantially liquid phase. Alternatively, the second end of the liquid withdrawal mechanism (22) may be integrally formed with the U-Tube mechanism (23).

[0030] An inner portion of the tank (2) is further provided with a set of baffles (24) located between the first level separation zone (14) and the second level separation zone (15). The baffles aid for diverting the contents of the reactor to enhance a mixing therein. The bottom zone of the tank may be additionally provided with a discharge valve (25) for withdrawing a substantially solid phase.

The tank (2) may be additionally provided with a manhole (26) at about the bottom zone (3) or at about the separation zone for servicing of the reactor.

[0031] The tank (2) is further provided with a heat exchanger (27) to maintain the temperature of the reactor (1) in the mesophilic (37±2°C) or thermophilic (55+2°C) range (as per the condition of the raw material) with aid of steam or hot water to get the enhanced performance. The tank is also provided with a pH adjusting mechanism (29) for maintaining a pH value of the slurry at about a predetermined value. The pH adjusting mechanism (29) comprises: • a pH measuring probe for measuring the pH value of the slurry; • a means for supplying a source of an acid to the slurry; • a means for supplying a source of a base to the slurry; and • a controller operably connected to the pH measuring probe for receiving therefrom a signal indicative of a pH value of the slurry and controlling an operation of the means for supplying the source of acid or the means for supplying the source of base so as to maintain the pH value of the slurry at about the predetermined value.

[0032] The reactor (1) is further provide with an biogas storage holder (30) for storing excess biogas. Entry of the biogas to the biogas storage holder may be restricted by a non-return valve (31).

[0033] Coming to the operation of the reactor, the slurry is pumped into the bottom zone (3) of the reactor (2) via the slurry inlet mechanism (5). The feed slurry is fed to the reactor (1) when the reactor (1) under atmospheric pressure. The inlet feed valve is closed after completion of the feeding and pressure inside the reactor builds up gradually due to the generation of biogas in the reactor (1). Slurry gets transported to top zone (4) from bottom zone (3) through the upward transporting and separating mechanism (7,8) provided inside the reactor (1) due to the differential pressure developed across the reactor (1). When the automatic pressure valve (28) opens at set pressure, slurry falls back to the bottom chamber through downer (9). Simultaneously biogas also travels to the bottom of the reactor (1) through gas inlet mechanism (6) and distribution system (13) . This phenomenon creates vibrant mixing inside the reactor. Under these conditions reactor (1) will be at atmosphere pressure and all the excess biogas is released to the biogas storage holder (30) through non-return valve (31). This action continuously takes place at regular intervals based on the predetermined pressure i.e. a pressure which is set as required to be maintained within the tank (2). The slurry is even distributed in the bottom zone (3) of the reactor (2) by the slurry distribution mechanism (12) and mixed with the anaerobic biomass. In the bottom zone (3) of the reactor (2) most of the organic components are converted into methane and carbon dioxide. Biogas is introduced in the bottom zone (3) of the reactor (2) via the gas inlet mechanism and is even distributed in the bottom zone (3) by us of the gas distribution mechanism (13). The biogas thus introduced into the bottom zone of the reactor along with the biogas thus produced tends to lift the biomass from the bottom zone (3) to the separation zone. Thus, in a sense the reactor functions as an anaerobic gas lift reactor without using any pumping mechanism for lifting the biomass. In the first level separation zone (14) the upwardly moving biomass is received by the first stage transporting and separating mechanism (16) and the same functions to separate the biomass into three phases namely a gas phase, a liquid rich phase and a solid rich phase. In the absence of the differential pressure, the solid rich phase because of its heavy mass tends to settle down and the liquid rich phase assisted by a part of the biogas is transported from the first level separation zone (14) to the second level separation zone (15). In the second level separation zone (15) the upwardly moving biomass is received by the second stage transporting and separation mechanism (17) and the same functions to separate the biomass into three phases namely a gas phase, a substantially liquid phase and a solid rich phase. In the absence of the differential pressure, the solid rich phase because of its heavy mass tends to settle down and the substantially liquid phase assisted by a part of the biogas is transported from the second level separation zone (15) to the top zone (4).

[0034] The substantially liquid phase collected at the top zone (4) and especially outside the second transporting and separating mechanism (17) can be withdrawn using the liquid withdrawal mechanism (22). Similarly, the gas phase separated inside the second transporting and separating mechanism is collected using the gas collection mechanism (11), which is located in the cap (20) which closes a top end of the second transporting and separating mechanism. Gas phase separated from the biomass at the first stage transporting and separating mechanism or otherwise in any part of the tank accumulates at the top zone (4) of the reactor (2) and can be withdrawn using the gas withdrawal mechanism (10a, 10b).

[0035] A downer (9) is provided within the tank (2) for operatively coupling the top zone (4) and the bottom zone (3) for enabling movement of a substantially liquid phase between the two zones. Without wishing to be constrained by any theory, it is believed that if a differential pressure across the bottom zone (3) and the top zone (4) is substantially high, a substantially quantity of the slurry and especially a liquid phase of the slurry automatically moves in a direction so as to compensate for the differential pressure. On the other hand, in the absence of the differential pressure, the slurry and especially, a solid rich phase of the slurry move downward. Such an action enables maintaining a proper operating condition within the tank. The transport mechanism (7,8), the downer (9) and the gas withdrawal mechanism (10a, 10b, 11) work in conjunction with each other such that certain amount of biogas leaves the reactor and certain amount of substantially liquid phase returns through the downer back to the bottom zone (3) of the tank (2). This action continuously takes place at regular intervals based on the predetermined pressure i.e. a pressure which is set as required to be maintained within the tank. The gas inlet mechanism (6) and/or the at least one gas withdrawal mechanisms (10a, 10b, 11) are controlled so as to maintain a predetermined pressure inside the tank (2). 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 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.

[0036] As is well known, solid organic wastes such as poultry litter, piggery manure, sewage sludge, water hyacinth, food waste, goat and sheep manure etc. contains high level of organic nitrogen and hence, during digestion of such waste, high amounts of ammonia and ammonium were produced due to anaerobic degradation of protein and amino acids present in these wastes. It has also been well documented that total ammoniacal nitrogen (TAN) generated during anaerobic degradation beyond the limit of 2000 -6000 mg/1 inhibits the process. Therefore, the type of organic wastes which are having high nitrogen content compared to carbon (in other words, having low C/N ratio) than optimally required face the challenge of ammonia inhibition in the biomethanation.

[0037] The reactor in accordance with the teachings of the present invention addresses the aforesaid challenge. It has been well established that ammonia within the anaerobic digestion process can be in two forms; ammonium ion (NH4+) or dissolved ammonia gas (NH3). Both forms are always in equilibrium with each other at particular pressure, the concentration of each depending on system pH as shown in equation below.

[0038] Whenever the system pH is 7.2 standard units or lower, equation equilibrium shifts toward the ammonium ion. At pH values greater than 7.2 standard units, the reaction shifts towards the gas phase. It is possible to disturb the equilibrium conditions at set pressure so that ammoniacal nitrogen in the liquid phase could be transferred to gas phase so that liquid ammoniacal nitrogen concentration could be reduced. Pressure inside the tank is controlled through the automatic valve mechanism (28) and pH inside the tank is controlled through the pH adjusting mechanism (29) so as to maintain a predetermined pressure and pH inside the tank (2). The predetermined pressure and pH is set such that the concentration of ammoniacal nitrogen in liquid phase is substantially less than the concentration of ammoniacal nitrogen in gas phase at that temperature (either mesophilic or thermophilic).

[0039] Also, the mixing phenomenon as attained in the tank because of the operation of the transporting mechanism (7, 8) and the downer (9) helps maintaining the pH so as to assist in scavenging the ammoniacal nitrogen from liquid phase in the reactor to gas phase. Thus, predominant part of ammoniacal nitrogen in the liquid phase gets transferred to gas phase and comes out of the reactor along with biogas. This reduces the liquid phase ammoniacal nitrogen in the reactor so that inhibition due to excess ammoniacal nitrogen is substantially reduced in the reactor.

[0040] The biogas formed in the reactor is withdrawn at a controlled rate via the gas withdrawal mechanism (10a, 10b, 11) which incorporate the automatic valve mechanism (28). While a part of the biogas is re-circulated, a part of the biogas can be withdrawn from the gas collection system (21) and held in a biogas holder (30) for use. A non-return valve (31) controls the movement of the biogas from the gas collection system (21) to the biogas holder (30). The non-return valve does not allow movement of the biogas from the biogas holder (30) to the gas collection system (21).

Bottom part of the reactor is provided with a manhole (26) to facilitate emergency discharge of accumulated inorganics that might enter into the reactor along with feed.

[0041] The reactor of the present invention functions as a continuous reactor and everyday known amount of slurry is fed to the bottom zone of the tank and equal quantity of the digested slurry is discharged from the top zone of the tank to a discharge tank. The production of methane can be estimated by methods known in the art. See Ch. 8, Metcalf & Eddy, Inc. (1991).

[0042] At the start of the reactor, organic solid waste is initially fed to the tank at a low flow rate to achieve a hydraulic residence time (HRT) of approximately 50 days. This is equivalent to a solid loading rate of 2 kg TS/m -day. After attaining steady state at this HRT, flow rate is increased in a stepwise manner to achieve an HRT of 24 days. Steady state in terms of destruction of volatile -3 solids (50-65%) and biogas production (0.55-0.6m / kg VS destroyed) is obtained at each flow rate before operating the digester at higher flow rate. The steady state operations reveal that digester could be operated at a solid loading rate of 4.2 kg TS/ m3.day with an HRT of 24 days.

[0043] Thus, it can be noticed that the reactor in accordance with the teachings of the present invention is a simple, reliable, inexpensive, and efficient anaerobic digester for treating organic solid wastes at a shortened residence time and higher organic loading rate. The rector in accordance with the present invention provides for enhancement in anaerobic digestion efficiency, enhancement in biogas production, enhancement in total solids loading rate, volatile solids loading rate and reduction in reactor volume without compromising on the methane production and volatile solids destruction rate for organic solid wastes or COD reduction. The reactor construction in accordance with the teachings of the present invention enables for an effective mixing of the contents of the reactor without incorporation of any further pumping mechanism. The reactor construction in accordance with the teachings of the present invention successfully addresses the challenge of ammonia inhibition in the biomethanation which can occur while digesting organic containing high nitrogen content as compared to carbon content (or having low C/N ratio). Since the reactor implements ammonia inhibition control mechanism (AICM), microbial consortium is retained at its optimum activity. Recirculation a part of the biogas thus produced to the gas inlet mechanism at suitable pressure enables for transfer of ammonia from the liquid phase to the gas phase.

[0044] Accordingly the invention provides a modified high rate anaerobic digester consisting of ammonia inhibition control mechanism (AICM) and internal mixing of reactor contents with recirculated biogas at set pressure. The reactor constructed in accordance with the teachings of the present invention is capable of handling the solid wastes having high ammonia (due to low C/N ratio of waste) so that microbial inhibition problem is eliminated. Solid organic wastes are treated at an HRT of 24 days, with 65% VS reduction, TS loading rates of 4.2 kg TS/ m3.day, VS loading rates of 3.3 kg VS/ m .day by generating biogas at the rate of 0.6 m /kg of VS reduced.

Ammoniacal nitrogen in the range of 500 to 1200 mg/L is maintained in the reactor due to specially designed inhibition control mechanism for the solid wastes having C/N ratio as low as 6. The present invention also provides improvement of existing solid loading rate and mixing of slurry.

The present invention eliminates the short circuit of feed slurry and usage of mechanical devices for gas mixing. 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. The design also prevents maintenance cost and eliminates possible plant shut down and provides trouble free operation. The other advantages of the AGR are possibility for scaling up to any level, elimination of scum formation, increase in the biogas production and methane content of the biogas.

[0045] Although not illustrated in figure 1, the reactor constructed in accordance with the teachings of the present invention also includes a feed preparation system with provision to add organic solid waste to the reactor vessel and arrangement for feeding calculated quantity of homogenized slurry to the feed tank of anaerobic gas lift reactor. The feed preparation system may be integrated with the reactor or the homogenized slurry may be transported from the feed preparation system to the reactor using pumps. The feed preparation system may be of a rectangular/ circular PPFRP / HDPE / RCC construction with suitable capacity depending on feed rate of organic solid waste and fitted with an air compressor to homogenize the slurry. The feed preparation system in a preferred embodiment has a slanting bottom with bottom discharge to remove the grits. The feed preparation system in a preferred embodiment has a lid and suitable overflow arrangement with damper to discharge the homogenized slurry to the reactor. In the normal course of action, digested slurry from digester (tank) is collected in gravity settler tank. This unit is suitably designed to receive calculated quantity of digested slurry from the digester (tank). The gravity settler can be in the form of a rectangular/ circular tank having suitable volume depending upon feeding rate to the reactor. The gravity settler has supporting structure with top mounted with mesh so that solids are retained on the top where as liquid is settled to the bottom. The liquid is pumped back (recycled) to feeding system for the preparation of feed slurry along with fresh water.

[0046] The functioning of the reactor of constructed in accordance with the teachings of the present invention is described with reference to the following examples, which are explained by way of illustration only and should not therefore be construed to limit the scope of the present invention.

Example 1: Anaerobic digestion of cow dung in conventional digester [0047] 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 total solids (TS) loading rate of 1 kg TS/ m3/day and HRT of 100 days. Subsequently, the TS 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 TS loading rate of 2.5 kg TS/ m3.day and HRT of 40 days.

Example 2: Anaerobic digestion of cow dung in reactor of the present invention [0048] The same experiments were conducted in the reactor of the present invention hereinafter referred to as anaerobic gas lift reactor (AGR) 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 (13% TS concentration) at the total solids (TS) loading rate of 1 kg TS/ m3.day and HRT of 100 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 TS loading rate of 4.2 kg TS/ m3.day and HRT of 24 days.

Example 3: Anaerobic digestion of poultry litter in conventional digester [0049] 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 total solids (TS) loading rate of 1 kg TS/ m3.day and HRT of 100 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 TS loading rate of 2.5 kg TS/ m3.day and HRT of 40 days.

Example 4: Anaerobic digestion of poultry litter in AGR

[0050] The same experiments were conducted in the AGR 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 (11.5 % TS concentration) at the total solids (TS) loading rate of 1 kg VS/ m3.day and HRT of 100 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 TS loading rate of 4.2 kg TS/ m3 .day and HRT of 24 days.

Example 5: Anaerobic digestion of press mud in conventional digester [0051] 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 press mud slurry (10% TS concentration) at the volatile solids (VS) loading rate of 1 kg VS/ m3.day and HRT of 100 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 TS loading rate of 2.5 kg TS/ m3.day and HRT of 40 days.

Example 6: Anaerobic digestion of press mud in AGR

[0052] The same experiments were conducted in the AGR 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 press mud slurry (15% TS concentration) at the total solids (TS) loading rate of 1 kg VS/ m3.day and HRT of 10 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 TS loading rate of 4.2 kg VS/ m3 .day and HRT of 24 days.

Example 7: Comparative evaluation of anaerobic digestion in conventional digester and AGR using cow dung as a feed.

[0053] The anaerobic digestion performed in example 1 and 2 using cow dung as a feed in conventional digester and AGR was compared to evaluate the benefits of the present invention.

Example 8. Comparative evaluation of anaerobic digestion in conventional digester and AGR using poultry litter as a feed.

[0054] The anaerobic digestion performed in example 3 and 4 using poultry litter as a feed in conventional digester and AGR were compared to evaluate the benefits of the present invention

Example 9. Comparative evaluation of anaerobic digestion in conventional digester and AGR using press mud as a feed.

[0055] The anaerobic digestion performed in example 5 and 6 using press mud as a feed in conventional digester and AGR were compared to evaluate the benefits of the present invention

The advantages of the present invention are • The main advantage of the present invention is reduction of hydraulic retention time (HRT) from 40 to 24days, which results in less volume of the digester per ton of organic solid waste to be treated compared to the conventional digester. • The other advantage of the present invention is the increase in solids loading rate from 2.5 to 4.2 kg TS/ (m3.day). • Another advantage of the present invention is the increase in volatile solids loading rate from 1.5 to 3.3 kg VS/ (m3.day). • Another advantage of the present invention is the increase in VS destruction rate from 40% to 65%. • Another main advantage of the present invention is that increase in biogas production from 0.4 m3/kg VS reduced to 0.6 m3/kg VS reduced. • Yet other advantage of the present invention is that 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). • Yet another advantage of the present invention is the intermittent mixing of digester slurry by using the biogas pressure. • Still other advantage of the present invention is de-alienation of scum formation problem, which is common in conventional digesters. • Still another advantage of the present invention is the increase in the biogas production and methane content in biogas. • Still yet other advantage of the present invention is elimination of short-circuit of feed in the digester. • Yet still other advantage of the present invention is the elimination of choking and clogging completely. • Yet still another advantage of the present invention is the reactor design suitable for a wide variety of organic solid wastes • Yet still another advantage of the present invention is the decrease in ammoniacal nitrogen concentration in the reactor from 2500 mg/L to 200 mg/L which will eliminate the toxicity due to high ammoniacal nitrogen concentration for solids having C/N ratio as low as 6.

Claims

1. A reactor for anaerobic digestion of a slurry comprising organic waste, said reactor comprising: • a tank defining a bottom zone, a separation zone and a top zone; • a slurry inlet mechanism located at about the bottom zone of the tank for feeding the slurry; • a gas inlet mechanism located at about the bottom zone of the tank for feeding gas so as to mix the slurry at about the bottom zone; • a transporting mechanism assisting in an upward movement of the slurry from the bottom zone to the top zone based on a differential pressure between the zones; • a downer operably connecting the top zone and the bottom zone for assisting in a downward movement of the slurry in the absence of the differential pressure; • a set of baffles located at about the separation zone for diverting contents of the slurry in the reactor; • a gas withdrawal mechanism for withdrawing a bio-gas produced within the tank, the gas withdrawal mechanism being operably connected to the gas inlet mechanism for recirculating at least a part of the bio-gas withdrawn; and • an automatic pressure sensitive valve mechanism being operably connected to the gas withdrawal mechanism and/or the gas inlet mechanism so as to maintain a predetermined pressure inside the tank.

2. The reactor as claimed in claim 1, wherein the slurry inlet mechanism is provided with a slurry distribution mechanism for supplying the slurry comprising organic waste in a distributed manner to the bottom zone.

3. The reactor as claimed in claim 1, wherein the slurry inlet mechanism is provided with valve mechanism for feeding the slurry to the tank under atmospheric conditions.

4. The reactor as claimed in claim 1, wherein the gas inlet mechanism is provided with a gas distribution mechanism.

5. The reactor as claimed in claim 1, wherein the separation zone comprises a first level separation zone and a second level separation zone.

6. The reactor as claimed in claim 1, wherein the transporting mechanism comprises a riser operably connecting the bottom zone and the top zone and an upward transporting and separating mechanism located at about the separation zone.

7. The reactor as claimed in claim 1, wherein the upward transporting and separating mechanism comprises: • a first stage transportation and separation mechanism located at about the first level separation zone for receiving the upwardly moving slurry, separating the same into a gas phase, a liquid rich phase and a solid rich phase and transporting the liquid rich phase from the first level separation zone to the second level separation zone; and • a second stage transportation and separation mechanism located at about the second level separation zone for receiving the upwardly moving liquid rich phase, separating the same into a gas phase, a substantially liquid phase and a solid rich phase and transporting the substantially liquid phase from the second level separation zone to the top zone.

8. The reactor as claimed in claim 7, wherein the first stage transportation and separation mechanism is in the form of an inverted funnel defining an open top end and the second stage transportation and separation mechanism is in the form of an inverted funnel defining a closed top end.

9. The reactor as claimed in claim 7, wherein the top end of the second stage transportation and separation mechanism is closed by a cap.

10. The reactor as claimed in claim 9, wherein the cap comprises a gas withdrawal mechanism for withdrawing the gas phase from second stage transportation and separation mechanism.

11. The reactor as claimed in claim 1, wherein a top end of the tank is provided with at least one gas withdrawal mechanisms for withdrawing the gas collected within the tank.

12. The reactor as claimed in claim 1, further comprising a gas collection system for collecting the gas withdrawn by said least one gas withdrawal mechanisms.

13. The reactor as claimed in claim 12, wherein the gas collection system is operatively coupled to the gas inlet mechanism for feeding at least a portion the collected gas to the gas inlet mechanism.

14. The reactor as claimed in claim 1, wherein the pressure and pH maintained within the tank is such that concentration of ammoniacal nitrogen present in the liquid phase is substantially less than concentration of ammoniacal nitrogen present in the gas phase under equilibrium conditions.

15. The reactor as claimed in claim 1, further comprising a liquid withdrawal mechanism located at the about top zone for withdrawing a part of the substantially liquid phase.

16. The reactor as claimed in claim 1, further comprising a liquid U-Tube mechanism for arresting escape of gas through liquid withdrawal mechanism while withdrawing liquid from the tank.

17. The reactor as claimed in claim 1, wherein the set of baffles is located between the first level separation zone and the second level separation zone of the tank.

18. The reactor as claimed in claim 1, further comprising a discharge valve at about the bottom zone of the tank for withdrawing a substantially solid phase.

19. The reactor as claimed in claim 1, further comprising a manhole located at about the bottom zone of the tank.

20. The reactor as claimed in claim 1, further comprising a heating mechanism operable to maintain a temperature of the slurry within a predetermined range.

21. The reactor as claimed in claim 1, further comprising a pH adjusting mechanism for maintaining a pH value of the slurry at about a predetermined value.

22. The reactor as claimed in claim 21, wherein the pH adjusting mechanism comprises: • a pH measuring probe for measuring the pH value of the slurry; • a means for supplying a source of an acid to the slurry; • a means for supplying a source of a base to the slurry; and • a controller operably connected to the pH measuring probe for receiving therefrom a signal indicative of a pH value of the slurry and controlling an operation of the means for supplying the source of acid or the means for supplying the source of base so as to maintain the pH value of the slurry at about the predetermined value.

23. A method for anaerobic digestion of a slurry comprising organic waste in a reactor, said method comprising: • feeding the slurry to a bottom zone of a tank, defining a bottom zone, a separation zone and a top zone; • feeding a gas to the bottom zone of the tank so as to mix the slurry at the bottom zone; • enabling an upward movement of the slurry from the bottom zone to the top zone based on a differential pressure between the zones by providing a transporting mechanism; • enabling a downward movement of the slurry in the absence of the differential pressure by providing a downer operably connecting the top zone and the bottom zone; • diverting the contents of slurry in the reactor by baffles located at about the separation zone; • withdrawing a bio-gas produced within the tank and recirculating at least a part of the withdrawn bio-gas using an automatic pressure sensitive valve mechanism so as to maintain a predetermined pressure inside the tank.