WO2008093044A1

WO2008093044A1 - Anaerobic digester

Info

Publication number: WO2008093044A1
Application number: PCT/GB2008/000202
Authority: WO
Inventors: David Stafford; Malcolm Hayes
Original assignee: Enviro Control Limited
Priority date: 2007-02-02
Filing date: 2008-01-22
Publication date: 2008-08-07
Also published as: GB2448113A; GB0702007D0

Abstract

An anaerobic digester (12) for organic microbial waste treatment apparatus (10), comprises a reaction chamber (22), an inlet for the supply of organic waste into the reaction chamber (22), an effluent outlet (26) for supplying effluent from the anaerobic digester (12) to an aerobic digester (14), a waste gas outlet (28) for waste gas produced in the reaction chamber (22) to flow out of the reaction chamber (22), two or more bio-gas discharge mixing devices (32) provided in the reaction chamber (22), and pumping apparatus (29a) for supplying mixed bio-gas to the bio-gas discharge mixing devices (32). The or each bio-gas discharge mixing devices (32) having a plurality of discharge outlets (34) for the discharge of mixed bio-gas into the organic waste in the reaction chamber (22). At least one of the mixing devices having first said discharge outlets (34) which are oriented to direct mixed bio-gas into the organic waste for mixing the waste, and at least the other mixing device having second said discharge outlets (34) which are oriented to direct mixed bio-gas at one or more interior surfaces (22b) of the reaction chamber (22) for scouring the or each said interior surface.

Description

ANAEROBIC DIGESTER

The present invention relates to an anaerobic digester, and more particularly to organic microbial waste treatment apparatus for treating organic waste having such an anaerobic digester.

It is known from GB2400369A to provide apparatus for and a method of treating organic waste. However, following observation and research, the known apparatus results in so-called 'dead spaces' in the digester in the anaerobic reaction vessel. To ensure proper mixing of digester, dislodgment of all settled solids and incorporating them into the digester liquid is vital in order to ensure a homogenous end result.

It is also important to maintain interior surfaces of the reaction vessels which are free or substantially free of caking and particulate deposition. This reduces the maintenance required, and more importantly retains as much solid material as possible within the digester liquid, thus contributing to the overall anaerobic activity within the anaerobic vessel.

The present invention seeks to overcome these problems.

According to a first aspect of the present invention, there is provided an anaerobic digester for organic microbial waste treatment apparatus, the anaerobic digester comprising a reaction chamber, an inlet for the supply of organic waste into the reaction chamber, an effluent outlet for supplying effluent from the anaerobic digester to an aerobic digester, a waste gas outlet for mixed waste bio-gas produced in the reaction chamber to flow out of the reaction chamber, two or more bio-gas discharge mixing devices provided in the reaction chamber, and pumping apparatus for supplying mixed bio-gas to the bio-gas discharge mixing devices, the or each bio-gas discharge mixing device having a plurality of discharge outlets for the discharge of mixed bio-gas into the organic waste in the reaction chamber, at least one of the mixing devices having first said discharge outlets which are oriented to direct mixed bio-gas into the organic waste for mixing the waste, and at least the other mixing device having second said discharge outlets which are oriented to direct mixed bio-gas at one or more interior surfaces of the reaction chamber for scouring the or each said interior surface.

Preferable and/or optional features of the first aspect of the invention are set forth in claims 2 to 17, inclusive.

According to a second aspect of the invention, there is provided organic microbial waste treatment apparatus for treating organic waste, comprising an anaerobic digester in accordance with the first aspect of the invention, an aerobic digester, and means for pumping effluent from the anaerobic digester to the aerobic digester.

The present invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which :

Figure 1 is a schematic view of organic microbial waste treatment apparatus, according to the second aspect of the invention and having a first embodiment of an anaerobic digester according to the first aspect of the invention;

Figure 2 is an enlarged diagrammatic side perspective view of the anaerobic digester shown in Figure 1, and showing bio-gas discharge mixing devices;

Figure 3 is a diagrammatic plan view of the mixing devices shown in Figure 2; Figure 4 is a diagrammatic side perspective view of a second embodiment of an anaerobic digester, in accordance with the first aspect of the invention;

Figure 5 is a diagrammatic plan view of the mixing devices shown in Figure 4; and

Figure 6 is a control system block diagram for controlling the organic microbial waste treatment apparatus, shown in Figure 1.

Referring firstly to Figure 1, there is shown organic microbial waste treatment apparatus 10 which comprises an anaerobic digester 12, an aerobic digester 14, a feed line 16 connected between the upper end of the anaerobic digester 12 and the lower end of the aerobic digester 14, and a feed pump 18 for pumping effluent from the anaerobic digester 12 to the aerobic digester 14 through the feed line 16. The anaerobic digester 12 comprises an insulated vessel 20 forming a reaction chamber 22, an inlet pipe 24 for supplying organic waste to the reaction chamber 22, an effluent outlet 26 which is in communication with the feed line 16 for the supply of effluent from the anaerobic digester 12 to the aerobic digester 14, and a waste gas outlet 28 at or adj acent to a roof of the reaction chamber 22.

Connected to the waste gas outlet 28 is a recycling pipe 29 which communicates with a recycle pump 29a. The recycle pump 29a draws the mixed waste bio-gas from the reaction chamber 22, through the recycling pipe 29, and supplies it to a gas inlet 30 at the base of the reaction chamber 22. The pump can be a compressor, blower, or any other suitable device for moving gas.

A plurality of independent bio-gas discharge mixing devices 32 are provided at and/or adjacent to the base of the reaction chamber 22. In the present embodiment, the base 22a of the reaction chamber 22 is conical or frusto-conical shape and, as can best be seen in Figures 2 and 3, three mixing devices 32a, 32b, 32c of increasing diameter are supported on the base in coaxially spaced relationship to also define an inverted conical or frusto-conical shape. The shape of the base of the reaction chamber 22 is beneficial in allowing draining of solids.

Each mixing device 32 is an endless, typically stainless steel, ring-shaped tube which is in fluid communication, via a controllable ball- valve (not shown), with the gas inlet 30. Each tube is held in stationary relationship relative to the reaction chamber 22, typically around 100 millimeters (mm) from the interior surface of the reaction chamber 22, and has a plurality of discharge outlets 34. First said discharge outlets 34a are oriented to direct mixed bio-gas (arrows A in Figure 3) into the organic waste for creating turbulence and thus mixing (arrows C in figure 1), and second said discharge outlets 34b are oriented to direct mixed bio-gas (arrows B in Figures 2 and 3) at one or more interior surfaces 22b of the reaction chamber 22 for scouring. Typically, the said first discharge outlets 34a are oriented in an up-down direction of the reaction chamber 22, and the second discharge outlets 34b are oriented in a sideways or radial direction. The discharge outlets 34 are typically holes having diameters of approximately 10 mm. The first discharge outlets 34a are formed in an upwardly or axially facing portion of the surface of the mixing device 32. The second discharge outlets 34b are formed in a side or outwardly radially facing portion of the surface of the mixing device 32. Discharge outlets which are angled between the axial and radial directions may also be advantageous in creating turbulent mixing.

Preferably, a first one 32a of the three mixing devices 32 has a diameter approaching that of the internal diameter of the reaction chamber 22. The second mixing device 32b has a diameter which is or substantially is two-thirds of the internal diameter of the reaction chamber 22; and the third mixing device 32c has a diameter which is or substantially is one-third of the internal diameter of the reaction chamber 22.

The orientation of the first and second discharge outlets 34a, 34b is also fixed relative to the reaction chamber 22, although it is feasible that the orientation is selectively controllable, for example by the use of motor drivable nozzles.

The anaerobic digester 12 also includes a settlement tube 36 against the inside wall of the reaction chamber 22. This has an inlet 38 above the base of the reaction chamber 22 and the mixing devices 32, and increases in cross-sectional dimensions as it extends upwards to the level at which effluent exits the chamber. The settlement tube 36 allows the digester contents to exit the chamber. The increasing cross-sectional area as the contents move upwards reduces the rate of upflow speed of the contained particles. The larger particles thus tend to slow down as well as fall back and remain in the tank for longer than ordinarily they would. Consequently, the retention time of solids is increased.

The arrangement of the anaerobic digester 12 is suitable for activity at all temperatures, including mesophilic (35°C) and thermophilic (56°C).

The aerobic digester 14 comprises an insulated vessel 40 forming an aerobic reaction chamber 42, a Venturi mixer 44, a liquid outlet 46, and an air outlet 48. The Venturi mixer 44 comprises a recycle tube 50 connecting the lower end of the aerobic reaction chamber 42 to the upper end of the reaction chamber 42, an air inlet tube 52 connected to the recycle tube 50 intermediate the ends thereof and a recycle pump 54 for recycling effluent from the aerobic digester 14 through the recycle tube 50 and past the inner end of the air inlet tube 52.

The line 16 is connected to the recycle tube 50 at the lower end thereof and upstream of the recycle pump 54.

Two temperature sensors 58, 60 are mounted in the aerobic reaction chamber 42.

A first one of the temperature sensors 58 is mounted at a position above a second one 60 of the temperature sensors, so that the temperature of the effluent at two levels in the aerobic reaction chamber 42 can be monitored.

A syphon break tube 62 is connected to the liquid outlet 46.

An anti-foam system 64 is included in each digester 12, 14 to counteract inherent and known foaming problems. Each anti-foam system 64 comprises a spray nozzle 66 mounted in an upstanding extension tube 68 at the apex of each reaction chamber 22, 42. The extension tube 68 has a viewing window 70 at its upper end and a gas vent 72 extending laterally and upwardly from the side of the upstanding extension tube 68. The nozzles 66 are supplied with an anti-foam liquid, which could be water or a mixture of water and an anti-foam agent, from a tank (not shown).

The nozzles 66 are used to evenly distribute the anti-foam liquid to the reaction chambers to physically disrupt the foam structure therewithin.

To further prevent or limit undesirable foaming in the anaerobic digester 12, a further bio-gas discharge mixing device 74 (shown by phantom lines), similar to those described above, can be provided partway between the above-mentioned mixing devices

32 and an upper surface of the reaction chamber 22 of the anaerobic digester 12. Typically, the further mixing device 74 is fixedly supported by the interior surface of the reaction chamber 22 at a position just below an upper surface of effluent in the reaction chamber 22. The further mixing device 74 is also fed, via a selectively operable valve, from the recycle pump 29a with the waste gas. Further first discharge outlets 34d for the recirculated gas are oriented to disrupt a surface of the effluent in the anaerobic reaction chamber 22, thus limiting foam production and accumulation, and also preventing or limiting coagulation and thus the formation of scum at the surface. Further second discharge outlets 34e can also be provided for scouring the interior surfaces of the reaction chamber 22, at and adjacent to the mixing device 32.

It is also considered feasible to provide yet further said bio-gas discharge mixing devices, as described above, at other positions between the plurality of mixing devices at and adjacent to the lower surface of the reaction chamber, and the upper surface of the reaction chamber.

Preferably, a reverse osmosis device 76 is provided downstream of the aerobic digester 14 and/or the anaerobic digester 12. The reverse osmosis device(s) 76 contain at least one semi-permeable membrane 78 which allows dirty effluent from the associated digester to be cleaned when passing through it. The contained, dissolved and suspended solids are removed allowing the clean water to pass through the membrane under pressure and be recovered. Reverse osmosis is capable of removing bacteria, salts and dissolved organics from the anaerobic or aerobic digester effluent. The removal of charged salts with reverse osmosis is helped by the natural electrical charge on the particles. Many of the organic particles are charged as well as the inorganic molecules. Thus both are removed, but especially the latter group.

A computer control system is utilised to enable remote monitoring and control of the apparatus. All pumps, motors, agitators, and other controllable components can be manually controlled locally in order to facilitate system check-out and start-up. Each device has a local automatic and manual selector switch which is interlocked into the logic of the control system. For safety, placing any device in manual mode locks out the automatic mode. Automatic sequencing for the anaerobic digester 12 is accomplished through the use of a programmable logic controller (PLC) in a control room. All analogue and digital I/O passes through a PLC, which allows all devices to be monitored and controlled automatically. All fixed sequences and timing are controlled from the PLC.

With the recycle pump 29a of the anaerobic digester 12 turned on, at least one ball valve is open, and thus the associated mixing device 32 is operating at all times. Subsequent other valves are opened before an open valve is closed. Valve sequencing and open times are chosen to facilitate optimal mixing and thus the minimisation or eradication of dead spaces. By way of example, two possible sequence control schemes for the three bio-gas discharge mixing devices 32a, 32b, 32c are shown in the tables below. If the valve is listed, it is open. If the valve is not listed, it is closed. The listed times are arbitrary, and are either manually set based on experience of the type of effluent in the anaerobic digester 12, or are automatically computer controlled with feedback via sensors placed within the anaerobic reaction chamber 22.

Mixing device turned on at time 0 Mixing device turned on at time 0 Valve Time (sees) Valve Time (sees)

1 0 1 0

1 60 2 0

2 60 1 60

2 75 3 60

2 135 3 75

3 135 2 75

3 150 2 135

3 210 2 150

1 210 1 150

3 270 1 165

1 270 1 225

2 270 3 225 3 250

Mixing device turned off at time 350 Mixing device turned off at time 350

An advisable scenario, in order to effectively as possible dislodge solids from dead spaces, is to make every other mixing event performed by the at least three bio-gas discharge mixing devices 32a, 32b, 32c have a random valve and time sequencing to cover all possible sequences.

Regarding the control of the apparatus 10, supervisory process control and data acquisition are accomplished through an IBM compatible computer (PC) operating under OS/2 or equivalent. The PC provides sufficient processing power to operate the control software and other programs in true multi-tasking mode. A hard disk provides local storage for programs and operating/laboratory data. The PC communicates with the PLC over an RS-232, or other suitable, interface using the MODBUS communications protocol or equivalent. A high speed internal modem provides remote communications capability for control and data exchange.

Presently, Windows RTM software provides a strong graphical user interface, compatible with a broad selection of hardware and software, and can provide limited multi-tasking and relatively supervisory control and data acquisition software. OS/2

Warp RTM also provides a strong graphical user interface, true multi-tasking and compatible with many Windows RTM applications and sophisticated process control software. The components of the system are required to ensure compatibility and smooth operation.

The selection of the supervisory control and data acquisition software was made in conjunction with that of the PC operating system.

Paragon TNT, an OS/2-based package is compatible with the hardware and software requirements, as well as offering a platform for future applications. Also, PC Anywhere software is used for remote communications. The modular approach employed in Paragon TNT permits independent development of process I/O, data management, control strategy, data acquisition, logging and reporting functions. This is important in allowing increasingly sophisticated protocols and levels of control to be implemented. The operator interface screen drawing utility, although not as sophisticated as those found in stand-alone illustration packages, allows functional screens, complete with animation, to be easily developed.

Communications software is an important element of the apparatus control system. The communications system must provide remote control capability, as well as the ability to up-load and down-load data between various locations. Selection criteria included operation system compatibility, stability, ease of use, ability to view both graphics and text base screens remotely, security features, and the ability to operate with both OS/2 and Windows-based call-in systems. The software, in conjunction with the internal modem, has proven to be a reliable means of both remote control and file transfer between OS/2 based systems, or equivalent. A scheduling utility is employed to load and unload the communications packages, allowing both software to utilize the same modem at different times of day.

Referring to the control system block diagram of Figure 6, the control system 80 allows for remote and local programming of the anaerobic and aerobic digesters 12, 14 to improve performance and to interface and utilise the digesters more effectively.

The software control is based on the specific digester design and desired functionality of the mixing and feeding regimes as well as the mixing devices and discharge outlet configurations. The control system 80 comprises a data storage device, typically being a computer 82, running, for example, the Microsoft XP Operating System, Opto-22 optically isolated I/O modules communicating via, for example Ethernet 83, with the data storage device, two Nema-4 cabinets 84a, 84b inside a control room house (separating logic and relay modules), and two Nema-4 outside cabinets 86a, 86b provided for field wiring connections. One outside cabinet 86a interfaces to digester analogue and digital I/O. The other outside cabinet 86b houses a set of manual lockout switches to field power wiring. Digester operators need only access the lockout switches and control computer.

A primary first screen is an animated operator screen schematic of the digester process with relevant process variable values. A second primary screen shows the current states of cyclical timing variables related to feeding, feed mixing, digester recirculation, digester mixing, effluent separation, and power generation watts. This second screen provides the means to initiate, pause, continue, and stop process control. A manual means of turning on and off digester functions is also available.

Secondary screens provide operating parameter setup, graphical display of feedback signals versus time, active alarms, a history of operator changes, and other system events, and a flexible data extraction screen for exporting to text files accessible to database and spreadsheet software, such as Access and Excel.

Separate software provided is an Ethernet driver for the Opto-22 equipment, and a database backup program.

Optional dynamic statistical control can be included or retrospectively included at a future date. This optimizes initial feed ramp up, adjustment of feed amounts in real time to account for available carbon- and/or hydrogen- and/or fatty-acids- and/or methane- fluctuations, and provides for feed shutoff if microbial response deteriorates as well as resuming feeding when microbial response returns to normal. Dynamic statistical control and feedback dispenses with the need for frequent and timely laboratory analysis of feed and digestate, and provides more accurate control information.

Dynamic control relies on on-line feedback of biogas flow, methane percent and pH values. This results in the need for a reliable and accurate flow meter, gas analyzer, and pH meter. An on-line free hydrogen monitor is an additional device that may be an asset to dynamic control. The parameters used to control the digester can be fed into the use of the bio-gas discharge mixing devices 32 and the discharge outlets 34, and their sequence of use within the digester. Referring now to Figures 4 and 5 of the drawings, there is shown a second embodiment of an anaerobic digester 112 forming part of organic microbial waste treatment apparatus 10 as described above. This anaerobic digester 112 comprises an insulated vessel 120 forming a reaction chamber 122. However, the lower surface of the reaction chamber 122 is flat, rather than an inverted cone as in the first embodiment. To this end, the three bio-gas discharge mixing devices 132a, 132b, 132c are positioned concentrically or substantially concentrically, and are thus coplanar.

The flat base of the reaction chamber 122 is beneficial in further reducing dead spaces.

The remaining parts are as described with reference to the first embodiment, and therefore further description is omitted.

It will be understood that, although the reaction chambers have a typically cylindrical side wall, other shapes are entirely feasible.

Furthermore, although the base of the reaction chamber can be an inverted cone, inverted frusto-conical, or flat, again other shapes are entirely possible, such as conical, frusto-conical or dome-shape.

Additionally, although the bio-gas discharge mixing devices are endless ring-shaped tubes, typically having a circular lateral cross-section, any suitable shape is possible. The mixing devices can be rectilinear, having ends, or simply arcuate with ends. The mixing devices can define circular or non-circular shapes, endless or with ends, and their lateral cross-sections can be circular or non-circular.

Combinations of shapes of mixing devices may be advantageous.

The mixing devices are intended to expel recirculated bio-gas back into the reaction chamber for mixing the contents. Therefore, the mixing devices can be considered to be pneumatic mixing devices. Although not shown, a reaction chamber contents recirculating circuit can be provided to circulate the liquid contents of the anaerobic digester. The recirculating circuit is typically external of the reaction chamber, and typically includes a pump and heat exchanger for imparting heat energy to the liquid contents of the reaction chamber via a heated water jacket, or other suitable device.

Although it is suggested that three bio-gas discharge mixing devices are utilised at the base of the reaction chamber, two mixing devices or more than three mixing devices can be provided. At least two mixing devices are required in order to provide suitable turbulence within the effluent, and thus adequate mixing. However, one of the mixing devices may only include discharge outlets which are directed at the interior surface or surfaces of the reaction chamber primarily, but not necessarily exclusively, for cleaning or scouring, and the other mixing device may only include outlets which are directed upwards, downwards and/or inwards for mixing.

It is thus possible to provide an anaerobic digester for organic microbial waste treatment apparatus which has greatly improved mixing, thus resulting in the minimization of dead spaces and a more homogeneous liquid. Efficiency is improved, and the build up of solids on interior surfaces is prevented or limited. It is further possible, by the use of the control system, to automatically optimise mixing sequences and timings, both locally and remotely.

The embodiments described above are given by way of examples only, and various other modifications will be apparent to persons skilled in the art without departing from the scope of the invention, as defined by the appended claims.

Claims

1. An anaerobic digester (12; 112) for organic microbial waste treatment apparatus (10), the anaerobic digester (12; 112) comprising : a reaction chamber (22; 122), an inlet (24) for the supply of organic waste into the reaction chamber (22; 122), an effluent outlet (26) for supplying effluent from the anaerobic digester (12; 112) to an aerobic digester, a waste gas outlet (28) for mixed waste bio-gas produced in the reaction chamber (22; 122) to flow out of the reaction chamber (22; 122), two or more bio-gas discharge mixing devices (32; 132) provided in the reaction chamber (22; 122), and pumping apparatus (29a) for supplying mixed bio-gas to the bio-gas discharge mixing devices (32; 132), the or each bio-gas discharge mixing device (32; 132) having a plurality of discharge outlets (34) for the discharge of mixed bio-gas into the organic waste in the reaction chamber (22; 122), at least one of the mixing devices (32; 132) having first said discharge outlets (34a) which are oriented to direct mixed bio-gas into the organic waste for mixing the waste, and at least the other mixing device (32; 132) having second said discharge outlets (34b) which are oriented to direct mixed bio-gas at one or more interior surfaces (22b) of the reaction chamber (22; 122) for scouring the or each said interior surface (22b).

2. An anaerobic digester as claimed in claim 1, wherein the or each said mixing device (32; 132) has both of the first and second discharge outlets (34a, 34b).

3. An anaerobic digester as claimed in claim 1 or claim 2, wherein the waste gas outlet (28) is in communication with the pumping apparatus (29a) for recirculating the mixed waste bio-gas and supplying it to the bio-gas discharge mixing devices (32; 132).

4. An anaerobic digester as claimed in any one of the preceding claims, wherein each bio-gas discharge mixing device (32; 132) is fixed relative to the reaction chamber (22; 122).

5. An anaerobic digester as claimed in any one of the preceding claims, wherein each discharge outlet (34) of the mixing devices (32; 132) has an orientation which is fixed relative to the reaction chamber (22; 122).

6. An anaerobic digester as claimed in any one of the preceding claims, wherein the bio-gas discharge mixing devices (32; 132) are arcuate.

7. An anaerobic digester as claimed in any one of the preceding claims, wherein the bio-gas discharge mixing devices (32; 132) are endless.

8. An anaerobic digester as claimed in any one of the preceding claims, wherein the bio-gas discharge mixing devices (132) are formed as concentric rings lying in a common plane.

9. An anaerobic digester as claimed in any one of claims 1 to 7, wherein the bio- gas discharge mixing devices (32) are formed as rings having different diameters, centres of the rings being aligned and spaced from each other.

10. An anaerobic digester as claimed in claim 9, wherein three bio-gas discharge mixing devices (32) are provided, the bio-gas discharge mixing devices (32) being arranged to define a cone or a frusto-cone.

11. An anaerobic digester as claimed in claim 10, wherein the cone or frusto-cone is inverted.

12. An anaerobic digester as claimed in any one of the preceding claims, wherein a further said bio-gas discharge mixing device (32; 132) is provided partway between the said two or more bio-gas discharge mixing devices (32; 132) and an upper surface of the reaction chamber (22; 122).

13. An anaerobic digester as claimed in any one of the preceding claims, further comprising computer control means (80, 82) for sequencing and timing operation of the bio-gas discharge mixing devices (32; 132).

14. An anaerobic digester as claimed in claim 13, wherein the computer control means (80, 82) includes a plurality of sequencing and timing operations which can be selected.

15. An anaerobic digester as claimed in claim 13 or claim 14, wherein the computer control means (80, 82) includes dynamic statistical feedback control of the anaerobic digester (12; 112) based on carbon- and/or hydrogen- and/or fatty-acids- and/or methane-fluctuation in the reaction chamber (22; 122), and/or microbial response.

16. An anaerobic digester as claimed in any one of claims 13 to 15, wherein the computer control means (80, 82) includes access means for local and remote operator access.

17. An anaerobic digester as claimed in any one of claims 13 to 16, wherein the computer control means (80, 82) includes a manual override for operating the bio-gas discharge mixing devices (32; 132).

18. Organic microbial waste treatment apparatus (10) for treating organic waste, comprising an anaerobic digester (12; 112) as claimed in any one of the preceding claims, an aerobic digester (14), and means (18) for pumping effluent from the anaerobic digester (12; 112) to the aerobic digester (14).