[0001] This invention relates to apparatus for anaerobic digestion of biodegradable waste
material.
[0002] In the design and operation of anaerobic digestion apparatus, mixing of the contents
of the tanks, which are often very large, is necessary to ensure good contact between
the bacteria and their food supply which is the effluent, i.e. biodegradable waste
material, to be treated.
[0003] The concentration of the solids material which should be maintained in suspension
in the tank during anaerobic digestion is often such, especially during start-up of
the process, that the solids material tends to settle fast to the floor of the tank
where it can compact and require a large force to disturb it and move it back into
suspension in the tank. Because of this much more energy is required to maintain in
suspension a low concentration of solids than is required with a high concentration
of solids which behaves more like a homogeneous liquid and can be kept mixed by using
relatively inexpensive gas lift pumps which use relatively little power.
[0004] ' In order for gas lift pumps to produce uniform liquid velocities of the order of
15 to 45 cms per second (0.5 to 1.5 feet per second) over the whole floor of the large
tank, very large energy inputs are required to overcome the friction and hydraulic
shock losses of moving these very large volumes.
[0005] An alternative method of preventing settling and compaction of the solids material
as a sludge on the bottom of the tank is to use a rotating scraper arm. Such scraper
arms are usually driven by a central torque tube from outside the tank roof by a reduction
gear drive system. For large tanks the central drive systems for the scraper arm involve
very large torques and are consequently very expensive.
[0006] The difficulty of mixing the anaerobic bacteria in the waste material in the digestion
tank is one of the important factors currently limiting the diameter of the digestion
tanks which, for this reason, are built higher and frequently involve very costly
foundations.
[0007] According to the present invention settling and compaction of sludge on the bottom
of the anaerobic digestion tank is substantially prevented by rotating an arm from
which gas is emitted at a plurality of positions along the length of the arm.
[0008] Further in accordance with the present invention there is provided apparatus for
anaerobic digestion of biodegradable waste material comprising a closed substantially
vertical cylindrical tank having a central axis, a gas delivery duct having an outlet
disposed near to the central axis of the tank and to the base of the tank, and a mixing
assembly located within the tank for rotation about the central axis of the tank,
the mixing assembly comprising a gas-holding member which has an open lower surface
and which defines an annulus concentric with the said central axis, a part of the
annulus defined by the gas-holding member being positioned above the outlet of the
gas delivery duct, and a gas distribution arm extending substantially radially outwardly
from the said gas-holding member towards the periphery of the tank and having exhaust
apertures spaced along the length of the arm for delivering gas from the gas-holding
member at different radial distances along the arm, means supporting the gas-holding
member and the gas distribution arm near the base of the tank, and drive means for
causing the mixing assembly to be rotated about the said central axis.
[0009] The drive means may comprise a rotatable member positioned within the tank and near
the periphery thereof at a similar level in the tank to the mixing assembly, a motor
located outside the tank and linked to the rotatable member by means enabling the
motor to drive the rotatable member, a wheel located concentrically with the axis
of the tank and rigidly connected to the gas-holding member and the gas distribution
arm, and an endless flexible drive member embracing the circumferences of the rotatable
member and the wheel for transmitting rotational movement from the rotatable member
to the wheel.
[0010] Alternatively, the drive means may comprise gas lift draught tubes mounted on the
gas distribution arm for receiving gas from the arm, each gas lift draught tube comprising
a vertical portion connected through a right angle bend to a horizontal portion. The
driving force is provided by a reaction force developed at the right angle bend. Preferably
each gas lift draught tube comprises two horizontal portions connected through right
angle bends to opposite ends of the vertical portion.
[0011] As a further alternative the drive means may comprise gas lift draught tubes mounted
on the gas distribution arm for receiving.gas from the arm and including a vertical
portion incorporating therein a propeller turbine, and means connecting the output
from the said turbine to individual drive wheels which support the gas distribution
arm.
[0012] Advantageously the drive means comprises a rotatable member positioned within the
tank above the level of liquid in the tank, and a motor located outside the tank and
linked to the rotatable member by means enabling the motor to drive the rotatable
member, and the supporting means comprises a torque transmitting cage suspended from
the rotatable member and carrying the mixing assembly.
[0013] The present invention will be further understood from the following detailed description
of three embodiments thereof which is made by way of example with reference to the
accompanying drawings in which:-
Figure 1 is a diagrammatic perspective view of apparatus for anaerobic digestion of
biodegradable waste material including one embodiment of mixing assembly comprising
a gas-holding member and a gas distribution arm,
Figure 2 is a perspective view of an alternative embodiment of mixing assembly including
a gas-holding member and a gas distribution arm, and
Figure 3 is a view similar to Figure lof another apparatus in accordance with the
present invention.
[0014] In the drawings the same or similar parts are designated by like reference numerals.
[0015] Referring to Figure 1 of the accompanying drawings there is shown a closed anaerobic
digestion tank indicated generally by the reference numeral 1, the tank comprising
a lower digester portion 2 and an upper gas storage portion 3. Near the base of the
lower digester portion 2 of the tank 1 there is provided a mixing assembly indicated
generally by the reference numeral 4, which is capable of rotation about a central
vertical member 5 the axis of which is concentric with the central vertical axis of
the tank 1.
[0016] The mixing assembly 4 is supported on supporting means constituted by a horizontal
wheel 6 which has pivotably mounted beneath its circumference a plurality of vertical
wheels 7 which contact the base of the tank 1. The mixing assembly 4 essentially comprises
a gas-holding member 8 and a gas distribution arm 9.
[0017] Both the gas holding member 8 and the gas distribution arm 9 are constituted by inverted
channel members. The gas-holding member 8 is arranged in a circle and thus defines
an annulus concentric with the axis of the central vertical member 5 and the central
vertical axis of the tank 1. The gas distribution arm 9 which is connected to and
extends radially from the outer circumference of the gas-holding member 8 includes
a plurality of cut-away portions 10 in the lower periphery of its inverted channel
member. The cut-away portions 10 may be U-shaped or V-shaped.
[0018] Advantageously, the gas is supplied to the gas-holding member 8 from the upper gas
storage portion 3 of the tank 1 by a pump 11 which pumps gas through a gas delivery
duct 12 having an outlet 13 placed vertically beneath a part of the annulus defined
by the gas-holding member 8. Accordingly,
[0019] 
portions 10 throughout the rotation of the mixing assembly 4.
[0020] An alternative embodiment of the mixing assembly including the gas-holding member
8 and the gas distribution arm 9 is shown in Figure 2 of the accompanying drawings.
This includes a horizontally extending arm 21 which is vertically above and substantially
parallel to the gas distribution arm 9. The arm 21 is mounted to the central vertical
member 5 by bearings 22 similar to the bearings 18 supporting the gas-holding member
8. The gas distribution arm 9 and the arm 21 together support gas lift draught tubes
23 which include a vertical portion 23a which is connected through a right angle bend
23b to a horizontal portion 23c. In the arrangements shown in Figure 2 the gas lift
draught tubes 23 also include a lower horizontal portion 23d which terminates in a
broader member 23e which has an aperture (not shown) in its lower surface.
[0021] In operation the gas distribution member 9 of Figure 2 delivers gas into the gas
lift tubes 23 which, being immersed in the lower digester portion of the tank, contain
liquids and solids material being subjected to anaerobic digestion. The emission of
the gas bubbles at the foot of the vertical portions 23a of the gas lift tubes 23
causes liquid-within these vertical portions to be raised vertically with a simultaneous
drawing in of further liquid or sludge through the apertures in the lower surfaces
of the broader members 23e and discharge of liquid from the upper horizontal portion
23c.
[0022] There is thus a continuous movement of liquid through the gas lift draught tubes
23 from the horizontal portion 23d into the vertical portion 23a and thence into the
horizontal portion 23c from which the liquid is expelled. The changes of direction
in the movement of the liquid at the right angle bends at the opposite ends of the
vertical portions 23a produce the reaction forces which cause the mixing assembly
of Figure 2 to turn on the bearings 18 and 22 about the central member 5 and so move
through the liquid and solids near the base of the lower digester portion to effect
mixing.
[0023] A gas lift draught tube 23 positioned on the mixing assembly at the end of the gas
distribution arm 9 remote from the central member 5 is advantageously of a greater
cross- sectional area than the gas lift draught tube positioned nearer to the central
member 5. Any desired number of gas lift draught tubes 23 can be provided in association
with the gas distribution arm 9.
[0024] The mixing assembly of Figure 2 may also be modified by the provision of one or more
cut-away portions 10 in the gas distribution arm 9, which cut-away portions are not
associated with gas lift draught tubes 23.
[0025] The mixing assembly of Figure 2 can be mounted on vertical wheels so that the weight
of the assembly including the gas distribution arm 9, the horizontal arm 21 and the
gas lift draught tubes 23 is supported by the wheels rather than by the central member
5. Alternatively, the mixing assembly of ligure 2 can be supported on bearings located
above the level of liquid under treatment, for example the bearings may be out side
and above the tank.
[0026] 
[0027] The gas lift draught tubes 23 could be constructed with only one of the horizontal
portions, that is to say either the horizontal portions 23c or the horizontal portion
23d, the reaction force at the single right angle bend in a plurality of gas lift
draught tubes mounted on the gas distribution arm 9 being sufficient to move the mixing
assembly.
[0028] As a further alternative, the gas lift draught tubes 23 of Figure 2 could be provided
without any horizontal portions 23c and 23d but with propeller turbines located in
the vertical portions 23a with drive means connecting the output from the turbines
to vertical wheels supporting the mixing assembly immediately beneath respective gas
lift draught tubes 23.
[0029] Referring now to Figure 3, theøeis shown another embodiment of apparatus according
to the present invention, and incorporating a mixing assembly comprising a gas-holding
member 8 and a pair of gas distribution arms 9 similar to the gas distribution arm
9 of Figure 1.
[0030] However, in the apparatus of Figure 3, the central vertical member 5, which is a
supporting column of concrete or steel, carries a disc 30 at a level which is below
the roof of the tank 1 but above the level of liquid in the tank 1. The disc 30, which
is made of the same material as the vertical member 5 is thus located in the upper
gas storage portion 3 of the tank 1. The disc 30 constitutes a platform within the
tank 1 on which an internally toothed ring gear wheel 31 is mounted on bearings. From
the outer periphery 32 of the gear wheel 31 there is suspended, by means of bolts
or other suitable fixing means 33, a torque transmission cage 34.
[0031] The torque transmission cage 34 carries the gas-holding member 8 and the gas distribution
arms 9 which are connected to the cage 34 by supporting wires 35. The gas distribution
arms 9 are therefor connected directly to the gas-holding member 8 for supply of gas,
and also to the cage 34 by the supporting wires 35.
[0032] The internally toothed ring gear wheel 31 is driven by a gear 38 from a standard
reduction gear motor 36 through any suitable gear means 37. The drive for the gear
wheel 31 may, conveniently, include a worm and wheel reducer gear.
[0033] The gas supply to the gas-holding member 8 of Figure 3 is essentially similar to
that described with reference to Figure 1. However, the gas-holding member 8 is provided
with four steady bearings 39 of lignum-vitae, or similar material, for maintaining
the suspended mixing assembly in position during rotation.
[0034] The embodiment of Figure 3 has the advantage over that of Figure 1 of accessibility
of the supporting means for maintenance and repair. Alternatively to the construction
illustrated the supporting means could be-external to the tank 1, for example on the
roof of the tank 1.
[0035] The arrangements described in accordance with the present invention are much less
costly to install than conventional mixing means, and the embodiments of Figures 1
and 3 have an additional advantage that much less power is required to provide both
scraping of the bottom of the lower digester portion 2 of the tank 1 and mixing of
the materials therein than in the conventional scraper and mixing assemblies.
[0036] With a conventional scraper arm rotating about a central axis the torque required
to rotate the scraper is large because it is moving through a thick sludge and propelling
the thick sludge towards the centre of the tank. With the gas-emitting scraper arm
of the present invention, the gas emitted from the arm 9 clears the sludge away from
the immediate vicinity of the scraper arm 9 with the result that, when the scraper
arm 9 is moving slowly (for example one revolution per hour), only a relatively small
torque is required to move the scraper arm through the liquid near the base of the
tank. It is because of this clearance of the thick sludge from the region of the scraper
arm that it is possible to drive the scraper arm with standard driving mechanisms
which are lighter and therefore less expensive than those used to drive prior art
rotating scraping arms. This applies not only to the drive mechanism which is located
outside the tank, but also applies to the torque transmission cage 34 or other torque
tube which transmits the torque to the gas-emitting scraper arm at the base of the
tank.
1. Apparatus for anaerobic digestion of biodegradable waste material comprising a
closed substantially vertical cylindrical tank (1) having a central axis, and a gas
delivery duct (12) having an outlet (13) disposed near to the base of the tank (1),
characterised in that there is located within the tank (1) for rotation about the
central axis of the tank (1), a mixing assembly (4) comprising an annular gas-holding
member (8) which has an open lower surface defining an annulus concentric with the
said central axis, a part of the annulus defined by the gas-holding member (8) being
positioned above the outlet (13) of the gas delivery duct (12), and a gas distribution
arm (9) extending substantially radially outwardly from the said gas-holding member
towards the periphery of the tank and having exhaust apertures(10) spaced along the
length of the arm (9) for delivering gas from the gas-holding member (8) at different
radial distances along the arm (9).
2. Apparatus according to Claim 1 in which the weight of the mixing assembly (4) is
supported on bearings above the level of liquid under treatment in the tank (1).
3. Apparatus according to Claim 1 or Claim 2 in which the

(4) is rotated about the central axis by drive means comprising gas lift draught
tubes (23) mounted on the gas distribution arm (9) for receiving gas from the arm
(9), each gas lift draught tube (23) comprising a vertical portion (23a) ccnnected
through a right angle bend (23b) to a horizontal portion (23c).
4. Apparatus according to Claim 3 in which each gas lift draught tube (23) comprises
two horizontal portions (23c, 23d) connected through right angle bends to opposite
ends of the vertical portion (23a).
:. Apparatus according to Claim 1 or Claim 2 in which the drive means comprises gas
lift draught tubes (23) mounted o, the gas distribution arm (9) for receiving gas
from the arm (9) and including a vertical portion incorporating therein a propeller
turbine, and means connecting the output from the said turbine to individual drive
wheels which support the gas distribution arm.
6. Apparatus according to Claim 1 or Claim 2 in which the mixing assembly is supported
by bearings on a platform (30) located within the tank (1) above the level of liquid
in the tank (1).
7. Apparatus according to Claim 2 in which the mixing assembly is driven by means
comprising a rotatable member (31) positioned within the tank (1) above the level
of liquid in the tank (1), a motor (36) located outside the tank (1) and linked to
the rotatable member (31) by means enabling the motor (36) to drive the rotatable
member (31), and a torque transmitting cage (34) suspended from the rotatable member
(31) and carrying the mixing assembly.