Stacked modules for anaerobic digestion

A compact vertical array of concentrically stacked and interfaced anaerobic digestion modules which comprise an anaerobic digestion sub-system. Each individual digestion module is an open-ended reaction vessel having an external flange on each end. Each pair of adjacent reaction vessels has a suitably sloped plate positioned at their interface and fastened to the bottom flange of the upper vessel and the top flange of the lower vessel. In this manner, each plate creates two individual digestion modules by serving as a bottom for the reaction vessel immediately above it and as a top for the reaction vessel immediately below it. Suitable openings are provided between adjacent digestion modules for transfer of digesting slurry from the module above to the module below. Each such opening is provided with a suitable valve for closing the opening during normal anaerobic digestion operations.

BACKGROUND OF THE INVENTION 
This invention relates to new and improved anaerobic digestion modules 
useful as components of a system for production of methane gas by 
anaerobic digestion of feed materials containing suitable organic 
materials. In particular, this invention relates to vertically stacked 
anaerobic digestion modules which comprise an anaerobic digestion 
sub-system suitable for use with the system disclosed in the closely 
related U.S. Patent Application Ser. No. 56,545, filed by David J. 
Fischer, et al. on July 11, 1979, for a System and Process for Anaerobic 
Digestion, which is hereby incorporated by reference. 
The system for production of methane gas by anaerobic digestion disclosed 
and claimed in Application Ser. No. 56,545 is comprised of a feed material 
pre-treatment sub-system, a slurry preparation sub-system, an anaerobic 
digestion sub-system, a gas collection sub-system, a gas storage 
sub-system, a sludge separation sub-system and a particle separation 
sub-system. The anaerobic digestion sub-system disclosed in that 
application is comprised of a plurality of individual digestion modules 
arranged to accommodate a multi-stage anaerobic digestion process. In the 
preferred embodiment disclosed in that application, each individual 
digestion module is a cylindrically shaped reaction vessel, such as a 
conventional fifty-five gallon drum, positioned on its side with its 
slurry inlet port at a slightly lower elevation than the slurry outlet 
port of the preceding digestion module. In addition, each individual 
digestion module is positioned on an incline with its slurry outlet port 
slightly lower than its slurry inlet port. 
The structural arrangement of digestion modules disclosed in Application 
Ser. No. 56,545 allows gravity transfer of the digesting slurry from one 
digestion module to the next successive digestion module. It can be 
readily appreciated that such a structural arrangement of digestion 
modules does not require pumps for transferring the digesting slurry. 
Also, with such a structural arrangement, no mechanical mixers are 
necessary for mixing the digesting slurry in each digestion module. 
Sufficient mixing occurs when the digesting slurry is transferred from one 
digestion module to the next successive digestion module. 
While an anaerobic digestion sub-system utilizing the structural 
arrangement of digestion modules disclosed in the Application Ser. No. 
56,545 has these and other advantages, an extensive structural frame is 
required for its vertical support. And, it is desirable to include an 
operator walkway as a component of the structural frame. Since the number 
of individual digestion modules in the anaerobic digestion sub-system 
disclosed in that application varies depending upon the particular feed 
material to be digested, a standard design for the structural frame for 
those modules is not possible. Furthermore, a structural frame of the type 
which is required to support those modules does not lend itself to 
fabrication in a manufacturing plant prior to field installation of the 
modules. For these reasons, expensive custom design and field fabrication 
will be required for most installations of anaerobic digestion sub-systems 
using the structural arrangement of digestion modules disclosed in 
Application Ser. No. 56,545. 
While an anaerobic digestion sub-system using the structural arrangement of 
digestion modules disclosed in Application Ser. No. 56,545 does not have 
to be installed inside a building, it is better practice to do so. First, 
it is difficult to maintain and control the temperature in the digestion 
modules if the sub-system is installed outside. Next, such a structural 
arrangement of digestion modules exposes a substantial surface area to the 
elements. Unless either expensive stainless steel construction or inside 
installation is used, frequent cleaning and maintenance of the modules and 
their structural frame will be required. 
It will be readily appreciated that the structural arrangement of digestion 
modules disclosed in Application Ser. No. 56,545 requires extensive piping 
for interconnection of the individual modules. Not only are expensive 
piping materials required, but also expensive field welding operations are 
required to fabricate the anaerobic digestion sub-system. In many 
countries where anaerobic digestion systems would be desired, it is often 
difficult to employ qualified field construction welders. 
It is desirable to have a plurality of anaerobic digestion modules which 
comprise an anaerobic digestion sub-system having all of the advantages of 
the structural arrangement of digestion modules disclosed in Application 
Ser. No. 56,545 without the foregoing and other disadvantages inherent in 
such a structural arrangement. Preferably, the operating characteristics 
of the resulting sub-system would be identical or superior to those of the 
sub-system disclosed in that application. It is also desirable to have a 
modular anaerobic digestion sub-system capable of being fabricated in a 
manufacturing plant and transported by barge, boat, rail or truck shipment 
to the site selected for installation of the anaerobic digestion system 
using the sub-system. And, of course, it is desirable for such a 
sub-system to be comprised of standard components which enable the system 
designer to easily vary the number of digestion modules in the sub-system 
without major perturbations in the normal operations of the manufacturing 
plant which fabricates such sub-systems. 
SUMMARY OF THE INVENTION 
This invention provides new and improved anaerobic digestion modules which 
overcome the disadvantages inherent in the structural arrangement of 
digestion modules disclosed in U.S. Patent Application Ser. No. 56,545, 
filed by David J. Fischer, et al. on July 11, 1979, for a System and 
Process for Anaerobic Digestion. The present invention overcomes those 
disadvantages without sacrificing the advantages of the structural 
arrangement of digestion modules disclosed in that application. With the 
present invention, standard components can be used to fabricate an 
anaerobic digestion sub-system having a plurality of digestion modules. 
Fabrication can be accomplished in a manufacturing plant and the completed 
sub-system can be transported by barge, boat, rail or truck to the site 
selected for the installation of the anaerobic digestion system using the 
sub-system. 
The present invention provides a plurality of vertically stacked and 
directly interconnected anaerobic digestion modules. Each individual 
digestion module is an open-ended reaction vessel having an external 
flange on each end. The individual reaction vessels are concentrically 
stacked and interfaced in a compact vertical array with a suitably sloped 
plate positioned between adjacent reaction vessels. Thus, each plate 
functions as a bottom for the digestion module which is located 
immediately above it and a top for the digestion module which is located 
immediately below it. 
Suitable openings are provided between adjacent digestion modules to 
function as a slurry outlet port for the upper digestion module and a 
slurry inlet port for the lower digestion module. During normal anaerobic 
digestion operations, a suitable valve closes each of these openings. When 
it is desired to transfer digesting slurry from one digestion module to 
the next successive digestion module, the valve closing the opening 
between the upper module containing the slurry to be transferred and the 
lower module to which transfer is desired is opened to allow the digesting 
slurry to flow from the upper module to the lower module. During such 
transfer operations, significant mixing of the digesting slurry takes 
place and solids which have settled during the retention period in the 
upper module are resuspended in the lower module. Various conventional 
valves, such as a slide valve or a ball valve, are suitable for this 
purpose. 
A coiled tube for circulation of steam or hot water is positioned in the 
interior of each digestion module to provide means for heating the 
digesting slurry in the module. The steam or hot water enters the coiled 
tube from a high temperature steam or hot water input manifold and exits 
the coiled tube into a low temperature steam or water discharge manifold. 
Each coiled tube has a suitable valve for controlling the flow of steam or 
hot water through the tube and maintaining the desired temperature. With 
the structural arrangement of the present invention, heat losses from the 
individual digestion modules are minimized. Thus, the heat necessary to 
maintain the desired temperature in each module is substantially less than 
would be necessary with other structural arrangements of digestion 
modules. And, of course, with this structural arrangement of digestion 
modules, substantial savings in heat insulation costs are realized. 
A gas sampling and discharge port having a three-way valve is provided near 
the top of each digestion module. Each of these ports is connected to a 
gas collection manifold. The system operator can use the three-way valve 
to either sample the gas production in the module or continuously transfer 
such gas to the gas collection manifold. If desired, a plurality of valves 
can be provided in the gas collection manifold to divert gases produced in 
individual modules or groups of modules to appropriate locations for 
storage or subsequent processing, or both, without mixing the gases 
produced in the individual modules or groups of modules. 
A sample port having a two-way valve is provided near the bottom of each 
digestion module to enable the system operator to sample the digesting 
slurry in the module. The sample withdrawn from each such port can be 
analyzed to provide chemical, physical, microbiological and enzyme 
distribution data necessary for control of the anaerobic digestion 
processes in each individual module. In addition, the sample port can be 
used to add appropriate materials to adjust the pH of the digesting slurry 
in the module and to add microbes, enzymes, catalysts and other materials 
suitable for enhancing methane gas production. 
A suitable top cover is provided for the top digestion module in the 
vertical array. The top cover has a slurry inlet port to transfer freshly 
prepared slurry from a suitable slurry preparation sub-system to the top 
or initial digestion module. A suitable bottom cover is provided for the 
bottom or final digestion module in the vertical array. The bottom cover 
has a slurry outlet port to transfer digested slurry to a suitable sludge 
separation sub-system. The vertical array of digestion modules is 
supported by a plurality of support posts. 
These and many other advantages, features and objects of the present 
invention will be apparent from the following brief description of 
drawings, description of the preferred embodiment and claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The vertically stacked anaerobic digestion modules of the present invention 
are illustrated in FIGS. 1, 2 and 3. Ten digestion modules are illustrated 
in FIG. 1, but the total number of digestion modules actually used for 
each individual anaerobic digestion sub-system will vary depending upon 
both the nature of the feed materials to be digested in the sub-system and 
the operating temperature of each individual module in the sub-system. 
Referring to FIG. 1, a plurality of digestion modules 10 are concentrically 
stacked and interfaced in a compact vertical array. Each module 10 is an 
open-ended cylindrically shaped reaction vessel having an upper flange 12 
and a lower flange 14. A conical-shaped plate 16 is positioned between 
adjacent digestion modules 10. With this arrangement, each plate 16 
functions as a bottom for the digestion module 10 immediately above the 
plate and as a top for the digestion module 10 immediately below the 
plate. Conventional fastening means, such as bolts and nuts, are used to 
fasten the opposed flanges 12 and 14 of adjacent digestion modules 10 and 
the plate 16 is positioned there between. 
Each conical-shaped plate 16 has an opening 18 through its center. The 
opening 18 functions as a slurry outlet port for the digestion module 10 
immediately above the plate 16 and a slurry inlet port for the digestion 
module 10 immediately below the plate 16. A conventional slide valve 20 is 
provided to close each opening 18 during normal anaerobic digestion 
operations. The slide valve 20 is operatively positioned below the opening 
18 with a pair of support brackets 22. 
Referring to FIG. 2, the valve 20 has an operating rod 24 connected thereto 
which passes through a conventional seal 26 positioned in a cylindrical 
opening through the wall 28 of the digestion module 10. It will be readily 
appreciated that this provides means for opening and closing the valve 20 
from the exterior of the digestion module 10. The valve 20 is maintained 
in an open mode when transferring digesting slurry from an upper digestion 
module 10 to the digestion module 10 immediately below. Accordingly, all 
transfers of digesting slurry are efficiently accomplished by gravity 
flow. And, the digesting slurry is mixed by the transfer thereby 
resuspending solids which settled in the upper module. 
While the digestion modules 10 which have been described and illustrated 
have circular cross sections, other geometric shapes are suitable for the 
modules 10. And, of course, the plates 16 need not be conical shaped. With 
respect to these features, the present invention requires only that each 
plate 16 have a downward slope which is sufficient in magnitude for 
efficient flow of digesting slurry during transfers of slurry from a 
digestion module 10 to the module 10 immediately below. For example, each 
plate 16 could be in the shape of a distorted cone having its low point 
located away from the center of the digestion module 10. Or, each plate 16 
could be a flat plate positioned on an incline to the horizontal. 
With many of these alternate embodiments, the opening 18 in the plates 16 
can be located nearer the outer walls of the digestion modules 10. In such 
cases, the operating rods 24 could be substantially shortened. And, of 
course, if the plates 16 are flat plates inclined to the horizontal, it 
may be desirable to locate the openings 18 through the side walls of the 
digestion modules 10 so that the valves 20 can be located outside of the 
digestion modules 10. This would, of course, greatly simplify valve 
maintenance procedures. It will be readily appreciated that many types of 
conventional valves, such as ball valves, can be substituted for the slide 
valves 20 which have been described and illustrated. 
To heat the digesting slurry in each digestion module 10, a coiled tube 30 
is positioned in the interior of the module. Either steam or hot water can 
be circulated through each coiled tube 30 in appropriate quantities to 
provide the necessary heat and to maintain the desired temperature. The 
steam or hot water enters the coiled tube 30 from a high temperature steam 
or water input manifold 32 and exits the coiled tube 30 into a low 
temperature steam or water discharge manifold 34. A suitable valve 36 is 
provided in the input side of the coiled tube 30 to control the flow of 
steam or hot water through the tube. It will be readily appreciated that 
other conventional heating means could be substituted for the coiled tubes 
30. Suitable insulation materials are installed around the exterior 
surfaces of each digestion module 10. 
Each digestion module 10 has a gas sampling and discharge port 38 near its 
top. A three-way valve 40 is provided in each gas sampling and discharge 
port 38. And, each gas sampling and discharge port 38 is connected to a 
gas collection manifold 42. If the three-way valve 40 is positioned in the 
appropriate mode, the system operator can sample the gas production in the 
digestion module 10. During normal anaerobic digestion operations, the 
three-way valve 40 will be positioned in an appropriate mode for 
continuous transfer of the gas produced in the module 10 to the gas 
collection manifold 42. 
A plurality of conventional valves can be provided in the gas collection 
manifold 42 to divert the gases produced in the individual digestion 
modules 10, or groups of modules, for storage or subsequent processing, or 
both, without mixing the gases produced in the individual digestion 
modules 10, or groups of modules. With this arrangement, less separative 
work is required for separation of gases produced in the modules into 
methane and carbon dioxide components 
A sample port 44 having a two-way valve 46 is positioned near the bottom of 
each digestion module 10. These sample ports 44 provide means for 
withdrawal of samples of digesting slurry by the system operator for 
subsequent chemical, physical, microbiological and enzyme distribution 
analysis. Such analyses are necessary to obtain data used to control the 
anaerobic digestion processes in each digestion module 10. Furthermore, if 
desired, the system operator can use the sample port 44 either to add 
appropriate materials to adjust the pH of the digesting slurry in the 
digestion module 10 or to add microbes, enzymes, catalysts and other 
materials suitable for enhancing methane gas production. 
A top cover 48 is provided for the top digestion module 10' in the vertical 
array. Conventional means, such as bolts and nuts, are used for fastening 
the top cover 48 to the top flange 12 of the top module 10'. The top cover 
48 has a slurry inlet port 50 for transfer of freshly prepared slurry from 
a suitable slurry preparation sub-system to the top digestion module 10'. 
A bottom cover 52 is provided for the bottom digestion module 10" in the 
vertical array. Conventional means, such as bolts and nuts, are used for 
fastening the bottom cover 52 to the bottom flange 14 of the bottom module 
10". The bottom cover 52 has a slurry outlet port 54 for transfer of 
digested slurry from the bottom digestion module 10" to a suitable sludge 
separation sub-system. If a suitable valve is not included in the inlet 
port of the sludge separation sub-system, valve 56 is provided in the 
slurry outlet port 54. Conventional means, such as bolts and nuts, are 
used to attach the bottom cover 52 to the top of a plurality of support 
posts 58 resting on a suitable support footing 60. While not absolutely 
required, the support posts 58 can be fastened to the support footing 60 
by conventional means, such as screws, if desired. In this manner, the 
vertical array of digestion modules 10 is supported by a single, 
inexpensive support stand. While a plurality of support posts have been 
illustrated, it will be readily appreciated that many other inexpensive 
support structures are suitable for this purpose. 
The vertical array of anaerobic digestion modules 10 which has been 
described is suitable for use as an anaerobic digestion sub-system of the 
type disclosed in U.S. Patent Application Ser. No. 56,545, filed by David 
J. Fischer, et al. on July 11, 1979, for a System and Process for 
Anaerobic Digestion. Operation of such a sub-system as a component of an 
anaerobic digestion system is fully described in that application 
While the present invention has been disclosed in connection with the 
preferred embodiment thereof, it should be understood that there may be 
other embodiments which fall within the spirit and scope of the invention 
as defined by the following claims.