WO1991019682A1

WO1991019682A1 - Improved method and apparatus for processing biodegradable organic material

Info

Publication number: WO1991019682A1
Application number: PCT/US1991/004422
Authority: WO
Inventors: Charles Currie; George M. Wilkins, Jr.; Ralph A. +Di Messing
Original assignee: Biodynamic Systems, Inc.
Priority date: 1990-06-21
Filing date: 1991-06-21
Publication date: 1991-12-26

Abstract

An improved method and apparatus for treating a biodegradable organic material in an aqueous medium to produce methane gas is disclosed. The method involves adjusting the pH and temperature of the material, flowing the aqueous medium under pressure through a hydrolytic-redox, immobilized microbe bioreactor (20) to form a reaction product, adjusting pH, pressure and temperature of said reaction product and then continuing the flow of the reaction product through an anaerobic, immobilized microbe bioreactor (30) whereby methane gas is evolved. The aqueous medium flow direction is reversible and may be recycled. This method also provides for the intermittent removal of carbon dioxide produced in the first bioreactor (20), where its presence is inhibitory, storage under pressure, and delivering the carbon dioxide at a constant, controllable rate to the second bioreactor (30), where it provides additional carbon for methane production and therefore increases the volume of methane gas produced. Any excess carbon dioxide is wasted to the atmosphere.

Description

IMPROVED METHOD AND APPARATUS FOR

PROCESSING BIODEGRADABLE ORGANIC MATERIAL

CROSS-REFERENCE TO RELATED APPLICATION

The present invention is a continuation-in-part of Application Ser. No. 07/418,269, filed October 6, 1989, currently pending, which is a continuation of Application Ser. No. 06/543,338, filed October 19, 1983, now abandoned. BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to organic waste processing and more particularly to an improved method and apparatus for processing biodegradable organic waste in an aqueous medium. It also relates to an improved method, using microbes immobilized on a porous organic support, for processing such organic biodegradable waste.

2. Description of the Prior Art

Numerous methods have been used for organic waste disposal and pollution control. Disposing of organic waste, as for example, by burial, land-fill, dumping at sea, and the like, have wreaked havoc with the environment and cleaning up the resultant toxic contamination has been very expensive.

other waste disposal methods include biological aerobic or anaerobic fermentation, thermophilic digestion, destructive distillation and incineration. In some instances, organic waste is converted into a source of energy and even to useful products. Thus W. J. Jewell et al. disclosed the use of anaerobic fermentation with the production of fuel in "Methane Generation from Agricultural Wastes: Review of Concept and Future Applications," Paper No. NA74-107, presented at the 1974 Northeast Regional Meeting of the American Society of Agricultural Engineers, West Virginia University, Morgantown, West Virginia, August 18-21, 1974. One of the most promising of the processes mentioned hereinabove is biological anaerobic fermentation, which has received considerable attention in recent years and is discussed by R. A. Messing in Biotechnology and Bioengineering, XXIV, 1115-1123 (1982) and in Genetic Engineering News. 2(3), 8-9 (May/June 1982). Current interest in biological anaerobic fermentation appears to be due, at least in part, to the development of the anaerobic filter. This is discussed by J. C. Young et al., J. Water Poll. Control Fed., 4 1 , R160 (1969); P. L. McCarty, "Anaerobic Processes," a paper presented at the Birmingham Short Course on Design Aspects of Biological Treatment, International Association of Water Pollution Research, Birmingham, England, Sept. 18, 1974; and J. C. Jennett et al., J. Water Poll. Control Fed., 47, 104 (1975).

The anaerobic filter is essentially a vertical column or tank usually containing rocks and having a film of microbes on the outer surface of the rocks. In the anaerobic filter, however, the waste is fed from the bottom of the column through the filter. Thus, the flow of waste is in an upward direction through the bed of rocks so that the bed is completely submerged. Anaerobic microorganisms accumulate in the void spaces between the rocks and provide a large, active biological mass. J. C. Young et al., supra, suggests that the effluent is essentially free of biological solids.

Current "state-of-the-art" single stage anaerobic reactors used in full scale waste treatment applications (similar to the anaerobic filter previously referenced) produce a biogas containing about 60 to 65% methane, 35 to 40% carbon dioxide and small amounts of other gases including hydrogen sulfide. Such reactors generally remove about 70% of the polluting load as described by a reduction in Chemical Oxygen Demand (COD). The effluent from these reactors contains a sludge which consists of unreacted waste solids and microbial cells.

A carbon balance analysis suggests that approximately one-half of the reacted carbon in an organic waste will be required for cell synthesis in the biological reactions of this process. If all of the carbon is reacted in some manner, the portion not required for cell synthesis is converted to carbon dioxide (which is a waste by-product of the acid forming bacteria), or to methane (which is a waste by-product of the methane forming bacteria).

Applying this carbon analysis to the current "state-of-the-art" reactors (similar to the anaerobic filter) reveals that between 15 and 25 percent of the carbon is converted to biogas. It follows that the same amount of carbon is used for new cell synthesis. Therefore, the remaining 50 to 70 percent of the carbon entering the reactor leaves with the aqueous medium as un-reacted sludge.

Further improvements in the use of immobilized microbes in anaerobic fermentation were taught by R. A. Messing in U.S. Patent No. 4,321,141 and in Application Ser. No. 07/418,269, filed October 6, 1989.

Messing's U.S. Patent No. 4,321,141 ("the '141 patent") discloses a method of processing biodegradable waste in an aqueous medium by serially passing aqueous medium containing organic waste under pressure through a first, hydrolytic-redox bioreactor and then through a second, anaerobic bioreactor. Each bioreactor contains microbes immobilized on an inorganic support. Both the first and second reactors of the processing method contain a porous inorganic support which is suitable for accumulation of a biomass. Carbon dioxide inhibits the metabolism and reproduction of the acid-forming microbes which are present in the first reactor. The aqueous medium is fed from the bottom of the first bioreactor and is forced under pressure to flow in an upward direction and the carbon dioxide evolved naturally bubbles in the same direction. Nonetheless, using the apparatus as taught by the '141 patent does not permit removal of any gas including the excess carbon dioxide from the first bioreactor.

Messing's Application Ser. No. 07/418,269 ("Messing application"), of which this is a continuation-in-part application, improved upon the '141 patent by providing a means for removing the carbon dioxide gas evolved in the first bioreactor where it is inhibitory and separately delivering the collected gas to the second bioreactor where it can serve as a carbon source for the production of methane (CH₄) and this technology was a major step forward in immobilized microbe reactors producing methane. However, uncontrolled introduction of carbon dioxide into the second bioreactor can produce adverse results, including fluctuation in the rate at which methane gas is produced and deviations in the percent methane present in the product gas.

Messing's Application Ser. No. 07/418,269 also provides for recirculation of the aqueous medium in the first reactor stage, but not in the second reactor stage. Recirculation provides a more uniform distribution and dispersal of components and characteristics of the aqueous waste medium.

Messing's reactor as described in Application Ser. No. 07/418,269 attempted to provide improved conditions in the individual stages for the respective acid-forming and methane forming microbes to flourish, for the purpose of improving yield from the biological reactions, by removing the CO. from the first stage and forwarding it to the second stage. Further discovery and methods were required to refine the Messing Methods of U.S. Patent No. 4,321,141 and U.S. Patent Application 07/418,289 to approach the theoretical yields depicted in the following table: PERFORMANCE THEORETICAL CURRENT STATE TWO STAGE METHODS CRITERIA RESULT THE ART RESULT OF THE PRESENT

INVENTION

COD Reduction 100 70 90

Gas Produced per

Kg COD Removed 1.5 cu m 0.6 cu m 1.2 cu m

Percent Methane

in Off-GAS 100 65 90+

Energy Produced 55,600 Btu 12, ,700 Btu 44,500 Btu per or or or kg COD Removed 14,000 Kg-Cal 3,,200 Kg-Cal 11,200 Kg-Cal

Estimated Composition

of Solids in Sludge

1. Ash 5% 5% 5% 2. Unreacted and

carbon compounds 0 60 10%

3. Microbial Cells 95% 35% 85%

The discovery and teachings of the invention have established that nearly 90 percent carbon conversion is possible and practical. Further, the methods of the teachings of the invention converts most of the carbon dioxide produced in stage one to methane as it is metered into stage two.

SUMMARY OF THE INVENTION

The present invention provides an improved method where the aqueous medium is initially passed through a hydrolytic-redox bioreactor, which contains a microbe population immobilized on a porous inorganic support, capable of hydrolyzing and oxidizing the organic waste material. The effluent from the first bioreactor is then passed through a second, anaerobic bioreactor which contains a second microbe population, also immobilized on a porous inorganic support, capable of converting the first reaction products into methane gas.

The method and apparatus of the present invention separates and isolates the two stages of biological reactions involved in anaerobic decomposition by preventing any back mixing of liquid flow between the two stages and preventing any back mixing of gases between the two stages. The biodegradable material is passed through a hydrolytic-redox first sealed bioreactor having a liquid lower portion containing microbes immobilized on a porous support whereby the first reaction product and a gaseous product, including carbon dioxide, are formed, and an upper portion wherein the gaseous product, including carbon dioxide, is collected. The liquid flow is continued by taking the first reaction product from the lower portion of the first bioreactor, and introducing the resultant solution into an anaerobic, immobilized microbe second sealed bioreactor including methanobacter wherein methane gas is formed as a reaction product as the organic material in the first reaction product is consumed. The gas produced in the first reactor is removed intermittently to reduce its inhibitory effect, stored under pressure, and metered continuously at a controlled rate into the second reactor wherein the carbon dioxide in the gas provides an additional source of carbon for conversion to methane. The first bioreactor is maintained at environmental conditions best suited to the acid forming bacterial reactions, and the second bioreactor is maintained at environmental conditions best suited to the methane forming reactions. The product gas, including methane, is collected from the second bioreactor and

the effluent from the second bioreactor is removed.

The present invention teaches how to avoid the problems resulting from uncontrolled delivery of carbon dioxide, and other gases, into the second reactor by providing a means for delivery of carbon dioxide at a continuous, controllable rate into the second reactor. The operating pressure in the first bioreactor stage is lowered to produce a more optimum regime for acid forming bacteria, by reducing the amount of CO₂ in solution.

The gases continuously produced in the first bioreactor are intermittently removed from the upper portion of the first reactor, accumulated under pressure, and delivered continuously at a controllable rate to the second reactor, where the presence of carbon dioxide in solution provides additional carbon for conversion to methane. Removal of gas from stage 1 and metering the gas into stage 2 serves to control the gas pressure and to optimize gas release from solution in stage 1, and provides an important element of isolation of the two stages.

The method and apparatus of the present invention provides for independent control of operating level in each reactor stage. While such controls may not be required in every waste treatment application, they provide a means of varying the proportion of total residence time in each reactor in order to achieve optimum operating conditions. Level adjustment in stage 2 is controlled by an elevated overflow device. Level in stage 1 is controlled by a level sensing device coupled to the feed pump. Flow between stage 1 and 2 is controlled by a pump which provides metering and prevents any back mixing of liquid between the two stages and is a second important element of isolating each reactor stage.

This method further permits the recycling of the organic material and directional flow reversal in each of the bioreactors.

This provides a means to assist gas release and to achieve excess sludge removal. Occasional reversal of the directional flow of the organic material also serves to maintain an even distribution of nutrients within each reactor stage. Use of recycle in both stages improves exposure of the microbes to the nutrients, increasing bacterial production. Recirculation in both stages also improves control of temperature and pH in the individual reactor stages.

Isolation of the two reactor stages, in accordance with the teachings of the present invention, permits the independent control of pH, pressure, volume, flow velocity, recirculation rate, flow direction, gas metering between the stages, retention time, and temperature of the aqueous medium within each of the bioreactors. These controls allow for optimization of breeding conditions for the microbes within the reactors, resulting in increased biomass, and thus provide more complete conversion of the available carbon in the organic waste to methane gas.

Further objects, features and advantages may be found in the following drawings, specifications and claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a block diagram of the apparatus of the invention. FIGURE 2 is a schematic representation of the apparatus of the invention and shows the first and second bioreactors and surrounding apparatus, including the carbon dioxide gas storage and regulation apparatus.

FIGURE 3 is a cross sectional view of the interior of reactors 20 and 30 of stages 1 and 2 respectively.

FIGURE 4 is a plot of COD Applied versus COD Removed, illustrating loading and removal rates over a ten month period.

FIGURE 5 is a plot of time versus percent methane produced in each stage indicating start-up performances and performance after pH adjustment in stage 2.

FIGURE 6 is a probability graph of Percent Values below Right Scale versus Liters Gas per kg COD Removed, showing Stage 1 Controls.

FIGURE 7 is a probability graph of pH versus Percent Methane Produced in the second bioreactor for the purposes of illustrating the improved conversion of waste to methane when pH control is added.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term "biodegradable" means only that a portion of the organic waste to be treated must be capable of being degraded by microorganisms. As a practical matter, at least 50 percent by weight of the organic waste will usually be biodegradable. It may be necessary or desirable, however, to utilize waste having substantially lower levels of biodegradable organic matter in the processing method of the present invention.

Thus, the organic waste or the aqueous medium containing such waste can contain non-biodegradable organic matter and inorganic materials, provided that the organic waste and aqueous medium are essentially free of compounds having significant toxicity toward the microbes present in either reactor, such as feed sources with high amounts of heavy metals.

In general, the nature of the aqueous medium is not critical. In most instances, water will constitute from about 80 to 98 percent by weight of the aqueous medium. Feed strength occurs from the particular process which generates the waste. The normal limit for waste treatment processes are about 60,000 mg/l COD. Some processes have great quantities of high strength wastes, notably fermentations and distillation processes (50,000-90,000 COD). Others have high strength wastes due to mistakes, bad batches, etc., in difficult to control processes. Still other high strength wastes are camouflaged by dilution with uncontaminated water. The present invention works well with relatively high strength wastes (30,000-150,000 COD), and it is projected that the reactor will produce as much energy in the form of high quality methane as it requires in the form of heat for temperature control and work for pumping when the feed strength is about 6,000 mg/l of COD. At higher COD levels, the reactor can be a significant net producer of usable energy products while treating the waste. The effluent from the present invention may be treated by a wide variety of conventional waste treatment methods. One of the possible uses of the present invention is as a pretreatment system for high strength wastes, prior to their discharge to municipal sewer systems.

The waste stream to be treated by the processing method of the present invention can frequently be used without any pretreatment, although it may be necessary to increase the pH of the mixture by the addition of a base, such as potassium carbonate, sodium hydroxide, triethylamine and the like. It may occasionally be desirable or necessary to dilute the waste stream with water and/or to separate out excessive amounts of solids or excessively coarse solids which might interfere with the pumping equipment necessary to move the aqueous medium through the processing apparatus of the present invention. Alternatively, solid or essentially non-aqueous organic waste can be diluted with water as desired.

The term "bioreactor" as used herein is a contraction of

"biochemical reactor" and therefore refers to the fact that the chemical transformations or conversions taking place therein are carried out by living organisms. The term "immobilized microbe bioreactor" is used to identify such living organisms as microbes which are in an immobilized state (as that term is used by those having ordinary skill in the art). "Bioreactor" and "reactor" are used interchangeably.

The two stage immobilized microbe reactor embodying the theory of the present invention separates the anaerobic process into two stages. The two stages of biodegradable wastes are: 1) a hydrolytic-redox stage in which acid-forming bacteria break down the molecules and produce CO₂, and other gases, in the process, 2) a methane-producing stage in which methanobacter convert available carbon to methane. These stages are synergistic in that the reaction products of the acid forming bacteria become the food supply of the methane formers. There is also a degree of antagonism present in that the respiration products of the bacteria become inhibitive as they accumulate, particularly when in solution. The present invention reduces the inhibitory effects by separating and isolating the stages, and increases the synergistic effect by providing the optimum conditions for each type of bacteria in the respective stages. Methane is inhibiting in stage 2, but only the methane in solution. Since methane is much less soluble than CO₂ in water, its inhibitory influence in stage 2 is much lower than the inhibitin influence of CO₂ in the first stage. The presence of CO₂ in the first stage is inhibitory to those bacteria which produce the reaction; thus, its removal enhances the reaction and the production of CO₂. The carbon in the CO₂ provides an additional source of carbon to the methanobacter in the second stage for conversion to methane. If CO₂ is fed to the methanobacter faster than they can utilize it, the excess CO₂ passes out of the reactor with the methane, thus diluting its quality. The isolation of the two stages permits the independent control of conditions for optimum release of gas in each stage and getting the reintroduced gaseous CO₂ into solution in the second stage. The gas removed from the first stage is stored under pressure and metered into the second stage at a continuous, controlled rate. The actual rate at which the gas from the first stage is metered into the methane-producing stage is determined by observation of the percentage of methane in the off gas while increasing flow of CO₂. When methane percentage starts to fall (indicating excess CO₂) the gas feed rate is cut back slightly to assure maximum conversion of CO₂ to methane. The teachings of the present invention show that virtually all of the CO₂ can be converted to methane, and practically none is lost due to overpressure in the receiver tank. The pressure in the receiver tank varies, increasing as the gas is collected from stage 1, and decreasing as the gas is metered into stage 2. The liquid levels in each stage (reactor) and the overall feed rate are controlled independently and establish relative volumes between the stages and thus the retention times in each stage. The temperature, pH and pressure in each stage (reactor) are controlled independently and provide optimization of the environment for each reaction. The use of immobilized microbes on controlled-pore media in both stages results in accelerated bacterial growth and reaction capability while providing greater stability against process upsets and permits quicker recovery from upsets. The use of recycle in both stages improves exposure of microbes to nutrients, increases bacterial reproductivity and reaction, and improves temperature control. The controllable (reversible) flow direction in both stages, for both recycle and feed, provides means of adjusting process flow to achieve or assist in gas release, excess sludge removal and distribution of bacterial mass and nutrients. The isolation of the two stages further permits independent pH control of each reactor. The present invention approaches complete utilization (approx. 90%) of the theoretically available carbon in the organic waste. This carbon is converted to either methane or cells which appear as a readily-separable solids in the effluent stream, and which (with certain feed sources) could provide a source of protein for animal feed.

Referring now to the FIGURES, FIGURE 1 is a block diagram of the apparatus of the invention, showing the relationship between the reactors and the path taken by the liquid effluent (solid lines), including the recirculation loops 3 and 5, and the gas formed (dotted lines), including the compressor 42, CO₂ receiver tank 40, and gas flow regulator/meter 48. The rate controlled pump 55 and the CO₂ system serve as physical barriers to isolate the two stages. The apparatus is detailed schematically in FIGURES 2 and 3 which, when taken together, illustrate an embodiment of the apparatus of the invention.

The biodegradable organic waste (hereinafter "waste") to be processed is introduced to the apparatus of the invention by placement in the feed tank 10, where the waste may be heated to a pre-determined temperature. Adjustment of the pH of the waste may also be accomplished while the waste is in the feed tank 10. The liquid and solid portions of the waste are kept from separating by means of an agitator or stirrer 18, which also acts to maintain an even temperature (and pH, if pH is adjusted here) in the feed tank 10. The waste is pumped from the feed tank 10 to the first reactor 20 by means of a first feed pump 53. The line feeding into the first reactor 20 is equipped with a check valve 57 in order to prevent the waste from backing up into the feed tank 10.

The reactors 20 and 30 include a closed, jacketed and insulated reaction vessel 162 having an interior array of microbial support media 150 held in place by a support grating 156 located in the lower liquid portion 152 of the reactor. Reactors 20 and 30 have liquid connections 160 functionally above and below the support media 150 located in the liquid space 152 of the reactors for use as inlet, outlet and recirculation ports as appropriate, and connections 158 in the gas space 154 for gas removal, pressure and level controls as may be necessary. The reactors contain a porous support 150 which is suitable for the accumulation of a biomass within the interior vessel. There is a connection between the inlet and the outlet lines such that a portion of the effluent is recycled through the reactors. The reactors have an outlet in the gaseous upper portion of the reaction vessel from which product gas is removed.

The waste is pumped into the reactor 20 so that the porous support 150 is completely submerged and there is at least an inch of liquid above the top of the porous support 150. The liquid effluent and the carbon dioxide gas produced by the reactions in the first reactor 20 exit the first reactor 20 by different means. The liquid effluent is pumped out of the first reactor and pumped into the lower portion of the second reactor by means of a second feed pump 55. This pump 55 provides flow control and metering through the system. It also provides an effective isolation of the liquid (aqueous medium) in each reactor stage by preventing any back mixing. Pumps suitable for this application include: positive displacement pumps, piston pumps, diaphragm pumps, gear pumps, screw pumps, progressive cavity pumps, etc..

The carbon dioxide gas produced by the reactions in the first reactor 20 accumulates in the upper portion of the reactor and is collected from the upper portion of the first reactor 20 and transferred by means of gas pump 42 to the carbon dioxide receiver tank 40 where it is stored under pressure until required in the second reactor 30, such that any back mixing of gases between the stages is prevented. The receiver tank 40 is equipped with a pressure gauge 44 and a relief valve 46 in order to prevent a dangerous build-up of pressure in the receiver tank 40 by venting to the atmosphere any excess CO₂ not being utilized in the second stage.

The gas transfer line (the inlet) from the first reactor 20 to the receiver tank 40 is provided with a manometer 50, which monitors the pressure of the gas in the first stage reactor 20 and controls gas pump 42 to maintain a low pressure range in reactor 20. The drip leg 59 having a drain petcock 61 extending from the bottom, is provided in order to catch and remove any liquid which may be accidentally removed with the gas.

From the receiver tank 40, the outlet is connected to the lower portion of the second reactor 30 via a flow regulator/meter 48 such that the gas is delivered at a continuous, controlled rate to the second reactor 30. The carbon dioxide transfer rate is controlled, automatically or manually, and adjustable by the flow regulator/meter 48. The actual CO₂ feed rate is determined by the total gas product of stage 2 and the percentage of CO₂ in the gas product. From the flow regulator/meter 48, the line to the second reactor 30 includes a loop 51 which extends at least twelve inches (thirty cm) above the second stage adjustable height overflow control 63 in order to prevent back flow of liquid into the gas line.

The second reactor 30 is similar in construction to the first reactor 20, including a porous support 150, except that there is an added inlet for the carbon dioxide. The liquid effluent from stage 2 exits though effluent collection system 63. As in the first reactor 20, the gas product, in this case primarily methane gas, accumulates in the upper portion of the reactor 30 and is collected therefrom.

Due to the flammable nature of the product gas, the line from the reactor 30 should be made of metal pipes conforming to safety standards and each section should be equipped with flame arrestors 67. The line from the second reactor 30 is also provided with a drip leg 59 having a drain petcock 61 extending from the bottom, in order to catch and remove any liquid which may be accidentally removed with the gas. From the drip leg, the product gas can be delivered through a gas meter 69 and then to a collector, or used. In either case, the gas line is provided with a pressure/relief regulator 65 which provides for the independent pressure control inside reactor 30. The product gas of stage 2 can be sample from sampling cock 71. The gas can be checked for percentage methane, percentage CO₂ and for the presence of hydrogen sulfide. The relative percentages of the two gases will determine the control of gas flow regulator/meter 48. The product gas of the present invention has not had any detectable amounts of hydrogen sulfide present. The burning of hydrogen sulfide is one of the causes of acid rain, therefore its presence in the product gas would be extremely detrimental to the quality of that product gas.

Heat can be applied to the feed tank 10 and reactors 20 and 30 by a heating system whereby water is heated in heaters 16 and circulated through a jacket 164 surrounding the tank 10 or reactors 20 and 30. As the heating system is a closed one, it is also equipped with an atmospheric expansion tank 12. This reactors 20 and 30 could also use their own, individual heating system. The temperature of the liquid inside the reactors 20 and 30 is monitored by means of a thermometer 130 which can be read from outside.

The reactors 20 and 30 are also equipped with a sight glass 120 on each reactor. For the reactor 20, a level controller 125 is mounted on the sight glass and is operably connected to the first feed pump 53. The level controller 125 is placed at the desired level in order to signal the first feed pump 53 to stop when the liquid has reached the desired level inside reactor 20 and to start when the liquid level falls below the desired level. For reactor 30, the level control is achieved by raising or lowering the physical position of collection system 63 on flexible tubing 205 until the level of the overflow pipe is at a level equal to the pressure setting of regulator 65. Collection system 63 then acts as a gravity overflow device. The controlled gas pressure in the reactor equals the liquid static head pressure of collection system 63. If the pressure is fixed and the collection system 63 is manually raised, the liquid level in the reactor will also rise and if the collection system 63 is lowered, the liquid level will fall. If the elevation of collection system 63 is fixed and the pressure is raised, the liquid level in the reactor will fall and if the pressure is lowered, the liquid level will rise. The reactors 20 and 30 are also equipped with a high level cut-off control 110. These level control systems provide for independent control of fluid level and relative retention time in each of the stages by increasing or decreasing the volume of liquid in each reactor.

The liquid effluent from the reactors 20 and 30 nay be recirculated via recirculation loops 3 which routes the effluent into the waste stream entering the reactors. The use of recycle in both stages improves exposure of microbes to nutrients, increase bacterial reproductivity and reaction, and improves temperature control. Recirculation pumps 4 individually control the recycle rates of each reactor. By increasing or decreasing the recycle rate of each reactor the relative flow velocity is independently controlled. The recycle rates are typically between 10-100 times greater than the over all system flow rate (controlled by pump 55).

The direction of recirculation can be independently reversed in each of the reactors, by valving or equivalent means. The controllable (reversible) flow direction in both stages, for both recycle and feed, provides means of adjusting process flow to achieve or assist in gas release, excess sludge removal, distribution of bacterial mass and nutrients.

The pH level of each reactor can be individually controlled. The pH of each reactor is monitored by pH elements 203 located at the outlets of each reactor. The pH elements 203 are connected to pH controllers 200 by metering acid or base as necessary from tanks 202 into recirculation loops 3 via pump 201. The independent control of pH in each stage allows for the maintenance of each reactor at optimum pH levels.

The two stage immobilized microbe reactor embodying the theory of the present invention was operated on corn waste from a food processor having a Chemical Oxygen Demand (COD) in the range of 12,000 to 15,000 mg/liter. Daily readings of critical performance criteria were taken, and all data have been included in the analysis of results.

Figure 4 is included to show that the reactor of this invention consistently removes about 90 percent of the COD load applied. These figures also show that the loading rates that cause a drop in COD removal efficiency have not been reached, since the removal ratio remains a straight line.

Figure 5 was generated by computer using raw data in an effort to see if there is a relationship between the volume of methane produced in each stage before and after pH control in each stage which was started around November 20. As can be seen, the methane production in stage 1 went down dramatically while the methane production in stage 2 increased bringing the overall production of methane to record highs.

In order to test the significance of this observation, a statistical analysis was done for all data observations of gas volume from stage 1 with pH 5.9 and under, and a second analysis with pH 6.0 and above. The results are shown on Figure 6.

Control Range A refers to the value of pH at 6.0 or higher.

The fact that the points form a fairly straight line indicates that the data are not skewed, and that the average gas production is over 1.2 cu meters for each kg of COD removed. The curve marked Control Range B refers to pH of 5.9 and below, and the variation at the right hand end suggests that a pH of 5.8 may be a better dividing line. Nonetheless, there was a good distribution of points otherwise, and the average production is between 600 and 700 liters for each kg of COD removed. The quantity of 600 liters of biogas per kg COD removed is equal to the performance which has been reported in the literature for the best currently available technologies. Thus, when the first bioreactor was operated at a pH of 5.8, only half of the volume of product gas was produced when operated at a pH of 6.0 or higher. Isolation of the reactor stages in accordance with the teachings of the present invention makes it possible to set the pH of aqueous medium to an optimum value such as 6.0 for corn waste. Isolation and control of stage 1 pH was shown to produce twice as much volume of product gas as the best available technologies.

operation of the two stage immobilized microbe reactor modified in accordance with the teachings of this invention proved that the gas produced in the first stage was primarily carbon dioxide, although small amounts of methane were present, especially during the first few months. This operation also proved that the gas produced in stage two was methane; and further proved that carbon dioxide in solution could be converted to methane through the bacterial reactions taking place.

A review of raw data suggested that pH in stage 2 was critical to methane production, and pH control was added to stage 2. Results were immediate and dramatic, with methane percentages in the gas collected jumping to over 70 percent, and rising steadily to 88 percent in less than one month. Figure 7 shows the result of a statistical analysis similar to the described above, but for the stage 2 data.

Control Range 3 on Fig. 7 represents pH of 7.0 and below. Control range 2 represents pH of 7.0 to 7.2, and Control Range 1 is for pH values of 7.3 and higher. Note that average methane percentages for Control Ranges 2 and 3 are in the 60 to 65 percent range, which is the exact range of methane in the biogas produced by the best available single stage technology of today. Control range 1 has produced average methane percentage in the 75 to 80 percent range, and has produced pipeline quality gas containing 90 to 95 percent methane as indicated in the table of page 3 of this specification. Furthermore, the product gas of the present invention shows no traces of hydrogen sulfide.

As previously indicated, both bioreactors of the present invention method contain a porous support 150 which is suitable for the accumulation of a biomass, such support preferably having controlled pore-size. Use of controlled-pore media in both reactors results in accelerated bacterial growth (and therefore reaction capability), provides greater stability against process "upsets" and, in the event of an "upset" occurring, permits quicker recovery and shortens "on-stream" time requirements.

As a matter of convenience, the porous support in the two bioreactors will be of the same type, although such is not required. The porous support in each bioreactor will preferably be a high surface area, recalcitrant, dimensionally-stable material which is suitable for the accumulation of a high biomass surface within a relatively small volume. More preferably, at least 70 percent of the pores of the support will have diameters at least as large as the smallest major dimension, but less than about five times the largest dimension, of the microbes present in the bioreactor. Most preferably, the average diameter of the pores of the support will be in the range of about 0.8 to 220 u.

As used herein, the expression "high surface area porous support" means a support having a surface area greater than about 0.01 m² per gram of support. In general, surface area is determined by inert gas adsorption or the B.E.T. method; see, e.g. Gregg. S. J. and K. S. W. Sing, "Adsorption, Surface Area, and Porosity," Academic Press, Inc., New York, 1967. Pore diameters, on the other hand, are more readily determined by mercury intrusion porosimetry; see, e.g., winslow, N.M. and J.J. Shapiro, "An Instrument for the Measurement of Pore-Size Distribution by Mercury Penetration," ASTM Bulletin No. 236, February 1959.

The support can be inorganic or organic and in general can be siliceous or nonsiliceous metal oxides which are amorphous or crystalline, or recalcitrant organic polymers with hydrophilic surfaces. Examples of siliceous materials include, among other, glass, silica, halloysite, kaolinite, cordierite, wollastonite, bentonite and the like. Examples of nonsiliceous metal oxides includes, among others, alumina, spinel, apatite, nickel oxide, titania and the like. The inorganic support also can be composed of a mixture of siliceous and nonsiliceous materials, such as alumina-cordierite. Cordierite and clay (i.e. halloysite and/or kaolinite) materials such as those employed in the examples are preferred. Examples of recalcitrant organic polymers are polyethylene, polypropylene, fluorocarbon polymers, each being modified with a hydrophilic surface layer. The organic support produced by Manville Corporation, Celite^(tm), is usable with the present invention.

For a more complete description of the inorganic support, see Application Ser. No. 833,278 filed Sept. 14, 1977, in the names of Ralph A. Messing and Robert A Oppermann, now U.S. Pat. No. 4,153,510.

As already indicated, the porous support in each bioreactor provides a locus for the accumulation of microbes. The porous nature of the support not only permits the accumulation of a relatively high biomass per unit volume of bioreactor but also aids in the retention of the biomass within each bioreactor.

As used herein, the term "microbe" (and derivations thereof) is meant to include any microorganism which degrades organic materials, e.g. utilizes organic materials as nutrients. This terminology, then, also includes microorganisms which utilize as nutrients one or more metabolites of one or more other microorganisms. Thus, the term "microbe," by way of illustration only, includes algae, bacteria, molds, and yeast, with bacteria being most preferred.

In general, the specific nature of the microbes present in each bioreactor is not critical so long as the first contains primarily acid forming bacteria and the second reactor contains primarily methane forming bacteria. It is only necessary that the biomass in each bioreactor be selected to achieve the desired results. Thus, such a biomass can consist of a single microbe species or several species, which species can either be known or unknown (unidentified).

Furthermore, the biomass in each bioreactor need not be strictly aerobic or strictly anaerobic, provided that the primary functions of the two bioreactors are consistent with their designations as hydrolytic-redox and anaerobic bioreactors, respectively. The term "primary function" as used herein means that at least 85 percent of the biomass in each bioreactor functions in accordance with the reactor designation. Stated differently, the demarcation line or zone between a hydrolytic-redox function and an anaerobic function is critical. For high concentrations of carbon compounds, it may be controlled by recycling and removal of carbon dioxide in the hydrolytic-redox stage.

As used herein, the term "hydrolytic-redox" refers to the function of the first bioreactor which is to break down any macromolecules present into smaller units, e.g. monomers and oligomers, by hydrolysis and oxidation-reduction reactions. In doing so, the first bioreactor also serves to deplete the aqueous medium of dissolved oxygen.

It should be apparent, therefore, that the first bioreactor is not an aerobic bioreactor as the term "aerobic" is used in the prior art. The aqueous medium is not aerated at all or even saturated with air or oxygen. As residual oxygen in the medium is depleted, however, at least some oxidation-reduction occurs aerobically. The aqueous stream to be processed is delivered from the hydrolytic-redox stage such that it allows the separation of excess carbon dioxide at the top of the stage, thus removing the inhibitory effect of that gas upon the acid-formers. Some or all of that carbon dioxide is then delivered to the anaerobic stage for reduction to methane.

Examples of microbes which can be employed in the hydrolytic-redox bioreactor include, among others, strict aerobic bacteria such as Pseudomonas fluorescens, Acinetobacter calcoaceticus, and the like; facultative anaerobic bacteria such as Escherichia coli, Bacillus subtilis, Streptococcus faecalis, Staphylococcus aureus,

Salmonella typhimurium, Klebsiella pneumoniae. Enterobacter cloacae, Proteus vulgaris, and the like; anaerobic bacteria such as Clostrjdjum butyricum. Bacteroides frazilis, Fusobacterium necrophorum, Leptotrichia buccalis, Veillonella parvula, Methanobacterium formicicum, Methanococcus mazei, Methanosarcina barkeri, Peptococcus anaerobius, Sarcina ventriculi, and the like; molds such as Trichoderma viride, Aspergillus niger, and the like; and yeasts such as Saccharorayces cerevisiae, Saccharomyces ellipsoideus, and the like. obviously, the hydrolytic-redox bioreactor should not contain either strict aerobes or strict anaerobes only.

Examples of microbes which can be utilized in the anaerobic bioreactor include, among others, facultative anaerobic bacteria, anaerobic bacteria, and yeasts such as those listed above. As already pointed out, the microbes employed in each bioreactor are selected on the basis of the results desired. If a particular product is not required, the choice of microbes can be made on the basis of waste conversion efficiency, operating parameters such as temperature, flow rate, and the like, microbe availability, microbe stability, or the like. If, on the other hand, a particular product is desired, the microbes typically are selected to maximize production of that product. By way of illustration only, the table below indicates some suitable combinations of microbes which will yield the indicated product.

TABLE II

Hydrolytic-Redox Bioreactor Anaerobic Bioreactor Product

Acetobacter aceti Methanobacterium

soehngenii Methane

Acetobacter peroxydans Methanobacterium

formicicum Methane

Acetobacter pasteurianus Hethanococcus mazei Methane Propionibacterium Methanobacterium

acidi-propionici thermoautrophicum Methane Bacillus macerans Methanobacterium

ruminantium Methane

Bacillus acetoethylicus Methanobacteriurn

mobile Methane

Erwinia dissolvens Methanosarcina

methanica Methane

Escherichia coli Methanosarcina barkeri Methane Klebsiella pneumoniae Methanococcus mazei Methane Trichoderma viride Methanococcus

vanneielli Methane

Aspergillus niger Propionibacterium

acidipropionici Methane

Saccharomyces cerevisiae Saccharomyces

cerivisiae Ethanol

Saccharomyces ellipsoideius Saccharomyces

ellipsoideius Ethanol

Aspergillus niger Clostridium

propionicum Propanol

Trichoderma viride Clostridium saccaroacetoper-butylicum Butanol

Escherichia coli Clostridium butyricum Hydrogen

In general, the microbes are introduced into each bioreactor in accordance with conventional procedures. For example, the bioreactor can be seeded with the desired microbes, typically by circulating an aqueous microbial suspension through the bioreactor. Alternatively, the microbes can be added to the waste stream at any desired point. In cases where the waste stream already contains the appropriate types of microbes, the passage of such waste through two bioreactors will in due course establish the requisite microbe colonies therein. Of course, the bioreactors can be assembled using porous supports having microbes immobilized thereon.

The second bioreactor optionally contains controlled-pore, hydrophobic organic membranes. As used herein, the terms "membranes" refers to either continuous-formed articles, or noncontinuous, the shape and dimensions of which are adapted to process requirements. Thus, the membranes can be flat or curved sheets, a three-dimensional article such as a rectangular or cylindrical tube, or a complex monolith having alternating channels for gas and aqueous medium. As a practical matter, the membranes most often will consist of two porous sheets to provide passage of aqueous medium between them. Gas diffusion occurs with carbon dioxide being delivered through the lower sheet and methane being removed through the upper sheet. Wall thickness is not critical, but must be sufficient to permit the membrane to withstand process conditions without deformation or breakage. In general, a wall thickness of at least about 1.0 mm is desired.

The membrane can be hydrophobic porous polyethylene, polypropylene or fluorocarbon.

The membrane must have a controlled porosity such that about 90 percent of the pores have diameters from about 100A to about 10,000A. Preferably, the pore diameter range will be from about 900A to about 9,000A, and most preferably from about 1,500A to about 6,000A.

It also should be apparent to one having ordinary skill in the art that the configurations of the two bioreactors are not critical to the processing method of the present invention. Thus, the present invention comprehends any configuration which is not inconsistent with the instant disclosure. Most often, the hydrolytic-redox bioreactor will be a conventional cylindrical or tubular reactor with a recycle mode, while the anaerobic bioreactor may be similar or a flat-bed type reactor.

Each bioreactor contains the porous support. Typically, such reactor is composed of any suitable material which is impervious to both gases and liquids. Suitable materials include, among others, glass, stainless steel, glass-coated steel, poly(tetrafluoroethylene), and the like. Each bioreactor optionally is jacketed. The jacket, if present, can be constructed from any of the usual materials, such as those listed for the bioreactors.

In the case of the second bioreactor, it optionally comprises the controlled-pore, hydrophobic organic membrane. In more general terms, each bioreactor normally will be shaped in such manner as to provide one or more channels for the passage of fluid. Where multiple channels are provided, such channels can provide independent flow of the fluid through such channels or they can be serially connected. The aqueous medium can flow through such channels or around such channels. Thus, the porous support can be constructed to form such channels or located around such channels. For example, given the cylindrical bioreactor already described, the porous support can be obtained in the form of a cylinder or tube. Hence, the aqueous medium can flow either through or around the cylinder or tube.

When the organic membrane is used in the second bioreactor, gaseous products or reactants will pass from or through the membrane. When the membrane is not used, gaseous products simply pass from the bioreactor liquid phase to a vapor or gas phase. Gas product removal, of course, is readily achieved by the various means known to those having ordinary skill in the art. Typically, the gaseous products are simply piped away from the second bioreactor. In other words, the gas space of the second bioreactor is connected to a gas collection means that is maintained at a pressure which is less than that of the second reactor.

While process temperatures are critical only to the extent that the microbes present in each reactor remain viable, as a practical matter the process of the present invention will be carried out at a temperature of from about 10°C to about 60°C. The first reactor preferably is maintained at an elevated temperature, i.e. a temperature above ambient temperature. The preferred temperature range for the first reactor under such circumstances is from about 30°C to about 40°C, while the second reactor is maintained at a lower temperature, preferably about 10°C lower.

The foregoing description has been directed to particular embodiments of the invention in accordance with the requirements of the Patent Statutes for the purposes of illustration and explanation. It will be apparent, however, to those skilled in this art that many modifications and changes will be possible without departure from the scope and spirit of the invention. It is intended that the following claims be interpreted to embrace all such modifications.

Claims

We claim: 1. An improved method for treating biodegradable organic waste material suspended or dissolved in an aqueous medium and producing methane gas as a by product, said method to comprise: a. separating and isolating two stages of biological reactions involved in anaerobic decomposition;

b. flowing said biodegradable material through a hydrolytic- redox first sealed bioreactor having a liquid lower portion containing microbes immobilized on a porous support whereby the first reaction product and a gaseous product, including carbon dioxide, are formed, and an upper portion wherein the gaseous product, including carbon dioxide, is collected; c. continuing the liquid flow by taking the first reaction product from the lower portion of the first bioreactor, and introducing the resultant solution into an anaerobic, immobilized microbe second sealed bioreactor including methanobacter wherein methane gas is formed as a reaction product as the organic material in the first reaction product is consumed;

d. removing the gas produced in the first reactor intermittently to reduce its inhibitory effect, storing the collected gas under pressure, and metering the gas continuously at a controlled rate into the second reactor wherein the carbon dioxide in the gas provides an additional source of carbon for conversion to methane;

e. collecting the product gas, including methane gas, from the second bioreactor; and

f. removing the effluent from the second bioreactor.

2. An improved method for treating biodegradable organic waste material suspended or dissolved in an aqueous medium and producing methane gas as a by product, said method to comprise:

a. separating and isolating two stages of biological reactions involved in anaerobic decomposition by preventing any back mixing of liquid flow between the two stages and preventing any back mixing of gases between the two stages; b. flowing said biodegradable material through a hydrolytic- redox first sealed bioreactor having a liquid lower portion containing microbes immobilized on a porous support whereby the first reaction product and a gaseous product, including carbon dioxide, are formed, and an upper portion wherein the gaseous product, including carbon dioxide, is collected; c. continuing the liquid flow by taking the first reaction product from the lower portion of the first bioreactor, and introducing the resultant solution into an anaerobic, immobilized microbe second sealed bioreactor including methanobacter wherein methane gas is formed as a reaction product as the organic material in the first reaction product is consumed;

f. removing the effluent from the second bioreactor.

3. An improved method for treating biodegradable organic waste material suspended or dissolved in an aqueous medium and producing methane gas as a by product, said method to comprise:

e. maintaining the first bioreactor at environmental conditions best suited to the acid forming bacterial reactions, and maintaining the second bioreactor at environmental conditions best suited to the methane forming reactions.

f. collecting the product gas, including methane gas, from the second bioreactor; and

g. removing the effluent from the second bioreactor.

4. The method of claim 3 further comprising immobilizing the microbes of both bioreactors on a porous high surface area, recalcitrant support.

5. The method of claim 3 further comprising providing the hydrolytic-redox, first bioreactor with facultative microbes and providing the methane-producing, second bioreactor with anaerobic microbes.

6. The method of claim 3 further comprising determining and controlling an independently reversible flow direction in each of the bioreactors.

7. The method of claim 3 further comprising determining and controlling an independent fluid level in each of the bioreactors.

8. The method of claim 3 further comprising determining and controlling an independent pressure level in each of the bioreactors.

9. The method of claim 3 further comprising determining and controlling an independent temperature level in each of the bioreactors.

10. The method of claim 3 further comprising controlling the effluent of each of the bioreactors such that the effluent can be recycled through the individual bioreactor.

11. The method of claim 3 further comprising determining and controlling an independent pH level in each of the bioreactors.

12. An apparatus for treating biodegradable organic waste material suspended or dissolved in an aqueous medium and producing methane gas as a by product, said apparatus to comprise:

a. a hydrolytic-redox first sealed bioreactor having a liquid lower portion containing microbes immobilized on a porous support whereby the first reaction product and a gaseous product, including carbon dioxide, are formed, and an upper portion wherein the gaseous product, including carbon dioxide, is collected; b. an anaerobic, immobilized microbe second sealed bioreactor serially linked to the first bioreactor, including methanobacter such that methane gas is formed as a reaction product as the organic material in the first reaction product is consumed;

c. means for delivering the biodegradable organic waste material to the first bioreactor;

d. means for continuing the liquid flow by taking the first reaction product from the lower portion of the first bioreactor, and introducing the resultant solution into the second bioreactor;

e. means for separating and isolating two stages of biological reactions involved in anaerobic decomposition by preventing any back mixing of liquid flow between the two reactors and preventing any back mixing of gases between the two reactors;

f. means for removing the gas produced in the first reactor intermittently to reduce its inhibitory effect;

g. means for storing the removed gas under pressure;

h. means for metering the stored gas continuously at a controlled rate into the second reactor wherein the carbon dioxide in the gas provides an additional source of carbon for conversion to methane;

i. means for collecting the product gas, including methane gas, from the second bioreactor; and

j. means for removing the effluent from the second bioreactor.

13. The apparatus of claim 12, further comprising means for maintaining the first bioreactor at environmental conditions best suited to the acid forming bacterial reactions, and means for maintaining the second bioreactor at environmental conditions best suited to the methane forming reactions.

14. The apparatus of claim 12, wherein each bioreactor comprises: a. a closed, jacketed and insulated reaction vessel;

b. an interior array of microbial support media held in place by a support grating located in the lower portion of said reaction vessel;

c. liquid connections functionally above and below said support media located in the lower portion of said reaction vessel for use as inlet, outlet and recirculation ports as appropriate;

d. a connection between the inlet and the outlet lines such that a portion of the effluent is recycled through the reactor; and e. an outlet in the gaseous upper portion of the reaction vessel from which product gas is removed.

15. The apparatus of claim 12 further comprising means for determining and controlling an independently reversible flow direction in each of the bioreactors.

16. The apparatus of claim 12 further comprising means for determining and controlling an independent fluid level in each of the bioreactors.

17. The apparatus of claim 12 further comprising means for determining and controlling an independent pressure level in each of the bioreactors.

18. The apparatus of claim 12 further comprising means for determining and controlling an independent temperature level in each of the bioreactors.

19. The apparatus of claim 12 further comprising means for controlling the effluent of each of the bioreactors such that the effluent can be recycled through the individual bioreactor.

20. The apparatus of claim 12 further comprising means for determining and controlling an independent pH level in each of the bioreactors.

21. The apparatus of claim 12 wherein the liquid effluent from the second reactor contains a sludge consisting of ash, small amounts of unreacted organic matter, and microbial cells.

22. The apparatus of claim 12 wherein the gaseous product from the second reactor is high quality methane containing less than 15 percent carbon dioxide and no hydrogen sulfide.