WO2008002448A2

WO2008002448A2 - Method of enhancing methane production from organic material

Info

Publication number: WO2008002448A2
Application number: PCT/US2007/014455
Authority: WO
Inventors: Thomas R. Punnett
Original assignee: Temple University - Of The Commonwealth Of Higher Education
Priority date: 2006-06-23
Filing date: 2007-06-21
Publication date: 2008-01-03
Also published as: WO2008002448A3

Abstract

The present invention includes a method for enhancing methane production from organic material. The method includes establishing baseline conditions of the organic material, which includes establishing baseline concentrations of at least four light metal cations in the organic material. The light metal cations include divalent and monovalent cations. The method further includes adjusting pH and alkalinity of the organic material. The method also includes adjusting the proportions of the light metal cations, which includes adjusting the ratio of the divalent to monovalent cations, adjusting the ratio of a first light metal cation to a second light metal cation, and adjusting the ratio of a third light metal cation to a fourth light metal cation.

Description

A METHOD OF MAXIMIZING METHANE PRODUCTION FROM ORGANIC MATERIAL

RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 60/815,954, filed June 23, 2006, the entire disclosure of which is incorporated herein by reference.

FIELD OF INVENTION

[0002] The present invention relates to methods of enhancing methane production from metabolizable organic material such as sewage sludge, coal, solid waste, and other waste by-products.

BACKGROUND OF THE INVENTION

[0003] Historically, optimizing methane production from organic material has focused on modifying only single aspects of the methane production process. For example, prior optimization has focused individually on (1) holding the pH constant pH at 7, (2) maintaining the temperature at 35°C, (3) adding a few cations that are considered nutrients, (4) altering physical arrangements such as including an acidic fermentation tank followed by a neutral fermentation tank, (5) inserting structures into the process, the structures having adsorbent surfaces that the microorganisms adhere to, thereby increasing organism/substrate contact, and (6) designing very large, complicated, and expensive sewage treatment plants in which the organic material is removed mainly by oxidation and methane production is minimized. In short, the prior attempts to optimize methane production failed to utilize simultaneous control of multiple operating conditions including light metal cation concentration, light metal cation ratios, pH, and alkalinity.

SUMMARY OF THE INVENTION

[0004] An embodiment of the present invention is a method for enhancing methane production from organic material. The method includes a step of establishing baseline conditions of the organic material, the step including establishing baseline concentrations of at least four light metal cations in the organic material. The light metal cations include divalent and monovalent cations. The method further includes adjusting pH and alkalinity of the organic material. The proportions of the light metal cations are also adjusted. The adjusting of the light metal cations includes adjusting the ratio of the divalent to monovalent cations, adjusting the ratio of a first light metal cation to a second light metal cation, and adjusting the ratio of a third light metal cation to a fourth light metal cation.

BRIEF DESCRIPTION OF THE DRAWING

{0005] For purposes of illustrating the invention there is shown in the drawing a presently preferred method; it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities particularly shown. [0006] FIG. 1 is a flow chart showing steps of one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0007] Generally, the method of the present invention optimizes methane production by adjusting and controlling multiple microbial growth conditions. The method can include a continuing application of a set of tests done to inform an operator how to adjust growth conditions in, for example, an anaerobic digester, so as to maximize microbial methane production and, preferably, keep that methane production at a constant, high level. A benefit of the method of the present invention is that increased methane production can be achieved regardless of the structural features of the facility housing the organic material.

[0008] The method of the present invention can be applied to all biological processes used to produce methane from waste material, e.g., plant and animal waste. The method can be applied to aerobic as well as to anaerobic streams of wastewater treatment plants ("WWTP"), waste streams of commercial WWTPs (e.g., waste streams from meat factories, fish culture ponds, animal farms, food processing facilities), coal beds, and buried garbage sites. Preferably, the method is applied to anaerobic waste streams at WWTPs having anaerobic digesters, e.g, single phase digesters, multiple phase digesters, and septic tanks. Included in the preferred application are WWTPs having structural modifications that contain mechanical devices designed to increase the contact between the organisms and the waste stream known as continuously stirred tank reactor (CSTR), upward-flow anaerobic sludge blanket (UASB) and all other designs known in the art. [0009) As shown in FIG. 1, the method of the present invention can include the following steps: (1) establishing a baseline set of microbial growth conditions in an organic material, including establishing baseline concentrations of at least four light metal cations, the light metal cations comprising divalent and monovalent cations; (2) optimizing pH and alkalinity of the organic material; and (3) adjusting proportions of light metal cations in the organic material. The adjustment of the proportions of the light metal cations can include adjusting the ratio of the divalent to monovalent cations, adjusting the ratio of a first light metal cation to a second light metal cation, and adjusting the ratio of a third light metal cation to a fourth light metal cation. [0010] The baseline set of microbial growth conditions can be established by sampling and measuring the organic material without modifying it, and then adopting the measurements as the baseline. Preferably, the baseline is established by modifying the growth conditions as they exist in the organic material by first sampling the organic material to determine the makeup of the organic material, and then modifying that makeup as necessary.

[0011] Preferably, a baseline set of microbial growth conditions is established by modifying growth conditions such as pH, alkalinity, and concentrations of at least four light metal cations. The at least four light metal cations are preferably sodium, potassium, magnesium, and calcium. Not all growth conditions must be modified to establish the baseline. For example, the baseline can be established by only modifying the concentrations of potassium and magnesium. Where the baseline concentrations of all four light metal cations are established, the concentrations are preferably established by first adding the light metal cations as metal hydroxides or soluble oxides to raise the pH of the organic material to about 5.4. The concentration of metal bicarbonates, which is one component of a final buffered balanced mixture, is minimal below this pH. A balanced mixture of the light metal cations can then be added as metal carbonates to raise the pH of the organic material to about 6.6. Finally, a balanced mixture of the light metal cations can be added as metal carbonates, bicarbonates (or metal hydroxides depending on the desired alkalinity) to maintain the pH of the organic material at about 6.6. The pH target of from about 5.4 to about 6.6 is merely preferred and is not mandatory. The balanced concentration of the four light metal cations is preferably about 20 mM sodium, about 7 mM potassium, about 7 raM calcium, and about 5 mM magnesium. [0012] In each of the steps of the method of the present invention, metal cations can be added as metal salts. The metal salts can be added to the organic material in any way known to one skilled in the art. For example, solid salts can be poured into an entry port two to five times a day. Multiple pourings per day are generally beneficial because reactor gases are pumped back through the reactor to mix the contents. Alternatively, concentrated solutions of the metal salts can be added manually or through metered pumps and mixed in the digester. Metered pumps are preferred if the method of the present invention are to be automatically controlled.

[0013] As part of the establishment of the baseline set of growth conditions, adequate micronutrients are preferably included in or provided to the organic material. There are at least 1 1 micronutrients required by microorganisms in micromolar or submicromolar concentrations in addition to iron, which should be supplied in a concentration of 10 to 40 micromolar. The micronutrients/submicronutrients are, roughly in decreasing order of the amount needed, zinc, copper, manganese, iodine, boron, molybdenum, cobalt, vanadium, tungsten, nickel, and selenium. The availability of micronutrients in the organic material is difficult to predict. A major problem is that sulfide is the major source of sulphur for most methanogens and at the same time is a precipitating agent for many heavy metals. In addition, when sulfate is reduced to sulfide by other anaerobic bacteria, its concentration can reach levels high enough to inhibit methanogens outright. Consequently, it is preferred that the sulfide concentration in the organic material is maintained in the range of about 200 mM to about 500 mM, amounts that support the growth of most methanogens.

[0014] Once the baseline set of microbial growth conditions is established, parameters such as alkalinity, pH, major divalent/monovalent cation ratio, individual ion pair ratios, and temperature can be optimized. Although the order in which the parameters are optimized is not critical, the preferred order of optimizing includes optimizing the pH and alkalinity first and then optimizing the proportions of light metal cations. [0015] To optimize the pH of the organic material, the following steps are preferably performed. The pH of the organic material is varied by changing the amount of the balanced light metal bases added so that the pH increases or decreases in steps of about 0.3 pH units. The pH of the digester is held at that level for a long enough time to insure that the rate of methane production reaches a constant, steady state value. This process of adjusting the pH and measuring the methane production rate can be repeated one or more times. The optimum pH can then be determined by comparing the amount of methane produced at each of the individual pH values. The magnitude of the pH adjustment is not critical. However, a value of about 0.3 pH units as an initial adjustment amount is preferred because it is convenient and is large enough to cause an appreciable change in the microbial metabolism. Smaller values can be used (e.g., 0.1 pH units) after the initial adjustments to fine tune the optimum pH value. The optimum pH value, once it has been determined, can be used as a starting pH for a digester which is being set up for the first time. The optimum pH value, once it has been determined, can be used as a starting pH for a digester that is being tuned up with present method. The optimum pH value, once it has been determined, can be used as the starting pH in a process of fixing a stuck digester.

[0016] To optimize the alkalinity of the organic material, the following steps are preferably performed. The light metal cations are added to the digester as hydroxides plus bicarbonates in about a 80/20 mixture keeping the pH at about 6.6. Once the desired mixture and pH are reached, the alkalinity can be measured and the methane production rate recorded. Next, the light metals are added as carbonates again keeping the pH at about 6.6. Once the pH stabilizes at the desired pH, the alkalinity and methane rate can again be determined. The resulting data (i.e., alkalinity v. methane production rate) can be used to calculate the dependence of methanogenesis on alkalinity by comparing the two sets of data to each other and to the values prevailing before the tests. The tests can be repeated, if necessary, to determine the alkalinity level at which the highest methane production rate is achieved (i.e., the optimal alkalinity). That optimal alkalinity is preferably maintained during further processing of the organic material. The two compositions given above, about 80%/20% hydroxides/bicarbonates and about 100% carbonates, are reasonable values but not mandatory for testing the system. Although not a necessary starting point, a preferred starting value for alkalinity is about 10 raM.

[0017] To optimize the divalent/monovalent cation ratio, the following steps are preferably performed. First, the ratio of divalent to monovalent cations is adjusted one or more times to identify an optimal ratio. The optimal ratio is the ratio that yields the maximum methane production rate. As used herein, the term "maximum methane production rate" is the rate of methane production that yields the highest or about the highest amounts of methane under specific conditions of the organic material. Preferably, the ratio of divalent to monovalent cations is modified systematically in steps of about 300% or more at first, and then in steps of about 100% or less for fine tuning. Each time the ratio is adjusted, the system is allowed to reach a steady state before the methane production rate is measured and before the next modification is performed.

[0018] To optimize the individual cation pairs, the ratio of divalent to monovalent cations is preferably maintained at the optimal ratio as determined in the previous step. The ratio of magnesium to calcium is adjusted one or more times to identify an optimal ratio as determined in the previous step. The optimal ratio is the ratio that yields the maximum methane production rate. Preferably, the ratio of magnesium to calcium cations is modified systematically in steps of about 100% or more at first, and then in steps of about 20% or less for fine tuning. Each time the ratio is adjusted, the system is allowed to reach a steady state before the methane production rate is measured and before the next modification is performed.

[0019] The ratio of sodium to potassium is then adjusted one or more times to identify an optimal ratio. The optimal ratio is the ratio that yields the maximum methane production rate. Preferably, the ratio of sodium to potassium cations is modified systematically in steps of 100% or more at first, and then in steps of 20% or less for fine tuning while the ratio of divalent to monovalent cations and the ratio of magnesium to calcium is maintained at relatively consistent levels. Each time the sodium/potassium ratio is adjusted, the system is allowed to reach a steady state before the methane production rate is measured and before the next modification is performed. [0020] If the concentration of salts needed for ionic balance is greater than that required for adjusting pH and/or alkalinity, the additional metal cations can be added as chlorides. Nitrates and phosphates should not be added. Sulfates should not be used unless a heavy metal toxicity problem requires their addition in small amounts to eliminate toxic effects or unless there is a sulfide deficiency.

[0021] The cations should be added with care in order to avoid putting physiological stress on the microbial complex by changing the growth conditions too rapidly. Increases of 100% to 150% in the concentration of a single cation or the magnitude of a cation ratio in one day are acceptable, but if the magnitude of change is greater, the additions should be stretched out to a period of two to five days depending on how great the change is. The slow addition allows time for those organisms that can adapt to the new conditions to do so or allow the organisms better suited to the new conditions to multiply and assume a dominant role.

[0022J This multiday protocol used with major changes in concentrations of the reagents raises the problem of approaching the desired new concentration asymptotically because of the continuing dilution of the tank contents by incoming wastes. To address the problem, a slight excess of the cation can be added in order to compensate for the dilution. For example, for a digester whose daily flow is 10% of its total volume, an extra 11% of the weight of the cation being added that day plus an additional 11% of the weight added in previous days, can be added to the digester. The value 11% is derived from the 10/1 ratio of the digester capacity to the average daily flow and is independent of both the number of days taken for the change in concentration and the choice of stepwise versus continuous addition of the cation. The small excess amount of the cation for one day (11%) will cause no significant stress on the microorganisms. The amount of excess cation added can be adjusted to compensate for different dilution rates which may be found in other WWTPs.

[0023] If several WWTPs located near each other in the same watershed are being optimized at the same time, adjustments described herein can be completed more quickly provided their waste streams are chemically similar. Adjustments may be done by holding the monovalent divalent ratio constant in one WWTP, the control, while lowering it in another and raising it in a third. If more than three WWTPs are involved, the additional plants are used to optimize the ratios of calcium to magnesium and sodium to potassium at the same time. Thus if enough WWTPs are involved, a single set of results obtained at one time can be used to reset all three ionic ratios as starting points for the next set of tests.

[0024] Preferably, when any process modification yields an increase in the methane production rate of about 50%, the recycling rate in the process should be reset to the rate necessary to produce about 90% of the new, increased methane production rate. [0025] Although other parameter values are contemplated to be within the scope of the method of the present invention, preferred parameter values include: monovalent to divalent cation ratio of about 1000/1 to about 1/5; sodium to potassium ratio of about 100/1 to about 1/5; calcium to magnesium ratio of about 10/1 to about 1/5; pH of about 4.0 to about 9.0; temperature of about 5°C to about 60⁰C; and recycling rate of 0% to about 90%. [0026] The method of the present invention can also be used with a stuck digester, i.e., a digester where large amounts of acid are introduced into the organic material so that the pH of the digester drops sharply, causing most, if not all, of the metabolic activity in the digester to cease, including methane production. With stuck digesters, balanced light metal hydroxides will be added in the same proportions as described above so that the pH of the digester is immediately brought back up to its optimal working value. The pH of the digester may then be kept constant by decreasing the amount of extra base added as the acid is washed out.

|0027J If the method of the present invention is used with a secondary digester, anaerobic effluent from the secondary digester includes, among other things, the balanced mixture of cations required by all microorganisms. The effluent can be aerated and pumped into the beginning of an aerobic waste stream at the plant to enhance the metabolic activity of the aerobic and the facultative bacteria as well as the fungi which are oxidatively decomposing waste products.

[0028] The method of the present invention can also include adding carbon dioxide to the organic material. The carbon dioxide can be added to the organic material before the baseline set of growth conditions is established, concurrently with the establishment of the baseline set of" growth conditions, or subsequent to establishing the baseline set of growth conditions. It is contemplated that increased carbon dioxide in the organic material can increase methane production. It is also contemplated that increased carbon dioxide can also aid as a buffer, allowing for greater stability in pH. [0029] Preferably, the carbon dioxide that is added is recycled carbon dioxide produced from the organic material during treatment, e,g., digestion in a primary digester. As the organic material is treated, e.g., in a primary digester, the organic material releases gases, e.g., methane, carbon dioxide, and hydrogen sulfide, that are generally termed head gases. The head gases can be released, captured and burned, or preferably, captured and recycled into the organic material. The head gases can be recycled into the organic material by any known recycling or recirculation system or plumbing such as PVC piping coupled with a pneumatic pump. Preferably, the head gases are recycled through the organic material when the head gases are produced in a quantity large enough to be pumped. The amount of recycled carbon dioxide can be plotted versus methane production. From this plot, it can be determined what level of recycled carbon dioxide produces the desired, e.g., highest, production of methane. If an increase in methane production occurs and reaches a concentration of carbon dioxide in the treatment container that is saturating, i.e., the addition of more carbon dioxide fails to yield any further increase in methane production, that concentration of carbon dioxide is then preferably maintained by recycling.

[0030] The concentration of carbon dioxide can be monitored and adjusted manually or by automation. Adjustments can be made based on parameters such as temperature, pressure, concentration of organics in the organic material, flow rate, and mixing rates. [0031] A major advantage of the method of the present invention is increased methane production. Increased methane production has several benefits. First, increasing methane production would result in increased fuel to power the treatment process as well as excess fuel that can be supplied to a local energy company for distribution to local residents and businesses. Second, increased methane production correlates to a reduction in the amount of organic pollutants in the effluent waste stream (i.e., lower chemical oxygen demand) and consequently increases the water quality of effluent receiving waters (e.g., streams, rivers, lakes). Third, increased methane production results in a solid waste from which the methane was produced having a large quantity of methanogens, which could be useful as nutritious compost for farmland after the material is heat treated. Fourth, increasing methane production could help to lessen a locality's reliance on fossil fuels, and could therefore help in improving air quality. All methane produced as a biogas and used as an energy source eliminates the need to produce and transport an equivalent amount of mineral natural gas. Where waste is not converted to methane, the waste is discharged in the form of organic matter which is ultimately oxidized to produce carbon dioxide in the environment. Efficient methane production from waste, therefore, means that a given amount of energy is produced with half as much energy source consumed and half as much carbon dioxide produced. A fifth benefit of increased methane production is that most of the carbon in the waste stream is converted to a useful gas with minimal production of anaerobic microorganisms that must be disposed of as biological waste. By contrast, in the more modern aerobic treatment plants, most of the waste stream is converted to carbon dioxide, a greenhouse gas, with the production of a much larger amount of aerobic microorganisms that must be disposed of as biological waste.

[0032] It will be appreciated by those skilled in the art, that the present invention may be practiced in various alternate forms and configurations. The previously detailed description of the disclosed embodiments is presented for purposes of clarity of understanding only, and no unnecessary limitations should be implied there from.

Claims

I claim:

1. A method of enhancing methane production from organic material, the method comprising: establishing baseline conditions of the organic material, including establishing baseline concentrations of at least four light metal cations in the organic material, the light metal cations comprising divalent and monovalent cations; adjusting pH and alkalinity of the organic material to enhance methane production from the organic material; and adjusting proportions of the light metal cations in the organic material to enhance methane production from the organic material, the adjusting comprising adjusting the ratio of the divalent to monovalent cations, adjusting the ratio of a first light metal cation to a second light metal cation, and adjusting the ratio of a third light metal cation to a fourth light metal cation.

2. A method according to claim 1, wherein the first light metal cation is magnesium, the second light metal cation is calcium, the third light metal cation is sodium, and the fourth light metal cation is potassium.

3. A method according to claim 2, wherein the step of establishing baseline concentrations of the at least four light metal cations in the organic material comprises: adding the light metal cations to the organic material as metal hydroxides or soluble oxides to raise the pH of the organic material to about 5.4; adding a mixture of the light metal cations to the organic material as metal carbonates to raise the pH of the organic material to about 6.6; adding a mixture of the light metal cations to the organic material as metal bicarbonates to maintain the pH of the organic material at about 6.6.

4. A method according to claim 2, wherein the step of adjusting the pH of the organic material comprises: adjusting the pH of the organic material one or more times by a predetermined amount by modifying the amount of light metal cations present in the organic material as metal bases to identify an optimal pH of the organic material, the optimal pH yielding a maximum methane production rate from the organic material.

5. A method according to claim 2, wherein the step of adjusting the alkalinity of the organic material comprises: measuring a first alkalinity of the organic material and a first methane production rate of the organic material; adding the light metal cations to the organic material as a combination of metal hydroxides and metal bicarbonates, while maintaining the pH of the organic material substantially constant; measuring a second alkalinity of the organic material and a second methane production rate of the organic material; adding the light metal cations to the organic material as metal carbonates, while maintaining the pH of the organic material substantially constant; measuring a third alkalinity of the organic material and a third methane production rate of the organic material; and determining the optimal alkalinity of the organic material, the optimal alkalinity consisting of the alkalinity relating to the highest of the first, second, or third methane production rates.

6. A method according to claim 2, wherein the baseline concentrations of the at least four light metal cations comprises about 20 mM sodium, about 7 mM potassium, about 7 mM calcium, and about 5 mM magnesium.

7. A method according to claim 2, wherein the step of adjusting proportions of the light metal cations comprises: adjusting the ratio of divalent to monovalent cations in the organic material one or more times to identify an optimal ratio, the optimal ratio yielding the maximum methane production rate of the organic material; maintaining the ratio of divalent to monovalent cations in the organic material at the optimal ratio; adjusting the ratio of magnesium to calcium in the organic material one or more times to identify an optimal ratio, the optimal ratio yielding the maximum methane production rate of the organic material; and adjusting the ratio of sodium to potassium in the organic material one or more time to identify an optimal ratio, the optimal ratio yielding the maximum methane production rate of the organic material.

8. A method according to claim 7, wherein the optimal ratio of monovalent to divalent cations is about 1000/1 to about 1/5.

9. A method according to claim 7, wherein the optimal ratio of sodium to potassium is about 100/1 to about 1/5

10. A method according to claim 7, wherein the optimal ratio of calcium to magnesium is about 10/1 to about 1/5.

1 1. A method according to claim 1, wherein carbon dioxide is added to the organic material.

12. A method according to claim 11, wherein the carbon dioxide comprises carbon dioxide recycled from the organic material.