Anaerobic process

Abstract

The invention relates to processes and equipment for treatment of a feedstock by anaerobic organisms to produce a methane containing biogas that can be used as a source of energy. The invention is particularly concerned with producing methane from a waste plant material such as produced by fermentation processes used in the alcoholic beverages industry, such as from brewing/distilling processes which employ grain material for fermentation.

Claims

1. A process for the anaerobic digestion of a substantially aqueous solution or an aqueous and oil two phase system containing organic material to produce a liquid output wherein the COD is reduced by more than 70% from the input COD, comprising: providing a feed of a substantially aqueous solution or aqueous and oil two phase system that has a COD concentration of 40 to 130 kg/m.sup.3 to an enclosed tank or lagoon containing a sludge bed of methanogenic microorganisms via an upward flow; subjecting the substantially aqueous solution, or aqueous and oil two phase system to methanogenesis to produce methane by digestion of the organic material in the enclosed tank or lagoon, wherein the liquid output from the enclosed tank or lagoon is not recycled in the process; producing a liquid output from the enclosed tank or lagoon with a COD reduced by more than 70% from the input COD; monitoring the content of at least one micronutrient in the feed and in the liquid output; and, adding the at least one micronutrient when the difference in micronutrient level between the feed and the liquid output is less than expected based on the amount of methane produced and the expected growth of the methanogenic microorganisms.

2. The process according to claim 1 wherein the substantially aqueous solution or aqueous and oil two phase system is a substantially aqueous spirit distillation effluent stream.

3. The process according to claim 1 wherein the substantially aqueous solution or aqueous and oil two phase system is a solution/system from cheese manufacture, sugar production, or vegetable processing.

4. The process according to claim 1 wherein the process is a continuous process.

5. The process according to claim 1 wherein the COD of the liquid output from the enclosed tank or lagoon is reduced by 90% or more relative to the input.

6. The process according to claim 1 wherein the methanogenesis is preceded by a separate acidogenic stage.

7. The process according to claim 1 wherein the hydraulic retention time in the enclosed tank or lagoon is from 5 to 15 days.

8. The process according to claim 1 wherein the at least one micronutrient is selected from the group consisting of iron, cobalt, nickel, vitamins and selenium.

9. The process according to claim 1 wherein the monitoring of the at least one micronutrient is by ICP mass spectroscopy.

10. The process according to claim 1 further comprising, collecting solid struvite (NH.sub.4MgPO.sub.4.6H.sub.2O) from the liquid output.

11. The process according to claim 10 wherein carbon dioxide is evolved from the liquid output to obtain the solid struvite.

12. The process according to claim 11 wherein the liquid output is filtered or subjected to a hydrocyclone process in order to isolate the solid struvite from the liquid output.

13. The process according to claim 11 wherein the liquid output following struvite removal is further processed by concentration of the liquid or evaporation to a solid product comprising nitrogen, phosphorus, and potassium.

14. The process according to claim 10 wherein the liquid output is conveyed along flexible tubing or piping, in order to prevent or minimize struvite from building up on an inner surface of a pipe or tube.

15. A process for the anaerobic digestion of a substantially aqueous solution or an aqueous and oil two phase system containing organic material to produce a liquid output wherein the COD is reduced by more than 70% from the input COD, consisting essentially of: providing a feed of a substantially aqueous solution or aqueous and oil two phase system that has a COD concentration of 40 to 130 kg/m.sup.3 to an enclosed tank or lagoon containing a sludge bed of methanogenic microorganisms via an upward flow; subjecting the substantially aqueous solution, or aqueous and oil two phase system to methanogenesis to produce methane by digestion of the organic material in the enclosed tank or lagoon, wherein the liquid output from the enclosed tank or lagoon is not recycled in the process; producing a liquid output from the enclosed tank or lagoon with a COD reduced by more than 70% from the input COD; monitoring the content of at least one micronutrient in the feed and in the liquid output; and adding the at least one micronutrient when the difference in micronutrient level between the feed and the liquid output is less than expected based on the amount of methane produced and the expected growth of the methanogenic microorganisms.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates schematically the operation of an anaerobic digestion process.

(2) FIGS. 2-4 show schematic flow diagrams of grain and malt whisky distillation processes and the waste liquid streams which are generated and may be subjected to anaerobic digestion in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE DRAWINGS AND SPECIFIC EMBODIMENTS

(3) The invention will now be further described with reference to specific embodiments and the accompanying drawings.

(4) The processing of an aqueous effluent stream containing soluble organic components derived from a distillery operation is now described. After fermentation of a water/grain mixture and distillation to remove the alcohol content, the aqueous waste was separated from solids (both spent grain and yeast were removed, typically by the use of filter presses, in this example).

(5) A typical composition of the aqueous grain distilling effluent stream is shown in Table 1 below. This is the soluble solids liquid stream as shown in FIG. 2. The data is based on a distillery operating with an alcohol fermentation process that operates with 65 to 70 degrees original gravity.

(6) FIGS. 3 and 4 show alternative liquid streams (pot ale and/or pot ale syrup) which may be employed in the process according to the present invention.

(7) TABLE-US-00001 TABLE 1 No. Distillery Soluble Effluent Stream Value a Light straw colour with cooked cereal nose. 1 Chemical Oxygen Demand 40,000 mg/l 2 Biological Oxygen Demand 20,000 mg/l 3 pH Range (natural) 3.7-4.2 4 Temperature of Feed to Acidogenic 38 deg C. Reactor- deg C. 5 Total Solids 5.0% 6 Total Suspended Solids 0.25% 7 Suspended Solids Type Trace cereal solids, yeast cells 8 Dissolved Solids 4.75% 9 Dissolved Solids Type Organic, Trace Mineral Ash 10 Dissolved Solids Breakdown - Protein - Circa. 15% Typical Compositional Data Carbohydrate - Circa. 70% Expressed as % on dry basis; Oil - Circa. 3% Typical ranges as +/5 to 10%. Organic Acids - Circa. 1% Glycerol - Circa. 3% Magnesium 250 mg/l Ammonium 60 mg/l Phosphate 300 mg/l Sulphur, 3 mg/l

(8) FIG. 1 shows schematically an apparatus suitable for a continuous anaerobic digestion process in accordance with the present invention. The apparatus can be of the form described in more detail in U.S. Pat. No. 6,395,173.

(9) An apparatus of this form was supplied for the digestion of a liquid of the type described in Table 1 above.

(10) The methanogenic stage has a process volume of about 8,700 m.sup.3. The hydraulic retention time in the methanogenic stage was expected to be of the order of 3 days as a significant recycle was anticipated as a requirement by the suppliers of the reactor. The preceding acidogenic process volume was 720 m.sup.3, giving a hydraulic retention time of the order of 24 hrs for that stage (no recycle is applied to the acidogenic stage).

(11) When operating in conventional fashion with a substantial recycle the apparatus had a design loading of 3.5 kg CODm.sup.3day.sup.1. In operation 70% of this COD loading was expected to be removed by the conventional process and the resultant biogas was expected to have a methane content of 60-75%.

(12) In practice when using a conventional level of recycle and without metals additions the process did not perform well. A high level of propionic acid was found in the recycling fluid and low natural bicarbonate alkalinity (typically, a negligible amount) was found in the methanogenic stage. At the same time the conversion of COD was poor and the quality of biogas a measured by methane content was also lowered. Thus operating as planned resulted in low efficiency.

(13) For example, during a period where a recycle stream was applied at 6:1 the propionic acid levels in the recycle were high (1-3 kgm.sup.3) and consequently had an inhibitory effect on the methanogenic stage of the anaerobic process. This recycle steam also contributes to the COD loading (methanogenic phase COD loading=COD from feed+COD from recycle stream). This combined flow (recycle and feed) also has an impact on the hydraulic retention time as discussed previously.

(14) COD breakdown and methane content of the biogas were generally low, both in the region of 65-70%.

(15) During operations with this recycle it was noted that there was no bi-carbonate alkalinity present in the reactor.

(16) The conventional response when poor performance such as this is found would be to increase the recycle or decrease the feed concentration, with a resulting loss in process throughput. However increasing the recycle even up to 12:1 did not improve the COD removal.

(17) Surprisingly once the recycle was reduced and ultimately stopped, to follow the process of the invention, a number of fundamental changes were observed. Bi-carbonate alkalinity rose rapidly to >4,000 mgl.sup.1, and the propionic acid levels in the output from the reactor reduced to less than 100 mg/litre. Improved COD reduction, better quality biogas and struvite crystallisation were also observed. The applicant is currently running 3 reactors in such a fashion and producing approximately 17,500 m.sup.3 biogas per day per reactor. This is equivalent to 11,300 m.sup.3 methane per day at a 65% methane amount in the biogas. The biogas produced is used to run two Jenbacher gas engines, which produce a total of 150 MW of electricity per day. These reactors have been run non-stop continuously for since 2009. It is expected that more gas and electricity could be generated, but the current limitation is the amount of feed available to provide to the generators, currently approximately 45 tonnes of COD is being removed per reactor per day

(18) As shown in FIG. 1, in operation in accordance with the invention, an aqueous waste stream feed 1, for example similar to that of table 1 above is passed into a balance tank 2 which provides a buffer volume of input to the anaerobic process and can smooth out inconsistencies in the output from upstream processes.

(19) The balance tank 2 serves as a buffer volume as it can allow the process to continue even if the feed 1 is interrupted or intermittent. The output 3 from the balance tank 2 (of the order of 50 m.sup.3 per hour) then enters an acidogenic tank 4 containing organisms where volatile fatty acids are produced and then broken down to acetic acid. The output 6 from the tank 4 (at a pH of the order of 3.5) is the input for a large two stage methanogenic process tank 8 which may be of concrete construction covered by a flexible sheet gas hood 10. Typically the process tank 8 will be buried or partially buried in the earth.

(20) The process tank 8 is configured to operate as an upflow anaerobic sludge blanket reactor (UASB), in this example with two stages.

(21) The UASB 8 is divided by a wall 12 (having passages for fluid to pass through) into two subunits, a high COD load first subunit 14 and a low COD load second subunit. A sludge blanket of methanogenic organisms 18 is provided in each sub unit 14,16 of the tank 8.

(22) In practice the output 6 from the acidogenic/acetogenic stage is fed upwards at several locations into tank 8 as in a conventional process at the input end of the tank i.e. into the high COD load first unit 14 (multiple inputs not shown for clarity). Baffles may be provided in the first unit 14 to aid mixing. Some of output 6 (generally a small amount) may also be fed to the low COD subunit, to maintain health of the microorganisms contained within which may be otherwise starved. The microorganisms in sludge blanket 18 digest the input 6 producing a biogas 20 containing 75% to 85% methane with the remainder being mostly CO.sub.2 with small amounts of H.sub.2S and ammonia. The biogas 20 can be purified (removal of the sulphur component) and then used as fuel, for example in a gas engine connected to a generator to produce electricity. Waste heat from the exhaust gases may in turn be used, for example to generate useful steam energy.

(23) The input 6 to UASB 8 flows gradually through, exiting via lamella separator 22 as an effluent stream 24 that can be discharged to drain following some minor clarification procedures, that may include removal of struvite. The struvite produced tends to crystallise out of the effluent stream 24 after it exits the lamella separator 22 possibly due to a raising in pH. Removal of struvite may be for example by use of one or more hydrocyclone separators indicated by box 26 placed before discharge to drain. Struvite production of some 2.5 tonnes per day can be realised in the described system using a feed similar to that in table 1, with the limiting factor to struvite production being input of magnesium from the feed. The lamella separator is used in the known manner to separate sludge carried out of tank 8 by the flow through of the process stream. The wet sludge from the lamella separator is normally returned to the tank 8, via a return line 28.

(24) The process described above differs from the conventional procedure by not recycling liquid effluent 24 at a high ratio (conventionally 3:1 to 12:1) via recycle line 30 for liquid from the lamella separator 22. In some circumstances a limited recycle may be employed, at up to 1:1 or even 2:1.

(25) The addition of metals, typically iron cobalt and nickel was shown to aid in COD removal and biogas production, the benefit being of the order of 10-15% of COD removal and the equivalent methane production equating to 350 m.sup.3 per tonne COD removed at STP. The metals addition was carried out in accordance with the third and fourth aspects of the invention as discussed above. The overall health of the sludge bed and robustness of the process was also improved.

(26) When operated as described above the process has been found to be stable and robust with a COD removal level of the order of 90% to 95%. The biogas quality is high, typically 60% to 75% methane. Other notable features of a process operated in this manner include:

(27) A loading rate of 5 kg's COD m.sup.3 day.sup.1 or more can be used;

(28) A natural high pH (7.2-7.4) consequent to a natural high bicarbonate level (>4000 mg per litre) that can be maintained without adjustment;

(29) The inhibitory propionic acid is absent or remains in low concentration (<50 mgl.sup.1).

Further Example

(30) Pilot plant and laboratory scale reactors of the upward flow anaerobic sludge blanket types were used to study the anaerobic digestion of whey.

(31) The digestion was carried out in two stages using separate acidogenic and methanogenic reactors. The reactors were of the cylindrical tower style.

(32) The whey was obtained from a local source and had a COD of circa 80,000 mg per litre.

(33) The COD loading rate was of the order of 5 m.sup.3 day.sup.1 or slightly more.

(34) When the anaerobic process was operated with typical recycle the COD removal was of the order of 75%.

(35) When the anaerobic process was operated without recycle a COD removal of the order of 80% was achieved with a methane yield of 0.33 m.sup.3 methane per kg of COD removed.

(36) The above tests were carried out without the addition of metals.