Biodigester in a glasshouse to maintain the temperature.
The glasshouse temperature reach 20 degree celcius.
Biogas is safe(?)
Blue flame from methane biodigester.
Low cost biogas digester
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Parameters to consider in running a biogas digester
The parameters are:
- Acidity - Anaerobic digestion will occur best within a pH range of 6.8 to 8.0.
- Carbon-nitrogen ration (C/N) - a carbon - nitrogen ratio of about 30 - 1 is ideal for the raw material fed into a biogas plant.
- Temperature control - A temperature between 32°C and 35°C has proven most efficient for stable and continuous production of methane. but the action of the digesting bacteria will decrease sharply below 16°C.
- Percentage of solids - Anaerobic digestion of organics will proceed best if the input material consists of roughly 8 % solids. In the case of fresh cow manure, this is the equivalent of dilution with roughly an equal quantity of water.
- Plant design - above or below ground. There are pros and cons with either types.
- Continuous/batch operation - there are pros and cons with either types.
- Stirring - stirring the slurry in a digester is always advantageous, if not essential.
- Gas collection - A non-return valve here is a valuable investment to prevent air being drawn into the digester, which would destroy the activity of the bacteria and provide a potentially explosive mixture inside the drum.
- The level of carbon dioxide and proportion of methane will give valuable information about the state of the fermentation process as well. Infrared sensors are the best means employed today for this purpose. The need for calibration is minimal or nonexistent and the small size, relatively low cost and minimal power consumption make them ideal for this type of application.
Construction of a Hestia biogas digester
A general rule is that the tank needs to be 50 times the size of the
daily input to allow for some space for gas to collect. If your input is
15 gallons of material per day, you’d need a 750-gallon tank.
Hestia biodigesters are approximately 5 by 7 feet wide by 5 feet deep, providing about 700 gallons of capacity. Slurry occupies about 600 gallons of this biodigester; the remaining space is for the gas that’s produced.
There’s an inlet for adding feedstock and an outlet for removing composted slurry.
A closed loop of PEX tubing in the bottom of the tank is plumbed to an on-demand water heater to add heat when the slurry temperature drops below 50°F—the temperature at which cryophilic methanogenic bacteria go dormant and stop producing gas.
If the climate is mild, it may be enough to build a hoop house over the tank to keep the slurry sufficiently warm in winter. Alternatively, the biodigester could be allowed to go dormant during the colder months.
More info: http://energez.blogspot.com/2012/04/video-hestia-home-biogas-plant.html
Hestia biodigesters are approximately 5 by 7 feet wide by 5 feet deep, providing about 700 gallons of capacity. Slurry occupies about 600 gallons of this biodigester; the remaining space is for the gas that’s produced.
There’s an inlet for adding feedstock and an outlet for removing composted slurry.
A closed loop of PEX tubing in the bottom of the tank is plumbed to an on-demand water heater to add heat when the slurry temperature drops below 50°F—the temperature at which cryophilic methanogenic bacteria go dormant and stop producing gas.
If the climate is mild, it may be enough to build a hoop house over the tank to keep the slurry sufficiently warm in winter. Alternatively, the biodigester could be allowed to go dormant during the colder months.
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Biogas digester troubleshooting
Anaerobic organisms require four conditions to produce gas effectively:
Too acidic. Generally if there’s a problem, it’s that the slurry is too acidic (pH below 7). If there is a lot of new, raw, green material placed in the digester or if too much material is added at once, the acid-forming bacteria have a field day. The methane bacteria are so annoyed by the high acid concentration, they simply can’t function. When this occurs, it can take a long time for the methane process to get underway naturally. Low pH is a constant risk and must be countered by plenty of carbon waste, such as leaves and straw, or wood ashes.
If a measured amount of new material—no more than one-fortieth of the total liquid volume of the tank—is added, then the new material has to be dilute enough not to upset the balance. At startup, though, there’s a lack of microorganisms and an inclination toward excessive acidity. Understanding this, we can see why some of the early literature on making methane states that the startup time can be anywhere from three weeks to three months.
I mentioned the acidity problem to a friend with whom I was working at the time. He said, “I make a lot of wine at home. Every once in awhile, I have the same problem. When I do, I add a little baking soda. It straightens out the condition right away.”
The baking soda added to my digester worked like a charm. Within three days, I had methane on the way. This is the secret for keeping your digester sweet and happy. Just add a little at a time until the pH is just right. If the pH keeps dropping, add baking soda periodically until the acid-forming bacteria are no longer producing excessive acid. Don’t be fooled if a lot of gas is produced. The baking soda itself will produce some carbon dioxide.
If the pH gets so low the digester “sours,” it is very difficult to revive and must be pumped out by a septic service and restarted. The Maitreya digester has not experienced this problem.
Too cold. You’ll need to know how hot the tank is, day to day, season to season. To eliminate the guesswork, install sensors both inside and outside the tank. Record these temperatures over a period of time. Then you will know how efficiently the tank is retaining heat, at what rate the temperature drops when no heat is added, and how much energy is needed to raise the temperature. If this is done, then a reliable calculation can be made of how much heat is needed to maintain working temperature if “free” heat is not available. Heat conservation, more than any other factor, determines whether a methane system will “fly” or not.
More info: http://www.homepower.com/articles/home-efficiency/equipment-products/home-cookin-homemade-biogas
- A constant temperature between 95 and 100°F (35 and 38°C). Gas is produced at lower temperatures, but only in very small amounts over an extended period of time.
- The exclusion of oxygen. A septic tank is not designed to keep out oxygen. Granted, some oxygen is excluded by virtue of the water in which the organic matter is transported, but a septic tank is not a sealed system.
- A gentle mixing action. The bacteria that produce methane either have to be transported to their nourishment or their nourishment has to be taken to them. Gentle motion accomplishes this.
- Anaerobic digestion will occur best within a pH range of 6.8 to 8.0. More acidic or basic mixtures will ferment at a lower speed. The introduction of raw material will often lower the pH (make the mixture more acidic). Digestion will stop or slow dramatically until the bacteria have absorbed the acids. A high pH will encourage the production of acidic carbon dioxide to neutralise the mixture again.
Too acidic. Generally if there’s a problem, it’s that the slurry is too acidic (pH below 7). If there is a lot of new, raw, green material placed in the digester or if too much material is added at once, the acid-forming bacteria have a field day. The methane bacteria are so annoyed by the high acid concentration, they simply can’t function. When this occurs, it can take a long time for the methane process to get underway naturally. Low pH is a constant risk and must be countered by plenty of carbon waste, such as leaves and straw, or wood ashes.
If a measured amount of new material—no more than one-fortieth of the total liquid volume of the tank—is added, then the new material has to be dilute enough not to upset the balance. At startup, though, there’s a lack of microorganisms and an inclination toward excessive acidity. Understanding this, we can see why some of the early literature on making methane states that the startup time can be anywhere from three weeks to three months.
I mentioned the acidity problem to a friend with whom I was working at the time. He said, “I make a lot of wine at home. Every once in awhile, I have the same problem. When I do, I add a little baking soda. It straightens out the condition right away.”
The baking soda added to my digester worked like a charm. Within three days, I had methane on the way. This is the secret for keeping your digester sweet and happy. Just add a little at a time until the pH is just right. If the pH keeps dropping, add baking soda periodically until the acid-forming bacteria are no longer producing excessive acid. Don’t be fooled if a lot of gas is produced. The baking soda itself will produce some carbon dioxide.
If the pH gets so low the digester “sours,” it is very difficult to revive and must be pumped out by a septic service and restarted. The Maitreya digester has not experienced this problem.
Too cold. You’ll need to know how hot the tank is, day to day, season to season. To eliminate the guesswork, install sensors both inside and outside the tank. Record these temperatures over a period of time. Then you will know how efficiently the tank is retaining heat, at what rate the temperature drops when no heat is added, and how much energy is needed to raise the temperature. If this is done, then a reliable calculation can be made of how much heat is needed to maintain working temperature if “free” heat is not available. Heat conservation, more than any other factor, determines whether a methane system will “fly” or not.
More info: http://www.homepower.com/articles/home-efficiency/equipment-products/home-cookin-homemade-biogas
Lab scale biogas digester research
Abstract. The effectiveness of cow dung for biogas production was investigated, using a laboratory scale 10L bioreactor working in batch and semi-continuous mode at 53oC. Anaerobic digestion seemed feasible with an organic loading of up to 1.7 kg volatile solids (VS)/L d and an HRT of 10 days during the semi-continuous operation. The averaged cumulative biogas yield and methane content observed was 0.15 L/kg VS added and 47%, respectively. The TS, VS and COD removals amounted to 49%, 47% and 48.5%, respectively. The results of the VS/TS ratio showed very small variation, which denote adequate mixing performance. However there was some evidence of ammonia inhibition probably due to the uncontrolled pH employed. The data obtained establish that cow dung is an effective feedstock for biogas production achieving high cumulative biogas yield with stable performance. The future work will be carried out to study the effect of varying organic loading rate on anaerobic digestion of cow dung in a semi-continuous mode.
Read full paper here: http://www.arpnjournals.com/jeas/research_papers/rp_2012/jeas_0212_635.pdf
Read full paper here: http://www.arpnjournals.com/jeas/research_papers/rp_2012/jeas_0212_635.pdf
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