Biogas :: The making of ...
A few weeks ago I went to a Biogas workshop which was held in an area not far from us. Here we learnt how to make a bio digester using commonly available materials.
For those of you who may not be aware, a Biogas digester is a device that is used to generate biogas (a mixture of methane, CO2 and hydrogen sulphide) from organic waste material. Biogas can be used for cooking, heating, lighting or even running a heat engine to generate mechanical or electrical power. About 2kgs of organic waste material is required to create about 1m3 of biogas. This could run one average size cooking burner for about 3 hours. It depends on various factors - some waste streams can generate far more gas than others, and some anaerobic bacteria better at producing methane than others. For the sake of this article, the above approximations hold roughly accurate.
The digestor has two main components, the digesting compartment (bottom tank) and the gas collector (top tank). In some cases these two tanks are combined into a single tank with a seperate gas collection chamber.
The gas is generated in the process of breaking down the organic material. It bubbles up in the digestor tank and is collected in the top tank. As the gas collects it displaces the water out of the top tank. This forces it either to rise up or (if it is fixed in place) will force the water down. The maximum pressure achievable under this design is the height of the water displaced. Once all the water is displaced, the biogas generated will just bubble out the sides of the top tank and escape into the environment.
Although methane is many times worse a greenhouse gas than CO2, it is important to note that decaying organic material in low oxygen concentrations already generate this gas. The process of harnessing this gas and burning it changes this methane into carbon neutral CO2.
The steps to making a biogas digester is as follows:
1/
First get 2 plastic water tanks, one larger than the other. In this case one was a 1000L and the other was a 2000L tank.
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Cut the top off the smaller tank first, this will be the gas collector of the bio digester.
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Invert the smaller tank over the larger one and mark out the perimeter with a marker. Try to get this as central as possible so it doesn't look weird.
4/
Lay the larger tank on its side and cut out the hole that has been marked. You can utilise power tools or manual alternatives - plastic isn't difficult to work with. In this case pilot holes were first drilled and then a jigsaw used to cut along the line.
5/
Now that the hole has been cut in the larger tank, a hole saw is used to create two large holes at the bottom of the large tank. Standard PVC drain pipes are used to create the inlet and outlet points as shown below. The hole saw size is chosen to a specific diameter that is required for the rubber seal to keep things tight and water proof. The workshop attempted to utilise commonly available materials and fittings to keep costs down.
6/
The smaller tank is also prepared with two small outlets at the top, again using standard PVC (irrigation) fittings. The gas that exits the top of the digestor is at very low pressure (no more than 2-3m of head of water), the pipe can easily do 30x that pressure. The reason why there are two outlets in this case is to stop the smaller tank falling into the larger tank, which apparently is very difficult to get out again - the elbow fittings stop this from happening.
7/
The larger tank is then carried to the site where it will be placed. It is then filled with garden waste and inoculated with anaerobic bacteria to help get the process started. This tends to occur naturally given enough time, however seeding the tank with this bacteria means the tank can quickly startup operation and become usable. Water is filled into the tank and the smaller gas collector tank is placed into the larger tank.
You will note that in this case, the bottom digestor has a diameter that's quite a bit larger than the top gas collector. This is not ideal as there will be side leakage of the generated gas. It is the price one has to pay for using standard available materials to do the job - rather than custom making the tanks. The gases escaping this particular digestor is however a small percentage of the overall gas collected. This setup uses a floating tank system, so the top tank will rise up as more gas is collected - and drop down again as the gas is used - so it is a bit like fuel gauge. Placing some weight on the top of the tank will pressurise the system if the gas needs to be delivered a longer distance.
There will be a follow up workshop at some stage in the future where we learn about how this digestor is performing, and perhaps even cook something on the BBQ from it.
I look forward to it ... :)
For those of you who may not be aware, a Biogas digester is a device that is used to generate biogas (a mixture of methane, CO2 and hydrogen sulphide) from organic waste material. Biogas can be used for cooking, heating, lighting or even running a heat engine to generate mechanical or electrical power. About 2kgs of organic waste material is required to create about 1m3 of biogas. This could run one average size cooking burner for about 3 hours. It depends on various factors - some waste streams can generate far more gas than others, and some anaerobic bacteria better at producing methane than others. For the sake of this article, the above approximations hold roughly accurate.
The digestor has two main components, the digesting compartment (bottom tank) and the gas collector (top tank). In some cases these two tanks are combined into a single tank with a seperate gas collection chamber.
The gas is generated in the process of breaking down the organic material. It bubbles up in the digestor tank and is collected in the top tank. As the gas collects it displaces the water out of the top tank. This forces it either to rise up or (if it is fixed in place) will force the water down. The maximum pressure achievable under this design is the height of the water displaced. Once all the water is displaced, the biogas generated will just bubble out the sides of the top tank and escape into the environment.
Although methane is many times worse a greenhouse gas than CO2, it is important to note that decaying organic material in low oxygen concentrations already generate this gas. The process of harnessing this gas and burning it changes this methane into carbon neutral CO2.
The steps to making a biogas digester is as follows:
1/
First get 2 plastic water tanks, one larger than the other. In this case one was a 1000L and the other was a 2000L tank.
Cut the top off the smaller tank first, this will be the gas collector of the bio digester.
Invert the smaller tank over the larger one and mark out the perimeter with a marker. Try to get this as central as possible so it doesn't look weird.
4/
Lay the larger tank on its side and cut out the hole that has been marked. You can utilise power tools or manual alternatives - plastic isn't difficult to work with. In this case pilot holes were first drilled and then a jigsaw used to cut along the line.
5/
Now that the hole has been cut in the larger tank, a hole saw is used to create two large holes at the bottom of the large tank. Standard PVC drain pipes are used to create the inlet and outlet points as shown below. The hole saw size is chosen to a specific diameter that is required for the rubber seal to keep things tight and water proof. The workshop attempted to utilise commonly available materials and fittings to keep costs down.
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| Note the use of a plug-in powered drill (rather than a cordless one) - this is because the hole saw is sizable and requires a significant amount of grunt to work. Careful however when the saw first catches on the plastic as it can twist the drill significantly and may injure your arm if you're not expecting it. |
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| An elbow with a screw on side cover outlet is used in the event that a blockage might need to be cleared. |
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| An additional valve is added in the event that lower tank needs to be drained for whatever reason. |
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| Here you can see the inlet pipe complete. The top of the pipe is fitted with a standard drain funnel that helps when adding new waste material in. The waste is mixed with water in a 1:1 proportion, so it flows well and maintains a good mix of water in the system. |
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| The process is repeated on the opposite side of the tank to create the outlet pipe, shown here. The liquid that exits the tank (called the digestate) is highly nutritious for plants, having ideal NPK (nitrogen, phosphorus and potassium) ratio mix that is very compatible for direct absorption by the plant's roots. |
6/
The smaller tank is also prepared with two small outlets at the top, again using standard PVC (irrigation) fittings. The gas that exits the top of the digestor is at very low pressure (no more than 2-3m of head of water), the pipe can easily do 30x that pressure. The reason why there are two outlets in this case is to stop the smaller tank falling into the larger tank, which apparently is very difficult to get out again - the elbow fittings stop this from happening.
The larger tank is then carried to the site where it will be placed. It is then filled with garden waste and inoculated with anaerobic bacteria to help get the process started. This tends to occur naturally given enough time, however seeding the tank with this bacteria means the tank can quickly startup operation and become usable. Water is filled into the tank and the smaller gas collector tank is placed into the larger tank.
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| A great group effort! |
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| The digestor gets its first feed and seeded with anaerobic bacteria. The inoculant was brought along by the guy running the workshop. It basically contained the digestate material from one of his other digesters, plus some secret herbs and spices?? He did suggest that bacteria used in sour dough could also be used. |
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| Just add water ... |
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| And finally, the smaller tank used as a gas collector goes on the top. Water is added via the inlet pipe until the tank is full. Note how the plastic elbows stop the top tank dropping into the bottom tank. |
There will be a follow up workshop at some stage in the future where we learn about how this digestor is performing, and perhaps even cook something on the BBQ from it.
I look forward to it ... :)
Hemon, I'm guessing that for optimum biological activity the digester should be kept warm but not allowed to get too hot?
ReplyDeleteWill all solid matter be digested in time or will there be a gradual build up of sludge at the bottom that has to be emptied?
In the diagram at top it looks like the digester is buried in ground but that would make it difficult to remove sludge if required.
What about the need to occasionally stir the contents so that bacteria have access to nutrients?
Ewen, apparently digesters in tropical areas have a increased rate of reaction and generate the gas much faster (though I'm not sure if they generate more gas or not for the same kg of feedstock) - so there is a temperature dependence. Not sure how hot the bacteria can survive but if the tank isn't insulated from the ground it will be at a relatively constant temperature if buried. One might choose to insulate the digester so that the heat released from the reactions sustains a higher rate of reaction as the temperature builds up? Or as suggested put the digester in a green house - then use the digestate to fertilize the plants in that green house, and biogas to bring light and heat to the green house at night :)
DeleteApparently all solid matter will evntually become suspended solids which will exit the digester in solution. It depends a bit on what you feed it, if there is too much woody stuff or earth in the mix you might have to get it pumped or drained. The moral of the story, keep the undigestables out and you shouldn't have too much to worry about. Pumping out a buried tank isn't difficult, just like buried septic tanks. However the problem with burying is that it needs to be strong enough for it - plastic may not be rated for this if not designed properly - especially seeing that the tank is weakened by the cut made in this case. A purpose molded plastic tank with appropriate bracing will be better buried - or perhaps a concrete tank for the digester with a plastic collector.
I think stiring might help but isn't necessary. Some researchers try to increase the rate of methane production for a better payback, however keeping things untouched seem to have decent enough rates?? (not sure). I think the larger biodigestor I visited in canterbury did have a stirrer in it but I think that was to break the surface tension of the top surface so that the methane can escape the liquids - in that case however they were using cow dung which has a low calorific value compared to raw undigested material (cows are very efficient, and some of that methane is released by the cow before it comes out).
I suppose in the end all the feed will be consumed given enough time. You could also use different bred bacteria for more efficient production - but again keeping things simple seems to be good enough not to bother. Anything with high sugars (fruits and veges from the kitchen or orchard) is ideal for higher methane production.
Will be good to see the outcome of the digestor once it gets going propely at out next follow up.
Hi Hemon, fascinating post. Question though, if the bottom tank is full of water, how does source material get in? As in, wont the inlet pipe be full of water and so anything placed in the inlet pipe just slowly sink down? Or is that exactly how it works?
ReplyDeleteHi Michael, thanks! The water level in the input pipe is as high as what is set by the output pipe level - so yes the way it works is that the whole bottom chamber is filled with water. When you add feedstock material in a 1:1 mix with water, you are putting in more fluids into the digester which then exits slowly at the output. The solids end up settling down where it starts breaking down. The output starts at the bottom to encourage suspended solid escaping the outlet with the outgoing fluids - which is the reason why the tank doesn't really need pumping out much. Also if the gas collector is fixed, any displaced water will also just exit the outlet.
DeleteAnother great article, Hemon!
ReplyDeleteIn researching biogas, I found a lot of references to farmers in Vietnam using it. Then I found this interesting paper on an alternative approach:
http://www.agrowingculture.org/2012/08/an-innovative-way-of-raising-pigs/
They suggest farmers use the feedstock in gasifiers instead of biodigesters, and go on to detail a whole system of yields that can flow from their model. It's not for everyone, but it's worth a read for some fascinating ideas.
Hi Darren, thanks for the link - I'll be sure to read it some times. I can see that gasifiers are useful for woody feedstocks where the biodigesters aren't able to work, and would require much less (a heck of a lot less) water - hence could be beneficial for just those reasons alone. They also create the materials for biochar production, so can be beneficial for the land as well. I'm certainly interested in gasifiers as well though - one step at a time ;) --- one thing I didn't want was only a gasifier as I would not feel comfortable piping CO into the kitchen for cooking. An external burner to heat water or running a genset might be worth considering however, but the gasifier route is more complex and hence will tend to be costlier as well.
Delete