“Biogas is the perfect solution to modern sustainable cooking.”
Biogas is a mixture of gasses that are produced as anaerobic bacteria break down organic matter. It is flammable, capable of producing a clean flame for cooking and heating, and it can be derived from as simple a feedstock as your kitchen rinse water.
A biogas digester is a container holding water and bacteria, which can be ‘fed’ organic matter to be broken down, and which collects the resultant biogas, to be tapped off either to a collector for storage, or directly for burning.
- Creates flammable gas
- Creates fertiliser
- Runs on kitchen rinse water
- Safe and Clean
- 200L blue plastic barrels
- plastic garbage bin
- PVC pipes and fittings
- PV hoses and fittings
- Silicone sealer
Filling my Biogas Digester
Feeding my Biogas Digester
Emptying my Biogas Digester
What else can I use to make a biogas digester?
How much scraps for how much biogas?
Will the biogas collector explode?
In March 2011 I set about designing my own home-scale biogas digester, and biogas collector, using 200L (44 gallon) blue plastic barrels. It was my aim to create a system capable of producing enough biogas to cook one meal per day, and therefore become the dominant energy source for cooking at my home.
A friend of mine had collected a number of 200 Litre blue plastic barrels from a local hospital, which had been used to store cleaning chemicals. They were sealed, with two screw-in lids at the top, and a slight dome shape top which became an important factor in my design.
A biogas digester requires an input pipe (a place to deposit fresh feedstock), and an output pipe of some kind (for contents to be displaced out of). Since there were already two screw-in lids which sealed perfectly, I decided to take advantage of them, and designed that both my input and output pipes should enter the barrel through the top of the barrel, through the lids, and the biogas output should also be located at the top of the barrel.
My design was different to others I had seen, and I wanted to be sure it would work, so I created a prototype biogas digester and collector using plastic bottles and straws. I mixed white vinegar with bicarb soda to create (rather quickly) carbon dioxide gas, to test how the physics of the system might work. This gave me a good platform of understanding, and I felt more confident to trial my design using the blue plastic barrels.
My biogas digester was designed to be gas-tight, so that no biogas could escape the unit except through the gas-out valve, which was a simple polytube irrigation valve. However, if for some reason pressure should build up in the system, it would simply push the contents out through one or both of the input/output pipes.
The input pipe ran through one of the lids of the blue plastic barrel, and extended almost to the bottom of the barrel. This meant that fresh feedstock would enter the digester at the bottom. The output pipe ran only to the centre of the barrel, which meant that as fresh feedstock entered, existing liquid would be displaced from the centre of the barrel, and exit the unit into an effluent bucket. Since solid particles of organic matter usually either float or sink, it would be mostly only liquid which comes out as effluent, leaving the larger particles in the digester to break down further.
I was able to set the output pipe almost at the level of entry into the barrel, which left a pocket inside the dome of the top of the barrel for biogas to collect, and it was at the very top of the barrel that I fixed my gas-out valve. This meant that at any given time there would be very little biogas stored in the digester – the digester would be almost completely filled with liquid, and almost all biogas would be expelled via the gas-out valve to the collector.
The collector was simply a blue plastic barrel with its top cut off, 3/4 filled with water, with a plastic garbage bin inverted and submerged, with a gas input and a gas output valve fitted to it. Biogas produced by the digester would collect in the garbage bin, and as the garbage bin filled, it would rise out of the water. If the unit became too filled with biogas, biogas would simply leak out the side.
Filling my biogas digester
I filled my biogas digester with water and about 20kg of cow manure. The idea is to breed the various kinds of intestinal bacteria from the manure, so that they can process feedstock the same way they do in a cow’s intestines – breaking it down into fertiliser and creating flammable gases. It took a week or so for the bacteria to settle in and begin digesting feedstock.
Feeding my biogas digester
Feeding my biogas digester is easy. I shut off the gas-out, and open the lids on the input and output pipes. As I pour my kitchen rinse water in, an equal volume of effluent pours out into a bucket, which I then take and pour into my garden.
Biogas is fart gas, the same as what a cow produces! It smells as you’d imagine, but when it is burnt the smell is the same as from a clean natural gas burner.
Burning biogas alone produces a large yellow sooty flame, which will turn the bottom of pots black, and will not heat very efficiently. It is when air is allowed to mix with the biogas just before it is burnt that a hot, clean, blue flame is produced. For this I created a bunsen-burner, to test the variables involved.
My bunsen burner is made from scrap pvc pipe, a spare polytube sprinkler fitting, and some circular wooden doweling fashioned into a gas restrictor. I made a few of these restrictors with different sized holes to experiment with, and drilled holes through a pvc collar into the main pipe, which allows air to enter and mix with the stream of biogas – also the collar can be rotated to limit air flow. It took me a few tests to find the most efficient combination of factors to get a nice hot flame.
I attached my home-made burner to the bottom of an old gas barbecue. Using the existing supports I can cook in a pot over the biogas flame. I place a weight of either 5kg or 10kg on top of the biogas collector bin to apply some pressure to the biogas. The 5kg weight allows for a modest flame capable of boiling a litre of water in about 15 minutes. The 10kg weight forces biogas out quicker, and a litre of water can be boiled in 10mins.
Emptying my Biogas Digester
During it’s first winter, I gave my digester diarrhoea! I had been feeding it fermenting/mouldy bread and I poured in some bokashi juice too. The bacteria inside the digester became unbalanced, and the wrong kind took over. Biogas production ceased and the effluent smelt like sickly diarrhoea. I decided I had to empty it and start over.
Most digester designs include a ‘sludge-out’ pipe toward the bottom, but mine does not have this. I developed a trick to displace most of the contents of my digester, so that the unit can be moved and emptied easily.
I disconnected the gas hose between the digester and the collector, leaving one end attached to the digester. By blowing into this hose, the pressure forces effluent out into the effluent bucket. 5L at a time I emptied bucket loads of diarrhoea into the garden. Since the PVC output pipe only extends halfway down the height of the barrel, I was only able to displace half the contents in this way, which left approx. 100L left in there. At this point a hand trolley could be used to shift the unit. I didn’t have a hand trolley, so I swapped the PVC input funnel fittings for the output pipe fittings and kept on blowing. Since the input pipe extends almost to the bottom of the unit, I was able to expel everything but the sludge at the bottom. Now I could unscrew the input and output pipes, and the gas-out valve and tip the barrel upside down into the garden. I used the ‘jet’ function on the hose to blast the sludge off the bottom.
I even went to the extent of flushing the barrel with eucalyptus oil and detergent, to clean and disinfect the unit. After much slushing and rinsing, I returned the unit to its corner and refilled it with cow manure and water. Biogas ensued for a couple of weeks, but the unit contracted diarrhoea again! It was still winter, so I’m not sure if temperature had anything to do with it. I gave up on my digester and left it. After some weeks of neglect, I noticed my collector had risen. I did a flame test and confirmed biogas was being produced. I checked the effluent and it smelt like good ol’ fresh cow manure again! I was back in business. I still don’t know exactly what happened, but the bacteria sorted itself out.
My biogas digester project was successful; the digester is capable of producing more than 30mins worth of biogas per day, and the collector can just hold this amount of biogas. However, I have decided not to go to the trouble of grinding/chopping/blending/mincing up kitchen scraps to fully feed the digester, and have instead been feeding it the rinse water from my kitchen which amounts to about 4L per day – a mix of water, liquids and food particles from rinsing cups and plates. From just this feedstock I have been generating about 15mins worth of biogas per day – enough to steam some vegies in the morning and make a cup of tea.
My system is limited by the collector – it can only store about 30mins worth of biogas, while the digester itself can produce more than double this if it is fed substantially. For my purposes, a larger collector is unnecessary, but I encourage anyone planning to do all their cooking on biogas, to consider multiple collectors or scaling up using larger containers.
Frequently Asked Questions
What else can I use to make a biogas digester?
Any large plastic container is fine. Do not use steel drums nor any metal parts for your design, as the hydrogen sulphide component of biogas will quickly corrode these parts. So far I’ve seen no deterioration of any of the parts I’ve used (PVC, Polyvinyl, Polyethylene) in contact with biogas and/or effluent. Consider olive barrels, or for a larger scale digester consider 1000L tanks (with cage).
How much scraps for how much biogas?
If, today, you ground/minced 1kg of mixed food scraps, and poured it with water into a digester, 24 hours later you would have enough biogas to cook for an hour. The next day you would still be collecting gas from this feedstock, so there is an overlapping return as you feed each day. I personally choose only to feed my digester the rinse water from our kitchen, which is about 4L per day, and this yields enough biogas to boil 1L of water once per day – enough to make tea or steam vegies. If I don’t cook on biogas one day, the next day I have enough to cook pasta or rice.
Will the biogas collector explode?
No. To explode your collector you would need to 1: mix air into the collector, 2: put the collector under pressure and 3: somehow create a spark inside. If for some reason you feel that these three events would occur simultaneously with your biogas digester, I recommend you do not persue this project. My collector is gas-tight, only under pressure when I’m cooking, and I couldn’t create a spark in there if I tried.
Well my friends, it has been more than two years since I first designed my biogas digester, and I consider it to be one of my most successful projects. I still use it every day to make my morning coffee:
This was a cheap portable gas stove, which I found in a dumpster bin (just when I was about to buy one!). I pulled out all non-essential parts and fed some polyvinyl irrigation tubing into the burner. Now I have a nice round flame to cook on, instead of a single bunsen-burner flame. I usually put an old tin can around my coffee machine (as a ‘pot skirt’) to keep the heat in, and act as a wind-break. I haven’t noticed any loss of efficiency between this burner and the last.
I’m still feeding my digester with rinse-water from the kitchen. When we wash our bowls and plates, first we rinse them and pour the rinse water into a bucket (no soap or detergent). Once the bucket is full, I pour it into the digester. I have daydreamed of an integrated system, where all water from the kitchen sink pipes directly into a digester, but for now I am used to the extra effort.
I pour the effluent around fruit trees in the backyard, and through summer last year this was all the watering they got. I haven’t noticed any ill-effects, and I’ve even poured the effluent on young seedlings and seen them flourish. I’m convinced it is a purely beneficial fertiliser.
I did an interesting test a few months ago, where I left 10kg weights on top of my collector (I usually only put them on when I’m cooking), and found the digester was able to handle this back-pressure, and the collector still rose. I then had an unrelated problem and stopped the test, but I’ll have to try again, because if the system can handle the back-pressure, then I won’t have to keep lifting the weights on and off. A minor convenience, but this would also mean that I could pipe the biogas all the way into the kitchen, and cook inside my house, without having to go outside and manage the weights. It also means I could make a second collector, and the weight of the garbage bin wouldn’t be a problem (the system might even be able to handle a third or fourth collector). I had just assumed that any amount of back-pressure would push effluent out, but it takes 20kg weights to do this. It was a question from a visitor which prompted me to do this test.
I’ve received a lot of comments and questions about my digester! I have been inspired by people’s ideas, and have tried to answer all questions as well as I can. I can safely say that everything I know about biogas and biogas digesters is here, in this article and especially in the comments. I have also been thrilled to have had Thomas Culhane visit and invite me to a facebook group which is all about biogas. It was his work on digesters which inspired my design. I encourage anyone who is interested in variations on design, dealing with cold winters, and community-scale digesters, or ANYTHING to do with biogas, to look him up on youtube, or join the conversation on Facebook.
If I modify my system in future, it will probably be to add an additional collector, so I can store more biogas and cook bigger meals. Along with this, I have an idea to design a bicycle powered grinder to mince up food scraps, so I can create more biogas. Perhaps the two projects will go hand-in-hand. I will post another update if I succeed.
Until then, thank you all for your support and interest, and good luck with your project!
Feeding a Biogas Digester (video)
Making a Coffee with Biogas (video)
“Involve different technologies into your backyard, by integrating.”