Biogas Digester

Design for My Biogas Digester

“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.

Benefits:

  • Creates flammable gas
  • Creates fertiliser
  • Runs on kitchen rinse water
  • Safe and Clean

Materials:

  • 200L blue plastic barrels
  • plastic garbage bin
  • PVC pipes and fittings
  • PV hoses and fittings
  • Silicone sealer

Jump to:

Planning
Designing
Filling my Biogas Digester
Feeding my Biogas Digester
Burning Biogas
Emptying my Biogas Digester
Summary
FAQ
What else can I use to make a biogas digester?
How much scraps for how much biogas?
Will the biogas collector explode?
Updates!
Videos!
Related Articles
Integration

Planning

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.

Designing

Design for my biogas digester, collector and burner

Design for my biogas digester, collector and burner

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.

The input pipe, gas-out valve and output pipe all fitted to the top of the barrel

The input pipe, gas-out valve and output pipe all fitted to the top of the barrel

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.

A prototype digester and collector, using vinegar and bicarb soda to produce and collect gas

A prototype digester and collector, using vinegar and bicarb soda to produce and collect gas

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.

The pink line shows the water level inside the digester. Note the gas-out valve is above this level

The pink line shows the water level inside the digester. Note the gas-out valve is above this level

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 (empty) is an inverted plastic garbage bin submerged in water

The collector (empty) is an inverted plastic garbage bin submerged in water

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

As I pour kitchen rinse water in, effluent is displaced into a bucket which goes straight on the garden

As I pour kitchen rinse water in, effluent is displaced into a bucket which goes straight on the garden

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.

Burning Biogas

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 pure (L) and mixed with air (R). The blue flame is clean and hot

Burning biogas pure (L) and mixed with air (R). The blue flame is clean and hot

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.

The holes allow air to mix with the jet of gas, after it passes through a restrictor

The holes allow air to mix with the jet of gas, after it passes through a restrictor

Gas restrictors made of wood. These force the biogas into a jet stream

Gas restrictors made of wood. These force the biogas into a jet stream

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.

A classic gas bbq, with biogas connected. A single clean flame heats a pot of water

A classic gas bbq, with biogas connected. A single clean flame heats a pot of water

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.

Summary

My biogas digester and collector full of gas

My biogas digester and collector full of gas

 

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.

Updates!

October 2013


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:

Modified

Camping stove modified for biogas, making my morning coffee :D

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!

Videos!

Feeding a Biogas Digester (video)

Making a Coffee with Biogas (video)

 

Related Articles

Integration

“Involve different technologies into your backyard, by integrating.”

 

78 comments on “Biogas Digester

  1. is there no gas flowing out through the outlet to the bucket together with the effluent?If no,how do you stop it from flowing in to that output but to the correct gas output?thanks

    • Hi Mokhele, thanks for your question. I think you are asking if biogas is escaping through the effluent out. It is not possible becuase the effluent pipe goes down into the digester, about halfway. Since the gas bubbles up to the top of the digester, there is no chance for it to escape. I hope this makes sense. I was not certain, so I did a test using bottles and straws.

  2. Hi sir how are you. I made biogas plant if 100 m3 capacity of plant . I want to use as a small genset for electrifity. I want clean biogas by removing g2s from biogas.could you please help in this matter. Thanks rahul dhawan

    • Hi Rahul,

      100m3 is a huge capacity! I have only learnt enough to make a home-scale digester (approx 1m3). I’m not sure what ‘g2s’ are. Biogas is made up of Methane, Carbon Dioxide and a small amount of Hydrogen Sulphide (which will corrode metal). If you want clean methane gas, you must take from the top of your storage (methane is lighter than CO2), and pass it through a ‘scrubber’, which is a pipe filled with steel shavings. The steel reacts with the hydrogen sulphide in the gas, then you will have pure methane. You can also bubble biogas through water which will react with CO2 to form carbonic acid, but this will require more pressure, and you must change the water regularly.

      I hope this helps.

      Good things,

      Shaun

    • Hi Chan, thanks for commenting.

      Good luck with your design, and let me know how it turns out – I would like to add a gas collector or two to my unit someday.

    • Hi Patrick. A biogas digester must contain water, feedstock (food scraps), bacteria (from manure) and very little oxygen (sealed container). If you have all of these ingredients at room temperature, you will produce biogas. Good luck!

  3. Shaun, very simple and informative article!

    I have a couple of questions. I am trying this in a miniature scale to see if I can produce gas at all.

    I am adding about 200 – 300 grams of mixed food and veg waste to a 2Litre bottle and fill it up with water to the top. I have a small tube with a valve attached to the cap (air tight). Here it is very sunny, so temo wouldn’t be a prob.

    1) Will this mixture work by itself? Or do I have to add manure like cow dung to start it off?

    2) I would like the gas it produces to burn for at least 10 seconds as proof that it worked. Will this setup produce at least such an amount of gas in 24hrs? If not, how long will it take?

    3) I do not intend to setup a secondary collector for now. I am going to let the 10second worth gas collect at the top of the digester bottle itself and release it and test it. Once it works, I plan to make a larger scale model with inverted bucket type collector. Do you think this will work for demo purposes?

    Thanks!

    • Hi Senthil,

      There are only some kinds of bacteria which produce methane gas. They are called ‘methanogens’ and they exist in a cow’s gut and also in other animals guts (even human). Manure is added to the mix to introduce these bacteria to the system. Without manure, other kinds of anaerobic bacteria will consume your food scraps, producing other gases which may not be flammable (such as CO2). If you don’t add manure I cannot guarantee success!

      Also, even if you do mix manure into the bottle, I cannot guarantee you will have flammable gas within 24hrs! There is a series of breakdown processes which occur before methanogens start working (yeasts ferment the food, making alcohols, acetogens turn alcohols into acetic acid, methanogens take acetic acid and produce methane gas). Digesters can take up to 2 weeks to produce methane. Up until then mostly CO2 is produced.

      I guarantee your project will work if you 1: mix cow manure into the bottle, 2: feed it a small amount to begin, 3: try burning the resultant gas each day until you see a flame. It could take a week.

      Finally, remember that biogas burns better when it’s mixed with air.

      Good Luck!

  4. Hi Shaun,

    Thanks for the information! I have created my setup, just waiting for manure; tough to find here in the city :D

    I plan to add 200g fresh manure + 200g veg and fruit waste. With some water for mashing them up, this should occupy about a litre. (Unfortunately, this is all the biomass I can get at the moment).

    I’ve decided to use a 20L bottle now. To remove air completely, I am going to fill up the rest of the 19L with water. Will this level of dilution cause any problems, other than possibly lower or slower yield?

    • Hi Senthil, sorry for the delay,

      If you cannot get cow manure, you can use humanure for your digester. I know, it’s a strange thought, but actually it is fine, so long as you are not sick at the time!

      You should know that methane gas is lighter than air, and air is lighter than co2. It’s likely then that methane should collect at the top of your container.

      Water is not a problem at all. The bacteria are ‘hydrophilic’, which means they love water, and operate well in water.

      Will you be collecting your biogas in a separate collector?

      Good things

      • Shaun, no problem!

        Yes, I’ve changed my plans; Now, digester is 20L bottle. Collector is a 1L bottle inverted on top of a 2L bottle containing water.

        I am thinking about the humanure thing; I am okay with it. But I’m wondering how my family will react, once they get to know what’s inside the bottle :D psychological thing. hehe.. :) But we have absolutely no problems with cow manure because handling it for household purposes has been part of our tradition for 1000s of years!

        I guess I am in a good position to get started now. Thanks for the help! I’ll let you know how it goes and post pics if possible.

        Cheers.

  5. Hi Shaun: I have just made a biogas digester based on your design. Just yesterday only i have charged it with cow dung and some rinse water. How much time it will take to produce gas after charging. I just wanted to know besides rinse water what else can be used as feedstock for the digester and what things i should avoid to put in. Also there is a little more space left in the digester after i charged it with cow dung and rinse water. is it okay to leave small amount of oxygen in the digester?

    • Hi Azmat,

      Thanks for your questions. They say it can take up to 3 weeks for a digester to start producing biogas. However, I had flammable gas 10 days after I filled my unit. I filled mine with one bag (20kg) cow manure and almost to the top with water. I started feeding it rinse water, which was about 4L per day, and I watched the collector for rising. First, co2 was produced, then biogas, so keep testing your gas with a flame until you see it burning. You don’t have to add anything to begin with – the manure and water will produce some biogas. The temperature will also affect the efficiency (warm temperatures work best)

      You can feed your digester much more than just rinse water. Rinse water gets me a ‘hobby’ amount of biogas – enough to boil some eggs and make a coffee each day, and it’s easy to collect and pour into the unit. You can grind up all your kitchen scraps, including bones, rotting meat, expired milk or cheese – all of the above. I have seen devices which fit under your sink drain and grind up anything that goes down. It’s best to mix the grindings with water and pour it into your digester. If you do this you will get far more gas – enough to cook on every day.

      If you decide to add ground/solid foods to your digester, you must be prepared that a sludge will build up at the bottom which will eventually block up your input pipe. Some people create a ‘sludge-out’ pipe at the bottom to empty their digesters. After more than a year of adding only rinse water to my digester, I haven’t had any problems with blockage (although I do add coffee grinds which might build up).

      The only thing I would caution a person not to put in a digester (though it may seem like a good idea at the time), is bokashi juice. I poured some in my digester thinking the anaerobic bacteria might add to the digestive bacteria in the system. My digester promptly stopped producing biogas, produced a big swell of co2, then the contents turned to diarrhoea!!! Those were dark times, and now I keep my bokashi buckets separate from my rinse water buckets.

      Finally, your digester will work best if it is filled with water (and some cow manure), but it doesn’t have to be 100% full, just so long as the pipes for input and output are submerged. You can be adding material for a while before the unit fills up and effluent starts displacing out. No problemo!

      Let me know how your system goes.

      Good things

      • Hi Shaun:

        Thanks for your detailed reply. I am now assured about my charging of the digester and if my digester is successful in producing the biogas all the credit goes to you, who designed the unit, and i have just implemented it based on it. Right now i have stopped feeding the digester until it rises with biogas. Being winter here it may take a bit more time than usual, but i will keep you informed how my digester is working from time to time and to get your inputs in managing it.

        I thank you again for your help and see you soon.

        Regards
        Azmat

  6. Hi Shaun,
    Thank you for your article. can we use poultry manure instead of cows or other mammals? do we need to have the manure fresh or we can use dry one?

    • Hi Mounes,

      Yes you can use chicken/poultry manure for your biogas digester. You can also use cow, pig or even human manure (humanure). I started my digester with cow manure which had been in a bag in the sun for a long time. It wasn’t completely dry, but almost dry. I don’t think dryness kills bacteria – they just go into stasis and wait to be moist again. I’ll bet your system will start fine with dry chicken manure. Let me know if it does!

        • Hi Scott,

          Not sure about that one. I suppose of you only fed chickens sterile grains, and never let them dig in the dirt, they may never ingest the right kind of bacteria.

          Nevertheless, whatever manure you use, its only there to inoculate your digester. After that, you don’t need to add manure again. I haven’t added manure to my digester for about a year.

          Good things

  7. Hi sir

    What the reason to collect bio gas in separate barrel with water? Any reaction to be take place there…. clarify me..

    • Hi Sabari,

      The main reason for storing biogas separately, is so the system has no smells. We have it right outside the house, and there is only smells when I feed the digester. There are some designs which have the collector bin inside the digester, but you will see the bin rise up with scum all around the sides. Flies would also be attracted to such a system. Also, having a separate container means the container can be larger, or there can be many containers storing gas from one digester.

  8. Good day Shaun,

    I do like the design as it is a solid drum and there will be no scum on the sides as you said.

    I have only one question to gas tightness. As the blue container has only 2 “Bung Tops” on top how do you make the gas outlet valve gas tight? You can’t put your hand in the drum. Do you cut a 1/2 inch thread in it?
    Cheers,
    Gus

    • Hi Gus,

      Thanks for your question. I’m not sure how standards vary for irrigation parts around the world, but I used a simple adaptor piece to screw into a hole I drilled in the top of the barrel, which then allowed the ‘gas-out’ valve to screw in nicely. The adaptor piece had a lip on it, and I was able to fit a rubber ring underneath the lip, such that when I screwed the adaptor into the barrel, the ring sealed it airtight.

      A couple things for your to consider. Since the gas is not under any pressure, a true gas-tight seal is not essential. Even sticky tape will allow you to test your digester. Also, if your parts are not exactly compatible, silicone can be used to seal all your seals. I had PVA glue first, so I could wet it and break seals to make changes, but after 6 months I sealed everything in silicone. Whatever you do, don’t use any metal parts, or the hydrogen sulfide in biogas will corrode the parts.

      Good luck!

  9. Dear Shaun
    thanks a lot for your informations.
    I am a MSc Student working on Biogas
    your weblog helps me a lot
    i will try to open a business based on biogas here in my country
    thanks agin

  10. Hi Shaun,

    We are conducting a project on biogas production preferably using a combination of cow dung and partially decomposed vegetable waste, and water in the ration of 1:1 in conical flask of 500 ml fitted with balloon. initially after 24 hours we found the balloons were inflated, but after 48 hours the balloon shrunk and after 72 hours there was inflation, and after 6 days the balloon was totally pulled (sucked)into the flask. can you please tell us why there was reduction in gas quantity on the 2nd day and total suction on the 6th day.
    smitha

    • Thank you Smitha, that is a fascinating experiment. Sorry for the delay, and I hope by now you have a balloon full of biogas!

      Once a digester is filled and fed, it can take up to three weeks before biogas is created. This is because different bacteria have to take turns digesting the feedstock, and methanogens (which create biogas) are third in line.

      First, yeasts consume the sugars in the feedstock, producing alcohol and carbon dioxide. This was probably the cause of the initial balloon inflation. This is the way beers and wines are brewed.

      Second, acetogens consume alcohols (and oxygen) and produce acids. This may explain the sucking in of the balloon. This is the way vinegar is made.

      Finally, methanogens are able to consume acids and produce methane gas, which is the flammable component of biogas.

      So you can see, methanogens have to wait their turn. Also, once methanogens are working, acetogens and yeasts are still working, which is why biogas is made up of methane and carbon dioxide.

      I hope this helps. It would be interesting to see if adding a bit of vinegar to the mix would allow methanogens to start earlier. Let me know how your experiment goes.

      Good thing

    • Hi Duncan,

      Yes, you can use a normal gas cooker, but you will have to remove the gas restrictor, which can usually be unscrewed. Make sure you can adjust the air intake, because biogas burns best when its mixed with air. Good luck!

  11. Hi Shaun,
    Thanks alot for your support.I just read in your article that this gas reacts with metal, wont it damage my gas cooker’s metal piping? The other question is why is your feed pipe funnel lidded?

    • Good thinking Duncan! Yes indeed the hydrogen sulphide in biogas will corrode many metals including brass, which is likely what your gas intake is made of. However, H2S is not corrosive to aluminium, so try protecting your metal with al-foil. Also, I don’t know how rapidly your cooker will corrode – it might not be an issue. You could experiment by putting some metal in your collector.

      For your second question, there is no need to have a lid on the feed pipe. If I forget to open the gas-out valve on the digester, biogas builds up and starts pushing effluent up both pipes. I did this once and at some time during the night the lid blew off and sank to the bottom of the collector! Also, effluent had been overflowing out the input pipe which made an ugly scene requiring a complete hose-down. I keep the lid on for tidiness and so as to not encourage flies. The lid on the output pipe is more useful, because it prevents any drips. With both lids in place, there can be no odours from the system, and the whole thing can be hosed down.

      • Thank you very much for the support. I am doing an experiment with 3 20litre cans based on your design. It is now 10 days old and hasnt seen any effect on the gas collector. Am trying to be patient.

  12. Hey man, great info. Im halfway through making a 1000ltr system, that i will be compressing into cylideres through the summer while its warm and methane production is high, and I will use the gas through the winter. I know the sulphur is corrosive to metal (my cyliders) Im trying to find the best way to SCRUB the gas so its (a) non reative and (b) more efficient. Im wondering what you think about water scrubbing for the c02 and swarf scrubbing for the sulphur, on a scale like 1000ltr …or event 4 or 5 1000ltr tanks, with metal cyliders and brass fittings?? Does this sound like a feasible eficient idea?, and could you foresee any limitations or problems?? BTW i have a bicycle geared up to a compresor moter and can store air in cyliders at High psi, so its all free energy hahaa

    • Hi Jayce,

      Awesome to read of your bicycle compressor. Be sure to stay within the psi limits of all your equipment (even hoses). As for your scrubber, bubbling through water to scrub co2 should be simple enough, because water is easy enough to reuse and replenish. I suggest dispersing the biogas as lots of small bubbles for a greater effect. Bear in mind too that biogas burns fine with the co2 left in, and so you will only be scrubbing to maximize your storage.
      As for hydrogen sulphide, you will have to find a regular source of an appropriate metal shavings, such as turnings from a steel fabricator. All you have to do is go overboard with your h2s scrubber, such that the shavings toward the end appear not corroded (measured against some control shavings). However, even if you regularly change out the shavings, I suspect some corrosion will still occur, just as some carbon dioxide will always be present after co2 scrubbing. This will only shorten the life of your set up, not render it useless.
      There was a man named Jean Pain who stored biogas in piles of car wheel tubes. There’s an interesting documentary about his methods somewhere.
      Also, there’s a group called ‘Solar Cities’ who have worked on using solar hot water systems to keep their digesters warm in winter.
      All the best, and let me know how your system works out!

  13. Hi Shaun,
    I’m encouraged & interested by your simple biogas design machine i just want to know if i can use a water buffalo(carabao)dung or horse dung in your biogas machine because there’s no cow dung in our village. Thank you very much

    • Hi Alfredo, Ive heard of cow, pig, sheep, chicken and human manure being used successfully. I will tell you a way you can test. If the animal’s fart stinks, the manure will work! This is because the sulphur smell of a smelly fart is a component of biogas, produced by methanogen bacteria. If the fart smells like sulphur, methanogens are at work! Interestingly, some humans have such unusual diets that methanogens never colonise their lower intestine! So human dung is not always reliable.

  14. Greetings Shaun :-)
    Many thanks for sharing your knowledge. I am considering adapting your design to make a continuously fed digester via a kitchen waste unit and a my toilet pan via a macerator pump [my diet makes me sulphur like ;).
    I want to ask,
    1. How should I size the system? Is there a rule of thumb you use?
    2. Should I plumb the sludge out into a composting bin to further break down the waste.
    I look forward to hearing your thought.

    Many thanks again

    • Hi Brookes,

      A continuously fed digester is a great idea. I would use gravity wherever possible, use large pipes for chunky inputs, and ensure I could flush/clean out the pipes if they become restricted. Check out IBC tanks.

      I dont have much experience with pumps, but I know pumps for greywater systems block up slowly, and you have to wear protective gear to clean them out/change filters. If there is a pump which can pump chunks without blocking up, that would be the pump id choose.

      As for toilet waste, i think its a great idea to collect what gas you can before passing it to the soil, but every precaution must be taken to ensure viruses and other pathogens cannot survive your process. Pumping sludge straight into a compost pile every time you use the toilet will not give the pile the time it needs to heat up, and sludge itself might be too wet to be composted immediately. If you wanna know about composting humanure, check out Joe Jenkins’ handbook.

      However, as with greywater systems, you could pump the sludge into mulch-covered trenches or basins in the yard. So long as nobody can accidentally come in contact with the sludge it should be an effective way to move the sludge on.

      Hope you get your system up and running. Let me know how it goes

  15. Hi, i have read your project and i learned so many things with your students, so me i would test it in my country (Nigeria) thanks.

    • Hi Shamsu,you’re most welcome. Good luck with your project in Nigeria. I hope you will share what you learn with everyone.

  16. nice job.just what i was looking for.i want to know if kitchen waste like rice,bean e.t.c can be added. the rinse water you added,does it contain soap?

    • Hi Ibrahim, you can grind up any kitchen waste, including rice, beans, bones, fish, old milk. If you are adding solids like this you will have sludge build up at the bottom of the digester, and you will have to empty the sludge. The rinse water I add doesn’t have soap, but a little soap is okay in a 200L digester. If you have a smaller digester, chemicals can cause more disruption.

  17. Hello Shaun
    I have read your project through and intend to build four Biogas digesters as a school science expo. In each I will decompose different Biomasses and compare the amounts of biogas produced. I have used your design in my planning and have decided on my own design.
    I will be using 20l paint buckets as the digesters and will fill them each with one of the following biomasses; horse manure, chicken manure,garden waste (grass and tree/leaf cuttings, and kitchen waste (food scraps)
    I am aware that a small amount of animal manure will have to be added to each digester to introduce the bacteria, and for this I will use fresh horse manure. (As I live close to a riding farm)
    Please could you advise on the correct mixture of waste to biomass I should use for my experiments. (ie. ? kg manure/waste + ? l water = most gas)
    I am using a 20 litre bucket and about a 5 or 10 l gas balloon for storage. Also how long do you suggest I should leave the digesters to produce the best results ?
    Thank You
    David

    • Hi David, what an exciting project!

      Your best friend for this project will be time – I hope you have at least some weeks to prepare. For an accurate demonstration of each biomass’s potential, you will have to have four stable digesters, and digesters can take up to a few weeks to stabilise (especially in cold weather).

      20L buckets is a good choice. I’ve seen biogas produced in a 2L bottle, and even inside a condom! But these systems will not necessarily stabilise.

      It’s true, if you throw manure, water and some kitchen scraps into a bucket and seal it, it will produce some biogas, but this is like throwing corn chips, cheese and salsa in the microwave and calling it nachos…

      My advice is to first prepare all digesters with water and horse manure only. You can add as much manure in as you like – less will take longer for the bacteria to multiply and fully occupy the digester, and more will likely get the digester producing biogas faster, but it then might take a long time to settle – so long as it settles it doesn’t matter. The digesters will first produce CO2, then go on to stabilise and produce biogas. Wait until the digesters stop producing gas before conducting your experiment. Don’t worry, I’ve left my digester for months without food, and it has happily produced again upon feeding.

      When you come to feed each digester with different materials, add no more than a handful of each, as any more could take a while longer to digest. A 20L digester should be able to handle a handful of feedstock well. Consider also that one handful of coffee grinds will produce a lot of gas – perhaps more than your balloon could hold, so you might like to have some time to optimise feed-in. Also it could take days for the digesters to finish processing their inputs, so allow for this time, and ensure the balloons don’t leak even slowly (like party balloons do).

      It would be interesting not only to know which feedstocks produce the most gas, but also whether some feedstocks produce gas faster than others. Nothing would say it better than time-lapse footage, but hey, now we’re dreaming!

      Good luck with your project, and let me know how it goes!

  18. Hi shaun, thanks for sharing this great knowledge, my question is, what are exactly ratios, cow dung to water and so on, after reading all your thread, you mentioned about putting 20 kilos of cow dung but didn’t say how many litres of water needed, will appreciate if you can point out about it,
    Maga

    • Hi Maga, thanks for your interest.

      Even a teaspoon of cow dung has thousands of bacteria. You could start even a large digester with this amount of cow dung. It will just take a LONG time for the bacteria to colonise the digester. So you see there is no necessary minimum.

      Also, you can fill your unit to the top with cow dung, then add only as much water as can fill the gaps, and your unit will produce biogas. This extreme will cause you problems because you cant add anything more! But you can see, to produce biogas there is no necessary maximum amount of cow dung.

      I started mine with a 20L bag of manure (in a 200L digester, so with about 180L water). But I could have started with half the amount or double the amount of cow dung. The dung is only there to inoculate the digester.

      Good Things

  19. hi Shaun, thanks a lot for ur great works. i just read it and i was inspired to try one. hope you get more ideas as time goes on.

  20. Hi Shaun,

    I was inspired my your model and went ahead and made one.But now I while reading your posts and comments again I realized I might have done some mistakes .
    1)I made the prototype with a 130 L plastic Drum and I left my gas collector tube open to Air as I dont want to collect from the initial stages.

    2)I have not filled up the cow manure and water up to the top .My outlet was above the level of the mix

    3) I have added the waste of grapes which was used for preparing wine.Will it make my drum acidic ?

    It has been more than two weeks now and when I am opening my inlet for feeding , it is producing rotten smell and I collected some slurry from the output pipe last day , it is of very foul smell.

    Could you please guide me where I have done wrong and what could be the corrective action ?

    • Hi Mat,

      Sorry to read of your troubles. I hope I can help.

      1) It is possible that with your gas-out valve left open, aerobic bacteria were able to populate your digester. I’ve never tried leaving mine open, but I do know that the kinds of bacteria which make your biogas are anaerobic. However, if this were the problem, if you close the gas-out valve, the anaerobic bacteria would take over and start producing biogas.

      2) I also didn’t fill my collector to the top with manure/water. Just enough to submerge the input/output pipes. After a week I was getting biogas. As I fed the digester I was slowly filling it up. It was good because I didn’t have to deal with effluent for a few weeks. I don’t see how you could have a problem there, so long as air is not getting in.

      3) I don’t recommend adding only one type of feedstock to a digester, unless the feedstock is only manure (which is a mixture of materials already). If you ate only grapes, you can imagine what will happen to you, and while it may seem like a silly suggestion, your digester is just like a stomach/colon! This doesn’t mean you need to feed your digester fresh organic fruits and vegetables to keep it healthy. They seem to work just fine with a mixture of kitchen scraps and rinse-water. I think you can still feed your digester lots of grapes, but you will have to add other things too, and perhaps less grapes than you first tried.

      I once fed my digester ‘bokashi juice’ from my fermented kitchen scraps, thinking that the extra bacteria might boost the digester. It turns out bokashi employs different bacteria, and it upset the balance in my digester, and gave it diarrhoea!!! The smell was terrible, and I tried emptying the digester, disinfecting, then starting again, but the problem came back! (disinfection failed). There was no biogas. I gave up, and stopped experimenting with the unit. After a month or so I noticed one day the collector was full. I did a burn test and it was biogas! I fed the digester, and the effluent smelled like fresh cow dung; balance had restored itself!

      I don’t know how lucky I was, but I suspect that if you add the right materials (mixed scraps, manure, water), and give it time, your unit will produce biogas.

      Good luck!

  21. Shaun, bravo! you seem not to seperate animal/ mammal waste from the process of making biogas. then am wondering why a landfill is able to produce gas in the methanogenic phase. this IS because landfills largely contain MSW and not animal waste.
    Also would like to know how often one should desludge.
    THANKS!

    • Hi Osbert,

      I have only used manure to start my biogas digester, and I haven’t added anymore, only kitchen rinse-water.

      Landfill gas is a complicated mix of gases, and different for different landfill sites. Even the temperature during the day will allow different biological activity to the night time, and so many different compounds and gases will be produced. I know there is methane produced in landfill, but I don’t know what are the best landfill materials for creating it. Some landfill sites capture the gases and use them to run the machinery at the site.

      You will have to empty your digester eventually, unless you design a ‘sludge-out’ valve/pipe towards the bottom. Sludge will build up as undigestible solids fall to the bottom, and they will build up much more quickly if you are feeding your digester a lot. Once the sludge blocks the bottom of the input pipe, you will have to empty your digester. You can buy yourself some time, by cutting the input pipe shorter, but the sludge will get there eventually, so having a method for emptying is recommended.

      With my design, you can empty much of the contents just by blowing into the gas-out pipe, back into the digester. This will push effluent out into your bucket. It may take a lot of back-and-forth with the bucket, but this will significantly reduce the weight of the unit (mine weighs 200kg+). Eventually you won’t be able to blow anymore out, and from there you’ll have to move the unit to a flat area of land, take out the pipes and pour it out. A hose helps to hurry the process along. It’s not a fun thing to do, and the smell is like fresh cow manure, but it’s excellent fertiliser, and you shouldn’t need to do it more than once every few years.

      Also, if you only feed your digester kitchen rinse-water, like I do, the sludge build up will take MUCH longer to block your system.

      Good things!

  22. Hello , …

    How to remove CO2 from Biogas ?
    Is there an easy and simple way to do this ?

    Sincerely , …

    • Hi Mohamed,

      There is a simple process for removing CO2 from biogas, but how to implement this process might be tricky.

      When biogas is bubbled through water, some of the CO2 reacts with water to produce carbonic acid. If you bubble biogas through enough water, you can remove ~all the CO2. This will make storage more effective.
      The problem is, to bubble biogas through water, you have to push the bubbles under the water pressure, and that water pressure will push back into your digester. The design for my digester cannot take a lot of back-pressure, either from the collector (which is why I always close the gas-out valve before I put weights on the collector), or from bubbling underwater (biogas goes directly into top of collector). Too much back-pressure, and effluent starts spilling out the effluent pipe.

      However, if you collect biogas first, then close the gas-out valve (from the digester), you could then apply pressure to the collector and push the gas through a CO2 scrubber. You might even be able to bubble it through the same scrubber twice or more, until you’ve eliminated the CO2, before storing it.

      Bear in mind you will have to change the water in your scrubber. Also, you should know that biogas burns just fine as it is. Scrubbing can save you ~40% storage space, but is not necessary.

      It is also true that methane is much lighter than CO2, and it might seem that methane will settle to the top of your collector, but I suspect these two are dissolved, much as how sugar can be dissolved in water (they do not separate easily).

      Good luck, and let me know how you are storing biogas, I am interested!

  23. Hey Shaun,

    Thank you for the great info and pictures. A few days ago I built a 200 L digester based on your design outside of Kathmandu, Nepal.

    I had a little trouble with regard to attaching the gas out valve. I haven’t been able to find the kind adapter you mentioned in the comment above, which allowed you to screw in the valve.

    So instead I drilled a 12 mm hole (slightly smaller than 1/2″), wrapped a 1/2″ hose barb with teflon tape to make it into a kind of plug, squirted that with some silicone, and then jammed it in. The result is flimsy but, for now, airtight.

    I have had difficulty finding standardized materials in Kathmandu. Can you suggest a better way to affix the valve using simple supplies?

    Best to you,

    Ben

    • Hi Ben,

      This type of digester is not designed to be under pressure, so even silicone will secure a gas-out valve, enough for the digester to function.
      To fix my gas-out valve, first I drilled a hole, smaller than was needed, then I used a sanding drill-bit to gently widen the hole, until the threaded piece could be screwed in tightly. I used a rubber ring around the piece, which was then squashed against the digester creating a seal. However, if you can force your gas-out valve piece into a tight hole, then silicone will seal it sufficiently.
      When I apply pressure to my collector (for cooking), I always have the gas-out valve on the digester closed. This ensures that pressure from the collector does not push back into the digester (which might cause the output pipe to overflow (messy!)

      I sealed all of my fittings first with a PVA glue, so I could wet it and break the seal to make changes. But I never had to make a change, so I committed to silicone for a more permanent seal.

      I hope this helps, and I hope you can get biogas all-year-round where you are!

      Good things

    • Hi Mwangi,

      The biogas in my collector is only enough to boil 1 Litre of water. If I finish all the gas, it will take 24 hours to fill the collector again. Does this answer your question?

      Yes, aged or dried manure still has the bacteria you need. It does not have to be fresh. I started mine with cow manure which was quite dry.

  24. Hi Shaun,
    Great information, thank you very much. I have a question, and a idea for your input when you get a chance.

    Q. How much pressure do you think the biogas can self pressurize an a tire tube with if you skipped the existing secondary gas storage before blowing water out the overflow?

    Idea on days you don’t need the gas, it would be cool to insert a tee and hook an old hand bike pump into the system. Then route that into to a tire tube. When you have extra gas, you or a family member could put a few pumps into a tube for compressed long term storage.

    Thanks,

    Roger Seher

    P.S. I just subscribed to you on youtube. Anyone who reads this should do the same:’)

    • Hi Roger,

      Thank you for your support :)

      With the gas-out valve on the digester open, I am able to place 15kg of weights on the collector, to create back-pressure on the digester. This did not cause effluent overflow, but 20kg would. Also, if I blow into the gas-out hose, I am easily able to cause overflow (I’ve used this method to empty the digester before). I’m quite sure this design will not suffice for inflating tyre tubes. However, additional collectors could be added, or a larger collector used, so long as the weight does not exceed this ~20kg capacity. I wonder also, if the output pipe could be extended, allowing perhaps ~25kg back-pressure?

      I like your thinking Roger! bicycle tyre tubes are something I could easily collect during rubbish collections (lots of people throw away bikes because they can afford cars). Even a bicycle itself could be used to compress biogas into the tubes for storage. This would also make biogas portable, perhaps for use with a camping stove.

      Also, because of your question, I have now discovered I can leave a 5kg weight on my collector, even when I’m not cooking, and the digester will still fill the collector without spilling. This will not only save me the effort of replacing the weight every time I cook, but it also means I can reroute biogas to my kitchen and cook without going outside, which is something I have hoped for.

      Thank you for sharing, and best of luck with your design!

  25. hello Mr.Shaun, me and my groupmates are having research 1 right now, we are BS Chemical Engineering students and we decided to fabricate a biodigester, using food and yard waste as a feed. and our design is inspired from your design, if you dont mind. but the problem is,in our study we should vary different pH’s in ranges 6.5-7.5. Do you have any suggestions on how would we able to control the pH? should we use lime or what? we havent come up with the title yet because we are kinda confused on how should we control the pH to rise or fall. and the deadline would be this coming august 20. Could we ask a help from you? thnx! :))

    • Hi Lissa,

      You can use any kind of vinegar or even lemon juice, to lower pH (more acid). And you can use lime, ash, pure soap or bicarbonate soda, to increase pH (more alkaline).

      Either way you will be causing imbalance in the digester (asking for trouble). If you make it too acidic, different kinds of bacteria will take over, you won’t produce biogas, and the contents will smell like diarrhoea! I dont know what will happen if you make to too alkaline, but im interested. Please let me know what you discover!

    • hello mr.shaun! thank you so much for the very informative respond. our study is that we are going to find out about at “what pH from range 6.5-7.5 would yield higher methane”.

      and also, is there a way on which we could measure the amount of methane produce?

      and if we install sludge out into our biodigester (since our feed is food and yard waste), does that also mean Sir that dung would be taken out? are we going to put a new dung again and wait for another 3 weeks?

      Good day-night sir

  26. good day mr. shaun :) my groupmates and i really appreciated your works in here.you’ve made us understand biodigesters easily as opposed to other sites..in our research 1, we decided to do it with biodigesters..would you oppose if we would model our biodigester after that of yours?thank you

    • Hi Mary, thank you for your comment. Feel free to base your design off mine; I would be honoured! Also, if you find any way to improve it, please let me know. Good luck

  27. Hello Mr. Shaun, i have a few questions.

    1.how do yo know if the collector is already filled with large amount of methane?

    2. if we install a “sludge output” on the biodigester, does it mean that if we remove the sludge, the dung will also be taken out of the digester? if that will happen, we’ll feed the biodigester again with dung?
    because our feed would be food and yard waste.
    If not, what is the best way to take out the sludge without taking out the dung?

    thank you so much sir.

    • 1. A burn test will show you if you’ve got biogas. Your digester will produce carbon dioxide first, then biogas once the methanogen bacteria start working. Remember biogas is a mix of gases, including ~40% CO2, so it will be difficult to measure exactly how much methane you have. However, if you measure the total amount of biogas, then deduct 40% you should have an estimate.

      2. A sludge out pipe could be messy. I would put it as close to the bottom as possible, and maybe dig a hole next to the digester so I could fit a bucket under the sludge out. Then I would take out the input pipe and use a long stick to stir the sludge around before I open the sludge out. Hopefully it would come out nicely without blockage. Yes manure would come out too, but remember the manure was only put in to introduce bacteria, and your digester should be filled with bacteria by now. I imagine you could empty half the digester, refill it with water and continue making biogas without a problem. Bacteria can double in a day, so at most your system might take a day before it starts producing again.

      Hope this helps

  28. thank you so much mr shaun. you’re such a great help to us. probably the greatest. The building would start next semester (Nov – March) and we would try to relay to you our experiments. thank you so much.

    hope your passion and kindness never tire! :)

  29. Hey Shaun its Me again, I really want to thank you for your insight into Biogas Digestors. If you remember, I conducted my Grade 9 science Expo on this topic… and I got into the county’s Regional Expo Competition for my school!! my research proved that Chicken Manure is the best biomass to use in digesters and produced the most methane.
    Although I used 20l digesters, I couldn’t create enough gas to cook with, (yet) but I aim to further my project one day. Maybe I could even power my house!!
    Hey, thank you again for your advice.
    David

    • Hi David, great to hear back from you, and congratulations! I hope my advice didn’t lead you too far astray, I knew you’d figure it out.

      I’m adding chicken manure to my system. I will post an update soon with my results, and what else I’ve learnt since this article was written.

      I love your ambition to run your home on biogas, that’s a nice vision to move toward.

      Good Things,
      Shaun

  30. Hi Shaun, great stuff, helped me understand the basic principles, I think! Very quickly, we bought ten acres of hillside in the Rift Valley a few months ago, great view, we want to turn it into a permaculture farm, water retention landscape, artists retreat cum yoga retreat (my wife is a sculptor and yogini, and my son a permaculture enthusiast) and general hippie hangout…. We thought immediately of biogas and talked to guys who are promoting dome underground systems, Africa Biogas Programme or something, then met a guy Dominic Kahungu who is creating systems using giant sausages of PVC, finally we heard about Takamoto, who’re doing systems using plastic tanks but they’re confined to a rural area the other side of Nairobi. Now I’ve read you, I think well DIY . Will keep you updated. If you should ever come out to Kenya, we’ll be happy to put you up and pick your brains ! It’s a wonderful country no matter what you may have heard. Our neighbors are Maasai pastoralists and we hope to introduce new ideas into the community while learning from them in turn. we asked our neighbor how many cows they had and they said, oh not many. How many ? Oh, just forty… Cheers

    • Hi Ali,
      This must be a very exciting time for you and your family! I have a friend in Kenya who has only told me good things.

      I can’t recommend a DIY biogas system enough, for the experience (and intimacy) before installing a larger/more permanent system. A DIY system doesn’t have to be connected to your home, and can become a demonstration model if you no longer need it. If you have the space, you could make a homescale system using IBC 1000L tanks. They’re used to transport chemicals internationally, so you should be able to find some wherever you are.

      You might even be able to pass your neighbours’ cow manure through your system for some extra biogas production!

      Good things

  31. Hello Mr. Shaun, is it possible to install a pH meter in in the digester? our design is based on yours.

    Do you have any suggestions on any research regarding anaerobic biodigester? we are having a hard time finding such.

    thank you.

    • Hi Lissa, thanks for your question. It would be interesting to see the changes in pH when a digester is first filled. I’ve only informally tested the pH of the effluent from my digester, finding it approximately neutral. I’m not sure how you could monitor pH in the digester, other than to test the effluent.

      As for research, I did a lot of searching online before I went ahead with my design. The best source of information I found was a fellow named Thomas Culhane on youtube. He’s explored many variations for home-scale and community-scale systems. There are industrial-scale systems in a few places in the world, and I’m sure their designs are based on strict science (and probably controlled inputs), but I’m not sure their data will be compatible with other systems.

      I find it quite amazing that such a diversity of micro-organisms can coexist in a sealed container. When a digester is first started, and food waste introduced, there must be quite a kerfuffle in there, before the situation settles. When in other areas of nature we see a tendency for systems to become more complex over time, it’s interesting to see that when the system is contained, after a kerfuffle, things tend to stabilise. I wonder if human systems will also reach a stability, once we’ve fully populated the earth!

      Anyway, good luck with your system!

  32. Hi Shaun, great post. We use insinkerator food grinders with our home biogas system and college biogas system (made from IBC Tote) and so we get lots of gas from the push of a button. I sympathized with your chopping up the food waste dilemma — we used to go through that 4 years ago when we started and then got the so called “garbage disposal” and life has been grreat since. We have a well attended open facebook group we would be honored to have you join our discussions in: Solar CITIES Biogas Innoventors and Practitioners. Hope to see you there sometime! Keep up the great work!

    • Hi Thomas! It is an honour to have your comment on this article! It is because of you that my digester and this article exist. It was your experimental design for a baby-poop digester, with all pipes connected through the lid which inspired my design. I was staring at my new-got blue barrel, with it’s two bungs and slightly curved top, trying to figure out how I could do the bare minimum to achieve a working digester.

      If I can add to your knowledge (which I’ve seen you share freely), I would only warn that if you ever come across the wonderful bio-technology called ‘bokashi’, which is great for preserving scraps without smells, it is NOT compatible with biodigester technology! The yeasts in the bokashi will throw out the balance in your system and give your digester diarrhoea!

      I have joined your group, and will encourage future commenters to do the same, as they have stretched my knowledge to its limit!

      Thank you for your efforts and your invitation.

      Good things

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