Biogas Digester




Intro: Biogas Digester

Part of the purpose of building the mobile food and apple grinder cart was to grind up kitchen scraps, garden leftovers, and even weeds for use in a biogas digester. I've been composting these things for years, but as I've read more about greenhouse gases and realized that methane is many times worse for the atmosphere than CO2, I began to think about capturing the methane that my household creates and doing something with it. I could just throw a tarp over the compost pile and light a match to the built-up gases every now and then, but a biogas digester would more efficiently convert the organic waste to methane, collect the methane, and provide a nutrient-rich compost liquid that I can use to water the garden.

Plus, I can use the methane to blow stuff up.

Part of the goal here too is to reuse materials I had cluttering up my garage and basement. Some I'd held on to with this project in mind, some I just happened to come across. Doing so probably wasn't cost-effective, given the number of plumbing adapters I had to buy to make X work with Y, but at least I cleared out some of the clutter. I won't get into too many specifics on dimensions for that reason - use your best judgment regarding materials if you plan on building your own digester.

Step 1: Prepare the Containers

I bought three different containers for this project: one with a removable lid, a 30-gallon drum, and a 50-gallon drum. All three are plastic (HDPE), and all three were sourced from a neighbor who specializes in second-hand barrels.

The removable lid container will be used for the digester itself, so it needs a feed pipe, a gas outlet, a drain, and an overflow provision. Both the 30- and 50-gallon drums will get their tops removed, and the smaller will fit inside the larger to trap the biogas.

Step 2: Add a Feed Tube to the Digester

I measured how far in from the edge of the lid to drill the hole to account for the gradual decrease in diameter toward the bottom of the barrel. I also eyeballed the depth so I could cut some 2-inch PVC to length - the bottom of the feed tube doesn't need to extend all the way down, just far enough to keep the end submerged and just short enough to allow whatever you dump down the tube to spread throughout the rest of the digester. I then fit a coupler to the top of the PVC and inserted a threaded adapter through the hole and into the coupler. PVC cement to keep it all together and some clear silicone to seal up the gaps (PVC cement does not work on HDPE, so it's not an option to simply glue the fill tube to the lid). A threaded male plug is easy enough to unscrew by hand when it comes time to fill.

I found a drum funnel that happened to have the same thread diameter and pitch as the PVC. Filling is much less messy with it, highly recommended.

Also, I built a stuffer using the cut-out round from the hole saw (ground down a bit on the edges) and an old broomstick. Whatever goes down the feed tube should be ground up finely enough not to get stuck in the tube, but this will help encourage what isn't. If the stuffer doesn't get the job done, a big long section of metal pipe will.

Step 3: Add the Drain Valve and Overflow Tube to the Digester

To prevent damage from freezing over during the winter, I want to be able to drain it, so I adde another hole toward the bottom for a garden hose-style drain. I entertained the idea of heating it throughout the winter using the heating element from an old dishwasher, but the ultimate goal here is to produce energy, not consume it.

The overflow tube goes toward the top using 3/4-inch PVC pipe and fittings. To keep the digester sealed, I added a valve (for when the fluid level hasn't yet reached the overflow tube) and a J-bend (for when it has). This will just dump out to a 5-gallon bucket for now, but I may add an overflow tank later on.

Note the upturned entrance to the overflow tube inside the digester. This ensures that only fluid will enter the tube and that the system doesn't airlock. Also note the multiple adapters it took to get from 3/4-inch PVC to 1-1/2-inch sink drain piping.

Step 4: Add the Biogas Outlet

I disassembled an old water softener system I got for free and ended up with a good amount of semi-rigid plastic tubing, fittings, and a valve that I figured would be perfect for the biogas outlet. Drilled another hole for the valve and threaded the adapter into it, then ran the line to another valve that will then connect to the collector.

Note the T fitting in the middle. That's acting as a placeholder for a sulfur scrubber that I intend to build later down the line. Biogas digesters produce plenty of methane, some CO2, and enough hydrogen sulfide to make the biogas stink. The sulfur isn't all that useful for us, so it'll be worth scrubbing out of the end product. Which my neighbors should appreciate.

As for the different-sized tubing, that's just a result of using what I had on hand. The larger tubing measures 3/8 inch and fits perfectly into compression fittings.

Step 5: Build the Biogas Collector

Cut the top off the 30-gallon and 55-gallon drums, add another drain toward the bottom of the latter, and add another gas outlet to the bottom of the former. Invert the smaller drum and stick it in the larger. Fill the 55-gallon drum about half full with water.

Step 6: Add the Biogas Inlet to the Collector

More 3/4-inch PVC. Starting with a 3/8-inch copper pipe and fitting salvaged from the used sink that formed the basis for the mobile grinder cart project, I then adapted that to a 90-degree elbow and then a pipe that extended to the bottom of the collector. Another 90-degree elbow takes it underneath the rim of the 30-gallon drum and toward the middle of the collector, then a third elbow directs the gas upward to bubble through the water.

Before I cemented it all together, I added a couple 1-inch T-fittings to the long pipe. As it fills with biogas, the 30-gallon drum will rise, but I wanted it to rise evenly and not wobble like a buoy, so I figured the biogas inlet pipe would also make for a good guide pole if I secured the drum to a couple T-fittings that would ride the inlet pipe up and down.

To secure the drum to the T-fittings without drilling into the drum (and thus creating potential gas leaks), I cut a couple sections of plastic-coated wire to about the circumference of the drum, looped the ends, and cinched the ends to the fittings via zip-ties routed through holes drilled in the T-fittings. With the wires as tight as possible, I then lowered the entire assembly into the 55-gallon drum. Another zip-tie through a couple holes drilled toward the top of the larger drum keeps the inlet pipe from flopping around.

Step 7: Connect the Digester to the Collector and Feed the Digester

Before moving the whole assembly into place, I made a nice little patio for it from old bricks. I then connected the line from the digester to the collector, opened the collector outlet valve, pushed as much trapped air as possible out from the collector, and then closed that valve before opening the two valves between the digester and collector.

As mentioned before, I intend to feed the digester with kitchen scraps and some weeds processed through the mobile grinder cart. Tap water typically contains enough chlorine to negate the bugs doing the digesting, so I'm feeding it with water from my basement dehumidifier. Eventually, I'll add some cow manure to really jumpstart the reaction.

Then, once I get the digester up to speed and the whole system purged of anything but biogas, the sky's the limit for what I can do with it. I'll likely run a small compressor to feed empty propane tanks for use in my barbecue, but I'm also considering buying a secondhand generator to convert to natural gas. Or, if anybody has experience building methane fuel cells, I'd love to hear from you!

Step 8: UPDATE: a Rough Winter

(Spring 2017) So I neglected to drain the digester and collector over the winter and thus both froze solid. Oops. I only lost the zip tie holding the gas outlet pipe in place on the collector, but the expand-o ice block in the digester popped the lid off and broke the water outlet pipe. The silicone seal on the compost inlet and the water outlet didn't hold up well either, so I scraped those off and replaced them with plumber's putty.

I didn't collect much biogas in the fall - not enough time and not enough warmth to really get the reaction going - so I'll build up the digester again this spring and hopefully generate a good amount of gas this summer.

Step 9: UPDATE: Improvements for Gas and Water Management

(Summer 2017) After fixing the winter's damage, I wasn't getting a sufficient amount of gas in the collector - like, none - so I started troubleshooting. It put off plenty of stank, so the digester was creating biogas; that meant I had leaks to address. Ultimately I identified two areas of improvement.

First, sealing - I replaced every point of entry/exit on the digester with a bulkhead fitting, and I added a water trap to the inlet so I wouldn't have to unscrew a cap every time I wanted to feed the digester, thus losing some built-up gas. I still get some minor leakage, but that's only when the digester is full to the brim with slurry.

Second, I took a commenter's advice and ditched the bubbler-style gas inlet to the collector. Now the gas simply feeds into the collector via what was once the gas outlet. There's a capped T-fitting in the gas line, so when it comes time to empty the collector, I can simply uncap that. I did find another use for the bubbler-style inlet: I ran the liquid overflow outlet from the digester via garden hose to the inlet. That way any gases that happen to escape via the overflow outlet (previous design let far too much gas escape) now go no farther thanks to the water trap formed by the droop of the garden hose. I had to raise the digester on some stout timbers I had laying around to allow gravity feed into the collector.

Note also that I took another commenter's advice and spray painted the digester black to take advantage of the direct afternoon sun on that side of the garage and to heat up the slurry within to aid in the digestion process.

Step 10: Whatcha Gonna Do With That Water Now?

With all the water now circulating through the collector tank, I decided to put an overflow outlet on it - with a bulkhead fitting and spigot, of course - and run the overflow liquid via garden hose to a big-ass PVC pipe that a previous owner of the house had used for channelling rainwater from the gutters away from the garage. Gutters are no more, so I extended the pipe to a water tank just below the pipe and just above my vegetable gardens, screened to prevent mosquitos but allow rainwater to collect in it. Another bulkhead fitting and spigot installed in the water tank will allow me to connect a drip irrigation system to the tank.

So far this spring, I've ground up most of the weeds from my gardens and flowerbeds (anything too stalky neither grinds up well nor breaks down well in the digester), mixed them with harvested rainwater or the water collected in my basement dehumidifier, and run them through the digester. Without any manure to jumpstart the process, I'm seeing far more rise to the collector than before. Next step will likely be to collect some manure from a nearby farm to really get the digester running.



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    20 Discussions


    Question 4 months ago on Step 5

    Hi! What is the use of water in the collector?


    6 months ago

    I love these digesters! I have one running in my basement which makes the temperature control issue much easier.

    Your design however has several fundamental flaws which can easily be fixed.

    1) The feed tube should extend down into (near the bottom, 1/3 from the bottom is a good starting point). That way, a) there is NO loss of gas when feeding. b) the stuff you put in starts at the bottom, as it decomposes and loses mass, it will become lighter and get shoved upwards toward the exit point. You already built a pushing rod which, while not absolutely necessary, can be used to help push everything in ensuring that the feed tube is full of nearly 100% water and no solids. This reduces residual gas production within the feed tube which could escape as part of the feed process.

    2) The exit tube should also extend down into the barrel with half way being optimal. Realize that some things are heavier than water and will collect at the bottom. Some things are lighter (oils, scum, etc.) and will collect at the top. What's in the middle is most likely to be nothing but water and that's what should be coming out as the overflow. The gas pressure (not much, but it is there) pushes the water up the pipe until it flows out. As you have experienced, your exit is so close to the top that the gas is pushing water out until it reaches the exit intake and then gas leaks into the air.

    Heating/controlling the temperature can be done a million ways. The key is in insulating everything. The bacteria which operate in the biogas digester are anarobic (they consider oxygen poisonous) and will produce no heat of their own. This is a contrast to aerobic bacteria found in a compost pile. Aerobic bacteria love oxygen (which is why you turn the compost) and they generate a fair amount of heat. Actually, some People have comined the two, basically putting the biogas digester in the middle of the compost pile so that the compost heats and insulates the biogas digester.

    How often you need to empty the digester depends on what you feed it. If you are feeding weeds, try to remove as much sand as possible first (a quick dunk in a water bucket works wonders) because the sand will settle to the bottom and never be digested. Oils are ok but not if they tend to solidify at low temperatures. For example, bacon grease doesn't work well and salad oil does.

    Sarah Lizard

    10 months ago

    The guys in the pakistan science club use truck inner tubes to store their gas. Then they just sit a brick on the tube if they want more pressure


    1 year ago

    Love this project. A few questions: Since 85 - 95°F is said to be the best temp for gas production, do you have a way to maintain that? Maybe paint the digester black? Also, your dip tube increases in diameter from the tubing to the PVC. Do you know how what pressure the gas has to be at to overcome the water column in the pipe? I'd be curious to see how tank pressure affects gas production. If it is detrimental, you might just affix a continuation of the inside of the smaller barrel, which would decrease the pressure needed to break the water seal. (Try blowing through a few feet of water with the tubing and then with 3/4" PVC to feel the difference!)

    2 replies

    RE: temperature. It warmed up here the last few days from the 50s to the 70s and I saw a big jump in production. Painting the digester black would indeed help, and I have a heating element from an outdoor dog bowl I'll eventually fix to the underside of the digester.

    RE: pressure. The digester as designed provides only one escape route for the gas, so it should eventually build enough pressure to break the water seal in the collector. Similar concept to an airlock used in fermenting beer or wine (


    RE: temperature. There are probably some passive things like paint or foil reflectors to keep the heat up, but I'm not sure how to maintain the optimal range without some kind of electronic controller. Electric would do it, but then you are paying to keep your compost warm. I wonder if you could do an Arduino solution with a temp sensor and solenoid to turn on a burner under the digester. Yes, it uses some of your gas, but it keeps it closed-loop.

    RE: pressure. The question is how much pressure will it take to break the seal. It takes 1 psi to break 27 inches of water column. Does 1 psi of back pressure affect the microbes? Do you want 1 psi of methane to bleed off of your digester when you break the seal to add material? A brewing airlock is designed to require very little pressure to break the seal, and a smaller diameter dip tube may help in this way. Just a thought. I'll be curious to hear an update with how much gas evacuates when you open the lid.

    Working on designing one of my own, and I really appreciate your project for ideas and experience.


    1 year ago

    So, have to put any thought to how to best store the gas? I mean, are you essentially just going to use the gas as it's produced, or are you going to compress the gas into a cylinder for storage/use at a later time?

    5 replies

    Reply 1 year ago

    I'd be a little nervous about compressing the biogas in this way. A lot can go wrong there. Can you verify no air mixture? Are you going to knock out the water vapor to avoid cylinder corrosion? You may want to look into any way to use it without compression before getting into that. Maybe pipe it to supplement your water heater or furnace first.


    Good questions. My wife's nervous about compressed methane around the house too. Definitely won't proceed until I'm sure it's safe.

    I am looking into how to compress the gas for later use, yes. If you have any ideas on methane compression and storage, I'd love to hear them.


    1 year ago
    This is the link

    1 reply

    I have always been curious about these. So the gases are produced as part of the digestion process. But isn't there also solid waste left over after the digestion takes place (like happens in animal food digestion)? So how often do you need to clean out the waste sludge?

    1 reply

    Good question. From everything I've read, digesters like this only need pumping out every few years. I'd presume that what's left over from digestion gets diluted in the liquid and only the real heavy solids settle out to the bottom. If that's the case, then by draining this every winter to avoid freeze-up will likely remove a good portion of the solids.

    Whatever the case, I'll update as I put this into use.


    1 year ago

    This is a great and simple way of doing it. I have done lots of research and there's a guy in BC that uses a food waste disposal unit and is much more complex. The only thing I can see as a precaution is where you build it as for us in the frozen north you would have to build a Something to keep the heat in so it won't freeze

    1 reply

    Would love to see the one built by the guy in BC if you have a link.

    As for freezing, I took that into consideration with the drains in both the digester and the collector.