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.