Biogas methane digester project

(Larry G) #1

I picked up some components for my methane digester project this weekend. I picked up a 275 gallon tote tank similar to this one for $50 (normally these run about $175 in my area, but this one is missing the cap on top, which I would not be using anyway:

I also got a ~30 gallon holding tank and submersible sump pump for $50 that will propbably eventualy be a primary stage to the big tank’s secondary. The pump works great, and fills a 5 gallon bucket in literally about 3 seconds.

A buddy is taking out a pond at his house that was half-arsed built by a previous home owner, so I got some 2" and 4" ABS drain pipe and some valves out of that for free.

More to follow as I build the project, I will try to document with costs, steps, and pics as well as I can.

Real Estate Paradigm Shift | oceanic business alliance | oceanic real estate | floating real estate |

Have you seen the IBC digester design at SolarCities?

Here’s the Facebook group:

Here’s a wiki on the design… :wink:

(Larry G) #3

That’s very close to what I’ve been thinking. Gonna use the existing 6" opening for feeding. This gives me the ability to use less-well-chopped material through the larger hole. Build a plunger like the press in a french press coffee maker to get the feed stock through the feed pipe into the main reservoir. Put a stop on the plunger shaft to keep it from going too far down and getting stuck. Cut the end of the fill pipe and the plunger at angles so feed stock has to slide off to the side and can’t just float back up.

Cut a new 2" hole for gas outlet on the opposite end from the effluent outlet. Mount an aquarium aerator pump inside this area through this new hole (need to research whether these things are explosion proof- still on my research checklist) Use that to re-circ CO2 through the water of the main reservoir by bubbling it down to the bottom on the end away from the effluent outlet. This will also help to stir the feed stock and assist in migrating sediment toward the effluent outlet.

Use the existing valve and outlet on the bottom for effluent/sediment removal.

(Larry G) #4

I’m also digging on the idea of a used garbage disposal macerator for inputs. At a later stage, perhaps.

I’m using a yard chipper for yard debris, but putting really soft messy stuff through it is unpleasant. Won’t even think about doggy-doo-doo going in that. I have about .3 acre of grass that gets cut regularly, lots of pine cones, lots of black berry brambles, weeds, and leaves that have to go somewhere.

Going to start the process out with that chipped debris, some dog-doo, the windfalls coming of my apple and pear trees, and probably jump-start the process with some septic enzymes and purging the air reservoir with the CO2 tank from my brewing/beer gear. I expect to start putting our paper and cardboard trash in there along with our kitchen waste (which is actually not that much.)


The better effluent zone would be about mid-way up the volume of the tank. Culhane has been working on and slowly modifying this system for a long time and is a professor at Mercy College(?) (Mercy something or other) and has had one, until recently, at home, for years. He’s THE go-to on this style. In put tends to do 2 things. Sink, or float. Heavy stuff will tend to digest and rise, while floating stuff tends to create scum, just like a septic tank.

You can stir with all of the gas, unless you have a specific method for separating the CO2. My system just tends to capture most of the CO2 in one place, since it is a multiple-chamber design.

If I can get mine finished, I have access to Llama and Alpaca poop, which is supposed to have more efficient critters in it. Llamas and Alpacas can do well on low-quality feed, so their microbes have to be hardier (that’s the theory, anyway).

People have actually started digesters with everything from baby diaper poop, to ground-up fish guts, mud from near-frozen lakes, and so on. Just need a source you trust and enough material to work with.

(Larry G) #6

Well, yes. I am looking for a safe way to simply pump the gas from the top of the reservoir (just one stage at this point in my plan) back through the water so that the CO2 is once again usable to the bacteria for increasing CH4 and decreasing CO2 in the output gas, as well as stirring the feed stock. With no significant oxygen in it, I can probably use basically any smal air pump to do it, but I would prefer one that is sparkless just in case some O2 leaks in there at some point.

I still need to do some more research to fully understand the multi-stage multi-reservoir designs. But doing is better than talking, so I am starting simple.


That looks fantastic :slight_smile: Best of luck with it.

Just wondering what you’d do with the end product, would you siphon off the gas for cooking or use it in some kind of fuel cell?


This is one of those respectable situations where the energy use of a process may exceed the energy output (aka parasitic losses). The same frustrating situation occurs in OTEC. Because what you need (in your last post) is physical motion of moving the carbon dioxide, connecting the pump to a floatie may work best, because if there’s anything the ocean does well, it’s keeping floaties moving around. But burning the methane in a generator to make electricity to power the motor to move the pump, well, that could require more methane than you get out of the digester.

I am only saying this in terms of evaluating the operation and efficency of the methane production, this is not a comment on the good fertiliser produced, or any other aspect of methane digesters. I am only saying that it would be a good thing, if you are engineering a digester to be put on a seastead or a boat, that you consider you have different options on land vs on water, and some parts of the process can be optomised for either. Note the waves may not be there to pump the CO the minute you need them, so some energy storage may be needed, and i don’t mean batteries. Using solar panels doesn’t count, altho it very well may work fine, because the digester may use more energy from solar than if you used only the solar to cook with and forgot about the digester.


Short answer: YES

Considering that Methane is the primary component of Natural Gas/LNG… It can be and has been used as fuel for almost every conceivable device that requires fuel for heat energy, from simple burners, to industrial engines, to fuel cells.

(Larry G) #10

For my initial experiments, I am planning to fuel a burner ring in a backyard fire pit. Purely recreational for now. One could cook on it.

A great many things don’t make economical sense in a civilized setting, yet may be the only option in other settings. Or may be worthwhile doing for the aesthetics or convenience (the cost not being justified by monetary or resource benefits, but by convenient consumption.)

(Jonas Smith) #11

I agree 100%. One of the big benefits of the biodigester is that it converts organic waste to fertilizer. Even if the amount of biogas generated is just enough to run the biodigester you still 1) get rid of all that organic waste in a clean and sustainable way and 2) have a source of high-grade fertilizer for use or export.

Edit: Here is a PDF describing a similar situation. The Pohlsche Heide MBT plant treats municipal waste, and they only put about 70%-80% of the digestible organic waste into the digester…it is a dry thermophilic digester so they redirect the watery stuff to a wastewater treatment area. This means they sacrifice the biogas generation from that 20%-30%, but they get savings by not having to de-water the digestate.

“Energetically, the plant does not compare so well, as other MBT plants treating residual wastes export significant amount of the renewable energy they produce. The system is designed to minimise water addition and wastewater treatment, and for these reasons not all of the organic waste is digested in the Dranco reactor. Therefore from an energy production point of view the plant does not produce as much biogas as it would if it were to digest the whole organic waste stream anaerobically. In this case the savings made on de-watering, wastewater treatment and electricity grid connection must have outweighed the potential income from electricity if the whole organic waste stream had been digested.”


A great many things don’t make economical sense in a civilized setting,
yet may be the only option in other settings. Or may be worthwhile doing
for the aesthetics or convenience (the cost not being justified by
monetary or resource benefits, but by convenient consumption.)

Afaik a few places do it on land and the sums work out, it’s just that there isn’t overwhelming pressure to do so like there will be for us.


For stirring, especially if you’re using a plastic chamber, you might be able to scale up the magnetic stirrers used in Chem labs. What goes into the chamber is basically a strong bar magnet, coated in Teflon. Just under the bottom of the chamber, is a device that creates a rotating magnetic field for the bar magnet to align to, and follow as it rotates. No seals or sparks to worry about; the switching to drive the coils is all solid state. Worth at least considering, I hope.


What is wrong with using a small gearbox, a length of driveshaft, and a paddle? I am thinking windshield wiper motor, 1/2 inch EMT, and anything resembling a paddle, bolted to the inside of the top of the digester (above the contents).

(Jonas Smith) #15

Just some helpful advice from the LinkoGas people:

These days experience has shown that taller, narrower digesters allow for better mixing, process dynamics and efficiency. Therefore if the plant was to be rebuilt, a different digester design would be used. The stirring/mixing efficiency was found to be vastly superior in the 3rd digester, which operated on the circular pattern pumping, as compared to the first two digesters which were stirred by a vertical shaft from the centre of the top of the reactor, with attached paddles.


(Larry G) #16

In the small size I am planning for my initial stage, I am not going to worry about stirring. At 275 gallons (just over a cubic meter) total volume, feeding it should provide enough stirring. This is a deliberate choice for simplicity over cost and complexity, even though it will probably result in a few percent less efficiency in output gas.

(Jonas Smith) #17

Viable method. This is the method used in Dranco bioreactors, as used in the Pohlsche Heide MBT plant in Germany:

The waste is mixed with a recycled portion of the reactor contents (removed at the bottom of the reactor) at a ratio of 1/3 fresh feed to 2/3 recyclate, and pumped back to the top of the reactor for re-introduction. In newly built Dranco reactors such as this one the inflow piping to the top of the reactor is inside the reactor vessel, rather than outside as in the Salzburg plant. This reduces heat loss, but could cause extra problems in the case of pipe corrosion/blockage etc. Similar to all other Dranco processes, a Putzmeister pump (Putzmeister, Germany) is used. These pumps, designed for heavy duty use in the cement industry, are used to pump feed to the top of the reactor, where it is introduced. Once pumped in to the top of the reactor, there is no internal mixing apart from the downward flow of the waste due to gravity. There is no internal or external heating, with the thermophilic operating temperature being solely controlled by steam addition to the influent stream. An advantage of this mixing system is that there are no moving parts inside the reactor, so no danger of blockage or malfunction leading to downtime. With more internal flow-pattern data it could possibly be argued that better waste/biomass contact could be achieved with more mixing, provided by an additional mixing system.


Another option for stirring would be a drill-powered paint-mixer on an extension, that would be sized smaller that the inlet and/or outlet.

(Larry G) #19

Ran into a minor snag. The IBC I picked up off of Craig’s List didn’t have a cap. This opening isn’t standardized with plumbing supplies. It appears to be a different thread and it’s inside dimension is nearly the outside dimension of a 6" ABS pipe.

I planned to use this opening for feeding because it gives me the largest opening without cutting a new hole that then needs to be sealed.

I’m hoping I can find a rubberized sleeve for 6" to 4" conversion that will stretch enough to fit it with a large hose clamp on the outside… The I can drop a 4" feed pipe through that nearly to the bottom and have it supported at the top by the sleeve. I would have preferred to have it all screwed together with teflon tape for gasketing.

Edit to add: I may have found a solution that doesn’t require any cutting, although it minimizes the feedstock opening:

This has standardized threading for the 2 x 2" openings (NPT is national pipe thread standard). that would mean easy, air-tight plumbing.

I picked up some 1/2" PVC fittings (less than a dollar each), and a 1/2" bulkhead fitting that will be my gas outlet (the most expensive of the plumbing parts so far at about $12). I plan to plumb this to a water valve (also PVC) and then out to an H2S filter and then on the other side of that neck it down to a standard 3/8" gas fitting with a gas valve (metal once the H2S is eliminated). I figure even if the PVC water valve leaks a little bit it will only be necessary while changing out the H2S filter, which is only an occasional need. But with the minimal pressure behind it, the PVC water valve should work fine.


If you need a cap, you might be able to make one, using an IMHO interesting plastic,

What’s very handy about this polymer is that is softens at hot-water temperatures, and can be easily hand formed, molded or reshaped – for instance to your IBC’s threads. If you try this, I’d do something like the following

  1. Melt/consolidate (get the air bubbles out.)

  2. Mold to a rough (slighly larger) cap shape.

  3. Grease the existing male threads.

  4. Mold the cap onto the greased threads, and let it cool in place.

It appears to be quite safe. I first learned about it from it’s use in US medical practice, to make small splints/supports for finger injuries.

It goes by many names at the retail level: instamorph, shapelock, friendly plastic, and thermomorph (the latter was the brand with the best price I found, when I last ordered some on Amazon.)

Hope this helps