Biodigestor - Fuel From Waste May 13,2017

Event Duration Dimensions ( 3 Years )

Reducing Your Energy Requirements – Bio Digester (Heat from Waste)


Earlier we had an article about composting (making humus materials / potting soil for gardening).  Today we will discuss the bio-digester which is similar in concept (turning waste into a resource).  While composting is an aerobic process (requires air with oxygen), the bio-digester is an anaerobic process (in the absence of air and oxygen).  If you don’t stir your compost, you might end up with anaerobic digestion.  Since the compost pile is open to the air, you won’t get any benefit from letting your compost pile go anaerobic. 

Your septic tank is an anaerobic digester, waste flows in and solids settle to the bottom and are broken down by anaerobic bacteria.  Waste products are methane (fuel) carbon dioxide and some smaller amounts of gases like ammonia, hydrogen sulfide (smells bad), etc.  But human waste is not the best material to use for a digester.  Cow manure is the preferred material particularly to start the digester and make sure you have the right methane producing bacteria.  Any animal waste will do, it just might take longer to get your methane producing bacteria culture going.  You can also add a methanogen starter culture of methane producing bacteria if you don’t have cow manure.  After getting the culture going, you can add food scraps (try to avoid fat and oils) and yard waste, dead leaves, wood shavings pretty much any organic material that you would put in your compost pile and even use spoiled meat (which you shouldn’t add to your compost). 




The nice thing is that you can build your own bio-digester and have gas (methane) to cook and heat even after the EOTWAWKI / SHTF.  I suggest starting with a 55-gallon plastic drum (image above) or a 1000 liter IBC (intermediate bulk container shown below) as a digester tank.  You should be able to get either one used for a reasonable price.  You then just have to plumb the tank with three pipes.



There are a several good videos on how to build one on Youtube such as:





First is your inlet pipe/tube where you add material.  This is a top-fed tube (you can cap it with a garbage disposal to grind up the material) with a funnel on top.  The bottom of the tube extends all the way to the bottom of the tank and you should cut the bottom at an angle (45 degrees or more) to facilitate the slurry exiting the tube and flowing across the bottom of the tank.  Since this tube transports undigested slurry and solids, it should be a large bore (2” or more).  You may want to have material on hand (that you add as needed) stored in a separate covered tank.  You can mix this material with water to make a slurry so materials are not in large clumps (they will be digested easier if they are broken up). You can even rig up a pump or other method of transferring the material into the digester so you don’t have to handle it (as it will be stinky and nasty). 

Next you will need an effluent tube.  There are several ways to plumb it, but you want this to pull liquid material from the middle of the tank (not the solids on the bottom where the anaerobic digestion is occurring or the gas on the top which you aim to harvest) with a shut off valve or handle at the end.  Basically, you will get liquid effluent out as you add material in.  This can be a smaller bore tube as long as it is not so small it restricts free flow when you open the valve.  The effluent can be transferred to your garden to fertilize your crops. 

The third tube is the methane capture.  This is connected to the top of the tank and captures the gases as produced – you can connect this to a bladder to store the methane and you can see how much methane you have as the bladder expands.  A smaller flexible tube is desirable so you can route the tube to your bladder or pressure tank.

The process works best at warmer temperatures so you might want to paint your tanks black and place them where they will get direct sunlight and absorb heat.  If the temperatures in your area go lower during the winter months – you might want to insulate the tanks or put them in a transparent enclosure (to limit heat loss from wind moving past the tanks but still permit light and heat to be absorbed). 

The biogas is roughly 55% methane and 45% carbon dioxide and will include some other gases (such as Nitrogen<10% and Hydrogen Sulfide <3%) that you want to remove through a system of scrubbers.  Hydrogen Sulfide (H2S) smells like rotten eggs and it can be toxic in high concentrations and is also corrosive to machinery.  It can be removed with iron compounds such as running the gas through a cylinder of steel wool.  Since carbon dioxide (CO2) will not burn, it is desirable to remove it also but not necessary for home applications.  CO2 can be removed by bubbling the gas through water under pressure (carbon dioxide easily dissolves in water). 

Instead of a bladder, you can set up a float system (the gas is captured by the float chamber and the float rises as it fills with methane) as a method of measuring how much gas you have available (see diagram above).  You pressurize the water tank by placing weights on the top of the float tank.  If you add lime or an alkaline to the water column, it will improve the efficiency of the CO2 and H2S removal.  Of course, you would have to replace the solution as it gets saturated.  Pressurizing the tank also facilitates flow to your point of usage.

Now if you want to spend some time and effort to purify your gas to 90% methane, you can start looking at using it in combustion engines so there is a way to have a working car after SHTF!

Make sure your appliances are configured for methane.  Conversion of an appliance from propane to methane requires several components be changed (not just the nozzles) so consult with a technician in this instance.  Or you can just buy appliances designed to use methane.   



How much methane can you get?  Your output is really going to depend on the materials you are adding (see chart above).  Yes, there is some science involved and researching this article took me back to some organic chemistry classes that I hoped to never see again. But rough estimates are a pound of cut grass should get you about 1.5 cubic feet of gas.  You need around 30 cubic feet to cook for an hour.  One cow can produce 140 pounds of manure per day (about 18 gallons in slurry) which would theorietically give you 85 cubic feet of biogas (equates to roughly 3 hours of cooking) but you have to run around the pasture to get this manure…..


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