Don’t Throw Away Your Wealth

Biogas Cooking

I met Dr. T.H. Culhane a couple of years ago at a National Geographic conference. He is on a lifelong mission to help the world’s poor, both urban and rural, bootstrap themselves out of poverty and improve the quality of their lives.

How?  He shows people how to avoid throwing away wealth.

Specifically, he teaches them that the food and bathroom waste they produced every day (about thirty percent of the energy they consume and most of the waste) can be transformed by a biogas digester into:

  • the fuel (methane) they can use to cleanly cook their food and
  • a composted slurry they can use to fertilize their crops.

DIY Biogas Digesters

Biogas digesters aren’t new.  They are in use in many places.  However, there are many, many more places, both intensely urban or spartan rural, that need them but don’t.

To give you some insight into how these systems work, here’s an example of an in ground biogas system that TH initiated in the Philippines (see TH’s blog for more detail on the entire project).   This system is set up so that the wife/kids can feed the digester biological waste every day and get methane gas and fertilizer in return (he teaches people to treat it as a family cow).

The system they are installing is essentially a septic tank with some minor tweaks that allow it to be manually fed and output methane/fertilizer.

To build these in ground tanks, they bought Chinese molds and shipped them in. You can see them below.   It cost TH about $8,500 to get them to the island and another $2,500 to get them past the bureaucrats.

The mold takes about two hours to stitch together. The majority of the labor is digging the hole it is going in and pouring the concrete. Once bought the mold can be used thousands of times.  Below you can see two tubes. One tube (near the kitchen) is for feeding the digester waste and one is for the fertilizer to flow to the garden.

One interesting twist on the installation is that TH builds vertical walls inside the tank using cinder blocks. Why? To provide more surfaces with a greater degree of temperature variation so that more bio films can form. Bio films are the most productive part of process. Here’s what it looks like (the photo is a bit blurry, but you can see the walls inside).

If cost is the primary factor, here’s an example of an above ground system.  Note the stand alone tank.  The blue bladder is to hold the methane produced.  Very simple.

Here’s how the methane is used.  Note the tube piping it in.

What’s the bottom line?

  • This system costs about $750 in materials, plus labor (mostly digging a hole and pouring concrete).
  • It generates 20 hours of cooking fuel a day (or 5 hours of electricity generation) per day, plus fertilizer.
  • It operates forever.

What’s the point of this example?

Don’t throw away wealth.

With some creative thinking, it’s possible to turn waste into wealth at the local level.

You can do this yourself (above ground) or with some neighbors (below ground) or you can contract for it.

If the community has the willpower, a municipal sewage system can use large biogas digesters to generate electricity — likely more than enough to operate a sewage treatment facility and power some community buildings.

Your not happy about throwing away wealth analyst,

John Robb

PS:  Here some of the ways biogas digesters reduce poverty and improve lives:

  1. It eliminates biological waste that can cause a health hazard.
  2. The methane produced burns cleanly inside the home (as opposed to wood/garbage smoke).
  3. It makes it possible for kids to go to school/play, since they aren’t required to spend 3-6 hours a day gathering fuel for the stove.  If they are buying bottled gas to cook, this saves them income.

PPS:  Bureaucratic road-blocks.   Here’s an interesting nugget from TH.  He found that municipal authorities and development agencies are often stopping people in developing countries from building DIY digesters for methane production.  Why?  They believe that they are dangerous.  So, the authorities have been telling people to shut them down and wait for electricity deliveries from a plant to be built-in the far future. TH maintains, and I think he is right about this, that the small volume of low pressure gas produced by these systems is safer than the alternatives available, and much healthier.

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  • Dan

    20 hours of cooking fuel per day sounds good. Is that with one household’s waste? Or is Culhane’s system village-scale? Another fun use of methane gas from human use is to fuel one’s car. Check out the bio bug car. The article says the waste from 70 homes would power the car for 10,000 miles. Again, I wonder how much fuel would be produce by one household per day. I’ve also looked at palm oil as fuel, of which one acre will produce over 700 gallons per year. That’s twice what I use annually and doesn’t take into account the fuel economy boost of diesel over my current unleaded vehicle. If the methane gas produced from the waste created by a family of four would meet or exceed the production of an acre of palm oil, it would have the advantage of not needing so much land dedicated to a single crop.

    Do you have more specific numbers?

    • johnrobb

      Dan, I’m going to write a more detailed report on this in the future. It’s a pretty rich topic. JR

  • http://Enteryouremailhere Bob Spencer

    I live in western Maine where almost nobody bothers with building permits. I wonder if I could just build a digester and nobody around here would really care.


    • johnrobb

      Bob, You are probably right. Certainly if it is DiY. JR

  • Wendy Kilpatrick Laubach

    You might be interested in the book “Humanure,” which focuses on the fertilizer part rather than on the cooking gas. Another advantage of this system, for those of us in the first world, is that it avoids our spectacular waste of potable water, in which we inexplicably use drinking water to transport waste solids several miles from each house to a municipal treatment plant: crazy, crazy, crazy, indefensible system. (Well, not my household, we have a septic tank.) In the U.S. it can be extremely tricky to get local authorities to sign off on any nonstandard onsite human waste treatment, but the technology absolutely is there to do it safely. The more squeamish probably never will get beyond using the fertilizer for non-food-bearing trees, though.

    PS to Bob Spencer: check out Joel Salatin’s bottomless chicken coops or “chicken tractors.” Chickens will eat garbage of all kinds, including meat products, and convert them to terrific fertilizer. They also devour any ant that gets anywhere near them, converting an anthill to a small, antless depression overnight. Free-range chickens are nice, if they’re safe from predators where you live, but here the raccoons, coyotes, and hawks would get them immediately if we didn’t keep them in a coop. The bottomless coop, which is moved to fresh ground from time to time, gives them a chance to tear up weeds and find new bugs and worms. And of course you get the eggs.

    • johnrobb

      Thanks Wendy. Will definitely give it a read. Also, definitely need to write a letter on bottomless coops/chicken tractors. JR

  • Happyhank

    A nice looking digestor. Would probably work with several neighbors providing the input. One family would not be able to operate a digestor of that size unless they had live stock or other source of digestable materials. In much of the USAit would also require added heat in the winter which would use up much of the gas that was produced. BTW Bob. chicken poop works very well for this purpose. Was a fellow in England ( Harold Bates IIRC) that used a smaller system to run his automobile.

  • ryan
  • Penny Pincher

    If one could be constructed that was for the most part invisible both from the road and from aerial spying drones, that would enable an enclave trying to live better than they were “allowed” to. Or perhaps if not completely invisible, it could be disguised as something else.

  • martin

    I work as an engineer designing industrial biogas plants. For me, those simple DIY approaches are always deeply fascinating.

    I’ll be back with some data re the gas yields when I got the data with me!

    • johnrobb

      Martin, That would be great. I should probably team up with you at some point to do a report on small scale/community biogas for resilience. JR

  • Dusty

    I would like to see more information about how to convert an existing septic tank to do the job.

  • Federico Mena Quintero

    How interesting! It sounds like they could spare the molds if they built the tank out of ferrocement. It’s probably a little more labor to build the ferrocement cage out of rebar/mesh than to just put the molds together.

    I’m intrigued by having both the waste input and fertilizer output pipes at the top of the tank. I had always seen designs for the output pipe (or door) at the bottom, so that the most fully cured waste always is at the bottom (thus precluding underground tanks, unless you open them once in a while to more or less empty them).

    I’m also intrigued by the smallest feasible tank one can have. John, didn’t you blog about a digester-in-an-oil-drum once? That would be about the perfect size for my home.

    • johnrobb


      Would love to do a letter on ferrocement construction. If you have some experience, would you want to write a tutorial on it that we could put up for comment?

      Bob Crosby had a design for a water treatment plant in a previous letter.


      • Federico Mena Quintero

        Thanks, John. I do have some experience with ferrocement, in building ceiling vaults – not tanks, however. There is a *lot* of information on ferrocement for vaults and housing forms in Steve Kornher, the owner of that site, was very kind to give me the initial “kick” to build concrete vaults at my home.

        I can certainly produce a basic description of how to build a room-sized ferrocement vault. The actual hand skills belong to the builders that worked on my house (i.e. troweling concrete), but I do have direct experience with laying down the rebar, laying and tying wire mesh, and *ahem* the proper technique to carry a concrete bucket onto your shoulder up a ladder :)

        • johnrobb

          Thanks Federico. I’ll give you a ping once I’ve read everything on Steve’s site. JR

    • martin

      The smallest feasible digester … let’s see.

      Lab scale digesters are typically 5 or 20 liters. They are only used for measurement purposes (testing gas yields and so on)

      No, to look at a small digester that actually does something – a rough estimate is 1,5 kWh/d energy use for cooking. We try to supply that with our digester. Using data for separatetely collected organic household waste from germeay (mostly kitchen leftovers, peels, etc.), You need 1,75kg/d wastes + maybe the same amount of water (for dilution, the wastes will be pretty thick, you need a liquid feedstock or gas bubbles will be trapped etc.)

      We try to keep the organic loading rate below 3,5 kgVS/d and arrive at a hydraulic retention tim of 40 days – on the short side.

      Then, you would need 135l of digester-volume.

      In terms of Volume, bigger is better. Bigger volumes mean more time for the bacteria to digest the feedstock, better thermal stability (Methane generating bacteria don’t like temperature changes) etc. In industrial applications, shorter times are used, but with better mixing and temperature control than you can probably manage at home.

      The urban farming guys (link above) use 3m³ of digester (when I understand the sizes of their their container correctly), no indication is given as to the input but it will be in a similiar range.

      40 days is shortish for a digestion at 40°C, cooler temperatures mean you need vastly more time.

      Keen calculators will have noted that the volume is smaller that 3,5l/d input times HRT 40 days – gas leaves the system and frees space, hence we need a slightly smaller volume

      I have not done the math on existing small scale digesters, in terms of retention times and dilution (if any) they use.

      • johnrobb

        Thanks much for the detailed input Martin. JR

      • Federico Mena Quintero

        Thanks for all the detail! This is totally worth experimenting with.

  • Ian

    First you say “It cost TH about $8,500 to get them to the island and another $2,500 to get them past the bureaucrats.” Then, “This system costs about $750 in materials, plus labor”. In what way did it cost $750? Was the rest of the $8,500 shipping?

    • johnrobb

      Two different things. The first cost is what it took to buy the mold. The mold can be used hundreds of times or sold. The second is the cost it takes to build a unit in materials and labor. JR

      • Ian

        Then your “bottom line” should have mentioned the cost of the mold separately, not left it out.

        • johnrobb

          Ian, Point taken. I was thinking in terms of marginal costs. The cost of the mold either counts toward that or not depending on what you do after you install your system. What can you do with it? I suspect I could sell the mold for what I spent on it. I also think I could rent the mold or actively use it as part of a business venture. JR

  • Doug

    I do business with waste water treatment plants. Many use the methane gas produced to heat their digesters (keep their bacteria warm) in cold weather. The methane produced isn’t enough and they also need to purchase natural gas. Methane is also a ‘dirty’ gas and need to be cleaned up before using in a home.

    Has this been addressed?

    Also, landfills have been using their methane gas to generate electricity for many years.

    The electric made is usually quite a bit beyond their own needs.

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