Homebuild digester and the pitfalls..

[mathewjenkinson]

Hi all,
1st off what an amazing site, very informative and i am so glad i stumbled across it.
To cut a long story short I am on a quest to turn me and my family, 'green' and 'off the grid'. As part of this challenge I have started to make biodiesel from waste cooking oil and have recently purchased an empty / wrecked house with the hopes of restoring this house to a 21C 'green' house.
I plan to use the waste from the toilets in the house, the bio-diesel by-product as well as the waste from a number of chickens I keep to feed the digester and then use the gas from the system as part of the CHP system as well as the cooking rings in the house to provide electricity, heat and cooking gas.

Im looking to make the system totally automated with a sort of continuos flow type system in place.
Can anyone recommend a set of plans I can begin to design from and would anyone care to share some of the 'watch out for this problem' stories with me?
Thanks very much,
Mathew.

[Bob]

Hi Mathew,

I think I'd start by trying to get a rough estimate of quantities of waste and gas you can produce. Based on the quantities you describe (toilet wastes from a single family residence, a few chickens and some glycerol) I'd guess you probably can't generate enough gas to justify the cost of a CHP system, but certainly enough for cooking, and maybe heating water. Have you tried my online calclator? (Caveat: It is somewhat dated, based on an older design that I have since redesigned, but it should still be useful to give you a rough order of magnitude idea.)

What I find myself recommending to most people is to first do some bench scale tests (one or more jars in a temperature controlled water bath) -- do some direct small-scale experimentation before committing to anything larger. Another advantage of that is what you learn about the digestion process, how to keep a larger system going, keep it from crashing, etc., all without a major investment.

[mathewjenkinson]

Hi Bob,
I think thats a fantastic idea - starting small to get an idea of the process involved.
I had a look at the calculator but it was very brief as I had a lot of other things to do with over the xmas period but I will be going back soon to study it more indepth.
One question that was bought up was, if an anaerobic digestion system was put in place connecting the toilets together, im assuming the use of bleach to clean the toilets is now out of the question? as the bacteria would be killed of? - if so how do you keep the toilets clean? lol

Thanks very much.
hope you had a good new years.
Mathew

[Bob]

how do you keep the toilets clean?

We used Shaklee products for years, until our local distributor moved away. After that we were able to find various other benign off-the-shelf products. But you're right. We didn't use bleach.

Here is a photo of a 4-digester test setup I built recently, to test the digestibility of glycerol. The cooler is insulated well enough that the aquarium pump alone (at right) is able to provide the heat necessary to maintain 95F water bath temperature. The top row of jars are the actual digesters. The 2nd set of jars (at bottom) are water traps. The black thing on the lower left jar is a pressure switch connected to a data collector that measures and records gas production. By having multiple digesters, all at the same temperature and operating conditions, we can do direct comparisons of different recipes. (Photo was taken during construction, still unfinished.)

cooler small IMG_0390.jpg (190.43 KiB) Viewed 5040 times

[mathewjenkinson]

wow, I dont presently have the ability to measure gas production.
But that is very impressive.
What are your recommendations and 'recipe' guidelines?

While I was traveling round Thailand, I went to see a project where a farmer is using the dung from his buffalo to feed the digester, he had more than enough gas to cook and heat water and told me that every year since he started to use the processed waste on his fields the quality of rice had improved year on year. I have some pictures of the setup somewhere, but it was essentially two tanks linked together and all made from HDPE plastic.
What kinds of build materials do you recommend using?

[mathewjenkinson]

I heard some commentary on the radio today about someone who claims that when paper is anaerobically digested it produces 4 times its paper weight in methane gas.. Is there any truth to this? Would you recommend putting shredded paper into an anaerobic digester?

[Bob]

...when paper is anaerobically digested it produces 4 times its paper weight in methane gas.. Is there any truth to this? Would you recommend putting shredded paper into an anaerobic digester?

I don't know about the weight of gas per weight of feedstock, but it can be calculated from basic chemistry -- if you know the reaction and the molar weight of the reactants.

Of more use, though, is the heat value of the gas generated from a given volume of feedstock. Yes, you can certainly add (some) papers (avoid clay-based, colored, with heavy metals in pigments, etc.) to your digester. In fact it is a useful carbon source if you need it to balance C:N ratios. Reasons not to would include problems with pumpability (lending itself to a batch or hybrid design). (Note that it is included in the online calculator.)

[mathewjenkinson]

Hi bob,

Im bringing this thread back to life!
Im now in a position to start experimenting with biogas production again.

Presently I have access to chicken manure as a feedstock.
My 1st experiment seems to not be going too well..
I have a 3 litre water container, it is filled with 80% chicken manure feedstock, the rest is topped up with water.
This container is connected to a gas collection container, and both are kept at 30C.
This experiment has been sitting now for approximately 6 days with no gas at all being produced.

My conclusion is that I havent got the right ratio of water to feedstock, is there a formula? or do you have a recommended jumping off point?
Thanks,
Mathew