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  #1  
Old 04-04-2013
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Biogas Producing Holding Tank

Hey,
I've been thinking how it could be a good use of a holding tank,
to collect the biogas that it produces, and use it for heating or cooking.

Making and using biogas is even more environmentally friendly than doing aerobic composting, while also providing a useful resource.

Depending on the size of boat and holding tank it could work.
Granted that most likely the biogas will only be an extra fuel.
it takes about 26lt of gas to boil 1lt of water [1],
perhaps less if using super-efficient stove and pot.

1 lt of waste produces about 1lt of biogas per day [2],
at 20C is takes at least 20 days to decompose [1].

if a person generates at least 2lt of waste a day,
of about 1.5 lt liquids and 0.5lt of solids,
and about 0.5lt for the flush of solids[3].
would need at least 50lt per person holding tank for 20 days.

I've read that average holding-tank is around 100lt,
perhaps if there were two, then one could ferment,
while the other fills up. And once it's done,
it would be compost grade slurry.
could get methane from both at same time.
200lt of gas per day would allow to cook 3 meals [1].
Though considering that those 200lt wont be full all the time would need some extra buffer.

Two 208lt (55gal) barrels would be enough for a family of 4, leaving plenty of extra gas.
Ideally however it would be some very efficient cookware and stove, as 4 people also eat more..

Could probably store them in the stern near the engine or something, similar to how intestines are near the rear.

Anyways, wondering what you guys think about it.

sources:
1. Project
2. The Complete Biogas Handbook: How to build a 55-gallon drum biogas digester
3. Gravity Discharge Toilets - Dometic
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Old 04-04-2013
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Re: Biogas Producing Holding Tank

On a very large boat it might be an option, but frankly I don't see this as a reasonable source of onboard energy, and the amount of space it would require would prohibit it on anything smaller than 50ish foot.
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Old 04-04-2013
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Re: Biogas Producing Holding Tank

You might also find all that "waste" to be a handy thing to have around in case of pirates attempting to board! A multi-use item! Just kidding around- sounds like a cool project. ~LL
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Old 04-05-2013
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Re: Biogas Producing Holding Tank

Cool idea, but I agree that there may not be room for it on most boats. Nevertheless, I say to try to figure it out.

Of course, keeping a holding tank from emitting foul odors is an art form among boaters. You would have to insure this plan didn't make it worse. It should be done naturally, but many/most try to kill their holding tank with noxious chemicals that would keep decomposition from happening too. Even flushing too much veg oil can film the top of a holding tank and prevent air circulation and aerobic bacteria proliferation.

All things to consider.
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Old 04-05-2013
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Re: Biogas Producing Holding Tank

Most holding tanks are in the 20-30 gallon range, which equates to ~50 litres (26.7 gal to be more precise). However, just because a holding tank can hold 30 gallons, they usually are not more than 2/3 full. You want to cram it to full capacity, because of the danger of overfilling, or any leakage.

Didn't the story line of Waterworld have something along these lines, using pig waste... or was it Mad Max III?

I am unclear on the ratio of methane produced per volume of black water, per unit of time. You seem to be equating volume of black water with volume of gas, or implying that the ratio is 1:1. I don't think that you will get that much. Also, if it takes 3 months to achieve a complete processing of the material, then it isn't going to be practical (what do you do with the rest of your effluent for the 3 months). I would also think that the temperature and composition (ratio of solid to liquid, pH, affect of Cl, etc.) of the black water would have a direct affect on the efficiency of the production from the bacteria.

I am not trying to discourage you, as I believe the idea has merit. If you get it to work, and patent it, you'll be rich and famous! However, I doubt that it will be simple.
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Last edited by eherlihy; 04-05-2013 at 09:10 AM.
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Old 04-05-2013
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Re: Biogas Producing Holding Tank

Two 55 gallon barrels? Ah, that's a fast 900 pounds to carry aboard, equal to the weight of four more crew.

There's a reason why methane tanks work on cattle farms and pig farms, and not at the local Holiday Inn.

If you think your numbers will work, buy a couple of drums, set it up at home, prove that you've got it right and it works. Then you can start taking it around to boat shows and selling the marine version.
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Old 04-05-2013
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Re: Biogas Producing Holding Tank

26.7 gallon is actually 101lt.

turns out it only produces a whole liter if at 30C,
at 20C it produces only 2/3 of a liter per day.
It is usually about 60% methane.

The recomended temperature seems to be 35C or above,
which would require insulation and some kind of heating.
most sustainable would be solar-thermal.

Air inhibits the production of biogas, so would need to make sure incoming matter didn't introduce any. potentially by having a small pre-holding tank, than drains into the main one via hose.
If it does have a lot of air in it, it'll be about a week before it starts producing biogas, since the aerobic processes have to eat it up.

the scientific literature is somewhat confusing in terms of processing time, it seems there are sources for it being 20-30 days for 20C, 35C and 55C.

Considering the space limitations, if 100lt is all that can fit aboard an average vessels, it's not going to be very useful for cooking.
Probably not going to be able to fully process it either,

However can still reduce the smells, by making use of the gas,
for lighting or extra heat, with a simple camping lantern, and a hose/vale coming from the top of your holding tank.
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Old 04-06-2013
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Re: Biogas Producing Holding Tank

Ah thread that started on April the first?

Ah no?

?
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Old 04-07-2013
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Re: Biogas Producing Holding Tank

Perhaps it was started April 1st, though I don't know how to check.
However it wasn't meant as a joke. Though perhaps it could be categorized as "toilet humour" lol.
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Old 04-08-2013
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Re: Biogas Producing Holding Tank

Maybe just having a permanent flame on the holding tank vent just may be a good idea anyway( permanent anchor light) At any rate, that may be one way to determine the output.
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