I think it's hard for most people to appreciate how easy it is to build your own home biogas
system.
Perhaps we want things to be more complicated than they are in order to somehow justify the
suffering we've had to endure all these centuries trying to live without domestic biodegesters.
Or perhaps we've simply kept the simple engineering principles out of the hands of the right people,
women and children particularly, people who would intuitively understand that the waste
from our kitchens and bathrooms can complete the growth cycle and become soil and food
and fuel once again, and would realize that all we are really building is a stomach with
a mouth and a throat and anus and a ureter.
This bio-mimicry is easy to understand if you think of a biodegester as a baby or as
an animal, a fire-breathing dragon if you like, rather than a machine.
But it is simple.
Here's how you do it, the Solar City's open-source way.
Get yourself an old IBC tank, an international bulk container.
You can find these used IBCs, these pallet-based, 1,000-liter plastic shipping containers, literally
all over the world.
Take a three-inch hole saw and drill three holes in the top of the tank as shown.
If you're in a place with no electricity and no power drill, take a piece of metal pipe
with a three-inch diameter and heat it up and melt the holes through the plastic.
We did this in the Dominican Republic with a three-inch metal sinks trainer.
Three inches is 76 millimeters for those of you doing metric.
This is the diameter of the outer flange of the two-inch uniseals we use.
Next, get yourself three two-inch uniseals, grease them up with plumber silicone, and
pop them into the holes.
Next, cut three pieces of two-inch or a 50-millimeter sewer pipe to a length of 1.5 meters.
Designate one of the pipes as the feeding pipe, and cut the bottom at a 45-degree angle
so that the food waste you put in can slide out easily into the bottom of the tank.
This gives the pipe a pointy end.
Sand any rough edges and soap up the pipe and the inner ring of the uniseal, and firmly
push the pipe through the uniseal twisting as you go, until it reaches the bottom of
the tank and stops.
Designate the second of your three pipes as the gas outlet pipe, and cut a hole so that
it will be just below the top of the digester when inserted through the second uniseal and
push to the bottom.
Yes you'll have to measure this one as the heights of your IBC can vary.
You can do this by putting the pipe in the hole before you put the uniseal in, and making
a mark with a permanent marker about 2 centimeters below the place where the top of the tank
touches the pipe.
Make sure you sand the edges of this hole smooth and flush so it doesn't cut or get
stuck on the uniseal on the way down.
Then push the gas outlet pipe into the tank through the uniseal.
We like to put the feeding pipe and the gas outlet pipe on the same side of the digester
as we find that this is where most of the food settles and most of the gas rises and
collects.
Designate the third of your two inch pipes, the slurry or effluent pipe, where the rich
organic liquid fertilizer or compost tea will exit.
We cut a hole in this last pipe, make sure you also sand the edges of this hole smooth
and flush so it doesn't cut or get stuck in the uniseal on the way down, so that when
it's pushed down to the bottom through the uniseal, the hole will be somewhere near the
middle of the tank.
We do this because bioactive solids are being digested at the bottom of the tank, mostly
carbs and protein, while lipids, oils and fats will rise and be digested at the top
of the tank, and we don't want to remove feedstock that is still energy rich.
By placing the outflow pipe on the opposite side of the tank, diagonally across from the
feeding pipe, we maximize the hydraulic retention time of the feeding material, that is, the
time it spends being digested in the tank, so we can win the maximum amount of energy
from it.
Since our digesters are a kind of plug flow digester design, the food we push in one
side of the tank is slowly shoved to the other side of the tank when new material is added,
but the fertilizer we force out with each feeding has had the best opportunity to get
digested, and because we draw it from the middle of the far end of the tank, it comes
out as a liquid that is nitrogen and nutrient rich, but doesn't clog pipes.
The sludge can stay in for years and years, and only rarely needs to be pumped out to
make a solid compost.
With these three pipes in place, you're basically done with the digester.
The rest is plumbing.
First, we want to take care of the gas outlet pipe.
The two inch pipe sticking up above the digester ensures that the gas outlet is well above
the water line and is wide enough that manure or other solids you may have put in to inoculate
your digester don't clog the pipe and stop the gas from coming out.
At the top of this pipe, we put a two inch to half inch PVC reducer, a half inch PVC
ball valve, an elbow, and a half inch barbed hose connector.
We don't glue these on in case we ever need to remove them to clear any possible clogs
should they arise.
For the fertilizer outlet pipe, we first cut off about five centimeters at the top and
then attach a two inch tee, a piece of two inch pipe, a two inch elbow, and a drain pipe
leading to the fertilizer bucket.
Every time you add ground up food waste and water to the system from the feeding end,
an equivalent amount of bioslurry will be forced out because the 90 degree part of the
tee is lower than the feeding pipe, so whenever it's filled, the fertilizer pipe will overspill
and drain into the bucket.
The open sky facing part of the tee ensures there won't be no vapor lock, otherwise you'll
get a siphoning effect, such as we experienced when we built one with an elbow instead of
a tee in Swaziland.
A siphon will drain your tank in no time, but with a tee the system automatically regulates.
The fertilizer that results is perfect for hydroponic and aeroponic gardens, diluted
with gray water 10 to 1, you should never have to buy fertilizer again.
Finally, we put a funnel onto the feeding pipe where ground up kitchen scraps, old banana
peels, and plate scrapings and whatnot are poured.
Unlike most compost piles, there is no food waste that you can't put in, fruit and vegetable
rinds, cooked food, meat and grease, you name it.
You just have to grind or mash it up and mix with water, dirty or soapy water is fine,
so that it will fit down the pipe without clogging.
In the Dominican Republic, we used 5 gallon water jugs cut in half as our funnels.
If you build your digester lower than your sink and install a food grinder, hook up the
outlet of the grinder directly to the feeding pipe, just don't forget to have a pipe or
hose attached to a tee to prevent a vacuum lock.
Of course, this digester doesn't store gas, so for that, you'll need some other form
of storage.
We tend to use PVC balloons or bladders, basically plastic bags connected to the tank with clear
plastic tubing, they work just fine.
In New York, we've built these digesters in two family basements and the gas is stored
outside in the bags with a tee line connecting the output to the kitchen stove.
Yes, biodegesters are perfectly safe to use in the home, and they are generally odorless
if fed correctly.
But if you do want to ensure there will never be any smells emanating from the pipes, pour
a thin sheen of vegetable oil or citronella in the feeding and outflow pipes every couple
of weeks and lightly cap them.
If you're building your digester outside, there are a couple of things to finish up.
If you're in a hot climate, paint your digester black.
For one thing, the digesters are animal stomachs with the same microbes that live in animals
doing the digestion, and they need to be kept at body temperature.
The sun will heat up a black digester and help keep it warm.
For another thing, the microbes must be kept in the dark.
If light gets into your digester, which it will through the white plastic of the tank,
algae will grow in the tank, produce oxygen, and kill off your anaerobic archaea.
You'll get lots of gas, but it will all be carbon dioxide.
So the tank should be opaque.
If you're building your solar city's digester outside in a cold climate, you need to insulate
it.
We've found that cheap styrofoam and stretch wrap can work okay if you put enough layers,
put your best off if you can get the thing foam sprayed, and in really cold climates,
you should insulate and put into a greenhouse, and even put pex heating coils inside connected
to a source of hot water, like a solar heater, or your shower or bath, using the grey water
as it comes out warm.
But that's the subject of another tutorial.
We hope you will join the solar city's evolution to make the benefits of biogas affordable
and simple for everyone.
It really is easier than you think.
So please, do try this at home.
