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Our biogas digester coming along...
For more photos click
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The time has come to install our septic tank. We’ve lived the past 6 months with an outdoor
compost toilet and a drain from the shower that goes into the gardens
somewhere, but a proper house with flushing loos needs a tank to deal with all
that waste.
What happens to our waste? Sewage – it’s so out of sight as to be out of
mind. It’s only when you are building a
house, or working with people displaced by war or disaster that you have to
deal with it.
How
this post is going to work (it’s way longer than planned, but hey)
First off I’m going to describe the standard sewerage disposal and
treatment systems that we would be expected to use around here, then explain
what we are doing and why. So bear with
me. I want to take you on a short journey,
the story of poo. I think it’s worth
talking about because we all have to deal with it, wherever people live, and
it’s the cause of countless deaths in poorer countries, billions are invested
to deal with it in richer nations, it pollutes our rivers and causes so many
problems – and despite all this it can be an incredible resource – for energy
production, soil enhancement and reforestation.
Then I’m going to talk about biogas and show a few examples. Then talk about the plan and biogas septic tank
we’re building outside our house in Portugal.
If you can’t face all this text and reading you can just skip to the
slideshow which is
here (all photos and few words
J).
Conventional
sewerage treatment
The world over, we treat sewage as a public health hazard, and go to
enormous lengths to get rid of it. In richer countries massive sewerage
networks are laid, linking each house to a treatment centre, that then deposits
“treated” remains into the sea, or rivers.
This systems works, but it costs fortunes to install and assumes a
fairly well organised and properly funded agency to maintain it all.
This is not the case in most of the world, where people have to deal
with it in other ways, which usually means open sewers, or houses depositing
their waste into their backyard, or into open drains running through the
community. Pretty gross and a
undoubtedly a source of water-borne diseases.
We live in a village of about 400 houses where there is no sewerage
system, so each house has their own septic tank. These usually consists of a
hole in the ground about 3 to 4m deep into which a column of concrete rings are
set, with a little gap between each one. When the sewage enters the solids sit
at the bottom and the fluids seep away through the gaps between these pipes
(which are usually around 1m diameter by the way). So basically these are soak away pits,
probably polluting the local ground-water with faecal chloroforms (something
you really want to avoid unless you fancy a bit of dysentery or cholera). It’s not an issue here in Portugal as people
don’t drink groundwater – it mostly comes from piped networks, is treated with
chlorine which kills these bugs and anyway, the earth and rock probably filter
out most of these bad guys before they get to the water table. Probably… The
problem is in places with a shallow water table, but that’s another story.
Most septic tanks have two chambers divided by a wall; as the solids
break down (micro-bacteria work their magic on the “solids”) they loose mass
and rise to the surface (think of the scum and dirty floaties you see in
minging beaches when you’re trying to catch a wave). Anyway, these bits float to the second tank
where an overflow takes away supposedly “clean” water. It’s not really clean, but
it’s cleaner than when it went it. This
waste or overflow water can be directed through a reed bed or an area filled
with plants to absorb the water and the nutrients that remain in it. Yes – there ARE nutrients. Something we generally tend to ignore. But most systems just redirect this waste
water back into a passing river or whatever.
When the solids build up too much (after a few years) a tractor come to
“de-sludge” the tank, which it then deposits on nearby fields to fertilise the
place. Gross? Not really, there’s mighty
nutrients in there, just like putting cow manure on the fields, but we’re
getting to that…
The problem
of methane production
As you probably know, sewage stinks, but that’s mostly the acids from
urine making the smell – not the methane, which is an odourless gas. All animal dung, including ours, produce
methane as it breaks down – which happens when billions of bacteria feed on the
stuff and it is they that produce the methane, not our poo. All living things give off gas: plants give
out oxygen, humans breath out carbon dioxide. These mega-bugs give off methane
(and a bit of carbon dioxide too).
Every septic tank leaks methane: it’s lighter than air and seeps out
through soil nice and easy. We don’t know it’s happening because it has no smell. I guess if you were to look at a community
through a gas-sensitive lens (like thermal imaging but for methane) you would
see plumes of the stuff heading sky-wards, from every backyard, or sewage
treatment plants – at least from those that don’t burn the stuff to cook or to
make electricity.
But you’ve probably heard that methane is a fairly potent green-house
gas; apparently it’s 23 times more powerful than carbon dioxide. Climate scientists are worried about melting
permafrost releasing billions of tonnes of methane which will further
accelerate global warming. Well, now
there’s 7 billion of us folk on-planet now, how much do we contribute every day
or year through our septic tanks?
I looked into this and read a recent study that found that septic tanks
provide around 100 kgs of CO2 equivalent to the atmosphere per
person, per year. Arguably this is
negligible, but given there’s so many of us it adds up: let’s take a population
of 10m people (that of Portugal, or about 5% of Pakistan). Using this study we can estimate that this
many people produce around 1m tonnes of CO2 (equivalent) per year,
equal to about 100,000 Hummers (fuel-hungry American SUVs).
Checking against a list of countries’ emissions it’s fairly shocking to
learn that this is equivalent to the annual emissions of the Maldives, or
Swaziland (or twice that of Liberia).
And that’s only from the methane emissions of 10m people’s poo. Take a
population of 1bn (India’s way past this point already) emissions from septic
tanks rise to 100m tonnes of CO2 equivalent per year, roughly equal to the
annual emissions of Belgium or Iraq.
Does this mean that all our sewage waste is a major greenhouse gas? It
looks like it, but I’d need to do more research to pin down these numbers. Has anyone else looked into this? If these
numbers are right surely people would be applying for carbon credits (i.e.
cash) to not emit this methane. (I
wonder if we could use these figures to raise carbon financing to build biogas
systems in Pakistan, where I work half of the time helping rebuild after
floods). Anyway, back to the case at
hand – biogas and our place.
Biogas – a
really cool and viable alternative?
Studying at the CAT a few years ago I came across biogas production, a
fairly straightforward system whereby the sewage tank (or septic tank) is airtight
(so no liquids or gas escape). A small
pipe is embedded into the top of the tank, which is round and dome shaped, so
there are no corners where pressure can cause ruptures. The gas in this case is a methane – carbon
dioxide blend and is flammable: it burns! In smaller systems it’s used for
cooking, in large, city-size plants there’s so much more gas it’s used to
generate electricity via a gas turbine.
I’ve heard that you can even connect this gas to the intake of a petrol
generator to make it run, or compress it and make transport gas. Or generate hot water for heating, showers or
whatever.
My friend Sarah Kent did loads of research (and her thesis!) on biogas
and told me about places in Nepal where for generations people had cut trees
for firewood till the place was completely denuded. Unable to survive there any
more people were starting to migrate to urban slums. So they introduced biogas plants, initially
working just on animal manure. These
provided enough gas for cooking; combined with reforestation they have been
able to deal with the problem in some communities.
Then I met David Fulford, of Kingdom Bioenergy, a former renewable energies
professor at Reading (UK) and for many years the biogas consultant for AshdenAwards. He worked in Nepal for years, introducing biogas design – and now there
are over 900,000 of small family or community sized plants working there.
I asked if he had a design we could use for human waste – in fact can it
work on our sewage? Given that most of the plants I’d seen worked with animal
manure only. He saw no reason why
not. But why not add in food waste, he
suggested.. And he then pointed me towards this amazing video of a system used
in India that works entirely on food waste.
To combine both human and food waste, David suggested we buy an
insinkerator (a kind of blender that fits below sinks, like in those American
horror movies). This would have to be
plumbed into the tank, so thought about well in advance. David kindly agreed to help us by designing a
basic plant (and has since been really helpful on many fronts!).
David also put me in touch with Govinda Devkota, a Nepalese biogas
specialist who has built thousands of these plants. Govinda sent some great technical guidance
material which has helped. (He’s
available for consultancies worldwide by the way!).
On-site
fertiliser too!
One of the best “outputs” from a biogas plant is the slurry – which is
“delivered” above ground by this system for use as fertiliser and is apparently
an incredibly good source of nutrients for trees or gardens. Our land has fairly acidic, sandy soil, quite
low in nutrients so we could really use this.
So could most of the planet as 80% of our agricultural land is now
effectively denuded by years of intensive chemical agriculture.
So in
summary:
- Biogas septic
tanks (digesters) deal with the problem of pollution to rivers or ground
water;
- They provide
a form of energy that would deal with environmental problems locally while
reducing global emission increases from human-based methane production.
- This energy
can be used to provide free, smokeless gas for cooking – freeing people up
in so many countries from the horror of fire-smoke, which WHO reports
kills over 2m people per year.
- The odourless
liquid effluent that biogas tanks deliver could solve the massive food
crisis affecting millions in poorer countries – by providing safe and
highly effective fertiliser to increase soil fertility, which people can
then use to grow more food at home and improve nutrition.
How our
biogas system should work
First off, here’s a picture of the plan David kindly drew up for
us.
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| The design we're using to build our biogas digester. Source - David Fulford, Kingdom Bioenergy |
So far this is what I’ve learned:
- When the sewage in the tank rises to the top of the
“digester pit” (see plan) it reaches the bottom of the dome “roof”. At that
point all the gas will be collected in the “gas dome”. As the gas pressure builds up here it will
“push down” on the liquid in the tank forcing it to “displace” into the slurry
reservoir, (to the right of the plan).
This body of water in the slurry reservoir will then push down on the
gas, providing it with about one bar of pressure – forcing it along the pipe
and to the place of end use (a biogas cooking ring for example). As the gas is used up, pressure drops and the
liquids will slowly return to finding their level again, until gas builds up
again. Get it?
- When the quantity of liquids in the tank increases,
the slurry reservoir will fill up quicker and eventually reach the top, where
there is an outlet. Here we will connect
a pipe and allow it to drain into a small holding tank we’re going to build,
where we’ll install a submersible sewage pump.
When this holding tank is nearing full the pump to engage and push the
waste material uphill, some 30m away to a bit of land we have where we’ll build
yet another tank, where we will add sawdust to help the slurry become more
solid and manageable for transporting around.
David told me about some women in India who added worms, which really
improved this process making it even better and more compost-like, which they
were then able to sell as a great fertiliser.
-
The temperature in the tank should remain fairly warm
to keep the multitudes of friendly bugs happy. This is around 30 degrees
C. Very warm in other words. True, but
they do generate their own heat as they work away in their anaerobic
(oxygen-free) environment down there.
The problem might be the cold earth in which they are surrounded. So we might try and insulate the tank, on top
at least. We also plan to put in a
heating coil connected to our hot water cylinder so if we every have any
“spare” heat we can share a bit of it with our billion-bug generator mates. I’ve included an electric wire so we can put
a thermostat in there too, to keep an eye on temperatures. I’m told that the
earth around here stays at around 17 degrees C, so we’ll see how they
cope.
- We shouldn’t put in too much water into the tank, but
too little isn’t good either. The amount
you get from a low-flush toilet is, David thinks, probably fine. When we wash food waste down the special sink
we can do so with warm water, and so decrease the cold shock factor for our
little buddies.
- We’ll need a dedicated sink for the food waste cutting
thing – we shouldn’t flush all our washing up water down there, obviously. So we’ve found a space near the kitchen where
we can have a special sink, but even then I think we should install a two-way
valve outside, which we will have to turn manually every time we want to direct
waste to the tank. That way we can still
use the sink to wash veg or boots or whatever, and direct that water to the
garden or elsewhere. Complicated? Not
really, just another thing to do and it’s not as if we don’t have enough of
those!
OK that’s all for now folks. I’ll try and keep posting updates as things
progress.
