A biogas digester septic tank for our home

Our biogas digester coming along...
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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 wordsJ).

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. 

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.