is mostly methane (around 60%) with carbon dioxide (around 40%)
and a little hydrogen and hydrogen sulphide. It is made by anaerobic
bacteria breaking down organic matter in the absence of oxygen (when
the organic matter is waterlogged – i.e. a slurry). Biogas
is generated naturally in the mud at the bottom of marshes; it is
called marsh gas, and often ignites. The process also occurs in
landfill sites, and in the digestive system of humans and other
animals (yes, farts are biogas).
But we can make it ourselves from plant and animal wastes, and even
human waste; soft material is better than twigs / woody material.
It can be burnt to drive a generator, or on a smaller scale, for
cooking or lighting gas lamps. Also, biogas engines have been developed
The equipment in which the organic matter breaks down anaerobically
is called a digester, and there is also some sort of storage container
for the gas produced. Raw biogas can be 'scrubbed' by passing it
through slaked lime, which removes most of the CO2
and increases its calorific value.
The two main types of digesters are the continuous and the batch.
Continuous digesters have a constant throughput of material, and
batch digesters extract the gas from a contained batch of material,
which is then emptied and a new batch added.
Biogas digesters are already widely used in developing countries,
especially India and China, as firewood for cooking becomes scarce.
By the end of the nineties, there were millions of small family
plants in India and China. In the West, digesters tend to be larger-scale,
taking animal slurries and human sewage. But they can be domestic-scale,
for individuals looking to reduce their dependency on fossil fuels.
what are the benefits?
CO2 emissions: because it is a substitute
for natural gas. Because CO2 from biogas is
from recently-alive plant matter (even if it was fed to animals),
it is part of a cycle – i.e. CO2 given
off by burning biogas is absorbed by plants that will provide future
reduces methane emissions: animal manures release
methane into the atmosphere – about 10% of methane emissions
in the US come from animals, according to one survey. When methane
is burnt it releases CO2, but methane is a
more potent greenhouse gas than CO2, so it
is a good idea to burn it rather than release it. However, it's
better for organic waste to be separated and put into an anaerobic
digester instead of collecting methane from landfill sites; and
it would save more energy if all organic waste, including paper,
was recycled instead of landfilled – plus it would prevent
leaching of contaminants into groundwater and soil.
reduces resource use: biogas doesn't need millions of miles
of pipes to deliver it, and doesn't need to be liquefied and shipped
across the world, with all the resources and energy that these things
entail. Plus it saves trees (for firewood). Natural gas is finite,
so won't last forever – and there will probably be wars for
it as it runs out.
creates two renewable resources: sewage sludge
and animal slurries usually end up as fertiliser anyway –
so it's better to obtain fuel from it first, and prevent runoff
and methane emissions at the same time – and you still get
fertiliser at the end of the process. It's the missing link for
those wanting to switch from fossil fuels – many people heat
their homes with wood and their water with solar, and get their
electricity from wind and solar – but cooking is a problem;
it's too expensive with electricity, and agas are expensive, take
a long time to fire up, will make your space too hot in the summer.
Gas is best, and now it can be done without gas bills.
nb: as with other biofuels, we think that the feedstock
(raw materials) should be waste material. We don't think it's a
good idea to set aside large areas of land for growing fuels when
much of the world doesn't have enough food (although the waste from
food crops is fine). See Biofuel
Watch. Also, large-scale digesters need to be fed by large operations
like factory farms or sewage plants. These bring their own problems,
such as hormones, treatment of animals, and energy-intensive transport
and chemicals. We think that the best solution is usually the smallest
scale possible – in this case the domestic scale. Why not
have a go?
can I do?
setting up: batch digesters based on some kind
of drum / container such as the one in the top photo are feasible
on the domestic scale. Continuous digesters are popular in Asia
(right) - an inlet and outlet pit with a concrete or steel gas container.
You can build your own – read a book, see our links page or attend a
sizing: in India, for a family of 8 with a few
animals (say 8-10 cows), a 10m³ digester is recommended, with
2 m³ gas storage. But a typical small family digester will
be around one cubic metre. For cooking and lighting, you don't need
much; every kg of biodegradable material will yield around 0.4 m³
(400l) of gas, and gas lights need around 100l per hour. 2 gas rings
for a couple of hours a day will use between 1-2 m³, so if
you have some livestock, plus kitchen and human waste, you can do
this easily. When it comes to driving any kind of engine (e.g. a
generator or a pump) it's a different matter, and is way beyond
the domestic-scale. How long you leave the material in a batch digester
depends on temperature (2 weeks at 50°C up to 2 months at 15°C).
The average is around 1 month, so gauge how much material you will
add each day, and multiply it by 30 to calculate the size of the
using: the waste input must be a slurry –
so add water if it's too solid. Try and keep the temperature over
30°C if possible; it generates a little heat, but in colder
countries the digester will need insulation and even a little extra
heat in the winter (which could be provided by some of the biogas).
A greenhouse is a good place for it.
safety: methane is explosive – see Adelaide
University's website for safety considerations.
- information, books, course, links etc.
of this factsheet (pdf)
biogas digester at Redfield. Waste material is put into the oil
drum, neoprene cover rises when full of gas, gas is tapped into
container (upside-down plastic drum with water seal) which rises
as more gas enters. When full, gas can be tapped off and used with
the little gas ring
cross-section of a type of digester used for animal and human waste
all over China and the far east. A compost toilet can be incorporated
into this system
on a family farm in India; there's no reason that they can't be
used successfully in the West too
anaerobic digester installed by Greenfinch
on a farm in Scotland