Biogas digester energy

Each kilogram of biodegradable material yields around 0.4 m³ (400l) of gas.

So in practice, in small scale waste to energy systems, if you have some livestock, plus kitchen and human waste you can meet your cooking and lighting needs easily:

• 2 gas rings for a couple of hours a day will use between 1-2 m³

• Gas lights need around 0.1 m3 (100l) per hour.

 

Driving any kind of engine (eg a generator or a pump) is, however, way beyond the domestic-scale. (Better to go for algal biodiesel!)

Biogas digester lay out

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Biogas digester disadvantages

Most practical to be generated and used at the source of the waste. This is because the energy needed to compress the gas for transport, or convert it into electricity is excessive, reducing the efficiency of biogas energy production.

For safety, basic precautions (see below) must be adhered to.

Biogas digester advantages

Makes good use of organic wastes. You can obtain fuel from sewage sludge and animal slurries first, and prevent runoff and methane emissions at the same time – and you still get fertiliser at the end of the process.

Is a clean, easily controlled source of renewable energy.

Uses up methane, a powerful greenhouse gas.

Reduces pathogen (disease agent) levels in the waste.

Residue provides valuable organic fertilizer.

Simple to build and operate.

Low maintenance requirements.

Can be efficiently used to run cooking, heating, gas lighting, absorption refrigerators and gas powered engines.

No smell (unless there’s a leak, which you’d want to know about and fix immediately anyway!).

Biogas digester temperature

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 digester.

While anaerobic digestion occurs between 32° F (0°C) and 150° F (65°C), the optimum temperature range for methane generating microbial activity is 85°F (29°C) to 95° F (35°C).

Little gas production occurs below 60°F (16°C). 

In colder climates placing the digester in a greenhouse, and perhaps using some of the methane to warm the system, are possible strategies.

Biogas digester size

If generating methane from manure, collect dung for several days to determine average daily dung production. 

On this basis, the appropriate size biogas digester plant can be calculated.

For example, where 55 kg of dung a day is available a 8 m3 plant is warranted; where it’s only 6 kg of dung a day, a 1 m3 plant will suffice.

For a family of 8 with a few animals (say 8-10 cows), a 10m³ digester is a commonly used size in India, with 2 m³ gas storage.

Biogas digester design

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