Frequently asked questions about Biogas

What is biogas?

Biogas is a gaseous mixture of mainly methane (CH4) and carbon dioxide (CO2) formed by the breakdown of organic matter in anaerobic conditions.

What is anaerobic?

The absence of oxygen is a condition referred to as anaerobic.

What is anaerobic digestion?

Anaerobic digestion (AD for short) is an overarching term used to describe the process whereby biogas is formed. Metabolic activity of anaerobic microorganism (bacteria and archaea) turns volatile organic matter into mainly CH4 and CO2.

Is anaerobic digestion the same as composting?

No. Composting, by definition, is an aerobic process., that means it relies on the presence of oxygen. The metabolic pathways of the microorganisms involved in composting consume oxygen and generate CO2.

How is anaerobic digestion different to other renewable energy?

Electricity generation from solar panels (photo voltaic) is only possible when there is sunshine. Wind turbines only produce electricity when the wind blows. Biogas can be stored and thus electricity can be generated on demand 24/7, making electricity generated from biogas baseload power. Furthermore, biogas can be upgraded making it equivalent to natural gas, meaning it can be used for many applications others than electricity generation. Anaerobic digestion is more often than not also a waste treatment technology, solving waste problems.

What can be used as input materials for anaerobic digestion?

Virtually all organic matter is suitable for anaerobic digestion, except for woody materials – containing lignin and cellulose. Examples of suitable feedstocks are:

  • Manure: dairy and beef cow manure/slurry, chicken manure, pig slurry, goat manure, sheep manure
  • Crop residue & invasive plants & energy crops: spoilt crops, peels (cassava, pineapples), straw, oil seed press cakes (sunflower, olive), water hyacinth, sisal, cactus, algae, grass, etc.
  • Industrial & food processing: fruit & vegetable peels, brewers spent grain & yeast, floor sweepings (sugar, flour), overdue show returns (frozen foods, fresh produce, meat, pasta, cereal, baked goods, etc), production overruns, out of specification products
  • Pure organics / food waste: vegetable market waste, kitchen waste
  • Slaughterhouse waste: stomach/rumen content, blood, fat, organs, DAF sludge, dead stock
  • Sewage sludge

How can biogas be utilized?

The two main utilisation options are electricity generation and upgrading biogas to bio-methane.

  • Biogas is a flammable gas thus it can be used as the fuel for reciprocating engines (like in cars) or gas turbines. The chemical energy embodies in the biogas is released during combustion is converted into mechanical energy which in turn drive generators to produce electricity.
  • Biogas is a mixture of mostly CH4 and CO2, by removing the bulk of the CO2 the remaining gas is virtually pure methane. This is referred to as bio-methane. Bio-methane can be compressed to make it more transportable.

What is digestate?

The volatile organic matter in the feedstock is partially broken down to form the biogas. Water and inorganic matter (minerals/ash) and undigestable organic matter remain and form what is referred to as digestate.

What are the by products of anaerobic digestion?

Feedstock is converted into biogas and digestate, these are the only two outputs of anaerobic digestion.

What happens to the nutrients?

As the inputs are organic materials they contain nutrients vital to life, such as nitrogen (N), potassium (K) and phosphorus (P), sulphur (S) and trace elements (various metals). These nutrients are not destroyed in the anaerobic digestion process, meaning the digestate is a valuable fertilizer.

Where do the microorganism come from that take part in the process?

Any organic waste, placed in an anaerobic atmosphere will start to decompose at hand of mircoorganism already present. To speed up the process, new biogas digesters are inoculated with seed material, usually from other anaerobic digesters or animal manure.

What types of digesters are there?

Digesters are typically described according to their fundamental operating principle. CSTR (continuously stirred tank reactor) and plug flow reactor are the two most common anaerobic digester, while fixed bed reactors (garage type) is a more seldom type. CSTRs are well mixed vessels, ensuring that the entire volume is homogenous. Plug flow digesters are not well mixed, to ensure that material fed in on one side is displaced towards the outlet as new material is added, this means the contents of a plug flow digester are not homogenous. Fixed bed reactors contain a stackable organic matter over which a biologically active leachate is peculated.

What is wet and dry anaerobic digestion?

Wet and dry AD is very ambiguous and inaccurate terms. Historically CSTRs were often referred to as wet digesters since the contents are flowable slurries. Plug flow digesters are often referred to as dry AD, however their contents are also flowable slurries, albeit less flowable then CSTRs.

At what temperature do digester operate at?

The relevant anaerobic microorganism thrive in two temperature bands, know as mesophilic 38-42°C and thermophilic (50-55°C) temperatures.