How Is Biogas Made?

Processed from a wide variety of different feedstocks, biogas is an environmentally friendly form of fuel that can be used to power vehicles or employed in many different industrial applications. It offers numerous advantages over traditional fuel sources, including reduced emissions of harmful contaminants like particulate matter and black carbon, as well as the fact that all materials used to create it are completely natural and renewable.

These materials can vary significantly depending upon the resources that are available, but can include biodegradable waste produced by consumers, businesses and industrial facilities, manure from livestock, crop residue, sludge from wastewater treatment facilities and out-of-date food from shops and restaurants. Regardless of the source, the materials involved undergo the same following process:

Step 1 – Crushing

The solid matter is crushed in order to achieve a uniform consistency. Nutrient-enriched water from a later stage of the process is mixed with the feedstock in order to reduce the proportion of solid materials to around 10% of the overall volume of the substance. This process is known as slurrification. Any unwanted waste particles, such as food packaging, are also filtered out at this stage.

Step 2 – Pre-digestion

Next, the slurrified feedstock is mixed with imported slurry in a pre-digester tank, where bacteria secrete enzymes capable of breaking down the substance into even smaller constituent parts. Prior to entering the digestion tank, the biomass must also be sanitised by heating it to 70°C for one hour to eliminate any harmful bacteria which may have survived the previous processes.

Step 3 – Anaerobic digestion

The biomass is now placed in a biogas reactor and heated to a temperature of around 37°C, since microbes prefer heat to thrive. Over a period of weeks, the microbes will feed on the carbohydrates, lipids and proteins in the biomass and turn them into carbon dioxide and methane. These gases are then harvested from the reactor via a spherical chamber at the top of the unit.

Step 4 – Digestate disposal

Any solid or liquid residue left over the from the anaerobic digestion process is sent to a post-digestion reactor, where it is prepared for use as organic fertiliser or for other agricultural purposes. The solids are separated from the liquids and processed for the aforementioned agricultural applications, while the liquids are returned to phase one of the process and mixed with the initial feedstock.

Step 5 – Purification

Before the biogas can be used as a fuel source, it must first be purified of any remaining contaminants. This is done by filtering the gas into columns and subjecting it to cascading water at very specific temperatures and pressures. It can also be purified via the use of carbon filters to arrive at a product which is comprised of at least 95% (and usually 98%) methane. And that’s it! The biogas is ready to use.