The process of anaerobic digestion (AD) of organic matter produces biogas. It mainly consists of methane (50-70%), carbon dioxide and a small amount of mixed gases including hydrogen sulphide. It is a renewable gas which gives a lot of flexibility for utilisation. Using combined heat and power engines (CHP), it burns to produce renewable electricity and heat. Alternatively, certain technologies can purify it by removing the carbon dioxide and other gases. This is known as biogas upgrading and the resulting gas is called biomethane which has multiple valuable uses. Thereafter, it can be injected into the gas grid or used as transportation fuel. As such, biomethane is one of the most important biofuels for our green future. Biogas production is truly vital to our future circular economy.
Anaerobic digestion is an environmentally-friendly, cost effective solution to process virtually all types of organic waste. These include food waste, farm waste (manures, slurries, etc.), food and drink production waste, garden waste and more. Furthermore, many farmers grow energy crops, such as energy maize and hybrid rye, specifically for AD. Some of them are break crops which grow in-between standard food crop cycles. This enables farmers to utilise their land during void periods, produce valuable crops which earn them income, improve soil condition and at the same time contribute to renewable development.
What Happens to Organic Waste?
At present most organic waste worldwide ends up in landfill sites. There, it subsequently decomposes and produces methane which escapes into the atmosphere causing vast pollution. Methane is an extremely potent gas which causes 30 times more pollution than an equivalent amount of carbon dioxide (CO2) as it can trap heat much more effectively. In addition, organic waste at landfills also generates leachate (wastewater) which presents many challenges. In many parts of the world landfills have no leachate containment measures. As a result, leachate drains freely into natural rivers and lakes causing severe water pollution.
More recently, mixed municipal solid waste (MSW), which includes organic waste, has been sent to incineration plants (the most common type of energy from waste plants). Incineration produces electricity and heat. Though effective for diversion from landfill, it is not a particularly efficient and environmentally-friendly method of processing organic waste.
Biogas Production – The Better Alternative
When organic waste is relatively pure or can be separated from mixed waste, anaerobic digestion is regarded by most as the best processing method. Biogas plants extract the energy content by converting it to useful biogas. Unlike other renewable technologies, such as wind and solar, biogas production is very efficient and also allows energy storage. Combined heat and power (CHP) engines can burn it straight away to produce renewable electricity and heat in a balanced way (not intermittent). Alternatively, upgrading it to biomethane (virtually pure methane) also allows energy storage, transportation and usage where and when necessary. Biomethane is useful in both gaseous and liquid form. It has similar properties to natural gas but it is a clean renewable biofuel. It has numerous benefits and plays an important part in making both gas grids and the transportation sector greener.
In addition to generating renewable energy, the by-product of the biogas production process is known as digestate. It is a nutrient rich natural fertiliser with numerous proven benefits for farming. Digestate replaces traditional chemical fertilisers, thus it saves costs to farmers and reduces pollution. As such, AD is effectively a zero to waste solution for organic waste.