Biogas: the energy value of waste

What a lack of oxygen does

Where organic materials are decomposing, but there is no oxygen, anaerobic digestion (deprived of air) takes place. It is a natural process in which bacteria feed on biomass, either a plant or an animal, producing biogas, i.e. a biofuel with a high energy content and high calorific value, which can be used as a renewable energy source.

In nature, biogas is produced in environments such as wetlands, soil, the sediments of rivers, lakes and oceans, and even in the digestive organs of animals. The result is a gas saturated with water, composed mainly of methane (CH4) – in a proportion that can reach up to 80% of the volume –, but also carbon dioxide (CO2) and traces of hydrogen (H2), nitrogen (N2), oxygen (O2) and hydrogen sulphate (H2S), among other elements.

Given the potential of this product to replace the consumption of natural gas or liquefied petroleum gas (LPG), both of which are extracted from non-renewable resources, and to also produce electrical and thermal energy, this chemical reaction is increasingly replicated under controlled conditions and with increasingly efficient and sustainable technologies. But it is still necessary to feed the micro-organisms with their preferred organic waste.

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Where biogas comes from and where it goes

In addition to its role in terms of energy transition, due to the use it makes of waste, biogas also contributes to a circular bioeconomy. Its production reuses, for example: agricultural waste such as bark, foliage, straw and crop residues; animal waste (a cow weighing 450 kilos produces an average of 360 kilos of manure a day); domestic and industrial waste and sewage; and forestry and wood waste.

Biogas can thus be recovered directly from landfills or water treatment plants, reducing pollution. And in agriculture, livestock farming and industry, as well as environmental benefits, biogas production systems help to reduce costs. Not only do they reduce the need to treat waste, but they also use it to produce energy.

Another benefit is that the solid residue that is produced from obtaining biogas can also be used as organic fertiliser and the liquid effluent can be applied as biofertiliser, as it contains nutrients such as nitrogen, phosphorus and potassium.

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A gas full of energy

The decarbonisation of the economy requires a greener gas system, since electrification is not a technically or economically suitable solution for all sectors. Biogas has the advantage that it can be purified to generate biomethane, which can be injected into the existing natural gas network or used as biofuel for vehicles, with an energy potential similar to that of natural gas and carbon monoxide emissions up to thirteen times lower than those of ordinary petrol.

In terms of energy equivalence, according to a 2015 survey by the National Ecological Foundation in Brazil, each cubic metre of biogas is the energy equivalent of:

• 0.40 kilos of LPG (cooking gas);
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• 0,61 a 0,70 litres of petrol;
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• 0,55 litres of diesel;
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• 0,80 litres de alcohol;
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• 1,25 a 1,43 kWh of electricity;
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• 1,6 a 3,5 kilos of firewood.

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Innovative circularity projects

The main reason for the increased use of biogas is its ability to turn what was once an environmental liability into an energy asset. In Portugal, in July 2022 the national gas network registered the introduction of biomethane produced from landfill waste for the first time, and the public strategy to support biogas was presented in January 2024.

LIFE Multi-AD 4 AgroSMEs is one of the projects being developed by Portuguese companies to recover biogas generated in water purification plants through the co-digestion of sewage sludge for industrial self-consumption and the recovery of biomethane for direct injection into the gas grid. This is an innovative automated system for the treatment of wastewater from small and medium-sized companies, based on technology patented by Bondalti Water, and two large-scale facilities operating in a brewery and a wastewater treatment plant shared by four factories in the food sector.

Multi-AD technology has made it possible to produce high-quality biogas with a methane content of over 80%, which is used as a source of thermal energy in the purification system itself, although it can also be used in other areas of the plant. In this way it contributes to the decarbonisation of industrial processes, reducing the carbon footprint associated with purification by more than 50% and reducing the cost of sludge management by around 80%.

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