Manure and slurry are underused energy resources

Despite their pungent odour, the dung and urine produced by cows while indoors are valuable raw materials. While farmers like to spread them on their fields as fertiliser, this can lead to a number of problems. In many places, it results in too many nutrients entering the air and water, causing habitats to become overfertilised and so endangering biodiversity. Moreover, in some regions such as eastern Switzerland, the amount of manure produced far exceeds the available field area. Some is even exported. "But hardly any of it is used for energy," says Vanessa Burg from the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL).

Burg is the co-author of a white paper that brings together the latest research findings on the use of animal manure for energy generation, with a view to their practical application. The paper was written as part of SCCER BIOSWEET, a multi-year energy research consortium supported by the Swiss federal government, which involved up to 15 academic research groups and dozens of implementation partners. "The research focused on biomass conversion processes that are already well developed and have high potential for market roll-out," explains the head of SCCER BIOSWEET, Oliver Kröcher of the Paul Scherrer Institute (PSI).

Energy and climate change mitigation

Switzerland currently has around 40,000 farmers but only 110 anaerobic digestion plants for farmyard manure, providing a total of 1,440 terajoules of energy in the form of methane gas. This corresponds to around 1.2% of the country's gas consumption. The untapped potential is vast, the authors conclude. A figure of 27,000 terajoules could be sustainably achieved, even taking into account the fact that not all manure can be fermented, for example that produced when cows are grazing outdoors.

According to the publication, there are three major benefits to using animal manure for energy rather than just as fertiliser:

• The biogas would replace fossil fuels.
• If the available manure were fermented to produce biogas, 0.8% of Switzerland's greenhouse gas emissions could be eliminated. Animal manure mainly releases methane (CH4), which is very damaging to the climate, and nitrous oxide (N2O).
• The solids left over from fermentation are packed with nutrients and could replace industrial artificial fertilisers.

"Biogas is highly versatile," says Burg, who conducts research into sustainable bioenergy sources at WSL. "It can be used to power cars or tractors, as well as to produce heat and electricity. The gas can even be stored and used when there's no wind or sun, for example at night and in winter."

Too few incentives

So given all these apparent advantages, why isn't all of Switzerland's solid and liquid manure being fermented? The main barriers are economic. One problem is that the sources are very unevenly distributed around the country, so transportation is required: most of the nation's manure is produced on the Swiss Plateau, in the cantons of Bern, Lucerne and St Gallen. A survey of farmers has found that many are positively disposed to the generation of energy from animal manure. However, there are obstacles in the form of high initial investments, too low energy prices and the complicated logistics involved in operating an anaerobic digestion plant. The smallest plants currently require manure from around 80 cows, while the average farm has just 27, and farmers tend to reject the idea of shared facilities.

In addition, the licensing procedures are often very lengthy, according to survey respondents. Furthermore, subsidies and surcharges, such as the pre-existing feed-in tariff, are often geared solely to electricity rather than gas production. "Gas is frequently overlooked in discussions about the energy transition," says Vanessa Burg. "We need to be promoting alternative sources of gas as well."

Another factor to consider is that technical innovations in the processes could make the plants more cost-effective, as the white paper also shows in detail. Pretreatments with microorganisms boost the energy yield, as does separating the solid and liquid components of animal waste. Also, the largely unused waste heat from the plants contains yet more energy. Taken together, these factors could make investments in anaerobic digestion plants more attractive to farmers. So-called hydrothermal processes are an attractive alternative to the fermentation of manure and slurry, since the biomass can be converted largely without transformation losses.

This could benefit power supply nationwide: because animal manure is mostly produced in winter, when cows are kept indoors, it could offset supply shortfalls during the colder months of the year. According to the white paper, this would make Switzerland less dependent on imports, especially of fossil fuels.

"The considerable potential of manure and slurry for the energy transition and CO2 reduction should encourage industry and policymakers to harness the technological opportunities and create the conditions to enable plants to be operated economically," says WSL bioenergy researcher Oliver Thees.

The BIOSWEET project was mainly funded by the Swiss Innovation Agency Innosuisse (formerly CTI).