Biogas from manure

Photo:Rolls-Royce Powersystems AG/flickr.com/ CC BY-NC-SA

Making biogas from manure results in reduced greenhouse gas emissions and the production of a renewable fuel. The technology is far from its full potential in Europe, but also has some serious limitations.

Producing biogas from manure has many advantages. Compared to present management of manure, increased biogas production can reduce greenhouse gas emissions (nitrous oxide and methane), while providing a renewable source of energy that can replace fossil fuels.

How widespread the technology is differs considerably between EU member states, also between countries with similar conditions. For example in 2012 Germany had around 7800 agricultural biogas plants in operation (plants processing only biomass included), while France only had around 100.

The European Biomass Association made in 2009 a rough estimate in different EU member states of the potential in terms of energy production (table). Even if there are limitations in terms of logistics and availability of suitable co-digestion materials is the potential far from being fully developed. In Denmark, one of the member states were biogas digestion is most spread still only 7 per cent of the manure is processed.

At present most of the manure digested to biogas is in the form of slurry (fluid animal manure). It is technically possible to make biogas also of solid manure, but the technology is not as developed. The process is slower, and it has so far been more difficult to make those systems economically viable, even with the subsidies available.

When slurry is produced, stored in slurry tanks and applied to agricultural land, methane and nitrous oxide is released into the atmosphere. The same comes to other types of manure. If the manure is fed through a biogas plant a good share of the carbon content is broken down to methane and later on when used as a fuel to carbon dioxide (one molecule of carbon dioxide affects the climate 25 times less than a methane molecule). Also nitrous oxide emissions from storage and fields will be reduced compared to a situation where manure is handled without degassing.

The digestate is also easier to handle as a fertilizer than untreated manure as it is more concentrated and the process makes nutrients more easily accessible to crops compared to the non-digested manure. The odour of the digestate is also less compared to untreated manure.

Dry matter, thus organically bound nitrogen, is degraded when manure is processed in a biogas plant. In that way substrate gets a lower content of organically bound nitrogen and a higher content of inorganically bound plant-available ammonium-nitrate. More than 80 per cent of the nitrogen in the digestate from manure is in the form of ammonium nitrate. This can be compared to 20-25 per cent for deep litter and 50-75 per cent for slurry.

This can lead to an increase to the nitrogen efficiency since more of the nitrogen can be absorbed directly by the plants compared to the untreated manure. However since the fraction of ammonia is higher there also a greater risk for ammonia losses. Another factor that contributes further is that the pH rises during the biogas process and a higher pH normally corresponds to an increased risk of ammonia emissions. Therefore it is particularly important that the digestate is not spread out-of growing season and directly incorporated into the soil. As manure, the digestate needs to be stored in a way that minimise ammonia losses, with a cover or even more preferably with an airtight lid. So with correct management ammonia losses for digestate is on the same level as manure.

One crucial factor for the sustainability of the technology is however that there is a need to add organic material to the slurry to make the process efficient. This could be organic waste fractions from food industry, public kitchens or households. However this resource could be limited, because of lacking infrastructure or high demand. Fore some organic waste fractions there might also be competition from other industries, e.g. as feed in fur farming.

Another option is to use biomass. Either straw, a residue from growing grains, or growing plants specifically for this purpose. Straw could be a limited resource since it could be burned and used directly for district heating and electricity production.

In Germany and Denmark maize has become a popular biogas plant, mainly to be processed by itself but also together with slurry. However many green groups criticize this development, as maize monocultures outcompetes farming for food and farming that contributes more to biodiversity. Other crops (beets, grains, grass, clover grass and Jerusalem artichokes) could also be used for this purpose, some of them may be better for biodiversity than maize, but the risk of unwanted land-use change and competition with food productions persists. 

Catch crops (sown after harvest of the main crop to prevent nitrogen leaching by incorporating excess nitrogen from the soil) might be a better option, since they are not in direct competition with food production. Therefore, harvesting of these crops (instead of mulching) can increase the available amount of additional biomass for the biogas production.

Deep litter (solid manure mixed with a lot of straw) is also a suitable component to mix with slurry, since it has a high content of dry matter and because the mechanical influence by the animals stamping has made the straw more degradable. This has also the advantage that the farmer can avoid applying deep litter to the fields.  Since nitrogen use efficiency for the digestate is higher than for deep litter, nitrogen losses to air and water are reduced. 

One disadvantage with the digestate as a fertilizer compared to both deep litter and slurry is the lower content of organic matter. This might lead to depletion of soil organic matter and in the long run loss of carbon stock. The use of straw and catch crops for biogas instead of incorporating the biomass directly into the soil might also redcue levels of organic matter. This balance needs to be looked into more in detail, especially in regions with already depleted soils.

Another limitation for the technology is logistics. Biogas plants tend to be most profitable in regions with large-scale industrial animal farms and high concentrations of animals, since there are enough raw materials for large reactors and transport distances are small. The long-term sustainability of this kind of farming is by reason questioned. And it is possible to argue that investments in biogas plants in some areas could lead to the conservation of an essentially unsustainable system.

This doesn’t mean that it is not possible to introduce biogas to at least medium-sized farms or in regions with moderate concentrations of animals. However, one must be aware that there are regions in Europe where the herds are too small and animal farms are too sparse for biogas to be a reasonable alternative.

Kajsa Lindqvist

This article partly based on an unpublished text by Jacob Sørensen and Bente Hesselund Andersen, developed in the project “Pathways to a Nordic food system that contributes to reduced emissions of greenhouse gases and air pollutants”.

Other sources:

A Biogas Road Map for Europe (2009) by European Biomass Association

Task 37 Biogas Country Overview (January 2014) IEA Bioenergy

Table: Estimation of biogas potential from manure for 2020 by European Biomass Association