Maurer & Müller (2012) found ammonia nitrogen (NH4-N) losses of 77 and 91% during the drying of two types of separtion solids of digestate. The loss of total N for the two types were 63 and 70%, respectively, which indicates that losses of organic bound N was low. Almost all N in the dried products were organic bound. The feedstock of the biogas plant was a mixture of residues from food and feed production, pig, and cattle slurries as well as energy plants.

The losses of NH4-N during drying can be reduced to 17-27% by acidifying the input material. However, in that case a simultaneous C loss of about 7-8% has been observed, equal to about one fifth of the organic matter.

Read more about Thermal drying.

A unit for drying digestate separation solids. The photo is from https://www.scolarisrl.com/en/digestate-dryers-manufacturers/.

The price of nitrogen fertilisers has increased during 2021 due to increasing energy prices, and the Russian attack on Ukraine by end of February 2022 has only worsened the situation. The result is that nitrogen fertilisers in April 2022 is about three times more expensive than at the beginning of 2021. Other mineral fertiliser nutrients, such as P and potassium (K) has increased dramatically in the same period due to the increasing energy prices. This situation raises the question whether we could utilise the nutrient in livestock manures better? Currently, around half of the nutrients in livestock manures are lost. A higher Nutrient Use Efficiency (NUE) of livestock manures would mean reduced environmental and climate impact of farming as well as less dependency on mineral fertilisers.

Agro Technology Atlas comprises scientifically validated descriptions of about 70 technologies for reducing the climate impact and environmental load of farming, and securing an optimal resource economy in primary food production. Among these are especially three groups of technologies that contributes to increased NUE in fertilisation with livestock manures*. These are:

• Slurry separation technologies, that are relatively cheap manure processing technologies with which slurry is separated into separation liquids with a high NUE of N and phosphorus (P), while the organic bound nutrients with low NUE are in the separation solids, that can be exported and for instance used as a valuable influent in an aerobic digestion process. Fertilisation with separation liquids rather than raw slurry would typically mean that the NUE of N is increased from 75% to 95%. In the case of P, the separation liquids contain a higher share of water-soluble P, and the total amount of P needed for fertilisation on a farm can be adjusted via the separation strength. Separation by use of a screw press will typically ensure that the amount of P in the separation liquids is complying with the needs for P fertilisation, and the P field and farm gate balances can thus in this way be eliminated or reduced to a minimum. 
• Anaerobic digestion is due to the fermentation processes degrading a share of the organic matter in the influent biomass, depending on the digestibility of this as well as the retention time and the process temperature among other factors. Also, the pH would typically be increased with 0.5 - 1 units during the process, which also has some impact. Generally speaking, anaerobic digestion increases the NUE for N with about 15-20%, whereas the NUE for phosphorus increases with much more, typically 50-60%. 
• Slurry acidification comprise three sub-technologies, namely in-field, in-tank and in-house acidification with each of them having characteristics that makes them most suited in specific situations. In-field and in-storage acidification will in general terms save the volatilisation of 15 kg N per ha via ammonia emissions, while this figure is about 30 kg per ha for in-house acidification because it  also reduces ammonia volatilisation from stables and stores, besides from the field spreading. These figures are given under the assumption that fertilisation happen with a dose of 30 tonnes of slurry per ha. Furthermore, the adjustment of the pH results in a higher availability of P in the acidified slurry, wherefore acidified slurry is an effective fertiliser where a high effect of the P fertiliser is required, such as in case of start fertilisation of maize.

A proverb says that "An accident rarely comes alone.". It is in this case fair to turn this around and say that "A good agro-environmental technology has several advantages." and all three mentioned technologies provides more benefits than the basis for an increased NUE of livestock manures. For example, an aerobic digestion is also providing additional energy production, and slurry acidification, especially in-house acidification, contributes to reduce greenhouse gas emissions from livestock farming.  

Please find the scientific substantiation for the above in https://www.agrotechnologyatlas.eu. For slurry acidification, see under "Other housing systems", "Storage systems" and "Spreading and outdoor handling".

* Whereas the NUE is not a standardised and fixed term, the used definition in this news post is that it is a percentage describing the share of the fertilised nutrient that is taken up by the crop. A NUE of 90% for nitrogen (N) is for example corresponding to a crop uptake of 90 kg N in case the crop is fertilised with 100 kg N.

Anaerobic digestion of livestock manures contributes to increased Nutrient Use Efficiency of livestcok manures and reduces the dependency of mineral fertilisers

The 'CNP flows interactive model' is for estimating and illustrating combined effects in the entire manure / crop / feed loop of on-farm measures related to CNP management at farms or in regions, such as the impacts of employment of various livestock manure processing technology. The tool is per default illustrating a NVZ baseline scenario, assuming livestok farming situated in a Nitrate Vulnerable Zone with the maximally possible number of animals with repect to the 170 kg N limit per ha for bringing livestock manure to the fields for fertilising, as given by the Nitrates Directive.

Read more about the CNP flows interactive model in this article at ResearchGate.

The 'CNP flows interative model' tool is developed by Organe Institute and made available at AgroTechnologyATLAS here.

The 'CNP flows interactive model' illustrates CNP flows in the manure / crop / feeding cycle at farms, including the location and magnitude of the flows.

Baltic Compact has issued a policy brief that through calculation of environmental services from biogas production, based on examples of using maize silage or cattle slurry as substrate, clearly shows, that biogas on basis of maize silage deteriorate nature and environment, while the opposite is the case if biogas is produced on basis of a waste material like cattle slurry.

You can download the policy brief here.

Subsidisation of biogas from maize silage not justified

AgroTechnologyATLAS and VERA Verification of environmental technologies for agricultural production - has initiated a collaboration to disseminate knowledge about agro-environmental technologies at an international level.

Peter Engel, head of the VERA secretariat says: The collaboration with AgroTechnologyATLAS is a good opportunity for the manufacturers of environmental technologies with a VERA Verification Statement to expose their technologies also to markets outside the VERA collaboration. Further it will help farmers and environmental authorities to get access to information about an even wider range of environmental technologies with a documented environmental efficiency.

VERA is collaboration between the national Danish, German and Dutch environmental and agricultural authorities. A VERA Verification Statement granted in one of these countries will also be recognized in the additional countries. A VERA Verification Statement is granted on the basis of a completed VERA test. The VERA tests are performed according to one of the five VERA test protocols for environmental efficient technologies for agricultural production.

VERA is planned to expand in order to comprise new types of technologies and to include more countries. The collaboration with AgroTechnologyATLAS will be a step in this direction.