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.
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