Nitrogen transformation during fermentation in agricultural biogas plants

The aim of the present study was to determine changes in the organic nitrogen fraction in the liquid fraction of the digestate during laboratory tests of mono- and co-digestion. Three agricultural wastes were tested: distillery residue, cattle slurry and corn silage. In the mono-fermentation tests, the initial total nitrogen concentration depended on the feedstock used. As a result of the hydrolysis and ammonification of the organic matter contained in the feedstocks, there was an increase in ammonium nitrogen concentration after 28 days of fermentation. Analysis of organic nitrogen fractions showed that different organic nitrogen fractions predominated in each of the feedstocks. Four co-digestion tests were performed with 20% and 80% inoculum content and the combination of feedstocks used in the mono-fermentation. Proper evaluation of the transformation of nitrogen forms for the tests with 20 and 80% inoculum participation required the calculation of concentrations from the mass balance, taking into account the effect of changes taking place in the inoculum itself (control test). For these tests, the initial concentrations of nitrogen forms are similar to those found in the mono-fermentation tests. A greater increase in ammonium nitrogen concentrations was found for the test with 80% inoculum than in the test with 20% (for the same feedstock). An increase in the removal efficiency of the dissolved organic nitrogen fraction (DON), and an increase for the colloidal (CON) and particulate organic nitrogen (PON) fractions after 28 days of co-digestion were observed. The trend of change was similar for 20% and 80% inoculum.

Automated summary

The aim of this study was to investigate the changes in the organic nitrogen fraction in the liquid fraction of digestate during mono- and co-digestion laboratory tests. Three agricultural wastes were tested: distillery residue, cattle slurry, and corn silage. In the mono-fermentation tests, the initial total nitrogen concentration varied depending on the feedstock used. Hydrolysis and ammonification of the organic matter in the feedstocks led to an increase in ammonium nitrogen concentration after 28 days of fermentation. Different organic nitrogen fractions were found to predominate in each feedstock. Co-digestion tests with 20% and 80% inoculum content and the same feedstock combination as the mono-fermentation were conducted. Evaluation of nitrogen transformation required calculating concentrations from the mass balance, considering the changes occurring in the inoculum itself. Increases in ammonium nitrogen concentrations were greater in the test with 80% inoculum compared to the test with 20%. Co-digestion for 28 days resulted in increased removal efficiency for dissolved organic nitrogen (DON), as well as increases in colloidal (CON) and particulate organic nitrogen (PON) fractions. The trend of change was similar for both 20% and 80% inoculum tests.