Quality and function of anaerobic digestion residues

The growing number of biogas plants in Europe has resulted in increased production of nutrient-rich digestate, which has potential as fertiliser on arable land. Many different organic materials can be degraded in the anaerobic digestion process, with most macronutrients and micronutrients retained in the digestate. Depending on the ingoing organic substrate and management of the biogas process, the nutrient content of digestate varies widely. It can also contain compounds such as heavy metals and organic pollutants that are potentially toxic to soil microorganisms. Previous studies on the effect of digestate on soil microorganisms and crop yield have yielded contradictory results, so further investigations are needed to determine its true fertiliser value. In this thesis, the fertiliser effect of different types of digestate originating from biogas plants operating with various substrates and operating parameters was determined by measuring: 1) general and specific soil microbial activity, 2) bacterial and archaeal community composition, 3) crop growth and 4) chemical and physical composition of different digestates. Soil respiration generally displayed a positive response to digestate addition, but soil respiration curves revealed differences in quality and quantity of organic carbon between digestate, pig slurry and cow manure. However, the total utilisation rate of the organic carbon in digestate, pig slurry and cow manure did not differ. Moreover, digestate showed both stimulatory and inhibitory effects on potential ammonia oxidation, while pig slurry and cow manure had a more consistent inhibitory effect. Addition of digestate to soil resulted in increased wheat yield compared with control soil and mineral fertiliser, but final yield was not as high as that from pig slurry. Digestate was also generally characterised by a higher content of ammonium and lower content of organic carbon than pig slurry and cow manure. Addition of digestate to soil resulted in changes in the microbial community structure, with less pronounced effect in sandy soils, but no change in diversity was detected. It can be concluded that the digestate from biogas plants has great potential as a fertiliser in crop production and does not seem to pose a greater risk of disturbing soil microorganisms than pig slurry and cow manure when spread on arable land.