MSc Christoph Lück, 2012

Digestate, the left-over of the substrate after anaerobic digestion in biogas reactors, is frequently used as fertilizer on arable fields. There is a risk that weed seeds that entered the reactor via manure and fresh or ensiled plant biomass survive the conditions in the biogas reactor and are distributed via the digestate. Here, I studied the probability that grass seeds survive the processes in the biogas production chain by determining the germinability and viability after exposure to ensiling in glass jars, anaerobic digestion in experimental batch reactors, or both. Grass species used were Cynodon dactylon, Digitaria ciliaris Pennisetum clandestinum, Lolium perenne, Alopecurus myosuroides, and Agrostis stolonifera. Some of these were selected on the basis of an anticipated high survival potential (endozoochory), identified by a literature search. It has been speculated that endozoochory selects for seed types that are resistant to the conditions in the intestinal tract of herbivores. These conditions are similar to those during anaerobic digestion. Tomato seeds were included for reference.

None of the grass or tomato seeds survived ensiling. Ensiling followed by anaerobic digestion also resulted in 100% mortality in tomato and all grasses. Exposure to 30 days of anaerobic digestion resulted in 100% mortality of all seeds, except in the case of C. dactylon where a certain percenatge of the seeds survived. The results show that the seeds of some grass species have the ability to survive anaerobic digestion.