Weed seed survival following ensiling and mesophilic anaerobic digestion in batch reactors

Westerman PR, Hildebrandt F & Gerowitt B (2012). Weed seed survival following ensiling and mesophilic anaerobic digestion in batch reactors. Weed Research 52 , 286–295. Summary Digestate, the semi‐solid leftover after anaerobic digestion in biogas reactors, is frequently used as a crop fertiliser. The ability of seeds from various weed species to survive anaerobic digestion was tested, because surviving seeds could be spread via the digestate. Experiments were conducted in experimental mesophilic batch reactors, either with or without ensiling as a pre‐treatment to simulate biomass storage in silage prior to use in reactors. The emphasis was on weed species whose seeds have a water‐impermeable layer, because these may be more thermoresistant. Species with hard seeds were more likely to survive ensiling (up to 98%) and mesophilic anaerobic digestion (up to 58%) compared with species whose seeds lack a water‐impermeable layer (≤1%). There was considerable variation in survival probability between and within species, and between silages and reactors. For hard‐seeded species, survival depended on the proportion of hard seeds and on the ability to remain hard‐seeded during treatment. Freshly harvested seeds lost their water impermeability and survived poorly. The survival of tomato seeds, an indicator species used in the sanitation of composts, was comparable with that of the best surviving species without physical dormancy, but was not comparable with that of hard‐seeded species. Tomato could therefore only represent species that are not hard‐seeded.