Weed seed survival during mesophilic anaerobic digestion in biogas plants

Westerman PR, Heiermann M, Pottberg U, Rodemann B & Gerowitt B (2012). Weed seed survival during mesophilic anaerobic digestion in biogas plants. Weed Research52, 307316. Summary Digestate, the semi-solid leftover after anaerobic digestion in biogas reactors, is frequently used as a crop fertiliser. The ability of seeds from five plant species (four weed species and one vegetable crop) to survive commercial biogas plants was tested, to assess the likelihood that these could be spread via digestate and to compare with results obtained from laboratory-scale reactors. Seeds were selected from among the best surviving species in experimental batch reactors in an earlier study. They were exposed for 19 days in two large-scale, commercial mesophilic biogas plants. The resulting exponential decay functions were estimated. The decimal reduction time, D, was estimated at 1.5 days for Abutilon theophrasti population D2003, 2 days for A. theophrasti population ES2008, 5.8 days for Malva neglecta, 4.7 and 19.7 days for Chenopodium album, and 1.29.1 days for Fallopia convolvulus. Given the differences in ranking of plant species, laboratory reactors are not necessarily a good model system for commercial reactors. Seeds could survive passage through biogas plants, although at (extremely) low numbers. If the objective is to eliminate all risks and dispose of all weed seeds, it will be necessary to either add extra sanitation steps, or to identify and manipulate the factors that are responsible for high seed mortality. However, it is currently unknown what these factors are.