Anaerobic digestion of sugar beet : fate of plant pathogens and gas potential

Abstract: Sweden and Europe aims at increasing the use of renewable energy. Biogas represents one way to reach this goal. Biogas, produced from organic waste or crop materials, can be used for production of heat and electricity and as fuel for vehicles. The biogas process is also advantageous as it mediate the recirculation of nutrient from waste products to arable fields. This can be achieved by spreading the nutrient rich bio manure, which is left after digestion, on arable fields. Organic material, used as substrate in a biogas process, can contain different contaminating organisms such as different fungi. Some fungi are plant pathogens and if these survive the biogas process and is spread on arable land they might infect the new crop, thus leading to reduced yields and an increased need for fungicides. If storage pathogens are spread they may survive on organic debris on the ground and damage the harvested crop during storage. Therefore it is important to evaluate potential risks when materials infected with plant pathogenic fungi are used as substrate in a biogas process. Furthermore, if fungi are killed, the biogas process offers an alternative way of using crops with not good enough quality for food or feed production. Presently, it is however at unclear what levels of gas production that can be reached with such "low quality" materials. The aim of this study was to investigate fate of plant pathogens during mesophilic anaerobic digestion and also to investigate gas production potential of infected and uninfected sugar beet, both fresh beet roots and those stored for one year. Survival studies were performed for three different sugar beet field pathogens, Aphanomyces cochlioides, Pythium ultimum and Rhizoctonia solani, causing emergence diseases, and for two different storage pathogens Fusarium culmorum and Botrytis cinerea. The gas production potential was determined in a batch test system started with inoculum from two different large scale biogas plants. The measurement of gas production potential showed that both uninfected fresh and stored sugar beets produced more methane (per g added Volatile Solid) than beet material infected by the different fungal pathogens. Survival studies performed with spores of Fusarium culmorum and Botrytis cinerea demonstrated a very short survival time, less than 2.5 hours. For two sugar beet pathogens, Aphanomyces cochlioides and Pythium ultimum, it was not possible to obtain the most resistant survival structure, the oospores, and the survival test was therefore performed with only mycelia and/or oogonia. Both these structures survived for a very short time. In order to predict the fate of these fungi in a biogas process, more studies are needed. However, even though it was not possible to test all fungal structures of interest, the results so far suggest that it is unlikely that fungi would pose a great problem in bio manure. Before the material is digested and used as bio manure it passes several steps, sanitation, anaerobic digestion, post-digestion, aerobic storage, with varied environments. Therefore, it seems unlikely that a fungus can adapt and survive through all of those steps. Conclusively, a biogas production process could be a good way to dispose of contaminated organic material. However, it is important to consider the lower methane yield when planning the biogas plant.