Process data, substrate and digestate samples from four dry AD plants were collected and analyzed. All plants used SSO as their main substrate but were operated at different temperatures (38–39, 42 and 54 °C). The microbial community analysis demonstrated that each process in the present study had different microbial profiles and that operating temperature strongly influenced the community structure. All processes operated at an ammonia level that is known to inhibit methanogens that directly use acetate for methane formation. The microbial community supported this and showed that a hydrogen-utilizing methanogen (Methanoculleus bourgensis) had an important role for efficient methane production in all processes. The hygenisation analysis indicated that a temperature >42 and <48 °C can be sufficient to reach pathogen reduction according to the ABP regulation. This means that at high ammonia level it might be possible to reach sufficient sanitation even at a high mesophilic/low thermophilic temperature. Being able to lower the process temperature slightly from 52–55 °C can give significant process advantages. However, to confirm this results more studies are needed. The theoretical calculations of heat demand for different process temperatures showed that if no pasteurization step was needed to achieve hygienic standards according to the ABP regulations for high mesophilic temperature (42 °C), 16-25 % less heat demand compared to mesophilic (39 °C) or thermophilic (52 °C) AD was achieved.
Dry plug flow anaerobic digestion (AD) is a relatively new technology for the biological treatment of food waste. To avoid costly external hygienisation, most Swedish dry AD plants wish to operate under thermophilic conditions. However, source separated organics (SSO) with high nitrogen content increase the risk for ammonia inhibition at higher temperatures. This project studied how the operating temperature affects microbial community composition, sanitation, process stability and energy efficiency at existing dry AD plants using food waste as main substrate.