Investigation of standardised and adapted inocula for biomethane potential tests

In biomethane potential (BMP) tests, different sources and origins of inocula might cause significant variations in the tests’ results, preventing from acquiring a harmonised and standardised evaluation of the different substrates. In this study, a method to produce standardised and storable inocula was elaborated. In anaerobic bioreactors, inoculum based on Dried Destined Grain Substrates (DDGS) pellets was initially produced as substrate under defined conditions. Freeze-drying was applied in the framework of this study as a long-term conservation method for the anaerobic inocula. In order to ensure a closed system with anaerobic conditions at all times, a test system was developed allowing accurate and representative mass-balance experiments. The test results demonstrated that inocula preservation and re-suspension in BMP tests is possible; the limit value of the expected methane yield of the positive and lyophilised DDGS-substrate- control, was reached. However, the lyophilized inoculum with DDGS as substrate showed a significant lower specific biogas production compared to the postive control. After the repeated addition of DDGS after 68 days of incubation, no lag phase was observed in biogas production in the lyophilized batch. In this case, a lower substrate degradation compared to the first step was observed. Therefore, further investigations are necessary to reduce the lag phase and to develop a two-stage BMP test in which the standardised and storable inoculum can be added at defined concentrations in capsule or tablet form.