Impact of nickel and cobalt on biogas production and process stability during semi-continuous anaerobic fermentation of a model substrate for maize silage

The importance of nickel and cobalt on anaerobic degradation of a defined model substrate for maize was demonstrated. Five semi-continuous reactors were operated for 250 days at 35 °C and a well-defined trace metal solution was added to all reactors. Two reactors each were limited regarding the concentration of Ni2+ and Co2+, respectively, for certain time intervals. The required nickel concentration was depending on the organic loading rates (OLR) while, for example, above 2.6 g ODM L−1 d−1 nickel concentrations below 0.06 mg kg−1 FM in the process significantly decreased biogas production by up to 25% compared to a control reactor containing 0.8 mg Ni2+ kg−1 FM. Similarly, limitation of cobalt to 0.02 mg kg−1 FM decreased biogas production by about 10%. Limitations of nickel as well as cobalt lead to process instability. However, after gradual addition of nickel till 0.6 mg and cobalt till 0.05 mg kg−1 FM the OLR was again increased to 4.3 g ODM L−1 d−1 while process stability was recovered and a fast metabolisation of acetic and propionic acid was detected. An increase of nickel to 0.88 mg kg−1 FM did not enhance biogas performance. Furthermore, the increase of cobalt from 0.05 mg kg−1 FM up to 0.07 mg kg−1 FM did not exhibit a change in anaerobic fermentation and biogas production.