Near-infrared spectroscopy (NIRS) for a real time monitoring of the biogas process

•The use of NIRS to detect changes in specific biogas process monitoring-parameters was investigated.•Changes in tendency in the substrate-specific parameters are very well identifiable.•Fluctuations in the biocenosis degradation products in a biogas plant can be detected precisely after load modification.•NIRS data provide information on process instabilities and critical digester states in a spatiotemporal high resolution. In this research project Near-infrared spectroscopy (NIRS) was applied to monitor the content of specific process parameters in anaerobic digestion. A laboratory scaled biogas digester was constantly fed every four hours with maize- and grass silage to keep a base load with an organic loading rate (OLR) of 2.5 kg oDM/m3 ∗ d. Daily impact loads with shredded wheat up to an OLR of 8 kg oDM/m3 ∗ d were added in order to generate peaks at the parameters tested. The developed calibration models are capable to show changes in process parameters like volatile fatty acids (VFA), propionic acid, total inorganic carbon (TIC) and the ratio of the volatile fatty acids to the carbonate buffer (VFA/TIC). Based on the calibration of the models for VFA and TIC, the values could be predicted with an R2 of 0.94 and 0.97, respectively. Moreover, the residual prediction deviations were 4.0 and 6.0 for VFA and TIC, respectively.