Transfer of a near infrared spectroscopy laboratory application to an online process analyser for in situ monitoring of anaerobic digestion

► We confirm the potential for robust in situ monitoring of anaerobic digestion. ► Total volatile fatty acid concentration can be visualised by near infrared spectroscopy. ► Validation of the models was performed on an independent spectra time series. ► Piecewise direct standardisation allows pooling of spectra from different instruments.