Mid‐Infrared Spectroscopy for Monitoring of Anaerobic Digestion Processes – Prospects and Challenges

To develop an online probe that is not only sufficiently robust but also able to measure crucial process variables in biogas plants is a tough challenge. Therefore, a mid infrared spectroscopic attenuated total reflection (ATR) probe and robust probe fitting were developed. To reduce maintenance of the probe to a minimum, a fully automated probe control, calibration after probe cleaning and analysis of the absorption spectra using Machine Learning were implemented. The relevant wavelengths in the Mid Infrared (MIR) spectrum for organic acids, total alkalinity, and ammonium nitrogen concentration were identified. Finally, intensive lab testing was carried out, followed by operation of the complete on-line measurement system at an industrial biogas plant. In order to improve signal strength and sensitivity, MEMS-based Fabry-Perot interferometers have also been investigated.