On-line near infrared monitoring of ammonium and dry matter in biosturry for robust biogas production: a full-scale feasibility study

Heterogeneous substrates fed into agricultural biogas plants originate from many sources with resulting quality fluctuations potentially inhibiting the process. Biogas yield can be substantially increased by optimisation of the organic dry matter load. In this study, near infrared spectroscopy was applied on-line in a re-circulating loop configuration operating identically as a full-scale setup. Ammonium could be modelled in the industrially relevant range 2.42 – 8.52 g L-1 with an excellent accuracy and precision, slope ~1.0, r2 = 0.97, corresponding toa relative Root Mean Square Error of Prediction (RMSEP) of 6.7 %. Also, dry matter in the similar plant relevant range 5.8 – 10.8 weight-percent could be predicted with acceptable accuracy (slope ~1.0, r2 = 0.83, and a relative RMSEP below 8.0 %. Based on these performance characteristics ,it was concluded that near infrared spectroscopy can be applied for optimising the efficiency of current and future biogas plants, as well as in biorefinery contexts converting heterogeneous bioslurry, energy crops, and wastes into value-added products. Adding model transfer capabilities, it is indicated that handheld instrumentation can play a vital role in bringing NIR technology directly in the field, and onto the plant floor – the implications for reliable biogas NIR process monitoring and control are significant.