Evaluation of agricultural feedstock-robust near infrared calibrations for the estimation of process parameters in anaerobic digestion

An external validation of near infrared (NIR) calibrations for the estimation of process parameters of anaerobic digestion (AD) is reported. Our results, which were obtained using slurry from 26 agricultural biogas plants that were fed with various types of energy crops and livestock residues, support the use of NIR spectroscopy for the screening of AD process parameters, even under challenging conditions. With a root mean square error of prediction (RMSEP) of 5g kg(-1) fresh matter (FM) for the volatile solids and of 0.29 kg(-1) FM for ammonium (NH4-N), both parameters were soundly estimated. Total volatile fatty acids (VFA), with a RMSEP of 0.99 kg(-)1 FM, confirmed the use of NIR analysis for the rapid detection of the accumulation of the total acidity in the slurry. For the tested samples, the alkalinity was found to be a function of the concentrations of NH4-N and total VFA and, hence, can be as well estimated with NIR spectroscopy with a precision of 1 g kg(-1) FM. By applying the models to a series of samples that were spiked with propionic acid, the specific estimation of this single acid was demonstrated.