Fig. 4

Linear Pearson correlation with 90% confidence band and corresponding Pearson coefficient of the conductivity at the outlet port with sCOD (A) and SCCA (B) depending on the feedstock for PFR1 (filled symbols) and PFR2 (open symbols). Each symbol represents the average measurement of one week. Measurements during start-up (W1–W6) and the first HRT after a feedstock change were excluded. The data points for PFR2 sCOD-conductivity with dominant lactic acid fermentation with 30% straw (w/w) are not included in the linear correlation. For 66% straw (w/w) sCOD-conductivity, weeks with dominant lactic acid fermentation are marked (LA) and the correlation coefficient excluding these points is presented in brackets, R² values can be found in Additional file 1: Table S3