Potential of multivariate statistical process monitoring based on the biogas composition to detect free ammonia intoxication in anaerobic reactors

•Free ammonia nitrogen (FAN) extreme intoxication was reached at ∼5 gFAN Lslurry−1.•Multivariate statistical process control (MSPC) models were built using only the biogas composition.•Recursively updated MSPC models detected a decrease of reactor performance early.•Early signs of process perturbation coincided to a drastic change in the reactor microbiome.•These models performance was negatively affected by a drastic shift in the reactor microbiome.