Introducing new monitoring indices from the headspace of biogas digester via e-nose: A case study

•The e-nose can be used to monitor the biogas digester by proper indices.•Recovery Time index is driven by e-nose data to keep digester in steady-state.•Updated reference point compensates the changes in condition of biogas digester.•Biogas digester is well run to treat food waste with irregular VS% by e-nose.•Feeding control in mono-digesting food waste help to prevent overloading.