NOVELTY - The method comprises monitoring an anaerobic digester (AD) (20) by: providing an AD offline extended Kalman filter (EKF) having an offline dynamic model of the AD and an AD online EKF having an online dynamic model of the AD, where the offline and the online dynamic models of the AD comprise states, process material balances, energy balances, bio-chemical reaction kinetics, estimated parameters and adapted model parameters that are a subset of the estimated parameters; and providing a historical operation data for the AD.    USE - The method is useful for operating an anaerobic digester, which is useful in a plant for treatment of wastewater (claimed).    ADVANTAGE - The method allows economic and stable operation of the anaerobic digester without any defects thus improving the treatment efficiency of the plant.    DETAILED DESCRIPTION - The method comprises monitoring an anaerobic digester (AD) (20) by: providing an AD offline extended Kalman filter (EKF) having an offline dynamic model of the AD and an AD online EKF having an online dynamic model of the AD, where the offline and the online dynamic models of the AD comprise states, process material balances, energy balances, bio-chemical reaction kinetics, estimated parameters and adapted model parameters that are a subset of the estimated parameters; providing a historical operation data for the AD, where the historical operation data comprises historical measured input data, historical measured output data and historical laboratory analysis data; identifying the estimated parameters of the offline dynamic model of the AD using the AD offline EKF and the historical operation data for the AD; importing the estimated parameters from the offline dynamic model of the AD into the online dynamic model of the AD; providing a real time operation data for the AD to the AD online EKF, where the real time operation data comprises of real time measured input data and real time measured output data of the AD; updating the adapted model parameters of the online dynamic model of the AD; estimating a model based inferred variables of the AD using the AD online EKF, the online dynamic model of the AD, the real time measured input data of the AD and the real time measured output data of the AD; and providing the adapted model parameters of the online dynamic model of the AD and the model based inferred variables of the AD to an operator of the AD. INDEPENDENT CLAIMS are included for:    (1) a method of operating a membrane bioreactor;    (2) a method of operating a wastewater treatment plant;    (3) a waste water treatment plant; and    (4) a system for monitoring and controlling a wastewater treatment plant.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of a AD.    Anaerobic digester (20)    AD control system (200)    Supervisory control system (201)    Low level control system (202)    AD extended Kalman filter (250)    Dynamic model of AD. (260)