Anaerobic digestion is an effective biotechnological process for treatment of different agricultural, municipal and industrial wastes. Use of mathematical models is a powerful tool for investigations and optimisation of the anaerobic digestion processes and for software sensors design as well. In this paper a new version, concerning the gas phase modelling of a simple mathematical model of the anaerobic digestion was developed and verified experimentally. The parameter identification was based on a set of 150 days of dynamical experiments in a laboratory bioreactor. Two step identification procedure to estimate 3 model parameters is presented. The results of 15 days of experiment in a pilot-scale bioreactor were then used to validate the model. On the base of the modified simple mathematical model of the AD software sensors using the differential algebraic approach were designed and studied by simulations.