NOVELTY - Monitoring biogas plants, comprises (i) measuring the concentration of phosphate ions or calcium ions, (ii) comparing the measured phosphate concentration with a predetermined reference value, forming the quotient of the phosphate concentration and calcium concentration and comparing it with the predetermined reference value, and (iii) recording the value, when the measured phosphate concentration exceeds the predetermined reference value by a factor of greater than or equal to 2 and/or the quotient of phosphate concentration and calcium concentration exceeds the predetermined reference value by a factor of greater than or equal to 1.5.    USE - The method is useful for monitoring biogas plants.    ADVANTAGE - The method detects the process fault much earlier and more clearly.    DETAILED DESCRIPTION - Monitoring biogas plants, comprises (i) measuring the concentration of phosphate ions or calcium ions during operation of the biogas plant, (ii) comparing the measured phosphate concentration with a predetermined reference value, forming the quotient of the phosphate concentration and calcium concentration and comparing it with the predetermined reference value, and (iii) recording the value, when the measured phosphate concentration exceeds the predetermined reference value by a factor of greater than or equal to 2 and/or the quotient of phosphate concentration and calcium concentration exceeds the predetermined reference value by a factor of greater than or equal to 1.5. An INDEPENDENT CLAIM is also included for a device for carrying out the above mentioned method, comprising at least one bioreactor, at least one process control unit for controlling bioreactors with at least one input and/or output unit, at least one sensor unit for quantitative determination of the phosphate and/or calcium concentration, at least one output unit for a sensor unit, and at least one metering unit for liquid and/or solids, which are controlled via the process control unit, to introduce liquid and/or solid material in the reactor, where the output unit is coupled to an input unit of the process control unit or the process control of the biogas plant is controllable, and the sensor unit transmits data from the biogas reactor to the process control unit