From theoretical calculations validated by the results of experiments, it was shown that the properties of the washing liquid that flows into biotrickling filters at livestock facilities determine the maximum ammonia removal efficiency (RE) that can be achieved. For a well-designed biotrickling filter not limited by hydrodynamic conditions, the ammonia driving force is mainly controlled by the pH, and also by the electrical conductivity (EC L), although to a lesser extent. With the washing liquid at a given temperature, therefore, the removal efficiency depends on the gaseous ammonia concentrations to be treated (C Gin ; usually lower than 20 mg NH3 m-3) and on the values of the pair (pH, EC L). Theoretical prediction of the maximum removal efficiency that can be achieved by simply measuring the properties of the washing liquid, in line or from samples, could be a useful diagnostic tool to identify possible failure of the apparatus and to manage the electrical conductivity to guarantee removal efficiency at a desired rate.