cycle is initiated when the ph value of the liquid running down from the trickling filter 5 falls below a predetermined value in this case the control system ensures that the moistening frequency and optionally the cycle time are automatically adjusted irrespective of the described control through the ph value the frequency of the spray cycles can also be controlled through the values for the total carbon content of the clean gas 10 issuing from the plant the total carbon content can be measured by a flame ionization detector fid in this method of control the spraying frequency is best increased when the elimination performance difference between input and output falls below a predetermined value a further development of the process is described in the following with reference to fig3 and 4 the moistened circular areas 17a to 17g associated with the spray cones 7 of the spray nozzles 6 on the surface of the trickling filter are shown diagrammatically in fig3 the surfaces 17a to 17g are not sprayed at the same time but instead with phase displacement the intermittent phase displaced moistening is illustrated in fig4 the seven different partial diagrams are associated with sections 17a to 17g on the surface of the trickling filter.