The temperature is a significant working parameter of biological waste water cleaning. The heat behaviour, described by temperature change of the water from put in to flowing-off, depends on the rate of biologic processes, the outside and water put-in-temperature, and also on the outside air temperature. To understand it has to be differential into a transmission heat flux, an air stream heat flux and an intern heat flux by biologic activities. The developed model will be proved by a real trickling filter in a working clarification plant. Predictions about the heat fluxes and derivations for operating characteristics are possible, for instance, change of CSV input concentration or using of air damping devices (closing of air registers).