Drying from a free water surface and from a waste activated sludge was studied in three separate experimental arrangements. Evaporation in a closed chamber with a surface area of 1.0 m2 was studied with temperatures between 20 and 60°C and air flows between 75 and 300 m3/hour. The rate of evaporation varied between 351 and 746 gram/m2 hour. The efficiency varied between 11 and 20%. To increase the efficiency the air-liquid area has to be enlarged. Evaporation from two pilot sludge drying beds was studied in an open air test lasting four months. One similar bed was tested in a controlled environment. The results showed that above a critical moisture content between 600 and 1100% evaporation from sludge equals the rate of evaporation from a free water surface. Below the critical moisture level the rate decreases rapidly.