Three different air-assist nozzles were investigated with a Phase-Doppler-Analyzer and high speed photography in a cold environment and then used in a pilot scale furnace at a thermal input of 200 kW. Waste water was simulated by a water - glycol mixture. Measurements showed temperatures above 1100 degrees C and homogeneous oxygen concentrations except close to the burner. With such conditions evaporation and, hence, atomization quality set the limits on burnout, as oxidizing reactions are fast. The Sauter Mean Diameter (SMD) did not correlate closely with burnout since SMD indicates a mean droplet size while performance is limited by the number of large droplets present. Better agreement was achieved for D-90% which is a parameter that represents the upper end of the size distribution. High speed photography is recommended to detect and measure the largest droplets in the spray. Their size correlated closely with the onset of incomplete burnout and is, therefore, an appropriate parameter to characterize atomization quality with respect to burnout.