The activated sludge laboratory model (Fig. 1) made of stainless steel, consisted of two parts. The volume of the aeration section was 12 L and the volume of the sedimentation section was 3 L. Wastewater was fed into the aeration section with a dosage pump. In the aeration section the water was aerated from the bottom with two diffusers and an air pump to maintain a dissolved oxygen concentration above 4 mg/L (for both the bacterial growth and the homogenous mixed liquor). Plastic pipes (5 mm diameter) were used to transfer water and air through the model. The aeration section was separated from the sedimentation section with a height adjustable screen having five holes with 5 mm diameter, from which the mixed liquor passed through the sedimentation section. The holes were placed carefully to prevent sedimentation solids from escaping. The settled sludge was returned to the aeration section passing under the adjustable screen.