Wastewater discharged from food processing plants still contains recoverable nutrition material such as protein and fat. Separation of these materials in wastewater not only contributes to product recovery but also reduces organic loading in wastewater treatment. Colloidal gas aphrons (CGA) is one promising technique for organic material removal. CGA are micro bubbles (10-100 mum) created by intense stirring of surfactant solution. In this work, selection of suitable nonionic edible surfactant was conducted by evaluating the stability and percentage of protein recovery. It was found that Tween 20 was a suitable surfactant. The stability of CGA was investigated by varying surfactant concentration, motor stirring speed, stirring time and concentration of NaCl as additive. The optimal condition for CGA preparation was 10 mM Tween 20 in distilled water, stirring speed of 5500 rpm and stirring time of 5 min. To simplify, lysozyme and beta-casein of different pI values were employed as substitutes of protein in wastewater. Factors affecting protein separation such as initial protein concentration, CGA volume ratio and pH were examined. It was demonstrated that hydrophobicity of Tween affected CGA stability and the electrostatic interaction between CGA and proteins played an important role in the separation. (C) 2003 Elsevier B.V. All rights reserved.