Kluyveromyces marxianus Y-8281 yeast culture was utilized for the biological treatment of deproteinated whey wastewater in a batch system. Removal of lactose was optimized by the utilization of response surface methodology, RSM. The empirical model developed through RSM in terms of effective operational factors of medium pH, temperature, lactose and ammonia concentrations was found adequate to describe the treatment of deproteinated whey. Through the analysis, medium pH and temperature were found to be the most significant factors and an increment in both had a positive effect on lactose utilization, while lactose and ammonia concentrations had the least weight within the ranges investigated. Based on contour plots and variance analysis, optimum operational conditions for maximizing lactose removal were found to be 31 degrees C, 45 g/L whey powder concentration, 4 g/L total ammonium salt concentration and medium pH 6. Under the optimum operating conditions determined, 95% lactose removal was achieved after an 18-h fermentation. (c) 2005 Elsevier Ltd. All rights reserved.