In the dairy industry, the use of acid, alkaline cleaners and sanitizers affects wastewater characteristics and typically results in a highly variable pH, which will bring an impact on the wastewater on-line treatment. In this study, the effect of pH on treatment of diary wastewater by nanofiltration was investigated using a rotating disk membrane module. With increase of pH, NF membrane permeability was improved but concentration polarization increased. Due to a stronger electrostatic repulsion between salt ions and membrane at higher pH, permeate conductivity decreased. However, because of membrane swelling effect at alkaline pH, the neutral organic solutes passed through membrane more easily, leading to an increase of Chemical Oxygen Demand (COD) in permeate. Membrane fouling could be greatly alleviated at pH of 10, and the fouling layer was mainly caused by the adsorption of acid radicals-protein aggregates, which also resulted in pH reduction and conductivity increase in retentate. Real dairy wastewaters with different pH were also treated and the results were very similar to those of model solution, while membrane fouling was very sensitive to feed pH when pH was larger than 8. These results from laboratory-scale tests can serve as valuable guide for process control in industrial applications. (C) 2011 Elsevier B.V. All rights reserved: