Model dairy waste waters (prepared from milk powder by dilution) were treated with ozone, and the effects of the ozonation time and the surfactant concentration on the flux, the membrane resistance, membrane fouling and gel formation were measured. It was found that the microflocculation effect of ozone may play a significant role at a higher gas flow rate, causing a decreased level of fouling and increased gel formation, while at a lower flow rate the effect of the degradation of large molecules wits more marked, causing a higher flux, but decreasing the retention.