Influence of temperature and cleaning on aromatic and semi-aromatic polyamide thin-film composite NF and RO membranes

Although cleaning removes fouling, it may change the properties of membranes and consequently influence their performance. We have evaluated the combined effect of acidic/alkaline cleaning and temperature on the performance of two aromatic membranes, one a reverse osmosis membrane (HR98PP) and the other a nanofiltration membrane (NF90), two semi-aromatic polypiperazine membranes (NF200, NFT-50) and a Desal-5DK nanofiltration membrane. At the retention minimum of each membrane, the KCl retention and the water permeability were in the order: NFT-50 < NF200 < Desal-5DK < NF90 < HR98PP and HR98PP < NF90 < NF200 < Desal-5DK < NFT-50, respectively. After nanofiltration at elevated temperatures acidic and alkaline cleaning increased the water permeability and decreased the retention of the Desal-5DK membrane. Similar behaviour was observed after acidic cleaning of the aromatic polyamide membranes and with alkaline cleaning of the semi-aromatic polyamide (polypiperazine) membranes. Increased hysteresis was seen in the membrane performance with increasing temperature. The hysteresis behaviour was dependent on the cleaning procedure and could be related to swelling and shrinkage of the active polyamide layer. At 40 °C the reverse osmosis membrane (HR98PP) showed the same performance as the NF90 nanofiltration membrane at 20 °C.