[Fouling reasons and cleaning methods of nanofiltration membrane filtrated with the effluent of membrane bioreactor].

In order to investigate the fouling reasons and cleaning methods of nanofiltration membrane fouled with the effluent of membrane bioreactor (MBR), FEG-SEM-EDS, FTIR, AFM and contact angle system, were applied to analyze the membrane fouling layer. The results showed that, the foulants included both organic and inorganic substances, which contained phosphorous (P) and magnesium (Mg). After investigations of membrane surfaces through different cleaning steps, it was showed that critic acid cleaning could remove Mg and most of P, which were inorganic; and subsequent sodium hydroxide cleaning could remove the residual P, which probably came from organic acid, organic phosphorous or protein. The roughness increased from 79.5 nm to 111.2 nm with fouling, but the hydrophilicity didn't change obviously according to the contact angle from 55.6 degrees to 62.1 degrees. But after acid cleaning, the roughness decreased to 51.9 nm and the contact angle changed to be 96.0 degrees, which showed the organic foulants were exposed and the surface changed to be hydrophobic. And after subsequent alkali cleaning, the roughness and the hydrophilicity were both close to those of the new membrane. These results concluded that acid cleaning followed by alkali cleaning could remove most of the foulants. It was also suggested that organic matters fouled the membrane in the initial stage, and inorganic matters aggravated the membrane fouling subsequently.