Ultrasound-assisted chemical cleaning of polyvinylidene fluoride membrane fouled by lactic acid fermentation broth

Ultrasound-assisted chemical cleaning was applied to remove foulants on polyvinylidene fluoride (PVDF) ultrafiltration membranes used to treat lactic acid fermentation broth. The cleaning effects of four chemicals, namely, deionized water, NaOH, NaClO, and a mixture of NaOH and NaClO, were compared through normalized permeate flux. The obtained results indicated that 45% of the normalized permeate flux can be achieved by cleaning with 1% of the mixture. This percentage can be enhanced to 97.5% through ultrasound. Scanning electron micrographs showed that deionized water can only remove part of foulants on the membrane surface, whereas the other three solutions can eliminate the surface foulants completely. Membrane fouling and flux decline were mainly caused by the foulants inside the membrane pores. Three-dimensional fluorescence spectrum analysis showed new fluorescent compounds generated by the oxidation effect of NaClO. Energy dispersive X-ray spectra showed that NaClO can decrease the oxygen content of the PVDF membrane.