In this investigation, nanofiltration membranes at various stages of filtration operation were subjected to chemical coagulation pretreatment and AOP pretreatment followed different cleaning solutions. The permeate flux decline patterns were compared for raw water, raw groundwater subjected to coagulation pretreatment, and AOP pretreatment prior to nanofiltration. The study also investigated the influence of cross-flow filtration rates on the fouling and flux reduction patterns. The permeate flux data provided sufficient information to determine which pretreatment strategy produced less membrane fouling and minimized permeate flux decline.
Additionally, the study evaluates morphological and/or structural damage occurring on membranes due to residual oxidant species and hydroxyl, peroxyl or other free-radical species generated during water treatment. This is an important consideration in membrane processes such as nanofiltration and reverse osmosis, because membranes fabricated from polymeric materials by sol-gel or other processes are prone to polymer decomposition and structural alteration. The techniques employed for evaluating fouling potential and morphological details of membrane surfaces include atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), and x-ray photoelectron spectroscopy (XPS).AFM studies provided information on membrane surface topography under different operating conditions, and XPS and FTIR provided details on numbers and types of organic functional groups on membrane surfaces attributable to different pretreatment techniques. The results provided useful information on several aspects: amelioration of membrane fouling (and permeate flux recovery) due to the application of cleaning agents, alteration of membrane surface characteristics attributed to membrane cleaning, and damage to membrane material caused by oxidative reactions associated with AOPs.
Keywords: Nanofiltration, pretreatment strategies, coagulation, advanced oxidation processes, natural organic matter, membrane fouling, surface characterization