Fouling And Its Effect On Rejection Of Micro-organic Pollutants During Nanofiltration For Advanced Treatment Of Wastewater

Nafiltration （NF） has found its application in advanced treatment of wastewatersince it can reject micro-organic pollutants that are harmful to human health. Theefficient application of NF has been hampered by membrane fouling, leading to reducedwater flux and compromised water quality. Therefore, understanding of NF membranefouling and its effect on rejection of micro-organic pollutants is of significantimportance. Combined membrane fouling of organics and metal ions was investigatedin this study. Using model foulants, this study gives an insight into the combinedmembrane fouling mechanisms, elucidates the effect of combined membrane fouling onrejection of micro-organic pollutants, and reveals the underlying mechanisms.Preliminary results obtained using real wastewater are also presented regardingmembrane fouling as well as its effect on the rejection of micro-organic pollutants.Metal ions in feed solution and membrane surface carboxyl group density are twokey factors affecting combined membrane fouling. The impact of Ca²⁺ions oncombined membrane fouling is dominated by bulk bridging and surface bridging.Increasing the Ca²⁺concentration in feed solution enhances both bulk bridging andsurface bridging. However, in contrast to surface bridging that usually aggravatesfouling, bulk bridging can retard fouling. No bulk bridging exists between Fe³⁺andfoulants with a more severe combined membrane fouling at a higher Fe³⁺concentration.Membrane surface carboxyl group density can be reduced by using a monomer thatcomprises less acyl chloride groups for NF membrane fabrication. Reduced surfacecarboxyl group density prevents surface bridging and thus benefits anti-foulingproperties of NF membranes.In order to demonstrate the effect of combined membrane fouling on rejection ofmicro-organic pollutants, the entire transport of micro-organic pollutants during NF wasdivided into aqueous transport and solid transport. The entire, aqueous and solidtransport is described by observed rejection, mass transfer coefficient and real rejection,respectively. At low Ca²⁺concentrations in feed solution, combined membrane foulingmarginally affects micro-organic pollutants’ observed rejections, mass transfercoefficients, and real rejections. At high Ca²⁺concentrations in feed solution, micro-organic pollutants’ mass transfer coefficients remain unchanged after membranefouling; the observed rejections decrease, resulting from the decreased real rejections ofthe fouled membrane.A rejection model for the fouled NF membrane was established to reveal themechanisms by which a fouling layer at the membrane surface influenced the rejectionof micro-organic pollutants. An inequality was derived from the rejection model whichcould be applied as a criterion to determine the role that the fouling layer played. Thiscriterion indicates that the effect of the fouling layer on micro-organic pollutants’rejection relates to its compactness, which can be reflected by its specific resistance.The variation of specific resistance with Ca²⁺concentrations in combined membranefouling indicate that the role the fouling layer played is to hinder the back-diffusion ofmicro-organic pollutants to the bulk solution. Membrane fouling at a higher Ca²⁺concentration results in a more compact fouling layer having higher resistance to theback-diffusion of micro-organic pollutants, and thus causes more reduction in the realrejection of micro-organic pollutants.With respect to the real wastewater system, membrane fouling accelerated withincreasing Ca²⁺concentrations in feed solution, suggesting that no bulk bridginghappened at all tested Ca²⁺concentrations. Membrane fouling enhanced the rejection ofmicro-organic pollutants. The enhancement of rejection by fouling was more significantfor the micro-pollutant with a smaller molecular weight and at a higher Ca²⁺concentration.