The phenomenon of flux decay in MD has been often observed in long-term operation and transmembrane flux declines as a consequence of fouling [43,44]. Membrane biofouling due to growth of microorganisms present in raw water often causes pore clogging, as well as an increase in pressure drop along the module. Chemical disinfection associated with UV treatment has been proposed to control biofouling; however, bacteria embedded on a polymeric surface show a significant resistance to biocides. The presence of fungi in the membrane pores was observed only on the feed side . Flux is also reduced by scaling, occurring whenever the concentration of dissolved salts and minerals overcomes the solubility limit. Solid precipitation on the membrane surface can lead to both pore clogging and pore wetting. The presence of particulate and colloids in the processed liquid can also induce fouling because these particles are preferentially trapped at the membrane-liquid interface by interfacial tension forces . In these cases, a prefiltering of the feed solution is generally sufficient to limit the flux decay effect . Membrane fouling is a severe problem particularly in foods concentration; again, a preliminary UF treatment for heavy fouling feeds can be useful in order to remove larger particles that could increases the viscosity of the stream through MD units .
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