Pretreatment for Rejection Increase and Fouling Prevention

Various processes are available for pretreatment of membrane processes. These methods are either aimed at rejection enhancement or fouling prevention. The majority of the techniques target different compounds, and thus need to be tailored to a specific water and application. 

Some of the available options are powdered activated carbon, UV/o2one, magnetite, lime softening, ion exchange, conventional media filtration, and coagulation with inorganic and polymeric coagulants. 

Filtration is required in some UF and in NF/RO processes. This is mainly because spiral wound modules cannot tolerate large particulates due to their very thin channels. Also, coarse particles may damage membranes under high shear conditions. Filters applied are usually in the 1 -25 pm range, aimed at suspended solids (Suratt (1993)). 

Conventional treatment or media filtration can reduce the suspended solids load by a factor of two (Suratt (1993)). This value seems very low and depends on the suspended solids characteristics. Gusses et at. (1997) stated that conventional treatment was not a sufficient pretreatment for NF. A combination of coagulation, ozonation, and biological treatment was a better alternative. Glucina et al. (1997) confirmed the inadequacy of conventional pretreatment for NF. UF pretreatment was more effective than MF in the long term regarding flux, while rejection of the NF membrane was independent of pretreatment. 

Qacangelo et al. (1995b)). The addition of a polyelectrolyte can also enhance ion rejection in NF (Levenstein et al. (1996)). 

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