Two-phase flow for fouling control in membranes

Abstract

The real challenge of the use of NF/RO spiral-wound membrane modules in water treatment is membrane fouling. Fouling problems in NF/RO systems are more complicated than in low pressure membrane processes, becaused fouling usually occurs on the nanoscale, combined with the complex geometry of spiral-wound membrane modules. Reducing membrane fouling is a must during operation of NF/RO membranes in order to minimize product loss and operational costs. Periodical cleaning is needed to relieve unwanted materials from the membrane surface and the feed channel. Physical reversible fouling in NF/RO processes is preferably prevented by a proper pretreatment (MF/UF/ozonation or activated carbon) and is traditionally removed by flushing (backflush, forward flush, reverse flush). Physical irreversible fouling (especially biofouling) needs to be tackled by a chemical cleaning. Yet, chemical agents are found to be ineffective to control biofouling and many studies revealed the survival of microbial cells after chemical cleaning. Technical and economical analyses demonstrated that gas/liquid two-phase flow cleaning is a promising technology to keep the membrane resistance at sufficiently low levels and to increase the membrane flux for many types of membrane processes: (i) microfiltration (MF), (ii) ultrafiltration (UF), (iii) nanofiltration (NF), (iv) reverse osmosis (RO), (v) membrane distillation (MD), (vi) electrodialysis (ED), and (vii) membrane bioreactors (MBR). Also it is applicable in a multitude of membrane module types: (i) flat/planar, (ii) tubular/capillary, (iii) hollow fiber and (iv) spiral wound membranes. The research presented in this PhD thesis of Yusuf Wibisono is to understand and optimize two-phase flow cleaning in spacer-filled membrane channels used as a model for spiral-wound membrane elements in relation to the following parameters: (i) feed types and concentrations; (ii) feed spacer geometry and orientation; (iii) feed pressure and velocity; (iv) gas/liquid ratio; and (v) feed spacer surface properties. The results showed that the feed type is by far the most essential factor contributing to the cleaning efficiency. The spacer geometry is ranked second, followed by the gas/liquid ratio and the liquid velocity, which both have an only very minor effect. In terms of a practical application, the operator should consider first the type of foulant prior to taking a decision on whether or not two-phase flow cleaning will be effective. Once the feed type is defined, the use of the highest gas/liquid ratio, the highest liquid velocity and the thickest feed spacer (diamond type) are recommended to achieve maximum two-phase flow cleaning efficiency.