Interactions between biofilms and NF/RO flux and their implications for control-A review of recent developments

Biofilm effects on NF/RO membrane flux have been extensively elucidated and evaluated in recent literature. These findings can be divided into two main categories: (1) Biofilm as a vertical obstacle contributed to the concentration polarization (CP) and fouling layer resistance (R _f). This directly decreased the permeate flux in the system. A comparison of these effects normalized to membrane resistance and membrane flux showed that CP was usually the primary parameter to affect the flux decline and caused ever more pronounced impacts as the feed water ionic strength increased. (2) Biofilm as a horizontal barrier to cross-flow caused increased axial pressure drop and flow maldistribution. The connection between the two categories is discussed. Biofouling effects from both of the above-mentioned categories have a combined and separate impact on the permeate in any given membrane installation, however, the relative magnitude of these effects will depend on the chemical composition of the feed and the pressure applied. Similarly the two components of flow in a membrane channel can each contribute to nutrient transport to the biofilm, and the relative magnitude of their contribution can have important implications for biofilm control. Latest findings on the effects of linear flow velocity, substrate concentration, and permeate flux are discussed in the framework of these two orientations. Lastly, recent ideas of biofilm control are discussed within the same framework.