Modeling the effect of biofilm formation on reverse osmosis performance: Flux, feed channel pressure drop and solute passage

A two-dimensional (2-d) mathematical model describing the effect of biofilm development on the performance of a spiral-wound reverse osmosis (RO) membrane device was developed. The micro-scale model combines hydrodynamics and mass transport of solutes (salt and substrate) with biomass attachment, biofilm growth and detachment due to mechanical stress induced by liquid flow in the feed channel. The model explains several experimental observations when operating at constant pressure: loss of permeate flux with increased salt passage in time and achievement of a quasi-steady state flux and biomass amount. The model also shows how the local balance between biofilm growth and detachment leads to irregular biofilm distribution in the feed channel and suggests places where most biomass accumulation is expected. Numerical simulations were performed in configurations without spacer or with different spacer geometries (submerged, cavity and zigzag). Three mechanisms were identified by which biofilms on RO membranes contribute to performance loss: (i) biofilm-enhanced concentration polarization; (ii) increased hydraulic resistance to trans-membrane flow; and (iii) increased feed channel pressure drop. For seawater and brackish water desalination, biofilm-enhanced concentration polarization appears to affect most the local flux. This modeling approach, combining computational fluid dynamics (CFD) with biofilm models allows a systematic study of biofouling in membrane systems. Moreover the approach is useful for improving feed spacer design and to evaluate operational conditions for minimum biofouling of reverse osmosis and nanofiltration (NF) membrane devices. © 2010 Elsevier B.V.