Microbial adhesion mechanisms in reverse osmosis and nanofiltration

Membrane biofouling has been found to occur extensively in nanofiltration (NF) and reverse osmosis (RO) membrane even after significant pretreatment (including conventional, membrane bioreactors, ultrafiltration/microfiltration) of the influent stream. It is hypothesized that concentration polarization, hydrodynamics, and membrane surface properties play a key role in microbial adhesion on NF and RO membranes. The hypothesis is based on the presumption that microbial adhesion on NF and RO membranes vary significantly from MF and OF membranes due to different surface chemistry (hydrophobicity), surface morphology (roughness), rejection of salt ions and dissolved organics. Moreover, the salt ions rejected on the membrane surface leads to the formation of a concentration polarization layer and is influenced by the hydrodynamics occurring with the membrane module (crossflow, presence of turbulent promoting spacers, permeation). Rejection of organics by NF and RO membranes lead to the formation of conditioning films and the interactions between the microorganism and the organic layer adsorbed on the membrane surface could dominate the initial adhesion process. The objective and focus of this study is to understand and elaborate the primary mechanisms involved in microbial deposition on NF and RO membranes including the study of clean and modified membrane property, hydrodynamics, and influence of organic adsorption.; In Chapter 1 an extensive literature review on the different mechanisms of initial adhesion and biofilm formation are summarized. In Chapter 2, the influence of hydrodynamics is reported in detail. Crossflow and permeation velocities played a significant role in the adhesion process. Membrane flow cell design and feed spacer configuration were found to significantly affect adhesion due to formation of stagnation zones. In Chapters 3 and 4 it was found that the membrane surface property such as roughness and surface chemistry of both clean and organic modified membranes played a key role in the adhesion mechanism. In Chapter 5, rapid deposition studies correlated well with long-term adhesion studies. Finally, it was found that long-term adhesion studies lead to the microbial cells being irreversibly attached to the membrane surface, hence only strong oxidizing agents were found to be effective in removing the attached microorganisms.