Bacterial Deposition and Fouling During Dead-End Microfiltration

Microfiltration membranes bring unique capabilities for municipal water treatment since they are capable of removing particulate matter and pathogenic protozoa such as Cryptosporidium and Giardia that are difficult to disinfect using chlorine. However, membrane fouling continues to impede their increased implementation for environmental separations. To date, much effort has already been spent on delineating the mechanisms by which inorganic colloids and natural organic matter cause fouling. However, there is much less information available on the mechanisms by which bacteria foul microfiltration membranes. The presentation will cover the entire filtration cycle including (i) short-term fouling when bacteria do not produce exopolymers and fouling can be quantitatively described by considering bacteria as if they were similar to inorganic particles of similar size and shape, (ii) mid-term fouling when a cake is formed on the membrane surface and its compressibility controls flux, and (iii) long-term fouling when bacterial secretions of EPS control their irreversible attachment to membranes, which determines the efficacy of backwashing and in turn fouling. 

During the early stages of constant pressure microfiltration, bacteria were observed to form patchy (non-uniform) deposits on membrane surfaces and the intermediate blocking law quantitatively modeled flux decline. Mechanistically, these heterogeneities arise either from non-uniform membrane surface porosity or stochastic initial binding of foulants.  Cake filtration was dominant over intermediate time frames. Statistically invariant values of cake specific resistance and compressibility were obtained for constant flux and constant pressure operation during bacterial microfiltration.  This suggests that biocolloids form cakes whose hydraulic resistance is dominated by a morphology that did not depend on their mode of deposition, confirming that the cake permeability was determined by the instantaneous pressure. Bacterial secretion of exopolymers over longer time frames were shown to play a major role in biofouling, which was comprehensively characterized using FTIR, XPS, and colorimetry. FTIR spectra revealed that one possible mechanism of biofilm formation to be polysaccharide O-acetylation.