n this study we investigated the removal of viruses with similar size and shape but with different
external surface capsid proteins by a bench-scale membrane bioreactor (MBR). The goal was to determine
which virus removal mechanisms (retention by clean backwashed membrane, retention by cake
layer, attachment to biomass, and inactivation) were most impacted by differences in the virus surface
properties. Seven bench-scale MBR experiments were performed using mixed liquor wastewater sludge
that was seeded with three lab-cultured bacteriophages with icosahedral capsids of ~30 nm diameter
(MS2, phiX174, and fr). The operating conditions were designed to simulate those at a reference, fullscale
MBR facility. The virus removal mechanism most affected by virus type was attachment to
biomass (removals of 0.2 log for MS2, 1.2 log for phiX174, and 3 log for fr). These differences in removal
could not be explained by electrostatic interactions, as the three viruses had similar net negative charge
when suspended in MBR permeate. Removals by the clean backwashed membrane (less than 1 log) and
cake layer (~0.6 log) were similar for the three viruses. A comparison between the clean membrane
removals seen at the bench-scale using a virgin membrane (~1 log), and the full-scale using 10-year old
membranes (~2e3 logs) suggests that irreversible fouling, accumulated on the membrane over years of
operation that cannot be removed by cleaning, also contributes towards virus removal. This study enhances
the current mechanistic understanding of virus removal in MBRs and will contribute to more
reliable treatment for water reuse applications.
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