NAMS 2002 Workshop - ICOM 2008

Membrane Fouling IV - RO & Desalination – 4
Friday July 18, 11:15 AM-11:45 AM, Moloka’i
Mechanisms of Marine Bacteria Adhesion to Seawater RO Membranes
X. Huang (Speaker), University of California Los Angeles, Los Angeles, California, USA -
xiaofeih@ucla.edu
E. Hoek, University of California Los Angeles, Los Angeles, California, USA
Biofouling is among the most problematic issues for seawater desalination by
reverse osmosis (RO) membranes. It is particularly difficult for seawater
applications because continuous chlorination of polyamide RO membranes is not
possible and because algal blooms result in periodic upsets to seawater quality
bringing increased biomass and assimilable organics into the RO system. The
high ionic strength of seawater virtually eliminates electrostatic double layer
interactions among foulants and membranes; hence, van der Waals and shortrange
interactions (acid-base, roughness, steric, metal-complexation, etc.) may
govern adhesion of bacteria and organic matter. Previous research in fresh and
brackish water applications suggests that membrane surface chemistry and
morphology govern colloidal fouling of RO membranes. Other research suggests
that calcium forms complexes between carboxylic acid functionality on foulants
and polyamide RO membranes - exacerbating flux decline and making the
membranes difficult to clean. We hypothesize that calcium-complexation will be
important for the initial rate of bacterial adhesion to seawater RO (SWRO)
membranes.
Our objective in this study is to elucidate the relative importance of van der
Waals interactions, acid-base interactions, surface roughness, and calciumcomplexation
on bacterial adhesion to SWRO membranes. We have selected a
model system comprising Halomonas pacifica (GFP), a common marine
bacterium, and two commercial polyamide composite seawater RO membranes.
The two membranes were selected because they represent a relatively
hydrophobic, rough membrane with significant carboxylic acid functionality at its
interface (Hydranautics SWC3+) and a relatively hydrophilic, smooth membrane
with little carboxylic acid functionality at its interface (FilmTec SWHR). The
former is expected to produce higher bacterial deposition rates due to attractive
acid-base interactions and its rough, carboxylic acid rich interface. The latter
membrane is expected to be relatively resistant to bacterial adhesion due to
repulsive acid-base interactions and its smooth non-carboxylated interface. In
some experiments, bacteria are dispersed in a real seawater matrix. In other
experiments, solution chemistry is systematically controlled by addition of
calcium and magnesium ions to NaCl solutions at seawater ionic strengths.