NAMS 2002 Workshop - ICOM 2008

Membrane Fouling IV - RO & Desalination – 5
Friday July 18, 11:45 AM-12:15 PM, Moloka’i
Optical Monitoring and Real-Time Digital Image Analysis of Mineral Scale
Formation on RO Membranes
M. Kim (Speaker), University of California Los Angeles, Los Angeles, California, USA
R. Rallo, Universitat Rovira i Virgili, Catalunya, Spain
E. Lyster, University of California Los Angeles
Y. Cohen, University of California Los Angeles - yoram@ucla.edu
Upgrading the water quality of inland brackish water by RO and NF memrmbane
processes is often limited in recovery due to the rise in the concentration of
sparingly soluble salts in the retentate stream to levels that exceed their solubility
limits. As a result, such mineral salts can precipitate in the retentate stream and
crystallize on the membrane surface resulting in surface scaling that reduces
permeate flux and ultimately damaging the membrane itself. Early detection of
membrane surface scaling and fouling is necessary for timely initiation of
fouling/scale mitigation steps. Present traditional measures of process
performance trends (primarily flux decline and salt passage) are used as indirect
indicators of the occurrence mineral scaling and fouling. Although numerous
methods of scale and fouling detection have been proposed, it is only recently
that real-time early detection of the onset of scale formation has become
possible. Direct visual observations and detection of mineral scale on RO/NF
membranes under high pressure have been made possible with an ex-situ scale
observation detector (EXSOD) along with digital image analysis. The EXSOD
system is an optically transparent high-pressure flat sheet membrane cell that
allows real-time digital imaging of the membrane surface under RO process
conditions. The EXSOD can be operated as a stand- alone laboratory RO system
or connected to an RO/NF plant such that the EXSOD receives a side- stream
from a tail element of the RO/NF plant and thus enable early detection of mineral
scale. In its stand-alone mode, the EXSOD system was recently redesigned to
enable real- time measurements of the kinetics of surface crystallization of
mineral salts and assessment of the efficiency of scale mitigation strategies. In
order to utilize the above approach, efficient on-line image analysis software was
developed assisted with neural networks algorithms to enhance image analysis
by providing image family groups to increase the accuracy of single crystal
analysis, surface area covered by scale and shape and thus crystal type
identification. Using the present approach, real-time evolution of surface scaling
was evaluated for RO/NF scaling by calcium sulfate and calcium carbonate.
Direct information on surface nucleation by mineral salt crystals and the rate of
single crystal growth was determined over a range of operating conditions and
different antiscalants, generating, for the first time, direct fundamental data on the
kinetics of surface mineral salt crystallization on RO/NF membranes. These