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Tailored Feed Pretreatment for
Primary and Secondary Desalination
| Successful application of desalination for
potable water production from brackish water requires effective feed
pretreatment in order to mitigate potential membrane fouling and scaling
problems – especially for the current class of commercial polyamide NF/RO
membranes. The choice of pretreatment strategy for every desalination
application is specific to the feed water composition, membrane properties, and
the desired product water recovery. For example, optimal addition of antiscalant
is not only a function of the feed water composition and recovery level, but
also of the membrane surface properties, salt rejection, and spiral wound
element spacer geometry. Since no pretreatment can be completely effective,
desalination membranes must possess certain fouling resistant characteristics,
as well as resistance to chemicals used in pretreatment and cleaning processes.
For example, NF/RO membrane tolerance to chlorine varies depending on the
membrane chemistry. Also, it has been shown that membrane surface chemistry and
morphology have fundamental impacts on antiscalant effectiveness. In order to
fully demonstrate the effectiveness of the fouling and scaling membranes, the
impact of pretreatment strategies involving microfiltration and chemical
additives are being studied at the WaTeR Center for a range of feed water
compositions expected in desalination of California brackish water. These
pretreatment strategies will be benchmarked using commercially available
desalination membranes and novel fouling-resistant and scaling-resistant
membranes being developed at the WaTeR Center.
Based on fundamental experimental studies and
process models, a systematic approach to optimization of feed pretreatment are
currently under development. Feed pretreatment options being considered as part
of the overall strategy include membrane filtration, precipitation softening,
advanced oxidation, pH control, and addition of flocculants and antiscalants.
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| Water feed pretreatment prior to primary
or secondary RO desalting. The specific required pretreatment
steps depend on the characteristics of the feed water (i.e., water
composition). The adequacy of feed pretreatment will be evaluated based
on membrane autopsies and diagnostic membrane flux decline studies with
the membrane RO concentrate and treated feed. |
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