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Membrane Systems Planning and Design 337
These types of questions should be considered as a part of the screening process before
selecting any membrane filtration systems to pilot. An additional consideration for MF/UF
systems is that because these systems are proprietary, the membrane modules vary in design
and are not presently interchangeable. Thus, a utility is limited to obtaining replacement
membranes from only one supplier after a system has been installed.
In some cases, screening tools for MF/UF membranes may be limited to experience at
other sites or chemical incompatibility. Therefore, pilot testing is an important consideration.
For NF/RO, proprietary software programs that are available from the various membrane
manufacturers can predict membrane system removal performance with a fairly good degree
of accuracy for a particular membrane product. Small-scale module testing may be an option
for screening some MF/UF membranes. Similarly, NF/RO membranes could be screened for
some parameters using flat-sheet studies or single element tests.
2.1.3. Scheduling
Scheduling is another important aspect of pilot test planning. One factor to consider when
scheduling a pilot test for surface waters is seasonal variations in water quality, since
turbidity, temperature, algae content, taste and odor, and other parameters can potentially
vary significantly throughout the year. A description of typical seasonal variation in surface
water quality is summarized below (1). Note that not all of the seasonal variations described
will be applicable for every site, or necessarily pertinent for all types of membrane filtration
systems.
Summer: Because user demand is typically highest in the summer, the filtration system
may also have to produce more water during this season. However, greater production is
facilitated by warmer water temperature. The degree to which a membrane process is affected
by a change in water temperature is related to the viscosity of the water. Since the viscosity of
the water is lower at higher temperatures, the membrane flux will likely be at its peak in this
season. Warmer water also facilitates enhanced algal growth that may be problematic for
membrane system operation. Taste and odor events may also manifest during the summer
months.
Autumn: In areas with hardwood cover in the watershed, autumn months typically bring an
increase in the organic content of surface water resulting from the decay of fallen leaves.
Turbidity may sharply increase, as well. Furthermore, cooler air temperatures and wind cause
surface waters in reservoirs to “turnover,” bringing deeper, more anaerobic water to the
surface and thus creating the potential for both taste and odor, and iron and manganese
problems.
Winter: Although winter months yield the coldest water temperatures and thus typically
the lowest membrane fluxes, the demand for water is typically at its lowest, as well. Both the
cold temperature and decrease in demand are likely to minimize required membrane flux in
this season. Most membrane manufacturers have membrane-specific temperature compensation
factors for cold-water operations.
Spring: Spring months usually yield increases in water temperature, as well as in the
potential for turbidity spikes related to run-off caused from snow melt. There is also potential