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Membrane Systems Planning and Design 339
consideration if the source water is limited, or alternatively if waste disposal is problematic.
In addition, chemical cleaning may be a labor-intensive operation that requires the
handling of harsh chemicals and produces a waste stream that may be difficult to dispose.
However, chemical cleaning is a necessary process associated with membrane filtration,
and establishing operating practices that extend the time between cleanings is one objective
of a membrane pilot study.
This section provides guidelines for balancing flux, productivity, backwash frequency
(where applicable), and chemical cleaning intervals during pilot testing. It is important to
note that in order for cause and effect to be analyzed properly only one process variable
should be changed at a time during the pilot study. Economic and time constraints often
dictate the duration of the pilot study and may not allow complete optimization of each
of these parameters. Therefore, it is important to understand which of these parameters is
most important for a particular application of membrane filtration and to structure the
pilot test protocol accordingly. Thus, the result of a pilot study is not necessarily the
“optimum” design, but rather a set of operational conditions that will result in feasible
and economic water treatment over the anticipated range of operating and source water
conditions.
2.2.1. Membrane Flux Optimization
Membrane manufacturers can recommend fluxes for particular applications and a given
water quality. Table 8.1 lists some of the important water quality data that should be provided
to membrane suppliers to facilitate a fairly accurate initial estimation of anticipated membrane
productivity.
If scheduling permits, it is typically advantageous to begin piloting MF/UF systems at a
conservative flux and then increase it based upon the rate of fouling observed. The flux may
be increased either after a chemical cleaning or during a filter run if the pilot unit has
undergone sustained operation with only a nominal increase in fouling. In subsequent filter
runs (i.e., between chemical cleanings) the pilot unit flux may be either increased if the
fouling rate of the previous run was still within acceptable tolerances, or decreased if the
fouling occurred at an unacceptably high rate. Note that the backwash frequency may also be
adjusted either during a filter run or between filter runs to minimize fouling at a particular
flux. This fine-tuning process of adjusting the flux and/or backwashing interval (if possible,
only one parameter should be adjusted at a time) between filter runs may be repeated to the
extent that budget and scheduling constraints allow, or to the point at which such adjustments
reach the point of diminishing returns. NF/RO performance can typically be accurately
gauged by the manufacturers based on water quality models; although it is often beneficial
to adjust the flux over a series of filter runs to optimize productivity based on pilot test results.
(Note, however, that manufacturers’ models do not predict fouling.) The pilot test protocol
should include the expected range of membrane fluxes and guidelines on how these fluxes
should be adjusted based upon the results of early testing. Models may also indicate chemical
pretreatment requirements necessary to control scaling on NF/RO membranes and thus to
help maintain acceptable fluxes.