Membrane and Desalination Technologies - TCE Moodle Website

366 N.K. Shammas and L.K. Wang
Table 8.6
Typical range of VCF values for various hydraulic configurations
Hydraulic configuration VCF
Deposition mode Dead-end 1
Suspension mode PFR 3–20
Crossflow 4–20
CSTR 4–20
5.1. Membrane Flux
The flux – the flow per unit of membrane area is one of the most fundamental considerations
in the design of a membrane filtration system, since this parameter dictates the amount
of membrane area necessary to achieve the desired system capacity and thus the number of
membrane modules required. Because the membrane modules represent a substantial component
of the capital cost of a membrane filtration system, considerable attention is given to
maximizing the membrane flux without inducing excessive reversible fouling, thereby minimizing
the number of modules required.
Typically, the maximum flux associated with a particular membrane filtration system is
determined during pilot testing, mandated by the state, or established via a combination of
these two in cases in which the state specifies a maximum operating flux based on the pilot
results. Independent of maximum flux, pilot testing is also commonly used to determine a
reasonable operating range that balances flux with backwash and chemical cleaning frequencies.
Because higher fluxes accelerate fouling, backwashing and chemical cleaning must
usually be conducted more frequently at higher fluxes. The upper bound of the range of
acceptable operating fluxes (provided this bound does not exceed the state-mandate maximum)
is sometimes called the “critical” flux, or the point at which a small increase in flux
results in a significant decrease in the run time between chemical cleanings. A membrane
filtration system should operate below this critical flux to avoid excessive downtime for
cleaning and the consequent wear on the membranes over time due to increased chemical
exposure.
The flux through a membrane is influenced by a number of factors, including:
1. Pore size (for MF, UF, and MCF membranes)
2. Module type (i.e., cartridge, hollow-fiber, spiral-wound, etc.)
3. Membrane material
4. Water quality
However, it is important to note that higher fluxes do not necessarily indicate that one
membrane is better than another for a particular application. Factors such as:
1. Estimated membrane life
2. Fouling potential
3. Frequency and effectiveness of chemical cleaning