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System Comparisons

While all types of membranes work well under proper conditions, choosing the most appropriate membrane for a given application still remains crucial. In many cases, selection is complicated by the availability of new types of membranes, applications or site-specific conditions. Bench and pilot tests are powerful tools for situations where process risks and uncertainties exist or the cost impacts from problems are potentially high.

Membrane classification standards vary considerably from one filter supplier to another. What one supplier sells as a UF product, another manufacturer calls an NF system. It is better to look directly at pore size, molecular weight cut off (MWCO) and applied pressure needed when comparing two membrane systems. MWCO (a measure of membrane pore dimensions) is a specification used by membrane suppliers to describe a membrane's retention capabilities.

Ultrafilter vs. Conventional Filter

UF, like RO, is a cross-flow separation process. Here liquid stream to be treated flows tangentially along the membrane surface, thereby producing two streams. The stream of liquid that comes through the membrane is called permeate. The type and amount of species left in the permeate will depend on the characteristics of the membrane, the operating conditions, and the quality of feed. The other liquid stream is called concentrate and gets progressively concentrated in those species removed by the membrane. In cross-flow separation, therefore, the membrane itself does not act as a collector of ions, molecules, or colloids but merely as a barrier to these species.

Conventional filters such as media filters or cartridge filters, on the other hand, only remove suspended solids by trapping these in the pores of the filter-media. These filters therefore act as depositories of suspended solids and have to be cleaned or replaced frequently. Conventional filters are used upstream from the membrane system to remove relatively large suspended solids and to let the membrane do the job of removing fine particles and dissolved solids. In UF, for many applications, no prefilters are used and UF modules concentrate all of the suspended and emulsified materials.

Concentration Polarization

When a membrane is used for a separation, the concentration of any species being removed is higher near the membrane surface than it is in the bulk of the stream. This condition is known as concentration polarization and exists in all UF and RO separations. The result of concentration polarization is the formation of a boundary layer of substantially high concentration of substances being removed by the membrane.

The thickness of the layer and its concentration depend on the mass of transfer conditions that exist in the membrane system. Membrane flux and feed flow velocity are both important in controlling the thickness and the concentration in the boundary layer. The boundary layer impedes the flow of water through the membrane and the high concentration of species in the boundary layer produces a permeate of inferior quality in ultrafiltration applications. Relatively high fluid velocities are maintained along the membrane surface to reduce the concentration polarization effect.