Microfiltration (MF), like ultrafiltration, is a pressure-driven membrane process. In the case of microfiltration, the pore size is typically in the 0.03- to 0.1-μm range. In this range, individual bacteria and viruses will pass through the membrane.

However, colloidal suspended solids, floc particles, and parasite cysts are prevented from passing. No removal of dissolved solids is accomplished by microfiltration membranes. Microfiltration membranes come in both in-line or cartridge filter arrangements and crossflow arrangements.

For cross-flow systems, the rejection rate is usually 5 to 10 percent, that is, 90 to 95 percent recovery. Higher recovery rates are feasible; however, the overall flux through the membrane is reduced. Cross-flow microfiltration membranes can be further subdivided into tubular and immersed-membrane types.

Tubular membranes are arranged such that the raw-water source is introduced under pressure in a tube that surrounds the membrane.Typically, the permeate water proceeds from the outside of the membrane into the lumen in the center of the membrane, where the permeate is then conducted back to a manifold for collection.

The solids remain on the outside of the membrane in the pressure tube and are periodically blown down from the system. Immersed membranes can be placed directly in a process tank where the permeate is removed through the membrane by a suction pump on the permeate collection manifold.This arrangement is possible because of the low transmembrane pressure exhibited by this type of membrane.

The flux through the membrane during operation is dependent on suspended solids in the feedwater as well as temperature (viscosity). A typical flux range for the membrane surface is 10 to 50 gal/day ⋅ ft2. Increasing transmembrane pressure also increases the flux.

Microfiltration membranes are typically periodically cleaned by backpulsing these membranes. This can be accomplished through a variety of ways, some incorporating air and some using just product water or water containing a small amount of hypochlorite. Some immersedmembrane systems also use diffused air to agitate the membranes and prevent solids from caking on the membrane surface.

Capital costs for microfiltration membrane system capacity range from $0.50 to approximately $1.00 per gallon per day.This range is primarily a function of solids concentration in the feed stream.Also note that membrane systems must be sized on peak flow rather than average daily flow, which can significantly affect the cost of a microfiltration membrane system.

Operational costs are primarily associated with power and vary from 50 to 120 hp/mgd. Pretreatment chemicals include coagulants, and when biofouling is a problem, hypochlorite solution can be used on some membranes for cleaning. Citric acid can also be used for membranes that do not tolerate chlorine. Microfiltration membranes are typically not compatible with polymer addition, which is not required, since even small “pin flocs” cannot permeate the membranes.

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