MICROFILTRATION

Cross-flow microfiltration

Microfiltration

The microfiltration process is a membrane solids separation technique that can be used to remove particles and suspended solids from for colloidal and suspended

solutions.

Two distinct flow configurations are commonly employed for microfiltration systems, Cross Flow Microfiltration and Dead End Microfiltration.

• Particle size in the range of 0.05-10 microns. On molecular weigh basis , the particles are separated or rejected. Passage of particles through membrane is usually a function of particle geometry (i.e; particles shape and size).

• Microfiltration is suited to separate larger sizes, such as suspended solids, particulates, and microorganisms and reduction of turbidity . Commercially available microfiltration membranes are made from a variety of materials including organic polymers, such as polypropylene, ceramics and metal alloys. systems are operated at much lower feed pressures than reverse osmosis or ultra filtration due to the greater effective pore size.

The Dead-end Process

• The complete feed flow is forced through the membrane and cumulated solids are trapped on the surface of the membrane until backwashing is performed.

• During backwashing, accumulated solids are flushed away from the membranes and are collected for disposal. Backwash volumes typically represent roughly 2 to 5 percent of the total influent feed stream.

Factors influencing performance are; • Raw water characteristics• Trans-membrane pressure• Temperature• Regular monitoring and maintenance.

PretreatmentA self backwashing 100 um strainer is often necessary to protect the membranes and moderate particulate loading. Depending on the raw water, a coagulant such as ferric chloride may be added to form pin flocs and help improve rejection.

Cross-Flow Microfiltration:With cross-flow filtration a constant turbulent flow along the membrane surface prevents the accumulation of matter on the membrane surface. The membranes used in this process are commonly tubes with a membrane layer on the inside wall of the tube. The feed flow through the membrane tube has an elevated pressure as driving force for the filtration process and a high flow speed to create turbulent conditions.

• The process is referred to as "cross-flow", because the feed flow and filtration flow direction have a 90 degrees angle. Cross-flow filtration is an excellent way to filter liquids with a high concentration of filterable matter. In cross-flow microfiltration (CFMF), the suspension is pumped tangentially over the filtration medium.

• Clear liquid permeates the filtration medium and is recovered as the permeate, while the solids accumulate at the filtration barrier to form a fouling layer, or cake.

• The cake, constituting an increase in hydraulic resistance, decreases the permeate flux.

• The tangential suspension flow tends to limit the growth of the cake termed as sweeping.

• Correspondingly, after an initial rapid decrease, the permeate flux levels off and either attains a steady-state, or exhibits a slow, long-term decline with time.

Examples of micro filtration applications

They are;

• Cold sterilization of beverages and

pharmaceuticals

• Clearing of fruit juice, wines and beer Separation

of bacteria from water (biological wastewater

treatment)

• Separation of oil/ water emulsions

• Pre-treatment of water for nano filtration or

Reverse Osmosis

• fermentation, broth clarification

• pre-treatment of water for nanofiltration