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.

Pretreatment

A 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