ULTRAFILTRATION UNITS

Description

Ultrafiltration is the separation of particles in the size range 0005 µm to 01 µm from liquid streams by forcing the bulk of the liquid through a synthetic membrane by imposing high pressure. In this way microorganisms, macromolecules and oil emulsions can be separated.

Ultrafiltration membranes are usually formed from polymers, such as PVC and cellulose acetate. Ceramics and metallic oxides can be used to form more robust constructions. The membranes are usually tubular, capillary or spiral wound and ultrafiltration units may be composed of a series of bundles of the hollow membranes.

The liquid is passed tangentially over the membrane at high velocity. This flow keeps the particulate material in suspension and minimises fouling and blinding of the active side of the membrane. Filtrate passes through the membrane under pressure and the reject is pumped to a concentration tank.

Application

Ultrafiltration is used for disinfection of liquids through the direct removal of bacteria, viruses and pyrogens (fragments of dead cells). Ultrafiltration is also appropriate for the removal of colloidal materials such as dyes and paints, and for the removal of oil (down to 10 ppm) from industrial effluent and aqueous cleaning fluids. The technology is also used in the pulp, paper, food and potable water industries.

Polymer membranes are suitable for flows of moderate viscosity at temperatures of 30-70 C and pressures of up to 3000 kPa. Metallic oxide membranes can be used with liquids up to 300 C, at a wide range of pH and up to 1000 kPa. They are relatively unaffected by cleaning fluids.

Ultrafiltration is also suitable for the recovery and recycling of certain products, such as lignin. As such, ultrafiltration can be used within a waste minimisation programme to prevent valuable products, or those that are costly to discharge, entering the waste stream.

Sizing

Ultrafiltration units require feed pumps, transfer pumps and recycle pumps. Heat energy may be required in the cleaning of the membrane. A typical ultrafiltration unit for treating oily effluent would consume about 20 kWh/m3 of treated liquid. Generally, trials would be required to select the most appropriate membrane for each application.