MICROFILTRATION UNITS

Description

Microfiltration is a method of separating particles of 01-50 µm from liquid. Microfiltration is used for water clarification and nonthermal disinfection in a wide variety of industries.

The microfilters are hollow fibres or tubes contained in a modular housing. Polymer fibre microfilters are perforated with micropores. Liquid is pressurised to 100-400 kPa, clarified filtrate passes through perforations in the walls of the tubes and particulate matter is retained. The liquid to be filtered can be introduced to the centre or the outside of the tubes - the filter surface being inside or outside. Polymer cloth tubes have larger perforations and the particulate matter builds up on the polymer support to form a dynamic filter. A small amount of time is required for the coat of solids to become effective; out-of-specification filtrate is recycled to the feed tank.

The flow of the liquid in the filter can either be perpendicular (dead end filtration), tangential (crossflow) or parallel to the tubes or fibres (continuous microfiltration). The latter aims to reduce the build up of particulate matter on the filter surface and prolong operating time. The reject matter is swept along the filter and concentrated.

The intervals between physical or chemical cleaning can be increased by backflushing the filter with water or pressurised gas. This forces the trapped solids out of the perforations. Continuous microfiltration adopts frequent backwashing to reduce the requirement for a tangential flow. Backflushing can be controlled manually or by a PLC. In cloth systems, brushing and temporary deformation of the tubes is used to enhance cleaning.

Application

A wide range of particles can be removed by microfiltration including heavy metal precipitates, coal dust, glass particles, algae and bacteria. Removing suspended solids can reduce the BOD, COD and turbidity of an effluent. Microfiltration tends to be used for the sterile filtration of low-volume high-value streams. It is also used to remove bacteria in vegetable processing and yeast from cider and wine.

Crossflow microfiltration tends to be used for food and industrial waste, where suspended solids concentrations are above 100 mg/l and where contamination is nonuniform. Dead end microfiltration tends to be more suitable for aqueous solutions with suspended solids concentrations of less than 100 mg/l.

Sizing

Microfiltration units handle flows from 1-1000 m3/hour. The energy requirement for microfiltration varies with application. The removal of low concentrations of larger particles may require 05 kWh/m3 whereas higher concentrations of finer particles could require up to 10 kWh/m3.