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Filtration of Microorganisms

Filtration of Microorganisms
Three types of filters exist for use in ventilation systems, pre-filters, HEPA
(High Efficiency Particulate Air) filters and ULPA filters (Fig. 13.5 a,b).

Filters of air Sanitation

A typical HEPA filter, such as the one shown in Fig. 13.5b will filter micron sized particles at about 95% efficiency. Some box or pleated type filters (Fig. 13.5a) can be as thin as 2-4 inches, or as wide as 8-12 inches. The picture at the right shows a bag type HEPA filter, which can extend upto 24 inches. Bag type filters typically have a lower pressure drop than the pleated or box type HEPA and are more efficient.

Illustration of a HEPA filter

HEPA filters are commonly found in hospital isolation rooms, operating theatres, and Level 3 and 4 containment facilities, as well as in industrial clean rooms.

HEPA filters are typically rated as 99.97% effective in removing dust and particulate matter above 0.3 micron in size, based on DOP (diocytl phthalate) testing usually performed by the manufacturer. In theory, HEPA filters should be highly effective against bacteria and fairly effective against viruses, but real world installations do not always achieve performance limits measured in laboratories.

HEPA filters consist of fine fibres as illustrated in Fig. 13.6. Materials vary, but generally these are made of synthetic fibrous materials. The principle of HEPA filtration is not to restrict the passage of particulate by the gap between fibres, but by altering the airflow streamlines. The airflow will slip around the fibre, but any higher-density bio-aerosols or particulate matter will not change direction so rapidly and, as a result of their inertia, will tend to impact the fibre. Once attached, most particulates will not be re-entrained in the air-stream.

 
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