Absolute filter

Synonyms, abbreviations and/or process names

—  HEPA-filter (High Efficiency Particle Air filter)
—  Surface filter
—  Pattern filter
—  Micro filter
—  Filter with large surface area

Removed components

—  Submicron dust (PM0,30 to PM0,12)
—  Very fine toxic, dangerous or radioactive substances, including most heavy metals, except mercury
—  Biological pollutants (batteries, pollen, mould traces, animal allergies, dust mites…)       




Process description

The to-be-cleaned gas stream is led into a chamber and then passed through a HEPA filter. HEPA stands for High Efficiency Particle Air. The filter material in a HEPA filter consists of very thin glass fibres which have been incorporated into paper. In order to realise the largest possible filter surface area, this glass fibre paper has been folded into a harmonica shape. This is necessary because the density of the glass fibre paper allows little air to permeate through it.  Thus a large surface area is needed to transfer a large quantity of air. 

The dust is left on the filter, but is not able permeate it. In other words, this is a surface filtration process. The dust layer left on the filter can have a fairly positive effect on dust-collection yield. However, over time the pressure drop on the filter becomes too great and it needs to be replaced.  HEPA filters are rarely cleaned because cleaning can result in the filter becoming damaged, thus causing leaks.


The most commonly used types are filter units, which are either rectangular or cylindrical.

—  In the rectangular version, the folded filter medium is placed in a wooden or metal frame (cartridge), through which the
      air is passed.
—  In the cylindrical version, the folded filter medium is placed in, and is supported by, a metal wire frame which is closed
      at one end with a metal cap. The air flows inside, passing through the cylinder.


Absolute filters have a very high dust-removal yield (more than 99.99 % for PM0,01 and more than 99.9999 % for PM0,1. They allow ultra-low dust concentrations to be realised.

In order to be recognised as a HEPA filter, the glass fibre paper must, in accordance with the DOP-test[1], be effective for at least 99.97% on 0.3 micron particles.  

Boundary conditions

—  Flow rate:       100 to 3 600 Nm³/h per module
—  Temperature:       Above the dew point of the gas flow
                                     < 200 °C (most commercial filters)
                                     < 530 °C (special heat-resistant versions)
—  Relative humidity:        < 95 %
—  Pressure:       atmospheric
—  In-coming concentrations:  maximum 30 g/Nm³, preferably lower
—  Absolute filters are not suitable for the removal of large dust concentrations and/or larger particles. These must be
      removed in advance by an earlier filtration phase, for example, a cyclone or vortex scrubber, to separate coarse dust
      and a fabric filter or compact filter to separate  > PM2,5.

Auxiliary materials

The used filter material (glass fibre paper).

Environmental aspects

Dust-laden filter elements are released as residue.

Energy use

Energy use (incl. ventilator) is low (< 0.1 kW for 1 000 Nm³/h).

Cost aspects

  • Investment
    1 800 – 2 400 EUR for 1 000 Nm³/h [2]
  • Operating costs
    —  Personnel costs:    ca. 2 500 EUR per year (2 hours per week) [2]
    —  Auxiliary and residual materials: 60 to 120 EUR per year for 1 000 Nm³/h (filter material) [2]

Advantages and disadvantages

  • Advantages
    —  Very high yield, low residual emissions (see efficiency)
    —  Can be used for fine particles
    —  Modular construction
    —  Low sensitivity for small variations in the gas stream
    —  Relatively simple to operate
    —  Few corrosion problems
  • Disadvantages
    Not suitable for the removal of wet substances
    —  Not suitable for high dust-loads (unless after filtration)
    —  Not suitable for gas stream containing bases
    —  Possible risk of explosion
    —  Need for filter element to be replaced regularly.


Absolute filters are suitable for cases requiring extensive removal of submicron particles or for cases concerning toxic or dangerous particles. It is possible to use them as the final step of a dust-removal system after, for example, an electrostatic or fabric filter.

Absolute filters are often used for inside air filtration in settings where high air-quality is required, for example, in operating rooms at hospitals or in production areas in the pharmaceutical, photographic or electronics sectors.


  1. Factsheets on Air-emission reduction techniques, www.infomil.nl, Infomil
  2. Common waste water and waste gas treatment and management systems in the chemical sector, BREF document, European IPPC Bureau, http://eippcb.jrc.es, 2002
  3. Air Pollution Technology Fact Sheets and Technical Bulletins, US EPA, http://www.epa.gov/ttn/catc/products.html, 1999-2002
  4. Supplier information

[1] DOP = dioctylphthalate. In the DOP test, dioctylphthalate particles with a controlled size of 0.3 microns are used as test gas.