Odour control: which filter for which odour

Odour in extracted, ventilation or process air usually comes from volatile organic compounds (VOCs), hydrogen sulphide (H2S) or ammonia (NH3). Which one it is differs widely between industries. That compound decides the filter medium.

Next step: Start the selector (which compound, how many m³/h)

What is in the air?

Often nobody knows which compound causes the odour. A single gas analysis is then only a snapshot: the make-up of the exhaust air can be different the next day. A test filter on site shows which medium works on your exhaust air, before the filter is designed. In this range, such a test filter runs on a side stream until the medium is saturated; that gives the service life and what it means for running costs.

Which technique fits which odour?

Activated carbon without impregnation for VOCs. The compounds stick physically to the carbon. According to the Dutch government's Informatiepunt Leefomgeving (iplo.nl), an activated carbon filter removes 80 to 95 per cent of the odour from the exhaust gas. Carbon works poorly in wet air: humidity must stay below 70 per cent, and you must prevent condensation in the filter.

Impregnated carbon for H2S and ammonia. The gas reacts with the impregnation and is bound almost irreversibly. For an acid gas such as H2S that is an alkaline impregnation, for an alkaline gas such as ammonia an acid one.

Chemisorption on alumina. Porous alumina granules with an impregnation that locks the gas in chemically. This medium is used in waste processing, among other places.

Scrubber. Here the gas dissolves in a liquid. With water, a scrubber removes 60 to 85 per cent of the odour; an alkaline-oxidising scrubber removes 80 to 90 per cent (iplo.nl). The alkaline-oxidising scrubber is used mainly in slaughterhouses, the food industry and at animal feed producers. Choosing between carbon and a scrubber: Activated carbon or scrubber.

Combination. With a high load, another technique, such as a biological filter, can sit before the carbon. See Activated carbon filters.

One central filter or several small ones?

In a local set-up, each source gets its own compact filter. See Filter installations. The largest single carbon filter in this range handles 50,000 m³/h; with filters in parallel or in series, that goes up to 100,000 m³/h.

Measuring odour and the rules

In the Netherlands there are no emission limit values for odour. Whether odour is acceptable is assessed against the odour load at homes and other sensitive locations. Which requirements apply to your site depends on the permit, the location and the process. Odour concentration is measured by olfactometry to NEN-EN 13725, by a measuring body accredited for it. If you measure before and after the filter, you can see how much odour it removes. See Measurements and analyses.

Odour by sector

At sewage pumping stations and water treatment plants, H2S is usually the source. We have separate sites with more depth on these: odour at sewage pumping stations and odour at wastewater treatment plants. For odour and process emissions in industry, such as the food and chemical industries, there is a site on industrial odour. For cooking odour in restaurants: odour filters for kitchens.

Ships. On board, H2S can form in the tanks that collect toilet waste water (black water tanks). Inline odour filters capture that odour.

Buildings near airports. For hotels and offices around an airport there are activated carbon filters for the air handling unit, against kerosene odour. See VOCs and formaldehyde.

Frequently asked questions

Is an activated carbon filter enough?

For VOCs it often is. For H2S and ammonia you need impregnated carbon. If the carbon alone fills up too quickly, a first stage that removes most of the load helps.

Can I rent an odour filter for a while?

Yes. Mobile activated carbon filter units are available for rent and can be in use within 2 to 3 days. See Filter unit rental.

Next step: Ask for a test filter