Technique

Description

Biofilter

The exhaust air is led through a filter bed of organic material, such as root wood or wood chips, coarse bark, compost or peat. The filter material is always kept moist by intermittent sprinkling of the surface. Dust particles and odorous air compounds are absorbed by the wet film and are oxidised or degraded by microorganisms living on the moistened litter material.

Bioscrubber (or biotrickling filter)

A packed tower filter with inert packing material which is normally maintained continuously wet by sprinkling water. Air pollutants are absorbed in the liquid phase and subsequently degraded by microorganisms settling on the filter elements. An ammonia reduction of between 70 % and 95 % can be achieved.

Dry filter

The exhaust air is blown against a screen made of e.g. multi-layered plastic placed in front of the end wall ventilator. The passing air is subject to strong changes of direction causing the separation of particles by centrifugal force.

Two-stage or three-stage air cleaning system

In a two-stage system, the first stage (wet acid scrubber) is usually combined with a bioscrubber (second stage). In a three-stage system, a first stage consisting of a water scrubber is usually combined with a second stage (wet acid scrubber), followed by a biofilter (third stage). An ammonia reduction of between 70 % and 95 % can be achieved.

Water scrubber

The exhaust air is blown through a packed filter medium by transverse flow. Water is continuously sprayed on the packing material. Dust is removed and settles in the water tank, which is emptied before refilling.

Water trap

The exhaust air is directed by ventilation fans down onto a water bath where dust particles get soaked. The flow is then redirected 180 degrees upward. The water level is topped up regularly to compensate for evaporation.

Wet acid scrubber

The exhaust air is forced through a filter (e.g. packed wall) where a circulating acid liquid (e.g. sulphuric acid) is sprayed. An ammonia reduction of between 70 % and 95 % can be achieved.