BIOFILTERS

Description

A biofilter is used to remove odours from gaseous emissions. Biofilters exploit the ability of certain microorganisms to break down organic, and some inorganic, odours.

In the biofilter, the microorganisms are provided with a substrate having a large adsorbent surface area, such as peat. In cases where pH control is required, calcium carbonate may be added. The microorganisms require a humid environment so the substrate is either sprayed with water or the gas is humidified. The gas may also require a dedusting pretreatment. The air to be treated is blown evenly throughout with a fan. The air can be introduced from below, through a plenum chamber, or from above. The latter system is designed to ensure even penetration of sprayed water. An open structure can be achieved by interlayering the peat with heather or by mixing in polystyrene.

Bacteria and fungi are seeded on the material or allowed to become established naturally. Seeding can be specific, where particular compounds are to be removed, or general, by applying activated sludge and nutrients. Once established, the microorganisms digest the odorous substances, converting them mainly to carbon dioxide and water. The treated gas stream is then vented to the atmosphere.

Biofilters are usually sited externally and can be open to the atmosphere or enclosed. The latter are easier to control, are more appropriate for specific pollutant removal and are better protected from the vagaries of the weather. Under controlled conditions, specific cultures of organisms can biodegrade certain pollutants typically regarded as difficult.

Application

Biofilters have had success in the removal of sulphurous smells from sewage treatment works and the food industry and VOCs from the plastics, textiles and motor industries. For example, biofilters are successful at removing toluene and xylene up to concentrations of 2000 mg/m3. Examples of specific microorganisms tackling specific gases are Nocardia for styrene vapours and Thiobacillus for hydrogen sulphide.

Sizing

Biofilters tend to have low capital and operating costs. For a typical biofilter, the electrical load of the fan and spray is about 4 kWh for every 10,000 m3 of air treated. Lower efficiencies are found at extremes of the size range and where the resistance of the associated ductwork is high.