Because of the absence of air and more particularly of oxygen, wastewater when it is collected becomes septic and can be extremely malodorous, sometimes directly at the network level and usually on arrival in the wastewater treatment plant.

At the treatment plant level, the storage and treatment of the sub-products resulting from the treatment of wastewater (sludge, scum, etc) can also release strong smells.

In the wastewater treatment plant, the nuisances are mainly related to the presence of sulphur compounds already partially formed in the network and, at a lower degree, to the presence of nitrogenous compounds.

The sulphuric compounds that form the majority of the concentrations in the foul air from the pre-treatment, and the thickening and dewatering of the sludge offer the highest olfactive thresholds. The nitrogenous compounds (ammonia, amines) are mainly present in the sludge treatment rooms.

To prevent the propagation to the surroundings of the odours produced in the treatment works, and to provide a healthy and safe environment to the operation personnel, all risks of releasing malodorous gas are minimized by using a biological odour removal system, called Alizair.

Alizair is a biological deodorisation process (bio filter) that uses the bio-filtration technique.

This process involves biological oxidation of odorous compounds in a liquid phase. It is particularly well suited to the treatment of gaseous effluent that is generally encountered in sewage / sludge treatment works and is designed to handle very high air flow rates. Alizair achieves very high filtration rates and ensures almost full removal of malodorous compounds.

Alizair is an air flow bio-reactor lined with a support material that is either mineral (e.g. Biodagene) or organic (peat, compost). The flow of air to be treated rises in the bio-reactor. A perforated floor supports the material while allowing air to evenly penetrate the material and the spray water to run out.

The air is fed into the bioreactors by centrifugal fans. Malodorous compounds are bio-degraded during filtration via specific bacteria fixed onto the support material. The composition of the air to be treated and the features of the plant determine the choice of support material to be used and the nature of the added nutrients.

Spraying the surface of the filter with raw water maintains a constant relative humidity in the bed, provides the nutrients needed for the bacterial activity and ensures evacuation of biological oxidation products such as sulphates.

The spray water recovered at the bottom of the reactor is either recirculated or returned to the inlet works.