Wastewater collection and treatment often involve the production of unpleasant smells.

In the sanitation network, the absence of air (and more precisely of oxygen) makes that wastewater become septic, and also malodorous. Gases are accumulated in the sewer headspace and emission to the open air can occur at the outlet in the treatment plant.

Wastewater and its by-product treatment (sludge, scum, etc.) also generate unpleasant smells.

The chemical species responsible of the olfactory nuisances are mainly Sulphide, Nitrogenous compounds and, sometimes, Volatile Organic Compounds (VOC).

The sulfide compounds mainly come from wastewater pretreatment, thickening and sludge dewatering stages. The nitrogenous compounds and the VOC are mainly from the sludge treatment.

To avoid the propagation of odors and provide the operating agents with a healthy and safe environment, the works are put in negative pressure, the polluted air is collected and undergoes a deodorization treatment via the Alizair™ process.


As it is biofiltration, Alizair™ is usually the least expensive odor removal solution and the easiest to operate.

Alizair™ oxidizes the malodorous substances (organic or inorganic) into inorganic and odorless compounds.

Alizair™ removes almost all the malodorous compounds. Its efficiency on mercaptans can be improved by the addition of downstream physical or chemical treatment.

Alizair™ requires low maintenance and reagent input. Its operating cost is among the lowest in the market.

Alizair™ is available in a wide range of flows.


Alizair™ is also available as compact units, ready to be used. There are four different sizes, with maximum flow ranging between 1800 and 6700 m³/h.


Alizair™ is a biological deodorization process using biofiltration.

It operates by bio-oxidation of compounds in the liquid phase.

As they pass through the filter, the malodorous molecules are biodegraded by specific micro-organisms on the support media.

The Alizair™ media rests on a perforated floor which participates in the uniform distribution of the polluted air in the structure. The air to be treated rises through it.

Regular watering maintains stable humidity in the filter, nutrients required for biological activity are injected and products from biological oxidation are removed (including sulfates).

The water recovered at the bottom of the reactor is recirculated or removed at the plant head.


Depending on the elements present in the air to be treated, the bio-deodorization operates with different types of micro-organisms, which imply different operating conditions.

Through its experience of biological processes, Veolia has been able to develop various packing media to optimize the process performance.

Alizair™ A

The best performances for hydrogen sulfide, other sulfides, ammonia and amine (RNH₂) removal are achieved by using autotrophic species.

In this case, the material used is Biodagene™, a non-degradable mineral. This is a granular material with high porosity.

A solution of nutritive elements is sprayed on the filter media to feed the purifying biomass. Generally, at least the following elements are required to ensure optimal operation:

  • A source of Carbon
  • A source of Nitrogen and Phosphorus

Usually, the watering with raw water on the filter media is enough to maintain the necessary nutrient and humidity levels.

The watering sequences of the filter bed are handled automatically depending on the air flow rate to treat. Sprinkling intensity also depends on the particle size of the present material.

Alizair™ A can achieve very high filtration rates (500 m³/m²/h) with treatment loads which can exceed 50g/h/m³ of medial.

Alizair™ H

When Volatile Organic Compounds (VOC) treatment is required, the filter and its operation are set up in order to enhance the development of heterotrophic species.

The granular medium used is made from peat. This variant of the process is called Alizair™ H.