The plant is entirely covered; the method of odour control is determined on the basis of the airspace contamination. The more contaminated air from the pre-cleaning phase, the pre-settlers and the sludge treatment facilities is deodorized by chemical treatment, while the less contaminated but much larger amount of air which comes from the airspace of biological reservoirs and the aeration of post-settlers is deodorized in biological odour control units.
This unit is made up of three washing towers connected in lines and is capable of cleaning approximately 75,000 Nm3 air per hour.
Their operation is fully automated. Spray water recovered from the bottom of the tower is either recirculated or returned to the host structure of the plant.
The biological breakdown of the unpleasant smelling compounds is carried out through filtration using a so-called biofilter, of special bacteria dispersed on a carrier material. The composition of the air to be cleaned and characteristics of the plant determine the amount of nutrient (e.g. phosphorous) to be added.
Spraying the surface of the filters maintains a constant relative humidity in the filter media bed, thus ensuring nutrients necessary for the activity of the bacteria and allowing for the removal of biologically oxidized end products (e.g. sulphates). Spray water collected at the bottom of the reactor will either be recirculated or returned to the water treatment line.
The biological odour treatment unit is capable of treating a volume between 215,000 m3/h and 250,000 m3/h. The particle size of the filter medium is between 3 and 6 mm. Six such biofilters can be found in the plant.