The most cost effective system to contain large volumes of air from biofilter units is an aluminium cover. This is made of modular walls with a waterproof plastic geomembrane which is welded on site. The modular system allows for easy change incover dimensions if necessary. The air retaining system should enclose the biofilter to prevent weather conditions affecting water distribution and odour dispersion. The cover allows air sampling and channeling to an extraction chimeney. The dispersion of odor improves by increasing the height of the discharge point and with high air velocity.