Composting plants, like all the plants where great masses of organic substances are managed and transformed, are the cause of bad smell emissions. All the processes of decomposition or suppression of volatile compounds are vectors of important olfactory stimuli. In the composting plants, above all, the reasons of particularly intense bad smell phenomena can be led back to the presence of critical situations of the process or plant engineering problems:
Presence of anaerobic bags in the heaps (not completely oxidized compounds of sulphur, carbon and nitrogen);
Scarce ventilation o not conveniently used by the biomass;
Improper use of the biomass turnings;
Wrong planning of the basic structure or a bad conduction of the plant.

Bio-filtration is a technology used to treat the gaseous emissions which are obliged to pass through a biologically active porous bed. The system is, in reality, a bed filled up with materials such as cortices, chopped wood, matured compound, peat, heather, etc. Many minerals have been tested, even the shells of the mussels, but in this specific case we should have to talk about bio-washing.
In practice, the microorganisms of a bio-filter complete the degradation of the initial organic substance of which the odorous components represent the intermediate stage of the degradation. If the environmental conditions are favourable to the life of the microorganisms for the presence of sufficient quantity of nutritive substance, humidity and temperature degree, the reproduction process of the same ones is obtained. It being understood that the factors that favour the bacterial multiplication vary for many species, the humidity has a fundamental importance as the microorganisms can only absorb a food substance from the watery phase. The colonization, therefore, such as the metabolic activities, occurs inside the “liquid bio-film”.

Odors are mostly spread because they are transported by supports with minimum dimensions (micro-dusts which are as small as thousandths/hundredths/tenths of micron) or by gasifying molecules.
Scrubbers are vertical towers where fumes or gases are washed.
The operation of these towers is based on the absorption principle; in fact, in these machineries the water-soluble polluting components, which are a means of transport for the odorant molecules, are transferred.
The most used absorbing liquid is water, which nevertheless limits the efficiency of the systems in presence of odor sources coming from poorly water-soluble compounds.
The most water-soluble molecules are: ammonia, alcohols, volatile fatty acids; the most hardly water-soluble ones are substances such as: amines, hydrogen sulphide, ketones, aldehydes.