A packed tower air scrubber is a reactor that has been filled with an inert or inorganic packing material. The packing material usually has a large porosity, or void volume, and a large specific area. Water is sprayed on top of the packed bed and consequently wetted. Contaminated air is introduced, either horizontally (crosscurrent) or upwards (counter-current), resulting in intensive contact between air and water, and enabling mass transfer from gas to liquid phase. A fraction of the trickling water is continuously recirculated; another fraction is discharged and replaced by fresh water (Melse and Ogink, 2005).
The mass transfer of ammonia (from air inlet to water) is regulated by the equilibrium reaction (below), that is influenced by pH and temperature, in a similar way than explained in Stripping Technology chart (section 7.4). To improve process efficiency, an acid reagent (mainly sulphuric acid) is usually added into water recirculation stream.
NH3(g)+H2O(l)↔NH3(aq)+H2O(l) ↔ NH4+(aq)+OH-(aq)
The efficiency of odour removal by an acid scrubber is the result of dissolution of the odorous compounds in the water phase and the water discharge rate. As the water solubility of odorous compounds may vary from very low to very high, odour removal efficiencies vary as well (Melse and Ogink, 2005).
The treatment of exausted air at farms is hardly used in practice in general. In Flanders there exist regulations obligating to this treatment. In other countries, the regulation demanded by authorities are based on minimum distances (from farm to population) to avoid odour problems. In a context where minimum distance can not be kept (business expansion), that technology can be demanded (Hahne and Varlop, 2001). Also air treatment may be of major importance for compliance with current and future PM10 and PM2.5 standards (particulate matter).
Ammonium salts are usually delivered to fertilizers production companies.