## 1 Introduction  
Air travel is considered to be one of the safest and fastest means of transport. The aviation industry stimulates the economic/cultural growth and creates many jobs. However, the day-to-day airport operations generate air, water, and soil pollution [1]. One of the main issues is the water runoff containing pollutants from runways, taxiways, washing and de-/anti-icing pads, trans-shipment points, fuel storage stations and hangars. These wastewater types contain petroleum-derived substances, surfactants, pavement de-icers, aircraft de-icing/anti-icing agents, and other organic and inorganic pollutants. In climate zones at risk of icing, the pollution caused by pavement de-icing poses a severe environmental problem. Urea, acetate, and sodium formate in the solid form, and acetate and potassium formate in the liquid form are the agents most commonly used for winter maintenance of airport pavements [2]. They release organic compounds and nitrogen into the environment while also increasing the salinity of water solutions and, thereby the salinity of areas adjacent to airports. Both the temperature [3] and the C/N ratio [4] are factors that significantly influence nitrogen conversion and contaminant removal in biofilters. Consequently, they have a major impact on the capacity to treat airport de-icing wastewater and protect the environment. The issue of the treatment of wastewater containing de-icing agents remains unresolved. Most airports are not equipped with a wastewater treatment system. Only a few airports in the world have wetlands, which effectively remove pollutants. Unfortunately wetlands create favorable environmental conditions for birds, which may endanger airport operations. Other rarely used solutions include filters with zeolite and perlite, media made of crushed clay and granular activated carbon, a mixture of granular activated alumina and porous concrete, granular activated lignite, half-burnt dolomite, and granular ferric hydroxides. Considering sustainable development principles, the best solution would be to use the filling made of waste materials, e.g., light weight aggregates prepared from fly ash from sewage sludge thermal treatment. It is characterized by a large specific surface, resistance to physicochemical factors, low heat conductivity, good phosphorus-sorption properties, and facilitates the deammonium process. These attributes provide good conditions for biofilm growth even at low temperatures [5,6]. The breakdown of urea into ammonium (NH\({}_{4}\)\({}^{+}\)) or ammonia (NH\({}_{3}\)) is a well-known process that occurs in both aerobic and anaerobic conditions at a wide range of pH, temperature, and C/N values [7]. The resultant processes of nitrogen reduction and oxidation affect the natural environment. The biological oxidation of NH\({}_{4}\)\({}^{+}\) or ammonia NH\({}_{3}\) to nitrate is known as nitrification. The nitrification of ammonium in a biofiliter is a two-step process in which ammonium or ammonia is first converted to nitrite (NO\({}_{2}\)\({}^{-}\)) and then to nitrate (NO\({}_{3}\)\({}^{-}\)). Its conversion to nitrite is mainly conducted by a group of obligatory autotrophic bacteria. Also, a few heterotrophs have been reported to carry out nitrification in the environment, but usually at much lower rates than autotrophic bacteria [8]. Nitrification allows converting a relatively immobile ammonium-nitrogen to highly mobile nitrate, affecting environmental quality [9]. The most effective pathway for N removal from wastewater is nitrification followed by denitrification [4]. The biological denitrification is conducted by denittifying microbes which use nitrate as a terminal electron acceptor, and organic and inorganic substances as electron donors and energy sources for sustaining the microbial growth [10]. In the course of autotrophic denitrification, microorganisms use sulfur compounds, hydrogen, and/or iron as energy sources, and carbon dioxide and hydrocarbons as carbon sources.