Biodegradation of aircraft deicing fluids in soil slurries

Aircraft de-icing fluids (ADF) containing propylene-glycol (PG) and other additives are used at Northern airport in winter. Potassium formate is also used for de-icing of runways and taxiways. Due to the mechanical removal of snow from the runways and to the drift of ADF from the aircrafts during take-off, the ADFs and formate are mixed with snow. The chemicals infiltrate the soil surface along the runways when the melting of snow begins in spring. It is important to ascertain that these chemicals do not contaminate groundwaters and surface waters. The unsaturated zone can act as a natural purification system, because PG and potassium formate are easily degraded by soil bacterial communities under aerobic conditions, whereas anaerobic degradation in groundwaters is considerably slower. Nevertheless, they still constitute a threat to the groundwater if biodegradation rates are not sufficient compared to infiltration velocities in the unsaturated zone. We present experimental data on the biodegradation of PG and formate under aerobic and anaerobic conditions. Soil samples from the Oslo International Airport at Gardermoen have been used as source of degrading microorganisms. Gardermoen Airport is built on Norway’s largest rain-fed aquifer and must operate on the condition that the groundwaters are not affected. All biodegradation tests were carried out in shaken flask reactors containing soil suspensions in PG or formate solutions. This is an extremely simplified system as compared to what actually occurs at the site, all phenomena related to mass transfer and to soil heterogeneities having been removed. In order to characterise and to quantify a complex reacting system such as that at hand, however, it is a universally acknowledged practice to measure the intrinsic kinetic parameters, once all the unnecessary complications have been removed. In order to afford the actual situation to be modelled in detail, the identification and measurement of the kinetic parameters is a necessary prerequisite. The rate and the extent of biodegradation has been found to be severely limited by many factors, such as temperature, biomass concentration, availability of additional nutrients (nitrogen and phosphorus sources) and of electron acceptors (oxygen, nitrate, sulphate, iron and manganese oxides).