Washing and maintaining aircraft can produce a lot of environmental contamination, especially in the form of heavy metals. Stockholm’s Arlanda airport has, for several years, been operating a purpose-designed, wash-water treatment plant to take care of this problem.
The wash-water cleaning plant at Arlanda airport was built in 1999, following a tightening of environmental emission regulations by the Swedish environmental authorities. Bengt Sj?gren, managing director of Aqua Konsult AB, the design consultants for the project, explains that, “the new regulations covered many metals which are commonly found on aircraft and in the airport environment, including cadmium, lead, copper, nickel, zinc and chromium. The restrictions they imposed on emission levels were quite strict.”
Chemical CocktailPeder Eklund, who’s a manager at SAS’s maintenance facilities at Arlanda, points out that, “All the SAS aircraft and vehicles based at Arlanda are regularly washed and maintained. SAS also washes and maintains aircraft and vehicles belonging to some other airlines at Arlanda, so, in effect, we handle the majority of the aircraft and vehicles at the airport. Obviously, the metals Sj?gren mentioned end up in the wash water. In addition, some of the maintenance work we carry out, such as plating and coating, produces small amounts of aluminum and chromium waste and – given our climate up here – we have to use significant amounts of chemicals for de-icing purposes, glycol for the aircraft and potassium acetate for the runways and taxiways. Some of these chemicals adhere to the aircraft and vehicles and also come out in the wash water.”
Prior to 1999, the wash water simply drained into the conventional sewage network, which delivered it to the municipal treatment plant. Part of the motivation behind the new regulations is to try to remove all heavy metal contamination on-site, so as to make treated domestic sewage acceptable as a fertilizer in general agriculture. The first step in building the on-site wash-water treatment plant was, therefore, to re-construct the drainage piping from the washing areas. The pipes now carry the water to a pumping station, which pumps it to the plant, all of which is now housed within SAS’s maintenance facilities.
Distillation PhaseThere are five inflow pipes into the wash-water treatment plant, each coming from different areas of the maintenance facility. “One of these flows in particular is heavily contaminated with heavy metals,” says Sj?gren. “It’s the flow originating from the area where aircrafts’ wings and wheels are washed – the brakes in particular can give rise to a lot of heavy metal contamination. Cadmium content can go as high as 12 milligrams per gallon, and there’s also, for example, lead and copper content.” Consequently, this particular inflow pipe is diverted to a distillation treatment to deal with its heavy metal content. The condensed water from this treatment then passes through the rest of the plant’s treatment phases.
The next phase for all the inflow is mechanical treatment. The heavy metals fall to the bottom of a sedimentation tank, while the non-soluble oil waste floating on the top is sucked away by a well-perforated suction tube on the surface. The next phase involves two mixing chambers. In the first, the pH is reduced from around 6 to between 2.5 to 3 by adding sulphuric acid. In the next chamber, the pH is raised to 10 by adding iron sulphate. These adjustments of the pH transform all the metals present into insoluble metal hydroxides. In the flocking chamber, a polymer is added to induce coagulation of these metal hydroxides.
The flocks are first settled out in a Nordic Water Lamella unit, with a surface area of 33 square feet. The treated water is then given a final polish by passing through a Nordic Water DynaSand unit. The DynaSand filter eliminates backwashing – fouled sand is continuously removed from the filter bed, washed and recycled without interruption to the filtration process. The filter is based on the counterflow principle. The water to be treated is admitted through the inlet distributor in the lower section of the unit and is cleaned as it flows upward through the sand bed, prior to its discharge through the filtrate outlet at the top. The sand containing the entrapped impurities is conveyed from the tapered bottom section of the unit by means of an airlift pump to the sand washer at the top. Cleaning of the sand commences in the pump itself, in which particles of dirt are separated from the sand grains by the turbulent mixing ac-tion. The contaminated sand spills from the pump outlet into the washer labyrinth, in which it is washed by a small flow of clean water. The impurities are discharged through the wash water outlet, while the grains of clean sand (which are heavier) are retained to the sand bed. As a result, the bed is in constant downward motion through the unit. Thus, water purification and sand washing both take place continuously, enabling the filter to remain in service without interruption.