Environmental treatment for aircraft wash-water

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 Sjögren, Managing Director of Aqua Konsult AB who were 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, and the restrictions they imposed on emission levels were quite strict, for example; cadmium was to be reduced to below 1 microgram per litre and chromium and copper to 50, respective 200 microgram per litre."

A chemical cocktail
Peder 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 Bengt mentioned end up in the wash water. In addition, some of the maintenance work we carry out, such as plating and coating, produce small amounts of aluminium 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 comes 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 at 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 phase
There 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 Bengt Sjögren. "It's the flow originating from the area where aircraft's 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 1-3 milligram per litre 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 suck off by a well-perforated suction tube on the surface. The next phase consists of 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 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 is polymer is added to induce coagulation of these metal hydroxides.

The flocks are then first settled out in a Nordic Water Lamella unit, with a surface area of 10 square metres. The treated water is then given a final polish by passing through a Nordic Water DynaSand unit. The great advantage of the Nordic Water Product's DynaSand filter is its elimination of the problem of downtime for backwashing. This problem has conventionally reduced the efficiency of sand filters.

The principle of the DynaSand filter
The DynaSand filter has overcome it by eliminating backwashing: fouled sand is continuously removed from the filter bed, washed and recycled without interruption to the filtration process. The DynaSand filter is based on the counterflow principle (see diagram). The water to be treated is admitted through the inlet distributor (1) in the lower section of the unit and is cleaned as it flows upward through the sand bed, prior to discharge through the filtrate outlet (2) at the top. The sand containing the entrapped impurities is conveyed from the tapered bottom section of the unit (3), by means of an air-lift pump (4), to the sand washer (5) 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 action. The contaminated sand spills from the pump outlet into the washer labyrinth (6), in which it is washed by a small flow of clean water. The impurities are discharged through the wash water outlet (7), while the grains of clean sand (which are heavier) are retained to the sand bed (8). 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.

Capacity margin
The plant has three DynaSand units, each with a filter area of 0.7 square metre, installed. Originally two of them were filled with activated carbon, but as Bengt Sjögren explains, "the carbon filters were used at the plant for the first year or so, but the results from the Lamella and the sand filter were so good that we were easily meeting the treatment requirements without the carbon filters. Of course, it's a great advantage, to have these two additional filters installed: they can be used as sand filters to increase the capacity of the plant, or, if environmental discharge regulations are tightened still further, we could start using them again as carbon filters - they give us a very reassuring margin of capacity for the plant."

Final disposal
The discharge water from the plant goes into the municipal sewage system: Bengt points out that, “it may still contain relatively high levels of BOD and COD, which the municipal treatment plant will take care of.” The solid wastes from the treatment process are all transported to a specialist hazardous waste disposal centre, run by a company called SAKAB, in the city of Örebro, in central Sweden. The sludge from the Lamella unit is first passed through a dewatering press. Peder Eklund comments that, “disposal of this costs SEK 2,000 a ton, while the distillation sludge, with its high cadmium content, costs between SEK 7 to 8,000.”

Also Oslo
The wash-water treatment plant has a design capacity flow of 4 cubic meters per hour, though Peder points out that the monthly flow is well below this at between 400 to 800 cubic metres. "We don't operate the plant continuously but, when it is running, we try to operate for as long a period as possible. The built-in primary buffer tank, with a capacity of 30 cubic metres, helps us to manage this. We've also got an 11 cubic metre emergency buffer. One of the main functions of this would be to divert incoming flow into a relatively safe area if it was contaminated with aviation fuel - though, thankfully, in the years that the plant's been running, this has never happened." Peder also revealed that Oslo's new Gardemøen airport, where SAS also has large maintenance facilities, has very similar wash-water treatment plant, also equipped with Nordic Water Lamella and DynaSand units.