The rain on planes
Major airport developments create large expanses of impermeable surfaces where, generally, natural catchments previously existed. Consequently, run-off increases both in quantity and in rate. David Hayward and Richard Barnard, directors of process and environmental management consultants Ashact Ltd, take off in search of contamination sources.
Stormwater contamination from major commercial airports can be well in excess of maximum permitted concentrations for direct discharge to small rivers. Treatment is often necessary. But of what?
De-icing agents remove ice from impermeable surfaces during aircraft movements and commonly involve ethylene, di-ethylene and tri-ethylene glycols. Other formulations, based on potassium acetate or urea, have also been used extensively. Potassium acetate-based chemicals have a lower biological oxygen demand (BOD) than glycol-based chemicals, and although more costly, are also relatively non-toxic to fish – the acetate rapidly degrading to carbon dioxide and water.
Urea, used in granular form, is an alternative de-icing agent for use on runways and hard standings and is cheaper than glycol-based de-icers. However, a major concern is its hydrolysis to ammonia which, in its un-ionised form (extent dependent on pH) is highly toxic to fish.
Anti-icers both remove ice and adhere to the treated aircraft surface during taxi-ing and take-off. Propylene glycol is widely used as the principal ingredient. Polymeric agents are included in the product to achieve the required adhesion prior to take-off speed being reached, although there are reports that some 80% of the applied anti-icer runs straight off. At some overseas airports, aircraft are successfully de-iced using only hot water, prior to a spray of anti-icing agent. This is an effective waste minimisation practice but has not however gained popularity in the UK due to operational drawbacks.
At small airports, the bulk of the de-icing fluid load arises from taxiways and runways. Consequently, the scope for pollution load reduction by a centralised de-icing unit is limited. At major airports, the operational constraints of a centralised de-icing unit for aircraft leading to potential air traffic congestion can be unacceptable. Another approach is to determine the possibility of disposal of the stormwater into large bodies of water, e.g. an estuary. In such situations, impact would be significantly less and treatment prior to disposal, other than for free oil interception and removal, might not be necessary.
Re-use of glycol de-icing chemicals used for taxiways and runways, after re-concentration, is a possibility, although recovery rates with standard membrane filtration (reverse osmosis) units are poor; also, the specification for aircraft de-icing may be too demanding to permit continued re-use in this manner.
Should on-site treatment prove to be necessary, aerobic biological treatment has been widely practised and can remove glycols over a wide range of concentrations. Both floating mechanical surface aerators (see photo) and fine-bubble (membrane-type) diffused-air systems have been successfully employed. However, it has to be said that the low temperature of contaminated stormwater in winter is not conducive to efficient aerobic treatment of glycols in isolation; balancing ponds at some airports have, in the past, frozen over during conditions of extreme cold.
As a possible alternative, the potential use of reed beds has generated interest. At some airports, it is feasible to pass the contaminated surface water generated in winter from a main balancing pond to a water storage lagoon where sensors detect contamination levels and trigger pumped diversion to the local municipal works for treatment together with domestic sewage.
Successful treatment of glycols in admixture (up to 200mg/l) with sewage has been reported, the glycols being effectively removed without inhibition to nitrification. However, treatment works need to be designed and operated to avoid risk of stable biological surface foams becoming established in activated sludge aeration tanks due to proliferation of certain species of filamentous bacteria.
Oil and fuel spillage
Airport operations pose a particular risk of spillage during refuelling. In addition, while major events such as tanker accidents are rare, the possibility of these and spillage from many other small sources must be allowed for in drainage system and outfall design.
Oil (including fuel spillage) removal is one of the primary control objectives at airports. However, the nature of the run-off involving high peak flows, usually means that interceptors at outfalls have to be very large to provide the necessary residence time for separation. The alternative approach is to employ a number of smaller interceptors to treat sub-catchments, although these have the draw-back of potentially being inundated if a large oil spill arises. The compromise solution is to use both methods.
Maintenance of aircraft can involve the use of hydraulic and lubricating oils and chemicals associated with metal finishing shops. Solvents are used widely and large quantities of kerosene waste can arise. Chemical bath concentrates will eventually become too contaminated and will require special disposal arrangements. Rinsewaters containing heavy metals and sometimes cyanide frequently require pretreatment prior to controlled discharge, usually to sewer. Aircraft washing is commonly confined to dedicated hangers where drainage facilities provided allow safe disposal off-site, again often to sewer.
Vehicle numbers at major airports are usually far greater than the number of aircraft. Whilst numerous vehicle washing operations could present a risk to surface water quality, provision of suitable facilities draining to foul sewers is relatively inexpensive. Automatic vehicle washing units can also, if required, be fitted with effluent treatment and recycling facilities, typically involving free oil and gross solids removal by gravity, sand filtration and activated carbon units.
For safety and aesthetic reasons, aircraft parking areas require regular cleaning. This operation commonly requires appreciable use of strong alkaline detergent solutions. At large airports with high levels of traffic and more frequent fuel spillages, there is a need for special facilities for the collection and disposal of such wastes.
In addition, large airports produce a wide range of other non-domestic wastewaters which require control and treatment, from sources such as industrial activity, water treatment plants and aircraft catering operations.
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