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As winter closes in and the temperature drops below freezing, airports
become awash with de-icing fluids. Up to 50 million litres of chemicals
are sprayed and slopped onto aircraft and runways each year to stop ice
forming on them. This is essential for safety, as iced-up wings may produce
too little lift to get a plane airborne, and aircraft could skid on an icy
runway. The trouble is that the fluids used are environmentally harmful:
they can contaminate waterways and ground water, killing fish and creating
toxic blooms of algae.

But this winter, departing passengers at Copenhagen airport may notice
that their aircraft halts briefly on a giant rubber mat as the wings and
fuselage are sprayed before takeoff. It is part of a trial to collect some
of the 1.5 million litres of de-icing fluid that are used there in a bad
winter. If successful, it could be taken up by many more airports around
Europe.

All Europe’s major airports take seriously the threat posed by de-icing
fluids to waterways and ground water. Two types of chemical are commonly
used: urea and glycols. Both work by lowering the freezing point of water.
A solution of urea, sprayed onto runways, effectively lowers the freezing
point of ice to -10 degreeC. Solutions of ethylene glycol and propylene
glycol, which are used on runways and aircraft, push the freezing point
of ice down to -13 degreeC and -59 degreeC respectively. Iced-up planes
must be treated before takeoff, and glycols are the only chemicals that
meet the stringent safety specifications for this. Heated hangers could
keep aircraft ice-free, but they are expensive and consume energy. For runways,
sand and salt are not alternatives, because sand blows away in jet blasts
and salt corrodes the aircraft. A new runway de-icer based on potassium
acetate is much less damaging to the environment, but costs up to six times
as much as urea.

The problems start once the preferred chemicals have done their job
and trickle off the runway. Urea breaks down into ammonia and then nitrates,
killing fish and encouraging the growth of algal blooms that deplete dissolved
oxygen. Up to 80 per cent of the glycol solution sprayed onto aircraft runs
straight off and onto the tarmac. When it eventually reaches watercourses
it too combines with oxygen, reducing the amount available to aquatic life.

DRAINING AWAY THE PROBLEM

Stockholm’s Arlanda Airport was one of the first to recognise the pollution
being caused: ‘About five years ago we noticed that large amounts of glycol
from Arlanda were eating up the oxygen in nearby waters,’ recalls Peter
Vikstrom, a hydrologist at the airport. After considering the options, the
management decided to embark on a two-year, £3.6 million project
to adapt the existing storm drains to collect de-icing fluids, and build
a new plant to treat airport sewage.

Excess glycol is now collected as it runs off aircraft, by retrieval
vehicles that work like giant vacuum cleaners. The intention is eventually
to recycle this, but for the time being it is transferred to other sewage
works where it is broken down by bacteria. The carbon dioxide thereby produced
is used to purify other waste products at the sewage works. The remaining
chemical de-icers, along with water from the runways, are transferred to
the airport sewage system. There they are mixed with domestic waste which
speeds up the process of biological degradation by increasing the temperature
as it decomposes. After a day or two the wastes, then about 60 per cent
degraded, are gradually fed into the national sewage system. Weekly monitoring
by airport staff of nearby waterways has shown a sixfold improvement in
water quality since the system was brought in. A similar system is being
installed at Brussels airport.

Other airport managements are tackling the problem with different techniques.
At Munich’s new airport, which was opened only last March, the design engineers
were able to consider environmental demands during construction. The result
is a unique system to deal with runoff. Buried a metre beneath each taxiway
is a waterproof barrier 20 metres wide, made of clay and synthetic fibre,
topped by a layer of gravel. This layer contains bacteria found naturally
in soil that can break glycol down to water and carbon dioxide. As contaminated
water drains off a taxiway, it runs horizontally through the gravel, being
slowed at intervals by sand barriers which ensure that the bacteria can
get to work before the liquid seeps to the edges and into the ground. Elsewhere
in the airport, use of de-icers is kept to a minimum. Because two runways
are available, most ice clearance can be done mechanically.

Aircraft at Munich are de-iced using a new system that resembles a
giant mobile car wash in which planes are sprayed with a mixture of glycol
and hot water. The excess that drips off is caught in a collection tank
and taken by lorry to a recycling station, where it is repurified. Up to
70 per cent of the glycol can be reused.

London’s main airports at Heathrow and Gatwick use balancing ponds,
where contaminated surface water is aerated to encourage the growth of bacteria
that will break down the chemicals. Unlike Germany and other European countries,
Britain imposes no absolute limits on the amount of pollutant that can be
discharged into waterways. Instead, the National Rivers Authority (NRA)
sets local limits designed to maintain set environmental quality standards
such as the level of oxygen in water.

Adapting existing airports to cut pollution by de-icers is not easy.
New drainage systems and storage tanks can be expensive, so the giant mat
being tested at Copenhagen airport is attracting the attention of those
looking for a cheaper alternative. Called RoMat, it was devised by two Danish
engineering students and is the first such mat in commercial use. At 72
metres by 59 metres, it is big enough for all aircraft except Boeing 747s.
The mat is made of rubber reinforced with steel and uses the normal slope
of the taxiway, which is about 1.5 per cent from the centre line, to drain
de-icer along a series of transverse grooves in its surface and through
a valve at the bottom of the slope into collecting tanks. The mat and tanks,
which hold 200 000 litres of liquid, together cost about £1 million.
Environmental considerations are the main reason for using the mat. Copenhagen
airport is close to the Baltic Sea, which is being given a comprehensive
cleanup. The RoMat could also improve safety. Because it can be installed
on a taxiway on the way to the runway, aircraft can be sprayed directly
before take-off. Other systems may need repeat applications if a flight
is delayed. This makes the mat particularly suitable for the climate of
northwest Europe where cold and damp often coincide. Under such conditions,
aircraft have to be de-iced immediately before take-off; any delay gives
condensation the chance to freeze on the wings and fuselage.

In principle, the best alternative to collecting runoff glycol would
be to use chemicals that do not harm the environment. In practice, with
no alternative aircraft de-icer on the market, the trend is towards a more
polluting formulation of glycol, called Type 2. While Type 1 contains only
glycol and water, a Type 2 mixture includes chemicals that help it stick
to the aircraft. Following several accidents in the US thought to have been
caused by icing-up after treatment with Type 1, use of Type 2 has increased
recently; in Europe, which has stricter safety standards, Type 2 has been
used for more than a decade.

There is a nonpolluting alternative for runways. Clearway 1, developed
by BP Chemicals in Britain, consists primarily of potassium acetate solution.
It was launched in Scandinavia in 1988 following two years of tests on its
effectiveness, corrosive qualities and environmental impact, to ensure it
conformed to European safety standards. The product is now used at about
55 airports and air bases worldwide. Barry Hurley of BP Chemicals believes
that 50 to 60 per cent of airports in cold climates will have switched to
Clearway 1 within the next few years.

Arlanda started using Clearway 1 this winter and is now monitoring its
impact in waterways and soil. So far no damage can be attributed to the
acetate-it breaks down easily to carbon dioxide and water, without using
much oxygen from waterways. Heathrow and Gatwick have also switched. In
Britain, the NRA is keen for Clearway 1 to be widely used, but accepts that
its cost may be a deterrent. But with Clearway 1 effective down to-60 degreeC,
Hurley remains enthusiastic: ‘When you look at the ice-melting performance,
it is cost-effective over a season because it is more effective. The price
per tonne doesn’t mean anything, because when your airport is closed it’s
³¦±ô´Ç²õ±ð»å.’

The problem of glycol for aircraft remains. Its large market is a clear
incentive for manufacturers to produce an environmentally acceptable replacement.
But the solution is not yet forthcoming.