De-icing Agents Volume 2 - Final Report
Report No DWI0448
It is estimated that typically 12,000 tonnes per year of glycol is purchased for antifreeze or de-icing activities. Approximately 4750 tonnes are used for airfield and aircraft de-icing, 7,200 tonnes for antifreeze in engine cooling systems and 50 tonnes for miscellaneous uses. While the antifreeze consumption represents the largest proportion, its impact on the environment is significantly less as any discharge to the environment is over the whole year, is spread over the whole urban area of the UK and in many instances will be discharged to combined or foul sewers.
Ethylene and diethylene glycol are toxic to humans but propylene glycol is relatively harmless. Recommended limits for glycols in drinking water are reported as 1 mg/l (USSR) and 0.14 mg/l (USA). Aquatic toxicity occurs at concentrations of approximately 100-1,000 mg/l for ethylene glycol and 1,000 mg/l for diethylene glycol and propylene glycol, although chronic effects such as loss of equilibrium in fish may occur at lower concentrations.
Glycols are biodegradable and exert a heavy oxygen demand, with BODs of the order of 300,000 mg/l for diethylene glycol, 800,000 mg/l for ethylene glycol and 1,000,000 mg/l for propylene glycol. Physico-chemical treatment methods considered to date do not seem practical. Biological treatment, at suitable temperatures, appears to be the most practical treatment method. Treatability testing of contaminated runoff has given rise to problems of bulking and foam formation in activated sludge systems. Prolonged aerated storage appears to be an effective treatment system.
Whilst reuse of glycol at centralised facilities for aircraft de-icing has been adopted at a few airports overseas, it does not eliminate pollution, due to de-icing of runways and taxiways. For several reasons it appears unlikely that centralised aircraft de-icing would be adopted in the UK.
At all major commercial and military airfields stormwater will be contaminated as a result of the use of chemicals for de-icing activities in winter. At most of the large civil and military airfields using glycols, peak concentration in stormwater may exceed 1,000 mg/l on occasions. At civil airports glycol is used for aircraft de-icing and frequently for runway and taxiway de-icing. At military airfields any glycol use is associated with de-icing of runways and taxiways.
Since at commercial airports de-icant chemicals are used as part of an industrial/commercial activity, contaminated stormwater is considered by several Regional Water Authorities (RWAs) to be an industrial discharge and consent conditions are starting to be applied. The consents are likely to vary depending on the location. Where airports are in a catchment upstream of a drinking water intake, or over an aquifer used for potable supply, the standards are likely to be fairly stringent; typically 20 mg/l glycol, or 15 mg/l BOD and 2 mg/l ammonia (generated as a result of the hydrolysis of urea).
Apart from the potential for impairing the quality of water to be abstracted for water supply, there is little evidence from RWAs and Scottish River Purification Boards of regular major impacts due to the use of glycols at airports. There are more reports on the impact of ammonia (from urea). The lack of impact of glycol in most cases is likely to be due to:
From the RWA point of view, they are bound by the requirements of COPA II and as such their records are open to inspection by the general public, pressure groups and other industrialists (or farmers) with direct discharges to rivers. They are also presently under pressure to improve river water quality. RWAs are also reluctant to attach a "percentage compliance" to an airport discharge as this means sampling programmes become extensive and costly and any legal action becomes extremely difficult and prolonged. It is open to discussion whether the criteria for water quality objectives in rivers which are not used for potable supply presently reflect the seasonal variation in glycol (ie BOD), which could perhaps be tolerated in winter in rivers when temperatures are low and flows are not at their minimum. However, the trend for European harmonisation of water quality standards together with a general tightening of standards would appear to preclude such an approach.
Whilst RWAs sympathise with the problem of stormwater contamination at airports, since it is associated with safety connotations, they cannot be expected to treat airports as special cases otherwise they would have difficulty in justifying the consent conditions applied to their own sewage treatment works or, perhaps more importantly, to other direct discharges (eg industry or farms). This control in England and Wales in the future will be the responsibility of the National Rivers Authority.
From the standpoint of the Airport Companies, they are faced with dealing with a problem which is largely outside their control. They do not know how severe and how prolonged the cold weather will be in many winters or the pattern of rainfall or snowfall. Furthermore, the range of winter weather that can be experienced is very wide and it would be onerous to provide treatment and/or disposal facilities that would be used to their full extent only once in every 5 or 10 years. However, even if facilities to deal with such frequencies are provided, they will inevitably going to fail in a very severe winter (which could theoretically occur shortly after commissioning).
Where facilities are designed on a 1 in 5 year or 1 in 10 year winter basis, they may involve airports in expenditures in the £1-10M range it fixed discharge standards of 20 mg/l glycol are imposed. Slight relaxation of standards to say 30 or 50 mg/l would not reduce expenditure; relaxation to several hundred mg/l on occasions would be necessary to have any major impact on cost implications for airports.
The Airport Companies will need considerable time to study their problem and produce a solution which is reliable and cost-effective. The layout and drainage systems of most airports have not been constructed with a view to separation and treatment of contaminated stormwater. Furthermore, there are also new de-icant chemicals being developed, for example acetate compounds which have only about 30% of the BOD of equivalent glycol usage and which, if deemed acceptable, may have a significant bearing on any treatment proposals. Trials were scheduled in UK early this year but the prolonged mild weather caused their postponement.
In the light of the foregoing summary, it is considered that a round-table discussion between the DoE and interested parties (or sub groups of the interested parties) would be beneficial to review the overall problem.
Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.