A reeding lesson from BAA
BAA's construction of a wetland treatment system, designed to deal with contaminated airfield runoff, has been praised for its environmental sustainability and innovation
Results showed that the reed beds treated glycol effectively, and provided a principle whereby BAA could size the system required. It was then costed and put in amongst the other technologies being reviewed. When the options were finally compared, reed beds proved most effective in terms of treatment performance and cost. Other technologies faced problems or were too expensive. Reverse osmosis for example, would have problems because of the impact hydrocarbons would have on the system. Reed beds demonstrated they could deal with glycol, hydrocarbons and metals which come off aircraft tyres and engine exhausts.
Heathrow Airport embarked on the construction of a wetland treatment system, bringing together a team to develop an innovative and environmentally sustainable scheme. Runoff from a large part of Heathrow Airport is drawn from two catchment areas, east and south. These produce a combined volume of nearly 90,000 m³. Potentially contaminated water is balanced and aerated before passing through one of two types of reed bed. Rafted reed beds are adopted for high hydraulic loading rates. Their performance is not compromised by larger quantities of water as rafts simply rise with the flow. There are also sub-surface reed beds, giving rise to high level microbiological treatment.
Partners on the scheme were Laing, TPS consult, Amec, Middlesex University, Binnie Black & Veatch, Turner & Townsend, Chris Blandford Associates and Penny Anderson Associates. These include consultants and contractors who are part of BAA's framework agreement as well as specialists. Peter Worrall, technical director at Penny Anderson Associates, explained how every member of the team was involved from the earliest stages of design, a critical factor when it came to construction. He believes that working as an integrated team helped reduce costs from £28M to £19M. The success of partnering was partly dependant on a no-blame culture, which fuelled team development and innovation.
Part of the team's goal was to drive sustainability at every opportunity. This included the use of recycled materials whenever possible and the adoption of certain building techniques. It took 18 months before the team found the perfect material for the sub-surface reed beds. They considered concrete, recycled brick and furnace slag before choosing gravel. The gravel used was being excavated for the construction of an olympic-size rowing lake. Although it is a non-renewable resource, the team saw it as a form of recycling. Innovative building techniques included the hire of steel formwork to assist with the construction of concrete walls. Traditionally, used timber in formwork is thrown away afterwards, whereas the steel frame was reusable. The team also used Bentonite, a clay powder, instead of concrete to line the reed beds. Bentonite hydrates and expands when water is added, to create a seal on the reed bed floor. Using concrete would have generated greater energy requirements and the production of CO².
Completed in summer 2001, the scheme has won awards for sustainable development and has been praised for its novel engineering and management techniques. In addition, the logistical challenges faced by the team led to a series of innovations. One of these involved the balancing reservoir in the eastern catchment and the need to separate clean and dirty water. Building a second reservoir was not an option as the site is an area of metropolitan importance for nature conservation. This led to the development of a floating, mobile, butyl curtain which splits the reservoir and allows dirty and clean water to be stored together. Its installation created a new set of logistical problems for the contractors who had to find the most practical way of handling the 200m-long curtain.
Transferring the water from the eastern reservoir to the newly constructed wetlands facility also presented a challenge. The obvious solution would have been to lay a 3km pipeline, but this would have caused considerable disruption to the A30. Instead, the team decided to tap into an existing fire main, which lies on the perimeter of Heathrow Airport, and draw off the water at an appropriate point near the reed bed system. The process took 18 months to develop as it was vital the fire fighters' working capacity was not compromised. A series of trials took place to look at whether the water would affect fire fighting equipment or the foam used. Results showed it did not alter the way the fire main operated and therefore it provided a relatively inexpensive way of transferring the water. In sustainability terms it was ideal.
In the lower reservoir on the eastern side, where clean water would be discharged to the River Crane, it was important for the team to ensure the water was in optimum condition, should any minor pollution occur. A series of wind turbines, a mechanism derived in the US, was installed for aeration treatment. The devices resemble large, floating drums, which revolve even at low wind speeds, pulling water from the bed of the reservoir to the surface. At the surface the natural absorption of oxygen can occur and the whole lake becomes well oxygenated without the need for electrical equipment or maintenance requirements.
At present there are no consent standards imposed upon the reed bed system. It will be closely monitored for three years after which time an appropriate standard will be agreed with the Environment Agency. "We modelled the quality and volume of water coming off the airfield and took the very worst case of de-icant pollution," said Worrall, "That was over 1,000 BOD. It can be reduced without a problem to 40 BOD." This self-imposed standard is a guide. Once in ten years the system will have to work at its optimum to get the water quality to 40 BOD. Generally results will be substantially lower. Worrall estimates below 10 BOD, maybe even 5 BOD.
BAA is still waiting for evidence to prove how effective the reed beds will
be. The system only went on-line on December 3, 2001. Worrall explained "It
is an event led process. In the first week there were de-icant events, but not
major ones. There hasn't been much rainfall, so there has not been much wash
off into the system." Initial results have been positive, but there is
still some way to go before some people in the industry will be convicned of
reed bed technology. "With a full winter's worth of operational data we
will be in a position to demonstrate the viability of this approach to surface
water management at airports," said Worrall.