Hanoi - Hanoi's Hoan Kiem Lake at the heart of the old city, is stagnant, thick with green algae and choked with sludge. It is also a historical treasure and the heart of Vietnamese nationalism. According to legend, 15th-century emperor Le Loi was rowing across this lake when a supernatural tortoise appeared, retrieved the magical sword the emperor had used in his successful revolt against China, and ascended to heaven - or dove to the depths of the lake, depending on the version.
Today, the lake is home to Vietnam's most famous turtle. The turtle may be the very last of a unique species, dubbed Rafetus leloii by biologist Ha Hinh Duc.
Or it may belong to a critically endangered species, Rafetus swinhoei, of which only three others are known to exist in Vietnam and China. Crowds gather at lakeside when the turtle is spotted, hoping to catch a glimpse of its head for good luck.
So when Peter Werner, a professor at the Technical University of Dresden, designed a project to use a robotic sediment collector to clean up the lake, he knew what to call it: the SediTurtle. A pilot run of the project, financed by Germany's Ministry of Research and Education in collaboration with the Hanoi Sewage and Drainage Company, began last week.
The SediTurtle, a submarine attached to cables which makes its way along the bottom sucking up organic sludge, is cleaning out a fenced-off 1,000-metre area of the lake. Hoses carry the sludge from the submarine to a set of humming 5-metre-high machines at the lakeside, which separate the contaminants from the water.
The hardest part of the cleanup job is protecting the famous turtle.
"The biodiversity in this lake is a special one," Werner said. "We have to do it step by step and very slowly, not to interfere with the turtle."
Most of Hanoi's 41 lakes are cleaned by draining the water and excavating the sludge. Many have been sealed with concrete bases, which is impossible at Hoan Kiem Lake, because it would kill the turtle.
But the lake is clogging up: in some places sludge has built up to within 40 centimetres of the surface, and algae is choking out other life.
The challenge is to suck out the sludge while preserving a functioning natural ecosystem, full of the fish and crabs the turtle feeds on, all inside a 11-hectare lake in the middle of Hanoi.
The area the submarine is clearing has been sealed with underwater fencing to keep the turtle away.
"If this turtle died, I am sure we would have a lot of big problems between Vietnam and Germany," Werner said.
The mechanical side of the project is run by Leonhard Fechter, head of a Berlin-based waste management company. Flanked by two German technicians in moustaches and blue overalls, Fechter supervises he huge, humming machines that churn out waste into long coiled hoses and a distillation tank, where water is purified with chemicals that take out more of the phosphorus and nitrogen.
The remaining water is 95 to 97 per cent pure. Any purer, and it would be bad for the turtle.
It took four months to ship the equipment to Vietnam, and another month to get it through customs. Fechter said customs officials could not understand what the machines were, or what their purpose was.
Werner said evaluating the results of the pilot sludge removal would almost certainly last into January.
"We have to take samples, to check the water quality in the lake, because the boundary conditions are different," Werner said. "The top of this sludge has a certain impact on water quality. After removal we have another layer of sludge which also has an influence on the water quality."
The project will have to find new funding as Germany's science ministry only funds the research phase.
It all seems a tremendous effort to protect one lonely turtle. A turtle who usually pokes his head out only at night or near dawn, when oxygen in the water is running low. The vast majority of Hanoians have never even seen the turtle.
But Werner has.
"I jog five times around the lake in the morning, six o'clock, and I saw it already three times," Werner said. "It's real, it's not a Loch Ness monster or something like that."
