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Wastewater plants extract nutrients from sewage(视频)

VOA 2012-10-25 10:16

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Sewage treatment plants around the world are beginning to put value on waste, turning it into a marketable resource.

Blue Plains Advanced Wastewater Treatment Plant in Washington is the largest facility of its kind in the world and is among the leaders in the effort.

On an average day, 1.4 billion liters of raw sewage flow into the plant, delivered from around the city through 2,000 kilometers of underground sewer pipes and pumping stations, an amount that could fill the nearby sports stadium.

The waste is loaded with nitrogen and other nutrients.

General Manager George Hawkins makes sure the discharge doesn’t pollute the river on which two million people in the region depend for their drinking water.

Limited discharge

By law, Blue Plains can dump only a small percentage of those nutrients into the Potomac River.

“We know that people buy nutrients all the time," Hawkins says. "So the question is how can we take things out of the water that we’re treating here and re-use it for the resource that it is?”

A massive $4 billion environmental program is under way at Blue Plains.

Among the three projects, two are mandated by the government, including one that will cut in half the amount of nitrogen and other waste nutrients discharged into the Potomac River.

The other is an underground tunnel to prevent accidental sewage overflow from its network of ageing pipes.

Sludge power

Engineer Chris Peot heads the solid waste program at Blue Plains, which is working on making better use of the mud-like sludge components of the wastewater stream.

He says by 2012 the four huge digesters now under construction will cook the sludge.

“The microbes in the digesters are methanogens and they help break down the organic matter and convert it into methane which can be collected and burned in a turbine which turns and creates electricity.”

The process will make enough methane gas to supply about one-third of the treatment plant’s electric power needs, a great savings given that Blue Plains is the city’s biggest electricity user.

The other half of the digested wastewater bio-solids will be sold as a nutrient-rich soil additive.

“If we blend it and make it look like a top soil we can market it to the public," Peot says. "It can certainly be a source of revenue, a source of good clean soils for restoration projects, for reducing run-off for tree planting.”

Ghana's green factory

Other sewage treatment plants in the United States and other developed countries have adopted this by-product technology, or are considering doing so. The idea is also generating interest in developing countries.

Amit Pramanik is an environmental engineer with the Water Environment Research Foundation. In a recent trip to Ghana, West Africa, he visited Kumasi, the nation’s second largest city, where only 10 percent of the residents are connected to a central sewer system.

Pramanik says the digester project under construction will help reduce waste that now flows largely into open pits or landfills. He says project managers hope it will also generate useful and commercially valuable by-products such as nutrient-rich soil additives, bio-diesel fuel and methane gas.

“Can you recover that gas and can you use that either for heating or cooking and so on?" he says. "Can you grow food crops with this? Can you recover the nitrogen and phosphorus and increase agricultural yields?”

Economically and environmentally sound

What’s attractive to the local community, Pramanik says, is the payback.

He says the model could be adopted anywhere that waste can be marketed as a profitable community resource. “So if you can get the community involved from the beginning and see that the community is benefiting from it, and the community sees that, they will take ownership.”

Pramanik says these new green factories make good sense for the local economy, for public health and for the environment.

Costly old infrastructure

George Hawkins at the Washington Water and Sewer Authority agrees, however he and other waste water managers in the United are faced with many challenges to put such new green factories in place. First, he says, their current system of pipes and pumping stations is old and requires on-going maintenance. “In most cities the system was put in at the turn of the century of the last century when cities were being formed. And, he adds, the fact is that we need a lot of revenue from our rate-payers to bring it back up to date.”

Hawkins predicts those rates will increase by double-digits annually for years to come.

Not only is this costly, but secondly operators must also comply with strict environmental rules. Under the Clean Water Act, passed 40 years ago and widely credited with protecting public health and the environment, industries are required to limit the waste that can go into the waterways.

Financial challenge

Many cash-strapped communities are finding it difficult to meet the law’s tough new standards, for what some, as George Hawkins told lawmakers at a recent hearing on Capitol Hill, believe are marginal environmental benefits to the waterway.

“We spend one billion dollars to get one tenth the protection of what we had spent 100 million dollars for in the past," he said. "And, it’s only going to get more expensive.

Joining Hawkins at the hearing was Lima, Ohio Mayor David Berger, who said his city doesn’t have all the funds it needs to comply with the new rules.

“These are unfunded mandates," Hawkins said. "So, if the Congress has passed these as mandates and orders for us to execute, then you must join us to pay for it. If the cash isn’t there or the political will isn’t there, then regulatory relief must come, and it has to come soon.”

While Republican Congressman Bob Gibbs told Berger that he agreed, that communities with ageing water and sewer systems need to be given some flexibility in complying with the law, Congress has actually moved to reduce funds for water projects from projects from $4.7 billion in 2009 to just $690 million in 2012.

Stalled Funding

Last year Democratic Congressman Tim Bishop introduced a bi-partisan bill to expand a fund that states could use to finance water projects.

In his remarks at the hearing, Bishop was clearly frustrated that the bill is stalled in Congress. “We’re going in the wrong direction here. And we are exacerbating a problem that all of you on the ground are trying to resolve.”

George Hawkins says he and other public utility mangers will continue to press the US Congress to provide the funds they need to comply with new clean water laws and maintain aging infrastructure. In the meantime, he urges the nation’s water treatment managers to pursue creative ways to turn wastewater into an income stream.