When the world’s largest working advanced digestion plant opened last month, it showed the power- hungry process of treating waste in the $360billion water industry can be self-sufficient in terms of energy use.
The Davyhulme facility that handles the sewage of 1.2 million people in Manchester today can export surplus power to the UK grid. It uses waste formerly dumped in the Irish Sea, generating renewable power on a scale no utility has done to date using that method.
The sludge recycling center runs on enough human waste to power 25,000 homes. It was built by Black & Veatch for United Utilities Group Plc, Britain’s largest publicly traded water company. Awarded IChemE’s international prize as “the most innovative green-energy scheme on Earth,” it renders waste into what engineers call “black gold.”
Davyhulme’s plant, using thermal hydrolysis technology, turns a problem waste stream into clean energy. It cuts fossil- fuel consumption, greenhouse-gas emissions and produces a sludge made into free fertilizer for farmers. In a water industry always looking for energy savings, the THP plant “revolutionized our whole sludge-treatment operation,” Lee Donnellan, manager of the site, said in an interview.
These kinds of improvements are just what 2,500 experts will discuss and dissect at the World Water Week conference in Stockholm starting Aug. 31.
The new technology at Davyhulme “really has improved the amount of energy you get from the sludge,” Donnellan said. “It’s a bigger bang for the buck” as more gas is generated “than ever before. That means really impressive power output from our CHP (combined heat and power) engines.”
The Warrington-based company sees “sludge as a valuable resource,” said Richard Lancaster, regional sludge manager for United Utilities. “We call it ‘black gold.’”
A solution had been sought because the amount of sludge produced each year across the utility’s North West region was huge — enough to fill the soccer stadium at Old Trafford where Wayne Rooney leads Manchester United 10 times over.
Engineers say Davyhulme’s configuration by the Manchester Ship Canal is unique. The THP facility is able to export sludge to agriculture as an enhanced treated product or via a 100- kilometer (62-mile) pipeline to United Utilities’ incineration facility at Widnes.
As the largest operational THP facility — the D.C. Water Blue Plains plant is not yet commissioned — Davyhulme has a capacity of 91,000 to 121,000 tons of dry solids a year.
The £105million ($175million) facelift to the facility by Black & Veatch, and a planned wastewater plant upgrade to treat larger volumes to a higher standard, will mean a cleaner ship canal as more refined water goes back into the waterway.
Davyhulme’s water-treatment works are already one of the largest in Europe — and historic. The sewage-treatment process known as activated sludge, now used worldwide to harness the power of micro-organisms, was developed 100 years ago at Davyhulme.
Sludge, once shipped offshore for disposal a century ago, has been banned at sea in Europe since 1998.
At the new site, sludge gets thickened in a centrifuge, fed into special silos, pumped further into a pulper for mixing and heating. Then thermal hydrolysis follows in the reactor for at least 30 minutes as the high temperature and pressure ensure no bugs survive.
Next, the sludge is depressurized in a flash tank that disintegrates the organic material into a more digestible form. After a heat exchange, the sludge is cooled to 40C (104F), and the digester comes into play.
The sludge is digested to generate biogas. Some goes for incineration while dry solids, what’s left after wastewater is cleaned, are exported to agriculture.
In the end, the pasteurization effect of thermal hydrolysis opened a “whole new land bank for us,” United Utilities said. That benefited grazing land for cows and sheep in the North West, which needed higher quality sludge for crops.
Basically the treatment works like a pressure cooker by heating sludge to 165 degrees C at high pressure. Then when suddenly depressurized, organic substances are “smashed,” according to the utility.
Harmful bacteria destroyed, the sewage sludge “food” becomes much more digestible, and 50% to 60% more volume of biogas is generated than by conventional techniques.
Energy-recovery technologies are such that biogas from digested sludge gets held in two giant green “gas bags” visible to motorists from Barton Bridge. These are Europe’s largest inflatable gas holders.
Each gas bag can hold 9,000 cubic meters of biogas produced by the sludge-recycling plant — the gas cleaned and used as fuel for five CHP engines and three steam boilers generates about 60 gigawatt-hours a year, according to the utility. Suppliers include General Electric’s Jenbacher gas engines.
Enough heat is recycled from CHP engines that the plant’s steam boilers rarely need to be fired up to heat the reaction vessels. That means almost all the gas produced is used for generating electricity.
The new advanced digestion facility, expected to cut operating costs about 45 percent, makes the power-consuming process of sewage treatment “virtually energy self- sufficient,” or 96% energy-neutral, said John Tattersall, Black & Veatch’s global water technology director.
The upgrades at the 254-acre site by Black & Veatch also reduce the volume of treatment by-product, or biosolids.
United Utilities, meanwhile, is capitalizing on its THP success with the world’s first sludge-powered data center, due to be completed at Davyhulme next month.