Thursday, October 21, 2010

Amazing Construction 6

Joseph Bazalgette
Once the enabling act was passed, the Metropolitan Board of Works could begin the task under the direction of its Chief Engineer, Joseph Bazalgette (1819-91). Most schemes for removing London's sewage had planned to carry it inland so it could still be used as fertilizer, but Bazalgette realised this was impractical. Instead, he planned to build main sewers to collect the contents of the existing sewers and take their contents east to be discharged into the Thames.

Bazalgette's scheme consisted of three major elements:
• the intercepting sewers
• the pumping stations and the outfall sewers
• the pumping stations at Beckton and Crossness.

Each of these involved major feats of engineering.

From the intercepting sewers to the pumping stations. On either side of the Thames, three main intercepting sewers captured the sewage from all existing sewers and carried them by gravity eastwards to the pumping stations at Abbey Mills (for the north bank) and Deptford (for the south).

To the Thames via the outfall sewers. The pumping stations raised the sewage so it could be carried by gravity to the Thames. It flowed to the river in the massive pipes of the outfall sewers. The outfall sewers were a massive feat of engineering. The Northern Outfall Sewer was more than 6.5 km (4 miles) long, and crossed the marshes east of the River Lea. For most of its length, the Northern Outfall Sewer was concealed within a massive embankment. Several bridges carried it across the River Lea and its branches. The scale of the work is still impressive today.

The Southern Outfall Sewer ran underground for much of its length from Deptford to Crossness. At Beckton and Crossness, the sewage was stored in huge reservoirs, from which it was released into the Thames at high tide.

The Thames Embankments. The Embankments on either side of the Thames were another benefit of Bazalgette's scheme. The idea of building embankments to reclaim marshy land along the riverbank had been debated for centuries, but little was done before Bazalgette.

His embankments concealed sections of his intercepting sewers, and avoided the huge cost and inconvenience of digging up central London streets. They reclaimed nearly 10 hectares (22 acres) of mud, and also concealed a section of the Metropolitan Railway, the world's first underground railway line.

Bazalgette’s Legacy. The whole scheme took seven years to complete. Bazlgette's achievement was amazing even by modern standards. When completed in 1865, London had 2100 km (1300 miles) of sewers, including 130 km (82 miles) of the main intercepting sewers. Almost a century and a half later, London still relies on Bazalgette's sewers.

Later improvements. The shortcomings of Bazalgette's scheme. Although Bazalgette's achievement was huge, his system simply shifted the problem downriver. As the sewage was discharged into the Thames untreated, problems were building up. Large mudbanks of sewage began to form downriver from the outfalls. Even worse, the sewage took a long time to clear the Thames, as incoming tides brought it back part of the way. The dreadful state of the river near the outfalls was highlighted by the Princess Alice tragedy of September 1878. The pleasure steamer sank after a collision with a collier in Galleons Reach, not far from Beckton.

Almost cut in half by the force of the impact, the Princess Alice sank in less than five minutes. The unfortunate passengers were thrown into the filthiest and most polluted stretch of the river.
Around 650 people died that evening in the biggest ever disaster on the Thames. Few of the victims died in the actual collision - most drowned in the toxic combination of raw sewage and industrial pollutants.

Chemical treatment. A Royal Commission gathered in 1882 to debate the next step. It recommended chemical treatment of the sewage. From 1887, the liquid effluent was separated from the solid sludge. Only the former was discharged into the river. The sludge was removed by special boats for disposal at sea. From 1887 to 1998, a fleet of sludge boats made regular journeys from Beckton and Crossness to Barrow Deep beyond the mouth of the Thames. Between 1915 and 1967, the nearby Black Deep site was also used for dumping sludge.

The sludge boats. Surprisingly, the best description of the work of London’s sludge boats comes from a sermon by the distinguished Methodist preacher William Sangster (1900-60). Always eager to use unconventional associations in his sermons, he used sludge disposal as a metaphor for divine forgiveness.

London has four sludge vessels. They are tankers really, with a cargo-carrying capacity of 1,500 tons. On every weekday tide, two of the sludge vessels set out laden with this unwanted and perilous matter and travel down the Thames to Black Deep, a depression on the bed of the ocean, fifteen miles off Foulness. When the vessel reaches Black Deep, the valves are opened and the complete cargo runs out in about twenty minutes. Down it goes, down into the salt aseptic sea. A dark stain spreads over the wake of the ship, but so wide is the ocean, and so deep the delivery, and so briny the sea that, within one hour, samples of water taken either from the surface of the sea, or the bed of the estuary prove to be completely innocuous. The sludge has gone, devitalized of all evil power, and never to be seen again…

Full circle - changes in the 1990s
Since the 1990s, the treatment of London's sewage has changed again. To comply with European Union legislation forbidding the dumping of sewage at sea, this method has been discontinued. Around half of London's waste is now incinerated at Crossness and Beckton. The electricity generated from the incineration is sufficient to power the treatment plants, and Beckton even has a surplus to sell back to the National Grid.

Almost half of London's sewage sludge is now sold in pellet form as a fertilizer for agriculture. Intriguingly, London's sewage has now come full circle, as much of it now fulfils the same functions as the night soil of centuries past.

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