Managing the Thames River
Imposing and clean as it is today, the Thames was once almost a dead river. But that was before Thames Water started its efficient sewage treatment operations.DATE 2020-11-11 AUTHOR Peter Rose
The Thames may not be the longest, widest or deepest river in the world; but it’s certainly among the most famous. It has been at the heart of English commerce since pre-Roman times, thanks to its strategic location and its navigable waters.
It is the principal reason why London grew up where it did and has contributed enormously to the capital city’s power and prestige throughout history.
Despite the rapid dismantling of the British Empire since the last war, the waters of the Thames still roll through the one of the richest individual regions in Europe. The finance houses, banks and other commercial offices that line its fringes generate such wealth that Greater London’s economy alone is larger than that of Russia and has played a major part in driving the British economy overall to its position as the fourth largest in the world. The Thames continues to play host to the palaces of the rich and powerful as well as the Houses of Parliament at Westminster; known as the Mother of parliaments.
For Londoners, the Thames has always exercised an emotional as well as commercial appeal. Warships for the fleet that defeated the Spanish Armada in the 17th Century were launched from Thames-side shipyards. The great warehouses that traded in the spices, coffee and tea from the Far East were built alongside the river with deep-water wharves capable of taking ocean-going vessels. Until the early 1800s, a flourishing fishing industry supplied the city’s homes and taverns with everything from salmon to lobster. Today, it supports a huge leisure industry based around hiking, boating and fishing.
From cesspit to good example
For many centuries the river was the capital city’s principal source of water for drinking, for industry and for disposing of its waste. In fact, the Thames was once almost a dead river, so heavily polluted by industrial and human effluent that little marine life could survive in it.
The danger to human health was acknowledged from the 1830s when filtering of the river waters was begun to remove some of the worst pollution. But it took several major outbreaks of cholera between the 1830s and the 1850s, during which up to 2000 Londoners a week were dying, to force the authorities to create one of the first proper sewage systems for a major city, an incredible engineering feat which took 10 years (1858–1868) to complete.
The authority originally established to develop and operate that sewage disposal system evolved into the privately-owned company that still manages the water and sewage needs of 13 million Londoners today. Now known as Thames Water and a member of the RWE Group, it is the third largest water-management company in the world, serving millions of customers across the globe.
Regularly rated as one of the UK’s most efficient water companies, its success can be measured not merely in financial but in environmental terms. Thanks to the company’s management programmes, the Thames has lost its reputation as a liquid cesspit and is now regarded as one of the cleanest metropolitan rivers in the world, supporting over 120 species of fish and other marine life.
Vital – but scarce – resource
The complexity and importance of this task is highlighted by a very curious statistic. Despite England’s image as a “green and pleasant land” and a reputation for almost constant rain, people living in London and the southeast of England actually enjoy access to less water than inhabitants of some of the world’s desert regions!
According to figures recently published by the UK Environment Agency, Londoners and their near-neighbours each use 58 000 gallons of water per head of population every year, whereas people in Syria have more than 95 000 gallons per annum at their disposal.
Managing such a scarce and vital natural resource is therefore a huge responsibility. Thames Water has been very careful in its selection of partners and suppliers to help in complying with its quality and environmental criteria. Since 1993, the UK arm of Alfa Laval has worked closely with Thames Water to develop and improve its systems for treating and disposing of sewage sludge; an essential task made more difficult by the fact that it has to be carried out in one of the world’s most densely-populated regions, where space is at a premium.
The way that the Alfa Laval – Thames Water relationship succeeds is probably best illustrated by the work currently underway in two treatment works west of London, Perry Oaks and Mogden. Between them, these two sites handle the sewage waste generated by the population of west London and its western suburbs. A 10 kilometre long pipeline links the sites, enabling sludge to be thickened and digested at Mogden and then pumped to Perry Oaks for final dewatering and subsequent recycling to agriculture.
Raw sewage must be treated in this way to meet strict environmental legislation governing its disposal. Until 1993, Perry Oaks STW used traditional sludge drying beds, in which gravity was used to settle out the solids content of the sludge. This was both a land-intensive and time-consuming method. In 1993, Alfa Laval designed and installed a new mechanical sludge dewatering plant comprising four DSNX 7050 decanter centrifuges and associated equipment. The purpose of the installation was to eliminate dependence on the settling lagoons and to produce a drier “cake” that would be easier to handle and dispose of mechanically.
Full capacity during relocation
Since 1999 all four decanters have been required to run almost continuously 365 days a year, 24 hours per day processing an average of 160 cubic metres of sludge every hour. In 2003, Perry Oaks handled a total throughput of 1.2 million cubic metres without a single, major breakdown.
As well as having one of the world’s most famous rivers on its door-step, Thames Water’s territory encompasses Heathrow, the world’s busiest airport. In fact, Perry Oaks occupies a part of the site for the new BAA/Terminal 5 (T5) for Heathrow Airport and the land is gradually being released for redevelopment as the T5 project progresses.
The efficiency of the Alfa Laval centrifuge plant has been a major factor in Thames Water’s ability to continue to operate the site within a smaller footprint. They have been able to maintain the dewatering plant at full capacity while relocating the original installation to the northern sector of the retained operational area.
To help accomplish the transition as smoothly as possible, Thames Water decided to install a fifth decanter centrifuge through which to route a fraction of the incoming sludge, thus enabling them to decommission each of the original decanters in turn.
They chose an Aldec G2, one of a range of new Smart Decanters that provides improved levels of sludge handling, drier cake and reduced energy usage. In fact, Perry Oaks was one of the first major plants in the world to install a G2 machine.
The Aldec G2 consumes less power than any other sludge decanter centrifuge currently available. Although the new machine is significantly smaller and lighter than its predecessors, it can cope comfortably with the 45 m3 per hour of sludge needed to facilitate the switch from the old to the new dewatering plant.
Steve Dymond is the project engineer responsible for overseeing this complex operation for Thames Water. He has worked closely with his counterparts within Alfa Laval for more than a decade, developing a strong working relationship and a system in which each company has been able to learn from the other.
“There is little question that our partnership with Alfa Laval has been beneficial,” he said. “We collaborated, for instance, to develop new sludge thickening systems for the Mogden site, based on their Mega-Duo thickeners. These have enabled us to deliver a product to Perry Oaks that is easier to handle.”
“Then, of course, we have had to work extremely well together at Perry Oaks to ensure that we release land as and when British Airports Authority need it for the Terminal 5 project without undermining the quality and reliability of the dewatering operation. It has been – and continues to be – a delicately balanced operation that would be difficult to accomplish without a healthy degree of mutual trust and confidence between all of the parties involved.”
The work at Perry Oaks is just one element in a complete re-development plan that also involves Thames Water eventually moving sludge dewatering operations to Iver South SDW, where a similar range of decanter centrifuges will operate at higher capacity. Based on their success at Perry Oaks, Steve Dymond says “Thames Water is confident the new site will meet the continuing demands for quality and environmental friendliness necessary to serve a quarter of the population of London.”