Mogden Sewage Treatment Works is a sewage treatment plant in the Ivybridge section of Isleworth, west London, England, formerly known as Mogden. Built in 1931–36 by Middlesex County Council and now operated by Thames Water, it is the third largest sewage works in the United Kingdom. The site covers .
It treats the waste water from three main sewers serving the northwest quarter of Outer London and two further ones from the south and south-west.
History Middlesex County Council Act
The Middlesex County Council Act 1931 is solely devoted to the scheme's construction and mitigations, comprising 78 sections and three schedules. Its work 1 is this works, works 2 to 5 are main sewers rising in Hendon, Uxbridge, Harrow and Staines.Middlesex County Council Act 1931: section 4 https://www.legislation.gov.uk/ukla/Geo5/21-22/32/pdfs/ukla_19310032_en.pdf The Teddington main also connects, as fed into the new sewer 5d from works of Twickenham Corporation.
The sewerage district (catchment) was defined as those districts named after: Brentford & Chiswick, Ealing, Feltham, Harrow, Hayes & Harlington, Hendon (both), Heston & Isleworth, Kingsbury, Ruislip-Northwood, Southall-Norwood, Staines, Sunbury-on-Thames, Teddington, Uxbridge, Wealdstone, Wembley, Yiewsley & West Drayton.Middlesex County Council Act 1931: First Schedule https://www.legislation.gov.uk/ukla/Geo5/21-22/32/pdfs/ukla_19310032_en.pdf The second schedule lists the compulsory purchases by plan reference. The third schedule sets the minimum standard of purity of 30 ppm of suspended solids in treated effluent and secondly to "not take up more than" 20 ppm of dissolved oxygen in five days at .
Treatment works The plant was built in 1931–36 for £1.7 million by Middlesex County Council, replacing 28 small sewage treatment facilities as part of the West Middlesex (Mogden-Perry Oaks) Main Drainage Scheme,Philip Howard, London's River, London: Hamilton, 1975, , p. 223.Bird Notes 26–28 (1954) 144. which had a total estimated cost of £5.25 million ().Middlesex County Council Act 1931: Preamble https://www.legislation.gov.uk/ukla/Geo5/21-22/32/pdfs/ukla_19310032_en.pdf The council enlarged the sewage operation at Mogden Farm, buying of it,Middlesex County Council Act 1931: section 12(2)(a) https://www.legislation.gov.uk/ukla/Geo5/21-22/32/pdfs/ukla_19310032_en.pdf after the public and Duke of Northumberland objected to a Syon Park proposal.The Municipal Journal and Public Works Engineer, 28 August 1936, p. 1670. of sewers were built to connect to it,Environmental Accomplishments to Date: A Reason for Hope: International Symposium II, July 16–18, 1974, ed. George M. Dalen and Clyde R. Tipton, Jr., The Environmental symposium series, Columbus, Ohio: Battelle Memorial Institute, , , p. 50. and the Duke of Northumberland's River was straightened out as a source of coolant.Mogden, Hidden London, retrieved 12 November 2013. The plant began operations in late 1935"Discussion of the Effect of Synthetic Detergents", The Institution of Civil Engineers, Proceedings 6 (1957) 398. and was formally opened with the rest of the scheme on 23 October 1936 by the then Minister of Health, Sir Kingsley Wood.Philip T. Sherwood, Heathrow: 2000 Years of History, Stroud: Sutton, 1999, , p. 33. In its first year of operation it treated an average of 60,020,000 gallons of sewage per day.Applied Chemistry Reports 23 (1938) 774.
From 1935 the treatment facilities at Mogden comprised storm water tanks, primary sedimentation, sewage aeration, final separation and sludge digestion. The plant was designed to treat three million cubic metres per day of sewage.Leslie B. Wood, The Restoration of the Tidal Thames, Bristol: Adam Hilger, 1982, , pp. 74–84. Treatment tanks at Mogden sewage treatment works, 1935ServiceNo.TypeStorm water tanks8RectangularPrimary sedimentation tanks8CircularSecondary sedimentation tanks4RectangularAeration tanksBattery A, 6
Battery B, 6
Battery C, 3RectangularFinal separating tanks52CircularSludge digestion tanks12Circular The works were expanded in 1962, 1989 and 1991 and upgraded in 1996–2002. The 1962 extension included commutators, grit chambers and pre-aeration tanks, together with the following additional tanks. Additional treatment tanks at Mogden sewage treatment works, 1962ServiceNo.TypeSedimentation tanks4CircularAeration tanksBattery C, 3RectangularFinal separating tanks12CircularSludge digestion tanks4Circular The most recent expansion, in 2011–13, increased the treatment capacity by more than half and increased the plant's size by , as part of the Thames Tideway Scheme to improve water quality in the River Thames. It was completed in summer 2013.Alix Culbertson, "£140m upgrade of Mogden Sewage works complete", Get West London, 9 May 2013.Maureen Gaines, "Thames Water opens upgraded Mogden STW", Water & Wastewater Treatment, 26 June 2013.
Technical approach When built, the Mogden works were very modern, one of the first large-scale applications of the activated sludge technique for sewage treatment.A. V. Koodie and I. J. Kirkaldy, "Uprating of Mogden Sewage Treatment Works", in Design, Operation and Economics of Large Wastewater Treatment Plants: Selected Proceedings of the 8th IAWQ International Conference on Design, Operation and Economics of Large Wastewater Treatment Plants, Held in Budapest, Hungary, 6–9 September 1999, Water Science and Technology 41.9, 2000, pp. 53–62, p. 53. The plant was also advanced in having a central control board for measurements and in using the methane generated by sewage treatment to generate electricity"Operation and Maintenance of Britain's Largest Dual-Fuel Engine Installation", The Oil Engine and Gas Turbine, January 1959, p. 346. and heat from the processing to heat the sludge. It was originally equipped with oil and spark-ignition gas engines;"Operation and Maintenance of Britain's Largest Dual-Fuel Engine Installation", p. 366. a heat and power plant was added in 1993. In 1974 the works were selling surplus methane for £12,000 a year ();"Methane—the most natural gas", New Scientist, 14 February 1974, p. 407. the latest upgrade enables them to meet 40% of their energy requirements from methane.
Initially, after approximately 25 days at Mogden, sludge was piped to settling ponds at the Perry Oaks Sludge Treatment Works near Heathrow Airport.Journal of the Institution of Civil Engineers 27–28 (1946) 377. In 1989 the Perry Oaks site was closed and the land reclaimed for Heathrow Terminal 5 with new sludge dewatering provided at nearby Iver South.Lev Nelik and Jim Brennan, Progressing Cavity Pumps, Downhole Pumps and Mudmotors, Gulf Pump Guides, Houston: Gulf, 2005, , pp. 67–68.
There is a plant for dosing the effluent with hydrogen peroxide before the outfall pipes. This is operated on the instruction of the Environment Agency if the stormtanks are overflowing and discharging into the effluent channel. The peroxide prevents low oxygen conditions developing in the tidal sections of the river that receive the flows. During dry weather with low flows the effluent from Mogden can constitute the main part of the flow in the river.
An effluent oxygenation plant was under construction in the late 2010s, consisting of an array of aerators that can be lowered into the effluent channel to increase the dissolved oxygen levels in the effluent. This would only operate during very dry weather to ensure that when flows in the river Thames are low oxygen levels in the Upper Thames Tideway are kept high enough to encourage salmonid migrations and ensure a healthy aquatic ecosystem. This will be the first such effluent aeration plant in the UK.B. Lodge and S. Spooner, "Development of a novel effluent aeration project at Mogden STW based on a water quality model for the Upper Thames Tideway", conference proceedings, AquaEnviro.
Finances Mogden is the namesake of the Mogden formula, used to estimate the cost of wastewater treatment.
Environmental complaints The Mogden works discharges effluent at Isleworth Ait, on the Tideway which is the western half of the Thames Estuary, centred on London's centre. Between 1956 and the completion of an expansion in 1962, some of this had not received secondary treatment.Advances in Water Pollution Research (1962) 845. Also during heavy rain, the plant was sometimes overwhelmed and released untreated sewage into the river; in summer 2011, 200,000 tonnes of untreated sewage from Mogden contributed to a large fish kill.Paul Newton, "Thames Water 'regrets' fish kill", Utility Week, 8 June 2011. Nearby residents have also complained about odour: the Borough of Hounslow served an odour abatement notice on Thames Water in 2001,Mogden sewage works, London Borough of Hounslow, retrieved 12 November 2013. and in 2011 complainants won a court judgement that the company had failed since 1990 to adequately manage odour and thereby violated human rights; £19,000 in damages to ten people were assessed."Victory For Residents Against Thames Water: Residents successful in 'David & Goliath' battle", St Margarets Community Website, 8 December 2011.Paul Newton, "Thames' failure to tackle sewage stink breached human rights, High Court rules" , Utility Week, 9 December 2011.
In January 2022 a report published by the Environmental Audit Committee found that Mogden discharged enough sewage to fill 400 Olympic-sized swimming pools on 3 and 4 October 2020. It added that during the whole of 2020, 3.5 billion litres of untreated sewage entered the Thames from Mogden - seven times as much as was dumped in 2016. Thames Water's 2025 documentary Thames Water: Inside the Crisis features Mogden employees talking about inadequate investment on maintenance and equipment upgrades making it difficult to prevent sewage discharges.Nils Pratley, "Thames Water on TV: pity the staff, this place is decrepit", The Guardian, 16 March 2025.
References
External links Mogden Sewage Treatment Works at Thames Water