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 Main Drainage Scheme. The council purchased Mogden Farm for the purpose after the public objected to the proposed site in Syon Park. of sewers were built to connect to it, and the Duke of Northumberland's River was diverted to flow through it as a source of coolant. The plant began operations in late 1935 and was formally opened with the rest of the scheme on 23 October 1936 by the then Minister of Health, Sir Kingsley Wood. In its first year of operation it treated an average of 60,020,000 gallons of sewage per day. 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. 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. 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.
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. 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 and heat from the processing to heat the sludge. It was originally equipped with oil and spark-ignition gas engines; a heat and power plant was added in 1993. In 1974 the works were selling surplus methane for £12,000 a year ; 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. 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. 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. There is also an effluent oxygenation plant under construction, due to be commissioned 2020. This consists 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.
Environmental complaints
The Mogden works discharge effluent at Isleworth Ait, on the Tideway in the upper reaches of the Thames Estuary. Between 1956 and the completion of an expansion in 1962, some of this had not received secondary treatment. 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. Nearby residents have also complained about odour: the Borough of Hounslow served an odour abatement notice on Thames Water in 2001, 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.
Mogden formula
The plant gave its name to the Mogden formula, a measure of the volume and strength of industrial effluent used in treatment pricing. The Mogden formula is: where: = charge per volume of effluent = reception and conveyance charge = primary treatment charge = additional volume charge for biological treatment = treatment and disposal charge where effluent goes to sea outfall = biological oxidation of settled sewage charge = chemical oxygen demand of effluent after one hour of quiescent settlement at pH 7 = chemical oxygen demand of effluent after one hour of quiescent settlement = treatment and disposal of primary sewage sludge charge = total suspended solids of effluent at pH 7 = total suspended solids of effluent The Mogden formula indicates that the more contaminated the wastewater or effluent is, and the greater the volume of suspended solids, the higher the charge will be for treating it.
