Thames Water awards Costain £150m wastewater upgrade program
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Costain has been selected to deliver around £150 million of upgrades across 15 Thames Water wastewater treatment sites as part of the utility’s AMP8 investment program.
The projects are spread across the Thames Valley and form part of Thames Water’s Wastewater Asset Assurance Programme. The work will include upgrades to inlet stations, storm tanks and other wastewater assets.
Delivery will take place during AMP8, the five-year regulatory period running from 2025 to 2030.
The sites will be split into eastern and western delivery areas. The eastern group covers Barkway, Brentwood, Long Reach, Little Hallingbury, North Weald and Standon. The western group includes Aldershot, Burghfield, Esher, Horley, Lightwater, Marlborough, Northleach, Waddesdon and Windsor.
For Costain, the award extends an existing relationship with Thames Water and adds to its work across the UK water sector.
For Thames Water, the contract comes as the company moves from investment plans toward project delivery across an aging and heavily used network.
A £150m contract sits within a much larger AMP8 program
The Costain award is one part of Thames Water’s wider investment plans for AMP8.
The utility plans to invest more than £20 billion between 2025 and 2030 as part of what it describes as its largest network upgrade in 150 years. Its plans cover water and wastewater infrastructure, leakage reduction, treatment assets, smart metering and measures to reduce pollution.
Thames Water’s November 2025 AMP8 delivery plan put allowed expenditure at £20.5 billion for the five-year period. It also said the level of investment represents a large increase from previous asset management periods.
Wastewater accounts for a major part of that spending.
A Thames Water information document for 2026-27 said £5 billion is planned for upgrades to hundreds of sewage treatment works. The program also includes work on stormwater systems intended to reduce the use of storm overflows.
A separate £840 million procurement program announced in 2025 covered 10 major capital projects across wastewater and water treatment sites. Those projects were designed to improve asset resilience, environmental performance and capacity for population growth.
Together, the figures show the scale of work moving through the UK water supply chain.
Utilities must now convert large investment allowances into designed, permitted and completed projects within fixed regulatory periods.
That creates demand for contractors with engineering, construction and program delivery experience as AMP8 activity increases.
Water companies now face the task of delivering the investment
The size of AMP8 creates a practical question for Thames Water and the wider industry: how much work can be completed within five years?
Thames Water has already identified delivery pressure as an issue.
Its AMP8 delivery plan said the increase in spending creates challenges, particularly in wastewater non-infrastructure work. The company said it had changed parts of its delivery model and its approach to suppliers in response.
That gives the structure of the Costain program added importance.
Managing 15 treatment sites as regional groups could allow engineering teams, project managers and suppliers to work across several sites rather than treat each upgrade as a separate project.
There is also a wider supply-chain challenge.
AMP8 has increased demand across the water sector at the same time. Contractors, engineering consultancies and specialist suppliers are being asked to support treatment works, pipelines, pumping stations, reservoirs and other infrastructure across several major utilities.
Costain has expanded its water-sector workload in recent years. Its contracts and frameworks include work with Thames Water, United Utilities, Severn Trent Water, Southern Water, Northumbrian Water and Yorkshire Water. The company has also said its strategy is focused on long-term infrastructure programs rather than isolated projects.
For asset owners, supplier capacity and project coordination could play an important role in whether AMP8 targets are met.
Price controls set the financial framework, but contractors and engineering teams still have to convert that funding into operating assets.
AMP8 is turning investment plans into construction work
Thames Water’s operating scale helps explain why wastewater upgrades are receiving significant attention.
The company treats around 4.3 billion liters of wastewater each day while supplying about 2.6 billion liters of drinking water. Its network includes more than 140000 kilometers of water pipes and sewers.
Its latest annual results also reported a 20% rise in annual capital investment to £2.7 billion for the year ended March 31, 2026. Thames Water said it was investing more than £7 million a day across its water and wastewater networks during that period.
There have been signs of operational improvement. The company reported an 18% fall in pollution incidents in 2025 compared with 2024, alongside a 27% reduction in serious pollution incidents.
However, infrastructure renewal remains a long-term task.
Thames Water is also working through a recapitalization process intended to support its AMP8 program. A proposal outlined in March 2026 included £3.35 billion of new equity and up to £6.55 billion of new debt, alongside proposed AMP8 expenditure of £20.4 billion.
That financial backdrop means individual contract awards can provide an indication of how planned spending is moving into delivery.
The £150 million Costain program is small compared with a five-year investment plan of more than £20 billion. Its 15-site scope, however, shows how AMP8 spending is being translated into individual projects across an operating network.
For engineering and construction companies, this points to sustained demand through the investment period.
The main test through 2030 will be whether utilities and their suppliers can turn record spending plans into measurable improvements in treatment capacity, asset condition and environmental performance.
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