Contract award

Costain takes 150 million pound Thames Water treatment upgrade

By Staff Writer  |  18 August 2026

Costain has been awarded work by Thames Water to upgrade strategic treatment assets across the Thames Valley, covering around 150 million pounds of upgrades at 15 wastewater treatment sites under the utility's Wastewater Asset Assurance Programme.

The award was announced on 17 August 2026. Costain will complete around 150 million pounds of essential upgrades to wastewater treatment assets across 15 sites, forming part of Thames Water's Wastewater Asset Assurance Programme. The work is to be completed during AMP8, the asset management period running from 2025 to 2030.

The scope covers modernising operations at inlet stations, improving the resilience of storm tanks, and other repairs, modifications and upgrades to wastewater systems. Thames Water describes the wider effort as its biggest upgrade of sewers and sewage treatment works in 150 years, with improvements planned at more than 250 sites across its region.

Costain will split the works into two regions, East and West, and has appointed a regional senior project manager for each. The eastern programme covers Barkway, Brentwood, Long Reach, Little Hallingbury, North Weald and Standon. The western programme covers Aldershot, Burghfield, Esher, Horley, Lightwater, Marlborough, Northleach, Waddesdon and Windsor, giving 15 treatment sites in total.

Matt Bateman, Water Sector Director at Costain, pointed to the company's existing relationship with the utility and its record on construction efficiencies, and said: "We'll work closely with Thames Water and our supply chain partners to deliver these essential infrastructure upgrades, that will build resilience into the region's critical wastewater system, efficiently and safely."

Tim Horton, Capital Delivery Director at Thames Water, said: "We are delivering the largest programme of investment in our wastewater network in more than 150 years, helping to improve resilience across our region and support a growing population." He added that Costain "will play an important role in upgrading key treatment assets across these 15 sites".

Costain was previously awarded contracts to upgrade strategic treatment assets at Markyate, Brickendon, Sandhurst and Selborne. It has also carried out clay compaction trials for The White Horse Reservoir, the new 150 million cubic metre reservoir in Oxfordshire intended to supply up to 15 million people.

What the practitioner should take from this

A programme of this shape is not a single project and should not be administered as one. Fifteen sites, two regions and a five year asset management period mean the commercial risk sits in the instruction and valuation machinery rather than in any one design. The practitioner acting on either side should establish early whether each site is a separate order carrying its own completion date and damages, or whether the sites are milestones within one obligation. The answer decides whether delay at Standon has any contractual effect on the account at Windsor, and it is a question far cheaper to settle at award than at final account.

Regulated water work carries a particular timing discipline. Expenditure has to fall within the asset management period for which it was allowed, so a programme described as running to 2030 has a funding horizon as well as a construction one. Where works slip beyond the period, the question of who carries the consequence is a matter of the contract terms rather than of regulatory sympathy, and the point is worth reading before the first site is mobilised.

The scope description also repeats a familiar source of argument. Repairs, modifications and upgrades to existing operational assets are, by their nature, defined by what is found rather than by what was drawn. On live treatment works the pre-existing condition of inlet stations and storm tanks is the ground on which most variation claims are built. The practitioner should look for the survey obligations, the allocation of risk in unforeseen physical conditions, and the notice periods that attach to each, because an existing asset that behaves differently from the record drawings is the ordinary case rather than the exception.

Nothing here suggests any difficulty on this programme, which has only just been awarded. These are the standard commercial features of multi site framework work on operational infrastructure, and the time to read them is now.