(Bloomberg) -- Thames Water customers will have contributed £540 million ($674 million) toward the construction of new sewerage infrastructure in London by the time it starts operating next year, the Financial Times reported, citing a study.

The utility’s 16 million customers pay a surcharge — currently £26 a year — that helps fund construction of the Tideway Project and will total more that half a billion pounds by March 2025, according to a study by Greenwich University.

Testing on the £4.5 billion tunnel, which will stretch for 25 kilometers (15.5 miles), is expected to start next week, with the removal of the deep underground walls that have kept it separate from the capital’s century-old sewerage system, the newspaper said.

Thames, the UK’s largest water and sewage utility, has come under pressure as investors pull back and its debts continue to pile. The company has proposed a 40% increase in its charges by the end of the decade in order to perform vital infrastructure improvements.

Thames Water does not own the project but will operate the tunnel.

London Tideway Project Nears Completion Amid Thames Water Woes

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