The project underlines the success of North West's overseas expansion programme. The company has already won four other international contracts this year.
The Bangkok project will supply the Thai capital with its first modern waste water treatment facilities.
Pollution and a rapidly expanding population have forced the government to act. At present raw sewage runs straight into the city's network of canals before reaching the Chao Phraya river.
North West won the contract at the head of a consortium that includes Siam Syntech Construction, Sino-Thai Engineering & Construction and OTV. The consortium, in which North West has a 50 per cent stake, beat off competition from seven other bidders.
Bob Thian, North West's chief executive, stressed that his company's role would be the design, project management and operation of the new facilities. Siam Syntech and Sino-Thai will be responsible for construction and will bear the financial risks of that operation.
'The reason for our success in winning these contracts overseas is that we have stuck rigorously to our core business of operating water and waste water utilities,' Mr Thian said. 'We are also in manufacturing, but even that is connected with making plant for water and waste water facilities.'
The attitude is in sharp contrast to some other privatised water utilities, which have diversified in an attempt to provide streams of non- regulated earnings.
Thames Water was forced to provide pounds 25m last week against problem contracts in the Third World.
Mr Thian stressed that the difference between North West and Thames was that his company was solely a project manager and operator whereas Thames was involved in construction.
He added that North West's strategy was for non-regulated business to provide 10 per cent of bottom-line profits within five years.
The Bangkok project runs for four years and will involve extensive micro-tunnelling to avoid the congestion that would result from open cutting. About 50km (31 miles) of interceptor sewers will be laid to pick up waste water from 430 outfalls. They will discharge into a five-kilometre tunnel that will carry the waste to the treatment plant.
Many of the pipes will be aligned beside or beneath the canal beds in what is expected to be the biggest pipe-jacking operation in the world, acording to North West.
North West's shares rose 8p to 538p.
Thailand's highly charged stock market is holding its breath for a late November public share offer by TelecomAsia Corporation, a private company franchised to install and service two million Bangkok telephone lines, Reuter reports.
Brokers said TA's partial flotation was keenly awaited by Thais and foreigners seeking a slice of the country's high-growth telecommunications industry.
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