The Sewers Project - which cost £160m - was officially opened by the NI Regional Development Minister Conor Murphy who 'launched' the major stormwater pumping station to mark its completion.
Work has been going on since 2006 to upgrade the existing sewer networks and reduce the risk of flooding.
Northern Ireland Water said the project is one of the biggest infrastructure schemes to be completed in Northern Ireland.
About 500 existing sewers have been upgraded at a cost of about £40m, while the storm-water management programme involved the construction of the network of tunnels at a cost of about £120m.
It is thought the new system will reduce the amount of pollution being put into the River Lagan by 85% and prevent streets from flooding.
The station will be able to pump the equivalent of an Olympic swimming pool in less than two minutes.
Speaking at the official opening of the new facility, the Minister said: "The completion of the Belfast Sewers Project is good news for everyone in the city and represents an important milestone in its history.
"By reducing the risk of out-of-sewer flooding and pollution of our local rivers, the project will help accommodate our expanding city and its needs well into the 21st Century.
"This project underlines our commitment to support growth and development across the region by improving vital infrastructure and highlights the importance of continuing to invest in our water and sewerage infrastructure," he said, noting that the project employed more than 300 personnel during construction work which included a tunnel network running at depths of up to 30 metres.
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The largest section spans four metres in diameter and stretches from Cromac Street to the pumping station at Duncrue Street.
NI Water Chief Executive, Laurence Mackenzie said: "This is a landmark achievement for NI Water, an achievement we wish to share with the traders, businesses and local community who worked in partnership with us throughout this project.
"This is the result of years of careful planning and the dedication of a skilled team working in tough conditions with cutting-edge technology. We are proud to now have a system that is recognised to be amongst the best in Europe."
Morgan Est, part of Morgan Sindall plc, were the leading engineering contractors on the Belfast Sewers Project.
They were responsible for the construction of all shafts and tunnels for the project. Morgan Farrans were the main contractor for this major investment, alongside NI Water's appointed project managers, Atkins.
Another point of note for engineers is that the depth of the Terminal Pumping station is 40m - which is greater than the height of the Albert Memorial Clock Tower.
Despite this week's opening event, the sewers actually enjoyed a high profile 'unofficial' launch when the BBC's Top Gear programme went 'round the bend' in Belfast's sewers.
The TV team sped through the cavernous sewers under the city streets just like in the film, The Italian Job as one of their most bizarre challenges.
Motoring guru Jeremy Clarkson took to Belfast's new six-mile sewer system last year with fellow Top Gear presenters Richard Hammond and James May in September last.
They drove through part of the six-mile, giant underground pipe from Cromac Street to Duncrue Street and a network of tributary tunnels - hailing it as one of the biggest civil engineering infrastructure projects in the UK.
Co Antrim based construction firm Farrans, who built the new sewers, said at the time that the building workers were very happy about the Top Gear team racing in their project after tunnelling work on the project to replace Belfast's Victorian-era sewerage system began in 2005.
Jason Barlow from Top Gear magazine said: "Basically what they want to try and do is emulate or recreate that famous scene in the Italian Job, which is just brilliant, where the Minis are escaping from the heist in Turin and they end up doing a loop-the-loop in the sewers."
The original movie scene saw a gang led by Charlie Croker, played by Michael Caine, hatch a plan to steal $4m in gold destined for banks in Turin before making their getaway in three Mini Coopers.
The city's sewerage system makes the ideal route for the little cars, which are chased through the tunnels by the Carabinieri.
This week's more staid event now marks the end of the massive contract and promises a major fillip for the city's infrastructure and water cleanliness.
(BMcC/GK)
