Tunnelling works for Singapore’s DTSS Phase 2 project completed

PUB, Singapore’s National Water Agency, has completed tunnelling works for the second phase of the Deep Tunnel Sewerage System (DTSS). A ceremony to mark this important milestone was held on Monday (21 Aug), attended by Minister for Sustainability and the Environment, Grace Fu, and more than 300 guests including pioneer engineers who started the DTSS project over two decades ago.

Phase 2 of the DTSS involves the construction of a 98-km-long network of deep tunnels and link sewers, as well as the future Tuas Water Reclamation Plant (Tuas WRP). These new infrastructure components extend the DTSS network to serve the western part of Singapore, including the downtown area and upcoming developments such as Tengah Town and Jurong Lake District.

A challenging project

The DTSS, described as Singapore’s mega underground superhighway for used water management, is an example of long-term water infrastructure planning and investment to benefit multiple generations. PUB said the project is designed to last for 100 years and costs around S$10 billion.

Due to its scale and complexity, the construction of DTSS had to be delivered in two phases. Phase 1 serves the eastern half of Singapore and was completed in 2008. Phase 2, which is ongoing and covers the western half of the country, is expected to be completed in 2026, with an estimated cost of S$6.5 billion.

A key component of DTSS Phase 2 is the future Tuas WRP, which will be equipped with advanced technology to treat two separate streams of domestic and industrial used water in an energy-efficient manner. Upon completion, this water reclamation plant will be co-located with the National Environment Agency’s (NEA) Integrated Waste Management Facility to form Tuas Nexus, Singapore’s first integrated used water and solid waste treatment facility that will be fully energy self-sufficient.

Spanning multiple tunnelling and construction contracts, the Phase 2 project has created 63-km-long link sewers with diameters of 0.3 to 3 m, and 35-km-long deep tunnels with diameters of 3 to 6 m at depths of between 35 and 55 m.

According to PUB, the project involved more than 5,000 workers at its construction peak. The tunnelling work, which commenced in 2019, was a challenging process through highly-built up areas that required deep expertise and significant amount of preparatory work.

The tunnelling works had to be carried out by five different contractors, using pioneering construction methods and smart technologies for safe and smooth operations. This served to greatly reduce disruption to above-ground infrastructure and the public.

Several new features have also been implemented for DTSS Phase 2 to ensure the integrity of the deep tunnels and ease of maintenance, added PUB. These include the use of microbiological-influenced corrosion (MIC) resistant concrete; isolation gates to allow for flow diversion; fibre optic cables for remote monitoring of a tunnel’s structural integrity; and air jumpers to control air flow within the tunnels.

Furthermore, the project utilised a smart tunnel monitoring system known as shaft and tunnel excavation monitoring system (STEMS). It provides real-time updates of the tunnel boring machine (TBM) locations and instrumentation readings, ensuring safe and smooth operations throughout the construction period with minimal disruption to the structures in the immediate vicinity.

Why is DTSS critical?

Conceived in the 1990s to transform Singapore’s used water management system, the DTSS is designed to meet the country’s long-term needs for used water collection, treatment, reclamation and discharge. It holds the key to enable PUB to reclaim and recycle water in an endless cycle, thus boosting its capacity to produce NEWater, Singapore’s third National Tap and a weather-resilient source of water.

PUB explained that prior to the DTSS, Singapore was divided into six used water catchment zones, each served by a water reclamation plant (WRP) and a total of over 130 pumping stations across the island. These pumping stations were needed to convey used water to the WRPs for treatment before the treated effluent was discharged to the sea. As Singapore’s population and economy grew rapidly, the government sought a more cost- and land-efficient solution for used water management.

Over two phases, the DTSS comprises a 206-km-long network of deep tunnels and link sewers that will convey used water entirely via gravity to three centralised water reclamation plants located in Changi, Kranji and Tuas. The conveyance of used water via gravity instead of energy-intensive pumping stations enhances the robustness and reliability of the used water collection system, said PUB. This also eliminates the risk of pollution in the rainwater catchments.

The development of the entire DTSS will allow PUB to reduce the overall land footprint of the used water system across Singapore by half, freeing up to 150 ha of land (nearly twice the size of the sprawling Singapore Botanic Gardens) for higher-value land use.

“As one of the most water-stressed countries in the world, the ability to effectively collect and recycle our used water in a closed water loop has been a game-changer in our quest for water security. The Deep Tunnel Sewerage System is not only an engineering feat, but a key pillar in strengthening Singapore’s water resilience to meet the long-term challenges of climate change and growing water needs,” said Goh Si Hou, chief executive of PUB.

“This has been made possible through the bold vision and innovation of our pioneers, and decades of meticulous planning and hard work by our planners, engineers and contractors. We look forward to the upcoming completion of our water reclamation and NEWater plants, which will realise the full potential of the DTSS in the years to come.”

Images 4 & 5: PUB, Singapore’s National Water Agency