Singapore Deep Tunnel Sewerage System

The Deep Tunnel Sewerage System (DTSS) was conceived as a long term solution to meet the needs for used water collection, treatment and disposal to serve the development of Singapore through the 21st Century.

In 2000, PUB started building the DTSS. Phase One of the DTSS comprises a 48 km north tunnel stretching from Kranji to Changi, a water reclamation plant with a capacity of 176 million gallons (800,000 cubic metres) per day, a 5 km sea outfall at Changi, and some 60 km of link sewers.
The DTSS main tunnels were primarily excavated by eight Tunnel Boring Machines (TBMs) with excavated diameters up to 7.2m. There were 23 shafts. The TBMs were all pressurised-face Earth Pressure Balance (EPB), including 4 TBMs as hybrid hard rock EPB TBMs excavating depths up to 55 m. Excavations were made in a wide range of ground conditions ranging from reclamation fills, soft clay, loose to cemented silty sands, decomposed rock weathered rock, fresh granite and sedimentary series. The groundwater table is very shallow due to very high tropical precipitation.

The DTSS will be constructed in two phases. The first phase of the project, is being implemented now. It is scheduled to be fully operational by end 2008. The second phase will consist of a deep tunnel to Tuas, a water reclamation plant there with a sea outfall into the Straits of Singapore, link sewers and an extension to the Changi Water Reclamation Plant. This phase will be implemented after the first phase when there is a requirement to handle more waste water arising from continuing population and commercial growth.

- Underground plants (sewage treatment)
- Sewerage systems

Project construction


- Pollution (esp. air pollution) is not acceptable
- Valuable surface space must be kept available or become available again
- Urban planning for combination of functions
Singapore is the most densely populated area in Asia. Land is scare and extremely valuable.
The existing several wastewater and 100+ pumping stations have buffer land zones for primarily odour control. Since both the wastewater treatment plants and pumping stations will be phased out with time and replaced by an enclosed deep underground conveyance system along with locating the new Changi Water Reclamation Plant at the far end of the island away from residences, odour will be eliminated as a nuisance.

The DTSS will free up prime land currently used to site the existing several wastewater treatment plants and many tens of pumping stations, as well as the buffer land surrounding the wastewater treatment plants. The compact design of the new Changi Water Reclamation Plant occupies about one-third of the land area required for a conventional plant. Ultimately about 290 hectares of land will be returned to the government and available for commercial/residential development.

At the same time, the centralisation of the used water treatment at Changi Water Reclamation Plant will be more cost effective than the existing plant. The area around this new plant will be developed commercially. It is also possible that the new plant, in the future, may accommodate overbuild commercial structures.

The DTSS main tunnels, as well as the link sewers are located primarily under existing expressways to accomplish multiple land usage.

More than half of Singapore's used water will eventually be directed through the North Tunnel section to the Changi Water Reclamation Plant for treatment. Part of the treated effluent will then be discharged through the deep sea outfall into Straits of Singapore. The rest of the effluent will be channeled to NEWater factories for recycling.

- Availability /Feasibility of certain key construction technologies
- Environmental impacts
- Advantages during construction phase (i.e. less obstruction) compared to other solutions
The project is unique not only due to its scale, complexity and timeline but also for the economic and environmental benefits it will allow the people of Singapore to realize.

- Increased capacity will be available for future long-term growth, the existing system was at capacity and expensive to expand and maintain, this was restricting growth for development.

- Undergrounding the system will release approximately 290 hectares of valuable land for more beneficial development.

- The new system will be easier and more economical to operate and maintain.

- Improved water quality in the Straits of Johor, and the Straits of Singapore increasing the attractiveness of waterfront development and water-based activities in northern coastal areas.

- Enhanced reliability and reduced risk of sewerage overflows, particularly in unprotected water catchment areas. Being on the equator, Singapore is subject to very high intensity rainfall all year round.

- Treated effluent will be reclaimed and reused or be discharged through deep-sea outfalls into the Straits of Singapore thereby eliminating the current discharge into the shallow Straits of Johor.

- The tunnel systems will work entirely by gravity, eliminating the need for the existing pumping stations. This provides benefits in reduced operations and maintenance costs and a reduction in electricity consumption.

- Replaced the need for 130 pumping stations – long term energy savings.