A city can hide pipes underground and still lose enormous amounts of land to the facilities needed to move and treat what those pipes carry.
Singapore’s older used-water system divided the island into several catchments, each served by its own water reclamation plant and a large number of pumping stations. The system worked, but every intermediate plant and pump needed land, power, maintenance access and protection from failure.
The Deep Tunnel Sewerage System, or DTSS, changes the geometry.
Instead of repeatedly lifting used water through many surface and near-surface stations, a network of link sewers feeds deep tunnels that use gravity to convey flows toward three large centralised water reclamation plants at Changi, Kranji and Tuas.
PUB describes the completed architecture as a 206-kilometre underground superhighway, costing around $10 billion and designed for a 100-year service life.
The public operating chain is: homes and premises discharge used water → local sewers collect it → link sewers connect the catchment → flows descend into deep tunnels → gravity carries them across the island → centralised WRPs treat them → suitable treated water is purified into NEWater → excess treated effluent is discharged safely.
Quick answer: why build one deep system?
- Gravity replaces much intermediate pumping: reducing energy use and the number of pump-dependent handoffs.
- Treatment is centralised: allowing large, compact WRPs to serve broad catchments.
- Land is released: PUB expects the full DTSS to halve the land footprint of the used-water system and free up to 150 hectares.
- Water recycling expands: large central plants provide a strong base for NEWater production.
- Rainwater catchments are better protected: deep gravity conveyance reduces reliance on pumping stations whose failure could create overflow risk.
- The network is built for generations: the system is planned around a century-long service horizon rather than one development cycle.
1. Used water and rainwater travel through separate networks
Singapore’s drains collect rainfall and route much of it toward reservoirs or the sea.
Used water from toilets, sinks, commercial premises and industry travels through the sewerage system toward treatment.
The DTSS belongs to the used-water side. Keeping these networks separate protects urban water catchments and ensures polluted flows receive treatment rather than entering stormwater drains.
2. The older system needed more than 130 pumping stations
Before DTSS, Singapore was divided into six used-water catchment zones served by six water reclamation plants and more than 130 pumping stations.
Pumps were needed because ordinary sewer gradients and local geography could not carry every flow to the treatment plants by gravity alone.
Every pump station solved a local elevation problem while adding another mechanical dependency to the islandwide chain.
3. DTSS uses depth to recover gravity
A sufficiently deep tunnel can maintain a gradual hydraulic fall over a long distance.
Local sewers and link sewers bring used water toward the deep-tunnel system. Once inside, the flow can continue toward the downstream reclamation plant mainly through gravity.
The system trades a difficult one-time tunnelling project for fewer repeated pumping steps over many decades.
4. Link sewers are the on-ramps
The deep tunnel cannot pass beneath every building.
Smaller and shallower sewers collect used water locally. Link sewers then connect those catchment flows into the main deep conveyance tunnel.
The architecture resembles a transport network: neighbourhood streets feed larger roads, and larger roads feed the underground superhighway.
5. Phase 1 connected Kranji to Changi
DTSS Phase 1 was completed in 2008.
PUB states that it comprises a 48-kilometre deep sewer tunnel running from Kranji to Changi, 60 kilometres of link sewers and the centralised Changi Water Reclamation Plant.
The first phase demonstrated that a large part of Singapore could be reorganised around long-distance gravity conveyance and compact central treatment.
6. Changi WRP concentrates major treatment capacity on 32 hectares
Changi Water Reclamation Plant is built on 32 hectares of reclaimed land.
PUB describes an initial treatment capacity of 800,000 cubic metres per day, with an ultimate expandable capacity of 2.4 million cubic metres per day.
Centralisation therefore does not mean one small plant replacing several large ones by magic. It means building a compact facility at a scale that can receive and process a much wider catchment.
7. Phase 2 extends the system across western Singapore
DTSS Phase 2 adds a 98-kilometre conveyance network serving the western half of Singapore, including the downtown area and future growth locations such as Tengah and Jurong Lake District.
The conveyance system consists of 49 kilometres of deep tunnels and 49 kilometres of link sewers feeding the future Tuas Water Reclamation Plant.
Tunnelling was completed in August 2023. PUB’s current March 2026 update states that the full DTSS Phase 2 network and Tuas WRP are scheduled to be commissioned progressively from 2027.
8. Tuas receives domestic and high-strength industrial flows separately
DTSS Phase 2 uses two separate deep conveyance streams toward Tuas WRP.
One carries domestic used water. The other carries high-strength industrial used water.
The separation allows the future plant to match treatment to the different characteristics of the incoming water rather than forcing every stream into one identical process from the beginning.
9. The Tuas plant is designed around two treatment modules
PUB states that the future Tuas WRP will treat domestic used water in a 650,000-cubic-metre-per-day module before further purification into NEWater.
A separate 150,000-cubic-metre-per-day industrial module will treat industrial used water to international water-quality standards before discharge to sea.
The plant therefore receives two hydraulic streams and returns two appropriately treated outcomes.
10. Centralisation frees surface land only after the old network can retire safely
Existing water reclamation plants and pumping stations cannot be removed merely because a new tunnel exists on a plan.
The deep tunnels, link sewers, treatment capacity, control systems and transition connections all have to be commissioned and proven.
Only then can older intermediate facilities be phased out without interrupting used-water service.
11. PUB expects the total land footprint to fall by half
When the entire DTSS is completed and the older intermediate network is progressively retired, PUB expects Singapore’s used-water land footprint to be reduced by about half.
Up to 150 hectares could be freed for other uses.
The tunnel therefore creates land indirectly. It does not add territory; it allows treatment and conveyance functions that once occupied many separate surface sites to be concentrated underground and at fewer central plants.
12. Gravity improves resilience by reducing pump dependence
Pumps require electricity, controls, maintenance and standby capacity.
A gravity sewer still needs monitoring and maintenance, but the water does not depend on a running motor at every intermediate lift point.
The DTSS therefore removes many repeated mechanical handoffs from the conveyance chain and reduces one class of overflow and catchment-pollution risk.
13. Deep tunnels create their own maintenance problem
A central tunnel can be efficient and difficult to access.
Used water creates a corrosive and hazardous environment. Flow cannot simply be stopped across half the island whenever a section needs inspection.
DTSS Phase 2 therefore incorporates additional resilience features learned from Phase 1.
14. Isolation gates make planned maintenance possible
PUB’s Phase 2 design includes tunnel isolation gates.
When maintenance is required, flow can be diverted temporarily through an alternative sewer route so repair work can proceed on an isolated section.
The gate converts one continuous tunnel into maintainable operational segments instead of forcing the entire network to remain one indivisible asset.
15. Corrosion protection is part of the century-long promise
Used-water environments can attack concrete and other materials over time.
Phase 2 includes improved tunnel lining and a secondary corrosion-protection layer to increase resilience.
A 100-year system depends less on the strength of new concrete on opening day than on how well its materials survive decades of chemical exposure and maintenance cycles.
16. Fibre-optic monitoring gives the tunnel a structural return path
PUB’s Phase 2 design uses fibre-optic cables to provide real-time updates on tunnel structural integrity.
The tunnel therefore does not disappear from institutional sight once the ground is reinstated above it.
Monitoring turns an inaccessible underground structure into an asset whose changing condition can still send evidence back to operators.
17. Airflow management controls the atmosphere inside the sewer network
Deep used-water tunnels contain air as well as liquid flow.
Pressure changes and odorous gases have to be managed at shafts and along the conveyance system.
Phase 2 includes purpose-built airflow management for better odour control and overall air quality along the sewer line.
18. DTSS is inseparable from NEWater
Large-scale water reclamation needs a reliable, concentrated supply of treated used water.
DTSS brings flows to central WRPs where treatment and advanced purification can be operated at scale.
The sewer tunnel is therefore not merely a waste-disposal system. It is the collection backbone of Singapore’s water-recycling loop.
19. Tuas Nexus joins used water with solid-waste energy
The future Tuas WRP will be co-located with NEA’s Integrated Waste Management Facility as Tuas Nexus.
PUB states that the integrated facility is designed to harness synergies among used-water treatment, solid-waste treatment, energy generation and resource recovery and to be fully energy self-sufficient.
Centralisation therefore creates another possibility: processes that would be too scattered to exchange resources efficiently can be planned as one industrial ecology.
20. A worked example: used water from a western new town
Imagine used water leaving homes in a growing western estate.
Local sewers collect the flow. Link sewers carry it toward the Phase 2 deep-tunnel network. Gravity moves it across the catchment toward Tuas WRP. Domestic flow enters the domestic treatment module and can later be purified into NEWater. Sludge and energy flows are managed through the treatment and Tuas Nexus systems.
The estate needs no large local water reclamation plant of its own because the tunnel connects it to a central one.
21. Common misconceptions
Misconception: DTSS is one empty pipe running directly from every home to Tuas.
No. local sewers and link sewers collect and feed used water into the deep-tunnel network.
Misconception: Gravity means the system needs no electricity.
No. treatment plants, local systems, controls and other infrastructure still use substantial energy; gravity principally reduces intermediate conveyance pumping.
Misconception: Phase 2 was fully operational when tunnelling ended in 2023.
No. lining, link-sewer, mechanical, electrical and plant works continued; PUB schedules phased commissioning from 2027.
Misconception: Centralisation removes all maintenance risk.
No. it changes the risk, making tunnel isolation, corrosion protection, airflow and structural monitoring especially important.
Misconception: The system exists only to dispose of sewage.
No. it is a core collection platform for treatment, reclamation and NEWater production.
22. The deeper idea: DTSS uses one deep line to retire many surface interruptions
The older network moved used water by repeating a local solution: collect, pump, move, pump again, treat in several places.
DTSS spends more engineering effort upfront to create a long continuous gravity path.
That path allows intermediate pumping stations and conventional plants to be phased out, consolidates treatment, supports water recycling and returns surface land to the city.
The tunnel is deep because Singapore is short of land. It is long because the land saving becomes possible only when many separate catchments can trust the same underground route.