18 June 2009

Where does our sewage go?

The Deep Tunnel Sewerage System phase 1 will be launched next week. The local media articles focus on the less squirmy aspects such as the Changi Water Reclamation Plant. But where does the sewage actually end up? As you might have guessed ... into the sea.

What is the Deep Deep Tunnel Sewerage System (DTSS)?
(from the PUB website)

Designed to save on land while catering for a rising population (which as of 2008 stands at 4.84million), 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 .

Link sewers intercept flows from existing sewers, pumping stations, and water reclamation plants. Intercepted flows are conveyed by gravity to the deep tunnel sewers.

Deep tunnel sewers convey flows by gravity to the two centralised water reclamation plants at the two ends of Singapore.

Water reclamation plants provide a high standard of treatment prior to discharging the treated effluent via the outfalls.

Outfalls convey treated effluent for deep-sea discharge via diffusers.

What happens to the sewage?
(from recent media articles on the wildsingapore news blog)

The treated wastewater is channelled to Changi Newater Factory on the rooftop of the reclamation plant. Here it is further purified through advanced membrane technologies. The processed water can be consumed by humans and is used in industry where high purity water is required. The Changi Water Reclamation Plan can treat 176 million gallons of water per day, about 320 Olympic-sized swimming pools.

Another unique feature of the reclamation plant is a Sludge Dryer, which reduces the volume of sludge to be disposed. The sludge will then be incinerated and used as landfill.

What happens to the rest of the stuff?
(from the PUB website)
Treated effluent from the Changi Water Reclamation Plant will be conveyed through the Changi Outfall and discharged through a series of diffuser heads to facilitate dilution and dispersion of the effluent in the receiving sea water.

Under the first phase of the DTSS, the Changi Outfalls will consist of 2 pipes about 5 kilometres long each. A shorter third pipe will be laid and terminated just offshore for future extension.

The land portion of the outfall pipes will consist of 3 concrete/steel pipes of 3 metres diameter each. The approximately 300 metres long land pipes will be installed in a trench and backfilled.

The sea portion of the pipes, which is approximately 4,900 metres long, will be laid in a dredged trench, upon a rock mattress and covered with graded rock. Above that, large size rock armours will be placed to protect the pipes from anchor damage as well as securing the pipelines in position against currents, storm surges etc.

The last section of the pipes will be a diffuser zone with a series of diffusers extending from the pipes to just above the sea bed. The diffusers are provided with concrete diffuser heads to protect them from ship anchors. They are at a depth of about 30m below the sea level.
from a 2008 presentation on the PUB website.

from a 2008 presentation on the PUB website.

Aerial view of the Changi Water Reclamation Plant taken in 2001 from the PUB website.

I can't seem to find an address for the Changi Water Reclamation Plant. But from Google Earth, this seems to be the Plant.And here is a wider view of the location.


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