August 2009, Vol. 21, No.8
In Singapore, Used Water is NEW
Singapore’s new Deep Tunnel Sewerage System conveys wastewater from homes and businesses through a 48-km-long (30-mi-long) tunnel sewer that runs 20 to 55 m (65 to 180 ft) below ground to a single, centralized plant for treatment.
Named the 2009 Water Project of the Year at the Global Water Awards held in April in Zurich, Switzerland, the $3.65 billion project is groundbreaking on many levels, according to Glen Daigger, senior vice president and chief technology officer at CH2M Hill (Englewood, Colo.). CH2M Hill has been working with PUB, Singapore’s national water agency, on the project’s planning, design, and construction since 1997.
“Most of the world’s biggest wastewater systems have been developed on a piecemeal basis,” Daigger explained. “They began with primary treatment. Then as technology evolved, they added secondary treatment and, later, advanced treatment.”
“The advantage in Singapore is that we got to start completely from scratch,” Daigger said. “That gave PUB the opportunity to gaze 100 years into the future and rethink everything from where and how ‘used water’ is treated to the visual aesthetics of the plant.” (PUB prefers the term “used water” to “wastewater,” Daigger said. “From their perspective, water is water,” he explained. “What’s critical is the quality of the product, not the source.”)
Completed in 2008, the Changi Water Reclamation Plant lies largely underground on a compact 55-ha (136-ac) site at the eastern edge of Singapore. Currently capable of treating about 800,000 m3/d (211 mgd), the new plant has the potential to triple in capacity through future expansion.
Wastewater treated at the Changi plant is discharged into the deep sea at the Straits of Singapore. Within a year or so, much of it will instead be routed to the 227,000-m3/d (60-mgd) Changi NEWater factory being constructed on the rooftop of the treatment plant. There, it will be purified further through microfiltration, reverse osmosis, and ultraviolet disinfection before being remarketed as NEWater, Singapore’s own brand of high-quality reclaimed water.
The 100-Year Plan
The most forward-looking water supply plans rarely look more than 50 years into the future. Singapore’s decision to look ahead an entire century had profound implications for its decision-making process, said Wah Yuen Long, PUB’s director of Water Reclamation. Among the most significant differences between now and then, he said, will be the small island nation’s population. Slightly larger in area than Chicago, the island of Singapore has 4.8 million residents within its 640 km2 (247 mi2). The population is projected to grow to about 7 million by the end of the century, according to Wah.
Meeting the water needs of this many people could pose a considerable challenge, Daigger explained, especially considering the nation’s limited water resources.
In the early 1990s, when PUB began its 100-year planning process, Singapore had two primary sources of water. Rainfall accounted for about half of the country’s water supply, with the remainder imported primarily from Malaysia.
Both sources pose long-term challenges, Daigger said. “Given Singapore’s small size, there is a limit to the amount of rainwater it can collect,” he explained.
“PUB would like to be able to purchase water from Malaysia as long as it’s available at a reasonable cost,” Daigger added. “But Malaysia is also growing, so it was important to PUB to identify other sustainable sources to meet the long-term needs of Singapore’s population.”
One of those new sources is seawater, which PUB is converts into potable water at a 114,000-m3/d (30-mgd) desalinization plant completed in 2005.
The other is “ultraclean” reclaimed water being produced at four NEWater factories that have been constructed around the country, similar to the one now being built atop the Changi Water Reclamation Plant. “Eventually, more than half of the wastewater in Singapore will be treated at Changi Water Reclamation Plant,” Wah said. When it is completed in 2010, the five NEWater plants together will provide for 30% of Singapore’s water needs, he said.
The Benefits of Consolidating Treatment
Given Singapore’s growing population and expanding water needs, it might seem the country needs more, not fewer, wastewater treatment plants. But looming population growth was one of the primary factors that fueled PUB’s decision to consolidate the country’s wastewater treatment at the Changi plant.
For most of the past 40 years, Singapore has operated six secondary treatment plants, Daigger said. Most were located inland and discharged primarily to the island’s shallow coastal waters.
“Designed with 1960s and 1970s technology, the plants were spread out around the island, were visually unappealing, and didn’t offer much in the way of odor control,” Daigger said. “They also took up valuable space in a country where land is at a premium.”
What’s more, Singapore’s wastewater collection system included more than 130 small wastewater pump stations, which added considerably to the system’s operation and maintenance expenses.
The decision to build the deep-tunnel system and consolidate treatment at the Changi site addressed all of these issues and more.
The new deep-tunnel system works entirely by gravity, eliminating the need for pumping stations and the risks of wastewater overflows. The old system’s many pump stations, in fact, are in the process of being phased out, with the land freed up for other development.
The new tunnel is also designed to require very little maintenance, according to Wah. “The tunnel has a corrosion protection layer, a sacrificial concrete layer, and thick precast segments that will protect it from wear and tear throughout its entire useful life,” he said.
Occupying less than one-third of the land needed for a conventional layout, the Changi plant looks more like a modern office complex than a wastewater plant, with most of its liquid treatment process located underground. Compact process tank configurations, including two-level stacked primary and secondary clarifiers, help make that possible.
The NEWater plant’s construction on the roof of the liquid processing plant reduces the required footprint for the combined plants and minimizes the pipelines needed to convey treated effluent.
The Changi plant also incorporates many energy- and labor-saving features in its solids processing design. Anaerobic digestion, for example, is used to help reduce solids volume. Methane gas produced in the digestion process is used to power thermal dryers that dry the digested solids, which are converted to pelletized fertilizer.
“At Changi, any resource that can be recovered, is recovered,” Daigger said.
From an operations and maintenance perspective, attention was focused on simplifying the mechanical and electrical equipment as much as possible, Wah said. “We also sought opportunities to automate the operation to minimize the number of people working,” he said.
Unlike Singapore’s older treatment plants, the Changi plant is designed for modern odor control, Daigger added. “Residents are much more sensitive to environmental and land use issues today than they were in the 1960s and 1970s,” he said. “This facility takes all that into account.”
Flow to the new plant is being transferred in phases, as existing water reclamation plants are decommissioned. Two plants already have been decommissioned, one each in 2008 and 2009. A third plant is scheduled to be decommissioned in 2011, according to Wah.
As these old plants are phased out, the land on which they are built will become available for new development, an added benefit of consolidation.
"To a certain extent, the decisions regarding timing are driven by the real estate market,” Daigger explained. “Given the current economy, now is not a great time for development, so the urgency to make the transition isn’t as great as it might have been a year or two ago.”
Not Finished Yet
When the Changi NEWater plant is completed next year, it will not mean the end of PUB’s construction program. The 100-year plan, according to Daigger, includes construction of a second deep-tunnel system and water reclamation plant in the coming decades to serve the western portion of Singapore, with the Changi plant serving the eastern half.
Meanwhile, CH2M Hill is working with PUB as it continues to plan how and when the old facilities will be phased out and new capacity added to the Changi plant.
PUB also continues to build public confidence in and acceptance of NEWater, which Wah said is treated to potable standards.
“Only a small amount of NEWater is currently blended with raw water, further treated and supplied to consumers as tap water,” Wah said. Most is used for nonpotable manufacturing processes and air-conditioning cooling towers in Singapore’s commercial buildings, he said, freeing up the available potable water for other purposes.
But as demand for water grows, the uses for this high-grade reclaimed water also are expected to grow. Top government officials have shown their support by drinking NEWater publicly, and endorsement from water experts have helped lend it credibility, Wah said. More than 16 million bottles of bottled NEWater have been distributed to date.
“Singapore is one of the few places in the world that has an overall integrated water management plan for the entire country,” Daigger said. “The Deep Tunnel Sewerage System and Changi Plant, along with PUB’s forward-looking approach to wastewater management, are all major steps toward ensuring the long-term sustainability of Singapore’s water resources.”
“This approach obviously requires a tremendous up-front investment,” Daigger said. “In the end, however, it pays.”
— Mary Bufe, WE&T
©2009 Water Environment Federation. All rights reserved.