Blog Article:
In an era of rising population, climate instability, and increasing pressure on natural resources, cities around the world are grappling with how to sustain their water supply. Yet one of the most densely populated cities on Earth quietly solved a major piece of this puzzle over 60 years ago — and almost no one outside the region knows about it.
Welcome to Hong Kong, where roughly 80% of toilets flush with seawater.
A Simple But Powerful Idea
Hong Kong pioneered a dual plumbing system that separates freshwater for drinking, cooking, and bathing from seawater used for flushing toilets. The concept was first implemented in the 1950s and has expanded across the city ever since.
How does it work? One set of pipes delivers treated freshwater for everyday use, while a separate network carries treated seawater from pumping stations and reservoirs to homes and buildings strictly for toilet flushing. It’s a system that, while requiring up-front infrastructure investment, saves millions of gallons of freshwater every day.
Why Seawater?
Hong Kong has limited natural freshwater resources. It relies heavily on water imported from mainland China via the Dongjiang River, supplemented by local reservoirs and rainfall. But these sources are not always reliable, especially in drought-prone years.
Seawater, by contrast, is abundant — Hong Kong is surrounded by it. Using it for flushing drastically reduces the city's demand for freshwater, preserving drinking supplies and reducing pressure on reservoirs and imported water contracts.
Does Saltwater Damage Infrastructure?
Using seawater isn’t as simple as just redirecting the ocean into your toilet. Salt is highly corrosive, so the system requires:
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Corrosion-resistant pipes and pumps (often plastic or coated metals)
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Special engineering at treatment facilities
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Regular maintenance to avoid salt buildup and equipment wear
Despite these challenges, Hong Kong has proven that these obstacles can be overcome with the right design and commitment.
What Happens to the Wastewater?
This is where the system gets even more interesting — and environmentally smart.
After being used in toilets, the seawater (now mixed with human waste and possibly other domestic wastewater) flows into the standard sewage system. It then undergoes full treatment at one of several sewage treatment works across the territory.
The largest and most advanced of these, Stonecutters Island Sewage Treatment Works, uses screening, sedimentation, biological processing, and disinfection to remove pollutants and pathogens from the sewage — even though it's salty. While the salinity slightly complicates treatment (some bacteria don’t thrive in saltwater), Hong Kong’s engineers have optimized systems to handle this efficiently.
After treatment, the effluent — now much cleaner but still slightly saline — is discharged into the sea through underwater outfalls. Since the water began as seawater, the discharge does not represent a loss of freshwater and poses minimal ecological disruption, especially with strict monitoring of water quality and pollutant levels.
No Freshwater Lost
Perhaps the most elegant aspect of this system is its closed-loop use of the ocean. Unlike most global cities where precious freshwater is flushed down the drain and into saltwater bodies, Hong Kong avoids that waste entirely. It treats the sea as both a resource and a receptor, allowing for high-efficiency urban water management without sacrificing public health or environmental integrity.
The Bigger Picture: Why Isn’t This Everywhere?
The reasons are mostly practical:
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Retrofitting a city to have a second set of plumbing is extremely costly and disruptive.
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Cities that aren’t coastal can’t use seawater.
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In places with abundant freshwater, there hasn’t been the same urgency to innovate.
But Hong Kong’s example is a reminder that bold infrastructure decisions can pay off for generations. With global climate pressures mounting and urban populations swelling, more cities may be forced to look at similar ideas — especially those already facing water shortages.
Final Thoughts
Hong Kong’s seawater flushing system isn’t flashy. It doesn’t make international headlines. But it quietly represents one of the most successful, large-scale urban water conservation strategies in the world.
By reimagining something as mundane as a toilet flush, Hong Kong has shown how smart infrastructure and long-term planning can create a more sustainable future — one flush at a time.
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