Hong Kong’s Dual Water System: How Seawater Is Used for Toilet Flushing

The 2024 wildfire in Los Angeles made many wonder: why can’t we use seawater to extinguish such fires?

There are several reasons. Seawater is corrosive to firefighting equipment, can damage vegetation and soil, and is not always accessible inland. These challenges indicate a growing concern: how can we preserve freshwater for emergencies like wildfires?

While wondering about alternative solutions, we discovered an intriguing approach in Hong Kong, where over 80 percent of the population is already served by a dual water system. But how does it work?

A City of Innovation and Constraints

In a city where space is limited and land prices are among the highest in the world, Hong Kong has had to innovate not only in housing but also in water management. One of its most remarkable achievements is a dual water supply system that separates freshwater from seawater, the latter used specifically for toilet flushing. Though rare globally, this system could be a creative and practical conservation strategy.

How It Works: A Two-System Approach

Hong Kong is one of the few places in the world with a dual plumbing infrastructure in most buildings:

* Freshwater system: Supplies kitchens, bathrooms (for bathing and handwashing), and laundry

* Saltwater system: Supplies seawater solely for toilet flushing

By separating these functions, the city reduces its demand for potable water, a limited and costly resource, by 20 to 30 percent.

Why Use Seawater?

Hong Kong lacks major rivers or lakes and relies heavily on imported freshwater from mainland China, particularly the Dongjiang (East River). To reduce this dependency and improve water security, the city began using seawater for flushing toilets as early as the 1950s, expanding it significantly by the 1970s.

Given Hong Kong’s coastal location, seawater is both plentiful and accessible, making the solution cost-effective and practical.

Infrastructure and Maintenance

To implement this system, Hong Kong built an extensive network of dedicated seawater pipelines and pumping stations. Each building typically has two water tanks and two separate plumbing systems: one for potable water and the other for flushing.

Seawater undergoes basic treatment before distribution to prevent pipe corrosion and eliminate harmful bacteria or marine organisms. Although it doesn’t need to be drinkable, it must meet sanitation and safety standards. The system is managed and maintained by the Water Supplies Department (WSD).

Environmental and Economic Benefits

* Water conservation: Saves approximately 270 million cubic meters of freshwater each year

* Cost savings: Flushing water is free or low-cost, encouraging its use

* Relief on freshwater supplies: Reduces pressure on local reservoirs and imported sources

Challenges and Limitations

Despite its success, the dual water system presents challenges:

* High construction costs: Doubling the plumbing infrastructure increases installation expenses

* Retrofitting older buildings: Difficult and costly to implement in existing structures

* Maintenance complexity: Operating two separate systems requires additional oversight, materials, and labor

Could It Be a Global Model?

While Hong Kong’s approach is unique and largely successful, it may not be easily replicated everywhere. Coastal access, existing infrastructure, and climate conditions are major limiting factors. There are also challenges such as potential impacts on marine ecosystems and the high cost of infrastructure development.

Still, in the face of growing climate stress and water scarcity, Hong Kong’s system presents an innovative example worth exploring, especially if we can develop solutions that support both people and the environment.

Sources:

https://www.wsd.gov.hk/.../seawater-for-flushing/index.html

https://water.hku.hk/.../HKUWaterCentre_Water...

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