City University of Hong Kong

Hong Kong's Desalination, can we afford it?

5 April 2017

Alicia AN, Assistant Professor, School of Energy and Environment, City University of Hong Kong.

Hong Kong (HK), while blessed with beautiful scenic terrain, has not been as well-endowed regarding natural resources – it has modest land suitable for agriculture, no significant rivers or forests, and only a handful of mineral resources. However, from what was remarked as a “barren rock with hardly a house upon it” by the British Foreign Secretary in 1841 at a time when HK’s population was about 4,000, HK has since then overhauled its landscape to be the home to 7 million people as a global financial hub.

Few may remember that, up to the 1960s, due to water shortage, water rationing was normal in HK. As the city evolved and expanded in rapid speed, HK implemented two measures to secure necessary water supply. First, it created a system for using seawater for toilet flushing and became the first in the world to introduce flushing with seawater without desalination. At present, more than 80% of the population is covered by this seawater flushing system. Second, HK began importing water from Mainland China. 

Recently, HK’s long term goal for water security listed six water resources, namely, Dongjiang water, local yield, seawater for flushing, seawater desalination, reclaimed water, and grey water re-use. These water resources will support the sustainable development by diversifying HK’s water supply. In particular, HK’s desalination project will soon be introducing the first stage of its proposed seawater desalination plant using reverse osmosis at Tseung Kwan O (TKO) Area 137. This plant will have a water production capacity of 135,000 cubic metres (m3) per day with provision for future expansion to produce up to 270,000 m3 per day.

The Tseung Kwan O desalination plant is not the first time HK has tried its hand at seawater desalination. The last of the first generation of desalination plants introduced in Hong Kong was closed in 1981 due to its high operation costs from excessive electricity consumption. These plants, which were demolished, used thermal desalination technology that requires 10 to 50 kwh to produce 1m3 water.  For reference, 1kwh is enough electricity to charge a laptop for 24 hours in addition to fully charging 200 cellphones. Then why has seawater desalination been brought back onto the table for HK?

The desalination technology which will be implemented at the Tseung Kwan O plant is seawater reverse osmosis (SWRO). While thermal desalination requires us to boil up the seawater, consuming a lot of energy in the process, RO is a membrane technology which utilizes the osmotic pressure of seawater to its advantage. The osmotic pressure of seawater is 350 psi (7 kg/cm2) so if as long as enough pressure is applied to overcome this osmotic pressure, it is possible to leave the salt behind and push water particles through an RO membrane, whose nano-sized pores can filter out any pollutants other than pure water.

Because SWRO is based on pressure, not heat, it only requires 3 to 6 kWh/m3 for operation. The re-introduction of a desalination plant in Hong Kong can be said to be a direct example of how the advance in technology can bring greater efficiency in the water and energy nexus – where the production of water requires energy and vice versa. However, is the lower energy consumption enough to make desalination an affordable option for HK?

To answer this question, we need to consider not only financial aspects but also our society’s values and the risks we face. Today, HK is confronted by a great number of challenges: increasing local water demand from population and economic growth; climate change and occurrence of extreme weather, severe drought, etc.; and, more than anything else, competition for water resource due to the rapid economic development in the Pearl Delta Area with other cities. If we see fresh water as a basic commodity, immediate action is called for. The growing concerns about water quality also call us to prepare ourselves for climate change and possible natural disasters based on demand and supply model. Personally, and based on government-issued reports, expanding our current water reservoirs will pose more negative impact, especially regarding ecology, on the environment than that which will be caused by the implementation of the desalination plant.

The cost for the desalination plant estimated by the HK Water Supplies Department is slightly higher than our neighboring countries. But the good news is that many discussions from various angles are being conducted to make the plant more affordable, such as Southeast New Territories Landfill (SENT) gas recovery, utilization of renewable energy, and new technology innovations.

Seawater desalination will be the next biggest contributor to HK’s water resource management after seawater toilet flushing, but what must precede the construction of the plant is to educate HK citizens that fresh water does not come by cheaply. We need to raise greater awareness of the scarcity and value of water. It is hoped that the education centre which will be built at the desalination plant will be a place where not only the advanced technology used in the plant is featured but also teaches the true value of the water we use.

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