Hong Kong's freshwater mainly comes from rainwater collected from local catchments and water imported from Guangdong, which account for about 20-30% and 70%-80% of Hong Kong's total freshwater supply respectively. However, the amount of water these two water sources can provide is affected by extreme weather, and water demand can also increase due to population and economic growth. Therefore, the Water Supplies Department (WSD) has been exploring and developing new water sources that are not susceptible to climate change, one of which is seawater desalination. This time, I have invited colleagues from the WSD to introduce the recently commissioned Tseung Kwan O Desalination Plant (hereinafter referred to as the TKODP) and share with us its reverse osmosis desalination technology.

Desalination to diversify water resources despite higher cost

Mr LAI Hon-lam, Engineer (Consultants Management Division) of the WSD, states that the department is keeping abreast of the latest developments in desalination technology. Feasibility studies conducted in 2002 and 2007 have confirmed that potable water produced by seawater desalination using reverse osmosis is in compliance with the Hong Kong Drinking Water Standards.

The TKODP can develop water resources that are not affected by climate change and diversify our water resources, ensuring the stability of Hong Kong's water supply and building resilience in Hong Kong’s water supply. However, due to the higher energy consumption required by the use of reverse osmosis technology to produce drinking water, the cost of seawater desalination for water supply will be higher than that of locally collected rainwater and Dongjiang water supply. For this reason, the two main water resources, namely, local collection and Dongjiang water, will not be replaced.

The first stage of TKODP can meet 5% of fresh water demand

The first stage of the TKODP has started to supply potable water since 22 December 2023. The water production capacity is about 135 000 cubic metres (m3) per day, with a provision for future expansion up to 270 000m3 per day that will meet around 5% to 10% of the overall fresh water demand in Hong Kong.

Located in Tseung Kwan O Area 137, the desalination plant is close to the seaside. The nearby seawater has low turbidity with relatively stable water quality, making it suitable for desalination.

Currently, water desalination has been adopted by more than 130 countries and regions around the world, including France, Germany and Spain. The reverse osmosis desalination technology, which is used by the TKODP, has been well-proven and accounts for more than 60% of global desalination production.

Four treatment processes to turn seawater into potable water, which is then fed into service reservoir to meet public demand.

Seawater entering the TKODP will undergo four main stages of treatment: screening, pre-treatment, reverse osmosis, and post-treatment. Extracted seawater will first be screened and pre-treated to prevent marine organisms from entering the facility and removing impurities and suspended solids. After that, reverse osmosis technology will be used to remove salt, impurities and micro-organisms from the pre-treated seawater.

Osmosis is a natural phenomenon where fresh water migrates from low salinity to high salinity. The principle of reverse osmosis is to apply excessive pressure on the seawater side to force water from the seawater through a semi-permeable membrane to the fresh water side so as to remove salt, impurities and micro-organisms to purify seawater.

After the reverse osmosis process, permeate will go through a post-treatment process where chemicals are added to adjust water hardness and pH levels, etc. to meet the Hong Kong Drinking Water Standards.

After all treatment processes, seawater becomes potable water, which will then be blended with treated water from the Pak Kong Water Treatment Works at the Tseung Kwan O Primary Fresh Water Service Reservoir, providing potable water for Sai Kung, East Kowloon and part of Hong Kong Island.

The technology uses a semi-permeable membrane to separate water and salt in pressure vessels

The first stage of the TKODP has 12 reverse osmosis racks with a total of 2 336 reverse osmosis pressure vessels installed in the racks. Each vessel contains seven spiral reverse osmosis membrane elements, and this is where the reverse osmosis process takes place.

To ensure drinking water safety in Hong Kong, the WSD has implemented a series of measures to monitor and strictly control the overall water quality produced by the TKODP.