The current water supply system in Hong Kong comprises three primary sources, including Dongjiang water imported from Guangdong, rainwater collected from local catchment areas, and seawater used for flushing purposes. Upon commissioning of the first stage of the Tseung Kwan O desalination plant which introduced desalinated seawater into Hong Kong’s water supply system at the end of last year, the Hong Kong's first water reclamation plant in Shek Wu Hui, North District, also started its operation on March 20 this year, thereby bringing in reclaimed water as another new water resource to the water supply system. A colleague from the Water Supplies Department (WSD) and "Water Save Dave" have been invited to introduce more information about reclaimed water and the newly constructed Shek Wu Hui water reclamation plant (SWHWRP).

Reclaimed water development: new opportunities to further conserve water resource

Mr Clayton LEI, Project Coordinator (Project Management) of the WSD, states that the department plans to supply reclaimed water in phases to certain areas of Sheung Shui and Fanling, as well as to the Kwu Tung North and Fanling North New Development Areas (NDAs). Reclaimed water refers to treated sewage effluent from sewage treatment works with further processing for non-potable uses. Similar to desalinated water, the production of reclaimed water is not affected by extreme climate change, and can diversify the overall portfolio of water resources.

Reclaimed water can be used for a variety of purposes, such as toilet flushing, landscape irrigation, and street cleansing. Seawater has been widely used for flushing in Hong Kong since the 1950s. At present, the WSD’s seawater supply network covers about 85% of the population in Hong Kong. However, as some of the inland areas (e.g. the North District) are distant from the seafront, it is energy intensive and less cost-effective for constructing many new infrastructure including seawater pipelines and pumping facilities in order to supply and transport seawater for flushing in these areas. As a result, residents in these areas are still using temporary mains fresh water for flushing. With Government subsidies, the first 30 cubic metres of fresh water for flushing purpose per domestic consumer is provided free of charge, calculated at 4-monthly intervals.

Sewage treatment level upgrade gives rise to the setting up of a water reclamation plant

In response to the population growth and the subsequent increase in sewage generated in Sheung Shui, Fanling, and the nearby NDAs, the Drainage Services Department commenced the in-situ reconstruction of the Shek Wu Hui Sewage Treatment Works (SWHSTW) in 2019. This project aims to increase the sewage treatment capacity and upgrade the sewage treatment level from secondary to tertiary standard in phases.

Leveraging the opportunity arising from the reconstruction of the SWHSTW, a water reclamation plant in Shek Wu Hui was constructed to further process tertiary-treated effluent by adding sodium hypochlorite, a chemical used to control bacterial levels, before producing reclaimed water. The WSD also adds a suitable amount of residual chlorine into the reclaimed water to maintain water quality during conveyance. Additionally, since reclaimed water is colourless and odourless, blue edible dye is added to the reclaimed water to allow consumers to distinguish it from fresh water, preventing misconnection of pipes or misuse.

Having entered into operation in March this year, the SWHWRP supplies reclaimed water to three schools and four housing estates in Sheung Shui at present. The WSD anticipates that upon full population intake in Kwu Tung North and Fanling North NDAs, the SWHWRP will supply water to approximately 520,000 individuals with a reclaimed water production capacity of up to 73,000 cubic metres per day upon its full operation. This initiative is estimated to save around 22 million cubic metres (mcm) of drinking water annually in Hong Kong, which is equivalent to the capacity of 8,800 standard swimming pools. Currently, the production cost of fresh water is around HK$10 per cubic metre, while the cost of reclaimed water is only HK$7.8 per cubic metre. The estimate of 22 mcm reclaimed water to replace fresh water for toilet flushing will equate to a saving of over HK$48 million each year.

The WSD will continue to review the development of reclaimed water supply. Wherever it is technically feasible and cost-effective, the department will extend reclaimed water supply to other new NDAs and districts that are still using fresh water for toilet flushing, including Hung Shui Kiu/ Ha Tsuen NDAs and the Yuen Long South development area, in order to further reduce the fresh water consumption. The Government is currently upgrading the existing Yuen Long Sewage Treatment Works into an effluent polishing plant with sewage treatment level at tertiary effluent polishing standard with a view to converting part of the treated sewage effluent to reclaimed water for non-potable use in the vicinity areas. The WSD also targets to expand the network coverage of using recycled water and seawater for toilet flushing from 85% to 90% of the total population in the long run.

Digital water development to achieve full automation of operations in water reclamation plants

The WSD is committed to promoting digital water and driving the digitalisation of water supply services, aiming to provide the public with quality, efficient, stable and secure water supply services. By adopting real-time monitoring instruments such as water quality analysers to comprehensively monitor the reclaimed water treatment process and collect data on water quality, the SWHWRP aligns with the digital water development and allows timely adjustments of sodium hypochlorite and blue edible dye levels in reclaimed water to ensure that the quality of reclaimed water complies with water quality standards.

The SWHWRP also applies digital twin technology to simulate, analyse, and learn from the production process of reclaimed water and the operation of pumping facilities using computer models. This technology assists in optimising levels of sodium hypochlorite and blue edible dye in the reclaimed water, along with suggesting the optimal periods to activate the pumping facilities, thereby enhancing operational efficiency towards full automation of operations in waterworks installations and reducing operational costs. Furthermore, the WSD has installed solar panels on the rooftop of the pumping station at the SWHWRP to generate renewable energy for use within the facility, and hence further facilitate energy saving and carbon emission reduction.

The grey water recycling system at the Anderson Road enhances the resilience of Hong Kong's water supply

Additionally, the WSD is actively developing grey water recycling system as an alternative source of recycled water. Grey water refers to the used water discharged from baths, showers, wash basins, kitchen sinks, and washing machines that can also be treated and reused for non-potable uses like toilet flushing. A grey water recycling system is currently under construction by the WSD at the Anderson Road Quarry development site to treat residents' grey water for toilet flushing purposes; it is expected to begin water supply in phases by the end of this year.

The WSD will continue to study and explore water resources which are not susceptible to the impacts of climate change, with a view to further strengthening the resilience of Hong Kong’s water supply, fostering sustainable development in the territory.