Desalination Plant at Tseung Kwan O

By the Water Supplies Department

The Tseung Kwan O Desalination Plant (TKODP) is one of the strategic infrastructures by which the Water Supplies Department (WSD), with a view to addressing the extreme effects of climate change, fosters climate resilience by providing a reliable supply of potable fresh water. Upon commissioning, the first stage of TKODP will have a potable water production capacity of 135 million litres per day (“Mld” in abbreviation), equivalent to about 5% of daily fresh water consumption in Hong Kong. The plant can be expanded for its second stage, with which it will be capable of providing an ultimate capacity of 270 Mld (that is, about 10% of daily fresh water consumption). TKODP is the first of its kind in Hong Kong to adopt the reverse osmosis (RO) technology to produce potable water for municipal use by desalinating seawater.

Figure 1: First Stage of TKODP

Currently, Hong Kong possesses two sources for fresh water supply: imported water from Dongjiang River in Guangdong and rainwater from local catchments. However, WSD is facing various challenges in the form of water supply stability and demand. These challenges have arisen out of population and economic growth, erratic rainfall patterns and severe droughts resulting from climate change, and the keen competitions for water resources due to the Pearl Delta Area’s rapid economic development.

To better prepare Hong Kong for these water supply challenges, WSD has been implementing the Total Water Management Strategy (the Strategy) since 2008. By emphasising the containment of water demand growth through the promotion of water conservation and the exploitation of new water resources, the Strategy aims to meet the forecasted fresh water demand up to 2040 and to bring with it an enhanced resilience. One of the key initiatives is the adoption of seawater desalination to provide a reliable supply of wholesome potable water not susceptible to climate change.

In December 2019, WSD commenced the design-buildoperate (DBO) for the first stage of TKODP at a contract sum of HK$9,018 million. The Plant was commissioned on 22 December 2023. With the adoption of RO technology, the desalination plant can remove the dissolved salts and impurities in seawater to produce potable water meeting the Hong Kong Drinking Water Standards (HKDWS). The Plant will supply potable water to Tseung Kwan O Fresh Water Primary Service Reservoir and be integrated into the existing fresh water supply system. Since water demand may vary, the Plant’s design is designed such that it may operate with a flexible capacity—from 25% to 100%.

Figure 2: Location of the first stage of TKODP

Brief overview of desalination over the world

It may appear, on the face of it, that water resources are abundant because water covers three-quarters of the earth’s surface. In reality, however, over 97% of the earth’s water appears as salt water in the oceans and around 2% is stored as fresh water in glaciers.¹ With over eight billion people in the world relying on the remaining 1% of fresh water that is readily accessible for human consumption, development of new water sources is of paramount importance. One method is to turn seawater into fresh water. Nowadays, over 150 countries adopt seawater desalination as part of their water supply.² While some of the countries are located in water-scarce regions, some less water-stressed countries such as Japan, Singapore and Spain also rely on this prominent desalination method to meet the ever-growing demand for water.

In Hong Kong’s case, WSD has confirmed, after conducting various feasibility studies, that seawater desalination with RO technology is feasible in terms of technical applicability, seawater quality, land availability and cost effectiveness. TKODP will be the first RO desalination plant producing drinking water in Hong Kong for municipal use.

Advanced seawater processing technology is utilised in TKODP for fresh water treatment. Thus tech-enabled, TKODP diversifies Hong Kong’s fresh water sources by serving as the new link connecting the sea with the city’s fresh water supply system.

Advanced seawater processing in TKODP

WSD adopts an advanced seawater processing system at TKODP, setting a precedent in Hong Kong for innovative desalination. Seawater undergoes the following steps to become potable water:

Figure 3: Treatment processes in TKODP

ActiDAFF

Pre-treatment of seawater at TKODP occurs in an integrated structure known as “ActiDAFF®”, which is a proprietary design developed by one of the joint venture members of the main contractor of the project from Spain.

ActiDAFF is a combination of dissolved air flotation (DAF) technology and filtration process. The DAF unit is particularly effective at removing low-density particles such as suspended particles, algae and natural organic matter that cannot be eliminated by the screens at the intake pipe and conventional sedimentation. This minimises the damage caused by potential foulants, such as particulates and biological materials, to the RO membranes during the desalination process. Ferric chloride is administered to agglomerate the suspended solids into larger flocs. The flocculated water flows through a distributor where it comes into contact with injected stream of air bubbles. The flocs are captured by the air bubbles and carried to the surface, where they are gathered by skimmer and conveyed to the sludge tank. With the large flocs removed, the seawater then descends to the filter media bed, where finer particles are removed. After that, the pre-treated seawater flows into the filtered water tank and is ready to undergo the RO process.

Such combined arrangement reduces the footprint of the Plant, thus sparing the land usage for other beneficial purposes. When compared to the traditional layout, where one structure serves only one purpose, this combined arrangement does a better job not only at reducing the energy consumption due to plant operation and hydraulic loss, but also at saving operational recurring cost and construction cost. Moreover, flexible operation is introduced where the air flotation process can be skipped, provided that the incoming seawater contains a below-threshold concentration of suspended particles. This further reduces the energy consumption.

Figure 4: Pre-treatment at ActiDAFF in TKODP

In TKODP, Reverse Osmosis (RO) technology is adopted for the first time in Hong Kong to remove 99.5% of salt and other impurities— and with an energy demand that is lower than the traditional thermal desalination technology.

Reverse Osmosis

TKODP adopts a “double pass” RO as its desalination approach, which is a well-proven and widely used technology for efficient desalination over the globe. RO is the process of forcing water from a concentrated solution to a less concentrated solution through a semi-permeable membrane under high pressure. This is in contradistinction to natural osmosis, in which fluid diffuses from a less concentrated solution to a more highly concentrated solution.

Figure 5: Mechanisms of natural osmosis (L) and reverse osmosis (R)

In TKODP, the RO process takes place in a series of white fibreglass pressure vessels (RO PV), each containing seven spiral RO membrane elements joined together. Each element is comprised of a semi-permeable membrane sheet of approximately 0.001 mm thick and 40 m long, rolled around the desalinated water collection tube at the centre to form a spiral membrane of 200 mm diameter. In TKODP, there are over 2,300 pressure vessels, containing over 16,100 membranes. In the 1st Pass RO, seawater is pumped through these membranes at 60 atmospheric pressure so that over 99.5% of salt, bacteria, viruses and other impurities are removed and the clean water (also known as “permeate”) enters the collection tube. Permeate from the front of the RO PVs is sent to Post-Treatment in normal operation, while the permeate from the rear is sent to the 2nd Pass RO for further RO treatment.

Figure 6: RO racks and RO pressure vessels in TKODP

On the other hand, the seawater concentrate, also known as brine, resulted from the RO process is dispersed back into the sea through a submarine outfall diffuser pipeline after recovering its residual energy. The submarine outfall diffuser pipeline is situated in an area of significant ocean flow so that the concentration of salt can quickly return to equilibrium in the ocean by mixing with seawater. An environmental impact assessment has been carried out prior to construction, confirming that there would be no noticeable increase in salinity at a distance of a few metres from the discharge point, hence protecting the local marine life.

Spatial and energy optimised desalination

The RO process is the major energy consumer in the entire membrane desalination process. Pre-treated seawater from ActiDAFF is pumped into the RO system at high pressure (HP). A pressure centre design configuration, where two small and two large HP pumps in lieu of the conventional layout of eight pumps for eight sets of RO racks, is adopted. Such arrangement achieves an efficiency of 89.5% while reducing footprint by 60%.

Positive displacement energy recovery devices (ERDs) are also in place to reduce the energy consumption for generating high pressure. The brine in the 1st Pass RO contains substantial residual pressure energy, which will be recovered by the ERDs. ERDs can recover up to 96% of the pressure energy in the brine, leading to the reduction of pumping energy by up to the 50% required for the booster pumps. In the 2nd pass RO, energy recovery turbines further reclaim residual energy from its reject stream to generate electricity.

Figure 7: ERD in 1st Pass RO desalination (L) and the simplified process flow of RO (R)

Figure 8: intake and outfall of TKODP

Innovative marine works during construction

Two sub-sea tunnels for the intake and outfall of the Plant respectively were constructed by tunnel boring machine (TBM) pipe-jacking method. The construction of the marine pits and tunnels involved innovative design approaches that successfully tackled various challenges, such as limited working space, tight construction programme and varying geological conditions. This is also the first project in Hong Kong to retrieve TBM under the sea.

Combined shaft (as jacking pit)

A single shaft—rather than two separate launching pits on land for the two tunnels—was constructed. A common wall separating the intake and outfall compartments was constructed at the combined shaft to serve as the TBM’s thrust wall and enable the launching of the two TBMs concurrently. Besides, the combined shaft also serves as a part of the permanent structure of the intake and outfall system of the Plant during operation. This arrangement results in significant reduction in the required footprint, construction time and cost.

Figure 9: Top-down view of the combined shaft, showing the jacking directions of the intake and outfall pipes

Intake shaft and outfall shaft (as receiving pits)

The 270-m-long outfall tunnel, with a diameter of 1.65 m, was constructed in rock while the 330-m-long intake tunnel, with a diameter of 2.5 m, was constructed in mixed ground. At the sea, temporary dry shaft and wet shaft were proposed for outfall and intake respectively in consideration of varying geological conditions and worker safety.

The dry shaft was a prefabricated steel caisson, 12 m in diameter and 20 m in height, reinforced by socketed H-piles installed around the perimeter. The caisson was embedded with a water leakage detection system in the bedrock below the sea. Seawater was then drained out, creating a mostly watertight condition within. The dry space within the shaft was maintained with standby pumps that switched on automatically when the water level threshold was reached. The caisson served two purposes: (i) a temporary working platform for receiving pit construction and TBM retrieval; and (ii) a separation shaft to safeguard marine life during TBM breakthrough of rock layers. In addition, the dry shaft allowed a safe working environment by limiting diving operations during construction.

Figure 10: Temporary outfall shaft at the sea

On the other hand, the wet shaft in mixed ground was a 15.5 m x 8 m shaft consisting of of pipe piles surrounded by sheet piles. Its mixed ground conditions, and the low rock head level, meant that it was extremely difficult to achieve cost-effective rock socketing with sheet pile for retainment. Therefore, the decision was made to leave the shaft “wet” (that is, seawater was not removed). Under wet condition, underwater connection works by divers was inevitable. Therefore, the wet shaft was designed and precisely prefabricated to be connected mostly by bolts and nuts in lieu of welding to reduce the duration of diving operation, hence enhancing construction safety and efficiency. While the wet shaft served as the receiving pit for TBM breakthrough and retrieval, it also enabled the later installation of the permanent Intake structure.

Figure 11: Temporary intake shaft at the sea

Subsea tunnelling

Pipe-jacking method (using TBM for micro-tunnelling) was selected to reduce potential impacts of the marine dredging works on water quality, marine ecology and fisheries of the sea. The biggest challenge of the TBM operations was to successfully retrieve the intake and outfall TBMs from, respectively, around 28 m and 20 m deep under the sea. In particular, the TBM had to be retrieved under a wet condition. To resolve this, a subsea recovery pipe module and an airlock chamber for hyperbaric intervention were installed at the rear of the TBM machine. As the TBM bored through the mixed ground, the airlock chamber enabled compressed air workers (CAWs) to carry out cutterhead maintenance by stabilising the surrounding pressure. Once the TBM machine was driven into the intake shaft, the “plug” at one end of the recovery module was left in and acted as waterstop to prevent seawater from flowing back into the tunnel. Concurrently, the divers connected the lifting system to the TBM before lifting up each compartment by crane barge. The whole underwater retrieval operation succeeded in its first attempt, achieving a new milestone in TBM history in Hong Kong.

Figure 12: Arrangement of the micro-tunnelling TBM showing its major components

Furthermore, to maximise operator safety against hyperbaric environment before and after the hyperbaric intervention operation, all CAWs were subjected to health check by appointed medical practitioners on site. During hyperbaric intervention, designated man-lock attendant maintained close contact with the CAWs and monitored the air quality inside. The project team also worked closely with the Labour Department and the Fire Services Department (FSD) to formulate safe working procedures for working under compressed air and facilitate the establishment and training of FSD’s Emergency Pressurised Team (EPT).

Figure 13: Intake TBM retrieval (top) and outfall TBM breakthrough (bottom)

Application of new technologies

On top of an advanced seawater desalination process and innovative construction of marine works, the project team seeks to improve safety and efficiency throughout the entire design-build-and-operate lifecycle with various smart applications.

Building Information Modelling / Management

In this project, an open BIM strategy is adopted, using ten different authoring tools in tandem. This enables our multi-disciplinary experts from over six geographical locations, such as Spain, Singapore, Japan, to work together under the centralised cloud-based platform from design to operation management.

Moreover, the project team diversifies BIM’s use by integrating it with other technologies. One example is BIM’s integration with augmented reality (AR), in which BIM models can be superimposed onto the real construction environment, providing an accurate sense of presence and immersion in a space yet to be built. This aids the visualisation for works inspection during construction and maintenance, and training for operational staff.

Figure 14: BIM 3D simulation of construction method statement (top) and AR application (bottom)

Figure 15: Delivery and installation of DfMA RO racks to RO building

Figure 16: Lifting of DfMA building external panels (L) and MiMEP of CO2 Tank (R)

Design for Manufacture and Assembly (DfMA) and Multitrade integrated Mechanical, Electrical and Plumbing (MiMEP)

In TKODP, the concepts of DfMA and MiMEP are actively applied. They involve designing the plant elements to be fabricated off-site before assembling them on-site, with an aim to enhance site safety, assure quality and reduce environmental impacts. The applications have also maximised construction efficiency and contributed essentially to the completion of the 4-year fast-tracked construction. Major components such as the RO racks, lime saturators, chemical dosing skids, open channel and building external wall panels were fabricated in factories, with site works proceeding simultaneously. The RO racks, in particular, sourced components from China, Spain, USA, Italy, India and South Korea, with final assembly being carried out in Nam Tong City in the Jiangsu Province of China at the height of the COVID-19 pandemic—from May 2021 onwards. The project team made extensive use of BIM to produce precise designs for shop drawings, aiding the manufacturer in smooth production. Despite disruptions from the COVID-19 outbreak, the RO racks arrived on site earlier than programmed and were accurately positioned into the RO Building in eight days, ensuring that the Plant can be commissioned on time.

Smart Site Safety System

Since 2020, TKODP has been one of WSD’s pilot projects in the implementation of Smart Site Safety System (SSSS). Great effort was made to introduce a series of smart safety devices and systems to enhance the working environment for worker safety. Some highlights include:

(i) Smart sensors

Over 100 sensors are being deployed to monitor high-risk construction activities and identify safety hazards. Examples include:

Tilt sensors at critical locations on a rock slope near the site boundary to provide advance warnings in cases of large boulders’ sudden movements

Smart lock for electrical boxes on site, openable only by the qualified electrician with the corresponding mobile application

A tag with QR code on all lifting appliance (LA) / lifting gear (LG) for easy access of data such as specification, inspection dates and certification validation period

Temporary cover monitoring system for openings in buildings, where unauthorised access is immediately detected and warned for quick reaction by the project team

(ii) Centralised Management Platform

The project team has developed a Centralised Management Platform and integrated it with the BIM models. It is an excellent real-time visualisation platform that collates control of a series of smart devices, including CCTV with artificial intelligent (AI) functions for safety management, LA/LG management system, moving plant monitoring system and GNSS. The integration of the BIM model in the platform provides a one-step hub to integrate and consolidate different data source into one common platform for monitoring the large volume of data from various sources.

(iii) Virtual Reality technology training kit

In the project, VR safety training for all personnel had been introduced at the start of the contract to enhance their induction training. The project took one step further with tailormade scenarios, integrating the project’s BIM model with the VR training to create an immersive and accurate sense of presence. This improves the users’ impression of the training, making them well-prepared for their upcoming tasks.

Figure 17: Centralised Management Platform at TKODP

Figure 18: VR Safety Training Centre at TKODP

Digital twin for operation

To improve plant performance, efficiency and operation flexibility, WSD has been proactive in exploring and implementing measures to reduce power consumption, optimise chemical dose, enhance operation cycle of consumable parts, and automate or optimise plant operation. Digital twin (DT) is one of the tools identified to achieve these objectives. DT is a virtual representation of an actual physical model, system or process with extensive integration of AI and machine learning.

WSD develops a DT to include the entire treatment processes of TKODP based on the plant design data and historical seawater quality. Prior to the commissioning of TKODP, simulated seawater quality based on historical data synchronised with seasonal changes will be fed into the DT for calibration and proof-of-concept.

During the operation phase of TKODP, the DT is applied where real-time operating data, in lieu of simulated data, will be made available and fed into the DT for machine learning and its fine-tuning. The more data are made available, the better the machine learning, the more reliable and accurate the DT prediction on the optimal dose of chemicals that meet the target water quality objectives.

Apart from generating recommendation on the optimal dose of chemicals, the DT of TKODP is capable of driving better operating decisions, reducing operating costs and resources, facilitating asset management as well as providing a safe training environment for the operators to experience the plant operation virtually, in conditions that rarely occur in daily operation, such as equipment failures, emergency shutdown and extreme water quality (for example, high turbidity or boron, algae bloom).

The drive to make provisions for renewable fresh water—by creating a sustainable desalination plant—gave rise to numerous enhancements that garner the Platinum rating in BEAM Plus New Buildings.

Sustainable water from sustainable development

BEAM Plus New Building

TKODP embraces sustainable development as early as the conceptual stage. One of the highlights is the adoption of BEAM Plus New Buildings (BPNB), a set of green building assessment tools specially designed to evaluate building sustainability in Hong Kong. TKODP had attained the highest achievable rating, the Platinum, in BPNB with a provisional score of 88.5%, significantly higher than 70%, the minimum requirement.

Notable achievements include:

• designing low-rise buildings as visual and spatial relief and integrating them well with future adjacent residential development and the surrounding countryside, with dedicated waterfront plaza for staff and public recreational uses and visual corridors to the sea;
• massively adopting soft landscaping, such as green roof, vertical greening and landscape garden to mitigate heat island effect arising from concrete building, reducing energy consumption in air-conditioning, facilitating drainage system and improving visual aesthetics; and
• Allocating over 1,800 solar panels on building rooftops to generate renewable energy and reduce energy use and carbon emission.

Figure 19: TKODP green roof (L) and blue PV panels (R)

Solar photovoltaics farm

Separate planning by WSD is underway for a large-scale, 10-MW solar photovoltaics (PV) farm on top of the landfill near the vicinity of the TKODP. The electricity generated from the PV farm will be used exclusively by the TKODP. This initiative falls in line with the HKSAR Government’s commitment to spearheading renewable energy development with a view to achieving carbon neutrality before 2050.

Figure 20: Technical visit (L) and school visit (R) by TKODP project team

Connection with the community and way forward

Educating visit

Not only is TKODP a strategic installation for water resource; it also serves as an educational hub to promote water conservation to the public. An interactive exhibition area in the TKODP is dedicated to showcasing its treatment process and its role in building water resilience. Through guided tours, the visitors can learn about water supply in Hong Kong, such as past endeavours to overcome droughts, threats to current water sources, application of desalination and details of RO process in TKODP. Through a series of inspection windows, visitors can directly view the ActiDAFF and RO equipment in a safe and comfortable manner. Immersive videos and interactive physical models are also included to help visitors to appreciate TKODP’s journey from design to construction.

Public promotion

Since TKODP’s construction stage, public promotion via school visits, project website, participation in local and international conferences, seminars and exhibitions has raised the public’s awareness and built their confidence in desalination technology. The way will also be paved for providing desalinated water for small pre-fabricated systems in the future development of outlying islands.

Future of TKODP

Stage 2 of TKODP, which already had its location reserved next to Stage 1, is now under planning. The success of TKODP shall be an important milestone for WSD's continuous efforts in diversifying water sources to better prepare for climate change and to contribute for the sustainable development of Hong Kong.

Acknowledgements

The WSD would like to thank the consultant, Binnies Hong Kong Ltd, the contractor AJC Joint Venture (comprising Acciona Agua SA, Jardine Engineering Corporation Ltd and China State Construction Engineering (Hong Kong) Ltd), and all other stakeholders for the collaborative efforts expended (and supports extended) by them in driving the success of the innovative and sustainable TKODP project.

References

1. United States Geological Survey (1993). Water: The Resource That Gets Used and Used and Used for Everything [Poster, Middle School Version].
2. Alix A, Bellet L, Trommsdorff C, and Audureau I (eds.) (2022). Reducing the Greenhouse Gas Emissions of Water and Sanitation Services: Overview of emissions and their potential reduction illustrated by utility know-how. IWA Publishing.