Yuen Long Effluent Polishing Plant
Content provided by the Drainage Services Department with the coordination of the HKIE Civil Division
Transformation into sustainable YLEPP
The existing Yuen Long Sewage Treatment Works (YLSTW), commissioned in 1984 now serves Yuen Long Town, Yuen Long Industrial Estate and Kam Tin area with treatment capacity of 70,000 m3/day at secondary sewage treatment level. To cope with the population growth and development needs, YLSTW will be upgraded in stages from its treatment capacity of 70,000 m3/day to 150,000 m3/day and be transformed into Yuen Long Effluent Polishing Plant (YLEPP) at tertiary treatment level to meet the more stringent discharge requirements, thus minimising adverse environmental impact on the ecological setting of Deep Bay.
Figure 1. Photomontage of YLEPP
YLSTW is located at the Yuen Long Industrial Estate within a cramped site of only 8 hectares. It has been operating for over 30 years and most of the facilities are reaching the end of their design life. To preserve the precious land resources for other uses by the community, YLSTW will be upgraded to the YLEPP within its existing footprint without the need of additional land. To achieve this, various advanced sewage treatment technologies, including lamella settler, Aerobic Granular Sludge (AGS), disc filer filtration, etc, were introduced to enable a compact plant design within a limited footprint.
To be in line with the commitment to decarbonisation in combating climate change, the Drainage Services Department have maximised the use of renewable energy in YLEPP striving towards carbon neutrality. The biogas generated from the treatment process are harvested to become fuel for the combined heat and power system, turning into electrical energy to support the daily operation of YLEPP. Large areas of solar photovoltaic panels will be installed on the building roof to maximise the use of solar energy. Treated sewage effluent will be reused and be used for cooling to further reduce the power and water demand. To integrate with the community, part of the plant will be designated as public co-use area including riverside promenade, view decks and education corridor for public enjoyment and promoting sustainability.
With the sensibly planned spaces in YLEPP and provision of high quality recreational spaces for the public, together with the renewable energy systems to be adopted, YLEPP reaches the BEAM Plus Neighbourhood Platinum rating which recognises the sustainability performance of YLEPP.
Overcoming the challenge
The YLEPP is located within Schedule Area No. 2 which is defined as an area where marble containing cavities may be present at depth. Based on the result of existing and project specific boreholes, the site is underlain by 15 m of superficial material consisting of fill, marine and estuarine deposits and alluvium below which is a layer of saprolite and partially weathered zone of varying thicknesses. The solid geology below the weathered zone indicates that towards the eastern portion of the site is underlain by marble/ interbedded marble whereas marble was generally absent in the remainder of the site.
Due to the presence of marble in the eastern portion of the site and in view of the relatively complex geology in the marble zone, a box raft foundation functioning as a compensatory raft was proposed to minimise the increase of vertical effective stress on marble surface. The majority of buildings outside the marble zone were designed using driven piles which are commonly used due to better settlement control.
YLEPP lies within a Wetland Buffer Area and is adjacent to some sites of conservation importance, such as Site of Special Scientific Interest, Priority Site for Enhanced Conservation, Mai Po Inner Deep Bay Ramsar Site in 300 m away, and Wetland Conservation Area Nam San Wai in 150 m away. Wetland provides a good environment to the waterbirds for a variety of activities which include feeding, breeding, nesting and moulting. Some of these sites are utilised by migratory waterbirds as their major stopover points during their migration process, particularly in winter.
Figure 2: Location of YLEPP and the neighbourhood
Figure 3: Location of YLEPP to Mai Po Inner Deep Bay Ramsar Site
The surrounding habitat of Shan Pui River next to YLEPP also supports abundance of nests of Chinese Pond Herons during breeding season, which starts from March to August every year. In this period, egretries could be very sensitive to surrounding disturbance.
Other than seasonal activities of the waterbirds, the wetland also attracts the waterbirds roosting in the area starting from evening time. The pre-construction study carried out in January 2021 identified two active night roosting areas, one at the mangrove strip located east and the other at the northeast of the YLEPP boundary.
To minimise the potential impacts of the project on these ecological sensitive receivers, a series of actions have been taken.
Figure 4: Various mitigation measures and construction constraints are in place to minimise the ecological impact of the construction on the waterbirds
Constraints on percussive construction
Percussive piling works and demolition using excavatormounted breakers are prohibited from November to March, which makes the progress of piling works more critical. Also, to minimise the impact on the ardeid night roosts adjacent to the YLEPP, no construction works with the use of powered mechanical equipment within 100 m from the identified active night roosts after sunset, i.e. 17:30 during dry season and 18:00 during wet season. In addition, demolition in the proximity of the egretry shall be avoided during egret’s breeding season. Some buildings were thus designed with box raft foundation so as to reduce the requirement of percussive piling works.
Further mitigation measures and monitoring
Having consulted relevant departments and green groups, further mitigation measures were implemented to minimise the noise and ecological impact of the project, including erection of a 4-metre high noise barrier with acoustic panels along the eastern, northern and western boundaries of YLSTW and erection of bird curtains along northern and eastern boundaries to block the bird’s views towards the construction works from the surrounding trees. Also, the site will implement tight control on light overspill and avoidance of direct lighting towards the ecologically sensitive areas. The project team, together with the Environmental Team and Independent Environmental Checker, have been keeping close monitoring on the effectiveness of the mitigation measures and identifying any unpredicted potential impacts on the surrounding ecological sensitive receivers.
During the course of the upgrading works, about half of the existing facilities of the YLSTW would be demolished while the other half would be kept in operation to maintain the sewage treatment service for the Yuen Long area. As there would be limited standby units available upon the demolition of half of the plants, it is crucial to ensure the remaining plant facilities are in good working condition to maintain a reliable sewage treatment service to comply with effluent discharge standards during the whole construction period of YLEPP. Therefore, proper overhaul works were carried out prior to demolition work.
Apart from the overhaul works, it was essential to plan and divert the process pipes and relocate the process equipment to isolate some of the existing treatment units for demolition while maintaining the operation of the remaining treatment units. The project team, consultant, contractors and plant operators jointly work together and optimise the construction sequences and diversion schemes to minimise the disturbance to YLSTW operation and facilitate the construction works at the same time.
“YLEPP adopts an advanced sewage treatment technology, the aerobic granular sludge (AGS) for biological treatment which provides a high efficiency solution with lower energy demand of. AGS also help YLEPP moving towards energy neutrality”
Advanced treatment technology - AGS
First full-scale AGS System in Hong Kong
YLEPP is the first wastewater treatment plant in Hong Kong adopting the advanced treatment technology of aerobic granular sludge (AGS) for biological treatment.
The AGS process utilises the unique characteristics of granular biomass working in aerobic conditions to treat the sewage. At the same time the characteristics of the granular biomass allow for anoxic conditions inside of the granules, enabling extensive simultaneous nitrification/denitrification during aeration. In addition, extra pre- and post-denitrification is achieved through manipulation of the amount of oxygen introduced during the treatment cycle.
The AGS process closely resembles the operation of Sequencing Batch Reactor (SBR) and each operating cycle of AGS process will mainly consist of three operations: 1) Fill/ Decant, 2) Aeration and 3) Fast Settling and Sludge Wasting.
In the Fill/Decant phase, the system will feed the appropriate portion of the primary effluent into the AGS reactor. As the reactor is equipped with fixed overflow weirs, supernatant is decanted simultaneously into the tertiary system.
During the Aeration phase of the operation cycle, the influent no longer enters the reactor, and the aeration system begins delivering oxygen to the reactor. The introduction of oxygen converts the reactor from an anoxic/anaerobic environment to an aerobic environment. In this phase Aerobic reactions take place in the outer part of granular biomass while anoxic condition exists in the middle of granular biomass.
After Aeration, the reactor enters the Fast Settling and Sludge Wasting phases. In these phases the influent does not enter the reactor. Also, the aeration system is turned off. The absence of flow and aeration activity produces an ideal quiescent environment in the reactor for settling to separate solids from the treated effluent.
Figure 5: Typical process cycle of AGS System
Advantages of AGS System
AGS improves and enhances biological nutrient (nitrogen and phosphorus) removal when compared to similarly loaded activated sludge. This is due to a significantly more effective substrate availability to PAO (for P-removal) and the potential to optimise the nitrification/denitrification in batch cycle compared to a continuously operating plant. The simple process configuration, automation and the resilience of the granules against less favourable process conditions make AGS one of the preferred treatment processes of YLEPP.
Compared to conventional secondary sewage treatment technologies, AGS requires less footprint as a result of high reactor biomass concentrations and the fast settling. AGS is comparatively energy efficient due to the reduced use of mechanical equipment, like mixers, recycle pumps, etc, thus, bring about the benefits of associated capital and operational cost savings.
Moreover, an important and significant advantage in addition to energy efficiency is that, in aerobic granular sludge, the overall concentration of biomass is high which enables extensive denitrification to take place without the addition of chemical carbon source.
In conclusion, AGS provides a high efficiency solution with lower energy demand of. AGS also help YLEPP moving towards energy neutrality.
Smart process control and optimisation
The AGS process is controlled through a dedicated software designed for the AGS treatment technology. This dedicated control system includes operating recipes (combining sequential operations and phases) and provides commands (based on information received from sensors/analysers) to the PLC which executes the process actions. The cycle parameters, e.g. scheduling of all the reactors, number of cycles per day, duration of each phase for a specific reactor, reacting environment, etc., will be automatically controlled by the SMART control system, which calculate the parameters based on real time influent strength, hydraulic and environmental conditions, the influent diurnal pattern of the plant, and historical data, etc. This dedicated control system is also highly transparent and provides easy and full insight to the operator about the status and next cycle steps of the various AGS reactors. Also the operator and supervisors have full ability to modify set-points, recipes and operation strategies.
Figure 6: Typical workflow for smart process control and optimisation
Pilot test before full-scale application
AGS is an emerging technology attracting lots of attention around the world but had not been tested in Hong Kong and its performance under local sewage condition was unknown. Therefore, a pilot test for AGS process had been conducted from 2015 to 2019 to evaluate its feasibility and performance before recommending for full-scale adoption in YLEPP.
The AGS Pilot Plant with treatment capacity of 1,000 m3/day was comprised of an influent feeding system, an aeration system, an AGS reactor, a discharge system, and a dedicated AGS controller. Sewage was fed into AGS reactor from the Influent Facility in batch mode and the final effluent would be discharged from the reactor after the completion of biological treatment. AGS operation resembled a sequencing batch reactor and treatment was provided in operating cycles. Within each cycle, the AGS controller would control the sequence and duration of the different operating phases, such as feeding, discharging (sludge), anoxic phase, aerobic and settling.
During the two-year testing period upon the commissioning of the AGS Pilot Plant under different operating scenarios, the AGS system had demonstrated good operational stability, robustness, treatment performance and adaptability to the characteristic to the local sewage. Apart from the treatment performance, sufficient information regarding E&M installation, testing and commmissioning, process startup and troubleshooting had been thoroughly studied and obtained from the pilot test as well.
Towards energy neutrality
Extensive provision of PV panels
YLEPP was designed to maximise its potential to utilise solar energy by installing more than 5,000 monocrystalline-type PV panels on the rooftop of most of the buildings of YLEPP for power generation, with the overall installation capacity greater than 2.0 MW. After commissioning, YLEPP will be one of the largest solar power generation facilities in Hong Kong.
Figure 7: Extensive provision of PV panels in YLEPP
Low energy sewage treatment process (AGS)
AGS is comparatively energy efficient due to the reduced use of mechanical equipment. Moreover, the dedicated AGS controller and the batch operation mode can allow AGS system utilise the aeration air more effectively than other conventional system.
Natural sunlight and ventilation in the building design
Some features have been adopted in building design for achieving energy neutrality in YLEPP. Glazing panel array on building wall allows neutral light to transmit and diffuse to building interior as supplemental lighting, hence minimises the dependency on artificial light and reduces energy use by lighting system. Shading created by cantilevered slab extending from all sides of building blocks direct sunlight to the interior and prevents excessive heat gain in buildings.
Reserved areas for co-digestion / imported sludge
Food waste is a great source of energy if it is co-digested with sewage sludge according to overseas experience and local studies. In YLEPP, an area with sufficient footprint has been reserved for collecting and handling about 260 wet tonnes of pre-treated food waste per day. These will provide the potential for YLEPP to co-digest the pre-treated food waste with sewage sludge to further boost up the biogas production.
Design towards energy neutrality
YLEPP was designed towards energy neutrality. To achieve this, the project team looks into multiple measures to minimise energy consumption and maximise energy generation.
To achieve energy saving, multiple measures were adopted from the perspective of process selection, equipment selection, and motor section; low hydraulic loss type equipment is proposed to minimise the fiction loss and the adoption of effluent pumping station which only operate for effluent discharge at high tide condition if necessary; and IE3 motors are preferred in the design of YLEPP due to its high energy efficiency. Power quality and energy management system also help to enhance energy performance and facilitate energy management in YLEPP.
On the other hand, numerous renewable energy installations have also been adopted in YLEPP to maximise its capacity of power output, including extensive provision of PV panel system, three high efficient CHP Generating Sets, effluent cooling system, etc.
“YLEPP adopts renewable energy installations to maximise its capacity of power output and at the same time achieves energy saving.”
Smart - Design, construction and process control
BIM adoption in design and construction stage
Building Information Modelling (BIM) technology has been widely adopted in YLEPP since the beginning of the project.
Civil and E&M design had been well collaborated via BIM technology and shared at the Common Data Environment for integration, clash analysis and finalisation for construction. Clashing of different trades of works can be identified at early stage and resolved in the BIM model which can in turn avoid abortive works and facilitate the construction. Besides, the BIM environment enables visualisation of the as-built record drawings of underground utilities, the on-site inspection pit/ trench excavation for the contractor to plan and review the sequence of works.
Figure 8 – Planning of E&M installation arrangement in BIM
Figure 9 – Coordination of new and existing utilities congested within limited road spaces and construction planning on utilities installation
In addition, 4D simulation was adopted to oversee the overall programme and site utilisation model was adopted to assist in the planning of site establishment, construction method and logistics.
Figure 10 – Simulation of construction sequence with the utilisation of BIM
The BIM model, integrated with other advanced technology, can provide a large variety of usage in the construction stage of this project. For example, the BIM model was adopted alongside the Virtual Reality (VR) technology to produce a virtual site walk for the user to imagine the end product, i.e. visit the new building before the building even been built.
Smart control and analysis in the operation stage
In the past, Sewage Treatment Works (STW) usually relied on experienced and skilful engineers and operators to optimise the treatment process, handle abnormal conditions, and perform troubleshooting when necessary. With the development of Artificial Intelligence (AI) technology, now the new generation of instrumentation, control, and automation system makes predictive control for STW, and machine learning for sewage treatment process optimisation became possible.
In YLEPP, the project team is exploring the application of smart control and analysis in sewage and sludge treatment processes to help make diagnostic, predictive and prescriptive decisions in the plant operation, with real-time anomaly detection on data streams to achieve low energy consumption, high treatment efﬁciency, and low operation cost.
“Creating a community of peace and harmony for man, bird and nature.”
Connect with the community
With the intention of creating a public institution for eco-learning through live experience, the redevelopment of YLEPP was designed to incorporate sustainable education elements.
A new secondary entrance for the general public, is located at the southeast corner of YLEPP whereby a new bridge is to be constructed connecting from Yuen Long Industrial Estate and the riverside promenade located directly at the east of YLEPP next to Shan Pui River.
Figure 11 – Secondary entrance to YLEPP for the general public
Along the promenade, visitors will be able to lean towards the building to view the operation of sewage treatment works through openings on the facades. With the blocks shifted around, these pocket spaces create interesting viewpoints that allow visitors to view particular components of the plant, both functional and communal. As visitors move along the promenade, they are free to meander through the line of trees and hunt for the salvaged mechanical parts that are planted along either side of the path. Moreover, some of the retained tree logs would be re-used to provide facilities such as wooden benches which will be placed on the sides of the path for use.
Figure 12 - Riverside promenade
Roof garden and bird hide
Located at the northeast corner of YLEPP, a roof garden with bird hide at the Tertiary treatment plant is designed as an open area for public recreational activities. Surrounded by native and seasonal faunas, a central lawn was formed with timber benches provided for small gatherings and rest. A narrow ring of path leading to the bird hide will have maximum control over the numbers of visitors going into the bird hide and create minimal disturbance to the nearby ecology.
Figure 13 - Roof garden and bird hide
The bird hide is designed in a circular form providing all round panorama view of the area and it is encircled with horizontal timber fins with a gradation of openings in the middle for viewing purposes.
The internal courtyard formed by the ring is another space that differs from the open quality of the public space. With shrubs growing within it creates an intimate space and a closer relationship with greenerys at an elevated height.
In YLEPP, a learning pathway has been developed for visitors and the general public to learn about sewage treatment process and the various renewable energies adopted in YLEPP. Visitors will be able to gather at the courtyard area of the Administration Building to be briefed and then walk around buildings to learn about the different sewage treatment processes and renewable energies through a series of ramps, elevated pathways and public decks and conclude the walk at the open area at the roof garden of the Tertiary treatment plant.
Figure 14 – Courtyard area of Administration Building
The YLEPP project has adopted various innovations to improve its sustainability performance and energy efficiency to strive towards carbon neutrality. YLEPP is the first full scale sewage treatment plants in Hong Kong adopting AGS. The number of PV panels had been maximised for solar power generation. Together with other renewable energy installation, YLEPP will be a tertiary sewage treatment works in Hong Kong that have the highest potential to achieve energy neutrality. All these could only happen with the intense communication, collaborative effort and partnering spirit amongst all parties to work towards a common goal.
The Design & Construction consultancy of YLEPP is the first consultancy in DSD adopting NEC3 PSC Option C and establish a co-location design office for DSD and consultant team. The construction contract for the main works of YLEPP also adopts NEC4 ECC Option C contract form. The spirit of partnering and collaboration was built since the schematic design stage, and will continue through the construction and towards the operation stage of YLEPP.
The Drainage Services Department (DSD) would like to thank the project designer and consultants, AECOM Asia Co. Ltd., and the contractor Paul Y. – CREC Joint Venture and their subcontractors for their collaborative effort with the project team in delivering this challenging project to transform the existing YLSTW into an innovative, energy-efficient and sustainable YLEPP.
Figure 15 – Establishment of Co-location Design Office at design stage
Figure 16: Collaborative effort of DSD, the project consultants, AECOM Asia Co. Ltd., and the contractor Paul Y. – CREC Joint Venture and their subcontractors in delivering this challenging project to transform the existing YLSTW into an innovative, energy-efficient and sustainable YLEPP