Hong Kong Harbour Area Treatment Scheme Stage I HATS is a massive infrastructure programme that

Abstract comprises four stages. Stage I of the HATS is targeted to deal with the worst polluted areas. Its catchment covers 1 The Harbour Area Treatment Scheme (HATS) is a major the whole of , extending from Tseung Kwan O Government initiative to improve the water quality in in the east to in the west, and the northeastern . It provides for the proper collection, part of Island. Under Stage I, sewage treatment and disposal of all wastewater discharging into generated by a population of 3.5 million people is the Harbour from the urban areas of Kowloon and Hong collected and transferred to a centralized sewage Kong Island. The Stage I of HATS, designed to serve a treatment works at via a deep tunnel population of 3.5 million of people, consists of a deep collector system for chemically enhanced primary tunnel sewage collection system and a sewage treatment treatment. plant with a design capacity for collecting and treating 1.7 million cubic metres of sewage generated from the urban The layout of HATS Stage I is shown in Figure 1 below areas of Kowloon and the northeastern part of Hong Kong and the Stage I works comprise the following major Island each day. components:

Construction of HATS Stage I commenced in April 1994 • Upgrading of seven existing preliminary treatment and completed in December 2001. The Stage I system is works; now in full operation and has brought about substantial • A 23.6 km deep tunnel collector system; improvement to the water quality of the Harbour. • A chemically enhanced primary sewage treatment Following the completion of the review conducted by the works at Stonecutters Island; and International Review Panel (IRP) in 2000, Government is • A 1.7 km marine outfall tunnel and a 1.2 km diffuser now planning for the implementation of further stages of pipeline HATS.

This paper mainly describes the key components of HATS Stage I to provide an overview of the Stage I scheme.

1. General Description of the HATS Stage I

Due to rapid development, the water quality of the marine waters of Hong Kong, especially its renowned Victoria Harbour, has deteriorated in recent years. The Harbour Area Treatment Scheme (HATS)1 is a major Government initiative to improve the water quality in Victoria Harbour and provide a safe and healthy environment for the people Figure 1- Layout of HATS Stage I of Hong Kong. It provides for the proper collection, treatment and disposal of all wastewater that discharges Construction of the Stage I works commenced in April into the Harbour from the urban areas of Kowloon and 1994. Works were packaged into 15 works contracts to . enable participation of both local and overseas contractors with the required specialist skills. Construction of the 1 Formerly known as Strategic Sewage Disposal Scheme (SSDS)

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Stonecutters Island Sewage Treatment Works (SCISTW) After preliminary treatment, the sewage is conveyed was completed in May 1997 and the plant has since been through a series of shafts and tunnels from the PTWs to put in operation, initially for treating flow from the Stonecutters Island for further treatment. Northwest Kowloon (NWK) catchment. Construction of the deep tunnel collection system has been an extremely The deep tunnel collector system is adopted to allow the challenging task involving mining through many sections shortest route to be chosen and more importantly, to of very difficult ground. All construction challenges have minimise the disturbance and nuisance to the public, the now been overcome and the tunnel system was environment, traffic, utilities, transport systems and successfully completed and commissioned in December building foundations during construction. 2001. The tunnel collector system is designed to operate mainly Completion of the HATS Stage I is not only a major as a series of inverted siphons with flow discharging into milestone in the HATS project, but also represents one of a vertical drop shaft at each PTW and raised from the the most remarkable engineering achievements ever tunnels by pumping stations provided at the downstream carried out in Hong Kong. ends at Kwun Tong and Stonecutters Island. This configuration results in the tunnels always flowing full in The following sections briefly describe the key a pressurized condition and thus minimises the head components of HATS Stage I. against which the sewage needs to be lifted by pumping from the tunnels to ground level. The operation of the 2. Upgrading of Existing Preliminary Treatment tunnel section from Tseung Kwan O to Kwun Tong is an Works exception where the existing pumping station at Tseung Raw sewage collected from each of the Stage I Kwan O has been upgraded and used to pump flow to catchments is firstly directed to the existing preliminary Kwun Tong. This section of tunnel therefore operates as a treatment works (PTWs) located along the harbour front rising main rather than an inverted siphon. for removing large solids and grit2 . These PTWs3, in existence for many years, have been modernized under Harbour Area Treatment Scheme - Stage I Key Components HATS Stage I to provide a high standard and reliability of Preliminary performance in removing grit and large solids from the Treatment Works sewage before discharging it into the deep tunnels via Riser Shaft drop shafts. This is necessary to ensure that a high degree Approach Channel of protection is provided to the HATS tunnel collector Drop Shaft system to prevent sediment deposition. The upgrading Tunnel works also include the provision of extensive instrumentation and control in order that the operation of each PTW can be remotely monitored and controlled Figure 2 - Key components of the tunnel system from the central control room on Stonecutters Island. A total of 23.6km of sewage tunnels, with diameters

3. Deep Tunnel Collector System ranging from 1.2m to 3.5m, have been formed at depths of up to 150m (equal to the height of a 50-storey building)

2 Before HATS Stage I, the preliminarily treated sewage was discharged by gravity through and a minimum rock cover of 30m above the tunnel outfalls of varying lengths to Victoria Harbour crown. These tunnels are the world’s deepest sewage

3 Seven modernised PTWs: Chai Wan, Shau Kei Wan, Tseung Kwan O, Kwun Tong, To Kwa tunnels and have a length of about twice that of the Mass Wan, , and PTWs

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Transit Railway line on Hong Kong Island. The tunnels Terminals. All these challenges have been successfully were excavated using hard rock tunnel boring machines, overcome and the tunnel system was completed and except for the short section between Kwai Chung and commissioned in December 2001. Tsing Yi where the drill and blast method was employed. The finished shafts and tunnels have been fully lined with 4. Pumping Stations concrete to enhance hydraulic efficiency of the system and at the same time prevent progressive deposition of When sewage arrives at Stonecutters Island, it is lifted sediments within the tunnels during operation. from the deep tunnels to the surface via the Stonecutters Island Main Pumping Station (SCIMPS) which is the largest underground sewage pumping station in Asia. The pumping station has an internal diameter of 50 metres and is more than 38 metres below ground. Construction of this facility involved deep excavation within a diaphragm wall.

SCIMPS is designed to handle a peak flow of 31.25 m3/s from the tunnel system. The pumping system comprises eight centrifugal pumps (six duty, two standby), each with

Figure 3 - Tunnel Boring Machine (TBM) their own motor, together with four variable speed drives. The following table provides a summary of the basic data The pumping station can therefore be operated in a way for the tunnel collector system: that allows the output of the pumps to match the sewage inflow from the tunnels. The discharge from the pumps Table 1 - Key Data of the Tunnel Collector System passes into a discharge channel below the roof of Depth Designed SCIMPS and flows by gravity to the headwork of the Finished below Peak Wet Tunnel Length Diameter nominal Weather sedimentation tanks. The pumping station is fully Drive (km) (m) sea level Flow enclosed and provided with facilities for odour control. (m) (m3/s)

126 – A 2.30 1.20 1.66 121 B 2.53 1.35 121 – 76 2.72 1.35 C 5.33 87 – 76 4.89 twin-pipes 143 – D 3.57 2.82 14.81 136 136 – E 5.50 3.54 21.80 125 132 – F 3.58 2.36 9.45 125 134 – G 0.78 2.21 7.36 132 Figure 4 - Stonecutters Island Main Pumping Station

Construction of these tunnels has been a very challenging The pump motors and control room are located in the task involving tremendous efforts in cutting through superstructure, which is a circular building with external various sections of faulted ground and traversing under curtain wall glazing to provide a high quality finish. An the built-up areas of the , the MTRC automated control system, the Distributed Control and underground railways (crossed at six locations), the Data Acquisition System (DCDAS), has been installed in , and Kwai Chung Container the pumping station to operate the entire Stage I system,

3 Works Digest Issue No.48 including the outlying PTWs and pumping stations, and the largest CEPT plant in the world. allow the operation of SCIMPS to be controlled in a way 5 1 which ensures maximum efficiency in the use of energy. Sedimentation Tanks Facility

Three other pumping stations have also been provided The sedimentation tanks facility at SCISTW consists of under HATS Stage I, namely the Kwun Tong Pumping three main processes: mixing, flocculation, and Station (KTPS), the North Pumping sedimentation. The incoming sewage first goes through a Station (NWKPS) and the Tseung Kwan O Pumping rapid mixing process whereby conditioning chemicals, Station (TKOPS). ferric chloride and a polyelectrolyte, are added to sewage and mixed through agitation by submerged mixers. The KTPS is a booster pumping station located at Kwun Tong conditioned sewage then passes into a series of to lift the flow from the tunnels serving the Chai Wan and flocculation tanks where aeration is used to keep the Shau Kei Wan PTWs and discharge it into the tunnel solids in suspension while a floc forms. This helps from Kwun Tong to Stonecutters Island. NWKPS is enhance the efficiency of the settling process further designed to handle the flow from the existing North West down the works. From the flocculation tanks the flow is Kowloon PTW on Stonecutters Island to the treatment distributed via a central distribution channel to the works where it is mixed with the flow from SCIMPS, double-tray primary sedimentation tanks where the whilst TKOPS is an existing pumping station modified tranquil flow regime would allow the floc to settle to the under HATS Stage I to pump flow from the Tseung Kwan base of each level of the tank and the scum to float to the O PTW through the tunnel to Kwun Tong. surface.

5. Stonecutters Island Sewage Treatment Works

Figure 6 - Sedimentation Tanks Facilities

Longitudinal collectors in each tank collect the sludge to a central hopper and the scum to an outer trough. The

Figure 5 - Layout of Stonecutters Island Sewage Treatment Works sludge is then directed to a sump by a similar cross

collector. Sludge and scum are drawn off from the tanks The Stonecutters Island Sewage Treatment Works and pumped to a separate facility for dewatering. (SCISTW), built on 10.6 hectares of reclaimed land, provides treatment to sewage conveyed through the The treated effluent from the sedimentation tanks tunnel collector system and NWKPS. SCISTW adopts a discharges over a downstream weir along the outer edge chemically enhanced primary treatment (CEPT) process of the tanks, into a culvert leading to the outfall tunnel for and a space-saving double-tray sedimentation tank design. disposal. The plant has a design capacity for treating an average sewage flow of 1.7 million cubic metres each day and is

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Thirty-eight double-tray sedimentation tanks have been houses ten (nine duty, one stand-by) high efficiency provided at SCISTW, each with a capacity of 50,700 centrifuges which lower the water content to make the m3/d at average dry weather flow. The detention time material suitable to be disposed of in landfill. Each through the tank is 90 minutes at this flow rate. centrifuge is rated at a throughput of 60.2 tds/d, operating at a speed of 2500 rpm. In addition, two prototype tanks which are hydraulically separated from the main tanks have also been built. The target minimum dry solids content of the sludge cake These prototype tanks are provided with separate rapid is 32%. In order to achieve this, the sludge is dosed with mixer, flocculation and other ancillary systems that can a polyelectrolyte to improve its dewaterability. The allow them to be used for conducting trials of different dewatering building also houses facilities to store, mix chemicals and dosing rates to determine the most and dilute polyelectrolyte to the required concentration. appropriate way to operate the SCISTW. Dewatered sludge cake is discharged from each 5.2. Chemical Dosing Facility centrifuge onto a conveyor system and transferred into holding silos before being discharged into containers for The chemical dosing facility provides storage and dosing transport by sea to the Government landfills at WENT facilities for the dosing of ferric chloride and polymer to and SENT. Odour control facilities have been the main and prototype sedimentation tanks in order to incorporated and sealed containers are used to minimise enhance sludge settling in the primary sedimentation nuisance to the public throughout the sludge treatment, process. The facility consists mainly of a ferric chloride handling and transportation processes. storage and pumping system, a polymer batching and pumping system and a dilution water system for the polymer.

Ferric chloride can be delivered to site in bulk liquid form by barge or truck tanker, as either a pure manufactured solution (up to 42% concentration) or as a by-product from another manufacturing process (as low as 32% concentration). The ferric chloride system is capable of dosing ferric chloride between 10 to 60 mg/L, with the Figure 7 - Disposal of sludge in sealed container normal range expected to be 20 to 40 mg/L. Polymer can be delivered by truck in dry form, in bulk shipping containers, or in pneumatic-unloading trucks, which is 5.4. Performance of SCISTW then transferred to two storage silos. The polymer feed Since its operation in May 1997, The CEPT plant at system is fully automatic and, using a batch system, it Stonecutters Island has shown excellent performance prepares a polymer solution of 0.1 percent to 0.5 percent results and is acclaimed by local and international experts of dry polymer by weight. as one of the world’s most efficient CEPT plants.

5.3. Sludge Dewatering and Disposal Facilities Since the commissioning of the tunnel system in Sludge and scum drawn off from the sedimentation tanks December 2001, average flows to the treatment works is pumped to adjacent holding tanks before being have increased to about 1,300,000 cubic metres per day, delivered into the dewatering building. This building which is in line with the predicted flows. The BOD and

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Suspended Solids (SS) removal rates are currently pipeline is 1204m. Each leg is tapered from an initial averaging 72% and 81%, which are significantly higher diameter of 3.25m to a final section of 1.5m diameter. than the design values of 35% and 70% respectively. This is done to improve the flow velocities and prevent Ferric chloride is being dosed at 10 mg/L instead of the deposition of material in the diffuser. A total of twenty- expected design dosage rate of between 20 and 40 mg/L, four diffusers are provided along the pipeline each and this smaller dosage rate has proved adequate to equipped with eight ports to achieve the required achieve the required effluent quality. dispersion performance.

6. Stage I Outfall The diffuser pipeline is protected from possible anchor damage by a layer of rock armour. In addition, each Treated effluent from SCISTW is discharged to the diffuser is equipped with a resilient polyethylene dome waters southwest of Stonecutters Island for dispersion designed to absorb the impact from ship anchors for with the tidal stream in the Western Dangerous Goods protection. Anchorage area via an outfall tunnel and a diffuser pipeline. 7. Benefits and Way Forward

The effluent flows by gravity through a twin cell box HATS Stage I has already been completed and put to full culvert and is discharged into a large open chamber operation since December 2001. It is now treating some connecting to the head of the drop shaft at the upstream 1.3 million cubic metres of sewage each day, representing end of the outfall tunnel. The outfall tunnel is 1.7km long about 70% of the raw sewage from the densely-populated and passes under the reclamation of Container Terminal areas around Victoria Harbour, and has brought about No. 8 and the Northern Fairway at a depth of about 100m. substantial improvement to the water quality of the It is the largest of the Stage I tunnels and was excavated Harbour. by tunnel boring machine through difficult ground conditions. The tunnel is lined with precast concrete The Government is now implementing a programme of segments to give a finished diameter of 5m and to provide trials and studies, recommended by the International for long-term durability and hydraulic performance. Review Panel following a review on HATS completed in 2000, with a view to formulating the way forward for the To transfer the flow from the further stages of HATS. The further stages of HATS aim tunnel to the diffuser at collecting and treating sewage from the northern and pipeline laid in a pre­ south-western parts of Hong Kong Island, and at dredged trench on the providing a higher level of treatment to sewage in order to seabed, two 3.25 metre meet all the water quality objectives. Upon completion of diameter riser pipes are the entire Scheme, all sewage generated from the main provided. The two steel urban areas on both sides of the Harbour will receive riser pipes, 75 metres long, adequate treatment and long-term improvement to the are installed and grouted water quality of the Harbour will ensue. into predrilled holes from a Figure 8 - Outfall Tunnel and Riser jack-up barge. Pipe

The diffuser pipeline comprises two sections of steel pipe from the central risers. The total length of the diffuser

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