An Unusual Island

Dear Harbour,

In our earlier letters, we shared with you how the HATS project began, why the centralised option was chosen, and how a labyrinth of deep sewage tunnels was successfully constructed despite enormous engineering difficulties. This crucial work took place from the early 1990s onwards as part of HATS Stage 1 to tackle increasingly serious pollution in your Harbour waters.

The centralised sewage treatment works on Stonecutters Island is the beating heart of HATS: fed by the deep tunnels that are its arteries, it treats the city’s sewage and discharges the treated sewage into the western approach of the Harbour.

Stonecutters Island

Since 1997, when Hong Kong became a Special Administrative Region of the People’s Republic of China, Stonecutters Island has housed naval facilities as well as a number of civil facilities, including the newly commissioned Stonecutters Island Sewage Treatment Works (SCISTW). Also on the island is the pre-existing North West Kowloon Preliminary Treatment Works (NWKPTW), which serves Sham Shui Po, Yau Ma Tei and Cheung Sha Wan.

Stonecutters Island was chosen to house the centralised sewage treatment works for an obvious reason: its central location in relation to all the Preliminary Treatment Works (PTWs) on both sides of the Victoria Harbour covered by the HATS plan keeps the length of the deep tunnels to a minimum. Although SCISTW was designed to ultimately serve the needs of 5.7 million people, the area designated for the entire treatment works was only 10.6 hectares, just about half the size of Victoria Park. The result is a world-class, compact and highly efficient sewage treatment works.

Overview of Main Pumping Station No.1

Stonecutters Island Sewage Treatment Works

The SCISTW is a key component of HATS Stage 1 and provides centralised chemically enhanced primary treatment (CEPT) to preliminarily treated sewage collected from the deep tunnel system. SCISTW is designed to handle average dry weather flow of 1.7 million cubic metres per day. HATS Stage 1 is intended to serve about 3.5 million people, with further HATS stages taking that number up to 5.7 million.

The SCISTW consists of six main facilities, all constructed during HATS Stage 1, including: Main Pumping Station No. 1, Sedimentation Tanks Facility, Chemical Dosing Facility, Sludge Dewatering Facility, Power Supply and Control System, and Stage 1 Outfall System.

Stonecutters Island Sewage Treatment Works HATS Stage 1 Layout Plan

Main Pumping Station No. 1

The large Main Pumping Station No. 1 (MPS1) is designed to lift the sewage from the deep tunnel system and deliver it to downstream treatment facilities above ground. For compactness, the MPS1 is circular, with a diameter of 50 metres and a depth of over 38 metres. The wet well is located at the periphery, while the central space houses the pumps, motors and control room. The pumping system comprises eight gigantic centrifugal pumps, together with four variable speed drives for energy efficient operation.

Main Pumping Station No.1 under construction

Sectional View of Main Pumping Station No.1

Main Pumping Station No. 1 inlet chamber

Main Pumping Station No. 1 motor hall

Sedimentation Tanks Facility

The Sedimentation Tanks Facility is designed with double-decked tanks so as to halve its footprint. The incoming sewage first goes through a rapid mixing process where ferric chloride is introduced. It then enters a flocculation zone where small particles are allowed to clump together with the aid of polymer to form floc for easier removal.

Following flocculation, pollutants in the sewage can be removed as sludge, while floatable solids can be removed as scum, leaving the effluent ready for discharge. The CEPT process at SCISTW is capable of removing about 80% of suspended solids and 70% of biochemical oxygen demand from the sewage, thus reducing bacteria growth.

Double-decked sedimentation tanks

Chemical Dosing Facility

The Chemical Dosing Facility provides storage and dosing facilities for adding ferric chloride and polymer to sewage for the subsequent sedimentation process. Highly automated, and capable of dosing ferric chloride from 10 milligrams per litre (mg/l) to 20 mg/l, the facility ensures round-the-clock supply of these essential chemicals for the CEPT process.

Chemical dosing facility

Ferric chloride storage tanks

Sludge Dewatering Facility

Sludge and scum collected during sedimentation are conveyed to the Sludge Dewatering Facility (SDF). When the sludge and scum, collectively called sludge, leave the sedimentation tanks, it has a dry solids content ranging from 2% to 4% by weight. Five sludge storage tanks are used to temporarily store the pre-dewatered sludge.

At the core of the SDF are ten centrifuges which dewater the wet sludge to a minimum of 32% dry solids by weight to meet the disposal requirement. Polymer is added as a coagulant to enhance the process. After dewatering, sludge cakes from the centrifuges are transported to one of the four sludge cake silos by conveyors before container loading. The sludge cakes were disposed at the South East New Territories Landfill and the West New Territories Landfill. The liquid removed during the dewatering process, which is known as centrate, is returned to the MPS1.

Sludge dewatering facility

Centrifuges in sludge dewatering facility

Power Supply and Control System

To ensure the smooth, round-the-clock operation of SCISTW, stable and uninterrupted power supply is vital. Power supply to SCISTW is a dual-feed system comprising two 11-kilovolt cables from different primary substations at Container Port Road and Ha Kwai Chung, an arrangement which ensures reliable electricity supply and minimises the risk of an outage due to cable damage.

The highly automated Distributed Control and Data Acquisition System controls the various process systems under HATS Stage 1 at SCISTW. Each independent facility control systems are linked by the System Information Network. Operator Work Stations at each facility allow operators to monitor the system operation and performance.

Overall, SCISTW is highly efficient considering its large treatment capacity versus the small number of staff required.

Control room in Main Pumping Station No. 1

Outfall System

Effluent gravitates to the outfall and is discharged to the tidal stream southwest of Stonecutters Island for dilution and dispersion. The outfall tunnel is 1.7 kilometres long with a 1,200-metre long diffuser pipeline.

For optimal diffusion effect, two sections of steel diffuser pipeline are extended from the central risers. The pipeline has a total of 24 diffusers, each with eight discharge ports, to maximise dispersion. It is also protected from possible ship anchor damage by a rock layer and by resilient polyethylene domes over the ports.

Longitudinal Profile of Outfall

Diffuser head with protective dome

Diffuser Details

North West Kowloon Pumping Station

Adjacent to the SCISTW is the pre-existing NWKPTW which serves Sham Shui Po, Yau Ma Tei and Cheung Sha Wan. As the NWKPTW is right next to the SCISTW, we built and commissioned in 1997 the North West Kowloon Pumping Station (NWKPS) under Stage 1 to convey the effluent from NWKPTW directly to the sedimentation tanks, without going through the deep tunnel system.

All Set on the Island

Thus, by late 2001 when all the key SCISTW facilities were completed and ready to roll, Stonecutters Island had been completely transformed. The island had become the “heart” of a powerful sewage treatment system that would prove highly effective in cleaning up our Harbour and improving the quality of life for millions.

Completed SCISTW in HATS Stage 1

POSTSCRIPT

“The E&M facilities of HATS have many characteristics. Its Main Pumping Station No. 1 (MPS1) for example, is the first pumping system in Hong Kong that lifts water from very deep tunnels. For the double-tray sedimentation tanks, a special engineering solution is required to pump away the sewage in the upper tray before maintenance of the lower tray can proceed. As to the sludge dewatering facilities, as Hong Kong’s sewage contains grit, the blades of the early centrifuges were seriously worn out. After discussions with the manufacturers, we re-designed the blades and replaced the centrifuges, thus solving the problem.”

“During Stage 2A design stage, we decided to construct the Main Pumping Station No. 2 (MPS2) to handle the newly increased sewage volume, and build an underground tunnel to connect MPS1 with MPS2. The tunnel will facilitate future maintenance when sewage from one pumping station can be diverted to the other to ensure continuous operation of the sewage treatment plant. In fact, we just completed a three-month major maintenance of the MPS1 at the end of 2018, the largest-scale maintenance exercise in 20 years. Both pumping stations and the interconnection tunnel performed exactly as planned in the exercise.”

Norman SIU Ka-kam

Former Assistant Director/E&M, DSD