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Billions of liters of sewage pass daily through a barely visible urban network, formed by deep tunnels, high-capacity pumps, chemical treatment, and special vessels, in a city where the lack of space led Hong Kong to concentrate part of the sanitation on Stonecutters Island.
Hong Kong concentrates on Stonecutters Island a station capable of treating up to 2.45 million cubic meters of sewage per day, a volume equivalent to about 2.45 billion liters.
The structure occupies a restricted area and is part of the Harbour Area Treatment Scheme, a program created to collect, treat, and disinfect the sewage generated on both sides of Victoria Harbour before discharge.
Designed for a city with limited land availability, the Stonecutters Island Sewage Treatment Works operates on about 10 hectares, an area compared to approximately half of Victoria Park.
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According to the Institution of Civil Engineers, the plant was sized to serve a population of up to 5.7 million people, considering the maximum capacity of the treatment system.
Before reaching the tanks, the sewage passes through preliminary facilities and travels through underground tunnels connected to the HATS system.
At Stonecutters Island, the flow is elevated by large pumps and proceeds to the treatment stages.
Deep tunnels carry sewage to the HATS system
The HATS replaced more dispersed discharges around the harbor with a centralized structure for collection, treatment, and disinfection.
The network receives sewage from urban areas of Kowloon and Hong Kong Island and directs it to Stonecutters Island.
This configuration reduced the direct discharge of sewage without adequate treatment into Victoria Harbour.
According to the Drainage Services Department, the system contributed to improving the water quality of the harbor after the implementation of the program’s stages.
The operation relies on deep tunnels that conduct the sewage to the station.
In publications about the HATS, the Drainage Services Department reports that the underground network totals 44.6 kilometers and reaches 163 meters below sea level at its deepest point.
Upon arrival at the plant, the flow is driven by Main Pumping Station No. 2.
The structure has an internal diameter of 55 meters, a depth of 40 meters, and houses eight large pumps.
Each pump has a nominal capacity of 4.4 cubic meters per second.
In the comparison used by agencies related to the system, this volume would allow filling an Olympic pool in approximately one minute.
Sewage treatment uses CEPT chemical process
After pumping, the sewage goes to sedimentation tanks that apply the chemically enhanced primary treatment, known by the acronym CEPT.
The process uses ferric chloride and polymer to promote the aggregation of particles and facilitate the separation of solids.
The mixture passes through rapid mixing chambers and flocculation tanks, where fine particles join into larger flocs.
Subsequently, the material is distributed through sedimentation tanks, where the heavier fraction settles at the bottom as sludge.
While the sludge is removed by collectors and directed to specific structures, foams and floating solids are removed from the surface.
The CEPT allows for the reduction of about 80% of suspended solids and 70% of the biochemical oxygen demand in the treated sewage.
The biochemical oxygen demand is an indicator used to measure the organic load present in the sewage.
In the case of the Stonecutters Island Sewage Treatment Works, the stage reduces part of the pollution before the effluent returns to the environment.
The station is not intended to produce drinking water.
Its function is to return to the harbor an effluent subjected to treatment and disinfection, with reduced solids, organic matter, and bacterial contamination compared to the material received.
Effluent disinfection occurs before discharge
After sedimentation, the effluent undergoes disinfection with sodium hypochlorite.
Then, the treated liquid remains for about 30 minutes in an effluent tunnel, the period expected for the product’s action before the neutralization stage.
Before discharge, the system applies sodium bisulfite to neutralize chlorine residues.
Only after this stage is the effluent sent to the submarine outfall, exiting on the west side of Victoria Harbour.
Disinfection reduced bacterial presence in the treated material, according to the Drainage Services Department.
The agency reports that the system removes more than 99% of E. coli, as well as 70% of biochemical oxygen demand and 80% of suspended solids.
Measurements cited by the department also record an improvement in the water quality of Victoria Harbour after the implementation of HATS.
Among the indicators mentioned are the reduction of bacterial contamination and the recovery of conditions associated with dissolved oxygen.
Sewage sludge is transported by ships to T·PARK
In addition to the liquid effluent, the treatment generates sludge removed from the sedimentation tanks.
On Stonecutters Island, this material undergoes dewatering in centrifuges, a stage that reduces moisture and increases the concentration of solids before transport.
The dewatering facility can handle about 1,200 tons of sludge per day, according to a technical publication by the Hong Kong Institution of Engineers.
The complex also has centrifuges and silos to store the sludge cakes produced at the station.
Hong Kong adopted maritime transport to take the dewatered sludge to T·PARK, in Tuen Mun.
Two vessels built for this route, called Clean Harbour 1 and Clean Harbour 2, transport the material in sealed containers.
The maritime route reduces the need for transport by land routes, according to the Hong Kong Institution of Engineers.
At T·PARK, the sludge is incinerated and the heat from the process is used to generate electricity.
T·PARK is designed to handle 2,000 tons of sludge per day.
The facility combines sludge reception, incineration, energy recovery, and treatment of gases generated in the process.
Stonecutters Island concentrates sanitation in a limited area
The limited area of the Stonecutters Island Sewage Treatment Works directly influenced the design of the station.
To concentrate treatment capacity in a smaller area, the plant uses sedimentation tanks in a two-story configuration.
The operation includes deep tunnels, pumping, chemical treatment, disinfection, dewatering, and maritime transport of the sludge.
This sequence allows a significant portion of Hong Kong’s sanitation to operate out of the everyday view of the city.
In Victoria Harbour, known for its skyline and vessels, the underground infrastructure is directly related to urban sewage control.
The material that exits through drains and pipes goes through an industrial chain before returning to the environment as treated effluent.
The Stonecutters Island Sewage Treatment Works represents a technical response to the challenge of treating large volumes of sewage in a city with limited land.
In Hong Kong, this operation combines tunnels, compact tanks, high-capacity pumps, and a disposal route that directs the sludge for energy recovery.
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