A centralised primary sewage treatment plant adopting a chemical process using ferric chloride and capable of handling 1.7 million cubic metres of sewage each day on average for serving a population of 3.5 million has been built and commissioned as part of the Harbour Area Treatment Scheme (HATS) Stage 1 in Hong Kong.

Stonecutters Island Sewage Treatment Works

As a core component of the HATS Stage 1, the Stonecutters Island Sewage Treatment Works (SCISTW) adopts a compact design with the employment of the latest technology and equipment. The treatment works, which is capable of handling a sewage peak flow of 39.8 m³/s, is accommodated in a small footprint of only 10.6 hectares.

The treatment works provides chemically enhanced primary treatment and is, on average, removing 80% suspended solids and 70% biochemical oxygen demand from the raw sewage. The design average dry weather flow and peak flow are 20 m³/s and 39.8 m³/s respectively.

Stonecutters Island Sewage Treatment Works - Main Pumping Station

Inside the treatment works, the sewage collected and conveyed from the urban areas is first pumped from the deep tunnels into the inlet works of the Main Pumping Station (capacity 31.3 m³/s) completed under HATS Stage 1. Apart from this, the Northwest Kowloon Pumping Station (capacity 8.5 m³/s) also pumps the sewage from the neighbouring Northwest Kowloon Preliminary Treatment Works through a pumping main to the inlet works.

Flocculation Tanks

Imported concentrated ferric chloride solution (30%-45% ferric chloride by weight) is stored in cylindrical storage tanks. As a coagulation reagent, it is injected at the inlet works at prescribed dosages for in-lined mixed with the sewage in flow in the rapid mix tanks. The sewage then passes through the flocculation tanks to allow formation of flocs, which are kept in suspension with the aid of air diffusers. After that, polymer, diluted with potable water, is injected into the sewage to facilitate deposition of flocs when the sewage passes through the sedimentation tanks.

Sedimentation

After flocculation, the sewage flows via the distribution channel into the rectangular sedimentation tanks. In order to save space, a double-decked configuration consisting of upper and lower compartments has been adopted in the design. When the sewage passes slowly (average velocity 0.6 m³/s) through the sedimentation tanks, the flocs settle at the bottom of the tanks as sludge and are removed by chain and flight sludge collectors into sludge hoppers. The lighter foam and floatable solids at the sewage surface are skimmed off by the same equipment.

The design surface loading rates of the sedimentation tanks at average dry weather flow and peak flow are 66 m³/m²/day and 132 m³/m²/day respectively. The average retention time of sewage is 1.3 hours.

Centrifuges inside the sludge dewatering building

The sludge collected from the hoppers of the sedimentation tanks, together with the floating scum, is pumped to sludge storage tanks prior to further treatment. In the treatment process, the raw sludge is first conditioned with polymer and then fed into centrifuges for dewatering up to a level acceptable for disposal at the landfill sites. The raw sludge drawn off from the sedimentation tanks with a water content of about 98% is dewatered to sludge cakes with a water content not higher than 68%.

Upon full commissioning of the HATS Stage 1, about 600 tonnes of dewatered sludge is produced and transported to landfill sites every day. Odour control facilities and sealed containers are used to minimise nuisance to the public throughout the treatment, handling and transportation process.

The treated effluent from the treatment works is currently discharged via an submanine outfall into the western approach of the Victoria Harbour. The effluent is dispersed into the tidal stream through 24 diffusers installed along the diffuser pipeline.

The operational control of the treatment plant is carried out by a sophisticated computer based system called Distributed Control and Data Acquisition System. The field instruments provided at selected locations transmit information on the sewage flow, system hydraulics and status of various aspects of the treatment process to a system control room for centralised monitoring and control. The system goes through a decision making programme and sends out signals to the various controlling devices to regulate the operation conditions.