Shatin Sewage Treatment Works Expansion and Upgrade, Hong Kong

Scheduled for completion in 2010, the latest programme of improvement at the Shatin Sewage Treatment Works – the largest wastewater plant in Hong Kong – will see its capacity extended and the disinfection regime enhanced and upgraded.

Capacity at the Shatin works will be increased to 340,000m³ a day to enable it to meet the rising demands of Shatin New Town, Ma On Shan New Town and the surrounding villages, while UV disinfection will be added to the plant to improve water quality. Hong Kong Drainage Services awarded the contract for the UV system to Jardine Engineering in June 2008.

The UV element cost – which also includes providing a similar facility at the Tai Po STW – amounts to some HK$124m. The whole expansion project budget is HK$2,425m.

Shatin Background

Located directly north of Kowloon, Shatin New Town is one of the fastest growing urban areas in Hong Kong's New Territories.

In the early 1970s, the area was a rural township and home to about 30,000 people; today there are more than 650,000 inhabitants and the burgeoning population has led to increasing demands on the water and wastewater infrastructure.

The Shatin works was designed to be rolled out in three stages. Stage I was first commissioned in 1982 with stage II following in 1986; in both cases, the original dry weather flow capacity was 205,000m³ a day.

In 1991, a plant upgrade improved nutrient removal, extending the original eight-hour aeration tank detention period by a further three hours, but this had the effect of decreasing the facility's capacity to 150,000m³ a day.

The subsequent stage II extension included the construction of new primary and sedimentation tanks, additional final sedimentation tanks, grit classifiers, an aerated grit channel, sludge and drainage pumping stations together with associated ancillary elements.

Work undertaken during stage III involved the rehabilitation and further extension of the plant to meet the effluent standards established by the Hong Kong Government. This comprised a series of wide ranging improvements including the grit removal system, chemical dosing, sludge collection and removal, aeration system, sludge thickening, sludge digestion and dewatering system, along with SCADA and process monitoring.

Stage III was completed in September 2004, having taken 41 months to complete.

The current project follows on from this, being designed to extend plant capacity further and improve water quality at Kai Tak Nullah – an urban stormwater drainage channel in south-east Kowloon, upstream of the old Kai Tak Airport.

Plant design

The plant design is conventional, preliminary treatment comprising coarse screening and grit removal. With the coarse solids excluded, the flow next enters the primary sedimentation tanks, allowing most of the suspended solids to settle out, the primary sludge being subsequently removed.

The design specification removal of 50% of the suspended solids and 40% of the BOD5 is achieved by the end of the primary sedimentation process.

Shatin uses a conventional activated sludge process to provide secondary treatment, with recently upgraded aeration diffusers to transfer oxygen to the biomass mix on a continuous basis.

Effluent from the aeration tanks flows to the final sedimentation tanks; the sludge is allowed to settle out and the return activated sludge cycled back to the aeration tanks, while the excess sludge is transferred to the sludge treatment facilities.

Sludge stabilisation at the plant involves anaerobic digestion. Primary and activated sludges are treated in mesophilic digesters running at a fairly constant 35°C, which break down the volatile organic matter to produce a stable sludge, which is subsequently dewatered via centrifuges before being transported off-site for disposal in landfill.

The biogas produced during the process fires an on-site generator to provide power for the plant and hot water to maintain digester temperature.

Once completed in December 2009, the new UV facility will provide the plant with a state-of-the art disinfection system to ensure improved effluent quality entering the Kai Tak Nullah – and specifically to reduce levels of E coli and other coliform bacteria.

Key players

Hong Kong Drainage Services Dept is the plant owner and awarded the UV disinfection contract to Jardine Engineering in June 2008. Other contractors involved in the work include Biwater, Leader Civil Engineering, Sun Fook Kong, Hydrotech Professional, Hang Kee Construction & Engineering, ATAL Engineering Ltd and the China Railway Engineering Corporation.