Central Waters

Victoria Harbour - an iconic symbol of Hong Kong

In this report, the Central Waters refers to four different Water Control Zones: the Victoria Harbour, Eastern Buffer, Western Buffer and Junk Bay WCZs. These make up some of the busiest shipping waters in the world, and together form the heart of Hong Kong's famous Victoria Harbour.

The Central Waters have faced some real challenges in the past. Wastewater from both sides of the harbour was discharged into it after just simple screening, leading to marine water low in dissolved oxygen and high in organic nutrients and sewage bacteria. By 2001, the catchment area for Victoria Harbour housed a population of four million, which was generating 1.7 million tonnes of wastewater daily and placing great pressure on the harbour’s marine environment.

Stonecutters Island Sewage Treatment Works, which now treats around three quarters of the wastewater discharged into Victoria Harbour

In response, at the end of 2001 the Government implemented the Harbour Area Treatment Scheme (HATS) Stage 1 and commissioned the Stonecutters Island Sewage Treatment Works (SCISTW). One of the largest of its kind in the world, the treatment works receives 1.4 million tonnes of wastewater daily from Kowloon and the eastern parts of Hong Kong Island, representing around 75% of all wastewater from the Victoria Harbour catchment area. The wastewater is given Chemically Enhanced Primary Treatment at the treatment works, which removes about 80% of the suspended solids and 70% of the biochemical oxygen demand (organic matter), and the effluent is then discharged at the western end of the harbour via a 1.7 km submarine pipe.

The HATS Stage 1 initiative quickly brought significant and sustained improvements to the water quality of the Central Waters, especially that of the eastern Victoria Harbour, Eastern Buffer and Junk Bay WCZs. Levels of dissolved oxygen increased by 10% in these areas, while levels of nutrients and organic pollutants fell by between 10% and 30%. Also falling have been levels of E. coli bacteria, which has led to improvements in water quality at the bathing beaches on the eastern end of Hong Kong Island. The long-term trend of increasing E. coli levels reversed in eastern Victoria Harbour from 2002, and in the mid-Harbour from 2003. Total inorganic nitrogen levels also went into significant decline after 2002, particularly in the eastern part of the Harbour.

Increases in levels of dissolved oxygen around Victoria Harbour since the implementation of HATS (a comparison of 2002-2005 with 2000-2001)

However, improvements are less noticeable in the western harbour area due to the fact that this is where the HATS Stage 1 scheme discharges the treated effluent. Because the effluent has not been disinfected, and the current treatment only reduces bacteria levels by half, E. coli levels are raised in the vicinity of the outfall. As a result the western harbour (Western Buffer WCZ and northern part of the Southern WCZ), including the water around Tsing Yi and the Tsuen Wan beaches, has experienced increased E. coli bacterial counts.

The Government plans to implement HATS Stage 2A, which aims to disinfect discharges from the Stonecutters Island Sewage Treatment Works by 2009 (subject to community support for the recovery of operating costs through sewage charges). If this goes ahead, the western part of the Central Waters area should enjoy similar improvements to those experienced in the east. In particular, this would prepare a number of Tsuen Wan beaches for opening which are currently closed due to poor water quality.

By 2005, the Eastern Buffer, Junk Bay and Western Buffer WCZs had all attained 100% compliance with the key WQOs, and the overall compliance of the Victoria Harbour WCZ was 83%. This was in sharp contrast with 2001 when WQO compliance only stood at 59%. High WQO compliance in Victoria Harbour has been maintained in the four years since HATS Stage 1 was implemented. This has also lifted the territory's overall WQO compliance rate to sit consistently at 85% or above since 2002.

Sediment

Heavy metals and organic contaminants are very persistent in marine sediment. Sediments in Victoria Harbour have an elevated organic content and are highly anoxic with low electrochemical potential (due to sewage), as well as being contaminated with heavy metals.

Sediment contamination levels in the Victoria Harbour WCZ have always been a problem, apparently building up over time as a result of discharges from a number of industries between the 1960s and the 1980s. Kwun Tong, To Kwa Wan and Tsuen Wan were all sites of concentrated industrial activity during these decades, and the waste from some of these industries-in particular printed circuit board manufacturers, electroplaters, and businesses involved in photo developing-included heavy metals which ended up in the harbour. The implementation of the Water Pollution Control Ordinance and the Waste Disposal Ordinance have substantially reduced the amount of this type of contamination entering the harbour. In addition, many of these industrial concerns shut down or relocated in the 1980s and 1990s. Those that remained were required to remove pollutants from their wastewater before discharge.

For the period 2001-2005, copper (one of the metals commonly used in the PCB and electroplating industries) was found in the sediment of all stations in Victoria Harbour at levels that exceeded the Upper Chemical Exceedance Limit. Meanwhile, four heavy metals affected sediment in Tsuen Wan Bay at levels above the Upper Chemical Exceedance Limit: these were copper (Cu), silver (Ag), chromium (Cr), and nickel (Ni). On the other hand, levels of trace organics (PCBs and PAHs) in Victoria Harbour were low. Only one station in Victoria Harbour, near Sai Ying Pun, recorded slightly higher levels of PCBs.