ENG-2004 — Page 364

314 The Environment

The first stage, which collects sewage from the urban areas of Kowloon, Tsuen Wan, Kwai Tsing, Tseung Kwan O and the north-eastern part of Hong Kong Island for treatment at a sewage treatment plant at Stonecutters Island, was brought into full operation at the end of 2001. In addition to the treatment plant, which some experts considered as one of the most efficient of its kind in the world, the first stage comprises a number of preliminary treatment works at collection nodes, 23.6 kilometres of conveyance tunnels up to 150 metres deep, and a tunnelled outfall which disperses the treated effluent into the western anchorage area away from core Victoria Harbour.

Since commissioning, the system has performed well. The Stonecutters Island Sewage Treatment Works, which employs a chemical treatment process, has been consistently removing about 70 per cent of the organic pollutants, 80 per cent of the solids and 50 per cent of the sewage bacteria expressed as E. coli. Since the first stage actually treats about 75 per cent of the wastewater generated in the harbour catchment, it has thus reduced the total pollution load in the harbour waters by about 50 per cent. About 600 tonnes of sludge that would otherwise have gone into that harbour each day, is now effectively being removed at the treatment works and sent to modern landfills for disposal. This has brought about marked improvements in water quality. In the harbour area waters, the levels of sewage bacteria have declined by some 50 per cent overall, dissolved oxygen has increased by 10 per cent and ammonia has declined by 25 per cent, helping to restore water quality to a more natural and ecologically healthier condition. It is also encouraging to note that some soft and hard coral communities, which are very sensitive to the quality of the marine water environment, have started to revive in Victoria Harbour, according to a survey done by a local marine conservation society.

The original proposal for the subsequent stages of the scheme envisaged collection of the remainder of the untreated sewage around the harbour (from the northern and western parts of Hong Kong Island) using a similar system of deep tunnels, centralised treatment at the works at Stonecutters Island, and eventual discharge of the treated effluent, after disinfection, through a long tunnelled outfall into the deep, oceanic waters to the south of Hong Kong.

This concept was reviewed in 2000 by a panel of international experts. In their report to the Government, the experts confirmed the use of deep tunnels and centralised treatment as an acceptable and cost-effective way of dealing with sewage. However, in the light of advances in sewage treatment technologies in recent years, they recommended that instead of pursuing the long outfall, the Government should consider upgrading the treatment level to a very high standard, and discharging the treated effluent into the harbour on a long-term basis.

The proposal to adopt a different technology and arrangement for the treatment of sewage raised a number of issues that required further study. The most important of these was whether the compact sewage treatment technology developed and applied in temperate climates could treat wastewater effectively under Hong Kong conditions, and whether it would be an environmentally sustainable proposition for