More advanced sewage treatment plants could have prevented 95,000 cubic metres of raw sewage from being discharged into the sea on Monday, a marine pollution expert says.
Professor Ho Kin-chung, dean of the Open University's school of science and technology, said that for the sake of the harbour, sewage plants and operations management must be upgraded.
This would include biological treatment that removes suspended solids and organic pollutants more efficiently, and improved networking among plants to ensure a backup in case of a system failure.
Only about 16 per cent of the city's sewage is treated this way.
Ho was responding to Monday's incident, in which Tuen Mun's Pillar Point treatment plant spewed sewage into the sea for six hours after a malfunction.
The leak prompted the closure of all 14 beaches in Tuen Mun and Tsuen Wan.
Ho said a more advanced secondary treatment facility would have provided more reserve tank space and more time for the plant operator to resolve the problem.
"They would not have had to discharge the waste out into the open so quickly," Ho said. "The lead time would have been a defence mechanism."
Ho warned that a similar malfunction at the main treatment works, on Stonecutters Island, could cause a much greater environmental disaster.
"If that plant breaks down like Pillar Point, we're toast," Ho said. "Not only will there be no more cross-harbour swimming, marine life will die off and Lamma Channel will be flooded with the waste of 3.5 million people."
Pillar Point and the Stonecutters plant provides only primary treatment - the filtering of suspended solids - with chemicals added for a better effect.
The more advanced secondary treatment process involves biologically purifying sewage by decomposing organic matter with micro-organisms. This would also require larger, upgraded facilities.
Ho said it was rare for sewage works to be centralised, as other cities generally spread out such facilities for safety and efficiency.
"In most places, once capacity reaches about 500,000 cubic metres per day, they'll have to transfer it or build another plant."
The contractor at Pillar Point might have breached an emergency response plan for the treatment works concerning official notification, the South China Morning Post reported yesterday.
Roy Tam Hoi-pong, president of environmental concern group Green Sense, said: "Such an important piece of public infrastructure like Pillar Point should not be left to a contractor to run."
The Environmental Protection and Drainage Services departments are investigating.