Sewage experts launch pilot to speed treatment

PUBLISHED : Monday, 29 December, 2008, 12:00am
UPDATED : Monday, 29 December, 2008, 12:00am

The processing capacity of sewage-treatment plants can be increased by 45 per cent just by tipping plastic granules into dirty water - which a government engineer likens to 'building high-rises for bacteria'.

The system is being tried for the first time in Hong Kong, although it has been used elsewhere for more than two decades.

The most critical and timeconsuming step in the biological sewage-treatment process is breaking down organic matter in the sewage with bacteria.

The system now in use works by keeping the bacteria in a suspension that limits the number of the micro-organisms in tanks and slows down the degradation process.

But pouring more than 30 million plastic granules - known as biofilm carriers - into a 210-cubic-metre sewage tank can provide more surface area for bacteria to grow, Drainage Services Department chief engineer Lo Kin-hung said.

'The more bacteria in the tanks, the faster they can degrade the organic matter in the sewage,' said Mr Lo, likening the process to 'building high-rises for bacteria to live'.

He said the technology could increase the plant's daily capacity from 5,800 to 8,400 cubic metres, without expanding the treatment facilities.

The new system - including the biofilm carriers and an air blower to supply oxygen to help the bacteria grow in the water - costs HK$3 million, much cheaper than building further plants.

The technology also helps cut the amount of nitrogen in the effluent, which reduces the incidence of red tides, said Mr Lo. Red tides are caused by sudden proliferation of algae, which live in the ocean and produce toxins that can kill marine life.

The effluent from the Stanley sewage plant would affect five beaches, including Stanley Main Beach.

Mr Lo said the technology could be applied to the seven plants in the city that used biological treatment. Hong Kong has more than 70 sewage-treatment plants.

The pilot will run until 2010.