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Reviving Our Fragrant Harbour

Members on the Environment Committee's tour of Stonecutters Island Sewage Treatment Works see firsthand how efforts to reduce the amount of sewage being dumped into Victoria Harbour are progressing

Five years ago, 1.5 million cubic meters of raw sewage sludge was being dumped into Hong Kong Harbour daily, making Victoria Harbour a disgrace to our world-class city ambitions and an environmental nightmare.

Today, the dreams of a cleaner harbour are starting to come true as half of the raw sewage from Hong Kong Island and certain areas in Kowloon now undergoes secondary treatment, a process that removes 85 percent of the pollutants and produces an effluent almost as clear as drinking water.

During the interim, billions of dollars have been spent to lay the world's deepest sewage pipe network to collect raw sewage from main urban areas of Kowloon and northeast Hong Kong Island and pump it to Stonecutters Island Sewage Treatment Works. This HK$2.04 billion plant, which went into operation in 1997, is a key element of stage 1 of the Strategic Sewage Disposal Scheme to reduce pollution in Victoria Harbour, under the Harbour Area Treatment Scheme (HATS).

"The total capacity of our pumping facilities is 39.75 cubic meters per second," Beaver Hui, Electrical & Mechanical Engineer, Sewage Treatment Division 2, of the Drainage Services Department, told 25 members who joined the Chamber's trip to the plant on October 30. "We are currently running at about 70 percent of capacity, and have reduced the amount of sewage being discharged into the harbour by 50 percent."

According to government figures, 95 percent of Hong Kong's population is served by sewage infrastructure. Treatment of most sewage before Stonecutters Island works, however, only involved "primary treatment," through which a process of sedimentation would remove around 30 percent of organic pollutants.  "Secondary treatment" treats sewage biologically, removing around 85 percent of organic pollutants.

"To give you an idea of how clean the water is after undergoing secondary treatment, it could be used to farm fish such as carp," Chu Wai-yuen, Senior Mechanical Inspector, Sewage Treatment Division 2, DSD, said.

The 25 members on the Environment Committee's tour were eager to see how the plant works, and how sewage is collected, treated and disposed of. Luckily for the group, a strong wind was blowing that day as members wandered along walkways over sewage treatment tanks.

Stonecutters island Sewage Treatment Works can chemically treat 1.7 million cubic meters of sewage per day, which passes through four main areas: pumping, sedimentation, chemical dosing and sludge treatment.

Mr Hui explained that sewage collected at preliminary treatment works in Kowloon and Hong Kong is first screened and degritted. It is then pumped through the world's deepest sewage pipe network into reception channels at the head of the sedimentation tank complex.

The sewage is then treated with ferric chloride and polymer, before passing through flocculation tanks designed to remove 70 percent of suspended solids and 35 percent of biochemical oxygen demand (BOD). While the primary treatment is a settling process, the secondary process uses oxygen to promote the growth of microorganisms which consume solids in the wastewater. The microorganisms increase their density and then settle out to the bottom of the tank. The whole process, from the time it enters the plant until it is discharged as secondary treatment sewage near Lantau Island takes around two and a half hours. Sludge and scum collected during treatment is shipped in sealed containers and dumped in landfill sites around Hong Kong.

James Graham, Chairman of the Chamber's Environmental Committee, who led the trip, said the facility represents a giant leap forward in the treatment of sewage in Hong Kong. But he stressed that a lot more still needs to be done.

Members questioned why treated water was not used to clean streets or for irrigation, and why sludge was dumped in landfills instead of being converted into fertiliser.

Mr Hui explained that it all boils down to money. The government heavily subsidises water and sewage charges in Hong Kong, paying about $2 for every $1 that Hongkongers pay.

"We could further treat the water and sludge to recycle it," he explained. "In Australia, for example, they burn the sludge to make paving flags. We could do that or turn it into fertilizer. The problem is these processes are very expensive and energy intensive and we just don't have the money to do that." 

Mr Graham said that it is very easy to criticise and say more should be done to further treat the effluent and even recycle it. "But you have to keep in mind that this [Stonecutters Island Sewage Treatment Works] is already a major achievement in reducing the amount of sewage being discharged into the harbour. That said, we also hope the government will do more to treat the sewage that is not currently treated."

For more details on the project, visit the Drainage Services Department's Web site at, http://www.dsd.gov.hk

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