Deep tunnels finally reach Stonecutter

Fourteen years ago Hong Kong embarked on a massive and radical scheme to deal with the problems of handling the sewage and wastewater from its rapidly expanding population.

The major urban areas of Hong Kong lie on either side of the channel between Hong Kong Island and Kowloon peninsula, which houses much of the industrial activity.

In 1987 the government commissioned a study which resulted in three key recommendations: legislation to control the quality of industrial wastes at source; upgrading of local sewerage systems; and, most important, the provision of a comprehensive sewerage network.

World's deepest outfall
This Strategic Sewage Disposal Scheme (SSDS) was based on the construction of a series of tunnels deep in the rock to intercept and transport the sewage from the existing smaller works to two primary treatment works.

In the early '90s the project was divided into four phases. Stage I consisted of tunnels from the north east point of Hong Kong Island along the southern and western edge of Kowloon, to a new sewage treatment plant on Stonecutter Island, off the western coast. After treatment, effluent would be discharged initially through an outfall to the west, but Stage II covered construction of a long sea outfall to the south. Stages III and IV related to a further series of deep tunnels along the northern coast of Hong Kong Island, plus a second sewage treatment plant housed in a series of caverns.

The Drainage Services Department of the Hong Kong Government was given responsibility for carrying out the scheme, aided by Montgomery Watson as consulting engineers

Land is not easy to come by in Hong Kong, and the site of the first of the new sewage treatment plants was limited to 10.6 ha. There was insufficient room for conventional sedimentation tanks, and it was proposed that the works should utilise an innovative system of 38 two-tray sedimentation tanks, each 60m long by 7m wide, and capable of handling a surface loading rate of 60 m3/m2 per day. Lime dosing to deal with toxic metal contamination from the many small industries in the Kowloon area was also considered, but by '94/95, CEPT using ferric chloride was employed as the permanent process for treating Stage I flow.

The SWT works on Stonecutter Island were commissioned in mid-1997, and by June 1999 the 1,750m long interim outfall tunnel had also been completed. Lying about 90m below the seabed this tunnel culminates in twin 3.25 m dia. riser shafts through which the effluent rises to a seabed pipeline equipped with a series of 24 diffusers.

Construction constraints
Construction of the network of deep collection and transfer tunnels to the new SWT was to prove less straightforward.

The Stage I tunnel, 23.5m in overall length, was divided into six sections, and lies some 80-150m below the surface. With the exception of the most northerly section which was constructed in drill and blast, all have been built using tunnel boring machines to minimise surface disruption.

The six sections were grouped into two contracts, both of which were won by a joint venture of the French company Campenon Bernard and Maeda of Japan, with fixed price bids of HK$490M and HK$694M. The contracts were awarded at the end of December 1994, with a completion date of mid-1997, and the contractor began work on 3 January 1995.

Disputes and delays
Progress, however, was disappointingly slow. As work fell further and further behind schedule relations between the Drainage Services Department and contractor became increasingly strained. The situation came to a head in mid-summer 1996, when the contractor stopped work, complaining that water inflows were much worse than had been allowed for, and it would be dangerous to continue. Prolonged negotiations failed to sort out the situation, and in December the contractor was removed from site.

The Drainage Services Department found itself faced with several tunnels in various stages of completion. The six tunnels were split into three contracts, and new bids were called from all those who had earlier expressed interest in the project.

Skanska took over the northern two, the central sections going to a joint venture of Gammon Construction and Kvaerner, and the most southerly sections being carried out by a joint venture of Paul Y Construction and Societa Esecuzione Lavori Idraulica SpA of Italy.

Stage I tunnels are now nearing completion, and by the end of this year it is expected that the sewage treatment works on Stonecutters Island, which has up to now been processing only about 25% of the anticipated flow, will be receiving its full design capacity.

The initial slow progress, negotiating procedures, rebidding process, and the necessary overhaul and refurbishment of the equipment which the new contractors were taking over has, however, inevitably led to serious delays, and costs, which were originally estimated at HK$6.8BN lion are currently up to HK$8.3BN.

Even at an earlier stage in the project there had been complaints from some groups that the levels of treatment proposed in the SSDS were insufficient, and criticisms of the Stage II proposal for a long sea outfall, which would have been the longest and deepest in the world. The delays, high costs and technical difficulties being experienced on the Stage I tunnels have added weight to the argument.

Following a second look at the scheme, an International Review Panel was set up to review the possible options, based on experience with Stage I.

The IRP reported on 30 November last year with new recommendations, suggesting that tertiary treatment facilities could be incorporated at the Stonecutter Island STW by using biological aerated filters (BAF). If this was done, effluent could be safely discharged through the existing outfall and the long sea outfall to the ocean south of Lamma Island would not be needed.

The panel also reviewed four options which combined a revised network of deep tunnels, BAFs and short outfalls for the later stages. This revised strategy would, suggested the IRP, result in faster completion and lower construction costs than the original proposals. The panel also suggested that a Design, Build and Operate approach be adopted for these subsequent works.

The report has been considered by Hong Kong's Legislative Council, which has taken a generally open- minded view of the proposals. However, it has announced that before making any further moves, some assumptions need to be tested - in particular the effectiveness of BAF technology with Hong Kong's saline sewage and, the levels of treatment needed to eliminate the need for the long sea outfall. Cost implications, the availability of land, and long-term effectiveness also have to be considered.

In March this year the Government of HKSAR announced it would invite manufacturers to offer pilot plant and a series of studies to assess the implications of the IRP's suggestions will be set up. The tests are expected to take about two years.

It also announced that it was dropping the term Strategic Sewage Disposal Scheme as no longer appropriate. The project has been renamed the Harbour Area Treatment Scheme or HATS.

Even working at only 25% of the anticipated flow, the plant on Stonecutter Island has been treating about 320,000m3/day of sewage, and reducing the suspended solids load to Victoria Harbour from 53 to 10t/day. When the Stage I tunnels are finally completed later this year, some 70% of the sewage entering Victoria Harbour will be being treated, and studies have been put in place to deal with the remainder.


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Waste & resource management

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