Overall Strategy and Programme

New Works

Operations and Maintenance of Sewerage Systems

Operations and Maintenance of Sewage Treatment Facilities

Operations and Maintenance of Submarine Outfalls

Harbour Area Treatment Scheme (HATS)

Ultra-Violet Radiation (UV) Disinfection Facility at Shek Wu Hui STW

Overall Strategy and Programme

Every day, over 2.3 million cubic metres of domestic and industrial wastewaters are generated in HK. This huge quantity of sewage continues to rise with the population growth and needs to be properly collected and treated before its disposal to the sea in order to protect the marine environment.

The long term strategy for collecting, treating and disposing of the wastewater generated by the community to meet the water quality objectives was established by the Sewage Strategy Study completed in 1989.

The study recommended substantial improvements to the public sewerage facilities for the whole territory. As the urban centres around Victoria Harbour generate more than 70% of sewage in the territory, the Harbour Area Treatment Scheme (HATS) Stage I is being implemented to address the water pollution problem in the harbour. Sewage is to be collected and transferred to a centralized sewage treatment plant at Stonecutters Island (SCI) by deep tunnels where it will receive chemically enhanced primary treatment (CEPT) before discharge to the western harbour.

Apart from HATS, improvements to the sewerage system in the territory are being carried out on a catchment-by-catchment basis in which the territory is divided into 16 Sewerage Master Plan (SMP) areas. In each of these areas, the existing sewerage network is extended and improved, and existing sewage treatment works (STWs) are upgraded or new facilities are constructed so as to ensure sufficient capacity for conveying and treating sewage generated today and in future developments.

New Works

DSD is responsible for implementing the sewerage improvement programme. Construction of the sewage treatment plant at Stonecutters Island and the outfall under HATS Stage I commenced in 1994 and have been operational since June 1997. Despite many difficulties encountered in the deep tunnelling works, all seven sewage collector tunnels were successfully excavated in November 2000. We are expecting full commissioning of whole Stage I by early 2002, by then the pollutants going into Victoria Harbour will be greatly reduced.

SMP works, which started in 1989, are progressing well. Two SMPs, namely the HK Island South and Chai Wan & Shau Kei Wan were completed, and the East Kowloon SMP is also largely completed. The other 13 SMPs are under various stages of design or construction, and are scheduled for completion in phases by 2009.

Upgrading of major STWs at Shatin, Tai Po and Shek Wu Hui will continue so as to keep pace with the developments and the latest water quality objectives. The upgrading works at Shek Wu Hui STW is scheduled for completion by 2001 while Stage III extension of Shatin STW had commenced in early 2001 and the planning work for Tai Po STW Stage V is well underway. However, in view of the latest population growth and housing demands, more sewage facilities and services would be required and reviews of the SMPs are being undertaken by EPD.

Brief descriptions of major sewerage projects are attached in Appendix D.

The total cost of the sewerage programme is about $31.4 billion, comprising $8.3 billion for HATS Stage I, $16.3 billion for SMP works and $6.8 billion for other associated works. So far, we have completed works amounting to $16.2 billion under the programme, another $15.3 billion of works is being planned for the next ten years. In 2000/01, the expenditure on sewerage projects was $1.8 billion.

Operations and Maintenance of Sewerage Systems

About 95% of the population is at present served by the public sewerage system with over 98% of the sewage produced being collected and treated. This system includes a sewerage network of over 1,320 km in length. As sewers are susceptible to blockage because of grease and solids in the sewage, DSD has implemented a preventive maintenance programme for carrying out regular inspection and cleansing of sewers to ensure that they function properly. In 2000, we have inspected 849 km of sewers, of which 569 km were cleansed, and removed 5,000 m³ of silt, costing a total of about $75 million.

To enhance the management of drainage and sewerage assets and the services to the public, DSD has completed digitization of all drainage records in 1996. This had enabled us to handle requests for drainage records efficiently. The system is being further enhanced so as to assist the planning of maintenance works and generate statistical reports on assets for better management of resources.

In 1997, we installed a Drainage Complaints Information System (DCIS) to replace the manual procedures for recording drainage complaints. All complaints received are directly input into the database so that progress on the follow-up actions can be monitored effectively. This system also serves to identify black spots for planning and review of the preventive maintenance programme.

Numbers of complaints on blockage of sewers received and dealt with in past years are shown in Appendix E.

As the Drainage Authority, DSD is responsible for vetting public and private developments with respect to their impacts on the sewerage system. We also provide advice, as necessary, on their connections to the public sewerage system. In 2000, we have processed 28,066 sets of sewerage-related submissions and carried out 182 connections for new developments.

Operations and Maintenance of Sewage Treatment Facilities

DSD is currently operating some 200 sewage handling facilities including over 130 pumping stations and about 60 STWs scattered over HK in which pollutants in sewage are removed by various treatment processes so to achieve the discharge requirements as specified in the discharge licences.

Location of major STWs is shown in Appendix F.

In terms of levels of treatment, preliminary treatment or screening basically removes large particles of 6 mm in diameter and above and grits of over 0.2 mm in size from the sewage. In primary treatment, further removal of pollutants is carried out by sedimentation. The CEPT plant at SCI makes use of chemical additives to speed up the sedimentation process and, hence, achieves higher and faster removal of pollutants than the normal primary treatment plants. Secondary treatment plants provide biological treatment to sewage in which organic matters are converted to stable materials by bacterial activities.

In 2000, we have treated a total of 858 million m³ of which 67% of sewage receives preliminary treatment, 14% receive CEPT treatment and 19% receive secondary treatment.

Prelimintary 67% CEPT 14% Secondary 19%

The sludge arising from the sewage treatment process is currently dewatered to a minimum of 30% dry solid content before final disposal at sanitary landfills. In 2000, a total of around 140,000 tonnes of dewatered sludge and 22,000 m³ of screenings and grit were disposed of.

In addition to all the proactive measures taken to ensure effective operation of our sewage treatment facilities, the department has designated engineers to carry out research and development work with a view to further improving our services, such as finding solutions for operational problems in sewage treatment, and keeping abreast of the latest technological development in the field.

To minimize breakdown of sewage treatment facilities, we have been applying advanced computer software to administer the day-to-day maintenance on the electrical and mechanical equipment in the major sewage treatment works. Proper maintenance of buildings and concrete structures in STWs is equally important and the task is managed by a dedicated team of engineers. Regular inspection and repair, if necessary, of structures is carried out to ensure integrity of structures and proper functioning of the plants. Apart from these, landscaping work including planting of 39,000 trees/shrubs, re-painting of 12 plants and cleansing of 180,000 m² of structures had been carried out during the year in order to upkeep and beautify the appearance of these plants.

Operations and Maintenance of Submarine Outfalls

There are a total of 43 submarine outfalls and two effluent disposal tunnels built for effective disposal of effluent. To prevent pollution of the receiving water bodies caused by the leakage of effluent through cracked submarine outfalls, inspections and maintenance of the outfalls are carried out regularly. One of the effective means adopted for monitoring the performance of the outfalls is by helicopter inspection of dye test on the outfall pipes. Dye tests on 37 effluent outall pipes at 25 plants have been carried out during the year, and 18 detected leakage were subsequently repaired. In addition, underwater inspection, hydrographic sounding survey, regular flushing and desilting of outfall have been carried out as scheduled in a comprehensive preventive maintenance programme.

Harbour Area Treatment Scheme (HATS)

In 1989, the Sewage Strategy Study recommended the Strategic Sewage Disposal Scheme [now renamed as HATS to collect and convey all wastewater from the urban areas surrounding Victoria Harbour through a deep tunnel system to one or two centralized sewage treatment works for treatment, before final disposal to the waters south of HK.

The scheme was divided into four stages for implementation. Stage I focused on collecting and conveying the sewage from Kowloon and northeastern HK Island to SCISTW for treatment, while Stages III and IV aimed to include the sewage from the northern and southwestern parts of HK Island. Stage II of the scheme was designed to provide a higher level of treatment for all Stages I and III/IV sewage and to discharge the effluent away from the harbour in order to meet the environmental standards.

Stages I works consisted of SCISTW, a 1.7-km long, 5-m diameter outfall tunnel and a 23.6-km long sewage conveyance system. The CEPT plant and the outfall were completed and have been in partial operation since May 1997. The chemical treatment process adopted at SCISTW achieves removal of pollutants close to 90% that of a secondary treatment plant. The tunnel system consists of seven deep tunnels of finished diameters from 1.2 to 3.54 m and depths at 76 to 143 m below sea level were designed to have a minimum sound-rock cover of 30 m. Except for the shortest tunnel, from Kwai Chung to Tsing Yi, which was constructed by drill and blast method, hard rock tunnel boring machines (TBMs) were used for excavating six other tunnels.

Construction of the deep tunnel system started in early 1995 but progress of the works were seriously disrupted in mid 1996 when the tunnel contractor unilaterally suspended all tunnelling works that finally led to forfeiture of the tunnelling contracts in December 1996. Subsequently, the remaining works were re-tendered and re-commenced in July 1997.

Tunnel progress was also affected by the adverse geological conditions encountered during excavation. Weak ground conditions associated with joints, faults, dykes and weathered zones in hard rock formation have caused significant ground stability problems and required extensive ground stabilization measures as tunnelling proceeded. In addition, these geological discontinuities often connected the tunnels to groundwater sources in the upper geological strata, leading to very large groundwater inflows into various tunnel sections that necessitated substantial water control measures including pre- or post-grouting and placing of in situ concrete lining.

Apart from forfeiture of contracts and adverse geological conditions, the extensive works involved in replacing and repairing the broken TBM drive shaft and the faulty spoil mucking systems had also caused major delays to the tunnelling operations.

Despite all these problems and difficulties, construction of Stage I tunnels achieved a major milestone in November 2000 by boring through the last tunnel. The target is to complete the entire Stage I works by the end of 2001, with full operation of the scheme achieved in early 2002. By then, over 900,000 m³ of sewage flows, currently entering the harbour each day, will be conveyed by the deep tunnels to SCISTW for chemical treatment before disposal. This will reduce greatly the amount of pollutants entering the harbour.

Ultra - Violet Radiation (UV) Disinfection Facility at Shek Wu Hui STW

To further improve the water quality of the receiving water at Deep Bay, the Environmental Protection Department (EPD) will impose a new requirement on E.Coli discharge in the licence of the Shek Wu Hui STW. The new requirement will be 1,500 E.Coli count/100 ml on 95 percentile basis with a monthly geometric mean of 100 E.Coli count/100 ml. In order to fulfil the discharge standard, it is necessary to install a new disinfection system in the plant.

Ultra-Violet (UV) radiation and chlorination have been widely used in DSD's small sewage treatment plants for disinfection of treated effluent. The effectiveness of UV disinfection and chlorination depends mainly on the UV transmittance and contact time of effluent respectively. With the more stringent requirement set by EPD on the total residual chlorine (TRC) in the effluent, there is a trend to shift the disinfection process from chlorination to UV radiation.

The new UV disinfection facility at the Shek Wu Hui STW, the first disinfection facility for major sewage treatment works in HK, has been installed under the Stage II upgrading project to meet the new discharge requirement. The facility employs a total of 520 medium pressure, high intensity UV lamps emitting light of a wavelength of 265 nm to disinfect the effluent by rearranging the genetic code (DNA) of microorganisms, thus preventing them from reproducing. It can disinfect an average effluent flow of 80,000 m³/day and a peak flow up to 10,000 m³/hour. To achieve a high disinfection efficiency, it is designed in such a way that the system will automatically adjust the number of operating lamps and UV intensity according to the pre-set effluent flow rate and transmittance.

The facility was successfully commissioned in April 2000 and its disinfection capability achieved so far is most satisfactory. Under normal operation, it can effectively reduce the E.Coli concentration in effluent to less than 50 count/100 ml (monthly geometric mean). The capital cost of the facility is about $24 million and its operation and maintenance cost (excluding the staff cost) is estimated to be $0.13/m³.