In my previous paper presented in the 21st WEDC Conference (Tang, 1995), the overall strategy used to tackle wastewater produced in the whole territory of Hong Kong was described. Also briefly mentioned in that paper
was a mega scale project named “Strategic Sewage Disposal Scheme” which is being undertaken in Hong Kong.
The scheme is not a solution to the water pollution problems in the whole territory but is only for solving the
problems at the most densely populated urban areas, viz, Kowloon Peninsula, northern part of Hong Kong Island,
and the harbour (named Victoria Harbour) separating Kowloon Peninsula and Hong Kong Island (see the rectangular frame in Figure 1). The current paper is to give a detail discussion on the strategic sewage disposal scheme (SSDS)

S.L. Tang (7182275)

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E   WASTEWATER AND ITS TREATMENT: TANG24422nd WEDC Conference New Delhi, India, 1996Strategic sewage disposal in Hong KongDr S.L. Tang, Hong KongREACHING THE UNREACHED: CHALLENGES FOR THE 21ST CENTURYIN MY PREVIOUS paper presented in the 21st WEDC Con-ference (Tang, 1995), the overall strategy used to tacklewastewater produced in the whole territory of HongKong was described. Also briefly mentioned in that paperwas a mega scale project named “Strategic Sewage Dis-posal Scheme” which is being undertaken in Hong Kong.The scheme is not a solution to the water pollution prob-lems in the whole territory but is only for solving theproblems at the most densely populated urban areas, viz,Kowloon Peninsula, northern part of Hong Kong Island,and the harbour (named Victoria Harbour) separatingKowloon Peninsula and Hong Kong Island (see the rec-tangular frame in Figure 1). The current paper is to give adetail discussion on the strategic sewage disposal scheme(SSDS).4 Stages in the SSDSThere are four stages in the SSDS. The 1st stage involvesthe construction of a deep tunnel system (at least 100mbelow ground level) collecting all sewage from the wholeKowloon Peninsula and the northeast part of the HongKong Island. The sewage collected will be treated prima-rily with chemical enhancement (called CEPT, chemicallyenhanced primary treatment) at a place named Stonecut-ter’s Island (Figure 2) (Hong Kong EPD, April 1995). Theeffluent will be discharged to the west side of VictoriaHarbour as an interim measure. The construction of the1st stage will be completed by June 1997, immediatelybefore the taking over of the territory by China. Theimplementation of this stage of the scheme has beenagreed on 20 December 1995 by both the Chinese Govern-ment and the Hong Kong/British Government (SCMP, 21December 1995). Agreements for the remaining 2nd, 3rdand 4th stages, however, have not yet been reached.Tentatively, the 2nd stage will involve the construction ofa long deep tunnel submarine outfall starting at theStonecutter’s Island which will supersede the interimoutfall of the 1st stage to dispose the effluent to deepocean waters 13km from Lamma Island in the SouthChina Sea. The 3rd and 4th stages will be the constructionof deep tunnel systems collecting sewage from the HongKong Island (except the northeast part of it which hasalready been completed in the 1st stage) to the same CEPTplant at the Stonecutter’s Island.Stage 1 of the SSDSIn the past, wastewaters collected from the KowloonPeninsula and the northern part of Hong Kong Islandwere discharged, with or without passing through pre-liminary treatment plants, directly to the Victoria Har-bour (Tang, 1984, 1995). As the harbour is badly pollutedby these untreated or preliminarily treated wastewaters,Figure 2. SSDS Stages 1, 2, 3, and 4Figure 1. Map of Hong KongE   WASTEWATER AND ITS TREATMENT: TANG245Deep tunnel conveyance of Stage 1In the SSDS, the existing preliminary treatment (or screen-ing) plants, after modifying or upgrading, will act asconnection nodes to the deep tunnel conveyance system(Figure 3) (SCMP, 1995). The most important aspect atthe preliminary treatment plants is to achieve high stand-ard of grit removal in order to avoid the deposition andaccumulation of solids in the tunnel conveyors. Afterpassing through screening plants, the flow will then beled to a vortex drop shaft through which it will enter intothe SSDS deep tunnels. Before it is led to the drop shaft,the flow will be measured using a magnetic flowmeter,as the control of the SSDS pumps will be dependent uponmeasurement of flows and levels at each connectionnode. The main tunnel system will be operated as a seriesof inverted syphons, with pumping stations to compen-sate the head losses within the system. In Stage 1, forexample, it will be operated in a surcharged condition asa series of “U-tubes” with flow discharging into a verticalvortex drop shaft at each preliminary treatment plant(i.e. connection node) and raised from the tunnels bypumping stations through rising shafts at the down-stream ends such as Kwun Tong and Stonecutter’s Island(Figure 3 and Figure 4) (AB2H Consultants, 1993). Pumpswill be operated automatically based on flow rates withinthe conveyance system. The upstream control methodwill be used, which means that the water level in theupstream shafts will be maintained rather constantlyat a high level using variable speed drives of thepumps.the SSDS has the purpose of rerouting these wastewatersto a deep tunnel system which ends at the CEPT plantlocated at the Stonecutter’s Island. The CEPT plant willremove 85 per cent suspended solids and 50 per cent BOD(CIWEM Hong Kong Branch, 1996). The use of chemicalsallows higher overflow rates in the sizing of the works,thus fewer tanks each with a greater throughput to handlethe design flow can be used. The selected chemical will beeither ferric chloride or ferric sulphate, and pilot trials arecurrently being carried out to determine the most appro-priate chemical. After treatment, the effluent will bedischarged to the Western Harbour through a 1.6km longand 5m diameter submarine outfall. This outfall is only aninterim one as it will be replaced by the Stage 2 outfall ofthe SSDS.Figure 4. Schematic hydraulic profileFigure 3. Stage 1 of the SSDSE   WASTEWATER AND ITS TREATMENT: TANG246Design criteriaThe design criteria of the SSDS are as follows:• Ensure that untreated sewage is not discharged to theconveyors.• Ensure adequate preliminary treatment to avoid sedi-ment deposition in the conveyance system.• Design conveyors (by selecting suitable diametersand gradients) to transport sediment so as to avoidsediment build-up.• Ensure a minimum of bed rock cover of 30m over thetop of the tunnels (this means that all tunnels are 100mto 150m below ground level).• Design conveyors to be capable of operating effi-ciently over a long period of time with minimummaintenance.• Ensure sufficient hydraulic capacity under maximumflow conditions.• Ensure sufficient hydraulic capacity at the designhorizon of year 2021.The average dry weather flows from 1997 to 2021 areestimated as follows (Pypun, 1995):Estimated sewage flow (m3/sec)1997 2001 2011 2021Stage 1 12.82 16.36 17.46 19.97Stages 3 and 4 - 4.00 4.18 4.27Total 12.82 20.36 21.64 24.24Contract packages and costs for Stage 1The total estimated costs at 1995 price for Stage 1 isHK$5.2 billions (ie. US$0.67 billion). There are 4 advanceworks contracts, 6 main civil works contracts and 5 E&Mworks contracts for Stage 1 (CIWEM Hong Kong Branch,1996). A summary of these contracts and their costs breakdown are shown below.Contracts Per cent ofstage 1 costsAdvance works• Piling for the foundation for the 2.0 per centsedimentation tanks.• Production shafts to gain access to 4.4 per centtunnels.• Diaphragm walls for Stonecutter’s 2.2 per centIsland main pumping station.• Site offices and facilities. 1.4 per centCivil works• Tunnelling (Chai Wan and Tseung 9.5 per centKwan O to Kwun Tong).• Tunnelling (Kwun Tong and Kwai 13.3 per centChung to Stonecutter’s Island).• Sedimentation tanks structure. 7.7 per cent• Stonecutter’s Island pumping station 6.0 per centbuilding and other infrastructure.• Sludge handling and loading 1.1 per centfacilities.• Interim outfall. 10.4 per centE&M works• Upgrading existing preliminary 17.7 per centtreatment plants.• Sludge dewatering facilities. 5.9 per cent• Chemical dosing facilities. 2.7 per cent• Pumping equipment for all pumping 8.1 per centstations.• E&M equipment for sedimentation 7.6 per centtanks.The estimated recurrent cost for stage 1 scheme will beapproximately HK$300 millions per year (ie. US$38.5millions per year). The recurrent cost will be recovered by“sewage charge” collected from citizens/residents of HongKong. The capital cost will not be recovered but will beborne entirely by the Hong Kong Government.Stages 2, 3 and 4 of the SSDSThese stages were proposed to be completed at the sametime (about 2003). Combined implementation of Stages 3and 4 was recommended and Stage 2 would remainseparate. The works included in Stages 3 and 4 are similarand combining them would result in earlier environmen-tal benefits and a more efficient implementation pro-gramme. Stage 2 ocean outfall was estimated to be of acapital cost of HK$3.6 billions (ie. US$0.46 billion) at 1992prices. This estimated cost for the ocean outfall (Figure 5)was based only on a likely location, since an agreementFigure 5. Stage 2 oceanic outfallE   WASTEWATER AND ITS TREATMENT: TANG247ment in the worst polluted areas as well as a long termstrategy of oceanic disposal to improve the water qualityin Hong Kong harbours as a whole. As a precaution,however, a site at Mount Davis (west tip of Hong KongIsland) is reserved to accommodate a secondary treat-ment plant (Figure 2) if the future monitoring of waterquality at the outfall reveals that such a plant is necessary.The additional cost of a secondary treatment plant will bein the order of HK$4 billions (ie. US$0.52 billion) at 1995prices (Hong Kong EPD, April 1995). This is also some-thing that the experts of the joint-venture engineeringconsultant company will look into.ReferencesAB2H Consultants, “SSDS Stage I - Final Consultant Re-port”, Environmental Protection Department, HongKong Government, January 1993.CIWEM Hong Kong Branch, “The Strategic Sewage Dis-posal Scheme, Stage I”, CIWEM Hong Kong BranchNewsletter, April 1996, p. 5-6.Hong Kong Environmental Protection Department,“Cleaning Up Our Harbour”, Hong Kong GovernmentPublication, April 1995.Pypun Engineering Consultants Ltd., “Review of Strate-gic Sewage Disposal Scheme Stage II Options”, Envi-ronmental Protection Department, Hong Kong Gov-ernment, April 1995.South China Morning Post (SCMP), “China Agrees toBlueprint for Sewage”, 21 December 1995.South China Morning Post (SCMP), “Strategic SewageScheme Plan Takes Off”, Journal of Hong Kong ProjectsDirectory, 1995, p. 52.Tang, S.L., “Hong Kong Wastewater - Now and Future”,Proceedings of the 21st WEDC Conference onSustainability of Water and Sanitation Systems, Kam-pala, Uganda, September 1995, p. 242-244.Tang, S.L., “Wastewater - Hong Kong and Taiwan”,Proceedings of the 10th WEDC Conference on Waterand Sanitation in Asia and the Pacific, Singapore, Au-gust 1984, p. 110-114.between the Chinese and the Hong Kong/British govern-ments has not yet been reached and the Stage 2 design hasnot yet been endorsed. The capital costs for Stages 3 and4 were estimated to be HK$2.2 billions (ie. US$0.28 bil-lion) at 1992 prices. The recurrent costs for Stage 2 andStages 3 and 4 were estimated to be HK$39 million peryear and HK$59 millions per year respectively.The decisions aheadThe effects on the water quality of the Victoria Harbour asa result of implementing Stage 1 of the SSDS were as-sessed using the Hong Kong Environmental ProtectionDepartment’s WAHMO computer model. It showed thata significant improvement in water quality within theharbour can be achieved. Considerable improvement interms of dissolved oxygen, E. coli, ammonia and inor-ganic matters was predicted. For Stage 2 of the SSDS, theenvironmental assessment of the impact of dischargesfrom the oceanic outfall has not been carried out.     Follow-ing the agreement between the Chinese and the HongKong/British governments made on 20 December 1995(SCMP, 21 December 1995), environmental assessmentwas proposed to be carried out at several oceanic outfalllocations to assess the potential and adverse impacts onthe receiving water quality attributable to the chemicallyenhanced primarily treated (CEPT) effluent discharged.A joint-venture engineering consultant company, con-sisting of both Hong Kong and Chinese experts, will beawarded the contract of carrying out an overall environ-mental assessment at the oceanic outfall locations.  Thejob will last for a period of 30 months, starting in 1996.The study includes the determination of influences on theecology, water quality of the surrounding environment,potential risks, capital cost and engineering feasibility.The author of this paper is of the opinion that theproposed CEPT with an oceanic disposal offers the bestvalue for money for the said pollutant removal efficiencyif the proposed environmental assessment does not findthe water quality around the outfall to be too unaccept-able. The scheme offers an early water quality improve-

Strategic sewage disposal in Hong Kong

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Strategic sewage disposal in Hong Kong

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