Contents

 

                                                                                                                                                                        Page

6                SEWERAGE AND SEWAGE TREATMENT IMPLICATIONS  1

6.1               Legislation, Standards and Guidelines  1

6.2               Existing Conditions  1

6.3               Methodology and Design Criteria  4

6.4               Sewage Flow Estimation  7

6.5               Proposed Development Sewerage Network  8

6.6               Construction, Operation and Maintenance Responsibilities  15

6.7               Impacts and Mitigations  15

 

Figures

Figure 6.1                  Existing, Planned and Proposed Sewerage Network Overall Plan

Figure 6.2                  Existing, Planned and Proposed Sewerage Network

(Sheet 1 of 9)

Figure 6.3                  Existing, Planned and Proposed Sewerage Network

(Sheet 2 of 9)

Figure 6.4                  Existing, Planned and Proposed Sewerage Network

(Sheet 3 of 9)

Figure 6.5                  Existing, Planned and Proposed Sewerage Network

(Sheet 4 of 9)

Figure 6.6                  Existing, Planned and Proposed Sewerage Network

(Sheet 5 of 9)

Figure 6.7                  Existing, Planned and Proposed Sewerage Network

(Sheet 6 of 9)

Figure 6.8                  Existing, Planned and Proposed Sewerage Network

(Sheet 7 of 9)

Figure 6.9                  Existing, Planned and Proposed Sewerage Network

(Sheet 8 of 9)

Figure 6.10                Existing, Planned and Proposed Sewerage Network

(Sheet 9 of 9)

Figure 6.11                Planned Sewerage Network Manhole Details

 

Figure 6.12                Western Portion of TCE Development

 

Appendices

Appendix 6.1             Sewage Flow Estimation

Appendix 6.2             InfoWorks Model User Manual

Appendix 6.3             MTRCL’s Requirements on Proposed Sewerage Works

Appendix 6.4             Responsibility Matrix

 

 

6                                SEWERAGE AND SEWAGE TREATMENT IMPLICATIONS

6.1                         Legislation, Standards and Guidelines

6.1.1                  Introduction

6.1.1.1           Apart from the legislative requirements under the WPCO, the following guidelines shall be referenced to for sewerage and sewage treatment implications of the Project:

·         EPD Report No.  EPD/TP 1/05 Guidelines for Estimating Sewage Flows (GESF) for Sewerage Infrastructure Planning Version 1.0

·         DSD Sewerage Manual Part 1 (Key Planning Issues and Gravity Collection System) and Part 2 (Pumping Stations and Rising Mains)

6.2                         Existing Conditions

6.2.1                  Introduction

6.2.1.1           The proposed Tung Chung New Town Extension (TCNTE) project falls within the North Lantau/ Airport Sewerage Catchment. The existing and planned sewerage infrastructure within and in the vicinity of the Study Area are briefly described in the following sections. Refer to Appendix 2.1 for the preliminary layout plan of Tung Chung East (TCE) and Tung Chung West (TCW) developments.

6.2.2                  Existing Sewage Treatment Works

6.2.2.1           Siu Ho Wan Sewage Treatment Works (SHWSTW) was a preliminary sewage treatment works when it was commissioned in 1996. It was subsequently upgraded under the Project PWP Item 4224DS “Outlying Islands Sewerage Stage 1 Phase 1C - Upgrading of Siu Ho Wan Sewage Treatment Plant” to chemically enhanced primary treatment (CEPT). The SHWSTW has a design Average Dry Weather Flow (ADWF) capacity of 180,000 m3/day according to information given by DSD.

6.2.2.2           An ultra-violet (UV) disinfection system was implemented in 2006 to receive the CEPT effluent and further improve the effluent quality. It consists of 2 screen inlet channels (1 duty and 1 standby) and 3 UV channels, and has an ADWF capacity of 90,000 m3/day and peak flow capacity of 1,875 l/s, which is based on the situation where all 3 UV channels are operating in parallel. Two spare screen channels and one spare UV channel have been reserved for future expansion. An effluent pumping station was also provided downstream of the UV disinfection system to ensure the effluent is discharged to the outfall during high tide condition and in case of insufficient hydraulic head. It has a peak flow capacity of 2,500 l/s and 2 spare channels are reserved for future expansion.

6.2.2.3           Currently SHWSTW receives sewage from Hong Kong International Airport (HKIA), Tung Chung, Disneyland and Penny’s Bay, Sunny Bay, Discovery Bay and Siu Ho Wan. The treated effluent is discharged into the marine waters of North Western Water Control Zone via a DN1840 submarine outfall.

6.2.3                  Existing and Planned Sewerage Network

6.2.3.1           The existing and planned sewerage network of Tung Chung is shown in Figure 6.1 to Figure 6.11. Currently, sewage from the west of Tung Chung is collected by Chung Mun Road Sewage Pumping Station (CMRSPS) and Chung Yan Road Sewage Pumping Station (CYRSPS), and is transferred to Tung Chung Sewage Pumping Station (TCSPS). TCSPS collects sewage from CYRSPS and Tung Chung Town Centre. It then conveys the sewage to SHWSTW via a DN1200 sewage rising main along Cheung Tung Road parallel with the North Lantau Highway.

6.2.3.2           CMRSPS was constructed under the Contract No. NL6/95 “Tung Chung Development Phase IIB – Main Works – Sewage Pumping Stations No. 1 and No. 2” and commenced its operation in 2000. It is located near the junction of Chung Mun Road and Yu Tung Road, and has the design peak flow capacity of 143 l/s. Currently CMRSPS collects sewage from adjacent schools and village houses. The sewage from CMRSPS is pumped into a DN350 rising main along Yu Tung Road and discharged into a DN375 sewer at the road junction with Chung Yan Road, and finally discharged into CYRSPS.

6.2.3.3           CYRSPS was also constructed under the Contract No. NL6/95 “Tung Chung Development Phase IIB – Main Works – Sewage Pumping Stations No. 1 and No. 2” and commenced its operation in 2000. It is located at the southbound of Chung Yan Road and collects sewage from the adjacent areas including Yat Tung Estate and other village houses, as well as that discharged from CMRSPS. It has a design peak flow capacity of 780 l/s. Discharge from CYRSPS flows through the DN700 rising main followed by a DN500 sewer directly to TCSPS.

6.2.3.4           TCSPS was constructed under Contract No. NL3/93 “North Lantau Sewage Treatment Works and Pumping Station.” It is located at Cheung Tung Road, and collects sewage from Tung Chung Town Centre and HKIA, as well as that discharged from CYRSPS. The existing capacity of the installed pump set at TCSPS is 1,840 l/s (with 2 duty pumps and 1 standby pump of each capacity 920 l/s). The sewage passes into a DN1200 rising main that runs along Cheung Tung Road, which is a service road that runs parallel to North Lantau Highway, and is then discharged to SHWSTW. A super-oxygenation facility with design ADWF capacity of around 45,000 m3/day was installed at the dry well in 2010 for the control of sewage septicity and odour of TCSPS by introducing a large amount of dissolved oxygen to the sewage. Space has been reserved within the TCSPS for additional coarse screens and pump sets. Additional pump sets to double the existing pumping capacity would provide for the designed maximum handling capacity of 3,680 l/s. It is noted that EPD has agreed to reserve 43,500 m3/day (ADWF) at TCSPS for the total sewage discharge from the expanded HKIA after completion of the 3RS project.

6.2.3.5           The existing DN1200 rising main between TCSPS and SHWSTW is about 6,169m long, and was commissioned in 1997 to convey sewage from TCSPS to SHWSTW along Cheung Tung Road parallel with North Lantau Highway.

6.2.3.6           The abovementioned existing rising main is currently the only means for conveying sewage from Tung Chung to SHWSTW and it is impractical to shut down the sewage rising main or divert sewage flow to facilitate inspection or maintenance. To enhance the operational reliability of the sewerage system, construction of additional sewage rising main(s), associated connection works at TCSPS and SHWSTW and also rehabilitation of the existing sewage rising main between TCSPS and SHWSTW will be undertaken by Agreement No. CE 6/2012(DS) “Construction of Additional Sewage Rising Mains and Rehabilitation of the Existing Sewage Rising Main between Tung Chung and Siu Ho Wan – Investigation, Design and Construction”. Under the latest report dated February 2015, it is proposed to construct a new DN1200 rising main with the existing DN1200 rising main after rehabilitation to be served as standby pipe for further maintenance operation. DSD has agreed to incorporate the sewage flow generated from TCNTE into their study under Agreement No. CE 6/2012(DS). As advised by DSD in April 2015, the proposed new rising main is scheduled for commissioning in June 2023, while rehabilitation of the existing rising main is scheduled for completion by end 2025.

6.2.3.7           Another planned development, Area 56, is also located southwest of TCE development and will be constructed by Hong Kong Housing Authority under separate project with a temporary sewage pumping station to temporarily discharge sewage derived from Area 56 to an existing sewer at Yi Tung Road. It was planned that the completed sewerage system of TCNTE will convey the sewage derived from Area 56, therefore it is assumed that Area 56 resides within the TCE sewerage catchment.

6.3                         Methodology and Design Criteria

6.3.1                  Objectives and Procedures

6.3.1.1           The following approach is adopted in carrying out this sewerage assessment:

·         Identify the scope of the development;

·         Determine the sewage generation of the development;

·         Identify the existing and planned sewerage systems within and in the vicinity of the proposed development boundary;

·         Examine the impact arising from new sewage generation from the proposed development on the existing sewage conveyance, pumping stations and treatment systems; and

·         Identify new and upgrading sewerage works.

6.3.1.2           The assessment has been carried out in accordance with the guidelines set out in EPD Report No. EPD/TP1/05 Guidelines for Estimating Sewage Flows (GESF) for Sewerage Infrastructure Planning Version 1.0 and DSD’s Sewerage Manual.

6.3.2                  Development Parameters

6.3.2.1           This sewerage impact assessment is based on the estimated population proposed in the RODP which is shown in Table 6.1 below. With an existing planned residential population of 124,000 in Tung Chung, an additional population of approximately 144,400 residents is proposed under TCNTE. The projected total residential population of Tung Chung would be approximately 268,400.

Table 6.1   Planned Population

Type

Tung Chung East

Tung Chung West

Residential Population

118,900

25,500

Employee (Commercial and GIC)

45,277

2,954

School (Student)

13,390

1,530

6.3.3                  Unit Flow Factors – Domestic Flows

6.3.3.1           Unit flow factor (UFF) used for domestic sewage flow estimation of residential population in the Study Area and North Lantau are shown in Table 6.2.

                          Table 6.2   Unit Flow Factors for Domestic Flows

Type

Unit Flow Factor
(m³/person/day)

Public Rental

0.190

Private R1

0.190

Private R2

0.270

Private R3

0.370

Private R4

0.370

Traditional Village

0.150

Modern Village

0.270

Institutional and special class

0.190

Temporary and non-domestic

0.150

Mobile residents

0.190

General Residential – Permanent housing in North Lantau (for catchment wide planning)

0.230

6.3.4                  Unit Flow Factors – Commercial and Institutional Flows

6.3.4.1           The unit flow factors for commercial and institutional flows due to commercial and employed population are shown in Table 6.3.

6.3.4.2           The total unit flow generated from an employee in a particular trade is the sum of the unit flow factor of the employee and the unit flow factor of commercial activities of a particular trade under consideration.

                          Table 6.3   Unit Flow Factors for Commercial and Institutional Flows

Type

Unit Flow Factor
(m³/employee/day)

Commercial Employee

0.080

Commercial activities

 

(a) Specific trades:

 

              J2

0.250

              J3

0.100

              J4

0.200

              J5

-

              J6

-

              J7

-

              J8

-

              J9

0.150

              J10

1.500

              J11

0.200

              J12

-

(b) General – territorial average

0.200

School student

0.040

6.3.5                  Unit Flow Factors – Industrial Flows

6.3.5.1           The unit flow factors for industrial flows due to manufacturing employees are shown in Table 6.4.

6.3.5.2           The total unit flow generated from an employee in a particular trade is the sum of the flows due to the employee and the unit flow factor for a particular trade under consideration.

                         Table 6.4    Unit Flow Factors for Industrial Flows

Type

Unit Flow Factor (m³/employee/day)

Industrial Employee

0.080

Industrial activities

 

(a)     J1 Manufacturing – Lantau Island (except Mui Wo)

0.450

6.3.6                  Peaking Factors

6.3.6.1           Peaking factors cater for seasonal/diurnal fluctuation and normal amount of infiltration and inflow. The peaking factors adopted in this study are shown in Table 6.5.

                         Table 6.5    Peaking Factors for Various Population Ranges

Population Range

Peaking Factor (including stormwater allowance) for facility with existing upstream sewerage

Peaking Factor (excluding stormwater allowance) for facility with new upstream sewerage

Sewers

< 1,000

8

6

1,000 – 5,000

6

5

5,000 – 10,000

5

4

10,000 – 50,000

4

3

> 50,000

Max (7.3/N0.15  , 2.4)

Max (6/N0.175  , 1.6)

Sewage Treatment Works, Preliminary Treatment Works and Pumping Stations

< 10,000

4

3

10,000 – 25,000

3.5

2.5

25,000 – 50,000

3

2

> 50,000

Max (3.9/N0.065  , 2.4)

Max (2.6/N0.065  , 1.6)

Notes:

N = Contributing population in thousands.

6.4                         Sewage Flow Estimation

6.4.1.1           The estimated sewage flows generated by TCE and TCW developments are summarised in Table 6.6. Detailed calculations are enclosed in Appendix 6.1.

                         Table 6.6    Sewage Flow Estimation from Tung Chung East and Tung Chung West Developments

Proposed Development

ADWF (m³/day)

Tung Chung East

43,070

Tung Chung West

7,010

6.4.1.2           In order to assess potential impacts to the sewerage network, the future sewage flows in the catchment of the sewage pumping stations (SPSs) and SHWSTW have been estimated based on the population forecast in the proposed development scheme as well as the 2009-based Territorial Population and Employment Data Matrix (TPEDM) within the catchment areas. Sewage projections from the interfacing 3RS project at HKIA have been obtained from the Hong Kong Airport Authority.

6.4.1.3           A comparison of the 2009-based and 2011-based TPEDMs population forecasts for residential, employment and schools reveals that the 2011-based TPEDM is generally lower than that of the 2009-based TPEDM. Therefore this study takes the more conservative approach to estimate the overall sewage flows using the 2009-based TPEDM.

6.4.1.4           A year-by-year flow build-up assessment has been conducted from Year 2011 to Year 2031, with TCNTE ultimate scenario occurring in Year 2031. The projected sewage flows at each SPS and SHWSTW are estimated and compared with the respective design capacities, and are summarized in Table 6.7 and Table 6.8 with calculations provided in Appendix 6.1.

6.4.1.5           The western portion of TCE development will undergo early population intake from Year 2023 to Year 2026, while the eastern portion of TCE development will undergo later population intake from Year 2027 to Year 2030. Separate sewerage schemes for interim stage (for population intake from Year 2023 to 2026) and ultimate stage (for population intake from Year 2027 to 2030) are proposed. Refer to Section 6.5 for more detail.

                         Table 6.7    Summary of Estimated Sewage Flows at Existing SPSs in Year 2031

Catchment [1]

ADWF (m3/day)

Peaking Factor

Peak Flow (l/s)

Design Capacity

(Existing Capacity)

(l/s, Peak Flow) [2]

CMR

SPS

 

Existing catchment (PDZs 246,266) + Portion of  TCW Development [3]

5,487

3.50

222

143

CYR

SPS

 

Existing catchment (PDZs 246,266,282,396)

 

14,260

3.01

497

780

TC

SPS

(Interim Stage – Year 2023 to 2026)

Existing catchment (PDZs 245,246,247,266,282,396) + TCW Development + TCE Development Interim Stage (western portion)

83,082

2.69

2,584

3,680

(1,840)

TC

SPS (Ultimate Stage – Year 2027 to 2030)

Existing catchment (PDZs 245,246,247,266,282,396) + TCW Development

 

 

75,047

2.71

2,350

3,680

(1,840)

Notes:

1.       Refer to 2009-based TPEDM for Planning Data Zone (PDZ) boundaries

2.       The design capacities of existing SPSs are given by DSD

3.       For purposes of this assessment, only TCW development areas in the vicinity of CMRSPS would contribute to its catchment; refer to Table 6.11 for description of these areas

 

Table 6.8      Summary of Estimated Sewage Flow at Existing STW in Year 2031

Catchment [1]

ADWF (m3/day)

Design Capacity

(Existing Capacity) 

(m3/day, ADWF) [2]

SHW

STW

 

Existing catchment (PDZs 239,241,245,246,247,265,266,282,327,395,396,398) + TCW Development + TCE Development Ultimate Stage (entire development)

144,585

180,000

(90,000)

Notes:

1.      Refer to 2009-based TPEDM for Planning Data Zone (PDZ) boundaries

2.      The design capacity of existing STW is given by DSD

6.5                         Proposed Development Sewerage Network

6.5.1.1           As a conservative design approach, this study examines a proposed sewerage network that is independent from the existing sewerage network so as to avoid impacts to existing gravity sewers. Figure 6.1 to Figure 6.11 shows the sewerage layout proposed under this study.

6.5.1.2           The hydraulic calculations of the proposed sewerage system are presented in Appendix 6.1.  Hydraulic assessment using mathematical simulation software “InfoWorks ICM” has been conducted and the results are presented in Appendix 6.2.

6.5.2                  Tung Chung East Development

Interim Stage – Year 2023 to Year 2026

6.5.2.1           The western portion of TCE development will undergo early population intake from Year 2023 to Year 2026, while the eastern portion of TCE development will undergo later population intake from Year 2027 to Year 2030. In order to serve the early population intake at the western portion, an interim stage sewerage scheme is proposed. Figure 6.12 below indicates the western portion of TCE development planned for early population intake.

 

 

 

 

 

 

 

 

 

 


Figure 6.12         Western Portion of TCE Development

6.5.2.2           A new sewerage system will be provided to serve the western portion of TCE development, and a new sewage pumping station (SPS) is proposed and designated “TCE West SPS.” All sewers and rising mains will be laid along existing and proposed roads or within drainage reserves. Table 6.9 summarizes the estimated pumping capacity required at TCE West SPS during the interim stage, which includes the provision of ultimate sewage flow derived from Area 56.

                         Table 6.9    Estimated Sewage Flow for Proposed Sewage Pumping Station during Interim Stage

SPS

Area Served

ADWF (m3/day)

Peaking Factor

Peak Flow (l/s)

TCE West SPS (Interim Stage)

New development areas in the western portion of Tung Chung East and Area 56

12,891

3.00

448

6.5.2.3           For the interim stage twin DN450 rising mains are proposed to divert flow from TCE West SPS along Ying Tung Road and Man Tung Road, crossing above the MTR Tung Chung Line tunnel.  They will then pass along an existing drainage reserve and below the Airport Express Line (AEL) and North Lantau Highway. In order to avoid disturbance to the AEL, North Lantau Highway and existing utilities, the twin rising mains are proposed to be installed within an existing pressure sewer subway tunnel that crosses below AEL and North Lantau Highway. After the tunnel, the rising mains will discharge to a proposed inlet pressure breaking chamber (manhole) within the TCSPS site compound and discharge to the inlet screen channel inside the TCSPS main building via twin gravity sewers. Refer to Figure 6.6 for details of the proposed connection.

Ultimate Stage – Year 2027 to Year 2030

6.5.2.4           To accommodate the planned population intake in the eastern portion of TCE development from Year 2027 to Year 2030, an ultimate stage sewerage scheme is proposed. A new sewerage system will be provided within the eastern portion of TCE development, and another sewage pumping station (SPS) is proposed and designated “TCE East SPS.” Sewage flow from TCE West SPS will be diverted by new twin DN450 rising mains and gravity sewers to the TCE East SPS. All sewers and rising mains will be laid along existing and proposed roads or within drainage reserves. Table 6.10 summarizes the estimated pumping capacities required at each SPS during the ultimate stage.

                         Table 6.10  Estimated Sewage Flows for Proposed Sewage Pumping Stations during Ultimate Stage

SPS

Area Served

ADWF (m3/day)

Peaking Factor

Peak Flow (l/s)

TCE West SPS (Ultimate Stage)

New development areas in the western portion of Tung Chung East and Area 56

12,891

3.00

448

TCE East SPS (Ultimate Stage)

All new development areas in Tung Chung East and Area 56

45,732

2.79

1,479

6.5.2.5           The proposed TCE East SPS, which will serve the entire TCE development, will transfer the sewage flow via proposed twin DN600 rising mains along Road P1 (Tung Chung – Tai Ho Section) and then along the footpath at the rear of the existing seawall at existing MTRCL Siu Ho Wan Depot. The rising mains will then cross below the MTR Tung Chung Line (TCL) tunnel and Airport Express Line (AEL) and North Lantau Highway, then along an existing drainage reserve and maintenance access road with discharge to SHWSTW. The crossing of twin rising mains below MTR TCL and AEL and the North Lantau Highway may be carried out by pipe jacking with jacking pit located within the MTRCL Siu Ho Wan Depot and receiving pit within existing drainage reserve area tentatively. The pipe jacking arrangement will be designed not to conflict with existing utilities. Precautionary and monitoring measures such as restricted working hours, horizontal pilot drilling and ground movement monitoring will be implemented. DSD has agreed that a combined vortex chamber will be constructed under Agreement No. CE 6/2012(DS) with service connections to accept the twin DN600 rising mains from TCE East SPS in future. CEDD plans to entrust to DSD the construction of the section of twin DN600 rising mains within the SHWSTW site compound. Refer to Figure 6.2 for details of the proposed connection.

6.5.2.6           During the course of this Study, liaison with MTRCL has been conducted and the design and construction of the proposed sewerage system will be in strict compliance with the general Railway Protection requirements and the requirement imposed by MTRCL as per the letter enclosed in Appendix 6.3. It is also noted that North Lantau Highway is a traffic sensitive route and is classified as Red Route. The traffic impact assessment and daytime ban requirements will be fulfilled according to HyD's Guidance Notes No.  RD/GN/021. To comply with MTRCL's and HyD's requirements, it is expected that night time construction works and trenchless construction method (such as pipe jacking) will be required. Particular attention should be paid in developing the detailed alignment, level and construction method of the proposed crossing in order to minimize impacts to traffic and MTRCL infrastructure. Restricted working hours and a construction noise permit will impose additional constraints on the construction works, and particular efforts will be made to plan the construction programme and specifications accordingly.

6.5.3                  Tung Chung West Development

6.5.3.1           The Tung Chung Stream runs inside the Tung Chung Valley with two tributaries (termed “East Stream” and “West Stream”). The downstream section of the East Stream is an engineered drainage channel. The East and West Streams combine within the TCW development area and discharge to Tung Chung Bay.

6.5.3.2           A new public sewerage system will be provided to serve the TCW development as well as to provide a village sewerage system for the existing unsewered village areas of Shek Mun Kap, Mok Ka, Nim Yuen, Lam Che, Ngau Au, Shek Lau Po, Ma Wan Chung and Wong Nai Uk. New sewers will be laid along existing and proposed roads, footways or drainage reserves.

6.5.3.3           To overcome the constraint posed by the presence of the East and West Streams, three new sewage pumping stations are proposed, two serving those developments west of the West Stream (designated “TCV West SPS” and “TCV North SPS”) and the third serving all of TCW development (designated “TCV East SPS”).

6.5.3.4           The TCV West SPS will deliver flow from proposed development areas TCV-3 and TCV-4 as well as existing villages of Mok Ka, Lam Che and Nim Yuen to the TCV East SPS via twin DN200 rising mains. Refer to Figure 6.8 to Figure 6.10 for the location of proposed developments in TCW.

6.5.3.5           The TCV North SPS will deliver flow from proposed development area TCV-8 and the existing village of Ngau Au to the TCV East SPS via twin DN200 rising mains.

6.5.3.6           Sewage arising from proposed development areas TCW-1, TCW-2, TCW-3, TCV-1, COM-1, COM-2, COM-3 and existing villages Ma Wan Chung and Wong Nai Uk will be discharged by proposed sewers laid along existing and proposed roads to Chung Mun Road Sewage Pumping Station (CMRSPS). The CMRSPS will be upgraded such that new stand-alone inlet chamber, screens, wet well and pump sets are provided to serve the proposed developments and two existing villages. Refer to Figure 6.9 for details of the proposed connection. The Upgraded CMRSPS will maximize the utilization of existing supporting infrastructure such as access road and power supply (upgrading may be required subject to detailed design), etc. so as to minimize the overall pumping station footprint. The Upgraded CMRSPS will discharge directly to TCV East SPS via new twin DN250 rising mains laid along Chung Mun Road.

6.5.3.7           The TCV East SPS will serve the entire TCW development including flows from Upgraded CMRSPS, TCV West SPS, TCV North SPS and existing villages including Shek Mun Kap, Mok Ka, Nim Yuen, Lam Che, Ngau Au, Shek Lau Po, Ma Wan Chung and Wong Nai Uk. The TCV East SPS will deliver flow with direct discharge to the TCSPS via new twin DN450 rising mains laid along Chung Mun Road and Yu Tung Road, thus bypassing the Chung Yan Road SPS. Table 6.11 summarizes the estimated pumping capacities required at the TCV West SPS, TCV North SPS, TCV East SPS, and Upgraded CMRSPS.

6.5.3.8           For noise control at proposed SPSs, relevant noise impact assessment and mitigation measures are presented in Section 4.6.

                         Table 6.11                 Estimated Sewage Flows for Proposed Sewage Pumping Stations serving Tung Chung West Development

SPS

Area Served

ADWF (m3/day)

Peaking Factor

Peak Flow (l/s)

TCV West SPS

New development areas TCV-3, TCV-4 and existing villages Mok Ka, Lam Che and Nim Yuen

551

4.00

 26

TCV North SPS

New development area TCV-8 and existing village Ngau Au

278

6.00

19

TCV East SPS

All new development areas in TCW and existing villages Mok Ka, Lam Che, Nim Yuen, Shek Mun Kap, Ngau Au, Shek Lau Po, Ma Wan Chung and Wong Nai Uk

 8,864

3.00

308

Upgraded CMRSPS

New development areas TCW-1, TCW-2, TCW-3, TCV-1, COM-1, COM-2, COM-3 and existing villages Ma Wan Chung and Wong Nai Uk

3,119

3.50

126

6.5.4                  Enhancements and Additional Provisions

6.5.4.1           Due to the key concerns of the ecological sensitivity of the Tung Chung Stream and the Tung Chung Bay, the following provisions are proposed to enhance the sewerage network reliability and minimize environmental impacts due to system failure or in case of emergency situations.

6.5.4.2           Twin rising mains would be provided for proposed discharge from all proposed SPSs and the Upgraded CMRSPS. It is proposed to use both mains as duty from economical and operational points of view. Should one of the duty mains be suspended from operation for maintenance, the remaining one would be arranged to deliver more flow at a higher velocity (exceeding 3m/s) during that period. Twin gravity sewers would also be provided at the downstream of the inlet break chamber for all proposed SPSs and the Upgraded CMRSPS.

6.5.4.3           It is proposed to adopt high density polyethylene (HDPE) pipe for proposed gravity sewers and rising mains. As gravity sewers will not be pressurized, they carry no risk of bursting. Further protection on proposed rising mains with concrete surround will be provided to prevent the rising mains from bursting. With the above proposed measures, bursting discharge is not expected and thus no adverse impact on water quality or ecology due to sewer bursting discharge is anticipated.

Additional Provisions for Tung Chung West Development

6.5.4.4           Taking into account the ecological sensitivity of the Tung Chung Stream, the occurrence of a possible emergency event at the SPSs in TCW (including TCV West SPS, TCV North SPS, TCV East SPS and Upgraded CMRSPS) has been considered due to the following risks: pump failure; rising main failure; and power failure.

6.5.4.5           To mitigate the risks of pump, rising main and/or power failure, several mitigation measures are proposed to cater for the emergency situations including a) 100% standby pumping capacity within each SPS, with spare pump up to 50% pumping capacity stockpiled in each SPS for any emergency use; b) twin rising mains; c) dual-feed power supply; d) emergency storage facilities up to 6-hours ADWF capacity; and e) emergency communication mechanism amongst relevant government departments.

6.5.4.6           Considering the possible emergency situations and respective risks, as well as the practicality of construction, the above mitigation measures are considered the most appropriate and practical measures to deal with the emergency situations of the proposed SPSs in TCW development.  Therefore, based on these provisions emergency discharge is not expected, and thus no adverse impact on water quality or ecology due to emergency discharge from the proposed SPSs in TCW development is anticipated.

Additional Provisions for Tung Chung East Development

6.5.4.7           The occurrence of any single emergency event at TCE East SPS and TCE West SPS has been considered due to the following risks: pump failure; rising main failure; and power failure.

6.5.4.8           To mitigate the risks of pump, rising main and/or power failure, several mitigation measures are proposed to cater for the emergency situations including a) 100% standby pumping capacity within each SPS, with spare pump up to 50% pumping capacity stockpiled in each SPS for any emergency use; b) twin rising mains; c) dual-feed power supply; d) emergency storage facilities up to 6-hours ADWF capacity; and e) emergency communication mechanism amongst relevant government departments.

6.5.4.9           Considering the possible emergency situations and respective risks, as well as the practicality of construction, the above mitigation measures are considered the most appropriate and practical measures to deal with the emergency situations of the proposed SPSs in TCE development.  Therefore, based on these provisions emergency discharge from the proposed SPSs in TCE is not expected, and thus no adverse impact on water quality or ecology due to emergency discharge from the proposed SPSs in TCE development is anticipated.

6.6                         Construction, Operation and Maintenance Responsibilities

6.6.1.1           Refer to Appendix 6.4 which sets out the requirements for the construction works, operation and maintenance responsibilities for the various proposed sewerage facilities.

6.7                         Impacts and Mitigations

6.7.1                  Introduction

6.7.1.1           The year-by-year sewage flow build-up at the sewage catchments of CMRSPS, CYRSPS, TCSPS and SHWSTW have been estimated and are presented in Appendix 6.1. The potential impacts on existing sewerage infrastructure and proposed mitigation measures are described in this section.

6.7.2                  Chung Mun Road Sewage Pumping Station (CMRSPS)

6.7.2.1           The existing pumping capacity of CMRSPS is inadequate to serve the increased sewage flows from new development areas TCW-1, TCW-2, TCW-3, TCV-1, COM-1, COM-2, COM-3 and existing villages Ma Wan Chung and Wong Nai Uk starting in Year 2023.  To avoid impact on the operation of the existing CMRSPS, it is proposed to provide new inlet chamber, screens, wet well and stand-alone pump sets to serve the additional sewage flows.

6.7.2.2           An existing 25m x 15m area of land immediately north of CMRSPS has been reserved for the CMRSPS upgrading works. The Upgraded CMRSPS will maximize the utilization of existing supporting infrastructure such as access road and power supply (upgrading may be required subject to detailed design), etc. so as to minimize the overall pumping station footprint.

6.7.2.3           As the sewage flow from portions of TCW development will be pumped by separate stand-alone pump sets at the Upgraded CMRSPS with discharge to the TCV East SPS, there will be no net increase in flow and thus no impact to the existing CMRSPS.

6.7.2.4           With the provision of mitigation measures presented in Sections 6.5.4.5 to 6.5.4.7 to cater for the emergency situations, emergency discharge from Upgraded CMRSPS is not expected and thus no adverse impact on water quality or ecology due to the emergency discharge is anticipated.

6.7.3                  Chung Yan Road Sewage Pumping Station (CYRSPS)

6.7.3.1           All sewage flow from proposed TCW development will be pumped by the TCV East SPS with direct discharge to TCSPS, thus bypassing the CYRSPS. Therefore there will be no net increase in flow and thus no impact to the CYRSPS.

6.7.4                  Tung Chung Sewage Pumping Station (TCSPS) and Rising Main (Interim Stage)

6.7.4.1           It is proposed to discharge the sewage arising from all proposed development in TCW as well as the western portion of TCE development to the TCSPS during interim stage from Year 2023 to Year 2026. It is noted that the flow build-up to TCSPS includes estimated flow based on the EIA assessment findings of the Hong Kong International Airport Three Runway System project.

6.7.4.2           As shown in the year-by-year flow build-up in Appendix 6.1, it is estimated that the existing pumping capacity of TCSPS (i.e. 1,840 l/s delivered by 2 duty pumps) would be exceeded in Year 2023 after the first population intake of TCNTE and the commissioning of the 3RS project at HKIA. To mitigate this impact, EPD has arranged with DSD to fit out the TCSPS to its designed maximum handling capacity (3,680 l/s) by Year 2023.

6.7.4.3           The TCSPS (Interim Stage) will discharge the proposed flows to SHWSTW, including those from TCE West SPS, TCV West SPS, TCV North SPS, TCV East SPS, and Upgraded CMRSPS with stand-alone pump sets.

6.7.4.4           Consent has been obtained from EPD to discharge the following sewage flow arising from the proposed TCNTE development to the public sewerage system:

a)         discharge not more than 7,010 m3/day of sewage flow arising from the TCW development to the TCSPS;

b)         discharge not more than 10,230 m3/day of sewage flow arising from the TCE development to the TCSPS up to Year 2026 or a later date agreed by EPD.

6.7.5                  Tung Chung Sewage Pumping Station (TCSPS) and Rising Main (Ultimate Stage)

6.7.5.1           From Year 2026, sewage from TCE West SPS will be diverted toward TCE East SPS for direct connection to SHWSTW, thus no more sewage from TCE development shall be discharged to TCSPS.  As such, the ultimate capacity at TCSPS of 3,680 l/s after completion of fitting out works is sufficient to serve the additional sewage flow derived from TCW development. EPD has arranged with DSD to fit out the TCSPS to its designed maximum handling capacity (3,680 l/s) by Year 2023.

6.7.5.2           The TCSPS (Ultimate Stage) will discharge the proposed flows to SHWSTW including those from TCV West SPS, TCV North SPS, TCV East SPS, and Upgraded CMRSPS.

6.7.5.3           The redirection of all the sewage flow arising from the TCE development away from the TCSPS for direct discharge to SHWSTW is scheduled for completion by Year 2026.

6.7.6                  Siu Ho Wan Sewage Treatment Works (SHWSTW)

6.7.6.1           As shown in the year-by-year flow build-up in Appendix 6.1, it is estimated that the projected flow (ADWF) in year 2031 at SHWSTW will reach 144,585 m3/day. While SHWSTW has a designed maximum handling capacity of 180,000 m3/day, some of its treatment units have not been fully fitted out to the designed maximum handling capacity.  EPD has arranged with DSD to fit out the remainder of the treatment units at SHWSTW to its designed maximum handling capacity (180,000 m3/day) by 2024 in order to cope with the projected sewage flows within its catchment.

6.7.6.2           As both the planned DN1200 rising main under Agreement No. CE 6/2012(DS) and the proposed twin rising mains under this Project will discharge to SHWSTW, a combined vortex chamber arrangement to accommodate the proposed twin DN600 rising mains from TCE East SPS in future has been explored. A preliminary arrangement amongst all the inlet rising mains is shown in Figure 6.2. Further details will be studied and agreed with DSD during detailed design stage. CEDD plans to entrust to DSD the construction of the section of proposed twin rising mains under TCNTE within SHWSTW in order to advance the construction of this section of rising mains.

6.7.6.3           Consent has been obtained from EPD to increase the total sewage flow arising from the TCE development to the SHWSTW from 10,230 m3/day to not more than 43,070 m3/day after Year 2026.

6.7.7                  Existing Sewers and Rising Mains

6.7.7.1           A new separate sewerage system will be provided to serve TCW and TCE developments. All sewage from proposed developments will discharge directly to proposed sewers, proposed and/or upgraded SPSs and to SHWSTW without discharging to existing sewers and rising mains other than TCSPS rising main as described above.  Therefore, the TCNTE development induces no impact to existing sewers and rising mains.

6.7.7.2           Agreement No. CE 6/2012(DS) proposes a new DN1200 rising main and rehabilitation of the existing DN1200 rising main between TCSPS and SHWSTW. DSD has agreed to incorporate the sewage flow generated from TCNTE estimated under this study into their study under Agreement No. CE 6/2012(DS) such that there will be sufficient capacity in the new and rehabilitated rising mains to cater for the sewage flow generated by TCNTE. Therefore, there is no adverse impact to the new and rehabilitated rising mains.

6.7.7.3           As gravity sewers will not be pressurized, they carry no risk of bursting. All proposed rising mains will be encased in concrete surround, it is expected that there will be no bursting of rising mains, and therefore there will be no impact to water quality or ecology due to sewer bursting discharge.

6.7.8                  Septicity and Odour Assessment

6.7.8.1           Qualitative study has been carried out to investigate the effect of sewerage and sewage characteristics on the potential to cause the sewage to turn septic and therefore lead to odour emission and corrosive damage in both sewage collection network and infrastructure at SHWSTW.

6.7.8.2           Septicity is caused under the anaerobic condition in the sewerage system after the dissolved oxygen is depleted by aerobic biological activity.  Anaerobic activity can then lead to the production of hydrogen sulphide (H2S) and potential to create odour nuisance.  The desertion rate of H2S increases with the concentration of organic substances in the sewage and generally with decreasing pH value, which can give rise to corrosion of the concrete infrastructure and metal components of the sewerage system.

6.7.8.3           The key parameters for formation of septicity is dependent upon the sewage strength, flow rate characteristics, length of the sewerage system, oxygen level and temperature.  Under this study, the proposed sewerage system is a separate system and mainly receives residential and commercial sewage and the system arrangement of the catchment is mainly relied on pumping with five new Sewage Pumping Stations (SPS) and upgrade of one existing SPS (refer to Figure 6.4, Figure 6.5 and Figure 6.9) plus some of the rising mains involved are considerably long as shown in Table 6.12.  Based on this conceptual design of sewerage arrangement, the septic issue in the sewage collection network is anticipated due to the following main reasons:

a)         The sewage in the proposed sewerage system is mainly domestic sewage with no dilution, which has a relatively high organic content favouring the septic issue;

b)         Long rising mains with the longest length of about 4km, which causes long sewage retention time (i.e. 2 to 2.5 hours at night time with minimum flow rates) with low oxygen content in the sewerage system; this condition favours anaerobic degradation of organic matter consequently turning sewage into septic;

c)         High variation of flow pattern resulting in long hydraulic detention time in the mains and pump wells in particular during night time period;

d)        Relatively high temperature in hot summer period.

                         Table 6.12  Approximate Length of Rising Mains Serving SPSs

Sewage Pumping Station

ADWF (m3/day)

Approximate Length of Rising Main (m)

TCV North SPS

278

200

TCV West SPS

551

200

TCV East SPS

8,864

2,500

Upgraded CMRSPS

3,119

300

TCE West SPS

12,891

1,500

TCE East SPS

45,732

4,000

6.7.8.4           All parameters mentioned above favour the generation of septic condition, therefore septicity control will be necessary in this sewage collection network.  Based on the preliminary qualitative septicity assessment, addition of chemicals such as nitrate salts or sodium chlorite to the sewage is considered as one of the feasible  septicity mitigation measures at all proposed sewage pumping stations in TCNTE, while further design details will be carried out at future detailed design stage to determine the need and extent of septicity mitigation measure, and to optimize the design so as to avoid septicity before resorting to the use of chemicals for mitigation. The most effective way for automatic septicity control is to monitor the dissolved sulphide level in the sewerage system at locations approved by DSD.  Potential odour nuisance and significant corrosion of sewerage system are anticipated when the sulphide level in sewage is above 2mg/L. This value can be potentially used to trigger the mitigation measure to control the septicity and H2S emission to acceptable levels.

6.7.8.5           Odour emission from the pump wells/ sumps will be anticipated.  For the odour control, all pumping stations should be enclosed in building with sufficient ventilation in order to maintain negative pressure preventing foul air escape from the building.  It is suggested that ventilation rate for the man and non-man access should be able to achieve at least 10 and 5 air changes per hour respectively. Wet well can be air sealed, the ventilated foul air should be treated by a deodourization system before discharging into the atmosphere. Relevant Odour Impact Assessment and corresponding mitigation measures are further presented in Section 3.5.