9.1
Introduction
9.1.1.1 This section presents the
assessment of impacts on the public sewerage, sewage treatment and disposal
facilities associated with the project, according to the requirements as specified in Section 3.4.7 and Appendix D2 of
the Study Brief (ESB-250/2012).
9.1.1.2 Additional sewage flow will be generated by the project. This section describes the
assessment of impacts associated with the additional sewage flow arising from operation
of the
project to the existing / planned sewerage system in North Lantau including the
sewerage catchments of Siu Ho Wan Sewage Treatment Works, and the
recommended appropriate mitigation
measures to mitigate any forecasted shortfalls in the sewerage system as a result of the project.
9.1.1.3
It
should be noted that the construction phase sewage issues
of the project, i.e., sewage generated by the construction workforce, are
addressed separately in Section
8.
9.2
Methodology
of Sewerage Impact Assessment
9.2.1
Assessment
Approach and Methodology
9.2.1.1
The assessment has been undertaken in accordance with the criteria and
guidelines for evaluating and assessing impacts on the downstream public
sewerage, sewage treatment and disposal facilities which would convey and
collect the sewage flow from the airport as stated in Section 6.5 in Annex 14
of the EIAO Technical Memorandum. The following approach and methodology have
been adopted to assess the impact on the existing / planned public sewerage
system and sewage treatment capacity arising from the project:
¡ Carry out a desktop study to collect relevant information for the assessment.
¡ Investigate and review the capacity of the existing and planned public sewerage networks and sewage treatment facilities in North Lantau.
¡ Review the maximum sewerage flows (i.e., maximum development flows) to be generated by the project in different phases.
¡ Study the need and assess the impacts of discharging sewage arising from the project to the existing / planned public sewerage systems in Tung Chung and Siu Ho Wan Sewage Treatment Works (SHWSTW).
¡
Formulate measures to mitigate
any forecasted shortfalls in the sewerage system as a result of the project and provide recommendations on
the design, operation and maintenance requirements for the proposed sewerage
system.
9.2.2
Design Reference
9.2.2.1
A desktop study was carried out to collect the relevant background
information for the assessment as summarised below:
¡ Revised Final Technical Paper (TP7) on Broad Engineering Assessment on Development Scheme Options under Agreement No. CE 32/2011 (CE) Planning and Engineering Study on the Remaining Development in Tung Chung – Feasibility Study (or the Tung Chung New Town Extension);
¡ Drainage Record Plans within the vicinity of the North Lantau from Drainage Services Department (DSD);
¡ Sewage flow generated from the Hong Kong International Airport (HKIA) in different phases;
¡
Proposed Development Layout
Plan of the project;
¡ Planning Study Report under Agreement no. CE 35/2006 for Kai Tak Development Engineering Study cum Design and Construction of Advance Works – Investigation, Design and Construction;
¡ Final Study Report of the Agreement No. CE32/2010 (CE) Site
Formation and Associated Infrastructural Works for Proposed Development of
Columbarium, Crematorium and Related Facilities at
¡ Final Study Report of the Agreement No. CE 1/97 for the Remaining Development in Tung Chung and Tai Ho Comprehensive Feasibility Study
¡
Environmental Impact
Assessment Report under the Agreement No. CE 60/96 for the Northshore
Lantau Development Feasibility; and
¡ The Technical Note on the Compilation of 2009-based Territorial Population and Employment Data Matrices (TPEDM).
9.2.3
Design
Standard Guideline
9.2.3.1
The assessment of the existing/planned downstream public sewerage networks and sewage
treatment facilities in North Lantau have been carried out based on
the following standards, guidelines,
manuals and reports:
¡ The Guidelines for Estimating Sewage Flows for Sewage Infrastructure Planning Version 1.0 published by EPD (GESF); and
¡ Sewerage Manual – Part 1 published by DSD (SM1).
9.3
Existing
and Planned Sewerage Condition
9.3.1
Existing Sewerage Network of the
Airport
9.3.1.1
The existing sewerage network at the airport comprises gravity sewers,
grey water pipelines, 11 nos. of sewage pumping stations and associated rising main which are maintained by AAHK. All the sewage flow within the existing
airport is collected locally and conveyed to a pumping station (Pumping Station
No. 1) to discharge all the sewage flow from the airport into the public sewerage system through a public sewage manhole no. FMH7042035 and then transferred to the Siu Ho Wan Sewage Treatment
Works (SHWSTW). As such, there is no sewage outfall at the
existing airport. The existing sewerage
system of the airport is shown in Drawing No. MCL/P132/EIA/9-001.
9.3.1.2 There is an
existing grey water system at the airport to collect and
convey grey water from terminal building kitchens, washroom sinks, and
aircraft catering and cleaning activities to a grey water treatment facility.
The capacity of the existing grey water treatment facility is 6,000 m3/day. The grey water is treated
with a portion used for landscape irrigation. The remaining treated grey water (i.e.,
surplus of treated
grey water) is currently overflowed to
the sewerage system within the airport
and then conveyed to the pumping station to discharge into public sewerage
system in North Lantau Island. It is understood that no
untreated grey water is currently discharged to the sewerage system as the
capacity of existing grey water treatment facility has not yet been
reached.
9.3.1.3 The
existing sewerage system was designed and maintained in accordance with the
standards and guidelines published by DSD. The existing sewerage network is
currently maintained and operated by AAHK.
9.3.2
Planned Sewerage Network for the Third Runway Development
9.3.2.1 For the proposed airport expansion, the planned sewage discharge scheme is to construct a new
sewerage system to collect the extra
sewage flow from the project and then convey it to the existing public sewerage manhole no.
FMH7042035 for discharge via the existing sewerage system in the airport.
9.3.2.2 The existing sewerage
system within the airport island will be further reviewed during
the detailed design stage and, if
necessary, upgraded to ensure the extra
sewage flow can be properly conveyed
to the public sewerage system in North Lantau. It is
anticipated that the capacities of the existing sewage pumping stations (PS1, PS2 and PS12f) are not
able to handle the increased flow volume collected from the network after Year
2023. The pumping stations
are required to be upgraded to increase
its capacity to cope with the future flow.
9.3.2.3 In addition to the possible modification of the existing sewerage
system, a new sewage pumping station will be needed to serve the additional
sewage capacity requirements of the third runway expansion. The new sewage
pumping station will be designed with spare capacity and at least one standby
pump will be provided to minimise the risk of service disruption due to
accidental pump failure. In the event of total pumping failure or burst sewer,
the contingency will be to transport the sewage by trucks to SHWSTW. With the
standby pumping capacity and contingency plan in place, emergency sewage
overflow system will not be required and there will be no emergency discharge
of sewage from the airport into the marine environment. Based on these provisions, there will be no discharge
of sewage effluent from HKIA into the marine environment, and no adverse water
quality impact due to sewage disposal is anticipated.
9.3.2.4 The planned sewerage system
will be designed in accordance with all the relevant standards and guidelines published by DSD. The planned and existing
sewerage network are maintained and operated by AAHK in accordance with the
Sewerage Manual published by DSD. In addition to continuing the odour control
arrangements currently undertaken by AAHK, maintaining the design maximum
retention time of the planned pumping station to not more than 2 hours,
monitoring the (hydrogen sulphide) H2S level once the 3RS is in operation and adoption of
active septicity management measures that can effectively contain any future
septicity problems will be included in the design for the planned 3RS sewerage
system. With the inclusion of active septicity management measures as part of
the design, maintenance and operation of the planned sewerage system, no
adverse impacts in respect of septicity and odour from the HKIA new sewerage
system are anticipated.
9.3.2.5 Due to the
scale of the project, there will be an interim operation phase after completion
of the land formation whereby the existing South Runway and the new third
runway will be in operation while the existing North Runway will be closed for
construction. The new airside fire station and the new air traffic control
tower will commence operation during the interim period,
however, the new sewerage system for the third runway facilities would be under
construction. Therefore, the sewage arising from these two facilities during
that period will be temporarily collected by a pump sump and transported to the
DSD’s sewerage treatment facilities by the licensed sewage collection
contractors at least seven times per day. The estimated sewage load from these new facilities would be
approximately 130 m3/day, therefore, no
adverse impacts to the existing sewerage system will be caused by this interim
arrangement. Upon completion of the new sewerage system for the third runway
area, all sewage flow generated within the HKIA will be collected and conveyed
via the sewerage network to the SHWSTW.
9.3.2.6 Based on the current scheme design, an additional grey water treatment
plant with a handling capacity of 700 m3/day
is proposed. More information about the
additional treatment plant is given in Chapter 8.
9.3.3
Existing Sewerage Network of Tung Chung
9.3.3.1
The existing sewerage network conveying the sewage flow from the
airport to Tung Chung comprises local gravity sewers between the airport
discharge manhole and the Tung
Chung Sewage Pumping Station (TCSPS)
rising main connecting TCSPS and SHWSTW. The locations of these sewerage infrastructures are shown in the Drawing No. MCL/P132/EIA/9-002.
9.3.3.2
The size of the
existing gravity sewers conveying the
sewage flow from the airport to TCSPS ranges from 1,050
to 1,500
mm in diameter as
shown in Drawing No. MCL/P132/EIA/9-003. The sewage catchment of
the gravity sewers mainly cover residential
establishments, commercial establishments and visitors in
Tung Chung areas.
9.3.3.3 The
TCSPS is currently served by a single rising
main of 1,200
mm in diameter and 6 km in length, which conveys the sewage generated from the
airport and Tung Chung to SHWSTW.
As advised by DSD, the
existing capacity of the installed pumpsets at
TCSPS is about 1,840 L/s, and the existing TCSPS has 3 nos. spare dry wells
reserved for the additional pumpset installation to
cater the future sewage flow.
9.3.4
Planned Sewerage Network in Tung Chung
9.3.4.1
The Planning Department (PlanD) and the
Civil Engineering and Development Department (CEDD) have jointly commissioned a consultancy study under the Agreement No. CE 32/2011 – Planning and Engineering
Study on the Remaining Development in Tung Chung. The study has
identified the development potential and opportunities for extension of the
Tung Chung New Town to the east and west, which may require a new sewerage network to support the development. The overall implementation programme for the proposed
Tung Chung New Town Extension (TCNTE) will be established after the reclamation
scale and infrastructure requirements have been determined. According to CEDD, the reclamation and infrastructure
works of the project will be carried out in phases starting from 2018 with the
anticipated first population intake in 2023/24. Also, subject to the approval
of EPD, CEDD is investigating the feasibility of conveying sewage collected
from TCNTE to the existing TCSPS and then pumped to SHWSTW for sewage
treatment. However, the associated sewerage network within the Tung Chung area
for sewage collection from TCNTE is still under planning and will be confirmed
at the later stage of their
study.
9.3.4.2 To enhance the operation
reliability of the sewerage system, a consultancy study on the construction
of an additional rising main from TCSPS
to SHWSTW is currently being carried out by DSD
under the Agreement No. CE 6/2012 –
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. According to the latest sewerage impact
assessment report from DSD, twin 1,200 mm diameter rising main will be adopted
for conveying the planned sewage flow from Tung Chung and the airport to the
SHWSTW. It
is planned to commence the construction in 2015 and complete the works by end 2022.
9.3.5
Existing/Planned
Sewage Treatment Works in North
Lantau
9.3.5.1 The sewage flow generated
from North Lantau, including Tung Chung, the airport and Hong Kong Disneyland, is
currently discharged to SHWSTW for treatment prior to
discharge to the sea through an
existing submarine outfall of 1,840 mm in diameter. The
SHWSTW has
been expanded and upgraded to Chemically
Enhanced Primary Treatment (CEPT) since March 2005. The CEPT facilities include six sedimentation tanks, a flocculation
and mixing tank, a sludge dewatering house, sludge buffer tanks, deodorization
facilities and chemical dosing facilities. The
allowable treatment capacity of these
facilities is 180,000 m3/day with a peak flow of 3,750 L/s. At the time of preparing this assessment, no further upgrading work for SHWSTW has been confirmed.
9.4
Assumptions
and Parameters Adopted for Assessment
9.4.1
Assessment
Scenarios
9.4.1.1 The assessment of impacts due to the project on the public
sewerage system in North Lantau has been carried out
for both baseline condition and ultimate design condition of the project as
summarised in Table 9.1. Where
appropriate, assessment for the interim
scenario has also been carried out to identify
the intermediate timeframe when the existing sewerage
system will become
saturated and
recommend the appropriate measures to
mitigate the impacts on the existing public sewage system in North
Lantau arising from the project.
Table 9.1: Assessment Scenarios
|
Scenario Year |
Description |
|
2012 |
Baseline
Condition |
|
2038 |
Ultimate Design Condition for 3RS (i.e., 15 years after the planned
commencement of operation of the 3RS in 2023) |
9.4.2
Global
Unit Flow Factors (GUFF) – Gravity Sewers in Tung Chung
9.4.2.1 Estimation of
the sewage flow in the affected gravity sewers in Tung Chung has been
undertaken according to the methodology as specified in GESF. Appropriate Global Unit Flow Factors
(GUFF) for domestic and commercial/institutional
activities have been selected for estimating sewage flows in the sewerage
catchments according to GESF respectively, as presented in Table 9.2.
9.4.2.2 With
reference to the Final Study Report of the Agreement
No. CE32/2010 (CE) Site Formation and Associated Infrastructural Works for
Proposed Development of Columbarium, Crematorium and Related Facilities at
Sandy Ridge Cemetery – Feasibility Study, a GUFF of 0.010 m3/day/person
is adopted for estimating
the sewage flow generated from visitors within the study area of
the project.
Table 9.2: Adopted Global Unit
Flow Factors (GUFF) for Gravity Sewers in Tung Chung
|
Category |
Unit (per) |
GUFF (m3/day) |
Data Source |
|
Domestic (catchment specific) |
|
|
|
|
Private Housing and Public Housing |
person |
0.230 |
GESF |
|
Commercial/Institutional |
|
|
|
|
Employed Population |
employee |
0.080 |
GESF |
|
Commercial Activities - General |
employee |
0.200 |
GESF |
|
Institutions/ Schools |
employee |
0.040 |
GESF |
|
Visitors for Cable Car
Terminal |
person |
0.010 |
Final Report of Agreement No. CE32/2010 |
9.4.3
Global
Unit Flow Factors (GUFF) – Sewage Treatment Works & Pumping Station
9.4.3.1 Estimation of
the sewage flow to the affected sewage treatment works and pumping
station in Tung Chung and North Lantau has been
undertaken according to the methodology as specified in GESF.
Appropriate
GUFF for domestic,
commercial, industrial and institutional activities have been selected in accordance with GESF, as shown in Table
9.3.
Table 9.3: Adopted Global Unit
Flow Factors (GUFF) for Sewage Treatment Works and Pumping Station
|
Category |
Unit (per) |
GUFF (m3/day) |
Data Source |
|
Domestic (catchment specific) |
|
|
|
|
Private Housing and Public Housing |
Person |
0.230 |
GESF |
|
Commercial/Institutional |
|
|
|
|
J2 – Electricity Gas & Water |
employee |
0.250 |
GESF |
|
J3 – Transport, Storage & Communication |
employee |
0.100 |
GESF |
|
J4 – Wholesale & Retail |
employee |
0.200 |
GESF |
|
J9 – Construction |
employee |
0.150 |
GESF |
|
J10 – Restaurants & Hotels |
employee |
1.500 |
GESF |
|
J11 – Community, Social & Personal Services |
employee |
0.200 |
GESF |
|
School Student |
person |
0.040 |
GESF |
|
Commercial Employee |
employee |
0.080 |
GESF |
|
Industrial Activities |
|
|
|
|
J1 – Manufacturing |
|
|
|
|
Industrial Employee |
employee |
0.080 |
GESF |
|
East Kowloon (overall), Sha Tin, Lantau Island (except Mui Wo) |
employee |
0.450 |
GESF |
9.4.4
Peaking Factors
9.4.4.1
According to GESF, a peaking factor should be applied in determining the
peak flow. As the service condition of the
existing sewerage systems within the study area is not
clear, the peaking
factors
that include stormwater allowance, which are more
conservative, have been adopted in this assessment. Based on GESF, the peaking factors adopted for determination of peak flows in sewers, sewage treatment works and pumping station
are based on different population ranges as shown in Table 9.4:
|
Population Range |
Peaking Factor (including stormwater
allowance) for Sewers |
|
<1000 |
8 |
|
1,000 – 5,000 |
6 |
|
5,000 – 10,000 |
5 |
|
10,000 – 50,000 |
4 |
|
>50,000 |
where: N = contributing population in
thousands; and Contributing population =
Calculated total average flow in m3/day divided by 0.27 m3/day |
|
Population Range |
Peaking Factor
(including stormwater allowance) for Sewage Treatment,
Preliminary Treatment Works and Pumping Stations |
|
<1000 |
4 |
|
10,000 – 25,000 |
3.5 |
|
25,000 – 50,000 |
3 |
|
>50,000 |
where: N = contributing population in thousands; and Contributing population = Calculated total average flow in m3/day divided by 0.27 m3/day |
9.5
Sewage Flow Estimation
9.5.1
Existing
and Projected Sewage Flow from the Airport
9.5.1.1
The maximum pumping flow rate of 628 L/s as recorded in 2012 for Pumping Station No. 1 (PS1), which collects all sewage flow
of the
existing
airport for discharge into the public manhole no. FMH7042035, has been adopted as the
existing peak
flow to assess the baseline condition of the downstream sewerage network. The Average
Dry Weather Flow
(ADWF), including the surplus of treated grey water, in PS1 in 2012 provided by
AAHK is about 18,100 m3 per day.
9.5.1.2
Based on the long term traffic forecast compiled by the International
Air Transport Association (IATA) for AAHK, various growth factors for the air
traffic
movements (ATM), numbers of passengers and quantities of cargo from 2012 to
2038 have been estimated for the project.
These growth factors were then applied to the baseline sewage flow in
2012 in order to estimate the sewage flow of
the project during the initial
operation year of 2023 and the ultimate design scenario year of 2038, the
results of which are summarised in Table 9.5. As explained in Section 9.3.1, overflow of the surplus treated grey water has
been included
in the estimates of current sewage flow and no untreated
grey water is currently discharged to the sewerage system. As the worst case scenario in this assessment,
however, it is conservatively assumed that starting from 2016 when the capacity
of existing grey water treatment works is predicted to be exceeded,
the untreated grey water would be discharged to the sewerage system. In other words, while it has been planned to
install an additional grey water treatment facility for the project, the worst
case scenario does not take into account the additional grey water treatment
capacity. Details of the sewage
estimation of airport are given in Appendix 9.1.
Table 9.5: Estimated Sewage Flows from the airport Adopted for Assessment
|
Scenario Year |
Approximate
Total ADWF of the Airport (m3/day)a |
Approximate Amount of
Untreated Grey Water of the Airport (m3/day)a |
Approximate Total ADWF of the
Airport under Worst Case Scenario (m3/day)a |
|
2012 (baseline condition) |
18,100 |
- |
18,100 |
|
2023b |
26,700 |
900 |
27,600 |
|
2038 (ultimate design condition) |
40,600 |
2,900 |
43,500 |
Note:
a. Approximate
values rounded to the nearest hundred.
b. The values in 2023
are provided to show the total ADWF arising from the airport during the initial
operation under worst case scenario, but are not for the purpose of assessment.
9.5.2
Existing
and Projected Sewage Flow for the Gravity Sewers Leading to TCSPS
9.5.2.1
In order to assess the potential impact on the local gravity sewers
between the airport discharge manhole and TCSPS, sewage flows in the existing sewers have been estimated based on the
population
and employment data within the sewage catchment areas of the concerned sewers, including the catchment
of Chung Mun Road Sewage Pumping Station (CMRSPS) and
Chung Yan Road Sewage Pumping Station (CYRSPS). It is identified that the
sewerage catchment of the gravity sewers comprises catchments L, K, M and the airport
and the corresponding catchment and sub-catchment plan are shown in Drawing No. MCL/P132/EIA/9-004 & MCL/P132/EIA/9-005
respectively.
9.5.2.2
The population and employment data of the concerned catchments have been
estimated based on the assumption that the developments
within the catchments are fully utilised. Information obtained
from private
sectors and the government departments such as the gross floor
area (GFA) of shopping malls and numbers of flats in private and
public housings have been adopted in the estimation. Due to
the lack of
employment information in some of the current government facilities such
as Police Station, Fire Station and Tung Chung Municipal Services Building, the planning data of such facilities
used in the
relevant previous
study, namely, Agreement No. CE 1/97 - Remaining
Development in Tung Chung and Tai Ho Comprehensive Feasibility Study, has been adopted
in this assessment.
9.5.2.3
Based on the aforementioned assumptions and information, the
sewage flows in different catchment areas of the gravity sewers connecting to
TCSPS have been estimated, as summarised in Table 9.6.
The locations for individual discharges at the gravity sewers are shown in Drawing No MCL/P132/EIA/9-003. Details of the
sewage flow
calculation are given in Appendix 9.2.
Table 9.6: Estimated Sewage Flows in Different Catchment
Areas in 2012
|
Catchment Area |
Approximate
ADWF in 2012 (m3/day)* |
|
Airport |
18,100 |
|
K (Existing ADWF of CYRSPS) |
8,800 |
|
L1 to L13 |
7,800 |
|
M1 to M10 |
7,200 |
|
Total to TCSPS |
41,900 |
*Note: Approximate
values rounded to the nearest hundred.
9.5.2.4
The sewerage catchments represented by the Planning Data Zones (PDZ)
system from the 2009-based TPEDM for the concerned gravity sewers and TCSPS are
245, 246, 266, 282 and 396. The estimated flow generated in these zones based
on the 2009-based TPEDM in Year 2012 and 2038 are summarised in Table 9.7. The total
flow of 41,900 m3/day in the relevant catchments (L, K, M and the
airport) in 2012 as estimated by the method
mentioned in Section
9.5.2.1 and 9.5.2.2
(see Table 9.6) is found
to be close to the total flow of 42,500 m3/day as estimated by
summing up the sewage flows in different PDZs (245, 246, 266, 282, 396 and
airport) in
the same year (see Table 9.7). Therefore, the method used to estimate the
sewage flow of the gravity sewers is considered as suitable for assessing the
impact of the concerned gravity sewers between the airport discharge manhole
and TCSPS.
Table 9.7: Estimated Sewage Flows of
Concerned PDZs for Local Gravity Sewer in 2012 and 2038
|
Planning Data Zones |
Approximate ADWF in 2012 (m3/day)* |
Approximate ADWF in 2038 (m3/day)* |
|
245, 246, 266, 282 and 396 |
24,400 |
37,500 |
|
247 (Airport) |
18,100 |
43,500 |
|
Total |
42,500 |
81,000 |
*Note: Approximate
values rounded to the nearest hundred.
9.5.2.5
As can be seen from Table
9.7, the
estimated sewage arising from the concerned PDZs (i.e., 245, 246, 266, 282
and 396)
will be increased by approximately 54 % from 24,400 m3/day
in 2012 to 37,500 m3/day in 2038.
Therefore, a conservative growth factor of 1.6 has been applied to the sewage flows in different catchments in 2012 for estimating the corresponding sewage flows in 2038, as summarised in Table 9.8.
Table 9.8: Estimated Sewage Flows in
Different Catchment Areasin 2038
|
Catchment Area |
Approximate
ADWF in 2038 (m3/day)* |
|
Airport |
43,500 |
|
K (Existing ADWF of CYRSPS) |
14,100 |
|
L1 to L13 |
12,500 |
|
M1 to M10 |
11,600 |
|
Total to TCSPS |
81,700 |
*Note: Approximate values
rounded to the nearest hundred.
9.5.2.6
As discussed in Section 9.3.4, details of the
proposed TCNTE and the associated
sewerage network are yet to be confirmed. Therefore, this proposed TCNTE has not been considered in
the assessment of local gravity sewers under the ultimate
design scenario.
9.5.3
Existing
and Projected Sewage Flow in TCSPS and SHWSTW
9.5.3.1 In order to assess the impact on TCSPS and SHWSTW
due to the project, the future sewage flows handled by TCSPS and SHWSTW are
estimated according to the population forecast in the airport and the sewerage
catchments of TCSPS and SHWSTW in different years.
9.5.3.2 The sewerage catchments represented by the Planning
Data Zones (PDZ) system from the 2009-based TPEDM are shown in Drawing No. MCL/P132/EIA/9-006.
The sewerage catchments of TCSPS and SHWSTW in terms of PDZ are presented in Table 9.9.
9.5.3.3 The
unit of person represents the total number of “Residents with Regular Residence
in HK” within the PDZ from the “Technical
Note on the Compilation of 2009-based Territorial Population and Employment
Data Matrices (TPEDM).
Table 9.9: Sewerage Catchments of TCSPS and SHWSTW by PDZ (excluding the project area)
|
Planning Data Zones |
Sewerage Catchments of TCSPS |
Sewerage Catchments of SHWSTW |
|
239 |
|
Yes |
|
241 |
|
Yes |
|
245 |
Yes |
Yes |
|
246 |
Yes |
Yes |
|
265 |
|
Yes |
|
266 |
Yes |
Yes |
|
282 |
Yes |
Yes |
|
283 |
Yes |
Yes |
|
284 |
|
Yes |
|
327 |
|
Yes |
|
395 |
|
Yes |
|
396 |
Yes |
Yes |
|
398 |
|
Yes |
9.5.3.4
Although the 2011-based TPEDM was issued in
July 2013, it
is considered more conservative to estimate the overall sewage flows
from PDZs
within the sewerage catchments of SHWSTW based on
the 2009-based TPEDM. Therefore, it is proposed to adopt the 2009-based TPEDM
for the
sewage flow estimation. Comparison of the key assumptions adopted in
the 2009-based
and 2011-based TPEDM is presented in Appendix 9.3.
9.5.3.5
The 2009-based TPEDM includes the population forecast in each PDZ in five
discrete years, namely, 2011, 2016, 2021, 2026 and 2031. The population forecasts for years not provided by the TPEDM between 2012 and
2038 are estimated by either interpolation or extrapolation from the data
of the available years. Moreover, adjustments to 2009-based TPEDM have been
made for the TCNTE and other proposed development within the sewerage
catchments of TCSPS and SHWSTW. The adjustments and assumptions adopted in
2009-based TPEDM are summarised in Table 9.10. Details
of estimating
the population forecasts in years between 2012 and 2038 are
shown in the Appendix
9.4.
Table 9.10: Adjustments and
Assumptions adopted in TPEDM 2009-based for Proposed Developments
|
Proposed Developments |
PDZs |
Adjustment to PDZ / Assumption
adopted |
|
TCNTE |
266,
282, 283 & 396 |
According to the latest assumption provided by the CEDD, the
sewage estimation for TCNTE is interpolated from 0 to 39,200 m3/day in between 2023 and 2027.
Therefore, the population forecast for TCNTE has been excluded from the
concerned PDZs for Year 2023 to 2038. The nil increase has been assumed from
Year 2023 to Year 2038 for Residential, Commercial and Educational
population. |
|
Public Rental Housing Development
at Tung Chung Area 39 |
246 |
Refer to revised population information
from Paper No. IDC/2013 from Islands District Council, the additional
population of 3,000 has been included from Year 2018 for the Public Rental Housing Development at Tung Chung Area
39. |
|
Lantau Logistic Park, Sunny Bay Development, Proposed Cross Boundary Transport
Hub above Siu Ho Wan Depot |
239
& 284 |
No update information has been
included for the concerned PDZs. The population assumption for these developments is based on the TPEDM
2009 (Revised Lantau Concept Plan released in May 2007). |
9.5.3.6
A domestic (Residential) population is made up of household
members who are either Usual Residents and Mobile Residents with
regular residence in Hong Kong within the Planning Data Zones (PDZ) from the 2009-based TPEDM (i.e. Residents with
regular residence in Hong Kong). The mobile residents and transients other
than usual residents present in Hong Kong (i.e. visitor) is excluded from the
domestic population as the GUFF for job type J10 as shown in Table 9.3 already
taken into account the sewage flow arising from customers and/or visitors.
9.5.3.7 Based on the population breakdown assigned to PDZ
of TPEDM and the appropriate GUFF, ADWF of individual PDZ that are within the
sewerage catchments of TCSPS and SHWSTW are estimated for the baseline and
ultimate design scenario years of 2012 and 2038. Table 9.11 summarises
the estimated ADWFs of PDZs and the airport.
Table 9.11: Estimated ADWF for the Relevant PDZs and airport in 2012 and 2038
|
Planning Data Zones |
Sewerage Catchments |
ADWF in 2012 (m3/day) |
ADWF in 2038 (m3/day) |
|
239 |
SHWSTW |
53 |
6,271 |
|
241 |
SHWSTW |
5,861 |
5,792 |
|
245 |
TCSPS & SHWSTW |
13,132 |
18,196 |
|
246 |
TCSPS & SHWSTW |
10,640 |
17,682 |
|
265 |
SHWSTW |
42 |
58 |
|
266 |
TCSPS & SHWSTW |
107 |
217 |
|
282 |
TCSPS & SHWSTW |
119 |
179 |
|
283 |
TCSPS & SHWSTW |
0 |
Included in TCNTE |
|
284 |
SHWSTW |
92 |
3,929 |
|
327 |
SHWSTW |
2,805 |
5,239 |
|
395 |
SHWSTW |
108 |
4,056 |
|
396 |
TCSPS & SHWSTW |
365 |
1,137 |
|
398 |
SHWSTW |
2 |
3,865 |
|
TCNTE* |
TCSPS & SHWSTW |
0 |
39,200 |
|
Airport |
TCSPS & SHWSTW |
18,100 |
43,500 |
*Note: ADWF arising from TCNTE for Year 2012
and Year 2038 is provided by CEDD
9.5.3.8 Based on the estimated
ADWF of the various PDZ and the airport, the total ADWF of TCSPS and SHWSTW in 2012
and 2038 are calculated, as summarised in the Table 9.12. Details of
the total sewage
flow estimation for TCSPS and SHWSTW in 2012 and 2038 are given in Appendix 9.5.
Table 9.12: Estimated ADWF of TCSPS and SHWSTW in 2012 and 2038
|
Sewage Facilities |
Approximate ADWF in 2012 (m3/day)* |
Approximate ADWF in 2038 (m3/day)* |
|
Tung Chung Sewage Pumping Station (TCSPS) |
42,500 |
120,200 |
|
Siu Ho Wan Sewage Treatment Works (SHWSTW) |
51,500 |
149,400 |
*Note: Approximate values
rounded to the nearest hundred
9.6
Assessment
of Impact to Existing/Planned Sewerage
and Sewage Treatment System
9.6.1
Overview
9.6.1.1
According to the information obtained from AAHK, the project will be in full-scale operation
in 2038 and thus the sewage flow in that year will be the ultimate design flow, which has been used to assess the impacts on the following key components of the sewerage and sewage treatment system:
¡ Gravity sewers between the airport discharge manhole and TCSPS;
¡ TCSPS and the associated rising main; and
¡ SHWSTW.
9.6.2
Gravity
Sewers from Airport Discharge Manhole to TCSPS
Basis of the Assessment
9.6.2.1
To assess the impacts on the existing gravity sewers from the airport discharge manhole, reference point
as shown in Table 9.10, to TCSPS (see MCL/P132/EIA/9-003), the estimated sewage flows from the airport for the scenario years of 2012 and 2038 (see Table 9.5) as well as the sewage flows from
different catchment areas in 2012 (see Table 9.6) are used as the basis. A summary of these estimated sewage flows adopted for the assessment is shown in Table 9.13. Details of the sewage flow calculation are given in Appendix 9.2.
Table 9.13: Estimated Sewage Flows for Assessment of the Gravity Sewer in 2012
|
Reference Point |
Discharge Manhole |
Sewerage Catchment |
Approximate ADWF (m3/day) |
Contributing Population |
Peaking Factor |
Peak Flow (L/s) |
|
|
A |
FMH7042035 |
Airport |
18,100 |
- |
3.0 |
628 |
|
|
B |
FMH7042030 |
L1, L2 & L4 |
984 |
3,644 |
6.0 |
68 |
|
|
C |
FMH7043286 |
L3, L5, L6, L7, L8, L9,
L10, L11,L12, L13 & K |
15,605 |
57,797 |
4.0 |
717 |
|
|
D |
FMH7043283 |
M1 to M10 |
7,138 |
26,437 |
4.0 |
330 |
|
9.6.2.2
According to the
latest information provided by the CEDD, the sewage arising from the
development of Tung Chung West and Tung Chung Village, part of TCNTE, is proposed
to be discharged to Point C (Manhole No.FMH7043286) which is from 0 to 10,450 m3/day in between year 2023 and year 2027.
As explained in Section 9.5.2.5, the estimated
sewage flows of concerned catchments L, M and K can be projected by applying
the growth factor to the corresponding sewage flows in 2012 for assessment of
ultimate design scenario, as summarised in Table 9.14.
Table 9.14: Estimated Sewage Flows for Assessment of the Gravity Sewer in 2038
|
Reference Point |
Discharge Manhole |
Sewerage Catchment |
Approximate ADWF (m3/day) |
Contributing Population |
Peaking Factor |
Growth Factor |
Peak Flow (L/s) |
|
A |
FMH7042035 |
Airport |
43,500 |
- |
3.0 |
1.536 |
1,510 |
|
B |
FMH7042030 |
L1, L2 & L4 |
1,511 |
5,598 |
5.0 |
1.536 |
87 |
|
C |
FMH7043286 |
L3, L5, L6, L7, L8, L9,
L10, L11,L12, L13 & K |
23,970 |
34,420 |
3.5 |
1.536 |
1,405 |
|
Tung Chung West & Tung Chung Valley |
10,450 |
||||||
|
D |
FMH7043283 |
M1 to M10 |
10,964 |
40,607 |
4.0 |
1.536 |
508 |
Hydraulic Assessment
9.6.2.3 The hydraulic assessment on the existing gravity sewers from the airport
discharge manhole to TCSPS is carried by the hydraulic model, InfoWorks
(CS Version 14.0), as required in the EIA Study Brief. Details of the modelling results for the existing
sewers are presented in Appendix 9.6, and are discussed below.
9.6.2.4 The sewerage model network of the concerned gravity
sewers was setup according to the drainage record plan published by DSD. The
ground level for the node of manhole is based on the information from the Light Detection and Ranging (LIDAR) survey data.
9.6.2.5 The pipe roughness coefficient is specified as an
equivalent roughness (ks) used by the Colebrook-White
equation. In this hydraulic assessment, the roughness coefficient or ks value used for the
existing sewer is taken as 6 mm to cater for
long term operational conditions of the sewer.
9.6.2.6 According to SM1, for
sewers of diameter up to 900 mm, it should be designed to achieve a self-cleansing velocity of 1.0 m/s in full pipe condition. The
baseline model has been checked against with this requirement for the scenario
of 2 times of ADWF. The overall velocity for the gravity sewer between airport
discharge manhole and TCSPS generally exceed 1.0m/s. Therefore, the velocity is sufficient to enable the sewage flow to achieve self-cleansing effect on the concerned sewer, hence, no sediment is assumed in the hydraulic model. The model result for checking of this
requirement is presented in Appendix 9.6.
9.6.2.7 As highlighted in SM1, local head losses arisen at
inlets, outlets, bends, elbows, joints, valves, manholes and other fittings are
usually small in relation to the pipeline head losses and are not normally
considered. Therefore, these head losses are not assumed in this
hydraulic assessment.
9.6.2.8 As summarised in Table 9.15,
the InfoWorks modelling results show that all the
assessed sewers would not be surcharged (i.e., with surcharge ratios well below
one) during the baseline scenario year of 2012.
Therefore, the existing
gravity sewers between the
airport discharge manhole and TCSPS are sufficient to receive all
the collected sewage flow from its sewerage catchments including the airport in
2012.
Table 9.15: Summary
of the Hydraulic Modelling Results for the Existing Gravity Sewers in 2012
|
Sewerage Catchments/ Upstream Manhole |
Downstream Manhole |
Pipe Diameter (mm) |
Hydraulic Modelling Results in 2012 |
|
|
Freeboard
at Peak
Flow (mm) |
Surcharge Ratio* |
|||
|
Airport |
FMH7042035 |
700 |
- |
- |
|
FMH7042035 |
FMH7042031 |
1,050 |
3,135 |
0.60 |
|
FMH7042031 |
FMH7042032 |
1,050 |
3,334 |
0.64 |
|
FMH7042032 |
FMH7042033 |
1,050 |
2,860 |
0.64 |
|
FMH7042033 |
FMH7042034 |
1,050 |
3,112 |
0.61 |
|
FMH7042034 |
FMH7042030 |
1,050 |
3,621 |
0.60 |
|
Catchment L1, L2 & L4 |
||||
|
FMH7042030 |
FMH7042061 |
1,050 |
4,217 |
0.60 |
|
FMH7042061 |
FMH7042060 |
1,050 |
3,833 |
0.59 |
|
FMH7042060 |
FMH7043290 |
1,050 |
4,261 |
0.54 |
|
FMH7043290 |
FMH7043289 |
1,050 |
4,274 |
0.59 |
|
FMH7043289 |
FMH7043288 |
1,050 |
4,820 |
0.59 |
|
FMH7043288 |
FMH7043287 |
1,050 |
5,270 |
0.56 |
|
FMH7043287 |
FMH7043286 |
1,050 |
7,931 |
0.50 |
|
Catchment L3, L5, L6,
L7, L8, L9, L10, L11,L12, L13 & K |
||||
|
FMH7043286 |
FMH7043285 |
1,350 |
7,459 |
0.56 |
|
FMH7043285 |
FMH7043284 |
1,350 |
7,416 |
0.50 |
|
FMH7043284 |
FMH7043283 |
1,350 |
8,202 |
0.45 |
|
Catchment M |
||||
|
FMH7043283 |
FMH7000680
(TCSPS) |
1,500 |
8,461 |
0.67 |
*Note: Surcharge ratio extracted from the InfoWorks
Model Result (i.e. surcharge ratio > 1: the pipe is
under surcharge condition)
9.6.2.9 In
the ultimate design scenario
year of 2038, there will be additional sewage flow of 25,400
(i.e., 43,500 –
18,100) m3/day
from the expanded airport discharging into the gravity sewers when compared to
the baseline scenario year of 2012. The hydraulic assessment results for Year 2038
are summarised in Table 9.16. It can be seen from the Table that some of
the existing gravity sewers would be surcharged from the design flow during the ultimate design scenario
(i.e., exceeded the “design capacity” of the sewer).
9.6.2.10 The predicted
freeboard of those surcharged sewers at peak flow during the ultimate design scenario year
of 2038 is summarized in Table 9.16.
Table 9.16: Summary
of the Hydraulic Modelling Results for the Existing Gravity Sewers in 2038
|
Sewerage Catchments/ Upstream Manhole |
Downstream Manhole |
Pipe Diameter (mm) |
Hydraulic Modelling Results in
2038 |
|
|
Freeboard at Peak Flow (mm) |
Surcharge Ratio* |
|||
|
Airport |
FMH7042035 |
700 |
- |
- |
|
FMH7042035 |
FMH7042031 |
1,050 |
1,778 |
2.00 |
|
FMH7042031 |
FMH7042032 |
1,050 |
2,150 |
2.00 |
|
FMH7042032 |
FMH7042033 |
1,050 |
1,880 |
2.00 |
|
FMH7042033 |
FMH7042034 |
1,050 |
2,308 |
2.00 |
|
FMH7042034 |
FMH7042030 |
1,050 |
2,954 |
2.00 |
|
Catchment L1, L2 & L4 |
||||
|
FMH7042030 |
FMH7042061 |
1,050 |
3,761 |
2.00 |
|
FMH7042061 |
FMH7042060 |
1,050 |
3,484 |
2.00 |
|
FMH7042060 |
FMH7043290 |
1,050 |
3,571 |
0.86 |
|
FMH7043290 |
FMH7043289 |
1,050 |
3,742 |
2.00 |
|
FMH7043289 |
FMH7043288 |
1,050 |
4,446 |
2.00 |
|
FMH7043288 |
FMH7043287 |
1,050 |
4,925 |
0.90 |
|
FMH7043287 |
FMH7043286 |
1,050 |
7,367 |
0.82 |
|
Catchment L3, L5, L6, L7, L8, L9, L10, L11,L12, L13, K, Tung Chung West &
Tung Chung Valley |
||||
|
FMH7043286 |
FMH7043285 |
1,350 |
6,992 |
0.96 |
|
FMH7043285 |
FMH7043284 |
1,350 |
6,982 |
0.84 |
|
FMH7043284 |
FMH7043283 |
1,350 |
7,754 |
0.78 |
|
Catchment M |
||||
|
FMH7043283 |
FMH7000680 (TCSPS) |
1,500 |
8,098 |
0.96 |
*Note: Surcharge ratio extracted from the InfoWorks
Model Result (i.e. surcharge ratio > 1: the pipe is
under surcharge condition)
9.6.2.11 In view of the above
assessment findings
and in accordance with SM1 that for design purposes the full bore flow at no
surcharge shall be taken as the design capacity of a sewer and sewers under
surcharge are not desirable, adequate
mitigation measure should be implemented in a timely manner to eliminate the surcharged condition. AAHK
therefore proposed to construct
a new gravity sewer with a diameter of 1,200 mm adjacent to the existing
gravity sewer (1,050 mm in diameter) and then to divert the sewage flow arising
from the airport and other sub-catchment in Tung Chung to the new gravity
sewer, as shown in Appendix
9.7. AAHK will consider to study the
feasibility to keep the proposed abandoned sewer (i.e., the existing gravity
sewer of 1,050 mm in diameter) in place as a spare sewer with an overflow
system for the emergency discharge subject to future deign of the new gravity
sewer by around 2022.
9.6.2.12 The sewer upgrading work
will be able to provide sufficient design capacity in the sewer in order to
deliver the sewage arising from the project to the TCSPS. The hydraulic
assessment for the upgrading proposal is summarized in Table 9.17 and Appendix 9.7. Design
and construction of the new gravity sewer shall be in accordance with relevant
guidelines as set out in the Project Administration Handbook for Civil
Engineering Works, Sewerage Manual published by DSD, DSD Technical Circular No.
2/2005 and DSD Practice Note No. 1/2010 to ensure smooth delivery and handing over
of the new sewer to DSD for the completion of works.
9.6.2.13 Since the concerned gravity sewer is projected to reach
full capacity by 2027, the sewer upgrading
work shall be completed by 2026 (allowing a buffer period of about one year
before the full capacity is reached), with the planning work to commence in
2022 (assuming one year for planning plus three years for design and
construction).
Table 9.17: Summary
of the Hydraulic Modelling Results for the Proposed Gravity Sewers in 2038
|
Sewerage Catchments/ Upstream Manhole |
Downstream Manhole |
Pipe Diameter (mm) |
Hydraulic Modelling Results in 2038 |
|
|
Freeboard
at Peak
Flow (mm) |
Surcharge Ratio* |
|||
|
Airport |
FMH01 |
700 |
- |
- |
|
FMH01 |
FMH02 |
1,200 |
2,652 |
0.80 |
|
FMH02 |
FMH03 |
1,200 |
3,315 |
0.78 |
|
FMH03 |
FMH04 |
1,200 |
3,909 |
0.71 |
|
Catchment L1, L2 & L4 |
||||
|
FMH04 |
FMH05 |
1,200 |
3,935 |
0.65 |
|
FMH05 |
FMH06 |
1,200 |
4,060 |
0.64 |
|
FMH06 |
FMH07 |
1,200 |
5,005 |
0.53 |
|
FMH07 |
FMH7043286 |
1,200 |
7,483 |
0.50 |
|
Catchment L3, L5, L6,
L7, L8, L9, L10, L11,L12, L13, K, Tung Chung West & Tung Chung Valley |
||||
|
FMH7043286 |
FMH7043285 |
1,350 |
6,998 |
0.88 |
|
FMH7043285 |
FMH7043284 |
1,350 |
7,129 |
0.78 |
|
FMH7043284 |
FMH7043283 |
1,350 |
7,949 |
0.74 |
|
Catchment M |
||||
|
FMH7043283 |
FMH7000680
(TCSPS) |
1,500 |
8,130 |
0.92 |
*Note: Surcharge ratio extracted from the InfoWorks
Model Result (i.e. surcharge ratio > 1: the pipe is
under surcharge condition)
9.6.2.14 While AAHK undertakes to implement and complete the
mitigation works for the affected gravity sewers by 2026, the discharge of
additional sewage will start upon commissioning of the project and the sewage
build up may be at a more rapid rate than that predicted. Therefore, it is recommended that AAHK should
monitor the sewage flow build-up as a part of the environmental monitoring and
audit (EM&A) for the project and start planning construction of the
upgrading works in 2022 or when the sewage flow in the affected gravity sewer
exceeds 80% of the design capacity of the sewer, whichever is earlier, so as to
ensure timely completion of the mitigation works before the flow would exceed
the design capacity of the sewer.
9.6.3
Tung
Chung Sewage Pumping Station (TCSPS) and the associated Rising Main
9.6.3.1 The potential impact on
TCSPS has been assessed based on the estimated sewage flows arising from the project in 2012 and 2038 as well as the population forecast from the 2009-based TPEDM within the
catchment areas of TCSPS (see Table 9.11). The estimated sewage flow handled by TCSPS are summarised in Table 9.18 and details of the sewage flow estimation are given in Appendix 9.5.
Table 9.18: Estimated Sewage Flows Handled by TCSPS in 2012, 2022 and 2038
|
Scenario Year |
Approximate ADWF from Relevant PDZs (m3/day)a |
Approximate ADWF from Airport (m3/day)a |
Total ADWF (m3/day) |
Contributing Population |
Peaking Factor |
Total Peak Flow (L/s) |
Existing Design Peak Flow of TCSPS (L/s) |
|
2012 (baseline condition) |
24,400 |
18,100 |
42,500 |
157,271 |
2.8 |
1,380 |
1,840 |
|
2022b |
34,400 |
23,100 |
57,500 |
212,937 |
2.8 |
1,832 |
|
|
2038 (ultimate design condition) |
76,700 |
43,500 |
120,200 |
444,854 |
2.6 |
3,648 |
Note:
a.
Approximate values rounded to the
nearest hundred.
b.
While 2022 is not one of the
assessment scenarios, the values in 2022 are provided to show that the design peak flow of TCSPS would be exceeded after that particular year.
9.6.3.2 As shown in Table 9.18, the total peak sewage flow from the airport and the relevant PDZ
will exceed the existing design peak flow of TCSPS (1,840 L/s) during the ultimate design scenario year of 2038.
9.6.3.3 By
estimating the total ADWF and peak flows between 2012 and 2038 , it can be found that the total peak
sewage flow from the airport and the relevant PDZ would exceed the existing design peak flow of TCSPS after 2022 (i.e., from 2023 onwards), subject to future
development of TCNTE, as presented in Table 9.18.
9.6.3.4 As mentioned in Section
9.3.4, a government project under the Agreement No.CE6/2012 is currently
underway to investigate, design and construct an additional sewage rising main
between TCSPS and SHWSTW to enhance the operation reliability of the sewerage
system. Under the project, it is planned to commence the construction in 2015 and complete the works by end 2022. According to the latest
information from DSD, twin 1,200 mm diameter rising main will be adopted for
conveying the planned sewage flow from Tung Chung and the airport to the
SHWSTW, which is sufficient for the estimated ultimate design sewage flow of
3,648 L/s. [Note: EPD has
agreed to reserve 43,500 m3 /day
(ADWF) at the TCSPS for the total sewage discharge from the expanded airport and AAHK will closely liaise with EPD and DSD to ascertain a smooth
interface with the upgrading works for TCSPS.] AAHK
will monitor the situation closely and take appropriate action to handle the
excess sewage arising in case there is any programme
mismatch.
9.6.4
Siu
Ho Wan Sewage Treatment Works (SHWSTW)
9.6.4.1 The potential impact on
SHWSTW has been assessed based on the estimated sewage flows arising from the project in 2012 and 2038 as well as the population forecast from the 2009-based TPEDM within the
catchment areas of SHWSTW (see Table 9.11). The estimated sewage flows handled by SHWSTW are summarised in Table 9.19 and details of the sewage flow estimation are given in Appendix 9.5.
Table 9.19: Estimated Sewage Flows Handled by SHWSTW in 2012, 2026 and 2038
|
Scenario Year |
Approximate ADWF from Relevant PDZ (m3/day)* |
Approximate ADWF from Airport (m3/day)a |
Total ADWF (m3/day)a |
Contributing Population |
Peaking Factor |
Total Peak Flow (L/s) |
Existing Treatment Capacity of SHWSTW |
|
|
Peak Flow (L/s) |
Daily Flow (m3/day) |
|||||||
|
2012 (baseline condition) |
33,400 |
18,100 |
51,500 |
190,462 |
2.8 |
1,650 |
3,750 |
180,000 |
|
2026b |
87,900 |
30,800 |
118,700 |
439,402 |
2.6 |
3,606 |
||
|
2038 (ultimate design condition) |
105,900 |
43,500 |
149,400 |
553,038 |
2.6 |
4,471 |
||
Note:
a. Approximate values rounded to the nearest
hundred.
b. While 2026 is not one of the assessment scenarios, the values in 2026 are provided to show that the design peak flow of SHWSTW would be exceeded after that particular year.
9.6.4.2
It can be seen from Table 9.19 that the estimated total ADWF and peak sewage flow to be handled by SHWSTW in 2026 would be approximately 118,700 m3/day and 3,606 L/s respectively, which are within the treatment capacity of the existing SHWSTW. However, the design peak flow (3750 L/s) of
the existing SHWSTW would be exceeded after 2026. The upgrading work for SHWSTW is considered necessary to cater for the ultimate design
peak flow of 4,471 L/s from the expanded airport and the relevant PDZ in 2038.
9.6.4.3
In view of the above assessment findings, it is considered that the
existing SHWSTW will not be able to handle estimated peak flows from the
project as well as other future development within the relevant PDZ after 2026.
It is understood that the upgrading works of SHWSTW will be carried out by the
relevant government departments before exceeding the capacity, and the scope and programme of the upgrading works are still under discussion among the relevant government departments
subject to future development within the catchment of SHWSTW including TCNTE. It is understood
that EPD will monitor the sewage flow build-up and coordinate the necessary
upgrading works for the SHWSTW when needed in due course.
9.6.4.4
With the upgrading of
SHWSTW to cater for the proposed development within North Lantau such as TCNTE,
the project and other developments of relevant PDZs, there is no need to establish any central pre-treatment facilities or separate
sewage treatment plant for the project.
9.7
Mitigation Measures
9.7.1
Planned
Sewerage System within the Expanded Airport Island
9.7.1.1 The planned sewerage
system will be designed in accordance with all the relevant standards and guidelines published by DSD. The planned and existing
sewerage network are maintained and operated by AAHK in accordance with the
Sewerage Manual published by DSD. In addition to continuing the odour control
arrangements currently undertaken by AAHK, maintaining the design maximum retention
time of the planned pumping station to not more than 2 hours, monitoring the H2S
level once the 3RS is in operation and adoption of active septicity management
measures that can effectively contain any future septicity problems will be
included in the design for the planned 3RS sewerage system. With the inclusion
of active septicity management measures as part of the design, maintenance and
operation of the planned sewerage system, no adverse impacts in respect of
septicity and odour from the HKIA new sewerage system are anticipated.
9.7.2
Gravity
Sewers from Airport Discharge Manhole to TCSPS
9.7.2.1 According to the InfoWorks modelling results,
it is predicted that some of the existing gravity sewers from the
airport discharge manhole to TCSPS would be under surcharge condition during
the ultimate design scenario year of 2038.
9.7.2.2 In view of the above
assessment findings and in accordance with SM1 that for design
purposes the full bore flow at no surcharge shall be taken as the design
capacity of a sewer and sewers under surcharge are not desirable, adequate mitigation measure should be
implemented in a timely manner to
eliminate the surcharged condition. AAHK therefore proposed to construct a new gravity sewer with a diameter
of 1,200 mm adjacent to the existing gravity sewer (1,050 mm in diameter) and
then to divert the sewage flow arising from the airport and other sub-catchment
in Tung Chung to the new gravity sewer, as shown in Appendix 9.7. AAHK will consider to study the feasibility
to keep the proposed abandoned sewer (i.e., the existing gravity sewer of 1,050
mm in diameter) in place as a spare sewer with an overflow system for the
emergency discharge subject to future deign of the new gravity sewer by around
2022.
9.7.2.3 This
sewer upgrading work will be able to provide sufficient design capacity in the
sewer in order to deliver the sewage arising from the project to the TCSPS. The
hydraulic assessment for the upgrading proposal is summarized in Table 9.17 and Appendix 9.7. Design and construction of the new gravity
sewer shall be in accordance with relevant guidelines as set out in the Project
Administration Handbook for Civil Engineering Works, Sewerage Manual published
by DSD, DSD Technical Circular No. 2/2005 and DSD Practice Note No. 1/2010 to
ensure smooth delivery and handing over of the new sewer to DSD for the
completion of works.
9.7.2.4 Since the
concerned gravity sewer is projected to reach full capacity by 2027, the sewer upgrading work shall be completed by 2026 (allowing a buffer
period of about one year before the full capacity is reached), with the
planning work to commence in 2022 (assuming one year for planning plus three years for design and construction).
9.7.2.5 While AAHK undertakes to implement and complete the
mitigation works for the affected gravity sewers by 2026, the discharge of
additional sewage will start upon commissioning of the project and the sewage
build up may be at a more rapid rate than that predicted. Therefore, it is recommended that AAHK should
monitor the sewage flow build-up as a part of the EM&A for the project and
start planning construction of the upgrading works in 2022 or when the sewage
flow in the affected gravity sewer exceeds 80% of the design capacity of the
sewer, whichever is earlier, so as to ensure timely completion of the
mitigation works before the flow would exceed the design capacity of the sewer.
Potential Secondary Environmental Impacts from
the Proposed Upgrading Works
9.7.2.6 The design and construction of new gravity sewer
for replacement of the existing sewer is anticipated to begin in early 2024 and
complete by 2026, which would not overlap with the planned construction period
of the 3RS project (i.e., 2015 – 2023).
The upgrading work is not of large scale, comprising a works area of
about 550 m in length and 3 m in width, and would generally involve the
following construction activities:
¡ Break up of road surface
¡ Excavation of soil material
¡ Laying of new sewers
¡ Steel fixing and concreting of manholes
¡ Laying of bedding material
¡ Backfilling and soil compaction
¡ Reinstatement of road surface