6
Sewerage and sewage treatment implications
6.1 Introduction
6.1.1 The potential sewerage impacts associated with construction and operation of the Project are presented in this section. The main objectives of this sewerage assessment include the followings:
¡¤ Review the existing sewerage condition of the Site based on available information;
¡¤ Assess the sewage generated from the Project;
¡¤ Outline the methodology adopted in this assessment;
¡¤ Assess any potential impact on the existing or planned sewerage facilities nearby due to the Project; and
¡¤ Propose mitigation measures and/or improvement works, if required, to minimize any potential sewerage impact from the Project.
6.2
Legislation, Standards and Guidelines
6.2.1 The standards and guidelines for assessing the existing sewerage and design of the proposed sewerage are listed in Table 6.1.
Table 6.1 List of Design Standards for Sewerage Design
|
Design Standards |
|
Hong Kong Buildings
Ordinance and Building Regulations |
|
Sewerage
Manual Part 1 and Part 2, Drainage Services Department (DSD¡¯s Sewerage
Manual) |
|
Guidelines
for Estimating Sewage Flows for Sewage Infrastructure Planning Version 1.0
(Report No. EPD/TP 1/05) (GESF), Environmental Protection Department |
6.3
Existing and Planned Conditions
Existing Sewage Treatment Works
6.3.1
The
Siu Ho Wan Sewage Treatment Works (SHWSTW), situated at about 420 meters from
the eastern boundary of SHD, is a chemically enhanced primary treatment works
(CEPT) with an ultimate design capacity of 180,000 m3/day[1]
and a design peak flow of 3,750 L/s. The catchment of SHWSTW covers areas in
North Lantau including Tung Chung, Tai Ho, Sunny Bay and the Hong Kong
International Airport (HKIA). The current mean daily average flow towards
SHWSTW is approximately 52,000 m3/day.
Existing and Planned
Sewerage Networks
6.3.2
The sewage inside the existing SHD is collected
by gravity sewers and rising mains. The sewage is then conveyed to an existing
pumping station within the SHD and ultimately pumped to existing manhole
FMH7022270 via an existing DN200 rising main crossing the North Lantau Highway
and Lantau Airport Railway along Shum Shui Kok Drive. The existing manhole
conveyed sewage from SHD, a DN150 rising sewer and a 300mm dia. gravity sewer
to existing DSD pumping station and the sewage is then pumped to SHWSTW. Total
sewerage discharge (ADWF) from the existing SHD is about 8.63 L/s (746 m3/day).
Estimating of the existing sewage is shown in Appendix 6.1,
while the existing sewerage system is shown in Figure No. NEX1062/S/SHD/ACM/Z10/180.
6.3.3 As the existing 1200mm dia. rising main along North Lantau Highway as shown in Figure No. NEX1062/S/SHD/ACM/Z10/180 is the only means of conveying sewage from Tung Chung to SHWSTW, construction of an additional 1200mm dia. rising main had commenced and rehabilitation of the existing mains would 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 ¨C Investigation, Design and Construction¡±.
6.3.4 According to the approved Environmental Impact Assessment of the Tung Chung New Town Extension (TCNTE EIA) (Register No.: AEIAR-196/2016), a twin 600mm dia. rising mains was proposed to transfer sewage from the planned Tung Chung East (East) Sewage Pumping Station to SHWSTW. Its alignment would follow the future Road P1 (Tung Chung ¨C Tai Ho Section) and then along the footpath at the rear of the existing seawall outside the SHD. The rising mains would then cross below the Tung Chung Line (TCL) tunnel and Airport Express Line (AEL) and North Lantau Highway, then along the existing eastern drainage reserve and maintenance access road with discharge to SHWSTW. The existing/planned/proposed sewerage network in vicinity of the project site are included in Figure No. NEX1062/S/SHD/ACM/Z10/181.
6.4
Assessment Methodology and Design Criteria
Assessment Methodology
6.4.1 The impact assessment has been carried out in accordance with the guidelines set out in GESF for Sewerage Infrastructure Planning Version 1.0 (Report No. EPD/TP1/05) and DSD¡¯s Sewerage Manual.
Development Parameters
6.4.2 For the purpose of this study, assumptions on the operation of SHO and Reprovisioned SHD have been made and summarised in Table 6.2, which are subjected to adjustment where necessary in detailed design stage.
Table 6.2 Development
Schedule
|
Employee/ Job Type |
Project
Element |
|
|
Reprovisioned
SHD (Floor
Area: 49,386 m2) |
SHO (Floor Area:
13,465 m2) |
|
|
J2 - Electricity, Gas & Water |
1,260 |
- |
|
J3 - Transport, Storage & Communication |
1,933 |
864 |
|
J4 - Wholesale & Retail |
- |
26 |
|
J10 - Restaurants & Hotels |
100 |
3 |
|
Contributing Population from Public toilet |
- |
419 |
|
Total Population |
3,293 |
1,312 |
Unit Flow Factors ¨C Commercial Flows
6.4.3 Commercial flows comprise flows from the employee and commercial activities which contain 11 job types as classified in GESF. The unit flow factors of commercial flows are shown in Table 6.3.
Table 6.3 Unit
Flow Factors for Commercial Flows
|
Employee/ Job Type(1) |
Unit Flow Factor (m3/head/day) |
|
Commercial Employee |
0.080 |
|
Commercial activities: |
|
|
J2 Electricity, Gas & Water |
0.250 |
|
J3 Transport, Storage & Communication |
0.100 |
|
J4 Wholesale &
Retail |
0.200 |
|
J10 Restaurants &
Hotels |
1.500 |
Note:
(1) Appendix V of
Guidelines for Estimating Sewage Flows for Sewage Infrastructure Planning
Version 1.0 (Report No. EPD/TP 1/05) refers.
Peaking Factors
6.4.4 The Peaking Factor (P) is used to determine peak flows which are flows resulting from a combination of factors including diurnal and seasonal flow variations of flow components and characteristic response of inflow and base flows to storm events. Peak flows could be determined from peaking factors and adequate safety margins could be provided during the planning of sewerage facilities.
Table 6.4 Peaking Factors
|
Population Range |
Peaking Factor
(including stormwater allowance) for facility with existing upstream
sewerage |
Peaking Factor
(excluding stormwater allowance) for facility with new upstream sewerage |
|
a) For 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 |
|
|
|
b) Sewage Treatment Works, Preliminary Treatment Works and Pumping Station |
||
|
<10,000 |
4 |
3 |
|
10,000 - 25,000 |
3.5 |
2.5 |
|
25,000 - 50,000 |
3 |
2 |
|
> 50,000 |
|
|
6.5
Sewage Flow Estimation
6.5.1 Sewage (ADWF) generated from the operation of SHO and Reprovisioned SHD are estimated to be about 3.25 L/s (281 m3/day) and 10.97 L/s (948 m3/day) respectively based on GESF. Details of the sewage estimation is shown in Appendix 6.2. The current sewage generated from the existing SHD is about 8.63 L/s (746 m3/day). The operation of SHO and Reprovisioned SHD will generate an additional sewerage flow of 5.59 L/s (i.e. 483 m3/day).
6.5.2 Based on the sewerage and sewage treatment implication study of Proposed Comprehensive Residential and Commercial Development atop SHD (SHD Topside Development), the estimated sewage flow (ADWF) generated from SHD Topside Development in Year 2038, the year of last population intake, will be 12,068 m3/day.
6.6
Cumulative
Impacts and Proposed Sewerage Network
6.6.1 For the SHO and Reprovisioned SHD, internal gravitational and pumping sewerage system shall be proposed to collect all wastewater from various buildings, SHO, as well as from tracks. A schematic plan showing the internal sewerage system within SHO and Reprovisioned SHD is presented in Appendix 6.3. The design and alignment will be subject to detailed design. All the sewage collected within the internal sewerage system will ultimately be conveyed to the proposed ultimate pumping station and discharged via twin rising mains of 200mm dia. to public sewerage system. The twin rising mains will be provided parallel to the Sewage Outfall Culvert beyond the Drainage Reserve and finally connected to public sewerage system via designated discharge manhole which is the existing manhole FMH7022270. The alignment plan and sections of the proposed sewage rising main is provided in Appendix 6.4.
6.6.2 The sewage generated from the SHD Topside Development will be conveyed to SHWSTW using a separate pumping system which will be constructed under SHD Topside Development. Separate sewerage systems for the Project and SHD Topside Development are deemed to be necessary for clear demarcation on the operation, management and maintenance responsibilities between railway facilities and SHD Topside Development.
6.6.3
The foul
water sump size will be designed to contain one hour ADWF of emergency storage
with provision of three pumps (two for
duty and one for standby in operation).
With respect to flow capacity, each of the duty pumps and the standby
pump solely would be able to handle the peak flow from SHO and Reprovisioned
SHD. The design capacity in ADWF and peak flow of the proposed ultimate sewage
pumping station will be 1,229m3/day and 58L/s respectively. Dual-feed
power supply will also be provided to the pumping station.
6.6.4
Either
ductile iron (DI) pipes or high-density polyethylene pipe (HDPE) and fittings
with sufficient strength to resist the pumping pressure and external load would
be adopted as the rising sewer. Thrust blocks, supports or relevant fittings
would be provided to avoid pipe failure such as pipe bursting and leakage.
6.6.5
Main
Control System would provide the human-machine interface for monitoring and
control of the pumping system through the integrated workstations. Under normal
operation, the pumps will operate autonomously according to the levels of the
respective wet wells. However, when the automatic control fails and any
condition occurs where the pumps are unable to cope with the amount of sewage,
the operator will be alerted to operate the pumps remotely from the integrated
workstation and handle the emergency situation. Regular maintenance and
checking of plant equipment will also be undertaken by qualified personnel to
prevent equipment failure. On-site staff for 24-7 emergency repair
service would be provided to handle emergence situations. The pumping
system is therefore able to resume operation generally within 1 hour after
emergency repair in the event of any incident. Tank away, if necessary, would
be conducted as a last resort for proper disposal at public sewage treatment
works. Operation and Maintenance (O&M) Manual will also be prepared to
standardize operation and maintenance procedures.
6.6.6
With
the implementation of precautionary measures recommended in Sections 6.6.3 to 6.6.5, no emergency discharge of sewage during the operation of SHO and Reprovisioned SHD is
anticipated.
6.6.7 Retention time of sewage in the pumping system and rising mains were estimated according to the schematic plan shown in Appendix 6.3, and the calculation is shown in Appendix 6.5 with results summarized in Table 6.5. Results indicated that the accumulating retention time of the sewage is slightly more than 2 hours. Variation of flow pattern may also result in longer detention time in particular during night time period. Based on the findings in Appendix 6.5, effective septicity control measures including active on-line monitoring of sewage septicity and active septicity treatment or dosage system will be provided by the SHO and Reprovisioned SHD to mitigate septicity problem that may arise in the downstream public sewerage system.
Table 6.5 Estimated
Sewage Retention Time for Each Internal Pumping System and Ultimate Pumping
Station
Pumping
|
ADWF
(m3/day)
|
Approximate Length of R.M. (m)
|
Sewage Retention Time (hours)
|
Accumulating Sewage Retention Time(2) (hours)
|
1
|
1,022
|
853
|
0.97
|
1.44(3)
|
2
|
166
|
204
|
0.58
|
0.58(4)
|
3
|
624
|
176
|
0.47
|
0.47(4)
|
4
|
91
|
418
|
0.97
|
0.97(4)
|
5
|
25
|
655
|
0.19
|
0.19(4)
|
Ultimate Pumping Station
|
1,229
|
1249
|
1.11
|
2.55(5)
|
Note:
(1) Schematic plan showing the internal sewerage system network within SHO and
Reprovisioned SHD adopted for estimation is provided in Appendix 6.3.
(2) Accumulating sewage retention time is the
summation of sewage retention time for the pumping system and its upstream
pumping system.
(3) Sewage from Pumping System 3 will be pumped to Ultimate Pumping Station
via Pumping System 1 and thus accumulating sewage retention time of Pumping
System 1 is the summation of 0.97 and 0.47 hours.
(4) For Pumping System 2, 3, 4 and 5, there are no upstream pumping system
and thus the accumulating sewage retention time at the pumping system is equal
to the sewage retention time of the individual pumping system.
(5) Total accumulating sewage retention time at ultimate pumping station is
taken as the summation of maximum accumulating sewage retention time of its
upstream pumping system and sewage retention time of the pumping station (i.e.
1.44 +1.11 = 2.55 hours).
6.6.8 The proposed sewerage layout plan for the Project is shown in Figure No. NEX1062/S/SHD/ACM/Z10/181. Calculation of the rising main for the SHO and Reprovisioned SHD is shown in Appendix 6.6.
6.6.9 Assessment on the public sewerage system downstream of FMH7022270 has been carried out. It is found that the existing sewer from FMH7022270 to FMH7022273 is under surcharged condition. In order to cater for the discharge of the sewage from SHO and Reprovisioned SHD to the sewage treatment works, modification works to upgrade the existing 300 mm dia. sewer to 375mm dia. are proposed and will be implemented by the Project Proponent to the satisfaction of EPD. Temporary sewerage diversion works will be required before the modification works to maintain the sewage flow. It is considered that, with implementation of the proposed modification works, there will be no adverse impact on the public sewerage system. The calculation is provided in Appendix 6.7.
6.6.10 The tentative last population intake year of SHD Topside Development is 2038. The Government has identified available capacity of about 13,400m3/day to accept the total sewage arising from SHD Topside Development, SHO and Reprovisioned SHD for treatment at SHWSTW. Detailed breakdown is provided in Table 6.6. Based on the findings of Table 6.6 above, there should be sufficient capacity at the SHWSTW to cater for the sewage generated from future SHO and Reprovisioned SHD.
Table 6.6 Estimation
of Cumulative Sewage Flow in 2038 (with developments)
Source of Sewage
|
Approx.
Estimated ADWF, m3/day
|
SHD Topside Development(1) |
12,100 |
SHO and Reprovisioned SHD |
1,229 |
Total
|
13,329
|
Available capacity in SHWSTW
|
13,400
|
Note:
(1) Reference was
made on Table 6.5a of SHD Topside Development EIA Report (July 2017).