This Section
presents an assessment of impact of sewage discharge to the existing sewerage
system in the vicinity of the proposed Desalination Plant and examines the
available capacity of the existing sewerage system in accordance with Section 3.4.5 of the Study Brief ESB-266/2013. If there is no existing sewerage system in
the vicinity to serve the desalination plant, the technical feasibility of
providing a sewage disposal scheme will be assessed. This Sewerage and Sewage Treatment
Implications Assessment (SSTIA) demonstrates that the proposed development is
viable in terms of its impact on the sewerage system.
7.1.2
The sewage flow discharge from the proposed
Desalination Plant is based on the following standards, guidelines and
reference for the sewerage and sewage treatment design:
·
Technical
Memorandum on Effluent Standards (Water Pollution Control Ordinance, Cap. 358
section 21);
·
Sewerage
Manual published by Drainage Services Department (DSD);
·
EPD Report No. EPD/TP
1/05 Guidelines for Estimating Sewage Flows for Sewerage Infrastructure
Planning Version 1.0; and,
·
Agreement
No. CE55/2007 (DS) – Village Sewerage in Tseung Kwan
O and Tuen Mun (Ka Loon
Tsuen and Lung Kwu Tan).
7.1.3
The objectives of this sewerage impact
assessment are to:
Ø
review and confirm whether the existing/planned
sewerage systems and sewage treatment works in TKO District will provide
adequate capacity for the Project;
Ø
identify the preliminary appropriate alignment
and layouts of the new sewerage to connect to the existing/planned/future
sewerage systems in TKO District ; and
Ø
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.
7.2.1
The
SSTIA was undertaken in accordance with the criteria and guidelines for
evaluating and assessing impacts on the downstream public sewerage treatment
and disposal facilities as states in Section
6.5 in Annex 14 of the Technical Memorandum on Environmental Impact
Assessment Ordinance (TM-EIAO). The following methodology was adopted for
this SSTIA:
·
Investigation
of the existing and planned sewerage facilities in the proposed Desalination
Plant development area.
·
Calculation
of the sewage flow generated from the proposed Desalination Plant.
·
Investigation
of sewage disposal strategy for the proposed Desalination Plant.
7.3.1
The proposed Desalination Plan is within an area
where no public sewerage connection is available. Based on the Government’s drainage record
plan, the existing sewer serves the Television Broadcasts Ltd, Tseung Kwan O Industrial Estate, and LOHAS Park. The collected sewage is then carried to the Tseung Kwan O Preliminary Treatment Works (PTWs) at Shek Kok Road. The
existing sewerage network and planned sewerage network are shown in Figures
7.1 & 7.2.
The quantity of wastewater
generated by the proposed Desalination Plant is calculated with reference to Appendix III of Guidelines for Estimating Sewerage Flows for Sewage Infrastructure
Planning (the Guidelines) ().
7.4.1
The
general layout plan of the proposed Desalination Plant is illustrated in Annex 7A. The proposed Desalination Plant consists of
an administrative building and other associated facilities. The sewage is
expected to be generated from administrative building for sanitary use, such as
toilet flushing and laundry. The sewage of the desalination plant will be
generated by two components, sewage from internal staff, and visitor and
neutralized chemical cleaning water from the Reverse Osmosis (RO) system. The sewage flow from the proposed
Desalination Plant is calculated based on the assumption listed in Table
7.1.
Table 7.1 Assumptions for
Sewage Flow Calculation
|
Parameters
|
Unit
|
|
Occupied rate of office
|
100%
|
|
Gross Floor Area
|
3,132 m2
|
|
No. of desalination plant staff
|
40 (1)
|
|
Unit flow staff (J2) (3)
|
0.33 m3/day
|
|
No. of Visitor
|
60 (1)
|
|
Unit Flow (Student) (2)
|
0.04 m3/day
|
|
RO Chemical Cleaning (m3/cleaning)
|
514.8
|
(1) The
maximum numbers of staff and visitor proposed by the consultant and agreed by
WSD.
(2)
Assume the Unit Flow generated by Visitor is equal to Student
(3) J2 – Employment categories and
Job Types for Electricity, Gas and Water
7.4.2
The
duration of the RO cleaning will take about 4-8 hours per cleaning and used cleaning
chemical will be neutralised before discharge to public sewer. The neutralised RO cleaning water will be
stored temporarily at the neutralisation tank and will be evenly discharged to
the public sewer over 4 days. The
average daily discharge by RO cleaning is about 128.7 m3/day.
7.4.3
The calculations of Average Dry Weather Flow
(ADWF) are shown in Table
7.2. The volume of sewage
that will be generated by the proposed desalination plant is approximately
144.3 m³/day.
Table
7.2 Estimated Sewage Flow
|
Catchment: Desalination Plant
|
Visitor
|
Employee (J2)
|
RO Chemical Cleaning
|
|
Number
|
60
|
40
|
-
|
|
Unit Flow (m3/day)
|
0.04
|
0.33
|
-
|
|
ADWF (m3/day)
|
60 x 0.04
= 2.4
|
40 x 0.33
= 13.2
|
128.7
|
|
ADWF (m3/s)
|
0.00003
|
0.00015
|
0.00149
|
|
Total Flow (m3/day)
|
2.4 + 13.2 +
128.7 = 144.3
|
|
Total Flow (m3/s)
|
0.00003 + 0.00015
+ 0.00149 = 0.00167
|
|
Contribution Population
|
(2.4+13.2)/0.27 +
128.7/0.27 = 58+477
|
|
Peak Flow (m3/s) (Peak factor = 3)(1)
|
(0.00018 x 3)+0.00149 = 0.00203
|
|
Peak Flow (m3/s) (Peak factor = 6)(2)
|
(0.00018 x
6)+0.00149 = 0.00257
|
(1) For
sewage treatment work (STWs), preliminary treatment works (PTWs) and pumping
station (PS) with population below 10,000 and excluding stormwater allowance
(2)
For sewer with population below 1,000 and excluding stormwater allowance
7.5.1
The proposed plant lies within the catchment of
the Tseung Kwan O PTWs and is in the vicinity of the
urban sewer network in Tseung Kwan O. The estimated sewage generated from the
proposed desalination plant is 144.3 m3/day for ADWF or at a peak
flow of 0.00203 (Peak factor (PF) = 3) m3/s (see Table
7.2). This is equivalent to
about 0.03% of the design capacity of the Tseung
Kwan O PTWs (design capacity of 6.85 m3/s ()). In view of the small volume of flow, it is
unlikely that the Tseung Kwan O PTWs will be
overloaded upon plant’s commissioning.
7.5.2
It is proposed that sewage generated from the proposed
desalination plant will be discharged by a new sewerage system (which may be
gravity sewer or pumping station with rising mains). The new sewerage system will be connected to
the existing 600 mm diameter trunk sewer along Wan Po Road. The estimated sewage generated from the
proposed desalination plant is 144.3 m3/day (ADWF), or a peak flow
of 0.00257 m3/s (PF =6) (see Table 7.2). This is equivalent to about 1.35% of the
design capacity of the existing sewer (design capacity of 0.19
m3/s ()). On this basis, the calculations indicate that
this small increase will not lead to overloading of the downstream sewers as
well as Tseung Kwan O PTWs.
7.5.3
Details of hydraulic calculation are enclosed in
Annex 7B.
7.5.4
A hydraulic model of the sewerage network of the
TKO catchment has been adopted to examine the hydraulic performance of the
downstream 600 mm diameter sewer along Wan Po Road under ultimate
scenario. The sewerage hydraulic model
was developed on the sewage catchment of Tsuen Kwan O under Agreement No. CE55/2007 (DS) – Village
Sewerage in Tseung Kwan O and Tuen
Mun (Ka Loon Tsuen and Lung Kwu
Tan). Under CE55/2007 (DS), various design scenarios were assessed,
which include design years of 2008, 2016, 2021, 2031 and ultimate development
scenario. The population estimate for the adopted hydraulic
model was based on the 2006-based Territory Population and Employment Data
Matrix (TPEDM). The ultimate development scenario of the hydraulic model
is adopted for the hydraulic calculation of this EIA Study.
7.5.5
Results from the hydraulic model indicate that
maximum depth along the downstream 600 mm diameter sewer arising from the
discharge of the proposed project would only be increased by approximately
0.003 m with sufficient freeboard (> 1m) along the sewer. Result tables and
hydraulic model are enclosed in Annex 7C.
7.5.6
The plant lies within the catchment of Tseung Kwan O PTWs.
Discharge of sewage directly via a sewerage system to the PTWs is a
feasible option and is in line with Government's sewage treatment and disposal
strategies. The rationale is that it
would tie in with the long term objective of the Victoria Harbour
water pollution control strategy to reduce the pollution load to Victoria Harbour.
7.5.7
The preliminary proposed strategy involves
construction of buried pipes to connect to the existing sewer at underneath Wan
Po Road by gravity sewer. Future sewer
network will have connection at an existing manhole (FMH4035449) under Wan Po
Road (Figure 7.2). Wayleaves
along the public road will be required for construction of the proposed
connection sewer. The full section of
the connection sewer will run along government land. It is not envisaged that requisition of
private lot would be required for construction nor future operation. In summary, the proposed strategy is
considered viable in terms of land.
7.5.8
The proposed strategy involves discharge to
public sewers via new sewer constructed.
In view of the small volume of flow, it will have insignificant
environmental impact as the full section of the sewer will be laid underground.
7.5.9
Construction of the section of gravity sewer
from the plant to connect with the existing public sewer (Figure 7.2) would
mainly involve conventional technology such as cut and cover excavation
techniques. There is no significant constraint in terms of complexity by
general civil contractors.
7.5.10
The
sewerage facilities provided as part of the proposed plant will be maintained
by the government after completion. No specific maintenance operations are
envisaged but it is considered desirable that the facilities will be inspected
regularly to ensure that these sewers can function properly. The maintenance
responsibility of those proposed sewerage facilities will be confirmed with
relevant authorities in the detailed design stage of this project.
7.6.1
As
discussed in Section 7.5, the sewage
flow generated from the proposed Desalination Plant is smaller than the flow
allowed in Tseung Kwan O PTWs and the existing
sewerage system are adequate for
the proposed development, thus no adverse impact to the local sewerage network
is anticipated, which should have sufficient capacity to cater for the sewage
flow generated from the proposed Desalination Plant.
7.6.2
The design of the gravity sewer should follow
the guidelines stipulated in:
·
Stormwater Drainage
Manual by DSD
·
Sewerage Manual Part 1 & Part 2 by DSD
·
Drainage Services Department Standard Drawings
7.6.3
The proposed sewerage system will be constructed
by WSD during the construction period and the system will handover to DSD for
further maintenance. The detailed design of the proposed sewerage system should
be circulated to DSD, EPD and other relevant parties for comment during
planning and detailed design stage to ensure acceptance by relevant parties. Access for sewers, equipment and personnel
for maintenance of the works should be adequately provided. The
maintenance responsibility of those proposed sewerage facilities will be
confirmed with relevant authorities in the detailed design stage of this project.
7.6.4
Some general design guidelines for the sewerage
system design are summarized as below.
7.6.5
The gravity sewer should be designed to
avoid under surcharge condition. 1m freeboard should be provided if surcharge
condition cannot be avoided. The flow velocity of the sewer should be not less
than 1m/s under full bore flow for self-cleansing purpose. The maximum velocity
should be limited to 3m/s. The structural and bedding design of the sewer
should in accordance with Section 6
of the Sewerage Manual Part 1.
7.6.6
The design of manhole should be in
accordance with Section 7 of Sewerage Manual Part 1 and manhole
should be provided at intersection of sewers, location where sewer changes
location and junction between different size/gradient of sewers.
7.7.1
With
the implementation of the proposed sewerage system according to the
requirements stated in Section 7.6,
no sewerage or sewage treatment implications are anticipated. No specific sewerage monitoring programme is
thus required for the proposed Desalination Plant.
7.8.1
The proposed strategy, discharge of sewage
directly via a sewerage system to the Tseung Kwan O
PTWs, is considered feasible in terms of regional sewerage strategy, land,
environmental impact and construction considerations. The proposed plant is
considered sustainable in terms of sewerage.
7.8.2
The configuration and alignment of the proposed
new sewerage system will be confirmed in the detail design stage of this
project, while the infrastructure design especially the road alignment and
sewerage system of TKO Area 137 is proposed.
Gravity sewer is a preferable option in term of cost-benefit
consideration by constructing the sewer from the plant to connect the existing
manhole.
7.8.3
Based on the above, there is no unacceptable
sewerage impact would be anticipated to the existing sewerage system arising
from the proposed Desalination Plant.