7.1
Introduction
This section presents the
assessment of impacts on the existing public sewerage facilities associated
with the Project, according to the requirements as specified in section 3.4.7
and Appendix D2 of the Study Brief (ESB-261/2013).
Additional sewage is
expected to be generated by the Project. This section is to assess the sewage
impacts to the existing sewerage system in Tai Shue Wan including the sewerage
catchments of Aberdeen Preliminary Treatment Works (APTW) due to the additional
sewage flow arising from the Project, and to recommend appropriate mitigation
measures to the sewerage system, if any, as a result of the Project.
7.2
Methodology
of Sewerage Impact Assessment
7.2.1
Assessment
Approach and Methodology
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 Project
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:
§ Collect relevant information for the assessment;
§ Investigate and review the capacity of the existing public sewerage networks and sewage treatment facilities in Aberdeen;
§ Assess the impact of discharging sewage arising from the Project to the existing public sewerage systems within the sewerage catchment of APTW; and
§
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.
7.2.2
Design Reference
A desktop study was
carried out
to collect the relevant background information for the assessment as summarised below:
§ Drainage Record Plans within the vicinity of the Tai Shue Wan from Drainage Services Department (DSD);
§ Sewage flow generated from the Project;
§ Proposed Development Layout Plan of the Project;
§ EIA Report under the Repositioning project; and
7.2.3
Design
Standard Guideline
The assessment of the existing/planned
downstream public sewerage networks and sewage treatment facilities in Aberdeen
Sewerage Catchment
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)
7.3
Existing
and Planned Sewerage Condition
7.3.1
Existing
Sewage Collection for Ocean Park
The Project is located at Tai Shue Wan, and it is within the Aberdeen
Sewerage Catchment, which also embodies South Bay, Repulse Bay, the eastern
part of the Ocean Park area, Wong Chuk Hang,
Aberdeen, Tin Wan and Chi Fu Fa Yuen. Sewage
generated within this catchment is conveyed to the Aberdeen Preliminary Treatment
Works (APTW) for treatment.
There are four main public sewage pipelines collecting sewage discharged from Ocean
Park. Three sewers are located at Shum Wan Road, one connection, and Nam Long Shan Road,
two connection, for the Tai Shue Wan side
of Ocean Park, and other is located at Wong Chuk Hang
Road.
The sewage generated from the Tai Sue Wan side of Ocean Park is
connected to the three existing sewers at one
connection on Shum Wan Road and two connections on Nam Long Shan Road. Part of the sewage generated at Tai Shue Wan
discharges to the 300mm diameter gravity sewer along Shum Wan Road and
then the pumping station located at Shum
Wan Road. The sewage is then pumped to a rising main and discharged to a sewer
located along Nam Long Shan Road and finally conveyed to the APTW. The Shum Wan
Road sewer serves the sewage generated from Ocean Park with a discharge of 162m3/day
as per the discharge license issued by EPD.
Another existing sewerage connection
at Nam Long Shan Area is to the 225mm diameter sewer and
450mm diameter at Nam Long Shan Road to collect the part of the sewage from the Tai
Shue Wan. The 225mm diameter sewer currently collects sewage generated from the
existing Ocean Park’s facilities of Marine
World with a discharge limit of 850m3/day as per
the discharge licence issued by EPD. The
450mm diameter sewer currently collects sewage generated from the new summit
area of Ocean Park including Funicular Building, Rainforest, Thrill Mountain
and Polar and the seawater usage for flushing and backwashing of the pools for
animals with approximately 1,062m3/day. The sewage was estimated
based on the actual water bill (709m3/day of fresh water) and
in-house seawater daily pumping record (353m3/day of seawater). Taking
into account the proposed change of opening hours of the existing Ocean Park
from “10:00hrs to 22:00hrs” to “09:00hrs to 23:00hrs”, it is estimated that the
sewage generated would be increased to approximately 1,239m3/day.
The existing sewerage connection
at Wong Chuk Hang Road allows sewage from the
existing Lowland area to the 300mm gravity sewer across the Wong Chuk Hang Road towards the APTW.
7.3.2
Planned
Sewerage Collection for Project
The sewage generated from
the Project and the existing facilities of Ocean Park at Tai Shue Wan will be
connected by a network within the Project area to a pump room
within the proposed Water Park building. Sewage will be temporarily collected
in a sump pit with a minimum size of 12m3 and then conveyed by twin rising main, 2
x 150mm diameter, up the hillside. Then it will be discharged ultimately to
the existing 450mm diameter sewer via the government sewerage manhole:
FMH7056262 at Nam Long Shan Area. The existing 450mm diameter sewer and the
proposed discharge location are shown in Figure 7.1.
According
to the latest planning, the chiller tank will be cleaned annually that a very
little amount of discharge with residual chlorine level at 5mg/L will be
conveyed to the sewerage system and diverted to APTW for treatment. Thus, it is
considered that the impact to the downstream sewerage system is minimal.
Moreover, a continuous release of
1.6L/s of spent cooling water will be generated during the Ocean
Park opening hours (0900 to 2300). The spent cooling
water generated will be reused on site for flushing purposes. Under the current design, a total amount of
573.13 m3/day of flushing water is required for the Project, which
will fully utilise the 69.12 m3 spent cooling water generated daily.
The spent cooling water for re-use will be
treated in accordance with the NSF 350 Standard in the 2012 USEPA
Guidelines for Water Reuse standard which is
along with the EMSD water quality objectives for re-use of spent cooling water
as flushing supply and summarised in Section
6.6. Hence, no spent cooling
water will be directly discharged, and no
impact to the existing sewerage system is anticipated
due the release of spent cooling water.
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 Ocean Park in accordance with the
Sewerage Manual published by DSD. The planned sewage pumping system is being designed in accordance with
the sewerage manual published by DSD, the requirement on self-cleaning velocity
will be achieved and the maximum retention time of the planned pumping station
will not be more than 2 hours to avoid the septic and odour problems.
7.3.3
Existing and
Planned Sewerage Treatment Facilities
Sewage collected
from the Ocean Park is conveyed by the existing sewerage systems to the APTW
for treatment. The treated effluent is ultimately discharged to sea for
disposal.
APTW is located at
Tin Wan Praya Road. The design capacity (Peak Flow)
is 3.06m3/s. Capacities of the individual treatment units are shown
in Table 7.1.
At the time of preparing this Section, no further
upgrading work for APTW
is confirmed.
Table 7.1: Existing Facilities of the APTW
|
Treatment Unit |
No. of Unit |
Capacity of Each Treatment Unit (m3/s) |
|
Pump Capacity |
3 Duty + 1 Standby |
3.06 |
|
Fine Screen |
2 Duty + 1 Standby |
3.06 |
|
Grit Trap |
1 Duty + 1 Standby |
3.06 |
7.4
Assumptions and Parameters Adopted for
Assessment
7.4.1
Assessment
Scenarios
The
assessment of impacts due to the Project on the public
sewerage system in APTW
sewerage catchment has been carried out
for both baseline condition and ultimate condition of the Project as
summarised in Table 7.2.
Table 7.2: Assessment Scenarios
|
Scenario Year |
Description |
|
2013 |
Baseline Condition |
|
2021 |
Ultimate
Condition for the Project |
7.4.2
Global
Unit Flow Factors (GUFF) – Sewage Treatment Works & Pumping Station
Estimation of the sewage flow to the affected
sewage treatment works and pumping station in
Aberdeen sewerage
catchment 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
7.3.
Table 7.3: Adopted Global Unit Flow Factors (GUFF) for Sewage Treatment Works &
Pumping Station
|
Category |
Unit (per) |
GUFF (m3/day) |
Data Source |
|
Domestic (catchment specific) |
|
|
|
|
Private Housing and Public Housing |
Person |
0.23 |
GESF |
|
Commercial |
|
|
|
|
Commercial Employee |
employee |
0.08 |
GESF |
|
S1 |
employee |
- |
GESF – J7 |
|
S3 |
employee |
0.25 |
GESF – J2 |
|
S4 |
employee |
0.15 |
GESF – J9 |
|
S5 |
employee |
- |
GESF – J5 |
|
S6 |
employee |
0.2 |
GESF – J4 |
|
S7 |
employee |
0.2 |
GESF – J4 |
|
S8 |
employee |
0.1 |
GESF – J3 |
|
S9 |
employee |
1.5 |
GESF – J10 |
|
S10 |
employee |
1.5 |
GESF – J10 |
|
S11 |
employee |
0.1 |
GESF – J3 |
|
S12 |
employee |
- |
GESF – J6 |
|
S13 |
employee |
- |
GESF – J6 |
|
S14 |
employee |
- |
GESF – J6 |
|
S15 |
employee |
- |
GESF – J12 |
|
S16 |
employee |
0.2 |
GESF – J11 |
|
S17 |
employee |
0.2 |
GESF – J11 |
|
S18 |
employee |
0.2 |
GESF – J11 |
|
S19 |
employee |
0.2 |
GESF – J11 |
|
Industrial |
|
|
|
|
Industrial Employee |
employee |
0.08 |
GESF |
|
S2 |
employee |
0.55 |
GESF |
|
Institutional |
|
|
|
|
School Student |
person |
0.04 |
GESF |
7.4.3
Peaking Factors
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, conservative
approach using the peaking
factors
including stormwater allowance are 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
7.4:
Table 7.4: Peaking Factors
|
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 |
7.5
Sewage Flow Estimation
7.5.1
Sewage
Generated by the Existing and Proposed Development
The
total sewage flow generated for the entire existing and planned Project as well
as the two future hotels that are under preliminary planning provided by the
Ocean Park is summarised in Table 7.5. Detailed estimation is shown in Appendix 7.4.
Table 7.5: Estimated Sewage Flows from Ocean Park Adopted for Assessment
|
Source |
Approximate
Total ADWF (m3/day) |
|
Existing
Ocean Park Facilities |
70.5 |
|
Proposed
Water Park Development (the Project) |
710.0 |
|
Future
Spa Hotel (under preliminary planning) |
142.2 |
|
Future
Fisherman’s Wharf Hotel (under preliminary planning) |
363.4 |
|
Total |
1286.1 |
7.5.2
Existing
and Projected Sewage Flow for Gravity Sewers along Nam Long Shan Road
Based on the aforementioned
assumptions and
drainage record plan, no planned sewage will be discharged to the existing 450mm
diameter sewer along Nam Long Shan Road. The sewage flows in different catchment
areas and the gravity sewers along Nam Long Shan Road have
been estimated, as
summarised in Table 7.6 and Table 7.7. The 1,239m3/day
ADWF for existing facilities of Ocean Park has taken into account the proposed
change of opening hours. The locations for individual discharges at the gravity sewers are shown in Figures 7.3 to 7.5. Details of the
sewage flow
calculation are given in Appendix 7.3.
Table 7.6: Estimated ADWF for the sewerage catchment
|
Catchment
No. |
ADWF (m3/day) |
Remark |
|
S1
(Existing Facilities of Ocean Park with the proposed change of opening hours
from “10:00hrs to 22:00hrs” to “09:00hrs to 23:00hrs” ) |
1239.0 |
From Ocean Park |
|
S2
(Project, existing Tai Shue Wan Facilities and the two future hotels) |
1286.1 |
From design consultant of project |
|
S3
(TWGHs David Trench Home for Elderly) |
160.9 |
From DSD |
|
S4
(Singapore International School) |
110.6 |
- |
|
S5
(Shue Yan Secondary School) |
89.8 |
- |
Table 7.7: Estimated ADWF and Peak flow for the gravity
sewers
|
Contributing
Catchment No. |
Discharged
Manhole |
ADWF
(m3/d) |
Contributing
Population |
Peaking
Factor |
Peak
Flow (l/s) |
|
S1 |
FMH7056267 |
1239.0 |
4589 |
6 |
86.1 |
|
S1 +
S2 |
FMH7056262 |
2525.1 |
9353 |
5 |
146.2 |
|
S1 +
S2 + S3 |
FMH7056229 |
2686.0 |
9949 |
5 |
155.5 |
|
S1 +
S2 + S3 + S4 + S5 |
FMH7056213 |
2886.4 |
10691 |
5 |
167.1 |
7.5.3
Existing
and Projected Sewage Flow in APTW
In order to assess the impact on APTW due to the Project,
the future sewage flow handled by APTW is estimated according to the population
forecast from the 2011-based TPEDM. The sewerage catchments represented by the
Planning Data Zones (PDZ) system from the 2011-based TPEDM are shown in Figure 7.2.
The sewerage catchments of APTW
in terms of PDZ included 40, 41, 43, 44, 45, 46, 47, 49, 349, 50, 51.
Based on the estimated
ADWF of the various PDZ within the sewerage catchment of APTW, the total ADWF discharged
to APTW in 2013 and 2021 are calculated
and
summarised in Table 7.8. Details of the
population
in each Aberdeen
Sewage Treatment Catchment and total sewage flow estimation for APTW in 2013 and 2021 are also
given in Appendix 7.2.
Table 7.8: Estimated ADWF for the Relevant PDZs within
the APTW sewerage catchment
|
Planning Data Zones |
ADWF in 2013 (m3/day) |
ADWF in 2021 (m3/day) |
|
40 |
1,319 |
1,153 |
|
41 |
1,064 |
923 |
|
43* |
211 |
290 |
|
44 |
4,825 |
5,070 |
|
45 |
2,310 |
2,191 |
|
46 |
5,805 |
5,914 |
|
47 |
16,343 |
16,302 |
|
49 |
8,497 |
12,038 |
|
349 |
2,258 |
2,783 |
|
50 |
1,486 |
1,456 |
|
51 |
3,268 |
3,614 |
|
Proposed Waterpark (Project) |
0 |
780.5 |
|
Future Spa Hotel (under
preliminary planning) |
0 |
142.2 |
|
Future Fisherman’s Wharf Hotel
(under preliminary planning) |
0 |
363.4 |
*
For PDZ 43, only Category School and S16 are assumed to contribute to APTW catchment.
7.6
Assessment
of Impact to Existing/Planned Sewerage
and Sewage Treatment System
7.6.1
Overview
The Project will be in full-scale operation before 2021 and thus the sewage flow in 2021 will be the ultimate flow. This ultimate flow is used to assess the impacts on the following key components of the sewerage system and sewage treatment works:
§ Gravity sewers along Nam Long Shan Road; and
§ APTW.
7.6.2
Gravity Sewers along
Nam Long Shan Road
It is proposed to connect the new development
sewerage to the 450mm sewer along Nam Long Shan Road. The existing 450mm
diameter along the Nam Long Shan Road has a capacity ranging from
0.17 m3/s to 0.84 m3/s. The utilization of the
450mm sewer for the catchments along Nam Long Shan Road including the project
is ranging from 10% to 84%. It is considered that the existing 450mm diameter
sewer capacity is sufficient to cater the sewage discharge arising from
Project. The comparison of the proposed sewage flow against the capacity of the
existing 450mm sewer along Nam Long Shan Road is shown in Appendix 7.3.
.
7.6.3
Downstream Sewerage System
The sewage flows arising from the Project will be conveyed to APTW
through approximate 2.5km long of 1500mm trunk sewer along Wong Chuk Hang Road. According to the assessment, the trunk
sewer generally has a capacity approximate 2.6 m3/s.
Thus, the sewage flow of 0.054 m3/s from the
Project only utilises approximately 2.1% of the
trunk sewer. It is considered that no adverse impact to the trunk sewer.
The potential impacts arising from the re-use of
spent cooling water for toilet flushing and the proposed disinfection system in
terms of temperature effect and residual chlorine would be on the existing
downstream sewerage system. It is expected that the spent cooling water will be
at a temperature of 38°C. According to Table 1 of the Technical Memorandum on
Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland
and Coastal Waters issued by EPD, it indicates the temperature standard 43°C
for effluent discharged into foul sewers leading into Government sewage
treatment plants. Temperature of the spent cooling water will be well below the
regulated one for effluent discharged into foul sewers. The expected residual
chlorine concentration at 5mg/l from annual cleaning process of the chiller
tank to discharge the sewage system would be acceptable and infrequent of once
per year. The potential impacts are therefore not anticipated to be
significant.
7.6.4
Aberdeen
Preliminary Treatment Works (APTW)
The potential impact on the APTW is assessed based on the estimated ultimate sewage flows arising from the Project as well as the population forecast from the 2011-based TPEDM within the catchment areas of the APTW (see Table 7.9). The estimated sewage flows handled by APTW are summarised in Table 7.9 and details of the sewage flow estimation are given in Appendix 7.1.
Table 7.9: Estimated Sewage Flows Handled by APTW in 2013 and 2021
|
Scenario Year |
Approxi-mate ADWF
from Relevant PDZ
(m3/day)* |
Approxi-mate ADWF from the Project and the two Future
Hotels (m3/day)* |
Total ADWF (m3/day) |
Catch-ment Inflow Factor* |
ADWF
x Catchment Inflow Factor (m3/day) |
Contri-buting Population |
Peaking Factor |
Total Peak
Flow (m3/s) |
Capacity of APTW |
|
|
|
Peak
Flow (m3/s) |
|||||||
|
2013 |
47,386 |
0 |
47,386 |
1.3 |
61,602 |
228,157 |
2.7 |
1.95 |
3.06 |
|
2021 |
51,733 |
1286.1 |
53,019 |
1.3 |
68,925 |
255,279 |
2.7 |
2.17 |
3.06 |
* For Catchment Inflow Factor, refer to the
Table T-4 of the GESF,
It can be seen from Table 7.9 that the estimated total ADWF and peak sewage flow, including that generated by the Project and
the future two hotels (under preliminary planning), to be handled by APTW in 2021 would be approximately 68,925 m3/day (ADWF) and 2.17 m3/s (Peak Flow) respectively, which are within the capacity of the existing APTW. It is concluded that the APTW has adequate
capacity to cater for the total flow generated by the Aberdeen sewerage catchment
in Year 2021 including the proposed works under the Project.
7.7
Mitigation Measures
7.7.1
Septicity
Septicity is caused by
increased retention time meaning the available dissolved oxygen and nitrate
consumed by the biomass and the effluent. In order to control the septicity of
sewage due to operation of sewage pumping facilities, the retention time of
sewage should be minimised. Use of appropriate pump rate should be considered
for reducing the time of retention of sewage. Direct injection of chemicals could
also be used to control septicity. The mitigation methods should be considered
under detailed design stage.
In order to prevent septicity occurring in the sewage pump pit and
rising mains, preventive measures will be adopted. These would include designing
the bottom of the sewage pump pit inclined to minimise the likelihood of sludge
accumulation and the sewage retention time in the pump pit not exceeding 30
minutes. Also, the rising main would be constructed from ductile iron pipes
with epoxy internal linings complying with BS EN 598:1995 and the design
minimum velocity within the rising mains would be 1m/s at full bore condition
as suggested in the Sewerage Manual Part 1.