Table of
Contents
2 Consideration
of Alternatives
2.2 Consideration
of Alternative Sites – Site Selection
2.3 Consideration
of Alternative Development Options
2.4 Consideration
of Alternative Construction Methods
2.5 Consideration
of Works Sequences
2.6 Selection of
Preferred Scenario
In accordance with Clause 3.3
in the EIA Study Brief (ESB-266/2013),
the following section presents a consideration of the alternatives for the
Project. The section has been divided into a discussion of the following:
·
Need of the Project;
·
Consideration of Alternative Sites;
·
Consideration of Alternative Development Options; and
·
Consideration of Alternative Construction Methods and Sequences of
Works.
Based on the above considerations, the Environmental Impact Assessment
(EIA) of the preferred alternative is presented in the subsequent sections.
Water shortage is a general phenomenon throughout the world. Many places
have shortage of freshwater for supporting the developments. Without exception,
Hong Kong is also facing this potential risk which is exaggerated with the
particular situation that the main freshwater supply is covered by a single
source of Dongjiang Water. Although under the Dongjiang Water Supply Agreement, the Government of
Guangdong Province agrees to supply up to an ultimate annual quantity of 1,100
million m3 of freshwater to Hong Kong, in extreme drought condition,
the water supply from this single source may be affected which will have a
direct impact to Hong Kong society.
In this case, it is of paramount importance to have a secure water
supply to sustain Hong Kong’s development. To better prepare Hong Kong for the
sudden unforeseeable uncertainties in this aspect, the Total Water Management
(TWM) strategy has been promulgated by the Hong Kong SAR Government in 2008.
The target of the TWM strategy is to diversify the freshwater resources to
minimize the impact on the society caused by the shortage of a particular
source.
Taking into account both the major source from Dongjiang
water and the past records of the local yield collected in water gathering
grounds, the Water Supply Department (WSD) has conducted a risk assessment of
freshwater resource adequacy in Hong Kong. The analysis shows that due to the
increase in water demand, a potential deficit of our freshwater resources of
maximum 39 million m3 per year is predicted.
To address the identified potential freshwater shortage, in the TWM
strategy, two alternative new water resources were identified and evaluated,
namely expansion of water gathering ground and reservoir storage and
implementation of desalination. The expansion of water gathering ground will
entail high land costs and undermine the conservation importance and
development potential of the areas concerned for protection of the water
quality. With this serious drawback, the TWM study concludes that the option of
expanding water gathering ground and reservoir is of very low priority for Hong
Kong. The TWM strategy was also presented in the 151st Meeting of
the Advisory Council on the Environment (ACE) held on 14 April 2008 which also
agreed that the expansion of water gathering ground might have negative impact
on the environment, in particular the ecology of some sensitive downstream
resources.
To this end, WSD has kept abreast of the latest developments in desalination
technology and prepared for the related planning and studies so that other
water sources can be tapped into in good time in case of water shortage. A
Feasibility Study (CE 71/2000 (WS)) and a Pilot Plant Study (CE 97/2002 (WS))
on developing desalination facilities in Hong Kong had been conducted in 2002
and 2007 respectively. Both studies confirmed the technical feasibility of
using desalination technologies as a reliable freshwater source to produce
potable water in compliance with the World Health Organisation
(WHO) standards. Furthermore, WSD has assessed the technical feasibility and
economic viability of building a medium-sized desalination plant in Hong Kong,
at a designated site in Tseung Kwan O (Area 137) to
cope with the risk of potential freshwater shortage. All the previous studies
show that constructing a desalination plant to provide potable water would be
an appropriate solution to alleviate the shortage of our freshwater resources.
Table 2.1 Comparison between Water
Ground Gathering Reservoir and Desalination Plant
Aspect
|
Water Ground Gathering Reservoir
|
Desalination Plant
|
Land Requirement
|
In
Hong Kong about one-third of the land serves as water gathering grounds. There
are limited additional land resources of Hong Kong for such use and it would
be difficult to find suitable sites for additional impounding reservoirs.
Development of such additional sites would require encroachment into
environmentally sensitive areas such as country parks and lowland natural
habitats. It is considered not
possible to expand the water ground gathering reservoir without increasing
the water gathering ground. This is
also keen competition of land for other uses.
|
A typical desalination
plant requires limited land (about 10 ha) and can easily be situated in less
sensitive areas such as reserved land for industrial use.
|
Cost
|
Entail
high cost for land resumption.
|
High capital costs and high level of electricity consumption. Energy efficiency measures can be incorporated into
project design to reduce operational cost.
|
Environmental benefits/ disbenefits
|
Expansion
of water gathering ground and reservoir storage will give rise to negative
impacts on ecology of sensitive downstream resources. Encroachment into country park may occur with loss of trees and associated
wildlife during the construction of the catchwater
in water gathering ground and reservoir.
|
RO
desalination plant is easy to operate with small footprint. Although
hypersaline brine will be discharged as a by-product of the desalination
process, the brine will be rapidly diluted to the salinity of ambient level.
As RO desalination would not have boiler, therefore no air emissions (e,g, NOx, SOx,
etc.) would be generated. Also, all noisy plants during operation phase will
be enclosed in building structure and this noise, if any, is minimized.
|
Supply stability
|
The yield in
water gathering ground is highly dependent on rainfall,
this option is not capable of dealing with climate change and is thus still
susceptible to rainfall variation which imposed impact to the water supply
reliability.
|
Seawater
resource is inexhaustible, it can accommodate different levels of demand and
provide alternative stable water resources
|
Conclusion
|
Very low
priority option for Hong Kong and thus is not considered as viable option
|
Proven technology and several reverse
osmosis desalination was built worldwide and thus is considered as viable
option
|
Table 2.2 Consultations and Comments
on the Desalination Plant
Consulted Party and
Meeting Date
|
Key Comments
|
Advisory
Committee on Water Resource and Quality of Water Supplies Meeting dated on 21
November 2014
|
·
Generally supported the construction of desalination plant
·
The brine generated from the desalination plant shall be properly
managed during operation
·
Understand that alternative methods proposed for brine management
cannot significantly reduce the volume of brine discharge from the plant
·
The feasibility of installing demonstrative scale of alternative
brine management for education purpose shall be further assessed
|
Sai
Kung District Council Meeting dated on 6 January 2015
|
·
Generally supported the construction of desalination plant
·
To ensure the produced water from desalination is safe for drinking
·
To ensure cost effectiveness and sustainability of the project
·
To address the environmental impact, in particular implication on
marine ecology and fisheries of surrounding water bodies
|
According to Hong Kong Observatory’s study report, climate change will,
in some instances, bring about more frequent extremely dry weather and increase
the likelihood of occurrence of consecutive droughts. This will not only affect
the local yield collected in Hong Kong, but also impact on the water resources
in Dongjiang which contributes 70 – 80% of the
freshwater demand in Hong Kong. Without the proposed Project, Hong Kong lacks
an alternative of freshwater resources and will be subject to water shortage
arising from severe droughts which impact the water supply reliability in Hong
Kong.
With the implementation of the Project, part of the freshwater supply will
be based on a guaranteed quantity from desalination which can release the water
stress on Dongjiang. The purpose and objective of
this Project is to present an appropriate solution to provide alternative
potable water source and alleviate the shortage of freshwater resources due to
climate change and subsequent adverse weather.
The proposed desalination plant can produce 135 million liter per day (Mld) of freshwater, which is
equivalent to 22% of the mean gross yield collected from water gathering grounds
over the past decade. After the expansion of the desalination plant to 270 Mld, this capacity will be
equivalent to 44% of mean gross yield collected from water gathering grounds
over the past decades. The proposed desalination plant can provide approximate
5% and 10% of contribution to the total freshwater demand in Hong Kong. The
proposed desalination plant is thus of paramount importance in supplementing
the fresh water resources in Hong Kong.
Prior to the
Project, various studies had been conducted for investigating the technical and
economic viability of desalination. In particular, the Study Agreement No. CE71/2000 – Feasibility Study
on Development of Desalination Facilities in Hong Kong (the “Feasibility
Study”) completed in 2002 identified various potential locations for the
desalination plant and assessed the suitability.
In the Feasibility
Study, several locations are shortlisted for constructing a new desalination
facility in Hong Kong, including Siu Ho Wan, Tuen Mun Area 38, Tsang Tsui Ash
Lagoon and Tseung Kwan O Area 137. The shortlisted
locations were assessed based on functionality, cost, environmental benefits/ disbenefits and regulatory/ social issues. Locations of these sites are depicted in Figure
2.1.
The site selection
criteria were further elaborated and confirmed in the Study Agreement No. CE97/2002(WS) – Pilot Plant
Study on Development of Desalination Facilities in Hong Kong (the “Pilot
Plant Study”) completed in 2007 which included a more detailed investigation in
the technical and financial viability of desalination using reverse osmosis
technology.
Findings from the Pilot Plant Study showed that the western waters of
Hong Kong is typically more turbid and has higher suspended solid (SS) levels
compared to that in the eastern side, due to influence of the Pearl River tidal
flows. On the other hand, the eastern waters of Hong Kong is oceanic in nature
with less turbidity and lower SS levels, and is remote from the influence of
the Pearl River flows. Since the eastern waters in Hong Kong is relatively
consistent in quality with relatively less variation in salinity, which is
beneficial to the operations of the desalination plant and should be put in a
higher priority for the development of desalination plant. In terms of
constructability and feasibility, Tseung Kwan O Area
137 is thus selected as the preferred location for the development of the
desalination plant. In 2012, a site of about 10 ha at Tseung
Kwan O Area 137 has been earmarked for the construction of a desalination plant
with an output capacity of 135Mld with provisions for future expansion to 270 Mld.
A summary description and evaluation of each short-listed location is
provided in Table 2.3.
Table 2.3 Potential Desalination
Plant Location and Evaluation Outcomes
|
|
A) Siu Ho Wan |
B) Tuen Mun Area 38 |
C) Tsang Tsui Ash
Lagoon |
D) Tseung Kwan O Area
137 |
|
Description |
The site is located on the northwest
coast of Lantau Island. |
The site is located in western New
Territories and it is a reclaimed land. |
The site is located within the
existing Tsang Tsui Ash Lagoon. |
The site is located at the southern
tip of the TKO Area 137 and it is a reclaimed land. |
|
Functionality |
Have less desirable quality of
seawater, but able to integrate with existing water supply system. |
Have less desirable quality of seawater
and not easy to integrate to water supply system. |
Have less desirable quality of
seawater and not easy to integrate to water supply system. |
Have acceptable quality of seawater
can integrate with existing water supply system. |
|
Cost |
Low opportunity and capital cost as
the land is owned by the government and the site is flat which give rise to
minor cost of building demolition. |
Low capital cost as the land has
already been formed but land use competition with other industries may increase
the opportunity cost. |
Low capital cost as the land has
already been formed but land use competition with other industries may
increase the opportunity cost. |
Low capital cost as the land has already
been formed but land use competition with other industries may increase the
opportunity cost. |
|
Environmental
benefits/ disbenefits |
2
The site is surrounded by residential
area with noise and water sensitive receivers and potential chlorine hazard.
The site does not involve habitat removal and thus the ecological impacts are
also minimal. However, potential visual impact may arise due to the new town
development in North Lantau area overlooking the site. 3
|
The site is situated in industrial area
and no or low noise impact, potential chlorine hazard and ecological impact.
Potential visual impact is identified as it is located by seafront and can be
viewed from sea. Across the Urmston Road Channel to
the northwest of the site Sha Chau and Lung Kwu
Chau marine park and Chinese White Dolphin feeding area is found and defined
as water sensitive receiver. |
Receiving water body is considered to
be topographically constrained with limited dispersive capacity. The site is
also in the vicinity of an oyster culture zone. |
The site has minimal impact on
landscape. As the land has been formed, extensive earthworks are not required
and thus further reduce the impacts on the surrounding environment. The site
is close to the sea and water distribution networks (Tseung
Kwan O Primary Fresh Water Service Reservoir) and to consumers to minimize
construction and land-use of pipelines and pumping efforts for water
distribution. |
|
Regulatory/
Social |
Government and occupied by government
department |
Government owned land and no
occupants. But it is close to river trade terminal, the development may have
impact on marine traffic. |
Land is owned by utility and required
land resumption |
A reclaimed land owned by Government. |
Based on the
evaluation in Table 2.3, Tseung Kwan O Area 137 was
selected as the most preferable site due to functionality aspect and relatively
fewer environmental disbenefits among the
short-listed sites.
A 10 hectares (ha)
land has been allocated for the Project at Tseung
Kwan O Area 137, and three options for the development of desalination plant
are considered:
Option
1 - Adopting the allocated land;
Option
2 – Locating the plant in rock cavern;
and,
Option 3 - Shifting the
footprint of plant site towards south west.
The
area required for Option 1 to 3 is shown in Figure 2.2.
For
Option 1, the allocated land is reclaimed land and currently occupied by Civil Engineering and Development Department (CEDD)
as public fill bank which has been formed to an appropriate level. As such, extensive site formation, which
induces extensive fugitive dust and construction waste generation, is not
anticipated for the Project. In
addition, the 10 ha allocated land is located outside the boundary of Clear
Water Bay Country Park. However, the
natural slope mitigation works would encroach into the Clear Water Bay Country
Park.
For
Option 2, the proposed cavern development for the desalination plant is located
within the natural slope in the Clear Water Bay Country Park next to the
allocated land (Figure 2.2). Since
cavern development involves blasting and vibratory construction method,
air-borne and ground-borne noise as well as construction waste surplus would be
anticipated. Furthermore, the area
requires for the same design capacity of the plant in cavern would be about 11
ha (i.e. larger than that for Option 1) as part of the plant components,
particularly chemical storage, would have to be situated outside the cavern for
safety purpose and thus lead to a larger project footprint. The above ground structures, such as portals
and ventilation shafts, may also involve construction in the Clear Water Bay
Country Park and thus lead to a larger project footprint.
For
Option 3, the footprint of the proposed desalination plant is set back towards
south west by 60-80m from the adjoining slope area to avoid the need of slope stabilization work at the nature slope
within the country parks area (Figure 2.2). The option would require the same area of
land as that for Option 1. However, the
10 ha site for the proposed desalination plant as mentioned in Option 1 was
reserved according to the Preferred Land Use Plan endorsed by Committee on Planning and Land Development (CPLD) in
2006. The shifting of the site boundary
of the desalination plant would adversely affect the planned land uses for the
adjoining areas and CPLD’s endorsement of the proposal would be required. Unless the required slope mitigation works
will involve irreversible damages to the landscape and rare species of
wildlife/flora in the Country Park, the alternative of shifting the site
boundary, to avoid any slope mitigation works to be carried out in the country
park area is not supported from land utilization of view.
The
summary of three options is shown in Table 2.4.
Table 2.4 Comparison of Plant
Footprint
|
Characteristics |
Option 1 Adopting the Allocated Land |
Option 2 Cavern Development |
Option 3 Shifting the plant footprint towards south west |
|
Land
Requirement |
~10ha |
~ 11ha (7ha – cavern development, 4 ha – Tseung Kwan O Area 137) |
~ 10 ha |
|
Environmental
benefits/ disbenefits |
· Plant site does not encroach into the Clear
Water Bay Country Park · Nuisances due to site formation is not
anticipated as the land has been formed by the existing fill bank |
· Air-borne and ground-borne noise in the
Clear Water Bay Country Park area during blasting and vibratory construction · Creates additional C&D surplus · The above ground structures, such as portals and ventilation shafts,
may also involve construction in the Clear Water Bay Country Park. |
· Plant site does not encroach into the Clear
Water Bay Country Park · Nuisances due to site formation is not
anticipated as the land has been formed by the existing fill bank · No slope stabilisation work within the
country park is required. |
|
Other issue |
Natural slope mitigation works would
encroach into the Clear Water Bay Country Park. |
Chemical storage required outside the
cavern because of safety consideration and leads to larger project footprint. |
The earmarked site for the desalination
plant is reserved according to the Preferred Land Use
Plan endorsed by CPLD in 2006. The shifting of the site boundary of the Plant
would adversely affect the planned land uses for the adjoining areas. |
|
Conclusion |
With minimum land requirement and
construction within the country park area is avoided, thus it is a preferred
option. |
Not viable option and thus not considered. |
Not viable option from land utilization point of view. |
In view of the above, Option 1 – adopting the allocated land is the preferred plant extent in terms of environmental benefits/ disbenefits and land utilization.
In accordance with the findings of the natural terrain hazard assessment
carried out in accordance with GEO Report No. 138, the natural slope adjoining the
plant area on the east side is deemed to impose landslide risk on the
development in the site. In order to
provide a safe environment for the development, landslide hazard mitigation
measures are proposed to alleviate the landslide risks. The recommended works
are based on consideration of four options:
Option 1: Set back proposed plant outside the influence
zone of potential landslides and rock falls from the natural slope
Option 2: Construct passive measures such as barriers
within the influence zone to contain potential landslides
Option 3: Adopt active protection measures for
stabilization works, including soil nailing and rock stabilization works, in
the natural slope area identified with risk of landslides and rock falls
Option 4: Adopt mixed use of active and passive
protection measures, such that debris and rock barriers and localized rock
stabilization of large unstable rocks and boulders and soil nailing at steep
portions above rock cliff
Option 1 is to set
back the plant and the ancillary infrastructures outside the influence zone of
the natural terrain. It will avoid slope
mitigation works on the natural hillside within the country park area. In
accordance with the guidance given in GEO Report No.138, this option requires
approximately 60 to 80m set back throughout the entire length of the plant site
adjoining the slope area, and thus sterilizing 4 ha of land along the toe of
the natural slopes. The required area of
set back is shown in Figure 2.3a.
Option 2 is to construct
passive defense measures, such as flexible barriers, within the influence zone
and at a distance of about 15m from the toe of the natural slope to catch
landslide debris, boulders and rock falls from the natural hillside and thus avoiding
slope mitigation works within the country park area. The required area of set
back is shown in Figure 2.3b.
Option 3 is to
adopt active stabilization measures by stabilizing the natural hillside slope
areas with potential landslide and rock fall hazards. The slope mitigation works will include
installing soil nails on the soil slopes and installing rock stabilization
works at the steep rock portions of the hillside in areas with risks for
landslides and rock falls throughout the natural slope. The areas of these
works are depicted in Figure 2.3c.
Option 4 is to
adopt a mixed use of active and passive stabilization measures by stabilizing
the localized natural hillside slope areas with large scale potential landslide
hazards, stabilizing the boulders with large scale rock fall hazards, and
providing flexible debris barriers to stop small scale rock fall hazards. This option minimizes the use of soil nails
and rock stabilization works on the slope area while protecting plant buildings
from small rock falls and debris run-out from potential landslides. The flexible debris barriers will be installed
in the lower portion of the slope but where it is effective, they will also be
installed within the influence zone in the plant site to minimise
the extent of works within the country park area. The extent of the proposed option is demonstrated in Figure
2.3d.
An evaluation of
the above options has been
undertaken and the findings are summarized in Table 2.5.
Table 2.5 Description
of Potential Natural Terrain Slope Mitigation Works and Evaluation Outcomes
|
Evaluation
Criteria |
Option
1 – |
Option
2 – |
Option
3 – |
Option
4 – |
|
Safety Protection from Landslide and Rock Fall Hazards |
Effective safety protection |
Incomplete protection, large
landslides and some boulder and rock falls on the upper portions of the natural
hill may still impact the development within original landslide impact zone
by overshooting the steep rock cliffs, and result in unacceptable residual
risk of landslides. |
Effective safety protection. |
Effective
safety protection. |
|
Land Utilization |
A strip of land (4 ha,
equivalent to 40% of the earmarked site area) shall be
reserved as no entry zone. |
A strip of land (0.8 ha,
equivalent to 8% of the earmarked site area) shall be
reserved as no entry zone. |
No set back is required. |
Two
strips of land (total 0.02 ha, less than 1% of site area) shall be reserved
as no entry zone. |
|
Potential Stabilization Area on the Hillside |
No stabilization works on the slope. Future maintenance works and repair works (usually soil nail and rock
stabilization works) will be needed when landslides occurs. |
No stabilization works on the slope. Future maintenance works and repair works (usually soil nail and rock
stabilization works) will be needed when failure occurs. |
Soil nailing and rock slope stabilization covers
approximately 3.3 ha of the hillside. |
Soil nailing and rock slope stabilization barriers covers
approximately 0.5 ha of the hillside. |
|
Encroachment of country park footprint |
0 ha |
0 ha |
3.3 ha |
0.49 ha |
|
Environmental
Benefits/ Disbenefits to Clear Water Bay Country
Park |
No
environmental disbenefits by the proposed slope
works. But future maintenance/ repair works
would lead to extensive disturbance to the natural habitats in the Clear
Water Bay Country Park when landslides and/or rock-falls occur. |
No
environmental disbenefits by the proposed slope
works. But future maintenance/ repair
works would lead to extensive disturbance to the natural habitats in the
Clear Water Bay Country Park when landslides and/or rock-falls occur. |
Relatively
large areas of clearance of vegetation on slope area will be required by the
slope mitigation works |
i) The slope works are localized to areas at the lower portion of the
slope and at the identified unstable boulders. Area with foundations of flexible debris
barriers will also be disturbed by the slope mitigation works ii)
Tree-felling can be avoided by careful designs of works around trees. Vegetation clearance can be reduced where
practicable. iii)
No extensive earthworks are required. iv)
Catering for alignment changes |
|
Construction Cost |
N/A |
Moderate |
High |
Moderate |
|
Maintenance /running cost |
Low |
Low |
Moderate |
Low |
Based on the
findings above, Option 1 would result in an extensive loss of usable land for
implementation of the project. The remaining area is not sufficient for the
development of the desalination plant at the Phase 1 design capacity of 135 Mld and the future expansion to 270 Mld. As discussed in Section 2.3.1 (a), the shifting of the site boundary to
southwest is considered not viable based on land utilization point of
view. The extension beyond the earmarked
site is not feasible due to planning and land zoning considerations.
Consequently, Option 1 is considered not viable.
Under Option 2,
there is a residual risk on the desalination plant caused by the overshooting
of landslide and boulder falls originated on the upper portions of the natural
hillside and is considered as an ineffective protection for the desalination
plant (refer to overshooting scenario in Figure 2.3b). Hence, Option 2 is not feasible due to the
unacceptable residual risk on the desalination plant and its operators from
major landslides and boulder fall hazards.
For both Options 1
and 2, without stabilization, the condition of the slope will deteriorate.
Landslides will occur during adverse weather conditions in future. Maintenance and repair works will be required
and provided as necessary. There are
still potential disturbance to natural habitats within Country Park area due to
maintenance and repair works of future landslides.
Option 3 employs
blanket slope stabilization works, including soil nailing and rock
stabilization works, will be required on the steep portions of the natural
slope covering 3.3 ha of the slope area.
Extensive disturbance to natural habitat within Country Park would be
unavoidable and thus this option is not considered due to the environmental
drawback.
Option 4 consists
of localized slope stabilization, localized boulder stabilization, and
localized flexible debris barriers. The
lengths of barriers on the natural slope have been optimized such that these
are located to contain potential landslides and boulder falls that may
overshoot the cliff tops. Lengths of
barriers are provided beyond the toe of the natural slope outside the Country
Park where there is no overlooking cliffs.
The works will provide effective safety protection from major landslides
and rock fall hazards. The localized
stabilization works on the slope area will be optimized to avoid the impact to
the existing vegetation and to minimize the impact to the Country Park as much
as practicable. Localized clearance of
vegetation to allow construction of the slope stabilization works will be
minimized, and no tree will be felled.
The extent is significantly less than that required in Option 3.
In view of the
less environmental disturbance and the effectiveness on safety protection,
Option 4 is recommended. In addition,
the above options for the natural terrain slope mitigation works were reviewed
by Civil Engineering Development Department under the Natural Terrain Hazard
Study (NTHS) Report for the Project. The
rationale of selecting the most appropriate option for slope mitigation works
were agreed by CEDD and the exact extent of the required mitigation works would
be subject to the detailed design. The
hazard mitigation works of Option 4 are demonstrated in Figure 2.4.
Because of the
shape of land earmarked for desalination plant, the 10 ha site shall be
optimized by the selection of treatment process and associated layout design
based on the technical requirement, design flow and flexibility of future
expansion. In addition, the scale and
size of above-ground structure of the Desalination Plant are optimized by
housing suitable facilities underground (i.e. intake / treated water pumps,
storage facilities, clarification basins and filtration facilities) to achieve
a better plant hydraulic and minimize the energy use in pumping and reduce
potential environmental impacts.
A chlorine store
shall be provided in the desalination plant for disinfection process. However,
chlorine storage within the Plant is categorized as a Potentially Hazard
Installation (PHI) under the Hong Kong Planning Standards and Guidelines and
will induce hazard to life to the surrounding citizen. The location of the
chlorine store shall be carefully designed in order to reduce the impact to the
surrounding environment.
Three locations of
the chlorine store were assessed, including:
Option 1 is locating the chlorine store near the northern boundary;
Option 2 is locating at around the center of the site; and,
Option 3 is locating at the southern boundary of the site.
The proposed locations for chlorine store are shown in the Figure
2.5 and the evaluation is demonstrated in Table 2.6.
Table 2.6 Comparison of Chlorine
Store Location within Desalination Plant
|
|
Option 1 Northern Boundary |
Option 2 Around Centre of the Site |
Option 3 Southern Boundary of the
Site |
|
Distance away from explosive offloading pier |
585m |
360m |
60m |
|
Distance away
from Existing offsite population (Assuming TKO industrial estate) |
915m |
1140m |
1440m |
Options 1 and 2 are located away from the CEDD explosive offloading pier
and this is found to have advantage over Option 3, as the explosive offloading
pier may impose external risk to the chlorine store building, for example leakage of chlorine drum or structural damage to the
chlorine store building.
Option 1, however, has a shorter separation distance between the
chlorine store and surrounding environment than Option 2. It is preferable to site hazardous
installation away from surrounding population to minimize potential hazards,
and so Option 2 was duly selected as the preferred chlorine store location.
Intake
Two intake options, namely Option 1 Offshore Open Intake in the western
side of Kwun Tsai, and; Option 2 – Subsurface Intake
System in Joss House Bay, were considered and they are illustrated in Figure
2.6. Evaluation outcomes are
summarized in Table 2.7.
Table 2.7 Comparison of Alignment
options for Submarine Intake
|
Characteristics |
Option 1 – Offshore Open Intake |
Option 2 – Subsurface Intake System |
|
Engineering |
The location of intake structure and
pipeline is selected such that the intake is adequately submerged at low
tide, protected from the damaging wave motion of storm, due to silt sediment.
This option also ensures stable feed water for desalination and subsequently
freshwater supply. |
Subsurface intake system requires to lay
larger number of perorated pipe under the seabed or vertical well along the
shoreline to ensure the water quality is good and stable. This method is
highly dependent on the geological characteristic of the local subsurface
media, However, based on the information available from geological testing and
conditions from the site, the transmissivity is expected to be very low and a
number of wells should be required which incur the spatial problem for
implementation. |
|
Environmental
benefits/ disbenefits |
To minimize the impingement and entrainment of planktonic organisms,
the intake is located 200-250m offshore where the productivity is relatively
low. Also, the backwash water from the desalination plant serves as cooling
water which further reduces the intake water volume. The intake pipe with corresponding intake structures will be
constructed for serving ultimate capacity which avoids the construction at
seabed during expansion in future. The diameter of intake pipes shall be
sized to maintain sufficient cleaning velocity, whilst maintaining a low
intake velocity to avoid infringement of the marine ecology. |
Extensive excavation of vertical well or
dredging under the seabed is required and subsequently generates extensive
excavated materials and increase of suspended solid level onshore during
dredging of seabed, which is anticipated to have higher coverage than that at
offshore. Also, the vertical wells require adequate spacing apart such that
yield/capacity is not impacted which leads to a larger project footprint. Since the system is difficult to maintain,
additional well or infiltration gallery will be required during operation and
this implies an extended construction duration (e.g. maintenance dredging). |
|
Safety |
Sufficient depth (at least 10m) of the
intake structure is required to avoid any obstruction to normal marine
traffic. |
This option is onshore construction, thus
impact of marine traffic is avoided. |
|
Conclusion |
Intake is designed to located 200-250m away
from the shore. The location is considered viable options in terms of
constructability and minimal impacts on marine ecology and fisheries. |
Extensive construction works on seabed or
along the shoreline is required and the system may be blocked by particle in
the sea and difficult to maintain and required additional dredging.
Therefore, the option is not preferred. |
Base on the evaluation above, Option 1 – Offshore Open Intake at Joss
House Bay is preferred due to constructability and minimum environmental
impacts.
Outfall
Two options for the outfall alignment were evaluated. Option 1 is discharging to Tathong Channel and Option 2 is discharging to Joss House
Bay. Both options are designed to
discharge as close to the tidal current as possible to enhance dispersion
process. The proposed alignment of both
options is provided in Figure 2.7. The evaluation outcome
of different outfall alignment options is presented in Table 2.8.
Table 2.8 Comparison of Alignment
options for Submarine Outfall
|
Characteristics |
Option 1– Tathong
Channel |
Option 2 – Joss House Bay |
|
Engineering |
The length and depth of the submarine
outfall with designed with due considerations on the discharge standards of
the marine waters of the Junk Bay, Eastern Buffer, Port Shelter, Mirs Bay and Southern Water Control Zones (WCZs). The
discharge location shall take into account the water depth, outlet velocity
of the currents and movement of water to allow sufficient mixing and dilution
upon discharge. |
The location of outfall structure is
selected such that the outfall is adequately submerged at low tide, to
protect from the damaging wave motion of storm. Also this outfall alignment
is close to tidal current to allow sufficient mixing and dilution upon
discharge. |
|
Environmental benefits/ disbenefits |
Located at Tathong Channel can provide
better tidal current for diluting the discharge. However, the proposed
location required longer outfall pipeline than option 2, this induced more
C&D waste surplus. |
Joss House Bay has relatively low fisheries productivity, and the
tidal current is sufficient to dilute the discharge. The outfall pipeline in
Joss House Bay would be shorter and thus require less excavation works and
minimize the waste generation. |
|
Safety |
Located at Tathong
Channel where is the main channel of marine traffic in Hong Kong, the
construction and operation of the outfall would disrupt the marine traffic. |
In comparison with Tathong
Channel, Joss House Bay is less busy in terms of marine traffic, and thus
minimize the obstruction to normal marine traffics. |
|
Conclusion |
This option is not preferred in terms of
marine traffic obstruction and the extensiveness of excavation works. |
Outfall designed to located 300-350m away
from the shore in Joss House Bay is considered viable option in terms of
constructability and minimal impacts on surrounding environment. |
By comparing the dispersion efficiency of the concentrate and the
associated environmental benefits of both options, Option 1 is relatively
better than Option 2 as it is closer to the tidal current. However, the proposed
site location is not next to the shoreline along the Tathong
Channel, and a longer outfall pipe will be required for Option 1 which results
in more C&D waste surplus during construction stage. Also, the Tathong Channel is the main channel of the marine traffic
in Hong Kong, the construction and operation and maintenance of the outfall may
cause disruption to marine traffic. Based on above evaluation, Option 2 is the
preferred option for the outfall.
Two options of the trunk feed alignment were studied and illustrated in Figure
2.8.
In Option 1, the alignment of new pipe will be laid along Wan Po Road, Po Hong Road Po
Lam Road North and Tsui Lam Road. In this option,
whole trunk main is proposed to be laid under the carriageway.
In Option 2, the proposed alignment runs from Wan Po Road and Chun Wang Street and
across Junk Bay to King Ling Road, Tong Yin Street, Po Shun Road, Po Hong Road,
Po Lam Road North and Tsui Lam Road. In this option,
submarine pipeline is required to be built for crossing Junk Bay from Chun Wang
Street to King Ling Road.
The
options were evaluated by the length of pipe, traffic impact, and environmental
impact and tabulated in Table 2.9.
Table 2.9 Description
of Potential Trunk Feed Alignment and Evaluation Outcomes
|
Characteristic |
Option 1 |
Option 2 |
|
Pipe Length |
9.3km |
9 km |
|
Traffic Impact |
Moderate
|
Low |
|
Environmental benefits/ disbenefits |
Although
there will be some obstruction along the existing Wan Po Road during
construction, this option only requires land-based construction in area of
relatively low ecological value. This avoids disturbance to marine ecological
environment and water quality. With the adoption of trenchless method, the
air and noise impacts associated with this option are largely minimized. In
addition, a treated water pumping station will be provided within the TKO
site for the desalination plant and no booster pumping station is anticipated
along the product water delivery pipeline alignment. This further reduces the
footprint of the project. |
This
option would construct a shorter truck feed system. However, about 2.2 km of
pipeline required to be constructed and potentially damage to the marine
ecology and affect the water quality in the surrounding region. In addition,
operation and maintenance of the submarine pipeline is relatively difficult.
The laying of submarine pipe would be longer than land-based underground pipe
of the same length. Thus, environmental impact for laying submarine pipe
should be longer than that of the land-based pipeline laying of the same
length. |
Although Option 2 can have a relative shorter pipe length and less
traffic impact on Wan Po Road, Option 1 is chosen as the preferred option for
the trunk main due to the drawbacks on marine ecology and water quality from
Option 2.
All the-state-of-the-art desalination technologies were considered in the
Project. Broadly speaking, two major desalination processes are commonly used
in large scale desalination applications around the world including seawater
reverse osmosis and multi-stage flash desalination.
Seawater Reverse
Osmosis (SWRO)
Seawater will be drawn from the seawater intake system for the
desalination process. Chlorine is dosed periodically into the intake seawater
for control of microbial growth at the intake and the associated screening
system.
Seawater will be delivered to the pre-treatment system for pre-treatment
by clarification followed by filtration prior to the Seawater Reverse Osmosis
(SWRO) process. Coagulant/polymer will be added to feed water for coagulation
and flocculation. Residual chlorine left over from the intake chlorination will
be removed by dechlorination process. Process waste
streams will be generated from the pre-treatment processes. The process waste
streams will include sludge from clarifiers and backwash waste from filters
(also known as residual streams).
High pressure feed pumps will drive the seawater through the RO system.
The pressurized seawater will be split into two streams, a low pressure
permeate stream (product stream) and a high pressure concentrate stream (RO
concentrate or waste stream) which is the rejected flow from the RO membranes
and required to be discharged. The permeate produced in the RO system will be
passed into the post-treatment system prior to pumping into the distribution
system for potable water uses. The RO membranes will require cleaning with
chemicals (i.e. clean-inplace or CIP) on periodic
basis. The waste generated from the RO cleaning process is neutralized before
disposal to public sewer.
Post-treatment processes will include disinfection using chlorine and
fluoridation, pH correction and stabilization via hydrated lime and carbon
dioxide dosing.
Multi-stage flash
desalination (MSF)
In the MSF process, seawater is heated in a vessel called the brine
heater. This is generally done by condensing steam on a bank of tubes that
carrying seawater and passing through the vessel. This heated seawater then
flows into another vessel, called a stage, where the ambient pressure is lower,
causing the water to boil.
The sudden introduction of the heated water into the chamber causes it
to boil rapidly, almost exploding or flashing into steam. Generally, only
a small percentage of this water is converted to steam (water vapour), depending on the pressure maintained in this
stage, since boiling will continue only until the water cools (furnishing the
heat of vaporisation) to the boiling point.
The vapour steam generated by flashing
is converted to fresh water by being condensed on tubes of heat exchangers that
run through each stage. The tubes are cooled by the incoming feed water going
to the brine heater. This, in turn, warms up the feed water so that the amount
of thermal energy needed in the brine heater to raise the temperature of the
seawater is reduced.
An MSF unit uses a series of stages set at increasingly lower
atmospheric pressures. The feed water could pass from one stage to another and
be boiled repeatedly without adding more heat. Typically, a MSF plant can
contain from 15 to 25 stages.
Evaluation of environmental benefits/ disbenefits
and feasibility of SWRO and MSF is presented in Table 2.10.
Table 2.10 Description of Potential
Desalination Technologies and Evaluation Outcomes
|
Characteristics |
Seawater Reverse Osmosis (SWRO) |
Multistage Flash (MSF) |
|
Land Requirement |
Modular (10 ha for this Study) |
Large |
|
Energy Consumption |
Moderate, 2.5-4.0 kWh/m3 (1.75 to 2.8 kg of CO2/m3) |
High, 12.7-15.0 kWh/m3 (8.9 to 10.5kg of CO2/m3) |
|
Engineering |
High pressure feed pumps will drive the seawater through the RO system.
The pressurized seawater will be split into two streams, a low pressure
permeate stream (product stream) and a high pressure concentrate stream (RO
concentrate or waste stream) which is the rejected flow from the RO membranes
and required to be discharged. The permeate produced in the RO system will be
passed into the post-treatment system prior to pumping into the distribution
system for potable water uses. It has high recovery of about 30% to 90%, i.e.
more effective in producing freshwater. |
Seawater is heated in a vessel called the brine heater. This is
generally done by condensing steam on a bank of tubes that carrying seawater
and passing through the vessel. MSF required high energy input for boiling
and poor recovery of 10% to 25%. |
|
Environmental benefits/ disbenefits |
Although the brine discharge would affect the water quality of
surrounding water bodies, the easy operation, small footprint, relatively
lower energy consumption (~30% to 70% lower than MSF) and high recovery would
promote the use of this technology and this also reduce the impacts to
surrounding habitats. Also, as no
boiling is required, impact of air quality would be minimized. The small
footprint also implies that the construction of the plant is not extensive, and
thus reduces the extent and duration of construction impacts. |
Due to boiling of water, generation of on-site air pollution by
burning fossil fuel is anticipated. Also, heated and concentrated seawater
will affect the water quality of surrounding water bodies when discharging |
The evaluation outcomes revealed that MSF technology requires high power
consumption, including electricity and fuel consumption. For fuel burning,
local emission of carbon dioxide, nitrogen oxides are anticipated which impose
environmental impact to the surrounding. Also MSF technology was applied in the
decommissioned Lok On Pai Desalination Plant which
was closed because of high power consumption of the operational method. In this
case, MSF should incur a relatively higher carbon footprint and induce a higher
environmental impact.
Compared with MSF, SWRO technology has definite advantages of less
energy consumption, no local emission of fossil fuel consumption and relatively
economic viable. Seawater Reverse Osmosis (SWRO) is thus adopted as the
preferred method for desalination process.
Table 2.11 Comparison of Chlorination
Option for Desalination Plant and Evaluation Outcome
|
|
Option 1 Chlorine Gas |
Option 2 Bulk Delivered Liquid Hypochlorite |
Option 3 Onsite Generation |
|
Energy
Consumption |
Low |
Low |
Moderate |
|
Environmental
benefits/ disbenefits |
Chlorine
gas leaks need to be dealt with scrubber system. |
Chlorine gas will be formed during the
accidental mix with acid |
Hydrogen gas is one of the by-products and mitigation
measures need to be in place. |
|
Source
supply/reliability |
Used
currently in all WSD WTWs. 90 days storage is typical, except in SWPS |
Not
currently used in WSD WTWs, except in SWPS. Storage limited to 30 days due to
deterioration/by-product formation |
Availability
of high purity salt with low bromide content is critical to minimize bromate
formation |
|
Other
issues |
N/A |
Chlorate:
Decay due to storage. Maximum chlorine
dose limited due to formation of above byproducts. The
quality of sodium hypochlorite depends on the manufacturer |
Chlorate
and bromate: Side reaction of the generation process. Maximum
chlorine dose limited due to formation of above byproducts. |
For marine pipeline construction, there are typically constructed by dredging
or trenchless method.
Based on the geological profile along the intake and outfall alignments, the rock head at the shores along and in the
vicinity of the proposed intake and outfall pipes alignments are exposed at or
close to the seabed, substantial dredging /rock breaking and backfilling is
necessary to restore the affected area to the original bathometry. The dredging
operation will give rise to considerable environmental impact to the water
quality and considered as not feasible.
The dredging extent for the option is demonstrated in Figure
2.9.
Trenchless method has been considered for the construction of the
proposed submarine outfall with a view to reduce potential impacts of the
marine dredging works on water quality, marine ecology and fisheries of the
Joss House Bay. The submarine outfall will be constructed by trenchless method,
i.e. Micro-tunneling or Horizontal Directional Drilling (HDD) method subject to
the final designed diameter of the conveyance pipe for intake and outfall. For HDD method, it involves drilling a pilot
hole and then progressively enlarging the hole using reaming tools in steps
until the required diameter is achieved.
Reaming is either done in the reverse direction to the pilot boring or
in the same direction (forward reaming).
Once the reamed hole has been fully formed, the pipeline is pulled
and/or pushed into the reamed hole. Micro-tunneling is a process remotely controlled Micro-Tunnel Boring Machine (MTBM) combined with the
pipe jacking system to directly install product pipelines underground in a
single pass. Jacking and reception shaft
at the opposite drive for retrieving the MTBM. The process is applicable to
tunnel diameter to about 2 to 3 meters.
The use of Micro-tunneling and HDD method is particularly suitable for
construction of the proposed submarine intake and outfall because marine
dredging and excavation would be minimal comparing with conventional submarine
pipeline installation methods by forming a trench in seabed along the entire
alignment of the proposed submarine pipe and backfilling after pipe laying.
Minor dredging of the seabed over a much smaller area at the end of the intake
and outfall pipe will, however, be required for constructing the reception
shaft of the MTBM and the installation of the intake structure and
diffuser. With the use of trenchless
method, the extent of dredging will be limited to ~ 50m for intake structure
and ~ 150 m for the diffuser, and the dredging extents are thus reduced considerably.
The extent of dredging for trenchless method is also demonstrated in Figure
2.9. This will effectively
reduce the dredging volume by more than half (reduced from ~ 18,000 m3
to ~ 6,330 m3) and greatly minimize the potential impacts on water quality, marine ecology
and fisheries associated with dredging activities. Therefore, trenchless method with localized
dredging is the preferred option for the installation of submarine pipelines,
intake and outfall. Evaluation of
submarine installation method is presented in Table 2.12 below.
Table 2.12 Comparison of Construction
Method for Submarine Pipelines, Intake and Outfall
|
Characteristics |
Option 1 – Conventional Dredging |
Option 2 – Trenchless method with localized dredging |
|
Engineering |
Based
on the geological profile along the intake and outfall alignment, the rock
head at the shores along and in the vicinity of the proposed intake and
outfall alignments are exposed at or close to the seabed, substantial
dredging /rock breaking and backfilling is needed to restore the affected
area to the original bathymetry. |
Horizontal
Directional Drilling (HDD) method involves drilling a pilot hole and then
progressively enlarging the hole using reaming tools in steps until the
required diameter is achieved. Reaming is either done in the reverse
direction to the pilot boring or in the same direction (forward reaming).
Once the reamed hole has been fully formed, the pipeline is pulled and/or
pushed into the reamed hole. Micro-tunneling is a process remotely controlled Micro-Tunnel Boring Machine
(MTBM) combined with the pipe jacking system to directly install product
pipelines underground in a single pass. Jacking and reception shaft at the
opposite drive for retrieving the MTBM. The process is applicable to tunnel
diameter to about 2 to 3 m. |
|
Environmental benefits/ disbenefits |
By adopting solely conventional
dredging for submarine installations, considerable impact to water quality
and generation of a significant amount of dredged materials would be
anticipated. |
The trenchless method can reduce the
dredging volume by more than half (reduced from ~ 18,000 m3 to ~
6,330 m3) and greatly minimize the potential impacts on water
quality, marine ecology and fisheries associated with dredging activities. |
|
Conclusion |
Not preferred in terms of environmental
drawbacks. |
Reduction in dredging extent would reduce the
potential impact on water quality due to submarine installations. Therefore,
this option is preferred. |
In addition, suitable rock fill material will be used to backfill the
standing platform of the intake structure and outfall diffuser. Rock fill is considered as the most suitable material
as it offers sufficient support and protection to the systems, and has low
fines content which reduces potential impacts on water quality, marine ecology
and fisheries during marine backfilling activities.
Since the freshwater rising mains will be constructed along the
carriageway to the TKOFWPSR, most of the delivery pipes will be constructed by
cut-and-cover method. Cut-and-cover method is a common construction method for
pipe laying by excavating the ground and backfill after laying the pipe. The maximum trench depth for cut and cover
will be about 3 m.
Cut-and-cover method is not feasible at
several road junctions along the alignment because of heavy traffic. Trenchless
method will be adopted at the designated locations. Pipe jacking and micro-tunnelling are the two
most preferable and effective trenchless methods for the proposed works.
A comparison of the environmental benefits
and disbenefits of the cut-and cover method and
trenchless method is summarized in Table 2.13 below.
Table 2.13 Comparison of Construction
Method for Trunk Feed System
|
Characteristics |
Cut- and-cover method |
Trenchless method |
|
Environmental benefits |
·
Relatively quicker technique for short sections of trench. ·
Possibility to reuse excavated materials or surplus fills from other
projects. ·
Catering for alignment changes. |
·
Surface works limited to the construction pits. Hence, reduced direct
impacts on habitats and vegetation. ·
The works are underground in nature and thus minimizing the
disturbance to sensitive receivers along the alignment. ·
Less PMEs is required with limited spoil to be disposed of compared
with C&C method. ·
The underground works will not be visible to the public and hence
reduced visual impacts. |
|
Environmental disbenefits |
·
Works may affect habitats adjacent to the proposed alignment. ·
All sensitive receivers along the alignment have the potential to be
affected. ·
More construction plant will be involved and this would generate
relatively more noise and dust impacts. ·
Larger amount of material handling due to excavation and backfilling. ·
More potential for construction run off due to open excavation. |
·
Sensitive receivers nearby the construction pits will be subject to a
longer period of environmental disturbance. ·
A wider pit is required compared to C&C method and sufficient area
may not be available at congested site. ·
Requires treatment of surplus bentonite before disposal. |
While trenchless
method may be used at the selected locations where cut-and-cover method is not
feasible, trenchless method is not suitable for wide-spread application because
of its limitations. Trenchless method is generally adopted for straight pipe
alignment. Construction of a working pit
will be required at each end of a trenchless pipe. Construction of additional
working pits is required for changing alignment direction. The length of a single trenchless pipe is
restricted because of the limitation of jacking force. Linking up the pipe segments will require
additional working pits. The working
pits have to be located at selected locations where adequate space is
available. Construction of working pits
typically requires installation of temporary structural supports and extensive
excavation. Sensitive receivers nearby the working pits will be subject to a
longer period of environmental disturbance.
Based upon the above comparison,
cut-and-cover method is proposed for the trunk main system because it is
relatively more flexible in managing the construction duration and engineering
constraint. Trenchless method will be considered in a particular location where
the cut-and-cover method is considered not feasible, including the
environmental impact imposed cannot be mitigated by the proper mitigation
measures.
The construction works involved in the proposed Desalination Plant
include Civil & Structural works, E&M works and Building Services
works. For foundation, piling is
required since the site is a reclamation area with deep level of rock head.
Bore-piling is preferred since it is a non-displacement piling method, which
produces less noise and ground vibration than the other displacement piling
methods. There is no alternative viable construction method due to site
constraint.
In order to suit with hydraulic requirement of the process, some
facilities are required to be constructed below the ground level and therefore,
excavation for underground structures like basements and pile caps is required.
Tradition excavation and cast-in-situ concreting will be adopted for pile caps,
basements and superstructures constructions.
For E&M and Building Services works, general fixing and installation
of treatment plants and facilities such as SWRO skid, high pressure pumps and
small sized utilities installations such as pipe-laying, ducting and cabling
will be conducted. As such, apart from Civil & Structural works, all the
works involved in the Desalination Plant are considered to create no adverse
impact to the environment.
As the fresh water rising mains overlap with Wan Po Road, it shall be
constructed in segments (approximately 40 m per workfront)
with limited number of concurrent workfront to
minimize disturbance to the local public and road users. In view of the potential noise impact, a
total of not more than four workfronts working
simultaneously would remarkably reduce the construction noise to the
surrounding but at the same time be able to deliver the Project per programme. The
construction of the Project is also planned to be implemented in multiple works
packages to reduce concurrent construction activities, which is regarded as one
of the effective approaches to reduce environmental impacts.
A comparison on sequence of works is presented in Table 2.14.
Table 2.14 Comparison of Works Sequence
|
|
Single workfront |
Multiple package with concurrent workfronts |
|
Environmental benefits |
·
Surface works limited to the 40m workfront,
and hence minimize the disturbance to the local public and road users. ·
Less PMEs is required for single workfront. ·
Catering for alignment changes. |
·
Reduce the overall duration of construction works and thus which
reduce the duration of potential environmental impacts to nearby sensitive
receivers. ·
The construction of the Project is also planned to be implemented in
multiple works packages to reduce concurrent construction activities, which
is regarded as one of the effective approaches to reduce environmental
impacts. |
|
Environmental disbenefits |
·
Sensitive receivers nearby the construction pits will be subject to a
longer period of environmental disturbance. ·
Longer overall project construction period with longer duration of
environmental impacts. |
·
More construction plant will be involved and this would generate
relatively more noise and dust impacts. |
For the process building in the plant area will be constructed by cast
in-situ method, all sub-structures and foundation will first be constructed and
followed by erection of falsework and formwork for
superstructure and concreting. The
construction sequence will be repeated until all process buildings are
constructed. No viable alternative is
considered.
The EIAO-TM specifies the
priorities for addressing impacts is avoidance and minimization. This
philosophy was referred to in designating the works construction programme by reducing the overall duration of construction
works.
The construction for the project will be
separated into two major contracts. Package A is for plant and other ancillary
facilities and Package B is for mainlaying of the
trunk feed. The Package A contract is scheduled to commence in Q3 2017
for completion of the construction in Q3 2020. The
Package B contract is scheduled to commence in Q2 2016 for completion of the
construction in Q4 2019. The major construction activities for the Project
would comprise site formation, excavation and backfilling, erection of formwork
and reinforcement, concreting, fabrication of steelwork, and testing and
commissioning.
Based on the review and consideration of project alternatives presented
in the preceding sections, the preferred alternative to be taken forward to
this EIA study is the provision of submarine pipelines for intake and outfall,
desalination plant by Reverse Osmosis, mitigation works of 0.49 ha of natural
slope within the Clear Water Bay Country Park by soil nailing, flexible
barriers and rock stabilization, and construction of a 9 km rising mains along
Wan Po Road. Both cut-and-cover method
and trenchless construction method will be adopted for the construction of
desalination plant and the truck feed system. The proposed submarine intake and
outfall will be constructed by trenchless method with minor seabed dredging for
the installation of the diffusers.
Within the proposed Desalination Plant, bored piling and in situ concreting will be used for
foundation and superstructure construction, respectively.
The selection of the preferred scenario is summarized in Table
2.15 and construction method has brought about a series of
environmental benefits to the Project, including:
·
The provision of alternative potable water source and alleviate the
shortage of freshwater resources due to climate change and subsequent adverse
weather;
·
The trunk feed system are proposed to be constructed underneath Wan PO
Road to minimize disturbance to sensitive receivers and natural habitats;
·
The mixed-use of soil nailing, flexible barrier and rock stabilization
for slope mitigation is localized in nature and thus has minimized the
disturbance to the natural habitats at the Clear Water Bay Country Park;
·
The alignment and length of submarine utilities are at sufficient
distances from sensitive receivers to reduce potential impacts on water
quality, marine ecology and fisheries;
·
The use of micro-tunnel boring machine for construction of the proposed
submarine utilities reduces the extent of seabed dredging and dredging volume,
thereby reducing the marine footprint of this project and the potential impacts
on water quality, marine ecology and fisheries; and
·
The recommended land-based construction methods are expected to avoid
prolonged construction duration and hence reduce potential disturbance to the
environment and the local public.
Table 2.15 Preferred Alternative for
the Proposed Desalination Plant Development in this EIA Study
|
|
Design |
Construction Method |
|
Desalination Plant |
||
|
Intake |
Offshore Open Intake |
Trenchless method and localized dredging |
|
Pre-treatment |
Two Stage Granular Media Filtration |
- |
|
Desalination |
Reverse Seawater Osmosis |
- |
|
Treatment Building in Desalination Plant |
- |
Foundation: Bored pilling Superstructure: in situ
concreting |
|
Outfall |
Submarine Outfall with Diffuser |
Trenchless method and localized dredging |
|
Natural Slope Mitigation |
||
|
Rock Slope |
- |
Mixed-use of flexible barrier, soil nail and rock stabilization. |
|
Trunk Feed System |
|
|
|
Pipeline of the rising main |
- |
Cut & Cover and Pipejacking & Microtunneling (if necessary) |