Table of Contents
The Project is classified as a Designated
Project under the Environmental Impact Assessment Ordinance (Cap. 499)
(EIAO). The works that are the subject
of the EIA Study include the construction and operation phases of the
Project. The key components of the
Project include the following:
·
Construction
of a new desalination plant in TKO Area 137 with a capacity of 135 million
liter per day (Mld),
expandable to 270Mld in the future (Schedule
2, Part I, Item E.2).
·
Natural slope mitigation works consists of soil
nailing, construction of flexible barrier and rock/boulder stabilization works
at the natural slope within the Clear Water Bay Country Park (Schedule
2, Part I, Q.1).
·
Storage
of chlorine, carbon dioxide and other chemicals on site (Schedule 2, Part I, Item K.13 – A dangerous good godown with a storage capacity exceeding 500 tonnes).
The
components and project layout for the proposed desalination plant will not be
procured until the Detailed Design Phase, which will commence once the EIA has
been approved. Therefore, a description
of the likely components, project layout and their installation is provided in
this EIA Report together with any alternatives (if necessary) and corresponding
methods of installation. The assessment
of environmental impacts will correspond to the component or process option
giving rise to the maximum perceive impact.
The
preferred scenario/ alternatives for the construction of desalination plant and
installation of submarine pipelines and truck feed system to be taken forward
in this EIA are described in Section 2. On the basis of the alternatives selection,
the layout plan for the proposed developments is shown in Figure 3.1. The desalination plant location, slope
mitigation works and pipeline route are illustrated in Figure 1.1, which are
indicative only and will be confirmed during the Detailed Design Phase of the
Project.
The proposed Tseung Kwan O
desalination plant is a Seawater Reverse Osmosis (SWRO) plant. SWRO is a mature and preferred technology,
which dominates the market due to its reliability and progressive reduction in
cost as the technology advances. The desalination plant comprises the
following major facilities:
·
Seawater intake system
·
Pre-treatment system
·
Reverse Osmosis (RO) treatment system
·
Post-treatment system
·
Backwash and chemical cleaning system
·
Sludge handling system
·
Concentrate discharge system
·
Auxiliary systems
Desalination
Plant
The construction works involved in this
project 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 then the other displacement piling methods.
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 Service 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.
Slope
Mitigation Works
The
natural slope overlooking the northeast boundary of the new desalination plant
at TKO Area 137 has a history of minor landslides and contains some potentially
unstable boulders. Landslide and boulder hazard mitigation
works including soil and rock stabilization works and provision of flexible
debris barriers may be required to protect the new desalination plant for the
landslide and boulder hazards from this slope.
Figure 3.2 shows the indicative area of potential landslide
slope mitigation works.
For
soil and rock stabilization works, temporary scaffolding, ladders and working
platforms will be erected to provide access to the slope face. Soil nails and rock dowels will be installed
in drill holes (typically 100 or 150mm diameter to a depth typical 4 to 8m for
soil nail; 75mm diameter to depths 3 to 5m for rock dowels) into the lower
portion of natural slopes within the country park area. Soil nails and rock dowels will be grouted,
and the soil heads will be excavated and concreted. Soil bags with applied seeds will be placed
on the affected soil slope. Some
localized loose boulders and rocks will be stabilized by rock dowelling and
providing buttresses on the base of the boulders/ rocks to prevent them from
initiating movements down the slope.
The
construction of flexible barrier at the toe and lower portion of the natural
slope will involve the erection of tensioned steel meshes that are 4 to 5 m
high across steel posts supported on foundation plates. The foundation (typically 600mm sq. and 500mm
deep) will be made of concrete and will be formed by drilling into soil/rock
using drill rigs erected on temporary working platforms. Similar foundation plates will also be
constructed for anchoring of stay wires.
The steel posts will be erected over the foundation plates by threading
the stay wires. The wire mesh will then
be pulled out and fixed to the top wire between the steel posts. The stay wires will also be tied from the top
of the posts to the foundation plates.
The
flexible barrier will align in alternative segments on either sides of the
intended alignment, such that gaps between segments of the barriers will allow
passage of wildlife. Temporary ladders
will be provided for accessing the works area.
In addition, construction materials will be lifted up using mobile
cranes (with arms up to 35m) located at the temporary working platforms in
front of the slope. No permanent access
will be provided for the slope mitigation works. Temporary ladders will be erected during
future maintenance works, if any.
Climbers,
creepers and suitable replanting will be provided on and around the flexible
barriers to restore the natural ground.
Recessed soil nail heads with soil bags will allow regrowth of
vegetation on the hillside surface. The
affected soil slope will be restored with hydro-seeding and planting in order
to provide greening effect. The detailed
scheme of slope mitigation works is illustrated in Figure 3.3.
While
such mitigation works may be required to be implemented within 0.49 ha of the
Clear Water Bay Country Park, the mitigation works will be avoided and
minimized as far as practicable to minimize any potential environmental impact
to the Country Park.
Trunk
Feed System
Since the fresh water 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 construction mainlaying for
the trunk feed system will be carried out for about 45 months. The fresh water main will be constructed in
small sections using open cut method.
The method involves soil excavation works and backfilling in small
sections (ie, 40 m) to minimize the volume of
excavated materials and number of construction plant to be deployed.
If the road sections or road junctions are not available for cut and
cover method because of heavy traffic, trenchless method, such as pipe jacking
by micro-tunnelling, will be adopted.
The proposed location for using trenchless method is indicated on Figure
3.4.
New freshwater mains must be cleaned and sterilized before being put
into operation. Typically, water mains are sterilized by chlorination. The purpose of chlorination is to disinfect
the water main, resulting in an absence of coliforms as confirmed by laboratory
analysis, before they are placed in service.
Chlorine solutions are normally used as the sterilizing agent. Procedures specified in WSD Departmental
Instruction No. 805: Mainlaying - Cleaning and Sterilization of Fresh Water
Mains will be followed for the main sterilization works. Water used to clean the pipes would be
properly treated to remove any solid flushed from the pipes before discharge to
public sewer. Chlorine solution would
also be dechlorinated to reduce total residual
chlorine level to below 1 mg/L before disposal to public sewer.
It is
anticipated that open onshore intake or submerged intake (approximately 200m –
250m long) and outfall pipes (approximately 300m - 350m long) will be
constructed. Intake and outfall will be
constructed with trenchless technologies.
While minor dredging works will be limited to ~50m x 50m for the
construction of intake structures and ~150m x 50m for the diffusers, dredging
works will be minimized as far as practicable to avoid or reduce any potential
environmental impacts. Furthermore,
prefabricated intake and outfall structures (e.g. intake openings and diffuser
heads) will be transferred and installed onsite.
Rock
fill materials with low fines content will be used to provide support and
protection to the systems.
Seawater
will be drawn from the seawater intake system for the desalination
process. Chlorine in form of
hypochlorite 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. 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 and
treated to appropriate level before discharge to public sewer.
Post-treatment
processes will include disinfection using chlorine and fluoridation, pH
correction and remineralisation via hydrated lime and
carbon dioxide dosing. A typical Process
Flow Diagram of a SWRO desalination plant is illustrated in Figure
3.5.
As
described above, a number of chemicals are required for the efficient and
effective operation of the desalination plant.
The actual chemical dosing regime will be assessed in Section 6 Water Quality of this EIA
study. Chemicals that may be dosed in
the water stream for main treatment are summarized in Table 3.1.
Table 3.1 Chemicals
that may be Dosed in the Potable Water Stream
Chemical |
Purpose |
Sodium Hypochlorite |
Pretreatment
- Bio growth control |
Ferric Chloride or Alum |
Pretreatment
- Coagulation |
Sulphuric Acid |
Pretreatment
- pH Adjustment |
Polymer |
Pretreatment
- Flocculation |
Sodium meta Bisulphite |
Pretreatment
- Dechlorination |
Antiscalant |
RO
membranes - Scaling control |
Sodium Hydroxide |
RO
membranes - pH adjustment (if required) |
Carbon Dioxide |
Post
Treatment - pH adjustment and remineralisation |
Hydrated Lime |
Post
Treatment - pH adjustment and remineralisation |
Sodium Silicofluoride |
Post
Treatment - Fluoridation |
Liquid Chlorine |
Post
Treatment - Final disinfection |
Polymer |
Sludge
Conditioning - Sludge Thickening/Dewatering |
In
addition to the above chemicals the cleaning chemicals for RO include sodium
hydroxide, citric acid, hydrochloric acid, and sodium laurel sulphate. Chemical
cleaning of RO membranes are typically carried out once a month. Such cleaning waste will be neutralized and
treated appropriately to stabilize the pH before discharge to public sewer.
Process
waste streams, including the concentrate produced as the result of RO process
and supernatants from residual processes will be discharged via the concentrate
discharge system to the outfall.
Wastewater from chemical cleaning would be disposed into public sewer,
which leads to the Tseung Kwan O Preliminary
Treatment Plant.
In
accordance with EPD Waste Disposal Ordinance, sludge generated from
clarification and filter backwash will be thickened and dewatered to 30% solids
content for subsequent disposal to landfill.
Traces of chemicals that are not captured by the sludge generated may be
found in the supernatants of these residual processes. Any residual chlorine left in the streams
will be quenched (or removed) using sodium meta bisulphite before discharge to the sea through the
outfall. RO concentrate will represent a
significant portion of these streams (i.e. about 90% of flow). Since RO concentrate is free from particulate
contaminants (which are removed in pre-treatment processes), the only
constituents that are present are soluble cations/anions. The key constituents that may be found in the
RO concentrate discharge stream blended with supernatants from residuals
streams are described in detail in Section
6.8.2 and the associated Annex 6A.
Table
3.2 presents a summary of the project details. The footprint of the proposed desalination
plant development at Tseung Kwan O Area 137 will be
approximately 10 ha, with the trunk feed system consisting of a 9.5 km of 1,200
mm in diameter of fresh water rising main along Wan Po Road. A maximum total marine dredging volume of
6,330 m3 (in situ) of marine sediments will be generated from the
two submarine utilities. Natural slope
mitigation works will be undertaken within 0.49 ha of the Clear Water Bay
Country Park.
Table 3.2 Summary
of Project Description
Details |
Preliminary Design
Information |
Submarine Utilities |
|
Area
of seabed affected by pipeline footprint |
|
·
Intake |
0.045
ha |
·
Outfall |
0.065
ha |
Marine
Dredging volume (in situ volume) |
|
·
Intake |
1,740
m3 |
·
Outfall |
4,590
m3 |
Rock
fill volume (in situ volume) |
|
·
Intake |
905
m3 |
·
Outfall |
4,320
m3 |
Length
of Submarine Utilities (m) |
|
·
Intake |
~
200 to 250 m |
·
Outfall |
~
300 to 350 m |
Diffuser |
|
·
Number of discharge port |
A
total of 36 discharge ports with diameter of 150 mm |
·
Length of diffuser |
152
m with port spacing of 4.2 m |
·
Configuration |
alternating
and inclined at 60° to horizontal |
Desalination Plant |
|
Area
of foundation occupied by the plant |
10
ha |
Building
parameters |
17
components with building height ranged between 2 m to 20 m |
Trunk Feed System |
|
Area
of land affected by the freshwater rising mains |
1.14
ha |
Excavation
volume |
42,733
m3 |
Length
of the freshwater rising main |
9,500
m |
Diameter
of the freshwater rising main |
1,200
mm |
Natural Slope Mitigation
Works |
|
Area
of slope mitigation works |
0.49
ha within the Clear Water Bay Country Park |
|
|
The
preliminary construction programme is provided in Figure
3.6.
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 clearance, excavation and backfilling, erection of
formwork and reinforcement, concreting, fabrication of steelwork, and testing
and commissioning.
Figure 3.6 Preliminary
Construction Programme of this Project
At
present the identified potential concurrent projects in sufficient proximity of
the proposed desalination plant and submarine utilities are the following
projects:
Cross
Bay Link (CBL), Tseung Kwan O (EP-459/2013), the project is to construct an approximately 1.8 km
long dual two-lane road mainly on viaduct with a footpath and a cycle
track. The Cross Bay Link is located
across Joss House Bay at > 3 km from the proposed desalination plant and
submarine pipelines. The preliminary
construction programme is anticipated between January
2017 and July 2020.
Tseung Kwan O – Lam Tin Tunnel and Associated
Works (EP-458/2013), the project is
to construct a dual two-lane highway connecting Tseung
Kwan O at Po Yap Road in the east with Trunk Road T2 in Kai Tak Development in
the west and Lei Yue Mun Road Underpass. The project involves a 4.8 km long highway
with about 3 km of the highway in the form of a tunnel. The Tseung Kwan O –
Lam Tin Tunnel will connect Cross Bay Link to form a new external road link to
meet the anticipated traffic flow in connection with further population intake
and development in Tseung Kwan O New Town. This project is located at > 4 km from the
proposed desalination plant and submarine pipelines. The preliminary construction programme is anticipated between February 2016 and April
2020.
Truck
Road T2 (EP-451/2013), the project is
to construct a dual two-lane trunk road of approximately 3 km long with about
2.7 km of the trunk road in form of tunnel between Cha Kwo
Ling and South Apron of the former Kai Tak Airport. The project is located at > 6 km from the
proposed desalination plant and submarine pipelines. The preliminary construction programme as per the approved EIA Report is between early
2015 and early 2020.
South
East New Territories (SENT) Landfill Extension (EP-308/2008/A), the project is to construct and operate of a
landfill extension of about 45 hectares with a target void space of about 6.5
million cubic metres on the southern side of the
existing SENT Landfill. The infrastructure
area of the project is located in the close proximity (about 100m) from the
proposed desalination plant. The
preliminary construction programme as per the
approved EIA Report is between early 2011 and late 2020. However, the programme
of the SENT Landfill Extension is uncertain at the moment, but it has been
considered as sensitive receivers, where appropriate in this EIA.
Fill
Bank at Tseung Kwan O Area 137 (EP-134/2002/K), the project is to construct, operate and remove a
temporary fill bank covering an area of approximately 104 hectares at Area 137
of Tseung Kwan O.
About 10 hectare of the fill bank area is earmarked for the proposed
desalination plant which lies along the northwest boundary of the Clear Water
Bay Country Park. CEDD has agreed to
hand over the 10 ha site cleared of stockpile for the construction of the
Project. Based on the latest
environmental permit, apart from the 10 hectares project area, the fill bank
will be operated until late 2018.
A
barging pier at TKO Area 137 for off-loading explosives is operated (TGLA No. SK 567) by the Mines Division of the CEDD. There is a
potential for the possible coexistence of the explosive off-loading facilities
with this Project. The potential impacts of the coexistence of the explosive
off-loading facilities and the desalination plant are assessed in the hazard to
life assessment (Section 13).
The
MTR Corporation Limited has currently set up a temporary explosive magazine for
its Shatin to Central Link and Kwun
Tong Line Extension projects at the TKO Area 137. Whilst the site for the
temporary explosive magazine encroaches considerably upon the reserved site for
the desalination plant, the site will be returned to WSD by late 2015 for the
construction of the desalination plant. As such no interaction with this
Project is expected.