Outlying Islands Sewerage Stage 2 –

South Lantau Sewerage Works

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3. PROJECT DESCRIPTION

3.1 The Project

The following components of the Project are regarded as Designated Projects under the

Environmental Impact Assessment Ordinance (Cap. 499) (EIAO):

A dredging operation which is less than 500 m from the nearest boundary of an existing

or planned coastal protection area (Item C.12 of Part I of Schedule 2 of EIAO);

Sewage treatment works with an installed capacity of more than 5,000 m3 per day and a

boundary of which is less than 200 m from the nearest boundary of an existing or

planned residential area, site of special scientific interest, site of culture heritage,

bathing beaches, etc. (Item F.2 of Part I of Schedule 2 of EIAO);

Sewage pumping stations (1)

with an installed capacity of more than 2,000 m3 per day

and a boundary of which is less than 150 m from an existing or planned residential area,

site of special scientific interest, site of culture heritage, bathing beaches, etc. (Item F.3

of Part I of Schedule 2 of EIAO);

An activity for the reuse of treated sewage effluent from a treatment plant (2)

(Item F.4

of Part I of Schedule 2 of EIAO); and,

A submarine sewage outfall (Item F.6 of Part I of Schedule 2 of EIAO).

The works that are the subject of the EIA Study include the construction and operation phases

of the Project.

3.2 Project Design

3.2.1 Purpose & Objectives of the Project

The proposed sewerage works will collect the sewage generated from the unsewered areas of

Shui Hau, Tong Fuk, Cheung Sha, San Shek Wan, Pui O and Ham Tin in South Lantau (i.e.

within the Project Catchment Area) and convey it to a proposed sewage treatment works at

San Shek Wan for treatment and disposal into outer bay of Pui O/ Chi Ma Wan via a

submarine sewage outfall. The proposed Project scheme is illustrated in Figure 1.1.

After implementation of the Project, a significant improvement on environmental hygiene is

expected along South Lantau coast. No additional pollutant load will be produced to the

Southern Water Control Zone taking into consideration the anticipated increase in population

growth and developments in the South Lantau area up to 2031.

3.2.2 Project Elements

The major elements of the Project include a village sewerage system, a trunk sewerage system

and a sewage treatment works. These elements are described in detail below. The general

layout plans of the Project are provided in Figures 3.2a-r.

(1) Only Cheung Fu Street sewage pumping station and San Shek Wan sewage pumping station.

(2) DSD considers that there will be no key changes on the scope of issues covered by the EIA Study Brief and has sought DEP's confirmation in this regard for

formality, pursuant to Clause 6.2 of the EIA Study Brief.

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Village Sewerage

The village sewerage works involve the construction of gravity sewers inside the unsewered

areas of Shui Hau, Tong Fuk, Cheung Sha, San Shek Wan, Pui O and Ham Tin in South

Lantau for collection of sewage generated from these unsewered areas. The village sewerage

will be connected to the trunk sewerage system described below.

The proposed village sewerage works include 225 mm to 300 mm diameters gravity sewers

and concrete manholes in villages and unsewered areas in South Lantau. Vitrified clay pipes,

unplasticized polyvinyl chloride (uPVC) pipes or precast concrete pipes will be used for the

gravity sewers.

Trunk Sewerage

The trunk sewerage works involve the construction of trunk sewers, sewage pumping stations

and twin rising mains (one duty and one stand-by) along South Lantau Road for conveyance

of sewage collected from the village sewerage described above to the proposed sewage

treatment works described below.

The proposed trunk sewers are 300 mm to 500 mm diameter vitrified clay pipes or concrete

pipes. The proposed rising mains are 100 mm to 200 mm ductile iron pipes. They are part of

the proposed trunk sewerage system and will be laid along the South Lantau Road and Chi

Ma Wan Road.

The proposed trunk sewerage system has six sewage pumping stations in Shui Hau, Tong Fuk,

Cheung Fu Street, Cheung Sha, San Shek Wan and Pui O. Each trunk sewage pumping

station will be enclosed by a single-storey superstructure (typical dimension of approximately

14 to 21m (L) x 8 to 16m (W) x 4 to 7m (H)) with underground wet wells, pump units,

storage tanks and chambers, and electrical and mechanical (E&M) equipment, which is an

unmanned sewerage facility. The inlet chamber and wet wells located underground and

enclosed with covers. The screenings handling area will be confined inside the superstructure

as far as possible to minimize odour nuisance to the surrounding air sensitive receivers. A

deodorization unit will be installed in order to reduce odour emission. Ventilation system will

be provided to ensure adequate air exchange within the sewage pumping stations.

For hygiene purpose, water points and water basins will be provided in the proposed sewage

pumping stations for general cleansing and use of the maintenance crew.

Standby units, e.g. pumps, dual power supply, etc., and an emergency bypass will be provided

in the sewage pumping station to handle unplanned events.

A generic layout plan of a sewage pumping station is provided in Figures 3.3a-h.

Sewage Treatment Works

The works involve construction of a sewage treatment works (STW) at San Shek Wan and its

associated submarine outfall for treatment and disposal of treated sewage.

The proposed San Shek Wan STW comprises an administration building and an operation

building. The operation building is a multi-storey superstructure with underground tanks and

chambers for sewage treatment. It also has an effluent pumping system for discharging

treated effluent to outer bay of Pui O/ Chi Ma Wan via the proposed submarine outfall. The

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superstructure of the administration building and operation building will be of an area of

about m 35m (L) x 8m (W) x 13m (H) and 45m (L) x 36m (W) x 13m (H). A preliminary

layout plan of the STW is provided in Figure 3.4a-f.

The sewage treatment works will consist of an inlet chamber, mechanical screen or other

protective device(s), grit removal system, biological treatment basins, membrane tanks,

reclaimed water storage tank, gravity thickeners, sludge holding tanks, filter presses,

disinfection system, chemical scrubbers, effluent pumping facilities, a submarine outfall,

associated pipe works, fittings, valves and chemical storage, and electrical and mechanical

(E&M) equipment. Air blowers are used for aeration system in the biological treatment

basins and membrane bioreactor system.

The screenings handling area will be confined inside the superstructure as far as possible to

minimize odour nuisance to the surrounding air sensitive receivers. A deodorization unit will

be installed in order to reduce odour emission. Ventilation system will be provided to ensure

adequate air change within the plant for man entry during routine servicing and maintenance

work.

For hygiene purpose, water points and water basins will be provided in the proposed sewage

treatment works for general cleansing and use of the operation and maintenance crew.

Standby units, e.g. pumps, dual power supply, etc., and an emergency bypass system will be

provided in the STW to handle unplanned events. The emergency bypass system will include

emergency storage tanks and a ~300 m gravity pipe for emergency coastal discharge of

untreated sewage directly into outer bay of Pui O/ Chi Ma Wan. The emergency bypass pipe

will be approximately 200 mm diameter high-density polyethylene (HDPE) underground pipe.

The proposed submarine outfall consists of a submarine outfall pipe running from the

proposed San Shek Wan STW to outer bay of Pui O/ Chi Ma Wan and a diffuser in Pui O Bay.

Treated effluent will be conveyed to outer bay of Pui O/ Chi Ma Wan through the submarine

outfall pipe and discharged to the sea through the diffuser. The submarine outfall pipe will be

approximately 350 mm diameter HDPE twin pipe and the diffuser is normally a pre-fabricated

unit.

3.2.3 Design Flow and Loads for Sewerage Facilities

The estimated annual average dry weather flow (ADWF) in the year 2011 is about 2,500

m3/day. Owing to the future development of South Lantau, it was anticipated that the flow

will build up to 5,800 m3/day in ultimate scenario (after the year 2036). Therefore, the design

flow (ADWF) for San Shek Wan STW will be 5,800m3/day.

The ultimate design flow rates of the San Shek Wan STW are summarized in Table 3.1. The

San Shek Wan STW has an installed capacity of 17,400 m3.

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Table 3.1 – Design Flow Rates of the San Shek Wan STW Type of Flow Flow (m3/day)

Annual Average Dry Weather Flow (ADWF) 5,800

Peak Wet Weather Flow (PWWF) (before biological treatment tanks) 17,400

PWWF (treatment units starting from biological treatment tanks) 11,600

Notes:-

Reference is made to DSD’s Sewerage Manual for the peaking factor of raw sewage. The biological treatment tanks would

be sized to cater for certain portion of the PWWF. Peaking factor for the treatment units is tentatively set as 2, which would

be further confirmed in Design phase with reference to local hydraulic data.

The proposed sewerage system will be designed for the ultimate scenario in which all areas

within the Project Catchment Area with potential sewage generation are developed and

occupied. The schematic flow diagram of the sewerage scheme of this Project is as below:

Based on the above, the design capacity of the proposed sewage pumping station and sewage

works are tabled as below:

Sewerage Facility Design Capacity

(m3/day)

Shui Hau Sewage Pumping Station 620

Tong Fuk Sewage Pumping Station 1,880

Cheung Fu Street Sewage Pumping Station 2,470

Cheung Sha Sewage Pumping Station 920

San Shek Wan Sewage Pumping Station 3,840

Pui O Sewage Pumping Station 1,960

San Shek Wan Sewage Treatment Works 5,800

Note: The majority sewage that San Shek Wan SPS collected is coming from Cheung Sha SPS and Cheung Fu Street SPS.

The installed capacity of San Shek Wan STW is the sum of the installed capacity of San Shek Wan SPS and Pui O SPS

which is 5,800 m3/day.

3.2.4 Treatment Objectives

Whilst the primary objective of the proposed sewage treatment works is to treat the sewage in

South Lantau for local disposal, in view of recent developments in sewage treatment

technology, which could produce high quality effluent, reuse of the treated effluent would

also be considered to minimize direct effluent discharge.

The Investigation Study of this Project has reviewed potential alternative disposal schemes

and non-potable reuse of treated effluent is deemed as a feasible option. Consequently, apart

from the marine disposal of the treated effluent via the submarine outfall, internal reuse of the

reclaimed water for non-potable use within the San Shek Wan STW is proposed. The use of

service water is primarily for washing of screens (headworks), process needs (for makeup

water for cleaning of membranes etc.), general housing keeping, toilet flushing, and irrigation

use within the STW. The consumption of the reclaimed water will account for approximately

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58 m3/day. The reclaimed water would be undergoing chlorination in the reclaimed water

tank before being conveyed to the effluent reuse units within STW. Small amount of liquid

bleach (sodium hypochlorite solution) (~60L) will be stored on-site. The reclaimed water

quality standard criteria has been proposed in Outlying Islands Sewerage Stage 2 – South

Lantau Sewerage Works – Investigation, Working Paper No. 2 – Alternative Disposal

Schemes and Effluent Reuse (Final), with reference to WSD water quality objectives for salt

water used for flushing (3)

, effluent standards from the Ngong Ping STW which is currently an

effluent reuse facility (4)

, effluent standards applied in Shek Wu Hui STW pilot study (5) (6)

and

United State Environmental Protection Agency (USEPA) guidelines for reclaimed water

quality for urban reuse. It should be noted that the proposed reclaimed water quality criteria

for this Project will be subject to further review in the Design phase of the Project.

Meanwhile, feasibility of other reuse schemes would be further explored in Design phase of

the Project and adopted, as far as practicable, subject to agreement with relevant authorities.

3.2.5 Effluent Quality Requirements

Based on the treatment objectives described in Section 3.2.4, the quality of the effluent

required from the proposed sewage treatment works is dictated by the following two major

considerations:

Discharge standards established by regulatory authorities, when effluent is discharged;

and

Standards established by regulatory authorities, when effluent is reused for non-potable

purposes.

Effluent Discharge Standards

The Technical Memorandum Standards for Effluents Discharged into Drainage and Sewerage

Systems, Inland and Coastal Waters published by the EPD under the Water Pollution Control

Ordinance (WPCO) specifies the acceptable standards of effluents discharging into drainage

systems and sewerage systems, inland waters and coastal waters. Effluent standards of the

proposed sewage treatment works will make reference to Table 10a – Standards for effluents

discharged into the inshore waters of Southern, Mirs Bay, Junk Bay, North Western, Eastern

Buffer and Western Buffer Water Control Zones of Cap 358. Relevant standards are

presented in Table 3.2.

Table 3.2 – Standards for effluents discharged into the inshore waters of Southern, Mirs Bay, Junk

Bay, North Western, Eastern Buffer and Western Buffer Water Control Zones described in the

Technical Memorandum Standards for Effluents Discharged into Drainage and Sewerage Systems,

Inland and Coastal Waters under the Water Pollution Control Ordinance Determinand Upper Limit

Flow Rate (m3/day) 14,500 ( = 2.5ADWF)

Suspended Solids 30

Biochemical Oxygen Demand (BOD) 20

Chemical Oxygen Demand (COD) 80

(3) WSD water quality objectives for salt water used for flushing

(4) Agreement. No. CE 29/2001 – Outlying Islands Sewerage Stage 1 Phase 1 – Ngong Ping Sewage Treatment Works and Sewerage Investigation,

Design and Construction

(5) Contract No. SS P320 – Design and Construction of the Redevelopment of Lo Wu Correctional Institution

(6) Agreement. No. CE 16/2004 (DS) – Demonstration Scheme on Reclaimed Water Used in the North District – Investigation

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Determinand Upper Limit

Total Nitrogen 30

Escherichia coli (count/100ml) 1,000

Note:

ADWF – Average Dry Weather Flow

All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated.

However, for the purpose of a worst-case assessment, the effluent discharge standards under

the WPCO may be adopted for assessment where appropriate (e.g. in water quality impact

assessment).

Effluent Reuse Standards

There are certain effluent reuse standards that are being practiced in Hong Kong. The effluent

quality standards adopted for the reuse at Ngong Ping STW (for toilet flushing and controlled

irrigation), North District (for toilet flushing, unrestricted irrigation & water features), Lo Wu

Correction Institution (for toilet flushing) and WSD’s water quality objectives for toilet

flushing have been reviewed to derive the proposed standards for the San Shek Wan STW

which are listed in Table 3.3.

Table 3.3 – Standards for Effluents for Reuse of Reclaimed Water at San Shek Wan STW Determinand Unit Proposed Reclaimed Water Quality Criteria

for San Shek Wan STW *

pH n/a 6-9 (d)

Turbidity NTU ≤ 2 (b-d)

Total Suspended Solids mg/L ≤ 10 (a)

Biochemical Oxygen Demand (BOD5) mg/L ≤10 (a)

Colour Hazen Unit ≤ 20 (a)

Ammonia Nitrogen mg/L ≤ 1 (a)

Threshold Odour Number T.O.N ≤ 100 (a)

Synthetic Detergents mg/L ≤ 5 (a)

Escherichia coli Cfu/100ml Not Detectable (b-d)

Dissolved Oxygen mg/L ≥ 2 (a)

Residual Chlorine mg/L ≥ 1 (d) (For Washing and Toilet Flushing)

mg/L ≤ 1 (d) (For Irrigation)

*Note: The effluent standards are adopted from:

(a) Agreement. No. CE 29/2001 – Outlying Islands Sewerage Stage 1 Phase 1 – Ngong Ping Sewage Treatment Works and Sewerage

Investigation, Design and Construction.

(b) Agreement No. CE 16/2004 (DS) – Demonstration Scheme on Reclaimed Water Uses in the North District –

Investigation

(c) Contract No. SS P320 – Design and Construction of the Redevelopment of Lo Wu Correctional Institution

(d) Guideline for Water Reuse (2004), USEPA

It is noted that the standards practiced for reuse are in general more stringent than the effluent

discharge standards under the WPCO. Hence, where applicable, the reuse standards will be

adopted as basis for developing the treatment process at the San Shek Wan STW.

3.2.6 Treatment Process & Preliminary Layout

Based on the latest review and investigation in this Project, Membrane Bioreactor (MBR) is

selected as the treatment process for the proposed San Shek Wan STW (see Section 2 for

details). A process flow diagram of the proposed MBR treatment plant is shown in Figure 3.5.

The headworks will comprise an influent wet well, flow monitoring, screens, screenings

washer and compactor, vortex grit removal system with grit classifier. Starting from the

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influent wet well, the sewage flow will be measured and pumped to the subsequent treatment

units. A splitter box will distribute influent from vortex type grit basin(s), where grit will

settle into a grit hopper, to the downstream anoxic / oxic biological treatment basins. In the

treatment basins, the influent will go through biological nutrient removal activated sludge

treatment process. The effluent will then flow to the membrane tanks for filtering out clean

and treated water from the mixed liquor. Returned activated sludge will be transported from

the membrane tanks back to the activated sludge process so as to maintain the Mixed Liquor

Suspended Solids (MLSS) content in the activated sludge process zone and prevent excessive

solid concentration of the membrane zone.

A monitoring system will be provided in STW to detect the productivity of membrances and

alert the maintenance crew to replace the flat sheet in a membrane immediately if it is

damaged. Since each membrane consists of large amount of flat sheet, the effluent quality will

not exceed the allowable limit immediately in case of the small damage to the flat sheet.

It is noted that effluent for marine disposal does not require chlorine disinfection as long as

the sewage treatment works can meet the effluent quality requirements with respect to

existence of pathogens. Liquid bleach (Sodium hypochlorite) will be fed into reclaimed water

storage tank for disinfecting and maintaining a suitable residual chlorine level on the portion

of reclaimed water for reuse. The reclaimed water storage tank will be provided with effluent

pumps for marine disposal, reclaimed water pumps for water reuse and non-potable water

pumps for process uses within the STW.

On-site sludge treatment will be provided through two process units, i.e. sludge thickener and

filter press. Gravity Thickener system is capable for thickening of Waste Activated Sludge

(WAS) up to approximately 3% solid content. The thickened sludge will flow by gravity to

the sludge holding tanks which provide temporary storage before transferring to the filter

presses, which will be operated in batch processing mode. The filter press system is capable

for producing 30% solid cake to meet the landfill disposal requirement.

The preliminary layout of the proposed San Shek Wan STW is enclosed in Figures 3.4a-f.

3.3 Project Construction

3.3.1 Village Sewerage

Traditional open cut method will be adopted for the construction of gravity sewers and

concrete manholes. Approximately 50,000 m3 of excavated materials will be generated from

the excavation works associated with the construction of village sewerage.

The works will be carried out in the narrow village alleys, accesses and open spaces of the

villages and unsewered areas. Where working space is limited and heavy vehicles or

construction plant are not accessible, hand-held tools or small-scale construction plant will be

used for construction activities such as excavation, pipe laying and backfilling.

The proposed sewer alignment will overlap with some existing septic tanks and thus it is

necessary to remove these septic tanks for construction of the proposed sewerage. The

proposed sewerage will be constructed and commissioned from downstream to upstream.

When the construction works is obstructed by an existing septic tank, the flow discharging to

the septic tank will be diverted to the new sewerage system and the septic tank will be

removed for further proceeding of the sewerage works.

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3.3.2 Trunk Sewerage

Trunk Sewers and Rising Mains

Open cut method or trenchless method (e.g. micro-tunnelling and hand-digging) will be

adopted for the construction of the trunk sewers and rising mains. The pipes will be

underneath South Lantau Road, except for some bridge crossing (for the rising mains) which

may be bridge-mounted type and exposed. Approximately 150,000 m3 of excavated materials

will be generated from the excavation works associated with the construction of trunk sewers

and rising mains.

The trunk sewers and rising mains will be constructed in sections and temporary traffic

arrangement will be implemented during construction to maintain the traffic flow of South

Lantau Road.

Sewage Pumping Stations

Traditional building method will be adopted for construction and the general construction

sequence is as follows:

Site clearance: this involves the removal of debris and vegetation by the use of

bulldozer and dump truck;

Site formation, excavation and construction of temporary lateral support: construction

activities will include breaking up of pavement by hydraulic breaker (for STW site

only), soil excavation by excavator to formation level, removal of debris and C&D

materials by dump truck, installation of temporary lateral support or sheetpiles with the

use of vibrating hammer, and filling of formation level and other fill area with

excavator, dump truck and power rammer;

Construction of substructures: this involves steel reinforcement fixing, formworking

and concreting with the use of concrete lorry mixer. Water pump and generator may be

used if ground water table is found to be high. Electric vibratory poker and generator

will be used for settling of concrete;

Backfilling: this involves equipment such as power rammer for compaction of soil

materials and hydraulic extractor for extraction of sheetpiles. Lorry will be used to

transport the filling materials to the site;

Construction of superstructure: this involves steel reinforcement fixing, formworking

and concreting with the use of concrete lorry mixer. Electric vibratory poker and

generator will be used for settling of concrete;

Installation of electrical & mechanical (E&M) services: these activities will be

undertaken mostly indoor or underground. Crane lorry will be used for transportation

of E&M installations. Apart from hand-made installations, some installations require

welding; and

Architectural and landscaping works: Small-sized lorry will be used for transportation

of façade and landscape materials. Welding may be required for some architectural

works.

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Approximately 2,670 m3 of excavated materials will be generated from the excavation works

associated with the construction of each sewage pumping station. Architectural and landscape

features will be proposed to decorate and blend the sewage pumping stations into their

adjoining environment.

3.3.3 Sewage Treatment Works

Buildings of STW

Traditional building method will be adopted for construction. The general construction

sequence described for the construction of sewage pumping stations (Section 3.3.2) will be

adopted. Approximately 30,000 m3 of excavated materials will be generated from the

excavation works associated with the construction of the STW. Architectural and landscape

features will be proposed to decorate and blend the sewage treatment works into their

adjoining environment.

Submarine Outfall

To minimize dredging in Pui O Bay, the submarine outfall pipe will be constructed by

Horizontal Directional Drilling (HDD) method from the STW.

Minor dredging of the seabed will be required for the installation of the diffuser. An area of

approximate 39m x 33m at the end of the outfall pipe will be dredged down to about -4.8mPD

by one closed grab dredger. There will be no excessive dredging. Approximately 4,700 m3 of

marine sediment will be dredged. The diffuser will be installed to the end of the submarine

outfall pipe in-situ on the seabed of Pui O Bay.

3.4 Project Operation

3.4.1 Sewerage System

The village sewerage system is a gravity sewer network in the villages and unsewered areas

for collection of sewage. The gravity sewer network will convey the collected sewage to the

trunk sewerage system.

The trunk sewerage system consists of trunk sewers, sewage pumping stations and rising

mains along South Lantau Road and Chi Ma Wan Road for conveyance of the collected

sewage from the village sewerage system to the sewage treatment works at San Shek Wan.

The village sewerage and trunk sewerage works are unmanned operation with regular

cleaning, checks and maintenance for efficient operation. The cleaning work consists of

mainly manual desilting and water jet cleansing.

3.4.2 Sewage Pumping Station

The sewage pumping stations will be designed as conventional wet well type pumping

stations. Incoming sewage will enter through screening device(s) to prevent large debris from

clogging the downstream sewage pumps. The sewage then passes into the wet well in which

pump units are located and is pumped out to rising mains leading to the proposed San Shek

Wan STW. Standby pump will be provided in the event of breakdown or maintenance.

The main items requiring periodic maintenance and inspection in the sewage pumping

stations will be the screening device(s) and wet well. Regular cleaning of the screening

device(s) will be necessary to prevent blockage by debris. Entrained grit tends to settle out in

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the wet wells thereby reducing the capacity of the wet wells and restricting sewage inflow.

The accumulated grit will be removed periodically along with any debris and multiple

compartments will be provided so that the pumping stations remain in operation while one of

the wells is being in maintenance. Facilities will be provided within the sewage pumping

stations for temporary storage of screenings in sealed drums or skips prior to removal for

disposal at landfills. The rate of solid waste generated from cleaning is about 1 m3 per day for

each of the sewage pumping stations. To reduce potential odour problem during operation

and maintenance within the sewage pumping stations, air inside those stations will be

collected and pass through an odour removal system before discharge into the atmosphere.

The used water from water points and water basins will be collected and discharged into the

wet well directly. Except during maintenance, the sewage pumping stations are unmanned

facilities.

To prevent sewage overflow from the sewage pumping stations due to plant failure, standby

pumps, dual power supply and emergency storage will be provided to the sewage pumping

stations. The standby pumps and dual power supply will maintain continuous operation of the

sewage pumping stations when the duty pumps or main power supply are down. In extreme

situation when both the duty and standby pumps are down or the dual power supply are failed,

the sewage entering the sewage pumping stations will be bypassed to the emergency storage

to prevent sewage overflow.

The SPSs and STW are all designed to have one standby unit (e.g. standby pumps and

standby treatment units) in case of equipment failure. Dual power supply will be in place in

case of power breakdown. In case of failure of dual power supply, emergency storage for

SPSs and STW would be in place to prevent emergency discharge. A 6-hour (ADWF)

emergency storage will be provided for each SPS. The storage capacity for each sewage

pumping station taking into account the transportation route and time of the repairing crew in

the case of emergency is shown as below:

Storage Capacity for Sewage Pumping Station

Cheung Sha SPS 230 m3

San Shek Wan SPS 343 m3

Cheung Fu Street SPS 148 m3

Tong Fuk SPS 315 m3

Shui Hau SPS 155 m3

Pui O SPS 490 m3

In addition to the sewage from their local catchments, some sewage pumping stations (i.e.

Tong Fuk SPS, Cheung Fu Street SPS and San Shek Wan SPS) will also receive sewage from

their upstream sewage pumping stations. When one of these sewage pumping stations is down,

sewage entering this sewage pumping station from its local catchment will be bypassed to its

emergency storage and all of its upstream sewage pumping stations will stop operation and

the sewage entering the upstream sewage pumping stations from their local catchments will

be bypassed to their emergency storages. It will then provide the required minimum 6 hours

of emergency storage to prevent sewage overflow, which has accounted for the required time

for travelling to and repairing of all the sewage pumping stations as confirmed by DSD.

Regular inspection and maintenance will be carried out by DSD’s maintenance crew which

will station in the San Shek Wan STW. During non-office hours, existing mobile team from

Ma Wan STW will be responsible to back up the maintenance crew in San Shek Wan STW.

With the implementation of above preventive measures, the possibility of sewage overflow

from the sewage pumping stations will be extreme low. However, for safety purpose, a safety

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outlet will be provided to each emergency storage to prevent flooding of the sewage pumping

station.

3.4.3 Sewage Treatment Works

The sewage treatment works collect the sewage from the sewage pumping stations for proper

central treatment. Membrane Bioreactor (MBR) is selected as the sewage treatment process

of the proposed San Shek Wan STW. Details of the treatment process are described in

Section 3.2.4. Treated effluent will be discharged to outer bay of Pui O/ Chi Ma Wan via a

submarine outfall.

The proposed sewage treatment works will be totally enclosed within a multi-storey operation

building. The entire treatment process will generally be fully automatic under a continuous

monitoring and control system. Batch processing with manned operation would be adopted

for the filter press units and disposal of sludge cake, in which frequency of processing would

be further investigated in the detailed design, based on the volume of incoming sewage.

Sludge generated in the treatment process (approximately 2.56 m3 per day) will go through a

thickening and dewatering process and will be stored properly in covered containers within

the plant structure to avoid odour nuisance, prior to removal for disposal at landfills or the

proposed Sludge Treatment Facility by trucks.

The used water from water points and water basins will be collected and discharged into the

inlet chamber for treatment. During the operation of the STW, about 5,800 m3 of treated

effluent will be discharged per day at the time of fully utilization of its design capacity which

will be after 2031.

To prevent sewage overflow from the sewage treatment works due to plant failure, standby

equipment, dual power supply and emergency storage will be provided to the sewage

treatment works. The standby equipment and dual power supply will maintain continuous

operation of the sewage treatment works when the duty equipment or main power supply is

down. In extreme situation when both the duty and standby equipment are down or the dual

power supply is failed, the sewage entering the sewage treatment works will be bypassed to

the emergency storage to prevent sewage overflow. In the case of dual power supply failure,

6-hour emergency storage tank and 6-hour equalization tank at STW will store the sewage

temporarily until the storage capacity is exceeded. When the STW is down, Pui O SPS will

stop conveying sewage to the STW and the sewage entering Pui O SPS will be bypassed to its

emergency storage. The STW will still receive sewage from other sewage pumping stations

(i.e. Shui Hau SPS, Tong Fuk SPS, Cheung Fu Street SPS, Cheung Sha SPS and San Shek

Wan SPS) but the sewage will be bypassed to the emergency storage of the sewage treatment

works.

Regular inspection and maintenance will be carried out by DSD’s maintenance crew which

will station in the San Shek Wan STW. During non-office hours, existing mobile team from

Ma Wan STW will be responsible to back up the maintenance crew in San Shek Wan STW.

With the implementation of above preventive measures, the possibility of sewage overflow

from the sewage treatment works and Pui O SPS will be extreme low. However, for safety

purpose, safety outlets will be provided to the emergency storages to prevent flooding of the

sewage treatment plant and Pui O SPS.

It is noted that as no treated effluent will be generated in the event of power failure, overflow

passage or temporary storage of treated effluent is not necessary.

Outlying Islands Sewerage Stage 2 –

South Lantau Sewerage Works

3-12 B&V / ERM

For regular cleaning and maintenance, screening and grit removal would be conducted inside

the STW. The rate of solid waste generated from STW is about 3 m3 per day.

3.5 Project Summary

Table 3.4 presents a summary of the project details.

Table 3.4 – Summary of Key Project Details Project Consideration Details

Proposed Sewerage Scheme Village sewerage works including gravity pipes and manholes in the

unsewered areas of Shui Hau, Tong Fuk, Cheung Sha, San Shek Wan, Pui O

and Ham Tin in South Lantau

Trunk sewerage works including trunk sewers and twin rising mains along

the South Lantau Road and Chi Ma Wan Road, and six sewage pumping

stations in Shui Hau, Tong Fuk, Cheung Fu Street, Cheung Sha, San Shek Wan

and Pui O

Sewage treatment works including a sewage treatment plant at San Shek Wan

and a submarine sewage outfall discharging into Pui O Wan

San Shek Wan Sewage Treatment

Works (STW)

Design flow = 5,800 m3/day Average Dry Weather Flow (ADWF)

Installed capacity = 17,400 m3

Treatment Objectives of San Shek Wan

STW

Marine disposal of treated effluent via the submarine outfall

Internal reuse of the reclaimed water for non-potable use within the San Shek

Wan STW

Treatment Process at San Shek Wan

STW

Membrane Bioreactor (MBR)

Shui Hau Sewage Pumping Station Design flow = 620 m3/day Average Dry Weather Flow (ADWF)

Tong Fuk Sewage Pumping Station Design flow = 1,880 m3/day Average Dry Weather Flow (ADWF)

Cheung Fu Street Sewage Pumping Station Design flow = 2,470 m3/day Average Dry Weather Flow (ADWF)

Cheung Sha Sewage Pumping Station Design flow = 920 m3/day Average Dry Weather Flow (ADWF)

San Shek Wan Sewage Pumping Station Design flow = 3,840 m3/day Average Dry Weather Flow (ADWF)

Pui O Sewage Pumping Station Design flow = 1,960 m3/day Average Dry Weather Flow (ADWF)

Project Footprint About 16 km of gravity pipes, 7.5 km of trunk sewers and 7.5 km of twin

rising mains

Ranged between 100 – 300 m2 for each sewage pumping station

About 280 m2 and 1,620 m2 for the administration building and operation

building of San Shek Wan STW (building area)

About 1,100 m for the length of submarine outfall

Construction Cut & Fill Requirements

o Gravity pipes, manholes, trunk

sewers and twin rising mains

Excavated materials = about 200,000 m3

Fill materials = about 169,000 m3

Surplus materials = about 31,000 m3

Outlying Islands Sewerage Stage 2 –

South Lantau Sewerage Works

3-13 B&V / ERM

Project Consideration Details

o Sewage pumping station (for a

total of six)

Excavated materials = about 16,000 m3 (about 2,670 m3 each)

Fill materials = about 7,300 m3

Surplus materials = about 8,700 m3

o Sewage Treatment Works Excavated materials = about 30,000 m3

Fill materials = about 2,000 m3

Surplus materials = about 28,000 m3

Area of Seabed Affected by

Installation of Outfall Diffuser

About 1,300 m2 (39m x 33m)

Marine Dredging Volume

(in situ volume)

About 4,700 m3

Project Operation Sludge generation = approx. 2.56 m3 per day

Treated Effluent discharged = 5,800 m3 per day

Other solid wastes from regular cleaning/ maintenance activities = 2,600 m3

per year

3.6 Project Planning & Implementation Programme

The proposed works to be implemented are expected to be packaged in three works packages

as follows:

Package A comprises the construction of a sewage treatment works at San Shek Wan

with submarine outfall, 1 no. of sewage pumping station in Pui O and sewerage to the

unsewered areas of Pui O area. Construction works is anticipated to commence in mid

2017 for completion in early 2023.

Package B the provision of sewerage to the unsewered areas of Ham Tin, San Shek

Wan, Cheung Sha Upper/Lower Village and Cheung Fu Street region. Construction

works is anticipated to commence in mid 2018 for completion in mid 2023. It involves

construction of village sewers, 3 nos. of sewage pumping stations in San Shek Wan,

Cheung Sha and Cheung Fu Street region, and the associated twin rising mains for

collection and conveyance of sewage to the proposed San Shek Wan STW for proper

treatment and disposal.

Package C comprises the provision of sewerage to the unsewered areas of Tong Fuk

and Shui Hau. Construction works is anticipated to commence in early 2019 for

completion in early 2024. It involves construction of village sewers, 2 nos. of sewage

pumping stations in Tong Fuk and Shui Hau, and the associated twin rising mains for

collection and conveyance of sewage to the proposed San Shek Wan STW for proper

treatment and disposal.

3.7 Interactions with other Projects

Major interfacing projects are summarized in Table 3.5. None of these projects are

considered as concurrent projects of this Project and hence they are not considered in this EIA.

Outlying Islands Sewerage Stage 2 –

South Lantau Sewerage Works

3-14 B&V / ERM

Table 3.5 – Major Interfacing Projects with This Project Project Title Project Scope Concurrent Project?

PWP No. 257WC – Siu Ho Wan Water

Treatment Works Extension – Pui O

Raw Water Pumping Stations and

Associated Mainlaying (WSD/Design)

A 1200mm diameter raw water main

along South Lantau Road in Pui O is

proposed under this Water Supplies

Department (WSD) project. Investigation

phase of the project was completed in

2010. There is no definite programme for

the raw water main.

No. Project programme is not

available for consideration at

time of reporting.

Agreement No. CE 38/2007 (CE) –

Planning and Engineering Study of

Private Housing Development at

Cheung Sha, Lantau – Feasibility

Study (CEDD/HKI&I)

This is a planning and engineering

feasibility study commissioned by Civil

Engineering and Development

Department (CEDD) for five potential

private housing development sites in

Cheung Sha. The feasibility study was

completed in 2010 and recommended

sites CSER 406 and CSER 407 for private

housing development. These two sites

had been recently sold to private

developer in 2011. The actual

development programme would have to

depend on the private stakeholders and

the project implementation mechanism.

No. Project programme is not

available for consideration at

time of reporting.

Agreement No. CE 11/2008 (WS) –

Replacement and Rehabilitation of

Water Mains Stage 4 (WSD/CM)

Water mains replacement and

rehabilitation works are proposed, under

Water Supplies Department (WSD)’s

Agreement No. CE 11/2008 (WS), in Pui

O, Ham Tin, Cheung Sha, Tong Fuk and

Shui Hau. The works involves

replacement of water supply pipelines in

the above villages and connection along

South Lantau Road. The construction

works for this project is scheduled to

commence in 2011 for completion in 2015.

No. Construction programme

is generally earlier than

majority of the South Lantau

Project.

CE 29/2008 (EP) – Engineering

Investigation and Environmental

Studies for Integrated Waste

Management Facilities Phase 1 –

Feasibility Study (EPD/IP)

A set of 132kV submarine electricity

cables was proposed by Environmental

Protection Department (EPD) to be

constructed underneath the seabed

between Shek Kwu Chau and Cheung Sha

under Agreement No. CE 29/2008 (EP)

and the project was gazetted under

Foreshore and Sea-Bed (Reclamations)

Ordinance (Cap. 127) in April 2011. The

project was in the status of feasibility

study and construction was scheduled

between 2013 and 2019.

No. This project is at sufficient

distance (>2km) from the

proposed submarine sewage

outfall.

Outlying Islands Sewerage Stage 2 –

South Lantau Sewerage Works

3-15 B&V / ERM

Project Title Project Scope Concurrent Project?

Agreement No. CE 15/2010 (DS) –

Upgrading of Cheung Chau and Tai O

Sewage Collection, Treatment and

Disposal Facilities (DSD/CM)

Under DSD’s Agreement No. CE 15/2010

(DS), the existing Cheung Chau and Tai O

sewage treatment works and their

associated sewerage networks are

proposed to be upgraded. The works

mainly comprise expansion and

upgrading of the existing Cheung Cha

STW and Tai O STW together with the

associated sewerage networks. The

construction works of this project is

tentatively scheduled to progressively

start in 2016-2017 for completion in 2021.

No. The proposed sewerage

works in Cheung Chau and Tai

O are at sufficient distances

(>4km and >5km respectively

for Cheung Chau and Tai O)

from the project area of South

Lantau sewerage works. In

addition, the locations of the

works in this project at Cheung

Chau and Tai O are

geographically separated from

the South Lantau sewerage

works by Chi Ma Wan

Peninsula and Lantau Peak

respectively.

Agreement No. 4600003274 – Cable

Route Consultancy Services along Chi

Ma Wan Road (CLP)

A set of 11kV electricity supply cable was

proposed by CLP Power Hong Kong

Limited (CLP) to be laid from the junction

of South Lantau Road and Chi Ma Wan

Road via Pui O Camp Site to the south of

Ham Tin and end at Chi Ma Wan

Peninsula, in order to provide sufficient

electricity supply to the Chi Ma Wan and

Chi Sun Correctional Institution, in which

expansion works are being planning.

Feasibility study of the project is

completed in 2010. Construction

programme of the works may hinge on

the funding of expansion works for those

two correctional institutions.

No. Project programme is not

available for consideration at

time of reporting.

RPIS IS009 – Construction of

Vehicular Access at Pui O San Wai

Tsuen, Lantau Island (HyD/Works)

A new vehicular access was proposed by

Highways Department (HyD) in Pui O

San Wai next to an existing river channel

for access from South Lantau Road to

uphill side of the village. The road

alignment is being reviewed by HyD and

there is no definite programme for the

works.

No. Project programme is not

available for consideration at

time of reporting.

RPIS IS005 – Construction of Car Park

at Pui O, Lantau Island (HyD/Works)

Construction of car park and footpath

improvement were proposed by

HyD/Works in the vicinity of Pui O Lo

Uk Tsuen. No definite programme for the

works is available during preparation of

this EIA Report.

No. Project programme is not

available for consideration at

time of reporting.