2.2
The Purposes, Objectives and Environmental Benefits of the Project
2.2.1.1 The Government plans to develop Yuen Long South Development Area (YLS
DA) which is a mega-scale and complex development aiming to provide land to
transform the degraded brownfield land to developable land contributing to meet
the territory’s medium to long-term need for housing development.
2.2.1.3 The environmental benefit of the Project is that the proposed Yuen
Long South Effluent Polishing Plant (YLSEPP) will provide sewage treatment to
the sewage generation from the population growth and development of YLS DA and
other developments in the North West New Territories (NWNT), which will prevent
pollution and minimize any impact to the discharging water body (Deep Bay)
compared to the case of having no sewage treatment. Without this Project,
alternative land shall still be sourced to meet such sewage treatment needs.
2.2.1.4 It is planned that there will be a Water Reclamation Facility and
its associated pumping facilities adjacent to YLSEPP where YLSEPP’s tertiary
effluent will be further treated as Reclaimed Water to meet flushing demand in
YLS DA, HSK/HT NDA and the surrounding new development areas. In addition,
there is future opportunity that YLSEPP’s tertiary effluent, would provide a
clean water source to revitalize the Yuen Long Nullah. Such high quality flow
would improve the scenic appearance of Yuen Long Nullah and would enhance the
ecological value to the nearby community. In addition, the surplus treated
effluent to Yuen Long Nullah could also serve as a back-up water for periodic
flushing of the Yuen Long Nullah Barrage under the Yuen long Nullah Barrage
Scheme.
2.3
The Need of the Project
2.3.1
Project Initiation
2.3.1.1
The YLS DA is generally rural in character with a mixture of land uses.
The predominant uses are brownfield operations including open storage yards,
warehouses, industrial workshops, etc. These brownfield operations are
intermingled with rural settlements and residential settlements, agricultural
land, livestock farms and vacant land. Proliferation of such brownfield sites
has resulted in degradation of the rural environment. There is a need to better
utilise the degraded brownfield land occupied by open storage yards,
warehouses, industrial workshops, etc for beneficial uses. Hence an effluent
polishing plant (EPP) is proposed to site at the YLS DA to optimise the
development potential to meet the territory’s needs, and to improve the local
living environment with infrastructure.
2.3.1.2
In 2011, Chief Executive announced out the “2011-12 Policy Address” to
explore the possibility of converting into housing land some 150 hectares of
agricultural land in North District and Yuen Long currently used mainly for
industrial purposes or temporary storage, or which is deserted.
2.3.1.3
In November 2012, Planning Department (PlanD)
and Civil Engineering and Development Department (CEDD) commissioned Agreement
No. CE 35/2012 (CE) “Planning and Engineering Study for Housing Sites in Yuen
Long South – Investigation” to examine the future land use, optimise the
development potential, and ascertain the feasibility for public and private
housing developments in the YLS area.
2.3.1.4
In 2020, Civil Engineering and Development Department (CEDD) and the
Planning Department (PlanD) promulgated the Revised
Recommended Outline Development Plan to provide a series of planning and technical
assessments for the optimization of the development intensity of some public
housing sites and sites for accommodating brownfield operations under Stages 1
and 2 of the Yuen Long South Development Area (YLS DA) and the increasing the
public housing component in the overall housing mix.
2.3.1.5
In 2020, the DSD has commissioned Agreement No. CE 6/2019 (DS) for the
investigation for HSKEPP and YLSEPP. The sewage flow projection and treatment
process for YLSEPP was reviewed. It was recommended to construct of a new
tertiary effluent polishing plant with a capacity up to 65,000 m3/day
of sewage from the YLS DA and the HSK NDA and other surrounding areas such as
Yuen Long Area 13 & 14 with due regard of the holistic sewage treatment and
disposal arrangement in the region.
2.3.2.1
Further review on sewage flow projections within the catchments of YLSEPP
have been conducted under this Assignment. Information on current and planned
catchment as well as planned developments within the catchments were obtained
from relevant government departments. The sewage flow projection principles
were based on the following guidelines:
(a)
Sewerage Manual, Part 1 (DSD, 2013)
(b)
Technical Paper No. EPD/TP1/05 – Guidelines for Estimating Sewage Flows
for Sewage Infrastructure Planning (EPD, 2005)
2.3.2.2
The population and planning data from Planning Department has been taken
into account for population projections. In addition, new development projects
and other sewerage and sewage projects were also included in the sewage flow
projections.
2.3.2.3
Due to the limited land supply of HSKEPP, the available footprint for
construction of HSKEPP cannot accommodate the required treatment facilities to
cater all the sewage from the catchment area. Therefore, as per the discussion
with DSD, CEDD and EPD, 16,000 m3/day of sewage from HSKEPP
catchment area would be diverted to YLSEPP for treatment. The sewage diversion
would start in Year 2038.
2.3.2.4
With consideration of the agreement no. CE 35/2012 (CE) – Planning and
Engineering Study for Housing Sites in Yuen Long South – Investigation and CE
36/2018 (CE) – Preliminary Technical Review on Potential Sites in Yuen Long
Areas 13 and 14 For Housing Development – Feasibility Study, the sewerage
catchment of the YLSEPP included YLS DA, YL Areas 13 and 14 and diverted flow
from HSK/HT NDA.
2.3.3
Organic Waste Co-digestion
2.3.3.1 As a sustainability consideration, co-digestion of organic wastes
with sewage sludge within YLSEPP will be adopted to enhance energy recovery
from the anaerobic digestion process. Food waste will be collected and pre-treated
in the Food Waste Pre-Treatment Facilities, which will be located out of 500m
from YLSEPP subject site boundary, before transported to YLSEPP for
co-digestion with sewage sludge. Additional facilities for organic wastes
co-digestion, including reception facilities, digesters and dewatering
facilities will be located within the YLSEPP’s footprint.
2.3.4
Scenario without the Project
2.3.4.1
The
projected Average Dry Weather Flow (ADWF) will be up to
65,000 m3/d. For the scenario without the implementation of YLSEPP,
there will be still a need to provide alternative sewerage and sewage disposal strategies or
facilities to collect and treat the sewage flow generated from YLS DA and other
developments in the North West New Territories (NWNT), before discharging to
the Deep Bay. This may lead to sourcing of alternative
land and the opportunity of
reusing treated effluent for the benefit of conserving freshwater may also be
lost.
2.3.5
Scenario with the Project
2.3.5.1
Upon the completion of YLSEPP, it will provide enough treatment capacity
for the sewage generated from YLS DA and its surrounding areas including HSK/HT
NDA and Yuen Long Area 13 &14. With sufficient treatment capacity and high
effluent quality, there are opportunities to handle sewage from future
developments within YLSEPP catchment.
2.3.5.2
YLSEPP will provide high level of treatment to minimize pollutant
discharge to Deep Bay. It will include co-digestion of organic wastes with
sewage sludge to enhance energy recovery and optimize land use. YLSEPP will
also cater for future planning of further treatment as reclaimed water (by
others), and provide further opportunity of river revitalization of Yuen Long
Nullah and back-up water for flushing of the Yuen Long Nullah Barrage Scheme.
2.4
Site Location, History and Scope of the Project
2.4.1.1 The proposed YLSEPP is located in adjacent to the end of Kung Um
Road. In the 1960s, the land was mainly agricultural and most of the
agricultural land was abandoned in 1990s. Currently, the existing land of the
site is mainly used for vehicle maintenance and logistic industry.
2.4.1.2 YLS DA has reserved about 4.6 hectares of land (including part of land OU3.3 and land G3.1) for the proposed YLSEPP, the location of which is shown in Figure 1.1. According to the YLS
DA’s Revised Recommended Outline Development Plan (RODP), YLSEPP is located at
the southern tip of YLS DA and is co-located with the planned Water Reclamation
Facilities and its associated pumping facilities at land OU3.3 It is bounded by
Ma Shan (Kung Um Shan) and Tai Lam Country Park at its south
west. There will be a planned reedbed and green belt to its north, and
planned government and institutional land use to its east side.
2.4.1.3 The Scope of this Project would only cover the construction of
YLSEPP of which the associated supporting structures, including the upstream sewage
pumping station, rising main for raw sewage, water
reclamation facilities and its associated pumping facilities, reedbed and
rising main for reclaimed water (as shown in Figure 1.1), are out of the study
brief of this EIA study. For avoidance of doubt, the site formation works of
the YLSEPP site (including land OU3.3 and G3.1) are also not covered in the
scope of this Project and the environmental impact of such works should be duly
reviewed in the separate EIA Report: AEIAR-215/2017- Housing Sites in Yuen Long
South.
2.4.1.4 The pre-treated food waste collected in YLSEPP would be transported
by EPD for subsequent co-digestion with sewage sludge. As such, no food waste
pre-treatment would be carried out in YLSEPP.
2.4.1.6 The proposed works of the Project comprise:
1.
Construction
of a sewage treatment plant with a maximum capacity of Average Dry Weather Flow
(ADWF) up to 65,000 m3/day;
2.
Construction
of sludge treatment facilities for treating sludge generated from YLSEPP and
other nearby sewage treatment works;
3.
Construction
of facilities for receiving and co-digesting pre-treated food or organic wastes;
4.
Construction
of effluent discharge pipe for the purpose of emergency discharge; and
5.
Associated
ancillary works.
2.4.1.7 The schematic layout for YLSEPP is presented in Figure 2.1. The design capacity and
effluent standards of YLSEPP are tabulated in Table 2.1.
Table 2.1 Design
Capacity and Effluent Standard of YLSEPP
|
YLSEPP
|
Remarks
|
|
Treatment Level
|
Tertiary
|
-
|
|
Treatment Capacity
|
65,000 m3/d
|
-
|
|
Effluent Standard
|
BOD5
|
10
mg/l
|
95 percentile
|
|
Suspended Solid
|
10 mg/l
|
95 percentile
|
|
NH3-N
|
2 mg/l
|
Annual average
|
|
Total Nitrogen
|
10 mg/l
|
Annual average
|
|
Total Phosphorus
|
1 mg/l
|
Annual average
|
|
E.
coli
|
100
no./100ml
|
Monthly geometric mean
|
2.4.2
Process Flow of Sewage
Treatment
2.4.2.1 Figure 2.2 illustrates
the process flow of sewage treatment at YLSEPP.
Coarse
Screen
2.4.2.2 Mechanically raked type coarse screens with inclined bars and a
clear bar opening will be provided. The coarse screens will protect the
downstream inlet pumps by removing large objects.
2.4.2.3 All coarse screen channels will be equipped with their own inlet and
outlet penstocks for channel isolation. The emergency bypass channel for coarse
screen system will be provided to overflow raw sewage from reception chamber to
the inlet pumps chamber in case of the clogging of coarse screen.
Fine Screen
2.4.2.4 Bar screen will be adopted as the fine screen technology. Screens
will be fully enclosed by polycarbonate cover for odour control. Removable
covers will be provided to facilitate maintenance of the screens. Access and
inspection openings with covers will also be provided at various locations to
facilitate safe entry to the channel interior for inspection and maintenance.
Grit Removal Facilities
2.4.2.5 Vortex grit separators will be provided for YLSEPP, which are
designed to effectively remove small particles from the sewage flow and also
designed to hydraulically accommodate the design peak flow.
2.4.2.6 After screened by the fine screens, sewage is directed to the vortex
grit separators through the respective penstock openings. Each vortex unit can
be isolated by closing the corresponding inlet/outlet gates. Vortex grit
separators allow grit separation from the screened effluent by inducing a
vortex pattern in the circular chamber, where the denser grit will gravitate
towards the bottom under centrifugal force.
2.4.2.7 Grit accumulated in the vortex hopper will be periodically pumped to
a grit classifier. The drainage flow generated from the grit classifiers will
be discharged to the screen effluent channel by gravity. The grit slurry is
settled, dewatered, and dropped into a screenings / grit conveyor and
eventually into a disposal bin.
Primary
Sedimentation
2.4.2.8 Plate settlers (i.e. Lamella) would be installed in the primary
sedimentation tank to remove the insoluble particles by sedimentation. Oil and
grease in sewage would be removed by scum trough while the sludge collected in the bottom of the
primary sedimentation tank would be pumped to holding tanks for subsequent
sludge treatment.
Biological
Treatment
2.4.2.9 Membrane bioreactors
would be provided as the biological treatment in YLSEPP. Each bioreactor would
be partitioned to three zones, the pre-anoxic zone, aerobic zone and
post-anoxic zone, through which the sewage will pass sequentially. In each
zone, the level of dissolved oxygen would be controlled to achieve carbon,
nitrogen and phosphorus removal with mechanical mixer or fine bubble diffusion
systems. Internal sludge recycle pumps
will also be installed to recycle the nitrified effluent in aerobic zone to
pre-anoxic zone for denitrification. The sewage will be filtered through
membrane modules to remove the suspended solids before passing to the
disinfection facilities. Surplus sludge accumulated in the process tanks would
be pumped to holding tanks for subsequent sludge treatment.
UV
Disinfection Facilities
2.4.2.10 UV channels with low pressure and high intensity UV lamps would be
provided as UV disinfection would to further reduce the concentration of E.coli.
of effluent before discharge.
Effluent
Discharge
2.4.2.11 In the ultimate scenario, the majority of treated tertiary effluent
would be discharged to the adjacent Water Reclamation Facilities for further
treatment. Part of the effluent would be used within YLSEPP (e.g.
pipe flushing) for internal plant operation and discharged to adjacent reedbed for
river revitalization.
2.4.2.12 However, the Water Reclamation Facility and reedbed are proposed
under CEDD's D&C consultancy for YLS DA Stage 2 Works, Phase 2 and Stage 3 Works respectively, all the treated effluent
would be discharged to Yuen Long Nullah as interim operation.
2.4.3
Process Flow of Sludge
Treatment
2.4.3.1 Figure 2.3 illustrates
the process flow of sludge treatment at YLSEPP.
Sludge
Thickening
2.4.3.2 Primary sludge and MBR secondary sludge will be collected from
lamella primary sludge sedimentation tanks and MBR membrane tanks respectively.
The collected sludge would be transferred to Sludge Blend Tank by pumping for
mixing. The mixed sludge would then be thickened by centrifuge before fed into
digesters. The centrifuge consists
of a rotating bowl where the sludge is fed from the centre
and the solid and liquid are separated by centrifugal force.
Pre-treated Food Waste
2.4.3.4 YLSEPP would receive pre-treated food waste from EPD via fully
enclosed tankers for co-digestion with sewage sludge. The pre-treated food
waste would be first accepted in the food waste bunker and diluted with treated
effluent of YLSEPP. The treated effluent of YLSEPP with pre-determinate
flowrate will be injected in the feed pipe of food waste feeding system and
mixed with the pre-treated food waste in the pipework by dynamic mixers. The
diluted food waste will then be stored in a Diluted Food Waste Preparation Tank
before being fed into digesters. Proper in-tank mixing will be provided.
Anaerobic Digestion
2.4.3.5 Thickened sludge and diluted food waste would be fed into digesters
and would be well mixed in the digestors for anaerobic digestion to reduce the
volatile solid content of the sludge. Waste heat from the CHP would be
collected to maintain the operation temperature of the digestors throughout the
digestion process to achieve optimum biogas production.
2.4.3.6 The digested sludge would be collected from the digesters in sludge
holding tanks for subsequent sludge dewatering process. The biogas obtained
from the digesters, will pass through iron sponge filters to reduce the H2S
content. The filter media reacts with the H2S content and binds the
sulphur in a stable solid form inside the filter. Thereafter, the biogas would
be transferred and stored in the biogas holders for utilization later for heat
and power generation with CHP and boiler. During normal operation, all biogas
will be consumed by the CHP units or boiler. Waste gas burners are provided for
emergency flaring of excess biogas during the rare case when the CHP system
and/ or boiler are not operational.
Combined
Heat and Power (CHP) Generation
Sludge
Dewatering
2.4.3.8 The digested sludge stored in the sludge holding tank would be
pumped to sludge centrifuge system to increase the dry solid content of sludge.
The centrifuge consists of a rotating bowl where the sludge is fed from the
centre and the solid and liquid are separated by centrifugal force. The dense
sludge cake is discharged from the bowl by a crew feeder and the dilute liquid
rejected from the centrifuge (i.e. centrate) would be collected. Coagulant and/
or flocculant is required to be inline dosed into the sludge feed to the
centrifuge for sludge conditioning.
2.4.3.9 To achieve the minimum dry solid content of dewatered sludge, a
portion of dewatered sludge, after dewatering via centrifuge system, would be
further dried using sludge dryers, which further reduces the water content
through evaporation by applying heat to the sludge. The heat is supplied in the
form of steam by a steam boiler that utilized biogas as combustion fuel for
steam production. The exhaust gas from the boiler combustion of biogas would
have similar composition as that of CHP units. A standby
boiler unit is provided to ensure the boiler system operators at the design
capacity in terms of biogas consumption rate to avoid flaring of excess biogas
by the waste gas burner. The standby unit will be put in operation if the
duty boiler is out of service. The evaporated water from the dried sludge will
be re-condensed at the cooling tower from which the collected water will be
combined with the dewatering centrifuge centrate to be further treated as
sewage by the side stream treatment. The dried sludge produced by the sludge
dryers would then be blended and mixed with dewatered sludge before sending out
to the Sludge Treatment Facilities (STF) for further processing.
Sidestream Treatment
2.4.3.10 The sewage collected from the
sludge dewatering and drying process would be treated by sidestream
anammox system to remove organics and ammonia content in the liquid stream before returning to the mainstream sewage treatment.
2.4.4
Deodorisation Facilities
Design
Approach of deodorization at YLSEPP
2.4.4.1
All the treatment units in YLSEPP, including channels and tanks, would
be fully covered to prevent the odour emission to the atmosphere causing
nuisance to adjacent sensitive receivers. Two-stage deodorization system would
be provided to control odour emission at individual treatment unit. Ventilation
fan would be provided to extract the odour gas from sewage and sludge treatment
units to deodorization units for treatment.
2.4.4.2
In two-stage deodorisation system, the odour gas would first be treated
by biotrickling filter followed by activated carbon filter for final polishing. Biotrickling filters make uses of bacteria or
other micro-organisms to transform odour to
non-odorous conditions and are commonly used to treat odour
with high H2S levels. They are recommended as odour
generated in those target odour extraction locations
contains high concentration of H2S and constant odour
loadings. This condition facilitates stable growth of micro-organisms in
biotrickling filters, and thus high H2S removal efficiency can be
maintained at all times with low operation and maintenance requirements. The
activated carbon filter is provided so that any remaining odour
after passing through the biotrickling filter is further minimized before the
treated air is released into the atmosphere.
Options
of Primary Treatment Process
2.5.1.1 Owing to limited space, a compact design of the primary
sedimentation tanks will be required for the YLSEPP. Thus, the following compact
primary treatment technologies have been shortlisted:
·
Primary clarifier with double-deck configuration
·
Primary clarifier with plate settler (Lamella clarifier)
·
Chemically enhanced primary treatment (CEPT)
2.5.1.2
Comparing the environmental benefits among the above options of primary
treatment process, primary clarifier with double-deck configuration and CEPT
would require a larger building footprint to accommodate all the necessary
E&M equipment which would induce more environmental impact such as noise
and dust during construction. The excavation for these two options is also more
extent which would induce a larger amount of C&D material and waste.
Moreover, CEPT would require more mixer for chemical mixing which would induce
noise during operation. Therefore, primary clarifier with plate settler
(Lamella clarifier), which is more compact in size, is selected as the primary
treatment process in YLSEPP.
Options
of Biological Treatment Process
2.5.1.3 The treatment option evaluation
section mainly focuses on the biological treatment processes as this is the
critical part in YLSEPP layouts.
2.5.1.4 A preliminary review of potential biological treatment processes,
including proven treatment processes, market availability, overseas and local
experiences and the emerging treatment process, have been conducted. Two
major options of biological treatment process for the YLSEPP were considered, including:
·
Conventional Activated Sludge (CAS)
·
Compacted-type technologies
2.5.1.5 Both CAS and the compacted-type technologies will provide biological
treatment capable of achieving the required effluent standards with UV
disinfection. Since the operation mode for both options is similar (e.g. both
of them require air bubble mixing with sewage for aerobic digestion), it is
considered that the environmental impact for two options during operation ,
such as noise, would not have significant difference. Nevertheless, CAS will require
larger footprint which could not be accommodated in the YLSEPP. The additional footprint
would be required if CAS is adopted, which will inevitably induce significant
environmental impacts.
2.5.1.6 It should also be noted that CAS will involve a considerably larger
volume of excavation works and, hence, significant increase in C&D
materials to be disposed of. In addition,
it will result in longer construction period during which more environmental
impacts would be arisen.
2.5.1.7 For the compacted-type technologies,
three processes were considered, comprising Moving Bed
Biofilm Reactor (MBBR), Aerobic Granular Sludge (AGS), Membrane Bioreactor
(MBR). Comparing the technologies,
MBBR and AGS have relatively larger footprint and excavation volume than MBR. MBR
is a more compact sewage treatment option. MBR would occupy the least footprint
and requires the lowest amount of excavation. The amount of TSS in the treated
effluent of MBR is also the lowest, which resulted in the lowest amount of
pollutant discharge to the Deep Bay. MBR’s high quality effluent is also favorable
for further reclaimed water treatment and production. Different compacted-type
technologies have their own environmental benefits and disbenefits.
2.5.1.8
Although those compacted-type
technologies would generally require higher power consumption and more O&M
requirements when compared to CAS, they have overall environmental benefits, in
particular during construction phase, and, thus, are recommended as preferred
option.
Options of Disinfection
Process
2.5.1.9
There are three common
disinfection technologies, namely ultraviolet (UV), chlorination and ozone
treatment. Chlorination is not recommended for YLSEPP as the treated effluent would
leave residual chlorine which is harmful to aquatic lives in the receiving
waterbody. Both ozone and UV leave no residuals in effluent that does not harm
the water life. Compared to UV disinfection, ozonation requires a larger
footprint and much higher power consumption, which is not as favorable as the UV option in terms of environmental
benefit. Therefore, UV is the most recommended for YLSEPP’s disinfection
application.
Options for Sludge
Treatment
2.5.2.1 Sludge cake generated from YLSEPP’s treatment of sewage sludge and organic
waste will be delivered to the Sludge Treatment Facility (STF) in Tuen Mun for incineration. Prior to conveyance to the STF, the
following handling options are considered:
·
Dewatering with prior anaerobic digestion including organic waste
co-digestion
·
Direct dewatering without digestion
2.5.2.2 Anaerobic digestion is recommended with the environmental benefits
of reducing the volume of sludge and food waste to be disposed of at STF and
allow energy recovery from biogas generation for utilization within YLSEPP. Also,
the organic contents in digested sludge would be much lower so as to minimize
the odour level in the downstream dewatering and offsite disposal process.
2.5.2.3 Emissions from biogas combustion were considered as minimum and
relevant impact assessment are included in Section 3. The environmental benefits would significantly
outweigh the disbenefits of the sludge digestion process.
2.5.2.4 As a sustainability consideration, co-digestion of organic wastes
with sewage sludge within YLSEPP is recommended to enhance biogas generation
and, hence, increase the generation of renewable energy for plant internal usage.
Additional facilities for organic wastes co-digestion, including reception
facilities, digesters and dewatering process, will be located within the YLSEPP’s
footprint.
Options for Sidestream Treatment
2.5.2.5 The following sidestream treatment
technology have been considered during the development of the plant design for
YLSEPP:
·
Anammox Technology
·
Ammonia Stripping
2.5.2.6 Anammox technology utilizes a low
energy ammonification process to convert aqueous ammonia to nitrogen gas. While
ammonia stripping technology heats the centrate/filtrate stream to convert
ammonia from liquid phase to gas phase. The air leaving the stripping column
will then be burned in a combustion chamber to convert the ammonia-laden air to
nitrogen and water before it is released to the atmosphere. In order to meet the emission standard for
NOx, additional NOx treatment unit will be added to further treat the ammonia
stripping exhaust gas. Compared with ammonia stripping technology, anammox uses
much less energy and produce less gaseous emissions, therefore it will be
selected for side-stream treatment in YLSEPP.
2.5.3
Deodorization Facilities
Options for
Deodorization System
2.5.3.1 Due to the constraint of footprint and height limit, the following
deodorization options have been considered during the development of the plant
design for YLSEPP:
·
A combination of one-stage and two-stage deodorization: to adopt two-stage deodorization system for areas with high odour concern
such as inlet works, primary treatment units, sludge treatment units and
organic waste reception facilities; and adopt one-stage deodorization system
for areas with less odour concern, such as biological treatment units and
tertiary treatment units.
·
Two-stage deodorization for all treatment processes
2.5.3.2 In the design of YLSEPP, two-stage deodorization for all treatment
processes including for treatment processes with lower odour emission rates
such as bioreactors for sewage treatment is adopted. Compared with a
combination of one-stage and two-stage deodorization, adoption of two-stage
deodorization for all treatment processes will achieve a higher odour removal
efficiency. This design will minimize the odour emission from YLSEPP as far as
possible.
2.5.4
Consideration of Layout
Options to Reduce
Environmental Impacts
Design Constraints and Environmental Factors
2.5.4.1 For developing of YLSEPP’s internal layout, considerations of numerous
engineering constraints and environmental factors have been made as below:
·
The locations, size and arrangement on new treatment facility is bounded
by recommended treatment options (Section
2.5.1 refers). Thus, compacted size
treatment facility is provided in the layout.
·
In green building consideration, energy recovery from biogas is
recommended (Section 2.5.2 refers).
Thus, chimneys from CHP is required in YLSEPP.
·
The required treatment capacity of 65,000 m3/day (Section 2.3.2 refers) would determine the size of
treatment units and likewise would determine the amount of odour emission and
the requirement on deodorization units to cater the odour nuisance due to
sewage treatment process.
·
Odour emission to be treated by deodourization units are determined
by the surface area of the treatment facilities, the air exchange flow rate and the odour emission strength according to odour
generation sources such as sewage and sludge tanks. Compacted treatment
facilities would assist in reducing the odour emissions with smaller tank sizes.
·
Odour extraction rates will be appropriately provided for man-access
areas with higher air exchange rates and for non-man-access odour enclosures
with lower air exchange rates to reduce the size of the deodourization units
necessary to treat the extracted foul air.
·
Foul air is treated as close to the emission source as possible with
decentralized deodourization units to reduce the footprint required for odour
extraction ductwork and to provide optimal deodourization treatment technology
for the various types of odour sources.
·
The layout of different treatment units is determined with due
considerations of not only the process requirements, but also environmental
factors, e.g. most of the air emission sources as well as deodourization units
are located with as much setback from the site boundary as possible to provide maximum
distance between the air emission sources and the sensitive receivers.
·
The scale and size of above-ground structures are determined by
striking a balance between the plant’s hydraulics and visual impacts to the
surroundings.
Consideration of Project Design and Layout
2.5.4.2 In order to avoid and minimize environmental impacts in terms of air
quality, noise, water quality, ecological, landscape and visual aspect, the
following major design and layout will be adopted in YLSEPP:
·
All
the treatment units will be covered and ventilated via deodorization units.
Thus, odour impact would be minimized.(Section
3 refers)
·
All
the treatment facilities and building will be ventilated with silencers at lourvers. Thus, fixed noise impact would then be minimized.
(Section 4 refers)
·
A
set of design measures will be installed to avoid and minimize the chance on
emergency discharge (Section 2.6 and Section
5 refers)
·
All
the treatment units and buildings are designed with due considerations on
minimizing the building heights by such means as adopting equipment that
requires low headroom. This is to ensure
all the aboveground structures would not be excessively bulky so as to minimize
the visual impacts. (Section 9
refers)
Consideration on the Amount of
Excavation
2.5.4.3 To reduce the amount of excavation within the Project, relaxation of
the height restriction for Land OU3.3 as stated in the YLS DA’s Revised
Recommended Outline Development Plan (RODP) (i.e. from +35.0 mPD to +40.0 mPD) for
construction of YLSEPP treatment facilities is adopted to minimise the
associated environmental impact such as the air quality and waste implication
due to excavation. Further relaxation of height limit (i.e. beyond +40.0 mPD) is not advisable with consideration of the visual
impact due to the project.
2.5.4.4 The design layout of YLSEPP taking into account the relaxation of
height restriction has already been adopted in this EIA report and the
environmental implication would be discussed in later sections.
Consideration of Locations for the
New Treatment Facilities
2.5.4.5 The locations of new treatment facilities are bounded by the inlet
sewage pipes and effluent outfall at the northern and south-eastern sides,
respectively. In order to minimize the
scale of construction, sewage inlet works would be located at the north side of
YLSEPP to minimise the distance of the incoming sewage rising mains and the effluent
outfall would be located close to the existing Yuen Long Nullah. As such, the
locations of new facilities are designed with inlet works at the north, primary
and biological unit in the middle and tertiary treatment units at the south of
YLSEPP. In addition, all the construction works and new facilities would be
within the site boundary to avoid additional project footprint. The new
treatment facilities arrangement is then designed under these boundary
conditions.
2.6
Consideration of Alternative
Designs to Minimize Emergency Discharges
2.6.1.1
Standby units for major treatment units and Electrical and Mechanical
(E&M) equipment will be provided in YLSEPP to minimize the occurrence of
emergency discharge.
2.6.1.2
The project site is subject to severe space constraints with very congested
arrangements in order to accommodate treatment and ancillary facilities of
YLSEPP. It is found infeasible to provide an emergency storage tank within the
YLSEPP site for storage of raw sewage during emergency situations. Nevertheless,
the reliability and robustness of YLSEPP is largely sufficient to avoid / minimize
the risk of emergency discharge, including the design measures as described
below.
Provision
of adequate standby units and peaking factors
Provision
of by-pass mechanism at coarse and fine screens
2.6.1.4
According to DSD’s record, an emergency discharge event from a sizable
STW was recorded in August 2014. This emergency discharge event was due to the malfunction
of fine screens at the inlet works. In order to avoid similar event occurring
at YLSEPP, in addition to providing standby units, by-pass mechanism will be provided
for both coarse screens and fine screens in the inlet works building. Influent
could be diverted to grit trap and lamella primary clarifier and emergency
discharge could be avoided/minimized if there is failure in coarse/fine
screens.
Provision
of reliable power supply
2.6.1.5
The CLP power supply to YLSEPP is stable and reliable. YLSEPP’s power supply system will be further
enhanced with dual power supply.
2.6.1.6
Apart from conventional power sources, alternative power supply will be
provided by the renewable energy sources within YLSEPP. One of the major
renewable energy sources will be from biogas stored in the gas holders. In case of power outage from CLP, the Combined Heat and Power units will generate
electricity by consuming the stored biogas.
Together with the
power generated by solar panels, the renewable energy could be able to drive
essential power demands for treatment processes for a duration of time. With such arrangements for YLSEPP’s power
supply system, the operation reliability of YLSEPP is further enhanced.
Provision
on interim emergency by-pass
2.6.1.7
Interim by pass after the primary clarifier
will be provided. In case there is failure in downstream treatment units such
as biological treatment or tertiary treatment, raw sewage could be treated by
primary solid/liquid separation and discharged via an interim by-pass. With
this provision, raw sewage by-pass could be avoided as much as possible.
Regular
maintenances and inspections
2.6.1.8
Regular maintenances and inspections to all treatment units, penstocks and
plant facilities will maintain a good operation condition of YLSEPP. As such, minimizing
the risk of malfunction and blockage of treatment units. The probability of
emergency discharge could also be minimized.
Application
of Emergency Response Plan
2.6.1.9
To provide a mechanism to minimize the impact of emergency discharges
and facilitate subsequent management of any emergency, the criteria and
procedures recommended in the “Contingency Plan for
Incidents Possibly Encountered in Sewage Treatment Facilities having a
Potential of Generating an Environmental Nuisance” by Sewage Treatment Divisions One &
Two of DSD should be referenced to formulate an Emergency Response Plan, which
shall be submitted prior to commissioning of YLSEPP. The plant operators of YLSEPP should carry
out necessary follow-up actions according to the procedures of the Emergency Response
Plan to minimize any impact on the identified WSRs due to emergency bypass.
2.6.1.10
In addition, in case of emergency discharge, the upstream pumping
station could be temporarily suspended, preventing sewage from entering to YLSEPP,
so to lower the risk of any emergency discharge.
2.7
Sustainability Consideration
2.7.1
Opportunities for Green
Building
2.7.1.1
To encourage low carbon and green STW, green building design is one of
the key tasks in this Assignment. The implementation of green features will be
subject to further studies during detail design stage of this Assignment. The
initial ideas of recommended green features are summarized below:-
Table 2.2 Initial Ideas on Green Features for YLSEPP
|
Green Features
|
Opportunities
|
|
Green roof
|
Relatively large areas of green roofs could be implemented at most
treatment facility buildings as most facilities are housed within buildings.
|
|
Renewable energy
|
Biogas from food waste co-digestion
will be utilized by Combined Heat and Power Units for generation of heat and power
as alternative energy source for plant operation.
Photovoltaics (PV) system will be provided at building rooftops.
|
|
Non-potable internal effluent reuse
|
Sewage will undergo tertiary treatment in YLSEPP. The current design
effluent quality standard is generally good enough for direct reuse for
non-potable use within YLSEPP. The treated effluent will be used as polymer
preparations within the treatment process and via distribution systems
without any human contact.
|
|
Materials
|
Selecting materials with lowest environmental impacts, from
local/regional sources, sustainable sources.
Provisions for porous pavement to limit the surface runoff.
|
2.8.1
Consideration of Site Formation
Works
2.8.1.1 The environmental implication of site formation work for both land
OU3.3 and G3.1 are covered in the EIA report no. AEIAR-215/2017 – Housing Sites
in Yuen Long South. As such, the consideration of alternative site formation
method is not included in this EIA report. Land OU3.3 and G3.1 refers to the
lot of land allocated in YLS DA’s revised RODP.
2.8.1.2 However, it is noted that the site formation level for the land G3.1
as shown in the EIA report no. AEIAR-215/2017 (i.e. 31.0 mPD)
will be changed to suit the latest proposed site formation level of YLSEPP.
Thus, an environmental review should be conducted by others (e.g. CEDD) to
assess the environmental impact resulted from this change.”
2.8.2
Consideration of Foundation
Methods
2.8.2.1 The foundation options are highly dependent on the ground
conditions. According to the available ground information, the site is generally
overlaid by fill, alluvium clay, completely decomposed granite (CDG) and
slightly/moderately decomposed granite (S/MDG). In considering the geotechnical
feasibility, the following foundation methods have been considered:-
·
Pre-bored socketed steel H
piles (for heavy-duty structures only)
·
Percussive piles
·
Shallow foundation (for
light-duty structures)
2.8.2.2 A comparison of individual foundation methods and its application
are presented in the below Table 2.3.
2.8.2.3 Since the proposed boundary of YLSEPP is currently located within
private land lots with numerous existing private structures, the area assessable
for ground investigation works is limited. As such, the technical feasibility
of different foundation options should be further reviewed at a later project
stage when more ground information can be gathered.
Light-duty
Structures
2.8.2.4 Piling foundation and shallow foundation are both the feasible
foundation options for light-duty structures in YLSEPP. In order to minimize
the disturbance to adjacent environment in terms of noise and vibration,
shallow foundation is adopted for light-duty structure to reduce the
environmental implication of the project during construction stage.
Heavy-duty
Structures
2.8.2.5 Based on the existing drilling record of ground investigation work,
it is considered that both percussive pile and bored pile are the feasible
foundation options for heavy-duty structures within YLSEPP. However, since the
stiffness of underground soil is undesirable for percussive pile, it is not the
preferable option due to anticipated long pile length leading to low economic
efficiency. Meanwhile, the tentative bed rock level is at about 20m below
ground level which allows the adoption of pre-bored socketed steel H piles for
heavy-duty buildings. Therefore, subject to the detailed design of YLSEPP,
bored pile would be the likely foundation options for those heavy structures in
YLSEPP.
2.8.2.6
In addition, it is recognised
that there are existing graves located at the south-western side of YLSEPP.
With this consideration, application of percussive piling will be avoided as
far as practicable to minimize the disturbance to existing structures and
prevent potential public complaint during Project’s construction stage. Bored
pile remains the preferrable foundation option for heavy-duty structures in
YLSEPP since it would generate less vibration and disturbance to the adjacent area.
However, it should also be noted that bored pile has its demerits of requiring
a comparatively longer construction period as well as generating much higher
volume of excavated materials to be disposed which would generate other
environmental impacts.
2.8.3
Construction Methods for Superstructures
2.8.3.1 Superstructures such as administration building, inlet works
building, tertiary treatment building, sludge treatment facilities and other
smaller facilities will be erected. These building structures will be of
typical reinforced concrete construction with architectural features, and
construction works generally include: i) formwork and falsework erection, ii)
rebar fixing, iii) concrete pouring and curing, iv) formwork striking and back
propping, v) installation of building services and vi) installation of large
E&M equipment.
2.8.3.2
Superstructures will adopt bottom-up construction by constructing the
ground floor slabs, beams, columns and walls from the low level and progressing
upwards to roof level.
2.8.3.3
Construction by precast or
prefabrication units will be adopted as far as practicable in order to minimize
the environmental impacts to the surroundings during construction phase.
2.8.4
Construction Sequences and Programme
2.8.4.1 In general, the construction of YLSEPP would be carried out under
the development of housing sites in Yuen Long South, in which construction of
nearby facilities would be carried out concurrently. Therefore, sensitive
receivers are not expected to be found in the vicinity of YLSEPP during
construction stage since it is surrounded by other construction sites.
Moreover, noting that the footprint of YLSEPP is located at and surrounded by
brownfield site (except for the south-western portion which is bounded by
hillside with existing graves), loss of habitats due to construction of this
Project is not anticipated if proper environmental mitigation measures are
adopted.
2.8.4.2 The major works of the Project include foundation works,
superstructure works and E&M equipment installation. Various construction
methods have been considered and the preferred methods have been selected,
taking into account the engineering feasibility, site constraints, programme
and environmental considerations. To minimize the potential ecological
disturbance, noisy activities including percussive piling works, if any, will
be avoided as far as possible.
Continuous fencing will also be erected along the site boundary prior to
the commencement of construction works.
2.8.4.3 With consideration of the flow build-up of YLSEPP, the YLSEPP should
start operating by 2032 to cater the incoming sewage from various catchment
area. The construction of YLSEPP would be implemented in one stage to achieve
the required treatment capacity before the ultimate design year.
2.8.4.4
Two approaches were considered
for planning the construction sequence, which are the concurrent construction
sequence and phased construction sequence. Concurrent construction sequence
involves various construction activities being carried out at the same time,
which can shorten the overall construction duration along with the duration of
resulted environmental impact. Phased construction sequence involves construction
activities being carried out one followed by another, which reduce the magnitude
of maximum cumulative environmental impact generated at the same time. However,
the construction duration would be prolonged. In view of limited construction
time frame, concurrent construction sequence is adopted.
2.8.4.5 Despite concurrent construction sequence would lead to environmental
drawback, the impact on the surrounding community should be minimal since
sensitive receivers are not expected to be found in the vicinity during the
construction of YLSEPP. Besides, compared to any mega development like Yuen
Long South Development Area, the scale of YLSEPP construction is relatively
small, the additional environmental impact generated by choosing concurrent
sequence over phased sequence shall be limited.
2.9
Construction Programme
2.9.1.1
The Project construction works are anticipated to commence in early 2028
with completion of the Project by 2032.
A tentative construction programme for the Project is provided in Appendix 2.1. This programme
provides the basis for the assessments presented in the EIA Report.
2.10.1.1
With a view to assess the cumulative impact of the Project, a list of concurrent
projects is identified based on the available information at the time of
submission of this EIA. It should be
noted that the implementation of individual projects would be subject to further
development and subsequent actions of the respective project proponents.
2.10.1.2
Table 2.4 and Figure 2.4 summarise the concurrent projects that would contribute to the
cumulative environmental impacts during construction and/or operational phase.
2.11.1.1 A public inspection
of the project profile was conducted from 2 to 15 February 2019 and public
comments were received. The main concerns and ways to address are outlined
below and presented in Appendix 2.2:
Main Concerns / Ways to
Address
· Watercourses
and their Riparian Zones Inside or In the Vicinity of the Proposed YLSEPP
The development
of YLSEPP including both the site formation and construction works would not bring
significant adverse impacts on the watercourses and their riparian zones. A
full-scale Ecological Impact Assessment has been conducted in this
Environmental Impact Assessment (EIA) under Section 8.
· Effluent
discharge to Deep Bay
YLSEPP has
adopted the highest sewage treatment standard of tertiary treatment in Hong
Kong. The water quality impact has been assessed in this Environmental Impact
Assessment (EIA) under Section 5.
2.11.1.2 With these preliminary comments and responses, further public
engagements were conducted with relevant stakeholders in 2021 to collect
their views on the project.
Green Groups Liaison
Meeting
2.11.1.3 The Project maintained a continuous communication with Green Groups
and their views were obtained through 3 sessions of online meetings conducted
on 13,14 and 17 May 2021 and one Green Group Liaison Meeting on 22 July 2021. Their
feedback and responses are recorded in Appendix
2.2.
Main Concerns / Ways to
Address
· Natural
Stream at the Southern Boundary of YLSEPP at G3.1
The natural
stream within the site boundary of YLSEPP at G3.1 will be maintained. The
layout and construction work of YLSEPP would not affect such natural stream.
Please refer to the landscape and visual impact assessment in Section 9 of this
EIA, particularly Figure 9.9 the Landscape and Visual Mitigation Plan where the
existing natural stream would be maintained and blended into YLSEPP’s landscape
design.
· Visual
Impact of YLSEPP to Hikers at Tai Lam Country Park
The landscape
and architectural design of YLSEPP has been developed with due considerations
of hikers at the nearby country park to ensure YLSEPP is blending into the
environment. The landscape and visual impact has been assessed in the EIA
report under Section 9.
· Potential Pollution due to the Operation of YLSEPP
YLSEPP has been
designed to minimize the environmental impact and potential pollution. Various
aspects of environmental impact assessment, including air, noise, water, waste
management etc have been conducted in this EIA.