Sewerage and Sewage
Treatment Implications
7.1.1.1
The proposed Yuen Long South (YLS) development scheme will generate
large amount of sewage flows due to the population and employment intake. The expected impact / implication on the
existing and planned sewerage systems and the proposals of sewerage
infrastructure and mitigation measures to support YLS development are discussed
in this chapter.
7.2
Legislation, Standards and
Guidelines
7.2.1.1
The following established principles and guidelines are adopted for
sewage flow estimation, assessment and evaluation of sewerage and sewage
treatment implications of the Project:
·
EPD’s Technical Paper
Guidelines for Estimating Sewage Flows (GESF) for Sewerage Infrastructure
Planning Version 1.0;
·
DSD’s Sewerage Manual Part 1 (Key Planning
Issues and Gravity Collection System) and Part 2 (Pumping Stations and Rising
Mains), Third Edition, May 2013;
·
Hong Kong Planning
Standards and Guidelines (HKPSG); and
·
EIAO-TM Annex 14.
7.3
Methodology and
Design Criteria
7.3.1
Objectives and
Procedures
7.3.1.1
The objectives and procedures of the sewerage impact
assessment are summarized as follows:
·
To conduct a desktop review to assess the
available capacities in the existing sewerage facilities, including committed
works, within and in the vicinity of the Study Area;
·
To derive the sewage flow and load projection
arising from the developments in the Study Area and the connected sewerage
catchment;
·
To examine the potential impact (if any) arising
from new (additional) sewage generation from the proposed development on the
existing sewerage infrastructure; and
·
To identify, evaluate and recommend the
necessary mitigation measures and an appropriate sewage collection, treatment
and disposal proposal for the PDA.
7.3.1.2
The
assessment has been carried out in accordance with the guidelines set out in
EPD’s Technical Paper Guidelines for Estimating Sewage Flows (GESF) for
Sewerage Infrastructure Planning Version 1.0 and DSD’s Sewerage Manual.
7.3.2
Unit Flow Factors
7.3.2.1
The following unit flow factors in Table 7.1 extracted from the
GESF are adopted:
Table 7.1 Unit Flow Factors for Different Land Uses
|
Land
Use as specified in GESF |
Unit
Flow Factor (m3/person/day) |
Remarks |
|
Residential – Public, Private (R1) |
0.19(1) |
Equivalent to RSc, R1(SSF)c, R2(SSF)c &
R1c in RODP |
|
Residential – Private (R2) |
0.27(1) |
Equivalent to R2c in RODP |
|
Residential – Private (R3), (R4) |
0.37(1) |
Equivalent to R3, R4, R5, R6 & OU(MU) in RODP |
|
Schools (E) |
0.04(2) for Students 0.08(2) for
Staff |
- |
|
Government, Institution or Community (G/IC) |
0.28(2) |
Equivalent to G, OU(RCP&SPS), OU(ESS), OU(STW) in RODP; Sum of
flows due to employee (0.08m3/person/day) & J11 related
activities (0.2m3/person/day) |
|
Commercial (C) |
0.28(2) |
Equivalent to
R1(SSF)c, R2(SSF)c, R2c & OU(MU) in RODP; Sum of flows due to employee
(0.08m3/person/day) & J4 related commercial activities (0.2m3/person/day) |
|
Industrial and Open Storage (I/OS) |
0.18(2) |
Equivalent to OU(SU)1, OU(SU)2, OU(S+W) & OU(OS) in RODP |
|
General Permanent Housing – Yuen Long, Mui Wo (EDA) |
0.25(1) |
The retained
existing developments embraces mixed
residential settlement including existing residential clusters, two existing non-indigenous
villages, three existing religious institutions and committed residential
developments |
Notes: (1) Referenced
from EPD GESF Table T-1
(2)
Referenced from EPD GESF Table T-2
7.3.3
Peaking
Factors
7.3.3.1
Peaking factors cater for seasonal/diurnal fluctuation
and normal amount of infiltration and inflow. The peaking factors shall be
adopted in accordance with EPD’s GESF as shown in Table 7.2.
7.3.3.2
Under normal condition, peaking factors (excluding stormwater allowance) are applicable to planning sewerage
facilities receiving flow from new upstream sewerage systems which essentially
have no misconnections and defects for infiltration. If the service conditions
of the upstream sewerage systems for the planning horizons under consideration
are unclear, peaking factors (including stormwater
allowance) shall be used. For design purpose, the peaking factors (including stormwater allowance) will be used.
Table 7.2 Peaking Factors for Various
Population Ranges
|
Population Range |
Peaking Factor (including stormwater allowance) for
facility with existing upstream sewerage |
Peaking Factor (excluding stormwater allowance) for facility with new upstream sewerage |
|
Sewers |
||
|
< 1,000 |
8 |
6 |
|
1,000 – 5,000 |
6 |
5 |
|
5,000 – 10,000 |
5 |
4 |
|
10,000 – 50,000 |
4 |
3 |
|
> 50,000 |
Max (7.3/N0.15, 2.4) |
Max (6/N0.175, 1.6) |
|
Sewage Treatment Works, Preliminary
Treatment Works and Pumping Stations |
||
|
< 10,000 |
4 |
3 |
|
10,000 – 25,000 |
3.5 |
2.5 |
|
25,000 – 50,000 |
3 |
2 |
|
> 50,000 |
Max (3.9/N0.065, 2.4) |
Max (2.6/N0.065, 1.6) |
Notes:
Referenced
from EPD GESF Table T-5
N = Contributing
population in thousands.
7.3.4
Unit
Load Factors
7.3.4.1
The
global unit load factors used to estimate the sewage loading from the proposed
developments are referred in Table 4 of the Sewerage Manual Part 1 as listed in
Table 7.3.
Table 7.3 Unit Load Factors for Different Land Uses
|
Trades |
Unit |
SS (kg/d) |
BOD (kg/d) |
COD (kg/d) |
TKN (kg/d) |
NH3-N (kg/d) |
E. coli (no./d) |
|
Domestic
(Residential) |
Person |
0.04 |
0.042 |
0.09 |
8.5x10-3 |
5 x10-3 |
4.3x1010 |
|
Commercial,
G/IC and Industrial (1) |
Employee |
0.059 |
0.087 |
0.173 |
9.2x10-3 |
4.8 x10-3 |
3.5x1010 |
|
Schools (2) |
Student |
0.034 |
0.034 |
0.07 |
6.7x10-3 |
4 x10-3 |
3.5x1010 |
Notes:
Referenced from Sewerage Manual Part 1 Table 4
(1) Unit Load Factors (Commercial, G/IC and
Industrial) is the sum of Unit Load Factors for commercial activities and
employed population.
(2) Unit Load Factors of Schools is
the Unit Load Factor of employed population.
7.3.5.1
There are a number of planned, on-going and completed sewerage
improvement works and sewage treatment projects within and in the vicinity of
the Study Area, as summarized in Table
7.4.
Table 7.4 Interfacing
Projects
|
Item
No. / Project Ref. |
Name
of Project |
Actual
or Programmed Completion
Date |
|
CE37/2012(DS) |
Improvement of Yuen Long Town Nullah (Town
Centre Section) – Stage 1 Improvement Works – D&C |
2026 (Programmed
Completion Date) |
|
DC/2013/10 (PWP
Item No.4411DS) |
“Design, Build and Operate San Wai Sewage Treatment Works – Phase 1” |
Sept
2020 (Programmed
Completion Date) |
7.4
Existing and Planned Sewage Infrastructure
7.4.1
Existing
Sewerage System
7.4.1.1
The YLS PDA is located within the sewerage catchment of San Wai
Sewage Treatment Works (SWSTW). The existing sewage generated within the catchment including the PDA is
currently conveyed sequentially to Ha Tsuen Sewage
Pumping Station (HTSPS) and
SWSTW for treatment. The effluent from SWSTW is then transported to the Northwest
New Territories effluent tunnel and the Urmston Road submarine outfall for deep
discharge to the North Western Water Control Zone (WCZ).
7.4.1.2
As
per DSD’s Agreement No. CE 43/2007 (DS) “Yuen Long and Kam
Tin sewerage treatment upgrade – Upgrading of San Wai
Sewage Treatment Works - Investigation, Design and Construction”,
HTSPS and SWSTW have existing capacities of 164,000m3/day (ADWF),
while the capacity of the Urmston
Road submarine outfall is 295,000m3/day. With further reference to
DSD’s Contract No. DC/2013/10, namely “Design, Build and Operate San Wai
Sewage Treatment Works – Phase 1” which has sought
EIA approval, the treatment capacity of the existing SWSTW is being upgraded to
7.4.1.3
At
close proximity to YLS PDA is the
sewerage catchment of Yuen Long Sewerage Treatment Works (YLSTW), covering the urban area
of Yuen Long and Kam Tin which provides sewage
treatment before discharge to Deep Bay in the north.
7.4.1.4
The YLSTW
is a secondary treatment works with a design capacity of 70,000m3/day
(ADWF) and a peaking factor of 3.
7.4.1.5
With reference to
DSD’s drainage record plans, the existing sewerage systems in and around the
PDA is shown on Figure 7.2 and described below:
·
For Tong Yan San Tsuen
(TYST), an existing public sewerage system collects sewage generated in TYST to
SWSTW. Sewage flows from
the western part of TYST is conveyed
in an east to west direction towards SWSTW via HTSPS, while the sewage flows from the eastern
portion is conveyed via the Ping
Shun Street SPS located immediately south
of the West Rail Long Ping Station.
·
For the eastern side of the PDA along Kung Um
Road, there is an intermediate Lung Tin Sewage Pumping
Station (LTSPS) situated at the junction of Yuen Long Highway and Kung Um Road,
transferring the sewage
collected from (i) the
existing sewers along Sham Chung Road and (ii) the planned sewers constructed under PWP Item No. 368DS to
the sewerage network leading to HTSPS and eventually to SWSTW. Currently, there is no
existing public sewerage system within eastern side of the PDA along Kung Um
Road and the sewage generated from the existing villages and open storage areas within these parts of the PDA are believed to be generally disposed of by means of private sewerage system
(i.e. septic tanks and soakaway
systems) or expedient
discharge to the Yuen Long Main Nullah.
7.4.1.6
A Yuen Long South trunk
sewerage
network under PWP Item No. 368DS and DSD’s Contract
No. DC/2009/08 “Construction of Yuen Long South Branch Sewers and Expansion of Ha Tsuen
Sewage Pumping Station” was
completed in Mar 2013,
which comprises of 4 nos. of
DN300-DN450 branch sewers along Kiu Hing Road, Shui Tsiu San Tsuen Road, Tai Tong
Road and Tai Shu Ha Road East and a new SPS at Shui Tsiu San Tsuen
Road as shown in Figure 7.2. It aims to discharge the sewage collected from unsewered
areas in Shap Pat Heung to the existing DN900 trunk
sewer along Sham Chung Road and subsequently towards SWSTW for treatment and
disposal.
7.4.2
Planned
Sewerage System
Yuen Long South Branch Sewerage
7.4.2.1
A planned Yuen
Long South branch sewerage system to collect the
sewage generated from Shap Pat Heung and Tai Tong Tsuen was being constructed for completion in 2016 under PWP Item Nos. 384DS and DSD’s Contract
No. DC/2012/05. The works comprise the construction of
gravity sewers for 6 unsewered areas, namely Nam Pin Wai,
Sai Pin Wai, Tsoi Uk Tsuen,
Wong Uk Tsuen, Ying Lung Wai and Tai Tong Tsuen, among
which the branch sewer in Tai Tong Tsuen as shown in Figure 7.2 was connecting
to the planned Yuen Long South trunk sewer constructed under
Contract No. DC/2009/08 mentioned above.
Upgrading of San Wai
Sewage Treatment Works
7.4.2.2
According to DSD’s Contract No. DC/2013/10
“Design, Build and Operate San Wai Sewage Treatment
Works – Phase 1” and the information from EPD, the upgrading of SWSTW will
increase its treatment capacity to an ADWF of 200,000m3/day in 2021
to meet the future population growth in the sewerage catchment and improve the
effluent quality through promoting its treatment level to chemically enhanced
primary treatment (CEPT) with ultraviolet disinfection. It is understood from EPD that the current proposal
for upgrading SWSTW has not considered the YLS PDA recommended under this
Study.
7.4.2.3
Although the Hung Shui Kiu New Development Area
(HSK NDA) are located within the sewerage catchment of SWSTW which is being
upgraded, according to information provided from CEDD, a new local STW in HSK
NDA will be provided to handle sewage generated from the NDA itself.
Yuen Long Effluent Polishing Plant
7.4.2.4
DSD is
implementing an upgrading works to increase the treatment capacity of YLSTW
from 70 000 m3/day to 150 000 m3/day while to
concurrently upgrade the YLSTW to an effluent polishing plant with tertiary
treatment level under CE 3/2015 (DS), Yuen Long Effluent Polishing Plant –
Investigation, Design and Construction. The project has been commenced in May
2016 and the anticipated date of completion is Nov 2028.
Improvement Works of Yuen Long Town Nullah (Town Centre Section)
7.4.2.5
DSD is
implementing the Rehabilitation of Yuen Long Nullah
(Town Centre Section) project under CE 37/2012 (DS), Improvement of Yuen Long Town Nullah
(Town Centre Section) – Stage 1 Improvement Works – Design and Construction. These include the
construction of a dry weather flow (DWF) interception system, scenic water
supply system and beautification works at the town centre section of Yuen Long Main Nullah. The dry weather flow interception system,
which will construct new DWF interceptors along Yuen Long Nullah
to convey flows downstream to YLSTW. The entire project is anticipated for
completion in 2020.
Other Planned Development with associated Sewerage
Works
7.4.2.6
There are also
numerous concurrent development projects within the vicinity of the Project
which involved associated sewerage works for discharging their sewage to
respective STW for treatment and disposal. These ongoing
developments include:
· CEDD’s Engineering
Works at Wang Chau, Yuen Long (Formerly, HKHA’s
Public Housing Development at Wang Chau);
· PlanD’s Housing Development at Kam Tin South MTRC West Rail Kam Sheung Road Station and Pat
Heung Maintenance Depot;
· Various development by private
developers;
· Sewage demands arising from
existing demands of Yuen Long STW Catchment, planned growth through the year of
population intake;
· Sewage demands
arising from the provision of public sewerage for unsewered
villages in Yuen Long areas; and
· Sewage demands arising from New
Territories North Development (i.e. Lok Ma Chau PDA and Ngau Tam Mei PDA and
San Tin area).
· CEDD and PlanD’s HSK NDA;
· Housing Development
in Tan Kwai Tsuen, Lam Tei; and
· Various development by private
developers.
7.5
Estimation of Sewage Flows and Loads
7.5.1
Sewage Flows
Estimation
Sewage Flows Generation from YLS PDA
7.5.1.1
The population intake for YLS will commence from 2027 until 2038 with
total population of about 88,000 people and employment of 10,500 people.
7.5.1.2
As per the methodology and technical approach described
in Section 7.3, the sewage flows generated from the YLS PDA at 2038 are estimated to be about 23,635 m3/day (ADWF) which is summarised in Table 7.5.
Table 7.5 Sewage Estimation for YLS PDA
|
Land
Use Type |
Parameter |
Remarks |
|
Residential |
||
|
Public Housing (RS,
R1(SSF), R2(SSF)) |
||
|
Population |
53,100 |
From Proposed Development Schedule |
|
Unit Flow Factor (m3/person/day) |
0.190 |
Table T-1, EPD's GESF |
|
ADWF (m3/day) |
10,089 |
|
|
Private Housing (R1c, R2, R2c, R3, R4, R5, R6, OU(MU)) |
||
|
Population (R1c) |
2,100 |
From Proposed Development Schedule |
|
Unit Flow Factor (m3/person/day) |
0.190 |
Table T-1, EPD's GESF |
|
ADWF (m3/day) |
399 |
|
|
Population (R2, R2c) |
15,900 |
From Proposed Development Schedule |
|
Unit Flow Factor (m3/person/day) |
0.270 |
Table T-1, EPD's GESF |
|
ADWF (m3/day) |
4,293 |
|
|
Population (R3, R4, R5, R6,
OU(MU)) |
14,400 |
From Proposed Development Schedule |
|
Unit Flow Factor (m3/person/day) |
0.370 |
Table T-1, EPD's GESF |
|
ADWF (m3/day) |
5,328 |
|
|
Population (EDA) |
2340 |
Existing Residential Population |
|
Unit Flow Factor (m3/person/day) |
0.250 |
Table T-1, EPD's GESF |
|
ADWF (m3/day) |
585 |
|
|
Education - Schools (E) |
||
|
No. of Schools |
8 |
Primary Schools |
|
Students per School |
765 |
Assuming each school has 30 classrooms with each
classroom 25.5 persons. Table 4, Chapter 3, HKPSG |
|
No. of Students |
6,120 |
|
|
Unit Flow Factor (m3/person/day) |
0.04 |
Table T-2, EPD's GESF |
|
No. of Staff |
504 |
From Key Development Parameters |
|
Unit Flow Factor (m3/person/day) |
0.08 |
Table T-2, EPD's GESF |
|
ADWF (m3/day) |
285 |
|
|
Government/Institution Community |
||
|
Community, Social Services (J11) |
||
|
No. of Employees |
744 |
From Key Development Parameters |
|
Unit Flow Factor (m3/employee/day) |
0.280 |
J11, Table T-2, EPD's GESF |
|
ADWF (m3/day) |
208 |
|
|
Public Utilities
(J2) |
|
|
|
No. of Employees |
160 |
From Key Development Parameters |
|
Unit Flow Factor (m3/employee/day) |
0.330 |
J2, Table T-2, EPD's GESF |
|
ADWF (m3/day) |
53 |
|
|
Commercial |
||
|
No. of Employees |
7,640 |
From Key Development Parameters |
|
Unit Flow Factor (m3/employee/day) |
0.280 |
J4, Table T-2, EPD's GESF |
|
ADWF (m3/day) |
2,139 |
|
|
Storage and Workshop + Open Storage |
||
|
No. of Employees |
1,421 |
From Key Development Parameters |
|
Unit Flow Factor (m3/employee/day) |
0.180 |
J3, Table T-2, EPD's GESF |
|
ADWF (m3/day) |
256 |
|
|
Summary |
||
|
Total ADWF (m3/day) |
23,635 |
|
Notes:
ADWF = Development Parameter x Unit Flow Factor
7.5.2
Pollutant
Loads Estimation
7.5.2.1
As per the methodology in Section 7.3 and the unit load factors in Table 7.3, estimation of pollutant loads arising from the proposed
PDA is detailed in Table 7.6.
Table 7.6 Projected Pollutant Loads for YLS PDA
|
Trades |
Development Parameter |
SS |
BOD |
COD |
TKN |
NH3-N |
E. coli |
|
(kg/d) |
(kg/d) |
(kg/d) |
(kg/d) |
(kg/d) |
(no./d) |
||
|
Domestic (Residential) |
87,840(1) |
3,514 |
3,689 |
7,906 |
747 |
439 |
37.8 x1014 |
|
Commercial, G/IC &
Industrial |
9965(2) |
588 |
867 |
1,724 |
92 |
48 |
3.5 x1014 |
|
Schools |
6624(3) |
225 |
225 |
464 |
44 |
26 |
2.3 x1014 |
|
Total |
|
4,327 |
4,781 |
10,093 |
883 |
513 |
44 x1014 |
Notes:
(1)
YLS PDA total
residential population
(2)
YLS PDA total
employment
(3)
YLS PDA total
students and staff
Pollutant Load = Development Parameter x Unit
Load Factor (Table 7.3)
7.5.2.2
With
the total projected ADWF of 23,635m3/day
and the projected pollutant loads as shown in Table 7.6, the average sewage concentration is estimated and shown
on Table 7.7.
Table 7.7 Projected
Pollutant Concentrations
|
Pollutant Concentration |
|||||
|
SS |
BOD |
COD |
TKN |
NH3-N |
E. coli |
|
(mg/L) |
(mg/L) |
(mg/L) |
(mg/L) |
(mg/L) |
(no./100ml) |
|
183.1 |
202.3 |
427.1 |
37.4 |
21.7 |
1.84 x107 |
Note:
Pollutant Concentration = Pollutant Load (Table
7.6) ÷ Total ADWF
7.6
Potential Impacts
to Sewerage systems and Recommended Mitigation Measures
7.6.1
Constraints of Existing Sewerage Catchments
7.6.1.1
Yuen Long, Kam Tin and HSK areas are undergoing rapid developments. Both the SWSTW and YLSTW have capacity
constraints to handle future catchment flows.
Major upgrading works to expand treatment capacities and upgrade
treatment levels of both the SWSTW and YLSTW are being implemented. However, none of the above upgrading sewage
treatment facilities can cater the proposed development of the YLS PDA.
7.6.1.2
To provide
protection to the inland and marine water quality of the Deep Bay WCZ, there is
a stringent requirement for “No Net Increase in Pollution Loading”, which requires the developments within the
Deep Bay catchment areas do not result in an increase in pollution loads to the
inland and marine waters. Sewage from
the sewerage catchment of YLSTW are being discharged to the Deep Bay WCZ after
treatment. If the sewage flows from the
YLS PDA are proposed to be discharged to Deep Bay, this requirement will need
to be complied with. Therefore, potential mitigation
measures and offsetting of pollution credits would be required.
7.6.2
Impacts to Existing and Planned Sewerage Systems
Existing Sewage Treatment Works
7.6.2.1
As
described in Section 7.4, the YLS PDA is located within the SWSTW sewerage
catchment, while at close proximity to the adjoining sewerage catchment of YLSTW.
Due to the presence of
other on-going major developments within these 2 sewerage catchments, the
sewage flow arising from these developments shall be considered collectively.
7.6.2.2
EPD has advised that both the upgraded
capacity of YLSTW (ultimately 150,000m3/day ADWF) and SWSTW (200,000m3/day ADWF) are inadequate to accommodate the flows from their respective sewerage catchments in 2031 and 2034 respectively. To accommodate the additional flow arising
from YLS, a new sewerage
infrastructure (including STW, SPS, sewers and rising mains) will
be required as presented in Section 7.6.3.
Existing Lung Tin Sewage Pumping Station
7.6.2.3
The existing Lung Tin Sewage Pumping Station
(LTSPS) at eastern side of the PDA along Kung Um Road, which transfers the sewage collected from (i) the existing DN900 sewers along Sham
Chung Road and (ii) the planned
sewers constructed under PWP Item No. 368DS to the sewerage network leading to
HTSPS and eventually SWSTW has been considered in the development of the RODP. As shown in Figure
7.1, the existing LTSPS will be retained in its present
location and unaffected by the proposed YLS PDA.
Existing Sewers at Tong Yan San Tsuen
7.6.2.4
The existing areas
at TYST are served by an existing public sewerage system of sewer sizes between
225mm and 600mm. To suit the new
development layout at TYST, these existing sewers would be replaced by the proposed sewerage system
of YLS.
7.6.3
Recommended Sewage Treatment and Disposal Strategy
Sewage Treatment by
On-site Sewage Treatment Works to Tertiary Level
7.6.3.1
Following numerous meetings,
liaison and coordination amongst CEDD, PlanD, EPD and DSD, it is proposed that the sewage generated
from the YLS PDA will be discharged to a new local STW, namely YLS STW for on-site
treatment, which is located to the southern end of the YLS PDA
(see Figure 7.1). The on-site STW will be designed to achieve a
treatment capacity of 24,000m3/day in ADWF and up to tertiary level
treatment standard to enable for reuse of treated sewage effluent (TSE) as
reclaimed water while the rest of sewage effluent is suggested to be treated by
secondary plus treatment (with UV disinfection and 75% nitrogen removal) and
discharged to the Urmston Road submarine outfall.
Reuse of TSE as Reclaimed
Water Reuse in YLS PDA, Existing Area and Adjacent Developments
7.6.3.2
The new STW will provide tertiary sewage
treatment to produce TSE of high quality for conversion into reclaimed water
that is suitable for reuse within the PDA and adjacent developments for
non-potable uses such as toilet flushing.
7.6.3.3 Moreover, as part of the urban design of the YLS
PDA, the existing Yuen Long Nullah (with some
sections to be partially decked to meet the traffic demand) adjacent to the
eastern side of the PDA along Kung Um Road will be revitalised by application
of both soft and hard landscaping treatments in order to improve its aesthetics
and to promote a water friendly culture within the YLS PDA. A small amount of
TSE discharged to Yuen Long Nullah can be regarded as
a continuous scenic water source as part of the waterscape provided within the
revitalised nullah.
7.6.3.4
However, as the TSE generated would be
excessive that it cannot be totally reused by YLS PDA itself, the tertiary TSE should be properly considered in view of
the no net increase in the pollution loading requirement of the Deep Bay WCZ.
7.6.3.5
As there are also some adjacent developments,
such as Long Bin, Tan Kwai Tsuen
and Tin Shui Wai, liaison
with the relevant project proponents and government departments regarding
adopting reclaimed water for non-potable uses (mainly for toilet flushing) are
being conducted.
Pollution Credit Offsetting by Existing Livestock Farms
7.6.3.6 There are currently 6 number of existing pig and
chicken farms scattered in YLS PDA and it is proposed that 5 of these livestock
farms would be removed. As it is understood that AFCD are implementing
Livestock Keeping License for operation of these livestock farms, the farm
operator/ owners shall treat their livestock waste to meet minimum discharge standards
as stipulated in the Waste Disposal (Livestock Waste) Regulations (Chapter
354A).
7.6.3.7 Hence the removal of the pollution loadings
generated from these livestock farms are identified as one of the distinct source
contributing to the pollution credit in Deep Bay. With some of these existing
livestock farms being removed by YLS, their associated pollution credit can
hence be released and offset by equivalent amount of pollution loading being
discharged from YLS STW to Yuen Long Nullah.
Further Polishing
of TSE by Reedbed
7.6.3.8 In the previous project namely Planning
and Engineering Study on Development of Lok
Ma Chau Loop and existing Tin Shui Wai Wetland Park, it has
been proven that reedbed is capable to further polish
the TSE by means of biological breakdown process. There are approximate 4 ha areas adjacent to
YLS STW in the RODP as shown in Figure 7.1, which would be developed into a reedbed. A
small amount of TSE from YLS STW will pass through the reedbed for further polishing before discharging into Yuen
Long Nullah.
7.6.3.9 However, due to the uncertain pollution removal
efficiency of the reedbed, its polishing function
would only be regarded as an extra provision and would not be considered in the
pollution load offset balance as shown in the following sections and Appendix 7.1.
Surplus TSE Export Disposal
Strategy
7.6.3.10
Having made reference to tertiary TSE
disposal strategy in HSK NDA, the surplus TSE will be discharged from YLS STW
to existing inlet chamber of the NWNT effluent tunnel at San Wai
for discharging to the Urmston Road submarine outfall for deep discharge to the
North Western WCZ.
7.6.3.11
With reference to the Environmental Impact
Assessment report approved in 2003 for Upgrading and Expansion of SWSTW,
effluent discharge of 246,000 m3/day from the upgraded SWSTW (Phase
1) equipped with Chemical-enhanced Preliminary Treatment (CEPT) with
disinfection, are within an acceptable level. Further study on approved
EP-464/2013 “Upgrading and expansion of
San Wai Sewage Treatment Works and Expansion of Ha Tsuen Pumping Station” and approved EIAR-203/2016 “Hung Shui Kiu New Development Area”, total quantities of effluent
discharge via the Urmston Road submarine outfall can be summarised in Table 7.8.
Table 7.8 Summary of Effluent Discharge from HSK NDA and SWSTW – Phase 1
|
SWSTW – Phase 1 with CEPT (m3 per day) |
Total TSE from HSK NDA to be discharged (with Secondary Plus Treatment) |
Discharge to the Urmston Road Submarine
Outfall (m3 per day) |
|
200,000 |
45,696 |
200,000
(CEPT)
+ 45,696 (Secondary Plus) |
Note: Secondary Plus Treatment is a combination of secondary treatment
with 75% nitrogen removal and UV disinfection.
7.6.3.12
Further studies have been carried out on
pollution loading of combined effluents and hydraulic capacity assessment of
the Urmston Road submarine outfall and the NWNT effluent tunnel. The summarised
results are shown in Table 7.9 and Table 7.10. It can be seen that
pollution loading from combined effluent of SWSTW, HSK STW and YLS STW are
within the EIA requirements and flow rate of combined effluent are within the
hydraulic capacity of the NWNT effluent tunnel and the Urmston Road submarine
outfall. Figure
7.5 shows the proposed sewer alignment from YLS
STW to inlet chamber of the NWNT effluent tunnel at San Wai
for discharging to the Urmston Road submarine outfall.
Table
7.9 Summary of Pollution Loading of Combined Effluents
|
Parameter |
Unit |
SWSTW |
HSK STW |
YLS STW |
Total Combined Effluent |
EIA No. 072/2003 |
|
E. Coli |
cfu/day |
4.00E+12 |
4.57E+10 |
1.00E+10 |
4.06E+12 |
4.92E+12 |
|
Total residual chlorine |
kg/day |
- |
- |
- |
- |
- |
|
Dissolved oxygen |
kg/day |
- |
- |
- |
|
- |
|
TSS |
kg/day |
11,000 |
1,371 |
300 |
12,671 |
13,530 |
|
Colour |
HU |
- |
- |
- |
- |
- |
|
Turbidity |
NTU |
- |
- |
- |
- |
- |
|
pH |
- |
- |
- |
- |
- |
- |
|
Threshold odour
number |
TON |
- |
- |
- |
- |
- |
|
BOD5 |
kg/day |
20,000 |
914 |
200 |
21,114 |
24,600 |
|
Ammonia nitrogen |
kg/day |
5,000 |
91 |
20 |
5,111 |
6,150 |
|
Total Inorganic Nitrogen (TIN) |
kg/day |
5,000 |
366 |
80 |
5,446 |
6,150 |
|
Synthetic detergents |
kg/day |
- |
- |
- |
- |
- |
|
Temperature |
degC |
- |
- |
- |
- |
- |
|
Chloride |
kg/day |
- |
- |
- |
- |
- |
Table
7.10 Summary of Hydraulic Capability
|
Effluent (m3/day) |
|
|
SWSTW
(CEPT)(1) |
200,000 |
|
HSK
STW (Secondary+)(2) |
45,696 |
|
YLS
STW (Secondary+) |
10,000 |
|
Total
Combined Effluent |
255,696 |
|
Capacity
of the NWNT Effluent Tunnel |
400,000 |
|
Capacity
of the Urmston Road Submarine
Outfall |
295,000 |
Note:
(1) Ref: Appendix 5A, approved EIA report for the Upgrading and
Expansion of San Wai STW and the Expansion of Ha Tsuen PS.
(2) Ref: Appendix 5.2, approved EIA report of HSK NDA
7.6.3.13
Alternatively, WSD is implementing a
saltwater supply system in the existing NWNT area including Tin Shui Wai and Yuen Long Town area.
It can be considered to extend the use of surplus reclaimed water as a supply
source to the new saltwater supply system. Liaison with WSD and Department of
Health (DH) on transferring surplus reclaimed water to Tan Kwai
Tsuen (TKT) saltwater service reservoir for being
utilised by its supply zone is being conducted concurrently.
Overall Sewage Treatment and Disposal
Strategy
7.6.3.14
As shown in the schematic Diagram 1, the
generated TSE volume from YLS STW would be 21,272m3/day
by assuming 90% conversion rate from the sewage, taking into account of the
sludge volume, system loss from the treatment process and transmission loss in
the distribution system.
Diagram 1 – Schematic Diagram of the overall sewage treatment and disposal strategy
Note: (1) With reference of EIA Report for Hung Shui Kiu New Development Area Planning and
Engineering Study – Investigation, minimum 90% of treated sewage can be reused.
7.6.3.15
As per the latest
development parameter, there will be 6,660m3/day
of water demand for toilet flushing in YLS that could be supplied by reclaimed
water (water demand calculation
for flushing toilet in YLS PDA can be referred to Appendix 7.2). For other potential reclaimed
water use in YLS such as
landscape irrigation, water make-up for water features and etc., as their water
demand are subject to seasonal variation and acceptance of other government
departments, they will not be considered in the overall reclaimed
water reuse plan at this
stage until further details of these infrastructures and acceptance by the
corresponding government has been sought.
7.6.3.16
As advised by the
project proponents of Long Bin and Tan Kwai Tsuen Development, their flushing water demand were
estimated to about 4,292m3/day (2,375m3/day
from Long Bin Development and 1,917m3/day from Tan Kwai Tsuen Development) in total.
As such, part of the reclaimed water, i.e. 4,292m3/day could be discharged to these development
for their flushing uses.
7.6.3.17
As
indicated in the schematic diagram above, a small amount of TSE
of about 570 m3/day could be
discharged into Yuen Long Nullah. This amount of flow
was calculated using in a conservative approach, thus the overall pollution to
Deep Bay is anticipated to be less than existing situation. The pollution
loading of 570m3/day of TSE being discharged
could be offset by the pollution credit gained from those existing livestock
farms (including 3 pig farms and 2 chicken farms) removed due to the
development of YLS PDA, which comply with the requirement for “No Net Increase
in Pollution Loading” into Deep Bay. For
more details including the quantification of pollution credit from existing
livestock farms and TSE being discharged
can refer to Appendix 7.1. On the contrary, if surplus TSE is not proposed to
be discharged to Yuen Long Nullah after removal of
living livestock farms, it is considered that it will result in an improvement
to the water quality of the downstream Deep Water Bay. Liaison with EPD on
further review the actual pollution loading generated from these livestock
farms and the exact of TSE to be discharged
to Yuen Long Nullah in compliance with the
requirement of “No Net Increase in Pollution Loading” into Deep Bay is being
conducted.
7.6.3.18
As mentioned in Section 7.6.3.10, the surplus
TSE can be conveyed to the existing inlet chamber of the NWNT effluent tunnel
at San Wai for discharging to the Urmston Road
submarine outfall at North Western WCZ.
The surplus TSE is suggested to be treated by secondary
plus treatment (with UV disinfection and 75% nitrogen removal).
7.6.3.19
Alternatively, liaison with WSD and DH is
being conducted such that 9,750m3/day
of reclaimed water would be
transferred to TKT saltwater service reservoir for being utilised by
its supply zone or modifying WSD existing saltwater supply system in existing
Tin Shui Wai area to
replace saltwater by reclaimed water for flushing use. However, DH expressed
their concern on potential associated human health risks, as they noted that
the existing salt water supply system also supplies to some existing
fire-hydrants in its supply zone, which could potentially be another route of
human exposure to reclaimed water. Further negotiation is ongoing
with WSD and DH concurrently.
7.6.4
Proposed
Sewerage System for YLS Development
7.6.4.1
A
new public sewerage network will be proposed to convey sewage flow from various
lots within YLS PDA to the proposed on-site STW for treatment and disposal. Figure
7.3 shows the preliminary layout of the proposed sewerage system
for the YLS PDA. The majority of alignment of the proposed trunk sewerage network
is either along the proposed roads or through amenities area. Drainage reserves
will be required for laying sewer pipes outside the roads such as through
amenities areas or open spaces.
7.6.4.2
Gravity
sewerage system will be provided as far as possible. To overcome topographical
constraints and to avoid excessive sewer depth, two new pumping
stations (SPSs) are proposed to convey the sewage flows to the proposed YLS
STW. The locations and catchments of the proposed SPSs are shown in Figure 7.3.
7.6.4.3
Table 7.11 to Table 7.13 show the accumulated projected sewage flow, peaking
factors and accumulated projected peak flows respectively for SPS1 and SPS2 for
Stage 1 to Stage 4. The breakdown calculation of sewage flow can be referred to
Appendix 7.3.
Table 7.11 Estimated Sewage Flows For Stage 1 to Stage 4
|
Facilities |
Accumulated Sewage Flow (m3/day) |
|||
|
Stage 1 |
Stage 2 |
Stage 3 |
Stage 4 |
|
|
SPS1 |
597 |
900 |
900 |
4,754 |
|
SPS2 |
2,534 |
8,643 |
19,773 |
23,627* |
Note:
* Small part of catchment will directly flow to STW by gravity pipe and thus
the accumulated sewage flow would not be identical to the total sewage flow as
shown in Diagram 1
Table 7.12 Peaking Factors for Proposed SPS
|
Facilities |
Peaking Factors (PF) |
|||
|
Stage 1 |
Stage 2 |
Stage 3 |
Stage 4 |
|
|
SPS1 |
4.0 |
4.0 |
4.0 |
3.5 |
|
SPS2 |
4.0 |
3.0 |
2.4 |
2.4 |
Table 7.13 Estimated Peak Flows for Stage 1 to Stage 4
|
Facilities |
Accumulated Projected Peak
Sewage Flow (l/s) |
|||
|
Stage 1 |
Stage 2 |
Stage 3 |
Stage 4 |
|
|
SPS1 |
28 |
42 |
42 |
193 |
|
SPS2 |
117 |
300 |
549 |
656 |
7.6.4.4
A network of water mains and a service
reservoir will be constructed for the reuse of TSE as reclaimed
water for non-potable uses such as toilet flushing within YLS PDA and adjacent
developments. The surplus TSE treated by secondary plus treatment can
be conveyed to the existing inlet chamber of the NWNT effluent tunnel at San Wai for discharging to the Urmston Road submarine outfall
at North Western WCZ. Alternatively, all reclaimed water could be treated up to
the proposed reuse quality and then be transferred to TKT saltwater service reservoir for being
utilised by its supply zone, which is under negotiation with WSD and DH.
7.6.4.5
Estimated volume
and target water quality standard of reclaimed water is summarised in Table 7.14 below.
Table 7.14 Estimated Volume of Reclaimed Water and Disposal
of TSE
|
Description |
Volume
(m3/day) |
|
Total
sewage flow |
23,635 |
|
Total
TSE Amount* |
21,272 |
|
Reuse
of TSE as reclaimed water and Discharging to Yuen Long
Nullah (Tertiary Treatment) |
11,522 |
|
Surplus
TSE to be discharged to the Urmston Road submarine outfall (Secondary Plus Treatment) |
9,750 |
Note:
*
With reference of
EIA Report for Hung Shui Kiu
New Development Area Planning and Engineering Study – Investigation, minimum
90% of treated sewage can be reused
7.6.4.6
The
treatment of reclaimed water up to the proposed reuse quality
presented in Table 7.15 will be
provided within the 4-ha footprint of YLS STW, including the chlorine contact
tank, chemical storage, reclaimed water storage and distribution pumps
connecting to the supply pipe network, as detailed in Figure 7.4. Adjacent to the YLS
STW, there is an adjoining reedbed for further
polishing of a small amount of TSE before
discharging to Yuen Long Nullah as mentioned in Section 7.6.3.
Table 7.15 Proposed Water Quality Standards of Reclaimed Water Reuse
|
Water Quality Parameter |
Unit |
Recommended
Standard for Flushing, Landscape Irrigation and DCS Make-up |
|
E. Coli |
cfu/100mL |
Not detectable |
|
Total residual
chlorine (TRC) |
mg/L |
>1 (out of treatment system) >0.2 (at
point-of-use) |
|
Dissolved oxygen (DO) |
mg/L |
>2 |
|
TSS |
mg/L |
<5 |
|
Colour |
HU |
<20 |
|
Turbidity |
NTU |
<5 |
|
pH |
- |
6 – 9 |
|
Threshold odour number (TON) |
TON |
<100 |
|
BOD5 |
mg/L |
< 10 |
|
Ammonia nitrogen |
mg/L |
<1 |
|
Synthetic detergents |
mg/L |
<5 |
Apart from TRC which
has been specified, the water quality standards for all parameters shall be
applied at the point-of-use of the system. HU = Hazen Unit. NTU = Nephelometric Turbidity Unit.
The parameters are
referenced to the target water quality standard adopted in the EIA Report for
Hung Shui Kiu New
Development Area Planning and Engineering Study
Sludge Handling and Disposal Routine
Operation in YLS STW
7.6.4.7
The
sewage sludge, being the by-product from the proposed YLS STW, will be
undergone anaerobic digestion, followed by dewatering in the STW to form sludge
cake. The sludge cake generated will be further transferred via sludge cake
tankers to Sludge Treatment Facility (STF) at Nim Wan
for further treatment. In order to minimise the public nuisance due to the
routine transportation of sludge cake tankers from the STW to Nim Wan, only sludge cake tankers with fully enclosed
container will be adopted for routine operations.
7.6.4.8
Apart
from direct disposal in STF, other alternative will also be explored should the
digestate be of suitable quality, including further
treatment/ polishing/ sanitisation for conversion to other products like
fertilisers or soil conditioner to support local agriculture/ horticulture
industry.
Proposed Staging Plan of YLS Sewerage System
7.6.4.9
The
YLS development will be developed into 4 stages. The proposed stage plan of YLS
Sewerage System to cope with the staged intakes of YLS are elaborated below:
|
|
Stage 1 |
Stage 2 |
Stage 3 |
Stage 4 |
|
Intake
Year |
2027-2029 |
2033 |
2038 |
2038 |
|
Total by
Phase (m3/day) |
2,534 |
6,109 |
11,139 |
3,854 |
|
Cumulative
Total (m3/day) |
2,534 |
8,643 |
19,782 |
23,635 |
7.6.4.10
The above detailed breakdown of sewage flows by
different Stages are calculated and summarised in Appendix
7.3.
7.6.4.11
Stage 1 (Year 2027): it is proposed that the 1,800m3/day sewage flow arising from YLS Stage 1 to be conveyed via the existing
Lung Ting Pumping Station (LTPS) to SWSTW from year 2027 to 2029 as an interim
arrangement. After the commencement of operation of YLS STW in 2030, the 1,800m3/day of sewage generated in the YLS stage 1 will be diverted back to YLS
STW for on-site treatment. In addition, the sewage from existing R5(EDA) will
also be conveyed to YLS STW after commissioning in Year 2030.
The Stage 1 sewerage work arrangement as aforementioned has sought EPD no
objection in principle. More details can be referred to Appendix 7.4.
7.6.4.12
Advance Stage 2 (Year
2030): YLS Stage 2 development will involve
majority part of the proposed sewerage system including the SPS and on-site YLS
STW as shown in Figure 7.3. The SPS and STW can be partially commission to cope with the flow in
Stage 2 with the flexibility for further expansion to cope with the flow in
subsequent stage.
7.6.4.13
Stage 3-4 (Year
2038): the proposed sewerage system for YLS Stage
3-4 developments will involve the construction of all proposed sewerage as
shown in Figure 7.3 and full commissioning of the proposed SPSs and on-site YLS STW.
7.6.5
Contingency
plan for the proposed SPS(s) and YLS STW
·
Twin rising mains would be
provided. Should one of the duty mains be taken out of operation, the remaining
one would still be able to deliver flow;
·
Standby pumps and treatment
facilities would be provided in case of unexpected breakdown of pumping and
treatment facilities such that the standby pumps and treatment facilities could
take over and function to replace the broken pumps and treatment facilities;
and
·
Dual electricity supply or
backup power supply facilities such as diesel generator would be provided in
case of power failure to sustain the function of pumping and treatment
facilities.
7.6.5.2
Under very unlikely extreme scenario, YLS STW and SPSs will still have
emergency discharge to the downstream water receiving body as described below.
Nevertheless, in order to minimise the potential water
quality impact in case of emergency discharge, the future operator will be
required to develop a Contingency Plan in the detailed design stage to deal
with the remote occurrence of emergency discharge. The Contingency Plan
should include the following items.
·
Locations of water bodies
or WSRs in the vicinity of the emergency discharges;
·
A list of relevant
government departments to be informed and to provide assistance in the event of
emergency discharge, including key contact persons and telephone numbers;
·
Reporting procedures
required in the event of emergency discharges; and
·
Procedures listing the most
effective means in rectifying the breakdown of the SPS in order to minimise the
discharge duration.
|
Scenario |
Discharge Route |
Receiving Water Body |
|
|
YLS STW not functioning |
All Raw Sewage -> Reedbed -> Deep Bay |
Deep Bay |
|
|
SPSs not functioning |
All Raw Sewage -> nullahs
near Shan Ha Road and Kiu Hing
Road -> Deep Bay |
Deep Bay |
|
|
The NWNT effluent tunnel / the Urmston Road submarine outfall not in operation (if surplus TSE would discharge to the inlet chamber of the NWNT
effluent tunnel at San Wai for discharging to the Urmston Road submarine outfall) |
Surplus TSE -> Reedbed ->
Deep Bay |
Deep Bay |
|
7.7
Conclusions and Recommendations
7.7.1.1
This chapter has reviewed the existing and planned sewerage
infrastructure within and in the vicinity of the Study Area. This chapter also
collected information of sewage collection, treatment and disposal arrangements
in the YLSTW and SWSTW sewerage catchments.
7.7.1.2
The sewage flows and loads generation from the YLS PDA are estimated and
the proposed sewage treatment and disposal strategy for YLS are accordingly
formulated, which involve discharging the YLS sewage flows by the proposed
sewerage system to a new on-site STW in the southern part of the PDA for
treatment, followed by the reuse of TSE as reclaimed water as non-potable uses
in PDA and adjacent developments.
7.7.1.3
Equivalent pollution loading from tertiary TSE in YLS STW (upon offsetting
by credit gained from the existing livestock farms) could be discharged to Yuen Long Nullah adjacent to the STW with
strict compliance to the requirement for “No Net Increase in Pollution
Loading”, while reedbed is provided to further polish
a small amount of TSE that discharge to Yuen Long Nullah and Deep Bay. The rest of
sewage effluent will be treated by secondary plus treatment and discharge to
the Urmston Road submarine outfall.
7.7.1.4
It should also be noted that the STW and the reclaimed water reuse activity are separate DPs to be
constructed and operated by the future operators. Separate EIA shall be
prepared and submitted to EPD for approval under the EIA Ordinance to apply for
the Environmental Permit.