Unit Load Factor
6.3.6
The global unit
load factors used to estimate the sewage loading from the proposed developments
are referred in Table 4 of the sewage Manual Part 1 (SM1) and are listed in Table 6‑4 below.
Table 6‑4 Global Unit Load
Factors
|
Trades
|
Unit
|
Percentage
(%)
|
SS (kg/d)
|
BOD
(kg/d)
|
COD
(kg/d)
|
TKN
(kg/d)
|
NH3N
(kg/d)
|
E.coli
(no./d)
|
|
Domestic
(Residential)
|
Person
|
100
|
0.04
|
0.042
|
0.09
|
8.5x10-3
|
5x10-3
|
4.3x1010
|
|
Commercial G/IC(1)
|
Employee
|
100
|
0.059
|
0.087
|
0.173
|
9.2 x10-3
|
4.8 x10-3
|
3.5 x1010
|
|
Industrial (R&D
and Adv. Manu)
|
Employee
|
10(2)
|
0.059
|
0.087
|
0.173
|
9.2 x10-3
|
4.8 x10-3
|
3.5 x1010
|
|
Flow
|
40(3)
|
559
(mg/L)
|
452 (mg/L)
|
1130
(mg/L)
|
62
(mg/L)
|
29
(mg/L)
|
3.5 x1010
|
|
flow
|
50(4)
|
800
(mg/L)
|
800 (mg/L)
|
2000
(mg/L)
|
100
(mg/L)
|
52
(mg/L)
|
3.5 x1010
|
|
Schools (5)
|
Student
|
100
|
0.034
|
0.034
|
0.07
|
6.7 x 10-3
|
3 x10-3
|
3.5 x1010
|
Note: References from Sewage Manual Part 1 Table 4
(1) Unit
Load Factors (Commercial, G/IC & Industrial) is the sum of Unit Load
Factors for commercial activities and employed population.
(2) It is
assumed 10% load from Industrial follow Commercial (Employed Population +
Commercial Activities)
(3) It is
assumed 40% load with similar characteristics to measured sewage at Tai Po
Industrial Estate
(4) It is
assumed 50% load could reach upper limits in WPCO TM Table 1
(5) Unit
Load Factors of Schools is based on the number of school students
Effluent
Polishing Plant
6.3.7
It is understood
that there is currently no spare capacity from YLSTW or SWHSTW to handle the
sewage generated from the Project, thus an on-site EPP is required, namely STLMC
EPP for on-site treatment. The on-site EPP will be designed up to tertiary
level treatment standard to enable for reuse of reclaimed water while the rest
of sewage effluent is suggested to discharge to the Deep Bay.
6.3.8
As discussed with
DSD, Moving Bed Biofilm Reactors (MBBR), Aerobic Granular Sludge (AGS) and Membrane
Bioreactor (MBR) sewage treatment process would be considered under the current
stage to achieve the discharge requirement.
6.3.9
It is expected
that EPP undergoes tertiary treatment level which is the highest level of
treatment for polishing the treated effluent from secondary treatment process.
This process typically comprises a combination of physical and biological
processes with the objective of further removing nutrients and suspended solids
in the sewage.
6.3.10
Table 6‑5 below shows the STLMC EPP Effluent
Discharge Standard which is reference to YLEPP standard and approved by the
EPD.
Table 6‑5 Treatment Standard of
STLMC EPP
|
Parameter
|
Unit
|
Design
Standard
|
|
5-day Biochemical
Oxygen Demand (BOD5)
|
mg/L
|
10 (95 percentile)
|
|
Total Suspended Solid
(TSS)
|
mg/L
|
10 (95 percentile)
|
|
Ammonia Nitrogen (NH3-N)
|
mg/L
|
2 (annual average)
|
|
Total Nitrogen (TN)
|
mg/L
|
10 (annual average)
|
|
Total Phosphorus (TP)
|
mg/L
|
1 (annual average)
|
|
Escherichia coli (E.
coli)
|
cfu/100ml
|
1001
(Upper limit)
|
Note: Monthly
geometric mean is adopted
6.3.11
Emergency by-pass
is proposed for the proposed on-site EPP and will convey the raw sewage to the
proposed drainage in case of emergency such as power failure at the proposed
on-site EPP.
6.3.12
The new EPP will
provide tertiary level sewage treatment to produce treated sewage effluent
(TSE) of high quality to the proposed water reclamation plant. The details are
in section 6.4.
6.4.1
The new EPP will
provide tertiary level sewage treatment to produce treated sewage effluent
(TSE) of high quality for conversion into reclaimed water that is suitable for
reuse within the Project for non-potable uses such as toilet flushing and
controlled irrigation. Based on the TSE water quality, the TSE has potential to
be utilized in district cooling system (DCS) make-up water.
6.4.2
Using TSE can
relieve the stress on freshwater supplies used for flushing and irrigation
purposes. Since recycled TSE provides a cost-efficient supply that decrease the
demands and stress on fresh water. Besides, since there are strict water
quality guidelines to control the effluent standard. It can prevent any
potential negative impacts or influence towards the public health and
environment sustainability.
6.4.3
The proposed
water quality standards for the TSE reuse is summarized below in Table 6‑6.
Table 6‑6 Proposed
Water Quality Standards of TSE Reuse
|
Water
Quality Parameter
|
Unit
|
Water
Quality
|
|
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
|
Note:
1.
Apart
from TRC which has been specified, the water quality standards for all
parameters shown above shall be applied at the point-of0use of the system. HU =
Hazen Unit. NTU = Nephelometric Turbidity Unit,
2.
The
parameters are referenced to Technical Specifications on Grey Water Reuse and
Rainwater Harvesting issued by WSD in May 2015, which is also referred to the
target water quality standard adopted in North East New Territories Area
project.
6.4.4
In general, the
following precautionary measures should be adopted for TSE reuse:
·
To avoid cross connection and
hence contamination, all pipes and fittings used for the TSE water supply and
distribution system should be purple in colour for distinguishing them from the
pipes and fittings used for the fresh water supply and distribution systems.
·
Regular checking/inspections of
the TSE supply and distribution systems for possible cross connection to the
fresh water supply and distribution system should be carried out. The use of
non-toxic dye may be adopted in the checking/inspections
·
Warning signs should be
permanently displayed where public access to TSE is possible (except for
toilets) notifying the employees, visitors and the public at large that treated
effluent is being used and is not suitable for drinking.
·
Storage of sodium hypochlorite
solution will be required and this is not a hazardous material. Thus, the
storage is not considered as Potentially Hazardous Installation (PHI)
6.4.5
The usual practice
to distinguish reclaimed water pipe work from potable pipework is by colour
code. Under Demonstration Scheme on Reclaimed Water Uses in the Northern
District, for example, purple/lavender-coloured pipes were used between SWHSTW
to respective user’s premises for easy differentiation from existing pipework.
6.4.6
Apart from that,
proper signage, promotion and education to the general public especially
potential local users of reclaimed water for landscape irrigation shall be
considered and implemented.
6.4.7
The main health
concern with TSE reuse is the small but definite risk of diarrhoeal diseases
associated with accidental ingestion of insufficiently treated TSE. With the
implementation of precautionary measures set out in above, and the adoption of
stringent health-based water quality standards for the TSE, significant
increase in health risk is not expected.
6.4.8
Reuse of TSE
offers following major benefits:
·
Savings in fresh water supply,
thus reduces the water to be diverted from the Dongjing River, and helps to
relief the fresh water stress in downstream Dongjiang River that is often in
needs of fresh water.
·
Savings in energy consumption to
convey the fresh water from the Dongjiang River, thus reduces the carbon footprint
and contributes to the substainabilities of the communities.
·
Savings in fresh water supply system
expansion requirement
·
Reduces the pollution to be discharged
to the environment.
6.4.9
TSE reuse has the
advantage of reducing wastewater discharge in the receiving water bodies thereby
reducing the pollution load to the environment. It also reduces demand on raw
water, which is scarce natural resource deserved for preservation to the
maximum extent practicable
Planned Sewerage Infrastructure
6.5.1
There is no
existing sewerage system within the vicinity of the Project. Further, the Project
fall within the sewage catchment area (SCA) of Yuen Long (YL STW). However,
both nearby Shek Wu Hui STW (SWH STW) or Yuen Long STW (YL STW) would not have
sufficient capacity to cater the additional sewage generated from the Project.
6.6
Estimation of Sewage Discharge
Estimated Sewage
Discharge from the Development
Residential Discharge
6.6.1
Based on the
design parameters and assumptions discussed in section 6.3, the estimated sewage discharge of Residential sites
in the Project is summarized in Table 6‑7.
Table 6‑7 Summary of
Estimated Sewage Discharge in Residential Site
|
Land Use
Type
|
Population
|
Total
ADWF (m3/day)
|
|
Village
|
70
|
20
|
|
Public Housing (R1)
|
104,000
|
19,719
|
|
Private Housing (R2)
|
24,000
|
6,495
|
|
Private Development
in Non-residential Land use (R2)
|
31,000
|
8,497
|
|
Sub-total
|
159,000
|
34,731
|
Note:
1.
The
Residential
accommodation includes OU(I&T)1.1.1, OU(I&T)1.1.2, OU(I&T)1.1.3, OU(MU)2.1.11,
OU(MU)1.2.1 and G.5.11
2.
Total
may not add up due to rounding
Commercial Discharge
6.6.2
Based on the
design parameters and assumptions discussed in section 6.3, the estimated sewage discharge of Residential sites
in the Project is summarized in Table 6‑8 below.
Table 6‑8 Summary of
Estimated Sewage Discharge in Commercial Site
|
Land Use
Type
|
Employment
|
Total
ADWF (m3/day)
|
|
Hotel (J10)
|
1,500
|
2,371
|
|
Advanced
Manufacturing-
Lab Staff(General)
|
22,8001
|
6,391
|
|
Advanced
Manufacturing-
Administration Staff (J6)
|
11,4001
|
913
|
|
R&D Wet Lab Staff
(J2)
3
|
21,5002
|
7,088
|
|
R&D Wet Lab
Administration Staff (J6) 3
|
5,4002
|
430
|
|
R&D Dry Lab Staff
(General)
3
|
43,0002
|
12,028
|
|
R&D Dry Lab
Administration Staff (J6) 3
|
10,7002
|
859
|
|
Retail (J4)
|
9,000
|
2,416
|
|
Dining (J10)
|
2,200
|
3,592
|
|
Entertainment (J11)
|
2,200
|
637
|
|
Data Centre (J6)
|
600
|
47
|
|
Office (J6)
|
7,700
|
618
|
|
Convention (J11)
|
700
|
187
|
|
Ancillary commercial (J3)
|
5,000
|
903
|
|
Logistic(J3)
|
6,200
|
1,107
|
|
GIC Facilities (J11)
|
12,500
|
3,501
|
|
Residential/ Talent
Accommodation (J11)
|
1,700
|
479
|
|
Sub-total
|
165,000
|
43,567
|
Note:
1.
Advanced
Manufacturing is divided into 2 categories, Admin Staff : Manufacturing Staff =
1 :2
2.
R&D
is divided into 4 categories, Dry Lab : Wet Lab = 2:1; Admin Staff : Lab Staff
= 1:4.The Unit Demand of R&D are reference to LMCL
3.
The
Unit Demand of R&D are reference to LMCL
4.
Total
may not add up due to rounding
Total Sewage Discharge
6.6.3
Based on the
design parameters and assumptions discussed in section 6.3, the estimated sewage discharge of Residential sites
in the Project is summarized in Table 6‑9.
Table 6‑9 Summary of
Total Sewage Discharge
|
Category
|
ADWF (m3/day)
|
|
Residential Discharge
|
34,731
|
|
Commercial Discharge
|
43,567
|
|
Other Contribution1
|
11,359
|
|
Sub-total of Proposed
Development
(with 10% allowance)
|
98,623
|
|
Flexibility for
receiving additional sewage flow from developments, existing villages, reject
water from WRP and centrate from FWPF
|
26,377
|
|
Total Sewage Discharge
|
125,000
|
Notes:
1.
Other
contribution includes the discharge from NOL, Sport Centre and Swimming Pool,
EPP, Schools, DCS, Private Hospital and Food waste pre-treatment plant.
6.7.1
Based on the
Global Unit Load Factors discussed in Section 4.6, the pollutant loads are
estimated from the proposed development and listed in Table 6‑10 below.
Table 6‑10 Projected
Pollutant Concentration
|
Trades
|
Development
Parameter
|
SS (kg/d)
|
BOD
(kg/d)
|
COD
(kg/d)
|
TKN
(kg/d)
|
NH3N
(kg/d)
|
E.coli
(no./d)
|
|
Domestic
(Residential)
|
159,977
|
6,340
|
6,720
|
14,400
|
1,360
|
800
|
6.68x1015
|
|
Commercial G/IC
|
45,813
|
2,700
|
3,990
|
7,930
|
420
|
220
|
1.60x1015
|
|
Industrial (R&D
and Adv. Manu)
|
107,801
|
16,650
|
14,960
|
45,080
|
2.920
|
732
|
3.77x1015
|
|
Schools
|
12,600
|
428
|
428
|
882
|
84
|
50
|
4.41 x1014
|
|
Total
|
326,191
|
15,891
|
20,512
|
41,855
|
2,857
|
1,588
|
1.27 x1016
|
Table 6‑11 Global Unit Load
Factors
|
Trades
|
Unit
|
Percentage
(%)
|
SS (kg/d)
|
BOD
(kg/d)
|
COD
(kg/d)
|
TKN
(kg/d)
|
NH3N
(kg/d)
|
E.coli
(no./d)
|
|
Domestic
(Residential)
|
Person
|
100
|
0.04
|
0.042
|
0.09
|
8.5x10-3
|
5x10-3
|
4.3x1010
|
|
Commercial G/IC(1)
|
Employee
|
100
|
0.059
|
0.087
|
0.173
|
9.2 x10-3
|
4.8 x10-3
|
3.5 x1010
|
|
Industrial (R&D
and Adv. Manu)
|
Employee
|
10(2)
|
0.059
|
0.087
|
0.173
|
9.2 x10-3
|
4.8 x10-3
|
3.5 x1010
|
|
Flow
|
40(3)
|
559
(mg/L)
|
452 (mg/L)
|
1130
(mg/L)
|
62
(mg/L)
|
29
(mg/L)
|
3.5 x1010
|
|
flow
|
50(4)
|
800
(mg/L)
|
800 (mg/L)
|
2000
(mg/L)
|
100
(mg/L)
|
52
(mg/L)
|
3.5 x1010
|
|
Schools (5)
|
Student
|
100
|
0.034
|
0.034
|
0.07
|
6.7 x 10-3
|
3 x10-3
|
3.5 x1010
|
Note:
References from
Sewage Manual Part 1 Table 4
(6) Unit
Load Factors (Commercial, G/IC & Industrial) is the sum of Unit Load
Factors for commercial activities and employed population.
(7) It is
assumed 10% load from Industrial follow Commercial (Employed Population +
Commercial Activities)
(8) It is
assumed 40% load with similar characteristics to measured sewage at Tai Po
Industrial Estate
(9) It is
assumed 50% load could reach upper limits in WPCO TM Table 1
(10) Unit
Load Factors of Schools is based on the number of school students
6.8.1
The design EPP
would take 125,000 m3/day. In order to enhance the system efficiency
and have wise land use planning, 2 phases implementation of EPP is recommended
and subjected to further liaison with DSD and EPD. The first phase capacity is
65,000 m3/day in 2031 while the second phase capacity is 125,000 m3/day
in 2035. All the civil works will be carried out in one go meanwhile the
E&M works will be separated in two phases. Besides, the co-location of EPP,
Food-Waste Pre-treatment Facilities (FWPF) and Water Reclamation Plant (WRP) is
suggested. Since the actual layout of the EPP is not finalised, EPP and FWPF are indicated in one color in
the land layout. The
land layout is shown in Figure 6‑1.
6.8.2
During operation
phase, the FWPF adopts mechanical treatment to food waste for impurities
removal and size reduction. The treated food waste will be delivered to the EPP
and then mixed with sludge. It is assumed that there are 47 tonne dry solids
per day combined. The sludge treatment involves sludge thickening, anaerobic
digestion, combined heat and power (CHP) generation and sludge dewatering. The
final disposal would be disposed to landfill or T-park. The disposal scheme
would be determined at detailed design stage.
Figure 6‑1 Location
Plan of EPP and Co-located Facilities
6.9.1
For food
waste/sewage sludge co-digestion at the proposed EPP, it is recommended to set
up a FWPF. Based on the population intake in the Project, a FWPF with minimum
100 wet t/d capacity is suggested by EPD. Therefore, 1 ha footprint should be
reserved for FWPF to cater development activities of the Project. In view of
facilitating the treatment process, FWPF will be set up in Site OU(EPP).5.3.
6.10
Proposed Sewerage Scheme
6.10.1
After considering
the relief in the Project, there is a need to establish 3 sewage pumping
stations because of the flat terrain on the east of the Project. Site OU.1.2, OU.3.2
and OU.5.7 are utilized as the sewage pumping station. Most sewage collected
within the internal sewerage system will be conveyed to the proposed pumping
station and discharged via twin rising mains to public sewerage system. A
schematic layout showing the locations of proposed sewage pumping stations is
shown in Figure 6‑2.
6.10.2
The preliminary
list of responsible parties of the proposed sewerage and sewage treatment
facilities is shown in Table 6‑12.
Table 6‑12 Preliminary list of
responsible parties of the proposed sewerage and sewage treatment facilities
|
Sewerage/sewage
treatment facilities
|
Parties
responsible for construction
|
Parties
responsible for maintenance
|
|
Twin rising mains
|
Sewerage Project Division
of Drainage Services Department
|
Mainland North
Division of Drainage Services Department
|
|
Sewage pumping
stations
|
Sewerage Project
Division of Drainage Services Department
|
Sewage Treatment
Division 1 of Drainage Services Department
|
|
Sewage treatment
facilities
|
Sewerage Project
Division of Drainage Services Department
|
Sewage Treatment
Division 1 of Drainage Services Department
|
Figure 6‑2 Proposed
SPSs location
6.11.1
The estimated
reclaimed water demand of the Project is 94,404 m3/day. The design capacity of the WRP is 112,500 m3/day.
The surplus reclaimed water could be exported for other potential developments nearby.
6.12.1
Under current
design, in the emergency event when there is a power failure of STLMC EPP, the
effluent will be discharged directly to Deep Bay control zone via Ngau Tam Mei
Channel. A new pipework. will be constructed to connect the proposed EPP and
Ngau Tam Mei Channel under San Tam Road. A schematic layout showing the
emergency bypass is shown in Figure 6‑3. For the normal discharge route, it will share the same
route with the proposed emergency bypass to make good use of the pipework.
Figure 6‑3 Proposed
Emergency Bypass
6.13.1
The sewage
discharge build up are summarized in Table 6‑13 below. The implementation phasing
will be further reviewed in detailed design stage and subjected to approval
from relevant departments.
|
Population intake
|
Total
Sewage Discharge with 10% allowance (cum/d)
|
Total
Sewage Discharge without 10% allowance (cum/d)
|
|
2031
|
13,856
|
12,596
|
|
2034
|
65,315
|
59,377
|
|
Post 2034
|
125,000
|
113,636
|
Table
6‑13 Sewage Discharge Build-up
6.13.2
Implementation
phasing of major sewerage infrastructures and co-located facilities of EPP are
listed in Table 6‑14.
Table 6‑14 Implementation Phasing
for Major Sewage related Infrastructure
|
Year
|
EPP
|
WRP
|
FWPF
|
Pumping
Station in Site OU(SPS)1.2
|
Pumping Station
in Site OU(SPS)3.2
|
Pumping
Station in Site OU(SPS)5.7
|
|
2031
|
~65,000 m3/d
|
112,500 m3/d
|
100 tpd
|
0.67 m3/s
|
1.17 m3/s
|
2.12 m3/s
|
|
2035
|
125,000 m3/d
|
|
|
|
|
|
6.14
Potential Impacts by the Proposed
Sewerage System
6.14.1
For SPSs and
rising mains serving new development areas, those facilities have to be
completed and commissioned early on the development but it can be years before
they receive the design flows from full occupancy of the site. This results in
low flows entering the wet well and long retention times in the rising main
give the opportunity for bacteria to multiply in the anaerobic conditions in
which formation of Hydrogen Sulphide would occur.
6.14.2
Odour impact are
one of possible impacts generated from the construction and operation of the
sewage system. The main odour nuisances can be attributed to the wet wells/
retention tanks of the SPSs and the sludge treatment process in EPP. Use of
chemical dosing to oxygen injection for septicity control will be considered in
the detailed design stage,
6.15.1
Sewage septicity
control measures shall be considered for the proposed SPSs and rising mains as
follows. Non-dosing solutions should be considered in prior to dosing solutions
6.15.2
At this preliminary
stage, direct injection of oxygen into the rising mains and pre-aeration in the
wet well of the pumping stations are adopted as the sewage septicity control
measures with details to be addressed in the detailed design stage.
6.15.3
Enclosed the
pumping station inside building structure is considered as an odour mitigation
measures. The structure is equipped with adequate odour control measures such
as scrubber and activated charcoal filter at the exhaust of the ventilation
system. The vent will be located away from air sensitive uses including the
proposed development itself.
6.15.4
The effluent from
STLMC EPP follows a stringent standard which won’t deteriorate the Deep Bay
water quality. As discussed in Table 6-6, the water quality standards of TSE reuse
can fulfil the discharge limits of pollutants in Standards for Effluents
Discharged into Drainage and Sewage System, Inland and Coastal Waters, the
Water Pollution Control Ordinance (CAP 358).
6.15.5
Given the
sensitivity of inner Deep Bay in term of water quality and ecology, extensive
effort will be expedited to avoid the occurrence for emergency discharge. In
order to achieve this, the design of STLMC EPP will be cautiously reviewed to
include additional provisions including as follows:
·
Applied peaking factors for all
major treatment units and electrical and mechanical equipment to a void
equipment failure;
·
Standby unit for all major
equipment 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
·
Back-up power for dual power
supply would be provided in case of power failure to sustain the function of
pumping and treatment facilities
6.16.1
There is no existing
sewerage system within the vicinity of the Project. The SWHSTW and YLSTW being
upgraded would not have sufficient treatment capacity to cater for the
additional sewage arising from the proposed development.
6.16.2
To cater the
sewage discharge in the Project, it is proposed to construct a tertiary EPP
with 125,000 cum/d capacity, WRP with 112,500 cum/d, FWPF with 100 wet t/d and three
SPSs in 2031. Reuse of reclaimed water is recommended for non-potable uses such
as toilet flushing and irrigation. The TSE for reclaimed water will be provided
from the STLMC EPP with tertiary treatment to ensure a higher water quality
standard. The rest of sewage effluent will be discharged to Deep Bay.
6.16.3
Based upon a
preliminary sewerage impact assessment as described in this section, it can be
concluded that the proposed development is sustainable from sewerage
collection, treatment and disposal prospective. There is no identified insurmountable
sewerage and sewage treatment implications arising from the Project.
