This Section outlines the existing sewerage
arrangements in the vicinity of the Development Site and examines the available
capacity of the existing sewerage system.
If there is no existing sewerage system in the vicinity to serve the
Site, the technical feasibility of providing a sewage disposal scheme will be
assessed. This Sewerage
and Sewage Treatment Implications Assessment (SSTIA) demonstrates that the
proposed development is viable in terms of its impact on the sewerage system.
7.2
Relevant
Legislative Requirements
The Development Site is currently on highland without
any development. It is accessible by
According to Section
11.1 of the Technical Memorandum of Standards
for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal
Waters published by Environmental Protection Department, no new effluents
are allowed in the following areas, regardless of the standards of the
effluents:
1.
Within
100 m of the boundaries of a gazetted beach in any direction, including rivers,
streams and storm water drains;
2.
Within
200 m of the seaward boundaries of a marine fish culture zone or a site of
special scientific interest, and within 100 m of the landward boundaries;
3.
In
any typhoon shelter;
4.
In
any marine park; and
5.
Within
100 m of a seawater intake point.
The Site falls in the vicinity of two SSSIs and no treated effluents are allowed to be discharged
within 100 m of the landward boundaries of the SSSIs.
The SSTIA was undertaken in accordance with the
criteria and guidelines for evaluating and assessing impacts on the downstream
public sewerage, sewage treatment and disposal facilities as states in section
6.5 in Annex 14 of the Technical
Memorandum on Environmental Impact Assessment process (Environmental Impact
Assessment Ordinance, Cap. 499, S.16).
The following methodology was adopted for this SSTIA:
·
Investigation
of the existing and planned sewerage facilities in the area of the development
site;
·
Calculation
of the sewage flow generated from the proposed development; and
·
Investigation
of three sewerage disposal options for the development site.
7.4
Existing Sewerage
Facilities and Planned Sewerage in the Area
The Development Site is within an area where no
public sewerage connection is available.
There is only a pubic toilet located at the end of
The nearby existing sewerage system is a sewer
network to the south of
The public sewer network will carry the sewage to Tai
Po Sewage Treatment Works (STW) located at the eastern part of the Tai Po
Industrial Estate. This STW provides
primary and secondary treatment. The
existing sewerage network is shown in Figure 7.1.
The quantity of wastewater generated by the proposed
development is calculated based on Appendix III of Guidelines for Estimating Sewage Flows for Sewage Infrastructure
Planning (the Guidelines) ([1]). The
sewage generated from the Nature Interpretation Centre, Multi-Cultural
Education Retreat cum Columbarium is calculated on the assumption of a maximum
of 200 visitors per day. The waste water
generated during special festivals such as Ching Ming or Chung Yeung, when
visitor numbers will be much higher, is discussed in Section 7.7.
The calculations of Average Dry Weather Flow (ADWF)
are shown as follow:
Maximum
No. of Visitors = 200
Global
unit flow rate = 0.080 m3/day (ref. 4(d)
of Appendix III of the Guidelines)
Maximum
ADWF = 200 x 0.080 m3/day = 16.0
m3/day = 0.185 l/s
The visitors will remain on the Site
during the open hours from 9:00am to 5:00pm.
The sewage generated by the visitors will be the water for flushing and
washing hands only. Since the behaviour
of visitors in terms of sewage generation is similar to the behaviour of
employees, a unit flow rate of 0.080 m3/day according to Clause 4(d)
of Appendix III of the Guidelines is used.
As there is no statutory requirement or guideline for
the assessment of sanitary fitments for religious places, the assessment of
sanitary fitments for cinemas as required in PNAP 297 is referenced. At least 10 female W.C., 4 male W.C. and 7
male urinals are assumed to be provided in the Multi-Cultural Education Retreat
and Columbarium for public use during the operation phase. The toilet facilities to be provided in the
Site are adequate to cater for a maximum of 200 visitors (including visitors to Nature Interpretation Centre but no visitors or
staffs of the Nature Interpretation Centre would stay overnight).
On a normal day, not more than 30 occupants will be
in the Multi-Cultural Education Retreat cum Columbarium & other facilities
stay overnight. The calculations of
Average Dry Weather Flow (ADWF) for occupant levels during normal periods are
as follows:
Normal No. of occupants
= 30
Global unit flow rate
= 0.370 m3/day (ref. 4(c)
of Appendix III of the Guidelines)
Maximum
ADWF = 30 x 0.370 m3/day = 11.1
m3/day = 0.128 l/s
As there will be no restaurant at the Site, it is
foreseen that the quantity of the sewage generated from other cleaning and
washing activities such as cleaning of staff quarters in a frequency of once a
week will be insignificant. Circulating
water will be used for the water cascade or water features. Therefore, no sewage will be generated by
these features.
During festival days, there will be a significant
increase in number of visitors in the Multi-Cultural Education Retreat cum
Columbarium. The estimated flow of
sewage generated is shown in Table 7.1
and is stipulated as follows:
Maximum Sewage Generated = 60.8 m3/day = 2.11
l/s (assumed 8 hours operation, 0900 - 1700)
Note that the above flow rate is based on a very conservative
estimation which assumes that the visitors will utilise the sanitary facilities
on a continuous basis during the open hours (from 9:00am to 5:00pm). The maximum capacity of a W.C. should be
limited to not more than 6L.
Table 7.1 Estimation
of Sewage Generated in One Festival Day
|
Data |
|
Total no. of female W.C. = 10 |
|
Total no. of male W.C. = 4 |
|
Total no. of male urinal = 7 |
|
Assumptions |
|
1.
Volume of flushing water for 1 W.C. = 6 L
(maximum) (1) |
|
2.
Volume of flushing water for 1 urinal = 1 L
(maximum) |
|
3.
Volume of water used by 1 person for
washing = 2 L (average) |
|
4.
Average duration of 1 female occupied a
W.C. = 2 min. |
|
5.
Average duration of 1 male occupied a W.C.
= 1 min. |
|
6.
Average duration of 1 male occupied a
urinal = 40 s |
|
7. Open
hours of the site in a day (9:00am - 5:00pm) |
|
Estimation of Total sewage Generated in One Festival Day |
|
Number of female using 1 W.C. in one hour = 60 min./ 2 min. = 30 |
|
Number of male using 1 W.C. in one hour = 60 min./ 1 min. = 60 |
|
Number of male using 1 urinal in one hour = 60 min./ (40/60)min. = 90 |
|
|
|
Total volume of sewage generated by female in 1 hour = 30 x (6+2)L x
10 = 2,400 L |
|
Total volume of sewage generated by male in 1 hour = {60 x (6+2)L x 4}
+ 90 x (1+2)L x 7} = 3,810 L |
|
|
|
Total volume of sewage generated by visitors in 1 day = (2,400 +
3,810)L x 8 hrs = 49,680 L (49.7 m3) |
|
|
|
Total number of visitors using toilet facilities in 1 day = (30 + 60 +
90) x 8 = 1,440 |
|
|
|
Total volume of sewage generated by occupants in 1 day = 11.1 m3
(see above) |
|
|
|
Total volume of sewage generated in one festival day = 49.7 + 11.1 = 60.8 m3 |
|
Note: (1) The 6 L flushing water for 1 W.C. is
commonly used in HK and only the water closets that actually have a flushing
water volume not more than 6L will be used |
Option 1: Discharge to the nearby existing sewage system (i.e.
Tai Po STW via public sewer at Tai Po Industrial Estate)
Under this option, it is necessary to construct
approximately 2 km of sewers of 225 mm in diameter running along Sha Lo Tung
Road with the downstream end connected to an existing sewerage manhole at Ting Kok Road (See Figure 7.2). Ultimately the sewage from the Site will be
discharged to Tai Po STW.
The impact on the Tai Po STW should be insignificant
after the commissioning of the Stage 5 upgrading works. The maximum sewage generated from the Site is
about 16+11.1=27.1 m3/day (ADWF), while the capacity of the Tai Po STW is about 120,000
m3/day (ADWF). The increase
in flow would be only about 0.023% of the capacity of the STW (ie 120,000 m3/day). It should be noted that the maximum sewage
volume generated from the Development Site will only happen on rare
occasions. Sewage volume during normal
periods will be about 40% of the maximum flow.
The ground level at the southern part of the Development
Site is about 203 mPD, while the ground level at the
northern part of the Development Site is about 184 mPD. A pumping station and rising mains are to be constructed
to deliver the sewage from the southern part of the Development Site in the
north.
The peak flow to be discharged into the downstream
sewerage system will be controlled by the capacity of the pumping system and
the size of the storage tank. Using a
larger storage tank can reduce the pumping rate as well as the sewage flow to
be discharged into downstream sewers without overloading the existing sewers in
the downstream area. We consider that
the maximum sewage generated from the Site is only about 27.1 m3/day
(ADWF). Assuming a storage tank of
minimum size of 198 m3, the average flow of the collected peak
sewage volume of 27.1 m3 to be discharged within a day to the
downstream sewers is about 0.00012 m3/s. The peak flow of the pumps installed is
proposed to be 0.0005 m3/s.
The minimum size of the existing downstream sewer is 225 mm with a
capacity of about 0.0155 m3/s.
The discharged sewage is only 3.2% of the capacity of the existing
minimum downstream sewer. Therefore, the
impact to the downstream sewer is considered insignificant. It is proposed to install a stand-by pump in
case of mal-function of the working pump occurs.
As the peak sewage generated from the Site
is not significant (only 27.1 m3/day (ADWF)) overloading of the
sewage system downstream and the Tai Po Sewage Treatment Plant (STW) can be
avoided through the use of a storage tank and pumping station. Please note that the capacity of the Tai Po
STW is about 120,000 m3/day (ADWF).
The increase in the flow to the Tai Po STW due to the Site is only 27.1
m3/day (ADWF) (assuming the peak flow condition). The increase in flow would be only about
0.023% of the capacity of the Tai Po STW. The impact to the Tai Po STW is
insignificant. The recommended option is
considered feasible and practical.
Option 2: On site sewage treatment plant and discharge to adjacent drainage system
On-site tertiary level sewage treatment facilities
are recommended in Option 2 for treating sewage from the Site to a standard
acceptable to Environmental Protection Department for discharge to the existing
drainage system.
As the Development Site is bounded by Fung Yuen
Valley SSSI to the south and Sha Lo Tung SSSI to the north, a 225 mm diameter stormwater rising main will be constructed at
A sewage treatment plant will be constructed within
the Development Site. The proposed location of the sewage treatment plant (STP)
is shown in Figure 7.3. The sewage treatment plant can be constructed
underground and entirely within an enclosure (eg.
beneath the landscaped area and within the Site) to minimize adverse
environmental impacts. The sewage
treatment plant could be a commercially available prefabricated package plant
with rotating biological contactor and chlorination disinfection
installation. The treatment rate is
estimated to be about 120 m3/day.
The effluents should be treated to acceptable
standards as stated in Table 7 in the “Technical
Memorandum on Standards for Effluents Discharged into Drainage and Sewerage
Systems, Inland and Coastal Waters” for discharge to the coastal waters of Tolo and Port Shelter Water Control Zones. However, there is an environmental concern
over the discharge of treated effluents into
Option 3: Deliver the sewage off Site by tanker
This option does not require the construction of a
pumping station within the Site or any rising main along
The normal sewage flow generated from the
Development Site is about 11.1 m3/day (ADWF) as estimated in Section 7.5. It is equivalent to about 77.7 m3
per week. Even for a maximum flow rate
of 27.1 m3/day (ADWF), the effluent volume to be stored per week
will be 189.7 m3. There would
be a need to construct a storage tank with a capacity of minimum 190 m3
to cater for the peak flow volume. The
stored sewage will be removed once a week.
During a normal week, it is estimated that 32 round trips of the tanker
in one day would be required to empty the tank (assuming 6 m3
capacity). If more sewage is to be
delivered, the delivery schedule could be split into several days in order not
to overload the existing public carriageway system. However, this option has the potential risk
of polluting the environment if leakage happens during the collection and
transportation of the sewage. It will
also increase the traffic load on the
Table
7.2 Summarises the
sewerage strategies for the proposed development.
Table 7.2 Sewage
Disposal Options
|
Sewerage
Disposal Strategy |
Option
1 Discharge
to the nearby existing sewerage system (i.e. Tai Po STW via public sewer at
Tai |
Option
2 On site
sewage treatment plant and discharge to adjacent drainage system |
Option
3 Deliver the
sewage off Site by tanker |
|
Implementation |
(1) Construct an
approximately 2000 m long 225mm diameter sewer from the Site running along (2) Construct Pumping
Station and rising mains at the southern part of the Site to pump the flow to
the south (ultimately to Tai Po STW). |
(1) Fung Yuen Valley
SSSI is located to the south of the Site and Sha Lo Tung SSSI is located to
the north of the Site. No new effluent is allowed to be discharged into the
vicinity of an SSSI. (2) Provide on-site
tertiary level sewage treatment plant capable of treating the sewage from the
Site to a standard acceptable (by EPD) for discharge to the drainage system
at (3) Tertiary treated
effluent is finally discharged to |
(1) Construct a storage tank
to store the sewage generated on the Site. (2) Deliver the stored
sewage off the Site by tanker at regular intervals. |
|
Possible Constraint and
Solution |
(1) Require a piece of
land to accommodate the pumping station and associated installation. (2) Possible conflicts
with existing utilities and other services under (3) Possible disturbance
to the existing traffic across (4) Through careful
route selection, appropriate temporary traffic management scheme and adoption
of suitable construction technique, the impact could be minimised. (5) Liaison with EPD and
DSD regarding any spare capacity to accommodate the sewage generated from the
Site and the programme for the completion of the Tai Po STW upgrading works. |
(1) Requires a piece of
land to accommodate the plant and associated installation. (2) The treatment plant
and the associated installation could be underground and entirely within an enclosure
to minimise adverse impacts. (3) Possible conflicts
with existing utilities and other services under (4) By careful route
selection, good design and implementation of temporary traffic management
scheme and adoption of good construction method. The impact could be minimized. |
(1) Require a piece of
land to accommodate the storage tank.
The land intake requirement is minimal. (2) Increases the
traffic load on (3) To be overcome by
regulating the sewage delivery schedule. |
|
Recommendation |
Recommended as the
preferred option |
Not recommended |
Not recommended |
7.7
Sewerage
and Sewage Treatment during Special Festivals
A maximum number of 7,400 visitors per day
are allowed to visit the Multi-Cultural Education Retreat and Columbarium
during Ching Ming or Chung Yeung Festival (refer to Section 3.8). It is foreseen
that sewage generated would be abruptly increased during these festival
days. At least 10 female W.C., 4 male
W.C. and 7 male urinals are assumed to be provided in the Multi-Cultural
Education Retreat and Columbarium for public use during the operation phase. According to Table 7.1, the estimated maximum sewage generated during festival
days is 60.8 m3/day.
This flow rate is based on a very conservative estimation which assumes
that the visitors will non-stop use the sanitary facilities during the open
hours (from 9:00am to 5:00pm).
Nevertheless, the proposed storage tank of minimum size of 180 m3
is considerably sufficient (with a buffer of approximately 3 days withholding
time to cater for the situation if the pumping facilities malfunctioned;
usually the public holidays during Ching Ming and Chung Yeung Festival do not
last more than three consecutive days) to cater for the abrupt increase in
generated sewage during festival days.
As the sewage flow rate of 0.0005 m3/s will be controlled by the pump, the
increase in flow to the Tai Po STW will be 43.2 m3/day,
i.e. 0.036% of the capacity of the STW.
It is concluded that the impact on the STW due to abrupt increase in sewage flow during festival days is insignificant. In addition, the sewerage system shall be of leak-proof design and the sewerage storage tank should also be emptied regularly in particular before, during and immediately after the festivals. A deodorizing facility, such as activated carbon, will be installed at the exhaust of the storage tank to control odour. However, it should be noted that the storage tank is only required during emergency or when the pumping facilities malfunctioned. The maintenance/repairing works will conducted over a period of less than 3 days. Therefore, the onsite storage of sewage will be temporary and infrequent. With the installation of a proper deodorizing facility and very infrequent temporary onsite storage of sewer, no potential odour problem will be envisaged.
The peak flow of the pump installed is proposed to be
0.0005 m3/s. The minimum size
of the existing downstream sewer is 225 mm with a capacity of about 0.0155 m3/s. The discharged sewage is only 3.2% of the
capacity of the existing minimum downstream sewer. Therefore, the impact to the downstream sewer
during festival days is considered insignificant. Also, it is expected that blockage of sewer
pipe is unlikely occurred in such a flow rate of the pump.
Approximately 3 portable toilets will be erected at the entrance of
7.8
Proposed
mitigation Measures
Assuming Option 1 is adopted, the following
mitigation measures are proposed:
·
Installation
of an approximately 2 km long 225 mm
diameter twin sewerage rising mains from the Development Site running along
·
Installation
of a stand-by pump (with a 100% standby
pumping capacity) in case of mal-function of the working pump;
·
Construction
of a storage tank (fitted with a level indicator
and a high level alarm system, designed and constructed to comply with the Buildings (Standards of Sanitary Fitments,
Plumbing, Drainage Works and Latrines) Regulations (Cap 123I) reg. 47A)
of minimum 180m3 capacity, to give approximately three days
withholding time over maximum visitor attendance over festival periods to cater
for the situation if the pumping facilities malfunctioned;
·
As a statutory requirement, the storage tank and all
sewers within the WGGs will be designed and
constructed to comply with the Buildings
(Standards of Sanitary Fitments, Plumbing, Drainage Works and Latrines)
Regulations (Cap 123I) reg. 47A to ensure that they are watertight. Through the circulation of drainage plans to
the Building Authority, other requirements of WSD on the design and details of
construction will be complied. As such,
the risk of leakage of sewage from the sewerage system is considered to be very
low, through the use of the best quality piping materials (typical example:
vitrified clay or PVC or concrete pipe, which are common materials for sewer
and are anti-corrosive; materials selected will subject to result of detailed
design of proposed sewerage) and provision of hatch box access covers within
chambers, and
·
A
deodorizing facility (e.g. activated carbon) will be installed at the exhaust
of the storage tank to control odour.
With the installation of a proper deodorizing facility and very
infrequent temporary onsite storage of sewer, no potential odour nuisance to
public is envisaged.
An assessment of the impacts of sewage generation
during festival days has also been considered.
With the following measures to manage the increase in sewage generation,
no adverse impacts to the sewerage system are expected:
·
Provision
of adequate permanent water closets and urinals in the complex;
·
Emptying
of the sewage storage tank before and immediately after the festivals; and
·
Provision
of portable toilets at the entrance of Sha Lo Tung road, next to
7.9
Conclusion and Recommendations
Taking into consideration the constraints of the
Development Site, and the existing and planned sewage network in the region,
three sewage disposal options for the development of the Site have been
investigated. In broad terms, the three
proposed sewage disposal options are discharging to Tai Po Sewage Treatment
Works, treatment on Site and removal off site.
Among the three options, the option of discharging
the sewage to Tai Po Sewage Treatment Works is recommended. The option of provision of on-site sewage
treatment plant and removal of the sewage off site by tanker are not
recommended. The recommended option is
considered feasible and practical given that the impacts to the downstream
sewer and the Tai Po Sewage Treatment Works are considered insignificant. The details of the recommended sewage
disposal scheme will be further confirmed at the detailed design stage and
close liaison will be carried out with relevant departments to approve the scheme.