Drainage Services Department
Hong Kong Special
of The People's Republic of China
Peng Chau Sewage Treatment
Investigation, Design and Construction
Agreement No. CE 83/2001(DS)
In December 1994, the Environmental
Protection Department (EPD) completed the Outlying Islands Sewerage Master Plan
(OISMP) Study (EPD, 1994). The areas studied in the OISMP include Lantau
Island, Lamma Island, Cheung Chau, Peng Chau and some other smaller islands to
the west and south of Hong Kong. Recommendations on the improvements and
extensions to the existing sewerage infrastructures were made.
The OISMP identified the need for
improvement of the water quality of marine waters close to Peng Chau. Further
to the OISMP, the Drainage Services Department (DSD) completed the Preliminary Project
Feasibility Study (PPFS) for Package K – Peng Chau Sewage Treatment Works
Upgrade. The study concluded that the existing capacity of the Peng Chau Sewage
Treatment Works (STW) is inadequate to meet the increase in flow due to the
proposed developments at Peng Chau. An upgrading and expanding of the existing
Peng Chau STW with an oceanic submarine outfall is therefore required. The
study also recommended to include Package K in the Outlying Islands Sewerage
Stage 1 Phase 2 Works.
In August 2002, the Drainage Services
Department (DSD) of the Hong Kong SAR commissioned CDM International Inc. (CDM)
under Agreement No. CE83/2001 (DS) to carry out investigations, environmental
impact assessments (EIA), design, tender, construction supervision and commissioning
of Package K – Peng Chau Sewage Treatment Works Upgrade (hereinafter called the
The submarine sewage outfall of this
Project is a “designated project” category F.6 under Schedule 2 of the
Environmental Impact Assessment (EIA) Ordinance (Cap. 499), and an
environmental permit is required for the Project. An EIA report in accordance with the requirements
under the EIA Ordinance and the EIA Study Brief is prepared.
The purpose of this EIA study is to
assess the environmental issues arising from the construction and operational
phases of the Project, and to recommend appropriate mitigation measures for all
identified adverse environmental impacts.
Two working phases were recommended
and the detailed scopes are:
Phase 1 Works:
construction of a new STW adjacent to the existing
STW comprising secondary treatment with nitrification, de-nitrification and
construction of two new submarine outfalls;
provision of de-odourization facilities;
provision of associated sludge treatment facilities;
extension of inlet pumping mains;
construction of equalisation tank, and
ii. Phase 2 Works:
demolition of the existing treatment work
Construction of sludge drying bed; and
Construction of remaining works.
Figure 2-1 illustrates the layout plan
of the proposed STW upgrade. Based on the preliminary construction programme,
the construction of the project is scheduled to commence in April 2005 and for
completion in November 2007.
Environmental Impact Assessment
The principle purpose of this EIA is
to assess the environmental acceptability of constructing and operating of the
proposed Peng Chau STW Upgrade including the associated submarine outfall and
emergency overflow outfall components. The study has been carried out in
accordance with the guidelines in the Technical Memorandum on Environmental
Impact Assessment Process (TMEIAP) as well as the EIA Study Brief.
The following environmental aspects
have been assessed:
Waste Management Implementation;
Marine Ecology; and
The findings of the assessment are
summarised in the following sections.
Representative air sensitive receivers
(ASRs) were identified in the EIA Study and are shown in Figure 3-1.
Dust generating activities were
identified and evaluated. If un-mitigated, construction of the Peng Chau STW
upgrade would have short term adverse impact on air quality, in the forms of
fugitive dust at the identified ASRs. The predicted construction dust impact
exceeds the 1-hour and 24-hour TSP criteria at some ASRs. Mitigation measures
including watering of on-site construction area are expected to limit fugitive
dust levels to acceptable levels. Model simulation results show that the 1-hour
and 24-hour TSP criterion could be met after implementing the recommended
The implementation of the Air
Pollution Control (Construction Dust) Regulation and good site practice during
construction phase are recommended.
During the operational phase of the
Project, all the potential odour generating facilities such as inlet works,
grit chamber, equalization tank, SBR, sludge thickener, sludge digester, screening
and grits storage area, return liquor pumping station and sludge drying bed
would be enclosed by air-tight covers. Odourous gas would be ventilated to the
deodourisation facility for further treatment before discharge. The
deodourisation facility would be capable of removing 99.5% of odour. During sludge transportation, it is
recommended that sludge should be carried by enclosed container to avoid
unacceptable odour nuisance.
With the above mitigation measures, no
unacceptable odour impact would be envisaged.
Representative noise sensitive
receivers (NSRs) were identified in the EIA Study and are shown in Figure 3-2.
Construction noise impact from STW
upgrade was evaluated. Key noise generating
activities include construction of pumping mains, treatment works upgrade and
demolition of old units, as well as outfall construction.
The predicted results show that the
construction noise standard of 75 dB(A) stipulated in TMEIAP is complied at all
identified NSRs from the proposed construction activities at all phases.
However, the construction of Peng Chau Helipad would be taken place
concurrently during the first year of construction when most noisy equipment
plants are scheduled to be operated. To minimise the cumulative noise impact, a
more stringent noise level of 72dB(A) is recommended to be observed by both
Noise mitigation measures are proposed
for the construction of STW upgrade. Two types of silencing PME are recommended
to reduce the noise impact. Noise levels at identified NSRs after mitigating
are all well below the proposed noise level of 72dB(A).
Exceedance of 72dB(A) noise level would occur by Helipad
construction activities at the worst identified NSR with the implementation of
mitigation measures. Scheduling of the STW construction have been properly
arranged to avoid the elevated cumulative noise impact at the worst identified
NSR. No exceedance of cumulative
noise level of 75dB(A) would be resulted within the concurrent construction
period of both projects.
No unacceptable residual impacts are
anticipated during the construction phase.
The major noise generating equipment
during operational phase are air blowers, ventilation fans and water pumps.
Operational noise prediction is carried out assuming all the noisy equipment
would be located outdoor without shielding. The predicted noise levels at
identified NSRs exceed the daytime/evening and night time operational noise
standard of 49dB(A) and 45dB(A), respectively.
However these noisy equipment are
designed to be enclosed inside their respective treatment buildings. A
reduction of 20dB(A) can be achieved if the building enclosures are built using
suitable material such as concrete with surface density of 25kg/m2.
With the proposed enclosure, the predicted noise levels during operational
phase at identified NSRs are reduced and would comply with the operational
noise standard during both daytime/evening and night time.
No residual operational noise impacts are
encountered after implementation of mitigation measures.
Water Quality Impact
Dredging activities for the
construction of submarine and emergency overflow outfalls would elevate the
suspended solids concentration in the marine environment. Assessment of
sediment dredging has been carried out by predicting the increase of sediment
concentration due to fine sediment lost to suspension.
High level of sediments which exceed
the WQO criteria are predicted in the vicinity of the study area where marine
ecological sensitive receiver corals are found. Mitigation measures are
proposed for dredging activities. It is recommended that closed grab dredger,
silt curtains with a minimum 75% solid reduction or higher are to be used and
reduction in dredging rate is to be implemented. With the implementation of
proposed mitigation measures, no exceedance of WQO criteria is predicted about
250m away from the dredged alignment.
Discharge Via Submarine Outfall
Water quality impacts from the
operation of Peng Chau STW upgraded have been assessed. Assessment of the near
field dispersion from discharge of treated effluent have been carried out by
using Visjet model for both dry and wet seasons. The modelling results show
that adequate initial dilutions can be achieved for all discharge scenarios
(planned average and peak average scenarios) at both seasons. The total
concentration of SS, ammonia, unionised ammonia and E.coli would comply with WQO standard after initial dilution.
Elevation in BOD concentration is not significant.
The total concentration of
BOD, SS, Ammonia, unionised ammonia and E.coli would comply with WQO standard
after initial dilution.
Although no WQ standard is available
for total residual chlorine (TRC- as a toxicant), USPEA criteria of 0.013mg/L
and 0.0075mg/L are adopted at the edge of initial dilution zone for acute and
chronic toxicity, respectively. The predicted results show that with TRC
reduction to 0.8mg/L, the proposed criteria can be met and water quality
impacts are not considered to be significant.
Total concentration of TIN is
predicted to exceed the WQO standard, owing to its high baseline concentration.
The dilution provided by the near field effect, however, allows the TIN
concentration to fall within the natural variation/fluctuation range of a water
quality monitoring station SM 10. The water quality impacts are thus considered
to be acceptable.
It is expected that operation of the
upgraded Peng Chau STW would be beneficial to the surrounding receiving water body.
The current Peng Chau STW provides secondary treatment to sewage collected from
residential developments at northern Peng Chau. Sewage from some village houses
is still being discharged untreated. Prior to the operation of the upgraded
STW, the Peng Chau Package K project would connect and divert the sewage from
the majority Peng Chau household to the STW. Less discharge of untreated sewage
would be anticipated. With the incorporation of denitrification process into
the upgraded Peng Chau STW, the discharge of total inorganic nitrogen into the
receiving water body would be substantially reduced. Long term improvement of water quality is anticipated.
Discharges of treated effluent via
emergency overflow outfall in the case of damage or blockage of submarine
outfall have been assessed. Visjet model was used for initial dilution
prediction and subsequent far field dilution was determined by using Brooks
equation. The predicted results show that adequate dilution can be achieved
within a few hours after initial dilution. The predicted concentration of SS,
ammonia, unionised ammonia and E.coli
would comply with WQO standards after far field dispersion. Concentration of
BOD and total residual chlorine would be insignificant. TIN remains as an
exceeding parameter due to high baseline concentration.
In the case of failure of both STW and
submarine outfall, untreated sewage would have to be discharged via the
emergency overflow outfall. Water quality impact from peak dry weather flow
scenario has also been assessed using the same methodology. The discharged effluent would have
significantly higher pollution load than the treated effluent and a longer time
is required for the effluent plume to achieve adequate far field dilution.
Concentrations of SS, ammonia and unionised ammonia would comply with WQO
standards. Concentrations of BOD and TKN would be insignificant. The
concentration of E.coli, however,
would still exceed the WQO criteria for secondary contact recreation zone.
Mitigation measures are recommended to minimise the chance of emergency
overflow of untreated effluent.
Review of previous sediment quality
survey reveals that the sediment quality may belong to Category L according to
the classification in ETWB TCW No 34/2002. A sediment sampling and analysis was
carried out to verify the speculation. Results of chemical screening analysis
confirmed that sediment in the proposed dredged area is uncontaminated and
belong to Category L . The dredged sediment would be suitable for open sea
Solid Waste Management
Waste generated by construction works
include workforce waste, maintenance and chemical waste and construction and
demolition material. With the implementation of appropriate mitigation measures
during the handling, collection and disposal of construction waste material,
the residual environmental impacts would be acceptable. The mitigation measures
can be enforced by incorporating them into a waste management plan as part of
the contract document. Environmental monitoring and audit would be necessary to
ensure the implementation of correct disposal requirements for the various
waste generated from construction works.
Dredging activities would be occurred
at the proposed outfall. Approximately 22,000 m3 of marine mud would
be dredged and disposed of. It is
recommended to avoid the leakage of the dredged mud into the marine environment
by implementing appropriate mitigation measures. The residual impact would not
be significant given the small amount of dredged mud to be handled and disposed
Waste generated during operational
phase of Peng Chau STW Upgrade would mainly be sludge from secondary treatment
units. Two sludge dewatering arrangements have been adopted. The digested wet
sludge would be barged away for centralised treatment and disposal of as the
duty operation procedures in the initial stage of STW operation. Sludge can also be dewatered on site by
drying bed to 30% solid content and for disposal of at designated landfill site
when necessary. The operation will
be reversed (i.e. drying bed for duty and export for standby) if the export
option turns out to be constrained by the treatment capacity of designated
dewatering facility or be less cost effective due to higher volume of wet
sludge to be barged away.
Marine Ecological Impact
The potential impact on marine
ecological communities would be associated with construction of submarine and
emergency overflow outfalls.
Impact assessments on the intertidal
community, subtidal soft bottom and subtidal hard bottom (including corals)
from habitat loss and sedimentation have been carried out. The overall impacts
are considered to be acceptable. No mitigation measures for marine ecology
would be required if construction water quality impacts are to be mitigated.
Zones of initial dilution (ZID) of the
effluent discharge are localized in the vicinity of outfall. In the wet season,
effluent quality after initial dilution for BOD, SS, unionised ammonia and E coli would not exceed the WQO. The
total residual chlorine concentration is also found to be complied with the
proposed water quality criteria if the discharge is reduced to 0.8mg/L.
Elevated TIN concentration is predicted but it is due to elevated background
TIN concentration in the Southern Water Control Zone. Benthic biota in the area
are not higher in diversity and abundance than other regional areas and there
are no significant coral communities in the ZID zone, impacts are therefore
minor and mitigation measures would not be required.
In the dry season the ZID extend
further from the outfall and significantly higher dilution are achieved. A
small portion of the higher valued soft bottom benthic habitat that exists to
the north west falls within the zone of initial dilution. However, water
quality would be indistinguishable from the background, thus the operational
impacts are anticipated to be insignificant and no mitigation measures would be
A baseline review was conducted on the
cultural heritage sites identification, with data supplement from geophysical
surveys at the proposed dredged area for outfall construction. There is no
cultural heritage sites of interest are identified and impacts arising from the
construction and operation of Peng Chau STW Upgrade is not expected.
ENVIRONMENTAL Monitoring and audit
A comprehensive environmental
monitoring and audit (EM&A) programme was recommended for implementation
during the construction and operation of the Project. The separate EM&A
manual covers various environmental issues including air quality, noise, water
quality, waste management and marine ecology.
The proposed Peng Chau Sewage
Treatment Works Upgrade would provide a high level of treatment to the sewage collected
from Peng Chau area. The treatment works upgrade is designed to improve the
water quality in the vicinity of Peng Chau by adopting a more stringent
discharge requirement for nitrogen and enhancing the dispersion of effluent
discharge by submarine outfalls.
This EIA has identified potential
environmental impacts due to construction and operational phase of Peng Chau
STW Upgrade. Assessments on air quality, noise, water quality, sediment
contamination, solid waste, marine ecology and cultural heritage have been
carried out. Cumulative environmental impacts from projects in the vicinity of
the study area have also been addressed.
Potential environmental and cumulative
impacts would be anticipated during construction phase if no mitigation
measures are to be implemented. Mitigation measures are also recommended to be
implemented during the operational phase to minimise the potential impact. No
cumulative operational phase environmental impacts are envisaged. No
unacceptable residual impacts would be anticipated.