Conclusion on Need for the ADF
2.14
The monitoring data shows that both the HATS
Stage 1 un-disinfected effluent discharge and local pollution sources are
contributing to the poor or very poor beach water quality at the seven Tsuen
Wan beaches. Before the beach management authority may consider
re-opening any of these beaches, it is necessary to improve beach water quality
to a level consistent with the present policy on beach management and community
aspirations.
2.15
As demonstrated in Section 5 of this EIA Report,
the proposed ADF will disinfect HATS effluent to bring beach water quality back
to pre-HATS Stage 1 conditions. The ADF, together with reduction of
localised un-treated wastewater discharges being implemented under the regional
sewerage schemes, will be essential to improving the Tsuen Wan beach water
quality and, consequently, re-opening of the beaches. From the historic
records as shown in Exhibit 2.2, three beaches, namely Anglers’, Ting
Kau and Approach, on the Tsuen Wan coast had been closed due to strong
influence of local pollution sources before the commissioning of HATS Stage
1. On top of the already poor water quality, un-disinfected effluent from
HATS Stage 1 commissioned in late 2001 has caused four other beaches on the
coast to close. They are, namely Lido, Casam, Hoi Mei Wan, and Gemini.
2.16
Based on the water quality modelling results
given in Section 5, provision of disinfection facilities at SCISTW after
implementation of ADF will keep the influence of the HATS effluent at the Tsuen
Wan Beaches to a very low level (≤ 15 E. coli per 100 ml). With
regard to the background (i.e., non-HATS) pollution sources, the government has
been implementing the local sewerage works along Castle Peak Road for
completion by 2009 in conjunction with the ADF to serve the unsewered villages and properties around Ting Kau, Sham Tseng and Tsing Lung Tau. As the
outfalls of Kwai Chung, Tsing Yi and Northwest Kowloon PTWs (which should have
an influence on the water quality of Tsuen Wan beaches at the pre-HATS stage)
have been decommissioned since the commissioning of HATS Stage 1 in 2001, it is expected that the beach water quality after the operation of the ADF would be improved to a level that is better than the pre-HATS Stage 1 condition to
facilitate early re-opening of the Tsuen Wan beaches.
2.17
Without disinfecting the HATS Stage 1 effluent,
the Tsuen Wan beach water quality cannot be restored even with the completion
of local sewerage works which can only arrest the local pollution
problems. The proposed ADF, in conjunction with the local sewerage works
being implemented, will bring about the necessary water quality improvements
needed for the re-opening of Tsuen Wan beaches at the earliest possible
moment. If the ADF were not implemented, the Tsuen Wan beach water quality
would remain jeopardised by the un-disinfected HATS Stage 1 effluent, until the
permanent disinfection facilities proposed under Stage 2A are commissioned. The current timetable for HATS Stage 2A is for substantial completion to be achieved by end 2014. This is about five years behind the target commissioning date of end 2009 for completing the ADF.
2.18
In conclusion, the provision of the proposed ADF
is an integral part of the sewerage programme to restore the water quality of
the Tsuen Wan beaches at the earliest possible moment. The E. coli levels
at the Tsuen Wan beaches during the ADF stage have been quantified by using mathematical
model. As the model input data such as the HATS flow rates and the background
pollution loading tended to be conservative to provide a margin of tolerance
(refer to Section 5.65 and Appendix 5.2 of this EIA report), it
is likely that the actual E. coli levels at the beaches could be lower
than that predicted in this EIA. This will be ascertained by monitoring
of the actual beach water quality. Once the actual monitoring
results confirm that the beach water quality has returned to an acceptable
level for swimming, arrangements to re-open the beaches for public enjoyment
will be instigated.
Consideration of Alternative Disinfection
Methods and Selection of Preferred Disinfection Technology
2.19
Disinfection is the destruction, inactivation,
or removal, to an appropriate level, of those microorganisms with the potential
to cause infection (and therefore harm to people), and refers to the use of a
technique designed specifically to reduce the number of viable infectious
microorganisms in an effluent. In this context, conventional primary
and/or secondary treatment are not considered as disinfection techniques,
although these forms of treatment do reduce concentrations of microorganisms in
sewage.
Project Specific Considerations
2.20
To achieve an optimal disinfection outcome for
HATS (including the ADF), the selection of disinfection technology must take
into accounts several project-specific conditions or requirements as well as
the unique environmental setting of SCISTW.
Scale of Facility
2.21
The SCISTW is a mega-sized facility serving
several million people. The quantity of effluent needing disinfection at
SCISTW will range from about 1.7million m3/day at 2009 (when the
proposed ADF is commissioned) to possibly around 2.5million m3/day
under ultimate Stage 2A/B conditions. This very large capacity of SCISTW and its pivotal role in marine
water quality protection means that any adopted disinfection technology must
have been proven reliable for application at similar scales.
Phased Implementation of HATS Stage2
2.22
Owing to the phased nature of the HATS Stage2
implementation programme, two types of effluent are envisaged at different
times. These are:
·
CEPT effluent,
which is being produced by SCISTW and will continue to be produced throughout
Stage 2A
·
Biologically treated (secondary) effluent, which will be produced by an upgraded SCISTW under HATS Stage 2B
2.23
CEPT and biologically treated effluents have
different characteristics. For example, un-disinfected CEPT effluent
typically has higher concentrations of microorganisms (including pathogens) and
suspended solids, and lower UV light transmittance when compared to
un-disinfected biologically treated effluent. Further, at present, ferric
chloride is used as coagulant in the CEPT process, and this is known to
interfere with the performance of UV radiation disinfection.
2.24
As performance and efficiency of disinfection
are related to effluent characteristics or quality, different disinfection
technologies may be optimal at different times of HATS. Therefore,
flexibility to suit varying effluent quality will need to be considered when
selecting the most appropriate disinfection scheme for HATS.
Early Improvement to Tsuen Wan Beach Water Quality
2.25
The discharge of un-disinfected sewage effluent
from SCISTW is contributing to unsatisfactory beach water quality at the Tsuen
Wan beaches. On this, the Report of the Public Accounts Committee (PAC)
of the Legislative Council on the Director of Audit’s Report No. 42 (2004) has
requested the Administration to “take into account the high bacteria level
of the effluent discharged from the Stonecutters Island Sewage Treatment Works
in planning the further stages of HATS, and in evaluating the options for
providing a permanent disinfection facility in the long term”.
2.26
In response to this request and trusting that
early restoration of the environmental quality should be pursued as far as
possible, the Government is proposing to advance the provision of part of the
permanent disinfection facilities under HATS Stage 2A at the earliest opportunity, rather than to wait until the completion of Stage 2A in late 2014. Therefore, the selected disinfection option for the ADF should be able to be implemented reasonably quickly (within two to three years).
Environmental Setting
2.27
Environmental setting is generally favourable
for effluent dilution or dispersion from the existing SCISTW marine
outfall. Key features of the SCI outfall include:
·
Beneficial use of the water body off SCI is
primarily “marine traffic”, meaning that there are virtually no sensitive
receivers in close proximity of the effluent discharge point
·
The ecological value of the inner harbour waters
is generally low
·
Water sensitive receivers (e.g., beaches, fish
farms, coral sites, marine parks, etc) are remote from the SCI outfall
·
Water currents are relatively high at or near
the SCI outfall, meaning that the effluent plume can be effectively diluted and
dispersed before it reaches the sensitive receivers
·
The receiving marine water facilitates die-off
of infectious microorganisms by virtue of natural environmental and biological
factors. These include temperature, solar radiation, salinity, natural
mortality, and the presence of protozoan flagellates.
Approach to Option Evaluation
2.28
With the above project-specific considerations
in mind, a comprehensive four-tier option evaluation has been conducted to
select the most appropriate disinfection technology for HATS (and the ADF),
considering environmental as well as non-environmental factors. The
evaluation process included long-listing and short-listing of technology
options, along with multi-criteria assessments with increasing levels of
sophistication.
2.29
In parallel, a review of local and international
disinfection practices was also conducted to provide general information for
reference by the project team.
2.30
Details of the option evaluation and review of
local and international practices are contained in Appendix 2.1. A summary of the key findings
are presented in the following paragraphs.
Multi-tiered, Multi-criteria Option Evaluation
Technology Long-listing
2.31
Drawing on established practices, input from the
public consultation sessions, and considering new or emerging disinfection
technologies, five groups of potential disinfection technologies (some with
sub-options) have been evaluated for SCISTW :
·
Chemical-based technologies:
o
Chlorination (and chlorine-based technologies)
o
Ozonation
o
Other non-chlorine based disinfection chemicals,
including mainly oxidants such as potassium permanganate, potassium ferrate,
hydrogen peroxide, peracetic acid, etc, and combinations of chemicals, such as
“ozone/hydrogen peroxide”
·
UV radiation
·
Membrane technologies
·
Artificial reefs
·
Extension of SCISTW outfall to enhance mixing
and dispersion of the effluent
Tier 1 Evaluation – Technical Feasibility
2.32
These initial options were evaluated against
feasibility requirements, in terms of functionality and proven-ness. In
summary, the Tier 1 evaluation showed that:
·
Chlorination and UV radiation are feasible, though they have different degrees of proven-ness for
application to a mega-scale project like HATS.
·
No other disinfection options are feasible for
HATS. It is acknowledged that some of these alternative technologies are
feasible for small-scale applications, but they are either unworkable or
unproven at the scale contemplated for HATS.
2.33
In conclusion, it was proposed to carry forward
both chlorination and UV radiation to the next stage of the evaluation.
Tier 2 - Confirmation of Environmental Acceptability
2.34
Before chlorination and UV radiation were
short-listed for further evaluation in Tier 3, an assessment of the fundamental
acceptability of the technologies was conducted with respect to the key aspects
of marine water quality and human/ecological health. The short-listed
technology will not be selected for further evaluation if its environmental
acceptability cannot be confirmed in this tier of evaluation.
2.35
With respect to chlorination, the combined
effect of source control measures (e.g., de-chlorination), effective initial
dilution/far field mixing, and absence of water sensitive receivers or uses in
the near field is that there should be no or negligible risk to human health or
aquatic lives. This has been confirmed by human health and ecological
risk assessments (see Sections 6 and 7), which have been based on precautionary
principle by adopting conservative approaches and assumptions, including:
l
Carry out laboratory analyses and include in
risk assessments of all the (9) chlorination by-products (CBPs) regulated
under US Drinking Water Standards plus all (25) chloro-organic compounds
in the priority pollutant list regulated under US National Pollutant Discharge
Standards, although these priority pollutants are not regulated due to the
concern of generation during chlorination process.
l
Adopt maximum effluent
and background contaminant concentrations (instead of mean values) to calculate
chemical of concern (COC) concentrations at exposure points (also see Section
6.37, 7.53 and 7.54)
l
Neglect the effect of possible chemical
degradation, volatilization and other mechanisms (also see Section 6.37, 7.53
and 7.54)
l
Adopt conservative incidental water ingestion
rate during swimming activities and seafood consumption rate (also see Section
6.37)
l
Include exposure scenarios of dropping into the
harbour once per year and frequent swimming at the edge of mixing zone, which
are unlikely to happen (also see Section 6.37)
l
Assume that aquatic organisms live their entire
lives at the mixing zone (also see Section 7.53)
l
Adopt conservative area use factors for marine
mammals (also see Section 7.54)
2.36
In addition, precautionary principle is followed
through by conducting Whole Effluent Toxicity Test (WETT) using
chlorinated/dechlorinated effluent at a higher chlorine dosage. The
results of the WETT provide assessment on combined effects of all the chemical
species present in the chlorinated/dechlorinated effluent including the
possible additive/synergistic/antagonistic interactive effects onto
representative local marine biota and fisheries (also see Section 7.18).
2.37
The findings of the human health and ecological
risk assessments are as follows:
·
Risks to human health and ecological resources
are low and within acceptable limits
·
No adverse impact on aquatic lives due to acute
and chronic toxicity
·
Chlorination disinfection is an environmentally
acceptable option for HATS, with respect to water quality, human and ecological
health risks
2.38
With respect to UV radiation:
·
Disinfection of sewage effluent by UV radiation
is generally considered environmentally benign. Aldehydes and other
oxidation by-products were found to exhibit a UV-dose response increase in
previous pilot studies using both low- and high-intensity UV systems, although
the elevated levels were found to be within the acceptable limits. UV pilot tests conducted at SCISTW also indicated that there
was no significant increase of any of the tested
compounds.
·
For the purpose of this assessment, it has been
considered that no or insignificant level of potentially toxic DBPs from UV
radiation would enter the receiving water body. Therefore, the risk on
human or ecological health due to UV radiation of effluent is insignificant or
negligible.
2.39
Overall, both chlorination and UV disinfection
are environmentally acceptable options for HATS, with respect to water quality,
human and ecological health risks.
Tier 3 Evaluation – Environmental Considerations
2.40
Subsequently, the two short listed options were
further developed. For chlorination, it was concluded that purchase of
sodium hypochlorite solution would be the most appropriate way of effecting
chlorination. For UV radiation, either low- or medium-pressure high
intensity UV lamps would be suitable for application at SCISTW.
2.41
In the Tier 3 evaluation, both options were
assessed against a comprehensive range of environmental criteria, including
fisheries, air quality, noise, landscape and visual impacts, waste management
implications, hazards to public, in addition to water quality, human health,
and marine ecology.
2.42
The analysis suggested that both disinfection
options would be environmentally acceptable. The relative performances of
each disinfection option against the environmental criteria are elaborated
below.
·
With respect to water quality improvement
(pathogen reduction), chlorination (purchase of sodium hypochlorite) would
be preferred to UV radiation. This is because the chlorination system
could be implemented much sooner than the UV system, with the potential to
bring about earlier improvement to water quality (reduction of pathogens) at the
Tsuen Wan beaches.
·
On human health, marine ecology, and
fisheries, the key concerns of chlorination are toxicity generated by total
residual chlorine (TRC) and formation of chlorination by-products (CBPs) (which
are mainly chlorinated organic compounds). It is noted that:
-
TRC can be removed from the effluent by
de-chlorination such that it would not pose a significant concern to human
health, marine ecology, or fisheries.
-
The formation of CBPs is related to the strength
of the sewage and the dosage of chlorine added to the wastewater. In the
case of HATS, chemical tests have confirmed that there are few CBPs in the HATS
effluents, and that the detected CBPs are of relatively low concentrations, and
found to be well within acceptable limits.
-
Further, hydrodynamic conditions are favourable
to effective mixing and dilution of the discharged effluent from SCISTW, while
the SCI outfall is located well away from human receivers, sensitive marine
resources, and fisheries resources.
-
The project-specific human and ecological health
risk assessments (based on conservative approaches) conducted as part of the
EIA study have demonstrated that the risks are low and are well within
acceptable criteria.
-
In conclusion, in the context of HATS, effluent
disinfection by chlorination poses low and therefore acceptable risk to human
health, marine ecology, and fisheries with the proposed dechlorination system,
precautionary design measures and accepted good operation practice.
·
Two groups of contaminants of concerns are associated
with UV radiation: mercury from the UV lamps and disinfection by-products
(DBPs) from the UV radiation process.
-
With respect to mercury, this could be released
into the effluent if the UV lamps break during operation and maintenance
activities. This is unlikely, however, given that proper procedures will
be in place.
-
Regarding DBPs from UV radiation of sewage,
there is relatively little information, as this subject is being
researched. Aldehydes and other oxidation by-products were found to
exhibit a UV-dose response increase in previous pilot studies using both low-
and high-intensity UV systems, although the elevated levels were within the
acceptable limits. UV pilot tests conducted at SCISTW also indicated there was
no significant increase of any of the tested compounds. This means that any
health risks due potential DBPs associated with UV radiation would be low.
-
Nevertheless, hydrodynamic conditions are
favourable to effective mixing and dilution of the discharged effluent from
SCISTW, while the SCI outfall is located well away from human receivers,
sensitive marine resources, and fisheries resources.
-
Again, in the context of HATS, effluent
disinfection by UV radiation is considered to pose low and therefore acceptable
risk to human health, marine ecology, and fisheries with precautionary
design measures or generally accepted good operation practice (e.g., storage
and handling of UV lamps etc).
·
With respect to air quality and noise,
neither disinfection option would generate any significant air and noise
emissions during operation. Therefore, either option is considered to
have no material air quality or noise impact.
·
Concerning landscape and visual impacts,
given the industrial setting of the area and the scale of disinfection plant
(either chlorination or UV radiation) relative to the SCISTW, it is considered
that either option would be acceptable without mitigation measures over and
above generally accepted good practice (e.g., sympathetic architectural
treatment).
·
Regarding waste management implications,
chlorination is preferred to UV radiation. This is because:
-
Application of UV radiation to CEPT effluent
(Stage 2A) would require switching the coagulant to alum (from ferric chloride) in the CEPT process. This, in turn, would lead to about 10%
extra sludge each day (about 100 to 130 wet tpd) requiring disposal at
landfills.
-
The capacity available for co-disposing sewage
sludge (with other solid wastes) at landfills is depleting rapidly. The
extra 10% of alum sludge is not a significant amount. To overcome this
problem, it may be necessary to pre-process the sludge to a condition that is
suitable for direct disposal at landfills.
-
UV lamps contain mercury, and spent UV lamps are
hazardous wastes requiring special disposal practice or re-processing by the
manufacturer. It has been estimated that on average thousands of spent UV
lamps would be generated each year under Stage 2A.
-
For Stage 2B, where biologically treated
effluent is involved, no extra sludge would be generated (as there would be no
need to use alum in the upstream CEPT process). However, spent UV lamps
will continue to be managed or disposed of as a hazardous waste.
·
With respect to hazards to public:
-
The chlorination option would involve sodium
hypochlorite and sodium bisulphite solutions. Given the nature of the
chemicals (neither substance is explosive or flammable), and the location of
SCISTW (which is isolated and remote from population centres), storage and
handling of these chemicals in SCISTW are not expected to pose any material
hazards to the public. Transportation of these substances to SCISTW will
be by purpose-built marine vessel or road tankers, and will follow standard HK
Fire Services Department requirements. Overall, it is expected that the
hazard posed to public during transportation of these substances would be low
with adoption of relevant Fire Services Department codes of practices.
-
The UV option would also pose a certain degree
of hazard to the public during transportation of spent UV lamps (which contain
mercury, a hazardous waste) from the disinfection plant to the disposal site
(e.g., the Chemical Waste Treatment Facility at south Tsing Yi).
Nevertheless, risks are considered acceptable with standard precautionary measures.
2.43
Overall, both chlorination and UV radiation
would comply with relevant environmental assessment criteria. Therefore,
both are considered preferred from an environmental perspective, as neither
option would lead to unacceptable or adverse environmental impacts
(effects). This also means that adoption of either chlorination or UV
radiation will avoid adverse environmental effects to the maximum practicable
extent.
2.44
Nevertheless, neither disinfection option is
superior to the other on all of the environmental aspects considered in the
Tier 3 evaluation. Each option has environmental benefits and disbenefits
relative to existing (i.e. baseline) conditions, as highlighted in Table 2.2.
Table 2.2 Summary of Environmental
Benefits and Disbenefits of the Disinfection Options
|
DISINFECTION
Option
|
benefits
|
disbenefits
|
|
Chlorination
|
§ Reduction of pathogen discharge from
un-disinfected effluent and the associated potential heath and safety
problems
§ Improvement to beach water quality
§ Can be installed at the earliest
opportunity (i.e. before end 2009)
|
§
Potential discharge of
chlorine residual, which is toxic to aquatic life, but the residual can be
controlled by the proposed dechlorination system to meet the TRC discharge
limits (0.2 mg/l at 95 percentile and 0.4 mg/l at maximum) as proposed in
Section 5).
§ Formation of potentially harmful
chlorinated organic compounds, but the short- and long-term residual risks to
human and ecological health have been found to be well within acceptable
limits
§ Potential hazard to life due to handling
and storage of disinfection chemicals, but this can be eliminated/minimized
upfront with precautionary design measures and the residual risk level is
well within acceptable criteria
|
|
UV radiation
|
§ Reduction of pathogen discharge from
un-disinfected effluent and the associated potential heath and safety
problems
§ Improvement to beach water quality
|
§ Generation of spent UV lamps which contain
mercury (which is a hazardous waste), but this will be controlled by recycle
and re-processing by the UV equipment suppliers
§ Possible mercury release into the effluent
when UV lamps break during operation and maintenance activities, but this is
unlikely given proper operation procedures will be in place
§ Formation of potentially harmful compounds
(e.g., aldehydes and other oxidation by-products) , but the residual risks to
human and ecological health have been found to be well within acceptable
limits
§ Generation of additional waste sludge from
the CEPT process due to the need to use alum instead of ferric chloride as
coagulant but the increase in sludge volume is slight (about 100 wet tpd).
|
2.45
To select a disinfection option for HATS, it was
necessary to proceed to Tier 4 evaluation, which has considered a broader range
of criteria (other than environmental aspects).
Tier 4 Evaluation- Consideration of Non-environmental
Factors
2.46
The two short listed options were further
evaluated in Tier 4 against four non-environmental criteria:
·
Cost, i.e.,
lifecycle cost, covering both capital, and operating and maintenance
components. The option with the lower the total lifecycle cost is
preferred.
·
Flexibility of
the disinfection system to cater for project risks or uncertainties, including,
for example, sewage flow and quality variations, delay to overall HATS
programme, etc. The more flexible option would be preferred.
·
Implementation issues, in terms of impact on existing SCISTW operations, and time
required to build and commission the disinfection facility.
·
Scale-Up Factor is
the capacity of the HATS disinfection plant relative to that of the largest
known and established plant using the same disinfection technology. It
may be regarded as an indirect measurement of the level of proven-ness of the
disinfection technology for application to HATS. The option with a
smaller scale up factor would be preferred, as the technical risks and
operational challenges to be managed or overcome would be smaller. As
HATS Stage 2 will be implemented in phases, the scale up factor is most
critical for disinfecting the type of effluent encountered at the initial stage
of HATS, i.e., CEPT effluent under the ADF and Stage 2A.
2.47
In essence, the Tier 4 evaluation has concluded
that:
·
Chlorination would be less costly than UV
radiation on a total lifecycle cost basis. This is because while the
operation and maintenance (O&M) costs of the chlorination and UV radiation
systems would be similar, the former would have a much lower capital cost, as
shown in Appendix
2.1. Note that:
-
The major capital cost item for the
chlorination/dechlorination system would be storage tanks and a dosing system,
which would be much lower than the cost of constructing a UV radiation plant of
equivalent capacity.
-
The major O&M cost item for chlorination would
be chemicals while for UV radiation it would be energy (electricity). For
HATS, these are comparable.
·
Chlorination would be more flexible to
deal with project risks such as lower (than expected) population (sewage)
growth, delay to project implementation due to socio-political factors,
variable effluent quality, etc. Specifically,
-
The quantity of sodium hypochlorite to be
purchased can be adjusted relatively easily to deal with these
uncertainties. That is, more or less sodium hypochlorite may be purchased
to suit actual sewage quantity and quality at any point in time.
-
On the other hand, the UV radiation plant will
have to be built to the largest anticipated capacity to deal with a CEPT-type
effluent under Stage 2A. Yet, about half of the UV plant capacity will become redundant as Stage 2B is implemented, as the quality of secondary effluent is
much better than CEPT effluent and will require a lower UV radiation
dose.
·
Chlorination would be quicker and easier
than UV radiation to implement at SCISTW, due to its relatively simple
construction (essentially only tanks and a dosing system). If a UV
radiation plant were to be implemented, an additional electrical substation
would be required to power the UV plant, and additional land (that is currently
not available) would need to be secured to accommodate the UV plant. The
time taken to provide these would be several years, meaning that improvement to
marine water quality will be delayed compared to applying chlorination at
SCISTW.
·
For HATS, developing a chlorination plant at
SCISTW would involve a smaller scale-up factor than UV radiation for the
critical case of disinfecting the CEPT effluent under the ADF and Stage 2A. This is elaborated below:
-
There is some experience
with applying UV radiation to disinfect CEPT effluent at several STWs in Hong Kong, e.g., Siu Ho Wan Phase 1 (96,000m3/d), Cyberport (10,800m3/d), and Sham Tseng (16,850m3/d).
-
Internationally, the largest known CEPT plant
using UV radiation disinfection is the Sand Island STW, Honolulu, USA (320,000m3/d), which is being commissioned at the moment.
-
The proposed ADF or Stage 2A CEPT disinfection plant at SCISTW (up to 2.5million m3/d) would be one to two orders of magnitude larger than these local or overseas CEPT STWs. Therefore,
applying UV radiation to HATS CEPT effluent would involve scaling up the UV
system by the corresponding orders of magnitude to deal with the challenges of
the local situation (e.g., effluent UV transmittance profile).
-
On the other hand, chlorination has been applied
to treat CEPT effluent at many large STWs, e.g., Deer Island STW, Boston, USA (1.8million m3/d) and Newtown Creek STW, New York, USA (1.2million m3/d). This scale
of application of chlorination (to CEPT effluent) is of the same order of
magnitude as HATS.
-
Overall, on a comparative basis, chlorination
would involve a much lower scale up factor. Hence, it is a relatively
more proven technology than UV radiation for disinfecting CEPT effluent for
HATS.
2.48
Overall, chlorination (purchase of sodium
hypochlorite solution) is preferred to UV radiation against the Tier 4
evaluation criteria.
Conclusions on Disinfection Alternatives
2.49
As concluded from the Tier 1 evaluation, the
feasible sewage disinfection options for HATS are chlorination and UV
radiation.
2.50
Based on the outcome of Tier 2 evaluation, it is
concluded that both chlorination (purchase of sodium hypochlorite) and UV
radiation are environmentally acceptable for HATS. Water quality and
ecological impacts can be controlled to well within established criteria
(contained in the Technical Memorandum of EIA Process).
2.51
A comparison of the relative environmental
performance of chlorination (purchase of sodium hypochlorite) and UV radiation
has been made in the Tier 3 evaluation. It is concluded that:
·
Neither chlorination nor UV radiation is
superior to the other on all of the environmental aspects. Nevertheless,
neither option would result in adverse environmental effects (or impacts
exceeding the relevant criteria in the TM).
·
Therefore, both chlorination (purchase of sodium
hypochlorite) and UV radiation are preferred from an environmental perspective.
2.52
For chlorination,
·
The environmental benefit is that it will bring
about significant reduction of pathogen discharge from un-disinfected effluent
and the associated potential heath and safety problems; and consequently
improvement to the Tsuen Wan beach water quality at the earliest opportunity
(by end 2009)
·
The environmental disbenefits are:
-
Potential discharge of chlorine residual, which
is toxic to aquatic life, but the residual can be controlled by the proposed
dechlorination system to meet
the TRC discharge limits (0.2 mg/l at 95 percentile and 0.4 mg/l at maximum) as
proposed in Section 5).
-
Formation of potentially harmful chlorinated
organic compounds, but the short- and long-term residual risks to human and
ecological health have been assessed to be well within acceptable limits
-
Potential hazard to life due to handling and
storage of disinfection chemicals, but this can be eliminated upfront with
precautionary design measures and the residual risk level is well within
acceptable criteria
2.53 For UV radiation,
·
The environmental benefit is that it will also
bring about significant reduction of pathogen discharge from un-disinfected
effluent and the associated potential heath and safety problems; and
consequently improvement to the Tsuen Wan beach water quality
·
The environmental disbenefits are:
-
Generation of spent UV lamps which contain
mercury (which is a hazardous waste), but this will be controlled by recycle
and re-processing by the UV equipment suppliers
-
Possible mercury release into the effluent when
UV lamps break during operation and maintenance activities, but this is
unlikely given proper operation procedures will be in place
-
Formation of potentially harmful compounds
(e.g., aldehydes and other oxidation by-products), but the residual risks to
human and ecological health have been found to be well within acceptable limits
-
Generation of additional waste sludge from the CEPT
process due to the need to use alum instead of ferric chloride as coagulant but
the increase in sludge volume is slight (about 100 wet tpd)
Preferred
Option
2.54
As discussed earlier, a multi-tier
multi-criteria option evaluation has been conducted to recommend a preferred
disinfection technology for HATS. The Tier 1 evaluation was a screening
test, and served to identify only those disinfection options that would be
technically feasible for HATS.
2.55
Environmental considerations have played a key
part in the technology selection process for HATS. These included:
·
In Tier 2 evaluation, the environmental
acceptability of both technically feasible options (UV radiation and
chlorination) in terms of water quality, human health, and ecological health
was confirmed before these were taken forward to the next stage of the
evaluation.
·
In Tier 3 evaluation, both UV radiation and
chlorination were assessed against nine environmental criteria, i.e.,
fisheries, air quality, noise, landscape and visual impacts, waste management
implications, hazards to public, in addition to water quality improvement
(pathogen reduction), human health, and marine ecology.
2.56
The Tier 3 evaluation has concluded that, from
an environmental perspective, both chlorination (purchase of sodium hypochlorite)
and UV radiation would be preferred. Neither option would result in any
adverse environmental effects.
2.57
Adoption of either option will avoid adverse
environmental effects to the maximum practicable extent, as either option will
comply with all relevant environmental criteria.
2.58
Nevertheless, neither disinfection option would
be superior to the other on all of the nine environmental criteria. There
were relative environmental benefits and disbenefits, depending on the criteria
under consideration, as elaborated above.
2.59
On “non-environmental” considerations,
chlorination does have advantages over UV radiation in the context of HATS
(including the ADF), in terms of a lower total lifecycle cost, a higher level of flexibility to deal with uncertainties and
project risks, easier and speedier implementation at SCISTW, and lower
technology scale-up factors and hence more proven
application in large-scale STWs. These
“non-environmental” advantages of chlorination also bring about significant
environmental benefits. In particular,
·
Adoption of UV radiation for the ADF (and Stage 2A), where CEPT effluent is involved, will result in substantial abortive works (hence capital investment and wastage of resources) at Stage 2B when biological treatment is
implemented.
·
Early commissioning of the disinfection plant at
SCISTW would bring about water quality improvement (pathogen reduction) at the
earliest opportunity.
·
Further, given the mega-size of SCISTW
(2.5million m3/d), and its pivotal role in marine water quality
protection, it would be prudent to minimise the technical risks and operational
challenges associated with system scale up.
2.60
Therefore, it is recommended that chlorination
(purchase of sodium hypochlorite solution) should be adopted as the disinfection
technology for HATS in preference to UV radiation.
2.61
In arriving at the above conclusion,
environmental factors have played a significant role, as manifested in the
conduct of Tiers 2 and 3 evaluations.
Review of Local and International Disinfection
Practices
2.62
As elaborated above, selection of sewage
disinfection technology is necessarily a project-specific exercise that must
consider a wide range of technical and non-technical constraints and issues
unique to the situation at hand. With this in mind, the objective of the
review of local and international disinfection practices was to provide
supplementary information for general reference by the project team. Appendix 2.1 contains the review, which is summarised below.
Local Review
2.63
In Hong Kong, there are over 260 sewerage
facilities. Eleven of these are municipal STWs employing primary,
secondary, or tertiary treatment. Most of these municipal STWs are less
than 100,000m3/day, with the exception of SCISTW (1.7million m3/d) and Shatin STW (340,000m3/d).
2.64
Disinfection of secondary effluent by either
chlorination or UV radiation has been practised for some time in Hong Kong. These include UV disinfection at Shek Wu Hui STW (80,000m3/d) and Sai Kung STW (22,000m3/d), both employing secondary treatment. At Stanley STW (11,600m3/d), the secondary effluent is disinfected using chlorination.
2.65
For CEPT effluent, there is some recent
experience with UV radiation disinfection at several STWs, including Siu Ho Wan CEPT Plant Phase 1 (96,000m3/day), Sham Tseng CEPT Plant (16,850m3/day), and Cyberport CEPT Plant (10,800m3/day).
2.66
To sum up, both chlorination and UV radiation
have been adopted at local STWs for either CEPT or secondary effluent, though
all these are at a scale much smaller than that anticipated for HATS (up to
2.5million m3/d). This point has been considered in the option
evaluation process, as mentioned previously.
International Review
2.67
The objective of the international survey was to
gather information on disinfection practices, being or likely to be adopted in
the near future, in coastal cities that discharge their treated effluents in
marine waters. It is cautioned that the conditions that led to the
adoption of a particular disinfection scheme or technology by an overseas
sewage treatment plant could be complex and might include a multitude of
technical and non-technical (e.g., social, political, etc)
considerations. As such, the review is to provide a general reference,
and it should not be taken as a definitive account of overseas trends or
practices.
2.68
Based on a set of criteria (e.g., population,
treatment requirements, level of infrastructure development, etc), 24 coastal
cities were selected for the survey, including eight (8) in North America, nine
(9) in Asia/Australia/New Zealand and seven (7) in Europe.
2.69
In these 24 cities, 132 sewage treatment works
(total capacity 38,688,000m3/day) were surveyed. Of the total effluent, 36% of it is discharged into receiving waters without
disinfection, 58% uses chlorination, 5% uses UV disinfection, and less than 1%
uses other technologies.
2.70
Of these 132 STWs, 13 were primary or CEPT STWs,
collectively treating about 6.8million m3 of effluent each
day. Of the surveyed primary STWs, the majority (69.4%, by volume) of the
effluent is discharged to receiving waters (mostly marine waters except two
were rivers) without disinfection. About 30.3% (by volume) of the
effluent is disinfected by chlorination, with the capacity of the largest STW
at about 1.2million m3/day. One small CEPT plant (capacity of
0.024million m3/day) uses UV radiation, accounting for about 0.4% of
the amount of effluent treated.
2.71
With respect to the 119 surveyed secondary or
tertiary STWs (totalling 31.9million m3 of effluent each day), about
29% (by volume) of the treated secondary/tertiary effluent is discharged to
receiving waters without disinfection. The
majority (65%, by volume) of the effluent is disinfected by chlorination, with
the capacity of the largest STW (which is in Deer Island, Boston, USA) at 1.8million m3/day. UV radiation is used to disinfect about 5% (by volume)
of the effluent, with the capacity of the largest plant (which is Ringsend STW,
Dublin, Ireland) at 0.64million m3/day. One plant in Japan (capacity 0.3million m3/day) uses a combined chlorination and ozonation process to
disinfect the treated effluent, prior to discharging it to the upstream of
water gathering catchments.
2.72
The statistics may also be presented in terms of
STW size. Of the 132 surveyed STWs, 48 were considered “large” (i.e.,
having a treatment capacity higher than 100,000m3/day). Collectively, these 48 large STWs treat about 36.6million m3 of effluent each day. The disinfection practices adopted in these large STWs
are summarised below:
·
About 36% (by volume) of the effluent is
discharged to receiving waters without disinfection.
·
Chlorination is used to disinfect about 60% (by
volume) of the effluent, with the capacity of the largest STW (which is a
secondary STW in the USA) at 1.8million m3/day.
·
UV radiation is used to disinfect about 3% (by
volume) of the effluent, with the capacity of the largest plant (which is a
secondary/tertiary STW in Ireland) at 0.64million m3/day
2.73
In conclusion, the review has found that
effluent disinfection is not practised at all of the surveyed STWs. Where
disinfection is employed, chlorination is the major technology (in terms of
volume of effluent treated) for either primary/CEPT, secondary, or tertiary
effluent. UV radiation is the next major
disinfection technology after chlorination. The review shows that it is
applied more to secondary/tertiary effluent than to primary/CEPT effluent.
2.74
With respect to disinfection practices being
applied currently or to be applied in the coming years, it was found that:
·
Of the disinfection facilities commissioned in
the surveyed cities in the last 10 years, some 75% of the installed capacities
use chlorination/dechlorination, whilst the remaining 25% use UV radiation for
disinfecting mostly secondary or tertiary effluents.
·
For those facilities to be commissioned in the
coming five years, the total flow capacity of the new UV radiation
installations is similar with that of new chlorination facilities.
·
Most of the UV facilities installed/to be
installed are for disinfection of secondary/tertiary effluents.
2.75
To sum up, overseas STWs are required to meet
effluent quality standards and/or comply with receiving water quality
objectives. Both chlorination (essentially sodium hypochlorite) and UV
radiation continue to be the preferred mainstream disinfection technologies for
municipal STWs.
2.76
Overall, there is not a simple, clear trend on
disinfection technology being adopted in future STWs. It would appear
that adoption of disinfection technologies is specific to local conditions and
requirements. This is consistent with the project-specific approach to
disinfection technology selection for HATS, as outlined in the preceding
sections
Project Elements
2.77
Key elements of the proposed ADF include:
·
Chlorination system - provision of a sodium
hypochlorite solution storage plant and associated dosing system, including:
- Six (4 duty and 2 standby) sodium
hypochlorite storage tanks (8m in diameter, 12.5m in height)
- One day tank for sodium hypochlorite
storage (capacity of about 100m3)
- Pipes in pipe trenches
- Switch room
- Chemical feeding and transfer system
- Bund wall (2m in height)
- ISO tank unloading area
- Sodium hypochlorite barge unloading
facility
·
Dechlorination system - provision of a sodium
bisulphite solution tank farm and associated dosing system, including:
- Two sodium bisulphite storage tanks (6m in diameter, 6.2m in height)
- Chemical feeding and transfer system
- Bund wall (1.5m in height)
- ISO tank unloading area
- Security fence and gate
2.78
A location plan of the proposed disinfection
facilities at SCISTW is shown in Figure 2.1.
Construction Method
2.79
The Project would involve the following major
construction activities:
l
Site formation & site establishment
l
Piling
l
Excavation and backfilling
l
Erection of formwork and reinforcement
l
Concreting
l
Fabrication of steelwork & installation of
E&M equipment
l
Testing and commissioning
Operation of the Disinfection Facilities
2.80
Under the recommended option of purchasing
sodium hypochlorite solution for the ADF, the existing effluent culvert system
will be used as the chlorine contact basin. Sodium hypochlorite solution
(w/v 10%-12%) as the disinfection agent will be purchased and delivered to SCISTW by a specially designed delivery barge and
unloaded at the berthing facility at an interval of 2 to 3 times a week (by
vehicles when weather or other conditions do not allow chemicals delivery by
sea). Sodium hypochlorite solution will be transferred to the storage
tanks in the chlorination plant through a designated and isolated piping system
laid within pipe trench. Then the chemical will be pumped from the
storage tanks to a day tank next to the flow
distribution chamber. The sodium hypochlorite
solution from the day tank will then combine with
carrier water (effluent) and dose to the flow distribution chamber through a
diffuser installed below effluent surface (see Figure
2.1). The maximum storage amount for
sodium hypochlorite solution on-site for the ADF operation is estimated to be
about 1,800,000L.
2.81
Sodium bisulphite solution (w/v 38%) will be used as the dechlorination agent and
dosed at the existing Chamber 15 prior to the outfall discharge (see Figures 2.1 and 2.2).
It will be purchased and delivered to SCISTW at an
interval of 1 to 2 time(s) a week only by road tankers and then unloaded to the
storage tanks. The chemical storage tanks for
sodium hypochlorite solution will be rubber-lined steel tanks and that for
sodium bisulphite solution will be fibreglass reinforced plastic tanks, and
both will be located inside bund walls (see Figures
2.3 and 2.2). The proposed
sodium hypochlorite, proposed sodium bisulphite, and existing ferric chloride
tank farms are physically separated from each other to prevent mixing of the
chemicals in case of failure of the storage tanks.
2.82
When designing a chlorination system, the key
objective is to minimize the use of chlorine and hence cost, as well as to
minimize the formation of chlorination by-products (CBPs). There are two
ways to achieve this objective: providing complete initial mixing between water
and chlorine; and controlling chlorine dosage by monitoring the residual
chlorine in the effluent.
2.83
To achieve complete mixing, sodium hypochlorite
from the day tank will combine with carrier water (effluent) and dose to the
flow distribution chamber through a diffuser installed below effluent surface
to enhance the mixing efficiency between chlorine and water. A better
mixing environment would result in better disinfection efficacy and thus
reducing the required chlorine dose.
2.84
For the control of chemical dosages, two
operation modes, namely Manual Operation Mode and Automatic Operation Mode are
developed. The control philosophy of two operation modes is basically the
same:
·
Sodium hypochlorite is used to disinfect CEPT
effluent, and its dosage is adjusted to maintain TRC levels at Chamber 15 (i.e.
outlet of disinfection system) of 2 – 4 mg/L
·
Sodium bisulphite is dosed to remove TRC before
discharge, and its dosage is adjusted to maintain zero TRC in discharge
·
Both hypochlorite and bisulphite dosing rates
are adjusted based on sewage flow rate and TRC level measured at Chamber 15
·
While laboratory tests have demonstrated that
dechlorination will occur within 15 seconds of adding of the bisulphite
solution to the chlorinated effluent, a dechlorination simulation
loop will be installed at Chamber 15 to monitor the de-chlorinated effluent TRC
and bisulphite level, and to confirm zero TRC level in discharge
·
Regular TRC measurement to ensure the
reliability of control
2.85
The difference between two operation modes is
the TRC measurement methods at Chamber 15, thereby the system’s response time
(chemical dosages adjustment) to the fluctuation of sewage characteristics then
TRC levels.
2.86
Under Automatic Operation Mode, chemical dosages
will be adjusted based on sewage flow rate and online TRC measurement at
Chamber 15. Dual TRC analyzers will be installed at Chamber 15 to monitor the TRC value in chlorinated effluent after travelling
through the effluent culvert and to provide continuous
measurement e.g. 15-min interval. Signals from
TRC analyzer and the flow data will be transmitted to the control system for
adjusting chlorine dosages in order to maintain the TRC of 2 – 4 mg/L at
Chamber 15, and for adjusting bisulphite dosages to ensure zero TRC
discharge. Set point alarms will be established to notify the operator in
case the TRC level at Chamber 15 drifting outside of the desired ranges
immediately. Upon notification of the alarm, the operator should adjust
the chlorine dosage to restore the TRC at Chamber 15 within the design
range. In this way, dechlorination chemical can
be adjusted based on the TRC data every 15 minutes. In addition, dual TRC analyzers and sodium bisulphite analyzers are
provided at the outlet of dechlorination simulation loop to confirm zero TRC in
discharge.
2.87
Under Manual Operation Mode, chemical dosages
will be adjusted based on sewage flow rate and manual TRC measurement at
Chamber 15. Regular TRC measurements will be conducted and dechlorination
chemical will be adjusted based on the measured TRC data. The operator should adjust the hypochlorite dosage when the TRC
measurement shifts outside the design range. A
higher dosage of dechlorination chemical would be provided to ensure zero TRC
in discharge during the Manual Operation Mode. This has allowed
sufficient safety margin to offset the potential fluctuation of TRC between the
hourly measurements. In addition, regular TRC and
bisulphite measurements will be conducted at the discharge of bisulphite
simulation loop to ensure the compliance of TRC discharge standards.
2.88
A schematic diagram of the control strategy is
shown in Figure 2.4.
Project Location
2.89
The proposed chlorination system of the
disinfection facilities would be located within the site boundary of the
existing SCISTW (Figure 2.1 refers).
The proposed dechlorination plant would be located adjacent to the existing
chamber no. 15 (Figure 2.1 refers) at
the western end of Container Port Road South. The land use to the north
of the SCISTW is industrial in nature and comprises Container Terminal no. 8,
the COSCO HIT Terminal Building, bus depot, shipyards and storage areas.
Adjacent to the south-eastern boundary of the SCISTW is the Government
Dockyard. The Ngong Shuen Chau Barracks is located at the south of Stonecutters Island. Route 8, which is under construction, is located adjacent to
the proposed dechlorination plant.
2.90
The discharge
point of the existing SCISTW outfall location was chosen at an area with low
ecological and fisheries value to avoid adverse impacts on key ecological and
fisheries resources. The outfall location is not at an area of high
marine traffic usage.
Continuous Public Involvement
2.91
Public consultations have been conducted for the
Project since an early stage of the EIA process. During December 2005,
public consultation meetings were conducted with green groups and academics and
professional institutions to introduce the EIA Study, brief the latest
progress, present the EIA findings and solicit their views and comments on the
EIA Study so that they could be considered in the EIA Study.
2.92
The list of green groups, professional bodies
and academics institutions that were invited to the consultation exercises is
presented in Table 2.3 while the consultation meetings conducted are
summarized in Table 2.4.
Table 2.3 Green Groups, Professional Bodies and Academics
Institutions Invited for Consultation Exercises
|
Green Groups
|
Academics Bodies / Professional
Institutions
|
|
World Wide Fund for Nature Hong Kong
|
Hong Kong
Institution of Engineers
|
|
Conservancy Association
|
Hong Kong
Institution of Environmental Impact Assessment
|
|
Green Power
|
Chartered Institution of Water and
Environmental Management
|
|
Hong Kong Marine
Conservation Society
|
University of Hong Kong
|
|
Friends of the Earth (HK)
|
Chinese University of Hong Kong
|
|
Earthcare
|
Hong Kong University of Science and Technology
|
|
Hong Kong Dolphin
Conservation Society
|
City University of Hong Kong
|
|
Green Student Council
|
Open University of Hong Kong
|
|
|
Hong Kong Polytechnic University
|
|
|
Hong Kong Baptist University
|
|
|
Marine Biological Association of Hong Kong
|
|
|
Hong Kong Institute of Planners
|
|
|
Hong Kong Institute of Architects
|
|
|
Hong Kong Institute of Surveyors
|
|
|
Association of Engineering Professionals
in Society Ltd
|
|
|
Hong Kong Project Management Exchange
Centre
|
Table 2.4 Summary of
Key Continuous Public Involvement
|
Date
|
Subject
|
|
1 December 2005
|
Public consultation discussion forum
with academics and professional institutions. Attended by following
parties:
Hong Kong Institution of Engineers
Hong Kong Institute of Environmental
Impact Assessment
Chartered Institution of Water and
Environmental Management
University of Hong Kong
Hong Kong University of Science and Technology
Hong Kong Institute of Vocational
Education
Marine Biological Association of Hong Kong
|
|
5 December 2005
|
Public consultation meeting with Green
Peace
|
|
5 December 2005
|
Public consultation meeting with Hong
Kong Marine Conservation Society
|
|
5 December 2005
|
Public consultation meeting with
Worldwide Fund for Nature Hong Kong
|
|
7 December 2005
|
Public consultation meeting with Green
Power
|
|
15 December 2005
|
Public consultation meeting with Hong
Kong Conservancy Association
|
|
2 March 2006
|
Public consultation discussion forum
with academics and professional institutions. Attended by following
parties:
Hong Kong Institution of Engineers
Hong Kong Institute of Environmental
Impact Assessment
Chartered Institution of Water and
Environmental Management
University of Hong Kong
Hong Kong University of Science and Technology
|
|
2 March 2006
|
Public consultation discussion forum
with non-government organizations. Attended by following parties:
Green Power
Worldwide Fund for Nature Hong Kong
|
|
11 May 2006
|
Public consultation meeting with
Earthcare
|
|
25 July 2006
|
Public consultation meeting with Hong
Kong Marine Conservation Society
|
|
12 October 2006
|
Public consultation discussion forum
with green groups, academics and professional institutions. Attended by
the following parties:
Hong Kong Institution of Engineers
Hong Kong Institute of Environmental
Impact Assessment
Hong Kong Project Management Exchange
Centre Limited
Chartered Institution of Water and
Environmental Management
University of Hong Kong
Conservancy Association
Green Power
World Wide Fund for Nature Hong Kong
|
2.93
Key comments and views received from the public
consultation forum held on 1 December 2005 are summarized in Table 2.5.
Table 2.5 Summary of Key
Comments/Views from the Public – December 2005
|
Concerned Parties
|
Key Comments / Views
|
Consultant’s Response
|
|
Green Groups
|
1.
Choice of
disinfection technologies:-
|
|
|
·
Alternative methods
of disinfection should be considered e.g. biofilters combined with artificial
reef
|
·
Disinfection
technology options evaluation to address biofilters
|
|
·
Consider
environmental considerations i.e. concerns on disinfection by-products of
chlorination
|
·
Assessed in the EIA
(Sections 5 to 9 and 12)
|
|
·
Consider
cost-effectiveness of different technologies
|
·
Cost-effectiveness
of different disinfection methods to be assessed and provided
|
|
·
Consider
alternative outfall options
|
·
EIA to address
alternative outfall options
|
|
2.
Environmental
concerns on disinfection:-
|
|
|
·
Potential adverse
impact on marine environment
|
·
Assessed in the
EIA
(Sections 5, 7 to 9)
|
|
·
Long term adverse
effects of disinfection facilities on marine environment
|
·
EIA to address
potential long term effects (Sections 5, 7 to 9)
|
|
·
Potential change in
benthic community near outfall
|
·
Assessed in the EIA
(Section 8) and confirmed by Post Project Monitoring
|
|
3.
Other concerns on
disinfection:-
|
|
|
·
Public health and
safety during delivery of chemicals or disposal of UV lamps
|
·
Assessed in the EIA
(Tables 6.3 and 6.10 of Appendix 2.1, Section 11)
|
|
·
Determine optimum
chemical dosage
|
·
Chlorine dosage
determined in the EIA (Table 5.18)
|
|
·
Address cumulative
impacts in water quality modeling
|
·
Cumulative impacts
from all point sources and non point sources to be included in the water
quality model for cumulative assessment
|
|
·
Interface between
different stages of HATS
|
·
Assessed in the EIA
(water quality impact as well as human health and ecological risk impact were
assessed in Sections 5 to 7)
|
|
Academics and Professional Institutions
|
1. Need for disinfection:-
|
|
|
l
Implement
biological treatment first and review need for disinfection
|
l
The purpose of ADF
is to reduce the bacteria level of the HATS discharge so as to enable
re-opening of the beaches in Tsuen Wan before completion of Stage 2A. The government has plans to implement Stage 2B (biological treatment), but current predictions indicate that the existing level of treatment is sufficient till
the end of the next decade. Hence, the timing for the implementation
Stage 2B is subject to a review to be conducted in 2010/11. Given the
lengthy time frame for various stages of HATS and the outcome of the public
consultation for HATS Stage 2, the public would like to see the re-opening of
the Tsuen Wan beaches as early as possible, and would not welcome the
proposal of delaying the implementation of ADF.
|
|
2. Choice of disinfection technologies:-
|
|
|
l
Alternative methods
of disinfection should be considered e.g. UV treatment, and their
cost-effectiveness
|
·
Cost effectiveness
of different disinfection methods to be assessed and provided
|
|
Concerned Parties
|
Key Comments / Views
|
Consultant’s Response
|
|
|
·
Consider
environmental considerations i.e. concerns on disinfection by-products of
chlorination
|
·
The EIA study has
included the following 5 tasks to address the CBP issue:
1)
Review literature on
formation of CBP during the disinfection process
2)
CBP testing – chlorination / dechlorination
process
3)
Water quality modeling – based on levels of
CBP determined during laboratory tests and would determine the size of the
mixing zone
4)
Whole Effluent Toxicity Testing (WETT) –
results would indicate the acute toxicity using 5 local species
5)
Ecological and human health risk assessment –
results would indicate the chronic toxicity
|
|
·
Secondary treatment
with higher removal of bacteria and other pathogens preferred
|
·
EIA Study (Section
3 of Appendix 2.1) addressed the effectiveness of disinfection technologies
in killing virus based on existing information
|
|
|
|
|
1.
Other concerns on
disinfection:-
|
|
|
·
Address cumulative
impacts i.e. effluent discharges from other STW
|
·
Assessed in the EIA
·
(S6.26 to 6.28 for
cumulative human risk impact and S7.41 to 7.43 for cumulative ecological risk
impact)
|
|
·
Expected chlorine
dosage
|
·
The EIA Study
(Table 5.18) to establish chlorine dosage rate and contact time
|
2.94
A second round of public consultation was held
on 2 March 2006 with the green groups and academics/professional institutions
to present the initial findings of the evaluation and selection of disinfection
technologies under the Project and to seek their views on the initial
findings. Key comments and views received from the public from March to
July 2006 are summarized in Table 2.6.
Table 2.6 Summary of Key
Comments/Views from the Public – March to July 2006
|
Concerned Parties
|
Key Comments / Views
|
Consultant’s Response
|
|
Green Groups
|
1.
Need of
disinfection
|
|
|
l
The need for
disinfection is acknowledged
|
l
Noted.
|
|
2.
Disinfection
Technology Selection
|
|
|
· Is chlorination the holistic approach
that provides the best interest to the public?
(adopt secondary treatment
earlier may be a better option)
|
·
Secondary treatment
is the ultimate goal for HATS
·
Chemically Enhanced
Primary Treatment (CEPT) is still an environmentally acceptable option at
present, which utilizes the limited financial resources of the community more
effectively
·
Chlorination is a
more effective disinfection option for CEPT effluent
|
|
3.
Environmental
Impact of Chlorination
|
|
|
·
Reduced diatom
population may increase other species (e.g. algae responsible for red tide)
|
·
No-Observable-Effect-Concentration
(NOEC) for diatom is 27.2% of Chlorinated/Dechlorinated effluent, at least
29x dilution at edge of Zone of Initial Dilution (i.e. effluent conc. is <
27.2% in marine environment). Therefore, there would be no significant
impact on diatom population in the receiving waters
|
|
·
Whole Effluent
Toxicity Test only considered the effect to single species, the interaction
between different species was not considered
|
·
The acute/chronic
toxicity of the most sensitive species was used to derive acute/chronic
toxicity unit
·
This approach is
used in local and overseas authorities
|
|
4.
Other Concerns
|
|
|
·
The potential
environmental cost of purchasing sodium hypochlorite from China (i.e. pollution produced during the generation process)
·
The Government
should aim at reducing the chemical additions to the marine waters
|
·
Electrochlorination/UV
disinfection has high power consumption, which also contributes to the air
pollution in Hong Kong
·
Can consider
requiring the chemical supplier to meet ISO 14000 standard
·
The operation of
the disinfection facilities will aim to reduce the dosage of chemicals as far
as practicable
|
|
Concerned Parties
|
Key Comments / Views
|
Consultant’s Response
|
|
Academics and Professional Institutions
|
1.
Disinfection
Technology Selection and Operation
|
|
|
·
SCISTW was
originally not designed to include disinfection in the treatment process
-
whether the bench
scale test findings can be achieved in real SCISTW operation?
|
l
There are potential
hydraulic issue i.e. short circuiting or flow imbalance of using the existing
effluent box culvert as chlorine contact chamber, as they are not originally
designed for disinfection facilities. Numerical modelling (Computerized
Fluid Dynamics modelling) has been carried out to ensure that the possible
differences between ideal laboratory and actual conditions can be assessed
and considered in the design
|
|
·
Is disinfection at
SCISTW needed to ensure Water Quality Objective at Tsuen Wan beaches are met?
-
Do other sources
largely contribute to the E-coli conc. at the beaches?
|
·
It was investigated
in details in the EIA Study (Section 5)
|
|
·
Is year-round
disinfection needed because there would be lower “infection risk” in winter
time
|
·
We need to ensure
that the beach can be opened in whole year so that swimmer can use the
beaches in both summer and winter
|
|
·
Any information
concerning the effectiveness of various disinfection technology to remove
pathogens?
|
·
Some studies on
virus and Cryptosporidium oocysts & Giardia cysts
(C&G), the effectiveness of chlorination and UV disinfection is similar
|
|
2.
Environmental
Impact of Chlorination
|
|
|
·
What will be the
risk at the emergency scenario (e.g. chlorination system failure,
dechlorination system failure?)
|
·
Interlock system
would be designed, the chlorination process will stop if dechlorination
failure occurs
·
The acute risk from
total residual chlorine would be investigated in the EIA Study (Section 5)
|
|
|
|
2.95
A third round of public consultation was held on
12 October 2006 with the green groups and academics/professional institutions
to present the findings of the EIA Study and to seek their views on the
findings. Key comments and views received from the public are summarized
in Table 2.7.
Table 2.7 Summary
of Key Comments/Views from the Public – October 2006
|
Concerned Parties
|
Key Comments / Views
|
Consultant’s Response
|
|
Green Groups
|
1.
Need of Project:
|
|
|
l
Was the closure of
Tsuen Wan beaches caused by the background E. coli level rather than the
un-disinfected HATS treated effluent?
|
·
Un-disinfected HATS
treated effluent contributed to additional E-coli level and caused the
deterioration of water quality at Tsuen Wan beaches.
·
To restore the
healthy state of the Victoria Harbour, both sources of E. coli at Tsuen Wan
beaches, un-disinfected HATS effluent and sewage from local discharges need
to be addressed by implementing disinfection facility at the SCISTW and local
sewerage project respectively.
|
|
2. Concern on
selected disinfection option:
|
|
|
l
How can the cost of
sodium hypochlorite be controlled? What would be the projection and
fluctuation of the cost of the chemical?
|
·
Sodium hypochlorite
solution to be used for the disinfection process is a rather common
chemical. There are many suppliers that are able to supply such
chemical and therefore there would be competition in the market. The
price of the chemical would be self-regulated by the competition among
suppliers.
|
|
l
Part of the disinfection
facility would be redundant when HATS Stage 2B commences
|
·
The selected
disinfection technology (i.e. direct purchase of sodium hypochlorite
solution) would be the most flexible approach that is applicable to the
conditions of HATS.
·
The proposed
technology does not involve very capital intensive facilities (only chemicals
storage, dosing and monitoring facilities to be constructed for the initial
years).
l
When HATS Stage 2B
is in operation, it only needs to decrease the chemical dosage and idling of
facility would be minimal, which minimizes abortive capital investment that
would not be utilized in HATS Stage 2B.
|
|
Concerned Parties
|
Key Comments / Views
|
Consultant’s Response
|
|
|
3. Implementation of HATS Stage
2B:
|
|
|
l
Any plan of earlier
implementation of Stage 2B
|
·
There are
uncertainties in population and sewage flow build-ups as well as many other
factors affecting the implementation of Stage 2B. The programme of
Stage 2B implementation would be reviewed at 2010/11, when more updated data
can be gathered.
|
|
Academics and Professional Institutions
|
1.
Need of Project:
|
|
|
l
Hong Kong can take the lead in disinfection
technology for the sewage effluent to improve water quality at the Region
|
l
Agreed to adopt
appropriate disinfection technology to minimize the bacterial content in the
sewage effluent.
|
|
l
What would be the
cost of implementing the disinfection project, and is it worth to spend that
amount of money to achieve the water quality objective (E coli. level) at the
beaches?
|
l
Based on the
findings of the disinfection option evaluation, chlorination was founded to
be an appropriate, reliable, flexible and a more cost-effective option for
the disinfection of HATS CEPT effluent. The recurrent cost of the
disinfection facility was estimated to be about HKD$80m in the initial years.
|
|
2.
Concern on selected
disinfection option:
|
|
|
l
The selected option
of using a large amount of sodium hypochlorite for effluent disinfection is
unique in the world and it may not be an economically sustainable solution.
|
·
The problem to be
solved is also a unique problem: need to disinfect the sewage generated from
the huge population around the Victoria Harbour, which would be more than 2
million m3 of effluent per day. A lot of effort has been put
to investigate and develop an appropriate solution to tackle the problem.
|
|
l
Adopting
chlorination as the disinfection technology for HATS would be a unique
project in the world in terms of its size and such project is considered not
providing best value for the money spent.
|
l
Various project
scenarios and the life cycle costs have been considered in the disinfection
option selection exercise. It was found that among the applicable
disinfection technologies to HATS, chlorination would be the most flexible
and cost-effective technology (i.e. best value for money).
|
|
Concerned Parties
|
Key Comments / Views
|
Consultant’s Response
|
|
|
l
Careful
consideration of the design and control of the dosing system of sodium
hypochlorite and sodium bisulphite needs to be given.
|
l
The operation and
control plan of the chemical dosing facility has been prepared, with
reference to a sewage treatment works at Boston, US which has similar scale
as SCISTW and uses chlorination (with dechlorination) for disinfection
|
|
3. Other comment / concern
|
|
|
l
Disinfection cannot
substantially improve other water quality parameters, but secondary treatment
can serve this purpose.
|
l
Addition of
disinfection treatment process will provide the necessary mitigation to
address the E. coli problem in the western part of Victoria Harbour. The Government has planned to review the programme of HATS Stage 2B based on the
trends of water quality, population and sewage flow build-ups in 2010/11.
|
Project Programme
2.96
It is intended that the construction works for
the Project will be packaged under one single contract and are tentatively
scheduled to commence in March 2008 for completion in September 2009. The
tentative construction programme is given in Figure
2.5. The major construction activities for the Project would
comprise site formation, excavation and backfilling, piling works, erection of
formwork and reinforcement, concreting, fabrication of steelwork, and testing
and commissioning. Construction works would take place at two locations
for the chlorination system and dechlorination plant.
Interactions with Other Projects
2.97
The planned diving rescue and diving training
centre for the Fire Services Department (FSD) would be located adjacent to the
eastern site boundary of the SCISTW. The general location plan of the FSD
diving rescue and diving training centre is shown on Figure 2.6. The construction works for
the FSD diving rescue and training centre are scheduled to start in December
2006 and with works completion in March 2009. By February 2009, the
construction activity involved for the proposed disinfection facilities at
SCISTW under the Project would be testing and commissioning, utility laying and
other minor works.
