5a.1.1.1 This section presents an assessment of the potential water quality impacts associated with construction and operation of the IWMF at the TTAL site. Recommendations for mitigation measures have been provided, where necessary, to minimise the identified water quality impacts to an acceptable level.
5a.2 Environmental Legislation, Standards and Guidelines
5a.2.1 Environmental Impact Assessment Ordinance (EIAO)
5a.2.1.1 The Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM) is issued by the EPD under Section 16 of the EIAO. It specifies the assessment method and criteria that need to be followed in EIA. Reference sections in EIAO-TM have provided the details of assessment criteria and guidelines that are relevant to the water quality impact assessment, including:
· Annex 6 Criteria for Evaluating Water Pollution
· Annex 14 Guidelines for Assessment of Water Pollution
5a.2.2 Water Pollution Control Ordinance (WPCO)
5a.2.2.1
The Water Pollution Control Ordinance (WPCO) provides
the major statutory framework for the protection and control of water quality
in
Table 5a.1 Summary of Water Quality Objectives for Deep Bay WCZ
|
Parameters |
Objectives |
Sub-Zone |
|
Offensive Odour, Tints |
Not to be present |
Whole zone |
|
Visible foam, oil scum, litter |
Not to be present |
Whole zone |
|
Dissolved Oxygen (DO) within 2 m of the
seabed |
Not less than 2.0 mg/L for 90% of samples |
|
|
Dissolved Oxygen (DO) within 1 m below
surface |
Not less than 4.0 mg/L for 90% of samples |
|
|
Not less than 5.0 mg/L for 90% of samples |
|
|
|
Depth-averaged DO |
Not less than 4.0 mg/L for 90% of samples |
|
|
Not less than 4.0 mg/L |
Yuen Long & Kam Tin (Upper and Lower)
Subzones, Beas Subzone, Indus Subzone, Ganges Subzone, Water Gathering Ground
Subzones and other inland waters of the Zone |
|
|
5-Day Biochemical Oxygen Demand (BOD5) |
Not to exceed 3 mg/L |
Yuen Long & Kam Tin (Upper) Subzone, Water Gathering Ground Subzones |
|
Not to exceed 5 mg/L |
Yuen Long & Kam Tin (Lower) Subzone
and other inland waters |
|
|
Chemical Oxygen Demand (COD) |
Not to exceed 15 mg/L |
Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone,
Ganges Subzone and Water Gathering Ground |
|
Not to exceed 30 mg/L |
Yuen Long & Kam Tin (Lower) Subzone
and other inland waters |
|
|
pH |
To be in the range of 6.5 - 8.5, change due
to waste discharges not to exceed 0.2 |
Marine waters excepting |
|
To be in the range of 6.5 – 8.5 |
Yuen Long & Kam Tin (Upper and Lower)
Subzones, |
|
|
To be in the range of 6.0 –9.0 |
Other inland waters |
|
|
To be in the range of 6.0 – 9.0 for 95%
samples, change due to waste discharges not to exceed 0.5 |
|
|
|
Salinity |
Change due to waste discharges not to
exceed 10% of ambient |
Whole zone |
|
Temperature |
Change due to waste discharges not to
exceed 2 oC |
Whole zone |
|
Suspended solids (SS) |
Not to raise the ambient level by 30%
caused by waste discharges and shall not affect aquatic communities |
Marine waters |
|
Not to cause the annual median to exceed
20 mg/L |
Yuen Long & Kam Tin (Upper and Lower)
Subzones, Beas Subzone, Ganges Subzone, Indus Subzone, Water Gathering Ground
Subzones and other inland waters |
|
|
Unionized Ammonia (UIA) |
Annual mean not to exceed 0.021 mg/L as
unionized form |
Whole zone |
|
Nutrients |
Shall not cause excessive algal growth |
Marine waters |
|
Total Inorganic Nitrogen (TIN) |
Annual mean depth-averaged inorganic
nitrogen not to exceed 0.7 mg/L |
|
|
Annual mean depth-averaged inorganic
nitrogen not to exceed 0.5 mg/L |
Outer |
|
|
Bacteria |
Not exceed 610 per 100ml, calculated as
the geometric mean of all samples collected in one calendar year |
|
|
Should be zero per 100 ml, calculated as
the running median of the most recent 5 consecutive samples taken between 7
and 21 days. |
Yuen Long & Kam Tin (Upper) Subzone,
Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground
Subzones |
|
|
Not exceed 180 per 100ml, calculated as
the geometric mean of the collected from March to October inclusive in one
calendar year. Samples should be taken at least 3 times in a calendar month
at intervals of between 3 and 14 days. |
|
|
|
Not exceed 1000 per 100ml, calculated as the
running median of the most recent 5 consecutive samples taken at intervals of
between 7 and 21 days |
Yuen Long & Kam Tin (Lower) Subzone
and other inland waters |
|
|
Colour |
Not to exceed 30 Hazen units |
Yuen Long & Kam Tin (Upper) Subzone,
Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground
Subzones |
|
Not to exceed 50 Hazen units |
Yuen Long & Kam Tin (Lower) Subzone
and other inland waters |
|
|
Turbidity |
Shall not reduce light transmission
substantially from the normal level |
|
|
Phenol |
Quantities shall not be sufficient to
produce a specific odour or more than 0.05 mg/L as C6 H5OH |
|
|
Toxins |
Should not cause a risk to any beneficial
uses of the aquatic environment |
Whole Zone |
|
Should not attain such levels as to
produce toxic carcinogenic, mutagenic or teratogenic effects in humans, fish
or any other aquatic organisms. |
Whole Zone |
Source: Statement of Water Quality Objectives (Deep Bay Water Control
Zone)
Table 5a.2 Summary of Water Quality Objectives for North Western WCZ
|
Parameters |
Objectives |
Sub-Zone |
|
Offensive Odour, Tints |
Not to be present |
Whole zone |
|
Visible foam, oil scum, litter |
Not to be present |
Whole zone |
|
Dissolved Oxygen (DO) within 2 m of the
seabed |
Not less than 2.0 mg/L for 90% of samples |
Marine waters |
|
Depth-averaged DO |
Not less than 4.0 mg/L |
Tuen Mun (A), Tuen Mun (B) and |
|
Not less than 4.0 mg/L for 90 % sample |
Marine waters |
|
|
pH |
To be in the range of 6.5 - 8.5, change
due to human activity not to exceed 0.2 |
Marine waters excepting Bathing Beach
Subzones |
|
To be in the range of 6.5 – 8.5 |
Tuen Mun (A), Tuen Mun (B) and |
|
|
To be in the range of 6.0 –9.0 |
Other inland waters |
|
|
To be in the range of 6.0 –9.0 for 95%
samples |
Bathing Beach Subzones |
|
|
Salinity |
Change due to human activity not to exceed
10% of ambient |
Whole zone |
|
Temperature |
Change due to human activity not to exceed
2 oC |
Whole zone |
|
Suspended solids (SS) |
Not to raise the ambient level by 30%
caused by human activity |
Marine waters |
|
Not to cause the annual med |
Tuen Mun (A), Tuen Mun (B) and |
|
|
Not to cause the annual med |
Inland waters |
|
|
Unionized Ammonia (UIA) |
Annual mean not to exceed 0.021 mg/L as
unionized form |
Whole zone |
|
Nutrients |
Shall not cause excessive algal growth |
Marine waters |
|
Total Inorganic Nitrogen (TIN) |
Annual mean depth-averaged inorganic
nitrogen not to exceed 0.3 mg/L |
|
|
Annual mean depth-averaged inorganic
nitrogen not to exceed 0.5 mg/L |
Marine waters excepting |
|
|
Bacteria |
Not exceed 610 per 100ml, calculated as
the geometric mean of all samples collected in one calendar year |
Secondary Contact Recreation Subzones |
|
Should be less than 1 per 100 ml,
calculated as the running med |
Tuen Mun (A) and |
|
|
Not exceed 1000 per 100 ml, calculated as
the running med |
Tuen Mun (C) Subzone and other inland
waters |
|
|
Not exceed 180 per 100 ml, calculated as
the geometric mean of all samples collected from March to October inclusive.
Samples should be taken at least 3 times in one calendar month at intervals
of between 3 and 14 days. |
Bathing Beach Subzones |
|
|
Colour |
Not to exceed 30 Hazen units |
Tuen Mun (A) and |
|
Not to exceed 50 Hazen units |
Tuen Mun (C) Subzone and other inland
waters |
|
|
5-Day Biochemical Oxygen Demand (BOD5) |
Not to exceed 3 mg/L |
Tuen Mun (A), Tuen Mun (B) and |
|
Not to exceed 5 mg/L |
Inland waters |
|
|
Chemical Oxygen Demand (COD) |
Not to exceed 15 mg/L |
Tuen Mun (A), Tuen Mun (B) and |
|
Not to exceed 30 mg/L |
Inland waters |
|
|
Toxins |
Should not cause a risk to any beneficial
uses of the aquatic environment |
Whole zone |
|
Waste discharge shall not cause the toxins
in water significant to produce toxic carcinogenic, mutagenic or teratogenic
effects in humans, fish or any other aquatic organisms. |
Whole zone |
|
|
Phenol |
Quantities shall not sufficient to produce
a specific odour or more than 0.05 mg/L as C6 H5OH |
Bathing Beach Subzones |
|
Turbidity |
Shall not reduce light transmission
substantially from the normal level |
Bathing Beach Subzones |
Source: Statement of Water Quality Objectives (North Western Water
Control Zone)
5a.2.3 Technical Memorandum on Effluents Discharge Standard
5a.2.3.1 Discharges of effluents are subject to control under the WPCO. The Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS), issued under Section 21 of the WPCO, gives guidance on permissible effluent discharges based on the type of receiving waters (foul sewers, storm water drains, inland and coastal waters). The limits control the physical, chemical and microbial quality of effluent. Any sewage from the proposed construction and operation activities must comply with the standards for effluent discharged into the foul sewers, inshore waters and marine waters of the Deep Bay WCZ provided in the TM-DSS.
5a.2.4.1 A practice note (PN) for professional persons was issued by the EPD to provide environmental guidelines for handling and disposal of construction site discharges. The ProPECC PN 1/94 “Construction Site Drainage” provides good practice guidelines for dealing with various types of discharge from a construction site. These include surface runoff, groundwater, boring and drilling water, bentonite slurry, water for testing and sterilisation of water retaining structures and water pipes, wastewater from building construction, acid cleaning, etching and pickling wastewater, and wastewater from site facilities. Practices outlined in the ProPECC PN 1/94 should be followed as far as possible during construction to minimize the water quality impact due to construction site drainage.
5a.3 Description of the Environment
5a.3.1.1 The construction of the IWMF has the potential to affect the inland watercourse of Tsang Kok stream within the Deep Bay WCZ. There is one EPD routine water quality monitoring station (DB8) along the Tsang Kok Stream. A summary of the monitoring data in 2010, which is the most recent monitoring data published on the EPD website at the moment of preparing this Report, is presented in Table 5a.3.
Table 5a.3 Summary of Inland Water Quality for Tsang Kok Stream in 2010
|
Parameter |
Unit |
DB8 |
|
Dissolved
oxygen |
mg/L |
8.5 |
|
pH |
|
8.1 |
|
Suspended
solids |
mg/L |
6 |
|
5-day Biochemical Oxygen Demand |
mg/L |
<1 |
|
Chemical
Oxygen Demand |
mg/L |
4 |
|
Oil
& grease |
mg/L |
<0.5 |
|
Faecal
coliforms |
cfu/100mL |
1500 |
|
E.coli |
cfu/100mL |
130 |
|
Ammonia-nitrogen |
mg/L |
0.03 |
|
Nitrate-nitrogen |
mg/L |
0.45 |
|
Total
Kjeldahl Nitrogen |
mg/L |
0.15 |
|
Ortho-phosphate |
mg/L |
<0.01 |
|
Total
phosphorus |
mg/L |
<0.02 |
|
Total sulphide |
mg/L |
<0.02 |
|
Aluminium |
µg/L |
155 |
|
Cadmium |
µg/L |
<0.1 |
|
Chromium |
µg/L |
<1 |
|
Copper |
µg/L |
1 |
|
Lead |
µg/L |
2 |
|
Zinc |
µg/L |
20 |
Notes:
1. Data source: River Water Quality in Hong Kong
in 2010.
2. Data presented are in annual medians of
monthly samples, except those for faecal coliforms and E.coli which are in annual geometric means. Figures in brackets are
annual ranges.
3. Figures in brackets are annual ranges.
5a.3.1.2 River water quality monitoring data at DB8 in 2010 showed the overall compliance of the Tsang Kok stream. Full compliance with WQOs were achieved for pH, suspended solids (SS), dissolved oxygen (DO), COD and BOD5. The water quality of this minor stream was reported to be excellent and free from point source pollution.
5a.3.2.1
The EPD water quality monitoring station DM4 and DM5 in
the Outer Deep Bay WCZ are the nearest monitoring stations in the vicinity of
the Project area (see Figure 5a.1). Monitoring
data collected at the
Table
5a.4 Summary of Marine Water Quality in Outer
|
Parameter |
Outer |
WPCO WQOs (in marine
waters) |
||
|
DM4 |
DM5 |
|||
|
Temperature (oC) |
24.6 (17.2 - 30.1) |
24.3 (17.4 - 29.3) |
Not more than 2 oC in daily
temperature range |
|
|
Salinity |
22.5 (15.6 - 29.7) |
25.6 (19.4 - 31.7) |
Not to cause more than 10% change |
|
|
Dissolved Oxygen (DO) (mg/L) |
Depth average |
5.9 (4.1 - 7.3) |
5.9 (4.4 - 7.3) |
Not less than 4 mg/L for 90% of the
samples |
|
Bottom |
5.7 (3.8 - 7.2) |
5.7 (4.0 - 7.3) |
Not less than 2 mg/L for 90% of the
samples |
|
|
Dissolved Oxygen (DO) (%
Saturation) |
Depth average |
80.0 (59 - 91) |
81.0 (64 - 92) |
Not available |
|
Bottom |
78.0 (56 - 89) |
79.0 (58 - 92) |
Not available |
|
|
pH |
7.7 (7.6 - 7.9) |
7.8 (7.5 - 8.0) |
6.5 - 8.5 (± 0.2 from natural range) |
|
|
Secchi disc Depth (m) |
1.1 (0.5 - 4.0) |
1.5 (0.7 - 8.0) |
Not available |
|
|
Turbidity (NTU) |
15.1 (9.4 - 21.0) |
13.5 (10.5 - 17.7) |
Not available |
|
|
Suspended Solids (mg/L) |
7.9 (4.3 - 14.5) |
5.8 (3.5 - 7.8) |
Not more than 30% increase |
|
|
5-day Biochemical Oxygen Demand (BOD5)
(mg/L) |
0.9 (0.4 - 1.6) |
0.8 (0.5 - 1.1) |
Not available |
|
|
Ammonia Nitrogen (NH3-N) (mg/L) |
0.305 (0.070 - 0.585) |
0.236 (0.056 - 0.547) |
Not available |
|
|
Unionised Ammonia (UIA) (mg/L) |
0.007 (0.004 - 0.009) |
0.006 (0.002 - 0.009) |
Not more than 0.021 mg/L |
|
|
Nitrite Nitrogen (NO2-N) (mg/L) |
0.186 (0.096 - 0.390) |
0.142 (0.037 - 0.297) |
Not available |
|
|
Nitrate Nitrogen (NO3-N) (mg/L) |
0.732 (0.345 - 1.250) |
0.580 (0.167 - 0.920) |
Not available |
|
|
Total Inorganic Nitrogen (TIN) (mg/L) |
1.22 (0.72 - 1.76) |
0.96 (0.47 - 1.46) |
Not more than 0.5 mg/L |
|
|
Total Kjeldahl Nitrogen (TKN) (mg/L) |
0.49 (0.33 - 0.68) |
0.41 (0.25 - 0.64) |
Not available |
|
|
Total Nitrogen (TN) (mg/L) |
1.40 (0.87 - 1.98) |
1.13 (0.62 - 1.58) |
Not available |
|
|
Orthophosphate Phosphorus (Ortho P) (mg/L) |
0.052 (0.029 - 0.064) |
0.041 (0.031 - 0.053) |
Not available |
|
|
Total Phosphorus (TP) (mg/L) |
0.08 (0.05 - 0.12) |
0.07 (0.05 - 0.10) |
Not available |
|
|
Silica (as SiO2) (mg/L) |
3.24 (1.75 - 4.50) |
2.59 (0.99 - 3.93) |
Not available |
|
|
Chlorophyll-a (µg/L) |
2.8 (1.1 - 9.5) |
2.5 (0.7 - 8.4) |
Not available |
|
|
E. coli (cfu/100 mL) |
78 (32 - 200) |
170 (28 - 450) |
Not available |
|
|
Faecal Coliforms (cfu/100 mL) |
160 (76 - 310) |
390 (120 - 1100) |
Not available |
|
Note:
1. Data source: 2010 Marine Water Quality in
2. Except as specified, data presented are
depth-averaged values calculated by taking the means of three depths: Surface,
mid-depth, bottom.
3. Data presented are annual arithmetic means
of depth-averaged results except for E.coli
and faecal coliforms that are annual geometric means.
4. Data in brackets indicate the ranges.
5a.3.2.2
According to the “2010 Marine Water Quality in
5a.4 Water Sensitive Receivers
5a.4.1.1 Two moderate sized streams are located at the southern part of the Ash Lagoon and discharge into a tidal channel to the east of the ash lagoon area (refer to Figure 5a.1). The lower reaches of stream W1 are routed through a man-made, tidally influenced channel to the south of the ash lagoons. Although the substrate of this channel is natural, the banks have been lined with geo-textile matting. The second stream (stream W2) drains into the tidal channel from the southeast. The section of stream flowing through the existing WENT Landfill site has been wholly channelized with concrete.
5a.4.1.2
Marine water sensitive receivers also include a
cooling water intake of the Black Point Power Station, secondary contact recreation
subzone at Black Point, as well as the coastal waters of
5a.4.1.3 Details of the ecological resources identified within the Study Area are provided in Section 7a.
5a.5.1.1
The Assessment Area as specified in the EIA Study Brief
covers an area within
5a.5.1.2 The water sensitive receivers that may be affected by various construction activities for the IWMF were identified. Potential sources of water quality impact that may arise during the construction and operation phase of the Project were described. All the identified sources of potential water quality impact were then evaluated and their impact significance determined. The need for mitigation measures to reduce any identified adverse impacts on water quality to acceptable levels was determined.
5a.6 Identification of Environmental Impacts
5a.6.1.1 The major construction works of the Project would be site formation, construction of facilities and construction of the access road. Potential water quality impact during construction phase of the IWMF would be occurred from:
·
Drainage and construction site runoff
during site formation and foundation piling;
· General construction activities;
· Accidental spillage and accumulation of solid wastes;
· Sewage effluent produced by on-site workforce;
· Release of pulverized fuel ash leachate from ash lagoon into the aquatic environment.
Drainage and Construction Site Runoff
5a.6.1.2 Runoff from the construction works area may contain increased loads of sediments, other suspended solids and contaminants. Potential sources of pollution from site drainage include:
·
Runoff and erosion from exposed soil surfaces, earth working areas and
stockpiles;
·
Release of grounting and cement materials with rain wash;
·
Wash water from dust suppression sprays; and
·
Fuel and lubricants from maintenance of construction vehicles and
mechanical equipment.
5a.6.1.3 Sediment laden runoff during site formation works, if uncontrolled, may carry pollutants (adsorbed onto the particle surfaces) into the nearby stream and coastal waters.
General Construction Activities
5a.6.1.4
Land-based construction works may have the
potential to cause water pollution. Various types of construction activities
would generate wastewater. These
include general cleaning and polishing, wheel washing, dust suppression and
utility installation. These types
of wastewater would contain high concentration of suspended solids. Wastewater would also be generated from
the accumulation of solid waste such as debris, rubbish, plastic package and
construction materials. If
uncontrolled, these would lead to deterioration in water quality.
Accidental Spillage
5a.6.1.5 Variety of chemicals would be used for carrying out construction activities. These chemicals may include petroleum products, spent lubrication oil, grease, mineral oil, solvent and other chemicals. Accidental spillages of chemicals in the works area may contaminate the surface soils. The contaminated soil particles may be washed away by construction site runoff causing water pollution.
Sewage Effluent
5a.6.1.6 Domestic sewage would be generated from the workforce during the construction phase. However, this sewage can be adequately treated by interim sewage treatment facilities, such as portable chemical toilets, which can be installed within the construction site.
Release of PFA Leachate from Ash Lagoon into the Aquatic Environment
5a.6.1.7 The IWMF will be located in the middle part of the existing ash lagoon area. The ash lagoons were constructed in the mid- to late 1980’s and were divided by bunds into the East, Middle and West Lagoons. Since 1989, the lagoons have been used for the storage of PFA, a by-product of the coal-burning. PFA is a fine, grey powder formed from the rock particles contained within coal, consisting mainly of silica, alumina and iron oxide.
5a.6.1.8 In 1997, China Light and Power (CLP) began to use the Middle Lagoon as part of its water collection and conservation system. The ash lagoon area is underlain by marine deposits which consist of fine grained material. Alluvium is present underneath the marine deposits. Depths of alluvium may vary from approximately 4.0 to 19.0m. The layer of alluvium is underlain by completely decomposed granite (CDG) with depths ranging from approximately 3.5 to 15.2m ([1]). There is liner, which was constructed of cementitious materials, at the bottom of the ash lagoons.
5a.6.1.9
The marine deposits in the sea wall location have been
removed prior to the sea wall construction. In order to prevent leakage
of PFA leachate through the sea wall to
5a.6.1.10
During construction phase of the Project, piling would
be applied for foundation construction. The piles would penetrate through
the base of the Middle Lagoon to the hard granite bedrock to support the
facility and the soil layer underneath the lagoon would be disturbed.
However, the piling activities are unlikely to cause significant changes in
geological structure of the lagoon site. The present of piles would
restrict the movement of groundwater in the soil layer. The low
permeability values of the marine deposits and alluvium underneath the PFA
layer would limit the seepage of PFA leachate. Leakage of PFA leachate
through the base of the Middle Lagoon to
5a.6.1.11 To evaluate the potential impacts of the PFA leachate to the nearby aquatic environment, the chemical characteristics of the PFA leachate and chemical toxicity data for aquatic life have been reviewed as detailed in Section 5a.7.
5a.6.2.1 Potential sources of water quality impacts generated from the operation of the Project include:
·
Wastewater generated from the Waste Treatment Process;
·
Sewage generated from floor & vehicle washing;
·
Sewage generated from the IWMF staff & visitors; and
·
Discharge of saline water from the proposed desalination plant.
Wastewater Generated from the Waste Treatment Process
5a.6.2.2 The IWMF will comprise a 3,000 tpd of moving grate incineration plant and a demonstration scale mechanical treatment plant of about 200 tpd capacity. Desalination plant may also be adopted as a water supply system in the IWMF. Wastewater will be generated from the mechanical treatment plant, the incineration plant and the desalination plant (if adopted) in the IWMF. No spent cooling water discharge is anticipated from the Project operation.
Mechanical Treatment Plant
Incineration Plant
5a.6.2.4 Wastewater will also be generated from various processes throughout the incineration plant including:
5a.6.2.5 Boiler - The practice of continuously removing a small percentage of boiler feed water from the boiler to maintain boiler water chemistry is referred to as boiler blowdown. Although the boiler steam cycle is essentially a closed-loop system, impurities can build up in the boiler which, over time, cause scaling and corrosion of the boiler tubes. These effects eventually lead to boiler tube failure. To reduce such problems, continuous boiler blowdown is employed. The hot blowdown water is passed through a heat exchange to recover heat before becoming a source a plant wastewater. The blowdown water is replaced with contaminant-free feed water make-up.
5a.6.2.6
5a.6.2.7 Boiler Feedwater Treatment System - The purpose of the boiler feedwater treatment system is to provide demineralized water for boiler make-up. Demineralized water is needed in the boiler to prevent scaling and corrosion due to mineral deposits. The treatment system typically involves filtering the feed water to remove suspended solids, and removing metals and minerals in a de-mineralizer. The de-mineralizer contains cation and anion exchangers are periodically regenerated using sulfuric acid and caustic soda respectively. Oxygen is removed from the demineralized water using a deaerator. Processed demineralized water is then stored in tanks and drawn off as needed for boiler feeder water, cooling water and other processes. The operation of the various filters, ion exchangers and deaerators requires periodic back flushing of the system to remove the collected contaminants from the treatment system. This process wastewater is then stored in a neutralization tank where appropriate amounts of acid or caustic are add to adjust the pH.
5a.6.2.9 Miscellaneous Blowdown Sources - Other processes that use process water can be minor sources of process water blowdown, these may include water cooled feed chutes, water cooled bearings, cooling water jacketing etc.
Sewage Generated from Floor & Vehicle Washing
5a.6.2.10 Approximately 31m3 of sewage would be generated daily during floor washing and vehicle washing in the IWMF. As the sewage would contain contaminants from MSW, treatment of the sewage will be required before disposal or reuse for other applications.
Sewage Generated from the IWMF Staff & Visitors
5a.6.2.11 The sewage generated from human activities in the IWMF would include the sewage from the IWMF staff and visitors, as well as the sewage generated from the canteen, and community facilities. It is estimated that approximate 96.25 m3/d sewage would be generated from the IWMF staff and visitors and the associated activities, as shown in Table 5a.4a.
Table 5a.4a Estimated Amount of Sewage Generated from the IWMF Staff & Visitors and the Associated Activities
|
Items |
No. of Employee or Visitor |
Unit Flow Factor (1) (m3/d/person) |
Flow (m3/d) |
|
Staff and Visitors |
|||
|
Staff of incineration plant and MT plant |
200 |
0.08 |
16.00 |
|
Staff of canteen |
25 |
1.50 |
37.50 |
|
Staff of community facilities |
20 |
0.35 |
7.00 |
|
Visitors |
450 |
0.06 |
27.00 |
|
Sub-total |
87.50 |
||
|
10% Contingency |
8.75 |
||
|
Total |
96.25 |
||
Note (1): The unit flow factors adopted to
estimate the sewage flow generated from the staff and visitors in the IWMF are
primarily based on the guidelines laid down in EPD’s Guidelines for Estimating
Sewage Flows for Sewage Infrastructure Planning Version 1.0 (GESF).
Reuse and Treatment of Wastewater Generated from Waste Treatment Process and Sewage from Floor & Vehicle Washing and the IWMF Staff & Visitors
5a.6.2.12 Table 5a.5 shows the estimated quantity and possible characteristic of wastewater generated from treatment process and sewage from floor & vehicle washing and the IWMF staff & visitors.
Table
5a.5 Estimated Quantity and Possible Characteristics of Wastewater
Generated from Treatment Process and Sewage from Floor & Vehicle Washing
and the IWMF Staff & Visitors
|
|
Flow (m3/d) |
pH |
BOD (mg/L) |
COD (mg/L) |
SS (mg/L) |
Temp. (℃) |
Chloride (mg/L) |
|
|
Incineration
Plant |
Miscellaneous Blowdown
Sources |
1 |
6-8 |
50 |
30 |
50 |
20 |
- |
|
Boiler Feedwater Treatment
System (Demineralizer Drain) |
30 |
9-11 |
- |
- |
20 |
20 |
3,000 |
|
|
Pump Leak Water |
10 |
6-8 |
- |
- |
50 |
20 |
50 |
|
|
Boiler and Evaporative |
80 |
10-12 |
- |
- |
50 |
50 |
50 |
|
|
Mechanical
Treatment Plant Drain |
5 |
6.5 - 9 |
6,000 – 8,500 |
20,000 - 25,000 |
9,000 - 10,000 |
- |
- |
|
|
Sewage
from Floor and Vehicle Washing |
Floor Washed Drain |
1 |
7-9 |
50 |
30 |
500 |
20 |
100 |
|
Vehicle Washed Drain |
30 |
6-8 |
300 |
200 |
500 |
20 |
100 |
|
|
Sewage
from the IWMF Staff & Visitors |
97 |
6-8 |
250 |
520 |
250 |
20 |
- |
|
· pH : 6 – 8
· BOD : 10 mg/L
· Turbidity : 2 NTU
· Total Coliform/100 mL : non-detectable
· Cl2 residual : 1 mg/L
5a.6.2.15 Because of the compacted area in TTAL, membrane bioreactor (MBR), which requires small footprint, is proposed for the IWMF for mainly human sewage treatment. Based on the above effluent standards and wastewater characteristics and quantity shown in Table 5a.5, the wastewater treatment facilities enclosed by the reinforced concrete structure under the reception hall of the incineration plant would occupy an area of about 2,000m2.
Desalination Plant
5a.6.2.16 If desalination plant is adopted as a water supply system in the IWMF, the brine water generated would be either discharged back to the sea where the seawater is collected for desalination or reused for ash quenching. The brine water drained from the desalination plant is concentrated seawater (about 1.7-1.8 time more concentrated than the raw seawater). The design flow of the desalination plant, if required will be about 1,520 m3 per day. The potential water quality impacts due to the discharge of saline water have been assessed by mathematical modelling as described in Section 5a.7.
5a.7 Prediction and Evaluation of Environmental Impact
Drainage and Construction Site Runoff
5a.7.1.1 Runoff from the construction works area may contain increased loads of sediments, other suspended solids and contaminants. At commencement of site formation works, exposed PFA will be covered with construction fill material and direct runoff of PFA from the project site would not be expected. As a good site practice, mitigation measures should be implemented to control construction site runoff and drainage from the works areas, and to prevent runoff and drainage water with high levels of suspended solids from entering the nearby water bodies. With the implementation of adequate construction site drainage and provision of sediment removal facilities as described in Section 5a.8.1.1, it is anticipated that unacceptable water quality impacts would not arise. The construction site drainage would be collected by the temporary drainage system installed by the Contractor and then treated on-site before discharging into the sea via silt removal facilities. Water pumped out from foundation piling would also be discharged into the sea via silt removal facilities. The Contractor would be required to obtain a license from EPD for discharge to the coastal waters.
General Construction Activities
5a.7.1.2 Land-based construction activities may generate wastewater and cause water pollution. Their impacts are likely to be minimal, provided that good construction practices and proper site management would be observed. Effluent discharge from temporary site facilities should be controlled to prevent direct discharge to the neighbouring water environment. It is anticipated that water quality impacts caused by general construction activities would be insignificant with adequate implementation of recommended mitigation measures.
Accidental Spillage
5a.7.1.3
Site drainage should be well maintained and good
construction practices should be observed to ensure that oil, fuels and solvents
are managed, stored and handled properly and do not enter the nearby water
streams. No adverse water quality
impacts are expected with proper implementation of the recommended mitigation
measures.
Sewage Effluent
5a.7.1.4 Domestic sewage would be generated from the workforce during the construction phase. However, this sewage can be adequately treated by interim sewage treatment facilities, such as portable chemical toilets, which can be installed within the construction site. It is unlikely that sewage generated from the site would have a significant water quality impact, provided that sewage is not discharged directly to the water environment, and chemical toilets are used and properly maintained.
Release of PFA Leachate from Ash Lagoon into the Aquatic Environment
5a.7.1.5 To evaluate the potential impacts of the PFA leachate to the nearby aquatic environment, the chemical characteristics of the PFA leachate and chemical toxicity data for aquatic life have been reviewed. The PFA leaching trial using seawater was conducted by Scott Wilson Kirkpatrick (1991) ([2]). The leaching trial result showed that the metals contents in the PFA varied with the type of coal and the length of PFA aging. Only low concentrations of potential contaminants were leached into seawater solution. The contaminants with the greatest tendency to leach into solution were found to be cadmium, chromium and aluminium. Fresh PFA tended to leach more metals compared to the lagooned PFA and was more variable among various coal types. Results from the lagooned PFA showed smaller variations and metal leaching was more consistent.
5a.7.1.6 Table 5a.6 shows the concentrations of different parameters from the lagooned PFA leaching trials. The major heavy metals released from the lagooned PFA were aluminium and chromium, with maximum concentrations of 900 and 300 mg/l respectively. The maximum cadmium concentration measured in the leaching trials was 4 mg/l. There was an uncertainty of the actual concentration of copper and nickel released from lagooned because of the high reporting limits. The analytical instrument for the seawater solution in the leaching trials was only available to detect copper concentration higher than 75mg/l and nickel higher than 25mg/l.
Table 5a.6 Comparison of Leaching Trial Results with the Background Levels and USEPA Water Quality Standards
|
Parameter |
Leaching Trial Results (mg/l) |
Background Concentration (mg/l) Note
1 |
USEPA Water Quality Standard for
Saltwater (mg/l) |
|
Aluminium |
900 |
132 |
n/a |
|
Chromium |
300 |
1.5 |
50 (210) Note 2 |
|
Cadmium |
4 |
< 0.05 |
9.3 |
|
Copper |
<75 |
< 5 |
2.9 |
|
Zinc |
30 |
6 |
86 |
|
Nickel |
<25 |
< 5 |
8.3 |
|
Iron |
20 |
145 |
n/a |
|
Lead |
6 |
0.9 |
8.5 |
|
Manganese |
3 |
17.5 |
n/a |
|
Selenium |
14 |
< 1 |
71 |
|
Arsenic |
3 |
1.2 |
36 |
Notes:
1. The background concentrations were based on the results measured around Black Point and Tap Shek Kok abstracted from Scientific Series, Chemical Analysis Report 20/91.
2. The value of 50 mg/l represents the standard for Chromium (VI) in saltwater and there is no standard for Chromium (III) in saltwater. The criterion for Chromium (III) in freshwater is 210 mg/l.
5a.7.1.7 Table 5a.6 also provided the concentration of trace metals measured around Black Point and Tap Shek Kok in 1991 as background concentration, as well as USEPA Water Quality Standards for Saltwater. Concentrations for aluminium, iron and manganese are not available in the USEPA standard. Comparison result of the leaching trial of these three parameters with the background concentration presented that both iron and manganese were below the background concentration, whiles aluminium concentration is about 7 times higher than background concentration. When diluted by the ambient seawater, the aluminium concentration would be indistinguishable from the background level within a short distance from the release point.
5a.7.1.8 To compare other parameters with the USEPA standard, the concentrations of all metals except chromium are below the USEPA standards. In the event that release of PFA leachate occurs, the potential water quality would be low. In fact, most of the ash would remain in the Middle Lagoon and there would be no off-site disposal of ash into the aquatic environment.
5a.7.1.9
A monitoring programme was conducted by CLP between
1987 and 1988 to monitor water quality at the location outside the Middle
Lagoon. The monitoring result is shown in Table 5a.7 indicating that there was no likely correlation between
the trace metal results measured outside the Middle Lagoon and at oyster buoy
and farm in
Table 5a.7 Results of CLP monitoring programme between 1987 and 1988
|
Parameters |
Monitored Average Concentrations (mg/l) |
Background Concentration (mg/l) |
|
|
At Location Immediately Outside the Middle Lagoon (Sep 1988 – Jan 1989) |
At Oyster Buoy and Farm in (Jun 1987 – Nov 1987) |
||
|
Cadmium |
0.09 |
0.41 |
<
0.05 |
|
Copper |
3.6 |
2.1 |
<
5 |
|
Lead |
2.3 |
1 |
0.9 |
|
Zinc |
7 |
23 |
6 |
|
Arsenic |
<
5 |
5.1 |
1.2 |
|
Selenium |
<
10 |
No
data available |
<
1 |
5a.7.1.10 Chemical toxicity data for aquatic life have been reviewed in order to evaluate the potential impacts of the PFA leachate. High concentrations of heavy metals can be detrimental to aquatic life. The effects of high concentration of metals may cause the changes in tissues, growth rates, blood chemistry, behaviour and reproduction of aquatic organisms. Fish can excrete excess heavy metals but bivalves cannot regulate excess heavy metals resulting in metal accumulation in the tissues.
5a.7.1.11
There are no relevant aquatic life criteria in
Table 5a.8 Comparison of the Leaching Trial Results with the USEPA Aquatic Life Criteria
|
Parameter |
Leaching Trial Results (mg/l) |
USEPA Aquatic Life Criteria (mg/l) |
|
Aluminium |
900 |
- |
|
Chromium |
300 |
- |
|
Cadmium |
4 |
8 |
|
Copper |
<75 |
2.9 |
|
Zinc |
30 |
76.6 |
|
Nickel |
<25 |
7.1 |
|
Iron |
20 |
- |
|
Lead |
6 |
5.8 |
|
Manganese |
3 |
- |
|
Selenium |
14 |
71 |
|
Arsenic |
3 |
50 |
5a.7.1.12 The water quality guidelines for general saltwater aquaculture uses adopted in the New Zealand Guidelines for Fresh and Marine Water Quality (Table 5a.9) and the UK Water Quality Standards for the Protection of Saltwater Life (Table 5a.10) are also applied to compare with the leaching trial results. Concentrations of aluminium, chromium, copper, iron and selenium are higher than the New Zealand Guidelines, while concentrations of chromium, cadmium and copper are higher than the UK Standards.
5a.7.1.13 Dilution for these metals could lower the concentrations to meet the New Zealand Guidelines and the UK Standards. The estimated dilution rates are shown in Table 5a.9 and Table 5a.10. In order to meet the New Zealand Guidelines, the highest dilution rate would be >90 for aluminium. Chromium and copper would require a dilution rate of >15 and iron and selenium require a dilution rate of >2. To meet the requirements of the UK Standards, the highest dilution rate is >20 for chromium, while required dilution rate for cadmium and copper are 1.6 and 20 respectively. The nearest oyster beds at Pak Nai are approximately 3 km away from the lagoons. The required dilutions are likely to be achieved for pollutants in the moving tidal current travelling for such a long distance. It is anticipated that the potential impacts to the nearby oyster beds would be insignificant.
Table 5a.9 Comparison
of the Leaching Trial Results with the
|
Parameter |
Leaching Trial Results (mg/l) |
|
Required Dilution to Meet the
Guidelines |
|
Aluminium |
900 |
< 10 |
> 90 |
|
Chromium
|
300 |
< 20 |
> 15 |
|
Cadmium |
4 |
< 5 |
- |
|
Copper |
<75 |
< 5 |
> 15 |
|
Zinc |
30 |
< 100 |
- |
|
Nickel |
<25 |
< 100 |
- |
|
Iron |
20 |
< 10 |
> 2 |
|
Lead |
6 |
< 20 |
- |
|
Manganese |
3 |
< 100 |
- |
|
Selenium
|
14 |
< 10 |
> 2 |
|
Arsenic
|
3 |
< 30 |
- |
Source: Australian
and
Table 5a.10 Comparison
of the Leaching Trial Results with the
|
Parameter |
Leaching Trial Results (mg/l) |
|
Required Dilution to Meet the Standards |
|
Aluminium |
900 |
- |
- |
|
Chromium |
300 |
15 |
>20 |
|
Cadmium |
4 |
2.5 |
>1.6 |
|
Copper |
<75 |
5 |
>15 |
|
Zinc |
30 |
40 |
- |
|
Nickel |
<25 |
30 |
- |
|
Iron |
20 |
1000 |
- |
|
Lead |
6 |
25 |
- |
|
Manganese |
3 |
- |
- |
|
Selenium |
14 |
- |
- |
|
Arsenic |
3 |
25 |
- |
5a.7.1.14
Table 5a.11 lists the
chemical toxicity data for aquatic life. LC50 (concentration
at which 50% mortality occurs) of the heavy metals for the species that could
be found in
5a.7.1.15 As the leakage through the base of the Middle Lagoon would not be significant, the PFA leachate in the Middle Lagoon is unlikely to cause unacceptable impact on the aquatic environment from an ecotoxicological point of view. The site conditions of Middle Lagoon during construction and operation phases would not be much different from the existing conditions. As most of the ash would remain in the Middle Lagoon and would not be disposed of into the aquatic environment, detailed ecotoxicological assessment and additional toxicity test are considered not necessary.
Table 5a.11 Chemical Toxicity Data for Aquatic Life
|
Parameter |
Leaching Trial Results (mg/l) |
Crassostrea gigas (Pacific Oyster) |
Mytilus edulis ( |
Oryzias laptipes (Medala, high-eyes) |
Scylla serrata (Green Crab) |
Crangon crangon (Common Shrimps) |
Artermia salina (Brine Shrimps) |
Amphiphods |
Polychaete Worm |
||||||||
|
LC50 |
Conc. (mg/L) |
LC50 |
Conc. (mg/L) |
LC50 |
Conc. (mg/L) |
LC50 |
Conc. (mg/L) |
LC50 |
Conc. (mg/L) |
LC50 |
Conc. (mg/L) |
LC50 |
Conc. (mg/L) |
LC50 |
Conc. (mg/L) |
||
|
Aluminium |
900 |
48h |
1000000 |
- |
- |
- |
- |
- |
- |
- |
- |
3d |
3100 |
- |
- |
96h |
405 |
|
Chromium |
300 |
- |
- |
- |
- |
96h |
120000 |
- |
- |
48h |
100000 |
24h 48h |
5300 3540 |
- |
- |
96h |
1000 |
|
Cadmium |
4 |
4h 96h |
85 19500 |
96h |
960 |
48h |
560000 |
- |
- |
96h |
460 |
24h 48h |
3100 1540 |
4d |
14.5 |
10d 28d |
83 39 |
|
Copper |
<75 |
14h 96h |
100 560 |
10d |
45 |
24h 48h |
610 410 |
- |
- |
48h |
10000 |
24h 48h |
800 440 |
- |
- |
4d 28d |
77 44 |
|
Zinc |
30 |
4d |
100 |
14d |
10 |
24h |
20000 |
- |
- |
48h |
100000 |
24h 48h |
4460 1700 |
- |
- |
28d |
350 |
|
Nickel |
<25 |
- |
- |
- |
- |
- |
- |
- |
- |
48h |
100000 |
48h |
162985 |
- |
- |
7d 10d |
7700 16090 |
|
Iron |
20 |
- |
- |
- |
- |
24h |
18500 |
- |
- |
48h |
33000 |
- |
- |
- |
- |
- |
- |
|
Lead |
6 |
- |
- |
105h 150h |
5000 500 |
24h 48h |
350000 205000 |
- |
- |
96h |
63000 |
24h 48h |
10000 5010 |
- |
- |
96h 28d |
7660 1430 |
|
Manganese |
3 |
- |
- |
- |
- |
24h |
1000000 |
- |
- |
48h |
3300 |
- |
- |
- |
- |
- |
- |
|
Selenium |
14 |
- |
- |
- |
- |
- |
- |
24h 72h |
68000 33000 |
- |
- |
- |
- |
- |
- |
- |
- |
|
Arsenic |
3 |
21d |
10 |
- |
- |
- |
- |
- |
- |
96h 192h |
96000 70000 |
24h |
1.3 umol/L |
- |
- |
96h |
7400 |
Note:
The media type of the underlined data is freshwater and the media type is marine water for the other data.
Source of information: Ecotox Database System
Wastewater from Waste Treatment Process and Sewage from Floor & Vehicle Washing and IWMF Staff & Visitors
5a.7.2.1 As discussed in Section 5a.6, the IWMF facilities would be designed with a net zero discharge of process and sanitary wastewater. A wastewater treatment plant would be provided on-site to treat high organic loading wastewater such as sewage from floor & vehicle washing (about 31m3/day) and from the IWMF staff & visitors (about 97 m3/day) for reuse in the incineration plant and the mechanical treatment plant or for washdown and landscape irrigation. The total amount of sewage to be treated is about 128m3/day. Therefore, the designed Average Dry Weather Flow (ADWF) of the on-site wastewater treatment plant is 128 m3/day. The bunker leachate / ash leachate from incineration plant (as described in Section 5a.6.2.8) would be highly polluted and would be conveyed to the incineration plant and co-incinerated with MSW. All other wastewater (i.e. low/nil organic loading wastewater coming from plant machinery such as demineralizer drain) only requires simple treatment such as sedimentation or neutralization or even not requires any treatment before being used for flue gas cooling in quench tower or ash quenching. Table 5a.12 shows the amount of water required for landscape irrigation and floor/vehicle washing as well as the amount of treated effluent generated from the wastewater treatment plant. All the treated effluent from the secondary wastewater treatment plant and wastewater with simple treatment or without any treatment would be fully reused without being discharged to the sea nearby. Therefore, no adverse water quality impact would be expected.
Table 5a.12 Amount of Water Demand for Landscape Irrigation and Washing
|
Description |
Amount |
|
Water Required for
Landscape Irrigation |
340 m3/d |
|
Water Required for Floor
/ Vehicle Washing |
31 m3/d |
|
Total Daily Demand of
Reclaimed Water |
371 m3/d |
|
Amount of Treated
Effluent |
128 m3/d(1) |
Note:
(1) Amount of treated effluent = 31
m3/d (floor and vehicle washing) + 97 m3/d (staff and visitors)
Discharge of Saline Water from Desalination Plant
5a.7.2.2 Approximately 1,520 m3/day of saline water would be generated from the proposed desalination plant and discharged to the sea. As the IWMF would be in 24-hour operation, continuous water supply will be required. Location of the discharge outfall is shown in Figure 5a.2. The peak saline water discharge rate is expected to be similar to the average discharge rate. The brine water drained from the desalination plant is just concentrated seawater (about 1.7 – 1.8 time more concentrated than the raw seawater) with a low discharge volume. There will be no temperature elevations in the brine water discharge as compared to the ambient water temperature. No biocides / anti-fouling chemicals (such as chlorine and C-treat-6) will be used for the proposed desalination plant. Instead, membrane would be backwashed frequently to prevent fouling problem. Backwash water would be either re-used or treated by secondary wastewater treatment plant provided on-site.
5a.7.2.3 A comparison of the characteristics of the saline water discharge with the standards for effluents discharged into the inshore waters of Deep Bay Water Control Zone is given in Table 5a.13 below.
Table 5a.13 Comparison of Saline Water Discharge from Desalination Plant with Effluent Discharge Standard
|
Saline Water Note 1 |
Discharge Standard Note 2 & 3 |
Compliance with Discharge Standard |
|
|
pH |
6 – 8 |
6 – 9 |
Yes |
|
Temperature (oC) |
16 – 29 |
45 |
Yes |
|
Suspended solids (mg/L) |
7 – 26 |
25 |
Yes |
|
BOD (mg/L) |
0.4 – 4 |
10 |
Yes |
|
Total Residual Chorine (mg/L) |
<1 |
<1 |
Yes |
Notes:
1. It is calculated based on the assumption
that the brine water produced is generally 1.7-1.8 times more concentrated than
raw seawater for SS and BOD. There
will be no temperature elevations in the saline water discharge as compared to
the ambient water temperature. The characteristics of the baseline seawater quality
are obtained from 2008 Marine Quality in
2. Discharge standard for flow rate of >1500
and ≤2000 m3/day based on Technical
Memorandum – Standards for Effluents Discharged into Drainage and Sewerage
Systems, Inland and Coastal Waters (TM-DSS).
3. The effluent discharge standards do not
specify a standard for salinity.
5a.7.2.4 The WQO stated that change of salinity due to human activity should not exceed 10% of ambient levels. Based on the assumption that the salinity in the effluent of the desalination plant will be raised 1.8 times of feedwater (ambient seawater), the required dilution to meet the WQO was calculated to be about 8 times. The near-field effluent dispersion model, namely the VISJET model, was used to simulate the impact of the saline water discharges. Key inputs to the near-field dispersion model including outfall configuration, ambient current speed, vertical density profile and effluent flow rate. The ambient current speed and vertical density profile were extracted from the far field hydrodynamic model output from the Delft3D Update Model developed under the EPD Study “Agreement No. CE 42/97 Update on Cumulative Water Quality and Hydrological Effect of Coastal Developments and Upgrading of Assessment Tool”. A monitoring point was set up in the hydrodynamic model of the Update Model near the proposed Project effluent point at northern boundary of Tsang Tsui Ash Lagoon. The far field hydrodynamic model of the Update Model is 3 dimensional with a total of 10 vertical water layers.
Table 5a.14 Density Profile at TTAL IWMF Desalination Plant Outfall
|
Depth from water surface (m) |
Density (kg/m3) |
|
|
Dry Season (D) |
Wet Season (W) |
|
|
0.21 |
1.0102 |
0.9971 |
|
0.64 |
1.0106 |
0.9973 |
|
1.06 |
1.0111 |
0.9987 |
|
1.49 |
1.0111 |
1.0009 |
|
1.91 |
1.0115 |
1.0034 |
|
2.34 |
1.0117 |
1.0041 |
|
2.76 |
1.0118 |
1.0041 |
|
3.19 |
1.0119 |
1.0042 |
|
3.61 |
1.0119 |
1.0044 |
|
4.04 |
1.0119 |
1.0095 |
|
4.25 |
1.0119 |
1.0095 |
Table 5a.15 Current Velocity Profile at TTAL IWMF Desalination Plant Outfall
|
Depth from water surface (m) |
Current Velocity (m/s) |
|||
|
Dry Season |
Wet Season |
|||
|
10%ile (dv10) |
90%ile (dv90) |
10%ile (wv10) |
90%ile (wv90) |
|
|
0.21 |
0.0795 |
0.7185 |
0.0993 |
0.6218 |
|
0.64 |
0.0719 |
0.7053 |
0.1020 |
0.6477 |
|
1.06 |
0.0672 |
0.6604 |
0.0955 |
0.6245 |
|
1.49 |
0.0820 |
0.6002 |
0.0929 |
0.5637 |
|
1.91 |
0.0677 |
0.4886 |
0.0896 |
0.5273 |
|
2.34 |
0.0631 |
0.4366 |
0.0973 |
0.4710 |
|
2.76 |
0.0658 |
0.3951 |
0.0945 |
0.4010 |
|
3.19 |
0.0595 |
0.3654 |
0.0909 |
0.3686 |
|
3.61 |
0.0556 |
0.3383 |
0.0875 |
0.3271 |
|
4.04 |
0.0497 |
0.2940 |
0.0717 |
0.2711 |
|
4.25 |
0.0497 |
0.2940 |
0.0717 |
0.2711 |
Table 5a.16 Summary of Proposed Model Runs
|
Model Run No. |
Run ID |
Density Profile |
Ambient Current Velocity |
|
1 |
D-dv10 |
D |
dv10 |
|
2 |
D-dv90 |
D |
dv90 |
|
3 |
W-wv10 |
W |
wv10 |
|
4 |
W-wv90 |
W |
wv90 |
Prediction and Evaluation of Near Field Modelling Results
5a.7.2.7 Table 5a.18 summarizes the results from the VISJET simulations. The predicted initial dilutions in Table 5a.18 were corrected with the background concentration build up due to the tidal effects. The basic assumption of any near field model is mixed with clean water. In actuality this is not true, particularly in a tidally mixed environment. The average tracer background build up concentrations were calculated from the far field Update model. The background build up was quantified by performing a conservative tracer run on the effluent. A conservative tracer, i.e. without decay or reaction, was used. The initial concentration of the tracer in the desalination plant seawall discharge outfall was set to be 1000 mg/l. It should be noted that the results from the grid cell into which the tracer is loaded is not representative of the true background build up as this cell will always contain the background build up plus the continuous tracer loading. Therefore, the necessary far field tracer results were taken from a cell located adjacent to the outfall grid cells. The average tracer results were predicted in both dry and wet seasons and were used for the background build up corrections. Table 5a.17 shows an example of the background build up correction (Run ID: W-wv10).
Table 5a.17 Example of Background Build Up Correction
|
Run ID |
Minimum Initial Dilution 1 |
Initial Tracer Concentration in Effluent 2
(mg/L) |
Average Tracer Concentration 3 (mg/L) |
Corrected Minimum Initial Dilution 4 |
|
(A) |
(B) |
(C) |
(D) |
|
|
W-wv10 |
15.6 |
1000 |
1.81 |
15.2 |
Note: 1. Minimum initial dilution predicted by VISJET model. This dilution occurred in the wet season (Run ID: W-wv10).
2. Effluent tracer concentration assumed in the far field modelling.
3. Average background build up concentration for dry season predicted by the far field model.
4. The average background build up concentration for dry season was used for the correction in this case as the minimum dilution occurred under the dry season scenario. Corrected Initial Dilution, (D) = (B) ÷ {[1 x (B) + ((A) – 1) x (C)] ÷ (A)}
Table 5a.18 Summary of Initial Dilutions Predicted at the Edge of ZID
|
Run ID |
Initial Dilution at the Edge of ZID 1 |
Corrected Initial Dilution at the Edge of ZID 2 |
Downstream Distance from Centre of the Outfall at the Edge of ZID
(m) 3 |
|
D-dv10 |
36 |
29 |
0.8 |
|
D-dv90 |
77 |
51 |
47 |
|
W-wv10 |
16 |
15 |
2.8 |
|
W-wv90 |
30 |
28 |
21 |
Notes: 1. Initial dilutions at the edge of the ZID calculated by VISJET model
2. Initial dilutions at the edge of ZID were corrected using the background build up concentration predicted by the far field Update model.
3. Definition of ZID is provided in Section 5a.7.2.6.
5a.7.2.8 As shown in Table 5a.18, the predicted minimum dilution rate is 15 which would occur in the wet season with the smallest ambient current velocity (W-wv10). The predicted minimum dilution rate of 15 is much greater than the required dilution rate of 8 times to meet the WQO. The closest identified sensitive receiver is about 10m away from the outfall (gorgonians). Table 5a.19 below shows the dilution rate at 10m away from the proposed outfall predicted under Scenarios D-dv90 and W-wv90 (both of these scenarios have a predicted downstream distance of more than 10m, refer to Table 5a.18). The model results indicated that a dilution rate of no less than 17 would be achieved at a downstream distance of 10m from the outfall, which is well above the required dilution of 8 times. Hence, no exceedance of WQO for salinity would occur at the closest sensitive receivers (gorgonians). It is therefore expected that the water quality impact due to the discharge of saline water from the desalination plant is negligible. Although the brine water discharge would unlikely contain any poisonous, noxious or polluting matter as it would be only a concentrate of seawater of nil / relatively low level of contamination, the project proponent will still confirm with the Regional Office of EPD on the need for applying a discharge license for the discharge of concentrated brine before the commencement of the IWMF Project.
Table 5a.19 Dilution Rate at 10 m away from TTAL IWMF Desalination Plant Outfall
|
Run ID |
Distance from Centre of the Outfall (m) |
Initial Dilution at 10m away from Centre of the Outfall |
Corrected Initial Dilution at 10m away from Centre of the Outfall |
|
D-dv90 |
10 |
24 |
21 |
|
W-wv90 |
10 |
18 |
17 |
Drainage and Construction Site Runoff
·
Boundaries of earthworks should be surrounded by dykes or embankments
for flood protection, as necessary.
·
Sand/silt removal facilities such as sand/silt traps and sediment basins
should be provided to remove sand/silt particles from runoff to meet the
requirements of the TM-DSS. The design
of efficient silt removal facilities should be based on the guidelines in
Appendix A1 of ProPECC PN 1/94, which states that the retention time for
silt/sand traps should be 5 minutes under maximum flow conditions. The detailed design of the sand/silt traps
shall be undertaken by the contractor prior to the commencement of
construction.
·
Water pumped out from foundation piles must be discharged into silt
removal facilities.
·
Measures should be taken to minimize the ingress of site runoff and
drainage into excavations. Drainage
water pumped out from excavations should be discharged into storm drains via
silt removal facilities.
·
During rainstorms, exposed slope/soil surfaces should be covered by a
tarpaulin or other means, as far as practicable. Other measures that need to be
implemented before, during and after rainstorms are summarized in ProPECC PN
1/94.
·
Exposed soil areas should be minimized to reduce potential for increased
siltation and contamination of runoff.
·
Earthwork final surfaces should be well compacted and subsequent
permanent work or surface protection should be immediately performed.
·
Open stockpiles of construction materials or construction wastes on-site
should be covered with tarpaulin or similar fabric during rainstorms.
·
All vehicles should be cleaned before leaving the works area to ensure
no earth, mud and debris is deposited on roads. An adequately designed and sited wheel
washing bay should be provided at every site exit. The wheel washing facility should be
designed to minimize the intake of surface water (rainwater). Wash-water should have sand and silt
settled out and removed at least on a weekly basis to ensure the continued
efficiency of the process.
General Construction Activities
Accidental Spillage
5a.8.1.5 Maintenance of vehicles and equipments involving activities with potential for leakage and spillage should only be undertaken within the areas which appropriately equipped to control these discharges.
5a.8.1.6 Oils and fuels should only be used and stored in designated areas which have pollution prevention facilities. All fuel tanks and storage areas should be sited on sealed areas in order to prevent spillage of fuels and solvents to the nearby watercourses. All waste oils and fuels should be collected in designated tanks prior to disposal.
· Suitable containers should be used to hold the chemical wastes to avoid leakage or spillage during storage, handling and transport.
· Chemical waste containers should be suitably labelled, to notify and warn the personnel who are handling the wastes, to avoid accidents.
· Storage area should be selected at a safe location on site and adequate space should be allocated to the storage area.
Sewage Effluent
Release of PFA Leachate from Ash Lagoon into the Aquatic Environment
5a.8.1.9 The past monitoring data showed that the water quality at the location outside the ash lagoon area was not affected by the PFA filling operation. The low permeability values of the marine deposits and alluvium underneath the PFA layer would limit the seepage of PFA leachate. The foundation construction of the IWMF is not likely to accelerate the release of PFA leachate through the base of the lagoon site.
Site Effluent
5a.8.2.1 The Project site will be equipped with an adequately sized wastewater treatment plant to provide treatment to some wastewater generated from the IWMF (mainly human sewage) for reuse in the incineration plant and the mechanical treatment plant or for washdown and landscape irrigation in the IWMF site. A “net zero discharge” scheme will be adopted during the operation of the IWMF.
5a.8.2.2 A small amount of brine water will be discharged into the marine water from the proposed desalination plant via a seawall discharge outfall at the northern boundary of the TTAL site. The potential water quality impact from the brine water discharge has been assessed to be negligible and therefore no mitigation measure specific to the brine water discharge is required.
Surface Runoff
5a.8.2.3 A pipeline drainage system will serve the development area collecting surface runoff from paved areas, roof, etc. Sustainable drainage principle would be adopted in the drainage system design to minimize peak surface runoff, maximize permeable surface and maximize beneficial use of rainwater.
5a.8.2.4 Oil interceptors should be provided in the drainage system of any potentially contaminated areas (such as truck parking area and maintenance workshop) and regularly cleaned to prevent the release of oil products into the storm water drainage system in case of accidental spillages. Accidental spillage should be cleaned up as soon as practicable and all waste oils and fuels should be collected and handled in compliance with the Waste Disposal Ordinance.
5a.9 Residual Environmental Impacts
5a.9.1.1
With
the implementation of the recommended mitigation measures for the construction
and operation phases of the proposed Project, no unacceptable residual impacts
on water quality are expected.
5a.10 Environmental Monitoring and Audit
5a.10.1.1
To
ensure no adverse water quality impact to the nearby stream due to the discharge of surface runoff and drainage from the works
areas, water quality monitoring of the Tsang Kok stream is
recommended during site formation. Marine water quality monitoring is
also recommended during foundation pilling of the IWMF to ensure that the
foundation construction would not cause unacceptable release of PFA leachate
into the
5a.11.1.1 The potential sources of water quality impact arising during the construction phase of the Project include construction site runoff and drainage, wastewater generated from general construction activities and sewerage from the workforce. With implementation of the recommended mitigation measures and site practices outlined in ProPECC PN 1/94, no unacceptable residual impacts on water quality are expected.
5a.11.1.2 During the operation phase of the Project, wastewater will be generated from the proposed incineration plant and mechanical treatment plant. An on-site wastewater treatment plant will be provided. All generated wastewater will be discharged to the on-site wastewater treatment plant and treated. The treated effluent from the wastewater treatment plant will be reused in the incineration plant and the mechanical treatment plant or washdown and landscape irrigation in the IWMF site. A “net zero discharge” scheme will be adopted during the operation of the IWMF.
5a.11.1.3 Saline water would be discharged from the proposed desalination plant in a low discharge rate. The saline water has been quantitatively assessed to be minor and acceptable. Adverse impacts on water quality due to the proposed saline water discharge would not be expected.