The project involves
installation of FGD plants to the two existing coal-fired Units L4 and L5 and
demolition of two existing light oil tanks on formed land. When the FGD plants
are in operation, limestone slurry is introduced to react with flue gas for removal
of SO2 in the FGD plants before emission to the atmosphere. As a result, wastewater filtrated from
the reacted limestone slurry, i.e. gypsum slurry, will be produced and it will
be directed to the existing wastewater treatment system.
This section examines the
potential water quality impact induced by the project and suggests the
applicable mitigation measures to minimise the adverse impacts, if any, on the
water quality.
The regulatory requirements
and standards to protect water quality are as follows:
·
Water Pollution Control
Ordinance (WPCO);
·
Technical Memorandum Standards for Effluents Discharged into
Drainage and Sewerage Systems, Inland and Coastal Waters (TM);
·
Environmental Impact Assessment Ordinance (Cap. 499. S.16),
Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM),
Annexes 6 and 14;
·
WPCO Licence
on Decantrate Tower of the Ash Lagoon; and,
·
Practice Note for Professional Persons on Construction Site
Drainage (PN 1/94)
The WPCO is
the legislation for the control of water pollution and water quality in Hong
Kong. Under the WPCO, Hong
Kong waters are divided into 10 Water Control Zones (WCZs). Each WCZ has a
designated set of statutory Water Quality Objectives (WQOs). The WQOs set
limits for different parameters that should be achieved in order to maintain
the water quality within the WCZs.
The Project is
located in the Southern WCZ. The
boundary for the WCZ is shown in Figure 4.1. The WQOs for the marine waters of the Southern WCZ,
which are presented in Table 4.1, are applicable as evaluation criteria
for assessing compliance of any effects from the discharges of the Project.
Table 4.1 Water
Quality Objectives for the Southern Water Control Zones
|
Water Quality Objectives |
Part or
parts of Zone |
|
A. ESTHETIC
APPEARANCE |
|
|
(a) Waste discharges shall
cause no objectionable odours or discolouration of the water. |
Whole Zone |
|
(b) Tarry residues, floating wood,
articles made of glass, plastic, rubber or of any other substance should be
absent. |
Whole Zone |
|
(c) Mineral oil should
not be visible on the surface. Surfactants should not give rise to a lasting
foam. |
Whole Zone |
|
(d) There should be no recognisable
sewage-derived debris |
Whole Zone |
|
(e) Floating, submerged and
semi-submerged objects of a size likely to interfere with the free movement
of vessels, or cause damage to vessels, should be absent. |
Whole Zone |
|
(f) Waste
discharges shall not cause the water to contain substances which settle to
form objectionable deposits. |
Whole Zone |
|
B. BACTERIA |
|
|
(a) The level of Escherichia
coli should not exceed 610 per 1000 mL, calculated as the geometric mean of all
samples collected in one calendar year. |
Secondary Contact Recreation
Subzones and Fish Culture Subzones |
|
(b) The level of Escherichia
coli should not exceed 180 per 100 mL, calculated as the geometric mean of
all samples 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. |
Bathing Beach Subzones |
|
C. DISSOLVED OXYGEN |
|
|
(a) Waste discharges shall not cause
the level of dissolved oxygen to fall below 4 milligrams per litre for 90% of
the sampling occasions during the year; values should be calculated as the
water column average (arithmetic mean of at least 3 measurements at 1 metre
below surface, mid-depth, and 1 metre above seabed). In addition, the
concentration of dissolved oxygen should not be less than 2 milligrams per
litre within 2 metres of the seabed for 90% of the sampling occasions during
the year. |
Marine waters excepting Fish
Culture Subzones |
|
(b) The dissolved oxygen level
should not be less than 5 milligrams per litre for 90% of the sampling
occasions during the year; values should be calculated as water column
average (arithmetic mean of at least 3 measurements at 1 metre below surface,
mid-depth and 1 metre above seabed). In addition, the concentration of
dissolved oxygen should not be less than 2 milligrams per litre within 2
metres of the seabed for 90% of the sampling occasions during the year. |
Fish Culture Subzones |
|
(c) Waste discharges
shall not cause the level of dissolved oxygen to be less than 4 milligrams
per litre. |
Inland waters of the Zone |
|
D. pH |
|
|
(a) The pH of the water should
be within the range of 6.5-8.5 units. In Marine waters addition, waste
discharges shall not cause the natural pH range to be excepting Bathing
extended by more than 0.2 units. |
Beach Subzones; Mui Wo (A), Mui Wo
(B), Mui Wo (C), Mui Wo (E), Mui Wo (F) Subzones |
|
(b) The pH of the water should
be within the range of 6.0-9.0 units. |
Mui Wo (D) Sub-zone and other
inland waters. |
|
(c) The pH of the water
should be within the range of 6.0-9.0 units for 95% of samples. In addition,
waste discharges shall not cause the natural pH range to be extended by more
than 0.5 units. |
Bathing Beach Subzones |
|
E. TEMPERATURE |
|
|
(a) Waste discharges shall not
cause the natural daily temperature range to change by more than 2.0 degrees
Celsius. |
Whole Zone |
|
F. SALINITY |
|
|
Waste discharges shall not cause
the natural ambient salinity level to change by more than 10%. |
Whole Zone |
|
G. SUSPENDED SOLIDS |
|
|
(a) Waste discharges shall
neither cause the natural ambient level to Marine waters be raised by 30% nor
give rise to accumulation of suspended solids which may adversely affect
aquatic communities. |
|
|
(b) Waste discharges shall not
cause the annual median of suspended solids to exceed 20 milligrams per
litre. |
Beach Subzones; Mui Wo (A), Mui Wo
(B), Mui Wo (C), Mui Wo (E), Mui Wo (F) Subzones |
|
(c) Waste discharges
shall not cause the annual median of suspended solids to exceed 25 milligrams
per litre. |
Mui Wo (D) Subzone and other Inland
Waters |
|
H. AMMONIA |
|
|
The ammonia nitrogen level should
not be more than 0.021 milligram per litre, calculated as the annual average
(arithmetic mean), as unionised form. |
Whole Zone |
|
I. NUTRIENTS |
|
|
(a) Nutrients shall not
be present in quantities sufficient
to cause excessive or nuisance growth of algae or other aquatic
plants. |
Marine
waters |
|
(b) Without limiting the generality of
objective (a)
above, the level of inorganic nitrogen should not exceed 0.1 milligram
per litre, expressed as annual water column average (arithmetic mean of at
least 3 measurements at 1 metre below surface, mid-depth and metre above
seabed). |
Marine
waters |
|
J. 5-DAY BIOCHEMICAL OXYGEN DEMAND |
|
|
Waste
discharges shall not
cause the 5-day biochemical oxygen demand to exceed 5 milligrams per litre. Inland
waters of thedemand to exceed 5 milligrams per litre. |
Inland Waters of the Zone |
|
K. CHEMICAL OXYGEN DEMAND |
|
|
Waste
discharges shall not
cause the chemical oxygen demand to exceed 30 milligrams per litre. Inland
waters of theexceed 30 milligrams per litre |
Inland Waters of the Zone |
|
L.
DANGEROUS SUBSTANCES |
|
|
(a) Waste discharges shall not cause the
concentrations of dangerous substances in marine waters to attain such levels
as to produce significant toxic effects in humans, fish or any other aquatic
organisms, with due regard to biologically cumulative effects in food chains
and to toxicant interactions with each other. |
Whole
Zone |
|
(b) Waste discharges of dangerous
substances shall not put a
risk to any beneficial uses of the aquatic environment. |
Whole
Zone |
All discharges from the
Lamma Power Station, including those from the emission control facilities, are
required to comply with the Technical Memorandum for Effluents Discharged
into Drainage and Sewerage Systems, Inland and Coastal Waters (TM) issued
under Section 21 of the WPCO. The TM defines discharge
limits for different types of receiving waters. Under the TM, effluents discharged into the drainage and
sewerage systems, inshore and coastal waters of the WCZs are subject to
pollutant concentration standards for particular discharge volumes. Any new discharges within a WCZ are
subject to licence conditions and the TM acts as a guideline for setting
discharge standards for inclusion in the licence.
For the discharges from the Lamma Power Station it is
appropriate to make reference to Table 10a Standards for Effluents Discharged
into the Inshore Waters of Southern, Mirs Bay, Junk Bay, North Western, Eastern
Buffer and Western Buffer Water Control Zones. Existing WPCO discharge licences have been issued for a
number of wastewater discharges from the existing power station, including the
cooling water systems, oil separators and wastewater treatment plant.
Annexes 6 and 14 of
the EIAO-TM provide general guidelines and criteria to be used in
assessing water quality issues.
The EIAO-TM recognises that it may not be possible to
achieve compliance with the WQOs in the vicinity of a wastewater
discharge. In this area, where the
initial dilution of pollutants takes place, there may be greater water quality
impacts than would be allowed by the WQOs. Such an area may be termed a ‘mixing zone’ and within this
area exceedence of the WQOs would not be deemed to be an adverse impact. In general, the criteria for acceptance
of a ‘mixing zone’ are that it must not impair the integrity of the water body
as a whole and must not damage the ecosystem or impact marine sensitive
receivers.
Whilst the technical circulars are non-statutory, they are
generally accepted as best guidelines in Hong Kong and have been adopted as
relevant for this assessment.
The WPCO licence
was issued to HEC in order to restrict the quality and quantity of the effluent
of FGD Wastewater Treatment Plant (WWTP) discharged into the ash lagoon. The effluent discharge limits are
applied for two situations, i.e. normal plant operation and during plant
maintenance. Tables 4.2 and 4.3
present the details.
Table 4.2 Effluent
Discharge Limits for FGD WWTP during Normal Plant Operation
|
Parameters |
Unit |
Maximum |
|
Flow Rate |
cu.m day-1 |
200 |
|
pH |
(pH units) |
6-9 |
|
Temperature |
oC |
40 |
|
Suspended Solids |
mg L-1 |
30 |
|
COD |
mg L-1 |
80 |
|
BOD |
mg L |
20 |
|
Iron |
mg L |
4 |
|
Barium |
mg L |
1.5 |
|
Mercury |
mg L |
0.005 |
|
Cadmium |
mg L |
0.005 |
|
Other Toxic Metals (individually) |
mg L |
0.4 |
|
Total Toxic Metals |
mg L |
0.8 |
|
Cyanide |
mg L |
0.1 |
|
Phenols |
mg L |
0.2 |
|
Sulphide |
mg L |
5 |
|
Total Phosphorus |
mg L |
8 |
|
Total Nitrogen |
mg L |
400 |
Table 4.3 Effluent Discharge Limits
for FGD WWTP during Plant Maintenance
|
Parameters |
Unit |
Maximum |
|
Flow Rate |
cu.m day-1 |
500 |
|
pH |
(pH units) |
6-9 |
|
Temperature |
oC |
40 |
|
Suspended Solids |
mg L-1 |
30 |
|
COD |
mg L-1 |
80 |
|
BOD |
mg L |
20 |
|
Iron |
mg L |
4 |
|
Barium |
mg L |
1.5 |
|
Mercury |
mg L |
0.003 |
|
Cadmium |
mg L |
0.003 |
|
Other Toxic Metals (individually) |
mg L |
0.4 |
|
Total Toxic Metals |
mg L |
0.8 |
|
Cyanide |
mg L |
0.1 |
|
Phenols |
mg L |
0.2 |
|
Sulphide |
mg L |
5 |
|
Total Phosphorus |
mg L |
8 |
|
Total Nitrogen |
mg L |
150 |
The ProPECC (PN 1/94)
issued by EPD provides some basic environmental guidelines for the handling and
disposal of construction site discharges to prevent or minimise construction
impacts on water quality.
4.3
Assessment Methodology
The retrofit methodology (see Section 1.5) has been
reviewed to assess the remoteness of the proposed project to existing and
committed Water Sensitive Receivers.
The WSRs were identified according to guidance provided in Hong Kong
Planning Standards and Guidelines (HKPSG).
Construction stages, sequence and duration were reviewed to
identify activities likely to impact upon identified WSRs and other water
courses. Following the
identification of WSRs and potential water quality impacts, the scale, extent
and severity of potential net (i.e. unmitigated) construction /operational
impacts were evaluated, taking into account all potential cumulative effects
including those of adjacent projects, with reference to the WPCO criteria.
Where net water quality impacts exceed the appropriate WPCO
criteria, practical water pollution control measures/mitigation proposals
will be identified to ensure compliance with reference to the WPCO criteria. Water quality monitoring and audit
requirements will be subsequently developed, if necessary, to ensure the
effectiveness of the water pollution control and mitigation measures.
4.4
Baseline Conditions and Water Quality
Sensitive Receivers
Construction
will be carried out on formed land where the drainage system has been well
established. During construction
activities, the primary sources of water quality impacts will be from
pollutants in site run-off. Pollutants, mainly suspended sediments, may also
enter receiving waters if the run-off is not adequately controlled
on-site. The potential sources of
impacts to water quality may be readily controlled by appropriate on-site
measures to minimise potential impacts as discussed in Section 4.6.
In order to evaluate the
water quality impacts resulting from the construction and/or operation of the
FGD plants on Lamma Island, the water sensitive receivers (WSRs) have been
identified in accordance with the HKPSG, which provides criteria for
identifying environmental factors influencing the proposed development.
The identified water
quality sensitive receivers close to (i.e. within 3 km) the Power Station and
around Lamma Island (Figure 4.2) include:
• Gazetted
Bathing Beaches: Hung Shing Ye Beach and Lo So Shing Beach.
• Water
Intakes: HEC Lamma Power Station Intake.
• Fish
Culture Zones: Lo Tik Wan Fish Culture Zone and Sok Kwu Wan Fish Culture Zone.
• Sites
of Ecological Interest: Pak Kok (corals present), Shek Kok Tsui (corals
present), Luk Chau (corals present), Ha Mei Wan (fish spawning ground) and
southern Lamma waters (Finless Porpoise, Chinese White Dolphin, green turtles
nesting site and proposed marine park).
SW Lamma 1 and SW Lamma 2, which are situated to the south of the works
area (Figure 4.2), have been selected
to assess any potential water quality impacts of this Project on the southern
Lamma waters. As long as the
impact on this part of the water is acceptable, the impact on the other parts
of the southern Lamma waters will be less and therefore also acceptable.
Table 4.4 shows that the
site is remote (i.e. > 1 km) from the WSRs except for the HEC Lamma Power
Station Intake.
Table 4.4 Distance
of the Site from the Water Quality Sensitive Receivers
|
Water Quality Sensitive
Receivers |
Minimum Distance |
ID |
|
|
from the Site (m) |
|
|
Pak Kok (Coral) |
3,020 |
SR1 |
|
Shek Kok Tsui (Coral) |
1,800 |
SR2 |
|
Luk Chau (Coral) |
3,110 |
SR3 |
|
Lo Tik Wan Fish Culture
Zone |
2,330 |
SR4 |
|
HEC Lamma Power Station
Intake |
200 |
SR5 |
|
Hung Shing Ye Beach |
1,360 |
SR6 |
|
Lo So Shing Beach |
2,010 |
SR7 |
|
Sok Kwu Wan Fish Culture
Zone |
2,480 |
SR8 |
|
Ha Mei Wan (Fish
Spawning Ground) |
1,500 |
SR9 |
|
SW Lamma 1 |
2,660 |
SR10 |
|
SW Lamma 2 |
3,920 |
SR11 |
The existing WWT system is designed to treat the effluent
from FGD plants by alkalization, precipitation and neutralization process
without removal of heavy metals apart from zinc and manganese. The plant capacity has been designed
based on 5x350MW FGD with a total capacity of 54 m3 hr-1
(2 streams each of 27 m3 hr-1) which can also cater for
supernatant from wastewater sludge centrifuge and other on-load equipment
drains. The schematic diagram of the FGD WWT is illustrated in
Figure 4.3 and the design capacity of
the WWT Plant is calculated as follows:
|
Constant flow
from 5 x 350MW (5 x 6.3 m3 hr-1): |
31.5
m3 |
|
Supernatant from
wastewater sludge centrifuge: |
16
m3 |
|
Other on-load equipment
drains: |
6.5
m3 |
|
Total: |
54 m3 |
Wastewater from FGD process is conveyed to the underground
wastewater storage sump pits equipped with agitator for temporary storage. When high level is reached, the
wastewater is pumped to the pH adjustment and coagulation tanks where caustic
soda is dosed to bring the pH to 9-10 for precipitation of metals as
hydroxides. After the pH
adjustment and coagulation tank, the wastewater is dosed with a polymer and is
brought to a thickener for settlement of suspended solids.
The water overflow from the thickeners is conveyed to a
primary treated water storage tank for further neutralization by hydrochloric
acid to decrease the pH to 6-9. In
case the pH is not correct or the turbidity of the wastewater is above a preset
limit, it would be recycled back to the storage sump pit for re-treatment.
The underflow
sludge is conveyed to two centrifuge feed tanks for temporary storage before
feeding into the centrifuges for dewatering. Dried sludge ejected from the centrifuges is temporarily
stored and discharged to barge through conveyors. The sludge is then taken away by contractor for reuse in the
building industry.
Neutralized
wastewater is temporarily stored in Rejected Treated Water Storage Tanks and
then is discharged to the existing Ash Lagoon. The Ash Lagoon will have sufficient capacity to store treated effluent from the FGD treatment
process. The discharge point for
effluent from the ash lagoon decantrate tower is presented in
Figure 4.4. Its quantity and quality is
closely monitored in accordance with the WPCO licence on Decantrate
Tower of the Ash Lagoon.
The decanted
water from the Ash Lagoon is then discharged through the Cooling Water Outfall
into the marine water. The
effluent of the Ash Lagoon is also monitored.
The details of
the monitoring results are presented in Section
4.5.2.
4.5
Water Quality Impact Assessment
Potential sources of impacts to water quality from the
construction activities are:
• Construction
runoff; and
• Sewage
effluents generated from the workforce.
Construction Runoff
Construction runoff from site areas may contain high loading
of suspended solids (SS) and contaminants. Potential water pollution sources
from construction site runoff include:
a)
Runoff and
erosion from site surfaces, earth working areas and stockpiles;
b)
Used water from
purging of the Light Oil Tanks; and
c)
Fuel, oil, solvents and
lubricants from maintenance of machinery and equipment.
Construction runoff may cause physical, biological and
chemical effects. Its physical effect can cause blockage of drainage channels
due to the deposits of increasing SS from the site. Chemical and biological effects are however highly dependent
on its chemical and nutritional contents.
Runoff containing significant amount of concrete and cement-derived
materials would lead to increasing turbidity and discoloration, elevation in
pH, and accretion of pH solids.
There will be no wastewater generated by the demolition of
oil tanks. The excavated top soil will
not be stored on site and will be removed offsite soon after the excavation due
to limited site works area.
Excavation is necessary for the construction of the piled
foundations of FGD booster fans, gas-gas heaters, gas ducts supports and the
shallow foundation of the Switchgear and Equipment Building. It is anticipated that the construction
runoff will not be significant. In
addition, the existing storm drains could catch the runoff to prevent the
run-off reaching the nearby marine water. With good practice of work, as stated
in ProPECC PN1/4, and appropriate mitigation measures, the construction
runoff should be minimal and unacceptable water quality impacts due to surface
runoff are not expected.
Sewage Effluents generated
from the Workforce
Sewage effluents will arise from the sanitary facilities
provided for the on-site workforce.
The characteristics of sewage would include high levels of 5-day
Biochemical Oxygen Demand (BOD5), Ammonia and E.coli counts.
The existing toilet facilities of the Power Station will be
provided for use by the workforce.
Additional sanitary facilities are therefore not required. Based on the
above, adverse impacts to water quality as a result of the sewage effluent
generated by the workforce are not expected to occur.
Potential source of impacts to water quality from the
operation of the FGD plants are as a result of filtrate generated from the
dewatering of gypsum slurry.
The limestone slurry is introduced to react with flue gas
for removal of SO2. Water will filter out from the gypsum slurry
after passing through the hydrocyclones.
It is then retained in the reclaimed water tanks before discharging to
the existing wastewater treatment plant (WWTP). The influent is characterised
as pH of 5-6 and SS of approximately 7% of the whole volume.
In line with the existing practice adopted for the Units L6,
L7 & L8 FGD plants, operational plant effluent from the proposed FGD plant
will be reused as far as possible for preparation of limestone slurry,
conditioning of PFA for offsite transportation, etc. to minimise discharge to
the existing WWTP.
The WWTP is equipped with a number of storage tanks/pits and
recirculation lines to temporary store the effluent for re-treatment if the
action limit is triggered. Chemical
tanks are usually sized based on 14 days usage for 5 x 350MW FGD Plants whereas
the transfer/dosing pumps are with 100% standby capacity. The sizing of the storage pits/tanks
are listed in Table 4.5.
Table 4.5 WWTP Plant Design Capacity
|
Equipment
Capacity |
Size |
Design |
|
Wastewater Storage Sump
Pit |
120 m3 x 2 |
2x50% for 5x350MW |
|
Rejected Treated Water
Storage Tanks |
260 m3 x 2 |
2x50% for 5x350MW |
Referring to the past record (October 2004 to September
2005) at the sampling point of the Rejected Treated Water Storage Tank (Table
4.6), the effluent generated is well below the licence limit. Note that not
all parameters require monitoring in accordance with the WPCO licence. In addition, there is no normal and
emergency plant maintenance discharge so far.
Table
4.6 Monitoring
Records of FGD Wastewater Treatment Plant Discharge to Ash Lagoon
|
|
Daily Average |
Concentration (Daily Average) |
||||||||||
|
Month |
Volume |
pH |
S.S. |
Ba |
Hg |
Cd |
Fe |
TP |
TN |
CN |
||
|
|
(m3 day-1) |
|
(mg L-1) |
(mg L-1) |
(mg L-1) |
(mg L-1) |
(mg L-1) |
(mg L-1) |
(mg L-1) |
(mg L-1) |
||
|
Licence Limit |
200 |
6-9 |
30 |
1500 |
5 |
5 |
4000 |
8 |
400 |
0.1 |
||
|
Oct 2004 |
43.4 |
8.3 |
17.7 |
69 |
<0.5 |
<1 |
240 |
<0.1 |
53 |
<0.01 |
||
|
Nov 2004 |
19.1 |
8.4 |
17.8 |
22 |
<0.5 |
<1 |
300 |
<0.1 |
200 |
<0.01 |
||
|
Dec 2004 |
24.9 |
8.1 |
18.7 |
31 |
<0.5 |
<1 |
440 |
0.2 |
150 |
0.02 |
||
|
Jan 2005 |
44.7 |
8.3 |
15.1 |
24 |
<0.5 |
<1 |
<50 |
0.2 |
130 |
<0.01 |
||
|
Feb 2005 |
26.6 |
8.3 |
18.4 |
138 |
<0.5 |
2 |
<50 |
<0.1 |
115 |
0.04 |
||
|
Mar 2005 |
53.6 |
8.3 |
19.3 |
280 |
<0.5 |
2 |
100 |
0.2 |
285 |
0.02 |
||
|
Apr 2005 |
56.1 |
8.4 |
19.1 |
741 |
<0.5 |
2 |
110 |
0.6 |
315 |
<0.01 |
||
|
May 2005 |
57.0 |
8.4 |
19.0 |
340 |
1.4 |
3.8 |
130 |
0.2 |
320 |
0.02 |
||
|
Jun 2005 |
42.1 |
8.4 |
17.9 |
313 |
<0.5 |
<1 |
<50 |
0.1 |
148 |
<0.01 |
||
|
Jul 2005 |
78.3 |
8.4 |
19.5 |
307 |
<0.5 |
<1 |
<50 |
<0.1 |
145 |
0.01 |
||
|
Aug 2005 |
89.8 |
8.4 |
18.6 |
273 |
<0.5 |
3 |
260 |
0.2 |
117 |
<0.01 |
||
|
Sep 2005 |
52.7 |
8.5 |
19.3 |
30 |
<0.5 |
<1 |
<50 |
0.3 |
87 |
<0.01 |
||
As the L4 &
5 FGD would adopt the same wet limestone-gypsum process, similar properties of
effluent would be generated with the following properties and the expected
influent and effluent are given in Table 4.7.
Table 4.7 Expected
Influent and Effluent Characteristics on FGD Wastewater Treatment Plan
|
Constituents |
Units |
Designed Influent for 5x350MW FGD |
Expected Influent for 5x350MW FGD |
Effluent at sampling point |
|
pH |
pH |
5~6 |
5~6 |
6~9 |
|
Temperature |
°C |
46 |
43~44 |
≤40 |
|
Suspended |
mg/L |
71,800 |
64,400~66,200 |
≤30 |
|
Solids |
|
|
|
|
Current design
capacity of the existing WWTP is 54 m3 hr-1, which is
designed to cater for effluent from 5 FGD plants each with a maximum effluent
discharge rate of 6.3 m3 hr-1. As the existing WWTP has spare capacity to cater for the
additional wastewater produced from the proposed retrofit project, it is
expected that effluent from the WWTP to the Ash Lagoon will meet the requirements
in the WPCO licence for the Ash Lagoon Decantrate Tower.
The decanted
water discharged from the Ash Lagoon is closely monitored. The monitoring results of the decanted
water discharged from the Ash Lagoon to the Cooling Water Outfall from October
2004 to September 2005 is presented in Table
4.8.
Table 4.8 Monitoring
Records of Ash Lagoon Discharged to Cooling Water Outfall
|
Month |
Volume |
SS |
Ba |
Hg |
Cd |
Fe |
TP |
TN |
CN |
|
|
(m3 day-1) |
(mg L-1) |
(mg L-1) |
(mg L-1) |
(mg L-1) |
(mg L-1) |
(mg L-1) |
(mg L-1) |
(mg L-1) |
|
Licence Limit |
20,000 |
30 |
200 |
1 |
1 |
600 |
5.0 |
20 |
0.01 |
|
Oct 2004 |
18589.7 |
19.0 |
62 |
<0.5 |
<1 |
190 |
<0.1 |
0.4 |
<0.01 |
|
Nov 2004 |
18581.3 |
18.9 |
22 |
<0.5 |
<1 |
360 |
<0.1 |
0.3 |
<0.01 |
|
Dec 2004 |
18594.2 |
19.6 |
33 |
<0.5 |
<1 |
410 |
0.1 |
0.5 |
<0.01 |
|
Jan 2005 |
7203.9 |
20.3 |
23 |
<0.5 |
<1 |
<50 |
0.1 |
0.1 |
<0.01 |
|
Feb 2005 |
0 |
* |
* |
* |
* |
* |
* |
* |
* |
|
Mar 2005 |
0 |
* |
* |
* |
* |
* |
* |
* |
* |
|
Apr 2005 |
0 |
* |
* |
* |
* |
* |
* |
* |
* |
|
May 2005 |
0 |
* |
* |
* |
* |
* |
* |
* |
* |
|
Jun 2005 |
7506.0 |
17.9 |
32 |
<0.5 |
<1 |
<50 |
0.2 |
2.1 |
<0.01 |
|
Jul 2005 |
14106.1 |
18.7 |
40 |
<0.5 |
<1 |
90 |
<0.1 |
1.3 |
<0.01 |
|
Aug 2005 |
14087.4 |
20.5 |
30 |
<0.5 |
<1 |
190 |
0.2 |
0.6 |
<0.01 |
|
Sep 2005 |
3756.0 |
16.1 |
27 |
<0.5 |
<1 |
50 |
<0.1 |
0.9 |
<0.01 |
|
Notes: * No sampling was taken due to no
discharge. |
|||||||||
The monitoring results show
that the concentrations of the effluent from Ash Lagoon are well below the
limit. In overall, the effluent
from Ash Lagoon is better in quality than that from the FGD WWTP. It has been shown that the additional
flow from the FGD plants is in compliance with the license and is
environmentally acceptable. On the
basis of the above results and the fact that the discharge point of the cooling
water outfall for the ash lagoon decantrate is directed away from WSRs (see
Figure 4.4), effluent from the Cooling
Water Outfall is not expected to pose any unacceptable adverse impacts on the
WSRs.
Based on the above, no
unacceptable impacts to WSRs are expected to occur as a result of either the
construction or operation of the FGD Retrofit.
General
Any construction effluent discharge should be diverted away
from embayed water such as the eastern waters of the working site where bathing
beaches are found.
Construction Runoff
Exposed soil areas should be minimised to reduce the
contamination of runoff and erosion.
As mentioned in Section 4.5.1, excavation is necessary for the
construction of the piled foundations of FGD booster fans, gas-gas heaters, gas
ducts supports and the shallow foundation of the Switchgear and Equipment
Building only. Silt removal
facilities, channels and manholes should be maintained and the deposited silt
and grit should be removed regularly to ensure they are functioning properly at
all times. Temporary covers (i.e.
tarpaulin) should also be provided to minimise the generation of high SS
runoff.
A licensed waste collector will be standby on-site to
collect the waste oils and other chemical waste collected from the demolition
of oil tank and oil separator as well as the used water from diesel oil tank
purging. The used water will then
be transported to a facility licensed to receive chemical waste, such as
Chemical Waste Treatment Centre at Tsing Yi.
In addition, control measures, including implementation of excavation schedules,
lining and covering of excavated stockpiles and contaminated soil treatment
areas, shall be implemented to minimize contaminated stormwater run-off from
the site. The contaminated run-off
water as well as leachate/seepage resulting from the contaminated soil
stockpiles and treatment shall be collected and treated to meet the WPCO
requirements prior to discharge.
Sewage Effluents
The existing toilet of the Power Station will be provided
for the construction workforce. No additional sanitary facilities will be
required and hence adverse impact is not anticipated.
Groundwater
Discharge
According to the Land Contamination Assessment (Section 2), exceedances of TPH were
detected in several groundwater samples.
As
groundwater is not used for either domestic or industrial purposes at the Site
and in the adjacent areas, remediation of TPH in the groundwater of the Site is
not considered necessary.
Groundwater extraction is not anticipated during the demolition and/or
construction programmes as the proposed excavations are at levels well above
the site groundwater table.
However, if groundwater is encountered during the demolition and/or
construction programmes and groundwater dewatering from the work areas is
required, the extracted groundwater will be collected, appropriately stored
on-site and recharged back to the underlying ground.
As mentioned in Section
4.5.2, the operational plant effluent from the FGD plants should be reused as
much as possible in order to minimise discharge to the WWTP. Maintenance of the WWTP should be
performed regularly to ensure the effluent from the WWTP would not exceed the
current requirements stipulated in the WPCO license for Ash Lagoon.
Any operational effluent discharge should be discharged to
the Ash Lagoon and the decanted water is then discharged through the Cooling
Water Outfall, which is away from embayed water such as the eastern waters of
the working site where bathing beaches are found
4.7
Summary of Environmental Outcomes and
Conclusion
The potential water quality impacts from the retrofit of FGD
plants have been assessed.
Assessment results indicate that no unacceptable water quality impacts
will arise from the construction activities provided that the recommended
mitigation measures are implemented.
The operation of the FGD plants will not
result in adverse water quality impacts on the water quality sensitive
receivers as the existing WWTP has adequate capacity to deal with the
additional FGD effluent.
4.7.1
Environmental Monitoring and Audit (EM&A) Requirements
Due to the small scale of the
demolition and construction works of the Project, and no adverse impacts
predicted, no EM&A is required for the Construction Phase.
Since the Project will not have
any unacceptable water quality impacts, no additional EM&A activities are
required, besides those already in place, such as those required by the
operation of the existing WWTP at Lamma Power Station.