5.8 Prediction and Evaluation of Environmental Impacts and Mitigation Measures

5.8.1 Construction Phase

Construction Site Runoff

5.8.1.1 The expansion of the San Wai STW may involve excavation and filling works. It is, however, estimated that the scale would be small. Excavation and filling activities generate stockpiles of excavated and fill materials. Exposure of unprotected ground surface to the atmosphere may lead to the loss of soil when a rainstorm occurs. Site runoff generated from a rainstorm would cause surface erosion and would carry high concentrations of sediment particles. Release of sediment-laden flow from the construction site would enter Tin Shui Wai Drainage Channel and eventually be discharged to Deep Bay.

5.8.1.2 Discharge of untreated site runoff may increase the suspended solids levels and turbidity in the channel and the Deep Bay waters. The impacts may include the reduction in light penetration into water for primary production and damage to aquatic organisms.

5.8.1.3 Wastewater would also be generated from other construction activities such as general cleaning, concreting, plastering, internal decoration, testing of water pipes, drilling and building construction. Water used for cleaning, polishing, wheel washing and dust suppression also contains high concentrations of SS and possibly a certain amount of grease and oil.

5.8.1.4 The recommended mitigation measures are described as follows:
· Exposed stockpiles should be covered with tarpaulin or impervious sheets before a rainstorm occurs.
· The exposed soil surfaces should also be properly protected to minimise dust emission.
· The stockpiles of materials should be placed in the locations away from the drainage channel so as to avoid releasing materials into the channel.
· Wheel washing facilities should be provided at site exits to ensure that earth, mud and debris would not be carried out of the works areas by vehicles.
· Provision of site drainage systems and treatment facilities would be required to minimise the water pollution.
· A discharge licence needs to be applied from EPD for discharging effluent from the construction site.
· The treated effluent quality is required to meet the requirements specified in the discharge licence.

Sewage from Workforce

5.8.1.5 Sewage would be generated from the workforce. Discharge of untreated sewage is illegal and is harmful to the aquatic environment. A "zero discharge policy" for Deep Bay has been implemented in Deep Bay catchment. Effluents discharged into Deep Bay are required to be properly treated prior to final disposal so as not to cause net increase in pollution load to Deep Bay. It is therefore necessary to implement suitable control measures so as to comply with the policy. Provision of chemical toilets may be required to collect sewage from workforce. The chemical toilets should be cleaned on a regular basis.

5.8.1.6 Wastewater generated from kitchens should be discharged to public foul sewers or collected in a temporary storage tank if connection to public foul sewers is not feasible. A licensed waste collector should be employed to clean the chemical toilets and temporary storage tank on a regular basis.

5.8.1.7 Since the potential water quality impact from sewage generation can be effectively controlled through management practices, it is anticipated that sewage generation during the construction period of the Project would not cause water quality impact to the Tin Shui Wai Drainage Channel and the Deep Bay waters.

Accidental Spillage of Chemicals

5.8.1.8 Illegal disposal and accidental spillage of chemicals in the construction site would cause soil contamination. This may have direct impacts on the groundwater quality. When the chemicals are washed away by site runoff to Tin Shui Wai Drainage Channel, the water quality in the channel and near the outlet of the channel would be polluted. Organic pollutants may have toxic effects on aquatic organisms in the affected areas.

5.8.1.9 Illegal disposal of chemicals should be strictly prohibited. Registration to EPD as a Chemical Waste Producer is required if chemical wastes are generated and need to be disposed of. Disposal of chemical wastes should be carried out in compliance with the Waste Disposal Ordinance. The Code of Practice on the Packaging, Labelling and Storage of Chemical Wastes published under the Waste Disposal Ordinance should be used as a guideline for handing chemical wastes.

5.8.1.10 The impact from accidental spillage of chemicals can be effectively controlled through good management practices. It is likely that the potential impact on water quality would be low.

Cumulative Construction Impacts

5.8.1.11 The other concurrent projects, which may affect the assessment area and cause cumulative impacts, include the Deep Bay Link project and Hung Shui Kiu New Development Area (HSKNDA) development.

5.8.1.12 The construction works for the Deep Bay Link project would be carried out from a land-based operation. Release of construction site runoff into Deep Bay may cause cumulative impacts with the San Wai STW project. Construction site runoff could be effectively controlled through the implementation of suitable mitigation measures including provision of site drainage systems and sedimentation facilities, routine monitoring of the effluent discharge quality and environmental audit. Generation of wastewater and sewage, and accidental spillage of toxic substances during the construction period could also be controlled by providing chemical toilets and/or wastewater treatment facilities, off-site disposal of wastewater/sewage and establishment of a spill response plan. The potential cumulative water quality impacts due to the Deep Bay Link project are expected to be low.

5.8.1.13 The HSKNDA development would involve a wide range of construction activities. Implementation of mitigation measures would minimise the construction phase impacts. Environmental monitoring and audit would be implemented for the HSKNDA project. It is not expected that there would be adverse cumulative impacts generated from these two projects.

5.8.2 Operational Phase

Discharge of Treated Effluents

5.8.2.1 The water quality model runs covered four treatment options including CEPT, CEPT with disinfection, secondary treatment with disinfection and secondary treatment with nitrogen removal and disinfection. The treated effluent was assumed to discharge through the outfall at Urmston Road. A scenario for baseline case without the Project was also included in the model runs.

5.8.2.2 Figures 5.2 to 5.8 show the contour plots for annual 90%ile depth-averaged DO, 90% bottom DO, depth-averaged TIN, SS, UIA, E. coli, and BOD5 for the baseline case (upper plot) and Option 1 (middle plot). The lower plot on each of the figures showed the absolute difference between the two scenarios (Option 1 relative to Baseline). Tables 5.5 to 5.7 summarise the baseline results for wet and dry seasons as well as the calculated annual values while Table 5.8 to 5.10 summarise the results for Option 1. The percentage differences of Option 1 relative to Baseline case are presented in Table 5.11.

Table 5.5 Wet Season Water Quality Results for Baseline

Indicator Point

DO

(mg/L)

Bottom DO

(mg/L)

TIN

(mg/L)

UIA

(mg/L)

SS

(mg/L)

E.coli

(count/100mL)

BOD5

(mg/L)

DM1

2.1

2.1

2.36

0.144

42.5

1717

5.86

DM2

2.5

2.5

1.77

0.082

32.5

585

3.06

DM3

3.3

3.3

1.15

0.024

21.2

94

1.03

DM4

3.5

3.2

0.91

0.008

16.8

85

0.59

DM5

3.8

3.8

0.79

0.004

17.6

348

0.49

NM1

4.9

4.9

0.53

0.002

7.3

5

0.24

NM2

4.6

4.6

0.62

0.002

8.7

57

0.28

NM3

4.5

4.6

0.63

0.003

9.3

30

0.29

NM5

4.1

4.2

0.72

0.004

12.9

381

0.40

NM6

4.0

3.9

0.76

0.003

12.8

62

0.39

NM8

4.2

4.0

0.74

0.003

11.5

9

0.34

C1

4.9

5.0

0.52

0.002

7.2

3

0.24

D1

3.9

3.8

0.80

0.004

15.2

293

0.47

E1

5.1

5.0

0.48

0.002

7.0

32

0.26

E2

5.0

5.0

0.50

0.002

6.9

24

0.24

E3

5.0

4.9

0.51

0.002

6.9

93

0.24

F1

2.1

2.1

2.48

0.159

43.1

4031

6.27

F2

4.4

4.4

0.90

0.010

18.0

4630

0.78

G1

4.0

4.1

0.75

0.004

13.4

518

0.42

H1

4.0

4.0

0.76

0.003

13.3

111

0.40

A1

0.4

0.4

3.52

0.247

63.0

2560

12.12

B1

4.4

4.0

0.76

0.004

12.9

41

0.40

B2

4.2

4.1

0.77

0.004

11.9

183

0.50

D2

4.3

4.4

0.67

0.003

10.6

64

0.31

D3

5.4

4.7

0.57

0.002

9.2

147

0.54

D4

4.2

4.2

0.70

0.003

10.7

10

0.32

L1

5.0

4.5

0.61

0.002

8.7

251

0.34

J1

5.0

4.6

0.60

0.002

8.8

197

0.43

E4

5.0

4.9

0.51

0.002

6.9

33

0.24

E6

5.0

5.0

0.50

0.002

6.9

17

0.24

E7

5.0

5.0

0.50

0.002

7.0

22

0.24

E8

4.8

4.6

0.63

0.002

8.6

146

0.31

E11

4.9

4.7

0.62

0.002

8.4

154

0.33

E13

4.6

4.5

0.63

0.002

8.9

88

0.31

F3

4.0

3.9

0.78

0.003

14.6

61

0.43

 

 Table 5.6 Dry Season Water Quality Results for Baseline

Indicator Point

DO

(mg/L)

Bottom DO

(mg/L)

TIN

(mg/L)

UIA

(mg/L)

SS

(mg/L)

E.coli

(count/100mL)

BOD5

(mg/L)

DM1

4.1

4.0

2.70

0.079

40.2

8791

7.56

DM2

4.7

4.7

1.85

0.044

28.9

3401

4.67

DM3

5.5

5.5

1.00

0.015

17.5

460

2.17

DM4

5.8

5.6

0.66

0.007

12.4

281

1.24

DM5

6.1

6.0

0.47

0.004

9.0

692

0.81

NM1

6.4

6.3

0.20

0.002

5.0

11

0.35

NM2

6.3

6.3

0.24

0.002

5.6

149

0.42

NM3

6.2

6.2

0.27

0.003

6.0

78

0.47

NM5

6.1

6.1

0.38

0.004

7.6

435

0.66

NM6

6.2

6.2

0.38

0.003

8.0

86

0.69

NM8

6.4

6.3

0.34

0.003

7.7

8

0.69

C1

6.4

6.3

0.20

0.002

4.9

9

0.34

D1

6.0

6.0

0.47

0.004

9.2

454

0.83

E1

7.4

7.3

0.14

0.002

5.1

1

0.56

E2

6.4

6.3

0.19

0.002

4.8

51

0.32

E3

6.3

6.3

0.19

0.002

4.8

146

0.33

F1

4.6

4.6

2.64

0.072

38.8

15210

7.58

F2

6.2

6.1

0.93

0.013

17.1

7527

2.20

G1

6.1

6.1

0.40

0.004

8.0

387

0.69

H1

6.1

6.1

0.40

0.003

8.1

177

0.70

A1

3.0

3.0

3.69

0.117

50.3

6715

10.17

B1

6.2

6.0

0.40

0.003

8.9

115

0.79

B2

6.2

6.0

0.40

0.004

8.3

602

0.75

D2

6.2

6.2

0.31

0.003

6.6

141

0.53

D3

7.2

6.8

0.18

0.002

7.6

453

0.99

D4

6.3

6.2

0.32

0.003

7.1

19

0.61

L1

6.4

6.1

0.22

0.002

6.7

174

0.58

J1

7.0

6.6

0.20

0.002

6.2

602

0.71

E4

6.3

6.3

0.19

0.002

4.8

83

0.34

E6

6.4

6.3

0.19

0.002

4.8

31

0.32

E7

6.3

6.3

0.19

0.002

4.8

31

0.33

E8

6.5

6.3

0.22

0.002

6.1

174

0.52

E11

6.6

6.3

0.21

0.002

7.0

168

0.71

E13

6.3

6.3

0.24

0.002

5.8

232

0.46

F3

6.1

6.1

0.43

0.004

8.6

174

0.75

 

Table 5.7 Annual Average Water Quality Results for Baseline

Indicator Point

DO

(mg/L)

Bottom DO

(mg/L)

TIN

(mg/L)

UIA

(mg/L)

SS

(mg/L)

E.coli

(count/100mL)

BOD5

(mg/L)

DM1

3.1

3.1

2.53

0.111

41.3

3885

6.71

DM2

3.6

3.6

1.81

0.063

30.7

1410

3.86

DM3

4.4

4.4

1.08

0.020

19.3

208

1.60

DM4

4.7

4.4

0.79

0.008

14.6

155

0.92

DM5

4.9

4.9

0.63

0.004

13.3

491

0.65

NM1

5.6

5.6

0.36

0.002

6.1

7

0.30

NM2

5.5

5.4

0.43

0.002

7.2

92

0.35

NM3

5.4

5.4

0.45

0.003

7.7

49

0.38

NM5

5.1

5.2

0.55

0.004

10.3

407

0.53

NM6

5.1

5.1

0.57

0.003

10.4

73

0.54

NM8

5.3

5.2

0.54

0.003

9.6

9

0.51

C1

5.7

5.7

0.36

0.002

6.0

5

0.29

D1

5.0

4.9

0.63

0.004

12.2

365

0.65

E1

6.2

6.1

0.31

0.002

6.0

6

0.41

E2

5.7

5.7

0.35

0.002

5.8

35

0.28

E3

5.7

5.6

0.35

0.002

5.8

117

0.29

F1

3.4

3.3

2.56

0.116

41.0

7830

6.93

F2

5.3

5.2

0.92

0.012

17.5

5903

1.49

G1

5.1

5.1

0.57

0.004

10.7

448

0.56

H1

5.1

5.0

0.58

0.003

10.7

140

0.55

A1

1.7

1.7

3.61

0.182

56.7

4146

11.15

B1

5.3

5.0

0.58

0.003

10.9

69

0.60

B2

5.2

5.0

0.59

0.004

10.1

332

0.63

D2

5.3

5.3

0.49

0.003

8.6

95

0.42

D3

6.3

5.7

0.38

0.002

8.4

258

0.77

D4

5.3

5.2

0.51

0.003

8.9

14

0.47

L1

5.7

5.3

0.41

0.002

7.7

209

0.46

J1

6.0

5.6

0.40

0.002

7.5

345

0.57

E4

5.7

5.6

0.35

0.002

5.9

52

0.29

E6

5.7

5.7

0.34

0.002

5.8

23

0.28

E7

5.7

5.7

0.35

0.002

5.9

26

0.28

E8

5.6

5.5

0.42

0.002

7.3

159

0.42

E11

5.8

5.5

0.42

0.002

7.7

161

0.52

E13

5.5

5.4

0.44

0.002

7.3

143

0.38

F3

5.1

5.0

0.60

0.003

11.6

103

0.59

Note: Values are calculated as average of wet and dry season results

      E.Coli values are calculated as geometric mean

 

Table 5.8 Wet Season Water Quality Results for Option 1

Indicator Point

DO

(mg/L)

Bottom DO

(mg/L)

TIN

(mg/L)

UIA

(mg/L)

SS

(mg/L)

E.coli

(count/100mL)

BOD5

(mg/L)

DM1

2.3

2.3

2.14

0.124

40.9

1497

5.35

DM2

2.6

2.6

1.65

0.071

31.6

558

2.88

DM3

3.4

3.4

1.11

0.021

20.8

94

0.99

DM4

3.6

3.3

0.90

0.007

16.6

91

0.58

DM5

3.8

3.8

0.79

0.004

17.5

426

0.48

NM1

4.9

4.9

0.52

0.002

7.3

5

0.24

NM2

4.6

4.6

0.61

0.002

8.7

61

0.27

NM3

4.5

4.6

0.63

0.003

9.3

38

0.29

NM5

4.1

4.2

0.72

0.004

12.8

488

0.38

NM6

4.0

4.0

0.76

0.003

12.7

80

0.38

NM8

4.2

4.0

0.73

0.003

11.4

11

0.34

C1

5.0

5.0

0.51

0.002

7.1

4

0.24

D1

3.9

3.9

0.79

0.004

15.2

346

0.46

E1

5.1

5.0

0.48

0.002

7.0

32

0.26

E2

5.0

5.0

0.50

0.002

6.8

25

0.24

E3

5.0

4.9

0.50

0.002

6.9

93

0.24

F1

2.4

2.3

2.14

0.127

40.7

2696

5.25

F2

4.4

4.4

0.87

0.008

17.7

2828

0.69

G1

4.1

4.1

0.75

0.004

13.3

659

0.41

H1

4.0

4.0

0.75

0.003

13.2

144

0.39

A1

0.6

0.5

3.21

0.221

60.7

2309

11.27

B1

4.4

4.1

0.76

0.004

12.8

45

0.39

B2

4.2

4.1

0.77

0.004

11.8

202

0.49

D2

4.4

4.4

0.66

0.003

10.6

80

0.31

D3

5.4

4.7

0.57

0.002

9.2

148

0.54

D4

4.3

4.2

0.70

0.003

10.7

13

0.32

L1

5.0

4.5

0.60

0.002

8.6

251

0.34

J1

5.0

4.6

0.60

0.002

8.8

198

0.43

E4

5.0

4.9

0.51

0.002

6.9

33

0.24

E6

5.0

5.0

0.49

0.002

6.8

17

0.24

E7

5.0

5.0

0.50

0.002

7.0

22

0.24

E8

4.8

4.6

0.62

0.002

8.5

148

0.31

E11

5.0

4.7

0.62

0.002

8.3

155

0.33

E13

4.6

4.5

0.63

0.002

8.9

92

0.30

F3

4.0

3.9

0.78

0.003

14.5

77

0.42

 

Table 5.9 Dry Season Water Quality Results for Option 1

Indicator Point

DO

(mg/L)

Bottom DO

(mg/L)

TIN

(mg/L)

UIA

(mg/L)

SS

(mg/L)

E.coli

(count/100mL)

BOD5

(mg/L)

DM1

4.1

4.1

2.51

0.073

37.8

7630

6.82

DM2

4.8

4.8

1.76

0.042

27.7

3232

4.38

DM3

5.6

5.5

0.97

0.015

17.1

457

2.09

DM4

5.8

5.7

0.65

0.007

12.3

302

1.22

DM5

6.1

6.0

0.47

0.004

9.0

863

0.79

NM1

6.4

6.3

0.20

0.002

5.0

12

0.35

NM2

6.3

6.3

0.24

0.002

5.6

156

0.42

NM3

6.3

6.2

0.27

0.003

6.0

97

0.47

NM5

6.1

6.1

0.38

0.004

7.6

555

0.64

NM6

6.2

6.2

0.38

0.003

7.9

110

0.68

NM8

6.4

6.3

0.34

0.003

7.7

10

0.68

C1

6.4

6.3

0.20

0.002

4.9

10

0.34

D1

6.1

6.0

0.47

0.004

9.1

545

0.81

E1

7.4

7.3

0.14

0.002

5.0

1

0.56

E2

6.4

6.3

0.19

0.002

4.8

52

0.32

E3

6.3

6.3

0.19

0.002

4.8

147

0.33

F1

4.6

4.6

2.35

0.063

35.2

10090

6.23

F2

6.2

6.2

0.89

0.012

16.6

4535

2.04

G1

6.1

6.1

0.40

0.004

7.9

489

0.67

H1

6.1

6.1

0.40

0.004

8.1

227

0.69

A1

3.1

3.1

3.29

0.104

45.7

4462

8.56

B1

6.2

6.0

0.40

0.003

8.8

131

0.78

B2

6.2

6.1

0.40

0.004

8.2

645

0.74

D2

6.2

6.2

0.31

0.003

6.5

172

0.52

D3

7.3

6.8

0.18

0.002

7.5

454

0.99

D4

6.3

6.3

0.32

0.003

7.0

24

0.61

L1

6.4

6.2

0.22

0.002

6.7

176

0.58

J1

7.0

6.6

0.20

0.002

6.2

603

0.71

E4

6.4

6.3

0.19

0.002

4.8

84

0.34

E6

6.4

6.3

0.19

0.002

4.8

31

0.32

E7

6.4

6.3

0.19

0.002

4.8

32

0.33

E8

6.5

6.4

0.22

0.002

6.0

176

0.52

E11

6.7

6.3

0.21

0.002

7.0

169

0.70

E13

6.4

6.3

0.24

0.002

5.7

244

0.46

F3

6.2

6.1

0.43

0.004

8.5

222

0.73

 

Table 5.10 Annual Water Quality Results for Option 1

Indicator Point

DO

(mg/L)

Bottom DO

(mg/L)

TIN

(mg/L)

UIA

(mg/L)

SS

(mg/L)

E.coli

(count/100mL)

BOD5

(mg/L)

DM1

3.2

3.2

2.33

0.098

39.3

3380

6.08

DM2

3.7

3.7

1.71

0.057

29.6

1343

3.63

DM3

4.5

4.5

1.04

0.018

18.9

207

1.54

DM4

4.7

4.5

0.77

0.007

14.4

166

0.90

DM5

5.0

4.9

0.63

0.004

13.2

606

0.63

NM1

5.6

5.6

0.36

0.002

6.1

8

0.30

NM2

5.5

5.4

0.43

0.002

7.1

97

0.35

NM3

5.4

5.4

0.45

0.003

7.6

61

0.38

NM5

5.1

5.2

0.55

0.004

10.2

521

0.51

NM6

5.1

5.1

0.57

0.003

10.3

94

0.53

NM8

5.3

5.2

0.53

0.003

9.6

11

0.51

C1

5.7

5.7

0.36

0.002

6.0

6

0.29

D2

5.0

4.9

0.63

0.004

12.1

434

0.63

E1

6.2

6.1

0.31

0.002

6.0

6

0.41

E2

5.7

5.7

0.35

0.002

5.8

36

0.28

E3

5.7

5.6

0.35

0.002

5.8

117

0.29

F1

3.5

3.5

2.25

0.095

37.9

5216

5.74

F2

5.3

5.3

0.88

0.010

17.1

3581

1.36

G1

5.1

5.1

0.57

0.004

10.6

568

0.54

H1

5.1

5.0

0.58

0.003

10.7

181

0.54

A1

1.8

1.8

3.25

0.163

53.2

3210

9.92

B1

5.3

5.0

0.58

0.003

10.8

77

0.59

B2

5.2

5.1

0.59

0.004

10.0

361

0.62

D2

5.3

5.3

0.49

0.003

8.6

117

0.41

D3

6.3

5.8

0.37

0.002

8.4

259

0.77

D4

5.3

5.2

0.51

0.003

8.9

18

0.46

L1

5.7

5.4

0.41

0.002

7.6

210

0.46

J1

6.0

5.6

0.40

0.002

7.5

345

0.57

E4

5.7

5.6

0.35

0.002

5.9

53

0.29

E6

5.7

5.7

0.34

0.002

5.8

23

0.28

E7

5.7

5.7

0.35