3.                  AIR QUALITY

 

Introduction

 

3.1.            Dust impact from the construction activities of the proposed sewers, pumping stations and the STW, and odour impact from the pumping stations and the STW have beenwere identified as the potential key environmental issues of concern during construction and operation phases of the project respectively.  This section presentedss the air quality assessment for the construction and operation phases of the Project.  

 

Environmental Legislation, Standards and Guidelines

 

3.2.            The design criteria related to air quality impact should make reference to the Technical Memorandum on EIA Process (TM-EIAEIAO-TM ), Hong Kong Planning Standards and Guidelines (HKPSG), the Air Pollution Control Ordinance (APCO) (Cap. 311) and the relevant Practice Note for Professional Persons (ProPECC Note).

 

3.3.            The APCO (Cap. 311) provides powersthe statutory authority for controlling air pollutants from a variety of stationary and mobile sources and encompasses a number of Air Quality Objectives (AQOs).  Currently AQOs stipulate concentrations for a range of pollutants, of which total suspended particulates (TSP) are relevant to this study.  The AQOs are listed in Table 3.1.

 

Table 3.1         Hong Kong Air Quality Objectives

 

Parameter	Maximum Average Concentration (µgm-3)1
	1-Hour2	24-Hour3	Annual4
TSP	5005	260	80

1           Measured at 298 K and 101.325 kPa.

2           Not to be exceeded more than three times per year.

3           Not to be exceeded more than once per year.

4           Arithmetic mean.

5           Not an AQO.  However, in addition to the established legislative controls, it is generally accepted that an hourly average TSP concentration of 500 µgm^-3 should not be exceeded.

 

3.4.            For construction dust, the limits for 1-hour and 24-hour TSP concentration at sensitive receivers are 500 µgm^‑3 and 260 µgm^-3 respectively in accordance with the criteria for evaluating air quality impact stipulated in Annex 4 of the EIAO EIAO-TM.

 

3.5.            The Air Pollution Control (Construction Dust) Regulation is effective from 16 June 1997.  Site formation is one of the processes controlled under this regulation.  Contractors and site agents are required to inform EPD and adopt dust reduction measures to minimize dust emission, while carrying out construction works, to the acceptable level.

 

3.6.            For odour assessment, 5 odour units (OU), averaged over 5 seconds, at odour sensitive receivers are required according to the criteriona stipulated in EIAO EIAO-TM.

 

Baseline Conditions

 

3.7.            The study area is a low-density population rural area.  No large-scale construction works would be expected in this area.  A quarry is located at the northern side of Picnic Bay, which may be potential air pollution source generating dust particulates.  A refuse transfer station, which is an open facility site, has been commissioned in early 2000.  However, according to the Initial EIA Report of Outlying Island Transfer Facilities, September 1993, no unacceptable dust and odour impacts would be expected due to the small scale of the station and distance from Air Sensitive Receivers (ASRs).

 

Sensitive Receivers

 

3.8.            Air Sensitive Receivers (ASRs) considered in both the construction and operation assessments included the existing residential houses, restaurants and recreational areas.  A description of the ASRs is summarized in Table 3.2.  Locations of representative ASRs are shown in Figure 3.1.  The separation distance between the ASRs and the sewer alignment, pumping stations and STW are indicated in Table 3.2.

 

Table 3.2        Description of Air Sensitive Receivers (ASRs)

 

ASRs	Description	Approximate horizontal distance from the sewage facilities (m)
		Sewer alignment	Pumping Station			STW
			P1A	P1B	P2
A01	Squatter house in Chung Mei Village	2	42	94	--	-
A02	Squatter house in Chung Mei Village	5	34	98	--	-
A03	Squatter house in Chung Mei Village	20.5	47	113	--	-
A04	Squatter house in Chung Mei Village	15.5	54	59	--	-
A05	Squatter house in Chung Mei Village	2	52	72	--	-
A06	Squatter house in Chung Mei Village	1	54	79	--	-
A07	Squatter house in Chung Mei Village	7	68	97	--	-
A08	Squatter house in Chung Mei Village	16.5	102	121	--	-
A09	Squatter house in Chung Mei Village	11.5	196	212	--	-
A10	Squatter house in Chung Mei Village	3	226	242	--	-
A11	Tin Hau Temple	4	209	137	353-	-
A12	17 Sok Kwu Wan Second Street	2	225	160	337353	-
A13	29 Sok Kwu Wan First Street	3	240	175	324337	-
A14	Restaurant seating area	2	258	194	308324	-
A15	Restaurant seating area	2.5	288	223	278308	-
A16	Restaurant seating area	2	322	256	246278	-
A17	Restaurant seating area	2	360	294	211246	-
A18	Restaurant seating area	2	392	326	182211	-
A19	Restaurant seating area	2	424	359	155182	-
A20	Playground	2	477	410	108155	-
A21	Playground	2	494	426	88108	-
A22	Playground	1.5	-	445	6088	-
A23	Playground	2	-	475	2060	-
A24	Football court	7.5	-	480	820	-
A25	Squatter house near Ta Shui Wan	2.5	-	-	518	499
A26	Squatter house near Ta Shui Wan	5.5	-	-	20351	343
A27	Squatter house near Ta Shui Wan	19.5	-	-	202203	344
A28	27 Sok Kwu Wan First Street	4.5	257	192	307202	-
A29	23 Sok Kwu Wan First Street	4	290	223	275307	-
A30	17 Sok Kwu Wan First Street	3.5	324	257	243275	-
A31	12 Sok Kwu Wan First Street	3.5	360	294	209243	-
A32	7 Sok Kwu Wan First Street	3	392	326	180209	-
A33	2 Sok Kwu Wan First Street	3	429	363	149180	-
A34	16 Sok Kwu Wan Second Street	3	249	181	314149	-
A35	21 Sok Kwu Wan First Street	2	299	231	264314	-
A36	13 Sok Kwu Wan Second Street	1	322	254	242264	-
A37	14 Sok Kwu Wan First Street	0.5	347	279	218242	-
A38	5 Sok Kwu Wan Second Street	2.5	378	309	189218	-
A39	3 Sok Kwu Wan First Street	2.5	424	356	148189	-
A40	Sitting-out area	1	448	380	126148	-
A41	Library	2	466	398	110126	-
A42	Squatter house in Chung Mei Village	2	73	53	--110	-

Remark: “ – “ means separate distance between the ASR and the pumping station/STW is larger than 500m.

 

 

Assessment Methodology

 

Construction Phase

 

3.9.            Fugitive Dust Model (FDM) (1993 version) has beenwas adopted to assess potential dust impacts from the construction activities.  1-hour average and 24-hour average of TSP concentrations have beenwere predicted at the representative ASRs.  Since the construction activities arewould be undertaken at ground level and underground level, the worst dust impact on the ASRs would be at ground floor of the ASRs.  The height of 1.5m above ground, which is the breathing level of human, was adopted for construction dust impact assessment.

 

3.10.        Prediction of dust emissions has beenwas based on emission factors from USEPA Compilation of Air Pollution Emission Factors (AP-42), 5^th Edition.  As a conservative simulation, general construction activities (including ground excavation for the underground sewer pipes and construction of the STW and pumping stations) and wind erosion of open sites have beenwere considered as the major dust emission sources from the construction works in this study.  As the dust generating activities or dust emission plants of the quarry are more than 500m from the study area, no cumulative dust impact on the representative ASRs iswould be expected.

 

3.11.        10-hour working day (08:00-18:00) has beenwas assumed for the construction works in the assessment.  Wind erosion of open sites would take place over the whole day.  It iswas assumed that construction of the STW, pumping stations and sewer pipes arewould be undertaken concurrently.

 

3.12.        Activities such as the installation of aboveground sewer pipes at discrete locations arewould not expected to generate significant dust emissions.  Therefore, construction of aboveground sewer pipes iswas not considered necessary to be included in the construction dust impact assessment.

 

3.13.        Detailed calculations of the emission factors are given in Appendix 3.1.

 

3.14.        A worst-case scenario was adopted in the modelling, which was represented by Pasquill stability classes D (for day-time) and F (for night-time) and wind speed condition of 1 ms^-1.  The following meteorological conditions have beenwere assumed in the modelling:

 

·             Wind speed                      :           1 m/s

·             Wind direction                  :           worst case

·             Resolution                        :           1°

·             Stability class                    :           D (day time) or F (night time)

·             Surface roughness:           1 m

·             Mixing height                    :           500 m

 

3.15.        A background TSP concentration of 87 mgm^-3 for rural area has beenwas added to the maximum 1-hour average and maximum 24-hour average results with the agreement of EPD.

 

3.1.A sample output file of the FDM model run is included in Appendix 3.2.

 

Operation Phase

 

3.16.        The main potential air quality impact during the operational phase of the project would be hydrogen sulphide gas (H₂S) arising from proposed STW and pumping stations.  The potential source of odours of pumping stations would be the wet wells and screen chamber, while sequence batching reactor (SBR) feed pump stationequalisation tank, sequence batching reactors, sludge holding tank and sludge dewatering room arewould be the potential sources of the STW.  The layout plan of the proposed STW is included in Appendix 3.3.

 

 

Emissions from STW excluding SBR FeedPump Station & Sludge Dewatering Room

 

3.17.        The emission calculation for the odour source from screens & equalization tank, sequencing batch reactors and sludge holding tank iswas in accordance with the odour assessment presented in North Lantau Development Topic Report TR23, Environmental Impact Assessment of the Tung Chung Main Sewage Pumping Station, Final Report and the Final Assessment Report of Outlying Islands Sewerage Stage 1 Phase 1 – EIA Study.  The emission rates arewere calculated based on two relationships: the established relationship between odour concentration andphysical factors and the volumetric emission flow rate compared with the rate of ventilation.  Thus emission rates arewere calculated based on the following two equations:

 

 

 

DF       =          1.6 x (T/10)^4.9 x (ORP + 200)^-0.59           Equation 1

 

E          =          DF x A x (V/3600) x Cf                                    Equation 2

 

where   DF       =          Odour concentration expressed as dilution factor, OUm^-3

            T          =          Temperature of sewage, Fehrenheit F

            ORP    =          Oxidation-reduction potential of sewage, mV

            E          =          Emission rate, OUs^-1

            A         =          Air volume of the emission source, m³

            V         =          Ventilation rate, air changes per hour

            Cf        =          Correction factor to adjust emission rates in the ratio of design

ventilation rate to that used in the derivation of Equation 1 (for 10 air changes per hour, Cf=0.26; for 5 air changes per hour, Cf=0.52)

 

 

3.18.        The sewage characteristics arewere obtained from the EngineerDrainage Services Department (DSD)engineer.  The sewage temperature iswas assumed to be 30°C in the assessment as a very worst-case scenario.  In accordance with the Outlying Islands Sewerage Stage 1 Phase I EIA Study, the fresh sewage ORP iswas 200 mV and the septic sewage ORP iswas 50 mV.  As a worst-case scenario, septic sewage iswas considered in the assessment.  In assessing the odour emission from an open diffusive source, the air volume of the emission source was taken as 0.5 metre above the sewage surface.  The equalisation tank, sequencing batch reactors and sludge holding tank, which arewould not be enclosed, arewere considered as an area source in the assessment.  Calculation of the odour emission factors is detailed in Appendix 3.3.

 

 

 

Emissions from Pumping Stations, SBR Feed Pump Station & Sludge Dewatering Room

 

3.19.        The odour emissions from pumping stations, SBR feed pump station and sludge dewatering room were provided by the Engineer and was approvedadvised by DSD.  The average H₂S inlet for the all pumping stations and sludge dewatering room of STW arewere assumed to be 5 ppm and10 ppm respectively.  No odour emission iswas expected at the effluent pumping station and effluent equalization tank as the sewage iswould be treated.   

 

Other Emissions from STW

 

3.20.        The other potential source of odour willwould be dewatered sludge removed from the sludge dewatering room.  The frequency for removal willwould likely be undertaken 1-2 times per week.  However, the separate distance between the sludge dewatering room and the ASRs is more than 200m, and the dewatered sludge willwould be stored and transported in a covered container and collection and transfer process confined to within the sludge dewatering room, adverse odour impacts willwould be not be expectedsignificant.

 

 

 

 

 

 

Dispersion Modelling

 

3.21.        The rural model dispersion option of the USEPA ISCST3 Model has beenwas used to predict 1-hour average odour concentrations at selected sensitive receivers, based on the topographical nature of the vicinity of the proposed pumping stations and STW.

 

3.22.        To convert the 1-hour averages to 3-minute averages, the power-law relationship has beenwas applied for different stability classes. For stability classes A to F, multiplying factors of 2.23, 2.23, 1.7, 1.38, 1.31 and 1.31 respectively have beenwere applied.  To further covert 3-minute averages to 5-second averages, a multiplying factor of 10 has beenwas applied for those hours with atmospheric stability classes A to B, and a factor of 5 for those hours with stability classes C to F.  This iswas in accordance with EIA Study Report, Sha Tin Sewage Treatment Work Stage III Extension, 1999.

 

 

3.23.        To simulate the worst-case scenario, 540 predefined separate meteorological conditions have beenwere used in the dispersion modelling.  The resolution on the wind direction has beenwas set to 10-degree increments and 5 wind speed conditions of 1ms^-1, 2ms^-1, 4ms^-1, 6ms^-1 and 8ms^-1.  The models have beenwere tested with Pasquil stability classes B and D for daytime hours and class F for nighttime hours.

 

3.24.        Referring to the engineering design, all pumping stations, SBR feed pump station and the sludge dewatering room arewould be all enclosed.  The outlet air from these facilities would be pumped out via the vent pipes.  Activated carbon filter or other deordorizationdeodorization facilities with equivalent removal efficiency Wet scrubber willwould be provided for sludge dewatering room and activated carbon will be used in the pumping stations as deodorization units.  It iswas assumed that the emissions from these plants are point sources in the assessment.  As advised by the EngineerDSD, tThe odour removal efficiency of both wet scrubber and activated carbon was sis 99.5% and 99.9% respectivelyin the assessment.  The specifications provided by the Contractor are attached in Appendix 3.3.

 

3.25.        Most of the representative ASRs are one or two storeys high, the potential odour impact at the levels of 1.5m and 4.5m for residential houses will bewere assessed, while the assessment point at the restaurant seating area and playground willwas considered to the height of 1.5m above ground only.   Sample computer output file is provided in Appendix 3.4.

 

 

Environmental Impact Identification, Prediction and Evaluation

 

Construction Phase

 

3.26.        Without any mitigation measures, predicted 1-hour and 24-hour average TSP concentrations at the representative ASRs are shown in Table 3.3.

 

 

 

 

 

 

 

 

Table 3.3          Predicted TSP concentrations at representative ASRs (Unmitigated)

 

ASRs	TSP Concentrations (µgm-3)
	Recommended Option
	1-hour	24-hour
A01	368	211
A02	325	193
A03	255	161
A04	251	159
A05	266	165
A06	282	172
A07	269	168
A08	214	143
A09	221	147
A10	244	156
A11	301	180
A12	313	186
A13	282	173
A14	329	194
A15	366	210
A16	364	209
A17	351	203
A18	267	168
A19	197	136
A20	312	186
A21	339	199
A22	349	203
A23	436	240
A24	530	279
A25	417	232
A26	255	161
A27	187	132
A28	330	194
A29	300	181
A30	306	183
A31	320	190
A32	285	175
A33	215	143
A34	211	142
A35	249	157
A36	227	148
A37	238	153
A38	333	195
A39	339	197
A40	349	203
A41	293	179
A42	251	160

* Background TSP concentration of 87 mgm^-3 is included

 

3.27.        Under the worst-case scenario, the predicted 1-hour and 24-hour average TSP concentrations at all ASRs would comply with the hourly average guideline level and daily average AQO limit, except exceedance of 1-hour and 24-hour TSP concentration predicted at ASR A24.

 

3.28.        In accordance with the assessment results, Tthe 1-hour average TSP concentration would ranges between 187-530 mgm^-3 and the 24-hour average TSP concentration would ranges between 132-279 mgm^-3.  The highest TSP concentration iswas predicted to occur at ASR A24, which iswas located next to the proposed pumping station P2.

 

3.29.        Since the predicted TSP concentration would exceed the guideline and AQO at one of representative ASRs, mitigation measures are required.

 

Operation Phase

 

3.30.        In view of the installation of the deodorization units at pumping stations and sludge dewatering room, the assessment results indicated that all existing representative ASRs would comply with odour criterion.  tThe predicted odour concentrations at representative ASRs are presented in Table 3.4. 

 

Table 3.4          Predicted Odour Concentrations at Representative ASRs

 

ASR	Odour Concentration (OU)
	1.5m	4.5m
A01	1.51.7	2.93.3
A02	1.01.4	2.53.0
A03	0.90.9	1.71.9
A04	0.90.8	1.41.6
A05	0.80.9	1.82.1
A06	0.80.8	1.41.6
A07	0.60.6	1.21.2
A08	0.50.4	1.00.8
A09	1.00.9	1.00.8
A10	0.50.4	0.40.4
A11	1.20.9	1.10.9
A12	0.30.3	0.50.3
A13	0.30.2	0.50.3
A14	0.40.3	-
A15	0.60.5	-
A16	1.51.3	-
A17	2.42.0	-
A18	1.81.5	-
A19	0.70.5	-
A20	1.00.5	-
A21	1.10.7	-
A22	1.50.9	-
A23	2.32.8	-
A24	2.94.3	-
A25	2.82.4	3.62.2
A26	1.31.0	1.31.0
A27	3.32.7	2.72.3
A28	0.30.2	0.40.2
A29	0.50.4	0.50.4
A30	1.41.2	1.31.1
A31	2.42.0	2.31.9
A32	2.01.7	2.01.6
A33	0.70.6	0.70.5
A34	0.30.2	0.30.2
A35	0.40.4	0.40.4
A36	0.90.8	0.80.7
A37	1.61.5	1.61.4
A38	2.52.1	2.32.0
A39	1.41.1	1.31.1
A40	0.80.5	1.00.5
A41	1.00.5	0.90.5
A42	1.00.7	1.11.1

 

3.31.        The results indicate that all ASRs comply with odour criterion.  The predicted odour concentration contours at 1.5m and 4.5m above local ground isare shown in Figures 3.2 and 3.3 respectively.  The predicted odour level in the vicinity of the proposed pumping stations and STW would exceed the acceptable level.  However, there areno existing and planned ASRs located within these areas. 

 

 

 

 

 

 

 

 

Mitigation of Adverse Environmental Impact

 

            Construction Phase

 

3.32.        In order to alleviate adverse dust impact on the ASR A24, it iswas proposed to install 2m high solid fences around the construction site of pumping station P2.  It is expected that tThe dust emission from general heavy construction activities would be reduced by 10% in accordance with Control Techniques for Particulate Emissions from Stationary Sources, Volume 1, USEPA (September 1982).

 

3.33.        With the installation of solid fences around the construction site of pumping station P2, no exceedance of 1-hour and 24-hour TSP iswould be expected in the assessment.  The concentrations of 1-hour and 24-hour TSP at the ASRs during mitigated scenario are summarised in Table 3.5.  The predicted 1-hour average and 24-hour average TSP concentration contours at 1.5m above local ground in the mitigated scenario are shown in Figures 3.43-3.54.  A sample output file (mitigated scenario) of the FDM model run is included in Appendix 3.2.

 

 

               

 

Table 3.5         Predicted TSP concentrations Concentrations at representative Representative ASRs (Mitigated)

 

ASRs	TSP Concentrations (µgm-3)*
	1-hour	24-hour
A01	368	211
A02	325	192
A03	255	161
A04	251	159
A05	266	165
A06	282	172
A07	269	168
A08	214	143
A09	221	147
A10	244	156
A11	301	180
A12	313	186
A13	282	173
A14	328	193
A15	364	209
A16	362	208
A17	351	203
A18	267	168
A19	192	134
A20	307	184
A21	329	195
A22	330	196
A23	373	214
A24	333	197
A25	394	223
A26	255	161
A27	187	132
A28	329	194
A29	300	181
A30	306	183
A31	320	190
A32	285	175
A33	210	141
A34	211	142
A35	249	157
A36	227	148
A37	238	153
A38	333	195
]A39	339	197
A40	349	203
A41	293	179
A42	251	160

* Background TSP concentration of 87 mgm^-3 is included

 

 

 

3.34.        Apart fromorm the above mitigation measure, adoption of following good site practices are recommended during construction for Recommended Option to ensure dust minimisation on-site.  Requirements stipulated in the Air Pollution Control (Construction Dust) Regulation shall be implemented during the construction activities.

 

(a)        Stockpiles of imported material kept on site shall be contained within hoardings, dampened and / or covered during dry and windy weather;

 

(b)        Material stockpiled alongside trenches should be covered with tarpaulins whenever works are close to village houses;

 

(c)        Water sprays shall be used during the delivery and handling of cement, sands, aggregates and the like.

 

(d)        Any vehicle used for moving cement, sands, aggregates and construction waste shall have properly fitting side and tail boards.  Materials shall not be loaded to a level higher than the side and tail boards, and shall be covered by a clean tarpaulin.

           

 

 

 

 

 

Operation Phase

 

3.35.        AAll pumping stations, sequence batching reactor (SBR) feed pump station and sludge dewatering room would be all enclosed and the outlet air from these facilities would be properly treated by deodorization facilities with 99.5% odour removal efficiency before discharge via vent pipes.  , nWith the implementation of all these measures, the assessment results showed that all ASRs would comply with the odour criterion.  o residual impact would be expected on the ASRs.  With the installation of the deodorization units at pumping stations and sludge dewatering room, the results show that all ASRs comply with odour criterion. Thus, further mitigation measures are not required.

 

Environmental Monitoring and Audit

 

3.36.        During construction phase, assessment predicted that construction dust impacts canwould be mitigated to acceptable levels and no residual impacts would occur.  However, it is recommended that environmental monitoring and audit (EM&A) be undertaken to ensure that the recommended mitigation measures are being implemented and are effective.  EM&A for odour during the operational phase iswould not be required.