2. AIR QUALITY

2.1 Introduction

2.1.1.1 Dust impacts would be the major air quality impacts during the construction phase of the project. During operational phase of the project, there would be potential odour impacts associated with the operation of the sewage treatment plant. The following sections detailed the approaches, criteria and guidelines on monitoring and managing dust and odour impacts as well as the associated event and action plans and the recommended mitigation measures.

2.2 Construction Phase Dust Monitoring

General

2.2.1.1 Dust impacts would be the major air quality impacts during the construction phase of the project. It is necessary to monitor the dust generates from the construction activities after timely implementation of the dust mitigation measures listed in this Manual. The purpose of monitoring is to ascertain that the dust levels would comply with the 1-hour average and 24-hour average Total Suspended Particulate (TSP) criteria at the sensitive receivers, and that the recommended mitigation measures are effective in suppressing dust levels.

2.2.1.2 The objectives of the monitoring are:

a. To identify the extent of construction dust impacts on nearby sensitive receivers;
b. To determine the effectiveness of the recommended dust mitigation measures to control dust from construction activities;
c. To recommend further mitigation measures where necessary; and
d. To ascertain that the dust levels would comply with the 1-hour average and 24-hour average Total Suspended Particulates (TSP) criteria at nearby sensitive receivers as defined in this Manual.

Air Quality Parameters

2.2.1.3 Monitoring and audit of the TSP levels should be carried out by the ET Leader to ensure that any deteriorating air quality could be readily detected and timely and appropriate action undertaken to rectify the situation.

2.2.1.4 1-hour and 24-hour TSP levels should be measured to indicate the impacts of dust on air quality. The TSP levels should be measured by following the standard high volume sampling method as set out in the Title 40 of the Code of Federal Regulations, Chapter 1 (Part 50), Appendix B. Upon approval of the ER, 1-hour TSP levels can be measured by direct reading methods which are capable of producing comparable results as that by the high volume sampling method, to indicate short event impacts.

2.2.1.5 All relevant data including temperature, pressure, weather conditions, elapsed-time meter reading for the start and stop of the sampler, identification and weight of the filter paper, other local atmospheric factors affecting or affected by site conditions and work progress of the concerned site etc. should be recorded down in details. A sample data record sheet is shown in Appendix B for reference.

Monitoring Equipment

2.2.1.6 High volume sampler (HVS) in compliance with the following specifications should be used for carrying out the 1-hour and 24-hour TSP monitoring:

a. 0.6-1.7m3/min (20-60 SCFM) adjustable flow range;
b. equipped with a timing/control device with +/- 5 minutes accuracy for 24 hours operation;
c. installed with elapsed-time meter with +/- 2 minutes accuracy for 24 hours operation;
d. capable of providing a minimum exposed area of 406cm2 (63in2);
e. flow control accuracy: +/- 2.5% deviation over 24-hour sampling period;
f. equipped with a shelter to protect the filter and sampler;
g. incorporated with an electronic mass flow rate controller or other equivalent devices;
h. equipped with a flow recorder for continuous monitoring;
i. provided with a peaked roof inlet;
j. incorporated with a manometer;
k. able to hold and seal the filter paper to the sampler housing at horizontal position;
l. easy to change the filter; and
m. capable of operating continuously for 24-hour period.

2.2.1.7 The ET Leader should be responsible for provision of the monitoring equipment. He/she should ensure that sufficient number of HVSs with an appropriate calibration kit are available for carrying out the baseline monitoring, regular impact monitoring and ad hoc monitoring. The HVSs should be equipped with an electronic mass flow controller and be calibrated against a traceable standard at regular intervals. All the equipment, calibration kit, filter papers, etc. should be clearly labelled.

2.2.1.8 Initial calibration of dust monitoring equipment should be conducted by the ET upon installation and thereafter at bi-monthly intervals. The transfer standard should be traceable to the internationally recognised primary standard and be calibrated annually. The calibration data should be properly documented for future reference by the concerned parties such as the IEC. All the data should be converted into standard temperature and pressure condition.

2.2.1.9 The flow-rate of the sampler before and after the sampling exercise with the filter in position should be verified to be constant and be recorded down in the data sheet (see Appendix B).

2.2.1.10 If the ET Leader proposes to use a direct reading dust meter to measure 1-hour TSP levels, he/she should submit sufficient information to the IEC to prove that the instrument is capable of achieving a comparable result as that of the HVS and may be used for the 1-hour sampling. The instrument should also be calibrated regularly, and the 1-hour sampling should be determined periodically by HVS to check the validity and accuracy of the results measured by direct reading method.

2.2.1.11 Wind data monitoring equipment should also be provided and set up at conspicuous locations for logging wind speed and wind direction near to the dust monitoring locations. The equipment installation location should be proposed by the ET Leader and agreed with the ER in consultation with the IEC. For installation and operation of wind data monitoring equipment, the following points should be observed:

a. the wind sensors should be installed on masts at an elevated level 10m above ground so that they are clear of obstructions or turbulence caused by the buildings;
b. the wind data should be captured by a data logger. The data recorded in the data logger should be downloaded periodically for analysis at least once a month;
c. the wind data monitoring equipment should be re-calibrated at least once every six months; and
d. wind direction should at least be divided into 16 sectors of 22.5 degrees each.

2.2.1.12 In exceptional situations, the ET Leader may propose alternative methods to obtain representative wind data upon approval from the ER and agreement from the IEC.

Laboratory Measurement /Analysis

2.2.1.13 A clean laboratory with constant temperature and humidity control and equipped with necessary measuring and conditioning instruments to handle the dust samples collected, should be available for sample analysis, and equipment calibration and maintenance. The laboratory should be HOKLAS accredited or other internationally accredited laboratory.

2.2.1.14 The IEC should conduct regular audit to the measurement performed by the laboratory so as to ensure the accuracy of measurement results. The ET Leader should provide the ER with one copy of the Title 40 of the Code of Federal Regulations, Chapter 1 (Part 50), Appendix B for his/her reference.

2.2.1.15 Filter paper of size 8"´10" should be labelled before sampling. It should be a clean filter paper with no pin holes, and should be conditioned in a humidity controlled chamber for over 24-hour and be pre-weighed before use for the sampling.

2.2.1.16 After sampling, the filter paper loaded with dust should be kept in a clean and tightly sealed plastic bag. The filter paper is then returned to the laboratory for reconditioning in the humidity-controlled chamber followed by accurate weighing by an electronic balance with a readout down to 0.1mg. The balance should be regularly calibrated against a traceable standard.

2.2.1.17 All the collected samples should be kept in a good condition for 6 months before disposal.

Monitoring Locations

2.2.1.18 Impact monitoring of dust shall be carried out at representative ASRs whenever there is ongoing construction work during construction phase of San Wai STW and Ha Tsuen PS. A total of four dust monitoring stations, two for San Wai STW and another two for Ha Tsuen PS, are proposed to set up at locations as shown in Figures 2.1 and 2.2.

2.2.1.19 Exact monitoring point for sensitive receivers shall be selected as close as is practical to the construction work boundary. The status and locations of air sensitive receivers may change after issuing this Manual. If such cases exist, the ET Leader should propose updated monitoring locations and seek approval from ER and agreement from the IEC.

2.2.1.20 When alternative monitoring locations are proposed, the following criteria, as far as practicable, should be followed:

a. At the site boundary or such locations close to the major dust emission source;
b. Close to the air sensitive receivers;
c. Proper position/siting and orientation of the monitoring equipment; and
d. Take into account the prevailing meteorological conditions.
2.2.1.21 The ET Leader should agree with the ER in consultation with the IEC the position of the HVS for installation of the monitoring equipment. When positioning the samplers, the following points should be noted:
a. A horizontal platform with appropriate support to secure the samplers against gusty wind should be provided;
b. No two samplers should be placed less than 2 meter apart;
c. The distance between the sampler and an obstacle, such as buildings, must be at least twice the height that the obstacle protrudes above the sampler;
d. A minimum of 2 metres of separation from walls, parapets and penthouses is required for rooftop samplers;
e. A minimum of 2 metre separation from any supporting structure, measured horizontally is required;
f. No furnace or incinerator flue is nearby;
g. Airflow around the sampler is unrestricted;
h. The sampler is more than 20 metres from the dripline;
i. Any wire fence and gate, to protect the sampler, should not cause any obstruction during monitoring;
j. Permission must be obtained to set up the samplers and to obtain access to the monitoring stations; and
k. A secured supply of electricity is needed to operate the samplers.

Baseline Monitoring

2.2.1.22 The ET Leader should carry out baseline monitoring at all of the designated monitoring locations for at least 14 consecutive days prior to the commissioning of the construction works to obtain daily 24-hour TSP samples. 1-hour sampling should also be done at least 3 times per day while the highest dust impact is expected. Before commencing the baseline monitoring, the ET Leader should inform the IEC of the baseline monitoring programme such that the IEC can conduct on-site audit to ensure accuracy of the baseline monitoring results.

2.2.1.23 During the baseline monitoring, there should not be any construction or dust generation activities in the vicinity of the monitoring stations.

2.2.1.24 In case the baseline monitoring cannot be carried out at the designated monitoring locations during the baseline monitoring period, the ET Leader should carry out the monitoring at alternative locations which can effectively represent the baseline conditions at the impact monitoring locations. The alternative baseline monitoring locations should be approved by the ER and agreed with IEC.

2.2.1.25 In exceptional case, when insufficient baseline monitoring data or questionable results are obtained, the ET Leader should liaise with the IEC and EPD to agree on an appropriate set of data to be used as a baseline reference and submit to ER for approval.

2.2.1.26 Ambient conditions may vary seasonally and should be reviewed at three monthly intervals. If the ET leader considers that the ambient conditions have been changed and a repeat of the baseline monitoring is required to be carried out for obtaining the updated baseline levels, the monitoring should be at times when the contractor's activities are not generating dust, at least in the proximity of the monitoring stations. Should change in ambient conditions be determined, the baseline levels and, in turn, the air quality criteria, should be revised. The revised baseline levels and air quality criteria should be agreed with the IEC and EPD.

Impact Monitoring

2.2.1.27 The ET Leader should carry out impact monitoring during construction phase of San Wai STW and Ha Tsuen PS. For regular impact monitoring, the sampling frequency of at least once in every six-days, should be strictly observed at all the monitoring stations for 24-hour TSP monitoring. For 1-hour TSP monitoring, the sampling frequency of at least three times in every six-days should be undertaken when the highest dust impact occurs. Before commencing the impact monitoring, the ET Leader should inform the IEC of the impact monitoring programme such that the IEC can conduct on-site audit to ensure accuracy of the impact monitoring results.

2.2.1.28 The specific time to start and stop the 24-hour TSP monitoring should be clearly defined for each location and be strictly followed by the field operator.
2.2.1.29 In case of non-compliance with the air quality criteria, more frequent monitoring exercise, as specified in the Action Plan in Section 2.3 of this Manual, should be conducted within 24 hours after the result is obtained. This additional monitoring should be continued until the excessive dust emission or the deterioration in air quality is rectified.

2.3 Event and Action Plan for Construction Phase Air Quality

2.3.1.1 The baseline monitoring results form the basis for determining the air quality criteria for the impact monitoring. The ET Leader should compare the impact monitoring results with air quality criteria set up for 24-hour TSP and 1-hour TSP. Table 2.1 shows the air quality criteria, namely Action and Limit levels to be used. Should non-compliance of the air quality criteria occurs, the ET Leader, ER, and Contractor should undertake relevant actions in accordance with the Action Plan as stated Table 2.2.

Table 2.2  Event/Action Plan for Air Quality (Dust) during Construction Phase.

2.4 Mitigation Measures During Construction Phase

2.4.1.1 The EIA Report has recommended dust control mitigation measures to minimise the impacts. The Contractor should be responsible for the design and implementation of these measures. If the recommended mitigation measures are not sufficient to restore the air quality to acceptable levels upon the advice of ET, the Contractor should liaise with the ET on some other mitigation measures, propose to ER for approval, and implement the mitigation measures.

2.4.1.2 In order to ensure that dust emission is minimised during the construction phase of the project, relevant dust control requirements set out in the Air Pollution Control (Construction Dust) Regulation should be met. The Contractor is required to adopt dust reduction measures when carrying out construction works.

2.4.1.3 Specific measures recommended for this Project include dust suppression by twice daily watering with complete coverage of all active construction areas and limit the construction vehicle travel speed on unpaved site areas to not more than 10 km per hour.

2.4.1.4 In particular, the mitigation measures listed below should be adopted where applicable. With the implementation of effective dust control measures, adverse dust impacts from the construction works of the Project is not expected.

Site clearance and demolition of existing structures

· The working area for the uprooting of trees, shrubs, or vegetation or for the removal of boulders, poles, pillars or temporary or permanent structures should be sprayed with water or a dust suppression chemical immediately before, during and immediately after the operation so as to maintain the entire surface wet;
· All demolished items (including trees, shrubs, vegetation, boulders, poles, pillars, structures, debris, rubbish and other items arising from site clearance) that may dislodge dust particles should be covered entirely by impervious sheeting or placed in an area sheltered on the top and the 3 sides within a day of demolition;

Site boundary and entrance

· Vehicle washing facilities including a high pressure water jet should be provided at every discernible or designated vehicle exit point;
· The area where vehicle washing takes place and the section of the road between the washing facilities and the exit point should be paved with concrete, bituminous materials or hardcores;
· Where a site boundary adjoins a road, street, service and or other area accessible to the public, hoarding of not less than 2.4m from ground level should be provided along the entire length of that portion of the site boundary except for a site entrance or exit;

Access road

· Every main haul road (i.e. any course inside a construction site having a vehicle passing rate of higher than 4 in any 30 minutes) should be paved with concrete, bituminous materials, hardcores or metal plates, and kept clear of dusty materials; or sprayed with water or a dust suppression chemical so as to maintain the entire road surface wet;
· The portion of any road leading only to a construction site that is within 30m of a discernible or designated vehicle entrance or exit should be kept clear of dusty materials;

Use of vehicle

· Immediately before leaving a construction site, every vehicle should be washed to remove any dusty materials from its body and wheels;
· Where a vehicle leaving a construction site is carrying a load of dusty materials, the load should be covered entirely by clean impervious sheeting to ensure that the dusty materials do not leak from the vehicle;
Excavation and earth moving
· The working area of any excavation or earth moving operation should be sprayed with water or a dusty suppression chemical immediately before, during and immediately after the operation so as to maintain the entire surface wet;
· Exposed earth shall be properly treated by compaction, turfing, hydroseeding, vegetation planting or sealing with latex, vinyl, bitumen, shotcrete or other suitable surface stabilizer within 6 months after the last construction activity on the construction site or part of the construction site where the exposed earth lies; and

Stockpiling of dusty materials

· Any stockpile of dusty material should be either covered entirely by impervious sheeting; placed in an area sheltered on the top and the 3 sides; or sprayed with water or a dust suppression chemical so as to maintain the entire surface wet.

2.4.1.5 If the above measures are not sufficient to restore the air quality to acceptable levels upon the advise of ET Leader, the Contractor should liaise with the ET Leader on some other mitigation measures, propose to ER for approval, and implement the mitigation measures.

2.5 Operational Phase Odour Monitoring

General

2.5.1.1 During operation of the expanded and upgraded San Wai STW and the expanded Ha Tsuen Pumping Station, routine monitoring should be undertaken to ensure that the air emissions from the facilities are in compliance with the 5 odour unit (OU) criteria stipulated in Annex 4 of the Technical Memorandum on Environmental Impact Assessment Process at nearby air sensitive receivers. It is suggested to start the operational phase odour impact monitoring 3 months after commissioning of the expanded and upgraded San Wai STW and the expanded Ha Tsuen Pumping Station.

2.5.1.2 Initially, H2S measurements and odour panel tests should be carried out at the site boundary and at nearby air sensitive receivers (ASRs). The purpose is to determine the correlation between H2S concentrations and odour units obtained from the odour panel tests. Once such correlation is established, H2S monitoring will be continued and H2S concentrations measured will be converted to equivalent odour units.

2.5.1.3 The monitoring programme can be discontinued after one year if there is no exceedance of the odour criteria. The ET should be responsible for the operational phase impact odour monitoring.

Monitoring Methodology

2.5.1.4 Monitoring and audit of the odour level should be carried out by the ET during the course of the odour generating activities to ensure that odour levels at the ASRs meet the requirements and timely action would be undertaken to rectify any exceedances.

2.5.1.5 According to Annex 4 of the EIAO-TM, an odour concentration as a result of odour emission of 5 OU m-3 or above based on an averaging time of 5 seconds at an ASR is considered an odour nuisance.

2.5.1.6 Hydrogen sulphide (H2S) is one of the main components of odour emissions. Since ambient H2S concentration can be readily monitored at the ASRs, it can serve as a surrogate indicator for sewage odours in addition to odour measurement using olfactometry techniques.

2.5.1.7 Prior to the operation of the odour generating activities, H2S measurements and odour panel tests should be carried out at the site boundary and at nearby ASRs. The purpose is to establish the averaged baseline H2S concentration conditions at each measurement position at the site boundary and at nearby ASRs. These baseline H2S concentrations will be used to establish the action levels of the odour EM&A program.

2.5.1.8 In addition, H2S measurements and odour panel tests should be carried out at the site boundary, at nearby ASRs, and at the exhaust of the deodourisation unit in order to determine the correlation between H2S concentrations and odour units obtained from the odour panel tests.

2.5.1.9 After the establishment of the correlation, only measurements of H2S concentrations at the site boundary, at nearby ASRs, and at the exhaust of the deodourisation unit are required during the operational phase of the STW and the PS. This is to indicate whether the odour concentrations are higher or lower than the baseline condition, and within the odour criteria.

2.5.1.10 Apart from the baseline and impact odour monitoring, regular odour patrolling in the vicinity of the STW and PS should also be conducted in a monthly interval during the operational phase to ensure that prompt actions would be taken whenever any excessive odour emissions are detected. The findings of the odour patrol should be properly recorded. If necessary, such as during initial operation stage or upon receipt of odour complaint, the regular odour patrolling should be carried out more frequently.

Monitoring Parameters

2.5.1.11 15-min H2S concentration (in parts per billion or in parts per million) should be measured at the site boundary, at nearby ASRs, and at the exhaust of the deodourisation unit using the equipment described below. Meteorological conditions including temperature, wind speed, wind direction and relative humidity should also be measured at the time of the monitoring.

2.5.1.12 For olfactometry analysis, air samples should also be collected for a period of 15-min at the selected locations in order to provide sufficient volume for olfactometric analysis. Odour concentrations should be expressed as OUm-3.
Monitoring Equipment and Methods

Hydrogen Sulphide Monitoring

2.5.1.13 Concentrations of H2S should be recorded using an H2S analyzers which utilises a gold film sensor for the detection of hydrogen sulphide. The instrument is controlled by microprocessor, ensuring rapid accurate analyses, and should be fitted with the following accessories:

· Data logger (to allow the instrument to operate unattended);
· Interface cable and interface software; and
· Data download and graphics service.

2.5.1.14 The instrument is capable of measuring H2S concentrations in the range 1ppb (1.4mgm-3) to 50 ppm (70 mg m-3) to an accuracy of ±6%. If the H2S concentrations at the site boundary or at the exhaust of the deodourisation unit are too high to be measured by this equipment, wet chemical method should be considered to analysis the H2S concentrations of the collected air samples in the laboratory. Meteorological information including wind speed, wind direction and temperature should be retrieved from the meteorological monitoring station.

Olfactometry Analysis

2.5.1.15 The odour concentration should be measured by a force-choice dynamic olfactometer in accordance with the Dutch National Standard Method (NVN2820).

2.5.1.16 Large plastic bags of 40 litres are usually used to collect air samples. To avoid adsorption onto or chemical reaction with the bag surface, bags and connection tubing in contact with odour-laden gas should be made of inert material, e.g. poly-tetra fluro-ethylene (PTFE) or Tedlar.

2.5.1.17 During the sampling, weather condition including wind direction, wind speed and temperature should be recorded. The collected samples should be transported to an odour laboratory as soon as possible and shall be analyzed within 24 hours. Qualified odour panelists should be selected and those participating in the odour testings should be screened by using a 50ppm of certified n-butanol standard gas.
Laboratory Analysis and QA/QC

Basic Principle of Odour Measurement

2.5.1.18 The odour concentration of a gaseous sample is measured using dynamic olfactometry with a panel of human assessors being the sensor. The odour panel normally composes of 6 to 8 persons. The odour concentration is measured by determining the dilution factor required to reach the detection threshold. The odour concentration at the detection threshold is 1 odour unit (OUm-3), which has a probability of 50% being detected under the conditions of the test. The odour concentration of the examined sample is then expressed in terms of multiples of one OUm-3 of neutral gas at standard conditions.

Panel Selection and Control

2.5.1.19 In order to ensure repeatability of the sensor, composed of individual panel members, their olfactory sensitivity should be within a narrow bandwidth. To achieve this aim, assessors with a specific sensitivity to a reference odour are selected to be panel members. The screening is on reference materials n-butanol with the concentration of 50ppm in nitrogen (v/v).

2.5.1.20 At least ten individual thresholds for the reference gas are collected for selection purposes. These data are collected in at least three sessions on separate days with a pause of at least one day between sessions. To become a panel member, the data collected for that assessor must comply with the following criteria:
· The geometric mean of the individual thresholds must fall between 20 and 80 ppb, when n-butanol standard gas is used; and

· A measuring history for each panel member is continuously recorded and their performance is compared with the selection criteria. If the panel member does not comply, he/she is excluded from all further measurements.

2.5.1.21 In order to control the quality of the results produced by panel members:

· Panel members must not eat or smoke for one hour prior to the session;
· Panel members should be in the odour room 15 minutes before measurements;
· Panel members should not use perfumes, after-shave lotions or any other fragrant essences before the session;
· Panel member should not attend a session if he/she has a cold, influenza or any other health problem, which will affect his/her nose;
· No panel member should be involved for more than 4 hours of odour testing. Within this period at least 2 ten minutes breaks for olfactory rest should be taken; and
· The odour panel should be housed in a room that is constructed of odour-free materials and is equipped with ventilation system to prevent build-up of odour in the room.

Instrument Calibration

2.5.1.22 Regular calibration of the olfactometer should be performed to check the accuracy and repeatability of its dilution settings and to establish its calibration history. The calibration may be performed using carbon monoxide as a tracer gas and a carbon monoxide monitor. The accuracy and repeatability of the olfactometer are calculated from two carbon monoxide concentrations with one measured at the sniffing port of the olfactometer and one being the certified carbon monoxide concentration.
Monitoring Locations

Monitoring Locations at ASRs

2.5.1.23 Odour sampling or H2S measurements should be carried out at representative ASRs nearest to the odour sources, in particular the exhaust point of the deodourisation unit. The ET Leader should propose monitoring locations and seek approval from the ER.

2.5.1.24 Odour sampling or H2S measurements should be taken outside the premises of the identified ASRs and these locations should preferably not be influenced by other nearby odour sources.

Monitoring Locations at Site Boundary

2.5.1.25 Odour sampling or H2S measurements should be undertaken at the site boundary downwind of the exhaust point of the deodourisation unit and the covered odour sources. The selected locations should be determined by the ET Leader and agreed with ER and EPD.

Monitoring Location at the Exhaust of the Deodourisation Unit

2.5.1.26 Odour sampling or H2S measurements should be undertaken at the exhaust point of the deodourisation unit. The selected location should be determined by the ET Leader and agreed with ER and EPD.

Baseline Monitoring

2.5.1.27 Odour and H2S monitoring should be taken prior to the operation of the odour generating activities in order to establish baseline odour and H2S concentrations and to set up the Action and Limit levels. These will be compared with the results obtained during the impact monitoring stage.

2.5.1.28 Odour baseline monitoring should consist of both odour sampling and H2S measurement. Sampling at the site boundary and at selected ASRs using olfactometry and an H2S analyser should be carried out simultaneously using the equipment and methodology described above. The purpose is to establish both the correlations between odour level (OUm-3) and H2S concentration, and the averaged baseline H2S concentration condition for each measurement position at the site boundary and at ASRs.

2.5.1.29 A 15-min sample should be collected every 3 hours for a duration of 24 hours at each of the monitoring locations. The purpose of sampling in 3-hour intervals and to cover a duration of a whole day is to capture the different atmospheric conditions at different time periods. If there is insufficient monitoring equipment or monitoring personnel, sampling/monitoring can be spread to 3 to 4 consecutive days as long as the different monitoring periods and each of monitoring locations are covered. Pairwise monitoring at ASRs and at site boundary should be carried out simultaneously. However, as the variation of odour concentration at site boundary should be less significant, timing of sampling or monitoring at the site boundary should be more flexible so as to accommodate the available resources for the monitoring programme.

2.5.1.30 In exceptional cases, when insufficient baseline monitoring data or questionable results are obtained, the ET Leader should liaise with EPD to agree on an appropriate set of data to be used as the baseline.

Impact Monitoring

2.5.1.31 The ET Leader should carry out the odour monitoring at all the designated monitoring locations during operation of the odour generating activities.

The First Set of Odour Monitoring

2.5.1.32 The first set of odour monitoring at the site boundary, at selected ASRs, and at the exhaust of the deodourisation unit should consist of both odour sampling and H2S measurement. Sampling at these locations using olfactometry and an H2S analyser should be carried out simultaneously using the equipment and methodology described above. The purpose is to establish the correlations between odour level (OUm-3) and H2S concentration for each measurement position.

2.5.1.33 A 15-min sample should be collected every 3 hours for a duration of 24 hours at each of the monitoring locations. The purpose of sampling in 3-hour intervals and to cover a duration of a whole day is to capture the different atmospheric conditions at different time periods. If there is insufficient monitoring equipment or monitoring personnel, sampling/monitoring can be spread to 3 to 4 consecutive days as long as the different monitoring periods and each of monitoring locations are covered. Pairwise monitoring at ASRs and at the site boundary should be carried out simultaneously. However, as the variation of odour concentration at the site boundary should be less significant, timing of sampling or monitoring at the site boundary should be more flexible so as to accommodate the available resources for the monitoring programme.

2.5.1.34 As the first set of odour sampling/monitoring is fundamentally to establish the correlation between OUm-3 and H2S, the timing of this event should be selected during the summer period, as far as possible, to capture the highest odour concentrations. Once the correlation between H2S concentrations and odour units is established, H2S monitoring should be continued during the subsequent odour monitoring and H2S concentrations measured should be converted to equivalent odour units. The degree of correlation should be constantly reviewed in order to ascertain that the most realistic correlation factor be used.

Subsequent Odour Monitoring

2.5.1.35 The subsequent odour impact monitoring should provide a continuation of the H2S monitoring at the site boundary, at selected ASRs and at the exhaust point of the deodourisation unit. Impact monitoring should be carried out every 3 months. If necessary, such as during initial operation stage or upon receipt of odour complaint, the subsequent odour impact monitoring should be carried out more frequently.

2.5.1.36 The same monitoring regime as the baseline monitoring should be followed, except that odour sampling and analysis by olfactometry is not required in the subsequent odour impact monitoring.

Odour Patrol

2.5.1.37 Besides the above odour monitoring, a simple and more frequent odour patrol should be carried out by the operators of the facilities on a monthly basis to identify any odour problem associated with the day-to-day operation of the facilities. Odour patrol means a simple judgement by an observer patrolling and sniffing around the facilities to detect any odour. This observer should be free from any respiratory diseases and not normally working at the facilities.

2.5.1.38 It is proposed to carry out the simple odour patrol at least twice a day at different hours of operation. If the plant has a distinct peak operation hours, then odour patrol should be conducted within such hours during which odour level is expected to be most obvious.

2.5.1.39 The observer has to follow a predetermined route which should normally be going from non-odorous to odorous area. The observer should patrol slowly along the route and use his olfactory senses to detect any odour.

2.5.1.40 The observer should bring along a log book to record the findings. The log book should be kept in the plant office where it can be inspected when necessary. The findings should include the following:

· the prevailing weather condition;
· the wind direction;
· location where odour is spotted;
· possible source of odour;
· perceived intensity of the odour; and
· duration of odour.

2.5.1.41 The perceived intensity is to be divided into 5 levels which are ranked in the descending order as follows:

· Extreme
· Strong
· Moderate
· Slight
· Not detectable

2.6 Event and Action Plan for Operational Phase Air Quality

2.6.1.1 The baseline monitoring results form the basis for determining the odour criteria for the impact monitoring. The ET Leader should compare the impact monitoring results with the odour criteria shown in Table 2.3, namely Action and Limit Levels. Should a non-compliance of the odour criteria occur, the relevant parties should undertake the relevant actions in accordance with the Event/Action Plan in Table 2.4.

Table 2.3  Action and Limit Levels for Operational Phase Odour Monitoring  

Table 2.4  Event/Action Plan for Operational Phase Odour Monitoring

2.7 Mitigation Measures During Operational Phase

2.7.1.1 In order to mitigate the potential odour impacts from the proposed San Wai STW to an acceptable level, it is recommended that all the major odour sources within the proposed STW namely the fine screen, the detritors, the sludge holding tanks, the solid handling house, the sludge dewatering house, the rapid mix and flocculation tank, and the primary sedimentation tanks should all be contained by within either building structures or enclosure to minimize direct emission of odour to the atmosphere.

2.7.1.2 Besides, all odour emissions from the above odour sources should be ventilated to a centralized deodorization unit. Based on the findings of the EIA study, the H2S removal efficiency of the deodourisation unit at San Wai STW should be 96% or better. The exhaust height and exit velocity of the treated air should not be less than 5m and 10m/s respectively.

2.7.1.3 For the proposed expanded Ha Tsuen Pumping Station, it is recommended that the air ventilated from the existing and the new wet wells should be treated by deodorizers. The H2S removal efficiency of the deodourisation units at Ha Tsuen PS should be 97% or better. The exhaust of the deodorisers should be located on the rooftop of the pumping station and the exit velocity of the treated air should not be less than 8m/s.

2.7.1.4 The two likely technologies for deodorization to be adopted for this project would be adsorption of odorous chemicals by granulated activated carbon (GAC) and srubbing of odorous air steam with chemical oxidants in a chemical scrubber. For example, chemicals such as sodium hydroxide, chlorine solution, and sodium hypochlorite etc are commonly used in scrubbers to oxidize odorous chemicals such as hydrogen sulphide and mercaptans.