The following environmental impacts have the potential to be generated during the construction and operation of the Project:
§ Dust and odour nuisance
§ Water quality impacts due to emergency discharge of untreated sewage or discharge of treated effluent during maintenance.
§ Solid waste management issues related to the construction and demolition materials and the generation of additional sludge.
§ Noise impacts due to the construction and operation activities.
§ Landfill gas hazards associated with the adjacent landfill site.
The assessment approach, key findings and recommendations of the Study are summarised in the following sections.
Potential emission of dust and odour from the TPSTW was identified as key air quality issues. A comprehensive assessment was undertaken to evaluate both the impact from the Project (Stage V) and the cumulative impact from the existing Stage I/II and IV and the proposed Stage V. Odour emission rates at the STW were estimated from appropriate empirical equations. The dispersion of odours was simulated using the AUSPLUME model, to predict the worst case impacts at the identified air sensitive receivers.
3.1.1 Construction Phase
A qualitative assessment was carried out to address the potential for dust nuisance during the construction phase. Given the small scale of construction works, dust impact at the air sensitive receivers (ASRs) is not expected to be significant. Construction works will follow the mitigation measures proposed in the Study and specified in the Air Pollution Control (Construction Dust) Regulation. An environmental audit programme is recommended to ensure the control measures are implemented effectively during the construction phase.
3.1.2 Operation Phase
Based on odour modelling results, mitigation measures are recommended for the Project. It is recommended that the mechanical screen, grit removal units and raw sewage channel be covered. The sludge treatment facilities including gravity sludge thickener, sludge digestion tank outlet chamber, sludge consolidation tanks, screening unit and wet well of sludge pumping stations should also be controlled by enclosure to ensure no leakage of odorous gas. The foul air from the sludge gravity thickener and sludge consolidation tanks would be treated by passing through deodorizer before being discharged into atmosphere. Odour emission from the Stage I/II and Stage IV primary sedimentation tanks could be controlled by covering the weir launders. In addition, chemical addition (e.g. addition of calcium nitrate) in sewage at Tai Yuen Sewage Pumping Stations No. 4before the sewage is conveyed to the Stage IV works of TPSTW could also effectively reduce the odour strength in inlet work channel and primary sedimentation tanks. Good house keeping, including regular maintenance and cleansing, should also be conducted to minimise odour generation. With the proposed mitigation measures, the residual odour impacts at the ASRs are expected to be in compliance with the odour criterion. A detailed odour monitoring programme is presented in the Environmental Monitoring and Audit (EM&A) Manual.
Potential water quality impacts associated with the Project include:
· Water quality impact within Victoria Harbour when treated effluent from TPSTW is discharged into Victoria Harbour via Kai Tak Nullah under the Tolo Harbour Effluent Export Scheme (THEES).
· Water quality impact within Tolo Harbour when untreated sewage or treated effluent is discharged into Tolo Harbour under emergency conditions or during shut-down of the THEES effluent tunnel for maintenance.
3.2.1 Construction Phase
An evaluation indicates that the potential impacts on water quality from general construction activities are minimal, provided that site drainage is well maintained and good construction practices are observed to ensure that litter, fuels, and solvents are managed, stored and handled properly. A comprehensive environmental audit program is recommended in the EM&A Manuel to be carried out during construction of the Project to ensure that the proposed site management practices are enforced.
3.2.2 Operation Phase
An assessment of water quality impacts due to operation of the Project has been made using the Delft3D model. Impacts are assessed over a series of one-year simulation periods for both Victoria Harbour and Tolo Harbour. The years of assessment are 2003 (baseline), 2010 (interim operation) and 2016 (full operation).
Despite conservative input parameters being used in the model, a comparison between the baseline and the operational water quality modelling results indicates that there will be no adverse impact on the marine water quality in Victoria Harbour during normal operation of the TPSTW in the operation phase of the Project.
The discharge of treated effluent during THEES maintenance and untreated effluent under emergency conditions would have potential water quality impacts on Tolo Harbour. Nonetheless, the modelling results indicate that, despite the very conservative nature of the assessment, the predicted increases in pollutant concentrations in Tolo Harbour are short-term. It is further noted that the frequency of such occurrences is very low. For the THEES tunnel maintenance, the anticipated frequency is once in five years. The duration of maintenance, assumed to be four weeks in the model, will in practice be shortened as far as possible, and the work will be conducted during low-flow periods or the winter months to avoid the "blooming" season of algae (normally from April to June) if practicable. For emergency bypass, according to DSD’s record, emergency discharge of untreated effluent had occurred only once in the past decade, with a duration less than three hours, due to power supply failure at the inlet works. To further minimize the likelihood of bypass, mitigation measures including dual or ring main power supply and standby treatment units and equipment will be provided under the Project. Thus the chance of emergency discharge would be extremely remote.
Despite the infrequent nature of Tolo Harbour discharge, an emergency contingency plan has been formulated to minimize its impact and facilitate subsequent management of the emergency. Close communication between DSD and WSD is an effective means to avoid adverse impact on the seawater intakes. No insurmountable water quality impact is expected provided that all the recommended mitigation measures are properly implemented.
To cope with the potential impact of treated effluent overflow from TPSTW (and STSTW) into Tolo Harbour during normal operation of the TPSTW in 2016, the inclusion of upgrading the Tai Po effluent pumping station into the public works programme is being undertaken by EPD. In the interim operation phase in 2010 after completion of the Phase I works, overflow at TPSTW may also occur occasionally during storm events. Modelling results indicate that the extent of such impact is only minor if present.
An EM&A programme is recommended to collect water quality information prior to and after commissioning of the Project. The monitoring results should be employed to identify areas for any further mitigation measures necessary to avoid, rectify and eliminate environmental damage associated with the Project.
In order to manage the various wastes from the construction and operation of the project, potential sources of waste were identified and their associated impacts were assessed. Wastes generated by the construction activities are likely to include construction and demolition material from the demolition works at the existing TPSTW site, and general refuse from the workforce. Provided that these identified waste are handled, transported and disposed of using approved methods and that the recommended good site practices are followed, adverse environmental impacts are not expected.
As the Project Site falls within the 250m Consultation Zone of Shuen Wan Landfill, a detailed qualitative landfill gas hazard assessment was conducted. The results of the risk assessment suggest that the overall level of landfill gas hazard is High.However, provided that the recommended protection measures are implemented properly, the health and safety of site workers/ personnel working at TPSTW will be safeguarded and there would be no adverse impact arisen from the proposed project.
The potential noise impact arising from daytime construction activities on representative noise sensitive receivers (NSR) located in the vicinity of the work sites at the TPSTW was assessed. With the adoption of quiet equipment and good site practices, no exceedance of noise standards at the representative NSR was predicted.
In order to ensure that the proposed mitigation measures for the control of dust, odour, water quality, waste, landfill gas hazard and noise impacts during both construction and operation phases are effective, a detailed schedule for the implementation of mitigation measures has been developed and an environment audit programme has been proposed. Details of the specific requirements for the operational issues are provided in a stand-alone Environmental Monitoring and Audit Manual.