Technical Memorandum
Annex 14
ANNEX 14 : GUIDELINES FOR ASSESSMENT OF WATER POLLUTION
1. General
1.1 The Annex describes the commonly adopted approaches and methodologies for assessment of water pollution arising from designated projects.
2. Aquatic System subject to Water Pollution Impact
2.1 In identifying and evaluating water pollution impacts on the aquatic environment, the following aspects shall be considered:
- Water as characterized in terms of:
- physical and chemical properties such as temperature, salinity, conductivity, pH, colour, dissolved oxygen, turbidity, suspended solids, and organic material concentration measured by 5-day Biochemical Oxygen Demand (BOD5), Chemical Oxygen Demand (COD) or Total Organic Carbon (TOC);
- pathogenic indicator organisms;
- toxic substances such as ammonia, heavy metals, residual chlorine, pesticides and industrial chemicals/by-products; and
- eutrophication related factors and indicators reflected by dissolved oxygen, nutrients, chlorophyll-a, frequency of red tide occurrence, and marked changes in density and composition of key phytoplankton groups such as diatoms and dinoflagellates.
- Sediments as characterized in terms of physical and chemical properties and constituents, including parameters such as pH, organic contents, nutrients, sulphide, toxic substances, etc.
3. Beneficial Uses Sensitive to Water Pollution
3.1 Existing or potential beneficial uses that are sensitive to water pollution shall include, but not be limited to:
- areas of ecological or conservation values including existing or gazetted proposed marine parks and marine reserves, aquatic systems of the sites of special scientific interest (SSSI), and existing or gazetted proposed country parks and special areas, wetlands, conservation area, mangroves and important freshwater habitats;
- areas for abstraction of water for potable water supply, aquaculture and irrigation;
- fish spawning and nursery grounds, fish culture zones, mariculture subzones including shellfish culture site and brackish/freshwater fish ponds;
- gazetted beaches and other secondary contact recreation areas;
- water abstraction for cooling, flushing and other industrial purposes;
- enclosed or sheltered water bodies including typhoon shelters, marinas and boat parks.
4. Assessment Approach
4.1 Assessment shall rely on the concept of assimilative capacity of the receiving water body and Water Quality Objectives (WQOs). Assimilative capacity will depend on the characteristics of each site, the type and number of discharges or activities as well as the beneficial uses in question. Evaluation of the assimilative capacity of the receiving waters shall take into account the relevant physical, chemical and biological processes. Sensitive receivers based on beneficial uses shall be identified and the water quality impact shall be assessed with reference to the WQOs or other relevant criteria covered in Annex 6. The implementation of the project shall not result in exceedance of the relevant WQOs for turbidity, suspended solids, temperature, salinity, pH, dissolved oxygen and bacteria for the beneficial uses to be protected for the water body.
4.2 For nutrient and prevention of undesirable algal bloom, as the level of total inorganic nitrogen (TIN) is largely influenced by the estuarine background inputs, the criteria may be assessed such that the discharge of wastewater or treated sewage effluent shall not cause any further deterioration by more than 30% of the annual average TIN levels, if the background levels have exceeded or are close to the established WQO, as set out in Section 1.3.2 of Annex 6. Alternatively, the nutrient requirement for any sewage treatment facilities can be met through adopting the acceptable treatment level set out in Section 2.1.2 of Annex 6, for the purpose of preventing undesirable algal bloom.
4.3 The discharge of wastewater and treated sewage effluent shall not cause any toxic impact that may affect aquatic life. The whole effluent toxicity criteria for such discharge should not exceed 0.3 Acute Toxic Unit (TUa) after the zone of initial dilution (ZID) and 1.0 Chronic Toxic Unit (TUc) after the mixing zone, respectively.
4.4 In evaluating water pollution impacts, both point and non-point sources of water pollutants shall be considered. Non-point pollutants refer to those substances which can be introduced into the receiving water body as a result of urban or rural runoff. Point sources are related to specific discharges from municipal or industrial facilities.
5. Assessment Methodology
5.1 Assessment methodology shall be site- and activity-specific. Assessment framework shall include the following elements:
Identification of Impact-causing Factors
5.2 It involves the identification and characterization of the impact-causing factors associated with a project. Information shall be based on specific features of the project, including coastline and river modifications, construction activities such as dredging and dumping, quality and quantity of wastewater and thermal discharges, changes in land-use and drainage, maritime wastes, waste disposal facilities and leachates, and non-point pollution sources. Consideration shall also include threat to aquatic life from exposure to toxic substances, and reduction in flushing or assimilative capacities of the water body.
Determination of the Impact Boundary
5.3 An essential first step in assessing the impact of an activity on the water body is the determination of the impact boundaries. The impacted area can be defined as the near-field and far-field. The near-field is where the initial dilution occurs and is determined by physical or hydrodynamic processes. The far-field refers to the subsequent, more complicated dilution which depends on water transport, physio-chemical processes, biological processes, etc. Estimating the impact area has to be carried out at the early stage of the assessment but may have to be revised in the light of information that emerges during the assessment process.
Baseline Study
5.4 It involves the compilation of existing information in the database characterising the relevant water body with emphasis on water quality parameters including turbidity, suspended solids, temperature, salinity, pH, dissolved oxygen, BOD5, COD, nitrogen, phosphorus, bacteria, etc. Field surveys shall be carried out to supplement existing information in situations when existing data are outdated or insufficient. Baseline study involves the development of a survey and sampling programme which shall cover aspects of meteorological, geological and hydrodynamic factors, water quality characteristics, and beneficial uses of the water body. The study should also consider changes that may arise from seasonal variations and impacts from other current or proposed developments in the area.
Impact Prediction and Assessment
5.5 Assessment shall make use of the scientific knowledge of near-field and far-field transport and dispersion of pollutants coupled with modelling and information obtained from the baseline study. Both construction and operation aspects of the project shall be considered. Assessment shall be based on quantitative techniques which can range from the use of simple mass balance approaches to sophisticated computer models. Models to be selected shall be well proven and be satisfactorily calibrated and verified with field data. The modelling capabilities and approach shall meet the relevant prevailing government requirements.
5.6 To conduct water quality modelling for effluent discharges from domestic and municipal sewage treatment facilities, reference should be made to the average effluent quality of relevant local sewage treatment facilities or the parameters and data given in the prevailing government guidelines or database.
5.7 The predictions will provide information which can be used as the basis for determining whether the aquatic resources and beneficial uses are at risk, or if there is any unacceptable impact on water sensitive receivers (WSRs) or beneficial uses as a result of implementation of the project.
Mitigation Measures
5.8 Mitigation shall aim to minimize any potential impact. Consideration shall also be given to opportunity to enhance existing conditions. The principle shall be to prevent rather than to rectify environmental damage at source. The approach shall be to minimize the risk of impairment to the beneficial uses, and to apply relevant solutions to prevent and rectify pollution problems.
Monitoring
5.9 Monitoring is generally conducted to gather information about compliance with regulations and licence requirements, model verification, and trends. Monitoring is required when there is uncertainty about the level, extent or duration of impacts, or the effectiveness of proposed mitigation measures. Monitoring provides the information for the validation process and the feedback needed for verifying the predictions and improving the monitoring programme as well as to justify any later changes to a project.
6. Activity/Project Specific Guidelines
Discharge of Wastewater and Treated Sewage Effluent
6.1 Wastewater discharges shall be pretreated to levels sufficient to protect the sewerage system downstream and the receiving water. The near-field and far-field effects shall be addressed by quantitative modelling techniques. Model for predicting the physical, chemical and microbiological processes which determine the transport and fate of pollutants associated with outfalls shall include initial dilution, effects of water stratification, advection towards shore, coliform die-off, dissolved oxygen depletion, dissolution of metals, particles settling, biotransformation, etc.
6.2 To control the organic and nutrient loads entering various waters with different environmental settings, the assessment criteria and approaches for discharges as detailed in Sections 1.3.2 and 2.1.2 of Annex 6 of this technical memorandum should be followed. Connection of wastewater discharges to public sewers leading to a public sewage treatment facility is always the preferred solution. Discharge into public sewerage systems shall not overload the hydraulic capacities nor contain substances that will cause damage to the sewerage systems.
6.3 Assessment of wastewater discharges shall address the potential toxicity to aquatic life. Toxic substances that may interfere with or pass through the treatment processes shall be controlled at source. For wastewater discharge of complex nature or containing constituents of unknown aquatic toxicity, the whole effluent toxicity test (WETT) in line with the prevailing guidelines and procedures shall be conducted to assess the potential toxicity to aquatic environment. The whole effluent toxicity criteria for the discharge should not exceed 0.3 TUa after the ZID and 1.0 TUc after the mixing zone, respectively. Sewage shall be discharged at a distance away from gazetted beaches, secondary contact recreation waters, fish culture zones and mariculture subzones. The use of chlorination disinfection shall be carefully evaluated as it can result in increase in effluent toxicity and has its own adverse effects on the aquatic environment. If disinfection by chlorination is unavoidable, the chlorinated effluent shall meet the relevant discharge standards or otherwise de-chlorination facilities shall be provided.
6.4 On-site treatment and disposal facilities shall include stand-by power and equipment and other provisions to prevent and minimize breaking down of the facilities, to facilitate rapid repair and to avoid by-pass of wastewater discharge. By-pass outfall designed to cope with emergency and unavoidable maintenance situations shall be located away from any WSRs. With the implementation of the above-mentioned standard preventive and mitigation measures as set out in the Sewerage Manual issued by the Drainage Services Department, discharges from facilities under emergency and unavoidable maintenance should be rare, and the associated water quality impact, if any, would be transient and insignificant. Water quality monitoring programmes covering pre-identified locations for the protection of WSRs should be developed for the unavoidable maintenance situations leading to the discharge of untreated sewage to the receiving water body.
Breakwaters, Reclamations and Other Works Involving Coastline and Bathometry Modifications
6.5 Assessment shall focus on the impacts on overall reduction in assimilative capacity of the affected flow channels, hydrology, and water quality of the water body within and outside the structures (e.g. typhoon shelter). Modelling shall be used to quantify these effects with a view to assessing potential water quality impact within and outside the structures as being acceptable.
Dredging, Sand Filling, and Dumping
6.6 Simulation modelling can be used to determine the short-term as well as the long-term fate of sediments. The size of the plume depends on type of dredging equipment used, quantities of sediments suspended and hydrodynamic conditions at the sites. The nature of the sediments is the first factor to consider to predict sediment suspension. When toxic or harmful constituents are found present in the sediments, the chemical effects shall also be addressed. Contaminants in the sediments shall be determined and analysed by bulk sediments, elutriate and pore water tests. In some special circumstances, assessment on the toxicity effects may be necessary. The principle in managing contaminated sediments is to minimize disturbance and isolate them from contact with the aquatic environment. If dredging cannot be avoided, a survey and sampling of potential contamination of bottom sediments shall be undertaken before dredging. The proposal should cover detailed assessment of the characteristics of the sediments, objective comparison of relevant alternatives for disposal, careful selection of site and disposal methods, and careful selection of dredging methods and equipment, while making reference to the prevailing government technical circulars and the guidelines under the London Convention as mentioned in Section 2.2.1 of Annex 6.
Thermal Discharges
6.7 Assessment shall be based on mathematical model studies using plume model to characterize the near-field and hydrodynamic and advection-diffusion model to characterize the far-field to predict the extent of the impacted area which can be defined by criteria based on temperature change and the residual of chemical additives (e.g. biocides, anti-fouling agents, anti-foaming agents, etc.) used. The assessment shall also cover and address cumulative impacts, if present. Mitigation shall include minimization of the use of chemical additives or by the use of alternative means of chemical dosage control.
Toxic Substances
6.8 Toxic substances can be classified into five subcategories: (a) non-metallic inorganic toxicants (e.g. ammonia, cyanide); (b) heavy metals and sub-metallic inorganic substances (e.g. mercury, cadmium); (c) easily degradable organic toxicants (e.g., volatile phenols, benzene); (d) refractory organic substances (e.g., DDT, PCBs, PAHs); and (e) radioactive substances. The Water Pollution Control Ordinance prohibits discharge or disposal of toxic and certain harmful substances into the water environment. The most effective and viable approach is to reduce at source the amount of these substances entering the sewer or discharging to the environmental waters. The four basic source control alternatives are pollution prevention, pretreatment, recycle and reuse.
6.9 If the project involves making a waste or wastewater discharge of complex nature or containing constituents of unknown aquatic toxicity, the WETT in line with the prevailing guidelines and procedures shall be included in the assessment to ensure that the discharge will not impose unacceptable toxicity impact on the receiving water environment.
Non-point Pollution Sources and Stormwater Discharges
6.10 Non-point or diffuse sources include inputs that are not point sources. Assessment shall include: (i) identifying these sources including surface runoff from construction sites, urban areas, livestock farms and agricultural lands; and (ii) quantifying the pollution levels where necessary. For prediction and assessment of the impacts on the aquatic environment, models shall be used where necessary and shall take into account the pollution loads from non-point sources.
6.11 The strategy to control non-point source pollution is to minimize the potential of pollutants coming into contact with rainfall or runoff. The most common source reduction measures include removal of expedient connections, prevention of illegal dumping of wastes, covering of chemical storage areas, prevention and containment of spills, minimization of chemical applications, catch basin cleaning, erosion control, and land use control. Devices designed to control pollution in a drainage system include, minimization of directly-connected impervious areas, provision of filter strips, trenches, road-side gully traps, petrol interceptors, dry weather flow interceptors, detention facilities, infiltration basins, swales, artificial wetlands, etc.
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