Environmental Impact Assessment Ordinance

Technical Memorandum

Annex 6


1. General Criteria

1.1 The Aquatic Environment

1.1.1 Criteria for protection of the aquatic environment against water pollution include consideration of the relevant aquatic components: water, sediments and aquatic life.

1.2 The Water Quality Objectives

1.2.1 Under the Water Pollution Control Ordinance (Cap. 358), the Water Quality Objectives (WQOs) are established in terms of measurements of physical, chemical and microbiological water quality in each Water Control Zone (WCZ) to achieve the required level of protection of the beneficial uses. Based on the beneficial uses, the WQOs can be broadly categorized as follows:

  1. Aesthetic Enjoyment: criteria concerning these aesthetic characteristics are general and descriptive. They depend on subjective senses of sight and smell. Criteria generally include:

    • not to cause objectionable odours or discolouration of the water;
    • not to cause visible matters on the water surface.

  2. Human Health: criteria concerning the quality of waters for bathing and secondary contact recreation uses, fish culture zones, mariculture subzones, and inland waters for abstraction of water for potable water supply. Key criteria include:

    • to limit the maximum levels of bacteria in waters used for, bathing and secondary contact recreation activities, fish culture zones, mariculture subzones, etc.

  3. Aquatic Life: criteria concerning protection of the water quality to maintain the integrity and balance of the aquatic ecosystem. Criteria include:

    • not to alter the physico-chemical properties e.g. turbidity, suspended solids, temperature, salinity, pH and dissolved oxygen of the water to such an extent that will cause unacceptable water quality impact on aquatic life;
    • to control nutrient inputs so as to help reduction of eutrophication or excessive algal growth;
    • to prevent concentration of toxic substances from reaching unacceptable levels.

  4. Industrial Use: criteria concerning prevention of deleterious chemicals, floatables and settleable matters affecting industrial uses of the water such as that for cooling systems, flushing water supply, etc.

1.3 The Mixing Zone Criteria

1.3.1 A mixing zone is therefore a region of a water body where the initial dilution of a discharge or input to the water body takes place and where water quality criteria can be exceeded. It generally consists of a zone of initial dilution (ZID), where initial mixing and dilution by momentum and buoyancy of the discharge occur, and the subsequent dilution zone which is extended to cover the secondary mixing from the ZID out to the edge of the mixing zone where water quality criteria need to be met.

1.3.2 It has been a well-established international practice to allow a mixing zone not fully meet all water quality criteria. The characteristics of a mixing zone such as the size, siting, shape and quality, depend on the quantities and properties of the effluent discharge and the characteristics of the receiving water body, and should be determined on a case-by-case basis. In general, the criteria for acceptance of a mixing zone are that:

  1. it shall not impair the integrity of the water body as a whole;
  2. it shall not endanger or diminish areas of sensitive beneficial uses, e.g. gazetted beaches and ecologically sensitive sites;
  3. it shall not result in the accumulation of substances to such levels as to produce unacceptable toxic effects in aquatic organisms;
  4. within a mixing zone the following basic water quality criteria shall be met

    • materials not in such concentrations that settle to form objectionable deposits;
    • floating debris, oil, scum, and other matter not in such concentrations that form nuisances; and
    • substances not in such concentrations that produce objectionable colour, odour, or turbidity.

  5. at the boundary of the mixing zone, the discharge of any effluent shall not cause the WQOs or applicable criteria for pH, 5-day Biochemical Oxygen Demand (BOD5), dissolved oxygen, suspended solids, ammonia nitrogen, bacteria and ecotoxicity to be exceeded; and
  6. for total inorganic nitrogen (TIN), which is set to prevent undesirable algal bloom, as the level is heavily influenced by the background seasonal estuarine input, the criteria should be assessed such that the discharge will not cause any further deterioration by more than 30% of the annual average levels, if their existing levels have exceeded or are close to the established WQO.

1.4 Cumulative Impacts

1.4.1 Cumulative impacts occur when multiple inputs of pollutants enter the same aquatic environment, leading to overlapping zones of influence, or where there is potential cumulative reduction in assimilative capacity caused by marked reduction in water exchange following sequential implementation of projects over a period of time. Criteria for evaluation are based on identification of the relevant pollution inputs and their impact zones, and consideration of the assimilative capacity of the water body that encompasses the relevant overlapping zones of influence over a certain period of time.

2. Activity / Project Specific Criteria

These criteria are to supplement and to be considered in conjunction with the General Criteria.

2.1 Discharges of Wastewater and Treated Sewage Effluent

2.1.1 It should be noted that the effluent standards of discharges are set by the Authority under the Water Pollution Control Ordinance . The purpose of the EIA study is to assess whether the proposed discharge can meet the criteria as stated in Section 1.3.2 through quantitative or qualitative analysis such as water quality modelling based on the hydrodynamics and assimilative capacity of the receiving water body, as well as the average effluent quality of the treatment level adopted.

2.1.2 Nutrients (i.e. TIN and phosphorus), though essential to aquatic life, may lead to eutrophication if over-enrichment occurs, causing increases in undesirable algal blooms. However, algal blooms in coastal waters (including Hong Kong) are generally controlled by a combination of natural hydrodynamic, weather and biological factors, such as water current, wind speed and direction, sunlight intensity, turbidity, temperature, water stratification, biological grazing or consumption, etc., in addition to nutrients. Moreover, the nutrient levels in Hong Kong waters are heavily influenced by the background seasonal estuarine input. Hence, it is acceptable to adopt an applicable benchmark treatment level as an alternative acceptance criteria to the TIN criteria and prevention of undesirable algal blooms set out in Section 1.3.2 for discharges of treated sewage effluent, which is a common practice worldwide. The acceptable treatment levels for discharges from sewage treatment facilities are listed in the table below. In general, submarine discharge outfall may not be necessary for discharges already undergoing secondary treatment plus disinfection, provided that its mixing zone would not encroach into water sensitive receivers (WSRs) such as gazetted beaches, fish culture zones, mariculture subzones, ecological sensitive sites, etc.

Water Control Zone/Waters
Receiving the discharge

Acceptable Sewage Treatment Level1

Tolo Harbour and Channel, Deep Bay2

Secondary treatment, nitrogen removal3, phosphorus removal4, and disinfection5

Other Water Control Zones

Secondary treatment, nitrogen removal3 and disinfection5


  1. Subject to feasibility investigation and water quality assessment, other alternative treatment level can be proposed and considered.
  2. The phosphorus removal requirement shall not be applied for temporary discharges into Deep Bay and Tolo Harbour, for example due to maintenance of the effluent export systems.
  3. For nitrogen removal, the target is 75% total inorganic nitrogen reduction with respect to the annual average influent nitrogen loads or concentrations. 
  4. For phosphorus removal, the target is 80% phosphorus reduction with respect to the annual average influent phosphorus loads or concentrations.
  5. Disinfection may not be required if membrane filtration is provided which can meet the relevant discharge standards for bacteria.

2.2 Dumping of Wastes

2.2.1 Criteria for acceptability and control of dumping of wastes in the aquatic environment are governed by the Dumping at Sea Ordinance (Cap. 466). The criteria laid down in the annexes to the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter (London Convention) and its Protocol also apply.

2.3 Stormwater Runoff

2.3.1 Criteria for control of diffuse pollution shall be based on measures to control pollution at source and to abate pollutants in the stormwater runoff. These criteria are to be met through the implementation of stormwater management practices which include, but not be limited to:

  1. erosion and sedimentation control;
  2. runoff quantity and quality control;
  3. identification and minimization of point source discharges;
  4. prevention of "first flush" pollution;
  5. avoidance of discharges into poor flushing areas except artificial wetlands designed for pollution abatement;
  6. filtration of polluted stormwater or diversion for further treatment.

2.4 Toxic and Prohibited Substances

2.4.1 The criteria are that there shall be no threat to aquatic life through control of toxic substances mainly at source by pollution prevention, pretreatment, and recycle and reuse. Substances that are toxic, persistent and accumulative in water, sediment or biota, and that cannot be rendered harmless by dilution, dispersion and other natural processes of the aquatic system and for which no numerical water quality criteria are available shall be controlled at source. Discharges of radioactive substances are prohibited.

2.4.2 The whole effluent toxicity criteria for discharge of treated sewage effluent should not exceed 0.3 Acute Toxic Unit (TUa) after the ZID and 1.0 Chronic Toxic Unit (TUc) after the mixing zone, respectively.