Advisory Council on the Environment

Ngong Ping Sewage Treatment Plant and Sewerage


(ACE-EIA Paper 13/2002)
For discussion

1. Purpose

This paper is to brief members of the ACE EIA Subcommittee of the project and to seek their views on the preferred treatment and effluent disposal option of the project.

2. Project Description

The aim of the project, which forms part of the Outlying Islands Sewerage Stage 1 Phase 1 works, is to provide sewage infrastructure to serve the unsewered Ngong Ping area on Lantau Island to meet the needs of Ngong Ping itself and the proposed Cable Car System which connects Tung Chung and Ngong Ping.

The project comprises the following three parts :-

(a) Ngong Ping Sewage Treatment Plant

The construction of a sewage treatment plant (STP) is to provide biological and tertiary treatment with disinfection to the sewage collected in the area. The following minimum standards of effluent discharged from Ngong Ping STW shall be met :-

Water Quality Parameter Minimum Effluent Standard
Biochemical Oxygen Demand, mg/L 10 (95%ile)
Suspended Solids, mg/L 15 (95%ile)
Ammonia Nitrogen, mg/L 1 (annual average)
Nitrate + Nitrite Nitrogen, mg/L 7 (annual average)
E.Coli. count/100mL 100 (geometric mean)

(b) Ngong Ping Trunk Sewer and Effluent Export Pipeline

  1. Laying of a trunk sewer and connection sewers to collect sewage generated at the cable car station and associated premises, as well as existing public toilets at Ngong Ping and Po Lin Monastery.
  2. Laying of an effluent export pipeline to convey the treated effluent from the proposed Ngong Ping STP to Tai O for discharge (see section 3.2 below for a brief description of why Tai O is chosen).

(c) Ngong Ping Village Sewerage

The construction of a new sewage collection network at Ngong Ping to cover the unsewered village premises in the area.

Parts (a) and (b) are scheduled for completion in 2005/2006 to tie in with the commissioning of the cable car system. As item (c) will involve land resumption, it is scheduled for completion by mid-2007.

3. Option Considerations

3.1 Potential Treatment and Disposal Options

The Ngong Ping area lies in the water gathering ground subzone of the North Western Water Control Zone (WCZ). The rivers and streams within the water gathering ground or country parks are classified as Group A inland waters in the Water Pollution Control Regulations, whereas those beyond the water gathering ground and country parks are Group B inland waters. The marine water bodies near to and farther away from the shoreline are classified as inshore waters and marine waters respectively.

The following table summarises the effluent standards(Note 1) for discharging into Group A inland waters, Group B inland waters, inshore waters and marine waters of North Western Water Control Zone. For comparison purpose, only the key water quality parameters prescribed for the effluent from the Ngong Ping STP are shown.

Water Quality Parameter Group AInland Waters Group BInland Waters Inshore Waters Marine Waters
Biochemical Oxygen Demand, mg/L 5 20 20 100
Suspended Solids, mg/L 5 30 30 100
Ammonia Nitrogen, mg/L 0.5 5 (see Note 2) (see Note 2)
Nitrate + Nitrite Nitrogen, mg/L 10 10
E.Coli. count/100mL <1 100 1000 4000

Note 1 : Standards for flow rate (in m3/day) ranges of 1000-2000 (Group A), 1500-2000(Group B) and up to 2000 (for Inshore and Marine)

Note 2 : There are no specified standards for Ammonia Nitrogen and Nitrate + Nitrite Nitrogen for Inshore and marine waters.

Five potential treatment and disposal options for the Ngong Ping sewerage system were considered :-

  1. Treatment at Ngong Ping and discharge locally to soakaways.
  2. Treatment at Ngong Ping and disposal at Group A inland waters within the water gathering ground or country park.
  3. Export of untreated sewage to existing or planned treatment facilities outside the water gathering ground or country park.
  4. Treatment at Ngong Ping and discharge to marine waters.
  5. Treatment at Ngong Ping and discharge at Group B inland waters or inshore waters.

Option (a) is considered not feasible as it would require 8,000m2 of land and introduce risk of groundwater contamination.

Option (b) is rejected due to uncertainty about the impacts of the effluent's residual pollutants on human health when the effluent flows into Shek Pik Reservior which serves the residents at Lantau South, Cheung Chau, Ping Chau and Hei Ling Chau throughout the year, as well as the western part of the Hong Kong Island during the wet season.

Option (c) is also considered undesirable because of concerns over potential leakage from the foul sewage pipeline running through water gathering ground and country park which are highly environmental sensitive areas.

For discharge to marine waters, Option (d), a somewhat lower level of treatment has been considered. It was however not favoured due to the similar concerns as described above regarding the potential leakage of pipeline through the water gathering ground and country park, which would justify a high level of treatment anyway. Furthermore, discharge of effluent to marine waters would require the construction of extended outfall. Construction of extended outfall may cause water quality impact due to the required dredging and filling operations.

Based on the above considerations, it is considered that Option (e) for treatment at Ngong Ping followed by disposal of effluent to Group B inland waters or inshore waters is a preferred treatment and disposal option.

3.2 Effluent Disposal Routes

Five routes have been considered for the export of highly treated effluent out of the Ngong Ping area (as shown on the attached drawing) :-

  1. Along the existing footpath east of Ngong Ping towards Tung Chung.
  2. Along the proposed footpath of the cable car route towards Tung Chung.
  3. Along the Ngong Ping-Sham Wat road towards Sham Wat.
  4. Along the Ngong Ping - Tai O road towards Tai O.
  5. Along Keung Shan Road towards Tung Wan.

(a) & (b) Towards Tung Chung

The two routes towards Tung Chung would be running close to environmentally sensitive sites. The route along the existing footpath will impinge upon the Ngong Ping Site of Special Scientific Interest, which is known to support the largest population of the protected Romer's Tree Frog on Lantau. The route along the proposed footpath of the cable car route will cut through the Lantau North Country Park with much of the route cutting through grassland and secondary broadleaved woodland and therefore the construction of the effluent export pipeline will aggravate the adverse impacts of the works on this area. Furthermore, new access roads would need to be constructed in conjunction with this option and this will further disturb the environment. The routes towards Tung Chung are therefore considered undesirable.

(c) Towards Sham Wat

The route to Sham Wat is also not preferred as the construction of an effluent pipeline would potentially affect the ecologically diverse Sham Wat stream and valley. Furthermore, Sham Wat Road is narrow and is the only access road between Sham Wat and Ngong Ping. Closure of the road for the construction of the effluent export pipeline will seriously affect emergency services.

(d) Towards Tai O

The route to Tai O mainly follows the existing two-lane road to Tai O and no major potential environmentally sensitive areas will be affected by this route. With the high treatment level of the effluent, this route is considered the best option from environmental point of view.

(e) Towards Tung Wan

The route along Keung Shan Road towards Tung Wan is near the Shek Pik Reservoir. The whole section of Keung Shan Road is within the water gathering ground and is situated to the immediate west of Shek Pik Reservoir. In view of its close proximity to the reservoir, there may be concern on the effect of any leakage/bursting of the pipe on the water of the reservoir. Birds and bat species of conservation interest have been recorded in this part of Lantau in the past. However, as the effluent pipe will be laid under the road the impact to ecology would be temporary and minimal.

Taking into account the potential human health risk and environmental impacts mentioned above it is considered that the Tung Wan route is less preferable than the Tai O route.

4. Key Findings of the EIA

A detailed EIA study has been carried out for the project based on local tertiary treatment at Ngong Ping and the disposal of effluent at Tai O. The EIA has examined potential environmental impacts of various nature including air quality, noise, water quality, waste management, ecology, fisheries, cultural heritage and landscape and visual impacts. The results of the assessments indicate that the environmental impacts identified were generally within acceptable environmental standards and residual impacts were not expected.

In particular, a water quality modelling has been performed to evaluate the impact of the exported effluent on Tai O creek and Tai O waters. The predicted water quality results comply very well with the Water Quality Objectives within the study area. Potential impacts to aquatic ecology and fisheries have also been examined. It has been found that the impacts will be minor, except that special attention needs to be paid to the stream south of Ngong Ping village which is the major breeding site of Romer's Tree Frog. It has been recommended that earthworks near this stream will not be done during the breeding season of Romer's Tree Frog from March to September. Furthermore, a waste management plan will be implemented to minimise waste generation as well as to ensure that all waste materials will be disposal of at designated waste disposal sites. An environmental monitoring and audit will be undertaken to ensure the implementation of all mitigation measures recommended in the EIA study.

Drainage Services Department
June 2002


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