2. OPTIONS REVIEW
2.1 Sewage Treatment Level for the Expanded and Upgraded San Wai STW
Under normal circumstances, the collected raw sewage would be pumped from the Ha Tsuen PS to San Wai STW for treatment. The treated effluent would be discharged through the NWNT Outfall. Four options of treatment level for the expanded and upgraded San Wai STW considered in the EIA are as follow:
Option 1 CEPT
Option 2 CEPT with disinfection
Option 3 Secondary treatment with disinfection
Option 4 Secondary treatment with nitrogen removal and disinfection
Based on the water quality impact assessment results, there
are no substantial differences of Option 2 from Options 3 and 4. Options 3 and 4
would further reduce BOD5 and nutrient levels in the receiving water. However,
the model results showed that the improvement of the water quality would be
Option 2 has an advantage over Option 1 due to the reduction in E. coli in the receiving water body. The model predictions indicated that there would be no adverse water quality impacts for the adoption of Option 2 as a preferred treatment option at San Wai STW.
The ecological and fisheries impact assessments indicate that with the adoption of Option 2, it will greatly improve water quality in Deep Bay, which in turn will enhance species richness of benthic fauna and the food base of migratory birds. Besides, the significant improvement in water quality with the adoption of Option 2 would have positive effects on the health of marine organisms including capture fisheries species and marine culture fish species.
Option 2, CEPT with disinfection, is thus identified as the preferred treatment option for the expanded and upgraded San Wai STW.
2.2 Alignment of Emergency Bypass Culvert
Four alternative alignments of the emergency bypass culvert have been proposed and are shown in Figure 3. Some sections of the proposed emergency bypass culvert will cross over the proposed HSKNDA and will laid under the local roads of the proposed HSKNDA. The exact alignment of the proposed emergency bypass culvert will thus be subject to modifications as per the road alignment of the proposed HSKNDA. In order to minimise the impact associated with the construction of the emergency bypass culvert, the proposed emergency bypass culvert will be constructed at the same time as the infrastructure works of the proposed HSKNDA.
In term of construction phase impacts, Alternative 4 would involve the shortest length and the smallest works area. Therefore, the dust, noise, water quality, ecology as well as visual and landscape impacts associated with the construction of Alternative 4 would be the lowest among the four alternatives. Whereas Alternative 3 alignment would encroach upon the Tseung Kong Wai potential site of cultural heritage with archaeological significance and thus is not preferable from a cultural heritage preservation point of view.
All the four alternatives will be constructed in the form of box culvert up to the connection point with the existing Tin Shui Wai Drainage Channel for Alternatives 1, 2, and 3, and with the existing Lo Uk Tsuen Drainage Channel for Alternative 4. As discussed in Section 1.3 above, the proposed emergency bypass culvert will only be used for discharging treated effluent from the proposed expanded and upgraded San Wai STW under emergency and maintenance situations. The residual sulphide content in the treated effluent would be minimal if any and thus odorous hydrogen sulphide emissions from the treated effluent flowing along the emergency bypass culvert would not be expected. Environmental impacts associated with the operation of the proposed emergency bypass culvert would be minimal if any.
Taking into account the potential construction and operational phases impacts of the four alternative alignments, Alternative 4 is identified as the preferred alignment option for the proposed emergency bypass culvert.