5.9 Evaluation of Residual Impacts The upgrading and expansion of San Wai STW would enhance the treatment efficiency and increase the capacity of the existing facilities. The effluent quality from the treatment option 2 or above would improve the water quality as compared with the current preliminary treatment process. The residual impacts are expected to the low.

5.10 Conclusions Based on the water quality impact assessment results, there were no substantial differences of Option 2 (CEPT with disinfection) 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 was small. In addition, the selection of CEPT with disinfection rather than the biological treatment options would not jeopardise the compliance of the WQOs for DO, TIN and UIA in North Western and Deep Bay WCZs. Meanwhile, Option 2 has an advantage over both baseline and 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. In addition, the proposed 4 options would significantly improve the water quality in the baseline water quality condictions. Disinfection would also satisfy the precautionary requirement for protection of dolphins and helps to protect nearby recreational areas.