Between 20 February and 4 March 2018, DSD completed the first stage of replacement of two large penstocks (measuring 1.4 metres (wide) x 4.2 metres (high) and 2.6 metres (wide) x 4.4 metres (high); and weighing 10 tonnes and 3 tonnes respectively) inside the Main Pumping Station No.1 (MPS1) of SCISTW. The maintenance works were being implemented in stages, and would involve five rounds of bypasses (including the first discharge which was completed). The whole project is scheduled for completion in 2020. DSD is reviewing the operational experience of the first round of bypass and environmental monitoring results for improving the future bypass arrangement to ensure safety of works, minimise the duration of bypass as far as possible and reduce environmental impacts. With a view to minimising the impacts of the maintenance works on the environment and the public, DSD appointed engineering consultants to conduct detailed water quality impact assessment when planning the project. A series of mitigation measures were in place during bypass and water quality was closely monitored.

Stage I works included detailed survey and dismantling of these two existing penstocks. Located at a unique position at 34 metres underground of the outermost part of MPS1, these penstocks were designed for regulation, where necessary, of the incoming sewage flow at the intersection between MPS1 and the deep sewage tunnels of the Harbour Area Treatment Scheme (HATS) through which sewage from seven HATS Stage 1 Preliminary Treatment Works (PTW) (Chai Wan, Shau Kei Wan, Tseung Kwan O, Kwun Tong, To Kwa Wan, Tsing Yi and Kwai Chung PTW) was collected. These two penstocks were in use since commissioning of HATS Stage 1 in 2001 and were coming to the end of their design life hence needing replacement. As MPS1 has to be closed to empty the accumulated sewage before technicians can enter to carry out the maintenance works, temporary sewage bypass is necessary at the seven HATS Stage 1 PTWs, i.e. the sewage after preliminary treatment will be directly discharged to Victoria Harbour through submarine outfalls without chemically enhanced primary treatment at SCISTW.

Since coming into operation in 2001, SCISTW has been collecting and treating sewage from both sides of the harbour every day. Following chemically enhanced primary treatment and disinfection, the effluent is discharged into the waters west of the harbour. As a result, the water quality of the harbour has been significantly improved.