Sewage must be preliminarily treated to remove large solids and grits to avoid deposition in the deep tunnels and to protect downstream facilities from damage or blockage. The existing eight PTWs at the northern and southwestern parts of Hong Kong Island are upgraded to cater for the technical requirements of HATS Stage 2A as well as population growth and development around the Island. In addition, architects and landscaping specialists are engaged to improve the aesthetics of the PTWs. Also, engineers are deployed to work out state of the art systems that ensure an odour free environment. Both bring lasting benefits to nearby residents.
Grit Removal Facilities
The key objectives of installing grit removal facilities at the PTWs are to avoid progressive accumulation of sediment in tunnel conveyance system and to eliminate coarse suspended solids which may cause excessive wear or clogging equipment. There is an impeller inside a grit trap to separate the heavy solids to the bottom of the trap. Grit trap can remove 95% of grits with size greater than 0.2mm.
The main purpose of screening is to remove large objects inside the sewage to protect the downstream equipment. There are two types of screens in the PTWs: coarse screen (with >40mm spacing) and fine screen (with 4mm spacing) for the removal of different sizes of objects to enhance the effectiveness and protection of screens.