Instream filtering system for the removal of suspended solids in King Yip Street Nullah, Kwun Tong

The preliminary design of the project “Revitalization of Tsui Ping River” recommended a few water quality improvement measures aiming to enhance the aesthetic appearance and environmental conditions of the existing King Yip Street Nullah in Kwun Tong, including the use of a cost-effective instream filtering system to remove suspended solids (SS) in dry weather flows. However, the effectiveness as well as operational and maintenance experience of instream filtering systems in Hong Kong is not well established. Thus, we conducted a site trial to ascertain its effectiveness before full-scale implementation. The results showed that the sand trap screened a lot of sandy materials during the site trial period, especially during rainfall events. The instream filtering system with a gravel size of 80−120 mm effectively removed SS and total organic carbon from the water. We recommend that for the instream filtering system to be implemented in other nullahs in Hong Kong, larger dimensions (length of 4−9 m) with possible flow control (0.3−1.5 m3/h) measures could be considered when the site conditions (water depth of 1.1−1.5 m and water turbidity level below 500 NTU) allow. Overall, this study provided indispensable guidance to formulate the future design of filter sizing criteria based on targeted rainfall events and specific hydrological objectives.