Response to Mr John Ashworth's letter published in the December 2018 issue of Hong Kong Engineer
I wish to respond to Mr John Ashworth's letter published in the December 2018 issue of Hong Kong Engineer regarding the lack of thermophilic anaerobic sewage sludge digestion under the Harbour Area Treatment Scheme (HATS).
Under HATS, sewage from the urban areas in Kowloon and Hong Kong Island is conveyed by deep sewage tunnels to the Stonecutters Island Sewage Treatment Works (SCISTW) for treatment. With only a 10 ha footprint, space at the SCISTW is barely adequate to accommodate facilities to treat 2.45 million cu m of sewage per day even after adopting an innovative double-deck design for the sedimentation tanks. There is just no additional space to house other facilities such as those for treatment and digestion of sludge to generate energy. Moreover, the saline chemical sludge from the SCISTW is not suitable for composting or soil improvement on landscaping. It is for these reasons that sludge from the SCISTW is dewatered and conveyed to the Sludge Treatment Facility (STF) at Nim Wan by two dedicated marine vessels.
The STF is essentially a waste-to-energy facility. It not only obviates the need for sludge to be disposed of at landfills, but also recovers energy from it, generating sufficient electricity not only for the operation of STF and its environmental education centre, but with the capability of exporting excess electricity.
Ir C C LEE
Chief Engineer/Harbour Area Treatment Scheme
Drainage Services Department