The effluent tunnel and disinfection system is an integral part of the Harbour Area Treatment Scheme (HATS) Stage 2A for effluent disposal and discharge licence compliance. The original scheme as proposed in the earlier phase of the study had adopted a conventional arrangement consisting of a short effluent tunnel, a large above-ground disinfection tank and connection culverts. After undertaking a value management (VM) exercise, an innovative alternative scheme which comprised an enlarged and lengthened effluent tunnel to provide sufficient retention time for disinfection was proposed, evaluated and adopted. This alternative scheme is an 8.5 m diameter, 880 m long effluent tunnel at 90 m below ground, conveying the effluent from the Stonecutters Island Sewage Treatment Works (SCISTW) to the new flow chamber towards south-west of SCISTW, and connecting to the existing outfall drop shaft. Various advantages in landuse planning, environmental, operation & maintenance (O&M) and life cycle cost (LCC) could be achieved.

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