volume plus, say, 80 million gallons of salt flushing water is basically what will be discharged into the sewers and means that we can no longer rely upon nature in terms of the tidal flow through the harbour to do our puri- fication for us.
Various degrees of treatment prior to the discharge of the effluent will, therefore, have to be given. This was foreseen some years ago when a pro- gramme was embarked upon to inter- cept the existing sewer outfalls and lead them by way of preliminary treat- ment works to submarine outfalls dis- charging well into the tidal flow.
At the same time a full scale inves- tigation of conditions in the main tidal streams into which the outfalls dis- charge was embarked upon to ensure that the level beyond which the na- tural cleansing properties of the sea would be impaired was not exceeded. Briefly this involves, inter alia, deter- mining the dissolved oxygen content of the sea and how much is used up under local condition in the oxidation
of the sewage discharged into it. If too much is used up, then not only is marine life killed but septic conditions
ensue.
Experimental plant
In addition to the monitoring of the Colony's waters and plans for treatment works, an experimental full scale treatment plant is under con- struction to serve Shek Wu Hui. This plant has been designed to try out three different methods of disposal and various degrees of treatment to as- certain what is the most appropriate and cheapest for Hong Kong, bearing in mind our shortage of land. Full scale treatment in the western manner requires large land areas but tests on the waters of Tide Cove and Tolo Har- bour indicate that in that locality full scale treatment must be used for any new concentrations of population.
Once again, therefore, the life of Hong Kong is centred upon the sea and the harbour and there is now as much competition for its resources as
there is for land. We purify its waters for drinking and our industry and then putrefy them with our subsequent waste. We build anchorages and berths for our shipping and fishing fleets and reclaim the natural protected inlets for building land. The process of reclama- tion either speeds up the current or, what may be worse still, stops it alto- gether.
It took the construction of a very sophisticated model of the Harbour by the United Kingdom Hydraulics Re- search Station and a series of tests therein which simulated the tidal con- ditions, to convince Hong Kong ship- ping interests that the reclamations in hand or planned would not spoil the harbour for shipping. A similar exer- cise for Aberdeen Harbour, however, has indicated that a very wide channel indeed will have to be left between Ap Lei Chau and Aberdeen to prevent pollution. Resettlement estates for 50,000 persons are planned for Ap Lei Chau and the problem now is -- must these be delayed until a bridge is built
Experimental full-scale desalinisation plant
26
Far East BUILDER, May 1971