compared with the previous year to 30,794, of which 18,080 were for chemical and 7,200 for bacteriological testing.
10.76. The Unit's water testing facilities and advisory services continued to be utilized by waterworks divisions concerned with the planning of new treatment works, extensions to existing works and for the experimental desalting plant. A total of 1,074 water samples were examined following requests by other government departments, commercial and industrial firms, and the general public.
Plover Cove Reservoir
10.77. The ecology and chemical quality continued to be closely monitored. Summer inflows reduced the salinity from 430 parts per million (p.p.m.) expressed as sodium chloride (NaCl) to 260 p.p.m. by November, the total of all dissolved salts then being 360 p.p.m. For the second year running, marked thermal and chemical stratification persisted between April and November and although there was some proliferation of blue-green algae from July to October, the overall quality of the stored water remained good. An interesting biological feature was that from December onwards the diatom Melosira replaced the hitherto dominant blue-green type of algae present. A further 275,000 fish fry were introduced during the year bringing the total number stocked to 1.375 million. A second Ecological Report on the reservoir covering the period August 1967 to December 1969 was produced. Steps were taken to curb the illegal practice of gill net fishing which is tending to increase as the population and size of fish grew.
Water Treatment
10.78. To provide advance indication of serious pollution or toxicity in the supply to the Sha Tin and Tai Po Tau treatment works, fish tanks were installed through which the raw water passes and the fish are kept under observation.
10.79. The conversion of existing filters from sand to layers of anthracite and sand to increase filtration efficiency and ratings continued, and investigations into the dosing of polyelectrolyte chemicals as additional aids to filtration were begun.
10.80. The infestation of salt water flushing distribution systems by marine organisms, which has been found to occur despite chlorination, was investigated and studies begun on possible alternative means of control.
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