Central platform to one group of four Greenleaf filters. Circular trough into which the clarified water flows can be seen
also added to the water at this point.
After passing over the gauging weir the water is discharged into the service reservoir, the function of which is two fold. First it provides a reserve against failure of supply and secondly it allows the works to operate with a steady throughput even though the demand in the supply area fluctuates to a very marked degree at various times of the day and night.
Instrumentation and quality control
Apart from metering the flow at inlet and outlet (the former instrument being coupled to the chlorine dosing equipment, and the latter to both chlorine and fluoride equipment), the pH value of the water is constantly measured and recorded from probes mounted in the inlet flume, accelator, filter inlet channel and 36 in. dia- meter outlet main. The residual chlorine in the water is measured at a point 50 ft. downstream of the gauge basin, and this too is indicated and recorded in the main control room.
For continuous visual observation of colour and turbidity, water is drawn from the inlet, accelator outlet and gauge basin and passed through cylin- drical glass clarity bowls in the control room. Beneath these bowls draw-off taps are provided so that samples may be taken for test purposes.
Chemical tests are undertaken in the laboratory to determine the most suitable dosing rates for alum and lime, depending on the very variable quality of the raw water, and to determine the various chemical characteristics of the raw, clarified and filtered water. Bacteriological tests of samples, taken at all stages of treatment and also from
the distribution system, are carried out at the central laboratory of the Water Quality Control Section at Sha Tin Treatment Works.
Warnings and other safeguards
In order to minimise the possibility of wastage of water, damage to the works, or danger to the operators, a number of alarm devices have been installed and other safety measures adopted.
Main control panel. The four glass bowls give a visual indication of the clarity of the water at inlet, after clarification, after fil- tration and at gauge basin
(iii) As stated, chlorine, alum, lime and fluoride are all fed into the water in solution, and for this purpose a supply of service water is pumped from the outlet gauge basin. If this water supply
and fluoride dosing equipment would should for any reason fail the chlorine
automatically shut down.
Breakdown of costs
(i)
(ii)
HK$ million
-
Site formation contract Wa Hing Construction Co. 1.02 Service reservoir- Nishimatsu Construction Co.
The main problems to be overcome are failure of the power supply, over- flow from one or more section of the works and dangers due to the nature of the chemicals in use. Considering (iii) Treatment works these possibilities individually:— (i) The works is provided with a diesel powered stand-by generator ca- pable of supplying sufficient power for all operations.
1.97
construction - Nishimatsu Construction Co.
1.53
(iv) Supply and erection of
treatment works plant-
Infilco
1.23
Staff quarters,
(ii) If, for any reason, the operator failed to wash the filter beds the level (v) of water in the bed would continue to rise until overflow occurred. To guard against this possibility alarm bells are provided in the central filter-control (vi) Misc. works by Water-
console and the main control room. These are actuated when the water level in any bed reaches the normal back- washing level.
(adjacent to pumping
station) - Chan Tao Contractor
0.23
works Office
0.10
Total for treatment works:
6.08
mains and distribution
mains
It will be noted from Fig. 2 that if (vii) Pumping plant, trunk the level rises above the throat of the back-wash siphon, the siphon would operate as an overflow weir.
The service reservoir is also provided with an overflow which will discharge to waste should the reservoir be over- filled. A float switch installed frac- tionally above top water level is coupled to an alarm in the main control room to warn the operator that overflow is about to commence.
Acknowledgement
4.29
11.00
This article was prepared by Mr. H.J. Woodthorpe, C.Eng, MICE, AMI Mun E, AMIWE, by kind permission of the Hon. A.M.J. Wright, CMG, Direc- tor of Public Works, Hong Kong.
Far East BUILDER, December 1968
41
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