GROUND
LEVEL
INLET MEASURING INSTRUMENTS
CHEMICAL DOSING PIPES
RADIAL COLLECTING
TROUGH
1
24′′₫ RAW WATER PIPE
FROM TAI PO TAU
PUMPING STATION
MOTOR OF SCRAPER AND ROTOR
CLEAR WATER TO RADIAL TROUGH
VACUUM TUBE
SIPHON BOX
INLET FUME
VACUUM TANK
SIPHON OPERATING
LÄÉ SLUDGE PIPE
GAUGING WER.
24′′O RAW WATER IMPE
FILTER MEDIA,
*** WASTE PIPE ¿CENTRE CORE OF FILTER
FRESH WATER PIPE
VENT PIPE
TO TAI PO TAU
SERVICE RESERVOIR '36'₫ FRESH WATER PIPE
FLOW MEASURING FLUME
Fig. 2. Diagram of filter process
After arriving at the site from Tai Po Tau pumping station the water passes through a measuring flume, the rate of flow being indicated on an instrument adjacent to the flume and also on a second instrument in the central control room within the stat- ion. Immediately downstream of the measuring point the water is dosed with chlorine, lime and aluminium sulphate (alum), all of which are intro- duced in solution form. The chlorine dose is automatically regulated by the flow of water and its function is threefold: first to substantially sterilize the incoming raw water, secondly to inhibit the development of algae in the clarifier and filter beds and thirdly,
to aid flocculation.
The alum reacts partly with the natural alkalinity in the raw water and partly with the lime dose to produce a 'floc' of aluminium hy droxide. The floc attracts and entraps suspended and colloidal particles, in- cluding organic matter, bacteria, silt and substances causing colour which would otherwise be difficult to settle. This coagulation of fine particles into larger and heavier particles facilitates the processes of sedimentation and filtration.
In order to obtain a satisfactory floc the water after the addition of the coagulating chemicals should nor- mally be slightly acidic, the lime dose being adjusted so as to maintain an optimum pH value of between 6.5
and 6.8.
From the inlet flume the chemical- ly dosed raw water passes to the 'accelator', which is a circular clarifier or sedimentation tank. The entry point is at the centre of the base under a conical steel hood. This is the mixing zone within which the chemi- cal reactions largely take place and in which the floc is formed. Rotating paddles are mounted under the hood to facilitate the mixing process.
The flow is upwards through the hood and then outwards through an annular slot and downwards into the outer settling zone. The heavy floc tends to settle towards the bottom
ACCELATOR
GREENLEAF FILTERS
of the tank carrying with it lighter particles collected en route. The clari- fied water rises in the outer zone and is collected by radial troughs.
Some of the settled floc is drawn back under the hood and re-circulated, thereby accelerating the formation of heavy particles and increasing the efficiency of the process.
Three pockets are provided in the floor into which the settled sludge is swept by a slowly rotating scraper. From these pockets draw-off pipes are led to a drainage sump, each pipe having a blow-off sludging valve at its end. The sludging valves operate on a cycle controlled by an adjustable clock mechanism which permits the period of opening and the time cycle between openings to be varied according to requirements. Too frequent opening would be wasteful of water and would also reduce the re-circulation of floc, but if the valves were operated too in- frequently the sludge accumulation would be too great, with the risk of floc being carried over into the radial col- lecting troughs.
Chemical feed pipes are also pro- vided in the central mixing zone as under certain circumstances it is de- sirable to apply the chemical dosing at
GAUGE BASIN
this point, instead of at the raw water inlet flume.
The clarified water flows through an open channel to the sand filter beds where the remaining suspended matter and any small particles of floc which have carried over from the clarifier are removed. Under the sand bed is a layer of pebbles and beneath this again is a false floor into which are fixed a large number of small rigid plastic tubes. It is through these tubes that the filtered water flows to the conduit beneath
the false floor. In order to prevent blockage by the pebble layer each tube is fitted with a mushroom shaped head.
There are eight filter beds, each bed having 1,500 of these tubes. This large number is required to ensure uniform filtration over the whole bed area.
As filtration proceeds the sand will gradually become clogged, and the head of water over the bed required to maintain a reasonably uniform through- put must progressively increase. A point is eventually reached where to increase this head further would result in overtopping the filter walls and at this stage the bed must be withdrawn from service and cleaned.
This is done by first draining the water above the bed down to a low
Accelator under construction. In the foreground is the scraper arm which is attached to the
central hood; the two rotate together
Far East BUILDER, December 1968
39
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