level. Compressed air, at 5 p.s.i. is then introduced under the false floor. This passes through the plastic tubes and is diffused by their mushroom heads. Continuing upwards it is further diffused by the pebble layer before entering the sand bed, so that finally it emerges from the top surface as a uniform pattern of small bubbles.
The purpose of the air scour is to agitate the sand particles and thereby produce a scrubbing action which loosens or removes the minute particles which have lodged in the bed during the filter run. This is continued for about two minutes at the end of which time filtered water from the operating beds is diverted into the conduit under the filter floor and following the pattern of the air-scour this flows upwards through the bed and away to waste, carrying with it the 'dirt' pre- viously filtered out, and leaving the bed once again in a clean condition ready for a further run.
The back-wash time is approximate- ly 7 minutes. The frequency of washing and the quantity of water used will depend on the quality of the raw
FROM ACCELATOR
FILTER MEDIA
water, but in general filter runs should be in the range 30 to 60 hours, and the quantity of wash water of the order of 2 to 4% of the total quantity filtered.
While the filtration and back- washing principles employed in the 'Greenleaf' filter are virtually the same as with other filter plant used in the colony, the details of construction dif- fer and back-washing operations are not started and stopped by the operation of sluice valves, but by siphonic means. The eight filters are arranged in two groups of four, each group being disposed around a central core which incorporates filter and back-wash siphons. Above the core is mounted a vacuum pump and vacuum tank. Vacuum pipes lead from the tank to the siphons which can be 'made' or broken by the operation of solenoid valves. The valve controls are situated in a central console between the two filter blocks.
At the commencement of the filter cycle water flows in from the 'accelator' to a circular channel around the central core. The appropriate con- trol button on the console is depressed
-LOW
VACUUM PIPE
VACUUM TANK
TO VACUUM PUMP
· INLET WEIR CHAMBER
INLET SIPHON
TOP OF BASIN
FREEBOARD
FILTER CELL
FILTER CELL
EFFLUENT CONTROL WEIA
UNDERDRAIN —
DRAIN
COMMON FILTERED WATER CHAMBER
COMMON FILTERED WATER CHAMBER
WATER TROUGH
MAX. FILTERING
HEAD
FILTERED WATER TO STORAGE OR SERVICE
Fig. 3. Bed on left is clean and head of water is therefore quite low, Bed on the right is in need of backwashing and head needed to maintain a constant throughput is at a maximum. Note: The head in the common filtered water chamber is the same for all beds. For the sake of clarity certain details have been simplified
PRETREATED
WATER FLUME
FILTER MEDIA
UNDERDRAIN
LOW
VACUUM PIPE
VACUUM TANK
TO VACUUM PUMP
DRAIN
WATER TROUGH
COMMON FILTERED WATER CHAMBER
EFFLUENT CONTROL WEIR
BACKWASH
HEAD
FILTERED WATER TO STORAGE OR SERVICE
Fig. 4, Backwashing: Bed on left is being backwashed, using filtered water from the other beds. Inlet siphon (A) has been 'broken', thus stopping normal inflow, while backwash siphon (B) has been actuated by means of the vacuum tank. Backwash water flows up from the common under-floor chamber, through the bed and away to waste
and the siphon box primed by vacuum. Water is then drawn from the circular channel through the siphon and flows down and onto the filter bed. (See Fig. 3).
The pressure under the filter floor is less than the total head of water on the filter due to the head lost in flowing through the bed. This loss will be small initially, when the bed is clean, but will progressively increase as the bed becomes dirty. The siphon box is meantime passing water onto the filter at a constant rate and thus the water level above the bed, in order to maintain equilibrium of inflow and outflow, gradually increases until it reaches a predetermined maximum. At this point a warning signal is given and an operator vents the siphon box thus cutting off the inflow. The level then gradually drops as the bed drains down, the rate of drain-off progres- sively diminishing as the head above the bed is reduced.
When the water level has dropped to that of the outflow weir the air. scour is commenced and on completion of this operation the back-wash process is initiated by evacuating the long siphon in the central core. (See Fig. 4 for details).
As this siphon commences its dis- charge the head of water above the bed is reduced still further, but the under- floor chamber of this bed is inter- connected with that of the other beds, thus when the pressure above the bed falls below the pressure in this chamber water begins to flow in the reverse direction up through the bed, fed directly by the filtration process in the other beds. The back-wash siphon discharges down the central core and into the waste pipe.
On completion of washing the siphon is vented at its crown and siphonic action
The inlet siphon box is then re-actuated and filtration begins again.
ceases.
From the filter beds the water flows to a gauge-basin where the rate of flow is again measured and recorded. At this point most if not all of the chlorine added initially to the raw water will have been absorbed, and a further dose of chlorine is now added to complete sterilization and to maintain a resi- duum in the final water, so as to guard against any risk of subsequent con- tamination of supply in the distribution system before it reaches the consumer's tap. To reduce corrosivity the pH value of the water is raised to between 8.5 and 8.8 by the addition of lime solution to the gauge-basin, and for fluoridation sodium silico fluoride is
40
Far East BUILDER, December 1968
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