FILTEREB
PAN WATER
WATER
TANK
Sectional illustration of the filtration plant
I
11
set
An auto-pneumatic booster pumping in duplicate maintains the highpres- sure water supply for valve operation and duplicate air compressors, with dual гр ceivers having a working pressure of 100 lbs. per sq. inch, are installed to provide the air required for filter agitation.
The wash water storage tank, housed ОП the top floor of the administrative block, is of the bolted pressed steel sec- tional type and has a capacity of 61,000 gallons.
where part of it is discharged into the ing asbestos-cement strainer tubes service reservoir being constructed at Lai Chi Kok and the remainder delivered to a pumping station at Kau Wa Kang which will raise it to another service reservoir being constructed near this station.
The supply is maintained by duplicate wash water transfer pumps each of 1,000 G.P.M. capacity and controlled through mercury switches fitted in the tank level transmitter. A Venturi meter is fitted in the wash main and an orific meter in the air main, both are fitted with transmitters and large double faced dial indicators will be sited in the upper operating gallery enabling the operator to clearly see from each control table the washing rate and air flow. In operation, should the de- mand of the service reservoirs fall below the output rate a consequent rise in the level of the pure water storage tanks will occur and master controls are fitted So that at predetermined levels the plant are will automatically shut down in stages. grouted into position over the central This control is effected through mercury collecting channel to form a complete tilt switches operating solenoids govern.
under non-metallic
collecting system. ing the outlet controllers of the filters in Each filter has a control table in the
groups of threes, with provision made upper operating gallery which is for the interchange of groups to varying brought control of valves, warning de- levels of control and from either half of The inlet measuring flume of the Par- vices, rate setter
and loss of head and the storage tank. The storage tank shall type with humped floor and side-con- rate of flow indicators and recorders. The also equipped with a level transmitter tracting walls in addition to measuring the valves used in filter cleansing are hydrau- indicating at the main instrument panel flow also controls the admission of chemi- lically operated from the table by selec- and also at the pumping station with ad- cals and sterilising agent. The head over tors linked to hydraulic switchgear with ditional warning light and alarm. the sill is transferred by balance pipe con- engraved strips indicating the movement nection to a stilling well housing a float and position of each valve throughout the operated transmitter in connection with operation. A loss of head through the indicators and recorder calibrated to give filter of 6 ft. is an indication that cleans direct readings in gallons per day. Further ing is needed. connection is made to float operated pro- portioning gear governing the chemical dosage which at all times is proportional to the raw water flow through the flume.
Each flocculating tank is fitted with two vertical spindle agitators rotating 2.5 R.P.M. and driven by electric motors through suitable reduction gear. The tanks are provided with wash out valves enabling precipated sludge to be drained away. Isolating gates and flap valves are
fitted so that either or both tanks can be by-passed
to
is
Duplicate tanks, constructed in reia- forced concrete, are provided for the pre- paration of both lime cream and alum solution. The lime is mixed by mechanic- al agitation and the alumina with com- This operation requires that the sand pressed air. Pumps are installed to lift bed be first agitated by air followed by a the chemicals to the apportioning gear reverse flow of filtered water. Both air sited in the central hall. Two 'Paterson' and wash water are applied through the chlorographs are also positioned in the underdrain system thereby ensuring an even distribution to obtain effective clean- sing of the filter media. In recent years the Paterson Engineering Company have introduced a method of syphon washing to secure considerable economy in wash water consumption.
central hall regulating and recording the supply of gaseous chlorine to the absorp tion tower sited immediately above the point of application in the lower operating gallery. The chlorographs are controlled through hydraulic counterpoises from the inlet flume and draw their supply from the adjacent chlorine store which is At Tsun Wan these, following the usual equipped with electric hoist and runway practice, are fitted in the vertical wall of for handling chlorine drums, a dormant The twelve filters are each fitted with the waste channel. A high velocity ver- indicating weighbridge and an automatic raw water inlet penstocks. Venturi outlet tical wash is first applied to thoroughly change-over panel ensuring а constant controllers with rate setters, slow start expand the media without loss of sand, supply of gas to the chlorographs. Provi- equipment and low level cut out; air inlet the syphons being non-operative. After sion is made for a constant inspection of valve, wash water valve, waste outlet washing in this manner for a little over the water throughout the treatment and penstock, by-pass valve and instruments one minute the syphons are put into samples are brought from the inlet flume, for indicating and recording rate of flow operation and a cross wash is given at a distribution channel and filtered channel and loss of head. The normal filtering lower rate, which ensures the sand being to clarity bowls incorporated in the main rate with all beds in use at 20 m.g.d. is thoroughly washed and, at the same time 80 gallons per sq. ft. per hour and with
an important point-practically all the one bed out of use for cleansing would dirty waste water over the top of the not exceed 87 gallons per sq. ft. per hour, media is removed. The filter bed is formed of several layers of carefully graded washed pebbles sup- porting more than a z ft. depth of graded
sand.
When the filter is brought into use the automatic slow start assembly takes over to ensure that the maximum filtration rate is not attained until the bed is con-
Pre-cast concrete header blocks carry- ditioned.
70
instrument panel.
of
water
of
the
A continuous sample also passing into supply is brought to a chlorine residual recorder to give a graphical chart record chlorine content throughout the duty hours. The 'Paterson' Residual Chlorine of Recorder is the result of many years careful research and experiment and in- corporates many novel and interesting features which combine to produce
an