TO WATER TREATMENT & ABSTRACTION
FILTRATION & CLARIFICATION
water is pumped into a filter via an inlet valve
& then flows through filter media where unwanted particles are removed from
The unwanted particles are trapped within or on top of the filter
media & over a period of time the filter starts to lose efficiency &
will become clogged & will require washing.
in an automatic plant the
filters will be washed after a predetermined time. if the plant is not automated
the filter wash will be done manually by an operator.
A typical wash sequence may be similar
to that shown below.
1 Close the inlet valve.
2 Drain the filter
down to a predetermined level
3 Close the outlet valve
4 Open the wash
water outlet valve
5 Open the air scour inlet valve
6 Run the air scour
here for a picture) for a predetermined amount
7 Close the Air Scour valve
8 Open the backwash inlet valve
9 Run the backwash pump for a predetermined amount of time
10 Stop the backwash
11 Close the wash water inlet valve
12 Close the wash water outlet
13 Allow the filter to settle for a predetermined amount of time
14 open the inlet valve
15 The filter will start to modulate the outlet
valve when the level dictates
Click here for
photos of valves
This description is very general &
any number of variations can occur depending on the design, type & configuration
of the filter
The diagram below shows the valve positions
when the filter is being washed & when the filter is in normal operation.
The outlet valve only goes fully open if there is a high input flow.
the inlet flow will be spread across several filters & to maintain the correct
level in the filters the outlet valve will open & close (modulate) to maintain
the correct level in the filter.
Level control is normally controlled by
one of two methods
A PID controller linked to a transducer or ultrasonic
a ball valve or float arrangement that opens & closes
the outlet valve hydraulically depending on level.
AIR FILTRATION (DAF FILTERS)
for photos of a DAF filter in operation
This type of filtration is almost a combined
Clarifier & filter in one.
Water is pumped into a flocculator tank via
inlet valves. Normally ferric sulphate (or a similar chemical) is added to the
water prior to the inlet The water enters the first tank which contains
paddles which ensure a good mix of water & chemical.
the water then
passes into the filter section where dissolved air is injected into the water
as it enters.
The dissolved air lifts the solids to the surface of the water
these solids are called floc & are the result of the ferric & solids
in the water combining. At predefined times a scraper runs across the top of
the filter & removes the accumulated floc
As with the rapid gravity
filter the water level in the filter is vital & the outlet valve is modulated
to control the level.
for a photograph of 3 pressure filters