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Principles of Filtration

Rapid Gravity Filters

Daf Filters

Principles of filtration

water is pumped into a filter via an inlet valve & then flows through filter media where unwanted particles are removed from the water.
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 compressor
(click here for a picture) for a predetermined amount of time
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 pump
11 Close the wash water inlet valve
12 Close the wash water outlet valve
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.
Normally 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 level transmitter
a ball valve or float arrangement that opens & closes the outlet valve hydraulically depending on level.



Click here 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. 



Click here for a photograph of 3 pressure filters