Sand filter terms
Constant pressure filtration
The driving force is constant throughout the filtration. Usually the full head of water is
applied at the start and not changed. If the filter cake is compressible, the filtration rate
may decline drastically.
Constant rate filtration
The driving force is relatively small at the start because there is little cake and little resistance.
The driving force is increased as needed to keep the rate constant. Usually a point is reached
where there is no longer any unused driving force (full head has been applied), so the rate
declines as the cake thickness increases and as the cake is compressed.
Declining rate filtration
There may be no constant rate that maximizes the amount of filtration for the entire operation.
A greater amount may result from using enough driving force to get a good rate early in
the run and employing driving force carefully as the cake builds up. This avoids compressing
the cake badly but matches the driving force to the conditions. Full head is applied later
in the run.
Weir box
An overflow device, roughly two meters high, where the level can be adjusted to apply the
desired driving force.
Cake filtration
Very commonly the fabric or granular medium has openings too large for effective filtration.
However, some particles are large enough to be strained out of the fluid, and these become
the finer covering with smaller passages that performs the filtration.
Depth filtration
Most of the capture is within the bed because the openings are too large for formation of
a cake at the surface. Particles strike an obstruction (the material that forms the bed) to
stick and be captured.
Pressure surges
A sudden increase in pressure can dislodge particles captured by depth filtration and carry
them deeper or even out of the bed to produce an unsatisfactory filtrate.
while in Porto, Nov. 96