Mechanisms of Filtration
How are particles
removed from water using filtration? Four
mechanisms have been found to be part of the filtration process -
straining,
adsorption, biological action, and absorption. Each mechanism
will be explained below.
Straining
Straining is the removal
of
particles from water by passing the water through a filter in which the
pores are smaller than the particles to be removed. This is the
most intuitive mechanism of filtration, and one which you probably use
in
your daily life. Straining occurs when you remove spaghetti from
water by pouring the water and spaghetti into a strainer.
The
picture below shows an example of straining in a filter. As you
can see, the floc cannot fit through the gaps between the sand
particles, so the floc are captured. The water is able to flow
through the sand, leaving the floc particles behind.
In the past, straining has been assumed to be very
important in the
filtration process. However, in many cases, the pores between
sand particles in
the filter are much larger than the particles captured by the
filter. It has been suggested that small particles become wedged
between sand grains as filtration occurs, making the pore spaces
smaller and allowing the filter to strain out yet smaller
particles.
However, a clean filter will produce clean water before any of this
pore size-reduction has occurred. Therefore, it is now believed
that
straining is not an important part
of most filtration processes.
Adsorption
The second, and in many cases the most important mechanism of
filtration, is adsorption. Adsorption
is the gathering of gas, liquid, or dissolved solids onto the surface
of another material, as shown below:
Coagulation takes advantage of the mechanism of adsorption when small
floc particles are pulled together by van der Waal's forces. In
filtration,
adsorption involves
particles becoming attracted to and "sticking" to the sand
particles. Adsorption can remove even very small particles from
water.
Biological Action
The third mechanism of filtration is biological action, which involves
any sort of breakdown of the particles in water by biological
processes. This may involve decomposition of organic particles by
algae, plankton, diatoms, and bacteria or it may involve microorganisms
eating each other. Although biological action is an important
part of filtration in
slow sand filters, in most other filters the water passes through the
filter too quickly for much biological action to occur.
Absorption
The final mechanism of filtration is absorption,
the soaking up of one
substance into the body of another substance. Absorption should
be a very familiar concept - sponges absorb water, as do towels.
In a
filter, absorption involves liquids being soaked up into the sand
grains, as shown below:
After the initial wetting of the sand, absorption is
not
very important in the filtration process.
