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.  

Straining

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:

Adsorption

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:
Absorption

After the initial wetting of the sand, absorption is not very important in the filtration process.