Surface Area


Surface area is a very important concept in wastewater treatment.  It influences the speed with with oxygen diffuses from air into water and the ability of bacteria to take up oxygen from the water.  The greater the surface area, the faster oxygen can move across the surface.



Aerators



The purpose of aerators is to transfer oxygen from the air to the water.  Aerators facilitate the movement of oxygen by increasing the surface area of water coming in contact with air.  

As the aerator breaks air or water up into small drops/bubbles or thin sheets, the same volume of either substance has a larger surface area.  Let's consider three different sets of water drops, drawn as squares to make it easy to measure the volume and surface area.  



The volume is the amount of water in each drop.  The drop on the left has a volume of four, as does the elongated drop in the center.  The small drops on the right each have a volume of one, so all four together have the same volume as each of the other two shapes. 

The surface area is the length of a line drawn all the way around an object.  The large drop has a surface area of 8, but the other two shapes have greater surface areas.  As you can see, a thin sheet of water (like the elongated drop) has an intermediate surface area.  When a drop of water is broken up into several smaller drops (like the four drops on the right), the surface area is greatly increased.  Air will diffuse into the small droplets much more quickly than into the large droplet due to the greater surface area in the small droplets.




Bacteria

Microorganisms take in oxygen through their surfaces.  Just as oxygen from the air can diffuse into water through the water's surface, oxygen enters a bacteria through its outer surface.  The larger the surface, the more oxygen the bacteria is able to take in.  

But a bacteria's oxygen requirement, how much oxygen it needs to survive, does not depend on the surface area. The oxygen requirement depends upon the mass, or bulk, of the organism.  Bacteria of the same mass and metabolism have about the same oxygen requirement.  The bigger the bacteria, the more oxygen it needs to survive.

The mass to surface area ratio is an important concept in a small microorganism:

Surface Area
Mass


The surface area to mass ratio was also important in aerator example given above.  Small water droplets had a large surface area to volume ratio (which you can think of as a surface area to mass ratio.)  As a result, more air was able to enter these small water droplets.  Larger water droplets had a lower surface area to volume ratio, so less air was able to enter the larger droplets.

The same concept applies to the oxygen uptake by bacteria.  Small bacteria have a large surface area to mass ratio, so they are able to take up a lot of oxygen compared to how much oxygen they need to survive.  Large bacteria, in contrast, have a small surface area to mass ratio.  So these large bacteria take up a lot less oxygen compared to how much oxygen they use up in their daily life.  

Since large microorganisms have a harder time taking up enough oxygen to survive, they have to live in water with a high oxygen concentration.  If the DO content of water drops below a certain point, only the smaller microorganisms will be able to survive.