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Name: Paul
Status: Student
Grade: 9-12
Location: CA
Country: United States
Date: February 2009


Question:
I was talking with my Dad about what he does which is water management. He said aeration is the best way to keep pond or lake water healthy and explained how it worked. I asked him how much water it moved in the lake and he was not sure if it could be calculated. He said the most common aerator he installs produces 10 cubic feet of air per minute. It does not work in ponds less than 5 ft deep. 1 cubic foot rising 10 ft seems like it would be an easier calculation. He called it displacement, as the air rose the water would move and suck water from the shore creating a circular flow. Does this help or is he right and it cant be calculated?



Replies:
Hi Paul,

I think the air transfer and water circulation can definitely be calculated, but you need to know a little bit more information. There are different types of aerators, but I am assuming you are talking about the air-sparger type that is placed on the bottom of a pond. This creates bubbles that rise from the bottom of the pond to the top. As they rise, some of the oxygen in the bubbles dissolves into the water surrounding them. The bubbles also drag water upward with them (viscosity, the molecular attraction between water molecules, is why water is dragged along upward with the bubble). Because the bubbles are dragging a bunch of water with them, a circulating current is created as water from the sides of the sparger moves in to replace the water dragged upward by the bubbles. This convection is also calculable. To calculate the mass transfer (oxygen from the bubbles into the water) and the convection (circulating water flow), you would minimally need to know the distribution of bubble sizes and the ambient oxygen concentration in the pond. If you want to get more complicated, you can take into account size effects such as non-spherical bubbles, pressure change, bubble depletion, shear, and other factors -- while this is probably much more than you want to do, these are active areas of research and in industrial practice.

Hope this helps,
Burr Zimmerman



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