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Aerated Pond Water Flow
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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