Airlift
By Kevin Thurston
From CAS Newsletter and Aquarist, Colorado Aquarium Society
Aquarticles.com
When I got my first aquarium, it came with a little box made of clear plastic. This box
was loaded with charcoal and glass wool and was called a filter. A hose was connected from
the air pump to a stem coming out of the box and the air came out of a tube next to the
stem. I couldn't see how this device could be a filter, but sure enough the material
inside got dirty and the water stayed clean. Some time later I acquired an outside box
filter. This filter had a siphon to get the water from the aquarium to the box. In a
separate compartment it had a tube with an air stem attached. The air hose attached to the
stem and the bubbles came out through the larger tube, which was bent back toward the
aquarium. This tube put the water back into the aquarium. This made the principle of
airlift a little more obvious, but it appeared that the bubbles were simply splashing the
water back into the aquarium and couldn't possibly be moving much water. In an attempt to
improve the performance of this filter, I tried reversing the mechanism and using the
airlift to put water into the filter and the siphon as a return. I was soon to be startled
at how fast that airlift could dump half the aquarium water on to the floor when the
siphon broke. Hobbyists who never had that experience tend to have a problem with
understanding just how much water is moved by airlift. For example, I once tried to
explain to a college graduate (with a degree in science, who was working in a fish store),
how an air operated reverse flow under gravel filter (previously on the market) worked.
After explaining the design to him, he insisted that it couldn't possibly work, despite my
assurance that I had used the device and it did indeed work. His insistence was based on
his failure to understand how an airlift works.
Airlift can be readily understood by considering what happens when a bubble is released
in water below the surface. The first obvious observation is that the bubble quickly rises
to the surface. What isn't so obvious is the fact that as the bubble rises; it does so
faster than the water can slip around the bubble. So there is a small column of water that
is being pushed ahead of the bubble, some of which is actually pushed above the surface.
So what about the water that does slip around the bubble? Well if we use a whole lot of
bubbles and surround them with a tube (or use a narrow tube with few large bubbles), the
water that slips past a bubble either lands on the column being pushed by the next bubble,
or it has trouble getting past the bubble due to the walls of the tube. So even though it
isn't obvious to the viewer, an airlift is capable of moving very large quantities of
water.
When the airlift principle is applied to under gravel filters, sponge filters, and
inside box filters you have to realize that there's a little more to the story. The water
is pushed into the filter by the water pressure at depth. The airlift then moves the water
out of the filter. The combination of water pressure at depth and airlift forms an
effective water pump mechanism. Even though this is a very effective water pump mechanism,
power heads have become an enormously popular replacement for air pumps because their
effect is more readily observed. Unfortunately they don't aerate as well as an airlift. As
far as aeration is concerned, there is a frequent misconception that in an airlift, the
bubbles mix with the water to create aeration. Aeration actually takes place at the
surface. If you observe an aquarium that has a few air stones or airlifts running in it,
you'll notice waves that agitate the surface with waves that propagate through out the
aquarium. It's this surface agitation that improves the aeration, not the mixing of the
bubbles with the water.
I use a lot of air operated filters because I have a regenerative blower which
generates more air than I can possibly use. I use sponge filters and inside box filters,
which run at a very high rate due to the plentiful air supply. This is not unusual, I know
a lot of hobbyists who use the same type of set-up. By using a combination of filter types
in conjunction with this set-up, I do not use any power heads or outside power filters.
I've been toying with the idea of designing and building an air powered canister filter,
which would essentially be a modification of an existing filter, but I've never actually
done it. The motivation for such a modification is in the fact that maintenance on filters
inside the aquarium is a little messier than on filters outside the aquarium. There are
still several different inside box filters being manufactured, but the best designed
inside box filter ever, was taken off the market just a few years ago. The Marineland box
filter had a box that would separate from the airlift and base assembly to make
maintenance easier. Since this one was discontinued, I tend to be indifferent to the brand
of the ones I buy these days. The one thing I do prefer is a larger lift tube for
increased water flow. The inside box filter is actually quite versatile. You can pack them
with any filter material you like. I've used them for peat filtration, carbon filtration
and just stuffed them full of cheap floss to get a quick cleanup of debris that was
stirred up. Still, I wish someone would manufacture an air driven outside box filter.
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