When we have been fiddling around with airlifts and measuring flow and diss=
olved oxygen we did notice some slight increases in flow and dissolved oxyg=
en when using diffusers. The diffusers we use(d) have fairly large pore siz=
e, even the PVC diffusers we use now have an opening 254 microns wide. I've=
typically supported the theory that gas exchange occurrs at the surface (a=
nd have run into all sorts of opposition because of that). However, it is o=
bvious that the airlift radically enhances the ability of an air pump to ra=
ise/maintain the rate of dissolved oxygen in water.
There is some literature/research supportive of this. I posted just a sampl=
ing of these in the files section of this forum under Airlift Science. They=
may be helpful or may confuse you even more but following are a couple exc=
erpts.
Air-Lift History
By Brian Sidney Johnson
Louisiana State University
gThe physical geometric configuration
of an airlift pump and method of air
injection (i.e. air diffusers or open
end pipe) could also influence airlift
pump operation. Discharge performance
for smaller diameter pumps has
also been enhanced through the use
of various distributed air injection
methods i.e. air stones, injector
plates, injector jackets (Khalil et al,
1999; Morrison et al, 1987). Loyless
(1995) was able to show that an increase
in the overall surface area of
gas bubbles within the draft tube of
an airlift (or number of bubbles per
volume of air injected) through the
use of an air stone could significantly
enhance performance. This phenomenon
was particularly true for oxygen
transfer into the water column due to
the increased amount of air/water
interface which resulted from the increased
surface area. However, these
techniques have only really been effective
in the bubble flow regime.
Therefore, these configuration modifications
were beyond the scope of
this study.
Airlifts by the very nature of their
design induce oxygen transfer into
the water that is being pumped.
Loyless (1995) studied the oxygen
transfer properties of 2 riser airlift
pumps for recirculating aquaculture
system (RAS) with lift height and
percent submergence as high as 12
and 75% respectively.h
Axial inhomogeneities in steady-state dissolved oxygen in
airlift bioreactors: predictive models
gCompared with conventional activated sludge
processes, oxygen transfer rates in airlift devices are up to
10-fold greaterh
Tim
Received on Fri Feb 03 2012 - 13:11:06 EST
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