Chemistry Reference
In-Depth Information
As the frequency increases, the pulsation and
collapse of the bubble occur more rapidly and more
radicals escape from the bubble. However, as the fre-
quency increases, the cavitation intensity decreases
and this reduces the yield of radicals and conse-
quently the number that reach the interface and
bulk solution.
a flow-through system with a capacity of 300 m
3
h
-1
.
The actual volume of treatment in the system was
2m
3
, with an active acoustic area of 2 m
2
. To inacti-
vate a large number of real types of plankton such
as Nauplii, Copopods and Rotifers, a specific power
of 0.05 kWh m
-3
proved to be satisfactory.
A continuing problem in water treatment is the
occurrence of algal blooms. Algae may be killed rel-
atively easily on exposure to ultrasound and with
a lightly 'polluted' system, which provides very little
attenuation to sound transmission, there is the
possibility of using high-frequency ultrasound (low
power emission and consumption). Such high fre-
quencies have been shown to give maximum activ-
ity, as shown through sonoluminescence, at the
interface between liquids and gases. Logically, then,
if a large number of small bubbles were introduced
into a field of high-frequency ultrasound there
would be a very large gas/liquid surface area for
cavitational activity and the bubbles themselves also
should provide 'seeds' for cavitation events. This is
the basis of an approach to algae removal and con-
trol proposed by the Belgian company Undatim.
In a trial involving the monocellular algae species
Scenedesmus Capricornutum
, some spectacular results
were obtained. A cell was constructed to treat water
at a rate of 2 m
3
h
-1
using an acoustic power of
450 W. At a temperature of 25°C and for a deep-
green, highly concentrated solution of the algae con-
taining some 4 ¥ 10
6
algal cells cm
-3
, a single pass
through the cell reduced the recovery threshold of
the microorganism by some 60%. This indicates that
this treatment, even operating at algal concentra-
tions that are far higher than might be encountered
in normal treatments, offers the potential not only
to kill the microorganism but also to severely restrict
its reproductive ability.
This ultrasonic anti-algae methodology has been
combined with an electromagnetic treatment to
provide a new water remediation technology for
cooling towers, known as Sonoxide [122]. This
process tackles two major problems of cooling
circuits: the build-up of algae (see above) and the
removal of hard-water scaling. The latter is affected
by changing the crystalline form of calcium carbon-
ate and causing it to precipitate out for easy removal.
In terms of chemical consumption, the process
removes the problem of heavy biocide dosage during
the irregular blooming conditions. It also means
a large saving in the continuous injection of the
4.2 Biological decontamination
Power ultrasound currently is under investigation
for use in the biological decontamination of water
[120]. Conventional methods of disinfection involve
the use of a bactericide, which for the large-scale
water industry may be chlorine, chlorine dioxide or
ozone. Current trends are towards the reduction in
quantity of the biocide used in sterilisation, but some
bacteria are capable of building up resistant strains
that may require more concentrated biocide. Power
ultrasound affords the opportunity of increasing the
efficiency of a biocide such as chlorine. The improve-
ment in the biocidal effect of chlorine is thought
to be the result of two major effects: a mechanical
breakdown of bacterial clumps or the material in
which the bacteria adhere, which will remove the
protection afforded to live bacteria in the centre of
the clumps and directly expose them to the biocide;
and ultrasound can increase the permeability of
the cell walls of the bacteria to the biocide and thus
increase its rate of uptake.
Although bacteria are difficult to kill with ultra-
sound in the absence of a bactericide, larger biolog-
ical contaminants such as plankton and algae can be
destroyed using low-power ultrasound without the
need for additional chemicals. This does provide a
real possibility for future development, with several
technologies approaching realisation.
Zooplankton often accidentally pass through the
purification cycle of a water treatment plant, leading
to re-germination and a clogging of filters located in
the water distribution system. It is important there-
fore to eliminate the plankton before the water
reaches the flocculation process. Such inactivation
can be achieved using power ultrasound through
the purely mechanical effects of acoustic cavitation,
which have been discussed above [121]. In order to
inactivate plankton a sound intensity of approxi-
mately 1 W cm
-2
and a high air content in the water
are especially effective. The economic viability of a
system for plankton treatment has been tested using
