Page 174 - Industrial Wastewater Treatment, Recycling and Reuse

148 Industrial Wastewater Treatment, Recycling, and Reuse

are recommended for use that will also give similar results as a single very
high frequency transducer, but with minimal erosion problems.

3.2.1.2.2 Effect of Irradiating Surface
In the acoustic cavitation reactor, most of the cavitational activity is present
near the vicinity of the irradiating surface, and the liquid present in that area
is only exposed to the cavitating conditions. The lower energy efficiency of
the acoustic cavitational reactor is mainly due to poor energy dissipation in
the surrounding liquid. Greater energy efficiency has been observed for
ultrasonic probes with higher irradiating surfaces (lower operating intensity
of irradiation), which results in uniform dissipation of energy (Gogate
et al., 2001). Thus, for the same power density (power input into the system
per unit volume of the effluent to be treated), power input to the system
should be through larger areas of irradiating surface. Flow cells having
transducers at multiple locations fitted at different heights and irradiating
planes can be used to increase the irradiation surface and hence the energy
transfer efficiency.

3.2.1.2.3 Intensity of Irradiation
The intensity of ultrasonic equipment is defined as the ratio of the system’s
power input to the transmitting area. Hence, intensity of irradiation can be
varied either by changing the power input to the system or by changing the
irradiating area of the transducers in the equipment. There exists a critical
intensity at which the beneficial effect of cavitation starts to occur. This is
due to the fact that the chemical reactions due to cavitation occur as a result
of the formation of a certain minimum number of free radicals. This number
of free radicals depends on the intensity of cavity collapse and the number of
cavitating events, which in turn depends on the operating intensity of irra-
diation. Thus, the intensity of irradiation should not be decreased below a
certain minimum cavitation intensity. Moreover, as said earlier, the two
methods by which intensity can be changed also play an important role
in the actual results. If the intensity is increased by increasing the power input
to the system (P/V of the system), there will be an increase in the number of
cavitation events; hence the cumulative pressure pulse (number of cavities
generated multiplied by the collapse pressure due to a single cavity) will
increase. In such a case, the degradation rates will be higher due to higher
overall magnitude of the pressure energy released. However, very high
power dissipation or larger intensity could result in lower cavitational