Introduction
Ultrasound irradiation is a novel advanced oxidation
process that has emerged as an answer to
the growing need for lower levels of contaminants
in wastewater (1, 2). The basis for the
present-day generation of ultrasound was established
as far back as 1880 with the discovery
of the piezoelectric effect by the Curies (3).
Cavitation phenomenon was first identified and
reported in 1895 (4). Destruction of microorganisms
by ultrasonic has been of considerable
interest since 1920’s when studies of Harvey
and Loomis were published. They showed that
heating injure the bacteria, but ultrasonic appeared
to have a greater effect (5). Since l945,
an increasing understanding of the phenomenon
of cavitation has developed coupled with significant
developments in electronic circuitry and
transducers (i.e. devices which convert electrical
to mechanical signals and vice versa). As a
result of this there has been a rapid expansion in
the application of power ultrasound to chemical
processes, a subject that has become known as
“Sonochemistry” (3, 6). In the 1960’s, research
concentrated on understanding the mechanisms
of ultrasonic interaction with microbial cells. Cavitation
phenomenon and associated shear disruption,
localized heating and free radical formation
were found to be contributory causes (7).
By 1975 it was shown that brief exposure to
ultrasonic lead to thinning of cell walls which was
attributed to release cytoplasm membrane from
the cell wall. Fecal coliforms inactivation most
likely results from a combination of physical and
chemical mechanisms which occur during acoustic
cavitation, so it is expected that higher intensities
will enhance inactivation rates. The correlation
of chemical reaction rates and ultrasonic
intensity has been reported previously. However,
for most processes, increase in process rate not
continues with higher sound intensities (8, 9).
Since 1990, several studies have focused on the
use of ultrasound to remove
organic xenobiotics
from water (10-13).

Sound theory
Most modern ultrasonic devices rely on transducers
which are composed of piezoelectric materials.
Such materials respond to the application
of an electrical potential across opposite faces
with a small change in dimensions. This is the
inverse of the piezoelectric effect. If the potential
is alternated at high frequencies, the crystal
converts electrical energy to mechanical vibration
(sound) energy. At sufficiently high alternating
potential, high frequency sound (ultrasound) will
be generated. When more powerful ultrasound
at a lower frequency is applied to a system, it is
possible to produce chemical changes as a result
of acoustically generated cavitation (3, 6). Frequencies
above 18 kHz are usually considered
to be ultrasonic. The frequencies used for ultrasonic
cleaning, range 20 kHz to over 100 kHz.
The most commonly used frequencies for industrial
cleaning are those between 20 and 50 kHz
(3, 14, 15). Ultrasound has wavelengths between
successive compression waves measuring roughly
10 to 10-3 cm. These are not comparable to molecular
dimensions (Fig. 1). Because of this mismatch,
the chemical effects of ultrasound cannot
result from a direct interaction of sound with
molecular species (6, 16).

Bubble cavitation
Ultrasound reactor technology (USRT) in a liquid
leads to the acoustic cavitation phenomenon
such as formation, growth, and collapse of bubbles
(cavitation), accompanied by generation of
local high temperature, pressure, and reactive
radical species (°OH , °OOH) via thermal dissociation
of water and oxygen. These radicals penetrate
into water and oxidize dissolved organic
compounds. Hydrogen peroxide (H2O2) is formed
as a consequence of °OH and °OOH radical recombination
in the outside of the cavitation bubble
(17-19). Concentration of HO° at a bubble
interface can be as high as 4x10-3 M, which is
108-109 times higher than that in the other advanced
oxidation processes. Pyrolysis of pollutants
could lead to radical formation and starting
chain reactions, e.g. degradation of carbon tetrachloride
(20):
The basis for ultrasound irradiation applications is
that acoustic cavitation can create a number of
mechanical, acoustical, chemical and biological
changes in a liquid (21, 22).
Bubbles form, grow and subsequently collapse
through compression-rarefaction cycles. Temperature
in collapsing bubbles can reach to 3000-
5000°K and pressure to 500-10,000 atm. Under
such extreme conditions, water molecules undergo
homolysis to yield hydroxyl radicals and
hydrogen atoms. Since oxidation by hydroxyl radical
is an important degradation pathway, amount
of the hydroxyl radicals present in the sonolysis
system is directly related to the degradation
efficiency (23).
There are two main mechanisms in sonolysis system
for pollutant decomposition:
Pyrolysis reactions in cavitation bubbles
Radical reactions by radical species (°H, °OH)
from water sonolysis.
These two mechanisms are as below (20):
In elastic media such as air and most solids,
there is a continuous transition as a sound wave
is transmitted. In non-elastic media such as water
and most liquids, there is continuous transition as long as the amplitude or loudness of the
sound is relatively low. As amplitude is increased
the magnitude of the negative pressure in the
areas of rarefaction eventually becomes sufficient
to cause the liquid to fracture because of the negative
pressure, causing a phenomenon known as
cavitation. Cavitation bubbles are created at sites
of rarefaction as the liquid fractures or tears because
of the negative pressure of sound waves
in the liquid. As the wave fronts pass, the cavitation
bubbles oscillate under influence of positive
pressure, eventually growing to an unstable size.
Finally the violent collapse of the cavitation bubbles
results in implosions, which causes radiation
of shock waves from the sites of the collapse.
The collapse and implosion of myriad cavitation
bubbles throughout an ultrasonically activated
liquid result in the effect commonly associated
with ultrasound (8, 24, 25). Thus, sonochemical
destruction of pollutants in aqueous phase
generally occurs as the results of imploding cavitation
bubbles and involves several reaction pathways
and zones such as pyrolysis inside the bubble
and/or at the bubble-liquid interface and hydroxyl
radical- mediated reactions at the bubbleliquid
interface and/or in the liquid bulk (26).

Types of acoustic cavitation
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Advantages and disadvantages
There are no additives introduced into the ultrasonic
system and no by products generated by
ultrasonic technology. Therefore, there are no anticipated
environmental concerns associated with
this technology (30).
In contrast to many other processes which are
negatively affected when suspended solids of effluent
increase, US efficiency may even improve
by increase of turbidity or suspended solids (31).
Although the technology has been shown to be
feasible on a small scale, the commercialization of
sonolysis is still a challenge, due to the high energy
requirement of the process (20).

Ultrasound applications
In recent years, considerable interest has been
shown in the application of ultrasound as an advanced
oxidation process for the treatment of
hazardous contaminants in water. Sonochemistry
has been demonstrated as a promising method
for the destruction of aqueous pollutants (32).
1- Applications of ultrasound in phenolic
effluents treatment
Phenol is one of the most abundant pollutants in
industrial wastewater (33). Phenol is released to
the environment from industries such as petroleum
refining, coal tar, steel, tanning, pesticides,
pharmaceuticals and etc (13, 34-36). Phenol has
attracted public attention due to its presence in
groundwater, rivers and drinking waters (13).
Phenol even in small quantities causes toxicity and
foul odor to the water. Most of the countries specify maximum allowable concentration of phenol
in effluent to be less than 1 ppm (37). Several
treatment methods such as chemical oxidation,
biological treatment, wet oxidation, ozonolysis and
activated carbon adsorption have been proposed
for the removal of phenol from industrial effluents.
In recent years advanced oxidation processes
(AOPs) was developed (38, 39). One of these
technologies is photolysis. This method is based
on supplying energy to chemical compounds as
radiation which is absorbed by reactant molecules
that can pass to excited states and have sufficient
time to promote reactions (40). Direct photolysis
has been always considered as one possible
alternative because it is possible for molecules of
most organic compounds to transform, to cleave
bonds and even to undergo complete destruction
in the presence of UV eradiation (41).
The photolysis degradation of the phenol at different
initial concentration in the range 1-100
mg L-1 was investigated by (42). Fig. 2 shows the
degradation of phenol as function of time. Time
required for complete degradation increased from
3 to 120 min when the initial concentration was
increased from 1 to 100 mg/L.

It is clearly shown that lower pH values favored
the phenol degradation. Degradation of phenol
attained 94% at pH 3, 91.5% at pH 5, 71% at
pH 9 and 62% at pH 11 (42). For photolysis of
phenol other researcher have reported that the
rate of degradation under acid condition were
faster than in alkaline condition (1, 40, 43). It is
found that D. magna is the most sensitive organism
to phenol (44), so authors also studied
phenol toxicity on D. magna. Results showed
that phenol is toxic to D. magna and resulted in
quite low LC50 values (LC50 96 h of 15.7% v/v),
24 and 48 h LC50
(% v/v) values ranged from
33.1 and 19.5 for phenol and 66.5 to 42.4 for
effluent mixture, respectively. Comparison of Toxicity
Unit (TU) between phenol and effluent toxicity
showed that TU value for effluent was 2.18
times lower than that obtain to phenol (according
to 48 h LC50). Thus, photolysis was able to
eliminate the toxicity of by-products formed during
the degradation of phenol (42). This reduction
was achieved by phenol degradation and
transformation of aromatics by-products to aliphatic
products by ring opening reactions (45).
Data of this study showed that bioassay can be
used as a suitable method for evaluation of the
efficiency of treatment procedures by ultraviolet
waves (42).
In other study phenol degradation was carried
out for 5 h irradiation time. Figure 3 shows the
variations of phenol concentration with time. Only
13% degradation of phenol has been observed
for 300 min sonication of 100 mg/L phenol solution
(46).

The experimental data from this study fitted well
with first order reaction rate equation. Initial rate
of ultrasonic degradation was high but later it
reduced substantially. It demonstrated that lower
pH values had favored the phenol degradation.
The maximum and minimum efficiencies of phenol
degradation were determined to be 37% and
19% at pH valuse of 3 and 11, respectively (46).
Potolysis degradation of phenol at different initial
concentrations in the range 1-100 mg/L was
investigated (47). Figure 3 shows degradation
of phenol by the photolysis process at different
pH. It was clearly showed that lower pH values
favored the phenol degradation. The degradation
of phenol attained 94% at pH 3, 91.5% at
pH 5, 71% at pH 9 and 62% at pH 11 (47). For
photolysis of phenol other researchers have reported
that the rate of degradation under acid
condition was faster than that in alkaline condition
(1, 42, 39). In this study, ionic species of
phenol is predominant when pH exceeds 10.0,
but molecular species predominates when pH is
less than the pKa. Fraction in molecular state of
phenol was larger when pH was smaller. Therefore,
it has been concluded that photolysis of
phenol is pH dependent and increases under more
acidic condilioas. This might be the reason why
lower pH favored the ultrasonic degradation of
phenol (47).
Francony and Petrier showed that the rates of
reactions involving hydroxyl radicals (H202 formation
and phenol degradation) have a maximum
value at 200 kHz compared with lower and
higher frequencies (20, 500 and 800 kHz) (11).
Goel and co-workers recognizedthat decomposition
rates of non-volatiles were lower than volatiles
(48).
Study on effect of temperature revealed that the
destruction rate of 1, 2-DCA (dichloroethane) is
almost independent of temperature (in the range
of 15-30° C) (49).
Influences of various factors, such as initial pH,
initial phenol concentrations and kinetic constant
on the UV degradation of phenol have been
studied (42). Also, they determined LC50 of the
aqueous phenol solution before and after photolysis
(reaction by-products) using Daphnia magna
as the test organisms.
The degradation of phenol by ultrasonic equipment
operating at 130 kHz has been studied (50). Also
influences of various factors, such as initial pH
and initial phenol concentrations on the ultrasonic
degradation of phenol and LC50 of an aqueous
phenol solution before and after sonication
using Daphnia magna as the test organisms
were studied. Phenol degradation was for 300
min irradiation time. Fig. 4 shows the change in
concentration of phenol over time. They observed
that initially the rate of ultrasonic degradation of
phenol is high but later it reduces substantially.
This can be explained by the fact that whatever
dissolved air is present in the solution, it is degassed
after the initial period of sonication resulting
in a decrease in the amount of hydroxyl
radicals generated.
It has been reported that 96% removal for phenol
(Co= 100 mg L-1) by a bath UV equipment
during 60 min irradiation (51). Also, 92% degradation
for phenol has been reported (1) by
means of UV at 254 nm (9 W) for initial phenol
concentration of about 1.06x10-4 mmol L-1 during
60 min.
In other study, the effects of low frequency ultrasound
(20 kHz) to remove organic contaminants
containing aromatic compounds such as
phenol (100 mg/L) in presence of catalysts and
alone was evaluated. Results showed that phenol
removal is about 10% after 180 min. Also the
main mechanism of phenol removal is through
reaction with oOH. Phenol removal efficiency
was increased using phenton process up to 85%
in 120 min (52).

2- Algae removal
A novel method to inhibit growth of algal population
is application of ultrasonic irradiation. Ultrasonic
irradiation in a liquid medium has been
used for many years to lyse biological cells. Ultrasonication
may have the potential to reduce
their capacity to float and control their buoyancy
there by reducing their concentration near the surface
of water bodies and reduction their growth
and survival. Ultrasonication may also inhibit or
reduce growth of algal population through its
affect on metabolic processes (53). Application
of ultrasonic irradiation to control algal population
was evaluated in the laboratory conditions
(54) and results showed that short exposure to
ultrasonic irradiation collapsed algae gas vacuoles,
which results in loss of buoyancy and regulating
ability and thus localizing the cells. By 30, 60, 90,
120 and 150 seconds of sonication, respectively
8.55, 35.22, 67.22, 90.67 and 100% of the algal
population were destroyed. Besides, results showed
that increasing of sonication time has a considerable
effect on algal removal. Results indicate
that there is no significant reduction in algal population
in less than 30 seconds contact time to
42 kHz but considerable reduction in control
can be expected at higher periods. Experiments
using Bransonic bath at 42 kHz for biological
decontamination of water show that destruction
of algal population occurs rapidly. It is concluded
that using this frequency 100% of the algal population
can be destructed in 150 sec (54).

3- Nematode Removal
There are more than 15,000 known species of
roundworms and several thousands of individual
nematodes. Conventional water treatment processes are not highly effective in nematodes removal.
Nematodes are very resistant to inactivation
by free chlorine and can pass through rapid
sand filters (55). One approach nematode inactivation
is ultrasonic (56).
In a research it has been shown that exposure to
ultrasonic irradiation results in destruction of nematodes.
12 min sonication destroys 100% of the
nematodes. Also results show that increasing of
sonication time has a considerable effect on nematode
removal. Results also indicate that there is
no significant kill of nematodes in less than 8 min
contact time to 42 kHz, but considerable levels in
control can be expected at higher periods. By 2,
4, 6, 8, 10 and 12 minutes of sonication, respectively
23.75, 42.50, 53.5, 82.25, 89.25 and 100%
of adults are destroyed, but by 2, 4, 6 and 8 min
of sonication, respectively 38.0, 50.5, 58.75 and
100% of the larva are destroyed (57).

4- Coliform Removal
Results of study showed that increasing of sonication
time has a significant effect on bacterial
kill. These results also indicate that there is no
significant kill of fecal coliforms in less than 20
min contact time to 42 kHz but considerable levels
all in activation can be expected at higher periods.
When ultrasonic bath is used to sonicate
smaller volumes of bacteria at low frequency, there
is a resultant in the intensity of ultrasonic entering
the system. Furthermore, this study showed removal
efficiency in 90 min was highest. On the
other hand, sonication of smaller volumes results
in more rapid kill. Fig. 5 summarizes results of
these experiments. As can be seen up to 99.95%
reduction in bacteria concentrations were achieved
with the majority of these reductions found to
occur in the 90 min (58).

Experiments show that it is possible to decrease
the number of organisms present in the water and
that the process depends on exposure time, frequency
and intensity of the ultrasound irradiation,
as well as on the type of organisms (8).
Effectiveness of ultrasonic in treatment of total
coliforms was studied (59). Results show that increasing
in sonication time has considerable effect
on bacterial kill. Also, there is no significant
kill of total coliforms in less than 20 min
contact time to 42 kHz but considerable levels
of inactivation can be expected at higher periods.
When ultrasonic bath is used to sonicate
smaller volumes of bacteria at low frequency,
there is a resultant in the intensity of ultrasonic entering
the system. The highest and lowest bacteria
reduction after sonication for 300 mL and
600 mL volumes were 99.94% and zero. Also,
for 800 mL volumes were 99.63% and zero, respectively.
Furthermore, this study showed that
removal efficiency in 90 min was highest. On the
other hand, sonication of smaller volumes produced a more rapid kill. Also, up to 99.84% reduction
in bacteria concentration was achieved
with the majority of these reductions found to
occur in the 90 min. they concluded that sonication
leads to formation of dead bacterial cells or
selectively destroying weak bacteria.
It was shown that by 5, 15, 20, 30, 40, 50, 60, 70,
80, 90 min of sonication, respectively 43.75, 78.61,
82.71, 85.62, 97.82, 98.99, 99.29, 99.50, 99.63
and 99.84% of the total coliforms are destroyed.
Besides, the results show that increasing the sonication
time has a significant effect on bacterial kill.
Results also indicate that there is no significant
kill of Total Coliforms in less than 20 min contact
time to 42 kHz but considerable levels of
inactivation can be expected at higher periods.
When ultrasonic bath is used to sonicate smaller
volumes of bacteria at low frequency, there is a
resultant in the intensity of ultrasonic entering the
system. According results the highest and lowest
bacteria reduction after sonication for 300 ml and
600 ml volumes were 99.94% and zero. Also, for
800 ml volumes were 99.63% and zero, respectively.
Fig. 6 summarizes the results. As can be
seen, up to 99.84% reduction in bacteria concentration
was achieved with the majority of
this reduction found to occur in the 90 min.

In another research the efficacy of various advanced
oxidation processes based on ultraviolet
and ultrasound irradiation to inactivate Escherichia
coli in sterile water and total coliforms (TCs) in
biologically treated municipal wastewater has been
studied (60). They found that H2O2-assisted UVA/
TiO2 photocatalysis (9 W lamp) could generally
lead to nearly complete E. coli destruction
in 20 min with the extent of inactivation depending
on the photocatalyst type and loading and oxidant
concentration. Also Low frequency (24-80
kHz), high power (150-450 W) ultrasound irradiation
was less effective than photocatalysis requiring
longer contact times (i.e. 120 min) for
E. coli inactivation (60).

5- Organic matters
Results of a study show that US reduces BOD5
of secondary effluent, but sanitation time had no
considerable effect on the efficiency of this treatment.
Suspended BOD5 was removed completely
(approximately 100%), however soluble BOD5
was increased in some cases. Efficiency of total
COD removal was determined to be 17-28%. Removal
of suspended COD is better accomplished
than SCOD. In this study most of COD removal
was accomplished in initial sonication time and
removal efficiency was not much increased by
time. Better organics removal from secondary effluent
is performed at 130 kHz compared with the
lower frequency. Efficiency of treatment in 60 min
sonication at the frequency of 35 kHz was about
24%, but raised to about 28% at 130 kHz. H202
formation at 130 kHz frequency was about 2.5
times higher than 35 kHz. In contrast to TCOD,
removal efficiency of suspended COD was better
at 35 kHz. Figs. 7 and 8 shows summary of this
study (61).