PURIFAST ‐ Newsletter
Issue n°2
Delivered by Next Technology Tecnotessile
Delivery date: January 20th, 2010
Welcome to issue No. 2 of “PURIFAST‐News”, semesterly
newsletter published within the project in order to keep
you informed about its current activities and initiatives, on
the main results achieved in industrial and mixed
wastewater treatment for water reuse.
In this issue:
Innovative Advanced Oxidation Processes for wastewater purification:
Ultrasonic Equipments.
A wide range of organic compounds is detected in industrial and municipal wastewater. Some of
these compounds (both synthetic organic chemicals and naturally occurring substances) pose
severe problems in biological treatment systems due to their resistance to biodegradation or/and
toxic effects on microbial processes. As a result, the use of alternative treatment technologies,
aiming to mineralize or transform refractory molecules into others which could be further
biodegraded, is a matter of great concern. Among them, advanced oxidation processes (AOPs)
have already been used for the treatment of wastewater containing recalcitrant organic
compounds such as pesticides, surfactants, colouring matters, pharmaceuticals and endocrine
disrupting chemicals.
The main mechanism of AOPs function is the generation of highly reactive free radicals. Hydroxyl
radicals (HO•) are effective in destroying organic chemicals because they are reactive electrophiles
(electron preferring) that react rapidly and nonselectively with nearly all electron‐rich organic
compounds. They have an oxidation potential of 2.33 V and exhibit faster rates of oxidation
reactions comparing to conventional oxidants such as hydrogen peroxide (H2O2) or Potassium
Pemanganate (KMnO4) Once generated, the hydroxyl radicals can attack organic chemicals by
radical addition (Eq. 1), hydrogen abstraction (Eq. 2) and electron transfer (Eq. 3) (SES, 1994). In
the following reactions, R is used to describe the reacting organic compound.
A great number of methods are classified under the broad definition of AOPs (Table 1).
Table 1 - Advanced oxidation processes available in the literature
Most of them use a combination of strong oxidizing agents (e.g H2O2, O3) with catalysts (e.g.
transition metal ions) and irradiation (e.g. ultraviolet, visible). Among different available AOPs
producing hydroxyl radicals, hydrogen peroxide/UV light process and Fenton’s reactions seem to
be some of the most popular technologies for wastewater treatment as shown by the large
amount of data available in the literature. A search in Scientific Publication databases revealed
that more than 600 papers have been published for the applications of these methods in
wastewater treatment during the last decade.
The proposed technologies showed some limits in their industrial exploitation because of:
‐
H2O2/UV light process is significant affected by the initial concentration of the target
compounds, the amount of H2O2 used, wastewater pH, presence of bicarbonate and
reaction time. Furthermore, it cannot utilise solar light as the source of UV light due to the
fact that the required UV energy for the photolysis of the oxidizer is not available in the
solar spectrum, then special reactors designed for UV irradiation are required. Moreover,
H2O2 has poor UV absorption characteristics and if the water matrix absorbs a lot of UV
light energy, then most of the light input to the reactor will be wasted.
‐
Fenton’s reaction (a mixture of ferrous iron – catalyst ‐ and hydrogen peroxide ‐ oxidizing
agent): the major parameters affecting Fenton process are solution’s pH, amount of
ferrous ions, concentration of H2O2, initial concentration of the pollutant and presence of
other ions. The optimum pH for Fenton’s reagent processes ranges from 2 to 4. At pH
higher than 4, the Fe2+ ions are unstable and they are easily transformed to Fe3+ ions,
forming complexes with hydroxyl. Moreover, under alkaline conditions H2O2 loses its
oxidative power due to its breakdown to oxygen and water Due to the above, wastewater
pH adjustment is usually needed before treatment with Fenton processes. Increase of
ferrous ions and H2O2 concentration results to an increase of degradation rate Fenton.
Oxidation of organic compounds is inhibited by phosphate, sulphate, fluoride, bromide and
chloride ions (Chloride and Sulphate are widely used in textile finishing). Inhibition by these
species may be due to precipitation of iron, scavenging of HO• or coordination to dissolved
Fe(III) to form a less reactive complex.
So it is clear that the main limitations related with the application of these technologies in
wastewater purification are mainly related with the relatively high operational cost of these due
to the use of costly chemicals and high energy consumption and the scavenging of the radicals
with non‐target substances. Based on the above, the main future challenges for using AOPs in
wastewater treatment could be the development of efficient and low cost materials to promote
sufficient treatment, the use of renewable energy sources, the adoption of strategies for
processes integration, the targeting of new classes of pollutants and the commercialization of
processes which have been so far used in the laboratory.
Within this scenario, the PURIFAST project is aiming to exploit the Ultrasonic Treatment for the
purification of Industrial and Mixed wastewater. In fact, the use of ultrasonic (US) irradiation with
or without oxidizing agents (such as hydrogen peroxide) could result to an increase of the
produced hydroxyl radicals, promoted by cavitation phenomenon at lower operational costs and
energy consumption.
What is Ultrasonic and Cavitation phenomenon?
Ultrasonics is the science of sound waves above the limits of human audibility. The frequency of a
sound wave determines its tone or pitch. Low frequencies produce low or bass tones. High
frequencies produce high or treble tones. Ultrasound is a sound with a pitch so high that it can not
be heard by the human ear. Frequencies above 18 kHz are usually considered to be ultrasonic. The
frequencies used for ultrasonic cleaning range from 20,000 cycles per second or KHz to over
100,000 KHz. The most commonly used frequencies for industrial cleaning are those between 20
KHz and 50KHz. Frequencies above 50KHz are more commonly used in small tabletop ultrasonic
cleaners such as those found in jewelry stores and dental offices and for chemical purposes. In
order to understand the mechanics of ultrasonics, it is necessary to first have a basic
understanding of sound waves, how they are generated and how they travel through a conducting
medium. The dictionary defines sound as the transmission of vibration through an elastic medium
which may be a solid, liquid, or a gas. A sound wave is produced when a solitary or repeating
displacement is generated in a sound conducting medium, such as by a "shock" event or
"vibratory" movement. The displacement of air by the cone of a radio speaker is a good example
of "vibratory" sound waves generated by mechanical movement. As the speaker cone moves back
and forth, the air in front of the cone is alternately compressed and rarefied to produce sound
waves, which travel through the air until they are finally dissipated. We are probably most familiar
with sound waves generated by alternating mechanical motion. There are also sound waves,
which are created by a single "shock" event. Shock events are sources of a single compression
wave, which radiates from the source. The compression generated by the sound source as it
moves propagates down the length of the spring as each adjacent coil of the spring pushes against
its neighbour. It is important to note that, although the wave travels from one end of the spring to
the other, the individual coils remain in their same relative positions, being displaced first one way
and then the other as the sound wave passes. As a result, each coil is first part of a compression as
it is pushed toward the next coil and then part of a rarefaction as it recedes from the adjacent coil.
In much the same way, any point in a sound conducting medium is alternately subjected to
compression and then rarefaction. At a point in the area of a compression, the pressure in the
medium is positive. At a point in the area of a rarefaction, the pressure in the medium is negative
(Figure 1).
Figure 1 – Compression and Rarefaction in a sound conductive medium
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,
however, 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 the sound wave in the liquid. As the wave fronts pass,
the cavitation "bubbles" oscillate under the influence of positive pressure, eventually growing to
an unstable size. Finally, the violent collapse of the cavitation "bubbles" results in implosions,
which cause shock waves to be radiated from the sites of the violent collapse. The concomitant
release of heat follows the formation of the so‐called ‘‘hotspots’’ reaching temperature and
pressure limits of 2000 C and 200 atm, respectively.
When water vapor, dissolved gas and/or organic substances are exposed to these extreme
conditions, bond rupture occurs. Then, sonolysis (Eq. 4) generates hydroxyl radical from water
dissociation:
(4)
During the process, three well determinate zones can be described: the cavitation bubble, the
supercritical interface and the bulk of the dissolution. For instance, the degradation of volatile
compounds is carried out in the gas phase of the cavitational bubble and/or in the interface of the
hotspot. Hydrophilic and non‐volatile compounds, on the contrary, are principally degraded once
the reactive radical species reach enough diffusion mass transfer in the aqueous phase.
To introduce ultrasonic energy into wastewater steams requires an ultrasonic transducer and an
ultrasonic power supply or "generator." The generator supplies electrical energy at the desired
ultrasonic frequency. The ultrasonic transducer converts the electrical energy from the ultrasonic
generator into mechanical vibrations.
Ultrasonic equipments
‐
US Generator
The ultrasonic generator converts electrical energy from the line, which is typically
alternating current at 50 or 60Hz to electrical energy at the ultrasonic frequency. The
ultrasonic generators realised by LAVO for their devices use solid state technology.
The effectiveness of ultrasonic equipment is ensure modulating or "sweeping" the
frequency of the generator output around the central operating frequency. In sweep
operation, the frequency of the output of the ultrasonic generator is modulated around a
central frequency which may itself be adjustable.
Various effects are produced by changing the speed and magnitude of the frequency
modulation. The frequency may be modulated from once every several seconds to several
hundred times per second with the magnitude of variation ranging from several hertz to
several kilohertz. Sweep may be used to prevent damage to extremely delicate parts or to
reduce the effects of standing waves in the tanks. Sweep operation may also be found
especially useful in facilitating the cavitation phenomenon.
‐
US transducer
Ultrasonic transducers convert alternating electrical energy to vibratory mechanical energy
by using piezoelectric effect in which certain materials change dimension when an
electrical charge is applied to them.
Electrical energy at the ultrasonic frequency is supplied to the transducer by the ultrasonic
generator. This electrical energy is applied to ceramic (piezoelectric element) in the
transducer which vibrate. These vibrations are amplified by the resonant masses of the
transducer and directed into the liquid through the radiating plate. All the transducers are
equipped with a air cooling system in order to prevent damage of the ceramics for long
operating period (more then 5 minutes).
In order to maximise Cavitation phenomenon, some parameters have to be optimised:
‐
Temperature is the most important single parameter to be considered in maximizing
cavitation intensity. This is because so many liquid properties affecting cavitation intensity
are related to temperature. Changes in temperature result in changes in viscosity, the
solubility of gas in the liquid, the diffusion rate of dissolved gasses in the liquid, and vapor
pressure, affecting cavitation intensity. In pure water, the cavitation effect is maximized at
approximately 30‐40°C, as the liquid starts to boil at the cavitation sites.
‐
Viscosity of a liquid must be minimized for maximum cavitation effect. Viscous liquids are
sluggish and cannot respond quickly enough to form cavitation bubbles and violent
implosion. The viscosity of most liquids is reduced as temperature is increased.
‐
Frequency of the waves. Generally Cavitation intensity is inversely related to Ultrasonic
Frequency. As the ultrasonic frequency is increased, cavitation intensity is reduced because
of the smaller size of the cavitation bubbles and their resultant less violent implosion.
Then, low frequency apparatus must be used for increase the effectiveness of the process
in the generation of hydroxyl radicals. The most suitable range, according to the radical
formation rate is achieved by applying a frequency in the range 150 – 500 kHz (Table 2).
‐
Acoustic power and Power Efficiency. Cavitation phenomenon is enhanced by the increase
in the Ultrasonic Power (Table 2).
Table 2 – Frequency and Power of LAVO Transducer linked with Radical formation rate
Equipment
US frequency
[kHz]
Acoustic Power
[W]
Electrical Power
[W]
Electrical Efficiency
[%]
Radical Formation Rate
[µmol/Lmin]
Sonicator Special
Sonogallery 350
Sonogallery 850
120 – 150
250 -350
500 - 850
33.2
75.4
10.7
48.0
105.0
101.0
69
72
10
3.47
6.46
0.24
For additional info please contact us at http://purifast.tecnotex.it/ or send an e‐mail to
[email protected]
PURIFAST Partners
Next Technology Tecnotessile
Società Nazionale di Ricerca r.l. (Italy)
www.tecnotex.it
Management of technical activities
University of Florence
Department of Civil Engineering (Italy)
www.dicea.unifi.it
Management of technical activities
University of Florence
Department of Mathematics (Italy)
www.math.unifi.it
Membrane system modelling
Gestione Impianti Depurazione Acque
S.p.A. (Italy)
www.gida‐spa.it
Mixed industrial/municipal WWTP
Tintoria King Colour SpA (Italy)
Industrial end‐user
Lavo s.r.l. (Italy)
www.lavo.it
Ultrasound Technology provider
inge AG (Germany)
www.inge.ag
Membrane Technology provider
IWW Rheinisch‐Westfälisches Institut für
Wasserforschung gemeinnützige GmbH
(Germany)
www.iwwonline.de
Membrane control system
Polymem SA (France)
www.polymem.fr
Membrane Technology provider