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Acoustic methods for biofouling control: A review

Authors:
  • Massey University, Albany, Auckland, New Zealand
  • Bancolor Aluminium Coil Coating

Abstract and Figures

Biological fouling is a significant problem to the shipping industry causing significant increases in fuel, maintenance, and downtime costs. Environmental concerns associated with toxic antifouling coatings have led to studies on alternative methods of biofouling control. This paper provides a literature review on laboratory and sea trial studies, which have used acoustic techniques for biofouling control. To the best of the authors׳ knowledge, this is the first in depth literature review on this topic. Applications of the reviewed studies have included the inhibition of biofouling on vessel hulls and pipes and also treatment of ballast water. The studies have used transducers operating in the audio and ultrasonic frequency range and sparkers. Variations were found in these acoustic parameters, which were reported to provide inhibition. Some have reported that low ultrasonic frequencies (about 20 kHz) may be optimal. The potential effect of marine life is considered. The use of ultrasonic frequencies for biofouling control appear to be more desirable than audio frequencies since they are outside the hearing range of most marine life. More studies are needed on this topic, which are well documented in terms of the parameters used and efficiency of the trials.
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Acoustic Methods for Biofouling Control: A Review
M. Legg
, M.K. Y¨
ucel, I. Garcia de Carellan, V. Kappatos, C. Selcuk, T.H. Gan
Brunel Innovation Centre1, Brunel University, Uxbridge, Middlesex, UB8 3PH, United Kingdom
Abstract
Biological fouling is a significant problem to the shipping industry causing significant increases in fuel, maintenance,
and downtime costs. Environmental concerns associated with toxic antifouling coatings have led to studies on alter-
native methods of biofouling control. This paper provides a literature review on laboratory and sea trial studies, which
have used acoustic techniques for biofouling control. To the best of the authors knowledge, this is the first in depth
literature review on this topic. Applications of the reviewed studies have included the inhibition of biofouling on
vessel hulls and pipes and also treatment of ballast water. The studies have used transducers operating in the audio and
ultrasonic frequency range and sparkers. Variations were found in these acoustic parameters, which were reported to
provide inhibition. Some have reported that low ultrasonic frequencies (about 20 kHz) may be optimal. The potential
eect of marine life is considered. The use of ultrasonic frequencies for biofouling control appear to be more desirable
than audio frequencies since they are outside the hearing range of most marine life. More studies are needed on this
topic, which are well documented in terms of the parameters used and eciency of the trials.
Keywords: Biofouling, antifouling, inhibition, ultrasonic, acoustic, sparkers, vessel hulls, pipes, ballast water,
marine structures
1. Introduction
Biological fouling, also called biofouling, is the un-
desirable formation of organisms on a surface immersed
in water. Biofouling build-up increases the drag force
caused by water flowing past the surface. It can cause
blockage of intake pipes and heat exchangers, and can
result in biocorrosion [82]. Biological fouling is a sig-
nificant problem for all marine structures such as ships,
oshore rigs and oceanographic sensors [27].
Biofouling has a significant economic cost for the ship-
ping industry. Biofouling can substantially increase ship
hull friction. Heavy calcareous fouling is calculated to
increase required shaft power by 86% as compared to a
hydraulically smooth hull at cruising speed [64]. Higher
Corresponding author
Email address: legg.mathew@gmail.com (M. Legg )
1bic@brunel.ac.uk
fuel consumption is required to compensate for this ef-
fect, which results in increased cost and pollution. Bio-
corrosion may also be caused by the biofouling, which
may aect the structural integrity of structures in contact
with water. There are maintenance costs, loss in opera-
tion time, and production of toxic waste associated with
addressing these biofouling problems [27]. There are dif-
ferent kinds of measures taken to combat the eects of
biofouling, which include antifouling coatings, see Figure
1. Environmental concerns associated with toxic antifoul-
ing coatings have led to studies into alternative methods
for biofouling control, which include acoustic techniques
[24, 82].
This paper was conducted as part of the Cleanship [17]
project. This project will perform trials on plates in a port
environment with the aim of investigating the use of ul-
trasonic waves for prevention and detection of biofouling
on ship hulls. Key information required for the project
were the operating parameters, such as frequency and
Preprint submitted to Ocean Engineering March 19, 2015
Figure 1: Photo of a ship with antifouling coating on the hull. Photo
provided by Lloyds Register.
power, which would provide the optimal biofouling con-
trol, while minimising the potential undesired eect on
marine life. A literature review on this topic was made to
address these questions. To the best of the authors knowl-
edge, this is the first in depth literature review performed
on studies using acoustic techniques for biofouling con-
trol.
In the following sections, biofouling and some non-
acoustic methods for its control are briefly described. A
literature review of acoustic methods for biofouling con-
trol is then presented. The applications include inhibit-
ing biofouling on vessel hulls and pipes and also treat-
ment of water including ballast water. These studies have
used transducers operating in the ultrasonic and audio fre-
quency range and also acoustic sparkers. An analysis of
the potential eect of acoustic biofouling techniques on
marine life is provided. A discussion then emphasises the
need for developing a methodology, which documents the
operating parameters used and the performance of the tri-
als so that a design of an eective system can be more
easily obtained.
2. Biofouling
Fouling, in general, can be defined as the accumula-
tion of organic or inorganic matter on a surface. Bio-
fouling is the formation of microorganisms, plants, and
other marine life on a surface in contact with water [82].
Upon immersion of a surface into water, a film composed
mainly of dissolved organic material begins to form al-
most immediately [27]. Next microorganisms begin to
colonise the surface in a layer referred to as microfouling.
These microorganisms include fungi, algae, bacteria, and
diatoms. This layer starts forming within hours of immer-
sion. Larvae of larger marine invertebrates such as bry-
ozoans, mussels, barnacles, and polychaetes also begin to
attach to the microfouled surface in a layer referred to as
macrofouling [13, 27]. The development of these biofoul-
ing layers is dependent on a combination of dierent en-
vironmental conditions such as salinity, temperature, con-
ductivity, pH, dissolved oxygen content, organic material
content, hydrodynamic conditions, currents, light, depth,
and distance from the shore [18]. Refer to Figure 2 for
two example photos of biofouling on ship hulls.
(a)
(b)
Figure 2: Example photos of fouling vessel hulls. Photos provided by
WRS Marine and Lloyd’s Register.
2
3. Non-Acoustic Methods for Biofouling Control
A broad spectrum, high-toxicity antifouling coating
system containing Tributyltin (TBT) compounds was de-
veloped in the mid-1950s [82]. This became a very suc-
cessful antifouling system, covering an estimated 70% of
the world fleet at one time [72]. Unfortunately, TBT sys-
tems adversely aect the environment. Copper, with the
addition of booster biocides, has replaced TBTs as the
main biocide ingredient in antifouling coatings [11, 27].
There are still environmental concerns associated with
copper based antifoulings [72]. There is, therefore, an in-
terest in developing environmentally friendly alternatives.
Naturally occurring antifouling substances may be ex-
tracted from a variety of natural sources and incorporated
into a paint matrix. These compounds would need to be
harvested or synthesized in large quantities at a commer-
cially viable price [82]. Fouling release coatings reduce
the attachment strength of the fouling, allowing them to be
more easily removed by water flow or mechanical clean-
ing. It has been reported that these coatings are relatively
expensive, exhibit poor adhesion to the substrate, and are
easily damaged [6, 70, 82]. Other non-toxic antifouling
techniques have included textured surface coatings, me-
chanical techniques, and electrical methods [15, 60, 82].
For pipes, other methods used include the use of chemi-
cals, acids, hot water, and UV techniques [27, 52, 82].
4. Acoustics Methods for Biofouling Control
Acoustic antifouling methods may provide a non-toxic
alternative for biofouling prevention. The applications
that have been studied include biofouling inhibition of
vessel hulls and pipes and treatment of ballast water. The
methods used may be divided into two groups; ultra-
sonic and audio range wave emission systems and acous-
tic sparkers (also called pulsers).
4.1. Marine Vessel Acoustic Antifouling Studies
4.1.1. Ultrasonic and Audio Biofouling Hardware
Studies relating to the application of preventing bio-
fouling on vessel hulls have used devices emitting me-
chanical waves in the ultrasonic (>20 kHz) and audi-
ble (20Hz - 20 kHz) frequency range. These devices
are generally composed of a signal generator or self os-
cillating circuit, power amplifier, and a transducer, see
Figure 3. Transducers used have included piezoelectric
transducers [7, 14, 22, 30–32, 41, 47, 65] and strips/films
[43, 51, 79], magnetostrictive transducers [66], and audio
speakers [56].
Figure 3: Diagram showing a basic acoustic antifouling system for a
vessel. A signal generator/power amplifier is used to drive a transducer
attached to a vessel’s hull causing it to vibrate.
Multiple transducers may be used together as an array
to optimize the overall gain. Placement geometry and
spacing of the transducers needs to be considered, due
to the eects of constructive and destructive interference.
Destructive interference may produce areas, referred to as
anti-nodes, where the acoustic energy will be a minimum,
potentially reducing the biofouling eect at these areas.
The position of the anti-nodes may change with frequency
[47, 56]. The antifouling eectiveness may be expected to
decrease with increasing distance from the transducer lo-
cations [66].
4.1.2. Ultrasonic Frequency Range Studies
Acoustic antifouling studies have been performed in
the ultrasonic frequency range [1–4, 16, 19, 21, 28–
32, 37, 41–43, 47, 50, 55, 65, 66, 69, 75]2. A number
of sea trials have been reported to have successfully used
lower ultrasonic frequencies (tens of kHz) for preventing
biofouling growth. Latour and Murphy stated that Wald-
2References [1, 3, 4, 16, 20, 21, 42, 50, 66, 75] have not been viewed
by the authors of this paper but have been mentioned by others in the
reviewed literature.
3
vogel [75] had reported that 16 foot aluminium boats vi-
brated at 25 – 55 kHz with 25 W input (several W/m2)
were “relatively free” of fouling. Similarly, Latour and
Murphy stated that Askel’band [1] had reported that mer-
chant ship hulls vibrated for periods of several years had
“reduced fouling levels”. These two references were not
able to be found by the authors so it is unclear what the
baseline for these reported reduced fouling levels were.
Sheherbakov et al. stated that by 1972 about 20 vessels
in the Soviet fleet had been equipped with ultrasonic an-
tifouling protection systems [66]. The hulls were vibrated
using oscillators fixed to the inner hull operating between
17 – 30 kHz at 200 W. They observed that fouling pre-
vention was evident, but that a stripped fouling pattern
occurred due to reduced vibration amplitude at the bulk-
heads and framing. At considerable distances from the
oscillator, where the oscillation acceleration level was be-
low 70 dB, they reported dense fouling.
A number of lab studies have investigated the eect of
ultrasound on barnacles using power levels high enough
to cause cavitation. In a lab study on barnacles, Kitamura
et al. investigated three dierent frequencies (19.5, 28,
and 50 kHz) and reported 19.5 kHz to be the most eec-
tive [41]. For 19.5 kHz, they stated that 4300 kPa.s (sound
pressure level multiplied by treatment duration) resulted
in 50% mortality in barnacle larvae, while 140 kPa.s treat-
ment resulted in 50% inhibition of larvae settlement. Guo
et al. used selected resonant ultrasound frequencies of 23,
63, and 102 kHz at 20 kPa to investigate its eect on bar-
nacle settlement inhibition [32]. Similarly to Kitamura
et al., they reported 23 kHz to be the most eective fre-
quency. By varying the cavitation threshold, while keep-
ing the same sound pressure level, they demonstrated that
cavitation had a significant barnacle settlement inhibition
eect. It was suggested that part of the observed fre-
quency dependence might be due to the cavitation thresh-
old being lower and implosions more powerful at lower
ultrasonic frequencies. Sub-cavitation level trials were
then performed where first the same sound pressure level
of 5 kPa and then the same transducer head displacement
of 10.05 nm was used for all three frequencies. In both
cases, they again reported 23 kHz to be the most optimal.
They concluded that this frequency dependence was not
simply related to cavitation eect but must also be related
to some additional factors, such as vibration of the surface
or acoustic waves in the water. They also demonstrated
that turning the equipment on and oin cyclical operation
mode achieved similar settlement inhibitory eects com-
pared to continuous mode. They suggested that 5 minutes
on and 20 minutes omight be suitable in terms of en-
ergy antifouling eciency and lifespan of the equipment.
Table 1 provides a brief summary of some ultrasonic bio-
fouling control studies [27].
4.1.3. Audio Frequency Range Studies
Acoustic antifouling studies have been performed
in the audible frequency range [7, 14, 20–22, 43].
Branscomb and Rittschof investigated the eects of low
frequency (15 - 45 Hz) sound waves on barnacle settle-
ment rates in laboratory studies [7]. They reported that
settlement inhibition was achieved, with 30 Hz being the
most ecient. The eectiveness was observed to reduce
after five days. In subsequent trials, however, other re-
searchers have not observed this antifouling eciency at
these low frequencies [14, 21]. In sea trials, Choi et al.
investigated the eect of low frequency (70 - 445 Hz) vi-
bration on biofouling [14]. In contrast to Branscomb and
Rittschof, no eect was observed, compared to a control,
for frequencies below 200 Hz. Above this, they reported
an increased deterrence in barnacle settlement rates with
increased vibrational frequency. This eectiveness for
barnacles lasted the period of the sea trial, which was over
98 days. Other forms of biofouling, including tubeworms,
bryozoans, ascidians, and algae, seemed to be unaected
by the applied excitation. In sea trials, Latour and Mur-
phy reported that a panel vibrated at 5 kHz was unfouled
after 5 months, while a panel lightly vibrated at 50 Hz
and a control panel were completely fouled by algae, bar-
nacles, mussels, and worms after a period respectively of
five and two months [43]. They reported similar results
for their trial on a fibreglass skivibrated at 5 kHz over
a 6 month period. They reported dierent mortality rates
for dierent life cycles with maximum mortality rate for
larvae. Refer to Table 2 for a brief summary of some au-
dio frequency acoustic antifouling studies.
4.1.4. Vessel Noise May Increase Biofouling
The audio frequency studies described above, reported
reduction of biofouling with the use of audio frequencies.
However, some recent studies have reported that gener-
ator sound, mainly composed of frequency components
4
Table 1: Example information from literature on ultrasonic frequency range acoustic treatment of biofouling. In this table, the citations have been
sorted by transmission frequency. Where more than one frequency was used, reported optimal frequency has been used for sorting.
Treatment
Types
Frequency Organism
Type
Power Treatment
Duration
Application Comments
Magnetostrictive
Transducer
17 kHz – 30
kHz
Biofouling 200 W Ship (fixed to
inner hull)
Prevention achieved.
Settlement inhibition only. [66] (1974)
Transducer 20 kHz Biofouling 1000 W Boat Biofoulers and other foulers removed.
Cleaning rate 4 – 6 cm/s. [47] (2011)
PZT transducer 23, 63, 102
kHz
Barnacle 9, 12, 22 kPa
pressure
30 – 300 s Laboratory 23 kHz optimal frequency at 22 kPa
for 30 s. Settlement inhibition.
Mortality observed only in long dura-
tion. Cavitation. [30, 32] (2011)
Transducer 20 – 25,
63 and 102
kHz
Barnacle 10.5 nm
substratum
vibration
and 5 kPa
pressure.
Continuous
and “5
min on 20
min o
treatment.
Laboratory 23 kHz optimal frequency. Settle-
ment inhibition. Intermittent signal
achieved same ecacy with continu-
ous signal treatment. Not cavitation.
[31] (2012)
PVF2piezo-
film strips
24 kHz Barnacles
worms,
mussels
2 A – 12
V. Accelera-
tion 0.004 – 1
G.
6 – 7 months Fiberglass
yacht hull. No
antifouling on
3 m2section.
No fouling observed. [43] (1981)
from 30 Hz to 2 kHz, emitted by ships at port can actu-
ally promote biofouling accumulation rather than prevent
it [49, 68, 78]. It was reported that it may even result in
faster growth and metamorphosis of some taxa. Stanley et
al. performed marine trials using panels in the sea, which
were vibrated by playing recordings of generator noise
emitted through a vessel’s hull in port [68]. They reported
that more than twice as many bryozoans, oysters, calcare-
ous tube worms, and barnacles settled and established on
surfaces with vessel noise compared to those without. It
was suggested that the noise from vessel hulls, having pre-
dominately components in the low to mid audio frequency
range (0.1 – 2 kHz), has characteristics similar to the pre-
ferred natural settlement habitats of these fouling species,
such as reefs. In marine trials on vessels, McDonald et al.
showed that there appeared to be spatial correlation in bio-
fouling with the intensity and frequency of the noise emit-
ted by the vessels generator [49]. They also performed
laboratory experiments where they reported that ascidian
Ciona intestinalis larvae showed significantly faster set-
tlement, metamorphosis, and larval survival rates when
exposed to underwater sound from a vessel generator.
4.2. Biofouling Prevention in Pipes
Pipes or heat exchanges, that take water from the sea,
lakes, or rivers, can experience problems with biofoul-
ing formation on their interior surface. There have been
studies relating to the inhibition of fouling in pipes us-
ing acoustic techniques. A few studies have been found,
which used ultrasound [5, 71], see Table 3. However, the
majority have used sparkers for biofouling inhibition.
4.2.1. Acoustic Sparkers
Figure 4: Diagram reproduced from reference [61] showing a basic
sparker system with an electrical discharge between electrodes in wa-
ter, which produces a vapor cavity bubble and a shock wave.
Acoustic sparkers, also referred to as pulsers, generate
impulsive, wide frequency bandwidth acoustic waves. A
sparker circuit generates a large voltage, which is stored
on a capacitor. This voltage is rapidly discharged between
two electrodes in water. By applying a voltage to the elec-
trodes, which is high enough to exceed water breakdown
5
Table 2: Example information from literature on audible frequency range acoustic treatment of biofouling.
Treatment
Types
Frequency Organism
Type
Power Treatment
Duration
Application Comments
Transducer 30 Hz Barnacle 20 hours Laboratory
dishes
Settlement inhibition only. [7] (1984)
PZT Transducer 70 – 445 Hz Barnacle Velocities 3
mm/s, 1.5
mm/s, and
0.75 mm/s
3 months Sea trials (vi-
brated panels)
Barnacles only aected by treatment.
Settlement inhibition only.
Increasing frequency and velocity am-
plitude increases inhibition level. No
eect below 200 Hz. [14] (2013)
PVF2film and a
vibrator
50 Hz and 5
kHz
Barnacles
worms,
mussels
0.05 G (5
kHz) and
0.005 G (50
Hz)
5.5 months Panels and
2 meter
fiberglass ski
Skiand 5 kHz panel unfouled after
5.5 month, 50 Hz and control panel
completely fouled after 5 month and 2
months respectively. [43] (1981)
Vessel gener-
ator noise and
speaker in bath
(lab)
Wide band-
width (30–
100 Hz)
dominate
Wide range
of fouling.
Lab study C.
intestinalis
larvae
127.5–140.6
dB re 1 µPa.
Reduces with
increased
frequency
24 hour (lab) Four 25 m
vessels and
lab trial
Biofouling higher on the fishing ves-
sel in the sites closest to the genera-
tor. In lab trial, the rates of settlement,
metamorphosis and survival are signif-
icantly increased in C. intestinalis lar-
vae when exposed to vessel noise [49].
(2014)
Speaker playing
vessel generator
noise
30 Hz –
2 kHz
dominant
Wide range. 128 dB re 1
µPa RMS.
Reduces with
increased fre-
quency
27 days Fiber-cement
panels (200 ×
200 mm) in
the sea
More than twice as many bryozoans,
oysters, calcareous tube worms and
barnacles settled and established on
surfaces with vessel noise compared to
those without. [68] (2014)
Figure 5: Diagram reproduced from reference [61] showing the experi-
mental setup used by Schaefer et al. to investigate biofouling control of
an intake pipe from a lake using a sparker.
level, the surrounding water may be vaporized causing an
acoustic shock wave, see Figure 4. Parabolic dishes have
been used as acoustic mirrors to direct the energy of the
pulse. A generic way of designing sparkers and how they
work is explained in detail in [12] and [26].
The use of sparkers for biofouling control has been doc-
umented in various patents and papers over the last few
decades. Virtually all the studies reviewed on sparkers
were related to the application of biofouling inhibition
of intake pipes of industrial facilities [46, 61, 62]. In an
investigation using a sparker to prevent fouling of Zebra
mussels on a 0.76 meter diameter intake pipe from a fresh-
water lake, Schaefer et al. report that the eective range
of mortality and settlement inhibition from a sparker were
respectively 1.5 and 26 meters [61], see Figure 5.
According to Walch et al., the eect of sparkers on bio-
fouling control has been attributed to cavitation and the
resulting acoustic shock wave. Sub-cavitation threshold
sparker treatments may manage to prevent fouling, but
generally fail to remove adhered organisms [74]. The ef-
fect of sparker induced cavitation on juvenile barnacles
was studied by Guo et al. using a high speed camera [28].
They reported that ultrasonic cavitation damaged the bar-
nacle shell. Newly attached barnacles were able to be re-
moved completely. Older barnacles were less easily re-
moved and left the base plate cement on the surface. The
shock wave induced by sparkers also provides inhibition.
Schaefer et al. suggested inhibition of mussels in a pipe
could be due to the shock wave causing the mussels to
close their shells and drift to the bottom of the pipe [61].
6
Table 3: Example information from literature on ultrasonic antifouling treatment of pipes. In this table, the citations have been sorted by transmis-
sion frequency. Where more than one frequency was used, reported optimal frequency has been used for sorting.
Treatment
Types
Frequency Organism
Type
Power Treatment
Duration
Application Comments
Piezoelectric
transducer
20 kHz Fungi, bacte-
ria, algae
600 W.
Amplitude
varied: in-
cluded 40%
and 20%
3 x 30 s
treatments
per day.
Heat ex-
changer tubes:
18 mm I.D x 1
m long.
Biofilm thickness reduction dependent
on maximum amplitude. 92% reduc-
tion at 40% amplitude. [5] (2000)
PZT Transducer 250 2000
kHz
Colonial
hydroid,
Garveia
fransiscana
6.2 W/cm2
sound inten-
sity
Pulse of 0.2
s, with 100
s inter-pulse
duration.
Intake pipes 250 kHz most energy ecient.
High frequency requires higher sound
intensity (power) for same results.
[71] (1983)
Refer to Table 4 for a summary of some acoustic sparker
treatment studies.
The fouling prevention ecacy may depend on the ap-
plied acoustic frequency; and the optimum antifouling
frequency may be species specific. A spark by nature pro-
duces a broad spectrum, typically ranging from about 10
Hz to 100 kHz, but may extend up to tens of MHz [9].
This broadband nature of sparkers could be beneficial in
terms of covering a wide range of organisms. However,
total energy is spread across the produced spectrum, po-
tentially meaning that the energy intensity in a desired
frequency range may not be large. Also, the intensity
of energy that arrives at a specific location is aected by
losses such as attenuation in the medium during propaga-
tion. Sound absorption in water generally increases with
increasing frequency. In contrast, for pipes, geometric ef-
fects may cause higher frequencies components of the sig-
nal to propagate further [9, 61]. It has been suggested that
a sparker could be designed to maximise the energy in the
desired frequency range [8, 9]. It appears that control over
the peak acoustic frequency generated by a sparker may
be able to be achieved by controlling parameters such as
capacitance, which aects the pulse length [9, 35, 54].
Sparkers require maintenance in the form of replacing
the electrodes, which erode over time [8, 9]. This erosion
is likely to be due largely to cavitation eects. If cavita-
tion occurs on the surface of the structure being protected
from biofouling, there is the potential that erosion of the
surface may occur. However, this may not be an issue es-
pecially if, as was the case in many references, the sparker
was not inside the pipe but in a separate enclosure onto
which the pipe was attached.
The references relating to biofouling control using
sparkers were almost entirely for the application of foul-
ing prevention of intake pipes [46, 61, 62]. No references
were found where sparkers were applied to the applica-
tion of biofouling protection of vessel hulls. In a pipe,
the shock wave induced by a sparker would be guided
by the structure causing it to propagate along the pipe.
However, for the application of a vessel hull, the shock
wave would be unconstrained causing much of the energy
to propagate away from the vessel. Therefore, sparkers
might be expected to be less ecient for vessel antifoul-
ing applications, than an ultrasonic system where the hull
is vibrated and the energy is likely to be more eciently
guided through the vessel structure. Sparkers also require
high power levels to function, although this is also true for
an ultrasonic device that functions using cavitation.
4.3. Treatment of Organisms Suspended in Water
Undesirable marine organisms are often spread to dif-
ferent regions through the emptying of vessel ballast wa-
ter tanks [36]. Several acoustic studies have investigated
the mortality rates of organisms suspended in water. In a
study relating to control of organisms in ballast water, Ga-
vand et al. showed that mortality of brine shrimp larvae,
cysts, and adults could be induced when exposed to son-
ication at 1.4 kHz for 20 minutes [22]. A number of lab-
oratory studies have investigated the eect of high power
ultrasound on barnacle survival rates [22, 65, 69]. Suzuki
and Konno used high power, pulsed ultrasonics between
28 kHz and 200 kHz and reported the higher frequency to
be more lethal to barnacle larvae [69]. Seth et al. (2010)
attempted to quantify the energy needed for barnacle lar-
vae destruction, using high power levels where cavitation
might be expected to occur. They reported that 20 kHz
7
Table 4: Example information from literature on acoustic sparker treatment of biofouling.
Treatment
Types
Frequency Organism
Type
Power Treatment
Duration
Application Comments
Sparker 100 Hz –
150 kHz
Zebra mus-
sels
5.5 kV, 968
J/pulse, 0.16
– 5.8 J/m2,
0.75 Hz
pulses rate
2 months Steel pipeline
115 m length.
0.76 m I.D.
0.04 MPa and 0.16 J/m2inhibit settle-
ment. 0.23 MPa and 5.8 J/m2adult
mussel mortality. 1.5 m from source
: mortality, 23 m from source: inhibi-
tion. [61] (2010)
Sparker Zebra mus-
sels
5 kV 3 months PVC pipes 4
m, 30 cm I.D.,
4.5 mm wall
thickness
53.7% of adult mortality after 5 weeks.
Estimated 9.3 weeks for 100% mortal-
ity. [46] (2000)
Sparker 10Hz-
100kHz
Micro- foul-
ing
17 kV dis-
charge
4 weeks Outside 5/8
inch titanium
pipe of 20ft.
Sea water.
95% inhibition of Microfoulers.
15 ft from source aected.
Removal and prevention achieved.
Flow rate: 1.8 ft/s. [8] (2003)
Sparker 10 kHz – 1
MHz
Slime 5 – 10 kV
discharge
4 weeks 5/8” titanium
pipe.
On pipe: Slime inhibition achieved.
Close to pipe: no inhibition achieved.
Flow Rate: 2 ft/s. [9] (1999)
Sparker Algae, bacte-
ria
12 – 15 kV, 4
W/ft2.
10 hour /day
for 10 days.
Centre of a 2
inch PVC pipe
6 – 8 inch close to source fully cleared.
10 feet aected by the source. Flow
rate: 0.5 ft/s. Prevention achieved. Re-
moval not achieved. Fouling occurs,
but slowly. [74] (2000)
at 0.0975 W/cm3can eectively pulverize barnacle larvae
within 45 seconds [65].
Holm et al. investigated the power levels and appli-
cation times required for 19 kHz ultrasound to produce
mortality of bacteria, phytoplankton (dinoagellate, di-
atom, cyanobacterium) and zooplankton (brine shrimp,
cladocerans, rotifers) for the application of ballast water
treatment [36]. They found that the ultrasonic treatment
eciency varied with the size of the test organism. Zoo-
plankton required only 39 s of 619 J/mL energy to cause
90% reduction in survival. In contrast, the smaller bac-
teria and phytoplankton required from 1 to 22 minutes at
31 to 1240 J/mL to achieve similar results. They sug-
gested that this eciency correlation with organism size
might be related to the fact that the occurrence of cavita-
tion microjet formation is dependant on the frequency of
sonication and the size of the particle. Particles smaller
than the size of the collapsing bubble cannot cause mi-
crojet formation. They concluded that a stand-alone ul-
trasonic treatment system for ballast water, operating at
19 – 20 kHz, may be eective for planktonic organisms
larger than 100 µm in size, but smaller planktonic organ-
isms such as phytoplankton and bacteria would require
treatment by an additional or alternative system. Refer to
Table 5 for more details on these studies.
These ultrasonic studies used high power levels and it is
likely that cavitation may have been a main cause of mor-
tality of the biofouling. However, this mechanism appears
to be unsuitable for the application of preventing biofoul-
ing forming on ships hulls where the sub-cavitation power
levels would be expected to be used, due to power limita-
tions and potential impact on marine life. Caution should,
therefore, be taken when comparing parameters used in
these studies. The same applies to the study performed
by Mazue et al. who developed a device for cleaning the
exterior of boat hulls in dock using high power ultrasonic
cavitation operating at 20 kHz [47].
A number of laboratory studies have investigated the
use of ultrasound for purifying water. Some of the organ-
isms investigated in water treatment studies, such as algae
and fungi, are also biofouling organisms, and hence some
of these studies have been included in this review. How-
ever, as in the previous section, high power ultrasound
is used in these studies, and it is likely that the mecha-
nism causing the organism mortality is related to cavita-
tion. Hence any comparison with vessel hull antifouling
applications should be treated with caution.
Lee et al. reported that 28 kHz was more eective in de-
8
Table 5: Example information from literature on acoustic treatment of organisms suspended in water. In this table, the citations have been sorted
by transmission frequency. Where more than one frequency was used, reported optimal frequency has been used for sorting.
Treatment
Types
Frequency Organism
Type
Power Treatment
Duration
Application Comments
H2O2and
ozone
Ultrasonic bath
1.4 kHz Algae and
shrimp (lar-
vae, cysts,
adults)
2 - 20 min Water treat-
ment
Combined treatments at 2 min yielded
mortality: Algae: 100%, Shrimp (lar-
vae, adult) (100%, 95%) Sonication
alone at 20 min yielded mortality at:
Algae: 35%, Shrimp (cysts nauplii,
adults) (55%, 100%, 85%). [22]
(2007)
Transducer 19-20 kHz Bacteria,
phytoplank-
ton and
zooplankton
6–1240
J/mL.
3 sec – 22
min
Laboratory Zooplankton (brine shrimp, Cladocer-
ans, rotifers) required 39 s of 619 J/mL
for 90% reduction in survival. Bac-
teria and phytoplankton (dinoagellate,
diatom, cyanobacterium) ranged from
1-22 min of 31-1240 J/mL. Concluded
19-20 kHz eective for planktonic or-
ganisms >100 µm. [36] (2008)
Transducer 19.5, 28.0,
50 kHz
Barnacle 240 W, 1.3
W/cm2. 0 –
8000 kPa.s.
5 – 90 s Laboratory 19.5 kHz optimal frequency. 4300 kP.s
gave 50% larvae mortality; 140 kPa.s
gave 50% larvae settlement inhibition.
Cyprids had greater mortality rates.
[41] (1995)
Transducer 20 kHz Barnacle 0.0975
W/cm3.
30, 50, 80.
110 W
45 s Laboratory Pulverization. Higher power reduced
required exposure duration for pulver-
ization. [65] (2010)
Ultrasonic bath 26 kHz Bacteria,
fungi, virus
1.1 – 3
W/cm2
30 min Water treat-
ment
P. Aeruginosa up to 80% killed.
B. Subtilis up to 75% killed.
S. Aureus up to 45% killed.
Ultrasound can kill fungi. [63] (1991)
Ultrasonic reac-
tor
28 , 21.5,
39.5, 84.4
kHz
Cyano- bac-
teria, algae
40, 120, 1200
W
30 min Water treat-
ment
28 kHz optimal frequency, 120 W, 3s,
80% algae settlement. [44] (2001)
Probe 20, 150,
410, 1007
kHz
Algae 30, 60, 90 W 20 min Water treat-
ment
Optimal frequency 150kHz at 30 W,
70% removal rate. [45] (2005)
Ultrasonic bath 20, 40, 580,
864, 1146
kHz
Algae 0.0015 –
0.0714
W/cm3.
Up to 30 min Water treat-
ment
Most algae reduction 21.05% at
864kHz at 0.0049 W/cm3over 30
min.
Most ecient algae reduction at 580
kHz. [38] (2010)
Homemade
multi-frequency
cell system
20, 80, 1320
kHz
Algae 32 and 80 W 5 min Water treat-
ment
1320 kHz was optimal. Algae eec-
tively removed by sonication. Gas
vesicle collapse. [83] (2006)
Piezoelectric
transducer
1 MHz Algae 3 W/cm2.15 min Water treat-
ment
30% cell destruction
Related with the cavitation generation.
[23] (1976)
PZT transducer 20 kHz and
1.7 MHz
Cyano-
bacteria
algae
14 and 70 W 5 min Water treat-
ment
1.7 MHz was optimal. Algae reduced
by 63% after 5 min ultrasonic irradia-
tion. [34] (2004)
9
creasing algae photosynthetic activity than 100 kHz [44].
In contrast, Joyce et al. investigated declumping and in-
activation of algae in water at ultrasonic frequencies of
20 and 40, 580, 864 and 1146 kHz [38]. They found
580 kHz to be the most ecient frequency for algae re-
duction. Low ultrasonic frequencies initially inactivated
algae cells, but also broke apart clumps of algae result-
ing in an increase in individual algae cell count. Ma et
al. reported that algae removal rate increased with power
[45]. Zhang et al. hypothesised that the main cause of al-
gae cell removal was the loss of buoyancy resulting from
ultrasound causing the collapse of the gas vesicles in the
algae [83]. Hao et al. stated that 1.7 MHz was more ef-
ficient than 20 kHz for causing algae settlement in water
[34]. They also associated this with collapsing of gas vesi-
cles in the algae and suggested the higher frequency might
be more ecient since it was closer to the resonance fre-
quency of a free bubble in water, which they calculated to
be in the order of 6.5 MHz. Purcell et al. investigated the
use of ultrasound on dierent types of algae and described
the optimal frequency to be species specific [58]. Refer
to Broekman et al. [10] for a brief review listing several
studies investigating the eect of using low and high ul-
trasonic frequencies, including a combination of both, for
water treatment. Table 5 provide a brief summary of some
literature using ultrasonic for algae and fungi control.
5. Potential Environmental Consideration of Acoustic
Antifouling Systems
Biofouling is a significant economic and environmen-
tal problem to the shipping industry. Acoustic antifouling
systems appear to have environmental advantages in terms
of not producing, for example, toxic leachates. However,
if these systems are inecient and do not control biofoul-
ing as eciently as a toxic paint, for example, the environ-
mental costs associated with increased fuel use or trans-
portation of non-indigenous species might be greater than
those associated with the biocides. Another environmen-
tal consideration is the potential impact of introducing
acoustic energy into the marine environment. This acous-
tic energy represents anthropogenic (man-made) noise,
which has the potential to have a negative impact on ma-
rine life.
5.1. Eects of Noise on Marine Life
Sound in the oceans can be divided into ambient or an-
thropogenic noise. Breaking waves, precipitation, and
marine life sounds are examples of ambient noise in
the marine environment. Shipping, sonar, seismic sur-
veying, pile driving, and dredging are examples of an-
thropogenic generated noise in the marine environment
[25, 76, 77, 81]. Refer to Table 6 for the frequency ranges
and sound pressure levels for several examples of under-
water sound.
Table 6: Examples of ambient and anthropogenic sound frequency and
sound pressure levels (SPL) in the marine environment. Data is taken
from references [25, 73, 76, 81].
Sound Source Frequency SPL
[kHz] [dB re. 1 µPa]
Shrimp snapping 0.7 – 30 183 – 189
Humpback whale song 0.03 – 8 144 – 186
Sperm whale click 0.1 – 30 160 – 180
Bottlenosed dolphin whistles 0.8 – 24 125 – 173
Precipitation 0.1 – 20 35
Seismic survey air gun 0.01 – 0.120 260 – 262
Military mid-frequency sonar 1 – 10 223 – 256 peak
Echosounder 1.5 – 36 235 peak
Large ship 0.05 – 0.5 180 – 190 rms
Dredging 0.1 – 0.5 168 – 186 rms
Sound appears to be a means for marine life to commu-
nicate, navigate, and detect other life [53, 59, 78]. Recent
studies have shown that low frequency noise from ves-
sel generators may be a settlement queue for biofouling
species, see Section 4.1.4 for more details. Over the last
few decades there has been increasing concern about the
eect of anthropogenic noise on marine life. If the noise
is within the hearing range of a life form, masking may
occur. The life form may be unable to detect, interpret, or
respond to biologically relevant sounds in the same fre-
quency range as the introduced sound [76]. Behavioural
eects may occur, such as the life form moving from its
current site; a possible feeding or breeding ground. If the
sound level is intense enough, there may be physical dam-
age to auditory or non-auditory tissue. Depending on the
extent of auditory damage, temporary or even permanent
hearing loss may occur. Death may occur in extreme cases
of body tissue damage [53, 67, 76].
Hearing thresholds have been measured for perhaps
only 100 of the many thousands of living fish species. The
majority of the fish species studied cannot hear sounds
10
Table 7: Results from studies on physical eect of sound sources on fish.
LFA refers to Low-Frequency Active, MFA refers to Mid-Frequency Ac-
tive, and TTS refers to Temporary Threshold Shift. The SPL dB values
are relative to 1 µPa.
Sound
Type
SPL Max
Freq.
Eect
[dB] [kHz]
Impulsive 203.6
rms
0.02 – 0.1 Snapper – permanent hearing
damage. (Air gun noise) [48]
(2003)
Non-
impulsive
193
rms
0.17 –
0.31
TTS for one set of rainbow
trout in one of two groups
of fish. No mortality. (LFA
sonar) [57] (2007)
Non-
impulsive
210 2.8 – 3.8 Rainbow trout - no eect
(outside their hearing range).
Catfish - TTS (24hr) dura-
tion. No fishmortality. (MFA
sonar) [33] (2012)
Non-
impulsive
115,
130,
150
2 20 Rainbow trout - hearing
sensitivity, growth, survival,
stress, and disease suscep-
tibility were not negatively
impacted. (Aquaculture
production noise) [80]
(2007)
above about 3-4 kHz and most of these can only hear up
to 1 kHz [53]. In contrast, cetaceans (whales, dolphins)
can detect sounds up to 22, 160, or 180 kHz depending on
the species. Pinnipeds (seals, etc.) can hear sounds in wa-
ter up to about 75 kHz [67]. Refer to reference [53] and
[40] for example graphs of fish, cetacean, and pinniped
hearing threshold levels in water. There have been stud-
ies investigating the physical eect of sound on fish, refer
to Table 7. For marine mammals, it has been suggested
that hearing losses may occur when noise is 80 dB above
the animal’s hearing threshold [39], and permanent tissue
damage for cetaceans and pinnipeds may respectively oc-
cur at sound pressure levels of 230 and 218 dB re. 1 µPa
over a 24 hour period [67]. However, it seems that there
is currently insucient data to provide accurate hazardous
exposure levels for marine mammals [76].
5.2. Acoustic Antifouling Marine Life Considerations
Antifouling systems that use audio frequencies may
have the potential to have negative impact to marine life
since they are in the frequency range of most marine
species. Acoustic biofouling methods that use sparkers
may generate audio frequency signals. These signal are
impulsive and have a broad bandwidth. Impulsive, broad
bandwidth noise may be more likely to cause auditory
tissue damage than continuous narrow bandwidth noise
[76]. However, since these sparkers are usually used in-
side pipes, this may have minimal undesirable eects, es-
pecially if the section of the pipe being treated for bio-
fouling is above water. Ultrasound frequencies should be
well above the hearing range of almost all fish and above
that of low frequency cetacean species [53, 67]. These
frequencies also have much higher attenuation rates com-
pared to audio frequencies and will, therefore, have sig-
nificantly reduced range of propagation through the wa-
ter. At moderate power levels, ultrasonic biofouling sys-
tems appear have less potential to eect marine life. More
study might be required to determine the safe operating
parameters for the use on exposed surfaces in the marine
environment, particularly in locations where mid to high
frequency cetaceans and pinnipeds frequent.
6. Discussion
Many of the reviewed trials were based on small-scale
lab studies looking at single species (predominantly bar-
nacles). Of the sea trials, most were not vessel-scale stud-
ies, but small vessels (yachts and skis) or small panels
placed in the sea. Most of the reviewed laboratory and
sea trial studies have reported successful biofouling inhi-
bition using acoustic techniques, mainly using low ultra-
sonic frequencies. A few recent studies have reported in-
creased levels of biofouling due to ship’s generator noise,
which is mainly composed of low audio frequency com-
ponents.
For high power ultrasound studies, cavitation appears
to be the main mechanism causing biofouling mortality
and inhibition. However, these high power levels do not
appear to be suitable for the application of biofouling in-
hibition on vessel hulls. For sub-cavitation power levels,
it is unclear what the mechanism is that can cause biofoul-
ing inhibition. The biofouling inhibition could be related
to the vibration itself or perhaps could be masking other
ambient or antropogenic noise that might attract biofoul-
ing organisms. More studies are needed to identity what
the mechanism is which causes acoustic techniques to in-
hibit or attract biofouling.
11
There has been some variation in the acoustic parame-
ters, which have been reported to be optimal for biofoul-
ing control, and limited information as to how ecient
the trials were at preventing fouling. There is also am-
biguity about the frequency dependant gain of the hard-
ware. The gain is the eciency of the hardware in con-
verting an electrical signal into acoustic vibration. This
eciency may vary with frequency. These factors make
comparisons and potential replication of these trials di-
cult. Variation in the antifouling eciency was reported
with the use of dierent ultrasonic frequencies. However,
since the relative gain of the acoustic antifouling hard-
ware (signal generator, power amplifier, and transducers)
at the frequencies used was generally not provided, it is
dicult to know if the antifouling eciency at a given
frequency was due to the gain of the hardware or because
this frequency was better at inhibiting biofouling forma-
tion. Similarly, often the acoustic energy used in a trial
was expressed in terms of the voltage or the power ap-
plied to the transducer. This is important information in
terms of assessing the economic viability of the technique.
However, the input electrical power used does not provide
information on how ecient the transducers were at con-
verting the electrical energy into vibration amplitude of
the substrate being protected from biofouling formation
or the sound pressure level of the water that the biofouling
was suspended in. It was also dicult to make compar-
isons with the studies on the eect of underwater noise on
marine life, which expressed the acoustic energy in terms
of sound pressure levels in dB relative to 1 µPa, either
at the source location, at a distance of a meter from the
source, or at the location of the marine organism.
The relative eectiveness of the acoustic antifouling tri-
als was also often dicult to evaluate. It was often said
that fouling “inhibition” had occurred or that the surfaces
were “relatively free” of fouling. However, without a
baseline such terms become ambiguous. Often no con-
trol was used and generally little quantitative information
was provided on the amount of accumulated biofouling
that occurred. Very little photographic documentation of
the biofouling trials was found.
Future studies would benefit from the development of
a methodology that would allow these studies to be con-
ducted in a way such that the fundamental operating pa-
rameters and performance characteristics are well defined.
This should allow multiple studies to be compared and
built upon, so that the design of an eective system could
be obtained. Choi et al. is an example of a study that doc-
umented the operating parameters and performance char-
acteristics in a more rigorous manner [14]. They investi-
gated the use of dierent low audio frequencies on mul-
tiple small panels with transducers attached in marine tri-
als. The materials and operational parameters were doc-
umented. Laser vibrometry was used to measure the ve-
locity of vibration of the panels in air as a function of
frequency. A non-vibrated control panel was used. The
dierent biofouling organisms attached were recorded.
Comparisons were made between the control and dierent
excitation frequencies and dierent panel velocities. The
number of barnacles that settled on the plates was doc-
umented and the fact that other species were unaected
by the acoustic treatment was stated. Photographic docu-
mentation of the results was included.
Future studies would benefit from the use of similar
methodology. In addition to the above methodology, it
would be beneficial to use a calibrated hydrophone to
measure the sound pressure level (dB re. 1 µPa) of the
water at a set distance from the source. This would pro-
vide an indication of the vibrational amplitude in the wa-
ter as a function of frequency. Measurements made at dif-
ferent times during the trial would enable the stability of
the acoustic excitation hardware to be determined. This
may be particularly important if resonant transducers are
used where a drift in operating frequency or the resonant
frequency of the transducer could cause the resulting vi-
brational amplitude to change during the trial period. It
also would enable comparisons to be made with marine
noise studies to determine the safe operating parameters
for marine life.
7. Conclusions
Biofouling has a significant economic cost for industry,
particularly the shipping industry. Biofouling results in
extra fuel and maintenance costs, loss in operation time,
and production of toxic waste associated with address-
ing biofouling problems. Acoustic antifouling methods
may provide a non-toxic alternative for biofouling preven-
tion (see above). This paper presented a literature review
on acoustic antifouling techniques. The potential impact
on marine life of dierent acoustic biofouling prevention
methods was also considered.
12
There has been a range of studies investigating the use
of acoustic techniques for the application of preventing
biofouling formation on vessel hulls. Ultrasound has been
reported to be eective at inhibiting the formation of bio-
fouling on surfaces suspended in water. However, quan-
tification of how successful this has been has been limited.
The majority of the studies have used frequencies from 17
to 30 kHz. There have been some reports that the lower ul-
trasonic frequencies such as 19 kHz may be more eective
for barnacle inhibition than higher ultrasonic frequencies
or audio frequencies. There have also been studies, which
have reported audio frequencies to cause some biofoul-
ing inhibition. However, other recent studies may indicate
that audio frequencies potentially up to 2 kHz should be
avoided since these may replicate ambient noise in natu-
ral settlement areas, such as reefs, and be a settlement cue
for some biofouling species. Also, audio frequencies are
in the hearing range of most marine life and, therefore,
may have the potential to have negative results. Although
more research needs to be performed, the reviewed liter-
ature indicates that lower ultrasonic frequencies may be
optimal for biofouling inhibition on vessel hulls.
Studies using acoustic techniques for the control of bio-
fouling in pipes were also reviewed. Several of these stud-
ies use ultrasound, but the majority use sparkers. Fouling
inhibition has been reported for distances up to 23 meters.
None of the reviewed sparker studies related to the ap-
plication of preventing fouling forming on a vessel’s hull
and no indication was found that it would be suitable for
this application. A range of studies were also reviewed
involving the use of acoustics to control biofouling organ-
isms suspended in water. In ballast water studies, low fre-
quency (i.e. 19 kHz) ultrasound cavitation was reported to
be more ecient at inducing mortality of larger biofoul-
ing zooplankton such as brine shrimp compared to smaller
organisms such as bacteria and phytoplankton. Several
studies on water treatment found higher ultrasonic fre-
quencies (several hundred kHz) to be more ecient than
lower ultrasonic frequencies at inducing mortality of al-
gae. It should be noted, however, that the application of
killing biofouling suspended in water is dierent from that
of trying to prevent the initial formation of biofouling on
a vessel hull where much lower power levels are likely to
be used.
Much of the reviewed studies have been small-scale lab
studies looking at single species. More photographically
documented trials should be performed to determine the
optimal and ecient operating parameters and practical
circumstances of use of an acoustic biofouling system that
would cover a wide range of fouling organisms, and also
study its safe use in the marine environment. Future stud-
ies would benefit from the development of a methodol-
ogy that would allow these studies to be conducted in a
way such that the fundamental operating parameters and
performance characteristics are well defined. This should
allow multiple studies to be compared and built upon, so
that the design of an eective system could be obtained.
Trials should be more scale-appropriate (i.e. vessel scale).
They should take into account factors such as variations
in hull form and shape, disruptive frequency sources (e.g.
vessel machinery) and seasonal eects (e.g. varying lev-
els of propagule pressure and species abundances related
to spawning periods).
8. ACKNOWLEDGEMENTS
The research leading to these results has received funding from the
European Union’s Seventh Framework Programme managed by REA-
Research Executive Agency http://ec.europa.eu/research/
rea([FP7/2007-2013]) for the project entitled “Prevention and de-
tection of fouling on ship hulls” CLEANSHIP, under grant agreement
no [312706], FP7-SME-2012-1 (http://cleanship-project.eu/).
CLEANSHIP is collaboration between the following organisations:
Brunel University, CERTH, Tecnalia, Enkon, Sofchem, WRS Marine,
InnotecUK, and Lloyds’s Register.
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Book
Due to the restrictions on TBT usage in antifouling paints since 2003 and its complete ban on all vessels in 2008 (IMO 2001), copper has been increasingly used as the main biocide ingredient in antifouling paint coatings. Copper is toxic to a wide range of aquatic organisms, which makes it an ideal biocide, preventing the colonisation of biofouling organisms on the vessel surface. There has been much concern from regulators and scientists that copper concentrations may become elevated in areas of high boating density such as marinas and estuaries with potential damaging effects on the animal and plant communities. In certain European countries, copper has been banned from use on recreational vessels, although so far this is restricted to inland freshwaters, many countries are beginning to re-evaluate current copper risk assessments in marine coastal waters. This chapter provides an outline of the concentrations of copper in the marine coastal environment as a result of its use as an antifouling biocide. The potential risk of copper to marine life has been evaluated with respect to copper bioavail ability, speciation and toxicity. The chapter outlines some of the shortfalls of current copper risk assessment and provides some suggestions for improvement