United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-84-062 Aug. 1984
&EPA Project Summary
V
Feasibility of Ultrasonic and
Other Methods for Direct
Measurement of Condenser
Biofouling
C. Richard Reeves, Wayne S. Seames, and Steve L. Winton
This project involved a literature
review and laboratory studies of the
potential of ultrasonic and other methods
for in-situ measurement of biofouling
on heat transfer surfaces (e.g., tubes) of
electric utility steam condensers.
Detection of the presence of biofouling
in steam condensers is important for
maintaining maximum heat transfer
efficiency and minimizing the addition
of chlorine (used to control biofouling)
to meet discharge regulations. Literature
relating to current industrial practices
and research underway was searched to
develop indirect and in-situ methods of
biofouling measurement. Most methods
are not sensitive enough to detect
biofouling in its early stages, when it is
easiest to control. A preliminary assess-
ment indicated that this shortcoming
might be avoided, using ultrasonics. An
evaluation of the sensitivity of ultrasonic
methods for this application confirmed
the possible feasibility of this approach,
but a number of questions were raised
because of the lack of testing with the
specific equipment needed, as well as
the lack of acoustic property data on
biofouling material. Samples of biofoul-
ing material, obtained from an operating
commercial condenser, were "grown"
in the laboratory. The acoustic properties
of the material were measured as being
close to those of water, but sufficiently
different than biofouling measurement
via ultrasonics might be feasible with
further equipment improvements.
This Project Summary was developed
by EPA's Industrial Environmental
Research Laboratory. Research Triangle
Park. NC. to announce key findings of
the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Steam/electric power plants utilize
steam/water condensers to remove heat
from and condense low pressure steam
following expansion in a steam turbine.
The cooling-water side of the condenser
can be operated in either a once-
through or recirculating mode. In either
case, a large quantity of cooling water is
pumped through the condensers. Cooling
waters may contain various materials that
can form inorganic and/or organic
deposits (referred to as "fouling") on
condenser tube walls and water boxes.
There are four general types of fouling: (1)
scale formation resulting from precipita-
tion of inorganic salts (e.g., calcium
carbonate, calcium sulfate, silicates); (2)
corrosion when insulating layers of metal
oxides are formed on tubes; (3) adherence
of paniculate matter (e.g., silt) on tube
surfaces; and (4) development of biological
growth (biofouling) on tube surfaces.
The accumulation of biofouling accele-
rates corrosion of metal surfaces and/or
impairs heat transfer from the condensing
steam to the cooling water flowing
through the condenser tubes. The inability
to remove heat from the condensing
steam results in higher condensing
steam temperature and a correspondingly
higher turbine back-pressure. Higher
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turbine backpressure limits the steam
flow through the turbine and directly
results in a loss of generating capacity.
Cooling water systems, especially in
the heat transfer tubes and inlet, outlet,
and turning heads of the condenser,
provide microbial organisms with a
warm, oxygenated, nutrient-rich environ-
ment in which to grow. When this growth
is uncontrolled, severe plugging of the
tubes occurs, reducing heat transfer. If
this situation persists, a generating unit
may have to reduce load or be shut down
to clean the condenser, resulting in a
costly reduction in generating unit avail-
ability and reliability.
To protect the condenser from biofouling,
a number of methods are commonly used
to either retard biological growth or
mechanically remove it before it reaches
a critical condition. Mechanical cleaning
methods have been used at a number of
plants and have met with some success;
however, most plants use a method
involving the addition of chlorine into the
condenser cooling water inlet to control
biofouling. High concentrations of resid-
ual chlorine are toxic to biofouling and
other aquatic organisms. The proper
doses of total residual chlorine can
usually control biofouling within tolerable
limits.
To protect other aquatic life, the EPA
has limited the chlorination period and
the chlorine concentrations in the
discharge from an individual generating
unit. Regulations allow a free residual
chlorine concentration of 0.1 mg/L
(average) and a maximum instantaneous
concentration of 0.5 mg/L for one 2-
hour period per day. Proposed New
Source Performance Standards require
zero discharge of total residual chlorine,
except where it is demonstrated that
biofouling cannot be controlled under
these conditions. In these cases, a
maximum total residual chlorine concen-
tration of 0.14 mg/L is allowed and
limited to 2 hours per day per discharge
point. These regulations emphasize the
need to minimize the use of chlorine by
close control of chlorine additions and
enhancement of the effectiveness of
each application.
In addition to an accurate chlorine
metering system, two critical measure-
ments are needed to properly control
biofouling at these reduced chlorine
concentrations: (1) accurate measure-
ment of residual chlorine, both at the
point of addition and at the condenser
outlet; and (2) accurate measurement of
condenser performance relative to the
chlorine dosage and biofouling. While
some problems still exist with the long
term reliability of continuous chlorine
analyzers, accurate measurement of
condenser performance is a more signif-
icant problem.
Calculating condenser performance is
complex, requiring the input of data from
both the water and steam sides of the
condenser. The calculation can be
modified to allow calculating an overall
heat transfer coefficient and its compo-
nent resistances to heat flow (e.g., the
fouling factor). Many plants automatically
monitor and calculate fouling factor
continuously with process computers.
However, the accuracy of this method is
highly dependent on the reliability and
sensitivity of the monitoring instruments
and upon plant maintenance practices.
Changes in load, steam flow rate, cooling
water and ambient temperatures, and
many other factors vary the calculated
fouling factor. This variation makes
fouling factor data difficult to interpret
and masks many of the subtle changes
during the initial stages of the biofouling
process.
More direct and simpler approaches to
monitoring of fouling are now being
developed. More accurate and reliable
detection methods could result in im-
proved generating unit availability. These
methods could greatly enhance the
effectiveness of low-level chlorination as
well as aid in meeting discharge regula-
tions. This project evaluated the feasibility
of some of these methods, with emphasis
on ultrasonics. The project consisted of:
Task 1, a literature search of current
industrial practices used to detect
biofouling in condensers and to identify
potential methods that appear promising
based on research results; Task 2, a
determination of the sensitivity of
ultrasonics for detecting biofouling and
the feasibility of this approach; and Task
3, a laboratory study to determine the
acoustic characteristics of biofouling
materials.
Task 1—Literature Review
Biofouling detection methods evalu-
ated and compared under this task are
shown in Table 1, along with information
on the development status and sensitivity
of each. Condenser cooling water pres-
sure drop, turbine backpressure monitor-
ing, and overall heat transfer efficiency
are the most common methods used for
detecting condenser tube fouling. How-
ever, the complexity of the factors
involved in their computation or meas-
urement results in very low sensitivity to
the early growth phase of biofilm devel-
opment. These methods indicate only
severe fouling accumulations and are,
therefore, of limited value to a chlorine
minimization program.
Three detection methods that are
potentially applicable to the detection of
biofouling were found during the survey:
1. The Monirex Fouling Device—This is
used to detect nonbiological fouling
of petroleum fractions in various
refining processes by a heat transfer
measurement. Its application to
biofouling would require additional
development, and sensitivity would
be limited as in other heat transfer
techniques.
2. Standard Plate Counts—These are
made from bacterial colonies that
develop under accelerated growth
conditions on a media that has been
exposed to the cooling water, thereby
indicating the presence of biofouling
organisms. Because correlating the
development of bacterial colonies
with biofouling accumulations in
condenser tubes would be difficult,
its usefulness is limited to supporting
other techniques.
3. Chemical/Biological Monitoring
Techniques - These utilize total
organic carbon (TOC) and adenosme
triphosphate (ATP) as indicators of
biological growth. ATP, found only in
living cells, is released to the cooling
water rapidly upon death. Monitoring
of ATP and TOC at the condenser
cooling water inlet and outlet is a
way to estimate biomass accumula-
tion in the condenser. The specialized
analytical equipment required to
monitor these parameters and the
batch nature of the analysis make
this method of biofouling detection
impractical at this time.
The new methods currently under
active development are sidestream reac-
tors, heat transfer resistance methods,
and ultrasonics. Sidestream reactors
consist of tubular and annular reactors
that attempt to simulate heat exchanger
conditions and are limited by their ability
to duplicate these conditions. These
techniques are in the early develop-
mental stage and require supplemental
microscopic and manual biomass accum-
ulation analysis to determine biofouling
accumulation thickness.
Heat transfer resistance methods have
been developed for detection of biofouling
of heat exchangers in Ocean Thermal
Energy Conversion (OTEC) systems.
These devices require highly skilled
technicians to operate and interpret
measurements. These devices are also
sidestream simulators, and their useful-
ness is limited by their ability to duplicate
-------
condenser operating conditions. Their
sensitivity to the early growth phase of
biofilm development is not well defined,
but appears to be effective in measuring
thickness in the 35-50 /urn range.
Ultrasonics is currently the only tech-
nique for direct measurement of biofoul-
ing film thickness under consideration.
Its application to thickness measurement
of materials is used extensively. Its high
accuracy in metal thickness measure-
ment in the 50 //m range closely corre-
lates to the 35-50 /urn detection range of
other biofouling detection methods.
Previous studies of the acoustic properties
of biofouling have indicated that detection
of biofilms less than 50 /urn in thickness
may be feasible.
Task 2 — Sensitivity of
Ultrasonics
This task involved a search of available
literature for: (1) sensitivity of ultrasonic
techniques in the detection of biofilm; (2)
determination of the acoustic properties
of biofouling materials; and (3) correlation
of chlorine effectiveness, biofilm thick-
ness, and ultrasonic detection.
The goal of this task was to determine
from available literature the feasibility of
ultrasonic technology for the detection of
biofouling and the advisability of continu-
ing the program with laboratory studies.
The information was grouped into four
categories: thickness measurements and
general applications, biofilm thickness
measurement, acoustic properties of
biomass, and probability of industrial
application.
Present Measurements by
Ultrasonic Techniques
Thickness measurement of metals is
one of the most common applications of
ultrasonic technologies in industry. Both
resonance and pulse-echo methods of
detection are utilized to measure metal
thicknesses and detect flaws. The most
common applications of the resonance
method for thickness measurement are
in the 0.05-in. (1270 urn) to 0.175-in.
(4445-/um) range with an accuracy of
±1.0 percent. Instruments using the
pulse-echo method of detection are
commercially available which measure
thicknesses of 0.1 in. (2540 /um) to 10 in.
(2.5 x 10s /urn) inch with a ±10 percent
accuracy. Most of these resonance and
pulse-echo instruments use frequencies
in the 1-5 MHz range.
Measurement of thicknesses in the 50
/urn-range requires the use of ultrasonic
waves with frequencies in the 10-30 MHz
range. A metal thickness of 50 //m has
been measured with an accuracy of +1
percent by the resonance method. This is
the smallest metal thickness measure-
ment recorded in the literature surveyed.
Details of the specific equipment used to
make this measurement were not found,
but it appears that measurements in this
range are within the measurement limits
of ultrasonic technology.
Biofilm Measurement by
Ultrasonic Techniques
Only one research paper was found on
measuring biofilm thickness ultrason-
ically: it discussed the theoretical analy-
sis of the potential of resonance and
pulse-echo methods for biofilm thickness
measurement. It concludes that mea-
surement of biofilms in the 10-30-//m
thickness range is possible by pulse-echo
and resonance methods at frequencies in
the 20-30 MHz range. However, the
accuracy of measurements in this range
had not been verified by laboratory test-
ing.
Table 1 . Summary of Biofouling Detection Methods
Developmental Status
of Method for Bio-
Detection Method fouling Detection
Ultrasonics
Condenser Pressure Drop
Tubular Reactor
Annular Reactor
Heat Transfer
Resistance Methods
Overall Heat Transfer
Efficiency
Turbine Backpressure
Carnegie-Mellon
University Device
Lockheed Missile and
Space Company Device
Monirex Fouling Device
Standard Plate Counts
Chemical/Biological
Monitoring
R&D
Commercial
R&D
R&D
Commercial
Commercial
R&D
R&D
Potential
Potential
Potential
Sensitivity
to Biological
Growth Phase"
1, E
E, P
N/A
N/A
E.P
E, P
Ec
Ec
E.P
N/A
N/A
Accuracy
±1% @ 50 fjm
N/A"
N/A
N/A
+4%
N/A
+ 10%
and
±12%*
3 x 10~5
fhr-°F-ft2J/Btue
N/A
N/A
N/A
Comments
Direct measurement
Common indirect detection method
Sidestream simulator
Sidestream simulator
Common indirect detection method
Most common indirect detection
method
Developed for ocean thermal
energy conversion
biofouling applications
Developed for ocean thermal
energy conversion
biofouling detection
Developed for petroleum process
fouling detection
Commonly used in evaluating bio-
fouling control agent effectiveness;
used for detection in cooling tower
systems
Commonly used in evaluating bio-
fouling control agent effectiveness
a/ - Induction phase or primary biofilm formation (biofilm thickness 0 to 40-50 umj.
E - Exponential accumulation phase (biofilm thickness greater than 35-45 /jm).
P - Plateau or steady-state phase.
bData not available.
^Overlap of induction and exponential phases may allow these methods to detect growth during the induction phase.
aFrom two different sources.
'To convert to metric equivalents, please use the following: 1°F = 9/5(°C + 32); 1 ft2 = 0.093 m2; and 1 Btu = 1.055 kj
-------
Acoustic Properties of
Biofouling Materials
Many of the acoustic properties of a
microbial slime (biomass), obtained from
a condenser using Chesapeake Bay water
for cooling, have also been determined,
using an ultrasonic interferometer which
uses the pulse-echo technique. Table 2
summarizes acoustic and other physical
properties of biofouling materials. Of the
acoustic properties presented, acoustic
impedance is of great importance when
considering the use of the pulse-echo
method for biofouling detection. The
acoustic impedance value in Table 2 is in
the same range as other biological
tissues of comparable specific gravity, as
shown in Table 3. Biofouling materials,
like other biological tissues, are composed
of up to 98 percent water. The high water
content strongly influences the acoustic
impedance of the materials and tends to
make acoustic impedance vary over a
small range. This correlation also con-
firms the validity of the biomass imped-
ance experimental measurement tech-
nique.
The acoustic impedance of biofouling
materials is only slightly higher (<5
percent) than that of water. This small
impedance difference could make it diffi-
cult to distinguish a biofilm/water
interface from a metal/water interface
because of the weak wave reflection from
the interface. This is confirmed by the
relative amplitude values of the reflected
signals from the biofilm/metal and
biofilm/water interfaces shown in Table
2. The large difference in the amplitudes
of the two interfaces makes resolution of
the weaker signal difficult. The poorer the
resolution, the more difficult it becomes
to measure the time interval between the
reflections. Measurement of this time
interval is crucial to accurate thickness
measurement.
The initial ultrasonic pulse can be
separated from the reflections by increas-
ing the transducer metal thickness and,
therefore, adding a time delay. However,
the reflected signals cannot be separated
and will result in a superimposed and
distorted signal curve, as shown in Figure
1. To measure the thickness of the
biofilm, the time between the reflected
signal amplitude peaks, T (see Figure 1),
must be discernible.
Other acoustic properties important to
ultrasonic thickness measurements by
the pulse-echo method are the attenua-
tion coefficient and the speed of sound
through the biofouling material Attenua-
tion is the measure of the sound energy
dissipated as it passes through a material.
The attenuation coefficient of biofouling
materials at 10 MHz is 1.3 dB/cm as
compared to 0.3 dB/cm for water.
Unfortunately, this higher attenuation
adds to the problem of detecting the
Table2. Acoustic and Other Physical Properties of Biofouling Materials
Acoustic Impedance of Biofouling
Material
Attenuation Coefficient
Relative Sound Speed of Biofouling
Material (Ratio of the speed of sound in
biomass to speed of sound in water)
Relative Density (specific gravity)
of Biofouling Material
Relative Amplitudes of Ultrasonic Echoes from Metal/Slime and Slime/Liquid Interface for the
following Metals'
3% Higher than seawater @ JO MHz or
approximately 1.57 x 105 g/crrf-sec
1 3 dB/cm @ 10 MHz
1.011 @ 10 MHz
1.02
Metal
Metal/Biofilm Interface
Relative Amplitude
Biofilm/Liquid Inter-
face Relative Amplitude
Aluminum
Titanium
Stainless Steel
0.837
0.895
0.937
4.6 x 10'
3.1 x 10'
1.9 x 10'
Table3. Acoustic Impedance of Biological Materials and Water @ 10 MHz
Material
Acoustic Impedance
g/cm2-sec
Specific Gravity
Water
Biofouling
Blood
Brain Tissue
Kidney
Liver
1.52 x W5
1.57 x 105
1.62 x 105
1.55-1.66 x 105
1.62 x 10s
1.64-1 68 x 10s
1.00
1.02
1.06
1.03
1.04
1.06
-10
u--20-
-50-
-60
T = O.3 x W'7 seconds
Metal/Biofilm Interlace
Signal
_, MetalV Biofilm/ Water
Combined Signal
Biofilm/Water Interface
Signal
0 123456
Signal Duration, 10~7 seconds
Figure 1. Predicted distortion caused by
the overlap of the time duration
of the reflections from metal/
biofilm and biofilm/water inter-
faces (theoretical signal curves
shown with dashes; TIS the time
lapse between interface reflec-
tions that must be measured in
order to detect the biofilm).
biofilm/water interface. The higher the
attenuation coefficient, the weaker the
reflected signal from an interface will be.
The speed of sound is only slightly higher
in biofouling materials than in water and
will not significantly impact the measure-
ment of biofilm thickness.
The pulse-echo method of thickness
measurement is very dependent on the
acoustic impedance of metal and biofoul-
ing materials and the ultrasonic wave
length. The resonance method of thick-
ness measurement correlates biofilm
thickness with the shift in the tube metal
resonance frequency. Therefore, the
resonance method of biofouling is much
less dependent on the individual acoustic
properties than is the pulse-echo method.
Task 3 — Measurement of
Acoustic Properties of
Biofouling Material
To determine the feasibility of ultrason-
ic detection of biofouling, laboratory
determinations of the acoustic impedance
of biofouling material were undertaken.
Since the acoustic properties of biofoul-
ing material and water are very similar,
the test work performed in this task
represents a worst case condition for
ultrasonic detection of biofilms. The
complex biofouling encountered in indus-
trial condensers may be more easily
detected than pure biological material
-------
due to the acoustic properties of contami-
nants trapped in the biomass. The
investigations were designed to deter-
mine if a reflection from the biofouling/
water interface can be detected with
state-of-the-art ultrasonic equipment.
Sample Collection and
Biofouling Growth
To conduct the acoustic characteriza-
tion study on representative fresh water
biofouling materials, samples from an
operating power plant condenser were
obtained from Public Service Electric and
Gas Company's Mercer Generating
Station, which uses Delaware River
water for cooling. This station has been
the site of testing of pilot-scale conden-
sers for biofouling measurement and
control by the Electric Power Research
Institute (EPRI). The station experiences
severe biofouling during the summer
months.
The samples were collected by plant
personnel during a unit outage. About 2
qt (1.9 liters) of biofouling sample was
collected and frozen prior to shipment.
The samples were a conglomeration of
biological materials and silt. Since there
was not enough biological material in the
sample that could be isolated for the
laboratory study, sufficient biomass had
to be grown. To do this, 10 gal. (37.9
liters) of Delaware River water was also
shipped to Radian to ensure that the
laboratory growth and nutrient availabil-
ity were comparable to the power plant
cooling water.
A laboratory apparatus was set up to
grow and maintain a healthy population
of biofouling microorganisms. It consisted
of a 10-gal. (37.9 liter) aquarium tank, a
plate glass sheet, a circulating pump,
aluminum test coupons, and an immer-
sion heater. The glass sheet was placed
in the aquarium at about a 30°angle. Wa-
ter in the aquarium was pumped and
distributed uniformly over the surface of
the inclined glass plate to provide a
flowing water film on the glass plate. Test
coupons were attached to the surface of
the glass plate to provide the growth and
accumulation points for the biofouling
materials. An immersion heater was
placed in the aquarium tank to maintain
the water at 76°F (24°C), which is
comparable to the cooling water outlet
temperature at the Mercer Station. To
simulate the utility condenser environ-
ment, the entire apparatus was enclosed
to exclude light. This system was designed
to encourage the growth of the microor-
ganisms that thrive under the condenser
temperature and nutrient conditions.
To compare acoustic properties of
biofouling material grown in the tank and
pure biofouling (slime) material, several
petri dishes containing an agar substrate
were innoculated with water from the
growth tank. Relatively pure cultures of
biofouling microorganisms were grown
in this manner. During the acoustic
characterization studies, both the pure
microorganisms grown in petri dishes and
tank-grown biofouling materials, which
used an agglomeration of living materials
with traces of silt, were evaluated.
The time required to accumulate
sufficient biofouling materials (approx.
100 ml) for the acoustic characterization
studies was approximately 3 months.
Growth on the aluminum test coupons was
extremely slow and had to be abandoned.
These coupons were to fit directly into the
ultrasonic testing assembly. As a result,
samples were scraped from the inclined
glass plate and aquarium walls. The
attachment of microorganisms to a tube
surface may have a significant impact on
the detection of a biofilm, but has
negligible impact on the acoustic imped-
ance measurements of this task.
Three samples were used in acoustic
characterization: (1) a water/biofouling
mixture from the growth tank (about 50
wt percent water), (2) a filtered biofouling
sample from the growth tank, and (3) a
pure bacterial sample from the petri dish
cultures. These three samples provided a
wide range of sample characteristics for
which acoustic properties could be
determined.
Summary of Results
Three test fixtures were used to
measure the relative sound speeds in
three samples of biofouling material (1) a
water/biomass mixture (approx. 50
percent free water) grown in a tank; (2)
dewatered biomass similar to the first
sample, except that most of the water
was removed by filtering; and (3) cultured
bacteria grown in a petri dish.
Each sample was tested in a different
test fixture. Although the fixtures were all
nominally of the same dimensions, they
were not identical: the slight differences
caused easily measurable differences in
the pulse transmit times. Consequently,
the transit times between different test
fixtures are not directly comparable.
However, the relative transit time of each
sample in a given test fixture with respect
to water in that same test fixture is a valid
comparison of the samples. Each fixture
was filled with water and tested to obtain
the reference for each sample.
The test results are summarized in
Table 4, which shows the measured delay
times (At) between the first and fifth
received pulses for each of the three
samples and for water in the test fixtures.
The temperatures at which the measure-
ments were made are also recorded
because sound speed in water is sensitive
to temperature. The uncompensated time
delay ratios and sound speed ratios are
computed first. A temperature correction
for the sound speed ratio is then computed
on the basis of 0.0016/°C temperature
difference between the water and bio-
fouling samples. This correction is
derived from the sonic velocity correction
for water at room temperature of 2.4
m/sec-°C divided by the sonic velocity of
1500 m/sec. Since the temperature of
the water and biofouling samples were
very close during the testing, this temper-
ature correction factor has little effect on
the sound speed ratio, as shown in the
last column.
The computed results are that the
sound speeds in the first two samples are
99.98 percent of the sound speed in
water. Since the experimental accuracy
is no better than 0.02 percent, it may be
concluded that the sound speed in these
two samples is indistinguishable from the
speed of sound in water. However, the
third sample (consisting of cultured
bacteria) had a sound speed 2.4 percent
greater than the sound speed in water.
This is much larger than the experimental
uncertainty, so the measured sound
speed difference is significant.
The acoustic impedance of the biofoul-
ing material is the product of its sound
speed and density. Similarly, the relative
acoustic impedance of the biofouling
material with respect to water is the
product of the relative sound speed and
relative density. This relative acoustic
impedance is important because it
determines the detectability of the
biofouling layer in contact with water by
ultrasonic inspection techniques.
The relative sound speed of the
biofouling material was measured to be
in the range of 1.0 to 1.024, depending on
the sample composition and preparation.
The density was measured using a
picnometer to be in the range of 1.0 for
water/biomass to 1.04 for dewatered
biomass. There was insufficient cultured
bacteria for a density determination. The
relative acoustic impedance of the
material is the product of the relative
density (specific gravity) and relative
sound speed.
The relative acoustic impedances
determined for each sample are presented
in Table 5.
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Table 4. Sound Speed Measurement Results
Sample
Water and
Biomass
Dewatered
Biomass
Cultured
Bacteria
A? Sample
V sec
5.55 @
24. 5° C
5.66 @
23.5°C
5.56 @
25.0°C
A? Water"
fj sec
5.54 @
25.5°C
5.65 @
24.5°C
5.70@
24. 7° C
Af Ratio
Sample/Water*
1.002
1.002
1.0975
Sound
Speed Ratio
Sample/Water*
0.998
0.998
1.025
Temperature
Correction
for Sound
Speed Ratio
0.0016
0.0016
-0.0005
Temperature
Corrected
Sound
Speed Ratio
0.999
0.999
1.024
a Af is the measured time dela y between the first and fifth pulses. Three different test fixtures were used to test the three bio fouling samples. Each water
Af at value is for the test fixture used with the sample in the same row of the table.
b/Vo temperature correction factor applied.
The results compare favorably with
those of another study that determined
an acoustic impedance of biomass
relative to sea water of 1.03.
Detection of Biofouling by
Ultrasonic Techniques
Ultrasonic techniques under consider-
ation for the detection of biofouling inside
a water-filled metal tube depend on the
biofouling material's having a different
acoustic impedance than water. The
interface between the biofouling material
and the water must present a distinct
change in impedance to the acoustic
signal to make this interface reflective.
A pulse-echo technique for detecting
biofouling is shown in Figure 2. A short
pulse is transmitted by a transducer into
the metal pipe wall. If biofouling is
present, as shown in the figure, the pulse
is reflected by the pipe/biofilm interface
(rpb) and by the biofilm/water interface
(rbw). The biofilm is detected and its
thickness measured by observing the
reflections of the pulse from the pipe
wall/water interface.
The relative amounts of energy, reflected
(r) and at an interface transmitted (t) past it,
are defined by the power coefficients (°c)
as follows:
(D
(2)
TableS. Relative Acoustic Impedance of Biofouling Materials
Sample
Water Biomass
Dewatered Biomass
Cultured Bacteria
Specific
Gravity
1.00
1.04
1.00"
Relative
Sound
Speed
0.999
0.999
1.024
Relative
Acoustic
Impedance
1.00
1.04
1.02
"Assumed to be the same as for water.
where p is the density of the material; c is
the speed of sound through the material;
their product, pc, is the acoustic imped-
ance of the material; and p (pipe), b
(biofilm), and w (water) indicate the
materials at the interface.
The flexibility of detecting biofilms
by the pulse-echo technique is determined
by the strengths of the rpb and rbw
reflections and the time delay between
them. The strengths of the reflections
are calculated by first calculating the
power.reflection and transmission coeffi-
cients at the interfaces using Equations
(1) - (4). An example is presented below
using the dewatered biomass from Table
5 and the properties of a stainless steel
pipe (tubing).
oc _ 1.04x1.5x105-4.7x10e2
rpb = 0 88
1.04 x 1.5x 105 + 4.7x106
g_1.5x108- 1.04x1.5x10°2
1.5x 10s + 1.04 x 1.5 x 105
= 3.8x10-"
tbw = 1 - °crbw = 0.9996
These coefficients indicate that at the
pipe/biofilm interface, 88 percent of the
pulse energy is reflected and 12 percent
is transmitted across the interface. Atthe
Transmitting
Transducer
Metal Delay
Pad
Water
Ipc = 1.5 x JO5}
Receiving
Transducer
Metal Pipe
Wall (Stainless Steel,
(pc = 4.7x 10*)
Biofilm Layer
fpc= 1.O4 x 1.5 x 10s)
i - ultrasonic energy pulse
/•pi, - reflection from pipe/biofilm interface
/"bw - reflection from biofilm/water interface
pc - acoustic impedance of the material
Figure 2. Detection of biofouling by reflec-
tion from biofilm layer interfaces
(pulse-echo method).
biofilm/water interface, only 0.004
percent of the energy is reflected.
By tracing the paths of the rpb and rbw
reflected signals in Figure 2, the received
signal levels can be calculated. Assuming
the incident signal (i in Figure 2) to have a
unity signal level in the metal pipe wall,
the rpb and rbw signal levels are:
Srpb = 0.88
Srbw = tpb (°crbw) (°ctpb) =
0.12 (0.0004) (0.12) = 5.5 x 10~6
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The srpb/srbw power ratio is 1.6 x 105 or
52 dB, where dB = 10 log (power ratio).
The times of arrival at the receiving
transducer of signals rpb and rbw differ by
the flight time through the biofilm layer,
r, which is determined by:
T - 2x/pc
r = 2x/(1.04)(1.5x 105)
where x is the thickness of the biofilm
layer in centimeters. For example, if x =
0.005 cm (50 A
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