# Biofouling Control in Cooling Water

Authors: T. Reg Bott
Journal: International Journal of Chemical Engineering
 (2009)

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## Abstract

An important aspect of environmental engineering is the control of greenhouse gas emissions. Fossil fuel-fired power stations, for instance, represent a substantial contribution to this problem. Unless suitable steps are taken the accumulation of microbial deposits (biofouling) on the cooling water side of the steam condensers can reduce their efficiency and in consequence, the overall efficiency of power production, with an attendant increase in fuel consumption and henceCO2 production. Biofouling control, therefore, is extremely important and can be exercised by chemical or physical techniques or a combination of both. The paper gives some examples of the effectiveness of different approaches to biofouling control.


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## Body

## 1. Introduction

The importance of good energy management is becoming far more demanding
than in the past. The substantial increase in energy costs and the threat of
climate change due to greenhouse gas emissions make it imperative that all
steps are taken to reduce energy usage. Of particular importance is the
effectiveness of cooling water, specially in the steam condensers of fossil fuel-fired power stations, in maintaining electrical energy output per tonne of
fuel at its highest possible level.Despite efforts to provide an effective design of heat exchanger and
careful attention to the maintenance of the design operating conditions, it is
likely that fouling on the water side of the heat exchangers will occur unless
suitable precautions are taken. The common practice of taking water from
natural sources such as rivers and lakes for cooling purposes means that it
will contain microorganisms, which will colonise the heat transfer surfaces,
to the detriment of cooling efficiency. The problem will be aggravated by the
fact that the temperature of the waterside surface in the heat exchanger is
usually close to the optimum temperature for maximum microbial growth. In
addition water from natural sources will contain nutrients from the breakdown
of naturally occurring organic material. Unless this bioactivity is controlled
the efficiency of the heat exchanger will be seriously reduced. In fossil fuel-fired power stations this will require additional fuel to be burnt to maintain
the required electricity output with an attendant increase in the emission of
greenhouse gases.  There are basically
two methods of controlling this problem, chemical and physical, that might be
used separately or in some sort of combination. Three conferences held in
Portugal, Melo et al. [1], Melo et al. [2] and International Water
Association and Collaborators [3] did much to stimulate interest is these
techniques and their effective use. In addition conferences organised byEPI
have also covered aspects of the associated technology. The most recent
conference was held in  Tomar, Portugal in 2007, organised by Müller-Steinhagen et al.  [4].


## 2. Chemical Control

In essence chemical control generally involves the use of biocides to kill
the microorganisms in the cooling
water, or biostats to reduce their activity. For many years the
preferred biocide was chlorine because it was relatively cheap and available.
However its use is becoming severely restricted on account of its detrimental
effects on the quality of the water discharged back to the natural environment
from where it was abstracted and its effect on other living creatures,
including humans. In contact with organic material in the natural environment
chlorine can form chloromethanes that are carcinogenic. An alternative to the
discharge of chlorine-contaminated water is to dechlorinate before disposal.
The techniques for dechlorination are likely to add considerably to the
operating costs. As a result efforts are
being made to provide alternatives and to be economical in their application. A
wide range of chemicals is used as a basis for biocides. They may be classified
as oxidising or nonoxidising. Oxidising agents include chlorine and chlorine
yielding chemicals, ozone, and hydrogen peroxide. Amongst the nonoxidising compounds
are amines, heavy metal compounds, aldehydes, organo-bromine compounds, and
isothiazolones.The so-called “environmentally friendly” biocides are generally those that have
a relatively short life after application, breaking down to innocuous products.
Simple examples are ozone which reverts to oxygen on decomposition and hydrogen
peroxide that breaks down to water. Alternatives to these inorganic chemicals
usually involve complex organic compounds that readily breakdown after
application. It also has to be appreciated that sometimes these breakdown
products are nutrients for microorganisms!.Clearly the objective is to remove any biofilm as it forms. In order to be
economical and effective, the technology of dosing has to be carefully
considered. The addition of chemicals to cooling water may be made in three
ways.(a)
Continuous. The maintenance of
a fixed concentration in the circulating water; the dose depending on the
concentration and species of the microorganisms present.(b)
Shock. An intermittent dose
of relatively high concentration maybe only once in 24 hours.(c)
Pulse. Involves dosing on
a fixed schedule but more frequently than shock dosing maybe once per hour, for
example.The method of dosing will, in general, depend on the season and the quality
of the water involved. Careful choice of dosing regime based on trials,
preferably in a pilot plant, will help maximise control and minimise cost. It
could be beneficial to employ a side-stream test section on the cooling water
system to optimise the dosing regime and to make comparisons between
alternative biocides.Some bacteria can develop a potential tolerance to an applied biocide,
particularly biocides that affect cell membranes. A change in the dosing regime
or a change in the biocide may be effective in overcoming this drawback.
Biocides that actually destroy the cell structure are more likely to be the
most effective.In pilot plant studies [5] using Pseudomonas
fluorescens as the biofllm forming speciesin single tubes, bulk water
flow velocities of 0.5 and 1.3m/s, and a proprietary biocide, the results
suggested that pulse dosing, based on comprehensive preliminary testing, is
likely to be the most effective regime. This observation is not surprising
since pulse dosing is the closest to continuous biocide application.It is generally considered that the season of the year could affect the
accumulation of biofouling since microbial activity will be greatest in the
summer when the temperature of the water will be higher than in the other
seasons. Some recent work  [6] suggests however that it
would not be advantageous to modify according to the season, the dosing pattern
of 0.2mg/L chlorine on a 550MW power station using sea water as the cooling
medium.An alternative to the use of environmentally friendly biocides to meet
discharge regulations is to have “zero discharge,”  that is,  to reuse the cooling
water, with only “make up” water to replace operating losses. It would allow
less costly biocides to be used but nevertheless such a procedure may be costly
in maintaining an overall acceptable quality of the circulating water.


## 3. Physical Control

In theory at least,
it should be possible to control biofouling by passing the water through the
exchanger at high velocity to increase the removal forces acting on the
biofilm. Although this may be feasible,  it is not really practical because
of the high energy requirement to overcome the large pressure drop involved.
There are a number of other physical techniques that may be applied to cooling
water systems to reduce the incidence of biofouling, but it has to be said that
many are still in the development stage and it remains to be seen if they are
practical and economically attractive.


### 3.1. Circulation of Sponge Rubber
Balls

The
use of circulating sponge rubber balls with the cooling water through the
tubes of steam condensers has been practiced with success, for many years. The
balls, having a diameter slightly larger than the internal diameter of the
tubes, wipe away any biofilm that begins to form. Cleaning is a random process,
so that the concentration of balls in the system has to be sufficiently high so
that every tube receives balls frequently enough to ensure effective cleaning.
In addition there has to be a replacement policy since the balls lose their
effectiveness after a time due to reduced diameter through wear.


#### 3.2. The Use of Inserts

Inserts
in tubes, originally intended to boost heat transfer, by the destruction of the
laminar sublayer on the tube surface are also capable of reducing the
incidence of biofouling at modest water velocities. Two basic types are
available:
(i) static inserts such as
“Hitran” wire wound inserts that break up the laminar sublayer by creating
turbulence and associated removal forces at the heat transfer surface and
(ii)
inserts such as those developed by total, that oscillate in response to the
water flow, thereby removing the developing biofilm by abrasion.Both
types of device will increase the pressure drop and hence the operating costs,
although this may be offset to some extent by suitable design. Data [7] on the effect of “Hitran” inserts on the accumulation of biofilm
consisting of the slime forming species P. fluorescens demonstrate
that although there is some scatter in the data, it is evident that the
presence of the insert reduces the accumulation of biofilm. In one test, after
700 hours operation, the reduction of biofilm accumulation compared to an
equivalent test with no insert present was approximately 33% with a water flow
velocity of 0.86m/s  and approximately 64% with a water velocity of 1.27m/s.


#### 3.3. The Use of Ultrasound

Ultrasound has the property of disturbing liquids and the structure of
solids. It is employed, for instance, to remove hard mineral deposits from heat
exchange surfaces exposed to high temperature combustion gases in boiler
installations. It has been demonstrated experimentally [8] that
biofilm accumulation (P. fluorescens) on the inside of tubes
can be reduced by the modest use of ultrasound of 20 kHz. Depending on the
amplitude (20 or 40%), the length (time), and frequency of groups of bursts of
ultrasound, the reduction in biofilm accumulation varied between 20 and 93%.
The water flow velocity was 1 m/s. Some further detail is given in Table 1.Table 1
Control of biofilm formation with different ultrasound
treatments.
Amplitude of
ultrasound %Number of
30-second burstsFrequency
of bursts each dayReduction of biofilm growth
compared with no treatment %203120.0203665.5201840.3A major drawback in respect of the use of ultrasound for the control of
biofouling is the initial cost of the equipment and its inclusion in heat
exchanger design.


#### 3.4. Circulation of Polymer
Fibres

Some preliminary work [7] on the use of Aramid fibres to control
biofouling suggests that the technique could be useful. Again using P.
fluorescens as the biofouling bacterium, it was shown that a
concentration of fibres of 100 ppm with a water velocity of 1.6 m/s is
adequate for effective control of biofilm formation. It was noteworthy that if
fibre addition was stopped, a biofilm soon began to form. Considerably more
work on the technology will be required however, before it can be applied effectively
to industrial cooling water. A major difficulty that would have to be faced
would be the recovery of the fibres prior to the final discharge of the water.
Fibre removal might also be necessary before passing the water through a
cooling tower in a recirculation system, to avoid potential accumulation and
associated blockage problems. Some preliminary work suggests that the fibres do
not accelerate the wear on the associated pumps.


### 4. Combined Chemical and Physical Control

It is possible that a combination of chemical and physical techniques of
control could be more effective than either solely applied.


#### 4.1. Ozone and Ultrasound

Experimental work using a combination of ozone and ultrasound over a four-week period [9], again using P.
fluorescens as the biofilm former and a water velocity of 1 m/s demonstrated
that this was a more effective control than ozone alone. During the first week
of a four-week period, no treatment was applied to allow a biofilm to develop.
The biofilm thickness at the end of that first week ranged from 45–60 μm.  Table 2 presents
some of the data obtained during the subsequent three weeks, demonstrating that
the application of the ultrasound (20 kHz at 20% amplitude) for three minutes
each day made a considerable difference to the retention of biofilm. The ozone
concentration in the water during these tests was very low, since water
containing an ozone concentration of around 2.2–2.8 mg/L was pumped into the
system to be mixed with the bulk water for only three hours each day. The data
contained in Table 2 indicate that the application of ultrasound made a
considerable reduction in the accumulated biofilm.Table 2
Mean biofilm thickness over a period of 28 days with
different treatments.
WeekBiofilm thickness for different tubesμmOzone onlyOzone + ultrasound226, 18, 17, 189, 8343, 15, 24, 2011, 124111, 49, 60, 517, 5


#### 4.2. Propriety Biocide and Inserts

Limited results [10] using Hitran inserts in conjunction
with a propriety biocide have demonstrated that whereas a biocide concentration
of around 50 mg/L would normally be required to control biofilm formation, in
the presence of inserts,  the concentration required could be as low as 10 mg/L. The conditions of these tests were similar to those quoted elsewhere in this
paper.


### 5. Concluding Remarks

This brief summary of investigations into the factors that influence
biofilm formation and perhaps more importantly the opportunities for control of
biofilm growth demonstrate how the application of chemical engineering and
associated technologies can help meeting the challenge of climate change and the
associated impact on the well-being of humanity.Conferences such as Chempor do much to spread the word about what is
possible through the application of chemical engineering principles.
Furthermore the “networking” that is a direct result of conference attendance
is extremely valuable in that it facilitates a combined approach to meeting the
challenge.


## 3.1. Circulation of Sponge Rubber
Balls

The
use of circulating sponge rubber balls with the cooling water through the
tubes of steam condensers has been practiced with success, for many years. The
balls, having a diameter slightly larger than the internal diameter of the
tubes, wipe away any biofilm that begins to form. Cleaning is a random process,
so that the concentration of balls in the system has to be sufficiently high so
that every tube receives balls frequently enough to ensure effective cleaning.
In addition there has to be a replacement policy since the balls lose their
effectiveness after a time due to reduced diameter through wear.


### 3.2. The Use of Inserts

Inserts
in tubes, originally intended to boost heat transfer, by the destruction of the
laminar sublayer on the tube surface are also capable of reducing the
incidence of biofouling at modest water velocities. Two basic types are
available:
(i) static inserts such as
“Hitran” wire wound inserts that break up the laminar sublayer by creating
turbulence and associated removal forces at the heat transfer surface and
(ii)
inserts such as those developed by total, that oscillate in response to the
water flow, thereby removing the developing biofilm by abrasion.Both
types of device will increase the pressure drop and hence the operating costs,
although this may be offset to some extent by suitable design. Data [7] on the effect of “Hitran” inserts on the accumulation of biofilm
consisting of the slime forming species P. fluorescens demonstrate
that although there is some scatter in the data, it is evident that the
presence of the insert reduces the accumulation of biofilm. In one test, after
700 hours operation, the reduction of biofilm accumulation compared to an
equivalent test with no insert present was approximately 33% with a water flow
velocity of 0.86m/s  and approximately 64% with a water velocity of 1.27m/s.


### 3.3. The Use of Ultrasound

Ultrasound has the property of disturbing liquids and the structure of
solids. It is employed, for instance, to remove hard mineral deposits from heat
exchange surfaces exposed to high temperature combustion gases in boiler
installations. It has been demonstrated experimentally [8] that
biofilm accumulation (P. fluorescens) on the inside of tubes
can be reduced by the modest use of ultrasound of 20 kHz. Depending on the
amplitude (20 or 40%), the length (time), and frequency of groups of bursts of
ultrasound, the reduction in biofilm accumulation varied between 20 and 93%.
The water flow velocity was 1 m/s. Some further detail is given in Table 1.Table 1
Control of biofilm formation with different ultrasound
treatments.
Amplitude of
ultrasound %Number of
30-second burstsFrequency
of bursts each dayReduction of biofilm growth
compared with no treatment %203120.0203665.5201840.3A major drawback in respect of the use of ultrasound for the control of
biofouling is the initial cost of the equipment and its inclusion in heat
exchanger design.


### 3.4. Circulation of Polymer
Fibres

Some preliminary work [7] on the use of Aramid fibres to control
biofouling suggests that the technique could be useful. Again using P.
fluorescens as the biofouling bacterium, it was shown that a
concentration of fibres of 100 ppm with a water velocity of 1.6 m/s is
adequate for effective control of biofilm formation. It was noteworthy that if
fibre addition was stopped, a biofilm soon began to form. Considerably more
work on the technology will be required however, before it can be applied effectively
to industrial cooling water. A major difficulty that would have to be faced
would be the recovery of the fibres prior to the final discharge of the water.
Fibre removal might also be necessary before passing the water through a
cooling tower in a recirculation system, to avoid potential accumulation and
associated blockage problems. Some preliminary work suggests that the fibres do
not accelerate the wear on the associated pumps.


## 3.2. The Use of Inserts

Inserts
in tubes, originally intended to boost heat transfer, by the destruction of the
laminar sublayer on the tube surface are also capable of reducing the
incidence of biofouling at modest water velocities. Two basic types are
available:
(i) static inserts such as
“Hitran” wire wound inserts that break up the laminar sublayer by creating
turbulence and associated removal forces at the heat transfer surface and
(ii)
inserts such as those developed by total, that oscillate in response to the
water flow, thereby removing the developing biofilm by abrasion.Both
types of device will increase the pressure drop and hence the operating costs,
although this may be offset to some extent by suitable design. Data [7] on the effect of “Hitran” inserts on the accumulation of biofilm
consisting of the slime forming species P. fluorescens demonstrate
that although there is some scatter in the data, it is evident that the
presence of the insert reduces the accumulation of biofilm. In one test, after
700 hours operation, the reduction of biofilm accumulation compared to an
equivalent test with no insert present was approximately 33% with a water flow
velocity of 0.86m/s  and approximately 64% with a water velocity of 1.27m/s.


## 3.3. The Use of Ultrasound

Ultrasound has the property of disturbing liquids and the structure of
solids. It is employed, for instance, to remove hard mineral deposits from heat
exchange surfaces exposed to high temperature combustion gases in boiler
installations. It has been demonstrated experimentally [8] that
biofilm accumulation (P. fluorescens) on the inside of tubes
can be reduced by the modest use of ultrasound of 20 kHz. Depending on the
amplitude (20 or 40%), the length (time), and frequency of groups of bursts of
ultrasound, the reduction in biofilm accumulation varied between 20 and 93%.
The water flow velocity was 1 m/s. Some further detail is given in Table 1.Table 1
Control of biofilm formation with different ultrasound
treatments.
Amplitude of
ultrasound %Number of
30-second burstsFrequency
of bursts each dayReduction of biofilm growth
compared with no treatment %203120.0203665.5201840.3A major drawback in respect of the use of ultrasound for the control of
biofouling is the initial cost of the equipment and its inclusion in heat
exchanger design.


## 3.4. Circulation of Polymer
Fibres

Some preliminary work [7] on the use of Aramid fibres to control
biofouling suggests that the technique could be useful. Again using P.
fluorescens as the biofouling bacterium, it was shown that a
concentration of fibres of 100 ppm with a water velocity of 1.6 m/s is
adequate for effective control of biofilm formation. It was noteworthy that if
fibre addition was stopped, a biofilm soon began to form. Considerably more
work on the technology will be required however, before it can be applied effectively
to industrial cooling water. A major difficulty that would have to be faced
would be the recovery of the fibres prior to the final discharge of the water.
Fibre removal might also be necessary before passing the water through a
cooling tower in a recirculation system, to avoid potential accumulation and
associated blockage problems. Some preliminary work suggests that the fibres do
not accelerate the wear on the associated pumps.


## 4. Combined Chemical and Physical Control

It is possible that a combination of chemical and physical techniques of
control could be more effective than either solely applied.


### 4.1. Ozone and Ultrasound

Experimental work using a combination of ozone and ultrasound over a four-week period [9], again using P.
fluorescens as the biofilm former and a water velocity of 1 m/s demonstrated
that this was a more effective control than ozone alone. During the first week
of a four-week period, no treatment was applied to allow a biofilm to develop.
The biofilm thickness at the end of that first week ranged from 45–60 μm.  Table 2 presents
some of the data obtained during the subsequent three weeks, demonstrating that
the application of the ultrasound (20 kHz at 20% amplitude) for three minutes
each day made a considerable difference to the retention of biofilm. The ozone
concentration in the water during these tests was very low, since water
containing an ozone concentration of around 2.2–2.8 mg/L was pumped into the
system to be mixed with the bulk water for only three hours each day. The data
contained in Table 2 indicate that the application of ultrasound made a
considerable reduction in the accumulated biofilm.Table 2
Mean biofilm thickness over a period of 28 days with
different treatments.
WeekBiofilm thickness for different tubesμmOzone onlyOzone + ultrasound226, 18, 17, 189, 8343, 15, 24, 2011, 124111, 49, 60, 517, 5


### 4.2. Propriety Biocide and Inserts

Limited results [10] using Hitran inserts in conjunction
with a propriety biocide have demonstrated that whereas a biocide concentration
of around 50 mg/L would normally be required to control biofilm formation, in
the presence of inserts,  the concentration required could be as low as 10 mg/L. The conditions of these tests were similar to those quoted elsewhere in this
paper.


## 4.1. Ozone and Ultrasound

Experimental work using a combination of ozone and ultrasound over a four-week period [9], again using P.
fluorescens as the biofilm former and a water velocity of 1 m/s demonstrated
that this was a more effective control than ozone alone. During the first week
of a four-week period, no treatment was applied to allow a biofilm to develop.
The biofilm thickness at the end of that first week ranged from 45–60 μm.  Table 2 presents
some of the data obtained during the subsequent three weeks, demonstrating that
the application of the ultrasound (20 kHz at 20% amplitude) for three minutes
each day made a considerable difference to the retention of biofilm. The ozone
concentration in the water during these tests was very low, since water
containing an ozone concentration of around 2.2–2.8 mg/L was pumped into the
system to be mixed with the bulk water for only three hours each day. The data
contained in Table 2 indicate that the application of ultrasound made a
considerable reduction in the accumulated biofilm.Table 2
Mean biofilm thickness over a period of 28 days with
different treatments.
WeekBiofilm thickness for different tubesμmOzone onlyOzone + ultrasound226, 18, 17, 189, 8343, 15, 24, 2011, 124111, 49, 60, 517, 5


## 4.2. Propriety Biocide and Inserts

Limited results [10] using Hitran inserts in conjunction
with a propriety biocide have demonstrated that whereas a biocide concentration
of around 50 mg/L would normally be required to control biofilm formation, in
the presence of inserts,  the concentration required could be as low as 10 mg/L. The conditions of these tests were similar to those quoted elsewhere in this
paper.


## 5. Concluding Remarks

This brief summary of investigations into the factors that influence
biofilm formation and perhaps more importantly the opportunities for control of
biofilm growth demonstrate how the application of chemical engineering and
associated technologies can help meeting the challenge of climate change and the
associated impact on the well-being of humanity.Conferences such as Chempor do much to spread the word about what is
possible through the application of chemical engineering principles.
Furthermore the “networking” that is a direct result of conference attendance
is extremely valuable in that it facilitates a combined approach to meeting the
challenge.



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*Source: 619873-2009-06-14.xml*