Cooling water systems, particularly open recirculating systems, provide
a favorable environment for the growth of microorganisms. Microbial growth
on wetted surfaces leads to the formation of biofilms. If uncontrolled,
such films cause fouling, which can adversely affect equipment performance,
promote metal corrosion, and accelerate wood deterioration. These problems
can be controlled through proper biomonitoring and application of appropriate
cooling water antimicrobials.
DIFFICULTIES DUE TO
Microbiological fouling in cooling systems is the result of abundant
growth of algae, fungi, and bacteria on surfaces. Once-through and open
or closed recirculating water systems may support microbial growth, but
fouling problems usually develop more quickly and are more extensive in
open recirculating systems.
Once-through cooling water streams generally contain relatively low levels
of the nutrients essential for microbial growth, so growth is relatively
slow. Open recirculating systems scrub microbes from the air and, through
evaporation, concentrate nutrients present in makeup water. As a result,
microbe growth is more rapid. Process leaks may contribute further to
the nutrient load of the cooling water. Reuse of wastewater for cooling
adds nutrients and also contributes large amounts of microbes to the cooling
In addition to the availability of organic and inorganic nutrients, factors
such as temperature, normal pH control range, and continuous aeration
of the cooling water contribute to an environment that is ideal for microbial
growth. Sunlight necessary for growth of algae may also be present. As
a result, large, varied microbial populations may develop.
The outcome of uncontrolled microbial growth on surfaces is "slime"
formation. Slimes typically are aggregates of biological and nonbiological
materials. The biological component, known as the biofilm, consists of
microbial cells and their by-products. The predominant by-product, extracellular
polymeric substance (EPS), is a mixture of hydrated polymers. These polymers
form a gel-like network around the cells and appear to aid attachment
to surfaces. The nonbiological components can be organic or inorganic
debris from many sources which have become adsorbed to or embedded in
the biofilm polymer.
Slimes can form throughout once-through and recirculating systems and
may be seen or felt where accessible. In nonexposed areas, slimes can
be manifested by decreased heat transfer efficiency or reduced water flow.
Wood-destroying organisms may penetrate the timbers of the cooling tower,
digesting the wood and causing collapse of the structure. Microbial activity
under deposits or within slimes can accelerate corrosion rates and even
perforate heat exchanger surfaces.
OF COOLING WATER
The microorganisms that
form slime deposits in cooling water systems are common soil, aquatic,
and airborne microbes (see Figure 26-1). These
microbes may enter the system with makeup water, either in low numbers
from fresh water sources or in high numbers when the makeup is wastewater.
Significant amounts may also be scrubbed from the air as it is drawn
through the cooling tower. Process leaks may contribute microorganisms
Bacteria. A wide variety of bacteria can colonize cooling systems.
Spherical, rod-shaped, spiral, and filamentous forms are common. Some
produce spores to survive adverse environmental conditions such as dry
periods or high temperatures. Both aerobic bacteria (which thrive in oxygenated
waters) and anaerobic bacteria (which are inhibited or killed by oxygen)
can be found in cooling systems.
Fungi. Two forms of fungi commonly encountered are molds (filamentous
forms) and yeasts (unicellular forms). Molds can be quite troublesome,
causing white rot or brown rot of the cooling tower wood, depending on
whether they are cellulolytic (attack cellulose) or lignin degrading.
Yeasts are also cellulolytic. They can produce slime in abundant amounts
and preferentially colonize wood surfaces.
Algae. Algae are photosynthetic organisms. Green and blue-green algae
are very common in cooling systems (blue-green algae are now classified
with the bacteria and are called cyanobacteria). Various types of algae
can be responsible for green growths which block screens and distribution
decks. Severe algae fouling can ultimately lead to unbalanced water flow
and reduced cooling tower efficiency. Diatoms (algae enclosed by a silicaceous
cell wall) may also be present but generally do not play a significant
role in cooling system problems.
Differences and Similarities
Although algae, fungi, and bacteria differ in many respects, they also
share many characteristics. These similarities and differences are important
in understanding biofouling and its control.
||Cell size differs according to the complexity of the cell structure.
The simpler bacteria and cyanobacteria are much smaller than molds, yeasts,
and other algae. Because of their faster metabolisms and rates of growth,
these smaller cells are able to reproduce much more rapidly.|
|All microorganisms require water for growth. Although they vary in
terms of absolute water requirements and ability to survive dry periods, an
active, viable microbial population cannot exist without water.|
|Most microorganisms growing in cooling systems are bound by a rigid cell
wall. The cell wall gives the organism its characteristic shape and
provides mechanical strength. Immediately inside the cell wall is a cell
membrane which functions as a permeability barrier for the cell. The barrier
allows the cell to concentrate desirable chemicals, such as nutrients, and
exclude or excrete toxic or unwanted chemicals, such as waste materials.
Concentration gradients of several orders of magnitude can be established
across the membrane. All cells must expend considerable amounts of metabolic
energy to maintain an optimal interior condition. One essential characteristic
of all microbes is the ability to preserve the necessary organization and
integrity of the cell in a hostile and changing environment.|
|All cells must obtain energy and chemical "building blocks" from
their environment in order to survive and grow. The ability of each type
of cell to fulfill this function in different environments is discussed in the
Among the essential building blocks used by microbial cells, and those needed
in largest quantity, are carbon, nitrogen, and phosphorus. Microbes differ in
the method they use to obtain carbon. Green algae, cyanobacteria, and certain
bacteria can utilize carbon dioxide as a sole carbon source and convert
("fix") it to cellular carbon compounds. Most bacteria, yeast, and
molds require preformed carbon compounds and use organic molecules that range
from very simple to very complex. In order to meet nitrogen requirements,
microbes "fix" atmospheric nitrogen or utilize amines, nitrites, and
nitrates present in the environment. Naturally occurring and synthetic inorganic
and organic phosphates can be used to meet microbial phosphate requirements.
Microbes have developed many ways to extract energy from their surroundings.
Algae and other photosynthetic organisms trap light energy from the sun.
Inorganic chemicals, such as ammonia, sulfur, and hydrogen, can be oxidized by
certain bacteria to release energy. More commonly, bacteria, yeasts, and molds
liberate chemical energy stored in organic compounds, such as sugars, proteins,
fats, oils, organic acids, and alcohols.
Aerobic organisms use oxygen to drive the oxidations that release chemical
energy. Anaerobes do not use oxygen but may substitute molecules such as sulfate
or nitrate in place of oxygen. In the anaerobic energy-yielding process, these
oxidizing molecules are reduced, forming sulfides or nitrogen gas. When no
acceptable oxidizer is available, some anaerobes can still generate energy,
although less efficiently, by coupling oxidation of one half of a substrate
molecule to reduction of the other half. Typically, the by-products of this
"fermentative" reaction are various organic acids. All microbes
extract and collect energy in small, usable packets. Once the energy is made
available, there are only minor differences in how it is used.
In the presence of sufficient nutrients, growth and reproduction can occur.
Bacteria and cyanobacteria multiply by binary fission, a process in which a cell
divides to form two identical daughter cells. Yeasts divide by budding, with a
mother cell repeatedly forming single, identical but much smaller daughter
cells. Filamentous molds grow by forming new cells at the growing tip of the
filament. Green algae can have several patterns of growth, depending on the
species, ranging from tip extension to production of several cells from a single
cell during one division cycle. As with other cell features, the complexity of
growth processes also increases with increasing cell size. Under optimal
conditions, some bacteria can double their numbers every 20 to 30 minutes, while
molds can take many hours to double in mass.
Microorganisms are also extremely adaptable to changes in their environment.
This characteristic is related to cell size and complexity. The simple forms
with minimal growth needs and fast growth rates can form many cell generations
within a few days. Slight random changes in cellular characteristics during
those generations can produce a new cell that is more capable of surviving in a
shifted environment. This new cell can soon dominate the environment. Many
microbes carry information in unexpressed form for functions to be performed
when needed for survival. Changes in the environment can activate this
information causing all members of a microbial population to achieve new
capabilities as a group, within a single generation.
Usually, cooling waters are not nutrient-rich, so microbes must expend a
great deal of energy transporting and concentrating nutrients inside the cell.
This process may spend energy resources already in short supply, but it is
necessary to allow the biochemical machinery to run at top speed. Because there
is strong competition for the available nutrients, those species most efficient
at concentrating their essential nutrients will have the opportunity to grow
most rapidly. The rate of growth will ultimately be limited by the nutrient
which first falls below an optimal concentration, but this will not necessarily
be the nutrient in the lowest concentration.
Chemicals applied to cooling systems may, at times, provide added sources of
the limiting nutrient and thus contribute to microbial growth in the systems.
Alterations of pH may shift a stable population balance to an unbalanced,
troublesome state. Although bacteria may be under control at neutral pH, a shift
to an acid pH may result in domination by molds or yeast. Because many algae
grow most abundantly at an alkaline pH, an attempt to reduce corrosion by
raising the pH can lead to an algal bloom.
Seasonal changes also affect growth patterns in cooling water systems.
Natural algal communities in a fresh water supply are quite dynamic, and the
dominant species can change rapidly with changing temperatures, nutrients, and
amounts of sunlight. Cyanobacteria can often be primary colonizers in a cooling
system. Seasonal changes which increase their numbers in the makeup water can
lead to an algal bloom in the system. In autumn, as falling leaves increase the
nutrient level and depress the pH, the bacterial population can increase at the
expense of the algal population.
Microbiologists recognize two different populations of microorganisms.
Free-floating (planktonic) populations are found in the bulk water. Attached
(sessile) populations colonize surfaces. The same kinds of microorganisms can be
found in either population, but the sessile population is responsible for
Much is known about the formation of biofilms on wetted surfaces such as heat
exchanger tubes. Microorganisms on submerged surfaces secrete polymers
(predominantly polysaccharides but also proteins),
which adhere firmly even to clean surfaces and prevent cells from being
swept away by the normal flow of cooling water. These
extracellular polymeric substances are hydrated in the natural state,
forming a gel-like network around sessile microorganisms. This polymer
network contributes to the integrity of the biofilm and acts as a physical
barrier hindering toxic materials and predatory organisms from reaching
the living cell (see Figure 26-2). Biofilm
polymers can also consume oxidizers before they reach and destroy microorganisms.
As a result, control of sessile microorganisms requires dosages many
times greater than required to control planktonic organisms.
Biofilms develop slowly at first, because only a few organisms can attach,
survive, grow, and multiply. As populations increase exponentially, the depth of
the biofilm increases rapidly. Biofilm polymers are sticky and aid in the
attachment of new cells to the colonized surface as well as the accumulation of
nonliving debris from the bulk water. Such debris may consist of various
inorganic chemical precipitates, organic flocs, and dead cell masses. Fouling
results from these accumulative processes, along with the growth and replication
of cells already on the surface and the generation of additional polymeric
material by these cells.
When fouling occurs, even mechanical cleaning does not remove all traces of
the biofilm. Previously fouled and cleaned surfaces are more rapidly colonized
than new surfaces. Residual biofilm materials promote colonization and reduce
the lag time before significant fouling reappears.
Biofilms on heat exchange surfaces act as insulating barriers. Heat exchanger
performance begins to deteriorate as soon as biofilm thickness exceeds that of
the laminar flow region. Microbes and hydrated biopolymers contain large amounts
of water, and biofilms can be over 90% water by weight. As a result, biofilms
have thermal conductivities very close to that of water and, in terms of heat
transfer efficiency, a biofilm is the equivalent of a layer of stagnant water
along the heat exchange surface.
In shell-and-tube heat exchangers, the resistance to heat transfer is least
in the turbulent flow of the bulk phase, slightly greater across the metal tube
walls, and greatest across laminar flow regions. As biofilm thickness increases,
so does the apparent thickness of the laminar flow region. Like water, biofilms
are 25 to 600 times more resistant to conductive heat transfer than many metals.
A small increase in the apparent thickness of the laminar region due to biofilm
growth has a significant impact on heat transfer. A thin biofilm reduces heat
transfer by an amount equal to a large increase in exchanger tube wall
thickness. For example, the resistance to heat transfer of a 1 mm thick
accumulation of biofilm on a low carbon steel exchanger wall is equivalent to an
80 mm increase in tube wall thickness.
Biofilms can promote corrosion of fouled metal surfaces in a variety of ways
(see Figure 26-3). This is referred to as microbially
influenced corrosion (MIC) and is discussed further in Chapter
25. Microbes act
as biological catalysts promoting conventional corrosion mechanisms:
|the simple, passive presence of the biological deposit prevents corrosion
inhibitors from reaching and passivating the fouled surface|
|microbial reactions can accelerate ongoing corrosion reactions|
|microbial by-products can be directly aggressive to the metal|
The physical presence of a biofilm and biochemical activity within the film
change the environment at the fouled surface. Differences between colonized and
uncolonized sites may promote a galvanic-like attack. Microbes consume oxygen
more rapidly than it can be transferred from the bulk solution, and areas
beneath the biofilm become anaerobic and anodic. Repassivation of colonized
surfaces is also hindered. Some microbes deprived of oxygen switch to
fermentative metabolisms and produce large amounts of organic acids. This can
result in local areas of low pH. Growth of anaerobes, such as sulfate-reducing
bacteria, is favored in low-oxygen environments. These bacteria can oxidize
hydrogen forming at the cathode and depolarize the corrosion cell. Their sulfide
by-products may be directly corrosive or may contribute further to the
electrochemical differential between fouled and unfouled sites.
In summary, microbes originating in the natural environment colonize cooling
systems by capitalizing on favorable environmental conditions. Cooling systems
are favorable environments for microorganisms because they contain water,
operate in acceptable temperature and pH ranges, and provide nutrients for
growth. Microbial attachment to surfaces in untreated systems produces deposits
which reduce equipment efficiency and can be highly destructive to cooling
CHOICE OF SLIME CONTROL
Because of the speed with which microbes can grow in cooling water systems,
frequent monitoring of these systems is essential for the identification of
developing problems. Vigilant monitoring of operating data can identify trends,
and periodic system inspections show whether or not fouling is occurring. Test
coupons and test heat exchangers may be used in operating systems to facilitate
monitoring without interrupting system operation.
Deposits collected from the cooling system can be analyzed in the laboratory
to determine their chemical composition and biological content. If a deposit has
a significant microbiological content, its causative agents should be identified
for treatment. The laboratory can identify the agents as predominantly algal,
bacterial, or fungal, either microscopically (see Figure
26-4) or by routine cultural isolation and identification.
Microbial counts can also be performed to determine whether populations
within the system are stable, increasing, or decreasing. Usually, planktonic
populations are monitored by means of the Standard Methods plate count
technique. However, not all organisms in the fouling process can be detected by
this method. Anaerobic bacteria, such as sulfate-reducers which can cause
under-deposit corrosion, are not revealed by aerobic cultural procedures.
Special techniques must be used to ensure detection of these organisms (see Figures
26-5 and 26-6). Figure 26-5. Figure
Sole reliance on bulk water counts will not provide sufficient information on
the extent of surface fouling. Results must be interpreted in light of operating
conditions at the time of sample collection. For example, in an untreated system
a healthy, stable biofilm population may be present while bulk water counts are
low, because few sessile organisms are being released from the fouled surface.
If an antimicrobial is applied, bulk water counts may actually increase
dramatically. This is due to disruption of the biofilm and sloughing of sessile
organisms into the bulk water.
For a better diagnosis, it is necessary to use microbial monitoring
techniques that allow more direct assessment of surface conditions. It is
possible to clean a known surface area and suspend removed organisms in a known
volume of sterile water. After this water is plated, back-calculation provides
an approximation of the number of organisms on the original surface.
Another technique involves monitoring biochemical activity on a surface of a
known area. A biofouled specimen is incubated with a suitable substrate. The
concentration of reaction product found after a specific contact time relates to
the numbers and health of organisms on the surface and consequently can be used
as a measure of biofouling.
Regardless of which target population or monitoring technique is used, a
single, isolated data point has little meaning. Various data must be compiled to
generate a profile of microbiological trends in the system. This record should
also include observations on equipment performance and operating conditions at
the time of sample collection, thereby providing a meaningful context for
interpretation of new data.
After it is determined that treatment is necessary to solve a fouling
problem, an effective product must be chosen. Preliminary choices may be made
only if the causative microbial agent is known, because the spectrum of activity
of all antimicrobials is not the same. Some effectively control algae but not
bacteria. For others, the reverse is true. For some, the activity spectrum,
determined by inhibition of radiolabeled nutrient uptake, is quite broad,
covering all common cooling water microbes (see Table 26-1 and Figure
Table 26-1. Antimicrobial efficacy.
*I50 values are the
concentrations that inhibit growth of the organism by 50% and are
determined by tuse of a 14C-labeled nutrient.
Knowledge of how different antimicrobials affect microorganisms is also
useful in choosing the appropriate treatment. Some kill the organisms they
contact. Others inhibit growth of organisms but do not necessarily kill them.
These biostats can be effective if a suitable concentration is maintained in a
system for a sufficient time (a continuous concentration is ideal).
A laboratory evaluation of the relative effectiveness of antimicrobials
should be performed. This helps to identify those likely to work against the
fouling organisms in the system and to eliminate those with little chance of
success. Because the goal of antimicrobial treatment is control or elimination
of biofilm organisms, it is helpful to conduct the evaluation with sessile
organisms found in deposits, as well as planktonic organisms in the flowing
The objective of any treatment program should be to expose the attached
microbial population to an antimicrobial dosage sufficient to penetrate and
disrupt the biofilm. Generally, cleanup of a fouled system requires higher
concentrations of intermittently fed treatment, while maintenance of a clean
system can be achieved with low-level continuous or semicontinuous feed. Given a
certain level of fouling, the shorter the exposure time allowed by system
operating conditions, the higher the required antimicrobial concentration.
Conversely, if exposure times are long, control of the same level of fouling may
be achieved with lower antimicrobial dosages.
Antimicrobial test results are most relevant when based on contact times
derived from the system which is to be treated. Because once-through contact
times are typically short, it can be very difficult to simulate these systems in
lab testing. The longer contact times associated with recirculating cooling
systems are easily duplicated in the lab.
In once-through systems, antimicrobials should be fed continuously to achieve
the necessary contact time. Often, only low levels of antimicrobial are
affordable on a continuous-feed basis. Semicontinuous treatment may be more
economical or may be required because of effluent restrictions. Such an
intermittent program for once-through systems must still be designed to achieve
an effective antimicrobial concentration throughout the system, using treatment
periods which range from minutes to hours per day.
Recirculating systems can also be treated continuously or intermittently,
although intermittent treatment programs are more common. The purpose of
intermittent treatment in these systems is to generate a high concentration of
antimicrobial which will penetrate and disrupt the biofilm and eventually
dissipate. When the treatment level drops below the toxic threshold, microbial
growth begins again. After a period of multiplication, new growth is removed
with another shock dose. As stated earlier, previously fouled surfaces can be
recolonized at an accelerated rate. Therefore, the period for growth and removal
may vary within a system, and certainly will vary among systems, even those
using the same water source.
The time at which an antimicrobial concentra-tion drops below a threshold
concentration in a recirculating system can be determined mathematically. Such
information can be very useful in planning a schedule for effective and
economical slime control. The threshold concentration desired should be
estimated from a toxicant evaluation. The theoretical depletion of the
antimicrobial from a system can be determined from the following formula:
BD x T
Log Cf = log Ci -
Cf = final concentration, ppm
Ci = initial concentration, ppm
BD = blowdown and windage loss, gpm
V = system capacity, gal
T = time, min
It is standard practice to repeat the shock treatment when Cf is
25% of Ci. On this basis, the time interval for antimicrobial
addition can be calculated as follows:
T = 1.385 x
Solving this equation for T will indicate how frequently a slug should be
added to the system, but this determination is only valid for 75% depletion or
The equations provided are not valid for the following compounds:
|compounds which are volatile and may be lost during passage over the tower|
|compounds which react with substances in the water (i.e., a demand)|
|compounds which degrade in water|
In the planning of a slime control program, any chemical demand of process
waters for the antimicrobial being used must also be considered. Failure to
allow for the chemical demand may prevent attainment of the necessary threshold
concentration and may lead to the failure of the treatment program. The
compatibility of the antimicrobial with other treatments added to the water
should also be considered.
Many system variables influence the behavior of microbes in the system, and
the effects of antimicrobials can also be influenced by these variables.
Therefore, careful consideration must be given to the determination of whether,
when, and where to treat a cooling water system.
Cost is a primary criterion for selecting a slime control program. It cannot
be determined without knowledge or estimation of the individual costs of
chemicals, feed equipment, and labor required to apply and monitor the program,
along with effluent treatment requirements. In addition, possible adverse
effects of implementing the program must be weighed against those which would
result if no treatment is made. Knowledge of component costs can help guide
implementation of the program. For example, if labor costs are high, it may be
more economical to feed an antimicrobial more frequently and reduce the amount
of monitoring required. Each system must be considered individually, and
seasonal adjustments may also be required.
OF NONOXIDIZING CHEMICALS USED IN COOLING SYSTEMS
Antimicrobials used for microbiological control can be broadly divided into
two groups: oxidizing and nonoxidizing. Oxidizers, such as chlorine and bromine,
are addressed in Chapter 27; nonoxidizers are discussed in the balance of this
Only a relative distinction can be made between oxidizing and nonoxidizing
antimicrobials, because certain nonoxidizers have weak to mild oxidizing
properties. The more significant difference between the two groups relates to
mode of action. Nonoxidizing antimicrobials exert their effects on
microorganisms by reaction with specific cell components or reaction pathways in
the cell. Oxidizing antimicrobials are believed to kill by a more indiscriminate
oxidation of chemical species on the surface or within the cell.
An understanding of the chemistries and modes of action of antimicrobials is
needed to ensure their proper use and an appreciation of their limitations.
Two characteristic mechanisms typify many of the nonoxidizing chemicals
applied to cooling systems for biofouling control. In one, microbes are
inhibited or killed as a result of damage to the cell membrane. In the other,
microbial death results from damage to the biochemical machinery involved in
energy production or energy utilization.
Quaternary ammonium compounds (quats) are cationic surface-active molecules.
They damage the cell membranes of bacteria, fungi, and algae. As a result,
compounds that are normally prevented from entering the cell are able to
penetrate this permeability barrier. Conversely, nutrients and essential
intracellular components concentrated within the cell leak out. Growth is
hindered, and the cell dies. At low concentrations, quats are biostatic because
many organisms can survive in a damaged state for some time. However, at medium
to high concentrations, quats can control the organisms.
Many antimicrobials interfere with energy metabolism. Because all microbial
activity ultimately depends on the orderly transfer of energy, it can be
anticipated that interference with the many energy-yielding or energy-trapping
reactions will have serious consequences for the cell. Antimicrobials known to
inhibit energy metabolism include the following:
All of these compounds are effective when applied in sufficient
concentrations. Dodecylguanidine salts also have surfactant properties, which
probably contribute to their effectiveness.
The exact sites or reactions which are affected by such metabolic inhibitors
are frequently unknown, although laboratory experiments may provide clues or
indirect evidence for specific mechanisms. Tin and other heavy metals in
sufficient concentrations cause proteins to lose their characteristic
three-dimensional structures, which are required for normal function. Some
antimicrobials, such as methylenebis(thiocyanate) (MBT) are believed to bind
irreversibly to biomolecules, preventing the sequential reduction and oxidation
these molecules must undergo in order to function.
Bromonitropropanediol (BNPD), a newer cooling water control agent, can be
shown to catalyze the formation of disulfide bonds (R-S-S-R) between sulfhydryl
groups (R-SH). Proteins contain sulfhydryls, and because enzymes are largely
protein, it is possible to speculate that the formation of disulfide bonds
between adjacent -SH groups may block enzyme activity. Many different enzymes
contain sulfhydryl groups, so this antimicrobial may affect a wide range of
microbial activities in addition to energy generation.
The mode of action of one common nonoxidizer cannot be categorized as either
a surface-active or metabolic inhibitor. The active, dibromonitrilopropionamide
(DBNPA), seems to behave somewhat like an oxidizing antimicrobial, reacting
quite rapidly with bacterial cells. Studies of the interaction of radioactively
labeled [14C]DBNPA with bacteria have shown that the 14C label never
penetrates the cell, as would be necessary for it to become involved
with energy metabolism. Instead, it binds strongly and rapidly to the
cell walls of the bacteria. However, analogous studies with [14C]-Beta-bromo-Beta-nitrostyrene
(BNS) have shown that the nitrostyrene penetrates the bacterial cell
and accumulates within at concentrations far above the external concentration.
Thus, although the mode of action of DBNPA is unknown, it most probably
is unlike the other mechanisms known for nonoxiding antimicrobials.
Some antimicrobials used in cooling systems are compounds that spontaneously
break down in water, thereby alleviating some potential environmental hazards.
This chemical breakdown is often accompanied by a reduction in the toxicity of
the compound. The compound can be added to the cooling water system, accomplish
its task of killing the microbes in the system, and then break down into less
noxious chemicals. Among the antimicrobials which have this attribute are BNS,
MBT, DBNPA, and BNPD.
The dynamics of microbial populations in cooling water systems are complex.
In situations where one microbial group or species dominates, fouling problems
can occur. In other instances, a balanced population mix can exist while no
fouling is evident. One explanation for such observations is that when balanced
populations coexist, they compete with each other for the available nutrients
and control each other's growth. When one group successfully displaces the
others, its growth can proceed without competition.
Because of such considerations, some proprietary antimicrobials are
formulated to contain more than one active. Proper blending of actives can
compensate for limitations in the spectrum of kill shown by one or more of the
actives. For example, if antimicrobial A is effective for bacteria but poor for
fungi, large amounts of A might have to be used to control potential fungal
problems. However, if antimicrobial B is fair for bacteria and good for fungi, a
combination of A and B would broaden the spectrum of control and thus be
preferable to high concentrations of A alone.
With no increase in the amount of antimicrobial used, the power of a blend
can exceed that expected from a simple additive effect. This greatly enhanced
performance or synergism is obtained from only certain combinations of actives.
Synergism allows microbial control at much lower combined concentrations of A
and B than could be achieved with either A or B alone. Use of products composed
of synergistic active blends can result in reduced treatment concentrations in
the blowdown water, as well as cost savings.
The spectrum of control can also be broadened by sequential feeding of
antimicrobials to a system: alternate feeding of two actives can have the same
outcome as blending of the actives for simultaneous feeding.
Another variation to be considered is the possible proliferation of resistant
microbes in the system. The resistant forms may arise spontaneously by mutation
within the cooling system but are much more likely to originate outside of the
system. The antimicrobial simply functions to reduce competition by nonresistant
forms and permits the unchecked growth of the newly introduced resistant
organisms. This is more likely to occur during treatment with a single
antimicrobial active ingredient, because the probability of a microbe being
resistant to more than one active is extremely low when the actives are
dissimilar. Sequentially added and synergistically blended antimicrobials are
probably equally effective in eradicating antimicrobial-resistant microbes from
a cooling system.
Development of an effective antimicrobial program is aided by an understanding
of the mode of action of the products, the system to be treated, and
environmental effects. All of these factors play a role in the selection
of an effective and economical microbiological control program that
is safe for the environment.