Microbiological growth or biofouling in secondary cooling systems and their control

Microbiological fouling in cooling systems is the result of growth of microbes, algae, fungi, and bacteria on surfaces. Such fouling is more extensive in open recirculating systems which scrub microbes from the air and, through evaporation, concentrate nutrients present in make-up water. As a result, microbe growth is more rapid. In addition to the availability of nutrients, factors such as temperature, pH range, and continuous aeration of the cooling water contribute to an ideal environment for microbial growth, as shown in Fig. 22. 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 are typically aggregates of biological and non-biological materials. Slimes can form throughout once-through and circulating systems. Microbial activity under deposits or within slimes can accelerate corrosion rates. Figure 23 illustrates a failure due to microbial accelerated corrosion.

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 non-living 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, together 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. A small

FIG. 21. Scale formation in and on heat exchanger tubes.

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 influence corrosion of fouled metal surfaces in a variety of ways (Fig. 24). Depending on the materials involved, the microbial reactions can accelerate ongoing corrosion processes, and even the byproducts of microbial reactions can also be aggressive to the metal. Additionally, biofilms can prevent corrosion inhibitors from reaching the metal surface for passivation. All these process are referred to as microbial influenced corrosion (MIC).

A slime control programme involves frequent monitoring of cooling systems. Regular monitoring of system data can identify trends, and periodic system inspections can reveal whether or not fouling is occurring. Test coupons and test heat exchangers can also be used in operating systems to facilitate monitoring without interrupting system operation. Deposits collected from cooling systems can be analysed in the laboratory to determine their chemical composition and biological content. If a deposit has a significant microbiological content, its contents should be identified for treatment. The laboratory can identify the agents as predominantly algal, bacterial or fungal, either microscopically 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. Sole reliance on bulk water counts will not provide sufficient information on the extent of surface fouling. Results must be

FIG. 22. Microbiological growth on tube surfaces.

FIG. 23. Failure caused by microbial accelerated corrosion.

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 [65].

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.

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 the 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 water.

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 semi-continuous feed. Given a certain level of fouling, the shorter the exposure time allowed by system operating conditions, the higher is the required antimicrobial concentration. Conversely, if exposure times are long, control of the same level of fouling may be achieved with lower antimicrobial dosages.

Circulating systems can be treated continuously or intermittently, although intermittent treatment programmes 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 re-colonized at an accelerated rate. Therefore, the period for growth and removal may vary within a system and will certainly vary among systems, even those using the same water source.

FIG. 24. Microbial influenced corrosion with consequent blocking of heat exchanger tubes.

In the planning of a slime control programme, 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 programme. The compatibility of the antimicrobial with other treatments added to the water should also be considered. Many system variables influence the behaviour 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.

Antimicrobials used for microbiological control can be broadly divided into two groups: oxidizing and non-oxidizing. Examples of oxidizers are chlorine and bromine. Only a relative distinction can be made between oxidizing and non-oxidizing antimicrobials, because certain non-oxidizers have weak to mild oxidizing properties.

The more significant difference between the two groups relates to mode of action. Non-oxidizing 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 non-oxidizing chemicals applied to cooling systems for biofouling control: in one mechanism, 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 are cationic surface-active molecules. They damage the cell membranes of bacteria, fungi and algae. At low concentrations, these compounds are biostatic because many organisms can survive in a damaged state for some time. However, at medium to high concentrations, quaternary ammonium compounds 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 organotins, bis (trichloromethyl) sulfone, methylenebis (thiocyanate) (MBT), Beta-bromo-Beta-nitrostyrene (BNS), dodecylguanidine salts, dibromonitrilopropionamide (DBNPA) and bromonitropropanediol (BNPD). All of these compounds are effective when applied in sufficient concentrations. Dodecylguanidine salts also have surfactant properties, which probably contribute to their effectiveness. 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 that have this attribute are BNS, MBT, DBNPA and BNPD [65].