Cooling Water:  System Impacts of Biofilm & Scale

By Taylor Robinson, Project Manager – Silver Bullet Science & Engineering Department

Evaporative cooling systems for both comfort cooling as well as various industrial processes face common issues associated with scale formation, corrosion, deposition and biofilm.

These challenges, if unmanaged, can significantly decrease system efficiency, increase operational costs, increase labor demand and limit the functional lifespan of various system components. For example, even a relatively small buildup of calcium carbonate scale can greatly limit the heat transfer efficiency of a heat exchanger as calcium carbonate is roughly 400 times less thermally conductive than copper. This effect is even more pronounced with biofilm which is considered to be four times less conductive than carbonate scales or up to 1,600 times less thermally conductive than copper. Microbiology is a potential common thread to scale formation, corrosion, and of course, biofilm. Effective microbial management can show positive impacts to each of the common cooling water challenges.

The performance and efficiency of a cooling tower system is directly proportional to the on-going water management practices conducted at the site. Scale formation, corrosion, deposition, biological fouling and potential risk of disease are common challenges associated with evaporative cooling systems and are the target areas of interest for water management. Evaporative cooling systems are incentivized to cycle up the concentration of their base water with less chemical corrosion inhibitor inputs and increase the efficiency of scale inhibitors when hardness is elevated in the makeup water. However, as a result of cycling up, scale, deposition and biofouling may occur more readily. In addition to physical plugging and fouling that can damage a cooling tower system, loss of efficiency in heat transfer can occur resulting in significant increases in energy demands and subsequent operational costs.

Mineral Scale

There are many dissolved and suspended constituents in most cooling waters including calcium, magnesium, iron, silica and others. As cooling towers cycle up, the chemistry and relative stability of the dissolved constituents can change, which may result in precipitated mineral deposits. Calcium carbonate is the most common type of scale found in cooling systems, but others including calcium phosphate, iron oxide, or magnesium silicate may also occur (Innovas, 2016). The formation of scale buildup and/or deposition depends on several factors including temperature, pH, and mineral concentrations. Magnesium and calcium have a unique solubility relationship with respect to temperature in which at higher temperatures, they become less soluble. This results in selective scale buildup on heat transfer surfaces where temperatures are higher (Innovas, 2016). As mineral scale is deposited onto heat transfer surfaces, their relative efficiency can be greatly reduced. For example, the thermal conductivity if copper is 384 W/m-K, whereas the thermal conductivity of calcium carbonate is 2.9 W/m-K (Keister, 2008).

Hardness Scale

Hardness scale includes minerals that incorporate calcium or magnesium at the cation and may include carbonate, bicarbonate or sulfate, for example, as the anion. As mentioned previously, calcium carbonate is the most commonly encountered scale in evaporative cooling systems. In nature, calcium carbonate can occur in a number of different crystalline forms, including calcite, aragonite, vaterite and dolomite, depending on impurities and the crystalline structure (Dobersek, 2007). The thermal conductivities of the various forms also change due to behavior of heat transfer through crystalline solids, which is described by molecular vibrations in the crystal lattice, creating lattice waves (phonons). The mechanism of heat transfer also accounts for the different conductive thermal properties between various other mineral scales. Convective heat transfer is also affected by the formation of hardness scale as the friction factor at the surface-bulk interface is altered (Characklis, 1981).

Iron Oxides

Iron can occur in both a soluble and insoluble form (ferrous iron II and ferric iron III, respectively). As soluble ferrous iron comes in contact with oxygen or other electron acceptors, it is oxidized to its insoluble ferric form and is often transformed into one of several oxides. Iron oxides tend to settle out of suspension and form deposits. These deposits can act as an insulating layer that limits the efficiency of heat exchangers. Iron oxide deposits can also act as a site for under deposit corrosion. The presence of iron oxide deposits can promote the growth of various species of acid-producing bacteria, further promoting the formation of corrosion cells (Lukanich, 1998).

Sulfurous Deposits

In most cooling systems, sulfur occurs in the form of an oxide as sulfate. Sulfate can potentially bind with several cations, depending on the water chemistry, to produce various scale deposits. These deposits have varying characteristics from formation rates, density and thermal conductivities, as can be seen in Table 1: Thermal Conductivities of Common Deposits and Various Materials. The thermal differences can be described by the variations in the crystal lattice, as previously outlined. In certain situations, anaerobic conditions can exist in cooling systems in deposit micro environments or in uncirculated system “dead zones”. In these anaerobic conditions, bacteria including Desulfovibrio spp. Or Clostridia spp. Can reduce sulfate into sulfides which present their own unique challenges (Flynn, 2009). Sulfides, such as hydrogen sulfide, are often toxic, flammable and highly corrosive with a characteristic foul odor.

Biological Fouling

Microorganisms are ubiquitous in nature and many cooling systems provide ideal environments for microbes to flourish. Not only are carbon and nutrient inputs available from the environment, but several of the commonly used treatment chemicals to manage scale and corrosion can also provide additional nutrients (ASHRAE, 2000). Common bacteria of interest include Legionella spp., Pseudomonas spp., Klebsiella spp. and other bacteria, viruses and protozoa (Liu, 2011). In addition to readily available nutrients, the cooling tower system is typically maintained between 25⁰C and 35⁰C (77⁰F to 95⁰F) with numerous surfaces to adhere in the fill and basin, for sessile species (Kusnetsov, 1993). Free floating planktonic cells may ultimately adhere to a surface and begin to produce an extracellular polymeric substance that contributes to a broader biofilm matrix (Fux, 2005).

Biofilm Formation

The formation of a biofilm occurs through four primary steps: exposure, organic adsorption, attachment and growth (Characklis, 1981). In the exposure phase, a clean and inert surface is exposed to a turbulent water flow which contains microorganisms, nutrients and organics. A thin film of organics is adhered to the surface in the organic adsorption phase. This organic film provides an anchoring site for bacteria to attach. In the third step, attachment, microbial cells are embedded in the organic film. In the final step, the attached microbes grow and produce an extracellular matrix which expands the biofilm and promotes the attachment of additional microbes. As the biofilm grows, eventually shearing may

Occur, in which case portions of the biofilm matrix are detached from the bulk mass and travel through the water stream where they eventually may seed a secondary biofilm elsewhere in the system.

Biofilm Impacts

The accumulation of biofilm can have several negative impacts to a cooling water system including physical plugging and fouling, under-deposit corrosion and can act as a harboring site for pathogenic bacteria (Innovas, 2016). Once established, biofilm can be difficult to remove as many traditional biocides are unable to penetrate past the surface to disinfect the underlying bacteria. Activity from aerobic bacteria matrixed into a biofilm can sufficiently consume the oxygen within the interior of the biofilm creating an anaerobic microenvironment. In this anaerobic zone, certain bacteria such as Sulfate Reducing Bacteria (SRB) can proliferate leading to localized corrosion, commonly referred to as under deposit corrosion or Microbiologically Induced Corrosion (MIC). Inorganic and organic acid production, development of differential cell sites, ammonia production, and sulfate reduction are all ways microorganisms can influence corrosion rates (Lukanich, 1998). Perhaps the most significant impact to biofilm in cooling systems is its low thermal conductivity which is considered to be up to four times as lower than mineral scales such as calcium carbonate (Chiang, 2016). Relative thermal conductivities can be seen in Table 1 (Flynn, 2009).

Table 1: Thermal Conductivities of Common Deposits

Biofilm deposits limits the thermal conductivity via two primary mechanisms: conductive and convective heat transfer. Conductive heat transfer is related to the thickness of the biofilm which separates, or insulates, the heat transfer surface from the bulk water stream. The low thermal conductivity of biofilm is very similar to that of water as can been seen in Table 1, which is appropriate as most biofilms are composed of 98% – 99% water by mass (Characklis, 1981). Convective heat transfer is related to the frictional resistance, or the friction factor, at the surface of the biofilm layer. In this case, convective heat transfer to the bulk fluid is limited as “micro-currents” are trapped within the rough peaks and valleys of the biofilm surface. This effect and the magnitude to which it occurs in thermal resistance, is dependent on the biofilm thickness in relation to both the thermal sublayer (close to the heat surface where conductive interaction occur, ≈22 μm thick) and the viscous sublayer (a boundary near the surface where flow is laminar and stratified, ≈44 μm thick) (Characklis, 1981). The decrease in relative thermal conductivity of biofilm and other deposits has an insulating effect that requires cooling system chillers to work harder to overcome the lack of heat transfer efficiency. This in-turn results on greater energy demands and associated costs as well as in creased mechanical strains that can result in more frequent system failures.

Biofilm Relation to Scale Formation

The inorganic composition of the bulk water plays a significant role in both the chemical composition and formation of scale and biofilm. The structural integrity, viscosity and density of a biofilm are directly related to the ratios and availability of calcium, magnesium and iron (silicon, aluminum and manganese can also make up a significant fraction) via their impact on intermolecular bonding within the biofilm matrix (Characklis, 1981). Divalent cations, such as calcium, are electrostatically attracted to the carboxylate functional group of the polysaccharides that make up the majority of the biofilm. As these ions are complexed into the biofilm matrix, they act to stabilize the biofilm via inter-polymer crosslinking and become more readily available to react with anions such as carbonate or phosphate that are present in the bulk water (Lukanich, 1998). This process results in a nucleation site for mineral scale to form and proliferate. The resulting complex would incorporate both mineral scale and biofilm into a structure that combines the operational challenges of each. In addition to the operational challenges, the combination of the inorganic and organic mass has several implications on the appropriate management and remediation protocols available.

Management Practices

Although fundamentally different processes, scale, corrosion and biofilm all potentially share a common thread with microbiology. Many scale complexes are associated with biomass that acts as a nucleation point and as a lattice for broader scale formation (Parker, 1995). Corrosion can occur through MIC in association with biofilm as described earlier. Biofilm formation is the response of various microorganisms reacting to specific environmental stimuli as an effort to proliferate in those conditions. It follows that appropriate measures to manage the microbial population will have a net positive impact on the major challenge areas associated with evaporative cooling systems. Increased system efficiency, decreased chemical demand, decreased maintenance demand and extended system lifespan are anticipated effects.



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