Corrosion, Scale, and Biofouling Control in Cooling Systems

Introduction to Corrosion, Scale, and Biofouling Control in Cooling Systems

Cooling Water System Fundamentals outlined the basics of cooling tower and heat exchanger design/operation. In this chapter, we will examine cooling water chemistry and treatment programs to maintain reliability throughout the cooling water network. Cooling systems require protection from corrosion, scaling, and microbiological fouling to maximize performance. 

Corrosion, scale, and biofouling control should be addressed collectively. A fourth, increasingly important factor is the potential environmental impact of water treatment chemistry, especially regarding chemicals that could appear in the plant discharge. Treatment programs that were once commonplace may no longer be allowed or may be severely restricted because of discharge regulations. 

Although treatment methods serve multiple purposes, one primary objective is to protect metal surfaces. In the following sections, we review the most common corrosion mechanisms and control methods.

Download Chapter as PDF

A Brief Review of Cooling System Metallurgy and Materials Makeup

The typical material for cooling system piping and many heat exchanger (HX) shells is mild carbon steel. HX tubes or plates may be of stainless steel, copper alloys, titanium, aluminum, or in some cases, expensive corrosion-resistant metals. Galvanized steel fasteners are often present in cooling towers, while smaller towers may be predominantly galvanized. Most large cooling towers have concrete basins, and some still have wooden structural components. Understanding all materials in a cooling system is crucial for choosing effective corrosion control methods.

Corrosion Cell

The driving force for corrosive reactions is the electrical potential between the electron acceptor (the corrosive medium) and the electron donor (the metal). The following example illustrates the fundamental corrosion process. A basic laboratory experiment involves immersing an iron bar in a hydrochloric acid (HCl) solution. Bubbles form along the submerged surface of the bar, and corrosion can be observed after a short period. Figure 7.2 summarizes the chemistry.

Fig. 7.2. Basic corrosion cell.

Three reactions explain the overall process:

1. Each iron atom at a corrosion site gives up two electrons (oxidizes) and transforms from a zero oxidation state to a +2 oxidation state, Fe⁰ → Fe²⁺ + 2e^– (Eq. 7-1). The Fe²⁺ (ferrous) ions migrate into the solution. These sites are known as anodes.
2. Electrons released at the anodes flow through the metal to other sites where they react with the hydrogen ions (reduction) of the acid to produce hydrogen gas, 2H⁺ + 2e^– → H₂↑ (Eq. 7-2). Reduction occurs at the cathodes.
3. Chloride ions (Cl^–) and ferrous ions migrate through solution to produce solvated ferrous chloride (FeCl₂) and complete the electrical circuit.

In this case, anodes and cathodes form along the metal surface. Many hydrogen bubbles immediately form, and shortly thereafter visible corrosion appears on the entire bar. This is an example of general corrosion, where anodes and cathodes shift constantly, and it is also a classic example of an oxidation-reduction or “redox” reaction.

Each metal has a unique tendency to gain or lose electrons, and the type of corrosive agent plays a significant role. The list of half-cell metal potentials as compared to the hydrogen half-cell potential is included below.

2H⁺ + 2e^– ⇌ H_2↑ E₀ = 0.00 V for a 1 molar solution by definition

The following table highlights the potentials of several of the most common metals.

Table 7-1. Half-Cell Potentials for Well-Known Metals as Compared to the Hydrogen Half Cell

From this table, we can derive several important concepts.

• The metals above hydrogen in the table will all corrode in acid.
  • Consider again the reaction shown in Figure 7.2. The table indicates that iron has a significant reaction potential when compared to acid.
• Copper (and typically its alloys) are on the “noble” side of the hydrogen half-cell potential, meaning that it remains stable in common mineral acids. Oxidizing agents are typically needed for corrosion.
• Silver and gold are two truly noble metals. Especially for gold, special solutions, e.g., aqua regia, are needed to dissolve the metal.
• This data is also important for determining the probability of galvanic corrosion when two dissimilar metals are coupled in a water environment. Galvanic corrosion is examined in more detail later.

Regarding the common structural material of many cooling water systems, mild streel, Figure 7.3 illustrates the influence of pH, i.e., the hydrogen ion concentration.

Figure 7.3. Influence of pH on mild steel corrosion rates.

Accordingly, most modern cooling system chemistry programs operate within an upper-7 to mid-8 pH range. In these conditions, a thin, passive CaCO₃ layer may form to further inhibit corrosion. However, as was briefly discussed in Cooling Water System Fundamentals and will be re-examined later in this chapter, calcium carbonate scale formation can be one of the most troublesome problems in cooling systems.

If acid was the only corrodent in a cooling water system, corrosion control would be straightforward. Many cathodic reactions may occur, but oxygen reduction is most common in neutral or alkaline solutions.

Figure 7.4. Oxygen-steel corrosion reaction.

The anodic reaction is the same as previously shown.

Fe⁰ → Fe²⁺ + 2e^‒ | Eq. 7-1

Oxygen is reduced at the cathode:

½O₂ + H₂O + 2e^‒ → 2OH^‒ | Eq. 7-3

The hydroxide ions combine with Fe²⁺ to form ferrous hydroxide:

Fe²⁺ + 2OH^‒ → Fe(OH)₂ | Eq. 7-4

In the oxygen-laden environment, additional reactions occur. First, ferrous hydroxide will continue reacting with oxygen to form ferric hydroxide:

2Fe(OH)₂ + ½O₂ + H₂O → 2Fe(OH)_3↓ | Eq. 7-5

The reactants shown in Equations 7-4 and 7-5 are in equilibrium with other iron oxide species, as shown below:

Fe(OH)₂ ⇌ FeO + H₂O | Eq. 7-6

Fe(OH)₃ ⇌ FeO(OH) + H₂O | Eq. 7-7

Eventually, these products dehydrate to rust, which is brown-colored and offers no protection to the underlying metal:

2FeO(OH) ⇌ Fe₂O_3↓ + H₂O | Eq. 7-8

It is also essential to recognize that, within the electrochemical process, cathodic reactions determine the rate of corrosion, whereas anodic reactions influence the specific form of corrosion that occurs. There are scenarios where a limited quantity of fixed anodes is present within a large cathodic environment. This combination can lead to severe localized corrosion and potentially rapid failures.

Upon examining Table 7.1, it is evident that magnesium and aluminum are positioned at the top, prompting consideration of their widespread application in infrastructure, including components for aircraft, electrical systems, beverage containers, and other uses. The elements are so reactive that during the production process, a tight oxide layer forms on the metal surface and protects the base metal from further corrosion. Only acids or strong alkalis will attack this protective layer, so during normal service the metals remain quite stable.

Several other important factors influence most corrosion mechanisms. These are outlined below.

Temperature and Dissolved Oxygen Concentration

Temperature commonly influences corrosion rate, and general guidelines suggest that the rate doubles with each 18°F (10°C) increase in fluid temperature. Additionally, the corrosion rate will, in general, double with each 10–20°F increase in metal skin temperature. Some processes may produce so much heat that molecular oxygen gas can form beneath deposits, exponentially influencing the corrosion process. The effect of temperature and oxygen concentration on corrosion rates is shown below.

Figure 7.6. General influence of temperature and dissolved oxygen on corrosion rate.

Types of Corrosion

We will now examine the most common types of corrosion in cooling systems. Some, such as general corrosion, often allow long material life and can be controlled with straightforward treatments. Others, like pitting, have been known to cause through-wall penetrations of pipes and other equipment within months, and sometimes even weeks. Expensive unit shutdowns and material replacement may be the result.

Pitting Corrosion

If corrosion becomes localized, permanent anodes will develop in a large cathodic environment. Pitting, or similar mechanisms as we shall observe, is the result, as shown below.

Figure 7.8. Pitting corrosion.

The corrosion rate may be the same as general corrosion, but damage is more serious because of rapid penetration of the metal at anodes. Numerous mechanisms or conditions can initiate pitting. Solids deposits on steel may produce oxygen-depleted areas. These spots are anodic to clean steel. Microbiological colonies can do the same and can also release corrosive compounds via metabolic processes. We will explore this phenomenon a bit later. Poor welding techniques can alter the chemical makeup of the metal at the weld location and increase corrosion susceptibility.

A common phenomenon with carbon steel is corrosion product (rust) accumulation over the pit.

Figure 7.9. An iron oxide tubercle covering a pit.

Reactions in a trapped liquid can raise acidity, increasing corrosion potential.

Figure 7.10. Chemistry under an iron oxide tubercle.

Chlorides or other anions diffuse into the pit to try to maintain charge neutrality, however, acidic conditions often remain. The deposits above the pit prevent bulk water corrosion inhibitors from re-passivating the metal surface within the pit.

The following figures illustrate additional pitting examples.

Figure 7.11a

Figure 7.11b.

Figure 7.11c.

7.11a is pitting that has penetrated the metal basin of a cooling tower. 7.11b shows corrosion products on the outside of a pipe that was generated by internal attack and resulting through-wall penetration. 7.11c illustrates another through-wall penetration.

For carbon steel pits covered or filled with corrosion products, if the material is black (typically due to the presence of magnetite (Fe₃O₄)), the pit is active and the corrosion process is ongoing.

Chloride pitting of stainless steels can occur in various settings, such as open recirculating systems where cooling tower evaporation (see Cooling Water System Fundamentals) leads to an increase in dissolved solids concentration. The two most popular stainless steels (SS) for steam surface condenser tubes are 304 and 316, and often one or the other is specified for a project without the designers giving thought to the chloride concentrations the materials will see. Stainless steels form an oxide coating that protects the base metal, but significant concentrations of chloride will penetrate the oxide layer and initiate pitting. For years, the recommended maximum chloride levels for these steels ranged from 500 ppm for 304 SS to 3,000 ppm for 316L SS at ambient temperature. Research has subsequently shown that these limits were too high, and one noted materials expert suggests 100 and 400 ppm, respectively, for clean tubes. Note the emphasis on clean tubes. Deposits exacerbate the corrosion potential. Additionally, temperatures above ambient greatly influence the potential for chloride-induced stress corrosion cracking (SCC) of austenitic stainless steels, which can be of huge concern in industries such as petroleum refining that have many heat exchangers for production of numerous compounds. SCC is examined shortly.

Resistance to chloride pitting is often a primary factor in heat exchanger materials selection. A well-known guide is the pitting resistance equivalent number (PREN) chart as shown below.

Figure 7.12. PREN chart. Illustration courtesy of Dan Janikowski, Plymouth Tube Company.

Higher-alloy materials beyond the 300 austenitic stainless series are recommended for cooling waters with appreciable chlorides. For highly brackish waters and seawater, the ferritic and super-ferritic alloys, e.g., SEA-CURE^®, are often required.

Manganese Influenced Pitting

A phenomenon that has plagued numerous water-cooled heat exchangers is manganese-influenced pitting, with many problems reported along the Ohio River. Even at concentrations as low as 0.02 ppm, dissolved manganese in cooling water can be oxidized to manganese dioxide (MnO2) by chlorination. A thin, varnish-like coating will appear on the heat exchanger surfaces.

Figure 7.14. Varnish-colored coating of manganese dioxide on a tube wall.

The manganese deposits are strongly cathodic to the underlying metal and can cause severe localized corrosion.

Figure 7.15a and b. Manganese-induced pitting on tubes subjected to periodic heavy chlorination.

304 and 316 SS are susceptible to manganese deposit corrosion, but it can also affect admiralty brass, aluminum brass, and cupro-nickel. Attack is likely if the manganese content of the deposit exceeds five percent. A concentration greater than 20 percent will result in severe pitting.

MnO₂ formation can be affected by factors such as elevated pH, aeration, and occasionally the catalytic properties of the metal surface. Furthermore, the MnO₂ layer can be oxidized to permanganate (MnO₄) by chlorine. Permanganate dissolves the base metal, and in the process is reduced back to MnO₂. The cycle repeats during each chlorination. Manganese corrosion is apparently not a major problem with mild steel, possibly because other corrosion products prevent manganese from forming a uniform and dense deposit.

Manganese deposition and corrosion control methods include limiting or eliminating oxidizing biocide feed (potentially by switching to non-oxidizing biocides), and application of an effective manganese stabilization program. Selection of corrosion resistant materials in the design phase is another approach.

Cavitation

Cavitation is a specific type of erosion corrosion that most commonly affects centrifugal pump impellers.

Figure 7.17. Severe cavitation damage of a pump impeller.

If insufficient pressure is available at the pump suction (inlet pressure is termed “net positive suction head (NPSH)”), bubbles in the water may collapse. The collapsing bubbles can generate very large, localized forces that strip protective oxide and damage the metal. Other potential locations for cavitation include valve discharge, regulators, orifices, or other locations of pressure drop. Potential cavitation issues should be addressed in the project design phase to ensure that pumps and other equipment have sufficient head pressure.

Galvanic Corrosion

Galvanic corrosion occurs when two different metals are in physical contact within the cooling water environment. Refer to Table 7-1, which lists the electrochemical potentials for several commonly used metals in cooling water infrastructure. When two metals are paired, the one with greater reactivity becomes anodic relative to the less reactive metal. The larger the separation in electrochemical potential, the greater the potential corrosion rate. A common example is shown in the figure below.

Figure 7.21. Galvanic corrosion of a steel pipe attached to a brass valve.

Galvanic cells can form as a result of chemical reactions that take place within a fluid. The manganese pitting corrosion mechanism outlined earlier was one example. Another common mechanism occurs in systems that have both steel and copper alloys, even if they are not physically connected. If the conditions allow some copper corrosion, the copper may plate out on the steel and produce a galvanic corrosion cell.

Figure 7.22. Galvanic corrosion induced by copper plating on steel.

This example illustrates the use of data from Table 7.1 by analyzing the reaction through the half-cell potentials.

A strong driving potential exists for this reaction.

The ideal solution to galvanic corrosion is to not have mixed metallurgies in cooling systems, but this arrangement is usually impractical. A key is having a small ratio of the more electronegative material, e.g., copper, in a system with a much larger amount of the electropositive material, e.g., steel. While galvanic corrosion can still occur, the large anode to cathode ratio ensures that the attack is spread out over a large area and does not cause severe damage.

In other cases, special fittings or connections may be employed to physically separate different metals.

Figure 7.23 illustrates the use of a di-electric union to separate copper from steel.

Figure 7.23. Blue plastic dielectric union separating copper and steel.

Microbiologically Induced (Influenced) Corrosion

Microbiologically induced corrosion (MIC) is a process in which microorganisms initiate, facilitate and/or accelerate corrosion reactions. Cooling systems provide an ideal environment for microorganisms to establish colonies and form slimy, mud-like deposits. (Additional details are provided in the microbiological control section later in this chapter.) In the first place, deposits can cause oxygen differential corrosion as outlined earlier. Beyond this issue, however, is that the metabolic processes of some microbes generate compounds that directly attack metals. For example, iron-oxidizing bacteria such as gallionella produce ferric chloride, which is known to accelerate pitting. Sulfate reducing bacteria (SRB) such as desulfovibrio desulfuricans extract oxygen from sulfate (SO₄) to produce hydrogen sulfide (H₂S). Sulfides in just about any form are quite corrosive to many metals.

MIC can leave smooth gouges in the metal surface as shown in Figure 7.25 below, and it can also cause rapid pitting of some materials, including the 300-series stainless steels.

Figure 7.25. MIC example

Copper Alloys

Copper alloys are the second most common material in cooling water systems, typically for heat exchanger tubes. They have a higher thermal conductivity than steel, are naturally toxic to many aquatic species, and are more resistant than steel to some corrosion mechanisms.

As demonstrated in Table 7.2, copper is considered noble relative to the hydrogen ion, and consequently, it exhibits minimal susceptibility to corrosion in acidic environments. However, oxygen is more reactive than H+, and in cooling water systems copper alloys will initially develop a cuprous oxide (Cu₂O) multi-layer of variable porosity, where copper exists in the +1 valence state.

2Cu + O₂ → Cu₂O | Eq. 7-9

Over time, and in the continued presence of oxygen or other oxidizing agents, further oxidation of the outer cuprous oxide layer can take place to produce a grayish-black layer of cupric oxide (CuO).

Cu₂O + ½O₂ → 2CuO | Eq. 7-10

In some cases, the cupric oxide layer may remain protective, but certain compounds, and most notably ammonia, can be very corrosive. It dissolves cupric ion through a mechanism known as d-orbital bonding.

Cu²⁺ + 4NH₃ → Cu(NH₃)₄²⁺ (aq) | Eq. 7-11

For makeup waters containing a significant concentration of ammonia, e.g., wastewater treatment plant effluent, corrosion of copper-alloy condenser or other heat exchanger tubes may be of significant concern. Copper-alloy corrosion has sometimes been problematic in steam systems where ammonia is utilized for condensate and feedwater pH control.

Another impurity that can cause enormous damage to copper alloys (and other metals) is sulfide, which, as has been noted may come from microbiological colonies that contain sulfate reducing bacteria.

Cu²⁺ + H₂S → CuS_↓ + H_2↑ | Eq. 7-12

Other potential sulfide sources include water that has been allowed to go septic, and from process leaks at refineries and similar plants. A startling example occurred a number of years ago, when the aging 90-10 Cu-Ni tubes in a steam surface condenser were replaced, only to have the new tubes fail from pitting attack within 18 months of commissioning. Investigation revealed that the tube fabricator utilized a lubricant containing sulfide, but did not remove the compound before shipment to the plant. The sulfide deposits did not wash off but burrowed into the metal in thousands of spots.

A different but very recognizable copper corrosion product is copper carbonate (CuCO₃). This bluish/green verdigris (also called patina) is often seen on weathered copper-alloy structures such as roofing, plaques and statues, and is typically part of the architectural design of the structures.

Figure 7.26. Statue of Liberty with blue/green patina.

The basic chemistry is:

2Cu + H₂O + CO₂ + O₂ → Cu(OH)₂ + CuCO_3↓ | Eq. 7-13

The color can be somewhat variable depending upon the degree of hydration of the film.

Aluminum

Referring to Table 7.1 from the beginning of this chapter, it may seem counterintuitive that aluminum, and in certain cases magnesium, are appropriate materials for construction, given their high reactivity. The key is that the high reactivity induces formation of a tight oxide layer that protects the metal underneath. Typical aluminum products include injection molds, engine blocks, radiators, cooling tower ladders and walkways, handrails, fan blades and other similar components. Aluminum is resistant to atmospheric corrosion and that aspect, coupled with its light weight, make it an excellent material for airplane components.

Aluminum is an amphoteric material meaning that it will corrode at low and high pH.
Aluminum can corrode in waters made alkaline with phosphate. Also, because aluminum is the more active electrochemically than steel, it will corrode when coupled to steel in cooling water environments.

Wood Degradation

Wood deterioration can generally be classified into three categories, physical, chemical, and microbiological.

• Physical
  • Excessively elevated cooling water temperatures greater than 140o F
  • Ice damage
  • Erosion
  • “Iron rot” from adjacent rusting metal fasteners (Figure 7.30)
  • Wet/dry cyclical damage
  • Mechanical Stress (Figure 7.31)

Figure 7.30. An example of iron rot on wooden components.

Figure 7.31. Excessive weight loading on distribution deck (36” header)

• Chemical
  • Elevated chlorine concentrations exceeding 1.0 ppm can delignify the wood. Lignin is the binder for cellulose fibers in wood but can be destroyed by a high halogen content.
  • Chemical injection lines placed near wooden structures. If these lines develop leaks, the concentrated chemicals can attack wood.
  • High pH values above 9.0. Elevated pH can be especially destructive when combined with high chlorine concentrations. Cooling water treatment programs are typically designed to operate below this pH.
• Biological (principally fungi-related)
  • Fungi are responsible for wood decay and destruction.
  • Fungi exist as yeasts (unicellular) and molds (multicellular-filamentous).
  • Fungi thrive in a slightly acid pH range of 5.5–6.5.
  • Fungi require high levels of organic carbon, as compared to bacteria. It follows that bulk water fungi proliferation is relatively rare, unless organic contamination is present, perhaps from hydrocarbon process leaks or if the makeup supply is municipal wastewater treatment plant effluent.

The following discussion outlines the three primary types of fungal wood rot.

Brown Rot: Brown rot, also known as dry rot, is more common in soft woods, and attacks beneath the preservative layer applied during fabrication. The fungi go after cellulose, leaving dark-colored lignins behind. It can penetrate deep into the wood.

Figure 7.32. Brown rot.

White Rot: White rot is more common in hard woods, and attacks lignins. It progresses more slowly than brown rot. The wood surface becomes soft and stringy and appears bleached. White rot can be controlled by surface fungicide treatments in early growth stages. The fungicide penetrates slowly into the wood.

Figure 7.33. White rot.

Soft Rot: Soft rot occurs only in water-washed areas and is confined to the surface in early stages. The attack is slower than white or brown rot. The surface appears cracked and light-colored when dry. Soft rot can be controlled by diligent cooling water microbiological treatment.

Figure 7.34. Soft rot.

Concrete Corrosion

Concrete was used to build large hyperbolic cooling towers at nuclear and some coal plants in the last century. Virtually no hyperbolic towers have been constructed in the last several decades and will not be considered further here. However, many of the large, mechanical draft towers in heavy industry and power have reinforced concrete basins.

Figure 7.35. Cooling Towers Constructed with Concrete Basins. Source: Rain Carbon Chemical Plant Prospect Survey – Jason Miklavcic – December 20, 2020.

Concrete is strong, poured on site, and can have a long life. Plants using sulfuric acid to control cooling water pH often face issues when the acid is injected undiluted into the basin. Commodity-strength sulfuric acid (93–98 percent concentration) has a density almost twice that of water and will sink rapidly to the basin floor if not diluted, where it can attack the concrete and concrete-reinforcing bars.

The primary method to minimize this damage is an acid dilution system, which externally mix acid and water. Distribution of the mixture is performed through a trough above the basin.

Figure 7.36. Acid dilution piping and trough above a cooling tower basin. Photo courtesy of Global Treat, Inc. of Spring, Texas.

Standard Portland cement can be attacked by waters with a high sulfate concentration (>1,500 ppm), which is possible in some cooling systems per the concentrating effect of the tower. This issue can be addressed in the design phase with careful calculation of the makeup water quality and the extent to which sulfate will concentrate when the tower is cycled up to normal levels. Conditions may require the use of Type V Portland cement, which has a reduced amount of tri-calcium aluminate, ordinarily one of the primary components of standard cement.

Before discussing corrosion control, we’ll first address the primary causes of deposition and scaling in cooling systems. Since water treatment programs typically combine corrosion and deposit inhibitors, these topics are often discussed together.

Restricted-Flow Issues

One common restricted-flow location, outlined in Cooling Water System Fundamentals, is cooling tower fill. High-efficiency film fill provides excellent heat transfer but at the price of a torturous flow path that reduces water velocity. Throttled flow to heat exchangers can also establish low-flow zones that collect solids. An often-overlooked item with large cooling systems are dead legs that can accumulate materials, including microbes.

Microbiological Fouling

Water and air are filled with microbes that can potentially form troublesome colonies throughout cooling systems. Fouling can occur very rapidly and potentially force unit derating or equipment shutdown within days of the microbial onset. The protective slime secreted by some microbes easily traps suspended solids that convert the material into a mud-like product.

Figure 7.37. Slime/silt fouling in a heat exchanger.

Process Contaminant Fouling

Many large industries have numerous heat exchangers. Heat exchanger leaks may introduce contaminants to the cooling water return to the tower. Particularly troublesome are oils and heavy hydrocarbons that can coat cooling system equipment.

Scale Formation

Makeup Water Pretreatment Methods included discussion about solubility products, and how when various dissolved ions reach a solubility limit, solids precipitation occurs. This is the mechanism behind scale formation in water systems.

Figure 7.38. Cross-section of a heat exchanger tube showing calcium carbonate scale formation.

In Makeup Water Pretreatment Methods, we learned that the most common precipitate in natural waters is calcium carbonate (CaCO₃), and how CaCO₃ precipitation chemistry can be used advantageously in lime softening reactions for makeup water treatment. The buildup of calcium carbonate scale in water systems, such as home plumbing, has long challenged water treaters and led to the development of modern scale-control chemistry. To review briefly, almost all natural waters contain dissolved calcium ions (Ca²⁺) and bicarbonate alkalinity (HCO₃^–). Because of various influences, including temperature, the ions will precipitate from solution. The following reaction is representative of this process.

Ca²⁺ + 2HCO₃^– + heat → CaCO_3↓ + CO_2↑ + H₂O | Eq. 7-14

Calcium carbonate has three polymorphs. Calcite is the thermodynamically most stable form, and comprises most natural deposits.

Figure 7.39. Calcite crystal

A less stable form is aragonite, which is mainly found in biosynthetic CaCO₃, such as shells and corals. The final structure is vaterite, which rarely occurs in nature, but plays an important transitional role in calcium carbonate formation from solution.

Calcium carbonate deposition was the driver for the development of the first programs to predict scale-forming (and corrosion) tendencies of water impurities. These developments are highlighted below.

Other Scales

Depending upon the chemistry of the makeup water, or how it changes when cycled up in a cooling tower, other mineral deposits are possible in cooling systems. Table 7-2 outlines the most common.

Table 7-2. Other Common Cooling Water Scale Deposits

Scaling issues, particularly those associated with sulfate and phosphate deposition, have primarily resulted from modifications or improvements to chemical treatment programs.

It’s important to note that the mineral compounds in Table 7-2 differ from CaCO₃, which contains the CO₃ anion—the conjugate base of H₂CO₃. While a detailed discussion of acids and bases is outside this book’s scope, the primary point is that carbonate deposits are typically removable with even dilute acid.

CaCO₃ + H₂SO₄ → Ca²⁺_(aq) + SO₄^2-_(aq) + H₂CO₃ | Eq. 7-21

H₂CO₃ ⇌ CO_2↑ + H₂O | Eq. 7-22

Thus, if acid is supplied in sufficient quantities with uniform contact, CaCO₃ deposits will entirely dissolve as the carbonate converts to carbon dioxide. Sulfuric acid feed to cooling tower makeup was, and in some cases still is, a common method to reduce alkalinity and lower the potential for CaCO₃ scale formation. Acid feed requirements are often not large enough to cause calcium sulfate precipitation, but the issue cannot be ignored.

Calcium Sulfate

An often problematic issue is gypsum (CaSO₄∙2H₂O) scaling, influenced by either elevated sulfate concentrations in the makeup or from acid treatment to remove carbonate.

Calcium sulfate has higher solubility than CaCO₃, as shown below.

Figure 7.40. CaCO3 and gypsum solubilities as a function of temperature.

The figure reveals that gypsum also exhibits reverse solubility, but not until temperatures reach approximately 105^o F.

A common general guideline suggests limits of 1,200 ppm calcium (mg/L as CaCO₃) and 1,200 ppm sulfate (mg/L as SO₄), or some multiple thereof, to prevent scale formation at normal cooling system temperatures in untreated water. Higher limits may be possible with chemical treatment, but these cases should be evaluated on an individual basis.

Calcium Phosphate

As will be outlined in greater detail in the next section, in the 1980s a major shift in chemical treatment of open-recirculating systems occurred with the adoption of inorganic and organic phosphate chemistry for both scale and corrosion control. Suddenly, tricalcium phosphate (Ca₃(PO₄)₂) deposition became a major problem at many facilities.

Besides tricalcium phosphate (Ca₃(PO₄)₂), other calcium phosphate phases can form in cooling water. It is often assumed that the thermodynamically stable hydroxyapatite (Ca₅(PO₄)₃(OH)) is a suitable prototype for scale prediction. It seems that during the precipitation of calcium phosphate, amorphous calcium phosphate (ACP) forms first followed by the nucleation and phase transformation of other compounds.

ACP → Precursor → Stable Phase

Calcium phosphate(s) solubility is strongly dependent on solution pH and temperature. All species show inverse solubility with respect to those two parameters. The scaling tendency of calcium phosphate is also dependent on other influences, including those from other metal ions. All factors must be considered when calculating scaling potential.

Silica/Silicates

The aqueous chemistry of silica is complex, and any number of precipitates may form depending upon temperature, pH, and other factors. Potential deposits include:

• Amorphous silica in cooler sections of the water system
• Metal silicates in warm/hot locations or at elevated pH
• Silicate minerals such as clay that are suspended in the water

Amorphous silica is simply SiO₂. Many surface waters contain low levels (<15 ppm) of silica, however, some groundwaters can have concentrations as high as 75–80 ppm. At ambient temperature, the silica saturation level is around 150 ppm, so as the concentration of silica increases to saturation and above, a polymerization process induces formation of colloidal silica that can attach to system surfaces. This deposition mainly occurs in the coolest locations, such as tower fill.

In the pH range of approximately 2.0–8.3, silica solubility is independent of pH, however, dissolved silica converts to silicate (SiO₃^–) as pH rises above 8.3. Silicates will precipitate with cations, most notably magnesium and calcium. These compounds exhibit inverse solubility with respect to pH and temperature, and will first accumulate in warm locations, i.e., heat exchangers. Silica and silicate scales are tenacious and difficult to remove. They are also strong insulators that significantly reduce heat transfer.

Some chemical treatment programs may allow operation with dissolved silica concentrations at or even above 200 ppm. However, thorough knowledge of the water chemistry is necessary to push beyond this concentration. For example, polyvalent ions, such as Zn²⁺ and Al³⁺, are surrounded by hydroxyl groups that can catalyze silica polymerization. Among all cations, magnesium has the greatest potential to induce silicate deposition.

Dissolved silica can be analyzed by ultraviolet/visible (UV/VIS) spectrophotometry via the molybdate method. Total silica measurement, including the colloidal form where silica exists as solid particles, requires more advanced techniques, such as inductively coupled plasma (ICP) or atomic absorption (AA) spectroscopy.

Polyphosphates

Polyphosphates contain multiple phosphorus atoms connected to each other through oxygen bridges as shown in Figure 7.42. Polyphosphates generally contain from 3–5 units, and have negatively charged oxygen atoms that attract cations including calcium and iron. This attraction effectively sequesters the cations, preventing them from forming deposits.

Figure 7.42. Polyphosphate structure with sequestered iron and calcium ions.

A “threshold concentration” is necessary to inhibit calcium carbonate scaling when concentrations are at saturation levels. Polyphosphate also combines with manganese. Sodium tripolyphosphate (Na₅P₃O₁₀), tetrasodium pyrophosphate (Na₄P₂O₇), and sodium hexametaphosphate ((NaPO₃)₆) are just some of the polyphosphates. Normally 2 to 5 ppm of polyphosphate is needed in a treatment program. Polyphosphates will hydrolyze and revert to orthophosphate, where various factors such as residence time and temperature influence the rate of reversion.

Polymer Developments

Development of polymers for crystal modification and sequestration have enhanced deposit control chemistry. Figure 7.44 illustrates various functional groups for important water treatment polymers.

Figure 7.44. Functional groups on deposit control polymers. HPA, hydroxypropyl acrylate; HPS, 2-hydroxypropylsulfonate; AMPS: 2-acrylamido-2-methylpropane sulfonic acid; TBA, tert-butyl acrylate.

Along with the functional groups, polymer structure and size have an influence on scale inhibition. Molecules of 500–15,000 daltons in size are common, but in some cases much larger polymers may work well. Some polymers were designed to control calcium carbonate, calcium sulfate, and iron-related deposits, and others to control the calcium phosphates that can emerge from phosphate/phosphonate treatment programs.

Advanced formulations may include co-, ter-, and quad-polymers that have several different functional groups to treat complex waters. The compounds inhibit scale formation through several mechanisms, including sequestration, crystal modification, and crystal dispersion.

Crystal Modification

Some polymers modify or distort incipient crystals.

Figure 7.45a and b. Orderly and distorted growth of calcium carbonate crystals.

Distorted crystals exhibit none of the needle-like or flat-faced crystals shown in Figure 7.45a, rather the structure is much more fragile, friable, and does not form large crystal grains. PA, PMA, and similar compounds are effective for controlling calcium carbonate.

Calcium phosphate scale control is more difficult, especially on heat transfer surfaces. Scale prevention may require co- or ter-polymers that include sulfonate groups.

Iron presents a challenge to polymer chemistry, as iron binds strongly to carboxylic and sulfonate sites, reducing their effectiveness for calcium sequestration. In many waters, though, the iron concentration is low and does not present significant problems.

Design Methods to Help Control Deposition

Where silt or macrofouling impacts heat exchanger performance, installation of backwash equipment may be beneficial, if the unit can come off-line periodically for cleaning.

Figure 7.46a. Piping arrangement for reverse flow flushing.

An additional modification is to install a manifold below the heat exchanger to air bump the shell side water, as illustrated in the drawing below.

Figure 7.46b. Piping arrangement for air bumping.

Calcium Carbonate – The Natural Corrosion Inhibitor

The earlier section on scale formation suggested that calcium carbonate is the most natural, and often most problematic deposit. However, when calcium and bicarbonate alkalinity concentrations exist in moderation, the presence of both can be beneficial. If the water has at least 50 ppm of calcium hardness and 50 ppm of alkalinity (both as CaCO₃), the constituents potentially offer some corrosion protection as a cathodic inhibitor. The key is that at the cathodes shown in Figure 7.47, the production of hydroxyl ions generates a localized region of elevated pH. This can induce formation of a light layer of calcium carbonate that inhibits electron transfer at the cathodes.

Polyphosphate

As noted, polyphosphate sequesters multivalent cations, like calcium and iron, to inhibit scaling. These complexes develop a net positive charge and migrate to cathodes to form a barrier deposit. The deposit blocks oxygen from the surface, reducing the corrosion current.

Environmental Concerns of Phosphate-Phosphonate-Zinc Chemistry

Of considerable and growing concern is phosphorus discharge to natural bodies of water, and the effects this discharge has on proliferation of toxic algae blooms.

Figure 7.48. A landsat satellite image of Lake Erie during a harmful algae bloom event from September 2017.

At many locations now, phosphorus discharge is limited if not entirely banned. Metals discharge, including zinc and copper, is also restricted. These restrictions are a major factor in movement away from phosphorus-based programs to alternative film-forming programs.

Filming Chemistry

As previously suggested, the key function of corrosion inhibitors is to protect metal surfaces. The former acid-chromate programs were excellent in this regard in that over time the chromate would react with the entire metal surface and establish continuity, as long as sufficient residual concentration was maintained in the cooling water. But the change to phosphate-phosphonate programs altered this methodology. Corrosion inhibition is largely accomplished by precipitation of solid products at cathodes and anodes. These deposits can be washed away, allowing localized corrosion. Conversely, overfeed may induce heavy precipitation of calcium phosphate and sometimes calcium phosphonates.

Based on several years of research, ChemTreat has developed a series of non-phosphate and non-zinc corrosion control programs (FlexPro^®). These programs interact with metal surfaces to form a reactive polyhydroxy starch inhibitor (RPSI) complex that operates independently of calcium, pH, or other water chemistry components.³ The chemistry establishes a direct protective layer on metal surfaces, unlike the phosphate/phosphonate programs that rely on deposition of reaction products to form protective barriers, which, as has been noted, can be difficult to control.

A classic example of the need for improved corrosion protection methods is shown in the following illustration of a two-pass tube-and-shell heat exchanger, whose cooling water at the time was treated with a traditional phosphate-phosphonate program.

Figure 7.49. Two-pass heat exchanger on a phosphate-phosphonate program just prior to a change in treatment chemistry.

At the inlet end of the heat exchanger (the lower tubes of this unit), corrosion was problematic. At the warmer outlet side (the top half), deposition and scale formation were troublesome. Thus, the original program was not effective at mitigating corrosion or deposition depending on location. A switch to RPSI chemistry eliminated both issues.

Figure 7.50. A similar heat exchanger on RPSI chemistry.

Nitrite

Nitrite (NO₂^‒), usually fed as sodium nitrite (NaNO₂), is an anodic inhibitor by virtue of the chemical reaction with iron hydroxide at anodes. Sodium nitrite is an inexpensive and safe chemical to handle. As shown in the following reactions, nitrite promotes the formation of a passive iron oxide layer on the metal surface.

9Fe(OH)₂ + NO₂ → 3Fe₃O₄ + NH₄ + 2OH + 6H₂O | Eq. 7-24

9Fe(OH)₂ + NO₂ → 3(Fe₂O₃) + NH₄ + 2OH + 3H₂O | Eq. 7-25

Common is the addition of an alkalinity builder/buffering agent to maintain pH within a mildly alkaline range.

Anodic inhibitors like nitrite are also known as “dangerous” inhibitors, because if residuals fall below threshold limits, a small number of anodes will develop in a large cathodic environment. Rapid pitting may occur. Accordingly, a common recommended nitrite residual range is 500–1,000 ppm to inhibit general corrosion and pitting. However, when used with other inhibitors such as molybdate, lower nitrite residuals may be satisfactory. A concern with nitrite is that it is an excellent nutrient for bacteria. Nitrobactera agilis can grow rapidly by converting nitrite to nitrate. A classic example comes from an automobile assembly plant, where nitrifying bacteria plugged the small, serpentine cooling water tubes in automatic welders. Oxidizing biocides are not suitable for microbiological control, as the oxidizers convert nitrite to nitrate. A non-oxidizing biocide (see discussion later in this chapter) may provide effective control.

Copper Alloy Corrosion Inhibition

Copper is a superb metal for heat transfer, and copper alloys have been selected for many heat exchanger tubes. While copper is a more noble metal than iron, significant corrosion is possible in certain environments. As previously noted, the combination of dissolved oxygen and ammonia can be particularly corrosive. The most popular corrosion control methods for years have utilized azoles, in another example of film-forming chemistry. Figure 7.52 illustrates the general effect.

Figure 7.52. Illustration of copper alloy corrosion inhibition by azoles.

Azoles bond with copper atoms on the metal surface via an active nitrogen group. The plate-like organic ring then forms a barrier to protect the metal from the bulk fluid. The most common azoles are listed below.

Tolyltriazole

Tolyltriazole (TTA – C₇H₇N_₃) is similar to BZT but with a methyl group added to the organic ring.

Figure 7.53. Structure of tolyltriazole.

The methyl group helps orient the molecule to establish a more uniform barrier film.

Another of the early azoles is 2-mercaptobenzothiazole (MBT), which has two sulfur groups in the nitrogen ring. One of the sulfur atoms also bonds with copper to form a thick passive film.

One difficulty with these original compounds is attack by oxidizing biocides, which, are necessary for microbiological control. Water treatment companies have developed halogen resistant azoles that contain additional side groups to resist oxidizer attack.

Recommended azole concentrations are usually maintained within a range of 1–10 ppm, and often 2–5 ppm.

In addition to bonding with the metal surface, azoles also form complexes with free copper ions in solution. So, dissolved copper contributes to the azole “demand,” which must be satisfied before surface filming can occur. However, if the azole is applied properly, any free copper should rapidly disappear and not normally be present thereafter. Azoles remain an important part of many FlexPro^® programs.

Additional Inhibitors

Some additional compounds may, in certain situations, serve as corrosion inhibitors. The next sections examine two of these.

Silicates

First used in 1921, sodium orthosilicate, or as the anion SiO₃^2‒, is a precipitation type anodic corrosion inhibitor similar to orthophosphate. Silicates have been popular inhibitors in potable water systems, as they are non-toxic and occasionally find use for cooling water applications.

Orthosilicate technically is a mixture of silica monomers that coexist with polymerized silica, aka polysilicate (Figure 7.54). The percentage of poly vs. ortho compounds depends on the original silicate product, the concentration of the silicate in water, the pH and holding time.

Figure 7.54. Polysilicate structure.

Orthosilicate is initially adsorbed onto anodes at the metal surface, where it reacts with iron to form a thin monomolecular layer.

Fe²⁺ + SiO₃^2- + 2H₂O → FeO∙Si(OH)₃↓ + ½H₂↑ | Eq 7-28

The layer blocks oxygen-metal contact and reduces corrosion. Eventually, the entire metal surface becomes covered, effectively isolating the metal from electrochemical reactions. Microscopic and x-ray examination of the silicate barrier indicates formation of two layers, with most of the silica in a surface layer adjacent to the water.

A common guideline is an initial silicate dosage to establish a 25-ppm residual above the background concentration. 30 to 60 days of operation at this level is typical. The concentration can then be reduced to 8–12 ppm above background for normal control. The film does not build on itself and will not form scale. However, treatment can be very problematic in open-recirculating systems or heat exchangers with high heat transfer because of the potential for magnesium or calcium silicate scale formation. Another potential foulant is aluminum silicate, which could result if alum is utilized as a coagulant for pretreatment clarification of the makeup water.

Borate

Sodium tetraborate pentahydrate (Na₂B₄O₇•5H₂O) is an anodic inhibitor that protects ferrous metals from corrosion. It appears that borate produces a ferric-borate layer on metal surfaces, encouraging the formation of ferric oxide which acts as a barrier to iron ion transport, particularly ferric ions, from the metal surface. As a relatively weak inhibitor, borate supplements other inhibitors like nitrite or silicate. A key feature of borates is a good buffering capacity at a mildly basic pH of 8.3 or slightly above.

Borates and boric acid can protect against various forms of wood degradation from fungi, however, the concentrations needed are much higher than those that can be reasonably maintained in recirculating cooling systems.

Galvanizing

Carbon steel galvanizing is common for many applications, including cooling tower structural components. Small galvanized cooling towers (see Figure 6.9 in the previous chapter) can often be ideal from a balance of structural stability versus cost standpoint.

Critical to long zinc life is proper conditioning of the galvanized coating to establish a protective dark gray zinc patina. This can be achieved by exposing galvanized components to the atmosphere for a period of several months. As previously shown in Figure 7.28, guard rails along roads and other zinc structures exposed to the atmosphere develop this patina naturally.

For cooling tower components that are installed without long-term atmospheric exposure, conditioning of the material for perhaps two months before the unit is placed in service is necessary to avoid the formation of “white rust.” Details of this chemistry are outlined later in the “Pre-Operational Cleaning and Passivation Section” of this chapter.

Microbiological Concerns

Cooling water systems provide an ideal environment, warm and wet, for microbiological growth. Bacteria can form colonies in many locations, fungi will attack cooling tower wood, and algae will proliferate in sunlit areas, particularly on the decks and other exposed areas. More advanced microorganisms, such as amoeba and protozoa, often multiply in established microbial colonies. These more complex organisms can enhance growth of Legionella bacteria.

To control microorganisms, EPA-registered biocides, both oxidizing and non-oxidizing, are routinely employed, sometimes in conjunction with a biopenetrant/biodispersant. This section discusses the common microbes that flourish in industrial water systems, and it outlines control strategies.

Fungi, Yeasts, and Molds

The term fungi includes both yeasts and mold. Fungi are a group of non-photosynthetic eukaryotic organisms that are larger in size than bacteria. Most fungi are aerobic and require oxygen for survival. Yeast, such as those for beer or winemaking, are facultative anaerobes. Fungi are important organisms for many waste treatment processes as they break down organic material. In industrial cooling water systems, these attributes can be harmful. The fungi Basidiomycetes and Ascomycetes can attack cooling tower wood.

Whereas yeast is a type of fungi that contains only single cells, molds contain multiple identical nuclei. The colonies are characterized by threadlike, multi-cellular (and sometimes colorful) filaments called hyphae. Molds are important industrially for production of antibiotics and cheese.

Fungi thrive in acidic environments and are less active in modern cooling water systems per typical operation within a mildly basic pH range. However, it is not uncommon to find fungi in biofilm deposits, which is suggestive of an acidic environment within the biofilm.

Figure 7.57. Filamentous fungi.

Microbes and Biofilms

As previously noted, microbes in cooling water are either free-floating or sessile. System conditions will in some measure dictate sessile development. For example, high-velocities may physically prevent microbes from settling. When microbes attach to surfaces, some organisms almost immediately begin to produce a protective, polysaccharide layer commonly termed biofilm or slime. Aerobes usually thrive in the outer layers of the biofilm (near the bulk fluid) while the anaerobes grow in areas of low oxygen concentration, i.e., at the base of the biofilm. The slime entraps other microorganisms, silt, corrosion products, and additional debris in industrial waters.

Figure 7.59. Biofilm structure.

Figure 7.60. Nutrient gradients in biofilm.

Figure 7.61. Bioslime. Photo credit: CBE-MSU Bozeman.

The protective biofilm makes sessile microorganisms more difficult to kill than planktonic microbes. Research performed by Professor Phil Stewart at Montana State University has shown that the movement of any molecule into a biofilm is limited by diffusion and reactivity. Thus, the extent of biocide penetration is limited by the reactivity of the molecule towards the protective slime. We will examine these concepts shortly. Ideally, a plant’s cooling/service water biocide feed system has been designed and is properly operated to kill planktonic organisms before they can establish colonies. However, many times the plant staff must deal with treatment of well-established colonies.

Higher Life Forms – Amoeba and Protozoa

As previously noted, if microbiological colonies become well established, higher life forms often emerge. Amoeba and protozoa are ubiquitous single-celled eukaryotic microbes. The protozoan cell may have specialized appendages such as cilia or flagella for motion.

Figure 7.62. Images of higher microbial life forms.

While many types of amoeba can cause human health problems, the relationship between amoeba, protozoa, and Legionella bacteria is well known. Legionella is the organism first discovered in 1976 when it infected people attending an American Legion convention in Philadelphia. Nearly three dozen people died, and many more became ill. It was traced to water vapor containing the bacteria in the exhaust plume of a cooling tower on the roof of the convention hotel. The vapor entered the intake of an air handling unit and spread through the hotel.

The link between Legionella and amoebae was first demonstrated by Tim Rowbotham, who showed that amoebae could serve as hosts for the Legionella. Soon after, Kurtz described the isolation of amoebae in cooling waters which contain Legionella, thereby lending further support to Rowbotham’s theory on the relationship between the two. A later report by Rowbotham then offered a mechanism for the growth and proliferation of Legionella within the amoebae species, Acanthamoeba polyphaga.

Figure 7.63. Amoeba/protozoa with Legionella inside.

Studies have found that Legionella can grow in and be released from higher organisms; however, effective design and operation of the biocide feed system is essential to reduce microbiological fouling. Another important measure is finding and eliminating all dead leg piping. Stagnant water can allow organisms to proliferate.

Amoeba and protozoa are much more resistant to the biocidal effects of halogens than the simpler microorganisms but are susceptible to several commonly used non-oxidizing biocides.

Microbiological Control

As a preliminary note to the following discussion of cooling water biocides, many countries, including the United States, Canada, and Mexico, have placed limits on the use of antimicrobial compounds in industrial settings. Product registration is required, which typically comes with limitations on how and where the product may be applied. Products with the same type and level of active components but with different trade names each require registration as separate entities. In the United States, registrations fall under the jurisdiction of the Environmental Protection Agency (EPA) and the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA).

In addition to federal registration, additional state/territory/province regulations may be placed on the use of antimicrobial products. Most companies – especially larger facilities – also require finalized processes before a new chemical can be applied. Plants also must comply with discharge limits for some compounds. In the U.S., these regulations are under the auspices of the National Pollution Discharge Elimination System (NPDES) created within the Clean Water Act (CWA) of 1972. The regulations require approved permits before a water stream with treatment chemicals, including biocides, can be discharged to the environment. Individual states must enforce the NPDES regulations but are free to impose more stringent guidelines if desired.

Oxidizing and Non-Oxidizing Biocides

For the vast majority of recirculating and once-through cooling waters, oxidizing biocides are the backbone of microbial control program, as the chemistry usually represents the most cost-effective method for maintaining system cleanliness. Harder to treat systems may also benefit from a supplemental non-oxidizing biocide, and/or the addition of a biopenetrant/biodispersant. The most common compounds in each category are discussed below.

Nonoxidizing Biocides

While oxidizing chemicals normally serve as the foundation of cooling water biocide programs, microorganisms can develop partial immunity, largely from the protective biofilms. Accordingly, in some applications feed of a non-oxidizing biocide on a periodic basis, e.g., once or twice per week for an hour or so, can help to control microbial growth. Whereas oxidizing biocides typically damage cell walls, the non-oxidizers penetrate cell walls to then react with compounds within the cell that are necessary for life. The following sections examine several of the most common non-oxidizers.

Bronopol

Figure 7.70. Structure of bronopol.

Bronopol is a halogenated diol that contains an electrophilic carbon atom. It is used extensively in water treatment applications and, like 2,2-dibromo-3-nitrilopropionamide below, has seen limited applications in the oilfield. Bronopol is particularly effective against Pseudomonas bacteria. The compound seems to function via different chemical mechanisms depending on whether conditions are aerobic or anaerobic. Bronopol is not a fast-acting biocide. The compound may release formaldehyde upon decomposition, but the formaldehyde is not responsible for its biocidal properties.

Bronopol will hydrolyze in aqueous solutions at varying rates depending on pH. The hydrolysis rate is much faster at alkaline pH. Increasing temperature increases the rate of hydrolysis at a given pH. The optimum pH range for bronopol efficacy is 5–9. The compound will react with and become deactivated by sulfides and bisulfite-based oxygen scavengers. Lastly, bronopol possesses the potential to release nitrite that, in the presence of secondary amines, can form nitrosoamines.

2,2-dibromo-3-nitrilopropionamide (DBNPA)
Figure 7.71. Structure of DBNPA

DBNPA is a halogenated amide that is widely used in water treatment and pulp and paper applications. In the oil field, DBNPA serves to treat makeup water for fracturing fluids. DBNPA’s biocidal action is very rapid, and residual concentrations quickly hydrolyze to less toxic by-products. Rapid hydrolysis can be advantageous if the wastewater stream discharges through a retention pond, allowing the residual to decompose without need for a deactivation chemical. DBNPA has proven to be effective in controlling microbiological fouling in high-purity makeup water systems, particularly reverse osmosis units.

The presence of two bromine atoms at the second carbon of DBNPA makes the site very electrophilic, which then reacts with the nucleophilic, sulfur-containing amino acids methionine and cysteine in microbial cells. The reaction is irreversible and the result is inactivation of the protein that contains these amino acids. Cell death ensues.

The optimum pH range for maximum DBNPA efficacy is 4.0–8.0. The hydrolysis rate increases with increasing pH, and the compound rapidly loses potency above 8.0 pH. Hydrolysis also increases with rising temperatures. DBNPA is deactivated by sulfides and bisulfite-based oxygen scavengers. The compound is not stable to UV light, and it reacts with ammonia.

Glutaraldehyde

Figure 7.72. Structure of glutaraldehyde.

Glutaraldehyde has been used extensively in industrial water treatment applications, including oil and gas operations, the paper industry, and for medical instrument sterilization. The chemical reacts with two of the twenty essential amino acids, lysine and arginine, necessary for cell function. Efficacy is optimal within a pH range of 7–10, but the compound is more stable at an acidic pH. Glutaraldehyde will react with bisulfite-based oxygen scavengers to form an aldehyde-bisulfite addition product. This reaction is reversible and at elevated temperatures (>60°C), the complex will break down to release the glutaraldehyde. The reaction of glutaraldehyde with amines or ammonium ions is not reversible and produces a byproduct with little to no biocidal efficacy.

Isothiazolones

Figure 7.73a and b. The two common isothiazolones for cooling water treatment.

Apart from cooling water applications, formulations of isothiazoline are, in very slight concentrations, a common anti-microbial agent in many detergents. The most common isothiazolone-based biocidal formulation for cooling water treatment has a 3:1 mixture of CMIT and MIT. The CMIT and MIT concentrations in registered products are either 1.5 percent or 4 percent active ingredient. Various stabilizers are part of industrial formations, including cupric nitrate, magnesium nitrate, and potassium iodate. Also available is a 1.5 percent active product stabilized with bronopol. Proper product selection is often influenced by the stabilizer. The isothiazolones are broad spectrum but slow-acting bactericides that also show good reactivity towards fungi.

Both CMIT and MIT are incompatible with hydrogen sulfide, or other sulfur-based nucleophiles. Therefore, if sulfur-reducing bacteria (SRB) are present, isothiazolones may not be the best non-oxidizer choice. Elevated pH (>9.5) will reduce CMIT, but MIT is stable at a pH above 10.0. Isothiazolones can be deactivated with sodium bisulfite at a 2:1 mole ratio to isothiazolone.

Quaternary Amines

Figure 7.74. General structure of quaternary amines.

Quaternary amines, or “quats” as they are commonly termed, have been utilized extensively in a wide variety of cooling and process water applications. The molecules are positively-charged, with four alkyl groups attached to a central nitrogen atom. One or more of the alkyl groups consists of a methyl, benzyl, decyl (C₁₀), coco (C₁₄), or soya (C₁₈) chain. Quats are commonly fed in combination with other biocides and are usually very cost effective. Some compounds also serve as filming-amine corrosion inhibitors.

Quats have surfactant properties that solubilize cell membranes, leading to cell damage and death. The compounds are especially effective when used in combination with other biocides that also attack cell walls.

Foaming is a concern with quaternary amines. The alkyldimethylbenzyl ammonium chloride (ADBAC) quat foams extensively, but low-foaming compounds such as didecyldimethyl ammonium chloride (DDAC) are now available. Foaming may be problematic in seawater systems where the foam can interfere with the operation of deaeration towers. The surfactant properties of quats can inhibit the separation of oil/water emulsions in oilfield production systems. Hard water can decrease the biocidal activity of quats, and quats can react with negatively-charged scale and corrosion inhibitors, which lowers efficacy.

Tetrakishydroxymethyl phosphonium sulfate (THPS)

Figure 7.75. Structure of THPS.

THPS is employed mostly in oil-field applications but has also been used in traditional water treatment systems, as well as some non-biocidal applications. THPS is unique in that it will kill microorganisms, reduce hydrogen sulfide concentrations, and react with and dissolve iron sulfide.

Cleaning and Sanitization of Microbiological Foulants

The evolution of high-efficiency cooling tower film fill led to significant improvement in tower performance. However, as already has been noted, with the increased surface area of high-efficiency fill comes increased potential for scale formation and fouling. The best method to clean tower fill depends on several factors, including safety concerns, system metallurgy, in-service vs. out-of-service cleaning, potential impact on plant operations, disposal options for the cleaning solution, impact on the environment, and the chemical and physical nature of the foulant.

Microbiological/Organic Deposit Matrices

Deposits where microbes or organics serve as the binder for the deposit matrix are characterized by a soft, often mud-like consistency. Unlike mineral scales, microbiological deposits tend to accumulate primarily in the middle of the fill. Water velocities directly under the spray nozzles are generally high enough to inhibit microbial adhesion in the upper layer. For this reason, microbiological fouling sometimes goes undetected because it is not visible from above. As the water velocity slows within the fill, microorganisms begin to colonize fill surfaces, acting as a filter for suspended solids. Fouling tends to be more intense in the middle than at the bottom because the last few inches of fill do not physically support a thick, soft deposit. The inability to clearly view microbiological deposition from either top or bottom, combined with the difficulty of inspecting the middle fill layers, often allows fouling to progress undetected until it has reached an advanced stage. Several techniques have been devised to monitor susceptible areas, including suspension of fill sections from load cells; cutting an access window into the tower casing to allow a middle section to be removed periodically for inspection; suspending a fill section beneath the main fill to allow it to be easily inspected and weighed. Each method has been partially successful, but none have proven to be totally satisfactory.

Several methods exist to remove biological-silt deposits from cooling tower fill. Hyperhalogenation is a common technique, but results are often disappointing. Potential corrosion of system components and the need to dechlorinate prior to discharge are additional important issues when considering this method.

Microbiological deposits typically have high water content and will shrink and detach from surfaces when thoroughly dried. However, effectively drying out cooling tower fill can prove problematic even with the help of fans and if the tower is located in a low humidity climate. Chlorine dioxide has also been used as a cleaner for cooling tower biofilms with some success. The most widely practiced and effective cleaning method for deposits with microbiological or organic binders is hydrogen peroxide (H₂O₂) because of its oxidizing strength and the physical action of the oxygen micro-bubbles produced as it reacts with organic deposits. The positive environmental aspects of hydrogen peroxide from the rapid breakdown to water and oxygen, and its ease of application are additional factors favoring peroxide as a fill cleaner. Typical dosages are in the range of 500–3,000 ppm active H₂O₂. Surfactant addition in low concentrations will help loosen deposits. Polymeric dispersants are often included to keep solids in suspension for wastewater discharge. Disposal procedures should be well-established to deal with discharge of these typically turbid waters.

Figures 7.76 a and b below illustrate the appearance of slime-silt deposits on moderately fouled high efficiency cooling tower fill before and after cleaning.

Figure 7.76a. Cross-fluted tower fill fouled with a slime-silt deposits prior to peroxide cleaning

Figure 7.76b. Cross-fluted tower fill after peroxide cleaning.

In cases where the deposit contains a high percentage of inorganics, the circulating water may become highly turbid. The project procedures should include provisions for disposal of the turbid wastewater following the cleaning process.

Non-chemical Treatment Devices

While traditional water treatment programs rely on various chemistries to control scale, corrosion, and microbiological fouling in industrial water systems; some companies market non-chemical technologies as green or environmentally-friendly alternatives. Examples include magnetic devices, pulsed-power systems, electrostatic systems, ultrasonic instruments, and hydrodynamic cavitation systems. Manufacturers claim that the devices can replace traditional chemical treatments.

At first, these claims were not rigorously tested to determine whether the devices were capable of the advertised capabilities. However, in April 2010 the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) published a research report on several of these technologies in pilot cooling towers. While magnet suppliers often do not make claims about microbiological control, the others frequently claim reduced microbiological fouling. The ASHRAE report indicated that none of the physical water treatment devices reduced microbiological populations in the pilot cooling tower systems, whereas traditional chlorine-based treatments were effective. A subsequent study confirmed that these same non-chemical devices could not reduce planktonic or sessile Legionella or overall microbiological populations in pilot towers.24

One non-chemical method that has demonstrated a significant degree of cooling water microbial control is ultraviolet light (UV). This technology continues to improve, and recent work has shown UV to be especially effective (with proper system design) for macrofouling control. A primary concern with UV is the ability to reliably deliver sufficient energy to large water volumes or to what extent UV penetration is influenced by water clarity. For example, turbidity from suspended solids can shield microorganisms from the UV energy. Recommended guidelines for selecting UV as part of a microbiological control program are:

• Turbidity: <5 NTU
• TSS: <10 mg/L
• Total hardness typically at or below a range of 125-200 mg/L, as hardness scaling on the quartz sleeves of the lamps can influence transmittance
• pH range: 6.0–9.0

UV dosage is proportional to intensity multiplied by residence time. Most microorganisms can be destroyed with a UV dose of approximately 10,000 µW-s/cm². A common recommendation is system design with a UV dosage of 30,000 µW-s/cm².

Chemical Control: Oxidizing Biocides

Chlorine is a strong biocide towards veligers, and even though many facilities are limited to two hours of chlorine or bleach feed per day, if the chemical is consistently fed every day, many veligers will succumb. In some cases, plants have received waivers from environmental regulators for continuous feed over several weeks to kill organisms, at a common free-residual chlorine concentration of 0.2–0.5 ppm. For Asiatic clam control, continuous chlorination for four weeks in the spring and fall during peak spawning periods can be effective. With zebra mussels, continuous chlorination through the summer may be effective. However, continous-feed permits are often denied per concerns about formation and discharge of halogenated organic compounds, generically termed trihalomethanes (THM).

Bromine is more toxic to adult and juvenile Asiatic clams than chlorine, and it is a more rapid oxidizer, which enhances intermittent application. In addition, bromine usually decays more rapidly in the environment, and is less corrosive to copper, copper alloys, and stainless steel than chlorine.

Continuous treatment with chlorine dioxide has proven effective for zebra mussel control. ClO₂ requires lower dosages and shorter exposure times than chlorine. A 0.2 ppm concentration offers 100% zebra mussel mortality in only eight days as compared to more than three weeks or more for chlorine. At 1.0 ppm residual, ClO₂ will deliver 99% mortality in a four-day exposure as compared to 14 days for chlorine. At higher water temperatures during summer, toxicity is enhanced and the kill-time reduced. Intermittent treatment with chlorine dioxide requires higher residual concentrations. A major advantage of chlorine dioxide is minimal production of THMs compared to chlorine. Disadvantages include chemical and equipment costs, and concerns about chemical storage. Deactivation of residual chlorine dioxide and chlorite – a byproduct from the process – prior to discharge may be required to ensure aquatic safety.

Physical Control Methods

Most plant intakes have screens or strainers to prevent large debris from entering the cooling system, and these screens can effectively prevent adult mollusk passage. However, the rough screens do little to prevent veliger infiltration and subsequent colony formation. Zebra mussels exhibit mortality at temperatures above approximately 104°F (40°C), which has occasionally been utilized at plants with a piping arrangement that allows cooling water discharge recirculation to the intake. The technique can be particularly effective in winter, when the mussels are acclimated to cold water. Other non-chemical methods that have given mixed results include electric current, ionization radiation, and desiccation (drying). Zebra mussels can live for up to three weeks out of water, which has been a factor in their spread across much of the country. They attach to recreational watercraft, and when the boats are transported from one body of water to another, the mussels infest the new source.

No single mechanical/physical method has proven to be completely effective in controlling or preventing macrofouling, which highlights the importance of well-designed and operated chemical treatment programs to minimize these pests. Once colonies become established, a plant shutdown may be required to remove them mechanically.

Microbe Field Tests

Dip Slides

Dip slides consist of a plastic carrier that is coated on two sides with solid agar culture media that may contain the same or different media on each side, each of which has metabolic indicators (dyes).

Figure 7.77. An aerobic dip slide.

Two-sided media increases the variety of microbes that can be identified, for example bacteria and fungi. A common incubation period for dip slides is two days at the manufacturer’s specified temperature. The planktonic population can then be estimated by comparing the mat of microorganisms that have grown on the agar nutrient to a reference chart.

Dip slides are easy to use and transport. However, varying agar types among manufacturers may not support all bacteria found in industrial water systems.

Petri Films

Petri films are flexible plastic plates that contain agar, nutrients and dyes to make microbes readily visible. Because of their thin size and ease of handling, Petri films are field or lab adaptable. Various Petri films are available for specific microbe identification, including aerobic bacteria, E. Coli, yeast, molds, and several others.

Figure 7.78. Petri films.

Adenosine Triphosphate Analyses

Adenosine Triphosphate (ATP) is the primary energy carrier for all living organisms. Because ATP plays a fundamental and important role in cellular energy production, metabolic regulation, and cellular signaling; numerous methods exist to measure this important compound. A common method utilizes an ATP-dependent enzyme, luciferase, to generate light. The light is captured by a detector known as a luminometer. The near-immediate reaction allows almost instant calculation of the microbiological concentration. The procedure involves mixing a water sample (some methods rely on concentrating the microbes in a sample using membrane filters) with a surfactant to lyse the cells, thereby releasing the ATP into solution. Luciferin and luciferase enzyme are then added with thorough mixing, followed by light measurement.

Monitoring of Oxidizing Biocides

Regular analysis of oxidizing biocide residual is extremely important to ensure consistent and proper dosages.

On-Line Chlorine Analysis

Mature, on-line oxidizing biocide measurement is available. A standard industry instrument is shown below.

Figure 7.81. A Hach CL17sc analyzer. Photo courtesy of Hach.

The analyzer extracts a sample, and per the DPD discussion above, injects the dye and buffer. Following the proper reaction time, the instrument displays the oxidizer concentration, flushes the spent sample, and extracts a new sample. Thus, the analyzer can provide round-the-clock analyses, separated only by the time needed for reagent reaction. On-line monitoring can be of great benefit in promptly detecting biocide feed system malfunctions or water chemistry changes that increase biocide demand. Similar to grab-sample DPD analyses, either a free- or total-residual reagent is available for the analyzer.

Chemical Feed, Deposition, and Corrosion Monitoring

The preceding sections outlined the prominent cooling water deposition and corrosion mechanisms, and control methods. The next sections outline deposition and corrosion monitoring techniques.

Cooling System Cleaning and Passivation

This section presents general passivation guidelines for cooling systems. Too often, design engineers and plant personnel view pre-operational cleanings and system passivation as minor or unnecessary items. In these cases, the procedure at most might consist of filling the system with water, hydrostatic testing (where applicable), and then unit startup. Frequently, the water treatment provider is brought into the discussion much too late to offer information and advice on the criticality of proper pre-cleaning and passivation. This discussion outlines general recommendations for common applications, with the understanding that every cooling system has individual characteristics and metallurgies. Consultation with reputable chemical treatment experts is necessary to develop precise cleaning and passivation procedures.

Cleanup Procedures for Non-Galvanized Towers

Cleanup and passivation steps can be performed together or in immediate succession. In most cases, the tower water system is first cleaned and flushed and then passivated with strong inhibitor concentrations prior to placing the system into operation. The following steps are common:

• Fill the tower sump and lines to normal operating levels and begin recirculating tower water at normal (operating) pumping rates.
• Process exchangers, coolers, and/or condensers should be by‐passed on the process side. All waterside inlets and outlets should be fully open.
• Add inhibitor chemicals after start of circulation. Chemicals can be pumped, or manually slug fed, into the tower sump or injected directly into any major supply or return water header.

In general, the chemicals employed for initial cleaning and passivation include agents to maintain a neutral or mildly alkaline pH, surfactants for oil and grease removal, and an anti-foaming agent. A pyrophosphate to maintain a pH of 10 may be beneficial for enhanced passivation.

After the chemicals have been added, the tower water should be circulated at normal operating rates for four to eight hours to ensure proper distribution and cleaning. Correctly-applied procedures will produce passivated steel.

Figure 7.92. Comparison of correct vs. incorrect passivation.

At process conclusion, the tower, cooling system piping, and heat exchangers should be drained and flushed as thoroughly as possible to remove oils, silts, and biological sludge from the system. Again, arrangements should already be in place to dispose of this wastewater.

If the cooling system will remain filled but on standby for more than three days, protection with a non-oxidizing biocide is recommended. A copper-free isothiazolin product is common. At a minimum, the tower water should be circulated for one to two hours weekly prior to startup, but 1–2 hours daily is ideal.

Pre-Operational Cleaning of a Closed Chiller Loop

Following an extended shutdown or new installation, flash rusting, and stagnant conditions can take a dramatic toll on the condition of chiller networks. Failure to properly clean and passivate a system prior to startup may result in repeated unplanned failures. If the opportunity exists to take photos of key equipment conditions before and after cleaning, the pictures will allow evaluation of cleaning efficacy. Complete knowledge of system metallurgy is a must for selection of the proper cleaning and passivating chemicals. Carbon steel typically constitutes the primary system metallurgy, but copper alloys and stainless steel may also be present. If aluminum components are in the system, special precautions are needed to protect this metal.

General Pre-Cleaning Procedure

1. Establish system circulation and flush the equipment to remove large debris at the maximum possible flow rate. Fill the system to no more than 80% design volume to allow room for thermal expansion and cleaning agent addition. Failure to provide for expansion can result in equipment failures.
2. Inject the cleaning/passivation products. These consist of alkaline detergents, surfactants, and pH buffers to remove oil and grease and other debris, and to passivate the metals. An azole may be included for copper alloy passivation. Circulate the cleaning solution through the exchangers for 12–36 hours to remove corrosion products, oil and grease, and to passivate all metals. Have antifoam chemicals available.
3. When the circulation period is complete, drain the system to remove the pre-cleaning chemicals, corrosion products, mill scale, dirt, etc. Flush all valves and inspect for dead legs. (Ideally, dead legs should be identified and eliminated whenever possible prior to cleaning, passivation, and normal operation.)
4. Add appropriate corrosion inhibitors and possibly a non-oxidizing biocide if the equipment will be in standby following the cleaning/passivation process.

The cleaning/passivation process is different for every application and may be very specific for certain facilities such as data processing centers.

Conclusion

Cooling water chemistry, and minimization of corrosion, scaling, and microbiological fouling, is a complex subject. Many factors influence these phenomena. It is ChemTreat’s hope that this chapter will provide useful information for plant owners, operators, and technical personnel in developing effective cooling water treatment programs.

References

1. W.D. Callister, Jr., Materials Science and Engineering:  An Introduction, 3rd Ed., John Wiley & Sons, 1994.
2. E-mail correspondence with Dan Janikowski of Plymouth Tube Company, January 2022.
3. Post, R., and Kalakodimi, P., “The Development and Application of Non-Phosphorus Corrosion Inhibitors for Cooling Water Systems”; presented at the World Energy Congress, Atlanta, Georgia, October 2017.
4. Corrosion Basics:  An Introduction. National Association of Corrosion Engineers, Houston, TX, 1984.
5. Deacon, J. (no date) The Microbial World: Thermophilic microorganisms, University of Edinburgh, School of Biological Sciences. University of Edinburgh, School of Biological Sciences. Available at: http://archive.bio.ed.ac.uk/jdeacon/microbes/thermo.htm (Accessed: November 22, 2022). 
6. J.W. McCoy, Microbiology of Cooling Water. Chemical Publishing Company, NY 1980, pp. 197–198.
7. What is the difference between mold & yeast? – quora (no date) Quora. Available at: https://www.quora.com/What-is-the-difference-between-mold-yeast (Accessed: September 1, 2022). 
8. Stewart, P.S., 1996. “Theoretical Aspects of Antibiotic Diffusion into Microbial Biofilm,” Antimicrobial Agents And Chemotherapy, Nov. 1996, p. 2517–2522.
9. U.S. Department of Health and Human Services. (n.d.). Legionnaires’ disease – about the disease. Genetic and Rare Diseases Information Center. Retrieved 2021, from https://rarediseases.info.nih.gov/diseases/6876/legionnaires-disease/ 
10. T. Rowbotham “Preliminary Report on the Pathogenicity of Legionella pneumophila for Freshwater and Soil Amoebae” J. Clin. Pathol. 1980, 33, 1179-1183.
11. J. B. Kurtz, C. L. R. Bartlett, U. A. Newton, R. A. White, and N. L. Jones “Legionella in Cooling Water Systems” J. Hyg. Camb. 1982, 88, 369-381.
12. E. E. Sutherland and S. G. Berk “Survival of Protozoa in Cooling Tower Biocides” J. Ind. Microbiol. 1996, 16, 73-78.
13. D. B. McIlwaine, J. Diemer, and S. Berk, “The Efficacy of Glutaraldehyde Against Legionella Harboring Protozoa”, Corrosion 98, Paper No. 98521, (San Diego CA, NACE 1998)
14. Environmental Protection Agency. (n.d.). EPA: National Pollutant Discharge Elimination System (NPDES). Retrieved November 22, 2022, from https://www.epa.gov/npdes 
15. Baron, C. D. “Novel, Mild Oxidant Improved Cooling Water Treatment Performance Relative to Traditional Oxidizers.” Cooling Technology Institute, March 2012
16. Baron, C. D. “Improve System Performance and Reduce System Corrosivity Using a Novel Biocide for Cooling Towers.” Cooling Technology Institute, February 2016
17. Rohm and Haas Biocides Technical Note, “Stoichiometry of KATHON WT 1.5% Microbiocide with Sodium Bisulfite”.
18. Merianos, J. J. 1991. “Quaternary Ammonium Antimicrobial Compounds” in Block, S. S., Disinfection, Sterilization, and Preservation, 4th ed., Lea & Febiger, Philadelphia, pp. 225 – 255, and references cited therein.
19. Petrocci, A. N., Green, H. A., Merianos, J. J., and Like, B., 1974. “The Properties of Dialkyl Dimethyl Quaternary Ammonium Compounds,” C. S. M. A. Proceedings of the 60th Mid Year Meeting, May 1974, pp 87 – 89.
20. R. M. Post, K. Emery, G. Dombrowski, and M. Fagan, “Effectively Cleaning Cellular Plastic Cooling Tower Fill”; from the 33rd Annual Electric Utility Chemistry Workshop, June 11-13, 2013, Champaign, Illinois.
21. K. Boudreaux, “Using Biodetergents to Recover Megawatts Cheaply and with Minimal Environmental Impact”, Power Plant Chemistry, 2015, Vol 17(3), pp. 168 – 177.
22. R. D. Vidic. S. M. Duda, and J. E. Stout, “Biological Control in Cooling Water Systems Using Non-Chemical Treatment Devices”, ASHRAE Project Number 1361-RP, Final Technical Report, April 2010.
23. J. E. Stout, S. M. Duda, and R. D. Vidic, “Efficacy of Non-Chemical Devices in Controlling Legionella: Results From A Model Cooling System” Cooling Technology Institute Paper No: TP11-08.
24. Khalanski, M. (1993). Testing of five methods for the control of zebra mussels in cooling circuits of power plants located on the Moselle River. Third International Zebra Mussel Conference, Toronto.
25. Matisoff, G., Brooks, G., and Bourland, I.B. (1996). Optimizing treatment processes of chlorine dioxide to adult mussels. J. A.W.W.A., August, 1996, pp. 93-106.
26. Roensch, F., “Clam and Mussel Control” ChemTreat Technical Bulletin #91, September 2004.

28. Elliott, P., “Pretreatment, Passivation, and Maintenance Recommendations for Galvanized Cooling Towers or Evaporative Condensers”, ChemTreat Technical Bulletin #82, April 2021.

Acknowledgements

The ChemTreat Water Essentials Handbook would not have been possible without the contributions of many people. See the full list of contributors.

Previous Chapter   Next Chapter