Circulating Water Treatment Guide: Scale, Corrosion & Microbial Control Explained

Industrial circulating cooling water systems are vital components in many processes. However, during operation, water evaporation and windage losses lead to the continuous concentration of circulating water. This results in increased salt content, a rise in anion and cation levels, and significant pH value changes, all contributing to water quality deterioration. Furthermore, the temperature, pH, and nutrient content of circulating water create a favorable environment for microbial proliferation, with cooling towers exposed to sunlight being ideal for algae growth. Effective circulating water treatment is therefore essential to manage scale formation, control corrosion, and inhibit microbial activity.

Common Problems in Circulating Water Systems

Several key issues can arise in circulating water systems if not properly managed:

• Scale Formation: The Silent Efficiency Killer

  As water circulates and evaporates in cooling systems, dissolved salt concentrations increase. When these concentrations exceed the solubility of certain salts, they precipitate and form hard deposits known as scale. Common types include calcium carbonate, calcium phosphate, and magnesium silicate. Scale is dense and significantly reduces heat transfer efficiency; a mere 0.6 mm layer of scale can decrease heat transfer coefficients by 20%. Proactive measures, such as utilizing Reverse Osmosis Systems to purify makeup water, can significantly mitigate scale buildup.

• Fouling: Beyond Simple Dirt

  Fouling is primarily caused by organic matter, microbial colonies and their secretions, silt, and dust suspended in the water. Unlike hard scale, foulants are typically softer but equally detrimental. They not only impede heat transfer but also promote under-deposit corrosion, thereby shortening equipment lifespan. Effective removal of these particulates is a key part of a comprehensive industrial water treatment strategy, often involving various filtration stages within a larger system.

• Corrosion: The Gradual Degradation of Assets

  Corrosion in circulating water systems, particularly of heat exchange equipment, is mainly electrochemical. It's driven by factors such as manufacturing defects in equipment, high dissolved oxygen levels, corrosive ions (e.g., Cl-, Fe2+, Cu2+), and biofilms formed by microbial secretions. The consequences of unchecked corrosion are severe, potentially leading to the rapid failure of heat exchangers and piping. Implementing a proper water treatment plant design is crucial for effective corrosion control.

• Microbial Slime: A Breeding Ground for Problems

  Circulating water often contains ample dissolved oxygen, optimal temperatures, and nutrient-rich conditions, making it highly conducive to microbial growth (bacteria, algae, fungi). Uncontrolled microbial proliferation can quickly lead to water quality degradation, foul odors, and discoloration (e.g., blackening). Cooling towers can suffer from extensive slime deposits, blockages, drastically reduced cooling efficiency, and intensified corrosion. Therefore, microbial control in water systems is a critical aspect of circulating water treatment. Solutions such as those found in our Sterilizer category, including UV sterilizers and ozone generators, can be highly effective in managing microbial populations.

The Menace of Microorganisms and Essential Monitoring

Microorganisms in cooling water systems originate from two main sources: airborne microbes drawn in during cooling tower operation and those present in the makeup water supply. Algae, under sunlight, perform photosynthesis using carbon dioxide and bicarbonate, releasing oxygen. A large algae bloom can thus increase dissolved oxygen, accelerating depolarization and corrosion processes.

Widespread microbial growth can cause circulating water to turn black and develop foul odors, polluting the environment. It also leads to the formation of significant slime deposits, which reduce cooling tower efficiency and can cause wood deterioration. Slime in heat exchangers lowers heat transfer rates, increases pressure drop, and can initiate severe under-deposit corrosion. Furthermore, these biofilms can shield underlying metal from corrosion inhibitors, rendering them ineffective. Some bacteria also produce metabolic byproducts that are directly corrosive. These issues collectively compromise the long-term, safe operation of circulating water systems, leading to substantial economic losses. Consequently, controlling microbial hazards is as critical as, if not more so than, managing scale and corrosion.

Monitoring microbial activity in circulating water can be achieved through these chemical analyses:

• Residual Chlorine (Free Chlorine): When using chlorine for disinfection, it's vital to monitor the appearance time and level of residual chlorine. High microbial loads significantly increase chlorine demand.
• Ammonia (NH3): Circulating water should ideally be ammonia-free. Its presence may indicate process leaks or atmospheric contamination. Investigate sources and monitor for nitrite, keeping ammonia below 10mg/l.
• Nitrite (NO2-): The presence of both ammonia and nitrite suggests nitrifying bacteria are converting ammonia. This dramatically increases chlorine demand, making it hard to achieve target residuals. Aim for NO2- levels below 1mg/l.
• Chemical Oxygen Demand (COD): Severe microbial proliferation increases COD because bacterial secretions add to the organic load. COD analysis helps track microbial trends; ideally, COD (KMnO4 method) should be below 5mg/l.

The damage caused by microorganisms in circulating water is extensive. Reactive measures after problems arise are often less effective and more costly, requiring large amounts of biocides. Therefore, proactive and comprehensive monitoring of microbial conditions is indispensable for effective cooling water treatment.

The Significance of Concentration Ratio (Cycles of Concentration)

The concentration ratio in a circulating water system refers to the degree to which dissolved solids in the water become concentrated due to evaporation and drift, benchmarked against the makeup water. It's a key comprehensive indicator of water quality control effectiveness.

A low concentration ratio means higher water consumption and blowdown volumes, and underutilization of water treatment chemical efficacy. A higher concentration ratio can reduce water usage and save on overall water treatment costs. However, an excessively high concentration ratio increases the propensity for scale formation, complicates scale and corrosion control, may lead to treatment chemical failure, and can hinder microbial control. Thus, maintaining an optimal and reasonable concentration ratio is crucial for balanced system operation. For more information on how effective systems contribute to cost savings, you can explore general solutions at Stark Water.

Understanding Scale Formation Mechanisms

Scale in circulating water systems forms from supersaturated dissolved components. Water contains various dissolved salts like bicarbonates, carbonates, chlorides, and silicates. Among these, dissolved bicarbonates such as calcium bicarbonate (Ca(HCO3)2) and magnesium bicarbonate (Mg(HCO3)2) are the most unstable and readily decompose to form carbonates. When cooling water rich in bicarbonates flows over heat exchanger surfaces, especially hotter areas, these salts decompose. If phosphate and calcium ions are present, calcium phosphate will also precipitate. Unlike many common salts, the solubility of calcium carbonate and calcium phosphate decreases as temperature increases. Consequently, on heat transfer surfaces, these sparingly soluble salts easily reach supersaturation and crystallize out of the solution. This tendency is exacerbated by low flow velocities or rough surfaces, leading to the deposition of these crystals as hard scale. Common scale components include calcium carbonate, calcium sulfate, calcium phosphate, magnesium salts, and silicates. Managing these scale-forming ions often involves pre-treatment and careful selection of water treatment system accessories and components like specific membranes or filter media.

Advanced Circulating Water Treatment Technologies

Selecting the right water treatment solutions is paramount, considering the specific characteristics of the enterprise's circulating water system, process conditions, and local water quality. By implementing measures such as precise chemical dosing programs, circulating water parameters can be maintained within an optimal range. This not only ensures the long-term, reliable operation of production equipment but also significantly enhances water use efficiency. The application of advanced circulating water treatment technology offers considerable economic benefits to businesses and positive environmental outcomes for society. Therefore, its adoption is highly necessary. Stark Water is committed to providing cutting-edge industrial water treatment technologies to address these challenges effectively.