Cycles of Concentration: The Number That Sets Blowdown

Legacy context

From the early days of industrial water treatment, the focus was always on precision. PolymerTech built its reputation not on guesswork, but on laboratory-backed analysis—measuring anions, cations, and conductivity to fine-tune chemical programs for cooling towers and boiler systems. That heritage of exact measurement is the foundation for every modern treatment strategy.

In any recirculating system, the goal is to reuse water efficiently. This is where the concept of cycles of concentration becomes central. Simply put, it is the ratio of dissolved solids in the blowdown water compared to the makeup water. As water evaporates in a cooling tower, minerals and impurities remain behind, concentrating over time. Managing this ratio is a delicate balance: higher cycles save water and reduce chemical usage, but if pushed too far, they can lead to scale formation or corrosion on heat transfer surfaces.

The analytical rigor that guided our early field representatives still applies today. Understanding the specific chemistry of your makeup water is the first step in determining how many cycles your system can safely achieve. It is not a fixed number but a dynamic target, influenced by temperature, system design, and the effectiveness of your treatment program. Getting that balance right is where operational efficiency begins.

What Cycles of Concentration Measures

Cycles of concentration (COC) is the technical term used to describe the mass flow relationship between the amount of system feed water and the amount of blowdown sent down the drain [1]. Also referred to as the concentration ratio, it is a direct measure of how many times dissolved minerals in the recirculating water have been concentrated relative to the incoming makeup water. In practical terms, if your makeup water contains 500 ppm of dissolved solids and your recirculating water contains 2,000 ppm, you are operating at four cycles of concentration.

The concept is straightforward: as water evaporates in the cooling tower, the dissolved solids remain behind in the basin. The recirculating water becomes progressively more concentrated with each evaporation cycle. The ratio between the concentration of these dissolved solids in the recirculating water and their concentration in the makeup water defines your operating cycles.

Deriving Cycles from Conductivity or Chloride

In field practice, cycles of concentration are typically monitored using conductivity or chloride measurements. Conductivity provides a convenient proxy for total dissolved solids because dissolved ions conduct electricity proportionally to their concentration. Chloride is often preferred as a tracer because it does not precipitate, volatilize, or get consumed by biological activity, making it a conservative species that accurately reflects concentration changes.

The calculation is a simple ratio:

Cycles of Concentration = Conductivity (or Chloride) in Recirculating Water ÷ Conductivity (or Chloride) in Makeup Water

For example, if the makeup water has a conductivity of 400 µS/cm and the recirculating water measures 1,600 µS/cm, the system is operating at four cycles. This ratio method assumes that the only mechanism adding dissolved solids to the recirculating water is evaporation, and that the only removal mechanism is blowdown. Drift—water droplets entrained in the air leaving the top of the tower or blown from the side by crosswinds—also removes dissolved solids but typically in much smaller quantities [1].

Relationship Between Evaporation, Blowdown, and Makeup

The mass balance of a cooling tower system ties together makeup water, evaporation, blowdown, and drift. Makeup water must replace all losses due to evaporation, leaks, or discharge [1]. At steady state, the amount of dissolved solids entering with makeup must equal the amount leaving with blowdown and drift.

Higher cycles of concentration are directly related to low levels of water loss from the system [1]. When you operate at higher cycles, you discharge less blowdown because the recirculating water can carry more dissolved solids before reaching the saturation limit. This means less makeup water is required to maintain the system inventory. Conversely, low cycles of concentration—meaning a high amount of blowdown in relation to the system feed—correlate to inefficient use of water in the system [2].

The relationship can be expressed simply: as cycles increase, blowdown volume decreases, and therefore makeup water demand decreases. This is why raising cycles is a primary water conservation strategy for cooling tower operations.

Why Raising Cycles Saves Water and Chemical

Operating at higher cycles reduces both water consumption and chemical usage. With less blowdown, you discharge less water, which directly reduces makeup requirements. The water savings are accompanied by chemical savings because treatment chemicals are consumed in proportion to the water being treated and discharged. When you blow down less frequently, you add less chemical to maintain the desired treatment residuals.

However, the savings are not linear. The marginal benefit of each additional cycle diminishes as cycles increase. Going from two to four cycles saves significantly more water than going from eight to ten cycles. The practical operating point depends on the specific water chemistry and treatment program.

Driving Toward Saturation

The fundamental limit on cycles of concentration is saturation. Scaling is the precipitation of dissolved mineral components that have become saturated in solution [4]. As cycles increase, the concentration of dissolved minerals in the recirculating water rises. Eventually, the solubility limits of sparingly soluble salts—particularly calcium carbonate, calcium sulfate, and silica—are approached or exceeded.

When these minerals precipitate, they form scale on heat exchange surfaces. Scale formation inhibits heat transfer because of the insulating properties of scale, and scale buildup will lower the efficiency of the system [4]. The factors that contribute to scaling tendencies include water quality, pH, and temperature [4]. Higher temperatures at heat exchange surfaces make scaling more likely because most mineral salts become less soluble as temperature increases.

The saturation point is not a fixed number. Other constituents in the water such as trace metals, natural organic matter, ligands, and phosphates can affect calcium carbonate precipitation rates and result in a higher or lower saturation pH [6]. This means the practical ceiling on cycles depends on the specific chemistry of your makeup water and the conditions in your system.

Treatment Chemistry Sets the Practical Ceiling

The practical maximum cycles of concentration for any cooling system is determined by the treatment chemistry program and the makeup water quality. Treatment chemicals are designed to address the four primary treatment concerns in open-recirculating cooling systems: corrosion, scaling, fouling, and microbiological activity [4]. These concerns are interrelated such that reducing one can have an impact on the severity of the other three [4].

Corrosion can be intensified by elevated levels of dissolved mineral content in the water and the presence of oxygen, both of which are typical of most cooling tower systems [4]. As cycles increase, the corrosive potential of the water generally increases. Scale inhibitors allow the system to operate at higher cycles by keeping minerals in solution beyond their natural saturation point. Dispersants prevent fouling by keeping suspended solids from settling on surfaces. Corrosion inhibitors protect metallurgy from the more aggressive water chemistry at higher cycles.

The ceiling is therefore a balance. Push cycles too high and you risk scaling or corrosion that damages equipment and reduces efficiency. Operate too low and you waste water and chemical. The optimal operating point is determined by your makeup water analysis, the metallurgy of your system, and the capabilities of your treatment program. Effective management of these conditions through proper treatment and filtration can optimize system performance, often resulting in moderate to significant energy and water savings [2].