Blowdown is the intentional removal of a portion of recirculating water from a cooling tower to prevent dissolved minerals from building up to damaging levels. As water evaporates from the tower during normal operation, the minerals it carried don’t evaporate with it. They stay behind, becoming more and more concentrated in the remaining water. Blowdown flushes out that mineral-heavy water so it can be replaced with fresh supply water, keeping the system in balance.
Why Minerals Build Up in the First Place
Cooling towers work by evaporating water to remove heat from a building or industrial process. The water that evaporates is essentially pure, leaving behind everything that was dissolved in it: calcium, magnesium, chloride, silica, and other minerals. Each cycle of evaporation concentrates those dissolved solids further, the same way boiling a pot of saltwater makes it saltier over time.
If nothing is done, the mineral concentration climbs until two things happen. First, minerals like calcium carbonate come out of solution and form hard, chalky scale on heat exchange surfaces, pipes, and tower fill. Scale acts as insulation, forcing the system to work harder and driving up energy costs. Second, the chemistry of the concentrated water shifts in ways that accelerate corrosion of metal components. Blowdown is the primary tool for keeping both problems in check.
Cycles of Concentration
The key metric for managing blowdown is called cycles of concentration (COC). This number tells you how many times more concentrated the dissolved solids in the recirculating water are compared to the fresh makeup water coming in. If your tower water has three times the mineral content of the supply water, you’re running at 3 COC.
Most chemically treated cooling towers operate between 4 and 6 COC, depending on the quality of the incoming water and how effective the chemical treatment program is. The COC is controlled directly by adjusting the blowdown valve, typically located at the bottom of the tower.
The relationship between COC and water use is not linear, which is important for anyone trying to save water. Consider a tower that evaporates 5,000 gallons in a given period. At 2 COC, blowdown equals the evaporation amount (5,000 gallons), and total water use is 10,000 gallons. At 4 COC, blowdown drops to about 1,667 gallons and total use falls to roughly 6,667 gallons. Pushing from 4 to 6 COC saves another 667 gallons of blowdown, but the returns get smaller with each step. Going from 6 to 8 COC only saves about 286 gallons. The biggest water savings come from moving out of the low COC range, while pushing toward very high COC yields diminishing returns and increases the risk of scale and corrosion.
How Scaling Potential Is Measured
Deciding how high to push the COC requires knowing when minerals will start dropping out of solution. Water treatment professionals use indices like the Langelier Saturation Index to estimate this threshold. The index compares the water’s actual pH to the pH at which calcium carbonate would begin to precipitate. A positive value means the water favors scale formation. A negative value means the water leans toward being corrosive. A value near zero indicates the water is roughly in equilibrium.
These indices don’t tell you how much scale is present. They only flag the potential. But they’re essential for setting blowdown rates because they help determine the maximum COC the system can handle before scale becomes a problem. A tower fed by hard, mineral-rich well water will need more frequent blowdown (lower COC) than one supplied with softer municipal water.
Where Blowdown Water Goes
Blowdown water contains elevated levels of dissolved minerals plus any chemical treatment additives (corrosion inhibitors, biocides, scale inhibitors) that were dosed into the recirculating loop. It can’t simply be dumped. In most cases, blowdown is discharged into the sanitary sewer system, where it flows to a municipal wastewater treatment plant.
That discharge is regulated under the Clean Water Act through the National Pretreatment Regulations. The EPA delegates enforcement to state environmental agencies, which in turn often hand authority to local utilities. Facilities that discharge cooling tower blowdown to a sewer system must comply with local limits on specific contaminants to prevent interference with wastewater treatment or pass-through of pollutants into receiving waterways. The exact limits vary by jurisdiction, so the requirements for a facility in one city may differ from those in the next.
Recovering and Reusing Blowdown Water
Because blowdown represents a significant portion of a cooling tower’s total water consumption, there’s growing interest in capturing and reusing it rather than sending it down the drain. Recovery systems typically route a portion of the blowdown through carbon filtration to strip out chlorine, then push it through reverse osmosis membranes that extract the dissolved minerals. The cleaned water, now with essentially zero hardness, is returned to the cooling tower loop.
A General Services Administration evaluation of one such system found that blowdown volume dropped by 60% when blowdown recovery was paired with a partial water-softening system on the makeup water side. The manufacturer estimates the combination could reduce blowdown by up to 93% under optimal conditions. These systems add upfront cost and maintenance, but for large commercial or industrial towers in regions with high water prices or supply constraints, the payback can be meaningful.
Practical Impact on Operations
For building operators and facility managers, blowdown is one of the main levers for balancing water efficiency against equipment protection. Too little blowdown and you get scale buildup, reduced heat transfer, and potential equipment failure. Too much and you waste water and chemicals. The goal is finding the sweet spot where COC is high enough to conserve water but low enough that the dissolved solids stay safely below the scaling threshold for your particular water chemistry.
Automated conductivity controllers make this easier by continuously measuring the dissolved solids in the recirculating water and opening the blowdown valve only when the concentration exceeds a set point. This prevents both over-blowing (wasting water) and under-blowing (risking scale). For systems still using manual or timer-based blowdown, switching to conductivity-based control is one of the simplest upgrades available for improving water efficiency.