Cooling towers reject heat by evaporating water continuously, and every litre evaporated has to be replaced with makeup water to keep the system operating at design capacity. The storage feeding that demand has different requirements to potable storage in almost every meaningful respect — sizing, water quality target, and material compatibility.
Where potable storage is typically sized around demand fluctuation and supply security, cooling tower makeup storage is sized around a continuous, calculable evaporation rate that scales directly with cooling load and ambient conditions. On a hot day with the plant running at full load, evaporation — and therefore makeup demand — can be substantially higher than on a mild day at partial load, and storage needs to buffer that variability reliably.
Evaporation, drift and blowdown together
Total makeup demand is the sum of three losses: evaporation (the primary heat-rejection mechanism and usually the largest component), drift (fine water droplets carried out with the exhaust air stream), and blowdown (water deliberately discharged to control dissolved solids concentration, since evaporation leaves minerals behind and concentrates them in the remaining water). Storage sizing needs to account for all three, not just the obvious evaporation loss.
Water quality targets differ from potable
Cooling water does not need to meet drinking water quality, but it does need to meet specific chemistry targets for the cooling system — controlled hardness, pH, and dissolved solids concentration to manage scaling and corrosion within the tower and heat exchanger surfaces. Storage and any associated treatment need to be specified around these cooling-system-specific chemistry targets, which are a different set of parameters entirely from potable water compliance.
Chemical dosing compatibility
Cooling systems are commonly dosed with corrosion inhibitors, scale inhibitors and biocides to manage water chemistry and biological growth — and the storage tank material needs to be compatible with whatever dosing regime the specific system uses. A tank material and coating selected without reference to the actual chemical treatment program can degrade prematurely in ways a standard potable water tank never would.
Segregation from potable supply is a genuine risk control
Where a site has both potable and cooling tower makeup storage, the two systems need clear physical segregation with backflow prevention at any interconnection point — cross-contamination risk between a chemically treated, biologically active cooling water system and a potable supply is a serious water safety issue, not a minor plumbing detail. This segregation should be verified at design and periodically re-checked, not assumed to remain correct indefinitely as a site is modified over time.
Cooling tower makeup storage looks like "just another industrial tank" from the outside, but its sizing, water chemistry target and material compatibility all need to be worked out against the cooling system's actual duty — not borrowed from a potable water specification.
| Check | Why it matters |
|---|---|
| Sizing based on evaporation, drift and blowdown together | Evaporation alone understates true makeup demand |
| Water chemistry targets matched to the cooling system | Different from potable compliance parameters entirely |
| Tank material compatible with the dosing regime | Chemical treatment can degrade an incompatible tank material |
| Physical segregation from potable supply, with backflow prevention | Prevents cross-contamination between the two systems |
Does cooling tower makeup water need to be potable quality?
Not necessarily. Makeup water needs to meet chemistry targets specific to the cooling system — hardness, pH and dissolved solids control for scaling and corrosion management — rather than drinking water standards. The exact requirement depends on the cooling system design and treatment program in use.
Why does blowdown affect how much makeup water storage is needed?
Blowdown is water deliberately discharged to control mineral concentration as evaporation leaves dissolved solids behind. It represents ongoing consumption alongside evaporation and drift, so all three losses together — not evaporation alone — determine actual makeup water demand and appropriate storage sizing.
Can potable and cooling tower storage share the same tank?
This is not recommended. The two systems have different water chemistry requirements and, if the cooling system is chemically dosed, a genuine cross-contamination risk if not properly segregated. Separate storage with backflow prevention at any interconnection is the safer and standard approach.
Specifying makeup water storage for a cooling tower system? PC Water Infrastructure can size and specify storage matched to your actual process demand.
Discuss Industrial Water Storage