Cooling Water Treatment: Scale, Corrosion and Biofouling is the chemical and mechanical management of open recirculating cooling water to control the three mechanisms that foul and destroy heat exchangers and cooling towers: mineral scale, metal corrosion, and biological growth. As water evaporates, dissolved solids concentrate, oxygen and nutrients enter, and warm wetted surfaces become an ideal habitat for microbes. A treatment programme keeps all three in balance so heat transfer stays efficient and metal loss stays predictable.
An open recirculating system rejects heat by evaporating a fraction of the circulating water. Pure water leaves as vapour, so every dissolved mineral, chloride and sulphate left behind concentrates with each pass. This is expressed as cycles of concentration, the ratio of a conserved species (often chloride or conductivity) in the circulating water to the same species in the make-up water. Higher cycles save water but push toward scaling, so the programme sets a target and holds it with controlled blowdown.
Scale is the deposition of sparingly soluble salts, most commonly calcium carbonate, on hot surfaces. It insulates tubes, throttles flow and shelters corrosion. The classic scaling predictor is the Langelier Saturation Index (LSI), defined as LSI = pH - pHs, where pHs is the pH at which the water is exactly saturated with calcium carbonate at the given calcium hardness, total alkalinity, total dissolved solids and temperature.
The Ryznar Stability Index, RSI = 2 x pHs - pH, is a related empirical index where values below about 6 signal a scaling tendency and values above about 7 signal a corrosive tendency. Both indices describe the calcium carbonate driving force only, not general corrosivity, so they support rather than replace direct corrosion monitoring.
A modern programme doses several product families that each target one mechanism, then trims them against measured water chemistry:
| Problem | Primary cause | Principal control |
|---|---|---|
| Calcium carbonate scale | Positive LSI, high hardness and alkalinity, high skin temperature | Scale inhibitor, acid or pH control, lower cycles |
| General and pitting corrosion | Dissolved oxygen, low pH, high chloride and sulphate, negative LSI | Corrosion inhibitor film, pH control, coupon monitoring |
| Biofouling (biofilm and slime) | Warm water, sunlight, nutrients, low or intermittent biocide | Oxidising plus non-oxidising biocide, dispersant, ORP control |
| Microbiologically influenced corrosion | Under-deposit anaerobic bacteria such as sulphate reducers | Biofilm control, dispersants, prevent deposits |
| Silt and suspended solids | Airborne dust scrubbed into the tower | Sidestream filtration, dispersant, adequate blowdown |
Warm, aerated, nutrient-rich cooling water grows bacteria, algae and fungi that form biofilm on tube walls and tower fill. Biofilm is a strong thermal insulator, cutting heat transfer far out of proportion to its thickness, and it shelters the metal beneath from bulk-water inhibitor. Under that film, oxygen-depleted zones let sulphate-reducing and other bacteria drive microbiologically influenced corrosion, producing deep localised pits that can perforate a tube while most of the surface stays clean. Control means holding a measurable biocide residual, alternating oxidising and non-oxidising chemistries, and using dispersants so a film never establishes.
Blowdown is the deliberate bleed of concentrated water to drain, replaced by fresh make-up, and it is the master control for cycles. As a first approximation, make-up equals evaporation plus blowdown plus drift, and cycles of concentration equals make-up divided by the sum of blowdown and drift. Running at three to six cycles is common; each added cycle saves water but tightens the scaling and corrosion window, so the achievable figure is set by make-up water chemistry. Uncontrolled deposits then drive heat exchanger fouling, which raises approach temperature and pumping load until the exchanger must be cleaned.
A routine programme tracks conductivity as a proxy for cycles, pH, calcium hardness and alkalinity for the LSI, inhibitor residual, and oxidising biocide by free residual or oxidation-reduction potential. Metal loss is confirmed with corrosion coupons, usually expressed in mils per year, and microbial load with dip slides or ATP testing. The table below lists representative targets that a site tunes to its own water and metallurgy.
| Parameter | Typical monitored range | Purpose |
|---|---|---|
| Cycles of concentration | 3 to 6 | Water and chemical economy versus scaling limit |
| pH | 7.5 to 9.0 | Balance corrosion against scale |
| Mild steel corrosion rate | Below about 5 mpy | Confirm inhibitor film performance |
| Free oxidising biocide residual | 0.2 to 1.0 mg/L | Suppress biofilm formation |
Because these checks recur on fixed intervals across many exchangers and towers, they suit a CMMS-driven schedule. Fabrico can hold the sampling routes, log readings against limits and trigger work orders when a coupon or dip slide drifts out of range. Tie the same discipline into cooling tower maintenance so mechanical cleaning and chemical control stay in step. Teams can book a Fabrico demo to see the routing in practice.
A positive LSI means the water is supersaturated with calcium carbonate and tends to deposit scale on hot surfaces. A negative LSI means the water is undersaturated and tends to be corrosive, and an LSI near zero is roughly at equilibrium.
Higher cycles concentrate hardness, alkalinity, chloride and sulphate. That pushes the water toward scaling and raises corrosive ion levels, so the maximum safe cycles figure is fixed by your make-up water chemistry and treatment, not by water cost alone.
Biofouling is the growth of biofilm from bacteria, algae and fungi. It insulates heat transfer surfaces and shelters the metal beneath from inhibitor, creating oxygen-starved zones where microbially influenced corrosion drives deep local pitting even while most of the surface stays clean.
At minimum: conductivity for cycles, pH, calcium hardness and alkalinity for the LSI, inhibitor residual, and an oxidising biocide residual or ORP. Confirm metal loss with corrosion coupons and microbial load with dip slides or ATP testing.
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