Scale, Corrosion and Biofouling: Building a Cooling Water Treatment Programme

Three failure modes, one water

Scale is dissolved hardness coming out of solution on the hottest surfaces, where it insulates exactly the metal you are trying to cool. Corrosion is metal returning to its oxide, accelerated by dissolved oxygen, chloride and low pH. Biofouling is a living film that insulates like scale, shelters corrosion underneath it and, in warm Gulf conditions, gives waterborne organisms somewhere to multiply.

They are coupled. Raising pH suppresses corrosion and promotes scale. Running the system harder to save water concentrates everything and worsens all three. Killing biofilm releases debris that fouls heat exchangers. A programme that optimises one number in isolation usually makes another worse.

Cycles of concentration: the number that drives the chemistry

Cycles of concentration is the ratio of dissolved solids in the circulating water to that in the make-up — in practice, measured as the ratio of conductivities. Every cycle you add saves make-up water and blowdown, and every cycle you add pushes hardness, chloride and silica closer to the point where they precipitate or attack metal.

Set the target cycles from a saturation calculation on your actual make-up analysis, not from a rule of thumb, then control blowdown automatically on conductivity. A programme is only meaningful once cycles are stable; dosing chemicals into a loop with uncontrolled blowdown wastes both the chemical and the diagnosis.

Scale control: antiscalants and dispersants

An antiscalant works by threshold inhibition — a few parts per million distort crystal growth enough to keep hardness salts in solution well past their normal saturation limit. Dispersants keep the particles that do form suspended so they leave with the blowdown rather than settling on a surface.

Selection depends on which salt is limiting: calcium carbonate is the usual constraint in Gulf make-up water, but calcium sulphate, phosphate and silica each need different chemistry, and silica in particular is unforgiving. Skid temperature and residence time matter too; an inhibitor that holds at 40 °C may not hold across a hot exchanger.

Corrosion control: match the inhibitor to the metallurgy

A loop containing mild steel, copper alloys and stainless steel needs an inhibitor package that protects all three — a copper-specific azole alongside a general steel inhibitor is the common approach. Galvanised steel in a new tower needs passivation before normal treatment begins.

Chloride is the parameter to watch in coastal and high-TDS make-up water: it attacks passive films on stainless steel, and its concentration rises directly with cycles. Corrosion coupons, changed and weighed on a fixed schedule, are still the most honest measure of whether the programme is working.

Microbiological control: oxidising and non-oxidising biocides

Oxidising biocides do the routine work. Bromine donors such as BCDMH hold up better than chlorine at the higher pH typical of cooling water and in the presence of ammonia, which is why tablet feeders are common on towers. Sodium hypochlorite remains a cost-effective oxidant where pH is controlled and dosing is continuous.

Non-oxidising biocides are used periodically, as a slug dose, to reach organisms sheltered inside biofilm that an oxidant cannot penetrate. Alternate between two unrelated non-oxidising chemistries to avoid selecting for resistant populations, and pair the programme with a dispersant so the killed film actually leaves the system.

Where a cooling tower serves an occupied building, the microbiological programme is a safety duty, not just an efficiency measure. Confirm the monitoring and record-keeping expected of your facility by the authority having jurisdiction, and keep the results.

Monitoring: the programme is the records

A working programme produces a log: conductivity and cycles, pH, free or total halogen residual, inhibitor residual, hardness and alkalinity, chloride, and microbiological counts at whatever interval your risk assessment sets. Corrosion coupons close the loop.

Test reagents, coupon racks and dosing pump spares belong on the same reorder list as the chemicals. The most common cause of a failed programme is not the wrong chemistry — it is a dosing pump that stopped and a test kit nobody restocked.

What to send us for a cooling water programme

• Make-up water analysis and system volume
• Recirculation rate, and the temperature across the heat exchanger
• Metallurgy in the loop, including any galvanised or copper components
• Current cycles of concentration and how blowdown is controlled
• Existing dosing equipment, feeders and tablet chlorinators
• Monitoring you already do — and what you are required to record

Frequently asked questions

How often should non-oxidising biocide be dosed?

It is dosed periodically rather than continuously, on a schedule set by your system's fouling tendency and monitoring results, and normally alternated between two chemistries. The correct interval comes from your microbiological counts, not from a fixed calendar rule.

Can I run more cycles of concentration to save water?

Sometimes, and it is usually worth calculating. The limit is set by the least soluble salt in your make-up water and by chloride attack on your metallurgy. Raise cycles only alongside a saturation calculation and a matched antiscalant, and watch corrosion coupons afterwards.