5 Warning Signs Your Cooling Tower Needs Biocide Treatment

Biological fouling in a cooling tower rarely announces itself with a single dramatic failure — it builds up quietly until efficiency, cost and compliance all start moving in the wrong direction at once. Here are the five signs worth acting on before it gets there.

1. Visible slime on the fill pack or basin

A biofilm layer feels slippery to the touch and often has a dull, slightly discoloured sheen. It usually shows up first in low-flow areas — basin corners, distribution deck edges, the underside of fill pack sheets — well before it's visible from a casual walk-by.

2. Unexplained drop in heat transfer efficiency

Biofilm insulates heat exchanger surfaces almost as effectively as mineral scale does. If approach temperature is creeping up and your scale inhibitor program looks fine on paper, biological fouling is one of the first things worth ruling out.

3. Musty or sulphurous odour near the tower

Certain bacteria common in poorly controlled cooling systems produce compounds with a distinctly unpleasant smell as a metabolic byproduct. An odour that wasn't there before is a reliable early signal, often showing up before slime is visible anywhere.

4. Localised corrosion or pitting under deposits

Biofilm creates oxygen-depleted pockets against metal surfaces, which is exactly the environment that drives microbiologically influenced corrosion (MIC). Pitting that shows up under a deposit, rather than spread evenly across a surface, is a classic MIC pattern.

5. Rising bacterial counts on routine testing

This is the one that shouldn't be optional. Regular dip-slide or lab testing for total bacteria count — and specifically for Legionella in systems with cooling tower drift exposure — is the only way to catch fouling before it becomes visible or starts affecting compliance.

What to do next

A single high reading doesn't always mean a system failure — it can mean a biocide feed pump losing prime, a recent shock-dose that wasn't followed through, or a seasonal shift in makeup water quality. The fix is usually a properly sequenced oxidising and non-oxidising biocide program with a bio-dispersant to break up existing biofilm, not just a higher dose of whatever's already being fed.