Controlling algae in a cooling tower

Short answer: Algae needs light, water and nutrients. A cooling tower supplies all three, so the practical controls are to exclude light where possible and to maintain an effective biocide and algaecide programme. Blocking light alone eliminates a large proportion of the problem.

Why algae grows

Unlike bacteria, algae require light to grow. That is why it appears in the parts of a cooling tower exposed to daylight:

• The distribution deck and open basin
• Louvres and outer surfaces
• Anywhere sunlight reaches wet surfaces

Enclosed pipework and heat exchangers do not grow algae, though they do grow bacteria.

Problems it causes

• Blocks nozzles and distribution holes, so water distribution becomes uneven
• Blocks strainers and fouls the fill
• Provides nutrients for bacteria — dead algae feeds bacterial growth downstream
• Reduces heat transfer where it accumulates
• Slippery surfaces — a safety hazard on walkways and ladders

Physical control — the most effective step

Because algae cannot grow without light, physical measures work extremely well:

• Cover the basin where practical
• Fit louvres that block direct sunlight while allowing airflow
• Shade the distribution deck
• Repair or replace missing panels — a single gap admitting sunlight creates a growth site

This is worth doing before increasing chemical dosing, because it addresses the cause rather than the symptom.

Chemical control

Algaecide

Products specifically formulated against algae. See our ZeroScale cooling tower algaecide.

Oxidising biocide

Chlorine and bromine based products also control algae as part of general microbiological control. See our oxidising biocide.

Non-oxidising biocide

Used in alternation to prevent resistance developing — see our biocide guide.

Why pH matters here too

As with bacterial control, oxidising biocide effectiveness drops sharply above pH 8. If your cooling water is running alkaline, the algaecide programme will underperform regardless of dose.

Control pH first — see our pH correction guide — then the chemical programme becomes effective.

Cleaning an established growth

Once algae is well established, dosing alone will not clear it:

1. Mechanical removal — brush and wash down accessible surfaces
2. Shock dose of biocide to kill what remains
3. Clean the basin thoroughly, removing settled debris
4. Clean strainers and nozzles
5. Then establish routine dosing to maintain the clean condition

Routine treatment maintains a clean system; it does not clean a dirty one.

Preventing recurrence

Side stream filtration

A proportion of the circulating water is continuously filtered, which removes suspended solids, dust and dead organic matter before they settle in the basin and feed further growth.

A disc filter is commonly used for this duty, since it can be cleaned and reused. See our disc vs screen guide.

Monitoring

• Visual inspection weekly — basin, deck, louvres and fill
• ORP — continuous, confirming biocide effectiveness
• pH — daily
• Dip slides — periodic microbiological check

Safety note

Cooling towers can harbour organisms of public health concern, particularly where biofilm and algae are allowed to establish. Beyond the operational cost, maintaining microbiological control is a health and safety responsibility.

Send us your tower details and we will recommend a complete programme from our cooling tower chemicals range.

Frequently asked questions