Controlling microbiological growth in a cooling tower with oxidizing and non-oxidizing biocides, and why a scheduled biocide feed bleeds first and then holds the bleed shut.
Biocides are hazardous by design. Read the product’s Safety Data Sheet and label before handling or setting a dose, and never mix an oxidizer with a non-oxidizing biocide in the same line or container. See Chemical Handling.
A cooling tower is warm, wet, aerated, and full of nutrients blown in with the air — ideal for bacteria, algae, and fungi. Left alone they form biofilm, which insulates heat-transfer surfaces, shelters corrosion underneath it, and harbours Legionella. Most programs use two kinds of biocide together.
| Oxidizing | Non-oxidizing | |
|---|---|---|
| Examples | Chlorine (bleach), bromine, chlorine dioxide | Isothiazolinone, glutaraldehyde, DBNPA, quaternary amines |
| How it is fed | Continuously, or in frequent short feeds, to hold a residual | As a slug: a large dose on a schedule, typically once or twice a week |
| How it is controlled | On ORP, or a timer checked against residual tests | On a schedule, dosed by system volume |
| Strength | Fast, broad kill; cheap | Penetrates biofilm; kills what the oxidizer misses |
| Weakness | Consumed by organics and ammonia; less effective at high pH | Needs contact time; bled out if the bleed opens too soon |
Alternating two different non-oxidizers from week to week keeps organisms from adapting to either.
ORP rises with oxidizer activity, so a Setpoint Control mode on an ORP probe, Falling, drives the oxidizer pump on when ORP drops below the setpoint. Choose the setpoint by testing: find the ORP that corresponds to the free halogen residual the program wants, then set it there. ORP depends on pH, so re-check the relationship if pH control changes. See ORP.
A slug feed runs in three phases.
Bleed the tower down before feeding. During the feed and lockout the bleed is held shut, so conductivity will climb unchecked for hours. Bleeding first — to a conductivity somewhat below the normal setpoint, or for a set time — gives the tower room to concentrate during the lockout without going over its limits.
Run the biocide pump for the time that delivers the dose. The dose is set by system volume, not make-up:
Product needed = system volume × target dose
Example. A system holding 5000 gallons, a target of 100 ppm of product: 5000 × 100 ÷ 1,000,000 = 0.5 gallons. A pump delivering 1 gal/h needs 30 minutes. Use the dose on the product label — it sets both the concentration and the contact time.
Hold the bleed shut for the contact time the label calls for — often several hours — so the biocide is not drained away before it has worked. Hold off the other chemical feeds too, if they would react with the biocide or waste it.
Why not just feed more? Without a lockout, a slug fed while conductivity is near the setpoint is partly bled straight out: you pay for biocide that goes down the drain.
Use a Calendar program. Calendar is in field testing — check it is available on the controller’s firmware.
| Part | Setting for the example |
|---|---|
| Schedule | Weekly, Monday, 02:00 |
| Before step | Pre-bleed — open the bleed until conductivity falls below 1600 µS/cm (80 % of a 2000 µS/cm setpoint), or 45 minutes, whichever comes first |
| Action | Timed output — biocide pump, 30 minutes |
| Lockouts | The conductivity bleed mode, with a trailing hold to cover the contact time; the inhibitor mode, if the product label requires it |
| Interlocks | Flow switch, and the biocide tank’s low level |
Keep one of these for each scheduled biocide, on site with the controller.
| Item | Program |
|---|---|
| Product and supplier | |
| Target dose (ppm) and contact time | |
| System volume (gal) | |
| Pump output (gal/h), calibrated on | |
| Schedule (days, start time) | |
| Pre-bleed: conductivity target, maximum time | |
| Feed time | |
| Bleed lockout time | |
| Other modes locked out |