Every refinery, IPP, fertiliser plant, and large textile mill in Pakistan runs at least one open recirculating cooling tower. None of them get the biocide programme right on the first try, and most of them never get it fully right. Here's how to think about it.
The microbial load you're actually fighting
A cooling tower at thermal steady state is a near-perfect bioreactor: 25–40°C, oxygenated, neutral-to-slightly-alkaline pH, organic loading from process leaks and air-scrubbed dust, and a constant supply of fresh nutrients in the makeup. Without biocide, sessile biofilm forms within days, planktonic counts climb past 10⁶ CFU/mL, and at least four problem populations establish:
- Slime-formers (Pseudomonas, Klebsiella, Aerobacter) — surface biofilm and heat-transfer fouling.
- Sulfate-reducing bacteria (SRB) — anaerobic pockets under biofilm; drive microbiologically-influenced corrosion (MIC) of carbon steel.
- Iron and manganese bacteria — co-deposit metals as tuberculation; couples to MIC under deposits.
- Legionella spp. — public-health risk, regulatory concern, and increasingly written into corporate HSSE standards for IPP and process tower operators.
The biocide classes you have to choose from
| Class | Examples | Mode | Strengths | Limitations |
|---|---|---|---|---|
| Halogen oxidising | Cl₂ (gas/hypochlorite), Br₂ (BCDMH, NaBr + oxidiser), ClO₂ | Oxidation of cell membranes and enzymes | Cheap, broad-spectrum, fast knockdown | Consumed by organics (halogen demand); ineffective against biofilm interior; corrosive to copper alloys above 0.5 ppm free residual |
| Non-halogen oxidising | Hydrogen peroxide, peracetic acid, ozone | Hydroxyl-radical oxidation | No DBP burden; effective at low pH; decomposes to benign residuals | Cost per active kg; storage/handling; not always compatible with phosphate corrosion-inhibitor chemistry |
| Non-oxidising, fast-acting | DBNPA, Glutaraldehyde | Rapid cell-membrane disruption | Penetrates biofilm; not consumed by halogen demand; effective in presence of ammonia and reducing organics | Short residual; not for continuous dosing |
| Non-oxidising, long-residual | Isothiazolinone (CMIT/MIT), Quaternary ammonium, THPS | Enzyme inhibition / lipid disruption | Multi-day residual; effective against SRB; biofilm-penetrating | Compatibility with anionic dispersants; resistance develops without rotation |
Why no single biocide wins
Halogens are cheap and fast, but their kill is surface-only — they get consumed by halogen demand before they penetrate biofilm. Non-oxidising biocides penetrate biofilm but cost 5–20× more per kilo and develop resistance over months of monotherapy. The honest answer is that a real cooling-water programme uses both, alternating in a rotation that defeats the resistance dynamic.
A practical rotation programme
This is the structure we recommend as a starting point for a typical Pakistani open recirculating tower with mixed metallurgy (carbon steel + copper + galvanised), pH 7.5–9.0, makeup conductivity 600–1500 µS/cm:
- Base oxidiser, continuous: sodium hypochlorite (or bromine via NaBr + hypochlorite) trimmed to 0.2–0.5 ppm free halogen residual at the tower basin. Hold for 4 hours per day minimum.
- Non-oxidising shock, alternating: AD-401 (isothiazolinone-based) at 50–100 ppm shock dose, once every 3–7 days, on a holding time of 4–8 hours before blowdown allowed.
- Second non-oxidiser, alternating: AD-402 (CMIT/MIT) or glutaraldehyde at vendor-specified shock dose, replacing the primary non-oxidiser every 2–4 weeks to prevent resistance.
- Anti-Legionella programme if required: monthly verification by ATP or culture; supplementary ClO₂ dosing if culture counts exceed action level.
Halogen demand and holding time — the two numbers operators forget
Halogen demand is the amount of oxidiser consumed by organics and reduced species in the water before any free residual remains. A tower drawing makeup from a canal or river can have a halogen demand of 2–5 ppm; one drawing from clean municipal water might be under 1 ppm. Without measuring it, you cannot set the right hypochlorite dose — you either underdose (no residual, no kill) or overdose (free residual >1 ppm, copper alloy attack accelerates).
Holding time is the contact time between biocide dose and the next blowdown event. For non-oxidising biocides, this should be at least 2 × the system half-life — meaning a 6-hour half-life biocide needs at least 12 hours between dose and blowdown. Operators routinely dose AD-401 and then blow it out via uncontrolled blowdown within hours, getting zero microbial control while paying full chemistry cost.
Monitoring you should be doing weekly
- Total bacteria (dipslide or ATP): baseline < 10⁴ CFU/mL; action level 10⁵.
- SRB (incubated vial): any positive at 28 days is an action signal.
- Free halogen residual (DPD): 0.2–0.5 ppm during halogen on-time, < 0.1 ppm off-time.
- ORP: 600–700 mV during oxidiser dosing; sub-450 mV indicates oxidiser exhaustion.
- Visual inspection of fill and basin: monthly. Biofilm sheen, deposit colour, and odour tell you more in 30 seconds than ATP tells you in 24 hours.
If your cooling tower is showing fouling on heat exchangers, microbial counts above action level, or unexplained corrosion under deposits, send us the chemistry log and we'll review the programme. See also AD-401 non-oxidising biocide, AD-402 isothiazolinone biocide, and AD-401 application on a Karachi IPP.