Every IPP, K-Electric station, Engro power island, and oil-fired plant in Pakistan runs at least one open recirculating cooling tower. The biocide programme is the single biggest determinant of how often the condenser comes off-line for cleaning, whether MIC takes a bite out of the carbon steel piping, and whether the plant ever has a Legionella exposure incident. None of these are subtle costs. All of them are downstream of the biocide selection.
Our previous post on oxidising vs non-oxidising biocide programmes covered the chemistry comparison. This post is the ranked product-list view: which biocides actually go into power-plant cooling programmes in 2026, who supplies them, and where AAS's AD-401 and AD-402 fit alongside Solenis, Dober, Lonza, Buckman, and Chardon Laboratories.
Biocide-family comparison at a glance
| Class | Type | Contact time | Continuous dose | Material compatibility | Residual measurement |
|---|---|---|---|---|---|
| NaOCl (12.5% bleach) | Oxidising halogen | 10–30 min | 0.2–0.5 ppm FAC | Attacks copper alloys above 1 ppm FAC; chlorinates organics | Free available chlorine (DPD) |
| Chlorine dioxide (ClO₂) | Oxidising | 5–15 min | 0.1–0.3 ppm | Better on copper alloys than chlorine; consumes more ammonia | ClO₂-specific colourimetric |
| Bromine (BCDMH / activated) | Oxidising halogen | 10–30 min | 0.3–0.5 ppm | Works at higher pH than chlorine; less reactive with ammonia | Total bromine |
| DBNPA | Non-oxidising, fast-acting | 30–120 min (degrades fast) | Slug dose only, 5–25 ppm | All metals; rapidly biodegradable | Hard to measure residual; dosed by time/concentration |
| Isothiazolone (CMIT/MIT) | Non-oxidising, residual | 4–24 hr | Slug dose 30–100 ppm; continuous 10–30 ppm | Most metals; sensitive to reducing agents (sulfite) | HPLC required for accuracy |
| Glutaraldehyde | Non-oxidising, SRB-active | 1–4 hr | Slug dose 50–200 ppm | All metals | Colourimetric / GC |
| THPS | Non-oxidising, low-tox | 2–6 hr | Slug dose 30–100 ppm | All metals; degrades to non-hazardous | Colourimetric |
1. Isothiazolone (CMIT/MIT): AAS AD-402 / Solenis isothiazolone
The non-oxidising backbone of most modern cooling-tower programmes. CMIT/MIT (5-chloro-2-methyl-4-isothiazolin-3-one + 2-methyl-4-isothiazolin-3-one) provides 2–5 day residual activity in a typical open recirculating system. AAS AD-402 (14% active CMIT/MIT) is the in-portfolio product — see AD-402 specifications. Comparable global products: Solenis Spectrus NX (similar active), Dober Polybor isothiazolone variants, Lonza Bardac and Bardac plus.
Use case: continuous low-dose in rotation with an oxidising biocide on a typical 3–7 day swap cycle. Residual time long enough that a single slug-dose protects through a weekend. Honest limitation: incompatible with sulfite-bearing dechlorination chemistry. Dose isothiazolone before SMBS injection or run them in separate side-streams.
2. Non-oxidising broad-spectrum: AAS AD-401 / Buckman BUSAN
Where isothiazolone is the residual-activity workhorse, AD-401 is the broad-spectrum knockdown product, a pale yellow liquid at 15–20% active, dosed at 25–50 ppm continuous, 50–150 ppm shock. Spectrum covers bacteria, algae, fungi, and sulfate-reducing bacteria (SRB). See AD-401 specifications. Comparable products: Buckman BUSAN 1192, Dober's polyglycol-amine-based biocides, Chardon Laboratories' specialty non-oxidisers.
Use case: rotation partner with AD-402 (different active class, prevents resistance development) and with the oxidising biocide. A typical power-plant programme runs continuous oxidiser (NaOCl or ClO₂) plus weekly alternating slug-dose between AD-401 and AD-402.
3. Sodium hypochlorite (NaOCl): commodity oxidising halogen
The default oxidiser on most Pakistani cooling towers, sourced from any chlor-alkali producer (Engro Polymer, Sitara). Continuous dose at 0.2–0.5 ppm free available chlorine, automated via ORP feedback. Honest limitations: attacks copper alloys above 1 ppm FAC (relevant if the condenser has admiralty brass tubes), chlorinates organics into halogenated by-products, and loses efficacy at pH above 8.0 as HOCl ionises to less-active OCl⁻.
Best fit: carbon-steel condensers, pH-controlled circulation, makeup water with low ammonia. Pair with non-oxidising biocides 25-50% of the time to control resistance and to handle the under-deposit SRB problem the oxidiser doesn't reach.
4. Chlorine dioxide (ClO₂)
The right oxidiser when NaOCl's limitations bite (copper-alloy condensers, high-ammonia makeup, biofilm-dominated fouling. ClO₂ penetrates biofilm better than chlorine and doesn't chlorinate organics. Generated on-site from sodium chlorite + acid + chlorine (or sodium chlorite + chlorine alone in two-precursor systems). Suppliers of generators and precursors: IDI Chlordioxide (idiclo2), PureLine, Bio-Cide International, Tomco₂.
Honest limitation: generator capex and operational complexity. The two-precursor or three-precursor system is more chemistry to manage than a simple NaOCl drum. Best fit: large IPP cooling towers (300+ MWe) where the biofilm-penetration advantage pays back in reduced condenser cleaning frequency.
5. Bromine (BCDMH / activated NaBr-NaOCl)
Bromine is the right halogen at pH 8.0–9.0 where chlorine loses activity. Common formats: solid BCDMH tablets in a feeder, or liquid activated bromine generated by reacting NaBr with NaOCl. Suppliers: Albemarle, ICL-IP, Lonza. Cost is higher than chlorine; supply chains into Pakistan are thinner.
Best fit: cooling circuits operating at elevated pH for corrosion control (typical refinery practice), or where the makeup water is already high-pH. Skip if the tower runs at neutral pH; chlorine is cheaper and equally effective there.
6. DBNPA: non-oxidising, fast-acting, fast-degrading
DBNPA (2,2-dibromo-3-nitrilopropionamide) is the fast-knockdown non-oxidiser. Half-life in cooling water is 2–4 hours, so it kills hard during the dose window and then disappears, useful for slug-dosing into systems with downstream discharge sensitivity, or as a pre-shutdown knockdown before draining. Suppliers: Dow (formerly), Solenis, Lonza, peer Chinese exporters.
Best fit: pre-CIP knockdown, slug-dose during low-flow events, side-stream sanitisation. Not a continuous-dose chemistry: too fast to degrade for sustained residual.
7. Glutaraldehyde + THPS: specialty SRB programmes
For cooling towers with sulfate-reducing bacteria problems (typical when there are dead-legs or under-deposit microbial pockets driving MIC), glutaraldehyde and THPS (tetrakis hydroxymethyl phosphonium sulfate) are the right specialty actives. Both target anaerobic populations the oxidiser doesn't reach. Suppliers: Solenis, Dober, Lonza on glutaraldehyde; THPS dominantly from Solvay and licensees.
Best fit: programme add-on for problem towers, not first-line chemistry. Typical use is monthly slug-dose at 50–200 ppm during otherwise-routine biocide rotation.
Programme architecture: how the chemistry stacks on a typical Pakistani power-plant tower
- Continuous oxidiser: NaOCl at 0.3 ppm FAC, automated by ORP. Switch to ClO₂ if the tower has copper-alloy condenser tubes or high-ammonia makeup.
- Non-oxidiser rotation: alternate weekly between AD-401 (broad-spectrum, 50 ppm slug) and AD-402 (CMIT/MIT, 50 ppm slug). Different active classes prevent resistance.
- Quarterly specialty knockdown: if SRB residuals appear on monthly testing, add glutaraldehyde or THPS slug-dose. If biofilm is visible on heat-transfer surfaces, add DBNPA before the next scheduled CIP.
- Residual monitoring: daily FAC (DPD test), weekly heterotrophic plate count, monthly SRB and Legionella PCR. Without the monitoring, the programme is guessing.
See the AD-401 application at a Karachi power plant for a worked example, and our power-plant cooling-tower programme for the full chemistry stack.
For a cooling-tower biocide programme review on a Pakistani IPP or utility station, including residual testing, rotation strategy, and chemistry supply, contact us. See also AD-401 specs, AD-402 specs, the oxidising vs non-oxidising chemistry comparison, and our power-plant cooling-tower programme.