A 0.001-inch deposit on cooling surfaces can drive fan and pump power up by ~10%. In pulp and paper mills awash in warm, fiber-laden water, a coordinated scale, corrosion, and biocide program keeps heat exchangers clean and towers efficient.
Pulp and paper mills rank among the heaviest industrial water users at about 54 m³ per tonne of product (watertechonline.com) (watertechonline.com). Roughly 85% of that is process-return water, which means cooling circuits routinely see warm, organically loaded flows — think cellulose fibers, lignin, and black liquor condensates — plus inorganic solids (watertechonline.com) (mitacoolingtechnologies.com).
In these conditions, a thin layer is expensive: even a ~0.001″ (0.025 mm) deposit can insulate heat exchangers enough to raise fan and pump power ~10% (wwdmag.com). Proper chemical control — scale and corrosion inhibitors plus biocides — holds surfaces near design cleanliness and keeps cooling towers on spec.
Scale inhibition under high heat loads
Mineral scale in these towers is primarily calcium carbonate, calcium sulfate, and silica. Control relies on threshold/antiscalant blends and pH management. Modern programs combine a phosphonate or chelant with a polymeric dispersant: for example, polyacrylates (≈2,000 molecular weight) at ~5–15 mg/L are cost-effective calcium inhibitors, while phosphonates such as HEDP or AMP at ~5–20 mg/L provide “excellent” calcium inhibition (pdfcoffee.com). Chelating organics (EDTA, NTA) and sulfonated polymers are also used at 5–20 mg/L as needed (pdfcoffee.com).
In practice, feeding ~10 mg/L polyacrylate plus 5–15 mg/L phosphonate both sequesters hardness and converts nascent crystals into non‑adherent forms (pdfcoffee.com) (pdfcoffee.com). Dispersants are often included to keep fibers and fine precipitates suspended. Programs of this type are supplied as cooling tower scale inhibitors and are normally metered with a dosing pump for accurate feed control.
The operating goal is high cycles of concentration (ratio of dissolved solids in recirculating water to makeup water) without breaching saturation limits. With conventional inhibitors, control is typical if the cycled Langelier Saturation Index (LSI, a scale‑forming tendency metric) stays below ~2.0; with state‑of‑the‑art polymers, SI of 3.0–3.5 can be tolerated without acid feed via aggressive inhibitors (pdfcoffee.com). If SI will exceed ~2.0–2.5, operators typically add acid or use softened makeup (pdfcoffee.com). Where hardness removal is preferred, a softener can trim Ca²⁺/Mg²⁺ to widen the scaling margin.
Higher cycles mean lower blowdown and fresh makeup. Boosting cycles from 3× to 6× (with inhibitors and blowdown controls) roughly halves makeup demand, cutting millions of liters per day in a medium‑sized mill. Performance on the ground is visible: near‑zero scale and deposit, with approach temperatures tracking design — versus the ~10% extra fan energy per 0.001″ of scale seen when fouling takes hold (wwdmag.com).
Corrosion control across mixed metallurgy
Pulp mill circuits mix carbon steel piping, steel exchangers, copper/brass or copper‑nickel tubes, and galvanized or FRP towers. Target corrosion rates are ≤1–2 mils/year (mil/yr = thousandth of an inch per year, ≈25–50 µm/year) on steel, which reflects an ~85–95% reduction from raw‑water rates when chemistry is balanced (pdfcoffee.com).
Blended programs typically include low‑level orthophosphate or polyphosphate (4–12 mg/L) to form protective calcium‑phosphate films; phosphonate esters (HEDP, AMP, PBTC at ~5–15 mg/L) add film formation and dispersancy (pdfcoffee.com) (pdfcoffee.com). These phosphate/phosphonate systems work best when calcium hardness exceeds ~50 mg/L and pH is about 8; otherwise, glassy silicates or organic inhibitors are used (pdfcoffee.com).
To protect copper alloys, azoles such as benzotriazole or tolutriazole are fed at ~2–8 mg/L (pdfcoffee.com). Other inorganic inhibitors supplement steel protection: zinc at 1–2 mg/L (often minimized for environmental reasons), silicates at ~6–12 mg/L, molybdate at 8–12 mg/L, and nitrates at 10–20 mg/L for aluminum defense (pdfcoffee.com) (pdfcoffee.com) (pdfcoffee.com). Most blends include at least two inorganic inhibitors (e.g., phosphate + molybdate or silicate) plus an organic film former. These formulations are typically supplied as corrosion inhibitors for cooling towers.
pH control anchors performance: 7.5–8.5 is common to optimize inhibitor films. Acid (e.g., H₂SO₄) is used cautiously; aggressive acidification will dissolve calcium carbonate scale (converting it toward calcium sulfate) but can accelerate corrosion (pdfcoffee.com).
Performance metrics are coupon‑based: <1–2 mil/yr loss on steel and <0.2 mil/yr on copper indicates ≥85% corrosion inhibition (pdfcoffee.com). Sustained results mean less maintenance and a longer life for mixed‑metal assets.
Dual‑biocide microbiological control
Warm, organics‑rich water is a ready habitat for bacteria, algae, and fungi, including Legionella. Biofilms (slimy microbial layers) insulate heat transfer and trigger localized corrosion cells. Best practice uses both oxidizing and non‑oxidizing biocides in rotation (chemicalprocessing.com), delivered via a biocide program tailored to the tower.
Oxidizing biocides include continuous or frequent dosing of chlorine (as NaOCl or stabilized on‑site generator feed), bromine (via NaBr/NaOCl), or chlorine dioxide. Maintain ~0.5–1.0 mg/L free‑halogen residual in the basin. Chlorine dioxide (typically fed by an on‑site generator) performs especially well in warm, high‑pH water by penetrating biofilms and forming fewer harmful byproducts. Periodic high‑dose bromine provides a more stable oxidizer at elevated pH, and stabilizers such as cyanurates or sulfamates can regulate halogen release (chemicalprocessing.com). Where facilities prefer generation on site, an electrochlorination system supplies chlorine as needed.
Secondary oxidizers improve biofilm penetration. Monochloramine — formed by adding a small ammonia slip to chlorine — has shown stronger activity against tower biofilms than free chlorine, with an ∼1 mg/L residual persisting longer (pubmed.ncbi.nlm.nih.gov). Physical pretreatment with ultraviolet on makeup water can also cut incoming microbial loads; in mills using UV, a compact ultraviolet unit limits organisms without chemicals.
Because microbes can adapt to a single oxidizer, non‑oxidizing agents are pulsed weekly or monthly: glutaraldehyde, DBNPA, isothiazolones, quaternary ammonium compounds, or THPS. One common tactic is a weekly “bump” of glutaraldehyde at ~5–10 mg/L (slug dosing) for long‑lasting action against sessile populations. Biofilm‑penetrating blends (e.g., peracetic acid–quat or mixed aldehydes) are cycled periodically. A representative regimen alternates continuous halogen (~1 mg/L) with weekly non‑oxidants (DBNPA/glutaraldehyde) and a monthly high‑intensity shock (chlorine dioxide or bromine) (chemicalprocessing.com) (pubmed.ncbi.nlm.nih.gov). Success is verified by routine plating/ATP with targets of <10^3–10^4 CFU/mL (CFU/mL = colony‑forming units per milliliter) and by the absence of visible slime.
Suspended solids management and side‑stream filtration
Pulp mill cooling water picks up entrained fibers, unbleached pulp fines, and inorganic solids. These particles fuel fouling and consume biocide. Side‑stream filtration/clarification has outsized payoff: fine cross‑flow filtration removes >90% of 10–20 µm particles, including most cellulose fines (wwdmag.com). In practice, cleaner water slows colloidal fouling and can cut biocide dosing ~30–35% for the same microbial control and corrosion outcomes (wwdmag.com). A compact side‑stream clarifier is often selected to intercept wood pulp solids without major civil works.
Cooling‑tower particulates are typically <5 µm (10^8–10^10 particles/mL), which traditional sand filters miss (wwdmag.com). Advanced media — cross‑flow microsand — or membranes trap sub‑micron debris, delaying fouling layers and shrinking Legionella habitat. In one field case, implementing fine filtration enabled a 35% reduction in chemical feed while maintaining target residuals (wwdmag.com). Where membranes are preferred, a skid‑mounted ultrafiltration system provides consistent removal of fines ahead of the tower. Dispersant polymers (e.g., polycarboxylates) also help keep fibers from settling; these are supplied as dispersant chemicals compatible with inhibitor programs.
Regulatory alignment and water savings
Indonesian pulp/paper effluent limits are stringent — BOD₃ ≤150 mg/L, COD ≤350 mg/L, TSS ≤150 mg/L (scribd.com). Cooling tower blowdown typically routes to the mill’s wastewater treatment, so chemical choices matter: use non‑toxic inhibitors (organic phosphonates, silicates, zinc at low levels) and avoid heavy metals or biocides that yield persistent organics. Chlorinated biocide byproducts are oxidized in effluent treatment, but high doses of organo‑chlorines can spike COD; mills therefore neutralize bleach with ultraviolet or through dechlorination as needed (scribd.com). Where dechlorination is required before discharge or reuse, facilities deploy a dechlorination agent, and for chemical stewardship many standard formulations are available as a consolidated cooling tower chemical package.
Phosphate‑based corrosion treatments can add nutrient load, so many mills — especially near sensitive water bodies — are moving to phosphate‑free chemistries (chemengonline.com). On the water balance, running higher cycles (enabled by inhibitors and controls) reduces blowdown volume by 20–30%, a reduction that quickly pays for treatment through lower water and sewer costs. Indonesian mills commonly capture evaporator condensate and reuse process flows; maximizing tower recycling at 3–7 cycles is consistent with this practice.
Where makeup disinfection is part of the permit strategy, compact UV systems lower biological loading without adding residual oxidants, aligning with effluent objectives.
Measured outcomes and operating continuity
With this coordinated program, steel corrosion falls below 1 mil/yr and copper below 0.2 mil/yr (pdfcoffee.com); heat‑transfer surfaces remain essentially clean (preventing those 0.001″ layers that impose ~10% fan/pump penalties), so measured approach or ΔT stays near design (wwdmag.com). Operators report 80–90% fewer cleaning outages and longer cycles between maintenance, which directly improves availability; when deep cleans are required, a scheduled cooling tower cleaning service minimizes downtime.
Energy‑wise, simply avoiding 1–2 mils (0.025–0.05 mm) of fouling translates to ~10–20% lower fan/pump power (wwdmag.com). Environmentally, reduced chemical bleed — notably lower phosphate/zinc than historical practice — keeps BOD, COD, and nutrients within permit (scribd.com).
Program summary and verification
The program integrates threshold scale control (polymer + phosphonate), blended corrosion inhibition (polyphosphate/silicate/azole/nitrate as required), and a dual‑biocide regimen (oxidizing + targeted non‑oxidizing). Dosing levels (mg/L) reflect industry practice and classic water‑treatment texts (pdfcoffee.com) (pdfcoffee.com), while operating checks (cycles, fouling rate, corrosion) verify success. The payoff is straightforward: near‑as‑new heat‑exchange efficiency, minimized blowdown, and compliance with Indonesian standards (scribd.com), avoiding the outages and energy penalties that unchecked scaling, corrosion, or biofouling would impose — an approach aligned with industry reviews and ANSI/ASHRAE‑grounded guidance (chemicalprocessing.com) (wwdmag.com) (wwdmag.com) (pubmed.ncbi.nlm.nih.gov).
