Palm‑oil mills run cooling towers under punishing loads: 30–40 °C ambient heat, hot process returns, and organics from fruit carryover. The fix is a tightly coordinated chemical program—scale control, corrosion inhibition, and a biocide regime—that sustains high cycles of concentration without fouling or failures.
Palm oil mills circulate warm water through towers and heat exchangers fed by variable-quality surface or well water. In open tropical towers, biofilms—over 90% water by weight—act as insulating layers, so even a thin slime severely inhibits heat transfer (watertechnologies.com) (watertechnologies.com). Veolia notes that deposit fouling “reduces the efficiency of heat transfer” and can seed oxygen-differential corrosion cells (watertechnologies.com).
In warm, wet, sunlit towers, microbial growth is rapid; neglected systems foul with algae and slime, raise biocide demand, and risk tube failure (chemicalprocessing.com) (chemicalprocessing.com). Without treatment, a few degrees of warming or a thin biofilm can cut condenser heat transfer by 5–15%.
Cycles of concentration (the ratio of dissolved solids in recirculating water to makeup) are the economic lever. Well-managed towers typically run at 5–8 cycles; fewer than 4 is wasteful, while >6–8 has diminishing ROI (watertechnologyreport.wordpress.com) (watertechnologyreport.wordpress.com). A chemical program should enable at least 5–6 cycles without scale or corrosion, and integrated nutrient control can even lower continuous chlorine usage by ~70% when phosphates are removed (chemengonline.com).
A coordinated package of cooling-tower chemicals—paired with accurate feed via a dosing pump—is the operating backbone in this setting.
Deposit control and scale inhibition
Primary inorganic scales in palm‑mill towers are CaCO3, CaSO4, silica, and iron oxides—many exhibit retrograde solubility, precipitating on the hottest surfaces (watertechnologies.com). At a typical cooling exit of ~40 °C, CaCO3 solubility is only ~50–80 mg/L; even moderate makeup hardness (~100 mg/L as CaCO3) will exceed solubility beyond ~2–4× cycles.
The control strategy combines threshold inhibitors and polymer dispersants. Common chemistries include phosphonates (e.g., HEDP, ATMP) plus acrylic/phosphino‑organic polymers that sequester Ca/Mg and prevent crystal growth; modern programs rely on acrylic copolymers to keep precipitates suspended (watertechnologies.com). In practice, a threshold inhibitor paired with a polymeric dispersant is standard; acrylic terpolymers stabilize colloidal silicas and carbonates (watertechnologies.com). Selecting and dosing scale inhibitors alongside dispersant polymers is the centerpiece.
In an Indonesian mill with source hardness ~50–150 mg/L, a robust program can sustain 6–8 cycles—e.g., Ca2+ ~300–600 mg/L—without scaling. Case data show threshold/dispersant blends raise tolerable hardness ~2–5×; one plant’s switch from phosphate to a regulated phosphorus‑free (RPSI) chemistry eliminated calcium‑phosphate fouling in plate heat exchangers (chemengonline.com). Eliminating inorganic phosphate feed removes a microbial nutrient and helps prevent scale.
Once scale tendency is controlled, keep LSI (Langelier Saturation Index, a qualitative indicator of calcium carbonate scaling potential) slightly negative with bulk pH ~7.5–8 and verify via conductivity trends and deposit coupons. Each additional cycle reduces makeup roughly 10–15%. Routine feed control using a dosing pump maintains setpoints and minimizes bleed.
Outcome/targets: maintain tower and exchanger surfaces essentially scale‑free over seasonal operation; aim for <0.5 mm/year uniform corrosion in parallel (see corrosion section) and zero visible scale on downstream finned coolers for at least 6–12 months.
Corrosion inhibition for mixed metallurgy
Cooling circuits in mills—mild steel piping and towers, some copper/brass components, stainless elements—operate at alkaline pH with dissolved oxygen. Corrosion deposits further reduce heat transfer and drive failures (watertechnologies.com).
Effective programs combine passivating (anodic) and precipitating (cathodic) inhibitors. For steel, nitrites or nitrates build Fe2O3 films; orthophosphate both passivates and precipitates as calcium orthophosphate, and zinc precipitates at cathodic sites—Veolia calls orthophosphate “valuable as both an anodic passivator and a cathodic precipitator” (watertechnologies.com). Typical feeds can include ~10–30 mg/L PO4³⁻ and ~1–5 mg/L Zn. RPSI programs replace stoichiometric zinc/phosphate with organic film inhibitors while achieving similar protective films (chemengonline.com) (watertechnologies.com).
Copper alloys require adsorption inhibitors—azoles such as tolyltriazole or benzotriazole—forming protective films at low dose (<1–3 ppm). Veolia notes azoles (and amines) “block the surface” via adsorption (watertechnologies.com). Maintain bulk pH ~7.8–8.5 and track alkalinity, sulfate, chloride, and inhibitor levels daily. In organic‑rich systems, occasional oxygen scavenging (e.g., sodium sulfite) can minimize pitting. Selecting a blended corrosion inhibitor compatible with the dispersant and biocide slate is essential.
Outcome/targets: well‑inhibited systems often operate at <0.1 mm/yr steel and <0.05 mm/yr copper (verified by corrosion coupons per ASTM G31 or probes). Case evidence shows nitrate/zinc replacement of chromate achieving steel rates <0.03 mm/yr during cyclical operation (chemengonline.com) (watertechnologies.com). The program should eliminate under‑deposit corrosion and hold metals within <10 mg/m² of corrosion product accumulation.
Microbiological control (oxidizers and non‑oxidizers)
Warm, sunlit, nutrient‑rich towers in the tropics rarely drop below 20–25 °C—ideal for algae, bacteria, fungi, and protozoa (chemicalprocessing.com). Biofilms behave like stagnant water layers; a 1–2 mm slime can impose the insulating effect of several centimeters of still water, with major heat‑transfer penalties (watertechnologies.com).
Best practice is a multi‑tier biocide program (chemicalprocessing.com): a continuous oxidizing residual (sodium hypochlorite, chlorine dioxide, or bromine) at ~0.5–2.0 ppm active halogen, plus periodic shocks; and a regular non‑oxidizing biocide pulse—e.g., glutaraldehyde at ~10–50 ppm for 1–4 h, one to two times per week—timed to microbial monitoring. Organisms can develop partial immunity to a single chemistry, so synchronized oxidizing and non‑oxidizing feeds remain standard (chemicalprocessing.com). A broad‑spectrum suite of cooling‑water biocides supports this rotation.
Reducing nutrients strengthens biocides. Eliminating phosphate dosing and removing oils/organics in makeup can lower bleach demand—one program saw ~70% less continuous chlorine after phosphate removal (chemengonline.com). Mechanical aids help: coarse filtration or settling, including passing makeup/bleed through an oil/water separator, strips free oils that fuel biofilms; an oil‑removal system aligns with this guidance. If foaming occurs, a targeted anti‑foam can be added without altering microbiology.
Outcome/targets: keep heterotrophic plate counts (HPC, a culture‑based measure of general bacteria) below ~10^4 CFU/mL and avoid visible slime; maintain oxidizer residuals by ORP or direct test and confirm low microbial activity via ATP (adenosine triphosphate) measurements—well‑treated systems can hold ATP below 100 pg/mL. Plants using combined oxidizer+glutaraldehyde regimes report clear water and minimal biofilm compared to untreated towers that foul within days (chemicalprocessing.com) (chemicalprocessing.com).
Automation, monitoring, and performance metrics
Coordination is twofold: monitoring and automated feed. Controllers/PLC‑linked analyzers can adjust bleed on conductivity (a proxy for cycles) and trigger inhibitor or biocide dosing in real time. A simple architecture—conductivity, pH, oxidant residual, and microbial counts (HPC/ATP)—sustains setpoints, with feed delivered by a dosing pump and supported by water‑treatment ancillaries.
Quantifying results anchors the ROI. If scale and biofouling go unchecked, heat‑transfer coefficients fall by ~10–30%, forcing towers to run harder; treated systems maintain a nominal thermal approach (e.g., 5–7 °C) and design fan power (watertechnologies.com) (watertechnologies.com). One mill holding 6× cycles saw a 50% reduction in makeup versus an untreated baseline. Another literature figure: cleaning a 1 mm scale deposit saved ~0.5% boiler fuel (watertechnologies.com). Avoiding unscheduled outages is a direct gain; as Brad Buecker notes, unchecked growth “can transform a well‑designed cooling tower into a poorly operating mess” (chemicalprocessing.com).
Discharge management and chemistry selection
In Indonesia, blowdown must meet effluent limits for BOD, COD, and oil content. Operating at higher cycles cuts blowdown volume, while selecting non‑chromate, environmentally considerate inhibitors and minimizing phosphate dosing reduces discharge risk—choices consistent with power‑sector practice (chemengonline.com) (watertechnologies.com).
Integrated outcome summary
The integrated plan for palm‑oil mill towers under heavy heat and organic load is threefold: threshold and dispersant scale inhibitors to run higher cycles without calcite/gypsum fouling; corrosion inhibitors that passivate steel and protect copper under alkaline, oxygenated conditions; and a robust oxidizing plus non‑oxidizing biocide regimen to suppress algae and biofilm. Case studies show these moves extend cleaning intervals, reduce bleed by tens of percent, and—after eliminating phosphates—can cut continuous chlorine needs by ~70% (chemengonline.com). Targets are explicit and auditable: cycles of concentration, corrosion rates, microbial counts, and thermal approach.
Sources and case references
Veolia Water Handbook on deposits and biofilms—“deposit fouling reduces the efficiency of heat transfer” and biofilms act like stagnant water layers (watertechnologies.com) (watertechnologies.com). Details on dispersant polymers and threshold inhibitors (watertechnologies.com). Corrosion mechanisms and inhibitors, including the dual role of orthophosphate and the move away from chromate (watertechnologies.com) (watertechnologies.com) (watertechnologies.com) and adsorption inhibitors for copper (watertechnologies.com).
Microbial risks and multi‑tier control from Brad Buecker and colleagues (chemicalprocessing.com) (chemicalprocessing.com). Case studies on phosphorus‑free programs and reduced chlorine demand, plus chromate replacements achieving <0.03 mm/yr steel corrosion (chemengonline.com) (chemengonline.com) (chemengonline.com). Cycles‑of‑concentration guidance and ROI boundaries (watertechnologyreport.wordpress.com) (watertechnologyreport.wordpress.com).
