The hard trade-offs behind keeping marine heat exchangers clean

Marine biofouling doesn’t just look bad on a tube bundle—it quietly taxes engines. Studies show biofilms can reduce heat‑exchanger efficiency by 20–50% (researchgate.net), forcing higher power input to maintain cooling. In one Aframax tanker case, underwater cleaning that removed fouling would save about US$2M in fuel over five years (rivieramm.com). Globally, losses from marine biofouling run to the billions per year (researchgate.net).

In practice, every percentage point of efficiency gain pays off in reduced fuel, maintenance downtime and greenhouse emissions (tethys-engineering.pnnl.gov) (rivieramm.com). That has made two issues—biofouling (organisms growing on wet surfaces) and scaling (mineral precipitation on heat-transfer walls)—priority risks in seawater cooling circuits.

Chemical antifoulants and the chlorine question

Chemical antifoulants (biocides—chemicals that suppress or kill biological growth) remain the most common control method. The oxidizing biocide of choice is chlorine or hypochlorite, which reacts with bromide in seawater to form hypobromous acid and related oxidants that attack microorganisms and detritus (evoqua.com). Chlorine programs are typically metered via accurate dosing pumps and supplied as part of broad oxidizing biocide regimes.

Two dosing regimens dominate practice: continuous low‑level injection and shock (intermittent high‑dose) treatment. Continuous dosing at the seawater intake (e.g., 0.5–2 mg/L free chlorine, with 1–2 mg/L typical) steadily suppresses biomass and keeps intake screens and tubes relatively clean (scribd.com). Periodic dosing can also be applied nearer the heat exchanger as 5–20 mg/L plumes for a few hours to purge fouling. The catch: many macrofouling animals—mussels, barnacles—sense oxidants and clamp shut, surviving extended chlorination; even a 30–60+ minute feed interruption allows them to reoxygenate and persist, so continuous low‑dose often fails to eradicate hard macrofouling (watertechnologies.com) (watertechnologies.com).

Shock chlorination trade-offs and discharge limits

Shock chlorination—periodic pulses at 10–50 ppm for several hours—has proven more effective against macro‑organisms in Navy and power‑plant experience (scribd.com). But high‑dose shocks often require system isolation or shutdown and strict safety controls. Chlorine is highly corrosive (forming hypochlorite, HOCl, and ClO₂⁻), consumes inhibitor chemicals and can promote pitting. Environmental guardrails are tightening: new guidance calls for <12–18 µg/L residual free chlorine at shoreline to protect 95–99% of marine life (info.awa.asn.au) (info.awa.asn.au). The trend is to minimize net chlorine discharge.

Electrolytic antifouling (onboard generation)

Electro‑chlorinators generate chlorine in situ from seawater using mixed‑metal‑oxide (MMO) electrodes. The chemistry matches bulk hypochlorite, but with safer logistics: no need to carry bleach or chlorine, automated dose matching to flow, and proven control of both micro‑ and macrofouling (evoqua.com). De Nora (Magneto) reports seawater electrolysis produces broad‑spectrum oxidants that “kill bacteria or algae and shellfish in the water and prevent attachment” (evoqua.com). Power demand typically runs 0.1–0.5 kWh/m³ treated, and on a ship electrical power is often cheaper and safer than transporting tons of chlorine. Many operators deploy packaged electrochlorination systems to achieve those safety and logistics benefits.

Impressed‑current metal‑ion release

Another electrolytic method applies DC to copper, aluminum or iron anodes in the circuit. The metal dissolves and releases Cu²⁺ or Al³⁺ at roughly 0.1–1 mg/L, toxic to settling organisms (“chemobrush” effect) and inhibitory to biofilm formation; nearby metal also receives local cathodic protection (corrosion-group.com). With no chemical handling and low maintenance, these systems are best for prevention rather than removal of heavy infestations (corrosion-group.com).

Non‑chemical options and complements

Ultrasonic waves, plasma and UV irradiation (ultraviolet light used to inactivate microorganisms) have been tested for fouling control. Thermal shock (recirculating hot water) works against mussels but cuts cooling capacity. In practice, electrolytic methods—chlorine generators or anode systems—offer the most practical “chemical‑free” alternative on vessels and can be complementary, for example pairing an electrolytic chlorine generator with low‑level copper‑ion dosing. Where UV is appropriate in side streams, compact UV systems offer 99.99% pathogen kill without chemicals.

Dispersants for slime and silt management

Dispersants and clarifiers (organic polymers that keep particulates suspended rather than deposited) are often added to cooling‑water programs. Common chemistries include polyacrylates and polymethacrylates that maintain particle suspension via electrostatic and steric effects (bluereef.co.za) (bluereef.co.za). A polyacrylate dispersant can adsorb to suspended fines, imparting like charges so particles repel and stay in suspension (bluereef.co.za) (bluereef.co.za).

One industrial example reports a bio‑dispersant “effectively cleans and disperses heavily fouled systems, restoring flow and clarity” (rxmarine.com). Dosing is typically continuous or slug at about 0.5–2 mg/L; these agents are non‑corrosive but add organic load that can feed microbes, so programs use them judiciously. Suppliers package these as dispersant chemicals for integration alongside oxidizing biocides.

Upstream screening and residual monitoring

Filtration—coarse screens upstream—should always precede biocide contact. Automated systems such as automatic intake screens limit debris ingress before oxidants or ions do their work. Monitoring residual chlorine or redox potential is critical to sustain target dosage—e.g., ~0.5 mg/L HOCl in operation—and to align with local discharge limits.

Seawater scaling mechanisms and risks

Seawater contains high hardness and alkalinity, typically 400–500 mg/L Ca²⁺ and over 1000 mg/L Mg²⁺, so scale formation is a major risk. The most common scales in marine heat exchangers are calcium carbonate (CaCO₃) and calcium sulfate (CaSO₄) (chemicalprocessing.com). As saturation rises with cycles of concentration (the ratio of dissolved solids in recirculating water to makeup water), calcium salts crystallize on tube walls. Antiscalants (threshold inhibitors that interfere with nucleation and crystal growth) prevent this by sequestering ions, disordering crystals, or keeping them small and dispersible—functions delivered by modern scale inhibitors designed for seawater.

Phosphorus‑based inhibitors: history and limits

Historically, phosphorus‑based treatments—polyphosphates (e.g., hexametaphosphate) and phosphonates like HEDP (1‑hydroxyethane‑1,1‑diphosphonic acid), ATMP (aminotri(methylenephosphonic acid)) and PBTC (phosphonobutane tricarboxylic acid)—have been the workhorses. They control CaCO₃ and CaSO₄ effectively at <5 mg/L and have a long history of success in seawater cooling (chemicalprocessing.com). Regulatory and public‑relations pressure is rising, however: discharged phosphates can stimulate algal blooms in coastal waters (chemicalprocessing.com). Indonesian regulations mirror global trends (Indonesia enforces IMO discharge standards), so many operators avoid high‑phosphate chemistries unless justified.

Polymeric, phosphorus‑free antiscalants

Phosphorus‑free polymeric inhibitors—polyacrylate/maleate copolymers, sulfonated acrylic polymers and polyaspartates—are increasingly popular. A maleic‑acrylic copolymer, for instance, adsorbs to growing CaCO₃ crystallites so they remain loose and flushable. Imported blends often fuse a low dose of phosphonate (e.g., HEDP) with acrylic polymers to broaden coverage (CaCO₃, CaSO₄, SiO₂). New formulations (e.g., polyether phosphate‑ethylene carboxylate, or FEUs) have raised limiting cycles in seawater: recent programs achieved cycles of concentration of about 3.0–3.1, roughly doubling reuse versus older schemes limited to ~1.5 cycles, equating to nearly 50% savings in makeup flow and pumping energy (chemicalprocessing.com).

Treatment design and environmental constraints

Typical programs continuously feed inhibitor to maintain a residual of about 0.5–5 mg/L, depending on hardness. Some sites apply slip‑stream softening (e.g., raising CO₂ or adding acid) to keep bicarbonates soluble, though most shipboard installations rely on antiscalants and manage cycles conservatively. A Fourier analysis of scaling risk (e.g., Langelier‑Saturation Index) guides inhibitor strength. Seawater systems rarely require separate corrosion inhibitors—the nickel alloys or titanium commonly used already resist corrosion (chemicalprocessing.com).

Operators must also manage effluent. Phosphonate inhibitors hydrolyze slowly and eventual breakdown may release phosphate (mdpi.com). Many “eco‑friendly” inhibitors are partially biodegradable; using non‑phosphorus inhibitors both prevents algal blooms and avoids potential future bans (chemicalprocessing.com) (chemicalprocessing.com). When working in Indonesian or sensitive waters, validate that antiscalants meet local effluent standards—the Indonesian Ministry of Environment’s water‑quality limits for total phosphorus are typically ≪0.5 mg/L.

Program integration and O&M outcomes

In the field, regimes combine methods: for biofouling, continuous low‑level electrochlorination to control slime with a once‑per‑shift shock dose if macrofouling risk is high; for solids, upstream screening; for minerals, a premix of dispersant and inhibitor (often polyacrylate plus phosphonate). Monitoring is central—digital saturation monitoring (pH and conductivity) can optimize inhibitor feed in real time (chemicalprocessing.com)—as is tracking chlorine residuals or redox to sustain about 0.5 mg/L HOCl in service.

Environmental policy shapes choices. Indonesia has ratified the 2001 IMO “Harmful Antifouling Systems” Convention, banning organotin coatings (Presidential Regulation No. 66/2014; peraturan.bpk.go.id), so antifouling discharges—even internal biocides—are scrutinized. Modern programs trend toward in‑situ generation (electrolysis) and biodegradable dispersants over persistent biocides. Results are tangible: one plant reported halving blowdown and water cost by raising cycles from 1.7 to ~3.1 using advanced inhibitors (chemicalprocessing.com) (chemicalprocessing.com). The value proposition is straightforward: less downtime and lower fuel spend, with compliance to environmental regulations (e.g., IMO/MARPOL).

Sources and metadata

Peer‑reviewed studies, industry data and government reports were analyzed. Key references include Mathew et al. (2020) and Riviera Maritime, with specialized handbooks and trade publications detailing macrofouling control and seawater treatment programs.

• Mathew N. et al., EcoDesign and Sustainability II (Springer, 2020) (tethys-engineering.pnnl.gov).
• “How to achieve a ‘clean’ fuel saving of US$2M between dry dockings”, Riviera Maritime (news content hub) (rivieramm.com).
• “Antifoulants and dispersants: How they are used in water treatment”, Blue Reef detail site (Benoni, South Africa; Apr. 22, 2025) (bluereef.co.za) (bluereef.co.za).
• “Electrolytic Antifouling”, Corrosion Group (Stavanger, Norway; webpage) (corrosion-group.com).
• Evoqua Industries (Magneto Special Anodes): “Seawater Electrolysis & Anti Bio‑Fouling” (product info flyer) (evoqua.com).
• I. Wallis & S. Chidgey, “New Guidance Values for Chlorine in Marine Waters”, Water e‑Journal 8(1) (AWA, 2021) (info.awa.asn.au) (info.awa.asn.au).
• Wright R., “Improve Treatment of Seawater for Cooling”, Chemical Processing (Jul. 29, 2020) (chemicalprocessing.com) (chemicalprocessing.com).
• Indonesian President Regulation No. 66/2014 (ratifying IMO’s 2001 “Harmful Antifouling Systems” Convention) (peraturan.bpk.go.id).
• Veolia Water Technologies, “Water Handbook – Chapter 28: Macrofouling Control” (current edition) (watertechnologies.com).