Scrubbers at Sea: The Cost, Chemistry, and Politics Behind Shipping’s SOx Fix

Shipping’s go-to workaround for IMO 2020 — exhaust gas cleaning systems (EGCS, often called “scrubbers”) — is a classic trade: clean air for complicated water and waste handling. Wet scrubbers spray water into exhaust to neutralize SO₂ into sulfates, with three main modes in play: open-loop (seawater in, washwater out), closed-loop (freshwater recirculated with caustic), and hybrid (switchable). Open-loop units are mechanically simpler with fewer parts (mdpi.com), while closed-loop systems add tanks, caustic dosing equipment, and residue handling (mdpi.com).

Both technologies routinely hit ~98–99% SOx removal — enough to meet the IMO 2020 0.5% sulfur cap or even 0.1% in Emission Control Areas (ECA, coastal zones with stricter limits) (mdpi.com).

Scrubber designs and operating modes

Open-loop scrubbers lean on the natural alkalinity of seawater; their efficiency drops in low-salinity or low-alkalinity waters (like the Baltic Sea) (mdpi.com). Closed-loop units maintain high pH with injected sodium hydroxide (NaOH, a strong base), achieving the same abatement at lower water flow (mdpi.com). Hybrids toggle between modes — closed where restrictions apply, open offshore — at the cost of added complexity and space (mdpi.com) (mdpi.com).

SOx removal performance and water alkalinity

Typical marine scrubbers remove ~98–99% of sulfur oxides (mdpi.com). Open-loop units inject large volumes of seawater — often tens of cubic meters per megawatt-hour (MWh, a unit of energy), around ~40 m³/MWh in practice — which demands powerful pumps (inameq.com). Because that removal hinges on raw seawater alkalinity, performance falls in low-alkalinity seas, and in very cold or fresh water open-loop scrubbing can become ineffective (mdpi.com).

Closed-loop systems dose caustic to hold pH constant and claim >98–99% removal under all conditions (mdpi.com). In practice, both designs can meet ECA limits; closed-loop offers consistency at the expense of chemical use.

Capital, energy draw, and chemical logistics

Open-loop scrubbers carry the lowest upfront cost — simple hardware, no caustic storage (mdpi.com). Closed-loop retrofits are 10–30% more expensive and occupy more space thanks to reactors, tanks, and dosing gear (mdpi.com). Hybrid systems cost even more. A typical mid-size ship retrofit sits in the low-single-digit millions of USD (varies with vessel size).

All wet scrubbers add backpressure to engines, nudging fuel use up — often by <1–3%. Open-loop seawater pumps can draw several kW, with high-capacity units needed to push ~40 m³/MWh (inameq.com). Closed-loop systems circulate smaller volumes but run slurry and dosing pumps continuously; net energy differences are minor compared with engine output.

On chemicals, open-loop rigs use none. Closed-loop systems consume several dozen-to-hundreds of kilograms of NaOH per day on a large engine, with caustic consumption often a few kg per tonne of fuel burned; EU guidance ties this to waste streams showing ~840 kg/day of closed-loop sludge for a 10 MW plant, versus ~24 kg/day for open-loop (eur-lex.europa.eu). That chemical usage can cost about ~$300–$600 per tonne of NaOH in some cases, and operators also cite ~500–1000 USD/ton NaOH as an ongoing expense in closed-loop operations (caustic supply, storage, and safe handling included) (argusmedia.com). Closed-loop dosing is typically automated with dedicated chemical dosing pumps.

Logistics bite too: some ports require land delivery of caustic, others ban bulk deliveries altogether, and crews must be trained to handle it safely (argusmedia.com). Maintenance runs higher on closed-loop units — corrosive sludge needs removal, and pH/flow sensors need regular calibration or replacement. Planning matters: DNV notes that operational planning (even down to how wash water flushes through systems) is a key factor in reliable performance (dnv.com). For retrofits, closed-loop and hybrid designs also require tank space and typically 6–10 weeks in drydock; open-loop installations need less extra tankage. Ancillary hardware such as storage tanks, piping, and monitoring gear often falls under supporting water-treatment ancillaries.

Waste volumes and discharge pathways

Open-loop scrubbing discharges its washwater back to the sea after filtering out sludge (mdpi.com). That means enormous volumes of mildly contaminated water. A global ICCT assessment estimates scrubber-equipped ships (≈3,600 vessels by end-2020) release on the order of 10 gigatonnes (Gt) of washwater per year — nearly equal in mass to the 11 Gt of cargo carried by world shipping annually (safety4sea.com). Container ships, bulk carriers, and tankers (75% of scrubber installations) account for ~70% of this discharge; cruise ships (only ~4% of scrubber ships) contribute ~15%, largely at berth (safety4sea.com).

Open-loop washwater carries dissolved sulfuric acid, sulfates, and captured pollutants (polycyclic aromatic hydrocarbons, heavy metal salts, nitrates, among others) (mdpi.com). Even after neutralization, studies warn it can depress local pH and concentrate toxins: one analysis estimates −0.004 to −0.010 pH-units per year at 15–35% scrubber penetration (mdpi.com), and another model suggests pH drops near Rotterdam of ~0.088 per year (versus ~50 years of acidification from climate) at high usage (mdpi.com). Before discharge, operators use filters — often cartridge-type filtration — to remove suspended solids.

Closed-loop systems retain most contaminants onboard. The recirculated water is continuously cleaned — via settling or filtration — and reused, with only a small “blowdown” (<0.1–0.5% of flow) discharged to sea (mdpi.com). Settling steps are commonly handled by equipment such as a clarifier. The bulk of captured sulfur and particulates accumulates as sludge (gypsum and spent alkali) that must be offloaded to port reception facilities. EU figures illustrate the scale: ~24 kg/day of solid waste from an open-loop scrubber versus ~840 kg/day for a closed-loop unit at ~10 MW (~40 t/day fuel burn) (eur-lex.europa.eu). Even a conservative ~100 kg/day is many times the open-loop rate; in practice, closed-loop ships often carry tens of cubic meters of caustic and generate on the order of 0.1–1 ton of waste per day, with port fees, added crew work, and storage space as ongoing burdens.

Bottom line: open-loop emits at sea (no solid waste to handle) but shifts pollution to marine ecosystems, while closed-loop concentrates waste onboard for shore disposal. Hybrids can run closed in port/protected zones and switch to open offshore, storing washwater for later and mitigating some local impacts (mdpi.com) (mdpi.com).

Regulatory map and fleet adoption

Because open-loop discharge is an industrial effluent, many coastal states restrict or ban it. Singapore, China, Malaysia (Fujairah), India, Bangladesh, and the UAE prohibit open-loop EGCS discharge in territorial waters; several European countries (Belgium, Ireland, parts of Norway) and U.S. states (Connecticut, Massachusetts, California, Hawaii) do too (argusmedia.com). Indonesia mandates ≤0.5% sulfur fuel on domestic voyages from January 2020 and allows scrubbers only if approved by the Directorate General of Sea Transportation (hubla.dephub.go.id) (hubla.dephub.go.id); its maritime authority also notes that “some countries have banned” open-loop washwater, requiring ships to hold it onboard for port reception (hubla.dephub.go.id). Closed-loop systems are generally permitted everywhere (only a neutralized bleed is discharged).

Scrubber uptake surged ahead of 2020: roughly 3,000 vessels — about 15% of world fuel use — had EGCS in service or on order by January 2020, rising to ~4,000 by March, according to DNV GL updates (offshore-energy.biz). Around 80–85% of these were open-loop, with the rest mostly hybrid and only a few hundred purely closed-loop units; one dataset cites 3,249 open, 678 hybrid, and 634 closed installations (offshore-energy.biz).

Bulk carriers (~1,100), container ships (~600), and oil/chemical tankers (~600) led adoption, and closed-loop setups remained a small minority (jurnalmaritim.com) (offshore-energy.biz). Demand tracked fuel economics: when heavy fuel oil–low sulfur fuel oil (HFO–LSFO) spreads were ~$200–$300/ton, payback looked attractive (dnv.com). As the spread narrowed post-2020 and open-loop bans expanded, new orders slowed in 2020–22.

Pros and cons at a glance

Open-loop scrubbers: Advantages include no reagents (only seawater), mechanical simplicity, and lower capital and operating costs (mdpi.com). There is also no onboard sludge to pay to dispose of. Disadvantages include very high seawater flow and pump power needs (inameq.com); effectiveness that depends on seawater alkalinity (mdpi.com); and washwater discharge (acidic sulfates, heavy metals, organics) that poses ecological risks (mdpi.com). Open-loop use is banned or restricted in many coastal waters (Singapore, China, EU ports, U.S. coasts, etc.), forcing ships to switch to low-sulfur fuel or fit hybrid/closed systems where required (argusmedia.com).

Closed-loop scrubbers: Advantages include allowance in ports and ECAs everywhere, high SOx removal even in low-alkalinity waters, and capturing pollutants onboard with only a minimal, neutralized bleed (mdpi.com) (mdpi.com). Disadvantages include higher capital and maintenance costs, continuous purchase of chemicals (e.g., ~500–1000 USD/ton NaOH), large sludge volumes (on the order of 0.1–1 t/day for a large vessel; ~840 kg/day at ~10 MW per EU estimates) and the need for safe storage and crew training (eur-lex.europa.eu) (mdpi.com). Complexity brings risk: DNV highlights that leakage/corrosion can cause downtime or fines in some cases (dnv.com).

Hybrid scrubbers: Advantages are flexibility — open offshore, closed or “zero-discharge” in sensitive zones — making them broadly compliant. Disadvantages are the highest capital cost, the largest footprint, and the combined complexity of both systems (mdpi.com) (mdpi.com).

Waste flows by the numbers

Open-loop ships collectively discharge washwater comparable to massive non-point pollution sources — ~10 Gt per year worldwide (safety4sea.com). Regional chemistry can shift: modelled pH drops near Rotterdam of ~0.088 per year at high scrubber penetration (vs ~50 years from climate-driven acidification) and −0.004 to −0.010 pH-units per year in other scenarios have been reported (mdpi.com). Closed-loop vessels instead generate concentrated waste streams: EU rules assume ~0.1–1 t/day of sludge for a ~10 MW engine, and in practice a single tanker might offload several tens of cubic meters of caustic sludge every docking (eur-lex.europa.eu).