Open-Loop Marine Scrubbers Cut Air Pollution by Discharging Acidic Washwater and Heavy Metals Directly Into Coastal Waters

Open-loop marine scrubbers reduce atmospheric sulfur emissions by spraying seawater into engine exhaust, but they achieve this by transferring the pollution directly into the ocean. The resulting washwater is highly acidic and laden with heavy metals, effectively trading air quality improvements for marine degradation.[10]

The mechanism was widely adopted following the International Maritime Organization's 2020 sulfur cap. The landmark regulation slashed the allowable sulfur content in marine fuels from 3.5 percent to 0.5 percent globally, forcing the shipping industry to drastically alter its emissions profile.[1]

Shipowners faced a stark choice: switch to expensive, distilled low-sulfur fuels or install exhaust gas cleaning systems. More than 5,800 vessels have since installed scrubbers to legally continue burning cheap, residual high-sulfur fuel oil.[6]

Open-loop systems account for approximately 81 percent of these installations. They are favored by operators because they are cheaper to install, require no onboard chemical storage, and continuously discharge their waste overboard rather than holding it for port disposal.[8]

The Chemistry of Washwater Discharge

Inside the scrubber tower, massive volumes of seawater are pumped through the hot exhaust stream. The natural alkalinity of the seawater neutralizes the sulfur dioxide gas, forming sulfurous and sulfuric acids in the liquid phase.[1]

This process demands a typical flow rate of 45 cubic meters of water per megawatt-hour of engine power. For a standard commercial bulk carrier, this translates to thousands of tons of water pumped, contaminated, and discharged daily.[1]

The effluent that exits the ship is fundamentally altered from the seawater that entered it. Research published in the Marine Pollution Bulletin found that scrubber washwater is, on average, 100 times more acidic than the surrounding ocean.[3]

"The scrubber washwater is found to be acidic with elevated concentrations of zinc, vanadium, copper, nickel, phenanthrene, and naphthalene," researchers noted in a comprehensive environmental assessment of the technology.[1]

The International Maritime Organization regulates the discharge for pH, turbidity, and polycyclic aromatic hydrocarbons. However, the current international guidelines do not set any limits for the heavy metals that wash out of the fuel.[6]

Heavy Metals and Marine Toxicity

High-sulfur fuel oil is a residual product of the refining process, meaning it concentrates the heavy metals naturally present in raw crude oil. When the fuel is burned, these metals aerosolize and are captured by the scrubber spray.[3]

Vanadium is the most abundant metal in the discharge, followed by iron, nickel, and zinc. A toxicological study published in Toxics found average vanadium concentrations of 5.55 milligrams per liter in scrubber effluent.[2]

These metals pose a severe and immediate threat to marine food webs. Ecotoxicological assessments demonstrate that even at dilutions of less than 1 percent, scrubber washwater causes acute toxicity in foundational marine organisms.[2]

Copepods, the microscopic crustaceans that form the base of the ocean's food chain, are particularly vulnerable. Exposure to the heavy metals in washwater impairs their survival and grazing rates, disrupting the broader ecosystem from the bottom up.[2]

"Discharges from both open-loop and closed-loop scrubbers are toxic to the tiny animals that support the marine food web," a 2023 environmental report submitted to the International Maritime Organization concluded.[7]

The Accumulation in Coastal Ports

While the open ocean can rapidly dilute these discharges, the environmental burden concentrates heavily in coastal waters and ports. Ships spend significant time idling, maneuvering, and loading in these confined, shallow environments.[5]

In 2022, ships equipped with scrubbers discharged over 88 million tonnes of washwater on Canada's Pacific Coast alone. Cruise ships, which frequently operate in sensitive coastal habitats, accounted for nearly half of that total volume.[5]

That localized discharge contained an estimated 26,000 kilograms of heavy metals. Because ports lack the rapid flushing currents of the open sea, these contaminants accumulate in the sediment and the local water column.[5]

Model calculations for the harbor docks in Antwerp, Belgium, demonstrated the severity of this localized impact. The continuous discharge from vessels reduced the ambient water pH and increased surface concentrations of vanadium by 46 percent.[1]

The acidification of harbor water compounds the broader, climate-driven acidification of the oceans. The localized pH drop in high-traffic ports can equal decades of climate-induced changes in a matter of months.[7]

The Regulatory Pushback and Local Bans

Faced with the ecological consequences of this pollution transfer, regional authorities are increasingly overriding the international framework. As of early 2023, more than 90 bans and restrictions on scrubber discharges had been implemented globally.[4]

China's Maritime Safety Administration prohibited washwater discharges in its inland river emission control areas and coastal ports in 2019. Vessels must switch to compliant low-sulfur fuels before entering these protected zones.[4]

Similar restrictions have been enacted in Singapore, parts of the United States, and several major European ports. These local bans force hybrid scrubber systems to switch to closed-loop mode or require ships to burn cleaner fuel.[4]

Closed-loop systems recirculate their washwater, treating it with sodium hydroxide to neutralize the acid. However, they still require periodic bleed-off discharges, which contain highly concentrated pollutants that must eventually be managed.[9]

"Untreated open-loop and hybrid scrubber effluent discharge is becoming more and more unpopular," noted Sahan Abeysekara, a principal specialist at Lloyd's Register, highlighting the shifting regulatory landscape for maritime operators.[8]

The Economic Calculation and Future Outlook

The initial economic case for installing scrubbers was overwhelming for the shipping industry. The massive price spread between high-sulfur fuel and compliant low-sulfur alternatives allowed shipowners to recoup their capital costs in just one to two years.[9]

In 2019 alone, the maritime industry saved an estimated €1.7 billion by utilizing scrubbers instead of switching fuels. This immense financial incentive drove the rapid proliferation of the technology despite the known environmental trade-offs.[9]

That economic advantage is now narrowing as the regulatory net tightens. The expanding network of local discharge bans requires vessels to carry and burn expensive low-sulfur fuels during port calls and coastal transits regardless of their equipment.[9]

Furthermore, the energy required to pump massive volumes of seawater through the scrubber system increases a vessel's overall fuel consumption. This operational penalty adds to the baseline cost of running the heavy machinery.[6]

The International Maritime Organization is currently evaluating revisions to its washwater guidelines. Environmental groups are pushing for comprehensive limits on heavy metals and stricter enforcement of existing parameters to protect coastal waters.[6]

Until a global consensus is reached, the maritime industry remains fractured. Open-loop scrubbers will continue to clean the air by polluting the sea, forcing operators to navigate an increasingly complex patchwork of local environmental protections.[10]