What is a scrubber?
Scrubbers — or Exhaust Gas Cleaning Systems (EGCS) — let vessel operators burn higher-sulfur fuel while still meeting emissions limits. The system draws seawater into the exhaust stream to strip out sulfur. But sulfur is not the only thing the water picks up. Heavy metals, oil residues, and polycyclic aromatic hydrocarbons (PAHs) also enter the wash water. Ships discharge much of this mixture directly into the sea, almost without treatment. Several of those compounds are long-lived and carcinogenic.
The regulatory background is straightforward. IMO sets a global sulfur cap of 0.50% m/m for marine fuels. Inside Emission Control Areas (ECAs) — the Baltic Sea, the North Sea, and the U.S. and Canadian coastal zones — the limit tightens to 0.10% m/m. Scrubbers let operators burn cheaper, higher-sulfur fuel while still meeting those limits.
Why the name “scrubber”? The system scrubs pollutants out of flue gas. The practical effect is simple: pollutants move from the air into the water
Three configurations:
Open-loop scrubber system
Open-loop scrubbers draw seawater from outside, pass it through the exhaust once, and discharge it overboard. They need high pump capacity because of the large water volumes involved. The system works only where seawater has enough natural alkalinity to neutralise acidic exhaust gas. Warm water, fresh water, and brackish water all fall short. The Baltic Sea, with its low salinity, sits outside the suitable range for open-loop operation.
Closed-loop scrubber system
Closed-loop scrubbers recirculate the same process water through repeated cleaning cycles and store it in onboard tanks. True zero-discharge closed-loop systems are rare. Most run with a bleed-off — a small, continuous release of process water. This bleed-off creates room to add sodium hydroxide, the base that keeps the system stripping sulfur oxides. Effluent volumes in closed-loop systems are small, typically a few cubic metres per hour. Pollutant concentrations, especially metals, run far higher than in open-loop effluent. Recirculation causes contaminants to accumulate. One benefit of recirculation: it helps separate PAHs, which tend to bind to particulate matter. Closed-loop systems cause less local acidification than open-loop ones, but the overall pollutant load can still be significant. When operators discharge bleed-off outside prohibition zones rather than at port, localised pollution hotspots can form.
Hybrid scrubber system
Hybrid scrubbers switch between open-loop and closed-loop operation through remotely controlled valves. In high-alkalinity open water, the system runs open-loop, without chemical additives. In low-alkalinity or restricted areas, it switches to closed-loop. This flexibility suits ships that trade across both open ocean and restricted coastal or port environments. The trade-off is cost: hybrid units carry a higher price tag than either single-mode alternative.
The Baltic Sea – a special case
The Baltic Sea deserves particular attention. Its semi-enclosed geography, low salinity, limited biodiversity, and slow water exchange make it highly sensitive to pollution. Contaminants persist there far longer than in open ocean environments. Research from Chalmers University of Technology found that scrubber wash water contributes up to 9% of certain carcinogenic PAH emissions in the Baltic. The number of Baltic states and ports restricting or banning scrubber discharges continues to grow. Operators should verify current rules before each call.
Scrubber rules in Polish ports
Certified open-loop scrubbers achieving 0.10% sulfur compliance may operate in Polish territorial waters, including anchorage roads.
However, within port areas, the Harbor Master’s Office guidelines ban the use of open-loop EGCS entirely. A vessel running an open-loop scrubber must notify the Harbor Master’s Office on VHF channel 14 on entry.