Open loop vs closed loop scrubbers: how each one works

The sulphur cap is 0.50%, or 0.10% in Emission Control Areas, and an approved scrubber is an equivalent way to comply. Compliance is checked as the SO2/CO2 ratio in the exhaust: 21.7 for 0.50% and 4.3 for 0.10%. Open loop uses the natural alkalinity of seawater, once through and overboard, and many ports ban or restrict it. Closed loop recirculates freshwater with caustic soda added, a small bleed-off is treated, and the sludge goes ashore. Caustic soda is corrosive, so wear full PPE. Hybrids switch between the two. Washwater pH, PAH and turbidity are monitored. If the scrubber is not back in compliance within about an hour, record it in the EGCS record book, report to the flag and port State, and be ready to change over to compliant fuel. Always follow the manual, your Chief Engineer and SMS.

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