EPA's Vessel General Permit and Small Vessel General

Exhaust Gas Scrubber Washwater Effluent. Exhaust gas scrubber washwater discharge
occurs as a result of cleaning the exhaust gas system on marine diesel engines. The washwater
discharge can be highly acidic, and can also contain traces of oil, polycyclic aromatic
hydrocarbons (PAHs), heavy metals and nitrogen. Washwater volumes of 2.8 million gallons
per day are estimated from a 10 MWh engine. While EPA anticipates very low numbers of
vessels will potentially be releasing this discharge, the risk of this discharge was analyzed in the
BE due to the very large volume and frequency of discharges, along with the potentially high
risk posed by the concentration and types of pollutant discharged.
Exhaust gas scrubbers would be used on large vessels such as oil and gas tankers, cargo/bulk
carriers, large ferries and medium and large cruise ships burning high-sulfur bunker oil (USEPA
2011b). The exhaust gas scrubbers remove sulfur dioxide generated by the vessel engines during
combustion of the bunker oil. Exhaust gas scrubber systems using seawater typically generate
45 m3 (11,900 gallons) of washwater per megawatt-hour (MWh) for scrubbing (MEPC 2008).
Closed freshwater exhaust gas scrubber systems have much smaller discharge rates than seawater
scrubbers. Freshwater closed loop scrubbers generate 0.1 to 0.3 m3/MWh (26 to 79
gallons/MWh) of scrubber washwater (MEPC 2008). To estimate exhaust gas scrubber
washwater flows for vessels like tankers, cargo/bulk carriers, ferries and cruise ships, EPA used
information on the engine sizes for each type of vessel.
Table 14 shows the range of engine sizes by vessel type and the estimated seawater and
freshwater exhaust gas scrubber washwater flow rates based on engine size. The size of the
propulsion engines for large cruise ships and large ferries was obtained from EPA’s Technical
Development Document for Exhaust Gas Scrubber Washwater Effluent (USEPA 2011b). EPA
assumed that the power requirements for propulsion engines on large ferries are similar to those
for medium cruise ships. For oil and gas tankers and cargo/bulk carriers, EPA used information
for the 80 MW (109,000 HP) engine onboard the Emma Maersk large container ship
(Technology 2011). EPA assumed that for smaller vessels such as tour boats, water taxis, fishing
boats and tug boats using low sulfur fuels will be a less expensive compliance alternative than
installing and operating exhaust gas scrubbers; therefore, exhaust gas scrubber washwater flows
for these vessel types were not estimated.
Table 14. Estimated Exhaust Gas Scrubber Washwater Discharge Rates used in EPAs
Analysis
Vessel Type Average
Propulsion
Power (MW)
Cargo/Bulk
Carriersa
Large Cruise
Ships
Medium Cruise
Ships
Estimated Saltwater
Exhaust Gas Scrubber
Washwater Discharge Rate
Estimated Freshwater
Exhaust Gas Scrubber
Washwater Discharge Rate
m3/hr gallons/hr m3/hr gallons/hr
80 3,600 951,000 16 4,227
45 2,025 535,000 9 2,377
4.8 216 57,000 1 264
250