EPA's Vessel General Permit and Small Vessel General

Table 23. Decision Process for those Vessel Discharges Not Evaluated in EPA's BE.
Vessel Discharge Description Number of
Vessels
Discharging
Boiler/Economizer
Blowdown
Motor Gasoline
and
Compensating
Discharge
Cathodic
Protection
accumulate and affect their operation. Although not subject to the VGP but
similar in operation to VGP vessels regarding this discharge, large naval
vessels can generate up to 244 gallons of washwater per day. Wash water can
include salts, lubricants, and combustion residuals.
Boiler blowdown occurs on vessels with steam propulsion or a steam generator
and is used to control the concentration of scaling constituents in boiler
systems. Boiler blowdown are infrequent, of short duration (seconds), in small
volumes, and at high pressure. The blowdown can contain water and steam or
sludge-bearing water at elevated temperatures (above 325°F). The discharge
can contain metals or boiler water treatment chemicals.
Motor gasoline is transported on vessels to operate vehicles and other
machinery. As the fuel is used, ambient water is added to the fuel tanks to
replace the weight. This ambient water is discharged when the vessel refills the
tanks with gasoline or when performing maintenance. Most vessels are
designed not to have motor gasoline and compensating discharge. The volume
of the compensating discharge is expected to range from less than 50 gallons to
up a few hundred gallons. The discharge can contain small amounts of fuel and
other fuel-related pollutants.
Nearly all vessels having steel hulls or metal hull appendages use cathodic
protection systems to prevent corrosion. Based on underwater hull inspections
and maintenance records, one-half of anodes are consumed after three years.
The primary pollutant released from cathodic protection is zinc. Average
pierside and underway zinc generation rates are 1.3 x 10-6 and 5.1 x 10-6 (lb
zinc/square foot of underwater surface area)/hr, respectively. Concentrations
modeled for military vessels using a mixing zone envelope approach indicate that zinc,
aluminum and mercury concentrations were below WQS within 0.5, 0.1 and 0.1 feet
from the vessels surface, respectively (USDOD/USEPA 1999).
Discharge
Volume
and
Frequency
Low Low Low
Amount and
Types of
Pollutants
Present in the
Discharge
Low Low Potentially
High
High Low Low
Non-oily Some larger vessels are expected to have some non-oily machinery discharges, Low Low Low
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