[Answer] What Are the Discharging Limits in Scrubber Wash-water?


With the Scrubber system gaining momentum in a majority of shipping companies, it has become extremely important that we understand the importance of the scrubbers system and how they function. The recent controversy over open-loop scrubbers discharge and the subsequent ban of open-loop scrubbers have brought back the focus on scrubber washwater.

The US Environmental Protection Agency Office of Wastewater Management has showcased the discharged limit guidelines in the scrubber washwater. Here’s an insight into that.

Discharge Limits Monitoring

The IMO guideline recommends pH, PAH concentration, turbidity and temperature
should be continuously monitored and recorded when the EGCS is operated in ports, harbors, or estuaries. In other areas, these parameters should be continuously monitored and recorded whenever the EGCS is in operation, except for short periods of maintenance and cleaning of the equipment.

Various Discharge Limit Parameters in Scrubbers

Monitoring pH

The washwater pH must meet one of the following requirements and the results recorded
in the vessel’s EGCS Technical Manual (ETM) as applicable (Resolution MEPC.184(59)):

  • The discharge washwater should have a pH of no less than 6.5 measured at the
    ship’s overboard discharge with the exception that during maneuvering and
    transit, a maximum difference of 2 pH is allowed between the ship’s inlet and
    overboard discharge
  • During commissioning of the unit(s) after installation, the discharged washwater
    plume should be measured externally from the ship (at rest in a harbor) and thedischarge pH at the ship’s overboard pH monitoring point should be recorded when
    the plume at a distance of 4 meters from the discharge point equals or exceeds a
    pH of 6.5.
  • This discharge pH, which is found to achieve a minimum pH of 6.5 in
    the washwater plume 4 meters from the ship, should become the overboard pH
    discharge limit.

Monitoring PAHs

The maximum continuous PAH concentration in the discharged washwater should not be
greater than 50 μg/L phenanthrene equivalence (PAHphe) above the inlet water PAH

  • For the purposes of this criteria, the PAH concentration in the washwater should
    be measured downstream of the water treatment equipment but upstream of any washwater dilution or other reactant dosing unit, if used, prior to discharge.
  • The 50 μg/L limit is normalized for a washwater flow rate through the EGCS unit of 45 tons/MWh.
  • For a 15-minute period in any 12-hour period, the continuous PAHphe concentration
    limit may exceed the limit described above by up to 100 percent, to allow for an abnormal start up of the EGCS (Resolution MEPC.184(59))

This limit should be adjusted for washwater flow rates, as shown in Table 1.

Monitoring Turbidity and Suspended Particulate Matter

The washwater treatment system should be designed to minimize suspended PM,
including heavy metals and ash.

  • The maximum continuous turbidity in washwater should not be greater than 25 formazin nephlometric units (FNU) or 25 nephlometric turbidity units (NTU) or equivalent units, above the inlet water turbidity.
  • However, during periods of high inlet turbidity, the precision of the measurement device and the time lapse between inlet measurement and discharge measurement are such that the use of a difference limit is unreliable.
  • Therefore, all turbidity difference readings should be a rolling average over a 15-minute period to a maximum of 25 FNU.
  • For the purposes of this criteria, the turbidity in the washwater should be measured
    downstream of the water treatment equipment but upstream of washwater dilution (or other reactant dosing) prior to discharge.
  • For a 15-minute period in any 12-hour period, the continuous turbidity discharge limit may be exceeded by 20 percent (Resolution MEPC.184(59)).


The washwater treatment system should prevent the discharge of nitrates beyond that
associated with a 12 percent removal of NOx from the exhaust, or beyond 60 mg/L (1 mM)
normalized for a washwater discharge rate of 45 t/MWh (similar to the adjustment tabulated above for PAHs), whichever is greater. All systems should be tested for nitrates in the discharge water.

Washwater Additives and Other Substances

An assessment of the washwater is required for those EGCS technologies which make
use of chemicals, additives, preparations or create relevant chemicals in situ.

Washwater Residue

Residues (sludge) generated by the EGCS should be delivered shoreside to adequate
reception facilities. Such residues should not be discharged to the sea nor incinerated onboard.

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Source: US Environmental Protection Agency