What are the washwater discharge criteria under MARPOL Annex VI?

Under IMO EGCS Guidelines (MEPC.184(59), updated by MEPC.340(77)), washwater must meet: pH ≥ 6.5 measured 4 metres from the discharge point; PAH concentration ≤ 50 µg/L PAHphe equivalent (phenanthrene equivalent); turbidity ≤ 25 NTU above ambient seawater (or 25 FTU above ambient if using FTU instruments); and a nitrate concentration limit applicable in special areas. The ship must carry continuous monitoring equipment and maintain records for three years.

What is the difference between open-loop and closed-loop scrubbers?

Open-loop scrubbers draw in seawater, spray it through the exhaust gas to absorb SO₂, and discharge the acidic washwater overboard continuously. Closed-loop scrubbers recirculate a fresh water/caustic soda (NaOH) solution; washwater is not continuously discharged — only a bleed-off is discharged or sent to the sludge tank. Hybrid scrubbers can switch between both modes; they operate in closed-loop in port or restricted areas and open-loop at sea.

Which ports and areas ban scrubber washwater discharge?

As of 2025, discharge bans apply in: the Chinese Emission Control Areas (ECAs) and most Chinese port waters (since 2019); Singapore port waters; UAE ports; the German Bight (Germany); Norwegian fjords (below 2 knots); California state waters (US); and several other ports globally. The list is growing — officers should consult the ITF/BIMCO port-specific guidance and the ship's EGCS Technical Manual before arriving in new jurisdictions. In ban areas the scrubber must be switched to closed-loop or the ship must burn compliant fuel.

What records must be kept for an EGCS?

The ship must maintain an EGCS Record Book (or equivalent section of the ORB) recording: the date and position of any mode change (open to closed or hybrid); pH, PAH, and turbidity readings at each monitoring point; volume of bleed-off discharged and residues landed ashore; any malfunction and corrective action; and the flag State's approved plan for closed-loop residue handling. MEPC.340(77) mandates continuous electronic logging of monitoring data.

Scrubber washwater discharge

Exhaust Gas Cleaning Systems (EGCS, commonly called scrubbers) allow ships to burn high-sulphur fuel oil while complying with the MARPOL Annex VI 0.50% global sulphur cap (in force 1 January 2020) and 0.10% ECA limit. The washwater produced by scrubbing exhaust gases must meet criteria set by IMO Resolution MEPC.184(59) as updated by MEPC.340(77), and is subject to a growing number of local bans.

Open-loop vs closed-loop scrubbers

· Open-loop: seawater pumped from sea chest through tower; acidic washwater discharged overboard. Only viable in waters with sufficient alkalinity (typically ocean seawater); not acceptable in low-alkalinity areas (Baltic, some river estuaries).
· Closed-loop: fresh water recirculated with NaOH dosing to maintain alkalinity; bleed-off discharged or collected; suitable for all waters. Higher CAPEX and operating cost (NaOH).
· Hybrid: software-controlled switchover between modes; uses open-loop at sea (cheaper) and closed-loop in port or ban areas.
· All types must be type-approved by the flag State administration under MEPC.340(77) guidelines; approval certificate carried onboard.

Washwater discharge criteria (MEPC.340(77))

· pH ≥ 6.5 measured at 4 m from the discharge point (or at the ship's side); instantaneous pH, not 15-minute average.
· PAH ≤ 50 µg/L PAHphe equivalent, measured continuously by UV fluorescence sensor.
· Turbidity ≤ 25 NTU above ambient background seawater turbidity (or ≤ 25 FTU); measured continuously.
· Nitrate: discharge limit of 60 mg/L NO₃⁻ in special areas (Baltic, North Sea ECAs); closed-loop bleed-off must comply.
· Monitoring data must be recorded electronically with GPS-timestamping and archived for three years.

Local discharge bans (2025 status)

· China: ECZ (Emission Control Zone) and most port anchorages — ban since January 2019; enforcement by MSA inspections.
· Singapore: port waters — ban since March 2020.
· Germany (German Bight/North Sea): national ban within 12 nm — in force since 2021.
· Norway: fjords and World Heritage sites (Geiranger, Nærøy) — speed and washwater restrictions.
· California (US): state waters ban since December 2020.
· UAE: port waters ban in Dubai and Abu Dhabi.
· Many additional ports list pending or newly enacted bans — check ITF EGCS tracker or BIMCO port agent before arrival.

Record-keeping and EGCS Technical Manual

· EGCS Technical Manual (ETM) approved by flag State: details operational limits, maintenance, monitoring calibration, and procedures for switching modes.
· EGCS Record Book (or annex to the Oil Record Book): log each mode change, port, monitoring readings, and any non-compliance event.
· PSC inspectors may download the electronic monitoring log and compare it with the BWRB and ORB for consistency; data manipulation is treated as falsification of an official document.
· In areas with discharge bans, the ship must demonstrate it was in closed-loop mode or burning compliant fuel; GPS track + monitoring log provides evidence.

Sources: MARPOL Annex VI (as amended); IMO Resolution MEPC.184(59) — EGCS Guidelines (2009); IMO Resolution MEPC.340(77) — Revised 2021 EGCS Guidelines. See also MARPOL Annex VI, Emission Control Area (ECA), MARPOL Annex I.