When You May Discharge — MARPOL Annex I Limits, Distances and Special Areas

1. The general principle

Warning: Operating rule

The flag State requirements, the M Notices in force, the IOPP Certificate and its supplement, the SMS and the Chief Engineer's orders override these notes. The overboard valve stays locked until the Chief Engineer authorises the discharge and is present for it.

MARPOL Annex I does not provide a general permission to discharge bilge water whenever a separator is fitted. For any proposed discharge, the ship must meet the applicable effluent, equipment, location, operating, and certification conditions at the same time:

• The effluent has to be clean enough — the oil content has to be within the limit for the area the ship is in.
• The ship has to meet the applicable operating conditions — including being en route where Regulation 15 requires it. There is no general 12-nautical-mile-from-land condition for machinery-space bilge discharge under Regulation 15.2.
• The equipment has to be working and monitoring — the separator or filter has to be in use, and the oil discharge monitoring and control system has to be in use with it.

A 15 ppm reading alone does not authorize discharge. Follow the vessel's approved procedures and applicable flag-State and coastal-State requirements.

2. Machinery space bilges, outside a Special Area

Under MARPOL Annex I Regulation 15.2, machinery-space bilge discharge from a ship of 400 gross tonnage and above is permitted only when all applicable conditions are met. The core conditions include:

1. The mixture is processed through oil-filtering equipment that meets Annex I Regulation 14.
2. The oil content of the effluent without dilution does not exceed 15 p.p.m.
3. The ship is en route.
4. The bilge mixture does not originate from cargo pump-room bilges on an oil tanker and is not mixed with oil cargo residues.
5. The discharge is not within an applicable Special Area; Regulation 15.3 and Regulation 14.7 add requirements there.

Regulation 15.2 does not impose a general 12 nautical mile distance for machinery-space bilge discharge. Do not confuse this with distance limits governing other MARPOL discharges, including oil-tanker cargo residues. “En route” and every other condition must still be satisfied.

3. Inside a Special Area

Within an Annex I Special Area, Regulation 15.3 requires compliance with Regulation 14.7 in addition to the relevant Regulation 15.2 conditions. This includes the applicable 15 ppm Bilge Alarm and automatic stopping/control arrangements. The approved equipment, ship's certificate supplement, and Administration instructions determine the arrangement fitted to the vessel.

There is no separate 12-mile band in the machinery-space bilge rule described in Regulation 15.2. Ships below 400 gross tonnage have separate treatment under Regulation 15.6; check the vessel's certificate and flag-State requirements rather than generalizing from the 400 GT rule.

4. Cargo and ballast residue from a tanker

Oil cargo residue from a tanker may be discharged only if every one of the following is satisfied:

• the ship is not in a Special Area;
• the ship is more than 50 nautical miles from land;
• the ship is on passage;
• the instantaneous rate of discharge is not more than 30 litres per nautical mile;
• the total quantity discharged is not more than 1/30,000 of the particular cargo carried — the older figure of 1/15,000 still applies to vessels built before the change.

The 50 nautical mile distance and tanker discharge-rate and total-quantity criteria belong to the separate oil-tanker cargo regime. They are not machinery-space bilge discharge limits.

5. The summary tables — memorise both

Machinery space bilges and ballast tanks:

Tanker cargo slops: more than 50 nautical miles from land, vessel en route, instantaneous rate not more than 30 litres per nautical mile, and the applicable total-quantity limit. This is a separate cargo-residue regime under Regulation 34; do not apply its distances or rates to machinery bilges.

For machinery-space bilges, remember the current operational criterion: effluent oil content not exceeding 15 p.p.m. without dilution, en route, through compliant equipment, with all other applicable conditions satisfied. There is no general 100 p.p.m. discharge allowance or 12-mile requirement in Regulation 15.2.

6. The Oil Record Book

The Oil Record Book is the legal record of everything that goes overboard and everything that does not. For machinery space operations it is kept by the engine room department, and it is required on all ships of 400 tons and above.

The items logged are:

1. ballasting and cleaning of oil fuel tanks;
2. discharge of dirty ballast or cleaning water;
3. disposal of sludge or oil residue;
4. discharge or disposal of bilges;
5. automatic bilge discharge;
6. failure of the oil discharge monitoring and control system, with details;
7. oil discharges, whether accidental or exceptional.

Every discharge through the separator is recorded with the quantity, the time and the ship's position, and signed by the operating officer, the Chief Engineer and the Master. A discharge made in a prohibited zone requires the quantity, source, time, position and date, signed by the Master and the Chief Engineer. The signature block is not a formality — it is the point at which responsibility is fixed on named individuals.