Where Sewage May Be Discharged — the 3 Mile and 12 Mile Rules
Where sewage may be discharged: the 3 and 12 nautical mile lines, treated versus untreated, holding tank rules, and the American Type I, II and III standards.
1. The discharge table — memorise this
Warning: Operating rule
The applicable MARPOL text, flag State and coastal-State requirements, the ISPP Certificate and its supplement, and the ship's approved procedures govern. Comminuted and disinfected sewage requires a distance greater than 3 nautical miles; untreated sewage requires a distance greater than 12. Special Area requirements may be stricter.
The rule is expressed as a table of distance against the degree of treatment, and it is worth learning in exactly that form.
Untreated sewage may only be discharged more than 12 nautical miles from the nearest land. Sewage comminuted and disinfected through an approved system may be discharged more than 3 nautical miles from land. Both routes require the ship to be en route; the untreated route also requires at least 4 knots and an approved discharge rate. Holding-tank sewage follows the moderate-rate, en-route, at-least-4-knot conditions in Annex IV. Do not treat the “4 knots” speed condition as a 4-mile distance limit.
| Type of sewage | 0–3 NM | More than 3 to 12 NM | Beyond 12 NM |
|---|---|---|---|
| Discharged from an approved sewage treatment plant | May be discharged, subject to no visible floating solids or discoloration, applicable approvals and any stricter local/Special Area rules | Same | Same |
| Comminuted and disinfected with approved equipment | Prohibited | Allowed when ship is en route; system must be approved | Allowed when ship is en route |
| Untreated sewage | Prohibited | Prohibited | Ship en route at not less than 4 knots; discharge at an approved moderate rate |
| Sewage retained in a holding tank | Distance depends on treatment: more than 3 NM if comminuted and disinfected by an approved system; more than 12 NM if untreated. In either case, stored sewage must be discharged at a moderate rate while en route at not less than 4 knots, at an Administration-approved rate. | ||
How to read the table
Three things fall out of it:
- The 3 nautical mile line applies to comminuted and disinfected sewage. Discharge is prohibited within 3 nautical miles; outside that limit the approved system and en-route conditions still apply.
- An approved sewage treatment plant has a separate route. Annex IV permits discharge subject to the plant's approval and no visible floating solids or discoloration, plus any stricter coastal-State or Special Area restrictions.
- Holding tank contents follow the applicable treatment route. Storage alone does not disinfect or treat sewage: use the 3 NM limit if it has been comminuted and disinfected, otherwise the 12 NM limit, and apply the additional moderate-rate, en-route and 4-knot condition to any stored sewage.
The 4 knots condition is a ship-speed requirement attached to untreated and holding-tank sewage discharge; it is not the 3 nautical mile distance rule. For comminuted and disinfected sewage, MARPOL requires the ship to be en route. Consult Annex IV Regulation 11 and the ship's approved arrangements for the exact discharge conditions.
Reference: IMO — Prevention of Pollution by Sewage from Ships, MARPOL Annex IV Regulation 11.
2. Annex IV
Annex IV of MARPOL 73/78 has the aim of regulating the disposal of sewage from ships internationally. Before it came into force, certain countries had already anticipated any internationally applied rules with their own national and regional controls — which is why older national standards still appear in historical maritime literature.
The ruling, as applied to vessels of over 200 gross tonnage with more than ten persons on board, would give a ten-year grace period to existing vessels. That grace period is why ships of a certain age were able to continue without a plant for years after the Annex was agreed.
3. The American types — the definitions to quote
American legislation defines three types of sewage treatment plant, and the definitions are the clearest statement anywhere of what different standards of treatment actually mean.
| Type | Requirement |
|---|---|
| Type I | Effluent with no visible floating solids and a coliform count of less than 1000 per 100 ml |
| Type II | Suspended solids not in excess of 150 mg per litre and a coliform count of less than 200 per 100 ml |
| Type III | Prevents the discharge overboard of treated or untreated sewage |
The three types form a ladder. Type I is the basic standard — visibly clean, and coliform reduced below 1000. Type II is the higher standard — it adds a quantitative limit on suspended solids and tightens the coliform limit to 200. Type III is not a standard of treatment at all; it is a retention system, and it is what the Elsan-type zero-discharge plants are built to satisfy.
Deadlines have been set for the installation of sewage treatment plant on vessels trading to American ports, so a ship trading to the United States has to meet these definitions regardless of her flag.
Standards & sources referenced
- MARPOL: International rules to prevent pollution from ships.
- Conventions index
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- What Sewage Is — the Definition, the Daily Volumes, and Why It Must Be Treated
- Aerobic and Anaerobic Breakdown — Why the Colony Dies and How Long Recovery Takes
- Where Sewage May Be Discharged — the 3 Mile and 12 Mile Rules
- The Sewage Treatment Plant — Aeration, Settling and Chlorination
- Retention, Holding and Zero-Discharge Systems — Holding Tanks, Vacuum Toilets and Elsan
- Effluent Quality — BOD, Suspended Solids and Coliform Count
- Sludge Disposal — Overboard at Sea, Sullage Tank, or Incinerator
- Troubleshooting the Sewage Plant — Ten Faults, Cause by Cause
- Maintenance of the Sewage Plant — Desludging, Aeration and the Daily Checks
- Safety on the Sewage Plant — Gas Hazards and Enclosed Space Entry