Where Sewage May Be Discharged — the 4 Mile and 12 Mile Rules
Where sewage may be discharged, in what condition, and at what distance from land.
Key Principles at a Glance 6 points
- Nothing is discharged within 4 nautical miles; untreated sewage only beyond 12; comminuted and disinfected sewage may go overboard between 4 and 12 miles at a moderate rate with the ship en route at not less than 4 knots.
- Sewage treated by an approved plant may be discharged at any distance, including inside 4 miles, provided there are no visible floating solids and no discoloration — that is the whole value of fitting the plant.
- Holding tank contents are treated as untreated for the purposes of the table: retaining sewage does not improve it.
- MARPOL Annex IV regulates sewage disposal internationally, and the ruling for vessels over 200 gross tonnage with more than ten persons on board gave existing vessels a ten-year grace period.
- American legislation defines three types: Type I, no visible floating solids and coliform under 1000 per 100 ml; Type II, suspended solids not over 150 mg/l and coliform under 200 per 100 ml; Type III, which prevents any discharge overboard at all.
- Type III is not a standard of treatment — it is a retention system, and it is what the Elsan-type zero-discharge plants are built to satisfy.
1. The discharge table — memorise this
The flag State requirements, the M Notices in force, the ISPP Certificate and its supplement and the Chief Engineer's orders override these notes. Nothing is discharged inside 4 nautical miles. Nothing untreated is discharged inside 12.
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 should only be discharged outside the 12 mile limit. Comminuted and disinfected sewage may be discharged as near as 4 miles from land. Nothing is discharged within the 4 mile limit.
| Type of sewage | 0–4 NM | 4–12 NM | Beyond 12 NM |
|---|---|---|---|
| Treated by an approved sewage treatment plant | No visible floating solids or discoloration | Same | Same |
| Comminuted and disinfected with approved equipment | Prohibited | Moderate rate, ship en route at not less than 4 knots | Allowed |
| Untreated sewage | Prohibited | Prohibited | Allowed |
| Sewage held in holding tanks | Prohibited | Prohibited | Moderate rate, ship en route at not less than 4 knots |
How to read the table
Three things fall out of it:
- The 4 mile line is absolute for comminuted and untreated sewage. Nothing of either kind goes overboard inside it.
- A properly treated plant changes the picture completely. Sewage treated by an approved plant may be discharged at any distance, including inside 4 miles, provided there are no visible floating solids and no discoloration. That is the whole value of fitting the plant — it is what allows a ship to operate in port and in inland waterways.
- Holding tank contents are treated as untreated for the purposes of the table. Retaining sewage does not improve it.
The ship en route at not less than 4 knots condition applies to the middle and last rows. It is a requirement about dispersal — the ship must be making way so that the discharge is spread rather than concentrated in one place.
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 textbooks and in older references.
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.