Module 17 - Marine Environment
Sewage and Waste Water
MCA MGN 599 Amendment 1 says the UK sewage provisions it describes apply to ships on international voyages over 400GT, or smaller ships certified to carry more than 15 people. It also says those regulations make no specific sewage provision for vessels below both thresholds. That distinction matters: the large-ship MARPOL equipment and discharge regime must not be presented as though every cruising yacht has the same legal configuration.
The same guidance gives a clear best-practice baseline for smaller pleasure vessels. Return waste to shore where possible; use a holding tank if fitted and empty it at a pump-out station. If shore disposal is impractical, its UK advice is to empty a holding tank only more than 3 nautical miles offshore in open sea where dispersion is effective. A yacht without a holding tank should avoid toilet use in poorly flushed estuaries, inland waterways, inlets and crowded anchorages by using shore facilities wherever possible.
The offshore advice is not a worldwide permission. Harbours, inland waterways, protected waters and other countries can prohibit or more tightly control discharge. Trace the system before departure, label valve positions, confirm capacity and pump-out availability, and make sure a crew member cannot accidentally select an overboard route in restricted water.
- Know the vessel's size, voyage status, certified capacity and sewage-system layout
- Use shore facilities and a holding tank where fitted
- Protect capacity in poorly flushed water and crowded anchorages
- Check harbour, inland, protected-area and foreign controls before operating an overboard valve
| Question | What the evidence says | Skipper action |
|---|---|---|
| Does the stated UK certification regime apply? | International-voyage ships over 400GT or certified for more than 15 people meet the described threshold | Confirm the vessel's actual status rather than borrowing a large-ship table |
| Typical smaller pleasure vessel | Use shore facilities and a holding tank where fitted | Plan capacity and pump-out before entering poorly flushed water |
| No practical shore disposal | MCA advice qualifies holding-tank emptying by more than 3nm, open sea and effective dispersion | Use only after confirming no tighter local or foreign rule applies |
| Harbour, inland waterway or foreign coast | Additional or different controls can apply | Check the current authority and set valves accordingly |
Optional quick check
Section 3 of 8
What is the strongest plan before entering a poorly flushed foreign estuary?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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