Sludge Disposal — Overboard at Sea, Sullage Tank, or Incinerator
Sewage sludge disposal: pumping overboard at sea clear of restricted areas, the sullage tank route in port, and burning sludge in the ship's incinerator.
1. The principle
Warning: Operating rule
The SMS and the Chief Engineer's orders override. Sludge is never discharged in port. It goes overboard at sea clear of restricted areas, or to a sullage tank, or to the incinerator.
Regular removal of sludge prevents impairment of operation. This is the reason sludge handling is a maintenance subject as much as a disposal subject — a plant whose sludge is not removed loses aeration volume and eventually loses its treatment capacity.
The sludge is pumped overboard when the vessel is at sea, clear of areas where restrictions are in force. Small quantities can be burnt in an incinerator.
Those are the two sea-going routes. Neither is available in port, which brings the third:
In port the sludge cannot be discharged, so it must be pumped to a sullage tank for disposal later. By using an incinerator to deal with excess sludge, a sullage tank may not be required.
That last sentence is the practical design trade. A ship with an incinerator capable of burning sewage sludge does not need a separate sullage tank for it — the incinerator is the disposal route. A ship without one has to retain.
2. The three routes
| Route | Where it can be used | Notes |
|---|---|---|
| Overboard at sea | At sea, clear of restricted areas | The normal route for a plant in regular service |
| Sullage tank, then shore | Anywhere, but the retention route for port | Adds tank capacity to the ship |
| Incinerator | At sea, with the Chief Engineer's permission | Removes the need for a sullage tank if capacity is adequate |
3. The incinerator connection
The incinerator should be able to burn solid matter from the sewage plant. Where the plant is arranged to feed it directly, a connection has to be made from the sewage plant to the pyrolysis chamber of the incinerator — a permanent installation rather than something improvised. The two plants are sometimes built as a single unit, and the arrangement is shown in Oily Sludge and Its Disposal, section 4.
The incinerator's safety devices, its operating procedure, and the certified burning capacity recorded on the IOPP supplement are common to all its duties — waste oil, sludge, rags, galley waste and sewage solids — and are dealt with in the oily water separator notes. The point that belongs here is simply that the sewage plant's solids are one of the wastes it is designed to take.
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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