What Sewage Is — the Definition, the Daily Volumes, and Why It Must Be Treated
What counts as sewage under MARPOL: the three limbs of the definition, why it must be treated, and how many litres per person per day a ship produces.
1. The definition — three parts
Warning: Operating rule
The flag State requirements, the M Notices in force, the ISPP Certificate and its supplement, the SMS and the Chief Engineer's orders override these notes. No untreated sewage is to be discharged to sea outside the regulation.
"Sewage" is a defined term, and the definition has three limbs. All three are regulated; a discharge that satisfies only two of them is still a discharge of sewage.
Sewage means:
- Drainage and wastes from toilets — but not including the wash water from basins and wash tubs.
- All drainage from medical premises, and this one does include the wash water from basins and wash tubs.
- Drainage from spaces containing living animals.
The exclusion in the first limb is worth noting because it catches people out. Water from a hand basin is not sewage. Water from a hospital sink is. The difference is not the water; it is where it came from.
The third limb applies to livestock carriers, and it is the reason such ships need treatment capacity out of proportion to their crew numbers.
2. Why it must be treated
Raw sewage discharged into restricted waters will eventually overwhelm the self-purification ability of the limited quantity of water available. In a closed dock the effect is plainly visible — black, sludgy water which, stirred by the ship's propeller, gives off an unpleasant smell.
The mechanism
Where the amount of sewage relative to the amount of water is small, dissolved oxygen in the water assists a bio-chemical — aerobic — action which breaks the sewage down into simple components and carbon dioxide. The water cleans itself, given time and oxygen.
The problem is that this capacity is finite and it is easily exhausted. The discharge of large quantities of raw sewage into restricted waters such as inland waterways and enclosed docks causes rapid depletion of the oxygen in the water, so that aerobic bacteria are unable to survive. When the self-purification ability is overwhelmed in this way, breakdown by putrefaction occurs — the anaerobic process, with its smell, its black appearance and its toxic gases.
So the ship has two choices: retain the sewage until it can be discharged where there is enough water to absorb it, or treat it on board to the point where the oxygen demand is removed. The plant in the next chapters is the second of those choices.
3. How much sewage a ship produces
The exact quantity is difficult to quantify, and the published figures differ by authority. Both sets are worth knowing, because the plant's rated capacity will be expressed against one of them.
| Source | Toilet waste | Washing water |
|---|---|---|
| European designers | 70 litres per person per day, including flushing water | 130 to 150 litres per person per day, including baths and laundries |
| US authorities | up to 114 litres per person per day | Twice the toilet figure |
Two things follow from the table.
First, the washing water is the larger problem by volume — roughly twice the toilet waste on the European figures and twice again on the American ones. This is the reason the definition of sewage excludes basin and wash tub water, and the reason a plant sized only on toilet waste will be overwhelmed.
Second, the flushing water dominates the toilet figure. A conventional toilet uses about 12 litres per flush; a vacuum toilet uses about 1. That single change reduces the volume of sewage the plant has to deal with by an order of magnitude, which is why the vacuum system is treated as part of the retention problem rather than as a subject of its own — see Retention, Holding and Zero-Discharge Systems, section 1.
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- OWS / Sewage / Marine Environment — 28 questions
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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