Aerobic and Anaerobic Breakdown — Why the Colony Dies and How Long Recovery Takes
Aerobic versus anaerobic sewage breakdown: why ships use aerobic treatment, what kills the bacteria, and why a dead colony takes 5 to 14 days to recover.
1. The distinction
Warning: Operating rule
The maker's manual and the PMS override. The plant is to be kept running at all times. A sewage plant that is shut down loses its bacteria, and regrowth takes days.
The breakdown of raw sewage in water is effected by aerobic bacteria if there is a relatively ample presence of oxygen, but by anaerobic bacteria if the oxygen has been depleted.
That is the whole of it, and everything else is consequence. The two processes take the same raw material and produce completely different results.
| Aerobic | Anaerobic | |
|---|---|---|
| Oxygen | Require free oxygen to survive | Only multiply in the absence of free oxygen — they use chemically bound oxygen instead |
| Products | Water, carbon dioxide, inert residue, and the energy to synthesise new bacteria | Water, carbon dioxide, methane, hydrogen sulphide and ammonia — this is putrefaction |
| Character of the effluent | Clean and clear | Black, turgid water; the products are noxious and toxic, and the effluent is of poor quality and highly corrosive |
| Gases produced | — | Toxic and flammable |
| On a ship | The basis of the sewage treatment plant | The principle of the septic tank — not suitable for sewage treatment on a ship |
The septic tank point is worth dwelling on, because it is the natural question. A septic tank works anaerobically, and it works perfectly well for a house with a large area of ground around it. On a ship it is unacceptable: the effluent is corrosive, the gases are toxic and flammable in a confined machinery space, and the smell is a habitability problem. The septic tank is a retention and digestion device, not a treatment plant.
2. Why the aerobic process is used on board
Ship sewage treatment uses the aerobic breakdown process, with the oxygen supply maintained by bubbling air through the water. The oxygen promotes the multiplication of the bacteria and the satisfactory decomposition of the wastes.
The mechanism is worth stating properly, because it explains the plant's behaviour. The bacteria build up a colony, using the sewage as food and the oxygen for their metabolism. The action results in a clean effluent liquid, which is then disinfected and discharged overboard.
Two consequences follow directly:
- The bacteria are a living colony, not a chemical reagent. They can be killed, and they take time to regrow.
- The oxygen supply is the colony's lifeline. Stop the air and the colony dies within hours, and what is left in the tank is a large volume of sewage with no treatment capacity at all.
The process is deliberately used ashore, in some shore sewage treatment works, to produce gas which is then used as fuel for internal combustion engines on the site. That is a reminder that anaerobic digestion is not simply a failure mode — it is a process with its own uses, just not on board a ship.
3. What kills the bacteria
This is the practical heart of the chapter. A sewage plant almost never fails mechanically. It fails biologically, and the causes are few and predictable:
- Oil or grease entering the system kills the useful bacteria. This is the single most common cause of a dead plant, and it is entirely preventable — it comes from galley grease, from machinery space oil reaching the sewage system, or from a cross-connection.
- Loss of the air supply. Without oxygen the colony dies and the tank goes anaerobic.
- A long shutdown. The bacteria die, and a new colony has to be started from nothing.
- A change of flushing liquid — for example when a ship moves from sea water flushing to fresh water flushing. The colony is adapted to its conditions and a drastic change harms it.
- A drastic change of temperature.
- Excess use of lavatory cleaning agents. The chemicals that clean the bowl also kill the bacteria in the tank.
The bacteria are sensitive to temperature, the type of water, and the regularity of flow. All three have to be kept roughly constant.
How long recovery takes
This is the figure that makes the whole argument. With the extended aeration system it can be 5 to 14 days before the plant is fully operational after a kill, because of the prolonged aeration necessary to produce the bacteria. Hence the plant should be kept operational at all times.
Regeneration can take several days, depending on the level of harm caused.
A ship with a dead plant is a ship with no sewage treatment for the duration of a voyage. That is why "keep it running" is an operating rule rather than a preference.
4. Why the settling tank is the danger point
Of all the compartments in the plant, the settling tank is the one that must never be left alone.
Activated sludge cannot be allowed to remain in the settling tank, because there is no oxygen supplied to that area, and in a very short time the collected sludge would become anaerobic and give off offensive odours. It is therefore continuously recycled to the aeration tank.
This is a good example of the aerobic/anaerobic distinction having a direct design consequence. The settling tank is, by its nature, an unaerated compartment. It is safe only because sludge passes through it rather than accumulating in it. Stop the recycle — a blocked air lift, for instance — and the settling tank becomes an anaerobic digester inside the plant.
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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