Aerobic and Anaerobic Breakdown — Why the Colony Dies and How Long Recovery Takes
The one distinction on which the whole subject turns.
Key Principles at a Glance 7 points
- Aerobic bacteria break sewage down when oxygen is relatively ample; anaerobic bacteria take over once it is depleted — and the two take the same raw material and produce completely different results.
- Aerobic breakdown gives water, carbon dioxide, inert residue and a clean, clear effluent; anaerobic breakdown is putrefaction, giving methane, hydrogen sulphide and ammonia, black turgid water and a corrosive effluent.
- The septic tank is a retention and digestion device, not a treatment plant, and it is unsuitable on board because the effluent is corrosive, the gases are toxic and flammable, and the smell is a habitability problem.
- Oil or grease entering the system kills the useful bacteria and is the single most common cause of a dead plant — and unlike a blower failure it gives no warning at all.
- A killed colony takes 5 to 14 days to become fully operational again with extended aeration, which is why the plant is kept running at all times.
- Activated sludge must never be left in the settling tank: the compartment is unaerated by design, so the sludge is continuously recycled to the aeration tank by air lift.
- Continuing to run a plant after an air supply failure propagates anaerobic bacteria and produces flammable, toxic gas in a confined space — the correct action is to stop, restore the air, and rebuild the colony.
1. The distinction
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.
The related hazard
Continuing to use a sewage plant after a failure of the air supply could result in the propagation of anaerobic bacteria and processes. The gases produced by anaerobic activity are dangerous, being flammable and toxic.
So the correct action on an air supply failure is not to keep running and hope. It is to stop, restore the air, and then re-establish the colony. Running an unaerated plant produces hydrogen sulphide and methane in a confined space.