Maintenance of the Sewage Plant — Desludging, Aeration and the Daily Checks

1. The list

• Desludge about every three months — pump out about three quarters of the aeration tank contents and refill with clean water.
• Keep the plant operational at all times. A shutdown kills the bacteria, and regeneration takes 5 to 14 days.
• Keep the air supply clean — the diffuser is porous material and blocks with dirty air.
• Check the aerators — they are removable through the side or end of the tank for replacement.
• Test the float switches and alarms — the high water level activating switch, the low water level activating switch, and the high water level alarm.
• Maintain the stand-by blower and the stand-by discharge pump in working order.
• Watch the sludge return — the settling tank must not be allowed to go anaerobic.
• Maintain the chlorinator and the 60 minute retention in the collection tank, and renew the calcium hypochlorite tablets.
• Check the surface skimmer and outlet weir at the centre of the settling tank.
• Check the holding tank vent, flame trap and coating.
• Keep oil and grease out of the system — it kills the bacteria.

2. The two items that matter most

Everything on the list is worth doing, but two items carry most of the risk.

Keeping oil and grease out

Oil or grease kills the useful bacteria, and it is the single most common cause of a dead plant. Unlike a blower failure, which announces itself, a slow ingress of grease produces no alarm — it simply produces a plant that gradually stops working, and by the time the effluent is visibly bad the colony is already gone.

The preventive work is therefore in the galley and in the machinery space, not in the plant. Grease traps, drip trays and clean bilges are sewage plant maintenance.

Keeping the sludge return working

The settling tank is the danger point of the plant. It is unaerated by design, so activated sludge left in it goes anaerobic in a short time and produces offensive odours and corrosive, toxic gas. The only thing preventing that is the continuous air lift return.

Checking the air lift is therefore a routine item, not a fault-finding step. A partially blocked air lift produces a plant that still works, but with a settling chamber slowly digesting.

3. The routine in tabular form