Maintenance of the Sewage Plant — Desludging, Aeration and the Daily Checks
Sewage treatment plant maintenance: three-monthly desludging of three quarters of the aeration tank, keeping the air supply clean, and testing float switches.
1. The list
Warning: Operating rule
The maker's manual and the PMS override these notes. Keep the plant operational at all times — a shutdown costs five to fourteen days of treatment capacity, whatever else is achieved.
- 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
| Interval | Action |
|---|---|
| Continuously | Keep the plant running; keep the air supply on; keep the sludge return working |
| Daily / weekly, per PMS | Check the effluent appearance; check the air supply and blower running; check for oil or grease ingress |
| Weekly to monthly, per PMS | Test the float switches and the high level alarm; check the surface skimmer and outlet weir |
| Monthly to quarterly | Check and renew the chlorinator tablets; verify the 60 minute retention; check the holding tank vent, flame trap and coating |
| About three-monthly | Desludge — pump out about three quarters of the aeration tank and refill with clean water |
| Per maker's interval / as found | Clean or renew the aerators; clean the diffuser; maintain the stand-by blower and discharge pump |
| On survey | Verify the plant against the ISPP Certificate and its supplement |
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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