The short answer
A sewage treatment plant should make a low, steady hum from its air blower and very little else. Most owners stop noticing it after a few days. If a plant is loud, rattling or vibrating, the blower is usually worn, badly mounted or housed somewhere that amplifies the sound, and each of those can be fixed.
Noise is a fair question to ask before buying one, especially if the plant will sit near a patio or a bedroom window.
Where the sound comes from
The blower is the only part of most domestic plants that runs constantly. It is a small air pump, usually in a housing on the tank lid or in a separate kiosk nearby, feeding air to the bacteria in the treatment chamber. It makes a gentle hum. You may also hear water moving or the odd click of a valve.
Plants that pump their effluent have a pump that runs for short spells, which you might hear briefly. Some designs, such as rotating biological contactors, have a slowly turning motor instead of a blower.
How loud is normal?
Manufacturers publish a noise figure for each model, usually measured at a set distance in decibels. Use that figure, measured at the same distance, to compare models and to judge where to put the plant. A healthy domestic blower in a closed housing is usually hard to hear from inside the house.
Noise drops with distance, and a wall, fence or hedge between the blower and a window helps further.
Why a plant gets louder
- Worn diaphragms. A blower running louder or hotter than usual is often the first sign that its diaphragms need replacing.
- Loose mounting. A blower sitting on a hard surface without rubber mounts, or touching the side of its housing, passes vibration into the lid and kiosk, which act like a drum.
- A damaged or missing housing lid. The cover muffles the sound, so a broken one makes a big difference.
- Failing bearings or a motor fault, in plants with rotating parts.
- Air leaks. A split hose or loose fitting can whistle.
Quietening a loud plant
Start with a service. Replacing worn diaphragms and fitting anti-vibration mounts fixes most noisy blowers. A better housing or an acoustic kiosk can help where the blower has to sit close to the house. In some cases the blower can be moved further away, with a longer air line run to the tank, though the manufacturer's limits on hose length apply.
Don't wrap the blower in blankets or seal its housing to muffle it. It needs air to breathe and to stay cool, and it will fail sooner if it overheats.
Thinking about noise when you install
The design stage is the easiest time to get noise right. Placing the blower away from bedrooms, patios and neighbours' windows, choosing a model with a low published figure, and using a proper housing all help. Building regulations already ask for the treatment plant's discharge to be sited at least 10 metres from other buildings, which tends to put some distance between the plant and the house.
What if a neighbour's plant is too loud?
Talk to them first, since the cause is usually a worn blower that is cheap to fix. If that does not work, persistent noise can be raised with the council's environmental health team, which deals with statutory nuisance.
Our treatment plant servicing includes checking the blower, diaphragms and mountings, and our repairs team can replace a noisy blower. If you are planning a new plant, see sewage treatment plant installation, and for other faults, common sewage treatment plant problems.
