How a Sewage Treatment Plant Works, Air and Disc Plants

A sewage treatment plant cleans the waste water from a home that is not on the mains sewer. It does it in three stages. The solids settle out first, then bacteria break down what is left while the plant feeds them air, and finally the last fine particles settle again so that only clean water leaves. That water is clean enough to go to a stream or ditch, or to the ground through a drainage field, which is the whole point of a plant and the reason it is allowed in places a septic tank is not.

Where plants differ is in how they get air to the bacteria. Some blow it in through a pump called a blower, and others turn a set of discs slowly through the waste so the bacteria growing on them are lifted into the air and lowered back in. We service both kinds, and both need a motor or a blower running all the time. If yours has stopped, smells or is alarming, our treatment plant servicing and repairs page explains what we check and how we get it running again.

Stage one, settling the solids

Everything from the house arrives at the first chamber. Heavy solids sink to the bottom as sludge, fats and lighter material float to form a crust, and the liquid in between moves on. Nothing clever happens here. It works the same way as the first chamber of a septic tank, and it is the part that fills up and needs emptying.

Because this chamber holds the sludge, it decides how often the plant has to be desludged. If it is left too long, solids carry over into the next stage, the bacteria are smothered, and the water leaving the plant gets dirtier. That is the single most common reason a plant that was working well starts to struggle.

Stage two, the bacteria do the work

The middle stage is where the cleaning happens. Bacteria that live on air feed on the dissolved waste and turn it into water, carbon dioxide and more bacteria. They are the plant. Everything else in the design exists to keep them alive and supplied with air, which is why a plant cannot simply be left to run on its own the way a septic tank can.

There are two main ways of doing it.

Air-pumped plants

In an air-pumped plant, a blower sits in a small housing on the lid or beside the tank and pushes air down to diffusers in the bottom of the chamber. The bubbles keep the water moving and carry oxygen to the bacteria, which live on plastic media fixed in the chamber or float freely in the water. The blower runs day and night. When it fails, the bacteria are starved of air, the plant goes septic and it begins to smell. The usual faults are a worn diaphragm in the blower, a blocked air filter or a split air line.

Rotating disc plants

A rotating disc plant, such as a Klargester BioDisc, does it the other way round. A row of large plastic discs sits on a shaft half in and half out of the waste water, and an electric motor turns them slowly through a gearbox. The bacteria grow as a film on the discs, feeding while they are under the water and taking in air while they are above it. There is no blower and no air pipe. The work falls on the motor, the gearbox, the drive sprocket and the bearings that carry the shaft, so those are the parts that wear and the parts we replace.

The photo at the top of this page is one of our engineers servicing exactly that kind of plant. A disc plant that has stopped turning is easy to spot, because the discs sit still with one half drying out in the air, and the film on them dies off quickly.

Some plants work in batches instead, filling a single chamber, aerating it, letting it settle and then pumping the clean water out on a timer. They have more pumps, timers and float switches to look after, but the job of the bacteria is the same.

Stage three, settling again

Bacteria that have done their work and fine particles still in the water drift into a final settlement chamber, sometimes called the clarifier. The water slows down, the last solids sink, and the clear water from the top leaves through the outlet. Some plants pump part of that settled material back to the start. A plant whose final water looks cloudy, grey or carries bits is telling you one of the earlier stages is not keeping up.

Where the clean water goes

A treatment plant can discharge to a stream or ditch that flows all year, which a septic tank must never do, or to the ground through a drainage field. Which one your plant uses depends on the site, and the General Binding Rules set out when it can do either without a permit from the Environment Agency, including the volume limits and the distances from sewers and protected sites. If the rules cannot be met, the Agency's guidance on the permits you need for a septic tank or treatment plant explains the next step.

New packaged plants are tested to BS EN 12566-3, the British Standard for packaged plants serving up to 50 people, and the binding rules expect a new plant to meet the standard in force when it was installed. A plant also has to meet the building regulations. Approved Document H covers where it can go and the need for a tanker to reach it for emptying.

What is the difference between a treatment plant and a septic tank compares the two side by side if you are not sure which you have.

Why a plant needs power and attention

The difference that matters most day to day is that a plant runs on electricity. If the power to it is switched off, tripped or cut in a storm, the blower or the motor stops and the bacteria start to suffer. A plant that has been off for a day or two needs time to recover once it is running again, and it can smell while it does.

The things that harm a plant are the same as for any private system, but the plant feels them more, because it depends on living bacteria. Large amounts of bleach, disinfectant and antibacterial cleaner, medicines poured away, fat down the sink, wipes and sanitary products, and a sudden surge of water such as a hot tub being drained all knock it back. A plant serving more people than it was sized for will struggle all the time.

Servicing and desludging

A plant needs two different kinds of visit, and they are easy to confuse.

• Desludging. The sludge that builds up in the first chamber is pumped out by a tanker. The binding rules ask for this before the tank goes past its capacity, and at least once a year or as often as the manufacturer says.
• Servicing. An engineer checks the mechanical and electrical parts, the condition of the bacteria, the flow through each stage and the quality of the water leaving the plant, and replaces worn parts before they fail.

The two are often done on the same visit. What to expect from treatment plant servicing walks through a service step by step, and how long does a treatment plant last explains how regular care stretches the life of the plant itself.

Never lean into or climb down into an open plant to look at a problem. The gases that build up in a tank can overcome a person very quickly, and the Health and Safety Executive treats a tank as a confined space. Lift the cover only to read an alarm or check the discs are turning, and leave the rest to whoever services it.

Signs your plant needs looking at

• A smell of drains or rotten eggs around the plant or the outlet
• The alarm sounding or a warning light on the control panel
• Silence where you would normally hear the blower hum or the discs turning
• Cloudy, grey or smelly water at the outlet, or sewage fungus in the ditch below it
• Slow drains or gurgling in the house
• Wet, green or sunken ground over a drainage field

Any of these is worth a call. A blower or a motor caught early is a part on a routine visit, while one that has been off for weeks can mean a plant that has to be emptied and started again.

Choosing, replacing or upgrading a plant

A plant is usually the answer when a septic tank can no longer discharge where it does, when a drainage field has failed on heavy ground, or when a new home has no sewer to connect to. Do I need a sewage treatment plant is a good place to start, and sewage treatment plant installation cost explains what moves the price of a new one. Our treatment plant installation page covers how we size, place and fit a plant, and new tank installations covers cesspools and septic tanks too. We work right across Kent and East Sussex.

Getting your plant looked at

A treatment plant is a small working sewage works in your garden, and it keeps working only while its bacteria are fed, aired and not drowned in sludge. Know which kind you have, keep the power on, watch for the signs above, and have it emptied and serviced on time.

KP Cesspool Services has over twenty years' experience with tanks and treatment plants across Kent and East Sussex, and you can read our reviews on Checkatrade. Call 01233 720725, email kpcesspoolservices@hotmail.com, or contact us to book a service or report a fault.