What is a sewage treatment plant and how does it clean the wastewater?

What is a sewage treatment plant?

vA sewage treatment plant has only one job to do. It must take all the wastewater from toilets, baths, washing machines and all other appliances that use water.  It must then turn it into clean water that is safe to discharge into the environment.

Sewage contains all kinds of pollutants,  including bacteria and chemicals. Some of these chemicals strip oxygen out of water, removing it for fish and other aquatic life that needs it to live. The wastewater must be re-qxygenated before it is discharged into rivers and streams.

We all flush toilets and create pollution and sewage treatment plants are essential in order to protect our environment and Public Health.

What Are The Stages Of Sewage Treatment?

Sewage Treatment refers to the process of removing contaminants, micro-organisms and other types of pollutants from wastewater.

Wastewater treatment is not done in a single chamber. It is treated in a multi-stage process by the Water Companies that make it safe to return to the environment and in multi-chambers in a private sewage treatment plant.

The wastewater undergoes several stages in its treatment process for Environmental Protection:


This involves the screening and removal of large debris, including sanitary towels, tampons, sticks, leaves, rags, wet-wipes, etc. from the raw sewage as these are not readily biodegradable by the process bacteria.  They are then dried and sent to landfill sites or incinerated.

It also involves the removal of sand and grit which is settled out in tanks and also removed to landfill sites.

 Primary Sewage Treatment

Primary wastewater treatment is another settlement stage.

The sewage is retained long enough to remove settleable solids.

Heavier solids sink to the bottom and form a sludge and fats rise to the surface and form a scum or crust.  This crust is colonised by aerobic bacteria (they breathe oxygen) and partially digested.

The liquid between these two layers drains off for processing in the Secondary treatment stage and the sludge and crust are removed periodically by a tanker lorry for further treatment.

 Secondary Sewage Treatment

Secondary treatment involves removing smaller organic matter that did not float or settle in the primary treatment tank.

This stage involves the introduction of oxygen to the sewage and uses aerobic bacteria to break down the remaining organic matter. This comes mainly from human body waste, in the liquid part of the sewage.

They also reduce the amounts of other contaminants, detergents, etc.

The effluent then transfers to a final settlement tank. The dead bacteria and humus are allowed to settle out in this chamber and this sludge is also removed periodically for further treatment.

The quality of the resulting effluent is usually regarded as safe for discharge to a watercourse, unless the watercourse is sensitive or already polluted. 

All sewage treatment plants must have an EN 12566-3 2005 certificate for acceptance by the Environment Agency.

Tertiary Treatment

This stage is required for sensitive discharge points eg. trout streams and bathing waters.

Tertiary treatment involves a further aerobic stage, followed by an oxygen depleted stage, which reduces nitrogen and phosphorous. It can also include a further disinfection process to remove bacteria and viruses.

Tertiary treatment methods include:

  • UV disinfection 
  • Chemical disinfection

Chemical disinfection usually involves the addition of Chlorine to the wastewater. Chlorine kills viruses and bacteria, but it is also harmful to the environment. It also makes the water smell, if not fully removed and alters the pH.  For this reason, dechlorination is needed before the sewage water can be discharged to rivers and streams.

UV disinfection does not involve the use of chemicals. It simply uses UV light to disinfect the effluent and is far more environmentally friendy. However, the equipment needed is costly and there is also the electrical consumption to consider.

UV light disinfection is primarily used to remove unwanted bacteria. It does not affect the water in any way and does not change the smell, pH or look of the final effluent.

WTE can design and supply a UV light disinfection system for any sewage treatment plant if required.

Private Wastewater Systems

Package sewage treatment plants do not usually involve the Pre-treatment or Tertiary treatment stages as the owners are more careful about what they put down the toilet and most watercourses can deal effectively with the minor level of contaminants left in the effluent. However, the VORTEX and QUANTUM sewage treatment plants do have the Pre-Treatment feature using a vibro-screen.

It is important to note that for sensitive streams, rivers and coastal bathing water discharge points, tertiary treatment is essential.

There are many different types of systems on the market; some produce much cleaner effluent than others, but ALL new sewage treatment plants and septic tank conversions must have an EN 12566-3 Certificate to be legal in the UK. 

Non-Electric sewage treatment plant systems are also available.

Read more about the wastewater treatment products and services we offer.