Sewage Treatment Processes Explained

The Wastewater Treatment Process

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

Wastewater, or raw sewage, is water that drains from toilets, sinks, showers, baths, dishwashers, washing machines and liquid industrial waste.  It is treated in a multi-stage process by the Water Companies that make it safe to return to the environment.

The wastewater undergoes several stages in its municipal 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

This is another settlement stage, during which mainly organic 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 for further treatment.

Secondary Sewage Treatment

This stage involves the introduction of oxygen to the sewage and uses aerobic bacteria to break down the organic matter, coming mainly from human body waste, in the liquid part of the sewage.  They also reduce the amounts of other contaminants, detergents, etc. and the quality of the resulting effluent is usually regarded as safe for discharge to a watercourse, unless the watercourse is sensitive or already polluted.  The dead bacteria and humus are allowed to settle out in a clarifying chamber and this sludge is also removed periodically for further treatment. 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.  Tertiary treatment involves a further aerobic stage, followed by an oxygen depleted stage, which reduces nitrogen and phosphorous. It can also include a disinfection process using UV light, chlorination or ozone treatment to remove bacteria and viruses.

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, 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.