After every flush of a toilet, the run of a tap and shower taken, wastewater flows into a vast sewage network covering an estimated 500,000 kilometres of pipes across the UK. Every day, over 11 billion litres of wastewater enter this system! That’s a lot of water to just let go to waste, and if we didn’t treat it our access to clean water would rapidly deplete.
Treating wastewater is essential not only for public health but also for preserving freshwater resources and ensuring a sustainable supply for future generations. At sewage treatment plants, wastewater is collected, treated, and safely returned to the water cycle, protecting both our health and the environment.
What is a Sewage Treatment Plant?
A sewage treatment plant collects, treats, and safely discharges wastewater back into the water cycle, playing a crucial role in sanitation infrastructure. These facilities, often large and highly automated, minimise human error and enhance overall efficiency. Today, they operate at peak performance, with 99% of plants meeting Environmental Agency (EA) regulatory standards – up from just 88% in the 1990s.
In areas where properties cannot connect to the main sewer system, such as remote locations or difficult terrain, small underground sewage treatment plants provide an alternative. Powered by electricity from the property, these compact systems enable effective wastewater treatment for individual residential or commercial sites to protect local environments.
How Does a Sewage Treatment Plant Work?
The sewage treatment plant is divided into four stages, preliminary, primary, secondary, and tertiary.
Preliminary:
Preliminary treatment is the first step in the wastewater treatment process. It enhances the efficiency of later treatment stages by removing coarse materials, grit, and other large debris through various screens. This increases process effectiveness and reduces the risk of damaging machinery.
Primary:
The primary treatment stage separates the solids from liquids in wastewater, preparing the water for more advanced treatment processes. Large sedimentation tanks (also called primary clarifiers) hold the water, and solids settle at the bottom of the tank due to gravity, forming a dense layer called sludge. Lighter materials like grease and oils rise to the surface, forming scum.
Sedimentation tanks typically retain wastewater for 1.5 to 2.5 hours, allowing sufficient time for most solids to settle without causing odours or other complications.
The clarified liquid is then discharged for further treatment, while the settled sludge is collected for processing.
After appropriate treatment, stabilised sludge can be used for fertiliser or soil conditioner in agriculture. Heavily contaminated sludge that cannot be safely reused is incinerated.
Secondary:
Secondary treatment is a biological process designed to break down organic contaminants in wastewater. Building on primary treatment, this stage uses microorganisms to degrade the remaining organic matter, greatly enhancing water quality for discharge.
In an aerobic process, wastewater is mixed with air in aeration tanks to encourage the growth of beneficial microorganisms. These microorganisms digest organic pollutants, converting them into simpler, harmless substances. Various methods can be used for this process, all producing significantly cleaner water.
In many cases, secondary treatment results in water that meets environmental standards for safe discharge into natural water bodies. However, if the water does not meet these standards or requires further purification for uses such as drinking, it undergoes additional processing in the tertiary treatment stage.
Tertiary:
Tertiary treatment is the final stage of wastewater processing, designed to improve water quality to meet high environmental and health standards. This stage is particularly critical when wastewater is discharged into protected waters such as bathing areas, shellfish waters, or sensitive ecosystems, where stricter regulations apply.
The type of tertiary treatment changes depending on the characteristics of the wastewater and the required discharge standards. For example, bathing water require disinfection whereas shellfish water will require nutrient removal.
- Microfiltration - Removes fine particles, microorganisms, and other impurities not captured during secondary treatment.
- Ion Exchange - Ions in the water, such as nitrates or heavy metals, are exchanged for other harmless ions. It is often used for water softening or removing specific contaminants.
- Activated Carbon Adsorption – Activated carbon attracts and holds organic compounds, chemicals, and pollutants, removing them from the water. This method is highly effective for eliminating trace organics and improving water taste and odour if reused for drinking.
- Disinfection - Pathogens are destroyed or inactivated using ultraviolet (UV) light or chemical disinfectants (e.g., chlorine or ozone).
Tertiary treatment ensures wastewater is treated to the highest quality, safeguarding public health and the environment.
The Discharge Process
Once the water has gone through the necessary treatment stages – preliminary, primary, secondary, and possibly tertiary – it is ready to be released back into the environment.
Before discharge, treated water undergoes a final inspection to it meets the required quality standards for its intended use.
Through underground pipes, the water is typically released into a natural water body, such as a river, lake, or ocean. The receiving water body should be able to dilute the treated water effectively and assimilate any remaining contaminants without harm to aquatic life or public health.
The Importance of Water Treatment Plants
Sewage treatment plants play a critical role in public health, environmental protection and sustainable development. In the UK, nearly 100% of drinking water comes from water treatment plants. Don’t worry! The quality of drinking water is strictly regulated to meet the Drinking Water Inspectorate (DWI) and World Health Organization (WHO) standards.
The ongoing investment and growth within the water industry and modernising these facilities are essential to support growing populations, combat pollution, and adapt to climate change.
