How Does a Wastewater Treatment Plant Work? Treatment Stages, UK Standards and Deployable Solutions
Every time you flush a toilet, empty a sink, or run a washing machine, that used water begins a journey most people never think about. It travels through a network of sewer pipes, joins the wider sewage system, and eventually arrives at a wastewater treatment plant. There, it is cleaned, treated, and returned safely to the natural water cycle.
In the UK, water companies and the Environment Agency hold wastewater treatment to strict standards. Whether you manage a rural property off the public sewer, operate an industrial site, or need a rapid deployable solution, understanding how a wastewater treatment plant works matters. This guide explains every treatment stage clearly, covers UK discharge standards, and shows when Butek Tanks' Modular Wastewater System (MWS) makes more sense than a fixed plant.
What Is a Wastewater Treatment Plant?
A wastewater treatment plant (also called a sewage treatment plant, sewage treatment works, or treatment works) is a facility designed to collect raw sewage and used water, remove harmful contaminants, and return treated wastewater safely to the environment.
The contaminants removed include solid matter, organic waste, pathogens, nutrients such as nitrogen and phosphorus, and in the case of industrial wastewater, heavy metals and chemical residues. The goal of every treatment plant, regardless of size, is the same: to protect water bodies, groundwater, and public health by making water safe before it re-enters the natural water cycle.
There are two broad categories. Large municipal treatment plants receive sewage from towns and cities via the public sewer and sewer network, processing millions of litres daily. Smaller on-site treatment systems serve individual properties, rural communities, or commercial sites not connected to the public sewer. Sites that also need large-scale effluent storage alongside their treatment system can pair any plant with Butek Tanks' Steel Water Tanks, available from 2m3 to 5,000m3 and fitted with specialist Liner Kits in Landflex ES, HDPE, Butyl, and EPDM to safely contain whatever is being stored.
What Are the Three Main Stages of Wastewater Treatment?
A wastewater treatment plant works by passing sewage through a series of carefully designed treatment stages. There are three main stages of wastewater treatment: primary treatment, secondary treatment, and tertiary treatment, typically preceded by a preliminary screening stage. Each treatment stage builds on the last, progressively improving water quality until the treated effluent meets the required standards for safe discharge.
Understanding these main stages explains not only how treatment plant work is structured, but also why each step is essential and what happens when one stage is skipped or fails.
Preliminary Treatment: Screening Raw Sewage
Raw sewage arriving at the treatment works via the sewer network first passes through screens. The purpose of this stage is to remove large solids: wet wipes, rags, plastics, leaves, and other debris that would damage the more sensitive equipment downstream. Grit removal tanks then slow the flow, allowing sand, gravel, and small stones to settle out.
Without this preliminary step, solid debris would clog pipework, wear down mechanical components, and disrupt the entire treatment process. The screened material is typically sent to landfill or incinerated.
Primary Treatment: Removing Solids and Sludge
Primary treatment involves large sedimentation tanks where the flow of sewage slows dramatically, allowing gravity to separate its contents. Heavier solid matter sinks to the bottom and accumulates as sludge. Fats, oils, and grease rise to the surface and are skimmed away.
This primary wastewater treatment stage removes approximately 60 to 70 percent of suspended solids from the incoming sewage and meaningfully reduces the biochemical oxygen demand (BOD) of the water before it moves forward. The sludge collected at this stage is pumped away for separate treatment, covered later in this guide.
The partially clarified water, now referred to as effluent, moves on to secondary treatment. Primary treatment alone is never sufficient for safe discharge. The effluent at this point still contains significant levels of dissolved organic matter, bacteria, and nutrients that require biological treatment to remove.
Agricultural operations that produce slurry alongside wastewater require purpose-built, certified storage to contain organic waste safely. Butek Tanks' Slurry Tanks are available from 280m3 to 1,250m3, certified to British and European structural standards including BS 5502, and lined with Landflex ES to resist corrosion and contamination.
Secondary Treatment: Biological Treatment of Organic Waste
Secondary treatment is the biological treatment stage and the heart of the entire wastewater treatment process. The partially clarified sewage enters a biological treatment zone where naturally occurring microorganisms consume and break down the dissolved organic pollutants that primary treatment cannot remove.
There are two commonly used secondary treatment processes in the UK.
Activated Sludge Process: Wastewater is pumped into aeration tanks where compressed air is continuously introduced. This oxygen-rich environment encourages aerobic bacteria to thrive and digest the organic matter in the sewage at speed. The process dramatically reduces BOD and suspended solids, producing significantly cleaner water. This method is used in the majority of large sewage treatment works and municipal treatment plants worldwide.
Biological Filter Beds and Rotating Biological Contactors: Wastewater trickles over or around media (aggregate, plastic, or rotating discs) colonised by bacteria. As the bacteria are alternately exposed to wastewater and air, they digest the organic contaminants. This approach uses less energy than activated sludge and is well suited to smaller on-site treatment systems.
After the secondary treatment process, the water passes through a final settlement tank where the bacterial biomass (now part of the sludge) settles out. The treated wastewater that remains is substantially cleaner and, in some circumstances, can be discharged to a watercourse at this point if it meets the required water quality standards. However, for many UK sites a further treatment stage is required.
Tertiary Treatment: Polishing Effluent to the Highest Standard
Not every wastewater treatment plant includes a tertiary treatment stage, but it is required when treated effluent will be discharged to sensitive water bodies such as rivers, bathing waters, or shellfish-producing coastal areas. The tertiary treatment stage is the final treatment step before the treated effluent is released to the environment.
This stage targets the fine suspended solids, residual nutrients, and pathogens that survive secondary treatment. Common tertiary treatment methods include:
Sand or membrane filtration: Removes fine suspended solids that passed through the secondary stage, producing cleaner water with lower turbidity.
UV disinfection: Ultraviolet light destroys remaining bacteria and viruses without chemicals, making it the most environmentally friendly disinfection option available.
Chemical dosing: Iron or aluminium salts precipitate and remove dissolved phosphorus, preventing eutrophication of receiving water bodies.
Chlorination: Kills remaining pathogens but requires dechlorination before the treated water can be safely discharged to surface water or a watercourse.
After tertiary treatment, the treated effluent meets the level of treatment required for safe discharge. Where effluent quality is high enough, treated waste water can also be reused for non-potable applications such as irrigation, toilet flushing, or industrial process water recycling, supporting more sustainable water management across a site.
What Happens to the Sludge?
Every treatment stage generates sludge as a by-product. Managing this sewage sludge is a critical part of the overall sewage treatment process and directly affects the efficiency and environmental compliance of the plant.
The most common sludge treatment approaches are:
Anaerobic digestion: Sludge is heated in sealed tanks where anaerobic microorganisms break down the organic matter and produce biogas (predominantly methane), which is captured to generate electricity and heat on-site.
Dewatering: Reduces the water content of the sludge through mechanical pressing or centrifugation, shrinking its volume and making it cheaper to transport.
Agricultural land application: Treated sewage sludge (biosolids) is spread on farmland as a nutrient-rich fertiliser, returning phosphorus and nitrogen to the soil and contributing to sustainable water management.
Incineration: Heavily contaminated sludge that cannot safely be applied to land is incinerated, with the resulting ash sometimes used in construction materials.
Effective sludge management is essential at every level of treatment and plays a direct role in the overall water quality outcomes a plant can achieve.
UK Discharge Standards: What Does a Treatment Plant Need to Achieve?
In the UK, wastewater treatment discharge standards are set and enforced by the Environment Agency in England, SEPA in Scotland, and Natural Resources Wales. These standards define the maximum concentrations of specific contaminants that treated effluent can contain before it is discharged to a watercourse, surface water, or drainage field.
A standard UK discharge permit typically specifies limits for biochemical oxygen demand (usually no more than 20mg/L), suspended solids (typically 30 to 100mg/L depending on the sensitivity of the receiving water), ammoniacal nitrogen (often 5 to 10mg/L), and phosphorus (as low as 0.25mg/L in protected areas).
The General Binding Rules (GBRs) govern small sewage discharges from systems treating domestic sewage from up to 13 people discharging less than 2m3 per day. Provided a system is correctly installed, maintained, and discharges to an appropriate drainage field or watercourse, no individual permit is required. For larger or more complex sewage discharges, a bespoke Environment Agency permit must be obtained.
Sewage treatment works serving sensitive locations must achieve stricter standards, often requiring full tertiary treatment including nutrient removal and UV disinfection. Failing to meet the required level of treatment risks enforcement action, fines, and legal liability for any water pollution caused.
Sewage Treatment Plant vs Septic Tank: What Is the Difference?
A septic tank provides primary wastewater treatment only. It separates solids from liquid using gravity and relies on a drainage field to complete the treatment process naturally in the soil. The effluent it produces is not clean enough for direct discharge to a watercourse.
A sewage treatment plant carries out the full treatment process on-site, including biological treatment and optionally tertiary treatment. The treated wastewater it produces is significantly cleaner and can, in most cases, be discharged directly to a watercourse under an Environment Agency permit.
Since January 2020, Environment Agency General Binding Rules prohibit septic tanks from discharging to surface water. For properties where a compliant drainage field is not viable, a sewage treatment plant is the legally required alternative.
When Is a Modular or Deployable Wastewater Treatment Solution the Right Choice?
Standard fixed wastewater treatment plants are built for permanent, planned infrastructure. But there are situations where speed, flexibility, or temporary capacity matter far more than permanence.
Planned maintenance: When an existing sewage treatment plant needs to go offline for upgrade or repair, a temporary treatment system keeps operations compliant and avoids sewage discharges to the environment.
Emergency response: Flooding, infrastructure failure, or unforeseen damage can leave a site without functioning treatment works within hours. A deployable system can be operational within days.
Remote or off-grid locations: New developments, construction sites, or rural communities with no access to the public sewer need on-site treatment that can be installed and operational quickly.
Scaling existing capacity: Where demand has outgrown an existing treatment plant, a modular addition can increase throughput without full plant replacement.
This is exactly where Butek Tanks' Modular Wastewater System (MWS) delivers. The MWS is a complete, deployable sewage treatment plant built on the activated sludge aeration process used in the majority of permanent sewage treatment works worldwide. It can be installed in as little as 7 to 14 days, treats up to 1,000m3 of wastewater per day, and produces treated wastewater that is 99.99% pathogen free.
The MWS treatment process covers primary settlement, aerobic digestion, liquid separation, UV or chlorination disinfection, and sludge treatment, making it a full multi-stage wastewater treatment plant in a modular, transportable format. It operates as a standalone system or connects to an existing plant to rapidly increase capacity with minimal downtime. For sites requiring covered effluent storage alongside the treatment system, our Roof Kits protect stored water quality and prevent contamination from debris, algae, and evaporation.
Frequently Asked Questions
How does a wastewater treatment plant work step by step?
Raw sewage arrives at the treatment works via the sewer network and passes through preliminary screening to remove large debris. In primary treatment, settlement tanks allow solids to sink as sludge and fats to rise as scum. Secondary treatment uses biological processes (aerobic bacteria) to break down dissolved organic pollutants. Where required, tertiary treatment removes fine solids, nutrients, and pathogens through filtration and disinfection. The resulting treated effluent is discharged safely to a watercourse or drainage field, and the sludge produced at each stage undergoes its own separate treatment.
What are the three main stages of wastewater treatment?
Primary treatment (removing solids and sludge), secondary treatment (biological treatment of organic matter), and tertiary treatment (final polishing, nutrient removal, and disinfection). Most plants also include a preliminary screening stage before these three main stages begin.
Can treated wastewater be reused?
Yes. After tertiary treatment including UV disinfection, treated wastewater can be safely reused for non-potable applications including irrigation, toilet flushing, and industrial process water use. Water recycling at this level supports sustainable water management and reduces demand on freshwater resources.
What happens to sewage sludge after treatment?
Sewage sludge is typically processed through anaerobic digestion (producing biogas for energy generation), dewatered to reduce volume, and then either spread on agricultural land as biosolids fertiliser or incinerated if contamination levels prevent land application.
Do I need a permit to discharge treated wastewater in the UK?
Small systems treating domestic sewage from up to 13 people and discharging less than 2m3 per day may be covered by the General Binding Rules, removing the need for an individual permit. Larger or more complex sewage discharges to a watercourse require a bespoke Environment Agency discharge permit with specific effluent quality conditions.
What is the difference between a sewage treatment plant and sewage treatment works?
They refer to the same type of facility. Sewage treatment works is the traditional UK term; wastewater treatment plant is the modern equivalent used internationally. Both describe a facility that treats raw sewage through multiple stages before discharge.
Conclusion
A wastewater treatment plant works by passing sewage through a structured treatment process, from preliminary screening and primary solids removal through biological secondary treatment to tertiary polishing and disinfection. Each treatment stage builds on the last, progressively improving water quality until the treated effluent is safe to return to the natural water cycle without causing water pollution or harm to water bodies.For properties, businesses, and sites that need wastewater treatment capacity quickly, temporarily, or in a location far from the public sewer, a modular approach changes what is possible. Whether you need a deployable sewage treatment plant, large-scale effluent storage, or specialist liner solutions for challenging liquid streams, contact the Butek Tanks team to discuss your requirements.