What is a Sewage treatment plant

A sewage treatment plant is a self-contained facility designed to treat raw domestic wastewater, typically in locations without access to mains drainage. These units are most often buried below ground and use biological processes to break down organic matter, converting sewage into treated effluent and sludge. The process relies on naturally occurring bacteria to degrade pollutants in a controlled environment, allowing for safe discharge of the final effluent into a watercourse or soakaway system.

Sewage treatment plants are commonly used in rural homes, small developments, businesses, caravan parks and other sites where connection to a public sewer is not feasible. Unlike septic tanks, which provide only basic primary treatment, sewage treatment plants carry out both primary and secondary treatment, and in some cases, additional polishing stages. This results in significantly cleaner effluent that meets regulatory standards for environmental discharge.

How a Sewage Treatment Plant Works

The treatment process in a sewage treatment plant is divided into several stages. Each stage is designed to progressively remove solids, organic material and harmful bacteria from the incoming wastewater.

1. Primary Treatment

This stage involves the initial separation of solids from the liquid waste. Wastewater flows into a settlement chamber where gravity allows heavier particles to sink to the bottom, forming sludge. Lighter materials such as grease and oils float to the top as scum. The clarified liquid in the middle moves on to the next treatment stage.

2. Secondary (Biological) Treatment

The clarified liquid enters an aeration chamber or biological zone where it is exposed to oxygen. Air is pumped into the chamber, encouraging the growth of aerobic bacteria which digest the organic matter remaining in the effluent. These microorganisms break down pollutants into less harmful substances such as carbon dioxide and water.

The biological treatment is typically achieved through one of the following methods:

• Activated sludge process: Air bubbles maintain a suspension of bacteria and organic matter, promoting rapid decomposition.

• Rotating biological contactors (RBCs): Discs rotate through sewage, allowing bacteria to attach to their surfaces and digest contaminants.

• Trickling filters: Wastewater is sprayed over a bed of media colonised by bacteria which degrade the waste.

3. Final Settlement

After biological treatment, the mixture enters a final chamber where any remaining suspended solids settle out. The clear, treated effluent at the top of this chamber is then suitable for discharge, either to ground or to a nearby stream or ditch, depending on environmental regulations.

4. Sludge Handling

The sludge collected during the primary and final stages must be removed periodically by a licensed waste carrier. This is typically required once or twice a year, depending on the plant’s size and the number of users.

Typical Components of a Sewage Treatment Plant

A modern packaged sewage treatment plant consists of several integrated components housed within a single compact unit. These include:

• Inlet chamber: Receives incoming sewage and allows initial flow calming.

• Primary settlement tank: Separates solids and reduces flow turbulence.

• Aeration or biozone chamber: Where aerobic bacteria break down organic pollutants.

• Final settlement tank: Allows treated effluent to be clarified before discharge.

• Air pump or blower: Supplies oxygen to support bacterial activity.

• Control panel: Manages air supply, alarms and system monitoring.

• Outlet pipe: Discharges the treated effluent to the approved receiving environment.

Some systems may also include UV disinfection units or filters for further polishing of the effluent if high-quality discharge is required.

Applications of Sewage Treatment Plants

Sewage treatment plants are used across a range of settings where connection to a mains sewer is not possible or practical. Typical applications include:

• Rural homes and cottages

• Small housing developments

• Holiday parks and campgrounds

• Schools, pubs and restaurants in remote locations

• Industrial estates and workshops

• Agricultural premises

In all of these cases, a sewage treatment plant offers a long-term, environmentally acceptable solution for onsite wastewater treatment.

Benefits of a Sewage Treatment Plant

Sewage treatment plants offer several advantages over other types of wastewater systems, especially in off-grid or rural settings:

• High level of treatment: Produces clear, low-odour effluent suitable for discharge to watercourses or the ground.

• Compact design: Units are often buried underground, requiring minimal space and visual impact.

• Low maintenance: Most systems need only periodic servicing and annual desludging.

• Environmentally friendly: Uses natural biological processes with minimal chemical input.

• Compliant with regulations: Helps meet the General Binding Rules and discharge standards set by the Environment Agency.

For small-scale developments, they offer a cost-effective and scalable alternative to extensive drainage infrastructure.

Maintenance Requirements

While sewage treatment plants are largely self-operating, routine maintenance is essential for reliable performance. Key maintenance tasks include:

• Annual servicing: Ensures that air pumps, alarms, diffusers and control systems are functioning correctly.

• Desludging: Carried out once or twice a year by a licensed contractor to remove accumulated solids.

• Effluent testing (if required): Some discharge permits require periodic sampling and laboratory analysis.

• Visual checks: Owners should inspect the system regularly for signs of blockages, odours or mechanical issues.

• Power supply: As most systems rely on electricity, a constant power supply must be maintained.

Neglecting maintenance can lead to odours, environmental pollution, mechanical failure and even prosecution in the case of unauthorised discharges.

Regulatory and Environmental Considerations

Sewage treatment plants in the UK must comply with environmental regulations overseen by the Environment Agency. Discharges from treatment plants fall under:

• The General Binding Rules: These allow discharge to ground (via a drainage field) or to surface water (e.g. stream or ditch) under specific conditions.

• Environmental Permits: Required if the system exceeds population limits, discharges to sensitive locations, or involves non-standard conditions.

• British Standard BS EN 12566: Applies to small sewage treatment units and covers design, testing and performance criteria.

Before installation, a site survey is often needed to assess ground conditions, water table level, proximity to watercourses, and soil percolation capacity. Installations must also comply with Building Regulations (Part H).

Differences Between a Sewage Treatment Plant and a Septic Tank

Though often confused, sewage treatment plants and septic tanks differ significantly in operation and output quality:

Due to stricter environmental rules, sewage treatment plants are now the preferred option for new installations where mains drainage is unavailable.

Challenges and Considerations

Although reliable, sewage treatment plants are not without challenges. These include:

1. Power dependency: Most systems need a constant electricity supply, making them vulnerable to outages.

2. Noise and vibration: From air blowers or pumps, though modern units are quieter than older models.

3. Initial cost: Higher capital investment compared to septic tanks or cesspools.

4. Installation complexity: Requires specialist groundworks and careful site assessment.

5. Environmental sensitivity: Poor installation or maintenance can still lead to pollution.

To overcome these issues, it is essential to choose a reputable system supplier, install the unit according to manufacturer instructions, and follow a maintenance schedule.

Conclusion

A sewage treatment plant is a compact, efficient and environmentally responsible solution for treating domestic wastewater where a mains sewer is not available. By using biological processes to purify wastewater, these systems produce high-quality effluent suitable for discharge into natural water bodies or the ground. While they require power and regular maintenance, their superior treatment capabilities make them the preferred choice under modern environmental standards. For homeowners, developers and businesses in rural or off-grid areas, a properly installed and maintained sewage treatment plant ensures compliance, protects the environment and supports long-term wastewater sustainability.