What is a Storm tanks

Storm tanks are specially designed storage structures used within sewerage systems or wastewater treatment works to temporarily hold excess stormwater during periods of high rainfall or peak hydraulic load. Their primary function is to prevent the immediate overflow of stormwater mixed with sewage into rivers, streams, or coastal waters, thereby reducing pollution, flooding, and system overload.

These tanks are a fundamental part of many urban drainage systems, especially those that use combined sewers — systems that convey both foul sewage and surface water runoff in a single pipe. During dry weather, flow volumes are manageable. However, when intense or prolonged rain occurs, the system may receive significantly more water than it can convey or treat in real time. Storm tanks provide a buffer, allowing this additional water to be stored and then gradually returned to the treatment process once capacity is available.

Purpose and Benefits of Storm Tanks

Storm tanks are implemented for several important reasons within modern drainage and treatment systems:

  • Preventing combined sewer overflows (CSOs): By holding excess water during storm events, storm tanks reduce the frequency and volume of discharges from CSOs into the environment.

  • Protecting wastewater treatment works: Treatment plants have a maximum flow capacity. Storm tanks prevent these facilities from being overwhelmed during peak flows.

  • Flood mitigation: Temporary storage of stormwater reduces the risk of backflows and flooding in upstream areas of the drainage network.

  • Pollution control: Retaining stormwater allows for later treatment, reducing the amount of contaminants discharged into receiving waters.

In essence, storm tanks contribute to both environmental protection and infrastructure resilience.

Where Storm Tanks Are Used

Storm tanks are typically installed in two main locations:

1. Within Sewer Networks

These in-sewer tanks are constructed along main interceptors or trunk sewers, often near CSO outfalls. They intercept high flows before they reach the outfall and store them until the main sewer has capacity again.

2. At Wastewater Treatment Works

Storm tanks at treatment plants temporarily hold inflows that exceed the design flow of the works. Once the peak subsides, the stored water is pumped back into the head of the works for full treatment.

The choice of location and design depends on factors such as local topography, sewer system configuration, population served and environmental sensitivity of nearby water bodies.

Types of Storm Tanks

Storm tanks vary in size, form, and operational complexity. The most common types include:

1. Offline Storm Tanks

In an offline arrangement, excess flow is diverted from the main sewer line into the tank when it reaches a trigger level. Once the storm has passed, stored water is returned to the sewer. This is the most common design due to operational flexibility and ease of maintenance.

2. Online Storm Tanks

Here, storm tanks form part of the main flow path. Water always passes through them, and excess volumes are simply retained temporarily. These are less common and are typically used where space is limited or where sediment settlement is desirable.

3. Covered vs Open Tanks

Storm tanks may be constructed below ground with concrete lids or above ground in open form. Covered tanks reduce odour and protect contents from further rainfall, while open tanks are sometimes used where cost is a concern or where visibility of infrastructure is not an issue.

Design Considerations

Storm tanks must be designed to operate effectively across a wide range of flow conditions and in line with national and local standards. Important factors include:

  • Volume requirements: Typically based on the size of the catchment area, rainfall intensity, sewer capacity and environmental discharge criteria

  • Inlet and outlet control: Often includes weirs, orifices, penstocks and automatic gates to regulate the flow in and out of the tank

  • Pump systems: Where gravity cannot return the stored water to the sewer, pumps are used for controlled return

  • Cleaning mechanisms: Many modern tanks include automatic flushing systems or tipping buckets to prevent sediment build-up

  • Overflow protection: In case the tank reaches full capacity, a controlled overflow point ensures water discharges in a managed manner

Designs must also account for long-term access, health and safety, resilience to blockages and integration with monitoring systems.

Operational Management of Storm Tanks

Proper operation of storm tanks is critical to ensure they perform as intended during rainfall events. Key operational aspects include:

  • Level monitoring: Sensors track the water level in real time and activate filling, emptying or alarm protocols

  • Telemetry systems: Allow for remote control and data collection, enabling operators to respond proactively to storms

  • Maintenance scheduling: Periodic inspections, sediment removal and equipment checks ensure reliability

  • Overflow recording: Some systems are required to record any discharge events and report them to regulatory agencies

In the UK, water companies are increasingly deploying Event Duration Monitors (EDMs) at storm tanks and associated overflows to provide transparency and ensure regulatory compliance.

Regulatory Context in the UK

The use and performance of storm tanks are governed by several environmental regulations and planning frameworks. Key aspects include:

  • Water Industry Act 1991: Places responsibility on sewerage undertakers to manage stormwater in a way that protects the environment

  • Urban Waste Water Treatment Regulations 1994: Requires appropriate storage at treatment works to limit pollution from storm discharges

  • Environment Agency (EA) Guidance: Specifies conditions for storm discharges, including storage requirements, event frequency and allowable volumes

  • Permit requirements: Discharges from storm tanks (e.g. overflows) may require permits with strict monitoring and reporting conditions

Storm tanks are also referenced in design standards such as Sewers for Adoption and WIS/WRc specifications for wastewater infrastructure.

Relationship to Other SuDS and Storage Elements

While storm tanks are typically considered part of conventional infrastructure, they share some similarities with sustainable drainage system (SuDS) storage features. However, important differences exist:

  • SuDS storage elements (e.g. detention basins, ponds, swales) are generally surface-based, decentralised, and aim to mimic natural processes

  • Storm tanks are engineered, centralised, and designed primarily for hydraulic buffering and pollution control

In some integrated urban drainage schemes, SuDS are used upstream to reduce inflows to storm tanks, improving their performance and reducing operational costs.

Challenges and Limitations

Despite their many advantages, storm tanks present several design and operational challenges:

  • Cost: Construction and installation of large tanks, especially underground, can be expensive

  • Space constraints: Urban environments may lack the space required for tanks of sufficient size

  • Odour and aesthetics: Without proper ventilation or covering, tanks can produce unpleasant smells or detract from local appearance

  • Maintenance intensity: Tanks must be regularly desilted and checked for mechanical reliability

  • Public perception: Some stakeholders may see storm tanks as a temporary fix rather than a sustainable long-term solution

Nevertheless, storm tanks remain a practical and effective part of stormwater management, particularly in built-up areas where options for surface-based solutions are limited.

Examples of Storm Tank Implementation

1. Combined Sewer Networks

Most UK cities, including London, Birmingham and Manchester, operate combined sewer networks with extensive storm tank infrastructure. These tanks play a critical role in reducing CSO discharges during wet weather.

2. Wastewater Treatment Works

At facilities such as Beckton STW (Thames Water) or Davyhulme STW (United Utilities), massive storm tanks provide thousands of cubic metres of additional capacity during storms, enabling the plant to handle inflows far exceeding average dry weather flow.

3. Industrial and Commercial Developments

Industrial estates, ports and logistics centres may incorporate on-site storm tanks to delay and regulate discharges to public sewers, especially where local regulations require flow control.

Conclusion

Storm tanks are a vital element of modern wastewater and drainage infrastructure, providing temporary storage for excess stormwater during periods of high rainfall. By buffering peak flows, protecting treatment works, and reducing pollution from combined sewer overflows, storm tanks contribute significantly to the resilience and environmental compliance of urban drainage systems.

As climate change leads to more intense rainfall events and urbanisation increases impervious surfaces, the need for effective stormwater management will only grow. Well-designed and properly maintained storm tanks will continue to play a crucial role in this effort — alongside greener, decentralised solutions — as part of an integrated approach to sustainable water infrastructure.