What is a Detention Tanks (Balancing Tanks)

Detention tanks, often referred to as balancing tanks, are integral elements within sewerage and surface water drainage systems. They are engineered structures designed to temporarily store excess water during periods of high flow, such as during intense rainfall or peak wastewater generation. By providing controlled attenuation of flow, these tanks help manage the hydraulic load on downstream infrastructure and reduce the risk of flooding, surcharging, and combined sewer overflows (CSOs).

Although similar in purpose to retention structures, detention tanks differ in that they are intended to empty after a storm event or flow surge, typically remaining dry or partially full between events. They are often installed underground, though above-ground or partially buried configurations may also be used depending on site constraints and design requirements.

Purpose and Function

The primary function of a detention tank is to provide temporary water storage during peak inflows, with controlled release into the downstream network once flows have returned to manageable levels. This delay in discharge ensures that the sewer or drainage system is not overwhelmed during high-intensity events.

Detention tanks play several critical roles within modern drainage systems:

  • Mitigating localised surface water flooding during storm events 
  • Preventing sewer surcharging by buffering inflow rates 
  • Reducing frequency and volume of CSOs in combined systems 
  • Managing runoff from impermeable surfaces such as roofs, car parks, and roads 
  • Facilitating compliance with planning conditions, particularly where discharge must be limited to greenfield runoff rates 

In many cases, detention tanks are used in conjunction with flow control devices, such as orifice plates, vortex controls, or penstocks, to regulate outflow and maintain system equilibrium.

Types of Detention Tanks

Detention tanks can be constructed using various materials and configurations, with selection based on factors such as required storage volume, available space, structural loading, and ease of maintenance. Common types include:

1. Geocellular (Modular) Tanks

Made of interlocking plastic units with a high void ratio, geocellular tanks are ideal for below-ground installations beneath roads, car parks, or landscaped areas. Wrapped in an impermeable membrane, they function effectively as sealed detention tanks, offering flexibility and rapid installation.

2. Concrete Tanks

Precast or in-situ reinforced concrete tanks are used for high-capacity storage in heavily trafficked or industrial areas. They offer excellent durability, structural strength, and longevity, often forming part of the permanent sewer infrastructure.

3. Oversized Pipes

Large-diameter concrete or plastic pipes laid horizontally act as cost-effective detention tanks, particularly in linear developments or where land availability is constrained. These systems are simple to install and maintain, especially when aligned with the pipe gradient.

4. Steel or Glass-Reinforced Plastic (GRP) Tanks

Steel and GRP tanks are typically used for above-ground or semi-buried installations. They may be prefabricated off-site and delivered for quick assembly. These tanks are common in temporary applications or where space and access are restricted.

Regardless of type, the tanks are designed with consideration for inflow management, outflow control, sediment deposition, and access for inspection and maintenance.

Hydraulic and Operational Considerations

Detention tanks are designed based on detailed hydraulic modelling, taking into account catchment size, rainfall intensity, soil infiltration, and downstream constraints. The storage volume required is determined by the need to limit outflow rates to a pre-defined level, typically a greenfield runoff rate or an agreed permitted discharge rate set by the local authority or sewerage undertaker.

Key hydraulic parameters in detention tank design include:

  • Design storm return period (commonly 1-in-30 or 1-in-100 years) 
  • Volume of attenuation required (m³) 
  • Time to empty (often within 24–48 hours) 
  • Controlled outflow rate (L/s) 
  • Self-cleansing velocity to minimise sediment build-up 

Many systems now incorporate real-time control (RTC), using sensors and automated valves to optimise storage use and dynamically adjust outflows in response to real-world conditions. This is particularly valuable in smart cities and integrated urban drainage systems.

Applications in Drainage Systems

Detention tanks serve a variety of applications across different types of developments and drainage systems:

  • Combined sewer networks: Balancing tanks reduce the risk of CSOs by intercepting excess combined sewage during rainfall events and releasing it gradually. 
  • Separate stormwater systems: They limit peak discharge to prevent downstream flooding and pipe overloads. 
  • Industrial facilities: Detention tanks manage washwater or stormwater runoff from process areas before controlled discharge. 
  • Residential and commercial developments: They provide compliance with planning and SuDS requirements, particularly where infiltration is not feasible. 
  • Highway drainage schemes: Tanks may be used to temporarily hold runoff from carriageways before release into watercourses or treatment systems. 

In all cases, detention tanks form part of a broader flow management strategy, often integrated with SuDS components like swales, basins, and infiltration trenches.

Regulatory Framework and Planning Policy

In the UK, the design and implementation of detention (balancing) tanks are governed by national and local policy frameworks, including:

  • Sewerage Sector Guidance (SSG): Defines adoptable standards for storage structures offered to sewerage undertakers. 
  • Lead Local Flood Authority (LLFA) guidance: Specifies requirements for surface water management, including allowable discharge rates and storage sizing. 
  • Building Regulations Part H: Addresses drainage design in buildings, including the use of attenuation for flood mitigation. 
  • Environment Agency regulations: Relevant where tanks discharge to controlled waters or interact with floodplains. 
  • National Planning Policy Framework (NPPF): Encourages sustainable development and integration of SuDS into all major planning proposals. 

Planning authorities typically require a Drainage Strategy or Flood Risk Assessment (FRA) that demonstrates how detention tanks, alongside other infrastructure, will ensure compliance with flood mitigation targets and environmental protection standards.

Operation, Maintenance, and Inspection

Detention tanks are passive systems but require routine inspection and maintenance to ensure continued performance. Accumulated sediment, debris, or failure of control devices can reduce effectiveness or cause structural or environmental problems.

Typical maintenance tasks include:

  • Visual inspections for signs of sedimentation, blockage, or structural damage 
  • Cleaning and flushing of chambers, particularly in areas with high particulate loads 
  • Checking flow control devices for functionality and clearance 
  • Maintaining access points and covers to ensure safe entry for authorised personnel 
  • Recording and reporting maintenance activity for regulatory and warranty purposes 

Access chambers, manholes, and inspection shafts must be designed into the system layout to facilitate these tasks, especially for underground or large-volume tanks.

Responsibility for maintenance may fall to:

  • The developer or landowner 
  • A management company or third-party maintenance provider 
  • A water company if the asset is adopted under SSG protocols 

The frequency of maintenance depends on tank usage, inflow quality, and system design, but annual inspections are generally recommended at minimum.

Benefits and Limitations

Detention (balancing) tanks offer several engineering and environmental advantages but must also be designed with awareness of potential limitations.

Key benefits:

  • Help meet planning conditions and regulatory discharge limits 
  • Mitigate surface water flooding in constrained urban environments 
  • Provide flexible underground storage, preserving surface use 
  • Easily combined with SuDS and real-time control systems 
  • Scalable and adaptable for different project sizes 

Common limitations:

  • Require routine inspection and potentially costly maintenance 
  • Do not provide water quality treatment unless combined with filtration or treatment systems 
  • Do not contribute to infiltration or groundwater recharge 
  • Risk of odour or stagnation if retention is prolonged 
  • Installation can be disruptive in retrofit scenarios, especially for large underground tanks 

Designers must weigh these factors when specifying a detention tank solution, ensuring it integrates seamlessly with other site drainage measures.

Integration with Sustainable Drainage (SuDS)

Although not inherently green or visible, detention tanks are commonly included in SuDS strategies where space is limited or where infiltration is not viable. They are particularly useful in:

  • Urban areas with high-density development 
  • Brownfield sites with contamination or poor soil permeability 
  • Projects with limited surface access where underground storage is the only option 

Where possible, detention tanks are paired with above-ground SuDS features to deliver holistic water management, combining quantity control with quality treatment, amenity, and biodiversity.

Conclusion

Detention tanks, also known as balancing tanks, are indispensable tools for managing peak flows in modern sewerage and surface water systems. Whether installed beneath city streets or integrated into housing developments, they serve a vital role in mitigating flood risk, protecting public infrastructure, and ensuring compliance with stringent planning and environmental standards.

With increasing rainfall intensity, urbanisation, and pressure on existing networks, the importance of well-designed and maintained detention tanks will only grow. For engineers, developers, planners, and asset managers, understanding the principles, benefits, and operational requirements of detention tanks is essential to delivering resilient, sustainable drainage infrastructure.