What is a Tank sewer

A tank sewer is a specially designed sewer that has a cross-sectional area greater than what is required for normal dry weather flow. The additional volume is intentionally created to provide in-line or near-line storage capacity for stormwater. In essence, a tank sewer serves a dual function: it acts as a conventional sewer under normal flow conditions, but also stores excess runoff during rainfall events, reducing the risk of downstream flooding and combined sewer overflows (CSOs).

Tank sewers are most commonly used in combined sewer systems, where both foul sewage and surface water runoff are transported in the same pipe network. In these systems, the challenge of managing increased flows during wet weather is significant. Rather than allowing peak flows to bypass treatment or cause flooding, tank sewers provide an integral solution by temporarily storing stormwater until the network can accommodate it.

This article explores the role, design, application, benefits, and limitations of tank sewers in modern drainage engineering.

The Need for Stormwater Storage in Sewer Systems

Urbanisation, climate change, and ageing infrastructure have significantly increased the burden on traditional sewer networks. In combined systems, rainfall events can lead to:

  • Hydraulic overloading of the sewer

  • Activation of storm overflows, discharging untreated sewage into watercourses

  • Basement or street flooding

  • Increased wear on pumping stations and treatment plants

These challenges are exacerbated by the prevalence of impermeable surfaces in towns and cities, which accelerate runoff and reduce natural infiltration. One response to these issues has been the development of storage-based attenuation strategies, with tank sewers forming a central part.

By increasing the internal volume of key sewer sections, tank sewers act as inline retention structures. They slow the passage of excess stormwater through the network, often in coordination with flow control structures, and release it later when capacity allows.

What is a Tank Sewer?

A tank sewer is a section of sewer with an intentionally oversized diameter or cross-sectional profile. Unlike conventional sewers, which are designed solely to convey flow at self-cleansing velocity, tank sewers are also designed to store a calculated volume of water during storm events.

There are two key elements to this function:

  1. Dry Weather Flow Handling
    During typical conditions, flow remains in the lower portion of the sewer. The sewer functions normally and allows for uninterrupted movement of wastewater.

  2. Stormwater Storage
    During heavy rainfall, when inflow exceeds the downstream network’s capacity, excess flow is held within the oversized section of pipe until conditions permit gradual release.

Tank sewers may be constructed using a range of materials and forms, including:

  • Circular or egg-shaped concrete or vitrified clay pipes

  • Precast concrete box culverts

  • Cast in-situ reinforced concrete tunnels

  • High-capacity plastic or composite-lined systems for specific applications

Where Are Tank Sewers Used?

Tank sewers are most often employed in urban combined sewer systems, but may also be found in:

  • Separate stormwater networks where temporary storage is needed before discharge

  • Downstream of critical flow control points, such as regulators or weirs

  • Upstream of pumping stations to reduce peak loads and extend pump life

  • In areas with limited surface space, where surface SuDS features are impractical

  • As a replacement for storm tanks or detention basins in retrofit scenarios

In large cities with complex sewer systems, tank sewers are often integrated into broader strategic storage schemes. They can also act as surge tanks, providing hydraulic buffering that protects downstream assets.

Design Principles of Tank Sewers

Designing a tank sewer requires careful hydraulic and structural analysis. Key factors include:

Hydraulic Design

  • Storage volume: Calculated based on rainfall intensity, contributing area, system capacity, and target discharge rates.

  • Flow regulation: Usually coordinated with flow control devices such as penstocks, orifices, or vortex valves to control inflow and outflow.

  • Self-cleansing velocity: Ensured through careful gradient design or flushing systems to avoid sedimentation during dry periods.

  • Freeboard and surcharge: Proper allowances are made for surcharge conditions and backflow risk.

Structural Considerations

  • Pipe strength: The large diameter and potential live loads require materials with high compressive strength.

  • Joint integrity: Watertight seals are essential to prevent infiltration and exfiltration.

  • Manhole and access provision: For maintenance, inspection, and emergency response.

In addition to these, control and monitoring systems may be integrated to track filling levels, control discharge rates, and interface with SCADA (Supervisory Control and Data Acquisition) systems in municipal networks.

Advantages of Tank Sewers

Tank sewers offer numerous benefits to system designers, operators, and urban planners. Key advantages include:

  • Dual functionality: They serve as both conduits and storage, maximising utility of below-ground infrastructure.

  • Reduced CSO frequency: By storing excess stormwater, they reduce the number and volume of overflows into watercourses.

  • Minimal land take: Being below ground, they are ideal for dense urban areas where surface storage is impractical.

  • Flexibility in retrofit projects: They can be installed within existing alignments or upgraded from current sewers.

  • Improved resilience: Provide buffering capacity that helps networks cope with extreme rainfall events.

These advantages make tank sewers a viable solution in both new developments and as part of sewer renewal and climate adaptation strategies.

Challenges and Limitations

Despite their benefits, tank sewers come with certain design, operational, and maintenance challenges:

  • High construction costs: Due to their size, material requirements, and depth.

  • Maintenance complexity: Sediment build-up, odour control, and confined space entry all require careful planning.

  • Risk of long retention: If flow is not properly managed, stored water can become anaerobic, leading to corrosion or gas generation.

  • Hydraulic balancing: Ensuring that stored water is released at the correct rate without causing downstream surcharging.

  • Limited treatment: Tank sewers provide temporary storage but do not treat water. Pollutants are still present unless combined with treatment solutions.

Proper design, integration with upstream SuDS measures, and regular maintenance are essential to mitigating these risks.

Maintenance and Operational Considerations

Tank sewers require a structured maintenance regime to ensure long-term performance. Tasks may include:

  • Routine inspection of structural integrity and access points

  • Sediment removal using jetting, vacuum extraction, or mechanical scrapers

  • Odour monitoring and control, especially in warm weather

  • Monitoring flow levels and volumes through telemetry systems

  • Emergency response planning in case of system blockage or overflow

Because of their underground and confined nature, maintenance activities must comply with confined space entry regulations and safety procedures, including gas monitoring, ventilation, and rescue planning.

Integration with Urban Drainage Strategies

Tank sewers are often one component of a broader urban drainage strategy. They are most effective when used in conjunction with:

  • Surface SuDS such as green roofs and permeable paving, which reduce inflow

  • Flow regulators and real-time control systems to optimise discharge

  • Pumping stations, which may control the rate of flow from storage

  • Treatment works, which must be protected from peak loads that could disrupt biological processes

The integration of tank sewers into intelligent drainage networks allows for more responsive and efficient stormwater management, improving the sustainability of urban infrastructure.

Real-World Examples

Numerous cities have successfully implemented tank sewers as part of flood prevention and CSO reduction programmes. Examples include:

  • London’s Thames Tideway Scheme, which incorporates large interceptor sewers that also serve as in-line storage tanks during storms.

  • Manchester’s sewer upgrades, where offline and inline storage pipes were introduced to reduce the frequency of storm discharges to the River Irwell.

  • Dublin’s drainage network, which uses oversize trunk sewers to accommodate intermittent storm surges in its mixed-use urban catchments.

These case studies demonstrate the scalability and versatility of tank sewer solutions in varied contexts.

Conclusion

Tank sewers represent a critical evolution in the design and operation of modern sewerage systems. By providing both conveyance and in-built storage, they offer a space-efficient, underground method for mitigating the impacts of heavy rainfall, protecting receiving waters, and extending the life and capacity of existing networks.

While not a universal solution, when properly integrated with flow control, maintenance plans, and complementary SuDS features, tank sewers deliver robust performance in both new and retrofitted systems. For engineers, planners, and utility operators facing the dual pressures of urban growth and climate uncertainty, the tank sewer is a key tool in building resilient and adaptable drainage infrastructure.