What is a Off-line tank

An off-line tank is a type of detention or storage facility that is not located directly in the main flow path of a drainage or sewer system. Instead, it is positioned alongside the primary network and is only brought into use during periods of elevated flow, such as during heavy rainfall or storm events. Off-line tanks play a critical role in reducing the risk of sewer overflows, surface flooding and downstream overloading by temporarily storing excess water and releasing it in a controlled manner after the event has passed.

These tanks are a key component of modern urban drainage strategies and are often integrated into both foul and stormwater networks to improve system resilience and environmental protection.

How an off-line tank works

Unlike in-line tanks, which are placed directly in the main flow channel, off-line tanks operate using a flow control mechanism that diverts excess water into the tank when the flow rate exceeds a set threshold. This is typically achieved using a weir, flow splitter, or throttling device that allows base flow to continue along the main pipe while diverting high flows into the tank.

Once the rainfall subsides and flow returns to normal, the stored water is gradually released back into the system through a controlled outlet such as an orifice, valve or pump.

The key features of an off-line tank include:

• Passive or mechanical flow diversion

• Detention and attenuation of peak flows

• Controlled discharge to avoid downstream overload

• Maintenance access for inspection and cleaning

Purpose and benefits

The primary function of an off-line tank is to manage excess flow volumes that cannot be handled by the main system during peak conditions. This helps to:

• Prevent surface water flooding in urban areas

• Reduce the frequency and volume of combined sewer overflows

• Protect downstream infrastructure from hydraulic overload

• Support compliance with environmental discharge limits

• Improve the performance of wastewater treatment works

Because the tank only activates under specific conditions, it remains empty or partially full most of the time, making it a flexible and low-maintenance solution for managing intermittent peak flows.

Applications

Off-line tanks are used in a variety of drainage and wastewater systems, including:

• Combined sewer systems, where foul and stormwater share the same pipes

• Surface water networks in highly urbanised areas

• Industrial or commercial sites with large impermeable areas

• Sustainable drainage systems (SuDS) as part of a wider attenuation strategy

• Retrofit projects where system capacity is limited but space is available nearby

They are particularly useful in locations where land constraints, hydraulic gradients or system layout make it impractical to install in-line tanks.

Design considerations

Effective design of an off-line tank requires careful planning and hydraulic modelling. Key factors to consider include:

• Volume of storage needed to manage a defined rainfall event (e.g. 1 in 30 or 1 in 100 year storm)

• Location of the flow diversion point within the network

• Flow control devices at the tank inlet and outlet

• Access for cleaning, inspection and maintenance

• Structural requirements to support ground or traffic loads

• Odour and ventilation management, especially in foul systems

• Overflow protection in the event that the tank reaches capacity

In most cases, off-line tanks are designed to operate automatically using gravity-based systems, though pumps may be included if elevation differences require mechanical discharge.

Materials and construction

Off-line tanks can be constructed using a variety of materials and methods depending on site constraints, required volume and long-term durability. Common options include:

• Precast concrete chambers

• Cast in-situ reinforced concrete tanks

• Modular geocellular crates with impermeable liners

• Glass-reinforced plastic (GRP) or polyethylene tanks

• Corrugated steel or modular steel structures for large-scale installations

The tank may be installed underground, at surface level or partially buried depending on available space and access requirements.

Maintenance and operation

Although off-line tanks are used intermittently, they require regular inspection and maintenance to ensure they are operational when needed. Maintenance tasks include:

• Removing silt, debris and sediment from the base of the tank

• Inspecting and testing flow control devices

• Checking for structural damage or infiltration

• Ensuring inlet and outlet points are not blocked

• Cleaning screens, grilles and valves where fitted

Access points, manholes and ladders should be included in the design to facilitate safe entry for maintenance personnel.

Integration with control systems

Some off-line tanks are equipped with sensors and telemetry systems to monitor water levels, flow rates and discharge events. These may be integrated into a wider network management system that allows operators to:

• Track system performance in real time

• Trigger alarms or automatic responses during high flow events

• Adjust pump or valve operation based on weather forecasts or inflow data

• Optimise discharge timing to reduce environmental impact

Such systems are particularly useful in complex or sensitive urban drainage networks where active control improves reliability and regulatory compliance.

Environmental benefits

Off-line tanks support environmental objectives by:

• Reducing pollution caused by sewer overflows

• Managing the quality and quantity of water discharged into watercourses

• Helping to meet discharge consents and stormwater control standards

• Providing a buffer against short-term extremes in rainfall or runoff

In some cases, tanks may also be integrated with water quality treatment processes such as settlement zones, filtration or disinfection prior to discharge.

Conclusion

An off-line tank is an effective and reliable tool for managing peak flows in modern drainage and sewer systems. By temporarily storing excess water during heavy rainfall and releasing it gradually after the event, off-line tanks reduce the risk of flooding, protect public infrastructure and help meet environmental targets.

Their flexibility, efficiency and ease of integration make them a preferred choice for many engineers, especially in retrofit or space-constrained environments. With proper design, maintenance and control, off-line tanks contribute significantly to sustainable and resilient urban water management.