How Do You Calculate Wet Well Storage?

Getting the wet well storage right is an important part of designing an adoptable pumping station.

The wet well needs sufficient volume to manage normal pump operation, while the wider drainage system must also provide the required emergency storage in the event of pump failure or loss of power.

The difficulty is that these are two different requirements. A wet well may have enough capacity to operate the pumps correctly but still fail to meet the emergency storage requirements of the adopting Water Authority or NAV.

For developers, consultants and groundworkers, this can become a significant issue if the storage calculations are not coordinated with the drainage levels, pump selection and civil design early enough.

What is the difference between operating volume and emergency storage?

Wet well storage is generally considered in two parts.

Operating volume is the usable capacity between the pump start and stop levels. It determines how frequently the pumps operate and how long they run during each cycle.

Emergency storage is the additional capacity needed to contain wastewater during interruptions to normal pumping, such as power failure or equipment faults.

Both are important, but they are calculated differently.

The operating volume is primarily influenced by pump duty, incoming wastewater flow and permitted starting frequency. Emergency storage is determined by the applicable design criteria and the arrangement of the upstream drainage system.

A pumping station therefore needs to be assessed as a complete hydraulic system rather than simply selecting a wet well large enough to contain a particular volume.

How is wet well operating volume calculated?

The operating volume is calculated by considering the relationship between incoming wastewater flow, pump discharge rate and the permitted number of pump starts.

For a simplified fixed-speed, single-pump arrangement, the minimum active operating volume can be estimated using:

V = Q × T / 4

Where:

• V = required operating volume
• Q = pump discharge rate
• T = minimum time between pump starts

For example, consider a pumping station with a discharge rate of 12 litres per second and an assumed maximum of six starts per hour.

Six starts per hour gives a minimum cycle time of 10 minutes, or 600 seconds.

The calculation would be:

V = (12 × 600) / 4 = 1,800 litres

This gives a preliminary operating volume of 1.8 m³ between the relevant pump start and stop levels.

The example assumes a single fixed-speed pump and constant incoming flow. The six-start limit is illustrative, not a universal adoption requirement.

For an actual adoptable pumping station, the calculation must also account for duty and standby arrangements, pump running time, control levels and the selected equipment.

Why pump cycling matters

If the operating volume is too small, the pumps may start and stop more frequently than intended.

Excessive cycling can increase wear on motors, starters and mechanical components, affecting long-term reliability.

Water UK’s Design and Construction Guidance (DCG) includes a maximum of 15 starts per hour for relevant adoptable pumping station designs, although individual adopting organisations and pump manufacturers may specify lower limits.

Increasing the wet well operating volume can reduce starting frequency, but that does not automatically make it the best solution.

A larger wet well can also increase wastewater retention time, potentially creating septicity, odour and maintenance concerns.

The aim is to achieve an appropriate balance between pump cycling, minimum running time and wastewater retention rather than simply maximising storage capacity.

How much emergency storage is required?

Emergency storage is a separate design consideration.

For qualifying foul pumping stations serving fewer than 500 dwellings, the Water UK DCG includes a minimum storage criterion of 160 litres per dwelling.

For example, a development of 150 homes would have an illustrative emergency storage requirement of:

150 × 160 litres = 24,000 litres (24 m³)

This does not mean the wet well itself must contain 24 m³ of additional storage.

Depending on the drainage arrangement and adopting organisation’s requirements, eligible emergency storage may be provided within upstream public sewers, chambers or specifically designed storage structures.

The relevant hydraulic levels are important. Storage must be available in an acceptable location without creating an unacceptable risk of wastewater backing up into upstream properties.

Simply increasing the wet well dimensions may not provide an acceptable solution.

This is why emergency storage calculations should be considered alongside incoming sewer levels and the wider civil design before the pumping station layout is fixed.

Why wet well sizing needs to be coordinated with the wider design

Wet well volume affects more than the chamber dimensions.

Changing the pump duty can alter the operating volume needed to manage pump cycling. Changes to incoming sewer levels can affect the wet well depth and available emergency storage. Increasing the chamber volume may affect wastewater retention and operating levels.

These decisions also influence the mechanical and electrical arrangement, including pump controls, alarms and standby operation.

The problem often comes when the wet well is treated as a fixed civil element before the hydraulic requirements have been fully established.

At Advantage, we work alongside developers, consultants and groundworkers to coordinate wet well sizing with the proposed pumping equipment, drainage design and requirements of the intended adopting organisation.

Our manufacturer-independent approach also gives us flexibility to select suitable equipment around the actual site conditions rather than designing the station around a predetermined pump range.

Adoption requirements should be considered early

For adoptable pumping stations, the future asset owner needs to be confident that the station will operate reliably and can be maintained throughout its working life.

Water Authorities can apply approved Local Practices alongside the national DCG, while NAVs may have different technical and operational requirements.

These can affect operating levels, emergency storage, pump cycling, telemetry, chamber arrangements and maintenance access.

Where the adopting organisation has not yet been confirmed, it is important to understand which options are being considered before the wet well design progresses too far.

A change in adoption requirements late in the project could affect the chamber dimensions, pumping equipment or wider drainage infrastructure.

Explore Water Authority & NAV Pumping Station Guidance

Working on a pumping station design?

If the wet well design is progressing but the operating volume, emergency storage or pump duty has not yet been fully resolved, this is a good point to involve Advantage.

Our engineers can work with your consultant and groundworker to review the hydraulic requirements and help coordinate the pumping station design before those decisions become difficult or expensive to change.

Discuss Your Pumping Station Design