What is a Overflow

Overflow refers to the discharge of sewage or wastewater into the environment before it has passed through a treatment process. This may occur as a result of system overload, equipment failure, blockages or extreme weather events. In some cases, overflows are controlled and intentional, while in others they are accidental and potentially hazardous. Regardless of the cause, sewage overflows can have serious environmental, public health and regulatory consequences.

Overflows are a significant concern in both combined and separate sewer systems, particularly in urban areas with ageing infrastructure or inadequate drainage capacity.

Types of overflow

Overflows are generally classified according to how and why they occur. The main types include:

  1. Combined Sewer Overflow (CSO)
    Occurs in systems where foul sewage and stormwater are carried in a single pipe. During heavy rainfall, the system may become overwhelmed, leading to excess flow being diverted directly into rivers, seas or watercourses through overflow structures. These are often permitted and monitored but still result in untreated discharge.

  2. Sanitary Sewer Overflow (SSO)
    Happens in separate foul sewer systems, usually due to blockages, equipment failure, or infiltration. These are typically unplanned and are considered pollution incidents requiring urgent attention.

  3. Stormwater overflow
    Results from excessive surface runoff during storms, where storage or attenuation systems exceed their design capacity and discharge directly into the environment.

  4. Internal property overflow
    Occurs when wastewater backs up into buildings due to blockages or surcharged public sewers. Though less environmentally damaging, it poses serious health risks to occupants.

Causes of overflow

Several factors contribute to the occurrence of overflows, both in public networks and private drainage systems. Common causes include:

  • Heavy rainfall exceeding drainage system design

  • Blockages from grease, wipes or debris

  • Groundwater infiltration during wet periods

  • Mechanical or electrical failure at pumping stations

  • Poorly maintained infrastructure

  • Illegally connected surface water drains

Urbanisation and climate change have increased the frequency and severity of overflow events by adding more impervious surfaces and increasing rainfall intensity.

Environmental and health impacts

When sewage overflows before treatment, it introduces pathogens, nutrients and pollutants directly into natural water bodies or the ground. The consequences can be severe:

  • Contamination of rivers, lakes and coastal waters

  • Oxygen depletion and harm to aquatic ecosystems

  • Spread of diseases such as E. coli, norovirus and hepatitis

  • Damage to recreational water use and fishing

  • Public health risks in areas with direct human contact

Repeated or poorly managed overflows can result in long term degradation of water quality and loss of biodiversity in receiving environments.

Monitoring and regulation

In the UK, sewage overflows are regulated by the Environment Agency, Natural Resources Wales or the Scottish Environment Protection Agency depending on location. Discharges from combined sewer overflows may be permitted under environmental permits but are subject to strict conditions.

Water companies are required to:

  • Monitor overflow frequency and duration using event duration monitors (EDMs)

  • Report overflows annually through public registers

  • Minimise the environmental impact through network upgrades and maintenance

  • Inform the public in real time about pollution risks, particularly near bathing waters

Unauthorised or excessive overflows are considered pollution incidents and may result in enforcement action, fines or legal proceedings.

Overflow mitigation strategies

Reducing the frequency and impact of overflows is a major focus for drainage and wastewater professionals. Strategies include:

  • Expanding or upgrading treatment works and storage capacity

  • Installing storm tanks and detention basins to hold excess flows

  • Introducing sustainable drainage systems (SuDS) to manage surface water

  • Separating foul and surface water systems in combined areas

  • Using real-time monitoring and control systems to optimise flow routing

  • Educating the public about what should and should not be flushed

Investment in infrastructure resilience and climate adaptation is essential to meet future demands and environmental targets.

Overflows in domestic settings

At the property level, overflows can occur due to blocked drains, failed private pumping systems or misconnected plumbing. These incidents may cause wastewater to back up through toilets, sinks or external gullies, posing health risks and requiring urgent intervention.

Property owners are responsible for the maintenance of internal drainage systems and should ensure:

  • Regular inspection and cleaning of private drains

  • Correct connection of appliances and gullies

  • Installation of non-return valves in flood-prone areas

In case of external sewer overflows affecting private property, the water company may be responsible depending on the location and cause.

Conclusion

Overflow is the uncontrolled or controlled discharge of untreated sewage into the environment. While some overflows are designed to protect infrastructure during extreme events, all forms of overflow pose risks to the environment, public health and water quality.

Effective monitoring, regulation, infrastructure investment and public awareness are essential to minimise the frequency and impact of overflows. As climate patterns shift and urban development continues, managing overflow will remain a priority for engineers, planners, water companies and environmental authorities.