What is a Infiltration to sewer

Infiltration to sewer refers to the unintentional entry of groundwater into a sewer or drainage system, typically through defective pipes, joints, manholes, or other compromised infrastructure. Unlike controlled inflows from domestic or industrial sources, infiltration introduces clean but unwanted water into foul or combined sewer networks, where it occupies capacity intended for wastewater and places unnecessary strain on treatment processes.

This phenomenon is especially common in older sewer systems, where time, soil movement, and groundwater pressure can create pathways for water to enter the network — often unnoticed until flow anomalies or performance issues arise.

How Infiltration Occurs

Groundwater can infiltrate a sewer system in various ways, particularly when components of the network have been compromised. Common points of entry include:

  • Cracked or fractured pipework caused by ageing, ground settlement, or vehicular loading
  • Defective joints between pipe sections that have separated or were poorly sealed during installation
  • Porous or degraded manhole structures, especially those built from brick or mortar in older networks
  • Unsealed lateral connections, including house drains or private sewers
  • Root intrusion, which both opens entry points and allows further groundwater tracking into the system

Infiltration typically increases during wet weather or when the groundwater table is elevated, and may fluctuate seasonally or in response to prolonged rainfall events.

Impacts of Infiltration to Sewer Systems

Although the infiltrating water is not contaminated, its presence in foul or combined sewer systems can have significant operational, environmental, and economic consequences:

  • Reduced network capacity: Infiltration consumes space within the pipes, reducing the ability of the system to carry actual wastewater.
  • Increased risk of sewer overflows: Especially in combined systems, where infiltration may cause surcharge or trigger combined sewer overflow (CSO) discharges to watercourses.
  • Higher treatment costs: Extra water must be pumped and treated at wastewater treatment works, increasing energy use and operational costs.
  • Diluted sewage: Lower concentrations of contaminants can impair biological treatment processes.
  • Asset stress and degradation: Prolonged wet conditions within the pipe network can contribute to accelerated deterioration of infrastructure.
  • Regulatory compliance risks: Excessive infiltration may result in breaches of discharge consents and environmental permits.

In regions with older infrastructure, infiltration can constitute a substantial proportion of dry-weather flow—sometimes exceeding 30%—which poses a persistent challenge for utilities.

Detection and Assessment

Detecting infiltration is inherently challenging due to its often hidden and variable nature. However, several investigative techniques are used to identify its presence and locate entry points:

  • Flow monitoring: Analysing diurnal flow patterns during dry and wet weather to identify abnormal base flows.
  • CCTV surveys: Visual inspection of pipes and joints to identify physical defects or water ingress.
  • Dye testing: Tracing the movement of coloured water introduced into surrounding soil or manholes to confirm infiltration.
  • Smoke testing: Although more commonly used to identify illegal inflow, it can help locate structural defects.
  • Correlation with groundwater levels: Comparing sewer flow increases to fluctuations in local groundwater levels.

A combination of these methods is typically required to develop an effective infiltration reduction strategy.

Control and Remediation Measures

Addressing infiltration to sewer involves a mix of repair, rehabilitation, and replacement strategies, tailored to the condition and accessibility of the affected infrastructure. Common measures include:

  • Cured-in-place pipe (CIPP) lining: A trenchless method that creates a seamless liner inside existing pipes, sealing cracks and joints.
  • Joint sealing or grouting: Injection of sealing compounds into defective pipe joints or manhole walls.
  • Manhole rehabilitation: Lining or recoating internal surfaces with impermeable materials.
  • Pipe replacement: In cases of severe damage or structural failure.
  • Lateral connection upgrades: Repairing or relining privately owned pipes that contribute to infiltration.

Prevention in new construction includes the use of watertight materials, correct bedding and backfilling practices, and stringent quality assurance during installation.

Regulatory and Management Context

In the UK, water and sewerage companies are legally responsible for managing infiltration under the Water Industry Act 1991 and environmental permitting regulations governed by the Environment Agency. They are expected to:

  • Maintain asset condition to minimise infiltration
  • Monitor flows to identify abnormal increases
  • Invest in network rehabilitation as part of Drainage and Wastewater Management Plans (DWMPs)
  • Demonstrate progress in reducing infiltration to protect water quality and reduce overflow incidents

Developers and property owners also have a role, particularly in ensuring that private sewers and lateral drains do not contribute to infiltration, either through poor installation or neglect.

Conclusion

Infiltration to sewer is a common but often underestimated challenge in wastewater and drainage systems. It represents the unwanted entry of groundwater into foul or combined sewers through structural defects and material failures. While the infiltrating water itself may be clean, its presence compromises the hydraulic and operational efficiency of the sewer network, increases treatment costs, and contributes to pollution risks. Through accurate detection, targeted rehabilitation, and responsible system design, infiltration can be controlled to ensure the resilience and performance of critical water infrastructure.