What is a Infiltration

In the context of drainage and sewerage systems, infiltration refers to the unintended entry of groundwater into a piped drainage or sewer network. This typically occurs through defects, cracks, faulty joints, or porous materials in pipes, manholes, or other underground infrastructure. Infiltration is distinct from inflow (which is the direct entry of surface water through manholes or downpipes) and is one of the primary contributors to excess flow in foul and combined sewer systems, particularly during wet weather or in areas with high water tables.

While the term “infiltration” can also describe natural percolation of water into soil, in this technical context it specifically concerns the undesirable intrusion of groundwater into systems designed to carry wastewater or stormwater.

Causes of Infiltration

Infiltration can occur in virtually any underground pipe system but is especially common in older or poorly maintained networks. Common causes include:

  • Cracked or fractured pipes due to ground movement, loading, or ageing
  • Displaced or poorly sealed joints between pipe sections
  • Deterioration of brickwork or mortar in older manholes or culverts
  • Tree root intrusion, which can widen existing defects
  • Poorly constructed or ageing lateral connections from private properties
  • Porous materials used in legacy systems, such as unlined concrete or clay

The volume of infiltrated water can fluctuate with seasonal groundwater levels, soil saturation, and recent rainfall, making it difficult to detect and quantify consistently.

Impacts of Infiltration

Although infiltration does not introduce pollutants in the way that inflow or industrial discharge might, it can still have significant operational, environmental, and financial consequences:

  • Overloading of foul and combined sewers, reducing system capacity
  • Increased risk of sewer flooding and combined sewer overflows (CSOs)
  • Higher volumes of flow to wastewater treatment plants, leading to increased energy usage and chemical dosing
  • Dilution of sewage, which can reduce treatment efficiency
  • Elevated maintenance costs, as pumps and treatment facilities must handle unnecessary volumes of clean water
  • Difficulty in network modelling, as variable infiltration masks true dry-weather flows

Infiltration is particularly problematic in combined sewer systems, where excess groundwater mixed with foul sewage may be discharged untreated during overflow events, posing environmental risks to watercourses.

Detection and Assessment

Identifying infiltration is a complex task, often requiring a combination of investigative methods:

  • Flow monitoring and diurnal flow analysis to detect anomalies during dry weather
  • CCTV surveys to visually inspect pipe condition and joint integrity
  • Dye testing to track water movement and confirm infiltration sources
  • Smoke testing (less common) to identify illegal connections and defects
  • Groundwater level correlation with observed increases in sewer flow

Engineers often use these techniques in combination to develop a comprehensive infiltration reduction strategy, particularly in areas with ageing infrastructure or persistent flooding.

Remediation and Prevention

Addressing infiltration typically involves rehabilitation or replacement of defective sewer sections. Common remediation methods include:

  • Pipe relining (CIPP – Cured-in-Place Pipe): A no-dig solution that seals joints and cracks with a resin-impregnated liner
  • Joint sealing and injection grouting: Used to restore watertightness at pipe connections
  • Excavation and pipe replacement: In cases of severe structural damage
  • Manhole refurbishment, including sealing of chamber walls and benching

Preventative measures may include the use of watertight pipe materials, modern jointing techniques, and quality assurance in construction to minimise defects that allow groundwater entry.

Regulatory and Design Considerations

In the UK, water companies are responsible for maintaining public sewers and reducing infiltration to improve treatment efficiency and reduce CSOs. Key regulatory drivers include:

  • Environment Agency permits that place limits on overflow frequency and pollution risk
  • Ofwat performance targets related to sewer flooding and asset management
  • Drainage and Wastewater Management Plans (DWMPs), which must consider infiltration as part of long-term planning

For new developments, best practice dictates the use of materials and construction standards that minimise the potential for infiltration. Additionally, private lateral drains, though often overlooked, can be significant contributors to infiltration and may fall under private or shared ownership responsibilities.

Conclusion

Infiltration is the unintentional and often hidden entry of groundwater into a drainage or sewer system. Though not immediately hazardous, it imposes a substantial burden on wastewater infrastructure by reducing system capacity, increasing treatment costs, and contributing to environmental pollution through overflow events. Detecting and controlling infiltration is an ongoing challenge for water utilities and engineers, requiring careful investigation, investment in rehabilitation, and continued vigilance in both public and private drainage assets. As climate patterns shift and groundwater levels fluctuate, managing infiltration remains a key element in building resilient, efficient, and sustainable wastewater networks.