What is a Inflow

Inflow refers to the entry of water or other liquids into a sewerage system, typically from surface-level or above-ground sources. It is most commonly associated with rainfall, stormwater runoff, or unauthorised connections, and is distinct from infiltration, which involves the unintended entry of groundwater through cracks or defects in underground pipes.

Inflow may include water from roof drains, yard gullies, manhole covers, sump pumps, foundation drains, or industrial processes, and it can rapidly increase the volume of flow in the sewer system — often exceeding its design capacity. Managing inflow is essential to maintaining the efficiency of foul and combined sewer networks and preventing sewer overflows, flooding, or treatment plant overloads.

Sources of Inflow

Inflow can originate from a variety of legal or illegal sources, including:

  • Surface water runoff entering through open or defective manhole covers
  • Roof and gutter drains that are misconnected to foul sewers instead of surface water systems
  • Yard and area drains connected directly to combined or foul networks
  • Foundation and basement drainage systems, especially in older properties
  • Stormwater runoff from paved surfaces, where surface drainage systems have been cross-connected to foul lines
  • Industrial effluent, either discharged under permit or introduced unlawfully

Unlike infiltration, which is gradual and influenced by groundwater levels, inflow tends to be immediate and volume-intensive, particularly during rainfall events.

Inflow vs. Infiltration

Though often discussed together as I/I (Inflow and Infiltration), these two phenomena differ in source and behaviour:

Parameter Inflow Infiltration
Source Above-ground or surface water Subsurface groundwater
Entry points Manholes, drains, roof downpipes Cracks, joints, pipe defects
Triggered by Rainfall, snowmelt, direct inputs High water table, wet ground
Flow pattern Sudden, during storm events Steady or slowly increasing
Prevention Proper system separation and design Asset maintenance and sealing

Understanding the difference is key to diagnosing problems and implementing the correct mitigation measures.

Impact on Sewer Systems

Excessive inflow can have serious operational and environmental consequences:

  • Overloaded sewer networks, especially in combined systems
  • Increased frequency of combined sewer overflows (CSOs) into watercourses
  • Flooding of basements and public areas due to backflows
  • Increased flow to wastewater treatment works, raising operating costs and energy use
  • Bypassing or short-circuiting of treatment processes, affecting effluent quality
  • Shortened asset lifespan, as pipes and pumps must handle volumes beyond their design capacity

Inflow also complicates hydraulic modelling and capacity planning, as the peaks are sharp, sudden, and difficult to predict without extensive monitoring.

Detection and Assessment

Identifying inflow typically involves a combination of methods:

  • Flow monitoring during wet and dry periods to detect rainfall-induced spikes
  • CCTV surveys to inspect for illegal connections or signs of rapid water ingress
  • Dye testing of surface drainage systems to identify cross-connections
  • Smoke testing, which reveals openings where surface air (or stormwater) can enter foul pipes
  • Rainfall correlation studies, comparing storm data to observed flow increases

Many water authorities run targeted inflow reduction programmes, especially in flood-prone or environmentally sensitive areas.

Control and Prevention

Mitigating inflow requires both infrastructure solutions and regulatory enforcement. Key strategies include:

  • Ensuring proper separation of foul and surface water systems in new developments
  • Correcting misconnections in existing buildings — a common issue in older housing stock
  • Raising manhole covers in flood-prone areas or using sealed/vented covers to prevent stormwater ingress
  • Disconnecting roof downpipes from foul sewers and redirecting to soakaways or SuDS features
  • Upgrading surface drainage infrastructure to reduce pressure on combined systems

On industrial sites, strict control of trade effluent discharges and compliance with discharge consents helps manage flow volumes and reduce system overloading.

Regulatory and Planning Context

In the UK, inflow management is governed by:

  • Building Regulations Part H, which set standards for separate drainage
  • Water Industry Act 1991, which gives water companies powers to inspect and regulate discharges
  • Environment Agency permits, which impose limits on CSOs and foul sewer capacity
  • Sustainable Drainage Systems (SuDS) guidelines, encouraging on-site surface water management to reduce inflow risk

Local authorities and developers are expected to demonstrate that new developments will not increase inflow into foul systems, often by employing attenuation, infiltration, or SuDS techniques.

Conclusion

Inflow is the entry of surface water or liquid waste into the sewer system from above-ground sources, often as a result of misconnection, rainfall, or industrial activity. It is a key concern in managing the performance and reliability of foul and combined sewers, particularly during storm events.