Sewer infiltration: why a sewer spills when it is not raining
TL;DR. Sewer infiltration is clean groundwater leaking into a foul sewer through cracked pipes and failed joints. Inflow is surface water entering through direct connections. Neither is sewage, and both consume the same capacity sewage needs, which is why a combined system can spill on a dry winter day. It is the one cause of overflow spills that gets worse quietly, over years, with nothing visible above ground.
Key takeaways
- There are 15,000 storm overflows in England, of which 13,350 discharge to inland rivers.
- Spills can be caused by blockages, operational issues and infiltration, where pipe joints and cracks let groundwater into the sewer and take up capacity.
- In wet winters, sewer capacity is sometimes consumed by these flows, causing overflows to operate more frequently and for prolonged periods.
- Infiltration can enter the public sewer through privately owned lateral connections on customers’ properties.
- Sewers can also receive inflows from watercourses and land drainage, taking capacity that storm runoff would otherwise use.
- Infiltration is seasonal and groundwater-driven, so it is visible in dry weather flow long before it causes a spill.
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Last updated: 25 August 2026
What is sewer infiltration, and how is it different from inflow?
Sewer infiltration is groundwater entering a foul or combined sewer through defects in the pipe itself: open joints, cracks, failed connections, porous brickwork in older sewers. Sewer infiltration is slow, diffuse and driven by the water table, so it rises through autumn and winter as the ground saturates and falls back through a dry summer.
Inflow is the fast version. It is surface water arriving through a direct route: a roof downpipe wrongly connected to the foul system, a gully cross-connection, a manhole cover in a hollow that takes standing water, a land drain teed into a sewer decades ago. Inflow appears within minutes of rain starting and stops shortly after it ends.
The distinction matters because the fixes are completely different. Sewer infiltration is a pipe condition problem solved by lining, sealing or replacing. Inflow is a connectivity problem solved by finding one wrong connection and disconnecting it. Confusing the two is how sites spend money relining a sewer that was never the issue.
Why does sewer infiltration cause overflow spills?
Because a sewer has a fixed hydraulic capacity and does not care what is filling it. Sewer infiltration fills it with clean water. The Storm Overflow Evidence Project, prepared for the Storm Overflow Taskforce and published on gov.uk, is blunt about the mechanism: spills can be caused by blockages, operational issues and infiltration, where pipe joints and cracks in sewers allow groundwater to enter taking up capacity.
The same report sets out the seasonal pattern. The presence of groundwater or rainfall induced infiltration can vary seasonally, and in wet winters sewer capacity is sometimes consumed with these flows, causing storm overflows to operate more frequently and sometimes for prolonged periods. Prolonged is the word that separates infiltration from a normal storm spill, which is short and rainfall-shaped.
There are 15,000 storm overflows in England, and 13,350 of them discharge to inland rivers. Every one of those has a permit written on the assumption that it operates as the result of heavy rainfall. Defra has set out its own programme for tackling storm overflow discharges, but an overflow spilling because the water table is high in February is not doing what its permit describes, and no amount of storage tank will fix a system that is full before the rain arrives.
Where does the water actually get in?
Sewer infiltration gets in partly on land nobody surveys. The same report notes that infiltration can enter the public sewer through privately owned lateral connections on customers’ properties, which puts a share of the problem behind boundaries the network operator cannot inspect. It also records that sewers can receive inflows from watercourses and land drainage, taking capacity that would otherwise be available for storm runoff from paved surfaces.
The Environment Agency’s Storm Overflow Assessment Framework is the process these permits are judged against. For an estate, a campus or a housing site, that list is usable rather than abstract. The candidates are private laterals under car parks and landscaping, roof drainage on buildings altered at some point in the last fifty years, land drains around playing fields and grounds, and manhole chambers sitting in low spots that pond in heavy rain.
How do you find sewer infiltration before it spills?
You look at the sewer when it is not raining. Sewer infiltration shows up in dry weather. That is the whole method, and it works because infiltration and inflow have different signatures in the dry weather flow trace, and both are visible months before either causes a spill.
- Establish the dry weather minimum, usually in the early hours, when genuine foul flow is close to nothing.
- Track that minimum across a season. If it climbs from autumn into winter and falls back in summer, that is infiltration following the water table.
- Watch what happens in the first fifteen minutes of rainfall on a dry day. A sharp step within minutes is inflow through a direct connection.
- Compare chambers upstream and downstream to bracket which length of sewer the extra flow is entering.
None of that requires anyone to enter the chamber. Level in a manhole or wet well is a proxy for flow, and AQUAIOT Radar reads it without contact, to within ±2 mm, at 60 GHz, through the foam and condensation that defeat other methods, with ATEX variants for hazardous chambers. Sitting a set of them along a suspect length turns a sewer infiltration guess into a bracketed section of pipe.
On the clean-water side of a site, the Clamp-on Ultrasonic Flow Meter does the reciprocal job. Comparing what a site takes in against what leaves in the foul sewer is how you tell sewer infiltration from a plain water leak feeding the drain. AQUAIOT’s sewer monitoring service applies the same radar sensing across wet wells, rising mains, manholes and overflows, reporting over encrypted cellular and LoRaWAN into the AQUAIOT Cloud with threshold alarms and exports.
Frequently asked questions
Is sewer infiltration the same as a leak?
It is the opposite direction. A water leak is treated water escaping from a pressurised pipe. Sewer infiltration is untreated groundwater entering a gravity sewer that is not pressurised, through cracks and open joints. One costs you water you paid for; the other costs you capacity, and eventually a spill.
Why is sewer infiltration worse in winter?
Because it follows the water table rather than the rainfall. Ground saturates through autumn and stays saturated through winter, so the head of water pressing on the outside of a defective pipe is higher for months at a time. That is why an infiltration-driven spill is prolonged rather than sharp, and why a dry winter can hide the problem entirely.
Can you fix inflow without relining the sewer?
Usually yes, and that is why separating the two matters commercially. Inflow comes through a specific wrong connection, so the fix is a disconnection or a rerouting rather than structural work on the pipe. Sewer infiltration is a condition problem and does need lining, sealing or replacement, which is an order of magnitude more expensive.
Who is responsible for infiltration through private pipes?
The Storm Overflow Evidence Project records that infiltration can enter the public sewer through privately owned lateral connections on customers’ properties, so a share of the source sits on private land. For an estate or a campus that is useful rather than awkward, because it means the pipework you can actually survey and fix is part of the answer.
Why this one is worth catching early
Most sewer problems announce themselves. A blockage backs up, a pump fails, a chamber surcharges. Sewer infiltration does none of that. It arrives as a slightly higher number at three in the morning, one winter, and then a slightly higher one the winter after, until a system that used to cope stops coping and the overflow starts running in weather nobody would call a storm.
The measurement that catches sewer infiltration is cheap and the remediation is not, which is the strongest argument there is for monitoring first. Speak to an AQUAIOT engineer about levelling a suspect length of sewer across a season before anyone specifies a liner.
By GP, Digital and IoT lead at AQUAIOT.