Sewers, drainage and stormwater

What is infiltration and inflow in a sewer network?

• Infiltration is slow and persistent. It tracks the water table, so it shows up in dry weather and worsens through winter.
• Inflow is fast and sharp. Flow rises within minutes of rainfall starting and falls away almost as quickly.
• The distinction sets the fix. Infiltration needs pipe rehabilitation. Inflow needs a wrong connection found and disconnected.
• Both raise spill risk. Capacity lost to clean water is capacity unavailable when a storm arrives.

How do you tell infiltration from inflow?

The two faults have genuinely different fingerprints in the data, which is why continuous flow and level monitoring separates them where a spot survey cannot.

Infiltration is a baseline problem. It lifts the dry weather flow across the whole day, including the small hours when almost nothing should be moving. It follows the season, rising as groundwater rises and easing through a dry summer. If a network runs high at 03:00 in February and normally at 03:00 in August, the difference is very likely infiltration.

Inflow is an event problem. It appears as a spike that correlates tightly with a rainfall record, often within minutes, and it decays quickly once the rain stops. Overlaying a rain gauge trace on a flow trace makes inflow obvious. The lag between the two tells you roughly how far upstream the wrong connection sits.

Why does I&I matter operationally?

Every litre of clean water in a foul sewer is paid for four times. It is pumped, it is conveyed, it is treated, and when the network runs out of headroom it contributes to a spill that has to be reported.

For a pumping station the effect is immediate and measurable. Pumps run more often and for longer, which raises energy cost and shortens the interval between mechanical failures. For a treatment works, hydraulic load rises without any matching rise in organic load, diluting the influent and making the process harder to control.

The reporting consequence is the one that tends to force action. England now monitors storm overflows for spill events across the network, and the resulting event duration data is published annually by the Environment Agency. A network carrying a large I&I burden spills earlier and more often than its design would suggest, and that shows up in the return.

How is I&I actually found?

The practical method is to instrument the network and let the data narrow the search before anyone lifts a cover.

Level and flow monitoring at manholes and key nodes gives a continuous picture of where flow is arriving from. Working down from the treatment works towards the catchment, each monitored branch either carries the anomaly or it does not, which halves the search area at every step. Adding rainfall telemetry lets the same dataset classify each anomaly as infiltration or inflow without a second visit.

Temperature is a useful secondary signal. Groundwater is markedly colder and more stable than sewage, so a persistent cold offset at a node is a strong infiltration indicator. Surface water arriving during a storm carries the temperature of the rain, which is a different signature again.

What fixes it once it is located?

Inflow is usually the cheaper win. A misconnected downpipe, a yard gully plumbed into the foul line, or a damaged cover admitting surface water can often be corrected as a small job once the location is known. The difficulty was never the repair, it was finding it.

Infiltration is a rehabilitation problem. Depending on condition that means lining, joint sealing, patch repair or replacement, prioritised by how much flow each defective length is admitting. This is where continuous data earns its cost, because it lets a limited rehabilitation budget go to the lengths admitting the most water rather than the lengths that happened to be surveyed most recently.