Infiltration, Inflow & Wet-Weather Flow

Infiltration and inflow, often called I&I, are major causes of excessive wet-weather flow in sanitary sewer systems. Although both introduce unwanted water into the collection system, they usually enter through different pathways and often produce different flow patterns.

Collection-system operators need to understand how groundwater infiltration differs from direct stormwater inflow, how rainfall affects flow, why I&I reduces available sewer and pump-station capacity, and how monitoring data can help identify problem areas.

What Is Infiltration?

Infiltration is groundwater entering the sanitary sewer through defects in the system.

Common sources include:

• cracked sewer pipe;
• defective joints;
• damaged service laterals;
• leaking manholes;
• root-damaged connections.

What Is Inflow?

Inflow is water entering the sanitary sewer through relatively direct connections or openings.

Common inflow sources include:

• roof drains;
• storm drains;
• sump pumps;
• foundation drains;
• yard drains;
• open or defective manhole covers.

Infiltration and Inflow Are Different

Infiltration commonly responds to groundwater conditions and may continue long after rainfall ends.

Inflow often responds rapidly to rainfall because stormwater enters the collection system directly.

Why I&I Matters

Excessive I&I can:

• consume sewer capacity;
• increase pump-station run time;
• increase treatment-plant hydraulic loading;
• contribute to sanitary sewer overflows;
• increase operating cost.

Sanitary Sewer Capacity

A sanitary sewer has limited hydraulic capacity.

When unwanted water enters, less capacity remains for actual wastewater.

Dry-Weather Flow

Dry-weather flow generally includes:

• normal customer wastewater;
• relatively steady groundwater infiltration.

Wet-Weather Flow

Wet-weather flow can include:

• normal wastewater;
• infiltration;
• direct inflow.

Rainfall Response

One of the most useful ways to understand I&I is to compare rainfall with sewer flow.

Rapid Flow Response

If sewer flow increases almost immediately after rainfall begins, possible causes include direct inflow sources such as:

• storm connections;
• roof drains;
• open manholes;
• sump pumps.

Delayed Flow Response

If flow rises more slowly after rain and remains elevated, groundwater infiltration may be important.

Long Recession Period

A sewer flow that remains elevated for many hours or days after rainfall can indicate groundwater entering through system defects.

Seasonal Infiltration

Infiltration can increase during periods of:

• high groundwater;
• snowmelt;
• extended wet weather.

Base Infiltration

Some systems experience groundwater infiltration even during dry weather.

This can create a relatively constant unwanted flow component.

Peak Inflow

Direct inflow can create very large short-duration peaks.

These peaks can quickly overload:

• gravity sewers;
• pump stations;
• treatment processes.

Wet-Weather Hydrograph

A flow hydrograph plots flow over time.

It can help operators see:

• normal daily flow patterns;
• rainfall response;
• peak flow;
• recession after rainfall.

Normal Daily Flow Pattern

Typical sanitary flow may show:

• low overnight flow;
• morning peak;
• midday variation;
• evening peak.

Rainfall Can Override Daily Patterns

During wet weather, I&I can become much larger than normal customer-use variations.

Peak-to-Average Relationship

A simple comparison is:

Peak Factor = Peak Flow ÷ Average Flow

Peak-Factor Example

Average flow is 2.0 MGD and wet-weather peak flow reaches 6.0 MGD.

Peak Factor = 6.0 ÷ 2.0

Peak Factor = 3.0

The peak flow is three times the average flow.

Flow Increase Example

Dry-weather flow is 2.5 MGD and storm flow reaches 5.5 MGD.

Approximate excess wet-weather flow is:

5.5 - 2.5 = 3.0 MGD

Percent Increase Example

Using the same values:

Percent Increase = (5.5 - 2.5) ÷ 2.5 × 100

Percent Increase = 120%

Collection-System Effects

High I&I can cause:

• higher sewer depth;
• surcharge;
• manhole overflow;
• customer backups;
• reduced hydraulic reserve.

Pump-Station Effects

Wet-weather flow can cause pump stations to:

• run longer;
• start more frequently;
• operate multiple pumps;
• approach maximum capacity.

Lead and Lag Pump Operation

If the lag pump runs much more frequently during rain, this can indicate increased wet-weather loading.

Wet-Well Level

Rising wet-well level during wet weather may indicate:

• incoming flow exceeds normal pump capacity;
• pump performance is reduced;
• force-main capacity is limited.

Treatment-Plant Effects

Excessive wet-weather flow can:

• reduce hydraulic detention time;
• increase clarifier loading;
• cause solids washout;
• increase pumping and treatment costs.

Dilution

Wet-weather flow can dilute wastewater concentration.

This means:

• flow may increase;
• BOD concentration may decrease;
• total mass loading may still change differently.

Concentration Is Not Enough

Operators should evaluate both:

• flow;
• concentration.

Mass Loading

A common relationship is:

Load, lb/day = Flow, MGD × Concentration, mg/L × 8.34

Wet-Weather Loading Example

Dry weather:

• flow = 2 MGD;
• BOD = 200 mg/L.

Load = 2 × 200 × 8.34 = 3,336 lb/day

Wet weather:

• flow = 4 MGD;
• BOD = 120 mg/L.

Load = 4 × 120 × 8.34 = 4,003.2 lb/day

Even though concentration decreased, total BOD load increased.

Manhole Inflow

Low-lying or damaged manholes can admit large volumes of stormwater.

Possible pathways include:

• cover holes;
• poor frame seals;
• flooded streets;
• damaged structures.

Roof Drains

Roof drains connected to sanitary sewers can create rapid storm-related inflow.

Sump Pumps

Sump pumps discharging to sanitary sewers can create significant inflow during periods of high groundwater.

Foundation Drains

Foundation drains can add substantial water when groundwater rises.

Cross Connections

Improper connections between storm and sanitary systems can create direct inflow.

Cracked Pipe

Cracked sewer pipe can allow groundwater infiltration, especially when the surrounding groundwater level is high.

Defective Joints

Joint defects can provide pathways for:

• groundwater;
• root intrusion;
• soil migration.

Service-Lateral Infiltration

Private or public service laterals can contribute significantly to total system infiltration.

Manhole Infiltration

Groundwater can enter through:

• cracks;
• barrel joints;
• pipe penetrations;
• frame areas.

Source Identification

No single method identifies every I&I source.

Operators may use:

• flow monitoring;
• rainfall monitoring;
• CCTV inspection;
• manhole inspection;
• smoke testing;
• dye testing;
• other field investigations.

Flow Monitoring

Flow monitors can be placed in selected sewer basins to compare wet-weather response.

Rain Gauges

Rainfall data should be collected with timing that can be compared with sewer-flow data.

Compare Similar Storms Carefully

Two storms with the same total rainfall can produce different sewer responses because of:

• rainfall intensity;
• storm duration;
• antecedent soil moisture;
• groundwater level.

Antecedent Conditions

A storm after several dry weeks may produce a different infiltration response than the same storm after a prolonged wet period.

Flow Monitoring by Basin

Dividing the collection system into monitored basins can help identify areas with the largest wet-weather response.

Nighttime Flow

Very high minimum nighttime flow can suggest significant infiltration where customer wastewater use should normally be low.

Nighttime Flow Is Not Pure Infiltration

Some legitimate wastewater flow continues overnight.

Operators should interpret nighttime flow with knowledge of system users.

Smoke Testing

Smoke testing can help identify pathways connecting the sanitary sewer to the surface.

Possible findings include:

• roof drains;
• yard drains;
• defective cleanouts;
• cross connections.

Smoke-Test Limitations

Smoke testing is more useful for some inflow sources than for groundwater infiltration through submerged defects.

Dye Testing

Dye can be used to determine whether a specific water source is connected to the sanitary sewer.

CCTV Inspection

CCTV can identify defects that may contribute to infiltration, including:

• cracks;
• open joints;
• roots;
• visible water entry.

Active Infiltration

Visible water entering through a joint or crack during CCTV inspection provides direct evidence of infiltration at that location.

Manhole Inspection

Inspect manholes for:

• leaking joints;
• cracks;
• poor frame seals;
• damaged covers;
• evidence of surface-water entry.

Wet-Weather Inspection

Some I&I sources are easiest to identify during or shortly after rain.

Safety During Storm Inspection

Wet-weather inspection can involve:

• traffic;
• flooding;
• slippery surfaces;
• surcharged manholes.

Do Not Open a Surcharged Manhole Carelessly

A pressurized or highly surcharged manhole can present serious hazards.

I&I and Sewer Rehabilitation

Possible corrective measures can include:

• pipe lining;
• joint sealing;
• manhole rehabilitation;
• spot repair;
• service-lateral repair;
• removal of improper inflow connections.

Not Every Repair Produces the Same Flow Reduction

Operators should prioritize based on:

• measured flow contribution;
• defect severity;
• overflow risk;
• cost;
• asset condition.

Verify Improvement After Repair

Post-rehabilitation monitoring should compare wet-weather performance before and after work.

Example: Flow Rises Immediately When Rain Starts

This pattern suggests direct inflow may be important.

Investigate:

• storm connections;
• roof drains;
• low manholes;
• sump pumps.

Example: Flow Rises Slowly After Several Hours of Rain

This can indicate groundwater infiltration increasing as surrounding soils become saturated.

Example: Flow Remains High Two Days After Rain

This pattern strongly suggests a groundwater-related infiltration component.

Example: Pump Station Starts Lag Pump Only During Storms

This indicates wet-weather flow is approaching or exceeding normal lead-pump capacity.

Example: Treatment Flow Triples but BOD Concentration Falls

This can occur because I&I dilutes wastewater while greatly increasing hydraulic flow.

Example: One Basin Has Much Larger Rainfall Response

Prioritize that basin for:

• flow investigation;
• manhole inspection;
• CCTV;
• smoke testing where appropriate.

Example: High Minimum Nighttime Flow

Possible causes include:

• groundwater infiltration;
• continuous industrial discharge;
• other legitimate nighttime flow.

Example: Flow Spike Lasts Only During Rain

Direct inflow is more likely than slow groundwater infiltration.

Example: New Manhole Area Shows Wet-Weather Flow Increase

Inspect:

• frame and cover;
• barrel joints;
• pipe penetrations;
• surface drainage.

Example: Rehabilitation Reduces Peak but Not Base Flow

The work may have removed inflow sources while substantial groundwater infiltration remains.

Example: Rehabilitation Reduces Base Flow but Peak Flow Remains High

The work may have reduced infiltration while direct stormwater inflow remains.

I&I Troubleshooting Strategy

Use several data sources together:

• rainfall;
• flow;
• pump-station run time;
• manhole levels;
• CCTV;
• field inspections.

Common I&I Mistakes

• Using infiltration and inflow as if they mean exactly the same thing.
• Looking only at total storm flow without reviewing timing.
• Ignoring groundwater conditions before the storm.
• Assuming every high wet-weather flow comes from public sewer mains.
• Ignoring service laterals and private connections.
• Using concentration changes without considering flow and mass load.
• Comparing storms only by total rainfall.
• Repairing visible defects without verifying actual flow reduction.
• Ignoring pump-station run-time trends.
• Waiting for overflows before investigating recurring wet-weather peaks.

A Practical Wet-Weather Flow Review

1. Review rainfall timing and intensity.
2. Review dry-weather baseline flow.
3. Review when sewer flow begins increasing.
4. Review peak flow.
5. Review how long flow remains elevated.
6. Review pump-station operation.
7. Review surcharge or overflow locations.
8. Identify basins needing further investigation.

A Practical Infiltration Review

1. Review minimum nighttime flow.
2. Review groundwater and seasonal conditions.
3. Review prolonged post-rain flow.
4. Inspect manholes.
5. Review CCTV for active leakage.
6. Review service laterals.
7. Prioritize defects contributing significant groundwater flow.

A Practical Inflow Review

1. Review rapid rainfall-flow response.
2. Inspect low-lying manholes.
3. Investigate roof and yard drains.
4. Investigate sump pumps and foundation drains.
5. Use smoke or dye testing where appropriate.
6. Correct confirmed improper connections.
7. Monitor later storms to verify improvement.

A Practical I&I Rehabilitation Review

1. Establish pre-repair flow data.
2. Identify the dominant I&I source.
3. Select the appropriate repair method.
4. Document completed work.
5. Monitor comparable wet-weather events afterward.
6. Compare peak, base, and recession flow.
7. Confirm whether the expected reduction occurred.

What to Remember for the Exam

• Infiltration is groundwater entering the sewer through defects.
• Inflow is water entering through relatively direct connections or openings.
• Cracks, leaking joints, manholes, and laterals are common infiltration sources.
• Roof drains, sump pumps, storm connections, and open manholes are common inflow sources.
• Inflow often causes rapid rainfall-related flow increases.
• Infiltration can rise more slowly and remain elevated after rainfall ends.
• Excessive I&I consumes sewer, pump-station, and treatment-plant capacity.
• Peak factor equals peak flow divided by average flow.
• Wet-weather dilution can lower concentration while total mass loading still increases.
• Mass load in lb/day can be calculated as MGD × mg/L × 8.34.
• High pump-station run time during storms can indicate excessive wet-weather flow.
• Flow hydrographs help show timing, peak response, and post-storm recession.
• Rainfall intensity, duration, groundwater, and antecedent moisture affect wet-weather response.
• High minimum nighttime flow can suggest infiltration but must be interpreted with actual nighttime wastewater use.
• Smoke testing is useful for identifying some direct inflow connections.
• CCTV can reveal cracks, joints, roots, and active groundwater entry.
• Manhole inspection can identify both infiltration and direct surface inflow.
• Post-rehabilitation monitoring is necessary to verify actual flow reduction.
• Fast flow response can suggest inflow, while prolonged elevated flow can suggest infiltration.
• Good I&I control combines rainfall data, flow monitoring, field inspection, CCTV, source testing, targeted repair, and post-repair verification.