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.

Combined Sewer Overflow (CSO) Basin Operations

A combined sewer system conveys sanitary wastewater and stormwater runoff through the same collection network. During heavy rainfall or snowmelt, combined flow may exceed available conveyance, storage, or treatment capacity.

A combined sewer overflow (CSO) is a discharge from a combined sewer system under conditions addressed by the system design, permits, and applicable operating requirements. It is not the same as an unintended sanitary sewer overflow (SSO) from a separate sanitary sewer system.

Purpose of CSO Storage and Equalization Basins

CSO basins may temporarily store combined wastewater during wet-weather flow peaks. Depending on the facility design, stored water may later be conveyed to a wastewater treatment facility as hydraulic capacity becomes available.

Some facilities also include screening, disinfection, or other treatment functions. Operators must follow the specific facility operating plan rather than assume that every CSO basin has identical equipment or discharge arrangements.

Monitoring Basin Conditions

Relevant operating information may include:

• Basin water level and available storage capacity.
• Incoming and outgoing flow rates.
• Rainfall intensity and forecasted wet-weather conditions.
• Pump, gate, valve, and flow-control status.
• High-level alarms and overflow indications.
• Screening, mixing, or other installed treatment equipment.
• Downstream conveyance and treatment capacity.
• Recorded discharge conditions and applicable reporting requirements.

Operating During Increasing Wet-Weather Flow

When wet-weather flow increases, operators should compare actual basin conditions with the approved operating procedures, available storage, and downstream system capacity.

Depending on facility design, authorized adjustments may involve gates, pumps, valves, or flow-control equipment to manage storage and conveyance.

Operators must not assume that increasing pump output is always appropriate. Additional flow may overload downstream facilities or conflict with approved operating limits.

Recovery After a Storm

As rainfall and incoming flow decline, the basin may be emptied or returned to its normal operating condition according to the approved operating plan.

Recovery operations should consider downstream capacity, pump limitations, accumulated solids, odors, equipment condition, and required cleaning or inspection.

Operators should document significant operating conditions, alarms, discharge events, maintenance needs, and corrective actions.

Troubleshooting Abnormal Conditions

If basin level rises more rapidly than expected, possible causes include unexpectedly high inflow, restricted discharge capacity, pump malfunction, incorrect gate or valve position, or instrumentation problems.

Operators should verify level and flow information, inspect accessible equipment safely, compare current observations with rainfall and operating trends, and follow established escalation procedures.

Any discharge or overflow condition must be evaluated against applicable permits, facility requirements, and reporting procedures. The existence of a CSO facility does not authorize unrestricted discharges.

Example: Rapid Basin Filling During a Storm

A CSO storage basin is filling faster than forecast, while downstream treatment flow approaches the approved operating limit.

The operator should verify rainfall, basin level, inflow, outflow, equipment status, and remaining storage capacity.

The appropriate response is to follow the approved wet-weather operating plan, coordinate with responsible personnel, and document conditions and actions. Increasing outflow without evaluating downstream capacity could create additional operational problems.

Exam reminder: CSO basin operation requires coordinated monitoring of wet-weather flow, storage capacity, equipment, downstream limitations, permit conditions, and post-storm recovery.

Combined Sewer Overflow Basin Operation and Wet-Weather Control

Combined sewer systems can carry both sanitary wastewater and stormwater. During significant rainfall, their flows may exceed the available conveyance or treatment capacity. Some systems use storage or retention basins to temporarily hold excess combined sewage and reduce uncontrolled overflow events.

Operators must follow the specific facility operating plan, applicable permit requirements, and approved procedures for routing, storing, treating, and releasing wastewater.

CSO Basin Components and Operating Objectives

A combined sewer overflow (CSO) storage basin may include inlet and diversion structures, gates, pumps, level instruments, overflow structures, screens, cleaning equipment, and outlet controls.

Its operating objective is to manage wet-weather flow within the approved hydraulic and environmental limits. A storage basin does not eliminate all overflow risks or increase the capacity of downstream treatment processes.

Monitor Basin Level and Available Storage

Monitor incoming flow, basin level, available storage volume, rainfall conditions, outlet capacity, and applicable alarms.

Compare the observed level trend with expected wet-weather inflow. Rapidly rising levels may require an authorized change in diversion or pumping arrangements.

Available storage depends on basin geometry and the current water level. Operators should use approved level-volume relationships rather than assuming that all basins have a constant volume per foot of depth.

Control Diversion Gates and Pumping

Follow approved operating sequences when changing diversion gates, pumps, and other hydraulic controls.

Verify the indicated equipment position or status and assess upstream and downstream hydraulic conditions before changing the flow path.

Increasing discharge from a storage basin without considering downstream capacity may cause surcharge or overload another part of the collection or treatment system.

Evaluate Overflow Conditions and Alarms

During storm events, monitor high-level alarms, pump failures, abnormal gate positions, loss of communications, and conditions indicating possible overflow.

Investigate conflicting instrument readings and use available independent information to determine whether the problem involves a sensor, control equipment, or actual hydraulic conditions.

If an overflow occurs or is imminent, follow the approved emergency response and reporting procedures. Do not assume that temporary storage eliminates regulatory reporting obligations.

Post-Storm Drawdown and Basin Recovery

After wet-weather inflow decreases, stored wastewater may need to be returned to the collection or treatment system at a rate compatible with downstream capacity.

Monitor basin drawdown, pumping equipment, discharge conditions, sediment accumulation, debris, and odor development.

Delayed drawdown can reduce storage capacity for the next storm. Excessively rapid drawdown can contribute to downstream hydraulic problems.

Follow facility procedures for cleaning and inspecting accessible basin components after the event.

Operational Example: Rising Basin Level

A CSO basin level continues rising even though the discharge pump is running. The operator should compare incoming and outgoing flows, confirm the actual pump condition, review valve and gate positions, and check downstream restrictions.

If incoming flow exceeds the permitted or available discharge capacity, the water level may continue increasing even when the pump is functioning correctly.

The appropriate response depends on available storage, downstream capacity, operating procedures, and current emergency conditions. Increasing pump speed is not automatically the correct solution.

Records, Compliance, and Performance Review

Document storm conditions, basin levels, flow trends, gate and pump operations, alarms, storage and drawdown events, observed overflows, and corrective actions.

Review repeated high-level conditions and recovery times to identify maintenance needs, hydraulic limitations, and opportunities for operational improvement.

Exam reminder: CSO basin operation requires coordinated control of inflow, storage, diversion, pumping, downstream capacity, overflow response, post-storm drawdown, and regulatory documentation.

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.