Sanitary Sewer Inflow and Infiltration Overflow Case Study

Sanitary sewer overflows during storms are often described as a capacity problem. That is only partly true. The engineering question is whether the collection system, lift station, force main, equalization volume, treatment headworks and operating response can pass the wet-weather flow that actually enters the sanitary boundary.

This case study follows a separated sanitary sewer district where dry-weather operation is stable, but a moderate storm produces a high-level alarm, emergency bypass and untreated overflow at an upstream lift station. The case is simplified for engineering learning. It is not a regulatory interpretation, sewer rehabilitation specification or substitute for site-specific hydraulic modelling.

The central decision is:

Is the overflow mainly caused by insufficient pumping capacity, insufficient equalization storage, excessive rainfall-derived inflow and infiltration, or a combination that requires both operational and source-control actions?

The answer requires event reconstruction. A single peak-flow number is not enough.

Case Context

The service area is nominally a separated sanitary sewer catchment. Roof drains, foundation drains and storm inlets should not be connected to the sanitary system, but several older neighborhoods have clay pipes, leaky manholes and private lateral defects. The wastewater treatment plant can accept short wet-weather surges, but the upstream lift station and force main are the controlling assets during this event.

During a six-hour rainfall event, operators observe:

• wet-well level rising above the high-level alarm;
• both duty pumps running continuously;
• flow at the force main meter reaching the lift-station operating limit;
• a bypass weir overflowing from the wet well to an emergency outfall;
• downstream treatment units still operating below biological process limits.

The initial debate is whether the plant needs more treatment capacity or whether the collection system must reduce wet-weather entry before more capital is added downstream.

Simplified Event Data

Quantity	Symbol	Value
rainfall depth over the event	P	42\ \text{mm}
event duration	t_e	6.0\ \text{h}
dry-weather flow expected during the same six hours	V_{DWF}	1560\ \text{m}^3
measured lift-station inflow during event	V_{in}	6300\ \text{m}^3
maximum sustainable pumped flow to force main	Q_{pump,lim}	900\ \text{m}^3/\text{h}
peak measured wet-weather inflow	Q_{peak}	1320\ \text{m}^3/\text{h}
equalization storage available at storm start	S_0	420\ \text{m}^3
observed overflow volume from event log	V_{overflow,obs}	510\ \text{m}^3
overflow suspended solids concentration	C_{TSS}	180\ \text{mg/L}
overflow biochemical oxygen demand concentration	C_{BOD}	150\ \text{mg/L}
overflow ammonia nitrogen concentration	C_{NH3-N}	24\ \text{mg/L as N}

The values are deliberately rounded. Real event reconstruction should use calibrated level sensors, pump run logs, drawdown tests, rainfall hyetographs, upstream flow meters, bypass level records, treatment-plant hydraulic limits, meter uncertainty and clock synchronization.

Step 1: Decompose the Wet-Weather Volume

Rainfall-derived inflow and infiltration is estimated by subtracting the dry-weather flow expected during the same period from the measured event inflow:

The rainfall-derived fraction of total event inflow is:

or:

Engineering Comment

The event is not a normal sanitary peak. About three quarters of the volume entering the lift station is rainfall-derived under this simplified baseline. That does not prove every cubic metre is illegal inflow; some may be groundwater infiltration through defects. It does show that dry-weather sanitary load is not the dominant driver of the overflow.

Step 2: Check Pumped Discharge Capacity

The pump station can sustainably discharge:

During the six-hour event, the maximum pumped volume is:

The volume that cannot be pumped during the event is:

Available equalization at the storm start absorbs part of this volume:

Engineering Comment

The calculated shortfall of 480\ \text{m}^3 is close to the observed overflow volume of 510\ \text{m}^3. The difference is credible because pump cycling, wet-well geometry, level-sensor uncertainty, bypass weir hydraulics, force-main head variation and time resolution can easily explain tens of cubic metres during a storm event.

This check supports the event log. The overflow is hydraulically plausible without invoking an undocumented equipment failure.

Step 3: Test the Peak Period, Not Only Total Volume

Volume balance can hide short-duration stress. During the worst two hours, measured inflow reaches:

The storage filling rate at the pump limit is:

With 420\ \text{m}^3 available at storm start, the available storage would be consumed in:

Engineering Comment

Even if the six-hour volume looks manageable with small assumptions, the peak period is not. At the observed peak, one hour is enough to consume the available storage. After that, any continuing excess must either overflow, back up into the collection system or be reduced by operational intervention. A capacity decision should therefore be based on the hydrograph, not only the daily total.

Step 4: Estimate Overflow Pollutant Load

Overflow load is calculated from:

where V is in \text{m}^3, C is in \text{mg/L} and 0.001 converts the result to kilograms.

For total suspended solids:

For biochemical oxygen demand:

For ammonia nitrogen:

Engineering Comment

The pollutant load calculation changes the event from a hydraulic nuisance into an environmental release. The overflow volume is not large compared with a full treatment plant day, but it is untreated and may discharge during high runoff when receiving-water oxygen, turbidity, bacteria and public-contact risks are already stressed.

The concentration values should not be treated as precise unless the sample timing is defensible. A grab sample near the beginning of an overflow may differ from a later diluted sample. A stronger event record uses flow-weighted sampling or at least documents sampling time, weather, bypass duration and uncertainty.

Step 5: Separate Equipment Capacity from Source Entry

The two duty pumps are drawdown-tested after the event. Each pump is close to its expected individual capacity, and the force main reaches the site operating head limit when both pumps run. That means replacing pump impellers alone may not solve the event; the force main and downstream hydraulic grade also constrain discharge.

The engineering team installs temporary upstream meters for the next comparable storm. The simplified peak-period results are:

Subcatchment	Area	Dry-weather flow	Wet-weather peak flow	Rainfall-derived increment
North old clay sewer	42\ \text{ha}	58\ \text{m}^3/\text{h}	420\ \text{m}^3/\text{h}	362\ \text{m}^3/\text{h}
South mixed-age neighborhood	34\ \text{ha}	52\ \text{m}^3/\text{h}	170\ \text{m}^3/\text{h}	118\ \text{m}^3/\text{h}
Industrial basin	18\ \text{ha}	75\ \text{m}^3/\text{h}	120\ \text{m}^3/\text{h}	45\ \text{m}^3/\text{h}
New subdivision	26\ \text{ha}	35\ \text{m}^3/\text{h}	60\ \text{m}^3/\text{h}	25\ \text{m}^3/\text{h}

Total rainfall-derived increment during the peak-metering snapshot is:

The North old clay sewer contribution is:

or:

Engineering Comment

The lift station is overloaded, but the overload is not uniformly distributed across the service area. One older subcatchment contributes about two thirds of the measured rainfall-derived peak increment. This does not mean every defect is there, but it makes the first rehabilitation campaign testable: inspect, smoke-test, dye-test, seal manholes, review private laterals and then re-meter the next storms.

Step 6: Screen Corrective Options

The team compares three possible actions.

Option	Direct effect	Technical risk
Add pump capacity only	May reduce overflow if force main and plant can accept higher peak flow.	Can transfer hydraulic stress downstream, increase surge risk and still leave the I/I source untreated.
Add equalization only	Stores excess during short peaks and releases later.	Requires land, odor control, cleaning, power, controls and available downstream capacity after the storm.
Reduce I/I in priority subcatchment and improve operations	Lowers wet-weather peak and volume at the source, while preserving storage margin.	Requires field evidence, private-side cooperation, construction quality and post-repair verification.

If the North old clay sewer increment is reduced by 25\%, the peak reduction is:

The peak storage filling rate would become:

The same 420\ \text{m}^3 of available storage would last:

This is better but not enough for the observed two-hour peak. A 45\% reduction in the North increment gives:

That still may not eliminate overflow for the worst event, but it materially reduces frequency and volume. If operators also pre-draw the equalization basin by another 180\ \text{m}^3 before forecast storms, available storage becomes:

With the 45\% North reduction:

Engineering Comment

The calculation does not prove a final design. It shows why a combined strategy is technically stronger than a single action. Source reduction lowers the hydrograph; pre-storm storage management buys response time; pump and force-main upgrades can then be sized against a smaller, better-understood wet-weather load.

Failure Modes Found

Failure mode	Evidence	Engineering implication
Rainfall-derived inflow through direct connections	Rapid flow rise aligned with rainfall intensity.	Smoke testing, dye testing and private inflow disconnection are justified.
Groundwater infiltration through pipe and manhole defects	Elevated flow remains after rainfall peak.	CCTV, manhole inspection and groundwater-level correlation are needed.
Lift-station discharge constraint	Both pumps run near expected output but wet well keeps rising.	Review force-main head, valve position, pump curve, air binding, check valves and downstream hydraulic grade.
Insufficient usable equalization	Storage consumed within about one hour at peak excess flow.	Operating rules should define pre-draw, high-level alarm response and available detention before storms.
Weak event evidence	Overflow volume depends on level log and bypass rating.	Calibrate meters, synchronize clocks and document bypass weir assumptions.
Corrective action aimed only at pumps	Source hydrograph remains unchanged.	Pump upgrades alone may move the problem downstream and leave compliance risk.

Decision and Release Basis

The event should be classified as a collection-system wet-weather capacity and I/I control problem, not simply as a wastewater treatment process failure. The immediate release decision is:

Do not close the corrective action with pump maintenance alone. Hold the event open until the utility has verified pump performance, reconstructed the hydraulic event, ranked subcatchments by rainfall response, implemented near-term operating controls and defined a post-rehabilitation metering target.

The recommended action package is:

1. confirm pump and force-main performance with drawdown testing and pressure trend review;
2. implement storm forecast pre-draw rules for available equalization storage;
3. inspect and smoke-test the North old clay sewer subcatchment first;
4. remove direct inflow sources before upsizing downstream assets;
5. repair manholes and high-defect pipe reaches using evidence from CCTV and rainfall-response metering;
6. re-run the hydraulic model with calibrated event data;
7. define an overflow reduction target in both volume and frequency;
8. verify performance over multiple storms rather than one favorable event.

Validation Plan

Validation should not rely on a single before-and-after photograph. A defensible record should include:

Evidence	Purpose
calibrated rain gauge or radar-adjusted rainfall record	establishes event basis and recurrence comparison
upstream temporary flow meters	separates source catchments and rainfall response
pump run, current and discharge pressure logs	confirms whether the station reached equipment or system limits
wet-well level and bypass rating curve	reconstructs overflow timing and volume
CCTV, smoke testing, dye testing and manhole inspection	identifies physical inflow and infiltration pathways
post-repair wet-weather metering over at least three storms	verifies whether the hydrograph changed
treatment plant hydraulic and process logs	checks that corrective actions do not transfer overload downstream
public notification and compliance record	preserves environmental and regulatory evidence

The validation metric should be event-normalized. Comparing one dry year after rehabilitation with one unusually wet year before rehabilitation can mislead. A better comparison relates rainfall depth, intensity, antecedent wetness, groundwater level and measured I/I response.

Common Mistakes

• Treating all wet-weather flow as unavoidable instead of separating direct inflow, groundwater infiltration and normal sanitary flow.
• Upsizing pumps before checking force-main head, downstream hydraulic grade, surge risk and treatment acceptance.
• Using daily average flow when the overflow is controlled by a one- or two-hour peak.
• Claiming storage capacity that is not actually empty when the storm begins.
• Ignoring private laterals, roof drains, foundation drains and leaky manholes because they sit outside the main pipe alignment.
• Reporting overflow volume without documenting the bypass weir, level sensor calibration and time basis.
• Judging rehabilitation success from construction completion rather than post-repair hydrograph evidence.

Engineering Takeaways

Sanitary sewer I/I control is a systems problem. The hydraulic arithmetic is simple, but the decision is not. A credible overflow investigation links rainfall, sewer defects, pump capacity, force-main hydraulics, storage availability, treatment acceptance, pollutant load, compliance evidence and verification.

The key lesson is that capacity and source control must be evaluated together. More pumping can reduce a wet-well alarm, but it does not remove stormwater from a sanitary system. Rehabilitation can reduce the source, but it must be proven with flow data under comparable storms. Engineering closure comes only when the post-action hydrograph, overflow record and downstream treatment evidence all support the same conclusion.