19.4 SSO Response, Flow Monitoring & Capacity Assurance

19.4 SSO Response, Flow Monitoring & Capacity Assurance

A sanitary sewer overflow (SSO) is untreated wastewater escaping the collection system before it reaches the treatment plant. It is a public health event, a regulatory violation, and the metric by which a collection program is judged.

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Causes of Overflows

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Responding to an Overflow

1. Respond immediately. Time is the largest single driver of volume.
2. Protect the public. Barricade, sign, and keep people and animals away from the discharge.
3. Stop the overflow — relieve the blockage, start a bypass pump, restore power, or restore pump operation.
4. Contain — dam, dike, or plug storm drain inlets so the discharge does not reach a wash or waterway.
5. Recover — vacuum standing wastewater back into the sewer where practical.
6. Clean and disinfect the affected area, and remove and dispose of contaminated debris.
7. Sample the receiving water where required.
8. Notify — see below.
9. Document — times, location, cause, estimated volume, receiving water, corrective actions, photographs.
10. Follow up — correct the cause and adjust maintenance frequency for that reach.

[!WARNING] Notification requirements are strict and the clock runs from awareness, not from the end of the event. Any unpermitted discharge that may endanger human health or the environment requires oral notification within 24 hours and a written report within 5 days. Additional immediate notification may be required to health authorities, downstream users, and the public where drinking water sources or recreational waters are affected.

Estimating Overflow Volume

Regulators require a volume estimate, and there are two accepted approaches:

Duration method: $\text{Volume (gal)} = \text{Flow rate (gpm)} \times \text{Duration (min)}$

Worked example. An overflow discharged at an estimated 25 gpm for 3 hours and 20 minutes (200 minutes).

$25 \times 200 = 5,000\text{ gal}$

Manhole overflow method: the rate of discharge from a manhole is estimated from the observed height of the plume above the rim using published tables, then multiplied by duration.

Other approaches include measuring the area and depth of a ponded discharge, and comparing plant influent records against expected flow. Whichever method is used, document the assumptions — a defensible estimate with stated assumptions is far better than an unexplained number.

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Flow Monitoring

Flow monitoring is how a utility learns how its system actually behaves, as opposed to how the model says it should.

[!IMPORTANT] Depth alone is not flow. A pipe flowing 60 percent full at 3 ft/s and the same pipe surcharged at low velocity can show similar depths and radically different flows. Paired depth and velocity measurement is what makes an area-velocity meter reliable, and it is why a level-only sensor is inadequate for quantifying I&I. This distinction is regularly tested.

Meter siting matters: avoid locations immediately downstream of a drop, a bend, a junction, or a lift station discharge, where the flow is not hydraulically uniform. Confirm meter accuracy periodically by an independent measurement, and clean sensors of grease and debris on a schedule.

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Quantifying Inflow and Infiltration

Infiltration is groundwater entering through defects — steady, seasonal, and tracking the water table. Inflow is surface water entering through direct connections — sharp, immediate, and tracking rainfall.

The basic analysis compares flow during a storm against a dry-weather baseline for the same day of the week, because weekday and weekend diurnal patterns differ:

$\text{RDII} = \text{Wet-weather flow} - \text{Dry-weather baseline}$

Worked example. A basin's dry-weather Tuesday flow totals 0.42 MG. On a rainy Tuesday it totals 1.05 MG.

$\text{RDII} = 1.05 - 0.42 = 0.63\text{ MG}$

That is 150 percent above baseline, which identifies the basin as a high-priority I&I candidate. Normalizing further — per inch of rainfall, per acre, or per inch-mile of pipe — allows basins of different sizes to be ranked against one another.

Follow-up investigation: smoke testing for inflow sources, night-time flow isolation for infiltration (measuring flow in the small hours when sanitary flow is minimal and what remains is largely groundwater), CCTV for defects, and manhole inspection.

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Capacity, Management, Operation and Maintenance

A CMOM program is the framework regulators expect a collection system utility to have in place. Its core elements:

Combined sewer overflow structures appear in the Need-to-Know Criteria as items collection operators monitor. Combined systems, which carry both sanitary sewage and stormwater in one pipe, are rare in Arizona but common in older eastern and midwestern cities, and the criteria are national. Operators check CSO regulator structures and equalization basins for blockage, verify that tide gates and flap gates operate, and record activation events, since CSO discharges are permitted but must be documented and minimized under the nine minimum controls.