15.2 Overflow Response, Bypass Pumping, CMOM Programs and Sewer Sulfide Control

Reporting an Overflow in Oregon

Overflow reporting duties come from the permit. Oregon's standard NPDES general conditions require 24-hour reporting of any noncompliance that may endanger health or the environment. Information is given orally within 24 hours of becoming aware of it: call the DEQ regional office during business hours, or the Oregon Emergency Response System (OERS) at 1-800-452-0311 outside business hours. A written submission within 5 days describes:

• the noncompliance and its cause;
• the exact dates and times, and whether it is continuing;
• steps taken or planned to reduce, eliminate and prevent recurrence.

Individual permits may add overflow-specific requirements, such as public notification, signs, sampling or monthly summaries with the DMR. Know your permit's wording.

Field Response Sequence

1. Safety first: traffic control, gas monitoring at open manholes, PPE.
2. Stop the source: clear the blockage, restart or bypass the pump station, or isolate the broken main.
3. Contain: sandbags, berms, plugging storm drains, vacuum trucks.
4. Estimate volume: duration × estimated flow rate, the count-the-manholes method, or upstream flow data. Photograph everything.
5. Protect the public: post warning signs at affected waters and properties, and notify downstream water suppliers if an intake could be affected.
6. Clean up: recover solids and liquid, wash down hard surfaces back to the sewer, and apply lime or disinfectant to soil where appropriate.
7. Sample receiving water when required (upstream, at and downstream of the discharge) until bacteria return to background.
8. Document and report: start and stop times, volume, cause, receiving water, actions and corrective measures.

Emergency Bypass Pumping

Bypass pumping moves flow around a failed pipe, a pump station under repair, or a rehabilitation work zone.

• Size pumps for peak wet-weather flow, not average flow, and provide a standby pump ready to start automatically or immediately.
• Use reliable plugs (inflatable or mechanical) with backup, and secure hoses and fittings. Discharge hoses at road crossings need ramps or buried crossings.
• Monitor continuously: levels upstream, fuel, pump operation and hose integrity. Night failures cause many overflows.
• Plan the shutdown sequence so the plugs are removed without surging the downstream system.

CMOM: Capacity, Management, Operation and Maintenance

EPA's CMOM framework organizes a collection system program into:

Many Oregon NPDES permits require utilities to keep collection system maintenance or overflow response plans in this spirit.

Combined Sewer Overflows

A few older Oregon cities have combined sewers, which carry stormwater and sewage in one pipe. Their permitted combined sewer overflow (CSO) outfalls discharge during large storms. Portland's Big Pipe project (completed in 2011) greatly reduced the city's CSOs. Operators of CSO systems monitor CSO structures and storage basins, keep regulators and tide gates working, and report overflow events as the permit requires.

Hydrogen Sulfide in Collection Systems

Generation: sulfate-reducing bacteria in the anaerobic slime layer convert sulfate to sulfide. Force mains are prime locations because they run full with no air, and long detention allows sulfide to build up. Warm temperatures and high-strength wastewater speed it up.

Release and corrosion: turbulence at drops, force main discharges and junctions releases H2S gas. Acidithiobacillus bacteria on moist crowns oxidize it to sulfuric acid, which dissolves concrete (crown corrosion) and attacks metal. H2S is also deadly to workers.

Track sulfide and pH at discharge points, monitor gas levels at problem manholes, and inspect concrete for corrosion. Exposed aggregate, missing cover over rebar and soft, pasty surfaces are signs of active attack.

Worked Example: Estimating Overflow Volume

A crew finds a manhole overflowing at about 25 gpm, estimated by timing a bucket fill at a nearby discharge point and comparing with reference charts. Records show it began 3 hours 20 minutes earlier.

• Duration = 200 minutes
• Volume ≈ 25 gpm × 200 min = 5,000 gallons

Report the estimate, the method used and the uncertainty. Refine it later with pump station run times, flow meter data or rainfall records, and update DEQ in the written report.