A Wet-Weather Bypass at a Plant Already Over Design Flow

Volumes (30-day month at 1.8 MGD = 54 MG total)

$$V_{bypass} = 0.4\ \text{MGD} \times \frac{11}{24}\ \text{d} = 0.183\ \text{MG}$$

$$V_{secondary} = 54 - 0.183 = 53.82\ \text{MG}$$

Using lb = mg/L × MG × 8.34:

Parameter	Secondary load (lb)	Bypass load (lb)	Total load	Combined conc.
CBOD5	19 × 53.82 × 8.34 = 8,528	120 × 0.183 × 8.34 = 183	8,711	8,711 ÷ (54 × 8.34) = 19.3 mg/L
TSS	27 × 53.82 × 8.34 = 12,119	95 × 0.183 × 8.34 = 145	12,264	27.2 mg/L
NH3-N	4.6 × 53.82 × 8.34 = 2,065	22 × 0.183 × 8.34 = 34	2,099	4.66 mg/L

Result: CBOD5 19.3 (limit 25) and TSS 27.2 (limit 30) comply. NH3-N at 4.66 mg/L exceeds the 4.0 mg/L limit - and note it already exceeded at 4.6 mg/L before the bypass. The bypass is not the cause of the ammonia violation, but it makes it worse and it is a separate unauthorized discharge.

What the DMR must show: the bypassed flow must be included in the monthly averages and reported (volume, duration, and the fact of the bypass). The plant cannot run a DMR that reports only the secondary effluent. It must also report the bypass itself as a noncompliance event, separately from the effluent limit exceedances.

Permissibility

Under 40 CFR 122.41(m), bypass - the intentional diversion of a wastestream from any portion of the treatment works - is prohibited unless all of the following hold:

1. It was unavoidable to prevent loss of life, personal injury, or severe property damage (economic loss alone does not count);
2. There were no feasible alternatives (e.g., storage, flow equalization, temporary chlorination/dechlorination of primary effluent, hauling, controlled discharge to a holding basin); and
3. The permittee was operating and maintaining the plant properly.

A bypass for essential maintenance may avoid notification only if it does not cause an exceedance - that exception is unavailable here, since raw-ish primary effluent was discharged and the ammonia limit was exceeded.

The plant's argument rests entirely on "protecting the clarifiers," which is property damage of the economic/operational kind, and the record shows 1.8 MGD average against 1.5 MGD design - a chronic capacity problem that undercuts proper-operation-and-maintenance. Expect the bypass to be treated as a violation, with the upset defense at 122.41(n) unavailable because the condition was not exceptional and was reasonably foreseeable given known I/I.

Reporting

• Anticipated bypass: written notice at least 10 days before.
• Unanticipated bypass: oral notice within 24 hours, written follow-up within 5 days.
• Report on the DMR as described above, plus any state SSO/bypass notification (many states have a 24-hour hotline).
• Notify the local health department about the beach 2.5 miles downstream; ammonia and pathogens are the concerns, and the beach is in active season. Consider posting/closure advice. Document the notification.

Finally, whether the 4.0 mg/L ammonia limit has any mixing-zone or TMDL basis matters: if a wasteload allocation exists, an ammonia exceedance may also be a TMDL implementation issue.

Diagnose the problem before buying concrete

• Average flow is 1.8 MGD against 1.5 MGD design - 20% over design in dry-ish weather. A new clarifier sized for peak wet weather does not fix a chronic base-flow problem.
• Peak was 4.6 MGD, a peaking factor of 4.6/1.8 ≈ 2.6 (3.1 against design). That is a classic RDII signature (rapid response to a 2.1-inch storm, 11-hour duration).
• Ammonia at 4.6 mg/L in June is a nitrification capacity problem - likely low SRT and cold/dilute conditions at high flow - not a clarifier surface-area problem.

Recommendation: sequence it, do not choose in the abstract.

1. Measure first. Sub-basin flow metering and rainfall-correlation over one wet season to quantify RDII by reach; smoke/dye testing and private-service inflow inspections. This is cheap and it tells you where the 4.6 MGD came from.
2. Then compare on cost per gallon removed. Sewer rehabilitation (lining, service lateral replacement, manhole sealing) is usually the lowest cost per gallon of peak flow removed and also protects baseline capacity; a clarifier is a large fixed cost that leaves the collection system leaking.
3. Address nitrification separately - evaluate SRT, aeration capacity, and seasonal nitrification; ammonia may need a different fix than either option.

Permit strategy

• Do not use a permit modification as a substitute for enforcing existing limits or as a way to paper over the bypass. Enforcement exposure for the June exceedances exists now.
• Pursue a capacity/consent-order path: a compliance schedule with enforceable milestones (flow metering, I/I reduction targets, design/construction), which is the standard mechanism when a permittee cannot meet limits on the current schedule.
• Ask whether the plant's design flow and the ammonia limit can be re-examined in a permit renewal with a defensible mixing zone and, if applicable, a TMDL-consistent wasteload allocation - but only with the collection-system work committed.
• If the city wants to rely on wet-weather blending of flows exceeding secondary capacity, that depends on explicit permit authorization and state approval; absent that, it is a bypass. Get it in the permit before relying on it.