Mill Brook WWTF: Wet-Weather Overflow with an Effluent in Exceedance
Mill Brook Wastewater Treatment Facility is a 0.30 MGD extended-aeration activated sludge plant serving 2,300 people, with a 4.5-mile gravity collection system, three lift stations, and an NPDES permit limiting CBOD5 to 25 mg/L and TSS to 30 mg/L as monthly averages, with 85% minimum removal for each. On April 12 a nor'easter drops 1.9 inches of rain in 20 hours. Plant flow reaches 0.95 MGD, the No. 2 lift station high-level alarm activates at 03:10, and at 05:40 an estimated 4,000 gallons of raw sewage overflows from a manhole on a residential driveway, running into a storm drain that discharges to Mill Brook about 300 feet away. Influent that day is TSS 240 mg/L and CBOD5 190 mg/L. The effluent monthly averages are TSS 38 mg/L and CBOD5 22 mg/L, with final pH 6.9 and DO 4.0 mg/L. The plant has no equalization basin and the clarifier blanket is running high.
1. Walk through your response to the overflow - on scene, then notifications and reporting.
Read the worked answer
On scene (first hours):
- Stop or reduce the source if you can (throttle or bypass around the problem reach, pump down the wet well, call in the vactor to relieve the line).
- Contain what is still flowing - sandbags, dikes, diversion - and recover as much as possible rather than letting it reach the brook.
- Lime or disinfect the affected driveway and ground surface, and keep people and pets off it.
- Sample Mill Brook upstream and downstream of the storm drain outfall, and take a chlorine residual where relevant.
- Post signage, notify the affected homeowner, and estimate the volume with times and observations rather than a guess made later.
Notifications: report to NH DES and the permitting authority as your NPDES permit and state rules require - the standard framework is a 24-hour oral or electronic notification followed by a written report within 5 days. Document the estimate, the cause, the receiving water, corrective actions, and whether a public water supply intake is downstream. This one goes into the annual report and often into the public notice record.
Do not dilute the story. The overflow and the effluent exceedance are separate compliance events; fixing one does not erase the other.
2. Using the numbers given, assess compliance and explain whether wet weather gives you a defense.
Read the worked answer
Do the math on both limits.
- TSS removal = (240 − 38) ÷ 240 = 84.2%. Required is 85%, so the removal limit fails as well as the 30 mg/L monthly average (38 mg/L).
- CBOD5 removal = (190 − 22) ÷ 190 = 88.4%, and 22 mg/L is under 25 mg/L, so CBOD5 passes.
So the TSS result is a violation on two independent bases. Ignoring the removal percentage because the concentration is the number everyone watches is a common and expensive mistake.
On the defense question: an upset or bypass defense is narrow. It generally requires that the event was beyond your control, that the facility was properly operated and maintained, and that you met the notification and reporting conditions in the permit and federal rules. Heavy rain alone does not qualify; a plant that was being run with a high clarifier blanket and no equalization invites the question of whether operation contributed. The honest course is to report the exceedance on the DMR with a written explanation of the wet-weather event and the operational response, rather than to argue the limit away.
Operational response: lower the clarifier blanket, adjust return and waste sludge rates to hold solids, check aeration capacity at the higher organic load, and watch for solids washout on the next storm.
3. The manager asks what to fund first to stop this from recurring. How do you prioritize in a collection system with limited money?
Read the worked answer
Rank by gallons of overflow prevented per dollar, not by visibility.
- Measure before you dig. Put flow monitors at key subareas and at each lift station for at least one wet season. You need to know whether the problem is infiltration (groundwater leaking in) or inflow (direct connections, roof leaders, sump pumps, area drains). They need different fixes.
- Lift station reliability first. High-level alarms with telemetry, backup power, redundant pumps, and adequate wet-well storage are usually the cheapest gallon of prevention available. An alarm that pages nobody at 03:10 is decoration.
- Then the smoke and dye testing, manhole inspection, and CCTV on the subarea the data flags - not the whole system at once. Illegal sump pump connections and roof leaders are typically found fast and removed cheaply.
- Then rehab (lining, manhole sealing, root control) where the data says the water is coming in.
Also fix the storage question: a small equalization basin or a flow-split at the headworks costs less than a treatment upgrade and protects the clarifier during the next storm. Put the SSO cost, the violation history, and the brook impact into the funding request - that is what makes the case for a state revolving fund project.