Wet-Weather Operations Strategy — The Operator's Playbook

The worst time to decide how to handle a storm is during the storm. A written wet-weather plan that the whole shift understands is the single best preparation. The plan should answer three questions in advance: at what flow do we take a specific action, who takes that action, and what's the priority order when several things need to happen at once.

Walk the plant against the plan before the wet season. Confirm equalization basins are empty and clean, the standby generator has been load-tested, all clarifiers are available (don't run a clarifier out of service into storm season), and the SCADA alarms you'd need are actually enunciating. A wet-weather plan in a binder that no one has rehearsed is barely better than no plan.

Know your real hydraulic capacity, not just the design number. The peak flow you can actually pass through without solids loss is usually lower than the nameplate because of current MLSS, clarifier condition, and channel hydraulics. A pre-season capacity check — what flow actually causes the first clarifier to fail — gives you a number you can act on.

Once flow climbs, the first priority is protecting the activated sludge inventory. A storm can wash out a meaningful fraction of the MLSS in hours if the clarifier is pushed past its solids-handling capacity. The biology is the slowest thing to rebuild — losing it turns a two-day event into a two-week recovery.

Lower MLSS ahead of the peak if you can, by increasing wasting a day or two before the storm is forecast to arrive, so the clarifier has headroom in its solids-loading rate. During the event, reduce RAS rate cautiously — too little RAS loses the blanket, too much overloads the clarifier hydraulically. The standard wet-weather adjustment is to back off wasting entirely (you need the inventory) and hold RAS at a rate that keeps the blanket in range without hydraulically overloading the clarifier.

Protect nitrification specifically. A high-flow dilution event can drop the F/M and SRT enough to stress nitrifiers even without biomass loss. After a storm, check effluent ammonia for a week — nitrification failure often shows up two or three days after the storm passed, not during it.

Use every bit of equalization you have. An empty basin ahead of a storm is worth far more than a half-full one, because the first gallon stored is the cheapest hydraulic relief you'll ever buy. If you have step-feed capability, use it to push flow to the downstream end of the aeration basin during peaks, keeping the upstream biomass in contact with influent longer while reducing peak loading on the front end.

Bypass decisions are permit decisions. If the plant has a permitted wet-weather bypass (common for peak excess flow), use it exactly as the permit describes and document flow, time, and duration. Do not improvise a bypass the permit doesn't authorize; an unpermitted bypass is an enforcement problem far more expensive than a treatment excursion.

Track influent vs. the sum of effluent plus storage to catch a silent failure. If the mass balance doesn't close during a storm, something is overflowing or bypassing the process you didn't intend — find it before the state finds it in your DMR.

Wet-weather SSOs usually happen in the collection system, not the plant, but they're the plant operator's notification responsibility in most states. Know your state's SSO notification rule — phone call within 24 hours, written report within 5 days, or whatever it is — and keep the contact list current. A missed SSO notification is one of the most common and most avoidable enforcement actions.

Coordinate with collection-system crews during the event. They're the ones who can throttle a problem lift station, redirect flow, or report a manhole overflow the plant never sees. A standing call frequency during storms (every two hours, say) keeps the two sides working the same event instead of discovering problems after the fact.

I/I is the real long-term fix. Every storm that overwhelms the plant is also evidence of I/I in the system. Track the peak-to-average flow ratio each event and trend it year over year; a rising ratio is the case for a sewer system evaluation survey and rehabilitation before the next storm pushes the plant past its limit.

Recovery is where good operators distinguish themselves. The temptation after a storm is to return immediately to normal wasting and aeration setpoints — but the biology has been through a shock and needs a ramp, not a snap-back. Bring wasting back gradually over several days, watching SVI and effluent turbidity, and don't return to the pre-storm SRT until at least one full sludge age of stable performance has passed.

Clean up before the next event. Drain and clean equalization basins used during the storm, restock chemical inventory that was drawn down, and document what worked and what didn't while it's fresh. A short post-event debrief — what was the peak, what failed first, what would we do differently — is the cheapest training you'll ever run and the input to next year's plan.

Watch for delayed effects. Inhibitory compounds washed in from the collection system (industrial, or just thefirst-flush trash) can show up as a process upset two to three days after the storm, exactly when the plant is being declared 'back to normal.' Keep checking OUR, effluent ammonia, and SVI for a week, not just until the flow returns to average.