Nitrification Slides After a New Industrial Discharge
A 1.8-mgd extended-aeration activated sludge plant has 1.2 MG of aeration volume, two 75-hp blowers with one on standby, and internal recycle for denitrification. Permit limits are 4.0 mg/L monthly average ammonia-N, 30 mg/L TSS and 30 mg/L BOD. Current conditions: influent flow 1.6 mgd at 260 mg/L BOD and 30 mg/L ammonia-N, water temperature 28 °C, basin pH 6.8, DO 0.4–0.9 mg/L, MLSS 4,900 mg/L, SVI 290 mL/g, clarifier blanket 30 inches against a 12-inch target, RAS at 60% of influent flow. Effluent ammonia is 8.9 mg/L and rising, TSS 26 mg/L, BOD 18 mg/L. Three weeks ago a food processor began discharging 40,000 gpd at roughly 3,000 mg/L BOD with noticeable sulfide odor, under no pretreatment requirement.
1. Build the mass balance and diagnose what is actually failing. Is this an organic overload?
Read the worked answer
Put the numbers together before you touch a valve.
- Base load: 1.6 mgd × 260 mg/L × 8.34 = 3,470 lb/day BOD
- Industrial load: 0.04 mgd × 3,000 mg/L × 8.34 = 1,000 lb/day BOD
- Total ≈ 4,470 lb/day
- Solids inventory: 1.2 MG × 8.34 × 4,900 mg/L ≈ 49,000 lb
- F/M = 4,470 ÷ 49,000 = 0.09
So this is not an organic overload. The failure is oxygen plus solids inventory.
DO at 0.4–0.9 mg/L starves the nitrifiers, which need roughly 2 mg/L and are the slowest growers in the basin. At 28 °C the water also holds less dissolved oxygen to begin with. Meanwhile SVI of 290 and a 30-inch blanket mean solids are washing over the clarifier weir, so sludge age - the one thing nitrifiers depend on - is falling while ammonia climbs.
The new discharge explains the bulking. High-strength, likely septic wastewater carrying sulfide selects for filaments such as Thiothrix and 021N. Confirm with a microscopic exam, and check DO probe calibration and actual blower output before concluding the biology is the whole story.
2. Lay out your corrective actions in priority order, including what you would and would not do about wasting.
Read the worked answer
Sequence matters more than effort.
- Restore oxygen. Clean and calibrate the probes, verify blower output, and bring the standby blower online to hold 2.0 mg/L at the end of the basin. Everything else is wasted motion until this is done.
- Stop making the problem. Sample the food processor's discharge, compare it to its permit or surcharge agreement, and require source control - sulfide and BOD slug control, pH adjustment, flow equalization, grease removal.
- Protect the biomass. Do not panic-waste. Cutting mixed liquor right now drives sludge age lower and costs you the rest of the nitrifiers. Use polymer to tighten the floc, maximize RAS within clarifier limits, and waste on a controlled schedule driven by a target SRT, not by blanket level alone.
- Watch alkalinity and pH. Nitrification consumes about 7.14 mg of alkalinity as CaCO3 per mg of ammonia-N oxidized - at 30 mg/L influent ammonia that is over 200 mg/L of alkalinity. At pH 6.8 you are close to stalling both nitrification and the clarifier. Feed caustic or lime before you drop below about 6.5, and check the internal recycle ratio so denitrification recovers some alkalinity.
3. What are your compliance and pretreatment obligations, and how do you document the ammonia exceedance?
Read the worked answer
The effluent number is the immediate compliance issue. Ammonia at 8.9 mg/L against a 4.0 monthly average is a violation you cannot fix today. Document the cause, the corrective actions taken and the expected recovery timeline, and call the state wastewater program before they call you. Most permits require reporting for anticipated violations and upset conditions; confirm the 24-hour and five-day reporting language in yours and the specific definition of 'upset' - an unauthorized industrial discharge that you failed to control may not qualify for that defense.
On the industrial side: the food processor is either outside its permit terms or operating under an agreement that never contemplated this load. Issue a written notice, start composite sampling with proper chain of custody, and require pretreatment or a compliance schedule with milestones. Verify whether the discharge is subject to categorical pretreatment standards for food processing.
Keep the record complete: influent and effluent lab sheets, DO and pH logs, blower run hours, wasting records, the microscopic exam notes, and the dates of every communication. If the permit is reopened or the violation is contested, that file is your case.