1Symptom
Treatment drops suddenly: effluent turbidity, BOD or ammonia rise within hours, the mixed liquor may change color or odor, foam may appear, and the microscope shows inactive or dead protozoa and dispersed floc. DO may rise because the biomass stops using oxygen.
2Check first
- Check DO: a sudden rise in DO at constant air is a strong warning sign.
- Look at the microscope: are stalked ciliates closed up or missing, rotifers inactive, floc breaking apart?
- Run an OUR test and compare with your normal value.
- Check pH in the influent and mixed liquor.
- Ask collections and pretreatment staff about unusual discharges, spills, or odors.
3Possible causes
- Industrial discharge: metals, solvents, cyanide, pesticides, cleaners, quaternary ammonia.
- Extreme pH (acid or caustic dump).
- High sulfide or septic slug.
- Chlorinated water or disinfection chemicals entering the sewer.
- High salt (sudden conductivity rise).
4Next investigation
- Save samples of influent and mixed liquor (refrigerated) for later analysis.
- Run conductivity, pH and if possible a respirometry test.
- Check SCADA trends for influent pH and conductivity.
5Corrective actions
- Isolate or divert the toxic flow if possible (equalization or a spare basin).
- Keep the remaining biomass: reduce or stop wasting, keep RAS running.
- Increase DO and protect nitrification.
- Notify pretreatment staff and your regulator as your permit requires.
- Reseed from another plant if the biomass is badly damaged.
6When to call for help
- You suspect a toxic discharge. Call your supervisor and pretreatment coordinator immediately.
- Effluent permit limits may be exceeded; notification timelines start now.
Make one change at a time, record it, and follow your plant procedures and permit. If you suspect a toxic discharge, a safety hazard or a permit violation, notify your supervisor right away.
This guide also appears on ActivatedSludge.com, our consulting site. Results are for training and operator use; always follow your plant’s procedures.