Clarifier Failure, Permit Exceedance, and a Confined Space
Northgate is a 0.9 MGD activated sludge plant with UV disinfection. Its SPDES permit sets monthly average limits of 30 mg/L TSS and 5.0 mg/L ammonia for April 1 through October 31. On July 8 the sludge collector in secondary clarifier #1 fails and the plant runs on clarifier #2 alone, at about 1.4 times its design solids loading. Over the next three days effluent TSS averages 48 mg/L and ammonia averages 9.2 mg/L. To keep the bar screens clear, a contractor is scheduled to remove grit from the influent channel, a permit-required confined space. Before ventilation, a four-gas meter at the channel cover reads 35 ppm hydrogen sulfide, 8% of the lower explosive limit, and 20.5% oxygen. The plant superintendent wants the grit removed today so the screens do not blind.
1. How do you bring TSS and ammonia back under control with one clarifier in service?
Read the worked answer
The two exceedances have different causes and need different levers.
Solids (TSS). The clarifier is at 1.4 times design solids loading, so the goal is to reduce the solids it must handle and improve its capture.
- Reduce mixed liquor suspended solids by increasing wasting, which lowers the solids load to the clarifier even though it temporarily shortens sludge age.
- Optimize return activated sludge rate: too low and the blanket rises; too high and you recycle solids and create hydraulic short-circuiting. Watch the sludge blanket, not just the flow.
- Add polymer or a coagulant aid to the secondary clarifier if the permit allows and jar data support it.
- Reduce inflow if possible, and check that industrial contributors are not slugging the plant.
Ammonia. 9.2 mg/L against a 5.0 mg/L limit means nitrification has been impaired, which is expected when solids wash out and sludge age drops.
- Check dissolved oxygen in the aeration basins first; nitrifiers are oxygen-sensitive and a low DO will show up as ammonia before it shows up as TSS.
- Check alkalinity and pH. Nitrification consumes roughly 7.1 mg of alkalinity as CaCO3 per mg of ammonia nitrogen oxidized, and a pH crash will stall it.
- Protect the nitrifier population: avoid over-wasting past the point where solids age falls below the nitrification requirement, and consider seeding from the healthy clarifier's return stream.
- Verify temperature and check for toxics from industrial users.
Immediate step. Notify the agency of the exceedances; do not wait for the process to recover before the 24-hour call.
2. Do you approve the confined space entry today? Justify the decision with the readings and controls.
Read the worked answer
No. The entry is not approvable as the space stands, and the superintendent's schedule pressure does not change the numbers.
The readings decide it. Hydrogen sulfide at 35 ppm exceeds the OSHA ceiling limit of 20 ppm, and it is above the level at which anyone should be in the space without supplied air. Hydrogen sulfide also deadens the sense of smell at concentrations near this range, so an odor-based judgment is worthless. Oxygen at 20.5% is normal and 8% of the lower explosive limit is within normal entry criteria, but one unacceptable parameter is enough to stop the entry. The IDLH for hydrogen sulfide is 100 ppm, which is not far above the reading.
Controls required before any entry:
- Continuous forced-air ventilation, with re-testing from outside the space until hydrogen sulfide is at an acceptable level and stable.
- A written permit reviewed and signed on site, with the space's hazards, isolation of influent flow, and lockout of the screens documented.
- Atmospheric monitoring by a calibrated four-gas meter worn by the entrant, not only by the attendant.
- An attendant stationed outside with communication, a retrieval system (tripod and winch), and a rescue plan; non-entry rescue is the default.
- If ventilation cannot hold levels down, supplied-air respiratory protection, which means the contractor's program must support it.
- Entry training and a confined space program for the contractor, verified before work begins.
Schedule. Blind screens can be cleaned with a bar-screen rake, a vacuum truck, or by reducing flow, none of which require entry. Offer those alternatives, and reschedule the grit removal for after ventilation and testing demonstrate the space is safe.