Bulking Sludge, Low DO, and a Septage Station at a 0.8-MGD Plant
Your 0.8-MGD extended-aeration plant has two 0.25-MG aeration tanks and two 60-foot-diameter clarifiers with 10-foot side water depth. Permit limits are 30 mg/L TSS, 25 mg/L CBOD5, and 2.0 mg/L ammonia (May-October). For three weeks, effluent TSS has run 30-45 mg/L while CBOD5 holds at 8-10 mg/L and ammonia at 0.4 mg/L. Current data: MLSS 4,300 mg/L, 30-minute settled sludge 1,050 mL/L in a 1-liter cylinder, DO 0.8-1.2 mg/L (one of three blowers is out for repair), clarifier sludge blanket at 4.5 feet, SVI not routinely calculated. Wasting is 6,000 gpd of waste sludge at 7,500 mg/L. Influent CBOD5 is 210 mg/L and flow is average for the season. Eight months ago the plant began accepting hauled septage, about 8,000 gpd, at a new receiving station with no equalization and no grit removal. The microscope shows abundant filamentous organisms extending from the floc into the bulk liquid; the analyst has not identified them past filaments. You are the chief operator.
1. Calculate SVI, MCRT, and F/M, and explain which of the classic bulking causes these numbers support.
Read the worked answer
SVI = 1,050 mL/L divided by 4.3 g/L = 244 mL/g. Anything above about 150 is bulking; 244 is severe.
Aeration basin solids = 4,300 mg/L x 0.5 MG x 8.34 = 17,930 lb. WAS mass = 6,000 gal x 8.34 x 7,500/1,000,000 = 375 lb/day. MCRT = 17,930 / 375 = about 48 days, and that ignores the clarifier inventory, so the true SRT is higher.
Influent CBOD load = 210 mg/L x 0.8 MGD x 8.34 = 1,400 lb/day. Septage at 8,000 gpd and roughly 4,000 mg/L CBOD adds about 267 lb/day, for 1,667 lb/day total. F/M = 1,667 / 17,930 = about 0.09 lb CBOD per lb MLSS per day, at the low end even for extended aeration.
The pattern is a low-F/M, low-DO filamentous bulge. A 48-day SRT at a low F/M selects for filaments that scavenge at low substrate - Type 1701, Microthrix, Haliscomenobacter, Nostocoida - and 0.8-1.2 mg/L DO selects for low-DO filaments and starves the nitrifiers. Septage adds two more risk factors: it is septic and sulfide-bearing, which can favor Thiothrix and Type 021N, and it arrives as an unmixed, ungritted slug that shocks the basin.
Confirmation matters: get a Gram stain and Neisser stain to identify the filament, because the fix differs. Thiothrix points at septage and sulfide; Microthrix points at grease and long SRT.
The effluent data confirm the mechanism: low CBOD5 and low ammonia with high TSS means solids are escaping the clarifier, not that treatment is incomplete.
2. Sequence corrective actions over the next two to four weeks. What do you change first, and what do you watch?
Read the worked answer
First, restore DO. Fix the blower and bring the basin to 2.0 mg/L at the outlet end. It is the cheapest, fastest action, it removes one of the two selection pressures, and it costs nothing in effluent quality. Hold it before changing anything else so you can see the effect.
Second, start a controlled wasting increase. To reach a 15-day MCRT at 17,930 lb of solids you need to waste about 1,195 lb/day, which at 7,500 mg/L is roughly 19,000 gpd - about three times the current wasting rate. Do not jump there in one day. Increase in 25-50 percent steps every few days, watching the clarifier blanket (hold it below about 2 feet), effluent TSS, and the 30-minute settle. Wasting pushes more solids to the clarifiers exactly when they are least able to handle it, and the plant is already over its 30 mg/L TSS limit.
Third, protect the permit while the fix works. Consider a polymer or coagulant feed to the clarifier, increase effluent monitoring, and document that the utility has notified the state and is taking corrective action. If effluent TSS stays above 30 mg/L, the plant is in violation and the record should show the cause and the response.
Fourth, address the selector. Long SRT alone will not fix a system with no plug-flow or anoxic zone. A selector - an anoxic or anaerobic compartment ahead of the aeration tank, or converting one pass to plug flow - is the durable fix and belongs in the capital plan.
Fifth, septage: install equalization and grit removal, meter and sample each load, set acceptance limits (pH, sulfide, CBOD, TSS, grease), and feed it at a controlled rate rather than dumping it.
3. The manager wants MCRT down to 8 days by next month. What is the risk, given the 2.0 mg/L ammonia limit?
Read the worked answer
Nitrification is an SRT problem before it is a DO problem. The minimum SRT for nitrifiers at 20 degrees C is roughly 5-8 days before you apply a safety factor; at 12 degrees C it climbs to about 10-13 days. An 8-day MCRT in warm weather would put you on the edge, and any cold snap, toxic slug, or septage shock would wash the nitrifiers out - and recovery takes weeks, not days.
The permit limit runs May through October, but the biomass has to survive the winter to nitrify again in May. Dropping to 8 days in the fall is exactly backwards.
A defensible target is 15-20 days, which is enough to control the low-F/M filaments without putting nitrification at risk, and the SVI/blanket response should be checked before going lower. Tie the wasting rate to the SVI and the clarifier blanket rather than to a fixed number.
Monitoring to add: daily effluent ammonia and a basin-profile ammonia (influent, mid-basin, outlet) so you see washout starting; weekly SVI and filament counts; daily clarifier blanket; and a DO log. Set a hard trigger - if effluent ammonia goes above 1.0 mg/L, stop wasting and restore inventory.
Present it as a range and a rationale, not a fixed number, and note that septage equalization is what makes any of these targets stable.