Troubleshooting Matrix

Pick the symptom you see. The panel shows what to check first, the likely causes and next steps, with a link to the full guide.

All symptoms at a glance

Symptom	What you see	Most common cause
High SVI	Sludge settles slowly and the blanket climbs. SVI above 150 mL/g and rising.	Filamentous bulking: low DO, low F/M, nutrient deficiency, septic influent or high grease feed favor filaments.
Pin Floc	Tiny, dense floc particles that settle fast but leave a hazy supernatant.	SRT too long for the current load, often after flows or loads drop seasonally.
Straggler Floc	Large, fluffy, light floc particles floating in the supernatant and over the weirs.	Low SRT and high F/M: young sludge with weak floc.
Filamentous Bulking	Filaments extend from floc and bridge between particles. SVI high, blanket fluffy.	Low DO filaments: S. natans, Type 1701, H. hydrossis. Often from aeration limits or high load.
Rising Sludge (Denitrification in the Clarifier)	Clumps of sludge float to the clarifier surface with small gas bubbles.	Nitrate in the mixed liquor plus long sludge holding time in the clarifier.
Low DO in the Aeration Basin	DO below setpoint, especially at peak load or in the first zone.	Aeration capacity limited: blower out of service, fouled diffusers, broken header or leaking piping.
Excessive DO and High Blower Demand	DO well above need and blower power higher than it should be.	Air needed for mixing exceeds air needed for oxygen at low load.
Poor Oxygen Transfer	Lots of air, little DO. Coarse bubbling or rising header pressure.	Diffuser fouling or scaling (biological slime, calcium, iron).
High Effluent Ammonia (Loss of Nitrification)	Effluent ammonia above normal or above the permit limit.	Low DO.
Low Alkalinity	Effluent alkalinity below about 50 to 70 mg/L, pH drifting down.	Full nitrification of a soft, low alkalinity influent.
Temperature Effects	Seasonal changes in nitrification, settling, DO and foaming.	Nitrifier growth rate roughly halves for every 8 to 10 C drop.
Toxic Upset Recognition	Sudden loss of treatment, dead protozoa, odd color or foam, OUR collapse.	Industrial discharge: metals, solvents, cyanide, pesticides, cleaners, quaternary ammonia.
Increasing Clarifier Blanket	Blanket depth growing day by day or during peak flow.	Thickening overload: too much mass for the clarifier area and the current settleability.
Solids Washout and Hydraulic Overload	Storm flows carry blanket solids over the weirs.	Peak flow beyond clarifier capacity for the current SVI.
RAS Problems	Thin RAS, plugged lines, uneven pump flows or flow meter errors.	RAS rate too high: thin RAS and high clarifier underflow loading.
Stable Brown Foam	Thick, greasy, chocolate brown foam that does not break down.	Nocardioform organisms, favored by long SRT, grease and warm water.
White Foam	Billowing, crisp white foam, often during startup or low MLSS.	Young sludge (low SRT, high F/M) with protein and surfactants not yet degraded.
Effluent TSS High	Effluent solids above normal or trending toward the limit.	Pin floc or dispersed growth.

This guide also appears on ActivatedSludge.com, our consulting site. Results are for training and operator use; always follow your plant’s procedures.