Filamentous Bulking

1Symptom

SVI is high (often above 150 to 200 mL/g) and the settleometer shows a loose, fluffy sludge that compacts slowly with a clear supernatant. Under the microscope, filaments extend from flocs and bridge between them.

2Check first

1. Rate filament abundance on a 0 to 6 scale at 100x. Abundance of 4 (very common) or more usually affects settling.
2. Do Gram and Neisser stains to narrow the filament type (for example Type 021N, Thiothrix, S. natans, Type 1701, M. parvicella, Nocardioforms).
3. Check DO in each zone and at different times of day, especially at peak load.
4. Check nutrients, influent septicity, F/M and grease loading.

3Possible causes

• Low DO filaments: S. natans, Type 1701, H. hydrossis. Often from aeration limits or high load.
• Low F/M filaments: M. parvicella, Type 0041, Type 0675, Type 1851. Common in long SRT, completely mixed basins.
• Septic wastewater and sulfide: Thiothrix, Type 021N, Beggiatoa.
• Nutrient deficiency: Type 021N, Thiothrix, also viscous bulking.
• Grease and oil: Nocardioforms and M. parvicella, usually with foam.

4Next investigation

• Match the dominant filament to its probable cause and then check that condition in your data.
• Review DO logs at 15 minute resolution for low periods; daily averages hide them.
• Measure soluble ammonia and orthophosphate leaving the aeration basin.

5Corrective actions

1. Correct the root cause: DO, nutrients, septicity, F/M gradient (selector), or grease control.
2. Manage clarifiers with RAS while the cause is corrected. The state point tool helps set RAS.
3. Selective RAS chlorination can knock back filaments extending outside the floc. Typical starting doses are 2 to 3 lb chlorine per 1,000 lb of MLSS inventory per day, applied where mixing is good, with close microscope checks. Stop when filaments retract.
4. Hydrogen peroxide or polymer can also help in the short term.
5. Expect recovery to take two to three SRTs after the cause is fixed.