| Filamentous Organism | Common Problem(s) | Primary Causes / Selectors | Notes / Typical Conditions |
| Microthrix parvicella | Bulking + Foaming | Low temperature (<15–20°C), high fats/oils/greases (FOG/lipids), long SRT/MCRT, anaerobic/anoxic zones | Very common in winter/cold weather; hydrophobic, causes stable brown foams |
| Nocardia spp. / Gordonia (Nocardial forms / GALO) | Foaming (primarily) | High FOG/oils/greases, warm temperatures, long SRT/high MLSS, low F/M | Branching filaments; causes thick, stable brown foams/scum; less bulking |
| Sphaerotilus natans | Bulking | Low dissolved oxygen (DO), high flow/shear, septic conditions, low F/M | Sheathed; common in low DO environments |
| Type 1701 | Bulking | Low DO, high flow/septic conditions (similar to S. natans) | Often grouped with S. natans; responds to similar selectors |
| Type 021N (Thiothrix-like) | Bulking | Septicity/sulfides, low nutrients (N/P deficiency), organic acids | Sulfur-related; common in septic influent |
| Thiothrix I & II | Bulking | Septicity/sulfides, low nutrients (N/P), organic acids | Sheathed; associated with reduced sulfur compounds |
| Nostocoida limicola (I, II, III) | Bulking | Septicity/low nutrients (esp. Type III), variable | Coiled/flexible; Types vary slightly in selectors |
| Type 0092 | Bulking | Low F/M, nutrient deficiency | Common globally; often in nutrient-stressed systems |
| Type 0041 | Bulking | Low F/M, low DO | Sheathed; frequent in low-loaded plants |
| H. hydrossis | Bulking | Low DO, nutrient deficiency | Rigid, toothpick-like; low DO indicator |
| Type 1863 | Foaming (rarely bulking) | Nutrient deficiency, FOG | Less common; associated with foaming in deficits |
Key Quick Tips for Operators
- Low DO → Increase aeration/DO levels; target >2.0 mg/L in bulk.
- Low F/M → Increase loading or waste sludge to raise F/M.
- Septicity/sulfides → Improve collection system aeration or add nitrates/oxidants.
- Nutrient deficiency (N or P) → Check influent BOD:N:P (~100:5:1); supplement if needed.
- High FOG → Pretreat upstream sources (e.g., grease interceptors); use selectors or bioaugmentation.
- Low temperature → Often seasonal; manage SRT and monitor closely in winter.
We suggest using Microbial Community Analysis (Molecular Testing) & advanced microscopic exam (phase contrast + Gram/Neisser stains) for positive ID. Both tests allow you to determine what is causing filamentous bulking and suggest the best control/prevention methods. This table covers ~80–90% of common issues in municipal plants; industrial systems may see more variation.