Type 1701 / Sphaerotilus natans (Low DO)
The classic low-DO filament; raising and holding DO above 2.0 mg/L typically resolves it within one to two sludge ages.
Thrives when DO drops below roughly 1.0-1.5 mg/L in zones where the filament is actively growing, out-competing floc-forming bacteria under low-oxygen conditions.
Visible under the microscope as long, unbranched filaments extending well outside the floc structure, often with visible sheaths.
Fix: raise and hold DO at 2.0+ mg/L across the aeration basin, verify probe calibration first since a fouled probe can mask a real low-DO condition.
Recovery typically takes one to two sludge ages once DO is corrected; resist the urge to add anti-filamentous chemicals as a substitute for fixing aeration.
Chlorination of RAS is a short-term chemical control option used at some plants during a severe bulking event, but it's non-selective and can damage floc-forming organisms too if overdosed — treat it as a stopgap while the underlying DO or F/M cause is corrected, not a permanent fix.
Low DO filaments frequently cluster near basin dead zones or areas with mixing/diffuser coverage gaps rather than uniformly across the whole basin — a spot DO survey across the basin footprint often reveals the specific zone driving the problem.
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