Thiothrix & Beggiatoa (Septicity / Sulfide)
Sulfur-oxidizing filaments pointing to septic influent or a collection-system problem upstream of the plant.
Identifiable under the microscope by visible internal sulfur granules, a distinguishing feature from other filament types.
Points to sulfide exposure from septic influent, long-detention force mains, or an anaerobic zone running too reducing.
Fix: address the upstream septicity, reduce collection-system detention time, improve pre-aeration, or add chemical sulfide control at problem pump stations.
Adjusting aeration basin DO or SRT alone won't resolve a collection-system-driven cause; the fix has to happen upstream.
Force main age and length correlate strongly with septicity risk — longer detention time in a warm-weather force main gives sulfate-reducing bacteria more time to generate hydrogen sulfide before the flow ever reaches the plant.
Point-of-discharge chemical addition (calcium nitrate or iron salts dosed at a problem pump station) is a common upstream fix when force main replacement or re-routing isn't practical in the near term.
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