Sphaerotilus natans (S. natans) is a large gram negative filament that causes lots of bulking in wastewater if proper conditions exist. It is usually easy to identify because of its false branching, and large sausage-shaped cells. False branchingis when two of the same filament get fused together by a sheath and continue to grow in opposite directions. Even though S. natans is Gram negative, its sheath can excreteEPS slimewhich can cause it to appear to have a gram positive sheath.
S. natans False Branching S. natans excreting slime with gram stain
S. natans can be a real threat to wastewater operators as it is difficult to subdue. Wasting of sludge is ineffective at controlling S. natans because the filaments are capable of anchoring themselves in a digester or aeration basin to avoid being washed out. This also causes S. natans to do well in conditions with high flow rates. S. natans tends to thrive in conditions of low dissolved oxygen, and low nutrients.
S. natans has a durable sheath which can continue to cause bulking even after the S. natans is dead. Luckily it is easy to prevent with good aeration, and balanced nutrients (BOD:N:P ratio of 100:10:1)
For those affected, Aquafix advised to applyOxyPaks XLandVitaStim Low F:Mto off set the filament's activity.