Which type of filamentous organism is known for causing poor settling in wastewater treatment?

Nocardia is a type of filamentous organism directly associated with causing poor settling behavior in activated sludge systems during wastewater treatment. These organisms can develop extensive filamentous growths that interfere with the normal aggregation and settling of flocs. As a result, they can create a bulky, unstable sludge that tends to float or remain in suspension during sedimentation processes, leading to issues such as sludge bulking. This characteristic makes Nocardia a prominent concern in wastewater treatment operations that must achieve efficient separation of solids from liquids.

The other organisms listed may have their own roles and effects in biological environments, but they are not typically noted for causing poor settling in the same manner as Nocardia. Understanding the mechanisms by which Nocardia affects sludge behavior is crucial for operators managing wastewater treatment facilities, as it guides approaches to control and mitigate these filamentous growths.