Filamentous organisms observed in activated sludge

Abstract Approximately 1100 activated sludge samples, mostly derived from purification plants with bulking sludge, have been investigated for the presence of filamentous organisms. Twenty-six types were distinguished which are preliminary classified into seven groups. The sheath-forming Gram-negative and Gram-positive bacteria are classified into Groups I and II, respectively. The organisms morphologically resembling some types of Cyanophyceae are included in Group III. The bacteria of Group IV are characterized by their tiny, curled filaments. The short, straight, multicellular bacteria are classified into Group V, whilst the organisms of Group VI are motile by a gliding activity. Finally, a number of additional types are collected in Group VII.