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Filamentous Chloroflexi species are often present in activated sludge wastewater treatment plants in relatively low numbers, although bulking incidences caused by Chloroflexi filaments have been observed. A new species-specific gene probe for FISH was designed and using phylum-, subdivision-, morphotype 1851- and species-specific gene probes, the abundance of Chloroflexi filaments were monitored in samples from 126 industrial wastewater treatment plants from five European countries. Chloroflexi filaments were present in 50% of the samples, although in low quantities. In most treatment plants the filaments could only be identified with phylum or subdivision probes, indicating the presence of great undescribed biodiversity. The ecophysiology of various Chloroflexi filaments was investigated by a suite of in situ methods. The experiments revealed that Chloroflexi constituted a specialized group of filamentous bacteria only active under aerobic conditions consuming primarily carbohydrates. Many exo-enzymes were excreted, e.g. chitinase, glucuronidase and galactosidase, suggesting growth on complex polysaccharides. The surface of Chloroflexi filaments appeared to be hydrophilic compared to other filaments present. These results are generally supported by physiological studies of two new isolates. Based on the results obtained in this study, the potential role of filamentous Chloroflexi species in activated sludge is discussed.

Excessive growth of filamentous bacteria in activated sludge wastewater treatment plants (WWTPs) can cause serious operational problems. With some filaments there may be the problem of bulking, where inadequate flocculation and settling of the biomass in the secondary clarifier results in a carryover of solids with the final treated liquid effluent. Their proliferation often encourages the development of stable foams on the surface of the reactors, and these foams may impact negatively on plant performance and operation. The availability of culture-independent molecular methods now allows us to identify many of the more common filamentous organisms encountered in WWTPs, which are phylogenetically diverse, affiliating to seven separate bacterial phyla. Furthermore, the extensive data published in the past decade on their in situ behaviour from the application of these culture-independent methods have not been summarized or reviewed critically. Hence, here, we attempt to discuss what we now know about their identity, ecophysiology and ecological niches and its practical value in better managing activated sludge processes. Some of this knowledge is already being applied to control and manage full-scale WWTPs better, and the hope is that this review will contribute towards further developments in this field of environmental microbiology.

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A comprehensive study of the ecophysiology of the filamentous Meganema perideroedes affiliated to the Alphaproteobacteria, possessing a "Nostocoida limicola Type II" filamentous morphology was conducted. This morphotype often causes serious bulking problems in activated sludge wastewater treatment plants, and hardly anything is known about its physiology. The study was carried out by applying a suite of in situ methods in an industrial activated sludge treatment plant with excessive growth of this species. The experiments revealed a very versatile organism able to take up a large variety of organic substrates under aerobic conditions. It had a remarkably high storage capacity forming polyhydroxyalkanoates from most substrates tested. When nitrate was present as e-acceptor, the number of substrates to be consumed by M. perideroedes was more restricted compared to aerobic conditions. With nitrite as e-acceptor, only acetate and glucose among the substrates tested could be assimilated and used for storage and possibly growth. This indicated that M. perideroedes might be able to denitrify under certain conditions, which is unusual for filamentous bacteria in activated sludge. No substrate uptake or storage was seen under anaerobic conditions. M. perideroedes was relatively hydrophobic, compared to other filamentous bacteria and microcolonies present in the sludge, indicating the presence of a hydrophobic sheath. Several excreted surface-associated exoenzymes were detected in the sludge, but M. perideroedes never showed any activity, except once after a breakdown in the production facility. This confirmed that M. perideroedes mainly grows on soluble substrates. Based on the studies of the ecophysiology of M. perideroedes, potential control strategies are suggested.