Compounds such as 2-methylisoborneol (2-MIB) and geosmin present in the final effluents of wastewater treatment plants may affect water treatment in downstream areas. To reduce the concentrations of 2-MIB and geosmin, we investigated the influence of the returned water from sludge treatment on their concentrations and behaviors during wastewater treatment in the reactor. The inflow of returned water from sludge treatment increased the geosmin concentration in the primary effluent (PE) making it higher than that in the influent. The dissolved 2-MIB concentration in the mixed liquor was higher than that in the PE. However, the geosmin concentration in the mixed liquor tended to be lower than that in the PE as the biological treatment progressed. Laboratory experiments indicated that the 2-MIB and geosmin concentrations varied in the reactor due to their release from activated sludge and removal by aeration and agitation. Our analyses revealed that the return activated sludge substantially impacts 2-MIB concentrations in secondary effluents. Moreover, the geosmin concentration in the reactor decreased, possibly due to removal by aeration and agitation.

Legionella is an important opportunistic pathogen in drinking water distribution and premise plumbing systems. This study investigated the potential growth of Legionella pneumophila during granular activated carbon (GAC) filtration, considering their viability states in the filter beds. Laboratory-scale biologically active GAC columns were used, and L. pneumophila cells were spiked with different viability states (culturable, viable but non-culturable (VBNC), and dead cells). The gene concentrations in the effluents were monitored for 70 days. In columns spiked with the culturable cells, high levels of L. pneumophila were detected in the effluents even after 70 days of operation, suggesting that the GAC filter bed could serve as an ecological niche for L. pneumophila colonization. However, when VBNC-cells were introduced, the levels of L. pneumophila in the effluents were significantly lower, though still higher than in the column spiked with dead cells. This suggested that the growth potential of L. pneumophila was influenced by their viability states in the influent water. These findings underscored the ecological potential for Legionella regrowth and emphasized the necessity of monitoring their behavior during GAC treatment, particularly when incomplete inactivation during ozonation is concerned.

The Eikelboom filamentous bacterial morphotype ‘Haliscomenobacter hydrossis’ is ubiquitous in Japanese and other wastewater treatment plants (WWTPs). However, it is still largely ‘identified’ microscopically on its distinctive morphology, and only one species Haliscomenobacter hydrossis is recognized currently. Thus, little is known about the true level of phylogenetic diversity among members of this morphotype. Here, using 16S rRNA gene amplicon data we reveal that populations of this morphotype belong to several clades. Fluorescence in situ hybridization (FISH) probes were designed against members of each of these morphotypes, together with a new probe more specific than those currently available to target members of the genus Haliscomenobacter. Japanese municipal WWTPs were surveyed to evaluate the abundances of ‘H. hydrossis’ morphotype using FISH analysis with the existing and newly designed FISH probes. Data showed that some ‘H. hydrossis’ morphotypes appeared to show suppressed growth in biological nitrogen removal processes, although process configuration seemed not to be a critical factor in determining their abundances. Their excessive growth appears to cause activated bulking incidents in WWTPs. These data may assist in extending our currently poor understanding of ‘H. hydrossis’ phylogeny and systematics and provide tools to study its global distribution.