Filamentous bulking is a common serious operational problem leading to deteriorated sludge settling that has long been observed in activated sludge biological wastewater treatment systems. A number of bacterial genera found therein possess filamentous morphology, where some have been shown to be implicated in bulking episodes (e.g., Ca. Microthrix), the impact of many others is still not clear. In this study we performed a survey of 17 Danish municipal wastewater treatment plants (WWTPs) with nutrient removal using 16S rRNA amplicon sequencing over a period of 13 years, where all known filamentous bacteria from 30 genera were analyzed. The filamentous community constituted on average 13 ± 6%, and up to 43% of total read abundance with the same genera common to all plants. Ca. Microthrix and several genera belonging to phylum Chloroflexi were among the most abundant filamentous bacteria. The effect of filamentous bacteria on sludge settling properties was analyzed using measurements of the diluted sludge volume index (DSVI). Strong positive correlations with DSVI were observed only for Ca. Microthrix and Ca. Amarolinea, the latter being a novel, recently characterized genus belonging to the phylum Chloroflexi. The bulking potential of other filamentous bacteria was not significant despite their presence in many plants. Low phylogenetic diversity was observed for both Ca. Microthrix and Ca. Amarolinea, making physiological characterization of individual species and potential development of control strategies more feasible. In this study we show that, despite the high diversity of filamentous phylotypes in Danish WWTPs, only few of them were responsible for severe bulking episodes.

丝状堆积是常见的严重操作问题，其导致污泥沉降恶化，这在活性污泥生物废水处理系统中早已观察到。其中发现的许多细菌属都具有丝状形态，其中一些已被证明与膨胀事件有关（例如，钙Microthrix），许多其他因素的影响尚不清楚。在这项研究中，我们对17个丹麦市政污水处理厂（WWTP）进行了一项调查，该调查在13年内使用16S rRNA扩增子测序去除了营养，分析了30属的所有已知丝状细菌。丝状群落平均占所有植物共有的同一属的平均阅读丰度的13±6％。钙Microthrix和属于Chloroflexi门的几个属是最丰富的丝状细菌。使用稀释的污泥体积指数（DSVI）的测量分析了丝状细菌对污泥沉降特性的影响。与DSVI的强正相关仅在钙 Microthrix和 钙Amarolinea，后者是一种新颖的，最近被归类为Chloroflexi门的属。尽管其他丝状细菌存在于许多植物中，但它们的膨大潜力并不显着。两者均观察到低的系统发育多样性钙 Microthrix和 钙Amarolinea，使单个物种的生理表征和控制策略的潜在发展更加可行。在这项研究中，我们表明，尽管丹麦污水处理厂的丝状系统型多样性很高，但只有极少数是造成严重膨大的原因。