Bulking sludge has been an operational problem of great interest in which filamentous bacteria (FB) have been shown to be the main cause. Recent evidence states that 90% of the sludge bulking cases is caused by the overgrowth of FB. A recent Belgian study revealed that 36% of the 90 studied industrial wastewater treatment plants (WWTPs) in Flanders had sludge settleability problems. However, no information related with the microbial population of industrial WWTPs in Belgium has yet been published.
The main aim of this study was to investigate the microbial ecology of activated sludge (AS) plants for a variety of industries, relevant for the region of Flanders, and to identify the external factors that have the greatest impact on the distribution of FB causing bulking sludge.
In this study, filamentous population was investigated in 31 WWTPs in Flanders, Belgium. FB were identified at the taxonomic level using 16S rRNA Amplicon Sequencing and morphologically classified using conventional microscopy techniques. Additionally quantitative PCR was used to quantify the most encountered FB.
FB constituted a significant part of the microbial community, ranging from 0.02% up to 13.8% read abundance. Our results revealed that Thiothrix, "Ca. Villigracilis" and Kouleothrix were the most encountered FB in the studied WWTPs. "Ca. Villigracilis" seems to serve as backbone structure for the flocs as reported in literature, while Thiothrix and Kouleothrix changed the floc structure resulting in low sludge settleability and high filamentous abundance index (FI). Our results reveal that 56% of the studied WWTPs had poor settling properties with dSVI values up to 448 mL/g.
Additionally, quantitative PCR (qPCR) assay was used to quantify Thiothrix population. qPCR revealed that Thiothrix ranged from –copies in the studied WWTPs. High abundance (–copies ) also coincided with high filamentous abundance according to FI and poor sludge settleability.
