Abstract
BackgroundBacterial communities are responsible for biological nutrient removal and flocculation in engineered systems such as activated floccular sludge. Predators such as bacteriophage and protozoa exert significant predation pressure and cause bacterial mortality within these communities. However, the roles of bacteriophage and protozoan predation in impacting granulation process remain limited. Recent studies hypothesised that protozoa, particularly sessile ciliates, could have an important role in granulation as these ciliates were often observed in high abundance on surfaces of granules. Bacteriophages were hypothesized to contribute to granular stability through bacteriophage-mediated extracellular DNA release by lysing bacterial cells. This current study investigated the bacteriophage and protozoan communities throughout the granulation process. In addition, the importance of protozoan predation during granulation was also determined through chemical killing of protozoa in the floccular sludge.ResultsFour independent bioreactors seeded with activated floccular sludge were operated for aerobic granulation for 11 weeks. Changes in the phage, protozoa and bacterial communities were characterized throughout the granulation process. The filamentous phage, Inoviridae, increased in abundance at the initiation phase of granulation. However, the abundance shifted towards lytic phages during the maturation phase. In contrast, the abundance and diversity of protozoa decreased initially, possibly due to the reduction in settling time and subsequent washout. Upon the formation of granules, ciliated protozoa from the class Oligohymenophorea were the dominant group of protozoa based on metacommunity analysis. These protozoa had a strong, positive-correlation with the initial formation of compact aggregates prior to granule development. Furthermore, chemical inhibition of these ciliates in the floccular sludge delayed the initiation of granule formation. Analysis of the bacterial communities in the thiram treated sludge demonstrated that the recovery of ‘Candidatus Accumulibacter’ was positively correlated with the formation of compact aggregates and granules.ConclusionPredation by bacteriophage and protozoa were positively correlated with the formation of aerobic granules. Increases in Inoviridae abundance suggested that filamentous phages may promote the structural formation of granules. Initiation of granules formation was delayed due to an absence of protozoa after chemical treatment. The presence of ‘Candidatus Accumulibacter’ was necessary for the formation of granules in the absence of protozoa.
Highlights
Bacterial communities are responsible for biological nutrient removal and flocculation in engineered systems such as activated floccular sludge
Development and microscopic observations of aerobic granular sludge Activated floccular sludge was used to seed the sequencing batch reactors (SBRs), which were operated under conditions optimal for the aerobic granulation process over a period of 11 weeks
The sludge volumetric index at 5 min (SVI5) of the floccular sludge increased from 190.8 ± 2.0 to 221.8 ± 5.4 mL g− 1, which indicated poor settling of the floccular sludge (Fig. 1b)
Summary
Bacterial communities are responsible for biological nutrient removal and flocculation in engineered systems such as activated floccular sludge. Bacteriophages were hypothesized to contribute to granular stability through bacteriophage-mediated extracellular DNA release by lysing bacterial cells This current study investigated the bacteriophage and protozoan communities throughout the granulation process. Aerobic granular sludge is a complex, human engineered ecosystem consisting of highly diverse and functional microbial communities that are utilized for specific biological functions [1, 2] These densely packed biofilm aggregates are typically developed from activated floccular sludge. The addition of AHLs to the SBR markedly increased the production of EPS, which mediates contact between bacterial cells [8, 9] Other biological factors such as predation have been demonstrated to enhance biofilm formation for several bacterial species [10,11,12]. Bacteriophages are highly abundant in engineered wastewater systems, appear to be active components of activated sludge systems and are able to infect both planktonic and biofilm associated bacterial cells [13,14,15]
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