Sludge bulking impact on relevant bacterial populations in a full-scale municipal wastewater treatment plant

Potential role of AgNPs within wastewater in deteriorating sludge floc structure and settleability during activated sludge process: Filamentous bacteria and quorum sensing

Excellent sludge floc structure and settleability are essential to maintain the process stability and excellent effluent quality during the activated sludge process. The underlying effect of silver nanoparticles (AgNPs) within wastewater on sludge floc structure and settleability is still unclear. The potential role of AgNPs in promoting filamentous bacterial proliferation and deteriorating sludge floc structure and settleability based on quorum sensing (QS) were investigated in this study. The results indicated that N-acyl homoserine lactose (AHL) concentration sharply increased from 23.56 to 108.41 ng/g VSS in the sequencing batch reactor with 1 mg/L AgNPs. AgNPs strengthened communication between filamentous bacteria, which triggered the filamentous bacterial QS system involving the synthetic gene hdtS and sensing genes traR and lasR. Filamentous bacterial proliferation was promoted by the triggered QS via positively regulating its cell cycle progression including chromosomal replication and divisome formation. In addition, extracellular protein production was obviously increased from 43.56 to 97.91 mg/g VSS through QS by regulating arginine and tyrosine secretion during filamentous bacterial proliferation under 1 mg/L AgNPs condition, which led to an increase in the negative charge and hydrophily at the cell surface. AgNPs resulted in an obvious increase in the surface energy barrier (WT) between bacteria. The change in the physicochemical properties of extracellular polymeric substance (EPS) induced by QS among filamentous bacteria obviously inhibited bacterial aggregation between filamentous bacteria and floc-forming bacteria under AgNPs condition, thus resulting in serious deterioration of the sludge floc structure and settleability. This study provided new insights into the microcosmic mechanism for the effect of AgNPs on sludge floc structure and settleability.

New insight into the granule formation in the reactor for enhanced biological phosphorus removal

While granulated activated sludge exhibits high productivity, the processes of granule formation are incompletely studied. The processes of granule formation and succession of communities were investigated in a laboratory sequencing batch reactor (SBR) under conditions for enhanced biological phosphorus removal (EBPR) using microbiological and molecular techniques. Active consumption of acetate, primarily by the phosphate-accumulating organisms (PAO), commenced at day 150 of cultivation. This was indicated by the high ratio of molar P-released/acetate uptake (0.73-0.77 P-mol/C-mol), characteristic of PAO. During this period, two types of granule-like aggregates formed spontaneously out of the activated sludge flocs. The aggregates differed in morphology and microbial taxonomic composition. While both aggregate types contained phosphorus-enriched bacterial cells, PAO prevailed in those of morphotype I, and glycogen-accumulating organisms (GAOs) were predominant in the aggregates of morphotype II. After 250 days, the elimination of the morphotype II aggregates from the reactor was observed. The subsequent selection of the community was associated with the development of the morphotype I aggregates, in which the relative abundance of PAO increased significantly, resulting in higher efficiency of phosphorus removal. Metagenomic analysis revealed a predominance of the organisms closely related to Candidatus Accumulibacter IС and IIС and of Ca. Accumulibacter IIB among the PAO. Based on the content of the genes of the key metabolic pathways, the genomes of potential PAO belonging to the genera Amaricoccus, Azonexus, Thauera, Zoogloea, Pinisolibacter, and Siculibacillus were selected. The patterns of physicochemical processes and the microbiome structure associated with granule formation and succession of the microbial communities were revealed.

Roles of high/low nucleic acid bacteria in flocs and probing their dynamic migrations with respirogram

Bacterial migration is crucial for the stability of activated sludge but rarely reported. The static distribution was explored by changes in bacteria concentration with extracellular polymeric substances (EPS) extractions. Next, denitrification and aeration were conducted as normal running conditions for examining the bacterial migration between floc-attached and dispersed growth. Above observations were further explored by conducting copper ion (Cu2+) shock as an extreme running condition. After extracting EPS, low nucleic acid (LNA) bacteria migrated from the sludge to the supernatant primarily, and high nucleic acid (HNA) bacteria remained in the residual sludge, suggesting that HNA bacteria mainly distributed inside the sludge while LNA bacteria outside the sludge. During the denitrification process, LNA bacteria migrated out of flocs, which increased by 6.94 × 106 events/mL in the supernatant. During the feast phase of aeration, LNA bacteria grew attached to flocs, causing the increased flocs diameter from 45.60 to 47.40 μm. During the following aerobic famine phase, LNA bacteria grew dispersedly, but HNA bacteria remained unchanged. However, a further severe famine phase drove HNA bacteria to be dispersed, breaking flocs with the decreased diameter from 48.10 to 46.50 μm. When the Cu2+ shock was employed, LNA and HNA bacteria increased but the LNA/HNA ratio decreased in the supernatant, indicating more HNA bacteria migrating to the dispersed phase. From a structural perspective, HNA bacteria distributed inside the sludge and functioned as the backbone of flocs, undertaking the maintenance of flocs stability primarily; while LNA bacteria distributed outside the sludge and functioned as filling materials, having a secondary influence on flocs stability. These processes were also probed by respirogram exactly, correlating the system-scale measurement and microscale migrations and providing an early warning signal under abnormal circumstances. The processed HNA-backbone theory is promising for regulating the stability of activated sludge based on bacterial migrations.

Microbial density-dependent viral dynamics and low activity of temperate phages in the activated sludge process

The ecological behavior of bacteriophages (phages), the most abundant biological entity in wastewater treatment systems, is poorly understood, especially that of temperate phages. Here, the temporal dynamics of lytic and temperate phages in a laboratory-scale activated sludge reactor with a sludge bulking issue was investigated using coupled sludge metagenomic and viromic analyses. The lysogenic fragments (prophages) identified were widely distributed in the reconstructed metagenome-assembled genomes (61.7%, n = 227). However, only 12.3% of the identified prophages experienced lysogenic-lytic switching, and the abundance contribution of prophages to free virus communities was only 0.02-0.3%, indicating low activity of temperate phages. Although the sludge community changed dramatically during reactor operation, no massive prophage induction events were detected. Statistical analyses showed strong correlations between sludge concentration and free virus and temperate phage communities, suggesting microbial density-dependent virus dynamics in the sludge microbiota.

New insight into soluble extracellular metabolites during sludge bulking process based on excitation-emission matrix spectroscopy and ultrahigh-performance liquid chromatography-mass spectrometry

Soluble extracellular metabolites (SEM) produced by microorganisms might significantly change during sludge bulking, which is a major operational problem caused by the excessive growth of filamentous bacteria. However, knowledge remains limited about the dynamics and potential role of SEM in the bulking of sludge. In this study, filamentous bulking was simulated in a laboratory-scale reactor and changes to SEM characteristics during the bulking process were investigated using excitation-emission matrix spectroscopy and ultrahigh-performance liquid chromatography-mass spectrometry. SEM components changed significantly at different phases of sludge bulking. Changes in SEM were closely correlated with the structure of the bacterial community. Based on the EEM profiles, significant increases in fulvic acid-like and humic acid-like substances in SEM were observed with the development of filamentous bulking. The degree of humification in SEM showed a clear increasing trend. Untargeted extracellular metabolomic analysis showed that the intensity of berberine and isorhamnetin in SEM increased significantly during the bulking phase, which might synergistically facilitate the development of filamentous bulking.

Seasonal microbial community dynamics complicates the evaluation of filamentous bulking mitigation strategies in full-scale WRRFs

The activated sludge wastewater treatment process has been thoroughly researched in more than 100 years, yet there are still operational challenges that have not been fully resolved. Such a challenge is the control of filamentous bulking caused by the overgrowth of certain filamentous bacteria. In this study, we tested different mitigation strategies to reduce filamentous bulking, caused by two common filamentous genera found in full-scale water resource recovery facilities (WRRF), Candidatus Microthrix and Candidatus Amarolinea. PAX dosing, ozone addition, hydrocyclone implementation, and the addition of nano-coagulants were tested as mitigation strategies in four parallel treatment lines in a full-scale WRRF over three consecutive years. Unexpectedly, the activated sludge settleability was not affected by any of the mitigation strategies. Some of the strategies appeared to have a strong mitigating effect on the two filamentous species. However, detailed analyses of the microbial communities revealed strong recurrent seasonal variations in all four lines, including the control line which masked the real effect. After removing the effect of the seasonal variation by using a time-series decomposition approach, it was clear that the filamentous bacteria were mostly unaffected by the mitigation strategies. Only PAX dosing had some effect on Ca. Microthrix, but only on one species, Ca. Microthrix subdominans, and not on the most common Ca. Microthrix parvicella. Overall, our study shows the importance of long-term monitoring of microbial communities at species level to understand the normal seasonal pattern to effectively plan and execute full-scale experiments. Moreover, the results highlight the importance of using parallel reference treatment lines when evaluating the effect of mitigation strategies in full-scale treatment plants.

Phosphorous removal and recovery from urban wastewater: Current practices and new directions

Phosphate rocks are an irreplaceable resource to produce fertilizers, but their availability will not be enough to meet the increasing demands of agriculture for food production. At the same time, the accumulation of phosphorous discharged by municipal wastewater treatment plants (WWTPs) is one of the main causes of eutrophication. In a perspective of circular economy, WWTPs play a key role in phosphorous management. Indeed, phosphorus removal and recovery from WWTPs can both reduce the occurrence of eutrophication and contribute to meeting the demand for phosphorus-based fertilizers. Phosphorous removal and recovery are interconnected phases in WWTP with the former generally involved in the mainstream treatment, while the latter on the side streams. Indeed, by reducing phosphorus concentration in the WWTP side streams, a further improvement of the overall phosphorus removal from the WWTP influent can be obtained. Many studies and patents have been recently focused on treatments and processes aimed at the removal and recovery of phosphorous from wastewater and sewage sludge. Notably, new advances on biological and material sciences are constantly put at the service of conventional or unconventional wastewater treatments to increase the phosphorous removal efficiency and/or reduce the treatment costs. Similarly, many studies have been devoted to the development of processes aimed at the recovery of phosphorus from wastewaters and sludge to produce fertilizers, and a wide range of recovery percentages is reported as a function of the different technologies applied (from 10-25% up to 70-90% of the phosphorous in the WWTP influent). In view of forthcoming and inevitable regulations on phosphorous removal and recovery from WWTP streams, this review summarizes the main recent advances in this field to provide the scientific and technical community with an updated and useful tool for choosing the best strategy to adopt during the design or upgrading of WWTPs.

Global warming readiness: Feasibility of enhanced biological phosphorus removal at 35 °C

Recent research has shown enhanced biological phosphorus removal (EBPR) from municipal wastewater at warmer temperatures around 30 °C to be achievable in both laboratory-scale reactors and full-scale treatment plants. In the context of a changing climate, the feasibility of EBPR at even higher temperatures is of interest. We operated two lab-scale EBPR sequencing batch reactors for > 300 days at 30 °C and 35 °C, respectively, and followed the dynamics of the communities of polyphosphate accumulating organisms (PAOs) and competing glycogen accumulating organisms (GAOs) using a combination of 16S rRNA gene metabarcoding, quantitative PCR and fluorescence in situ hybridization analyses. Stable and nearly complete phosphorus (P) removal was achieved at 30 °C; similarly, long term P removal was stable at 35 °C with effluent PO₄^3-_P concentrations < 0.5 mg/L on half of all monitored days. Diverse and abundant Candidatus Accumulibacter amplicon sequence variants were closely related to those found in temperate environments, suggesting that EBPR at this temperature does not require a highly specialized PAO community. A slow-feeding strategy effectively limited the carbon uptake rates of GAOs, allowing PAOs to outcompete GAOs at both temperatures. Candidatus Competibacter was the main GAO, along with cluster III Defluviicoccus members. These organisms withstood the slow-feeding regime, suggesting that their bioenergetic characteristics of carbon uptake differ from those of their tetrad-forming relatives. Comparative cycle studies revealed higher carbon and P cycling activity of Ca. Accumulibacter when the temperature was increased from 30 °C to 35 °C, implying that the lowered P removal performance at 35 °C was not a direct effect of temperature, but a result of higher metabolic rates of carbon (and/or P) utilization of PAOs and GAOs, the resultant carbon deficiency, and escalated community competition. An increase in the TOC-to-PO₄^3--P ratio (from 25:1 to 40:1) effectively eased the carbon deficiency and benefited PAOs. In general, a slow-feeding strategy and sufficiently high carbon input benefited a high and stable EBPR at 35 °C, representing basic conditions suitable for full-scale treatment plants experiencing higher water temperatures.

Quantification of Biologically and Chemically Bound Phosphorus in Activated Sludge from Full-Scale Plants with Biological P-Removal

Phosphorus (P) is present in activated sludge from wastewater treatment plants in the form of metal salt precipitates, extracellular polymeric substances, or bound into the biomass, for example, as intracellular polyphosphate (poly-P). Several methods for a reliable quantification of the different P-fractions have recently been developed, and this study combines them to obtain a comprehensive P mass-balance of activated sludge from four enhanced biological phosphate removal (EBPR) plants. Chemical characterization by ICP-OES and sequential P fractionation showed that chemically bound P constituted 38-69% of total P, most likely in the form of Fe, Mg, or Al minerals. Raman microspectroscopy, solution state ³¹P NMR, and ³¹P MAS NMR spectroscopy applied before and after anaerobic P-release experiments, were used to quantify poly-P, which constituted 22-54% of total P and was found in approximately 25% of all bacterial cells. Raman microspectroscopy in combination with fluorescence in situ hybridization was used to quantify poly-P in known polyphosphate-accumulating organisms (PAO) (Tetrasphaera, Candidatus Accumulibacter, and Dechloromonas) and other microorganisms known to possess high level of poly-P, such as the filamentous Ca. Microthrix. Interestingly, only 1-13% of total P was stored by unidentified PAO, highlighting that most PAOs in the full-scale EBPR plants investigated are known.

New insight into filamentous sludge bulking: Potential role of AHL-mediated quorum sensing in deteriorating sludge floc stability and structure

The microcosmic mechanisms underlying filamentous bulking remain unclear. The role of extracellular polymeric substances (EPS) governed by quorum sensing (QS) in deteriorating sludge floc stability and structure during filamentous bulking and the feasibility of using quorum quenching (QQ) to maintain sludge floc stability and structure and sludge settling were investigated in this study. The results indicated that the concentration of C₆HSL increased from 22.08±3.22 ng/g VSS to 81.42±5.98 ng/g VSS during filamentous bulking. The filamentous bacteria gradually evolved the hdtS gene related to the synthesis of C₆HSL with increases in the population density. Triggered QS by filamentous bacteria proliferation induced variation in the composition and structure of EPS within the sludge flocs. The proteins (PN) content of the EPS increased evidently from 40.06 ± 2.41 mg/g VSS to 110.32 ± 4.32 mg/g VSS, and the polysaccharides (PS) content slightly increased during filamentous bulking. The upregulated proteins in the EPS led to a decrease in the relative hydrophobicity of the sludge and an increase in negative surface charge. The α-helix/(β-sheet+random coil) ratio evidently increased from 0.76 to 0.99 during filamentous bulking, revealing that the proteins were tightly structured, which prevented the exposure of inner hydrophobic groups. The total energy of the interaction (W_T) between bacteria increased during sludge bulking, which resulted in the weakening of sludge aggregation. Variation in the physicochemical properties of EPS induced by QS in the filamentous bacteria markedly restrained adhesion between the filamentous bacteria and floc-forming bacteria. The production of PN in the EPS and the expression of the hdtS gene were inhibited by vanillin, which served as a QS inhibitor. The W_T between bacteria with 50 mg/L of vanillin basically did not change. Filamentous bulking was significantly inhibited by the addition of vanillin. Therefore, QQ is a potential strategy for the prevention and control of filamentous bulking. This study provides new information regarding the microcosmic mechanisms of filamentous bulking.

A Multi-View Image Feature Fusion Network Applied in Analysis of Aeration Velocity for WWTP

The instability of the aeration system brings a significant challenge to the management of wastewater treatment plants (WWTP). Using image recognition methods to monitor aeration conditions accurately and enhance management efficiency is a promising way to solve this problem. To improve the efficiency of aeration condition identification and provide support for troubleshooting, we propose a method for aeration velocity condition identification based on a multi-view image feature fusion network (MVNN). Firstly, an experimental platform for simulating aeration tanks is established, and two cameras are used to acquire aeration images from different perspectives. Secondly, an image data set with 10 aeration velocity gradients is constructed and applied to the network’s training. Finally, the MVNN is used to extract and fuse the features of aeration images, and the model’s performance is evaluated on the dataset. Experiments show that the average accuracy of the method is over 98.3%, and the AUC of aeration identification is above 0.98, which indicates that the model has the potential for practical application in WWTP.

Candidatus Kaistella beijingensis sp. nov., Isolated from a Municipal Wastewater Treatment Plant, Is Involved in Sludge Foaming

Biological foaming (or biofoaming) is a frequently occurring problem in wastewater treatment plants (WWTPs) and is attributed to the overwhelming growth of filamentous bulking and foaming bacteria (BFB). Biological foaming has been intensively investigated, with BFB like Microthrix and Skermania having been identified from WWTPs and implicated in foaming. Nevertheless, studies are still needed to improve our understanding of the microbial diversity of WWTP biofoams and how microbial activities contribute to foaming. In this study, sludge foaming at the Qinghe WWTP of China was monitored, and sludge foams were investigated using culture-dependent and culture-independent microbiological methods. The foam microbiomes exhibited high abundances of Skermania, Mycobacterium, Flavobacteriales, and Kaistella. A previously unknown bacterium, Candidatus Kaistella beijingensis, was cultivated from foams, its genome was sequenced, and it was phenotypically characterized. Ca. K. beijingensis exhibits hydrophobic cell surfaces, produces extracellular polymeric substances (EPS), and metabolizes lipids. Ca. K. beijingensis abundances were proportional to EPS levels in foams. Several proteins encoded by the Ca. K. beijingensis genome were identified from EPS that was extracted from sludge foams. Ca. K. beijingensis populations accounted for 4 to 6% of the total bacterial populations in sludge foam samples within the Qinghe WWTP, although their abundances were higher in spring than in other seasons. Cooccurrence analysis indicated that Ca. K. beijingensis was not a core node among the WWTP community network, but its abundances were negatively correlated with those of the well-studied BFB Skermania piniformis among cross-season Qinghe WWTP communities. IMPORTANCE Biological foaming, also known as scumming, is a sludge separation problem that has become the subject of major concern for long-term stable activated sludge operation in decades. Biological foaming was considered induced by foaming bacteria. However, the occurrence and deterioration of foaming in many WWTPs are still not completely understood. Cultivation and characterization of the enriched bacteria in foaming are critical to understand their genetic, physiological, phylogenetic, and ecological traits, as well as to improve the understanding of their relationships with foaming and performance of WWTPs.

Efficiency of sulfamethoxazole removal from wastewater using aerobic granular sludge: influence of environmental factors

The effects of adsorption, sulfamethoxazole (SMX) content, chemical oxygen demand (COD), and dissolved oxygen (DO) are recognized to be crucial for SMX removal in the aerobic granular sludge (AGS) system. Therefore, we investigated the impact of adsorption and these three different environmental factors on the SMX removal loading rate and removal efficiency of an AGS system, and determined the differences in microbial community composition under different environmental conditions. Adsorption was not the main SMX removal mechanism, as it only accounted for 5% of the total removal. The optimal SMX removal conditions were obtained for AGS when the COD, DO, and SMX concentrations were 600 mg/L, 8 mg/L, and 2,000 µg/L, respectively. The highest SMX removal efficiency was 93.53%. Variations in the three environmental factors promoted the diversity and changes of microbial communities in the AGS system. Flavobacterium, Thauera, and norank_f_Microscillaceae are key microorganisms in the AGS system. Thauera, and norank_f_Microscillaceae were sensitive to increases in SMX concentrations and beneficial for degrading high SMX concentrations. In particular, Flavobacterium abundances gradually decreased with increasing SMX concentrations.

Bacteriophage ecology in biological wastewater treatment systems

Biological wastewater treatment (BWT) is currently the most widely applied approach for treating wastewater. The performance of BWT systems depends on the complex microbial communities they support. Although bacteriophages (phages), which are the viruses that infect prokaryotes, are recognized as the most abundant life entities, understanding of their ecological roles in BWT systems remains limited. Here, we review recent progress in phage-associated researches in BWT systems, including the interactions between phage and host, polyvalent phages, the influence of phage activity on BWT performance, and the potential applications of phage-based control for sludge bulking/foaming and pathogens. The challenges and perspectives of phage ecology are also outlined, which are expected to provide implications for future research and applications.Key points• Phage-host interactions in BWT systems are summarized• Impacts of phage activities on BWT performance• Potential applications of phages in BWT systems.

Low Global Diversity of Candidatus Microthrix, a Troublesome Filamentous Organism in Full-Scale WWTPs

Candidatus Microthrix is one of the most common bulking filamentous microorganisms found in activated sludge wastewater treatment plants (WWTPs) across the globe. One species, Ca. M. parvicella, is frequently observed, but global genus diversity, as well as important aspects of its ecology and physiology, are still unknown. Here, we use the MiDAS ecosystem-specific 16S rRNA gene database in combination with amplicon sequencing of Danish and global WWTPs to investigate Ca. Microthrix spp. diversity, distribution, and factors affecting their global presence. Only two species were abundant across the world confirming low diversity of the genus: the dominant Ca. M. parvicella and an unknown species typically present along with Ca. M. parvicella, although usually in lower abundances. Both species were mostly found in Europe at low-to-moderate temperatures and their growth was favored in municipal WWTPs with advanced process designs. As no isolate is available for the novel species, we propose the name "Candidatus Microthrix subdominans." Ten high-quality metagenome-assembled genomes recovered from Danish WWTPs, including 6 representing the novel Ca. M. subdominans, demonstrated high genetic similarity between the two species with a likely preference for lipids, a putative capability to reduce nitrate and nitrite, and the potential to store lipids and poly-P. Ca. M. subdominans had a potentially more versatile metabolism including additional sugar transporters, higher oxygen tolerance, and the potential to use carbon monoxide as energy source. Newly designed fluorescence in situ hybridization probes revealed similar filamentous morphology for both species. Raman microspectroscopy was used to quantify the in situ levels of intracellular poly-P. Despite the observed similarities in their physiology (both by genomes and in situ), the two species showed different seasonal dynamics in Danish WWTPs through a 13-years survey, possibly indicating occupation of slightly different niches. The genomic information provides the basis for future research into in situ gene expression and regulation, while the new FISH probes provide a useful tool for further characterization in situ. This study is an important step toward understanding the ecology of Ca. Microthrix in WWTPs, which may eventually lead to optimization of control strategies for its growth in this ecosystem.

Distribution and diversity of filamentous bacteria in wastewater treatment plants exhibiting foaming of Taihu Lake Basin, China

Foaming caused by filamentous bacteria in activated sludge (AS) is a common phenomenon in municipal wastewater treatment plants (WWTPs) in Taihu Lake Basin of South China. In this study, total bacterial and filamentous bacterial communities were comprehensively characterized in AS and foams from eight municipal WWTPs by high-throughput sequencing technology. Results showed that alpha diversities of total bacterial communities in foams were obviously lower than those in AS samples. The bacterial community structures were significantly different between WWTPs rather than sample types (AS vs. foam). For most WWTPs, the Actinobacteria phylum was highly enriched in foams and the most abundant genera in foams were common mycolata. Sixteen filamentous bacteria were identified against the improved bulking and foaming bacteria (BFB) database. Abundance and composition of BFB in different WWTPs and different sample types were significantly different. 'Nostocoida limicola' I Trichococcus and Microthrix were generally dominant in AS samples. The dominant BFB in foams were associated with Microthrix, Skermania, Gordonia, and Mycobacterium. A new Defluviicoccus spp. in cluster III was identified in severe and continuous foams. Moreover, dominant BFB in stable and continuous foams with light level in one typical WWTP were diverse, even, and dynamic. Bacterial co-occurrence network analysis implied that the bacterial community of AS was more sensitive to disturbance than that of foam.

Factors influencing Candidatus Microthrix parvicella growth and specific filamentous bulking control: A review

Candidatus Microthrix parvicella has been frequently detected as the dominant filamentous bacteria in bulking sludge and thus seriously affects the stable operation of activated sludge processes. The extremely low growth rate of Ca. M. parvicella and its sensitivity to environmental variations greatly limit the development of effective techniques to control filamentous bulking. Based on previous investigations, a variety of restrictive substrates, operating and culture conditions, environmental factors and other potential inhibitors have varying degrees of impact on the growth of this microorganism. This review systematically summarizes the key factors affecting Ca. M. parvicella growth with a focus on the influencing mechanism. Recent filamentous bulking control strategies are also critically reviewed and discussed. Additionally, research needs for the next few years are proposed with the aim of establishing effective and specific control strategies for filamentous sludge bulking.

Effects of interchange ratio on sludge reduction and microbial community structures in an anaerobic/anoxic/oxic process with combined anaerobic side-stream reactor

Oxic-settling-anaerobic (OSA) process is effective in minimizing sludge production, by inserting an anaerobic side-stream reactor (ASSR) in the recycling bypass. Interchange ratio (IR), the quantity ratio of sludge entering the ASSR to the sludge in the main stream reactors, is one of the most important parameters for OSA process. In the present study, a laboratory-scale anaerobic/anoxic/oxic (A²/O) process combined with an ASSR (A²/O-ASSR) was operated for 366 days in parallel with a conventional A²/O process to investigate the effects of IR on sludge reduction. IR was assigned values of 5%, 8%, 10%, and 15%, and the A²/O-ASSR process achieved 14.0%, 16.0%, 24.1%, and 13.7% of sludge reduction, respectively. At the optimum IR of 10%, high through-put sequencing analysis showed that the microbes responsible for pollutant removal and ubiquitous in wastewater treatment remained predominant in the two systems, and slow-growing microbes related to hydrolysis, nitrogen and phosphorus removal increased in the A²/O-ASSR process, which probably played a key role in sludge reduction. 40.6-58.6% of sludge reduction was caused by sludge decay in the ASSR. The tiny amount of extracellular polymeric substance released in the A²/O-ASSR process was subthreshold to cause remarkable sludge reduction.

The microbial community in filamentous bulking sludge with the ultra-low sludge loading and long sludge retention time in oxidation ditch

Sludge bulking is a major problem that restricts the development of the activated sludge process. The microbial community responsible for sludge bulking varies depending on water quality and operational conditions. This study analysed the microbial community of bulking sludge in oxidation ditch with ultra-low sludge loading and long sludge retention time using high-throughput sequencing. The study found that the relative abundance of bacterial genus Saprospiraceae_norank was the highest in bulking sludge, reaching 13.39-28.83%, followed by Comamonadaceae_unclassified, Ardenticatenia_norank and Tetrasphaera, with the relative abundance of 4.59-11.08%, 0.52-16.60% and 0.17-8.92% respectively. In contrast, the relative abundance of bacteria that easily caused sludge bulking including Microthrix (0.54-2.47%), Trichococcus (0.32-1.71%), Gordonia (0.14-1.28%), and Thiothrix (0.01-0.06%) were relatively low. Saprospiraceae_norank was predominant and induced sludge bulking in oxidation ditch. The relative abundance of fungal genus Trichosporon was the highest in bulking sludge, reaching 16.95-24.98%, while other fungal genera were Saccharomycetales_unclassified (5.59-14.55%), Ascomycota_norank (1.45-13.51%), Galactomyces (5.23-11.23%), and Debaryomyces (7.69-9.42%), whereas Trichosporon was the dominant fungal genus in bulking sludge. This study reported that excessive Saprospiraceae_norank can induce sludge bulking for the first time, which provides important knowledge to control sludge bulking.

Factors affecting SVI in small scale WWTPs

This paper analyses factors associated with bulking in 195 small scale wastewater treatment plants (WWTPs) in Estonia. Operational data from each plant were collected and analysed statistically. The key factors associated with bulking were infiltration into sewage pipes, the type and purpose of process reactor, operational practices and influent characteristics. Both anaerobic fraction and volumetric fraction of the anaerobic reactor compared to the aerobic reactor resulted in a positive correlation with sludge volume index (SVI) <150 ml/g values. Good operation and maintenance practice as well as an operator's competence play a crucial role in bulking prevention. Using the 30 minute settling test (V₃₀) as the single process control parameter can mislead an operator's judgement in process control strategies and cause effluent violations. Misjudgements in process control decisions can lead to unwanted conditions in small WWTPs (e.g. excessive chemical addition favoured bulking). Use of instrumentation, control and automation helped to keep the process conditions more stable and reduce the probability of bulking. Analyses of variance showed that the factors associated with Microthrix parvicella growth were long solids retention time (SRT), low food-to-microorganism ratio (F/M) and lack of carbon content compared against nitrogen and phosphorus contents.

The impact of powdered keramsite on activated sludge and wastewater treatment in a sequencing batch reactor

As the efficiency of biological wastewater treatment in activated sludge systems can be increased if powdered mineral materials are added, the work described here investigated the effect of powdered keramsite on activated sludge and wastewater treatment in a sequencing batch reactor on the laboratory scale. Specifically, experiments were carried out on two sequencing batch reactors, with the reference system being the classical SBR with conventional activated sludge, while the second system involved a K-SBR (Keramsite - Sequencing Batch Reactor) with powdered keramsite added with a view to enhancing the effectiveness of the activated sludge. The concentration of powdered keramsite in the latter reactor was maintained at 0.75 g/L. The results confirmed the greater efficiency of biological wastewater treatment processes where activated sludge was supported by the keramsite.

Monitoring, isolation and characterization of Microthrix parvicella strains from a Chinese wastewater treatment plant

Microthrix parvicella is a filamentous bacterium that frequently causes severe bulking events in wastewater treatment plants (WWTPs) worldwide. In this study, sludge properties and dynamics of filamentous bacteria in a Beijing WWTP seasonally suffering from M. parvicella bulking were continuously monitored over a duration of 15 months, and the correlations between M. parvicella and operating parameters were evaluated. The predominance of M. parvicella was observed at low temperatures (14-18.8 °C) with the relative abundance of around 30% (estimated by both qPCR and FISH analysis). Using micromanipulation technology, 545 filaments of M. parvicella were micromanipulated from bulking sludge (SVI > 180 mL g^-1) on six different media. After 3-month purification and enrichment, six strains, phylogenetically closely related to Candidatus Microthrix parvicella, were successfully acquired on R₂A medium (20 °C) in pure cultures. Considering the limitation and extremely slow growth rate of M. parvicella filaments, newly isolated strains represent valuable sources for further investigations on the physiology and behavior of this filamentous bacterium, with the focus on the establishment of bulking control strategy.

Isolation and Genomic Characterization of an Acinetobacter johnsonii Bacteriophage AJO2 From Bulking Activated Sludge

A novel Podoviridae lytic phage AJO2, specifically infecting Acinetobacter johnsonii, was isolated from bulking activated sludge. The one-step growth experiment showed that the latent period and burst size of AJO2 were estimated to be 30 min and 78.1 phages per infected cell, respectively. The viability test indicated that neutral conditions (pH 6-8) were table for AJO2 survival, while it was sensitive to high temperature (≥60°C) and ultraviolet treatment (254 nm). Genomic sequencing revealed that the AJO2 had a linearly permuted, double-stranded (ds) DNA consisting of 38,124 bp, with the G+C content of 41 mol%. A total of 58 putative open reading frames (ORFs), 11 pairs of repeats and 11 promoters were identified. The AJO2 genome had a modular gene structure which shared some similarities to those of A. baumanii phages. However, genomic comparative analysis revealed many differences among them, and novel genes were identified in the AJO2 genome. These results contribute to subsequent researches on the interaction between bacteriophages and hosts in wastewater treatment, especially during the bulking period. Additionally, the newly isolated phage could be a good candidate as a therapeutic agent to control nosocomial infections caused by A. johnsonii.

Control strategy for filamentous sludge bulking: Bench-scale test and full-scale application

Sludge bulking caused by the overgrowth of filamentous bacteria, especially Microthrix parvicella, has been observed in WWTPs worldwide during low-temperature periods. In this study, the impacts of sludge load on the in situ growth of M. parvicella and sludge settleability were first evaluated at 15 °C over a period of 500 d using a bench-scale anaerobic-anoxic-aerobic reactor fed with raw sewage from a full-scale WWTP. When the reactor was operated at a sludge load of 0.07 ± 0.015 kg Chemical Oxygen Demand (COD) (kg MLSS·d)^-1 for 120 d, the sludge volume index (SVI) increased gradually from 85 mL g^-1 to 157 mL g^-1, and the abundance of M. parvicella quantified by qPCR and FISH methods also increased from 0.42% to 4.63% and 1.56%-13.59%, respectively. When the sludge load was further reduced to 0.04 ± 0.004 kg COD (kg MLSS·d)^-1, the SVI value varied in a narrow range of 135-164 mL g^-1 over a duration of 280 d, while the M. parvicella abundance increased to the maximum values of 10.13% (qPCR) and 18.53% (FISH), respectively. When the sludge load was increased to 0.12 ± 0.016 kg COD (kg MLSS·d)^-1, filamentous abundance and SVI were reduced to 1.06% (qPCR) and 105 mL g^-1 within 100 d, suggesting that it might be possible to control the growth of M. parvicella by keeping the sludge load above 0.1 kg COD (kg MLSS·d)^-1. The feasibility of the strategy was further validated in the same WWTP. It was found that the SVI and filamentous abundance in winter were successfully controlled for two successive years at below 120 mL g^-1 and 7% (FISH), respectively, when the sludge load was maintained at 0.14 ± 0.04 kg COD (kg MLSS·d)^-1 by adjusting sludge discharge, proving that this sludge-load-based strategy could be an efficient approach to control filamentous bulking.

Seasonal occurrence, source evaluation and ecological risk assessment of polycyclic aromatic hydrocarbons in industrial and agricultural effluents discharged in Wadi El Bey (Tunisia)

Polycyclic aromatic hydrocarbons are of great concern due to their persistence, bioaccumulation and toxic properties. The occurrence, source and ecological risk assessment of 26 polycyclic aromatic hydrocarbons in industrial and agricultural effluents affecting the Wadi El Bey watershed were investigated by means of gas chromatographic/mass spectrometric analysis (GC/MS). Total PAHs (∑ 26 PAH) ranged from 1.21 to 91.7 µg/L. The 4- and 5-ring compounds were the principal PAHs detected in most of 5 sites examined. Diagnostic concentration ratios and molecular indices were performed to identify the PAH sources. Results show that PAHs could originate from petrogenic, pyrolytic and mixed sources. According to the ecotoxicological assessment, the potential risk associated with PAHs affecting agricultural and industrial effluents ranged from moderate to high for both aquatic ecosystem and human health. The toxic equivalency factor (TEF) approach indicated that benzo[a]pyrene and benz[a]anthracene were the principal responsible for carcinogenic power of samples.

Linking nitrous oxide emissions to population dynamics of nitrifying and denitrifying prokaryotes in four full-scale wastewater treatment plants

Ammonia-oxidizing bacteria (AOB), ammonia oxidizing archaea (AOA) and N₂O-reducing denitrifiers were measured by quantitative real-time PCR (qPCR) in activated sludge samples from four full-scale wastewater treatment plants (WWTPs) in South Spain, and their abundances were linked to the generation of N₂O in the samples using multivariate analysis (Non-metric multidimensional scaling, MDS, and BIO-ENV). The average abundances of AOA remained in similar orders of magnitude in all WWTPs (10⁶ copies amoA/L activated sludge mixed liquor), while significant differences were detected for AOB (10⁵-10⁹copies amoA/L) and N₂O-reducers (10⁷-10¹⁰copies nosZ/L). Average N₂O emissions measured in activated sludge samples ranged from 0.10 ± 0.05 to 6.49 ± 8.89 mg N₂O-N/h/L activated sludge, and were strongly correlated with increased abundances of AOB and lower counts of N₂O-reducers. A significant contribution of AOA to N₂O generation was unlikely, since their abundance correlated negatively to N₂O emissions. AOB abundance was favoured by higher NO₃^- and NO₂^-concentrations in the activated sludge.

Factors affecting the growth of Microthrix parvicella: Batch tests using bulking sludge as seed sludge

Sludge bulking caused by the overgrowth of filamentous bacteria, particularly Microthrix parvicella, is one of the challenges for the stable operation of municipal wastewater treatment plants (WWTPs). The driving forces for the development of sludge bulking, however, have not been well understood because of the extremely low growth rate of M. parvicella. In this study, batch experiments were performed using bulking sludge (sludge volume index (SVI), around 185mLg^-1) from a full-scale WWTP as the seed sludge to investigate the influences of carbon source, anaerobic/aerobic alternation condition and temperature on the growth of M. parvicella. The qPCR results showed that the use of oleic acid as carbon source, anaerobic/aerobic alternation treatment and low temperature (13°C) were favorable conditions for maintaining the dominance of M. parvicella in the tested activated sludge. Under these conditions, the SVI values remained at comparatively high values of 170.5mLg^-1, 162.5mLg^-1 and 129.5mLg^-1 after operation for approximately two months, and the relative abundances of M. parvicella were 36.7%, 9.74% and 34.07%, respectively, in comparison with the initial values of 33.04%, 29.29% and 54.66%. However, the relative abundances of M. parvicella decreased to 0.86-4.44%, 0.7% and 4.94%, respectively, under the conditions of other carbon sources, aerobic-only treatment and a temperature of 20°C. The FISH analysis gave a similar result. This study was performed with mixed sludge under controlled operating conditions, which provided a valuable information for the pure culture of M. parvicella and further investigations on its physiology and metabolism.

Phage-host associations in a full-scale activated sludge plant during sludge bulking

Sludge bulking, a notorious microbial issue in activated sludge plants, is always accompanied by dramatic changes in the bacterial community. Despite large numbers of phages in sludge systems, their responses to sludge bulking and phage-host associations during bulking are unknown. In this study, high-throughput sequencing of viral metagenomes and bacterial 16S rRNA genes were employed to characterize viral and bacterial communities in a sludge plant under different sludge conditions (sludge volume index (SVI) of 180, 132, and 73 ml/g). Bulking sludges (SVI > 125 ml/g) taken about 10 months apart exhibited similar bacterial and viral composition. This reflects ecological resilience of the sludge microbial community and indicates that changes in viral and bacterial populations correlate closely with each other. Overgrowth of "Candidatus Microthrix parvicella" led to filamentous bulking, but few corresponding viral genotypes were identified. In contrast, sludge viromes were characterized by numerous contigs associated with "Candidatus Accumulibacter phosphatis," suggesting an abundance of corresponding phages in the sludge viral community. Notably, while nitrifiers (mainly Nitrosomonadaceae and Nitrospiraceae) declined significantly along with sludge bulking, their corresponding viral contigs were identified more frequently and with greater abundance in the bulking viromes, implying that phage-mediated lysis might contribute to the loss of autotrophic nitrifiers under bulking conditions.

Microbial community analysis of simultaneous ammonium removal and Fe3+ reduction at different influent ammonium concentrations

This study investigates the impacts of influent ammonium concentrations on the microbial community in immobilized heterotrophic ammonium removal system. Klebsiella sp. FC61, the immobilized species, has the ability to perform simultaneous ammonium removal and Fe³⁺ reduction. It was found that average ammonium removal rate decreased from 0.308 to 0.157 mg/L/h, as the influent NH₄⁺-N was reduced from 20 to 10 mg/L. Meanwhile, at a total Fe³⁺ concentration of 20 mg/L, the average Fe³⁺ reduction removal efficiency and rate decreased from 44.61% and 0.18 mg/L/h, to 27.10% and 0.11 mg/L/h, respectively. High-throughput sequencing was used to observe microbial communities in bioreactor Samples B1, B2, and B3, after exposure to different influent NH₄⁺-N conditions. Results show that higher influent NH₄⁺-N concentrations increased microbial richness and diversity and that Klebsiella sp. FC61 play a functional role in the simultaneous removal of NH₄⁺-N and Fe³⁺ reduction in bioreactor systems.

Temporal dynamics of bacterial communities and predicted nitrogen metabolism genes in a full-scale wastewater treatment plant

Dynamics of bacterial communities and nitrogen metabolism genes in a full-scale WWTP as revealed by Illumina sequencing and PICRUSt.

Detailed comparison of bacterial communities during seasonal sludge bulking in a municipal wastewater treatment plant

In this study, pyrosequencing combined with clone library analysis, qPCR, and fluorescent in situ hybridization (FISH) were performed to identify detailed changes of bacterial and filamentous bacterial communities in activated sludge (AS) in 3 types of typical AS samples: sludge bulking (B-AS), excessive bulking (EB-AS), and non-bulking (N-AS). Sludge bulking resulted in a decrease in total bacterial numbers from (6.4 ± 0.18) × 10⁸ gene copies/mL in N-AS to (2.4 ± 0.22) × 10⁸ in EB-AS and a decrease in bacterial diversity from 2757 OTUs in N-AS to 2217 OTUs in EB-AS. With the occurrence of sludge bulking, Actinobacteria and Firmicutes increased sharply, whereas Proteobacteria, which was the predominant phylum in N-AS, decreased markedly. In addition, Nitrospirae, a major lineage of the nitrite-oxidizing bacteria, had quite a low abundance in EB-AS (0.15%), while it was relatively high in N-AS (1.17%). On the other hand, filamentous bacteria accounted for 28.77% and 5.72% of total sequences in EB-AS and N-AS, respectively. More interestingly, 11 types of filamentous bacteria were always present in 3 types of typical AS samples from different stages of sludge bulking, and most of them enriched in EB-AS compared to N-AS. It is noteworthy that, in addition to the frequently reported filamentous bacteria such as Candidatus M. parvicella and Tetrasphaera, novel filamentous species of Trichococcus might exist in this bulking WWTP. Our results reveal that sludge bulking are derived from diverse taxa, which expands previous understanding and provides new insight into the underlying complications of the bulking phenomenon in AS.

New insight into influence of mechanical stirring on membrane fouling of membrane bioreactor: Mixed liquor properties and hydrodynamic conditions

Although membrane bioreactor is widely used in wastewater treatment, the problem of membrane fouling remains to be resolved. This paper focused on the influence of mechanical stirring on membrane fouling. Ammonium removal decreased with viscous bulking when stirring rates slowed down. Trans-membrane pressure increased more rapidly when the stirring rate decreased. The resistance of the gel layer increased significantly under low stirring rates, which indicated that the fouling rates of MBR in different stages were attributed to gel layer variation. The proportion of small particles increased when stirring rates slowed down. Furthermore, 16S rRNA gene amplicon sequencing showed that Proteobacteria and Actinobacteria were dominant in the mixed liquor. The relative abundance of Actinobacteria increased from 41% to 50% in the entire experiment. The computational fluid dynamics model was used to simulate the fluid flow characteristics. The model indicated velocities and directions of the fluid flow changes with different stirring rates.

Seasonal Changes in Bacterial Communities Cause Foaming in a Wastewater Treatment Plant

Bio-foaming is a major problem in solid separation in activated sludge (AS) wastewater treatment systems. Understanding the changes in bacterial communities during sludge foaming is vital for explaining foam formation. Changes in bacterial communities in the foam, corresponding foaming AS, and non-foaming AS in a seasonal foaming wastewater treatment plant (WWTP) in Northern China were investigated by high-throughput pyrosequencing and molecular quantification-based approaches. We found that bacterial communities of the foam and the corresponding foaming AS were similar but markedly different from those of the non-foaming AS. Actinobacteria was the predominant phylum in the foam and the corresponding foaming AS, whereas Proteobacteria was predominant in the non-foaming AS. Similar to the results of most previous studies, our results showed that Candidatus "Microthrix parvicella" was the predominant filamentous bacteria in the foam and the corresponding foaming AS and was significantly enriched in the foam compared to the corresponding foaming AS. Its abundance decreased gradually with a slow disappearance of sludge foaming, indicating that its overgrowth had a direct relationship with sludge foaming. In addition to Candidatus M. parvicella, Tetrasphaera and Trichococcus might play a role in sludge foaming, because they supported the changes in AS microbial ecology for foam formation. The effluent water quality of the surveyed plant remained stable during the period of sludge foaming, but the microbial consortia responsible for nitrogen and phosphorus transformation and removal markedly changed compared to that in the non-foaming AS. This study adds to the previous understanding of bacterial communities causing foaming in WWTPs.

Bacterial communities in full-scale wastewater treatment systems

Bacterial metabolism determines the effectiveness of biological treatment of wastewater. Therefore, it is important to define the relations between the species structure and the performance of full-scale installations. Although there is much laboratory data on microbial consortia, our understanding of dependencies between the microbial structure and operational parameters of full-scale wastewater treatment plants (WWTP) is limited. This mini-review presents the types of microbial consortia in WWTP. Information is given on extracellular polymeric substances production as factor that is key for formation of spatial structures of microorganisms. Additionally, we discuss data on microbial groups including nitrifiers, denitrifiers, Anammox bacteria, and phosphate- and glycogen-accumulating bacteria in full-scale aerobic systems that was obtained with the use of molecular techniques, including high-throughput sequencing, to shed light on dependencies between the microbial ecology of biomass and the overall efficiency and functional stability of wastewater treatment systems. Sludge bulking in WWTPs is addressed, as well as the microbial composition of consortia involved in antibiotic and micropollutant removal.

Evaluating the influence of wastewater composition on the growth of Microthrix parvicella by GCxGC/qMS and real-time PCR

This study underlines the significance of long chain fatty acid (LCFA) content in wastewater influents as an influencing factor promoting the growth of Candidatus 'Microthrix parvicella' (M. parvicella), the most common filamentous bacteria causing foam in activated sludge systems worldwide. Quantification of M. parvicella by real-time polymerase chain reaction (real-time PCR) and analysis of LCFAs by means of two-dimensional gas chromatography coupled with mass spectrometry (GCxGC/qMS), involving solid phase micro-extraction (SPME) to enhance sensitivity, were combined for the first time as a monitoring tool. The results indicate a highly significant correlation between the abundance of M. parvicella and the total LCFA loading (r = 0.96) and linolenic acid C18:3 (r = 0.98) in particular. Additionally, comparison of slope values for the direct correlations of all significant LCFAs found in the analyses showed that the influence of LCFAs on M. parvicella growth increases with an increasing degree of unsaturation of carbon chains. These findings suggest that by removing lipid compounds from the incoming waters, substrate availability would be limited for M. parvicella.