Ecophysiology of polyphosphate-accumulating organisms and glycogen-accumulating organisms in a continuously aerated enhanced biological phosphorus removal process

Mechanisms of endogenous and exogenous partial denitrification in response to different carbon/nitrogen ratios: Transcript levels, nitrous oxide production, electron transport

Nitrite as an important substrate for Anammox can be provided by partial denitrification (PD). In this study, endogenous partial denitrification (EdPD) and exogenous partial denitrification (ExPD) sludge were domesticated and their nitrite transformation rate reached 74.4% and 83.4%, respectively. The impact of four carbon/nitrogen (C/N) ratios (1.5, 3.0, 5.0 and 6.0) on nitrous oxide (N2O) emission and denitrification functional genes expression in both PD systems were investigated. Results showed that elevated C/N ratios enhanced most denitrification genes expression, but in EdPD, high nitrite levels suppressed nosZ genes expression (from 9.4% to 1.4%), leading to increased N2O emission (0 to 3.4%). EdPD also exhibited lower electron transfer system activity, resulting in slower nitrogen oxide conversion efficiency and more stable nitrite accumulation compared to ExPD. These findings offer insights for optimizing PD systems under varying water quality conditions.

The phylogeny, ecology and ecophysiology of the glycogen accumulating organism (GAO) Defluviicoccus in wastewater treatment plants

This comprehensive review looks critically what is known about members of the genus Defluviicoccus, an example of a glycogen accumulating organism (GAO), in wastewater treatment plants, but found also in other habitats. It considers the operating conditions thought to affect its performance in activated sludge plants designed to remove phosphorus microbiologically, including the still controversial view that it competes with the polyphosphate accumulating bacterium Ca. Accumulibacter for readily biodegradable substrates in the anaerobic zone receiving the influent raw sewage. It looks at its present phylogeny and what is known about it's physiology and biochemistry under the highly selective conditions of these plants, where the biomass is recycled continuously through alternative anaerobic (feed); aerobic (famine) conditions encountered there. The impact of whole genome sequence data, which have revealed considerable intra- and interclade genotypic diversity, on our understanding of its in situ behaviour is also addressed. Particular attention is paid to the problems in much of the literature data based on clone library and next generation DNA sequencing data, where Defluviicoccus identification is restricted to genus level only. Equally problematic, in many publications no attempt has been made to distinguish between Defluviicoccus and the other known GAO, especially Ca. Competibacter, which, as shown here, has a very different ecophysiology. The impact this has had and continues to have on our understanding of members of this genus is discussed, as is the present controversy over its taxonomy. It also suggests where research should be directed to answer some of the important research questions raised in this review.

Advanced nitrogen removal from real municipal wastewater by multiple coupling nitritation, denitritation and endogenous denitritation with anammox in a single suspended sludge bioreactor

Achieving advanced nitrogen removal based on anammox for treating mainstream municipal wastewater in a single suspended sludge bioreactor is a challenging research topic. In this study, multiple coupling nitritation, denitritation and endogenous denitritation with anammox (PNA-(E)PDA) was simultaneously achieved in a 10 L step-feed bioreactor, which enhanced stable nitrogen removal. After 223 days of operation, the total nitrogen concentrations of the influent and effluent were 70.7 ± 6.1 and 4.3 ± 1.8 mg/L, respectively, when treating municipal wastewater even at C/N ratio of 2.24 with only 5 h of aerobic time (DO: 0.5-0.8 mg/L). After the evolution of nitritation/anammox to PNA-(E)PDA, the contribution of anammox to nitrogen removal increased to 78.6% and the anammox activity increased from 4.3 ± 0.2 to 15.2 ± 0.7 mg NH₄⁺-N/gVSS/d. qPCR results showed that the abundance of anammox bacteria increased from 4.1 × 10⁹ to 4.5 × 10¹⁰ copies/ (g VSS). High-throughput sequencing further revealed that the relative abundance of Candidatus Brocadia, the dominant anammox genus, increased from 0.09 to 0.46%. Based on the strong competitiveness of anammox on nitrite, this novel PNA-(E)PDA process provides a potential strategy for enriching anammox bacteria in municipal wastewater treatment plants.

An investigation into the optimal granular sludge size for simultaneous nitrogen and phosphate removal

An aerobic granular sludge (AGS) pilot plant fed with a mixture of acetate amended centrate and secondary effluent was used to investigate the optimal granule size range for simultaneous nitrification and denitrification (SND) and ortho-phosphate removal. The anaerobic phase was mixed to understand how AGS will perform if integrated with a continuous flow activated sludge system that cannot feed the influent through the settled sludge bed. Five different granule size fractions were taken from the pilot (operated at DO setpoint of 2mgO₂/L) and each size was subjected to activity tests in a well-controlled lab-scale AGS reactor at four dissolved oxygen (DO) concentrations of 1, 2, 3, and 4 mgO₂/L. The size fractions were: 212 - 600 µm, 600 - 1000 µm, 1000 - 1400 µm, 1400 - 2000 µm, and >2000 µm. The smallest size range (212 - 600 µm) had the highest nitrification and phosphate removal rates at DO setpoints from 1 - 3 mgO₂/L, which was attributed to the higher aerobic volume fraction in small granules and hence a higher abundance of phosphorus accumulating organisms (PAO) and ammonia oxidizing bacteria (AOB). In comparison, large granules (>1000 µm) had 1.4 - 4.7 times lower ammonia oxidation rates than the smallest size range, which aligned with their lower AOB abundance relative to granule biomass. The granules with the highest anoxic volume fraction had the highest abundance of nitrite reductase genes (nir gene) but did not show the highest specific nitrogen removal rate. Instead, smaller granules (212 - 600 and 600 - 1000 µm), which had a lower nir gene abundance, had the highest specific nitrogen removal rates (1.2 - 3.1 times higher than larger granules) across all DO values except at 4 mgO₂/L. At a DO setpoint of 4 mgO₂/L, nitrite production by ammonia oxidation (ammonia monooxygenase) exceeded nitrite reduction by nitrite reductase in granules smaller than 1000 µm, in addition, some denitrifying heterotrophs switched to oxygen utilization in deeper layers hence suppressing denitrification activity. At the DO range of 2 - 4 mg/L, granular size had a greater effect on nutrient removal than DO. Therefore, for AGS developed at an average DO setpoint of 2 mgO₂/L, selecting for size fractions in the range of 212 - 1000 µm and avoiding DO values higher than 3 mgO₂/L can achieve both a higher nitrogen removal capacity and energy savings. This study is the first to investigate the influence of different DO values on SND and biological phosphorus removal performance of different aerobic granular sludge sizes.

Development of novel denitrifying nitrite accumulation and phosphorus removal (DNAPR) process for offering an alternative pretreatment to achieve mainstream Anammox

For achieving mainstream anaerobic ammonium oxidation (Anammox), there is a need to achieve organic carbon and phosphorus removal meanwhile supplying nitrite (NO₂^--N). Based on this demand, a novel anaerobic/anoxic/aerobic operated denitrifying nitrite accumulation and phosphorus removal (DNAPR) process was proposed for treating synthetic municipal and nitrate (NO₃^--N) wastewaters simultaneously (volume ratio of 5:1). By adjusting influent composition, discharging anaerobic-end supernatant, shortening anoxic duration, and adding a short aerobic stage, DNAPR process achieved promising and stable nitrate-to-nitrite transformation (78.35%) and phosphorus removal (98.34%) performance. Moreover, effluent with chemical oxygen demand of 16.63 mg/L, nitrite of 54.16 mg/L, orthophosphate of 0.37 mg/L, and nitrite to ammonia ratio of 1.3 were finally obtained after 141-day operation. Microbiological analysis showed that Thauera (34.9%) and unclassified_f_Rhodobacteraceae (6.79%) were both responsible for DNAPR. Therefore, DNAPR, serving as promising alternative pretreatment, might possess significance for achieving mainstream Anammox.

Achieving deep-level nutrient removal via combined denitrifying phosphorus removal and simultaneous partial nitrification-endogenous denitrification process in a single-sludge sequencing batch reactor

The feasibility of coupling denitrifying phosphorus removal (DPR) with simultaneous partial nitrification-endogenous denitrification (SPNED) was investigated in a single-sludge sequencing batch reactor for deep-level nutrient removal from municipal and nitrate wastewaters. After 160-day operation, the DPR process simultaneously reduced most PO₄^3--P and NO₃^--N anoxically, and the SPNED process achieved further total nitrogen (TN) removal at low dissolved oxygen condition with TN removal efficiency of 90.8%. The effluent NH₄⁺-N, PO₄^3--P and TN concentrations were 1.0, 0.1 and 7.2 mg/L, respectively. Microbial analysis revealed that Dechloromonas (6.7%) dominated DPR process, whereas the gradually enriched Nitrosomonas (4.5%) and Candidatus Competibacter (6.8%) conducted SPNED process accompanied with sharply eliminated Nitrospirae (1.4%). Based on these findings, a novel strategy was proposed to achieve further nutrient removal in conventional treatment through integrating the DPR-SPNED process. As a result, ∼100% of extra carbon and ∼10% of oxygen consumptions would be reduced with satisfactory effluent quality.

Evaluating the potential for sustaining mainstream anammox by endogenous partial denitrification and phosphorus removal for energy-efficient wastewater treatment

This study demonstrated a novel process configuration for sustaining mainstream anammox by integrating the anammox and endogenous partial denitrification-and-phosphorus removal (EPDPR) in two-stage sequencing batch reactors (SBRs). In the EPDPR-SBR, high nitrate-to-nitrite transformation (68.2%) and P removal (99.3%) were achieved by adjusting the anaerobic/anoxic/aerobic durations and influent nitrate concentration, providing a suitable NO₂^--N/NH₄⁺-N (∼1.37) for subsequent anammox reaction. In the Anammox-SBR, ∼95% of TN was removed without external carbon and oxygen demands. Satisfactory effluent quality (∼6 mgTN/L and 0.2 mgP/L) achieved in the integrated EPDPR/anammox opens a new window towards the energy-efficient wastewater treatment. Microbial analysis further revealed that Dechloromonas (1.6-9.6%) and Candidatus Competibacter (6.4-5.8%) respectively conducted P removal and NO₂^--N production (79.2%) from NO₃^--N denitrification in the EPDPR-SBR, whereas Candidatus Kuenenia (7.0-29.7%) dominated NO₂^--N and NH₄⁺-N removal (91.3% and 99.5%) in the Anammox-SBR, with 10 genera identified as denitrifying bacteria (0.6-8.1%) further reduced 18.9% of the produced NO₃^--N.

Achieving advanced nitrogen removal from low C/N wastewater by combining endogenous partial denitrification with anammox in mainstream treatment

Successful application of mainstream anammox would be favorable for energy- and resource-efficient sewage treatment. This study presents a new strategy to achieve mainstream anammox, which combined with endogenous partial denitrification (EPD) for treating sewage wastewater. In this EPD-Anammox system, nitrite was stably produced by EPD with a nitrate-to-nitrite transformation ratio of 80%. Through adjusting the volume exchange ratio of EPD-reactor after anaerobic reaction, a suitable NO₂^--N/NH₄⁺-N ratio of ∼1.20 for anammox reaction was achieved. Further, results showed a stable, high nitrogen removal efficiency (90%) with an effluent total nitrogen of 5.8 mg N/L under low C/N (∼2.9). Anammox contributed 49.8% of the overall nitrogen removal owing to the steady nitrite supply from EPD. Denitrifying glycogen-accumulating organisms (GAOs, 36.6%) having potential for endogenous denitrification and Candidatus Brocadia (34.6%) were respectively dominated in the EPD-SBR and anammox-UASB and responsible for the high nitrite accumulation and anammox reaction.

Culture-Dependent and -Independent Identification of Polyphosphate-Accumulating Dechloromonas spp. Predominating in a Full-Scale Oxidation Ditch Wastewater Treatment Plant

The oxidation ditch process is one of the most economical approaches currently used to simultaneously remove organic carbon, nitrogen, and also phosphorus (P) from wastewater. However, limited information is available on biological P removal in this process. In the present study, microorganisms contributing to P removal in a full-scale oxidation ditch reactor were investigated using culture-dependent and -independent approaches. A microbial community analysis based on 16S rRNA gene sequencing revealed that a phylotype closely related to Dechloromonas spp. in the family Rhodocyclaceae dominated in the oxidation ditch reactor. This dominant Dechloromonas sp. was successfully isolated and subjected to fluorescent staining for polyphosphate, followed by microscopic observations and a spectrofluorometric analysis, which clearly demonstrated that the Dechloromonas isolate exhibited a strong ability to accumulate polyphosphate within its cells. These results indicate the potential key role of Dechloromonas spp. in efficient P removal in the oxidation ditch wastewater treatment process.

Characterisation of Phosphate Accumulating Organisms and Techniques for Polyphosphate Detection: A Review

Phosphate minerals have long been used for the production of phosphorus-based chemicals used in many economic sectors. However, these resources are not renewable and the natural phosphate stocks are decreasing. In this context, the research of new phosphate sources has become necessary. Many types of wastes contain non-negligible phosphate concentrations, such as wastewater. In wastewater treatment plants, phosphorus is eliminated by physicochemical and/or biological techniques. In this latter case, a specific microbiota, phosphate accumulating organisms (PAOs), accumulates phosphate as polyphosphate. This molecule can be considered as an alternative phosphate source, and is directly extracted from wastewater generated by human activities. This review focuses on the techniques which can be applied to enrich and try to isolate these PAOs, and to detect the presence of polyphosphate in microbial cells.

Re-appraisal of the phylogeny and fluorescence in situ hybridization probes for the analysis of the Competibacteraceae in wastewater treatment systems

Members of the family Competibacteraceae are common in wastewater treatment plants (WWTPs) designed for enhanced biological phosphorus removal (EBPR) and are putatively deleterious to the process of P removal. Their ability to accumulate large amounts of polyhydroxyalkanoates is also suggested to be of potential commercial interest for bioplastic production. In this study we have updated the 16S rRNA-based phylogeny of the Competibacter and the Plasticicumulans lineages. The former is delineated by 13 clades including two described genera; 'Ca. Competibacter' and 'Ca. Contendobacter'. The oligonucleotide probes used for detection of the family by fluorescence in situ hybridization (FISH) were re-evaluated and designed for coverage of these clades. Surveys of full-scale WWTPs based on 16S rRNA gene amplicon sequencing and FISH analysis indicate that a number of member clades always coexist, with their relative abundances varying substantially between and temporally within plants. The hypothesis that these differences are based on niche partitioning is supported by marked phenotypic differences between clades. An in-depth understanding of the ecology of the family requires further studies of the metabolism of individual clades in situ. The proposed phylogeny and FISH probes will provide the foundation for such studies.

Metabolic characteristics of a glycogen-accumulating organism in Defluviicoccus cluster II revealed by comparative genomics

Glycogen-accumulating organisms (GAOs) may compete with phosphate-accumulating organisms (PAOs) for short-chain fatty acids (VFAs) in anaerobic polyhydroxyalkanoates (PHA) synthesis, but no consequently aerobic polyphosphate accumulation in enhanced biological phosphorus removal (EBPR) process, thus deteriorating the EBPR process. They are detected frequently in the deteriorated EBPR process, but their metabolisms are still far from our comprehensions for there is seldom pure culture. In this study, a nearly complete draft genome of a GAOs in Defluviicoccus cluster II, GAO-HK, is recruited from the metagenome of activated sludge in a full-scale industrial anoxic/aerobic wastewater plant. Comparative genomics reveal similar metabolisms of PHA and glycogen in GAOs of GAO-HK, Defluviicoccus tetraformis TFO71 (TFO71) and Competibacter phosphatis clade IIA (CPIIA), and PAOs of Accumulibacter clade IIA UW-1 (UW-1) and Tetrasphaera elongata Lp2 (Lp2). Although there are similar gene cassettes related with polyphosphate metabolism in these GAOs and PAOs, especially for Defluviicoccus-relative bacteria and UW-1, ppk1 in GAOs are diverse from those in the identified PAOs, implying the difference of polyphosphate metabolism in GAOs and PAOs. Additionally, genes related to the dissimilatory denitrification are absent in TFO71 and GAO-HK, implying that additional nitrate or nitrite may favor PAOs over Defluviicoccus-relative GAOs. Therefore, PAOs suffering from competition of Defluviicoccus-relative GAOs might be rescued with the additional nitrate/nitrite, which is important to improve the stability of EBPR processes.

Low acetate concentrations favor polyphosphate-accumulating organisms over glycogen-accumulating organisms in enhanced biological phosphorus removal from wastewater

Glycogen-accumulating organisms (GAOs) are thought to compete with polyphosphate-accumulating organisms (PAOs) in enhanced biological phosphorus removal (EBPR) wastewater treatment systems. A laboratory sequencing batch reactor (SBR) was operated for one year to test the hypothesis that PAOs have a competitive advantage at low acetate concentrations, with a focus on low pH conditions previously shown to favor GAOs. PAOs dominated the system under conventional SBR operation with rapid acetate addition (producing high in-reactor concentrations) and pH values of 7.4-8.4. GAOs dominated when the pH was decreased (6.4-7.0). Decreasing the acetate addition rate led to very low reactor acetate concentrations, and PAOs recovered, supporting the study hypothesis. When the acetate feed rate was increased, EBPR failed again. Dominant PAOs and GAOs were Candidatus Accumulibacter phosphatis and Defluviicoccus Cluster 2, respectively, according to fluorescent in situ hybridization and 454 pyrosequencing. Surprisingly, GAOs were not the immediate causes of PAO failures, based on functional and population measurements. Pyrosequencing results suggested Dechloromonas and Tetrasphaera spp. may have also been PAOs, and additional potential GAOs were also identified. Full-scale systems typically have lower in-reactor acetate concentrations than laboratory SBRs, and so, previous laboratory studies may have overestimated the practical importance of GAOs as causes of EBPR failure.

Improved biological phosphorus removal performance driven by the aerobic/extended-idle regime with propionate as the sole carbon source

Our previous studies proved that biological phosphorus removal (BPR) could be achieved in an aerobic/extended-idle (AEI) process employing two typical substrates of glucose and acetate as the carbon sources. This paper further evaluated the feasibility of another important substrate, propionate, serving as the carbon source for BPR in the AEI process, and compared the BPR performance between the AEI and anaerobic/oxic (A/O) processes. Two sequencing batch reactors (SBRs) were operated, respectively, as the AEI and A/O regimes for BPR using propionate as the sole substrate. The results showed that the AEI-reactor removed 2.98 ± 0.04-4.06 ± 0.06 mg of phosphorus per g of total suspended solids during the course of the steady operational trial, and the phosphorus content of the dried sludge was reached 8.0 ± 0.4% after 56-day operation, demonstrating the good performance of phosphorus removal. Then, the efficiencies of BPR and the transformations of the intracellular storages were compared between two SBRs. It was observed that the phosphorus removal efficiency was maintained around 95% in the AEI-reactor, and about 83% in the A/O-reactor, although the latter showed much greater transformations of both polyhydroxyalkanoates and glycogen. The facts clearly showed that BPR could be enhanced by the AEI regime using propionate as the carbon source. Finally, the mechanisms for the propionate fed AEI-reactor improving BPR were investigated. It was found that the sludge cultured by the AEI regime had more polyphosphate containing cells than that by the A/O regime. Further investigation revealed that the residual nitrate generated in the last aerobic period was readily deteriorated BPR in the A/O-SBR, but a slight deterioration was observed in the AEI-SBR. Moreover, the lower glycogen transformation measured in the AEI-SBR indicated that the biomass cultured by the AEI regime contained less glycogen accumulating organisms activities than that by the A/O regime.

Research advances and challenges in the microbiology of enhanced biological phosphorus removal--a critical review

Enhanced biological phosphorus removal (EBPR) is a well-established technology for removing phosphorus from wastewater. However, the process remains operationally unstable in many systems, primarily because there is a lack of understanding regarding the microbiology of EBPR. This paper presents a review of advances made in the study of EBPR microbiology and focuses on the identification, enrichment, classification, morphology, and metabolic capacity of polyphosphate- and glycogen-accumulating organisms. The paper also highlights knowledge gaps and research challenges in the field of EBPR microbiology. Based on the review, the following recommendations regarding the future direction of EBPR microbial research were developed: (1) shifting from a reductionist approach to a more holistic system-based approach, (2) using a combination of culture-dependent and culture-independent techniques in characterizing microbial composition, (3) integrating ecological principles into system design to enhance stability, and (4) reexamining current theoretical explanations of why and how EBPR occurs.

Filamentous members of cluster III Defluviicoccus have the in situ phenotype expected of a glycogen-accumulating organism in activated sludge

The in situ ecophysiology of alphaproteobacterial filamentous Cluster III Defluviicoccus present in enhanced biological phosphorus removal (EBPR)-activated sludge systems was evaluated using FISH-MAR and histochemical staining methods. These organisms, sharing the Nostocoida limicola morphotype, are known to be responsible for serious episodes of activated sludge bulking. The data presented here also demonstrate an ability to assimilate short-chain fatty acids and synthesize poly-β-hydroxyalkanoates (PHA) anaerobically, and then utilize this stored PHA under aerobic conditions, but with no corresponding synthesis of polyphosphate. These features are consistent with an in situ phenotype of glycogen-accumulating organisms (GAO), populations thought to lower the efficiency of EBPR systems by outcompeting polyphosphate-accumulating organisms (PAO) for substrates in their anaerobic feed phase. Survey data indicate that these GAO are as commonly seen as the known PAO in full-scale EBPR-activated sludge systems, which suggest that they might play important roles there, and therefore should not be viewed just as laboratory curiosities.

Phylogeny and in situ identification of a novel gammaproteobacterium in activated sludge

Failure of a continuously aerated sequencing batch reactor (SBR) pilot plant-enhanced biological phosphorus removal (EBPR) process, designed to remove phosphorus from the clarified effluent from a conventional non-EBPR wastewater treatment plant, was associated with the dominance (c. 50% of the biovolume) of gammaproteobacterial coccobacilli. Flow cytometry and subsequent clone library generation from an enriched population of these Gammaproteobacteria showed that their 16S rRNA genes were most similar to partial clone sequences obtained from an actively denitrifying SBR community, and from anaerobic : aerobic EBPR communities. Under the SBR operating conditions used here, these cells stained for poly-beta-hydroxyalkanoates, but never polyphosphate. Applying FISH probes designed against them in combination with microautoradiography showed that they could also assimilate acetate 'aerobically'. FISH analyses of biomass samples from the full-scale treatment plant providing the pilot plant feed showed that they were present there in high numbers. However, they were not detected by FISH in laboratory-scale communities of the same aerated laboratory-scale EBPR process even when EBPR had failed, or from several full-scale EBPR plants or other activated sludge processes.