Key features of successful BNR operation

Effect of biofilm thickness on the activity and community composition of phosphorus accumulating bacteria in a moving bed biofilm reactor

Can biofilms enhance the rates of phosphorus removal in wastewater treatment? In order to narrow the scientific gap on the effect of biofilm thickness on the activity and microbial community of phosphorus-accumulating bacteria, this study investigated biofilms of 30 to 1000 µm thickness in a moving bed biofilm reactor. Measurements on 5 different biofilm carriers showed that biomass-specific phosphorus release and uptake rates increased as a function of biofilm thickness for biofilms thinner than about 110 µm but were lower for thicker biofilms of about 550-1000 µm. The reduced phosphorus uptake and release rates in the thickest biofilms can result from substrate mass transfer limitations whereas the low activity in the thinnest biofilms can be related to a too high turnover rate in the biofilm due to heterotrophic growth. Additionally, the microbial ecology of the different biofilms confirms the observed phosphorus uptake and release rates. The results from the full-length 16S rRNA gene sequencing of the bacterial community showed that the thicker biofilms were characterized by higher relative abundance (40-58%) of potential phosphorus accumulating genera Zoogloea, Acinetobacter, Dechloromonas and Ca. Accumulibacter. In contrast, the thinner biofilms were dominated by the genus Ferribacterium (34-60%), which might be competing with phosphorus-accumulating bacteria as indicated by the relatively high acetate uptake rates in the thinner biofilms. It is concluded that there is an optimal biofilm thickness of 100-500 µm, at which the phosphorus accumulating bacteria have the highest activity.

Anaerobic zone Functionality, Design and Configurations for a Sustainable EBPR process: A Critical Review

Stringent discharge phosphorus limits and rising urge to reach very low effluent total phosphorus concentrations have challenged the available technologies to further remove phosphorus. The significance of Enhanced Biological Phosphorus Removal (EBPR) process may have been overshadowed by the design and operation limitations. These scarcities mainly root back to the lack of knowledge and understanding of fundamental mechanisms, design standards, and operational guidance. Anaerobic biomass fraction design and operation as a primary driving force for biological phosphorus removal process is commonly outweighed by aerobic and total plant sludge retention operation and design criteria. This paper tends to critically review the different perspectives of mainstream and side-stream EBPR processes and to particularly target contrasting views on hydrolysis and fermentation rates as well as anaerobic condition implementation and magnitude. Subsequently, from distinct point of views, knowledge gaps are comprehensively discussed to eventually recognize the advances and drawbacks aimed to reach a sustainable EBPR process.

Enhancing resource recovery via cranberry syrup waste at the Wisconsin Rapids WRRF: An experimental and modeling study

The Wisconsin Rapids Wastewater Treatment Plant (WRWWTP) is faced with a more stringent effluent phosphorus requirement that will drive capital investment between 2020 and 2025. The facility will need to achieve a monthly average value of 0.36 mg L^-1 of total phosphorus (TP). While the facility has sufficient influent carbon to drive a conventional enhanced biological phosphorus removal (EBPR) configuration, the existing infrastructure makes the addition of influent selector zones cost prohibitive. Underutilized aeration basin capacity was repurposed for testing return activated sludge (RAS) fermentation. The WRWWTP began pilot testing of RAS fermentation in April 2021. The facility moved through a series of operational setpoints to optimize phosphorus removal in a sidestream RAS (SSR) configuration, including RAS diversion, decrease of DO in aeration basins and chemical dosing shutoff. One of the key implementations was the addition of cranberry syrup waste to provide additional carbon for RAS fermentation, converting the process to a SSR plus carbon (SSRC) configuration. By the end of the testing period, effluent total phosphorus was averaging less than 0.4 mg L^-1 with no chemical addition. A model was developed in the SUMO platform and was used to capture orthophosphate trends during the testing period. The model investigated microbial population dynamics and found that the operational changes including RAS diversion, chemical dosing shutoff and cranberry syrup waste addition impacted the enrichment of phosphorus accumulating organisms (PAO). After performing a sensitivity analysis on hydrolysis parameters, the predicted hydrolysis rate around 1.8-1.9 mg COD g VSS^-1 hr^-1 was found to match the batch rate testing data. This is the first study where cranberry syrup waste was used to successfully enhance EBPR performance, resulting in 90% TP removal. While further research is needed regarding the composition of the waste matrix and the microbial community composition, this expands the routes for resource recovery in the field of wastewater treatment.

Enrichment and Aggregation of Purple Non-sulfur Bacteria in a Mixed-Culture Sequencing-Batch Photobioreactor for Biological Nutrient Removal From Wastewater

Mixed-culture biotechnologies are widely used to capture nutrients from wastewater. Purple non-sulfur bacteria (PNSB), a guild of anoxygenic photomixotrophic organisms, rise interest for their ability to directly assimilate nutrients in the biomass. One challenge targets the aggregation and accumulation of PNSB biomass to separate it from the treated water. Our aim was to enrich and produce a concentrated, fast-settling PNSB biomass with high nutrient removal capacity in a 1.5-L, stirred-tank, anaerobic sequencing-batch photobioreactor (SBR). PNSB were rapidly enriched after inoculation with activated sludge at 0.1 gVSS L^-1 in a first batch of 24 h under continuous irradiance of infrared (IR) light (>700 nm) at 375 W m^-2, with Rhodobacter reaching 54% of amplicon sequencing read counts. SBR operations with decreasing hydraulic retention times (48 to 16 h, i.e., 1-3 cycles d^-1) and increasing volumetric organic loading rates (0.2-1.3 kg COD d^-1 m^-3) stimulated biomass aggregation, settling, and accumulation in the system, reaching as high as 3.8 g VSS L^-1. The sludge retention time (SRT) increased freely from 2.5 to 11 days. Acetate, ammonium, and orthophosphate were removed up to 96% at a rate of 1.1 kg COD d^-1 m^-3, 77% at 113 g N d^-1 m^-3, and 73% at 15 g P d^-1 m^-3, respectively, with COD:N:P assimilation ratio of 100:6.7:0.9 m/m/m. SBR regime shifts sequentially selected for Rhodobacter (90%) under shorter SRT and non-limiting concentration of acetate during reaction phases, for Rhodopseudomonas (70%) under longer SRT and acetate limitation during reaction, and Blastochloris (10%) under higher biomass concentrations, underlying competition for substrate and photons in the PNSB guild. With SBR operations we produced a fast-settling biomass, highly (>90%) enriched in PNSB. A high nutrient removal was achieved by biomass assimilation, reaching the European nutrient discharge limits. We opened further insights on the microbial ecology of PNSB-based processes for water resource recovery.

Long-term simulation of a full-scale EBPR plant with a novel metabolic-ASM model and its use as a diagnostic tool

This study evaluates the predictive capacity of the META-ASM model, a new integrated metabolic activated sludge model, in describing the long-term performance of a full-scale enhanced biological phosphorus removal (EBPR) system that suffers from inconsistent performance. In order to elucidate the causes of EBPR upsets and troubleshoot the process accordingly, the META-ASM model was tested as an operational diagnostic tool in a 1336-day long-term dynamic simulation, while its performance was compared with the ASM-inCTRL model, a version based on the Barker & Dold model. Overall, the predictions obtained with the META-ASM without changing default parameters were more reliable and effective at describing the active biomass of polyphosphate accumulating organisms (PAOs) and the dynamics of their storage polymers. The primary causes of the EBPR upsets were the high aerobic hydraulic retention times (HRTs) and low organic loading rates (OLRs) of the plant, which led to periods of starvation. The impact of these factors on EBPR performance were only identified with the META-ASM model. Furthermore, the first signs of process upsets were predicted by variations in the aerobic PAO maintenance rates, suggesting that the META-ASM model has potential to provide an early warning of process upset. The simulation of a new viable operational strategy indicated that troubleshooting the process could be achieved by reducing the aerated volume by switching off air in the first half of the aeration tank. In this new strategy, the META-ASM model predicted a simultaneous improvement in the biological phosphorus (P) and nitrogen (N) removal due to the enhancement of the hydrolysis and fermentation of the mixed liquor sludge in the new unaerated zone, which increased the availability of volatile fatty acids (VFAs) for PAOs. This study demonstrates that the META-ASM model is a powerful operational diagnostic tool for EBPR systems, capable of predicting and mitigating upsets, optimising performance and evaluating new process designs.

Survey of full-scale sidestream enhanced biological phosphorus removal (S2EBPR) systems and comparison with conventional EBPRs in North America: Process stability, kinetics, and microbial populations

Sidestream EBPR (S2EBPR) is an emerging alternative process to address common challenges in EBPR related to weak wastewater influent and may improve EBPR process stability. A systematic evaluation and comparison of the process performance and microbial community structure was conducted between conventional and S2EBPR facilities in North America. The statistical analysis suggested higher performance stability in S2EBPR than conventional EBPR, although possible bias associated with other plant-specific factors might have affected the comparison. Variations in stoichiometric values related to EBPR activity and discrepancies between the observed values and current model predictions suggested a varying degree of metabolic versatility of PAOs in S2EBPR systems that warrant further investigation. Microbial community analysis using various techniques suggested comparable known candidate PAO relative abundances in S2EBPR and conventional EBPR systems, whereas the relative abundance of known candidate GAOs seemed to be consistently lower in S2EBPR facilities than conventional EBPR facilities. 16S rRNA gene sequencing analysis revealed differences in the community phylogenetic fingerprints between S2EBPR and conventional facilities and indicated statistically higher microbial diversity index values in S2EBPR facilities than those in conventional EBPRs. PRACTITIONER POINTS: Sidestream EBPR (S2EBPR) can be implemented with varying and flexible configurations, and they offer advantages over conventional configurations for addressing the common challenges in EBPR related to weak wastewater influent and may improve EBPR process stability. Survey of S2EBPR plants in North America suggested statistically more stable phosphorus removal performance in S2EBPR plants than conventional EBPRs, although possible bias might affect the comparison due to other plant-specific factors. The EBPR kinetics and stoichiometry of the S2EBPR facilities seemed to vary and are associated with metabolic versatility of PAOs in S2EBPR systems that warrant further investigation. The abundance of known candidate PAOs in S2EBPR plants was similar to those in conventional EBPRs, and the abundance of known candidate GAOs was generally lower in S2EBPR than conventional EBPR facilities. Further finer-resolution analysis of PAOs and GAOs, as well as identification of other unknown PAOs and GAOs, is needed. Microbial diversity is higher in S2EBPR facilities compared with conventional ones, implying that S2EBPR microbial communities could show better resilience to perturbations due to potential functional redundancy.

Side-stream enhanced biological phosphorus removal (S2EBPR) process improves system performance - A full-scale comparative study

To address the common challenges in enhanced biological phosphorus removal (EBPR) related to stability and unfavorable influent carbon to phosphorus ratio, a side-stream EBPR (S2EBPR) process that involves a side-stream anaerobic biological sludge hydrolysis and fermentation reactor was proposed as an emerging alternative. In this study, a full-scale pilot testing was performed with side-by-side operation of a conventional anaerobic-anoxic-aerobic (A2O) process versus a S2EBPR process. A comparison of the performance, activity and microbial community between the two configurations was performed. The results demonstrated that, with the same influent wastewater characteristics, S2EBPR configuration showed improved P removal performance and stability than the conventional A2O configuration, especially when the mixers in the side-stream anaerobic reactor were operated intermittently. Mass balance analysis illustrated that both denitrification and EBPR were enhanced in S2EBPR configuration, where return activated sludge was diverted into the anaerobic zone to promote fermentation and enrichment of polyphosphate accumulating organisms (PAOs), and the influent was bypassed to the anoxic zone for enhancing denitrification. A relatively higher PAO activity and total PAO abundance were observed in S2EBPR than in A2O configuration, accompanied by a higher degree of dependence on glycolysis pathway than tricarboxylic acid cycle. No significant difference in the relative abundances of putative PAOs, including Ca. Accumulibacter and Tetrasphaera, were observed between the two configurations. However, higher microbial community diversity indices were observed in S2EBPR configuration than in conventional one. In addition, consistently lower relative abundance of known glycogen accumulating organisms (GAOs) was observed in S2EBPR system. Extended anaerobic retention time and conditions that generate continuous and more complex volatile fatty acids in the side-stream anaerobic reactor of S2EBPR process likely give more competitive advantage for PAOs over GAOs. PAOs exhibited sustained EBPR activity and delayed decay under extended anaerobic condition, likely due to their versatile metabolic pathways depending on the availability and utilization of multiple intracellular polymers. This study provided new insights into the effects of implementing side-stream EBPR configuration on microbial populations, EBPR activity profiles and resulted system performance.

Improvement of partial nitrification endogenous denitrification and phosphorus removal system: Balancing competition between phosphorus and glycogen accumulating organisms to enhance nitrogen removal without initiating phosphorus removal deterioration

The novel partial nitrification endogenous denitrification and phosphorus removal (PNEDPR) process can achieve deep-level nutrient removal from low carbon/nitrogen municipal wastewater without extra carbons. However, its performance is limited by long hydraulic retention time (HRT) and low specific endogenous denitrification rate (r_NO2). This study aimed at investigating the effects of two improving strategies on PNEDPR. One was decreasing both anaerobic and anoxic reaction time for shortening HRT from 55 h to 17.5 h. The other was temporarily discharging orthophosphate-rich supernatant for balancing the competition between phosphorus and glycogen accumulating organisms to further raise r_NO2 without deterioration of phosphorus removal. Results revealed that, desirable nutrient removal was obtained, as average effluent concentrations of total nitrogen and orthophosphate were 8.4 and 0.5 mg/L with their average removal efficiencies of 86.8% and 90.9%. High-throughput sequencing analysis revealed that, Candidatus_Competibacter conducted nitrogen removal endogenous denitrification and Candidatus_Accumulibacter and Tetrasphaera ensured phosphorus removal.

Removal of anti-inflammatory/analgesic pharmaceuticals from urban wastewater in a pilot-scale A2O system: Linking performance and microbial population dynamics to operating variables

In this study, the removal rates of eight anti-inflammatory and/or analgesic pharmaceuticals, AIAPs (acetaminophen, ibuprofen, naproxen, ketoprofen, diclofenac, codeine, indomethacin and propyphenazone) were assessed in a pilot-scale A²O system (including anaerobic/anoxic/aerobic zones), long term operated during two experimental phases using different sets of environmental conditions and operating parameters. qPCR was used to quantify the absolute abundances of total Bacteria, total Archaea, mycolic-acid containing filamentous Actinobacteria (Mycolata) and Fungi within the activated sludge microbial community developed in the system. Multivariate analyses and Spearman correlation coefficients were used in search of significant links among the removal rates of the AIAPs, the abundances of the targeted microbial groups in the activated sludge, and the changes of environmental/operating variables in the A²O system. Improved removal efficiencies of several of the AIAPs analyzed (acetaminophen, ibuprofen, naproxen, ketoprofen) were correlated to higher organic load in the influent water, higher concentration of mixed liquor suspended solids (MLSS), lower temperature and lower food-to-microorganisms ratio (F/M). Removal efficiencies of several pharmaceuticals correlated with increased abundances of Mycolata in the A²O system, pointing at this group of bacteria as candidate key players for AIAPs removal in activated sludge.

Multilevel correlations in the biological phosphorus removal process: From bacterial enrichment to conductivity-based metabolic batch tests and polyphosphatase assays

Enhanced biological phosphorus removal (EBPR) from wastewater relies on the preferential selection of active polyphosphate-accumulating organisms (PAO) in the underlying bacterial community continuum. Efficient management of the bacterial resource requires understanding of population dynamics as well as availability of bioanalytical methods for rapid and regular assessment of relative abundances of active PAOs and their glycogen-accumulating competitors (GAO). A systems approach was adopted here toward the investigation of multilevel correlations from the EBPR bioprocess to the bacterial community, metabolic, and enzymatic levels. Two anaerobic-aerobic sequencing-batch reactors were operated to enrich activated sludge in PAOs and GAOs affiliating with "Candidati Accumulibacter and Competibacter phosphates", respectively. Bacterial selection was optimized by dynamic control of the organic loading rate and the anaerobic contact time. The distinct core bacteriomes mainly comprised populations related to the classes Betaproteobacteria, Cytophagia, and Chloroflexi in the PAO enrichment and of Gammaproteobacteria, Alphaproteobacteria, Acidobacteria, and Sphingobacteria in the GAO enrichment. An anaerobic metabolic batch test based on electrical conductivity evolution and a polyphosphatase enzymatic assay were developed for rapid and low-cost assessment of the active PAO fraction and dephosphatation potential of activated sludge. Linear correlations were obtained between the PAO fraction, biomass specific rate of conductivity increase under anaerobic conditions, and polyphosphate-hydrolyzing activity of PAO/GAO mixtures. The correlations between PAO/GAO ratios, metabolic activities, and conductivity profiles were confirmed by simulations with a mathematical model developed in the aqueous geochemistry software PHREEQC.

Long-term impact of anaerobic reaction time on the performance and granular characteristics of granular denitrifying biological phosphorus removal systems

Removal of nitrogen and phosphorus (P) from wastewater is successfully and widely practiced in systems employing both granular sludge technology and enhanced biological P removal (EBPR) processes; however, the key parameter, anaerobic reaction time (AnRT), has not been thoroughly investigated. Successful EBPR is highly dependent on an appropriate AnRT, which induces carbon and polyphosphate metabolism by phosphorus accumulating organisms (PAOs). Therefore, the long-term impact of AnRT on denitrifying P removal performance and granular characteristics was investigated in three identical granular sludge sequencing batch reactors with AnRTs of 90 (R1), 120 (R2) and 150 min (R3). The microbial community structures and anaerobic stoichiometric parameters related to various AnRTs were monitored over time. Free nitrite acid (FNA) accumulation (e.g., 0.0008-0.0016 mg HNO2-N/L) occurred frequently owing to incomplete denitrification in the adaptation period, especially in R3, which influenced the anaerobic/anoxic intracellular intermediate metabolites and activities of intracellular enzymes negatively, resulting in lower levels of poly-P and reduced activity of polyphosphate kinase. As a result, the Accumulibacter-PAOs population decreased from 51 ± 2.5% to 43 ± 2.1% when AnRT was extended from 90 to 150 min, leading to decreased denitrifying P removal performance. Additionally, frequent exposure of microorganisms to the FNA accumulation and anaerobic endogenous conditions in excess AnRT cases (e.g., 150 min) stimulated increased extracellular polymeric substances (EPS) production by microorganisms, resulting in enhanced granular formation and larger granules (size of 0.6-1.2 mm), but decreasing anaerobic PHA synthesis and glycogen hydrolysis. Phosphorus removal capacity was mediated to some extent by EPS adsorption in granular sludge systems that possessed more EPS, longer AnRT and relatively higher GAOs.

The effect of volatile fatty acids (VFAs) on nutrient removal in SBR with biomass adapted to dairy wastewater

This study aims to determine the effect of volatile fatty acids on nitrates and orthophosphate removal in a sequencing batch reactor (SBR) with activated sludge biomass adapted to process dairy wastewater. The research also determine whether it is the type of fatty acid applied that is responsible for the effectiveness of denitrification and dephosphatation at varying nitrate:orthophosphate ratios, or whether these processes are additionally affected by the presence of microorganisms that have adapted to the specific carbon composition of the wastewater being treated. At the beginning of an operating cycle SBRs were dosed with VFAs to provide a source of carbon. A comparative analysis was performed of nitrate and orthophosphate removal at initial nitrate concentrations of 1.22, 7.3 and 15.2 mgN(NO3)L⁻¹. Doses of fatty acids were approximately 10.5 mg⁻¹COD·mgP(PO4). They consisted of acetic, propionic, butyric, isobutyric, valeric, isovaleric and caproic acids. Increases of nitrate concentration from 1.22 to 15.2 mg N(NO3)L⁻¹ were observed to reduce the quantity of removed orthophosphate depending on the fatty acid applied, from 7.2-9.2 mgP(PO4)L to 4.5 - 6.7 mgP(PO4)L. Every increase in the removed nitrates by 5.0 mgN(NO3)L⁻¹ was accompanied by a decrease in the removed orthophosphate of around 1 mgP(PO4)L⁻¹. The reactor containing acetic acid was found to remove the highest amount of orthophosphate irrespective of the nitrates concentration. Acids present in significant amount in dairy wastewaters (i.e. acetic, propionic and butyric) were more effective source of carbon in the denitrification process compared to low concentration acids.

Effect of anaerobic HRT on biological phosphorus removal and the enrichment of phosphorus accumulating organisms

The purpose of this research was to develop a better understanding of the dynamic effects of anaerobic hydraulic retention time (HRT) on both enhanced biological phosphorus removal (EBPR) performance and enrichment of phosphorus accumulating organisms (PAOs). The research was conducted using laboratory-scale sequencing batch reactors inoculated with mixed microbial consortia and fed real wastewater. Exposing microorganisms to extended anaerobic HRTs is not recommended for EBPR configured systems. In this research, however, longer anaerobic exposure did not negatively affect performance even if volatile fatty acids were depleted. Further, extended anaerobic HRTs may positively affect phosphorus removal through enhanced aerobic uptake. The EBPR consortia also appear to maintain reserve energetic capacity in the form of polyphosphate that can be used to survive and grow under variable operational and environmental conditions. Finally, the tested EBPR systems yield mixed microbial consortia enriched with PAOs (specifically Candidatus Accumulibacter phosphatis) at approximately 7.1 to 21.6% of the total population.

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.

'Candidatus Accumulibacter' gene expression in response to dynamic EBPR conditions

Enhanced biological phosphorus removal (EBPR) activated sludge communities enriched in 'Candidatus Accumulibacter' relatives are widely used in wastewater treatment, but much remains to be learned about molecular-level controls on the EBPR process. The expression of genes found in the carbon and polyphosphate metabolic pathways in Accumulibacter was investigated using reverse transcription quantitative PCR. During a normal anaerobic/aerobic EBPR cycle, gene expression exhibited a dynamic change in response to external acetate, oxygen, phosphate concentrations and probably internal chemical pools. Anaerobic acetate addition induced expression of genes associated with the methylmalonyl-CoA pathway enabling the split mode of the tricarboxylic acid (TCA) cycle. Components of the full TCA cycle were induced after the switch to aerobic conditions. The induction of a key gene in the glyoxylate shunt pathway was observed under both anaerobic and aerobic conditions, with a higher induction by aeration. Polyphosphate kinase 1 from Accumulibacter was expressed, but did not appear to be regulated by phosphate limitation. To understand how Accumulibacter responds to disturbed electron donor and acceptor conditions, we perturbed the process by adding acetate aerobically. When high concentrations of oxygen were present simultaneously with acetate, phosphate-release was almost completely inhibited, and polyphosphate kinase 1 transcript abundance decreased. Genes associated with the methylmalonyl-CoA pathway were repressed and genes associated with the aerobic TCA cycle exhibited higher expression under this perturbation, suggesting that more acetyl-CoA was metabolized through the TCA cycle. These findings suggest that several genes involved in EBPR are tightly regulated at the transcriptional level.

Robustness of sludge enriched with short SBR cycles for biological nutrient removal

In this study, it is proposed that short sequencing batch reactor (SBR) cycles select and maintain a robust and active biomass, able to cope with typical disturbances occurring in wastewater treatment plants. In order to test this hypothesis, an SBR system was subjected to COD, N and P shock loads. It was shown that the sludge enriched in the SBR operated with short cycles was able to rapidly recover from the tested disturbances. COD and N removal recovered within 1-2 days for shock loads of 10 times the standard concentration. The P removal took up to 2-3 sludge ages to fully recover from the COD spike, but the enhanced biological phosphorus removal (EBPR) performance was still able to be totally re-established after each of the tests, even in theoretically adverse conditions for the growth of polyphosphate accumulating organisms.

The microbiology of phosphorus removal in activated sludge processes-the current state of play

This review discusses critically what we know and would like to know about the microbiology of phosphorus (P) removal in activated sludge systems. In particular, the description of the genome sequences of two strains of the polyphosphate accumulating organism found in these processes, Candidatus 'Accumulibacter phosphatis', allows us to address many of the previously unanswered questions relating to how these processes behave, and to raise new questions about the microbiology of P removal. This article attempts to be deliberately speculative, and inevitably subjective, but hopefully at the same time useful to those who have an active interest in these environmentally very important processes.