Inhibition of anaerobic phosphate release by nitric oxide in activated sludge

Improving nutrients removal of Anaerobic-Anoxic-Oxic process via inhibiting partial anaerobic mixture with nitrite in side-stream tanks: role of nitric oxide

A side-stream tank which was in parallel with the anoxic tank was used to improve the performance of the Anaerobic-Anoxic-Oxic process. The partial mixtures from the anaerobic tank were injected into the side-stream tank with the initial nitrite nitrogen (NO₂^--N) concentrations of 10 mg/L and 20 mg/L. When the initial NO₂^--N concentration in the tank was 20 mg/L, total nitrogen and total phosphorus removal efficiencies of the A²/O process increased from 72% and 48% to 90% and 89%, respectively. 2.23 mg/L of nitric oxide (NO) were observed in the side-stream tank. The abundance of Nitrosomonas sp. and Nitrospira sp. were varied from 0.98% and 6.13% to 2.04% and 1.13%, respectively. The abundances of Pseudomonas sp. and Acinetobacter sp. were increased from 0.81% and 0.74% to 6.69% and 5.48%, respectively. NO plays an important role for improving the nutrients removal of the A²/O process in the side-stream nitrite-enhanced strategy.

Comparison of anaerobic phosphorus release from activated sludge with three carbon sources

Three carbon sources, namely sodium acetate, sewage and effluent, were used to simulate the process of phosphorus release in an actual sewage treatment plant, in order to explore the phosphorus release performance of the sludge, the relationship between phosphorus release and chemical oxygen demand (COD) in sewage, and the stability of phosphorus-rich sludge. The results showed that the type and concentration of carbon sources had significant effects on the rate of phosphorus release, reaction equilibrium time and phosphorus release amount. When sodium acetate was used as the carbon source, the phosphorus release rate reached 12.54 mg P (g VSS·h)^-1, and tended to be stable at 4.0 hours. The phosphorus release amount in the first 2.5 hours accounted for 36.88% of the total phosphorus in the sludge. When sewage was used as the carbon source, the phosphorus release rate did not reach equilibrium even at 5.0 hours, and the phosphorus release amount increased by 1.0 mg L^-1 for every 10.5 mg L^-1 increase sewage COD within a certain range. When effluent was used as the carbon source, the visual phosphorus release amount was only 0.83 mg L^-1 after standing for 24 hours and the actual phosphorus release amount was 17.98 mg L^-1. These conclusions can provide technical support for the optimization of phosphorus removal in sewage treatment plants.

Influence of zinc oxide nanoparticles on anaerobic digestion of waste activated sludge and microbial communities

The influence of long-term exposure of zinc oxide nanoparticles (ZnO NPs) to waste activated sludge on anaerobic digestion and microbial communities was studied. The exposure concentrations were 0, 30, 60, 90, 120, and 150 mg g-1-volatile suspended solids (VSS) (dry). ZnO NPs inhibit the degradation of macromolecular organic matter and the reduction of VSS in waste activated sludge during anaerobic digestion. Only slight effects on the activities of protease, cellulase, acetated kinase, and coenzyme F420 were found at ZnO-NP concentrations of less than 30 mg g-1-VSS, whereas the activities of these three enzymes were adversely affected in a dose-dependent manner when the ZnO NP concentrations were increased from 30 mg g-1-VSS to 150 mg g-1-VSS. High-throughput sequencing analysis revealed that ZnO NPs had an adverse influence on the archaeal community diversity but increased the bacterial community diversity to some extent. High-throughput sequencing analysis also revealed that ZnO NPs resulted in different shift trends in the archaeal and bacteria community structure at phylum, class, and genus levels. ZnO NPs have negative impacts on the Euryarchaeota community, which plays a significant role as methanogens in the anaerobic digestion. In addition, ZnO NPs could increase the relative abundance of Clostridia and Bacteroidia, playing an important role in hydrolysis during the anaerobic digestion.

Passive phosphorus capture in biofiltration context: nitrate impact on the performance

Research on the development of a passive phosphorus entrapment process characterized by biofilters with active wood-based media impregnated with iron hydroxide has been conducted. Phosphorus removal was done by sorption which includes adsorption, exchange of ions and precipitation. Experiments were performed in order to investigate the effect of nitrate, generally present at the end of secondary treatment, on the phosphorus removal performance. Columns tests were performed with anaerobic activated wood-based media and immersion over a period of 150 days. Columns were fed for 32 days with a synthetic solution of 5 mg P L^-1. Different concentrations of nitrate (5, 10 and 25 mg N-NO₃ L^-1) were then applied on three columns (C₂, C₃ and C₄), column C₁ serving as a control. Results showed total phosphorus (TP) removal efficiencies of 96.9%, 81.7%, 70.6% and 75.7%, respectively, for C₁, C₂, C₃ and C₄. Addition of nitrate increases the oxidoreduction potential (ORP). This results in an inhibition of the reductive dissolution, characterized by a decrease in the release of ferrous ions. Simultaneous denitrification occurs within the columns. It is both biological and chemical through the oxidation of ferrous ions by NO₂, produced during biological denitrification. Furthermore, bacterial identification tests have highlighted the presence of iron-related bacteria (Pseudomonas, Thiobacillus, Enteric bacteria, e.g. E. coli), slym forming bacteria, sulphate reducing bacteria and denitrifying microorganisms such as Pseudomonas and E. bacteria in biofilters.

Impact of solid residence time (SRT) on functionally relevant microbial populations and performance in full-scale enhanced biological phosphorus removal (EBPR) systems

Investigations of the impact of solid residence time (SRT) on microbial ecology and performance of enhanced biological phosphorus removal (EBPR) process in full-scale systems have been scarce due to the challenges in isolating and examining the SRT from other complex plant-specific factors. This study performed a comprehensive evaluation of the influence of SRT on polyphosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs) dynamics and on P removal performance at Clark County Water Reclamation District Facility in Las Vegas, USA. Five parallel treatment trains with separated clarifiers were operated with five different SRTs ranging from 6 to 40 days. Microbial community analysis using multiple molecular and Raman techniques suggested that the relative abundances and diversity of PAOs and GAOs in EBPR systems are highly affected by the SRT. The resultant EBPR system stability and performance can be potentially controlled and optimized by manipulating the system SRT, and shorter SRT (<10 days) seems to be preferred. PRACTITIONER POINTS: Phosphorus removal performance and kinetics are highly affected by the operational solid residence time (SRT), with lower and more stable effluent P level achieved at SRT < 10 days. Excessive long SRTs above that needed for nitrification may harm EBPR performance; additionally, excessive long SRT may favor GAOs to dominate over PAOs and thus further reducing efficient use of rbCOD for EBPR. Microbial population abundance and diversity, especially those functionally relevant PAOs and GAOs, can impact the P removal performances, and they are highly dependent on the operational solid residence time. EBPR performance can be potentially controlled and optimized by manipulating the system SRT, and shorter SRT (≤10 days) seems to be preferred at the influent rbCOD/P ratio and environmental conditions as in the plant studied.

Metabolic Response of "Candidatus Accumulibacter Phosphatis" Clade II C to Changes in Influent P/C Ratio

The objective of this study was to investigate the ability of a culture highly enriched with the polyphosphate-accumulating organism, “Candidatus Accumulibacter phosphatis” clade IIC, to adjust their metabolism to different phosphate availabilities. For this purpose the biomass was cultivated in a sequencing batch reactor with acetate and exposed to different phosphate/carbon influent ratios during six experimental phases. Activity tests were conducted to determine the anaerobic kinetic and stoichiometric parameters as well as the composition of the microbial community. Increasing influent phosphate concentrations led to increased poly-phosphate content and decreased glycogen content of the biomass. In response to higher biomass poly-phosphate content, the biomass showed higher specific phosphate release rates. Together with the phosphate release rates, acetate uptake rates also increased up to an optimal poly-phosphate/glycogen ratio of 0.3 P-mol/C-mol. At higher poly-phosphate/glycogen ratios (obtained at influent P/C ratios above 0.051 P-mol/C-mol), the acetate uptake rates started to decrease. The stoichiometry of the anaerobic conversions clearly demonstrated a metabolic shift from a glycogen dominated to a poly-phosphate dominated metabolism as the biomass poly-phosphate content increased. FISH and DGGE analyses confirmed that no significant changes occurred in the microbial community, suggesting that the changes in the biomass activity were due to different metabolic behavior, allowing the organisms to proliferate under conditions with fluctuating phosphate levels.

The nature of the carbon source rules the competition between PAO and denitrifiers in systems for simultaneous biological nitrogen and phosphorus removal

The presence of nitrate in the theoretical anaerobic reactor of a municipal WWTP aiming at simultaneous C, N and P removal usually leads to Enhanced Biological Phosphorus Removal (EBPR) failure due to the competition between PAO and denitrifiers for organic substrate. This problem was studied in a continuous anaerobic-anoxic-aerobic (A2/O) pilot plant (146 L) operating with good removal performance and a PAO-enriched sludge (72%). Nitrate presence in the initially anaerobic reactor was studied by switching the operation of the plant to an anoxic-aerobic configuration. When the influent COD composition was a mixture of different carbon sources (acetic acid, propionic acid and sucrose) the system was surprisingly able to maintain EBPR, even with internal recycle ratios up to ten times the influent flow rate and COD limiting conditions. However, the utilisation of sucrose as sole carbon source resulted in a fast EBPR failure. Batch tests with different nitrate concentrations (0-40 mg L(-1)) were performed in order to gain insight into the competition for the carbon source in terms of P-release or denitrification rates and P-release/C-uptake ratio. Surprisingly, no inhibitory or detrimental effect on EBPR performance due to nitrate was observed. A model based on ASM2d but considering two step nitrification and denitrification was developed and experimentally validated. Simulation studies showed that anaerobic VFA availability is critical to maintain EBPR activity.

The role of nitrite and free nitrous acid (FNA) in wastewater treatment plants

Nitrite is known to accumulate in wastewater treatment plants (WWTPs) under certain environmental conditions. The protonated form of nitrite, free nitrous acid (FNA), has been found to cause severe inhibition to numerous bioprocesses at WWTPs. However, this inhibitory effect of FNA may possibly be gainfully exploited, such as repressing nitrite oxidizing bacteria (NOB) growth to achieve N removal via the nitrite shortcut. However, the inhibition threshold of FNA to repress NOB (∼0.02 mg HNO2-N/L) may also inhibit other bioprocesses. This paper reviews the inhibitory effects of FNA on nitrifiers, denitrifiers, anammox bacteria, phosphorus accumulating organisms (PAO), methanogens, and other microorganisms in populations used in WWTPs. The possible inhibition mechanisms of FNA on microorganisms are discussed and compared. It is concluded that a single inhibition mechanism is not sufficient to explain the negative impacts of FNA on microbial metabolisms and that multiple inhibitory effects can be generated from FNA. The review would suggest further research is necessary before the FNA inhibition mechanisms can be more effectively used to optimize WWTP bioprocesses. Perspectives on research directions, how the outcomes may be used to manipulate bioprocesses and the overall implications of FNA on WWTPs are also discussed.

Process optimization by decoupled control of key microbial populations: distribution of activity and abundance of polyphosphate-accumulating organisms and nitrifying populations in a full-scale IFAS-EBPR plant

This study investigated the abundance and distribution of key functional microbial populations and their activities in a full-scale integrated fixed film activated sludge-enhanced biological phosphorus removal (IFAS-EBPR) process. Polyphosphate accumulating organisms (PAOs) including Accumulibacter and EBPR activities were predominately associated with the mixed liquor (>90%) whereas nitrifying populations and nitrification activity resided mostly (>70%) on the carrier media. Ammonia oxidizer bacteria (AOB) were members of the Nitrosomonas europaea/eutropha/halophila and the Nitrosomonas oligotropha lineages, while nitrite oxidizer bacteria (NOB) belonged to the Nitrospira genus. Addition of the carrier media in the hybrid activated sludge system increased the nitrification capacity and stability; this effect was much greater in the first IFAS stage than in the second one where the residual ammonia concentration becomes limiting. Our results show that IFAS-EBPR systems enable decoupling of solid residence time (SRT) control for nitrifiers and PAOs that require or prefer conflicting SRT values (e.g. >15 days required for nitrifiers and <5 days preferred for PAOs). Allowing the slow-growing nitrifiers to attach to the carrier media and the faster-growing phosphorus (P)-removing organisms (and other heterotrophs, e.g. denitrifiers) to be in the suspended mixed liquor (ML), the EBPR-IFAS system facilitates separate SRT controls and overall optimization for both N and P removal processes.

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.

Bacterial community and "Candidatus Accumulibacter" population dynamics in laboratory-scale enhanced biological phosphorus removal reactors

"Candidatus Accumulibacter" and total bacterial community dynamics were studied in two lab-scale enhanced biological phosphorus removal (EBPR) reactors by using a community fingerprint technique, automated ribosomal intergenic spacer analysis (ARISA). We first evaluated the quantitative capability of ARISA compared to quantitative real-time PCR (qPCR). ARISA and qPCR provided comparable relative quantification of the two dominant "Ca. Accumulibacter" clades (IA and IIA) detected in our reactors. The quantification of total "Ca. Accumulibacter" 16S rRNA genes relative to that from the total bacterial community was highly correlated, with ARISA systematically underestimating "Ca. Accumulibacter" abundance, probably due to the different normalization techniques applied. During 6 months of normal (undisturbed) operation, the distribution of the two clades within the total "Ca. Accumulibacter" population was quite stable in one reactor while comparatively dynamic in the other reactor. However, the variance in the clade distribution did not appear to affect reactor performance. Instead, good EBPR activity was positively associated with the abundance of total "Ca. Accumulibacter." Therefore, we concluded that the different clades in the system provided functional redundancy. We disturbed the reactor operation by adding nitrate together with acetate feeding in the anaerobic phase to reach initial reactor concentrations of 10 mg/liter NO(3)-N for 35 days. The reactor performance deteriorated with a concomitant decrease in the total "Ca. Accumulibacter" population, suggesting that a population shift was the cause of performance upset after a long exposure to nitrate in the anaerobic phase.

Microbial population response to changes of the operating conditions in a dynamic nutrient-removal sequencing batch reactor

A nutrient-removal sequencing batch reactor operated with short anaerobic/aerobic cycles was subjected to different operating conditions, namely, cycle length, feeding pattern and feed composition. The changes in microbial population, as well as the contribution of microbial groups to the total nutrient removal, were estimated using the kinetic parameters obtained in this study. Denitrifying polyphosphate-accumulating organisms (DPAOs) were detected in the system, representing a fraction of 23% of phosphorus-accumulating organisms (PAOs). The results suggest that DPAOs and non-DPAOs are different microorganisms. The presence of nitrate in the feed stimulated DPAOs to predominate over non-DPAOs. Feeding the reactor with a mixture of organic substrates also stimulated DPAOs. Glycogen-accumulating organisms (GAOs) were likely to be present in the system and their development over PAOs was apparently favoured by increasing the aeration time and feeding during the aerobic phase. In contrast, the presence of propanoate in the feed apparently favoured PAOs over GAOs.

Comparison between biological and chemical treatment of wastewater containing nitrogen and phosphorus

The present work compared chemical and biological treatment methods to achieve the most efficient treatment for the reduction or elimination of phosphorus and nitrogen from mixed industrial-domestic wastewaters. Batch chemical precipitation by ferric chloride and aluminum sulfate (alum) and a continuous biological suspended growth system were investigated as well as the optimum operating conditions. Concerning chemical treatment, Alum generally achieved a higher removal efficiency percentage for the investigated pollutants compared with FeCl(3) at their optimum pH and dose, especially with chemical oxygen demand (COD). FeCl(3) treatment achieved success only with phosphorus removal, while none of the COD, 5-day biochemical oxygen demand (BOD(5)), total nitrogen (TN) and N-NH(3) achieved acceptable treatment and remained above the maximum permissible limits (MPL). Thus, for such wastewaters, alum is more efficient than FeCl(3). Biological treatment exhibited higher efficiencies, particularly towards nitrogen. TN removal increased by increasing the flow rate to 30-60 l/day. N-NH(3) removal was effective at the slowest flow rate and decreased with increasing flow rate, while an opposite trend was recorded for N-NO(3). At all flow rates, phosphorus levels were below the accepted MPL for discharging into natural systems. Moreover, there was a general trend for the proposed biological treatment to achieve a high removal efficiency for BOD(5) and COD, bringing them to acceptable levels to be released into watercourses safely, especially at the slowest flow rates. Thus, integration between the proposed chemical and biological treatment is highly recommended, producing high-quality effluents acceptable by the environmental law.

The microbiology of biological phosphorus removal in activated sludge systems

Activated sludge systems are designed and operated globally to remove phosphorus microbiologically, a process called enhanced biological phosphorus removal (EBPR). Yet little is still known about the ecology of EBPR processes, the microbes involved, their functions there and the possible reasons why they often perform unreliably. The application of rRNA-based methods to analyze EBPR community structure has changed dramatically our understanding of the microbial populations responsible for EBPR, but many substantial gaps in our knowledge of the population dynamics of EBPR and its underlying mechanisms remain. This review critically examines what we once thought we knew about the microbial ecology of EBPR, what we think we now know, and what still needs to be elucidated before these processes can be operated and controlled more reliably than is currently possible. It looks at the history of EBPR, the currently available biochemical models, the structure of the microbial communities found in EBPR systems, possible identities of the bacteria responsible, and the evidence why these systems might operate suboptimally. The review stresses the need to extend what have been predominantly laboratory-based studies to full-scale operating plants. It aims to encourage microbiologists and process engineers to collaborate more closely and to bring an interdisciplinary approach to bear on this complex ecosystem.

Role of polyphosphate kinase in biofilm formation by Porphyromonas gingivalis

In order to assess the role of polyphosphate kinase (PPK) in the physiology of Porphyromonas gingivalis, a ppk gene mutant, CW120, was constructed and characterized. P. gingivalis was demonstrated to synthesize short-chain polyphosphate (polyP) but not long-chain polyP. CW120 failed to survive in the stationary phase as well as the parental cell did, and it was attenuated in biofilm formation on polyvinylchloride and glass surfaces. Furthermore, the complementation by insertion of an intact copy of the ppk gene into the mutant CW120 restored its biofilm formation and stationary-phase survival. These results suggest that PPK may be important for incorporation of these organisms into subgingival plaque in the human oral cavity.

Atypical polyphosphate accumulation by the denitrifying bacterium Paracoccus denitrificans

Polyphosphate accumulation by Paracoccus denitrificans was examined under aerobic, anoxic, and anaerobic conditions. Polyphosphate synthesis by this denitrifier took place with either oxygen or nitrate as the electron acceptor and in the presence of an external carbon source. Cells were capable of poly-beta-hydroxybutyrate (PHB) synthesis, but no polyphosphate was produced when PHB-rich cells were incubated under anoxic conditions in the absence of an external carbon source. By comparison of these findings to those with polyphosphate-accumulating organisms thought to be responsible for phosphate removal in activated sludge systems, it is concluded that P. denitrificans is capable of combined phosphate and nitrate removal without the need for alternating anaerobic/aerobic or anaerobic/anoxic switches. Studies on additional denitrifying isolates from a denitrifying fluidized bed reactor suggested that polyphosphate accumulation is widespread among denitrifiers.