Phosphorus Removal System Research Articles

The Occurrence and Role of Tetrasphaera in Enhanced Biological Phosphorus Removal Systems

The application of enhanced biological phosphorus removal (EBPR) in wastewater treatment plants (WWTPs) has commonly been utilized worldwide. However, the optimum efficiency has not been realized over the past decades, prompting many studies and publications. The limitations, especially comprehension of the abundance and actual potential of polyphosphate-accumulating organisms (PAOs), are not fully understood. Recently identified putative PAOs, Tetrasphaera, present a vast metabolic versatility compared to Candidatus Accumulibacter. The characterisation of Tetrasphaera unique abilities to utilize various carbon substrates, volatile fatty acids production and consistent high abundance, presents potential boosts towards the process efficiency improvement. This paper provides the existing knowledge on the physiology, morphology and genetic description of PAOs with a special attention to the current state of research on Tetrasphaera and its potential. In addition, process conditions and their influence on the microbial activities in EBPR systems are discussed.

Response of performance, sludge characteristics, and microbial communities of biological phosphorus removal system to salinity

The effects of salinity on highly enriched polyphosphate- or glycogen-accumulating organisms (PAOs or GAOs) have been revealed, which is meaningful but idealized. In this study, three salinity levels (0.5%, 1.0%, and 0.75%) were sequentially adopted in a PAOs and GAOs coexisted biological phosphorus removal (BPR) reactor within 150 days. Compared to a slight decrease of phosphorus removal efficiency (PRE) under 0.5% salinity (from 96.09% to 73.68%), doubled salinity (1.0%) resulted in a lengthy recovery period and a sharp PRE decline (13.89%), and the PRE was merely kept at 27.39% even through salinity was decreased to 0.75% hereafter. Salinity was also found to stimulate more extracellular protein secretion, resulting in sludge volume index reduction (<32.87 mL/g) and particle size enlargement (222.78 μm on average). Hyphomicrobium (0.96%–1.76%) and unclassified_f_Rhodobacteraceae (4.72%–13.33%) could resist certain salinity and conduct BPR, but better salt-tolerant Candidatus_Competibacter eventually became the predominant genus (>40%).

Metabolism transformation and free nitrous acid tolerance of denitrifying polyphosphate accumulating organisms under different intracellular polyphosphate levels

Polyphosphate (Poly-P) is a key compound in the known metabolism of denitrifying polyphosphate organisms (DPAOs). This study investigated the efficiency of endogenous denitrification, metabolic changes, and tolerance to free nitrous acid (FNA) of DPAOs at different levels of Poly-P. A high C/P ratio (100:1) made DPAOs transform from polyphosphate accumulating metabolism (PAM) to glycogen accumulating metabolism (GAM), and the nitrate (NO3--N) reduction rate of DPAOs increased significantly in GAM mode. Accumulibacter IA, IIC, and IID were the main dominant DPAOs, and their synergy and the increase of PHA may be responsible for the high NO3--N reduction efficiency. The DPAOs under GAM mode had better tolerance to FNA, and N2O reductase was destroyed due to the FNA influence, leading to a large accumulation of N2O. Flow cytometry results showed that FNA caused DPAOs to over-consume ATP and depolarize, thereby inhibiting their growth. And FNA disrupted the membrane integrity and esterase activity of DPAOs, which may result in early apoptosis. The metabolic transformation and FNA tolerance of DPAOs under GAM mode ensure the stable operation and performance recovery of denitrifying phosphorus removal systems, which demonstrates the DPAOs-dominated biological phosphorus removal has wide application prospects.

Rapid start-up and stable operation of an aerobic/oxic/anoxic simultaneous nitrification, denitrification, and phosphorus removal reactor with no sludge discharge

An anaerobic/aerobic/anoxic mode simultaneous nitrification, denitrification, and phosphorus removal system was visited for enhanced low-strength wastewater treatment and dramatic in situ sludge reduction. Results showed that rapid start-up was achieved with conventional activated sludge after 15 days, with effluent ammonia nitrogen, total nitrogen, total phosphorus, and chemical oxygen demand being 0.25, 7.89, 0.12, 24.37 mg/L, respectively. A two-stage biomass growth rate was observed with the sludge yield of 0.285 (day 1–50) and 0.017 g MLSS/g COD (day 51–110) without sludge discharge. Dynamics of bacterial community has been identified with outstanding accumulation of Candidatus_Competibacter up to 29.06 %, which contributed to both simultaneous nutrients removal and sludge reduction. Further analysis via PICRUSt2 revealed the main pathway of nitrogen metabolism, while proposed mechanism for phosphorus removal with no sludge discharge was analyzed from the intracellular and extracellular perspectives. Overall, this study provided guidance and reference for the development and application of A/O/A-SNDPR technology.

Upgrading integrated fixed-biofilm activated sludge (IFAS) system into separated two-sludge denitrifying phosphorus removal system: Nutrient removal and microbial structure

An integrated fixed-biofilm activated sludge (IFAS) system was upgraded into a separated two-sludge denitrifying phosphorus removal system to treat domestic sewage with a low carbon/nitrogen ratio (C/N = 3.6). The system was operated in IFAS mode for 135 days and then converted to the separated two-sludge mode to perform denitrifying phosphorus removal for 110 days. With denitrification and phosphorus removal conducted using “single carbon source”, the experimental results show that the total inorganic nitrogen (TIN) and phosphorus (P) removal efficiencies were improved by 13.4% and 36.5%, respectively. As the activated sludge unit and biofilm unit were separated, Dechloromonas (4.10%) and Candidatus Accumulibacter (1.56%) were enriched in activated sludge system, while the nitrifiers Nitrospira and Nitrosomonas occupied the biofilm with proportions of 19.6% and 5.68%, respectively. The separated two-sludge denitrifying phosphorus removal system was characterized by higher nutrient removal and functional bacteria enrichment.

Performance enhancement and population structure of denitrifying phosphorus removal system over redox mediator at low temperature

The development of denitrifying polyphosphate accumulating organisms (DPAOs) presents a strategy to carbon competition between denitrifying bacteria and phosphorus removing bacteria. However, low temperature inhibits the rate of enzyme-catalyzed and substrate diffusion during denitrifying phosphorus removal (DPR). Therefore, the present study assessed the addition of NQS (100 μmol/L) for enhancing the removal of TP and TN in DPR reactors operated at alternating anaerobic and anoxic phases and different influent phosphate concentrations. The results showed that the removal efficiency of TP and TN in NQS-DPR system at 10 °C were 99.9% and 42.0%, respectively, which were 2.1 and 2.0 times higher than that of DPR system. Adding NQS significantly alleviated the increase of pH under anoxic condition and decreased the ORP value of the reactor, which in turn enhanced the PHAs accumulation process. The determination of functional genes (nirK, narG and phoD) showed that Dechloromonas, Lentimicrobium, and Terrimonas were the dominant functional bacteria in NQS-DPR system at 10 °C with the relative abundance of 3.09%, 2.99% and 2.28%, respectively. This study can provide valuable information for the effects of the addition of the redox mediator on denitrifying phosphorus removal technology.

Phosphorus removal and recovery: state of the science and challenges.

Phosphorus is one of the main nutrients required for all life. Phosphorus as phosphate form plays an important role in different cellular processes. Entrance of phosphorus in the environment leads to serious ecological problems including water quality problems and soil pollution. Furthermore, it may cause eutrophication as well as harmful algae blooms (HABs) in aquatic environments. Several physical, chemical, and biological methods have been presented for phosphorus removal and recovery. In this review, there is an overview of phosphorus role in nature provided, available removal processes are discussed, and each of them is explained in detail. Chemical precipitation, ion exchange, membrane separation, and adsorption can be listed as the most used methods. Identifying advantages of these technologies will allow the performance of phosphorus removal systems to be updated, optimized, evaluate the treatment cost and benefits, and support select directions for further action. Two main applications of biochar and nanoscale materials are recommended.

An effective titanium salt dosing strategy for phosphorus removal from wastewater: Synergistic enhancement of chemical and biological treatment

Titanium salt coagulant, as a new type of water treatment agent, has been widely studied, but most researches do not consider its effect on the biological treatment. In this study, different doses of TiCl4 were added to the biological phosphorus removal (BPR) system to investigate the impact of TiCl4 on BPR. The results showed that the addition of TiCl4 not only significantly reduced the phosphorus concentration in effluent (below 0.5 mg/L), but also kept it stable. Moreover, the sedimentation performance of activated sludge was improved, which was superior to the control group. According to the results of flow cytometry (FCM), a small amount of TiCl4 significantly improved the bioactivities, but excessive dosage caused inhibition. When the dosage of TiCl4 below 20 mg/L, polyphosphate accumulating metabolism (PAM) was strengthened. In addition, the richness of microbial community and the relative abundance of Candidatus Accumulibacter clades also increased. However, when the dosage reached 60 mg/L, the relative abundance of Candidatus Competibacter increased and the BPR system was deteriorated. This study suggests that the addition of appropriate concentration of TiCl4 can realize the synergistic enhancement of biological and chemical phosphorus removal in sewage treatment.

Effect of granular matrix on functional flora in EBPR system

Enhanced biological phosphorus removal (EBPR) is a very mature phosphorus removal technology, but there is still a problem of unstable treatment effect in the operation of actual wastewater treatment plants, so the study on the effect of phosphorus removal performance still needs to be further studied. Because there are a lot of granular matrix in municipal wastewater, it occupies an important proportion in the total Chemical Oxygen Demand (COD) of municipal wastewater, and the fluctuation of granular matrix concentration is also related to the instability of phosphorus removal effect. Therefore, it is of practical guiding significance to further explore the influence of granular matrix on the functional bacteria community of phosphorus removal in the enhanced biological phosphorus removal system. The acetic acid was used as a carbon source to enrich and cultivate phosphorus accumulating bacteria. After the enrichment and the system was stable, soluble starch was added into the water to study the effects of granular matrix on phosphorus removal performance and functional bacteria community in EBPR system. The results showed that acetic acid was beneficial to the proliferation of phosphorus-accumulating bacteria in the enrichment stage, and the phosphate removal rate of the system could reach more than 99%. After the system is stabilized, granular matrix with different concentrations is added to the system in three stages, and the phosphate removal rate of the system gradually decreases. With the increasing of granular matrix concentration in the influent, the phosphorus removal performance of sludge decreased gradually, the maximum phosphorus release rate and maximum phosphorus uptake rate of sludge decreased, and the acetic acid utilization rate ΔP/ΔHAc value also decreased. Fluorescence in situ hybridization was used to measure the changes of functional bacteria in activated sludge under different conditions. With the increase of the concentration of soluble starch, the proportion of PAOs gradually decreased, and the proportion of GAOs gradually increased. Granular matrix exists in the internal of floc sludge and cannot be utilized by PAOs, which results in inhibition of the growth of PAOs and weakened activity. This indicates that the higher concentration of granular matrix will lead to a decrease in the proportion of PAOs in EBPR system, and then lead to a decrease in the phosphorus removal activity of activated sludge, and ultimately reduce the phosphorus removal effect of EBPR system.

Exploration of the metabolic flexibility of glycogen accumulating organisms through metatranscriptome analysis and metabolic characterization

Glycogen accumulating organisms (GAOs) are closely related to the deterioration of enhanced biological phosphorus removal systems. However, the metabolic mechanisms that drive GAOs remain unclear. Here, the two-thirds supernatant of a reactor were decanted following the anaerobic period to enrich GAOs. Long-term monitoring demonstrated that the system was stable and exhibited typical characteristics of GAOs metabolism. Acetate was completely consumed after 60 min of the anaerobic phase. The level of glycogen decreased from 0.20 to 0.14 g/gSS during the anaerobic phase, whereas the level of glycogen significantly increased to 0.21g/gSS at the end of the aerobic period. Moreover, there was almost no phosphate release and absorption in the complete periods, thus confirming the successful construction of a GAOs enrichment system. Microbial community analysis demonstrated that Ca. Contendobacter was among the core functional genera and showed the highest activity among all of the communities. Furthermore, our study is the first to identify the involvement of the ethyl-malonyl-CoA pathway in the synthesis of polyhydroxyvalerate via croR, ccr, ecm, mcd, mch and mcl genes. The Embden-Meyerhof-Parnas (EMP) pathway was preferentially used via glgP. Furthermore, the glyoxylate cycle was the main source of ATP under anaerobic conditions, whereas the tricarboxylic acid cycle provided ATP under aerobic conditions. aceA and mdh appeared to be major modulators of the glyoxylate pathway for controlling energy flow. Collectively, our findings not only revealed the crucial metabolic mechanisms in a GAOs enrichment system but also provided insights into the potential application of Ca. Contendobacter for wastewater treatment.

Pilot demonstration of nitrogen removal from municipal wastewater by vacuum membrane distillation

Nitrogen in municipal wastewater must be removed prior to discharge to the environment. However existing energy intensive aerobic processes used for nitrogen removal currently do not capture this important resource. Instead, nitrogen capture in the form of ammonia using a lower energy technology such as the membrane distillation; so far mostly tested in lab-scale, could be a significant improvement. This study reports a vacuum membrane distillation (VMD) pilot trial conducted at a municipal wastewater treatment plant to uniquely demonstrate ammonium removal and recovery to meet effluent discharge targets. The pilot plant was located in two streams on-site in the plant's anaerobic digester liquor handling facilities. The first trialed stream was on the existing phosphorus removal lime dosed effluent, and the second stream was effluent from an experimental struvite phosphorous recovery operation. In the post-lime scenario, VMD recovered N as ammonium sulphate solution and achieved up to 93% reduction. In the post struvite scenario, the VMD system achieved up to 85% reduction within the duration of the demonstration. With VMD driving force derived from volatility of the compound of interest, which is reliably calculated via Henry's law, the ammonia mass transfer coefficient across the membrane was 0.16 kg/m2/h/bar (±37% RSD) over the trial period, being lower than typical best case laboratory test results. Overall, the nitrogen removal and recovery process via VMD was proven technically viable on actual plant effluent leaving existing lime dosed and potential future struvite phosphorus removal systems. However, further studies are required to assess the economics of VMD for nitrogen removal compared with other nitrification-denitrification methods used in wastewater treatment.

Carbon uptake bioenergetics of PAOs and GAOs in full-scale enhanced biological phosphorus removal systems

This work analyzed, for the first time, the bioenergetics of PAOs and GAOs in full-scale wastewater treatment plants (WWTPs) for the uptake of different carbon sources. Fifteen samples were collected from five full-scale WWTPs. Predominance of different PAOs, i.e., Ca. Accumulibacter (0.00–0.49%), Tetrasphaera (0.37–3.94%), Microlunatus phosphovorus (0.01–0.18%), etc., and GAOs, i.e., Ca. Competibacter (0.08–5.39%), Defluviicoccus (0.05–5.34%), Micropruina (0.17–1.87%), etc., were shown by 16S rRNA gene amplicon sequencing. Despite the distinct PAO/GAO community compositions in different samples, proton motive force (PMF) was found as the key driving force (up to 90.1%) for the uptake of volatile fatty acids (VFAs, acetate and propionate) and amino acids (glutamate and aspartate) by both GAOs and PAOs at the community level, contrasting the previous understanding that Defluviicoccus have a low demand of PMF for acetate uptake. For the uptake of acetate or propionate, PAOs rarely activated F1, F0- ATPase (< 11.7%) or fumarate reductase (< 5.3%) for PMF generation; whereas, intensive involvements of these two pathways (up to 49.2% and 61.0%, respectively) were observed for GAOs, highlighting a major and community-level difference in their VFA uptake biogenetics in full-scale systems. However, different from VFAs, the uptake of glutamate and aspartate by both PAOs and GAOs commonly involved fumarate reductase and F1, F0-ATPase activities. Apart from these major and community-level differences, high level fine-scale micro-diversity in carbon uptake bioenergetics was observed within PAO and GAO lineages, probably resulting from their versatilities in employing different pathways for reducing power generation. Ca. Accumulibacter and Halomonas seemed to show higher dependency on the reverse operation of F1, F0-ATPase than other PAOs, likely due to the low involvement of glyoxylate shunt pathway. Unlike Tetrasphaera, but similar to Ca. Accumulibacter, Microlunatus phosphovorus took up glutamate and aspartate via the proton/glutamate-aspartate symporter driven by PMF. This feature was testified using a pure culture of Microlunatus phosphovorus stain NM-1. The major difference between PAOs and GAOs highlights the potential to selectively suppress GAOs for community regulation in EBPR systems. The finer-scale carbon uptake bioenergetics of PAOs or GAOs from different lineages benefits in understanding their interactions in community assembly in complex environment.

(Micro) nanoplastics promote the risk of antibiotic resistance gene propagation in biological phosphorus removal system

Microplastics (MPs), nanoplastics (NPs) and antibiotic resistance genes (ARGs), as emerging pollutants, have been frequently detected in wastewater treatment plants. However, the behavior of phosphorus and ARGs under MP and NP (MP/NP) pressure in biological phosphorus removal (BPR) system is still unknown. This study investigated the effects of MP/NPs on phosphorus removal and ARGs propagation in BPR system. Results showed that MP/NPs had no influence on phosphorus removal, but significantly promoted the amplification of tetracycline resistance genes (TRGs). Moreover, the TRG abundance were more facilitated by MPs than NPs, and the TRGs of efflux pump and enzymatic modification mechanism were mainly enriched. Meanwhile, MP/NPs increased the transmission risk of multiple resistance genes and mobile gene elements (MGEs). Microbial communities demonstrated the main polyphosphate accumulating organisms shifted from Acinetobacter to unclassified_Gammaproteobacteria, which explained why phosphorus removal efficiency was unaffected with MP/NP addition. Correlation analysis revealed there was no significant correlation between ARGs and MGEs (intI1 and intI2), but the abundances of potential hosts of ARGs were significantly increased with MP/NP addition, implying microbial community structure changes rather than gene horizontal transfer was the main factor promoting ARG propagation under MP/NP pressure.

Microbial population changes and metabolic shift of candidatus accumulibacter under low temperature and limiting polyphosphate.

This study explored the microbial population dynamics of Accumulibacter (Acc) at low temperature and metabolic shift to limiting polyphosphate (Poly-P) in enhanced biological phosphorus removal (EBPR) system. The Accumulibacter-enriched EBPR systems, fed with acetate (HAc) and propionate (HPr) at 10 ± 1 °C respectively, were operated for 60 days in two identical SBR reactors (SBR-1 and SBR-2). The phosphorus removal performance in two systems was stable at 10 ± 1 °C, while the microbial community structure changed. Compared with the population structure in seed sludge, Accumulibacter clades reduced in the HAc system, while Acc I increased significantly in the HPr system. Low temperature was beneficial to the formation of granular sludge in the EBPR system, and the sludge granulation in the HAc system was more homogeneous than that in the HPr system. Accumulibacter in the HPr system can get ATP through glycogen accumulating metabolism (GAM) under limiting Poly-P condition at 10 ± 1 °C, while that in the HAc system cannot. This work suggests that poly-P levels can affect the metabolic pathway of Accumulibacter in EBPR systems under low temperature.

Coagulants put phosphate-accumulating organisms at a competitive disadvantage with glycogen-accumulating organisms in enhanced biological phosphorus removal system

Enhanced biological phosphorus removal (EBPR) process is susceptible to the changed operation condition, which results in an unstable treatment performance. In this work, long-term effect of coagulants addition, aluminum salt for the reactor R1 and iron salt for the reactor R2, on EBPR systems was comprehensively evaluated. Results showed that during the initial 30 days’ coagulant addition, effluent chemical oxygen demand and phosphorus can be reduced below 25 and 0.5 mg·L-1, respectively. Further supply of metal salts would stimulate microbial extracellular polymeric substance excretion and induce reactive oxygen species accumulation, which destroyed the cell membrane integrity and deteriorated the phosphorus removal performance. Moreover, coagulants would decrease the relative abundance of Candidatus Accumulibacter while increase the relative abundance of Candidatus Competibacter, leading phosphors accumulating organisms in a disadvantage position. The results of this work might be valuable for the operation of chemical assisted biological phosphorus removal bioreactor.

Oligotyping and metagenomics reveal distinct Candidatus Accumulibacter communities in side-stream versus conventional full-scale enhanced biological phosphorus removal (EBPR) systems

Candidatus Accumulibacter phosphatis (CAP) and its clade-level micro-diversity has been associated with and implicated in functional differences in phosphorus removal performance in enhanced biological phosphorus removal (EBPR) systems. Side-stream EBPR (S2EBPR) is an emerging process that has been shown to present a suite of advantages over the conventional EBPR design, however, large knowledge gaps remain in terms of its underlying ecological mechanisms. Here, we compared and revealed the higher-resolution differences in microbial ecology of CAP between a full-scale side-stream EBPR configuration and a conventional A2O EBPR process that were operated in parallel and with the same influent feed. Even though the relative abundance of CAP, revealed by 16S rRNA gene amplicon sequencing, was similar in both treatment trains, a clade-level analysis, using combined 16S rRNA-gene based amplicon sequencing and oligotyping analysis and metagenomics analysis, revealed the distinct CAP microdiversity between the S2EBPR and A2O configurations that likely attributed to the improved performance in S2EBPR in comparison to conventional EBPR. Furthermore, genome-resolved metagenomics enabled extraction of three metagenome-assembled genomes (MAGs) belonging to CAP clades IIB (RCAB4-2), IIC (RC14) and II (RC18), from full-scale EBPR sludge for the first time, including a distinct Ca. Accumulibacter clade that is dominant and associated only with the S2EBPR configuration. The results also revealed the temporally increasing predominance of RC14, which belonged to Clade IIC, during the implementation of the S2EBPR configuration. Finally, we also show the existence of previously uncharacterized diversity of clades of CAP, namely the clades IIB and as yet unidentified clade of type II, in full-scale EBPR communities, highlighting the unknown diversity of CAP communities in full-scale EBPR systems.

Study on the effectiveness of an independent biological phosphorus removal system based on immobilized biological fillers nitrogen removal system in municipal wastewater

This study focused on the problem that simultaneous nitrogen and phosphorus removal cannot be achieved in current urban sewage treatment systems. To address this, a pilot system was developed to first use immobilized biologically active fillers to remove nitrogen, and then use activated sludge to independently remove phosphorus. The goal was to maximize the effectiveness of biological phosphorus removal. This study mainly aimed at analyzing the post-independent phosphorus removal characteristics and mechanisms; the composition of the phosphorus removing bacteria; and the contribution rate of the system. The effluent PO43--P of the system remained stable at 0.15–0.2 mgL−1, with the phosphorus removal anaerobic tank maintaining a "micro-release" state. When the hydraulic retention time (HRT) of the aerobic phosphorus removal tank was reduced to 1 h, the contribution rate of the aerobic phosphorus removal bacteria decreased from 28.42% to 16.68%; the contribution rate of the denitrifying phosphorus removal bacteria increased from 72.58% to 83.32%; and the abundance of the dominant bacteria Dechloromonas and Thaurea significantly increased. The phosphorus removal system provides a new approach for deep phosphorus removal in main municipal wastewater systems.

Effect of Chemical Oxygen Demand Concentration on Nutrient Removal in Simultaneous Nitrification, Denitrification and Phosphorus Removal System in High-Altitude Areas

The application of biological nitrogen and phosphorus removal processes in high-altitude areas faces severe challenges due to low temperature, low atmosphere pressure and low oxygen concentration. In this study, a simultaneous nitrification, denitrification and phosphorus removal (SNDPR) system was operated under low atmosphere pressure. The chemical oxygen demand (COD) concentrations in influent were decreased from 300 mg/L (stage I) to 200 mg/L (stage II), corresponding to the low COD concentration of sewage in high-altitude areas. The removal of COD and total phosphate was efficient at the H1 reactor (72 kPa). The removal rates of COD and total phosphate were 94.08% (stage I), 90.66% (stage II) and 98.43% (stage I), 99.34% (stage II), respectively, which were similar to L1 (100 kPa). The removal rates of total inorganic nitrogen and simulation nitrification and denitrification were from 81.21% (stage I) and 59.48% (stage I) to 72.86% (stage II) and 31.95% (stage II), respectively, which were also improved compared to L1. Cycle experiment results indicated that the activity of phosphorus accumulating organisms was enhanced, while the ammonia oxidation process was inhibited under low atmosphere pressure.

Combining Different Aerobic/Anoxic Durations with Zoned Sludge Discharge to Optimize Short-cut Nitrification Denitrifying Phosphorus Removal Granules in Domestic Sewage

In this study, domestic sewage was used to inoculate mature short-cut nitrification denitrifying phosphorus removal particles, which were cultivated and matured under artificial water. The operation of the short-cut nitrification denitrifying phosphorus removal system was optimized using different aerobic/anoxic durations combined with zoned sludge discharge. The results showed that regulating the aerobic/anoxic duration, in combination with zoned sludge discharge, can realize the stable operation of the system. In the later stable period, the effluent COD concentration was below 50 mg·L-1, the effluent TN concentration was below 15 mg·L-1, the TN removal rate reached about 83% and remained stable, the effluent P concentration was below 0.5 mg·L-1, and the average P removal rate was 93.72%. At the same time, zoned sludge discharge (70% top sludge and 30% bottom sludge) can be used to screen out microorganisms, maintain good nitrosation and phosphorus removal performance, limit the particle size distribution, and ensure the growth advantages of AOB and DPAOs. Increases in the anoxic duration improved the growth rate of anoxic heterotrophic bacteria, causing them to secrete more EPS and ensuring granular sludge improvements and continued stability.

Shortening SRT of Intermittent Gradient Aeration to Realize Nitrogen and Phosphorus Removal in Short-range SNEDPR System

This study explored the effect of sludge retention time (SRT) on ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) under intermittent gradient aeration, as well as the effect of the short-range nitrification endogenous denitrification phosphorus removal system on the treatment of low C/N ratio domestic sewage. In this study, an SBR reactor was used to cultivate aerobic granular sludge, using actual domestic sewage as the influent substrate. As the SRT decreased from 50 d to 30 d, the specific ammonia oxidation rate increased from 3.16 mg·(g·h)-1to 4.38 mg·(g·h)-1, and the specific nitrite oxidation rate decreased from 3.4 mg·(g·h)-1to 1.8 mg·(g·h)-1. The activity of NOB decreased by about 44%, resulting in short-range nitrification within the system. With an SRT of 30 d, the maximum nitrite accumulation was 6.93 mg·L-1. Because the reduced SRT led to a slight decrease in sludge concentration within the system, an aeration reduction strategy was adopted after 40 d, according to the DO curve. When the final SRT was 30 d, the effluent COD concentration was 40.76 mg·L-1, the TN concentration was 12.4 mg·L-1, the TP concentration was 0.31 mg·L-1, and the simultaneous removal of C, N and P was realized. Thus, a stable short-range nitrification endogenous denitrification phosphorus removal system was finally obtained. At the same time, the EPS content of aerobic granular sludge was negatively correlated with SRT, the protein content increased from 66.7 mg·g-1 to 95.1 mg·g-1, and the polysaccharide content remained in the range of 12.1-17.2 mg·g-1, indicating that the decreased SRT had a great effect on the protein content. With an SRT of 30 d, the PN/PS value was maintained at approximately 6.2, and the structural stability of aerobic granular sludge can be maintained under such conditions.