Influence of HRT and carbon source on the enhancement of nutrient removal in an Anaerobic-Oxic-Anoxic (AOA) system

The eutrophication and increase in toxicity promoted by the continuous or abundant supply of nutrients in water bodies threaten the safety of drinking water and human health. In this regard, this study proposes the investigation of wastewater treatment focusing on the simultaneous removal of nitrogen and phosphorus in the anaerobic-oxic-anoxic (AOA) system. The AOA system was operated in three different stages to verify the influence of the external carbon source addition in the anoxic reactor and the reduction of hydraulic retention time (HRT) in the anaerobic and oxic reactors for nutrient removal optimization. Results showed that the best performance of the AOA system on nutrient removal was obtained in Stage 3, with the reduction of the HRT in the anaerobic and oxic reactors (HRT = 4 h) while maintaining HRT of 6.4 h in the anoxic reactor with no addition of the external carbon source. Under these conditions, the average removal efficiencies reached 98% for Chemical Oxygen Demand (COD), 88% for Total Ammonia Nitrogen (TAN), 81% for Total Kjeldahl Nitrogen (TKN), and 70% for Total Phosphorus (TP). The results also demonstrate that the highest phosphorus removal efficiency was achieved in the anoxic reactor, thus indicating the occurrence of denitrifying phosphorous removal by Denitrifying Phosphate Accumulating Organisms (DNPAOs). This configuration was efficient regarding the simultaneous removal of nitrogen and phosphorus; besides, the advantages of this system include robust configuration and excellent performance on the nutrient removal.

[In-situ Phosphorus Removal Activity and Impact of the Organic Matter Concentration on Denitrifying Phosphorus Removal in Sludge Aggregates]

In this work, the redox potential, dissolved oxygen, and phosphate microelectrodes were used to quantitatively study the in-situ activity of dephosphorization bacteria and the impact of the organic matter concentration on denitrifying phosphorus removal in sludge aggregates in a sequencing batch reactor. The results showed that the maximum net volume release rate of phosphorus was 3.29 mg·(cm³·h)^-1 in the initial anaerobic sludge aggregates, which was approximately 3 times the maximum net volume uptake rate of phosphorus at the initial anoxic stage. The release rate of phosphorus clearly decreased at the final anaerobic stage, and the maximum net volume release rate of phosphorus was only half of that at the initial anaerobic stage. At the final anoxic stage, the maximum net volume uptake rate of phosphorus decreased to 0.14 mg·(cm³·h)^-1, and the phenomenon of secondary phosphorus release occurred in the deep area below 1800 μm. When the concentration of COD decreased from 350 mg·L^-1 to 250 mg·L^-1 and 150 mg·L^-1, the maximum net volume release rate of phosphorus of dephosphorization bacteria decreased from 3.27 mg·(cm³·h)^-1 to 2.44 mg·(cm³·h)^-1 and 2.01 mg·(cm³·h)^-1, respectively, and the rapid uptake area of phosphorus narrowed to the surface of the sludge aggregates.

[Start-up and Optimization of Denitrifying Phosphorus Removal in ABR-MBR Coupling Process]

The feasibility of the denitrifying phosphorus removal process in the ABR-MBR system with no sludge reflux and high concentration of seeding activated sludge (25 g ·L^-1, in MLSS) in the ABR was investigated. The characteristics of the microbial community in the denitrifying phosphorus removal compartment were also evaluated. The denitrifying phosphorus removal function was achieved by gradually increasing the reflux ratio (R) from 0% to 200%. During the stable operation, the average removal rates of COD, PO₄^3--P, and TN in the system were 88.28%, 54.45%, and 61.93%, respectively. When the influent loading rate, NO_x^--N reflux ratio, and hydraulic retention time (HRT) of ABR and MBR were 0.8 kg ·(m³ ·d)^-1, 150%, and 9 h and 3.3 h, respectively, the average VFA concentration of 80.58 mg ·L^-1, ρ(NO₂^--N)/ρ(NO₃^--N) reflux ratio of 1.68, and PO₄^3--P and TN removal rates of 64.94% and 62.95% were obtained. The short-cut nitrification denitrifying phosphorus removal was achieved in the ABR-MBR system. Batch tests showed that denitrifying phosphorus removal bacteria (DPAOs) were the main functional bacteria in the ABR, with anaerobic phosphorus release and anoxic phosphorus uptake of 3.73 mg ·L^-1 and 10.22 mg ·L^-1, respectively. High throughput sequencing results showed that Proteobacteria and Bacteroidetes were the dominant phyla in the phosphorus removal compartment, accounting for 23.49%-53.66% and 16.55%-21.78% of the total phyla, respectively. Thauera, Thiothrix, Pseudomonas, norank_ f_Rhodocyclaceae, and unclassification_ f_Rhodocyclaceae in Proteobacteria, and Sphingobacteriales in Bacteroidetes were the potential denitrifying phosphorus removal microorganisms.

[Simultaneous Short-Cut Nitrification-Denitrification Phosphorus Removal Granules Induced by Phosphorus Removal Granules]

This paper investigated domestic sewage with a low C/N ratio. Mature phosphorus removal granules were inoculated to cultivate granules with a simultaneous short-cut nitrification and denitrification function. The characteristics of nitrogen and phosphorus removal of this process were analyzed. Results show that AOB can be enriched by prolonging the sludge age for 30 days with an aeration intensity of 5 L·(h·L)^-1 and shorter aeration time (140 min), whereas the simultaneous nitrification and denitrification ability could not be improved. The nitrogen loss increased at the aerobic time when aeration intensity was reduced by 3.5 L·(h·L)^-1 and aeration time was prolonged by 200 min. The aeration time was further optimized to restrain the transformation of NO₂^- to NO₃^-, and finally the effluent of TP < 0.5 mg·L^-1 and TN < 15 mg·L^-1. During the process of the system function transformation from phosphorus removal to nitrogen and phosphorus removal, the phosphorus release decreased, however PAOs still played a dominant role (60%) in the process of internal carbon storage. Batch experiments showed that DPAOs that can utilize nitrite as an electron acceptor accounts for 52.43% in the total PAO_S, which alleviated the pressure of the carbon source and improved the simultaneous nitrogen and phosphorus removal.

Effect of Varying Nitrate Concentrations on Denitrifying Phosphorus Uptake by DPAOs With a Molecular Insight Into Pho Regulon Gene Expression

Bacterial Pho regulon is a key regulator component in biological phosphorus-uptake. Poly-phosphate accumulating bacteria used in enhanced biological phosphorus removal (EBPR) system encounter negative regulation of the Pho regulon, resulting in reduced phosphorus-uptake from phosphorus-replete waste effluents. This study demonstrates possible trends of overcoming the PhoU negative regulation, resulting in excessive PO4 3--P uptake at varying concentrations of NO3 --N through denitrifying phosphorus removal process. We investigated the Pho regulon gene expression pattern and kinetic studies of P-removal by denitrifying phosphate accumulating organisms (DPAOs) which are able to remove both PO4 3--P and NO3 --N in single anoxic stage with the utilization of external carbon sources, without the use of stored polyhydroxyalkanoate (PHA) and without any anaerobic-aerobic or anaerobic-anoxic switches. Our study establishes that a minimum addition of 100 ppm NO3 --N leads to the withdrawal of the negative regulation of Pho regulon and results in ∼100% P-removal with concomitant escalated poly-phosphate accumulation by our established DPAO isolates and their artificially made consortium, isolated from sludge sample of PO4 3- -rich parboiled rice mill effluent, in a settling tank within 12 h of treatment. The same results were obtained when a phosphate rich effluent (stillage from distillery) mixed with a nitrate rich effluent (from explosive industry) was treated together in a single phase anoxic batch reactor, eliminating the need for alternating anaerobic/aerobic or anaerobic/anoxic switches for removing both the pollutants simultaneously. The highest poly-phosphate accumulation was observed to be more than 17% of cell dry weight. Our studies unequivocally establish that nitrate induction of Pho regulon is parallely associated with the repression of PhoU gene transcription, which is the negative regulator of Pho regulon. Based on earlier observations where similar nitrate mediated transcriptional repression was cited, we hypothesize the possible involvement of NarL/NarP transcriptional regulator proteins in PhoU repression. At present, we propose this denitrifying phosphorus removal endeavor as an innovative methodology to overcome the negative regulation of Pho regulon for accelerated unhindered phosphorus remediation from phosphate rich wastewater in India and the developing world where the stringency of EBPR and other reactors prevent their use due to financial reasons.

[Start-up and Stable Operation of CANON Coupled with Denitrifying Phosphorus Removal]

A process coupled completely autotrophic nitrogen removal over nitrite (CANON)with denitrifying phosphorus removal in a modified anaerobic baffled reactor (ABR) coupled with a membrane bioreactor (MBR), inoculated with ordinary activated sludge, was proposed for treating artificial wastewater with ammonia 200 mg·L^-1 and COD/TN=1. This experiment studied the start-up of the process and its nitrogen and phosphorus removal efficiency by controlling the recycle ratio and increasing it from 50% to 200% step by step, with a temperature of (25±1)℃ and pH of 7.5±0.2. The results showed that the anaerobic part in the ABR consumed 70% COD, and resulted in a quick start-up of partial-nitrification at 21 d under low DO and high ammonia nitrogen. Then, by controlling the intermittent aeration (exposure stop ratio:2 h:2 h, DO 0.3-0.4 mg·L^-1), the start-up of the CANON part in the coupling process was successfully achieved at 132 d, such that the concentration of nitrates in the electron acceptor of the ABR anoxic section increased steadily, and finally the coupling process started successfully at 160 d. With stable operation, the TN removal load in the MBR reached 0.22 kg·(m³·d)^-1, and the average removal efficiency of COD, TN, and PO₄^3--P was 87.0%, 90.4%, and 81.8%, respectively. The batch experiment estimated that the denitrifying phosphate accumulating organisms (DPAOs) using nitrates as electron acceptors in the ABR accounted for 68% of the phosphate accumulating organisms (PAOs). The DPAOs, ammonia-oxidizing bacteria (AOB), and anaerobic ammonium oxidizing bacteria (AnAOB) have been developed in the system and have good simultaneous nitrogen and phosphorus removal efficiency.

[Effect of Sludge Retention Time and pH on the Denitrifying Phosphorus Removal Process]

In this work, the effects of the sludge retention time (SRT, 35, 25, or 15 d) and pH (7.5, 8.0, 8.5) on denitrifying phosphorus removal were investigated using denitrifying phosphorus bacteria (DPBs) enriched in a sequencing batch reactor (SBR). The results indicated that shortening the SRT from 35 d to 25 d resulted in a decrease in the mixed liquor volatile suspended solids (MLVSS) from 2821 to 2301 mg·L^-1, while the sludge loading rate (F/M) increased from 0.256 kg·(kg·d)^-1to 0.312 kg·(kg·d)^-1. Although the quantity of net phosphorus release and net phosphorus uptake decreased at this stage, the rates of anaerobic phosphorus release, anoxic phosphorus absorption, and denitrification reached their highest levels with values of 25.07, 15.92, and 9.45 mg·(g·h)^-1, respectively, due to the increased sludge activity. Consequently, the phosphorus content of the sludge increased from 4.78% to 5.33%, and the removal rate of PO₄^3--P was stable at above 95% with an average effluent PO₄^3--P concentration below 0.5 mg·L^-1. When the SRT was further shortened to 15 d, the MLVSS decreased to values as low as 1448 mg·L^-1, and the proportion of DPBs in the phosphorus accumulating organisms (PAOs) decreased from 82.4% to 65.7%, indicating that the DPBs were gradually washed out from the system due to the excessively short SRT. At this stage, the phosphorus content of sludge decreased to 3.43%, while the rates of phosphorus release, phosphorus absorption, and denitrification also decreased to some extent. When the pH was increased (7.5-8.0), the anaerobic phosphorus release rate and the anoxic phosphorus absorption rate also increased, and reached 25.86 mg·(g·h)^-1 and 16.62 mg·(g·h)^-1, respectively, at a pH of 8.0. When the pH exceeded 8.0, the phosphorus removal efficiency dropped rapidly, supposedly due to phosphorus chemical precipitation.

[Effect of Influent C/N Ratio on the Nutrient Removal Characteristics of SNEDPR Systems]

To determine the performance of nitrogen and phosphorus removal within a simultaneous nitrification endogenous denitrification system (SNEDPR), an extended anaerobic/low aerobic (dissolved oxygen:0.5-2.0 mg·L^-1)-operated sequencing batch reactor (SBR) was fed with simulation wastewater. The SBR was initiated under a constant influent C/N ratio of 10, with the simultaneous enrichment of polyphosphate-accumulating organisms (PAOs). It was then investigated at different influent C/N ratios of 10, 7.5, 5, and 2.5. The experimental results indicated that, when the influent C/N ratio was 10, SNEDPR could be successfully started up. The effluent PO₄^3--P and total nitrogen (TN) concentrations were 0.1 mg·L^-1 and 8.1 mg·L^-1. PO₄^3--P efficiency, TN efficiency, and SNED efficiency were 99.79%, 89.38%, and 58.0%, respectively. When the influent C/N ratio increased from 5 to 10, the nitrogen and phosphorus removal performance of the system improved with PRA, and SNED efficiency increased from 16.0 m·L^-1 and 48.0% to 24.4 mg·L^-1 and 69.2%, respectively. When the C/N ratio was 10, the TN and PO₄^3--P removal efficiencies increased to 94.5% and 100%, respectfully. When the C/N ratio was decreased to 2.5, the nitrogen and phosphorus removal performance of the system decreased. The PRA and SNED efficiencies were only 1.36 mg·L^-1 and 10%, respectively. During the stable phase of the system (C/N ratio were 10, 7.5 and 5), SNED efficiency reached to 85.9%, with the average effluent concentration of NH₄⁺-N, _x^--N, and PO₄^3--P being 0.0, 8.1, and 0.1 mg·L^-1, respectively.

[Start-up and Stable Operation of ABR-MBR Denitrifying Phosphorus Removal Process]

The nitrogen and phosphorus removal characteristics during the start-up and the long-term operational stability of an anaerobic/anoxic (A/A) ABR coupled aerobic MBR system treating low C/N domestic wastewater were investigated. The results showed that the denitrifying phosphorus bacteria (DPBs) were successfully enriched within 46 d by controlling the nitrate recycling ratio (increasing from 150% to 300%), with a temperature of 30℃±2℃, volume loading rate of 0.8 kg·(m³·d)^-1 and sludge reflux ratio of 80% in the ABR, sludge retention time (SRT) in the denitrifying phosphorus removal functional area of 25 d, and the dissolved oxygen (DO) of 1-2 mg·L^-1 in the MBR. The net phosphorus release and phosphorus uptake of DPBs reached 20.56 mg·L^-1and 27.74 mg·L^-1, respectively. Batch tests demonstrated that about 84.8% of phosphorus-accumulating organisms (PAOs) could use NO₃^--N as an electron acceptor for denitrifying phosphorus removal. After 50 d of stable operation after the successful system start-up, the average removal rates of COD, NH₄⁺-N, TN, and PO₄^3--P were 91.8%, 99.0%, 71.5%, and 94.2%, respectively. The results also suggested that 0.83 mg·L^-1NO₃^--N was consumed per 1 mg·L^-1 PO₄^3--P removed during the denitrifying phosphorus removal, indicating that the simultaneous nitrogen and phosphorus removal was achieved in the ABR-MBR system.

[Effect of Different Sludge Retention Time (SRT) Operations on the Nutrient Removal Characteristics of a SNEDPR System]

This study focuses on the nitrogen (N) and phosphorus (P) removal characteristics in a simultaneous nitrification endogenous denitrification and phosphorus removal (SNEDPR) system operating at different sludge retention time (SRT). Four extended anaerobic/low aerobic (dissolved oxygen:0.5-1.0 mg·L^-1)-operated sequencing batch reactors (SBRs) fed with municipal sewage were studied at different SRT of 5, 10, 15, and 25 d. The experimental results show that a shorter SRT at an SRT ≥ 10 d enhances the competitive advantage of PAOs in the system and an efficient phosphorus removal performance of the SNEDPR system was achieved at a SRT of 10 d and 15 d. Especially at an SRT of 10 d; the average P_{PAOs, An} was 68.4%, the PRA and PUA reached 31.9 and 34.3 mg·L^-1, respectively. The nitrification performance of the system was not affected by SRT changes. The most efficient nitrogen removal performance was achieved at a SRT of 15 d, with a high average TN removal and SNED efficiencies reaching 89.6% and 71.8%, respectively. At a SRT ≥ 10 d, the COD removal performance of the SNEDPR system was also not affected by SRT changes. The COD removal efficiencies were higher than 78%. However, when the SRT was shortened to 5 d, the C, N, and P performances of the system worsened due to the loss of biomass; the SNED and PO₄^3--P removal efficiencies were as low as 5.7% and 0.5%, respectively. In addition, at an SRT=15 d, the sludge-settling performance of the system was the best. The SV and SVI were 20% and 64 mL·g^-1, respectively, and the sludge concentration increased with the extension of the SRT. Under long SRT (25 d) operation, the system showed a good resistance to shock loads, but the sedimentation performance of the sludge deteriorated.

[Effect of NOx--N Recycling Ratio on Denitrifying Phosphorus Removal Efficiency in the ABR-MBR Combined Process]

Based on the coupling of the ABR process and the MBR process, a novel combined ABR-MBR process, including biophase separation, liquid circulation, and functional linkage, was developed to achieve simultaneous carbon, nutrient, and phosphorus removal when treating domestic wastewater with low carbon/nitrogen ratio and to obtain the best combination of ABR, providing a quality carbon source, and MBR, achieving shortcut nitrification by optimizing hydraulic retention time (HRT). The influence of NO_x^--N recycling ratio on nitrogen and phosphorus removal was investigated at NO_x^--N recycling ratios of 100%, 200%, 300%, and 400%, respectively. The experimental results under different conditions showed that the efficiency of denitrifying phosphorus removal in the ABR was found to increase with increasing NO_x^--N recycling ratio from 100% to 300% but decreased when the NO_x^--N recycling ratio was 400%. Shortcut nitrification was achieved by controlling the low dissolved oxygen (DO) concentration ranges from 0.3 to 1.0 mg·L^-1 with the short HRT of 3 h in the MBR reactor. The nitrite accumulation ratio was above 60%, when the NO_x^--N recycling ratio was 300%. Meanwhile, shortcut denitrifying phosphorus removal (where NO₂^--N mainly acted as the electron acceptor for denitrifying phosphorus removal) was achieved and played the dominant role in phosphorus removal.

[Effect of Volume Loading Rate (VLR) on Denitrifying Phosphorus Removal by the ABR-MBR Process]

The effect of volume loading rate (VLR) on denitrifying phosphorus removal was investigated in a continuous-flow ABR-MBR combined process treating domestic wastewater to arrive at optimum process parameters. In the experiment, the VLR of the ABR was set at 0.76, 1.01, 1.51, and 2.27 kg·(m³·d)^-1. The removal of carbon, nitrogen, and phosphorus in the system and the effect of the VLR in the MBR on nitrification performance were observed for each VLR of the ABR. The results showed that under the condition when the VLR of the ABR was 1.51 kg·(m³·d)^-1, the amount of COD removal in the A2 chamber was the largest, and shortcut nitrification was achieved in the MBR when the VLR of the MBR was 1.51 kg·(m³·d)^-1. Meanwhile, the removal efficiency of NH₄⁺-N and TN reached more than 90% and 72%, respectively, the anaerobic P-release and anoxic P-uptake were 7.41 mg·L^-1and 15.42 mg·L^-1, respectively, and the concentration of PO₄^3--P in effluent was lower than 0.5 mg·L^-1, which indicated that the shortcut nitrification was more conducive to strengthening the performance of denitrifying phosphorus removal in the ABR-MBR system.

[Nitrite Type Denitrifying Phosphorus Removal Capacity of Cycle Activated Sludge Technology Processes Under Different Inducing Patterns]

A modified cyclic activated sludge technology (CAST) reactor was utilized to investigate the phosphorus and nitrogen removal performance under different inducing patterns in this experiment. The results show that nitrite addition under anoxic conditions has a more inhibitory effect on the denitrifying phosphorus removal performance of the sludge. The phosphorus removal performance of the system was least effective when nitrite dosage was 5 mg·L^-1. Compared to an anoxic addition system, the CAST system is more stable under aerobic addition conditions. The phosphorus removal properties have a slight fluctuation during each initial operating condition when the nitrite concentrations are 5, 10 and 15 mg·L^-1, respectively. However, the phosphorus removal rate was observed to recover quickly and remain stable at more than 95% after acclimatizing for 10, 6, and 34 days, respectively. The effluent phosphorus concentration was less than 0.5 mg·L^-1 in all cases. It was also found that the phosphorus removal performance deteriorated drastically when the nitrite dosage was 20 mg·L^-1. Nevertheless, the nitrite type denitrifying phosphorus uptake capacity of the sludge was 10.4 times greater than that of the sludge before acclimatizing, suggesting that the phosphorus performance deterioration due to nitrite addition could be relieved and long-term addition is beneficial to enriching denitrifying phosphorus accumulating bacteria using NO₂^- as an electron acceptor. Moreover, the sludge settling performance was found to be effective and the sludge concentration decreased continuously when adding a certain concentration of nitrite under aerobic conditions, which is of significant for sludge reduction.

[Effects of Influent C/N Ratios on Denitrifying Phosphorus Removal Performance Based on ABR-MBR Combined Process]

An ABR-MBR integrated reactor based on a combination of the anaerobic baffled reactor (ABR) with the microbial phase separation and membrane bioreactor (MBR) with high-effect entrapment was constructed and the circulation and interactivity of the combined process were examined by adding nitrate recycling and sludge reflux. By increasing the influent COD to adjust the COD/TN ratio, the influence of the mechanism on the denitrifying phosphorus removal performance under the condition of continuous-flow was investigated. The results showed that the average effluent concentration of soluble phosphorus under different influent C/N conditions were 0.22, 0.34, 0.39, 0.42, and 2.45 mg·L^-1 and the low influent C/N ratio was beneficial to phosphate removal. When the influent C/N was 4.8-6.0, the average removal rates of COD, TN, and soluble PO₄^3--P were more than 87%, 76%, and 93%. In addition, when the influent C/N ratio was 3.6-6.0, the removal of TN was proportional to the anoxic phosphorus uptake of ABR and conducive to the removal of TN after increasing the influent COD concentration. Higher C/N ratios of the influent improved the removal of TN at this stage. Finally, the C/N ratio of 6 was suggested to achieve the simultaneous removal of nitrogen and phosphorus.

[Optimization of Denitrifying Phosphorus Removal Performance Based on ABR-MBR Combined Process]

An integrated process based on combination of the anaerobic baffled reactor(ABR)-membrane bioreactor(MBR) was adopted to treat domestic sewage with low C/N ratio. In order to realize the function of highly efficient denitrifying phosphorus removal, nitrate recycling ratio and sludge recycle ratio were optimized in this study. The results indicated that the optimized denitrifying phosphorus removal efficiency was achieved under the conditions of organic loading rate of 2.0 kg·(m³·d)^-1 in ABR, total hydraulic retention time (HRT) of ABR-MBR at 9 h, the SRT at 15 d, sludge reflux ratio of 100%, and nitrate recycling ratios set to 300%. The average removal rates of TN and soluble PO₄^3--P were 84% and 94%, the amount of phosphorus removed by denitrifying accounted for 87% of the total phosphorus removed, and the average effluent concentration for TN and soluble phosphorus were 12.98 mg·L^-1 and 0.43 mg·L^-1 respectively.