Dissolved Oxygen Control Research Articles

Calibration and validation for a real-time membrane bioreactor: A sliding window approach

The paper presents a novel model calibration and validation strategy of membrane bioreactor (MBR) for wastewater treatment. The approach is based on a dynamic model of the activated sludge process and it consists simultaneously on estimating the model’s parameters and computing the dissolved oxygen control input. Activated sludge model No. 1 (ASM1) has been widely used to describe the biological process of activated sludge processes. However, most system states and parameters within ASM1 are not easily obtained, hence not applicable for model calibration and validation. In this work, a reduced-order model presented herein serves as a tool for predicting the dynamic behavior of the MBR plant. The model contains only 4 measurable states, where 13 parameters need to be identified. To reduce the complexity of the calibration, the static sensitivity analysis is performed to select the sensitive parameters. The selected parameters are identified based on directly measurable real-time data obtained from the plant. In addition, the dissolved oxygen is also maintained at a certain level to mimic the real-time control behavior. Model calibration is achieved based on a sliding window optimization problem, which searches for the optimal parameters set and control variables during each identification cycle. Different datasets sampled for the same MBR plant have been used for model validation. Both calibration and validation results are evaluated by several performance indexes, which indicates an acceptable correspondence with the experimental data. The developed model can be employed for process state estimation and control purpose as well as design issues for MBR systems.

Auto-Control Technique Using Gradient Method Based on Radial Basis Function Neural Networks to Control of an Activated Sludge Process of Wastewater Treatment

Dissolved oxygen (DO) concentration is a key variable in the activated sludge wastewater treatment processes. In this paper, an auto control strategy based on Euler method and gradient method with radial basis function (RBF) neural networks (NNs) is proposed to solve the DO concentration control problem in an activated sludge process of wastewater treatment. The control purpose is to maintain the dissolved oxygen concentration in the aerated tank for having the substrate concentration within the standard limits established by legislation of wastewater treatment. For that reason, a new proposed control strategy based on gradient descent method and RBF neural network has been used. Compared with RBF neural network PI control, the obtained results show the effectiveness in terms of both transient and steady performances of proposed control method for dissolved oxygen control in the activated sludge wastewater treatment processes.

Event-triggered PID control for wastewater treatment plants

This paper is concerned with the dissolved oxygen control problem in the wastewater treatment plants (WWTPs) based on a dynamic event-triggered PID control strategy (D-EPID). The aim of the paper is to develop a simple event-triggered PID strategy for practical application. The developed strategy differs from existing PID controllers in that, it updates the control input only when certain triggering conditions are satisfied, while existing controllers update the control input at each sampling instant. The latter controller may consume extra energy even there is no need to update the control input. The triggering condition is designed based on the deviation of tracking error and an auxiliary variable. The stability analysis is conducted theoretically via designing an appropriate Lyapunov function, which cannot be provided by existing PID control methods. The controller is designed for improving the control performance of WWTPs by regulating one significant variable, i.e., oxygen concentration at some set points. Simulations based on the BSM1 platform are conducted to verify the effectiveness of the proposed control strategy. Simulation results show that for our proposed control method, the updating number of the control input can be decreased greatly while maintaining the desired tracking performance.

On the control of coolant parameters in long-life facilities

When nuclear power plants with heavy coolants are brought to operating mode as well as during their operation, it is necessary to control and maintain the oxygen content in the coolant within the specified limits. As a rule, the oxygen content in metal melts is controlled by sensors based on solid oxygen-ionic electrolytes. The article presents an analysis of the methodological aspects of dissolved oxygen control in non-isothermal circulating loops with metal coolants, using such sensors. It is shown that in the presence of dissolved loop wall materials and suspensions of their various oxides in the coolant, control over the values of the oxygen activity and concentration calculated for a pure coolant is in general unjustified. The authors present the experimental results of the distribution of oxidation potentials along the loop depending on the coolant temperature, obtained during long-term tests of cladding samples in a lead melt in two circulation facilities – SM2-M and TsU1-M – which differ in principal methods for maintaining specified oxygen conditions. In the low temperature region, the experimental values of the oxidation potential in both facilities are lower than those calculated for pure lead, which leads to a difference by two or more times of the calculated oxygen concentrations for the regions of the loop with Тmin and Тmax, i.e., the so-called oxygen ‘non-isoconcentration’ is observed along the loop. In deoxidation mode during hydrogen ejection into the coolant, the oxidation potential in the loop changes in a complex way, and it makes no sense to talk about the oxygen concentration. It is concluded that in long-life facilities, the coolant parameters for oxygen must be controlled not by the calculated oxygen activity or concentration but by the oxidation potential in the maximum temperature region. To obtain the correct values of the oxidation potential, measurements should be carried out in temperature-stable modes of throughout the facility.

A pretreatment method of wastewater based on artificial intelligence and fuzzy neural network system

A pretreatment method of industrial saline wastewater based on Artificial Intelligence based fuzzy neural network analysis was proposed to improve the pretreatment accuracy of industrial saline wastewater. This method uses a four-layer AI fuzzy neural network model and proposes a graded fuzzy neural network model for pretreatment method of industrial saline wastewater, it includes input layer, fuzzification layer, fuzzy logical layer and output layer, and designs the framework and calculation mode of the fuzzy function block and the neural network module. Finally, the dynamic simulation experiments of dissolved oxygen control in the fifth zone and nitrate nitrogen control in the second zone are carried out based on the simulation benchmark model (BSM1) platform. The experimental results show that this approach can effectively raise the adaptive control accuracy of the system compared with PID, feed forward neural network and conventional recurrent neural network.

Oxygen transfer rate affect polyhydroxybutyrate production and oxidative stress response in submerged cultures of Rhizobium phaseoli

Oxygen is a key factor during free-state growth of Rhizobium phaseoli or symbiosis with common bean roots. Five different initial oxygen transfer rates (OTRI) were fix by dissolved oxygen control and constants stirrer speed rates to determine the growth of R. phaseoli in a lab-scale bioreactor. The biomass did not presented differences in the maximum concentrations for OTRI of 1.0, 1.7 and 5.9 mmol l−1 h−1. However, a sublethal effect was observed for 11.9 and 20.2 mmol l−1 h−1, where lower biomass, kinetic and stoichiometric parameters were determined. Catalase activity as an oxidative stress indicator was approximately two-fold greater with 20.2 mmol l−1 h−1 than lower OTRI. Polyhydroxybutyrate (PHB) production did not present differences in the maximum concentration. Nevertheless, an increase in PHB accumulation, yield on biomass and specific productivity were determined at the higher OTRI, suggesting a defense mechanism against oxidative stress induce by the enhancement of the oxygen transfer rates.

In situ startup of a full-scale combined partial nitritation and anammox process treating swine digestate by regulation of nitrite and dissolved oxygen

A challenge during the startup of the combined partial nitritation and anammox process is how to balance dissolved oxygen control and nitrite accumulation for converting partial nitritation into anammox, maintaining stable partial nitritation and promoting growth of anammox bacteria. An innovative regulation strategy of nitrite dosing and dissolved oxygen control in this study was developed to achieve the rapid startup of a full-scale combined partial nitritation and anammox reactor within 77 days and the total nitrogen removal rate of reactor was 0.097 kg N/kgMLSS·d−1, and the activity and gene copy concentration of anammox bacteria reached 0.307 kg N/kgMLVSS·d−1 and 7.79 × 109 copies/gMLVSS, respectively. Microbial community analysis revealed that Candidatus_Kuenenia and Nitrosomonas were the dominant nitrogen transformation bacteria with an abundance of 2.49% and 14.86%, respectively. This study offers a new method for rapid startup and spreading application of the full-scale anammox process.

Impacts of dissolved oxygen control on different greenhouse gas emission sources in wastewater treatment process

Activated sludge wastewater treatment plant (WWTP) has been recognized as one of the main contributors of greenhouse gas (GHG). Several technologies have attempted to control GHG in WWTP but have resulted in limited impacts and caused secondary pollution. Recently, often calculating total GHG as an evaluation indicator, the GHG emissions from different GHG sources have rarely been investigated. In this study, focusing on the automation of WWTP process, an alternative solution is proposed to explore the effects of control implement on GHG reduction. Firstly, simulating with the influent data of 14-days stormy weather, based on the recognized benchmark simulation model No 1 (BSM1) that accounts for the reaction kinetics and material balances of microorganism, the on-site GHG emission models of four sub-processes in WWTP, including CO2 from endogenous decay, biochemical oxygen demand removal, nitrification, and N2O from denitrification, are developed, and the off-site CO2 and CH4 emissions caused by the sludge disposal, CO2 from energy for aeration, pumping and chemical usage, are also estimated. The simulation results show that the overall averaged GHG of 844.01 kg CO2-eq/h are emitted, where 25.83% emits from the on-site sub-processes and 74.17% discharges from the off-site sources. Subsequently, the traditional proportional-integral (PI) control strategy is applied to control the concentrations of dissolved oxygen (DO) in three aerated tanks. With the control actions, the DO-PI control systems manage to reduce the GHG emissions by 15.53 kg CO2-eq/h averagely, and the GHG reduction mainly comes from the sources of aeration and nitrification, which are reduced by 11.67 kg CO2-eq/h and 4.64 kg CO2-eq/h, respectively. But, there are few influences on the other sources. Summarily, the impacts of DO-PI control on the GHG emissions from different sources of WWTP are investigated, revealing that the automatic control system can be a practical and viable approach to achieve the GHG reduction in WWTP, which caters to the sustainable development of WWTP.

Design of water quality monitoring system for aquaculture ponds based on NB-IoT

In order to promote the development of aquaculture informatization and monitor aquaculture ponds more accurately and conveniently, this article has developed a water quality monitoring system for aquaculture ponds based on the narrow band internet of things (NB-IoT) technology. This system realizes remote collection and data storage of multi-sensor processor information (temperature, pH, dissolved oxygen (DO) and other environmental parameters), as well as intelligent control and centralized management of breeding ponds. The system uses STM32L151C8 microcontroller and sensor terminal real-time acquisition, such as temperature, pH value, dissolved oxygen. It realizes data aggregation and transmission over a long distance to the Internet of things (IoT) telecom cloud platform through the technology of NB-IoT. The software called Keil implement the data format design of wireless communication module and data transmission. Java is used to develop background monitoring applications for accessing cloud platform, controlling underlying devices and local data processing. It can not only send hypertext transfer protocol (HTTP) requests to monitor cloud platform data, but also issue commands to the underlying control module to control the startup and shutdown of equipment such as aerator. The system was implemented and tested in ChangZhou, JiangSu Province, China. The experimental results showed that the system can obtain water quality parameters in time. The temperature control accuracy is maintained at ±0.12℃, the average relative error is 0.15 %, the dissolved oxygen control accuracy is maintained within ±0.55mg/L, the average relative error is 2.48 %, the pH control accuracy is maintained at ±0.09, and the average relative error is 0.21 %. The system has stable overall operation, real-time and accurate data transmission, which can meet the actual production needs and provide strong data and technical support for further water quality regulation and aquaculture production management.

Automatic Dissolved Oxygen Control to Oreochromis Fish’s Crop into Geomembrane Tank

Huila is a Colombian state with a high production of Oreochromis fish, its contribution to national production is of 53%, that is distributed in 338 hectares of ponds on land and floating cages. The environmental and climatic characteristics of the region allow having dissolved oxygen production in the ponds between 4ppm and 12ppm during the day, but at night the situation is unfavorable, since the amount of dissolved oxygen can decrease up to 1ppm while carbon dioxide increases. Therefore, it is necessary to have adequate oxygenation equipment and systems to prevent delayed in fish growth and to decrease death rates. This article presents the design and implementation of an automatic dissolved oxygen control system by manipulation of a water recirculation flow that operates in parallel with an industrial oxygen generator. The implemented system tracks and records the temperature and oxygen variables present in the geomembrane tank to evaluate the process evolution for different periods of the fish development cycle. The data was acquired using an Atlas Scientific dissolved oxygen sensor kit and a DS18B20 temperature probe they send the data directly to a Raspberry Pi that transmits by wireless the information collected from the process to the SISCEFA web server and a mobile application through which users can observe the data traceability. The dissolved oxygen concentration was maintained within the threshold established and the fish rate death decrease.

Enhanced Energy Saving in Wastewater Treatment Plant using Dissolved Oxygen Control and Hydrocyclone

Abstract Water and energy are precious commodities. With the rising population, their demand is escalating day by day. If we talk about water, groundwater is the primary freshwater source for private colonies, apartments and institutions in India. The present media reports forecast that 21 Indian cities will be running out of groundwater by 2020. Reuse of treated water from wastewater treatment for non-domestic applications is one of the best alternatives to control the scarcity of fresh water. The conventional Wastewater Treatment Plants (WWTPs) have limited capacity and are energy-intensive. The existing WWTP of the LNM Institute of Information Technology (LNMIIT) is retrofitted to achieve the required capacity and quality for reuse. In this paper, some of the novel techniques like hydrocyclone and smart aeration control have been adopted for energy saving per unit of wastewater treated. Hydrocyclone was designed and installed for primary sludge separation. The air supplied in the bioreactor was controlled through the smart aeration control system consisting of Variable Frequency Drive (VFD), Dissolved Oxygen (DO) sensor and Programmable Logic Controller (PLC). Under this process, the installation of VFD enhanced the power factor by 17.15%, thus creating energy saving of 17.49%. The DO control with VFD in the bioreactor reduced the electrical energy consumption for aeration by 66.75%. The installation of hydrocyclone as a primary sludge separator with DO control saved the electrical energy for aeration by 71.46%. The developed system provided stable operation with enhanced aeration energy efficiency and treatment capacity.

Quantifying the Benefit of a Dynamic Performance Assessment of WWTP

In this work a comprehensive analysis of the environmental impact of the operation of a wastewater treatment plant (WWTP) using different control strategies is carried out considering the dynamic evolution of some environmental indicators and average operation costs. The selected strategies are PI (proportional integral) control schemes such as dissolved oxygen control in the aerobic zone (DO control), DO control and nitrates control in the anoxic zone (DO + NO control) and regulation of ammonium control at the end of aerobic zone (Cascade SNHSP) commonly used in WWTPs to maintain the conditions that ensure the desired effluent quality in a variable influent scenario. The main novelty of the work is the integration of potential insights into environmental impact from the analysis of dynamic evolution of environmental indicators at different time scales. The consideration of annual, bimonthly and weekly temporal windows to evaluate performance indicators makes it possible to capture seasonal effects of influent disturbances and control actions on environmental costs of wastewater treatment that are unnoticed in the annual-based performance evaluation. Then, in the case of periodic events, it is possible to find solutions to improve operation by the adjustment of the control variables in specific periods of time along the operation horizon. The analysis of the annual average and dynamic profiles (weekly and bimonthly) of environmental indicators showed that ammonium-based control (Cascade SNHSP) produce the best compromise solution between environmental and operation costs compared with DO control and DO + NO control. An alternative control strategy, named SNHSP var Qcarb var, has been defined considering a sequence of changes on ammonium set-point (SNHSP) and carbon dosage (Qcarb) on different temporal windows. It is compared with DO control considering weekly and bimonthly profiles and annual average values leading to the conclusion that both strategies, Cascade SNHSP and SNHSP var Qcarb var, produce an improvement of dynamic and annual average environmental performance and operation costs, but benefits of Cascade SNHSP strategy are associated with reduction of electricity consumption and emissions to water, while SNHSP var Qcarb var strategy reduces electricity consumption, use of chemicals (reducing external carbon dosage) and operation costs.

Dissolved Oxygen Control in Biological Wastewater Treatments with Non-Ideal Sensors and Actuators

The improvement of the dissolved oxygen control is one of the main objectives in the research works on control of wastewater treatment plants. In the research literature, most of the works are base...

Wet Metal Recess Process for Enabling Scaling Boosters

Accompanied with the downscaling of semiconductor devices, many challenges arise in the semiconductor fabrication process. One of the biggest challenges is securing patterning margin as dimension size of semiconductor structures shrink. To overcome these challenges, scaling boosters are proposed, such as FSAV (Fully Self-Aligned Via) and SAGC (Self-Aligned Gate Contact) [1-2]. The common requirement for these scaling boosters is the metal line recess prior to upper interconnect layer formation. For these metal recess processes, the recess amount uniformity is required (within wafer, pattern to pattern), since recess amount nonuniformity results in electrical resistance variation. Also, surface roughness control is important to minimize electron scattering at the surface, which also results in resistance increase. The concept of Cu recess using a cyclic process comprising of wet oxidation and oxide removal steps for controlled metal recess was previously reported [3]. In this study, the process for applying the above-mentioned requirements for advanced interconnects has been further optimized. In a cyclic process comprising of wet oxidation and metal oxide removal, 3% H2O2 and 0.05% HF (both at room temperature) were used. It was found that the Cu and Co recess amounts were impacted significantly depending on the pattern width when the H2O2 – HF cyclic process was applied. For Cu, the recess amount was around 10 nm for 440 nm Cu line width; however, the recess amount increased to 35 nm for 20 nm Cu widths. This phenomenon can be explained by the difference of grain sizes as a function of pattern widths. Cu grain size is known to be smaller when patterned in smaller trenches [4], which results in a higher density of the grain boundaries. It is believed that an increase in grain boundary results in a faster reaction rate due to the greater lattice disorder compared to the bulk structure; therefore, Cu and Co recesses are estimated to proceed faster in smaller trenches compared to larger trenches. This leads to nonuniformity of the recess amount when different trench widths exist on the same surface. To mitigate this effect, different etchant chemistries were evaluated. By using organic acids like acetic acid or citric acid, the recess amount uniformity between different CD sizes improved drastically. Since acetic acid and citric acid have higher molecular volumes compared to HF, they are less susceptible to grain boundary lattice disorders. As a result, the recess amount range between different CD sizes (20 nm ~ 440 nm) reduced from 26 nm (H2O2 - HF) to 3 nm (H2O2 – Acetic acid). This process is effective not only for Cu recess, but also for Co recess. Another factor for recess amount uniformity control is dissolved oxygen level in etchant chemistry. It has been reported that dissolved oxygen control is a key factor for controlling metal etching amount in acidic chemistry [5]. It will also be shown that both dissolved oxygen and atmospheric oxygen impact on Cu and Co recess during the organic acid step. Finally, the optimized metal recess process for Cu/Co enabling scaling boosters for advanced process nodes will be proposed.

Systematic evaluation of high-throughput scale-down models for single-use bioreactors (SUB) using volumetric gas flow rate as the criterion

Developing representative bioreactor scale-down models is a critical part in process development and characterization. Here we present a systematic analysis of high-throughput scale-down models (ambr®250, 250 mL) developed for 500 L or 2000 L single-use bioreactors using engineering approaches. A main scaling criterion, vvm (volume of gas per volume of liquid per minute), was studied for scale-down model development after analyzing bioreactor hydrodynamic environment and gas transfer characteristics. Two different processes with distinguished peak cell densities (12–14 vs. 20–25 × 106 cells/mL) for a monoclonal antibody (mAb) were evaluated. We demonstrated that scaling-down using similar vvm as the criterion is feasible in reproducing large-scale gas transfer characteristics for both cell densities, though more challenging in high-density conditions. Furthermore, a range of pCO2 levels could be generated through vvm modulations. The same vvm approach was successfully applied to two other mAbs. Practical aspects of ambr®250 operations, especially the impact of background air sparge rate and antifoam addition on dissolved oxygen control and gas transfer, were also discussed. Understanding and managing gas transfer gaps between small- and large-scale bioreactors is expected to streamline scale-up effort, and equally importantly scale-down activities for process characterization.

Study and Application of Self-Adaptive Fuzzy PID Control in Dissolved Oxygen Control of Wastewater Treatment

In view of the typical nonlinear, time-varying and large lag characteristics of dissolved oxygen (DO) concentration control in wastewater treatment, it is difficult to obtain a good control performance by using conventional proportional-integral-derivative (PID) control. In order to greatly improve the control effect, designing a self-adaptive fuzzy PID controller. Through analysis and reasoning can obtain the optimal control effect under the optimal membership. MATLAB simulation and application show that compared with the conventional PID control, the self-adaptive PID controller accelerates the response speed of the dissolved oxygen control system and reduces the overshoot. This control method achieves a better control performance in the actual sewage treatment central control system, improves the sewage treatment efficiency, and has the value of popularization and application.

A model free dissolved oxygen controller for industry effluent using hybrid variables measuring technique

<p>The present scenario of the world relays on water scarcity, enormous amount of water is needed for people but the world has pollution which makes a great effect. Water is polluted due to various reasons such as industrial waste, sewage, and global warming and oil pollution and mainly polluted due to industrial effluent waste water. The main objective of this paper is to control the dissolved oxygen of the industrial waste water by measuring turbidity and dissolved oxygen (DO). Turbidity is measured through the intensity of the light through the medium in Nephelometric unit (NTU) and dissolved oxygen is measured in PPM through amperometic electrode method. The controller is designed to improve the quality of the effluent water through the aeration process. The very low concentration of dissolved oxygen (DO) level is too harmful to the aquatic ecosystem and it pollutes the water to the maximum extent. This measurement method of turbidity has an enhanced idea which processed with the software and it is monitored. The controller is designed as such with the condition of inverse proportionality of water turbidity and dissolved oxygen, which contributes an additive advantage to the concept design.</p>

Effective biodegradation of chicken feather waste by co-cultivation of keratinase producing strains

BackgroundChicken feather, a byproduct of poultry-processing industries, are considered a potential high-quality protein supplement owing to their crude protein content of more than 85%. Nonetheless, chicken feathers have been classified as waste because of the lack of effective recycling methods. In our previous studies, Bacillus licheniformis BBE11-1 and Stenotrophomonas maltophilia BBE11-1 have been shown to have feather-degrading capabilities in the qualitative phase. To efficiently recycle chicken feather waste, in this study, we investigated the characteristics of feather degradation by B. licheniformis BBE11-1 and S. maltophilia BBE11-1. In addition, in an analysis of the respective advantages of the two degradation systems, cocultivation was found to improve the efficiency of chicken feather waste degradation.ResultsB. licheniformis BBE11-1 and S. maltophilia BBE11-1 were used to degrade 50 g/L chicken feather waste in batches, and the degradation rates were 35.4% and 22.8% in 96 h, respectively. The degradation rate of the coculture system reached 55.2% because of higher keratinase and protease activities. Furthermore, cocultivation was conducted in a 3 L fermenter by integrating dissolved oxygen control and a two-stage temperature control strategy. Thus, the degradation rate was greatly increased to 81.8%, and the conversion rate was 70.0% in 48 h. The hydrolysates exhibited antioxidant activity and contained large quantities of amino acids (895.89 mg/L) and soluble peptides.ConclusionsCocultivation of B. licheniformis BBE11-1 and S. maltophilia BBE11-1 can efficiently degrade 50 g/L chicken feather waste and produce large amounts of amino acids and antioxidant substances at a conversion rate of 70.0%.

A hierarchical state estimation and control framework for monitoring and dissolved oxygen regulation in bioprocesses.

The integration of state estimation and control is a promising approach to overcome challenges related to unavailable or noisy online measurements and plant-model mismatch. Extended Kalman filter (EKF) and moving horizon estimator (MHE) are widely used methods that have complementary features. EKF provides fast estimation and MHE optimal performance. In this paper, a novel hierarchical EKF/MHE approach combined with a dynamic matrix controller (DMC), denoted as EKF/MHE-DMC, is proposed for process monitoring and dissolved oxygen control in airlift bioreactors. The approach is successfully tested in simulated cultivations of Escherichia coli for recombinant protein production, considering specific scenarios of step set point tracking, step disturbance rejection, plant-model mismatch, and measurement noise. Results also show that, given a model that describes the measured dissolved oxygen precisely, as assumed in this study for the in silico experiments, the EKF/MHE-DMC approach is able to estimate the cell, protein, substrate, and dissolved oxygen concentrations based only on the measurement of the latter, reducing the estimation error by 93.8% when compared to a benchmark case employing EKF and DMC. The general structure of the proposed EKF/MHE-DMC framework could be adapted for implementation on other relevant bioprocess systems employing their derived process models.

Partial Nitrification and Denitrification of Low C/N Ratio Sewage Based on Zoning Oxygen and Dissolved Oxygen Control

Poor nitrogen removal from municipal sewage is mainly due to insufficient carbon source and low C/N ratio. The A2/O pilot plant was established to investigate the accumulation rate of nitrous nitrogen and the removal of nitrogen pollutants by adjusting the ratio of anoxic/aerobic zoning and dissolved oxygen levels in the aerobic zone. The results showed that when DO is 2.0-2.5 mg·L-1, changing the ratio of anoxic to aerobic zoning had little effect on the reaction system, and it was difficult to realize partial nitrification. When DO is 0.5-0.8 mg·L-1, VAnoxic:VAerobic=1:1, this is the best working condition of the system. The accumulation rate of nitrous nitrogen at the end of aerobic zone is stable at more than 62%, and the total nitrogen of effluent is reduced to 9.0 mg·L-1, which can achieve the goal of deep denitrification. Analyzing the apparent activity of nitrifying bacteria, it was found that the SAOR and SNOR (according to N/VSS calculation) were 0.14 g·(g·d)-1 and 0.04 g·(g·d)-1, respectively, under the optimum conditions. The difference between them was more obvious than that in other stages of the experiment, that is, the higher inhibition of NOB activity was the direct reason for the increase of nitrite accumulation rate. Illumina MiSeq sequencing showed that the number of NOB in this stage was significantly lower than that in other stages. Intermittent OUR method was used to analyze the composition of carbon sources at the inlet and outlet of the anoxic zone. The results showed that short-cut nitrification and denitrification could save 27.3% of the carbon sources under the optimal operating conditions. The biodegradable COD consumption in the anoxic zone was 63.6%, which was much higher than that in other stages.