Internal Recycle Ratio Research Articles

Effect of internal and external recycle ratios on the nutrient removal efficiency of anaerobic/anoxic/oxic (VIP) wastewater treatment plant

Abstract Due to the disposal of different wastewater into the water bodies, the rate of surface water pollution is increasing. The virginia initiative plant (VIP), one of the most efficient and economical wastewater treatment systems, was assessed. The experiments were carried out by a laboratory-scale VIP system used for this study, with a flow rate of 100 L/day and a solid retention time rate estimated at 10 days. The system works on three different ratios for internal rotation (100, 150, and 200%) and three for external rotation (80, 90, and 100%), and the effective volumes were 20, 40, and 60 L for anaerobic, anoxic, and oxic reactors, respectively. The results showed that the VIP system achieved the best removal efficiency of organic matter represented by COD, phosphorous, and ammonia (86, 94, and 93%, respectively). The impact of internal and external rotation ratios was tested by removing COD, phosphorous, and ammonia. The percentages of internal rotation significantly affect the biological removal of nitrates. The relationship between them is inverse, while the percentages of external rotation significantly impact the biological removal process of phosphorus. The relationship between them is positive, whereas the internal and external rotation percentages did not considerably affect the efficiency of removing both ammonia and COD. According to the research results, internal and external rotation ratios enhanced the removal efficiency of phosphorus and nitrates. The VIP system proved to be an effective method for domestic wastewater treatment with a flow conforming to Iraqi standards for draining wastewater with all organic matter, phosphorous, and nitrogenous compounds to rivers.

Influence of growth media and internal recycle rate on nitrogen removal from high-strength wastewater in a low footprint anoxic-upflow/aerobic process

This research investigated the use of integrated fixed-film media in comparison to suspended-growth in a low-footprint upflow biological nitrogen removal (BNR) process to find a robust solution for treatment of high-strength dairy farm wastewater (DFW). Using real DFW and synthetic wastewater (SWW), two juxtaposed anoxic upflow bioreactor/aerobic systems (UBR-A) were operated under different internal recycle ratios (R) of 3 and 4 in both modes. Performance of the UBR-A system was sensitive to R, particularly in suspended-growth. The denitrification efficiency (ηD) and nitrate removal rates in this mode were significantly lower (p < 0.05) at R= 4 compared to R= 3 with both real DFW and SWW. With DFW, this was observed despite a higher COD/NO3-N of 20–50. In integrated fixed-film, ηD at R= 4 significantly improved to ∼95 % with both feeds. Whereas a COD/NO3-N > 9 was required to offset the imbalance caused by increased NO3-N loading at R= 4. Modeling of the denitrification potential controlled by R (PD-IR) and COD availability (PD-COD) showed that denitrification reciprocated with PD-IR. From the multiple regression analysis, nitrate removal was linearly correlated with growth medium, R, and wastewater type (p < 0.00001) accounting for 43 % of the variation. Nitrification efficiency was consistently higher > 90 % under all conditions but specific nitrification rates nearly doubled in integrated fixed-film mode. Correspondingly, the total nitrogen removal increased from 67 % to 80 % in DFW-system and 59–84 % in SWW-system at R= 4 compared to suspended growth at the same R. The study results add to the understanding of integrated media in BNR processes for high-strength wastewater treatment.

Mechanism insights into the nitrogen removal of bio-augmented AAO: Specifically focusing on the nitrogen metabolic pathways and microbial taxa-functional genes associations

Rapid loss of inoculant microorganisms and the slow-growth of autotrophic bacteria pose significant challenges to bio-augmentation technology. Given the low-cost of coal ash and its greater compatibility with nitrifiers, it was utilized as a carrier to maintain bio-augmentation performance and enhance the AAO's nitrogen removal. Results showed the bio-augmented system operated stably, and immobilized coal ash improved the efficiencies of NH4+-N from 67.6 ± 10.8 % to 94.3 ± 3.0 % and TIN from 63.2 ± 9.2 % to 84.3 ± 4.3 %. An in-depth analysis indicated bio-augmentation altered the microbial community structures and increased the relative abundance of bacterial associated with nitrogen removal, significantly increasing the abundance of Nitrosomonas and Nirospira. In addition, bio-augmentation enhanced the activity of nitrogen removal enzymes, especially amoABC, hao, narGH, nirK, and nosZ transporter genes, according to predicted function profiles. Network analysis between microorganisms and N-metabolic functional genes revealed bio-augmentation improved the complexity of network-structure and the sources of N-metabolic functional genes, thereby enhancing functional stability. Return sludge flow was the most significant factor in regulating the microbial community composition associated with N removal in the bio-augmented AAO system, followed by DO and internal recycle ratio. The aforementioned results obtained from the present study will enable the application of immobilized coal ash in wastewater treatment plants to improve N removal efficiency.

Plant-wide modeling of a metropolitan wastewater treatment plant to reduce energy consumption and carbon footprint.

A real metropolitan wastewater treatment plant (RWWTP) serving a population equivalent of 1.55 million was modeled to reduce energy consumption and carbon footprint (CFP). An approach was proposed to handle the dilution factor and partial aeration due to discontinuous air diffuser locations in the Bardenpho-5 configuration. Various operational, structural, and configurational modifications were evaluated. Results indicated that management scenarios might provide conflicting outcomes for different targets. Reduced energy consumption may not result in lower CFP at the same time. Moreover, operational changes that would impact total nitrogen (TN) concentrations and N2O release may significantly impact CFP. A policy of using a modified Bardenpho-5 process with reduced internal recycle (IR) ratio, waste activated sludge (WAS), and return activated sludge (RAS) flow rates provided the lowest CPF. Modified Bardenpho-5 process and replacing belt thickeners with gravity thickeners supplied the highest savings in energy consumption. Overall, up to 14% and 20% reductions were possible in the energy consumption and CFP of the plant, respectively. The RWWTP may save up to 10% in energy expenses annually by operational modifications.

Nitrous oxide (N2O) emissions from a pilot-scale oxidation ditch under different COD/N ratios, aeration rates and two shock-load conditions

Nitrous oxide (N2O) generated from wastewater treatment plants (WWTPs) has drawn attention due to its high emission load and significant greenhouse effect. In the present study, N2O emissions from a pilot-scale Carrousel oxidation ditch under various chemical oxygen demand (COD) to nitrogen ratio (COD/N) and aeration rates were systematically investigated. The highest N2O emission factor was 0.142 ± 0.013%, at COD/N of 5 and aeration rate of 1.8 m3 h−1, which was much lower than the majority of previous studies. The results could be attributed to the high internal recycle ratio of the oxidation ditch process which lightened the burden of influent load to the system. The profiles of N2O emissions and dissolved N2O concentration along the channels showed a distinct spatial variation that N2O emissions primarily occurred in the aeration zones due to the air stripping effect. However, both the aeration and anoxic zones contributed to N2O generation due to autotrophic nitrification (AN), which was considered to be the main N2O generation process. In addition, two simulated shock-load conditions, ammonia overload shock and aeration failure shock, were carried out to explore the response of the biological nitrogen removal (BNR) system. The results indicated that both shock-loads lead to excessive N2O emissions, especially at higher aeration rates, which could be explained by the improved N2O generation by AN process during the shock-load period. This study offered new insights into the role of operational parameters to N2O emission and the alternative approach for N2O mitigation during both the steady-state operation and shock-load conditions in the oxidation ditch process.

Fate of aromatic amines through decolorization of real textile wastewater under anoxic-aerobic membrane bioreactor

Abstract The presence of residual dyes and their biotransformation products in textile wastewater is considered the main challenge of wastewater treatment plants. Many biological treatment methods have been used for treatment of textile wastewater. However, incomplete dye mineralization produces colorless Aromatic Amines (AAs) which are considered carcinogenic and highly toxic to aquatic organisms. This study assessed the removal efficiency and the fate of AAs in the treatment of real textile wastewater by anoxic-aerobic MBR. The fate of 22 AAs in both aqueous and sludge phases was investigated under different Internal Recycle ratio (IR). Most AAs possessing one aromatic ring and an Electron Donating Group (EDG) such as (-NH2) and (-CH3) are highly biodegradable under aerobic condition (above 80%), whereas AAs with 2 aromatic rings and strong Electron Withdrawal Group (EWG), such as azo bond and (-Cl) exhibited low to moderate removal efficiency (below 75%). The ∑AAs increased by 91% (from 205.1 to 393.7 ng/L) in the anoxic tank (IR=2.0), owing to the unique bacterial diversity under high IR operation scheme. On the other hand, ∑AAs in the anaerobic tank (IR=0.0) increased by 6.1% (from 489.26 to 519.1 ng/L), whereas, the ∑AAs in the aerobic tank remained constant at 50 ng/L in both cases. Most of AAs accumulated in sludge under reducing condition due to the refractory behavior of AAs in this environment, whereas hydrophobic AAs were slightly accumulated in aerobic sludge.

Effect of Internal Recycle Ratio on Nitrogen Removal Efficiency in a Three-stage ANAMMOX Process for Treating Reject Water

Objectives:In order to improve the stability of the ANAMMOX process, a three-stage ANAMMOX process combined with denitritation/nitritation process was operated, and effect of internal recycle ratio on the nitrogen removal efficiency was investigated.Methods:For the treatment of supernatant from anaerobic digester, a three-stage ANAMMOX process which combined as anoxic/oxic reactor and subsequent ANAMMOX reactor was installed. The reactor was designed with a working volume of 2.5 m&lt;sup&gt;3&lt;/sup&gt;. The internal recycle ratio was considered as the main operating parameter, and the performance of nitrogen removal in the ANAMMOX process was evaluated.Results and Discussion:As the internal recycle ratio was increased from 0 to 1.5Q, reduction of the nitrogen load increased to 23.5 ± 3.8%, and the nitrogen load reduced by internal recycle was 0.22 ± 0.04 kgN/m&lt;sup&gt;3&lt;/sup&gt;/d. Since the ratio of NO&lt;sub&gt;2&lt;/sub&gt;-N/NH&lt;sub&gt;3&lt;/sub&gt;-N introduced in the ANAMMOX process is very important, considering this together, the most suitable internal recycle ratio was determined to be 1.5Q. As a result of introducing the effluent from the A/O process into the ANAMMOX process, the nitrogen removal rate could be stably maintained at 1.27 kg/m&lt;sup&gt;3&lt;/sup&gt;/d.Conclusions:In the ANAMMOX process in conjunction with the A/O process including internal recycle, the A/O process represented an effect of reducing the influent nitrogen load and nitrite concentration, and the ANAMMOX process was able to effectively remove residual nitrogen.

Performances of Anoxic- Aerobic Membrane Bioreactors for The Treatment of Real Textile Wastewater

&lt;p&gt;Wastewater from textile industry is considered one of the major environmental challenges due to the large volume of highly colored, polluted and toxic effluent. This study investigated the treatability of real textile wastewater by pilot-scale anoxic-aerobic Membrane Bioreactor (MBR) system without sludge wasting for operation period of 100 days. The proposed system was investigated under different Internal Recycle (IR) ratios and the impact of IR ratio on Total Organic Carbon (TOC), Total Nitrogen (TN) and Color removals were examined. Under IR ratios between anoxic and aerobic tanks of 0.0, 0.5 and 2.0, the respective average removal efficiency of TN was 20.9%,53.4% and 71.7%, whereas average color removal of 81%, 85% and 88%, respectively was noted. The results indicated that increase of recycle ratio from 0.5 to 2.0 enhanced TN removal to about 71% and color removal to above 85%. The IR between anoxic and aerobic tanks has a significant role in TN and color removal due its effect on the development of bacterial communities. On the other hand, the results indicate over 93% TOC removal, which was independent of IR ratio.&lt;/p&gt;&#x0D;

EM 담체를 사용한 침지여상에서 담체의 충전율과 내부순환율이 질소, 인 제거에 미치는 영향

EM 담체를 사용한 침지여상에서 담체의 충전율과 내부순환율이 질소, 인 제거에 미치는 영향

Intelligent Control/Operational Strategies in WWTPs through an Integrated Q-Learning Algorithm with ASM2d-Guided Reward

The operation of a wastewater treatment plant (WWTP) is a typical complex control problem, with nonlinear dynamics and coupling effects among the variables, which renders the implementation of real-time optimal control an enormous challenge. In this study, a Q-learning algorithm with activated sludge model No. 2d-guided (ASM2d-guided) reward setting (an integrated ASM2d-QL algorithm) is proposed, and the widely applied anaerobic-anoxic-oxic (AAO) system is chosen as the research paradigm. The integrated ASM2d-QL algorithms equipped with a self-learning mechanism are derived for optimizing the control strategies (hydraulic retention time (HRT) and internal recycling ratio (IRR)) of the AAO system. To optimize the control strategies of the AAO system under varying influent loads, Q matrixes were built for both HRTs and IRR optimization through the pair of &lt;max reward-action&gt; based on the integrated ASM2d-QL algorithm. 8 days of actual influent qualities of a certain municipal AAO wastewater treatment plant in June were arbitrarily chosen as the influent concentrations for model verification. Good agreement between the values of the model simulations and experimental results indicated that this proposed integrated ASM2d-QL algorithm performed properly and successfully realized intelligent modeling and stable optimal control strategies under fluctuating influent loads during wastewater treatment.

Experimental and analysis study on dispersion of phases in an Ebullated Bed Reactor

The effectiveness and performance of industrial hydro-processing Ebullated Bed Reactors (EBRs) are highly dependent on the bed hydrodynamics and operating conditions. In present work, hydrodynamics of EBRs was studied in a cold model experimental setup using air–water–solid particles system. Pressure gradient method and Residence Time Distribution (RTD) technique were used to estimate the individual holdups, and dispersion coefficients in the lab-scale ebullated bed column. System Hydraulic Efficiency (HEF) was also estimated. The results showed that liquid internal recycle ratio, which characterized the EBRs, has a predominant effect on the individual holdups and dispersion coefficients. Empirical correlations were developed for prediction of phase holdups, and dispersion coefficients with good accuracy.

Effect of internal recycle ratio on the denitrification process and nirS-containing bacteria of an anaerobic/anoxic/oxic (A²/O) wastewater treatment process

Effect of internal recycle ratio on the denitrification process and nirS-containing bacteria of an anaerobic/anoxic/oxic (A²/O) wastewater treatment process

Effect of operating parameters on the performance of wastewater treatment plant (Case study: The south of Tehran wastewater treatment)

Despite the fact that there are wastewater treatment plants (WWTPs) currently operational across Iran and great advances have been made in this area, there are still problems in the design, construction, and operation of WWTPs with large nonlinear systems, varying flow rates, and pollution charges. The objective of this study was to investigate the effect of operating parameters including the return activated sludge (RAS) ratio, internal recycle (IR) ratio and dissolved oxygen (DO) concentration in an activated sludge system for the Modules 5&6 of the Southern Tehran WWTP. This study designed and simulated a plant based on the activated sludge model No.1 (ASM1) to determine the factors affecting wastewater treatment systems; then, the kinetic parameters were measured. The kinetic parameters such as the yield coefficient (Y), decay coefficient (Kd), maximum specific growth rate (K), and saturation constant (Ks) were in the range of 0.303-0.331g/g, 0.030-0.033d-1, 1.65-1.93d-1 and 37.6-44.92mg/l, respectively. The RAS ratios, IR ratios, and DO concentration varied from 0.2 to 2, 1 to 3.5, and 0.27 to 3.54 mg/l, respectively. The amount of RAS had the greatest impact on the effluent. The amounts of IR and DO concentration had no significant effect on the concentration of the five-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), and total suspended solids (TSS) in the effluent. After the RAS, the amount of IR had the most direct effect on reducing the effluent total nitrogen (TN) concentration. As a result, the overall removal efficiency increased up to 75% when the IR rate was 200% of the influent flow rate, the RAS rate was 90% of the influent flow rate, and the DO concentration in the first aeration unit was 2 mg/l considering the aeration cost. Therefore, proper operating parameters can provide the best quality of effluent that meets environmental standards.

Enhancing the Water Accounting and Vulnerability Evaluation Model: WAVE.

Due to the increasing relevance of analyzing water consumption along product life cycles, the water accounting and vulnerability evaluation model (WAVE) has been updated and methodologically enhanced. Recent data from the atmospheric moisture tracking model WAM2-layers is used to update the basin internal evaporation recycling (BIER) ratio, which denotes atmospheric moisture recycling within drainage basins. Potential local impacts resulting from water consumption are quantified by means of the water deprivation index (WDI). Based on the hydrological model WaterGAP3, WDI is updated and methodologically refined to express a basin's vulnerability to freshwater deprivation resulting from the relative scarcity and absolute shortage of water. Compared to the predecessor version, BIER and WDI are provided on an increased spatial and temporal (monthly) resolution. Differences compared to annual averages are relevant in semiarid and arid basins characterized by a high seasonal variation of water consumption and availability. In order to support applicability in water footprinting and life cycle assessment, BIER and WDI are combined to an integrated WAVE+ factor, which is provided on different temporal and spatial resolutions. The applicability of the WAVE+ method is proven in a case study on sugar cane, and results are compared to those obtained by other impact assessment methods.

Nitrogen removal performance and microbial community of an enhanced multistage A/O biofilm reactor treating low-strength domestic wastewater.

The low-strength domestic wastewater (LSDW) treatment with low chemical oxygen demand (COD) has drawn extensive attention for the poor total nitrogen (TN) removal performance. In the present study, an enhanced multistage anoxic/oxic (A/O) biofilm reactor was designed to improve the TN removal performance of the LSDW treatment. Efficient nitrifying and denitrifying biofilm carriers were cultivated and then filled into the enhanced biofilm reactor as the sole microbial source. Step-feed strategy and internal recycle were adopted to optimize the substrate distribution and the organics utilization. Key operational parameters were optimized to obtain the best nitrogen and organics removal efficiencies. A hydraulic retention time of 8h, an influent distribution ratio of 2:1 and an internal recycle ratio of 200% were tested as the optimum parameters. The ammonium, TN and COD removal efficiencies under the optimal operational parameters separately achieved 99.75 ± 0.21, 59.51 ± 1.95 and 85.06 ± 0.79% with an organic loading rate at around 0.36kg COD/m3d. The high-throughput sequencing technology confirmed that nitrifying and denitrifying biofilm could maintain functional bacteria in the system during long-period operation. Proteobacteria and Bacteroidetes were the dominant phyla in all the nitrifying and denitrifying biofilm samples. Nitrosomonadaceae_uncultured and Nitrospira sp. stably existed in nitrifying biofilm as the main nitrifiers, while several heterotrophic genera, such as Thauera sp. and Flavobacterium sp., acted as potential genera responsible for TN removal in denitrifying biofilm. These findings suggested that the enhanced biofilm reactor could be a promising route for the treatment of LSDW with a low COD level.

Effect of feed characteristics and operational conditions on treatment of dairy farm wastewater in a coupled anoxic-upflow and aerobic system

This study investigated the removal of nitrogen and organic matter from dairy farm wastewater (DFW) in a hybrid anoxic-upflow bioreactor and aerobic (UBR-A) system. Due to variable and complex composition of real DFW, system performance and behaviour was compared with synthetic wastewater (SWW) in two identical pilot-scale UBR-A systems. Both systems were operated for 334 days first under internal recycle ratios (R) of 2, 3 and 4 and then at solids retention times (SRT) of 140–150, 65–75 and 30–40 days at uniform R of 3. Performance with both wastewaters was comparable with higher nitrification efficiencies ( > 90%) and COD removal (>80%) under all conditions, and lower denitrification efficiencies at R > 3. DFW exhibited significantly lower nitrification/denitrification rates, biomass yield, VSS/TSS ratio and a higher inert COD fraction showing that kinetics, sludge characteristics and organic matter degradation were not analogous. The optimum internal recycle ratio was found as R of 3 at a total hydraulic retention time (HRT) of 4 days whereas at different SRTs, performance efficiencies were mostly comparable however nitrification and denitrification rates significantly increased at lower SRT of 30–40 days.

Nutrients Removal from Domestic Wastewater in Basrah City (Southern Iraq) using Combined A2/O Bio Contact Oxidation Technology

The nutrients removal from domestic wastewater in Basrah City (Southern Iraq) by using the technology of up flow A2-O Bio Contact Oxidation with bypass flow was studied in this research. The treatment system was designed in order to treating 100 L/day of domestic wastewater by using a laboratory scale Anaerobic-Anoxic-Aerobic Moving Bed Biofilm Reactors (MBBRS) in series form with effective volume equal to 15 L, 15 L, and 30 L respectively. Kaldnes polyethylene media (K1) was used in this study in order to achieve attached biofilm process with filling ratio equal to 30% for both anaerobic anoxic MBBRS. and with filling ratio 50% for aerobic MBBR. After the biofilm was developed in the carriers the system was operated under internal recycle ratio equal to 100%. bypass ratio equal to 40%. and under 3 different external recycle ratio (25%, 50%, and 100%). The experimental results showed that the optimal value of the external recycle ratio was equal to 50 %, which the average values of removal efficiencies of COD, NH4+-N, TN and TP were equal to 98.15 %, 97.16%, 82.12%, and 93.39 % respectively. Under the condition of the optimum value of external recycle ratio the average concentration of the dissolved oxygen in both anoxic and aerobic reactors were equal to 0.151 mg/L, and 3.57 mg/L respectively, and the temperature of water wras controlled in the range of 35° C in the anaerobic reactor and in the range of 30°C in both anoxic and aerobic reactors by used temperature controlled.

Influence of Internal and External Recycle on Nitrogen Removal in Compact Bioreactor

Internal Recycle ratios (IR) were alternated and operated to determine its efficiency on nitrate removal within a SMALL FOOTPRINT bioreactor. Determining a suitable recycle ratio is essential to justify the prerequisites required for operative optimization. In order to achieve considerable nitrogen reduction, the reactor was designed with primary (pre) and secondary (post) anoxic chambers, combined aeration chamber for nitrification and carbon removal and final settler to settle the biomass prior to discharge. The nitrate oxidized in aeration chamber was recycled back to the pre-anoxic chamber. The residual nitrate flows to the post anoxic chamber for denitrification. Each IR ratio of 6, 4 and 0, was operated with distinct influent Chemical Oxygen Demand (COD) to Total Kjeldahl Nitrogen (TKN) ratios of 18.9, 14.9 and 10.9, respectively. The IR of 6 with COD/TKN ratio of 18.9 attained maximum Total Nitrogen (TN) removal efficiency of 90.7%. The overall average total nitrogen (TN) = 4.0 mg/L in effluent was achieved, signifying an average of 9.5% efficiency more than the process without nitrate recycle, with average effluent TN = 7.7 mg/L.

Impact of internal recycle ratio on nitrous oxide generation from anaerobic/anoxic/oxic biological nitrogen removal process

To evaluate the effects of internal recycle ratio on nitrous oxide (N2O) generation, we set up three laboratory-scale anaerobic–anoxic–oxic (A2O) wastewater treatment processes, with internal recycle ratios of 100%, 200%, and 300%, respectively. Total nitrogen (TN) removal was markedly enhanced from 45.8% to 61.9%, as the internal recycle ratio increased from 100% to 300%. N2O generation was increased from 3.47 to 9.81×10−2mg/L during the treatment process, with the anoxic section showing the largest N2O increment from denitrification. This phenomenon is attributed to the increased amount of nitrate (NO3−-N) substrate available for denitrification, due to the increased volume of internal recycle liquid, as well as the increased amount of oxygen, which could restrain the activity of nitrous oxide reductase brought to the anoxic section. Nitrous oxide reductase was more sensitive to oxygen than nitrate and nitrite reductases. Microorganism analysis indicated that the population of nosZ gene-containing bacteria was only slightly affected by the recycle ratio. However, the number of nosZ gene copies decreased as the internal recycle ratio increased from 100% to 300%; this result reveals noticeable decreases in the denitrification capacity of the system for reducing N2O to N2.

Impact of the Anoxic Volume Ratio on the Dynamics of Biological Nitrogen Removal Under Extended Aeration Conditions

The study provided a critical appraisal of the extended aeration process as a single-sludge system for nitrogen removal, emphasizing its inherent deficiencies. For this purpose, the system was designed first using the prescribed procedure in the German practice, ATV A-131. The design used the basic data reported in different studies related to conventional characterization and chemical oxygen demand (COD) fractionation defining the biodegradation characteristics of domestic wastewater. A critical appraisal of the design was made with emphasis on the fate of biodegradable COD and oxidized nitrogen in the anoxic phase by process modeling and evaluation. The results obtained were evaluated using basic stoichiometry and mass balance for major nitrogen fractions. The A-131 design based on a total sludge age of 20 days defined a system with a hydraulic residence time of 1.2 days where half of the volume was operated under anoxic conditions; the effluent nitrate concentration was reduced to 8.3 mg N/L with an internal recycle (nitrate) ratio of 4.9. Model evaluation of the prescribed design indicated that oxidized nitrogen was totally consumed within the first 25–30 % portion of the anoxic volume. The remaining volume was forced to operate under anaerobic conditions, where no appreciable endogenous decay would occur. ATV A-131 procedure, relying on empirical coefficients and expressions, was neither consistent with process stoichiometry nor justifiable by modeling. Evaluations based on modeling and process stoichiometry revealed significant inherent weaknesses of extended aeration for providing a sustainable basis for nitrogen removal.