Theoretical Hydraulic Retention Time Research Articles

Performance of loach to reduce clogging matter in vertical-flow constructed wetlands: Hydraulic evaluation and metabolic processes

Extracellular polymeric substances (EPS) secreted by microorganisms cause constructed wetlands (CWs) clogging, which is a major challenge for the long-term stable operation of CWs. This study provided a comprehensive and in-depth study of the hydraulic evaluation and metabolic processes of loach (Misgurnus anguillicaudatus) in mitigating clogging. The results showed that the loach reduced the clogging content in the substrate layer and improved the hydraulic performance of the system. The loach system increased the initial porosity in the sand layer by 11.52 %, 25.11 %, and 28.15 %, and the initial porosity in the gravel layer by 15.49 %, 15.85 %, and 13.83 %, respectively. Loach also increased the actual and theoretical hydraulic retention time (HRT), improved the connectivity between different layers, and reduced the “dead zone” area of the system. The analysis of EPS components revealed that the daily EPS removal amount of each loach was 36.64 mg L-1, in which S-EPS, LB-EPS, and TB-EPS were 21.80, 8.02, and 6.82 mg L-1d-1, respectively. Direct feeding and the indirect coordination with microorganisms effectively reduced the contents of polysaccharides and proteins in EPS, and increased the proportion of humic acid and nucleic acid. The clogging characteristics analysis also confirmed that loach decreased tyrosine and tryptophan fluorophores groups and reduced the proportion of protein or peptide substances in the macromolecules in EPS. In addition, loach increased the activities of dehydrogenase, fluorescein diacetate, and the relative abundance of Chlorobiaceae in the sand layer, which was conducive to the hydrolysis of clogging matters with the metabolic activity of microorganisms. Loach increased the relative abundance of denitrifying microorganisms such as Aeromonadaceae, Rhodocyclaceae, and Hydrogenophilaceae in the gravel layer, which enhanced the denitrification process of the system and was conducive to pollutant transformation and clogging matter degradation. In conclusion, this study proposes a novel, eco-friendly strategy for preventing CW clogging in wetland animals.

Performance evaluation of modified Living Wall garden for treating septic tank effluent.

The Living Wall (LW) garden system has been employed as a post-treatment system to improve the effluent quality of septic tanks. This improvement primarily involves reducing nutrient levels, as well as facilitating the removal of organic matter and solids in accordance with effluent discharge guidelines. The objective of this study was to investigate the treatment performance of the LW system connected to a septic tank, along with an examination of the microbial communities within the LW units. A laboratory-scale LW system, comprising LW1, LW2, and LW3 units, was employed. The system was fed with effluent obtained from septic tanks and varied by theoretical hydraulic retention time (HRT) of 6, 12, and 24h. The TCOD, SCOD, TSS, TVS, TKN, and TP removal efficiencies of the LWs were achieved at 62 ± 24, 42 ± 19, 72 ± 21, 66 ± 15, 80 ± 15, and 58 ± 21%, respectively. To classify microbial communities in the soil and gravels collected from each LW unit, the Illumina MiSeq System Sequencer was employed. Nitrospirota was consistently found in all LW units, aiding in the conversion of nitrogen. Fusobacteriota were detected in specific layers of the LW units, indicating varying oxygen levels in the LW system.

What is the best design approach for estimating effluent concentrations from horizontal flow treatment wetlands: the use of volumetric (kV) or areal (kA) removal rate coefficients?

Effluent concentrations from horizontal flow (HF) treatment wetlands can be estimated by using the Tanks-In-Series model for describing hydraulics and first-order removal rate coefficients for describing pollutant removal. In the design of conventional wastewater treatment plants, volumetric removal rate coefficients (kV) are traditionally used in conjunction with the theoretical hydraulic retention time. Areal removal rate coefficients (kA) coupled with the applied areal hydraulic loading rate are widely used in the literature. Despite this, supporting evidence of its appropriateness is scarce in the literature. The objective of this study is to investigate the adequacy of both approaches by analyzing the influence of liquid depth on kV and kA. Data from 74 HF wetlands were collected, covering biochemical oxygen demand and chemical oxygen demand, and diverse types of influents (raw sewage and primary, secondary and tertiary effluents). For these conditions, kV decreased with depth of the wetland system. Regression analyses between depth and removal rate coefficients were performed, and the equations indicated that kV was approximately related to the inverse of depth, while kA was almost independent of depth. These findings endorse the utilization of the areal-based approach for design purposes. The volumetric-based approach can also be used, but the value of kV must be provided together with the depth being considered.

Manganese oxidation and prokaryotic community analysis in a polycaprolactone-packed aerated biofilm reactor operated under seawater conditions.

Biogenic manganese oxides (BioMnOx) have been receiving increasing attention for the removal of environmental contaminants and recovery of minor metals from water environments. However, the enrichment of heterotrophic Mn(II)-oxidizing microorganisms for BioMnOx production in the presence of fast-growing coexisting heterotrophs is challenging. In our previous work, we revealed that polycaprolactone (PCL), a biodegradable aliphatic polyester, can serve as an effective solid organic substrate to enrich Mn-oxidizing microbial communities under seawater conditions. However, marine BioMnOx-producing bioreactor systems utilizing PCL have not yet been established. Therefore, a laboratory-scale continuous-flow PCL-packed aerated biofilm (PAB) reactor was operated for 238days to evaluate its feasibility for BioMnOx production under seawater conditions. After the start-up of the reactor, the average dissolved Mn removal rates of 0.4-2.3mg/L/day, likely caused by Mn(II) oxidation, were confirmed under different influent dissolved Mn concentrations (2.5-14.0mg/L on average) and theoretical hydraulic retention time (0.19-0.77day) conditions. The 16S rRNA gene amplicon sequencing analysis suggested the presence of putative Mn(II)-oxidizing and PCL-degrading bacterial lineages in the reactor. Two highly dominant operational units (OTUs) in the packed PCL-associated biofilm were assigned to the genera Marinobacter and Pseudohoeflea, whereas the genus Lewinella and unclassified Alphaproteobacteria OTUs were highly dominant in the MnOx-containing black/dark brown precipitate-associated biofilm formed in the reactor. Excitation-emission matrix fluorescence spectroscopy analysis revealed the production of tyrosine- and tryptophane-like components, which may serve as soluble heterotrophic organic substrates in the reactor. Our findings indicate that PAB reactors are potentially applicable to BioMnOx production under seawater conditions.

Enhanced methane production in an up-flow microbial electrolysis assisted reactors: Hydrodynamics characteristics and electron balance under different spatial distributions of bioelectrodes

Compared with common anaerobic digestion, microbial electrolysis has been proven feasibly to accelerate biodegradation and methanogenesis with the advantages of effective electron flow regulation. However, its actual application and scale-up required a full understanding and further investigation on electrode size and distribution. For making full use of the space of the integrated reactor and improve methane recovery, an effective interior configuration was significant. In this work, three types of reactors with different cathode spatial distributions, that is, different cathode space ratios (ratio of cathode surface area to reaction region volume), were studied to form a good flow pattern for obtaining high methane production. Tracer experiments and numerical simulation were employed simultaneously for understanding the hydrodynamics characters of the interior flow field. The results showed that by increasing the cathode space ratio to 1.33 cm2/cm3 and 2 cm2/cm3, respectively, better flow patterns with the residence time of 1.336 times and 1.363 times of theoretical hydraulic retention time could be obtained. The stacked structure of nickel meshes was beneficial to prolong the contact time of contaminant and improve the mass transfer. Increasing the cathode space ratio could also enhance the electrochemical performance. Considering the organic removal, methane recovery, electrons generation, and material consumption, the recommended cathode space ratio was 1.33 cm2/cm3. With this structure, COD removal efficiency reached 93.2 ± 1.9% and 94.1 ± 1.5%, methane production rate reached 332.0 and 334.8 mL CH4/L reactor/day, and methane yield was 171.3 and 246.4 mL CH4/g COD under the HRT of 24 h and 36 h, respectively.

Field-scale anaerobic baffled reactor for domestic wastewater treatment: effect of dynamic operating conditions

Abstract A 45 m3/d field-scale anaerobic baffled reactor (ABR) was studied for its performance to remove carbonaceous organic content and suspended solids under a dynamic hydraulic loading rate (HLR) and organic loading rate (OLR). Sludge granulation effect was also assessed with and without sand as a bio-carrier aided with poly aluminium chloride. ABR was subjected to a significant variation in HLR (0.26 to 7.72 m3/m2.d) and OLR (0.03 to 8 kg COD/m3.d). Tracer study showed flow-through time was 50% of theoretical hydraulic retention time. The initial compartments of ABR were more effective for the removal of organic carbon. An overall CODTotal removal of 60 to 90% was possible for OLR in the range 1 to 8 kg COD/m3.d irrespective of low/high HLR. OLR dominated the performance of ABR compared to HLR. The compartmentalized nature of ABR was visualized through a two-phase system of anaerobic digestion as alkalinity increased while VFA decreased from the first to last compartment even under dynamic conditions. Sludge granulation with sand and PAC increased the size of granule from 629 to 1,471 μm, decreased sludge depth by 20% and enhanced CODTotal removal within a month. ABR is sturdy to sustain low/high HLR with low/high OLR conditions without impairing CODTotal removal efficiency significantly.

Optimization of mixing in a Chinese dome digester for tropical regions

Optimization of mixing in a Chinese dome digester for tropical regions

Evaluation of liquid and solid phase mixing in Chinese dome digesters using residence time distribution (RTD) technique

The Residence Time Distribution (RTD) technique was applied to evaluate mixing of liquid and solid phases in laboratory scale Chinese dome digesters mixed via hydraulic variation. To achieve this purpose, six laboratory scales digesters with different mixing modes and two total solids (TS) concentrations using appropriate tracers were studied over a theoretical hydraulic retention time (HRT) of 30 days. The three different mixing modes were impeller, unmixed and hydraulic mixing, each at influent concentration of ca. 7.5 and 15% TS concentrations. The mode of mixing strongly affected the effective or actual residence time (ta) and directly influenced the percentage of dead zones. The Chinese dome digesters had more dead volumes than the impeller mixed reactors and less than the unmixed reactors. This implied that mixing was more efficient in the impeller mixed reactors followed by the hydraulic mixed reactors and then the unmixed reactors, irrespective of the TS concentration. There was a clear relation between the RTD results and anaerobic digestion performance viz. methane production. There is need to optimize the hydraulic variation in the Chinese dome digester to reduce dead zones while also optimizing the effective residence time (ta).

Effect of hydraulic retention time on a continuous biohydrogen production in a packed bed biofilm reactor with recirculation flow of the liquid phase

The present paper reports on results obtained from experiments carried out in a laboratory-scale anaerobic packed bed biofilm reactor (APBR), with recirculation of the liquid phase, for continuously biohydrogen production via dark fermentation. The reactor was filled with Kaldnes® biofilm carrier and inoculated with an anaerobic mesophilic sludge from a urban wastewater treatment plant (WWTP). The APBR was operated at a temperature of 37 °C, without pH buffering. The effect of theoretical hydraulic retention time (HRT) from 1 to 5 h on hydrogen yield (HY), hydrogen production rate (HPR), substrate conversion and metabolic pathways was investigated. This study indicates the possibility of enhancing hydrogen production by using APBR with recirculation flow. Among respondents values of HRT the highest average values of HY (2.35 mol H2/mol substrate) and HPR (0.085 L h−1L−1) have been obtained at HRT equal to 2 h.

Comparison of the treatment performance of a high rate algal pond and a facultative waste stabilisation pond operating in rural South Australia.

South Australian community wastewater management schemes (CWMS) treat wastewater using waste stabilisation ponds before disposal or reuse. This study compared the performance of a facultative pond, 6,300 m2, 27.5 d theoretical hydraulic retention time (THRT), with a high rate algal pond (HRAP) operated at depths of 0.32, 0.43 and 0.55 m with THRT equivalent to 4.5, 6.4 and 9.1 d respectively. Both ponds received influents of identical quality, differing only in quantity, and were operated in similar climatic conditions. The depth of HRAP operation had only a minor influence on treatment performance. The study showed that the quality of the treated effluent from the HRAP was equivalent to that of the facultative pond, 5-day biochemical oxygen demand removal >89%, NH4-N removal 59.09-74.45%. Significantly, Escherichia coli log10 reduction values by the HRAP, 1.74-2.10, were equivalent to those of the facultative pond. Consequently, HRAPs could replace facultative ponds within CWMS while maintaining treated effluent quality. The benefit would be halving the surface area requirement from 4.2 m2 capita-1 for the facultative pond to between 2.0 and 2.3 m2 capita-1, depth dependent, for an HRAP, with significant attendant reductions in the capital costs for construction.

Denitrifying Bioreactors Resist Disturbance from Fluctuating Water Levels

Nitrate can be removed from wastewater streams, including subsurface agricultural drainage systems, using woodchip bioreactors to promote microbial denitrification. However, the variations in water flow in these systems could make reliable performance from this microbially-mediated process a challenge. In the current work, the effects of fluctuating water levels on nitrate removal, denitrifying activity, and microbial community composition in laboratory-scale bioreactors were investigated. The performance was sensitive to changing water level. An average of 31% nitrate was removed at high water level and 59% at low water level, despite flow adjustments to maintain a constant theoretical hydraulic retention time. The potential activity, as assessed through denitrifying enzyme assays, averaged 0.0008 mg N2O-N/h/dry g woodchip and did not show statistically significant differences between reactors, sampling depths, or operational conditions. In the denitrifying enzyme assays, nitrate removal consistently exceeded nitrous oxide production. The denitrifying bacterial communities were not significantly different from each other, regardless of water level, meaning that the denitrifying bacterial community did not change in response to disturbance. The overall bacterial communities, however, became more distinct between the two reactors when one reactor was operated with periodic disturbances of changing water height, and showed a stronger effect at the most severely disturbed location. The communities were not distinguishable, though, when comparing the same location under high and low water levels, indicating that the communities in the disturbed reactor were adapted to fluctuating conditions rather than to high or low water level. Overall, these results describe a biological treatment process and microbial community that is resistant to disturbance via water level fluctuations.

Evaluation of Water Distribution and Oxygen Mass Transfer in Sponge Support Media for a Down-flow Hanging Sponge Reactor

A down-flow hanging sponge (DHS) reactor has been developed for sewage treatment, mainly in developing countries. This novel reactor employs polyurethane sponge material as a support medium, which promises a proliferation of a large amount of biomass, offering excellent pollutant removal capability. Three types of sponge medium were evaluated with respect to water distribution and oxygen mass transfer. Water was supplied to the device, which consisted of 40 pieces of sponge media connected in series, and a tracer experiment was carried out. The ratios of actual hydraulic retention time (HRT) to theoretical HRT were in the range of 25-67% depending on the type of support medium. By supplying deoxygenated water from the top of the device, the overall volumetric oxygen transfer coefficient, KLa, was evaluated. Despite the non-aerated conditions, the KLa values of the support media were very high, in the range of 0.56-4.88 (1/min), surpassing those of other mechanically aerated processes. Furthermore, it was found that the suspended solids (SS) concentration in the influent played a role in increasing the actual HRT/theoretical HRT ratio, suggesting that managing the influent SS concentration is prerequisite for preventing clogging problems in the DHS.

Influence of hydraulic loading rate, simulated storm events and seasonality on the treatment performance of an experimental three-stage hybrid constructed wetland system

An experimental hybrid system based on an anaerobic reactor followed by three stages of different constructed wetland configurations was evaluated when operating under a high hydraulic loading rate (HLR=0.27md−1, considering the area of the VF beds) for one year, which corresponds to four times the nominal hydraulic loading rate, with the purpose of reducing the specific area required. Moreover, in order to assess its buffer capacity, a major storm event was simulated by increasing the HLR 10 times during 1h. A tracer experiment was also performed to determine the experimental hydraulic retention time (HRT). The system consisted of a hydrolytic upflow sludge blanket (HUSB) reactor followed by two alternating 1.5m2 vertical subsurface flow, a 2m2 horizontal subsurface flow and a 2m2 free water surface constructed wetlands operating in series. The system achieved very high values of removal of solids, organic matter and nutrients (82, 93, 96 and 75% for COD, BOD5, TSS and NH4-N, respectively). Removal of PO4-P and SO42− were though fairly low, of 11 and 10%, respectively. There was a seasonal effect in the system for parameters whose removal highly depends on biodegradation, being enhanced under warmer conditions (98 and 92% removal of BOD5 and NH4-N in summer vs. 87 and 67% removal of BOD5 and NH4-N in winter). The experimental HRT of the entire system was of about 38h, which is greater than the theoretical HRT (28h). During the simulation of the storm event removal efficiencies did not vary significantly from the ones obtained under normal conditions (average of 83, 99 and 80% for COD, TSS and NH4-N removal, respectively). The system showed a very good buffer capacity coping with sharp fluctuations in flow to be treated, showing to be an adequate solution for wastewater treatment in small communities. The specific area requirement under the long-term operation showed to be as low as 2m2/PE.

Kinetics of nitrate reduction with the packed bed iron bipolar electrode

This study investigated the kinetics of nitrate reduction with the zero-valent iron bipolar (ZVI-BP) electrode. The kinetics of the nitrate reduction was considered with respect to the initial concentration of nitrate and the flow rate. Increasing the initial concentration of nitrate enhanced the nitrate reduction rate and there was the proper flow rate (20mL/min) to achieve the highest nitrate reduction rate (6.2h−1). But increasing the initial nitrate concentration rather decreased conversion ratio of nitrate to ammonia from 54.5% to 45.9% because of the ammonia oxidation to nitrogen gas as electro-catalytic oxidation. The highest reaction rate was obtained when the experimental hydraulic retention time (HRT) was analogous to the theoretical HRT. The current efficiency was over 100% because of low current density and the redox reaction with Fe2+/Fe3+ redox couple. The corrosion of the ZVI-BP electrodes was characterized by using the X-ray Diffraction (XRD) and the field emission scanning electron microscope (FE-SEM). Magnetite was the major corrosion product and it had potential to enhance the nitrate reduction. The modeling of nitrate reduction had similar trend with experimental data and the verification of the modeling parameter will help the up-scale design of the system.

Tertiary nitrification using moving-bed biofilm reactor: a case study in Tunisia.

In this study, the effect of operational conditions on biofilm development and nitrification in moving-bed biofilm reactor (MBBR) was investigated. The reactor was operated in a continuously fed regime during 170days and with theoretical hydraulic retention time of 7h, respectively. The presence of chemical oxygen demand (COD) increased the time required to form stable nitrifying. Subsequent stepwise increase of influent COD caused an increment in total polysaccharide (PS) and protein (PN) content, which was accompanied by an attachment of the biofilm, as shown by atomic force microscope (AFM). PS and PN concentrations proved to be good indicators of biomass development and attachment in MBBR system. Reactor was operated and water quality was characterized before and after treatment. Parameters including pH, 5-day biochemical oxygen demand (BOD5), total suspended solids (TSS) (COD), PN, PS, and fecal bacteria in both raw and treated wastewater were monitored during the treatment. The removal rates of ammonium-nitrogen (NH4 (+)-N), BOD5, COD, and TSS are 95, 67.5, 69.2, and 73.33%, respectively. The average bacterial reduction between the inlet and the outlet was of the order of 5±1 logarithmic units for fecal coliforms. AFM showed that distinct biofilm and extracellular polymeric substances were formed in biofilm was thicker in the 70days than in the 30days. These results showed that the consumption rate for each substrate increased parabolically with biofilm thickness due to the increased amount of biomass Thus, MBBR can serve as a promising technology for wastewater treatment and can be scaled up for small communities in the developing countries.

Enhancement of methane production with horse manure supplement and pretreatment in a full-scale biogas process

The increased demand for renewable energy resources worldwide has lead to a strong interest in biomass for energy and heat production. However, the use of energy crops competes with human food production for limited available arable land. Therefore, it is necessary to develop alternate feedstocks for anaerobic digestion and increase the use of agricultural residues and by-products. In this work, the usability of straw-based horse manure was investigated in a full-scale biogas plant over a period of 160 days. Additionally, for the improvement of the methane production, a mechanical disintegration device was tested. The results of this long-term study indicate that the digestion of horse manure is not sufficient without further disintegration. The pretreatment of the substrates caused an increase in specific methane production of approximately 26.5%. The determination of the degradation efficiency resulted in an almost complete degradation of the disintegrated substrates during the theoretical hydraulic retention time of 80 days. Regarding these results, the energy demand for the pretreatment is negligible. Therefore, the anaerobic digestion of lignocellulosic materials with an appropriate pretreatment is the suggested method for a sustainable and economically viable energy production.

Taenia eggs in a stabilization pond system with poor hydraulics: concern for human cysticercosis?

The objective of this study is to compare the removal of Taenia eggs to the removal of Ascaris eggs in a wastewater stabilization pond system consisting of three ponds in series, where the hydraulic residence time distribution has been characterized via a tracer study supported by computational fluid dynamics modeling. Despite a theoretical hydraulic retention time of 30 days, the peak dye concentration was measured in the effluent of the first pond after only 26 hours. The smaller-sized Taenia eggs were detected in higher concentrations than Ascaris eggs in the raw wastewater. Ascaris eggs were not detected in the pond system effluent, but 45 Taenia eggs/L were detected in the system effluent. If some of these eggs were of the species Taenia solium, and if the treated wastewater were used for the irrigation of crops for human consumption, farmers and consumers could potentially be at risk for neurocysticercosis. Thus, limits for Taenia eggs in irrigation water should be established, and precautions should be taken in regions where pig taeniasis is endemic. The results of this study indicate that the theoretical hydraulic retention time (volume/flow) of a pond is not always a good surrogate for helminth egg removal.

Performance and behaviour of planted and unplanted units of a horizontal subsurface flow constructed wetland system treating municipal effluent from a UASB reactor

A system composed of two horizontal subsurface flow constructed wetlands operating in parallel was evaluated for the post-treatment of UASB (upflow anaerobic sludge blanket) reactor effluent, for a population equivalent of 50 inhabitants per unit. One unit was planted with cattail (Typha latifolia) and the other was unplanted. The study was undertaken over a period of 4 years, comprising monitoring of influent and effluent constituents together with a full characterization of the behaviour of the units (tracer studies, mathematical modelling of chemical oxygen demand (COD) decay, characterization of solids in the filter medium). The mean value of the surface hydraulic load was 0.11 m(3)m(-2)d(-1), and the theoretical hydraulic retention time was 1.1 d in each unit. Using tracer tests with (82)Br, dispersion number (d) values of 0.084 and 0.079 for the planted and unplanted units were obtained, indicating low to moderate dispersion. The final effluent had excellent quality in terms of organic matter and suspended solids, but the system showed low capacity for nitrogen removal. Four-year mean effluent concentration values from the planted and unplanted units were, respectively: biochemical oxygen demand (BOD(5)): 25 and 23 mg L(-1); COD: 50 and 55 mg L(-1); total suspended solids (TSS): 9 and 9 mg L(-1); N-ammonia: 27 and 28 mg L(-1). The COD decay coefficient K for the traditional plug-flow model was 0.81 and 0.84 d(-1) for the planted and unplanted units. Around 80% of the total solids present in the filter medium were inorganic, and most of them were present in the interstices rather than attached to the support medium. As an overall conclusion, horizontal subsurface flow wetlands can be a very suitable post-treatment method for municipal effluents from anaerobic reactors.

Low-temperature anaerobic digestion of swine manure in a plug-flow reactor

A low-temperature (25°C) anaerobic eight-compartment (PF01 to PF08) cascade reactor simulating a plug-flow reactor (PFR) treating pig manure was monitored for a year. The bioreactor was fed at an average loading rate of 2.4±0.2 g of total chemical oxygen demand (TCOD) per litre of reactor per day for a theoretical hydraulic retention time (HRT) of 67±7 d. An average of 79% of TCOD was removed from pig manure (converted into biogas and in sediments), whereas specific methane yields ranging from 397 to 482 NL CH4 kg−1 VS (148.6 to 171.4 NL CH4 kg−1 TCOD) were obtained. After 150 d, fluctuating performances of the process were observed, associated with solids accumulation in the upstream compartments, preventing the complete anaerobic digestion of swine manure in the compartments PF01 to PF04. Low-temperature anaerobic PFR represents an interesting alternative for the treatment of pig manure and recovery of green energy. Further investigations regarding a modified design, with better accumulating solids management, are needed to optimize the performance of this low-temperature PFR treating pig manure.

Enhancement of denitrification in a down-flow hanging sponge reactor by effluent recirculation

A down-flow hanging sponge reactor, constructed by connecting three identical units in series, was applied to the treatment of artificial wastewater containing phenol and ammonia under high salinity conditions (10.9 g-Cl(-)/L). The theoretical hydraulic retention time (HRT) of each unit was 4 h (total HRT = 12 h). To enhance denitrification by effluent recirculation, the effluent recirculation ratio was increased in increments ranging from 0.0 to 2.0. The concentration of total ammonia nitrogen (TAN), NO2-N, and NO3-N in the final effluent as a proportion of the TAN in the influent was determined to calculate the unrecovered, or denitrification, proportion. The denitrification proportion of the reactor was equivalent to 19.1 ± 14.1% with no effluent recirculation; however, this was increased to 58.6 ± 6.2% when the effluent recirculation ratio was increased to 1.5. Further increasing the effluent recirculation ratio to 2.0 resulted in a decrease in the denitrification proportion to 50.9 ± 9.3%. Activity assays of nitrification and denitrification, as well as 16S rRNA gene sequence analysis, revealed that denitrification occurred primarily in the upper sections of the reactor, while nitrification increased in the lower sections of the reactor. Gene sequence analysis revealed that denitrification by Azoarcus-like species using phenol as an electron donor was dominant.