Environ. Sci. Technol. 2010, 44, 7245–7253
Partial Nitrification/Denitrification Can Be Attributed to the Slow Response of Nitrite Oxidizing Bacteria to Periodic Anoxic Disturbances M . K O R N A R O S , * ,† S . N . D O K I A N A K I S , § A N D G . L Y B E R A T O S †,‡ Department of Chemical Engineering, University of Patras, 1 Karatheodori Street, GR 26500 Patras, Greece, Institute of Chemical Engineering and High Temperature Chemical Processes, GR 26504 Patras, Greece, and Technological Educational Institute of Crete, School of Agricultural Technology, Stavromenos, GR 71004 Heraklion Crete, Greece
Received February 18, 2010. Revised manuscript received June 2, 2010. Accepted June 8, 2010.
AOB
(ammonium oxidation) NH+ 4 98 NO2
NOB
(nitrite oxidation) NO2 98 NO3
Denitrification is a five-step process in which nitrate is reduced to nitrite and finally to molecular nitrogen: NO3 f NO2 f NO f N2O f N2 (denitrification)
During the past decade several new nitrogen removal processes have been developed to treat municipal and industrial wastewaters with medium or high ammonium load. Special attention has been focused on partial nitrificationdenitrification (PND) (1–3), either as a stand-alone process or coupled with Anammox (4), such as OLAND (5), CANON (6), and other processes. In the stand-alone PND process, ammonium oxidation takes place up to the level of nitrite (via AOB) and then nitrite is denitrified by denitrifying bacteria (DB) to molecular nitrogen: AOB
DB
NH+ 4 98 NO2 98 N2 (PND)
This work aims to assess and model the behavior of both ammonium (AOB) and nitrite (NOB) oxidizing bacteria during the transitionfromcompletelyanoxictoaerobicconditions.Anenhanced aerobically grown culture containing AOB and NOB was subjected to anoxic conditions of varying durations from 1.5 to 12 h before its exposure to aerobic conditions. Experiments were carried out in both continuously stirred tank reactor (CSTR) and batch type reactors. Although the AOB did not exhibit any impact in their performance following the anoxic disturbance, the NOB were seriously inhibited presenting a period of reduced growth rate, which was proportional to the duration of the disturbance. This finding proves the previously postulated mechanism (NOB inhibition under periodic aerobic/anoxic operation) for achieving nitrogen removal via the partial nitrification/denitrification (PND) process as demonstrated in lab- and pilot-scale operating conditions. A mathematical model was developed to describe with sufficient accuracy the performance of AOB and NOB under aerobic, anoxic, and transient conditions in both CSTR and batch type systems. The model is able to describe the inhibitory effect of anoxic exposure to NOB by assuming enzyme deactivation (under anoxic conditions) and reactivation (adjustment of the NOB enzymatic mechanism) under aerobic conditions. The presented kinetic model is quite simple and general and therefore may be used for predicting the performance of mixed growth biological systems operating via the PND process.
1. Introduction The most widely used process for biological ammonium removal is the biological nitrification-denitrification (ND) system that converts ammonium into molecular nitrogen. More precisely, in the nitrification process, ammonium is converted to nitrite by ammonium-oxidizing bacteria (AOB) and nitrite is oxidized to nitrate by nitrite-oxidizing bacteria (NOB): * Corresponding author phone: +30 2610 997418; fax: +30 2610 969556; e-mail:
[email protected]. † University of Patras. § Technological Educational Institute of Crete. ‡ Institute of Chemical Engineering and High Temperature Chemical Processes. 10.1021/es100564j
2010 American Chemical Society
Published on Web 06/28/2010
Compared to conventional N-removal, PND presents significant advantages, as it theoretically saves approximately 25% of electron acceptor (oxygen) for nitrification, 40% of electron donor (organic carbon) for denitrification, and achieves a lower sludge production (1–3, 7). In addition, it has been reported (7, 8) that nitrite reduction enhances the denitrification rate by 63% with a 33-35% lower sludge production in nitrification process and 55% in denitrification process. Moreover, PND can contribute to the reduction of CO2 emissions by 20% due to the denitrification from nitrite instead of nitrate (8). Many researchers have reported numerous important factors that make it possible to accomplish partial nitrification-denitrification, such as dissolved oxygen (DO) concentration, pH, temperature, free ammonia (FA) concentration, and free nitrous acid (FNA) concentration (9–15). Among the various methods that have been proposed for partial nitrification, the exploitation of the time-lag exhibited by NOB under alternating aerobic/anoxic conditions is by far preferable, as it does not require any addition of chemicals or extreme growth conditions (e.g., temperature), but a simple manipulation of the operating conditions. The key point, according to this strategy, is to enhance the nitritification process, while inhibiting or suppressing, at the same time, the nitratification process to obtain a mixed liquor biomass enriched in AOB and poor in NOB. Depending on the difference in the effective growth rates of AOB and NOB, the effluent of the system can range from basically nitrite to a mixture of ammonium and nitrite and also a mixture of nitrite and nitrate with low ammonia concentration (16, 17). If the nitratification process is inhibited to an extent that the minimum SRT required for NOB growth is higher than the current SRT of the system, then NOB will be washed out and the effluent will not contain nitrate. Our research team has demonstrated (3) that using a frequent enough switching between aerobic and anoxic conditions in a sequencing batch reactor (SBR) and an aerobic to anoxic phase ratio specific to the treated wastewater, it is possible to achieve PND, i.e., partially oxidize ammonia to nitrite (aerobically) and then subsequently reduce nitrite to nitrogen gas (under anoxic conditions), without producing VOL. 44, NO. 19, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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nitrate. The proved methodology was successfully transferred from the lab-scale SBR treating synthetic wastewater to a pilot-scale SBR system (18), which was constructed and operated in the Wastewater Treatment Plant of Patras (Western Greece) treating raw municipal wastewater. The success of this process has been attributed to the postulation that the NOB probably exhibit a period of reduced growth rate under aerobic conditions following an anoxic phase, but there has been no proof that this is indeed the case. The aim of this study was to assess the validity of this postulation to explain our findings from previous studies (3, 18, 19), and to model the “delay” effect exhibited by the NOB during transition from anoxic to aerobic conditions. The overall nitrification system behavior is modeled for both (a) a continuously stirred tank reactor (CSTR), in which mixed (suspended and attached) growth conditions prevail and (b) a batch reactor.
2. Materials and Methods 2.1. Preparation of Enhanced Cultures of AOB and NOB. A grab sample of mixed liquor from the oxidation ditch of the University of Patras (Western Greece) Wastewater Treatment Plant was collected and used as inoculum for the preparation of enhanced cultures of AOB and NOB. The procedure followed to this aim was based on feeding the inoculum with a selective autotrophic culture medium. For the NOB, 10 mL of sludge was added to a shaking flask (dark conditions were maintained using a foil flask cover to prevent photosynthetic organisms growth) containing 90 mL of a synthetic mineral medium (20) that contained the following (g/L): NaNO2, 0.30; NaHCO3, 1.0; K2HPO4, 10.52; KH2PO4, 4.72; and 1 mL/L of trace elements with the following composition (g/L): FeSO4 · 7H2O, 1.0; MgSO4 · 7H2O, 1.0; CaCl2 · 2H2O, 0.25; Na2MoO4 · 2H2O, 0.25; H3BO3, 0.1; and conc. H2SO4, 5 mL/L. Therefore, the initial concentration of nitrogen in the shaking flask was about 55 mg/L. The flask was incubated in a shaking bath (120 rpm) at 25 °C. Air was supplied to the agitated culture through an air pump at a constant flow rate to secure fully aerobic conditions. During the course of incubation, the concentration of NO2--N in the culture broth was monitored. After complete oxidation of nitrite nitrogen was observed (10-15 days), 10 mL of the culture broth was transferred to a new shake flask containing 90 mL of fresh medium. This procedure was repeated three times. Small aliquots of the flask were then spread on agar plates. The medium in agar plates consisted of the inorganic salts used in the synthetic mineral medium, containing also agar-agar at 18 g/L as solidifying medium. The agar plates were incubated for 3-4 weeks at 25 °C. Colonies were picked and restreaked until visually pure isolates were obtained. Cells were transferred from the agar plates to liquid media and cultivated in flasks for about a month and the final culture was grown and maintained in a draw-and-fill aerobic reactor (working volume 2 L) fed with the same synthetic mineral medium. The temperature of the reactor was automatically controlled at 25 ( 0.3 °C. The suspension medium was agitated by a magnetic stirrer. The pH in the reactor was maintained constant at 7.3 ( 0.2 by the buffering capacity of the phosphate salts (KH2PO4 and K2HPO4) used. For the preparation of the AOB enhanced culture the same procedure was followed regarding the use of culture medium and agar solid medium as those for NOB, in which, however, NaNO2 was substituted with (NH4)2SO4. In this case an enhanced nitrifying culture was developed where both AOB and NOB coexisted in the final reactor due to the synergistic effect of their growth (the product of AOB is the substrate for NOB growth). 2.2. Experiments with Enhanced AOB and NOB Cultures for the Determination of Kinetic Parameters. Batch experiments were performed using 250-mL Erlenmeyer (glass) 7246
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flasks. All batch experiments were conducted in a shaking bath at 25 °C. Air was supplied to the agitated culture through an air pump at a constant flow rate to secure fully aerobic conditions. The same medium used for the preparation of enhanced cultures, containing appropriate amounts of nitrite nitrogen (for NOB) and ammonium nitrogen (for AOB), was added to the active cultures of bacteria and their behavior was monitored. The nitritification and the nitratification rates were estimated by measuring the oxidation rate of ammonium and nitrite nitrogen and the concentration of volatile suspended solids in triplicates. The pH of the culture medium in all experiments was maintained in the range of 7.20-7.70 because of the buffering capacity of the KH2PO4 and K2HPO4 salts used. Two CSTR plexiglas reactors were also operated at different retention times to determine the model kinetic parameters. The reactor working volumes were 1254 mL for NOB cultivation and 1220 mL for the mixed AOB+NOB culture. The reactors were fed with the synthetic mineral medium using an electronically controlled peristaltic pump (ColePalmer). Different retention times were achieved by modifying the volumetric feed flow rate in each CSTR. Temperature, air, agitation, and pH were maintained at preset values via a thermocouple controller, an air pump, a magnetic stirrer, and the buffer capacity of the culture medium, respectively. The main procedure followed for the determination of kinetic parameters under conditions of mixed (suspended and attached) growth is analytically described in ref 20. 2.3. Experiments for Studying the Slow Response of NOB during Transition from Anoxic to Aerobic Conditions. Both CSTR and batch reactors were used for studying the “delay” (period of reduced growth rate) exhibited by NOB under aerobic conditions following an anoxic phase. The idea of those experiments was to expose the NOB (contained in the enhanced AOB+NOB culture) for different predetermined periods of time, either in a CSTR or batch reactor, to anoxic conditions and then re-establish aerobiosis to study their behavior. To this end, six different time intervals of anoxic exposure were selected, namely 1.5, 3.5, 6, 9, and 12 h. All experiments conducted in the CSTR were in fact consecutive disturbances since each exposure of the culture to anoxic conditions was followed by an aerobic period, long enough for the system to reach again steady-state (or quasi steady-state) conditions, before applying the next anoxic exposure. For each batch experiment two 500-mL Erlenmeyer (glass) flasks were filled with 250 mL from an actively growing enhanced AOB+NOB culture. It should be mentioned that argon gas was used for an initial intense sparging to degas the culture medium from dissolved oxygen, and afterward at a minimum flow just to ensure anoxic conditions inside the reactors during the anoxic phase. The reactors’ performance was documented by measuring pH, DO, ammonium, nitrite, and nitrate. Samples were taken from the reactors and immediately filtered through a glass-fiber filter (Whatman, GF/F grade; pore size diameter 0.7 µm) and then through a 0.22-µm nylon filter before chemical analysis. 2.4. Chemical Analysis. Ammonium and nitrite nitrogen were determined spectrophotometrically according to the phenate (4500-E) and the colorimetric method (4500-B), respectively (21). For nitrate nitrogen, ion chromatography was used (DX300, Dionex Corp.). The volatile suspended solids (VSS) were measured according to the 2540-D.E method (21). The pH and dissolved oxygen (DO) were monitored using a Hanna electrode (HI 8224) and a Russell electrode, respectively.
3. Experimental Results The effect of exposure of AOB and NOB to anoxic conditions on the nitritification and nitratification processes was
FIGURE 1. Exposure of the enhanced AOB+NOB culture to anoxic conditions for 12 h in a CSTR system. Experimental and simulated concentration profiles for (a) ammonium, (b) nitrite, (c) nitrate, (d) dissolved oxygen, and (e) AOB during the transition from anoxic to aerobic conditions; (f) predicted density of active attached AOB and NOB; (g) predicted evolution of NOB maximum specific growth -1 rate. S1,F ) 70.24 mg NH+ 4 -N/L and D ) 1.18 d . studied by conducting experiments in both batch and CSTR reactors. Six different anoxic time periods were selected to be implemented prior to complete aerobiosis, namely
1.5, 3.5, 6, 9, and 12 h. The obtained experimental results regarding the time course of ammonium, nitrite, nitrate, and dissolved oxygen, as well as the simulated growth of VOL. 44, NO. 19, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Exposure of the enhanced AOB+NOB culture to anoxic conditions for 1.5 h in a CSTR system. Experimental and simulated concentration profiles for (a) ammonium, (b) nitrite, (c) nitrate, (d) dissolved oxygen, and (e) AOB during the transition from anoxic to aerobic conditions; (f) predicted density of active attached AOB and NOB; (g) predicted evolution of NOB maximum specific growth -1 rate. S1,F ) 70.13 mg NH+ 4 -N/L and D ) 1.18 d . AOB and NOB for the extreme cases of 1.5 and 12 h for the batch and the CSTR experiments are presented in Figures 1 to 4. The experimental behavior of each culture can be 7248
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compared with the simulated one (using the model described in the next section) as illustrated by the use of lines in each figure diagram.
FIGURE 3. Exposure of the enhanced AOB+NOB culture to anoxic conditions for 1.5 h in a batch system. Experimental and simulated concentration profiles for (a) ammonium, (b) nitrite, (c) nitrate, (d) dissolved oxygen, and (e) AOB and NOB during the transition from anoxic to aerobic conditions; (f) predicted evolution of NOB maximum specific growth rate. Figure 1 shows the response of a nitrifying CSTR to a 12-h exposure to anoxic conditions. As seen, nitrite accumulates and it takes more than 8 h before the nitrite levels fall to their steady-state values. Figure 2 shows the response of a CSTR to a 1.5-h anoxic disturbance. Again nitrate accumulation is observed lasting for approximately 3 h. According to the experimental observations, no NOB were detected in the liquid phase of the culture medium, at least to a measurable concentration level, in all CSTR experiments. This observation was based on batch tests conducted periodically in which 200 mL was carefully removed from the reactor’s mixed liquor containing suspended biomass. Ammonium and nitrite consumption rates were measured after addition of 50 mL of fresh medium containing either ammonium or nitrite. Although ammonium consumption was observed in any case, nitrite consumption was not detected. Thus nitrite oxidation in the CSTR is assumed to take place mainly by NOB attached to the vessel walls.
Figures 3 and 4 demonstrate the response of batch cultures to 1.5 and 12 h, respectively. Again, nitrite accumulates during the disturbance.
4. Mathematical Model Development The mathematical model that was developed to describe the performance of the enhanced AOB+NOB culture in a CSTR is described by the following equations: µm1S1 SO dx1 A x + x′ ) -Dx1 + dt Ks1 + S1 SO + KSO1 1 V 1
(
)
(1)
SO dS1 1 µm1S1 A x + x′ ) D(S1,F - S1) dt Y1 Ks1 + S1 SO + KSO1 1 V 1
(
) (2)
µ2S2 SO dx2 A x + x′ ) -Dx2 + dt Ks2 + S2 SO + KSO2 2 V 2
(
)
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(3)
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(
)
dS2 µm1S1 SO A 1 - iNB x + x′ ) -DS2 + dt Y1 Ks1 + S1 SO + KSO1 1 V 1 SO 1 µ2S2 A x + x′ Y2 Ks2 + S2 SO + KSO2 2 V 2 (4)
(
(
(
)
)
)
dS3 µ2S2 SO A 1 - iNB x + x′ ) -DS3 + dt Y2 Ks2 + S2 SO + KSO2 2 V 2
(
) (5)
dSO ) KLa(SO,SAT - SO) + D(SO,FEED - SO) dt S1 SO µm1 A x + x′ YO,1 S1 + KS1 SO + KSO1 1 V 1 µ2 S2 SO A x + x′ YO,2 S2 + KS2 SO + KSO2 2 V 2
(6)
SO KSO3 dµ2 + R(µm2 - µ2) ) -Kdµ2 dt KSO3 + SO KSO3 + SO
(7)
(
(
)
)
where the biomass density in the biofilm (x′) is expressed (20) as: xi′ ) LmaxFC,i )
[
]
2Ds,jYi,j S µm,iFC,i B,j
0.5
FC,i )
[
2Ds,jYi,jFC,i µm,i
]
0.5
0.5 SB,j
(8)
Because only one of these substrates may be the limiting one, the biofilm thickness Lmax is calculated for every substrate and the minimum biofilm depth is considered as Lmax (22) for the calculation of both x 1′ and x 2′. The presented mathematical model was developed stepwise: first based on fitting nitratification and then nitritification. The experimental data (characteristics of steady-state conditions) obtained from the operation of a CSTR system, in which the NOB were cultivated at various hydraulic retention times, were used for the estimation of all kinetic parameters involved in the model. The development of the model for nitratification, based on the Topiwala and Hammer expression (23), is described in detail in our previous work (20). This model was extended to
FIGURE 4. Exposure of the enhanced AOB+NOB culture to anoxic conditions for 12 h in a batch system. Experimental and simulated concentration profiles for (a) ammonium, (b) nitrite, (c) nitrate, (d) dissolved oxygen, and (e) AOB and NOB during the transition from anoxic to aerobic conditions; (f) predicted evolution of NOB maximum specific growth rate. 7250
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TABLE 1. Kinetic Parameters Used in the Proposed Model and Overall Reactions for Ammonium and Nitrite Oxidation symbol µm1 Y1 Ks1 Ds1 FC1 KSO1 YO,1
definition
value
unit
AOB maximum specific growth rate of AOB 0.20 yield of AOB on ammonium 0.23 affinity constant for ammonium 1.82 diffusion coefficient of ammonium in the biofilm 0.0006 biomass (AOB) concentration in the biofilm 58,936 affinity constant of AOB for oxygen 0.45 yield of AOB on oxygen 0.07 overall reaction for ammonium oxidation:
1/d mg biomass/mg NH4+-N mg NH4+-N/L dm2/d mg/dm3 mg O2/L mg biomass/mg O2
35NH4- + 46O2 + 4CO2 + HCO3- f C5H7O2N + 34NO2- + 33H2O + 68H+
µm2 Y2 Ks2 Ds2 DSO FC2 KSO2 YO,2 Kd R KSO3
NOB maximum specific growth rate of NOB 0.19 yield of NOB on nitrite 0.23 affinity constant for nitrite 0.24 diffusion coefficient of nitrite in the biofilm 0.0006 diffusion coefficient of oxygen in the biofilm 0.01512 biomass (NOB) concentration in the biofilm 93,508 affinity constant of NOB for oxygen 0.83 yield of NOB on oxygen 0.15 deactivation constant of NOB under anoxic conditions 0.00066 adaptation parameter of NOB to aerobic conditions 0.0026 dissolved oxygen half saturation constant for NOB adaptation to 0.01 aerobic conditions overall reaction for nitrite oxidation:
1/d mg biomass/mg NO2--N mg NO2--N/L dm2/d dm2/d mg/dm3 mg O2/L mg biomass/mg O2 dimensionless dimensionless mg O2/L
35NO3- + 10.5O2 + 5CO2 + H+ + 3H2O f C5H7O2N + 34NO3-
simulate the whole process of nitrification and to include the “delay” effect exhibited by the NOB during the transition from anoxic to aerobic conditions (expressed by eq 7). This effect was attributed to the deactivation of a critical enzyme for NOB growth, and this was modeled with first-order kinetics (with Kd deactivation constant), since according to the extended review of Sadana (24) this is the most common case. It was thus assumed that the maximum specific growth rate of the NOB (µm), being directly proportional to the activity of this critical enzyme, decreases accordingly (due to NOB deactivation) and the decrease depends also on the duration of the anoxic phase exposure. In the sequel, when the NOB are exposed again to aerobic conditions this enzyme is reactivated and the maximum specific growth rate of NOB tends to adjust and reach its maximum value which is defined as µm2. To mathematically describe the observed period of reduced growth rate, during the adjustment of NOB to the aerobic environment, the approach followed by Wang and Stephanopoulos (25) and also by Kornaros et al. (26) was used for the estimation of the specific growth rate of NOB. Therefore an adaptation term (with a as the adaptation parameter) has been incorporated into the second part in the right side of eq 7. The definition and values of the model’s kinetic parameters are given in Table 1. Based on the estimated values of yield coefficients, the overall stoichiometric equations, including cell synthesis, for both nitritification and nitratification were determined and are given also in Table 1. C5H7O2N is the empirical formula for both types of microbial cells, AOB and NOB. Both equations are in agreement with literature (i.e., 27, 28). For the evaluation of the product Ka (volumetric masstransfer coefficient) in the mass-balance of dissolved oxygen the “dynamic gassing out” method was used (29, 30). The value of the KLa that was calculated was 0.905 min-1. SO,SAT for the specific culture medium was estimated to be 7.40 mg O2/L. The value of iNB (content of nitrogen in biomass) was taken from Activated Sludge Model No.1 (ASM 1) (31) equal to 0.086 g N/g cell COD or 0.122 g N/g cell dry weight.
The experimental data presented in Figure 1 were used for estimating the value of Kd (shown in Table 1) in eq 7 provided that all other model parameters had already been calculated using either steady state data from the CSTR operating with NOB and the enhanced AOB+NOB or specifically designed batch tests for each parameter. The model was then used to simulate the CSTR behavior in all other experiments, i.e., 1.5, 3.5, 6, and 9 h anoxic phase. As shown in Figure 2 which presents, indicatively, the results for the experiment of 1.5 h anoxic phase, the agreement obtained between experimental and simulated (theoretical) behavior is excellent. The same mathematical model (eqs 1-8) was then used to describe the performance of the enhanced AOB+NOB culture under batch conditions. In this case the model is described by the following set of mass balances (eqs 9-15) by removing the influent and effluent terms and adding eq 11 that describes NOB growth under batch (short-term) suspended-growth conditions:
(
SO µm1S1 dx1 x ) dt Ks1 + S1 SO + KSO1 1
(9)
SO dS1 1 µm1S1 x )dt Y1 Ks1 + S1 SO + KSO1 1
(10)
dx2 SO µ2S2 x ) dt Ks2 + S2 SO + KSO2 2
(11)
)
µm1S1 SO dS2 1 - iNB x ) dt Y1 Ks1 + S1 SO + KSO1 1 SO 1 µ2S2 x (12) Y2 Ks2 + S2 SO + KSO2 2
(
)
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µm1 dSO S1 SO x ) KLa(SO,SAT - SO) dt YO,1 S1 + KS1 SO + KSO1 1 µ2 S2 SO x YO,2 S2 + KS2 SO + KSO2 2 (14)
dµ2 KSO3 SO ) -Kdµ2 + R(µm2 - µ2) dt KSO3 + SO KSO3 + SO
Ds,i Kd KLa Ks1 Ks2 KsO1 KsO2
(15) Because of the very short duration of these experiments no bacterial growth was observed on the reactor walls and therefore all terms expressing the contribution of attached biomass in equations (1-7) were excluded in the new set of equations (9-15). The model parameters values indicated in Table 1 were used for simulating the batch experiments. Comparing the experimental data with the simulated behavior shown in Figures 3 and 4 illustrating the culture performance after 1.5 and 12 h exposure in anoxic conditions prior to aerobiosis, excellent agreement can be easily concluded.
KsO3
5. Significance of Work
SO,FEED
This work represents actual experimental proof that NOB are indeed slow in adapting to aerobic conditions, following one (as in batch experiments) or consecutive (as in CSTR experiments) anoxic disturbances and the delay in their recovery is a strong function of the duration of each disturbance. A general dynamic kinetic model was developed for the overall nitrification process as an extension of simpler models developed for describing the oxidation of nitrites under completely aerobic conditions. This model has the exceptional ability to predict with sufficient accuracy the behavior of both AOB and NOB under aerobic, anoxic, and transient operating conditions as those prevailing in all common WWTPs. Experimental data obtained from both CSTR reactor operation and batch experiments were used for the validation of the presented model. The effect of exposing the NOB to anoxic conditions and their subsequent inhibition because of the presumed deactivation has been described mathematically. The delay in the growth and activity of NOB following their alternating exposure to anoxic and aerobic conditions may be exploited and has already been proven by our group to be effective for achieving PND under both lab- (3, 19) and pilot-scale (18) conditions. The proposed method of nitrate-bypassing is in essence the only acceptable means of bypassing, because the alternative technologies require addition of chemicals, and have as a prerequisite close monitoring and control of the process pH and dissolved oxygen, while maintaining the reaction mixture at elevated temperatures is energyconsuming during the treatment of high-volume wastewaters, such as municipal wastewaters. The proposed model may be used for designing appropriate PND operating strategies. It may be also used for the retrofit of existing units, improving their performance significantly, without any significant costs since PND is only based on proper manipulation of aerating conditions.
SO,SAT
Acknowledgments We thank the European Social Fund (ESF), Operational Program for Educational and Vocational Training II (EPEAEK II) and particularly the Program IRAKLEITOS, for funding the above work.
Appendix A Nomenclature wetted surface area of the reactor (dm2) dilution rate (d-1)
A D 7252
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Lmax S1 S2 S3 SO
SB,i S1,F V x1 x1′ x2 x2′ Y1 Y2 YO,1 YO,2 R µm1 µm2 µ2 FC1 FC2 iNB
diffusion coefficient of component “i” in biofilm (dm2/d) NOB deactivation kinetic constant (dimensionless) volumetric oxygen transfer coefficient (min-1) ammonium half saturation constant (mg/L) nitrite half saturation constant (mg/L) dissolved oxygen half saturation constant for AOB (mg/L) dissolved oxygen half saturation constant for NOB (mg/L) dissolved oxygen half saturation constant for NOB adaptation to aerobic conditions (mg/L) maximum active thickness of the biofilm (dm) ammonium concentration in the culture medium (mg/L) nitrite concentration in the culture medium (mg/L) nitrate concentration in the culture medium (mg/L) dissolved oxygen concentration in the culture medium (mg/L) dissolved oxygen concentration in the feed of the CSTR (mg/L) dissolved oxygen concentration in the liquid at equilibrium with the gas phase (mg/L) concentration of component “i” in the bulk liquid (mg/L) ammonium concentration in the feed of the CSTR (mg/L) working volume of the reactor (L) ammonium oxidizing bacteria (AOB) concentration in the culture medium (mg/L) density of active AOB attached to the reactor’s walls (mg/dm2) nitrite oxidizing bacteria (NOB) concentration in the culture medium (mg/L) density of active NOB attached to the reactor’s walls (mg/dm2) growth yield coefficient for ammonium oxidation (g biomass/g NH4+-N) growth yield coefficient for nitrite oxidation (g biomass/g NO2--N) growth yield coefficient for oxygen consumption by AOB (g biomass/g O2) growth yield coefficient for oxygen consumption by nitrite oxidizing bacteria (g biomass/g O2) adaptation parameter (dimensionless) maximum specific growth rate of AOB (d-1) maximum specific growth rate of NOB (d-1) specific growth rate of NOB (d-1) AOB concentration in the biofilm (mg/dm3) NOB concentration in the biofilm (mg/dm3) nitrogen content of biomass (mg N/mg cell dry weight)
Subscripts i culture type, either 1 for AOB or 2 for NOB j limiting substrate for biofilm growth either ammonium, nitrite or dissolved oxygen
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