The kinetics of nitrogen removal and biogas production in an anammox non-woven membrane reactor

Bioresource Technology 101 (2010) 5767–5773

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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

The kinetics of nitrogen removal and biogas production in an anammox non-woven membrane reactor Shou-Qing Ni a,*, Po-Heng Lee b, Shihwu Sung b,* a b

School of Environmental Science and Engineering, Shandong University, Jinan, Shandong 250100, China Department of Civil, Construction and Environmental Engineering, Iowa State University, Ames, IA 50011-3232, USA

a r t i c l e

i n f o

Article history: Received 24 December 2009 Received in revised form 14 February 2010 Accepted 16 February 2010 Available online 25 March 2010 Keywords: Anammox Kinetics Model Substrate removal Nitrogen gas production

a b s t r a c t The anammox non-woven membrane reactor (ANMR) is a novel reactor configuration to culture the slowly growing anammox bacteria. Different mathematical models were used to study the process kinetics of the nitrogen removal in the ANMR. The kinetics of nitrogen gas production of anammox process was first evaluated in this paper. For substrate removal kinetics, the modified Stover-Kincannon model and the Grau second-order model were more applicable to the ANMR than the first-order model and the Monod model. For nitrogen gas production kinetics, the Van der Meer and Heertjes model was more appropriate than the modified Stover-Kincannon model. Model evaluation was carried out by comparing experimental data with predicted values calculated from suitable models. Both model kinetics study and model testing showed that the Grau second-order model and the Van der Meer and Heertjes model seemed to be the best models to describe the nitrogen removal and nitrogen gas production in the ANMR, respectively. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Anaerobic ammonium oxidation (anammox) is a novel, autotrophic and cost-effective alternative to the traditional biological nitrogen removal process (Pynaert et al., 2004; Strous and Jetten, 2004), which was originally discovered in a denitrifying fluidized-bed reactor treating effluent from a methanogenic reactor (Mulder et al., 1995). The anammox bacteria oxidize ammonia to nitrogen gas using nitrite as an electron accepter under anoxic conditions, and their growth occurs by carbon dioxide fixation (Strous et al., 1998). The discovery of the anammox process brought revolutionary changes to the conventional biological nitrogen removal from wastewater. Some unique characteristics make the anammox consider to be a promising and sustainable process (Abma et al., 2007), such as no need for aeration and addition of external carbon sources (Chamchoi et al., 2008) and low biomass yield in this autotrophic process. While the newly discovered anammox process opens up new possibilities for nitrogen removal from wastewater, the major obstacle for the implementation of the anammox process is the slow growth rate of the anammox microorganisms (Strous et al., 1998), which makes the anammox process be difficult to apply for practical wastewater treatments (Abma et al., 2005). In order to fulfill practical application of the anammox pro* Corresponding authors. Tel.: +86 531 8836 5062; fax: +86 531 8836 4513 (S.Q. Ni); tel.: +1 515 294 3896; fax: +1 515 294 8216 (S. Sung). E-mail addresses: [email protected] (S.-Q. Ni), [email protected] (S. Sung). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.02.074

cess, many researchers focus on the obtainment of anammox bacteria. Many studies were carried out to enrich the anammox organisms, either by different methods such as biofilm or granulation (Ni et al., 2010), or by different types of reactors such as sequencing batch reactor (Strous et al., 1998), fluidized-bed reactor (Mulder et al., 1995), membrane bioreactor (MBR) (van der Star et al., 2008) and rotating biological contactor (Egli et al., 2001). Among these different reactors, the anammox non-woven membrane reactor (ANMR) is a novel reactor configuration to enrich anammox biomass (Ni et al., 2010). The reactor was developed by connecting a set of non-woven membrane module, which also served as an effluent port, with an anaerobic reactor. The membrane module was installed outside the reactor, which is different from the immerged membrane reactors. Unlike conventional MBR, wastewater circulated in the membrane module and the biofilms grew on the membrane interior surface. A large amount of the suspended biomass could remain in the reactor by filtration through the non-woven membrane and biofilm, resulting in improvement of the effluent quality and enhancement of the solid retention in the reactor. The ANMR combined a short HRT and stable SRT. After over eight months of operation, the purity (percentage of anammox cells in the community) of the anammox bacteria in the reactor was quantified to be 97.7% (Ni et al., 2010). The cost effective ANMR was shown to be suitable for the slowly growing anammox bacteria. Process modeling is widely used to describe and to simulate the performance of biological processes. The process kinetics provides

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a rational basis for process analysis and control, as well as dealing with operational and environmental factors affecting substrate utilization rates. It is possible to optimize reactor performance based on the kinetics study. Although anammox process has been studied for a long time by different researchers, little attention has been paid to substrate removal kinetics of anammox process (Jin and Zheng, 2009), and none was done on the nitrogen gas production kinetics. The novel ANMR could develop both biofilm on the interior surface of the membrane and biomass aggregates in the anaerobic reactor (Ni et al., 2010), which was different from the normal biofilm reactor (Jin and Zheng, 2009). The application of the present models in the description and simulation of the ANMR was unknown. Since nitrogen gas production is an important parameter for anammox treatment systems, the nitrogen gas production kinetics should also be addressed, which can be used to control and predict the treatment plant performance and to optimize the plant design. In this study, different mathematical models including the first-order substrate removal model, the Grau second-order substrate removal model, the modified Stover-Kincannon model, the Monod model, and the Van der Meer and Heertjes model were used to study the process kinetics of the novel ANMR. The objective of this study was to evaluate different kinetics models for describing the substrate removal and biogas production of the ANMR. 2. Kinetics approaches As mentioned above, four different substrate removal models, including the first-order substrate removal model, the Grau second-order substrate removal model, the modified Stover-Kincannon model and the Monod model, were used to evaluate the nitrogen removal kinetics. Two different models, including the modified Stover-Kincannon model and the Van der Meer and Heertjes model, were applied to study the nitrogen gas production kinetics.

2.1.1. First-order substrate removal model When the first-order substrate removal model was applicable to the reactor, the change rate of substrate concentration in the complete mixed system could be expressed as follows (Jin and Zheng, 2009):

dS QSi QSe ¼   k 1 Se dt V V

ð1Þ

Since the change rate (dS/dt) is negligible under pseudo-steady-state condition, the equation can be derived as:

S i  Se ¼ k1 Se HRT

ð2Þ

where Si and Se are influent and effluent total nitrogen concentrations (g/L), k1 is the first-order substrate removal rate constant (1/ d), HRT is the hydraulic retention time (day), Q is the inflow rate (L/d) and V is the reactor volume (L). So the unknown k1 can be derived from the slope of the line by plotting ((Si  Se)/HRT) versus Se in the above equation. 2.1.2. Grau second-order substrate removal model The general equation of a second-order model is illustrated as below (Grau et al., 1975):



  dS Se ¼ k2 X dt Si

ð3Þ

By integration and linearization, the above equation can be expressed as follows:

ð4Þ

If Si/(k2X) is considered as a constant and (Si  Se)/Si is replaced by the total substrate removal efficiency (E), Eq. (4) can be modified as follows:

HRT ¼ a þ bHRT E

ð5Þ

where a equals to Si/(k2X), a and b are constants, E is the total nitrogen removal efficiency (%), k2 is the Grau second-order substrate removal rate constant (1/d) and X is the biomass concentration in the reactor (g/L). 2.1.3. Modified Stover-Kincannon model The Stover-Kincannon model (Stover and Kincannon, 1982) was initially used to predict the attached-growth biomass performance in a rotating biological contactor. Later, the model was modified and widely applied to describe and predict the bioreactor performance (Isik and Sponza, 2005; Kapdan and Aslan, 2008; Yu et al., 1998). The original model equation is QS dS U max ð A i Þ ¼ dt K B þ ðQSA i Þ

ð6Þ

where dS/dt is the substrate removal rate (kg/m3/d), Umax is the maximum utilization rate constant (kg/m3/d), KB is the saturation value constant (kg/m3/d) and A is the disc surface area (L). If the surface area (A) is replaced by the reactor volume (V), the Stover-Kincannon model can be modified as follows (Yu et al., 1998): QS dS U max ð V i Þ ¼ dt K B þ ðQSV i Þ

ð7Þ

The Stover-Kincannon model considers dS/dt as a function of substrate loading rate at steady state as in Eq. (8):

dS Q ¼  ðSi  Se Þ dt V

2.1. Substrate removal kinetics



Si HRT Si ¼ HRT þ S i  Se k2 X

ð8Þ

So Eq. (7) can be illustrated as follows: K V

B V 1 ¼ Umax þ QðSi  Se Þ QSi U max

ð9Þ

2.1.4. Monod model The Monod model is widely used to describe microbial growth (Monod, 1949) as illustrated in Eq. (10).

dS kXSe ¼ dt K s þ Se

ð10Þ

where k is the maximum rate of substrate utilization (1/d), and Ks is the half saturation concentration (g/L). If the substrate removal rate is replaced by Eq. (8) and Eq. (10) can be expressed as follows:

XV 1 Ks 1 ¼ þ QðSi  Se Þ k k Se

ð11Þ

2.2. Nitrogen gas production kinetics 2.2.1. Van der Meer and Heertjes model This model was used to determine methane gas production (Van der Meer and Heertjes, 1983). In this study, this model was applied to evaluate the nitrogen gas production of the ANMR, as shown in Eq. (12).

GN ¼ ksg Q ðSi  Se Þ

ð12Þ

where ksg is the Van der Meer and Heertjes kinetic constant (mL/ mg), and GN is the nitrogen gas production (L/d).

S.-Q. Ni et al. / Bioresource Technology 101 (2010) 5767–5773

2.2.2. Modified Stover-Kincannon model The methane production rate can also be modeled in terms of substrate removal (Yu et al., 1998). It depends on the substrate inflow rate and substrate loading rate. So Eq. (7) can be modified as follows:

N¼

Nmax NB þ

  QSi V

 

ð13Þ

QSi V

where N is the specific nitrogen gas production rate (L/L/d), Nmax is the maximum nitrogen gas production rate (L/L/d), and NB is a constant (g/L/d). As QSi/V is the total nitrogen loading rate (NLR), the above equation can be expressed as follows:

1 NB 1 1 ¼ þ N Nmax NLR Nmax

ð14Þ

3. Methods 3.1. ANMR reactor and operation With an equivalent pore size of 1 lm, six single tubular membranes (1 cm internal diameter, 56 cm length, 0.5 cm material thickness and 0.1 m2 total filtration area) were connected with a 4 L anaerobic reactor to constitute the ANMR (Fig. 1). The cost effective tubular non-woven fabric membrane which is the same material for making disposal masks, diaper liners, etc. was provided by the KNH Enterprise Co., Taipei, Taiwan. This membrane module has low packing density and is less susceptible to fouling than the capillary or hollow fiber modules. The non-woven membrane was used for enhancing anammox biomass retention, forming a bioflim on the interior surface of the membrane, and separating the effluent from the biomass. A level sensor was installed to maintain efficient volume in the reactor. Back flushing and mechanical cleaning without any implementation of chemicals were carried out as needed. Inside-to-out flow separation and its cross flow shear force were driven by a peristaltic pump (MasterFlex, Cole-Parmer Instrument, Vernon Hills, IL, USA). The biomass and wastewater were circulated through the inside of the non-woven membrane connected to the reactor using a recycling pump. The transmembrane pressure (TMP) was regulated in a range of 0.5–3 psi. The initial sludge was from a pilot-scale SBR reactor treating the digester supernatant. The ANMR was adopted to study the enrichment culture of anammox bacteria and a very pure culture was obtained (Ni et al., 2010). After the enrichment study, some biomass was taken out to do other study and the reactor was further used

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for kinetics analysis. At that time, the biomass concentration in ANMR was 1.52 g VSS/L. In this study, the reactor was continuously fed with the synthetic mineral media at ambient temperature of 20–25 °C. The vessel was stirred at 150 rpm by a six-bladed turbine. During the kinetics study, the operation parameters were not changed until a pseudo-steady-state was reached. The initial ammonium and nitrite concentrations were 115 and 153 mg N/l, respectively. They were increased gradually according to research trends. The effluent was filtered by the membrane module driven by a peristaltic pump, which was controlled by a liquid level sensor placed within the reactor. Unlike the conventional MBR, anammox biofilm formed on the interior surface of the membrane modules which would enhance the removal efficiency of ammonium and nitrite. The biofilm thickness was controlled by the cross flow shear stress from the peristaltic pump, which also served as the filtration driving force. The HRT was set at 3 days and reduced gradually to 0.6 day. The HRT was calculated through dividing the reactor volume by the inflow rate. At each HRT, in order to reach the steady state, the reactor was run at least 3 turnovers of HRT. Anaerobiosis of the reactor was achieved by flushing with argon gas. The pH in the reactor was adjusted to 7.5–8.0 with CO2 gas. More details about the reactor and operation were described by Ni et al. (2010). 3.2. Synthetic wastewater The composition of synthetic wastewater was (g/L): KHCO3 0.5, KH2PO4 0.0272, MgSO47H2O 0.18, CaCl22H2O 0.12 and 1 mL trace elements solutions I and II (van de Graaf et al., 1996). Ammonium and nitrite were added in the required amounts in the form of NaNO2 and (NH4)2SO4. The trace elements solution I contained (g/L): EDTA 5 and FeSO4 5; and trace elements solution II contained (g/L): EDTA 15, ZnSO47H2O 0.43, CoCl26H2O 0.24, MnCl24H2O 0.99, CuSO45H2O 0.25, NaMoO42H2O 0.22, NiCl26H2O 0.19, NaSeO410H2O 0.21 and H3BO4 0.014. The synthetic wastewater was deoxygenated by flushing with argon gas before feeding to the reactor. 3.3. Analysis The samples were not taken until a pseudo-steady-state of each operational condition was achieved. Nitrite and nitrate concentrations in the samples were determined by ion-chromatography (DX 500, Dionex, Sunnyvale, CA, USA). Ammonium was measured by selective electrode according to Standard Methods (APHA, 1998). The pH was determined via a calibrated pHS-25 acidimeter and a

Fig. 1. Schematic diagram of the anammox non-woven membrane reactor (ANMR) for kinetics study.

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general purpose pH electrode (Thermo Fisher Scientific, Waltham, MA, USA). SS and VSS were determined by the weighing method after being dried at 103–105 °C and burnt to ash at 550 °C (APHA, 1998). The gas produced in the reactor was collected at the top, and measured with a wet-type laboratory gas meter (Schlumberger, Sugar Land, TX, USA). The gas was sampled as needed by using 1 mL gastight syringe (Hamilton, Reno, NV, USA). Gas composition was measured with a GC-350 (GOW-MAC Instrument, Bethlehem, PA, USA) equipped with a column and a thermal conductivity detector, operated at 70 °C and 200 °C, respectively.

4. Results 4.1. Reactor performance In order to obtain the kinetic parameters of different models, the ANMR was run at different HRTs from 2.9 days to 0.6 days with synthetic wastewater. The ammonium and nitrite concentrations in the influent were increased from 115 and 153 mg N/L to 297 and 313 mg N/L, respectively. The initial NLR was 107 mg N/L/d. With the increase of the biomass activity and the decrease of the HRT, the NLR increased to the highest 746 mg N/L/d gradually. The results obtained at steady conditions during kinetics study at different HRTs are illustrated in Fig. 2. As the HRT was decreased from 2.9 days to less than one day, total nitrogen (TN) removal efficiencies decreased from 89.9% to the lowest 80.7%. Meanwhile, ammonium removal efficiencies dropped from 99.3% to the lowest 82.6%. However, the decrease of HRT did not affect the nitrite removal significantly. During the whole operation period, almost 100% of nitrite was removed from the wastewater. This indicated that nitrite was the limiting nutrient in this study (Lopez et al., 2008) and the bacteria grew at a growth rate which was lower than the maximum specific one. During the study, the gas produced in the reactor was collected and measured by a gas meter. Over 99% (v/v) of the gas production was composed of N2 and no CH4 and CO2 were detected. As the anammox bacteria can convert ammonium and nitrite to nitrogen gas, the gas production rates are directly related to the conversion rate in the reactor, and can be used as a parameter for the control of the process performance (Mulder et al., 1995). In the experimental period, the nitrogen gas production increased gradually from 228 mL/d to 2133 mL/d with the decrease of HRT. The decrease of HRT resulted in higher NLR so that more substrate was available for the conversion by anammox process, leading to higher gas production.

Nitrogen removal efficiency (%)

100

90

80

TN + NH4 -N

70

-

NO2 -N 60

0.5

1.0

1.5 2.0 HRT (d)

2.5

3.0

Fig. 2. Reactor performance at different HRTs during the kinetics study operation.

4.2. Substrate removal kinetics 4.2.1. First-order substrate removal model The first-order substrate removal kinetics of the ANMR treating high nitrogen wastewater is shown in Fig. 3A. The first-order kinetic constant k1 was calculated as 5.3051 per day from the model line by plotting (Si  Se)/HRT versus Se. The correlation coefficient (R2) was 0.7066. 4.2.2. Grau second-order substrate removal model Grau second-order model coefficients were determined by plotting Eq. (5) (Fig. 3B). The values of a and b were calculated to be 0.1054 and 1.1101 from the intercept and slope of the plot line in Fig. 3B. The R2 of the second-order kinetic model was 0.9954. The Grau second-order substrate removal rate constant (k2) could then be obtained from the equation a = Si/(k2X). 4.2.3. Modified Stover-Kincannon model At steady state, the modified Stover-Kincannon model was applied to simulate nitrogen removal kinetics of the ANMR (Fig. 3C). If V/(Q(Si  Se)) is plotted against V/(QSi), KB/Umax is the slope and 1/Umax is the intercept point. The KB and Umax constants were calculated to be 8.98 kg/m3/d and 7.89 kg/m3/d. This indicated that the maximum total nitrogen removal rate was 7.89 kg/m3/d in this anammox reactor. The correlation coefficient is 0.9986, confirming the applicability of the modified Stover-Kincannon model. 4.2.4. Monod model A plot of Eq. (11) gave the necessary information for the Monod kinetic coefficients. From Fig. 3D, the values of the maximum rate of substrate utilization (k) and half saturation concentration (Ks) were 0.1693 per day and 0.1922 g/L, respectively. The R2 of the regression was 0.5993, being much lower than correlation coefficients of other models. 4.3. Nitrogen gas production kinetics Although nitrogen gas production is an important parameter for anammox process, seldom studies have been done on the nitrogen gas production kinetics. In this study, two models were used to evaluate the gas production kinetics in the ANMR. 4.3.1. Van der Meer and Heertjes model In this study, the Van der Meer and Heertjes model was modified to simulate nitrogen gas production from methane gas production. In this model, the nitrogen gas production is related to kinetic constant, inflow rate of the reactor and nitrogen removal during the process. By plotting Q(Si  Se) against GN, the kinetic constant (ksg) of gas production was determined from the slope of the regression line (Fig. 4A). The ksg was calculated as 0.7148 mL/mg N removed. 4.3.2. Modified Stover-Kincannon model Yu et al. (1998) found that the methane production was a function of the total substrate loading rate. In this study, it was supposed that the nitrogen gas production could be modeled in terms of substrate removal and was dependent on the total NLR. Therefore, the nitrogen gas production could be modeled by similar kinetics of substrate removal as in Section 3.2.3. In this model, the nitrogen gas production is related to kinetic constant, inflow rate of the reactor and applied total NLR. In order to determine the kinetic constant, the inverse of the N2 production rate was plotted against the inverse of the NLR (Fig. 4B). The intercept and slope of the plot line resulted in 1/Nmax and NB/Nmax, respectively. The maximum specific nitrogen gas production rate (Nmax) and proportional constant (NB) were determined to be 0.4211 L/L/d and 0.1476 g/L/d, respectively.

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A

0.6

3.0

0.5 y = 5.3051x - 0.08986 2 R = 0.7066

0.4

2.5 HRT/E (d)

(Si-Se)/HRT (g/L/d)

B

3.5

0.3 0.2

1.5

0.1

1.0

0.0 0.02

y = 1.1101x + 0.1054 2 R = 0.9954

2.0

0.5 0.04

0.06

0.08 Se (g/L)

0.10

0.12

0.5

1.0

1.5 2.0 HRT (d)

2.5

3.0

8

14

C

D 6

XV/Q(Si-Se ) (g/g/d)

10

V/Q(Si-Se ) (L d/g)

12

7

8 6

y = 1.1375x + 0.1267 2 R = 0.9986

4

y = 1.1348x - 5.9054 2 R = 0.5993

5 4 3

2 2 7.0

0 0

2

4

6 8 V/QSi (L d/g)

10

12

7.5

8.0

8.5

9.0 9.5 1/Se (L/g)

10.0

10.5

11.0

Fig. 3. Substrate removal model plot for nitrogen removal in the ANMR. (A) The first-order model, (B) the Grau second-order model, (C) the modified Stover-Kincannon model and (D) the Monod model.

2500

A 2000

6

y = 714.8x + 18.01 2 R = 0.8795

1/N (L d/L)

GN (mL/d)

1500

B

7

1000

5

4

y = 0.3506x + 2.3746 2 R = 0.5832

3

500

2 0 0.0

0.5

1.0

1.5 2.0 Q(Si-Se) (g/d)

2.5

3.0

3.5

0

2

4

6 8 1/NLR (L d/g)

10

12

Fig. 4. Gas production model plot for nitrogen gas production in the ANMR. (A) The Van der Meer and Heertjes model and (B) the modified Stover-Kincannon model.

5. Discussion In this study, four substrate removal models and two gas production models were applied to model the reactor process. All kinetic coefficients obtained from the models are summarized in Table 1. The data showed that the modified Stover-Kincannon

model and the Grau second-order model were more applicable to predict the nitrogen removal performance of the ANMR than the first-order model and the Monod model. The Grau second-order model and the modified Stover-Kincannon were proved appropriate for anaerobic reactors by other researchers (Buyukkamaci and Filibeli, 2002; Isik and Sponza, 2005; Yu et al., 1998). The low cor-

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Table 1 Summary of kinetic constants in different substrate removal and gas production models. Kinetic models Substrate removal

First-order Grau second-order Modified StoverKincannon Monod

Gas production

Van der Meer and Heertjes Modified StoverKincannon

Kinetic constants

Values

R2

k1 (1/d) a (1/d) b KB (kg/m3/d) Umax (kg/m3/ d) k (1/d) Ks (g/L) ksg (mL/mg N)

5.3051 0.1054 1.1101 8.98 7.89

0.7066 0.9954 0.9954 0.9986 0.9986

0.1693 0.1922 0.7148

0.5993 0.5993 0.8795

NB (g/L/d) Nmax (L/L/d)

0.1476 0.4211

0.5832 0.5832

relation coefficients of the first-order model and the Monod model indicated that these two models were not suitable for the ANMR reactor with fair degree of precision (Table 1). The Monod model was proposed as an empirical model to describe microbial reactor (Monod, 1949), and it was the most widely used model for different reactors and wastewaters (Isik and Sponza, 2005; Yu et al., 1998). However, this study found that the Monod model was not appropriate for describing the anammox process kinetics in the ANMR. The major difference between the modified Stover-Kincannon model and the Monod model is the introduction of NLR (QSi/V) to the Stover-Kincannon model. Even the kinetics of the anaerobic process are more complicated than the aerobic process, the StoverKincannon model was applicable to the kinetic analysis of the ANMR. This model is capable of predicting substrate removal at any loading conditions, no matter which order kinetics. More important, this model did not model substrate diffusion, hydraulic dynamics and other parameters, which may be important to the reactor performance but were difficult to measure at the present study. From the analysis of different substrate models, the Grau second-order model and the modified Stover-Kincannon model may be applied to predict the performance of this novel anammox reactor. The maximum total nitrogen removal rate (Umax) turned out to be 7.89 kg/m3/d in the anammox reactor from the analysis of modified Stover-Kincannon model, suggesting that the ANMR reactor in this study possessed a good nitrogen removal potential. By introducing the values of KB and Umax to Eq. (9), the effluent nitrogen concentrations can be predicted by Eq. (15). Also, from the Grau second-order model, the formula for predicting effluent nitrogen concentrations for the ANMR is given in Eq. (16).

Se ¼ S i 

7:89Si  

ð15Þ

! 1 1:1101 þ 0:1054 HRT

ð16Þ

8:98 þ

Se ¼ Si 1 

QSi V

Nitrogen gas production is an important parameter to monitor the anammox process. The special nitrogen production rate should relate to the substrate loading rate or the substrate removal rate. In the Van der Meer and Heertjes model, the nitrogen production is a function of nitrogen removal rate, while in the modified StoverKincannon model the nitrogen production rate has a direct relationship with the NLR. Model comparison was listed in Table 1. The maximum nitrogen gas production rate was estimated to be 0.4211 L/L/d by the modified Stover-Kincannon model with low R2 value of 0.5832. From the Van der Meer and Heertjes model, nitrogen gas production constant was 0.7148 mL/mg N removed with high R2 value. The Van der Meer and Heertjes model was more appropriate to predict the nitrogen gas production than the modified Stover-Kincannon model, indicating that the nitrogen gas production in the anammox process was more related to the substrate removal rate. Then, the nitrogen gas production can be predicted by Eqs. (17) and (18) by introducing kinetic constants to the Van der Meer and Heertjes model and the modified Stover-Kincannon model.

GN ¼ 0:7148Q ðSi  Se Þ 0:4211NLR N¼ 0:1476 þ NLR

ð17Þ ð18Þ

In order to test the validity of the models, experimental data were compared with predicted values calculated from models. As shown in Table 2, for nitrogen removal models, both the Grau second-order model and the modified Stover-Kincannon model were acceptable to predict the effluent TN concentration. A good linear relationship was obtained between the experimental effluent nitrogen concentrations and values predicted by models. The linear relationship between the observed nitrogen concentrations and those calculated from the modified Stover-Kincannon model (y = 0.9841x + 5.9080, R2 = 0.9271) was lower than the one obtained from the Grau second-order model (y = 0.9860x + 6.4720, R2 = 0.9445), indicating that the Grau second-oreder model was slightly more suitable for nitrogen removal kinetics in the ANMR. For nitrogen gas production, values predicted by the Van der Meer and Heertjes model were closer to the experimental measured ones than those predicted from the modified Stover-Kincannon model (Table 2). Meanwhile, a good linear relationship between the experimental nitrogen gas production and predicted gas production by the Van der Meer and Heertjes model was obtained (y = 0.6938x + 133.2, R2 = 0.9578). The linear relationship between the experimental data and predicted values by the modified Stover-Kincannon model (y = 0.0640x + 213.9, R2 = 0.8995) was found to be lower. From modeling test, the present substrate removal model could predict the nitrogen removal from the wastewater efficiently, while the biogas production model could not simulate the nitrogen gas production in the anammox reactor accurately. Other study (Kuscu and Sponza, 2009) generated the same conclusion. In practice, in order to control and predict the treatment plant performance and to optimize the plant design, the substrate removal

Table 2 Comparison of predicted and experimental data for nitrogen removal and nitrogen gas production kinetic models. HRT (d)

Effluent TN (mg/L) Experimental value

1.8 1.7 1.6 1.0 0.93 0.85

39.5 56.4 73.3 102.0 102.4 112.8

Nitrogen gas production (mL/d) Predicted value

Experimental value

Grau second-order

Modified Stover-Kincannon

41.6 73.4 70.0 109.4 103.7 120.3

39.4 74.6 69.8 109.8 100.7 119.8

478 926 672 1920 2095 2133

Predicted value Van der Meer and Heertjes

Modified Stover-Kincannon

395 746 706 1473 1431 1754

219 280 280 340 339 352

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models were more appropriate. On the other hand, more study should be carried out to develop new nitrogen gas production model or to modify the current models. 6. Conclusions We have evaluated that the Grau second-order model and the modified Stover-Kincannon model were appropriate to describe the nitrogen removal of the novel ANMR. Even though the correlation coefficient of the modified Stover-Kincannon model was higher than that of the Grau second-order model, model testing indicated that the Grau second-order model was slightly more suitable for nitrogen removal kinetics in the ANMR. The nitrogen gas production in this study was more related to the substrate removal rate than the NLR. Both the kinetic study and model validity testing concluded that the Van der Meer and Heertjes model could be used to predict the nitrogen gas production in the ANMR. Acknowledgements Shouqing Ni was financially supported by State-Sponsored Scholarship Program from the China Scholarship Council (CSC). The authors thank Jun Meng and the anonymous reviewers for their contributions to improvement of the manuscript. References Abma, W.R., Mulder, J.W., van Loosdrect, M.C.M., Strous, M., Tokutomi, T., 2005. Anammox demonstration on full scale in Rotterdam. In: The Proceedings of Third IWA Leading-Edge Conference and Exhibition on Water and Wastewater Treatment Technologies at Sapporo. Abma, W.R., Schultz, C.E., Mulder, J.W., van der Star, W.R.L., Strous, M., Tokutomi, T., van Loosdrecht, M.C.M., 2007. Full-scale granular sludge anammox process. Water Sci. Technol. 55 (8–9), 27–33. APHA, AWWA, WEF, 1998. Standard Methods for the Examinations of Water and Wastewater, 20th ed. American Public Health Association, Washington DC, USA. Buyukkamaci, N., Filibeli, A., 2002. Determination of kinetic constants of an anaerobic hybrid reactor. Process Biochem. 38, 73–79. Chamchoi, N., Nitisorvut, S., Schmidt, J.E., 2008. Inactivation of anammox communities under concurrent operation of anaerobic ammonium oxidation (anammox) and denitrification. Bioresour. Technol. 99, 3331–3336.

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