Model-based evaluation of temperature and inflow variations on a partial nitrification–ANAMMOX biofilm process

Water Research 36 (2002) 4839–4849

Model-based evaluation of temperature and inflow variations on a partial nitrification–ANAMMOX biofilm process Xiaodi Haoa,b,1, Joseph J. Heijnena, Mark C.M. Van Loosdrechta,* a

Kluyver Laboratory for Biotechnology, Department of Biochemical Engineering, Delft University of Technology, Julianalaan 67, 2628 BC, Delft, Netherlands b The Research Centre for Ecological Economics and Environmental Technology, Shanxi University of Finance and Economics, Taiyuan, Shanxi 030006, People’s Republic of China Received 20 April 2001; received in revised form 11 December 2001; accepted 17 May 2002

Abstract A mathematical model describing nitrification (nitritification plus nitratification) and anaerobic ammonium oxidation (ANAMMOX) combined in a biofilm reactor was developed. Based on this model, a previously proposed one-reactor completely autotrophic ammonium removal over nitrite (CANON) process was evaluated for its temperature dependency and behaviour under variable inflow. The temperature-dependency of growth rates of the involved organisms is described by an Arrhenius-type equation. If temperature decreases, the activities of the involved organisms decrease. This means that thicker biofilms are needed or the ammonium surface load (ASL) to the biofilm should be decreased to maintain full N-removal at lower temperatures. Although the growth rate of nitrite oxidisers is higher than that of ammonium oxidisers at lower temperatures, these organisms can be effectively competed out due to a lower oxygen affinity. Variable inflow or dissolved oxygen (DO) concentration negatively affect the N-removal efficiency due to an unbalance between applied ASL load and required oxygen concentration. A variation of the dissolved oxygen concentration in a small range (70.2 g O2/m3) has no significant influence on the process performance, which means that requirements on electrode sensitivity and a DO control scheme are not too stringent. A variable ASL has obvious influence on the process performance, at both constant and variable DO. A good adjustment of DO in accordance with the variable ASL is needed to optimise the N-removal efficiency. At T ¼ 201C, an N-removal 2 efficiency of 88% is possible at ASL=0.5 g NHþ 4  N/m d, in a biofilm of at least 0.7 mm thickness and a DO level of 3 0.3 g O2/m in the bulk liquid. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: CANON; ANAMMOX; Temperature; Ammonium surface load (ASL); Nitrogen removal

1. Introduction The combination of partial nitrification and anaerobic ammonium oxidation (ANAMMOX) has a high poten*Corresponding author. Tel.: +31-15-278-1618; fax: +3115-278-2355. E-mail address: [email protected] (M.C.M. Van Loosdrecht). 1 Present address: The R&D Centre for Sustainable Environmental Technology, Beijing University of Civil Engineering and Architecture, 1 Zhanlan Road, Beijing 100044, People’s Republic of China.

tial for more sustainable ammonium removal processes [1]. A combined process with two separate reactors in series for partial nitritification (SHARON) and ANAMMOX has been studied with sludge recycle liquor 3 (1–1.5 kg NH+ 4 -N/m ) from the WWTP RotterdamDokhaven [2]. After ammonium was aerobically oxidised for 53% to nitrite in a SHARON reactor without any biomass retention, the effluent (an ammoniumnitrite mixture) from the SHARON reactor was used as an influent for an SBR unit of ANAMMOX. All nitrite in the influent to the SBR unit was reduced into dinitrogen gas with ammonium as electron donor under anaerobic conditions. This process does not require

0043-1354/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 3 - 1 3 5 4 ( 0 2 ) 0 0 2 1 9 - 1

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X. Hao et al. / Water Research 36 (2002) 4839–4849

Nomenclature ANAMMOX ASL b CANON

anaerobic ammonium oxidation ammonium surface load 2 (g NHþ 4  N/m d) endogenous respiration rate (1/d) completely autotrophic N-removal over nitrite

organic carbon (COD), produces almost no sludge and uses only 40% of the aeration energy required for a conventional nitrogen removal process. Alternatively, a one-reactor ammonium removal process with partial nitrification and ANAMMOX combined in a biofilm reactor seems competitive with respect to the investment (engineering, construction and materials) costs [3]. This process has been entitled ‘CANON’ (completely autotrophic N-removal over nitrite, [4]). Other research groups study similar processes [5] often under different names, i.e. the OLAND process [6] and aerobic deammonification [7,8]. To evaluate potential scale up of the CANON process, a mathematical model describing simultaneous nitritification (aerobic ammonium oxidation), nitratification (aerobic nitrite oxidation) and ANAMMOX was already developed [9,10]. At a constant temperature of 301C, key kinetic, biofilm and process parameters relevant to the CANON process were evaluated by model simulations conducted in AQUASIM [11]. It was shown that kinetic parameters and biofilm characteristics such as density and porosity were, in general, insensitive towards the total effluent concentrations [9]. This is caused by the fact that the processes are essentially diffusion limited. Biofilm thickness influences the N-conversion when the biofilm depth is limited mainly because of a too small anoxic biofilm volume, limiting the space for growth of Anammox organisms. Dissolved oxygen (DO) and ammonium surface load (ASL) were identified as the two key process factors governing the behaviour of a CANON process. The performance of the CANON process depends on the competition among the involved organisms. Nitrite oxidisers should be competed out by ammonium oxidisers (competition for oxygen) and Anammox organisms (competition for nitrite). Since the autotrophic organisms involved in the CANON process have different growth rates and temperature coefficients, change of temperature can therefore result in changing the process performance. A microbiological study on ANAMMOX [12] indicated that the Anammox organisms favoured high temperatures with 40731C as the optimal temperature range. This means that a lower temperature decreases the activity of Anammox organisms. Also, the activities of ammonium and nitrite oxidisers will decrease at lower temperatures. Above

DO Eact LF m R T y

dissolved oxygen (g O2/m3) activation energy (J/mol substrate) biofilm thickness (mm/mm) growth rate of bacteria (1/d) gas constant (8.32 J/mol K) temperature (1C/K) temperature coefficient (dimensionless)

approximately 201C, ammonium oxidisers grow faster than nitrite oxidisers, whereas below this temperature the reverse is true. In simulation of temperature dependency, correctly determining temperature coefficients ðyÞ is very important. In the model, there are two types of kinetic constants: affinity (Ks ) and rate (m and b) constants. In general, affinity constants are considered to be related to the binding of a substrate to its transport protein. Hence, Ks -values resemble equilibrium constants that are relatively temperature-independent in a small temperature range [13]. Growth (m) or decay (b) values are typical temperature dependent rate constants. Since the Anammox activity was found to obey the Arrhenius law in the temperature range between 201C and 401C [12], the Arrhenius law can be used to describe the temperature dependency of Anammox organisms. Also, the Arrhenius law is suitable for temperature-dependency of ammonium and nitrite oxidisers. In reality, a constant inflow of wastewater is very rare unless a buffer tank is used. Raw wastewater from either industrial or domestic sources is often variable on both flow rate and concentrations. Variable inflow directly leads to changing ASL to the biofilm in the CANON process. The previous study revealed that an ASL was associated with an optimal DO level for maximal Nremoval efficiency [9]. Effectively, the ASL needs to be stoichiometrically related to the oxygen mass transfer to the biofilm. Practically, this is achieved by adjusting the DO level in the bulk liquid to the ASL. If DO cannot be carefully regulated when ASL changes, the N-removal efficiency in the CANON process might be reduced. From the engineering point of view, it is essential to ascertain the influence of variable ASL on the performance of the CANON process. For potential scale up of the CANON process, a study of the temperature dependency and effect of variable flow rates is essential. Experimental evaluation would take extremely long times because of the slow growth rate of anammox bacteria and the slow response of the biofilm population distribution on changes in process conditions. Each different process condition would have to be tested for at least 6–9 months to be representative. Therefore, a model-based study was initiated. In a previous study [9] we have shown that the biological parameters such as affinity constants are not very

X. Hao et al. / Water Research 36 (2002) 4839–4849

sensitive towards the model results. The (well-known) diffusion and mass transfer processes dominate the conversion rate in the biofilm process. This makes sure that the predictions will give a good qualitative insight in the CANON process. The results cannot be interpreted in a fully quantitative sense, however, the results can be used for process evaluation and directing pilot-plant research on the critical aspects of the process.

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is then 0.094, 0.061 and 0.096 1/K, respectively, in the temperature range of interest. With the determined temperature coefficients, the growth and endogenous respiration rates of nitritification, nitratification and Anammox at the standard temperature (201C) are listed in Table 1. The other kinetic parameters such as yields have been given in a previous study [9]. 2.2. Variable ammonium surface load (ASL)

2. Methodology 2.1. Temperature coefficients (y) The effect of temperature on a rate constant relative to a standard temperature (normally 293K) can be expressed by the modified Arrhenius equation: rT ¼ r293 exp½Eact ð293  TÞ=R293T:

ð1Þ

For small temperature differences, the product of ‘293T’ (absolute temperature) does not change much, and so this relation can be simplified to an expression relative to a relative standard temperature: rT ¼ r293 exp½yð293  TÞ;

Eact ; R293T

2.3. Simulation approach

ð2Þ

where rT is the reaction rate at temperature T; r293 the reaction rate at the standard temperature (2931K), Eact the activation energy (J/mol substrate), R the gas constant (8.32 J/mol K), T the temperature (K), and y the temperature coefficient (1/K). Comparing Eq. (2) with Eq. (1), the expression for y can be obtained as y¼

A constant ammonium surface load of 2 g NHþ 4  N/ m d was applied for the scenario study of temperaturedependency, which is close to the applied ASL in pilotplant tests reported by Hippen et al. [8], Siegrist et al. [5] and Helmer et al. [7]. Around the constant ASL, a dynamic ASL with both variable flow rate and ammonium concentration was designed for the scenario study of variable diurnal ASL [ASLv in Fig. 1(a)]. For 2 the dynamic ASL, the mean ASL of 2 g NHþ 4  N/m d [ASL2 in Fig. 1(a)] remained. 2

ð3Þ

when Eact is known, and y can be thus calculated. The activation energies of nitritification, nitratification and Anammox are given as 68, 44 and 70 kJ/mol substrate, respectively [14]. The corresponding y-values

The simulation model was introduced previously [9]. It contains the stoichiometry and kinetic equations for ammonium oxidisers, nitrite oxidisers and anammox bacteria. The kinetic equations were expanded by including a temperature dependency on the growth rates. The model was integrated in the biofilm compartment of AQUASIM [11]. Anammox bacteria were found to be active at temperatures between >101C and 431C (optimum 40731C, [12]), and so simulations were conducted in the same temperature range for this scenario study of temperature-dependency. Since the Anammox activity is less than predicted by the Arrhenius law at 101C [12], the model was only used in a temperature range of 15–401C.

Table 1 Growth rates and endogenous respiration rates and their temperature dependency for nitritification, nitratification and ANAMMOX at 201C Symbol

Definition

Value (1/d)

Temperature coefficient (1/K)

Aerobic ammonium oxidisers (XNH ) mmax Maximal growth rate of XNH NH BNH Aerobic endogenous respiration rate

0.80 0.050

0.094 0.094

Aerobic nitrite oxidisers (XNO ) mmax Maximal growth rate of XNO NO bNO Aerobic endogenous respiration rate

0.79 0.033

0.061 0.061

Anaerobic ammonium oxidisers ANAMMOX (XAN ) mmax Maximal growth rate of XAN AN bAN Aerobic endogenous respiration rate

0.028 0.001

0.096 0.096

X. Hao et al. / Water Research 36 (2002) 4839–4849

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2.5

2 1.6

2 1.2

1.5 1

0.8

DO in bulk (g O2/m3)

3 ASL (g N/m2.d)

2.4

2.4 ASL2 ASLv DOp

DO in bulk (g O2/m3)

4 3.5

DO1.3 DOs DOp DOc

2 1.6 1.2 0.8

0.5 0 (a)

0.4

0.4 0

4

8

12

16

20

0

24

Time (hours)

(b)

4

8

12

16

20

24

Time (hours)

Fig. 1. Applied variations in the diurnal ammonium surface load (ASL) and dissolved oxygen (DO) concentrations in the simulations. Exact description of each line can be found in the text: ASL2: constant loading rate; ASLv: variable laoding rate; DO1.3: constant dissolved oxygen concentration; and DOp,c,s: variable dissolved oxygen concentration.

The previous study [9] indicated that there was a minimal biofilm thickness at a defined ammonium surface load and a defined temperature. To find out a relation between temperature and minimal biofilm thickness (LF), simulations with different biofilm thickness were run at different temperatures and a constant 2 ASL of 2 gNHþ 4  N/m d. The performance of the CANON process at a temperature of 201C was evaluated at a defined (standard) biofilm thickness of 0.7 mm, by changing 2 the ammonium surface load (0.25–4 g NHþ 4  N/m d). In practice, a variable ammonium surface load causes a change of DO in the reactor. In the case of the variable ASL shown in Fig. 1(a), even a well-controlled DO tends to have a changing profile (DOp) in reality, which is close to an upper and lower boundary in the DO controller. A changing DO profile might have influence on the performance of the CANON process. For this reason, the changing DO profile (DOp) and an additional sine-type DO profile [DOs in Fig. 1(b)] were used for simulating the DO influence on the N-removal efficiency. The sinus DO function was used to represent fluctuations around a set-point. Carefully regulating DO towards matching the variation in ASL could make the N-removal efficiency optimal. A strictly controlled DO profile (DOc) is shown in Fig. 1(b), making the oxygen flux to the biofilm in accordance to the ammonium flux. Variable ASL and DO can be simulated with two approaches. The first approach uses variable ASL and DO inputs (a long-term dynamics) from the beginning to the end of a simulation until a steady state is reached. This requires a long calculating time. Alternatively, the simulation can be first run with the constant (average) ASL and DO to reach a steady state, after which the variable ASL and DO inputs are used for simulation of a 24-h period (a short-term dynamics). In this way, a

quick calculation could be achieved. Details of simulations refer to the previous study [9].

3. Results and discusion Simulations with the long- and short-term dynamics indicated that no significant difference existed in the final results as soon as a steady state was reached. The biomass population in the biofilm was not different under the long-term dynamics. This is probably caused by the fact that the fluctuating conditions occurred on a much smaller time scale than that for biofilm growth. As under the optimal condition with constant ASL and DO (the means), nitrite oxidisers are still effectively competed out. Therefore, the second approach of simulation (i.e. a long-term constant calculation followed by a short-term dynamic calculation) was used for simulating dynamic ASL and DO. Koch et al. [10] simulated a lab-scale batch experiment of ANAMMOX at a moderate temperature (201C) with a similar model. They experimentally evaluated the Anammox activity on ammonium and nitrite in different batch reactors using biomass from a nitrifying rotating biological contactor [5]. Koch et al. [10] used in their simulation model a high maximal growth rate for the Anammox bacteria (0.08 1/d). Their model did not successfully fit the measured data from the batch experiment if the experimentally evaluated fraction of Anammox bacteria (normally 20–25% of the total particulate COD predicted from their on-site observation) was used. An Anammox fraction of only 5% of the total particulate COD was able to fit the measured data. If, however, the experimentally determined maximal growth rate of 0.028 1/d (201C) is used [12] the model can properly describe all the measured data and the fraction of anammox bacteria in the biofilm.

X. Hao et al. / Water Research 36 (2002) 4839–4849

3.1. Temperature dependency

this does not directly favour Anammox activity. Fig. 2(c) reveals that the biomass amount of Anammox organisms was not enough to achieve optimal Nremoval even though Anammox organisms developed over the whole biofilm depth [Fig. 2(c)]. Under this condition, a thicker biofilm (or lower loading rate and DO) would be needed for an optimal Anammox activity. At 301C, the activity Anammox bacteria increases. At the optimal DO for 201C (0.6 g O2/m3), an N-removal efficiency of 47% is obtained at 301C [Fig. 2(b)]. This is not the maximal efficiency for 301C, but is higher than the N-removal efficiency (35%) obtained at 201C. Fig. 2(d) indicates that at 301C Anammox organisms has not developed over the whole biofilm depth at DO=0.6 g O2/m3. This means that due to the increased conversion rate the biomass amount of Anammox organisms was not limiting the conversion. At the optimal DO (0.8 g O2/m3) for 301C, the biofilm thickness (0.7 mm) was just enough to contain all required Anammox organisms, and so the maximal N-removal efficiency (74%) was achieved. For comparison, the concentrations of the different nitrogen forms at different temperatures as function of

T=20˚C

Effluent N in bulk (g N/m3)

Effluent N in bulk (g N/m3)

The simulation results for the conversions at 201C and 2 301C (ASL=2 g NHþ 4  N/m d; LF=0.7 mm) are shown in Fig. 2. At first glance, the performance of the CANON process is heavily limited at the lower temperature [Fig. 2(a)]. Compared to the good process performance (74% for the maximal N-removal efficiency) at 301C [Fig. 2(b)], the maximal N-removal efficiency (only 35%) at 201C decreases by about 40% [Fig. 2(a)]. At 201C, the maximal N-removal efficiency occurs at DO=0.6 g O2/m3, accompanied by an ammonium removal efficiency of 40%. Although accumulation of nitrite gradually increases and a part of ammonium remains above DO=0.6 g O2/m3, the formation of dinitrogen gas tends to decrease. Obviously, this is directly related to a lower Anammox activity at the lower temperature. At DO=1.6 g O2/m3, a peak of nitrite up to 47 g N/m3 occurs due to the lower Anammox activity and a limited growth of nitrite oxidisers below DO=1.6 g O2/m3 [Fig. 2(c)]. The simulations at 201C illustrate that at a lower temperature accumulation of nitrite in the biofilm can occur but

80 NH4 NO2 NO3 N2 Tot. N

60

40

20

0 1.5

1

2

2.5

3

DO conc. in bulk (g O2 /m3)

60

40

20

14000

T=20˚C

DO=0.6 g O2 /m3

12000 X_AM X_NH

10000

X_ NO

8000

X_I

6000 4000 2000 0 0

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200

300

400

500

Biofilm thickness (µm)

600

0

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700

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DO=0.6 g O2 /m3

12000 10000 8000 6000 4000 2000 0 0

(d)

1

DO conc. in bulk (g O2 /m3)

(b) Biomass concentration (g COD/m3)

0.5

(a) Biomass concentration (g COD/m3)

T=30˚C

80

0 0

(c)

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100

200

300

400

500

600 700

Biofilm thickness (µm)

Fig. 2. Simulation results for the CANON process operated under constant conditions: (a) 201C, (b) 301C, (c–d) effect of applied oxygen content on population distribution of the biofilm.

X. Hao et al. / Water Research 36 (2002) 4839–4849

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strongly depending on the biofilm thickness, ASL, growth rates and biofilm density. The optimal DO at which the maximal N-removal efficiency occurs as function of temperature is depicted in Fig. 4, at a constant biofilm depth (0.7 mm) and a constant ASL (2 g 2 NHþ 4  N/m d). It shows that the optimal DO increases with increasing temperature. 3.2. Influence of biofilm thickness Simulations with different biofilm thickness (0.2– 4 mm) were run at variable temperature and constant 2 ASL (2 g NHþ 4  N/m d). With increasing biofilm thickness, a maximal production of dinitrogen gas (i.e. the maximal activity of Anammox bacteria) occurred at an increasing DO level in the bulk liquid. The relations among temperature, biofilm thickness, maximal Nremoval efficiency and optimal DO are shown in Fig. 5. As shown in Figs. 5(a–e), the N-removal efficiency increases with biofilm thickness as long as the biofilm depth is limiting. This reflects that with increasing biofilm thickness the space for ANAMMOX increases.

80

NH2− in effluent (g N/m3)

NH4+ in effluent (g N/m3)

DO in the bulk liquid are depicted in Fig. 3, at a constant biofilm depth (LF=0.7 mm) and a constant 2 ASL (2 g NHþ 4  N/m  d). Ammonium removal increases with increasing temperature [Fig. 3(a)]. Simultaneously, also the conversion of nitrite to nitrate increases with temperature [Fig. 3(c)]. Due to the fact that with decreasing temperature the oxygen penetration depth increases, the space for Anammox organisms reduces. Moreover, the growth rate of Anammox organisms decreases with lowering temperature. Consequently, less dinitrogen gas is produced [Fig. 3(d)] and more nitrite accumulates [Fig. 3(b)] at lower temperatures. The general effect observed is that the optimal DO level for the maximal N-removal efficiency (or Anammox activity) is at the point where ammonium is almost maximally oxidised. In a temperature range of 15–201C, the conversion rate of nitrite oxidisers is faster than that of ammonium oxidisers. The generally observed lower affinity of nitrite oxidisers for oxygen [15–19] made that the ammonium oxidisers still effectively outcompeted the nitrite oxidisers on oxygen. The effect of DO on the observed N-concentrations is

NH4in T=40 C

60

T=35 C T=30 C T=25 C

40

T=20 C T=15 C

20

0

(a)

0.5

1

1.5

2

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20

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0

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DO level in bulk (g O2 /m3) 80

1

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NH3− in effluent (g N/m3)

60

0 0

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60 .5 40

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DO level in bulk (g O2 /m3)

3

0

(d)

0.5

1

1.5

2

2.5

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DO level in bulk (g O2 /m3)

Fig. 3. Influence of temperature (different line types) and dissolved oxygen in the CANON process on (a) ammonium effluent, (b) nitrite effluent, (c) nitrate effluent and (d) dinitrogen gas, expressed as if it is in the effluent. Biofilm thickness: 0.7 mm; ASL: 2 g 2 3 NHþ 4  N/m d;and influent ammonium: 80 g/m .

X. Hao et al. / Water Research 36 (2002) 4839–4849

3.3. Influence of ammonium surface load

1 90 0.8 70 0.6 50

0.4 DO

0.2

N removal (%)

Optimal DO in bulk (g O2/m3)

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30

N removal NH4 removal

0

10 10

20

30

40

Temperature (˚C) Fig. 4. Predicted N-removal and optimal DO in the CANON process as a function of the temperature. Biofilm thickness: 2 0.7 mm; ASL: 2 g NHþ 4  N/m d.

Above a certain optimal biofilm depth, the N-removal efficiency is not further improved. This reveals that there should be an optimal biofilm thickness to achieve the maximal N-removal efficiency for each temperature. This is because a thicker biofilm above the optimal depth cannot contribute to improved N-removal, instead a higher DO level is required. In other words, a larger biofilm thickness generally contains a higher inert fraction and thereby a decreased volumetric activity of the biofilm, which has requires a higher DO to maintain the same N-removal efficiency. For this reason, a proper point on each plateau of the N-removal efficiency in Figs. 5(a)–(e) can be seen as the critical point to which an optimal biofilm depth corresponds. The relations between temperature, optimal biofilm depth (LF), maximal N-removal efficiency and optimal DO are shown in Fig. 5(f). From this figure, it can be concluded that temperature strongly affect the optimal biofilm depth to achieve a maximal N-removal efficiency. A lower temperature requires a thicker biofilm associated with a higher DO level in the bulk liquid. The exact thickness in reality required will also be related to the biofilm density. Here a biofilm density of 50 kg COD/m3 was used as a typical value for autotrophic biofilm. This density will depend on the choice of biofilm reactor, however, proper correlation between type of reactor and biofilm density are not available. This requirement of a larger biofilm thickness at lower temperatures might make the use of granular sludge less feasible. Granules of up to 3–4 mm radius are needed. With such large granules, the specific surface area (300–400 m2/m3) of the biofilm becomes similar as for moving bed or fixed bed reactors.

As already concluded above, a thin biofilm has a limited capacity for the activity of Anammox bacteria. This is, however, associated to the ASL. Fig. 5(a) reveals that at 201C a biofilm up to 2 mm thick is optimal at 2 ASL=2.0 g NHþ 4  N/m d with a maximal N-removal efficiency of 74%, associated with an optimal DO=2.5 g O2/m3. A thicker biofilm does not further improve the N-removal efficiency but needs a higher DO level. This reflects that the capacity of Anammox activity was limited at this lower temperature. In other words, 2 ASL=2.0 g NHþ 4  N/m d was too high to achieve a better N-removal efficiency at this temperature. If a higher N-removal efficiency is required at a lower temperature, lowering ASL might be an alternative. Several simulations with different ASL (0.25–4 g 2 NHþ 4  N/m d) were run at a constant temperature (201C) and a fixed biofilm depth (FL=0.7 mm). The maximal N-removal efficiency and associated DO at each ASL are shown in Fig. 6. Clearly, lowering ASL largely benefits N-removal for 2 201C. At ASL=0.5 g NHþ 4  N/m d, the N-removal reaches to 88% (94% for the ammonium removal). Further, the N-removal of 92% (98% for the ammonium removal) can be achieved at ASL=0.25 g 2 NHþ 4  N/m d. As shown in Fig. 6, lower ASL is coupled with lower DO. It is thus concluded that at a lower temperature a lower ASL and a lower DO can give an optimal N-removal. However, such low DO might be difficult to be properly controlled in large-scale reactors. 3.4. Influence of variable ASL in relation to DO variation Fig. 5(c) shows that a biofilm of 1 mm thick is optimal to achieve the maximal N-removal of 82% (94% for the 2 ammonium removal) at ASL=2 g NHþ 4  N/m d and 301C, which is associated with the optimal DO level of 1.3 g O2/m3. Therefore, this biofilm was used here for the simulations of the effect of variation of operational parameters. A previous study [9] and the present simulations have shown that the DO range for the maximal N-removal efficiency is narrow; a DO deviation of 70.2 g O2/m3 from the optimal level can easily reduce the N-removal efficiency by about 5–15%. This seems to require sensitive DO electrode for an exact control of DO level in practice. To ascertain what actually happens for the process performance if DO cannot be accurately measured by the sensors of electrode, a varying DO input [as a sine-function, DOs in Fig. 1(b)] was applied 2 at a constant ASL of 2 g NHþ 4  N/m d, with a maximal 3 DO deviation of 0.2 g O2/m from the optimal level of 1.3 g O2/m3 [Fig. 7(a)]. Fig. 7(a) shows that peaks of nitrite occur at the highest DO level and a peak of ammonium appears at the lowest DO level. On the

X. Hao et al. / Water Research 36 (2002) 4839–4849 5

T=20 ˚C

3

70

2 50 1

N removal

N removal (%)

90 4

DO in bulk (g O2/m3)

90

N removal (%)

3

T=25 ˚C

2.5 2

70 1.5 1

50

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NH4 removal

DO in bulk (g O2/m3)

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DO level

30 0.5

1.5

2.5

3.5

0 4.5

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Biofilm thickness (mm)

(a)

0.8

1.2

1.6

0 2.4

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T=35 ˚C

T=30 ˚C

90

1 50 0.5

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N removal (%)

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T=40 ˚C

2.6

0.6 50 0.3

30 0

(e)

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1 LF DO N removal NH4 removal

0.6 0.2

0 1.2

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90 2.2

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(f)

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N removal (%)

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Optimal LF (mm) DO in bulk (g O2 /m3)

N removal (%)

1.2

DO in bulk (g O2 /m3)

90

60

40

50 50

Temperature (˚C)

Fig. 5. Relations between temperature, biofilm thickness, N-removal efficiency and DO in the CANON process. (a-e) varying 2 temperature; and (f) optimal conditions for each temperature. Biofilm thickness: 0.7 mm; ASL: 2 g NHþ 4  N/m d.

average, the sine-type DO input decreased the Nremoval efficiency by only 2% (1% for the ammonium removal), as listed in Table 2. The simulations with a variable ammonium surface load [Fig. 1(a)] were run with three different DO inputs. A constant DO level of 1.3 g O2/m3 was too high for low ASL (before 8:00), resulting in high ammonium oxida-

tion so that a peak of nitrite is formed in Fig. 7(b). Between 8:00 and 16:00, a peak of ASL [Fig. 1(a)] required a DO level higher than 1.3 g O2/m3 to oxidise more ammonium to nitrite. A limiting DO actually made ammonium left over, resulting in a peak of ammonium. Clearly, nitrite was a limiting substrate for ANAMMOX in that period. This case led to a mean

X. Hao et al. / Water Research 36 (2002) 4839–4849

decrease of 10% on the N-removal efficiency, demonstrating the obvious influence of variable ASL on the process performance. 100

0.8

N removal (%)

80

0.6

60 0.4 40 0.2

20

0

DO in bulk (g O2/m3)

N removal NH4 removal DO level

0 0

2

1

4

3

5

ASL (g N/m2 .d) Fig. 6. Influence of ASL on the N-removal efficiency and the optimal DO level in the CANON process at 201C; and biofilm thickness 0.7 mm.

In reality, DO in the bulk liquid generally changes along with variable ASL. During a high ASL, the DO will be at the lower limit while at a low ASL the DO will be at the high limit. The DO control is generally applied within a certain window (a minimum and maximum DO level). To approach such a practical DO variation, a variable DO input [DOp in Fig. 1(b)] was applied together with the variable ASL [Fig. 1(a)]. Two larger peaks of nitrite and ammonium appear in Fig. 7(c), and the process performance is significantly influenced. An average decrease of 14% on the N-removal efficiency occurred in this case (Table 2). An even higher decrease on the N-removal efficiency for a real DO change caused by variable ASL could be expected. If in practice it is feasible to control the DO exactly on the requirement of the momentary N-load, this might lead to a relative high N-removal efficiency along with variable ASL. A controlled DO input, which matches ASL in a practical way [DOc in Fig. 1(b)], was applied for this purpose. The profile of the N-conversion shown in Fig. 7(d) indicates that the controlled DO input is indeed of benefit for increasing the N-removal efficiency. With a simple three-phase regulation of DO as shown in 80

NH4 NO3

ASL2 + DOs

NO2 N2

60

40

20

N conc. in effluent (g N/m3)

N conc. in effluent (g N/m3)

80

0 0

4

8

12

16

20

60

40

20

0

4

(b)

8

12

16

20

24

Time (hours) 80

80

ASL v + DOc

ASL v + DOp

N conc. in effluent (g N/m3)

N conc. in effluent (g N/m3)

ASLV + DO1.3

0

24

Time (hours)

(a)

60

40

20

0

60

40

20

0 0

(c)

4847

4

8

12

16

Time (hours)

20

0

24 (d)

4

8

12

16

20

24

Time (hours)

Fig. 7. Influence of variable ASL and DO on the total effluent concentrations: (a) constant ASL, variable DO; (b) variable ASL, 2 constant DO; (c-d) variable DO and ASL; the variations are given in Fig. 1. Biofilm thickness 1 mm; average ASL 2 gNHþ 4  N/m d; indicates the maximal production of dinitrogen gas achievable at the constant ASL and DO.

X. Hao et al. / Water Research 36 (2002) 4839–4849

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Table 2 Mean total nitrogen removal under constant and variable ammonium surface load (ASL) and different dissolved oxygen concentrations (DO). The applied variations are given in the referenced figures Mean removal efficiency

NHþ 4 (%) Total N (%) a

2 Constant ASL2 (2 g NHþ 4  N/m  d)

Variable ASLv [Fig. 1(a)]

DO1.3a ( )

DOs [Fig. 7(a)]

DO1.3 [Fig. 7(b)]

DOp [Fig. 7(c)]

DOc [Fig. 7(d)]

94 82

93 80

90 72

87 68

91 78

DO=1.3 g O2/m3.

Fig. 1(b), the N-removal efficiency of 78% approaches to the maximum (82%) which is achievable at the constant ASL and DO (Table 2). Clearly, careful regulation of DO is needed to optimally operate the CANON process at variable ASL. The simulations clearly show that a good DO control, adjusting the DO to the ASL is essential. This means that good DO electrodes and a proper maintenance will be required. Small fluctuations from the optimal DO setpoint are not critical, mainly because the population dynamics are relatively slow. However, a deviation from the average required set-point will only be detected after some time, because of the slow response in the biofilm population. This difference in time constants in the system makes the design of a good control scheme not easy. A two-step system [2] where in one reactor ammonium is partially converted to nitrite and in a second reactor the Anammox process takes places does not require such high demands on the process control. However, this system needs two reactors to be constructed, i.e. higher investment costs. A final choice of which process option is most optimal will depend probably heavily on local conditions and constraints (already existing equipment, type of operators on the plant, etc.).

4. Conclusions A mathematical model was used to evaluate the influence of variable temperature and ammonium surface load (ASL) on a full autotrophic N-removal process in an aerated biofilm reactor. Although the results cannot be interpreted as fully quantitatively correct, they give a good qualitative description of this novel treatment concept. The simulations indicate that at a constant ASL lower temperatures strongly negatively affect the N-removal efficiency of the CANON process due to a limited activity of Anammox bacteria at lower temperatures. At a constant temperature and a defined ASL, there is always an optimal biofilm depth to achieve the maximal N-removal efficiency. At a defined ASL, a lower

temperature needs a thicker biofilm and a higher DO to maintain the N-removal efficiency at a high level. At a defined biofilm thickness, alternatively, a lower temperature needs a lower ASL and a lower DO for a better N-removal efficiency. Variable ammonium loading rates and dissolved oxygen concentrations negatively affect the N-removal efficiency of the CANON process. If the oxygen variations are small (e.g. 0.2 mg O2/l) around the optimal set-point there is no significant influence on the process performance. A diurnal variable ammonium load has a more negative influence on the process performance, at both a constant and a variable DO level profile caused by the variable ASL. An unbalance between applied and required DO is responsible for the decreased N-removal efficiency. For practical applications, careful control of dissolved oxygen along with a variable ammonium load is absolutely needed to obtain a good N-removal efficiency.

Acknowledgements This study is within the framework of a joint research between Dutch and Chinese universities, which is financed by the Royal Dutch Academy of Sciences (KNAW: 01cdp003). Sincere thanks are due to Prof. Dr. J.-Z. Sun and Prof. M.-L. Wang in SUFE for their promotion of the project in China.

References [1] Jettten MSM, Horn SJ, van Loosdrecht MCM. Towards a more sustainable municipal wastewater treatment system. Water Sci Technol 1997;35(9):171–80. [2] van Dongen U, Jetten MSM, van Loosdrecht MCM. The SHARONs-ANAMMOXs process for treatment of ammonium rich wastewater. Water Sci Technol 2001;44: 153–60. [3] van Loosdrecht MCM, Jetten MSM. Method for treating ammonia-comprising wastewater. PCT/NL97/00482, 1997.

X. Hao et al. / Water Research 36 (2002) 4839–4849 [4] Strous M. Anammox, nitrification. In: Microbiology of anaerobic ammonium oxidation. Ph.D. thesis, The Netherlands, 2000. 2000. p. 63–81. ISBN 90-9013621-5. [5] Siegrist H, Reithaar S, Koch G, Lais P. Nitrogen loss in a nitrifying rotating contactor treating ammonium-rich wastewater without organic carbon. Water Sci Technol 1998;38(8–9):241–8. [6] Kual LP, Verstraete W. Ammonium removal by the oxygen-limited autotrophic nitrification-denitrification system. Appl Environ Microbiol 1998;64:4500–6. [7] Helmer C, Kunst S, Juretschko S, Schmid MC, Schleifer K-H, Wagner M. Nitrogen loss in a nitrifying biofilm system. Water Sci Technol 1999;39(7):13–21. [8] Hippen A, Rosenwinkel K-H, Baumgarten G, Seyfried CF. Aerobic deammonification: a new experience in the treatment of wastewaters. Water Sci Technol 1997;35(10): 111–20. [9] Hao X, Heijnen JJ, van Loosdrecht MCM. Sensitivity analysis of a biofilm model describing a one-stage completely autotrophic nitrogen removal (CANON) process. Biotechnol Bioeng 2002;77(3):266–77. [10] Koch G, Egli K, van der Meer JR, Siegrist H. Mathematical modelling of autotrophic denitrification in a nitrifying biofilm of a rotating biological contactor. Water Sci Technol 2000;41(4–5):191–8. [11] Reichert P. AQUASIM 2.0—computer program for the identification and simulation of aquatic systems. Dubendorf, . Switzerland: EAWAG, 1998, ISBN 3 906484-16-5.

4849

[12] Strous M, Kunen JG, Jetten MSM. Key physiology of anaerobic ammonium oxidation. Appl Environ Microbiol 1999;65:3248–50. [13] Heijnen JJ. Bioengineering of microbial growth. In: Flickinger MC, Drew SW, editors. Encyclopedia of bioprocess technology: fermentation, biocatalysis, and bioseparation. New York: Wiley, 1999. p. 267–91 ISBN 0-471-13822-3. [14] Jetten MSM, Strous M, van de Pas-Schoonen KT, Schalk J, van Dongen LGJM, van de Graaf AA, Logemann S, Muyzer G, van Loosdrecht MCM, Kunen JG. The anaerobic oxidation of ammonium. FEMS Microbiol Rev 1999;22:421–37. [15] Jayamohan S, Ohgaki S, Hanaki K. Effect of DO on kinetics of nitrification. Water Supply (Sweden) 1988;6:141–50. [16] Laanbroek HJ, Gerards S. Competition for limiting amounts of oxygen between Nitrosomonas europaea and Nitrobacter winogradsky grown in mixed continuous cultures. Arch Microbiol 1993;159:453–9. [17] Sheintuch M, Tartakovsky B, Narkis N, Rebhun M. Substrate inhibition and multiple states in a continuous nitrification process. Water Res 1995;29(3):953–63. [18] Siegrist H, Gujer W. Demonstration of mass transfer and pH effects in a nitrifying biofilm. Water Res 1987;21: 1481–7. [19] Stevens DK, Berthouex PM, Chapman TW. Dynamic model of nitrification in a fluidised bed. J Environ Eng 1989;115(5):910–29.