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The lab-scale up-flow anaerobic biofilm (UABF) reactor was successfully operated for the treatment of synthetic wastewater with high nitrogen load by anammox (anaerobic ammonium oxidation) process. During the entire period of operation (the nitrifying biomass period in this reactor was up 120 days) at 35 ±1 °C, the reactor was tested under different hydraulic retention times (HRTs) and substrate concentrations. The operational strategy consisted of both increasing the ammonium and nitrite concentrations from 60 to 700 mg N/L and from 80 to 920 mg N/L, and decreasing the hydraulic retention time from 24 to 6 h, at each step. Respectively, the highest removal efficiency of ammonium and nitrite achieved was 91 and 93%. Consequently, due to their acceptable performance for nitrogen removal in previous researches, the modified Stover-Kincannon and the Grau second-order models were selected from the different kinetic models for the evaluation of the performance process in the UABF reactor. According to the experiment results, the model validity testing showed that the Stover-Kincannon model was a little more appropriate for the description of the nitrogen removal in the UABF reactor, even though both models gave high correlation coefficients (R2 = 0.999).
Keywords: Anammox, Nitrogen removal, Kinetics, Stover-Kincannon, Grau, model
Due to lower operation cost and energy consumption, the use biological treatment processes are recommended in order to treat the effluents with high-concentrations of ammonium before discharging them into the water resources. There are a number of biological treatment processes used for ammonium removal that include: shortcut nitrification-denitrification ammonium removal over nitrite (SHARON), oxygen-limited autotrophic nitrification-denitrification (OLAND), complete autotrophic nitrogen removal over night (CANON), and anaerobic ammonium oxidation (ANAMMOX) (Paredes et al., 2007; Yang and Zhou, 2008). Among the above mentioned processes, Anammox is an initiative process that was discovered in the 1990s and it has been known to show noticeable results regarding the removal of ammonium and nitrogenous compounds from wastewater (Dongen and Tollner, 2003). In this process, ammonia in the presence of nitrite as the electron acceptor oxidizes to nitrogen gas under anoxic conditions (Mulder et al., 1995).
Anammox is an autotrophic process that consumes less than 40% oxygen without requiring an organic carbon source for denitrification. It also oxidizes 55-60% of ammonium to nitrite and the remaining of ammonium will be oxidized with nitrite at a reasonable time during this process (Siegrist et al., 1998).
The application of anammox process for ammonia removal has been developed for wastewater treatment of many different reactors, such as landfill leachate in a continuous reactor (Liang and Liu, 2008), nitrous organic wastewater in ASBR reactors (Jin-ping et al., 2006), anaerobic digester supernatant in SBR reactor (Vazquez-padin et al, 2009), supernatant with high concentrations of ammonium in MBBR (Szatkowska et al., 2007), and etc. But few studies have been done to evaluate and determine the substrate removal kinetics of the anammox process (Jin and Zheng, 2009). Only two studies have evaluated the different kinetics models by describing the nitrogen removal by the anammox process, which were carried out in an anammox non-woven membrane reactor (Ni et al., 2010) and in an anammox up-flow filter (Jin and Zheng, 2009).
Process modeling can be applied to control and evaluate the function of the treatment plant operation, while optimizing the plant design and up scaling the pilot plant investigations (Izi and Sponza, 2005). Although different kinetic models were used to study the substrate removal in anaerobic biological process, the two above mentioned models seem to be the best for describing nitrogen removal (Ni et al., 2010; Jin and Zheng, 2009).
The purpose of this research was to investigate the feasibility of the of the anammox process application in the up-flow anaerobic biofilm reactor for the treatment of synthetic wastewater with high concentrations of ammonium and nitrite. Furthermore, another purpose of this study was to determine the kinetics of the anammox process by the modified Stover-Kincannon and Grau second order models for describing the nitrogen removal in UABF reactor.
2. Kinetic approach
2.1. The modified Stover-Kincannon model
The initial formula Stover-Kincannon model for the rotating biofilm reactor (RBC) is: (Stover and Kincannon, 1982)
Where A expresses the total disc surface area on which there is immobilized biomass concentration. Umax represents the maximum removal rate of substrate (g/L d) and KB is the constant of saturation value (g/L d). In this model, the suspended biomass concentration is compared with the attached biomass that is supposed to be negligible. If instead of the disc surface area (A) we insert the reactor working volume (V), the original Stover-Kincannon model will be modified as follows: (Yu et al., 1998)
The Stover-Kincannon model, at steady state, can be shown as in Eq. (3):
Where dS/dt is the removal rate of substrate (mg/L d), Q is the flow-rate (L/d), V is the reactor effective volume (L), and S0 is the influent substrate concentration (mg/L) and Se is the effluent substrate concentration (mg/L).
In equation (2) the shape of linear equation can be illustrated as the following:
Respectively, Umax and KB can be calculated via intercept and slope of the line.
The substrate balance for the reactor can be expressed as the following:
Replacing equation (3) in above equation gives:
So, this equation via substitution of the kinetic constants Umax and KB can be used for any concentration of effluent substrate according to:
Or gain volume required reactor:
2.2. Second-order substrate removal model
The general equation of a second-order kinetic model applied by Optaken (1982), Grau et al. (1975) is shown in Eq. (9):
The above equation can be linearized to obtain:
If the second term of the right part of Eq. (10) is admitted as a constant, Eq. (10) will be obtained:
Expresses the substrate removal efficiency and is symbolized as E. therefore, the last equation can be:
3. Materials and methods
3.1. UABF reactor operation
A lab-scale up-flow anaerobic biofilm (UABF) reactor in continuous mode was used for nitrogen removal of synthetic wastewater by the Anammox process (Fig. 1). The bioreactor was inoculated with 400 ml of granule sludge (Provided from an UASB plant). The culture of the anammox sludge was prepared by decreasing the COD/N ratio in the influent step, where it lasted approximately 120 days (Mosquera-Corral et al., 2005). The lab-scale bioreactor with the effective volume of 1.8 L consisted of a double wall plexiglass cylindrical column (25 cm high), which was comprised of: an inner cylinder (Internal diameter of 11 cm and outer diameter 12 cm) and outer cylinder as water jacket (Internal diameter of 14 cm and outer diameter of 15 cm). Temperature was kept at 35 ± 1 °C by a set including a recycling pump used for pumping the hot water from the water tank to the outer cylinder (according to Fig. 1), and a thermostat was installed on the water tank. The body of the reactor was covered with a dark cover to prevent light penetration and algal growth. The Plastic media was filled to 50% of the total reactor volume which was bee-cell 2000. This type of media was used as a biofilm support material due to its large surface area (650 m2/ m3) and cost-effective when compared with other packing media, and high porosity (pore volumes up to 87%).
Initially, the bioreactor was operated in a continuous mode with the synthetic wastewater flow rate of 1.8 L/d. The pH was kept at a range of 7.5-8.0 by adding sodium bicarbonate. The hydraulic retention time was adjusted to 24 h. Respectively, The initial ammonium and nitrite concentrations were 60 and 80 mg N/L. The operational trend consisted of increasing the concentration of influent nitrogen and decreasing the HRT stepwise.
3.2. Synthetic wastewater
The composition of the synthetic wastewater that was used for lab-scale up-flow bioreactor was (g/L) (Van de Graf et al., 1996): NaHCO3, 1.25; KH2PO4, 0.027; CaCl2.2H2O, 0.3; MgSO4.7H2O, 0.3. The Ammonium and nitrite in the form of (NH4)2SO4 and NaNO2 were used during certain ranges as the main influent substrates. 1 ml/l of any trace element solution was added to the above composition. Concentration of the trace elements in each of the solution was as following: (g/L)
Trace element NO. 1: EDTA 5.0, FeSO4 5.0.
Trace element NO. 2: EDTA 15.0, ZnSO4.7H2O 0.43, CoCl2 .6H2O 0.24, MnCl2 .4H2O 0.99, CuSO4 5H2O 0.25, NaMoO4.2H2O 0.22, NiCl2.2H2O 0.19, Na.SeO4 .10H2O 0.21, H3BO4 0.014, and NaWO4.2H2O 0.050.
The ammonium, nitrite and nitrate concentrations were measured according to standard methods, (APHA, 2005). The Ammonium and nitrite were analyzed by the colorimetric method and nitrate was measured by the spectrophotometric method. Respectively, the pH and dissolved oxygen (DO) were measured by a portable pH meter (Metrohm, model 826) and a portable DO meter (Eutech, model 1500). The biomass of the ammonium oxidizing culture was measured as expounded in standard methods (2005). The soluble COD was measured calorimetrically by closed reflux methods. Each examination was run in triplicate.
4. Result and discussion
4.1. Process performance
As illustrated in Fig. 2 (A, B), the initial 8 weeks of the first operation period can be considered as the acclimatization period (from 120 to 175 days) for the microbial population. During this period of the experiment, due to variation in sludge culture medium, the ammonium removal efficiency was between 11 and 16%, but nitrite removal efficiency (from 117 to 140 day) was between 85 and 91%. However, from day 140 to 160, the biomass encountered a shock of loading rate, and the nitrite removal efficiency decreased from 85 to 45%. Respectively, the removal efficiency of nitrite and ammonium improved from day 160 to 180 and onward. These results more or less agree with the results reported by Yang et al. (2009). Under these conditions, predominant bacteria (for example nitrifying bacteria) may be killed due to the conversion of organic nitrogen to ammonium (Chamchoi and Nitisovut, 2007) but after time (60 days); it caused the adaptation of bacteria with an environment of seed sludge, therefore, the ammonium and nitrite removal efficiency was improved. During the operation period, the influent ammonium and nitrite concentrations gradually increased from 60 and 80 mg N/L to 700 and 920 mg N/L. Consequently, HRT was decreased from 24 to 6 h. Due to the gradual decrease of HRT and increase of nitrogen loading rate to the highest 5 g N/L d, ammonium removal efficiency reduced from 93% (from day 181) to 86% (to day 290). However, from day 270 to 295 (in the higher ammonium concentrations), the anammox bacteria could not reduce ammonia and nitrite concentrations due to the increase of nitrogen loading rate, which could probably be toxic to ammonium oxidizing bacteria under anoxic conditions. As a result, ammonium and nitrite concentrations increased in the effluent. During the entire run time, the maximum ammonium and nitrite removal efficiency was achieved at 91 and 93%, respectively. The results are strongly similar to the findings of Lopez et al. (2008) and Ni et al. (2010). Also, when nitrate was observed in the effluent, the concentration did not significantly increase.
The nitrogen conversion rate, determined as the sum of ammonium and nitrite removal rate, obtained in the range between 0.762 and 5.64 g N/L d, respectively.
4.2. Physical specification
The color of sludge changed in during the entire process period, so that color of sludge before cultivation was a dark color. After the ammonium and nitrite removal in the UABF reactor, at the initial concentration of ammonium 60-210 mg N/L, the color of the biomass was grizzle or ashen in color. Also, in high concentrations of ammonium, at approximately 250 mg/L a maize color, orange or light red was created, which is similar to the anammox cultures expounded in previous studies (Van Dongen et al., 2001a, b; Isaka et al., 2006). However, the color of sludge was fixed at the highest concentration of 400 mg/L, which indicate the application of the anammox while processed in high concentrations.
4.3. Calculation of nitrogen removal rate by anammox bacteria
Nitrogen removal rate by anammox bacteria in the UABF reactor was determined based on the stoichiometry reaction of the anammox process found in the Equation below:
= (Influent ammonium - effluent ammonium) + (effluent nitrite) + (effluent nitrate)
Concentrations of influent and effluent ammonium, nitrite and nitrate were at mg N/L, respectively. As illustrated in Fig. 3, the ANR increased (exponential increasing) from 0.232 to 2.964 g N/L d while the NLR increased from 0.195 to 2.8 g N/L d for along time.
4.4. Evaluation of the process performance in UABF reactor by the kinetic models
The modified Stover-Kincannon model and Grau second-order model were used to evaluate the anammox process performance in the UABF reactor. During the kinetic study, the HRT was adjusted at 24 h and gradually reduced to 6 h, whereas the flow rate increased from 1.8 L/d to 7.2 L/d. Also, the influent ammonium and nitrite concentrations were increased from 350 and 470 mg N/L to 700 and 924 mg N/L, respectively. Consequently, The NLR increased gradually from 0.835 g N/L d to 6.496 g N/L d. The results obtained from experiments during the kinetic study, indicated a meaningful relationship between the removal efficiency and the hydraulic retention time. As the HRT decreased, total nitrogen concentration in effluent increased from 0.07 g/L to 0.2 g/L. Also, the removal efficiency decreased stepwise from 91.6 to 81.8%. The effect of the nitrogen loading rate on the substrate removal rate is depicted in Fig. 4. According to Fig. 4, with the increase of nitrogen loading rate, the substrate removal rate was increased from 0.765 to 5.316 kg/m3 d and the nitrogen removal efficiency decreased to 81%.
4.4.1. Modified Stover-Kincannon model
Fig. 5A shows the kinetic coefficients of the modified Stover-Kincannon model obtained from the plot of V/[Q(Si-Se)] vs. V/(QSi). In this plot, V/[Q(Si-Se)] represents the inverse of the removal rate and V/(QSi) indicates the inverse of the total substrate loading rate. Values of the saturation value constant (KB) and the maximum substrate removal rate (Umax) were determined as 38.107 kg/m3 d and 35.71 kg/m3 d from the slop and intercept of Fig. 5A, respectively. The correlation coefficient is 0.9993, upholding the capability of the modified Stover-Kincannon model.
4.4.2. Grau second-order substrate removal model
In order to obtain the kinetic coefficients, Eq. (12) was plotted in Fig. 5B. The values of (a) and (b) were calculated to be 0.037 and 1.056 according to the intercept and slope of the straight line on the graph. The correlation coefficient of the second-order model was 0.9992. The formula for predicting effluent nitrogen concentrations for the UABF reactor is given by:
Fig. 6 illustrates the experimental data obtained in this study vs. the predicted values for the effluent total nitrogen obtained by the modified Stover-Kincannon and the Grau second-order models in UABF reactor, which were calculated by Eqs. (7) and (14), respectively. It can be observed that the predicted data is in agreement with the experimental data (R2=0.978 and 0.956 by the modified Stover-Kincannon and the Grau second-order models respectively) that indicate the Stover-Kincannon model was more appropriate for nitrogen removal kinetic in the UABF reactor.
Table 1 compares the constants obtained from the modified Stover-Kincannon model and Grau second-order model in the previous studies with coefficients determined in this research. In the current study, KB and Umax values are larger than those calculated by Jin and Zheng (2009) and Ni et al, (2010). Presumably, this variation in value depends on influent wastewater composition, the type of activated culture applied to the reactor, the type of reactor and so on. The authors believe this indicates that the UABF reactor has a higher capability for the treatment of high-strength wastewaters.
The anammox process performance in the up-flow anaerobic biofilm reactor was evaluated at different NLRs and HRTs using synthetic wastewater with high loading rates. Consequently, kinetic parameter analyses of the reactor was carried out according to the experimental data to determine the most appropriate model in describing the nitrogen removal in the UABF reactor. Two kinetic models including the modified Stover-Kincannon and Grau second-order model were used for the bio-kinetic modeling of the UABF reactor. According to the results, even though the correlation coefficients of both models were high (R2 = 0.999), evaluation of the model by comparing experimental results and predicted values indicated that the modified Stover-Kincannon model was a little more suitable for nitrogen removal in the UABF reactor.