Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization

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Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization A.A.L. Zinatizadeh a,∗, E. Ghaytooli b a b

Water and Wastewater Research Center (WWRC), Applied Chemistry Department, Faculty of Chemistry, Razi University, Kermanshah, Iran Department of Chemical Engineering, Faculty of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran

a r t i c l e

i n f o

Article history: Received 12 December 2014 Revised 24 February 2015 Accepted 26 February 2015 Available online xxx Keywords: Simultaneous nitrification and denitrification Moving bed biofilm reactor (MBBR) Packing media Process optimization

a b s t r a c t In this study, a moving bed biofilm reactor (MBBR) was examined removing organic carbon and nitrogen from municipal wastewater through simultaneous nitrification and denitrification (SND) process. To assess the process performance, two numerical independent variables, hydraulic retention time (HRT) and dissolved oxygen (DO), and one categorical variable, type of packing media, were selected. The effects of the numerical variables were investigated at three levels, 4, 8 and 12 h for HRT and 2, 3 and 4 mg/L for DO, while two levels of the categorical variable (Ring form and Kaldnes-3) were examined. The packing media used were different in the structure and geometry but similar to specific surface area (500 m2 /m3 ). The experiments were carried out at two parallel reactors. The process was analyzed and modeled by monitoring 10 dependent responses. Maximum COD removal efficiency was found to be 85 and 88%, respectively, for the system with Ring form and Kaldnes-3 at HRT of 12 h and DO of 4 mg/L. The results showed that the system with Ring form could achieve more TN removal efficiency than that of the process with Kaldnes-3, indicating that anoxic condition is favored with Ring form due to its structure geometry. In all the conditions tested, nitrite oxidizing bacteria (NOB) was dominant species. It implies more nitrite production from the ammonia oxidizing bacteria (AOB) activity which leads to increase NOB growth. The maximum denitrification rates for Ring form and Kaldnes-3 were obtained 90 and 70 mg N/L.d, respectively at DO of 3 mg/L and HRT of 8 h. Overall, the changes in the system with Ring form media at different conditions have been more compared to that in the system with Kaldnes-3, indicating more stability of the system operated with Kaldnes-3. As a conclusion, the biofilm in the Ring form showed lower stability compared to that of the system with Kaldnes-3. © 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Increasing rapid rate of urbanization and population leads to produce large amount of wastewater. Adverse environmental impacts and enactment of more stringent legislation force the wastewater producers to treat wastewater before discharging to the environment [1–3]. Ammonium and nitrate concentrations in water environment persuade eutrophication and deplete dissolved oxygen due to activity of the ammonia oxidizing bacteria [4–7]. Therefore, removal of such substances from wastewaters in order to reduce their harm to the environment is of very much importance. Several biological and physico-chemical treatment methods have been developed to remove nitrogen from wastewater. Among these methods, biological treatment of wastewater can be a cost-effective

∗

Corresponding author. Tel.: +98 9188581130; fax: +98 8334274559. E-mail address: [email protected], [email protected] (A.A.L. Zinatizadeh).

and environmental friendly method, and more compatible with existing plant facilities and operation [8–10]. Biological removal of nitrogenous compounds from wastewater involves combination of two processes of nitrification and denitrification. Nitrification transforms ammonia to a more oxidized nitrogen compound such as nitrite or nitrate by two different groups of microorganisms, ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB). The denitrification process, in which organic compounds are used as electron donors, is the most common form of denitrification [11–13]. Biological processes based on suspended biomass are effective for removal of organic carbon and nutrient from municipal wastewater plants. However, this technology is limited by some problems such as sludge settle ability, so need for large reactors, settling tanks and biomass recycling [14–16]. Also, biofilm processes have proved to be reliable for reduction of chemical oxygen demand (COD) and nutrients without some of the problems of activated sludge processes [17,18]. Biofilm reactors are especially useful when slowly growing organisms like nitrifiers have to be kept in a wastewater treatment process [15,19].

http://dx.doi.org/10.1016/j.jtice.2015.02.034 1876-1070/© 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: A.A.L. Zinatizadeh, E. Ghaytooli, Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.02.034

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So it is necessary to find alternatives which have the advantages of the activated sludge process without its disadvantages. Moving bed biofilm reactor (MBBR) is an efficient alternative for organic carbon and nitrogen removal, which combine the advantages of both the activated sludge process and a biofilm reactor by incorporating free-floating carriers that provide a large surface area for colonization with no need for biomass recycling [20–22]. Other advantages of MBBR include the reduction in space as compared to traditional activated sludge system, ease in upgrade of existing facilities, low head loss in comparison with submerged filter configurations, fewer requirements for cleaning or backwash, increased solid retention time for slowly growing organisms, and more rapid recovery from extreme loading conditions [23–26]. Most of the literature regarding MBBR is based on nitrification and denitrification in two separate reactors. However, recent studies have revealed that these two important steps can occur concurrently in the same reactor that has been termed simultaneous nitrification and denitrification (SND) [12,27]. SND offers several advantages, compared to the nitrogen removal through conventional nitrification and denitrification; (1) there is no need for either two separate tanks operated in series, or intermittent aeration in a single tank, thus continuous effluent output can be achieved with a smaller footprint; (2) it utilizes less carbon about 22–40% which subsequently decreases sludge yield by 30%; (3) it needs less alkalinity as the alkalinity is produced during denitrification, and (4) it consumes less energy due to less aeration requirement [28–31]. Quan et al. (2012) [1] studied the effects of packing rates (20%, 30%, and 40%) of polyurethane foam (PUF) and hydraulic retention time (HRT) (5, 7 h) on the performance of a MBBR in terms of carbon and nitrogen removal from urban synthetic wastewater. From this study, the packing rate showed little influence on the COD removal efficiency. 96.3% ammonium removal efficiency was achieved at HRT of 5 h in 40% packing rate reactor, while it was 37.4% in 20% packing rate. In another research work carried out by Martín-Pascual et al. [3], three MBBRs with different carriers (Aqwise, Kaldnes, BIOCONS) were operated and the influence of HRT (5, 10, 15 h) and the filling ratio (20, 35, 50) on the process performance for treatment of municipal wastewater was investigated. The highest COD removal efficiencies for carriers were obtained 56.97%, 58.92% and 46.13%, respectively, for Aqwise, Kaldnes, BIOCONS under HRT of 15 h and filling ratio of 50%. Simultaneous nitrification and denitrification (SND) in a MBBR was studied by Chu et al. [11]. In this study, biodegradable polymer (PCL) was used as both solid carbon source and biofilm carriers to remove nitrogen from wastewater with a low C/N ratio. In average, 74.6% TN removal efficiency was obtained at HRT of 18.5 h. In another work done by the authors [8], the effects of two types of packing media, polyurethane foam (PUF) and biodegradable polymer polycaprolactone (PCL), at different HRT (14, 16, 24, 40 h) on the removal of organics and nitrogen from wastewater with a low C/N ratio were evaluated. The PUF carriers showed better performance in terms of TOC and ammonium removal efficiency (90 and 65% versus 72% and 56%, respectively). Kermani et al. [15] employed a moving bed biofilm process in series with anerobic, anoxic and aerobic units in four separate reactors for nutrients’ removal. SCOD, total nitrogen and phosphorus removal efficiencies obtained to be 96.9, 84.6 and 95.8%, respectively. Shore et al. [32] examined a MBBR as a tertiary treatment step for ammonia removal at high temperature (35–45 °C). A successful removal of ammonia (>90%) from both the synthetic and industrial wastewaters was reported.

In this study, a moving bed biofilm reactor (MBBR) as a single reactor was applied to remove carbon and nutrients simultaneously from a real municipal wastewater. A general factorial design was employed to describe and model the variation trends of 10 significant responses, i.e., removal of TCOD, TN, total Kjeldahl nitrogen (TKN), N-organic, and phosphorus (TP), the content of effluent nitrate, total effluent turbidity, and population of heterotrophic, nitrifying and denitrifying bacteria as a function of two independent numerical variables, HRT and DO, and one categorical variable. 2. Materials and methods 2.1. Wastewater characteristics Wastewater was collected from the influent of the Bisuton’s municipal wastewater treatment plant, Kermanshah, Iran. The samples were stored in a cold room at 4 °C until use. This storage technique had no observable effect on its composition. The wastewater characteristics are shown in Table 1. COD:N:P ratio of the Bisuton’s municipal wastewater was almost 100:10:1. In order to investigate the capacity of the MBBR system in phosphorous and nitrogen removal, the composition of the wastewater manually reached to the level of 100:20:3. To adjust the ratio of COD:N:P to the level of 100:20:3, ammonium chloride and potassium di-hydrogen phosphate were added as nitrogen and phosphorus sources, respectively. 2.2. Bioreactor configuration The experimental rig along with schematic diagram of the moving bed biofilm reactor (MBBR( used in this study is shown in Fig. 1. Two plexi-glass bioreactor columns were fabricated with an internal diameter of 13 cm and height of 70 cm with a total volume of 9.3 L. The working volume in each bioreactor was 2.2 L with height of 16.9 cm. A 2-L settling tank without sludge recycling was used for each bioreactor. Each bioreactor was packed with a different carrier (Kaldnes-3 in R1 and Ring form in R2) with packing rate 50%. Both carriers were made of high density polyethylene but with different shapes (Fig. 1), providing specific surface area equal to 500 m2 /m3 . The carrier elements are moving freely within the reactor and kept in circulation by the air introduced at the bottom of the reactor. A gas flow meter was used to control the air flow rate for adjustment of dissolved oxygen (DO) level. 2.3. Bioreactor operation procedure The operation of the two MBBRs was started by inoculating activated sludge taken from aerobic tank at the Bisuton’s municipal wastewater treatment plant, Kermanshah, Iran. The initial sludge concentration was 3.0 g total suspended solids (TSS)/L. In order to develop biofilm on the packing media, the bioreactors were operated in batch mode for five weeks. During the start-up, the retention time and DO were set at 24 h and 1.5 mg/L. The biofilm formed on the surface of the packing is shown in the Fig. 1. After the bioreactor start-up, wastewater was continuously fed to the bioreactors from the top of the bioreactor using a peristaltic pump. The carrier elements are moving freely within the reactor, kept in circulation by the air introduced at the bottom of the reactor using an air pump. The MBBR bioreactor was operated under room temperature (25 ± 2 °C). In this stage, Bisuton’s municipal wastewater after

Table 1 Characteristics of Bisuton’s municipal wastewater. Parameters

COD (mg/L)

BOD5 (mg/L)

TN (mg/L)

TP (mg/L)

TSS (mg/L)

pH

Amount

300–500

150–250

30–50

2–4

100–300

6.5–7.5

Please cite this article as: A.A.L. Zinatizadeh, E. Ghaytooli, Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.02.034

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3

Fig. 1. Experimental setup.

addition of nitrogen and phosphorus with COD:N:P ratio of 100:20:3 was pumped into the bioreactors at a hydraulic retention time (HRT) of 8 h. This flow was continued until a steady-state condition was achieved. In steady-state condition, the MBBR bioreactor was operated according to the experimental conditions designed with two independent effective variables, i.e., HRT (4–12 h) and DO (2–4 mg/L). The experimental runs were designed using design expert software (DOE) (Stat-Ease Inc. version 7.0) as described in Section 2.4. It is noted that all the responses in second stage were discussed based on the predicted values obtained from the models of the DOE. 2.4. Experimental design and mathematical model The design of the experiments and statistical analysis of data were carried out by Design Expert Software (version 7.0). HRT and DO as two independent numerical variables along with type of packing as a

Table 2 Experimental range and levels of the variables. Name of variable

Level of variables

A = DO (mg/L) B = HRT (h) C = Type of packing

2 4 Ring form

3 8

4 12 Kaldnes-3

categorical factor were selected in the experiments design. The range and levels of the variables used in this study are shown in Table 2. On the basis of the factorial design, 13 experiments for each of the packing type (including 4 factorial points, 4 axial points, 1 center point and 4 replications of the center point) were designed. Table 3 shows experimental conditions applied in this study. The amount of biofilm per unit reactor, TCOD removal efficiency, TN removal efficiency, effluent N-organic, ammonia, nitrite and nitrate concentrations, TP removal efficiency, TSS removal efficiency, effluent turbidity, and population

Please cite this article as: A.A.L. Zinatizadeh, E. Ghaytooli, Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.02.034

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A.A.L. Zinatizadeh, E. Ghaytooli / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–14 Table 3 Experimental conditions applied in this study. Run

Factor 1 A: DO concentration (mg/L)

Factor 2 B: HRT (h)

3. Results and discussion 3.1. MBBR specification

Factor 3 C: Type of packing

The MBBRs are characterized by the quantity and quality of the biomass attached on the moving media. In the MBBR, microbial community is developed as attached biofilm and the biomass is somehow not controllable. Thus, before dealing with the process performance, biomass concentration at different operating conditions in the both MBBRs is presented in Fig. 2a and b. As is seen in the figures, the biomass concentration had a decreasing trend as DO was decreased from 4 to 2 mg/L. The results showed that higher biomass washout was caused by combination of two independent factors: decrease in organic load and decrease in DO. It is noted that the effect of the DO showed to be more than HRT. However, the change in the biomass concentration is analyzed and modeled in the following sections.

1 1

4 4

4 4

Ring form Kaldnes-3

2 2

4 4

8 8

Ring form Kaldnes-3

3 3

4 4

12 12

Ring form Kaldnes-3

4 4

3 3

4 4

Ring form Kaldnes-3

5 5

3 3

8 8

Ring form Kaldnes-3

6 6

3 3

8 8

Ring form Kaldnes-3

7 7

3 3

8 8

Ring form Kaldnes-3

8 8

3 3

8 8

Ring form Kaldnes-3

3.2. Process analysis and modeling

9 9

3 3

8 8

Ring form Kaldnes-3

10 10

3 3

12 12

Ring form Kaldnes-3

11 11

2 2

4 4

Ring form Kaldnes-3

12 12

2 2

8 8

Ring form Kaldnes-3

13 13

2 2

12 12

Ring form Kaldnes-3

3.2.1. Statistical analysis Central composite design (CCD) as one of the methods under response surface methodology (RSM) was selected to explore the relationship between the process responses and the variables. Table 4 represents the list of the three independent variables (A, B and C) in the terms of actual units, and the experimental data obtained for the 7 responses. Twenty-six experimental runs were carried out in accordance with Table 4 by CCD. Table 5 illustrates the reduced models in terms of coded factors with significant model terms and analysis of variance (ANOVA) results for the responses. In this study, various responses were investigated with different degree polynomial models for data fitting (Table 5). In order to quantify the curvature effects, the data from the experimental results were fitted to higher degree polynomial equations, i.e. two factor interaction (2FI), quadratic, etc. The finalized model terms in the equations are those which remained after the elimination of insignificant variables and their interactions. The actual and the predicted plots for the responses are shown in Fig. 3. The figure showed good agreement between the actual and the modeled data. The values obtained from the ANOVA analysis determine the rank of the significance degree. For each response, the P-value was computed to determine the significance of the model terms. The statistical results indicate that the corresponding models and the individual coefficients are more significant [35]. In Table 5 and 7 models were developed with the very low probability values (ࣘ0.0001). This implies that the model terms were significant for all the models. The lack-of-fit values are also insignificant (>0.05) for all the responses studied. The fit of the models were further verified by the coefficient of determination, R2 , and adjusted R2 between the experimental and the modeled values. Table 5 proves that the coefficient of determination, R2 and adjusted R2 are near to each other and larger than 0.8 except for TP removal efficiency. Adequate precision measures the signal to noise ratio. A ratio greater than 4 is desirable. In all the cases, the value of adequate precision was around 13.5–27.57. This value indicates adequate model discrimination. As a result, except for TP removal efficiency, the other proposed models could be adequately used to describe the responses under a wide range of operating conditions. Detailed analysis on the models is presented in the following sections.

of heterotrophic and nitrifying bacteria were measured or calculated as response. The experimental data obtained was used to determine the coefficients of the polynomial model (Eq. (1)), [33].

Y = β0 + βi Xi + βj Xj + βii Xi 2 + βjj Xj 2 + βij Xi Xj + · · ·

(1)

where i and j are the coefficients of linear multi-degree-and β is the correlation coefficient. P value with 95% confidence level was considered to evaluate the effectiveness of the model terms. 2.5. Analytical methods The concentrations of COD, biological oxygen demand (BOD), TN, TKN, nitrate and nitrite, NH4 -N, TP, TSS were determined by using standard methods [34]. Organic nitrogen was measured by deducing the ammonia nitrogen from TKN. For COD, a colorimetric method with closed reflux method was developed. Spectrophotometer (DR 5000, Hach, Jenway, USA) at 600 nm was used to measure the absorbance of COD samples. TKN and NeNH4 were determined by TKN meter Gerhardt model (Vapodest 10, Germany). The DO concentration in wastewater was determined using a DO probe. DO meter was supplied by WTW DO Cell OX 330, electro DO probe, Germany. Turbidity was measured by a turbidity meter model 2100 P (Hach Co., USA). Possible counts of ammonium oxidizer bacteria (AOB), nitrite oxidizers bacteria (NOB) were determined by most probably number (MPN) method. List of microbial cells by MPN methods was carried out in three kinds of growth limited culture mediums, namely medium for AOB, and NOB bacteria. The dry weight of the attached biofilm per unit of wetted surface area of the packing (X) was determined by drying a packing at 80 °C for 24 h before and after biofilm attachment. In this procedure, a packing taken out from the middle part of the bioreactor was dried and weighed. It was subsequently cleaned to remove the attached biofilm, followed by drying and weighing again. This was done twice, before and after each set of experiment. No significant difference in the amount of attached biomass was noted.

3.2.2. Attached biomass concentration Biomass concentration in the MBBR indicates treatment capacity of the system removing C and N, depending on the biofilm thickness. As shown in Table 5, the biomass concentration is a function of A, B, AC, BC, B2 . Fig. 4a and b shows the effect of the variables on the response for two packing media. As can be seen in Fig. 4a and b, by increasing DO at a constant HRT, the response was increased. At this condition, the required oxygen is provided for the inner biofilm

Please cite this article as: A.A.L. Zinatizadeh, E. Ghaytooli, Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.02.034

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5

Attached biomass concentration (mg VSS/L)

7000 DO = 2 mg/L

DO = 3 mg/L

DO = 4 mg/L

6000 5000 4000 3000 2000 1000 0 1

2

3

4

5

6 7 8 Run number

9

10

11

12

13

(a) Attached biomas concentration (mg VSS/lit)

4500 DO = 4 mg/L

DO = 2 mg/L

DO = 3 mg/L

4000 3500 3000 2500 2000 1500 1000 500 0 1

2

3

4

5

6

7 8 Run number

9

10

11

12

13

(b) Fig. 2. Biomass concentration in the MBBR for different packing media: (a) Ring form and (b) Kaldnes-3. Table 4 Experimental conditions and results. Run

Factor 1 A: DO concentration (mg/L)

Factor 2 B: HRT (h)

Factor 3 C: c

Response 1 biomass Response 2 concentration COD removal (mg/L) (%)

Response 3 TN removal (%)

1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 12 12 13 13

4 4 4 4 4 4 3 3 3 3 3 3 3 3 3 3 3 3 3 3 2 2 2 2 2 2

4 4 8 8 12 12 4 4 8 8 8 8 8 8 8 8 8 8 12 12 4 4 8 8 12 12

Level 1 c Level 2 c Level 1 c Level 2 c Level 1 c Level 2 c Level 1 c Level 2 c Level 1 c Level 2 c Level 1 c Level 2 c Level 1 c Level 2 c Level 1 c Level 2 c Level 1 c Level 2 c Level 1 c Level 2 c Level 1 c Level 2 c Level 1 c Level 2 c Level 1 c Level 2 c

5200 ± 120 3050 ± 112 5800 ± 170 4100 ± 0.198 3800 ± 150 3200 ± 115 3600 ± 190 2900 ± 170 3200 ± 112 3100 ± 190 3000 ± 190 3300 ± 190 2500 ± 180 2900 ± 100 3300 ± 190 3000 ± 100 2600 ± 125 2500 ± 90 2750 ± 80 2150 ± 85 1900 ± 60 2200 ± 50 1500 ± 80 1890 ± 75 1100 ± 90 1750 ± 95

41 38 42 43 29 26 51 41 51 45 47 43 50 44 48 40 45 39 40 33 39 41 42 47 35 30

78 80 80 84 82 86 62 57 72 74 71 73 73 75 72 72 73 76 74 79 54 50 58 53 55 65

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.7 0.9 0.7 0.8 0.7 0.3 0.9 0.5 0.7 0.6 0.7 0.6 0.7 0.7 0.9 0.7 0.8 0.7 0.7 0.65 0.5 0.7 0.7 0.7 0.8 0.6

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.9 0.7 0.8 0.7 0.9 0.7 0.7 1.4 1.3 0.7 0.7 0.7 0.7 0.7 0.7 1.2 1 0.9 0.8 0.7 0.7 1 0.9 0.8 0.7 1.4

Response 4 effluent NO3 concentration (mg/L) 24 30 25 31 36 45 17 25 21 28 22 30 24 32 22 33 20 30 28 35 7.9 9.7 3.16 1.8 10 19

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4

Response 5 TSS removal (%) 79.5 86.1 79.7 86.8 55 0.03 0.03 89 0.03 0.03 0.03 83 0.03 84 0.03 0.03 0.03 0.03 61. 7 0.03 87.3 0.03 0.03 0.03 88.7 0.03

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.03 0.05 0.05 0.05 0.05 71 85.1 0.05 84.9 89.8 77.9 0.05 84 0.05 70 82 75 85 0.05 81.4 0.05 95.2 90.2 94.8 0.05 93.9

Response 6 effluent turbidity (NTU) 2 15 11 7 9 14 8 7 15 5 10 11 7 10 12 6 1 13 5 12 9 1 17 14 15 6

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.6 0.6 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.7 0.7 0.7 0.6 0.6 0.6 0.7 0.7 0.7

Response 7 TP removal (%)

−6 ± −6 ± 18 ± 18 ± 0 0 −10 ± 18 ± −17 ± −17 ± −14.7 ± −15 ± −15.5 ± −17 ± −14 ± −17 ± −14.5 ± −11 ± −5 ± −11 ± 38 ± 29 ± 13 ± 13 ± −2 ± −10 ±

0.7 0.7 0.7 0.7

0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7

Please cite this article as: A.A.L. Zinatizadeh, E. Ghaytooli, Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.02.034

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Table 5 ANOVA results for the models of the Design Expert 6.0.6 for studied responses. Response

Modified equations with significant terms

Probability (P-value)

R2

Adj. R2

Adeq. precision

S.D.

CV

Press

Probability for lack of fit

Attaced biomass concentration (mg VSS/L) COD removal (%) TN removal (%) Effluent NO− 3 −N concnetration (mg/L) TSS removal (%)

+3058.57 + 1092.50 A − 391.67B − 340.83AC + 216.67BC − 308.57B2

<0.0001

0.9

0.89

26.9

315.8

10.80

4.1E+06

0.096

+71.64 + 12.92 A + 5.00 B + 2.17BC − 3.14B2 +47.57 − 4.91 B − 2.10C − 5.99 A2 − 6.42 B2 +29.46 + 11.51 A+5.73 B+2.56C+2.99AB − 9.36A2

< 0.0001 <0.0001 <0.0001

0.91 0.9 0.92

0.9 0.89 0.89

27.57 21 18.72

3.32 2.33 4.16

4.73 5.90 15.67

384.96 183.70 924.23

0.07 0.14 0.097

+87.14 − 7.67 A − 5.89B + 3.79C − 4.96AB − 5.99B2 +11.71 + 1.50A − 2.23C+3.12 AB − 5.21A2

< 0.0001

0.85

0.81

20.25

4.09

4.84

584.88

0.27

<0.0001

0.81

0.78

15.69

2.11

22.65

162.97

0.48

−12.17 − 4.89A − 7.75 B + 11.61AB + 20.94A2

<0.0001

0.75

0.7

13.5

8.94

35.61

264.43

0.07

Effluent turbidity (NTU) TP removal (%)

layers, avoiding the biofilm sloughing. From the model, the increasing effect of DO is resulted from the direct effect of A with positive coefficient and interactive effect of AC with negative coefficient. It is noted that the decreasing effect of AC was more for the Ring form media (Fig. 4a). It is also noticed that the effect of DO variation on Kaldnes-3 type media has been less compared to the Ring form media which can be because of the physical structure of the packing and smaller holes in the Kaldnes-3 form relative to the Ring form. In both the systems, an increase in HRT caused a decrease in the response as a result of less organic load. Overall, the changes in the system with Ring form media at different conditions have been more compared to that in the system with Kaldnes-3, indicating more stability of the system operated with Kaldnes-3. As a result, the oxygen and feed mass transfer is restricted in the system with Ring form media, limiting the biomass growth. 3.2.3. COD removal For overall evaluation of the system in the removal of organic compounds, COD removal efficiency was monitored throughout the experiments. As presented in Table 5, the response is a function of the studied variables as a modified quadratic model. According to the model, BC is the only interactive term. In the system with Kaldnes-3, the interaction of BC results in an intensive positive effect of HRT on the response. It is noted that the effect of DO on the response is only as a first order term. While, HRT effect showed as the first and second order and interactive effect with type of packing. Fig. 5a and b represents the effects of the numerical variables on the COD removal efficiency for Ring form and Kaldnes-3, respectively. As shown in Fig. 5a and b, by increasing DO from 2 to 4 mg/L, the response was increased in both the systems with the same trend. However, the increasing effects of the variables were more for the system with Kaldnes-3 media. Maximum COD removal efficiency was found to be 85 and 88%, respectively, for the system with Ring form and Kaldnes-3 at HRT of 12 h and DO of 4 mg/L. The minimum values for the response were 54 and 50%, respectively, for the system with Ring form and Kaldnes-3 at HRT of 4 h and DO of 2 mg/L. From the experimental data, both variables (HRT and DO) showed increasing effect which might be due to an increase in the aerobic heterotrophic bacteria (as discussed in Sections 3.1 and 3.2.2). The results showed that the simultaneous increase or decrease in the variables had direct intensive effect compared to their direct first order effect, indicating an interactive effect of the variables on the response. As can be seen in the model (Table 5), the increasing effect of A and B on the response is shown with positive coefficients, +12.92 and +5, respectively, indicating more dependency of the response to DO (A) in comparison with HRT (B) [22,36]. Similar work is performed by Pascual et al. in a MBBR treating municipal wastewater with different packing percentage and HRT at constant DO. A maximum

COD removal efficiency of 50% was obtained in this study [3]. In another study, two MBBRs in series (combination of anoxic and aerobic zones) were used for treatment of synthetic municipal wastewater and 98% COD removal efficiency was obtained at HRT of 20 h. It is noted that applying two MBBRs involves more energy consumption and cost. 3.2.4. Nitrogen removal Total nitrogen (TN) removal. Nitrogen removal in a single bioreactor is occurred under simultaneous nitrification and denitrification (SND) process using autotrophic nitrifiers and heterotrophic denitrifiers [37]. So, the required condition for SND process must be provided. Since a biofilm is formed in the MBBR and anoxic condition can be developed, TN removal was monitored at different operating conditions applied. The model for TN removal efficiency was a function of B, C, A2 and B2 as shown in Table 5. Fig. 6a and b shows the effects of the variables (DO and HRT) on TN removal efficiency for two used packing media. From Fig. 6a and b, by increasing DO and HRT, respectively, from 2 to 3 mg/L and from 4 to 8 h, the response increased. In this condition, oxidation potential for conversion of ammonia to nitrate was intensified and also availability of the substrate and oxygen for the microorganisms favored the microbial growth. The microbial growth is shown in Fig. 4a and b. In this condition, the anoxic condition for the denitrification process was provided and more nitrogen is removed. Whereas, further increase in DO and HRT, respectively from 3 to 4 mg/L and 8 to 12 h, caused a decrease in the response. In this condition, with an increase in the variables nitrification is encouraged while denitrification process is limited as a result of low feeding [15,16]. The results showed that the system with Ring form could achieve more TN removal efficiency than that of the process with Kaldnes-3, indicating that anoxic condition is favored with Ring form due to its structure geometry (Fig. 1). In this study, maximum TN removal efficiency was obtained to be 50 and 46%, respectively for the systems with Ring form and Kaldnes3 at DO of 2.5–3 mg/L and HRT of 5.5–7.5 h. In a similar research work, a TN removal efficiency of 53% was reported at initial nitrogen concentration of 25−30 mg/L, HRT of 7 h and packing percentage of 40% [38]. It must be noted that the relatively low TN removal was attributed to the high initial nitrogen concentration applied in this study (C:N:P of 100:20:3) which was to explore the maximum capacity for simultaneous nitrification and denitrification reactions in the MBBR. The ratio of C:N:P for Bisuton’s Municipal wastewater was 100:10:1 in average. Based on the results obtained, in case the municipal wastewater is used, TN removal efficiency higher than 90% could be achieved. The populations of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) were measured at three different operating conditions with DO of 3 mg/L and HRT of 4, 8 and 12 h. The

Please cite this article as: A.A.L. Zinatizadeh, E. Ghaytooli, Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.02.034

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A.A.L. Zinatizadeh, E. Ghaytooli / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–14

Predicted vs. Actual

Design-Expert® Software Biomass concentration, mg/l

Design-Expert® Sof tware COD remov al, %

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Design-Expert® Sof tware Ef f luent NO3 concnetration, mg/l

55.00

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17.00

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Fig. 3. Actual versus predicted values for (a) attached biomass concentration, (b) COD removal efficiency, (c) TN removal efficiency, (d) effluent nitrate concentration, (e) TSS removal efficiency, (f) effluent turbidity, and (g) TP removal efficiency.

Please cite this article as: A.A.L. Zinatizadeh, E. Ghaytooli, Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.02.034

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5200 4075 2950 1825 700

12.00 4 10.00

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A.A.L. Zinatizadeh, E. Ghaytooli / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–14

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41.5

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A: DO concentration, mg/L

B: HRT, h 2.00

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Fig. 6. Three dimensional plot for TN removal efficiency: (a) with Ring form and (b) with Kaldnes-3.

Please cite this article as: A.A.L. Zinatizadeh, E. Ghaytooli, Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.02.034

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A.A.L. Zinatizadeh, E. Ghaytooli / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–14 Table 6 AOB and NOB results at selected conditions. Operating condition

Test

Log(MPN)

4

AOB NOB

5.63 12.04

8

AOB NOB

5.36 9.04

12

AOB NOB

5.36 9.04

DO (mg/L)

HRT (h)

3

respectively at DO of 3 mg/L and HRT of 8 h. Also, the maximum removal of the organic nitrogen with the both packing media was found at HRT of 4 h and DO of 2 mg/L, where the biofilm thickness was more which favors anoxic development. In contrast, ammoniacal nitrogen was maximized at high HRT and DO, where maximum nitrification was obtained. Effluent nitrate. The nitrate concentration in the effluent was measured to evaluate the nitrification and denitrification process at different operating conditions. This response was found to be a function of A, B, C, AB and A2 as shown in Table 5. In the model, A, B, C, and AB had increasing effect while A2 showed decreasing effect. DO was more effective variable relative to HRT on the response. Fig. 8a and b illustrates the effects of the variables on the response in the two systems. As it is seen, the trend of changes in the response was similar for both packing media. However, the response values for the Kaldnes-3 were longer compared to the Ring form. In areas at DO in the range of 2–3 mg/L showed more increasing effect relative to the range of 3–4 mg/L. It was attributed more denitrification in the lower range of DO, leading to nitrate consumption and less effluent nitrate concentration [8]. Whereas by increasing HRT, the biofilm thickness decreased and denitrification process is restricted. Simultaneous increase in HRT and DO results in an increase in the response comparison of types of packing materials which showed that the increasing effect of DO on the response was more for Kaldnes-3 relative to Ring form. Due to the structure of Kaldnes-3, the biofilm

Nitrogen concentration (mg N/L)

experimental results are presented in Table 6. In all the conditions tested, NOB was dominant species. It implies more nitrite production from the AOB activity which leads to increase in NOB growth. The highest amounts of the AOB and NOB were obtained at HRT of 4 h where maximum organic loading rate is applied, implying more favored condition for heterotrophic bacteria. The nitrogen fraction (organic and ammoniacal nitrogen, nitrite and nitrate) in influent and effluent at different operating conditions is shown in Fig. 7a and b. The figures were drawn according to the number of experiments in Table 2. The difference between each couple column at an experiment indicates nitrogen removed as a result of denitrification and cell synthesis. On this basis, the maximum denitrification rates for Ring form and Kaldnes-3 were obtained 90 and 70 mg N/L.d,

DO = 4 mg/L

100

DO = 2 mg/L

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N-org

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Nitrogen concentration (mg N/L)

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N-NO2 N-NO3 N-NH4 N-org

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2

3

4

9

5

6

7

8

9

10

11

12

13

Run number

(b) Fig. 7. Nitrogen fractionation at influent and effluent at different operating conditions: (a) Ring form and (b) Kaldnes-3.

Please cite this article as: A.A.L. Zinatizadeh, E. Ghaytooli, Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.02.034

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Ef f luent N O3 c onc net rat ion, m g/ l

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(b)

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101

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93.5

TSS removal, %

TSS removal, %

Fig. 8. Three dimensional plot for effluent nitrate: (a) with Ring form and (b) with Kaldnes-3.

74

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Fig. 9. Three dimensional plot for TSS removal efficiency: (a) with Ring form and (b) with Kaldnes-3.

grown on the packing is exposed to a higher DO concentration compared to the Ring form [39].

3.2.5. TSS removal efficiency Since the biofilm sloughing in the attached systems causes occasional instability, total suspended solids were measured as an indicator for the system stability. Fig. 9a and b shows TSS removal efficiency as a function of the numerical variables for two systems, Ring form and Kaldnes-3. From the model obtained for the response (Table 5), A, B, B2 and AB showed a decreasing impact while C had an increasing effect. This is confirmed with the trend of changes in the response in the figures. By decreasing DO, the biomass concentration was decreased which was attributed to more biofilm sloughing. As the sloughing increases, the microbial flocs are formed and the settling of the suspended solids in the settling tank is improved, increasing TSS removal efficiency. In a similar research, treating slaughterhouse in a MBBR, TSS removal efficiency after settling tank was 97% [40]. In another research, treating domestic wastewater, a value of 76.5% was reported for the response [41].

3.2.6. Effluent turbidity Turbidity is an indicator for the system stability. The model obtained to describe the variation of effluent turbidity (Table 5) shows that the response is a function of A, C, AB and A2 . Fig. 10a and b depicts the effluent turbidity for Ring form and Kaldnes-3, respectively. As is observed in the figures, the response for Ring form was larger than that of the Kaldnes-3, which might be due to the Ring form structure.

3.2.7. TP removal efficiency Biological phosphorous removal process is done in two steps; anaerobic and aerobic, releasing phosphorous and accumulating poly hydroxy butyrate (PHB) by phosphate accumulating organisms (PAOs) in the anaerobic zone and absorb in phosphorous in the aerobic zone [15]. This must be noted that in the attached systems like MBBR, TP removal cannot be controlled as the SRT in such systems is very long and the biomass discharge is not controllable. The too long SRT in the attached growth systems causes PAOs accumulation in the system and subsequently occasional phosphorus release at effluent even more than the influent level. However, as different

Please cite this article as: A.A.L. Zinatizadeh, E. Ghaytooli, Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.02.034

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12

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TP removal, %

Fig. 10. Three dimensional plot for effluent turbidity: (a) with Ring form and (b) with Kaldnes-3.

9

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Fig. 11. Three dimensional plot for TP removal efficiency: (a) with Ring form and (b) with Kaldnes-3.

operating conditions were studied in the MBBR and in some cases local anerobic conditions are created, therefore, the TP removal efficiency was monitored to assess the system performance in terms of phosphorus removal. Negative values obtained for the response prove the aforementioned point. According to Fig. 11a and b, the maximum response was found at the low DO and HRT, indicating anaerobic– aerobic environment is provided at this condition. In a similar work carried out by Wang and his colleague, TP removal in a MBBR with municipal wastewater was reported 9–14% [7]. 3.3. Process optimization Graphical optimization was used to explore optimum conditions in the design space of the variables studied. Graphical optimization produces an overlay plot of the contour graphs to display the area of feasible response values in the factor space. The shaded area in the overlay plots is the region that meets the proposed criteria. As presented in Fig. 12, the process optimization was performed in three

steps with different criteria of the process responses (COD, TSS, and TN removal efficiency and effluent turbidity). The values determined as the criteria are given in Table 7. Comparison of Fig. 12a and b with Fig. 12c and d shows that with a decrease in the effluent turbidity from 15 to 10 NTU, MBBR with Ring form (c) could not meet the criteria determined as the optimum conditions. While for the system with Kaldnes-3 (Fig. 12b and d), an optimum region was found in the HRT of 5–8 h and DO of 2.8–3.5 mg/L. In the third step (Fig. 12e and f), as is seen in the figures, Ring form did not show any optimum region whereas the system with Kaldnes-3, a narrow region in high values of DO > 3.5 mg/L and medium amount of HRT (4–6 h) was obtained as the optimum conditions. 3.4. Biofilm developed on the packing media (morphology of the attached biofilm) Scanning electron microscope (SEM) images were used to study the morphology of the biofilms developed on the packing media (Ring

Please cite this article as: A.A.L. Zinatizadeh, E. Ghaytooli, Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.02.034

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A.A.L. Zinatizadeh, E. Ghaytooli / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–14

Design-Expert® Software

Design-Expert® Software

Overlay Plot

Overlay Plot

Overlay Plot

Overlay Plot

12.00

COD removal, % TSS removal, % TN removal, % Design Points

12.00

COD removal, % TSS removal, % TN removal, % Design Points

TSS removal, %: 80

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Actual Factor C: c = Level 1 of C

10.00

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(a) Design-Expert® Software

TSS removal, %: 80 Effluent Turbidity, NTU: 10

10.00

10.00

Effluent Turbidity, NTU: 10 TSS removal, %: 80

X1 = A: DO concentration, mg/L X2 = B: HRT, h Actual Factor C: c = Level 2 of C

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COD removal, % TSS removal, % TN removal, % Effluent Turbidity, NTU Design Points

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3.50

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(c) Design-Expert® Software

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(f)

Fig. 12. Overlay plot for the process optimization: (a, c, e) Ring form and (b, d, f) Kaldnes-3.

Please cite this article as: A.A.L. Zinatizadeh, E. Ghaytooli, Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.02.034

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A.A.L. Zinatizadeh, E. Ghaytooli / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–14 Table 7 The optimization criteria for chosen response. Figure

Response

Limit

Unit

11a and b

TCOD removal TSS removal TN removal Effluent turbidity TCOD removal TSS removal TN removal Effluent turbidity TCOD removal TSS removal TN removal

>70 >80 >45 <15 >70 >80 >45 <10 >80 >85 >40

% % % NTU % % % NTU % % %

11c and d

11e and f

13

form and Kaldnes-3). The images are taken from the samples after preparation and fixation of the microbial films. Fig. 13 shows the biofilm formed. As is seen in the figures, the filamentous bacteria are the dominant species which have formed a matrix structure of the biofilm. From the images, the microbial population in the system with Kaldnes-3 was more which was confirmed by the biomass concentration data. Since, a part of the raw wastewater content was a colloidal matter and a fraction of the particles was not biodegradable, they are attached to the biofilm and caused a decrease in the biological activity and an increase in the biofilm sloughing. This phenomenon is shown in Fig. 14 for the system operated with Ring form. This finding was a reason for the lower stability of the biofilm in the Ring form compared to the Kaldnes-3.

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 13. SEM images of the biofilm developed on the used packing media (a–c) Kildnes-3, and (d–f) Ringform.

(a)

(b)

Fig. 14. SEM images of the influent suspended solids trapped in the biofilm developed on Ringform media.

Please cite this article as: A.A.L. Zinatizadeh, E. Ghaytooli, Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.02.034

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A.A.L. Zinatizadeh, E. Ghaytooli / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–14

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Please cite this article as: A.A.L. Zinatizadeh, E. Ghaytooli, Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.02.034