PERFORMANCE PREDICTION OF ROTATING BIOLOGICAL CONTACTOR IN WASTEWATER TREATMENT APPLICATIONS

Transcription

1 PERFORMANCE PREDICTION OF ROTATING BIOLOGICAL CONTACTOR IN WASTEWATER TREATMENT APPLICATIONS Dr. Amer D. Zmat College o Engineering AL-Qadisiya University Dr. Ali H. GHAWI College o Engineering AL-Qadisiya University Abstract: Bioilm models were used to model and predict the perormance o Rotating Biological Contactors (RBC) in wastewater treatment plants. Assessment techniques have been adopted to evaluate this method in terms o hydraulic loading and Biochemical Oxygen Demand (BOD) removal. The results revealed that RBC not only is eicient, but also proven to be odor controlling reactor. RBC provides eicient mixing and reduces time o retention to minimum, so that a great reduction o area o treatment plants will be achieved. Key word: BOD, COD, organic material, wastewater, bioilm. تخمين الا داء للملامسات البايولوجية الدوارة في محطات معالجة المياه الثقيلة الدآتور عامر دحام زماط آلية الهندسة جامعة القادسية الدآتور علي هادي غاوي آلية الهندسة جامعة القادسية الخلاصة:. ( bio-disks ). BOD. 164

2 Nomenclature A kx a lumped parameter which is equal to D, mg L 1 Aw surace area o the disk, m 2 A ds the slope or z= 0 as expressed to be a linear correlation with S B, m 1 dz B ds the intercept or z= 0 dz as expressed to be a linear correlation with S B, kg m 4 D K i Ks K L Lt Q Ra S S 0 S B Se diusion coeicient o substrate within the bioilm, cm 2 h 1 constant or inhibition, mg L 1 saturation constant, mg L 1 maximum speciic substrate consumption rate, s 1 bioilm thickness, mm length o the reactor, m low rate, m3 d 1 substrate consumption rate by the microbes attached on the disc, kg m 2 s 1 substrate concentration, mg L 1 substrate inluent concentration, mg L 1 liquid phase substrate concentration, mg L 1 substrate eluent concentration, mg L 1 Introduction Rotating biological contactor (RBC) has been widely used or the secondary treatment o domestic and industrial wastewaters. The RBC consists o a series discs attached to a common shat. The wastewater is ed into the contactor with a certain low rate. All the discs are partially submerged into the wastewater. When the discs are continuously rotated by the shat, a subsequent submergence and exposure to atmosphere will take place. Thus, the microbial ilm on the disc that is initially in contact with the nutrients o the wastewater phase and then with the oxygen in the atmosphere. Hence, the organic compounds in the wastewater would serve as the nutrients or the microbes to digest and grow. By such periodical operation, the microbes would grow and a certain sludge ilm thickness would be obtained. The RBC is used because o its advantages such as high speciic surace area, high activated sludge concentration, better sludge settling, process stability, and low maintenance and power consumption. Beneield and Randall [1] described the design o the biological treatment processes or wastewaters [1]. Two pilot-scale RBCs with PE discs arranged in our stages were used [2] or the treatment o 2-nitrophenol or 2-chlorophenol contained synthetic wastewater. Opatken and Bond [3] treated a leachate with high concentration o ammonianitrogen ( mg/l) by the nitriication process with a pilot-scale RBC. Dierent experiments were conducted to determine the operating parameters o the RBC treatment system. Buisman et al. [4] compared three dierent bioreactor systems or the removal o sulphide containing wastewater. 165

3 Stirred reactor, biorotor reactor and uplow reactor were used or comparison. The biomass support materials o Rasching rings and polyurethane were also compared. It is obvious that the bioilm disc is very important or the RBC perormance. Brower and Barord [5] introduced dierent biological ixedilm systems in their report [5]. In 1978, a theoretical model or RBC systems was provided so that process design criteria or a pilotplant RBC process can be established and be compared with the activated sludge process [6]. Since RBC is composed o a series o discs with microbial growth in a ilm. The ilm model is certainly important or it. Mass transer problems across the ilm should also be taken into consideration. The steady-state bioilm kinetic models were proposed or both conditions o deep and shallow bioilms [7]. In the adopted model, mainly Monod growth model with substrate diusion along single dimension was assumed. The ilm was assumed to be o a planar orm. The substrate lux was also discussed. The deep bioilm model with Monod assumption or completely mixed and plug low bioilm reactors was analyzed [8]. Such bioilm reactors were ound to be extremely sensitive to the surace parameters. Models are based on Monod growth kinetics. Activated sludge reactors and rotating biological contactors demonstrated that both suspended and attached growths can be eective methods or treating the wastewater as described above. Sagy and Kott [9] examined ecal coliorm bacteria and Salmonella typhimurium die-o in an experimental RBC which received settled domestic sewage rom the city main sewer. The behavior o the microbes on the bioilm is airy important. Most bioilms are ormed by mixed species. Gupta and Gupta used a mixed culture aerobic bioilm to remove carbon and nitrogen rom a synthetic domestic sewage [10]. RBC is an essential treatment process or treating industrial wastewater. Meanwhile, it is also a very interesting system or theoretical analysis. Hence, in this work, we established the model or the bioilm system based on both Mond and substrate inhibition mechanisms. With the models, the operation conditions as well as the number o the bioilm discs can be analyzed Developed O Mathematical Model The bioilm reactor being considered has a length o L t and a low direction parallel to the horizontal axis as shown in Fig. 1. The schematic representation o a bioilm is shown in Fig. 2. It was assumed that the luid was well-mixed in the radial direction and the substrate consumption rom the suspended microbial can be negligible when compared to that o the attached bioilm. Consider an ininitesimal volume in a bioilm reactor, by the mass balance or a shell, (1) where V t is the volume o wastewater in the bioilm reactor, L t is the length o the reactor, R a is the substrate consumption rate by the microbial attached on the disc, S B is liquid phase substrate concentration, and Q is the low rate. I we divide both sides o equation 1 by y, we obtain 166

4 (2) I the substrate consumption rate by the attached microbial on the disc is equal to the substrate diusion rate at the surace o the ilm, then, (3) where D is the diusion coeicient o substrate within the bioilm, S is the substrate concentration in the bioilm, and z is the axis along the bioilm thickness. Under steady state assumption (4) The ollowing boundary conditions were implemented to solve equation The boundary conditions in case o thick bioilm: For the case o thick bioilm with Monod kinetics µ = µ m S/Ks + S. (5) Where kx A = and k is the maximum speciic substrate consumption rate [t 1 ]. D For the case o the thick ilm with substrate inhibition kinetic (1) I K i /K s < 4, then (6) 167

5 (2) I K i /K s > 4, then (3) I Ki/Ks = 4, then (7) (8) Substituting the above equations (Eqs. (5)-(8)) into Eq. (4), we obtain the ollowing results, Without substrate inhibition: (9) With substrate inhibition: (1) I K i /K s < 4, (2) I K i /K s > 4, (10) (3) I K i /K s = 4, (11) (12) The above equations are all irst-order ordinary dierential equations. They can be solved or the numerical solution o S B z=l, which is the exit substrate concentration, S e, by the ourthorder Runge- Kutta method. 168

6 2. The boundary conditions in case o thin bioilm: ds S and z= 0 can be solved by inite dierence and the exit substrate concentration S B z=l dz can be urther obtained. Based on the above theorem and assumptions, the eluent substrate concentration can be computed. During the computation process, several operation conditions, which draw inluence on the perormance o the bioilm reactor, such as inluent substrate concentration, length o reactor, total surace area o the bioilm, low rate, numbers o reactors in series, ilm thickness should be provided. The growth and diusion parameters related to the bioilm such as K s, K i, k, X, and D should be given as well. For the thin bioilm, under Monod kinetics assumption, when the substrate concentration in liquid phase is higher, the substrate consumption rate R a would be higher as well. For substrate inhibition kinetics assumption the result is similar, which is higher substrate consumption rate or higher liquid phase substrate concentration. However, when the substrate concentration is too high, substrate inhibition would cause the decreasing o substrate consumption rate. It can be inspected rom Fig. 3 that under certain range o S B, ds ds z=0 (R a ) is a linear unction o S B or the case o thin ilm. Thereore, z= 0 can be dz dz expressed as ds = a S z= 0. B + b dz where a is the slope and b is the intercept. a and b are dependent on the range o S B. Provided that the liquid phase substrate concentrations were S B, 1 and S B, 2, then, (13) (14) (15) 169

7 Combining Eqs. (14) and (15), a and b can then be obtained. Substituting Eq. (13) into Eq. (4), then, (16) (17) Results And Discussion Six stages bioilm reactors or wastewater treatment was considered in the ollowing analysis. Both Monod kinetic and substrate inhibition kinetic were applied in this study. The length o the bioilm reactor was set to be 5.0 m. The total surace area or treatment was 5,000 m2. Kinetic parameters or cell growth were K s = 100 mg/l and X = 20,000 mg/l. The diusivity o the bioilm was D = 0.02 cm 2 /h. Flow directions o the inluent eed were set to be parallel to the horizontal axis o the reactor. For low direction parallel to the axis, the bioilm reactor maybe regarded to be a plug low reactor (PFR). The BOD removal eiciency was evaluated in terms o inlet substrate concentration at dierent hydraulic loadings, as shown in Fig. 4. The results show that higher hydraulic loadings exhibit higher BOD removal eiciency at low inlet substrate concentration whereas, lower hydraulic loadings can result in very low BOD removal eiciency at higher inlet substrate concentration. Fig. 5 shows that increasing the number o bio-disks can steeply reduce the eluent substrate concentration or dierent inlet substrate concentration. These results may be used by designers to determine the required number o disks or speciic inlet and outlet constraints. Fig. 6 shows that law inlet substrate concentration slightly aect the substrate removal eiciency when hydraulic loading increased, while high inlet concentration can cause signiicant drop in the removal eiciency as a threshold value in exceeded. It can be seen rom Fig. 7 that low inlet substrate concentration is very sensitive to the number o bio-disks, whereas, higher inlet concentration show gradual response to the number o bio-disks. The COD removal in terms o hydraulics loadings was compared to BOD removal. Figure 8 below was adopted rom [11] or comparison. The eect o hydraulic loading on COD removal eiciency was also adopted rom [11 ] to be compared with the BOD removal eiciency. Figure 10 shows that dissolved oxygen is steeply declined as the thickness o the bioilm increases. This interprets that the process is diusion-controlled. The submergence ratio o bio-disks was investigated in terms o the dissolved oxygen and number o stages o the process ( Fig. 11). Submergence ratios between 0.4 and 0.6 were ound to appropriate and eective than higher and lower ratios. Conclusion 170

8 Based on Monod kinetics and substrate inhibition kinetics or the bioilm, the model or the substrate removal eiciency o a 6 stage bioilm reactor was established. The inluence o eeding rate, hydraulic loading and organic loading rate (hydraulic retention time) was investigated and discussed. RBC technology was ound to be cost eective in wastewater treatment as it considerably reduces both the size o equipments and the number o unit operations. Reerences [1] Beneield, L. D. and C. W. Randall, Biological Process Design or Wastewater Treatment, Prentice Hall, New York, U.S.A. (1980). [2] Tokuz, R. Y., Biodegradation and Removal o Phenols in Rotating Biological Contactors, Water Sci. Technol., 21(12), 1751 (1989). [3]Opatken, E. J. and J. J. Bond, RBC Nitriication o High Ammonia Leachates, Environ. Prog., 10(1), 60 (1991). [4] Buisman, C. J., B. Wit, and G. Lettinga, Biotechnological Sulphide Removal in Three Polyurethane Carrier Reactors: Stirred Reactor, Biorotor Reactor, and Uplow Reactor, Water Res., 24(2), 245 (1990). [5] Brower, J. B. and C. C. Barord, Biological Fixed-Film Systems, Water Environ. Res., 69, 487 (1997). [6] Clark, J. H., E. M. Moseng, and T. Asano, Perormance o a Rotating Biological Contractor under Varying Wastewater Flow, J. Water Poll. Control Fed., 50, 896 (1978). [7] Rittmann, B. E. and P. L. McCarty, Model o Steady-State-Bioilm Kinetics, Biotechnol. Bioeng., 22(11), 2343 (1980). [8] Suidan, M. T. and Y. T. Wang, Uniied Analysis o Bioilm Kinetics, J. Environ. Eng., 111(5), 634 (1985). [9] Sagy, M. and Y. Kott, Eiciency o Rotating Biological Contractor in Removing Pathogenic Bacteria rom Domestic Sewage, Water Res., 24(9), 1125 (1990). [10] Gupta, A. B. and S. K. Gupta, Simultaneous Carbon and Nitrogen Removal in a Mixed Culture Aerobic RBC Bioilm, Water Res., 33(2), 555 (1999). [11] Kossay K. Al-Ahmady" Eect o Organic Loading on Rotating Biological Contactor Eiciency"Int. J. Environ. Res. Public Health, 2(3), (2005). 171

9 Figure 1. Schematic represent o a bioilm reactors. Figure 2. Schematic representation o a bioilm. Figure 3. Representation o bioilm reactors (Plug Flow Reactor PFR) 172

10 Figure 4: Substrate (BOD) removal eiciency with respect to inlet concentration under dierent low rates and 3 mm bioilm active thicknesses. Figure 5: Eluent substrate concentration with respect to number o bioilm disks under dierent substrate inlet concentration 173

11 Figure 6: Eect o hydraulic loadings on the substrate removal eiciency at law and high inlet substrate concentrations. Figure 7: Eect o the number o bio-disks on substrate removal eiciency at dierent inlet 174

12 Figure 8: Eect o hydraulic loading on % COD removal. Figure 9: Eect o organic loading on the COD removal eiciency 175

13 Figure 10: Concentration proiles o organic substrate, and dissolved oxygen inside the bioilm in RBC stages Figure11: The DO concentration in bulk liquid under varying submergence ratio in RBC stages 176