A Kinetic Model for Autotrophic Denitrification using Sulphur:Limestone Reactors

Transcription

1 Water Qual. Res. J. Canada, 2003 Volume 38, No. 1, Copyright 2003, CAWQ A Kinetic Model for Autotrophic Denitrification using Sulphur:Limestone Reactors ASHREF DARBI AND THIRUVENKATACHARI VIRARAGHAVAN* Faculty of Engineering, University of Regina, Regina, Saskatchewan, Canada The kinetics of autotrophic denitrification of groundwater by Thiobacillus denitrificans in a sulfur:limestone upflow reactor was examined in order to predict effluent concentrations. Experiments were performed using water containing 60 and 90 mg NO 3 N/L and sulfur and limestone with average particle size of 3.5 mm. Results clearly showed that nitrate was completely removed from 60 and 90 mg NO 3 N/L influent concentrations. The results showed that the autotrophic denitrification rates in sulfur:limestone reactors can be described by half-order kinetics. The half-order reaction rate constants for the entire media were estimated at 1.34 and 1.54 mg 1/2 /L 1/2 h for influent concentrations of 60 and 90 mg NO 3 N/L, respectively. Key words: autotrophic denitrification, nitrate removal, Thiobacillus denitrificans, sulfur, limestone, drinking water, kinetics Introduction Numerous water agencies face problems of increasing concentrations of nitrate in groundwater. The main reason for increasing nitrate concentrations in groundwater is the extensive application of artificial fertilizers and animal manure in agriculture. The contamination of groundwater by excessive concentrations of nitrate is a significant public health problem. Infant methemoglobinemia is known to occur when nitrate-nitrogen in drinking water exceeds 10 mg/l (Packham 1992). The Canadian drinking water standard for nitrate is 10 mg/l as nitrate-nitrogen (Guidelines for Canadian Drinking Water Quality 1996). A wide range of physico-chemical processes such as ion exchange, reverse osmosis and electrodialysis, biological denitrification and chemical denitrification, are currently in use or under development for the removal of nitrates from drinking water (Kapoor and Viraraghavan 1997). Methanol is often added for nitrate removal by heterotrophic denitrification since the availability of organic carbon is limited in drinking water (Hoek et al. 1987). As an alternative, autotrophic denitrification using Thiobacillus denitrificans can reduce nitrate to nitrogen gas while oxidizing elemental sulfur to sulfate. Limestone is used to maintain the ph and provide an inorganic carbon * Corresponding author; t.viraraghavan@uregina.ca

2 184 DARBI AND VIRARAGHAVAN source for the bacteria. The reaction proceeds as follows (Batchelor and Lawrence 1978; Schippers and Kruithof 1987): 55S + 50NO H 2 O + 20CO 2 + 4NH + 4 4C 5 H 7 O 2 N + 25N SO H + (1) Further practical application of autotrophic denitrification requires a kinetic model. Data on concentration profiles available from experiments made by Sutton (1973) at 20 C showed that the data fitted the half-order reaction and gave volume removal rates in the range of 0.08 to 0.14 mg 1/2 /L 1/2 min and surface removal rates between 1.3 and 2.2 x 10-3 mg 1/2 /L 1/2 min for 13-mm ceramic intalox saddles. The scatter of the data at other temperatures made conclusions on the order of reaction difficult (Harremoës 1976). Harremoës and Riemer (1975) examined a large number of profiles with different hydraulic loadings on a pilot-scale downflow filter with a 3- to 4-mm gravel medium. When the removal rate per unit volume was plotted against the nitrate concentration, they found that a half-order reaction fitted the data well. Batchelor and Lawrence (1987) developed a kinetic model to describe continuous flow reactors with elemental sulfur biomass slurries. LeCloirec (1985) developed a mathematical model for denitrification by Thiobacillus denitrificans in a sulfur:limestone (S:L) reactor. However, none of these models are easily applicable to predict nitrate profiles and effluent nitrate concentrations in the upflow sulfur:limestone reactor. Koenig and Liu (2001) conducted experiments using autotrophic denitrification of synthetic wastewater by Thiobacillus denitrificans in upflow sulfur packed-bed reactors. The experimental results showed that autotrophic denitrification by the biofilm in the reactors can be described by a half-order kinetic model. A batch study made by Koenig and Liu (2002) showed that in the case of autotrophic denitrification with sufficient alkalinity (initial alkalinity of 680 mg/l as CaCO 3 ), nitrate concentration decreased linearly with time and autotrophic denitrification followed first-order kinetics. The objectives of this study were to examine the use of a suitable kinetic model for upflow sulfur:limestone reactors, and to determine the kinetic constants for reactors with different nitrate-nitrogen concentrations and compare the results with other studies. Thiobacillus denitrificans Culture Materials and Methods Thiobacillus denitrificans (ATCC 23642) was grown in a medium as described by Lampe and Zhang (1996). The composition of the medium was 6 g/l Na 2 S 2 O 3.5H 2 O, 3 g/l KNO 3, 1.5 g/l NaHCO 3, 1.5 g/l Na 2 HPO 4, 0.3 g/l KH 2 PO 4, 0.4 g/l MgSO 4.7H 2 O and 1 ml/l trace nutrient solution. The composition of the trace nutrient solution was

3 SULPHUR: LIMESTONE REACTORS mg/l K 2 HPO 4, 5.74 mg/l NH 4 Cl, 1 mg/l MgCl 2.6H 2 O, 1 mg/l MnSO 4.H 2 O, 1 mg/l CaCl 2 and 1 mg/l FeCl 2.6H 2 O. The stock culture was inoculated into 1 L of medium, flushed with nitrogen and incubated at room temperature for 7 to 14 d. Column Studies A(2:1) sulfur:limestone (mass/mass) reactor (Fig. 1) was used in the column study. This same reactor had a nitrate removal rate of approximately 100% at the end of one year of operation. The column was under continuous operation with a flow rate of 2 ml/min. The results showed that the nitrate removal efficiencies were almost exactly the same over a period of six months, indicating that the reactor had reached steady-state conditions. Earlier detailed column studies have shown that 2:1 (mass/mass) is the optimum S:L ratio; under a hydraulic retention time (HRT) of 13 h, nitrate concentration of 60 mg NO 3 N/L was reduced to less than 5 mg NO 3 N/L (Darbi et al., Unpublished). Tap water was used to prepare two different initial nitrate concentrations (60 and 90 mg NO 3 N/L); the sulfur and limestone particles both had a mean particle size of 3.5 mm. Characteristics of the tap water are shown in Table 1. The reactor was fed continuously in the upflow mode by peristaltic pumps. All experiments were conducted at room temperature of 21 ± 1 C. Samples were collected for analysis from the influent, the three sampling ports and the effluent. Fig. 1. Upflow fixed-bed column reactor.

4 186 DARBI AND VIRARAGHAVAN Table 1. Tap water characteristics Parameters Tap water ph 7.5 Conductivity, µs/cm 534 NO 3 N, mg/l 0 NO 2 N, mg/l 0 Cl -, mg/l 18 SO 4, mg/l Hardness, mg/l as CaCO Alkalinity, mg/l as CaCO TDS, mg/l 230 Specific surface areas of sulfur and particles were determined using a Flowsorb II 2300 manufactured by Micromeritics Instrument Corporation, Georgia, U.S.A. Single point surface area measurements were carried out using a gas mixture of 29.0 M % nitrogen and 71.0 M % helium. Liquid nitrogen was used to set the temperature for adsorption of nitrogen gas by the samples. Specific gravity and porosity of the media were determined to be 2.03 and 0.42, respectively, using methods described in American Society for Testing and Materials (ASTM 1991). Kinetics As the Monod saturation constant (Ks) for autotrophic denitrification was reported to be low (Claus and Kutzner 1985; Batchelor and Lawrence 1987), the intrinsic reaction inside the biofilm can be taken as zero-order. Since the penetration of substrate in the pores of biofilm is less than fully effective, a zero-order reaction in the biofilm becomes a half-order reaction at the surface of biofilm (Koenig and Liu 2001). Assuming the filter as a plug flow reactor: C A = rv Y Q (2) where C is concentration, Y is distance from the entrance, Q is flow rate, A is cross-sectional area, and r v is removal rate per unit volume of the filter. A partially efficient biofilm will result in a half-order reaction: r v = k 1/2 v C 1/2 (3)

5 SULPHUR: LIMESTONE REACTORS 187 where k 1/2 v is the half-order reaction constant per unit volume of the reactor. C = kc 1/2 t (4) (5) 1/2 1/2 Ce = C k 1/2 v t H (6) where C e is effluent concentration, C 0 is influent concentration, and t H is the empty bed residence time = AH/Q (H = height of reactor). The profile should be a straight line in a plot of C 1/2 versus t H. The half-order reaction rate per unit volume can be calculated from the slope of the line. The specific surface area of the filter media is ω which was estimated by multiplying the specific surface area of the particles by the factor (1-ε) where ε is the porosity of the reactor. k 1/2 v = ω k 1/2 a (7) Where k 1/2 a is the half-order surface reaction rate of the biofilm and ω the specific surface area of the reactor media. Results and Discussion Column Studies The column contained sulfur and limestone media with a ratio of 2/1 mass/mass and all kinetic constants calculated were for all the media. The limestone provided buffering capacity and was the major inorganic carbon source for the S:L system (Flere and Zhang 1999). Figure 2 shows clearly that nitrate was completely removed under both initial concentrations of 60 and 90 mg NO 3 N/L. The figure shows the reduction of nitrate concentration as a function of residence time. A plot of square root of nitrate concentration versus empty bed residence times shows a straight line. Plotting C in mg 1/2 /L and T in h gives the following dimensions for the volume rate k 1/2v = mg 1/2 /L 1/2 h. Figure 3 provides a plot between the square root of nitrate concentration and HRT using the data from Fig. 2. The relationship was found to be linear, with a correlation coefficient of nearly 1.0. Using equation 7 the half-order reaction rate constants for reactor media can be estimated. Table 2 shows the k 1/2a to be and mg 1/2 /dm 1/2 h for influent concentrations of 60 and 90 mg NO 3 N/L, respectively.

6 188 DARBI AND VIRARAGHAVAN Fig. 2. Nitrate concentration profile through the column for 60 and 90 mg NO 3 N/L. Fig. 3. Square root of nitrate concentration versus HRT for 60 and 90 mg NO 3 N/L.

7 SULPHUR: LIMESTONE REACTORS 189 Table 2. Comparison of half-order reaction rate constants for autotrophic denitrification a Type of Medium k1/2v ω k1/2a Refer. medium size, mm Porosity mg 1/2 /L 1:2 h dm 2 /dm 3 mg 1/2 /dm 1/2 h T C This study S+L ±1 This study S ±1 Koenig and Liu (2001) S S S Koenig and Liu (1997) S a A S:L ratio of 2:1 by mass would correspond to a S:L ratio by volume of about 2.5:1, i.e., 71% of the media volume consists of sulfur. Ak1/2v value of 1.45 for the mixed column would therefore correspond to a k1/2v value of 2.17 on the basis of a 100% sulfur column.

8 190 DARBI AND VIRARAGHAVAN The plots shown in Fig. 3 clearly demonstrate the applicability of the half-order reaction rate model. Table 2 also shows the data from other sources in the literature. The results from this study are comparable to literature values where these rates are defined in terms of surface. In this study, the reactor contained sulfur and limestone with a ratio of 2:1 (mass/mass); in other studies, the medium was only sulfur. The concentrations used in this study were 60 and 90 mg NO 3 N/L, while in the Koenig and Liu (2001) studies, the concentrations were different and the porosity in this study was slightly different. Consequently, different half-order reaction rate constants were obtained. The theoretical requirement of alkalinity for complete denitrification based on equation 1 is 427 mg/l as CaCO 3. The autotrophic denitrification with sufficient alkalinity (680 mg/l as CaCO 3 ) followed first-order kinetics (Liu and Koenig 2002). When the initial alkalinity decreased to 217 and 113 mg/l as CaCO 3 the slopes of nitrate profiles became lower indicating a gradual decrease in the denitrification rates (Liu and Koenig 2002). In these experiments, the result showed that the alkalinity in the raw water was 145 mg/l as CaCO 3 and increased to 196 mg/l as CaCO 3 after it passed through the bioreactor. These levels of alkalinity were insufficient to allow complete denitrification in the system. Based on calculations using data from other sources in the literature with filter sizes ranging from laboratory to full scale, media size from 3 to 28 mm, porosity from 0.4 to 0.9, with different shapes and with temperature from 6 to 27 C, surface rates varied from 1 to 10 x 10-3 mg 1/2 /dm 1/2 min. This indicates that all filters functioned on the same principle: zeroorder reaction in pores that are only partly effective, resulting in overall half-order reaction kinetics (Harremoës 1976). Based on the analysis of the data with zero-, half- and first-order reaction kinetics, it was also found that the half-order reaction rate fitted the data well and the correlation coefficient was higher in this case than with the other reaction orders (Table 3). Conclusions The half-order reaction model can be applied to describe a sulfur:limestone autotrophic denitrification system. Nitrate concentra- Table 3. Correlation coefficient of the reaction rates Zero order Half order First order Column 60 mg NO 3 N Column 90 mg NO 3 N

9 SULPHUR: LIMESTONE REACTORS 191 tions of autotrophic denitrification using sulfur:limestone reactor can be estimated using a half-order reaction equation: C e 1/2 = C 0 1/2 1 k 1/2 v t H 2 where k 1/2v for the entire media in the column study is 1.34 and 1.54 mg 1/2 /L 1/2 h for 60 and 90 mg NO 3 N/L, respectively. The half-order reaction rate constant depends on the specific surface area of the reactor media. k 1/2a is and 0.20 mg 1/2 /dm 1/2 h for influent concentrations of 60 and 90 mg NO 3 N/L, respectively, for the column reactor. References American Society for Testing and Materials (ASTM) West Conshohocken, Pennsylvania, USA. Batchelor B, Lawrence AW Autotrophic denitrification using elemental sulfur. JWPCF 50: Claus G, Kutzner HJ Physiology and kinetics of autotrophic denitrification by Thiobacillus denitrificans. Appl. Microbiol. Biotechnol. 22: Darbi A, Viraraghavan T, Butler R, Corkal D. Unpublished. Flere JM, Zhang TC Nitrate removal with sulfur-limestone autotrophic denitrification processes. J. Environ. Eng. 125: Guidelines for Canadian Drinking Water Quality Prepared by Federal- Provincial subcomittee on drinking water of the Federal-Provincial advisory committee on environmental and Occupational Health. Authority of Minister of National Health and Welfare. 6 th ed. Harremoës P, Riemer M Report on pilot experiments on down-flow filter denitrification. Report from Department of Sanitary Engineering, Technical University of Denmark, Lyngby. Harremoës P The significance of pore diffusion to filter denitrification. J. Water Pollut. Control Fed. 48: Hoek JP, van der Klapwijk A Nitrate removal from groundwater. Water Res. 21: Lampe DG, Zhang TC Evaluation of sulfur-based autotrophic denitrification. Proceeding of the HSRC/WERC Joint Conference on Environment. Albuquerque, N.M., May p Kapoor A, Viraraghavan T Nitrate removal from drinking water-review. J. Environ. Eng. 123: Koenig A, Liu LH Autotrophic denitrification of nitrified leachate in sulphur packed-bed reactors, p In S. Margherita di Pula (ed.), Proceedings SARDINIA 97 Sixth International Landfill Symposium, Cagliari, Italy, October. Koenig A, Liu LH Kinetic model of autotrophic denitrification in sulphur packed-bed reactors. Water Res. 35: Koenig A, Liu LH Use of limestone for ph control in autotrophic denitrification: batch experiments. Process Biochem. 37:

10 192 DARBI AND VIRARAGHAVAN LeCloirec P Mathematical model of denitrification on sulfur-calcium carbonate filters. Chem. Eng. J. 3:B9 B18. Packham R Public health and regulatory aspects of inorganic nitrogen compounds in drinking water. Water Supply 10:1 6. Schippers CJ, Kruithof CJ Removal of nitrate by slow sulfur/limestone filtration. Aqua 5: Sutton PM Continuous biological denitrification of wastewater. M. Eng. Thesis, McMaster University, Hamilton, Ontario, Canada.