Indian Journal of Chemical Technology Vol. 19, January 2012, pp. 32-36
Continuous removal of chromium from tannery wastewater using activated sludge process—Determination of kinetic parameters Debabrata Bera, Parimal Chattopadhyay & Lalitagauri Ray* Department of Food Technology & Biochemical Engineering, Jadavpur University, Kolkata 700 032, India Received 10 May 2011; accepted 22 December 2011 The activated sludge has been acclimatized with chromium (VI) ions from 5 mg/L to 20 mg/L. It is observed that the chromium removal percentage, COD removal and mixed liquor suspended solid (MLSS) decrease with the increase in dilution rate. Maximum food to microorganisms ratio (F/M) is found to be 0.19. When initial metal ion concentration increases from 5 mg/L to 20 mg/L, the chromium removal decreases from 95.58% to 63.85% (dilution rate 0.041h-1). Keywords: Activated sludge process, Biosorption, Chromium, Kinetic parameters, Tannery wastewater
Environmental pollution due to development in technology and structural shift towards increased industrialization is one of the important problems of this century1. The presence of toxic heavy metal in aqueous streams, arising from the discharge of untreated metal containing effluents into water bodies, is one of the most important environmental issues2. Chromium is one of the heavy metal contaminants which is present in trivalent and hexa-valent forms. Hexa-valent chromium is more toxic than trivalent forms and carcinogenic also3-6. Tannery waste contains 80-250 mg/L chromium, whereas safe value for drinking purpose is 0.05 mg/L and recommended value for discharge7 is less than 5 mg/L. The metallic species are non biodegradable and therefore persist indefinitely, accumulating in living tissues throughout the food chain8. Conventional methods9,10 for removal of heavy metals from wastewater have significant disadvantages including incomplete metal removal, need for expensive equipment and monitoring systems, high reagents, and energy requirements11-13. Biosorption is an alternative technique for heavy metal removal. The search for alternative and innovative treatment techniques has focused attention on use of biological materials such as algae, fungi, yeast and bacteria for removal and recovery technologies and has gained importance during recent years because of the better performance and low cost ________ *Corresponding author. E-mail:
[email protected]
of these biological materials14-16. A few investigators used living organisms for this purpose17-19. Among bioremediation processes, activated sludge system is the superior one as physical parameters except temperature are not acute problem. This cheap and continuous process also reduces BOD and COD from large amount of industrial wastes. The present study deals with the removal of Cr(VI) and reduction of COD of synthetic tannery wastewater as well as original tannery wastewater by activated sludge reactor and determination of biokinetic parameters which are valuable in designing and scaling up of activated sludge reactor. Experimental Procedure Organism
A mutated bacterial strain Bacillus cereus M116 isolated and identified in our laboratory was used for the present study20,21. The strain was maintained on nutrient agar at 30°C for 24 h and stored at 4°C. The inoculum medium was composed of beef extract 1.0 g/L, yeast extract 2.0 g/L, peptone 5.0 g/L, and NaCl 5.0 g/L and pH 6.0. One loopful of Bacillus cereus M116 was transferred in several flasks each containing 50 mL medium in 250 mL Erlenmeyer flasks and incubated at 30°C, 120 rpm for 24 h. After 24 h fermentation, broth containing the biomass was used as seed culture for the activated sludge reactor. Wastewater source
The synthetic wastewater containing Cr(VI) ion range 5-20 mg/L in the tap water and original tannery
BERA et al.: CONTINUOUS REMOVAL OF CHROMIUM FROM TANNERY WASTEWATER
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wastewater, collected from tannery near Park Circus, Kolkata, were used for the present study. Bench scale continuous reactor
The reactor was made of polyacrylamide (Perplex) sheet and divided into two chambers, viz. the aeration chamber and settling chamber linked at the bottom. A baffle was placed at the middle position to leave an opening of 1/4 inch at the bottom. Working capacity of the reactor was 25 L (Reactor size 30 inch × 30 inch × 30 inch). Air was introduced through a Sparger. Wastewater was fed continuously from a constant head feed reservoir by means of gravity and overflow in the effluent reactor22. The flow rate of the inlet to the reactor was controlled. Schematic diagram of the reactor is shown in Fig. 1. No recycle of the sludge was provided in the experiment.
Fig. 1—Schematic diagram of continuous stirred tank reactor (not to the scale)
Seed acclimatization
The seed (24 h cell suspension of Bacillus cereus M116 having cell concentration 1.6 g/L) was added in the reactor in 5% (v/v) concentration (i.e. 1250 mL of cell suspension with 5 mg/L initial Cr(VI) concentration). Some nutrients, viz dextrose 2.0%, (NH4)2HPO4 0.1% and sodium nitrate 0.02%, were added on each alternate day as a source of carbon, nitrogen and phosphorus required for survival of organism. Then from second day, mixed liquor suspended solid (MLSS) was estimated daily. For a given experiment, a constant concentration of MLSS was maintained by adjusting MLSS concentration daily and withdrawing calculated amount of sludge when required. Treatment procedure for synthetic waste
The influent wastewater was allowed to flow to the aeration chamber from a reservoir vessel by gravity at a flow rate necessary to obtain the desired residence time in the aeration chamber. Flow rate was controlled using needle valve. The reactor was operated until steady state conditions were achieved. Attainment of steady state was assumed when COD of effluent became stable for a particular influent metal ion concentration at a particular flow rate. Cr(VI) ion concentration in influent was varied in the range 5-20 mg/L, whereas dilution rates were maintained at 0.014, 0.021 and 0.041 h-1. The pH of the activated sludge reactor was kept in the range 6.5-7.0 throughout the experiment. A synthetic wastewater containing Cr(VI) ion (5-20 mg/L) was used as the feed solution. COD of the influent was
maintained at 820 mg/L. Potassium dichromate solution was used as the source of Cr(VI). Treatment procedure for original tannery wastewater
An industrial tannery wastewater used in this study was collected from tannery near Tiljala, Kolkata, India. The original tannery wastewater was filtered to remove the insoluble materials present in the wastewater. The COD and chromium concentration of the wastewater were determined and found to be 6400 mg/L and 80 mg/L respectively. To maintain the COD value at 800 mg/L and chromium ion concentration at 10 mg/L original waste water was diluted to 8 fold before performing the experiments in activated sludge system. The activated sludge reactor was operated at dilution rate 0.014 h-1, and microorganisms were acclimatized for 12 days. During acclimatization period, samples were collected at definite time period and rate of Cr(VI) removal was determined to calculate the time to reach steady state. After reaching steady state, samples were collected at every 24 h and analyzed. Analytical method
Flow rate was measured by collecting liquid at certain time interval to find out the volume collected per unit time. MLSS and COD were measured following standard methods23. The effluent sample was collected and centrifuged at 5500 rpm for 15 min and clear supernatant was used for the determination of COD and concentration of chromium using atomic absorption spectrophotometer (Chemito Technologies
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INDIAN J. CHEM. TECHNOL., JANUARY 2012
Pvt. Ltd. India, Model No.AA-203, wave length 357 nm, slit width 0.5 nm). Results and Discussion The experiments were been performed at an average daily temperature of 30°-32°C and MLSS range of 4325-5119 mg/L. During the operation, pH of the effluent was nearly constant (6.5-7.0). The influent characteristics were as follows: average COD 820 mg/L, pH 6.5 and temperature 30-32°C. Performance summery
Cr(VI) uptake by microorganisms from influent containing different concentrations of Cr(VI) was calculated separately using the following equation: % Chromium uptake = [(C0-C) / C0 ] × 100 where C0 is the Cr(VI) concentration (mg/L) in influent; and C, the Cr(VI) concentration (mg/L) in effluent. Activated sludge system
The reactor containing tap water (23.75 L) having some dissolved nutrients as mentioned above was inoculated with 1250 mL (5%) cell suspension ( 24 h growth) of Bacillus cereus M116 having 1.6 g biomass/L. MLSS of the sludge system was estimated from second day of operation. Nutrients were added at alternate day upto ninth day. After ninth day of operation MLSS was found to be stable at a range of 4325-5199 mg/L (observed upto twelfth day). Then synthetic wastewater containing 5 mg/L Cr(VI) ion was supplied to the activated sludge reactor at a dilution rate of 0.014 h-1. COD removal efficiency and percentage of metal removal were estimated each day. After ninth day it becomes stable (94%) and the same efficiency was observed upto twelfth day. Then the synthetic wastewater containing 10 mg/L Cr(VI) ion was supplied to the same reactor at the same flow rate. COD removal efficiency and percentage of metal removal were determined daily. It became stable on ninth day and the operation was continued upto twelfth day. Maximum Cr(VI) removal efficiency was found to be 84%. After that in the same way experiment was carried out with synthetic wastewater containing 20 mg/L Cr(VI) ion. 72% metal removal was observed. The activated sludge reactor worked (after stabilization of MLSS) continuously for 36 days. The same experiment was performed by varying the flow rate (viz. at the dilution rate of 0.021 and
0.041 h-1) separately and the effect of flow rate using different Cr(VI) ion concentration (viz. 5, 10, 20 mg/L) on metal ion removal efficiency was also observed. Chromium (VI) removal was highly dependant on different dilution rate. At higher dilution rate of 0.041h-1, Cr(VI) uptake was very low due to the less contact of metal ion with biomass. Another possible reason for the reduction of sorption capacity could be the changed ion-sorption properties of microorganisms established at the higher dilution rates. It has been suggested that the growth conditions are important in the production of metal-complexing proteins and exo-polymers which are known to be effective in the binding of heavy metals on sludge surface. Reduction in sorption capacity with the increase in dilution rate was also attributed to a possible change in bio-kinetic properties of microorganisms which is in accordance with lower COD and Cr(VI) removals observed at higher dilution rates. Furthermore the reactor was not stable enough even at steady state, indicating the possible wash out of some microorganisms from the reactor resulting in a lower MLSS concentration. At the same time effluent COD decreased exponentially with the increase in hydraulic retention time. In Fig. 2, specific substrate removal rate (q) was plotted against effluent substrate concentration at different initial metal ion concentration (5, 10 and 20 mg/L). The intercept on X axis represents non biodegradable portion of substrate (Sn). The values were found to be 138.29 (16.86%), 301.88 (36.81%) and 257.12 (31.36%), since COD was utilized as a measure of substrate concentration. In case of original tannery waste the value was 250 (31.25%). Chen and
Fig. 2—Plot of specific substrate utilization rate vs. effluent COD at different metal ion concentrations
BERA et al.: CONTINUOUS REMOVAL OF CHROMIUM FROM TANNERY WASTEWATER
Hasimoto24 found that 58.4% of the dairy waste was not biodegradable which was consistent with long term digestion. They also found that non degradable portion of substrate was 36.2% for dairy waste. In Fig. 3, 1/q was plotted against 1/(Se-S0). This is a Lineweaver plot which follows the following equation: 1/q = Ks / qmax. 1/(Se-S0) + 1/qmax where Ks is the half velocity constant (mg/L); qmax maximum substrate utilization rate; and Se and S0, the equilibrium and initial substrate concentrations (mg/L). From the slope and intercept, Ks and qmax were calculated. The values are found to be Ks = 7017.2, 526.19 and 53.68 mg/L and qmax = 4.19, 0.539 and 0.22 day-1 respectively for C0 of 5 , 10 and 20 mg/L respectively. At the same time for industrial wastewater, these values were 3832.68 mg/L and 0.295 day-1 respectively. In Fig. 4, specific substrate utilization rate (q) was plotted against specific growth rate (µ). Yield coefficient (Y) and endogenous decay coefficient (Kd) were calculated using the equation µ = Yq - Kd. The values were Y = 0.835, 0.77 and 0.64 mg MLSS/mg substrate and Kd = 0.15, 0.02 and 0.024 day-1 respectively for C0 of 5, 10 and 20 mg/L. From the value of Y and qmax, maximum growth rate can be predicted using the equation µmax = Y.qmax. The values of µmax from the study were obtained as 3.498, 0.415 and 0.14 day-1 for C0 of 5, 10 and 20 mg/L respectively. Using original tannery wastewater, Y, Kd and µx were found to be 0.915 mg MLVSS/mg substrate,
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0.014 day-1 and 0.269 day-1 respectively (Fig. 5). In case of Monod model, the constants µmax, Ks and Y are required. It may be concluded from the present study that precise consistency is not the characteristics of heterogeneous populations. In Fig. 6, per cent removal of COD was plotted against F/M ratio. It was observed that with the decrease in F/M ratio, per cent COD removal is increased upto a certain level. About 75% removal (maximum) was observed at F/M ratio of 0.04, when C0 was 5 mg/L. When tannery wastewater was used, per cent removal of COD decreased with F/M ratio after 0.045. A maximum 65.25% COD removal was possible at F/M ratio of 0.045 when initial chromium concentration was 10 mg/L. Cr(VI) concentration decreased from 10 mg/L to 1.8 mg/L in 24 h. Orozco et al.25 reported hexavalent chromium removal using aerobic activated sludge systems added with powdered activated carbon. With the increase in initial Cr(VI) concentration, Cr(VI) removal rate and
Fig. 4—Effect of specific growth rate on specific substrate removal rate
Fig. 3—Plot of 1/q vs (Se – Sn ) at 0.014 h-1 dilution rate
Fig. 5—Specific growth rate vs specific substrate removal rate (original tannery wastewater)
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References 1 2 3 4
5 6 7
8 Fig. 6—Effect of F/M on COD removal efficiency at 0.014 dilution rate
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RE value decreased reflecting loss of metabolic activity of the activated sludge due to toxicity of Cr(VI), however this inhibition was less in systems with activated carbon. Lee et al.26 reported the treatment of Cr(VI) containing wastewater by addition of powdered activated carbon to the activated sludge process. Conclusion (i) In this study, per cent removal of COD decreases with increasing hydraulic retention time and the maximum 65.2% removal is possible. (ii) Maximum 84% Cr(VI) removal is possible when influent metal ion concentration and dilution rate are 10 mg/L and 0.014 h-1. (iii) Ks, qmax, Y & Kd are 7017.2 mg/L, 4.19 day-1, 0.835 mg MLVSS/mg substrate and 0.15 day-1 respectively for synthetic tannery waste at 0.014 h-1. The respective values for original tannery wastewater are 832.68 mg/L, 0.295 day-1, 0.915 mg MLVSS/mg substrate and 0.014 day-1.
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Acknowledgement Authors gratefully acknowledge University Grants Commission, Govt. of India for financial support to carry out the research work.
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