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Effect of influent pH and alkalinity on the removal of chlorophenols in sequential anaerobic–aerobic reactors
Bioresource Technology 100 (2009) 1881–1883
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Effect of influent pH and alkalinity on the removal of chlorophenols in sequential anaerobic–aerobic reactors Partha Sarathi Majumder, S.K. Gupta * Centre for Environmental Science and Engineering, Indian Institute of Technology, Powai, Mumbai 400 076, India
a r t i c l e
i n f o
Article history: Received 13 August 2008 Received in revised form 2 October 2008 Accepted 3 October 2008 Available online 18 November 2008 Keywords: 2-Chlorophenol 2,4-Dichlorophenol Upflow anaerobic sludge blanket Rotating biological contactor
a b s t r a c t This study was carried out to determine the effect of influent pH and alkalinity on the performance of sequential UASB and RBC reactors for the removal of 2-CP and 2,4-DCP from two different simulated wastewaters. The performance of methanogens at low (<6.0) to high (>8.0) pH values and at sufficiently high alkalinity (1500–3500 mg/l as CaCO3) is described in this paper. Sequential reactors were capable of handling wastewaters with influent pH, 5.5–8.5. However, with influent pH 7.0 ± 0.1 UASB reactor showed best performance for 2-CP (99%) and 2,4-DCP (88%) removals. Increase in alkalinity/COD ratio in the influent (>1.1) caused gradual decrease in the chlorophenol removal in UASB reactors. The UASB reactors could not tolerate wastewater with higher alkalinity/COD ratio (2.6) and showed significant deterioration of its performance in terms of chlorophenols removal achieving only 74.7% 2-CP and 60% 2,4-DCP removals, respectively. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Chlorophenols are extensively used chemicals for the synthesis of pesticides, herbicides and dyes. Recent studies were carried out for the removal of chlorophenols in sequencing batch reactor (Kargi et al., 2005), hydrogenotrophic, gas-permeable, silicone membrane bioreactor (Chang et al., 2003) and fixed bed reactor (Bajaj et al., 2008). In the present study UASB and RBC reactors were chosen as anaerobic and aerobic reactors, respectively for the sequential biological treatment of chlorophenols. The pH and alkalinity have important roles to play in the aerobic and anaerobic biological treatment of wastewater. In aerobic biological oxidation pH in the range of 6.0–9.0 is tolerable (Metcalf and Eddy, 2003). Brás et al. (2005) reported that in anaerobic process limited pH range of 6–8 is required for the growth of methanogens. During aerobic biological oxidation alkalinity has an important role in nitrification–denitrification. During nitrification–denitrification process 3.57 g alkalinity as CaCO3 is consumed per gram of NHþ 4 –N removal (Metcalf and Eddy, 2003). Anaerobic wastewater treatment requires high alkalinity and failure of anaerobic bioreactor takes place in the absence of sufficient alkalinity. Wilcox et al. (1995) reported that for successful and stable operation of anaerobic digester, the required bicarbonate alkalinity should be 1000–3000 mg/l as CaCO3. Several studies have been carried out to assess the performance of anaerobic reactors at
* Corresponding author. Tel.: +91 22 25767853; fax: +91 22 25723480. E-mail addresses: [email protected] (P.S. Majumder), skgupta@iitb. ac.in (S.K. Gupta). 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.10.002
low to moderate alkalinity/COD ratio. The optimum ratios in these studies were 0.4–1.3 (González et al., 1998; Ag˘dag˘ and Sponza, 2005). The investigations on the effect of alkalinity for wastewater treatment in anaerobic–aerobic reactor systems containing chlorophenols and high alkalinity/COD ratio are limited. The present study brings out two important observations about the activity of methanogens at low (<6.0) to high (>8.0) influent pH and at higher alkalinity (alkalinity/COD ratios 1.1–2.6) for the treatment of wastewaters containing chlorophenol. 2. Methods 2.1. Experimental set up Sequential UASB-I and RBC-I and UASB-II and RBC-II reactors were used for the removal of 2-CP and 2,4-DCP, respectively. The details of the dimensions of the reactors are mentioned elsewhere (Majumder and Gupta, 2007). Hydraulic retention time (HRT), F/M ratio and solid retention time (SRT) of UASB-I and UASB-II reactors were 12 h, 0.22 g COD/g VSS d and 188 d, respectively. The HRT, F/ M ratio and SRT of RBC-I reactor was 23 h, 0.16 g COD/g VSS d and 246 d, respectively. The HRT, F/M ratio and SRT of RBC-II reactor was 28.8 h, 0.13 g COD/g VSS d and 102 d, respectively. 2.2. Experimental protocol This study was carried out at three different influent pH values viz. 5.5 ± 0.1, 7.0 ± 0.1 and 8.5 ± 0.1. The feed composition of the UASB reactors was as follows: CH3COONa 3H2O, 3.0 g/l; NH4Cl,
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P.S. Majumder, S.K. Gupta / Bioresource Technology 100 (2009) 1881–1883
91.7 mg/l; K2HPO4, 17.45 mg/l; KH2PO4, 7.45 mg/l; CaCl2 2H2O, 200 mg/l; trace metal solution, 1 ml/l and chlorophenol (2-CP or 2,4-DCP) 30 mg/l. Sodium bicarbonate was not added in the influent of pH 7.0 ± 0.1 and pH 5.5 ± 0.1. Influent of pH 5.5 ± 0.1 and 8.5 ± 0.1 was prepared with the addition of 1.1 N HCl (1.5 ml/l) and 1 N NaOH (1.25 ml/l) + NaHCO3 (300 mg/l) to the normal feed, respectively. The composition of supplementary feed of RBC reactors (2 l/d) was as follows (mg/l): 6800, CH3COONa 3H2O; 900, NH4Cl; 36.23, K2HPO4; 15.53, KH2PO4; 1.0, MgSO4 7H2O; and 2 ml/l trace metal solution. Composition of trace metal solution for UASB and RBC reactors are mentioned elsewhere (Majumder and Gupta, 2007). The performance of sequential reactors was evaluated at different alkalinity to COD ratios viz., 1.1:1, 1.5:1, 2.1:1 and 2.6:1. The feed composition of the UASB reactors was as follows: CH3COONa 3H2O, 3.0 g/l; NH4Cl, 91.7 mg/l; K2HPO4, 17.45 mg/l; KH2PO4, 7.45 mg/l; CaCl2 2H2O, 200 mg/l; NaHCO3, (0.85–5.2) g/l; trace metal solution, 1 ml/l and chlorophenol (2-CP or 2,4-DCP) 30 mg/l. The composition of supplementary feed of RBC reactors was same as used for the study at different pH. Reactors were operated for 25–30 days under pseudo-steady-state condition in both the studies. 2.3. Analytical methods Analysis of COD, pH, alkalinity, phenolic compounds, SS and VSS was done as per the procedures given in Standard Methods (APHA, 1998). Methods for estimation of other parameters and their statistical analysis are same as given elsewhere (Majumder and Gupta, 2008). 3. Results and discussion
(F = 2.58, P < 0.01) removal. Significant difference was observed (F = 34.83, P < 0.01) for COD removal in RBC-I reactor. ANOVA test showed that there was significant difference for 2,4-DCP (F = 486.74, P = 0) and COD (F = 1143.43, P = 0) removal in UASBII reactor and also for 2,4-DCP (F = 54.13, P < 0.01) and COD (F = 273.35, P = 0) removal in RBC-II reactor. It implied the change in influent pH affected the removal of chlorophenols and COD in UASB reactors. Statistical analysis of data showed very good correlation of 2-CP (0.99) and 2,4-DCP (0.98) with COD removal in UASB-I and UASB-II reactors, respectively as well as for RBC-I (0.89) and RBC-II (0.97) reactors. Effluent pH and alkalinity of all the reactors showed good correlation (R2 between 0.71 and 0.99). Variation of pH along the height of UASB-I and UASB-II reactors was measured during the treatment of wastewater with two extreme influent pH of 5.5 ± 0.1 and 8.5 ± 0.1. A gradual increase of the pH was observed within the reactor with increase in the height of reactor. This type of gradual increase in the pH within the reactor has been reported by Buyukkamaci and Filibeli (2004). It can be said from the above study that influent pH of simulated wastewaters did not cause any significant effect on the performance of the sequential reactors. This is contrary to the study of Brás et al. (2005) who reported that the methanogens are survived best within the pH range of 6–8. However, Taconi et al. (2007) observed that the methanogenesis took place even at very low pH of 4.5. This might be due to the fact that irrespective of influent pH, whether it was acidic or basic, the pH of the wastewater within the UASB reactors was always maintained within the specified range. This could be due to the buffering capacity within the UASB reactors. Kalyuzhnyi et al. (1997) reported similar findings. Reactors were found to be suitable for the treatment of all the wastewaters in the present study. However, UASB reactors showed better performance in terms of removal of both chlorophenols and COD at neutral pH.
3.1. Effect of pH 3.2. Effect of alkalinity Table 1 shows the performance of sequential reactors operated at different influent pH. Removal of 2-CP in UASB-I reactor at influent pH 5.5 ± 0.1, 7.0 ± 0.1 and 8.5 ± 0.1 was 89.7 ± 0.7%, 99 ± 0.3% and 95.3 ± 0.3%, respectively and corresponding removal of COD in UASB-I reactor was 95 ± 0.5%, 99 ± 0.4% and 97.1 ± 0.3%, respectively. RBC-I reactor showed 96.7–100% and 94.5–99.5% removal of 2-CP and COD, respectively. Removal of 2,4-DCP in UASB-II reactor was 79 ± 0.7%, 88 ± 0.7% and 81.4 ± 0.7% at influent pH 5.5 ± 0.1, 7.0 ± 0.1 and 8.5 ± 0.1, respectively. Removal of COD in UASB-II reactor was 93.4 ± 0.4%, 99.5 ± 0.1% and 96.4 ± 0.3% at influent pH 5.5 ± 0.1, 7.0 ± 0.1 and 8.5 ± 0.1, respectively. RBC-II reactor showed 95.2–100% and 93.6–100% removal of 2,4-DCP and COD, respectively. ANOVA test showed that there was significant difference for 2CP (F = 699.00, P = 0) and COD (F = 209.39, P = 0) removal in UASB-I reactor at three different pH values of wastewater. However, in RBC-I reactor there was no significant difference in 2-CP
Table 2 shows the performance of sequential reactors operated at different alkalinity to COD ratio for treatment of chlorophenol containing wastewaters. Removal of 2-CP in UASB-I reactor was 97.3 ± 0.3%, 95.3 ± 0.3%, 93 ± 0.3% and 74.7 ± 0.3% at alkalinity/COD ratio of 1.1, 1.5, 2.1 and 2.6, respectively. Removal of COD in UASB-I reactor was 98.7 ± 0.3%, 97.4 ± 0.2%, 95.7 ± 0.4% and 95.5 ± 0.4% at alkalinity/COD ratio of 1.1, 1.5, 2.1 and 2.6, respectively. RBC-I reactor showed 100% and 94.5–98.4% removal of 2-CP and COD, respectively. Similarly, the removal of 2,4-DCP in UASB-II reactor was 86.7 ± 0.3%, 83.3 ± 0.3%, 78.4 ± 0.3% and 60 ± 0.3% at alkalinity/COD ratio of 1.1, 1.5, 2.1 and 2.6, respectively. Removal of COD in UASB-II reactor was 98.3 ± 0.2%, 97.3 ± 0.3%, 96.6 ± 0.4% and 95.3 ± 0.4% at alkalinity/COD ratio of 1.1, 1.5, 2.1 and 2.6, respectively. RBC-II reactor showed 98.1–100% and 93.8–96.9% removal of 2,4-DCP and COD, respectively.
Table 1 Performance of sequential reactors for 2-CP and 2,4-DCP wastewater treatments at different pH. Pollutant
Operating condition (Inf. pH)
COD (mg/l) Inf. UASB
Eff. UASB
Eff. RBC
Chlorophenol (mg/l) Inf. UASB
Eff. UASB
Eff. RBC
Inf. UASB
Eff. UASB
Eff. RBC
Inf. UASB
Eff. UASB
Eff. RBC
2-CP
5.5 ± 0.1 7.0 ± 0.1 8.5 ± 0.1
1315 ± 14 1319 ± 17 1324 ± 14
65 ± 7 13 ± 5 38 ± 4
13 ± 3 2±2 7±3
30.0 ± 0.2 30.1 ± 0.1 30.0 ± 0.1
3.1 ± 0.2 0.3 ± 0.1 1.4 ± 0.1
0.1 ± 0.1 BDL BDL
5.5 ± 0.1 7.0 ± 0.1 8.5 ± 0.1
7.9 ± 0.1 8.1 ± 0.1 8.2 ± 0.1
9.3 ± 0.1 9.1 ± 0.1 9.3 ± 0.1
671 ± 12 911 ± 11 1189 ± 12
940 ± 12 974 ± 14 1276 ± 14
1115 ± 12 1021 ± 10 1370 ± 15
2,4-DCP
5.5 ± 0.1 7.0 ± 0.1 8.5 ± 0.1
1317 ± 17 1306 ± 17 1324 ± 14
87 ± 5 6±1 47 ± 4
30 ± 4 BDL 15 ± 2
30.0 ± 0.1 30.0 ± 0.1 30.1 ± 0.1
6.3 ± 0.2 3.6 ± 0.2 5.6 ± 0.2
0.3 ± 0.1 BDL 0.2 ± 0.1
5.5 ± 0.1 7.0 ± 0.1 8.5 ± 0.1
8.2 ± 0.1 8.2 ± 0.1 8.3 ± 0.1
9.3 ± 0.1 9.1 ± 0.1 9.3 ± 0.1
669 ± 17 912 ± 10 1186 ± 11
1037 ± 12 987 ± 12 1264 ± 12
1142 ± 18 1024 ± 7 1395 ± 12
Inf: Influent, Eff: Effluent, BDL: Below detectable level.
pH
Alkalinity (as mg CaCO3/l)
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P.S. Majumder, S.K. Gupta / Bioresource Technology 100 (2009) 1881–1883 Table 2 Performance of sequential reactors for 2-CP and 2,4-DCP wastewater treatments at different alkalinity. Pollutant
Alkalinity/COD ratio
COD (mg/l)
Chlorophenol (mg/l)
Inf. UASB
Eff. UASB
Eff. RBC
Inf. UASB
Eff. UASB
2-CP
1.1 1.5 2.1 2.6
1350 ± 20 1343 ± 23 1345 ± 27 1342 ± 21
17 ± 4 34 ± 3 57 ± 5 60 ± 5
6±5 8±4 16 ± 3 15 ± 3
30.0 ± 0.1 30.1 ± 0.1 30.0 ± 0.2 30.0 ± 0.2
0.8 ± 0.1 1.4 ± 0.1 2.1 ± 0.1 7.6 ± 0.1
2,4-DCP
1.1 1.5 2.1 2.6
1350 ± 28 1356 ± 25 1342 ± 23 1357 ± 25
22 ± 3 37 ± 4 45 ± 5 63 ± 5
13 ± 3 16 ± 2 13 ± 5 27 ± 3
30.1 ± 0.1 30.0 ± 0.1 30.1 ± 0.2 30.0 ± 0.2
4.0 ± 0.1 5.0 ± 0.1 6.5 ± 0.1 12.0 ± 0.1
Alkalinity (as mg CaCO3/l)
pH
Eff. RBC
Inf. UASB
Eff. UASB
Eff. RBC
Inf. UASB
Eff. UASB
Eff. RBC
BDL BDL BDL BDL
1499 ± 11 2006 ± 12 2751 ± 11 3508 ± 12
1551 ± 15 2099 ± 11 2779 ± 6 3900 ± 8
1618 ± 21 2145 ± 12 2818 ± 12 4841 ± 10
8.0 ± 0.1 8.2 ± 0.1 8.5 ± 0.1 8.7 ± 0.1
8.1 ± 0.1 8.4 ± 0.1 8.7 ± 0.1 8.9 ± 0.1
9.2 ± 0.1 9.3 ± 0.1 9.5 ± 0.1 9.8 ± 0.1
BDL BDL BDL 0.1 ± 0
1505 ± 12 2002 ± 12 2758 ± 12 3513 ± 13
1566 ± 20 2121 ± 17 2810 ± 10 3870 ± 16
1630 ± 16 2141 ± 13 2855 ± 12 4933 ± 12
8.0 ± 0.1 8.2 ± 0.1 8.5 ± 0.1 8.7 ± 0
8.1 ± 0.1 8.4 ± 0.1 8.6 ± 0.1 9.0 ± 0.1
9.1 ± 0.1 9.3 ± 0.1 9.6 ± 0 9.8 ± 0
Inf: Influent, Eff: Effluent, BDL: Below detectable level.
ANOVA test showed that there was significant difference for 2CP (F = 15408.49, P = 0) and COD (F = 234.68, P = 0) removal in UASB-I reactor. However, in RBC-I reactor there was no significant difference in 2-CP (F = 0.80, P = 0.5) removal. Significant difference was observed (F = 14.52, P < 0.01) for COD removal in RBC-I reactor. ANOVA test showed that there was significant difference for 2,4-DCP (F = 11250.68, P = 0) and COD (F = 164.68, P = 0) removal in UASB-II reactor. Significant difference was not observed for 2,4-DCP removal (F = 0.57, P = 0.6) in RBC-II reactor. Significant difference was observed for COD removal (F = 32.14, P < 0.01) at four different alkalinity in RBC-II reactor. It implied the change in alkalinity affected the removal of chlorophenols and COD in UASB reactors. Statistical analysis of data showed very good correlation of 2-CP (>0.95) and 2,4-DCP (>0.95) with COD removal in the reactors. Effluent pH and alkalinity of all the reactors showed good correlation (>0.94). Increase in alkalinity caused the deterioration of performance of UASB reactors. This deterioration was less significant when alkalinity/COD ratio was increased from 1.1 to 2.1 but was more significant at alkalinity/COD ratio of 2.6. Both the UASB reactors showed poor performance and showed only 74.7 ± 0.3% and 60 ± 0.3% removal of 2-CP and 2,4-DCP, respectively at alkalinity/ COD ratio of 2.6. Although removal of chlorophenols decreased significantly at alkalinity/COD ratio 2.6 but significant deterioration of COD removal was not observed. Isßık and Sponza (2005) reported that NaHCO3 did not cause significant effect on the removal of COD. It was evident from the results that at higher alkalinity/COD ratios (2.1 and 2.6) dechlorination were inhibited but methanogenesis was not affected significantly. When Vancomycin inhibitor was used in the dechlorination of 2,4,6-TCP similar inhibition of only dechlorination but not of methanogenesis was observed by Perkins et al. (1994). The selective inhibition of methanogenesis might be due to the combined effect of high pH and antimicrobial activity of sodium bicarbonate. Antimicrobial activity of sodium bicarbonate was also observed by Corral et al. (1998). This study shows the optimum alkalinity/COD ratio for the treatment of wastewater containing chlorophenol in UASB reactors is 1.1. However, sequential reactors were capable of handling the above wastewater thus producing extremely good quality treated effluent. 4. Conclusions Following conclusions can be drawn from the above studies: Sequential UASB and RBC reactors could easily handle wastewaters with influent pH of 5.5–8.5. However, UASB reactor showed best performance at neutral pH.
The optimum alkalinity/COD ratio for the treatment of chlorophenolic wastewaters in UASB reactors was 1.1. However, sequential reactors produced very high quality effluents even at high influent alkalinity/COD ratio of 2.6. Significant selective inhibition on dechlorination was observed at very high alkalinity/COD ratio of 2.1–2.6 but inhibition on methanogenesis was not very significant.
References Ag˘dag˘, O.N., Sponza, D.T., 2005. Effect of alkalinity on the performance of a simulated landfill bioreactor digesting organic solid wastes. Chemosphere 59 (6), 871–879. APHA, AWWA, WPCF, 1998. Standard Methods for the Examination of Water and Wastewater, twentieth ed. American Public Health Association, Washington, DC. Bajaj, M., Gallert, C., Winter, J., 2008. Anaerobic biodegradation of high strength 2chlorophenol-containing synthetic wastewater in a fixed bed reactor. Chemosphere 73 (5), 705–710. Brás, R., Gomes, A., Ferra, M.I.A., Pinheiro, H.M., Gonçalves, I.C., 2005. Monoazo and diazo dye decolourisation studies in a methanogenic UASB reactor. J. Biotechnol. 115 (1), 57–66. Buyukkamaci, N., Filibeli, A., 2004. Volatile fatty acid formation in an anaerobic hybrid reactor. Process Biochem. 39 (11), 1491–1494. Chang, C., Tseng, S., Chang, C., Ho, C., 2003. Reductive dechlorination of 2chlorophenol in a hydrogenotrophic, gas-permeable, silicone membrane bioreactor. Bioresour. Technol. 90 (3), 323–328. Corral, L.G., Post, L.S., Montville, T.J., 1998. Antimicrobial activity of sodium bicarbonate. J. Food Sci. 53, 981–982. González, J.S., Rivera, A., Borja, R., Sánchez, E., 1998. Influence of organic volumetric loading rate, nutrient balance and alkalinity: COD ratio on the anaerobic sludge granulation of an UASB reactor treating sugar cane molasses. Int. Biodet. Biodeg. 41 (2), 127–131. Isßık, M., Sponza, D.T., 2005. Effects of alkalinity and co-substrate on the performance of an upflow anaerobic sludge blanket (UASB) reactor through decolorization of Congo Red azo dye. Bioresour. Technol. 96 (5), 633–643. Kalyuzhnyi, S.V., Martinez, E.P., Martinez, J.R., 1997. Anaerobic treatment of highstrength cheese-whey wastewaters in laboratory and pilot UASB-reactors. Bioresour. Technol. 60 (1), 59–65. Kargi, F., Uygur, A., Baskaya, H.S., 2005. Para-chlorophenol inhibition on COD, nitrogen and phosphate removal from synthetic wastewater in a sequencing batch reactor. Bioresour. Technol. 95 (15), 1696–1702. Majumder, P.S., Gupta, S.K., 2007. Removal of chlorophenols in sequential anaerobic–aerobic reactors. Bioresour. Technol. 98 (1), 118–129. Majumder, P.S., Gupta, S.K., 2008. Effect of carbon sources and shock loading on the removal of chlorophenols in sequential anaerobic–aerobic reactors. Bioresour. Technol. 99 (8), 2930–2937. Metcalf, L., Eddy, H., 2003. Wastewater Engineering: Treatment and Reuse, fourth ed. Tata McGraw-Hill, New Delhi. Perkins, P.S., Komisar, S.J., Puhakka, J.A., Ferguson, J.F., 1994. Effects of electron donors and inhibitors on reductive dechlorination of 2,4,6-trichlorophenol. Water Res. 28 (10), 2101–2107. Taconi, K.A., Zappi, M.E., French, W.T., Brown, L.R., 2007. Feasibility of methanogenic digestion applied to a low pH acetic acid solution. Bioresour. Technol. 98 (8), 1579–1585. Wilcox, S.J., Hawkes, D.L., Hawkes, F.R., Guwy, A.J., 1995. A neural network, based on bicarbonate monitoring, to control anaerobic digestion. Water Res. 29 (6), 1465–1470.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Effect of influent pH and alkalinity on the removal of chlorophenols in sequential anaerobic–aerobic reactors Partha Sarathi Majumder, S.K. Gupta * Centre for Environmental Science and Engineering, Indian Institute of Technology, Powai, Mumbai 400 076, India
a r t i c l e
i n f o
Article history: Received 13 August 2008 Received in revised form 2 October 2008 Accepted 3 October 2008 Available online 18 November 2008 Keywords: 2-Chlorophenol 2,4-Dichlorophenol Upflow anaerobic sludge blanket Rotating biological contactor
a b s t r a c t This study was carried out to determine the effect of influent pH and alkalinity on the performance of sequential UASB and RBC reactors for the removal of 2-CP and 2,4-DCP from two different simulated wastewaters. The performance of methanogens at low (<6.0) to high (>8.0) pH values and at sufficiently high alkalinity (1500–3500 mg/l as CaCO3) is described in this paper. Sequential reactors were capable of handling wastewaters with influent pH, 5.5–8.5. However, with influent pH 7.0 ± 0.1 UASB reactor showed best performance for 2-CP (99%) and 2,4-DCP (88%) removals. Increase in alkalinity/COD ratio in the influent (>1.1) caused gradual decrease in the chlorophenol removal in UASB reactors. The UASB reactors could not tolerate wastewater with higher alkalinity/COD ratio (2.6) and showed significant deterioration of its performance in terms of chlorophenols removal achieving only 74.7% 2-CP and 60% 2,4-DCP removals, respectively. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Chlorophenols are extensively used chemicals for the synthesis of pesticides, herbicides and dyes. Recent studies were carried out for the removal of chlorophenols in sequencing batch reactor (Kargi et al., 2005), hydrogenotrophic, gas-permeable, silicone membrane bioreactor (Chang et al., 2003) and fixed bed reactor (Bajaj et al., 2008). In the present study UASB and RBC reactors were chosen as anaerobic and aerobic reactors, respectively for the sequential biological treatment of chlorophenols. The pH and alkalinity have important roles to play in the aerobic and anaerobic biological treatment of wastewater. In aerobic biological oxidation pH in the range of 6.0–9.0 is tolerable (Metcalf and Eddy, 2003). Brás et al. (2005) reported that in anaerobic process limited pH range of 6–8 is required for the growth of methanogens. During aerobic biological oxidation alkalinity has an important role in nitrification–denitrification. During nitrification–denitrification process 3.57 g alkalinity as CaCO3 is consumed per gram of NHþ 4 –N removal (Metcalf and Eddy, 2003). Anaerobic wastewater treatment requires high alkalinity and failure of anaerobic bioreactor takes place in the absence of sufficient alkalinity. Wilcox et al. (1995) reported that for successful and stable operation of anaerobic digester, the required bicarbonate alkalinity should be 1000–3000 mg/l as CaCO3. Several studies have been carried out to assess the performance of anaerobic reactors at
* Corresponding author. Tel.: +91 22 25767853; fax: +91 22 25723480. E-mail addresses: [email protected] (P.S. Majumder), skgupta@iitb. ac.in (S.K. Gupta). 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.10.002
low to moderate alkalinity/COD ratio. The optimum ratios in these studies were 0.4–1.3 (González et al., 1998; Ag˘dag˘ and Sponza, 2005). The investigations on the effect of alkalinity for wastewater treatment in anaerobic–aerobic reactor systems containing chlorophenols and high alkalinity/COD ratio are limited. The present study brings out two important observations about the activity of methanogens at low (<6.0) to high (>8.0) influent pH and at higher alkalinity (alkalinity/COD ratios 1.1–2.6) for the treatment of wastewaters containing chlorophenol. 2. Methods 2.1. Experimental set up Sequential UASB-I and RBC-I and UASB-II and RBC-II reactors were used for the removal of 2-CP and 2,4-DCP, respectively. The details of the dimensions of the reactors are mentioned elsewhere (Majumder and Gupta, 2007). Hydraulic retention time (HRT), F/M ratio and solid retention time (SRT) of UASB-I and UASB-II reactors were 12 h, 0.22 g COD/g VSS d and 188 d, respectively. The HRT, F/ M ratio and SRT of RBC-I reactor was 23 h, 0.16 g COD/g VSS d and 246 d, respectively. The HRT, F/M ratio and SRT of RBC-II reactor was 28.8 h, 0.13 g COD/g VSS d and 102 d, respectively. 2.2. Experimental protocol This study was carried out at three different influent pH values viz. 5.5 ± 0.1, 7.0 ± 0.1 and 8.5 ± 0.1. The feed composition of the UASB reactors was as follows: CH3COONa 3H2O, 3.0 g/l; NH4Cl,
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91.7 mg/l; K2HPO4, 17.45 mg/l; KH2PO4, 7.45 mg/l; CaCl2 2H2O, 200 mg/l; trace metal solution, 1 ml/l and chlorophenol (2-CP or 2,4-DCP) 30 mg/l. Sodium bicarbonate was not added in the influent of pH 7.0 ± 0.1 and pH 5.5 ± 0.1. Influent of pH 5.5 ± 0.1 and 8.5 ± 0.1 was prepared with the addition of 1.1 N HCl (1.5 ml/l) and 1 N NaOH (1.25 ml/l) + NaHCO3 (300 mg/l) to the normal feed, respectively. The composition of supplementary feed of RBC reactors (2 l/d) was as follows (mg/l): 6800, CH3COONa 3H2O; 900, NH4Cl; 36.23, K2HPO4; 15.53, KH2PO4; 1.0, MgSO4 7H2O; and 2 ml/l trace metal solution. Composition of trace metal solution for UASB and RBC reactors are mentioned elsewhere (Majumder and Gupta, 2007). The performance of sequential reactors was evaluated at different alkalinity to COD ratios viz., 1.1:1, 1.5:1, 2.1:1 and 2.6:1. The feed composition of the UASB reactors was as follows: CH3COONa 3H2O, 3.0 g/l; NH4Cl, 91.7 mg/l; K2HPO4, 17.45 mg/l; KH2PO4, 7.45 mg/l; CaCl2 2H2O, 200 mg/l; NaHCO3, (0.85–5.2) g/l; trace metal solution, 1 ml/l and chlorophenol (2-CP or 2,4-DCP) 30 mg/l. The composition of supplementary feed of RBC reactors was same as used for the study at different pH. Reactors were operated for 25–30 days under pseudo-steady-state condition in both the studies. 2.3. Analytical methods Analysis of COD, pH, alkalinity, phenolic compounds, SS and VSS was done as per the procedures given in Standard Methods (APHA, 1998). Methods for estimation of other parameters and their statistical analysis are same as given elsewhere (Majumder and Gupta, 2008). 3. Results and discussion
(F = 2.58, P < 0.01) removal. Significant difference was observed (F = 34.83, P < 0.01) for COD removal in RBC-I reactor. ANOVA test showed that there was significant difference for 2,4-DCP (F = 486.74, P = 0) and COD (F = 1143.43, P = 0) removal in UASBII reactor and also for 2,4-DCP (F = 54.13, P < 0.01) and COD (F = 273.35, P = 0) removal in RBC-II reactor. It implied the change in influent pH affected the removal of chlorophenols and COD in UASB reactors. Statistical analysis of data showed very good correlation of 2-CP (0.99) and 2,4-DCP (0.98) with COD removal in UASB-I and UASB-II reactors, respectively as well as for RBC-I (0.89) and RBC-II (0.97) reactors. Effluent pH and alkalinity of all the reactors showed good correlation (R2 between 0.71 and 0.99). Variation of pH along the height of UASB-I and UASB-II reactors was measured during the treatment of wastewater with two extreme influent pH of 5.5 ± 0.1 and 8.5 ± 0.1. A gradual increase of the pH was observed within the reactor with increase in the height of reactor. This type of gradual increase in the pH within the reactor has been reported by Buyukkamaci and Filibeli (2004). It can be said from the above study that influent pH of simulated wastewaters did not cause any significant effect on the performance of the sequential reactors. This is contrary to the study of Brás et al. (2005) who reported that the methanogens are survived best within the pH range of 6–8. However, Taconi et al. (2007) observed that the methanogenesis took place even at very low pH of 4.5. This might be due to the fact that irrespective of influent pH, whether it was acidic or basic, the pH of the wastewater within the UASB reactors was always maintained within the specified range. This could be due to the buffering capacity within the UASB reactors. Kalyuzhnyi et al. (1997) reported similar findings. Reactors were found to be suitable for the treatment of all the wastewaters in the present study. However, UASB reactors showed better performance in terms of removal of both chlorophenols and COD at neutral pH.
3.1. Effect of pH 3.2. Effect of alkalinity Table 1 shows the performance of sequential reactors operated at different influent pH. Removal of 2-CP in UASB-I reactor at influent pH 5.5 ± 0.1, 7.0 ± 0.1 and 8.5 ± 0.1 was 89.7 ± 0.7%, 99 ± 0.3% and 95.3 ± 0.3%, respectively and corresponding removal of COD in UASB-I reactor was 95 ± 0.5%, 99 ± 0.4% and 97.1 ± 0.3%, respectively. RBC-I reactor showed 96.7–100% and 94.5–99.5% removal of 2-CP and COD, respectively. Removal of 2,4-DCP in UASB-II reactor was 79 ± 0.7%, 88 ± 0.7% and 81.4 ± 0.7% at influent pH 5.5 ± 0.1, 7.0 ± 0.1 and 8.5 ± 0.1, respectively. Removal of COD in UASB-II reactor was 93.4 ± 0.4%, 99.5 ± 0.1% and 96.4 ± 0.3% at influent pH 5.5 ± 0.1, 7.0 ± 0.1 and 8.5 ± 0.1, respectively. RBC-II reactor showed 95.2–100% and 93.6–100% removal of 2,4-DCP and COD, respectively. ANOVA test showed that there was significant difference for 2CP (F = 699.00, P = 0) and COD (F = 209.39, P = 0) removal in UASB-I reactor at three different pH values of wastewater. However, in RBC-I reactor there was no significant difference in 2-CP
Table 2 shows the performance of sequential reactors operated at different alkalinity to COD ratio for treatment of chlorophenol containing wastewaters. Removal of 2-CP in UASB-I reactor was 97.3 ± 0.3%, 95.3 ± 0.3%, 93 ± 0.3% and 74.7 ± 0.3% at alkalinity/COD ratio of 1.1, 1.5, 2.1 and 2.6, respectively. Removal of COD in UASB-I reactor was 98.7 ± 0.3%, 97.4 ± 0.2%, 95.7 ± 0.4% and 95.5 ± 0.4% at alkalinity/COD ratio of 1.1, 1.5, 2.1 and 2.6, respectively. RBC-I reactor showed 100% and 94.5–98.4% removal of 2-CP and COD, respectively. Similarly, the removal of 2,4-DCP in UASB-II reactor was 86.7 ± 0.3%, 83.3 ± 0.3%, 78.4 ± 0.3% and 60 ± 0.3% at alkalinity/COD ratio of 1.1, 1.5, 2.1 and 2.6, respectively. Removal of COD in UASB-II reactor was 98.3 ± 0.2%, 97.3 ± 0.3%, 96.6 ± 0.4% and 95.3 ± 0.4% at alkalinity/COD ratio of 1.1, 1.5, 2.1 and 2.6, respectively. RBC-II reactor showed 98.1–100% and 93.8–96.9% removal of 2,4-DCP and COD, respectively.
Table 1 Performance of sequential reactors for 2-CP and 2,4-DCP wastewater treatments at different pH. Pollutant
Operating condition (Inf. pH)
COD (mg/l) Inf. UASB
Eff. UASB
Eff. RBC
Chlorophenol (mg/l) Inf. UASB
Eff. UASB
Eff. RBC
Inf. UASB
Eff. UASB
Eff. RBC
Inf. UASB
Eff. UASB
Eff. RBC
2-CP
5.5 ± 0.1 7.0 ± 0.1 8.5 ± 0.1
1315 ± 14 1319 ± 17 1324 ± 14
65 ± 7 13 ± 5 38 ± 4
13 ± 3 2±2 7±3
30.0 ± 0.2 30.1 ± 0.1 30.0 ± 0.1
3.1 ± 0.2 0.3 ± 0.1 1.4 ± 0.1
0.1 ± 0.1 BDL BDL
5.5 ± 0.1 7.0 ± 0.1 8.5 ± 0.1
7.9 ± 0.1 8.1 ± 0.1 8.2 ± 0.1
9.3 ± 0.1 9.1 ± 0.1 9.3 ± 0.1
671 ± 12 911 ± 11 1189 ± 12
940 ± 12 974 ± 14 1276 ± 14
1115 ± 12 1021 ± 10 1370 ± 15
2,4-DCP
5.5 ± 0.1 7.0 ± 0.1 8.5 ± 0.1
1317 ± 17 1306 ± 17 1324 ± 14
87 ± 5 6±1 47 ± 4
30 ± 4 BDL 15 ± 2
30.0 ± 0.1 30.0 ± 0.1 30.1 ± 0.1
6.3 ± 0.2 3.6 ± 0.2 5.6 ± 0.2
0.3 ± 0.1 BDL 0.2 ± 0.1
5.5 ± 0.1 7.0 ± 0.1 8.5 ± 0.1
8.2 ± 0.1 8.2 ± 0.1 8.3 ± 0.1
9.3 ± 0.1 9.1 ± 0.1 9.3 ± 0.1
669 ± 17 912 ± 10 1186 ± 11
1037 ± 12 987 ± 12 1264 ± 12
1142 ± 18 1024 ± 7 1395 ± 12
Inf: Influent, Eff: Effluent, BDL: Below detectable level.
pH
Alkalinity (as mg CaCO3/l)
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P.S. Majumder, S.K. Gupta / Bioresource Technology 100 (2009) 1881–1883 Table 2 Performance of sequential reactors for 2-CP and 2,4-DCP wastewater treatments at different alkalinity. Pollutant
Alkalinity/COD ratio
COD (mg/l)
Chlorophenol (mg/l)
Inf. UASB
Eff. UASB
Eff. RBC
Inf. UASB
Eff. UASB
2-CP
1.1 1.5 2.1 2.6
1350 ± 20 1343 ± 23 1345 ± 27 1342 ± 21
17 ± 4 34 ± 3 57 ± 5 60 ± 5
6±5 8±4 16 ± 3 15 ± 3
30.0 ± 0.1 30.1 ± 0.1 30.0 ± 0.2 30.0 ± 0.2
0.8 ± 0.1 1.4 ± 0.1 2.1 ± 0.1 7.6 ± 0.1
2,4-DCP
1.1 1.5 2.1 2.6
1350 ± 28 1356 ± 25 1342 ± 23 1357 ± 25
22 ± 3 37 ± 4 45 ± 5 63 ± 5
13 ± 3 16 ± 2 13 ± 5 27 ± 3
30.1 ± 0.1 30.0 ± 0.1 30.1 ± 0.2 30.0 ± 0.2
4.0 ± 0.1 5.0 ± 0.1 6.5 ± 0.1 12.0 ± 0.1
Alkalinity (as mg CaCO3/l)
pH
Eff. RBC
Inf. UASB
Eff. UASB
Eff. RBC
Inf. UASB
Eff. UASB
Eff. RBC
BDL BDL BDL BDL
1499 ± 11 2006 ± 12 2751 ± 11 3508 ± 12
1551 ± 15 2099 ± 11 2779 ± 6 3900 ± 8
1618 ± 21 2145 ± 12 2818 ± 12 4841 ± 10
8.0 ± 0.1 8.2 ± 0.1 8.5 ± 0.1 8.7 ± 0.1
8.1 ± 0.1 8.4 ± 0.1 8.7 ± 0.1 8.9 ± 0.1
9.2 ± 0.1 9.3 ± 0.1 9.5 ± 0.1 9.8 ± 0.1
BDL BDL BDL 0.1 ± 0
1505 ± 12 2002 ± 12 2758 ± 12 3513 ± 13
1566 ± 20 2121 ± 17 2810 ± 10 3870 ± 16
1630 ± 16 2141 ± 13 2855 ± 12 4933 ± 12
8.0 ± 0.1 8.2 ± 0.1 8.5 ± 0.1 8.7 ± 0
8.1 ± 0.1 8.4 ± 0.1 8.6 ± 0.1 9.0 ± 0.1
9.1 ± 0.1 9.3 ± 0.1 9.6 ± 0 9.8 ± 0
Inf: Influent, Eff: Effluent, BDL: Below detectable level.
ANOVA test showed that there was significant difference for 2CP (F = 15408.49, P = 0) and COD (F = 234.68, P = 0) removal in UASB-I reactor. However, in RBC-I reactor there was no significant difference in 2-CP (F = 0.80, P = 0.5) removal. Significant difference was observed (F = 14.52, P < 0.01) for COD removal in RBC-I reactor. ANOVA test showed that there was significant difference for 2,4-DCP (F = 11250.68, P = 0) and COD (F = 164.68, P = 0) removal in UASB-II reactor. Significant difference was not observed for 2,4-DCP removal (F = 0.57, P = 0.6) in RBC-II reactor. Significant difference was observed for COD removal (F = 32.14, P < 0.01) at four different alkalinity in RBC-II reactor. It implied the change in alkalinity affected the removal of chlorophenols and COD in UASB reactors. Statistical analysis of data showed very good correlation of 2-CP (>0.95) and 2,4-DCP (>0.95) with COD removal in the reactors. Effluent pH and alkalinity of all the reactors showed good correlation (>0.94). Increase in alkalinity caused the deterioration of performance of UASB reactors. This deterioration was less significant when alkalinity/COD ratio was increased from 1.1 to 2.1 but was more significant at alkalinity/COD ratio of 2.6. Both the UASB reactors showed poor performance and showed only 74.7 ± 0.3% and 60 ± 0.3% removal of 2-CP and 2,4-DCP, respectively at alkalinity/ COD ratio of 2.6. Although removal of chlorophenols decreased significantly at alkalinity/COD ratio 2.6 but significant deterioration of COD removal was not observed. Isßık and Sponza (2005) reported that NaHCO3 did not cause significant effect on the removal of COD. It was evident from the results that at higher alkalinity/COD ratios (2.1 and 2.6) dechlorination were inhibited but methanogenesis was not affected significantly. When Vancomycin inhibitor was used in the dechlorination of 2,4,6-TCP similar inhibition of only dechlorination but not of methanogenesis was observed by Perkins et al. (1994). The selective inhibition of methanogenesis might be due to the combined effect of high pH and antimicrobial activity of sodium bicarbonate. Antimicrobial activity of sodium bicarbonate was also observed by Corral et al. (1998). This study shows the optimum alkalinity/COD ratio for the treatment of wastewater containing chlorophenol in UASB reactors is 1.1. However, sequential reactors were capable of handling the above wastewater thus producing extremely good quality treated effluent. 4. Conclusions Following conclusions can be drawn from the above studies: Sequential UASB and RBC reactors could easily handle wastewaters with influent pH of 5.5–8.5. However, UASB reactor showed best performance at neutral pH.
The optimum alkalinity/COD ratio for the treatment of chlorophenolic wastewaters in UASB reactors was 1.1. However, sequential reactors produced very high quality effluents even at high influent alkalinity/COD ratio of 2.6. Significant selective inhibition on dechlorination was observed at very high alkalinity/COD ratio of 2.1–2.6 but inhibition on methanogenesis was not very significant.
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