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Hybrid reactor for priority pollutant nitrobenzene removal
ARTICLE IN PRESS
Water Research 37 (2003) 4331–4336
Hybrid reactor for priority pollutant nitrobenzene removal Partha Sarathi Majumder, S.K. Gupta* Centre for Environmental Science and Engineering, Indian Institute of Technology, Powai, Mumbai 400076, India Received 30 October 2001; accepted 24 July 2003
Abstract The performance of a hybrid reactor, comprising of trickling filter and activated sludge process, in treating nitrobenzene wastewater was investigated. Acetate induced cells of mixed consortia was acclimatized with gradual increase of nitrobenzene concentration up to 90 mg/l in 100 days using sodium acetate as co-substrate and considering COD and nitrobenzene concentration as paramount parameters for assessing the growth of biofilm and acclimation. A removal of 60–95.80% COD and 80–90.23% nitrobenzene was observed during acclimation. During hydraulic retention time (HRT) studies, the optimum HRT was found to be 29.55 h at which a maximum of 95.83% COD and 97.93% nitrobenzene removal was observed. Other studies included optimization of C:N ratio, substrate:co-substrate ratio, effect of shock loading and estimation of volatilization losses. The optimum C:N ratio was found to be 100:20 at which maximum 97.93% removal of nitrobenzene was observed. At optimum HRT (29.55 h) and optimum C:N ratio (100:20) optimum substrate:co-substrate ratio was found to be 1:33. From the shock load studies it can be concluded that the system can withstand shock load up to two times of usual nitrobenzene concentration. A loss of 9.44% nitrobenzene was observed due to volatilization and mass balance gave an efficiency of 87.49% biological removal of nitrobenzene. r 2003 Elsevier Ltd. All rights reserved. Keywords: Nitrobenzene; Trickling filter; Activated sludge process; Hybrid reactor
1. Introduction Anthropogenic activities cause the massive use of natural resources and these lead to pollution. Among domestic wastewater, industrial wastewater and run-off from agricultural field industrial wastewater is more important because of the presence of both organic and inorganic compounds. Sixty-five classes of such compounds are considered as hazardous [1] and 129 organic and inorganic compounds have been designated as ‘‘priority pollutant’’ by USEPA on the basis of their known or suspected carcinogenicity, mutagenicity, teratogenicity or high acute toxicity [2]. Nitrobenzene is one of the nitroaromatic compounds having considerable industrial importance. It has been *Corresponding author. Tel.: +91-22-5767853; fax: +91-225723480. E-mail address: [email protected] (S.K. Gupta).
estimated that about 19 million lb of nitrobenzene is released into the environment annually [3]. It causes vomiting, skin and eye irritation, and headache. Its continuous exposure leads to liver damage and anemia. If the concentration exceeds 2 mg/l in the waste then it is declared as hazardous waste (USEPA). Because of its toxicity, Nitrobenzene has been placed as one of the 129 priority pollutants. Among various physical, chemical and biological methods for removal of nitrobenzene the biological treatment option is most cost effective. Sewage effluent under anaerobic condition converts nitrobenzene to aniline but no aromatic amine was detected under aerobic condition [4]. Dickel et al. [5] reported that nitroaromatics are gratuitously converted to the corresponding aromatic amines in reductive condition. Marvin-Sikkema and Bont [6] reported four probable pathways of degradation of nitrobenzene. Fixed film system has higher biomass population and longer solid
0043-1354/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0043-1354(03)00436-6
ARTICLE IN PRESS 4332
P.S. Majumder, S.K. Gupta / Water Research 37 (2003) 4331–4336
added, which was prepared by using dextrose (3.0 g/l), ammonium nitrate (0.685 g/l), dipotassium hydrogen phosphate (0.067 g/l) and trace metal solution. Trace metal solution was contributing 100 mg Na—EDTA, 4.4 mg ZnSO4, 11.0 mg CaCl2, 10.12 mg MnCl2 4H2O, 10.0 mg FeSO4 7H2O, 2.2 mg (NH4)6Mo7O24 4H2O, 3.22 mg CoCl2 5H2O, 3.14 mg CuSO4 5H2O, and 0.228 mg H3BO3 per liter in the feed [9].
retention time (SRT). Secondary treatment processes combining both fixed growth (trickling filter (TF)) and suspended growth (activated sludge) systems are becoming very popular. These processes offer the simplicity of operation and process stability of a fixed growth system with the high effluent quality associated with suspended growth system. This also offers economic advantage in comparison to the other operation [7]. Misra and Gupta [8] examined the potential of hybrid biological reactor for treatment of wastewater containing trichloroethylene. This paper presents the development of acclimated biofilm on support matrix and suspension and the study of the hybrid reactor’s performance at different HRT, C:N ratio, substrate:co-substrate ratio, and under shock loads.
2.2. Experimental set-up The hybrid reactor (Fig. 1) consisted of a TF and an aeration tank (AT) giving a combination of attached growth and suspended growth systems similar to reactors used by Misra and Gupta [8] was employed. A portion of settled sludge from clarifier was recycled to the AT. At five different HRTs of AT 4.4, 5.6, 8.5, 9.25 and 11.9 h SRTs were 7.2, 9.0, 16.8, 19.4 and 22.1 d, respectively. TF consisted of PVC raschig rings as support media had a volume of 6.0 l (0.1 m 0.1 m 0.6 m). Total effective surface area and specific surface area of the TF were 1.29 m2 and 216.5 m2/m3, respectively. The void volume of TF was 5.5 l. AT had a volume of 10 l (0.2 m 0.2 m 0.25 m). Its effective volume was adjusted to 2.5, 5.0 7.5 and 10.0 l by four sampling ports at a distance of 0.625 m from the other adjacent port. Out of four sampling ports only lower two were used to adjust the volume of the reactor at 2.5 and 5.0 l during HRT studies. Diffused aerators were used for aeration at 2–2.2 l/min aeration rate. Two aerators were used for aeration.
2. Materials and methods 2.1. Seed culture An actively grown culture was collected from a reactor in our laboratory used for maintaining seed culture. The culture was maintained in a nutrient medium of dextrose, ammonium nitrate, dipotassium hydrogen phosphate and trace metal solution with a ratio of carbon, nitrogen, and phosphorous as 100:20:1. Every alternate day aeration was stopped for half an hour, supernatant was discarded from the reactor after settlement of biomass and feed/nutrient media (3 l) was
Influent
Trickling Filter
Sampling Port for Aeration Tank
Effluent to clarifier
Diffuser Effluent Activated sludge unit Clarifier
Returned Activated Sludge
Wastage
Fig. 1. Schematic diagram of the hybrid reactor.
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2.3. Experimental procedure The system was operated with an influent flow rate of 7 ml/min and C:N:P ratio of feed was maintained at 100:20:1 during acclimation period. Above mentioned seed culture was taken in reactor and was acclimatized to 90 mg/l of nitrobenzene (Synthesis grade 99% pure, E. Merck, India) and this was done by increment of influent nitrobenzene concentration to 90 mg/l. Acclimatization was started with 4 mg/l nitrobenzene and was increased to 20 mg/l nitrobenzene concentration by increment of 4 mg/l of nitrobenzene in each increment in 8 days, then increased to 25 mg/l and after that it was increased to 85 mg/l of nitrobenzene by an increment of 15 mg/l in each increment, in 10 days. At last influent nitrobenzene concentration was increased from 85 to 90 mg/l for a period of 10 days. System took 100 days for acclimation. Acclimation was confirmed by a steadiness of effluent COD and nitrobenzene concentration. The influent simulated wastewater used for HRT studies was containing 90 mg/l nitrobenzene, 3.00 g/l sodium acetate, 0.36 g/l NH4Cl, 0.0392 g/l K2HPO4, 0.0168 g/l KH2PO4, 0.001 g/l MgSO4. 7H2O, and trace metal solution [9]. Five HRT studies were conducted at 14 h (TF 9.6 h+ASP 4.4 h), 18 h (TF 12.4 h+ASP 5.6 h), 24.99 h (TF 13.09 h+ASP 11.90 h), 27.20 h (TF 18.70 h+ASP 8.50 h), and 29.55 h (TF 20.3 h+ASP 9.25 h) to optimize the system in terms of surface loading with respect to nitrobenzene and COD removal. Higher HRTs were chosen because of recalcitrant nature of nitrobenzene. HRT was varied by varying influent flow rate from minimum 4.51 to maximum 9.54 ml/min. Effective volume of TF was 5.5 l in all cases and effective volume of AT was 5 l in case of 24.99 h HRT and 2.5 l in all other cases. Studies were also carried out at four different C:N ratios of 100:15, 100:20, 100:25, and 100:30 to obtain an optimum C:N ratio for maximum nitrobenzene removal. The substrate nitrobenzene concentration was kept around 90 mg/l in the simulated wastewater and C:N ratio was varied by varying NH4Cl concentration. Trace metal solution was also added in the feed. The performance of the reactor was evaluated at four different substrate:co-substrate ratio of 1:8, 1:16, 1:33, and 1:48. The substrate:co-substrate ratio was varied by using different concentration of sodium acetate. Substrate nitrobenzene concentration was kept around 90 mg/l in the simulated wastewater and trace metal solution was added in the feed. Trace metal solution contributed different metal concentration in the same amount as in HRT studies. The performance of the reactor was also evaluated at two different shock loadings. The substrate concentrations at two shock loads were 135 and 180 mg/l corresponding to 1.5 and 2 times the concentration of nitrobenzene used during HRT studies. During all the above mentioned studies reactor temperature varied
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from 25 C to 30 C. MLSS of the AT was maintained around 4000–4200 mg/l for all studies. 2.4. Analytical methods Influent and effluent BOD, COD, pH, dissolved oxygen (DO), suspended solids (SS), NH+ 4 -N, NO2 -N, 3 NO3 -N, TKN, PO4 -P, alkalinity were measured. Ammonia nitrogen was measured by nesslerization [10], nitrate nitrogen by brucine sulfate [11], nitrite nitrogen by diazotization, TKN by macro-kjeldahl and phosphate phosphorous by stannous chloride method [12]. Effluent SS was SS of clarified effluent. DO meter (YSI model 58, USA) was used for DO measurement. Nitrobenzene concentration was analyzed by gas chromatography (GC) (Model Sigma 2000 capillary chromatograph, Perkin-Elmer) using 15% SE-30 chromosorb, 80/100 WHP, 2 m 1/800 SS column at an oven temperature 180 C. Injector and detector temperature were kept as 250 C each. Nitrogen was used as carrier gas and the flow rate was maintained at 20 ml/min. Samples were injected in aqueous phase of 2 ml in volume [13].
3. Results and discussion 3.1. Effect of HRT HRT study was carried out to see its effect on the degradation of nitrobenzene. Reactor performance was evaluated at five different HRTs, viz., 29.55, 27.20, 24.99, 18, and 14 h. Substrate loading rate on TF at five different HRTs was 0.1062, 0.1154, 0.1649, 0.1741 and 0.2247 kg/m3/d, respectively. During HRT studies C:N ratio was maintained at 100:20 and substrate:cosubstrate ratio was kept at 1:33. As the HRT decreased the concentration of nitrobenzene in the influent increased from 1.86 to 15.56 mg/l (Table 1) corresponding to 97.93–82.71% removal. On increasing HRT percentage removal of nitrobenzene increased except for 11.9 h HRT of ASP. This may be due to the fact that influent nitrobenzene concentration of ASP was higher at this HRT, which suppresses the biodegradation because of its toxicity. Intermediates were not identified in this study but it was expected to follow the degradation pathway proposed by Nishino and Spain [14,15], and Jung et al. [16]. It is assumed that the observed nitrobenzene removal in TF followed first-order kinetics according to the following expression: In ðCeff =Cinf Þ ¼ kt;
ð1Þ
where Ceff is the effluent nitrobenzene concentration and Cinf is the influent nitrobenzene concentration. Here k is nitrobenzene removal rate coefficient and t is the
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Table 1 Average steady-state performance data at different HRT (C:N=1:20, substrate:co-substrate=1:33) Parameter
HRT (h) 29.55 (20.3 TF, 9.25 ASP)
27.20 (18.7 TF, 8.5 ASP)
24.99 (13.09 TF, 11.9 ASP)
18.00 (12.4 TF 5.6ASP)
14.00 (9.6 TF, 4.4 ASP)
I COD 1440 BOD 580 Nitrobenzene 90 NH+ 102 4 -N NO ND 3 -N ND NO 2 -N PO3 10.2 4 -P DO 1.9 pH 7.6 Alkalinity 800 SS —
T
E
I
T
E
I
T
E
I
T
E
I
T
E
374 150 9.7 54 41.6 0.9 2.3 1.2 8.75 840 97
60 30 1.86 5.9 65 1.2 1.2 5.6 8.94 890 41
1440 580 90 103 ND ND 10.2 1.9 7.65 820 —
400 155 11.9 57.7 36.7 1.01 2.45 1.2 8.75 865 96
92 40 3.18 10.6 53 1.25 1.25 5.4 8.94 910 42
1412 580 90 103 ND ND 10.2 1.8 7.62 810 —
459 175 26.3 74 25 1.0 2.5 1.1 8.44 865 95
81 65 4.2 12.9 35 1.2 1.33 5.3 8.67 900 42
1440 570 90 102 ND ND 10.1 1.9 7.64 800 —
477 175 27.7 76.5 20 0.9 2.5 1.2 8.47 860 97
176 50 11.08 10.5 60 0.6 1.3 5.1 8.63 900 42
1440 570 90 105 ND ND 10.1 1.8 7.68 810 ––
512 190 32.4 78.3 22 0.8 2.65 1.2 8.49 855 96
235 55 15.56 27.8 52 0.6 1.35 5.0 8.79 905 43
All concentration are in mg/l except for alkalinity which is in mg CaCO3/l and pH. I=influent concentration; T=concentration in effluent of TF; E=concentration in effluent of activated sludge process; ND=not detectable. Table 2 Nitrobenzene removal rate constants Tricking filter HRT (h)
k1 (/d)
20.30 18.70 13.09 12.40 9.60 Standard deviation
2.63 2.59 2.25 2.28 2.55 70.18
Activated sludge process Maximum rate of substrate utilization (k) Half-velocity constant (KS )
0.0385 (/d) 12.96 mg/l
reaction time. Table 2 shows nitrobenzene removal rate coefficients k1 in TF at five different HRTs. Removal of nitrobenzene in ASP follows the rate equation: xy KS 1 ¼ þ ; So S kS k
and consumption due to biological growth. Nitrogen balance shows that 23.4–52.3% nitrogen removal occurred at different HRTs in the reactor. Reactor performance showed very poor SND activity in TF. This may be due to toxicity of nitrobenzene. During HRT studies 86.66–88.23% PO3 4 -P removal was achieved for an influent PO3 4 -P concentration of 10.1–10.2 mg/l. This can be attributed to the phosphate precipitation in the biofilm [17] caused by the increase in pH observed in this study. BOD in the effluent varied from 30 to 65 mg/l. Increase in alkalinity and pH was observed in the effluent. The suspended solid in the clarified effluent varied from 41 to 43 mg/l. Lowest effluent concentration of nitrobenzene at 29.55 h HRT suggests that nitrobenzene is a recalcitrant compound. In this process bacteria initially degrades sodium acetate, which induces the secretion of required enzymes [14–16]. These enzymes probably lead to the oxidative or reductive pathway for the biodegradation of nitrobenzene as mentioned earlier.
ð2Þ
where X is the concentration of microorganisms (mg/l), y is the hydraulic detention time (d), So is the Influent substrate concentration (mg/l), S is the effluent substrate concentration (mg/l), KS is the half-velocity constant (mg/l), k is the maximum rate of substrate utilization (/d). Intercept of plot of X y=ðSo SÞ vs. 1=S gives values of k and KS ; which were found to be 0.0385/d and 12.96 mg/l, respectively. Significant nitrogen removal mechanisms in the reactor are simultaneous nitrification and denitrification (SND) in biofilm of TF, stripping of ammonia in the AT
3.2. Effect of C:N ratio During C:N ratio studies HRT and substrate:cosubstrate ratio was maintained at 29.55 h and 1:33, respectively. The C:N ratio is a vital parameter for biodegradation as this establishes the ratio of heterotrophs to autotrophs in biomass [9]. At higher C:N ratio heterotrophs are dominating species and cause lower nitrification. This is clearly evident from Table 3. Effluent NO 3 -N concentration at four different C:N ratio was 51, 65, 67.5 and 71.6 mg/l. With the decrease in C:N ratio from 100:20 to 100:30, the effluent COD increased and this was because of the fact that at higher
ARTICLE IN PRESS P.S. Majumder, S.K. Gupta / Water Research 37 (2003) 4331–4336
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Table 3 Average steady-state performance data at different C:N ratio (HRT=29.55 h, substrate:co-substrate=1:33) Parameter
C:N ratio 100:15
COD BOD Nitrobenzene NH+ 4 -N NO 3 -N NO 2 -N TKN PO3 4 -P DO pH Alkalinity SS
100:20
100:25
100:30
I
T
E
I
T
E
I
T
E
I
T
E
1440 580 90 76 ND ND 84 9.1 1.9 7.51 710 —
394 155 16.05 48.3 25.6 0.78 49.5 1.8 1.1 8.64 760 98
88 40 3.88 0.8 51 0.31 1.2 0.8 5.6 8.72 810 42
1440 580 90 102 ND ND 111 10.2 1.9 7.6 800 —
374 150 9.7 54 41.6 0.9 54.5 2.3 1.2 8.75 840 97
60 30 1.86 5.9 65 1.2 6.0 1.2 5.6 8.94 890 41
1440 570 90 127 ND ND 135 9.1 1.9 7.69 910 —
482 160 16.85 65.5 41 0.96 67.5 1.9 1.1 8.77 975 97
164 40 4.12 20.2 67.5 1.3 20.4 1.0 5.7 8.94 1030 42
1448 590 89.75 151 ND ND 162 9.1 1.9 7.71 1020 —
530 170 17.81 70 47 1.15 72 1.85 1.1 8.79 1075 96
266 45 4.31 29.25 71.6 1.4 29.5 0.92 5.6 8.95 1120 43
All concentration are in mg/l except for alkalinity which is in mg CaCO3/l and pH. I=influent concentration; T=concentration in effluent of TF; E=concentration in effluent of ASP; ND=not detectable.
Table 4 Average steady-state performance data at different substrate:co-substrate ratio (HRT=29.55 h, C:N=100:20) Parameter
Substrate:co-substrate ratio 1:08
COD BOD Nitrobenzene NH+ 4 -N NO 3 -N NO 2 -N PO3 4 -P DO pH Alkalinity SS
1:16
1:33
1:48
I
T
E
I
T
E
I
T
E
I
T
E
500 200 89.75 103 ND ND 9.0 1.9 7.24 150 —
180 55 15.87 55 37 0.86 1.85 1.2 8.52 190 97
60 ND 4.56 5.25 70 1.12 0.89 5.5 8.72 240 42
800 390 90 102 ND ND 9.1 1.9 7.55 450 —
240 100 13.23 52.2 40.8 0.95 1.87 1.2 8.61 500 98
96 ND 2.20 4.8 69 1.25 1.1 5.7 8.92 540 42
1440 580 90 102 ND ND 10.2 1.9 7.6 800 —
374 150 9.7 54 41.6 0.9 2.3 1.2 8.75 840 97
60 30 1.86 5.9 65 1.2 1.2 5.6 8.94 890 41
2080 810 90 105 ND ND 9.1 2.0 7.62 1200 —
672 240 9.1 59 35.2 0.84 1.71 1.1 8.56 1260 98
320 55 1.84 10 54 1.3 1.2 5.6 8.79 1320 43
All concentration are in mg/l except for alkalinity which is in mg CaCO3/l and pH. I=influent concentration; T=concentration in effluent of TF; E=concentration in effluent of activated sludge process; ND=not detectable.
C:N ratio heterotrophs are dominating species causing COD reduction in the effluent. The optimum C:N ratio was 100:20 at which maximum removal of nitrobenzene (97.93%) and COD (95.83%) were observed. The influent and effluent samples from TF and AT were analyzed for all other parameters and they showed similar trends as was observed in HRT study. 3.3. Optimization of substrate:co-substrate Ratio This study was conducted to find out the optimum substrate:co-substrate ratio at which maximum efficiency of the reactor can be obtained. Table 4 shows the
experimental results obtained during this study. During this study HRT and C:N ratios were 29.55 h and 100:20 respectively. The effluent nitrobenzene concentration were 4.56, 2.20, 1.86, and 1.84 mg/l at substrate:cosubstrate ratio of 1:8, 1:16, 1:33, and 1:48 corresponding to 94.91%, 97.55%, 97.93%, and 97.95% removal, respectively. Effluent COD concentrations were 60, 96, 60, and 320 mg/l corresponding to 88%, 88%, 95.83%, and 84.61% removal, respectively. Considering the removal of nitrobenzene and COD it can be said that the optimum ratio was 1:33 at which removal of nitrobenzene was considerable and removal of COD was higher than at other ratios.
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3.4. Effect of shock loading
*
The performance of the system under shock load was studied at different substrate concentration of 135 and 180 mg/l corresponding to 1.5 and 2 times of acclimated concentration of nitrobenzene at substrate loading rate 0.159 and 0.2125 kg/m3/d, respectively on the TF. HRT, C:N ratio and substrate:co-substrate ratio were 29.55 h, 100:20 and 1:33, respectively. During shock loading studies only the effluent COD and nitrobenzene concentration were measured. The effluent COD value in the first shock load study was 206 mg/l while effluent nitrobenzene concentration was 4.39 mg/l corresponding to 86.22% and 96.74% removal, respectively. The Effluent COD in the second shock load study was 235 mg/l and nitrobenzene was 6.35 mg/l corresponding to 84.64% and 96.47% removal, respectively. This shows that the removal efficiency of the system was not completely inhibited even at high concentration of nitrobenzene and it can be concluded that the reactor can withstand shock loads up to 2.0 times the original concentration of nitrobenzene.
*
3.5. Volatilization estimation The volatilization losses were determined by adopting static headspace analysis technique. The influent concentration of nitrobenzene was kept around 90 mg/l and from gas chromatography analysis the effluent concentration of nitrobenzene was found to be around 81.50 mg/l. This study showed that the removal of nitrobenzene by volatilization is 8.50 mg/l corresponding to 9.44% of influent nitrobenzene concentration and removal due to the biodegradation was 87.49% of the influent nitrobenzene concentration. Metabolites were not identified in this case also.
4. Conclusions This study demonstrated the possibility of using hybrid reactor containing mixed consortia of biofilm for the treatment of wastewater having nitrobenzene. Based on this study following conclusions can be drawn: *
*
*
The effluent concentration of nitrobenzene can be brought below the regulatory level of 2 mg/l by this process. The optimum HRT, C:N ratio, substrate:co-substrate ratio were found to be 29.55 h, 100:20 and 1:33, respectively. The removal of nitrobenzene was 97.93% at the influent nitrobenzene concentration of 90 mg/l at an optimum HRT, C:N ratio and substrate:co-substrate ratio.
*
The pH of the effluent was found to be higher than the influent and it was found to be 8.6–8.9. The hybrid reactor withstood shock loads up to two times (180 mg/l) of acclimated concentration of nitrobenzene. The loss of nitrobenzene due to the volatilization was found to be 9.44%.
References [1] Kobayashi H, Rittmann BE. Microbial removal of hazardous organic compounds. Environ Sci Technol 1992; 16:170–80. [2] Metcalf and Eddy Inc. Wastewater engineering: treatment, disposal and reuse. New Delhi: Tata McGraw Hill Publishing Company; 1996. [3] Haigler BE, Spain JC. Biotransformation of nitrobenzene by bacteria containing toluene degradative pathways. Appl Environ Microbiol 1991;57(11):3156–62. [4] Hallas LE, Alexander M. Microbial transformation of nitroaromatic compounds in sewage effluent. Appl Environ Microbiol 1983;45(4):1234–41. [5] Dickel O, Kanekar JC, Knackmuss HJ. Biodegradation of nitrobenzene by a sequential anaerobic aerobic process. Biodegradation 1993;4:187–94. [6] Marvin-Sikkema FD, de Bont JAM. Degradation of nitroaromatic compounds by microorganisms. Appl Microbiol Biotechnol 1994;42:499–507. [7] Harison JR, Daigger GT, Filbert JW. A survey of combined trickling filter and activated sludge processes. Res J Water Pollut Control Fed 1984;56:1073–9. [8] Misra C, Gupta SK. Hybrid reactor for priority pollutant trichloroethylene removal. Water Res 2001;35(1):160–6. [9] Radhakrishnan L. Removal of priority pollutant: nitrobenzene by algal-bacterial system in rotating biological contactor. M. Tech. Dissertation, IIT Bombay, 1997. [10] APHA. Standard methods for the examination of water and wastewater. Washington, DC: American Public Health Association; 1985. [11] APHA. Standard methods for the examination of water and wastewater. Washington, DC: American Public Health Association; 1976. [12] APHA, AWWA and WPCF. Standard methods for the examination of water and wastewater. Washington, DC: American Public Health Association; 1998. [13] Patil SS, Shinde VM. Biodegradation studies of aniline and nitrobenzene plant wastewater by Gas Chromatography. Environ Sci Technol 1988;22(10):1160–5. [14] Nishino SF, Spain JC. Degradation of nitrobenzene by a Pseudomonas pseudoalcaligenes. Appl Environ Microbiol 1993;59(8):2520–4. [15] Nishino SF, Spain JC. Oxidative pathway for the biodegradation of nitrobenzene by a Comamonas sp. Appl Environ Microbiol 1995;61(6):2308–13. [16] Jung KH, Lee JH, Kim HS. Biodegradation of nitrobenzene through a hybrid pathway in Pseudomonas putida. Biotechnol Bioeng 1995;48:625–30. [17] Arvin E, Kristensen GH. Phosphate precipitation in biofilms and flocs. Water Sci Technol 1983;15:65–8.
Water Research 37 (2003) 4331–4336
Hybrid reactor for priority pollutant nitrobenzene removal Partha Sarathi Majumder, S.K. Gupta* Centre for Environmental Science and Engineering, Indian Institute of Technology, Powai, Mumbai 400076, India Received 30 October 2001; accepted 24 July 2003
Abstract The performance of a hybrid reactor, comprising of trickling filter and activated sludge process, in treating nitrobenzene wastewater was investigated. Acetate induced cells of mixed consortia was acclimatized with gradual increase of nitrobenzene concentration up to 90 mg/l in 100 days using sodium acetate as co-substrate and considering COD and nitrobenzene concentration as paramount parameters for assessing the growth of biofilm and acclimation. A removal of 60–95.80% COD and 80–90.23% nitrobenzene was observed during acclimation. During hydraulic retention time (HRT) studies, the optimum HRT was found to be 29.55 h at which a maximum of 95.83% COD and 97.93% nitrobenzene removal was observed. Other studies included optimization of C:N ratio, substrate:co-substrate ratio, effect of shock loading and estimation of volatilization losses. The optimum C:N ratio was found to be 100:20 at which maximum 97.93% removal of nitrobenzene was observed. At optimum HRT (29.55 h) and optimum C:N ratio (100:20) optimum substrate:co-substrate ratio was found to be 1:33. From the shock load studies it can be concluded that the system can withstand shock load up to two times of usual nitrobenzene concentration. A loss of 9.44% nitrobenzene was observed due to volatilization and mass balance gave an efficiency of 87.49% biological removal of nitrobenzene. r 2003 Elsevier Ltd. All rights reserved. Keywords: Nitrobenzene; Trickling filter; Activated sludge process; Hybrid reactor
1. Introduction Anthropogenic activities cause the massive use of natural resources and these lead to pollution. Among domestic wastewater, industrial wastewater and run-off from agricultural field industrial wastewater is more important because of the presence of both organic and inorganic compounds. Sixty-five classes of such compounds are considered as hazardous [1] and 129 organic and inorganic compounds have been designated as ‘‘priority pollutant’’ by USEPA on the basis of their known or suspected carcinogenicity, mutagenicity, teratogenicity or high acute toxicity [2]. Nitrobenzene is one of the nitroaromatic compounds having considerable industrial importance. It has been *Corresponding author. Tel.: +91-22-5767853; fax: +91-225723480. E-mail address: [email protected] (S.K. Gupta).
estimated that about 19 million lb of nitrobenzene is released into the environment annually [3]. It causes vomiting, skin and eye irritation, and headache. Its continuous exposure leads to liver damage and anemia. If the concentration exceeds 2 mg/l in the waste then it is declared as hazardous waste (USEPA). Because of its toxicity, Nitrobenzene has been placed as one of the 129 priority pollutants. Among various physical, chemical and biological methods for removal of nitrobenzene the biological treatment option is most cost effective. Sewage effluent under anaerobic condition converts nitrobenzene to aniline but no aromatic amine was detected under aerobic condition [4]. Dickel et al. [5] reported that nitroaromatics are gratuitously converted to the corresponding aromatic amines in reductive condition. Marvin-Sikkema and Bont [6] reported four probable pathways of degradation of nitrobenzene. Fixed film system has higher biomass population and longer solid
0043-1354/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0043-1354(03)00436-6
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added, which was prepared by using dextrose (3.0 g/l), ammonium nitrate (0.685 g/l), dipotassium hydrogen phosphate (0.067 g/l) and trace metal solution. Trace metal solution was contributing 100 mg Na—EDTA, 4.4 mg ZnSO4, 11.0 mg CaCl2, 10.12 mg MnCl2 4H2O, 10.0 mg FeSO4 7H2O, 2.2 mg (NH4)6Mo7O24 4H2O, 3.22 mg CoCl2 5H2O, 3.14 mg CuSO4 5H2O, and 0.228 mg H3BO3 per liter in the feed [9].
retention time (SRT). Secondary treatment processes combining both fixed growth (trickling filter (TF)) and suspended growth (activated sludge) systems are becoming very popular. These processes offer the simplicity of operation and process stability of a fixed growth system with the high effluent quality associated with suspended growth system. This also offers economic advantage in comparison to the other operation [7]. Misra and Gupta [8] examined the potential of hybrid biological reactor for treatment of wastewater containing trichloroethylene. This paper presents the development of acclimated biofilm on support matrix and suspension and the study of the hybrid reactor’s performance at different HRT, C:N ratio, substrate:co-substrate ratio, and under shock loads.
2.2. Experimental set-up The hybrid reactor (Fig. 1) consisted of a TF and an aeration tank (AT) giving a combination of attached growth and suspended growth systems similar to reactors used by Misra and Gupta [8] was employed. A portion of settled sludge from clarifier was recycled to the AT. At five different HRTs of AT 4.4, 5.6, 8.5, 9.25 and 11.9 h SRTs were 7.2, 9.0, 16.8, 19.4 and 22.1 d, respectively. TF consisted of PVC raschig rings as support media had a volume of 6.0 l (0.1 m 0.1 m 0.6 m). Total effective surface area and specific surface area of the TF were 1.29 m2 and 216.5 m2/m3, respectively. The void volume of TF was 5.5 l. AT had a volume of 10 l (0.2 m 0.2 m 0.25 m). Its effective volume was adjusted to 2.5, 5.0 7.5 and 10.0 l by four sampling ports at a distance of 0.625 m from the other adjacent port. Out of four sampling ports only lower two were used to adjust the volume of the reactor at 2.5 and 5.0 l during HRT studies. Diffused aerators were used for aeration at 2–2.2 l/min aeration rate. Two aerators were used for aeration.
2. Materials and methods 2.1. Seed culture An actively grown culture was collected from a reactor in our laboratory used for maintaining seed culture. The culture was maintained in a nutrient medium of dextrose, ammonium nitrate, dipotassium hydrogen phosphate and trace metal solution with a ratio of carbon, nitrogen, and phosphorous as 100:20:1. Every alternate day aeration was stopped for half an hour, supernatant was discarded from the reactor after settlement of biomass and feed/nutrient media (3 l) was
Influent
Trickling Filter
Sampling Port for Aeration Tank
Effluent to clarifier
Diffuser Effluent Activated sludge unit Clarifier
Returned Activated Sludge
Wastage
Fig. 1. Schematic diagram of the hybrid reactor.
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2.3. Experimental procedure The system was operated with an influent flow rate of 7 ml/min and C:N:P ratio of feed was maintained at 100:20:1 during acclimation period. Above mentioned seed culture was taken in reactor and was acclimatized to 90 mg/l of nitrobenzene (Synthesis grade 99% pure, E. Merck, India) and this was done by increment of influent nitrobenzene concentration to 90 mg/l. Acclimatization was started with 4 mg/l nitrobenzene and was increased to 20 mg/l nitrobenzene concentration by increment of 4 mg/l of nitrobenzene in each increment in 8 days, then increased to 25 mg/l and after that it was increased to 85 mg/l of nitrobenzene by an increment of 15 mg/l in each increment, in 10 days. At last influent nitrobenzene concentration was increased from 85 to 90 mg/l for a period of 10 days. System took 100 days for acclimation. Acclimation was confirmed by a steadiness of effluent COD and nitrobenzene concentration. The influent simulated wastewater used for HRT studies was containing 90 mg/l nitrobenzene, 3.00 g/l sodium acetate, 0.36 g/l NH4Cl, 0.0392 g/l K2HPO4, 0.0168 g/l KH2PO4, 0.001 g/l MgSO4. 7H2O, and trace metal solution [9]. Five HRT studies were conducted at 14 h (TF 9.6 h+ASP 4.4 h), 18 h (TF 12.4 h+ASP 5.6 h), 24.99 h (TF 13.09 h+ASP 11.90 h), 27.20 h (TF 18.70 h+ASP 8.50 h), and 29.55 h (TF 20.3 h+ASP 9.25 h) to optimize the system in terms of surface loading with respect to nitrobenzene and COD removal. Higher HRTs were chosen because of recalcitrant nature of nitrobenzene. HRT was varied by varying influent flow rate from minimum 4.51 to maximum 9.54 ml/min. Effective volume of TF was 5.5 l in all cases and effective volume of AT was 5 l in case of 24.99 h HRT and 2.5 l in all other cases. Studies were also carried out at four different C:N ratios of 100:15, 100:20, 100:25, and 100:30 to obtain an optimum C:N ratio for maximum nitrobenzene removal. The substrate nitrobenzene concentration was kept around 90 mg/l in the simulated wastewater and C:N ratio was varied by varying NH4Cl concentration. Trace metal solution was also added in the feed. The performance of the reactor was evaluated at four different substrate:co-substrate ratio of 1:8, 1:16, 1:33, and 1:48. The substrate:co-substrate ratio was varied by using different concentration of sodium acetate. Substrate nitrobenzene concentration was kept around 90 mg/l in the simulated wastewater and trace metal solution was added in the feed. Trace metal solution contributed different metal concentration in the same amount as in HRT studies. The performance of the reactor was also evaluated at two different shock loadings. The substrate concentrations at two shock loads were 135 and 180 mg/l corresponding to 1.5 and 2 times the concentration of nitrobenzene used during HRT studies. During all the above mentioned studies reactor temperature varied
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from 25 C to 30 C. MLSS of the AT was maintained around 4000–4200 mg/l for all studies. 2.4. Analytical methods Influent and effluent BOD, COD, pH, dissolved oxygen (DO), suspended solids (SS), NH+ 4 -N, NO2 -N, 3 NO3 -N, TKN, PO4 -P, alkalinity were measured. Ammonia nitrogen was measured by nesslerization [10], nitrate nitrogen by brucine sulfate [11], nitrite nitrogen by diazotization, TKN by macro-kjeldahl and phosphate phosphorous by stannous chloride method [12]. Effluent SS was SS of clarified effluent. DO meter (YSI model 58, USA) was used for DO measurement. Nitrobenzene concentration was analyzed by gas chromatography (GC) (Model Sigma 2000 capillary chromatograph, Perkin-Elmer) using 15% SE-30 chromosorb, 80/100 WHP, 2 m 1/800 SS column at an oven temperature 180 C. Injector and detector temperature were kept as 250 C each. Nitrogen was used as carrier gas and the flow rate was maintained at 20 ml/min. Samples were injected in aqueous phase of 2 ml in volume [13].
3. Results and discussion 3.1. Effect of HRT HRT study was carried out to see its effect on the degradation of nitrobenzene. Reactor performance was evaluated at five different HRTs, viz., 29.55, 27.20, 24.99, 18, and 14 h. Substrate loading rate on TF at five different HRTs was 0.1062, 0.1154, 0.1649, 0.1741 and 0.2247 kg/m3/d, respectively. During HRT studies C:N ratio was maintained at 100:20 and substrate:cosubstrate ratio was kept at 1:33. As the HRT decreased the concentration of nitrobenzene in the influent increased from 1.86 to 15.56 mg/l (Table 1) corresponding to 97.93–82.71% removal. On increasing HRT percentage removal of nitrobenzene increased except for 11.9 h HRT of ASP. This may be due to the fact that influent nitrobenzene concentration of ASP was higher at this HRT, which suppresses the biodegradation because of its toxicity. Intermediates were not identified in this study but it was expected to follow the degradation pathway proposed by Nishino and Spain [14,15], and Jung et al. [16]. It is assumed that the observed nitrobenzene removal in TF followed first-order kinetics according to the following expression: In ðCeff =Cinf Þ ¼ kt;
ð1Þ
where Ceff is the effluent nitrobenzene concentration and Cinf is the influent nitrobenzene concentration. Here k is nitrobenzene removal rate coefficient and t is the
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Table 1 Average steady-state performance data at different HRT (C:N=1:20, substrate:co-substrate=1:33) Parameter
HRT (h) 29.55 (20.3 TF, 9.25 ASP)
27.20 (18.7 TF, 8.5 ASP)
24.99 (13.09 TF, 11.9 ASP)
18.00 (12.4 TF 5.6ASP)
14.00 (9.6 TF, 4.4 ASP)
I COD 1440 BOD 580 Nitrobenzene 90 NH+ 102 4 -N NO ND 3 -N ND NO 2 -N PO3 10.2 4 -P DO 1.9 pH 7.6 Alkalinity 800 SS —
T
E
I
T
E
I
T
E
I
T
E
I
T
E
374 150 9.7 54 41.6 0.9 2.3 1.2 8.75 840 97
60 30 1.86 5.9 65 1.2 1.2 5.6 8.94 890 41
1440 580 90 103 ND ND 10.2 1.9 7.65 820 —
400 155 11.9 57.7 36.7 1.01 2.45 1.2 8.75 865 96
92 40 3.18 10.6 53 1.25 1.25 5.4 8.94 910 42
1412 580 90 103 ND ND 10.2 1.8 7.62 810 —
459 175 26.3 74 25 1.0 2.5 1.1 8.44 865 95
81 65 4.2 12.9 35 1.2 1.33 5.3 8.67 900 42
1440 570 90 102 ND ND 10.1 1.9 7.64 800 —
477 175 27.7 76.5 20 0.9 2.5 1.2 8.47 860 97
176 50 11.08 10.5 60 0.6 1.3 5.1 8.63 900 42
1440 570 90 105 ND ND 10.1 1.8 7.68 810 ––
512 190 32.4 78.3 22 0.8 2.65 1.2 8.49 855 96
235 55 15.56 27.8 52 0.6 1.35 5.0 8.79 905 43
All concentration are in mg/l except for alkalinity which is in mg CaCO3/l and pH. I=influent concentration; T=concentration in effluent of TF; E=concentration in effluent of activated sludge process; ND=not detectable. Table 2 Nitrobenzene removal rate constants Tricking filter HRT (h)
k1 (/d)
20.30 18.70 13.09 12.40 9.60 Standard deviation
2.63 2.59 2.25 2.28 2.55 70.18
Activated sludge process Maximum rate of substrate utilization (k) Half-velocity constant (KS )
0.0385 (/d) 12.96 mg/l
reaction time. Table 2 shows nitrobenzene removal rate coefficients k1 in TF at five different HRTs. Removal of nitrobenzene in ASP follows the rate equation: xy KS 1 ¼ þ ; So S kS k
and consumption due to biological growth. Nitrogen balance shows that 23.4–52.3% nitrogen removal occurred at different HRTs in the reactor. Reactor performance showed very poor SND activity in TF. This may be due to toxicity of nitrobenzene. During HRT studies 86.66–88.23% PO3 4 -P removal was achieved for an influent PO3 4 -P concentration of 10.1–10.2 mg/l. This can be attributed to the phosphate precipitation in the biofilm [17] caused by the increase in pH observed in this study. BOD in the effluent varied from 30 to 65 mg/l. Increase in alkalinity and pH was observed in the effluent. The suspended solid in the clarified effluent varied from 41 to 43 mg/l. Lowest effluent concentration of nitrobenzene at 29.55 h HRT suggests that nitrobenzene is a recalcitrant compound. In this process bacteria initially degrades sodium acetate, which induces the secretion of required enzymes [14–16]. These enzymes probably lead to the oxidative or reductive pathway for the biodegradation of nitrobenzene as mentioned earlier.
ð2Þ
where X is the concentration of microorganisms (mg/l), y is the hydraulic detention time (d), So is the Influent substrate concentration (mg/l), S is the effluent substrate concentration (mg/l), KS is the half-velocity constant (mg/l), k is the maximum rate of substrate utilization (/d). Intercept of plot of X y=ðSo SÞ vs. 1=S gives values of k and KS ; which were found to be 0.0385/d and 12.96 mg/l, respectively. Significant nitrogen removal mechanisms in the reactor are simultaneous nitrification and denitrification (SND) in biofilm of TF, stripping of ammonia in the AT
3.2. Effect of C:N ratio During C:N ratio studies HRT and substrate:cosubstrate ratio was maintained at 29.55 h and 1:33, respectively. The C:N ratio is a vital parameter for biodegradation as this establishes the ratio of heterotrophs to autotrophs in biomass [9]. At higher C:N ratio heterotrophs are dominating species and cause lower nitrification. This is clearly evident from Table 3. Effluent NO 3 -N concentration at four different C:N ratio was 51, 65, 67.5 and 71.6 mg/l. With the decrease in C:N ratio from 100:20 to 100:30, the effluent COD increased and this was because of the fact that at higher
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Table 3 Average steady-state performance data at different C:N ratio (HRT=29.55 h, substrate:co-substrate=1:33) Parameter
C:N ratio 100:15
COD BOD Nitrobenzene NH+ 4 -N NO 3 -N NO 2 -N TKN PO3 4 -P DO pH Alkalinity SS
100:20
100:25
100:30
I
T
E
I
T
E
I
T
E
I
T
E
1440 580 90 76 ND ND 84 9.1 1.9 7.51 710 —
394 155 16.05 48.3 25.6 0.78 49.5 1.8 1.1 8.64 760 98
88 40 3.88 0.8 51 0.31 1.2 0.8 5.6 8.72 810 42
1440 580 90 102 ND ND 111 10.2 1.9 7.6 800 —
374 150 9.7 54 41.6 0.9 54.5 2.3 1.2 8.75 840 97
60 30 1.86 5.9 65 1.2 6.0 1.2 5.6 8.94 890 41
1440 570 90 127 ND ND 135 9.1 1.9 7.69 910 —
482 160 16.85 65.5 41 0.96 67.5 1.9 1.1 8.77 975 97
164 40 4.12 20.2 67.5 1.3 20.4 1.0 5.7 8.94 1030 42
1448 590 89.75 151 ND ND 162 9.1 1.9 7.71 1020 —
530 170 17.81 70 47 1.15 72 1.85 1.1 8.79 1075 96
266 45 4.31 29.25 71.6 1.4 29.5 0.92 5.6 8.95 1120 43
All concentration are in mg/l except for alkalinity which is in mg CaCO3/l and pH. I=influent concentration; T=concentration in effluent of TF; E=concentration in effluent of ASP; ND=not detectable.
Table 4 Average steady-state performance data at different substrate:co-substrate ratio (HRT=29.55 h, C:N=100:20) Parameter
Substrate:co-substrate ratio 1:08
COD BOD Nitrobenzene NH+ 4 -N NO 3 -N NO 2 -N PO3 4 -P DO pH Alkalinity SS
1:16
1:33
1:48
I
T
E
I
T
E
I
T
E
I
T
E
500 200 89.75 103 ND ND 9.0 1.9 7.24 150 —
180 55 15.87 55 37 0.86 1.85 1.2 8.52 190 97
60 ND 4.56 5.25 70 1.12 0.89 5.5 8.72 240 42
800 390 90 102 ND ND 9.1 1.9 7.55 450 —
240 100 13.23 52.2 40.8 0.95 1.87 1.2 8.61 500 98
96 ND 2.20 4.8 69 1.25 1.1 5.7 8.92 540 42
1440 580 90 102 ND ND 10.2 1.9 7.6 800 —
374 150 9.7 54 41.6 0.9 2.3 1.2 8.75 840 97
60 30 1.86 5.9 65 1.2 1.2 5.6 8.94 890 41
2080 810 90 105 ND ND 9.1 2.0 7.62 1200 —
672 240 9.1 59 35.2 0.84 1.71 1.1 8.56 1260 98
320 55 1.84 10 54 1.3 1.2 5.6 8.79 1320 43
All concentration are in mg/l except for alkalinity which is in mg CaCO3/l and pH. I=influent concentration; T=concentration in effluent of TF; E=concentration in effluent of activated sludge process; ND=not detectable.
C:N ratio heterotrophs are dominating species causing COD reduction in the effluent. The optimum C:N ratio was 100:20 at which maximum removal of nitrobenzene (97.93%) and COD (95.83%) were observed. The influent and effluent samples from TF and AT were analyzed for all other parameters and they showed similar trends as was observed in HRT study. 3.3. Optimization of substrate:co-substrate Ratio This study was conducted to find out the optimum substrate:co-substrate ratio at which maximum efficiency of the reactor can be obtained. Table 4 shows the
experimental results obtained during this study. During this study HRT and C:N ratios were 29.55 h and 100:20 respectively. The effluent nitrobenzene concentration were 4.56, 2.20, 1.86, and 1.84 mg/l at substrate:cosubstrate ratio of 1:8, 1:16, 1:33, and 1:48 corresponding to 94.91%, 97.55%, 97.93%, and 97.95% removal, respectively. Effluent COD concentrations were 60, 96, 60, and 320 mg/l corresponding to 88%, 88%, 95.83%, and 84.61% removal, respectively. Considering the removal of nitrobenzene and COD it can be said that the optimum ratio was 1:33 at which removal of nitrobenzene was considerable and removal of COD was higher than at other ratios.
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3.4. Effect of shock loading
*
The performance of the system under shock load was studied at different substrate concentration of 135 and 180 mg/l corresponding to 1.5 and 2 times of acclimated concentration of nitrobenzene at substrate loading rate 0.159 and 0.2125 kg/m3/d, respectively on the TF. HRT, C:N ratio and substrate:co-substrate ratio were 29.55 h, 100:20 and 1:33, respectively. During shock loading studies only the effluent COD and nitrobenzene concentration were measured. The effluent COD value in the first shock load study was 206 mg/l while effluent nitrobenzene concentration was 4.39 mg/l corresponding to 86.22% and 96.74% removal, respectively. The Effluent COD in the second shock load study was 235 mg/l and nitrobenzene was 6.35 mg/l corresponding to 84.64% and 96.47% removal, respectively. This shows that the removal efficiency of the system was not completely inhibited even at high concentration of nitrobenzene and it can be concluded that the reactor can withstand shock loads up to 2.0 times the original concentration of nitrobenzene.
*
3.5. Volatilization estimation The volatilization losses were determined by adopting static headspace analysis technique. The influent concentration of nitrobenzene was kept around 90 mg/l and from gas chromatography analysis the effluent concentration of nitrobenzene was found to be around 81.50 mg/l. This study showed that the removal of nitrobenzene by volatilization is 8.50 mg/l corresponding to 9.44% of influent nitrobenzene concentration and removal due to the biodegradation was 87.49% of the influent nitrobenzene concentration. Metabolites were not identified in this case also.
4. Conclusions This study demonstrated the possibility of using hybrid reactor containing mixed consortia of biofilm for the treatment of wastewater having nitrobenzene. Based on this study following conclusions can be drawn: *
*
*
The effluent concentration of nitrobenzene can be brought below the regulatory level of 2 mg/l by this process. The optimum HRT, C:N ratio, substrate:co-substrate ratio were found to be 29.55 h, 100:20 and 1:33, respectively. The removal of nitrobenzene was 97.93% at the influent nitrobenzene concentration of 90 mg/l at an optimum HRT, C:N ratio and substrate:co-substrate ratio.
*
The pH of the effluent was found to be higher than the influent and it was found to be 8.6–8.9. The hybrid reactor withstood shock loads up to two times (180 mg/l) of acclimated concentration of nitrobenzene. The loss of nitrobenzene due to the volatilization was found to be 9.44%.
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