- Email: [email protected]
Development of an expanded-bed GAC reactor for anaerobic treatment of terephthalate-containing wastewater
water research 43 (2009) 417–422
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Development of an expanded-bed GAC reactor for anaerobic treatment of terephthalate-containing wastewater Hiroshi Tsuno*, Masasumi Kawamura Department of Urban and Environmental Engineering, Graduate School of Engineering, Kyoto University, Kyoto-Daigaku-Katsura C1, Nishikyo-ku, Kyoto 615-8540, Japan
article info
abstract
Article history:
An expanded-bed granular activated carbon (GAC) anaerobic reactor was developed to treat
Received 23 March 2008
terephthalate-containing wastewater. Terephthalate inhibits biological anaerobic degra-
Received in revised form
dation of terephthalate and methane production when present at a concentration of more
5 September 2008
than 150 mg/L. In the GAC anaerobic reactor developed here, degradation of terephthalate
Accepted 27 October 2008
and other organic compounds occurred smoothly and stably with removal and methane
Published online 6 November 2008
fermentation ratios of more than 90% under a chemical oxygen demand (COD) loading rate of 4 kg COD/(m3 d) and a terephthalate loading rate of 1 kg terephthalate/(m3 d).
Keywords:
ª 2008 Published by Elsevier Ltd.
Anaerobic treatment Biofilm Expanded-bed reactor Granular activated carbon Terephthalate
1.
Introduction
Purified terephthalic acid (1,4-benzenedicarboxylic acid) is produced in large quantities to use as a raw material in the manufacture of polyethylene terephthalate bottles, textile fibers, and polyester films (Macarie et al., 1992). Wastewater containing terephthalate and acetate as the main organic constituents is discharged during the terephthalate production process. The wastewater is generally treated using an aerobic biological treatment process because biological degradation of terephthalate readily occurs under aerobic conditions, but little degradation occurs under anaerobic conditions. Recently, however, much attention has been given to anaerobic digestion technologies because of their advantages, which include a decrease in excess sludge production, lower energy consumption, and the ability to recover energy
from the process. A few studies have shown the possibility of biological degradation of terephthalate under anaerobic conditions (Macarie et al., 1992; Kleerebezem et al., 1997). However, stable and effective reactors are required, and few trials have been performed using a biofilm reactor with attached growth medium for microbes, such as polypropylene net and granules, and a two-stage reactor (Kleerebezem et al., 1999, 2005; Chen et al., 2004). To date, no successful reactor applications have been developed. Some researchers discussed inhibition of anaerobic digestion by terephthalic acid, but contradictory results have been reported. Its inhibition of anaerobic degradation has been found at concentrations of 500 mg/L by Kuang and Wang (1994) and 2160 mg/L by Macarie et al. (1992). On the other hand, there are also researches which showed no serious inhibition to any of species involved in terephthalic acid
* Corresponding author. Tel.: þ81 75 383 3350; fax: þ81 75 383 3351. E-mail addresses: [email protected] (H. Tsuno), [email protected] (M. Kawamura). 0043-1354/$ – see front matter ª 2008 Published by Elsevier Ltd. doi:10.1016/j.watres.2008.10.046
418
water research 43 (2009) 417–422
degradation at the commonly found concentration as high as more than 5000 mg/L (Pereboom et al., 1994; Kleerebezem et al., 1997; Fajardo et al., 1997). However, it can be concluded to be important to take the inhibition by terephthalic acid to each step of its anaerobic degradation, such as acidogenesis and methanogenesis, into consideration. An expanded-bed granular activated carbon (GAC) anaerobic reactors have been developed to treat wastewater containing hazardous, more or less recalcitrant and/or inhibitory chemicals (Suidan et al., 1983a,b; Tsuno et al., 1996, 2006). The reactors are characterized by a combination of physical and biological removal mechanisms, i.e. adsorption onto GAC and biological degradation by microorganisms growing on the GAC. The adsorptive function of GAC contributes towards the reduction in the aqueous concentration of inhibitory chemicals to levels below the threshold of inhibition, and aids stability in treatment performance against shock loads of the influent. The accumulation of microorganisms in the reactor, having the ability to degrade the target chemicals and bring about the regeneration of the GAC adsorptive capacity, is also expected because of the long-term retention of the chemicals at a low concentration and long solids retention times (SRT) of the microbes present. In this study, an expanded-bed GAC anaerobic reactor for high rate and stable treatment of terephthalate-containing wastewater was developed and its operating conditions were investigated experimentally.
Gas
Effluent Gasbag
Flange
Constant temperature circulator
Influent P
P Flow meter
2.
Materials and methods
The adsorptive characteristics of terephthalate on GAC used in this study were examined. A given weight of GAC (0–7 g/L) was added to 1 L of solution containing a given concentration of terephthalate (43–430 mg/L). The concentration of terephthalate in the solution was determined after mixing with a magnetic stirrer at 100 rpm for 10–100 h. A control test, in which no GAC was added to the solution, was also conducted. A schematic diagram of the expanded-bed GAC anaerobic reactor developed for the continuous treatment is shown in Fig. 1. The reactor was made of a thick Plexiglas tube with a diameter of 10 cm and a volume of 10 L. 1.5 kg GAC with a particle size of 0.9–1.1 mm was placed in the reactor as the attached growth medium. The liquid in the reactor was circulated from the top part to the bottom to expand the GAC medium by 25%, resulting in an expanded-bed volume of 4.3 L. Target wastewater to be treated was fed into the circulation line at a given flow rate. The water temperature in the reactor was maintained at 30 C using a hot water jacket fitted around the reactor. The gas produced was collected in a gasbag at the top of the reactor. The target wastewater for treatment was synthetic wastewater (Run 1) and actual wastewater discharged from the purified terephthalate production process (Runs 2 and 3). The synthetic wastewater contained only terephthalate and acetate as organic elements, and nutrients and minerals, as shown in Table 1. The actual wastewater was prepared by 10 times dilution of a terephthalate-synthesizing step wastewater with tap water and addition of acetate to give a concentration of 1000 mg/L to make the constituent similar
Recirculation pump
Fig. 1 – Schematic diagram of experimental apparatus: expanded-bed GAC anaerobic reactor.
as that of the actual wastewater from the total terephthalate production process. Hence, benzoate and other organics were present in the actual wastewater, in addition to terephthalate and acetate. Nutrients and minerals were also added to the wastewater to arrive at a composition shown in Table 1.
Table 1 – Composition of synthetic wastewater. Organics Terephthalic acid Sodium acetate
80–450 mg (given amount) 500–1000 mg (given amount)
Nutrients K2HPO4 KH2PO4 NH4Cl MgCl2$6H2O CaCl2$2H2O
0.35 g 0.23 g 0.50 g 0.41 g 0.25 g
Vitamins and mineralsa Tap water
Trace 1L
a Vitamins & minerals: Pyridoxine-HCl, 0.1 (mg); ZnSO4$7H2O, 0.3 (mg); Folic acid, 0.02; MnCl2$4H2O, 0.1; Biotin, 0.02; H3BO3, 0.9; Thiamine HCl, 0.05; CoCl2$6H2O, 0.6; Riboflavin, 0.05; CuCl2$2H2O, 0.03; Nicotinic acid, 0.05; NiCl2$6H2O, 0.06; Calcium pantothenate, 0.05; Na2MoO$2H2O, 0.1; p-Aminobenzoic acid, 0.05; Cystein-HCl, 300; Thioctic acid, 0.05.
419
water research 43 (2009) 417–422
3.
Results and discussion
A Freundlich-type isotherm curve of adsorption for terephthalate is shown in Fig. 2. It shows that terephthalate is
20
10
q*, mg TA/g-GAC
At the start of the continuous treatment experiment for the synthetic wastewater (Run 1), 100 mL of sludge was obtained from the anaerobic digestion tank in the Toba municipal sewage treatment plant of Kyoto City, Japan, and used to inoculate the reactor. For the actual wastewater treatment experiments (Run 2 and Run 3), effluent from the reactor operated in Run 1 was used to inoculate each reactor to seed them with microorganisms that had the ability to degrade terephthalate anaerobically. The operational conditions are summarized in Table 2. The continuous experiment was carried out for each target wastewater with increasing organic loading rate by decreasing hydraulic retention time (HRT) and/ or increasing concentrations of organics as well as supplemental materials in the influents. In Run 1, basic operational parameters were experimentally investigated using the synthetic wastewater. In Run 2, confirmation of anaerobic terephthalate degradation and the applicability of the reactor to the actual wastewater treatment were examined. Replicability was confirmed in Run 3 by increasing the organic loading rate more rapidly than in Run 2. During each experiment, water samples were taken at the inlet and outlet of the reactor once or twice a week, and the terephthalate and COD concentrations were measured. The gas production rate was calculated each day using the data from the volume and composition of the gas collected in the gasbag (gas analyzer CGT-7000, Shimadzu). COD measurements were based on the Standard Method (APHA, AWWA, and WPCF Standard Method, 1985), and terephthalate was measured by a spectrophotometric method at 254 nm and confirmed using a liquid chromatographical method (Fajardo et al., 1997). Oxidation–reduction potential (ORP) and pH values during good stable performance of treatment were in the range from 250 to 400 mV and from 7.5 to 8.0, respectively.
q* = 6.45 × Ce0.085 R2 = 0.976 1 0.1
1
10
100
1000
Ce, mg TA/L Fig. 2 – Isotherm curve of adsorption for terephthalic acid (TA) on GAC. Ce: concentration of TA in liquid phase; q*: adsorbed amount of TA on GAC.
adsorbed well on GAC and that the GAC reactor can be effective. The treatment characteristics of terephthalate-containing wastewater in Run 1 are shown in Fig. 3. After the 40th day of continuous operation, methane gas production started and increased gradually. From these results, terephthalate concentration in the effluent was considered to be under 6 mg/L, controlled first by GAC adsorption and then through biodegradation. However, treatment performance deteriorated after an increase in the terephthalate concentration in the influent from 85 to 430 mg/L and, eventually, effluent terephthalate concentration increased to the same level as that in the influent and methane production stopped (Phase 1). The influent terephthalate concentration was decreased to 220 mg/L on the 153rd day and then gradually
Table 2 – Experimental conditions in continuous treatment. Parameters
Run 1 Phase 1
Wastewater
Run 2
Run 3
Phase 2
Synthetic wastewater
Actual wastewater
Operation duration, d
153
1350
665
400
Organics in influent Terephthalic acid, mg/L Sodium acetate, mg/L COD, mg/Lb
85–430 500–1000 580–1510
220–430 1000 1170–1660
63–430 0–1000 230–2050
100–440 1000 900–2030
HRT, da
2.2
2.2–0.48
2.2–0.48
2.2–0.48
COD loading rate gCOD/(kg-GAC d) kgCOD/(m3 d)a
0.63–1.9 0.22–0.66
1.5–8.3 0.51–2.9
0.29–12 0.10–4.1
1.2–12 0.42–4.0
a Based on GAC expanded-bed volume. b Total COD including the other organics in addition to terephthalic acid and acetate.
420
700
Terephthalic acid, mg/L
Phase1
Phase2
600 500 400 300 Effluent
200
Influent 100 0 0
200
400
600
800
1000
1200
1400
2000
Methane production rate, TA removal rate
water research 43 (2009) 417–422
1 Methane production rate, L/d TA removal rate, mgTA/d
0.5
0 0
100
200
COD, mg/L
300
400
500
Terephthalic acid, mg/L
1500
Fig. 4 – Relationship among terephthalic acid (TA) concentration, methane production rate and terephthalic acid removal rate. (Methane volume is the value under 0 8C and 1 bar.)
1000 Influent Effluent (total) 500
Effluent (soluble)
0 0
400
600
800
1000
1200
1400
Theoretical value for organics* Theoretical value for acetate*
4
Value measured
The inhibition effects of terephthalate concentration in the reactor liquid on terephthalate removal rate and methane production rate are shown in Fig. 4 for data during Phase 1, and before the recovery in Phase 2. The acute inhibitory effects on both terephthalate removal rate and methane production rate occurred when its concentration was above
3 2
1.2
1 0 0
200
400
600
800
1000
1200
1400
Operation time, days Fig. 3 – Treatment performance in Run 1. (*Calculated from loading rate. Methane volume is the value under 0 8C and 1 bar.)
TA removal rate, kg/(m3 • d)
Methane production rate, L/d
5
200
1 0.8 0.6 0.4 Run1 Run2
0.2
Run3 0
0
0.2
0.4
0.6
0.8
1
1.2
TA loading rate, kg/(m3 • d) 4.5
COD removal rate, kg/(m3 • d)
increased to a final concentration of 430 mg/L by checking the treatment performance (Phase 2). Treatment performance had gradually recovered by the 300th day; terephthalate concentration and COD concentration in effluent had decreased to less than 2 and 30 mg/L, respectively, and the methane production rate also recovered to levels almost equal to, and sometimes, greater than the theoretical one calculated from the influent loading rate of terephthalate and acetate. The surplus methane production showed the methane fermentation of terephthalate adsorbed and accumulated on the GAC during the former days. This phenomenon was observed during each increase in the loading rate. After recovery, good operational performance, shown by a terephthalate concentration of less than 11 mg/L and COD concentration of less than 120 mg/L, was maintained during whole of Phase 2, except for some minor fluctuations, until a COD loading rate of 2.9 kg COD/(m3 d) was reached.
4 3.5 3 2.5 2 1.5
Run1
1
Run2 Run3
0.5 0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
COD loading rate, kg/(m3 • d) Fig. 5 – Relationship between loading rate and removal rate in COD and terephthalic acid (TA) (plots show mean ± standard deviation).
CH4 measured and calculated from the removed other organics than TA, L/(L • d)
water research 43 (2009) 417–422
2
4.
Run1 measured Run2 measured Run3 measured Run1 calculated from others Run2 calculated from others Run3 calculated from others
1.5
0.5
b From the other organics
0
0
0.5
1
Conclusions
The applicability of an expanded-bed GAC anaerobic reactor was experimentally verified for treatment of terephthalatecontaining wastewater using both, artificially prepared and actual wastewater. Its operational conditions were investigated and the main results are summarized as follows.
a From terephthalate
1
421
1.5
CH4 calculated from total organics removed, L/(L • d) Fig. 6 – Relationship between measured and calculated methane production rate (mean ± standard deviation. Methane volume is the value under 0 8C and 1 bar.)
150 mg/L, although this value was much smaller than the values found by the other researches (Kuang and Wang, 1994; Macarie et al., 1992; Pereboom et al., 1994; Kleerebezem et al., 1997; Fajardo et al., 1997). These results mean that terephthalate inhibits microbial groups associated with both first step of terephthalate degradation and methane production even from acetate. It is, therefore, important to operate the reactor at a terephthalate concentration below 150 mg/L. Degradation of terephthalate and the other organics included in the actual wastewater occurred smoothly and stably until the COD loading rate and terephthalate loading rate reached 4 kg COD/(m3 d) and 1 kg terephthalate/(m3 d), respectively, in Run 2 and Run 3, through a gradual increase of loading rate, while effluent terephthalate concentration was kept below 150 mg/L. Fig. 5 shows the relationships between loading rate and removal rate associated with COD and terephthalate. The slope of this graph indicates the removal ratio of all organics. In all runs, that is in Run 1 fed with artificial wastewater and Runs 2 and 3 fed with actual wastewater, more than 90% of both COD and terephthalate were stably removed with increasing COD and terephthalate loading rates up to 2.9 and 0.75 kg/(m3 d), respectively, in Run 1, and 4 and 1 kg/(m3 d), respectively, in Runs 2 and 3. This confirms the anaerobic degradation of terephthalate and applicability of the reactor in the treatment of actual wastewater. The results were replicable. The relationship between the calculated methane production rate from removed total COD and measured methane production rate is shown in Fig. 6. The measured rate is similar to the calculated one, which suggests that COD was removed through biological reactions. The methane production rate calculated from the removed organics, other than terephthalate, is also plotted against the one calculated from removed total COD in Fig. 6. The ratios between methane production rates by terephthalate degradation and by the degradation of other organics (corresponding to (a) and (b) in Fig. 6, respectively) correspond well to the composition ratio between terephthalate and the other organics in the influent, suggesting the simultaneous degradation and methane fermentation of organic substances.
(1) There was good adsorption of terephthalate onto the GAC. This followed the Freundlich-type isotherm curve. (2) Terephthalate has an inhibitory effect on biological anaerobic degradation of terephthalate and methane production. The threshold concentration of inhibition was 150 mg terephthalate/L. (3) In the expanded-bed GAC anaerobic reactor developed in this study, terephthalate and the other organics included in both artificial (Run 1) and actual wastewater (Runs 2 and 3) were smoothly and stably degraded until they reached a COD loading rate and terephthalate loading rate of 2.9 kg COD/(m3 d) (Run 1) and 4 kg COD/(m3 d) (Runs 2 and 3), and 0.75 kg terephthalate/(m3 d) (Run 1) and 1 kg terephthalate/(m3 d) (Runs 2 and 3), respectively, by gradually increasing the loading rate. (4) More than 90% of both COD and terephthalate were stably removed. A stable methane fermentation ratio of more than 90% was also achieved. (5) GAC adsorption ability contributed well to the stability of the treatment performance, especially in the start-up stage and just after an increase in loading rate. This stability mechanism was demonstrated by a surplus methane production rate greater than the theoretical one calculated from the loading rate. (6) The ratios between methane production rates by terephthalate degradation and by degradation of other organics correspond well to the composition ratios of them in the influent, which suggests the simultaneous degradation and methane fermentation of organic substrates.
references
APHA, AWWA, WPCF, 1985. Standard Method, 16th ed. Chen, C.-L., Macarie, H., Ramirez, I., Oimos, A., Ong, S.L., Monroy, O., Liu, W.-T., 2004. Microbial community structure in a thermophilic anaerobic hybrid reactor degrading terephthalate. Microbiology 150, 3429–3440. Fajardo, C., Guyot, J.P., Macarie, H., Monery, O., 1997. Inhibition of anaerobic digestion by terephthalic acid and its aromatic byproducts. Water Science and Technology 36 (6–7), 83–90. Kleerebezem, R., Montier, J., Pol, L.W.H., Lettinga, G., 1997. Anaerobic pre-treatment of petrochemical effluents: terephthalic acid wastewater. Water Science and Technology 36 (2–3), 237–248. Kleerebezem, R., Ivalo, M., Pol, L.W.H., Lettinga, G., 1999. Highrate treatment of terephthalate in anaerobic hybrid reactors. Biotechnology Progress 15, 347–357. Kleerebezem, R., Beckers, J., Pol, L.W.H., Lettinga, G., 2005. Highrate treatment of terephthalic acid production wastewater in a two-stage anaerobic bioreactor. Biotechnology and Bioengineering 91 (2), 169–179.
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Kuang, X., Wang, J., 1994. Anaerobic biodegradability of terephthalic acid and its inhibitory effect on anaerobic digestion. Journal of Environmental Sciences 6 (2), 180–188. Macarie, H., Noyola, A., Guyot, J.P., 1992. Anaerobic treatment of a petrochemical wastewater from a terephthalic acid plant. Water Science and Technology 25 (7), 223–235. Pereboom, J.H.F., De Man, G., Su, Y.T., 1994. Start-up of full scale UASB reactor for the treatment of terephthalic acid wastewater. In: Poster Papers, the 7th International Symposium on Anaerobic Digestion, Cape Town, South Africa, 23–27 January, pp. 307–312.
Suidan, M.T., Cross, W.H., Fong, M., Calvent, J.W., 1983a. Anaerobic carbon filter for degradation of phenols. ASCE EE 107 (3), 563–579. Suidan, M.T., Strubler, C.E., Kao, S.W., Pfeffer, J.T., 1983b. Treatment of coal gasification wastewater with anaerobic filter technology. Journal – Water Pollution Control Federation 55 (10). Tsuno, H., Kawamura, M., Somiya, I., 1996. Anaerobic degradation of pentachlorophenol (PCP) in biological expanded-bed reactor. Water Science and Technology 34 (5–6), 335–344. Tsuno, H., Kawamura, M., Oya, T., 2006. Application of biological activated carbon anaerobic reactor for treatment of hazardous chemicals. Water Science and Technology 53 (11), 251–260.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Development of an expanded-bed GAC reactor for anaerobic treatment of terephthalate-containing wastewater Hiroshi Tsuno*, Masasumi Kawamura Department of Urban and Environmental Engineering, Graduate School of Engineering, Kyoto University, Kyoto-Daigaku-Katsura C1, Nishikyo-ku, Kyoto 615-8540, Japan
article info
abstract
Article history:
An expanded-bed granular activated carbon (GAC) anaerobic reactor was developed to treat
Received 23 March 2008
terephthalate-containing wastewater. Terephthalate inhibits biological anaerobic degra-
Received in revised form
dation of terephthalate and methane production when present at a concentration of more
5 September 2008
than 150 mg/L. In the GAC anaerobic reactor developed here, degradation of terephthalate
Accepted 27 October 2008
and other organic compounds occurred smoothly and stably with removal and methane
Published online 6 November 2008
fermentation ratios of more than 90% under a chemical oxygen demand (COD) loading rate of 4 kg COD/(m3 d) and a terephthalate loading rate of 1 kg terephthalate/(m3 d).
Keywords:
ª 2008 Published by Elsevier Ltd.
Anaerobic treatment Biofilm Expanded-bed reactor Granular activated carbon Terephthalate
1.
Introduction
Purified terephthalic acid (1,4-benzenedicarboxylic acid) is produced in large quantities to use as a raw material in the manufacture of polyethylene terephthalate bottles, textile fibers, and polyester films (Macarie et al., 1992). Wastewater containing terephthalate and acetate as the main organic constituents is discharged during the terephthalate production process. The wastewater is generally treated using an aerobic biological treatment process because biological degradation of terephthalate readily occurs under aerobic conditions, but little degradation occurs under anaerobic conditions. Recently, however, much attention has been given to anaerobic digestion technologies because of their advantages, which include a decrease in excess sludge production, lower energy consumption, and the ability to recover energy
from the process. A few studies have shown the possibility of biological degradation of terephthalate under anaerobic conditions (Macarie et al., 1992; Kleerebezem et al., 1997). However, stable and effective reactors are required, and few trials have been performed using a biofilm reactor with attached growth medium for microbes, such as polypropylene net and granules, and a two-stage reactor (Kleerebezem et al., 1999, 2005; Chen et al., 2004). To date, no successful reactor applications have been developed. Some researchers discussed inhibition of anaerobic digestion by terephthalic acid, but contradictory results have been reported. Its inhibition of anaerobic degradation has been found at concentrations of 500 mg/L by Kuang and Wang (1994) and 2160 mg/L by Macarie et al. (1992). On the other hand, there are also researches which showed no serious inhibition to any of species involved in terephthalic acid
* Corresponding author. Tel.: þ81 75 383 3350; fax: þ81 75 383 3351. E-mail addresses: [email protected] (H. Tsuno), [email protected] (M. Kawamura). 0043-1354/$ – see front matter ª 2008 Published by Elsevier Ltd. doi:10.1016/j.watres.2008.10.046
418
water research 43 (2009) 417–422
degradation at the commonly found concentration as high as more than 5000 mg/L (Pereboom et al., 1994; Kleerebezem et al., 1997; Fajardo et al., 1997). However, it can be concluded to be important to take the inhibition by terephthalic acid to each step of its anaerobic degradation, such as acidogenesis and methanogenesis, into consideration. An expanded-bed granular activated carbon (GAC) anaerobic reactors have been developed to treat wastewater containing hazardous, more or less recalcitrant and/or inhibitory chemicals (Suidan et al., 1983a,b; Tsuno et al., 1996, 2006). The reactors are characterized by a combination of physical and biological removal mechanisms, i.e. adsorption onto GAC and biological degradation by microorganisms growing on the GAC. The adsorptive function of GAC contributes towards the reduction in the aqueous concentration of inhibitory chemicals to levels below the threshold of inhibition, and aids stability in treatment performance against shock loads of the influent. The accumulation of microorganisms in the reactor, having the ability to degrade the target chemicals and bring about the regeneration of the GAC adsorptive capacity, is also expected because of the long-term retention of the chemicals at a low concentration and long solids retention times (SRT) of the microbes present. In this study, an expanded-bed GAC anaerobic reactor for high rate and stable treatment of terephthalate-containing wastewater was developed and its operating conditions were investigated experimentally.
Gas
Effluent Gasbag
Flange
Constant temperature circulator
Influent P
P Flow meter
2.
Materials and methods
The adsorptive characteristics of terephthalate on GAC used in this study were examined. A given weight of GAC (0–7 g/L) was added to 1 L of solution containing a given concentration of terephthalate (43–430 mg/L). The concentration of terephthalate in the solution was determined after mixing with a magnetic stirrer at 100 rpm for 10–100 h. A control test, in which no GAC was added to the solution, was also conducted. A schematic diagram of the expanded-bed GAC anaerobic reactor developed for the continuous treatment is shown in Fig. 1. The reactor was made of a thick Plexiglas tube with a diameter of 10 cm and a volume of 10 L. 1.5 kg GAC with a particle size of 0.9–1.1 mm was placed in the reactor as the attached growth medium. The liquid in the reactor was circulated from the top part to the bottom to expand the GAC medium by 25%, resulting in an expanded-bed volume of 4.3 L. Target wastewater to be treated was fed into the circulation line at a given flow rate. The water temperature in the reactor was maintained at 30 C using a hot water jacket fitted around the reactor. The gas produced was collected in a gasbag at the top of the reactor. The target wastewater for treatment was synthetic wastewater (Run 1) and actual wastewater discharged from the purified terephthalate production process (Runs 2 and 3). The synthetic wastewater contained only terephthalate and acetate as organic elements, and nutrients and minerals, as shown in Table 1. The actual wastewater was prepared by 10 times dilution of a terephthalate-synthesizing step wastewater with tap water and addition of acetate to give a concentration of 1000 mg/L to make the constituent similar
Recirculation pump
Fig. 1 – Schematic diagram of experimental apparatus: expanded-bed GAC anaerobic reactor.
as that of the actual wastewater from the total terephthalate production process. Hence, benzoate and other organics were present in the actual wastewater, in addition to terephthalate and acetate. Nutrients and minerals were also added to the wastewater to arrive at a composition shown in Table 1.
Table 1 – Composition of synthetic wastewater. Organics Terephthalic acid Sodium acetate
80–450 mg (given amount) 500–1000 mg (given amount)
Nutrients K2HPO4 KH2PO4 NH4Cl MgCl2$6H2O CaCl2$2H2O
0.35 g 0.23 g 0.50 g 0.41 g 0.25 g
Vitamins and mineralsa Tap water
Trace 1L
a Vitamins & minerals: Pyridoxine-HCl, 0.1 (mg); ZnSO4$7H2O, 0.3 (mg); Folic acid, 0.02; MnCl2$4H2O, 0.1; Biotin, 0.02; H3BO3, 0.9; Thiamine HCl, 0.05; CoCl2$6H2O, 0.6; Riboflavin, 0.05; CuCl2$2H2O, 0.03; Nicotinic acid, 0.05; NiCl2$6H2O, 0.06; Calcium pantothenate, 0.05; Na2MoO$2H2O, 0.1; p-Aminobenzoic acid, 0.05; Cystein-HCl, 300; Thioctic acid, 0.05.
419
water research 43 (2009) 417–422
3.
Results and discussion
A Freundlich-type isotherm curve of adsorption for terephthalate is shown in Fig. 2. It shows that terephthalate is
20
10
q*, mg TA/g-GAC
At the start of the continuous treatment experiment for the synthetic wastewater (Run 1), 100 mL of sludge was obtained from the anaerobic digestion tank in the Toba municipal sewage treatment plant of Kyoto City, Japan, and used to inoculate the reactor. For the actual wastewater treatment experiments (Run 2 and Run 3), effluent from the reactor operated in Run 1 was used to inoculate each reactor to seed them with microorganisms that had the ability to degrade terephthalate anaerobically. The operational conditions are summarized in Table 2. The continuous experiment was carried out for each target wastewater with increasing organic loading rate by decreasing hydraulic retention time (HRT) and/ or increasing concentrations of organics as well as supplemental materials in the influents. In Run 1, basic operational parameters were experimentally investigated using the synthetic wastewater. In Run 2, confirmation of anaerobic terephthalate degradation and the applicability of the reactor to the actual wastewater treatment were examined. Replicability was confirmed in Run 3 by increasing the organic loading rate more rapidly than in Run 2. During each experiment, water samples were taken at the inlet and outlet of the reactor once or twice a week, and the terephthalate and COD concentrations were measured. The gas production rate was calculated each day using the data from the volume and composition of the gas collected in the gasbag (gas analyzer CGT-7000, Shimadzu). COD measurements were based on the Standard Method (APHA, AWWA, and WPCF Standard Method, 1985), and terephthalate was measured by a spectrophotometric method at 254 nm and confirmed using a liquid chromatographical method (Fajardo et al., 1997). Oxidation–reduction potential (ORP) and pH values during good stable performance of treatment were in the range from 250 to 400 mV and from 7.5 to 8.0, respectively.
q* = 6.45 × Ce0.085 R2 = 0.976 1 0.1
1
10
100
1000
Ce, mg TA/L Fig. 2 – Isotherm curve of adsorption for terephthalic acid (TA) on GAC. Ce: concentration of TA in liquid phase; q*: adsorbed amount of TA on GAC.
adsorbed well on GAC and that the GAC reactor can be effective. The treatment characteristics of terephthalate-containing wastewater in Run 1 are shown in Fig. 3. After the 40th day of continuous operation, methane gas production started and increased gradually. From these results, terephthalate concentration in the effluent was considered to be under 6 mg/L, controlled first by GAC adsorption and then through biodegradation. However, treatment performance deteriorated after an increase in the terephthalate concentration in the influent from 85 to 430 mg/L and, eventually, effluent terephthalate concentration increased to the same level as that in the influent and methane production stopped (Phase 1). The influent terephthalate concentration was decreased to 220 mg/L on the 153rd day and then gradually
Table 2 – Experimental conditions in continuous treatment. Parameters
Run 1 Phase 1
Wastewater
Run 2
Run 3
Phase 2
Synthetic wastewater
Actual wastewater
Operation duration, d
153
1350
665
400
Organics in influent Terephthalic acid, mg/L Sodium acetate, mg/L COD, mg/Lb
85–430 500–1000 580–1510
220–430 1000 1170–1660
63–430 0–1000 230–2050
100–440 1000 900–2030
HRT, da
2.2
2.2–0.48
2.2–0.48
2.2–0.48
COD loading rate gCOD/(kg-GAC d) kgCOD/(m3 d)a
0.63–1.9 0.22–0.66
1.5–8.3 0.51–2.9
0.29–12 0.10–4.1
1.2–12 0.42–4.0
a Based on GAC expanded-bed volume. b Total COD including the other organics in addition to terephthalic acid and acetate.
420
700
Terephthalic acid, mg/L
Phase1
Phase2
600 500 400 300 Effluent
200
Influent 100 0 0
200
400
600
800
1000
1200
1400
2000
Methane production rate, TA removal rate
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1 Methane production rate, L/d TA removal rate, mgTA/d
0.5
0 0
100
200
COD, mg/L
300
400
500
Terephthalic acid, mg/L
1500
Fig. 4 – Relationship among terephthalic acid (TA) concentration, methane production rate and terephthalic acid removal rate. (Methane volume is the value under 0 8C and 1 bar.)
1000 Influent Effluent (total) 500
Effluent (soluble)
0 0
400
600
800
1000
1200
1400
Theoretical value for organics* Theoretical value for acetate*
4
Value measured
The inhibition effects of terephthalate concentration in the reactor liquid on terephthalate removal rate and methane production rate are shown in Fig. 4 for data during Phase 1, and before the recovery in Phase 2. The acute inhibitory effects on both terephthalate removal rate and methane production rate occurred when its concentration was above
3 2
1.2
1 0 0
200
400
600
800
1000
1200
1400
Operation time, days Fig. 3 – Treatment performance in Run 1. (*Calculated from loading rate. Methane volume is the value under 0 8C and 1 bar.)
TA removal rate, kg/(m3 • d)
Methane production rate, L/d
5
200
1 0.8 0.6 0.4 Run1 Run2
0.2
Run3 0
0
0.2
0.4
0.6
0.8
1
1.2
TA loading rate, kg/(m3 • d) 4.5
COD removal rate, kg/(m3 • d)
increased to a final concentration of 430 mg/L by checking the treatment performance (Phase 2). Treatment performance had gradually recovered by the 300th day; terephthalate concentration and COD concentration in effluent had decreased to less than 2 and 30 mg/L, respectively, and the methane production rate also recovered to levels almost equal to, and sometimes, greater than the theoretical one calculated from the influent loading rate of terephthalate and acetate. The surplus methane production showed the methane fermentation of terephthalate adsorbed and accumulated on the GAC during the former days. This phenomenon was observed during each increase in the loading rate. After recovery, good operational performance, shown by a terephthalate concentration of less than 11 mg/L and COD concentration of less than 120 mg/L, was maintained during whole of Phase 2, except for some minor fluctuations, until a COD loading rate of 2.9 kg COD/(m3 d) was reached.
4 3.5 3 2.5 2 1.5
Run1
1
Run2 Run3
0.5 0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
COD loading rate, kg/(m3 • d) Fig. 5 – Relationship between loading rate and removal rate in COD and terephthalic acid (TA) (plots show mean ± standard deviation).
CH4 measured and calculated from the removed other organics than TA, L/(L • d)
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2
4.
Run1 measured Run2 measured Run3 measured Run1 calculated from others Run2 calculated from others Run3 calculated from others
1.5
0.5
b From the other organics
0
0
0.5
1
Conclusions
The applicability of an expanded-bed GAC anaerobic reactor was experimentally verified for treatment of terephthalatecontaining wastewater using both, artificially prepared and actual wastewater. Its operational conditions were investigated and the main results are summarized as follows.
a From terephthalate
1
421
1.5
CH4 calculated from total organics removed, L/(L • d) Fig. 6 – Relationship between measured and calculated methane production rate (mean ± standard deviation. Methane volume is the value under 0 8C and 1 bar.)
150 mg/L, although this value was much smaller than the values found by the other researches (Kuang and Wang, 1994; Macarie et al., 1992; Pereboom et al., 1994; Kleerebezem et al., 1997; Fajardo et al., 1997). These results mean that terephthalate inhibits microbial groups associated with both first step of terephthalate degradation and methane production even from acetate. It is, therefore, important to operate the reactor at a terephthalate concentration below 150 mg/L. Degradation of terephthalate and the other organics included in the actual wastewater occurred smoothly and stably until the COD loading rate and terephthalate loading rate reached 4 kg COD/(m3 d) and 1 kg terephthalate/(m3 d), respectively, in Run 2 and Run 3, through a gradual increase of loading rate, while effluent terephthalate concentration was kept below 150 mg/L. Fig. 5 shows the relationships between loading rate and removal rate associated with COD and terephthalate. The slope of this graph indicates the removal ratio of all organics. In all runs, that is in Run 1 fed with artificial wastewater and Runs 2 and 3 fed with actual wastewater, more than 90% of both COD and terephthalate were stably removed with increasing COD and terephthalate loading rates up to 2.9 and 0.75 kg/(m3 d), respectively, in Run 1, and 4 and 1 kg/(m3 d), respectively, in Runs 2 and 3. This confirms the anaerobic degradation of terephthalate and applicability of the reactor in the treatment of actual wastewater. The results were replicable. The relationship between the calculated methane production rate from removed total COD and measured methane production rate is shown in Fig. 6. The measured rate is similar to the calculated one, which suggests that COD was removed through biological reactions. The methane production rate calculated from the removed organics, other than terephthalate, is also plotted against the one calculated from removed total COD in Fig. 6. The ratios between methane production rates by terephthalate degradation and by the degradation of other organics (corresponding to (a) and (b) in Fig. 6, respectively) correspond well to the composition ratio between terephthalate and the other organics in the influent, suggesting the simultaneous degradation and methane fermentation of organic substances.
(1) There was good adsorption of terephthalate onto the GAC. This followed the Freundlich-type isotherm curve. (2) Terephthalate has an inhibitory effect on biological anaerobic degradation of terephthalate and methane production. The threshold concentration of inhibition was 150 mg terephthalate/L. (3) In the expanded-bed GAC anaerobic reactor developed in this study, terephthalate and the other organics included in both artificial (Run 1) and actual wastewater (Runs 2 and 3) were smoothly and stably degraded until they reached a COD loading rate and terephthalate loading rate of 2.9 kg COD/(m3 d) (Run 1) and 4 kg COD/(m3 d) (Runs 2 and 3), and 0.75 kg terephthalate/(m3 d) (Run 1) and 1 kg terephthalate/(m3 d) (Runs 2 and 3), respectively, by gradually increasing the loading rate. (4) More than 90% of both COD and terephthalate were stably removed. A stable methane fermentation ratio of more than 90% was also achieved. (5) GAC adsorption ability contributed well to the stability of the treatment performance, especially in the start-up stage and just after an increase in loading rate. This stability mechanism was demonstrated by a surplus methane production rate greater than the theoretical one calculated from the loading rate. (6) The ratios between methane production rates by terephthalate degradation and by degradation of other organics correspond well to the composition ratios of them in the influent, which suggests the simultaneous degradation and methane fermentation of organic substrates.
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