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ORIGINAL ARTICLE
Int J Env Health Eng 2012,  1:35

Anaerobic biodegradation of ethylene dichloride in an anaerobic sequential batch reactor


1 Department of HSE, Bandar Imam Petrochemical Industries, Khouzestan, Iran
2 Environment Research Center; Department of Environmental Health Engineering, IUMS, Isfahan, Iran
3 School of Environment and Energy, Science and Research Branch, Islamic Azad University of Tehran, Tehran, Iran
4 Environment Research Center; Department of Biostatistics and Epidemiology, School of Health, IUMS, Isfahan, Iran

Date of Web Publication27-Aug-2012

Correspondence Address:
Mohammad Mehdi Amin
Environment Research Center, Isfahan University of Medical Sciences, Hezar-Jerib Avenue, Isfahan
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2277-9183.100137

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  Abstract 

Aims: The efficiency of an anaerobic sequencing batch reactor (ASBR) in ethylene dichloride ( EDC) and chemical oxygen demand (COD) removal at different operational conditions was evaluated.
Materials and Methods: Biological EDC and COD removal was studied in a laboratory scale ASBR. The ASBR was seeded at the start-up with granular anaerobic sludge of sugarcane industry and operated at different organic loading rates (OLR), EDC loading rates, and influent concentration of COD and EDC.
Results: During start-up period, COD removal efficiencies of above 80% were selected for reactor adaptation to achieve steady state during 48 days of operation. Maximum COD removal efficiency was 95% with an influent COD concentration of 1700 mg/L at 0.5 gCOD/L.d, and the efficiency rapidly dropped with increasing influent COD concentrations and OLR. When the EDC loading rate was adjusted between 0.6 to 4.7 gCOD/L.d, the EDC removal efficiencies were 95% and 46%, respectively, with influent EDC concentrations of 2000 and 16000 mg/L at the end of EDC loading stage. The kinetic study showed that the EDC and COD removal by ASBR followed the second order kinetic.
Conclusions: Based on the results of this study, the ASBR process is successfully applicable for biodegradation of the COD and EDC (>90%) in wastewater treatment. The kinetic study showed that, at same time, ASBR capable to removing COD rather than EDC.

Keywords: Anaerobic biodegradation, ASBR, ethylene dichloride, kinetic, volatile fatty acids


How to cite this article:
Nadi A, Fatehizadeh A, Hassani AH, Marasy MR, Amin MM. Anaerobic biodegradation of ethylene dichloride in an anaerobic sequential batch reactor. Int J Env Health Eng 2012;1:35

How to cite this URL:
Nadi A, Fatehizadeh A, Hassani AH, Marasy MR, Amin MM. Anaerobic biodegradation of ethylene dichloride in an anaerobic sequential batch reactor. Int J Env Health Eng [serial online] 2012 [cited 2019 Nov 17];1:35. Available from: http://www.ijehe.org/text.asp?2012/1/1/35/100137


  Introduction Top


Ethylene dichloride (EDC) (Cl-CH 2 -CH 2 -Cl) or 1,2-dichloroethane is a synthetic chemical, which has no known natural sources. EDC is dominantly used as an intermediate in the synthesis of vinyl chloride and is also used in the production of chlorinated solvents such as trichloroethene, tetrachloroethene, and 1, 1, 1-trichloroethane. [1] EDC is one of the major halogenated organic pollutants that were detected in groundwater and industrial effluents. [2] EDC is a known carcinogen due to the conversion into chloroacetaldehyde, which is considered to have mutagenic properties. [2],[3] Other health effect of EDC consists of strongly irritating to the eyes and upper respiratory tract of man and on the central nervous system (CNS). [4]

Due to persistence and toxicity of EDC, it is subjected to degradation by physico-chemical or biological methods rather than phase change (e.g., adsorption on sludge or gas stripping). [1]

Anaerobic digestion is a controlled biodegradation process that converts organic matter in wastewater into biogas. Conventional digesters used for wastewater treatment include continuously stirred tank reactors (CSTR) and plug-flow reactors. [5],[6],[7] In these two types of digesters, hydraulic retention time (HRT) equals solid retention time (SRT) and active biomass is removed from the digester in the effluent on a daily basis. [6] The HRT needs to be long enough to ensure a sufficient SRT in the digester so that a viable bacteria population necessary for complete anaerobic digestion process is maintained. The minimum SRT varies with the digester temperature and generally decreases with the increase of temperature. [6],[8]

A conventional anaerobic sequential batch reactor (ASBR) is operated with intermittent cycles of four stages: Feeding or loading process of liquid influent, anaerobic biological reactions, biomass sedimentation, and effluent discharge with removal of sludge when necessary. [9],[10] ASBR has been used for treating high-strength wastewaters (dairy, brewery, piggery, petrochemical and landfill leachate) [11],[12],[13],[14],[15] as well as for low-strength ones (domestic wastewater). [16],[17] The main advantages of ASBR application are: (i) No short circuit, (ii) High efficiency for both chemical oxygen demand (COD) removal and gas production, (iii) No primary and secondary settles, and (iv) Flexible control. [13],[18]

The purpose of this study was to evaluate the efficiency of an anaerobic sequencing batch reactor (ASBR) in COD and EDC removal at different operational conditions.


  Materials and Methods Top


Anaerobic Sequence Batch Reactor Set-Up

A glass ASBR with a working volume of 2.5 L, an internal diameter of 10 cm, and height of 32 cm was used to carry out the experiments. A schematic of the ASBR set-up is given in [Figure 1].
Figure 1: Experimental setup of the ASBR used in this study (1: ASBR, 2: Substrate reservoir, 3: Feed pump, 4: PLC, 5: Bath room, 6: Effluent reservoir, 7: Magnetic stirrer, 8: Electronic valve, 9: Oil reservoir, 10: Oil pump, 11: Solenoid valves and 12: Gas meter)

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According to previous studies, all the experiments were performed at 35 ± 0.5°C by circulating warm oil around the reactors. [19]

Substrate and Seed Sludge

The synthetic substrates consist of EDC and acetic acid as main and auxiliary substrate, respectively. The nutrients and trace elements with following composition were used: CaCl 2 (0.008 mg/L), CoCl 2 (0.051 mg/L), FeCl 3 .6H 2 O (2.465 mg/L), NaHPO 4 .7H 2 O (2.237 mg/L), KH 2 PO 4 (0.874 mg/L), NaHCO 3 (25.7 mg/L), FeSO 4 .7H 2 O (5.1 mg/L) and MgSO 4 .7H 2 O (3.6 mg/L). The system was inoculated with granular anaerobic sludge obtained from the anaerobic digester of the sugarcane industry wastewater treatment plant (KeshtoSanate Ahwaz, Iran). The sludge is typically dispersed or granulates with 20120 mg/L of volatile suspended solids (VSS), 28750 mg/L of suspended solids (SS) and VSS/SS ratio of 0.7.

Reactor Start-Up and Operation

The phase duration and operation condition of ASBR are set according to previously described details by Ghasemian, et al., [20] demonstrated in [Table 1]. For adaptation of microorganisms, achieving an acceptable COD removal (>80% removal) and obtaining the steady state condition, the start-up phase consisting of two stages: First, the reactor was operated with an OLR of 0.5 gCOD/L.d for 30 days of operation. At second stage (31-48 days), the OLR was sharply augmented and ASBR was operated at OLR level of 1 gCOD/L.d. In this time, the input EDC concentration to the reactor was zero. [Table 2] shows the main operating parameters of the ASBR system.

Chemicals

The EDC was obtained from Merck Co, Germany. All other chemicals were of analytical grade and used without further purification.

Analytical Methods

The pH and COD were analyzed according to the Standard Methods for the Examination of Water and Wastewater. [21] EDC was determined by gas chromatograph (Tekmar Dohrmann 3800). The liquid sample was filtered through a 0.45 μm membrane filter and injection into the column and directly analyzed in gas chromatograph equipped with capillary column (Thermo TR-VI 30 m × 0.32 mm × 1.8 mm). The purge and trap (P and T) was used in order to water removal from samples. Nitrogen was used as a carrier gas (20 mL/min) with electron capture detector (ECD) as a detector. Injector temperature and detector temperature were 250°C and 280°C, respectively. The column oven temperature program was initially 70°C, ramped up at 10°C/min to 150°C, holding for 1 minute, and ramped up at 25°C/min to 280°C. Volatile fatty acids (VFA) in the effluent were measured by injecting 2 mL of filtered acidified samples through gas chromatograph (Agilent Techno 7890A. 5975C) equipped with flame ionization detector (GC-FID) using a 10% free fatty acid phase. The analysis was carried out at an oven temperature of 150°C, injector temperature of 180°C and detector temperature of 250°C. Nitrogen was applied as a carrier gas at a flow rate of 20 mL/min.


  Results Top


Start-Up and Acclimation

The start-up and acclimation stage was prolonged to 48 days with OLR of 0.5 to 1 gCOD/L.d and EDC concentration equal to zero. This stage was performed to achieve stable condition and acceptable COD removal efficiency. [Figure 2] shows the results of acclimation stage at various OLR. The maximum COD removal efficiency was obtained with 1 gCOD/L.d at the end of 48 days' period.
Figure 2: Removal of COD and pH solution variation during acclimation (COD: 1700 mg/L and pH: 7 ± 0.04)

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Chemical Oxygen Demand (COD) Removal

The experiment lasted for 107 days. The time courses of COD in influent and effluent and OLR are shown in [Figure 3]. Four distinct phases are indicated with 1.7, 3.4, 6.8, 10.21 and 13.6 g/L of COD in the influent. The results showed that with increasing initial concentration of COD, the COD removal efficiency was decreased.
Figure 3: The variation of CODInf, CODEff and organic loading rate (OLR) during the experiment

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Ethylene Dichloride Removal

The results on EDC removal via ASBR investigated as a function of initial concentration of EDC and EDC loading rate are depicted in [Figure 4]. The results showed that with increasing initial concentration of EDC and EDC loading rate, the COD removal efficiency declined. After 43 days of ASBR operation, the concentration of EDC in the synthetic wastewater was decreased to 0.083 g/L (95% removal efficiency) when EDC initial concentration of 2 g/L and EDC loading rate of 0.59 gEDC/L.d was used.
Figure 4: The fluctuation of EDCInf, EDCEff and organic loading rate (OLR) during the experiment

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Volatile Fatty Acid Profile

The effect of OLR on the variation of VFA content and pH solution is plotted in [Figure 5]. At the beginning of the changing in OLR, the pH decreased to <7 and then was augmented to >8. At this time, the VFA concentration was descended. In addition, the high OLR resulted in the enhancement of VFA concentration.
Figure 5: The variation of pH solution and volatile fatty acid (VFA) in ASBR operation

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Typical patterns of VFA concentration profile as function of pH solution in ASBR are shown for all OLRs in [Figure 6]. According to [Figure 6], with increasing pH up to 7.5, the VFA content was increased. After this point, VFA concentration was significantly decreased and then arises.
Figure 6: Variation of volatile fatty acid (VFA) content as function of pH solution for all organic loading rates (OLR)

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Performance of ASBR in an Operation Cycle

Profiles of operational parameters in a cycle are presented in [Figure 7]. The result showed that the COD and EDC removal increased from 19% to 73% and 20% to 75%, respectively, when HRT varied from 0.5 h to 24 h. During this period, the pH of the solution was increased from 7 to 8.92 and followed by gradual decrease to 8.14.
Figure 7: Performance of ASBR on (a) COD removal, (b) EDC removal and (c) pH variation in a cycle of operation

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  Discussion Top


ASBR Performance on COD and EDC Removal

During the start-up period, COD concentration in the effluent was kept at 1700 mg/L whereas the OLR was promptly increased from 0.5 to 1 gCOD/L.d. Correspondingly, the COD removal efficiency was more than 80% after 40 days ASBR operation. In this time, the pH value increased gradually from 7 to 8.83. This result revealed that methanogenic bacteria populations are dominated and capable to consume VFA as a substrate source.

According to [Figure 3] and [Figure 4], there was significant fluctuation on OLR and EDC loading rate in the experiment, and the COD and EDC removal efficiency were varied. In each stage of OLR and EDC loading rate, with increasing SRT, the COD and EDC removal was increased. In first stage of OLR and EDC loading rate, increasing the SRT from 49 to 92 days resulted in the COD and EDC removal efficiency to increase from 31% to 95% and 36% to 95%, respectively. A gradual decrease in COD removal was observed with OLR increases during the experimental phase. The COD removal efficiencies decreased from 95% to 48% for OLR fluctuation from 0.5 to 4 g COD/cycle.

Variations in COD and EDC removal efficiencies at the same volumetric loading rates were due to the variations in specific loading rates. [22] The results clearly show that the presence of bacterial species cause COD and EDC biodegradation. With variation of OLR and EDC loading rate, COD and EDC removal efficiency fluctuated. These changes in COD and EDC removal indicate an increase in biological activity, which would also suggest an increase in pH effluent, as observed in past studies. [12] Theoretically, feeding material to the microbial reactor can be removed by three possible mechanisms: (i) Adsorption onto the sludge, (ii) biological degradation and (iii) stripping into the gas phase. [23],[24],[25]

Under anaerobic conditions, most of the wastewater COD is converted to methane emitted into the gas phase. In bio-sludge, the main transformation of EDC was carried out by dichloro elimination reactions, resulting in the production of ethane (65-70%). Another type of reductive dechlorination mechanism, reductive hydrogenolysis has produced only small amounts of ethane (less than 1%). A conversion of EDC into carbon dioxide cannot be excluded. [1],[26] EDC changed very little without the addition of electron donor. [1],[3] In some previous studies, it has been mentioned that species such as Xanthobacter, Pseudomonas, Desulfobacteriur and Mycobacterium are capable to biodegrade the chlorinated hydrocarbons. [11],[27],[28],[29]

Profile of VFA Content

VFA is an intermediate and indicator in the anaerobic process. Because the acid-producing bacteria grow more quickly than the acid-consuming bacteria on readily biodegradable organics, it will provoke accumulation of these acids, mainly at the beginning of a cycle, which may lead to pH reduction. Previous studies have shown that acetic, propionic, valeric, caproic, formic and butyric acids were the major constituents of VFA. [30],[31]

Therefore, it is critical to balance the VFA production and utilization in order to keep a low VFA concentration in a cycle. [13] VFAs are not directly used by methanogens, but serve as substrates for proton-reducing syntrophic bacteria. [31]

From [Figure 5], VFAs were mostly consumed in the ASBR system. At each stage of OLR, with increasing of SRT, the VFA content was decreased. In first stage of OLR, VFA concentration was measured 48 mg/L instead of 286 mg/L and at end of period.

Temperature significantly affected the VFA degradation with higher VFA degradation occurring at a lower reaction temperature. However, this does not seem to result in significantly higher production of methane. This obvious imbalance may be attributed to less conversion efficiency of VFAs to methane at the lower temperature than at the higher temperature. [32]

The Variation of pH Solution in ASBR

The amounts of pH solution reflected the acid concentration results, which accumulated acid the most rapidly, reaching the lowest pH during the beginning of OLR changing [Figure 5].

Anaerobic reactions are highly pH dependent. The optimal pH range for methane producing bacteria is 6.8-7.2, while acid-forming bacteria can stand under more acidic pH values. [17] In initial anaerobic digestion, with increasing the amount of VFA, the pH was declined. This fact is related to accumulation of VFA, at beginning, methanogenic bacteria are unable to consume the high amount of VFA produced by acid production bacteria. [9]

After transition to methanogenic conditions, pH values increased and total alkalinity concentrations tended to decrease because methanogens utilized the available VFA as substrate. [30] On the other hand, the bicarbonate alkalinity (BA) generation and the low values of VFA in the previous periods were considered indicators of the balance between acidogenesis and methanogenesis. [10]

Adequate alkalinity, or buffer capacity, is necessary to maintain a stable pH in the anaerobic reactor for optimal biological activity. Tchobanoglous, et al. mentioned that an alkalinity level ranging from 1000 to 5000 mg/L as CaCO 3 is necessary. [33] This link between VFA removal and pH increase was observed by Van Gulck, et al. and Lozecznik, et al. [12],[34]

Kinetic Study

Variations in composition and concentration of materials in the reactor are the main factors in the purification of water and wastewater. To completely describe a reactor system and its design, reaction rates that occur in the reactor must be specified because these rates directly affect the reactor size. Therefore, the study of reaction kinetics to predict pollutant removal rates is very important in designing and modeling the treatment process. [33]

Determination of the kinetics of the ASBR process on COD and EDC removal reactions is necessary to estimate an optimum time required for the removal reaction.

A kinetic analysis was conducted by fitting the time-course performance data with zero, first, and pseudo-second-order kinetic equations as shown in [Table 3] and [Figure 8], where r c is the rate of conversion, k 0 , k 1 , and k 2 are reaction rate coefficients, t is time, and C 0 and C are the initial and final concentration of the constituent in the liquid, respectively.
Figure 8: Kinetics study of EDC and COD removal in a cycle of ASBR operation (a) Zero order, (b) First order and (c) Second order

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The data were properly correlated (with a higher R 2 than the other models) with the second-order kinetic model revealing that the second-order model can successfully simulate the COD and EDC removal in the ASBR (R 2 > 0.92). Second-order reaction showed that the reaction progressed at a rate proportional to the second power of the reactants initial concentrations. According to [Table 3], the second-order kinetic constant (k 2 ) related to the COD removal was higher than that for the EDC removal (0.036 versus 0.034). It means that, at the same time, ASBR was capable to reduce COD more than EDC.

The results of this study clearly showed that the ASBR was able to achieve high removal efficiencies related to organic matter and EDC. The authors suggest that the ASBR technology as a simple, efficient and reliable method performs a good potential and could be an alternative method for the treatment of wastewater containing EDC.


  Acknowledgments Top


The authors wish to thank to Research Center of Bandar Imam Petrochemical Industries for financial support of this research project.

 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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