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Study on treatment of coking wastewater by biofilm reactors combined with zero-valent iron process
Journal of Hazardous Materials 162 (2009) 1423–1429
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Study on treatment of coking wastewater by biofilm reactors combined with zero-valent iron process Peng Lai a , Hua-zhang Zhao a , Ming Zeng b , Jin-ren Ni a,∗ a b
Department of Environmental Engineering, Peking University, The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China Beijing Gaia Environmental Technology Co., Ltd., Beijing 100871, China
a r t i c l e
i n f o
Article history: Received 6 March 2008 Received in revised form 11 June 2008 Accepted 11 June 2008 Available online 19 June 2008 Keywords: Coking wastewater Biofilm Zero-valent iron (ZVI) Chemical oxygen demand (COD) Ammonia nitrogen (NH3 -N)
a b s t r a c t Experiments were conducted to investigate the behavior of the integrated system with biofilm reactors and zero-valent iron (ZVI) process for coking wastewater treatment. Particular attention was paid to the performance of the integrated system for removal of organic and inorganic nitrogen compounds. Maximal removal efficiencies of chemical oxygen demand (COD), ammonia nitrogen (NH3 -N) and total inorganic nitrogen (TIN) were up to 96.1, 99.2 and 92.3%, respectively. Moreover, it was found that some phenolic compounds were effectively removed. The refractory organic compounds were primarily removed in ZVI process of the integrated system. These compounds, with molecular weights either ranged 10,000–30,000 Da or 0–2000 Da, were mainly the humic acid (HA) and hydrophilic (HyI) compounds. Oxidation–reduction and coagulation were the main removal mechanisms in ZVI process, which could enhance the biodegradability of the system effluent. Furthermore, the integrated system showed a rapid recovery performance against the sudden loading shock and remained high efficiencies for pollutants removal. Overall, the integrated system was proved feasible for coking wastewater treatment in practical applications. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since coal is the main energy resource in China, coking wastewater produced from coal industry becomes one of the most severe environmental problems [1]. As reported, coking wastewater is composed of complex inorganic and organic contaminants such as ammonia, cyanide, thiocyanide, phenolic compounds, polynuclear aromatic hydrocarbons (PAHs), polycyclic nitrogen-containing aromatics, oxygen- and sulfur-containing heterocyclics and acyclic compounds [2–4]. High concentration of phenolic compounds, which are identified by the US EPA as the prior pollutants [5], is the one of main characteristics of coking wastewater. Meanwhile, excess discharge of ammonia can trigger severe eutrophication in receiving watercourses [6,7]. Furthermore, most of above pollutants are refractory, toxic, mutative and carcinogenic [8,9], and may produce serious environmental and ecological impacts [10–12]. Biological treatment is by far the most widely applied and costeffective process for wastewater treatment. However, due to a certain number of refractory and inhibitory pollutants in coking wastewater, COD and NH3 -N are poorly removed by the conventional activated sludge process. In China, anoxic–oxic (A–O),
∗ Corresponding author. Tel.: +86 10 62751185; fax: +86 10 62756526. E-mail address: [email protected] (J.-r. Ni). 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.06.034
anaerobic–anoxic–oxic (A1 –A2 –O) and sequencing batch reactor (SBR) processes have been extensively investigated for coking wastewater treatment [13–15]. Unfortunately, these aforementioned processes are not efficient enough in practice to meet the National Discharge Standard of China (GB8978-1996). Recently, it is reported the biofilm reactor system plays important roles in the detoxification of hazardous organic contaminants such as volatile aromatic hydrocarbons, chlorinated solvents and phenolic aromatics [12,16–19]. In our previous study, a special biofilm reactor system showed a well biodegradable performance for removal of COD and NH3 -N in coking wastewater treatment [20]. ZVI process is a new physico-chemical method developing quickly in recent decades. Due to the strong reductive capacity, ZVI can transform the structures and minimize the toxicity of hazardous pollutants [21–23]. In our previous research [24], ZVI process was found to be more feasible for advanced treatment of coking wastewater than other physico-chemical processes, taking the treatment effect and cost into consideration. Furthermore, ZVI process enhanced the biodegradability of effluent obviously, which could help to improve the biodegradation ability of biofilm reactor system when they were combined. This study aims to explore an efficient treatment of actual coking wastewater by combining biofilm reactor system and ZVI process. Special attention was paid to the performance of the integrated system for removal of COD, total organic carbon (TOC) and
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Table 1 Characteristics of the influent in the integrated system Parameter
Unit
pH COD TOC NH3 -N NO2 − -N NO3 − -N
mg l−1 mg l−1 mg l−1 mg l−1 mg l−1
a
Concentration 8.2–8.5 1590.4–2105.6 555.0–710.9 232.0–289.5 –a 44.1–51.9
Not detected.
biochemical oxygen demand (BOD5 ) with full consideration of the transformation among NH3 -N, nitrite nitrogen (NO2 − -N) and nitrate nitrogen (NO3 − -N). Moreover, gas chromatography/mass spectrum (GC/MS) analysis was conducted to indicate the removal of organic compounds in the different reactors. Observation and analysis of surface change of the iron materials in Reactor ZVI were conducted with environmental scan electron microscope (ESEM) and energy dispersive X-ray fluorescence spectrometer (EDX). Furthermore, molecular weight distribution (MWD) analysis and fractionation of organic compounds by XAD resin were applied to study the characteristics of organic compounds removal by ZVI process in the integrated system. 2. Materials and methods 2.1. Materials 2.1.1. Wastewater The raw coking wastewater was collected in a coking factory, located in Shanxi Province, North China. The raw wastewater was appropriately diluted with tap water before treatment and the parameters of actual influent were shown in Table 1. 1.5 g l−1 of NaHCO3 and 0.2 g l−1 of KH2 PO4 were added to the influent to supply enough inorganic carbon and nutrients for microorganisms. Moreover, other trace metals were added as reported by Sarfaraz et al. [5]. 2.1.2. Microorganisms The microorganisms initially applied for the biofilm reactors were collected in Shanxi Coke Corporation, which were the activated sludge in the aerated tank of the coking wastewater treatment plant. When ZVI process was combined, the biofilm reactor system has been operated for about 1 year. 2.1.3. Integrated treatment system Fig. 1 shows the process of the integrated treatment system, which consisted of the biofilm and ZVI reactors. The biofilm reactor system is composed of one anaerobic reactor (A) and two aero-
bic reactors (O1 and O2 ), which were detailedly described in the previous study [20]. In the present study, Reactor A was mainly used for denitrification in the integrated system. The self-made FPUFS patent carriers were used in the three biofilm reactors as the support materials for biofilm formation, which were described and applied in earlier studies [20,25]. Reactor ZVI has an effective volume of 12 l, which was filled with about 30 vol% of industrial iron and some activated carbon (Fe/C = 3). Industrial iron consisted of filings and shavings which were largely free from visible rust and retained a metallic glaze. In order to eliminate the adsorption effect, the activated carbon was saturated with the wastewater sample before use. In Reactor ZVI, air diffusers and inflators were used to mix the reaction system. Descriptions about addition and pretreatment for iron and activated carbon could refer to literature [24]. 2.2. Experimental procedure Before ZVI process was combined, the biofilm reactor system for coking wastewater treatment has been continuously operated for about 1 year. In the integrated system, HRT for each biofilm reactor was adjusted to 15 h and the total HRT for the biofilm system was 45 h. In Reactor ZVI, 3–6 l of the biological effluent was retained for reaction and the excess effluent was discharged. Furthermore, recirculation of ZVI effluent to Reactor A was employed with the ratio adjusted to 3. Moreover, the produced sludge in Reactor ZVI was removed every 2 weeks. In this study, the effluent quality of each reactor was detected every 2 or 3 days continuously. To study the stability of the integrated system against the sudden loading shock of the influent variation, the concentrations of COD and NH3 -N in the influent were increased to a jump change to about 10,000 and 1200 mg l−1 for 24 h. Then, the influent was returned to the normal level and the effluent quality was detected to examine the recovery ability of the integrated system. 2.3. Analytical method Wastewater constituents, such as NH3 -N, NO2 − -N and NO3 − N, were determined with the China national standard methods. COD, BOD5 , TOC, pH and Fe3+ were measured by the potassium dichromate oxidation method (Hach heating system, Hach Corporation, USA), the respirometric method (OxiTop IS6, Germany), Multi TOC/TN 3000 Analyzer (Analytik Jena AG Corporation, Germany), pH meter (pH-201, Hanna Corporation, Italy), and atomic absorption spectrophotometer (AAS FL, Analytik Jena AG Corporation, Germany), respectively. GC/MS and MWD analyses were conducted according to the former procedures [24]. Observation and analysis of iron surface were conducted with the special system (FEI QUANTA 200F, USA) of ESEM and EDX. The fractionation
Fig. 1. Flow chart of the integrated treatment system.
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Fig. 2. Fractionation procedure of organic compounds in the integrated system.
procedure of humic acid (HA), fulvic acid (FA) and hydrophilic (HyI) compounds by XAD-8 resin was used similarly to the other published researches [26–28], which was shown in Fig. 2. According to the procedure, different TOC concentrations of HA, FA and HyI compounds could be calculated as follows: TOCHA = TOC1 − TOC2 ; TOCFA = TOC2 − TOC3 ; TOCHyI = TOC3 . 3. Results and discussion 3.1. Efficiencies of the integrated system After treatment by the integrated system, COD of the final effluent was steadily reduced to about 100 mg l−1 , which could meet the requirement of the National Discharge Standard of China (GB89781996). The COD change in the influent and the ZVI effluent was shown in Fig. 3. The average COD removal efficiency of the integrated system was 94.2% during the operation period, which was much higher than that of the biofilm reactor system. In the single biofilm reactor system, COD of the biological effluent was more than 200 mg l−1 and the maximal COD removal efficiency was 90.2% [20]. In the integrated system, the maximal COD removal efficiency was up to 96.1% and COD of the final effluent was reduced to 76.2 mg l−1 . It is clear that ZVI process played an important role for organic compounds removal as it was combined with the biofilm reactor system. Fig. 4 shows the variations of COD, TOC and BOD5 in the different reactor effluent. As it could be seen, 81.1% of COD and 76.1% of TOC were removed in Reactor A. Since the concentration of DO in the recirculated ZVI effluent was 4–6 mg l−1 , part of organic compounds would be biodegraded by aerobic microorganisms at the lower location of Reactor A. Besides aerobic biodegradation, denitrification and dilution of the recirculation were of significance to the reduction of the organic compounds in Reactor A effluent.
Fig. 4. Variation of COD, BOD5 and TOC in the different reactors.
TOC removal efficiencies of Reactor O1 , O2 and ZVI were 38.7, 12.5 and 30.4%, respectively. It was indicated that Reactor O1 and ZVI removed most part of organic compounds from Reactor A. From BOD5 change in Fig. 4, it could be found that BOD5 was increased when treated by ZVI process. Moreover, BOD5 /COD could be calculated, which varies from 0.58 in the influent to 0.45, 0.16, 0.09 and 0.22, respectively, in the subsequent reactors. BOD5 /COD decreased when treated by biofilm reactors but increased after ZVI process. Because of the oxidation–reduction function of ZVI process, some refractory organic compounds were transformed or removed, and the biodegradability of effluent was enhanced, which caused the increase of the BOD5 value and BOD5 /COD. Meanwhile, other similar studies proved the increase of biodegradability by ZVI process [24,29,30]. The concentrations of different inorganic nitrogen compounds in the integrated system were shown in Fig. 5. Since the dominant inorganic nitrogen compound in coking wastewater was ammonia, TIN was defined as the sum of NH3 -N, NO2 − -N and NO3 − -N herein. As it could be found from Fig. 5(a), the removal of NH3 -N occurred in Reactor A, O1 and O2 and the concentration of NH3 -N was almost the same between Reactor O2 and ZVI. For the dilution function of recirculation, the concentration of NH3 -N in Reactor A was decreased to 100 mg l−1 below and the residual NH3 -N was biodegraded by nitrification in Reactor O1 and O2 , which could be proved by the increased NO2 − -N and NO3 − -N shown in Fig. 5(b) and (c). From Fig. 5(d), it was indicated most of TIN was removed in Reactor A and O1 , which could be explained by denitrification of NO2 − -N and NO3 − -N. Interestingly, some TIN disappeared in Reactor O1 , which reflected the simultaneous nitrification and denitrification (SND) existed in the biofilm reactor system. No use was found for the ZVI process to removal of inorganic nitrogen compounds, which implied that these compounds were removed by nitrification and denitrification occurred in the biofilm reactors. In the 60 days of operation, NH3 -N of ZVI effluent was always less than 10 mg l−1 and the maximal removal efficiency was up to 99.2%, which was much higher than the performance of single biofilm reactor system in the previous study [20]. The maximal removal efficiency of TIN was 92.3% and the effluent from Reactor ZVI was only composed of low concentrations of NH3 -N and NO3 − -N. Some other residual refractory organic compounds could be removed in the ZVI process, which in turn enhanced the biodegradation of inorganic nitrogen compounds. 3.2. Organic compounds removal in the integrated system
Fig. 3. COD variation in the influent and in the ZVI effluent.
Organic compounds in the influent as well as the effluent from different reactors of the integrated system were analyzed by GC/MS. Table 2 listed the main relative abundance of the influent and efflu-
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Fig. 5. Variation of inorganic nitrogen compounds in the different reactors.
ent from Reactor A, O1 , O2 and ZVI (>1% of integration area). It could be concluded that the main organic compounds of low-molecular weight in the influent were phenolic compounds, which held more than 80% of total integration area. Treated by the integrated system, there were only a few phenolic compounds detected in Reactor A and there were not any organic compounds detected in other reactors. The aerobic biofilm reactors were found to be efficient to remove phenolic compounds, which agreed with the findings by other investigators [12,16,18,20]. Because of the high efficiency for phenolic compounds removal, the integrated system showed a well performance for organic compounds removal during the operation period. 3.3. Characteristics of organic compounds removal by ZVI process In the integrated system, some refractory organic compounds from Reactor O2 could be removed by ZVI process (Fig. 4). Fig. 6 showed the variations of MWD in the biological effluent
(TOC = 80.2 mg l−1 ) and the ZVI effluent (TOC = 44.4 mg l−1 ). It was indicated that considerable part of organic compounds in the biological effluent was high-molecular weight matter, especially in range of 10,000–30,000 Da. From the comparison of MWD in Fig. 6, it was found TOC concentrations in different molecular weight ranges were removed by ZVI process, mostly in ranges of 10,000–30,000 Da and 0–2000 Da. As it could be seen, there were still some organic compounds in different ranges of molecular weight in the effluent. As discussed in Section 3.2, these residual organic compounds could not be analyzed by GC/MS method. The fractionation of organic compounds by XAD-8 resin between the effluent from Reactor O2 and ZVI was shown in Table 3. It was found the biological effluent from Reactor O2 was mainly composed of FA and HyI compounds, which were 39.2 and 50.7% of the total TOC. In some other studies, humic substances (HA and FA) were found to be the considerable part of refractory organic compounds remaining in the effluent after the wastewater treated by biological systems [27,28,31]. After treatment by ZVI process, HA, FA
Table 2 Main organic compounds analysis in the influent (I) and effluent from different reactors (Reactor A, O1 , O2 and ZVI) Organic compounds
I
A
O1
O2
ZVI
Organic compounds
I
A
O1
O2
ZVI
Phenol 2-Methylphenol 4-Methylphenol 2,5-Dimethylphenol 2,4-Dimethylphenol 2-Ethylphenol 4-Ethylphenol
10.4a 13.0 29.5 1.0 9.7 1.4 2.7
6.2 48.6 3.6 8.8 26.7 – –
–b – – – – – –
– – – – – – –
– – – – – – –
3-Ethylphenol 3,4-Dimethylphenol 2,3-Dimethylphenol 3-Ethyl-5-methylphenol 2,3,6-Trimethylphenol 2,3-Dihydro-1H-Inden 2-Naphthalenol
3.7 7.9 1.7 2.2 1.1 1.8 1.8
– – – – – – –
– – – – – – –
– – – – – – –
– – – – – – –
a b
Percentage of relative integration area. Not detected.
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Table 4 Mass percentage of elements on the surface of the iron analyzed by EDX Element
Prepared iron
Used iron
C Si Fe O
18.7 0.8 80.4 –a
10.6 0.4 48.5 40.5
a
Fig. 6. MWD in the biological effluent and the ZVI effluent.
Table 3 TOC concentrations of HA, FA and HyI compounds of Reactor O2 and ZVI Component −1
HA (mg l ) FA (mg l−1 ) HyI (mg l−1 )
Reactor O2
Reactor ZVI
5.8 22.6 29.2
3.1 18.0 17.1
Not detected.
observed on the surface of the iron after ZVI process. It could be concluded that the corrosion of iron surface occurred in ZVI process, which was caused by oxidation–reduction reaction. The produced oxidation–reduction potential could transform the structures and decrease the toxicity of some refractory organic compounds, and consequently enhanced the biodegradability of the effluent. Furthermore, some reddish-yellow iron rust was produced on the surface of the submerged iron materials, which indicated the zerovalent iron was transformed to iron oxides and hydroxides in ZVI process. Table 4 shows the change of elements on the surface of iron. As it could be seen, O element was the main component newly produced of the corroded iron, which proved the transformations of the iron materials. In Reactor ZVI, some iron was proved to be corroded and much sludge was produced, which could consist of iron rust and complexes of iron hydroxides and organic compounds. Based on the following equations: 2Fe − 4e → 2Fe2+
and HyI compounds were all partly removed with different removal efficiencies. TOC removal efficiencies for the three fractions were 46.6, 20.4 and 41.4%, respectively, which revealed that ZVI process was more efficient for removal of HA and HyI compounds. In the ZVI effluent, the residual organic compounds mainly consisted of FA and HyI compounds, which could not be analyzed by GC/MS. Moreover, these HA, FA and HyI compounds in Reactor O2 and ZVI existed in the different ranges of molecular weight as discussed above. 3.4. Removal mechanism analysis of ZVI process The coagulation, precipitation and oxidation–reduction were found to be the main mechanisms for organic compounds removal in ZVI process [24]. The surface change of iron materials in Reactor ZVI was shown in Fig. 7. The initial surface of iron was smooth and retained a metallic glaze. However, there were some cracks
O2 + 4e + 2H2 O → 4OH 2+
Fe
(1) −
−
+ 2OH → Fe(OH)2
(2) (3)
4Fe(OH)2 + O2 + 2H2 O → 4Fe(OH)3
(4)
2Fe(OH)3 → Fe2 O3 + 3H2 O
(5)
the transformations of zero-valent iron in Reactor ZVI could be revealed. Furthermore, the pH value was increased from 8.0 in the Reactor O2 effluent to 9.4 in the ZVI effluent, indicating the increase of produced OH− in ZVI process as described in Eq. (2). Higher pH in Reactor ZVI would be corresponding to lower concentration of Fe3+ (3.7 mg l−1 herein), and much corroded iron was transformed to oxides or hydroxides (as precipitants). The initial produced amorphous Fe(OH)2 and Fe(OH)3 in ZVI process would act as coagulants which removed some organic compounds by coagulation mechanism. Eq. (5) indicates that the produced Fe(OH)3 was unstable, which could be broken down to Fe2 O3 . Furthermore, Fe2 O3 was not
Fig. 7. ESEM photo of the iron materials in Reactor ZVI. Left: 1500× prepared iron not used. Right: 1500× iron in Reactor ZVI after operation period.
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SND phenomenon was found in the aerobic biofilm reactor of the integrated system, the maximal TIN removal efficiency was 92.3% with very low concentrations of NH3 -N and NO3 − -N. Moreover, GC/MS analysis showed the integrated system was also good for removal of phenolic compounds. It was found that ZVI process at the end of integrated system was efficient for removal of some refractory organic compounds with molecular weight ranged either 10,000–30,000 Da or 0–2000 Da, which were mainly the HA and HyI compounds. The ZVI process could also enhance the biodegradability of the effluent through oxidation–reduction and coagulation. Another eminent advantage of the proposed integrated system is its high recovery ability to sudden shock loading, which is of significance to the practical applications. Acknowledgements Fig. 8. Variation of the effluent after the shock loading.
the real chemical structure of the iron oxide but the mixture of complex iron oxides. Lepidocrocite (␥-FeOOH), goethite (␣-FeOOH) and akaganeite (-FeOOH), etc., were found to be the corrosion products under different conditions in the report of Huang and Zhang [32]. Therefore, the main removal mechanisms in ZVI process were oxidation–reduction and coagulation, accompanied with the corrosion of the iron materials and the produce of complex iron oxides and hydroxides. 3.5. Response to sudden shock loading of the integrated system The response of the integrated system after receiving the shock loading was shown in Fig. 8. After 24 h for high loading influent operated, the concentrations of COD and NH3 -N in the final effluent of the integrated system were increased to more than 200 and 30 mg l−1 , respectively. Although both COD and NH3 -N removal efficiencies were recovered rapidly after the shock loading, the former seemed to be recovered faster than the latter. In the proposed system, it took 3 days for recovery of COD removal but 5 days for NH3 -N removal. Since the autotrophic nitrifiers were more susceptible to environmental changes than heterotrophic microorganisms, the recovery of NH3 -N removal efficiency was always slower. Moreover, ZVI process could remove part of increased refractory organic compounds, which could help the recovery of the integrated system for removal of COD and NH3 -N in short time. When the integrated system returned to the normal situation, the concentrations of COD and NH3 -N were reduced to less than 150 and 10 mg l−1 , which indicated that the high performance for pollutants removal of the system could not be weakened by sudden increase of the influent loading. As a result, the integrated system, which combined the biofilm reactors with ZVI process together, was more efficient in actual application for coking wastewater treatment than the single biological treatment process. 4. Conclusions A novel integrated system was established by combining three biofilm reactors and a ZVI process for coking wastewater treatment. Experiments showed this system had much better performance in pollutants removal than the single biofilm system or the separated ZVI process. The maximal efficiencies for COD and NH3 -N removal in the integrated system were up to 96.1 and 99.2%, respectively. Moreover, the average COD and NH3 -N after integrated treatment were 96.6 and 5.1 mg/l, respectively, which could generally meet the Grade 1 as required by National Discharge Standard of China (GB8978-1996).
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Study on treatment of coking wastewater by biofilm reactors combined with zero-valent iron process Peng Lai a , Hua-zhang Zhao a , Ming Zeng b , Jin-ren Ni a,∗ a b
Department of Environmental Engineering, Peking University, The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China Beijing Gaia Environmental Technology Co., Ltd., Beijing 100871, China
a r t i c l e
i n f o
Article history: Received 6 March 2008 Received in revised form 11 June 2008 Accepted 11 June 2008 Available online 19 June 2008 Keywords: Coking wastewater Biofilm Zero-valent iron (ZVI) Chemical oxygen demand (COD) Ammonia nitrogen (NH3 -N)
a b s t r a c t Experiments were conducted to investigate the behavior of the integrated system with biofilm reactors and zero-valent iron (ZVI) process for coking wastewater treatment. Particular attention was paid to the performance of the integrated system for removal of organic and inorganic nitrogen compounds. Maximal removal efficiencies of chemical oxygen demand (COD), ammonia nitrogen (NH3 -N) and total inorganic nitrogen (TIN) were up to 96.1, 99.2 and 92.3%, respectively. Moreover, it was found that some phenolic compounds were effectively removed. The refractory organic compounds were primarily removed in ZVI process of the integrated system. These compounds, with molecular weights either ranged 10,000–30,000 Da or 0–2000 Da, were mainly the humic acid (HA) and hydrophilic (HyI) compounds. Oxidation–reduction and coagulation were the main removal mechanisms in ZVI process, which could enhance the biodegradability of the system effluent. Furthermore, the integrated system showed a rapid recovery performance against the sudden loading shock and remained high efficiencies for pollutants removal. Overall, the integrated system was proved feasible for coking wastewater treatment in practical applications. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since coal is the main energy resource in China, coking wastewater produced from coal industry becomes one of the most severe environmental problems [1]. As reported, coking wastewater is composed of complex inorganic and organic contaminants such as ammonia, cyanide, thiocyanide, phenolic compounds, polynuclear aromatic hydrocarbons (PAHs), polycyclic nitrogen-containing aromatics, oxygen- and sulfur-containing heterocyclics and acyclic compounds [2–4]. High concentration of phenolic compounds, which are identified by the US EPA as the prior pollutants [5], is the one of main characteristics of coking wastewater. Meanwhile, excess discharge of ammonia can trigger severe eutrophication in receiving watercourses [6,7]. Furthermore, most of above pollutants are refractory, toxic, mutative and carcinogenic [8,9], and may produce serious environmental and ecological impacts [10–12]. Biological treatment is by far the most widely applied and costeffective process for wastewater treatment. However, due to a certain number of refractory and inhibitory pollutants in coking wastewater, COD and NH3 -N are poorly removed by the conventional activated sludge process. In China, anoxic–oxic (A–O),
∗ Corresponding author. Tel.: +86 10 62751185; fax: +86 10 62756526. E-mail address: [email protected] (J.-r. Ni). 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.06.034
anaerobic–anoxic–oxic (A1 –A2 –O) and sequencing batch reactor (SBR) processes have been extensively investigated for coking wastewater treatment [13–15]. Unfortunately, these aforementioned processes are not efficient enough in practice to meet the National Discharge Standard of China (GB8978-1996). Recently, it is reported the biofilm reactor system plays important roles in the detoxification of hazardous organic contaminants such as volatile aromatic hydrocarbons, chlorinated solvents and phenolic aromatics [12,16–19]. In our previous study, a special biofilm reactor system showed a well biodegradable performance for removal of COD and NH3 -N in coking wastewater treatment [20]. ZVI process is a new physico-chemical method developing quickly in recent decades. Due to the strong reductive capacity, ZVI can transform the structures and minimize the toxicity of hazardous pollutants [21–23]. In our previous research [24], ZVI process was found to be more feasible for advanced treatment of coking wastewater than other physico-chemical processes, taking the treatment effect and cost into consideration. Furthermore, ZVI process enhanced the biodegradability of effluent obviously, which could help to improve the biodegradation ability of biofilm reactor system when they were combined. This study aims to explore an efficient treatment of actual coking wastewater by combining biofilm reactor system and ZVI process. Special attention was paid to the performance of the integrated system for removal of COD, total organic carbon (TOC) and
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Table 1 Characteristics of the influent in the integrated system Parameter
Unit
pH COD TOC NH3 -N NO2 − -N NO3 − -N
mg l−1 mg l−1 mg l−1 mg l−1 mg l−1
a
Concentration 8.2–8.5 1590.4–2105.6 555.0–710.9 232.0–289.5 –a 44.1–51.9
Not detected.
biochemical oxygen demand (BOD5 ) with full consideration of the transformation among NH3 -N, nitrite nitrogen (NO2 − -N) and nitrate nitrogen (NO3 − -N). Moreover, gas chromatography/mass spectrum (GC/MS) analysis was conducted to indicate the removal of organic compounds in the different reactors. Observation and analysis of surface change of the iron materials in Reactor ZVI were conducted with environmental scan electron microscope (ESEM) and energy dispersive X-ray fluorescence spectrometer (EDX). Furthermore, molecular weight distribution (MWD) analysis and fractionation of organic compounds by XAD resin were applied to study the characteristics of organic compounds removal by ZVI process in the integrated system. 2. Materials and methods 2.1. Materials 2.1.1. Wastewater The raw coking wastewater was collected in a coking factory, located in Shanxi Province, North China. The raw wastewater was appropriately diluted with tap water before treatment and the parameters of actual influent were shown in Table 1. 1.5 g l−1 of NaHCO3 and 0.2 g l−1 of KH2 PO4 were added to the influent to supply enough inorganic carbon and nutrients for microorganisms. Moreover, other trace metals were added as reported by Sarfaraz et al. [5]. 2.1.2. Microorganisms The microorganisms initially applied for the biofilm reactors were collected in Shanxi Coke Corporation, which were the activated sludge in the aerated tank of the coking wastewater treatment plant. When ZVI process was combined, the biofilm reactor system has been operated for about 1 year. 2.1.3. Integrated treatment system Fig. 1 shows the process of the integrated treatment system, which consisted of the biofilm and ZVI reactors. The biofilm reactor system is composed of one anaerobic reactor (A) and two aero-
bic reactors (O1 and O2 ), which were detailedly described in the previous study [20]. In the present study, Reactor A was mainly used for denitrification in the integrated system. The self-made FPUFS patent carriers were used in the three biofilm reactors as the support materials for biofilm formation, which were described and applied in earlier studies [20,25]. Reactor ZVI has an effective volume of 12 l, which was filled with about 30 vol% of industrial iron and some activated carbon (Fe/C = 3). Industrial iron consisted of filings and shavings which were largely free from visible rust and retained a metallic glaze. In order to eliminate the adsorption effect, the activated carbon was saturated with the wastewater sample before use. In Reactor ZVI, air diffusers and inflators were used to mix the reaction system. Descriptions about addition and pretreatment for iron and activated carbon could refer to literature [24]. 2.2. Experimental procedure Before ZVI process was combined, the biofilm reactor system for coking wastewater treatment has been continuously operated for about 1 year. In the integrated system, HRT for each biofilm reactor was adjusted to 15 h and the total HRT for the biofilm system was 45 h. In Reactor ZVI, 3–6 l of the biological effluent was retained for reaction and the excess effluent was discharged. Furthermore, recirculation of ZVI effluent to Reactor A was employed with the ratio adjusted to 3. Moreover, the produced sludge in Reactor ZVI was removed every 2 weeks. In this study, the effluent quality of each reactor was detected every 2 or 3 days continuously. To study the stability of the integrated system against the sudden loading shock of the influent variation, the concentrations of COD and NH3 -N in the influent were increased to a jump change to about 10,000 and 1200 mg l−1 for 24 h. Then, the influent was returned to the normal level and the effluent quality was detected to examine the recovery ability of the integrated system. 2.3. Analytical method Wastewater constituents, such as NH3 -N, NO2 − -N and NO3 − N, were determined with the China national standard methods. COD, BOD5 , TOC, pH and Fe3+ were measured by the potassium dichromate oxidation method (Hach heating system, Hach Corporation, USA), the respirometric method (OxiTop IS6, Germany), Multi TOC/TN 3000 Analyzer (Analytik Jena AG Corporation, Germany), pH meter (pH-201, Hanna Corporation, Italy), and atomic absorption spectrophotometer (AAS FL, Analytik Jena AG Corporation, Germany), respectively. GC/MS and MWD analyses were conducted according to the former procedures [24]. Observation and analysis of iron surface were conducted with the special system (FEI QUANTA 200F, USA) of ESEM and EDX. The fractionation
Fig. 1. Flow chart of the integrated treatment system.
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Fig. 2. Fractionation procedure of organic compounds in the integrated system.
procedure of humic acid (HA), fulvic acid (FA) and hydrophilic (HyI) compounds by XAD-8 resin was used similarly to the other published researches [26–28], which was shown in Fig. 2. According to the procedure, different TOC concentrations of HA, FA and HyI compounds could be calculated as follows: TOCHA = TOC1 − TOC2 ; TOCFA = TOC2 − TOC3 ; TOCHyI = TOC3 . 3. Results and discussion 3.1. Efficiencies of the integrated system After treatment by the integrated system, COD of the final effluent was steadily reduced to about 100 mg l−1 , which could meet the requirement of the National Discharge Standard of China (GB89781996). The COD change in the influent and the ZVI effluent was shown in Fig. 3. The average COD removal efficiency of the integrated system was 94.2% during the operation period, which was much higher than that of the biofilm reactor system. In the single biofilm reactor system, COD of the biological effluent was more than 200 mg l−1 and the maximal COD removal efficiency was 90.2% [20]. In the integrated system, the maximal COD removal efficiency was up to 96.1% and COD of the final effluent was reduced to 76.2 mg l−1 . It is clear that ZVI process played an important role for organic compounds removal as it was combined with the biofilm reactor system. Fig. 4 shows the variations of COD, TOC and BOD5 in the different reactor effluent. As it could be seen, 81.1% of COD and 76.1% of TOC were removed in Reactor A. Since the concentration of DO in the recirculated ZVI effluent was 4–6 mg l−1 , part of organic compounds would be biodegraded by aerobic microorganisms at the lower location of Reactor A. Besides aerobic biodegradation, denitrification and dilution of the recirculation were of significance to the reduction of the organic compounds in Reactor A effluent.
Fig. 4. Variation of COD, BOD5 and TOC in the different reactors.
TOC removal efficiencies of Reactor O1 , O2 and ZVI were 38.7, 12.5 and 30.4%, respectively. It was indicated that Reactor O1 and ZVI removed most part of organic compounds from Reactor A. From BOD5 change in Fig. 4, it could be found that BOD5 was increased when treated by ZVI process. Moreover, BOD5 /COD could be calculated, which varies from 0.58 in the influent to 0.45, 0.16, 0.09 and 0.22, respectively, in the subsequent reactors. BOD5 /COD decreased when treated by biofilm reactors but increased after ZVI process. Because of the oxidation–reduction function of ZVI process, some refractory organic compounds were transformed or removed, and the biodegradability of effluent was enhanced, which caused the increase of the BOD5 value and BOD5 /COD. Meanwhile, other similar studies proved the increase of biodegradability by ZVI process [24,29,30]. The concentrations of different inorganic nitrogen compounds in the integrated system were shown in Fig. 5. Since the dominant inorganic nitrogen compound in coking wastewater was ammonia, TIN was defined as the sum of NH3 -N, NO2 − -N and NO3 − -N herein. As it could be found from Fig. 5(a), the removal of NH3 -N occurred in Reactor A, O1 and O2 and the concentration of NH3 -N was almost the same between Reactor O2 and ZVI. For the dilution function of recirculation, the concentration of NH3 -N in Reactor A was decreased to 100 mg l−1 below and the residual NH3 -N was biodegraded by nitrification in Reactor O1 and O2 , which could be proved by the increased NO2 − -N and NO3 − -N shown in Fig. 5(b) and (c). From Fig. 5(d), it was indicated most of TIN was removed in Reactor A and O1 , which could be explained by denitrification of NO2 − -N and NO3 − -N. Interestingly, some TIN disappeared in Reactor O1 , which reflected the simultaneous nitrification and denitrification (SND) existed in the biofilm reactor system. No use was found for the ZVI process to removal of inorganic nitrogen compounds, which implied that these compounds were removed by nitrification and denitrification occurred in the biofilm reactors. In the 60 days of operation, NH3 -N of ZVI effluent was always less than 10 mg l−1 and the maximal removal efficiency was up to 99.2%, which was much higher than the performance of single biofilm reactor system in the previous study [20]. The maximal removal efficiency of TIN was 92.3% and the effluent from Reactor ZVI was only composed of low concentrations of NH3 -N and NO3 − -N. Some other residual refractory organic compounds could be removed in the ZVI process, which in turn enhanced the biodegradation of inorganic nitrogen compounds. 3.2. Organic compounds removal in the integrated system
Fig. 3. COD variation in the influent and in the ZVI effluent.
Organic compounds in the influent as well as the effluent from different reactors of the integrated system were analyzed by GC/MS. Table 2 listed the main relative abundance of the influent and efflu-
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Fig. 5. Variation of inorganic nitrogen compounds in the different reactors.
ent from Reactor A, O1 , O2 and ZVI (>1% of integration area). It could be concluded that the main organic compounds of low-molecular weight in the influent were phenolic compounds, which held more than 80% of total integration area. Treated by the integrated system, there were only a few phenolic compounds detected in Reactor A and there were not any organic compounds detected in other reactors. The aerobic biofilm reactors were found to be efficient to remove phenolic compounds, which agreed with the findings by other investigators [12,16,18,20]. Because of the high efficiency for phenolic compounds removal, the integrated system showed a well performance for organic compounds removal during the operation period. 3.3. Characteristics of organic compounds removal by ZVI process In the integrated system, some refractory organic compounds from Reactor O2 could be removed by ZVI process (Fig. 4). Fig. 6 showed the variations of MWD in the biological effluent
(TOC = 80.2 mg l−1 ) and the ZVI effluent (TOC = 44.4 mg l−1 ). It was indicated that considerable part of organic compounds in the biological effluent was high-molecular weight matter, especially in range of 10,000–30,000 Da. From the comparison of MWD in Fig. 6, it was found TOC concentrations in different molecular weight ranges were removed by ZVI process, mostly in ranges of 10,000–30,000 Da and 0–2000 Da. As it could be seen, there were still some organic compounds in different ranges of molecular weight in the effluent. As discussed in Section 3.2, these residual organic compounds could not be analyzed by GC/MS method. The fractionation of organic compounds by XAD-8 resin between the effluent from Reactor O2 and ZVI was shown in Table 3. It was found the biological effluent from Reactor O2 was mainly composed of FA and HyI compounds, which were 39.2 and 50.7% of the total TOC. In some other studies, humic substances (HA and FA) were found to be the considerable part of refractory organic compounds remaining in the effluent after the wastewater treated by biological systems [27,28,31]. After treatment by ZVI process, HA, FA
Table 2 Main organic compounds analysis in the influent (I) and effluent from different reactors (Reactor A, O1 , O2 and ZVI) Organic compounds
I
A
O1
O2
ZVI
Organic compounds
I
A
O1
O2
ZVI
Phenol 2-Methylphenol 4-Methylphenol 2,5-Dimethylphenol 2,4-Dimethylphenol 2-Ethylphenol 4-Ethylphenol
10.4a 13.0 29.5 1.0 9.7 1.4 2.7
6.2 48.6 3.6 8.8 26.7 – –
–b – – – – – –
– – – – – – –
– – – – – – –
3-Ethylphenol 3,4-Dimethylphenol 2,3-Dimethylphenol 3-Ethyl-5-methylphenol 2,3,6-Trimethylphenol 2,3-Dihydro-1H-Inden 2-Naphthalenol
3.7 7.9 1.7 2.2 1.1 1.8 1.8
– – – – – – –
– – – – – – –
– – – – – – –
– – – – – – –
a b
Percentage of relative integration area. Not detected.
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Table 4 Mass percentage of elements on the surface of the iron analyzed by EDX Element
Prepared iron
Used iron
C Si Fe O
18.7 0.8 80.4 –a
10.6 0.4 48.5 40.5
a
Fig. 6. MWD in the biological effluent and the ZVI effluent.
Table 3 TOC concentrations of HA, FA and HyI compounds of Reactor O2 and ZVI Component −1
HA (mg l ) FA (mg l−1 ) HyI (mg l−1 )
Reactor O2
Reactor ZVI
5.8 22.6 29.2
3.1 18.0 17.1
Not detected.
observed on the surface of the iron after ZVI process. It could be concluded that the corrosion of iron surface occurred in ZVI process, which was caused by oxidation–reduction reaction. The produced oxidation–reduction potential could transform the structures and decrease the toxicity of some refractory organic compounds, and consequently enhanced the biodegradability of the effluent. Furthermore, some reddish-yellow iron rust was produced on the surface of the submerged iron materials, which indicated the zerovalent iron was transformed to iron oxides and hydroxides in ZVI process. Table 4 shows the change of elements on the surface of iron. As it could be seen, O element was the main component newly produced of the corroded iron, which proved the transformations of the iron materials. In Reactor ZVI, some iron was proved to be corroded and much sludge was produced, which could consist of iron rust and complexes of iron hydroxides and organic compounds. Based on the following equations: 2Fe − 4e → 2Fe2+
and HyI compounds were all partly removed with different removal efficiencies. TOC removal efficiencies for the three fractions were 46.6, 20.4 and 41.4%, respectively, which revealed that ZVI process was more efficient for removal of HA and HyI compounds. In the ZVI effluent, the residual organic compounds mainly consisted of FA and HyI compounds, which could not be analyzed by GC/MS. Moreover, these HA, FA and HyI compounds in Reactor O2 and ZVI existed in the different ranges of molecular weight as discussed above. 3.4. Removal mechanism analysis of ZVI process The coagulation, precipitation and oxidation–reduction were found to be the main mechanisms for organic compounds removal in ZVI process [24]. The surface change of iron materials in Reactor ZVI was shown in Fig. 7. The initial surface of iron was smooth and retained a metallic glaze. However, there were some cracks
O2 + 4e + 2H2 O → 4OH 2+
Fe
(1) −
−
+ 2OH → Fe(OH)2
(2) (3)
4Fe(OH)2 + O2 + 2H2 O → 4Fe(OH)3
(4)
2Fe(OH)3 → Fe2 O3 + 3H2 O
(5)
the transformations of zero-valent iron in Reactor ZVI could be revealed. Furthermore, the pH value was increased from 8.0 in the Reactor O2 effluent to 9.4 in the ZVI effluent, indicating the increase of produced OH− in ZVI process as described in Eq. (2). Higher pH in Reactor ZVI would be corresponding to lower concentration of Fe3+ (3.7 mg l−1 herein), and much corroded iron was transformed to oxides or hydroxides (as precipitants). The initial produced amorphous Fe(OH)2 and Fe(OH)3 in ZVI process would act as coagulants which removed some organic compounds by coagulation mechanism. Eq. (5) indicates that the produced Fe(OH)3 was unstable, which could be broken down to Fe2 O3 . Furthermore, Fe2 O3 was not
Fig. 7. ESEM photo of the iron materials in Reactor ZVI. Left: 1500× prepared iron not used. Right: 1500× iron in Reactor ZVI after operation period.
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SND phenomenon was found in the aerobic biofilm reactor of the integrated system, the maximal TIN removal efficiency was 92.3% with very low concentrations of NH3 -N and NO3 − -N. Moreover, GC/MS analysis showed the integrated system was also good for removal of phenolic compounds. It was found that ZVI process at the end of integrated system was efficient for removal of some refractory organic compounds with molecular weight ranged either 10,000–30,000 Da or 0–2000 Da, which were mainly the HA and HyI compounds. The ZVI process could also enhance the biodegradability of the effluent through oxidation–reduction and coagulation. Another eminent advantage of the proposed integrated system is its high recovery ability to sudden shock loading, which is of significance to the practical applications. Acknowledgements Fig. 8. Variation of the effluent after the shock loading.
the real chemical structure of the iron oxide but the mixture of complex iron oxides. Lepidocrocite (␥-FeOOH), goethite (␣-FeOOH) and akaganeite (-FeOOH), etc., were found to be the corrosion products under different conditions in the report of Huang and Zhang [32]. Therefore, the main removal mechanisms in ZVI process were oxidation–reduction and coagulation, accompanied with the corrosion of the iron materials and the produce of complex iron oxides and hydroxides. 3.5. Response to sudden shock loading of the integrated system The response of the integrated system after receiving the shock loading was shown in Fig. 8. After 24 h for high loading influent operated, the concentrations of COD and NH3 -N in the final effluent of the integrated system were increased to more than 200 and 30 mg l−1 , respectively. Although both COD and NH3 -N removal efficiencies were recovered rapidly after the shock loading, the former seemed to be recovered faster than the latter. In the proposed system, it took 3 days for recovery of COD removal but 5 days for NH3 -N removal. Since the autotrophic nitrifiers were more susceptible to environmental changes than heterotrophic microorganisms, the recovery of NH3 -N removal efficiency was always slower. Moreover, ZVI process could remove part of increased refractory organic compounds, which could help the recovery of the integrated system for removal of COD and NH3 -N in short time. When the integrated system returned to the normal situation, the concentrations of COD and NH3 -N were reduced to less than 150 and 10 mg l−1 , which indicated that the high performance for pollutants removal of the system could not be weakened by sudden increase of the influent loading. As a result, the integrated system, which combined the biofilm reactors with ZVI process together, was more efficient in actual application for coking wastewater treatment than the single biological treatment process. 4. Conclusions A novel integrated system was established by combining three biofilm reactors and a ZVI process for coking wastewater treatment. Experiments showed this system had much better performance in pollutants removal than the single biofilm system or the separated ZVI process. The maximal efficiencies for COD and NH3 -N removal in the integrated system were up to 96.1 and 99.2%, respectively. Moreover, the average COD and NH3 -N after integrated treatment were 96.6 and 5.1 mg/l, respectively, which could generally meet the Grade 1 as required by National Discharge Standard of China (GB8978-1996).
This work was supported by the National Basic Research Program of China under grant No. 2006BAB04A14. Financial support from the National Nature Science Foundation of China with Grant No. 20607001 is also gratefully acknowledged. We would like to express our sincere thanks to Professor Alistair Borthwick for his kind help in English editing. References [1] J.L. Wang, X.C. Quan, L.B. Wu, Y. Qian, H. Werner, Bioaugmentation as a tool to enhance the removal of refractory compound in coke plant wastewater, Process Biochem. 38 (2002) 777–781. [2] R.G. Luthy, V.C. Stamoudis, J.R. Campbell, W. Harrison, Removal of organic contaminants from coal conversion process condensates, J. Water Pollut. Control Fed. 55 (2) (1983) 196–207. [3] Y. Qian, Y. Wen, H. Zhang, Efficiency of pre-treatment methods in the activated sludge removal of refractory compounds in coke-plant wastewater, Water Res. 28 (1994) 701–710. [4] M. Zhang, J.H. Tay, Y. Qian, X.S. Gu, Coke plant wastewater treatment by fixed biofilm system for COD and NH3 -N removal, Water Res. 32 (2) (1998) 519–527. [5] S. Sarfaraz, S. Thomas, U.K. Tewari, L. Iyenga, Anoxic treatment of phenolic wastewater in sequencing batch reactor, Water Res. 38 (2004) 965–971. [6] J.P.Y. Jokela, R.H. Kettunen, K.M. Sormunen, J.A. Rintala, Biological nitrogen removal from municipal landfill leachate: low-cost nitrification in biofilters and laboratory scale in-situ denitrification, Water Res. 36 (2002) 4079–4087. [7] J. Barbier Jr., L. Oliviero, B. Renard, D. Duprez, Catalytic wet air oxidation of ammonia over M/CeO2 catalysts in the treatment of nitrogen-containing pollutants, Catal. Today 75 (2002) 29–34. [8] B.R. Lim, H.Y. Hu, K. Fujie, Biological degradation and chemical oxidation characteristics of coke-oven wastewater, Water Air Soil Pollut. 146 (2003) 23–33. ˜ [9] I. Vázquez, J. Rodríguez, E. Maranón, L. Castrillón, Y. Fernández, Simultaneous removal of phenol, ammonium and thiocyanate from coke wastewater by aerobic biodegradation, J. Hazard. Mater. B137 (2006) 1773–1780. [10] N.G. Azhar, D.C. Stuckey, The influence of chemical structure on the anaerobic catabolism of refractory compounds: a case study of instant coffee waste, Water Sci. Technol. 30 (1994) 223–232. [11] Y.M. Li, G.W. Gu, J.F. Zhao, H.Q. Yu, Anoxic degradation of nitrogenous heterocyclic compounds by acclimated activated sludge, Process Biochem. 37 (2001) 81–86. [12] S.H. Hosseini, S.M. Borghei, The treatment of phenolic wastewater using a moving bed bio-reactor, Process Biochem. 40 (2005) 1027–1031. [13] H.Q. Yu, G.W. Gu, L.P. Song, Posttreatment of effluent from coke-plant wastewater treatment system in sequencing batch reactors, J. Environ. Eng. 123 (3) (1997) 305–308. [14] H.Q. Yu, G.W. Gu, L.P. Song, The effect of fill mode on the performance of sequencing-batch reactors treating various wastewaters, Bioresour. Technol. 58 (1996) 46–55. [15] M.W. Lee, J.M. Park, Biological nitrogen removal from coke plant wastewater with external carbon addition, Water Environ. Res. 70 (5) (1998) 1090–1095. ˜ Biodegradation of phenolic [16] G. González, G. Herrera, M.T. García, M. Pena, industrial wastewater in a fluidized bed bioreactor with immobilized cells of Pseudomonas putida, Bioresour. Technol. 80 (2001) 137–142. [17] L. Bertin, M. Majone, D. Di Gioia, F. Fava, An aerobic fixed-phase biofilm reactor system for the degradation of the low-molecular weight aromatic compounds occurring in the effluents of anaerobic digestors treating olive mill wastewaters, J. Biotechnol. 87 (2001) 161–177. [18] S.M. Borghei, S.H. Hosseini, The treatment of phenolic wastewater using a moving bed biofilm reactor, Process Biochem. 39 (2004) 1177–1181. [19] A.V. Vinod, G.V. Reddy, Simulation of biodegradation process of phenolic wastewater at higher concentrations in a fluidized-bed bioreactor, Biochem. Eng. J. 24 (2005) 1–10.
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