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The pretreatment by the Fe–Cu process for enhancing biological degradability of the mixed wastewater
Journal of Hazardous Materials 164 (2009) 1392–1397
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
The pretreatment by the Fe–Cu process for enhancing biological degradability of the mixed wastewater Jin-Hong Fan ∗ , Lu-Ming Ma The National Engineering Research Center for Urban Pollution Control, Tongji University, Shanghai 200092, China
a r t i c l e
i n f o
Article history: Received 6 October 2006 Received in revised form 18 April 2008 Accepted 16 September 2008 Available online 7 October 2008 Keywords: The Fe–Cu process Biological treatment Biodegradability Bio-refractory wastewater treatment
a b s t r a c t The Fe–Cu process in combination with cyclic activated sludge system (CASS) was used to treat the mixed wastewater composed of industry wastewater and urban sewage in this work. The results showed that the pretreatment by the Fe–Cu process removed 20% of CODcr and 32% of total phosphorus (TP), which reduced the loading rate of the subsequent biological treatment. Mean while, biodegradability of the wastewater was enhanced, which created favorable condition for the subsequent biological treatment. The formation of heavy, lumpy or granular, absorbent, enriched with microorganisms bio-ferric activated sludge with good setting performance promoted degradation of various refractory organic contaminants. The increase by 10 times of nitrifying and denitrifying bacteria counts (total biofilm biomass increased by 59%) and 0.4 of pH value enhanced the biological nitrification and denitrification to ensure the final effluent NH3 –N and TN to be 8 and 20 mg/L, respectively. Agglomeration, passivation and clogging of iron were not observed in three months of continuous operation. Furthermore, the consumption of iron was low. All these led to an easy maintenance and low operating cost. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Wastewater from pharmaceutical factories, paper mills, textile processing plants, machinery plants and chemical plants are an important source of environmental contamination. Organic contaminants from these industries are known to be largely nonbiodegradable in aerobic conditions [1–24]. So, it is necessary to conduct pretreatment for the mixed wastewater before conventional aerobic bioprocess. Anaerobic hydrolysis-acidification is the very one of pretreatment processes, it may enhance biodegradability of the water to an extant. Besides, its reactive conditions are comparatively mild, but it is directed mainly at insoluble macromolecular weight organics, such as protein, starch, fat and etc [1–3]. Zero-valent iron (Fe0 ) reductive technology has also been extensively studied and used to enhance biodegradability of chemicals and in-situ remediate chemical-contaminated groundwater on a lab scale [4–16]. For example, zero-valent iron was employed in the reduction of nitroaromatic compounds [4–7], decolorization of dyes [8–13] and dechlorination of chlorinated organic compounds [14–16]. However, there exist several problems to use it in industrial wastewater pretreatment. Firstly, the treatment capacity is low. Secondly, the agglomeration and clogging phenomena appear
easily and make iron lost. Finally, the iron is hard to be reused and the cost is too high. In addition, Fe–C pretreatment process is widely used to improve subsequent biological treatment. There are two types of filling materials in such reactors, one is composed only of castiron scrap, and the other is of a mixture of cast-iron scrap and inert carbon particles such as graphite, active carbon or coke. Numerous galvanic cells are formed between Fe with low potential and C with high potential in wastewater acting as electrolyte, electrode reactions and their consequent electrochemical redox, electrophoretic deposition, electrochemical flocculation actions occur, biodegradability of contaminants is enhanced, chroma and CODCr are removed as a result. Meanwhile, properties of activated sludge are promoted by participation of Fe2+ and Fe3+ in the electron transfer in bio-oxidase [17–18]. The electrode reactions can be expressed figuratively: • Anode (Fe):
Fe − 2e → Fe2+ ,
E
Fe2+ Fe
= −0.44 V
(1)
Cathode (C):
∗ Corresponding author. Tel.: +86 21 65984569; fax: +86 21 65983602. E-mail address: [email protected] (J.-H. Fan). 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.09.115
2H+ + 2e → 2[H] → H2 ↑,
E
H+ H2
= 0V
(2)
J.-H. Fan, L.-M. Ma / Journal of Hazardous Materials 164 (2009) 1392–1397
O2 + 2H2 O + 4e → 4OH− ,
E
O 2 H2 O
= 0.41 V
(3)
1393
2. Experimental 2.1. Wastewater sample and the corresponding characteristics
O2 + 4H+ + 4e → 2H2 O,
E
O 2 H2 O
= 1.23 V
(4)
Virtues of the Fe–C process have been found since it came into study and use, as particularized as follows: low operating cost, iron scrap is plentiful and cheap; high efficiency, enhancing biodegradability of wastewater facilitates subsequent biological treatment; wide application, it can be applied to treating wastewater from different industries; with small footprints, the reactor is of simple construction. However, the treatment by this technology is usually carried out under acidic condition (pH 2–3), i.e. a large amount of acid has to be added to adjust wastewater’s pH, mean while, Fe0 is dissolved fast. In order to ensure neutral pH range required for subsequent biological treatment or final discharge, an amount of precipitates is produced in the system after adjusting pH to 8–9, which increases loading rate to sludge dewatering and disposal. Moreover, the treatment process is complicated. Aeration is necessary for the reaction according to Eqs. (2)–(4) and thus the treatment efficiency decreases tremendously in time due to agglomeration and passivation of iron. In order to overcome the above-mentioned shortcomings of the Fe–C process, our research team has developed the Fe–Cu process [19–24]. A small amount of copper is added directly to a conventional Fe–C reactor. First, copper increases the potential difference between Fe anode and C cathode, which further raises efficiency of the electrode reactions. More important, some refractory organics can be reduced directly on the surface of copper. Secondly, since aeration is not required for the treatment Fe0 and Fe2+ will not react with oxygen substantially to produce a great amount of iron-based sludge, iron consumption decreases. As a result, maintenance is also facilitated. Thirdly, this technology covers wider pH range: the nitrobenzene-containing water responded well to the reduction by the Fe–Cu process when pH changed from 2 to 10.5. Summarizing the above-mentioned, pretreatment by the Fe–Cu process is feasible to enhance biodegradability of wastewater and improve subsequent biological treatment with a view to its treatment effect and cost. Therefore, the Fe–Cu process in combination with CASS was used to treat the mixed wastewater in this work. The feasibility of the pretreatment by the Fe–Cu process was explored, the removal rate of CODcr , TP and the like parameter was studied, and enhancement of biodegradability of the water and improvement of setting performance of activated sludge and biofilm biomass were discussed. Finally, the cost-effectiveness of the Fe–Cu process was analyzed. This work provided a theoretical base and technical parameters for bio-refractory wastewater treatment.
The mixed wastewater from an urban sewage treatment plant located in Sichuan Province of China, consists of industrial wastewater and domestic sewage, among them the former accounts for 2/3 and is mainly from pharmaceutical factories, paper mills, dye factories, machinery plants and chemical plants with the proportion of 40:20:20:12:8. Nearly all biologically degradable contaminants in the industrial water are already removed after pretreatment in their own plants and factories, so the mixed wastewater contains a large number of bio-refractory organics, such as chlorinated hydrocarbons (mainly dichloromethane), azo dyes (vat dyes, disperse dyes, mordant dyes, direct dyes, neutral dyes), nitroaromatics, surfactants; moreover, the mixed wastewater changes very often in quality and CODcr, NH3 –N, TP and pH were 386–751, 100–146, 4.35–13.7 and 7.45–8.19 mg/L, respectively.
2.2. Apparatus and materials The schematic diagram of experimental apparatus is shown in Fig. 1. The Fe–Cu bed of PVC plate (80 cm × 20 cm × 25 cm) is a plug flow reactor. The reactor with a working volume of 20 L was divided into anterior inflow region, back outflow region, upper region filled with Fe and Cu and the nether sludge discharge region. Three baffles were setup in the upper and low parts of the reactor separately in the direction of water flow to force water to advance in a wavelike manner, so as to avoid its short circuit. In the nether sludge discharge region a set of aeration pipes was laid evenly for backwashing the reactor. Iron scrap was thoroughly mixed with Cu pieces in a desired proportion (10:1–6:1, w/w) and held them down so that the observed density reached ∼0.5 kg/L. The mixture was then placed in the reactor to form a fixed Fe–Cu bed. The reactor was covered with a lid, but not sealed. The Fe–Cu bed remained lower than water to avoid short circuit. In order to intensify and accelerate mass transfer between solid and liquid phases and the electrode reactions the effluent from the Fe–Cu bed was returned with a reflux ratio of 2:1–5:1. The Fe–Cu reactor operated continuously for about 3 months with 1 h of hydraulic retention time. The CASS bioreactor of PVC plates (83 cm × 34 cm × 40 cm) is with effective water depth of 25 cm and working volume of 70 L. The entire bioreactor was divided into two regions with a volume ratio of 1:4: the first one was anoxic and the second one was aerobic. Two baffles were set up on the left and right of the anoxic region. In the aerobic region a water-outfall was installed to discharge supernatant fluid and a sludge outfall was installed to control sludge age. The suspended carrier of polypropylene (Ø 25 mm × 25 mm) has a specific surface area of 224 m2 /m3 , density of 0.96–0.99 g/cm3 , throwing ratio of 30%. The experiment was conducted with 20 h of
Fig. 1. Schematic diagram of experimental apparatus.
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Fig. 2. CODcr removal by the Fe–Cu process.
HRT and 100% sludge reflux ratio. A running cycle last 8 h, thereunto 2.0 h for inflow, 5.5 h for microporous aeration, 1.5 h for sedimentation and 1.0 h for discharge. Both influent and effluent volumes in a running cycle were 28 L, which means both influent and effluent have a ratio of 40%. The running cycle was controlled automatically. The measured biomass in the bioreactor was 2.5 g/L and the dissolved oxygen was 4 mg/L when the system was in a steady operation.
3Fe + XN = NX + 4H2 O → 3Fe2+ + XNH2 + X NH2 + 4OH−
2.3. Analytical methods
ArNO2 + 6e + 4H2 O
CODcr , BOD5 , NH3 –N, MLSS, TP and total Fe (Fe2+ and Fe3+ ), MNP (Most Probable Number) were determined using standard methods [25]. pH was measured using a pH/ISE with a Ross sure-flow pH electrode, DO was registered with a portable DO meter. Bioferric activated sludge morphology was analyzed by the scanning electron microscopy (SEM) (S-2360N, Hitachi, Japan). Drop of the surface layer of 500 mL sludge was measured after 30 min sedimentation (SV30 ). Weighing biofilm biomass was carried out after drying 2 h at 105 ◦ C.
Consequently, biodegradability of the wastewater was enhanced, which created favorable conditions for the subsequent biological treatment. The ratio of BOD5 /CODCr was used as a reference to assess biodegradability of the wastewater before and after pretreatment by the Fe–Cu process. BOD5 /CODCr were measured and calculated several times, when the system was in operation. The results indicated that the ratio of BOD5 /CODcr increased from a range of 0.13–0.18 to a range of 0.28–0.34 after the pretreatment by the Fe–Cu process.
3. Results and discussion
3.1.2. TP removal TP removal rate by the Fe–Cu process achieved 32% (Fig. 4). TP was partially removed from the system due to the formation of the precipitates produced by the reaction of Fe2+ with PO4 3− . Furthermore, Fe(OH)2 formed by Fe2+ under alkaline condition could adsorb a mass of PO4 3− and remove it by flocculation and sedimentation [27]. The optimal pH for TP removal was around 8 [28], the rise of pH value in the Fe–Cu process probably favored PO4 3− pre-
3.1. Pretreatment effect of the Fe–Cu process 3.1.1. CODcr removal and BOD5 /CODCr enhancement Fig. 2 showed the effect of CODCr removal by the Fe–Cu process. The results showed the average concentration of the influent and effluent CODcr were 545 and 439 mg/L, respectively, i.e. an average CODCr removal rate of 20% was achieved, and consequently, the organic load of the subsequent biological treatment was reduced. CODCr removal was mainly due to coagulation of the nascent state Fe (OH)2 and Fe (OH)3 formed by Fe2+ —the corrosion product of Fe0 , such as flocculation of ferric hydroxide colloid on surfactants [26](Fig. 3). Moreover, suspended contaminants were filtrated and adsorbed by the Fe–Cu bed. Additionally, the contaminants with strong electron-drawing groups, such as nitro-group and azo-group contained in nitroaromatics and azo dyes were reduced to amino-group, chlorinated hydrocarbons were dechlorinated directly on the copper surface as well as upon undergoing redox-reaction with Fe0 and Fe(II) [12,13,19–24]. Different reactions mainly including the oxidation of Fe0 and the reduction of H2 O and contaminants occurred in the Fe–Cu process: Fe–2H2 O → Fe2+ + H2 + 2OH− 2+
2Fe + 3H2 O + XClx → 2Fe
Fig. 3. Flocculation of ferric hydroxide colloid on surfactants.
Cu cathode
XClx + 2e + H2 O
−→
XN = NX + 6e + 4H2 O
XHClx−1 + Cl− + OH−
Cu cathode
−→
XNH2 + X NH2 + 4OH−
Cu cathode
−→
ArNH2 + 6OH−
(5) −
−
+ 3OH + H2 + XHClx−1 + Cl
(6)
Fig. 4. TP removal by the Fe–Cu process.
(7) (9) (10) (11)
J.-H. Fan, L.-M. Ma / Journal of Hazardous Materials 164 (2009) 1392–1397
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Fig. 7. Improving CODcr biological removal. Fig. 5. Comparison of SCS between two kinds of sludge.
cipitation. The experimental results showed that the original pH value of the mixed water ranged between 7.45 and 8.19, after the pretreatment by the Fe–Cu process it increased by 0.4 on average. As expressed by Eqs. (5)–(11), OH− was generated on the reduction of H2 O and different contaminants in aqueous solution, which caused rapid rise of pH. However, the formation of Fe(OH)2 and Fe(OH)3 (as Eqs. (12)–(14)) buffered the variation of pH value [21]. As a result, pH merely grew by 0.4: Fe2+ + 2OH− → Fe(OH)2 Fe anode
Fe2+ − e + 3OH− −→ Fe(OH)3 Fe anode
Fe(OH)2 − e + OH− −→ Fe(OH)3
(12) (13) (14)
3.2. Improvement of the subsequent biological treatment 3.2.1. Improvement of sludge’s setting performance The settlement curve of sludge (SCS) was plotted for the combined technology “the Fe–Cu process + CASS” and CASS technology separately (Fig. 5) under the same condition. As shown in Fig. 6, compression of sludge in the combined process occurred 4 min after settling, in the CASS process compression occurred 8 min after
settling. Compression of the former was 4 min ahead of the latter, which means the sludge produced in the Fe–Cu + CASS process settled faster than that in CASS process, which raised utilization ratio of the bioreactor [29]. The SCS of the combined process always located below that of CASS process. Additionally, SV30 of the sludge produced in the combined process was 15% lower than that in the CASS process. All these led up to a fact that the sludge produced in the combined process had a better settling performance. It is mainly because Fe2+ formed by Fe0 corrosion in the Fe–Cu process entered the CASS bioreactor, which promoted electron transfer in biological oxidation, bacterial multiplication and enzyme secreting, and improved biological activity of sludge [30,31]. Fe(OH)2 and Fe(OH)3 floccules formed by Fe2+ under alkaline and aerobic conditions react with highly absorbent microorganisms, and as a result, thick and compact granular bio-ferric activated sludge was produced as a product in the bioreactor (Fig. 6). The experimental results showed that the dissolved Fe (including Fe2+ and Fe3+ ) in the effluent of Fe–Cu process was 18 mg/L, much less than 170 mg/L Fe2+ , which was produced in the treatment of dyestuff wastewater containing 700–1100 mg/L CODcr by the Fe–C process for 1 h (pH ∼2.0), merely 3% of 700 mg/L Fe2+ that was dissolved under aerobic conditions of 5 L/min air volume [32]. And, it was about 10% of 175 mg/L Fe2+ , which was produced in the treatment of papermaking wastewater containing 526–731 mg/L CODcr by the Fe–C process for 10 min under aerobic conditions of 0.4 L/min air volume and of pH ∼3.0 [33]. Therefore, the consumption of iron was reduced leading to easy maintenance and low operating cost. 3.2.2. Improvement of CODCr biological removal Fig. 7 showed that CODCr removal rate in the first 12 days by CASS biotreatment declined fast; in the next 15 days, CODCr removal rate rose gradually due to performing the Fe–Cu process pretreatment before the biological treatment. The mixed wastewater contained many biorefractory organic contaminants, which badly affect the activated sludge in CASS and reduced the CODCr removal efficiency. The biodegradability of the wastewater was enhanced greatly after the pretreatment by the Fe–Cu process and formation of the highly absorbent enriched with microorganisms and organics bio-ferric active sludge; accordingly, removal of biorefractory CODCr was facilitated.
Fig. 6. SEM micrographs of bio-ferric activated sludge (3000×).
3.2.3. Improvement of biofilm biomass and biological nitrification-denitrification The results indicated that average biofilm biomass of a single suspended carrier in the single CASS process was 0.97 g, while 1.54 g in the Fe–Cu + CASS process. And the total biofilm biomass of reactor increased by 59% after the pretreatment of Fe–Cu process. Possible
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reasons were as follows: most bio-refractory organics were reductively transformed to easily biodegradable compounds by Fe–Cu process; Fe2+ , Fe3+ —the corrosion product of Fe0 played electron transfer mediator in the bio-oxidation–reduction to enhance bacteria reproduction and enzyme secretion [30], generated charge neutralization with negatively charged polypropylene to improve negatively charged bacteria growth on the suspended carriers. Correspondingly, nitrifying and denitrifying bacteria counts (Most Probable Number, MPN) increased by about 10 times to enhance biological nitrification and denitrification capacity. Furthermore, pH value (an average of 0.4) of the effluent increased after the Fe–Cu pretreatment to provide sufficient alkalinity for the biological nitrification. Consequently, the final effluent NH3 –N and TN were 8 and 20 mg/L respectively, i.e. a total NH3 –N removal rate of 93.3% was achieved.
ber, MPN) increased by about 10 times to ensure the final effluent NH3 –N and TN to be 8 and 20 mg/L, respectively. Agglomeration, passivation and clogging of iron were not observed in 3 months of continuous operation. Furthermore, the consumption of iron was low. All these led to an easy maintenance and low operating cost and demonstrated that the Fe–Cu process was a promising technology for refractory biodegradable organics-containing wastewater pretreatment. Acknowledgments Thanks to Xia-hui Zhi and Wen-ying Xu for their helpful contributions. This study has been financially supported by 863 Hi-Tech Research and Development Program of the Peoples Republic of China and Everbright Environment Innovation fund.
3.3. Cost-effectiveness analysis References The Fe–Cu process was effective and stable in the mixed wastewater pretreatment. CODcr , TP, NH3 –N removal rate increased by more than 22%, 35%, 5%, respectively, compared with the single CASS process. Agglomeration, passivation and clogging were not observed in the Fe–Cu bed after three months of continuous operation, and the surface properties of fillings almost remained unchanged, which spelled their good metallicity. Furthermore, the process featured low operating costs including iron consumption and electricity consumption. According to measured, the iron consumption was less than 0.05 kg/ton wastewater and the corresponding cost was only 0.05 yuan (RMB). The electricity consumption was about 0.12 kWh/ton wastewater and the corresponding cost was 0.06 yuan (RMB). A Fe–Cu reaction tank was used for pretreatment of 60,000 m3 /day mixed chemical wastewater in Taopu industrial area, in January 2006, at the Putuo District in northwest of Shanghai, China and the long-term operating effect is good. All these led to a promising technology of Fe–Cu process for refractory wastewater pretreatment. 4. Conclusions The pretreatment by the Fe–Cu process removed 20% of CODcr and 32% of total phosphorus (TP), which reduced the loading rate of the subsequent biological treatment. Meanwhile, BOD5 /CODCr ratio was increased by about one time, which created favorable condition for the subsequent biological treatment. The effects may be attributed to the following factors: (1) coagulation of organic contaminants by the nascent state Fe (OH)2 and Fe (OH)3 ; (2) filtration and adsorption of suspended contaminants by the Fe–Cu bed; (3) reduction of biorefractory organic pollutants with strong electron-drawing groups directly on the copper surface; (4) reduction of biorefractory organics by Fe0 and Fe(II); (5) sedimentation and flocculation of PO4 3− by Fe2+ , Fe(OH)2 and hydrolyzates. 18 mg/L of Fe2+ and Fe3+ formed by Fe0 corrosion in the Fe–Cu process entered the CASS bioreactor, participated in the electron transfer in biological oxidase to enhance bacteria reproduction and enzyme secretion and generated charge neutralization with negatively charged polypropylene to improve negatively charged bacteria growth on the suspended carriers. Additionally, Fe(OH)2 and Fe (OH)3 floccules formed next under alkaline and aerobic conditions combined with floc forming bacteria to form bio-ferric activated sludge. As a result, the total biofilm biomass increased by 59% and the bio-ferric activated sludge with good setting performance promoted the biological removal of CODCr. .Meanwhile, the nitrifying and denitrifying bacteria counts (Most Probable Num-
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
The pretreatment by the Fe–Cu process for enhancing biological degradability of the mixed wastewater Jin-Hong Fan ∗ , Lu-Ming Ma The National Engineering Research Center for Urban Pollution Control, Tongji University, Shanghai 200092, China
a r t i c l e
i n f o
Article history: Received 6 October 2006 Received in revised form 18 April 2008 Accepted 16 September 2008 Available online 7 October 2008 Keywords: The Fe–Cu process Biological treatment Biodegradability Bio-refractory wastewater treatment
a b s t r a c t The Fe–Cu process in combination with cyclic activated sludge system (CASS) was used to treat the mixed wastewater composed of industry wastewater and urban sewage in this work. The results showed that the pretreatment by the Fe–Cu process removed 20% of CODcr and 32% of total phosphorus (TP), which reduced the loading rate of the subsequent biological treatment. Mean while, biodegradability of the wastewater was enhanced, which created favorable condition for the subsequent biological treatment. The formation of heavy, lumpy or granular, absorbent, enriched with microorganisms bio-ferric activated sludge with good setting performance promoted degradation of various refractory organic contaminants. The increase by 10 times of nitrifying and denitrifying bacteria counts (total biofilm biomass increased by 59%) and 0.4 of pH value enhanced the biological nitrification and denitrification to ensure the final effluent NH3 –N and TN to be 8 and 20 mg/L, respectively. Agglomeration, passivation and clogging of iron were not observed in three months of continuous operation. Furthermore, the consumption of iron was low. All these led to an easy maintenance and low operating cost. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Wastewater from pharmaceutical factories, paper mills, textile processing plants, machinery plants and chemical plants are an important source of environmental contamination. Organic contaminants from these industries are known to be largely nonbiodegradable in aerobic conditions [1–24]. So, it is necessary to conduct pretreatment for the mixed wastewater before conventional aerobic bioprocess. Anaerobic hydrolysis-acidification is the very one of pretreatment processes, it may enhance biodegradability of the water to an extant. Besides, its reactive conditions are comparatively mild, but it is directed mainly at insoluble macromolecular weight organics, such as protein, starch, fat and etc [1–3]. Zero-valent iron (Fe0 ) reductive technology has also been extensively studied and used to enhance biodegradability of chemicals and in-situ remediate chemical-contaminated groundwater on a lab scale [4–16]. For example, zero-valent iron was employed in the reduction of nitroaromatic compounds [4–7], decolorization of dyes [8–13] and dechlorination of chlorinated organic compounds [14–16]. However, there exist several problems to use it in industrial wastewater pretreatment. Firstly, the treatment capacity is low. Secondly, the agglomeration and clogging phenomena appear
easily and make iron lost. Finally, the iron is hard to be reused and the cost is too high. In addition, Fe–C pretreatment process is widely used to improve subsequent biological treatment. There are two types of filling materials in such reactors, one is composed only of castiron scrap, and the other is of a mixture of cast-iron scrap and inert carbon particles such as graphite, active carbon or coke. Numerous galvanic cells are formed between Fe with low potential and C with high potential in wastewater acting as electrolyte, electrode reactions and their consequent electrochemical redox, electrophoretic deposition, electrochemical flocculation actions occur, biodegradability of contaminants is enhanced, chroma and CODCr are removed as a result. Meanwhile, properties of activated sludge are promoted by participation of Fe2+ and Fe3+ in the electron transfer in bio-oxidase [17–18]. The electrode reactions can be expressed figuratively: • Anode (Fe):
Fe − 2e → Fe2+ ,
E
Fe2+ Fe
= −0.44 V
(1)
Cathode (C):
∗ Corresponding author. Tel.: +86 21 65984569; fax: +86 21 65983602. E-mail address: [email protected] (J.-H. Fan). 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.09.115
2H+ + 2e → 2[H] → H2 ↑,
E
H+ H2
= 0V
(2)
J.-H. Fan, L.-M. Ma / Journal of Hazardous Materials 164 (2009) 1392–1397
O2 + 2H2 O + 4e → 4OH− ,
E
O 2 H2 O
= 0.41 V
(3)
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2. Experimental 2.1. Wastewater sample and the corresponding characteristics
O2 + 4H+ + 4e → 2H2 O,
E
O 2 H2 O
= 1.23 V
(4)
Virtues of the Fe–C process have been found since it came into study and use, as particularized as follows: low operating cost, iron scrap is plentiful and cheap; high efficiency, enhancing biodegradability of wastewater facilitates subsequent biological treatment; wide application, it can be applied to treating wastewater from different industries; with small footprints, the reactor is of simple construction. However, the treatment by this technology is usually carried out under acidic condition (pH 2–3), i.e. a large amount of acid has to be added to adjust wastewater’s pH, mean while, Fe0 is dissolved fast. In order to ensure neutral pH range required for subsequent biological treatment or final discharge, an amount of precipitates is produced in the system after adjusting pH to 8–9, which increases loading rate to sludge dewatering and disposal. Moreover, the treatment process is complicated. Aeration is necessary for the reaction according to Eqs. (2)–(4) and thus the treatment efficiency decreases tremendously in time due to agglomeration and passivation of iron. In order to overcome the above-mentioned shortcomings of the Fe–C process, our research team has developed the Fe–Cu process [19–24]. A small amount of copper is added directly to a conventional Fe–C reactor. First, copper increases the potential difference between Fe anode and C cathode, which further raises efficiency of the electrode reactions. More important, some refractory organics can be reduced directly on the surface of copper. Secondly, since aeration is not required for the treatment Fe0 and Fe2+ will not react with oxygen substantially to produce a great amount of iron-based sludge, iron consumption decreases. As a result, maintenance is also facilitated. Thirdly, this technology covers wider pH range: the nitrobenzene-containing water responded well to the reduction by the Fe–Cu process when pH changed from 2 to 10.5. Summarizing the above-mentioned, pretreatment by the Fe–Cu process is feasible to enhance biodegradability of wastewater and improve subsequent biological treatment with a view to its treatment effect and cost. Therefore, the Fe–Cu process in combination with CASS was used to treat the mixed wastewater in this work. The feasibility of the pretreatment by the Fe–Cu process was explored, the removal rate of CODcr , TP and the like parameter was studied, and enhancement of biodegradability of the water and improvement of setting performance of activated sludge and biofilm biomass were discussed. Finally, the cost-effectiveness of the Fe–Cu process was analyzed. This work provided a theoretical base and technical parameters for bio-refractory wastewater treatment.
The mixed wastewater from an urban sewage treatment plant located in Sichuan Province of China, consists of industrial wastewater and domestic sewage, among them the former accounts for 2/3 and is mainly from pharmaceutical factories, paper mills, dye factories, machinery plants and chemical plants with the proportion of 40:20:20:12:8. Nearly all biologically degradable contaminants in the industrial water are already removed after pretreatment in their own plants and factories, so the mixed wastewater contains a large number of bio-refractory organics, such as chlorinated hydrocarbons (mainly dichloromethane), azo dyes (vat dyes, disperse dyes, mordant dyes, direct dyes, neutral dyes), nitroaromatics, surfactants; moreover, the mixed wastewater changes very often in quality and CODcr, NH3 –N, TP and pH were 386–751, 100–146, 4.35–13.7 and 7.45–8.19 mg/L, respectively.
2.2. Apparatus and materials The schematic diagram of experimental apparatus is shown in Fig. 1. The Fe–Cu bed of PVC plate (80 cm × 20 cm × 25 cm) is a plug flow reactor. The reactor with a working volume of 20 L was divided into anterior inflow region, back outflow region, upper region filled with Fe and Cu and the nether sludge discharge region. Three baffles were setup in the upper and low parts of the reactor separately in the direction of water flow to force water to advance in a wavelike manner, so as to avoid its short circuit. In the nether sludge discharge region a set of aeration pipes was laid evenly for backwashing the reactor. Iron scrap was thoroughly mixed with Cu pieces in a desired proportion (10:1–6:1, w/w) and held them down so that the observed density reached ∼0.5 kg/L. The mixture was then placed in the reactor to form a fixed Fe–Cu bed. The reactor was covered with a lid, but not sealed. The Fe–Cu bed remained lower than water to avoid short circuit. In order to intensify and accelerate mass transfer between solid and liquid phases and the electrode reactions the effluent from the Fe–Cu bed was returned with a reflux ratio of 2:1–5:1. The Fe–Cu reactor operated continuously for about 3 months with 1 h of hydraulic retention time. The CASS bioreactor of PVC plates (83 cm × 34 cm × 40 cm) is with effective water depth of 25 cm and working volume of 70 L. The entire bioreactor was divided into two regions with a volume ratio of 1:4: the first one was anoxic and the second one was aerobic. Two baffles were set up on the left and right of the anoxic region. In the aerobic region a water-outfall was installed to discharge supernatant fluid and a sludge outfall was installed to control sludge age. The suspended carrier of polypropylene (Ø 25 mm × 25 mm) has a specific surface area of 224 m2 /m3 , density of 0.96–0.99 g/cm3 , throwing ratio of 30%. The experiment was conducted with 20 h of
Fig. 1. Schematic diagram of experimental apparatus.
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Fig. 2. CODcr removal by the Fe–Cu process.
HRT and 100% sludge reflux ratio. A running cycle last 8 h, thereunto 2.0 h for inflow, 5.5 h for microporous aeration, 1.5 h for sedimentation and 1.0 h for discharge. Both influent and effluent volumes in a running cycle were 28 L, which means both influent and effluent have a ratio of 40%. The running cycle was controlled automatically. The measured biomass in the bioreactor was 2.5 g/L and the dissolved oxygen was 4 mg/L when the system was in a steady operation.
3Fe + XN = NX + 4H2 O → 3Fe2+ + XNH2 + X NH2 + 4OH−
2.3. Analytical methods
ArNO2 + 6e + 4H2 O
CODcr , BOD5 , NH3 –N, MLSS, TP and total Fe (Fe2+ and Fe3+ ), MNP (Most Probable Number) were determined using standard methods [25]. pH was measured using a pH/ISE with a Ross sure-flow pH electrode, DO was registered with a portable DO meter. Bioferric activated sludge morphology was analyzed by the scanning electron microscopy (SEM) (S-2360N, Hitachi, Japan). Drop of the surface layer of 500 mL sludge was measured after 30 min sedimentation (SV30 ). Weighing biofilm biomass was carried out after drying 2 h at 105 ◦ C.
Consequently, biodegradability of the wastewater was enhanced, which created favorable conditions for the subsequent biological treatment. The ratio of BOD5 /CODCr was used as a reference to assess biodegradability of the wastewater before and after pretreatment by the Fe–Cu process. BOD5 /CODCr were measured and calculated several times, when the system was in operation. The results indicated that the ratio of BOD5 /CODcr increased from a range of 0.13–0.18 to a range of 0.28–0.34 after the pretreatment by the Fe–Cu process.
3. Results and discussion
3.1.2. TP removal TP removal rate by the Fe–Cu process achieved 32% (Fig. 4). TP was partially removed from the system due to the formation of the precipitates produced by the reaction of Fe2+ with PO4 3− . Furthermore, Fe(OH)2 formed by Fe2+ under alkaline condition could adsorb a mass of PO4 3− and remove it by flocculation and sedimentation [27]. The optimal pH for TP removal was around 8 [28], the rise of pH value in the Fe–Cu process probably favored PO4 3− pre-
3.1. Pretreatment effect of the Fe–Cu process 3.1.1. CODcr removal and BOD5 /CODCr enhancement Fig. 2 showed the effect of CODCr removal by the Fe–Cu process. The results showed the average concentration of the influent and effluent CODcr were 545 and 439 mg/L, respectively, i.e. an average CODCr removal rate of 20% was achieved, and consequently, the organic load of the subsequent biological treatment was reduced. CODCr removal was mainly due to coagulation of the nascent state Fe (OH)2 and Fe (OH)3 formed by Fe2+ —the corrosion product of Fe0 , such as flocculation of ferric hydroxide colloid on surfactants [26](Fig. 3). Moreover, suspended contaminants were filtrated and adsorbed by the Fe–Cu bed. Additionally, the contaminants with strong electron-drawing groups, such as nitro-group and azo-group contained in nitroaromatics and azo dyes were reduced to amino-group, chlorinated hydrocarbons were dechlorinated directly on the copper surface as well as upon undergoing redox-reaction with Fe0 and Fe(II) [12,13,19–24]. Different reactions mainly including the oxidation of Fe0 and the reduction of H2 O and contaminants occurred in the Fe–Cu process: Fe–2H2 O → Fe2+ + H2 + 2OH− 2+
2Fe + 3H2 O + XClx → 2Fe
Fig. 3. Flocculation of ferric hydroxide colloid on surfactants.
Cu cathode
XClx + 2e + H2 O
−→
XN = NX + 6e + 4H2 O
XHClx−1 + Cl− + OH−
Cu cathode
−→
XNH2 + X NH2 + 4OH−
Cu cathode
−→
ArNH2 + 6OH−
(5) −
−
+ 3OH + H2 + XHClx−1 + Cl
(6)
Fig. 4. TP removal by the Fe–Cu process.
(7) (9) (10) (11)
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Fig. 7. Improving CODcr biological removal. Fig. 5. Comparison of SCS between two kinds of sludge.
cipitation. The experimental results showed that the original pH value of the mixed water ranged between 7.45 and 8.19, after the pretreatment by the Fe–Cu process it increased by 0.4 on average. As expressed by Eqs. (5)–(11), OH− was generated on the reduction of H2 O and different contaminants in aqueous solution, which caused rapid rise of pH. However, the formation of Fe(OH)2 and Fe(OH)3 (as Eqs. (12)–(14)) buffered the variation of pH value [21]. As a result, pH merely grew by 0.4: Fe2+ + 2OH− → Fe(OH)2 Fe anode
Fe2+ − e + 3OH− −→ Fe(OH)3 Fe anode
Fe(OH)2 − e + OH− −→ Fe(OH)3
(12) (13) (14)
3.2. Improvement of the subsequent biological treatment 3.2.1. Improvement of sludge’s setting performance The settlement curve of sludge (SCS) was plotted for the combined technology “the Fe–Cu process + CASS” and CASS technology separately (Fig. 5) under the same condition. As shown in Fig. 6, compression of sludge in the combined process occurred 4 min after settling, in the CASS process compression occurred 8 min after
settling. Compression of the former was 4 min ahead of the latter, which means the sludge produced in the Fe–Cu + CASS process settled faster than that in CASS process, which raised utilization ratio of the bioreactor [29]. The SCS of the combined process always located below that of CASS process. Additionally, SV30 of the sludge produced in the combined process was 15% lower than that in the CASS process. All these led up to a fact that the sludge produced in the combined process had a better settling performance. It is mainly because Fe2+ formed by Fe0 corrosion in the Fe–Cu process entered the CASS bioreactor, which promoted electron transfer in biological oxidation, bacterial multiplication and enzyme secreting, and improved biological activity of sludge [30,31]. Fe(OH)2 and Fe(OH)3 floccules formed by Fe2+ under alkaline and aerobic conditions react with highly absorbent microorganisms, and as a result, thick and compact granular bio-ferric activated sludge was produced as a product in the bioreactor (Fig. 6). The experimental results showed that the dissolved Fe (including Fe2+ and Fe3+ ) in the effluent of Fe–Cu process was 18 mg/L, much less than 170 mg/L Fe2+ , which was produced in the treatment of dyestuff wastewater containing 700–1100 mg/L CODcr by the Fe–C process for 1 h (pH ∼2.0), merely 3% of 700 mg/L Fe2+ that was dissolved under aerobic conditions of 5 L/min air volume [32]. And, it was about 10% of 175 mg/L Fe2+ , which was produced in the treatment of papermaking wastewater containing 526–731 mg/L CODcr by the Fe–C process for 10 min under aerobic conditions of 0.4 L/min air volume and of pH ∼3.0 [33]. Therefore, the consumption of iron was reduced leading to easy maintenance and low operating cost. 3.2.2. Improvement of CODCr biological removal Fig. 7 showed that CODCr removal rate in the first 12 days by CASS biotreatment declined fast; in the next 15 days, CODCr removal rate rose gradually due to performing the Fe–Cu process pretreatment before the biological treatment. The mixed wastewater contained many biorefractory organic contaminants, which badly affect the activated sludge in CASS and reduced the CODCr removal efficiency. The biodegradability of the wastewater was enhanced greatly after the pretreatment by the Fe–Cu process and formation of the highly absorbent enriched with microorganisms and organics bio-ferric active sludge; accordingly, removal of biorefractory CODCr was facilitated.
Fig. 6. SEM micrographs of bio-ferric activated sludge (3000×).
3.2.3. Improvement of biofilm biomass and biological nitrification-denitrification The results indicated that average biofilm biomass of a single suspended carrier in the single CASS process was 0.97 g, while 1.54 g in the Fe–Cu + CASS process. And the total biofilm biomass of reactor increased by 59% after the pretreatment of Fe–Cu process. Possible
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reasons were as follows: most bio-refractory organics were reductively transformed to easily biodegradable compounds by Fe–Cu process; Fe2+ , Fe3+ —the corrosion product of Fe0 played electron transfer mediator in the bio-oxidation–reduction to enhance bacteria reproduction and enzyme secretion [30], generated charge neutralization with negatively charged polypropylene to improve negatively charged bacteria growth on the suspended carriers. Correspondingly, nitrifying and denitrifying bacteria counts (Most Probable Number, MPN) increased by about 10 times to enhance biological nitrification and denitrification capacity. Furthermore, pH value (an average of 0.4) of the effluent increased after the Fe–Cu pretreatment to provide sufficient alkalinity for the biological nitrification. Consequently, the final effluent NH3 –N and TN were 8 and 20 mg/L respectively, i.e. a total NH3 –N removal rate of 93.3% was achieved.
ber, MPN) increased by about 10 times to ensure the final effluent NH3 –N and TN to be 8 and 20 mg/L, respectively. Agglomeration, passivation and clogging of iron were not observed in 3 months of continuous operation. Furthermore, the consumption of iron was low. All these led to an easy maintenance and low operating cost and demonstrated that the Fe–Cu process was a promising technology for refractory biodegradable organics-containing wastewater pretreatment. Acknowledgments Thanks to Xia-hui Zhi and Wen-ying Xu for their helpful contributions. This study has been financially supported by 863 Hi-Tech Research and Development Program of the Peoples Republic of China and Everbright Environment Innovation fund.
3.3. Cost-effectiveness analysis References The Fe–Cu process was effective and stable in the mixed wastewater pretreatment. CODcr , TP, NH3 –N removal rate increased by more than 22%, 35%, 5%, respectively, compared with the single CASS process. Agglomeration, passivation and clogging were not observed in the Fe–Cu bed after three months of continuous operation, and the surface properties of fillings almost remained unchanged, which spelled their good metallicity. Furthermore, the process featured low operating costs including iron consumption and electricity consumption. According to measured, the iron consumption was less than 0.05 kg/ton wastewater and the corresponding cost was only 0.05 yuan (RMB). The electricity consumption was about 0.12 kWh/ton wastewater and the corresponding cost was 0.06 yuan (RMB). A Fe–Cu reaction tank was used for pretreatment of 60,000 m3 /day mixed chemical wastewater in Taopu industrial area, in January 2006, at the Putuo District in northwest of Shanghai, China and the long-term operating effect is good. All these led to a promising technology of Fe–Cu process for refractory wastewater pretreatment. 4. Conclusions The pretreatment by the Fe–Cu process removed 20% of CODcr and 32% of total phosphorus (TP), which reduced the loading rate of the subsequent biological treatment. Meanwhile, BOD5 /CODCr ratio was increased by about one time, which created favorable condition for the subsequent biological treatment. The effects may be attributed to the following factors: (1) coagulation of organic contaminants by the nascent state Fe (OH)2 and Fe (OH)3 ; (2) filtration and adsorption of suspended contaminants by the Fe–Cu bed; (3) reduction of biorefractory organic pollutants with strong electron-drawing groups directly on the copper surface; (4) reduction of biorefractory organics by Fe0 and Fe(II); (5) sedimentation and flocculation of PO4 3− by Fe2+ , Fe(OH)2 and hydrolyzates. 18 mg/L of Fe2+ and Fe3+ formed by Fe0 corrosion in the Fe–Cu process entered the CASS bioreactor, participated in the electron transfer in biological oxidase to enhance bacteria reproduction and enzyme secretion and generated charge neutralization with negatively charged polypropylene to improve negatively charged bacteria growth on the suspended carriers. Additionally, Fe(OH)2 and Fe (OH)3 floccules formed next under alkaline and aerobic conditions combined with floc forming bacteria to form bio-ferric activated sludge. As a result, the total biofilm biomass increased by 59% and the bio-ferric activated sludge with good setting performance promoted the biological removal of CODCr. .Meanwhile, the nitrifying and denitrifying bacteria counts (Most Probable Num-
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