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Enhanced dewaterability of textile dyeing sludge using micro-electrolysis pretreatment
Journal of Environmental Management 161 (2015) 181e187
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
Enhanced dewaterability of textile dyeing sludge using microelectrolysis pretreatment Xun-an Ning*, 1, Weibin Wen 1, Yaping Zhang, Ruijing Li, Jian Sun, Yujie Wang, Zuoyi Yang, Jingyong Liu School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 April 2015 Received in revised form 13 June 2015 Accepted 20 June 2015 Available online xxx
The effects of micro-electrolysis treatment on textile dyeing sludge dewatering and its mechanisms were investigated in this study. Capillary suction time (CST) and settling velocity (SV) were used to evaluate sludge dewaterability. Extracellular polymeric substances (EPS) concentration and sludge disintegration degree (DDSCOD) were determined to explain the observed changes in sludge dewaterability. The results demonstrated that the micro-electrolysis could significantly improve sludge dewaterability by disrupting the sludge floc structure. The optimal conditions of sludge dewatering were the reaction time of 20 min, initial pH of 2.5, Fe/C mass ratio of 1/1, and the iron powder dosage of 2.50 g/L, which achieved good CST (from 34.1 to 27.8 s) and SV (from 75 to 60%) reduction efficiency. In addition, the scanning electron microscope (SEM) images revealed that the treated sludge floc clusters are broken up and that the dispersion degree is better than that of a raw sludge sample. The optimal EPS concentration and DDSCOD to obtain maximum sludge dewaterability was 43e46 mg/L and 4.2e4.9%, respectively. The destruction of EPS was one of the primary reasons for the improvement of sludge dewaterability during microelectrolysis treatment. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Textile dyeing sludge Micro-electrolysis Dewaterability Extracellular polymeric substances Sludge disintegration
1. Introduction With the expansion of the textile industry in China, textile dyeing sludge production is increasing steadily in the processes of treating wastewater (Dos Santos et al., 2007; Nguyen and Juang, 2013). Currently, the disposal of textile dyeing sludge is a great challenge for the wastewater treatment plant because transportation and disposal of sludge may account for up to 60% of the total operation expenses (Qi et al., 2011). Sludge dewatering is of major importance during wastewater treatment because it lowers the cost of sludge transportation and disposal by separating the water from the sludge flocs and reducing the sludge volume (Guan et al., 2012). It is generally accepted that the formation of sludge flocs is based on interactions among microbial polymers, organic particles, inorganic particles, and filamentous bacterial strains, which are glued together by extracellular polymeric substances (EPS) (Neyens et al., 2004; Bala et al., 2010). Polysaccharides and
* Corresponding author. E-mail addresses: [email protected] (X.-a. Ning), [email protected] (W. Wen). 1 These authors contributed to the work equally. http://dx.doi.org/10.1016/j.jenvman.2015.06.041 0301-4797/© 2015 Elsevier Ltd. All rights reserved.
proteins have been recognized as the major components of EPS, which are identified as the key to the improve dewaterability of sludge because of its strong affinity for water (Frølund et al., 1996). Hence, it is believed that sludge dewatering efficiency can be promoted by means of the quick attack on the EPS and the bacterial cells trapped in sludge flocs. Various methods, including thermal (Neyens et al., 2003), AOP (Tony et al., 2008; Liu et al., 2013), ultrasonication (Appels et al., 2008; Dewil et al., 2006; Feng et al., 2009), biological treatment (More et al., 2010), microwave conditioning (Yu et al., 2009), have been developed to improve sludge dewaterability have been developed to improve sludge dewaterability. However, the application of these methods has been limited by factors including the complexity of implementation, the toxicity of the chemicals and high-energy consumption for the operation, and the increase in sludge volume. Therefore, it is urgent to explore a new method with low capital investment and environmental risk. The micro-electrolysis technology was first used in the pretreatment of dyeing wastewaters in the 1970s (Yang et al., 2009). Currently, this technology has been widely applied in industrial wastewaters such as antibiotic (Liu et al., 2005), olive mill (Kallel et al., 2009), and coking wastewater (Liu et al., 2012a,b). In recent
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years, the micro-electrolysis technology has shown an enormous potential for disposing organic wastes. Organic pollutants, such as phenol (Guan et al., 2012), p-nitrophenol (Tang et al., 2012), propionitrile (Lai et al., 2013), nitrobenzene (Li et al., 2011) and organic dyestuff (Huang et al., 2013; Zhang et al., 2014), can be oxidized by oxidants and radicals produced during the micro-electrolysis process. The advantages of the micro-electrolysis technology include: (1) high process efficiency, (2) simple reactor construction (3) moderate operating costs, and (4) the availability of moderate cost, plentiful raw materials (Ju, 2011). Raw materials, activated carbon and iron chips are employed to be the electrolytic materials of micro-electrolysis. When the mixture of activated carbon and iron chips contacts acid wastewater (electrolyte solution), there are numerous macroscopic galvanic cells formed between the particles of carbon and iron. The electrode reactions can be represented as follows (Yang et al., 2009; Ruan et al., 2010; Cheng et al., 2007): Iron anode (oxidation): Fe(s) e 2ee / Fe2þ(aq), Eq (Fe2þ/Fe) ¼ 0.44 V Fe2þ(aq) e ee / Fe3þ(aq), Eq (Fe3þ/Fe2þ) ¼ þ0.77 V Carbon cathode (reduction): 2Hþ(aq) þ 2ee / 2H / H2(g), Eq (Hþ/H2) ¼ 0 V In the presence of oxygen: O2(g) þ 4Hþ(aq) þ 4ee / 2H2O, Eq (O2/ H2O) ¼ þ1.23 V O2(g) þ 2Hþ(aq) þ 2e / H2O2, Eq (O2/ H2O2) ¼ þ0.68 V O2(g) þ 2H2O þ 4ee / 4OHe(aq), Eq (O2/OH) ¼ þ0.40 V Ferrous ions (Fe2þ) are rapidly released into the solution because of the dissolution of iron scraps. Subsequently, H2O2 is generated and combined with Fe2þ to form Fenton's reagents, which, theoretically, can generate OH with powerful oxidizing abilities, resulting in the degradation of EPS and the improvement of the sludge dewatering property (Ju et al., 2011; Neyens et al., 2003; Ning et al., 2014). Based on the advantage and analysis mentioned above, microelectrolysis is likely an effective, moderate cost and novel technology for dewatering textile dyeing sludge. To the best of our knowledge, there has been no detailed report that uses microelectrolysis to improve the sludge dewaterability. This paper aims to investigate the effect of micro-electrolysis treatment on the physicochemical features of textile dyeing sludge. Sludge samples were treated with different electrolysis parameters (reaction time, initial pH mass ratio of iron powder to carbon and the dosage of iron powder). Capillary suction time (CST) and settling velocity (SV) were measured to define the optimal parameters for enhancing sludge dewaterability. The EPS concentration and sludge disintegration degree (DDSCOD) of the sludge sample supernatant were measured to analyze the relationship between sludge dewaterability and disintegration. The mechanism behind the changes observed in sludge disintegration and dewaterability was also discussed. 2. Materials and methods 2.1. Sludge samples The sludge samples collected from a textile dyeing wastewater treatment plant (Dongguan City, Guangdong province, China) were
subsequently settled for 24 h to acquire thickened sludge samples. The thickened samples were stored in plastic containers and were placed in a refrigerator at 4 C prior to use. The fundamental properties of the sludge samples are listed in Table 1. 2.2. Apparatus A bench-scale experiment was performed in the self-made micro-electrolysis reactor, which is depicted schematically in Fig. 1. The reactor was made of a transparent synthetic glass column (Ф9 cm 14 cm), and the electromagnetic vibrating air pump (YT-304, air pressure 0.013 MPa, air delivery rate 4.5 L/min) was used to generate oxygen and mix the granular activated carbon (GAC) and iron powder together. In addition, 400-mL textile dyeing wastewater sludge was employed to investigate the sludge dewaterability. The mean particle size of the commercial GAC (Tianjin Fucheng Chemical Reagent Factory) was approximately 4 mm. Prior to use, GAC was immersed in the sludge for 48 h to avoid the interference of adsorption. The commercial iron powder was obtained from Chengdu Kelong Chemical Reagent Factory, particle size was approximately 0.08e0.10 mm and the iron content was 98%. 2.3. Experimental procedure A 400-mL sludge sample was transferred to the microelectrolytic reactor, and the pH was adjusted with sulfuric acid (H2SO4) and measured with a pH meter (pHS-3C, LEICI, China). The micro-electrolysis treatment conditions were optimized by the single-factor experiments, and the main influence factors including reaction time (0 min, 5 min, 10 min, 15 min, 20 min, 25 min and 30 min), initial pH (2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0), mass ratio of iron powder to GAC (1/2, 2/3, 1/1, 4/3, 2/1, and 4/1) and the dosage of iron powder (1.25 g/L, 2.50 g/L, 3.75 g/L, 5.00 g/L, 6.25 g/L, and 7.50 g/L) were investigated. The sludge dewaterability was evaluated in terms of CST, SV and the viscosity of sludge. To further understand the mechanism behind the observed changes in sludge dewaterability, the EPS concentration and DDSCOD of the supernatant sample were also measured. All experiments were conducted at room temperature, each experiment was performed in triplicate, and the average values and the standard deviations were obtained. 2.4. Analytical methods Sludge dewaterability was determined by the CST, which was obtained using a standard apparatus (304 M, Triton, UK). The SV of the sludge sample was obtained by measuring the sludge volume change in a 100-ml cylinder (100 mL, ARROW) after a 30-min settlement. The viscosity of sludge was measured by a viscosity analyzer (NDJ-8S, Changji, China). Proteins and polysaccharides were selected to characterize the EPS concentration of the sludge supernatant, which were determined spectrophotometrically using a T6 UV/visible
Table 1 Characteristics of the raw sludge sample. Parameters
Unit
Average value
pH Moisture content Settling velocity Soluble chemical oxygen demand Protein Polysaccharide Capillary suction time
% % mg/L mg/L mg/L S
6.70 98.68 99 77.53 1.37 11.94 59.60
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Fig. 1. Schematic diagram of the micro-electrolysis experimental apparatus: 1, electromagnetic vibrating air pump, 2, synthetic glass column, 3, iron powder, 4, granular activated carbon, 5, textile dyeing sludge, 6, aerator.
spectrophotometer (PGeneral, China). Proteins were determined by the Coomassie Brilliant Blue G-250 method, using casein as the standard, and its absorbance was measured at 595 nm (Mecozzi et al., 2005). Polysaccharides were stained with anthrone, and its absorbance was measured at 625 nm (Riesz et al., 1985) by using glucose as the standard. The DDSCOD was defined as the ratio of soluble chemical oxygen demand (SCOD) increment by microelectrolysis treatment to the maximum possible SCOD increment (Ning et al., 2014), which could be calculated as follows:
DDSCOD ð%Þ ¼
SCOD SCOD0 100 TCOD SCOD0
where TCOD is the total COD of the untreated sludge and the SCOD and SCOD0 values were the soluble COD of the treated and untreated sludge samples, respectively. The COD of the samples filtered through a 0.45-mm membrane was referred to as SCOD. The morphology of untreated or micro-electrolysis treated sludge was obtained by the scanning electronic microscope (SEM, Se3400N, Hitachi, Japan). Prior to observation, the samples were rapidly frozen in liquid nitrogen and then vacuum dried.
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20 min, Fe/C mass ratio of 1/1 and iron powder dosage of 5.00 g/L). Compared with the raw sludge, the CST and SV values increased by 27.5% and 59.7% when the pH increased from 2.5 to 4.0. An additional increase in pH only resulted in a slight increase in the CST and SV values. Thus, the optimal pH value was 2.5. As shown in Fig. 2c, the sludge SV value was initially 98% and substantially decreased with an increase in the Fe/C mass ratio. The lowest SV value for the Fe/C mass ratio of 1/1 was 55% at the reaction time of 20 min, initial pH of 2.5, and iron dosage of 5.00 g/L. However, when the Fe/C mass ratio further increased, the SV value rapidly increased and even exceeded that of the unconditioned sludge. In addition, the CST value of the untreated sludge decreased from 43.9 to 39.3 s for the micro-electrolysis treated sludge (Fe/C mass ratio of 1/1). When the Fe/C mass ratio further increased, the CST increased significantly. So, the optimal Fe/C mass ratio was 1/1. The effect of the iron powder dosage on sludge dewaterability under the given conditions (reaction time of 20 min, initial pH of 2.5, and Fe/C mass ratio of 1/1) is shown in Fig. 2d. As the iron powder dosage increased from 1.25 to 2.50 g/L, the SV and CST values decreased from 93 to 80% and 48.0 to 43.2 s, respectively. However, both the SV and CST values increased as the iron dosage exceeded 2.50 g/L and nearly reached to the initial values at the iron powder dosage of 6.25 g/L. Thus, 2.50 g/L was the optimal iron powder dosage. As a consequence, the optimal micro-electrolysis conditions were the reaction time of 20 min, initial pH of 2.5, and Fe/C mass ratio of 1/1, iron powder dosage of 2.50 g/L. The sludge dewaterability was improved at first and then became worse during the micro-electrolysis process. This finding was relatively consistent with that of Liu et al. (2012a,b). One possible explanation is that intracellular and extracellular materials are released into the solution by disruption of the floc structure and integrity at certain micro-electrolysis conditions. However, many fine particles, which had a negative effect on sludge dewatering, were also produced during the micro-electrolysis process. Therefore, when the reaction time was shorter or longer than a certain value, the sludge dewaterability worsened (Fig. 2a). Similar trends were also observed in Fig. 2b, c, and d. These findings were essentially in agreement with those of Yu et al. (2009), who observed that 900 W and 60 s were ideal microwave conditions to yield maximum dewaterability of municipal sludge, and an increase in the contact time and energy level did not achieve a better sludge dewaterability.
3. Results and discussion
3.2. Microscopic structure of sludge
3.1. Effect of the micro-electrolysis conditioning parameters on sludge dewaterability
To confirm the morphological changes of sludge, the microscopic observation of sludge sample was visualized (Fig. 3). The differences in sludge appearance were obvious. Fig. 3a shows that the floc clusters of the raw sludge are aggregated and are relatively smooth, while Fig. 3b shows that the sludge flocs are broken up and the floc clusters are dispersed after the micro-electrolysis treatment. This result is attributed to the effect of micro-electrolysis, which disrupts the sludge floc structure and its integrity by generating OH. The intracellular substances including EPS and cell interstitial water are released into the solution, which might cause the dewaterability of sludge improving (Yuan et al., 2011a,b). A similar conclusion was drawn by Ahn et al. (2009).
To optimize the operating conditions for sludge dewatering, a series of experiments were conducted with different reaction times, initial pHs, Fe/C mass ratios and iron powder dosages, respectively. As presented in Fig. 2a, the effect of the micro-electrolysis reaction time on CST and SV values of sludge was investigated. When the initial pH, mass ratio of iron powder to GAC (Fe/C) and iron powder dosage were set at 3.0, 1/1, and 5.00 g/L, respectively, the CST and SV values decreased rapidly as the reaction time increased from 0 to 20 min. The CST value decreased from 34.1 to 27.8 s, and the SV value decreased from 75 to 60%. However, a negative effect of micro-electrolysis treatment occurred when the reaction time continued. As the micro-electrolysis time exceeded 20 min, both the CST and SV values started to increase. Therefore, 20 min was selected as the optimal reaction time. As shown in Fig. 2b, a higher pH value led to worse dewaterability at a certain micro-electrolysis condition (reaction time of
3.3. Relationship between EPS and sludge dewaterability 3.3.1. Changes in EPS concentration following micro-electrolysis treatment In activated sludge, 70e80% of the extracellular organic carbon exists in the form of saccharides and proteins (Dignac et al., 1998). With the help of these compounds, water is retained and water-
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Fig. 2. Effect of micro-electrolysis treatment variables on sludge dewaterability. (a) Reaction time, (b) pH value, (c) Fe/C mass ratios, (d) Iron powder dosage.
binding capacity of the sludge floc matrix is enhanced significantly (Jin et al., 2004). The initial concentration of polysaccharides and proteins in the sludge supernatant were 10.17 mg/L and 2.04 mg/L, respectively. Fig. 4 shows that polysaccharides and proteins
increase to certain peak values (40.77 mg/L, 5.98 mg/L) at first as the reaction time increases, and then they decrease. At the reaction time of 20 min, polysaccharides and proteins increased by 75.1% and 65.9%, respectively, compared to the raw sludge. This is
Fig. 3. Morphological structure images of sludge. (a) raw sludge, (b) sludge treated by micro-electrolysis.
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Fig. 4. Effect of micro-electrolysis reaction time on EPS concentration in the sludge supernatant (The polynomial regression equation for ‘Reaction time Polysaccharide þ Protein’: y ¼ 0.08831x2 þ 3.47903x þ 7.64733, R2 ¼ 0.92499, the polynomial regression equation for ‘Reaction time Polysaccharide’: y ¼ 0.07704x2 þ 3.16934x þ 5.279, R2 ¼ 0.93576, the polynomial regression equation for ‘Reaction time-Protein’: y ¼ 0.01561x2 þ 0.46212x þ 2.01054, R2 ¼ 0.9661).
probably related to the effect of micro-electrolysis on sludge disintegration by generating OH, which disrupted cell membranes, releasing the intracellular polysaccharides and proteins into the extracellular matrix and increasing the concentrations of polysaccharides and proteins (Ju et al., 2011; Ning et al., 2014). When the reaction time was less than 20 min, the effect of microelectrolysis was so weak that cell membranes could not be disrupted completely. As reaction time exceeded the certain value (20 min), polysaccharides and proteins were oxidized by a large number of OH radicals, which were characterized as highly reactive and non-selective. Moreover, quadratic relationships between reaction time and the EPS concentration (R2 ¼ 0.9250) are obtained in Fig. 4. There were high correlation coefficients between the reaction time and individual EPS components, polysaccharides and proteins (R2 ¼ 0.9358, R2 ¼ 0.9387, respectively). This indicated that the micro-electrolysis treatment presented a high efficiency on sludge disruption, thus increasing the EPS concentration significantly. 3.3.2. Effect of EPS on CST and viscosity With the increase of EPS concentration, the viscosity of sludge increased, and its dewaterability was improved (Wang et al., 2006, Chen et al., 2001). Therefore, information regarding EPS contributes to understanding the exact roles of EPS in controlling dewaterability properties and reveals the mechanisms for improving sludge dewaterability. Additionally, the viscosity of sludge was measured to further analyze how sludge dewaterability was affected by EPS concentration. The relationship between EPS concentration and sludge dewaterability, however, is not always directly proportional. This means that the increase in EPS concentration did not always result in the enhancement in sludge dewaterability. It was clearly observed that as the EPS concentration increased from 12 to 36 mg/L, the CST value decreased sharply from 34.1 to 28.3 s. When the EPS concentration further increased, the decrease in sludge dewaterability concentration was negligible. A similar conclusion was obtained by Yuan et al. (2011a,b). Fig. 5 also shows that EPS concentration exhibited a proportional correlation with sludge viscosity. As the EPS concentration increased from 12 to 46 mg/L, the sludge
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Fig. 5. Effect of supernatant EPS concentration on CST and viscosity (The polynomial regression equation for ‘EPS-CST’: y ¼ 0.00688x2 0.58402x þ 40.32983, R2 ¼ 0.96262, the polynomial regression equation for ‘EPS- viscosity’: y ¼ 0.00207x2 þ 0.25204x 0.24658, R2 ¼ 0.97578).
viscosity continuously increased. The reason is that the sludge flocs were disrupted at a relatively short reaction time, leading to a large amount of EPS release into the soluble phase, which resulted in an increase in sludge viscosity and in an improvement of sludge dewaterability. However, as the reaction time further increased, cells were destroyed completely and intercellular materials were released into the solution, sludge dewaterability deteriorated (Ning et al., 2014). The lowest CST value corresponds to the optimum value of EPS concentration at which sludge dewaterability was maximized. Therefore, according to Fig. 5, the optimum EPS value was calculated as approximately 43e46 mg/L, which was higher than that of unconditioned sludge. These results also agreed with the result reported by Yuan et al. (2011a,b) and Houghton et al. (2001), who both found that there was an optimum EPS concentration for maximum sludge dewaterability. The optimum value of viscosity corresponding to a minimal CST value was calculated as approximately 6e7 mPa s. The effect of EPS concentration on sludge dewatering could be observed by the correlation between CST/viscosity and EPS concentration with different reaction times (R2 ¼ 0.9626 and R2 ¼ 0.9757). These findings were in accordance with the result achieved by Houghton and Stephenson (2002), who found a strong quadratic relationship between CST value and the EPS concentration of digested sludge (R2 ¼ 0.9687), and with the result of Wang et al. (2006), who reported a correlation between CST value and EPS concentration of sewage sludge (R2 ¼ 0.9223). In addition, the correlations between EPS concentration and viscosity were consistent with those reported by Sanin (1994) and Forster (2002), who observed that the viscosity and EPS concentration of industrial sludge and sewage waste sludge had a positive correlation. It confirmed that increasing the EPS content results in an increase in the viscosity to some extent, thus reducing the CST value.
3.4. Relationship between DDSCOD and sludge dewaterability 3.4.1. Changes in DDSCOD following micro-electrolysis treatment The effect of micro-electrolysis reaction time on sludge disruption was investigated by measuring the DDSCOD of the supernatant (Fig. 6). DDSCOD increased from 2.4 to 4.9% at the first 20 min because floc structure was broken up and filamentous bacteria were exposed (as revealed by the SEM). When the reaction
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Fig. 6. Effect of reaction time on DDSCOD (The polynomial regression equation for ‘Reaction time-DDSCOD’: y ¼ 0.00709x2 þ 0.28619x þ 1.64527, R ¼ 0.9175).
time increased to 20 min, the floc structure was completely disrupted so that intracellular and extracellular biopolymers, including proteins and polysaccharides, were released into the soluble phase from sludge flocs. However, when the reaction time increased further, DDSCOD decreased to 3.6% at 30 min. This might be due to the oxidation of the carbohydrate, EPS and other organic substances by highly reactive and non-selective OH radicals generated in the micro-electrolysis process (Yuan et al., 2011a,b). Similar changes in sludge disintegration following microwave treatment and ozonation were observed by Yu et al. (2009) and Zhang et al. (2009).
3.4.2. Effect of DDSCOD on CST and viscosity A quantity of the organic matter in sludge flocs was released, which is reflected by the variation of DDSCOD. DDSCOD represents sludge disintegration, which could affect the viscosity of sludge and influence sludge dewaterability ultimately (Zhang et al., 2007). The relationship between DDSCOD and CST/viscosity is shown in Fig. 7.
On one hand, the CST value decreased sharply when DDSCOD increased to less than 2.3e4.2%. When DDSCOD increased from 4.2 to 4.9%, the CST value was maintained at approximately 28 s. It was relatively the same as those reported by Yu et al. (2011). On the other hand, the effect of DDSCOD on dewaterability was also indirectly reflected by the variation of viscosity. As the DDSCOD of sludge increased from 2.3 to 4.9%, the sludge viscosity continuously increased. Therefore, an optimal DDSCOD exists for obtaining maximum sludge dewaterability. Fig. 7 shows that the optimum value of DDSCOD and viscosity are approximately 4.2e4.9% and 6e7 mPa s, respectively. As a result, a proper DDSCOD and viscosity were essential to enhance sludge dewaterability. Sludge flocs were broken up and organic matter in sludge flocs was released by the effect of micro-electrolysis, resulted in an increase in sludge viscosity and an improvement in sludge dewaterability. The effect of DDSCOD on sludge dewaterability could also be observed from the correlation between DDSCOD and CST/viscosity under different micro-electrolysis times (R2 ¼ 0.9082 and R2 ¼ 0.9584, respectively). These results were in agreement with the finding achieved by Li et al. (2009), who found that there was a strong relation between dewaterability of municipal sludge and DDSCOD under ultrasonic condition. 4. Conclusions The effect of micro-electrolysis on the dewaterability of textile dyeing sludge was subjected to the reaction time, initial pH, Fe/C mass ratio and iron powder dosage. The micro-electrolysis treatment provided high efficiency on sludge dewatering and disruption based on SCT, SV EPS, and DDSCOD results. The optimal parameters were obtained: the reaction time of 20 min, initial pH of 2.5, Fe/C mass ratio of 1/1 and iron powder dosage of 2.50 g/L. The EPS concentration of the sludge supernatant and DDSCOD played a vital role in the observed changes in sludge dewaterability. The optimal EPS concentration (43e46 mg/L) and DDSCOD (4.2e4.9%) were beneficial for obtaining maximum sludge dewaterability. By considering the integrated characteristics of micro-electrolysis (cheap material, easy operation), high dewatering efficiency and sludge disruption degree, micro-electrolysis technology has great application potential in the pretreatment of textile dyeing sludge. Acknowledgments This research was supported by the Key Project of Technology Innovation of the Department of Education of Guangdong Province (No. 2012CXZD0021) and the National Natural Science Foundation of China (No. 51308132). References
Fig. 7. Effect of DDSCOD on CST and viscosity (The polynomial regression equation for ‘DDSCOD-CST’: y ¼ 0.99777x2 9.01054x þ 48.34261, R2 ¼ 0.90818; the polynomial regression equation for ‘DDSCOD-viscosity’: y ¼ 0.2223x2 þ 3.08457x 2.76649, R2 ¼ 0.95839).
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
Enhanced dewaterability of textile dyeing sludge using microelectrolysis pretreatment Xun-an Ning*, 1, Weibin Wen 1, Yaping Zhang, Ruijing Li, Jian Sun, Yujie Wang, Zuoyi Yang, Jingyong Liu School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 April 2015 Received in revised form 13 June 2015 Accepted 20 June 2015 Available online xxx
The effects of micro-electrolysis treatment on textile dyeing sludge dewatering and its mechanisms were investigated in this study. Capillary suction time (CST) and settling velocity (SV) were used to evaluate sludge dewaterability. Extracellular polymeric substances (EPS) concentration and sludge disintegration degree (DDSCOD) were determined to explain the observed changes in sludge dewaterability. The results demonstrated that the micro-electrolysis could significantly improve sludge dewaterability by disrupting the sludge floc structure. The optimal conditions of sludge dewatering were the reaction time of 20 min, initial pH of 2.5, Fe/C mass ratio of 1/1, and the iron powder dosage of 2.50 g/L, which achieved good CST (from 34.1 to 27.8 s) and SV (from 75 to 60%) reduction efficiency. In addition, the scanning electron microscope (SEM) images revealed that the treated sludge floc clusters are broken up and that the dispersion degree is better than that of a raw sludge sample. The optimal EPS concentration and DDSCOD to obtain maximum sludge dewaterability was 43e46 mg/L and 4.2e4.9%, respectively. The destruction of EPS was one of the primary reasons for the improvement of sludge dewaterability during microelectrolysis treatment. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Textile dyeing sludge Micro-electrolysis Dewaterability Extracellular polymeric substances Sludge disintegration
1. Introduction With the expansion of the textile industry in China, textile dyeing sludge production is increasing steadily in the processes of treating wastewater (Dos Santos et al., 2007; Nguyen and Juang, 2013). Currently, the disposal of textile dyeing sludge is a great challenge for the wastewater treatment plant because transportation and disposal of sludge may account for up to 60% of the total operation expenses (Qi et al., 2011). Sludge dewatering is of major importance during wastewater treatment because it lowers the cost of sludge transportation and disposal by separating the water from the sludge flocs and reducing the sludge volume (Guan et al., 2012). It is generally accepted that the formation of sludge flocs is based on interactions among microbial polymers, organic particles, inorganic particles, and filamentous bacterial strains, which are glued together by extracellular polymeric substances (EPS) (Neyens et al., 2004; Bala et al., 2010). Polysaccharides and
* Corresponding author. E-mail addresses: [email protected] (X.-a. Ning), [email protected] (W. Wen). 1 These authors contributed to the work equally. http://dx.doi.org/10.1016/j.jenvman.2015.06.041 0301-4797/© 2015 Elsevier Ltd. All rights reserved.
proteins have been recognized as the major components of EPS, which are identified as the key to the improve dewaterability of sludge because of its strong affinity for water (Frølund et al., 1996). Hence, it is believed that sludge dewatering efficiency can be promoted by means of the quick attack on the EPS and the bacterial cells trapped in sludge flocs. Various methods, including thermal (Neyens et al., 2003), AOP (Tony et al., 2008; Liu et al., 2013), ultrasonication (Appels et al., 2008; Dewil et al., 2006; Feng et al., 2009), biological treatment (More et al., 2010), microwave conditioning (Yu et al., 2009), have been developed to improve sludge dewaterability have been developed to improve sludge dewaterability. However, the application of these methods has been limited by factors including the complexity of implementation, the toxicity of the chemicals and high-energy consumption for the operation, and the increase in sludge volume. Therefore, it is urgent to explore a new method with low capital investment and environmental risk. The micro-electrolysis technology was first used in the pretreatment of dyeing wastewaters in the 1970s (Yang et al., 2009). Currently, this technology has been widely applied in industrial wastewaters such as antibiotic (Liu et al., 2005), olive mill (Kallel et al., 2009), and coking wastewater (Liu et al., 2012a,b). In recent
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years, the micro-electrolysis technology has shown an enormous potential for disposing organic wastes. Organic pollutants, such as phenol (Guan et al., 2012), p-nitrophenol (Tang et al., 2012), propionitrile (Lai et al., 2013), nitrobenzene (Li et al., 2011) and organic dyestuff (Huang et al., 2013; Zhang et al., 2014), can be oxidized by oxidants and radicals produced during the micro-electrolysis process. The advantages of the micro-electrolysis technology include: (1) high process efficiency, (2) simple reactor construction (3) moderate operating costs, and (4) the availability of moderate cost, plentiful raw materials (Ju, 2011). Raw materials, activated carbon and iron chips are employed to be the electrolytic materials of micro-electrolysis. When the mixture of activated carbon and iron chips contacts acid wastewater (electrolyte solution), there are numerous macroscopic galvanic cells formed between the particles of carbon and iron. The electrode reactions can be represented as follows (Yang et al., 2009; Ruan et al., 2010; Cheng et al., 2007): Iron anode (oxidation): Fe(s) e 2ee / Fe2þ(aq), Eq (Fe2þ/Fe) ¼ 0.44 V Fe2þ(aq) e ee / Fe3þ(aq), Eq (Fe3þ/Fe2þ) ¼ þ0.77 V Carbon cathode (reduction): 2Hþ(aq) þ 2ee / 2H / H2(g), Eq (Hþ/H2) ¼ 0 V In the presence of oxygen: O2(g) þ 4Hþ(aq) þ 4ee / 2H2O, Eq (O2/ H2O) ¼ þ1.23 V O2(g) þ 2Hþ(aq) þ 2e / H2O2, Eq (O2/ H2O2) ¼ þ0.68 V O2(g) þ 2H2O þ 4ee / 4OHe(aq), Eq (O2/OH) ¼ þ0.40 V Ferrous ions (Fe2þ) are rapidly released into the solution because of the dissolution of iron scraps. Subsequently, H2O2 is generated and combined with Fe2þ to form Fenton's reagents, which, theoretically, can generate OH with powerful oxidizing abilities, resulting in the degradation of EPS and the improvement of the sludge dewatering property (Ju et al., 2011; Neyens et al., 2003; Ning et al., 2014). Based on the advantage and analysis mentioned above, microelectrolysis is likely an effective, moderate cost and novel technology for dewatering textile dyeing sludge. To the best of our knowledge, there has been no detailed report that uses microelectrolysis to improve the sludge dewaterability. This paper aims to investigate the effect of micro-electrolysis treatment on the physicochemical features of textile dyeing sludge. Sludge samples were treated with different electrolysis parameters (reaction time, initial pH mass ratio of iron powder to carbon and the dosage of iron powder). Capillary suction time (CST) and settling velocity (SV) were measured to define the optimal parameters for enhancing sludge dewaterability. The EPS concentration and sludge disintegration degree (DDSCOD) of the sludge sample supernatant were measured to analyze the relationship between sludge dewaterability and disintegration. The mechanism behind the changes observed in sludge disintegration and dewaterability was also discussed. 2. Materials and methods 2.1. Sludge samples The sludge samples collected from a textile dyeing wastewater treatment plant (Dongguan City, Guangdong province, China) were
subsequently settled for 24 h to acquire thickened sludge samples. The thickened samples were stored in plastic containers and were placed in a refrigerator at 4 C prior to use. The fundamental properties of the sludge samples are listed in Table 1. 2.2. Apparatus A bench-scale experiment was performed in the self-made micro-electrolysis reactor, which is depicted schematically in Fig. 1. The reactor was made of a transparent synthetic glass column (Ф9 cm 14 cm), and the electromagnetic vibrating air pump (YT-304, air pressure 0.013 MPa, air delivery rate 4.5 L/min) was used to generate oxygen and mix the granular activated carbon (GAC) and iron powder together. In addition, 400-mL textile dyeing wastewater sludge was employed to investigate the sludge dewaterability. The mean particle size of the commercial GAC (Tianjin Fucheng Chemical Reagent Factory) was approximately 4 mm. Prior to use, GAC was immersed in the sludge for 48 h to avoid the interference of adsorption. The commercial iron powder was obtained from Chengdu Kelong Chemical Reagent Factory, particle size was approximately 0.08e0.10 mm and the iron content was 98%. 2.3. Experimental procedure A 400-mL sludge sample was transferred to the microelectrolytic reactor, and the pH was adjusted with sulfuric acid (H2SO4) and measured with a pH meter (pHS-3C, LEICI, China). The micro-electrolysis treatment conditions were optimized by the single-factor experiments, and the main influence factors including reaction time (0 min, 5 min, 10 min, 15 min, 20 min, 25 min and 30 min), initial pH (2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0), mass ratio of iron powder to GAC (1/2, 2/3, 1/1, 4/3, 2/1, and 4/1) and the dosage of iron powder (1.25 g/L, 2.50 g/L, 3.75 g/L, 5.00 g/L, 6.25 g/L, and 7.50 g/L) were investigated. The sludge dewaterability was evaluated in terms of CST, SV and the viscosity of sludge. To further understand the mechanism behind the observed changes in sludge dewaterability, the EPS concentration and DDSCOD of the supernatant sample were also measured. All experiments were conducted at room temperature, each experiment was performed in triplicate, and the average values and the standard deviations were obtained. 2.4. Analytical methods Sludge dewaterability was determined by the CST, which was obtained using a standard apparatus (304 M, Triton, UK). The SV of the sludge sample was obtained by measuring the sludge volume change in a 100-ml cylinder (100 mL, ARROW) after a 30-min settlement. The viscosity of sludge was measured by a viscosity analyzer (NDJ-8S, Changji, China). Proteins and polysaccharides were selected to characterize the EPS concentration of the sludge supernatant, which were determined spectrophotometrically using a T6 UV/visible
Table 1 Characteristics of the raw sludge sample. Parameters
Unit
Average value
pH Moisture content Settling velocity Soluble chemical oxygen demand Protein Polysaccharide Capillary suction time
% % mg/L mg/L mg/L S
6.70 98.68 99 77.53 1.37 11.94 59.60
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Fig. 1. Schematic diagram of the micro-electrolysis experimental apparatus: 1, electromagnetic vibrating air pump, 2, synthetic glass column, 3, iron powder, 4, granular activated carbon, 5, textile dyeing sludge, 6, aerator.
spectrophotometer (PGeneral, China). Proteins were determined by the Coomassie Brilliant Blue G-250 method, using casein as the standard, and its absorbance was measured at 595 nm (Mecozzi et al., 2005). Polysaccharides were stained with anthrone, and its absorbance was measured at 625 nm (Riesz et al., 1985) by using glucose as the standard. The DDSCOD was defined as the ratio of soluble chemical oxygen demand (SCOD) increment by microelectrolysis treatment to the maximum possible SCOD increment (Ning et al., 2014), which could be calculated as follows:
DDSCOD ð%Þ ¼
SCOD SCOD0 100 TCOD SCOD0
where TCOD is the total COD of the untreated sludge and the SCOD and SCOD0 values were the soluble COD of the treated and untreated sludge samples, respectively. The COD of the samples filtered through a 0.45-mm membrane was referred to as SCOD. The morphology of untreated or micro-electrolysis treated sludge was obtained by the scanning electronic microscope (SEM, Se3400N, Hitachi, Japan). Prior to observation, the samples were rapidly frozen in liquid nitrogen and then vacuum dried.
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20 min, Fe/C mass ratio of 1/1 and iron powder dosage of 5.00 g/L). Compared with the raw sludge, the CST and SV values increased by 27.5% and 59.7% when the pH increased from 2.5 to 4.0. An additional increase in pH only resulted in a slight increase in the CST and SV values. Thus, the optimal pH value was 2.5. As shown in Fig. 2c, the sludge SV value was initially 98% and substantially decreased with an increase in the Fe/C mass ratio. The lowest SV value for the Fe/C mass ratio of 1/1 was 55% at the reaction time of 20 min, initial pH of 2.5, and iron dosage of 5.00 g/L. However, when the Fe/C mass ratio further increased, the SV value rapidly increased and even exceeded that of the unconditioned sludge. In addition, the CST value of the untreated sludge decreased from 43.9 to 39.3 s for the micro-electrolysis treated sludge (Fe/C mass ratio of 1/1). When the Fe/C mass ratio further increased, the CST increased significantly. So, the optimal Fe/C mass ratio was 1/1. The effect of the iron powder dosage on sludge dewaterability under the given conditions (reaction time of 20 min, initial pH of 2.5, and Fe/C mass ratio of 1/1) is shown in Fig. 2d. As the iron powder dosage increased from 1.25 to 2.50 g/L, the SV and CST values decreased from 93 to 80% and 48.0 to 43.2 s, respectively. However, both the SV and CST values increased as the iron dosage exceeded 2.50 g/L and nearly reached to the initial values at the iron powder dosage of 6.25 g/L. Thus, 2.50 g/L was the optimal iron powder dosage. As a consequence, the optimal micro-electrolysis conditions were the reaction time of 20 min, initial pH of 2.5, and Fe/C mass ratio of 1/1, iron powder dosage of 2.50 g/L. The sludge dewaterability was improved at first and then became worse during the micro-electrolysis process. This finding was relatively consistent with that of Liu et al. (2012a,b). One possible explanation is that intracellular and extracellular materials are released into the solution by disruption of the floc structure and integrity at certain micro-electrolysis conditions. However, many fine particles, which had a negative effect on sludge dewatering, were also produced during the micro-electrolysis process. Therefore, when the reaction time was shorter or longer than a certain value, the sludge dewaterability worsened (Fig. 2a). Similar trends were also observed in Fig. 2b, c, and d. These findings were essentially in agreement with those of Yu et al. (2009), who observed that 900 W and 60 s were ideal microwave conditions to yield maximum dewaterability of municipal sludge, and an increase in the contact time and energy level did not achieve a better sludge dewaterability.
3. Results and discussion
3.2. Microscopic structure of sludge
3.1. Effect of the micro-electrolysis conditioning parameters on sludge dewaterability
To confirm the morphological changes of sludge, the microscopic observation of sludge sample was visualized (Fig. 3). The differences in sludge appearance were obvious. Fig. 3a shows that the floc clusters of the raw sludge are aggregated and are relatively smooth, while Fig. 3b shows that the sludge flocs are broken up and the floc clusters are dispersed after the micro-electrolysis treatment. This result is attributed to the effect of micro-electrolysis, which disrupts the sludge floc structure and its integrity by generating OH. The intracellular substances including EPS and cell interstitial water are released into the solution, which might cause the dewaterability of sludge improving (Yuan et al., 2011a,b). A similar conclusion was drawn by Ahn et al. (2009).
To optimize the operating conditions for sludge dewatering, a series of experiments were conducted with different reaction times, initial pHs, Fe/C mass ratios and iron powder dosages, respectively. As presented in Fig. 2a, the effect of the micro-electrolysis reaction time on CST and SV values of sludge was investigated. When the initial pH, mass ratio of iron powder to GAC (Fe/C) and iron powder dosage were set at 3.0, 1/1, and 5.00 g/L, respectively, the CST and SV values decreased rapidly as the reaction time increased from 0 to 20 min. The CST value decreased from 34.1 to 27.8 s, and the SV value decreased from 75 to 60%. However, a negative effect of micro-electrolysis treatment occurred when the reaction time continued. As the micro-electrolysis time exceeded 20 min, both the CST and SV values started to increase. Therefore, 20 min was selected as the optimal reaction time. As shown in Fig. 2b, a higher pH value led to worse dewaterability at a certain micro-electrolysis condition (reaction time of
3.3. Relationship between EPS and sludge dewaterability 3.3.1. Changes in EPS concentration following micro-electrolysis treatment In activated sludge, 70e80% of the extracellular organic carbon exists in the form of saccharides and proteins (Dignac et al., 1998). With the help of these compounds, water is retained and water-
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Fig. 2. Effect of micro-electrolysis treatment variables on sludge dewaterability. (a) Reaction time, (b) pH value, (c) Fe/C mass ratios, (d) Iron powder dosage.
binding capacity of the sludge floc matrix is enhanced significantly (Jin et al., 2004). The initial concentration of polysaccharides and proteins in the sludge supernatant were 10.17 mg/L and 2.04 mg/L, respectively. Fig. 4 shows that polysaccharides and proteins
increase to certain peak values (40.77 mg/L, 5.98 mg/L) at first as the reaction time increases, and then they decrease. At the reaction time of 20 min, polysaccharides and proteins increased by 75.1% and 65.9%, respectively, compared to the raw sludge. This is
Fig. 3. Morphological structure images of sludge. (a) raw sludge, (b) sludge treated by micro-electrolysis.
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Fig. 4. Effect of micro-electrolysis reaction time on EPS concentration in the sludge supernatant (The polynomial regression equation for ‘Reaction time Polysaccharide þ Protein’: y ¼ 0.08831x2 þ 3.47903x þ 7.64733, R2 ¼ 0.92499, the polynomial regression equation for ‘Reaction time Polysaccharide’: y ¼ 0.07704x2 þ 3.16934x þ 5.279, R2 ¼ 0.93576, the polynomial regression equation for ‘Reaction time-Protein’: y ¼ 0.01561x2 þ 0.46212x þ 2.01054, R2 ¼ 0.9661).
probably related to the effect of micro-electrolysis on sludge disintegration by generating OH, which disrupted cell membranes, releasing the intracellular polysaccharides and proteins into the extracellular matrix and increasing the concentrations of polysaccharides and proteins (Ju et al., 2011; Ning et al., 2014). When the reaction time was less than 20 min, the effect of microelectrolysis was so weak that cell membranes could not be disrupted completely. As reaction time exceeded the certain value (20 min), polysaccharides and proteins were oxidized by a large number of OH radicals, which were characterized as highly reactive and non-selective. Moreover, quadratic relationships between reaction time and the EPS concentration (R2 ¼ 0.9250) are obtained in Fig. 4. There were high correlation coefficients between the reaction time and individual EPS components, polysaccharides and proteins (R2 ¼ 0.9358, R2 ¼ 0.9387, respectively). This indicated that the micro-electrolysis treatment presented a high efficiency on sludge disruption, thus increasing the EPS concentration significantly. 3.3.2. Effect of EPS on CST and viscosity With the increase of EPS concentration, the viscosity of sludge increased, and its dewaterability was improved (Wang et al., 2006, Chen et al., 2001). Therefore, information regarding EPS contributes to understanding the exact roles of EPS in controlling dewaterability properties and reveals the mechanisms for improving sludge dewaterability. Additionally, the viscosity of sludge was measured to further analyze how sludge dewaterability was affected by EPS concentration. The relationship between EPS concentration and sludge dewaterability, however, is not always directly proportional. This means that the increase in EPS concentration did not always result in the enhancement in sludge dewaterability. It was clearly observed that as the EPS concentration increased from 12 to 36 mg/L, the CST value decreased sharply from 34.1 to 28.3 s. When the EPS concentration further increased, the decrease in sludge dewaterability concentration was negligible. A similar conclusion was obtained by Yuan et al. (2011a,b). Fig. 5 also shows that EPS concentration exhibited a proportional correlation with sludge viscosity. As the EPS concentration increased from 12 to 46 mg/L, the sludge
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Fig. 5. Effect of supernatant EPS concentration on CST and viscosity (The polynomial regression equation for ‘EPS-CST’: y ¼ 0.00688x2 0.58402x þ 40.32983, R2 ¼ 0.96262, the polynomial regression equation for ‘EPS- viscosity’: y ¼ 0.00207x2 þ 0.25204x 0.24658, R2 ¼ 0.97578).
viscosity continuously increased. The reason is that the sludge flocs were disrupted at a relatively short reaction time, leading to a large amount of EPS release into the soluble phase, which resulted in an increase in sludge viscosity and in an improvement of sludge dewaterability. However, as the reaction time further increased, cells were destroyed completely and intercellular materials were released into the solution, sludge dewaterability deteriorated (Ning et al., 2014). The lowest CST value corresponds to the optimum value of EPS concentration at which sludge dewaterability was maximized. Therefore, according to Fig. 5, the optimum EPS value was calculated as approximately 43e46 mg/L, which was higher than that of unconditioned sludge. These results also agreed with the result reported by Yuan et al. (2011a,b) and Houghton et al. (2001), who both found that there was an optimum EPS concentration for maximum sludge dewaterability. The optimum value of viscosity corresponding to a minimal CST value was calculated as approximately 6e7 mPa s. The effect of EPS concentration on sludge dewatering could be observed by the correlation between CST/viscosity and EPS concentration with different reaction times (R2 ¼ 0.9626 and R2 ¼ 0.9757). These findings were in accordance with the result achieved by Houghton and Stephenson (2002), who found a strong quadratic relationship between CST value and the EPS concentration of digested sludge (R2 ¼ 0.9687), and with the result of Wang et al. (2006), who reported a correlation between CST value and EPS concentration of sewage sludge (R2 ¼ 0.9223). In addition, the correlations between EPS concentration and viscosity were consistent with those reported by Sanin (1994) and Forster (2002), who observed that the viscosity and EPS concentration of industrial sludge and sewage waste sludge had a positive correlation. It confirmed that increasing the EPS content results in an increase in the viscosity to some extent, thus reducing the CST value.
3.4. Relationship between DDSCOD and sludge dewaterability 3.4.1. Changes in DDSCOD following micro-electrolysis treatment The effect of micro-electrolysis reaction time on sludge disruption was investigated by measuring the DDSCOD of the supernatant (Fig. 6). DDSCOD increased from 2.4 to 4.9% at the first 20 min because floc structure was broken up and filamentous bacteria were exposed (as revealed by the SEM). When the reaction
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Fig. 6. Effect of reaction time on DDSCOD (The polynomial regression equation for ‘Reaction time-DDSCOD’: y ¼ 0.00709x2 þ 0.28619x þ 1.64527, R ¼ 0.9175).
time increased to 20 min, the floc structure was completely disrupted so that intracellular and extracellular biopolymers, including proteins and polysaccharides, were released into the soluble phase from sludge flocs. However, when the reaction time increased further, DDSCOD decreased to 3.6% at 30 min. This might be due to the oxidation of the carbohydrate, EPS and other organic substances by highly reactive and non-selective OH radicals generated in the micro-electrolysis process (Yuan et al., 2011a,b). Similar changes in sludge disintegration following microwave treatment and ozonation were observed by Yu et al. (2009) and Zhang et al. (2009).
3.4.2. Effect of DDSCOD on CST and viscosity A quantity of the organic matter in sludge flocs was released, which is reflected by the variation of DDSCOD. DDSCOD represents sludge disintegration, which could affect the viscosity of sludge and influence sludge dewaterability ultimately (Zhang et al., 2007). The relationship between DDSCOD and CST/viscosity is shown in Fig. 7.
On one hand, the CST value decreased sharply when DDSCOD increased to less than 2.3e4.2%. When DDSCOD increased from 4.2 to 4.9%, the CST value was maintained at approximately 28 s. It was relatively the same as those reported by Yu et al. (2011). On the other hand, the effect of DDSCOD on dewaterability was also indirectly reflected by the variation of viscosity. As the DDSCOD of sludge increased from 2.3 to 4.9%, the sludge viscosity continuously increased. Therefore, an optimal DDSCOD exists for obtaining maximum sludge dewaterability. Fig. 7 shows that the optimum value of DDSCOD and viscosity are approximately 4.2e4.9% and 6e7 mPa s, respectively. As a result, a proper DDSCOD and viscosity were essential to enhance sludge dewaterability. Sludge flocs were broken up and organic matter in sludge flocs was released by the effect of micro-electrolysis, resulted in an increase in sludge viscosity and an improvement in sludge dewaterability. The effect of DDSCOD on sludge dewaterability could also be observed from the correlation between DDSCOD and CST/viscosity under different micro-electrolysis times (R2 ¼ 0.9082 and R2 ¼ 0.9584, respectively). These results were in agreement with the finding achieved by Li et al. (2009), who found that there was a strong relation between dewaterability of municipal sludge and DDSCOD under ultrasonic condition. 4. Conclusions The effect of micro-electrolysis on the dewaterability of textile dyeing sludge was subjected to the reaction time, initial pH, Fe/C mass ratio and iron powder dosage. The micro-electrolysis treatment provided high efficiency on sludge dewatering and disruption based on SCT, SV EPS, and DDSCOD results. The optimal parameters were obtained: the reaction time of 20 min, initial pH of 2.5, Fe/C mass ratio of 1/1 and iron powder dosage of 2.50 g/L. The EPS concentration of the sludge supernatant and DDSCOD played a vital role in the observed changes in sludge dewaterability. The optimal EPS concentration (43e46 mg/L) and DDSCOD (4.2e4.9%) were beneficial for obtaining maximum sludge dewaterability. By considering the integrated characteristics of micro-electrolysis (cheap material, easy operation), high dewatering efficiency and sludge disruption degree, micro-electrolysis technology has great application potential in the pretreatment of textile dyeing sludge. Acknowledgments This research was supported by the Key Project of Technology Innovation of the Department of Education of Guangdong Province (No. 2012CXZD0021) and the National Natural Science Foundation of China (No. 51308132). References
Fig. 7. Effect of DDSCOD on CST and viscosity (The polynomial regression equation for ‘DDSCOD-CST’: y ¼ 0.99777x2 9.01054x þ 48.34261, R2 ¼ 0.90818; the polynomial regression equation for ‘DDSCOD-viscosity’: y ¼ 0.2223x2 þ 3.08457x 2.76649, R2 ¼ 0.95839).
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