Insights into the respective role of acidification and oxidation for enhancing anaerobic digested sludge dewatering performance with Fenton process

Bioresource Technology 181 (2015) 247–253

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Insights into the respective role of acidification and oxidation for enhancing anaerobic digested sludge dewatering performance with Fenton process Weijun Zhang, Peng Yang, Xiaoyin Yang, Zhan Chen, Dongsheng Wang ⇑ State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, 18, Shuangqing Road, Beijing 100085, China

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a b s t r a c t Digested sludges generally exhibit poorer dewaterability than activated sludges. This study investigated the effects of acidification and oxidation on EPS properties and dewaterability of anaerobic digested sludge in Fenton treatment in order to unravel the underlying mechanism of sludge conditioning. The results indicated that sludge dewatering property was improved after acidification treatment. Meanwhile, fluorescence analysis revealed that the protein-like substances were effectively removed from sludge bulk after acidification treatment. Acidification and Fenton oxidation showed a significant synergetic effect in enhancing sludge dewatering process. Solubilization and decomposition of bound EPS occurred synchronously during Fenton conditioning. Oxidation process is very likely to play a more important role in sludge conditioning than Fenton coagulation. According to pilot test, Fenton treatment performed much better in cake moisture content reduction than chemical conditioning with traditional inorganic coagulants. Additionally, full-scale application of Fenton conditioning will not have detrimental effects on performance of wastewater treatment system. Ó 2015 Elsevier Ltd. All rights reserved.

The management of wastewater sludge, now often referred to as biosolids, accounts for a major portion of the cost of the wastewater treatment process and represents significant technical

http://dx.doi.org/10.1016/j.biortech.2015.01.003 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.

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1. Introduction

E-mail address: [email protected] (D. Wang).

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⇑ Corresponding author. Tel./fax: +86 10 62849138.

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Keywords: Anaerobic digested sludge Dewaterability Extracellular polymeric substances Fenton oxidation

Fenton oxidation

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Article history: Received 27 September 2014 Received in revised form 28 December 2014 Accepted 3 January 2015 Available online 7 January 2015

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Fenton oxidation

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from sludge bulk after acidification treatment.  Acidification and Fenton oxidation showed a significant synergetic effect.  Oxidation played a more important role than coagulation in Fenton conditioning.  Extracellular polymers degradation was the major mechanism of sludge conditioning.

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h i g h l i g h t s

challenges. Reducing the amount of sludge produced and improving the dewaterability are hence of paramount importance to cut transportation and disposal cost (Niu et al., 2013; Zhai et al., 2012). Activated sludges are generally hard to dewater due to high content organic materials (Yuan et al., 2011). It was reported that extracellular polymeric substances (EPS) accounted for 60–80% of total sludge mass (Liu and Fang, 2003; Vaxelaire and Cézac, 2004). The distribution and abundance of EPS have a significant

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influence on sludge dewatering property (Liu and Fang, 2003). Houghton et al. (2001) stated that sludge dewaterability was mainly affected by the EPS content, and there was an optimum level of EPS at which the sludge exhibited maximum dewaterability. In addition, EPS also had an influence on charge properties, moisture content of dewatered sludge cake and floc stability and so on (Mikkelsen and Keiding, 2002). Of main components in EPS, protein-like substances were believed to play more important in sludge dewatering than polysaccharide and humic acid (Liu and Fang, 2003). Murthy and Novak (1999) suggested that high protein content was detrimental to sludge dewatering. Prior to dewatering most sludges are always conditioned with chemicals in order to improve performance of sludge dewatering devices. Addition of traditional chemical conditioners (inorganic salt coagulants and organic polymers) can agglomerate the small fines and compress EPS structure of the sludge colloids to form large flocs through charge neutralization and bridging, which can be more easily separated from the water (Niu et al., 2013). It was noted that EPS are highly hydrated and are able to bind a large volume of water (Houghton et al., 2001), but traditional chemical conditioners are ineffective to remove the bound and intercellular water in sludge flocs (Neyens et al., 2004). In recent years, many advanced sludge treatment (AST) processes have been developed in order to improve sludge dewatering and to facilitate handling and ultimate disposal (Neyens and Baeyens, 2003a). Neyens et al. (2004) demonstrated that AST improve the sludge dewaterability through decomposing and/or changing EPS properties to reduce bound water content. Established AST methods include photoFenton/Fenton oxidation (Neyens and Baeyens, 2003a), acid/ alkaline treatment (Neyens et al., 2004), thermal treatment (Neyens and Baeyens, 2003b) and enzymatic treatment (Ayol, 2005) or their integrated processes. Fenton oxidation is known to be dependent on in situ producing a non-selective strong hydroxyl radical which can effectively destruct various refractory organic matters at ambient condition (Neyens and Baeyens, 2003a). Many studies reported that Fenton reagents are able to break activated sludge floc and solubilize EPS, promoting the conversion from bound water to free water (Neyens and Baeyens, 2003a,b; Neyens et al., 2004; Pham et al., 2010). In addition, the ferric hydroxo complexes produced from Fenton process could act as coagulants to further enhance dewatering performance of biosludge (Neyens and Baeyens, 2003a). Liu et al. (2012) found that sludge conditioning with combined Fenton’s reagent and skeleton builders was an efficient mean to achieve deep dewatering. Fenton treatment resulted in partial destruction of EPS and decrease in sludge floc size. Then lime and Portland cement were added to serve as skeleton builders to transmitted the stresses to the internal parts of flocs and provide channels for water release under high pressures (Liu et al., 2013). As mentioned above, many previous studies reported that sludge (activated or digested) flocculation and dewaterability could be greatly improved via peroxidation treatment, but the responsible mechanism is not fully understood. Therefore, the objectives of this paper were to: (1) get more comprehensive insights into the respective role of acidification and oxidation in anaerobic digested sludge conditioning with Fenton reagents; (2) investigate the effects of acidification and Fenton oxidation on solubilization and chemical properties of digested sludge EPS in detail; (3) examine the dewatering or drying performance of digested sludge after Fenton conditioning through pilot test; (4) evaluated the potential impacts of Fenton conditioning on influent quality and operation of wastewater treatment plant was also evaluated. In addition, the present study is able to provide information on improvement of digested sludge treatment and management with Fenton process in practical application.

2. Methods 2.1. Source and properties of digested sludge Sewage sludge was obtained from anaerobic digested reactors of the biggest wastewater treatment plant in Beijing city. It treats approximately 1,200,000 m3 of wastewater daily by anaerobic/ anoxic/oxic process. Sample was stored at 4 °C after sampling. The sludge characteristics can be found in Table 1. All the chemicals used in this study were of analytical grade. 98% Sulfuric acid and sodium hydroxide (Sinopharm Chemical Reagent, China) was used to adjust pH of the sludge samples. Ferrous sulfate (FeSO4 7H2O) and hydrogen peroxide (H2O2, 30 wt.%) were purchased from Sinopharm Chemical Reagent of China. 2.2. Sludge conditioning with Fenton oxidation A 200 mL of sludge sample was added in a 500 Erlenmeyer flask. The beaker was placed in thermostatic bath (25 ± 1 °C). And then appropriate amounts of ferrous sulfate heptahydrate salt (FeSO4 7H2O) were added in the beaker at varying pH values. An aliquot of H2O2 (30% (w/v)) was spiked in the sludge sample under vigorous stirring using a magnetic stirrer. Finally, the reaction was stopped by raising the pH of the solution to 7 after 2 h stirring. The suspension was centrifuged at 5000g for 15 min, and the supernatant was used for TOC and fluorescence analysis later. Each experiment was performed in duplicate. 2.3. Analytical methods 2.3.1. Dewatering test The dewaterability of the sludge flocs was measured with a capillary suction time (CST) instrument (Model 319, Triton, UK) equipped with an 18 mm diameter funnel and Whatman No. 17 chromatography-grade filtration paper. In addition, centrifugal dewatering test was conducted to get a further understanding into the effects of Fenton treatment on dewatering efficiency. The sludge conditioned with Fenton reagents was transferred and centrifuged at 5000g for 30 min, and then the moisture content of sediments was measured according to standard method. 2.3.2. Three-dimensional excitation emission matrix (3-DEEM) spectra analysis The sample was diluted with Milli Q water until concentration of DOC was below 10 mg/L. 3-DEEM spectra were measured by a Hitachi F-4500 fluorescence spectrophotometer with an excitation range from 200 to 400 nm at 10 nm sampling intervals and an emission range from 280 to 500 nm at 10 nm sampling interval. The spectra were recorded at a scan rate of 12,000 nm/min, using excitation and emission slit bandwidths of 10 nm. Each scan had 37 emission and 27 excitation wavelengths. As stated by Sheng and Yu (2006), pH had significant effect on EEM analysis, so solution pH was adjusted to be neural prior to analysis in order to remove the interference. 2.3.3. Other indicators Zeta potential and turbidity was measured using Zetasizer2000 (Malvern Instruments Ltd, Malvern, UK) and 2100 N Turbidimeter (Hach, USA) respectively. The dissolve organic carbon (DOC) in the filtrate was analyzed using TOC analyzer (Shimadzu, Kyoto, Japan). pH was measured by a pHS-3C (Shanghai, China) pH meter, which was calibrated using pH 7.01 and pH 9.18 buffers. The sludge floc size was determined by using Mastersizer 2000 (Malvern, UK). The d0.5 value mean that 50% of the particles measured were less than or equal to the size stated. Other sludge

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W. Zhang et al. / Bioresource Technology 181 (2015) 247–253 Table 1 Characteristics of anaerobic digested sludge. Indicator

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97.8

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7.49

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0.63

293.1

parameters, including total suspended solids (TSS), volatile suspended solids (VSS), ammonium nitrogen (NH+4) and total phosphorus (TP) were analyzed following APHA (2005). 3. Results and discussion 3.1. Effect of acidification on sludge properties 3.1.1. Effect of acidification on sludge dewaterability and floc size It can be seen from Fig. 1 that CST was reduced from 200 s to 157 s and 100 with decrease in pH from 7 to 4 and 3 respectively. Extracellular polymeric substances (EPS) of biological sludge were mainly composed by protein and polysaccharide. Sludge particles always carry negative charge due to the ionization of functional groups, such as carboxyl, amino and phosphate etc (Liu and Fang, 2003). High negative surface charge density can cause electrostatic repulsion, preventing destabilization and flocculation sludge particles. As depicted in Fig. S1, zeta potential of sludge particles increased with rise in pH and reach ‘‘isoelectric point’’ at pH around 3, indicating dissociation constant (pKa) of EPS reached minimum (Liu and Fang, 2003). Acidification treatment caused protonation of negatively charged chemical groups of EPS, leading to stabilization and flocculation of colloidal sludge system. As a result, the sludge floc size reached the maximum at pH of 3. In addition, reduction of floc size was observed with further decrease in pH from 3 to 2, which might be attributed to solubilization and breakage of sludge floc under strong acid condition. 3.1.2. Effect of acidic treatment on solubilization of organic materials in sludge Fig. S2 showed that no significant variation in DOC content was observed at pH values above 4. DOC concentration was increased from 120 mg/L to 140.05 mg/L and 175 mg/L by decreasing pH to 3 and 4 respectively. This result further confirmed that EPS was dissolved and released to sludge bulk under strong acid environment. 3-DEEM has been proven to be a very useful tool to characterize EPS of biological origins (Zhen et al., 2012; Zhu et al., 2012). The effect of pH on 3-DEEM spectra of soluble EPS was presented in 80 200

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Fig. 2. Two dominant fluorescent peaks (Peak A (k ex/em = 280/ 335), Peak B (k ex/em = 225/335)) were detected at pH above 4. Both of them were associated with protein-like substances (Chen et al., 2003). It was worthy to note that new two peaks (Peak C (k ex/em = 330/410) and Peak D (k ex/em = 275/425)) could be observed as the solution pH was lowered to below 3. Peaks C and D were always related to humic substances and fulvic acid respectively. In addition, it can be seen from Fig. 2 that the fluorescent intensities of peaks A and B weakened by 78.5% at pH of 2 in comparison to at neutral condition, while that of peaks C and D were increased by 55.7% and 77.1% respectively. It was reported that humic acids and fulvic acids contained a larger proportion of high-molecular fractions as well as a higher proportion of hydrophobic fractions which can readily bind with heavy metal ions to form complex firmly embedding within sludge floc interior. Strong acid would cause breakage of sludge floc and dissolution of metal ions, consequently resulted in release of humic acids and fulvic acids. Fluorescence region integration (FRI) was conducted to obtain a semi-quantitative assessment of pH effects on chemical characteristics of soluble EPS. As depicted in Fig. S3, peaks at shorter wavelengths (<250 nm) and shorter emission wavelengths (<350 nm) are associated with simple aromatic proteins such as tyrosine and tryptophan (Regions I and II). Peaks at intermediate excitation wavelengths (250–280 nm) and shorter emission wavelengths (<380 nm) are related to soluble microbial byproduct-like material (Region IV) while peaks located at the excitation wavelengths (200–250 nm) and the emission wavelengths (>380 nm) represent fulvic acid-like substances (Region III). Peaks at longer excitation wavelengths (>280 nm) and longer emission wavelengths (>380 nm) are related to humic substances (Region V) (Chen et al., 2003; Guo et al., 2014). The influence of pH on dissolved biopolymers located in different fluorescent regions was presented in Table 2. It was found that the cumulative fluorescent intensity of aromatic protein I and II and soluble microbial byproducts (actually is protein-like substances) weaken by 74.6%, 75.1% and 70.0% with decrease in solution pH from 7 to 2, while that of humic substances and fulvic acid were enhanced by 45.7% and 39.1% respectively. In addition, we also found that acidic treatment resulted in decline of fluorescent intensity of protein-like substances (aromatic protein I & II and SMP), which could not be recovered when the pH of sludge was readjusted to neutral. This observation indicated that chemical characteristics of proteins underwent irreversible change, which is always known as ‘‘protein denaturation’’. The fluorescent response of tryptophan-like proteins was mainly originated from their indole group which was unstable and could be readily damaged though protonation in presence of strong acid (Joule and Mills, 2010). According to many reports, acidic treatment was used to extract the EPS from activated sludge (Sheng et al., 2005, 2010). However, it was noticeable that acidic treatment significantly destroyed the chemical structure of protein and caused analytical errors, so this method is not advisable. Again, protein-like substances in soluble EPS were believed to be the major factor affecting sludge dewatering property (Yu et al., 2008; Zhang et al., 2014a). Hydrolysis and aggregation of proteins might be the main mechanism for sludge dewaterability improvement in acid environment.

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Table 2 Influence of pH on EEM FRI of soluble EPS. pH

Aromatic protein I

Aromatic protein II

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SMP

Humic acid-like

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18,927 17,426 38,791 51,194 57,759 74,549

25,384 22,646 50,433 69,588 79,397 101,969

32,022 25,951 24,604 23,706 20,864 19,512

37,157 30,520 67,748 94,732 107,274 123,674

114,914 86,152 58,725 66,531 71,918 78,884

3.2. Digested sludge conditioning with Fenton’s reagents 3.2.1. Optimization of Fenton process for sludge conditioning The effects of reaction conditions on sludge dewaterability can be found in Fig. S4. It can be seen from Fig. S4(a) that conditioning efficiency of Fenton process was significantly enhanced as pH was decreased from 7 to 4, and no obvious improvement was observed with further drop in pH. It is well known that Fenton oxidation was strongly dependent on reaction pH. Ferric ion starts to hydrolyzeprecipitate and lose its catalytic activity via reactions (2) and (3) at the pH greater than 4 (Zhang et al., 2013, 2014b). Fenton’s oxidation that is possessed of the advantages of both hydroxyl radical oxidation and hydrolyzed iron coagulation processes (Neyens and Baeyens, 2003a):

Fe2þ þ H2 O2 ! Fe3þ þ  OH þ OH

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As presented in Fig. S4(b), CST reached the minimum at 18 s when the [Fe2+]/[H2O2](mol/mol) was 0.1, and further increase of molar ratio resulted in slight decline of sludge dewatering performance. When the [Fe2+]/[H2O2](mol/mol) was less than 0.1,

the oxidative reaction was inhibited (Zhang et al., 2014b). The ferrous ions reacted very rapidly with hydrogen peroxide to produce hydroxyl radicals (reaction (1)), and then ferric ions were reduced to ferrous ions by hydrogen peroxide via reactions (3) and (4). However, the rate of oxidation in the second stage (ferric system) was much slower than in the first one due to the low regeneration rate of ferrous from ferric ions (Zhang et al., 2013). On the other hand, Ferrous can react with H2O2 to generate OH, but it can also act as scavengers of OH through reaction (4), leading to decreased conditioning efficiency of Fenton process (Pignatello et al., 2006). 

OH þ Fe2þ ! Fe3þ þ OH

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From Fig. 4(c), minimum CST value of 16 s was achieved at the H2O2 dosage of 0.3% (v/v). Again, as depicted in Fig. S5, dewatered sludge cake moisture was decreased with the dosage of Fenton’s reagents. Cake moisture was reduced from 88.2% to 75.6% at the H2O2 dosage of 0.9% (v/v) in comparison to unconditioned sludge. As mentioned above, since the EPS is the main constituent of activated sludge and it has a high water-holding capacity. The EPS compression was the major mechanism of sludge conditioning with traditional inorganic flocculants which are incapable to remove intercellular water inside sludge flocs (Niu et al., 2013).

W. Zhang et al. / Bioresource Technology 181 (2015) 247–253

However, Fenton process performed well in destroying and dissolving the EPS fractions, consequently converting bound water into free water (Buyukkamaci, 2004; Lu et al., 2003).

3.2.2. Influence of reaction conditions on sludge floc morphology and solubilization of bound EPS Effects of Fenton oxidation conditions on average sludge floc size and dissolved organic carbon (DOC) were presented in Fig. 3.

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It can be seen from Fig. 3(a) that both sludge floc size and DOC reached the maximum at pH of 3. Fenton oxidation exhibited the strongest activity at pH of 3 and hence it was most efficient to degrade the bound EPS. As a result of EPS structure breakage, electrostatic repulsion between sludge particles weakened, leading to particles aggregation and growth. The maximum EPS solubilization efficiency was achieved at the [Fe2+]/[H2O2] molar ratio in the range from 1:5–1:20, and the DOC was increased to 180 mg/ L–185 mg/L (Fig. 3(b)). When the [Fe2+]/[H2O2] molar ratio was more than 1:5, EPS solubilization efficiency of Fenton process was reduced due to the scavenging effect of overdosed ferrous ions. Furthermore, as mentioned earlier, Fenton’s reagents can serve the dual function of oxidation and coagulation. Addition of excessive ferrous ions might cause charge reversal and restabilization of colloidal system and thus reduction of floc size distribution. By contrast, ferrous ions were not very sufficient to catalyze oxidative reaction at [Fe2+]/[H2O2] molar ratio below 1:20, hence Fenton process was less effective to dissolve the EPS. No obvious change in DOC concentration was observed at the 30% H2O2 doses less than 0.2% (v/v), while DOC content increased almost linearly with H2O2 dosage (shown in Fig. 3(c)). It was very likely that Fenton’s reagent reacted more readily with soluble biopolymers and could not break down bound EPS at low doses. The impact of Fenton treatment on sludge morphology was presented in Fig. S6. The microscopy analysis revealed that the raw sludge had a porous and irregular structure and the EPS was distributed as glue in the voids between sludge cells with different shapes. After Fenton conditioning, most of the tiny sludge particles disappeared, and sticky EPS fraction was effectively solubilized and destroyed. Furthermore, the sludge floc became denser and more uniform due to Fenton coagulation. As shown in Fig. 4, Fenton process could effectively solubilize the sludge and decomposed the protein-like substances (Peaks A and B) at acid conditions (pH < 4), acidification and oxidation exhibited a significant synergistic effect. It was very interesting to note that DOC content was significantly higher at acid conditions and no obvious variation in EEM spectra could be detected, indicating that release and degradation of EPS were synchronous. Table 3 presented the effects of Fenton oxidation on FRI of soluble EPS at different pHs. Cumulative fluorescent intensities of aromatic protein I and II, fulvic acid, SMP and humic substances were reduced by 77.4%, 81.4%, 65.0%, 85.3% and 55.1% at pH of 2 compared to that at neutral condition. Since the sticky protein-like substances were proven to be the key constituent affecting sludge dewaterability (Yu et al., 2008), the breakage and oxidative degradation of them was the major mechanism for sludge dewaterability improvement in Fenton conditioning process. In addition, it was found that DOC content was greatly increased with increasing of H2O2 dosage while no obvious variation in EEM spectra was detected (Fig. S7). From Fig. S8, it can be seen that high performance size-exclusion chromatography analysis also confirmed that the intensities of all peaks were significantly reduced after acidification and Fenton treatment, especially the organics with high molecular weights were effectively destroyed. 3.3. Pilot test of high-pressure dewatering of anaerobic digested sludge using Fenton reagents as conditioners

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The 400 L of digested sludge was homogenized by stirring at 200 rpm for 5 min in a 1000 L conditioning reactor equipped with a paddle agitator. At first, the pH of sludge was adjusted to around 4 with 98% sulfuric acid (H2SO4). A desired amount of 20% (w/v) ferrous sulfate (FeSO4) solution was dosed into the reactor at the stirring speed of 200 rpm. Then, 30% (w/v) H2O2 was added and mixed with sludge and stirred at 200 rpm for a further 60 min. Lastly, the industrial-grade lime (Ca(OH)2) was used to adjusted

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Table 3 Influence of pH on EEM FRI of DOM in sludge supernatant after Fenton conditioning. pH

Aromatic protein I

Aromatic protein II

Fulvic acid-like

SMP

Humic acid-like

2 3 4 5 6 7

15,311 17,426 17,877 42,983 52,220 67,788

16,694 19,649 20,827 47,021 67,287 89,885

9913 11,343 10,398 17,911 23,379 28,299

15,788 17,182 22,743 52,262 78,594 107,107

31,028 31,867 31,354 45,210 56,863 69,174

the sludge pH to 7. Filtration compression dewatering was conducted with a diaphragm plate-and-frame filter press (shown in Fig. S9) equipped with 6 plates. Finally, the sludge was dewatered by the filter press consisting of a 90 min pressing phase with a pressure of 0.6–0.9 MPa and a 30 min filter plate expanding phase with a pressure of 1.5 MPa. The moisture content and appearance of sludge cake were given in Figs. S10 and S11 respectively. Cake moisture content was reduced to 58.2% as only H2O2 of 0.1% was added. In addition, further increase in Fenton reagent doses resulted in continual decrease of cake moisture content. It should be pointed out that when dosages of ferric chloride (FeCl36H2O) and polyaluminium chloride (Al2O3 content = 28%) were 10% and 8% (optimal dosages based on CST test), the cake moisture content was decreased to 65.2% and 59.4% respectively. These results confirmed that Fenton process performed much better in dewatering than traditional conditioning methods. In order to evaluate of influence of Fenton conditioning on influent qualities on wastewater treatment, the DOC, NH+4-N and total phosphorus (TP) were determined. Fig. S12 showed that no significant variation in organic content was observed at the H2O2 dosage of less than 3% (v/v). Ammonium concentration in digested liquid was very high at 560 mg/L, which could be attributed to decomposition of proteinlike substances. Fenton process had no obvious effect on NH+4 concentration, since it was relatively resistant to chemical oxidation. TP was decreased from 40.2 mg/L to 6.2 mg/L when the dosage of H2O2 and ferrous were 0.1% (v/v) and [Fe2+]/[H2O2](mol/mol) = 0.1

respectively, but it didn’t change much with further increase in Fenton reagent dosages. Neyens et al. (2004) demonstrated that the biorefractory biopolymer with high molecular weights (MW) in sludge supernatant could be converted more biodegradable organics (organic acids) after Fenton treatment. They are very likely to enhance biological denitrification and improve nitrogen removal efficiency. Therefore, it can be inferred that full-scale application of Fenton process in sludge conditioning should have no detrimental effects on operation and performance of WWTP.

4. Conclusion This study attempted to investigate the effects of acidification and Fenton oxidation on physiochemical properties and dewaterability of digested sludge. It was observed that protein-like substances in SEPS were greatly destroyed after acidification treatment. Acidification and Fenton oxidation showed a significant synergistic effect in improving sludge dewatering and drying performance. Pilot-scale filter press dewatering test revealed that Fenton treatment was much more effective in reducing cake moisture content than traditional conditioners. Furthermore, the operating performance of wastewater treatment system won’t be negatively impacted by Fenton conditioning. These results indicated that Fenton conditioning was very promising in digested sludge treatment.

W. Zhang et al. / Bioresource Technology 181 (2015) 247–253

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